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Bifunctional catalytic activity of 12-Tungstophosphoric acid impregnated to different supports for esterification and oxidation of Benzyl alcohol: A Comparative study on catalytic activity and kinetics Anjali Uday Patel, and Sukriti Singh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie400953d • Publication Date (Web): 09 Jul 2013 Downloaded from http://pubs.acs.org on July 29, 2013

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Industrial & Engineering Chemistry Research

Bifunctional catalytic activity of 12-Tungstophosphoric acid impregnated to different supports for esterification and oxidation of Benzyl alcohol: A Comparative study on catalytic activity and kinetics Anjali Patel* and Sukriti Singh Polyoxometalates and Catalysis Laboratory, Department of Chemistry, Faculty of Science, M.S. University of Baroda, Vadodara 390002, India *Corresponding Author: Telephone: +91- 265 2795552, *E-mail: [email protected]. Abstract The bi-functional catalytic activity of 30 % 12-Tungstophosphoric acid impregnated to MCM-41 (TPA3/MCM-41) and Zirconia (TPA3/ZrO2) was explored for acid catalysis as well as oxidation reactions. The effect of various reaction parameters such as, molar ratio, amount of catalyst, time and temperature were studied to optimize the conditions for maximum conversion. The catalysts show the potential of being used as recyclable catalytic materials after simple regeneration without significant loss in conversion. TPA3/MCM-41 shows high activity in terms of % conversion towards esterification as well as oxidation reaction as compared to that of TPA3/ZrO2. High catalytic activity of TPA3/MCM-41 may be due to its porosity, higher surface area as well as high total acidity. Keywords: Bi-functional catalyst, 12-Tungstophosphoric acid, MCM-41, Zirconia, Kinetic study

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1. Introduction Benzyl alcohol is a promising molecule in organic chemistry as numbers of organic transformations are possible. The huge amount of applications of alcohols has recently increased the interest in the synthesis of variety of aromatic compounds by esterification via. acid catalysis1,2 and synthesis of various aldehydes, ketones and acids via. selective catalytic oxidation processes. 3, 4 Benzyl acetate has found its usage in artificial essences, food, pharmaceuticals and as a base solvent for some flavor compounds. 5 The chemical synthesis of benzyl acetate is carried out by acetoxylation of toluene by using inorganic catalysts, which produces lot of side products. 6 A literature survey shows that, many solid acid catalysts such as zeolites 7, ion exchange resins 8, Bronsted acidic ionic liquids. 9,10 NbCl5/Al2O3 11 and metal coated nanaoparticals 12 have been found to be efficient catalysts in benzyl alcohol esterification. Another pathway to convert benzyl alcohol into useful chemical is by selective oxidation, to give benzaldehyde. Benzaldehyde is a very valuable chemical that has widespread applications in perfumery, dyestuff, agro and other chemical industries etc. 13 Production of benzaldehyde is the first step in oxidation of benzyl alcohol and further oxidation leads to the production of benzoic acid with side reactions leading to the other products like toluene, dibenzyl acetal, and benzyl benzoate. 14 Benzaldehyde is commercially produced as a by-product from the benzoic acid industry, oxidation of toluene in organic solvents and hydrolysis of benzyl chloride which often contains traces of chlorine impurities and copious waste is generated in this process. 15 These procedures suffer from a lack of selectivity, use of organic solvents, toxicity of the reagents, and waste production, which requires an environmentally benign alternative. Environmental concerns have forced the chemical industries to re-evaluate many of its processes to reduce or eliminate the formation of by-products. Hence, the ideal system for

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“green” oxidation is the use of molecular oxygen or hydrogen peroxide as the primary oxidant together with recyclable catalysts. The significant advantage of this method is that the main coproduct is water, thus avoiding the production of highly toxic organic residues. Consequently from the view point of demands as well as significance of acid and oxidation reactions, it would be more beneficial if a bi-functional catalyst could be developed for esterification as well as oxidation of benzyl alcohol. Recently, supported HPAs have attracted much attention as economically and environmentally benign catalyst due to their acidity as well as redox properties and also for having number of advantages over traditional catalysts It was found that very few reports are available on esterification of benzyl alcohol with acetic acid 16,17 and oxidation of benzyl alcohol over supported heteropolyacids. 18,19 To the best of our knowledge bifunctional catalytic activity over 12-tungstophophoric acid (TPA) impregnated to MCM-41 and Zirconia has not been evaluated for benzyl alcohol. The synthesis and catalytic activity of 12-tungstopsphoric acid impregnated to MCM-41 and hydrous Zirconia in esterification, transesterification and alkylation has been carried out by our group. 20,21 In order to perform a new contribution to the field of ecofriendly bifunctional catalysis, we report here liquid phase esterification as well as oxidation of benzyl alcohol, over environmentally benign, 12-tungstophosphoric acid impregnated to different supports, MCM-41 and Zirconia. The effect of various reaction parameters such as mole ratio, catalyst amount, time, and temperature was studied to optimize the conditions for maximum conversion. Also the catalyst was regenerated and reused up to four cycles. A detailed kinetic study was carried out for esterification of benzyl alcohol with both the catalysts. The rate constants as well as the activation energy for the reaction were determined. The results were correlated with the nature of support.



