Probing the Catalytic Efficiency of Supported Heteropoly Acids for Esterification : Effect of Weak Catalyst Support Interactions

Supported heteropoly acids are an interesting class of solid acid catalysts which possess flexible structure and super acidic properties essentially required for the oil-based biodiesel production. In this study, a series of catalysts containing 25 wt.% of heteropolytungstate (HPW) supported on various clays or SiO2 were prepared, and their catalytic efficiency was evaluated for esterification of acetic acid with heptanol. -e as-prepared catalysts were characterized by various techniques including FT-IR spectroscopy, thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, and BET. -e catalytic efficiency of both bulk and supported HPW catalysts for the esterification activity strongly depends on the type of support and amount of catalyst; the bulk HPW catalyst and the catalyst supported by kaolinite with 25 wt.% of HPW exhibited highest activity. In order to study the effect of temperature on conversion, all the catalysts were subjected to different reaction temperatures. It was revealed that esterification activity of both bulk and supported HPW catalysts strongly depends upon the temperature variations of the reaction. Besides, the effect of leaching of active sites on the catalysts performance for biodiesel production was also evaluated by inductively coupled plasma studies (ICP).-e kaolinite-supported catalyst (25%HPW/kaolinite) demonstrated higher amount of leaching which is also confirmed by the significant decrease in its catalytic activity when it is used for the second time. However, the higher activity demonstrated by HPW/kaolinite maybe because of some homogeneous reaction indicating a weak catalyst support interaction (WCSI) resulting in the leaching of the catalyst during the test. Furthermore, the effects of other reaction variables such as catalyst loading and reaction time on the conversion of acetic acid were also studied.


Introduction
Exhaustion of the world petroleum reserves and increasing demand for renewable energy sources have made biofuels an attractive alternative that can reduce the consumption of the traditional fossil fuels [1].Besides, the emissions of greenhouse gasses from the burning of fossil fuels possess serious threat to both environment and human life [2].Recently, biodiesel has generated immense interest as a suitable alternative to fossil fuels, which is nontoxic and biodegradable renewable fuel [3].Biodiesel belongs to a class of fatty acid methyl ester, which is usually obtained from various natural resources including vegetable oils and animal fats [4].ese natural feedstocks are typically converted to biodiesel through a transesterification process in the presence of suitable catalysts [5].Biodiesel does not contain any harmful substances and produces less dangerous emission to the environment and thus significantly contributes to minimize the emissions of toxic gases such as SO 2 , SO 3 , and CO [6].Besides, biodiesel also contains higher lubricity, fewer emissions of carcinogenic particulate matter, and ease of storage, handling, and transport compared to conventional fossil fuel [7].
Biodiesel is commonly produced by transesterification of triglycerides of refined/edible types of oils using alcohol in presence of an acid or a basic catalyst [8].Typically, the transesterification of vegetable oils and animal fats is carried out by using homogeneous basic catalysts, such as KOH and NaOH, at mild temperatures [9].Similarly, due to their Brønsted acidity, homogeneous acid catalysts such as HCl and H 2 SO 4 are more appropriate for triglycerides from inedible oil, which promotes electrolytic activation of substrates [10].Usually, homogeneous basic catalysts are more preferred over their acidic counterparts' due to their better activity, especially for triglycerides with low free fatty acid (FFA) contents [11].However, homogeneous catalysts usually suffer from various problems including saponification, long transesterification reaction times, and corrosion.Apart from this, homogeneous catalysts are difficult to separate and recover after the reaction, which significantly increases the cost and seriously affects the economic viability of the process [12].
On the contrary, heterogeneous catalysts offer better alternative to their counterparts due to their ease of separation from the reaction medium [13,14].In addition, heterogeneous catalysts are less expensive, are easily available, possess longer durability, are easy to regenerate, and are more stable at high temperatures and pressure [15].erefore, advancement in the field of heterogeneous catalysis for the development of sustainable and efficient catalysts is highly desirable to minimize the cost of biodiesel production [16].So far, several studies have been reported on biodiesel-related esterification reactions using various heterogeneous catalysts.For instance, Park et al. [17] examined the tungsten oxide zirconia, sulfated zirconia, and Amberlyst-15 catalysts for the conversion of used vegetable oils (VOs) to fatty acid methyl esters (FAMEs).Among them, tungsten oxide zirconia was a promising heterogeneous catalyst for the production of biodiesel fuels from used VOs because of high activity in the conversion over 93% and no leaching of WO 3 in the esterification reaction.Still, the production of biodiesel is currently more expensive when compared to conventional diesel fuel because of its high raw material and production costs.To minimize the production cost of biodiesel, proper feedstock selection is highly required.Although the biodiesel feeds such as animal fats or used oils are cheaper, they contain high free fatty acid (FFA) when compared to without FFA VOs.However, the consumption of less expensive, high FFA acid feedstocks such as used oils in biodiesel production generally require large amount of catalysts and increased purification cost.Supported catalysts, such as dodecatungstophosphoric acid, dodecatungstosilicic acid, and dodecamolybdophosphoric acid, on acid-activated bentonite have been reported for esterification of acetic acid with primary, secondary, and tertiary butanol.[18].
To overcome the aforementioned problems, the FFAs are usually converted to alkyl esters in the presence of different types of acid catalysts including mineral acids, mesoporous materials, and supported catalysts, such as H 2 SO 4 , HF, anionic resins, and heteropoly acids [19] ] have attracted considerable attention.Due to their easy availability and cost-effectiveness, these catalysts offer more efficient and cleaner processes [20].However, solid HPAs are usually suffering from deactivation during esterification reactions due to the formation of carbonaceous deposits (coke) on the catalyst surface.Notably, the conventional regeneration methods of burning coke at high temperatures at 500-550 °C are not applicable to HPAs-based catalysts due to their inefficient thermal stability.In addition, the HPAs-based catalysts also possess low surface area (1-10 m 2 /g) and solubility in various polar solvents [21].erefore, HPAs are generally dispersed on various supports to enhance their activity and make them more ecofriendly and less soluble solid acid catalysts with high thermal stability.So far, various types of supports including silica, active carbon, MCM-41, and clays with large surface area have been successfully applied for this purpose [22,23].ese supports not only enhance the active sites of the catalysts but also inhibit the formation of coke [5].
e acidity and catalytic activity of supported HPAs are strongly dependent on various parameters including the type of the support, amount of HPA loading, and conditions of pretreatment.
erefore, detail investigations of these parameters are highly desirable to enhance the catalytic activity of the HPAs-based solid acid catalysts.
In this study, heteropolytungstate (HPW) supported on various clays or SiO 2 was prepared, and their catalytic efficiency was evaluated for esterification of acetic acid with heptanol (cf.Scheme 1).Detail investigations were carried out to determine various factors responsible for the catalytic efficiency of both bulk and supported HPW catalysts for the esterification reactions.e as-prepared HPAs-based supported catalysts were characterized using various microscopic and analytical techniques including X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis, and scanning electron microscopy (TEM).

