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

Article abstract of DOI:10.1039/c2gc35225g

A new porous zinc phosphonate material (HZnP-1) has been synthesized via the reaction between p-xylenediphosphonic acid and anhydrous ZnCl₂ under hydrothermal and mildly acidic conditions (pH ～ 5) in the absence of any structure directing agent. The phenyl group of this material has been sulfonated with concentrated sulfuric acid to obtain sulfonic acid functionalized material HZnPS-1. Powder X-ray diffraction (XRD), field emission scanning electron microscopy (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, b = 8.74, c = 14.62 A, α = β = γ = 90° for HZnP-1, and a = 10.65, b = 13.52, c = 15.30 A 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 both reactant and 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, such as drastic conditions (high reaction temperature) and requirement of hazardous organic solvents.

Full text of DOI:10.1039/c2gc35225g

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Magnetically seperable organocatalyst Catalytic conversion of biomass
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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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Organicꢀinorganic 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,
Kolkataꢀ700 032, INDIA, *Corresponding author. Eꢀmail: msab@iacs.res.in
‡Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2ꢀ1
Yamadaoka, Suita, 565ꢀ0871, JAPAN
Abstract
A new porous zinc phosphonate material (HZnPꢀ1) has been synthesized through the
reaction between pꢀxylenediphosphonic 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
13
functionalized material HZnPSꢀ1. Powder XRD, FE SEM, N2 sorption, solid state C 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 HZnPꢀ1, and
a=10.65 Å, b=13.52 Å, c=15.30 Å and α=β=γ=90 for HZnPSꢀ1. HZnPSꢀ1 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 reactantꢀcumꢀsolvent at room
temperature. The green and ecoꢀfriendly 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
phaseꢀtransfer 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 ecoꢀfriendly methodology for the production of bioꢀfuel 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 byꢀproduct. 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 reactantꢀcumꢀsolvent 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 used1,4ꢀ
benzenediphosphonateꢀbis(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 m2gꢀ1.
Here we have synthesized pꢀxylenediphosphonic acid24 via phosphorylation of the respective
aromatic dibromide and employed it as the bridging organophosphorous precursor. The new
orthorhombic zinc(II) phosphonate (HZnPꢀ1) formed by using this precursor showed flakeꢀlike
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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HZnPSꢀ1 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 Xꢀray diffraction pattern of HZnPꢀ1
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 HZnPꢀ1 is 1371.48 Å3 with very low standard deviation. For the
sulphonated material, HZnPSꢀ1, due to the incorporation of sulphonic acid into the porous
architecture the cell parameters and the unit cell volume changes. The diffraction peaks of
HZnPSꢀ1 (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 (HZnPSꢀ1) remains
same after sulphonation, but the change in particle morphology and size was clearly noticeable.
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FE SEM images of HZnPꢀ1 and the sulphonated material HZnPSꢀ1are shown in Figure 3. As
seen in the figure for HZnPꢀ1 thin flakeꢀlike particles having dimensions of ca.350ꢀ450 nm long
and ca. 30 nm in width are uniformly distributed throughout the specimen. However, the
sulphonated material HZnPSꢀ1 showed much bigger brickꢀlike particle morphology with clear
crystal edges. The average length, height and width of HZnPSꢀ1 particles are 5.08 ꢁm, 1.32 ꢁm
and 1.66 ꢁm, respectively.
Hierarchical porosity. The N2 adsorption/desorption isotherms of HZnPꢀ1 are shown in Figure
4. BrunauerꢀEmmettꢀTelller (BET) surface area, average pore diameter and pore volume of
HZnPꢀ1 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 slitꢀlike pores.31
A
strong capillary uptake for the adsorbed gas at relatively high pressure (0.70ꢀ0.90) suggest the
presence of appreciable amount of mesopores in the framework. The mesoporosity of the
material can originate from the aggregation of the flakeꢀlike particles and inter particle voids.23b
The BET surface area of HZnPꢀ1 is 105 m2gꢀ1 with total pore volume of 0.39 cm3gꢀ1.
