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).