The role of the electrostatic potential, electric field and electric field gradient on the acidity of AFI and CHA zeotypes

We consider the role of short- and long-range interactions in determining the acidity of microporous solids. Within the context of an interatomic forcefield approach and a correct treatment of long-range electrostatics we demonstrate the role of the electrostatic potential, electric field and electric field gradient in determining the vibrational frequency of a Brønsted acid centre. We demonstrate the applicability of these concepts for AlPO4-5, AlPO4-34, SSZ-24 and SSZ-13, in whose unit cells a single Brønsted acid site was introduced in the four different oxygens. Furthermore, we show, by using the energy required to elongate the OH bond—essentially the first stage of proton transfer—as a measure of acidity, how the vibrational frequency is related to the acid strength.