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Acidity and Characterization of 12-Tungstophosphoric Acid Supported on Silica-Alumina

Abstract

This work deals with preparation and characterization of H3PW12O40 (H3PW) supported on silica-alumina. Impregnation of H3PW (15, 20, 30 and 40 wt.%) on commercial silica-alumina support in acidic aqueous solution is effective for preparing this catalyst keeping its Keggin structure, according to different methods of characterization. The catalysts were tested in a model reaction of acetic acid with ethanol and 30 wt.% H3PW/SiO2-Al2O3 had the highest activity under the conditions: catalyst calcination at 300 ºC, temperature of 100 ºC, acetic acid:ethanol molar ratio of 2:1 and catalyst:acetic acid mass ratio of 10 wt.%. The reaction yield was 79 and 100% selectivity for ethyl acetate over three reutilizations, for reaction time of 2 h. The calculated total acid site distribution was 0.299 mmol g-1 (97% of the theoretical probed by pyridine), and most of these (0.236 mmol g-1) were Brønsted weak-medium strength (pyridine desorption between 300 and 500 ºC).

Keywords:
supported heteropolyacid; 12-tungstophosphoric acid; silica-alumina; ethanol; esterification of acetic acid with ethanol


Introduction

An unique class of compounds known as polyoxometalates (POMs) is one of the many areas of inorganic chemistry that is developing rapidly.11 Long, D. L.; Burkholder, E.; Cronin, L.; Chem. Soc. Rev. 2007, 36, 105. The synthesis of large clusters has been considered to be a notable advancement in this field due to their possible applications in nanoscience.11 Long, D. L.; Burkholder, E.; Cronin, L.; Chem. Soc. Rev. 2007, 36, 105.

2 Long, D. L.; Tsunashima, R.; Cronin, L.; Angew. Chem. Int. Ed. 2010, 49, 1736.
-33 Ito, T.; Polichem. 2012, 6. Because of the wide range of physical and chemical properties of these materials, they have been used in many technological fields as demonstrated by several reviews in the last decade.11 Long, D. L.; Burkholder, E.; Cronin, L.; Chem. Soc. Rev. 2007, 36, 105.,44 Heravi, M. M.; Sadjadi, S.; J. Iran. Chem. Soc. 2009, 6, 1.

5 Misono, M.; Catal. Today 2009, 144, 285.

6 Kiricsi, I.; Appl. Catal., A 2003, 256, 1.
-77 Hill, C. L.; Chem. Rev. 1998, 98, 1.

In particular, these materials have been utilized in catalytic applications. Keggin-type POMs have been used in both laboratory-scale reactions and industrial processes.88 Kozhevnikov, I. V. In Catalysis by Polyoxometalates, vol. 2; Roberts, S. M., ed.; John Wiley & Sons Ltd. : Chichester, England, 2002. A key catalytic compound is 12-tungstophosphoric acid (H3PW12O40, H3PW), which has been heterogenized by anchoring it to a variety of supports (e.g., silica, alumina, zirconia, niobia).99 Dias, J. A.; Caliman, E.; Dias, S. C. L.; Paulo, M.; de Souza, A. T. C. P.; Catal. Today 2003, 85, 39.

10 Caliman, E.; Dias, J. A.; Dias, S. C. L.; Prado, A. G. S.; Catal. Today 2005, 107-108, 816.

11 Oliveira, C. F.; Dezaneti, L. M.; Garcia, F. A. C.; de Macedo, J. L.; Dias, J. A.; Dias, S. C. L.; Alvim, K. S. P.; Appl. Catal., A 2010, 372, 153.
-1212 Caliman, E.; Dias, J. A.; Dias, S. C. L.; Garcia, F. A. C.; de Macedo, J. L.; Almeida, L. S.; Microporous Mesoporous Mater. 2010, 132, 103. From POMs, H3PW is the strongest heteropolyacid (HPA), and its solid acidity can be finely controlled by heterogenization.1313 Moffat, J. B.; Metal-Oxygen Clusters: The Surface and Catalytic Properties of Heteropoly Oxometalates; Kluwer: New York, USA, 2001.

A challenge in the technological application of supported HPA is to avoid leaching in polar solvents due to their high solubility when it is used in batch reactions.55 Misono, M.; Catal. Today 2009, 144, 285. Thus, a proper support that strongly interacts with the Keggin anion will enable the HPA specific surface to be enhanced, thereby increasing the accessibility of its acidic protons and keeping small nanocrystals on the surface. This idea has long been discussed in the literature,1414 Augustine, R. L.; Tanielyan, S. K.; Mahata, N.; Gao, Y.; Zsigmond, A.; Yang, H.; Appl. Catal., A 2003, 256, 69. and despite some controversial results, it is generally accepted that the change of experimental conditions of preparation (e.g., solvent, temperature of calcination and method of impregnation) modifies the degree of interaction between the phases.88 Kozhevnikov, I. V. In Catalysis by Polyoxometalates, vol. 2; Roberts, S. M., ed.; John Wiley & Sons Ltd. : Chichester, England, 2002. For example, H3PW has been used to improve the activity of the chiral complex [Ru(BINAP)] (BINAP: 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) for the asymmetric hydrogenation of β-ketoesters.1515 Ahn, S. H.; Choi, M. S.; Im, J. S.; Sheikh, R.; Park, Y. H.; J. Mol. Catal., A 2013, 373, 55. Whereas there is a strong interaction between the H3PW and the metal complex,1414 Augustine, R. L.; Tanielyan, S. K.; Mahata, N.; Gao, Y.; Zsigmond, A.; Yang, H.; Appl. Catal., A 2003, 256, 69.,1515 Ahn, S. H.; Choi, M. S.; Im, J. S.; Sheikh, R.; Park, Y. H.; J. Mol. Catal., A 2013, 373, 55. an effective immobilization of the HPA on the alumina surface is required in order to keep the whole catalyst heterogeneous.

Silica-alumina is an important industrial material for various applications; for example, it is used in fluid catalytic cracking (FCC) catalysts and as support for other active phases.1616 Pieta, I. S.; Ishaq, M.; Wells, R. P. K.; Anderson, J. A.; Appl. Catal., A 2010, 390, 127. It is an amorphous mixed oxide network of interconnected silica and alumina tetrahedra that has both Brønsted and Lewis acid sites. The strength, distribution and number of those sites depend mostly on the amount of aluminum, which determines the catalytic and adsorptive properties of silica-alumina materials.1616 Pieta, I. S.; Ishaq, M.; Wells, R. P. K.; Anderson, J. A.; Appl. Catal., A 2010, 390, 127.,1717 Leydier, F.; Chizallet, C.; Chaumonnot, A.; Digne, M.; Soyer, E.; Quoineaud, A. A.; Costa, D.; Raybaud, P.; J. Catal. 2011, 284, 215.

Catalysts based on HPA supported on silica-alumina have not been widely explored in the literature mainly due to the supposed decomposition of the Keggin structure on the surfaces of aluminum oxide derivatives.1818 Rao, K. M.; Gobetto, R.; Iannibello, A.; Zecchina, A.; J. Catal. 1989, 119, 512. However, it has been demonstrated that the decomposition of the Keggin anion can be controlled by utilizing the correct solvent and preparation method.1010 Caliman, E.; Dias, J. A.; Dias, S. C. L.; Prado, A. G. S.; Catal. Today 2005, 107-108, 816.,1919 Pizzio, L. R.; Cáceres, C. V.; Blanco, M. N.; Appl. Catal., A 1998, 167, 283.

