Organic–inorganic hybrid porous sulfonated zinc phosphonate material: efficient catalyst for biodiesel synthesis at room temperature

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www.rsc.org/greenchem

1463-9262(2010)12:9;1-U

ISSN 1463-9262

Cutting-edge research for a greener sustainable future

www.rsc.org/greenchem Volume 12 | Number 9 | September 2010 | Pages 1481–1676

COMMUNICATIONLuque, Varma and BaruwatiMagnetically seperable organocatalyst for homocoupling of arylboronic acids

CRITICAL REVIEWDumesic et al.Catalytic conversion of biomass to biofuels

Green ChemistryD

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http://dx.doi.org/10.1039/c2gc35225g

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Organic-inorganic hybrid porous sulfonated zinc phosphonate material:

efficient catalyst for biodiesel synthesis at room temperature

Malay Pramanik, Mahasweta Nandi, Hiroshi Uyama and Asim Bhaumik*

Highly porous new organic-inorganic hybrid zincphosphonate material has been synthesized by

using p-xylenediphosphonic as organophosphorous precursor and its surface sulphonated

material showed outstanding catalytic activity in biodiesel synthesis at room temperature.

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Organic-inorganic hybrid porous sulfonated zinc phosphonate material:

efficient catalyst for biodiesel synthesis at room temperature

Malay Pramanik,† Mahasweta Nandi,‡ Hiroshi Uyama‡ and Asim Bhaumik*,†

†Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur,

Kolkata-700 032, INDIA, *Corresponding author. E-mail: [email protected]

‡Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1

Yamadaoka, Suita, 565-0871, JAPAN

Abstract

A new porous zinc phosphonate material (HZnP-1) has been synthesized through the

reaction between p-xylenediphosphonic acid and anhydrous ZnCl2 under hydrothermal condition

at mildly acidic condition (pH~5) in the absence of any structure directing agent. Phenyl group

of this material has been sulfonated with concentrated sulfuric acid to obtain sulfonic acid

functionalized material HZnPS-1. Powder XRD, FE SEM, N2 sorption, solid state 13C CP MAS

and 31P MAS NMR, and FT IR spectroscopic tools are employed to characterize these materials.

The crystal structures of both the materials are indexed corresponding to the new orthorhombic

phases with unit cell parameters a=11.00 Å, b=8.74 Å, c=14.62 Å, α=β=γ=90 for HZnP-1, and

a=10.65 Å, b=13.52 Å, c=15.30 Å and α=β=γ=90 for HZnPS-1. HZnPS-1 showed outstanding

catalytic activity and high recycling efficiency for the synthesis of different biodiesel compounds

via esterification of long chain fatty acids by using methanol as reactant-cum-solvent at room

temperature. The green and eco-friendly catalytic system described herein can overcome the

problem faced by the existing catalytic systems known in biodiesel synthesis, like drastic

conditions (high reaction temperature) and requirement of hazardous organic solvents.

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Keywords. Biodiesel; hybrid metal phosphonate; mesoporosity; esterification; surface acidity.

Introduction

Biodiesel1 has attracted huge public and scientific attention today, driven by factors such

as the need for increased energy security,2 oil price hikes and concern over the greenhouse gas

emission from fossil fuels.3 Thus, from energy as well as environmental point of view, biofuels

can be one of the best alternatives. Further, being a renewable source of energy, free from

hazardous components (sulfur and aromatic compounds)4 and no need for any further major

modification for use in the actual compression and ignition engine,5 the demand for biodiesel is

increasing every day. Chemically biodiesels are produced via esterification of long chain fatty

acids (lipid) with an alcohol.6 However, the production of these esters has encountered a lot of

technical and mechanical problems.7 Traditionally, the esterification reaction is carried out in

organic solvents in the presence of concentrated sulphuric acid as the catalyst under refluxing

conditions. Catalyst separation, product isolation, energy consumption and extraction of water

present in the azeotropic mixture with the organic solvent from the reaction medium are major

problems in the synthesis of biodiesel.8 Therefore many alternative strategies have been

proposed to overcome these problems. Several catalysts have showed the potential to replace

sulphuric acid, like biocatalyst lipase,9 molecular sieve,10 solid acid,11 ion exchange resin12 and

phase-transfer catalyst.13 But in most of these cases the reactions are carried out at moderately

high temperature.14 In recent years porous solid acid based heterogeneous catalysts are attracting

increasing interest in biodiesel synthesis.15 MOF encapsulated Keggin heteropolyacid,16 acid

functionalized hybrid silica,17 Brönsted acidic ionic liquids bearing sulfonic acid group18 have

been successfully employed for the esterification reactions very recently. But in most of these

cases the reactions are carried out under drastic conditions in the presence of hazardous organic


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solvents. Thus, a green and eco-friendly methodology for the production of bio-fuel is highly

desirable.19 Further, there are only very few reports on the synthesis of biodiesels at room

temperature.20 The main problem of this reaction is the reversibility of the esterification

process.12 The ester molecules produced during the reaction could be easily hydrolyzed in the

presence of water, which is formed as a by-product. So our aim is to design a new heterogeneous

porous catalyst, which can easily accommodate the water molecules formed in the reaction

mixture. This will facilitate the forward reaction at room temperature in the presence of a bare

minimum amount of alcohol, which could act as a reactant-cum-solvent in the whole process.

