Facile Synthesis of Ethyl Acetate over ZrO 2 .TiO 2 Mixed Oxide Supported Vanadium Boasted Phosphomolybdic Acid Catalyst at Room Temperature

Initially, mixed oxide of ZrO 2 .TiO 2 support was synthesized by the sol-gel method, and then after vanadium was incorporated, phosphomolybdic acid (VPMA) was synthesized by the hydrothermal method. VPMA supported on ZrO 2 .TiO 2 support of various VPMA loadings that were synthesized by the impregnation method. Te structure, surface area, acidity, and morphology of the materials were analysed by X-ray difraction, BETsurface area, ammonia TPD studies, and SEM analysis, respectively. Te XRD result shows crystallites of zirconia, titania, and Keggin ion of VPMA catalysts, and also, below 20 wt% of VPMA loading, crystallites of Keggin ion are not observed. BET surface area analysis reveals that increase of VPMA loading, surface area was sharply declined at higher loadings. Ammonia TPD analysis fnds that acidity is inclined with VPMA loadings. SEM studies reveal that bulk formation of the active phase is observed at higher loadings. Esterifcation of acetic acid was tested over VPMA/ ZrO 2 .TiO 2 catalyst that exhibited higher activity during reaction than pure VPMA catalyst. Tese fndings were well correlated with surface area and acidity loadings.


Introduction
Polyoxometalates (POMs) are well-known solid acid catalysts because of their tunable acid and redox properties of materials for various kinds of reactions [1,2].In recent years, POMS gained more attention due to industrial applications of diferent acid and oxidation reactions [3][4][5][6][7][8][9][10][11][12][13][14][15].POMs are strong Brønsted acid materials.However, their major limitation is their low surface area, which has an impact on their catalytic property during reaction.Te limitation of lower surface area of pure POMs can be minimized by while promoting active POMs on support of single or mixed oxide results higher surface area as well as acidity can boast the catalytic performance [5][6][7][8][9].Vanadium containing POMs were studied in various reactions such as dehydration, condensation, and other reactions [3,16,17].It is also important that support makes the active phase more stable, as well as more number of active sites are important for higher catalytic activity during the reaction.In recent times, ZrO 2 .TiO 2 mixed oxide was used as support in various reactions [18][19][20].In the view of several organic conversions studied over V modifed phosphomolybdic acid (VPMA) catalyst, we studied esterifcation of acetic acid over VPMA supported on mixed ZrO 2 .TiO 2 catalyst at room temperature.
Te catalytic esterifcation of acetic acid to alkyl acetate has a marked importance in both applied and basic research which is because of the huge applications of ester in industrial and academic usages.Ethyl acetate or alkyl acetate is synthesized by using mineral acid such as sulphuric or phosphoric acid, which is a corrosive process due to reactors that were damaged during the course of reaction.Inaddition, using larger quantities of sulfuric acid as a catalyst causes coke toform during the production of ethyl acetate.Another disadvantage of usingsulfuric acid as a catalyst is that it produces acetaldehyde.Consequently,acetaldehyde's competition with the other reactants reduces the ester yields [21][22][23].However, sulphuric acid materials are corrosive, and isolation of the product is not easy and eco-friendly.Terefore, the development of a catalyst is eco-friendly and not corrosive and easy isolation of the product is challenging for researchers and scientists.
In this present work, studied esterifcation of acetic acid over zirconia and titania mixed oxide supported vanadiummodifed phosphomolybdic acid catalysts.Te aim of this study is the efect of VPMA on ZrO 2 .TiO 2 support during the reaction.We also studied the various reaction parameters such as amount of catalyst, mole ratio of reactants, time on stream, and diferent alcohols to optimize the higher activity towards the product of ethyl acetate.Te purpose of the present study is to estimate the acidity of VPMA on ZrO 2 .TiO 2 and mark a relation between catalytic performance and acidity.
2.2.Methods.ZrO 2 .TiO 2 mixed oxide support was synthesised by using zirconium (IV) isopropoxide and titanium (IV) isopropoxide as a precursor.About 1 : 1 mole ratio of zirconium (IV) isopropoxide and titanium (IV) isopropoxide is initially mixed to get a homogeneous mixture and then hydrolysation by slow addition of 20 ml of distilled water results into a white precipitate.Tis is washed with distilled water, dried, then calcined at 300 °C for 3 h, and labelled as ZrO 2 .TiO 2 support.Vanadium-substituted PMA (VPMA � H 4 PMo 11 VO 40 ) was synthesized by the hydrothermal method according to the literature method [3].Te various VPMA loadings from 10 to 50 wt% VPMA supported on ZrO 2 .TiO 2 by the impregnation method and thereafter calcined at 200 °C for 3 h.

