Effect of the Porosity and Acidity of Hierarchical Al-BEA Zeolites on the Catalytic Activity in α-Pinene Isomerization

Hierarchical Al-BEA zeolites were prepared by hydrothermal treatment of a concentrated reaction mixture containing a highly dispersed silicon source. The hierarchical zeolites obtained consist of aggregated crystallites and contain, along with zeolite micropores, also intercrystallite mesopores. With a decrease in the H2O/Si ratio from 15 to 5.5 and in the Si/Al ratio from 12.5 to 10, the crystallite size decreases to 13 nm. This is accompanied by an increase in the mesopore volume and in the external and total specific surface area of the samples. The catalytic properties of the hierarchical zeolites in the α-pinene isomerization were studied. The initial rate of the α-pinene consumption increases with an increase in the concentration of strong Brønsted acid sites and is the highest for the commercial Al-BEA zeolite with the ratio Si/Al = 12.5. The α-pinene conversion increases with an increase in the external specific surface area of the zeolites and reaches 91% after 24 h of the reaction for the hierarchical zeolite with the ratio Si/Al = 9, prepared in the concentrated reaction mixture. The selectivity of camphene formation decreases and the selectivity of limonene formation increases with an increase in the Brønsted-to-Lewis acid site ratio. The highest camphene and limonene yields, equal to 45 and 31%, respectively, are reached when using the sample with the most developed mesoporosity and the Brønsted-to-Lewis acid site ratio of 0.9.