Controlling the Isolation and Pairing of Aluminum in Chabazite Zeolites Using Mixtures of Organic and Inorganic Structure-Directing Agents

Abstract

Synthetic methods are reported to prepare chabazite (CHA) zeolites (SSZ-13) at fixed elemental composition but with different arrangements of their framework Al atoms (Al–O(−Si–O) x –Al) between paired ( x = 1, 2) and isolated ( x ≥ 3) configurations. SSZ-13 zeolites (Si/Al = 15–30) that crystallized in hydroxide media and in the presence of only N, N, N -trimethyl-1-admantylammonium cations (TMAda + ) contained only isolated framework Al, evident in their inability to exchange divalent Cu 2+ or Co 2+ cations that titrate paired Al sites. Addition of Na + to the zeolite crystallization medium, with total cationic charge ((Na + + TMAda + )/Al) and other synthesis parameters held constant, crystallized SSZ-13 zeolites at a fixed Si/Al ratio but with paired Al densities that increased linearly with the Na + content incorporated into the crystalline solids. Crystallization media containing equimolar amounts of Na + and TMAda +, but varying Al content, crystallized SSZ-13 at different compositions (Si/Al = 5, 15, 25) with the paired Al density expected if Al atoms were distributed randomly subject to Löwenstein’s rule. The relative ratio of organic (TMAda + ) and inorganic (Na + ) cations in the crystallization medium, which determines its cationic charge density, systematically influences the arrangement of isolated and paired Al, which determines the anionic charge density in the zeolite framework. The synthetic strategy of using mixtures of organic and inorganic structure-directing agents extends concepts of charge density mismatch in zeolite crystallization, which have been exploited previously to discover routes to new framework topologies and compositions, to systematically control the atomic arrangement in a given zeolite at fixed composition and, in doing so, provide new methods to introduce structural and catalytic diversity.