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CATALYSIS DEVELOPMENT TO ACHIEVE SUSTAINABLE SYNTHESIS OF ADIPIC ACID PRODUCTION WITH HYDROGEN PEROXIDE AS OXIDANT A REVIEW A Dissertation submitted to the University of Manchester for the degree of the Master of Science in the Faculty of Engineering and Physical Science 2010 ABDIL HALIMIS STANI SCHOOL OF CHEMICAL ENGINEERING AND ANALYTICAL SCIENCE

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Page 1: Master's Dissertation

CATALYSIS DEVELOPMENT TO ACHIEVE SUSTAINABLE

SYNTHESIS OF ADIPIC ACID PRODUCTION WITH HYDROGEN

PEROXIDE AS OXIDANT – A REVIEW

A Dissertation submitted to the University of Manchester for the

degree of the Master of Science in the Faculty of Engineering and

Physical Science

2010

ABDIL HALIMIS STANI

SCHOOL OF CHEMICAL ENGINEERING AND ANALYTICAL

SCIENCE

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TABLE OF CONTENTS

TABLE OF CONTENTS .................................................................................................................................. 2

LIST OF TABLES .............................................................................................................................................. 4

LIST OF FIGURES ............................................................................................................................................ 5

ABSTRACT ......................................................................................................................................................... 6

ACKNOWLEDGEMENT ................................................................................................................................. 7

DECLARATION ................................................................................................................................................. 8

COPYRIGHT STATEMENT ........................................................................................................................... 9

CHAPTER 1 - INTRODUCTION ............................................................................................................... 10

1.1Background ......................................................................................................................................... 10

1.2 Objectives and the Scope of the Review ............................................................................... 11

CHAPTER 2 – THE CONVENTIONAL PRODUCTION OF ADIPIC ACID.................................... 14

2.1 Introduction ...................................................................................................................................... 14

2.2 Two-step Oxidation of Cyclohexane ........................................................................................ 14

2.3 Phenol Route ..................................................................................................................................... 16

2.4 Environmental Concerns ............................................................................................................. 17

CHAPTER 3- OXIDATION OF THE CYCLOHEXANE TO OL/ONE BY USING

HOMOGENEOUS CATALYSTS ................................................................................................................. 19

3.1 Introduction ...................................................................................................................................... 19

3.2 Iron and Manganese Based Catalysts ...................................................................................... 20

3.2.1 Non-heme Mononuclear Iron and Manganese Complexes .............................. 20

3.2.2 Non-heme Binuclear Iron and Manganese Complexes .................................... 23

3.2.3 Non-heme Hexanuclear Iron Complexes ........................................................... 24

3.2.4 Heme Iron Complexes ......................................................................................... 24

3.2.5 Iron Salts .............................................................................................................. 25

3.3 Copper Based catalysts ................................................................................................................. 26

3.4 Vanadium Based Catalysts .......................................................................................................... 29

3.5 Hetero- Metallic Complexes ........................................................................................................ 30

3.5 Polyoxometallates (POMs) .......................................................................................................... 30

3.6 Hybrid System Metal Complexes - Polyoxometallates .................................................... 33

3.7 An Overview of Homogeneous Catalysis Development .................................................. 33

3.7.1 Summary of the Recent Research Progress ...................................................... 33

3.7.2 Future Prospect in Homogeneous Catalysis Development .............................. 34

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CHAPTER 4 – OXIDATION OF CYCLOHEXANE TO OL/ONE BY USING HETEROGENEOUS

CATALYSTS .................................................................................................................................................... 40

4.1 Molecular Sieve Catalysts ............................................................................................................ 40

4.1.1 Titanium Based catalysts .................................................................................... 41

4.1.2 Vanadium Based Catalysts .................................................................................. 41

4.1.3 Cobalt, Iron and Chromium Based Catalysts ..................................................... 43

4.1.4 Cerium Based Catalysts ...................................................................................... 44

4.1.5 Germanic Faujasite ............................................................................................. 45

4.2 Carbon Nitride Polymer ............................................................................................................... 46

4.3 Heterogenizing Metal Complexes into Solid Materials .................................................... 46

4.4 An Overview of Heterogeneous Catalysis developments ............................................... 49

4.4.1Summary of the Recent Research Progress ....................................................... 49

4.7.2 Future Prospect in Heterogeneous Catalysis Development ............................ 51

CHAPTER 5 – DIRECT OXIDATION OF CYCLOHEXENE TO ADIPIC ACID............................. 53

5.1 Biphasic reacting System (Phase-Transfer catalysts) ...................................................... 53

5.2 Molecular Sieves catalysts ........................................................................................................... 56

5.3 An Overview: Summary and Future Prospect of Direct Oxidation of Cyclohexene

to Adipic Acid ........................................................................................................................................... 57

CHAPTER 6 - CONCLUDING REMARKS AND OUTLOOK ............................................................. 61

GLOSSARY ....................................................................................................................................................... 63

REFERENCES ................................................................................................................................................. 65

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LIST OF TABLES

Table 1-1 Principles of green chemistry ............................................................................. 11

Table 1-2 Common oxidising agent used in the chemical industries ...................... 12

Table 3-1 Comparison of activities of copper catalyst for the cyclohexane

oxidation to Ol/One............................................................................................................ 28

Table 3-2 Cyclohexane oxidation catalysed by Polyoxometalates ........................... 31

Table 3-3 Comparison of catalysis performance with cobalt naphtalene versus

various homogeneous catalysts .................................................................................... 39

Table 4-1 Summary of heterogeneous catalysis for the cyclohexane oxidation to

Ol/One ..................................................................................................................................... 45

Table 4-2 Summary of the cyclohexane oxidation catalysed by immobilised

metal complexes .................................................................................................................. 47

Table 5-1 Cyclohexene oxidation via water-organic bi-phase catalytic system in

free organic solvent reactions ........................................................................................ 55

Table 5-2 Oxidation of Cyclohexene to adipic acid catalysed by molecular sieves

.................................................................................................................................................... 57

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LIST OF FIGURES

Figure 2-1 Conventional route of commercial production of adipic acid .............. 16

Figure 3-1 Oxidation mechanisms for hydrocarbon in the presence of iron

complex – hydrogen peroxide ........................................................................................ 21

Figure 3-2 In-situ preparation of Schiff-Base ligand ..................................................... 22

Figure 3-3 Molecular structures of complex [Fe (mqmp) (CH3OH) Cl2] ................ 23

Figure 4-1 Summary of researches on heterogenous catalysis developmen ....... 50

Figure 5-1 Direct oxidation of the cyclohexene to adipic acid ................................... 53

Figure 5-2 Reaction mechanisms of the direct oxidation of cyclohexene to adipic

acid ........................................................................................................................................... 53

Figure 5-3 Summary of possible synthetic pathways to AA from direct oxidation

of cyclohexene ...................................................................................................................... 59

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ABSTRACT

Adipic acid is a valuable chemical intermediate used for production of a wide range of materials. Currently, the major adipic acid production uses two-step oxidation of cyclohexane, which proceeds using air and nitric acid as oxidants. However it is carried out under harsh operating conditions, requires cumbersome separations, and accounts for 5-8% of the total global anthropogenic emission of N2O. Hydrogen peroxide is an attractive alternative oxidant because it is capable of carrying the reaction under mild conditions and producing benign by-products. This paper therefore will discuss the catalysis development to achieve a more sustainable synthesis of adipic acid production using hydrogen peroxide as oxidant. This review consists of three main parts. The first two parts are concerned with the homogeneous and heterogeneous catalysis for the oxidation of cyclohexane to the cyclohexanol-cyclohexanone, whilst the last part concentrates on one-step oxidation of cyclohexene to adipic acid. Development of various ligands for homogeneous systems has facilitated significant improved of activities, but the catalysts are often unstable. As for heterogeneous catalysis, many transition metals substituted molecular sieve catalysts shown to be active, but they sometimes lead to leaching of metal. New families of catalysts, e.g. rare earth metals, germanium faujasite and carbon nitride polymer have been shown to be more stable. The biggest barrier to overcome is to combine the advantages of homogeneous and heterogeneous catalysts. Direct synthesis of adipic acid from cyclohexene can be carried out using molecular sieve or biphasic transfer catalysts. This pathway is advantageous because it eliminates one step process and avoids the N2O emission, but the drawback is the relatively cost of feedstock and hydrogen peroxide. It can be concluded that adipic acid synthesis using hydrogen peroxide is unlikely to substitute the current technologies because of low efficiencies of hydrogen peroxide utilisation arise in many cases. However, research in this field remains to attract a great interest and would not close the possibility of implementation in commercial level in the future.

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ACKNOWLEDGEMENT

I would like to thank and praise to Allah Almighty for each of His generous

blessing and may peace and blessings upon Prophet Muhammad SAW, my

family for their everlasting support and my friends for our colourful days in

Manchester.

I express my sincere gratitude to my supervisor, Dr. Sven L.M Schroeder, which

has reviewed my dissertation and broadened my knowledge.

I would also thank to Centre for Environmental Technology BPPT Indonesia for

giving me permissions in continuing my study.

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DECLARATION

I declare that no portion of the work referred to in the dissertation has been

submitted in support of an application for another degree or qualification of this

or any other university or other institute of learning.

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COPYRIGHT STATEMENT

Copyright in this text of this dissertation rests with the author. Copies (by any

process) either in full, or extract, may be made only in accordance with

instruction given by the author. Details may be obtained from the appropriate

Graduate Office. This page must form part of any such copies made. Further

copies (by any process) of copies made in accordance with such instructions

may not be made without the permission (in writing) of the author.

The ownership of any intellectual property rights which may be described in

this dissertation is vested in the University of Manchester, subject on any prior

agreement to the contrary, and may not be available for use by third parties

without the written permission of the University, which will prescribe the terms

and conditions of any such agreement.

Further information on the conditions under which disclosures and

exploitations may take place is available from the Head of the School of

Chemical Engineering and Analytical Science.

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CHAPTER 1 – INTRODUCTION

1.1 Background

Adipic acid, a straight-chain dicarboxylic acid, is one valuable chemical as it is a

raw material for the production of Nylon 6.6 and its derivatives can be used in a

wide range of application such as resins, plasticisers, food acidulant, lubricant,

and polyurethanes (1). In 2008 the global production capacity of adipic acid

was achieved at about 2.6 million a metric tonne of which at about 60% of

adipic acid is used for production of Nylon 6.6 and fibres, whereas

polyutherethanes accounted for almost 24% of total consumption (2). Adipic

acid market is growing rapidly with annual global growth is nearly 3%, whilst

demand is growing in the range of 5% to 6% annually by 2010 (3).

The important milestone of adipic acid production began in around 1935

when DuPont team invented Fibre 6.6 through laboratory research and then

commercially introduced as Nylon 6.6 in 1938 (4). In around 1939 the first

commercial adipic acid was operated in West Virginia by using phenol as a raw

material, however the increase of phenol price in around 1942 led to start up a

new route via two-step oxidation of cyclohexane (1).

This method is produces a relatively high yield and the most

economically viable route, however it potentially generates considerable

amount of nitrous oxides, which account for 5-8% of the total global

anthropogenic emission of N2O or at about 400, 00 metric tonnes N2O emission

per year (5). Moreover the presence of homogeneous catalyst, and/or corrosive

solvent initiator is likely to result in cumbersome separations and leaching of

metals. Furthermore the oxidation of cyclohexane possess inherent hazard

during the processes due to harsh operating conditions.

Although the installation of N2O abatement technologies is likely to

decrease the N2O emissions with the efficiency range is 90% to 99% reduction

of N2O emissions (6), however the prevention of waste is strongly preferred

over the treatment or clean up after it generated. Moreover the concepts of

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green chemistry and cleaner production have been widely introduced to replace

‘command and control’ approach for environmental protection since around

1990 (7). The twelve principles of green chemistry are shown in Table 1 (8).

These offers a standpoint of the greener technologies can be viewed and now

become important guidelines to reduce or eliminate the hazardous compounds

in the design, manufacture, and application of chemical products (9).

Table 1-1 Principles of green chemistry (8)

1.2 Objectives and the Scope of the Review

Oxidising agents may affect the formation of product and the rate

transfer of oxygen to the substrate (10). The selection of oxidant, therefore,

becomes a relevant factor in accomplishing greener processes. A number of

different types of oxidants are displayed in Table 1. It can be seen that several

oxidising agents such as KMnO4 and CrO3, are not interesting to be exploited

because they may result in toxic salts (11), whereas N2O and HNO3 should be

avoidable due to its potential aerial emission pollutant by-products. NaClO may

offer relatively high oxygen content, but in several occasions, ClO- can form toxic

1. It is better to prevent waste than to treat or clean up waste after it formed 2. Synthetic method should be designed to maximise the incorporation of all materials

used into the final products 3. Wherever practicable, synthetic methodologies should be designed to use and

generate substances that posses little or no toxicity to the human health and the environment

4. Chemical products should be designed to preserve efficacy of function while reducing toxicity

5. The use of auxiliary substances (e.g. solvents, separation agents, etc.) should be made unnecessary wherever possible and, innocuous

6. Energy requirements should be recognised for their environmental and economic impacts and should be minimised. Synthetic methods should be conducted at ambient temperature and pressure

7. A raw material of feedstock should be renewable rather than depleting wherever technically and economically practicable

8. Unnecessary derivatisation (blocking group, protection/deprotection, temporary modification of physical/chemical processes) should be avoided whenever possible

9. Catalytic reagents (as selective as possible) are superior to stoichiometric reagents 10. Chemical products should be designed to preserve efficacy of function while

reducing toxicity 11. Analytical methodologies need to be developed to allow for real-time, in-process

monitoring and control prior to the formation of hazardous substances

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and carcinogenic chloro-carbons (11). C6H5IO, on the other hand, is frequently

effective in metal catalysed oxidation, but it is quite expensive (11).

Hydrogen peroxide, ozone, molecular oxygen and tert-butyl peroxide

seem to be the most interesting choices to be exploited because their high

oxygen content and leave no harmful residuals. Molecular oxygen would most

often to be preferred because of the economical advantage arising from its

abundance in air, but O2 commonly requires high temperatures and pressures

to be activated (10). Tert-butyl peroxide has quite high solubility relative to

hydrogen peroxide and molecular oxygen. However it contains a relatively low

active oxygen species (12). Ozone, O3, is a powerful oxidant, but rarely

investigated (13) since the cost of its production is prohibitive. Hydrogen

peroxide, therefore, remains the most attractive oxidant from a combined

environmental and economic point of view. The potential advantages intrinsic

in the use of hydrogen peroxide are quite evident due to its ability to carry out

reaction in the relatively mild conditions, producing a benign by-product

(water) while having high oxygen content.

