Effect of Aluminum Siting in H‑ZSM‑5 on Reaction Barriers

We investigate the influence of acidity
and confinement for different
aluminum T-site substitutions in H-ZSM-5 using reactions related to
the methanol-to-olefin (MTO) process as examples. We use density functional
theory at the PBE-D3 level to study all 12 different T-sites existing
in the MFI framework. We find that transition-state energies vary
by about 20 kJ/mol with the commonly employed T12 site having some
of the lowest barriers. A large part of the energetic differences
can be ascribed to differences in dispersion forces due to the surrounding
framework, as also evidenced by smaller and uncorrelated differences
in calculated heats of adsorption of ammonia. Our analysis shows that
taking the T12 site as a computational active site model will yield
reaction barriers that are among the lowest of all T-sites available.