Hydrocarbons In Zeolites Research Articles

Mechanistic differences between methanol and dimethyl ether in zeolite-catalyzed hydrocarbon synthesis

Water influences critically the kinetics of the autocatalytic conversion of methanol to hydrocarbons in acid zeolites. At very low conversions but otherwise typical reaction conditions, the initiation of the reaction is delayed in presence of H2O. In absence of hydrocarbons, the main reactions are the methanol and dimethyl ether (DME) interconversion and the formation of a C1 reactive mixture-which in turn initiates the formation of first hydrocarbons in the zeolite pores. We conclude that the dominant reactions for the formation of a reactive C1 pool at this stage involve hydrogen transfer from both MeOH and DME to surface methoxy groups, leading to methane and formaldehyde in a 1:1 stoichiometry. While formaldehyde reacts further to other C1 intermediates and initiates the formation of first C-C bonds, CH4 is not reacting. The hydride transfer to methoxy groups is the rate-determining step in the initiation of the conversion of methanol and DME to hydrocarbons. Thus, CH4 formation rates at very low conversions, i.e., in the initiation stage before autocatalysis starts, are used to gauge the formation rates of first hydrocarbons. Kinetics, in good agreement with theoretical calculations, show surprisingly that hydrogen transfer from DME to methoxy species is 10 times faster than hydrogen transfer from methanol. This difference in reactivity causes the observed faster formation of hydrocarbons in dry feeds, when the concentration of methanol is lower than in presence of water. Importantly, the kinetic analysis of CH4 formation rates provides a unique quantitative parameter to characterize the activity of catalysts in the methanol-to-hydrocarbon process.

Effect of Temperature on the Adsorption of Short Alkanes in the Zeolite SSZ-13—Adapting Adsorption Isotherms to Microporous Materials

Understanding the diffusion and adsorption of hydrocarbons in zeolites is a highly important topic in the field of catalysis in micro- and mesoporous materials. Especially, the properties of alkanes in zeolites have been studied extensively. A theoretical description of these processes is challenging, because two interactions are involved: the alkane physisorbs to the zeolite wall and chemisorbs weakly to the active centers. At room temperature, the alkane remains physisorbed almost all the time, but the chemical bond to the active sites is regularly broken. In this work, we study this behavior using ab initio molecular dynamics simulations for the adsorption of methane, ethane, and propane in SSZ-13, the zeolite with the smallest unit cell, at temperatures of 250, 275, 325, and 350 K. We find a temperature dependence of the adsorption energy and the probability of the alkane to be close to the active site, which corresponds to chemisorption. We derive a temperature-dependent expression for these probabilities or active site coverages, which have the energy difference between physisorbed and chemisorbed state as the main variable. The methodology derived in this work will be highly useful in correlating static electronic structure calculations to finite temperature coverages, which, following the Sabatier principle, is a key step to understand the performance of catalysts under reaction conditions and a prerequisite to computationally design such materials.

Physisorption and Chemisorption of Hydrocarbons in H-FAU Using QM-Pot(MP2//B3LYP) Calculations

The DFT parametrized zeolite force field in the QM-Pot program is extended with carbon−carbon, carbon−hydrogen, and alkoxy bond describing parameters. The extended force field has been combined with B3LYP and with MP2 as the high-level quantum mechanical (QM) method to simulate the physisorption and chemisorption of ethene, isobutene, 1-butene, 1-pentene, and 1-octene in H-FAU (Si/AlF = 95) and for physisorption of 1-pentene, n-pentane, 1-octene, and n-octane in all silica FAU. The new parametrization predicts more stable chemisorption complexes than physisorbed π complexes, but with smaller chemisorption energies which are more reliable as shown by comparison with experimental results and with accurate hybrid MP2:DFT calculations. An embedded cluster size study shows that, due to the importance of the stabilizing van der Waals part in the MM contribution of the cluster, QM-Pot(MP2//B3LYP) calculations yield more reliable physisorption and chemisorption energies of hydrocarbons in zeolites than QM-Pot(B3L...

