A Multiple-Process Equilibrium Analysis of Silica Gel and HZSM-5

Abstract

Adsorption isotherms are reported for HZSM-5 and silica gel using a series of gas adsorptives at several temperatures above their critical temperature. The data are analyzed with a new multiple equilibria adsorption model producing equilibrium constants (Ki), capacities (ni), and enthalpies (−ΔHi) for each of the processes. Unlike the amorphous adsorbents studied earlier, which contain a distribution of pore sizes, zeolitic materials have uniform pore dimensions. This uniformity provides a test of our interpretation of the number of processes required in the isotherm fits of the multiple equilibrium analysis, MEA. As expected from the two pores in the structure of HZSM-5, most adsorbates require two processes (K1,ads and K2,ads) to fit the adsorption isotherms. HZSM-5 is compared to amorphous carbonaceous adsorbents, revealing fundamental differences in their behavior. Small cylindrical channels in HZSM-5 lead to an unfavorable entropic contribution from restrictions imposed by adsorptive packing and inte...