Theoretical investigation of the olefin cycle in H-SSZ-13 for the ethanol-to-olefins process using ab initio calculations and kinetic modeling

Abstract

The formation of the hydrocarbon pool (HCP) in the ethanol-to-olefins (ETO) process catalyzed by H-SSZ-13 is studied in a kinetic model with ab initio computed reaction barriers. Free energy barriers are computed using density functional theory (DFT) and post-Hartree–Fock methods with a complete basis set extrapolation applied to a hierarchy of periodic and cluster models. The kinetic model includes ethanol (EtOH) dehydration to ethene as well as olefin ethylations up to hexene isomers and the corresponding cracking reactions. Ethylation of ethene and of products thereof leads only to even-numbered olefins, while cracking can lead to propene and thus initiate the formation of olefins with an odd number of carbon atoms. During EtOH dehydration at 473.15 K we observe diethyl ether (DEE) formation for a short period of time where the DEE selectivity decreases monotonically with increasing EtOH conversion. At 673.15 K we find that EtOH dehydration occurs much faster than ethylation of the formed ethene, which takes considerably longer due to higher free energy barriers. Hexene isomers form on the same time scale as butene, where branched isomers are favored with 2-methyl-pentene isomers contributing most to the formation of propene through cracking. As in the methanol-to-olefins (MTO) process, the most relevant alkylation pathway is the stepwise mechanism via surface alkoxy species (SAS) on the zeolite catalyst. A comparison of ethylation with methylation barriers of up to heptene isomers forming nonene and octene isomers, respectively, shows that ethylation barriers are lower by around 11 kJ mol⁻¹ on average.