高等学校化学学报 ›› 2021, Vol. 42 ›› Issue (9): 2878.doi: 10.7503/cjcu20210282

• 物理化学 • 上一篇    下一篇

电中性团簇MCu2Ox(M=Cu2+, Ce4+, Zr4+)上甲烷和二氧化碳直接合成乙酸的理论研究

钟声广, 夏文生(), 张庆红, 万惠霖   

  1. 厦门大学化学化工学院, 固体表面物理化学国家重点实验室, 醇醚酯化工清洁生产国家工程实验室, 福建省理论与计算化学重点实验室, 厦门 361005
  • 收稿日期:2021-04-25 出版日期:2021-09-10 发布日期:2021-09-08
  • 通讯作者: 夏文生 E-mail:wsxia@xmu.edu.cn
  • 基金资助:
    国家重点研发计划项目(2019YFE04400);国家自然科学基金(21373169);教育部创新团队发展计划项目(IRT1036)

Theoretical Study on Direct Conversion of CH4 and CO2 into Acetic Acid over MCu2Ox(M = Cu2+, Ce4+, Zr4+) Clusters

ZHONG Shengguang, XIA Wensheng(), ZHANG Qinghong, WAN Huilin   

  1. State Key Laboratory of Physical Chemistry of Solid State Surface,National Engineering Laboratory for Green Chemical Productions of Alcohols?Ethers?Esters,Key Laboratory of Theoretical and Computational Chemistry of Fujian Province,College of Chemistry and Chemical Engineering,Xiamen University,Xiamen 361005,China
  • Received:2021-04-25 Online:2021-09-10 Published:2021-09-08
  • Contact: XIA Wensheng E-mail:wsxia@xmu.edu.cn
  • Supported by:
    the National Key R&D Program, China(2019YFE04400);the National Natural Science Foundation of China(21373169);the Program for Innovative Research Team in University of Ministry of Education, China(IRT1036)

摘要:

采用密度泛函理论(DFT)方法研究了电中性团簇MCu2Ox(M=Cu2+, Ce4+, Zr4+; x=3, 4)的特性及其对甲烷和二氧化碳直接合成乙酸反应的影响. 结果表明, 团簇催化的反应由甲烷C—H活化、 二氧化碳插入引起C-C偶联、 CH3COO转向和氢迁移4步构成. 前两步为关键步骤, C—H和C-C各自与团簇活性位点间形成四中心结构并推动反应进行. 电子自甲烷流出到团簇, 再流入二氧化碳, 使甲烷的C—H和二氧化碳的C=O得以活化, 继而驱动C-C偶联. Ce, Zr引入至氧化铜团簇中后, 团簇由原有的六元环结构衍变为六元环Ⅰ、 掺杂原子分别位于中心和端末的双四元环Ⅱ和Ⅲ 3种结构. 团簇结构和电子自旋均会影响反应的进行. 低自旋团簇有利于甲烷 C—H活化, 而高自旋团簇则有利于C-C偶联; 在3种掺杂团簇结构中, 处于三重态的结构Ⅲ团簇可以较好地兼顾C—H活化和C-C偶联. 通过比较相同结构发现, Ce, Zr掺杂调变了氧化铜团簇活性位点的局域电荷, 虽使其对甲烷C—H活化的能力略有下降, 但却显著降低了C-C偶联反应的活化自由能垒, 从而促进了反应的进行. 掺杂原子Zr的助剂作用比Ce要大.

关键词: 氧化铜改性, 甲烷, 二氧化碳, 乙酸, 密度泛函理论

Abstract:

Transformation and utilization of methane and carbon dioxide are important to chemical industry of natural(shale) gas and environmental protection. The properties of electroneutral clusters MCu2Ox(M=Cu2+, Ce4+, Zr4+x=3, 4) and their influences on direct conversion of methane and carbon dioxide to acetic acid were investigated by means of density functional theory(DFT). The clusters-catalyzed reaction consists of C—H activation of methane, C-C coupling by carbon dioxide insertion, CH3COO rotation, hydrogen transfer. The first 2 steps are essential to the proceeding of the whole reaction, in which C—H and C-C interacted with the active sites of the clusters, respectively, leading to the formation of 4-centered transition states. During this process, electrons transferred to the clusters from methane, and then transferred to carbon dioxide, which activated both C—H bond of CH4 and C=O bond of CO2, and then drove C-C coupling. With introducing of Ce or Zr, the clusters displayed the structure not only with 6-membered ring(I), but also double 4-membered ring with Ce or Zr located in the center(II) or end(III). The structures and electron spin states of the clusters are associated with the reactivity. It was found that the clusters with low spin states preferred to C—H activation, and the ones with high spin states tended to C-C coupling. Among the doped clusters with three types of structures, the cluster III at triplet state well matches C—H activation with C-C coupling in the targeted reaction. Comparing the clusters with/without doping for the same structure and electron spin state, the local charge at active sites was changed with the doping of Ce or Zr into copper oxide clusters. This weakened slightly the ability of the clusters in activating C—H bond of methane, but decreased significantly Gibbs free barrier for C-C coupling, and then drove well the overall reaction. The doped Zr showed better promoting roles than Ce in this process.

Key words: Modified copper oxide, Methane, Carbon dioxide, Acetic acid, Density functional theory

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