Descriptor-based Theoretical Design Of Single-site Catalysts For Direct Oxidation Of Methane To Methanol

In order to achieve carbon peaking and carbon neutrality goals,the accelerated development of efficient and clean natural gas utilization technologies is a major energy strategic demand for China,which plays an important role in adjusting the energy distribution,solving the energy shortage,and reducing oil import dependence and environmental problems.The direct oxidation of methane to methanol as the liquid fuel and valuable chemical is one of the most ideal conversion pathways.Therefore,the design and development of novel catalysts to achieve the efficient conversion of methane to methanol directly at low temperatures with high activity and selectivity is of great fundamental scientific significance and application prospect.An in-depth understanding of the catalytic reaction mechanism and the nature of the catalytic effect at the atomic scale can provide important theoretical guidance and scientific basis for the rational design of new high-efficiency catalysts.In this dissertation,density functional theory(DFT)calculations were used to study enzyme-mimicking single-site confined catalytic systems,including the catalysts of dual-atom M-BTC(M = Mn,Fe,Co,Ni,and Cu)metal-organic frameworks(MOFs),organic ligand-deficient dual-atom Cu-BTC MOFs,dual-atom Cu doped in graphene with different N coordination numbers,and ZSM-5 loaded mononuclear Fe synergistic with proximity Br?nsted acid sites(BAS).The influences of the metal type of active centers,the coordination environment(the number of organic ligands and N atoms),and the synergistic effect on key intermediates and rate-determining steps in different reaction stages of methane to methanol were investigated at the atomic scale.The structure-activity relationships,corresponding screening windows,and regulation rules between descriptors and final catalytic performance were established and understood.The main research contents and conclusions are as follows:(1)The structure-activity relationships of dual-atom M-BTC(M = Mn,Fe,Co,Ni,and Cu)MOFs for the direct oxidation of methane to methanol were investigated.In the catalytic reaction,the H2O2 activation energy increases and the methane activation energy decreases linearly with increasing group number(Mn to Cu),and the rate-limiting step transitions from the methane activation(Mn,Fe,and Co)to the H2O2activation(Ni and Cu).Based on the active site formation energy(ΔEO),linear relationships between it and adsorption energies of key reaction intermediates,reaction energies and activation energies of H2O2 dissociation and methane activation can be established.A volcano curve between the methane activation rate and ΔEO was further constructed,and the screening window was determined(ΔEO is-1～0 e V,where the active site formation energy is in equilibrium with the methane activation energy).Electronic structure analysis quantitatively demonstrated that the d-band center as an electronic descriptor for the redox properties of M-BTC(M = Mn,Fe,Co,Ni,and Cu)is linearly related to ΔEO.A volcano curve between the methane activation rate and the d-band center,and the screening window(-2～-1 e V)based on the d-band center were established to directly screen active sites.It is demonstrated that the d-band center descriptor can be applied to M-BTC with 4d transition metals,where only Rh-BTC meets the screening criteria.Ab initio dynamics molecular(AIMD)simulations,binding energy calculations,and charge analysis of the highest active Ni-BTC proved its excellent stability.(2)The effect of the organic ligand defect in the dual-atom Cu-BTC on the direct oxidation of methane to methanol was investigated.The results show that organic ligands in BTC act as oxidants and stabilize Cu metal centers.As the number of organic ligands decreases,the oxidation ability of the ligand environment in coordinatively unsaturated BTC(CUS-Cu-BTC)decreases compared with Cu-BTC.Meanwhile,the transfer charge from Cu sites decreases,and the valence state of Cu metal centers changes from +2 to +1.In the catalytic reaction,CUS-Cu-BTC has a lower oxidation state and unique spatial configuration,which strongly reduces the active site formation energy(ΔEO),while the methane activation energy changes less.Therefore,the rate-limiting step changes from active site formation to methane activation,and the whole energy barrier is significantly reduced.The electronic analysis