Influence of temperature on CO2 separation using fluorescent Zn(II)-based MOF

https://doi.org/10.3390/molecules27123845

“We then investigated the sorption capacity of CO₂ and CH₄ gases onto QUF-001 as depicted in Figure 6. High-pressure experiments were performed at 298 and 318 K isotherms. A complete adsorption–desorption cycle passed through stepwise pressure increases and decreases with each adsorbate was carried out from vacuum to 50 bars and back to vacuum at the end of the measurements to observe the hysteresis behavior. At each isotherm, there were a total of 12 adsorption and 8 desorption data points collected for QUF-001, which are presented in Figures S6–S9. At first glance, all of those curves demonstrated a smooth increasing trend with increasing pressure. However, adsorption and desorption data points showed almost the same values, which showed that there was no hysteresis and no significant changes occurred in the samples during the overall pressure loop. We noticed that in order to obtain reliable data, peripheral conditions such as humidity, ambient pressure, and temperature had to be considered [7]. To prevent potential irreversible structure collapse and reduction of the surface area as well as pore volume due to moisture within the measurement chamber, samples were degassed. Furthermore, a Drierite column was used to pre-dry the gases. A similar sorption–desorption overlapped trend of variation was observed while studying a (NH₄)₂Mg(H₂P₂O₇)₂•2H₂O) single-crystal sample with CO₂ and CH₄ at isotherms 298 and 318 K [62]. Figures S9 and S10 confirm the expected thermodynamic trend of variation, namely that the sorptivity of both used gases decreased upon increasing the temperature and increased upon increasing the pressure. However, all of the isotherms with CO₂ and CH₄ with QUF-001 were completely reversible and the absence of hysteresis confirmed the advantages of reusability and cost efficiency of this MOF under primary vacuum [3,7,63]. Sorption results showed that CO₂ (1.6413 mmol/g) sorption was significantly lower than that for CH₄ (8.0907 mmol/g) at temperature 298 K and pressure of 50 bars, which meant that QUF-001 had higher affinity to capture CH₄ as compared to CO₂. Clear evidence of QUF-001 CH₄ sorption selectivity over CO₂ gas at each temperature and pressure showed that the use of this MOF may be beneficial for the chemical and petroleum industries in terms of CH₄ separation from CH₄/CO₂ mixtures [7]. From Figure 6, at temperature 298 K, between 45 to 50 bar pressure, the sorption curve seems flattened. Typically, MOFs, covalent organic frameworks and covalent organic polymers follow type IV adsorption isotherms, showing finite multi-layer adsorption corresponding to complete filling of the capillaries and pores [64]. The adsorption isotherm profiles (Figure 6) rather fell within the type III behavior, indicating weak substrates and the formation of multilayers. Here, there was no flattish region in the curve assuming lack of a monolayer. In one of our studies, the CO₂ sorption capacities of Rb₂Co(H₂P₂O₇)₂•2H₂O were higher than those of the currently investigated sample, but CH₄ sorption efficiency was 3.5-fold better in QUF-001. Although there was no clear superiority among either CO₂, CH₄ or other gases for MOF structures, the trend in ranking the sorption performance of such gases showed that the CO₂ capture performance of MOFs was higher than that for methane [65,66,67]. We also showed a similar trend through gas sorption demonstrations on MOF-5 previously [68,69]. Moreover, on comparing the CO₂ sorption data of this work with hydroxy metal carbonates M(CO₃)_x(OH)_y (M = Zn, Zn-Mg, Mg, Mg-Cu, Cu, Ni, and Pb) [70], Rb₂Co(H₂P₂O₇)₂•2H₂O showed higher values at 35 bar and 318 K, although the hydroxy metal carbonates were measured at 316 K.”

“Figure 6. Plots of the CO₂ (violet squares) and CH₄ gas (green cycles) adsorption behavior of QUF-001and the uptake of these two gases as a function of the pressure, at two different temperatures of 298 K (a) and 318 K (b).”