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

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• 作者：Amber JandaBess Vlaisavljevich ; Berend SmitLi-Chiang Lin ; Alexis T. Bell
• 刊名：Journal of Physical Chemistry C
• 出版年：2017
• 出版时间：January 26, 2017
• 年：2017
• 卷：121
• 期：3
• 页码：1618-1638
• 全文大小：1375K
• ISSN：1932-7455

文摘

Monte Carlo simulations are used to systematically investigate the effects of structural topology on the thermodynamics of n-alkanes adsorbed at Brønsted protons in zeolites having one-dimensional channel systems. In zeolites without cages, the enthalpy and entropy of adsorption (ΔH_ads-H⁺ and ΔS_ads-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 (ΔA_ads-H⁺) is weak because the changes in ΔH_ads-H⁺ and TΔS_ads-H⁺ largely cancel. The addition of cages having a largest-cavity diameter (LCD) greater than the PLD increases both ΔH_ads-H⁺ and ΔS_ads-H⁺. Replacing channels with cages of the same diameter does not change ΔS_ads-H⁺ significantly when the PLD is similar to the alkane length but decreases both ΔH_ads-H⁺ and ΔA_ads-H⁺ because of the greater surface area of cages relative to channels. The selectivity to adsorption via a central C–C 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 ΔA_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 TΔS_ads-H⁺ and ΔH_ads-H⁺ that occur upon changing only one structural parameter.