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2. Experimental 2.1. Materials All chemicals used were of A.R. grade. H3PW12O40.nH2O (Loba chemie, Mumbai), ZrOCl2 .8H2O (Zirconium oxychloride), liquor ammonia, CTAB (Cetyltrimethyl ammonium bromide), TEOS (Tetraethyl orthosilicate), benzyl alcohol, acetic acid, hydrogen peroxide and dichloromethane were used as received from Merck. 2.2. Synthesis of the Supports (MCM-41 and ZrO2) MCM-41 was synthesized by following procedure reported by us. 20 Surfactant (CTAB) was added to the very dilute solution of NaOH with stirring at 60 0C. When the solution became homogeneous, TEOS was added drop wise, and the obtained gel was aged for 2 h. The resulting product was filtered, washed with distilled water, and then dried at room temperature. The obtained material was calcined at 550 0C in air for 5 h and designated as MCM-41. Hydrous zirconia was prepared by following the same method as reported by us. 21 Aqueous ammonia solution was added to aqueous solution of ZrOCl2 .8H2O up to pH 8.5. The precipitates were aged at 100 0C over a water bath for 1 h, filtered, washed with conductivity water until chloride free water was obtained and dried at 100 0C for 10 h. The obtained material is designated as ZrO2. 2.3. Synthesis of the Catalysts (TPA Impregnated to MCM-41 and ZrO2). Two series of catalysts containing 10-40% of TPA impregnated to MCM-41 and Zirconia were synthesized by incipient impregnation method. One g of MCM-41 was impregnated with an aqueous solution of TPA (0.1/10-0.4/40 g/mL of double distilled water) and dried at 100 0C for


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10 h. The obtained materials were designated as TPA1/MCM-41, TPA2/MCM-41, TPA3/MCM41, and TPA4/MCM-41, respectively. A series of catalysts containing 10-40% of TPA impregnated to ZrO2 was synthesized following the same method. 2.4. Characterization A comprehensive study on the characterizations of both the synthesized catalysts can be found in our earlier publications. 20,21 In the present article FT-IR, BET, n-butylamine titration acidity, and XRD measurements are given for reader’s convenience. Elemental analysis (EDS) was carried out using JSM 5910 LV combined with INCA instrument. FT-IR spectra of the samples were obtained by using the KBr wafer on the Perkin Elmer instrument. Adsorption-desorption isotherms of samples were recorded on a Micromeritics ASAP 2010 surface area analyzer at -196 0C. From adsorption-desorption isotherms surface area was calculated using the BET method. The total acidity for all the materials has been determined by n-butylamine titration. The XRD pattern was obtained by using PHILIPS PW-1830. The conditions used were as follows: Cu Kα radiation (1.5417 A0), scanning angle from 10 to 600. 2.5. N-Butyl amine acidity determination A 0.025 M solution of n-butylamine in toluene was used for estimation. The catalyst weighing 0.5 g was suspended in this solution for 24 h and excess base was titrated against trichloroacetic acid using neutral red as an indicator. This gives the total acidity of the material. 2.6 Catalytic Activity 2.6.1. Oxidation reaction



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Oxidation reaction was carried out in a batch type reactor, by taking 25 mg catalyst with different mole of alcohol and 30% aqueous H2O2 at 90 oC. After completion of the reaction, the product was extracted with dichloromethane, dried with magnesium sulphate and analyzed on Gas Chromatograph (Shimatzu-2014) using RTX-5 capillary column. Product recognition was done by comparison with standards and finally by a Gas Chromatography-Mass Spectrometer (GC-MS). 2.6.2 Esterification reaction The esterification of benzyl alcohol with acetic acid (mole ratio 1:3) was carried out in a batch reactor provided with Dean-Stark apparatus, magnetic stirrer, and a guard tube. The reaction mixture was refluxed at 100 0C for different time intervals. The obtained products were analyzed by means of gas chromatograph (Shimatzu-2014) using a capillary column (RTX-5). Products were identified by comparison with the authentic samples and finally by gas chromatography-mass spectroscopy (GC-MS). 3. Results and discussion 3.1. Catalyst characterization FT-IR of MCM-41 (Fig.1a) shows a broad band in the region of 1300-1000 cm-1 corresponding to asymmetric stretching of Si–O–Si. A symmetric stretching at 966 cm-1 corresponds to vibration of Si–OH. The band at 801 and 458 cm-1 are due to symmetric stretching and bending vibration of Si-O- Si, respectively. The typical bands for TPA (Fig. 1e) are seen at 1088, 987, and 800 cm-1 corresponding to P-O, W=O and W-O-W stretching, respectively. FT-IR spectra of TPA3/MCM-41 (Fig.1b) is almost same as that of MCM-41. The absence of respective FT-IR bands of TPA in TPA3/MCM-41 may due to the overlapping of TPA bands with that of support.


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TPA3/ZrO2 shows bands at 812 cm-1, 964 cm-1 and 1070 cm-1 (Figure 1c) corresponding to the symmetric stretching of W-O-W, W=O and P-O, respectively, indicating the retainment of the Keggin unit even after supporting on ZrO221. Figure.1 The data for surface area, elemental analysis, and total acidity by n-butyl amine titration are presented in Table 1. Specific surface area strongly decreased for catalysts compared to the supports. Also on incorporating TPA, surface area values decreased for both MCM-41 and ZrO2 based catalysts, which is as expected and may be due to the blocking of the pores. The decrease in surface area is the first evidence of chemical interaction between TPA and the supports. N-Butyl amine acidity values indicate that MCM-41 is fairly acidic as compared to ZrO2 which is as expected (Table1). An increase in the acidity of the catalysts as compared to that of supports is due to the presence of TPA, which possesses strong Bronsted acidity. Table 1 The XRD patterns of the supports and catalysts are shown in Fig. 2. The XRD pattern of MCM-41 shows a sharp peak at around 2θ=20 and a few weak peaks in 2θ = 3-50, which indicated a well-ordered hexagonal structure of MCM-41 (Figure 2a). Further the absence of characteristic peaks of crystalline phase of TPA indicates that TPA is finely dispersed inside the hexagonal channels of MCM-41.The XRD pattern of TPA3/ ZrO2 (Fig. 2b) shows no crystalline peak corresponding to TPA which indicates that TPA is finely dispersed on to the surface of ZrO2. Figure 2 3.2. Oxidation Reaction



