(583ab) First-Principles Calculations of Isomerization of 2-Pentene Over Beta Zeolite

Introduction

Skeletal isomerization of paraffins is one of the most economical and effective methods to increase the octane rating of gasoline. Because of the large pore size and high acidity, beta zeolite was found to have the capability of acting as a good catalyst for isomerization reactions. [1] In pentane isomerization, n-pentane is first adsorbed on the metallic active sites (e.g., Pt) and dehydrogenated to form pentene. Then, pentene is transferred from the metallic sites to the acid sites of zeolites, where pentene is isomerized into isopentene. Finally, isopentene is diffused back to the metallic sites and hydrogenated to yield i-pentane. In the bifunctional mechanism, hydrogenation and dehydrogenation steps are very rapid, and skeletal isomerization is the rate-determining step.

Extensive experimental studies on the reaction mechanism for n-alkane isomerization have been performed [2-4], which indicated that n-pentane and larger hydrocarbons isomerized via a monomolecular mechanism both on zeolites and on sulfated zirconia. In this mechanism, the isomerization reactions might occur along two pathways: the one-step alkyl shift pathway and the “protonated cyclopropane” pathway. However, it is difficult to identify the dominant reaction pathway between the two hypotheses from purely experimental considerations.

Our density functional theory calculations present the adsorption configurations of all possible reactants, products, and intermediates, transition states participating in the skeletal isomerization of 2-pentene over beta zeolite. The reaction heats and activation energies for the elementary steps involved are calculated to construct the potential energy diagram for the overall reaction. According to the energetic span theory, the dominant reaction pathway is eventually identified.

Computational details

Density functional theory calculations have been performed by using the VASP code [5]. Exchange and correlation of the Kohn-Sham theory were treated with the generalized gradient approximation (GGA) in the formulation of PBE functional. The interactions between valence electrons and ion cores were represented by Blöchl’s all-electron-like projector augmented wave method (PAW). A plane wave energy cutoff of 400 eV was used in the present work. The BEA structure was represented as a unit cell with a tetragonal space group of P4122.

Results and discussion

Five possible reaction pathways for 2-pentene isomerization over beta zeolite are examined. It is found that methyl shift and ethyl shift mechanisms have the close effective reaction barriers (1.09 eV and 1.10 eV). The most possible reaction pathways follow the out-of-plane dimethyl-cyclopropane (DMCP) mechanism and the near-in-plane DMCP mechanism. For the out-of-plane DMCP mechanism, the effective activation energy is calculated to be 0.83 eV, a little higher than that for the near-in-plane DMCP mechanism (0.77 eV). Our calculated results compare closely to the previously reported energy barrier of 74 kJ/mol for the DMCP mechanism over ZSM-22. In addition, the ring opening steps in the DMCP mechanisms are much easier than ring-closure steps.

Conclusion

The isomerization of n-pentene via the one-step alkyl shift mechanism is more difficult to occur because of the formation of unstable carbenium ions. The isomerization of n-pentyl is believed to take place following the protonated cyclopropane (PCP) mechanism. The calculated effective reaction barriers for the near-in-plane DMCP mechanism and the out-of-plane DMCP mechanism are 0.77 eV and 0.83 eV, respectively, much lower than the other examined reaction pathways. Hence, the out-of-plane DMCP mechanism and the near-in-plane DMCP mechanism are the dominant reaction pathways for 2-penetene isomerization over beta zeolite.

References

[1] Shetty, S.; Kulkarni, B. S.; Kanhere, D. G.; Goursot, A.; Pal, S. J. Phys. Chem. B 2008, 112, 2573-2579.

[2] Luzgin, M.V.; Stepanov, A. G.; Shmachkova, V. P.; Kotsarenko, N. S. J. Catal. 2001, 203,273-280.

[3] Li, X. B.; Iglesia, E. J. Catal. 2008, 255,134-137.

[4] Miyaji, A.; Echizen, T.; Li, L. S.; Suzuki, T.; Yoshinaga, Y.; Okuhara, T. Catal today. 2002, 74,291-297.

[5] Kresse, G.; Hafner, J. Phys. Rev. B 1993, 48, 13115–13118.

Acknowledgements

This work is supported by Natural Science Foundation of China (No. U1162112, 21003046).