Catalyst Preparation.
All the supported HPW catalysts including HPW-S, HPW-K, HPW-B, and HPW-M are prepared via the wet impregnation method by using 25 wt.% of HPW.For this purpose, required amount of HPW is dissolved in ethanol by constant stirring, and subsequently, the respective amount of support (e.g., in case of HPW-S, silica was added) was slowly added to the HPW solution.e resultant solution was left for aging for 24 h to form slurry.
e solvent was removed in a rotary evaporator.After this, no further pretreatments were applied to any of the materials, which were stored in air prior to analysis and reaction testing.

2
Journal of Chemistry

Esteri cation of Acetic Acid.
Liquid-phase esteri cation reactions were carried out in a 50 ml glass reactor (roundbottom two-neck ask) equipped with a condenser and a magnetic stirrer at di erent temperatures (70, 90, and 110 °C) under atmospheric pressure.e total weight of reaction mixture (acetic acid + heptanol) was 10 g.Dodecane (2% w/w) was added as a GC internal standard.Catalyst (0.1 g and 0.02 g) was added to reaction mixture.e reaction was carried out for 3 h.To monitor the reaction, samples (0.5 ml) of the reaction mixture were taken periodically, every twenty minutes.e sample for the analysis was separated from the catalyst by centrifugation.e samples were analyzed by gas chromatography.

