Catalytic Conversion Of Lignin And Its Model Compounds To Arenes And Alkanes

As a natural resource containing abundant aromatics,lignin can be considered as a good substitute for production of aromatic hydrocarbons from non-renewable fossil oil.This dissertation focuses on the catalytic conversion of lignin and lignin-derived dimers and monomers to arenes and alkanes.The new hydrodeoxygenation route of conversion ofb-O-4 to arenes is designed and the key steps in this route are also intensively investigated.The hydrothermal stability and hydrodeoxygenation activity of catalysts are carefully considered and such targeted catalysts are thus designed.New strategies for increasing the yields of light alkanes via suppressing the condensations in the depolymerization of lignin is proposed and the hierarchical catalyst whose pore size is consistent with the lignin particle size is successfully synthesized and it obtains nearly theretical light alkane yield from the hydrodeoxygenation of lignin.It contains the following parts:（1）Catalytic conversion of lignin dimers and monomers to arenes at mild conditionsIn the third chapter,firstly,the mild reaction conditions at 240℃ and 2 bar H₂ as well as the metal catalysts of Ru/C are screened and confirmed for the selective hydrogenolysis of the C-O bond of lignin dimerb-O-4 to phenol and ethylbenzene in water.Subsequently,combined with the kinetics ofb-O-4 conversion on Ru/C,the hydrodeoxygenation of intermediate phenol to benzene with the preservation of ethylbenzene is implemented by the combination of Ru/C and solid acid catalyst.Then,the designed hydrodeoxygenation route can also be applied to other lignin-derived dimers likea-O-4和4-O-5 with high aromatic products selectivity,indicating that this new reaction route is subjected to the conversion of main dimers in lignin.Not only is the new reaction route developed but also the mechanisms of its key steps are studied.The starting step of this cascade reaction is selective hydrogenolysis of the C-O bond of guaiacol without hydrogenating the benzene ring,and the high hydrogenolysis selectivity is achieved by screening different metal sites and regulating the distribution of activated hydrogen on the catalyst under appropriate dynamic conditions.The second step of this cascade reaction is selective hydrodeoxygenation of intermediate phenol to benzene without hydrogenation of ethylbenzene.The hydrogenolysis and hydrogenation rates of the combined metallic and acidic catalysts arrive at a balance,that is,the low hydrogenation rate of ethylbenzene,the proper rates of hydrogenation of phenol to cyclohexanol,dehydration of cyclohexanol to cyclohexene,and dehydrogenation of cyclohexene to benzene.Only in this way can the cascade reaction work smoothly,and the preferred mixed catalyst is Ru/Zirconium sulfate（SZ）.DFT calculation and experimental results indicate that the key to the second step is the stronger adsorption heat of phenol than that of other intermediates on the Ru/SZ catalyst.Because the stronger adsorption of phenol on the Ru metal surface can not only contribute to the desorption of the other intermediates like ethylbenzene,cyclohexanene,and activated hydrogen by occupying their adsorption sites but also form a hydrogenation-dehydration cycle by accepting the activated hydrogen generated by the dehydrogenation of cyclohexene.Guaiacol is the richest structure in the lignin and lignin-derived phenolic oil,and its hydrodeoxygenation to arenes have been studied widely.However,the problems in these studies are either the poor hydrothermal stability of the catalysts in the condition of the high selectivity of arene products if the reaction is conducted in the water or the easy coking of the catalysts with the slow hydrodeoxygenation rates if the reaction is performed in the gas phase.On these two questions,the detailed investigations of the improvement of the hydrothermal stability of catalysts in the fourth chapter and the increase of the deoxygenating rates in the hydrodeoxygenation of the guaiacol in the fifth chapter are carried out.