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.^[1] 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 and 10Mo/SiO₂). The molybdenum loading was also adjusted for the Al₂O₃ support between 1 and 20 wt%. X-ray absorption spectroscopy demonstrated that the oxidation state of Mo in the 10Mo/Al₂O₃ decreased as the reduction temperature increased. 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. Interestingly, the change in conversion from introducing Mo on a SiO₂ was very pronounced relative to Mo on Al₂O₃. The 1Mo/Al₂O₃ actually had higher conversion than the higher weight loading samples. 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 Mo weakens the acid strength of the Al₂O₃ while also enhancing the conversion.
The acidity of the materials was characterized with pyridine and deuterated acetonitrile (CD₃CN) infrared spectroscopy. CD₃CN spectra showed the strength of the Lewis acid sites (LAS), and 10Mo/Al₂O₃ had the strongest sites. Pyridine adsorption measurements showed that 1 wt% Mo/Al₂O₃ had the highest Lewis acid site concentration. The pyridine measurements were also done before and after the reduction step, and it was shown that the reduction step resulted in a higher concentration of strong sites that can retain pyridine at 450 °C in vacuum. This further confirms that the reduction step strengthens acid sites.

Giada Innocenti

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