Characterization. X-ray di raction (XRD) patterns of
the powdered samples were recorded on Ultima IV (Rigaku, Japan) X-ray powder di ractometer using Cu Kα radiation (λ 1.5418 Å ). e intensity data were collected over a 2θ range of 10-70 °. e FT-IR spectra were recorded on a Per-kinElmer FT-IR Spectrometer spectrum using the KBr disc method in the range of 400-4000 cm −1 .ermogravimetric analyses (TGA) were performed at temperatures between 100 and 600 °C using Shimadzu TGA-50.Surface area measurements were performed by using Brunauer-Emmett-Teller (BET) method using a Micromeritics Surface Area and Porosity (ASAP 2020 V3.01 E) instrument.Elements analyses were performed through inductively coupled plasma/mass spectrometer (ICP/MS) by PerkinElmer (NexION 300 D) instrument.e conversion of the catalytic products was analyzed using GC, 7890A, Agilent Technologies Inc., equipped with a ame ionization detector (FID) and a 19019S-001 HP-PONA column.

XRD Analysis.
In order to study the e ect of support on the catalytic activity of HPAs-based acid catalyst for the esteri cation reactions, various acid catalysts are prepared by using di erent supports.
e catalysts such as 25% HPW/SiO 2 (HPW-S), 25% HPW/kaolinite (HPW-K), 25% HPW/bentonite (HPW-B), and 25% HPW/montmorillonite (HPW-M) are synthesized by wet impregnation methods using 25 wt.% heteropolytungstate (HPW) in each catalyst.All these catalysts are applied for the catalytic esteri cation of acetic acid with heptanol to produce heptyl acetate.Furthermore, the activities of these catalysts are also compared with catalytic activities of bulk HPW and Amberlyst@ 15 which were procured commercially.Initially, XRD spectra of all the supported catalysts are measured to con rm whether the HPW is present in the crystalline form or dispersed when supported.As shown in Figures 1(a  °which clearly corresponds to the typical Keggin structure of HPW, hence pointing towards the intact structure of HPW on the SiO 2 support after impregnation [24]. ese reflections in the corresponding XRD pattern also confirm the crystalline form of HPW in both HPW-S and HPW-K with a low intensity, indicating a possible spread of the catalyst on the support.Figures 1(c) and 1(d) exhibit the XRD spectra of HPW-B and HPW-M, respectively.e absence of reflections belonging to the HPW in the XRD spectrum of HPW-B potentially indicates the amorphous state of the HPW and points towards either the random distribution of HPW on the surface of bentonite or possible incorporation of HPW into the pores of the support [25].However, the presence of a single intense peak at 2θ � 27 °in the XRD spectrum of HPW-M indicates a good dispersion of HPW on the support which possibly led to a significant decrease in the intensity of the HPW reflections [26].

FT-IR Analysis.
FT-IR is a promising technique to determine the structures of clay-supported HPW catalysts.Particularly, the absorption bands due to structural OH (∼3700 cm −1 ) and Si-O (800 to 1100 cm −1 ) groups often play an important role for determining the structure.e FT-IR spectrum of the HPW-S is presented in Figure 2(a), which exhibited two bands around 983 and 895 cm −1 , respectively, while the bands around 1105 and 811 cm −1 are attributed to the Keggin structure.Notably, the characteristic bands of HPW also appear in the same range from 800 to 1100 cm −1 which may overlap with the peaks of SiO 2 [27].For instance, three typical absorption peaks of HPW appear at 1072 cm −1 (P-O), 961 cm −1 (W�O), and 884 cm −1 (W-Oc-W) [28].e presence of characteristic bands of both SiO 2 and HPW with slight shifts, due to their interactions, clearly demonstrates that HPW in the sample still retains the Keggin structure.Similarly, the IR spectrum of HPW-K (cf. Figure 2(b)) showed bands at 810, 985, 1080, and 1120 cm −1 pointing towards the presence of Keggin structure which remains intact after the impregnation process.Typically, the IR spectrum of kaolinite shows Si-O stretching and bending as well as OH bending absorptions in the 1300-400 cm −1 range.However, the Si-O stretching vibrations of kaolinite give several well-resolved strong bands in the 1120-1000 cm −1 region.erefore, the absorption around 1080 cm −1 in the HPW-K overlapped with the strong absorption of the kaolin peak around 1120 cm −1 .ese results are consistent with those reported in earlier study [29].
In the case of HPW-B, the primary structure of support was identified by five characteristic IR bands between 600 and 3000 cm −1 , namely, the broad stretching band at 1052 to 1118 cm −1 was attributed to the Si-O stretching vibration, and the shoulder at 926 cm −1 was indicative of -OH groups bonded to Al atoms.On the contrary, the bands at 524-820 cm −1 were assigned to Si-O and Si-O-Al vibrations.As shown in Figure 2(c), the prominent peak at 820 cm −1 was attributed to interactions between HPW and the support [5].Besides, the FT-IR spectra of the HPW-M sample in Figure 2(d) exhibited the band at 815 cm −1 which can be attributed to the formation of Si-O-W bonds, which may be due to the chemical interactions between the HPW and montmorillonite (MMT).erefore, in the spectrum of the MMT sample, the band at 795 cm −1 was attributed to the symmetric stretching frequency of Si-O-Si [30].

