The relative stability of the H3PO4 in each extra-framework
site is evaluated by calculating the binding energy (BE):
Ebind
¼ {E(H3PO4/HZSM-5) E(H3PO4) E(HZSM-5)}
Where E(H3PO4/HZSM-5), E(H3PO4) and E(HZSM-5) represent
the electronic energy for the optimized H3PO4/HZSM-5 cluster,
the isolated H3PO4 and the HZSM-5 cluster, respectively.
The deprotonation energy (DE) is calculated to estimate the
acidity of the bridging hydroxyl groups, which is expressed as
the total energy difference between the initial (neutral) and the
deprotonated (anionic) forms of the HZSM-5 or H3PO4/HZSM-5
clusters. The DE value cannot be directly measured experimentally.
The difference between the DE values of two different
OH groups can be determined from the difference in the
binding energies of a probe basic molecule.40