（2）Developing the hydrothermally stable and highly selective catalysts for converting of guaiacol to benzeneIn the fourth chapter the screening of different Ru based catalysts in the hydrodeoxygenation of guaiacol and a series of recycling tests and characterizations results demonstrate that Ru/HZSM-5 is the catalyst with the best hydrothermal stability and highest arene products selectivity.The reaction results of different Ru based catalysts manifest that the hydrogenation of the benzene ring of guaiacol is prone to occur on the Ru supported on materials with Lewis acid or without acid,while the high selectivity of hydrogenolysis of C（sp²）-OMe in the guaiacol is achieved by the Ru based catalysts with the combination of Br?nsted and Lewis acid.The carbon balance in this catalytic system is at around 98.6%.The reaction route in the water follows selective hydrogenolysis of guaiacol to phenol and methanol and hydrogenolysis phenol to benzene,while this is different from the reaction route of selective hydrogenolysis of guaiacol to catechol,hydrogenolysis catechol to phenol and hydrogenolysis phenol to benzene occurred in the gas phase.This difference is attributed to the following three reasons:1.DFT calculation results indicate that the guaiacol is horizontally adsorbed on the Ru surface,and the C（sp³）-OAr bond in comparison with the other two C-O bonds is away from the metallic Ru site as a result of the sp³ bonding carbon atom in the methoxy group.2.With the increase of reactant concentrations from 1 wt%in the aqueous phase,50 wt%in the aqueous phase,100wt%in the gas phase,and 100 wt%in the liquid phase,the selectivity of catechol in the products also increases,while the selectivity of phenol decreases.This is ascribed to the higher probability of hydrogenolysis of the weakest C（sp³）-OAr with more accessibilities between Ru and guaiacol when the guaiacol concentrations increase.3.DFT calculations demonstrate that the bonding energies of the three different C-O bonds in the guaicol is C（sp²）-OH(466 KJ?mol^-1),C（sp²）-OMe(409-421 KJ?mol^-1),and C（sp³）-OAr(262-276 KJ?mol^-1).In the gas phase the first dissociation starts from the weakest C（sp³）-OAr bond,while in the aqueous phase the prior hydrogenolysis of C（sp²）-OMe takes place because of the greater decline of C（sp²）-OMe bond energy deduced by the interactions between the oxygen in the guaiacol and the protons ionized by the high-temperature water.Therefore,these three explanations basically figure out the differences of hydrogenolysis of C-O bonds of guaiacol in the aqueous and gas phase.（3）Developing the bimetallic RuNi/HZSM-5 catalyst for efficiently converting of guaiacol to benzeneIn the fifth chapter we aim at developing better catalysts to further increase the hydrodeoxygenation rates of guaiacol.In the fourth chapter the kinetic studies of guaiacol and its derived compounds indicate that the rate-determining step in the cascade hydrodeoxygenation of guaiacol is the hydrogenolysis of guaiacol to phenol,and this is because of its low reaction rate caused by the low hydrogen pressure.The strategy to improve the reaction rate is to synthesize the catalyst with more active sites.Different kinds of bimetallic Ru-M/HZSM-5 catalysts are synthesized and applied to the hydrodeoxygenation of guaiacol in the water,and the results demonstrate that RuNi/HZSM-5 is the most effective catalyst for selectively hydrodeoxygenation.A series of characterizations such as XRD,TEM,HR-TEM,TEM-Mapping,TEM-EELS,XAFS,and DFT calculations show that the RuNi nanoparticles distributed on HZSM-5 are separately dispersed with the independent Ru and Ni nanoparticles instead of the formation of RuNi alloys phase.H₂-TPR,CO-IR,and XPS results indicate that there are interactions between Ru and Ni nanoparticles.Different amounts and different kinds of RuNi active species are synthesized by changing different Ru/Ni ratios and reduction temperatures,respectively.The testing results on these catalysts tell us that the interacted Ru and Ni nanoparticles on HZSM-5 are the real active sites for the hydrodeoxygenation of guaiacol in the water.The average distances of Ru and Ni nanoparticles are critically regulated by changing the mixed and compressed ways of Ru/HZSM-5 and Ni/HZSM-5,and it is confirmed by H₂-TPR,TEM-EELS,and SEM-EELS results.The hydrogenolysis results tested on these catalysts show that the shorter the distances between the Ru and Ni nanoparticles are,the higher the hydrogenolysis rates will be,and this may be due to the possible hydrogen spillover occurred on Ru/HZSM-5.Experimental data demonstratethatindependentNi/HZSM-5remainsinactiveforthe hydrodeoxygenation of guaicol in the water,while the hydrodeoxygenation rate is significantly increased on the RuNi/HZSM-5.That is because the Ni nanoparticles are not able