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Download This Article PDF Format Chemical Science View Article Online REVIEW View Journal | View Issue Kinetic effects of molecular clustering and solvation by extended networks in zeolite acid Cite this: Chem. Sci.,2021,12, 4699 catalysis Jason S. Bates and Rajamani Gounder * Reactions catalyzed within porous inorganic and organic materials and at electrochemical interfaces commonly occur at high coverage and in condensed media, causing turnover rates to depend strongly on interfacial structure and composition, collectively referred to as “solvent effects”. Transition state theory treatments define how solvation phenomena enter kinetic rate expressions, and identify two distinct types of solvent effects that originate from molecular clustering and from the solvation of such Received 11th January 2021 clusters by extended solvent networks. We review examples from the recent literature that investigate Accepted 17th February 2021 reactions within microporous zeolite catalysts to illustrate these concepts, and provide a critical appraisal DOI: 10.1039/d1sc00151e of open questions in the field where future research can aid in developing new chemistry and catalyst Creative Commons Attribution-NonCommercial 3.0 Unported Licence. rsc.li/chemical-science design principles. 1. Introduction details of solvent effects in catalysis,13 because their reaction environments can be systematically varied via synthetic and Catalytic reactions that occur at high pore occupancy within post-synthetic changes to pore topology, size, and chemical zeolites and porous materials are of both practical and funda- functionality. This structural diversity can be leveraged to mental interest. Practical applications include biomass elucidate new reactivity paradigms such as mechanisms 14–16 conversions,1–4 selective oxidations of hydrocarbons and involving solvated and mobilized cation active sites or 5–7 8–12 This article is licensed under a oxygenates, and alkene oligomerization. Zeolites provide solvent-mediated charge interactions that inuence free energy 17,18 a well-dened materials platform to probe the mechanistic landscapes. These ndings regarding the effects of solvation on the kinetics and mechanisms of catalytic reactions in zeolites have broader implications both for heterogeneous Open Access Article. Published on 24 February 2021. Downloaded 9/30/2021 11:05:41 PM. Charles D. Davidson School of Chemical Engineering, Purdue University, 480 Stadium catalysts in condensed phases (e.g., Fischer–Tropsch Mall Drive, West Lafayette, IN 47907, USA. E-mail: [email protected] Jason S. Bates received his B.S. Rajamani Gounder is the Larry in Chemical Engineering from and Virginia Faith Associate the University of Kansas in 2014 Professor in the Davidson School and his PhD in Chemical Engi- of Chemical Engineering at Pur- neering from Purdue University due University. He received his in 2019 under the supervision of B.S. in Chemical Engineering Rajamani Gounder. He is from the University of Wisconsin currently an NIH postdoctoral in 2006 and his PhD in Chem- fellow in the department of ical Engineering from the Chemistry at the University of University of California, Berke- Wisconsin under the supervision ley in 2011; he completed of Shannon S. Stahl. His a postdoctoral appointment at research explores the funda- the California Institute of Tech- mentals of heterogeneous catalysis in the areas of renewable nology in 2013. His research interests include studying the feedstock conversion, energy, and organic synthesis. fundamentals and applications of catalysis for energy and the environment, focusing on converting conventional and emerging carbon feedstocks to fuels and chemicals, and automotive pollu- tion abatement. © 2021 The Author(s). Published by the Royal Society of Chemistry Chem. Sci.,2021,12, 4699–4708 | 4699 View Article Online Chemical Science Review synthesis,19–22 organic synthesis23), and for adjacent sub-elds of uid-phase species and adsorbed intermediates. Following the catalysis where solvation at interfaces or within conning treatment of Foley and Bhan38 (cf. eqn (2.1.12)): X X environments is critical, including catalysis at electrochemical G À G ¼ X G À X G 24–28 29–31 – TS;app IS;app RC;i TS;i TRC;j j (2) interfaces and within enzymes and metal organic i¼TS j¼species frameworks (MOFs).32,33 Solvation alters the free energy land- scapes of reactions that occur in solution, including situations where solvent rearrangement occurs along reaction coordi- Thus, the apparent free energy of activation for a multistep nates,34 and where non-equilibrium solvation dynamics moti- reaction network reects a weighted average of the standard- vate revisions of “classical” organic reaction mechanisms.35 state Gibbs free energies of the individual species and