Study on Ru-B/MIL-53(AlxCr1) Catalysts for Partial Hydrogenation of Benzene to Cyclohexene

Metal-organic frameworks (MOFs) have attracted enormous research interests not only because of their merits such as high specific surface area, high porosity, and regular pore channels, but also due to their peculiarities of extremely abundant chemical and structural diversity and tunability. In this work, we synthesized MIL-53(Al) and MIL-53(Cr) containing one coordination metal and the novel MIL-53(AlxCr1)(x= 1, 2, 3, and 4) MOFs containing two coordination metals as the supports for the Ru-B/MIL-53 catalysts, which were prepared by the facile impregnation- chemical reduction method. In the challenging partial hydrogenation of benzene to cyclohexene, it is revealed that the Al/Cr ratio had pronounced influences on both the initial hydrogenation rate (r(0)) and the initial selectivity to cyclohexene (S-0). In general, MIL-53 containing a higher fraction of Al affords a higher r(0), while MIL-53 containing both Al and Cr is conducive to a higher S-0 than either MIL-53(Al) or MIL-53(Cr) containing only one coordination metal. On the Ru-B/MIL-53(Al3Cr1) catalyst exhibiting the highest selectivity to cyclohexene, the r(0) and S-0 were 9.2 mmol/(min.g) and 71%, respectively. The best Ru-B/MIL-53(Al3Cr1) catalyst and the Ru-B/MIL-53(Cr) catalyst displaying the lowest selectivity to cyclohexene were comparatively characterized to have an insight into the difference in their catalytic performance. It is found that while both catalysts had similar Ru/B molar ratio, electronic property, and microstructure, the Ru-B/MIL-53(Al3Cr1) catalyst had higher active surface area (Sact), smaller and more highly dispersed Ru-B nanoparticles (NPs), and stronger metal-support interaction than the Ru-B/MIL-53(Cr) catalyst. The smaller Ru-B NPs could not only provide more active sites for the hydrogenation of benzene, but also be beneficial to the formation of cyclohexene. By further optimization of the reaction conditions, at 180 degrees C, H-2 pressure of 5.0 MPa, and using 100 mu L of ethanolamine as the modifier, a cyclohexene yield of 29% was obtained over the Ru- B/ MIL- 53( Al3Cr1) catalyst.