Adsorption of hydrocarbons in zeolites from molecular simulations. The alkane–ferrierite system revisited

The adsorption of linear alkanes in zeolite ferrierite is reinvestigated using a recently developed forcefield based on the anisotropic united atom (AUA) potential scheme with no futher readjustment. The simulation model is able to reproduce the subtle changes in adsorption site occupancies of n-alkanes (n = 3–7) in ferrierite in good agreement with experimental data.

Development of a Transferable Guest−Host Force Field for Adsorption of Hydrocarbons in Zeolites. II. Prediction of Alkenes Adsorption and Alkane/Alkene Selectivity in Silicalite

Alkene pure components and alkane/alkene binary mixtures adsorption isotherms are computed by grand canonical Monte Carlo simulations. The anisotropic united atom potential optimized from experimen...

Catalytic Conversion of Hydrocarbons in Zeolites from First Principles

AbstractFor Abstract see ChemInform Abstract in Full Text.

Development of a transferable guest–host force field for adsorption of hydrocarbons in zeolites : I. Reinvestigation of alkane adsorption in silicalite by grand canonical Monte Carlo simulation

Grand canonical Monte Carlo simulation of several linear and branched alkanes adsorption in silicalite-1 zeolite has been performed using the anisotropic united atoms AUA-4 potential that had been developed earlier by Ungerer and co-workers to predict several bulk equilibrium properties of various hydrocarbons (P. Ungerer, C. Beauvais, J. Delhommelle, A. Boutin, B. Rousseau and A. H. Fuchs, J. Chem. Phys., 2000, 112, 5499). The zeolite potential parameters for the CH2 and CH3 groups were optimized using butane experimental adsorption data only. The use of combination rules enabled to predict the adsorption properties of other alkanes (methane, ethane, propane, butane, isobutane, pentane, 2-methylbutane, hexane, 2-methylpentane, heptane, 2-methylhexane, octane, and nonane) without any further readjustment of the force field parameters. In the temperature range 277 to 374 K a good agreement was found between simulation and experiments, as well as with the previous simulations results of Smit and coworkers. The effectiveness of the transferability of the present force field is demonstrated by the fact that adsorption properties of CH4 and other molecules containing a CH group were successfully predicted, although these groups were not introduced in the fitting procedure.

Influence of Framework Flexibility on the Adsorption Properties of Hydrocarbons in the Zeolite Silicalite

Computer simulations of the adsorption of hydrocarbons in zeolites are usually performed using rigid zeolite frameworks. This allows for the use of grid interpolation techniques to compute the hydrocarbon−zeolite interaction very efficiently. In this paper, we investigate the influence of the framework flexibility on the adsorption properties of hydrocarbons adsorbed in the zeolite silicalite. We find that at low loading, the influence of the framework flexibility on the heat of adsorption and the Henry coefficient is quite small. However, for molecules such as isobutane and heptane with inflection behavior, the influence at high loading seems to be much larger.

Catalyic conversion of hydrocarbons in zeolites from first principles

Abstract The application of the density functional techniques to processes of the conversion of hydrocarbons in zeolites has been reviewed. The conversion of hydrocarbons over zeolites is an important industrial process. The microscopic steps of the conversion, however, are still not satisfactorily understood. In order to examine reaction pathways, both static and molecular dynamics density functional theory (DFT) calculations have been performed. Simulated structural and spectral properties compare reasonably with experimental data. Comparison of energies of physisorption and chemisorption indicates possible reaction channel of the conversion through the chemisorption at the specific O-sites of the zeolite.

Monte Carlo simulations in zeolites

In this short review, the applications of Monte Carlo simulations to the study of the adsorption and diffusion of hydrocarbons in zeolites are discussed. We focus on those systems for which the conventional molecular simulation techniques, molecular dynamics and Monte Carlo, are not sufficiently efficient. In particular, to simulate the adsorption and diffusion of long-chain hydrocarbons, novel Monte Carlo techniques have been developed. Here we discuss configurational-bias Monte Carlo (CBMC) and kinetic Monte Carlo (KMC). CBMC was developed to compute the thermodynamic properties. KMC is applied to compute transport properties. The use of these methods is illustrated with examples of technological importance.