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A detailed study was carried out on oxidation of benzyl alcohol by varying different parameters such as molar ratio of alcohol to H2O2, catalyst amount, reaction time, and reaction temperature in order to optimize the conditions using environmentally benign oxidant H2O2. In oxidation of benzyl alcohol, Benzaldehyde is obtained as a major product along with some amount of benzoic acid as an over oxidation byproduct (Scheme 1). The oxidation reaction of benzyl alcohol was carried out by changing the molar ratio of benzyl alcohol to H2O2 from 1:1 to 2:1, as shown in Table 2 with 25 mg of the catalyst for 24 h at 80 oC over both the catalysts. With increase in the concentration of H2O2, an increase in the conversion of benzyl alcohol was observed. It was found that the maximum conversion was obtained with 1:3 molar ratio. 24 and 22 % conversion with 90 and 98 % selectivity for BA (Benzaldehyde) was obtained with TPA3/MCM-41 and TPA3/ZrO2, respectively. Hence, optimization of the conditions was carried out with 1:3 molar ratio of Benzyl alcohol: H2O2. Table 2 The data for effect of the amount of catalyst on benzyl alcohol can be seen in Figure 3. The catalyst amount was varied in the range 10 mg - 40 mg while keeping the other parameters constant. It is seen from the figure that the catalytic activity increases initially up to 25 mg and then becomes constant with further increase in the amount of catalyst. It is known that in company of polar molecules, heteropolyacids display pseudo liquid behavior 2 in which catalytic activity is directly proportional to the active species present in the catalyst. Additional, no significant increase in conversion after 25 mg was observed. It is seen that after 25mg that there is no change in the conversion. As we take total 10 mmol of alcohol in oxidation reaction which is fixed each time and the amount of catalyst is increased. So it might be possible that with the increase in the amount of catalyst, new active sites might not be available for attack of the


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reactants and there might be a possibility that the active sites (TPA) might not get rid of the product molecules. The obtained results are in good agreement with the proposed explanation. It is also seen that in all cases Benzaldehyde (98%) is obtained as a single selective product for TPA3/ZrO2 and 90% selectivity was obtained for TPA3/MCM-41. The marked change in the selectivity might be due to nature of the support. Figure 3. The effect of temperature on benzyl alcohol conversion was studied in the temperature range from 60-100 0C, keeping other parameters fixed (molar ratio 1:3, catalyst amount 25 mg, reaction time 24 h) and results are presented in Figure 4. The results show that conversion increased with increase in temperature from 60 to 80 oC. Only a small increase in conversion was observed on increasing temperature from 90 to 100 oC. So 90 oC was found most favorable for obtaining highest conversion for benzyl alcohol. Figure 4. The effect of reaction time is shown in Figure 5. It was observed that the benzyl alcohol conversion increases with reaction time. With increase in the reaction time, the conversion of both benzyl alcohol and selectivity for benzoic acid also increases in both the catalysts. This may be due to the fact that with increasing reaction time benzaldehyde undergoes further oxidation and gives benzoic acid, which decreased the selectivity of benzaldehyde. Highest selectivity for benzoic acid is observed in TPA3/MCM-41. After 24 h, the conversion remains constant. This observation is due to the fact that the catalyst activation and attainment of equilibrium requires time. Once the equilibrium is attained, the conversion becomes almost constant. As seen from Figure 5, both the catalysts achieve maximum conversion in 24 h. Figure 5.



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The optimum conditions are, mole ratio to Benzyl alcohol to H2O2= 1:3; catalyst amount= 25 mg, temperature= 90 oC, time= 24 h. 3.2.1. Heterogeneity test For the thorough evidence of heterogeneity, a test 22 was carried out by filtering catalyst from the reaction mixture at 90 oC after 6 h and the filtrate was allowed to react up to the completion of the reaction (10h). The reaction mixture of 6 h and the filtrate were analyzed by gas chromatography. No change in the % conversion or the % selectivity was found indicating the present catalyst falls into category C 22 and it could be proved that the catalysts are truly heterogeneous. The results are presented in supplementary Table 1. The results under optimized conditions for both the catalysts are presented in supplementary table 2. It is seen from the results that there is no significant change in % conversion over both the catalysts. However a significant change in % selectivity is observed. The difference in the selectivity can be explained on the basis of the type of supports used. MCM-41 is more acidic than ZrO2 (Table 1). The selectivity for benzoic acid might be attributed to the acidity of MCM-41 which facilitates the over oxidation of Benzaldehyde to give benzoic acid. 3.2.2. Comparison with reported catalysts Table 3 represents the comparative data with other catalysts for oxidation of benzyl alcohol with H2O2. Jian et.al. have reported the use of phase transfer modified PW12O40 in presence of solvent for liquid phase oxidation of benzyl alcohol. 14.3 % conversion was obtained at 0.08mmol catalyst amount and 80 oC. 19 Oxoperoxo molybdenum (VI) and tungsten (VI) complexes with 1-(20-hydroxyphenyl) ethanone oxime 23 have been used as catalyst in presence