Characterization
Techniques.X-ray powder difraction patterns of the samples were obtained with a model: D8 difractometer (Advance, Bruker, Germany) using Cu Kα radiation (1.5406 Å) at 40 kV and 30 mA.Te measurements were recorded in steps of 0.0450 with a count time of 0.5 s in the range of 2-65 °.Temperature-programmed desorption (TPD) studies of NH 3 were conducted on the Auto Chem 2910 (micromeritics, USA) instrument.Te ammonia concentration in the efuent stream was monitored with the thermal conductivity detector, and the area under the peak was integrated using the software GRAMS/32 to determine the amount of desorbed ammonia.BET surface area analysis was performed on the Auto Chem 2910 analyzer.Pyridineadsorbed FT-IR spectra of samples were analyzed by using the FT-IR Nicolet 670 spectrometer with the KBr disc method at room temperature.Te SEM analyses of samples were recorded on S-4800 microscopy.

Catalytic Esterifcation.
Te catalysts were tested for esterifcation of acetic acid with ethanol in a 50 mL round bottom fask at room temperature.In the experiment (Scheme 1), 10 mmol of acetic acid, 20 mmol of ethanol, and 0.1 g of the catalyst were employed in the fask with stirring.Te reaction mixture was monitored by using a gas chromatograph with a fame ionization detector.Te recycled catalyst is also tested for the same reaction.
Te Keggin ion density of VPMA/ZrO 2 .TiO 2 catalysts was calculated by the following equation: Te Keggin ion density results provide the information about the number of Keggin anions per square nanometer of the zirconia and titania mixed oxide surface.It is calculated according to the active phase VPMA loading and the catalyst surface area.Subsequently, each Keggin cluster (KU) occupies around 1.44 nm 2 [5] and the theoretical monolayer of KU with 0.69 KU nm −2 is formed.Te Keggin ion densities of synthesized samples are in the range of 0.28 to 2.25 which shows that ∼20 VPMA/ZrO 2 .TiO 2 catalysts are well below the theoretical monolayer coverage (0.69 HPA/nm 2 ).

Journal of Chemistry
Tese fndings indicate that the VPMA is highly dispersed over support surface at lower active phase loadings than higher loadings.Tis is due to covering and plugging of pores by the large Keggin ion leads to decrease in the surface area of the catalysts.Tus, the BET results (Table 1) are suggesting that at lower active phase loadings, the active phase Keggin ion density is well below the theoretical monolayer and is highly dispersed on the support.Tis result clearly demonstrates that at 50 wt% loading, the BET surface area is signifcantly reduced than other loadings which provide bulk formation of VPMA observed at higher loadings.
Te acidity, BET surface area, and Keggin ion density results are tabulated in Table 1, and ammonia TPD profle is shown in Figure 2. Tese acidity measurements reveal that acidity values are inclined from 10 to 40 wt % of VPMA and minimized at 50 wt% loading.Te BET surface area results provide information about the surface area which is signifcantly minimized at higher VPMA loadings, and this fnding well correlates with Keggin ion density observed with respect to active phase loadings.Tus, acidity is declined at higher loading which is due to the bulk formation of VPMA on support, leading to the blockage of the pores of the support.Tis result number available acidic site was minimized at higher loading.
To investigate the nature of acidic sites by pyridine, adsorbed samples were recorded on the FT-IR spectrometer (Figure 3).Te various wt% of VPMA on ZrO 2 .TiO 2 catalysts exhibits 3 characteristic IR bands at 1532-1535, 1482-1485, and 1442-1443 cm −1 that were attributed to Brønsted (B), Brønsted + Lewis (B + L), and Lewis (L) acidic sites, respectively.Tis fnding reveals that higher amount of Brønsted acidic sites than Lewis acidic sites were observed at higher loadings than lower loadings.
Te morphology of catalysts is analyzed and is shown in Figure 4. Tese fndings indicate pure VPMA which shows cylindrical morphology, whereas 20 wt% active phase loadings on support were evenly distributed, and at 40 wt%, it shows small crystallites of VPMA observed on support.However, at 50 wt%, loading on support reveals bulk or larger crystallites that were observed during SEM analysis.Tis SEM analysis well correlates with XRD fnding as well as BET surface area of the catalysts.
Initially, the blank and catalytic reaction is performed for the esterifcation of acetic acid with ethanol, and results are tabulated in Table 2. Te blank reaction without catalyst produces 1.5% yield of ethyl acetate.Whereas, VPMA catalyst exhibits 58% yield towards ethyl acetate.In the case of various VPMA loadings on mixed oxide support, 40 wt % VPMA loading behaves higher yield 93% of ethyl acetate than other loadings.Tese results were strongly inclined with acidity, amount, and number of Brønsted and Lewis acidic sites (Figure 3) of the catalysts as well as the surface area and Keggin ion density of the catalysts.Te higher amount of acidity/more number of B & L acidic sites of the catalysts favours the better catalytic yield of ethyl acetate.Tis is in turn of lower crystalinity which was observed at lower VPMA loading than higher loadings correlates with BET surface area of the catalyst as well as SEM analysis.Terefore, at 40 wt% loading behaves more number of     Journal of Chemistry Brønsted as well as Lewis acidic sites (Figure 3) than other loadings, boasting the acetic acid conversion towards ester formation during the course of reaction.Te efect of mole ratio of acetic acid to ethanol was performed over 40 wt% VPMA/ZrO 2 .TiO 2 catalyst (Table 3).Tese results indicated that ester yield increased from 78 to 93% with an increase in the mole ratio of acetic acid to ethanol from 1 : 1 to 1 : 2, respectively.Whereas, higher amount of ethanol used during course of reaction results drop in the ethyl acetate formation is due to further dilution of acetic acid noticeably.

