Rajamani Gounder

Catalytic Consequences of Spatial Constraints and Acid Site Location for Monomolecular Alkane Activation on Zeolites

The location of Brønsted acid sites within zeolite channels strongly influences reactivity because of the extent to which spatial constraints determine the stability of reactants and of cationic transition states relevant to alkane activation catalysis. Turnover rates for monomolecular cracking and dehydrogenation of propane and n-butane differed among zeolites with varying channel structure (H-MFI, H-FER, H-MOR) and between OH groups within eight-membered ring (8-MR) side pockets and 12-MR main channels in H-MOR. Measured monomolecular alkane activation barriers depended on catalyst and reactant properties, such as deprotonation enthalpies and proton affinities, respectively, consistent with Born-Haber thermochemical cycles that define energy relations in acid catalysis. Monomolecular alkane cracking and dehydrogenation turnovers occurred with strong preference on acid sites contained within smaller 8-MR pockets in H-MOR, while rates on sites located within 12-MR channels were much lower and often undetectable. This strong specificity reflects transition states that are confined only partially within 8-MR pockets; as a result, entropic gains compensate for enthalpic penalties caused by their incomplete containment to give a lower free energy for transition states within small 8-MR side pockets. These effects of entropy are stronger for dehydrogenation, with a later and looser transition state, than for cracking in the case of both propane and n-butane; therefore, selectivity can be tuned by the selective positioning or titration of OH groups within specific environments, the number of which was assessed in H-MOR by rigorous deconvolution of their infrared spectra. Specifically, cracking-to-dehydrogenation ratios for propane and n-butane were much smaller and terminal-to-central C-C bond cleavage ratios for n-butane were much larger on 8-MR than on 12-MR acid sites as a result of partial confinement, a concept previously considered phenomenologically as pore mouth catalysis. These marked effects of spatial constraints and of entropic factors on acid site reactivity and selectivity, also inferred for MFI from titration of OH groups by Na(+), have not been previously proposed or recognized and appear to be unprecedented in hydrocarbon catalysis. These findings and their conceptual interpretations open opportunities for the design of microporous solids by the rational positioning of acid sites within specific channel locations and with predictable consequences for catalytic rates and selectivities.

Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites

The relationships among the macroscopic compositional parameters of a Cu-exchanged SSZ-13 zeolite catalyst, the types and numbers of Cu active sites, and activity for the selective catalytic reduction (SCR) of NOx with NH3 are established through experimental interrogation and computational analysis of materials across the catalyst composition space. Density functional theory, stochastic models, and experimental characterizations demonstrate that within the synthesis protocols applied here and across Si:Al ratios, the volumetric density of six-membered-rings (6MR) containing two Al (2Al sites) is consistent with a random Al siting in the SSZ-13 lattice subject to Löwensteins rule. Further, exchanged Cu(II) ions first populate these 2Al sites before populating remaining unpaired, or 1Al, sites as Cu(II)OH. These sites are distinguished and enumerated ex situ through vibrational and X-ray absorption spectroscopies (XAS) and chemical titrations. In situ and operando XAS follow Cu oxidation state and coordination environment as a function of environmental conditions including low-temperature (473 K) SCR catalysis and are rationalized through first-principles thermodynamics and ab initio molecular dynamics. Experiment and theory together reveal that the Cu sites respond sensitively to exposure conditions, and in particular that Cu species are solvated and mobilized by NH3 under SCR conditions. While Cu sites are spectroscopically and chemically distinct away from these conditions, they exhibit similar turnover rates, apparent activation energies and apparent reaction orders at the SCR conditions, even on zeolite frameworks other than SSZ13.

Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu‐SSZ‐13

Operando X-ray absorption experiments and density functional theory (DFT) calculations are reported that elucidate the role of copper redox chemistry in the selective catalytic reduction (SCR) of NO over Cu-exchanged SSZ-13. Catalysts prepared to contain only isolated, exchanged Cu(II) ions evidence both Cu(II) and Cu(I) ions under standard SCR conditions at 473 K. Reactant cutoff experiments show that NO and NH3 together are necessary for Cu(II) reduction to Cu(I). DFT calculations show that NO-assisted NH3 dissociation is both energetically favorable and accounts for the observed Cu(II) reduction. The calculations predict in situ generation of Brønsted sites proximal to Cu(I) upon reduction, which we quantify in separate titration experiments. Both NO and O2 are necessary for oxidation of Cu(I) to Cu(II), which DFT suggests to occur by a NO2 intermediate. Reaction of Cu-bound NO2 with proximal NH4(+) completes the catalytic cycle. N2 is produced in both reduction and oxidation half-cycles.

