A computational study on heteroatom distribution in zeotype materials

SSZ-13 and SAPO-34 are a zeolite and a SAPO, respectively, both having the CHA topology. We present computational studies of the relative stabilities of a pair of acid sites in these materials as a function of their separation. The most stable configurations are found when the acid sites are next nearest neighbors, viz. separated by only one other T-atom. Replacing the acidic protons with a distributed charge leads to the most stable geometry involving a maximum distance between the two sites, in accordance with Dempsey’s rule. Unsurprisingly, we find a negative correlation between calculated stability and calculated Brønsted acid strength. The results afford novel interpretation of earlier spectroscopic studies.

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► Acid sites in a zeolite and a SAPO tend to form next-nearest-neighbor pairs (NNNP).
► This only applies to calcined materials.
► When charge is delocalized by coke or template, NNNPs disappear.
► NNNP sites are less acidic than discrete sites.