6738 Chem. Commun., 2012, 48, 6738–6740 This journal is c The Royal Society of Chemistry 2012

Cite this: Chem. Commun., 2012, 48, 6738–6740

Hybrid porous tin(IV) phosphonate: an efficient catalyst for adipic acid

synthesis and a very good adsorbent for CO2 uptakew

Arghya Dutta,aMalay Pramanik,

aAstam K. Patra,

aMahasweta Nandi,

bHiroshi Uyama

b

and Asim Bhaumik*a

Received 30th March 2012, Accepted 9th May 2012

DOI: 10.1039/c2cc32298f

A new porous organic–inorganic hybrid tin phosphonate material has

been synthesized hydrothermally, which shows a Brunauer–Emmett–

Teller surface area of 723 m2g�1

and it adsorbs 4.8 mmol g�1

CO2

at 273 K and 5 bar pressure. The material also shows remarkable

catalytic activity in one-pot liquid phase oxidation of cyclohexanone

to adipic acid under eco-friendly conditions.

Adipic acid is an important chemical for the production of

nylon-6,6, which is an essential polymeric material for our

daily needs. Most of the industrial processes for the production

of adipic acid involve nitric acid oxidation of cyclohexanol or

cyclohexanol–cyclohexanone mixtures.1 But in this process,

nitrous oxide (N2O) comes out to be an unavoidable chemical

waste, which contributes significantly to global warming. So a

green synthetic route for the production of adipic acid is highly

desirable. Sato et al. reported for the first time a sodium

tungstate catalyzed direct oxidation of cyclohexenes to adipic

acid by using hydrogen peroxide as the oxidant.2 To date most

of the known reactions have involved tungsten based catalysts3

and either hydrogen peroxide4 or other peroxides as oxidants.5

On the other hand, adipic acid can be synthesized via selective

hydrogenation of trans,trans-muconic acid.6 But the cost

involved in the synthesis of the catalysts as well as the catalytic

reactions is a matter of concern for these catalytic routes to be

industrially attractive. Although there are a few reports on

homogeneous catalysis for the conversion of cyclohexane to

adipic acid with molecular oxygen as oxidant,7 in most of the

cases organic solvents, and sometimes high pressure reactors,

are needed to make the process feasible. Apart from these, it is

always desirable to have a heterogeneous catalyst for the ease

of separation of the product from reaction medium. In this

context it is pertinent to mention that microporous and

mesoporous materials are known for long as good heterogeneous

catalysts for different types of eco-friendly catalytic reactions.8

As a subclass of porous materials, organic–inorganic hybrid

porous solids represent a fascinating class of materials, which

can impart the rigidity of inorganic solids as well as the

functionality of organic building blocks.9 Introduction of porosity

along with multi-functional organic groups makes them important

contributors in the fields of gas storage/adsorption,10 catalysis,11

sensing,12 selective separation/ion-exchange,13 etc.

On the other hand, emission of greenhouse gases due to the

combustion of fossil fuels has been a matter of major environ-

mental concern in recent times. There are several reports on

metal–organic frameworks,14 covalent organic frameworks,15

porous carbons,16 polymers17 and amine modified silica18

materials exhibiting high CO2 adsorption. But to date CO2

adsorption on phosphate based porous materials has been

very rare.19 Herein, we report for the first time a hybrid porous

tin(IV) phosphonate (HMSnP-1) material with 723 m2 g�1

surface area and good catalytic activity for the one-pot

oxidation of cyclohexanone to adipic acid in aqueous medium

just by purging the reaction vessel with a balloon filled with air.

This material also shows very high CO2 uptake at 273 K and 5 bar

pressure. Pentaethylenehexamine-octakis-(methyl phosphonic acid)

hexadecasodium salt solution (PEHMP) was used as the

phosphonate source and cetyl trimethylammonium bromide

(CTAB) was used as the structure directing agent (SDA).

Hybrid tin phosphonate materials reported so far in the

literature are mostly microporous in nature20 and their syntheses

mostly followed non-templating pathways. There are also reports

on bothmicroporous21 andmesoporous22 tin phenylphosphonates

using sodium dodecylsulfate (SDS) as the structure directing agent.

