Molecular Modeling of Interactions in Zeolites: An Ab Initio Embedded Cluster Study of NH3 Adsorption in Chabazite

Activation of Molecular O2 on CoFe2 O4 (001) Surfaces: An Embedded Cluster Study

Dioxygen activation pathways on the (001) surfaces of cobalt ferrite, CoFe2 O4 , were investigated computationally using density functional theory and the hybrid Perdew-Burke-Ernzerhof exchange-correlation functional (PBE0) within the periodic electrostatic embedded cluster model. We considered two terminations: the A-layer exposing Fe2+ and Co2+ metal sites in tetrahedral and octahedral positions, respectively, and the B-layer exposing octahedrally coordinated Co3+ . On the A-layer, molecular oxygen is chemisorbed as a superoxide on the Fe monocenter or bridging a Fe-Co cation pair, whereas on the B-layer it is adsorbed at the most stable anionic vacancy. Activation is promoted by transfer of electrons provided by the d metal centers onto the adsorbed oxygen. The subsequent dissociation of dioxygen into monoatomic species and surface reoxidation have been identified as the most critical steps that may limit the rate of the oxidation processes. Of the reactive metal-O species, [FeIII -O]2+ is thermodynamically most stable, while the oxygen of the Co-O species may easily migrate across the A-layer with barriers smaller than the associative desorption.

Vapor-Phase Beckmann Rearrangement of Oxime Molecules over H-Faujasite Zeolite

The Beckmann rearrangement (BR) plays an important role in a variety of industries. The mechanism of this reaction rearrangement of oximes with different molecular sizes, specifically, the oximes of formaldehyde (H(2)C=NOH), Z-acetaldehyde (CH(3)HC=NOH), E-acetaldehyde (CH(3)HC=NOH) and acetone (CH(3))(2)C=NOH, catalyzed by the Faujasite zeolite is investigated by both the quantum cluster and embedded cluster approaches at the B3LYP level of theory using the 6-31G (d,p) basis set. To enhance the energetic properties, single point calculations are undertaken at MP2/6-311G(d,p). The rearrangement step, using the bare cluster model, is the rate determining step of the entire reaction of these oxime molecules of which the energy barrier is between 50-70 kcal mol(-1). The more accurate embedded cluster model, in which the effect of the zeolitic framework is included, yields as the rate determining step, the formaldehyde oxime reaction rearrangement with an energy barrier of 50.4 kcal mol(-1). With the inclusion of the methyl substitution at the carbon-end of formaldehyde oxime, the rate determining step of the reaction becomes the 1,2 H-shift step for Z-acetaldehyde oxime (30.5 kcal mol(-1)) and acetone oxime (31.2 kcal mol(-1)), while, in the E-acetaldehyde oxime, the rate determining step is either the 1,2 H-shift (26.2 kcal mol(-1)) or the rearrangement step (26.6 kcal mol(-1)). These results signify the important role that the effect of the zeolite framework plays in lowering the activation energy by stabilizing all of the ionic species in the process. It should, however, be noted that the sizeable turnover of a reaction catalyzed by the Brønsted acid site might be delayed by the quantitatively high desorption energy of the product and readsorption of the reactant at the active center.

A charge-scaling implementation of the variational electrostatic projection method

Two new charge-scaling methods for efficient modeling of the solvated macromolecular environment in hybrid QM/MM calculations of biological reactions are presented. The methods are extensions of the variational electrostatic projection (VEP) method, and allows a subset of atomic charges in the external environment to be adjusted to mimic, in the active dynamical region, the electrostatic potential and field due to the large surrounding macromolecule and solvent. The method has the advantages that it offers improved accuracy, does not require the use of a three-dimensional grid or auxiliary set of fitting points, and requires only minor molecular simulation code modifications. The VEP-cs and VEP-RVM+cs methods are able to attain very high accuracy (relative force errors of 10(-7) or better with appropriate choice of control parameters), and take advantage of a recently introduced set of high-order discretization schemes and Gaussian exponents for boundary element solvation and VEP methods. The methods developed here serve as potentially powerful tools in the arsenal of computational techniques used in multiscale computational modeling problems.

Hydrogen Bond vs Proton Transfer in HZSM5 Zeolite. A Theoretical Study

The interaction of a large set of bases covering a wide range of the basicity scale with HZSM5 medium-size zeolites has been investigated through the use of two model clusters, namely 5T and 7T:63T. The 5T cluster has been treated fully ab initio at the B3LYP level, whereas the 63T cluster has been treated with the ONIOM2 scheme using the B3LYP:MNDO combination for geometry optimizations and B3LYP:HF/3-21G for adsorption energies. The optimized geometries of the different hydrogen bond (HB) and ion pair (IP) complexes obtained with both models are rather similar. However, there are significant dissimilarities as far as the adsorption energies are concerned, in particular when dealing with IP clusters whose intrinsic stability is largely underestimated when the simpler 5T model is used. 5T clusters could be used to obtain reasonable estimates of adsorption energies provided these are scaled by a factor of 1.1 for HB complexes and 1.4 for IP complexes. The zeolite cavity favors the proton transfer process, similarly to that found by third polar partners in gas-phase HB trimers. The intrinsic basicity of the base and its adsorption energy within the zeolite are correlated. From this correlation, is possible to conclude that, in general, bases with proton affinities (PA) larger than 200 kcal mol(-1) should lead to the formation of IPs, whereas bases with PA smaller than this value should form HB complexes.