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of solvent (Acetonitrile) for oxidation reaction. Here higher conversion was obtained but the molar ratio of alcohol and H2O2 is 25:100, reaction time is 24 h. Table 3 All the above mentioned reactions use higher mole ratio of alcohol to oxidant and solvent is needed. These reports also do not mention recycling and reusability of the catalyst. The present catalyst is promising for oxidation of benzyl alcohol under mild reaction conditions. 3.3. Esterification of Benzyl alcohol The esterification of alcohol is an equilibrium-limited reaction. In order to overcome the equilibrium limitation, generally esterification of alcohol is carried out by taking acetic acid in excess in order to favor the forward reaction. In order to optimize the conditions a detailed study was carried out for esterification of benzyl alcohol. The effect of mole ratio of alcohol to acid was studied by using 0.15 g of the catalyst and was subjected to react for 1 h at 100 oC. It is seen from Figure 6 that upon increasing the concentration of acid to 2 times, higher conversion was obtained. For both the catalysts benzyl alcohol conversion increased with increase in the Benzyl alcohol: Acetic acid molar ratio, and becomes almost constant after 1:2 molar ratio. Hence, 1:2 molar ratio was optimized for obtaining maximum conversion of 87% and 79% for TPA3/MCM-41 and TPA3/ZrO2, respectively. Figure 6. The effect of the amount of catalyst on alcohol conversion was investigated. Increasing catalyst amount from 50 to 200 mg resulted in promoting the alcohol conversion. It was observed from Figure 7, that the conversion increased with increase in catalyst amount and reaches a maximum of 87% and 79% for TPA3/MCM-41 and TPA3/ZrO2, respectively for 0.150 g catalyst.



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However with a further increase in the amount of catalyst after 0.150 g, there is no considerable increase in conversion, which may be due to the blocking of the active sites. Figure 7. In order to determine the most favorable temperature, the reaction was carried out at different temperatures 70-110 oC, keeping other parameters fixed (molar ratio 1:2, catalyst amount 150 mg, reaction time 1 h) and results are presented in Figure 8. The results show that conversion increases with increase in temperature from 70 to 90 oC. Only a negligible improvement in conversion was observed on increasing temperature from 100 to 110 oC. So the temperature of 100 oC was found optimized for the maximum conversion of benzyl alcohol. Figure 8. The effect of reaction time on conversion was studied. It was observed (Fig. 9) that there is an increase in conversion with an increase in reaction time. As the reaction time was prolonged, further esterification of benzyl alcohol was enhanced. The alcohol conversion was 96 % in 2 h for TPA3/MCM-41 and 90% for TPA3/ZrO2.In all the cases, both the catalysts give single selectivity (100%) towards benzyl acetate. Figure. 9. The optimum conditions are, mole ratio to Benzyl alcohol to acetic acid= 1:2; catalyst amount= 150 mg, temperature= 100 oC, time= 2 h. The difference in % conversion may be attributed to the nature of supports. Higher % conversion is observed in the case of TPA3/MCM-41which may be due to the high surface area as well as high acidity of MCM-41(Table 1). 3.3.1. Heterogeneity test


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For the rigorous proof of heterogeneity, a test 22 was carried out by filtering catalyst from the reaction mixture at 100 oC after 30 min and the filtrate was allowed to react up to the completion of the reaction (2 h). The reaction mixture of 30 min and the filtrate were analyzed by gas chromatography. No change in the % conversion or the % selectivity was found indicating the present catalyst falls into category C. 22 The results are presented in supplementary Table 3. 3.3.2. Comparison of catalytic activity with reported catalysts Table 4 represents the comparative data with other catalysts for esterification of benzyl alcohol with acetic acid. Halligudi et al.17 have reported the use of silicotungstic acid supported to Zirconia for the esterification of benzyl alcohol. 11.2% conversion was obtained with 0.25 g of catalyst after 3 h of reaction at 100 oC. The same group 17 has also reported a 59% conversion of benzyl alcohol with silicotungstic acid/zirconia supported SBA-15 in same reaction conditions. Ligand coated platinum nanoparticles 12 as heterogeneous catalyst has been used to give 85 % conversion in 5 h. Lactam-based Brønsted-acidic ionic liquids 9 gave 83% conversion with good selectivity towards benzyl acetate. All the above mentioned reactions use higher amount of catalyst at longer reaction time and conversions are lower as compared to the present catalyst. The present catalyst is a promising one for esterification of alcohols, with high conversion and 100 % selectivity for benzyl acetate. Table 4 3.4. Recycling of the Catalysts The catalysts were recycled in order to test its activity as well as stability. The catalysts were separated from the reaction mixture only by simple filtration. The first washing was given



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with dichloromethane (for oxidation) and conductivity water (for esterification) to remove unreacted reactant molecules, and then drying at 100 0C, and the recovered catalyst was charged for the further run. There is no appreciable change in the % conversion of benzyl alcohol in esterification as well as oxidation reaction using the regenerated catalysts up to four cycles (Figure 10). Figure 10. 3.5. Leaching test and Characterization of regenerated catalysts Any leaching of the active species from the support makes the catalyst unattractive and hence it is needed to study the stability as well as leaching of TPA from the support. TPA can be quantitatively characterized by the heteropoly blue color, which is observed when reacted with a mild reducing agent such as ascorbic acid .24 In the present study, this method was used for determining the leaching of TPA from the support. Standard samples containing 1-5% of TPA in water were prepared. To 10 ml of the above samples, 1 ml of 10% ascorbic acid was added. The mixture was diluted to 25 ml. The resultant solution was scanned at λ max of 785 cm for its absorbance values. A standard calibration curve was obtained by plotting values of absorbance vs percent concentration. 1 g of catalyst with 10 ml double distilled water was refluxed for 24 hours. Then 1 ml of the supernatant solution was treated with 10% ascorbic acid. Development of blue color was not observed indicating that there was no leaching. The same procedure was repeated with alcohols and the filtrate of the reaction mixture after completion of reaction in order to check the presence of any leached TPA. The absence of blue color indicates no leaching of TPA. The leaching of W from TPA3/MCM-41 and TPA3/ZrO2 was conﬁrmed by carrying out an analysis of the used catalyst (EDS) as well as the product mixtures (AAS). The values of EDS