On the other hand, there are very few reports on the synthesis of porous zinc(II)

phosphonate by using aromatic biphosphonic acid as the spacer group.21 Organophosphorous

spacers could be utilized for the synthesis of a variety of porous phosphonate materials, which

are largely utilized in many efficient catalytic reactions.22,23 Recently Shimizu et al. have used1,4-

benzenediphosphonate-bis(monoethyl ester) as the organic spacer group for the production of

porous zinc phosphonate.24 The BET surface area of the resulting material was only 4.5 m2g-1.

Here we have synthesized p-xylenediphosphonic acid24 via phosphorylation of the respective

aromatic dibromide and employed it as the bridging organophosphorous precursor. The new

orthorhombic zinc(II) phosphonate (HZnP-1) formed by using this precursor showed flake-like

crystal morphology and hierarchical large scale porosity under mild hydrothermal conditions.

Further, the benzene ring in the zinc phosphonate framework can be sulfonated to increase the

surface acidity of the porous architecture. The crystalline sulphonic acid functionalized hybrid

porous zinc phosphonate material thus formed showed outstanding catalytic activity in the

esterification of long chain fatty acids (lipids) at room temperature. To the best of our knowledge


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HZnPS-1 is one of the very rare heterogeneous catalysts, which showed excellent catalytic

activity in the synthesis of biodiesel at room temperature.

Results and Discussion:

Crystal structure of HZnP-1 and HZnPS-1. The powder X-ray diffraction pattern of HZnP-1

is shown in Figure 1. The diffraction peaks can be assigned very well with a new orthorhombic

phase having unit cell parameters a=11.00 Å, b=8.74 Å, c=14.62 Å and α=β=γ=90° (Table 1S).

The unit cell volume of HZnP-1 is 1371.48 Å3 with very low standard deviation. For the

sulphonated material, HZnPS-1, due to the incorporation of sulphonic acid into the porous

architecture the cell parameters and the unit cell volume changes. The diffraction peaks of

HZnPS-1 (Figure 2) could also be assigned to an orthorhombic phase with unit cell parameters

a=10.65 Å, b=13.52 Å, c=15.30 Å,α=β=γ=90° and unit cell volume V=2203.27 Å3 (Table 2S).

This increment in unit cell volume is in quite good agreement with the results on the

sulphonation of benzene ring of related porous framework material.28 The increase in unit cell

volume suggests that the sulphonic acid groups are not entrapped into the porous framework,

rather functionalization of the aromatic ring at the surface of the catalyst has taken place.29 The

crystal plane parameters (hkl) and corresponding d spacing values for the two materials are given

in supporting information (Tables 1S and 2S, respectively). The peaks are assigned using the

REFLEX software and the standard deviations of the unit cell parameters and volumes of the

unit cell are determined by using CELSIZ program. The very small deviations of the calculated

data from the experimental values agree well with estimated orthorhombic crystal phases for

both the materials.

Electron microscopic analyses. Although, the crystal phase of the material (HZnPS-1) remains

same after sulphonation, but the change in particle morphology and size was clearly noticeable.


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FE SEM images of HZnP-1 and the sulphonated material HZnPS-1are shown in Figure 3. As

seen in the figure for HZnP-1 thin flake-like particles having dimensions of ca.350-450 nm long

and ca. 30 nm in width are uniformly distributed throughout the specimen. However, the

sulphonated material HZnPS-1 showed much bigger brick-like particle morphology with clear

crystal edges. The average length, height and width of HZnPS-1 particles are 5.08 µm, 1.32 µm

and 1.66 µm, respectively.

Hierarchical porosity. The N2 adsorption/desorption isotherms of HZnP-1 are shown in Figure

4. Brunauer-Emmett-Telller (BET) surface area, average pore diameter and pore volume of

HZnP-1 sample were estimated from the adsorption/desorption isotherms. The isotherms can be

classified as typical type IV, characteristic of mesoporous materials together with a H3 type

hysteresis loop,30 indicating substantial textural mesoporosity with narrow slit-like pores.31 A

strong capillary uptake for the adsorbed gas at relatively high pressure (0.70-0.90) suggest the

presence of appreciable amount of mesopores in the framework. The mesoporosity of the

material can originate from the aggregation of the flake-like particles and inter particle voids.23b

The BET surface area of HZnP-1 is 105 m2g-1 with total pore volume of 0.39 cm3g-1.

