| Methane is the main greenhouse gas in addition to carbon dioxide. The emissionof a large quantity of methane to the atmosphere is harmful to the climate and theozone layer. Hence, it has become the great problem to control methane emissions inthe word. Coal mine is one of the largest methane emission sources. It is very difficultto enrich methane gas due to the low concentration and involved safety problems.Therefore, it is urgent to design the novel porous materials with high selectivity andadsorption capacity and low desorption energy consumption of ventilated minemethane to reduce the content of methane in coal mine.Activated carbon or various types of molecular sieves have become the mostsuitable porous materials for methane adsorption because of their unique structures,large surface areas, and high thermal stability. Practically speaking, it has proved thatactivated carbon or various types of molecular sieves with large surface areas,including the high percentage of micropores, mesopores, and macropores, can noteffectively adsorb methane molecules because methane molecular radium is only3.82. The results reported in the literature have shown that the gas adsorptionperformance of porous materials is associated not only with the structures and surfaceareas but also with the channel polarity, pore sizes, and pore volumes.To improve and further enhance their pressure swing adsorption (PSA)performance of methane and nitrogen, we prepared spherical, rod-like, and polyhedralsilica SBA-15and SBA-16with ordered or wormhole-like mesopores under thehydrothermal conditions; Na-Beta adsorbents were obtained via the ion-exchangingroute with several metal cations; the porous metal-organic frameworks based onmultidentate ligands were synthesized. The physicochemical properties of theas-prepared materials were characterized by means of techniques, such as X-raydiffraction (XRD), scanning electron microscopy (SEM), N2adsorption-desorption(BET), temperature-programmed desorption (TPD), X-ray fluorescence spectroscopy(XRF), inductively coupled plasma-atomic emission spectrometry (ICP-AES), singlecrystal diffraction, and thermogravimetric analysis (TGA). The pressure swingadsorption behaviors for methane and nitrogen separation of these materials wereinvestigated. The main results obtained in the present investigations are as follows:(1) By changing the surfactant types and controlling the synthesis conditions, spherical, rod-like, and polyhedral silica SBA-15and SBA-16with ordered orwormhole-like mesopores were fabricated under the hydrothermal conditions. It isshown that the rod-like SBA-15sample with an ordered mesoporous structure andrelatively large pore volume exhibited the best PSA performance for the separation ofmethane and nitrogen, with the separation coefficient (CH4/N2) being up to6.84. ThePSA efficiency for methane and nitrogen separation of such kinds of mesoporoussilicas was associated with the pore volume, ordered porosity, and particlemorphology.(2) M-Beta porous adsorbents were obtained via the ion-exchanging route withseveral cations (H~+, Mg~2+, Sr~2+, Ba~2+, Cu~2+, Co~2+, Ni~2+, and Ce~3+) to study their effectson the adsorption behaviors of methane and nitrogen. It is shown that the Sr-Betamolecular sieve exhibited the best PSA performance for the separation of methane andnitrogen, whereas the maximal adsorption capacity for methane and nitrogen could beachieved for the Ba-Beta sample. For adsorbents containing alkaline-earth metalcations, the nitrogen adsorption capacity increased with increasing the cationic size. Itis confirmed that the pore size and channel acidity are the main factors in influencingthe PSA performance for the separation of methane and nitrogen in0~1000mmHgrange, whereas the surface area is the main factor in influencing the PSA performancefor the methane and nitrogen in2000~6000mmHg range. Furthermore, for the strongacid sites, the acidic amount of the adsorbent fellow a decreasing order of Ba-Beta>Sr-Beta> Co-Beta> Mg-Beta> Ce-Beta> Na-Beta> H-Beta> Ni-Beta, and themethane adsorption capacity decreased according to the same sequence. The more thestrong sites, the more is the methane adsorption capacity. Based on the results, weconclude that the adsorption capacity of methane is in concord with the sequence ofincreasing the strong acid sites.(3) Novel one-and two-dimensional metal-organic materials were synthesizedusing the solvent diffusion and hydrothermal method with pyridine-3,5-dicarboxylicacid, isonicotinic acid or pyridine-2,6-dicarboxylic acid as multidentate ligands, andCo, Ni, Mn and Cu as central metal ions,(4) Novel metal-organic materials were synthesized via the hydrothermal routeusing pyridine-2.6-dicarboxylic acid as ligand, and rare earth metal and alkaline earthmetal Ca or Sr hydroxides as precursors. X-ray single-crystallography reveals that twonovel coordination polymers were obtained when the molar ratio ofpyridine-2,6-dicarboxylate acid, Ln~3+, and Ca~2+was different. The Ln-organic polymer exhibited an one-dimensional chained structure. However, the novel Ln-Caheterometal-organic compounds displayed three-dimensional frameworks withone-dimensional channel. The XRD and TGA results indicate that the guest watermolecules played a role in supporting the pore structure, and the dehydration of thesematerials could induce serious collapses of the frameworks. Under the similar reactionconditions, the obtained metal–organic materials were different due to the differentcoordination modes of various alkaline earth metals and even to the coordination withthe same ligand. Ln-Sr-organic coordination polymers exhibited one-dimensionalribbon-like structure obtained solvethermally as compared to the Ln-Ca-organicframeworks.(5)[bis(3-nitryl-4-hydroxyl-pyridine-6-carboxylic) copper] dihydrate was insitu generated by using the hydrothermal method via the decarboxylation and nitrationof4-hydroxyl-pyridine-2,6-dicarboxylic acid ligand with copper nitratetrihydrate asthe metal source.(6) Novel metal-organic compound was synthesized via the solvethermal routewith isonicotinate and terephthalic acid as ligand and Zn~2+as center ion. X-raysingle-crystallography reveals that metal-organic compound displayed athree-dimensional framework with one-dimensional channel. The XRD and TGAresults indicate that the removal of DMF molecules in channel did not induce thecollapsing of the framework even when the sample was in vacuum dried at150oC for3h. The compound possessed good thermal stability. The Ba-Beta molecular sieveexhibited better PSA performance in methane adsorption capacity in the range of0~1000mmHg. The higher the pressure, the more is the methane adsorption capacityof the compound than that of Ba-Beta. Compared to metal-Beta zeolites, thecompound showed better PSA performance for nitrogen adsorption capacity andbetter methane adsorption selectivity in the range of1000~6000mmHg.
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