Single-gas sorption isotherm
Pure component equilibrium adsorption isotherms for C2H2, C2H4 and CO2 were measured at 298 K up to 1 bar, as presented in Figure 3a. The C2H2 uptake on Zn2(bpy)(btec) reached 93.5 cm3cm−3 at 298 K and 1 bar, significantly higher than that of C2H4 and CO2 at the same conditions. Notably, through size sieving by the appropriate aperture, Zn2(bpy)(btec) presents the C2H2 uptake of 93.5 cm3 cm−3, very low C2H4 uptake of 9.1 cm3cm−3 and low CO2 uptake of 28.9 cm3 cm−3 (298 K and 1 bar), giving an excellent uptake ratio of C2H2 over C2H4 (10.31) and CO2(3.23), which are the highest values among the indicated MOFs (Supporting information, Table S2 and S3).
To compare the separation properties of Zn2(bpy)(btec) with other top-performing MOFs, its ideal adsorbed solution theory (IAST) selectivity of C2H2 over C2H4 and CO2 were calculated on their single-component isotherms (Figure 3a). The gas mixtures were selected as C2H2/C2H4(1:99, v/v) and C2H2/CO2(50:50, v/v) at a total gas pressure of 1 bar and 298 K, to mimic the composition of the industrial purification process. As seen in Figure 3b, c, Zn2(bpy)(btec) exhibits an extraordinarily high selectivity of over 107.8 for the C2H2/C2H4mixture and 33.3 for the C2H2/CO2 mixture, notably higher than the previous benchmark SIFSIX-2-Cu-i (44.54)14, M’MOF-3a (24.03)30, CPL-1 (26.8)31 and ELM-12 (14.8)32 for C2H2/C2H4separation, and FeNi-M’MOF (24)33 HOF-3a (21)34, UTSA-74 (9)35, TIFSIX-2-Cu-i (6.5)36 and for C2H2/CO2 separation.
In industrial applications, energy cost in the regeneration process is also an unavoidable problem and should be taken into consideration. The interactions between the adsorbents and C2H2 are evaluated by calculating the isosteric heats of adsorption (Qst), which is measured from the single component isotherms at different temperatures (Figure 3d). The calculated Qst of Zn2(bpy)(btec) at zero coverage for C2H2 is 28.7 kJ/mol (Figure S4), which is much lower than those values reported in other MOFs with open metal sites such as MOF-74 series (47 kJ/mol for Fe-MOF-74).12 Thus, the molecular sieving based adsorption not only avoided excessive temperature fluctuations during the adsorption process but also decreased the cost and energy requirement in the regeneration process, which indicates advantages of Zn2(bpy)(btec) in real industrial applications. In addition, comparing with other top-performing materials (Table S2, S3), Zn2(bpy)(btec) exhibits the lowest total C2H4 and CO2 loading in normal conditions (Figure 3e), and less co-adsorption will lead to a higher C2H2purity in the generation process. Besides, this material exhibits the highest C2H2/C2H4uptake ratio (10.31) and C2H2/CO2 uptake ratio (3.23), as well as extraordinarily high selectivity (107.8, 33.3) (Figure 3f), which makes it one of the most promising materials for the efficient C2H2 separation from C2H4 or CO2 mixtures.