jp5b09571_si_001.pdf (1.31 MB)
An Adsorption Study of CH4 on ZSM-5, MOR, and ZSM-12 Zeolites
journal contribution
posted on 2015-12-31, 00:00 authored by Yiwei Zhang, JingYe Yu, Yu-Hao Yeh, Raymond. J. Gorte, Srinivas Rangarajan, Manos MavrikakisCH4 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.