Table 1-2 Common oxidising agent used in the chemical industries (11; 14)

In a number of studies, catalytic approaches have been attempted to

develop greener and more sustainable process for adipic acid productions. This

research paper is, therefore, intended to make a comprehensive review of the

Oxidant Active oxygen content (%) Product

O2 100 Nothing or H2O

O2/reductor 50 H2O

H2O2 47 H2O

N2O 36.4 N2

O3 33.3 O2

KMnO4 30.4 Mn(II) salts

HNO3 25 NOx

CrO3 24 Cr(III) salts

NaOCl 21.6 NaCl

CH3COOH 21.1 CH3COOH

tBuOOH 17.8 tBuOOH

C5H11NO2 (NMO) 13.7 C5H11NO (NMM)

KHSO5 10.5 KHSO4

mClC6H4COOOH 9.3 mClC6H4COOH

Me3SiOOSiMe3 9 Me3SiOOSiMe3

NaIO4 7.5 NaIO3

PhIO4 7.3 PhI

C6H5IO 7.3 C6H5I

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development of catalysis to obtain adipic acid in high yields in a manner that is

environmentally friendly and economically viable in the presence hydrogen

peroxide as oxidant. We also limit the scope of this review to cyclohexane

oxidation and single step oxidation of cyclohexene to produce adipic acid.

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CHAPTER 2 – CONVENTIONAL TECHNOLOGIES FOR

ADIPIC ACID PRODUCTION

2.1 Introduction

Cyclohexane (C6H12) and phenol (C6H5OH) are the common raw materials from

which adipic acid is produced. Two-step oxidation of cyclohexane accounts for

about 95% of worldwide production and is therefore the common method for

the commercial production of adipic acid (15). Adipic acid production based on

phenol, on other hand, is rarely used due to several reasons, which include the

availability of starting materials, plant size and capital investment (16). Figure

2-1 presents the commercial pathways of adipic acid production.

2.2 Two-step Oxidation of Cyclohexane

The commercial operation of this process was first introduced in 1942 as

a batch oxidation process, but as manufacturing process demand grew

considerably; productivity became an important consideration. As a result, a

continuous oxidation has become the preferred method of operation (1). The

basic technology underpinning this route remains similar that to introduced in

the early production plants. The variation of current commercial production is

commonly associated with the manufacturing of KA oil- also referred to “KA

Mixture” or Ol/One- (17). The term KA oil arises from the fact that it contains

cyclohexanone, a ketone (K), and cyclohexanol, which is an alcohol (A) (18).

In the first step, cyclohexane is oxidised by air in the liquid phase to form

cyclohexanone-cyclohexanol at temperatures of 125 to 165°C at pressures of 0.8

to 1.5 MPa (19). This range of temperature achieve favourable reaction rates,

whilst operating pressures above 0.8 MPa are required in order to maintain

cyclohexane in the liquid phase. Without a catalyst or an initiator, this step can

be done within 30 min at 165°C; however the presence of soluble metal

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catalysts (e.g. cobalt naphthenate, cobalt 2-ethylhexanoate, manganese or

chromium naphthenate) and, additionally, aldehyde or ketone initiators can

shorten the reaction to about 5 min. The economical process is normally used at

conversions between 6% to 9% in order to achieve selectivity in the range of

60% to 80% (1; 20). The low conversion of cyclohexane is necessary to prevent

over-oxidation since KA oil reacts more readily than cyclohexane, inviting

further reaction to form cyclohexyl radicals (1; 21).

The unreacted cyclohexane, then, is separated and recycled by

azeotropic distillation with steam or vacuum distillation (1; 16). Significant

improvement of the process has been made since Baskirov and co-workers

revealed in the late 1950s the importance of using boric acid (HBO2) to improve

hydrocarbon oxidation (22). This method was subsequently adopted by many

companies such as Halcon Inc, IFP, Stamicarbon Technology and Institut

Francais du Petrole. With the aid of boric acid, more satisfactory selectivity

(85% to 90%) at conversion levels of about 4% to 15% can be achieved (1; 16).

The overriding reason of this improvement is the formation of cyclohexyl

borate that is capable of protecting cyclohexyl group, thus, the formation of

cyclohexyl radicals as intermediate can be avoided. However the advantages of

this technology are to some extent offset by the necessity to synthesise boric

acid, to treat cyclohexane-boric acid slurries and to recover boric acid through

crystallisation, centrifugation, and dehydration (1). Nevertheless both methods

(with and without boric acid) are commonly used in the current commercial

productions.

In the second stage, the KA mixture is converted to adipic acid by

oxidation with nitric acid in the presence of copper and vanadium catalysts. As

nitric acid is used as the oxidising agent, it is reduced to NO2, NO, N2O and N2.

The manufacturing plant is only capable of capturing NO2 and NO, whilst N2O

and N2 are considered as nitric acid loss as they can not be recovered within the

system (5). This process is not only very rapid, but also highly corrosive and

exothermic; thus heat removal and control of reaction are necessarily required.

In order to accomplish the oxidation process, a high proportion of nitric acid

must be maintained. The suitable ratio of nitric acid and KA mixture to complete

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oxidation process is at about 40:1(1; 16). Adipic acid yields of 93% to 95% are

typically achieved by this route (23).

Benzene

Phenol Cyclohexane

Cyclohexanol

Cyclohexanone

(KA mixture)

Hydrogenation

OH

nitric acid oxidation

+H2

Cyclohexanol

OH

+

O

HOOC

Adipic Acid

HOOC

Co, V

Co, Ni, Cr

Co, V

Co, Cr, MnHBO2

air oxidation air oxidation

Figure 2-1 Conventional route of commercial production of adipic acid (1; 16)

2.3 Phenol Route

Phenol can be hydrogenated to either cyclohexanol or cyclohexanone-

cyclohexanol in the molten state, a reaction that usually takes place in the

presence of nickel, copper or chromium catalyst at around 150°C and 0.3 MPa

(1; 16). The selection of catalyst and operating condition can affect the

formation of cyclohexanol or cyclohexanone (16). Cyclohexanone is not

commonly used to produce adipic acid as it leads to a low yield. It is widely used

for the preparation of caprolactam, thus the hydrogenation process is designed

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to produce a high selectivity to cyclohexanol. With proper control and catalyst,

no less than 99% phenol can be converted to cyclohexanol for the selectivity

range between 97% to 99%, whilst the small amount of unconverted product

can be removed through distillation (16).

2.4 Environmental Concerns

There are a number of environmental issues associated with the conventional

two-step oxidation of cyclohexane. The issues are mainly associated with the

use of boric acid or homogeneous Co, Cr or V metal salt catalysts and the HNO3

as oxidising agent, which can lead to both large amounts of atmospheric and

liquid wastes.

In the first step, oxidation process having inherent hazard since it is carried

out in the relatively harsh condition (high temperature and pressure) and

flammable and explosive potential of vapour mixture of cyclohexane with gas

containing oxygen (1). This step commonly forms catalyst and organics waste

from ketone-alcohol purification. Oily water, low pH streams containing adipic

acid, boric, glutaric and succinic acid with copper, vanadium and sulphuric acid.

Several special control techniques, therefore, may be required to handle these

wastes (24):

- Ion exchange systems to remove organic salts such as copper or

vanadium salts from organic catalyst

- Evaporation and crystallisation to recover boric acid and other by-

products

- High efficiency biological treatment to treat remaining wastes

- Optimised phase separation and extraction with incineration of

organic phase to reduce organic loads

These processes are typically consumes a large amount of energy because

considerable amounts of solids need to be separated, decomposed and boric

acid has to be recycled. Thus the operational costs of this process are commonly

high (20). Moreover, the leaching of heavy metal (from homogeneous cobalt

catalyst) is still frequently unavoidable (25) or, in case of using boric acid as

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promoter, slurry contains mixture of cyclohexane boric-acid also becomes a

great concerns (1).

In the second stage, the KA mixture is catalytically (copper, vanadium salts)

oxidised with nitric acid. This process is highly exothermic, and the presence of

nitric acid solutions of organic acid, particularly with vanadium, may lead to

extremely corrosive conditions (1). Several issues associated with this step are:

- The process releases considerable amount of nitrous oxide from the

stripping columns and crystallisers which can be estimated to produce at

about 0.3 kg N2O per kg of adipic acid (26). If N2O is re-used, it can be

utilised either by burning at high temperatures in the presence of steam

to manufacture adipic acid or using N2O to selectively oxidise benzene to

phenol. If N2O is released to the atmosphere, either catalytic

decomposition or thermal destruction is required as end-pipe

techniques. Both heat from exothermic reaction and combustion process

may be used to raise or produce steam [18]. Other airborne pollutants

involved are adipic acid particulates generated from drying and

handlings, organic material originating from feedstock, absorbers and

purification columns on the KA section and fugitive-hydrocarbon

emitted through normal venting in the hydrocarbon-containing tanks (1;

24).

- Strong odour is potentially produced from acid and handling storage

since the process produces 10 to 20% by-product such caproic, adipic,

valeric, butyric, acetic and propionic acid. Furthermore there are

difficulties to separate and utilise the by-product obtained, and large

amount of base typically required to neutralise the acid (20). Other

liquid wastes are released from ketone-alcohol catalyst, plant cleaning,

non-volatile organic residues and organic recovery tails from KA mixture

production, boric acid sweepings, oxidiser residues, caustic wash

residues, ketone alcohol sump dredging and wastes on shutdown such as

tar-contaminates sand (24).

Considering these impacts above, there are challenges for engineer and scientist

currently is to find out more attractive and environmentally friendly routes that

can address both economic and technical point of view.

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CHAPTER 3- OXIDATION OF THE CYCLOHEXANE TO

OL/ONE BY USING HOMOGENEOUS CATALYSTS

3.1 Introduction

The selective oxidation of cyclohexane is a challenging problem due to its inert

C-H bonds (27). The classical process uses relatively high temperatures and

pressures to initiate the cleavage of C-H Bond. In the first step of the oxidation,

the conversion of cyclohexane must be maintained in the range of 4% to 7% in

order to prevent over oxidation and affords selectivity up to 90% (11; 16; 27).

The concentration of a KA mixture of 0.4 mol/L and alcohol/ketone ratio of

60:40 is typically attained after 40 min (27). Besides its low conversion, the

reaction commonly leads to undesirable features such as CrIII salts or boric acid

and their problematic disposal (1). The greener routes of cyclohexane oxidation

therefore is expected to find an ‘ideal’ catalyst that is stable and capable of

producing approach 100% conversion and selectivity toward Ol/One in

relatively milder conditions.

Scuchardts et al (27) and Cavani et al (3), in their reviews regarding the

synthesis of adipic acid, pointed out that oxygen was the most preferred

terminal oxidant to achieve an environmentally and economically sustainable

process for the cyclohexane oxidation to Ol/One. This the case because of its

lower price, higher yields and due to the fact that the reaction can be carried out

in the free solvent reaction. These advantages may be offset with the harsh

condition required to carry out the reaction. The oxidation under hydrogen

peroxide is much less energy intensive than that of air or molecular oxygen. The

reaction temperature can be as low as 60°C, or even at room temperature, and

pressures as low as 1 atm. This chapter will review greener oxidation of

cyclohexane to cyclohexanol-cyclohexanone (Ol/One) using various

homogeneous catalytic system such as various metal salts, metal complexes,

and polyoxometallates in the presence of hydrogen peroxide as oxidant.

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3.2 Iron and Manganese Based Catalysts

3.2.1 Non-heme Mononuclear Iron and Manganese Complexes

A review regarding of selective oxidation of hydrocarbons in biomimetic non-

heme iron and manganese catalysts using hydrogen peroxide was published by

Tanase and Bouwman in 2006 (28). Enzymatic monoxygenase is capable of

oxidising several hydrocarbons and halocarbons (11; 29); biomimetic systems

therefore have been used in attempts to perform the reaction under relatively

mild conditions. Both non-heme and heme- (e.g. porphyrin group bearing) iron

and manganese complexes are often to be used to stimulate the enzymatic

action of organic substrates oxidation (28). Besides requiring low energy, these

non-heme metal–hydrogen peroxide systems have inherent environmental

advantages since dioxygen atoms is split yielding one O atom that is added to a

hydrocarbon molecule to produce a hydroxyl group, whilst the other atom O is

used to produce water, a benign by-product (30). The substrate oxidation

proceeds as follows (28)

RH + H2O2 ROH + H2O (3.1)

Manganese and iron (either mononuclear or binuclear) complexes are

common metals used to conduct the reaction with hydrogen peroxide due to

their accessibilities. Acetonitrile or acetone is required as an organic solvent in

order to increase efficiency of hydrogen peroxide utilisation, but acetonitrile is

frequently preferred because it facilitates higher efficiency and avoids hazards

associated with the reaction between hydrogen peroxide and acetone, which

can produce highly explosive acetone peroxide adduct as a by-product (32). The

possible reaction patterns of cyclohexane oxidation with hydrogen peroxide by

mononuclear iron complexes fall into two pathways. On one hand uncontrolled

hydroxyl radicals as follows (eq. 3.2 and 3.3) (28)

FeII + HOOH FeIII + HOO• + HO- (3.2)

FeIII + HOOH FeII + HOO• + HO+ (3.3)

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On the other, the formation of high valent metal iron based oxidant can occur as

presented in figure 3-1 below (28)

LFeII + H2O2

LFeIV = O

LFeV = O

HO-

+

LFeIII - OOHRH

ROH

+HOo

Figure 3-1 Oxidation mechanisms for hydrocarbon in the presence of iron

complex–hydrogen peroxide (28)

The homolytic cleavages in the equations 2 and 3 are commonly recognised as

the Fenton reaction (14). Various mechanisms have been observed, which

depend on operating conditions and compete in different ways (33). The high

valent LFeIV=O species, however, is the key feature of oxidation catalysed by

non-heme metal complex-H2O2 since homolysis of the O=O bond may able to

generate short lived HO• that is rapidly reacted to the cyclohexane to form

alcohol as predominant product (28). An A/K ratio > 1, therefore, indicates that

short lived HO• is formed by metal-based oxidant.

