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.

Ru–B/MIL-53(AlCr) affords an exceptionally high turnover frequency (TOF) of 6.4 s−1for hydrogenation of benzene to cyclohexane under mild reaction conditions.

A series of metal-organic framework (MOF) materials were synthesized together with the corresponding amorphous Ru-B/MOF catalysts, which were prepared by the impregnation-chemical reduction method. These materials were subsequently evaluated for the first time as catalysts for the partial hydrogenation of benzene to cyclohexene. The results for the initial hydrogenation rate (r(0)) for the different catalysts followed the trend Ru-B/MIL-53(Al)> Ru-B/MIL-53(Al)-NH2> Ru-B/UIO-66(Zr)> Ru-B/ UIO-66(Zr)-NH2> Ru-B/ MIL-53(Cr)> Ru-B/ MIL-101(Cr)>> Ru-B/ MIL-100(Fe), whereas the initial selectivity for cyclohexene (S-0) was of the order of Ru-B/MIL-53(Al)approximate to Ru-B/MIL-53(Cr)>Ru-B/UIO-66(Zr)-NH2>Ru-B/MIL-101(Cr)>Ru-B/MIL-53(Al)-NH2>Ru-B/UIO-66(Zr)approximate to Ru-B/MIL-100(Fe). The Ru-B/MIL-53(Al) catalyst exhibited the highest r(0) and S-0 values of 23mmol center dot min(-1)center dot g(-1) and 72%, respectively. The characterization results demonstrated that the Ru- B amorphous alloy nanoparticles were highly dispersed on MIL-53(Al) with the average diameter of 3.2 nm. In contrast, the Ru-B nanoparticles on MIL-100(Fe) had an average diameter of 46.6 nm. The smaller Ru-B nanoparticles not only provided more active sites for the hydrogenation to occur, but could also be beneficial in the formation of cyclohexene. The reaction conditions were further optimized for the Ru- B/MIL-53(Al) catalyst. At 180 degrees C under a H2 pressure of 5 MPa, a cyclohexene yield of 24% was obtained, highlighting the potential of MOF materials as catalyst supports for the partial hydrogenation of benzene.

The particle size effect on partial hydrogenation of benzene to cyclohexene over Ru/ZrO2 catalysts is reported. Uniform Ru nanoparticles (NPs) with a tunable particle size from 2.4 to 5.4 nm were synthesized by a polyol reduction method and deposited on ZrO2. The catalysts were characterized by ultraviolet-visible absorption spectroscopy (UV-Vis), N2 physisorption, H2 chemisorption, temperature-programmed desorption of H2 (H2-TPD), powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). It was found that the type of polyol and the concentration of additive (sodium acetate trihydrate) imposed remarkable effect on the particle size of Ru. A distinct particle size effect occurred in partial hydrogenation of benzene. With the size of the Ru NPs increasing, the hydrogenation activity of benzene increased, and the initial selectivity ( S 0) to cyclohexene showed a volcanic-type variation tendency, which revealed that the optimal Ru size for obtaining the highest S 0 is 4.4 nm. The Ru/ZrO2 catalyst reduced by 1,2-propanediol exhibited the highest S 0 (82%) and the yield of cyclohexene (39%). Based on the characterization results, the size effect of Ru on the activity and selectivity is discussed.

Partial hydrogenation of benzene to cyclohexene is attractive in terms of feedstock accessibility, atomic economy, and operational simplicity. Herein, a series of Ru/ZrO2 catalysts were prepared by post-treatment of a binary Ru-Zn/ZrO2 catalyst using 5-30wt% NaOH aqueous solutions. Alloying between Ru and Zn was evidenced for the catalyst post-treated only by water (Ru/ZrO2-0). Alkaline post-treatment removed metallic Zn, forming smaller Ru nanoparticles. Concomitantly, the hydrophilicity of the catalysts was increased and maximized on the 10wt% NaOH-treated catalyst (Ru/ZrO2-10). In partial hydrogenation of benzene, the Ru/ZrO2-0 catalyst displayed the highest turnover frequency but the lowest initial selectivity to cyclohexene, whereas the Ru/ZrO2-10 catalyst exhibited the highest initial selectivity (86%) and yield of cyclohexene (51%) among the catalysts investigated. A quantitative relationship between the initial selectivity to cyclohexene and the hydrophilicity of these catalysts was identified, which rationalizes the significant impact of alkaline post-treatment on selectivity enhancement in partial hydrogenation of benzene to cyclohexene over Ru/ZrO2 catalysts.

Ru-B/ZrO2 catalysts using monoclinic, amorphous, and tetragonal ZrO2 as supports were prepared and used for liquid-phase hydrogenation of benzene to cyclohexene. It is identified that both the Lewis acid sites and the Bronsted acid sites existed on monoclinic ZrO2 (ZrO2-M), while there were only Lewis acid sites on amorphous (ZrO2-A) and tetragonal ZrO2 (ZrO2-T). The amount of acid sites on ZrO2-T was the lowest. In liquid-phase hydrogenation of benzene to cyclohexene, the Ru-B/ZrO2-T catalyst exhibited the highest selectivity and yield of cyclohexene, with the maximum yield of cyclohexene being 47%. These results suggest that for ZrO2-supported Ru-B catalysts, the lower was the amount of acid sites on ZrO2, the higher was the selectivity to cyclohexene. Also, the presence of the Bronsted acid sites on ZrO2 is probably adverse to the selectivity toward cyclohexene.