Abstract
Lactic acid can be prepared by isomerization of renewable dihydroxyacetone over acid catalysts. However, the activities of Lewis acid and Brønsted acid sites in dihydroxyacetone isomerization are poorly understood. We prepared catalysts by heat treatment of ZSM-5. The heat treated ZSM-5 exhibited a greater Lewis acid site density and enhanced selectivity toward lactic acid. Dihydroxyacetone dehydration to the intermediate pyruvaldehyde was readily formed at 140 °C without added catalysts. Lewis acid sites were needed to convert pyruvaldehyde to lactic acid. Moreover, the Lewis acid site density was consistent with the order of catalytic performance, which suggested that the Lewis acid sites were the active sites for pyruvaldehyde rehydration. Conversely, the Brønsted acid sites were key in formation of unwanted product from pyruvaldehyde. These findings highlight the potential use of commercial zeolites as adjustable solid Lewis acid catalysts in biomass conversion reactions in which Lewis acid sites are needed.
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