Functional Metal-Organic Frameworks For Selective Hydrogenation Of Carbon Dioxide

The research goal of catalysis in the 21st century is to obtain 100%selectivity of the target product.The development of highly efficient and selective catalysts has always been the focus of chemists.Efficient conversion of carbon dioxide into fuels or value-added chemicals can not only reduce the greenhouse effect caused by global CO2 emissions,but also provide an alternative to replace fossil fuels.The activity and selectivity of the CO2 hydrogenation catalyst are closely related to the strong metal support interaction(SMSI)between the support and the active metal centers.The carrier can be used not only to stabilize the ultra-small metal nanoparticles and even single-site(atomic-level)catalytic centers,but also to form a unique interface structure to synergistically produce highly active interfacial sites.However,the structures of traditional oxide supports are complicated with low utilization efficiency of metal atoms and low density of active sites.These structures also present challenges for spectroscopic characterization,since active interfaces only constitute a small fraction of the materials,causing spectral signals overwhelmed by the irrelevant bulk materials.On the other hand,compared to metal/oxide catalysts,the homogeneous catalysts have well defined structures,but their CO2 hydrogenation activities are limited by low solubility of hydrogen in solution and the deactivation due to aggregation of small molecules.Preparation of better defined heterogeneous catalysts is thus one of the research direction under extensive investigation.Built from metal cluster secondary building units(SBUs)and functional organic linkers,metal-organic frameworks(MOFs)have been explored as useful molecular materials for many potential applications.MOFs as a carrier can help to construct highly efficient heterogeneous catalysts with well defined structures.The assembled porous molecular solids can also serve as catalysts for CO2 hydrogenation at the solid-gas interface.The solubility of hydrogen in solvent will no longer be a constraint on the catalytic rates.Moreover,site isolation of the embedded molecular catalysts will prevent bimolecular decomposition from happening,thus stabilizing the catalysts.In this thesis,we adopt self-assembly techniques to obtain active centers on sub-nanometer scale and/or active sigle-site molecular catalysts in MOF supports,carrying out systematic studies on the catalytic activity,product selectivity and turn-over mechanism of CO2 hydrogenation.In addition,we also plan to search new molecular catalysts that are unstable under traditional homogeneous catalytic conditions but highly active on the solid-gas/solid-gas-liquid interface.These well-defined MOF catalysts achieve 100%product selectivities and show synergetically catalytic effect with SMSI in CO2 hydrogenation.Firstly,the interfaces of Cu/ZnO and Cu/ZrO2 play vital roles in the hydrogenation of CO2 to methanol by these composite catalysts.Surface structural reorganization and particle growth during catalysis deleteriously reduce these active interfaces,diminishing both catalytic activities and MeOH selectivities.Here we report the use of pre-assembled bpy and Zr6(μ3-O)4(μ3-OH)4 sites in UiO-bpy metal-organic frameworks(MOFs)to anchor ultrasmall Cu/ZnOx nanoparticles,thus preventing the agglomeration of Cu NPs and phase separation between Cu and ZnOx in MOF cavity-confined Cu/ZnOx nanoparticles.The resultant Cu/ZnOx@MOF catalysts show very high activity with a space-time yield of up to 2.59 kgMeOH gCu-1 h-1 and 100%selectivity for CO2 hydrogenation to methanol and high stability over 100 hours.These new types of strong metal-support interactions between metallic nanoparticles and organic chelates/metal-oxo clusters offer new opportunities in fine-tuning catalytic activities and selectivities of metal nanoparticles@MOFs.Hydrogenation of CO2 to formate is usually catalyzed by homogeneous catalysts in basic solutions,but its efficiency is limited by low solubility of hydrogen in water.Here we report the design of molecular iridium catalysts immobilized in metal-organic frameworks(MOFs)that were positioned in the condensing chamber of a Soxhlet extractor for efficient CO2 hydrogenation.Droplets of hot water seeped through the MOF catalyst to create dynamic gas/liquid interfaces which maximize the contact of CO2,H2,H2O,and the catalyst to achieve a high turnover frequency of 410 h-1 under atmospheric pressure and at 85 ℃.H/D kinetic isotope effect measurements and density functional theory calculations revealed concerted proton-hydride transfer in the rate-determining step of CO2 hydrogenation,which was difficult to unravel in homogeneous reactions due to base-catalyzed H/D exchange.Furthermore,CO2 can be converted to alkyl formates as value-added products via tandem esterification and hydrogenation on bifunctional solid catalysts.We report here the assembly of the required distinct active sites in the same nano-cavity of a designer metal-organic framework(MOF),including basic pyridine sites for CO2 activation and metallic Pd nanoparticles(Pd NPs)for hydrogenation.Alkyl formate can be generated from CO2 via a hydrogenation-esterification pathway,in which CO2 is first hydrogenated to formic acid followed by esterification with alcohol,as widely reported in literature.Here we found,however,that the presence of pyridine sites in the MOFs lead to a kinetically more favorable and selective esterification-hydrogenation pathway for the formation of alkyl formate,in which CO2 is converted to a mono-alkyl pyrocarbonate-pyridinium salt intermediate on the pyridine sites before being hydrogenated on Pd surfaces to form alkyl formate.This MOF hybrid exhibits an excellent catalytic activity of 1333 μmol/gcat/h and 93.5%selectivity for ethyl formate(HCO2Et).The turnover number of HCO2Et based on Pd atoms is up to 475,higher than those of other catalysts previously reported under similar conditions.Finally,Selective hydrogenation of CO2 to ethanol(EtOH)is of great interest but is quite challenging,due to the requirement of both forming one C-C bond and keeping one C-O bond.Here we constructed cooperative CuⅠ sites on the secondary building units of a MOF based on Zr12 connection nodes.These CuⅠcenters are spatially close to undergo bimetallic oxidative addition for hydrogen activation.These adjacent Cu1 sites together with a nearby alkaline cation are also responsible for C-C coupling to produce EtOH.Strikingly,the catalyst with Cs+ cation(Zri2-bpdc-CuCs)gave 100%selectivity to EtOH,which is unprecedented for Cu based CO2 hydrogenation catalyst.The turnover number(TON)of this catalyst is 490 in 10 hours at 2 MPa of CO2/H2 = 1/3 and 100 ℃,and is 4080 in 10 hours when supercritical CO2 is used as the solvent at 250 bar of CO2,50 bar of H2 and 85 ℃.Such well-defined catalytic sites on MOF SBUs bring in new insights of Cu based catalysts for CO2 hydrogenation to C2+ oxygenates.