КИНЕТИКА И КАТАЛИЗ, 2010, том 51, № 3, с. 401-405
УДК 542.128:546.18378-325:546.831-31
SUPPORTED HETEROPOLYACIDS: SYNTHESIS, CHARACTERIZATION AND EFFECT OF SUPPORTS ON ESTERIFICATION REACTIONS
© 2010 V. Brahmkhatri, A. Patel*
Department of Chemistry, Faculty of Science, M. S. University of Baroda, Vadodara, India *E-mail: aupatel_chem@yahoo.com Received 02.01.2009
12-Tungstophosphoric acid supported onto silica was synthesized by impregnation. The supports and synthesized catalysts were characterized for chemical stability, ion exchange capacity, thermogravimetric analysis, differential scanning calorimetry, FT—IR, and BET surface area. The catalytic activity was evaluated for liquid phase esterification reactions. The catalyst was regenerated and reused. The best catalyst was calcined at different temperatures and its catalytic activity was also evaluated for esterification reactions under optimized conditions. Further, obtained results are compared with 12-tungstophosphori acid supported onto zirconia in order to see effect of acidic nature of support on catalytic activity as well as thermal stability of the catalyst.
In recent times supported heteropolyacids (HPAs) have been gaining importance as acid catalyst, especially for reactions of esterification [1—6], alkylation [7—12] and acylation [8, 13—16], as they posses a number of advantages such as high catalytic activity and selectivity, high surface area, high thermal stability over traditional corrosive and hazardous liquid acid catalyst, easy separation from reaction mixture and possibility of their repeated use.
In case of supported HPAs, it is known that support does not play always merely a mechanical role but it can also modify the catalytic properties of the HPAs. Various supports (see [5, 6]) have been used for supporting HPAs. As a contribution towards the same, we have established the use of 12-tungstophosphoric acid (TPA) supported onto Al2O3 (neutral support) and ZrO2 (acidic support) for some organic reactions such as esterification of primary and secondary alcohols [17], tert-butylation of phenols and cresols [18, 19]. The obtained promising results encouraged us to explore the catalytic activity of TPA supported onto SiO2, more acidic support than ZrO2. The main aim of present work is to see the effect of more acidic support (SiO2) on catalytic activity as well as on thermal stability of TPA.
The present work consists of synthesis and characterization of TPA supported onto SiO2. The support and catalysts were characterized by chemical stability, ion exchange capacity, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), FT-IR, and BET surface area. The catalytic activity was evaluated for the esterification of primary alcohols (ethanol, «-propanol, «-butanol) with acetic acid. The best catalyst was calcined at different temperatures (300, 400, and 500°C) and evaluated for esterifi-cation reactions under optimized conditions. To see the effect of support, the obtained results are
compared with TPA supported onto ZrO2. The difference in the catalytic activity was correlated with the nature of support.
EXPERIMENTAL
Materials
All chemicals used were of A.R. grade. H3PW12O40 • ■ «H2O ("Lobachemie, Mumbai") and silica (60-120 mesh size, "SD Fine Chemicals", Mumbai) was used as received. Ethanol, propanol, «-butanol, and glacial acetic acid were obtained from "Merck" and used as received.
Sy«thesis of the catalyst
TPA was supported onto SiO2 by impregnation method. A catalyst containing 30% TPA was synthesized by impregnating 1 g of SiO2 with an aqueous solution of TPA (0.3 g/30 ml of bidistilled water) with stirring for 35 h and dried at 100°C for 10 h. The obtained material was designated as TPA3/SiO2.
The same process was followed for the synthesis of series of supported TPA containing 20 and 40% of TPA (0.2 or 0.4 g, 20 or 40 ml ofbidistilled water). The obtained materials were designated as TPA2/SiO2 and TPA4/SiO2, respectively. The best catalyst TPA3/SiO2 was calcined at 300, 400 and 500°C in air for 5 h and the resulting materials were designated as C3-TPA3/SiO2, C4-TPA3/SiO2 and C5-TPA3/SiO2, respectively.
Characterizatio«
Detail characterization of TPA3/ZrO2 can be found in our earlier publications [17, 20]. The present study
Table 1. Ion exchange capacity (IEC) values of the catalysts
Catalyst IEC, meq/g
SÍO2 0.21
TPA2/SÍO2 0.57
TPA3/SiO2 0.62
TPA4/SiO2 0.69
Weight, % 100
98 96 94 92
100 200 300 400 500 600
Temperature, °C
Fig. 1. TGA of TPA (1) and TPA3/SiO2 (2).
20 column. Turnover number (TON) is defined as mole substrate reacted per mole of the catalyst and was calculated using following formula:
numbers of moles of substrate reacted
TON =
number of moles of catalyst
includes detail characterization of TPA supported onto SiO2. Different mineral acids and bases were used for checking the chemical stability of the material.