BET Analysis.
e surface areas of bulk HPW, HPW-S, HPW-K, HPW-B, and HPW-M were determined by the BET method from nitrogen adsorption-desorption isotherms which is summarized in Table 1 [31].e texture (surface area, pore diameter, and pore volume) of bulk Keggin HPAs is well documented in the literature [32].Table 1 shows that the catalyst HPW-S possesses relatively high surface area which is followed by HPW-M, whereas HPW-K and HPW-B showed almost similar and low surface area.In terms of pore diameter, HPW-S possessed larger pores size which is followed by HPW-B, whereas HPW-K and HPW-M have smaller pores.e isotherms of silica-supported HPW (HPW-S) demonstrate a type IV pattern with a hysteresis loop (type H3) indicating the presence of nonuniform size and/or shape of mesopores [33,34].e N 2 adsorption-desorption isotherm of HPW-K showed type II isotherm which demonstrated that these materials possessed microporous structures.However, HPW-B and HPW-M demonstrate a type IV which points toward the presence of mesoporous materials with pores shapes of conical and cylindrical, respectively [35].

Catalytic Activity.
Esterification of acetic acid with 1-heptanol was performed using a three-neck flask under atmospheric pressure at 90 °C.To prevent the evaporation of reactants, a cooling condenser was used.Initially, desired amount of acetic acid and catalyst were taken; the reaction  mixture was heated, when the temperature reached 90 °C, and then required amount of 1-heptanol and dodecane were added, and the reaction was continued for three hours.To compare the catalytic activities of as-prepared catalysts, all reactions were performed under similar conditions by varying the catalysts.e esteri cation activity as a function of amount of the catalyst was studied, and the results are presented in Figure 3(a).It was revealed that the conversion increased by increasing the amount of catalyst (cf.Table 2).
In case of bulk HPW, a conversion of 72.12% was obtained by using 0.025 g of catalyst in 140 min, which is reached to 100% when the amount of the catalyst was increased to 0.1 g.However, using same amount of Amberlyst@15 (0.1 g), a maximum conversion of 76.3% could be obtained under similar set of conditions.Notably, all the supported catalysts have demonstrated less conversion using higher amount of the catalyst (0.1 g) when compared to the bulk HPW.However, among various supported catalysts, HPW-K has exhibited higher conversion of 75%, which is followed by HPW-M, HPW-S, and HPW-B, respectively, as shown in Table 2. To study the e ect of temperature on the esteri cation of acetic acid, several reactions are carried out at di erent temperatures such as 70, 90, and 110 °C using the supported catalysts, and the results are presented in Figure 4.It is found that the conversion of acetic acid increased with increasing the temperature and amount of the catalyst, and higher conversion is obtained when HPW-K is used.All the results are summarized in Table 3.
Furthermore, an attempt was made to reuse the asprepared supported catalysts, including HPW-K, HPW-B, and HPW-M for the esteri cation of acetic acid.e regeneration of the catalysts after 3 h reaction time was conducted by simple ltration, washing with water for several times followed by calcination at 200 °C in an oven, and they were used in the esteri cation reactions with a fresh reaction mixture.Notably, HPW-S was excluded for this study due to its low catalytic activity towards the acetic acid conversion even after using it in large amount at higher temperature.Indeed, this result is also corroborated from the literature [36].It is reported that, although SiO 2 exhibits large surface area (also observed in this study, SiO 2 demonstrates a large surface area of 218 m 2 /g compared to other supported catalysts), it is a frequently used support for the HPW catalyst; however, SiO 2 -supported HPW often possess poor thermal stability, due to weak interaction between HPW and SiO 2 [37].Commonly, thermal decomposition of supported Keggin-type HPW catalysts may lead to a considerable loss of acidity, hence causing a decrease in the catalytic activity.us, the low catalytic activity of HPW-S towards the esteri cation of acetic acid could be explained from this behavior.
However, the other supported catalysts such as HPW-K, HPW-B, and HPW-M, which demonstrated a conversion of 100, 35, and 96% in the rst run, have showed decreased catalytic activity of about 32, 27, and 42% in the second run, respectively, as shown in Figure 5. e decreased catalytic activities of the supported catalysts for the 2nd run can be attributed to leaching of the catalysts [38].Apart from this, the reuse of the solid catalysts is also governed by their extent of leaching in the reaction medium.With the aim of quantifying the amount of the catalyst which is being leached, solutions were analyzed by inductively coupled plasma (ICP), and the leaching percentages of the samples are summarized in Table 4. Indeed, the results obtained show the leaching of a fraction of all the supported catalysts.However, HPW-K exhibits higher amount of leaching when compared to HPW-B and HPW-M which also explains its decreased activity in the second run.It also  possesses a small surface area and low pore size.Furthermore, literature reports have revealed that the higher leaching of HPW-K can also be attributed to a complete breakdown of the crystalline structure of the leached kaolin compared to the parent kaolin, which leads to signi cant loss of the structure that facilitated the breakdown of kaolin's octahedral AlO 6 structure [39].Due to this, the crystalline nature of the HPW-K catalyst, which is demonstrated by its respective XRD, deteriorates at the expense of increased amorphous phase.Indeed, HPW-K has showed best catalytic activity among all the catalysts in the rst run (100% conversion of acetic acid) due to its superior crystalline nature and better dispersion of HPW on the surface of kaolinite.

