Vapor Phase Hydrodeoxygenation Of M-Cresol Over Ni Based Catalysts

The depletion of petroleum resource and the environmental pollution are two major problems under current society.Development of reproducible and environment friendly biomass resources to produce fuels and valuable products is one of the important ways to solve those problems.Fast pyrolysis of lignin could get the crude bio-oil,which mainly contains phenolics compounds.Catalytic hydrodeoxygenation(HDO)is a key step to upgrade bio-oil to compatible with today's hydrocarbon fuels,which has attracted wide attention around the world.Currently,HDO reaction was mainly focused on model phenolic compounds,like phenol or anisole which contains the characteristic-OH or-OCH₃ functional group,to screen catalyst and investigate the reaction mechanism.However,most of the researches are focused on noble metal catalysts,the active site and the reaction mechanism are not very clear.In this work,we use m-cresol as a model compound and focus on non-noble metal Ni based catalysts,aiming to design high performance Ni based catalyst for HDO of phenolics,as well as understand the active site and reaction mechanism.We start at understanding the structure sensitivity of HDO of m-cresol.It is well known that the metal particle size influences its surface sites distribution,like terrace,step,and corner sites.These different surface sites have different local environment,which may have different ability for chemical adsorption,chemical bond breaking/formation,thus may influence the catalytic performance.Here,we prepared four 5 wt.%Ni/SiO₂ catalysts with the average Ni particle size of 2,5,10 and 22 nm respectively,and studied their HDO performance for m-cresol conversion at 300? and 1 atm H₂.m-Cresol conversion goes through three parallel pathways,i.e.,selective deoxygenation to toluene,phenyl ring hydrogenation to3-methylcyclohexanone and 3-methylcyclohexanol,and C-C hydrogenolysis to CH₄and phenol.Decreasing the Ni particle size promotes m-cresol conversion and enhances deoxygenation selectivity,while inhibits C-C hydrogenolysis.DFT calculations found that the defected Ni surface facilitates phenol adsorption,and stabilizes the transition state during C-O breaking,thus promotes deoxygenation to toluene.Whereas the terrace Ni surface is required to form the transition state during C-C hydrogenolysis,thus showing high hydrogenolysis activity.In sum,decreasing the Ni particle size is an effective way to improve the HDO performance of Ni based catalysts.Based on this rule,we applied nickel phyllosilicate derived Ni/SiO₂ for HDO of m-cresol.This method results a highly uniformly dispersed and stable Ni/SiO₂ catalyst.It shows significantly enhanced activity and deoxygenation selectivity,compared with the impregnation method derived Ni/SiO₂.Decreasing the Ni particle size is indeed an effective way to promote deoxygenation of m-cresol to toluene,however,even though the Ni particle size decreases to 2 nm,the defected Ni surface still shows 123 k J/mol high activation barrier for C-O cleavage.One persuasive reason is that the-OH group in phenol is repulsion by Ni surface.Here,we introduce oxophilic Re to Ni/SiO₂ system.Ni and Re tends to form surface Ni Re alloy with electron transfer from Ni to Re after reduction.DFT calculation suggests that the O in phenyl could form a bond with Re while the phenyl ring contact with Ni site.This adsorption structure lengthens the C-O bond,and lowers the C-O breaking energy to 96 k J/mol.Thus,the Ni Re/SiO₂catalyst shows much higher activity,and significant enhancement in toluene selectivity.Besides,it also effectively inhibits C-C hydrogenolysis side reaction.Since the Re addition breaks the surface Ni ensemble size,and lowers the Ni electron density which inhibit the formation of transition state during C-C hydrogenolysis and facilitate desorption of toluene,avoiding its further hydrogenolysis.In addition,the reaction network for m-cresol conversion was established over Ni/SiO₂ and Ni Re/SiO₂.There are three initial parallel pathways:deoxygenation,hydrogenation and C-C hydrogenolysis.C-C hydrogenolysis happens on unsaturated compounds with phenyl ring,instead of saturated compounds,since dehydrogenation is prior to C-C cleavage.The turnover frequency for CH₄ formation follows an order of benzene>>toluene>m-cresol,indicating that C-C hydrogenolysis is accelerated after deoxygenation and/or demethylation.Increasing reaction temperature promotes both deoxygenation and hydrogenolysis.However,toluene selectivity keeps lower over Ni/SiO₂ at 340?,but much higher over Ni Re/SiO₂.Since Re addition not only promotes deoxygenation to toluene but also facilitates it desorption and inhibits C-C hydrogenolysis.The above studies indicate that decreasing the Ni particle size is a good way to promote m-cresol deoxygenation to toluene,whereas introducing Re to Ni/SiO₂could further decrease the C-O breaking barrier.However,none of those ways could completely inhibit C-C hydrogenolysis,which consumes large amount of H₂,and causes vast C loss.Here,we found that Mo O_x modified Ni surface not only shows magnitude high activity than Ni/SiO₂,but also exhibit extremely high deoxygenation selectivity to toluene at 250-350?.More importantly,it shows no activity for C-C hydrogenolysis even at 350?,and very lower phenyl ring hydrogenation selectivity at 250?.A detailed characterization indicates that Ni and Mo tend to form Ni Mo O₄after calcination.Interestingly,after reduction at 500?,contrary to the formation of Ni Mo alloy,Ni cluster separate out with Mo O_x partially decorating on Ni surface and geometrically and chemically modified Ni surface.The synergistic effect between Ni and optimized amount of surface Mo O_x is essential to maximize the deoxygenation activity while completely inhibit C-C hydrogenolysis.W modified Ni/SiO₂ catalysts show similar HDO behavior like Ni Mo/SiO₂catalysts.However,these two catalysts have very different active structure.After calcination at 500?,rather than forming Ni WO₄,Ni and W connected by(Ni-O)₂-W(=O)₂.This structure reduces to surface Ni W alloy and W₃O,accompanied with electron transfer from Ni to W.It is proposed that both Ni W alloy and Ni-WO_x interface are the active deoxygenation site.The co-impregnation method and the W/Ni molar ratio of 1 are essential to maximize the Ni-W interaction,showing the highest deoxygenation activity,while completely inhibit C-C hydrogenolysis to CH₄.In addition,the Ni W catalyst calcined at 750? could form Ni WO₄,however,it shows same HDO performance as that calcined at 500?.