【作者】 李旭；

【导师】 马隆龙；

【作者基本信息】 中国科学技术大学 ， 动力工程及工程热物理， 2023， 博士

【摘要】 在碳中和背景下,促进能源供给与生态环境的协调发展是各学科领域关注的重要课题。生物质的高效转化是推动能源结构向清洁低碳转型的有效途径之一。木质素是木质纤维素生物质三大组分中唯一具备芳环结构的化合物,具有制备高附加值化学品、高性能燃料和新型环保材料的潜力。因此,开展木质素脱除、解聚及衍生物转化的全链条利用技术研究,对于促进生物质全组分综合利用十分重要。天然木质素化学结构复杂,在预处理脱除过程中易发生改变,影响后续催化解聚转化。而当前的预处理研究更侧重于高效分离体系的研发,关于木质素在微观尺度下的脱除规律探索并不充分,对预处理机理认识不深刻,无法形成系统的理论作为支撑指导后续高值利用。此外,木质素是苯基丙烷单元通过C-O键和C-C键连接构成的三维高分子聚合物,具有丰富的含氧官能团,对木质素酚类衍生物进行催化加氢脱氧精炼的过程通常在高温高压和强还原剂参与下进行,这不利于工业大规模应用和环保需求。设计经济环保的催化体系实现木质素解聚以及木质素酚类衍生物的低温高效转化,是具有重要价值和深刻意义的研究课题。针对上述问题,本文以常见的生物质资源玉米秸秆为原料,开展有机溶剂预处理时木质素在细胞壁中的迁移规律研究,深化对预处理机理的认识。开发高效催化体系将分离出的有机溶剂木质素解聚为木质素衍生酚,进一步加氢脱氧为高附加值产品。主要内容及结论如下:首先以玉米秸秆为原料,开展了酸性二氧六环有机溶剂体系脱除木质素的研究,得到了预处理中木质素微区迁移规律与化学结构变化的关联机制。结果表明预处理温度为70℃时,木质素的脱除优先从次生细胞壁开始,反应进行到75 min,脱除率变化明显,在5min内脱除率增加10.8%,木质素的脱除发展到复合包间层。木质素化学结构特征在反应75 min前后变化明显,包括单体组成（S/G）、连接键数量（β-O-4、β-β、β-5）以及分子量,说明木质素在细胞壁中的分布差异导致其脱除难易程度的不同,这使预处理不同阶段回收的木质素化学结构具有差异。为验证分离得到的木质素结构单元变化趋势及解聚特性,本文设计Pt/C-杂多酸协同体系催化解聚分离得到的有机溶剂木质素（DOLs）。木质素中典型的C-O结构（4-O-5、α-O-4和β-O-4）能够在Pt/C协同磷钨酸的催化作用下断键。在240℃、2 h的反应条件下,对二氧六环预处理60 min和120 min后回收的木质素（DOLs-60和DOLs-120）进行解聚,单体收率分别为15.9%和17.4%,说明DOLs具有较高的反应活性,可以被解聚为大量单体。此外,DOLs-60解聚产物中G单元的相对含量较高,说明解聚产物的单体种类与DOLs结构有关。在完成木质素解聚的基础上,本文围绕木质素酚类衍生物的加氢脱氧转化开展了进一步研究。开发出有机基团修饰的TiO₂（B）纳米片,成功制备了5%Pt/TiO₂（B）催化剂,在180℃、1MPa 10%H₂/N₂条件下实现了丙基苯酚的完全转化,丙苯选择性为73%。通过与不同载体的Pt基催化剂进行对比,发现载体能够影响Pt的粒径大小进而影响催化剂对底物选择性脱氧的活性。Pt/TiO₂（B）表面的Pt纳米颗粒平均粒径为1.49 nm,同时存在孤立单原子,说明TiO₂（B）上的有机基团有利于Pt颗粒的高度分散,能够协助提高Pt/TiO₂（B）的活性。此外,金属-载体强相互作用促进了 Ti³⁺缺陷的产生,有助于丙基苯酚的高效催化脱氧。本研究为通过修饰载体提高催化剂催化活性提供了新思路。对以Pt/TiO₂（B）催化剂为核心的催化脱氧体系进一步优化,建立了近室温条件下的酚类化合物催化加氢脱氧制烷烃反应体系。开发了表面Pt物种存在状态不同的Pt/TiO₂（B）催化剂,在50℃、1 atm H₂的条件下实现了苯酚到环己烷和环己醇的完全转化。发现Pt纳米颗粒和Pt单原子同时存在最有利于苯酚加氢脱氧。对反应进行原位监测,发现TiO₂（B）载体上稳定的有机基团有利于关键中间体的生成。通过DFT方法计算基元反应能垒,结合中间体的验证实验推断出可能的反应机理。即通过键能较低的烯醇中间体C-O键断裂实现温和条件下苯酚的加氢脱氧。本研究开发的催化体系为木质素酚类衍生物高效加氢脱氧提供节能环保的方案。更多还原