Journal of Chemistry
Te efect of catalytic amount of 40 wt% VPMA/ ZrO 2 .TiO 2 catalysts was tested, and results are tabulated in Table 4.It can be seen from Table 4, the ethyl acetate formation increases from 68% to 93% when catalytic amount of 0.05 g changes to 0.1 g, respectively.However, there is no signifcant change in ethyl acetate when higher amount of catalyst is used.
Te formation of yield of ethyl acetate with respect to reaction time is monitored over 40 wt% of VPMA on mixed oxide support, and results are tabulated in Table 5. Te time on stream analysis indicated that increase in the conversion with time reaches maximum 95% ethyl acetate yield at 36 h at room temperature, and further reaction time equilibrium yield is maintained.Te catalytic performance of acetic acid with various alcohols was studied at the same reaction conditions, and the results are tabulated in Table 6.Tese fndings reveal that formation of ester is declined with increase of carbon chain of the alcohol.Terefore, methanol produced higher yield than other alcohols used during the reaction.
Te catalyst stability during reaction was examined by the recycling experiment which is shown in Figure 5. Before going to perform recycling, after the reaction catalyst and reaction mixture were centrifuged and then separated and dried in an oven at 100 °C which were thereafter calcined at 200 °C for 3 h in air.Now, the regenerated catalyst was tested for the same reaction.Te catalytic conversion reveals that the catalyst exhibits stable activity during the 3 cycles of reactions (Figure 5).Tus, the catalyst shows stable conversion or yield during regeneration studies.

Reaction Mechanism.
In order to fnd the formation, esters form acetic acid and ethanol through mechanism over Brønsted and Lewis acidic sites which is shown in Scheme 2. Ethyl acetate was produced via two pathways, and those are

6
Journal of Chemistry path A and path B, respectively.In path A, Brønsted acidic sites or protons making coordination with a lone pair of oxygen in carbonyl group makes carbonyl carbon which is more electron-defcient.After that, a lone pair on oxygen in alcohol is attacked on electron-defcient carbonyl carbon that leads to loss of water molecule to produce ethyl acetate.
In path B, similar to path A, the Lewis (L) acidic sites pull the carbonyl carbon more electron-defcient and attack of lone pair on oxygen in alcohol at carbonyl carbon favours the formation of ethyl acetate.
Te comparison of the present study with literature of acetic acid esterifcation over various catalysts is shown in Table 7. Alsalme et al. [27] studied the esterifcation of acetic acid with ethanol at 90 °C to produce 75 yield of ester.Whereas, Sekine et al. [28] carried out esterifcation of acetic acid over AEI-type zeolite membrane at 90 °C to produce 84% yield.Gurav et al. [29] reported acetic acid esterifcation in the presence of 20 and 30 wt% DPTA/K10 at 100 °C, showing 89.8 and 90% ethyl acetate yields, respectively.However, in the present study, 40 wt% VPMA/ZrO 2 .TiO 2 exhibits 93% yield towards ethyl acetate at room temperature.Te VPMA/ZrO 2 .TiO 2 catalyst provides better catalytic yield than other catalysts even when the reaction operated at room temperature.

Conclusions
Vanadium boasted the phosphomolybdic acid catalyst supported on ZrO 2 .TiO 2 oxide-supported catalysts which efciently conducted selective esterifcation of acetic acid to ethyl acetate at room temperature.40 wt% VPMA/ZrO 2 .TiO 2 catalyst exhibits higher catalytic performance during the course of reaction than other VPMA loadings.Te catalytic functionality well reveals with structural and acidic properties of the catalyst.Te XRD fnding clearly indicated that the active phase of VPMA is highly dispersed at lower loadings as well as crystallites of Keggin ion are observed at higher loadings.TPD of ammonia data reveals that acidity is inclined with VPMA loading on support.BET surface area analysis shows that a higher surface area of the catalyst provides lower Keggin ion density.SEM images clearly illustrated that bulk formation of VPMA on support was observed at higher loadings.Pyridine-adsorbed FT-IR spectra suggest that Brønsted and Lewis acids favour the formation of ethyl acetate

Table 3 :
Catalytic results of various mole ratios of reactants over the 40 wt% VPMA/ZrO 2 .TiO 2 catalyst a .

Table 7 :
Comparison study of various catalysts with VPMA/ZrO 2 .TiO 2 catalyst for esterifcation of acetic acid.Te recycle studies show stable catalytic functionality up to 3 cycles during the reaction.