Effect of postsynthesis preparation methods on catalytic performance of Ti-Beta zeolite in ketonization of propionic acid

Heteroatom metal-zeolite, with heteroatom metal (oxide) dispersed in the zeolite framework, enables the reactions typically catalyzed by bulk metal oxide counterpart but with maximized the atomic utilization. However, the distribution of heteroatoms in the framework is strongly influenced by the preparation method. In this work, Ti-Beta zeolites were postsynthesized using the recrystallization (Ti-Beta-Re) and solid-state ion-exchange (Ti-Beta-SSIE) methods. Characterizations showed that more Ti species were incorporated in the tetrahedral framework site of Ti-Beta-Re to form Lewis acid sites than in that of Ti-Beta-SSIE. In addition, intra-crystallite mesopores were developed in Ti-Beta-Re due to partial dissolution of BEA framework during alkaline etching and subsequent recrystallization of the etched framework. These zeolites were then tested for ketonization of biomass derived propionic acid at 350 °C and atmospheric pressure. The turnover frequency of ketonization was found to be 25% higher on Ti-Beta-Re (0.25 min−1) than on Ti-Beta-SSIE (0.20 min−1), whereas the apparent activation energy of 80 kJ/mol on Ti-Beta-Re is lower than 95 kJ/mol on Ti-Beta-SSIE. Moreover, the selectivity of 3-pentanone kept > 95% on Ti-Beta-Re while it dropped to ∼63% on Ti-Beta-SSIE with increase of reaction time at a space time of 2 h. This work demonstrated that the tetrahedrally coordinated Ti species are the active sites and the presence of intra-crystallite mesopores facilitates selective ketonization of carboxylic acids.