Zeolites are widely used as catalysts for the processing of petroleum to produce transportation fuels, the synthesis of a wide variety of chemicals, and for the abatement of automotive emissions. These applications have stimulated an interest in describing the mechanism and kinetics for zeolite-catalyzed reactions using theoretical methods. This Mini-review summarizes the author's efforts towards this goal. It is shown that accurate predictions of adsorption and activation enthalpies and entropies requires that several criteria be met. The first is a correct description of the structure of the catalytically active center, as well as the portion of the zeolite framework immediately surrounding the active center and that located far from the active center. Second, the level of density functional theory (DFT) must be sufficiently high to account for the effects of dispersive interactions between the adsorbate, the active center, and the immediately surrounding zeolite atoms. Third, dispersive and coulombic interactions between the atoms in the vicinity of the active center and the balance of the zeolite framework must also be accounted for. It is shown that these conditions can be met using hybrid quantum mechanics/molecular mechanics (QM/MM) together with a high-level exchange-correlation functional and a large basis set. The success of our QM/MM approach is illustrated for reactions of light alkanes in H-MFI, as well as other protonated zeolites, and in Ga/H-MFI. We show that for low temperatures (<400 K), the QM/MM approach gives good predictions of molecular adsorption enthalpies and activation enthalpies for elementary reactions. This is also true for higher temperatures (>400 K) if the effects of configuration are considered using a correction obtained from configurationally biased Monte Carlo (CBMC) calculations. Calculations of the molecular adsorption entropy and the activation entropy for elementary reactions are more difficult to predict accurately. Application of the quasi-rigid rotor harmonic approximation overpredicts the loss of entropy of adsorption from the gas phase, particularly for zeolites containing large cavities and channels. CBMC corrections capture this deviation well for molecular adsorption and for early transition states resembling the adsorbed state but are inadequate for late transition states involving two loosely associated fragments.

Passive NO x adsorbers (PNA) using Pd/zeolites have emerged as a promising solution for the reduction of cold-start emissions from vehicle exhaust. However, the nature of the active sites and the mechanisms underlying NO x adsorption in Pd/zeolites remain a subject of ongoing investigation. In this study, we employ quantum chemical simulations to investigate the structure of Pd species in cation-exchange sites at isolated Al and Al pairs in the 6-ring and 8-ring of the CHA framework, before the introduction of NO x . Our calculations show that the speciation of Pd in these exchange sites strongly depends on the precise Al arrangement within the framework, as well as the operating conditions. Ionically dispersed Pd is found to be the most favorable species over a wide range of oxidizing and reducing conditions. Small oligomers of PdO and metallic Pd do not appear to be competitive at either isolated Al or Al pairs. Notably, our calculations show that ion exchange sites other than next-next-nearest neighbor Al pairs in the 6-ring will be preferentially occupied by Pd + instead of Pd 2+ . The stability of Pd + in the zeolite environment is an interesting contrast with its rareness in molecular Pd compounds. Nonetheless, a detailed analysis of the electronic structure shows that predicted Pd oxidation states are consistent with chemical intuition for all complexes investigated in this study. We also discuss the potential ambiguity in Pd characterization provided by typical experimental techniques such as XANES, EXAFS and UV-VIS, and highlight the need for additional EPR spectroscopy studies to further elucidate the initial Pd speciation in zeolites for PNA applications.

The oxidation state (OS) of metals and ligands in inorganic complexes may be defined by carefully curated rules, such as from IUPAC, or by computational procedures such as the effective oxidation state (EOS) or localized orbital bonding analysis (LOBA). Such definitions typically agree for systems with simple ionic bonding and innocent ligands but may disagree as the boundary between ionic and covalent bonds is approached, or as the role of ligand noninnocence becomes nontrivial, or high oxidation states of metals are supported by heavy dative bonding, and so on. This work systematically compares IUPAC, EOS, and LOBA across a series of complexes where OS assignment is challenging. These systems include high-valent transition metal oxides, transition metal complexes with noninnocent ligands such as dithiolate and nitrosyl, metal sulfur dioxide adducts, and two transition metal carbene complexes. The differences in OS assignment by the three methods are carefully discussed, demonstrating the synergy between EOS and LOBA. In addition, a clarity index for LOBA OS assignments is introduced that provides an indication of whether or not its predictions are close to the ionic-covalent boundary.

In this paper, we review the importance of long-range zeolite framework interactions in theoretical predictions for a variety of zeolite-catalyzed processes, and we show why such interactions must be determined accurately in order to reproduce experimentally measured adsorption and activation energies. We begin with an overview of the different strategies that have been used to account for long-range coulombic and dispersive interactions of zeolite framework atoms with species adsorbed at an active site. These methods include full periodic-DFT calculations and multi-layer hybrid techniques. Electrostatic interactions are observed to have a more significant impact than dispersive interactions on the geometries of ion-pair transition states and adsorbed species. Stabilization of the TS relative to reactant complexes is also dictated by electrostatic interactions. Dispersion effects are found to significantly stabilize both transition and reactant states for adsorbed species, especially those which have dimensions that provide good fits within the zeolite pore or cavity. We also show that the relevance of particular active site configurations can be missed, if the effects of long-range interactions are neglected. As a case in point, we demonstrate that a site previously considered inactive for ethane dehydrogenation, [GaH2]+ may in fact be more active than previously thought, when the impact of long-range interactions on the predicted activation energy is taken into account. Finally, the use of hybrid quantum mechanics/molecular mechanics approaches on extended, finite zeolite clusters has emerged as an accurate, highly cost-effective, and versatile alternative towards overcoming some of the present-day limitations of periodic calculations.

