Synergistic roles of B/L acids and hierarchical micro-mesoporous structures for the unexpected isomerization of β-pinene over dual-modified MOR zeolite by inorganic/organic bases

Abstract The isomerization of β-pinene is an important reaction to synthesize spices, such as camphene, limonene, terpinene, and terpinolene, among which terpinene and terpinolene are particularly difficult to be obtained. Herein, MOR zeolite modified by both inorganic base (NaOH) and organic base (TEAOH) was conducted, which showed high catalytic activity for the isomerization of β-pinene to high-added value spices of terpinene and terpinolene. Different from the zeolite catalyst modified by solely inorganic base or organic base, both high conversion of β-pinene and high selectivity for terpinene and terpinolene could be achieved over the dual-modified zeolite catalyst. Based on the various characterizations, catalytic reaction results and comparison studies, the roles of inorganic base and organic base were revealed and possible reaction mechanism was described. The modification by inorganic base was mainly to introduce additional mesopores in the MOR zeolite, which had important influence on the conversion of β-pinene through accelerating the diffusion and mass transfer of various bulky organic molecules in the reaction system. The modification by organic base was mainly to tune the Bronsted/Lewis (B/L) acid property, which had important impact on the selectivity of products. In contrast, the zeolite modified by solely inorganic base mainly showed Lewis acidity, which led to camphene and limonene as the main products; the zeolite modified by solely organic base mainly showed Bronsted acidity, which produced camphene, terpinene and terpinolene as the main products. The dual-modified zeolite catalyst by inorganic/organic bases could achieve 99.5% conversion of β-pinene with the total selectivity for terpinene and terpinolene as high as 70.9%. In addition, the catalyst showed high reaction-recycling stability, which could maintain the catalytic performance up to 10 recycles.