Abstract
To account for thermal and entropic effects caused by the dynamics of the motion of the reaction intermediates, ethanol adsorption on the Brønsted acid site of the H-ZSM-5 catalyst has been studied at different temperatures and ethanol loadings using ab initio molecular dynamics (AIMD) simulations, infrared (IR) spectroscopy, and calorimetric measurements. At low temperatures (T ≤ 400 K) and ethanol loading, a single ethanol molecule adsorbed in H-ZSM-5 forms a Zundel-like structure where the proton is equally shared between the oxygen of the zeolite and the oxygen of the alcohol. At higher ethanol loading, a second ethanol molecule helps to stabilize the protonated ethanol at all temperatures by acting as a solvating agent. The vibrational density of states (VDOS), as calculated from the AIMD simulations, are in excellent agreement with measured IR spectra for C2H5OH, C2H5OD, and C2D5OH isotopomers and support the existence of both monomers and dimers. A quasi-harmonic approximation (QHA), applied to the VDOS obtained from the AIMD simulations, provides estimates of adsorption free energy within ∼10 kJ/mol of the experimentally determined quantities, whereas the traditional approach, employing harmonic frequencies from a single ground state minimum, strongly overestimates the adsorption free energy by at least 20∼50 kJ/mol. This discrepancy is traced back to the inability of the harmonic approximation to represent the contributions to the vibrational motions of the ethanol molecule upon confinement in the zeolite.
Original language | English |
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Pages (from-to) | 7172-7182 |
Number of pages | 11 |
Journal | Journal of Physical Chemistry C |
Volume | 120 |
Issue number | 13 |
DOIs | |
State | Published - Apr 21 2016 |
Externally published | Yes |
Funding
This article is dedicated to our colleague Dr. Bruce Garrett on the occasion of his 65th birthday. He has achieved significant impact in his career with his contributions to our understanding of how dynamical processes and anharmonicity contribute to rate theories as well as served as a dedicated steward of PNNL's fundamental science programs over the past decade. K.A., M.F.R., and G.B.M. were supported by the Long Term Structural Methusalem Funding by the Flemish Government, grant no. BOF09/01M00409. M.S.L., V.A.G., R.R., and J.A.L. were supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. PNNL is a multiprogram national laboratory operated for DOE by Battelle. Computational resources were provided at W. R. Wiley Environmental Molecular Science Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research located at PNNL, the National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory and the Stevin Supercomputer Infrastructure at Ghent University.