Synthetic Control of Framework Aluminum Distribution in Zeolites and Consequences for Acid and Redox Catalysis

Synthetic Control of Framework Aluminum Distribution in Zeolites and Consequences for Acid and Redox Catalysis

John R. Di Iorio¹, Young Gul Hur², Rajamani Gounder^1*

¹School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907

² Department of Chemical and Biological Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul 136-701, South Korea

*rgounder@purdue.edu

Zeolites contain different arrangements of framework Al atoms (Al−O(−Si−O)_x−Al) between paired (x = 1, 2) and isolated (x ≥ 3) configurations, defined functionally by their ability to exchange cationic species of different valence (e.g., Mⁿ⁺ = Cu²⁺, [CuOH]⁺) and with different stoichiometry (Mⁿ⁺/Al) quantifiable using methods to selectively titrate residual protons with NH₃ [1]. CHA zeolites synthesized in hydroxide media (Si/Al = 15-30) using only N,N,N-trimethyl-1-adamantylammonium cations (TMAda⁺) as the organic structure directing agent (SDA) contain only isolated Al atoms [2], which exchange monovalent (e.g. [CuOH]⁺, Na⁺, NH₄⁺) but not divalent (e.g., Co²⁺, Cu²⁺) cations. Addition of inorganic Na⁺ cations to zeolite synthesis solutions as a cooperative SDA, with other synthesis parameters held constant (e.g., Si/Al, pH, total cationic charge: [Na⁺+TMAda⁺]/Al), crystallized SSZ-13 zeolites of fixed Si/Al ratio but with numbers of paired Al sites that increased linearly with the Na⁺ incorporated in the crystalline product. Equimolar amounts of Na⁺ and TMAda⁺ in the synthesis solution, but with varying Si/Al ratios (5-25), crystallized SSZ-13 zeolites with a paired Al density consistent with a random Al distribution subject to Löwenstein’s rule [3]. Manipulating the ratio of high charge density inorganic (Na⁺) and low charge density organic (TMAda⁺) cations in crystallization media, which determines the solution cationic charge density, systematically influences the distribution of framework Al atoms between paired and isolated sites, which determines the anionic charge density of the zeolite framework. We have used mixtures of organic and inorganic SDAs in zeolite crystallization media and charge density mismatch concepts to systematically control the distribution of framework Al atoms in CHA and other zeolite frameworks. The catalytic consequences of paired and isolated Al arrangements in zeolites, which influence Brønsted acid strength and extraframework cation speciation, are investigated using the acid-catalyzed dehydration of methanol to dimethyl ether [4] and Cu-catalyzed NOx selective catalytic reduction with NH₃ [5]. These synthetic techniques provide routes to systematically control the atomic arrangement of Al heteroatoms and introduce structural and catalytic diversity into zeolites of a given topology at fixed elemental composition.

[1] Di Iorio, J. R., Bates, S. A., Verma, A. A., Delgass, W. N., Ribeiro, F. H., Miller, J. T., Gounder, R., Topics in Catalysis, 2015, 58, 424-434

[2] Di Iorio, J. R., Gounder, R., Chemistry of Materials, 2016, 28, 2236−2247

[3] Bates, S. A., Verma, A. A., Paolucci, C., Parekh, A. A., Anggara, T., Yezerets, A., Schneider, W. F., Miller, J.T., Delgass, W. N., Ribeiro, F. H., Journal of Catalysis, 2014, 312, 87-97

[4] Jones, A. J., Carr, R. T., Zones, S. I., Iglesia, E., Journal of Catalysis, 2014, 312, 58-68

[5] Paolucci, C., Parekh, A. A., Khurana, I., Di Iorio, J. R., Li, H., Albarracin-Caballero, J. D., Shih, A., Anggara, T., Delgass, W. N., Miller, J.T., Ribeiro, F. H., Gounder, R., Schneider, W. F., Journal of the American Chemical Society, 2016, DOI: 10.1021/jacs.6b02651