20 Pizzio, L. R.; Vázquez, P. G.; Cáceres, C. V.; Blanco, M. N.; Appl. Catal., A 2003, 256, 125.
-2121 Rao, P. M.; Wolfson, A.; Kababya, S.; Vega, S.; Landau, M. V.; J. Catal. 2005, 232, 210. In fact, H3PMo12O40 and H3PW12O40 supported on SiO2-Al2O3, prepared by different synthetic routes, performed satisfactorily in phenol tetrahydropyranylation2222 Romanelli, G.; Vázquez, P.; Pizzio, L.; Quaranta, N.; Autino, J.; Blanco, M.; Cáceres, C.; Appl. Catal., A 2004, 261, 163. and in the syntheses of azlactones.2323 Romanelli, G.; Autino, J. C.; Vázquez, P.; Pizzio, L.; Blanco, M.; Cáceres, C.; Appl. Catal., A 2009, 352, 208. H4SiW12O40 supported on silica-alumina with different Si/Al ratios was tested in gas-phase glycerol dehydration, and showed good yields for acrolein.2424 Kim, Y. T.; You, S. J.; Jung, K. -D.; Park, E. D.; Bull. Korean Chem. Soc. 2012, 33, 2369. Moreover, other studies on supported HPA on various aluminosilicates (e.g., zeolites, clays, molecular sieves) have appeared in the literature, indicating that the Keggin anion can be stabilized on these surfaces.2525 Jin, D.; Gao, J.; Hou, Z.; Guo, Y.; Lu, X.; Zhu, Y.; Zheng, X.; Appl. Catal., A 2009, 352, 259.

26 Bokade, V. V.; Yadav, G. D.; Appl. Clay Sci. 2011, 53, 263.
-2727 Nandhini, K. U.; Mabel, J. H.; Arabindoo, B.; Palanichamy, M.; Murugesan, V.; Microporous Mesoporous Mater. 2006, 96, 21. By stabilizing the Keggin anion, it is possible to produce nanocrystals of supported HPA, which are the major goal for most catalytic reactions.

This report details a wide-ranging study of H3PW/SiO2-Al2O3 catalysts. A series of materials with various H3PW loadings was prepared, and their structural properties were characterized by Fourier transform infrared spectroscopy (FTIR), 31P magic angle spinning nuclear magnetic resonance (31P MAS NMR), X-ray powder diffraction (XRD) and low-temperature N2 sorption. In addition, their reactivities were probed by studying: acidity by gaseous pyridine adsorption, leaching in ethanol and thermal stability under air flow by thermogravimetric and differential thermal (TG/DTA) analyses. Finally, these catalysts were employed in a model reaction of esterification of acetic acid with ethanol to produce ethyl acetate, which is an important reagent in industrial manufacturing, in order to correlate acidic strength, distribution and proposed reaction mechanism.

Experimental

Synthesis of H3PW supported on silica-alumina

H3PW used in the impregnations was obtained from Sigma. Silica-alumina (Aldrich, 12 wt.% Al2O3) was pre-activated at 550 ºC for 12 h to transform it from its ammoniacal form to its protonic form. The x wt.% H3PW/SiO2-Al2O3 materials, where x is the H3PW loading (15, 20, 30 and 40 wt.%; mass H3PW/(mass H3PW + mass SiO2-Al2O3)), were prepared by impregnation in an aqueous acid solution (0.1 mol L-1 HCl, Vetec). Specifically, calculated amounts of H3PW and SiO2-Al2O3 were placed in a 100 mL glass round-bottom flask with 10 mL of the acid solution per g of silica-alumina, and the suspension was maintained at 80 ºC under constant stirring until the solvent was completely dried. Then, the resulting materials were ground to fine particles and calcined in a muffle furnace (model EDG-3PS) under static air conditions at 200, 300 and 400 ºC for 4 h (heating rate of 10 ºC min-1). The catalysts were characterized by elemental analysis using energy dispersive X-ray fluorescence (EDX) spectrometer (Shimadzu EDX 720 with a rhodium X-ray tube). The X-ray fluorescence (XRF) spectra were collected under vacuum (< 45 Pa). The analyses were performed on powders placed inside the sample holder using a polypropylene film. The H3PW content was calculated based on W analysis determined by the QualiQuant method of fundamental standards. The nominal values of 15, 20, 30 and 40 wt.% were really 14.5; 20.3; 29.6 and 39.8 wt.%. As the actual values were close to the nominal ones, for simplicity the nominal values were used in this article.

Characterization of H3PW/SiO2-Al2O3

The structures of these materials were characterized by FTIR, 31P MAS NMR, XRD and nitrogen sorption. Their reactivities were probed by gas-phase pyridine adsorption, H3PW leaching in ethanol and thermal stability measurements under air.

The FTIR spectra of the samples were obtained using 1% of solid in dried KBr pellets in a Thermo Scientific Nicolet 6700 spectrophotometer (deuterated triglycine sulfate (DTGS) detector). The collected spectra used 256 scans with a 4 cm-1 resolution at 25 ºC.

The XRD patterns were obtained on a Bruker D8 FOCUS using a copper tube (Cu Kα, λ = 1.5418 Å at 40 kV and 30 mA). A 2θ step of 0.05º and an integration time of 1 s step-1 were used to scan 2θ values between of 5 and 60º.

The 31P MAS NMR spectra were acquired in two apparatuses; (i) for the samples calcined at 200 ºC: a Mercury Plus Varian of 7.05 T (300 MHz for 1H) spectrometer with a Varian 7 mm CP/MAS (cross polarization) probe (zirconia rotor). The following conditions were set: frequency of 121.4 MHz, single-pulse excitation of 8.0 μs, acquisition time of 0.25 ms, recycle delay of 10 s, no 1H decoupling, MAS rate of 5 kHz, and minimum of 512 acquisitions. Signals were referenced to 85% phosphoric acid (δ 0 ppm); (ii) for the samples calcined at 300 ºC: a Bruker Avance III HD Ascend of 14.1 T (600 MHz for 1H) spectrometer with a 4 mm CP/MAS probe (zirconia rotor). The following conditions were set: a frequency of 243.1 MHz, single-pulse excitation (4.75 μs duration), acquisition time of 0.1 ms, recycle delay of 10 s, no 1H decoupling, MAS rate of 10 kHz and minimum of 128 acquisitions. Signals were referenced to a secondary standard NH4H2PO4 (δ 0.9 ppm) relative to 85% phosphoric acid (δ 0 ppm). The spectra were adjusted to be referenced to H3PO4.

The textural properties (specific surface area (SBET) and pore volume) were calculated from nitrogen adsorption isotherms obtained at -196 ºC on a Micromeritics model ASAP 2020C instrument. The SBET value was calculated from the adsorption branch in the relative pressure range of 0.06-0.20, whereas the total pore volume was obtained from the total amount of N2 adsorbed at pressures close to unity. The materials were calcined at 200 ºC for 4 h and then degassed under vacuum at 120 ºC for 12 h before the measurements.

TG/DTG curves were obtained in a simultaneous TG-DSC equipment (TA Instruments, model SDT 2960), with heating rate of 10 ºC min-1, from 25 up to 800 ºC under different atmospheres: (i) for stability study, it was used synthetic air (80.0 ± 0.5% N2 and 20.0 ± 0.5% O2) flow of 110 mL min-1; and (ii) for determination of acidity, it was used N2 (99.999%) flow of 100 mL min-1.

Gas-phase pyridine was adsorbed on the catalyst using a custom-made glass fixed bed reactor, using an in situ experiment. The supported samples were first dried at 200 ºC for 4 h under vacuum in the reactor and then cooled to 100 ºC. Gaseous pyridine diluted in dry N2 was subsequently flowed over the samples for 1 h. The reactor was then heated and maintained at 150 ºC under vacuum for 1 h to remove all physically adsorbed pyridine. Finally, the samples with chemisorbed pyridine were analyzed immediately by TG/DTG and FTIR, in separated experiments.

