Supported Heteropolyacid Catalysts Research Articles

Catalytic Chemistry of Supported Heteropolyacids and Their Applications as Solid Acids to Industrial Processes

This article reviews recent research and development of supported heteropolyacid (HPA) catalysts; focusing on the acidic and catalytic properties. First the basic knowledge of solid HPAs is provided briefly to facilitate understanding of heterogeneous catalysis of HPAs. Secondly, the structure as well as the physical and chemical properties of supported HPA catalysts is described. Especially the layer structure of HPA dispersed on the surface of SiO2 and the changes in the surface acidity with the loading level of HPA are discussed. For this purpose, temperature programmed desorption of benzonitrile devised by Okuhara and Kamiya’s group is useful. This method is capable to assess the surface acidity of the supported HPA, which controls the catalytic activity for the reactions proceeding by surface-type catalysis. Then, two new industrial processes developed by Showa Denko K. K. are described; (i) production of ethyl acetate from ethylene and acetic acid and (ii) oxidation of ethylene to acetic acid. Supported HPA catalysts are utilized for both processes, which were recently much improved by finely controlling the catalysts and reaction conditions.

Chemical Immobilization of Heteropolyacid Catalyst on Inorganic Mesoporous Material for use as an Oxidation Catalyst

Recent progress on the chemical immobilization of heteropolyacid (HPA) catalyst on inorganic mesoporous material is reported in this review. Mesostructured cellular foam silica, mesoporous carbon, and nitrogen-containing mesoporous carbon were used as supporting materials. The mesoporous materials were modified to have a positive charge, and thus, to provide sites for the immobilization of HPA catalyst. By taking advantage of the overall negative charge of heteropolyanion, the HPA catalyst was chemically immobilized on the surface-modified mesoporous material as a charge-compensating component. Characterization results showed that the HPA catalyst was finely and molecularly dispersed on the surface of mesoporous material via strong chemical immobilization, and that the pore structure of mesoporous material was still maintained even after the immobilization of HPA catalyst. The supported HPA catalysts were applied to the model vapor-phase ethanol conversion, 2-propanol conversion, and methacrolein oxidation reactions. The supported HPA catalyst showed a better oxidation catalytic activity than the unsupported HPA catalyst in the model reactions. The enhanced oxidation catalytic performance of the supported HPA catalyst was attributed to the finely dispersed HPA catalyst, which was chemically immobilized on the positive site of mesoporous material by sacrificing its proton (Bronsted acid site). The HPA catalyst chemically immobilized on mesoporous material served as an excellent oxidation catalyst.

FTIR and reaction studies of the acylation of anisole with acetic anhydride over supported HPA catalysts

The behaviours of two heteropoly acid (HPA) catalysts, one supported on a commercial silica and the other on a silica–zirconia mixed oxide, were compared in the acylation of anisole with acetic anhydride. Both supported catalysts and the latter support, but not the silica, were active for the reaction, although all underwent a deactivation process before achieving complete conversion of anisole to p-methoxyacetophenone. The greatest yield to p-methoxyacetophenone was obtained for HPA/SiO 2 while the extent of deactivation was in the order HPA/SiO 2–ZrO 2 > SiO 2–ZrO 2 > HPA/SiO 2. Although acetate species accumulated on the SiO 2–ZrO 2 and HPA/SiO 2–ZrO 2 catalysts during the reaction, these species are not thought to play a role in the deactivation process. The relative extent of deactivation was consistent with the calculated ratios of adsorption coefficients which determine the strength of adsorption of product to that of anisole and indicate that product inhibition is greatest in the case of HPA/SiO 2–ZrO 2. Calculated absorption coefficients for pyridine adsorbed on the samples failed to distinguish between the nature of the HPA species dispersed on the two different supports.

