Influence of Acidity on the Methanol-to-DME Reaction in Zeotypes: A First Principles-Based Microkinetic Study

Acidity is considered a key factor in zeotype-based catalysts.
Here, the effect of acidity in the methanol-to-DME reaction is investigated
using first-principles calculations and microkinetic modeling, thereby
establishing a connection between acididity and kinetics. The CHA,
MFI, and BEA frameworks are investigated, and the acidity of the Brønsted
hydroxyl group is varied by exchanging a T-site Si with Al, B, Ga,
and Fe in the zeolites, along with SAPO-34, Mg-AlPO-34, Zn-AlPO-34,
and Ti-AlPO-34 zeotypes with the CHA structure, and as a result, the
Brønsted hydroxyl group spans a wide range of acidity. Clear
trends in adsorption and transition-state energies are found and by
means of linear regression, we obtain scaling relations of relevant
energies that are later used as input in a mean-field steady-state
microkinetic model. This study confirms that both the shift in frequency
of the Brønsted hydroxyl stretch, ΔfOH, caused by adsorption of CO and the ammonia adsorption energy,
ΔEammonia, on the Brønsted
site are equivalent descriptors for the acidity of the Brønsted
acid site and the reactivity of the different zeotypes relevant for
the methanol-to-DME reaction. It further shows that a full microkinetic
model is needed to accurately describe the reaction over the whole
range of temperatures. However, if focusing on low temperatures, where
the associative mechanism is dominating the reaction, a simple rate-determining
step model is actually able to describe the results with satisfying
agreement (deviation of the rate by less than a factor of two).