Studies of Adsorption Characteristics in Supercritical Fluids by FTIR and Spectroscopic Measurements

本文采用红外光谱和色谱两种不同的方法研究了超临界流体中的吸附特性。在光谱测量中，用超临界CO_2稀释有机溶剂组成的吸附体系在大孔硅胶（SG800）上，从常压到15 MPa，在313.2 K下获得了红外光谱。我们测量的有机溶剂有甲醇、丙酮、三乙胺、乙腈、四氢呋喃和乙醚。值得注意的一个重要结果是，除三乙胺体系外，所有sylanol氢键OH带的强度在20-50 MPa左右达到最大值，然后随着压力的增加而减小。红外观测所得的特征曲线与用蒙特卡罗模拟计算的超临界CO_2稀释苯在石墨壁裂隙中的吸附等温线基本一致。分子模拟已经提出了影响超临界流体吸附特性的两个因素：在超临界流体中溶解度的增加和溶剂（CO_2）分子的竞争吸附随压力的增加。本文利用工业泵新建成的超临界流体色谱仪，测定了超临界CO_2中丙酮和苯在SG800上的吸附等温线。得到的洗脱曲线显示，在不同的注入量下，一个大的尾迹形成了一个共同的包络线，这表明：(a) ECP（特征点洗脱）法适用于获得溶质的吸附等温线；(b) SG800具有能量不均匀的表面，在理论分析中应该考虑到这一点。在本工作中得到的结果可以总结如下。(1)色谱测得的丙酮等温线与红外光谱测得的等温线定性吻合。(2)高压区吸附等温线下降幅度较大，这可能是由于在高竞争吸附区，范德华力对吸附的贡献较小。(3)苯在CO_2中的吸附等温线与丙酮相似，但苯分子不与硅胶上的OH基团形成氢键，而丙酮分子有氢键的红外光谱证据。少

The present work has been intended to investigate the adsorption characteristics in supercritical fluids by using two different methods, Infrared spectroscopy and Chromatography. In spectroscopic measurements, IR spectra were obtained for adsorption systems composed of an organic solvent diluted in supercritical CO_2 on a large-pore Silica Gel (SG800) from atmospheric pressure to 15 MPa at 313.2 K.The organic solvents we measured are methanol, acetone, triethylamine, acetonitrile, tetrahydrofuran, and diethyl ether. A most important result to be noted is that all the intensities of hydrogen-bonded OH bands of sylanol, except for a system of triethylamine, showed a maximum at about 20-50 MPa and then they dcreased with increasing pressure. The characteristic curves of the IR observations are almost the same as the adsorption isotherms calculated by Monte Carlo simulations for a system of benzene diluted in supercritical CO_2 in slitpores of graphitic walls. The molecular simulations hav … More e already suggested two factors for adsorption characteristics in supercritical fluids : that is, an increase in solubility into a supercritical fluid and an increase in competitive adsorption of the solvent (CO_2) molecules with increasing pressure.A supercritical fluid chromatograph, newly constructed in the present work by use of commercial pumps, has been used for measuring the adsorption isotherms of acetone and benzene on SG800 in supercritical CO_2. The elution curves obtained showed a large tailing forming a common envelope for different amount of injections, which indicated that (a) the ECP (Elution by Characteristic Point) method was applicable for obtaining the adsorption isotherms of solutes and (b) the SG800 has energetically heterogeneous surfaces that should be taken into consideration for a theoretical analysis.The results obtained in the present work may be summarized as follows. (1) The isotherm curves of acetone obtained from the chromatographic measurements qualitatively coincide with the isotherms obtained from the IR spectroscopy. (2) The adsorption isotherms of the chromatographic measurements decrease more in the high-pressure region, which may be attributable to the less contribution of the van der Waals force on adsorption in the region of high competitive-adsorption. (3) The adsorption isotherms of benzene in CO_2 resemble those of acetone though benzene molecules do not show hydrogen-bonding with OH groups on silica gel while acetone molecules show an IR spectroscopic evidence of hydrogen-bonding. Less