Uncommon Synergy between Adsorption and Diffusion of Hexane Isomer Mixtures in MFI Zeolite Induced by Configurational Entropy Effects

While n-hexane (nC6) can adsorb at any location within the MFI zeolite pore network, configurational considerations cause the branched isomer 2-methylpentane (2MP) to locate preferentially at the channel intersections. For adsorption of nC6/2MP mixtures, infrared microscopy (IRM) measurements show that the adsorption selectivity favors the linear isomer by about one order of magnitude when the total mixture loading, Theta(t), exceeds four molecules per unit cell at which all intersection sites are fully occupied. The IRM data are in quantitative agreement with configurational-bias Monte Carlo (CBMC) simulations. IRM monitoring of the transient uptake of nC6/2MP mixtures within crystals of MFI exposed to step increases in the pressures shows that the configurational entropy effect also leaves its imprint on the uptake characteristics. For operating conditions in which Theta(t) > 4, increase in the 2MP partial pressure in the bulk gas phase leads to a reduction in the 2MP uptake; this reduction leads to a concomitant and synergistic increase in the diffusivities of both isomers due to reduced extent of intersection blocking