Study On The Cascade Reactions From Furfural To γ-valerolactone

With the massive consumption of non-renewable fossil resources and the increasing environmental pollution,the development of new technologies for producing chemicals and fuels from renewable resources has become one of the major challenges facing the current chemical research field.Among various sustainable resources（e.g.,solar energy,biomass,and wind）,biomass is a carbon-containing renewable resource in nature,which makes it an ideal alternative for fossil-based chemicals and fuels.As one of the most important biomass-derived industrial chemicals,furfural can further produce high value-added product gamma-valerolactone（GVL）.One-pot cascade transformation of furfural intoγ-valerolactone is mainly composed of two groups of transfer hydrogenation（CTH）reactions and one ring-opening reaction,in which transfer hydrogenation is the key step.In this study,we successfully synthesized functional catalytic materials with Meerwein-Ponndorf-Verley（MPV）activity using natural building blocks and metal zirconium and finally realized the one-step cascade conversion of furfural into GVL on a single catalyst.In addition,the effects of ligand substitution and electronic effects on the activity of Schiff base zirconium Lewis acid-mediated transfer hydrogenation were also be evaluated.the main conclusions of this paper are as follows:1.An amorphous quercetin-zirconium catalyst was synthesized using quercetin and ZrCl₄as raw materials.The catalyst showed excellent catalytic performance for MPV reaction,it can efficiently convert furfural（FUR）to furfuryl alcohol under mild conditions（120?°C）,with a high FAL yield of 97.9%after 1.5 h creation.The systematic characterization confirmed that there are abundant Lewis acid-base(Zr⁴⁺-O^2--)sites in zirconium quercetin,which also has good generality for MPV reduction of other carbonyl compounds.Kinetic studies revealed that the activation energy of the MPV reduction of FF was as low as 55.6?k J/mol,accounting for the high reaction rate.Moreover,Qu-Zrshowed good recyclability for at least six reaction cycles.2.A novel bifunctional mesoporous zirconium-alizarin（Zr-AZN）hybrid was successfully synthesized by using a template-free self-assembly method.Systematic characterizations demonstrated that the catalyst possessed abundant acid-base couple sites（density:0.47 vs 0.79mmol/g,acid/base molar ratio:0.59）and high porosity.Zr-AZN exhibited superior catalytic activity toward Meerwein-Ponndorf-Verley（MPV）reduction of different biomass-derived carbonyl compounds compared to other catalysts,especially of ethyl levulinate（EL）toγ-valerolactone（GVL）,in quantitative yields（96%）with high GVL formation rate of1205?μmol?g^-1?h and TOF of 0.47?h^-1.Theoretical calculations show that the structure of Zr-AZN with the highest catalytic activity is the complexation that occurs at the 1,2-dihydroxyl（catechol）group,instead of the 1-hydroxy-9-keto group.Furthermore,it also presents a higher activity and selectivity and a lower computed apparent activation free energy（19.4 vs.29.3 kcal/mol）for the hydrogenation of EL compared to the Sn-AZN.3.A multifunctional Zr-containing catalyst（FM-Zr-ARS）was successfully synthesized by a modulated hydrothermal synthesis route.Systematic characterization results supported the presence of robust porous inorganic-organic frameworks stabilized by the strong coordination interaction of Zr⁴⁺with oxygen-rich functional groups in Alizarin red S（ARS）.Moreover,the-O-Zr-O-network in the FM-Zr-ARS structure formed a rich content of acid-base sites.In addition,the inherent sulfonic groups in ARS made the FM-Zr-ARS hybrids possess Br?nsted acid sites.Therefore,under the synergistic catalysis of the multiple functional sites,FM-Zr-ARS showed remarkably high catalytic activity forγ-valerolactone（GVL）production from levulinate esters and furfural.Finally,72.4%and 97.7%yields of GVL were obtained in the conversion of furfural and ethyl levulinate,respectively,after 8 h of reaction at 160oC.On the basis of the role of different functional sites,a plausible catalytic mechanism was presented for the conversion of biomass-derived furfural to GVL.4.To get in-depth knowledge of the ligand substituent electronic effects on the catalytic activity of the catalyst,a series of Schiff base Zrcatalysts with different phenyl substituents were designed and synthesized.The experiments and molecular modeling calculations were performed to establish the structure–reactivity relationship with various ligand substituents of ZrCl₂-Sal（ph）en-X（X=H,CH₃,OCH₃,Br,NO₂,Cl）.The influence of the phenyl group bridged by the Schiff base ligand diamine on the reactivity of the Lewis acid metal center and the effect of the two remaining axial coordination vacancies in the planar array on the reactivity of the catalysts was also investigated by theoretical calculations.Significant findings:A clear linear free-energy relationship（LFER,r²=0.97/0.93）was found between Hammettσ_p value of the phenoxyl side group substituent X and the rate K_X or reaction barrier of the hydrogenation of ethyl levulinate.Lewis acid sites are stronger in catalysts with an electron-withdrawing group close to the Zrsite.In addition,the auxiliary ligands at two axial sites connected to the Zrsite have a more significant impact on the catalyst activity.Among the three axial ligands（Cl,OH and Oi Pr）,the-OH ligand with a smaller size endows additional basicity of catalyst,which is beneficial to the activation and dissociation of the hydroxyl group in the 2-propanol at the Lewis acidic sites(Zr⁴⁺)and basic sites(O^2-),thus increasing the reaction rate of the Meerwein-Ponndorf-Verley（MPV）reaction of ethyl levulinate.