Different kinds of ligands for the metal centre have to be prepared and

designed to enhance the selective oxidation of alkenes. These ligands play an

important role in controlling the reactivity of metal ions by binding one or two

metal centres and donating electron to achieve higher oxidation states (28). In

the case of iron and manganese complexes, the ancillary N-donor and O-donor

ligands are able to stabilise high-valent manganese species in order to fasten C-

H bond activation through ‘hydrogen abstraction’ (28). Current developments

emphasise on modifying ligands properties or synthesising ligands that are

stable under oxidation condition without suffering extensive degradation as

well as capable of donating electron (28).

Oxidation of cyclohexane using pyridine-based pentadentate ligands

(N4py) has been reported by Feringa and co-workers (34). Using acetonitrile as

the solvent, the selectivity of the complex [Fe(N4py)(CH3CN2](ClO4)2 gave a

moderate conversion based on oxidant (31%) at 25°C. Iron complexes and new

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ligands based tetra dentate ligands with an amide donor inside the molecule,

bis(2-pyridyl)methyl-2-pyridylcarboxamide (tpcaH) (35), gave a relatively low

turnover and efficiency (12%). Better results were obtained by Chen and Que

(36) using a [Fe(bpmen)(CH3CN)2](ClO4)2 [Bpmen = N,N’-dimethyl-N,N’-bis(2-

pyridylmethyl)-1,2-diaminoethane] catalyst. The complex is capable of

oxidising cyclohexane and leading to the corresponding alcohols and ketones in

a conversion up to 63% with Ol/One ratio of 8.

Fairly good results have also been reported by Fernandes et al (37) using

iron complex [Fe (gma) (PBu3)] [where H2 (gma) = glyoxal-bis (2-

mercaptoanil)], bearing an {FeN2S2} centre. A yield of 11.7 % was obtained

under mild conditions (25°C), after 4 h in acetonitrile; with oxidant to

cyclohexane ratio of 1.2. A co catalyst was used to carry out the reaction that

performs under atmosphere of N2 and O2 (1 atm).

The most impressive achievement of oxidation cyclohexene in the

presence of mononuclear metal complexes was achieved recently by Nayak and

co-workers (38). They used a coordination of transition metal iron (III) and a

Schiff-base ligand, 2-methoxy-6-((quinolin-8-ylimino) methyl)phenol (mqmp).

It can easily be synthesised in-situ from the reaction between o-vanillin and 8-

aminoquinoline as presented in Figure 3-2.

OHCNH2

N

OCH3HO

MeOH

+N

N

OCH3HO

(MQMPH)

Figure 3-2 In-situ preparation of Schiff-Base ligand (38)

The structure of the catalyst is depicted in Figure 3-3. Three donor atoms

of the ligand mqmp- (O, N, N) surrounding the iron (III) centre result in a rigid

coordination. A total cyclohexane conversion and selectivity toward KA oil up to

91% can be attained in the presence of [Fe (mqmp) (CH3OH) Cl2] catalyst in

acetonitrile at 50°C after 24 h, with an oxidant to cyclohexane ratio of 1.5. A

mechanistic pathway study using DMSO as hydroxyl-radical scavenger

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indicated that highly active hydroxyl radicals were only involved in the least

part of the reaction.

Figure 3-3 Molecular structures of complex [Fe (mqmp) (CH3OH) Cl2] (38)

3.2.2 Non-heme Binuclear Iron and Manganese Complexes

Binuclear iron and manganese complexes are designed to mimic methane

monoxygenase (MMO). Pioneering work on the oxidation reaction of various

hydrocarbon catalysed by binuclear iron complexes was carried out by the

researchers of the University of California in the early 1990’s (39). The iron

complexes [Fe2O(OAc)Cl2(bpy)2] and [Fe2O(OAc)(tmima)2](ClO4)3, (tmima =

tris[(I-methylimidazol-2-yl)methyl]amine were tested to oxidise cyclohexane at

ambient temperatures. However the reactions exhibited low activities and

relativities with maximum turn over number (TON) of 15 and efficiencies as

low as 8%. A better result was achieved by microwave assisted radiation in the

presence of Fe complexes containing BMPA (bis-(2-pyridylmethyl) amine) and

BMPA-derivative ligands (40). The experiment showed that the complexes of

[Fe(BMPA)Cl3] was the most active systems, giving after 25 min at 160°C under

100 W microwave irradiation, substrate conversions of 16.9% with the

following product distribution: 28.9% cyclohexanol, 25.7% cyclohexanone and

34.5% adipic acid. Binuclear manganese complexes, on the other hand, have

been extensively tested by Shul’pin et al (41). The manganese(IV) complex

catalyst with 1,4,7-trimethyl-1,4,7-triazacyclononane afforded a cyclohexane

conversion of 46% at 25°C after 2 h, but the efficiency was still relatively low

(30%). More recently Shul’pin (42) suggested the use of pyrazine-2, 3-

dicarboxylic acid (2, 3-PDCA) as co-catalyst to accelerate the reaction.

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3.2.3 Non-heme Hexanuclear Iron Complexes

Trettenhahn et al (43) reported the catalysis performance of the hexanuclear

iron p-nitro benzoate [Fe6O3 (OH) (p-NO2C6H4COO) 11(dmf)4] with an [Fe6 (µ3-O)

3((µ2-OH)] 11+ core. A fairly sophisticated yield of about 30% was obtained in

acetonitrile with temperature control (6.5, 15.4, 25.4 and 38.8°C) after 1 h.

Although this metal complex performs notable activities, an unfavourable

feature of this system is a high ratio of HP-Cyclohexane (10 to 40 mmol) that is

required to achieve such a yield. These operating conditions therefore seem to

be rather cost prohibitive for commercial application.

3.2.4 Heme Iron Complexes

In 1987, Mansuy (44) reported that cytochrome P-450 can catalyse the

oxidation of organic substrate through a monoxygenase reaction. The reaction

is given in equation 3.4 as follows (44)

RΗ + Ο2 + 2Η+ + 2e P -450 RΟΗ + Η2Ο

In the presence of two electrons and two protons, one atom of dioxygen is

inserted into an organic substrate, whilst another atom forms water as by-

product. As iron porphyrin is an active site of cytochrome P-450, the selective

oxidation of cyclohexane catalysed by metalloporphyrin system has been a

great interest for chemical scientist. The reaction catalysed by cytochrome P-

450 in the presence of hydrogen peroxide (metalloporphyrin/O2/H2O2 system),

being analogue with biological system for hydrocarbon oxidation, can be called

a shunt system because it utilises oxidising agent to donor an active oxygen

species (45).

Unlike non-heme metal complexes; molecular oxygen or t-butyl hydro

peroxide (TBHP) are more often to be used as oxidising agent in this reaction. A

number of publications reported unsatisfactory results of cyclohexane and

hydrocarbon oxidation catalysed by metalloporphyrin-hydrogen peroxide

systems. For instance, the catalysis performance of [Fe (TPP) Cl]

(TPP=tetraphenylphorphyrin), R4PFeCl, and R4MnPFeCl (R= Complexes of

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meso-tetrakis (3, 5-ditert-butyl-4-hydroxypenyl) porphyrin) that only gave a

yield of Ol/One below 6% (46; 47).

The reason of this poor performance has been delineated by Giorgio

Strukul (14) and later reinvestigated by E.I Karasevich and Y.K Karasevich (45).

They described the mechanistic pathway of the shunt system and demonstrated

the dismutation of hydrogen peroxide in water and dioxygen by heme iron

complexes that derive from the production of hydroxyl radical due to the

homolytic cleavage of the peroxide O-O bound. The generation of HO• then

would result in mutagenic or lethal event to biological system, hence lead to

iron porphyrin self destruction (14; 45).

Two methods have been developed in attempt to enhance the catalysis

stability and prevent self oxidative destruction of the

metalloporphyrin/O2/H2O2 system (14; 45; 48)

(i) Inserting substituent in the porphyrin ring in order to

increasing the electrovity of iron porphyrin

(ii) Absorbing ionic metalloporphyrin on an ion-exchange resin or

using supported iron porphyrin in solid support (not

encapsulated).

The first approach has been developed by Moreira et al using Tetrakis

(2,3,4,5,6-pentafluorophenyl)porphyrin ([Fe(TFPP)]+) and Iron(III) meso-

tetrakis(2,3,5,6-tetrafluoro-N,N,N,-trimethyl-4-anilinium) porphyrin

([FeTF4TMAPP]5+). They showed higher catalytic activities instead of iron

porphyrin that do not bear electron withdrawing substituent in the mesophenyl

rings. The second approach will be discussed further in chapter 4.2.

3.2.5 Iron Salts

The pioneering work of catalysis study upon oxidation of cyclohexane using

hydrogen peroxide in the presence of iron salts (without added ligand) has been

reported by Sawyer (49) in the mid 1990’s and later reinvestigated by Shul’pin

et al (50) in 2004. The reaction took places at room temperature in acetonitrile

using FeCl3 as catalyst, but afforded a very low conversion (3.2% based on

H2O2) while efficiency is as low as 40% after 3 h reaction. More recently the

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oxidation of cyclohexane using hydrogen peroxide in the presence iron salts, Fe

(ClO4)2, at a relatively higher temperature (50°C) has been studied by the

researchers at Leiden University (33). The reaction performed at 50°C in

acetonitrile with a total conversion (100%) of cyclohexane and selectivity of

97% can be attained after 23 h. Interestingly the same conversion and

selectivity can be achieved within 2 h when the oxidation was performed under

argon atmosphere. The significant catalytic improvements seem to be

attributed to the operating variations (temperature and Ar at atmospheric

pressure).

3.3 Copper Based catalysts

A number of different types of metal complexes have been widely studied as

catalyst for the oxidation of cyclohexane under mild conditions. Copper is one

interesting compound to be investigated due to its abundance present in nature

(51; 52; 53). The summary of peroxidative oxidation of cyclohexane toward

Ol/One is presented in Table 3-1. The biomimetic system is extensively

investigated using copper as the metal centre to coordinate with a various type

of ligands since copper is found in the active sites of a number of enzymes and

plays substantial roles in the living system (51; 52; 53). For instance, an

interesting oxidation properties of copper is reported by A.C Sylva et al using

(51) BMPA (bis-(2-pyridilmethyl)amine) ligand incorporated Cu forms a

mononuclear complex [Cu(BMPA)Cl2] and the four-centre complex

{[Cu(BMPA)Cl2][Cu(BMPA)( H2O)Cl][Cu(BMPA)Cl][CuCl4]}. It has been found

that complex {[Cu(BMPA)Cl2][Cu(BMPA)(H2O)Cl][Cu(BMPA)Cl][CuCl4]} exhibits

a much higher activities and yield rather than complex [Cu(BMPA)Cl2]. It

resulted in a yield up to 68% after 24 h at room temperature. The observation

suggested that the more unit of copper in the complex, the higher yield can be

obtained; it also raised the possibility of an important role of [CuCl4]-. The

species [CuCl4]- is more easily reducible and oxidisable and thus capable of

producing more active catalysis system.

Detoni et al (54) reported oxidation of cyclohexane catalysed copper

complex with a phenanthroline ligand. It was found that the higher Lewis

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acidity results in higher catalysis performance. The catalytic activity of the 1,10-

phenantroline Cu(II) complexes followed the order: [Cu(phen)2Cl]Cl.5H2O >

[Cu(phen)Cl2] ≥ [Cu(phen)3]Cl2. The optimum operating temperature is at about

70°C, allowing a yield of 67% after 24 h under Ar atmosphere. Besides Ol/One,

adipic acid and intermediate cyclohexyl peroxide were also formed in this

reaction. It is clearly obvious that the temperature greatly affected the

formation of the product since the cyclohexyl peroxide is decomposed rapidly at

higher temperature and adipic acid can be obtained directly at the range of 50

to 80°C.

A marked growth of catalysis activity was also observed by addition of

nitric acid in the medium. Kirillov et al (52) tested copper complex with various

nuclearity (mono-, di-, tri-, tetra- and polynuclear copper) to oxidise

cyclohexane. These reactions proceeded on acidic medium and formed biphasic

liquid catalysis system at room temperature. The activity and stability of copper

complex over polydentate triethanolaminate (H3tea) were investigated under

various operating parameters such as amount of cyclohexane, reaction times,

oxidants, solvents and nitric acid concentrations. The best result was obtained

in the presence of [Cu2(H2tea)2{m-C6H4(COO)2-1,4}n] · 2nH2O catalyst. A 37%

yield of Ol/One can be attained after 6 h reaction in acetonitrile with

HP/Cyclohexane ratio of 16. The catalyst did not suffer significant decrease of

activity after it being reused 5 times. Its performances seem to be strongly

dependent on the acidity of the medium, oxidant and solvent. It has been shown

that the reactions practically do not proceed or only gave negligible yield in the

absence or low content of nitric acid and solvent. The acidic medium to carry

out the process may also play an important role in enhancing catalytic process

through proton transfer step and preventing rapid decomposition of HP to

water and oxygen. The choice of acetonitrile as a solvent offered a number of

advantages including: (i) capability to solubilise cyclohexane and Ol/One (ii)

stability under reaction condition (iii) similar boiling point with the reactant,

thus facilitating an easy recirculation of cyclohexane and solvent mixture.

A more environmentally friendly novel catalyst introduced by Silva et al

(53) is a water soluble scorpionate complex. The hydro solubility of the catalyst

may offer a possibility to replace any organic solvent as the oxidation of

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28

cyclohexane can be carried out in the pure aqueous media. The catalyst can be

synthesised from a commercially available metallic salt, CuCL2 and

functionalized scorpionate 2,2,2-tris(1-pyrazolyl)ethanol and 2,2,2-tris(1-

pyrazolyl)ethyl methanesulfonate to form the corresponding water soluble

Cu(II) complexes [CuCl2{HOCH2C(pz)3}] and [CuCl2{CH3SO2OCH2C(pz)3}]2.

Although this catalyst is relatively less effective and gave low yield, this process

possesses a strong green attribute as it also can proceed at room temperature.

