Theoretical Study On The Hydrothermal Stability And Structure-activity Relationship Of Metal-organic Framework Materials In Catalytic Reaction

Metal-organic frameworks(MOFs)are constructed by both inorganic nodes(metal ions/clusters)and organic ligands via coordination bonds.The structural diversity,ultrahigh surface area,and facile tailorability as well as the high crystalline nature make the MOFs,in particular,the single metal sites(SMSs)upon MOFs and MOF-derived materials,to be widely used in heterogeneous catalytic reaction processes.Up to now,there are two types of MOF based catalysts with active SMSs.The first type SMSs are intrinsically created at the metal nodes and metallolinkers within MOF host frameworks during pre-synthesis and in situ synthesis.Abundant SMSs have been created over metal nodes in the pristine and defective MOFs,which are also called as open metal sites(OMSs).The second type SMSs are built via the addition of various funtional groups over the coordinatively unsaturated metal nodes or organic linkers that encapsulates a variety of metal moieties,typically including inorganic metal species and organometallic complexes.In the present work,the hydrolytic stability and catalytic performace for two types of SMSs over MOFs including M3(BTC)2(M=Cu,Co,Mn,Ni,and Zn),which have high-density M2+OMSs at the paddlewheel M-M node,and the functionalized MOF-808 with the flexible ethylenediaminetetraacetic acid(EDTA)groups for achoring dual metal sites have been systematically invesigated using density functional theory(DFT)calculations and ab initio molecular dynamics(AIMD)simulations.CO oxidation and electrochemical reduction of CO2(CO2RR)are used as the probe reactions,respectively.The major work is described as follows:(1)The hydrolytic breakdown mechanism of a Cu-Cu metal node at Cu3(BTC)in the presence of water molecules has been investigated using DFT calculations and AIMD simulations.AIMD simulation trajectories show that the presence of water molecules induces pronounced oscillations of all Cu-O bonds at the Cu-Cu node,leading to the precursor state for the initial Cu-O bond breaking.The hydrolytic breakdown mechanism that involves water adsorption,ligand displacement,and water dissociation steps under various water concentrations has been studied using DFT.It had been found that Cu-O bond breaking via ligand displacement with water substitution is quite facile.The complete detachment of two ligands at the Cu-Cu node needs at least three water molecules.The detaile hydrolytic breakdown mechanism provides fundamental insights into the structural collapse of the Cu3(BTC)framework;(2)The structural and hydrolytic stabilities of coordinatively unsaturated,paddlewheel M3(BTC)2(M=Cu,Co,Mn,Ni,and Zn)have been studied using DFT-based simulations combined with experimental measurements.AIMD simulations of isostructural metal-substitution analogues of M3(BTC)2 show that the structural oscillation intensity relies heavily on the metal type,where the skeleton of Ni3(BTC)2 and Zn3(BTC)2 dramatically vibrates.The presence of water molecules further induces oscillations of all coordinatively unsaturated M-O bonds at the metal node to some extent,leading to the precursor state for the initial M-O bond breaking.For the hydrolytic breakdown of M3(BTC)2 that involves adsorption,substitution,and dissociation steps,the hydrolytic stability trend is predicted as follows:Cu3(BTC)2>Co3(BTC)2>Mn3(BTC)2>Ni3(BTC)2>Zn3(BTC)2,which is consistent with our experimental observation;(3)Three model catalysts,i.e.,the pristine HKUST-1,the defective HKUST-1 with one linker displaced by a hydroxyl group(HKUST-DE1),and the defective HKUST-1 with one linker completely removed(HKUST-DE2)are used to understand how the OMSs and the defective site affect CO oxidation reaction mechanisms and kinetics.Theoretical DFT calculations indicate the CO oxidation activity follows the order of HKUST-DE2>HKUST-DE1>HKUST-1.The presence of dangling OH group over HKUST-DE1 provides a new reaction pathway of forming COOH intermediate that further accelerates the CO oxidation reaction.CO oxidation is pronouncedly promoted by both steric and electronic effects derived from the missing linker over HKUST-DE2,which is largely due to the formation of Cu-C-O-O-Cu five-membered ring intermediate on the reduced Cuδ+/Cuδ+(δ<2)dimer.(4)MOF-808 has been functionalized with the flexible ethylenediaminetetraacetic acid(EDTA)groups,which are used to anchor vairous single-atom metals(SAMs)for emhanced catalytic performance.A total of 21 pairwise SAM combinations of six metals(M=Fe,Cu,Ni,Pd,Pt,Au)has been chosen to identify the bimetallic MOF-808-EDTA-M1-M2 catalyst with the best CO2RR activity and selectivity.The MOF-808-EDTA-FeFe can reduce CO2 to CH4 at-0.55 V and various C2 products at-0.87 V versus reversible hydrogen electrode.While the MOF-808-EDTA-FePt maintains a low limiting potential(-0.66 V)for C2 formation mechanisms.The spatial configurations of M1-M2 dual metal sites are dynamic in terms of dM1-M2 and 02N-2N during CO2RR reaction,which consistently stabilize the C1 and C2 intermediates in their optimal states.