同步辐射真空紫外光电离质谱研究碳氢燃料催化热裂解机理

Active or regenerative cooling technology with endothermic hydrocarbon fuels is one of crucial issues in the development of thermal protection system for hypersonic aircrafts. To meet the cooling requirement of Ma 6 flight, zeolite catalyst is generally taken as a crucial method to accelerate the pyrolysis and thus to obtain enough heat sink of hydrocarbon fuel. Consequently, catalytic pyrolysis mechanism of hydrocarbon fuel over the wall-coated zeolites has to be elucidated for maximizing heat sink of hydrocarbon fuel by controlling reaction pathway. .In this proposal we attempt to identify the products and radical intermediates in the catalytic pyrolysis of hydrocarbon fuels using synchrotron-radiation VUV photoionization mass spectrum (SVUV-PIMS), and then to elucidate several scientific problems relevant to the reaction mechanism by analyzing the reaction pathway of those species. Problems to be studied include as follows: establishing the experimental methodology for studying catalytic pyrolysis of hydrocarbon fuel with SVUV-PIMS technique, involving reaction temperature, catalyst loading and residence time, etc.; elucidating the roles of acidic sites (espcially Lewis acidic sites) of zeolites in catalyitc pyrolysis using n-docane and HZSM-5 as model compounds and catalyst; exploring the possible combination mechanism between surface catalytic cracking reactions and bulk radical pyrolysis reactions in catalytic pyrolysis of hydrocarbon fuel. .The objective of this proposal is to provide more information on the catalytic pyrolysis mechanism of hydrocarbon fuel in view of the advantages of SVUV-PIMS in detecting the radical intermediates, which is very important for the development of active cooling technology for hypersonic aircrafts. In this way the fruits of the research will be exposed to a broader audience in petroleum chemical industry, for example, producing ethylene from catalytic pyrolysis of C4 or heavy paraffic fractions.

吸热型碳氢燃料主动冷却技术是高超声速飞行器热防护的重大关键技术之一。为满足马赫6飞行冷却要求，利用催化剂加速裂解是提高碳氢燃料热沉的重要途径。其中，催化热裂解机理是认识和调控反应途径的必须解决的基础科学问题。本项目拟采用同步辐射真空紫外单光电离质谱（SVUV-PIMS）技术，跟踪催化热裂解产物与自由基中间体的历程，研究催化热裂解的复杂反应机制。主要内容包括：建立SVUV-PIMS研究碳氢燃料催化热裂解的实验方法；选用正癸烷和HZSM-5分子筛为模型燃料和催化剂，探索催化热裂解中分子筛酸位（特别是L酸位）的催化作用机理；研究碳氢燃料催化热裂解反应过程中表面催化反应与自由基反应两类复杂裂解机理的组合机制。本项目旨在利用SVUV-PIMS探测自由基中间体的优势，力图阐明碳氢燃料催化热裂解机理相关的基础科学问题，对于碳氢燃料主动冷却技术以及催化热裂解制乙烯等相关领域，具有重要的理论意义和应用价值。

应用SVUV-PIMS技术开展了exo-TCD在温度范围900-1600 K的低压热解实验。通过检测和鉴定热解过程中出的主要产物（自由基及部分异构体），结合量子化学理论计算，开发了包含316个物质和807个反应的exo-TCD低压热解详细反应动力学模型。该模型能够对低压实验数据进行预测，对比模拟预测和实验数据发现该低压模型具有很好的适用性。通过ROP和敏感性分析讨论了exo-TCD热解以及产物生成的主要途径。通过自主研发设计的高温固定床实验平台，系统研究了473-873K低压条件下，固体酸催化正戊烷裂解的反应机理。随着反应温度的不断升高，通过观察目标产物的出现顺序，确定了单分子质子化裂解路径的相对难易程度；考虑到反应路径中涉及到的物质的相对稳定性和浓度，确定了氢转移反应各个路径的相对重要性；与此同时，指出C4+和C5+碳正离子β裂解反应中，各个路径的相对重要性。基于正戊烷催化热裂解的反应机理分析，明确产物的主要生成路径，简化了正戊烷催化裂解的反应网络。在873-1073K低压条件下，开展正戊烷催化热裂解的相关实验研究。研究融合了自由基路径和碳正离子路径的催化热裂解反应机理。进一步的裂解反应动力学分析表明，催化热裂解的活化能接近191 KJ•mol-1，明显高于催化裂解所需要的活化能（109 KJ•mol-1）；在催化热裂解反应中，随着反应温度的不断升高，自由基路线不断得到强化，甚至可能会逐渐超越碳正离子路径，占据主导地位；此外，通过调节反应温度和反应空速，可使自由基路径和碳正离子路径之间产生协同效应，且在参数kc/kt接近2时最为显著。