Improved Catalytic Performances of the NaOH-Treated ZSM-22 Zeolite in the 1-Butene Skeletal Isomerization Reaction: Effect of External Acid Sites

Abstract

In this paper, a series of alkaline-treated ZSM-22 zeolite samples were prepared by treating the parent ZSM-22 zeolite using NaOH aqueous solution with different concentrations. By investigating the effects of alkaline treatment on the parent ZSM-22 zeolite, we discovered that the alkaline treatment contributed to the reduction of Brønsted acid sites due to the coverage of extra-framework Al on its external surface. In addition, it was found that the alkaline-treated samples were favorable to the improvement of the isobutene yield and selectivity, while these features appeared to be low for the subsequent acid-washed counterparts in the skeletal isomerization reaction of 1-butene. These results indicate that the catalytic performance of ZSM-22 zeolite is related to reduced amounts of Brønsted acid sites in it. To further reveal the reasons for the promoted catalytic performances of the alkaline-treated ZSM-22 series zeolites, we studied the properties of coke deposited on the two series of samples using Raman spectroscopy and thermogravimetric analysis and mass spectrometry (TG/MS-TPO). It was shown that the carbon deposited on the alkaline-treated series samples was mainly distributed at the outer surface, while the coke was distributed to a relatively lesser extent at the exterior surface for the acid-washed series samples. Moreover, by partially passivating outer acid sites of the parent zeolite, the selected alkaline-treated zeolite, and acid-washed zeolite, their isobutene selectivities were all improved with the decrease in outer acid sites. These phenomena confirmed that the improved catalytic performances of the alkaline-treated samples are related to their decreased external Brønsted acid site density, which further demonstrated that the high isobutene yield and selectivity in the skeletal isomerization reaction of 1-butene is realized via the monomolecular reaction pathway of 1-butene.