Conversion of fructose into methyl lactate over SnO2/Al2O3 catalystin flow regime

Abstract

In recent years, numerous researchers have focused on the development of catalytic methods for processing of biomass-derived sugars into alkyl lactates, which are widely used as non-toxic solvents and are the starting material for obtaining monomeric lactide. In this work, the transformation of fructose into methyl lactate on Sn-containing catalyst in the flow reactor that may be of practical interest was studied. The supported Sn-containing catalyst was obtained by a simple impregnation method of granular γ-Al₂O₃. The catalytic experiments were performed in a flow reactor at temperatures of 160-190 °C and pressure of 3.0 MPa. The 1.6-9.5 wt. % fructose solutions in 80 % aqueous methanol were used as a reaction mixture. It was found that addition to a reaction mixture of 0.03 wt. % potassium carbonate leads to the increase in selectivity towards methyl lactate on 15 % at 100 % conversion of fructose. Products of the target reaction С₆Н₁₂О₆ + 2СН₃ОН = 2С₄Н₈О₃ + 2Н₂О were analyzed using ¹³C NMR method. The following process conditions for obtaining of 65 mol % methyl lactate yield at 100 % fructose conversion were found: use of 4.8 wt. % fructose solution in 80 % methanol, 180 °С, 3.0 МПа and a load on catalyst 1.5 mmol C₆H₁₂O₆/ml_cat/h at contact time of 11 minutes. The catalyst productivity is 2.0 mmol C₄H₈O₃/ml_cat/h and the by-products are  1,3-dihydroxyacetone dimethyl acetal (20 %) and 5-hydroxymethylfurfural (10 %). It should be noted that a racemic mixture of L- and D-methyl lactates has been obtained by conversion of D-fructose on the SnO₂/Al₂O₃ catalyst. The SnO₂/Al₂O₃ catalyst was found to be stable for 6 h while maintaining full fructose conversion at 55–70 % methyl lactate selectivity. After regeneration the catalyst completely restores the initial activity.