| Carbon-based precious metal catalysts are widely used in liquid-phase hydrogenation reactions. Research on their structure-activity relationships could be conducive to develop high-performance hydrogenation catalysts.Carbon nanotube (CNT) was chosen as one model support. Ru nanoparticles (NPs) were loaded on the inner or the outer surface of CNT, respectively, with the same size distributions (1-3 nm). Heat treating at 700 ? in N2 flow could hardly change their sizes but make Ru electron rich. Benzene,p-chloronitrobenzene (p-CNB) and cinnamaldehyde (CAL) were chosen as model substrates to evaluate the hydrogenation performance of catalysts. Results indicated that, the electron-rich Ru would be favorable for the adsorption of -NO2 which is a strong electron-withdrawing group, but unfavorable for the activation of benzene which is an electron-donating molecule, and it could improve the selectivity of C=O in cinnamaldehyde hydrogenation. Due to the distinction of electronic structures among the three molecules, the confinement effect in CNT derived from electronic effect has different influence on these liquid-phase hydrogenation reactions.Graphene was chosen as another model support. Functionalized graphene obtained from thermal exfoliation of graphite oxide at low temperature (200 ?) under air atmosphere was used to support Ru NPs (1-5 nm) with the method of incipient wetness impregnation. Compared with CNT-supported Ru catalyst, Ru NPs loaded on graphene are more electron-deficient, and thus doubled the specific activity of benzene hydrogenation (1302 h"1 vs.622 h"1). Meanwhile, Ru NPs loaed on graphene are more flat than that loaded on CNT, and thus doubled the specific activity of CAL hydrogenation (119 h-1 vs.52 h-1) due to its favorable planar adsorption, but rendered a twofold reduction in specific activity of p-CNB hydrogenation (93 h-1 vs.174 h-1) due to its unfavorable adsorption on such flat Ru NPs.Hydrothermal synthesis method was adopted to prepare graphene-supported Ru nanocatalyst (1?4 nm), and the catalyst was employed to catalyze the conversion of levulinic acid (LA) at low temperature (50 ?) in aqueous phase. It performed relatively high hydrogenation activity (661 h-1), while the main product was the hydrogenated intermediate 4-hydroxyvaleric acid (HVA) with a 82% yield in 40 minutes. In order to promote the dehydration of HVA to form y-valerolactone (GVL), the catalyst was covalently functionalized with the aryl diazonium salt of sulfanilic acid. The introduced strong acid sites could effectively catalyze the dehydration of HVA to form GVL at low temperature and not affect the hydrogenation activity of Ru NPs. A 82% yield of GVL was obtained in 40 minutes, and 100% yield can be reached by prolonging the reaction time to 80 minutes.Graphene-supported Ir nanocatalyst (1-4 nm) was prepared by the hydrothermal synthesis method, and it was used to catalyze the selective hydrogenation ofp-CNB and CAL. The influences of hydrothermal conditions on the hydrogenation performances of catalysts were investigated. Results indicated that, reduction temperature affected the surface chemistry of graphene. The higher the temperature is the more nonpolar its surface is, and thus it is easier for the adsorption of CAL through ?-? non-covalent interaction, so catalysts prepared at higher temperature performed the higher activity of CAL hydrogenation. Oppositely, the lower the temperature is the more polar its surface is, and hydrogen-bonding interaction with nitro groups could improve the accessibility of p-CNB to the surface of catalyst, so the reverse trend was present in the activity of p-CNB hydrogenation. |
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