(509bg) Molybdenum Oxide Supported Catalysts for Aldol Condensation

Aldol condensation is an important C-C bond forming reaction for the upgrading of smaller oxygenates (C2, C3) derived from biomass. Previously, we have shown that bulk molybdenum oxide (MoO₃) can catalyze the aldol condensation of ethanol and acetaldehyde when the MoO₃ has been reduced in hydrogen at high temperatures. One major disadvantage for practical catalytic applications is the low surface area of MoO₃ (~2 m²/g). To develop catalysts with a higher density of active sites, we studied MoO_x supported on high surface area oxides (Al₂O₃ and SiO₂) for the aldol condensation of acetaldehyde.

Catalysts were synthesized using incipient wetness impregnation at loadings of 10 wt% molybdenum (10Mo/Al₂O₃ and10Mo/SiO₂). The molybdenum loading was also adjusted for the Al₂O₃ support between 1 and 20 wt%. Steady state reaction studies showed the selectivity was predominately to crotonaldehyde across all catalysts. Conversion to crotonaldehyde was highest over 10Mo/Al₂O₃, while the bare Al₂O₃ support had a slightly higher conversion than 10Mo/SiO₂. Additionally the deactivation of these catalysts was significant as shown by time on stream profiles. Temperature programmed oxidation-MS analysis of the spent catalysts revealed the Al₂O₃ had the most strongly adsorbed species as shown by the highest temperature for peak desorption. The 10Mo/Al₂O₃ had a lower temperature peak suggesting the molybdenum weakens the acidity strength of the Al₂O₃ while also enhancing the conversion.

The acidity of the materials was characterized with adsorption of pyridine and deuterated acetonitrile (CD₃CN) followed by infrared spectroscopy. CD₃CN spectra showed the strength of the Lewis acid sites (LAS), and 10Mo/Al₂O₃ had the strongest sites. Experiments were done before and after the reduction step, and the increase in the vibrational frequency of CD₃CN on LAS after reduction suggests this pretreatment helps strengthen the site. Pyridine adsorption measurements showed that 1 wt% Mo/Al₂O₃ had the highest Lewis acid site concentration.