Surface chemistry of MgO/SiO2 catalyst during the ethanol catalytic conversion to 1,3-butadiene: in-situ DRIFTS and DFT study

1,3-Butadiene is an important commodity chemical and new, selective routes of catalytic synthesis using green feedstocks, such as ethanol, is of interest. For this purpose, surface chemistry of MgO/SiO2 catalyst synthesized using wet-kneading was explored during the reaction of ethanol and the corresponding reactive intermediates, including acetaldehyde, crotonaldehyde, crotyl alcohol using temperature programmed in situ DRIFT spectroscopy combined with DFT calculations. Ethanol adsorption yielded several physisorbed and chemisorbed surface species. Acetaldehyde exhibited high reactivity to form crotonaldehyde. However, aldehyde intermediates resulted in strongly bound surface species stable even at high temperatures, assigned to surface acetate, and/or 2,4-hexadienal or polymerized acetaldehyde. Crotonaldehyde was reduced by ethanol to yield crotyl alcohol via MPV mechanism. Crotyl alcohol, on the other hand, showed to be very reactive and yield two different species on the surface, namely physisorbed and deprotonated that would further desorb as 1,3-BD. Presence of gas phase hydrogen containing molecules, such as ethanol, proved to be key in several reactive steps, including acetaldehyde condensation step and crotonaldehyde reduction. Altogether, these data suggested complex reactive interactions between the surface hydroxyl groups, gaseous reactants and surface bound reactive intermediates during the 1,3-BD formation. Future work is needed to correlate vapor phase product evolution with the transient reactive surface intermediates to examine trends leading to higher overall 1,3-BD selectivity.