Microwave synthesis of a nanoporous hybrid material, chromium trimesate
Microwave synthesis of a nanoporous hybrid material, chromium trimesate
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DOI：
10.5012/bkcs.2005.26.6.880
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发表时间：
2005
影响因子：
1.7
通讯作者：
S. Jhung;Jin-Ho Lee;Jong‐San Chang
中科院分区：
文献类型：
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作者：
S. Jhung;Jin-Ho Lee;Jong‐San Chang
近年来，纳米多孔材料的研究领域得到了极大的拓展，出现了金属-有机骨架(MOF)、多孔配位聚合物或有机-无机杂化材料，它们是由金属离子通过骨架中的有机桥联配体连接而成的一类最受关注的材料。无机和有机相结合的特性吸引了人们对新型杂化材料的研究，这些杂化材料具有潜在的应用，包括气体储存、催化、分离和分子识别。最近，Ferey和他的同事报道了一种新型的杂化材料，三偏酸铬(命名为MIL-100)，它具有分级孔系统(微孔：5-9A；介孔：25-30A)，具有非常高的朗缪尔比表面积。多孔性杂化材料的合成主要通过水热或溶剂热合成，一般需要几天的时间。微波技术由于在水热条件下需要几天的时间来快速合成纳米多孔材料而受到越来越多的关注。该技术在合成多孔材料方面的潜在优势包括相选择性、窄的粒度分布和易于控制的形貌，以及快速结晶。然而，尽管微波合成不仅是有机分子的，而且是无机材料的，但微波技术还没有应用于纳米孔杂化材料的合成。在本文中，我们首次报道了微波合成有机-无机杂化材料多孔三偏酸铬的成功结果。多孔三甲酸铬(MIL-100)是在水介质中合成的，与以前报道的方法类似，只是使用了微波辐射作为加热源。反应物的摩尔比为1.0Cr：0.67H3BTC(苯三元酸)：2.0Hf：290H2O。将反应物混合物装入特氟龙高压灭菌器，密封并放置在微波炉(MARS-5，CEM)中。将高压灭菌器加热到220℃的反应温度，并保持预定时间。用X-射线粉末衍射仪测定了合成样品的结构和结晶度。利用热分析仪获得了气流中的热重分析图。采用体积法进行了吸附实验。图1显示了在微波辐射下，通过在220℃下改变晶化时间来合成的三偏酸铬的X射线衍射图。微波辐射合成的MIL-100样品的X射线衍射谱与常规水热合成的MIL-100样品在220℃下4d合成的样品相吻合。然而，用微波法合成的三偏铬材料在晶化2 h前仍含有未反应的金属铬物种。以铬为原料的三偏酸铬微波合成4h结晶率为44%，与常规合成4d结晶产率45%相当。图2的TGA图谱显示标题化合物的稳定性高达270℃。用TGA测定的热稳定性与以前的结果非常相似。微波加热合成的三偏铬的氮气吸附-脱附等温线与MIL-100(数据未显示)的吸附-脱附等温线非常相似，代表了两种方法合成的杂化材料的永久孔隙率和相似的孔结构。对于氮气的多层覆盖，我们估计其表观比表面积和孔体积分别为1700m/g和0.97mL/g。根据氢的吸附等温线(图3)，在-196℃和1大气压下，氢的吸附容量约为150毫升/克(STP)，这与各种MOF和多孔杂化材料的吸附结果相当。预吸附氢在-196℃解吸后的再吸附等温线与新鲜样品上的吸附等温线相同，表明三偏铬中不存在化学吸附中心
Recently, the domain of nanoporous materials has been enlarged very much to the development of porous hybrid materials designated as metal-organic frameworks (MOF), porous coordination polymers or organic-inorganic hybrids which are the most recently highlighted class of materials consisting of metal ions linked together by organic bridging ligands in the framework. The attraction of combining properties from both inorganic and organic components has led to a quest of research toward new hybrid materials with potential applications including gas storage, catalysis, separation, and molecular recognition. Very recently, Ferey and co-workers have reported a novel hybrid material, chromium trimesate (designated as MIL-100), which has a hierarchical pore system (micro: 5-9 A; mesoporous: 25-30 A) with a very high Langmuir surface area. Syntheses of the porous hybrid materials have been carried out mainly by hydrothermal or solvothermal synthesis generally in a period of several days. Microwave techniques have attracted growing attention for the rapid synthesis of nanoporous materials requiring several days to prepare under hydrothermal conditions. Potential advantages of this technique in the synthesis of porous materials include phase selectivity, narrow particle size distribution and facile morphology control besides fast crystallization. However, microwave technique has not been applied to the synthesis of nanoporous hybrid materials yet even though the microwave syntheses of not only organic molecules but also inorganic materials have been often studied. In this communication, we report the first successful result on the microwave synthesis of porous chromium trimesate as an organic-inorganic hybrid material. The porous chromium trimesate (MIL-100) was synthesized in aqueous media similar to the previous reported method except using microwave irradiation as a heating source. The molar composition of reactant mixture was 1.0 Cr: 0.67 H3BTC (benzene tricarboxylic acid): 2.0 HF: 290 H2O. The reactant mixture was loaded in a Teflon autoclave, which was sealed and placed in a microwave oven (Mars-5, CEM). The autoclave was heated to the reaction temperature of 220 C and kept for a predetermined time. The structure and crystallinity of the synthesized samples were determined by X-ray powder diffraction. The TGA pattern was obtained with a thermal analyzer in the air flow. The sorption experiments were carried out volumetrically. Figure 1 shows the XRD patterns of as-synthesized chromium trimesate obtained by varying crystallization time at 220 C under microwave irradiation. The XRD patterns of the samples obtained from microwave irradiation are well consistent with the pattern of MIL-100 synthesized for 4 days at 220 C using conventional hydrothermal heating. However, the chromium trimesate materials synthesized using microwave method contained unreacted metallic chromium species until 2 h of crystallization. The crystal yield of the chromium trimesate based on chromium from microwave synthesis for 4 h is 44%, which is comparable with the result of 45% in the conventional synthesis for 4 days. The TGA profile of Figure 2 reveals a stability of the title compound up to 270 C. The thermal stability determined with TGA are very similar to the previous results. The chromium trimesate synthesized by microwave heating shows nitrogen adsorption-desorption isotherm very similar to the isotherm of MIL-100 (Data not shown), representing the permanent porosity and similarity of the pore structure of the hybrid materials synthesized by both methods. For multiplayer coverage of N2, we estimate the apparent BET surface area and pore volume to be 1700 m/g and 0.97 mL/g, respectively. From H2 adsorption isotherm (Figure 3), the adsorption capacity of hydrogen at –196 C and 1 atm is estimated to be about 150 mL/g (STP), which is comparable with the results adsorbed on various MOFs and porous hybrid materials. The re-adsorption isotherm after the desorption at –196 C of the pre-adsorbed hydrogen is same as the adsorption isotherm on the fresh sample, representing that there is no chemisorption site in the chromium trimesate