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 Bronsted acidic sites leads to the formation of a pi-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 pi-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).