Study On The Structural Design Of Tungsten Carbides And Their Tandem Catalysis For Hydrodeoxygenation Of Lignin-oxygenates

Over the last few years,the rapid development of industry accompanies with exploding demand of fossil energy,which posts the issue of energy security under the spotlight.Lignocellulose is an abundant and renewable resource.The production of biofuels and value-added chemicals from lignocellulose can alleviate the dependence on fossil resources and reduce the environmental pollution.The primary pyrolysis of lignocellulose to lignin-derived oxygenates such as bio-oils is regarded as the simplest and the most cost-effective pathway.However,the high O content can result in in low heating value,high viscosity and poor stability.Such bio-oils contain large amount of oxygenates including phenolics,acids,esters,furans and ketones,and they are hardly for further utilization.Hydrodeoxygenation(HDO)is an efficient way to extensively remove the O-containing functional groups and produce value-added chemicals and bio-oils with improved energy density,which is the one of the most important processes to upgrade lignin-derived oxygenates.Specifically,it is of great importance to develop cost-effective catalysts with superior performance and prolong duration for HDO of lignin-derived oxygenates.To tackle the challenge in the activation and transformation of C-O bonds in these oxygenates,this dissertation chooses guaiacol(GUA)as the typical model substrate.Structure of tungsten carbides and their tandem catalysis were strategically designed on basis of the goal products,such as phenol,cyclohexanol,cyclohexane and so on.The subtle relationship between the structure and performance and proposed possible reaction pathways were studied,aiming to get deeper understanding in the transformation of lignocellulose.The main content is shown as follows:1.Evenly dispersed tungsten carbide nanoparticles on carbon spheres were synthesized by integrated pyrolysis and carbon-thermal reduction.The phase compositions of WxC@CS catalyst are controlled.Among them,WxC@CS-3h with typical compositions(W2C/WC=43/57)exhibits an impressive catalytic capacity to convert aryl ethers.For example,HDO of GUA can be fully complete with a selectivity of 92.7%towards phenol.The results also show that tungsten carbide display regio-selective hydrogenolysis of inert aryl ether C-O bonds rather than aliphatic C-O bonds.2.Tungsten carbide catalysts with different phase compositions including W@CS,W2C@CS,W1.25C@CS and WC@CS were constructed to investigate the relationship between structure and catalytic performance in GUA HDO.Among these catalysts,W2C@CS cleaves all kinds of C-O bonds without regioselectivity,which is due to the electronic properties and extensive exposure of tungsten sites.W1.25C@CS with similar composition of WxC@CS-3h displays superior selective conversion of GUA towards phenol.Based on the results of a series of characterizations including XRD,TEM,TPD,XPS,HS-LEIS analysis,it is found that the carbide structure and electronic property change with the increase of carburization.The insertion of C atoms into the parent tungsten allows the electron transfer from W to C species,affecting the activation of H2 and GUA molecule.Further reconstruction of commercial WC from inert phase to active catalysts demonstrates that there is an intimate correlation between the phenol space time yield(STY)and the surface C/W atomic ratio.The optimal catalytic performance was obtained when the C/W atomic ratio is 7.2,achieving the max phenol STY of 13.6 mmol gcat-1 h-1.3.Transition metal carbides always easily deactivate in heterogeneous catalysis and it remains challenging to retain the sintering-resistant properties for them in redox reactions,such as hydroprocessing.The deactivation of WxC@CS in GUA conversion was found and extensive investigated in this section.Particle sintering and agglomeration were identified as the main reasons.To resolve the issue,an integrated strategy to synthesize well-defined and anti-sintering p-WxC/CNT was proposed by assembling the metal precursor on carbon nanotubes(CNTs),wrapping a thin polymeric layer(RF),and following controllable carburization.The RF serves as a soft carbon source to modulate the metal/carbon ratio in carbide phases and incorporates amorphous carbons around the particles to prevent from sintering.As consequence,the p-WxC/CNT exhibits a high stability in cleavage of aryl C-O bond in GUA for more than 150 h.It also achieves superior performance in selective cleavage of the aryl C-O bonds in lignin-derived oxygenates,including anisole,dimethoxylphenol,and diphenyl ether,with robust lifespan.4.We propose an interoperated catalytic strategy that separates C-O hydrogenolysis and hydrogenation on different active catalysts,which achieves high oxygen removal in lignin-derived oxygenates.The flexible use of WxC@CS for cleavage of C-O bonds and Ni/CNT catalyst for hydrogenation of aromatic rings enables tunable production of cyclohexane and cyclohexanol from GUA.Such integrated dual catalysts with closed proximity lead to superior HDO performance in anisole,dimethoxylphenol,and diphenyl ether.The superior HDO activity of actual bio-oils into liquid alkanes can also be achieved with a high mass and carbon yield of 27.9 and 45.0 wt%,respectively.5.We design a bimetallic Ni-Fe/CNT for HDO of GUA.Rational incorporation of Ni and Fe achieves high catalytic performance in GUA conversion towards phenol or cyclohexane while Ni and Fe alone exhibit poor activities.In contrast,cyclohexane and phenol can be obtained over the Ni5-Fe1/CNT and Ni1-Fe5/CNT catalysts,respectively.The bimetallic Ni-Fe/CNT shows a synergistic effect on GUA conversion.Ni species facilitate the activation of H2,and Fe species are beneficial for the activation of O-containing functional group due to its strong oxophilicity.Moreover,the catalyst displays a significant particle size effect that smaller particles achieve higher HDO performance.The introduction of Fe species into to Ni-based catalysts is beneficial for higher dispersion of metal species.Consequently,the catalysts with suitable Ni/Fe atomic ratio with smaller particle sizes perform better activity in HDO of GUA,leading to high selectivity to cyclohexane or phenol.