Corresponding pore size distribution employing non-local density functional theory (NLDFT)

model is shown in the inset of Figure 4. The pore size distribution showed peak centered at 5.78

nm. Upon sulfonation of the benzene ring the surface area and pore volume of the material is

decreased to 40 m2g-1 and 0.21 cm3g-1, respectively, in HZnPS-1, whereas the pore size

distribution profile showed decrease in pore width to 3.89 nm. Decrease in pore width after

sulfonation suggested the functionalization of the phenyl rings preceded at the periphery of the

pores.


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Thermal stability. To investigate the thermal stability of these phosphonate materials (HZnP-1

and HZnPS-1) TG-DTA analyses have been carried out. For HZnP-1, first decrease in weight

(ca. 0.81 %) up to 319 K corresponds to the loss of water molecules in the framework. The

second and third weight losses (together ca. 13.6 %) up to 656 K are probably due to the burning

of C-C and C-P bonds present in the framework. On the other hand, for HZnPS-1, first two

weight losses (together ca. 5.6 %) are probably due to the loss of two types of water molecules

present in the framework. First weight loss could be attributed to the physically adsorbed water

present at the pore surface and second one due to the chemisorbed water molecule having non-

covalent interaction with free sulphonic acid group. The third weight loss (ca. 23.3 %) could be

attributed to cleavage of C-C, C-P, C-S bonds and the burning of organics present in the

framework and it is continued up to 700 K. From the thermal analysis data it can be suggested

that both the material have considerably high thermal stability.

Spectroscopic studies. The FT IR spectra of the materials HZnP-1 and HZnPS-1 are shown in

Figure 5. For HZnP-1 (Figure 5a), the peak at 570 cm-1 is the characteristic stretching vibration

of Zn-O group. The set of bands at 990 and 1085 cm-1 can be assigned to stretching vibration of

the tetrahedral CPO3 groups. Two characteristic peaks at 1418 cm-1 and 1523 cm-1 could be

attributed to the presence of benzene ring in the porous framework. The peak at 1623 cm-1

corresponds to the bending vibration of water molecules. Two bands at 2925 and 3025 cm-1 can

be assigned to the symmetric and asymmetric stretching vibration of –CH2 groups whereas the

broad peak centered at 3413 cm-1 reveals the presence of defect P–OH groups and free water

molecules bound to the surface of the framework. Thus the FT IR spectroscopic analysis

suggested the presence of p-xylenediphosphonicspacer group in porous zinc phosphonate

framework. For HZnPS-1 (Figure 5b), all peaks of HZnP-1 has been retained, which suggested


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that upon sulfonation the main framework remains intact. An additional band at 748 cm-1 is

generated, which could be assigned to the aryl-sulphur stretching frequency. Apart from that

vibration bands at 1005 and 1080 cm-1 are originated corresponding to the symmetric and

asymmetric stretching of (S=O) bond.32 The broadening of the peak centered around 3000 cm-1 is

probably due to the presence of defect –OH as well as hydroxyl group present in free sulphonic

acid.

Solid State MAS NMR studies. 13C CP MAS and 31P MAS NMR experiments often provide

useful information regarding the chemical environment around the C and P nuclei and the

presence of organic functional groups in metal phosphonate frameworks. In the Figure 6, 13C CP

MAS NMR of HZnP-1 is shown. The signals at 131.7 and 128.4 ppm are due to the C1 and C2

carbon of the spacer group in the porous matrix.33Signals at two different chemical shifts 35.5

and 34.7 ppm for the benzylic carbon provide important information about the asymmetric

environment of the two carbon atoms (C3 and C4). These results indicate that the p-

xylenediphosphonic acid group is covalently grafted in between the inorganic composite

framework. On the other hand, the 31P MAS NMR spectrum of HZnP-1 sample (Figure 7) shows

one strong signal at 32.3 ppm. This could be attributed to the ((OH)(OZn)2P-C8H8–

P(OZn)2(OH))species.34 Up-field shift in the 31P signal further suggested the presence of p-

xylenediphosphonic acid moiety in the framework.

Molecular formula of the catalyst. From the TGA-DTA analysis of the sample (HZnPS-1) it is

confirmed that the sulphonated framework contains 23.3 % organics including sulfur. CHNS

analysis data further revealed their contents as: 11.2 % carbon, 3.4 % hydrogen, 0 % nitrogen

and 7.8 % sulfur. Atomic Absorption Spectroscopy (AAS) indicate the presence of 13.2 % zinc.

Hence the molar ratio of Carbon (C): Zinc (Zn): sulfur (S) is 0.93:0.44: 0.48 and the molecular


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formula of the catalyst should be LZn4S4, x H2O (where L= p-xylenediphosphonic acid and x is

the number of adsorbed water molecules per unit of formula weight). Thus the molecular

formula of the catalyst can be written as C8H12O18P2S4Zn4, xH2O.

