Solid acid catalysts based on H3PW12O40 heteropoly acid: Acid and catalytic properties at a gas–solid interface

Solid acid catalysts prepared by supporting 15 wt%H3PW12O40 heteropoly acid (HPA) on TiO2, ZrO2 and Nb2O5 with a sub-monolayer HPA coverage were characterised at a gas–solid interface, regarding their acid properties and chemical structure of HPA on the catalyst surface and compared to “standard” HPA catalysts such as bulk and silica-supported H3PW12O40 and Cs2.5H0.5PW12O40. In contrast to the parent acid, H3PW12O40, possessing strong Brønsted acid sites, the catalysts supported on TiO2, ZrO2 and Nb2O5 have both Brønsted and Lewis acid sites, with the latter mainly originating from the oxide support. The strength of acid sites in these catalysts is weaker than that in H3PW12O40 and Cs2.5H0.5PW12O40. The catalytic activity (turnover frequency) in gas-phase isopropanol dehydration decreases in the order: H3PW12O40 > Cs2.5H0.5PW12O40 > 15%H3PW12O40/SiO2 > 15%H3PW12O40/TiO2 > 15%H3PW12O40/Nb2O5 > 15%H3PW12O40/ZrO2, which is in line with the acid strength as determined by NH3 adsorption calorimetry. Ammonia adsorption calorimetry, 31P{1H} MAS NMR and FTIR indicate increasing interaction between support and HPA in the following order of supports: SiO2 < TiO2 < Nb2O5 < ZrO2.