Figure 11. C2H4 adsorption isotherms in a linear (A) and logarithmic scale (B) and C2H6 adsorption isotherm in a linear (C) and logarithmic scale (D) on the UiO-66 composites at -58°C (blue symbols) and -73°C (red symbols) in the pressure range 0−100 kPa
To further explore the effects of composites texture and chemistry on the adsorption of small gas molecules, the adsorption of C2H4 and C2H6 was measured on UiO-66 and on UiO-66-nGr2 (Figure 11). Ethane and ethylene, besides having slightly different kinetic molecular sizes (σ for C2H4 and C2H6 is 0.416 and 0.444 nm, respectively [42]), also differ in chemical properties with the former being more basic due to the presence of a π-bond [32]. Their adsorption was measured at – 58 and – 73°C and the results are summarized in Table 2. In this case, building the composite significantly increased the amounts of both C2H4 and C2H6 adsorbed compared to those on UiO-66, and, on UiO-66-nGr2, ~ 42% more of both gases were adsorbed at 100 kPa. Since the differences in the overall porosity between these two samples are not so pronounced (the porosity of the composite is only about 25 % higher), this difference might be caused by different extents of pore filling processes and in the accessibility of the pores. The affinity of hydrocarbons to adsorb on the graphite/graphite-UiO-66 interface might play a role. The biggest difference in the porosity between UiO-66 and the composite is in pores with sizes > 0.7 nm, and these pores seem to govern the amount of both gases adsorbed.
The comparison of the isosteric heats of C2H4 and C2H6 adsorption is presented in Figure 8B. Q sto for C2H4 is 26.1 and 24.7 kJ/mol on UiO-66 and on the composite, respectively. For C2H6, these values are 28.8 and 27.6 kJ/mol, respectively. The heats are consistent with those reported by Wang and co-workers [25]. The decrease inQ sto with the coverage indicates that pores of various sizes are involved in the adsorption process, especially for ethane, those that are first occupied being the narrowest, for which the adsorption potential is the strongest. Therefore, the slightly higher heats at low surface coverage on UiO-66 might be due to the volume of pores with size 0.5-0.7 nm that is higher in pure UiO-66 than in the composite. Then, with the progress of the adsorption process, the octahedral pores and these related to defects are filled and the composite has higher volume of such pore than has UiO-66.
The addition of nanographite had a positive effect on the gases adsorbed either by an increase in the narrow porosity or by an increase in the specific interactions with the nanographite phase, and could be a strategy to increase the gas adsorption capacity of MOFs. An exceptional hydrogen storage has indeed been achieved with SNU-70, UMCM-9, and PCN-610/NU-100, but analysis of trends revealed the existence of a volumetric ceiling at ∼ 40 g H2L−1[43]. Surpassing this ceiling is proposed as a new capacity target for hydrogen adsorbents. In the same study, it was concluded that usable capacities in the highest-capacity materials are negatively correlated with density and volumetric surface area. Instead, the H2 capacity is maximized by increasing gravimetric surface area and the porosity, which have been enhanced by nanographite addition in the present study.