Hierarchical hafnium modified silicious Beta zeolite by tailoring Lewis acid sites for efficient conversion of acetone to isobutene

With growing demand of isobutene, acetone to isobutene conversion is becoming a promising route from non-oil feedstocks, as acetone could be obtained from biomass fermentation or pyrolysis processes. Herein, we developed a series of Hafnium (Hf) modified silicious Beta zeolite as highly efficient catalysts in the acetone to isobutene conversion via tetraethyl ammonium hydroxide (TEAOH) post treatment. A joint characterization based on solid-state ¹³C CPMAS NMR spectroscopy with acetone-2-¹³C adsorption, FTIR spectra with deuterated acetonitrile adsorption and NH₃-TPD reveals that the amount of Lewis acidic Hf sites enhances significantly and, the ratio of open Hf sites to closed Hf sites rises up after the treatment of TEAOH solution, which plays pivotal role in the conversion of acetone to isobutene. In addition, TEAOH, serves a dual role as base and template agent, could create mesopores and simultaneously recrystallize the dissolved zeolitic species, thus reducing the restrictions on mass transfer and improving the accessibility of active sites. Appropriate concentration of TEAOH solution is essential to precisely obtain a balance between dissolution and recrystallization processes and an optimum TEAOH solution of 0.5 mol/L could exhibit the highest activity toward acetone to isobutene conversion and the isobutene yield is ca. 20 % higher than the parent Hf/Beta catalyst via a traditional impregnation method. This work provides a facile method to construct hierarchical Hf-BEA catalyst for the conversion of acetone to isobutene.