Table 3-1 Comparison of activities of copper catalyst for the cyclohexane oxidation to Ol/One

a in Molar bYield include cyclohexylperoxide (CHHP)

Catalyst

Cat.

µmol

mmol HP:

mmol ane

Solvent (ml)

T, t oC, h

Conv. (%)

Selectivity Ol/One

(%)

Yield Ol/One

(%) Ref

[Cu(BMPA)Cl2] 0.8a 0.8/0.8a MeCN (n.d) RT, 24 n.d n.d 1.2,AA1.5 (51)

{[Cu(BMPA)Cl2][Cu(BMPA) (H2O)Cl][Cu(BMPA)Cl]

[CuCl4]} 0.8a 0.8/0.8a MeCN (n.d) RT, 24 n.d n.d 66.9, AA 2 (51)

[Cu(phen)3]Cl2 0.8a 0.8/0.8a MeCN (n.d) 70, 24 n.d 79, AA 7 61b (54)

[Cu(phen)2Cl]Cl 0.8a 0.8/0.8a MeCN (n.d) 70, 24 n.d 85, AA 6 67b (54)

[Cu(phen)3]Cl2 0.8a 0.8/0.8a MeCN (n.d) 70, 24 n.d 93, AA 2 60b (54)

[Cu2(H2tea)2 (N3)] 25 10/0.63 MeCN (5)

+

RT, 6 n.d n.d 27.7 (52)

[Cu2(H2tea)2(H4C6H4COO)2] ·2H2O

25 10/0.63 MeCN (5)

RT, 6 n.d n.d 15.4 (52)

[Cu2(H2tea)2((CH3)4C6H4COO)2

·2H2O

25 10/0.63 MeCN (5)

RT, 6 n.d n.d 17 (52)

[Cu2(H2tea)2

(Cl3C6H4COO)2]·2H2O 25 10/0.63

MeCN (5)

RT, 6 n.d n.d 14.6 (52)

[Cu2(H2tea)2(XC6H4COO)2] ·2H2O

25 10/0.63 MeCN (5)

RT, 6 n.d n.d 14.7 (52)

[OCu4(tea)4-(BOH)4][BF4]2 25 10/0.63 MeCN (5)

RT, 6 n.d n.d 37.2 (52)

[Cu2(H2tea)2{m-C6H4(COO)2-1,4}n] · 2nH2O

25 10/0.63 MeCN (5)

RT, 6 38.5 100 38.5 (52)

[CuCl2{HOCH2C(pz)3}] 10 10/0.63 MeCN (3)

20, 6 n.d n.d 23.4 (53)

[CuCl2{HOCH2C(pz)3}] 10 10/0.63 H2O 20, 6 n.d n.d 0.09 (53)

[CuCl2

{CH3SO2OCH2C(pz)3}]2 10 10/0.63

MeCN (3)

20, 6 n.d n.d 2 (53)

[CuCl2

{CH3SO2OCH2C(pz)3}] 10 10/0.63 H2O 20, 6 n.d n.d 0.4 (53)

Cu3 (PO4)2 131 24/8 MeCN (10)

+

65, 12 10.5 49.3 5.2 (55)

Cu2P2O7 166 24/8 MeCN (10) 65, 12 58.6 63.1 37 (55)

Cu5 (P3O10)2

60 24/8 MeCN (10)

+

65, 12 73.1 40.7 29.8 (55)

CuNO3 25 10/0.63 MeCN (5)

RT, 6 6 90 5.4 (52)

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Besides using copper complex systems, a number of researchers also

developed interest to test the oxidation of cyclohexane catalysed by copper

salts. Recently Y.Du et al (55) disclosed results of cyclohexane oxidations over

three copper salts under various types of solvents, temperatures and organic

acid initiators. The best result was obtained in the presence of copper

pyrophosphate, Cu2P2O7, acetonitrile and temperature of 65°C, with a

conversion of 58.8% and selectivity toward KA up to 100% oil after 12 h. The

modest hydrophobicity of the catalyst is believed to play an important role in

this process since it is capable of enhancing desorption of polar product,

cyclohexanone and cyclohexanol, from the active sites once they are formed.

3.4 Vanadium Based Catalysts

Researchers of U.S Environmental Protection Agency (56) have tested catalysts

containing vanadium ions to carry out the oxidation of cyclohexane under mild

conditions. The calcined vanadium phosphorus oxide (VPO) catalyst, with a P/V

ratio of 1.1, exhibited vanadyl pyrophosphate (VO)2P2O7 as the predominant

phase and also low intensity peaks characteristic of VOPO4 phase. The reactions

took place in homogeneous system in acetonitrile with the hydrogen peroxide

to cyclohexane ratio of 4 and under nitrogen pressure. It afforded a conversion

of 70% and 100% selectivity toward Ol/One after 4 h. When the reaction time

was 8 h, the conversion of cyclohexane increased to 84%.

OH

+

O

[V5+]

[V4+] H2O

H2O2

Figure 3-4 Oxidation pathway over VPO catalyst (56)

The mechanistic pathway elucidates the advantage of using nitrogen

pressure over oxygen air (oxygen). The appearance of V5+ sites derives from

pyrophosphate frameworks that are stabilised by the excess phosphorus at the

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surface. As depicted in Figure 3-4, the species V5+ plays a key role as a dynamic

oxidising centre; however the presence of oxygen may hinder the reduction of

V5+ species back to V4+.

3.5 Hetero- Metallic Complexes

In 2006 Nesterov et al (57) introduced an innovative catalysis system using

inorganic coordination compounds bearing Fe/Cu/Co metals and a Ni bearing

Cu/Co core that can be prepared by self assembly. A 45% yield of Ol/One was

achieved for a HP/Cyclohexane molar ratio of 3 or 5 after 6 h in the presence of

[FeCuCo(L)3(NCS)2(MeOH)]2.3.H2O (H2L=diethanolamine) complex in

acetonitrile, whereas complex [Ni(H2L)2][CuCo(H2L)(L)2(NCS)]2(A)2 {A =NCS or

Br} failed to exhibit satisfactory results. It only produced a yield of 8% at the

same operating conditions. The dissociation of the Ni Centre from Cu/Co core

seems to be the overriding reason of this poor performance. This experiment

not only pointed towards a fundamental role of iron and the synergistic effect of

Fe/Cu/Co to enhance the catalytic activities. It was also found that the greater

HP/Cyclohexane molar ratio, the higher yield and faster reaction time can be

achieved; but that ratio below 5 would lead to over oxidation and reduce the

yield of product.

3.5 Polyoxometallates (POMs)

Polyoxometallates (POMs) is incorporation of metal-oxygen compound which

forms a unique cluster (58). The crystalline structures of POMs made up of a

polyhedral cage structure or framework anions, which is balanced by positive

charges that are external to the cage (58). Being analog of metalloporphyrins,

POMs have a rigid coordination sites surrounding the metal centre and have the

common formula (XM12O40)3- (11; 27). The molecular compounds as the

structural unit include isopoly acids, heteropoly acid and their salts. The

advantages of using POMs in oxidation reaction are the stability under reaction

conditions and the possibilities to substitute metal in peripheral centre without

losing the structural integrity. Keggin type polyoxometalates is the most studied

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the alternative system to oxidise cyclohexane. For example, the use of

dimanganese and diiron substituted polyoxometallates to facilitate the

oxidation of cyclohexane using hydrogen peroxide at 83°C has been reported by

Mizuno et al (59). Under these conditions, a selectivity toward Ol/One up to

100% with an HP efficiency of 95% was obtained in the presence of diiron

substituted [γ-SiW10{Fe(OH2)}2O38]6-.

Table 3-2 Cyclohexane oxidation catalysed by Polyoxometalates

a the rest of product is cyclohexyl hydro peroxide (CHHP)

The high selectivity towards product is believed to be due to the non-

involvement of hydroxyl radical pathway during the reaction. Dimanganese-

substituted [γ-SiW10Mn2O38]6- and non-substituted Keggin anion α-SiW12O404-,

in contrast, showed no significant activity. Even though this system is carried

out with a HP/Cyclohexane molar ratio equal to 1 and showed no significant

Catalyst

Cat. µmol

mol ane: mol HP

Solv. (ml) T, t

°C , h

Conv. Ol/One

(%)

Select. Ol/One

(%)

Yield Ol/one

(%) Ref

[γ-SiW10{Fe(OH2)}2O38]6- 8 1/1 MeCN (6) 83, 96 66 100 66 (59)

[α-SiW11Fe(OH2)O39]5- 8 1/1 MeCN (6) 83, 96 7 100 7 (59)

[α-SiW9{Fe(OH2)}3O37]7 8 1/1 MeCN (6) 83, 96 5 100 5 (59)

[γ-SiW10Mn2O38]6- 1.5 1/1 MeCN (6) 83, 96 <1 100 <1 (59)

[α-SiW12O40]4- 1.5 1/1 MeCN (6) 83, 96 <1 100 <1 (59)

Co4(PW9)2 1.5 4/1 MeCN(1.5) 40, 12 91 100 91 (60; 61)

Mn4(PW9)2 1.5 4/1 MeCN (1.5) 40, 12 98 100 98 (60; 61)

Fe4(PW9)2 1.5 4/1 MeCN (1.5) 40, 6 98 48 47 (60; 61)

Fe4(PW9)2 1.5 4/2 MeCN (1.5) 40, 12 100 30 30 (60; 61)

(TBA)4 H3 PW11 O39 39 19/39 MeCN (10) 80, 12 35 93a 32.6 (62)

(TBA)4 HPW11 CuO39 40 19/29. MeCN (10) 80, 12 11 87a 9.6 (62)

(TBA)4 HPW11 Co(H2O)O39.2H2O

38 19/39 MeCN (10) 80, 12 5 100 5 (62)

(TBA)4 HPW11 Mn(H2O)O39.3H2O

38 19/39 MeCN 10) 80, 12 8 100 8 (62)

(TBA)4 HPW11 Ni(H2O)O39.H2O

39 9/39 MeCN (10) 80, 12 20 100 20 (62)

(TBA)4 HPW11 Fe(H2O)O39.2H2O

42 19/39 MeCN (10) 80, 9 76 25a 19 (62)

TBA4H2BFe(H2O)W11O39·H2O

1.5 2/1 MeCN (1.5) 80, 12 87 68a 59.2 (63)

TBA4H2BFe(H2O)W11O39·H2O

1.5 4/1 MeCN (1.5) 80, 6 99 47a 46.5 (63)

TBA4H2BFe(H2O)W11O39·H2O

1.5 4/1 MeCN (1.5) 80, 12 98 43a 42.1 (63)

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decrease of activity in the second running, however the long duration of

reaction (96 h) is likely to be a major drawback of this system.

Cyclohexane oxidation catalysed by Keggin type polyoxometallates-

hydrogen peroxide systems (60; 61) was reported with catalytic performance

of tetrabutylammonium (TBA) salts of Keggin-type polyoxotungstates

[PW11O39]7- and [PW11M(L)O39]7- (where Mm+ is a first row transition metal

including Fe, Cu, Mn, or Ni and L is H2O or CH3CN). Conversions in order of 5%

to 20% were observed when Cu, Mn or Ni was used, but interestingly only

cyclohexane and cyclohexanol were produced as the final products. When the

reaction was carried out in the absence of transition metals, the heteroplyanion

PW11 showed a higher conversion but it also produce CHHP as by-product.

CHHP may decompose to cyclohexanol and cyclohexanone but carboxylic acid

was also generated as by-product at the end of the process. Using iron, a quite

high conversion was obtained, but resulted in a significant amount of cyclohexyl

hydro peroxide (CHHP); accounting for 75% of total product. The longer

reaction time would result in significant higher conversion, but led to the

formation of a complex mixture with a lower selectivity. The efficiency of H202

for iron substituted-POM, however, is as high as 100%.

Later it was disclosed that iron metal substituted Keggin-type

tungstoborates TBA4H2BFe (H2O) W11O39·H2O (62) and HP/Cyclohexane ratio

of 2mmol/1mmol, a conversion of 87% could be obtained, with an Ol/One

product accounting for 68% and efficiency of H2O2 being more than 98%. When

the ratio of HP/Cyclohexane was increased to 4 /1mmol, a much higher

conversion was observed (99%), which led to a substantial decline in the H2O2

efficiency. More recently sandwich-type tungstophosphates B-α-

[M4(H2O)2(PW9O34)2]10−, MII = Co, Mn and [FeIII4(H2O)2(PW9O34)2]6−

(abbreviated as M4(PW9)2) were shown to catalyse the same reaction (63). The

manganese substituted (PW9)2 is capable of converting the cyclohexane up to

98% and exhibited 100% selectivity toward Ol/One after 12 h at relatively mild

conditions. Fe substituted (PW9)2, on the other hand, also exhibited high

selectivity with conversion up to 100% and turnover number more than 1300

(mmol oxidised product per mmol catalyst) after 12 h; however, the cyclohexyl

peroxide was formed and accounts for 70% of total product. Catalysis stability

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33

test using infrared spectra, unfortunately, indicated that the residue that the

catalysts Mn, Co and Fe substituted POMs suffering extensive degradation. A

summary of the oxidation of cyclohexane to Ol/One catalysed by POMs is

presented in Table 3-2.

3.6 Hybrid System Metal Complexes – Polyoxometallates

Recently Mirkhani et al (64) disclosed an interesting novel strategy for the

preparation of metalloenzymes by covalent linkage to a Keggin type

polyoxometallates (POM).K8SiW11O39. They incorporated Copper (II) salen

(where H2salen=N, NO-bis (salicylidene) ethylenediamine) with K8SiW11O39. Cu

salen-POM proved to enhance the reaction, affording the cyclohexanone as the

predominant product (87%) and a low proportion of cyclohexanol (13%). At

80°C, with HP/Substrate ratio of 6, high conversion and turnover frequency

were obtained, reaching 45% and 2.35, after 10 h reaction. This result is higher

compared to the corresponding unhybridised Cu salen. The attachment of POM

is believed able to avoid the formation of the µ-oxo bridged complex that may

lead to decreased activity of Cu salen.