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analysis of fresh as well as reused catalysts are given in Table 1. Analysis of used catalysts did not show any appreciable loss in W content as compared to the fresh catalyst. Analysis of the product mixtures shows that if any W was present it was below the detection limit, which corresponds to less than 1 ppm. These observations strongly suggest that there is no leaching of any active species form the support. Further the reused catalysts were characterized for FT-IR, surface area, XRD and acidity measurements. The FT-IR data for the fresh as well as the reused catalysts are represented in Figure 11. No appreciable shift in the FT-IR band position of the reused catalyst compared to fresh one indicates the retention of Keggin-type TPA into MCM-41 as well as onto ZrO2. There is a slight decrease in the value of surface area of the reused catalyst R-TPA3/MCM-41, which might be due to the pore blocking. Surface area of R-TPA3/ZrO2 remains unchanged (Table 5). Wide angle XRD also shows no change in the property of recycled catalysts (Figure 12). Further the n-butyl amine acidity values of reused catalysts shows no significant change as compared to the fresh catalysts. Hence there is no deactivation of catalyst. Table 5 Figure 11. Figure 12. 3.6. Control experiments Control experiments were carried out using supports as well as TPA under the optimized conditions (Supplementary Table 4). Esterification and oxidation reaction of benzyl alcohol was carried out without any catalyst under optimized conditions. It can be seen from the supplementary Table 4 that MCM-41 and ZrO2 are not much active towards the esterification and oxidation of benzyl alcohol, indicating that the catalytic activity is due to TPA only. The same



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reaction was carried out by taking the active amount of TPA (5.7 mg in oxidation, 35 mg in esterification). The obtained results are almost same as that of supported catalyst indicating that TPA is the real active species. Thus, we are successful in anchoring TPA to supports, without any significant loss in activity and hence in overcoming the traditional problems of homogeneous catalysis. 4. Kinetics A detailed kinetic study was performed for esterification of benzyl alcohol over TPA3/MCM-41 and TPA3/ZrO2. In all the experiments, reaction mixtures were analyzed using gas chromatography at a fixed interval of time. The plot of ln Co/C vs time (Figure13) shows a linear correlation of alcohol consumption with respect to time. With an increase in reaction time there is a gradual and linear decrease in the alcohol concentration over both the catalysts. These observations indicate the esterification of benzyl alcohol follows first order dependence with respect to time. Figure13. It was observed from Figure14, that the rate of reaction increases with an increase in the catalyst concentration. The plot of reaction rate vs catalyst concentration (Figure14) also shows a linear relationship for the TPA3/MCM-41 and TPA3/ZrO2 catalyst. Figure 14. 4.1. Estimation of Activation Energy (Ea) The graph of ln k vs 1/T was plotted (Figure15), and the value of activation energy (Ea) was determined from the plot. The kinetic parameters such as pre-exponential factor (A), and activation energy (Ea) for esterification reaction of benzyl alcohol were calculated. As the esterification reactions are temperature sensitive, the effect of temperature on the esterification of


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benzyl alcohol was also studied by varying the temperature between 343 and 383 K, keeping the alcohol: acetic acid ratio of 1:2 and catalyst concentration of 12 x 10-3 mol. As the temperature increases from 343 to 373 K, the rate constant increases. This may be due to the activation of the catalytic species with an increase in temperature. Figure 15. It is important to recognize whether the reaction rate is diffusion limited/mass transfer limited or it is truly governed by the chemical step where the catalyst is being used to its maximum capacity. It is reported that the activation energy for diffusion limited reactions is as low as 10-15 kJ mol−1. 25 In the present reaction conditions activation energy for TPA3/MCM-41 was found to be 40 kJ mol-1 and 43 kJ mol-1 for TPA3/ZrO2 and hence, in both the case the rate is truly governed by the chemical step. 4.2. Effect of support. It is known that in the case of mesoporous support, textural properties of support affect the catalytic activity. The obtained difference in catalytic activity and product selectivity in oxidation reaction of benzyl alcohol may be due to the nature of supports. The surface area of MCM-41 is higher than that of ZrO2 and the same trend was observed for the catalysts. Also the total acidity of TPA3 /MCM-41 is higher than TPA3 /ZrO2 catalyst, even though the % loading of TPA is same for both the supports. So, in the case of oxidation, MCM-41 based catalyst shows 90 % selectivity towards benzaldehyde and other product was benzoic acid. The increase in selectivity for benzoic acid in TPA3/MCM-41 as compared to TPA3/ZrO2 might be due to over oxidation of Benzaldehyde to benzoic acid. This small effect might be due to the Pore structure as well as acidity of MCM-41 which facilitates the oxidation of Benzaldehyde in presence of water.



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5. Conclusion A versatile bi-functional solid catalytic system comprising of 12-tungstophosphoric acid, MCM-41 and Zirconia, has been introduced for acid catalysis as well as oxidation reactions of benzyl alcohol. The significance of the present work lies in obtaining excellent conversions in esterification as well as in oxidation reaction, over both the catalysts. High conversion for both the types of reactions over TPA3/MCM-41 was correlated with high surface area and acidity of MCM-41. The activation energy is low in both the cases and the rate is truly governed by a chemical step. Also, both catalysts show potential for being used as recyclable bifunctional catalytic materials after simple regeneration without any significant loss in the conversion. ACKNOWLEDGMENT We are thankful to Department of Science and technology (DST), Project. No.SR/S5/GC01/2009, New Delhi, for the financial support. One of the authors Ms. Sukriti Singh is thankful to the same for the fellowship. CORRESPONDING AUTHOR Telephone: +91- 265 2795552,E-mail: [email protected]. SUPPORTING INFORMATION The results of heterogeneity test for oxidation reaction, Heterogeneity test for esterification reaction, Comparison of both the catalysts under optimized conditions for oxidation reaction, Control experiments for Benzyl alcohol reactions. This material is available free of charge via the Internet at http://pubs.acs.org/.