But surface areas of these materials are much lower compared to

the present report. Interestingly, the HMSnP-1 material showed

three different types of pores. Apart from template assisted

mesopores, the material also showed micropores due to cross-

linking of the ligand and interparticle porosity originating

from aggregation of nanoparticles. Scanning electron micro-

scopic (SEM) images (Fig. 1a and b) show that there are

tiny spherical particles of approximately 70 nm in diameter

almost uniformly aggregated throughout the material. From

the high resolution transmission electron microscopic (TEM)

images (Fig. 1c and d) it is clear that there are wormhole-

like micropores in the framework of the material together

with randomly distributed mesopores of dimension ca. 3 nm

aDepartment of Materials Science, Indian Association for theCultivation of Science, Jadavpur, Kolkata 700 032, India.E-mail: msab@iacs.res.in; Fax: +91-33-2473-2805

bDepartment of Applied Chemistry, Graduate School of Engineering,Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan

w Electronic supplementary information (ESI) available: Completeexperimental details including catalysis, XRD, solid state MASNMR and FT-IR spectra, N2 adsorption isotherm, 1H, 13C NMRand FT-IR spectra of adipic acid, CO2 adsorption and catalystrecycling. See DOI: 10.1039/c2cc32298f

ChemComm Dynamic Article Links

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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 6738–6740 6739

(ESIw, Fig. S1: SEM, TEM and STEM images). Graphical

representation of three types of pores is shown schematically

in Scheme 1.

In the small angle powder X-ray diffraction pattern (ESIw,Fig. S2), the as-synthesized and the extracted materials show

broad peaks at 2y = 2.881 and 2.981, respectively, indicating

weak short-range ordering of mesopores. This shift in 2y valueis due to the contraction of pore aperture during template

removal and it indicates an average inter-pore spacing of 2.9 nm

for the extracted sample. The broadness of the peaks could be

attributed to aggregation of nanoparticles on the nanoscopic

length scale. In the wide angle PXRD pattern (ESIw, Fig. S3),there are some broad and low intense diffraction peaks which

may be due to the presence of a tin oxide (cassiterite) phase as

impurity. Elemental analysis on HMSnP-1 suggests a C/N/Sn

molar ratio of 17.68 : 5.70 : 4.00 in the hybrid tin phosphonate

material HMSnP-1. This observation is in agreement with the

C/N molar ratio of 18 : 6 in the organophosphorus precursor

PEHMP (ESIw, Scheme S1). Thermogravimetric analysis (ESIw,Fig. S4) suggests a total of 21% weight loss in the temperature

range of 400 K to 1073 K and it is nearly the cumulative weight

percentage of C, H and N as obtained by CHN analysis. From

these elemental and thermal analyses data the material can be

formulated as Sn4(C18H36N6O24P8)�xH2O. Detailed characteriza-

tion of framework structure and bonding was performed by using

FT-IR and solid state MAS NMR spectroscopic studies (ESIw,Sections S6 and S7: Fig. S5–S7).

The material shows a Brunauer–Emmett–Teller (BET) surface

area of ca. 723 m2 g�1 and a pore volume of 0.87 cc g�1. The

N2 adsorption–desorption isotherm (ESIw, Fig. S8) at 77 K

shows that a considerable amount of nitrogen has been adsorbed

under the relative pressure below 0.01, indicating microporosity

in the framework. The pore size distribution was calculated by

applying non-local density functional theory (NLDFT) and

the material shows a hierarchical pore system with three

different types of pores (ESIw, Fig. S9) and this has been

retained after repeated catalytic cycles (ESIw, Fig. S10 and S11).

The micropores of diameter 1.39 nm originated via cross-linking

of the multidentate ligand PEHMP with Sn(IV) sites. In the

intermediate relative pressure (P/P0) region of 0.02 to 0.4, the

adsorption isotherm shows gradual increase in N2 uptake,

indicating some amount of multilayer adsorption along with

very small hysteresis. The latter could be due to the existence of

large mesopores in the material arising from the interaction of

the framework with the CTAB molecules.23 Further increase in

N2 uptake at higher P/P0 is attributed to interparticle pores.

The cross linking and the templating mechanism have been

explained in ESIw, Section S9 (ESIw, Fig. S12). The surface

area of HMSnP-1 is much higher than the previously reported

microporous tin phosphonate materials.20 Often the combination

ofmicropores andmesopores in the samematerial is advantageous

from the catalytic view point because of enhanced accessibility of

micropores and increased mass transport in the mesoporous

channels compared to microporous material alone. On the other

hand textural porosity due to aggregation of small particles gives

better access to these framework mesopores.24 Thus, the presence

of three types of pores in HMSnP-1 could be useful for improving

catalytic processes.

