Strain Development of Selective Adsorption of Hydrocarbons in a Cu-ZSM-5 Crystal.

Abstract

Zeolites are 3D aluminosilicate materials having subnanometer pore channels. The Lewis basic pores have charge-balancing cations, easily tuned to metallic ions as more chemically active sites. Among the ion-exchanged zeolites, Cu2+ ion-exchanged ZSM-5 (Cu-ZSM-5) is one of the most active zeolites with chemical interactions of Lewis basic compounds. Even though the chemical interactions of hydrocarbons with Cu2+ sites in Cu-ZSM-5 have been tremendously studied in the category of zeolite catalysts, it is not yet thoroughly investigated how such interactions affect the structural lattice of the zeolite. Hydrocarbons with different chemical properties and their relative size can induce lattice strain by different chemical adsorption effects on the Cu2+ sites. In this work, we investigate the internal deformation of the Cu-ZSM-5 crystal using Bragg coherent X-ray diffraction imaging during the adsorption of four hydrocarbons depending on the alkyl chain length, the existence of a double bond in the molecule, linear structure versus benzene ring structure, and so forth. In the three-carbon system (propane and propene), relatively weak chemical adsorption occurred at room temperature and 100 °C, whereas strong adsorption was observed over 150 °C. For the six-carbon system (n-hexane and benzene), strong strains evolved in the crystal by active chemical adsorption from 150 °C. The observations suggest that propene and propane adsorb at the Cu2+ sites from the outer shell to the center with increasing temperature. In comparison, n-hexane and benzene adsorb at both parts at the same temperature. The results provide the internal structural information for the lattice with the chemical interactions of hydrocarbons in the Cu-ZSM-5 zeolite and help to understand zeolite-based chemisorption or catalysis research.