N-Butene Conversion on H-Ferrierite Studied by 13C MAS NMR

Abstract

13C MAS NMR analysis of the hydrocarbon products formed from the selectively 13C-labeled n-but-1-ene on zeolite ferrierite (H-FER) in a closed batch reactor revealed the following successive steps of the olefin conversion with temperature increase from 300 to 823 K: a double-bond-shift reaction, scrambling of the selective 13C label in the formed n-but-2-ene, oligomerization (dimerization), conjunct polymerization, formation of condensed aromatics, and formation of the simple aromatics. Arguments in favor of either bimolecular or pseudo-monomolecular mechanisms are provided, excluding at the same time the monomolecular isomerization of n- to isobutene on a fresh sample. The arguments are based on selective label redistribution in the n-but-2-enes, the impossibility of the existence of isobutene inside the pores of the zeolite under static conditions and the observation of n-but-2-enes oligomerization (dimerization). Conjunct polymerization leads to the formation of alkyl-substituted cyclopentenyl cations (CPCs), which can serve as an intermediate for pseudo-monomolecular isomerization. Carbonaceous deposits (polycyclic aromatics), which deactivate the catalyst in the isomerization reaction, are formed from the CPCs. Polycyclic aromatics are transformed into simple aromatics with methane and ethane evolution at 823 K.