Density Functional Study of the Mechanism of the Beckmann Rearrangement Catalyzed by H-ZSM-5: A Cluster and Embedded Cluster Study

The mechanism of the Beckmann rearrangement (BR) catalyzed by the ZSM-5 zeolite has been investigated by both the quantum cluster and embedded cluster approaches at the B3LYP level of theory using the 6-31G(d,p) basis set. Single-point calculations were carried out at the MP2/6-311G(d,p) level of theory to improve energetic properties. The embedded cluster model suggests that the initial step of the Beckmann rearrangement is not the O-protonated oxime but the N-protonated oxime. The energy barriers derived from the proton shuttle of the N-bound to the O-bound isomer are determined to be approximately 99 and approximately 40 kJ/mol for the embedded cluster and quantum cluster approaches, respectively. The difference in the activation energy is due mainly to the effect of the Madelung potential from the zeolite framework. The next step is the rearrangement step, which is the transformation of the O-protonated oxime to be an enol-formed amide compound, formimidic acid. The activation energy, at the rearrangement step, is calculated to be approximately 125 and approximately 270 kJ/mol for the embedded cluster and quantum cluster approaches, respectively. The final step is the tautomerization step which transforms the enol-form to the keto-form, formamide compound. The energy barrier for tautomerization is calculated to be 123 and 151 kJ/mol for the embedded cluster and quantum cluster approaches, respectively. These calculated results suggest that the rate-determining step of the vapor phase of the Beckmann rearrangement on H-ZSM-5 is the rearrangement or tautomerization step.

Nature of the Metal−Support Interaction in Bifunctional Catalytic Pt/H-ZSM-5 Zeolite

The metal-support interaction of a dispersed Pt atom on H-ZSM-5 zeolite has been investigated by using an embedded cluster and cluster models with the density functional theory/B3LYP functional method. We found that the Pt atom interacts with a Brønsted proton and a nearby framework oxygen. Interaction with the framework oxygen causes electron transfer from the zeolite to the Pt atom. Concurrently, a Brønsted proton stabilizes the Pt atom on the zeolite surface by withdrawing excess electron density from the Pt atom. These charge transfers result in a zero net charge on the Pt atom while changing its orbital occupation. The binding energy of Pt on the Brønsted acid was 15 kcal/mol. Inclusion of the Madelung potential by Surface Charge Representation of the Electrostatic Embedded Potential method (SCREEP) had small effects on structure and charge density of Pt/H-ZSM-5 but it shifted the stretching mode of CO toward a higher frequency by almost 40 cm(-1). The frequency shift of absorbed CO calculated with embedded cluster models was from 8 to 11 cm(-1) red shift, compared to 20 cm(-1) red shift from experiment. This implies that not only the electronic state of the Pt atom but also the Madelung potential of the support is responsible for the observed small red shift of CO on the Pt-H-ZSM-5.

Theoretical ONIOM2 Study on Pyridine Adsorption in the Channels and Intersection of ZSM-5

The structures of the acid sites in the channels and intersections of H-, Li-, and Na-ZSM-5 (ZSM = zeolite socony mobil) and their interactions with pyridine molecule have been computed by using three corresponding models containing 22 tetrahedral sites. The calculated adsorption energies of pyridine in the intersection regions of H-, Li-, and Na-ZSM-5 are 197.0, 172.5, and 122.3 kJ/mol, respectively, in good agreement with the respective experimental values of 200 +/- 5, 155-195, and 120 kJ/mol, while those in the straight and sinusoidal channels are much smaller (157.9 and 127.6, 152.2 and 149.4, and 150.4 and 109.9 kJ/mol, respectively). These indicate that the most probable adsorption site for pyridine in ZSM-5 is the acidic site located in the intersection region. The structural parameters of the adsorption complexes show that the acidic proton in the three models of H-ZSM-5 has been transferred to the nitrogen of pyridine, while in alkali cation-exchanged ZSM-5, the coordination of the alkali cation to the nitrogen atom of pyridine dominates the overall interaction. In addition, the adsorption complexes were further stabilized by the long-range electrostatic interaction between the positively charged pyridine hydrogen atoms and the negatively charged lattice oxygen atoms of the zeolite framework. In the intersection regions of H-, Li-, and Na-ZSM-5, the coordination energy of the charge-compensating cation to the pyridine nitrogen amounts to 58, 60, and 68% of the total adsorption energy, respectively, while another 42, 40, and 32%, respectively, is due to long-range electrostatic interactions. This indicates that the zeolite lattice framework surrounding the adsorption site has important contributions to the adsorption energy of the pyridine molecule.

