Experimental and Theoretical Assessments of Aluminum Proximity in MFI Zeolites and Its Alteration by Organic and Inorganic Structure-Directing Agents

The molecular structure and cationic charge density of organic and inorganic structure-directing agents (SDAs) influence the siting and arrangement of Al substituted in zeolite frameworks. Yet, developing robust synthesis-structure relations for MFI zeolites is difficult because of the complexities inherent to its low-symmetry framework (12 unique tetrahedral sites), which generates a large combinatorial space of Al-Al site pairs to exhaustively model by density functional theory (DFT) and quantify by experiment. Here, we develop an experimental protocol to reproducibly quantify Co2+-titratable Al-Al site pairs in MFI with saturation uptakes validated by corroborating spectroscopic and cation site balance data. Using tetrapropylammonium (TPA(+)) as the sole SDA, MFI zeolites were crystallized with varying Al contents (Si/Al = 37-185; 0.52-2.52 Al per unit cell) within a composition range consistent with charge density mismatch theory and the ocdusion of one TPA(+) per channel intersection with fractions of paired Al (0.0-0.34) that increased with bulk Al content. DFT calculations performed using a 96 T-site MFI unit cell containing either an isolated Al site (all 96 configurations) or various Al-Al site pairs (1773 out of 13 680 total configurations), charge balanced by one or two TPA(+), respectively, reveal the dominant influence of electrostatic interactions between the cationic N of TPA(+) and the anionic lattice charge on Al siting energies. Together with DFT calculations of Co2+ exchange energies at Al-Al site pairs, theory predicts that two TPA(+) cations confined within adjacent channel intersections can form many Al-Al site pair ensembles that are Co2+-titratable, rationalizing the considerable presence of paired Al sites in MFI samples crystallized using only TPA(+). The use of TPA(+) and Na+ as co-SDAs in the synthesis gel, while varying the Na+/TPA(+) ratio (0-5) at a constant SDA/AI ratio ((TPA(+) + Na+/Al = 30), crystallized MFI with a similar bulk Al content (Si/AI approximate to 50) but varying fractions of Al in pairs (0.12-0.44). Separate crystallization experiments performed using charge-neutral organic SDAs, either pentaerythritol or a mixture of 1,4-diazabicydo[2.2.2]octane and methylamine, together with Na+ to compensate for framework Al, crystallized MFI at similar bulk Al content (Si/AI approximate to 50) but with lower fractions of Al in pairs (<0.14). Among MFI samples crystallized with an organic SDA and Na+ as a co-SDA, the number of paired Al sites formed generally increased with the co-occluded Na+. content on the zeolite, a synthesis-structure relation that resembles our prior observations on CHA zeolites. The combined theoretical and experimental approach used here provides a microscopic model to define and quantify Al-Al site pairs in MFI, which can be adapted to do so for other framework topologies. These findings highlight how such Al siting models can be exercised to quantitatively characterize zeolite materials to develop synthetic strategies that can predictably vary their framework Al arrangements and catalytic and adsorption properties in turn, as shown here for samples of essentially constant bulk composition by exploiting mixtures of organic and inorganic SDAs during hydrothermal crystallization.