Influence and stability of the surface density of MoOx on TiO2 in deoxydehydration: structure–activity correlations

Abstract

Titania supported MoO_x catalysts containing varying surface densities of Mo were prepared and well-characterized to identify the most effective MoO_x species for the deoxydehydration (DODH) reaction of 1,4-anhydroerythritol (1,4-AHE), and eventually close the gap between experimental observations and theoretical predictions. Structure–activity correlations revealed isolated MoO_x species to be more active than oligomeric/polymeric/crystalline MoO_x species. Decreasing activity with increasing Mo surface density proved to be a result of the increase in strong acidity of the catalysts. These acid sites favored side reactions involving dehydration reaction and ketal formation. Comparative studies with dried and calcined catalysts showed that calcination moderately improves the selectivity of isolated MoO_x species while it decreases the selectivity of polymerized MoO_x species. This was due to the increased Brønsted acidity of the catalyst, a consequence of calcination. Brønsted acid sites were not present on isolated MoO_x species. Regarding leaching, the calcination only benefited the isolated species as the leached species were DODH inactive while for polymerized species, due to their higher loading, the leached species were DODH active. The use of a merely dried catalyst exposed significant leaching of catalytically active species into the reaction medium in the early hours of the reaction. Recycle studies with the most active catalyst demonstrated the stable loading of Mo on TiO₂, carbon deposition, and possible acidity regeneration during heat treatments.