Study On Transfer Hydrogenation Of Furfural To Furfuryl Alcohol Catalyzed By Cu-based Hydrotalcite Catalysts

The decreasing fossil fuel resources and the prominent environmental pollution problems have prompted researchers to seek green and renewable alternative energy sources.Biomass resources have received wide attention for their advantages of large reserves,green,renewable and sustainability.Compared with fossil resources,biomass resources contain relatively high oxygen and low hydrogen-carbon.Therefore,biomass resources can obtain more chemicals and fuels with high added value and high energy density through catalytic conversion.Furfural（FAL）is considered as one of the most valuable platform compounds in the world,furfural can generate intermediates with high value-added compounds such as furfuryl alcohol,tetrahydrofurfuryl alcohol,2-methylfuran,etc.By hydrogenation,oxidation and condensation reactions,among which furfuryl alcohol（FOL）is the most important hydrogenation product of furfural,which is an important fine chemical intermediate widely used in synthetic resin,polyester,fuel and medicine,etc.At present,the demand is large,but the production capacity is insufficient,therefore,the preparation of furfuryl alcohol by efficient catalytic furfural has important research value and broad application.The current hydrogenation of furfuryl alcohol by furfural can be classified into two categories according to the hydrogen source,one is the direct hydrogen supply,this type of hydrogen source has limitations such as transportation and operational safety,and the cost is high.The second is solvent in-situ hydrogen supply,mainly using the solvent in the reaction system as the hydrogen donor,and the hydrogen in the hydrogen donor is removed and transferred to the unsaturated groups in the reactants under the action of the catalyst,which in turn leads to catalytic transfer hydrogenation（CTH）,and in the process of hydrogen transfer hydrogenation,the most typical reaction mechanism is the MPV reaction.MPV reaction is the hydrogen transfer behavior caused by the dominant Lewis acid-base site in the catalyst,a six-membered ring intermediate transition state consisting of the acid-base site and the hydrogen donor and reactants is formed during hydrogen transfer,which in turn drives the catalytic hydrogenation efficiency.In situ hydrogen supply via solvent is a hot topic of research because of its simple and easy to control operation,low cost,high safety and high selectivity for products,which has significant advantages over H₂ hydrogen supply.Based on this,the preparation of furfuryl alcohols by hydrogen transfer hydrogenation of furfural was carried out using alcohols as solvent and hydrogen donor.The non-precious metal Cu-based catalyst has good catalytic performance in the hydrogen transfer hydrogenation（CTH）process,because of the electrostatic repulsion of the 3d bond of Cu,which can make the lone pair of electrons of the carbonyl oxygen outside the furfural ring adsorbed vertically on the catalyst surface,and this adsorption configuration can catalyze the MPV reaction of FAL efficiently.The hydrotalcite-like catalysts are rich in Lewis acid-base sites and can be effectively regulated due to their overall structure consisting of positively charged nanoscale main body lamellae and interlayer anions.Therefore,based on green catalytic transfer hydrogenation,hydrotalcite-like catalysts containing different Lewis acid-bases were designed in alcohol solvents to study the influence law of hydrotalcite acid-base and its composition on catalytic performance,optimize the reaction process parameters in FAL hydrogen transfer hydrogenation to FOL,combine with catalyst performance evaluation and its structural characterization,establish the conformational relationship of catalysts,and reveal the FAL to FOL The MPV reaction mechanism of FAL to FOL can be realized under mild conditions for the efficient hydrogenation of furfural to prepare furfuryl alcohol.