Synthetic Methods to Influence Framework Al Arrangements in CHA Zeolites and Consequences for NOx Selective Catalytic Reduction

Casey  Jones, Purdue University

John R. Di Iorio¹, Casey Jones¹, Sichi Li², Edward Kunkes³, Subramanian Prasad³, Ahmad Moini³, William F. Schneider², Rajamani Gounder¹*

¹ Charles D. Davidson School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907, USA

² Department of Chemical and Biomolecular Engineering, University of Notre Dame, 182 Fitzpatrick Hall, Notre Dame, IN 46556, USA

³ BASF Corporation, Iselin, NJ 08830, USA

The use of organic and inorganic cations as structure-directing agents (SDAs) in zeolite crystallization provides a mechanism for influencing the arrangement of framework Al atoms (Al-O(-Si-O)x-Al) in isolated (x ≥ 3) or paired (x = 1,2) configurations. We explore this using chabazite (CHA), a high symmetry framework containing a single crystallographically-unique T-site, which facilitates using computational approaches together with experimental ones to investigate how the molecular and electronic structures of SDA molecules affect the Al distribution. Protocols for titration of paired Al sites in CHA have been developed using divalent Cu²⁺ and Co²⁺ cations and have been validated using UV-visible spectroscopy and by titration of residual Bronsted acid sites after cation exchange using ammonia. Computational energies of divalent cation siting at different Al configurations in CHA are consistent with experimental titrations. CHA zeolites (Si/Al = 15) synthesized using only the low charge-density organic N,N,N-trimethyl-1adamantyl (TMAda⁺) cations as the SDA contain primarily isolated Al, as only one adamantyl group (~0.7 nm diam.) can occupy each CHA cage (~0.7 nm diam.). Partial replacement of TMAda⁺ with Na⁺ during synthesis, keeping all other variables constant, results in the formation of Al pairs that correlate with the amount of Na⁺ incorporated within crystalline products. Density functional theory calculations are used to compute the relationship between cation size and charge and interactions with Al-Al pair configurations to explain titration and Al siting effects. We also discuss the effects of framework Al distribution on extraframework Cu ion speciation and density, and their kinetic behavior for the selective catalytic reduction (SCR) of NOx with ammonia.