Influence of Tetrapropylammonium and Ethylenediamine Structure-Directing Agents on the Framework Al Distribution in B–Al–MFI Zeolites

Abstract

We report synthetic protocols to independently influence composition and crystallite sizes of MFI zeolites, properties that contribute to Thiele moduli, and the proximity of Al heteroatoms at fixed composition (Si/Al ∼ 50). Crystallite sizes decrease with the addition of non-catalytic B heteroatoms in B–Al–MFI zeolites. Using only tetra-n-propylammonium (TPA+) as a structure-directing agent (SDA) leads to occlusion of one TPA+ per channel intersection and a finite percentage of framework Al atoms (20–40%) in proximal configurations. In contrast, using mixtures of ethylenediamine (EDA) and TPA+ (EDA/TPA+ = 15) leads to incorporation of EDA that displaces some TPA+ and to nearly all (>95%) Al in isolated configurations. These findings indicate how adding B and EDA to zeolite synthesis solutions provides a route to crystallizing MFI zeolites of similar composition but with framework Al distributed at varying relative proximity, or with similar Al distributions but different diffusion path lengths.