Catalytic Conversion Of Glucose To Lactic Acid And Lactate Over Modified Sn-beta Catalysts

The production of value-added chemicals from renewable biomass resources is of great importance to the sustainable development of human society and the abatement of carbon emission.Sn-beta,knew as an active heterogeneous catalyst for the production of lactic acid and lactate from biomass-derived sugars,has attracted widespread attention in recent years.Based on our previous research,we prepared modified Sn-beta catalysts via ion exchange with alkaline earth metal cations and evaluated the performance in the conversion of glucose to lactic acid.Compared to the original Sn-beta,the catalytic activity was greatly improved,and the yield of lactic acid was increased up to 58%.The promotional effect was also more profound than that from alkali metal modified Sn-beta.During the mechanism study,it was proposed that the addition of alkaline earth metals can better stabilize the consequent reaction intermediates.Such a synergistic effect,thereby,leads to effectively promote the retroaldol condensation reaction.After that,the crystal size effect of the catalyst was explored.By adjusting the crystallization time,we prepared Sn-beta catalysts with different sizes.With the increase of the crystal size,the yield of lactic acid increased accordingly.This is because the catalyst with larger size enables the substrate to react more fully inside the pores,thus reducing the occurrence of side reactions occurred from those intermediates being diffused in the bulk acidic solution.In addition,the counterpart catalysts prepared by other modification methods can also greatly increase the lactic acid yield from glucose reaction.Upon the initial wet impregnation method,the best catalytic effect was achieved with the loading of Ca and Mg at 0.5 wt%where the lactic acid yield reached up to 68%.However,in the aqueous solution,the stability of the modified Sn-beta was always poor,regardless of varied preparation methods.By characterizing the structure and element content before and after the reaction,the reason for the deactivation is mainly due to the destruction of the catalyst structure and the leaching of alkaline earth metal.However,this stability dilemma confronted in aqueous solution appears not a big concern for the production of methyl lactate from glucose in organic solvent like methanol.There was no significant leaching of incorporated metal species in the recycled catalysts,apparently thanks to the avoidance of organic acid products and the use of organic solvent that leave H+barely available in the bulk solution to exchange with alkaline earth cation of the catalyst.In order to meet the low energy consumption requirements of industrialization.Based on previous research,we further explored the performance of Sn-beta modified with Mo,which can significantly enhance the catalytic activity at low temperature(100℃).The yield of methyl lactate can reach 16%.Upon the addition of Mg,the yield was further improved.An optimum yield of 35%of methyl lactate was obtained at a reaction temperature of 100℃,when the loadings of Mo and Mg were 3 wt%and 0.5 wt%respectively.The increase of reaction temperature to 160℃ will lead to a yield up to 51%.Extensive experiments have been conducted to explore the possible mechanism.It was proposed that Mo can catalyze the occurrence of retro-aldol condensation at low temperature,which was further enhanced by the presence of neighboring alkaline earth metal as a co-catalyst through a synergistic effect of Lewis acid-base similar to that between Ca/Mg and Sn mentioned above.The Sn-Mo-Mg catalyst also exhibited good stability during the reaction,which enables the catalyst a great potential in industrial application.