Catalysis: We have used various long chain mono and bicarboxylic acids for esterification

reactions at room temperature and the results are given in Table 1. The yield of the esterified

products was measured through net weigh after drying and the compounds were characterized by

1H and 13C NMR spectroscopy. When the reaction was carried out in the absence of catalyst

(oleic acid as the representative fatty acid) the yield of the reaction was only 5.0 % (Table 1,

entry 8, conversion was measured by GC). Usually the long chain fatty acids are esterified at

high temperature (338 - 351 K) in the presence of concentrated sulphuric acid as the catalyst.35

Since it is very difficult to separate the homogeneous catalyst from the reaction mixture,36 it is a

big challenge to remove the in-situ formed water from the esterification reaction, which is

present in the azeotropic mixture.37 As seen from Table 1, the sulphonated zinc phosphonate

HZnPS-1 acts as a very efficient heterogeneous catalyst for the production of methyl esters of

long chain fatty acids at room temperature using methanol as reactant-cum-solvent.

Role of methanol. It is pertinent to mention that for the esterification reaction over HZnPS-1 we

have used little excess methanol (stoichiometrically) than the corresponding fatty acids. So the

obvious question arises whether the term ‘solvent-free’ is applicable here or not? To understand

the role of methanol, we have studied few more controlled reactions (Table 2). When we have

used acid : methanol in 1:1 stoichiometry, then also the conversion is quite appreciable (81% for

lauric acid). Further, when we have used lauric acid : methanol =1:2.5 (minimum amount of

methanol to dissolve the solid fatty acids at room temperature), the conversion reaches up to 95.0

wt% (studied for lauric acid only) at room temperature in 24 h. Hence, we can conclude that


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methanol plays the role of reactant-cum-solvent in the biofuel production over this hybrid

catalyst. Then the question comes why should we use excess methanol for this reaction? The

answer is, if we use stoichimetric amount of acid and methanol (1:1) then some practical

problem arises. After completion of the reaction excess fatty acid, methanol, product ester and

the catalyst forms a viscous mixture. The purification procedure is little bit difficult now.

Chloroform was added to the round bottom flask and the catalyst was filtered out and the organic

part was repeatedly washed with 2% sodium bicarbonate (NaHCO3) solution to remove the

excess fatty acids. The final ester was collected by vacuum evaporation of the organic fraction.

Simultaneously as the boiling point of methanol is quite low (338 K) and it evaporates slowly

from the upper surface of the reaction medium at room temperature (298 K). So the maintenance

of stoichiometry (especially 1:1) throughout 24 h of the reaction is a challenging problem and we

have to use an extra water cooled condenser. Thus, in this case we have to use a constant source

of cold water and as the catalyst get stick to the inner wall of the round bottom flask, we have to

use low boiling organic solvents (chloroform, alcohol) to remove the catalyst from the round

bottom flask. But if we use excess methanol then we do not need to use any other equipments or

solvents rather than just a cork-fitted round bottom flask. The excess methanol used in the

reaction can be reused again in the next run. The boiling point of methanol is quite low so the

removal of excess methanol from the reaction medium is not a problem but its presence in little

excess than the stoichiometry can easily overcome these practical problems. Especially the

purification of the biofuel product becomes much easier when we have used excess methanol in

the reaction mixture.

Further, to check the versatility of HZnPS-1 as an acid catalyst we have carried out the

transesterification38 reaction over HZnPS-1 at room temperature. The results of


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transesterification reaction are shown in Table 3. As seen from this table that HZnPS-1 is highly

reactive in the transesterification of ethyl cyanoacetate. Respective methyl ester yield was 81.5

wt%, whereas for ethyl chloroacetate and ethyl acrylate respective yields were low: 41.2 and

35.1, respectively. In both the cases separation of catalyst from the reaction mixture is very

simple. Further, since the system is devoid of additional organic solvent formation of azeotropic

mixture with the by-product water can be avoided.

Reuse of the catalyst. The catalyst was recovered from the reaction mixture by filtration and

was washed with methanol and acetone repeatedly to remove the polar and non-polar substrates

from the surface of the catalyst (HZnPS-1). Before the recycling test, the catalyst was dried

overnight at 333 K and the procedure was repeated for five times. The reusability of the catalyst

was tested by using oleic acid as the reference fatty acid (Figure 8). As seen from Figure 8, the

product yields for various cycles were very consistent suggesting high catalytic efficiency of

HZnPS-1 in biodiesel synthesis reactions. Further, to test whether any sulfonate groups are

leached out from HZnPS-1 during the reaction, in one reaction cycle the catalyst was separated

out from the reaction mixture after 6 h through filtration. At this stage the yield of the product

ester was 60.4 %. Then the reaction was continued without the catalyst at room temperature for

another 24 h. After 24 h the product was analyzed and the conversion was found to increase by a

very marginal amount (yield 62.0 %). This result confirms that during the catalytic reaction no

leaching of the sulfonate group takes place.

Possible explanation for catalytic behavior. While designing the catalyst our primary objective

is to introduce a sulphonic acid group into the solid hybrid zinc phosphonate porous architecture.

