Room temperature methoxylation in zeolites: insight into a key step of the methanol-to-hydrocarbons process

Neutron scattering methods observed complete room temperature conversion of methanol to framework methoxy in a commercial sample of methanol-to-hydrocarbons (MTH) catalyst H-ZSM-5, evidenced by methanol immobility and vibrational spectra matched by ab initio calculations. No methoxylation was observed in a commercial HY sample, attributed to the dealumination involved in high silica HY synthesis

Host-guest and guest-guest interactions between xylene isomers confined in the MIL-47(V) pore system

The porous MIL-47 material shows a selective adsorption behavior for para-, ortho-, and meta-isomers of xylenes, making the material a serious candidate for separation applications. The origin of the selectivity lies in the differences in interactions (energetic) and confining (entropic). This paper investigates the xylene-framework interactions and the xylene-xylene interactions with quantum mechanical calculations, using a dispersion-corrected density functional and periodic boundary conditions to describe the crystal. First, the strength and geometrical characteristics of the optimal xylene-xylene interactions are quantified by studying the pure and mixed pairs in gas phase. An extended set of initial structures is created and optimized to sample as many relative orientations and distances as possible. Next, the pairs are brought in the pores of MIL-47. The interaction with the terephthalic linkers and other xylenes increases the stacking energy in gas phase (-31.7 kJ/mol per pair) by roughly a factor four in the fully loaded state (-58.3 kJ/mol per xylene). Our decomposition of the adsorption energy shows various trends in the contributing xylene-xylene interactions. The absence of a significant difference in energetics between the isomers indicates that entropic effects must be mainly responsible for the separation behavior

Challenges for the theoretical description of the mechanism and kinetics of reactions catalyzed by zeolites

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

Impact of long-range electrostatic and dispersive interactions on theoretical predictions of adsorption and catalysis in zeolites

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

Understanding Brønsted-Acid Catalyzed Monomolecular Reactions of Alkanes in Zeolite Pores by Combining Insights from Experiment and Theory

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

Characterization of Isolated Ga3+ Cations in Ga/H-MFI Prepared by Vapor-Phase Exchange of H-MFI Zeolite with GaCl3

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

Characterization of Isolated Ga3+ Cations in Ga/H-MFI Prepared by Vapor-Phase Exchange of H-MFI Zeolite with GaCl3

Ga/H-MFI was prepared by vapor-phase reaction of GaCl with Brønsted acid O-H groups in dehydrated H-MFI zeolite. The resulting [GaCl ] cations in the as-exchanged zeolite are treated in H at 823 K to stoichiometrically remove Cl ligands and form [GaH ] cations. Subsequent oxidation in O and characterization by IR spectroscopy and NH -temperature-programmed desorption (TPD) suggests that, for Ga/Al ratios ≤0.3, Ga exists predominantly as [Ga(OH) ] -H cation pairs and to a lesser degree as [Ga(OH)] 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)] 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 Ga is present predominantly as [Ga(OH) ] cations (or [Ga(OH) ] -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 GaO species. Upon reduction in H , the oxidized Ga species produce [Ga(OH)H] -H cation pairs, [GaH ] -H cation pairs, and [GaH] cations. Computed phase diagrams indicate that the thermodynamic stability of the reduced Ga species depends sensitively on temperature, Al-Al interatomic distance, and H and H O partial pressures. For Ga/Al ratios ≤0.2, it is concluded that [GaH ] -H cation pairs and [GaH] cations are the predominant species present in Ga/H-MFI reduced above 673 K in 10 Pa H and in the absence of water vapor. 3 2 2 2 2 3 2 2 2 x 2 2 2 2 2 2 + + 3+ + + 2+ 2+ 3+ + + + 3+ + + + + 2+ 3+ + + 2+