Construction Of Non-precious Metal Oxide-Based Nanocatalysts And Their Applications In Borrowing Hydrogen Reactions

The construction of the carbon-carbon（C-C）bond and carbon-nitrogen（C-N）bond is the basic content of organic synthetic chemistry.The various coupling reactions used in the formation of traditional C-C and C-N bonds usually have environmentally unfriendly leaving groups,such as halides or boron.It is known that these processes have produced waste discharge,energy waste,and other problems,which brought serious pollution to the environment.Borrowing hydrogen reactions,also known as automatic hydrogen transfer reactions,have received much attention due to their inherent high efficiency,which allows alcohols or amines to be used as alkylating agents,respectively producing water or ammonia as the only by-products.Borrowing hydrogen reaction is a tandem process,which generally goes through the following process:alcohols or amines dehydrogenation under the action of the base,the resulting compound condensation to nucleophilic reagent form an unsaturated intermediate,then the unsaturated compound hydrogenation reduction to get the final product.The earliest research in this field mainly used ruthenium,iridium,and other noble metal-organic complexes as catalysts,but the metal-organic complex catalysts cannot be recovered,and most of the metal-organic complexes are sensitive to oxygen and complex to synthesize.With the popularity of the concept of green chemistry,the heterogeneous catalyst has the characteristics of low cost,reusable,and easy to recover,so the application of heterogeneous catalyst in borrowing hydrogen reaction system is more favorable.The development of new heterogeneous catalysts to improve selectivity and catalytic efficiency has become one of the research focuses of organic synthesis.However,there are some problems such as low selectivity,limited reaction rate,and complex catalyst preparation.To solve the above problems,combined with our research group on borrowing hydrogen reactions in recent years,this paper designed and synthesized a series of metal oxide-based catalysts,using simple and cheap substrates to efficiently synthesize a series of ketone compounds,amine compounds,and a variety of N-methyl compounds.The main research results are as follows:The dehydrogenation step on different heterogeneous catalyst surfaces will affect the selectivity of borrowing hydrogen reaction.Firstly,a composite catalyst of metal oxideγ-Al₂O₃ supported Cu nanoparticles with acid-base sites was prepared in this paper and used for the cross-coupling borrowing hydrogen reaction of secondary alcohols.In this paper,the catalytic activity and catalytic mechanism of Cu/γ-Al₂O₃ nanocomposite catalyst for secondary alcohol cross-coupling reaction were investigated,and the mechanism of the synergistic effect between metal and metal oxides was found.The composition and valence state of the catalyst were studied by XRD,SEM,and XPS.The pore structure of the catalyst was detected by BET.The TPD test explained that the catalyst could remove and store hydrogen in the reaction.The catalytic borrowing hydrogen reaction results show that the highest catalytic yield of 98%can be achieved whenγ-Al₂O₃ is loaded with only 0.5%elemental Cu,and 15 kinds of substrates can be extended to achieve a higher yield.The composite catalyst showed high catalytic stability and remained active after repeated use five times.The structure of the catalyst did not change.The reduction steps of borrowing hydrogen reaction can be catalyzed by different heterogeneous catalysts on the surface.After successfully preparing metal oxide co-catalyzed borrowing hydrogen reaction with metal nanoparticles,based on this research,the direct application of metal oxide catalyst to catalyze borrowing hydrogen reaction will be deepened.Based on the understanding of the surface properties of metal oxides and the borrowing hydrogen reaction,it is found that Zr O₂ has the potential to catalyze both alcohol dehydrogenation and unsaturated double-bond hydrogenation.Zr O₂ nanoparticles were prepared and used to catalyze a series of borrowing hydrogen reactions.The results show that metal oxide nanoparticles Zr O₂ have excellent catalytic borrowing hydrogen ability.The synthesis of Zr O₂ was confirmed by XRD,SEM,and TEM.The pore structure of the catalyst was detected by BET.Zr O₂ can also catalyze the construction of the C-C bond and C-N bond in borrowing hydrogen reactions.The highest yield was 97.5%,and 24 substrates were expanded.Zr O₂ nanoparticles showed high catalytic stability and retained their original activity after five reuses.Based on the fact that the reduction step can be catalyzed by metal oxide in the hydrogenation reaction,the hydrogenation reduction step can be catalyzed by different metal oxide surfaces.At the same time,the hydrogenation reaction can be catalyzed by metal oxidation from thermal catalysis to photocatalysis.In this paper,a p-n type Cu₂O/Ti O₂photocatalyst was designed to catalyze borrowing hydrogen reaction.Cu₂O/Ti O₂photocatalyst was used to solve the problem that the slow speed of the reduction step in the photocatalytic hydrogen borrowing reaction hindered the whole borrowing hydrogen reaction.At the same time,it is found for the first time that the imine reduction reaction can be carried out on the surface of Ti O₂.The components and valence states of the catalyst were confirmed by XRD,SEM,EDX,TEM,and XPS.The pore structure of the catalyst was tested by BET.Finally,the ultraviolet-visible spectrum and Motty-Schotty test proved that the catalyst was a p-n type photocatalyst.The fluorescence spectra test further explained the reason for the high efficiency of the catalyst.The photocatalytic N-methylation rate reported in this section is double the highest rate reported.When Ti O₂ is only supported by 0.5%Cu₂O,the highest catalytic yield can reach 99.1%,and 30 kinds of substrates are expanded and catalyzed.Cu₂O/Ti O₂ showed high catalytic stability,maintaining the original activity and structure unchanged after repeated use five times.Based on previous work,further explore the influence of solvent in heterogeneous catalysis.Cu/Zr O₂ catalyst formed in situ in the reaction solvent 1-octanol.The mechanism studies show that Cu（0）nanoparticles act as hydrogen transfer active sites to degrade alcohols and adsorb aryl bromine in the reaction system.XRD,SEM,TEM,and XPS confirmed that the elemental Cu was formed in situ and loaded on the surface of Zr O₂.BET detected the pore structure of the catalyst.Cu/Zr O₂ is effective for both debromination and dechlorination.Cu/Zr O₂ showed high catalytic stability in the 1-octanol solvent,which could be reused 5times without significant degradation of catalytic activity.