Proton exchange of small hydrocarbons over acidic chabazite: Ab initio study of entropic effects

The proton-exchange reaction of a series of short hydrocarbons over an acidic zeolite (chabazite) was studied using periodic density functional theory (DFT) calculations. It was found that the chain length of hydrocarbons does not have a significant effect on the height of the potential-energy barrier. The experimentally observed regioselectivity between methyl and methylene groups in propane and between methyl and methine groups in isobutane was shown to be an entropic effect. In addition to the direct H-exchange, a mechanism mediated by a methylpropene molecule recently suggested by experimentalists was explored. It was found that entropy plays a very important role in driving the reaction.

[1]  A. Stepanov,et al.  Activation of Small Alkanes on Solid Acids. An H/D Exchange Study by Liquid and Solid-State NMR: The Activation Energy and the Inhibiting Effect of Carbon Monoxide , 1999 .

[2]  Alexis T. Bell,et al.  Density Functional Theory Study of Proton Mobility in Zeolites: Proton Migration and Hydrogen Exchange in ZSM-5 , 2000 .

[3]  Eric Darve,et al.  Calculating Free Energies Using a Scaled-Force Molecular Dynamics Algorithm , 2002 .

[4]  P. Fleurat‐Lessard,et al.  Tracing the minimum-energy path on the free-energy surface. , 2005, The Journal of chemical physics.

[5]  K. Fukui.  The path of chemical reactions - the IRC approach , 1981 .

[6]  G. Kresse,et al.  From ultrasoft pseudopotentials to the projector augmented-wave method , 1999 .

[7]  S. Nosé.  A unified formulation of the constant temperature molecular dynamics methods , 1984 .

[8]  Hafner,et al.  Ab initio molecular dynamics for open-shell transition metals. , 1993, Physical review. B, Condensed matter.

[9]  Jackson,et al.  Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation. , 1992, Physical review. B, Condensed matter.

[10]  Blöchl,et al.  Projector augmented-wave method. , 1994, Physical review. B, Condensed matter.

[11]  Jeffery L. White,et al.  Alkane C-H bond activation in zeolites: evidence for direct protium exchange. , 2006, Journal of the American Chemical Society.

[12]  G. G. Stokes.  "J." , 1890, The New Yale Book of Quotations.

[13]  János G. Ángyán,et al.  Brønsted Acid Sites in HSAPO-34 and Chabazite: An Ab Initio Structural Study , 1998 .

[14]  J. Lercher,et al.  Alkane sorption in molecular sieves: The contribution of ordering, intermolecular interactions, and sorption on Brønsted acid sites , 1997 .

[15]  G. Kramer,et al.  Reactivity Theory of Zeolitic Broensted Acidic Sites , 1995 .

[16]  Free energy from molecular dynamics with multiple constraints , 2000 .

[17]  J. Ángyán,et al.  London dispersion forces by range-separated hybrid density functional with second order perturbational corrections: the case of rare gas complexes. , 2007, The Journal of chemical physics.

[18]  G. Ciccotti,et al.  Constrained reaction coordinate dynamics for the simulation of rare events , 1989 .

[19]  G. Henkelman,et al.  A dimer method for finding saddle points on high dimensional potential surfaces using only first derivatives , 1999 .

[20]  Bruce A. Murtagh,et al.  Computational Experience with Quadratically Convergent Minimisation Methods , 1970, Comput. J..

[21]  J. Engelhardt,et al.  The Activation of CD4 for H/D Exchange Over H-Zeolites , 2002 .

[22]  G. Ciccotti,et al.  Numerical Integration of the Cartesian Equations of Motion of a System with Constraints: Molecular Dynamics of n-Alkanes , 1977 .

[23]  Hoover,et al.  Canonical dynamics: Equilibrium phase-space distributions. , 1985, Physical review. A, General physics.

[24]  A. Bell,et al.  Efficient methods for finding transition states in chemical reactions: comparison of improved dimer method and partitioned rational function optimization method. , 2005, The Journal of chemical physics.

[25]  Xiaobo Zheng,et al.  Reactivity of isobutane on zeolites: a first principles study. , 2006, The journal of physical chemistry. A.

[26]  R. A. Santen,et al.  Carbonium ion formation in zeolite catalysis , 1994 .

[27]  G. Kramer,et al.  Understanding the acid behaviour of zeolites from theory and experiment , 1993, Nature.

[28]  B. M. Fulk.  MATH , 1992 .

[29]  Robert J. Davis,et al.  A density functional theory study of the alkylation of isobutane with butene over phosphotungstic acid , 2006 .

[30]  Hafner,et al.  Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium. , 1994, Physical review. B, Condensed matter.

[31]  Wang,et al.  Accurate and simple analytic representation of the electron-gas correlation energy. , 1992, Physical review. B, Condensed matter.

[32]  J. Lercher,et al.  On the role of the pore size and tortuosity for sorption of aalkanes in molecular sieves , 1997 .

[33]  C. Mota,et al.  Reactivity of Alkanes on Zeolites: A Theoretical ab Initio Study of the H/H Exchange , 1999 .

[34]  J. Valyon,et al.  H/D exchange between CD4 and the OH-groups of H-zeolites and γ-Al2O3 , 2001 .