Effect of support composition on the structural and catalytic properties of Ru/AlOOH-SiO2 catalysts for benzene selective hydrogenation

Benzene selective hydrogenation to cyclohexene, which has been attracting attention in terms of atomic economy, feedstock accessibility, and operational facilitation, has been investigated on Ru catalysts supported on AlOOH-SiO2 composites with varying Al/Si ratios aiming at unveiling the effect of support acidity. Various techniques such as Fourier transform infrared spectroscopy of adsorbed pyridine, transmission electron microscopy, and extended X-ray absorption fine structure spectroscopy were employed to comprehensively characterize the catalysts. It is confirmed that the amount of Lewis acid sites increased steadily with the increase in the Al/Si molar ratio. In benzene selective hydrogenation, the turnover frequency of benzene linearly correlated with the amount of acid sites, while the cyclohexene selectivity showed a volcano-type trend, achieving a maximum of 81%. Kinetic investigation revealed that the Lewis acid sites promoted the rate constants of both the benzene hydrogenation and cyclohexene hydrogenation steps to inequable extents. The resulting ratios of the rate constants evolved parallel to the variation in the cyclohexene selectivity, which elucidated the volcano-type relationship between the cyclohexene selectivity and the amount of Lewis acid sites.