Here due to the presence of free sulphonic acid and phosphonic acid (as seen from the solid state

NMR study) in the material HZnPS-1, the catalytic surface is highly hydrophilic and possesses


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strong acidity. These two unique properties of the material are crucial for its success in the room

temperature esterification reaction of long chain fatty acids. Removal of water molecules from

the reaction mixture is a tedious process as it forms an azeotropic mixture with the solvent.38

Here we have tried to overcome this problem by carrying out the reaction in the absence of any

externally added solvent. When long chain fatty acids are dissolved into methanol in the

presence of the catalyst (HZnPS-1) the polar carboxylic acid group (-COOH) is adsorbed to the

hydrophilic surface of the material. Free sulphonic acid and phosphonic acid present at the

surface of the catalyst creates a slight positive charge (δ+) on the carbonyl carbon by protonating

the adjacent oxygen atom. Then the nucleophilic methanol (MeOH) attacks the carbonyl carbon

having very high electrophilicity, and consequently the methyl esterified product and water are

formed in the reaction. The polarity of the esterified products is diminished compared to the

reactant viz. the free fatty acids. Thus, the final products get easily desorbed from the hydrophilic

surface to the reaction medium (methanol). On the other hand, the water molecules formed as a

by-product could be removed from the homogeneous reaction phase by adsorption on the highly

hydrophilic surface of the catalyst and thus the reaction became unidirectional.

Water storage capacity. The organic-inorganic hybrid porous sulfonated zinc phosphonate

surface plays crucial role as water scavenger during the esterification reaction. Observed water

adsorption capacity of the sulfonated HZnPS-1 catalyst is 117.0 wt% at 298 K, which is

exceptionally high. Very high concentration of the surface phosphonate and sulphonate groups

could be responsible for high water adsorption capacity. Under experimental condition in the

reaction between 1 mmol of each of the lauric acid and methanol, 0.95 mmol of water is

produced (taking 95% conversion, Table 1). This corresponds to 17.1 mg H2O. 10 mg catalyst

used for this reaction would theoretically adsorb 11.7 mg water (considering 117 wt% water


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adsorption capacity), which is ca. 68.5% of the total water produced during this esterification

reaction. Thus major amount of water produced during the reaction has been dragged away by

the hydrophilic catalyst surface and this water-scavenging effect could be responsible in driving

the esterification reaction towards the forward direction.

Conclusion

We can conclude that a new hierarchical porous zinc phosphonate has been designed by

using tailor made precursor p-xylenediphosphonic acid as a spacer group under hydrothermal

conditions in the absence of any structure directing agent. Further, it has been sulphonated under

very mild condition to obtain its suphonated derivative HZnPS-1. Both the materials resemble a

well-defined orthorhombic crystal structure with certain variation in their morphologies. While

HZnP-1 shows a flake-like morphology, HZnPS-1 is composed of bigger brick-like particles

with clear crystal edges. HZnPS-1 behaves as an excellent catalyst for the production of

biodiesel at room temperature. Biodiesels are superior alternatives of fossil fuels due to the

diminishing stock of the later as well as the global warming associated with them due to

emission of various greenhouse gases upon their combustion. Thus our novel hybrid porous zinc

phosphonate material, HZnPS-1 can play significant role for the large scale production of

biofuels.

Experimental Section:

Preparation of p-xylenediphosphonic acid (H2O3P-C8H8-PO3H2). In the typical synthetic

procedure (Scheme 1), 3.0 g (0.011 mol) α,α'-dibromo-p-xylene and 4.312 g (0.026 mol)of

freshly distilled triethylphosphite were taken in a round bottom flask fitted with a water cooled

condenser. The flask was purged with N2 gas and heated at 433 K for 2.5 h. The crude diethyl

phosphate ester (3.7 g) was collected by vacuum distillation and the ester product was


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hydrolyzed by 6N HCl for 72 h. The hydrolyzed product was dried in a rotary evaporator to

collect the crude diphosphonic acid (2.47 g). The crude product was purified by crystallization

from water and characterized by 1H, 13C and 31P NMR spectroscopy.26 The yield of the

diphosphonic acid was 81.7 %.

Synthesis of porous Zinc(II) phosphonate, HZnP-1. In a typical synthesis of HZnP-1, 2.66 g

of p-xylenediphosphonic (10 mmol) was dissolved in 20 ml distilled water and the pH of the

solution was raised to ca.5.0 by adding aqueous sodium hydroxide (6N) solution. In another

beaker 3.40 g of the Zn(II) precursor, ZnCl2 (25 mmol) was dissolved in 10 ml of distilled water.

