| The catalytic conversion of renewable biomass and its derivatives to produce high value-added chemicals and fuels is of great significance to the sustainable development of mankind.Lignocellulosic biomass is composed of three components,cellulose,hemicellulose and lignin.Glucose as the basic unit of cellulose,can be converted into many important biomass-based platforms and fuels,such as fructose,5-hydroxymethylfurfural（HMF）,levulinic acid（LA）,γ-valerolactone（GVL）over the different catalystic process.Heterogeneous catalysts have the advantages of easy separation,easy to regulate the active sites,recyclable use and environmental protection.The development of green and efficient heterogeneous catalysts is one of the research hotspots to realize the industrial production of bio-based chemicals.Graphitized carbon nitride（g-C3N4）possesses many advantages,such as low raw material price,simple preparation process,non-toxicity,unique electronic structure,stable physical and chemical properties,and easy modification.It could be modified to enhance multiple functionalizations for catalytic use and catalytic performance.The application of g-C3N4 in the catalytic conversion field of biomass still needs to be further explored,so this paper constructs several g-C3N4-based catalysts for the catalytic conversion of glucose and its derivatives.While characterization of the physicochemical structure and the distribution and properties of active sites of the catalyst were explored.Finally,the reasonable catalytic reaction pathway and mechanism were proposed based on the experimental results.（1）Glucose isomerization to fructose is a crucial step for the efficient production of fuel and valuable chemicals from renewable carbohydrates.Series of acid-base bifunctional catalyst aluminum-doped graphitized carbon nitride（g-C3N4）catalyst（Al-UCN）were constructed by blending thermal polymerization and applied to the isomerization of glucose to fructose.Series of characterization techniques such as XRD,TEM,FT-IR,XPS,NMR,CO2/NH3-TPD were used to analyst the physico-chemical properties catalysts.The results of characterization confirmed that aluminum species was incorporated into the cavity structure of g-C3N4 via nitrogen-aluminum coordination,which can be used as the acidic catalytic site,combined with the basic site provided by the nitrogen-containing group of g-C3N4.Al-UCN catalysts could catalyze the isomerization of glucose through the synergistic effect of acid and base.The results of the glucose catalytic reaction experiment showed that the yield of fructose was 48.29%and the yield of levulinic acid was 14.9%withγ-valerolactone（GVL）solvent as the reaction medium at 160oC for 3 h.The selectivity of high value-added products is up to 93.9%,almost reach to the enzymatic catalytic efficiency.The yield of fructose is almost comparable to the quantitative yield（ca.50%）of the biocatalytic route.Combined with the catalyst characterization and experimental results,the main catalytic active sites of the Al-UCN catalyst are the Lewis acid six-coordinate aluminum species（Al[6]）and the basic sites provided by the nitrogen-containing groups of g-C3N4 itself.The Lewis acid catalytic pathway dominates and catalyzes the glucose isomerization reaction efficiently through acid-base synergy.Finally,the catalyst was recycled and the yield of fructose remained at about 48%after five consecutive uses.The results of the cycling experiments of the catalysts show that the Al-UCN catalysts have excellent cycling stability.（2）A phosphorus-doped graphitized nitriding carbonic acid catalyst（P-UCN）was prepared by phosphorus substitution,and its catalytic performance in the dehydration of fructose to 5-hydroxymethylfurfural（HMF）was investigated.The physical and chemical structures of the prepared catalysts were studied by means of XRD,TEM,FT-IR,XPS,NMR and other characterization methods.The results showed that phosphorus atoms successfully replaced the Corner Carbon and the Bay Carbon is firmly anchored in the g-C3N4 skeleton structure,which can overcome the disadvantage that the active group is easy to fall off in the traditional sulfonic acid-based catalyst.In dimethyl sulfoxide（DMSO）solvent,the yield of HMF could reach 91.7%at the optimum fractionation conditions（160°C with 2 h）.The NH3-TPD characterization results indicated the appropriate amount of phosphorus doping is beneficial to improve the acidity of the catalyst,thereby promoting the dehydration of fructose into HMF.In addition,the P-UCN catalyst has good cycle stability and can still maintain high catalytic activity after five consecutive cycles.（3）A serise of Cu-Ru bimetallic catalyst（Cu-Ru/Zr O2-SO3-Si O2）supported by composite support Zr O2-SO3-Si O2 was prepared by impregnation method for the efficient catalytic hydrogenation of ethyl levulinate（EL）into fuel moleculeγ-valerolactone（GVL）.Using a low-carbon alcohol as a hydrogen source,the in-situ hydrogenation of ethyl levulinate is carried out,and the reaction system avoids the use of external hydrogen sources（such as hydrogen,formic acid,etc.）,and has the advantages of safety,simplicity,convenience,and high efficiency.The structure and interaction of the catalyst were studied by a series of characterizations.The results showed that the interaction of bimetallic Cu-Ru was beneficial to the dispersion of the metal particles on the support and the regulation of the electronic state of the metal.The composite oxide support（Zr O2-SO3-Si O2）showed better hydrogenation performance than single oxide support（Zr O2,Si O2）,which may be owing to sulfur element in composite support could act as electron donor to regulate Cu-Ru metal The electronic state of the particles,and accordingly,has an important influence on the hydrogenation activity of the catalyst.When Cu-Ru/Zr O2-SO3-Si O2-450 was used as catalyst and isopropanol as hydrogen solvent,the EL could be comoletely converted to GVL at 180oC for 12h.In addition,a variety of typical biomass-based compounds（such as furfural,benzaldehyde,benzophenone,phenylacetaldehyde,etc.）containing carbonyl groups（mainly aldehyde groups and ketone groups）were selected for catalyst hydrogenation performance tests.The catalyst has excellent suitability for selective catalytic hydrogenation,but it does not perform well in the hydrogenation of the acidic substrate levulinic acid.（4）A graphitized carbon nitride supported metal ruthenium catalyst（Ru/g-C3N4）was constructed to catalyze the hydrogenation of the acidic substrate levulinic acid（LA）to prepare the fuel molecule GVL.The Ru metal particles interact with the nitrogen atoms in the g-C3N4support to anchor Ru on the surface of the g-C3N4 support,which increases the stability of the catalyst,especially the acid resistance.Besides,compared with traditional Ru/C catalysts,Ru/g-C3N4 catalysts have the advantages of low metal usage,high catalytic activity,and stable structure and properties.In addition,it was found that the g-C3N4 supports prepared using different precursors had an important effect on the particle size and electronic state of the supported ruthenium metal,which in turn affected their catalytic activity.Compared with the catalyst Ru/MCN obtained by using melamine precursor,the catalyst Ru/UCN prepared from urea has higher catalytic activity.The LA was converted into GVL,using isopropanol as the solvent and Ru/UCN as the catalyst,at 100oC for 12h,the 99.8%GVL yield achieved.The XRD,FT-IR and TEM characterization results of the catalyst show that the Ru metal particles are more uniformly dispersed on the UCN support and the metal particle size is smaller;XPS characterization shows that the electron-rich Ru0 appeared on the Ru/UCN catalyst,which could perform excellent hydrogenation activity.The study of the reaction mechanism showed that the Ru/UCN catalyst catalyzed the hydrogenation of LA to GVL with a single reaction path,which could effectively avoid the generation of by-products.The above studies show that the g-C3N4-based composite catalyst has a variety of anchoring sites.The Al,P,Ru and other active sites could be anchored into of g-C3N4 to achieve a variety of functional modifications,under the condition of ensuring the integrity of its basic structure.Therefore,it has the potential to be exploited for multi-step tandem reaction of raw biomass. |
