(101d) Cooperation and Competition between Organic and Inorganic Structure Directing Agents Influences the Aluminum Arrangement in CHA Zeolites

The arrangement of anionic Al centers in silica-based zeolite frameworks (Al−O(−Si−O)x−Al) that are present in either isolated (x ≥ 3) or paired (x = 1, 2) configurations [1] can be influenced by manipulating the cationic charge density of the structure directing agents (SDAs) used during hydrothermal crystallization. One approach to influencing the cationic charge density of the zeolite synthesis solution is to use a combination of a low-charge density organic SDA and a high-charge density inorganic SDAs [2]. Here, we investigate the consequences of fixing the total cation and heteroatom content (SDA/Al), but varying the relative ratios of the high and low-charge density SDAs used and the identity of the inorganic SDA used, on the framework Al arrangement in chabazite (CHA) zeolites, which are used as commercial catalysts for the reduction nitrogen oxides with ammonia in diesel engine aftertreatment.

In CHA zeolites (Si/Al ≥ 12), isolated Al sites are predominantly formed when samples are crystallized in the presence of only N,N,N-trimethyl-1-adamantylammonium (TMAda⁺), consistent with thermogravimetric analysis of crystalline CHA products that indicate occupancy of each CHA cage with one TMAda⁺ [3]. The systematic replacement of TMAda⁺ with Na⁺ in the synthesis medium results in crystallization of CHA zeolites with fractions of framework Al in paired configurations (two Al in one six-membered ring, as quantified by Co²⁺ titration [3]) that increase concomitantly with amount of Na⁺ co-occluded with TMAda⁺ in crystalline CHA products [3]. DFT calculations using a 72 T-atom CHA unit cell were used to estimate the stability of different framework Al-Al arrangements when compensated by different SDA cations [4]. Ab initio molecular dynamics (AIMD) simulations reveal facile rotation within a CHA cage of TMAda⁺ about its vertical axis through the quaternary nitrogen center, but energetically unfavorable rotation about its perpendicular axis. DFT-computed energies of zeolite lattices charge-compensated by a single TMAda⁺ cation indicate that attractive electrostatic forces dictate the stability of single Al (Si/Al = 35) substitutions, favoring Al incorporation at lattice positions located within a 0.6 nm radius of the quaternary nitrogen in TMAda⁺. DFT calculations containing two TMAda⁺ in adjacent cages and different arrangements of two framework Al atoms (Si/Al = 35) indicate that TMAda⁺ disfavors the formation of Al-O-Al linkages and Al−O−Si−O−Al in 4-membered rings, but does not preferentially stabilize any specific Al-Al sites at longer separation. The stability of different Al-Al arrangements in the presence of TMAda⁺ and different alkali cations (e.g., Na⁺) was also investigated using AIMD to study how occlusion of inorganic SDAs influences the arrangement of Al in CHA zeolites, and provides evidence that co-occlusion of different SDAs within the same CHA cage energetically favor the formation of specific Al-Al arrangements. We will also discuss experimental and theoretical findings into how other organic and inorganic SDA combinations either favor or disfavor co-occupancy within CHA void spaces, and the resulting consequences on framework Al distribution. These insights provide strategies to crystallize CHA zeolites of fixed elemental composition, but systematic variations in the arrangement of their framework Al centers.

References

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