Korean Chemical Engineering Research

The Korean Institute of Chemical Engineers (한국화학공학회)
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A Computational Study on the Adsorption Characteristics of Hydrocarbons (Propylene, n-Butane and Toluene) by uing Cation-exchanged ZSM-5 Zeolites

  • Lee, Hyun Chul (Department of Chemical Engineering, Pukyong National University) ;
  • Kim, Kyung Min (Department of Chemical Engineering, Pukyong National University) ;
  • Choi, Sung Il (Department of Chemical Engineering, Pukyong National University) ;
  • Kim, Yong Ha (Department of Chemical Engineering, Pukyong National University) ;
  • Woo, Hee Chul (Department of Chemical Engineering, Pukyong National University) ;
  • Won, Yong Sun (Department of Chemical Engineering, Pukyong National University)
  • Received : 2018.09.06
  • Accepted : 2018.10.30
  • Published : 2018.12.01
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Abstract

A hydrocarbon trap (HT) plays an important role of controlling vehicle emissions in the so-called cold emission period by holding hydrocarbons until three way catalysts (TWCs) are thermally activated. In this study, we have investigated the adsorption characteristics of cation (H, La, K, and Ag)-exchanged ZSM-5 zeolites for hydrocarbons (propylene, n-butane, and toluene) by DFT (density functional theory)-based computational chemistry. Cation exchange is to improve the hydrothermal stability of zeolites and their adsorption capacity, thereby rendering cation-exchanged zeolites promising materials for HT. The idea of cluster approximation makes the calculation of adsorption energies superbly efficient in computation. The results showed that Ag-exchanged ZSM-5 would be the best for the adsorption of all three adsorbates, without often encountered Ag oxidation in experiments. Besides, the hydrothermal stability of La-exchanged ZSM-5 was confirmed from the change of geometrical parameters by cation exchange, and it showed good adsorption capacity for propylene and toluene. Hydrogen-exchanged ZSM-5 was also good for hydrogen adsorption, but had poor hydrothermal stability.

Keywords

HHGHHL_2018_v56n6_909_f0001.png 이미지

Fig. 1. The largest pore of the 3-dimenstional ZSM-5 structure (left) and the extracted highly symmetric cluster representing the largest pore (right).

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Fig. 2. Geometrically optimized cation-exchanged clusters; (a) H-ZSM-5, (b) La-ZSM-5, (c) K-ZSM-5, and (d) Ag-ZSM-5.

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Fig. 3. Geometrical parameters of cation-exchanged zeolites based on geometrically optimized cluster structures.

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Fig. 4. The binding energy calculation for the toluene adsorption on H-ZSM-5. Gray and white balls in toluene denote carbons and hydrogens, respectively.

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Fig. 5. Geometrically optimized molecular structures of adsorption complexes; (a) propylene on H-ZSM-5, (b) toluene on H-ZSM-5, (c) propylene on La-ZSM-5, and (d) toluene on La-ZSM-5. Gray and white balls denote carbons and hydrogens, respectively.

Table 1.Comparison of calculated adsorption energies between adsorbates and cation-exchanged zeolites. The energies are in the unit of kcal/mol

HHGHHL_2018_v56n6_909_t0001.png 이미지

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