CO2 is one of the major contributors in greenhouse gases.

Thus considerable research efforts have been focused on

developing carbon dioxide sequestering materials. Due to the

presence of framework nitrogen in the organophosphorus

ligand PEHMP, we were interested in examining the CO2

adsorption property of the material HMSnP-1. It shows CO2

adsorption of 4.8 mmol g�1 at 273 K under 5 bar pressure

(Fig. 2). This adsorption capacity is much higher compared to

previously reported phosphonate based metal–organic framework

materials.19 At 298 K, the material adsorbed 1.4 mmol g�1 CO2

under 5 bar pressure and 0.91 mmol g�1 under ambient conditions.

When this value was compared with the previously reported

ordered titanium phosphonates,25 it was seen that TiP materials

having surface area more than 1000 m2 g�1 showed little better

CO2 adsorption under similar experimental conditions but

those having surface area around 500 m2 g�1 or so had lower

Fig. 1 Scanning electron microscopic images of HMSnP-1 (a and b).

HR TEM images of HMSnP-1 sample (c and d).

Scheme 1 Graphical presentation of three types of pores.

Fig. 2 CO2 adsorption–desorption isotherm of HMSnP-1 at 273 K.

Adsorption points are marked by filled symbols and desorption points

by empty symbols.

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6740 Chem. Commun., 2012, 48, 6738–6740 This journal is c The Royal Society of Chemistry 2012

adsorption capacity. This is because high surface area as well as

the nature of organic moiety in the organic–inorganic hybrid

framework plays a crucial role in influencing the CO2 adsorption

capacity of a material. When recyclability of HMSnP-1 for CO2

adsorption was tested, the material showed almost the same

adsorption capacity after 3 cycles and the difference in the results

falls in the range of experimental error (ESIw, Fig. S13).Tin containing microporous materials are well known for

catalyzing Baeyer–Villiger oxidation of cyclohexanone.26 This

has motivated us to explore the possibility of Baeyer–Villiger

oxidation over HMSnP-1 without using any peroxide as

oxidizing agent. So without using any peroxide we tried to carry

out BV oxidation in aqueous medium, using cyclohexanone as the

substrate. Surprisingly, a white coloured crystalline compound

was obtained as the final product and the compound was

characterized to be adipic acid. Yield of the product was calculated

to be 74% with respect to the initial cyclohexanone.

The same reaction was carried out using mesoporous tin

phosphate27 as the catalyst to verify the role of the organic

functionality of HMSnP-1. After the same reaction time, the

yield of adipic acid was found to be 22%. Probably the

presence of free amine in the spacer group of the material

helps to stabilize the keto–enol tautomerization reaction and

the enol form is stabilized by the metal (tin) present in the

porous framework. Simultaneously, the metal present in the

framework activates the molecular oxygen and helps to form

the cyclic six membered transition state, which further undergoes

rearrangement to form a cyclic ester. Under the reaction conditions

the cyclic ester gets hydrolyzed to form 6-hydroxo-hexanoic acid,

which further oxidizes to adipic acid. A proposed mechanism is

shown in Scheme 2. While using methanol as solvent, 6-hydroxo-

methyl hexanoate is obtained as the final product. Probably due to

the lower solubility of molecular oxygen in methanol, further

oxidation of the primary alcohol could not proceed in methanol.28

The catalytic reaction was repeated over five cycles and there was

no significant loss of catalytic efficiency (ESIw, Fig. S14). FT-IRspectroscopy (ESIw, Section S6) and N2 adsorption (ESIw, SectionS9) experimental results of the catalyst show that the framework

bonding and porous structure are retained after repeated catalytic

cycles and this is the reason behind the limited loss in catalytic

activity of our hybrid tin phosphonate material.

Hierarchically porous organic–inorganic hybrid tin phos-

phonate material HMSnP-1 has been synthesized hydrothermally

by using PEHMP as the organophosphorus precursor and CTAB

as the structure directing agent. This novel material showed

excellent catalytic activity in direct one-pot oxidation of cyclo-

hexanone to adipic acid using molecular oxygen under liquid phase

conditions and very good CO2 adsorption capacity at 273 K.

AD, MP and AKP thank CSIR, New Delhi, for their

respective senior research fellowships.

Notes and references

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Scheme 2 Possible mechanistic pathway for the synthesis of adipic

acid over HMSnP-1 in the presence of O2.

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