非平衡等离子体与催化协同常温一步转化CH4/CO2制乙酸及机理研究

CH4 and CO2 are not only the main greenhouse gases, but also the key feedstocks for C1 chemistry. Currently, the effective utilization of CH4 and CO2 have turned into a hot and challenging issue. Direct conversion of CH4 and CO2 to acetic acid (CH3COOH) is an ideal reaction with 100% atomic efficiency. At low temperature, however, this reaction suffers double restrictions from dynamics and thermodynamics due to excellent stability of CH4 and CO2 molecules. At high temperature, however, production of CH3COOH is restrained since it is more active than feed gas. In this project, we propose a strategy of introducing a non-volume work into feed gas, i.e., synergy between non-equilibrium plasma and catalyst, to promote CH4/CO2 reaction for CH3COOH at room temperature. However, low conversion (CH4 and CO2) and unsatisfied selectivity (CH3COOH) are obtained in preparatory experiment, which are mainly caused by poor activity of catalysts at room temperature. To overcome this key problem, this project focuses on design of highly active catalysts through bifunctional construction by combining metal sites and acid sites and dielectric regulation by introducing materials with varied dielectric constant. Combining with reaction performance, a structure-activity relationship of catalyst with reaction will be obtained, and it will guide further optimization of catalyst to enhance the synergy between plasma and catalyst and to improve catalytic performance. In addition, during CH4/CO2 reaction process, in situ diagnostics (e.g., OES, DB-FTIR) will be used to detect and monitor the intermediates in plasma zone and on catalysts surface, which are crucial to reveal the quantity-activity relationship of intermediates with reaction performance. Finally, the reaction mechanism will be clarified based on the structure-activity relationship and the quantity-activity relationship.

CH4和CO2均是温室气体，也是一碳化学重要原料，两者有效利用是当前研究热点和难点。由CH4和CO2出发制乙酸是原子利用率100%的理想反应，但因两者化学性质稳定，低温反应受动力学和热力学双重限制，而高温反应不利于乙酸生成。为了突破动力学和热力学双重限制，本项目提出向反应体系引入非体积功的策略，即利用非平衡等离子体和催化剂协同作用，常温一步转化CH4和CO2制乙酸，但因低温时催化剂活性低，导致原料转化率和乙酸选择性低。为攻克该关键问题，本项目兼顾催化剂催化功能和电化学性，重点研制金属中心和酸中心协同的双功能催化剂，调控其介电常数，建立其组成结构与反应性能的构效关系，指导催化剂优化，增强催化剂与等离子体协同效应，提高原料转化率和乙酸选择性；利用发射光谱、双光束红外光谱等技术原位诊断等离子体区非稳态物种和催化剂表面吸附态物种，建立活性物种与反应性能的量效关系，再结合构效关系阐明反应机理。

CH4和CO2是大气中主要温室气体，也是一碳化学与化工重要原料，由两者出发直接制乙酸是原子经济性100%的理想反应，是学术界和工业界关注的焦点和难点。然而，该反应处于起步阶段，相关催化剂研究极少，并且催化活性位不明。本项目以外场强化常规催化过程为技术手段，突破反应热力学限制，成功在温和条件下将CH4和CO2一步转化为乙酸。项目聚焦等离子体催化CH4/CO2制乙酸的双功能催化剂研发、放电反应器结构优化和添加气研究。通过双功能催化剂反应性能评价及其物化性质分析和原位等离子体区活性物种鉴别，揭示催化剂活性组分价态（Cu和Au）、酸性质（B酸和L酸）、催化剂制备方法、放电反应结构（板板式和线筒式）和添加气（H2O）显著影响反应性能。具体是，（1） 在铜基催化剂和金基催化剂中，Cu+和Au3+有利于乙酸生成，Cu2+和Au+有利于醇类生成；（2）HZSM-5分子筛具有高浓度B酸中心显著促进乙酸生成，13X分子筛只有L酸中心则显著促进醇类生成；（3）金属-B酸中心协同的双功能催化剂是等离子体催化CH4/CO2制乙酸催化剂得最佳活性位构成，离子交换法是Cu/HZSM-5和Au/HZSM-5催化剂的最优制备方法；（4）线筒式放电反应器对乙酸生成有利，板板式反应器和添加气H2O显著抑制乙酸生成，但大幅度提高醇类选择性；（5）在最优条件下，获得的CH4和CO2转化率分别为23%和27%，液态产物总选择性50%，其中乙酸选择性约为30%。（6）基于反应性能与催化剂物化性质间构效关系，本项目提出了四种乙酸生成路径。相关研究为后续等离子体催化CH4/CO2制乙酸应用研究提供重要理论基础，为温和条件下CH4和CO2利用提供一种新途径。