功能化MOFs固载磷钨酸催化剂的可控组装及定向催化水解纤维素特性研究

It is the primarily condition of the biomass energy or platform chemicals production from directionally hydrolyzing of cellulose to glucose. The difficulty and hotspot of this research field is to hydrolysis cellulose to glucose directionally and reduce the further degradation of glucose produced from cellulose hydrolyzing. According to the effect of the acid catalytic sites and absorption sites of the catalysts on cellulose hydrolysis and glucose degradation, the new organic framework materials (MOFs) is innovatively adopted to design and synthesize solid catalysts in this project. The aim of this project is to obtain the method for functionalized metal-organic frameworks loading phosphotungstic acid (PTA) synthesis, one kind of solid catalysts, which are able to hydrolyze plant fibers to glucose directionally. The rule of crystal growth of MOFs with large pore channels and its effects on the crystal structures and the stability are investigated through regulating and controlling the synthesis system of MOFs. Moreover, the mechanism of organic ligands with various alkalinity on the stability and catalytic activity of PTA loading on MOFs, the role of strongly electronegative groups of organic ligands in selective absorption between cellulose and glucose and in cellulose directional conversion and the mutual interaction between supramolecular structures of vegetable fibers and the catalysts are all deeply studied to provide theoretical foundation for success in the design and synthesis of the catalysts capable of directionally transforming cellulose into glucose. This project is of great significance in the field converting plant fibers into high-value chemicals and increasing the efficiency of vegetable fibers directional conversion to energy and platform chemicals.

纤维素定向水解转化为葡萄糖是其用于制取生物质能源或平台化合物的前提，减少纤维素水解过程中葡萄糖进一步降解副反应是实现纤维素定向转化为葡萄糖的难点和研究热点。项目根据纤维素固相催化水解过程中酸催化活性中心和吸附中心对纤维素水解及葡萄糖降解副反应的影响，创造性将新型有机骨架材料(MOFs)用于固相催化剂的设计组装，旨在研究获得可实现植物纤维定向转化为葡萄糖的功能化MOFs固载PTA催化剂。通过调控MOFs组装体系研究大孔道MOFs的晶体生长规律及其对晶体结构及稳定性的影响，研究有机配体碱性官能团对PTA在MOFs固载稳定性及其催化活性的作用机制，探讨催化剂强电负性官能团对纤维素和葡萄糖差异化吸附性能及纤维素定向转化的影响，解析植物纤维超分子结构与催化剂间相互作用机制，为实现纤维素定向转化为葡萄糖催化剂的设计及提高植物纤维定向转化效率提供理论基础，项目对发展植物纤维高值化利用技术具有重要意义。

本项目以纤维素为研究对象，进行MOFs固载PTA催化体系的设计合成及其催化水解纤维素定向转化为葡萄糖的机理研究。结果表明，当合成溶剂为DMF，金属离子浓度小于0.16 M，合成时间为16 h时，可以获得结构稳定的MIL-101材料。将Ni离子引入MIL-101（Cr）结构的金属有机骨架中可以进一步优化MOFs材料的水稳定性能，在不同pH值溶液中，Ni/MIL-101（Cr）具有较高的耐水解性。采用表面活性剂模板法、延长有机配体法和调节剂诱导缺陷法对MOFs载体扩孔后，发现MOFs载体的孔径越大，可进入载体内部与PTA充分接触的微晶纤维素越多，葡萄糖的产率越高。碱性官能团TA-NH2的加入使样品均保持较高的热稳定性，在固载PTA后MOF仍保持完整的Keggin结构。由于-NH2与PTA间的静电作用逐渐增强，导致样品的酸强度降低，酸密度降低，有效降低催化剂参与水解反应的PTA损失量，保持PTA的固载稳定性及酸催化活性。活性官能团-Cl的嫁接不会改变催化剂的晶型结构和PTA的固载量，且催化剂的-Cl含量与所获葡萄糖产率间表现出显著线性关系，高氯含量有利于提高葡萄糖产率，促进纤维素定向转化的实现。以PTA@MIL-101-NH2-Cl为催化剂，-Cl基团能通过与纤维素羟基形成氢键O-H…Cl选择性吸附纤维素，实现纤维素定向转化为葡萄糖。反应前后纤维素键能较强的分子间氢键O(6)H…O(3’)含量由25.95%下降到10.12%，结晶度由80.63%降低到37.79%。-Cl通过破坏纤维素分子间氢键以及将其转化为键能较低的分子内氢键两种方式降低纤维素的结晶度，有效降低纤维素的水解难度。研究结果为MOFs负载PTA的固相纤维素水解催化剂的设计合成提供理论指导，对发展植物纤维能源化技术，提高植物纤维向能源物质定向转化效率具有重要意义。