Green Construction Of Highly Efficient Catalysts For In Situ Hydrogenation Of Biomass Derivatives

Catalytic conversion of renewable biomass and its derivatives to biofuels and high-value chemicals is of great significance to the sustainable development of human society.Furfural,5-hydroxymethylfurfural,vanillin,cinnamaldehyde and other biomass-derived platform molecules are considered to be the bridge between biomass and renewable biofuel and chemicals.However,the current catalytic systems have some disadvantages,such as low catalytic activity,low product selectivity,harsh reaction conditions,complex catalyst preparation process,expensive and non-renewable raw materials,and the use of a large number of toxic reagents.In this doctoral dissertation,a series of green catalysts with simple but environmental-friendly preparation process and high catalytic activity were developed by using cheap biomass derivatives or wastes as raw materials.Their structure-activity relationship in the conversion of biomass derivatives and catalytic reaction mechanism was also studied.The main research contents are as follows:1.Preparation of MnO_x/N-doped carbon aerogel catalyst and its application in furfural hydrogenationA sustainable route to in-situ synthesize a monolithic MnO_x/N-doped carbon aerogel catalyst（Mn-NCA）by pyrolyzing MnO（OH）₂-cellulose aerogel precursor based on an alkali–urea aqueous system was developed.The as-obtained Mn-NCA furnish highly efficient catalytic activity for transfer hydrogenation of a broad scope of biomass-derived aldehydes,yielding 90～100%conversion and 64～100%selectivity to corresponding alcohols under mild conditions in an oven without agitation.Combination of controlled experiments and detailed characterizations indicates that the superior performance of Mn-NCA is attributed to the monolithic three-dimensional（3D）hierarchical porous architecture and the synergistic effects between homogeneously dispersed MnO_x nanoparticles（NPs）and urea-derived basic sites.The monolithic feature of Mn-NCA exhibits more excellent dispersibility and separability compared to conventional centrifugation and filtration techniques in powdery catalytic system.Moreover,a possible reaction mechanism is proposed.2.Preparation of ultrafine Co₃O₄/N-doped carbon nanofiber catalyst and its application in cinnamaldehyde hydrogenationBacterial cellulose was used to construct highly dispersed Co₃O₄ nanocatalysts embedded within nitrogen-doped carbon nanofibers（Co₃O₄/NCNF）.Benefiting from the nanofibrous confinement strategy,urea-assisted carbonation process and mild nitrates decomposition process,the cobalt precursor was transformed into ultrasmall and homogeneous Co₃O₄ nanoparticles（NPs,ca.1.57 nm）,which is to our knowledge the smallest value among reported supported Co₃O₄ materials.The as-obtained Co₃O₄/NCNF exhibits superior catalytic activity for selectively hydrogenation of bioderivedα,β-unsaturated aldehydes with 2-propanol as H-source,yielding 90～100%conversion under mild conditions.Controlled experiments and detailed characterizations revealed that the three-dimensional nanofibrous porous structure can be favourable for the improved diffusion and mass transfer,while the uniform distribution of ultrafine Co₃O₄ NPs and urea-derived abundant basic sites exhibit synergism in the adsorption and activation of reactants,which contributes to excellent catalytic performance.3.Preparation of zirconium–lignosulfonate polyphenolic polymer and its application in furfural hydrogenationLignosulfonate,a waste by-product from the paper industry,was simply assembled with Zr Cl₄ under non-toxic hydrothermal conditions for scalable preparation of Zr-containing polyphenolic biopolymer catalysts（Zr-LS）.Systematic characterizations indicated that the strong coordination between Zr⁴⁺and phenolic hydroxyl groups in lignosulfonate led to the formation of strong Lewis acid-base couple sites(Zr⁴⁺-O^2-)and porous inorganic-organic framework structure（mesopores centered at 6.1 nm）,while the inherent sulfonic groups in lignosulphonate could serve as Br?nsted acidic sites.The cooperative role of these versatile acid-base sites in Zr-LS afforded excellent catalytic performance for Meerwein-Ponndorf-Verley（MPV）reaction of a broad range of bioderived platform chemicals under mild conditions（80°C）,especially of furfural（FF）to furfuryl alcohol（FA）,in quantitative yields（96%）with high FA formation rate of 9600μmol g^-1 h^-1 and TOF of4.37 h^-1.Isotopic labelling experiments demonstrated direct hydrogen transfer from theα-C of2-Pr OH to theα-C of FF on acid-base sites was the rate-determining step.Moreover,Zr-LS showed good recyclability for at least seven reaction cycles.4.Preparation of hafnium–lignosulfonate polyphenolic polymer and its application in5-hydroxymethylfurfural hydrogenationLignosulfonate were used as building block to coordinate with different metal ions(Hf⁴⁺,Zr⁴⁺,Fe³⁺,Al³⁺,Zn²⁺)and thus a series of inorganic-biopolymer hybrids（M-Lig S）were prepared by hydrothermal self-assembly method.The resulting Hf-Lig S hybrid with strong Lewis acid-base couple sites,moderate Br?nsted acidic sites from the inherent sulfonic groups in Lig S exhibited the best catalytic activity for CTH of 5-hydroxymethylfurfural（5-HMF）with 2-propanol（2-Pr OH）in high yields（90%）under mild reaction conditions（100°C in 2 h）.This robust bifunctional acid-base Hf-Lig S is also demonstrated to be effective in one-step reductive etherification of 5-HMF to 5-[（1-methylethoxy）methyl]-2-furanmethanol（MEFA）,a potential biomass-derived fuel additive,with 95%yield.Kinetic studies revealed that the activation energy for CTH of 5-HMF was 62.25 k J/mol,accounting for the high reaction rate.Due to the strong interactions between Hf⁴⁺and phenolic hydroxyl groups,Hf-Lig S was highly stable and could be reused without significant decline in activity.5.Preparation of nitrogen doped graphene encapsulated cobalt catalyst and its application in vanillin hydrodeoxygenationA scale-up and sustainable method to fabricate gram-quantities of highly dispersed cobalt nanocatalysts sheathed in multilayered N-doped graphene（Co@NG）was developed by using biomacromolecule carboxymethyl cellulose（CMC）as raw material.The ionic gelation of CMC,urea and Co²⁺ions lead to the uniformly dispersion and chelation of different species,consequently resulting in the formation of high distributed Co nanoparticles（NPs）（10.91 nm）with N-enriched graphene shells in solid-state thermolysis process.The usage of urea as non-corrosive activation agents can introduce a porous belt-like nanostructure and abundant doped nitrogen.Among all the prepared catalysts in this chapter,the optimized Co@NG-6exhibited excellent catalytic activity（99%yield of 2-methoxy-p-cresol）for base-free transfer hydrodeoxygenation（THD）of lignin molecule vanillin using bioderived formic acid as H source at 160°C for 6 h.The poisoning tests and electron paramagnetic resonance spectra（EPR）verified that the strong interaction between N atoms and encapsulated Co NPs provided synergistic effects,which were essential for the outstanding catalytic performance of Co@NG-6.The deuterium kinetic isotope effect study clearly demonstrated the formation of Co-H^-viaβ-hydride elimination and protonation was rate-determining step,and the protic N-H⁺and hydridic Co-H^-were considered to be active intermediate species in THD reaction.Furthermore,Co@NG-6 was highly stable for recycling owing to graphene shells preventing Co NPs from corrosion and aggregation.