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CO2 capture using a single-ligand ultra-microporous MOF

DOI: 10.1126/sciadv.150042

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Fig. 2 Experimental and simulated adsorption isotherms and HOA plots.

(A) Experimental H2 and N2 isotherms. (B) CO2 adsorption isotherms carried out on 1 at different temperatures (filled circles, adsorption; open circles, desorption). For CO2 at 195 K, the simulated adsorption isotherm is shown. (C) HOA for CO2 in 1 as a function of the CO2 loading determined from a virial fit to isotherms collected at temperatures ranging from −25° to 30°C. HOA determined from GCMC simulations at 25°C are also shown. (D) Experimental and simulated gas adsorption isotherms for CO2 at 298 K (0 to 10 bar).”
“The N2 adsorption isotherm at 77 K is given in Fig. 2A and yields a Brunauer, Emmett, and Teller (BET) surface area of 945 m2/g. Despite the modest surface area, 195-K uptake is notably higher than that of most other ultra-microporous MOFs (table S1). Figure 2B depicts the CO2 adsorption isotherms over a range of temperatures with a total uptake of 11, 5.5, and 3.6 mmol/g at 195, 273, and 303 K, at 1 bar, respectively. The density functional theory (DFT) model of the 195-K CO2 adsorption branch indicated a bimodal pore distribution in the ultra-microporous regime (3.5 and 4.8 Å, fig. S11). A positron annihilation lifetime spectroscopy (PALS) measurement was also carried out to establish the ultra-microporous character of 1 (48). The PALS was recorded on a MeOH exchanged sample that was activated at 100°C for 24 hours (Supplementary Materials). The spectra were least-squares fit with the program PosFit (fig. S12) (49), using three lifetime components, as shown in table S3. The fit to the 1.2- to 1.4-ns component of the ortho-positronium (o-Ps) annihilation yielded a spherical pore size of 3.9 to 4.4 Å, which is fairly consistent with the values obtained from the single-crystal structure of 1 and from the 195-K CO2 data.”

The CO2 HOA in 1 were determined via both virial fits and a DFT model using isotherms collected at −25°, −10°, 0°, +10°, and +30°C. The virial fit presented in Fig. 2C shows that 1 has the zero-loading HOA value of 34 kJ/mol and this falls down to a value of 26 at ~2 mmol/g loading and settles down at a moderate 28 kJ/mol at higher loadings. Both models showed a similar trend (fig. S13).
For an ultra-microporous material, 1 has an exceptionally high CO2 saturation capacity of 10.8 mmol/g (195 K), which suggested that it may also have a high CO2 uptake capacity at high pressure and high temperature—conditions relevant to precombustion H2 purification. The adsorption pressure of PSA systems used in precombustion CO2 capture is typically 5 to 40 bar and occurs at elevated temperatures usually around 40°C (1223). To explore this, we first performed grand canonical Monte Carlo (GCMC) simulations (see the Supplementary Materials for details), which predicted a high uptake capacity of 8.2 mmol/g at 10 bar and 298 K. Because the simulated and experimental HOA and 195-K CO2 adsorption isotherms were in good agreement with one another (Fig. 2, B and C), this inspired us to measure the high-pressure CO2 adsorption. Figure 2D reveals that the simulated and experimental CO2 adsorption between 1 and 10 bar and 298 K are in excellent agreement. In addition, the high-pressure H2 adsorption revealed that 1 did not show any appreciable H2 uptake even at 35 bar (fig. S16).
For an ultra-microporous material, the exceptional CO2 uptake capacity of 1 near the saturation limits demands a molecular-level investigation of the adsorption sites to understand how 1 can accommodate such a large amount of CO2. To study this, we examined the nature and location of the binding sites within Ni-4PyC via simulation. The GCMC simulations that are used to generate the adsorption isotherms also yield probability distributions of the guest molecules that can be used to locate the binding sites. We have performed a similar analysis on a ZnAtzOx MOF and found there to be excellent agreement between the computed CO2 binding sites and those determined from crystallography (32). The low temperature saturation limit of 10.8 mmol/g determined experimentally corresponds to approximately 28 CO2 molecules per unit cell. Figure 3 shows the location of the strongest 30 binding sites, with binding energies ranging from −24.0 to −32.8 kJ/mol. These were calculated by geometry optimizing a single CO2 molecule in the empty MOF, starting from the CO2 position identified from the maxima of the probability distributions.”

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