REFERENCES (1) Schmitt, M.; Scala, C.; Moritz, P. ; Hasse, H. n-Hexyl Acetate Pilot Plant Reactive Distillation with Modified Internals. Chem. Eng. Process. 2005, 44, 677. 18 ACS Paragon Plus Environment

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(2) Shanmugam, S.; Vishwanathan, B. ; Vardarajan, T.K. Esterification by solid acid catalysts-a comparison, J.Mol.Catal.A.Chem. 2004,223,143. (3) Tundo , P.; Romanelli , G. P.; Vázquez , P. G.; Aricò, F. Multiphase oxidation of alcohols and sulfides with hydrogen peroxide catalyzed by heteropolyacids. Cat. Comm. 2010, 11 1181. (4) Tojo, G.; Fernández, M. Oxidation of Alcohols to Aldehydes and Ketones. first ed. Springer, New York, 2006. (5) Zaidi, A.; Gainer, J.L.; Carta, G.; Fatty acid esterification using nylon-immobilized lipase. Biotechnol Bio-Eng. 1995,48,601. (6) Komatsu, T.; Inaba, K.; Uezno, T.; Onda, A.; Yashima, T. Nano-size particles of palladium intermetallic compounds as catalysts for oxidative acetoxylation. Appl.Catal. A: Gen. 2003,251,315. (7) Kirumakki , S. R.; Nagaraju , N.; Narayanan, S. A comparative esterification of benzyl alcohol with acetic acid over zeolites Hb, HY and HZSM5. Appl. Catal. A: Gen. 2004, 273, 1. (8) Ali, S. H.; Merchant, S.Q.; Kinetic Study of Dowex 50 Wx8-Catalyzed Esterification and Hydrolysis of Benzyl Acetate. Ind. Eng. Chem. Res. 2009, 48 ,2519. (9) Zhou, H.; Yang, J.; Ye, L.; Lin, H.; Yuan, Y.; Effects of acidity and immiscibility of lactambased Brønsted-acidic ionic liquids on their catalytic performance for esterification. Green Chem.2010,12, 661. (10) Joseph, T.; Sahoo, S.; Halligudi, S.B. Bronsted acidic ionic liquids: A green, efficient and reusable catalyst system and reaction medium for Fischer esterification. J. Mol. Catal.A: Chem. 2005, 234, 107.



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(11) Barbosa , S.L.; Hurtado , G. R.; Klein , S. I.; Junior , V. L.; Dabdoub , M. J.; Guimaraes, C. F. Niobium to alcohol mol ratio control of the concurring esterification and etherification reactions promoted by NbCl5 and Al2O3 catalysts under microwave irradiation. Appl. Catal. A: Gen. 2008 ,338 ,9. (12) Ghosha , A.; Stellacci , F.; Kumar, R. New mixed ligand coated platinum nanoparticles for heterogeneous catalytic applications. Catal. Today. 2012, 198, 77 . (13) Ulmann’s, Encyclopedia of Industrial Chemistry, 5th ed.; VCH: Weinheim, 1985, p 469. (14) Li, G.; Enache, D. I.; Edwards, J. K.; Carley, A. F.; Knight, D. W.; Hutchings, G. J. Solvent-free oxidation of benzyl alcohol with oxygen using zeolite-supported Au and Au-Pd catalysts. Catal. Lett. 2006, 110, 7. (15) McGrath, D. V.; Grubbs, R.H.; Ziller, J. W. Aqueous ruthenium(II) complexes of functionalized olefins: the x-ray structure of Ru(H2O)2 (.eta.1(O):.eta.2(C,C0)-OCOCHCHdCHCH)2. J. Am. Chem. Soc.1991, 113, 3611. (16) Heravi, M. M.; Behbahani, F. K.; Bamoharrama, F. F.; H14 [NaP5W30O110]: A heteropoly acid catalyzed acetylation of alcohols and phenols in acetic anhydride. J. Mol. Catal A: Chem. 2006, 253, 16. (17) Sawant , D. P.; Vinu, A.; Justus, J.; Srinivasu , P.; Halligudi, S.B. Catalytic performances of silicotungstic acid/zirconia supported SBA-15 in an esterification of benzyl alcohol with acetic acid. J.Mol. Catal,2007, 276, 150. (18) Li, J.; Hu, S.; Luo, S.; Ch, J.P. Chiral Amine-Polyoxometalate Hybrids as Recoverable Asymmetric Enamine Catalysts under Neat and Aqueous Conditions. Eur. J. Org. Chem. 2009, 132.


