CO_(2)作为化学品原料的高效利用途径:与烷烃耦合反应

An efficient way to use CO_(2)as chemical feedstock by coupling with alkanes

CO_(2)作为碳资源的规模化高附加值利用是实现其减排的重要方向.然而,由于其热力学稳定,以CO_(2)为原料高效转化为大宗化学品一直是一个巨大的挑战.工业上普遍以富氢的石脑油为原料生产相对缺氢的烯烃和芳烃产品,但其存在原料和目标产品之间的碳氢不平衡问题.理论上,采用CO_(2)与富氢的烷烃耦合,可以改善二者的平衡关系,提高目标产物选择性,同时实现CO_(2)资源化利用.已有研究采用CO_(2)与烷烃反应,将CO_(2)转化为CO并减少氢气的生成,但CO_(2)的碳原子没有进入烃类产物中.本文系统研究了酸性分子筛催化的CO_(2)与烷烃耦合反应,大幅提高了芳烃选择性,证实部分CO_(2)中的碳原子直接进入了芳烃产品中.本文以H-ZSM-5为催化剂,对比研究了正丁烷、正戊烷和正己烷在He和CO_(2)气氛中的转化反应,并详细研究了反应温度、CO_(2)/n-butane比例、接触时间以及分子筛酸量等条件对耦合反应的影响.结果表明,CO_(2)的引入可大幅促进芳烃的生成,同时甲烷和乙烷等小分子烷烃的生成受到抑制.在优化反应条件下,CO_(2)/n-butane比例为0.475时,CO_(2)和n-butane转化率分别可达17.5%和100%,芳烃选择性高达80%.使用^(13)C同位素标记的CO_(2)与n-butane进行耦合反应,发现芳烃产物中含有部分13C同位素标记的碳原子,表明这部分碳原子来自CO_(2)中的碳.对耦合反应后的催化剂进行溶积碳并采用色质谱分析,发现大量甲基取代的内酯和甲基取代的环烯酮等含氧物种.同位素标记实验结果表明,这些含氧中间体由CO_(2)与烃类耦合转化生成.通过设计实验验证了反应途径,即CO_(2)与碳正离子反应得到环内酯,环内酯进一步转化为甲基环烯酮,甲基环烯酮转化为芳烃产物.提出了H-ZSM-5催化CO_(2)与烷烃耦合反应的机理,并采用密度泛函理论计算了耦合反应机理各步骤的能垒,结果验证了耦合反应机理的可行性.综上,本文提出的耦合反应为CO_(2)碳资源的大规模直接利用提供了一条有效的途径,具有广阔的应用前景.

The most promising method to eliminate CO_(2)is to find large‐scale and value‐added applications of CO_(2)as a carbon resource.However,the utilization of CO_(2)as feedstock for basic chemicals has long been a great challenge owing to its high thermodynamic stability.Herein,we report the coupling conversion of CO_(2)with light alkanes over the HZSM‐5 zeolite with much higher aromatic selectivity than light alkanes as the only reactant.A CO_(2)conversion of 17.5%and n‐butane conversion of 100%with aromatic selectivity of 80%could be achieved by the coupling reaction at the CO_(2)to n‐butane ratio of 0.475,in which CO_(2)not only acted as an agent for balancing hydrogen in the reaction but also partly(~25%)incorporated into the aromatic products.Methyl‐substituted lactones(MLTOs)and methyl‐substituted cycloalkenones(MCEOs)were identified as key intermediates during the coupling reaction.13C isotope labeling experiments,^(13)C solid‐state NMR,in‐situ diffuse reflectance infrared Fourier transform spectroscopy,and density functional theory(DFT)calculations revealed that CO_(2)could react with carbonium ions generated from alkane cracking to form MLTOs,which could further get converted into MCEOs,thus generating aromatic compounds.This coupling reaction provides guidance for the direct utilization of CO_(2)to produce value‐added chemicals with the simultaneous transformation of light alkanes.