Conclusion
is work demonstrates that, among various supported Keggin-type heteropolytungstate (HPW) catalysts, the HPW supported on kaolinite (HPW-K) is much more active and stable than those on silica, bentonite, montmorillonite, and bulk HPW for the esteri cation of acetic acid.
e high   From the catalytic data obtained, it can be concluded that the catalytic activity does not solely depend on the surface area of the support as HPW-K shows high catalytic activity, although its surface area is comparable to HPW-B.e higher activity demonstrated by HPW-K maybe because of the weak catalyst support interaction resulting in the leaching of the catalyst during the test, resulting in some homogeneous reaction.is is further supported by substantial lowering of the catalytic activity in the second run for HPW-K and HPW-M.However, for HPW-B which does not show a very high catalytic activity in the first run, the lowering of the activity in second run is less pronounced.
is reflects in the elemental analysis carried out for the catalysts before and after the reaction, which shows lesser catalyst loss for HPW-B compared with HPW-K and HPW-M.Furthermore, it was revealed that esterification activity of both bulk and supported HPW catalysts strongly depend upon the temperature variations of the reaction.is type of comparative study of various supports and its findings could be important for the model reactions for the production of biodiesel, in view of searching for more effective supports with improved thermal stability to survive repeated regenerations. )

Scheme 1 :
Scheme 1: Schematic representation of the preparation of clays or SiO 2 -supported heteropolytungstate (HPW) catalysts and their catalytic reaction for the esteri cation of acetic acid with heptanol.

Figure 3 :
Figure3: (a) Esteri cation of acetic acid with heptanol in the presence of HPW-K, HPW-M, and HPW-B using 0.02 g of catalysts.(b) Same esteri cation reactions carried out using 0.1 g of the catalyst.Notably, HPW-S showed extremely low catalytic activity when 0.02 g of the catalyst was used.

Figure 5 :
Figure 5: Reuse of supported HPW catalysts for the esteri cation of acetic acid with heptanol in the presence of (a) HPW-K, (b) HPW-B, and (c) HPW-M, and (d) data shown in the form of table.

Table 1 :
e texture of various supported HPW catalysts obtained from nitrogen adsorption.

Table 2 :
Esteri cation of acetic acid with heptanol at 90 °C using di erent amount of catalysts (0.02 and 0.1 g).

Table 3 :
E ect of temperature on the catalytic activities of supported HPW catalysts for the esteri cation of acetic acid with heptanol.

Table 4 :
Surface area and elemental analysis of tungsten (W) in the catalyst for esteri cation of acetic acid with heptanol.catalytic activity of HPW-K is attributed to the effective Brønsted acid sites of the catalysts and preservation of the HPW Keggin structure in highly dispersed states in HPW-K.