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

Abstract

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.

Reactivity of Surface Lewis and Brønsted Acid Sites in Zeolite Catalysis: A Computational Case Study of DME Synthesis Using H-SSZ-13

“…Depending on the temperature and partial pressures, oxygenates such as water, methanol, and dimethyl ether (DME) adsorb at the BAS through the formation of a hydrogen bond. Recent studies also focus on aluminum siting − as well as the effect of different Si/Al ratios. − The reactivity of surface BAS, however, has rarely been studied, although with a crystal size of a few nanometers to micrometers, , zeolite particles as well as 2D zeolites − offer a large surface area. At the surface, substitution of a silicon atom at a single silanol group followed by dehydration leads to a threefold coordinated aluminum atom (a structural motif not present in BAS), which generally acts as a Lewis acid site (LAS, see Scheme ).…”

Multilevel quantum mechanical calculations show the role of promoter molecules in the dehydration of methanol to dimethyl ether in H-ZSM-5

“…94 Our calculated intrinsic barriers using PBE+D3 alone give similar results to those calculated in the literature. 94,95 For example, using PBE+D3 we calculate an intrinsic barrier of 128 kJ mol -1 for the formation of a surface methoxy species, Smith et al calculates an intrinsic barrier of 143 kJ mol -1 , 95 whilst Kilburn et al calculates an intrinsic barrier of 119 kJ mol -1 . 94 Di Iorio et al have investigated the same dehydration reaction in a different zeolite 25 , CHA.…”

Cooperative Effects of Active Sites in the MTO Process: A Computational Study of the Aromatic Cycle in H-SSZ-13

“…Most commonly, Brønsted acidity is introduced by substitution of a silicon atom of the zeolite framework with an aluminum along with a proton compensating the charge difference. Both the location of the aluminum substitution in the framework and the aluminum content (i.e., the Si/Al ratio) have been shown to influence the reactivity of Brønsted acid sites. − It is thus not surprising that computational studies showed that Brønsted acid catalyzed reaction barriers change by as much as 20 kJ/mol with acid site location and aluminum content. − In the theoretical studies investigating proximate acid sites, the effect of an additional site is generally indirect, i.e. it changes the reactivity by modifying the potential energy surface, but it does not directly take part in the chemical reaction so that the reaction mechanism remains essentially identical. − This is in contrast to Cu-substituted zeolites employed in NOx selective catalytic reduction, where mobile Cu-species were proposed to react in a cooperative fashion, thus changing the reaction mechanism when compared to a single site …”