（1）To investigate the effect of different acid bases in hydrotalcite-like catalysts on the reaction of furfural catalytic transfer hydrogenation to furfuryl alcohol MPV and to reveal the reaction mechanism.Hydrotalcite-like catalysts of CuAl,ZnAl,MgAl,CuMgAl and CuZnAl with different acid-base were designed.The results showed that the Cu Mg Al catalyst with higher Lewis basicity exhibited excellent catalytic performance,and the yield of FOL reached 80.3% in 4 h of reaction at 160 ℃ with ethanol as solvent.Combined with the catalyst characterization,it is speculated that the reason for the excellent catalytic performance of the catalyst is the synergistic catalysis of the acid and base sites and the metal Cu site on the catalyst surface.The base site can significantly promote the dehydrogenation behavior in the methylene group of the hydrogen donor ethanol molecule,and then the base with its adjacent Lewis acidic site can promote the formation of the intermediate six-membered ring transition state of the MPV reaction,and the active metal Cu site can be used to absorb and activate the carbonyl and alcohol-derived hydroxyl oxygen in furfural and promote H-transfer.（2）From the results of the previous chapter,it is known that the basic sites in the catalyst have a significant effect on the catalytic performance of the catalyst.In order to further investigate the effect of Lewis acid-base on the catalytic performance of the catalyst,the Mg²⁺/Al³⁺ molar ratio in the CMA hydrotalcite-like catalyst was effectively regulated under the constant Cu content,and then the acid-base was modulated,and then the Mg²⁺/Al³⁺ molar ratio on the catalytic hydrogen transfer hydrogenation of furfural to prepare furfuryl alcohol.The catalysts with different molar ratios of x:y=2:1,x:y=3:1,x:y=4:1,x:y=5:1 and x:y=6:1 were prepared according to n(Mg²⁺): n(Al³⁺)=x:y,respectively.The results showed that the catalysts exhibited excellent catalytic performance when the Mg²⁺/Al³⁺ molar ratio was 6:1,and the yield of furfuryl alcohol was as high as 88.8% when the reaction was carried out at 160 ℃ for 4 h.Combined with the catalyst characterization results,it was concluded that the excellent catalytic performance was attributed to the high content of Cu⁰,Cu⁺ and Lewis basic sites on the catalyst surface CMA and the high synergistic catalytic ability among these active sites.During the catalytic reaction,Cu⁰ is used to adsorb and activate the carbonyl group in furfural and the hydroxyl oxygen in the ethanol hydrogen donor,Lewis basic sites are used to adsorb the hydroxyl hydrogen in the hydrogen donor,and Lewis acidic sites are used to adsorb the hydroxyl oxygen in the hydrogen donor and the carbonyl oxygen in furfural,thus catalyzing the hydrogen transfer behavior of furfural efficiently.（3）From the results of the above two parts of the study,it is clear that the basic sites on the catalyst surface are the dominant active sites to achieve hydrogen transfer hydrogenation of furfural.In order to further improve the basicity of the catalysts to enhance their catalytic efficiency,this chapter uses CuAl hydrotalcite-like as the parent material,adds melamine in its preparation,introduces nitrogen atoms with high electronphilicity（electron-rich to provide basic sites）,and prepares x:y=3:1;x:y=2:1;x:y=1:1;x:y=1 :2,etc.With four different ratios of Nitrogen-doped carbon materials CuAl hydrotalcite-like catalysts（CuAl/NC）,which were used for hydrogen transfer catalyst hydrogenation of furfural.The results showed that the content of doped NC material in the catalysts had a significant effect on the catalyst structure,properties and its catalytic efficiency for catalytic hydrogen transfer hydrogenation of furfural.When the molar ratio of CuAl:NC was 1,the catalyst surface had more basic sites,and active metal Cu sites,and the catalyst exhibited better catalyst performance,and the conversion of furfural and the yield of target product were 73.0% and 67.7%,respectively,at 4 h of reaction at170 ℃,which had significant advantages over the present literature.Combined with the catalyst characterization results,it was concluded that the reason for the significant improvement of catalytic activity and selectivity with the introduction of NC material into the CA catalyst was that there was a strong interaction between the active metal Cu and NC material,and under this strong interaction,at the interface between the active metal Cu and NC thin layer,the d electrons on the metal Cu were transferred to the NC thin layer sheet,so that the NC thin layer had a more abundant electron-rich basicity in the basic site and the synergistic catalysis of the active metal Cu site and acid site significantly improved its catalytic reaction efficiency for the hydrogen transfer hydrogenation of furfural to furfuryl alcohol.