Acidity, i.e., quantification of acid sites, was obtained by temperature programmed desorption (TPD) of pyridine using TG/DTG detection, by the difference between the curves of each catalyst before and after pyridine adsorption. Details of this calculation are in the Supplementary Information (SI) section and in the literature.2828 Santos, J. S.; Dias, J. A.; Dias, S. C. L.; de Macedo, J. L.; Garcia, F. A. C.; Almeida, L. S.; de Carvalho, E. N. C. B.; Appl. Catal., A 2012, 443-444, 33.

The qualitative nature of acid sites was achieved by FTIR. Pellets with 10 wt.% of chemisorbed pyridine catalyst were prepared in a N2 glove box, placed in a holder, and taken straight to the FTIR spectrometer. The spectra were recorded at 25 ºC.

Leaching tests of H3PW supported on silica-alumina (calcined at 200, 300 and 400 ºC) were performed with ethanol as solvent. The supported catalyst was evaluated by adding 50 mL of ethanol (Vetec, 99.8%, previously dried over activated 3 Å molecular sieves) to approximately 0.02 g of the solid at 25 ºC. The resulting slurry was kept under magnetic stirring for 1 h. Every 10 min, the stirring was stopped to remove an aliquot using a 1.0 mL Hamilton gas-tight syringe with a membrane filter in the tip (0.45 μm, 13 mm diameter). The H3PW concentration, corrected for the withdrawal volume, was measured by placing the sample in a 1 cm (0.5 mL) quartz cuvette and using a Beckman DU 650 UV-Vis spectrophotometer at a wavelength of 263 nm (maximum of the [PW12O40]3- absorption band). The value was obtained using the following analytical curve for H3PW in ethanol (from 2 × 10-6 up to 4 × 10-5 mol L-1):

(1) Absorbance = 4 . 2709 × 10 4 [ H 3 PW ] + 0 . 02542 , R 2 = 0 . 9998 R 2 is the correlation coefficient

This method is able to detect 3-6 ppm of H3PW, considering up to 3 times the lowest absorbance (A) reading in the curve (i.e., A = 3 × 0.02542 = 0.07626).

Catalytic esterification using H3PW/SiO2-Al2O3

The activity of the materials was evaluated in a model reaction of esterification of acetic acid (Vetec, 99.7%) with ethanol (Vetec, 99.8%). Acetic acid was used as supplied, and ethanol was placed in activated 3 Å molecular sieves for at least 48 h before use. The reactions were performed in mini-reactors with capacities of 5 and 10 mL to which 1.00 and 2.00 g of acetic acid, respectively, were added. Ethanol was added to obtain molar ratio of 2:1, 1:1 or 1:2 (acetic acid:ethanol). Before the reaction, the catalyst was activated in a muffle furnace at 200 ºC for 2 h. The following parameters were optimized sequentially: (i) amount of H3PW (15, 20, 30 and 40 wt.%) supported on silica-alumina; (ii) temperature (80 and 100 ºC); (iii) acetic acid:ethanol molar ratio (2:1, 1:1 and 1:2); (iv) catalyst:acetic acid mass ratio (2, 5, 10 and 15 wt.%); and (v) the reaction time (1, 2, 4 and 6 h). After each reaction, the system was allowed to cool to room temperature, the catalyst was separated by centrifugation, and the liquid phase was analyzed by gas chromatography (GC-17A Shimadzu with a flame ionization detector (FID) and polydimethylsiloxane column (CBPI PONA-M50-042)). The selectivity was 100% for ethyl acetate in all cases. The yield was obtained by the integration of the area peak of ethyl acetate and calculated with its standard analytical curve. Further experiments at reaction times of 15, 30, 45 and 60 min were performed and allowed the calculation of the turnover frequency (TOF), according to the literature.2828 Santos, J. S.; Dias, J. A.; Dias, S. C. L.; de Macedo, J. L.; Garcia, F. A. C.; Almeida, L. S.; de Carvalho, E. N. C. B.; Appl. Catal., A 2012, 443-444, 33. A curve of yield of the product (mmol) versus time (s) was plotted for each catalyst in order to determine the initial activity, and each curve was fitted to a first-order polynomial function. The obtained function was differentiated, evaluated at t = 0 and the value divided by the total number of acid sites achieved by TPD of pyridine, i.e., TOF = mmol ethyl acetate (EA) mmol sites-1 s-1.

Results and Discussion

Structure and stability of the catalysts

The stability of the Keggin structure on supports containing aluminum is a major concern and source of controversy in the literature. FTIR spectra of the H3PW/SiO2-Al2O3 catalysts were obtained (Figure 1) to verify the presence of the Keggin structure by identifying the H3PW fingerprint bands. The silica-alumina support has absorption bands in the 1200-450 cm-1 region, which are assigned as follows: 1087 cm-1, νas(Si-O); 802 cm-1, νas(Si-O-); 578 cm-1, external δ(Si-O-); and 462 cm-1, δ(Si-O-Si).2828 Santos, J. S.; Dias, J. A.; Dias, S. C. L.; de Macedo, J. L.; Garcia, F. A. C.; Almeida, L. S.; de Carvalho, E. N. C. B.; Appl. Catal., A 2012, 443-444, 33. On the other hand, the main bands of H3PW are located at 1080, 983, 893 and 798 cm-1 and correspond to P-O, W = O, W-Ocorner-W and W-Oedge-W asymmetric stretching vibrations, respectively.88 Kozhevnikov, I. V. In Catalysis by Polyoxometalates, vol. 2; Roberts, S. M., ed.; John Wiley & Sons Ltd. : Chichester, England, 2002. Because of the strong absorption of the support in the same region of the fingerprint bands of H3PW, it is difficult to assign such bands due to overlaps, especially for lower H3PW loadings. However, it can be noted that the actual bands at 981 and 892 cm-1 became more visible with increasing H3PW loading. Even though quantitative analysis by FTIR using KBr pellets is not possible, such bands suggest the presence of the Keggin structure. Thus, it can be established that FTIR spectroscopy can partially prove the maintenance of the Keggin structure of H3PW supported on silica-alumina, as stated elsewhere.1818 Rao, K. M.; Gobetto, R.; Iannibello, A.; Zecchina, A.; J. Catal. 1989, 119, 512.

Figure 1
FTIR spectra (1% KBr pellets) of x wt.% H3PW/SiO2-Al2O3 after calcination at 200 ºC for 4 h.

XRD patterns of supported H3PW and the respective parent solids (i.e., silica-alumina and H3PW) are compared in Figure 2. Silica-alumina is a known amorphous solid and presents only a large halo with maximum around 22-23º (2θ). In contrast, H3PW, treated at 300º for 4 h, showed a crystalline pattern compatible with the cubic structure (space group Pn3m) with cell parameter a = 12.16 Å, which is related to a hexahydrate crystalline phase.2929 Brown, G. M.; Noe-Spirlet, M. R.; Busing, W. R.; Levy, H. A.; Acta Crystallogr. Sect. B: Struct. Sci. 1977, 33, 1038. The most intense reflections for H3PW are 2θ at 10.4º (110), 25.4º (222) and 34.5º (332). The crystallites of hydrated H3PW showed domain sizes greater than 50 nm by calculation using Scherrer's equation of that reflections.3030 Klug H. P.; Alexander, L. E.; X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials; John Wiley & Sons: New York, USA, 1962.

Figure 2
XRD patterns of x wt.% H3PW/SiO2-Al2O3 after calcination at 200 ºC for 4 h.

