We
combine experiment and theory to investigate the cooperation
or competition between organic and inorganic structure-directing agents
(SDAs) for occupancy within microporous voids of chabazite (CHA) zeolites
and to rationalize the effects of SDA siting on biasing the framework
Al arrangement (Al–O(−Si–O)
x
–Al, x = 1–3) among CHA zeolites
of essentially fixed composition (Si/Al = 15). CHA zeolites crystallized
using mixtures of TMAda+ and Na+ contain one
TMAda+ occluded per cage and Na+ co-occluded
in an amount linearly proportional to the number of 6-MR paired Al
sites, quantified by Co2+ titration. In contrast, CHA zeolites
crystallized using mixtures of TMAda+ and K+ provide evidence that three K+ cations, on average, displace
one TMAda+ from occupying a cage and contain predominantly
6-MR isolated Al sites. Moreover, CHA crystallizes from synthesis
media containing more than 10-fold higher inorganic-to-organic ratios
with K+ than with Na+ before competing crystalline
phases form, providing a route to decrease the amount of organic SDA
needed to crystallize high-silica CHA. Density functional theory calculations
show that differences in the ionic radii of Na+ and K+ determine their preferences for siting in different CHA rings,
which influences their energy to co-occlude with TMAda+ and stabilize different Al configurations. Monte Carlo models confirm
that energy differences resulting from Na+ or K+ co-occlusion promote the formation of 6-MR and 8-MR paired Al arrangements,
respectively. These results highlight opportunities to exploit using
mixtures of organic and inorganic SDAs during zeolite crystallization
in order to more efficiently use organic SDAs and influence framework
Al arrangements.
NO
x
selective catalytic reduction (SCR)
with NH3 on Cu-zeolites is a commercial emissions control
technology for diesel and lean-burn engines. Mitigating low-temperature
emissions remains an outstanding challenge, motivating an improved
understanding of the reaction mechanism, active site requirements,
and rate-determining processes at low temperatures (<523 K). In
this Perspective, we discuss how operando spectroscopy
provides crucial information about how the structures, coordination
environments, and oxidation states of Cu active sites depend on reaction
conditions and sample composition; when combined with kinetic measurements,
such operando data provide insights into the Cu site
and spatial density requirements for reduction and oxidation steps
relevant to the Cu(II)/Cu(I) SCR redox cycle. Isolated Cu ions coordinated
to zeolite oxygen atoms ex situ become coordinated
to NH3
in situ and dynamically interconvert
between mononuclear and binuclear NH3-solvated Cu complexes
to catalyze SCR turnovers. We conclude with future research directions
that can benefit from combining quantitative kinetic measurements
with operando spectroscopy.
Catalysis science is founded on understanding the structure, number, and reactivity of active sites. Kinetic models that consider active sites to be static and noninteracting entities are routinely successful in describing the behavior of heterogeneous catalysts. Yet, active site ensembles often restructure in response to their external environment and even during steady-state catalytic turnover, sometimes requiring non-mean-field kinetic treatments to describe distance-dependent interactions among sites. Such behavior is being recognized more frequently in modern catalysis research, with the advent of experimental methods to quantify turnover rates with increasing precision, an expanding arsenal of operando characterization tools, and computational descriptions of atomic structure and motion at chemical potentials and timescales increasingly relevant to reaction conditions. This review focuses on dynamic changes to metal active site ensembles on zeolite supports, which are silica-based crystalline materials substituted with Al that generate binding sites for isolated and low-nuclearity metal site ensembles. Metal sites can become solvated and mobilized during reaction, facilitating interactions among sites that change their nuclearity and function. Such intersite communication can be regulated by the zeolite support, resulting in non-single-site and potentially non-mean-field kinetic behavior arising from mechanisms of catalytic action that combine elements of those canonically associated with homogeneous and heterogeneous catalysis. We discuss recent literature examples that document dynamic active site behavior in metal-zeolites and outline methodologies to identify and interpret such behavior. We conclude with our outlook on future research directions to develop this evolving branch of catalysis science and harness it for practical applications. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 12 is June 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Spectroscopic, titrimetric, and gas-phase product analysis methods reveal a six-electron process for NH3-assisted reduction of mononuclear Cu(ii) sites to Cu(i) in Cu-CHA zeolites of different Cu(ii) site speciation and density.
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