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.