Breakthrough separation
Furthermore, we performed actual breakthrough experiments on Zn2(bpy)(btec) to establish the feasibility of C2H2/C2H4and C2H2/CO2separations, in which C2H2/C2H4(v/v, 1/99) and C2H2/CO2(v/v, 50/50) mixtures were used as feeds to mimic the industrial process conditions (Figure 4 and S6). As shown in Figure 4a, c, only after several minutes, C2H4 or CO2 first eluted through the bed to yield a polymer-grade gas and the purity of C2H4monitored at the outlet was >99.9999%. Then, after a long period of time (130 min for C2H2/C2H4, 50 min for C2H2/CO2), C2H2 broke through from the adsorption bed. During this process, C2H2 was clearly captured by Zn2(bpy)(btec), with the concentration of the impurities decreased to lower than 1 ppm for C2H4 and 10 ppm for CO2. Polymer-grade C2H4 and high purity of CO2 were directly collected at the outlet. From the desorption curves in Figure 4b, d, it can be found that adsorbates were fully desorbed from the materials in 20 minutes. Due to the high C2H2/C2H4and C2H2/CO2 uptake ratios and selectivities, a high purity of C2H2 over 98.5 % (b) or 92.5 % (d) were generated, respectively.
The kinetic ad/desorption rate of C2H2was also investigated in Figure S8 and S9, showing that Zn2(bpy)(btec) has a high kinetic ad/desorption rate, and therefore that C2H2 can be saturated or fully desorbed in about ten minutes. In addition, the desorption and recycling measurements revealed that this material can maintain its C2H2 capture ability and high selectivity in several repeated adsorption and separation cycles (Figure 4b, d, and S7). The breakthrough time remains almost unchanged during five cyclic breakthrough experiments, confirming the good recyclability of this material for the C2H2/C2H4and C2H2/CO2separation.
Generally, in the real production process, the feed gas also contains some other components, which is a tremendous challenge in the recovery of C2H2 from such refinery gas. Therefore, we investigated the single-component adsorption isotherms of some main components of refinery gas (CH4, C2H2, C2H4, C2H6, C3H4, C3H6 and C3H8) and common gas components (CO2, H2, N2 and O2) as seen in Figure 5 a, b. Based on the well-matched pore cavity of Zn2(bpy)(btec), this material selectively adsorbs C2H2 and blocks most other molecules to achieve an admirable sieving effect (Fig. 4a). The uptakes of most of the gases were below 10 cm3cm−3 at 1 bar and 298 K. Furthermore, the gas mixtures separation process was carried out on this molecular sieve material for the recovery of C2H2 from the simulated steam cracking mixtures. As can be seen from the breakthrough curves, most of the mixture gases eluted through the fixed bed quickly and a trace amount of C2H2 was totally adsorbed during the long retention time. In the generation process, the adsorbent was fully desorbed in 20 minutes and obtained high purity C2H2 (> 98 %). The multi-component separation and desorption tests show strong evidence of the efficient C2H2 separation ability of Zn2(bpy)(btec) materials, which has initiated the promise of MOF materials for this very important industrial application.