TY - JOUR
T1 - Characterization of acidic OH groups in zeolites of different types
T2 - An interpretation of NH3-TPD results in the light of confinement effects
AU - Hunger, Bernd
AU - Heuchel, Matthias
AU - Clark, Louis A.
AU - Snurr, Randall Q.
PY - 2002/4/18
Y1 - 2002/4/18
N2 - Ammonia TPD experiments have been carried out on protonated forms of the zeolites FAU, FER, MFI, and MOR to investigate the acid strength dependence on OH group location and Al content. Effective adsorption energy distributions are derived using a regularization method and corrected to achieve a truer measure of acid strength using nonspecific interaction energies from atomistic Monte Carlo simulations. The correction energies include all interactions not associated with the OH groups and range from 18 to 37 kJ/mol, depending on the region and the zeolite structure. In the FAU structure, we find a bimodal distribution of adsorption energies in both the supercages and the sodalite cages, with the stronger acid sites being more common in the supercages. In contrast, essentially the same acid strength for different regions in MOR is seen, though the acid strength for those sites in the side pockets may be slightly lower. For FER and MFI, the TPD gives unimodal distributions and therefore prohibits distinction between different regions. In agreement with the model of Barthomeuf et al., we see an apparently linear decline (60 kJ/mol) in acid strength beyond a certain aluminum framework density. However, we see a smaller but consistent increase (16 kJ/mol) in acidity before this threshold, despite model predictions that it should be constant.
AB - Ammonia TPD experiments have been carried out on protonated forms of the zeolites FAU, FER, MFI, and MOR to investigate the acid strength dependence on OH group location and Al content. Effective adsorption energy distributions are derived using a regularization method and corrected to achieve a truer measure of acid strength using nonspecific interaction energies from atomistic Monte Carlo simulations. The correction energies include all interactions not associated with the OH groups and range from 18 to 37 kJ/mol, depending on the region and the zeolite structure. In the FAU structure, we find a bimodal distribution of adsorption energies in both the supercages and the sodalite cages, with the stronger acid sites being more common in the supercages. In contrast, essentially the same acid strength for different regions in MOR is seen, though the acid strength for those sites in the side pockets may be slightly lower. For FER and MFI, the TPD gives unimodal distributions and therefore prohibits distinction between different regions. In agreement with the model of Barthomeuf et al., we see an apparently linear decline (60 kJ/mol) in acid strength beyond a certain aluminum framework density. However, we see a smaller but consistent increase (16 kJ/mol) in acidity before this threshold, despite model predictions that it should be constant.
UR - http://www.scopus.com/inward/record.url?scp=0037129023&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=0037129023&partnerID=8YFLogxK
U2 - 10.1021/jp012688n
DO - 10.1021/jp012688n
M3 - Article
AN - SCOPUS:0037129023
SN - 1089-5647
VL - 106
SP - 3882
EP - 3889
JO - Journal of Physical Chemistry B
JF - Journal of Physical Chemistry B
IS - 15
ER -