(100c) Mechanistic Insights on the Dehydration of Polyols over Bronsted Acid Sites
AIChE Annual Meeting
Monday, November 14, 2022 - 1:06pm to 1:24pm
In this contribution, we probe the selective conversion of hydroxyl groups in polyethylene-vinyl alcohol (EVOH)âa common co-polymer for oxygen-barrier functionalityâusing polyalcohols as the model molecules. The DFT calculations are performed to investigate the reaction kinetics of two neighboring hydroxyl groups over propylsulfonic functionalized amorphous silica. The study of elementary steps shows that the dehydration of the first hydroxyl group occurs via four plausible routes, in which the âdirectâ dehydration reassembling the E2 mechanism to trans-allylic alcohol is more favored. The âindirectâ pathway initiates with the formation of alkoxide intermediate through the SN1 mechanism as the rate-determining step, which subsequently deprotonates following E1-like mechanism to form allylic alcohols. Limited by the reaction coordinates, the dehydration of the second hydroxyl group proceeds through the similar âindirectâ pathway to form conjugated diene products. The isomerization of allylic alcohols to ketones exhibits higher activation barriers compared to the dehydration. Furthermore, AIMD simulations in the presence of explicit water reveals that the proton-shuttling among solvent molecules and the adjacent acid site can change the energetics of reactions. These results provide guidelines for tuning the rate and selectivity of conversion in the real polymeric system by precisely manipulating the time on stream, acid site density, and solvent concentration.