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 MoOx 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.

Molybdenum oxide supported catalysts for aldol condensation

ACS Fall 2021

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