In Situ 13C Solid‐State NMR and Ex Situ GC–MS Analysis of the Products of tert‐Butyl Alcohol Dehydration on H‐ZSM‐5 Zeolite Catalyst

Abstract

AbstractThe hydrocarbon products that are formed upon dehydration at 296–673 K of tert‐butyl alcohol (tBuOH), adsorbed on H‐ZSM‐5 zeolite in concentrations equal to that to active AlOHSi sites in the catalyst, have been analyzed by 13C solid‐state MAS NMR and GC–MS. To facilitate 13C NMR analysis, the alcohol selectively labeled with 13C isotope in the COH group was used. It was found that tBuOH transforms to the adsorbed C8 butene dimers plus a trace amount of alkanes at 296 K. Butene dimers exist inside HZSM‐5 pores in the form of interconverting adsorbed octene, octyl silyl ether, and octyl carbenium ion; octyl silyl ether is the main adsorption state. Fluxionality of the carbenium ion form provides a pathway for isomerization of the highly branched hydrocarbon skeleton of the intial alcohol to the predominantly linear one in the adsorbed butene dimer. The driving force for the isomerization into the linear structure is the shape selectivity induced by the small size of the zeolite channels. At 373 K the adsorbed butene dimers further crack into species that contain an average of about 6.5 carbon atoms, in addition to further alkanes. At 448 K the adsorbed C3–C7+ paraffins become the predominant hydrocarbon products observed with both in situ 13C NMR and ex situ GC–MS. Simultaneously, a mixture of adsorbed polyenes is formed. According to 13C CP/MAS NMR, polyenes exist in the zeolite pores in the form of rather stable cyclopentenyl cations. At 573–673 K adsorbed cyclopentenyl cations further transform into a mixture of condensed and simple aromatics and then into xylenes and toluene. Simultaneously, paraffins crack further to give mainly C3–C4 paraffinic species at 573 K and propane at 673 K.