【Abstract】 Promoting the coordinated development of energy supply and ecological environment under the context of carbon is an important issue of concern in various fields and disciplines.The efficient conversion of biomass is an effective pathway for promoting the transition of energy structure towards clean and low-carbon.Lignin is the only compound with an aromatic ring structure among the three major components of lignocellulosic biomass,and it has the potential to produce high value-added chemicals,high-performance fuels,and new environmentally friendly materials.Therefore,conducting comprehensive research on lignin removal,depolymerization,and derivative conversion to enhance the overall utilization value of lignin components is essential to promote the development of biomass resource utilization technologies.The chemical structure of natural lignin is complex and prone to alteration during the pretreatment process,which affects the subsequent catalytic depolymerization conversion.Current pretreatment research is more focused on the development of efficient separation systems,and there is insufficient exploration of the removal laws of lignin at the microscopic scale.Moreover,there is a lack of profound understanding of the pretreatment mechanisms,resulting in a failure to establish a systematic theory to guide the subsequent high-value utilization.In addition,lignin is a three-dimensional polymer composed of phenylpropane units connected by C-O bonds and C-C bonds,with abundant oxygen-containing functional groups.The catalytic hydrodeoxygenation refining of lignin phenolic derivatives is usually carried out under high temperature,high pressure,and strong reducing agents,which is not conducive to industrial largescale applications and environmental protection requirements.Therefore,designing an economically and environmentally friendly catalytic system to achieve the depolymerization of lignin macromolecules and the low-temperature and highefficiency conversion of lignin phenolic derivatives is a research topic of great value and significance.To address the aforementioned issues,this paper focuses on the common biomass resource of corn stover and conducts research on the migration patterns of lignin in the cell wall during organic solvent pretreatment,in order to deepen our understanding of the pretreatment mechanism.Additionally,an efficient catalytic system is developed to depolymerize the lignin separated by the organic solvent into lignin-derived phenols,which are further hydrogenated and deoxygenated into high value-added products.The main content and conclusions are presented as follows.Firstly,using corn stalks as raw material,the removal of lignin in an acidic dioxane/water solvent system was studied,and the correlation mechanism between the migration pattern of lignin in the cell wall microstructure and chemical structure changes during pretreatment was obtained.Results showed that at a pretreatment temperature of 70℃,the removal of lignin firstly occurred in the secondary cell wall and became significant after 75 min,with a 10.8%increase in removal rate in the first 5 minutes.The removal of lignin progressed towards the compound middle lamella.Through experimentation,changes in lignin chemical structure characteristics were observed at 75 min,including monomer composition（S/G）,linkage numbers（β-O-4,β-β,β-5）,and molecular weight.It was further explained that the differential distribution of lignin in the cell wall led to different degrees of removal difficulty,resulting in differences in the chemical structure of recovered lignin at different stages of pretreatment.To verify the trends in lignin structural unit changes and depolymerization properties,this study designed Pt/C-heteropolyacid synergistic system to catalyze the depolymerization of lignin recovered by organic solvents pretreatment（DOLs）.The typical C-O structures（4-O-5,α-O-4,and β-O-4）can be cleaved under the catalytic of Pt/C and phosphotungstic acid.Under the reaction conditions of 240℃ and 2 h,DOLs60 and DOLs-120 obtained by recovering lignin after pretreating with acidic dioxane for 60 min and 120 min were depolymerized.The monomer yields were 15.9%and 17.4%,respectively,indicating that DOLs have high reaction activity and can be depolymerized into a large number of monomers.In addition,the higher relative content of G unit in the depolymerization product of DOLs-60 proved that the types of monomers in the depolymerization product are related to the structure of DOLs.Building on the completion of lignin depolymerization,this paper further investigates the hydrodeoxygenation（HDO）of lignin phenolic derivatives.An organic group-modified TiO₂（B）nanosheet was developed and used to prepare the 5%Pt/TiO₂（B）catalyst,which achieved complete conversion of propylphenol at 180℃ and 1 MPa 10%H₂/N₂,with a selectivity to propylbenzene of 73%.Comparison with Ptbased catalysts on different supporters confirmed that the supporter can affect the size of Pt particle,thus influencing the catalytic activity for HDO.The Pt nanoparticles on the surface of Pt/TiO₂（B）had an average particle size of 1.49 nm and isolated single atoms,demonstrating that the organic groups on TiO₂（B）favored the high dispersion of Pt particles and improved the activity of Pt/TiO₂（B）.It was also found that the strong metal-supporter interaction promoted the formation of Ti³⁺ defects,which facilitated the efficient catalytic deoxygenation of propylphenol.This study provides a new avenue for enhancing the catalytic activity of catalysts by modifying the supporter.Based on the previous chapter,we further optimized the catalytic deoxygenation system with Pt/TiO₂（B）catalyst as the core and established a catalytic HDO system for phenolic compounds under near-room temperature conditions.Complete conversion of phenol to cyclohexane and cyclohexanol was achieved under 50℃ and 1 atm H₂.Preparation of Pt/TiO₂（B）catalyst with different states of surface Pt species demonstrated that coexistence of nanoparticles and single atoms was most favorable for the Pt species involved in phenol HDO.In-situ monitoring of the reaction confirmed that stable organic groups on TiO₂（B）support were favorable for the generation of key intermediates.Possible reaction mechanisms were inferred through DFT calculations of elementary reaction barriers and validation experiments of intermediates.Specifically,phenol hydrogenation deoxygenation under mild condiction was achieved by breaking the C-O bond of low-energy enol intermediates.This catalytic system provides an energy-saving and environmentally friendly solution for efficient HDO of lignin-derived phenolic.更多还原