Preparation Of Molybdenum Carbide And Hydrogenolysis Of C-O Bond Of Guaiacol

Transition metal carbides,as a kind of important catalytic materials,have been widely concerned in the field of heterogeneous catalysis.Due to the incorporation of carbon atoms in the interstitial position,the transition metal carbide has a much higher state density near the Fermi level,which gives the material the electron configuration and catalytic behavior similar to precious metals.A large number of studies have shown that the crystal structure,site occupation,surface termination and metal/carbon defects on the bulk phase or surface of transition metal carbides are structural factors that affect their catalytic performance.These metal carbides exhibit unique catalytic performance comparable to precious metal catalysts in hydrogenation,dehydrogenation,isomerization,and hydrogenation deoxygenation reactions.Therefore,to expand and deepen the catalytic properties and structure-activity relationship of transition metal carbides has important research significance and broad application prospects.In this dissertation,the hydrodeoxygenation(HDO)of guaiacol(a model compound of lignin)to phenol was selected as a target reaction by using molybdenum carbides as the catalysts.The works focused on the preparation,performance and structure-activity of the catalysts.The main achievements are obtained as follows.A temperature-programmed reduction method was used to prepare the bulk molybdenum carbide catalysts at different heating rates.The catalyst sample was denoted as MoxC-T(T means heating rate,℃/min).Through XRD characterization,it is found that the crystal phases of the catalysts are related to the heating rates of carburization.Among them,the catalyst obtained by carburization with heating rate at 1℃/min is pure phase of Mo2C.With the increase of heating rate,the characteristic peaks of MoO2 and Mo appear gradually,except the characteristic peak of Mo2C,which indicates that the carburization degree of carbides is reduced with the increase of heating rate.The catalysts MoxC-5 and MoxC-10 obtained by carburization of 5℃/min and 10℃/min have the miscibility of MoO2,Mo and Mo2C simultaneously.The molybdenum carbide catalysts were then subjected to evaluate the performance of guaiacol HDO.The results show that the catalysts with higher carburization degree(MoxC-1 and MoxC-2.5)present higher guaiacol conversion(90%and 92%,respectively)with higher selectivity to benzene but lower selectivity to phenol.Among them,the selectivities of 36%benzene and 45%phenol are obtained over the catalyst MoxC-1;and those of 56%benzene and 26%phenol are obtained over the catalyst MoxC-2.5.The above results indicate that these two molybdenum carbide catalysts have strong hydrogenolysis ability.For the MoxC-5 and MoxC-10 catalysts,a higher phenol selectivity but lower benzene selectivity can be obtained,showing phenol selectivity at approximately 60%and 80%over MoxC-5 and MoxC-10,respectively.Thereafter,the effects of reaction temperature on the performance of MoxC-1 were investigated.The results show that,as the reaction temperature increases,the selectivity of phenol decreases and the selectivity of benzene increases,evidencing that the reaction temperature is closely related to the product selectivity.The results showed that the bulk molybdenum carbide catalyst exhibited catalytic hydrolysis ability for different C-O bonds in the guaiacol HDO reaction,resulting in phenol and benzene with low selectivity.On the basis of above results,a series of supported molybdenum carbide catalysts were prepared by wetness impregnation followed by temperature-programmed carburization method for the guaiacol HDO.The results show that the selectivity to phenol over several carriers-supported molybdenum carbidesis greatly improved.Among them,the supported molybdenum carbide catalyst with carbon nanotube(CNT)as the carrier has the best catalytic performance.The experimental results also show that different precursors have different crystal phases of molybdenum carbide.The catalysts obtained by directly carburizing ammonium molybdate/CNT precursorare consisted of MoC and Mo2C phases,which are denoted as im-MoCx/CNT.The catalysts obtained by calcination followed by carburization of ammonium molybdate/CNT precusor contain Mo2C phase only,which are denoted as im-cal-MoCx/CNT.Comparing the HDO reaction performance of im-10MoCx/CNT and im-cal10MoCx/CNT catalysts(both Mo loading is 10 wt%),it is found that the phenol selectivity of the two is almost the same each other(83-84%),but the guaiacol conversion over im-10MoCx/CNT catalyst is 66%,which is twice higher that of im-cal10MoCx/CNT catalyst(33%).But the conversion rate of im-10MoCx/CNT catalyst is twice that of im-cal-10MoCx/CNT,66%and 33%respectively.The characterization results of H2-TPD and CO-TPD show that im-10MoCx/CNT and im-cal-10MoCx/CNT have similar ability to activate guaiacol,but im-10MoCx/CNT shows stronger activation of H2.The ability to promote the conversion of guaiacol to produce phenol.It is worth noting that the catalysts im-cal-10MoCx/CNT and MoxC-2.5 exhibit different product selectivities in the catalytic reaction of guaiacol HDO.Combined with a series of characterizations such as XPS,it was found that although im-cal-10MoCx/CNT and MoxC-2.5 catalysts have similar crystal structure(XRD),their surfaces have different C/Mo atomic ratios.The valence of Mo on the surface of bulk MoxC-2.5 catalyst is lower,showing more metallic properties,and exposing more Mo metal sites,which is conducive to the adsorption and activation of Ar-OCH3 and Ar-OH bonds,resulting in phenol and benzene product.The im-cal-10MoCx/CNT with a suitable C/Mo atomic ratio can efficiently hydrogenolyze the Ar-OCH3 bond to obtain highly selective phenol.By optimizing the carburization conditions,it is found that the im-MoCx/CNT catalyst prepared at a carburization temperature at 700℃ for 2 h has the best HDO reactivity.The influence of different loadings on im-MoCx/CNT catalyst was further investigated.XRD characterization showed that with the increase of loading,the Mo2C phase in the catalyst showed an increasing trend.Under optimized reaction conditions,the im-5MoCx/CNT catalyst with a Mo loading of 5 wt%exhibited the highest phenol selectivity(91%).However,the XRD characterization of the catalyst im-10MoCx/CNT showed that it began to appear a more obvious mixed phase of MoC and Mo2C,and its phenol selectivity was not much different from that of the catalyst im-5MoCx/CNT,so its stability was investigated.The results showed that during the 150 h reaction,the selectivity of phenol remained basically stable(about 80%),but the conversion rate slowly decreased from 48.2%to 30.5%,indicating that the catalyst was deactivated to a certain extent during the reaction.