An Adsorption Study of CH₄ on ZSM-5, MOR, and ZSM-12 Zeolites

Abstract

CH 4 adsorption was studied experimentally and theoretically on ZSM-5, MOR, and ZSM-12 zeolites using calorimetric measurements at 195 K and plane wave DFT calculations. Differential heats measured on four different H-ZSM-5 samples were determined to be 22.5 ± 1 kJ/mol, independent of Brønsted site density or defect concentration. However, DFT calculations performed using various functionals and on the most stable Brønsted site indicated that CH 4 should bind to this site by an additional 1−7 kJ/mol, a discrepancy that is due to the inability of standard DFT methods to capture hydrogen-bonding effects accurately with CH 4 . Differential heats for CH 4 in MOR were 30 ± 1 kJ/mol at low coverages, falling to 25 kJ/mol for coverages above one molecule per 8-membered-ring side pocket, while differential heats on ZSM-12 were initially 22.5 kJ/mol, decreasing to 21 kJ/ mol with coverage. DFT calculations on the siliceous form of the zeolites were able to predict these values within 5 kJ/mol in most cases. The results indicate that CH 4 is an excellent probe molecule for characterizing the pore structure of zeolites.

Adsorption of Small Alkanes on ZSM-5 Zeolites: Influence of Brønsted Sites

“…An important example is that of simple alkanes wherein the adsorption thermochemistry depends on the dispersive interactions with the zeolite wall and the noncovalent interactions with the acid site. In our previous study, we showed that interactions of CH 4 with the Brønsted sites in H-ZSM-5 do not contribute significantly to the heats of adsorption; differential heats were found to be independent of coverage or zeolite Si/Al 2 ratio. However, there are both experimental , and theoretical , reports that the contributions to the heats of adsorption from interactions with Brønsted sites are significant for larger alkanes.…”

“…Finally, most real zeolites have significant concentrations of vacancy defects, observable as nested silanols; and these have a profound effect on the adsorption of polar molecules, like water . Although vacancy sites do not appear to affect CH 4 adsorption, CH 4 adsorption was also not affected by the presence of Brønsted sites.…”

Adsorption properties of acetylene, ethylene and ethane in UiO-66 with linker defects and NO2 functionalization

“…At lower temperatures, such as 195 K, the partition function occupancy is more separated by the different adsorption sites, allowing one to distinguish the adsorption energy associated with each type of site. [56][57][58][59][60] Measurements at higher temperatures (293 K) typically represent some average binding enthalpy from the different adsorption sites in the MOF and complicate the analysis. In fact, partition-function occupancyprobability calculations suggest that it is likely impossible to observe minority adsorption site enthalpies that are 5 kJ/mol higher than the majority adsorption sites at room temperature.…”

Probing Gas Adsorption in Zeolites by Variable-Temperature IR Spectroscopy: An Overview of Current Research

“…To that endeavor, precise characterization of the adsorption sites and the determination of the gas-solid interaction modes and corresponding interaction energy are prime requirements. Moreover, a similar strategy can also be used in the search for cost-effective adsorbents for large-scale (reversible) storage and delivery of fuels such as methane and hydrogen; which constitutes a present-day strategic issue in the transportation sector [ 14 , 15 , 16 , 17 , 18 , 19 , 20 ]. Aside from the foregoing usage, increased knowledge of the nature and layout of active adsorption sites is also highly relevant to the wide-ranging industrial application of zeolites as catalysts in several fields, such as the petrochemical industry, methanol to olefin conversion, catalytic production of specialty chemicals [ 21 , 22 , 23 , 24 , 25 , 26 , 27 ] and CO 2 methanation [ 28 , 29 ], to mention only a few examples.…”