Corresponding pore size distribution employing nonꢀlocal 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 m2gꢀ1 and 0.21 cm3gꢀ1, respectively, in HZnPSꢀ1, 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 (HZnPꢀ1
and HZnPSꢀ1) TGꢀDTA analyses have been carried out. For HZnPꢀ1, 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 CꢀC and CꢀP bonds present in the framework. On the other hand, for HZnPSꢀ1, 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 nonꢀ
covalent interaction with free sulphonic acid group. The third weight loss (ca. 23.3 %) could be
attributed to cleavage of CꢀC, CꢀP, CꢀS 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 HZnPꢀ1 and HZnPSꢀ1 are shown in
Figure 5. For HZnPꢀ1 (Figure 5a), the peak at 570 cmꢀ1 is the characteristic stretching vibration
of ZnꢀO group. The set of bands at 990 and 1085 cmꢀ1 can be assigned to stretching vibration of
the tetrahedral CPO3 groups. Two characteristic peaks at 1418 cmꢀ1 and 1523 cmꢀ1 could be
attributed to the presence of benzene ring in the porous framework. The peak at 1623 cmꢀ1
corresponds to the bending vibration of water molecules. Two bands at 2925 and 3025 cmꢀ1 can
be assigned to the symmetric and asymmetric stretching vibration of –CH2 groups whereas the
broad peak centered at 3413 cmꢀ1 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 pꢀxylenediphosphonicspacer group in porous zinc phosphonate
framework. For HZnPSꢀ1 (Figure 5b), all peaks of HZnPꢀ1 has been retained, which suggested
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that upon sulfonation the main framework remains intact. An additional band at 748 cmꢀ1 is
generated, which could be assigned to the arylꢀsulphur stretching frequency. Apart from that
vibration bands at 1005 and 1080 cmꢀ1 are originated corresponding to the symmetric and
asymmetric stretching of (S=O) bond.32 The broadening of the peak centered around 3000 cmꢀ1 is
probably due to the presence of defect –OH as well as hydroxyl group present in free sulphonic
acid.
13
31
Solid State MAS NMR studies. C CP MAS and P 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 HZnPꢀ1 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 pꢀ
xylenediphosphonic acid group is covalently grafted in between the inorganic composite
framework. On the other hand, the 31P MAS NMR spectrum of HZnPꢀ1 sample (Figure 7) shows
one strong signal at 32.3 ppm. This could be attributed to the ((OH)(OZn)2PꢀC8H8–
P(OZn)2(OH))species.34 Upꢀfield shift in the 31P signal further suggested the presence of pꢀ
xylenediphosphonic acid moiety in the framework.
Molecular formula of the catalyst. From the TGAꢀDTA analysis of the sample (HZnPSꢀ1) 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= pꢀxylenediphosphonic 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 inꢀsitu formed water from the esterification reaction, which is
present in the azeotropic mixture.37 As seen from Table 1, the sulphonated zinc phosphonate
HZnPSꢀ1 acts as a very efficient heterogeneous catalyst for the production of methyl esters of
long chain fatty acids at room temperature using methanol as reactantꢀcumꢀsolvent.
