乙醇制浆木素超临界选择性定向解聚机制及定点封端抑制逆转行为研究

Ethanol pulping is convinced to be the technology with the most promising industrialization prospect in the pollution-free pulping at present, and the bottleneck restricting its industrialization process is the high value application of ethanol pulping black liquor lignin. Lignin depolymerization by supercritical fluid has the advantages of higher depolymerization products yield, controllable fluid thermodynamics properties and so on. But, there are many problems about the depolymerization mechanism needed to be clarified in the supercritical multiphase fluid system, such as how to promote lignin depolymerization, how to selectively regulate depolymerizing product composition, how to prevent the reverse repolymerization and so on. With ethanol pulping black liquor lignin as the material in this proposal, reaction course of ethanol pulping lignin depolymerizaiton into small molecular fragments caused by supercritical fluid and selective breaking mechanism of key chemical active points will be elucidated, and the activity points of active depolymerized lignin intermediates being prone to reverse repolymerizaiton behavior and the fixed-point capping mechanism to them by capping agent will be studied, and the selective breaking mechanism of lignin catalyzed by primary base metals and the regulation approach controlling will be investigated, and the complex regulatory network to depolymerize lignin directionally into the target products in the eutectic system of supercritical fluid, capping agent and catalyst, and the complete response mechanism of "auto-catalyzed ethanol pulping - pulping black liquor lignin separation - selective breaking and fixed point capping regulation - directional generation of target product" will be established, which will provide theoretical support for the depolymerization technology and reactor design and optimization.

乙醇制浆被认为是目前无污染制浆中最具工业化前景的方法，而限制其工业化过程的瓶颈是黑液中乙醇制浆木素的高值化利用问题。超临界流体解聚木素时具有流体热力学特性可调控、解聚产物收率高等优点，但超临界流体体系如何促进木素解聚、如何定向调控解聚产物组成、如何抑制逆转重聚发生等机制亟待明晰。本课题以乙醇制浆木素为原料，拟明确超临界流体促使乙醇制浆木素解聚成小分子片段反应历程及关键化学活点选择性断裂机制；拟明确解聚木素活性中间体易发生逆转重聚行为活性点位置及封端剂对其定点封端机制；拟明确重要金属催化木素选择性断键机制及调控解聚程度途径；建立超临界流体、封端剂、催化剂共溶体系下定向解聚生成目标产物的复杂调控网络路径，构建"自催化乙醇制浆--制浆黑液木素分离--选择性断键及定点封端调控--定向生成目标产物"完整应答机制，为解聚工艺及反应器设计和优化提供理论支撑。

近年来，利用生物质生产可再生燃料已经逐渐被视为一种有希望的能源战略，以期部分取代日益减少的不可再生化石能源。木质纤维原料三大主要成分（纤维素、半纤维素和木素）中纤维素和半纤维素转化为可替代燃料技术已趋于完善。但由于木素自身结构的复杂性和分子内较强的化学连接键，虽然其占地球总非化石类有机碳的30%，是为数不多的含有芳香族结构的可再生资源，但至今仍没有有效的转化手段将木素转化为高价值化学品和替代燃料。为了面对这一挑战，本项目以木素选择性催化制备芳香平台分子化合物为目标，以木素超临界溶剂解聚、递进式改性酸性ZSM-5催化解聚制备高价值酚类平台分子分布特性与规律、反应历程机制为核心，提出木素在溶剂中酸/碱催化制备芳香平台分子化合物的相关方法和理论，为高效、廉价催化木素解聚制备生物质基化学品提供理论基础。以杨木自催化乙醇制浆木素为原料，在不同超临界流体如丙酮、四氢呋喃、乙醇和乙醇/水中解聚木素制备芳香平台分子，发现超临界乙醇可以作为氢源直接参与解聚反应。综合不同超临界流体中酚类产物分布与得率，发现超临界乙醇/水（50%乙醇）体系比较适合木素解聚成单体目标产物，适宜的反应温度在300℃左右。采用递进式改性酸性ZSM-5研究思路，合成了含不同SiO2/Al2O3摩尔比HZSM-5、纳米改性HZSM-5、多级孔道改性HZSM-5和Co负载多级孔道HZSM-5等递进式系列催化剂。通过递进式HZSM-5系列改性解聚木素，可以得到不同产物分布和得率的酚类化合物，如HZSM-5中SiO2/Al2O3摩尔比为70时，G、S、H型结构的C4位没有侧链的产物（C6P）和C4位置有饱和或者未饱和的脂肪族侧链产物（A4P）得率较高；纳米改性HZSM-5和多级孔道改性的HZSM-5均具有更强的侧链加氢作用，对芳基醚键均裂后形成的单体C4位选择性加氢形成饱和脂肪族侧链（单体4-烷基产物）更有利；Co负载的多级孔道改性HZSM-5除了能产生较高得率的单体4-烷基产物外，还产生了大量的C4位侧链连有羰基类产物。从而提出了一种以递进式催化剂改性调控木素产物种类分布的新型调控路径。