(513b) Characterization of Partially Reduced Polyoxometalate Catalysts Using Ammonia Adsorption Microcalorimetry and Methanol Oxidation Studies

Selective oxidation of hydrocarbon reactions continue to play an important part of the chemical industry resulting in an ongoing investigation of the heterogeneous catalysts that accomplish these reactions. Because of the opportunity to finely control their structure and composition, heteropoly acids and their salts are commonly investigated for their ability to perform selective oxidation reactions. These studies are commonly focused on selective oxidation of light hydrocarbons such as propane and butane to products such as acrylic acid and maleic acid due to the availability of these feed stocks and the industrial usefulness of the products, particularly in producing polymers. A recent advance in this area has used niobium and pyridine salts of molybdophosphoric or molybdo(vanado)phosphoric acid (NbPMo12pyr or NbPMo11Vpyr) as precursors to highly active catalysts for the selective oxidation of propane and butane. The productivity of maleic or acrylic acid from these catalysts significantly exceeds that of the current VPO or MoVNbTeO catalysts. These catalysts are activated by heating in an inert atmosphere to produce partially decomposed, partially reduced polyoxometalate structures. Selective oxidation reactions require bifunctional catalysts: an acid/base property to activate the hydrocarbon and a redox property to perform the selective oxidation. At this point, it is unclear if either or both of these properties are responsible for the unique behavior of these polyoxometalate catalysts.

A series of catalysts containing each of the basic features of the catalyst were prepared. These included H3PMo12O40 (denoted as PMo12), a pyridine exchanged molybdophosphoric acid (PMo12pyr), a niobium exchanged molybdophosphoric acid (NbPMo12), and a niobium and pyridine exchanged molybdophosphoric acid (NbPMo12pyr). These catalysts were pretreated by heating to 420°C in flowing He. Ammonia microcalorimetry was used to measure the acid/base properties of the catalysts. Ammonia was dosed on the catalysts to determine ammonia adsorption strength and specific ammonia uptake. Methanol oxidation studies were also used to probe the acid/base and redox properties of the catalysts. A mixture of methanol, oxygen, and helium were fed to the reactor and reactor effluent was monitored using a GCMS.

Ammonia microcalorimetry results show that PMo12 had the lowest initial heat of adsorption at approximately 80 kJ/mol and the lowest total uptake of approximately 110 ìmol/g. The two pyridine exchanged samples showed similar initial heats of adsorption of 120 kJ/mol. However, at higher coverages (in excess of 200 ìmol/g) The NbPMo12pyr catalyst showed a constant 80 kJ/mol heat of adsorption versus approximately 50 kJ/mol for the PMo12pyr catalyst. The niobium exchanged sample showed a slightly lower initial heat of 110 kJ/mol and intermediate higher coverage heat of 60 kJ/mol. Thus the pyridine appears to result in slightly higher initial heats of ammonia adsorption, but the addition of Nb contributes to higher strength adsorption sites at higher coverages.

For the methanol oxidation reaction, the primary product from NbPMo12pyr is dimethyl ether. According to the literature, dimethyl ether is representative of the acidic character of the catalyst. This agrees well with the strong ammonia heat of adsorption for this catalyst at high coverages. However, this contrasts with literature reports for MoO3 which has strong redox characteristics and mild acidic characteristics. Nb2O5 is reported to have negligible redox characteristic and very mild acidic characteristics. Thus, the partially reduced polyoxometalate catalysts appear to function as more than just localized fragments of the bulk oxide structure. Methanol oxidation studies of the remaining catalysts are underway to better define the contribution of each feature to the activity.