Diffusion of hydrocarbons in confined media: Translational and rotational motion

Diffusion of monatomic guest species within confined media has been understood to a good degree due to investigations carried out during the past decade and a half. Most guest species that are of industrial relevance are actually polyatomics such as, for example, hydrocarbons in zeolites. We attempt to investigate the influence of non-spherical nature of guest species on diffusion. Recent molecular dynamics (MD) simulations of motion of methane in NaCaA and NaY, benzene in NaY and one-dimensional channels AlPO4−5, VPI−5 and carbon nanotube indicate interesting insights into the influence of the host on rotational degrees of freedom and orientational properties. It is shown that benzene in one-dimensional channels where the levitation parameter is near unity exhibits translational motion opposite to what is expected on the basis of molecular anisotropy. Rotational motion of benzene also possesses rotational diffusivities aroundC 6 and C2axes opposite to what is expected on the basis of molecular geometry. Methane shows orientational preference for 2+ 2 or 1 + 3 depending on the magnitude of the levitation parameter.

Adsorption of linear hydrocarbons in zeolites: A density-functional investigation

An extensive first-principles periodical study of adsorption properties of linear hydrocarbons in zeolites is presented. The applicability of density-functional theory to weak interactions is inspected within both local-density (LDA) and generalized-gradient (GGA) approaches for C1 to C6 linear hydrocarbons. The LDA adsorption energies are due to the overbinding ∼2.5 times larger than the GGA values. A compact diagram is constructed showing the increase of the adsorption energy with the length of the adsorbed molecule and with the concentration of acid sites in the zeolite support. The flow of the electron density induced by the adsorption indicates that the adsorption on the acid site is realized through the hydrogen bonding between the OH group and the CH3 group. The pattern of the reconstructed bonding, however, is more complex than that of the simple hydrogen bond. The regions of redistributed electron density within the adsorbed molecule are spread over the whole CH3 group and the adjacent C–C bond. The off-centering of the reconstructed regions from atomic positions is in good agreement with recent C13 measurements, showing only slight variation of chemical shifts with the hydrocarbon length for both proton-free and the protonated forms of zeolites.

The generalized Maxwell–Stefan model for diffusion in zeolites:: sorbate molecules with different saturation loadings

Using the Maxwell–Stefan approach, expressions have been derived for the diffusion of mixtures of hydrocarbons in zeolites wherein the individual components have different saturation loadings. This Maxwell–Stefan diffusion model, in combination with the Ideal Adsorbed Solution (IAS) theory and the single-component adsorption isotherms, provides a superior, qualitative and quantitative, prediction of the permeation fluxes of ethane/methane and propane/methane mixtures through a silicalite-1 membrane. The difference in adsorption saturation loadings becomes apparent in the entropy effects in the mixture adsorption.

Effects of Coadsorbed Water on the Mobility of Polyaromatic Hydrocarbons in Zeolite Y

Triplet excited states of pyrene, phenanthrene, and naphthalene have been prepared in zeolite KY by pulsed laser excitation and the influence of coadsorbed water on quenching of these triplet states by ferrocene and the immobile ferrocene derivative ferrocenylmethytrimethylammonium cation (FcMN) has been studied by transient absorption spectroscopy. To make these measurements, probe loadings in KY have been kept deliberately low such that quenching involves migration of molecules from one supercage to another. In all cases, maxima in bimolecular triplet quenching rates versus the number of H2O per supercage (Nsc) were observed at Nsc ≈ 4, a value which coincides with the number of H2O molecules that fill the sodalite cages. For pyrene quenching by FcMN, the effect of 4 H2O/sc was particularly dramatic, where the quenching rate increased by 3 orders of magnitude relative to dehydrated KY. Above Nsc = 4, further additions of H2O lowered the rate of quenching. The rate of triplet quenching by FcMN, where quenching results from motion of the aromatic probes in their triplet state to FcMN, generally followed the trend 3Np* > 3Ph* ≫ 3Py*. This trend, as well as the influence of coadsorbed H2O on quenching, have been explained by the effects of adsorption and molecular size on diffusion of these molecules in KY.

