Preparation Of Highly Dispersed And Supported Ni-Ru Catalysts And Their Application In Catalytic Hydrogenolysis Of Lignin

As an abundant and renewable resource,biomass is alternative to fossil-based feedstocks for producing clean fuels and chemicals,and hence efficient utilization of biomass has attracted much attention.Lignin,a main component of biomass,is the by-product of paper industry or biorefinery and has not been well utilized.Lignin is the only renewable source rich in aromatic rings.Arenes,phenols,and other chemicals can be obtained by selective hydrogenolysis of the aryl ether bonds in lignin,which will greatly improve the economics of paper industry or biorefinery process.However,due to the stubborn structure of lignin,developing highly active heterogeneous catalysts is a key for the efficient hydrogenolysis of lignin.In recent years,layered double hydroxides（LDHs）with the characteristics of adjustable cation and anion,adjustable acidity and alkalinity and topological transformation is widely employed as precursors for preparing highly dispersed supported catalyst.Therefore,in this paper,a variety of highly dispersed Ni-Ru/Al₂O₃ catalysts were prepared with LDHs as precursors for efficient hydrogenolysis of lignin.NiRuAl-LDH was firstly prepared by a coprecipitation method,and Ni-Ru/Al₂O₃ catalyst was obtained after calcination and reduction.Analyses with X-ray diffraction,high resolution transmission electron microscopy,X-ray photoelectron spectroscopy and N₂ adsorption and desorption show that the catalyst has a mesoporous structure and the active metal was highly dispersed on the support.In addition,electron transfer from Ni to Ru was also found.Phenoxyethylbenzene（PEB）,phenylbenzyl ether（BPE）and diphenyl ether（DPE）were selected as lignin model compounds to investigate the hydrogenolysis activity of the catalyst.The results exhibit that Ni_40%-Ru_5%/Al₂O₃ catalyst can efficiently catalyze the breakage of aryl ether bonds using isopropanol as hydrogen source,and its performance is much better than that of commercial Ru/C.For example,BPE was completely converted at 130 ℃ after 3 h with toluene and cyclohexane being the most.The products can be controlled by adjusting the reaction temperature and time.High temperatures and long reaction time favour the hydrogenation to produce cycloalkanes and cyclohexanols,while low temperatures and short reaction time facilitate the formation of arenes and phenols.Kinetics studies indicate that the activation energies of the different model compounds toward catalytic transfer hydrogenolysis were determined to be 90.6（BPE）,126.7（PEB）and 172.2 kJ/mol（DPE）,which are much lower than the dissociation energies of the corresponding aryl ether bonds.In view of the excellent catalytic activity of Ni_40%-Ru_5%/Al₂O₃ for the model compounds,the effects of solvent,temperature and time on the Ni_40%-Ru_5%/Al₂O₃-catalyzed hydrogenolysis of enzymatic hydrolysis ignin（EHL）were examined.The results display that isopropanol is the most suitable solvent and high temperature leads to the repolymerization of the product accompanied by coking,which was detrimental to the catalytic hydrogenolysis of EHL.In addition,the optimum reaction temperature and time are 220 ^oC and 4 h,respectively,with a methanol soluble portion（MSP）yield of 58.12%and relative monomer/dimer content of 64.8%.The analyses with two-dimensional gas chromatography-time of flight mass spectrometer,two-dimensional heteronuclear single quantum coherence nuclear magnetic resonance（2D-HSQC NMR）and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer（ESI-FTICRMS）show that most of the aryl ether bonds in EHL were broken under the action of Ni_40%-Ru_5%/Al₂O₃,and the efficient hydrogenolysis of EHL was achieved at low temperature.The MSP mainly contains phenolic compounds,providing a reference for the efficient conversion of lignin to phenolic compounds.In order to achieve the modulation of EHL hydrogenolysis products and further improve the efficiency of EHL catalytic hydrogenolysis,Ni-Ru/Al₂O₃-T catalysts with higher dispersion were prepared using an exogenous method to introduce Ru according to the property of hydrotalcite topological transformation.The catalytic hydrogenolysis activity of the catalysts for various model compounds was investigated and the results demonstrate that Ni-Ru/Al₂O₃-T was more active than Ni_40%-Ru_5%/Al₂O₃ under the same conditions.In addition,the catalyst was more stable and easily separable.More importantly,the catalytic hydrogenolysis of EHL at high temperature（300 ^oC）significantly suppressed the formation of coke,obtaining MSP yield up to 71.96%and significantly improving the degree of hydrogenolysis.Elemental analysis,2D-HSQC NMR and ESI-FTICRMS indicate that a deep hydrogenation saturation of MSP occurred at high temperature,suggestting that the catalyst is expected to achieve efficient and directional conversion of lignin.