Dynamic multinuclear sites formed by mobilized copper ions in NOx selective catalytic reduction

X-ray vision spies copper on the move Copper ions in zeolites help remove noxious nitrogen oxides from diesel exhaust by catalyzing their reaction with ammonia and oxygen. Paolucci et al. found that these copper ions may move about during the reaction (see the Perspective by Janssens and Vennestrom). Zeolite catalysts generally fix metals in place while the reacting partners flow in and out of their cagelike structures. In this case, though, x-ray absorption spectroscopy suggested that the ammonia was mobilizing the copper ions to pair up as they activated oxygen during the catalytic cycle. Science, this issue p. 898; see also p. 866 Copper ions can move about and pair up in a zeolite framework as they catalyze nitric oxide removal from diesel exhaust. Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia (NH3), but the low-temperature rate dependence on copper (Cu) volumetric density is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calculations to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an oxygen (O2)–mediated CuI→CuII redox step integral to SCR. Electrostatic tethering to framework aluminum centers limits the volume that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst.

Effects of Partial Confinement on the Specificity of Monomolecular Alkane Reactions for Acid Sites in Side Pockets of Mordenite

The location of Brønsted acid sites within zeolites influences catalytic rates and selectivities when their diverse intrachannel environments stabilize transition states to different extents. Mordenite zeolites in the proton form (H-MOR) contain acid sites located within two general environments: eight-membered ring (8-MR) side pockets and 12-MR main channels. The location of these acid sites can be determined by rigorous deconvolution of OH infrared bands and by titration with molecules of varying size, allowing catalytic turnover rates to be described in terms of the respective contributions from sites within these two locations. We have shown previously that monomolecular cracking and dehydrogenation of propane and n-butane occur preferentially within constrained 8-MR pockets, where transition states and adsorbed reactants are only partially confined. Such configurations lead to entropy gains that compensate for the weaker binding of partially confined structures to give lower free energies for transition states within 8-MR pockets. For n-alkanes, monomolecular dehydrogenation reactions show greater specificity for 8-MR locations than cracking and also show higher activation barriers, predominantly because (C-H-H) species involved in transition states for dehydrogenation reactions are less stable than the (C-CH) carbonium ions in cracking transition states (proponium; n-butonium). Activation entropies were also higher for n-alkane dehydrogenation than for cracking, consistent with crossing potential curve descriptions of charge transfer reaction coordinates using, which indicate that transition states with higher energies are looser and occur later along the reaction coordinates. Thus, it seems plausible that reactions involving later and looser transition states, with more fully formed ion pairs, benefit preferentially from entropy gains caused by partial confinement within 8-MR side pockets. The electrostatic underpinnings of these entropy benefits resemble those for proton-transfer and electrontransfer reactions in solvated systems, for which the entropies for molecular and charge reorganization are essential in stabilizing the ion pairs formed upon charge transfer. Here, we probe and extend these concepts of 8-MR pocket specificity in ion-pair stabilization to monomolecular reactions of branched alkanes. We show that isobutane cracking has a stronger preference for reaction within 8-MR locations in MOR than does dehydrogenation, in sharp contrast with the trends for n-alkane reactions. Transition state energies are higher for isobutane cracking than for dehydrogenation, consistent with the less stable cations formed upon protonation of C C bonds instead of the tertiary C H bond in isobutane. We propose that, as for monomolecular n-alkane dehydrogenation, isobutane cracking shows a stronger preference for reaction on 8-MR acid sites than does dehydrogenation because it involves later and looser transition states, which benefit more strongly from entropy gains arising from partial confinement. The fraction of the Brønsted acid sites located within 8MR pockets varies widely (10–80%) among H-MOR samples prepared by partial exchange of H with Na and also among H-MOR samples of different provenance. Rate constants for monomolecular isobutane cracking (per total H; 748 K; Figure 1 a) and dehydrogenation (Figure 1b) increased monotonically as the fraction of the protons located within 8-MR pockets increased. As for n-alkanes, these data show that both reactions occur preferentially on sites located within 8-MR pockets. Isobutane cracking-to-dehydrogenation rate ratios increased with increasing 8-MR H fraction, in contrast with those measured for propane and n-butane (Figure 2); thus, cracking shows a stronger kinetic preference for 8-MR sites than dehydrogenation for isobutane reactants (700–780 K; Section S.1, Supporting Information). The rate constants for isobutane dehydrogenation and cracking on 8MR and 12-MR acid sites were extracted from their respective dependences on the number of sites at each location for each temperature (Section S.2, Supporting Information). At 748 K, dehydrogenation rate constants were approximately 7 times larger on 8-MR than on 12-MR sites, while cracking rate constants were not detectable on 12-MR sites (Table 1). Monomolecular alkane activation involves carbonium ion-like transition states 12] formed by interactions of adsorbed reactants (Az) with Brønsted acid sites (H ); adsorbed reactants are in quasi-equilibrium with those in the extracrystalline gas phase (Ag; Scheme 1). Reaction rates [Eq. (1)] are first-order in alkane pressure (PA), where kint and