Role of methanol. It is pertinent to mention that for the esterification reaction over HZnPSꢀ1 we
have used little excess methanol (stoichiometrically) than the corresponding fatty acids. So the
obvious question arises whether the term ‘solventꢀfree’ 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 reactantꢀcumꢀsolvent 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 corkꢀfitted 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 HZnPSꢀ1 as an acid catalyst we have carried out the
transesterification38 reaction over HZnPSꢀ1 at room temperature. The results of
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transesterification reaction are shown in Table 3. As seen from this table that HZnPSꢀ1 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 byꢀproduct 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 nonꢀpolar substrates
from the surface of the catalyst (HZnPSꢀ1). 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
HZnPSꢀ1 in biodiesel synthesis reactions. Further, to test whether any sulfonate groups are
leached out from HZnPSꢀ1 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 HZnPSꢀ1, 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 (HZnPSꢀ1) 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
byꢀproduct 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 organicꢀinorganic hybrid porous sulfonated zinc phosphonate
surface plays crucial role as water scavenger during the esterification reaction. Observed water
adsorption capacity of the sulfonated HZnPSꢀ1 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 waterꢀscavenging 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 pꢀxylenediphosphonic 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 HZnPSꢀ1. Both the materials resemble a
wellꢀdefined orthorhombic crystal structure with certain variation in their morphologies. While
HZnPꢀ1 shows a flakeꢀlike morphology, HZnPSꢀ1 is composed of bigger brickꢀlike particles
with clear crystal edges. HZnPSꢀ1 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, HZnPSꢀ1 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'ꢀdibromoꢀpꢀxylene 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
1
from water and characterized by H, 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 HZnPꢀ1, 2.66 g
of pꢀxylenediphosphonic (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 5.0ꢀ6.0 by adding
additional 6N aqueous sodium hydroxide solution. The reaction mixture was stirred overnight
and then the content was transferred into a Teflonꢀlined 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 (HZnPꢀ1) was 4.8 g.
Synthesis of the sulphonated material, (HZnPS-1). HZnPꢀ1 was sulfonated following
literature procedure.27 In a typical synthesis, 3.15 g HZnPꢀ1 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 HZnPSꢀ1 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. Acidꢀbase titration with
excess NaOH solution revealed that our hybrid porous sulfonated material has acid strength of
4.67 mmolgꢀ1. Considering all phosphonate groups are coordinated with Zn sites, the material
have free 4.67 mmolgꢀ1 free –SO3H groups. Due to such high concentration of acid sites this
HZnPSꢀ1 material is highly hydrophilic and potential to be exploited as acid catalyst in
esterification and transesterification reactions.
Characterization techniques. Powder Xꢀray 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 Niꢀfiltered Cu Kα (λ = 0.15406 nm)
radiation. A Jeol JEM 6700 field emission scanning electron microscope (FE–SEM) with an
energy dispersive Xꢀray 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 TAꢀSDT Qꢀ600 under N2 flow.
FT IR spectra of the samples were recorded using a Nicolet MAGNAꢀFT IR 750 spectrometer
1
Series II. H NMR experiments were carried out on a BrukerDPXꢀ300/500 NMR spectrometer.
Solid state MAS NMR studies have been carried out using a Chemagnetics 300 MHz CMX 300
spectrometer. Bulk Zn content in HZnPꢀ1 and HZnPSꢀ1 samples was measured through chemical
analysis by using a Shimadzu AAꢀ6300 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=2.276ꢀ2.245 (2H, t); δ=1.595ꢀ1.552 (2H, m);
δ=1.252ꢀ1.223 (16H, m);δ=0.855ꢀ0.828 (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
1HNMR (500 MHZ, CDCl3) δ=3.626 (3H, s); δ=2.279ꢀ2.249 (2H, t); δ=1.598ꢀ1.570 (2H, m);
δ=1.254ꢀ1.225 (20H, m); δ=0.859ꢀ0.832 (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=2.273ꢀ2.243 (2H, t);δ=1.593ꢀ1.564 (2H, m);
δ=1.249ꢀ1.218 (24H, m); δ=0.854ꢀ0.826 (3H, t); 13C NMR (500 MHZ, CDCl3) δ=174.40; 51.51;
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
1HNMR (500 MHZ, CDCl3) δ=3.622 (3H, s); δ=2.275ꢀ2.245 (2H, t); δ=1.596ꢀ1.553 (2H, m);
δ=1.253ꢀ1.221 (28H, m); δ=0.857ꢀ0.830 (3H, t); 13C NMR (500 MHZ, CDCl3) δ=174.32; 51.42;
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=5.325ꢀ5.288 (2H, m); 3.626 (3H, s); 2.291ꢀ2.241 (2H, t); 2.028ꢀ
1.949 (4H, m); 1.161ꢀ1.565 (2H, m); 1.274ꢀ1.229 (20H, m); 0.870ꢀ0.827 (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=2.221ꢀ2.171 (4H, t); δ=1.533ꢀ1.487 (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=2.262ꢀ2.258 (4H, t); δ=1.588ꢀ1.561 (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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References
1
(a) H. Baumann, M. Buhler, H. Fochem, F. Hirsinger, H. Zoebelein, J. Falbe, Angew. Chem. Int. Ed.
