Liquid-phase dehydration of propylene glycol using solid-acid catalysts

In this work we combine experiments with Density Functional Theory (DFT) calculations to investigate the heterogeneous dehydration of propylene glycol. The reactions were carried out with pure, liquid propylene glycol over MFI-framework zeolite catalysts or the mesoporous sulfonic-acid resin Amberlyst 36Dry. When Amberlyst 36Dry was used, propylene glycol dehydrated to form propionaldehyde with 77% selectivity. All of the propionaldehyde further reacted with propylene glycol to form a cyclic acetal. The final products consisted of 78% acetal, 13% dipropylene glycol, and the remaining 9% was composed of acetone and a cyclic ketal formed from acetone. The zeolite catalysts demonstrated significantly higher selectivity toward dipropylene glycol compared to Amberlyst 36Dry. Furthermore, the zeolite had a lower conversion to cyclic acetals, improving the selectivity toward C3 products, acetone and propionaldehyde. DFT calculations confirmed that propionaldehyde is the favorable product in both catalysts, since it can be formed either through dehydration of the secondary hydroxyl group or via dehydration of the primary hydroxyl group with a concerted pinacol rearrangement. However, in the case of zeolites, the cyclic acetals experience steric hindrance since their size is comparable to that of the zeolite pores. Thus we argue that the cyclic acetals produced over the zeolite catalyst were formed homogeneously from the C3 products which diffused out of the zeolite pores.

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► Carbonyl-containing dehydration products showed high conversion to cyclic acetals.
► Zeolite yields broader distribution of dehydration products than Amberlyst.
► Product distribution in each catalyst explained using ab-initio calculations.
► Differences in cyclic acetal conversion relate to the pore sizes of the catalyst.
► Selectivity in the zeolite is governed by stability of chemisorbed intermediates.