Preparation, Structural Characterization And Hydrogen/Methane Premselectivity Of New Metal Organic Framework/Porous Anodic Alumina Composite Membrane

As a kind of green energy, hydrogen has been considered to meet therequirements of human sustainable development. Generally, hydrogen is produced bydecomposition of methane, because there is only a small amount of hydrogen innature. During productive process, the key technology is how to separate purehydrogen from hydrogen and methane mixtures. At present, because of its lowselectivity, as well as high energy consumption and high cost, it is difficult to separatepure hydrogen from the mixed gases by pressure swing adsorption. In contrast totraditional separation technologies, Membrane-based gas separation technology is ofhigh efficiency and plays an increasingly important role in industrial processes.However, traditional membrane materials are difficult to meet the development of thistechnology. Polymer membranes are prone to be plasticized or swollen, leading to alow efficiency of gas separation. With high thermal stability, inorganic membranes,such as carbon molecular sieve membrane, could maintain stability at hightemperature, however, the uncontrollable pore structure limits its further development.Thus it is urgent and necessary develop a new type of membrane material to separatepure hydrogen during industrial production.In the end of the twentieth century, metal-organic framework （MOF）, which arecrystalline hybrid materials composed of metal ions and organic ligands, have beendeveloped rapidly and used widely, such as gas-sorption properties depend on theirlarge porosity and designability of structures. Till now, there are few studies reportedabout MOF composite membranes, much less than their gas separation properties. Asideal composite membranes to separate hydrogen from mixed gases, it is necessary tocombine the thermal stability and high selectivity of metal organic framework with the mechanical strength characteristics of inorganic materials. This research couldexpand the utilization of MOF/PAAM composite membranes in the field of gasseparation, especially for hydrogen separation. And it could promote the developmentof new MOF materials as well. Results of this research are as follows:1. The synthetic processes and structural characterizations of new metal organicframeworks.As ideal MOF materials to separation H₂and CH₄, it should exhibit highseparation performance with the pore structure, moreover, the pore size is between thetwo molecule diameters of H₂and CH₄. In addition, thermal stability is equallyimportant for them when the temperature ranges from373K to473K. In this research,we synthesized eight compounds using five ligands. The experimental conditionswere as follows: concentration was0.003-0.05mol/L, solvent was H₂O or CH3OH,synthetic temperature was in the range298-453K, reaction time was from four toseven days.1) Compound1to5were proved to be zero dimensional compact structures bysingle crystal diffraction method. Compound6belongs to monoclinic system with onedimensional structure. Along the C axis, compound6forms one dimensional porewith the aperture0.5nm. Compound7is compact structure. Compound8belongs totetragonal system with the one dimensional pore along the C axis. The aperture is0.38nm. Among eight new MOF crystal materials, both compound6and8have porestructures, but the aperture of compound6is1.3times larger than the kinetic diameterof CH₄. Thus only compound8meets the requirement of pore size.2) With nitrogen protected, it is possible to maintain a stable structure below473K by gravitational thermal analysis.3) Gas adsorption behavior of single component gas on compound8wasinvestigated. The result shows that at303K, gas adsorption of H₂is zero with pressurefrom0-1000mbar, because of the weak interaction between the H₂molecule and MOFmaterials, although the H₂molecule can diffuse into the pores. Maximum adsorption capacity of CO2is2wt%. At273K and303K, the gas adsorption of CH₄is zero,because the kinetic diameter of CH₄is larger than the crystal aperture. Thisphenomenon confirms that compound8possesses obvious molecular sievingproperties.2. Preparing PAAM and testing gas transmittance of hydrogen and methane.For high selective MOF composite membrane to separate H₂and CH₄, idealinorganic substrate materials are required to have strong surface adhesion with theMOF materials, strong mechanical strength and good gas permeability, all of whichare decided by the aperture and thickness of substrate. In general, nickel net, porousceramics sheet, breathable stainless steel sheet and PAAM are used as substratematerials for preparing composite membrane. The MOF composite membrane wereprepared by “in suit” method in this work. The result shows that the ideal inorganicsubstrate materials is PAAM. With the thicknesses of77.4and pore of25nm, thestrong mechanical strength and gas permeability meet the requirements. The gastransmission rates of H₂and CH₄are5.60710-5mol/Pa·m2·s and2.38510-5mol/Pa·m2·srespectively.3. Preparing MOF/PAAM composite membranes and measuring gastransmittance of hydrogen and methane.The ideal MOF/PAAM composite membranes should have the followingperformance: high gas separation efficiency, high thermal stability, uniform thickness,high compressive strength and high gas permeability. The results are shown in details:1) The quality of new MOF/PAAM composite membrane which is prepared bytablet vacuum method is influenced by many factors, for example the concentration ofcoating solution, operating temperature and different nickel salt. This experimentshows that the best concentration of coating solution is0.2-0.7percentage. When thetemperature raises from413K to433K, the thickness of composite membranedecreases from450μm to200μm. Gap appears among the crystal particles when thetemperature is453K. The thickness of the composite membrane is200μm using Ni（CH3COO）2.4H₂O as nickel source,however, crystal packing appears through thecross-section of composite membrane.2) Considering the quality of composite membrane, we choose one compositemembrane to test gas permeability. This composite membrane is consisted of twoparts, one is the MOF membrane with thickness of450μm, another is PAAM withthickness of77.4μm. The compressive strength of this composite membrane is greaterthan10barrer. The gas transmission rates of H₂is1.44×10-9mol/Pa·m²·s at298K andalmost keeps as constant with pressure under0.1-0.3Mpa. At the same condition, CH₄can not penetrate through this composite membrane, so the gas transmission rates ofCH₄is zero.3) In range of298-373K, gas transmission rates of H₂decrease with theincreasing temperature. This MOF/PAAM composite membrane shows highselectivity of H₂and CH₄,for example at298K, the gas transmission rates of H₂is1.38×10^-9mol/Pa·m²·s, but the gas transmission rate of CH₄is zero.Based on this research, the MOF/PAAM composite membrane is expected to beapplied in the production of high purity H₂through decomposition of CH₄, because ofits high thermal stability and high compressive strength, especially high gasseparation efficiency. The scientific innovation in this research is that we prepared thefirst new MOF/PAAM composite membrane by tablet vacuum method in the world.The main research feather is that we obtain the preparation technology parameters ofMOF/PPA, such as thickness, temperature, and so on. It will provide necessarytheoretical and technical foundation of MOF/PPAM in production of high pure H₂industrial technology developments.