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Theoretical Study of the Adsorption of the Butanol Isomers in H-ZSM-5

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journal contribution
posted on 05.05.2011, 00:00 by Cuong M. Nguyen, Marie-Françoise Reyniers, Guy B. Marin
The adsorption of the four butanol isomers (1-BuOH, i-BuOH, 2-BuOH, and t-BuOH) at the Al12−O24(H)−Si12 intersection site in H-ZSM-5 is investigated using the periodic [DFT-D] approach in which a damped interatomic potential (D) is included within the periodic density functional theory (pbcDFT) calculations to account for dispersive van der Waals (vdW) interactions. Both the pbcDFT and pbc[DFT-D] methods yield largely similar results for the geometry of adsorbed complexes. The pbc[DFT-D] adsorption energies of the butanols are similar to those obtained using the pbcDFT with an add-on single-point dispersion-energy correction (pbcDFT + D). The adsorption strength of butanols decreases in the following order: 1-BuOH (−160 to −164 kJ mol−1) > 2-BuOH (−155 to −157 kJ mol−1) > t-BuOH (−151 kJ mol−1) > i-BuOH (−147 kJ mol−1). On the basis of a partial Hessian vibration analysis of the most stable physisorbed and chemisorbed complexes, reported experimental infrared (IR) spectra could be interpreted by assuming an equilibrium between physisorbed and chemisorbed minima on a flat potential energy surface. The extent to which the zeolite proton is transferred upon physisorption of the butanols increases from 1-BuOH to t-BuOH in agreement with an increase of the proton affinity of the alcohols. At the straight channel, the hydrogen bond strength ranges from −73 to −88 kJ mol−1 while at the zigzag channel, the hydrogen bond strength for 2-BuOH (−91 kJ mol−1) is somewhat larger as compared to 1- and i-BuOH (−68 to −72 kJ mol−1). Upon chemisorption, the four butoxonium ions experience similar Coulomb and hydrogen bonding interactions. Steric constraints and vdW interactions exerted by the zeolite frame are found to be the dominant factors in governing the adsorption strength of butanols in H-ZSM-5. At the straight channel, the steric constraints increase slightly by +15 kJ mol−1 in going from 1-BuOH to t-BuOH while in the zigzag channel a remarkably more pronounced increase of +63 kJ mol−1 is observed. Neither for physisorption nor for chemisorption of i-BuOH and t-BuOH, there is an energetic preference for one of the channels because in both channels vdW interactions counterbalance steric constraints. In contrast, for 1-BuOH and 2-BuOH vdW interactions prevail leading to an energetic preference of some 11−14 kJ mol−1 for both physisorption and chemisorption in the zigzag channel. Accounting for the formation enthalpy of the gas-phase butanols, the stability of the adsorbed complexes of t-BuOH and 2-BuOH is predicted to be some 15−20 kJ mol−1 higher than those of 1-BuOH and i-BuOH.