transi- The prevalence of solvent effects in heterogeneous catalysis tion states involved in elementary steps. The terms in eqn (1) in condensed media has motivated developing quantitative and (2) readily illustrate, without requiring further simplica- kinetic, spectroscopic, and theoretical assessments of solvent tion, how molecular clustering and solvation inuences rates. structures and their interactions with reaction intermediates The GTS;app term re ects the mechanistic pathway by which the and transition states. In our research, we have found it useful to reaction proceeds, and thus captures both the chemical identity conceptually delineate “solvent effects” into two parts: (i) and the thermodynamic stability of the kinetically relevant s molecular clustering, which describes the local structures and transition states within the summation (XRC,i 0), which depends on their size, molecularity, and solvation by the molecularity of reaction intermediates and transition states bonded closely to the active center, and (ii) the solvation of such surrounding environment. The GIS;app term similarly depends clusters by extended solvent networks, which considers the on the structures of the most abundant reactive intermediates s thermodynamic consequences of the arrangements of solvent (MARI; XTRC,i 0), also referred to as the catalyst resting state, molecules and charged moieties in response to reactant which are oen solvent-reactant clusters stabilized at active adsorption, transition state formation, and connement within sites within micropores. The thermodynamic activities (aj)of ffi chemical species contain activity coe cients (gj) that re ect Creative Commons Attribution-NonCommercial 3.0 Unported Licence. porous environments. This conceptual division is manifested mathematically in transition state theory treatments of kinetic solvation when their Gibbs free energies depart from those at rate expressions that provide a quantitative basis to measure standard state in the limit of low surrounding pore occupancy. solvent effects, as we discuss in Section 2. Examples from the Kinetic, spectroscopic, and theoretical interrogation of the recent literature that illustrate these two distinct effects are apparent IS and TS under reaction conditions is required to concisely reviewed in Sections 3 and 4, and an outlook is derive mechanism-based rate expressions as a more specic provided in Section 5 to identify opportunities for future embodiment of eqn (1) that enable quantifying the kinetic research into the fundamentals of solvation in zeolite catalysis. effects of solvation. Knowledge of the apparent IS and TS under a given set of This article is licensed under a conditions enables writing the overall reaction as an apparent rate-determining step of the form: IS / TS, where the IS 2. Kinetic implications of solvation collectively describes the MARI species and the additional uid- phase species required to traverse from the IS to the TS. Fluid- Open Access Article. Published on 24 February 2021. Downloaded 9/30/2021 11:05:41 PM. Quantitatively precise descriptions of solvation require reliable phase species may have both positive or negative stoichiometric measurements of the catalytic turnover rate, because transition coefficients (by convention, species that are consumed en route state theory formalisms enable rigorously relating turnover to the apparent TS have negative stoichiometric coefficients) if rates to molecular-level properties of bound adsorbates, tran- MARI species of the IS contain spectating components and thus sition states, and solvent molecules. Thus, turnover rates must require desorption of molecules to form the TS. Quasi- be measured in kinetic regimes uncorrupted by transport equilibrium between the IS and TS can be expressed as an phenomena, and normalized by the number of catalytically equilibrium constant: 36 Y YÀ Á relevant active sites (L). Upon satisfying these conditions, ‡ nj nj K ¼ aj ¼ C‡g‡ Cjgj (3) measured turnover rates, which are proportional to the j js‡ concentration of transition states,37 can be expressed in terms ffi of the free energies of the apparent initial state (IS) and tran- where nj are the stoichiometric coe cients of the species j that / sition state (TS), according to recently derived transition state comprise the apparent rate-determining step (IS TS), Cj are 38 theory-based treatments of multistep reaction networks (cf. their concentrations, and gj are their thermodynamic activity ffi eqn (2.2.13) in that work): coe cients. In the case of a single MARI species (assumptions: ¼ ¼ Â ¼ Y nMARI 1; CMARI qMARI L L), the turnover rate can be kBT kBT ÀðG ÀG Þ=RT ÀXTRC;j rTST ¼ C‡ ¼ e TS;app IS;app aj expressed by solving eqn (3) for C‡, and substituting this h h (1) j expression into eqn (1): k
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