The XRD pattern of the supported H3PW is very similar to silica-alumina, except for a large reflection present at 7.9º (2θ) (d = 1.1 nm). The presence of this reflection has been interpreted in the literature as the X-ray scattering by non-ordered hydrated polyanions of H3PW.2121 Rao, P. M.; Wolfson, A.; Kababya, S.; Vega, S.; Landau, M. V.; J. Catal. 2005, 232, 210. These polyanions are present at the surface either as isolated molecular moieties or as small clusters containing few Keggin units. The calculated crystalline domain sizes from broadening of that peak were about 2 nm. A cluster of this size could contain about 4 Keggin units and may contain more hydrogen-bonded water molecules per unit than conventional bulk H3PW.2121 Rao, P. M.; Wolfson, A.; Kababya, S.; Vega, S.; Landau, M. V.; J. Catal. 2005, 232, 210. Therefore, the XRD data presented an indication for the existence of H3PW adsorbed on silica-alumina in the form of hydrated surface molecular species or of small nanoclusters containing few Keggin units. Nonetheless, definitive information about their molecular structure could not be extended. To clarify the structural integrity of the Keggin anion after the adsorption on the support, it was conducted additional 31P MAS NMR spectroscopy at different calcination temperatures.

31P MAS NMR spectroscopy can establish the presence of the Keggin structure for supported H3PW more definitively than FTIR and XRD. Figure 3a shows the spectra of the supported x wt.% H3PW calcined at 200 ºC, as well as the hexahydrate (-15.2 ppm) and the anhydrous H3PW (-11.1 ppm). For that, at 15 wt.% H3PW loading, there are two signals: one at -12.9 ppm and another at -11.2 ppm. For higher loadings (20, 30 and 40 wt.%), there is only a single resonance at -12.9 ppm. These results showed that the signal of the Keggin structure was displaced from the original hexahydrate condition to a lower degree of hydration, as a function of calcination. This is sustained by similar results obtained for H3PW supported on alumina and niobia.1010 Caliman, E.; Dias, J. A.; Dias, S. C. L.; Prado, A. G. S.; Catal. Today 2005, 107-108, 816.,1212 Caliman, E.; Dias, J. A.; Dias, S. C. L.; Garcia, F. A. C.; de Macedo, J. L.; Almeida, L. S.; Microporous Mesoporous Mater. 2010, 132, 103. Formation of lacunary species ([PW11O39]7-) is disregarded because no characteristic signal at -10.4 ppm is present.3131 Pope, M. T.; Heteropoly and Isopoly Oxometalates; Springer-Verlag: Berlin, Germany, 1983. Thus, this corroborates to an interaction between the Keggin structure and the support, which gave rise to the shift in the main hydrate H3PW Keggin signal and an additional signal. According to literature,3232 Lefebvre, F. J.; Chem. Soc. Chem. Commun. 1992, 756.

33 Legagneux, N.; Basset, J. -M.; Thomas, A.; Lefebvre, F. J.; Goguet, A.; Sá, J.; Hardacre, C.; Dalton Trans. 2009, 2235.
-3434 Newman, A. D.; Brown, D. R.; Siril, P.; Lee, A. F.; Wilson, K.; Phys. Chem. Chem. Phys. 2006, 8, 2893. analyses by FTIR, Raman, X-ray photoelectron spectroscopy (XPS), 1H and 31P MAS NMR and extended X-ray absorption fine structure (EXAFS) revealed the formation of species like [(≡SiOH2)3PW12O40], [(≡SiOH2)xH3-xPW12O40] or [(≡Si)x+ H3-xPW12O40- + H2O] on silica surface, remaining stable up to 300 ºC. Depending on the thermal treatment, the highly acidic protons reacted with a protonated silanol, leading to the evolution of water and the formation of Si-O-W bonds.3333 Legagneux, N.; Basset, J. -M.; Thomas, A.; Lefebvre, F. J.; Goguet, A.; Sá, J.; Hardacre, C.; Dalton Trans. 2009, 2235.

Figure 3
31P MAS NMR spectra of x wt.% H3PW/SiO2-Al2O3 referenced to 85% H3PO4: (a) samples calcined at 200 ºC for 4 h and obtained at 7.05 T, and (b) samples calcined at 300 ºC for 4 h and obtained at 14.1 T.

An additional evidence for that arose from the spectra of the supported samples calcined at 300 ºC (Figure 3b). The spectra of supported H3PW showed similar shifts to -13.2 ppm, as well as other broad signals at -11.5 and -11.0 ppm. As it is known in the literature,3535 Uchida, S.; Inumaru, K.; Misono, M.; J. Phys. Chem. B 2000, 104, 8108. the higher calcination temperature, the lower hydration degree of the Keggin structure, limited by stability temperature for the Keggin anion. Thus, anhydrous H3PW exhibited a single signal around -10 to -11 ppm.88 Kozhevnikov, I. V. In Catalysis by Polyoxometalates, vol. 2; Roberts, S. M., ed.; John Wiley & Sons Ltd. : Chichester, England, 2002.,1212 Caliman, E.; Dias, J. A.; Dias, S. C. L.; Garcia, F. A. C.; de Macedo, J. L.; Almeida, L. S.; Microporous Mesoporous Mater. 2010, 132, 103. It is worth to note the differences on the characteristic signals of the anhydrous, hydrated and supported H3PW/silica-alumina. Pure hydrated H3PW shows a single signal (ca. -14 up to -16 ppm, according to many reports in the literature) and the full width at half maximum (FWHM) is about 45 Hz; anhydrous H3PW has a single signal (ca. -10 to -11 ppm) with linewidth around 95 Hz; supported on silica-alumina, the signals of H3PW were growing broader (15, 20, 30 and 40 wt.% H3PW/SiO2-Al2O3 calcined at 200 ºC, FWHMs were 50, 90, 102 and 111 Hz for the signal at -12.9 ppm, respectively). The same trend in FWHM was observed when the materials were calcined at 300 ºC. In principle, the line broadening observed in the supported H3PW on silica-alumina depends on the dipolar interaction between 31P and 27Al nuclei, which is not averaged to zero by magic angle spinning of a quadrupolar nucleus as Al. Nonetheless, it is stated that this dipolar interaction is not the only responsible for that broadening based on the results observed for supported 12-molybdophosphoric acid,1818 Rao, K. M.; Gobetto, R.; Iannibello, A.; Zecchina, A.; J. Catal. 1989, 119, 512. which was much broader than those of supported H3PW. Thus, the broadening of the 31P NMR signals as a function of calcination temperature may be taken as an indication of an interaction of H3PW and silica-alumina support. Moreover, 31P MAS NMR spectrum of a mechanical mixture of H3PW and silica-alumina (e.g., 20 wt.% H3PW) gave rise to a similar spectrum of pure H3PW (single signal at -15.2 ppm and FWHM of 46 Hz), as already observed for mechanical mixtures with silica.99 Dias, J. A.; Caliman, E.; Dias, S. C. L.; Paulo, M.; de Souza, A. T. C. P.; Catal. Today 2003, 85, 39.

The nature of interaction between H3PW and silica-alumina can be inferred based on the knowledge of silica-alumina surface. The characteristics of acid sites on amorphous silica-alumina have been debated for a long time in the literature.1717 Leydier, F.; Chizallet, C.; Chaumonnot, A.; Digne, M.; Soyer, E.; Quoineaud, A. A.; Costa, D.; Raybaud, P.; J. Catal. 2011, 284, 215.,3636 Nielsen, B. P.; Onuferko, J. H.; Gates, B. C.; Ind. Eng. Chem. Fundam. 1986, 25, 337.

