催化学报 ›› 2020, Vol. 41 ›› Issue (12): 1906-1915.DOI: 10.1016/S1872-2067(20)63627-0

• 论文 • 上一篇    

CeO2(110)还原表面上受阻路易斯酸碱对在合成气直接转化中的作用

黄正清, 李腾昊, 杨伯伦, 常春然   

  1. 西安交通大学化学工程与技术学院, 陕西省能源化工过程强化重点实验室, 陕西西安 710049
  • 收稿日期:2020-03-07 修回日期:2020-04-15 出版日期:2020-12-18 发布日期:2020-08-14
  • 通讯作者: 常春然
  • 基金资助:
    国家自然科学基金(91645203,21603170);中国博士后科学基金(2018T111034);中央高校基本科研业务费(xtr0218016,cxtd2017004);陕西省科技创新团队项目(2019TD-039);王宽诚教育基金会;陕西省高校科协青年人才托举计划.

Role of surface frustrated Lewis pairs on reduced CeO2(110) in direct conversion of syngas

Zheng-Qing Huang, Teng-Hao Li, Bolun Yang, Chun-Ran Chang   

  1. Shaanxi Key Laboratory of Energy Chemical Process Intensification, School of Chemical Engineering and Technology, Xi'an Jiaotong University, Xi'an 710049, Shaanxi, China
  • Received:2020-03-07 Revised:2020-04-15 Online:2020-12-18 Published:2020-08-14
  • Supported by:
    This work was supported by the National Natural Science Foundation of China (91645203, 21603170), the China Postdoctoral Science Foundation (2018T111034), the Fundamental Research Funds for the Central Universities (xtr0218016, cxtd2017004), the Shaanxi Creative Talents Promotion Plan-Technological Innovation Team (2019TD-039), the K. C. Wong Education Foundation and the Young Talent Fund of University Association for Science and Technology in Shaanxi.

摘要: 随着石油资源的日益枯竭,寻找非石油路线生产低碳烯烃的新途径显得十分重要.将天然气、煤以及生物质等经合成气(H2和CO)转化为低碳烯烃是一条具有前景的路线.近年来,双功能氧化物-分子筛(OX-ZEO)催化剂催化合成气直接制备低碳烯烃引起了国内外的广泛关注.由于CO活化并生成中间物种与C—C偶联分别在氧化物和分子筛上发生,OX-ZEO过程突破了费托合成中ASF产物分布的限制,低碳烯烃选择性显著提高.虽然实验方面已经取得了大量进展,但是OX-ZEO过程仍然存在一些关键问题,特别是金属氧化物中氧空位的作用,以及关键中间体是乙烯酮或甲醇的反应机理仍然不清楚.因此,本文通过密度泛函理论(DFT)计算来解决上述两个问题,对典型的可还原金属氧化物CeO2表面上的合成气直接转化进行了理论研究.
计算结果表明,CeO2(110)表面上的氧空位通过形成受阻路易斯酸碱对(FLP),在活化H2和CO中起着关键作用.H2在FLPs上经过异裂分解,形成与O原子结合的质子以及与Ce原子结合的氢负离子,其反应活化能仅为0.01eV,并且稳定在FLP位点的氢负离子将是CO加氢的关键活性物种.在FLP上,CO通过与FLP的碱性位点(O原子)结合形成CO22-实现活化,其吸附能为-1.68eV.随后,我们在FLP上探索了四种合成气的转化途径,其中两种容易形成乙烯酮,另外两种倾向于产生甲醇,该结果恰好可以解释实验上关于此两种中间产物的报道.通过计算反应条件下的反应速率常数,我们对四条反应路径进行了比较,发现从两个CO*经过表面C—C偶联形成OCCO*,并最终形成乙烯酮是最占优势的反应路径,并且在FLPs位点上CO*或CHO*的C—C偶联比CH2O*的加氢更容易发生.此外,我们发现相邻的双FLP位点在表面C—C偶联形成乙烯酮反应中至关重要,主要由于相邻的双FLP位点可以使CO*或CHO*同时吸附,因而有助于表面C—C偶联发生.总之,本文首次揭示了表面FLP或氧空位在活化H2和CO中的作用,以及形成乙烯酮或甲醇的反应机理,从而为OX-ZEO催化剂催化合成气直接转化提供了机理认识.

关键词: 合成气转化, 低碳烯烃, 氧化物-分子筛, 受阻路易斯酸碱对, CeO2

Abstract: Direct syngas conversion to light olefins on bifunctional oxide-zeolite (OX-ZEO) catalysts is of great interest to both academia and industry, but the role of oxygen vacancy (Vo) in metal oxides and whether the key intermediate in the reaction mechanism is ketene or methanol are still not well-understood. To address these two issues, we carry out a theoretical study of the syngas conversion on the typical reducible metal oxide, CeO2, using density functional theory calculations. Our results demonstrate that by forming frustrated Lewis pairs (FLPs), the VOs in CeO2 play a key role in the activation of H2 and CO. The activation of H2 on FLPs undergoes a heterolytic dissociative pathway with a tiny barrier of 0.01 eV, while CO is activated on FLPs by combining with the basic site (O atom) of FLPs to form CO22-. Four pathways for the conversion of syngas were explored on FLPs, two of which are prone to form ketene and the other two are inclined to produce methanol suggesting a compromise to resolve the debate about the key intermediates (ketene or methanol) in the experiments. Rate constant calculations showed that the route initiating with the coupling of two CO* into OCCO* and ending with the formation of ketene is the dominant pathway, with the neighboring FLPs playing an important role in this pathway. Overall, our study reveals the function of the surface FLPs in the activation of H2 and CO and the reaction mechanism for the production of ketene and methanol for the first time, providing novel insights into syngas conversion over OX-ZEO catalysts.

Key words: Syngas conversion, Light olefins, Oxide-zeolite, Frustrated Lewis pairs, CeO2