shows that the reduction of organic ligands in CUS-Cu-BTC leads to the weakness of ligated oxygen atoms to bind Cu metal centers,and the d-band center is elevated,which promotes the oxidation process to form oxygen active species.Meanwhile,the reduction of organic ligands has little effect on the electronic state of the formed Cu-O active sites,so the change in methane activation energy is small.It is further demonstrated that the conclusion based on CUS-Cu-BTC is also applicable in other metal CUS-M-BTC(M =Mn,Fe,Co,and Ni),and these CUS-M-BTC(M = Mn,Fe,Co,Ni,and Cu)will form a new volcano curve of the methane activation rate with significantly higher summits than M-BTC(M = Mn,Fe,Co,Ni,and Cu).Therefore,the introduction of new variables of the coordination number of organic ligands in CUS-M-BTC(M = Mn,Fe,Co,Ni,and Cu)will break the previous structure-activity relationships in M-BTC(M = Mn,Fe,Co,Ni,and Cu).(3)The effect of the coordination number of N-graphene-confined dual-atom Cu centers on the direct oxidation of methane to methanol was investigated.The results show that the ligand environment of dual-atom Cu centers acts as the oxidant,and the oxidation property is enhanced with the increase of the N coordination number.Meanwhile,the oxidation state of Cu centers is elevated and the stability is improved.In the catalytic reaction,as the N coordination number increases,the N2 O activation energy linearly becomes larger and the CH4 activation energy linearly decreases,and the rate-limiting step changes from CH4 activation energy(Cu-4N)to N2 O activation energy(Cu-6N and Cu-8N).The redox ability of Cu-4N,Cu-6N,and Cu-8N is measured by the oxygen atom adsorption energy(ΔEO),which increases linearly with the increase of the N coordination number.Meanwhile,the oxidation property is enhanced and the reduction property is weakened,which is linearly related to the reaction energy and the activation energy of N2 O cleavage and CH4 activation.The methane activation rate increases first and then decreases with the increase of coordination number.Due to the redox ability of Cu-6N is between Cu-4N and Cu-8N,the methane activation rate of Cu-6N is the highest.The chemical origin of the effect of the N coordination number was revealed quantitatively by the electronic structure and chemical bond analysis.With the increasing N coordination number,the charge limitation effect of Cu centers by the ligand environment and the total Cu-N bond strength are enhanced.Meanwhile,the Cu oxidation state is elevated and the d-band center is linearly decreased.These will lead to the weaker Cu-O bond strength in the active site,corresponding to the weaker reduction property and the stronger oxidation property of catalysts to change the final catalytic performance.Meanwhile,it is demonstrated that the coordination number of supports and the metal type together determine the redox property of the whole reaction site.(4)The reaction mechanism of mononuclear Fe sites with adjacent Bronsted acid sites(BAS)in ZSM-5 synergistically promoting the direct oxidation of methane was investigated.In the stage of active site formation,BAS provides the hydrogen proton to Fe-OOH intermediate species and the chemical bond competition of the newly formed O-H bond can effectively weaken the peroxide bond strength.It will significantly lower the energy barrier of O-O bond cleavage to generate Fe(V)=O active species.In the stage of methane activation,the vacant BAS can provide additional charge compensation for the Fe(V)central atom,resulting in the highest activity of Fe(V)=O.The electronic structure analysis proves that the vacant BAS can promote the reduction of the energy of the lowest unoccupied molecular orbital(LUMO)in the Fe(V)=O active site and the charge transfer in the C-H bond homolytic cleavage.Meanwhile,the O-2px and O-2py orbitals can effectively stabilize the hydrogen atom of methane to reduce the methane reaction energy and activation barrier.In the stage of product generation,the vacant BAS can accept the hydrogen atom of H2O2 molecules to regenerate BAS and promote the combination of OOH radicals with CH3 radicals to form CH3 OOH,which is more favorable compared with the CH3 OH generation.Therefore,the BAS acts as the co-catalyst and functions as the proton cycle center,which synergistically interacts with the mononuclear Fe active site to promote the overall reaction performance of methane direct oxidation,in agreement with the experimental results.