37 Crépeau, G.; Montouillout, V.; Vimont, A.; Mariey, L.; Cseri, T.; Maugé, F.; J. Phys. Chem. B 2006, 110, 15172.
-3838 Zaki, M. I.; Hasan, M. A.; Al-Sagheer, F. A.; Pasupulety, L.; Colloids Surf., A 2001, 190, 261. However, different reports using diffuse reflectance Fourier transformed infrared spectroscopy (DRIFTS), density functional theory (DFT) and their combination have clarified it.1717 Leydier, F.; Chizallet, C.; Chaumonnot, A.; Digne, M.; Soyer, E.; Quoineaud, A. A.; Costa, D.; Raybaud, P.; J. Catal. 2011, 284, 215. It has been proposed that silica-alumina surface is formed by isolated silanols and/or hydrogen-bonded bridge silanols.1717 Leydier, F.; Chizallet, C.; Chaumonnot, A.; Digne, M.; Soyer, E.; Quoineaud, A. A.; Costa, D.; Raybaud, P.; J. Catal. 2011, 284, 215.,3939 Braga, V. S.; Dias, J. A.; Dias, S. C. L.; de Macedo, J. L.; Chem. Mater. 2005, 17, 690. The highest frequencies of OH vibrations are found for Si-OH groups (isolated silanols) and O-Si-OH—Al groups (called aluminic pseudo-bridging silanols).1717 Leydier, F.; Chizallet, C.; Chaumonnot, A.; Digne, M.; Soyer, E.; Quoineaud, A. A.; Costa, D.; Raybaud, P.; J. Catal. 2011, 284, 215. Thus, it may be suggested that the interaction of H3PW Keggin structure can take place preferentially at these sites, since most of the metal oxides interact to O-H surface sites of different supports.3939 Braga, V. S.; Dias, J. A.; Dias, S. C. L.; de Macedo, J. L.; Chem. Mater. 2005, 17, 690.,4040 Dias, J. A.; Rangel, M. C.; Dias, S. C. L.; Caliman, E.; Garcia, F. A. C.; Appl. Catal., A 2007, 328, 189.

Nitrogen physisorption isotherm obtained for SiO2-Al2O3 was analyzed by plotting the t-curve, i.e., amount of N2 adsorbed versus thickness (t).4141 Gregg, S. J.; Sing, K. S. W.; Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press Inc. : London, Great Britain, 1982. The linear fit from the latter showed a straight line passing through the origin, which indicates a multilayer adsorption of N2 on the solid surface (Figure S1, in the SI section). The absence of low or high pressure deviations in the t-curve evidenced that SiO2-Al2O3 porosity is not related to the presence of micropores or cylindrical mesopores, respectively.4141 Gregg, S. J.; Sing, K. S. W.; Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press Inc. : London, Great Britain, 1982. Indeed, adsorption/desorption isotherms showed a type IV curve and type H2 hysteresis, which indicated capillary condensation in macropores or inkbottle-type mesopores.

The main textural properties of the prepared materials also revealed how H3PW and SiO2-Al2O3 interacted with each other. The nature of the support and the conditions used in the catalyst synthesis are important parameters for improving the stability of H3PW supported materials.99 Dias, J. A.; Caliman, E.; Dias, S. C. L.; Paulo, M.; de Souza, A. T. C. P.; Catal. Today 2003, 85, 39.

10 Caliman, E.; Dias, J. A.; Dias, S. C. L.; Prado, A. G. S.; Catal. Today 2005, 107-108, 816.

11 Oliveira, C. F.; Dezaneti, L. M.; Garcia, F. A. C.; de Macedo, J. L.; Dias, J. A.; Dias, S. C. L.; Alvim, K. S. P.; Appl. Catal., A 2010, 372, 153.
-1212 Caliman, E.; Dias, J. A.; Dias, S. C. L.; Garcia, F. A. C.; de Macedo, J. L.; Almeida, L. S.; Microporous Mesoporous Mater. 2010, 132, 103.,4242 Braga, P. R. S.; Costa, A. A.; de Freitas, E. F.; Rocha, R. O.; de Macedo, J. L.; Araujo, A. S.; Dias, J. A.; Dias, S. C. L.; J. Mol. Catal. A: Chem. 2012, 358, 99. The commercial silica-alumina showed 489 m2 g-1 (SBET) and 0.69 cm3 g-1 of pore volume (Pv). Isotherms and t-curves of the supported H3PW are presented in Figures S2 to S5 (in the SI section). SBET and Pv of the H3PW/SiO2-Al2O3 materials decreased linearly (r2 = 0.992 and 0.994, respectively) as the H3PW loading increased from 15 to 40 wt.% (Table 1). This behavior was related to the impregnation of H3PW on SiO2-Al2O3 surface and porous system. The actual decrease of SBET with H3PW loading was also analyzed by considering the amount of SiO2-Al2O3 in the materials. For example, the sample containing 40% of H3PW has only 60% of SiO2-Al2O3, which would be accountable for a surface area of 293 m2 g-1 (normalized based on the SBET for pure SiO2-Al2O3). The actual SBET value obtained for the sample with 40% of H3PW was 240 m2 g-1, resulting in a decrease of only 53 m2 g-1. Table 1 shows the normalized (SN) and decreased (SD) calculated specific surface values for all the samples, indicating that an approximated constant decrease of surface area was reached around 30% of impregnation. These results and the theoretical coverage on the support indicated that an increase of H3PW above 30% (0.20 coverage) may result in some H3PW agglomeration, see Table 1.

Table 1
Textural properties of x wt.% H3PW/SiO2-Al2O3

The stability of H3PW/SiO2-Al2O3 was evaluated by TG/DTG curves under air flow and pure H3PW and curves for silica-alumina (Figure S6a-b, in the SI section) were provided for comparison. The TG/DTG curves for H3PW show three thermal events.4040 Dias, J. A.; Rangel, M. C.; Dias, S. C. L.; Caliman, E.; Garcia, F. A. C.; Appl. Catal., A 2007, 328, 189. The first mass loss, between 25 and 100 ºC, was due to release of physically adsorbed water and some of the hydrating water molecules, forming H3PW∙6H2O. The temperature range and maximum might vary based on the initial amount of water in the material.4040 Dias, J. A.; Rangel, M. C.; Dias, S. C. L.; Caliman, E.; Garcia, F. A. C.; Appl. Catal., A 2007, 328, 189. The second mass loss occurred between 120 and 240 ºC (maximum at approximately 170 ºC) and corresponded to the formation of the anhydrous catalyst. Finally, the third step occurred over a wide temperature range (approximately 380-600 ºC), which is related to the decomposition of the Keggin anion by formation of the anhydride (PW12O38.5) followed by total transformation in the oxides (P2O5 and WO3).4040 Dias, J. A.; Rangel, M. C.; Dias, S. C. L.; Caliman, E.; Garcia, F. A. C.; Appl. Catal., A 2007, 328, 189. The location of the maximum (usually between 450 and 470 ºC) cannot be precisely determined because of the wide peak width. On the other hand, supported H3PW/SiO2-Al2O3 presented similar curves (Figure S6c-f, in the SI section), but showed basically two thermal events. The first mass loss occurred between 50-250 ºC (maximum at about 100 ºC), which is associated to the release of physically adsorbed water and hydrating water molecules attached to the protons in the secondary structure of HPA. A second notable mass loss took place between 400-600 ºC (maximum at 510 ºC), which can be assigned to fragmentation of Keggin anion followed by the total decomposition of H3PW at about 600 ºC. It can be noted that the supported HPA may be considered slightly more stable than pure H3PW, and probably silica-alumina underwent some dehydroxylation, which also contributed to the mass loss processes.

A last point should be addressed to the stability of H3PW. The utilized commercial silica-alumina had a Si/Al ratio of 6.2. Structural studies confirmed that the Keggin anion was not destroyed, consistent to our previous report1010 Caliman, E.; Dias, J. A.; Dias, S. C. L.; Prado, A. G. S.; Catal. Today 2005, 107-108, 816. and others2525 Jin, D.; Gao, J.; Hou, Z.; Guo, Y.; Lu, X.; Zhu, Y.; Zheng, X.; Appl. Catal., A 2009, 352, 259.

