兼具催化解聚与异构化功能双酸型离子液体的构建及催化纤维素醇解制备乙酰丙酸酯的研究

Levulinate esters, as the very important commodity chemicals, have been wildly used in numerous industries. Direct catalytic alcoholysis of lignocellulose into levulinate esters is one of the most important ways to produce levulinate esters. Traditionally, the cellulose alcoholysis process is mainly carried out in consecutive four steps, viz., depolymerization of cellulose into alkyl glucosides (rate control step), isomerization of alkyl glucosides into alkyl fructosides (rate control step), dehydration of alkyl fructosides into 5-alkoxymethylfurfural, and alcoholization of 5-alkoxymethylfurfural into levulinate esters and formate esters with the presence of Brønsted acidic catalysts. However, Brønsted acid is not the efficient catalyst for cellulose alcoholysis because the products of alcoholysis are broadly distributed due to its weak ability in catalytic isomerization of alkyl glucoside. Therefore, based on the designability of ionic liquids, this project plans to construct a series of Brønsted-Lewis dual acidic ionic liquids to further improve the selectivity to levulinate esters from the alcoholysis of cellulose by combining the Brønsted acid functionalized ionic liquids with the metal chlorides (Lewis acid) which have the catalytic capacity for the isomerization of glucose into fructose. In these dual acidic ionic liquid catalysts, by changing the types of the Brønsted and Lewis acids and adjusting their composition, the synergy mechanism between Brønsted acid and Lewis acid sites will be studied and clarified. In the meantime, the alcoholysis process will be optimized through the correlation of the alcoholysis efficiency with the process factors such as catalyst dosage, solvent composition and dosage, reaction temperature, etc. Finally, the alcoholysis mechanism will be proposed by the correlation of the product distribution with the structure and composition of the Brønsted acidic (cations) and Lewis acidic (anions) sites as well as the structure of feedstocks. The study can lay a solid foundation for the theoretical and experimental research on the efficient alcoholysis of cellulose and other biomass into levulinate esters in industry.

乙酰丙酸酯是非常重要的生物质基化学品，并得到广泛应用。纤维素催化醇解是制备乙酰丙酸酯的重要途径，且醇解过程主要分为解聚、异构化、脱水转化及醇化四步，其中解聚和异构化为控速步骤。由于传统Brønsted酸催化中间体烷基葡萄糖苷异构化能力较弱，不利于纤维素的高效醇解。因此，为了弥补Brønsted酸催化异构化能力的不足，并结合离子液体可设计性特点，本项目拟将对葡萄糖具有催化异构化功能的金属氯盐（Lewis酸）与Brønsted酸离子液体氯盐组合，构建同时含有Brønsted酸和Lewis酸中心的双酸型离子液体催化剂，耦合催化解聚与异构化功能。通过探讨Brønsted酸和Lewis酸结构、含量及过程因素对纤维素醇解的影响，揭示Brønsted酸与Lewis酸协同催化机制以及优化醇解工艺参数；同时分析醇解中间体和产物分布，阐释纤维素醇解过程机理。研究结果可为纤维素醇解工业化应用提供理论依据及指导。

将可再生的纤维素等生物质资源催化转化为高附加值的乙酰丙酸酯类平台化学品是获得替代化石基化工产品的重要途径之一。催化剂的设计合成是生物质高效催化转化的关键所在，离子液体的“量体裁衣”特点为纤维素等碳水化合物的定向转化催化剂的制备提供了理想的设计思路。基于此，本项目完成内容和取得的主要成果包括：（1）设计合成了系列B酸型、L酸型以及B-L双酸型离子液体催化剂，并完成定向催化纤维素醇解制备乙酰丙酸酯等平台化学品的研究，获得阴、阳离子协同催化效率高的离子液体组成，及其与目标产物结构特点之间的关联规律；（2）重点研究了纤维素醇解转化过程关键中间体5-羟甲基糠醛（HMF）的绿色催化合成，获得了HMF高效制备的氯基（—Cl）非催化反应体系，以及低毒高效的氯化锂/异丙醇（LiCl/i-PrOH）催化体系；（3）研究了阳离子含有氯基（—Cl）的功能化离子液体氯盐对纤维素转化为乙酰丙酸的促进作用，初步揭示了离子液体中阳离子（—Cl）与阴离子（Cl¯）对纤维素转化的协同作用机制；（4）针对均相催化体系难于回收使用的缺点，构建了以钇（Y3+）为催化活性中心，市售的ZSM-5分子筛为载体的多相催化剂Y-ZSM-5，探索了该催化剂对果糖基碳水化合物醇解制备乳酸甲酯的催化活性，获得了乳酸甲酯高效催化合成的多相催化体系；（5）鉴于酸性离子液体对纤维素等碳水化合物的催化活性，考察了该类离子液体催化转化果业废弃物荔枝渣的性能，构建了适合荔枝渣催化液化的离子液体催化剂体系，实现了荔枝渣废弃物的高值化利用。在本项目的资助下，发表学术论文9篇，其中SCI/EI收录6篇；申请中国发明专利4件，授权1件；培养硕士研究生2名。本项目的研究内容及获得的成果可为生物质基化学品的绿色催化合成提供借鉴，有利于我国“碳达峰”、“碳中和”目标的早日实现。