This is a response to the paper published by S. A. Kadam, H. Li, R. F. Wormsbacher, A. Travert, Chem. Eur. J. 2018, 24, 5489. Key consistencies between our reported results and those reported in this work are also highlighted.

This study investigates NO adsorption on Pd-exchanged chabazite (Pd/H-CHA), a promising passive NOx adsorber (PNA) for capturing cold-start NOx emissions of gasoline- and diesel-powered vehicles. Temperature-programed desorption (TPD) and IR spectroscopy are combined with theoretical calculations to elucidate how and where NO is stored, and how water and O2 affect this process. NO adsorption on Pd/H-CHA produces two TPD features, around 423 and 753 K, and IR bands centered at 1860 and 1810 cm−1. Calculated NO stretching frequencies and maximum-desorption temperatures reveal that Pd2+ and Pd+ sites are responsible for these low- and high-temperature features, respectively, and that while the IR feature at 1810 cm−1 is due to NO adsorption on Pd+, the 1860 cm−1 feature contains contributions from both weakly-bound NO on Pd2+ and more strongly bound NO on Pd+, consistent with experimentally observed effects of water and O2.

Ga/H-MFI was prepared by vapor-phase reaction of GaCl3 with Brønsted acid O-H groups in dehydrated H-MFI zeolite. The resulting [GaCl2]+ cations in the as-exchanged zeolite are treated in H2 at 823 K to stoichiometrically remove Cl ligands and form [GaH2]+ cations. Subsequent oxidation in O2 and characterization by IR spectroscopy and NH3-temperature-programmed desorption (TPD) suggests that, for Ga/Al ratios ≤0.3, Ga3+ exists predominantly as [Ga(OH)2]+-H+ cation pairs and to a lesser degree as [Ga(OH)]2+ cations at low Ga/Al ratios (∼0.1); while both species are associated with proximate cation-exchange sites, calculated free energies of formation suggest that [Ga(OH)]2+ cations are more stable on cation-exchange sites associated with NNN (next-nearest neighbor) framework Al atoms than on those associated with NNNN (next-next-nearest neighbor) framework Al atoms. Ga K-edge X-ray Absorption Near Edge Spectroscopy (XANES) measurements indicate that, under oxidizing conditions and for all Ga/Al ratios, all Ga species are in the +3 oxidation state and are tetrahedrally coordinated to 4 O atoms. Fourier analysis of Ga K-edge Extended X-ray Absorption Fine Structure (EXAFS) data supports the conclusion that Ga3+ is present predominantly as [Ga(OH)2]+ cations (or [Ga(OH)2]+-H+ cation pairs). For Ga/Al ratios ≤0.3, wavelet analysis of EXAFS data provide evidence for backscattering from nearest neighboring O atoms and from next-nearest neighboring framework Al atoms. For Ga/Al > 0.3, backscattering from next-nearest neighboring Ga atoms is also evident, characteristic of GaOx species. Upon reduction in H2, the oxidized Ga3+ species produce [Ga(OH)H]+-H+ cation pairs, [GaH2]+-H+ cation pairs, and [GaH]2+ cations. Computed phase diagrams indicate that the thermodynamic stability of the reduced Ga3+ species depends sensitively on temperature, Al-Al interatomic distance, and H2 and H2O partial pressures. For Ga/Al ratios ≤0.2, it is concluded that [GaH2]+-H+ cation pairs and [GaH]2+ cations are the predominant species present in Ga/H-MFI reduced above 673 K in 105 Pa H2 and in the absence of water vapor.

Acidic zeolites are effective catalysts for the cracking of large hydrocarbon molecules into lower molecular weight products required for transportation fuels. However, the ways in which the zeolite structure affects the catalytic activity at Brønsted protons are not fully understood. One way to characterize the influence of the zeolite structure on the catalysis is to study alkane cracking and dehydrogenation at very low conversion, conditions for which the kinetics are well defined. To understand the effects of zeolite structure on the measured rate coefficient (k_app ), it is necessary to identify the equilibrium constant for adsorption into the reactant state (K_ads-H+ ) and the intrinsic rate coefficient of the reaction (k_int ) at reaction temperatures, since k_app is proportional to the product of K_ads-H+ and k_int . We show that K_ads-H+ cannot be calculated from experimental adsorption data collected near ambient temperature, but can, however, be estimated accurately from configurational-bias Monte Carlo (CBMC) simulations. Using monomolecular cracking and dehydrogenation of C₃ -C₆ alkanes as an example, we review recent efforts aimed at elucidating the influence of the acid site location and the zeolite framework structure on the observed values of k_app and its components, K_ads-H+ and k_int .

The front cover artwork is provided by Bell and co-workers. The image shows a butane molecule adsorbed at a Brønsted acid site inside the pores of zeolite H-MFI, surrounded by the products of monomolecular cracking and dehydrogenation reactions catalyzed by these sites. Read the full text of the Minireview at 10.1002/cphc.201701084.