26 Bokade, V. V.; Yadav, G. D.; Appl. Clay Sci. 2011, 53, 263.
-2727 Nandhini, K. U.; Mabel, J. H.; Arabindoo, B.; Palanichamy, M.; Murugesan, V.; Microporous Mesoporous Mater. 2006, 96, 21. about the stability of HPAs in the presence of aluminum containing supports. However, the surface interactions and acidity could still be finely tuned to optimize the catalyst for efficient performance in a given reaction.

Catalyst: reactivity, acidity and recycling

Short-chain esters are widely used as solvents and in the production of chemicals for chemical and pharmaceutical industries.4444 Sakamuri, R. In Encyclopedia of Chemical Technology; Othmer, K., ed.; John Wiley & Sons: London, England, 2003. Alkyl ester production is most frequently achieved by the esterification of carboxylic acids with alcohols using a homogeneous Brønsted acid catalyst such as sulfuric acid. However, when a liquid mineral acid is used as the catalyst, an additional neutralization step is usually required, contributing to catalyst degradation and chemical waste generation.4545 Bhorodwaj, S. K.; Pathak, M. G.; Dutta, D. K.; Catal. Lett. 2009, 133, 185. Currently, the development of heterogeneous catalysts is important for creating more environmentally friendly processes. Therefore, the heterogeneous catalytic esterification of acetic acid with ethanol was optimized using the H3PW/SiO2-Al2O3 materials. This can be an alternative route to traditional Fischer esterification either in batch or even in fixed bed reactors.

A preliminary study was performed to check the best calcination temperature of the catalysts, and thus the highest possible activity of the catalysts. 30 wt.% H3PW/SiO2-Al2O3 was calcined at 200, 300 and 400 ºC, and subjected to the leaching test with ethanol. The results indicated that no leaching was detected under the UV-Vis method for the tested at temperatures of 300 and 400 ºC of calcination. Thus, a calcination treatment at 300 ºC was adopted since it is known that this temperature is efficient to keep acidity and increase the interaction of H3PW on the support.99 Dias, J. A.; Caliman, E.; Dias, S. C. L.; Paulo, M.; de Souza, A. T. C. P.; Catal. Today 2003, 85, 39.

10 Caliman, E.; Dias, J. A.; Dias, S. C. L.; Prado, A. G. S.; Catal. Today 2005, 107-108, 816.

11 Oliveira, C. F.; Dezaneti, L. M.; Garcia, F. A. C.; de Macedo, J. L.; Dias, J. A.; Dias, S. C. L.; Alvim, K. S. P.; Appl. Catal., A 2010, 372, 153.
-1212 Caliman, E.; Dias, J. A.; Dias, S. C. L.; Garcia, F. A. C.; de Macedo, J. L.; Almeida, L. S.; Microporous Mesoporous Mater. 2010, 132, 103.

Next, the optimal H3PW loading was determined using the following reaction conditions: a temperature of 100 ºC, reaction time of 4 h, acetic acid:ethanol molar ratio of 2:1 and catalyst:acetic acid mass ratio of 10 wt.%. Figure 4 shows the results, which indicate that 30 wt.% H3PW/SiO2-Al2O3 was the most active catalyst (approximately 78.3% ethyl acetate yield with 100% selectivity). In addition, the reaction with 30 wt.% H3PW/SiO2-Al2O3 was repeated under the same conditions, but at 80 ºC. The yield decreased to approximately 54.8%; thus, 100 ºC was determined to be the optimal temperature. It should be noted that using pure silica-alumina, yield was about 42% under the same experimental conditions (first point in Figure 4).

Figure 4
Effect of the H3PW loading (x wt.%) on the esterification of acetic acid with ethanol catalyzed by x wt.% H3PW/SiO2-Al2O3. Conditions: 100 ºC, 4 h, 2:1 acetic acid:ethanol molar ratio, 10 wt.% catalyst:acetic acid mass ratio.

Nevertheless, to compare the intrinsic activity of each catalyst, the reaction was measured at a low conversion range, i.e., below 1 h reaction time. The curves of ethyl acetate yield versus time are shown in Figure S7 (in the SI section). The yield is practically linear with increasing H3PW loading, and no mass or diffusion effect could be noted. Thus, the calculated TOF was based on the curve fitting of the plot to a first-order polynomial function. This function was differentiated, evaluated at t = 0 and divided by the number of total acid sites obtained by TPD of pyridine for each catalyst (Table 2). It can be confirmed that the most active catalyst is the 30 wt.% H3PW/SiO2-Al2O3.

Table 2
Acidity and catalytic properties of H3PW supported on silica-alumina

The acetic acid:ethanol molar ratio (2:1; 1:1 and 1:2) in the esterification reaction was varied to determine its effect on the catalyst activity. The catalytic runs were performed at 100 ºC for 4 h using 10 wt.% of 30 wt.% H3PW/SiO2-Al2O3. Figure 5 shows that the highest ethyl acetate yield (78.3%) was achieved when the molar ratio was 2:1. The yield decreased to 63.4 and 56.0% when the molar ratio was 1:2 and 1:1, respectively. The esterification is an equilibrium reaction, and the products are formed when one of the reactants is used in excess or when one of the products is removed from the reactor. The increase in the ethyl acetate yield when acetic acid was in excess could be attributed to the contribution of acetic acid to the self-catalyzed reaction. However, the kinetics of self-catalyzed esterification reactions is usually negligible, and a catalyst must be used to improve the kinetics and yield.4646 Pereira, C. S. M.; Pinho, S. P.; Silva, V. M. T. M.; Rodrigues, A. E.; Ind. Eng. Chem. Res. 2008, 47, 1453. A blank experiment without any catalyst showed only 24% yield (first point in Figure 6). It has been suggested that an excess of alcohol in the esterification reduces the ester production by blocking the catalyst active sites.4747 Das, J.; Parida, K. M.; J. Mol. Catal. A: Chem. 2007, 264, 248.,4848 Chakraborty, A. K.; Basak, A.; Grover, V.; J. Org. Chem. 1999, 64, 8014. The results in this report are also in agreement with a recent catalytic esterification study using H3PW-TaO5 nanocomposite, which obtained the highest conversion using a 2:1 acetic acid to ethanol molar ratio.4949 Leilei, X.; Yihang, G.; Huiru, M.; Jianguo, G.; J. Wuhan Univ. Technol. Mater. Sci. Ed. 2013, 28, 580. Also, a study of this reaction with the same 2:1 molar ratio, using 30 wt.% H3PW/montmorillonite K10 under approximately similar experimental conditions of our report, yielded 90% ethyl acetate, but at 8 h reaction.5050 Gurav, H.; Bokade, V. V.; J. Nat. Gas Chem. 2010, 19, 161.

Figure 5
Effect of the acetic acid:ethanol molar ratio on the esterification reaction catalyzed by 30 wt.% H3PW/SiO2-Al2O3. Conditions: 100 ºC, 4 h, 10 wt.% catalyst:acetic acid mass ratio.

Figure 6
Effect of the catalyst mass on the esterification of acetic acid with ethanol catalyzed by 30 wt.% H3PW/SiO2-Al2O3. Conditions: 100 ºC, 4 h, 2:1 acetic acid:ethanol molar ratio.

