Skeletal Isomerization of Butene in Fixed Beds. 1. Experimental Investigation and Structure−Performance Effects

Abstract

An experimental investigation of structure-performance effects in zeolite catalyzed skeletal isomerization has been carried out. Two structurally different, medium-pore-size zeolites (H-TON and H-FER) with similar acidities were compared in butene skeletal isomerization. While both catalysts proved to be efficient in the test reaction, their deactivation behavior differed substantially. H-FER exhibited significantly higher initial isobutene yields and selectivities, and the catalyst performance was also more stable with time-on-stream. H-TON, on the other hand, needed prolonged operation times in order to achieve product distributions comparable to those of H-FER. The predominant route to isobutene was found to be the monomolecular one, with the bimolecular paths of butene largely responsible for byproduct formation. H-TON was more selective toward disproportionation and, owing to its slightly larger pore dimensions, hydrogen transfer products. H-TON was also observed to be less sensitive to feed reactant than H-FER was, confirming previous theoretically calculated structure effects. Coke belonging to both aliphatic and aromatic families was detected over both zeolites, although the coke formed on H-FER was overall heavier and more aromatic in nature. The tested zeolites could be regenerated by burning of the carbonaceous deposits in air, and both catalysts regained almost all of their initial activities.

Interaction of C3–C5Alkenes with Zeolitic Brønsted Sites: π-Complexes, Alkoxides, and Carbenium Ions in H-FER

“…Proton-exchanged zeolites play an important role in the petrochemical industry, fine chemical production, and oil refining. − Based on the acidity of their Brønsted sites, they are successfully used as catalysts for a range chemical reactions. − One application is hydrocarbon transformation reactions such as skeletal isomerization of linear alkenes. The activity of zeolites in this reaction is based on their ability to donate a proton to alkenes, forming positively charged carbenium ions as transition structures or intermediates.…”

Mechanistic Insights into the Pore Confinement Effect on Bimolecular and Monomolecular Cracking Mechanisms of N-Octane over HY and HZSM-5 Zeolites: A DFT Study

“…Three possible mechanisms have been proposed for the skeletal isomerization of hydrocarbon, monomolecular, , bimolecular, and pseudo-monomolecular mechanisms. Among these three mechanisms, the monomolecular mechanism is currently considered to be the predominant mechanism for the isomerization, whereas the bimolecular mechanism is mainly responsible for the formation of byproducts. , …”

Zeolites and Other Structurally Microporous Solids as Acid–Base Materials

“…101 Most authors seem to converge today on the monomolecular mechanism. 102,103 On the other hand, it has been shown that monobranched compounds may diffuse much better in the 10 MR larger channels of FER structure than in the 8 MR smaller ones, where access of branched compounds is strongly hindered. 92 Compounds with the tert-butyl group appear to be unallowed to enter both channels, at least at room temperature.…”