Hierarchically organized zeolites are materials retaining the crystalline order and associated functionality of bulk zeolites while also integrating a multilevel pore network. Here, the authors review the raft of techniques applied to characterize their crystal, pore and active site structures.
The adsorption of ammonia on HKUST-1 (a metal-organic framework, MOF) and HKUST-1/graphite oxide (GO) composites was investigated in two different experimental conditions. From the isotherms, the isosteric heats of adsorption were calculated from the Clausius-Clapeyron equation following the virial approach. The results on HKUST-1 were compared with those obtained using molecular simulation studies. All materials exhibit higher ammonia adsorption capacities than those reported in the literature. The ammonia adsorption on the composites is higher than that measured separately on the MOF component and on GO. The strong adsorption of ammonia caused by chemical interactions on different adsorption sites is evidenced by the trends in the isosteric heats of adsorption. The molecular simulations conducted on HKUST-1 support the trends observed experimentally. In particular, the strong chemisorption of ammonia on the metallic centers of HKUST-1 is confirmed. Nevertheless, higher adsorption capacities are predicted compared with the experimental results. This discrepancy is mainly assigned to the partial collapse of the MOF structure upon exposure to ammonia, which is not accounted for in the simulation study.
In this study, biochars obtained by either direct carbonization or hydrothermal treatment of corn stems were used as precursors of two series of activated carbons (ACs). The pore size distributions (PSDs) of biochars and ACs were evaluated from N 2 and CO 2 adsorption isotherms using models based on the two-dimensional version of the non-local density functional theory (2D-NLDFT). We showed that more detailed carbon PSDs might be obtained from simultaneous (dual) gas analysis of both N 2 and CO 2 isotherms than from single isotherms. The dual gas method showed a peak at about 0.4 nm which was not detected by the single N 2 analysis. This fact is related to the restricted diffusion of N 2 into very narrow micropores at low temperatures and pressures. By modifying the lower pore width limit w min in the nitrogen model, we obtained an excellent fit of the dual model to both isotherms for all studied samples, which demonstrates the reliability and robustness of this refined method. Finally, we demonstrated that this approach would allow measuring N 2 isotherms at relative pressures starting at about 0.001 rather than at 10-6 , which would save time without losing resolution of the calculated PSD.
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