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(19) Weng, Z. H.; Wang, J. Y.; Jian, X. G. Efficient oxidation of benzyl alcohol with heteropolytungstate as reaction-controlled phase-transfer catalyst. Chinese Chem. Lett. 2007, 18 ,936. (20) Brahmkhatri, V.; Patel, A. Biodiesel Production by Esterification of Free Fatty Acids over 12-Tungstophosphoric Acid Anchored to MCM-41. Ind. Eng. Chem. Res. 2011, 50, 6620. (21) Patel, S.; Purohit, N.; Patel, A. Synthesis, characterization and catalytic activity of new solid acid catalysts, H3PW12O40 supported on to hydrous zirconia. J. Mol. Catal. A: Chem. 2003, 192 ,195. (22) Sheldon, A.; Walau, M.; Arends, I.W.C.E.; Schuchurdt, U. Heterogeneous catalysts for liquid-phase oxidations: philosophers stones or Trojan horses? Acc. Chem. Res.1998, 31,485. (23) Gharah , N.; Chakraborty , S. ; Mukherjee , A. K.; Bhattacharyya, R. Oxoperoxo molybdenum (VI)- and tungsten(VI) complexes with 1-(20-hydroxyphenyl) ethanone oxime: Synthesis, structure and catalytic uses in the oxidation of olefins, alcohols, sulfides and amines using H2O2 as a terminal oxidant. Inorg. Chim. Acta . 2009, 362, 1089. (24) Yadav, G.D.; Bokade, V.V, Novelties of Heteropoly Acid Supported on Clay: Etheriﬁcation of Phenethyl Alcohol with Alkanols. Appl. Catal. A: Gen. 1996, 147, 299. (25) Bond, G.C. Heterogeneous Catalysis: Principles and Applications, Oxford Chemistry Series, 1974 (Chapter 3, p. 49). Figure captions Scheme1: Oxidation of Benzyl alcohol to Benzaldehyde. Figure 1. FT-IR of (a) MCM-41(b) TPA3/MCM-41(c) TPA3/ZrO2 (d) ZrO2 and (e) TPA. Figure 2. XRD pattern of (a) MCM-41,TPA , TPA3/MCM-41and (b) ZrO2 and TPA3/ZrO2



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Figure 3. Effect of catalyst amount on oxidation, Benzyl alcohol: H2O2 1:3; reaction time 24 h, BA= Benzaldehyde. Figure 4. Effect of temperature on oxidation, Benzyl alcohol: H2O2 1:3; reaction time 24 h. Figure 5. Effect of time on oxidation, Benzyl alcohol: H2O2 1:3; catalyst amount 25 mg; reaction temp.90oC. Figure 6. Effect of molar ratio of Benzyl alcohol: acetic acid on esterification, Reaction conditions; amount of catalyst 0.15 g, reaction temperature 100 0C, reaction time 1 h. Figure 7. Effect of amount of catalyst on esterification, Reaction conditions; molar ratio of benzyl alcohol: acetic acid-1:2, reaction temperature 100 0C, reaction time 1 h. Figure 8. Effect of temperature on esterification, Reaction conditions; molar ratio of benzyl alcohol: acetic acid-1:2, reaction time 1 h. Figure 9. Effect of reaction time on esterification; Reaction conditions; molar ratio of benzyl alcohol: acetic acid-1:2, amount of catalyst 0.15 g, reaction temperature 100 0C. Figure 10 . A. Recycling of the catalysts , molar ratio Benzyl Alcohol: H2O2 1:3, amount of catalyst 25 mg, reaction time 24 h, reaction temperature 90 0 C.B. molar ratio Benzyl Alcohol: acetic acid 1:2, amount of catalyst 0.15 g, reaction time 2 h, reaction temperature 100 0 C.

Figure 11. FT-IR of (A) (a) Fresh TPA3/ZrO2 ,(b) Recycled R-TPA3/ZrO2; (B)(a)TPA3/MCM41 (b) R-TPA3/MCM-41. Figure 12. Wide angle XRD of (a) R-TPA3/MCM-41(b) TPA3/MCM-41(c) R-TPA3/ZrO2 (d) TPA3/ZrO2. Figure 13. First order plot for esterification of benzyl alcohol over TPA3/MCM-41 and TPA3/ZrO2 Figure 14. Plot of reaction rate vs catalyst concentrations. Figure 15. Arrhenius plot for determination of activation energy.


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Table heading Table 1 Textural and acidity properties of all the catalysts. Table 2 % Conversion and % Selectivity with different mole ratio for oxidation of benzyl alcohol. Table 3 Comparison of both the catalysts under optimized conditions for oxidation reaction. Table 4 Comparison with the reported catalysts for esterification of benzyl alcohol a. Table 5 Textural, elemental and acidity properties of fresh as well as recycled catalysts.

Table.1 Textural and acidity properties of all the catalysts.

(m2/g)

n-butyl amine acidity (mmol/g)

MCM-41

659

0.82

TPA1/MCM-41

400

1.28

TPA2/MCM-41

372

1.32

TPA3/MCM-41

360

1.41

TPA4/MCM-41

350

1.48

ZrO2

170

0.62

TPA2/ZrO2

182

0.92

TPA3/ZrO2

146

1.10

TPA4/ZrO2

123

1.20

Surface area

Catalysts

Table 2 % Conversion and % Selectivity with different molar ratio for oxidation of benzyl alcohol 23 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Catalyst

Page 24 of 39

Molar ratio

% Conversion

% Selectivity to BA

1:1

10

95

1:2

18

95

1:3

24

90

1:4

24

85

2:1

17

90

1:1

8

99

1:2

15

99

1:3

22

98

1:4

20

96

2:1

15

95

TPA3/MCM-41

TPA3/ZrO2

%Conversion is based on benzyl alcohol; Molar ratio; benzyl alcohol: H2O2, amount of the catalyst is 25 mg; reaction time is 24 h.