The behavior of the higher yield with excess of acetic acid was consistent to other mechanism studies in the literature.5151 López, D. E.; Suwannak, K.; Goodwin Jr., J. G.; Bruce, D. A.; Ind. Eng. Chem. Res. 2008, 47, 2221. It has been proposed a single-site mechanism (Eley-Riedel) in which the adsorbed carboxylic acid reacts with the alcohol (methanol or ethanol) in both gas/liquid-phase, describing the kinetics very satisfactorily.5151 López, D. E.; Suwannak, K.; Goodwin Jr., J. G.; Bruce, D. A.; Ind. Eng. Chem. Res. 2008, 47, 2221. Briefly, the H3PW/SiO2-Al2O3 catalyst furnished a proton to react with the acyl group of acetic acid, allowing it to be more electrophylic, followed by the substitution of an O-H group by an O-Et group from ethanol (nucleophile) and the release of a water molecule after the formation of a tetra-coordinated intermediate.

The effect of the amount of catalyst was subsequently tested. Figure 6 presents the influence of the catalyst mass on the ethyl acetate production at 100 ºC over 4 h using an acetic acid:ethanol molar ratio of 2:1. As the amount of catalyst was increased up to 10 wt.% (based on the acetic acid mass), the ethyl acetate yield considerably increased, 59.7, 76.0, 78.3 and 79.4%, for 2.5, 5, 10 and 15 wt.% of catalyst, respectively. Repeating the experiment at lower times (1 h reaction), it was obtained 69.2, 73.3 and 74.4% for 5, 10 and 15% of catalyst, respectively. Figure 6 could be divided in two linear sections: up to 5% and from 5 to 15%. It is clear that above 5%, there is only a slight linear increase of yield with the amount of the catalyst. Actually, above a mass ratio of 5 wt.%, the yield was nearly constant. Nonetheless, based on the whole curve (2.5 to 15%), the increased yield is not linear with the amount of catalyst. These results indicate that the esterification of acetic acid with ethanol depends on the catalyst amount, i.e., the number of acid sites, to some extent, below 5 wt.%. According to the literature,4747 Das, J.; Parida, K. M.; J. Mol. Catal. A: Chem. 2007, 264, 248. the increase in the ethyl acetate yield with the number of acid sites suggests that the reaction rate depends on the catalyst concentration. The non-linear increase in the product yield between 2.5 to 15 wt.% of the catalyst might indicate mass-limiting diffusion. As in the range of 5 to 15 wt.% catalyst, the yield was approximately constant, 10 wt.% was chosen to be the optimal amount of catalyst. As presented, the study of yield versus time (Figure S7, in the SI section), below than 1 h, with 10 wt.% catalyst showed a linear fit for the different x wt.% H3PW/SiO2-Al2O3 catalysts, confirming no mass diffusion effects under these experimental conditions.

Finally, the ethyl acetate yield was measured as a function of time under the optimized conditions. Figure 7 shows that the product formation increased between 1 and 2 h of reaction time (70 to 79%). The yield was approximately constant at longer reaction times of 4 and 6 h (80 ± 3%). These results demonstrate that most of the catalysis occurred within approximately 1 h, because control experiments performed under the same conditions (i.e., 1 h reaction) without the catalyst or with only silica-alumina resulted in product yields of approximately 19 and 36%, respectively.

Figure 7
Yield as a function of the reaction time for the esterification of acetic acid with ethanol catalyzed by 30 wt.% H3PW/SiO2-Al2O3. Conditions: 100 ºC, 2:1 acetic acid:ethanol molar ratio, 10 wt.% catalyst:acetic acid mass ratio.

In order to explain the experimental activity results of the different catalysts, their acidities were quantified. Acidity is an important catalyst property and ultimately has a significant impact on the activity in the esterification reaction. Thus, the active catalysts were studied by measuring their gas-phase pyridine adsorption/desorption capacity.

Supporting H3PW on silica-alumina makes the original strong Brønsted sites of the heteropolyacids weaker, which is expected since part of the protons of H3PW reacts with silica-alumina most basic surface sites causing a leveling effect.99 Dias, J. A.; Caliman, E.; Dias, S. C. L.; Paulo, M.; de Souza, A. T. C. P.; Catal. Today 2003, 85, 39.,1010 Caliman, E.; Dias, J. A.; Dias, S. C. L.; Prado, A. G. S.; Catal. Today 2005, 107-108, 816. The profiles of the desorption curves (Figure S8, in the SI section) confirmed the above statement. TPD-Py of H3PW showed a very narrow peak centered at 538 ºC, which means that the acid sites are very uniform in strength (range of desorption between 300 to 600 ºC). It is interesting to note that pyridine probed practically all protons (93%) of H3PW (Table 2). This profile is very similar to one obtained by TPD-NH3.5252 Okuhara, T.; Mizuno, N.; Misono, M.; Adv. Catal. 1996, 41, 113. On the other hand, the TPD-Py profiles for the supported catalysts resemble more the one obtained for silica-alumina itself, but with much less broad peaks. The curves for 15 and 20 wt.% showed two maxima, whereas the ones for 30 and 40 wt.% have broad peaks with maximum about 450 ºC. It can be noted in all cases that most of pyridine is released up to 600 ºC. This is consistent with other studies involving pyridine and other amines adsorbed on silica-alumina and zeolites.5353 Parrilo, D. J.; Adamo, A. T.; Kokotailo, G. T.; Gorte, R. J.; Appl. Catal. 1990, 67, 107.,5454 Tittensor, J. G.; Gorte, R. J.; Chapman, D. M.; J. Catal. 1992, 138, 714. Above that temperature, pyridine and other amines usually react with the solid acids forming different species as detected by TPD/MS.5353 Parrilo, D. J.; Adamo, A. T.; Kokotailo, G. T.; Gorte, R. J.; Appl. Catal. 1990, 67, 107.,5454 Tittensor, J. G.; Gorte, R. J.; Chapman, D. M.; J. Catal. 1992, 138, 714. It is worth noting that the new distribution of acid site strengths might contribute for the good activity of the supported catalysts.

The strength distribution of the acid sites of H3PW supported material is provided in Figure 8. It can be noted that the majority of sites for 30 and 40 wt.% supported H3PW arose between 300 and 500 ºC (79 and 89% of sites, respectively), whereas the catalysts with 15 and 20 wt.% in the same range of temperature showed 71 and 66% of sites, respectively. These data (Tables S1 and S2, in the SI section) revealed the presence of acid sites of weak to medium strengths that should be responsible for most of the measured activity. Table 2 shows the experimental versus theoretical amount of acid sites based on the total H+ presented on the surface support originating from HPA. It can be observed that the total amount of sites probed by pyridine corresponded to 97 and 91% of sites for 30 and 40 wt.% supported H3PW, respectively, whereas for 15 and 20 wt.%, the total amount surpasses the theoretical calculation. This is because the pyridine molecule is reacting not only with the protons of H3PW, but also to acidic sites on silica-alumina surface on the lower surface coverage samples. As the H3PW loading increases, the acidic surface sites of the support become less accessible because higher dispersion and coverage are reached. The maximum yield for ethyl acetate observed in Figure 4 demonstrated that 30 wt.% H3PW/SiO2-Al2O3 might have the greatest accessibility for the reactants. This enhanced access to the acid sites is corroborated by the highest dispersion of this amount of H3PW, according to BET specific area, i.e., a true monolayer may have been reached.

Figure 8
Distribution of the acid sites for x wt.% H3PW/SiO2-Al2O3 determined by TG/DTG analysis of pyridine desorption between 300 and 600 ºC.

The nature of the acid sites of H3PW supported on silica-alumina was also examined by FTIR (Figure 9). The pyridinium absorption bands at 1540 and 1488 cm-1 are compatible with the presence mainly of Brønsted sites,5555 Parry, E. P.; J. Catal. 1963, 2, 371. which are dependent on the amount of H3PW supported material (as also confirmed by TG calculations). Silica-alumina shows Brønsted and Lewis sites, depending on Si/Al ratio, according to literature.3838 Zaki, M. I.; Hasan, M. A.; Al-Sagheer, F. A.; Pasupulety, L.; Colloids Surf., A 2001, 190, 261. We have observed in our sample of protonic silica-alumina low Lewis acidity, since its characteristic absorption at 1450 cm-1 is very weak.5555 Parry, E. P.; J. Catal. 1963, 2, 371.,5656 Topsoe, N. -Y.; Pedersen, K.; Derouane, E. G.; J. Catal. 1981, 70, 41. Thus, the observed enhanced activities compared to silica-alumina are due to the occurrence of stronger Brønsted sites from H3PW.