3.7 An Overview of Homogeneous Catalysis Development

3.7.1 Summary of the Recent Research Progress

A number of various homogeneous catalysts for peroxidative oxidation of the

cyclohexane to Ol/One other than Co complexes have been reported in the

recent literature. A summary is presented in Table 3-3. The homogeneous

system based on heme iron complexes of H2O2 was found to be ineffective to

carry out the oxidation of cyclohexane. The self-destruction of the catalysts

within the operating conditions due to homolytic cleavage of the peroxide O-O

bond and low ability to produce satisfactory yield of Ol/One make this system

uninteresting for further investigation; whilst the rapid developments of

various ligands in the non-heme iron and copper complexes lead to significant

progress of peroxidative oxidation of cyclohexane. Schiff base ligand is one of

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the impressive works that demonstrated the 100% conversion of cyclohexane

and that selectivity more than 90% can be achieved, but the time of reaction (24

h) is too long. Acidic medium proved to be enhancing the catalysis reaction by

copper complexes with triethanolamine (H3tea). These copper catalysts can be

prepared self assembly and produce reproducible results, but the use of large

amount of nitric acid in order to form acidic medium may not represent a step

forward for a greener process. A marked growth of catalysis process was also

observed in heterotrimetallic Fe/Cu/Co complexes, but the efficiency of H2O2 is

as low as 9%.

The use of metal salts of copper, vanadium or iron salts may provide an

attractive alternative as those can be used without added ligand and showed

fairly good activities. However some of them require inert atmosphere.

Compared with biomimetic metal complex catalyst; POMs, on the other hand,

exhibits more promising performance and offered a number of intrinsic

advantages such as higher thermal stabilities and facilitation of rapid and

reversible multi-electron redox transformation under operating conditions.

These may lead to very efficient oxidant utilisation and substrate conversion

(66). Unfortunately the catalysts are often unstable, suffering decomposition

under operating conditions.

3.7.2 Future Prospect in Homogeneous Catalysis Development

Impressive results have been achieved, but those cutting-edge experiments can

be reviewed critically from economical, technical and environmental point of

view. Based on economical and technical consideration, it is generally found

that the catalysis development recently only focuses on activity, selectivity and

stability of the catalysts, but gives no adequate elucidation in terms of the

efficiency of hydrogen peroxide utilisation. As amplified by Scuchardt et al (27),

the relative prices of hydrogen peroxide (US$ 0.58/kg) compared to adipic acid

(US$ 1.34-1.5/kg) (65) is critical, and a poor efficiency of the peroxidative

oxidation (below 40%) would attract higher cost of the oxidant. Unfortunately,

it is quite often the case a that high ratio of cyclohexane-hydrogen peroxide (2

to 6) is frequently necessarily required to achieve higher yield of Ol/One;

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efficiency of HP utilisation is frequently not determined (or yield based on

oxidant). Furthermore, the limitation of this catalyst in many cases derives from

the fact that the catalyst suffers extensive degradation under reaction

conditions. From environmental perspective, it is sometimes apparent that the

catalysis reactions under experiments seems to be environmentally friendly, the

material and energy used to synthesis and prepare the catalyst often involve

unsustainable processes. Another drawback is the requirement of solvent to

carry out the reaction. Since contrast polarity between cyclohexane and

hydrogen peroxide, at which cyclohexane is highly hydrophobic and hydrogen

peroxide is strong hydrophilic (55), a solvent therefore is essentially needed in

order to facilitate homogeneity for those reactants. Unfortunately traditional

volatile organic solvent with high vapour pressure and toxicity were quite often

used in the reported literature. Acetonitrile, for example, possesses high toxicity

to the environment due to its aquatic persistence and potentially leads to

bioaccumulations. Moreover since acetonitrile is a common compound that

widely used in chemical industry, the high demand and tight supply of this

chemical may result in higher price in the future (67).

A number of substantial features therefore may need to be considered

and improved to make the development of homogeneous catalyst more

economically and environmentally sustainable in the future. First factor is

performing reaction in green or free solvent conditions. A various alternative

reaction medium may consist of room temperature ionic liquids, supercritical

carbon dioxide, or solvent free conditions (68). The use of supercritical CO2 may

offer advantages in terms of the possibility of recycling catalyst and minimising

the loss of catalyst. This is particularly important when high cost metal and

ligands are used. The presence of supercritical carbon dioxide may also lead to

higher mass transfer rate and the improvement of selectivity of peroxidative

oxidation of cyclohexane has been reported by Olsen et al. However since this

process is also energy intensive and hindered with high price of CO2 - the

commercial application may come into reality if yields obtained under this

reaction are much superior to those obtained under conventional liquid-phase

condition. The same benefits may possibly be obtained in the presence of ionic

liquids. Since ionic liquids have no vapour pressure and can be generated by

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combining a wide range of anions and cations; they could replace the

conventional organic solvents. But it is also noteworthy than the manufacturing

of ‘greener’ solvents such as ionic liquid should not involve a metathesis step

that may decline their green attribute to some extent (69). The toxicity, thermal

stability, biodegradability properties of a solvent also should be fully

determined. The latter alternative has investigated by Sylva et al (53). With a

hydro soluble complex catalyst, the oxidation can be carried out in pure

aqueous media, but the yield is quite low. The further challenge therefore is to

find a novel water-soluble catalyst that capable of producing more competitive

yield.

A second feature is minimising cost, complexity and toxicity of reactant.

There are a number of drawbacks that are restraining the application of the

oxidation of cyclohexane to Ol/One catalysed by metal complex-HP system.

Those limitations include most of metal complexes are often very expensive,

frequently involved complicated preparation and only few data available

regarding the potential toxicity of the ligands to the environment. For instance,

in case of a ligand synthesise for metalloporphyrin or non heme complex

catalyst. It is generally found that synthesis of more robust ligands can be

achieved by halogenations (11) that may use considerable amount of

chlorinated compounds or acidic solutions, thus producing a catalyst that is not

more environmentally sustainable compared with those of simple metal salt

catalysts. The simple method of a ligand synthesis such as in-situ preparation of

Schiff base complex as demonstrated by Nayak and co-workers (38) or other

easy and simple preparation seems to be more favourable for future

developments. In addition as homogeneous catalysts are soluble in the reaction

mixture, from an environmental perspective, the toxicity level of the metals

should be taken into account. For example, the high toxicity of vanadium salts

may make it unacceptable to develop homogeneous system catalyst.

Third, designing suitable reactors for catalyst recovery has yet been

sufficiently addressed. If costly catalyst is inevitable, designing appropriate

reactors and processes to recover both metals and ligands is likely to be a future

challenge (70). One of the methods of separation technologies is heterogenised

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homogenous catalyst that is discussed in the chapter 5. A general scheme to

improve the reusability for homogeneous catalysis is presented in Figure 3-5.

Fourth, investigating the mechanistic pathway of catalysis reaction is

required. Although outstanding progress of cyclohexane oxidation has been

achieved in the recent decade, however the experimental data regarding the

mechanistic pathway of catalysis reaction are still limited. In many cases,

particularly for biomimetic system catalyst, the materials are often unstable and

suffering extensive degradation under reaction conditions. The further

mechanistic studies to distinguish intra- from intermolecular mechanisms

therefore seem to be substantial to achieve better understanding of catalysis

process (28). This is particularly important to elucidate how exactly the

underpinning catalysis works, what the reasons of the catalysis

decompositions/deactivation are, and how to improve the catalysis

performance.

Homogeneous Catalysts

Continuousrecycle

Recycle afterwork-up

On heterogeneoussupport

With liquidsupports

Ionicliquid

SupercriticalCO2

water

Catalyst lost inproduct

Figure 3-5 A general scheme to improve the reusability of homogeneous catalysts (3; 67)

Last, improving the efficiency of H2O2 utilisation is of paramount of

importance. With a low ratio of HP/Cyclohexane, the efficiency of H2O2

utilisation is relatively higher, but the yield of Ol/One is commonly not

satisfactory. Increasing the ratio of HP/Cyclohexane proved to be able to results

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in higher conversion, but it can be problematic. It frequently decreases the

efficiency of H2O2 utilisation (63) and may lead to unselective oxidation due to

over oxidation (57), and more importantly, it is obviously constrained by cost of

the HP. Optimisation of the reaction to find an appropriate ratio of

HP/Cyclohexane or method maximise oxidant efficiency seems to be essential.

Furthermore it is also substantial the catalysis performance in terms of both

yield based on substrate and efficiency of HP (sometimes called yield based on

oxidant) need to be considered more.

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Table 3-3 Comparison of catalysis performance with cobalt naphtalene versus various homogeneous catalysts

a Currently used in commercial application with oxygen or air as terminal oxidant bTurn over number (mmol oxidised product per mmol catalyst) cTurn over frequency (mmol oxidised product per mmol catalyst per hour)

Catalytic property

Co or Mn Saltsa

(1; 3; 27)

FeP (46)

Fe Complex (38)

Cu complex (52)

V salt (56)

Fe Salt (33)

Cu Salt (55)

Fe/Cu/Co

(57) POMs (63)

Hybrid Cu Salen-POMs

(64) Temp.°C 150-180 RT 50 RT 65 50 65 RT 40 80

Pressure (atm) 10-20 1 atm (Ar) 1 atm 1 atm 1 atm (N2) 1 atm (Ar) 1 atm 1 atm 1 atm 1 atm Time 40 mins 2 h 24 h 6 ho 4 h 2 h 12 h 6 h 6 h 10 h

Conv. ane (%) 5-7 n.d 100 38.5 70 87 58.6 n.d 98 47 Select. Ol/One

(%) 75-80 n.d 90.5 100 100 100 100 n.d 48 100

Yield Ol/One(%) 3.8 – 5.6 5.53 90.5 38.5 70 87 37 45 47 47 Ol:One 60:40 3.5:2 39.5 : 51 23.4 : 14.2 37 : 63 45 : 42 37 :63 40:3 14 : 34 13 : 87 HP/ane - 1 1.5 16 4 1.5 3 2 6

HP eff. (%) - n.d n.d n.d n.d n.d n.d 8.9% 72% n.d TONb 47 n.d 380 308 n.d n.d 47 657 2.35l

Solvent - MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN Ligand - - Schiff-Base Triethanolamine - - - Diethanolamine - Schiff-Base

Co-catalyst/ additives

- - - HNO3 - - - HNO3 - -

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CHAPTER 4 – OXIDATION OF CYCLOHEXANE TO OL/ONE

BY USING HETEROGENEOUS CATALYSTS

4.1 Molecular Sieve Catalysts

The difficulties to separate the catalyst from the mixture KA oil may be

eliminated if the catalyst can be in a heterogeneous system. The development of

heterogeneous catalysis system therefore may eliminate the cons of

homogeneous system, which, when used on conventionally, results in a process

with unavoidable leaching of metal salts or acid slurry during the oxidation of

cyclohexane to Ol/One. Moreover, continuous processing is easily implemented

with a heterogeneous system since the catalyst may be reusable and give

reproducible performance (11; 70). The use of molecular sieves catalysts may

offer distinct advantages due to their exchangeability, reactant or product

selectivity, thermal stability and reusability (70).

The term molecular sieve refers to a material containing a crystalline

structure and a range of compositions which are capable of performing shape-

selective adsorption and reaction properties (71) The development of

heterogeneous catalysis of this cyclohexane oxidation in the last two decades

predominantly using active transition metals such as Ti, V, Co, Cr, and Fe that

incorporated in a variety of molecular sieves structures (e.g. APO, MCM-41,

HMA, SBA etc) by hydrothermal synthesis. Few researchers also reported the

catalytic performance of transition metal oxide such as V2O5–TiO2 prepared via

the sol-gel method. A summary of literature reporting on the heterogeneous

oxidation of cyclohexane with hydrogen peroxide is given in Table 4-1.

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4.1.1 Titanium Based catalysts

Since the discovery of a microporous solid silicate structure materials,

especially those containing SiO2 and TiO2 modified by isomorphous substitution

of Si (IV) with Ti (IV), TS-1; extensive studies have been attempted to

investigate TS-1 in various catalytic reactions (72). TS-1 can be prepared via

hydrothermal crystallisation of a silicon–titanium gel containing

tetrapropylammonium salt (73). The reaction of cyclohexane in the presence of

TS-1 in acetone showed stable activity, resulting in cyclohexane conversion of

6.3% and 64.9% selectivity toward KA oil after 5 h (74). In the other

experiments using acetic acid as a solvent, TS-1 showed higher activities,

leading to a conversion of cyclohexane and selectivity toward Ol/One of 16 and

79%, respectively, after 4 h reaction. In this case, the author pointed out the

ability of acetic acid to facilitate the complexation of active sites within the

titanium structure to enhance catalytic performance (73). However, some

titanium leaching was observed during the experiment.

A number of Ti-based catalysts were also tested by several researchers.

A mixed phase material made up of both micro porous and meso porous Si/Ti

materials (Ti-MMM-1) exhibited fairly good cyclohexane oxidation with acetone

as a solvent (75). The most excellent result of heterogeneous Ti-Based system,

however, was disclosed by Selvam and Mohapatra (76) using meso porous Ti-

MCM-41 and Ti-HMA. Ti substituted hexagonal mesoporous aluminophosphates

(HMA) with a narrow pore distribution and meso pores diameter of 2.9 nm. It

exhibited a conversion and selectivity more than 90% at 100°C after 12 h in the

presence of acetic acid as a solvent and MEK initiator. At the same operating

conditions, Ti-MCM-41 showed activities only slightly lower than that Ti-HMA.

4.1.2 Vanadium Based Catalysts

Selvam and Dapurkar (77) investigated the synthesis and catalytic activity of

various vanadium sources. The hydrothermal synthesis using different Si/V

involving incipient wetness and calcination led to pore diameter of VMCM-1 in a

range of 2.4 – 2.7 nm. The activity of these systems is greatly affected by the

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sources of vanadium, which determines the extent of vanadium incorporation in

the mesoporous matrix and thus catalytic performance. It has been found that

tetravalent vanadium is preferred instead of pentavalent vanadium since it

forms monomeric VO43- and exhibits maximum vanadium incorporation in the

mesoporous matrix MCM-41. It afforded a conversion and selectivity to Ol/One

more than 95% after 12 h in acetic acid. The catalytic reusability test showed

that V-MCM-1 were more stable than VS-1 and V205/MCM-41, which suffer

leaching of active vanadium ions from the matrix. With the same range of pore

size diameter (2.5-2.7 nm), a vanadium framework substituted HMA (V-HMA)

was performing catalysis activities as high and stable as V-MCM-41 (78). The

presence of both isolated V4+/V5+ ions in tetrahedral framework sites may be

attributed to its stable activity upon cycling. One of reason behind the

decomposition of the catalyst is caused by the presence of water as by-product

and accelerates by carboxylic acid.