Table 3 Comparison with the reported catalysts for oxidation of benzyl alcohol a. Solvent

Conversion

Products/

mL

%

selectivity

10

14.3

BA

[19]

10

63c

BA

[23]

25:100:24:0.025b: 80 10

65c

BA

[23]

TPA3/MCM-41

10:30:24:25:90

-

24

BA/90

Present work

TPA3/ ZrO2

10:30:24:25:90

-

22

BA/98

Present work

Catalyst [PW12O40]3-

Reaction Conditions 20:18:1.8:0.08b:80

Reference

BTMA PPh4[MoO(O2)2 25:100:24:0.025b:80 (HPEOH)] PPh4[WO(O2)2 (HPEOH)]

a

Benzyl alcohol (mmol): hydrogen peroxide (mmol); reaction time (h): amount of catalyst (mg);

BA = benzaldehyde.b mmol of catalyst, c % Yield Table 4 Comparison with the reported catalysts for esterification of benzyl alcohol a. 24 ACS Paragon Plus Environment

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Catalyst

Reaction

Conversion

Conditions 15STA/22.4

Products/

Reference

selectivity

1:1:3:0.25:100

59

BAc/97

[17]

15STA/ZrO2

1:1:3:0.25:100

11.2

BAc/96

[17]

MPSA–Pt

1:1:5:2:80

85.5

BAc/>99

[12]

IL[CP][CH3SO3]

20:20:4:4b:60

83.8

BAc/100

[9]

TPA3/MCM-41

1:2:2:0.15:100

96

BAc/100

Present work

TPA3/ ZrO2

1:2:2:0.15:100

90

BAc/100

Present work

ZrO2/SBA-15

a

Molar ratio alcohol: acid; reaction time (h); amount of catalyst (g): temperature (oC)

BAc=Benzyl Acetate

Table 5 Textural, elemental and acidity properties of fresh as well as recycled catalysts. Elemental analysis (weight %) W

Catalyst By

P

Theoretical

By

Surface

n-butyl

area

amine

(m2/g)

acidity (mmol/g)

Theoretical

O

Si/Zr*

TPA3/MCM-41

53.9

27.8

18.0

19

0.30

0.32

360

1.41

R-TPA3/MCM-41

53.9

27.9

17.8

19

0.30

0.32

350

1.41

TPA3/ZrO2

31.9

49.6*

17.6

17.6

0.40

0.38

146

1.10

R-TPA3/ZrO2

32.0

49.6*

17.4

17.6

0.38

0.38

145

1.08

EDS

EDS



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Scheme1: Oxidation of Benzyl alcohol to Benzaldehyde

Figure 1. FT-IR of (a) MCM-41(b) TPA3/MCM-41(c) TPA3/ZrO2 (d) ZrO2 and (e) TPA.


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Figure 2 XRD pattern of (a) MCM-41,TPA, TPA3/MCM-41and (b) ZrO2 and TPA3/ZrO2



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Figure 3. Effect of catalyst amount on oxidation, Benzyl alcohol: H2O2 1:3; reaction time 24 h, BA= Benzaldehyde.


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Figure 4. Effect of temperature on oxidation, Benzyl alcohol: H2O2 1:3; reaction time 24 h.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 5. Effect of time on oxidation, Benzyl alcohol: H2O2 1:3; catalyst amount 25 mg; reaction temp.90oC.


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Figure 6. Effect of molar ratio of Benzyl alcohol: acetic acid on esterification, Reaction conditions; amount of catalyst 0.15 g, reaction temperature 100 0C, reaction time 1 h.



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Figure 7. Effect of amount of catalyst on esterification, Reaction conditions; molar ratio of benzyl alcohol: acetic acid-1:2, reaction temperature 100 0C, reaction time 1 h.


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Figure 8. Effect of temperature on esterification, Reaction conditions; molar ratio of benzyl alcohol: acetic acid-1:2, reaction time 1 h.



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Page 34 of 39

Figure 9. Effect of reaction time on esterification; Reaction conditions; molar ratio of benzyl alcohol: acetic acid-1:2, amount of catalyst 0.15 g, reaction temperature 100 0C.


Page 35 of 39

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Figure 10 . A. Recycling of the catalysts , molar ratio Benzyl Alcohol: H2O2 1:3, amount of catalyst 25 mg, reaction time 24 h, reaction temperature 90 0 C.B. molar ratio Benzyl Alcohol: acetic acid 1:2, amount of catalyst 0.15 g, reaction time 2 h, reaction temperature 100 0 C.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

A

Page 36 of 39

B

Figure 11. FT-IR of (A) (a) Fresh TPA3/ZrO2 ,(b) Recycled R-TPA3/ZrO2; (B)(a)TPA3/MCM41 (b) R-TPA3/MCM-41


Page 37 of 39

Figure 12. Wide angle XRD of (a) R-TPA3/MCM-41(b) TPA3/MCM-41(c) R-TPA3/ZrO2 (d) TPA3/ZrO2

1.6

log Co/C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.4

TPA3/MCM-41

1.2

TPA3/ZrO2

1.0 0.8 0.6 0.4 0.2 0.0 0

30

60 90 120 Reaction time (min)

150

180

Figure 13. First order plot for esterification of benzyl alcohol over TPA3/MCM-41 and TPA3/ZrO2



0.45 0.40 Reaction Rate x10-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 39

TPA3/MCM-41 TPA3/ZrO2

0.35 0.30 0.25 0.20 0.15 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 Catalyst quantity mmol

Figure 14. Plot of reaction rate vs catalyst concentrations for esterification of benzyl alcohol.


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-5.00 -5.20 -5.40 -5.60

lnK

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-5.80 -6.00

TPA3/MCM-41

-6.20

TPA3/ZrO2

-6.40 -6.60 0.00250

0.00260

0.00270

0.00280

0.00290

1/T

Figure 15. Arrhenius plot for determination of activation energy, for esterification of benzyl alcohol.


Bifunctional Catalytic Activity of 12 ... - ACS Publications

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