Design Of A Catalytic System Towards Liquid-phase Hydrogenation Tandem Reactions And Investigations On The Solvent Effect

The liquid phase hydrogenation tandem reaction plays a crucial role in fundamental and applied research areas,including organic synthesis,fine chemical industry and energy industry,due to its exceptional advantages in terms of superior efficiency,environmental friendliness and economic viability.However,it is still a very challenging task to coordinate multiple active sites and regulate the compatibility of multi-step reactions within a single reaction system,so as to construct an efficient liquid phase hydrogenation tandem system.In recent years,researchers have focused on developing tandem catalysts with enhanced selectivity and optimizing reaction conditions to facilitate multi-step tandem reactions.Despite a lot of research progress in this field,several problems and challenges still need to be addressed:1)limited understanding of the intrinsic active site and structure-activity correlation of catalysts hinders the improvement of reaction efficiency and selectivity;2)determining a common operational window for multi-step reactions proves challenging,which makes it difficult to effectively control the catalytic behavior of hydrogenation tandem reactions;3)The mechanism of solvent effect on liquid phase hydrogenation reactions and the understanding of catalytic reaction mechanism need to be further studied.Therefore,how to design and prepare new catalysts,and optimize the whole reaction process to enhance the efficiency of the coupling reaction,and then improve the performance of liquid phase hydrogenation tandem reaction,is a research topic with important scientific significance and practical application prospects.In this dissertation,a topological transformation approach was employed to prepare three types of highly efficient tandem catalysts using layer double hydroxides（LDHs）as catalyst precursors.This approach enables precise control over the structure of catalytic active sites,resulting in enhanced performance in three liquid-phase hydrogenation processes:the hydroalkylation of benzene,the hydrogenation tandem reaction of furfural,and the hydrogenation rearrangement reaction of furan derivatives.Additionally,by virue of a solvent effect strategy,the pathway of the hydrogenation tandem reaction was further coordinated and optimized.The structure-property correlation,along with the solvent effect,was investigated using a combination of in situ experimental techniques and computational methods,and the catalytic reaction mechanism was unveiled.This dissertation offers some theoretical basis and practical exploration for the design,preparation of new catalysts,reaction optimization in liquid phase hydrogenation tandem reaction systems.The main research contents are outlined as follows:1.Bifunctional HPW-Ni catalysts for the hydroalkylation of benzeneThe heteropoly acid H₃PW₁₂O₄₀（HPW）supported on Ni-based catalysts（denoted as x HPW-Ni/MMOs,x=mass ratio of HPW/Ni）were synthesized through a two-step method involving the topotactic transformation of NiAl-LDHs followed by an impregnation-reduction treatment.The optimal sample（0.3HPW-Ni/MMOs）demonstrates good catalytic performance in benzene hydroalkylation,achieving a yield of cyclohexylbenzene（CHB）at 41.2%（conversion:70.7%;selectivity:58.3%）,and maintaining stability over six cycles.A comprehensive investigation including HR-TEM,XPS,XANES and in situ FT-IR confirms the immobilization of HPW onto the surface of Ni nanoparticles（～14.0 nm）,accompanied with electron transfer from W atom to adjacent Ni atom.Structural-property correlation studies based on in situ FT-IR,XPS,TPD as well as control experiments validate that the HPW-Ni interface structure in 0.3HPW-Ni/MMOs provides unique adsorption sites for benzene and CHE with a moderate adsorption strength,serving as intrinsic active centers for this reaction.Benzene molecule undergoes partial hydrogenation at the Ni site located at the HPW-Ni interface to produce CHE;subsequently,CHE experiences alkylation reaction with another benzene molecule adsorbed at the interfacial Br?nsted acid site of HPW.This study offers fundamental insights into the metal-acid synergistic catalytic hydroalkylation reaction,which can be extended to the design and preparation of other high-performance hydrogenation tandem reaction catalysts.2.Solvent-switching strategy for the chemoselectivity of furfural hydrogenation on fully-exposed Pt clustersA heterogonous catalyst composed of fully-exposed Pt clusters immobilized on CoAl mixed metal oxides support（denoted as Pt_n/CoAl-MMOs）was prepared through loading Pt species on CoAl-layered double hydroxides（LDHs）precursor followed by a reduction treatment.The resulting Pt_n/CoAl-MMOs catalyst exhibits a prominent catalytic performance towards liquid phase hydrogenation reaction of furfural（FAL）.Most strikingly,a switchable hydrogenation chemoselectivity is achieved via a facile solvent regulation within one catalytic system:tetrahydrofurfuryl alcohol（THFA;yield:91.4%）,furfuryl alcohol（FA;yield:97.7%）,2-methylfuran（2-MF;yield:92.1%）and furan（FU;yield:90.8%）are obtained in ethanol,dioxane,isopropanol and n-hexane solvent,respectively.Fine-structure characterizations including AC-HAADF-STEM,in situ CO-DRIFTS and XAFS verify that single-atomic-layer thick Pt clusters（?0.76 nm）are stabilized on CoAl-MMOs via Pt-Pt,Pt-Co and Pt-O bonds.Based on experimental studies（in situ FT-IR and TPSR-Mass）and DFT calculations,we substantiate that solvent molecules exert a decisive impact on the adsorption configuration of FAL via changing the solvent-catalyst and/or substrate-catalyst interaction,which ultimately determines the hydrogenation pathway,key intermediate and final product.This work presents a facile solvent-dependent product-switching strategy within a single catalytic system,which shows potential applications in customizing hydrogenation selectivity in tandem reactions.3.Pt₁Co_nsingle-atom alloy for the aqueous-phase hydrogenation rearrangement of furan derivativesA Pt₁Co_nsingle-atom alloy（SAA）catalyst was prepared via a facile two-step procedure:the preparation of a CoAl-(Pt Cl₆^2-)-LDHs precursor followed by a reduction process.The Pt₁Co_nSAA catalyst exhibits remarkable catalytic performance in the aqueous-phase hydrogenation-rearrangement reaction of furfural（FAL）to cyclopentanol（CPL）,with a yield of 99%and a conversion frequency（TOF）value of 2257 h^-1,surpassing that of reported heterogeneous catalysts.A comprehensive investigation involving HAADF-STEM,XPS and XAFS verifies that isolated Pt atoms are stably dispersed on the surface of Co nanoparticles via Pt-Co coordination.A collaborative investigation including reaction dynamics,in situ FT-IR spectroscopy and isotope-label tracing experiments confirms that FAL undergoes a five-step tandem reaction pathway to produce CPL.In this process,water molecules not only participate as partial hydrogen donors in the hydrogenation process but also contribute to the reconstruction of the C-O bond on the side chain of furan ring.Kinetic analysis and DFT calculation further reveal that the Pt-Co interfacial site markedly diminishes the energy barrier associated with the rate-determining step（cyclopentanone hydrogenation）by facilitating the activation adsorption of carbonyl,thereby significantly enhancing catalytic performance.Based on an in-depth exploration of the mechanism,this study further optimizes the reaction efficiency,making valuable contributions to the development of an efficient and environmentally friendly aqueous-phase hydrogenation tandem reaction system.