Structure-directing study of 1-methylimidazolium-based dication with tetramethylene as spacer length in the synthesis of microporous silicoaluminophosphates†
In this work we study the structure-directing effect of 1-methylimidazolium-based dications that have four methylene units as the spacer length (4BI), as Organic Structure-Directing Agent (OSDA) in the synthesis of silicoaluminophosphate (SAPO) microporous molecular sieves. Results showed the occurrence of three different phases depending on the mineralizing agent (fluoride or hydroxide). When the synthesis is carried out in hydroxide medium, the OSDA tends to favor the crystallization of AEI and AFO frameworks, with the latter appearing only under specific synthesis conditions. By contrast, under concentrated fluoride conditions, this OSDA induces the formation of the triclinic chabazite (CHA) SAPO-34(t) material. Considering that this cation (4BI) should be too bulky to be occluded in both the CHA and AEI cavities, molecular simulations were carried out to study the locations and conformations of the dications in these structures. In the case of CHA, computational results showed a most likely location for 4BI cations spanning two adjacent chabazite cavities, with two imidazolium rings of adjacent cations confined in a single cavity in a stacked sandwich-like configuration. By contrast, the location of the 4BI cations within the AEI framework involves a lower incorporation of one imidazolium ring within each cavity, again with the organic cations spanning two adjacent cavities, but avoiding π–π imidazolium stacking interactions. In both CHA and AEI frameworks, our results suggest that the alkyl chain of the 4BI cations passes through 8-ring windows connecting adjacent cavities.