Dehydrogenation activity and structure of polyaniline supported heteropolyacid catalysts

Polyaniline(PAN) supported H6P2W18O62(PW) , H3PMo12O40 (PMo) and H4PMo11VO40(PMoV) catalysts were prepared and their activities for hexanol conversion were tested. IR, XRD, ICP and SEM measurements proved that the heteropolyacids (HPA) could be supported on this type of polymer. The PAN supported HPA catalysts exhibit higher redox activities and low acid-base activities for the hexanol conversion. The redox activities increase with increasing amount of the heteropolyacid. Substitution of Mo ion by V ion results in an increase of redox activities of the catalysts.

Dispersion measurement of heteropoly acid supported on KIT-1 mesoporous material

Heteropoly acid (HPA) catalysts supported on KIT-1 mesoporous material were prepared with different loadings and characterized using XRD, UV–VIS and N 2 adsorption. Supported HPA was dispersed mainly on the inner surface of pores, resulting in the disappearance of XRD peaks. The diffraction peaks of HPA from the KIT-1 mesoporous material loaded with HPA of 41 wt% was very weak compared to those of physical mixtures of HPA and mesoporous material. The remaining adsorption amount of 1-butene on the supported HPA catalysts revealed the exposed amount of HPA molecules, which is useful in determining the HPA dispersion. HPA dispersion of supported HPA catalysts was useful to interpret their catalytic activity in the skeletal isomerization of 1-butene.

Oxide supports for 12-tungstosilicic acid catalysts in gas phase synthesis of MTBE

A series of simple and mixed oxides: SiO 2, AlPO 4, SiO 2-Al 2O 3, TiO 2, kaolin and γ-Al 2O 3 differing by their basicities as characterised by effective negative charge on oxygen calculated according to Sanderson’s theorem was used for the preparation of supported H 4SiW 12O 40 catalysts. Using gas phase synthesis of MTBE as the catalytic test reaction it has been shown that on the supports of highest basicity, i.e. γ-Al 2O 3 and kaolin the heteropolyacid was decomposed resulting in the catalyst which exhibited only low activity. On the other hand, the least basic oxide SiO 2 gave the most active catalysts. However, the sequence of activities differed from that of basicities by the fact that the activity of TiO 2-supported catalyst was at the coverage with H 4SiW 12O 40 Θ=0.25 nearly as high as SiO 2 supported one and at Θ=1.0 TiO 2-supported catalyst was the best. This indicates that the basicity of the support is only one of the factors determining the activity of supported heteropolyacid catalysts in acid–base type reactions. On the other hand good correlation was obtained between the catalytic activity and the neutralisation heat of the acid sites on the surface of catalyst which was determined using thermometric titration with n-butylamine solution in toluene.

Chemical modification of metal oxide carrier's surfaces with a silane coupling agent and an application of the method to catalyst preparation

Several metal oxides were modified with γ-anilinopropyltrimethoxysilane (AnPS). Variations in the physicochemical properties with the modification were investigated in detail. The adsorptivity of 12-tungstophosphate anion (PW12) was shown to be greatly improved in the carriers such as SiO2, TiO2, and Al2O3, which are porous and have a high surface area, but it was not increased in SnO2 and MgO, which have a small surface area and a high isoelectric point. With TiO2-carrier, quantitative studies on the adsorptivity were carried out further, and the interaction modes of PW12 on the AnPS-modified surfaces were discussed based on FT-IR and XPS analyses. The isotherm attained quite steeply to a monomolecular layer of PW12. The FT-IR bands ascribed to the anilino group deformed considerably and the N1s-peak in XPS shifted along with the PW12-adsorption. From the findings it was strongly suggested that PW12 was effectively fixed through an acid–base interaction on the AnPS-TiO2. Catalytic activity of PW12 fixed on AnPS-TiO2 for 2-propanol dehydration as a test reaction were examined; in the pretreatment at 300°C the activity was limited to a low level but after the treatment at 450°C it increased in proportion to the PW12-loading on the AnPS-TiO2 surfaces. Thus, it was inferred that PW12 was quite regularly dispersed over the modified surface. In conclusion, it is suggested that the modification technique is applicable to the preparation of a metal oxide–supported heteropolyacid catalyst.