Mechanism of intra- and inter-molecular C[dbnd]C bond formation of propanal on Brønsted acid sites contained within MFI zeolites

Abstract

Kinetic and chemical titration studies are used to unravel the reaction pathways and catalytic requirements for propanal deoxygenation over Brønsted acid sites contained within MFI zeolites. Propanal deoxygenation in the absence of external hydrogen source is initiated via primary and competitive pathways of inter- and intra-molecular CC bond formation that involve bimolecular coupling of propanal and uni-molecular deoxygenation steps, respectively. The inter-molecular CC bond formation proceeds via mechanistic steps resembled the acid-catalyzed aldol condensation reactions in the homogeneous phase, and its reactive collision frequencies increase with increasing propanal pressure. The reaction is initiated by keto–enol tautomerization of propanal to form small concentrations of propenol. The propenol undergoes kinetically-relevant nucleophilic attack to protonated propanal, the most abundant surface intermediates, to create the inter-molecular CC bond. The competitive uni-molecular deoxygenation step involves kinetically-relevant hydrogen transfer from hydrogen-donating agents and occurs at rates that remain invariance with propanal pressure. Hydrogen-donating agents are aliphatic rings produced from consecutive inter-molecular CC bond formation and ring closure events and donate hydrogen via dehydrogenation steps to increase their extent of unsaturation. Hydrogen-donating events must kinetically couple with the direct hydrogen insertion step on propanal to satisfy the deoxygenation stoichiometry and form propanol, which upon dehydration evolves predominantly propene, thus preserving the carbon backbone. Water as a by-product prevents binding of larger, inactive carbonaceous species on acid sites and inhibits the inter-molecular CC bond formation step by increasing the reverse rate of this step. Water, however, does not alter the net rate for intra-molecular CC bond formation, because of its irreversible nature. An increase in the rate ratio for intra- over inter-molecular CC bond formation upon the addition of 3-methyl-1-pentene, an effective hydrogen-donating agent, confirms the kinetic relevance of the hydrogen transfer step for propene formation. These findings on the different kinetic dependencies for the competitive reactions and their mechanistic interpretations provide the operating strategies to tune the reaction pathways, manipulate the extent of hydrogen transfer, and tailor the distributions of larger oxygenates and alkenes during propanal deoxygenation reactions.