Microwave-assisted Green Synthesis Of Supported Ruthenium-based Catalysts And Their Catalytic Performance

With the growing shortage of fossil energy and environmental problems inevitably deteriorating even further, biomass as a kind of green renewable resource has become a global research hotspot. Both of levulinic acid and succinic acid are important platform molecules for biomass transformation, and their hydrogenation products are high value-added, which is a chemical bridge connecting biomass and petroleum processing.Ruthenium-based catalysts were prepared via a one-step solvent-free microwave-assisted thermolytic process with Ru3(CO)12and Pt(acac)2as precursors. Microwave heating system can make temperature almost the same everywhere, which is the advantage for the uniform dispersion of metal particles and stabile formation of catalysts structure. The obtained catalysts showed excellent catalytic performance in hydrogenation of cinnamaldehyde (CMA), levulinic acid (LA) and succinic acid (SA).Microwave irradiation provides the energy for the catalysts preparation and improves the rate of solid phase reaction, which is conducive to the growth of metal seed. These Ru catalysts were characterized by XRD, TEM, HRTEM and FT-IR. The Ru particles have been transformed into single crystals with the disappearance of twins after a5min irradiation, successive structural rearrangement of multiply twinned particles near grain boundaries leads to the annihilation of the twins. The ratio of Ru single crystals to all nano-particles can be increased to92%when the irradiation was carried out for5min. The themolysis process of Ru12(CO)3was deduced as follows:Ru3(CO)12→Ru (CO)x(1<x<4)→Ru.Both of Ru/CNT and Ru/FCNT showed excellent selectivity in hydrogenation of CMA. As the Ru loading is increased, Ru will also have a good chance to be found highly dispersed, which consequently leads to high CMA conversion. The surface functionalization of carbon nanotubes results in larger ruthenium nanoparticles and lower dispersion. The Ru/FCNT samples exhibited superior selectivity toward C=O hydrogenation. The catalysts with a high amount of oxygen surface groups may repel the benzene ring from the support surface in that way hampering the C=C bond to approach the metal surface. Therefore, only the C=O bond can approach the metal resulting in a higher selectivity for C=O bond hydrogenation.A sustainable y-valerolactone (GVL) has been produced through an aqueous phase hydrogenation of biomass-derived LA in the presence of supported Ru catalysts, which were prepared without solvent by microwave-assisted thermolytic method. The effects of catalyst support, reaction solvent, pressure, temperature and LA initial concentration are investigated to find the optimum mild conditions for high GVL yield.5wt.%Ru/AC catalyst presents a more superior catalytic performance compared with Ru/CNT, Ru/FCNT, Ru/y-Al2O3-MW and Ru/y-Al2O3-IM at100℃,2.00MPa with0.10g/mL LA concentration in water solution owing to the higher dispersion of metallic Ru over coconut shell activated carbon. The hydrogenation performance of the five catalysts decreased in the order;Ru/AC> Ru/CNT≈Ru/FCNT> Ru/γ-Al2O3-MW≈Ru/γ-Al2O3-IM. A series of reactions were carried out to investigate solvent (water, cyclohexane, anisole, methanol, ethanol and acetone) effects upon the hydrogenation of LA. Water which is an abundant and non-toxic solvent can minimize by-products formation and avoid the resulting separation problems. GVL can be produced with good yield of>99%under optimum conditions, which can provide a green, renewable platform for biotransformation.The aqueous phase hydrogenation of SA to y-butyrolactone (GBL),1,4-butanediol (BDO) and tetrahydrofuran (THF) which are of great commercial value was investigated using Ru-Pt bimetallic catalysts. There are no big differences in SA conversion among Ru/CNT, Ru80Pt20/CNT, Ru50Pt50/CNT, Ru20Pt80/CNT and Pt/CNT. Ru80Pt20/CNT had a higher selectivity to GBL about94%and Pt/CNT gave THF about40%. Ru had good catalytic activity for dehydration cyclization from SA to GBL, while Pt exhibited excellent catalytic hydrogenation performance from BDO to THF. The reactions were conducted three times in succession with Ru/CNT, Ru50Pt/CNT and Pt/CNT, all of them exhibited excellent cycle stability with the activity about38%without metal loss.