Engl.,1988, 27, 41ꢀ62. (b) L. SohandJ. Zimmerman, Green Chem., 2011, 13, 1422ꢀ1429.
2
(a) M. Z. Jacobson, Energy Environ. Sci., 2009, 2, 148–173. (b) N. Armaroli, V. Balzani, Angew.
Chem. Int. Ed. 2007, 46, 52–66.
3
4
H. J. Berchmansand S. Hirata, Bioresour. Technol., 2008, 99, 1716–1721.
(a) K. KohseꢀHoeinghaus, P. Osswald, T. A. Cool, T. Kasper, N. Hansen, F. Qi, C. K. Westbrook
and P. R. Westmoreland, Angew. Chem. Int. Ed. 2010, 49, 3572–3597; (b) K. Srilatha, T.
Issariyakul, N. Lingaiah, P. S. Sai Prasad, J. Kozinski and A. K. Dalai, Energy Fuels, 2010, 24,
4748ꢀ4755.
5
6
(a) M. Stocker, Angew. Chem. Int. Ed. 2008, 47, 9200–9211; (b) Y.C. Sharma, B. Singh and S.N.
Upadhyay, Fuel, 2008, 87, 2355–2373.
E. Lotero, Y. Liu, D. E. Lopez, K. Suwannakarn, D. A. Bruce and J. G. Goodwin, Jr., Ind. Eng.
Chem. Res. 2005, 44, 5353ꢀ5363.
7
8
M. G. Kulkarni, R. Gopinath, L. C. Meher and A. K. Dalai, Green Chem. 2006, 8, 1056–1062.
(a) A. C. Cole, J. L. Jensen, I. Ntai, K. L. T. Tran, K. J. Weaver, D. C. Forbes and J. H.
Davis, Jr., J. Am. Chem. Soc. 2002, 124, 5962ꢀ5963 (b) H. Shen, H. Shi, H. Wu and J. Xiang,
Chinese Pat. CN101406816ꢀA ; CN101406816ꢀB (c) F. Ma and M. A. Hanna, Bioresour.
Technol. 1999, 70, 1–15.
9
(a) T. Stoveken, A. Steinbuchel, Angew. Chem. Int. Ed. 2008, 47, 3688–3694. (b) A. R. M. Yahya,
W. A. Anderson, M. MooꢀYoung, Enz. Micro. Technol.,1998, 23, 438ꢀ450.
10 S. R. Kirumakki, N. Nagarajuand S. Narayanan, Appl.Catal. A: Gen., 2004, 273, 1–9.
18

Page 29 of 44
Green Chemistry
View Online
11 X. Chen, Z. Xu and T. Okuhara, Appl. Catal. A: Gen., 1999, 180, 261ꢀ269.
12 S. Blagov, S. Parada, O. Bailer, P. Moritz, D. Lam, R. Weinand and H. Hasse, Chem. Eng. Sci.,
2006, 61, 753–765.
13 L. Gang, L. Xinzong and W. Eli, New J. Chem., 2007, 31, 348ꢀ351.
14 (a) J. Dhainaut, J. P. Dacquin, A. F. Lee and K. Wilson, Green Chem., 2010,12, 296ꢀ303; (b) P. S.
Nigam and A. Singh, Prog. Ener. Comb. Sci., 2011, 37, 52ꢀ68; (c) K. White, N. Lorenz, T. Potts, W.
R. Penney, R. Babcock, A. Hardison, E. A. Canuel and J. A. Hestekin, Fuel, 2011, 90, 3193ꢀ3199.
15 (a) S. K. Das,M. K. Bhunia, A. K. Sinha and A. Bhaumik, ACS Catal., 2011, 1, 493ꢀ501. (b) B.