The synthesis gel was prepared by adding the former solution to the highly acidic metal salt

solution and maintaining the pH of the final solution within the range of 5.0-6.0 by adding

additional 6N aqueous sodium hydroxide solution. The reaction mixture was stirred overnight

and then the content was transferred into a Teflon-lined pressure vessel and treated

hydrothermally at 393 K for 1 day. Then the resulting product mixture was centrifuged, the solid

was washed with distilled water for several times and finally dried at room temperature. The pH

of the final solution was acidic in nature. Weight of the dried product (HZnP-1) was 4.8 g.

Synthesis of the sulphonated material, (HZnPS-1). HZnP-1 was sulfonated following

literature procedure.27 In a typical synthesis, 3.15 g HZnP-1 was suspended in 25 ml dry ether

and heated at 313 K in a round bottom flask equipped with a dropping funnel. The flask was

purged with N2 gas and a mixture of 5 g of sulfuric acid in 15 ml of dry ether was taken in the

dropping funnel. The acidified ether solution was added drop wise to the suspension over a

period of 3 h. Then the sulphonated product was isolated, washed with cold water for three times

to remove the excess sulphuric acid adhering to the product. The sulphonated HZnPS-1 material


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thus obtained was dried under high vacuum and thoroughly characterized before carrying out the

catalytic reactions.

Calculation of acid strength in the sulphonated material, HZnPS-1. Acid-base titration with

excess NaOH solution revealed that our hybrid porous sulfonated material has acid strength of

4.67 mmolg-1. Considering all phosphonate groups are coordinated with Zn sites, the material

have free 4.67 mmolg-1 free –SO3H groups. Due to such high concentration of acid sites this

HZnPS-1 material is highly hydrophilic and potential to be exploited as acid catalyst in

esterification and transesterification reactions.

Characterization techniques. Powder X-ray diffraction patterns were recorded on a Bruker D8

Advance SWAX diffractometer operated at 40 kV voltage and 40 mA current. The instrument

was calibrated with a standard silicon sample, using Ni-filtered Cu Kα (λ = 0.15406 nm)

radiation. A Jeol JEM 6700 field emission scanning electron microscope (FE–SEM) with an

energy dispersive X-ray spectroscopic (EDS) attachment was used for the determination of

morphology of the particles and its surface chemical compositions, respectively. Nitrogen

adsorption/desorption isotherms were obtained by using a Beckmann Coulter SA 3100 surface

area analyzer at 77 K. Prior to gas adsorption, samples were degassed for 3 h at 423 K under

high vacuum. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the

samples were carried out in a TGA instrument thermal analyzer TA-SDT Q-600 under N2 flow.

FT IR spectra of the samples were recorded using a Nicolet MAGNA-FT IR 750 spectrometer

Series II. 1H NMR experiments were carried out on a BrukerDPX-300/500 NMR spectrometer.

Solid state MAS NMR studies have been carried out using a Chemagnetics 300 MHz CMX 300

spectrometer. Bulk Zn content in HZnP-1 and HZnPS-1 samples was measured through chemical

analysis by using a Shimadzu AA-6300 atomic absorption spectrophotometer. Before the


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analyses suitable stock solutions were prepared by digesting the solid powder in aqueous

solution containing minimum amount of HNO3 followed by their slow evaporation.

Catalytic reactions. In a typical catalytic reaction, 1 mmol of the long chain fatty acids were

dissolved into 10 ml methanol and 5 wt% of the catalyst (with respect to the fatty acid) was

added to the mixture under stirring condition at room temperature. The progress of the reactions

was monitored by analysis of the reaction mixtures by thin layer chromatography (TLC) at

various time intervals. It took about 24 h to complete the esterification reactions and the catalyst

was removed from the reaction mixture by filtration, regenerated and used for five consecutive

cycles to study the recycling efficiency. After completion of the reaction there was no trace of

excess acid in the reaction medium (confirmed through thin layer chromatography). As the esters

have much higher boiling point then the excess methanol present in the reaction medium the

purification of the ester products were very easy. The fatty acid esters were collected by simple

vacuum evaporation of methanol from the reaction mixtures.

However, when we have carried out the esterification reaction in the presence of

stoichiometric amount of acid and methanol (acid:methanol=1:1) then after 24 h of the reaction,

chloroform was added into the reaction mixture to facilitate the separation process. For the

transesterification reaction, 1 mmol of the mother esters were dissolved into 1 ml methanol and 5

wt % of the catalyst (with respect to fatty acids) was added to the reaction mixture and stirred at

room temperature for 24 h. The conversions in the transesterification reactions were measured

through gas chromatographic analysis.

1H and

13C NMR chemical shifts for different long chain fatty acids’ methyl esters reported

in Table 1 (respective spectra are shown in the supporting information):


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Entry 1

1HNMR (500 MHZ, CDCl3) δ=3.622 (3H, s); δ=2.276-2.245 (2H, t); δ=1.595-1.552 (2H, m);

δ=1.252-1.223 (16H, m);δ=0.855-0.828 (3H, m);13C NMR (500 MHZ,CDCl3) δ=174.31; 51.41;

34.19; 32.01; 29.70; 29.55; 29.43; 29.36; 29.26; 25.06; 27.76; 14.14.

