多级孔分子筛酸性调控及催化糖转化制备乳酸酯研究

Lactic acid (LA) and alkyl lactates are widely used in food, commodity, chemical, pharmaceutical and plastic industries. Presently, the major production process of LA and alkyl lactates is the fermentation of carbohydrates. However, the space-time yield of this method is limited, and large amounts of sulfuric acid are consumed with large amounts of salt waste as byproducts during fermentation. Therefore, it is desiderated to develop a novel and effective method to produce LA and alkyl lactates from carbohydrates. This project selects the production of alkyl lactates from cheap and abundant glucose as a model reaction. The reaction is very complex which composed of multiple reaction steps such as isomerization between aldose and ketose, retro-aldol to trioses, dehydration, addition, and etc. Both Lewis (L) and Brønsted (B) acid sites are needed for accomplishing the reaction. Therefore, hierarchical molecular sieves with adjustable L and B acidity are employed to fulfill the reaction in this project. The product selectivity will be controlled through adjustment of L and B acidity. Meanwhile, the efficiency of mass transfer will be improved through adjusting the pore structure of molecular sieves. So the further reaction of the desired product to other byproducts will be reduced, and the catalytic activity and selectivity for the desired product of the hierarchical molecular sieves will be improved. Based on the results of the research, the relationship between the acidity and pore structure of molecular sieves and their catalytic performace in the conversion of carbohydrates to alkyl lactates will be found. A high-efficiently catalytic reaction system for the transformation of carbohydrates to alkyl lactates will be established.

乳酸及乳酸酯在食品、化妆品、化学品、制药及塑料工业被广泛使用。当前，乳酸及乳酸酯主要通过糖发酵制备，该法时空收率有限，且消耗大量硫酸，产生大量废盐。因此，亟需开发由糖制备乳酸及乳酸酯的高效新方法。本项目以廉价、丰富的葡萄糖转化制备乳酸酯为模型反应进行研究，该反应是一个包含醛酮糖异构、反羟醛缩合、脱水、加醇-异构等多种反应类型的多步反应过程，需要Lewis(L)酸和Brønsted(B)酸的共同催化，且易发生副反应。因此，申请人提出用L酸和B酸酸性可调的多级孔分子筛催化该反应。通过L酸和B酸酸性的调控，实现反应产物的调控；通过孔结构的调控，提高传质效率，避免目标产物进一步转化成其它副产物，从而提高多级孔分子筛的催化活性和对目标产物的选择性。通过本项目的研究，建立分子筛的酸性质和孔结构与催化糖转化制备乳酸酯反应性能之间的关系，开发出高效的催化反应体系。

乳酸及乳酸酯在食品、化妆品、制药、塑料工业及合成化学中被广泛使用。采用化学催化转化法将糖转化为乳酸及乳酸酯具有重要的科学意义和应用价值。糖转化为乳酸酯是一个包含醛酮糖异构、反羟醛缩合、脱水、加醇-异构等多种反应类型的多步反应过程。本项目采用分子筛为催化剂，考察了分子筛的L/B酸性和孔结构对糖转化为乳酸酯的影响，建立了分子筛催化剂与糖类转化制备乳酸酯反应之间的构效关系。发现强L酸位尤其是Snn+作为L酸位有利于乳酸酯生成，强B酸位催化副反应的进行，不利于乳酸酯生成。对于三碳糖转化，弱B酸位促进糖脱水生成中间产物丙酮醛，而有利于乳酸酯生成；对于六碳糖转化，弱B酸位不利于六碳糖反羟醛缩合生成三碳糖，所以不利于乳酸酯生成。三维十二元大微孔结构有利于反应扩散传质，所以有利于乳酸酯生成，介孔的存在会更进一步促进这一过程。通过研究，建立了糖类转化制备乳酸酯的高效催化反应体系，在多级孔Sn-USY分子筛上，1,3-二羟基丙酮（DHA）在甲醇中转化，在室温（25 oC）反应24 h，DHA转化率和乳酸甲酯（MLA）收率分别达到98%和96%。在Au/Sn-USY催化剂上，甘油可直接氧化-异构化为乳酸酯，160 oC反应10 h，得到79%的MLA收率。无氟体系、低浓度模板剂条件下制备的多级孔Sn-Beta分子筛催化葡萄糖在甲醇中转化，160 oC反应10 h，MLA收率达到58%。本项目的系统研究深化了对含羰基化合物例如糖的催化选择性转化的认识，为进一步构建高效催化糖转化为乳酸酯催化剂体系提供了科学基础。