Studies On Active Sites Of Ru-Based Catalysts Toward The Selective Hydrogenation Of Benzene

Cyclohexene is one of the most important commercial petrochemicals, which is widely used as a raw material for the production of medicine, pesticides, feed additives, polyester and other value-added fine chemicals. The production of cyclohexene by selective hydrogenation of benzene has attracted considerable attention due to its advantages in inexpensive feedstock, atomic economy and operational simplicity. Ru-based catalysts have been proved as one promising candidate toward this reaction. Nevertheless, three key scientific and practical issues are still needed to be solved:1) the inhibition of further hydrogenation of cyclohexene is difficult due to the difficulties in tuning the fine-structure of active sites; 2) detailed knowledge on the correlation between active sites structure and catalytic performance is rather lacking, which retards the rational design and fabrication of efficient catalysts; 3) the commonly used corrosive additives result in serious problems in separation and equipment maintenance. In this work, three novel Ru-based catalysts are fabricated based on the principles of optimal exposure of active sites, enhanced dispersion of active sites as well as the synergistic catalysis between Ru active site and support, which exhibit excellent catalytic behavior toward selective hydrogenation of benzene. Based on various experimental and computational methods, the reaction kinetics, reaction mechanism as well as the structure-property correlation have been systematically investigated. This work not only rationalizes the significant influence of modulations of active sites on the hydrogenation selectivity, but also provides three proming, green and eco-friendly catalytic processes for selective hydrogenation of benzene to give cyclohexene. The detailed contents are listed as follows:1. The effect of active sites exposure for Ru-based catalysts on the selective hydrogenation of benzeneSeries of Cu-decorated Ru catalysts supported on MgAl-LDH (denoted as RuxCuy/MgAl-LDH) were prepared by a facile two-step procedure involving the co-reduction of RuCl3-impregnated CuMgAl-LDH precursors, followed by the rehydration of the resulting RuxCuy/MgAl-MMO to RuxCuy/MgAl-LDH. CO-TPD and in situ CO-FTIR techniques reveal the preferential deposition of Cu on the surface low-coordinated sites of Ru NPs. The resulting Cu-decorated Ru catalysts exhibit tunable catalytic selectivity to cyclohexene, and the best catalytic behavior can be achieved over the Ru1.0Cu0.5/MgAl-LDH catalyst (cyclohexene yield of 44.0% at 150℃ and 5.0 MPa) without the use of any additives. Kinetic investigations and DFT calculations further demonstrate that the low-coordinated Ru atoms possess a high surface free energy, which is unfavorable for the desorption of the formed cyclohexene; an appropriate coverage of the low-coordinated Ru by inactive Cu gives rise to a largely enhanced selectivity toward cyclohexene. This work gives a better understanding of the significant influence of active sites exposure for Ru-based catalysts on the selective hydrogenation of benzene.2. The effect of active sites dispersion of Ru on the selective hydrogenation of benzeneHerein, we reported the design and fabrication of uniform and ultrafine Ru-B amorphous alloy nanocatalyst supported on titanate nanosheets (denoted as Ru-B/TNS) via a facile two-step procedure involving the confined synthesis of Ru-B NPs in titanate nanotubes (TNT) followed by unwrapping the tube to sheetlike titanate (TNS). HRTEM images display the resulting Ru-B NPs are highly dispersed on the titanate nanosheets (mean particle size:2.5 nm), which largely enhances the dispersion and exposure of defects of Ru active sites revealed by H2-TPD and EXAFS techniques; moreover, XPS measurement demonstrates a large density of surface alloying B and a strong electron transfer from B to Ru, favorable for the formation and desorption of cyclohexene. Eventually, an excellent catalytic performance (cyclohexene yield of 50.7% at 150℃ and 5.0 MPa) can be achieved over the Ru-B/TNS without any additives. The surfactant-free confined synthesis and additive-free catalytic system make the Ru-B/TNS catalyst a promising candidate for the selective hydrogenation of benzene.3. The metal-substrate synergistic effect on the selective hydrogenation of benzeneA porous TiO2-capped Ru/TiO2 catalyst (denoted as Ru/TiO2@pTiO2) was fabricated by a facile two-step procedure involving a modification of titania layer on Ru/TiO2, followed by the removal of pore generator to form porous TiO2 layer. By virtue of the selective diffusion of hydrogen molecule within the porous TiO2, benzene can be hydrogenated on the surface of TiO2 to give cyclohexene, avoiding the deep hydrogenation of cyclohexene on highly-active Ru. The catalytic evaluations give an excellent cyclohexene yield of 75.1% over the Ru/TiO2@pTiO2 catalyst in the absence of any additives, which is the highest value to the best of our knowledge. The hydrogenation process of benzene studied by in situ FTIR technique show that benzene adsorbed on the TiO2 surface can effectively converts to cyclohexene by spillover hydrogen from metallic Ru. Spillover hydrogen-catalyzed selective hydrogenation of benzene based on the Ru-TiO2 synergistic effect displays the following merits for the improvement of cyclohexene selectivity: (i) TiO2 possesses an excellent hydrophilicity and moderate interaction with cyclohexene, which facilitates the desorption of cyclohexene formed on its surface; (2) the hydrogenation of cyclohexene to cyclohexane needs a much higher active energy on the TiO2 suface compared with that on the Ru surface revealed by DFT calculations, which decreases the the probability of further hydrogenation of cyclohexene. Therefore, based on the synergistic catalysis of metallic Ru and support TiO2, this work provides a new and efficient hydrogenation route for green conversion of benzene to cyclohexene.