An Examination of Confinement Effects in High-Silica Zeolites

The environment provided by the zeolite channels has been examined in high-silica zeolites having the MFI (ZSM-5), CHA (Chabazite), MOR (Mordenite), TON (ZSM-22), MTW (ZSM-12), FER (Ferrierite), and FAU (Faujasite) structures. Calorimetric measurements of CH 4 and O 2 at ∼210 K showed that structure affects the adsorption properties in a manner which depends on the pore dimensions. The differential heats for CH 4 at low coverage were 28 kJ/mol in FER, 27 kJ/mol in TON, 25 kJ/mol in MOR, 21 kJ/mol in MFI, 20.5 kJ/mol in MTW, 19.5 kJ/mol in CHA, and 14 kJ/mol in FAU. However, calorimetric data for acetonitrile in the acidic forms of these zeolites gave heats that were independent of structure within experimental error, with differential heats for the 1:1 adsorption complexes all being 100 ± 10 kJ/mol. A possible explanation lies in the additional orientation-dependent interaction resulting from the hydrogen bonding. These topology-sensitive, orientational interactions are observable in the temperature-sensitive, methyl-proton, NMR line shapes, where it is found that the barriers to reorientation of 1:1 adsorption complexes are much higher in small-pore zeolites. The implications of these measurements to “confinement” effects in catalysis are discussed.