SYNTHETIC METHODS TO CONTROL ALUMINUM PROXIMITY IN CHABAZITE ZEOLITES AND CONSEQUENCES FOR ACID AND REDOX CATALYSIS

Description

<p>Zeolites
contain distinct Brønsted acid site (H⁺) ensembles that arise from
differences in the arrangement of framework Al atoms (Al−O(−Si−O)x−Al) between
isolated (x ≥3) and paired (x=1,2) configurations, the latter defined by their
ability to exchange certain divalent cations (e.g., Cu²⁺, Co²⁺).
Manipulation of the synthesis conditions used to prepare MFI zeolites has been
proposed to influence the proximity of framework Al atoms, but in a manner that
is neither determined randomly nor by any simple predictive rules. Moreover, the
effects of proton proximity have been studied for hydrocarbon catalysis in MFI
zeolites, but interpretations of catalytic phenomena are convoluted by effects
of the distribution of framework Al atoms among different crystallographic
tetrahedral sites (T-sites) and diverse pore environments (i.e., confining
environments) present in MFI. This work instead focuses on the chabazite (CHA)
framework, which contains a single crystallographically-distinct lattice
tetrahedral site (T-site) that allows clarifying how synthesis conditions
influence Al proximity, and in turn, how H⁺ site proximity
influences catalysis independent of T-site location. </p>

<p> Selective quantification of the
number and type of H⁺ site ensembles present in a given zeolite
allows for more rigorous normalization of reaction rates by the number of active
sites, but also for probing the number and identity of active sites on
bifunctional catalysts that contain mixtures of Brønsted and Lewis acid sites. Gaseous
NH₃ titrations can be used to count the total number of protons on small-pore
CHA zeolites, which are inaccessible to larger amine titrants (e.g., pyridine,
alkylamines), and can be used to quantify the exchange stoichiometry of extraframework
metal cations (e.g., Cu²⁺, [CuOH]⁺) that are stabilized at
different framework Al arrangements. Additionally, paired Al sites in CHA zeolites
can be titrated selectively by divalent Co²⁺ cations, whose sole
presence is validated by measuring UV-Visible spectra, counting residual
protons after Co²⁺ exchange, and titration of paired Al with other
divalent cations (e.g., Cu²⁺). These different titration procedures
enabled reliable and reproducible quantification of different Al arrangements,
and recognition of the effects of different synthetic methods on the resulting arrangement
of framework Al atoms in CHA zeolites. </p>

<p>Upon
the advent of this suite of characterization and titration tools, different
synthetic methods were developed to crystallize CHA zeolites at constant
composition (e.g., Si/Al = 15) but with systematic variation in their paired Al
content. The substitution of N,N,N-trimethyl-1-adamantylammonium (TMAda⁺)
cations for Na⁺ in the synthesis media (Na⁺/TMAda⁺<2),
while holding all other synthetic variables constant, resulted in CHA zeolites
of similar composition (Si/Al = 15) and organic content (ca. 1 TMAda⁺
per cage), but with percentages of paired Al (0-44%) that increased with the
total amount of sodium retained on the zeolite product. This result suggests
that sodium atoms are occluded near the ammonium group of TMAda⁺ leading
to the formation of a paired Al site. Replacement of Na⁺ by other
alkali cations in the synthesis media allowed for the crystallization of CHA (Si/Al
= 15) at higher ratios of alkali to TMAda⁺than accessible by Na⁺,
likely due to the ability of different alkali cations to favor (or inhibit)
crystallization of other zeolite phases. Incorporation of different alkali
cations during CHA crystallization influences the formation of paired Al sites
in different ways, likely reflecting the nature of different alkali to
preferentially occupy different positions within the CHA framework. <i>Ab initio</i> molecular dynamics simulations
were used to assess the stability of various Al-Al arrangements in the presence
of combinations of alkali and TMAda⁺ cations, and provide
thermodynamic insight into electrostatic interactions between cationic
structure-directing agents that stabilize paired Al sites in CHA. </p>