Entry 2


δ=1.254-1.225 (20H, m); δ=0.859-0.832 (3H, t); 13C NMR (500 MHZ, CDCl3) δ=174.31; 51.42;

34.18; 32.03; 29.78; 29.75; 29.70; 29.56; 29.46; 29.36; 29.26; 25.06; 22.78; 14.15.

Entry 3

1HNMR (500 MHZ, CDCl3) δ=3.619 (3H, s); δ=2.273-2.243 (2H, t);δ=1.593-1.564 (2H, m);


34.27; 32.13; 29.89; 29.87; 29.81; 29.66; 29.58; 29.47; 29.36; 25.16; 22.89; 14.25.

Entry 4



34.19; 32.05; 29.81; 29.78; 29.71; 29.58; 29.48; 29.38; 29.28; 25.07; 22.79; 14.17.

Entry 5

1HNMR (300 MHZ, CDCl3) δ=5.325-5.288 (2H, m); 3.626 (3H, s); 2.291-2.241 (2H, t); 2.028-

1.949 (4H, m); 1.161-1.565 (2H, m); 1.274-1.229 (20H, m); 0.870-0.827 (3H, m); 13C NMR (300

MHZ, CDCl3) δ=174.29; 130.03; 51.45; 34.14; 32.00; 29.85; 29.76; 29.68; 29.62; 29.41; 29.24;

29.21; 29.11; 27.29; 27.23; 25.70; 25.02; 22.76; 14.15.

Entry 6

1HNMR (300 MHZ, CDCl3) δ=3.558 (6H, s); δ=2.221-2.171 (4H, t); δ=1.533-1.487 (4H,

m);δ=1.20192 (8H, s); 13C NMR (300 MHZ,CDCl3) δ=174.12; 51.313; 33.936; 28.961; 24.810.


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Entry 7

1HNMR (500 MHZ, CDCl3) δ=3.583 (6H, s); δ=2.262-2.258 (4H, t); δ=1.588-1.561 (4H, m); 13C

NMR (500 MHZ, CDCl3) δ=173.81; 51.55; 33.70; 33.51; 24.42; 24.37; 24.22.


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Table 1. Esterification of different long chain fatty acids over HZnPS-1.a

Entry Fatty acid Conversion (%) Weight (mg) of the

corresponding methyl ester

1 CH3-(CH2)10-COOH 95 203

2 CH3-(CH2)12-COOH 94 227

3 CH3-(CH2)14-COOH 95 256

4 CH3-(CH2)16-COOH 93 277

5 CH3(CH2)7CH=CH(CH2)7COOH 90 266

6 HOOC-(CH2)8-COOH 95 218

7 HOOC-(CH2)4-COOH 92 160

8b CH3(CH2)7CH=CH(CH2)7COOH 5.0 14

aReaction conditions: 1 mmol fatty acid, 10 ml methanol, 5 wt.% catalyst, HZnPS-1 with respect

to the fatty acid, reaction temperature = 298 K, reaction time = 24 h.

bReaction was carried out in the absence of any catalyst.


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Table 2. Esterification of different long chain fatty acids in stoichimetric reaction condition

(acid:methanol=1:1) over HZnPS-1.1

Entry Fatty acid Conversion (%) Weight (mg) of the

corresponding methyl ester

1 CH3-(CH2)10-COOH 81 520

2 CH3-(CH2)12-COOH 81 588

5 CH3(CH2)7CH=CH(CH2)7COOH 84 783

1Reaction conditions: 3 mmol fatty acid, 3 mmol methanol, 5 wt.% catalyst (HZnPS-1) with

respect to the fatty acid, reaction temperature = 298 K, reaction time = 24 h.


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Table 3. Transesterification of different esters over HZnPS-1.1

Entry Substrate Conversion (%)

1 Ethyl cyanoacetate 81.5

2 Ethyl chloroacetate 41.2

3 Ethyl acrylate 35.1

1Reaction conditions: 1 mmol of substrate ester, 1 ml methanol, 5 wt.% catalyst (HZnPS-1) with

respect to the esters, reaction temperature = 298 K, reaction time = 24 h.

For gas chromatographic analysis see supporting information.


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Caption for figures

Figure 1 Powder X-ray diffraction pattern of HZnP-1. Inset: enlarged portion.

Figure 2 Powder X-ray diffractionpattern of HZnPS-1. Inset: enlarged portion.

Figure 3 FE SEM image of HZnP-1 (a) and HZnPS-1(b).

Figure 4 N2 adsorption and desorption isotherms for HZnP-1. Adsorption points are marked

by filled circles and those for desorption by empty circles. NLDFT pore size

distribution is shown in the inset.

Figure 5 FT IR spectrum of HZnP-1 (a) and HZnPS-1 (b).

Figure 6 13C CP MAS NMR spectra of HZnP-1.

