Anharmonicity and Confinement in Zeolites: Structure, Spectroscopy, and Adsorption Free Energy of Ethanol in H-ZSM-5

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 C 2 H 5 OH, C 2 H 5 OD, and C 2 D 5 OH 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.

Theoretical Determination of Size Effects in Zeolite-Catalyzed Alcohol Dehydration

“…Note that the previous calculations on the adsorption enthalpies used the periodic DFT-D method and a cluster model. The adsorption enthalpy of ethanol in H-ZSM-5 calculated in this work with 18 T sites incorporated in the QM region was −23.9 kcal mol −1 , which is in good agreement with Alexopoulos's [49] experimental measurement of −21.3 kcal mol −1 . Our result was also consistent with the series of calculations performed by Van der Mynsbrugge [50] using M062X/6-31+G(d) and PBE/6-31+G(d) (with various dispersion corrections) and a large cluster model (46 T sites) as well as periodic functional calculations of H-ZSM-5.…”

“…They reported values for the adsorption of ethanol on H-ZSM-5 at 400 K ranging from −23 to −34 kcal mol −1 [50]. Alexopoulos et al [49], Expt. = Experimental results; Van der Mynsbrugge et al [50], PBE-D = PBE-Dispersion; Lee et al [51], pbc = periodic boundary conditions; Nguyen et al [52], QHA = quasi-harmonic approximation; HA = harmonic approximation.…”

Structure and solvation of confined water and water–ethanol clusters within microporous Brønsted acids and their effects on ethanol dehydration catalysis

“…In the locally stable ethanol monomer cluster ðEW * 5 Þ, the proton remains solvated for >98% of the simulation time, indicating that larger water clusters more effectively solvate protons away from the zeolite framework. Two ethanol molecules are sufficient to solvate the H + away from the zeolite framework without water, 108 and thus the H + remains solvated nearly 100% of the simulation time across the entire range of water coverages (n ¼ 1-6 H 2 O, m ¼ 2). The H + remains solvated as H 3 O + in (C 2 H 5 OH) 2 (H + )(H 2 O) n species rather than being shared between the two C 2 H 5 OH in a protonated dimer, which we surmise leads to additional free energy penalties to reorganize positive charges and localize them at the ethyl group in S N 2 transition states that form diethyl ether and water, as investigated next.…”

Energetic Effects of a Closed System Approach Including Explicit Proton and Electron Acceptors as Demonstrated by a Mononuclear Ruthenium Water Oxidation Catalyst

“…However, in this trajectory bonds are formed and broken, and therefore one cannot assume that the change in zero‐point energy will be negligible. For this reason the vibrational density of states was obtained from the CPMD trajectory files,, as shown in Figure S5. The zero‐point energy contribution from the changed Ru−O bond, the broken O−H bond, and the formed H + solv /[H 5 O 2 ] + complex would amount to a correction of only −0.04 eV, which is small in comparison to the uncertainty in the change in KS energy.…”