Zhao, L. Greiner and W. Leitner, Chem. Commun. 2011, 47, 2973 – 2975.
16 L. H. Wee, S. R. Bajpe, N. Janssens, I. Hermans, K. Houthoofd, C. E. A. Kirschhocka and J. A.
Martens, Chem. Commun., 2010, 46, 8186ꢀ 8188.
17 M. L. Testa, V. L. Parola and A. M. Venezia, Catal. Today, 2010, 158, 109–113.
18 X. Z. Li , Q. Lin and L. Ma, Ultrason.Sonochem.,2010, 17, 752–755.
19 W. Thitsartarn and S. Kawi, Green Chem. 2011, 13, 3423ꢀ3430.
20 (a) V. W. d. G. Silva, L. O. Laier and M. J. D. Silva, Catal. Lett., 2010, 135, 207–211; (b) V. R.
Chintareddy, H. A. Ho, A. D. Sadow and J. G. Verkade, Tetrahedron Lett., 2011, 52, 6523ꢀ6529.
21 J. Liang and G. K. H. Shimizu, Inorg. Chem., 2007, 46, 10449ꢀ10451.
22 (a) S. H. Huang, C. H. Lin, W. C. Wu and S. L. Wang, Angew. Chem. Int. Ed., 2009, 48, 6124 –
6127; (b) A. Milo and R. Neumann, Adv. Synth. Catal., 2010, 352, 2159ꢀ2165.
23 (a) M. Deng, Y. Ling, B. Xia, Z. Chen, Y. Zhou, X. Liu, B. Yue and H. He, Chem. Eur. J. 2011, 17,
10323–10328.(b) M. Pramanik, M. Nandi, H. Uyama and A. Bhaumik, Catal. Sci. Technol. 2012, 2,
613ꢀ620
.
19

Green Chemistry
Page 30 of 44
View Online
24 S. S. Iremonger, J. Liang, R. Vaidhyanathan and G. K. H. Shimizu, Chem. Commun., 2011, 47,
4430–4432.
25 T. W. T. Muesmann, C. Zitzer, A. Mietrach, T. Kluner, J. Christoffers and M. S. Wickleder, Dalton
Trans.2011, 40, 3128–3141.
26 C. Volkringer, T. Loiseau, N. Guillou, G. Ferey, M. Haouas, F. Taulelle, E. Elkaim and N. Stock,
Inorg. Chem., 2010, 49, 9852ꢀ9862.
27 (a)D. Chandra, N. Mukherjee, A. Mondal and A. Bhaumik, J. Phys. Chem. C, 2008, 112,8668–
8674; (b) M. Kruk and M. Jaroniec, Chem. Mater. 2001, 13, 3169ꢀ3183.
28 V. V. Kutarov, Y. I. Tarasevich, E. V. Aksenenko and Z. G. Ivanova, Rus. J. Phys. Chem. A, 2011,
85,1222–1227.
29 L. Peng , A. Philippaerts , X. Ke , J. V. Noyen , F. D. Clippel , G. V. Tendeloo , P. A. Jacobs and B.
F. Sels, Catal. Today, 2010, 150, 140ꢀ146.
30 M. Paul, N. Pal, P. R. Rajamohanan, B. S. Rana, A. K. Sinha and A. Bhaumik, Phys. Chem. Chem.
Phys. 2010, 12, 9389–9394.
31 (a) Z. Wu , Z. Liu , P. Tian , Y. He , L. Xu , Y. Yang , Y. Zhang , X. Bao , X. Liu and X. Liu,
Microporous Mesoporous Mater.,2005,81, 175–183; (b) K. Sarkar, T. Yokoi, T. Tatsumi and A.
Bhaumik, Microporous Mesoporous Mater.2008, 110, 405ꢀ412
.
