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

Abstract

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 Co 2+ -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 occlusion 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 Co 2+ 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 Co 2+ -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/Al ratio ((TPA + + Na + )/Al = 30), crystallized MFI with a similar bulk Al content (Si/Al ≈ 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-diazabicyclo[2.2.2]octane and methylamine, together with Na + to compensate for framework Al, crystallized MFI at similar bulk Al content (Si/Al ≈ 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 arrang...

A priori control of zeolite phase competition and intergrowth with high-throughput simulations

“…Only distributions within 100 kJ/mol of the most stable configuration (ID 28) are displayed. The calculations indicate that the adsorption of Co 2+ in d6runits with paired aluminum sites is the most favorable configuration, for which the Co-exchange is known to be favorable(80). The calculated energies are in quantitative agreement with previous studies(43).…”

Interzeolite Conversion and the Role of Aluminum: Toward Generic Principles of Acid Site Genesis and Distributions in ZSM-5 and SSZ-13

“…Especially for the low-symmetry MFI this is important, as it was recently calculated theoretically that numerous (5-or 6-membered rings based) acid site duo's have feasible exchange energies for divalent cobalt. 10 To validate our probe, we demonstrate the presence of almost solely Co 2+ (H2O)6 after room temperature DCC exchange on variable samples. Diffuse reflectance UV-vis absorption spectra (Figure S8) demonstrate the presence of Co 2+ ions as hexa-aqua complexes (absorption band with a maximum at 19 400 and a shoulder at 20 400 cm -1 ) 83,84 and suggests the absence of significant amounts of cobalt oxides (typically between 25.000-33.000 cm -1 ) 83,85 in both a crystalline sample…”

“…Most recent studies on short-range Al distributions have deduced 'thermodynamic' synthesis outcomes based on localized charge balances at the cage level between a positively charged (organic) structure-directing agent (O)SDA and the negatively charged framework from either aluminum or defect sites. [8][9][10][11][12][13][14][15] Furthermore, Al separation rules such as Löwenstein rule 16 or Dempsey's rule 17 and the intrinsic thermodynamic preference of Al for particular T-sites (framework dependent) 18 also seem to influence the short-range Al distribution in synthesized zeolites, despite the strong influence of kinetic processes during zeolite crystallization. 19,20 The use of alternative Si and Al sources −and other recipe alterations− may provoke Al distribution outcomes unexpected from a charge-balancing perspective.…”

On the Acid Sites of Phosphorous Modified Zeosils

“…The T12-site is the most frequently chosen site for aluminum substitution due to its presence at the interception of the straight and sinusoidal channels of MFI [71][72][73] and the lowest energy among all 12 T-sites, 74,75 T2-site is also located at the interception of channels 75,76 and is energetically comparable to T12-site for aluminum incorporation. 75 Additionally, the energetically most favorable T-site for the P-active site is unknown. As such, calculations were performed and compared on both T2 and T12-sites for both set of sites (Al-, and P-) to choose the most representative T-site for adsorption energy calculations.…”