“Lignin-first”策略下镁碱催化原生木质素定向氧化为小分子有机酸的机制研究

The feasibility of fractionation and valorization of protolignin in lignocellulosic biomass determines the economy and sustainability of the overall utilization of biomass. Simultaneous separation and degradation of protolignin to low-molecular-weight organic acids such as formic acid and acetic acid by oxidation is a green and facile process design. But this process is hindered by serious side reactions; for instance, inert carbon-carbon bond condensation and deep oxidation to low-value carbon dioxide. According to the lignin-first strategy and the technique of cooking with active oxygen and solid alkali (CAOSA), a novel process for the selective oxidation of protolignin into low-molecular-weight organic acids is developed for which the proposed Magnesium-based catalysts can supply the reaction system with a regulated micro-alkaline environment. Initially, the Magnesium-based catalysts will be designed, prepared, characterized and screened to reveal the correlation between various preparation conditions of catalysts and their surface morphologies, chemical compositions, alkali site types and strength. Subsequently, interactions between degradation products selectivity and properties of Magnesium-based catalyst as well as structures of protolignin are going to be elucidated by monitoring structural changes of lignin during delignification process, investigating the distribution and evolution of products, and analyzing breaking mechanism of bonds for model molecules with a simplified lignin structure. Eventually, on the basis of the calculation of theoretical simulation and the kinetic models, the mechanism of Magnesium-based catalyst to inhibit inert condensation and deep oxidation pathways will be proposed. The following catalyst redesigning and reaction system regulating will be explored to optimize the selectivity of oxidative protolignin separation. The present study can potentially provide the theoretical references for the lignin-first approach to full utilization of lignocellulosic biomass.

原生木质素的分离和转化效率决定着生物质整体利用的经济性和可持续性，将其同步氧化分离并转化为小分子有机酸是一条相对绿色且简洁的思路，但如何抑制氧化分离过程中惰性碳碳键缩合和深度氧化为二氧化碳途径是难点。基于“lignin-first”策略和固体碱活性氧蒸煮技术，本项目拟在镁碱催化的可控微碱环境中将原生木质素定向氧化为小分子有机酸。首先通过对镁碱的设计、合成、表征和筛选，明确催化剂制备条件与其形貌、组成、碱位点类型以及碱强度关系；接着通过表征反应过程中木质素结构变化，分析小分子降解产物分布与演变规律，解析具有典型结构的木质素模型分子断键机制，进而把握镁碱特性与木质素结构以及反应选择性之间的联动关系；最后结合理论模拟计算与动力学模型，阐明镁碱催化对惰性缩合和深度氧化反应途径的抑制机制，探索催化剂再设计和反应体系调控方法，以期提高原生木质素氧化分离转化选择性，为木质素优先利用提供思路和理论参考。

微碱环境氧化脱木质素是一种相对绿色高效的木质素组分分离转化思路，本项目在新型固体碱活性氧水相蒸煮技术基础上，通过处理微碱制浆废液得到甲酸乙酸，成功实现了惰性缩合木质素的氧化利用，大幅提高了以甲酸和乙酸为代表的小分子有机酸得率，甲酸和乙酸的产率最高基于竹粉干重产率为21.5%，基于黄液中总碳的产率为57.0%，基于原生木质素（27.6%）的产率为77.7%；借助模型物反应的手段，模拟木质素不同化学结构和化学键在微碱环境下的反应过程，探明了氧化分离过程中碳碳键惰性缩合反应和深度氧化为二氧化碳反应途径的抑制机制，提出了微碱环境下木质素非酚结构的两种独特降解途径，结果表明，在微碱性条件下，非酚类结构的降解程度（1%-2%）低于酚类结构（3%-20%），但确实会发生降解；通过微碱弱化水热法高选择抽提半纤维素木聚糖，显著提高半纤维素组分分离选择性和利用附加值的同时，降低脱木质素固体碱用量，还可联产户外重组竹板，结果表明，可以控制竹材中木聚糖的水解，并保持竹材的力学性能，采用适当的碱抑制方法，木聚糖的最大产率可达16.8%，与炭化竹板相比，脱糖竹板的关键力学性能显著提高2～4倍，在相同条件下，脱糖竹板的防腐防霉性能优于炭化竹板。

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