Another developments of vanadium substituted molecular sieve was

reported by Jermy et al (79) using three dimensional (3-D) cubic V-KIT-6. The

catalyst was successfully synthesised with a surface area at about 1000 m2/g

and narrow size distribution of pore diameter (5-6 nm). It has been

demonstrated that the higher vanadium content enhances the concentration of

weak Bronsted acid sites due to the presence of tetrahedral vanadium. The

activity of cyclohexane oxidation increased with the vanadium content due to

higher acidity, however the excess vanadium content implies on the presence

V2O5 species at the external surface that may decrease the efficiency of the

oxidant. The optimum ratio of Si/V was found to be 49 (1.89% wt vanadium),

which affords a cyclohexane conversion of 83.6% with Ol/One selectivity of

93.8% at 100°C in acetonitrile and MEK initiator after 9 h.

Moderate yields were obtained in the presence of incorporation of two

transition metal ions in the APO framework, (Co-V) APO-5 catalyst (80). The

cyclohexane oxidation was very selective, but the conversion was rather low.

Furthermore, although the catalyst (Cr-V) APO-5 lost approximately 25% wt

after five catalytic cycle, it did not suffer from significant decrease of catalytic

performance. This fact indicates that leached metal ions may play a

fundamental role in retaining the catalyst activity.

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Transition metal oxides also attract great interest and are the subject of

intensive research. For instance, a mixed oxide, V2O5–TiO2, has been reported

by Choukchou-Braham and his co-workers (81). The catalyst was successfully

prepared and synthesised by an acid-catalysed sol-gel process, which affords

materials containing three morphology types including: small agglomerated

grains of about 20 nm, smooth middle-seized grains of about 85 nm and smooth

large grains of about 300 nm diameters. Acetonitrile, acetic acid and methanol

were tested as solvents to carry out the oxidation of cyclohexane with hydrogen

peroxide. Best results were obtained with acetic acid as a solvent and acetone as

initiator presented with a conversion of 8% and selectivity of 76% towards

cyclohexanol after 8 h at 70°C.

4.1.3 Cobalt, Iron and Chromium Based Catalysts

A number of authors reported catalytic activities of cyclohexane oxidation using

heterogeneous cobalt based catalyst. Selvam and Mohapatra (82) compared the

catalytic properties of cobalt framework-substituted HMA, cobalt containing

mesoporous silicate (Co-MCM-41), microporous cobalt aluminophosphate

(CoAPO-5), and cobalt silicate (Co/S-1). Using a Co-HMA as catalyst, they

obtained a conversion of 92.4% with the selectivity toward Ol/One of 96.5%

after 12 h. Under the same conditions, Co MCM-41, Co APO-5, and Co S-1

showed activities lower than that Co-HMA. It was found that hydrothermally

synthesised Co-HMA proved to be a stable heterogeneous catalyst since only a

negligible amount of leaching of cobalt was observed.

The catalytic performance of cobalt doped mesoporous SBA type silica

materials, Co-SBA 15 and Co-SBA 3 also have been studied recently (83). The

larger pore diameter of Co-SBA-15 (4.5nm) compared to that those of Co-SBA-

15 (3.6 nm) and Co-MCM-41 (2.8 nm) is believed to play a key role in enhancing

selectivity toward desired product (Ol/One). Co- SBA 3 gave a nearly 100%

selectivity to Ol/One at about 91% conversion, in 8 h reaction time at 100°C.

Moreover, when the filtered Co-SBA-3 was reused, its activity was not

significantly diminished.

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The incorporation of cobalt into mesoporous crystalline structured

titanium, followed by cogellation method using 1-dodecylamine as template, led

to formation of a Co-MTiO2 catalyst with average pore diameter of 10.8 nm. It

afforded a 93.5% yield of Ol/One and turnover number of 136, using acetic acid

as a solvent (84). The recycling of the catalyst in successive experiments gave

reproducible results. A lower activity, on the other hand, was observed in the

Co/TiO2 catalyst due to dispersion of Co on the surface of TiO2 instead of the

inside pore channel of TiO2 by impregnation.

The investigation of catalytic properties of iron metal substituted

molecular sieve is somewhat difficult due to the instability of iron species in the

matrix during heat treatment. Selvam and Mohapatra (85) therefore used a

number of different characterisation methods to investigate the stability of

trivalent iron in the hexagonal frameworks. It has demonstrated that the

dislodgement of trivalent iron from the framework structure during calcination

lead to high thermal stability of Fe-HMA catalyst. A yield of 84.1% with turn

over number of 756 can be obtained after 12 h in acetonitrile. Since the catalyst

retained its structured and porosity after recycling, the repeated running

exhibited reproducible results and no significant leaching of active iron species

was observed during catalytic stability test (85).

Satisfactory results were also achieved using chromium doped HMA and

MCM-41 (86). The ability of mesoporous silicates and aluminophosphates to

stabilise chromium in the framework led to stable catalyst that do not suffer

leaching under reaction conditions. The calcined Cr HMA possesses an average

pore size of 2.8 nm and showed an excellent selectivity to Ol/One (100%) after

12 h reactions at 100°C in acetic acid.

4.1.4 Cerium Based Catalysts

Unlike transition metals, the investigation of catalytic properties of rare earth

element has been rather limited. One example was disclosed by researchers at

Yunnan University (87). Cerium-doped MCM-41 catalyst showed a conversion

of cyclohexane up to 94.6% and predominantly produced cyclohexanol at 100°C.

At reaction temperature of 50°C, it was found that cyclohexanone was the major

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product. More interestingly, the catalyst was recyclable since it showed stable

performance during repeated running.

Table 4-1 Summary of heterogeneous catalysis for the cyclohexane oxidation to Ol/One

Catalyst

Cat.

(g) mol HP/

mol ane

Solv. T, t

oC, h

Conv.

ane(%)

Select. to

Ol/One (%)

Yield

Ol/One

(%)

Ref

Na-GeX 0.1 2 without 70, 3 66.5 100 66.5 (88)

Sulphated GeX 0.1 2 without 70, 1.5 42 100 42 (88)

Ce-MCM-41 0.4 2 CH3COOH 100, 12 94.6 89 84.1 (87)

Cr-HMA 0.05 1 CH3COOH 100, 12 93.5 100 93.5 (86)

Cr-MCM-41 0.05 1 CH3COOH 100, 12 95.6 97.1 92.8 (86)

TS-1 0.1 1.3 (CH3)2CO

77, 5 6.3 64.9 4.1 (74)

TS-1 0.27 3 CH3COOH 60, 4 16 79 12.6 (73)

Ti-MMM-41 0.1 3 (CH3)2CO

95, 8 9.2 89.8 8.2 (75)

Ti-MCM-41 0.05 1 CH3COOH (+MEK) 100, 12 90 95 85.5 (76)

Ti- HMA 0.05 1 CH3COOH (+MEK) 100, 12 88 99 87.1 (76)

V2O5–TiO2 0.03 0.27 CH3COOH 70, 8 7 76 5.3 (81)

Co-HMA 0.05 1 CH3COOH (+MEK) 100, 12 92.4 96.5 89.1 (82)

Co-MCM-41 0.05 1 CH3COOH (+MEK) 100, 12 58.2 87 50.6 (82)

Co-APO-5 0.05 1 CH3COOH (+MEK) 100, 12 33.5 83.7 28 (82)

Co-S-1 0.05 1 CH3COOH (+MEK) 100, 12 28 85.3 23.9 (82)

Co-SBA-3 0.1 2.2 CH3COOH 100, 8 91.6 99.9 91.5 (83)

Co-SBA-15 0.1 2.2 CH3COOH 100, 8 42.8 91.8 39.3 (83)

Co-MTiO2 0.15 2 CH3COOH 100, 12 ˜100 93.5 93.5 (84)

Co-TiO2 0.15 2 CH3COOH 100, 12 43.7 85.2 37.2 (84)

Co-MCM-41 0.1 2 CH3COOH 100, 12 87.4 88.6 77.4 (83)

Fe-HMA 0.05 1 CH3COOH 100, 12 84 93 78.1 (85)

VMCM-41 0.05 1 CH3COOH 100, 12 99 96.4 95.4 (77)

V205-MCM-41 0.05 1 CH3COOH 100, 12 97.3 93.9 91.4 (77)

VS-1 0.05 1 CH3COOH 100, 12 36.8 97.6 35.9 (77)

V-HMA 0.1 1 CH3COOH (+MEK)

100, 12 95 94.4 89.7 (78)

V-KIT-6 0.1 4 CH3COOH (+MEK)

100, 9 83.6 93.8 78.4 (79)

(Cr-V)APO-5 0.1 1 (CH3)2CO

60, 40 10 90 9 (80)

(CH3)2CO

CH3COOH (+MEK)

4.1.5 Germanic Faujasite

A research of catalytic activities of a germanic near faujasite and its sulphated

form molecular sieve has been reported by Pârvulescu et al (88). The Na-GeX

catalysts was synthesised from Al-Ge gels at low temperature, whilst sulphated

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Ge-X can be prepared by treating Na-GeX with NH4SO4 and then the sample

heated gradually up to 400°C. Impressively, The Na-GeX systems exhibited an

excellent catalytic activity. A cyclohexane conversion of 66% and 100%

selectivity toward KA oil can be attained after 3 h at temperature of 70°C under

autogenic pressure and free solvent reaction. The H2O2 efficiency was also fairly

good (58.7%); whilst the sulphatation of Na-GeX able to enhance the catalytic

activity, but the H2O2 efficiency decreased to 25%. The successive experiments

showed that catalyst did not suffer significant decrease of activity.

4.2 Carbon Nitride Polymer

Some work has recently focussed on carbon nanostructures due to its wide

range of application. An example is bulk CxNy materials. Wang et al (89)

reported a novel metal free-catalyst using boron- and fluorine enriched

mesoporous polymeric carbon nitride (CNBF) that can be environmentally

synthesised in the presence of an ionic liquid. This catalyst preparation

potentially replaces the conventional method of removing silica structures that

typically involving metathesis step using aqueous hazardous ammonium

bifluoride (NH4HF2) or hydrogen fluoride (HF). Impressively, this polymer

proved to be a heterogeneous catalyst since it can be separated easily by simple

filtration and gave reproducible result without loss of significant activity when

repeated catalyst was used. The oxidation is highly selective; with a 100% of

Ol/One can be attained after 4 h in acetonitrile, but conversion is as low as

5.7%.

4.3 Heterogenising Metal Complexes into Solid Materials

Latest catalysis developments that have been recently investigated are

immobilisation of homogenous catalysts into supported solid material or

inorganic matrixes in order to improve the recoverability and reusability of the

catalysts and to produce more stable active site species. A summary of research

development in this field is presented in Table 4-2.

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The encapsulation of metal complexes in solid support is, however, often

leads to unsatisfactory results. For example, the cyclohexane and hydrocarbon

oxidation catalysed by iron porphyrin encapsulated in zeolite [FeIII(TPP)Y] or

in silica matrix (FePES) showed poor performance and produced a yield of

Ol/One below 1% (46; 48)[24c, 49]. Moreira et al [24c] and Olsen et al [49] then

suggested the problem associated with the encapsulation of iron heme complex

as follows:

- The oxygen rebound mechanism required for oxidation of the inert

cyclohexane may be impeded with the polar environment of the FeP

active site

- The encapsulation of the metalloporphyrin catalyst led to an electronic

effect that capable of hindering the reaction

Table 4-2 Summary of the cyclohexane oxidation catalysed by immobilised metal complexes

Catalyst mol HP: mol ane

Solvent T, t

oC,h

Conv. ane (%)

Select. Ol/One(%)

Yield Ol/One(%)

HP Εff(%)

ref

FeIII(TPP)- Zeolite Y 1 MeCn ΡΤ, 2 n.d n.d 0.72 n.d (46)

[Co2([Η]8-Ν4Ο4)]-ΝαΥ 2 MeCn 60, 2 18.6 100 18.6 n.d (90)

[Νi2([Η]8-Ν4Ο4)]-ΝαΥ 2 MeCn 60, 2 3.7 100 3.7 n.d (90)

[Cu2([H]8-N4O4)]a-NaY 2 MeCN 60, 2 39.8 100 39.8 n.d (90)

[Mn2([H]8-N4O4)]a-NaY 2 MeCN 60, 2 8.7 100 8.7 n.d (90)

Fe-Montmorillonite 0.05 DCM-MeCN RT, 24 n.d n.d n.d 13 (91)

Mn-Montmorillonite 0.05 DCM-MeCN RT, 24 n.d n.d n.d 8 (91)

Fe-Silica 0.05 DCM-MeCN RT, 24 n.d n.d n.d 16 (91)

Mn-Silica 0.05 DCM-MeCN RT, 24 n.d n.d n.d <5 (91)

Salen VO complex-MCM-41

4 MeCN 60, 12 45.5 100 45.5 n.d (92)

Fe Complex - Zn/Al LDH 1 MeCN 70, 8 45.5 100 45.5 37.8 (93) aAverage value after 12 times reuse

Better results were obtained through encapsulated catalysts in non heme

metal complexes. Salavati-Niasari et al (90) reported the preparation and

catalytic performance of Mn (II), Co (II), Ni (II) and Cu (II) and complexes of

octahydro-schiff base immobilised in nanopores of zeolite in the presence of

acetonitrile solvent. Copper complex has found to be the most active system,

whereas cobalt, manganese and nickel exhibited weaker activities. It is

noticeable that the temperature greatly affect the catalysis performance,

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showing the most selective and stable performance in the presence of [Cu2

([H]8-N4O4)]-NaY catalyst at 60°C. Compared with [Cu2([H]8-N4O4)], without

encapsulation with zeolite framework, this catalytic performance is relatively

lower. These results agree with the other experiments that demonstrated the

decrease activity of encapsulated metal complex [24c, 49]. This disadvantage

may be offset by the benefit of the recyclability of the catalyst. It has been

shown that the catalyst shows excellent activity after 4 times running. In this

case the authors hypothesised that:

(i) The nanocavities are capable of immobilising the complexes

(ii) The formation of inactive species in the nanocavities can be

diminished due to steric effect of zeolite framework

(iii) The interaction of zeolite framework with zeolite lattice.