Hydrophobic microporous and mesoporous oxides as Brønsted and Lewis acid catalysts for biomass conversion in liquid water

The use of heterogeneous catalysts in liquid water, even at the moderate temperatures (<523 K) typical of most condensed-phase biomass conversion processes, is often fraught with issues related to structural instability and to active site inhibition caused by deactivation mechanisms that differ from those prevalent in the gas phase at higher temperatures. For porous silica-based oxides, one strategy to address these issues is to design or functionalize oxide surfaces with hydrophobic moieties or domains. Hydrophobic moieties can be present either at external crystallite surfaces or within the internal porous voids where most active sites typically reside. Both extracrystalline and intracrystalline hydrophobic environments can prevent the condensation of bulk water within internal void spaces and thus alleviate any transport restrictions its presence may cause, while only intracrystalline environments can influence the kinetic effects of molecular water at active sites. As a result, hydrophobic environments at both external and internal crystallite surfaces can have fundamentally different consequences for reactivity, in spite of the phenomenological similarities of their effects on observed reaction rates. The conceptual distinction between these two forms of hydrophobicity, together with accurate assessments of transport and kinetic contributions to measured reaction rates, can inform the placement of hydrophobic domains at appropriate locations in porous solids to cause predictable changes in reactivity. This mini-review discusses these concepts within the context of recent studies that have used hydrophobic Bronsted and Lewis acidic microporous and mesoporous oxides in catalytic reactions of biomass-derived molecules in liquid water and biphasic water–organic mixtures.

Solid State NMR Characterization of Sn-Beta Zeolites that Catalyze Glucose Isomerization and Epimerization

High resolution, multi-nuclear solid state nuclear magnetic resonance (NMR) characterizations are carried out in order to obtain insights into the structural features of Sn-beta zeolites that catalyze glucose isomerization or epimerization reactions in water and methanol solvents. In particular, we focus on investigating the local structural changes to catalytically-active framework Sn sites of different 119Sn-labeled beta zeolites, including the calcined, dehydrated, rehydrated, and post-sugar isomerization catalysis forms. Magic angle spinning (MAS) and cross polarization MAS (either from 1H or 19F) 119Sn NMR spectra provide evidence for changes to the local framework Sn coordination in the presence of water and sugar molecules, and provide insights into structural features of adsorbed intermediates that may be relevant in sugar isomerization reaction pathways.

Structural and kinetic changes to small-pore Cu-zeolites after hydrothermal aging treatments and selective catalytic reduction of NOx with ammonia

Three small-pore, eight-membered ring (8-MR) zeolites of different cage-based topology (CHA, AEI, RTH), in their proton- and copper-exchanged forms, were first exposed to high temperature hydrothermal aging treatments (1073 K, 16 h, 10% (v/v) H2O) and then to reaction conditions for low temperature (473 K) standard selective catalytic reduction (SCR) of NOx with ammonia, in order to study the effect of zeolite topology on the structural and kinetic changes that occur to Cu-zeolites used in NOx abatement. UV-visible spectra were collected to monitor changes to Cu structure and showed that band intensities for isolated, hydrated Cu2+ cations (∼12 500 cm−1) remain constant after hydrothermal aging, but decrease in intensity upon subsequent exposure to low temperature SCR reaction conditions. Standard SCR rates (per Cu, 473 K), activation energies, and reaction orders are similar between Cu-AEI and Cu-CHA zeolites before and after hydrothermal aging, although rates are lower after hydrothermal aging as expected from the decreases in intensity of UV-visible bands for Cu2+ active sites. For Cu-RTH, rates are lower (by 2–3×) and apparent activation energies are lower (by ∼2×) than for Cu-AEI or Cu-CHA. These findings suggest that the RTH framework imposes internal transport restrictions, effectively functioning as a one-dimensional framework during SCR catalysis. Hydrothermal aging of Cu-RTH results in complete deactivation and undetectable SCR rates, despite X-ray diffraction patterns and Ar micropore volumes (87 K) that remain unchanged after hydrothermal aging treatments and subsequent SCR exposure. These findings highlight some of the differences in low temperature SCR behavior among small-pore Cu-zeolites of different topology, and the beneficial properties conferred by double six-membered ring (D6R) composite building units. They demonstrate that deleterious structural changes to Cu sites occur after exposure to hydrothermal aging conditions and SCR reactants at low temperatures, likely reflecting the formation of inactive copper-aluminate domains. Therefore, the viability of Cu-zeolites for practical low temperature NOx SCR catalysis cannot be inferred solely from assessments of framework structural integrity after hydrothermal aging treatments, but also require Cu active site and kinetic characterization after hydrothermally aged zeolites are exposed to low temperature SCR reaction conditions.

