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

Berend Smit
American Chemical Society, 2017
Journal paper

Abstract

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.

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Berend Smit
American Chemical Society, 2017
Journal paper

Abstract

Official source

https://infoscience.epfl.ch/record/226968?ln=en

About this result

Related concepts (11)

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Related publications

Effects of Zeolite Structural Confinement on Adsorption Thermodynamics and Reaction Kinetics for Monomolecular Cracking and Dehydrogenation of n-Butane

Berend Smit

The effects of zeolite structure on the kinetics of n-butane monomolecular cracking and dehydrogenation are investigated for eight zeolites differing in the topology of channels and cages. Monte Carlo simulations are used to calculate enthalpy and entropy changes for adsorption (Delta Hads-H+ and Delta Sads-H+) of gas-phase alkanes onto Bronsted protons. These parameters are used to extract intrinsic activation enthalpies (Delta H-int(double dagger)), entropies (Delta S-int(double dagger)), and rate coefficients (k(int)) from measured data. As Delta Sads-H+ decreases (i.e., as confinement increases), Delta H-int(double dagger) and Delta S-int(double dagger), for terminal cracking and dehydrogenation decrease for a given channel topology. These results, together with positive values observed for Delta S(int)(double dagger)ASitt, indicate that the transition states for these reactions resemble products. For central cracking (an earlier transition state), Delta H-int(double dagger) is relatively constant, while Delta S-int(double dagger), increases as Delta Sads-H+ decreases because less entropy is lost upon protonation of the alkane. Concurrently, selectivities to terminal cracking and dehydrogenation decrease relative to central cracking because Delta S-int(double dagger) decreases for the former reactions. Depending on channel topology, changes in the measured rate coefficients (k(app)) with confinement are driven by changes in kit or by changes in the adsorption equilibrium constant (Kads-H+). Values of Delta S-int(double dagger) and Delta H-int(double dagger) are positively correlated, consistent with weaker interactions between the zeolite and transition state and with the greater freedom of movement of product fragments within more spacious pores. These results differ from earlier reports that Delta H-int(double dagger) and Delta S-int(double dagger) are structure-insensitive and that k(app) is dominated by Kads-H+. They also suggest that Delta Sads-H+ is a meaningful descriptor of confinement for zeolites having similar channel topologies.

American Chemical Society2016

Experimental' measurements of the rate coefficient (k(app)) and apparent enthalpies and entropies of activation (Delta H-app arid Delta S-app) for alkane cracking catalyzed by acidic zeolites can be used to characterize the effects of zeolite structure and alkane size On the intrinsic enthalpy and entropy of activation, Delta H-int(double dagger) and Delta S-int(double dagger). To determine Delta H-int(double dagger) and Delta S-int(double dagger), enthalpies and entropies of adsorption, Delta Hads-H+ and Delta Sads-H+, must be determined for alkane molecules moving from the gas phase to Bronsted add sites at reaction temperatures (>673 K). Experimental values of Delta H-app and Delta S-app must also be properly defined in terms of Delta Hads-H+ and Delta Hads-H+. We report here a method for determining Delta Hads-H+ and Delta Sads-H+ in which the adsorption site is represented by a fixed volume that includes the proton. Values of Delta Hads-H+ and Delta Sads-H+ obtained from Monte Carlo simulations are in good agreement with values obtained from experimental data measured at 300-400 K. An important feature of the simulations, however, is their ability to account for the redistribution of alkane adsorbed at protons in different locations with ineteasing temperature. Values of Delta H-int(double dagger) and Delta S-int(double dagger), for the cracking of propane through n-hexane, determined from measured values of k(app) and Delta H-app and simulated: values of Delta Hads-H+ and Delta Sads-H+, agree well with values obtained independently from quantum mechanics/molecular mechanics calculations. Application of our method of analysis reveals that the observed increase in kapp with increasing n-alkane size is due primarily to a decrease in Delta H-int(double dagger) with increasing chain length and that Delta S-int(double dagger) is independent of chain length.

American Chemical Society2015

Effects of Zeolite Structural Confinement on Adsorption Thermodynamics and Reaction Kinetics for Monomolecular Cracking and Dehydrogenation of n-Butane

Berend Smit

American Chemical Society2016