Effect of metal modification of titania and hydrogen co-feeding on the reaction pathways and catalytic stability in the acetone aldol condensation

A stable performance of TiO2 catalysts for gas-phase acetone aldol condensation was observed when reduced metals were added (Pt or Ni, 1.5 wt%) and the reactions were conducted in presence of hydrogen. In both cases, the resulting metal-loaded catalysts are stable for 10 h, whereas continuous deactivation is observed for the parent TiO2 catalyst (573 K). Both the activation of the H-2 molecule by metal nanoparticles and the change of the catalytic surface by metal insertion (in the case of Ni-loaded catalyst) enable suppressing oligomerization (by hindering enolates formation) and the strong adsorption of intermediates (by decreasing the concentration of high-strength acid-basic active sites), respectively. More interestingly, these metals allow to tune the selectivity of the reaction. Indeed, the Ni-loaded titania catalyst is highly selective for the synthesis of alpha,beta-unsaturated ketones (selectivity to unsaturated C6 and C9 species >98%, at similar to 12% acetone conversion), whereas the Pt-loaded one is highly selective to the formation of saturated C6 and C9 ketones (MIBK and DIBK, with selectivities >95% at similar to 42% acetone conversion). The catalytic activity and stability of the two materials (Ni/TiO2 and Pt/TiO2) in both absence and presence of H-2 are compared between them and with those of the parent TiO2. The results obtained by the reaction gas-phase analysis are supplemented through different solid characterization techniques (i.e., CO2-TPD and NH3-TPD, HRTEM, XPS, TPO, and DRIFTS). (C) 2019 Elsevier Inc. All rights reserved.