Figure 7 31P MAS NMR spectra of HZnP-1.

Figure 8 Recyclability test of the catalyst by using oleic acid.

Scheme 1 Reaction pathway for the synthesis of p-xylenediphosphonic acid

organophosphorous precursor.


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26

Figure 1 [Pramanik et al.]

10 20 30 40 50

33 36 39

(420)

(413)

(006)

(025)

(314)

(205)(411)

(131)(130)(031)

(303)

(600)

(240)

(431)

(207)

(141)(140)

(225)(312)

(302)

(014)

(300)

(120)(211)

(003)

(200)

(201)

(100)

Intensity (a.u.)

2θθθθ in degree


Gre

en C

hem

istr

y A

ccep

ted

Man

usc

rip

t

Dow

nloa

ded

by D

uke

Uni

vers

ity o

n 28

May

201

2Pu

blis

hed

on 2

4 M

ay 2

012

on h

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pubs

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.org

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27


10 20 30 40 50

22 23 24 25

(221)

(131)

(032)

(123)

(222)

Intensity (a.u.)

2θθθθ in degree

(110) (002)

(120)

(121) (013)

(211)

(212)

(246)

(037)

(261)

(117)(107)

(007)

(422)

(403)

(250)

(402)

(234)

(331)(015)

(005)

(041)(231)

(040)


Gre

en C

hem

istr

y A

ccep

ted

Man

usc

rip

t

Dow

nloa

ded

by D

uke

Uni

vers

ity o

n 28

May

201

2Pu

blis

hed

on 2

4 M

ay 2

012

on h

ttp://

pubs

.rsc

.org

| do

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225G

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28


a

b


Gre

en C

hem

istr

y A

ccep

ted

Man

usc

rip

t

Dow

nloa

ded

by D

uke

Uni

vers

ity o

n 28

May

201

2Pu

blis

hed

on 2

4 M

ay 2

012

on h

ttp://

pubs

.rsc

.org

| do

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29


0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

N2 Volume (cc/g)

Relative Pressure (P/P0)


Gre

en C

hem

istr

y A

ccep

ted

Man

usc

rip

t

Dow

nloa

ded

by D

uke

Uni

vers

ity o

n 28

May

201

2Pu

blis

hed

on 2

4 M

ay 2

012

on h

ttp://

pubs

.rsc

.org

| do

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30


4000 3000 2000 1000

b

Transmittance (a.u.)

Wave number (cm-1)

a


Gre

en C

hem

istr

y A

ccep

ted

Man

usc

rip

t

Dow

nloa

ded

by D

uke

Uni

vers

ity o

n 28

May

201

2Pu

blis

hed

on 2

4 M

ay 2

012

on h

ttp://

pubs

.rsc

.org

| do

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31


0 50 100 150 200

P

P

C1

C2

C3C4

Chemical Shift (ppm)

*

*

*

*


Gre

en C

hem

istr

y A

ccep

ted

Man

usc

rip

t

Dow

nloa

ded

by D

uke

Uni

vers

ity o

n 28

May

201

2Pu

blis

hed

on 2

4 M

ay 2

012

on h

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.org

| do

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32


-100 0 100 200

Chemical Shift (ppm)

* *

* *


Gre

en C

hem

istr

y A

ccep

ted

Man

usc

rip

t

Dow

nloa

ded

by D

uke

Uni

vers

ity o

n 28

May

201

2Pu

blis

hed

on 2

4 M

ay 2

012

on h

ttp://

pubs

.rsc

.org

| do

i:10.

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225G

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33


0 5 10 15 20 250

20

40

60

80

100

Conversion (%)

Time (h)

Run1

Run2

Run3

Run4

Run5


Gre

en C

hem

istr

y A

ccep

ted

Man

usc

rip

t

Dow

nloa

ded

by D

uke

Uni

vers

ity o

n 28

May

201

2Pu

blis

hed

on 2

4 M

ay 2

012

on h

ttp://

pubs

.rsc

.org

| do

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C35

225G

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34

Scheme 1 [Pramanik et al.]

Br

Br

+ P(OEt)3

(EtO)2OP

(EtO)2OP

+ EtBr

(0.0113 mole) (3gm)

(0.0259 mole) (4.312 gm)

(3.7 gm)

2.30 h

433 k

(EtO)2OP

(EtO)2OP

+ HCl(6N)Refluxed

15 h

(HO)2OP

(HO)2OP

(2 gm)(3 gm)


Gre

en C

hem

istr

y A

ccep

ted

Man

usc

rip

t

Dow

nloa

ded

by D

uke

Uni

vers

ity o

n 28

May

201

2Pu

blis

hed

on 2

4 M

ay 2

012

on h

ttp://

pubs

.rsc

.org

| do

i:10.

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/C2G

C35

225G

View Online


Documents

Organic–inorganic hybrid porous sulfonated zinc phosphonate material: efficient catalyst for biodiesel synthesis at room temperature