Immobilisation of homogeneous catalysts by other methods also has

been reported by a number of authors. The structure of inorganic solids is

capable of preventing the intermolecular aggregation of the active surface of

homogenous catalyst due to their rigid structure (72). Covalent bonding is one

of the simple methods to achieve this purpose. For instance, an interesting

supported catalyst based on incorporation of iron and manganese porphyrin

and aminofunctionalized montmorillonite K10 or silica by covalent binding has

been recently investigated by the researchers from Universidade de São Paulo

(91). Mineral clay is selected because of its wide range of chemical properties,

its ability to expand and undergo pillarisation, its intercalation chemistry and

ion change properties (91). Unfortunately the yield of cyclohexanone and

cyclohexanol of these supported metallophorphyrin-HP system is by far lower

compared to the catalysis system in the presence PhIO as terminal oxidant. The

authors assumed that the difficulty with supported MePs-H2O2 system is the

fact that the inorganic matrix can result in peroxide dismutation. In addition,

the low proportion of HP/Cyclohexane and low temperature during the

experiment may become other significant reasons of this poor performance.

Recently a better result was obtained by using a salen oxovanadium complex

immobilised in MCM-41 via multi grafting method (92). The uniformity of the

mesoporous catalyst MCM-41 allows the bulky cyclohexane molecule to diffuse

with minimum steric hydrance, resulting in a conversion of 45.5% with an

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excellent selectivity (100%) after 12 h at 60°C. More recently Parida et al (93)

reported the oxidation of cyclohexane with iron-Schiff base complex

intercalated Zn-Al layer double hydroxide (LDH). This system, unlike the

encapsulated method, interestingly showed higher performance compared to

the homogenous iron-Schiff base complex (without immobilisation). The

authors suggested the well separation and isolation of metal centres from each

other is capable of enhancing the oxidation reaction. But in case of homogenous

system, the formation of inactive oxo dimer species may responsible for the

lower catalytic performance.

4.4 An Overview of Heterogeneous Catalysis development

4.4.1 Summary of the Recent Research Progress

Heterogeneous oxidation of the cyclohexane to Ol/One, transition metals (e.g.

cobalt, iron, or chromium) and rare earth metal (cerium) substituted

mesoporous HMA, SBA-3, SBA 15, KIT and MCM-41 showed active and stable

performances. Titanium and vanadium ions are also active and stable, but much

resistant to leaching in some cases. The physical properties of the catalyst (e.g.

pores size) and the operating conditions (temperature, solvents) greatly affect

the catalytic performance of molecular sieves catalyst. In many cases, acetic acid

has found to be the most suitable solvent to carry out the reactions, but

crystalline faujasite and its sulphated form, Na-GeX and Sulphated Ge-X,

remarkably are capable of performing reactions under free solvent conditions.

The emerging field of catalytic properties using boron- and fluorine-containing

mesoporous carbon nitride polymers also shows fairly good results. This metal

free catalyst is active and stable under the reaction conditions.

Heterogenising homogeneous catalyst through immobilisation is also

extensively investigated. Covalent bonding is the simplest method to achieve

this, but the performances are not satisfactory. Better results were attained

when the catalyst was placed inside of a zeolite or a mesoporous material. This

is particularly successful for nonheme metal complexes, but failed to exhibit

satisfactory results for metalloporphyrin catalysts. In addition, in most cases,

the activities of the attached catalyst are somewhat reduced, but uncommon

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results achieved in the presence of Fe(III)-Schiff base intercalated Zn-Al layer

double hydroxide. The catalyst performance was slightly better than that

homogeneous metal complex. However the H2O2 efficiency is rather low. A

summary of literature reporting on the heterogeneous oxidation of cyclohexane

with hydrogen peroxide is displayed in Figure 4.1

Heterogeneouscatalysts

Molecular sieves

Heterogenisationmetal complexes on solid support

Transition metals

Rare earthmetals

Other metals

Boron, fluorine-carbon nitride

polymer(CNBF)

Ti based catalystsTS-1, Ti-MCM-41, Ti-HMA

V2O5-TiO2

V based catalystsV-MCM-41, V-HMA,

VS-1, V-KIT-6

Co based catalystsCo-HMA, Co-S-1, Co-APO-5

Co-SBA-3, Co-SBA-15Co-MTiO2, CoTiO2,

Co-MCM-41

Fe based catalystsFe-HMA

Cr based catalystsCr-HMA, Cr-MCM-41

Covalent bonds

Intercalation

Multi grafting

Encapsulation

Non-metals

Cerium based catalystCe-MCM-41

Germanium based catalystsNa-Gex, Sulphated Ge-X

Fe Complex - Zn/Al LDH

Fe-montmorilloniteMn-montmorilloniteFe-Silica, Mn-Silica

Salen VO complex-MCM-41

[Cu2([Η]8-Ν4Ο4)]-ΝαΥ

[Ni2([Η]8-Ν4Ο4)]-ΝαΥ

[Co2([Η]8-Ν4Ο4)]-ΝαΥ

[Mn2([Η]8-Ν4Ο4)]-ΝαΥ

FeIII(TPP)- Zeolite Y

Figure 4-1 Summary of researches on heterogeneous catalysis development

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4.7.2 Future Prospect in Heterogeneous Catalysis Developments

Generally there are a number of pros and cons found in the oxidation of

cyclohexane to Ol/One catalysed by heterogeneous system, particularly

transition metal substituted molecular sieves catalysts. First, low stability of the

catalysts under operating conditions was observed in some cases. The examples

are TS-1, VS-1, APO-5 and V MCM-41 that are much less resistant to leaching. As

pointed out by Centi et al (11), leaching of metal may lead to irreversible

catalyst deactivation and it also may greatly affect all of the observed catalysis

activity. The development of alternative metals such as germanic faujasite and

rare earth materials is likely to be a good alternative in the near term.

Second, the preparation and synthesis of many heterogeneous catalysts

based on silicates-aluminates such as MCM-41, HMA, and TS-1 may involve

energy-intensive hydrothermal procedures. The physical properties of the

catalysts such as pore size, surface area and pore volume seem greatly affect the

catalytic performance of molecular sieves catalysts. The formation of pores,

however, involves drying or calcining precipitates of hydrous oxides that is

carried out under extreme conditions and rather time consuming. Development

of new materials of the heterogeneous catalysts that can be environmentally

synthesised, e.g. carbon nitride polymers, seems to have attracts interest for the

future research.

Third, like the homogeneous system, the selection of the solvent plays

fundamental roles in determining the polarity of medium. Various initiators

also have to be found capable of enhancing the reactions. The catalysis reactions

therefore usually require traditional solvents and additives such as acetonitrile,

acetone and methyl ethyl ketone; with an exception of crystalline faujasite Na-

geX and its modified sulphated form that perform the oxidation of cyclohexane

under free solvent reaction. Lastly, the experimental data of H2O2 efficiency is

also little bit scarce.

Some of those disadvantages could be cancelled out by the notable

achievements of research progress in the recent years. Reproducible result of

the repeated catalysts can be an important feature for the industrial application.

Compared with homogeneous catalysts, the distillation used to separate

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reaction products and unused reactants from the homogeneous conventional

cobalt catalyst is typically energy-intensive process and requires high range of

temperatures. This is definitely crucial factor that may result in high

downstream cost. This problem can be eliminated by using heterogeneous

system since catalyst-product separation can be easily done by simple filtration.

However, since the activity that can be obtained with molecular sieve catalyst is

typically inferior to that homogeneous catalyst; this type of catalyst seems to be

not favoured for recent industrial application. The improvement to allow the

homogeneous catalyst separation, recovery and recycle through immobilisation

is likely to be a trend and more preferable in the future development since it

can combine the advantages of homogeneous and heterogeneous catalysts. The

method immobilisation and support material seem to be greatly affected the

activity and the stability of the catalysts. The important milestone of developing

chemically homogeneous catalyst but physically heterogeneous catalyst has

achieved in a recent study. Fe(III)-Schiff base intercalated Zn-Al layer double

hydroxide catalyst succeed in demonstrating that the intercalation does not

reduce the activity of the bound catalyst.

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CHAPTER 5 – DIRECT OXIDATION OF CYCLOHEXENE TO

ADIPIC ACID

5.1 Biphasic reacting System (Phase-Transfer catalysts)

Noyori and co-workers (5) suggested an innovative route to adipic acid

by oxidising cyclohexene with 30% hydrogen peroxides via phase-transfer

catalytic reaction. Hydrogen peroxide is a moderate inorganic oxidant. It can not

form homogenous solutions with most organic substrates; therefore phase

transfer catalyst is required to conduct the reaction. Noyori reported the use of

[CH3(n-C8H17)3N]HSO4 as a phase transfer catalyst (PTC), and sodium tungstate,

Na2WO4 to carry out the reaction as shown in Figure 5.1

+ 4 H202

Cyclohexene

Na2WO4

[CH3(n-C8H17)N]HSO4

Adipic Acid

HOOC

HOOC

Figure 5-1 Direct oxidation of the cyclohexene to adipic acid (5)

The intermediates to define the reaction mechanism involves 6 steps

(Figure 5-2) including epoxidation, hydrolysis of epoxide, ring opening, alcohol,

Baeyer-Villiger oxidations and followed by oxidation to obtain anhydride. In the

last step, hydrolysis of anhydride produces adipic acid at about 93% yield. This

reaction can be done without the presence of any organic and halide solvent.

[H20] [O]

OH OH

O[O]

O

[O] [H20]

O

O

OH

O

O

O

[O]OH

Adipic Acid

HOOC

HOOC

Figure 5-2 Reaction mechanisms of the direct oxidation of cyclohexene to adipic acid (5)

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Even though organic solvents are not used in this process, the co-

catalysts, quaternary ammonium compounds, are not environmentally benign.

Moreover, phase transfer catalysis is a relatively expensive process for

industrial application. Further research therefore suggested the replacement of

phase-transfer catalysts with a complex catalyst peroxytungstate,

[W(O)(O2)2L(2)]2-, using oxalic acid as a ligand (94). Oxalic acid is able to

perform reaction phase-transfer. It forms an ‘oil phase’ of peroxytungstate-

oxalic acid complex to prevent immiscibility between cyclohexane and the

catalysts. The stronger the acidity, the more oleophilic a peroxytungstate-

organic complex system can be formed, and the higher yield can be produced. It

has been shown that an adipic acid yield of 86 % can be obtained after 24 h in

the presence of 0.5% mol (relative to the amount of cyclohexene)

peroxytungstate-oxalic acid complexes catalyst. The yield will increase to 96.6%

by increasing the amount of catalyst to 1% mol. For amount of catalyst up to

1.5% mol, an adipic acid yield of 93.5% can be produced after 8 h. When the

same experiment was carried out using amphiphilic oxodiperoxo tungsten

complex (WO(O2)2·2QOH) with 8-quinolinol (QOH) as a ligand, an adipic acid

yield of 89.8% was obtained after 24 h (95).

Success with peroxometallates has stimulated investigations of

surfactant type peroxytungstates and peroxomolybdates (96) in order to

stabilise the emulsion droplet and achieve more satisfactory activities. The

dimeric anion of polyoxometallates, [M2O3(O2)4]2−(M = W and Mo), served as a

catalyst, and was combined with a long chain lipophilic cation. It possessed

phase-transfer function in the W/O emulsion system to form surfactant type

catalyst (STC) and tuning the hydrophile-liphophile balance of surfactant. The

experimental results showed that the activities of peroxytungstates,

[C16H33N(CH3)3]2[W2O3(O2)4] and [πC5H5NC16H33]2[W2O3(O2)4, gave 77.8% and

78.3% yields, respectively. Molybdenum analogous catalysts,

[C16H33N(CH3)3]2[Mo2O3(O2)4 and [C16H33N(CH3)3]2[Mo2O3(O2)4] resulted in low

activities, affording adipic acid yields of 0% and 15.2% after 20 h.

Despite reports of higher yields of adipic acid under laboratory

conditions, the use of these synthesised surfactant type polyoxometallates are

still expensive for commercial application. The emulsion may also result in

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problematic handling. Another alternative is offered by using a small molecule,

glicine, as a ligand. Heteropoly complexes, glycine

phosphotungstate [HGly]3[PW12O40] · 5H2O exhibited an excellent yield of adipic

acid (95.1%) after 12 h and it has been shown that the reaction is strongly

dependent of pH value of the solution (97).

Table 5-1 Cyclohexene oxidation via water-organic bi-phase catalytic system in free organic solvent reactions

Catalyst catalyst (mmol)

T (oC)

Reaction time (h)

Yield of AA (%)

Ref

[CH3(n-C8H17)3N]HSO4 and Na2WO4

1 75-90 8 93 (5)

[W(O)(O2)2L(2)]2- 0.5A 94 24 86 (94) [W(O)(O2)2L(2)]2- 1A 94 24 96.6 (94) [W(O)(O2)2L(2)]2- 1.5A 94 8 93.5 (94) [W(O)(O2)2L(2)]2- 2A 94 8 94.2 (94)

WO(O2)2·2QOH 0.9 100 10 88.1 (95) WO(O2)2·2QOH 0.9 110 10 86.0 (95) WO(O2)2·2QOH 0.9 90 24 89.8 (95)

[C16H33N(CH3)3]2[W2O3(O2)4] 0.6 90 20 77.8 (96) [πC5H5NC16H33]2[W2O3(O2)4 0.6 90 20 78.3 (96)

[C16H33N(CH3)3]2[Mo2O3(O2)4 0.6 90 20 0 (96) [C16H33N(CH3)3]2[Mo2O3(O2)4] 0.6 90 20 15.2 (96)

[HGly]3[PW12O40] · 5H2OA 0.2 90 12 95.1 (97) (NH4)6Mo7O24 · 4H2O 1.2 70-90B 0.75 0 (98)

(NH4)3PMo12O40 · xH2O 1.2 70-90B 0.75 0 (98) Na3[P(Mo3O10)4] · xH2O 1.2 70-90B 0.75 0 (98)

[CH3(n-C8H17)3N]HSO4 and Na2WO4

1.2 70-90B 0.75 21 (98)

(NH4)6H2W12O40 . xH2O 1.2 70-90B 0.75 30 (98) Na3[P(W3O10)4] · aq 1.2 70-90B 0.75 40 (98)

3Na2WO4. 9WO3 1.2 70-90B 0.75 45 (98) (NH4)6Mo7O24 0.17 90 6 90 (100)

A= in % mol (relative to the amount of cyclohexene) B=under microwave heating programs

Another strategy for improving the catalytic performance was proposed

by Freitag et al (98). The abovementioned Noyori oxidation system was

reinvestigated under microwave (MW) assisted radiations. The microwave

energy is capable of enhancing reaction as a 21% yield of adipic acid was

rapidly formed within 45 min. Furthermore, a various commercially available

tungsten salts and molybdenum salts were also tested to substitute sodium

tungstate. It has been demonstrated that sodium polytungstate, 3Na2WO4.9WO3,

afford the highest yield; resulting a 45% yield of adipic acid at the same

operating conditions. Some of molybdenum salts and ammonium paratungstate

did not produce adipic acid. The major difficulties of the coupling of microwave

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energy for commercial applications are the constraint to the energy input and

the irradiation time to control and setting of the reaction parameters (99).

