Study Of Hydrodeoxygenation Of Lignin Derivatives To Value-Added Chemicals On Fe-and W₂C-Based Catalysts

Renewable biomass has been considered as an alternative feedstock to fossils for sustainable production of fuels and chemicals.Lignin is an important component of biomass resources,which can potentially been converted to energy carriers due to its higher ratio of carbon/oxygen and energy density.At present,the direct utilization of lignin-derived monomers,which contain a large number of oxygen groups,is not cost-effective.Hydrodeoxygenation(HDO)process is one of the approaches to remove the oxygen in lignin-derived monomers.In HDO processes,hydrogen could be consumed in both hydrogenation of benzene rings and hydrogenolysis of C-O bond while it is highly desired to selectively cleave the C-O bond to minimize the hydrogen consumption.It has been reported that Fe and W2C both show the excellent activity in C-0 bond cleavage.However,the deactivation of Fe-based catalysts by water limits their applications.Though Pd can effectively improve the oxidation resistance of Fe,search for alternatives to precious metals such as Pd is highly desired.Compared to Fe-based catalysts,W2C shows higher catalytic activity and selectivity.In particular,monolayer-dispersed W2C tends to exhibit the properties of precious metals.This dissertation focuses on the study of the effect of Pt/Fe ratio in stabilizing Fe from being oxidized by water,the role of nitrogen doped carbon materials on the stability of Fe against water oxidation,and the structure and function relationship of W2C using HDO of lignin-derived m-cresol as a probe reaction.The objective is to provide scientific basis for the development of efficient and stable catalysts in lignin hydrodeoxygenation reaction.The research results of this dissertation are summarized as follows:1.A series of Pt-Fe bimetallic catalysts(containing 0.02 wt.%,0.05 wt.%,and 0.1 wt.%Pt)with uniform Fe particle size were prepared by a co-precipitation method.The addition of Pt increased the amount of H2 adsorption on the surface of Fe,which resulted in an improvement in activity and the antioxidation stability of Fe catalysts.A minimum Pt/Fe atomic ratio of 1/13 was found to be able to maintain the Fe stability and the oxidation resistance mechanism might be explained by the fact that the active H species from H2 dissociation on Pt can effectively spillover and cover the entire surface of the Fe catalyst,which promotes the removal of oxygen species on the surface of Fe,thus effectively avoids the deactivation of the Fe catalyst.2.Fe/NC-x catalysts with doped nitrogen concentrations ranging from 2.0 wt.%to 7.8 wt.%were successfully prepared by adjusting the ratio of glucose and the nitrogenous precursor.Higher nitrogen content results in smaller particle size of Fe,which is related to the interaction between Fe and nitrogen species.Oxyphilic capacity of Fe is found to inversely proportional to nitrogen content,and reduced oxyphilic capacity favors the desorption of oxygen species in the process of HDO reaction.We further found that a lower nitrogen content leads to a higher content of skeleton nitrogen species(mainly including pyridine-N and pyrrolic-N functional groups),which promotes the Fe catalyst to maintain high reaction stability and water oxidation stability.Increased nitrogen content can lead to increased amino-N species,which is inclined to suffering from wastage during the hydrodeoxygenation reaction,resulting in poor stability of Fe catalysts.The stability of Fe catalysts against water oxidation would be improved by adjusting the environment of coordination nitrogen species around Fe.3.xWO3/SiO2 samples with different loadings were prepared by grafting method,and its derived xW2C/SiO2 catalysts with various W2C particle sizes were formed by carbonization.With the increase of the W2C size,the turnover frequency(TOF)values of catalysts gradually increased,and the TOF value of pure W2C sample was the highest.Kinetic studies further showed that HDO reaction is zero-order with respect to m-cresol,and the relationship between reaction rate and partial pressure of hydrogen follows the Langmuir-Hinshelwood mechanism,revealing that the reaction rates on W2C with different sizes were mainly determined by dissociation and adsorption capacity of H2 on W2C and intrinsic catalytic activity to cleavage C-O bond.