The Use of the Logarithmic Adsorption Isotherm for the Determination of the Micropore-Size Distribution

The determination of the micropore-size distribution of microporous materials is of major importance in the field of adsorption and catalysis. This new method to derive the micropore-size distribution for Pillared Clays (PILC) is based on the logarithmic adsorption isotherm. When a N(2)-adsorption isotherm is replotted on a log P/P(0) scale, a curve representing the adsorption potential is obtained. Because the adsorption potential is highly dependent on the pore-size, this curve contains all the information on the pore structure. These curves often exhibit steps, large increases in volume adsorbed by small changes in adsorption potential. These are thermodynamically related to the filling of pores of a certain size. The relative pressures where these steps appear are dependent on the pore-size. Taking the derivative, dV/dlog (P/P(0)), the location of these steps becomes more clear. These derivative curves give a lot of information on the pore-size distribution. The presence of several maxima and their sharpness informs about the number of different pore-sizes present and the homogeneity. Assuming that N(2)-adsorption in the micropores is ruled by the amount of N(2)-layers which can fit in, the micropore range can be subdivided in five pore groups adsorbing 1-5 layers. By locating the main maxima in the derivative curves of pillared clays with known but different slit widths, we can correlate a pore-size range to a certain P/P(0) range where these pores will fill. In this way a micropore-size distribution based on the adsorption potential can be constructed.