Study On The Preparation And Catalytic Application Of Novel Bimetallic Coordination Polymers

Metal ions and organic ligands are assembled by coordination bonds to form metal-organic frameworks（MOFs）or coordination polymers（CPs）.In view of the diversity of structure and function,MOFs/CPs have become one of the most active fields of materials and chemistry,and gained a quick development in the design of new structure,preparation method and application.In order to realize the synergy and integration of different metals,and improve the designability of coordination polymers,the preparation of bimetallic coordination polymers with two uniformly distributed metal elements has attracted ever-increasing attention.In addition,MOFs/CPs have been proved to be the ideal template for the preparation of carbon-based or metal-based nanomaterials.Therefore,the preparation and application of bimetallic coordination polymers and derived functional materials have become one of the hot topics in the field of nanomaterials.In this work,firstly,zeolite imidazolate framework-8（ZIF-8）was used as template,hydrothermal reaction with appropriate amount of Cu（NO₃）₂ and organic ligand trimesic acid（H₃BTC）.The bimetallic coordination polymers CuZn-BTC with coordination bonds linking Cu^Ⅱ,Zn^Ⅱand organic ligand（H₃BTC）was prepared successfully.The simple preparation method based on the ZIF-8 template had the characteristics of relatively short preparation period and the obtained bimetallic coordination polymer possessed two uniformly distributed metal elements and rod-shaped morphology.In addition,the novel method overcame the disadvantages of the traditional"one pot hydrothermal method"and"post synthesis modification/exchange"method for the preparation of CuZn bimetallic MOFs/CPs.The formation mechanism of the bimetallic coordination polymers CuZn-BTC was then explained by SEM and XRD:Using ZIF-8 as sacrificial template,the adding of Cu（NO₃）₂·3H₂O resulted in the Cu²⁺hydrolysis reaction,generated proton acid H⁺and the solution became acidic.ZIF-8 could be degraded rapidly under the condition of proton acid and hydrothermal reaction,resulting in irregular ZIF-8 intermediate.In the process of continuous decomposition,the ZIF-8 intermediate with high reactivity was reassembled with Cu²⁺and H₃BTC to form a new framework,and the CuZn-BTC bimetallic coordination polymers was formed.Subsequently,the prepared CuZn-BTC bimetallic coordination polymer was used as the precursor to obtain CuO/ZnO catalyst by calcination.The slow decomposition of the organic ligand in the calcination process inhibited the rapid growth of particle size and the sintering of nano-particles.With the help of Cu,Zn metal nodes in the bimetallic coordination polymers,the highly ordered CuO and ZnO in the calcined sample sufficiently dispersed and completely separated with each other.The catalyst showed good reduction performance and surface basic sites distribution.The well-separated Cu and ZnO in catalyst after reduction possessed relatively concentrated particle size distribution and generated a higher amount of Cu-ZnO interfacial structure.For methanol synthesis from CO₂ hydrogenation,the CCZB-3 catalyst derived from pure phase CuZn-BTC bimetallic coordination polymers precursor（CZB-3）bad high CO₂ conversion rate and and showed excellent stability during the continuous evaluation of 280 h.The Cu and ZnO still existed in the form of nano-particles in the catalyst after evaluation,no obvious particle size growth and sintering phenomenon were observed.The structural characteristics of uniformly dispersed and well-separated did not change significantly.A large number of interfaces formed between the catalyst components are maintained.Therefore,the catalysts prepared by the pyrolysis of CuZn-BTC bimetallic coordination polymers have good stability and excellent catalytic performance.The CCZB-3 catalyst derived from CuZn-BTC bimetallic coordination polymers precursor was compared with co-precipitation catalyst CCZ-P in characterization and evaluation of catalysts.There was no significant difference between the two groups in reduction performance,basic sites distribution,pore structure and so on.However,the interface structures of Cu-ZnO in these two catalysts after reduction were significantly different.The interfacial structure ofCCZB-3 catalyst derived from CuZn-BTC bimetallic coordination polymers precursor originated from well-separated,evenly distributed and fully contacted Cu,ZnO oxide particles.While the interfacial structure of the co-precipitation catalyst is the formation of Cu nanoparticles packaged with dispersed ZnO in graphite-like morphology.The results showed that the oxygen deficient composite metal oxide Cu_x Zn_1-xO_y could be observed in the two kinds of interface structures,but the partial oxidation degree of copper in the CCZB-3 catalyst was higher,which indicated that the interaction between Cu and ZnO was stronger at the interface.The TOF value of methanol production in CCZB-3 was significantly higher than that of co-precipitation catalyst,and showed a significant upward trend with the increase of temperature.While the TOF value of methanol production in co-precipitation catalystdid not change significantly with the increase of temperature.Therefore,the uniformly dispersed and well-separated interface structures of Cu-ZnO in CCZB-3 derived from bimetallic coordination polymers was propitious to improve the activity of methanol active sites.Therefore,the unique structural feature of the MOFs would provide unprecedented opportunities in the preparation of catalysts as precursors,and interface and its existence form were important factors to affect the catalytic performance.Finally,selecting two kinds of zeolite imidazolate frameworks ZIF-67 and ZIF-8 as the starting template material,CoMn-BTC and ZnCr-BDC bimetallic coordination polymers were successfully prepared via hydrothermal reaction with corresponding metal salts and organic ligands.The results show that the preparation method of bimetallic coordination polymers in this paper had general applicability.