to activate the dissolved hydrogen with low concentrations in the water,while Ru can easily dissolve the low-concentration hydrogen to the activated H·,the generated activated H·is then transferred to the surrounding Ni nanoparticles with the aid of HZSM-5 support,and the Ni nanoparticles recover the hydrogenolysis activity with the H·adsorbed on the surface.This will greatly increase the numbers of active sites,therefore the Ni sites are more likely to be activated with reduced hydrogen spillover distances.H₂-TPR indicate that the reduction temperature of NiO is dramatically decreased in the presence of metallic Ru nanoparticals and H₂-TPD-MS results manifest that HZSM-5 is a wonderful carrier to transfer and store the H·dissolved on the metallic sites.The two evidences both prove the existence of hydrogen spillover in this catalytic system.Detailed characterizations of RuNi/HZSM-5 indicate that there are many Ni islands on the HZSM-5,which are inactive.The highly uniform distribution of independent Ru and Ni nanoparticles are synthesized by first dipping of Ru precursors and second dipping of Ni precursors into HZSM-5 different from simultaneous dipping of Ru and Ni precursors into HZSM-5.It is confirmed by Py-IR,H₂-TPR,and CO-IR results and the hydrogenolysis activity is further increased on modified RuNi/HZSM-5.（4）Designing the hierarchical Ni/ASA catalyst to highly convert lignin to alkanesIt was reported that the yields of light alkanes from catalytic conversion of lignin were far from the theoretical yield.This is probably due to the possible condensations caused by aldol condensation and electrophilic addition reaction in the presence of active groups of carbonyls and benzene rings from the lignin depolymerized intermediates.Eventually the new C-C bonds are formed,which are difficult to be further cleaved.Such condensations maybe resulted from the poor contact between the metallic sites and the lignin macromolecules in the heterogeneous catalysis,so in the sixth chapter,we propose a strategy to restrain the repolymerizations by fast hydrogenation of active groups（carbonyls and benzene rings）to inactive groups（hydroxyl and saturated rings）in the course of hydrodeoxygenation of lignin.The Ni based catalysts with different porous structures such as micropore,macro-micropore,open macro-micropore,and macro-mesopore are synthesized by post-treatment of silicalite-1 with alkali and hydrolysis of（C₄H₉O）₃·Al and C₄H₁₂O₄Si.The results show that the yield of the light alkanes is greatly improved when the pore size of the synthesized catalyst is matched with lignin particle size.Nearly theoretical alkane product yield can be obtained and large amounts of lignin can be converted on the Ni/ASA with macro-and meso-pore structures.The effects of different porous Ni based catalysts on hydrodeoxygenation of lignin to alkanes are tracked and compared by a series of characterizations such as in-situ IR,and NMR,MS,and GPC combined with the kinetics.The characterization results show that in the macro-mesoporous catalyst（Ni/ASA）,the peripheral ether bonds in the lignin structure are firstly cleaved to produce monomer and oligomer,the benzene ring and carbonyl groups in the monomer is swiftly hydrogenated to“stable”saturated ring and hydroxyl groups in the alcohol,the alcohol continue to be hydrodeoxygenated to alkanes,while the benzene rings in the oligomer are also fastly hydrogenated to saturated rings in the saturated polycyclic alcohol,the polycyclic alcohol can be cleaved to monomer alcohol,the alcohol is also hydrodeoxygenated to alkane.However,in the microporous catalyst（Ni/Silicalite-1）the peripheral ether bonds in the lignin structure are also firstly cleaved to monomer and oligomer,but the subsequent hydrogenation rate of active groups of monomer and oligomer is quite low.These results indicate that different porous Ni based catalysts not only alter the route of hydrodeoxygenation of lignin,but also the hydrodeoxygenation rates of active groups to inactive groups in the lignin and its depolymerized intermediates,and thus breaking the balance of depolymerization and repolymerization of lignin.The ultraviolet research shows that the faster adsorption rates and greater amounts of lignin are obtained on the Ni/ASA in comparison with Ni/Silicalite-1,demonstrating that the matched size of the pore in Ni/ASA and lignin particle is conclusive to the adsorption of lignin macromolecule on Ni/ASA.