| Supported heteropoly acid catalysts, including PW/SiO2, PW/USY, PW/SBA-15 and PW/HM, were prepared by impregnation at ambient temperature. Their physicochemical properties such as the dispersion of PW, specific surface area, pore structure and acidity were characterized by X-ray diffraction (XRD), N2 adsorption isotherm, infrared-ray spectrum (IR), thermogravimetry-differential scanning calorimetry (TG-DSC) and temperature programmed desorption of ammonia (NH3-TPD). Based on the characterizations, the catalytic behavior was first tested in alkylation of naphthalene with isopropanol, and the correlation of the surface properties of the catalysts and their activity and selectivity has been discussed in detail.It is found that the pore structure and acidity of catalysts are the key factors that influence the catalytic performances, and the catalysts with larger acid amount, stronger acidity and wider pore sizes exhibit higher activity and selectivity for β-IPN and β,β′-DIPN. The shape-selectivity for 2,6-DIPN is higher at the expense of their activities over the catalysts with smaller pore diameter because of the shape selectivity of catalyst pore.PW/SiO2 catalysts with high PW loadings have larger acidic amount with stronger acidity, and PW could disperse highly on the surface of SiO2 until the concentrate of PW is as high as 30% by weight. Tests of isopropylation of naphthalene over PW/SiO2 indicate that the optimum of PW loadings, activation temperature, reaction temperature, mole ratio of isopropanol to naphthalene is 30%, 473 K, 473 K, 3∶1, respectively. The distribution of product as a function of reaction time demonstrates that this reaction is a consecutive reaction over this catalyst, and Low concentration of reactants shows the high selectivity for β,β′-DIPN.XRD results indicate that PW/USY with various PW loadings retain the high specific surface area and micropore structure of USY with abundant secondary pore system. When the PW loading is as high as 30%,PW could highly disperse on the surface of USY because of the interaction of molecular PW and secondary pore wallsof USY. Comparing to pure PW, thermal stability of PW supported on USY gets great improvement measured by TG-DSC. On the other hand,both Br?nsted and Lewis acid sites are observed on PW/USY catalysts. Comparing to pure PW or USY, PW/USY catalysts possess larger amount of acid sites. This kind of catalyst, PW/USY, with dualism of heteropoly acid (acidity) and zeolite (pore system) exhibits excellent activity at low reaction temperature and selectivity for β-IPN and β,β′-DIPN. As far as 10%PW/USY is concerned, its suitable activation temperature is 573 K, and the lengthened reaction time is favorable to isopropylation of naphthalene.SBA-15, which possesses high specific surface area, and thermal and hydrothermal stability, is a mesoporous silica molecular sieve with uniform pore diameter between 46～300 ?. When PW is loaded on SBA-15, the resulted catalyst still retains the high surface area and PW could highly disperse on the surface of SBA-15 until PW loading is as high as 60%. Thermal stability of PW/SBA-15 decreases at low and medium PW loadings, but increases slightly at high PW loadings because of the interaction between PW and silanol groups on the surface of SBA-15. PW supported on SBA-15 retains intact Keggin structure at the range of 20%～80% of PW loadings. Similar to the catalysts supported on other kinds of supports, acidity of PW/SBA-15 has been improved with the increase of PW loadings. Reaction results in isopropylation of naphthalene over PW/SBA-15 catalysts demonstrate that suitable activation temperature (573 K), high reaction temperature and PW loadings result in high catalytic activities and selectivity for β and β,β′-DIPN.Catalytic activities decrease remarkable over PW/SiO2, PW/USY or PW/SBA-15 catalysts after several reaction cycles of isopropylation of naphthalene, probably due to the carbon deposition on the catalyst active acid sites arising from the improved catalyst acidity b... |
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