An Adsorption Study of CH4 on ZSM-5, MOR, and ZSM-12 Zeolites
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University of Pennsylvania Philadelphia United States
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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 -1kJmol, independent of Broensted site density or defect concentration. However, DFT calculations performed using various functionals and on the most stable Broensted site indicated that CH4 should bind to this site by an additional 1 to 7 kJmol, 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 kJmol at low coverages, falling to 25 kJmol for coverages above one molecule per 8-membered-ring side pocket, while differential heats on ZSM-12 were initially 22.5 kJmol, decreasing to 21 kJmol with coverage. DFT calculations on the siliceous form of the zeolites were able to predict these values within 5 kJmol in most cases. The results indicate thatCH4 is an excellent probe molecule for characterizing the pore structure of zeolites.
- Physical Chemistry