Even though the application of heteropoly compound and

polyoxometallates in two-phase catalytic systems have been shown to result in

high yields, a highly dispersed emulsion is formed during reaction that behaves

like a homogenous catalyst and is therefore difficult to be recycled. This may

results in significant catalyst loss because of the complexity of the recovery

process. In order to allow easier separation of the catalyst from the product

mixture, designing a suitable reactor to immobilise the phase-transfer catalyst

may remove this disadvantage. Recently, researchers at the University of

Calabria (100) described a symmetric hydrophobic membrane (M3),

characterised by a high water contact angle (R > 110°) for both layers. It

facilitated the oxidation of cyclohexene based on (NH4)6Mo7O24 catalyst and

succinic acid as a ligand. The experiments showed that selectivity toward adipic

acid of 90% can be obtained at 90°C after 6 h. The pros of this reactor are: (i)

providing the compartmentalisation of two reaction phases by means of

polymeric membranes that result in contacting cyclohexene with the catalytic

active species in the aqueous phase (ii) optimisation of catalytic performance in

terms of H2O2 efficiency (iii) separation of the product from organic phase can

also be achieved by this configuration. However membrane stability could be an

important issue for commercial applications. A summary of cyclohexene

oxidation by biphasic catalytic systems is presented in Table 5.1

5.2 Molecular Sieves catalysts

As heterogeneous systems offer the possibility of continuous processing, a

number of researchers have sought another method for liquid phase oxidation

of cyclohexene by using molecular sieve catalysts. A summary of these

experiments is presented in Table 5-2. The researchers at the University of

Cambridge (101) disclosed a result of cyclohexene oxidation using a molecular

sieve catalyst based on TAPO-5 under free-organic solvent conditions.

Cyclohexane conversion of 100% with the selectivity toward adipic acid of

30.3% was obtained after 72 h at 80°C. Better results were obtained by

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Taiwanese researchers (102) using a periodic mesoporous catalyst (PMS) based

on a mixed-valence oxotungsten–silica mesoporous structure (WSBA-15)

incorporating tetrahedral tungsten units. They obtained a conversion of

cyclohexane up to 100% after 13 h with selectivity toward adipic acid of 30%.

The selectivity to adipic acid was increased at about 45% after 30 h.

However because these results do not address the possibility of recycling

of catalysts, Timofeeva et al (103) investigated catalytic activity and stability by

using titanium- and cerium containing mesoporous silicate TI-MMM-2 and Ce-

SBA-15. The catalyst from the first run was filtered and washed and used in the

second catalytic cycle. The results show that the yield of adipic acid decreased

considerably in the second catalytic cycle, despite the fact that 100%

conversion of cyclohexane can be reached. For TI-MMM-2, the yield of adipic

acid decreased from 15% to 5%; also the yield of adipic acid decreased

considerably from 10% to 1% in the presence of Ce-SBA-15 (103). In addition,

both catalysts suffer leaching of metal due to interaction of the surface Ti sites

with the reaction products. Partial loss of mesoporous structure and

destruction of the active sites due to aggressive reaction medium and polar

reaction products result in irreversible catalyst deactivation (103).

Table 5-2 Oxidation of Cyclohexene to adipic acid catalysed by molecular sieves

Catalyst

Weight (g)

T °C

t (h) Ene Conv. (%)

Selectivity (%) HP. Eff (%)

Ref. AA Diol Othera

TAPO-5 0.5 80 24 50 13.1 64.4 22.5 - (101) TAPO-5 0.5 80 72 100 30.3 30 39.7 - (101)

WSBA-15 0.2 85 13 100 30 59.5 10.5 - (102) WSBA-15 0.2 85 30 100 45.9a 42.8 11.3 - (102)

TI-MMM-2b 0.2 80 24 100(100) 15(5) 8(27) 77(68) 76(59) (103) Ce-SBA-15b 0.2 80 72 100(100) 10(1) 12(15) 78(84) 67(55) (103)

aother products including 2-cyclohexene-ol, 2-cyclohexenone, 2-hydroxycyclohexanone, cyclohexandione. cyclooxoheptane-1-ol-6-one and adipic acid anhydride. b Filtered catalyst was used in the second run

5.3 An Overview: Summary and the Future Prospect of Direct Oxidation of

Cyclohexene to Adipic Acid

For the direct oxidation of cyclohexene to adipic acid, biphasic reacting system

(phase transfer catalyst) and molecular sieves catalyst have been extensively

investigated to carry out the reaction. In biphasic reacting system tungsten or

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molybdenum is the typical metals salts used as catalysts. Since the pH plays a

fundamental role in determining solubility in two phases, a ligand therefore

substantially required to prevent immiscibility of the reactants. The presence of

surfactants may replace the use of a ligand because its properties may lower the

interfacial areas. However, the generated emulsions may result in problematic

handling problem. The development of a membrane reactor may resolve the

problem of separating reactant and product. Microwave assisted radiation, on

the other hand, successfully reduces the reaction time and affords a yield of

45% after 45 min.

The use of molecular sieves mesoporous Si, Ti catalysts, whereas, is less

satisfactory since catalyst deactivation is frequently found; thus it can not afford

reproducible results.

However, the major disadvantage of this route is, as shown in the

stoichiometric reaction, that the formation a mole of adipic acid route will

require 4 moles of hydrogen peroxides (figure 2), the relatively high cost of

hydrogen peroxide, thus, is likely become the major drawback of commercial

application (27) The new development of synthesis of hydrogen peroxide

lowering the cost and to afford commercially viable solutions in small- to

medium size plants therefore seems to be crucial to make this process become

more attractive (104). The indirect anthraquinone process, which is only

economically applicable for relatively large-scale production, accounts for 95%

of global total H2O2 production. However, recently there is a significant progress

of inexpensive and more environmentally benign production based on direct

synthesis of H2O2 in the presence of Pd and Au-Pd catalyst that are capable of

affording selectivity more than 80% (105).

Another drawback of this process relates to feedstock availability. As

cyclohexene is not a readily available raw material, the production of

cyclohexene must be synthesised from another materials. Cyclohexene can be

obtained from (27; 106):

(i) The dehydration of cyclohexanol

(ii) The oxidative dehydrogenation of cyclohexane

(iii) Partial hydrogenation of benzene and

(iv) Dehydrohalogenation of cyclohexyl aldehydes.

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The first method may not be preferred as cyclohexanol can be oxidised directly

to yield adipic acid, whereas the oxidative dehydrogenation of cyclohexane

involves energy-intensive process and the yield is typically not high. Tavolaris

and Keane (107) proposed an innovative strategy through

dehydrohalogenation of cyclohexyl aldehydes. Despite having the potential

advantages of recycling halogenated compound (27), this process suffers

serious catalytic problem due to coke formation during the reaction. The most

viable option is partial hydrogenation of benzene developed by Asahi Chemical

(108; 109). This method is currently implemented for commercial use and

affords yield of cyclohexane up to 60%. However this process is also somewhat

problematic since the hydrogenation to cyclohexene (ΔG° = −98 kJ/mol, at 298

K) is thermodynamically favoured instead of the partial hydrogenation to

cyclohexene (ΔG° = −23 kJ/mol) (106).

°

Phase transfer catalysts

HOOC

Adipic Acid

HOOC

OHCl

Cyclohexene

OxidativeDehydrogenation

PartialHydrogenation

Dehydrohalogenation Dehydration

Benzene Cyclohexane CyclohexanolCyclohexyl halides

Direct (one-step) oxidation

Molecular sieve catalysts

in commercial use

in research phase

Figure 5-3 Summary of possible synthetic pathways to AA from direct oxidation of cyclohexene (27; 106)

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Nevertheless this innovative route abides by the rule of green chemistry

and would eliminate the conventional pathway via cyclohexane or cyclohexanol

and thus it also avoid both atmospheric and non-atmospheric waste formation

generated from two steps oxidation of cyclohexane. Since study in this area is

still active, the step toward a new and sustainable adipic acid synthesis from

this route remains possible, but it is highly dependent on the research progress

of the preparation of hydrogen peroxide and cyclohexene. A summary of

possible synthetic pathways to adipic acid from this route is presented in Figure

5-3.

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CHAPTER 6 - CONCLUDING REMARKS AND OUTLOOK

The oxidation of the cyclohexane to Ol/One and direct oxidation of cyclohexene

to adipic acid in the presence of hydrogen peroxide have been reviewed. The

use of homogenous catalyst offers advantages in terms of possibilities to tune

the performance, through modifying the properties of ligands. It can gain better

level of understanding on a molecular level and generally results in better

activities compared to the solid or heterogeneous catalyst (3). However, these

catalyst–ligand systems seem to be quite expensive and they can be unstable

under reaction conditions. The preparation of transition metal substituted

molecular sieve catalysts, on the other hand, is typically time consuming and

energy intensive, but it would not result in a considerable problem as long as

the catalyst can be easily recovered and give reproducible result. However,

leaching of metal, arise in some cases, can lead catalytic deactivation.

Nevertheless research in this area is still active, apparently driven by the

potential enormous benefit of realising success and encouraged by

development of various new classes of catalysts. The examples are germanic

faujasite, rare earth materials, and carbon nitride nanomaterials.

The biggest barrier to overcome is to combine the advantages of

homogeneous catalyst and heterogeneous catalyst. There are two alternatives

to answer this challenge; either immobilise homogenous catalyst on

heterogeneous support or with liquid support such as water, supercritical fluid

or ionic liquids. The latter method may also offer great advantage in terms of

substitute the use of traditional and ‘unacceptable’ solvents such as acetonitrile.

The further studies related to support materials and methods of immobilisation

are likely to attract great interest in near future.

Direct oxidation of cyclohexene, on the other hand, is also attractive

because it eliminates one step process and avoids the nitrous oxide emissions to

the atmosphere and aqueous nitric acid waste and handling. Biphasic transfer

catalysis is the most promising subject of research in this field. Maximising

catalyst hydrophobicity/oleophilicity is the successful key in this biphasic

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reaction. It can be achieved by modifying ligands to achieve desirable properties

that are able to prevent immiscibility between cyclohexene and catalyst.

Another approached proposed to improve the contact between reactants are

the use of a membrane reactor and surfactants type catalysts. Microwave

assisted radiation, on the other hand, is hugely beneficial in reducing reaction

times, but a detail cost-benefit analysis may also be required. This route is,

however, prohibitive due to the relatively cost of feedstock and hydrogen

peroxide.

In today adipic acid industry, the current production technologies are

considered ‘mature’, where extensive research efforts is not expected to

substitute the conventional methods. Furthermore the nature of hydrogen

peroxide that can easily decompose to water and oxygen under the reaction

conditions may decline the efficiency of hydrogen peroxide utilisation; thus

hinder its industrial application. However the oxidation under hydrogen

peroxide is much less energy intensive than that of air, molecular oxygen or

nitric acid; and the profit margin can be obtained by significant catalysis

performances. Therefore, the possibility of implementing synthesis of adipic

acid using hydrogen peroxide at the commercial level is still widely open;

although to date, it seems to be highly unlikely.

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GLOSSARY

AA adipic acid

Ane cyclohexane

APO aluminium phosphate

A/K alcohol/ketone

BMPA (bis-(2-pyridilmethyl)amine)

Bpmen N,N’-dimethyl-N,N’-bis(2-pyridylmethyl)-1,2

diaminoethane]

CNBF Carbon nitride boron fluorine

CHHP cyclohexyl peroxide

Conv. conversion

DMSO dymethyl sulfoxide

DCM dichloromethane

Eff. efficiency

Ene cyclohexene

gma glyoxal-bis (2-mercaptoanil)

HMA hexagonal mesoporous aluminophosphates

HP hydrogen peroxide

H3tea triethanolaminate

h hour(s)

IL ionic liquid

KA ketone/alcohol

KIT Korea Advanced Institute of Science and Technology

LDH layer double hydroxide

MCM mobil composition of matter

MeCN acetonitrile

MEK methyl ethyl ketone

MMM microporous mesoporous material

MMO methane monoxygenase

MW microwave

min minute(s)

mqmp 2-methoxy-6-((quinolin-8-ylimino) methyl)phenol

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NCS N-Chlorosuccinide

n.d not determined

Ol/One cyclohexanol/cyclohexanone

PDCA Pyrazine-2, 3-dicarboxylic acid

Phen phenanthroline

POMs Polyoxometallates

Pz (1-pyrazolyl)

TAPO titanium aluminophosphate

Temp. temperature

TBHP t-butyl hydro peroxide

TF4TMAPP (2,3,5,6-tetrafluoro-N,N,N,-trimethyl-4-anilinium)

porphyrin

Tmima tris[( I-methylimidazol-2-yl)methyl]amine]

TON turnover number

tpcaH bis(2-pyridyl)methyl-2-pyridylcarboxamide

TPP tetraphenylphorphyrin

SBA Santa Barbara amorphous

Select. Selectivity

Solv. Solvent

STC surfactant type catalyst

QOH 8-quinolinol

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REFERENCES

1. McKetta, J.J. Encyclopedia of Chemical Processing and Design. New York :

Marcel Dekker, 1977.

2. Sri Consulting. World Petrochemicals Report on Adipic Acid. [Online] 2010.

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