Probe Molecule Kinetic Studies of Adsorption on MCM-41

Gases and vapors of varying dimensions were used as probes to investigate the adsorption processes on MCM-41 and precipitated silica, prepared by the same method as for MCM-41, but without the template. The adsorption kinetics of nitrogen, carbon dioxide, n-nonane, and α-pinene were studied for different amounts of preadsorbed gas/vapor as a function of relative pressure. The adsorption kinetics follow a linear driving force model for changes in surface coverage up to the adsorption isotherm plateaus. The variation of rate constant with relative pressure for MCM-41 and precipitated silica showed marked differences associated with adsorption in mesoporous structure, which are attributed to alignment of molecules in the mesopores leading to a pore blocking effect. The results are discussed in terms of differences in the adsorption mechanisms of gas/vapor diffusion in porous materials.