乙醇在MIL-101上的吸附相平衡及其吸附机理

主要研究了MIL-101材料对乙醇的吸附性能和吸附机理。采用水热合成法制备了MIL-101(Cr),并分别应用N₂静态吸附、X射线粉末衍射(PXRD)、傅里叶红外光谱(FTIR)等分析手段对MIL-101晶形结构、孔隙结构参数进行分析表征。应用静态吸附法测定乙醇和水蒸气在不同温度下的吸附等温线,并讨论乙醇吸附在MIL-101(Cr) 4种吸附位的机理,根据吸附等温线估算出乙醇和水蒸气在MIL-101上的等量吸附热,并测试了乙醇在MIL-101上的吸附循环性能。研究表明,在298 K下,MIL-101的乙醇吸附容量为20.3 mmol·g^-1,远高于传统吸附材料。在低压下MIL-101对乙醇的吸附量高于水蒸气的吸附量,这是由于乙醇的偶极矩和分子动力学直径均比水大,使得乙醇分子在孔道中受到更大吸附力场作用;在低吸附量范围,乙醇在MIL-101上的等量吸附热要高于水蒸气的等量吸附热。在较高吸附压力条件下,主要发生多层吸附或孔填充,受吸附剂的孔容限制效应,尺寸越大的分子被吸附的物质的量会越少,由于乙醇的动力学直径(0.45 nm)大于水分子的动力学直径(0.268 nm),所以在较高吸附压力下乙醇在MIL-101上吸附量要小于水蒸气的吸附量。多次吸附脱附等温线测试显示MIL-101具有良好的乙醇吸附循环性能。

Adsorption equilibrium and mechanism of ethanol on MIL-101(Cr)

The adsorption performance and adsorption mechanism of ethanol on MIL-101(Cr) were investigated. MIL-101(Cr) was synthesized using a hydrothermal synthesis method, and characterized using N₂ adsorption measurements, powder X-ray diffraction, and Fourier transform infrared spectroscopy. The adsorption isotherms of ethanol and water vapor were measured by static adsorption method at different temperatures, and ethanol adsorption mechanisms on four types of adsorptive sites in MIL-101 were discussed. The isosteric heats of ethanol and water adsorption were calculated according to these isotherms. The adsorption and desorption of ethanol on MIL-101 were evaluated. Results show that adsorption capacity of MIL-101(Cr) for ethanol is up to 20.3 mmol·g^-1 at 298 K, much higher than those of some traditional adsorbents. At low pressure, the adsorption capacity of MIL-101 for ethanol is higher than that for water vapor, which could be ascribed to larger dipole moment and dynamic diameter of ethanol compared to water molecule. In addition, the isosteric heat of ethanol adsorption on MIL-101 is higher than that of water vapor adsorption. At high pressure, multilayer adsorption or cage filling occurs, so the molecule with large dynamic size will fill within MOFs with less moles than that with small dynamic size due the limitation of pore volume of MOFs. As a consequence, the adsorption capacity of MIL-101 for ethanol is lower than that for water vapor because the dynamic diameter of ethanol (0.45 nm) is larger than that of water molecule (0.268 nm). The ethanol adsorption and desorption isotherms show that ethanol adsorption on MIL-101 is highly reversible.