Abstract
We investigated the hydrogen-adsorption properties of four zeolites (FAU-, MFI-, *BEA- and MOR-type zeolites) with various Si/Al ratios and charge cations (H+, Li+ or Na+), focusing particularly on the measurements of isosteric heat of adsorption (Qst). The profiles of Qst describe the heterogeneity of the zeolite surface. Zeolites with narrower pore channel (2.6 ∞ 5.7 A) exhibit higher initial Qst values than zeolites with wide pore channel (7.4 ∞ 7.4 A), because of the strong dispersion interaction between hydrogen molecules and the surface of the pore channel. Li-exchanged zeolites exhibit high initial Qst values; however, the 6-membered ring zeolite frameworks adsorb less hydrogen at 77 K and 0.1 MPa than Na-exchanged zeolites, because Li cations, unlike larger Na cations, can penetrate the ring, and thus become less accessible to hydrogen molecules. The enhancement of hydrogen uptake for Li-exchanged zeolites correlates with the amount of Li+.
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