Characterization of selective oxidation catalysts from polyoxometalate precursors using ammonia adsorption microcalorimetry and methanol oxidation studies

摘要

Phosphomolybdic acid (H3PMo_(12)O_(40)) along with niobium, pyridine and niobium/pyridine exchanged phosphomolybdic acid compounds were prepared. These compounds were converted to selective oxidation catalysts by pre-treating to 693 K in an inert atmosphere. As shown previously, the active catalyst consists of partially decomposed, partially reduced Keggin units and MoO_x fragments with some MoO_x fragments collected around the Nb. The amount of surface Mo species reduced to the 5+ oxidation state varied among the catalysts. Ammonia adsorption microcalorimetry and methanol oxidation studies were carried out to investigate the acid sites strength and the acid/base/redox properties of each catalyst. The addition of niobium, pyridine or both increased the ammonia heat of adsorption by 30-40 kj/mol and the total ammonia uptake. The catalyst with both niobium and pyridine demonstrated the largest number of strong sites. For the parent H3PMo_(12)O_(40) catalyst, methanol oxidation favors the redox product (～95% selectivity). However, catalyst deactivation occurs. The presence of niobium results in similar selectivity to redox products (～93%) but also results in no catalyst deactivation. Incorporation of pyridine to the precursor compound, in contrast, changes the selectivity to initially favor the acid product (～62%). Again, the catalyst deactivated and selectivity changed during deactivation to favor the redox product (～55%). Finally, the inclusion of both niobium and pyridine results in strong selectivity to the acid product (～95%) while also showing no catalyst deactivation and stable selectivity. Specific activity for the niobium and pyridine exchanged catalyst for the methanol oxidation reaction was twice any other catalyst. Selectivity to acid products was correlated with the amount of reduced surface Mo species. Thus, the presence of pyridine appears to enhance the acid property of the active site in the catalyst while niobium appears to stabilize the active site.