Isomerization kinetics of small hydrocarbons in confinement
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Isomerization kinetics of small hydrocarbons in confinement Erik E. Santiso · Marco Buongiorno Nardelli · Keith E. Gubbins
Received: 4 May 2007 / Revised: 10 August 2007 / Accepted: 22 October 2007 / Published online: 8 November 2007 © Springer Science+Business Media, LLC 2007
Abstract Chemical reactions are often carried out in nanostructured materials, which can enhance reactions due to their large specific surface area, their interactions with the reacting mixture and confinement effects. In this work, we present a systematic study of the effect that the geometrical restrictions imposed by the pore walls can have on reactions that involve a three dimensional rearrangement of the atoms in a molecule. In particular, we consider the isomerization of three 4-membered hydrocarbons—n-butane, 1-butene and 1,3-butadiene confined in carbon nanopores of slit geometry. Our results illustrate the fact that, in the molecular sieving limit, the reaction rates change as the double exponential of the pore size (Santiso et al., in J. Chem. Phys., 2007a, submitted), and therefore the transition rates in nanopores can be many orders of magnitude different from the corresponding bulk values. These results can be used as a guideline for the molecular-level design of improved catalytic materials.
E.E. Santiso () · K.E. Gubbins Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695, USA e-mail: [email protected]
Keywords Chemical reactions · Confinement · Carbon · Density functional theory · Variational transition state theory Abbreviations A Frequency factor ‡ A Scaled frequency factor, A ≡ A exp(−E∞ /kB T ) a.u. Atomic unit (for length, 1 a.u. = 1 Bohr radius = 0.5291772108 Å) Eov Energy due to the overlap of the electron clouds k Rate constant k∞ Rate constant for a reaction occurring in the bulk phase kB Boltzmann’s constant (1.3806503 × 10−23 J/K) P Overlap energy parameter P Scaled overlap energy parameter, P ≡ P /kB T r Pore width R2 Pearson’s correlation coefficient squared T Absolute temperature (K) E ‡ Activation energy barrier ‡ E∞ Activation energy barrier for a reaction occurring in the bulk phase ρ Molecular size parameter
1 Introduction M. Buongiorno Nardelli Department of Physics, North Carolina State University, Raleigh, NC 2769, USA M. Buongiorno Nardelli CCS-CSM, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA E.E. Santiso · M. Buongiorno Nardelli · K.E. Gubbins Center for High-Performance Simulation (CHiPS), North Carolina State University, Raleigh, NC 27695, USA
Chemical reactions are often carried out in nano-structured materials, which can enhance reaction rates and equilibrium yields through many different effects. Some of these effects include an increase in the contact area between the catalyst and the reactive mixture, additional catalytic effects due to interactions with the support, selective adsorption of the reactants and/or products, and geometric constraints, among others. An understanding of the individual role of each one
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