Impact of confinement on zeolite cracking selectivity via Monte Carlo integration

Monte Carlo integration methods employing a classic molecular mechanics force field were used to probe the impact of confinement on the selectivity of the n-hexane/3-methylpentane test reaction in 12 different zeolites. The simulations estimated relative cracking rates and predicted the constraint index (CI) selectivity for both monomolecular and bimolecular cracking mechanisms. By comparing with experimentally measured values of the CI, information on the dominant mechanism responsible for the observed selectivity was obtained. For small-pore zeolites, the monomolecular mechanism dominated, with the measured CI attributed to reactant selectivity based on preferential adsorption. For large-pore zeolites, the CI selectivity was attributed to confinement effects imposed on the bimolecular transition states. For zeolite structures with intermediate-sized pores, such as MTW (ZSM-12) and TON (ZSM-22), the simulations indicated that both reaction mechanisms may be operative. The experimentally observed reduction in the CI with temperature in MFI (ZSM-5) can be explained in terms of the relative importance of steric and entropic factors in the stabilization of the bimolecular transition state. A change in mechanism from bimolecular to monomolecular, as postulated in the literature, is not necessary to explain the experimentally observed temperature dependence of the CI.

[1]  S. Zones,et al.  The Constraint Index test revisited: anomalies based upon new zeolite structure types , 2000 .

[2]  V. Grondelle,et al.  Kinetics of Hydro-isomerization ofn-Hexane over Platinum Containing Zeolites , 1997 .

[3]  R. Snurr,et al.  Monomolecular cracking of n-hexane on Y, MOR, and ZSM-5 zeolites , 1999 .

[4]  V. Kazansky,et al.  Quantum-chemical study of the nonclassical carbonium ion-like transition states in isobutane cracking on zeolites , 1996 .

[5]  P. B. Weisz,et al.  Transport and reactivity of hydrocarbon molecules in a shape-selective zeolite , 1981 .

[6]  V. Kazansky.  Adsorbed carbocations as transition states in heterogeneous acid catalyzed transformations of hydrocarbons , 1999 .

[7]  R. Snurr,et al.  The roles of acid strength and pore diffusion in the enhanced cracking activity of steamed Y zeolites , 1999 .

[8]  S. Zones,et al.  Inverse shape selectivity in molecular sieves: Observations, modelling, and predictions , 1993 .

[9]  J. Nicholas,et al.  Density functional theory studies of zeolite structure, acidity, and reactivity , 1997 .

[10]  Keith J. Laidler,et al.  Theories Of Chemical Reaction Rates , 1969 .

[11]  B. Smit,et al.  The Shape Selectivity of Paraffin Hydroconversion on TON-, MTT-, and AEL-Type Sieves , 1999 .

[12]  J. Sauer.  Structure and reactivity of zeolite catalysts: Atomistic modelling using ab initio techniques , 1994 .

[13]  A. T. Hagler,et al.  Consistent force field studies of intermolecular forces in hydrogen-bonded crystals. 2. A benchmark for the objective comparison of alternative force fields , 1979 .

[14]  A. Corma,et al.  A Theoretical Study of the Mechanism of the Hydride Transfer Reaction between Alkanes and Alkenes Catalyzed by an Acidic Zeolite , 1998 .

[15]  R. A. Santen,et al.  A quantum-chemical study of adsorbed nonclassical carbonium ions as active intermediates in catalytic transformations of paraffins. I. Protolytic cracking of ethane on high silica zeolites , 1994 .

[16]  A. Corma,et al.  Isomerization and disproportionation of m-xylene : Selectivities Induced by the Void Structure of the Zeolite Framework , 1988 .

[17]  G. Grest,et al.  Influence of intracrystalline diffusion in shape selective catalytic test reactions , 1998 .

[18]  Edward J. Maginn,et al.  A biased grand canonical Monte Carlo method for simulating adsorption using all-atom and branched united atom models , 1999 .

[19]  W. Haag,et al.  Kinetics and Mechanism of Paraffin Cracking with Zeolite Catalysts , 1991 .

[20]  S. Ernst,et al.  Large pore molecular sieves: Chapter 5 Catalytic test reactions for probing the pore width of large and super-large pore molecular sieves , 1994 .

[21]  E. Derouane.  Zeolites as solid solvents , 1998 .

[22]  W. Haag,et al.  Catalysis by crystalline aluminosilicates: Characterization of intermediate pore-size zeolites by the “Constraint Index” , 1981 .

[23]  M. E. Leonowicz,et al.  MCM-22: A Molecular Sieve with Two Independent Multidimensional Channel Systems , 1994, Science.

[24]  Alexis T. Bell,et al.  Transition-state studies of xenon and sulfur hexafluoride diffusion in silicalite , 1991 .

[25]  R. A. Santen.  The cluster approach to molecular heterogeneous catalysis , 1997 .

[26]  J. Martens,et al.  Estimation of the void structure and pore dimensions of molecular sieve zeolites using the hydroconversion of n-decane , 1984 .

[27]  M. Dewar,et al.  Ground States of Molecules. 38. The MNDO Method. Approximations and Parameters , 1977 .

[28]  Alexis T. Bell,et al.  Prediction of low occupancy sorption of alkanes in silicalite , 1990 .

[29]  Alexis T. Bell,et al.  Sorption Thermodynamics, Siting, and Conformation of Long n-Alkanes in Silicalite As Predicted by Configurational-Bias Monte Carlo Integration , 1995 .