32 J.M. Marchetti and A.F. Errazu, Biomass and Bioenergy. 2008, 32, 892ꢀ895.
33 D. J. ColeꢀHamilton, 2003, 299, 1702ꢀ1706.
34 Y. Yan, U. T. Bornscheuer and R.D. Schmid, Biotechnol. Bioeng.,2002, 78, 31ꢀ34.
35 C. B. DirkꢀFaitakis and K. T. Chuang, Ind. Eng. Chem. Res.,2004, 43, 762ꢀ768.
36 1HNMR (500 MHz, D2O) δ= 3.067 (s, 2H, CH2); δ= 3.133 (s, 2H, CH2); δ= 7.211 (s, 4H); 13C NMR
20

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(500 MHz, D2O) δ= 33.86; 34.89; 129.94; 31P NMR (500 MHz, D2O) δ=24.24.
37 N. K. Mal, A. Bhaumik, M. Matsukata and M. Fujiwara, Ind. Eng. Chem. Res., 2006, 45, 7748ꢀ
7751.
38 (a) R. Jothiramalingam and M. K. Wang, Ind. Eng. Chem. Res., 2009, 48, 6162–6172 (b) P. K.
Ghosh et al, US Pat., 7,666,234, 2010.
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Table 1. Esterification of different long chain fatty acids over HZnPSꢀ1.a
Weight (mg) of the
Entry
Fatty acid
Conversion (%)
corresponding
methyl ester
1
2
3
4
5
6
7
8b
CH3ꢀ(CH2)10ꢀCOOH
CH3ꢀ(CH2)12ꢀCOOH
95
94
95
93
90
95
92
5.0
203
227
256
277
266
218
160
14
CH3ꢀ(CH2)14ꢀCOOH
CH3ꢀ(CH2)16ꢀCOOH
CH3(CH2)7CH=CH(CH2)7COOH
HOOCꢀ(CH2)8ꢀCOOH
HOOCꢀ(CH2)4ꢀCOOH
CH3(CH2)7CH=CH(CH2)7COOH
aReaction conditions: 1 mmol fatty acid, 10 ml methanol, 5 wt.% catalyst, HZnPSꢀ1 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 HZnPSꢀ1.1
Weight (mg) of the
Entry
Fatty acid
Conversion (%)
corresponding
methyl ester
1
2
5
CH3ꢀ(CH2)10ꢀCOOH
CH3ꢀ(CH2)12ꢀCOOH
81
81
84
520
588
783
CH3(CH2)7CH=CH(CH2)7COOH
1Reaction conditions: 3 mmol fatty acid, 3 mmol methanol, 5 wt.% catalyst (HZnPSꢀ1) 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 HZnPSꢀ1.1
Entry
Substrate
Conversion (%)
1
2
3
Ethyl cyanoacetate
Ethyl chloroacetate
Ethyl acrylate
81.5
41.2
35.1
1Reaction conditions: 1 mmol of substrate ester, 1 ml methanol, 5 wt.% catalyst (HZnPSꢀ1) 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
Powder Xꢀray diffraction pattern of HZnPꢀ1. Inset: enlarged portion.
Powder Xꢀray diffractionpattern of HZnPSꢀ1. Inset: enlarged portion.
FE SEM image of HZnPꢀ1 (a) and HZnPSꢀ1(b).
Figure 1
Figure 2
Figure 3
Figure 4
N2 adsorption and desorption isotherms for HZnPꢀ1. Adsorption points are marked
by filled circles and those for desorption by empty circles. NLDFT pore size
distribution is shown in the inset.
FT IR spectrum of HZnPꢀ1 (a) and HZnPSꢀ1 (b).
13C CP MAS NMR spectra of HZnPꢀ1.
Figure 5
Figure 6
Figure 7
Figure 8
Scheme 1
31P MAS NMR spectra of HZnPꢀ1.
Recyclability test of the catalyst by using oleic acid.
Reaction pathway for the synthesis of pꢀxylenediphosphonic acid
organophosphorous precursor.
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Figure 1 [Pramanik et al.]
33
36
39
10
20
30
40
50
2
θ
in degree
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Figure 2 [Pramanik et al.]
22
23
24
25
10
20
30
40
50
2θ
in degree
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Figure 3 [Pramanik et al.]
a
b
28