Adsorption of Linear and Branched Alkanes in the Zeolite Silicalite-1

Department of Chemical EngineeringUniVersity of Amsterdam, Nieuwe Achtergracht 1661018 WV Amsterdam, The NetherlandsDepartment of Chemical EngineeringDelft UniVersity of Technology, Julianalaan 1362628 BL Delft, The NetherlandsReceiVed December 22, 1997Understanding the adsorption of hydrocarbons in zeolites is aprerequisite for optimizing catalytic processes using these materi-als.

In‐situ CIR‐FTIR study of the diffusion of supercritical hydrocarbons in zeolite L

AbstractCylindrical internal reflectance infrared spectroscopy (CIR‐FTIR) was used for the determination of the transport properties of hydrocarbons in zeolite L at high temperatures and pressures. This was the first time that the intracrystalline diffusivity of hydrocarbons in microporous media could be determined under supercritical conditions by using a dynamic technique. The study investigated the self‐diffusion of n‐heptane, the counterdiffusion of n‐hexane and 1‐hexene, and the codiffusion of n‐heptane and pyxlene in zeolite L under subcritical and supercritical conditions. The diffusion time constants obtained for the single and binary systems showed that the diffusion within the pore was reduced significantly under supercritical conditions (i.e., liquidlike behavior) as the hydrocarbon density within the pore was increased.

Simulation of adsorption and diffusion of hydrocarbons in zeolites

Molecular simulations are used to investigate the energetics and siting of linear and branched alkanes in the zeolite silicalite. The calculated heats of adsorption of the branched alkanes are in good agreement with the experimental data. The simulations show a striking difference between the behaviour of linear and branched alkanes. The linear alkanes are relatively free to move in all channels of the zeolites. The branched alkanes are trapped with their CH group in the intersection of the zig-zag and straight channels of silicalite. This trapping of the branched alkanes suggests that diffusion of these molecules is an activated process; most of the time the molecule is located in the intersection but, occasionally, it hops from one intersection to another. The straight and zig-zag channels form a barrier for the diffusion. We present some preliminary calculations of this hopping rate, from which the diffusion coefficient can be calculated. These preliminary results are in reasonable agreement with experimental data.

PFG NMR tracer exchange measurements of xenon in zeolites

Understanding mass transport phenomena in zeolites at a molecular level is of prime importance to the study of zeolite catalysis. Among the techniques applicable, PFG NMR has been shown to provide vital information about diffusion of hydrocarbons in zeolites. 2 Here this technique is used to study xenon as a probe molecule, because of its chemical inertness and the large differences in chemical shift of the xenon atoms in the sorbed and gas phase.

Adsorption and Diffusion of Aromatic Hydrocarbons in Zeolite Y by Molecular Mechanics Calculation and X-ray Powder Diffraction

Adsorption and Diffusion of Aromatic Hydrocarbons in Zeolite Y by Molecular Mechanics Calculation and X-ray Powder Diffraction

P.f.g. n.m.r. study of the influence of the exchangeable cations on the self-diffusion of hydrocarbons in zeolites

The molecular mobility of methane and n-hexane adsorbed in X- and A-type zeolites with different calcium contents is studied by pulsed-field gradient (p.f.g.) n.m.r. In contrast to sodium, the calcium ions are found to be of substantial influence on the intracrystalline diffusivities. As a consequence of this interaction, the diffusivities of methane in NaCaA and NaCaX are found to be quite close to each other in both the absolute values and the dependence on temperature and concentration. For n-hexane, the diffusivities in NaCaA are significantly below the diffusivities in NaCaX. This can be attributed to the larger critical diameter of the n-hexane molecules, which is of the order of the diameters of the windows between adjacent supercages in zeolite A. The proposed model of molecular interaction is confirmed by comparison with the results of MD simulations and by n.m.r. tracer desorption measurements with zeolite alpha.