Effects of Pore and Cage Topology on the Thermodynamics of n-Alkane Adsorption at Bronsted Protons in Zeolites at High Temperature

Monte Carlo simulations are used to systematically investigate the effects of structural topology on the thermodynamics of n-alkanes adsorbed at Bronsted protons in zeolites having one-dimensional channel systems. In zeolites without cages, the enthalpy and entropy of adsorption (Delta Hads-H+ and Delta Sads-H+) at fixed pore-limiting diameter (PLD) generally increase (become less negative) as the ratio of the minimum to maximum channel diameter decreases, and are lowest when this ratio equals 1 (corresponding to approximately circular cross sections). The effect of a change in diameter ratio on the free energy of adsorption (Delta A(ads-H+)) is weak because the changes in Delta Hads-H+ and T Delta Sads-H+ largely cancel. The addition of cages having a largest-cavity diameter (LCD) greater than the PLD increases both Delta Hads-H+ and Delta Sads-H+. Replacing channels with cages of the same diameter does not change Delta Sads-H+ significantly when the PLD is similar to the alkane length but decreases both Delta Hads-H+ and Delta A(ads-H+) because of the greater surface area of cages relative to channels. The selectivity to adsorption via a central CC bond vs a terminal bond when cages are absent is smallest for PLDs near the alkane length and, when cages are present, is even lower when the LCD exceeds the alkane length. This effect is attributed to more rotation of the alkane in cages vs channels. The results show that Delta A(ads-H+) at 773 K can be tuned by manipulating a characteristic dimension (LCD, PLD) and topology (e.g., adding cages) simultaneously, in order to circumvent the compensating changes in T Delta Sads-H+ and Delta Hads-H+ that occur upon changing only one structural parameter.