Conversions of various pentoses and hexoses into methyl lactate has been demonstrated for the Sn-Beta catalyst. It is found that pentoses are converted to methyl lactate in slightly lower yields (∼40%) than what is obtained for hexoses (∼50%), but higher yields of glycolaldehyde dimethyl acetal are observed for the pentoses. This finding is in accordance to a reaction pathway that involves the retro aldol condensation of the sugars to form a triose and glycolaldehyde for the pentoses, and two trioses for hexoses. When reacting glycolaldehyde (formally a C2-sugar) in the presence of Sn-Beta, aldol condensation occurs, leading to the formation of methyl lactate, methyl vinylglycolate and methyl 2-hydroxy-4-methoxybutanoate. In contrast, when converting the sugars in water at low temperatures (100 °C), Sn-Beta catalyses the isomerisation of sugars (ketose–aldose epimers), rather than the formation of lactates.
Electronic supplementary information (ESI) available: Detailed HPLC
analysis procedure, sugar isomerization distributions, carbon balances,
NMR spectra and MS fragmentation patterns of key components. See