A transmission infrared cell design for temperature-controlled adsorption and reactivity studies on heterogeneous catalysts.

A design is presented for a versatile transmission infrared cell that can interface with an external vacuum manifold to undergo in situ gas treatments and receive controlled doses of various adsorbates and probe molecules, allowing characterization of heterogeneous catalyst surfaces in order to identify and quantify active sites and adsorbed surface species. Critical design characteristics include customized temperature control for operation between cryogenic and elevated temperatures (100-1000 K) and modified Cajon fittings for operation over a wide pressure range (10-2-103 Torr) that eliminates the complications introduced when using sealants or flanges to secure cell windows. The customized, hand-tightened Cajon fittings simplify operation of the cell compared to previously reported designs, because they allow for rapid cell assembly and disassembly and, in turn, replacement of catalyst samples. In order to validate the performance of the cell, transmission infrared spectroscopic experiments are reported to characterize the Brønsted and Lewis acid sites present in H-beta and H-mordenite zeolites using cryogenic adsorption of CO (<150 K).

Catalytic Alkylation Routes via Carbonium‐Ion‐Like Transition States on Acidic Zeolites

Brønsted acid sites in zeolites catalyze alkene hydrogenation with H2 via the same kinetically-relevant (C-H-H) + carboniumion-like transition states as those involved in monomolecular alkane dehydrogenation. Reactions between C3H6 and H2 selectively form C3H8 (>80 % carbon basis) at high H2/C3H6 ratios (>2500) and temperatures (>700 K). Ratios of C3H8 dehydrogenation to C3H6 hydrogenation rate constants (718–778 K) were identical on H-FER, H-MFI, and H-MOR zeolites and equal to the equilibrium constant for the stoichiometric gas-phase reaction, consistent with De Donder non-equilibrium thermodynamic treatments of chemical reaction rates. 3] The seemingly fortuitous extensions of the principle of microscopic reversibility and the De Donder relations beyond their rigorous descriptions of chemical reaction dynamics at equilibrium and far from equilibrium but at identical (T, Pj), respectively, reflect the persistence of the same single kinetically-relevant step and the prevalence of unoccupied H sites at the different conditions used to measure forward and reverse rates. By inference, larger alkanes should also form in direct alkene-alkane addition steps via the same (C-C-H) carboniumion-like transition states involved in monomolecular alkane cracking. These chemical processes differ from alkylation mechanisms prevalent on liquid and solid acids (e.g. , HF, H2SO4, Hzeolites) and superacids (e.g. , HF-SbF5, HF-TaF5), which are mediated by carbenium-ion chain carriers that terminate as alkanes via hydride transfer. Carbonium-ions contain threeatom/two-electron centers 7] and have been posited to mediate the formation of C3H8 in reactions of CH4-C2H4 mixtures on superacids at the low temperatures (<573 K) required for favorable alkylation thermodynamics. Here, we provide definitive kinetic and isotopic evidence that catalytic CH4-C2H4 alkylation reactions are mediated by the same transition states involved in monomolecular alkane cracking, even on zeolitic Brønsted acid sites at high temperatures (>700 K). Monomolecular alkane cracking routes prevail at high temperatures and low concentrations of alkene products ; they involve late (C-C-H) carbonium-ion-like transition states in kinetically-relevant C C bond cleavage steps and unoccupied H sites as most abundant surface intermediates (MASI). 7, 12–16] Minority species adsorbed on H sites are in quasi-equilibrium with gas phase reactants and products, leading to monomolecular C3H8 cracking rates given by Equation (1):