Zeolites are common catalysts for multiple industrial applications, including alcohol
dehydration to produce
olefins, and given their commercial importance, reaction mechanisms in
zeolites have long been proposed and studied. Some proposed reaction mechanisms for alcohol
dehydration exhibit noncyclic carbocation intermediates or transition states that resemble carbocations, and several previous studies suggest that the tert-butyl
cation is the only noncyclic
cation more stable than the corresponding chemisorbed species with the
hydrocarbon bound to the framework
oxygen (i.e., an alkoxide). To determine if carbocations can exist at high temperatures in
zeolites, where these catalysts are finding new applications for biomass vapor-phase upgrading (∼500 °C), the stability of carbocations and the corresponding alkoxides were calculated with two ONIOM embedding methods (M06-2X/6-311G(d,p):M06-2X/3-21G) and (PBE-D3/6-311G(d,p):PBE-D3/3-21G) and plane-wave density functional theory (DFT) using the PBE functional corrected with entropic and Tkatchenko-Scheffler van der Waals corrections. The embedding methods tested are unreliable at finding minima for primary carbocations, and only secondary or higher carbocations can be described with embedding methods consistent with the periodic DFT results. The relative energy between the carbocations and alkoxides differs significantly between the embedding and the periodic DFT methods. The difference is between ∼0.23 and 14.30 kcal/mol depending on the molecule, the model, and the functional chosen for the embedding method. At high temperatures, the pw-DFT calculations predict that the allyl, isopropyl, and sec-butyl
cations exhibit negligible populations while acetyl and tert-butyl
cations exhibit significant populations (>10%). Moreover, the periodic DFT results indicate that mechanisms including secondary and tertiary carbocations intermediates or carbocations stabilized by adjacent
oxygen or double bonds are possible at high temperatures relevant to some industrial uses of
zeolite catalysts, although as the minority species in most cases.