<p> Using these synthetic procedures to
prepare CHA zeolites of similar composition, but with varied arrangements of
framework Al, the catalytic consequences of framework Al arrangement were
investigated using acid and redox catalysis. The low-temperature (473 K) selective
catalytic reduction of NOx with NH₃ (NH₃-SCR) was
investigated over Cu-exchanged CHA zeolites containing various Al arrangements.
Cu cations exchange as both divalent Cu²⁺ and monovalent [CuOH]⁺
complexes, which exchange at paired and isolated Al sites, respectively, and
turnover with similar SCR rates (473 K). <i>In
situ</i> and <i>operando</i> X-ray
absorption spectroscopy (XAS) were used to monitor the oxidation state and
coordination environment of Cu as a function of time and environmental
conditions. Rationalization of these experimental observations by first-principles
thermodynamics and <i>ab initio</i>
molecular dynamics simulations revealed that both Cu²⁺ and [CuOH]⁺
complexes are solvated by NH₃ and undergo reduction to Cu⁺
upon oxidation of NO with NH₃. Cu⁺ cations become mobilized
by coordination with NH₃ under reaction conditions (473 K,
equimolar NO and NH₃ feed), and activate O₂ through a
dicopper complex formed dynamically during reaction. These results implicate
the spatial density of nominally site-isolated Cu cations and, in turn, the
arrangement of anionic framework Al atoms that anchor such cationic Cu
complexes, influence the kinetics of O₂ activation in selective
oxidation reactions, manifested as SCR rates (per 1000 A³) that
depend quadratically on Cu density (per 1000 A³) and become
rate-limiting processes in practice at low temperatures.</p>

<p>Furthermore,
first-order and zero-order rate constants (415 K, per H⁺) of
methanol dehydration, a probe reaction of acid strength and confinement effects
in solid Brønsted acids, are nearly one order of magnitude larger on paired
than on isolated protons in CHA zeolites, reflecting differences in prevalent
mechanisms and apparent enthalpic and entropic barriers at these different
active site ensembles. Yet, these differences in rate constants and activation
parameters at isolated and paired protons do not persist within larger pore
zeolites (e.g., MFI). <i>In situ </i>IR
spectra measured during steady-state methanol dehydration catalysis (415 K,
0.05-22 kPa CH₃OH) reveal that surface methoxy species are present
in CHA zeolites containing paired protons, but not in CHA zeolites containing
only isolated protons or MFI zeolites, providing evidence that sequential dehydration
pathways via methoxy intermediates become accessible on paired protons in CHA.
Density functional theory is used to provide atomistic detail of confined
intermediates and transition states at isolated and paired protons in CHA and
MFI zeolites, indicating that paired protons in CHA preferentially stabilize
dehydration transition states that are partially-confined within the 8-membered
ring (8-MR) of CHA. These findings provide evidence that catalytic diversity
for the same stoichiometric reaction among zeolites of fixed structure and
composition, even for frameworks containing a single T-site, can be introduced
deliberately through synthetic control of the atomic arrangement of matter. </p>

Links & Downloads

1. 10.25394/pgs.7406831.v1

Tags

Zeolites

Catalysis

Synthesis

Chabazite

Selective Catalytic Reduction

Methanol Dehydration

Additional Fields

Identifer	oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/7406831
Date	16 January 2020
Creators	John R. Di Iorio (5929640)
Source Sets	Purdue University
Detected Language	English
Type	Text, Thesis
Rights	CC BY 4.0
Relation	https://figshare.com/articles/SYNTHETIC_METHODS_TO_CONTROL_ALUMINUM_PROXIMITY_IN_CHABAZITE_ZEOLITES_AND_CONSEQUENCES_FOR_ACID_AND_REDOX_CATALYSIS/7406831