HZSM-5分子筛上乙基叔丁基醚合成反应的理论研究

采用ONIOM (B3LYP/6-31G(d,p):UFF)分层计算的方法, 研究了HZSM-5 分子筛上乙醇和异丁烯合成乙基叔丁基醚(ETBE)的反应机理. 通过反应物在HZSM-5 分子筛上吸附性质的研究发现, 乙醇与分子筛酸性位相互作用形成氢键, 而异丁烯则作用在Brönsted 酸位上形成π配位吸附. 确定反应物吸附位置后, 进一步探索反应机理, 结果表明: HZSM-5分子筛上乙醇和异丁烯合成乙基叔丁基醚的反应为协同反应, 并且, 反应物吸附顺序的不同对反应过程存在一定的影响. ETBE合成反应的最优途径以反应物同时吸附形成的复合物作为起点. 在反应过程中, 形成π配位的H原子向异丁烯分子中不饱和双键的端位C原子靠近, 被吸附的乙醇分子中的O原子向异丁烯双键中的另一个C原子靠近, 直到形成C－O键, 生成ETBE. 这一过程中, 原有的质子H加成到异丁烯的端基C上形成C－H键, 而原醇羟基中的H和B酸位附近的O原子作用形成新的酸性位. 相应的协同反应的最低的反应势垒为25.14 kJ·mol^-1.

Theoretical Study on the Synthesis of Ethyl Tertiary Butyl Ether over HZSM-5 Zeolite

The formation mechanism of ethyl tertiary butyl ether (ETBE) from ethanol and isobutene catalyzed by HZSM-5 has been investigated using the ONIOM (B3LYP/6-31G(d,p):UFF) method. The calculation results of the reactants adsorbability reveal that the interaction between ethanol and the acidic sites on HZSM-5 leads to the formation of hydrogen bonds. The interaction between isobutene and Bro nsted acidic sites leads to the formation of a π-complex. It is subsequently found that the mechanism of the ETBE formation from ethanol and isobutene catalyzed by HZSM-5 is a concerted reaction, and that the order of reactant adsorption onto HZSM-5 affected the reaction. The favorable pathway is based on the complex formed by the simultaneous adsorption of ethanol and isobutene, in which the H atom of the π-complex is transferred to the C atom of the C=C in isobutene, and the O atom of the adsorbed ethanol is transferred to the other C atom of the C=C to form the C-O bond. In this process, the proton of the acidic sites adds to the C=C bond forming the C-H bond, and the H atom of the ethanol hydroxyl interacts with acidic sites, generating a new proton. The corresponding lowest energy barrier was 25.14 kJ · mol -1 .