Figure 9
FTIR spectra of x wt.% H3PW/SiO2-Al2O3 with pyridine adsorbed.

Coupling these acidity results with the activity of the catalysts in the esterification, it can be inferred that the combination of high dispersion, good interaction and adequate strength of the 30 wt.% H3PW/SiO2-Al2O3 acidic sites were probably responsible for the highest activity of this catalyst in the series. This catalyst also presented the highest turnover frequency (TOF) as calculated in Table 2. The low coverage of that catalyst (only 21% of the total specific surface of the support) associated with the strong interaction on the silica-alumina sites might be responsible for the good stability and reusability of this catalyst.

Another important issue is the leaching of H3PW supported. It is known that H3PW is very soluble in ethanol and reasonably soluble in acetic acid and ethyl acetate.5757 Izume, Y.; Hasebe, R.; Urabe, K.; J. Catal. 1983, 84, 402.,5858 Alsalme, A.; Kozhevnikova, E. F.; Kozhevnikov, I. V.; Appl. Catal., A 2008, 349, 170. Therefore, the catalyst solubility in ethanol was measured by UV-Vis because this method is very sensitive to the presence of the Keggin structure in solution.1111 Oliveira, C. F.; Dezaneti, L. M.; Garcia, F. A. C.; de Macedo, J. L.; Dias, J. A.; Dias, S. C. L.; Alvim, K. S. P.; Appl. Catal., A 2010, 372, 153.,5858 Alsalme, A.; Kozhevnikova, E. F.; Kozhevnikov, I. V.; Appl. Catal., A 2008, 349, 170. The 30 wt.% H3PW/SiO2-Al2O3 catalyst was calcined at different temperatures (200, 300 and 400 ºC) and then added to ethanol to form a slurry. The slurry was monitored by UV-Vis for 1 h (sampling solution at every 10 min) to detect the Keggin anion in solution. In all the tests, the absorbance measurements were below the limit of detection, indicating that no leaching of H3PW occurred. These results suggested that the esterification reaction proceeded by heterogeneous catalysis. This behavior of H3PW supported material is in agreement with others that have detected very low leaching under adequate preparation conditions.5959 Scroccaro, K. I.; Tanobe, V. O. A.; Côcco, L. C.; Yamamoto, C. I.; Wypych, F.; Quim. Nova 2012, 35, 1343.,6060 Sepúlveda, J. H.; Vera, C. R.; Yori, J. C.; Badano, J. M.; Santarosa, D.; Mandelli, D.; Quim. Nova 2011, 34, 601.

To verify that the catalysis was truly heterogeneous, the optimized esterification reaction was repeated three times. The 30 wt.% H3PW/SiO2-Al2O3 catalyst was recovered by centrifugation, washed with ethanol, dried in an oven at 100 ºC and calcined at 200 ºC before reuse. The reaction time was 2 h, and the average yield was 79 ± 3% (Figure 10). The yield was high and nearly constant during the recycling tests, indicating that catalyst leaching was probably very low up to three cycles. The catalyst recovery between cycles was 95 ± 2%. The recovered catalyst was subject to FTIR and XRD analyses that showed the same characteristics of the fresh sample, as described in Figures 1 and 2. Elemental analysis was conducted in this 30 wt.% catalyst, and showed 29.6 wt.% (fresh) and after three recycles decreased to 29.2 wt.%.

Figure 10
Catalytic runs for the esterification of acetic acid with ethanol catalyzed by 30 wt.% H3PW/SiO2-Al2O3 calcined at 300 ºC. Conditions: 100 ºC; 2 h reaction; 2:1 acetic acid:ethanol molar ratio; 10 wt.% catalyst:acetic acid mass ratio.

An additional test, which has been proposed in the literature,5858 Alsalme, A.; Kozhevnikova, E. F.; Kozhevnikov, I. V.; Appl. Catal., A 2008, 349, 170. was performed to verify the level of leaching that could take place. A reaction with the 30 wt.% H3PW/SiO2-Al2O3 was conducted (under the same optimized conditions) for 1 h. The reaction was stopped, and after separation of the solid catalyst by centrifugation, it was left more 1 h at the same conditions (i.e., stirring at temperature of 100 ºC). The measured yield (2 h) was 70.1%, which was about the same yield measured in the reaction after catalyst separation (1 h), i.e., 70.0%. This demonstrates that no soluble species is responsible for any improvement in the reaction kinetics, after removing the solid acid. Also, the solution was tested for the presence of Keggin anions by the UV-Vis method and no detectable signal was observed. Thus, all these experimental data are in agreement with very low leaching under this method and conditions of evaluation. The leaching tests were also in agreement with 31P MAS NMR results that pointed to a strong interaction of H3PW with silica-alumina surface. Coupled with the low coverage and the high accessibility degree of pyridine for the protons (97%), it might point to that only relatively fine overlayers of nanocrystals were formed on silica-alumina surface.

Conclusions

This article presents the synthesis and characterization of a heterogeneous 12-tungstophosphoric acid (H3PW) supported on silica-alumina (H3PW/SiO2-Al2O3) as catalyst. It was demonstrated that impregnating the support with H3PW (15, 20, 30 and 40 wt.%) in an acidic solution (0.1 mol L-1 HCl) produces a catalyst with the Keggin structure as shown by FTIR and mainly 31P MAS NMR. Thus, the Keggin structure can be maintained on oxide supports having aluminum, when an appropriate preparation method is used. Based on the results obtained by leaching and 31P MAS NMR, it is supposed a strong interaction with the hydroxyl sites, i.e., isolated and hydrogen-bonded bridge silanols on the silica-alumina surface. From all the catalysts prepared (15, 20, 30 and 40 wt.% H3PW supported on SiO2-Al2O3), 30 wt.% H3PW/SiO2-Al2O3 was the most active for the esterification of acetic acid with ethanol. The optimized reaction conditions were: temperature of 100 ºC, acetic acid:ethanol molar ratio of 2:1, catalyst:acetic acid mass ratio of 10 wt.% and calcination of the catalyst at 300 ºC. The yield for three consecutive 2 h reactions was 79 ± 3% with a selectivity of 100% (ethyl acetate) under these conditions, and the average catalyst recovery was 95 ± 2% (obtained by centrifugation). This catalyst had a Brønsted acid site concentration of 0.299 mmol g-1 (mostly weak to medium), indicating that about 97% of the theoretical acid sites (0.309 mmol g-1) were accessible, based on the amount of supported H3PW.

Supplementary Information

Supplementary data are available free of charge at http://jbcs.org.br as a PDF file.

https://minio.scielo.br/documentstore/1678-4790/sDBHj9wvHpJ93nkx7qSDYbS/c9386ce7f80b94e6c362c391da42d9bd63bd506f.pdf

Acknowledgments

We acknowledge CNPq for research, CAPES for Master, Doctorate and DPP/ProIC/CNPq for undergraduate scholarships. We also thank UnB/DPP/IQ, FINATEC, FAPDF, MCT/CNPq, CAPES, Petrobras, FINEP/CTPetro and FINEP/CTInfra for financial support.

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Publication Dates

  • Publication in this collection
    Feb 2017

History

  • Received
    03 Mar 2016
  • Accepted
    14 June 2016
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