Insight into the effects of acid characteristics on the catalytic performance of Sn-MFI zeolites in the transformation of dihydroxyacetone to methyl lactate

Abstract

To investigate the effects of the acid characteristic of Sn-MFI zeolite on catalytic performance in the transformation of dihydroxyacetone to methyl lactate, multiple characterization and experimental methods were employed. Firstly, the transformation between Lewis and Brönsted acid sites was initially confirmed by 31P-TMP NMR method, owing to the introduction of CH3OH by framework Sn species to donate protons. It is found that Lewis acid sites are cooperative with Brönsted acid sites to boost the primary reactions of dihydroxyacetone (DHA) to methyl lactate (ML). Yet, too high ratio of Brönsted acid sites to Lewis acid sites favors the generation of side product pyruvic aldehyde dimethyl acetal (PADA). That is because Brönsted acid sites majorly catalyze the dehydration of DHA, the addition of pyruvaldehyde (PA) with methanol and the addition of hemiacetal (HA) with methanol to PADA, whereas Lewis acidic framework Sn sites are responsible for the isomerization of HA to ML, which can be further verified by the isotopic labeling experiments. Therefore, this study provides a definite and systematical insight on the impacts of acid property on product selectivity.