介孔吸附剂表面分形分析

Surface Fractal Analysis of Mesoporous Adsorbents

用自溶液中吸附的方法测定了介孔硅胶和活性炭的分形维数D .结果表明 :( 1)硅胶自四氯化碳或环已烷中吸附脂肪醇、酮、酯和含氧芳香化合物 ,活性炭自水中吸附芳香化合物的等温线均服从Langmuir方程 .用极限吸附量nsm 和表观分子面积σa,根据方程lognsm=-D2 logσa+常数 ,可计算出分维D .nsm 和σa 是根据Langmuir方程和吸附剂比表面数据求出的 .( 2 )用液相吸附法求出的介孔硅胶的分维值与由气体吸附法求出的值相同 ,并均近于 2 .当表面曲率半径大于吸附分子大小时 ,吸附剂表面可是低分维的 .( 3 )由液相吸附法得到的介孔活性炭的分维也近似为 2 .可能的解释是 ,活化作用改变了原始炭的微孔结构 ,使其成为介孔和大孔 .

The fractal dimension D values of the mesoporous silica gel and activated carbon have been determined by the method of adsorption from solutions. The results are as follows. (1) The adsorption isotherms of aliphatic alcohols, ketones, esters and oxy aromatic compounds from carbon tetrachloride or cyclohexane on silica gel, and those of aromatic compounds from water on activated carbon can be represented by the Langmuir equation. The fractal dimension D can be calculated by the limiting adsorbed amount, n s m, and the apparent molecular across section, σ a, based on log n s m=- D 2log σ a+constant. The n s m and σ a were calculated with the Langmuir equation and the specific surface area of adsorbents. (2) The fractal dimension D values of the mesoporous silica gel derived by the method of adsorption from solutions are the same as those from gases, which are ca. 2. As long as the radius of the surface curvature is larger than the size of adsorbed molecules, the surface of adsorbent may have low fractal dimension. (3) The obtained fractal dimension D value of the mesoporous activated carbon derived by the method of adsorption from solutions is also close to 2. A possible explanation is that progressive activation changes the microporous structure of the original charcoal gradually to a meso/macroporous structure. Then the mesoporous activated carbon can be also characterized by low fractal dimension.