Supported Bifuctional Catalyst And Its Application In Methanol Halide-free Carbonylation/Dehydrogenation Coupling Reactions

As an important chemical raw,methanol can be obtained from a wide range of carbon resources such as coal,natural gas,biomass,etc.However,in the face of the global methanol overcapacity,it is urgent to develop a broader downstream chemical production process.In addition to the commericalized methanol-to-olefins（MTO）process,methanol carbonylation to acetic acid（AA）and methyl acetate（MA）,can also alleviate the above situation.Homogeneous Rh/Ir complex and iodine promoters（like CH₃I）are commonly used in the carbonylation process,which brings unignorable problems such as high costs,equipment corrosion and complicated separation process,despite of their high acetyls yields.Heterogeneous halide-free carbonylation of methanol can avoid these problems,but its catalyst systems always suffer from the poor performance and stability.The synthesis of MA via methanol dehydrogenation coupling is another industrially attractive reaction,which has unique advantages over the traditional methanol carbonylation route:（1）methanol as the only feed can avoid the use of CO and iodine promoters;（2）using relatively cheap metals;（3）under mild reaction conditions（<250 ^oC,ambient pressure）;（4）co-producing hydrogen.However,this reaction involves dehydrogenation,C-C and C-O coupling multiple steps,thus making it hard to achieve an efficient MA production from methanol.In this thesis,on the basis of the synthesis strategy of“metal-solid acid”with bifunctional catalysis,a facile Si O₂-supported Ni-Mo O_x catalyst is developed via impregenation method for structure-activity relationship study of vapor-phase halide-free methanol carbonylation.Furthermore,Lewis acidic zeolites（Sn-Beta,B-Silicalite-1）supported Cu catalysts with Cu-Sn（B）synergistic catalysis are developed for methanol dehydrogenation coupling.The main contents are summarized as follows:（1）Ni-Mo Ox bifunctional catalyst on Si O2 for vapor-phase halide-free methanol carbonylation:Insight into synergistic catalysis between Ni and Mo OxThe vapor-phase halide-free methanol carbonylation to synthesize AA and MA is an ideal reaction,but very chanllenging.Herein,a promising Ni-Mo O_x bifunctional catalyst for methanol carbonylation is developed by H₂-reduction of Ni O-Mo O₃/Si O₂.The catalyst performance is strongly dependent on the calcination/reduction temperature.Over the preferable Ni Mo-350-600/Si O₂ catalyst obtained by calcining at 350 ^oC and subsequently reducing at 600 ^oC,methanol conversion of 4.2%and acetyls space-time yield of 1.37 mol kg^-1_cat h^-1 are achieved with 22.1%selectivity to acetyls at 290 ^oC and 3 MPa.Co-existence of Ni⁰ and Mo O_x（especially Mo O₂）markedly decreases formation of dimethyl ether（DME）and reversely increases acetyls formation,in nature due to the catalyst acidity modulation as well as the partial electron transfer from Ni⁰ to Mo O_x tuning the CO adsorption strength on Ni⁰site.（2）Cu/Sn-Beta catalyst enabling one-step synthesis of methyl acetate from methanol alone via dehydrogenation couplingMethyl acetate（MA）can be directly synthesized via methanol dehydro-coupling reaction,which is attractive but urgently calling for a groundbreaking catalyst.Herein,a Cu/Sn-Beta bifunctional system with Sn-Cu synergistic catalysis is reported for the title reaction.Over the preferable 3Cu/Sn-Beta-30 catalyst（Si/Sn molar ratio of 33,Cu loading of 3 wt%）,a maximum MA space time yield of 1.4 mol _MA kg^-1_cat h^-1 is obtained with 4.1%methanol conversion and 56.4%selectivity to MA at 220 ^oC and ambient pressure.Electron transfer from Cu species to framework-confined Sn occurs and thus increases and stabilizes Cu⁺sites.The framework Sn-Cu⁺pair with Lewis acidity can convert a methanol molecule into bridged-adsorbed formaldehyde species（CH₂O*）with the assisted dehydrogenation of Cu⁰.Such CH₂O*speices is subsequently C-C coupled with CH₃O*adsorbed on another Sn site to form CH₃CHO*,accompanied by the migration of the H atom left at Cu⁺site to the Sn-O site（formed by C-O bond cleavage of CH₃O*）to form Sn-OH which subsequently recovers into initial state.The CH₃CHO*ultimately condenses with CH₃O*adsorbed on Cu⁰ site to form MA.（3）Superior bifunctional Cu/BS-1 catalyst for highly selective methanol dehydrogenation coupling to methyl acetateMethanol dehydrogenation coupling to MA is an industrially promising reaction.However,it is still challenging to achieve efficient MA synthesis due to the reaction complexity.Herein,a borosilicate（B-Silicalite-1）supported Cu catalyst with Cu-B Lewis acidic synergy was developed for the title reaction.The optimal 1Cu/BS-1-60can achieve about 5%methanol conversion and 75%MA selectivity with a maximum MA space-time yield of 2.33 mol kg^-1_cat h^-1 at 200 ^oC and 0.1 MPa.Cu²⁺is rapidly reduced into active Cu⁰/Cu⁺species during the reaction,and the occurrence of electron transfer from Cu to framework B can stabilize and form more Cu⁺.More notably,the framework B with trigonally-coordinated state（B₃）is partly transformed into tetrahedrally-coordinated state（B₄）via protonation.These increased B₄ species will aggravate the production of DME and MF,while the B₃ species with active-Si O-B-（OH）₂ structure together with Cu enables the directional conversion of methanol to MA.In situ FT-IR observes that the surface CH₂O*species adsorbed on the skeletal（HO）₂（Si O）B-Cu⁺Lewis acid pair are enriched as the reaction temperature increased,and this observation combined with DFT micro kinetic analysis jointly indicates that the formation of CH₃CHO*（rather than CH₂O*）via C-C coupling over the skeletal（HO）₂（Si O）B-Cu⁺pair is the rate-determining step,while the formed CH₃CHO*can facilely couple with another CH₃O*to generate MA*.