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

CH₄ 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 CH₄ 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 CH₄. Differential heats for CH₄ 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 CH₄ is an excellent probe molecule for characterizing the pore structure of zeolites.