Figure 6 Adsorption selectivity of MOF-74 and py-MOF-74 predicted by IAST for the binary mixture with the specific molar ratio at 298 K.
3.3 | Selective adsorption of C4 olefine on py-MOF-74
Considering the basic nature of dangling N atom of pyrazine, we envisage that py-MOF-74 with narrow pores could enable selective adsorption of more acidic but smaller olefine instead of paraffine. Accordingly, pure-component isotherms were collected at 298 K to investigate the selective adsorption properties of C4 olefine on py-MOF-74 and their separation potential of C4 olefine from paraffine, processes that are relevant for petrochemical industries. Figure 7a reveals Type-I isotherms of olefine and paraffine on MOF-74 and py-MOF-74. MOF-74 displays a tiny adsorption difference between n-butene and n-butane. As a result of the block effect of pyrazine, the absolute capacity of gas molecules reduces, which is similar with the CO2adsorption result. However, the adsorption difference of py-MOF-74a for n-butene and n-butane becomes obvious. According to the IAST prediction (Figure 7b, Table S5-S7), the optimal ideal adsorption selectivity of n-butene over n-butane appears at py-MOF-74a (Figure 7b), which is distinct from the CO2 selective adsorption results. For the selective adsorption of the larger C4 olefine, on the one hand, py-MOF-74a with the lowest pyrazine loading content and thereby suitable pore space displays affinity to more acidic n-butene over n-butane. On the other hand, the narrowing pore space of py-MOF-74a, compared with the parent framework, could distinguish the molecular difference of n-butene and n-butane (kinetic diameter, 0.446 nm versus 0.469 nm)35, widening the adsorption gaps between these two molecules. Therefore, the adsorption selectivity of n-butene/n-butane increases from 0.7 (MOF-74) to 3.4 (py-MOF-74a).