Synthesis And Oxygen Adsorption-desorption Properties Of Metal-organic Framewok Materials With Unsaturated Metal Sites

Coal bed methane belonging to the standard low-carbon resources exists in the coal seams and it’s main component is methane. In the coal mining process of China, a large number of low concentration coal bed methane was directly discharged into the atmosphere, which not only results in a serious waste of resources, but also brings up great pressure to environmental protection, so the enrichment and utilization of low concentration coal bed methane will be of great significance. The O2removing is a key step of low concentration coal bed methane recycling.Metal-organic frameworks materials (MOFs) with unsaturated metal sites have great potential and advantages of the selective adsorption of O2. Accordingly, in this paper, we choose the MOFs with unsaturated metal sites as the research objects,[Mn(NDC)(DEF)]n, Mn3(NDC)3(DMF)4, Mn5(btac)4(μ3-OH)2(EtOH)2·DMF·3EtOH·3H2O, Mn/dobdc, Ni/dobdc, Co/dobdc and Mg/dobdc (MOF-74) were synthesized by solvent-thermal synthesis method under different conditions. In the experiment, we investigated in detail the influence of the reaction time, the synthesis temperature, solvents, material ratio of the sample to obtain the optimum reaction conditions, and their structure, morphology and thermal stability were analysed by X-ray difrractometer(XRD), scanning electron microscopy(SEM) and thermogravimet-ric analysis(TG), and their oxygen adsorption-desorption properties were tested. The main contents and conclusions are listed as follows:(1)The optimal synthesis conditions of the sample [Mn(NDC)(DEF)]n are: n(M2+):n(2,6-ndc)=0.5mmol/0.5mmol, DEF=3.5ml, at105℃for24h; the optimal synthesis conditions of the sample Mn3(NDC)3(DMF)4are: n(M2+):n(2,6-ndc)=0.5mmol/0.5mmol, DMF=7ml, at110℃for24h; the optimal synthesis conditions of the sample samples Mn5(btac)4(μ3-OH)2(EtOH)2·DMF·-3EtOH·3H2O are:n(M2+):n(H2btac)=0.5mmol/0.4mmol, DMF/EtOH/H2O=4ml/lml/4ml,at200℃for72h.The O2adsorption isotherms of Mn5(btac)4(μ3-OH)2(EtOH)2DMF·3EtOH·-3H2O indicated that the sample bind O2by chemisorption at low pressure, and their sorption capacities were obviously decreased upon lowering the temperature, finally completely converted to physical adsorption. The O2isotherm of the sample for a second cycle reveals a greatly reduced capacity, this is mainly due to the differences in electron density of the respective Mn centers with different coordination environments. Much more electron density around the Mn centers will occur the irreversible O2chemisorption, and the optimal electron density will favor the reversible O2chemisorption. However, the [[Mn(NDC)(DEF)]n and Mn3(NDC)3(DMF)4， the O2adsorption isotherms of the two samples belonged to the typical Langmuir-type form, as befits the physical adsorption o f O2on the framework surface. It is indicated that the carboxylate oxygen atoms around the unsaturated Mn centers provide so little electron density that partial charge transfer from the Mn centers to the O2molecules could not occur.(2)The optimal synthesis conditions of the sample Mn/dobdc are: n(M2+):n(dobdc)=0.6mmol/0.2mmol, DMF/EtOH/H2O=6ml/0.5ml/0.5ml, at120℃for24h; the optimal synthesis conditions of the sample Ni/dobdc are: n(M2+):n(dobdc)=0.75mmol/0.375mmol, THF/H2O=5ml/5ml, at110℃for72h; the optimal synthesis conditions of the sample Co/dobdc are:(M2+):n(dobdc)=0.75mmol/0.375mmol, THF/H2O=5ml/5ml, at110℃for72h; the optimal synthesis conditions of the sample Mg/dobdc are:n(M2+):n(dobdc)=0.463mmol/0.14mmol, DMF/EtOH/H2O=7.5ml/0.5ml/0.5ml, at125℃for20h.The O2adsorption isotherms of Mn/dobdc indicated that the sample bind O2by reversible chemisorption at low pressure, and their sorption capacities were obviously decreased upon lowering the temperature, finally completely converted to physical adsorption. The O2isotherms of the sample for a second cycle reveals a same capacity with the first cycle. However, the O2adsorption isotherms of the other three samples belonged to the O2physical adsorption. It is indicated that the nature of the unsaturated metal atom has a great influence on the O2chemisorption among the MOFs having the same structure.