The ion exchange capacity (IEC) was determined by using the following formula:
IEC (in meq/g) = _ (normality of NaOH) x (volume of NaOH)
weight of material Thermogravimetric analysis of the samples was carried out on METTLER TOLEDO STAR SW 7.01 instrument. Differential scanning calorimetry of the samples was carried out on METTLER S RSW810. The FT-IR spectra of the samples (fresh and calcined) were obtained by using KBr wafer on Perkin—Elmer spectrometer. Adsorption—desorption isotherms of samples were recorded on a Micromatries ASAp 2010 surface area analyzer at —196°C. From adsorption-desorption isotherms surface area was calculated using BET method.
Esterification reactions
The esterification reaction was carried out in a 50 ml glass reactor provided with a double walled air condenser, Dean—Stark apparatus, magnetic stirrer and a guard tube. Dean—Stark apparatus was attached to a round bottom flask to separate the water formed during the reaction. The reaction mixture was heated at 80°C for 4 h. The obtained esters were analyzed on a gas chromatograph (Nucon-5700) using a Carbowax
RESULTS AND DISCUSSION
Characterization of catalysts
The catalyst comprising TPA supported onto ZrO2 was synthesized in our laboratory and well characterized by us earlier. From our earlier study, it was found that catalyst containing 30% loading of TPA onto ZrO2 was the best. The main characterization of TPA3/SiO2 and TPA3/ZrO2 like ion exchange capacity, surface area measurement and thermal stability are presented in Table 3.
Chemical stability of material plays an important role. The material having solubility in water and/or in acidic media may not be very useful as a catalyst. Therefore the chemical stability of the material has been checked in different mineral acids (HCl, H2SO4, HNO3) and bases (NaOH, Na2CO3) up to 4 M concentration. The present catalyst TPA3/SiO2 shows no change in color or form indicating their stability.
Table 1 shows values of ion exchange capacity. These values give an idea about the acidity of materials. It is an indirect way to determine the Bronsted acidity of materials. It is observed from the Table 1 that value of IEC increases as the amount of TPA supported onto SiO2 increases.
In order to check the stability, the catalyst was studied for thermogravimetric analysis. The TGA of TPA (Fig. 1) shows initial weight loss up to 210°C is due to loss of adsorbed water and water of crystallization. After that there is no weight loss up to 475°C. The weight loss between 475 to 500°C may be due to the decomposition of TPA. The TGA of TPA3/SiO2 shows 6.6% weight loss within 50—100°C temperature range which is due to loss of adsorbed water and there is no appreciable change in weight till 500°C indicating increase in the stability of the TPA. This decrease in percentage weight loss indicates the presence of chemical interaction between the support, SiO2 and TPA.
This can be further supported by DSC. DSC of TPA3/SiO2 (Fig. 2) shows an endothermic peak in the region of 80—100°C indicating the loss of adsorbed water molecule. The DSC of TPA3/SiO2 did not show any endothermic peak up to 500°C indicating no decomposition of supported material. TGA and DSC indicate that TPA is thermally stable up to 500°C when supported onto silica.
FT-IR spectrum of SiO2 (Fig. 3a) shows broad band at of 3464 cm-1, attributed to asymmetric hy-droxo (-OH) stretches and two bending vibrations at 1636 and 1370 cm-1 corresponding to H-O-H and O-H-O, respectively. In addition to these bands, FT-
SUPPORTED HETEROPOLYACIDS
403
Heat flow, mW
-1
(a)
-2
-3
-4
50 100 200 300 400 500
Temperature, °C
i_i_i_i_i_i
0 5 15 25
35 45
Time, min
TPA3/SiO2
4000 3200 2400 1800 1400 1000 600
(b)
Fig. 2. DSC of TPA3/SiO2.
IR spectrum of TPA3/SiO2 (Fig. 3a) shows bands at 1088, 987 and 800 cm-1 which correspond to W-O-W bending, W—O and P—O symmetric stretching, respectively. The positions are in good agreement with those reported earlier [21, 22]. Further, no appreciable shifting in the band positions (Fig. 3b) of calcined catalyst ( C3-TPA3/SiO2, C4-TPA3/SiO2, C5-TPA3/SiO2) were observed as compared to TPA3/SiO2, indicating that the TPA keeps its Keggin type structure unde-graded up to 500°C when supported onto SiO2.
The values of BET surface area of SiO2 and TPA3/SiO2 were found to be 500 and 234 m2/g respectively. It is known [23] that there may be decrease in the surface area in case of the supported catalyst in which oxides are used as supports. Decrease in surface area [24] could be explained due to support pore blocking by active phase.
Esterification reactions
Esterification is a straight forward reaction subjected to general Bronsted acid catalysis. The yields can be increased by increasing the concentration of either alcohol or acid. In a practical means, it is desirable to obtain maximum yield for economic reasons, the re-actant that is usually less expensive is taken in excess. In present study corresponding acid is taken in excess.
Esterification of n-butanol
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