Adsorption of NH3 and H2O in Acidic Chabazite. Comparison of ONIOM Approach with Periodic Calculations

The adsorption of NH(3) and H(2)O in acidic chabazite has been studied with the B3LYP method within the cluster approach (5T, 48T clusters) and the periodic approach adopting a Si/Al = 11/1 chabazite and a basis set of polarized double-zeta quality. The 5T cluster has been treated fully ab initio at the B3LYP level whereas the 48T cluster has been treated with the ONIOM2 scheme using B3LYP as the high level of theory and the MNDO, AM1, and HF/3-21G methods as low levels of theory. Periodic calculations show that the adsorption of NH(3) in acidic chabazite takes place through an ion pair (NH(4)(+)-CHA(-)) structure, the computed adsorption energy being -32 kcal/mol. The adsorption of H(2)O leads to a hydrogen bonded (H(2)O-HCHA) complex with the computed adsorption energy of -20 kcal/mol. All ONIOM combinations provide similar structures to those obtained with periodic calculations. Adsorption energies, however, are sensitive to the low level used, especially for NH(3). The ONIOM B3LYP:HF/3-21G method is the one that provides more satisfactory results. Present results show that, for larger zeolites, the ONIOM scheme can be successfully applied while drastically reducing the cost of a fully ab initio treatment.

Selectivity of a model zeolite ring over hydrocarbons with different symmetry, travelling with different orientations and speeds

We explore the selectivity of a model zeolite ring over representative hydrocarbons of crude oil. The model ring consists of 7 silicon tetrahedral units and one chemically active aluminum site through which hydrocarbons with symmetries varying from almost spherically symmetric to linear chains (1D), planar (2D), and pyramidal (3D) structures diffuse. The selectivity is further investigated when the hydrocarbons travel with different orientations and speeds. The semiclassical Born-Oppenheimer molecular dynamics approximation is used to characterize the chemical dynamics, as well as to determine the energetics and reaction products. The simulations reveal noticeable differences in energy profiles and charge populations. Our results are important to understand aspects of mass transport and some of the factors that control the catalytic activity in zeolites.

Adsorption and vibrational spectroscopy of ammonia at mordenite: Ab initio study

The adsorption of ammonia at various active centers at the outer and inner surfaces of mordenite, involving Brønsted acid (BA) sites, terminal silanol groups, and Lewis sites has been investigated using periodic ab initio density-functional theory. It is shown that ammonia forms an ammonium ion when adsorbed at strong BA sites. The calculated adsorption energies for different BA sites vary in the interval from 111.5 to 174.7 kJ/mol depending on the local environment of the adduct. The lowest adsorption energy is found for a monodentate complex in the main channel, the highest for a tetradentate configuration in the side pocket. At weak BA sites such as terminal silanol groups or a defect with a BA site in a two-membered ring ammonia is H bonded via the N atom. Additional weak H bonds are formed between H atoms of ammonia and O atoms of neighboring terminal silanol groups. The calculated adsorption energies for such adducts range between 61.7 and 70.9 kJ/mol. The interaction of ammonia with different Lewis sites is shown to range between weak (DeltaE(ads)=17.8 kJ/mol) and very strong (DeltaE(ads)=161.7 kJ/mol), the strongest Lewis site being a tricoordinated Al atom at the outer surface. Our results are in very good agreement with the distribution of desorption energies estimated from temperature-programmed desorption (TPD) and microcalorimetry experiments, the multipeaked structure of the TPD spectra is shown to arise from strong and weak Brønsted and Lewis sites. The vibrational properties of the adsorption complexes are investigated using a force-constant approach. The stretching and bending modes of NH(4) (+) adsorbed to the zeolite are strongly influenced by the local environment. The strongest redshift is calculated for the asymmetric stretching mode involving the NH group hydrogen bonded to the bridging O atom of the BA site, the shift is largest for a monodentate and smallest for a tetradentate adsorption complex. The reduced symmetry of the adsorbate also leads to a substantial splitting of the stretching and bending modes. In agreement with experiment we show that the main vibrational feature which differentiates coordinatively bonded ammonia from a hydrogen-bonded ammonium ion is the absence of bending modes above 1630 cm(-1) and in the region between 1260 and 1600 cm(-1), and a low-frequency bending band in the range from 1130 to 1260 cm(-1). The calculated distribution of vibrational frequencies agrees very well with the measured infrared adsorption spectra. From the comparison of the adsorption data and the vibrational spectra we conclude that due to the complex adsorption geometry the redshift of the asymmetric stretching is a better measure of the acidity of an active sites than the adsorption energy.