(627e) 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