4. Conclusions
The adsorption of different small gas molecules allowed to clarify that the addition of even small amounts of nanographite phase, targeting 1, 2 and 6 wt. % in the composites during the synthesis of UiO-66, resulted in a marked (~ 30 %) increase in the porosity. Ultramicropores narrower than 0.5 nm and pores wider than 0.7 nm were formed and they probably have their origin in defective UiO-66 deposited on the nanographite particles. These defects increased markedly the adsorbed amounts of H2, CO2, C2H4 and C2H6 on the composites compared to those on the parent UiO-66. Even though no direct dependence of the porosity on the amount of nanographite in the composites was found when the texture was analyzed by N2 adsorption, the increase in the quanatity of hydrogen adsorbed with an increase in the nanographite content might indicate the existence of ultramicropores only accessible to H2. The isosteric heat of CO2adsorption at zero surface coverage almost linearly increased with the nanographite content, suggesting the strong adsorption of CO2 on the interface. In the case of H2, its amount depended on the amount of nanographite suggesting the extensive effects of the interface or the improved diffusion of H2 to the smallest pores owing to the assembly of UiO-66 particle “walls” on the nanographite components. In the case of ethane and ethylene adsorption, besides an increased porosity of the composite, the interactions of these hydrocarbons with nanographite phase likely contributed to the marked increase in the amount adsorbed on the composite (42%). The trends in the heats of adsorption (higher on pure UiO-66 than on the composite) suggested that these molecules adsorbed in tetrahedral, octahedral, and defects-related pores. In particular, the addition of nanographite appears as an especially interesting strategy to increase H2 adsorption capacity of MOFs.
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