Extra Unsaturated Metal Centers of Zirconium‐Based MOFs: a Facile Approach towards Increasing CO₂ Uptake Capacity at Low Pressure

Abstract

Developing adsorption materials to capture CO 2 at low pressure has attracted great attention due to the low CO 2 partial pressure in fuel gases. Herein, zirconium-based metalorganic frameworks (PCN-X) are successfully synthesized and open metal sites (Fe 3+ and Al 3+ ) are incorporated into the PCN-X MOFs. The results indicate that the CO 2 adsorption capability is enhanced through the strong interaction between extra open metal centers and CO 2 molecules at low pressure. Meanwhile, [a] CO 2 Adsorption Experiments and Kinetics Measurements: The CO 2 adsorption equilibrium data were obtained with a BElSORPmax instrument (MicrotracBEL, Japan) through a static volumetric method under low pressure (0-1.0 bar). Temperatures were set as three fixed values (298 K, 313 K, and 328 K). Adsorbent samples (about 0.1 g each) were used for tests; for pretreatment, samples were placed in vacuo at 150°C for 8 h. CO 2 , with 99.999 % purity, from Nanjing Special Gases Company, was used as received. The CO 2 adsorption kinetic data were recorded with an intelligent gravimetric analyzer (IGA-100, Hiden) at 298 K and 1 bar. Adsorbent samples (about 0.02 g each), which were pretreated at 150°C for 4 h, were used for pure CO 2 adsorption. The mass increase of the samples was recorded over time, until adsorption equilibrium was reached, for the calculation of the kinetic rate constant.

Trends in Solid Adsorbent Materials Development for CO2 Capture

“…To increase CO 2 uptake on Zr-MOFs under post-combustion conditions, Li et al (2018) studied the structure and performance of porphyrinic Zr-MOFs (known as PCN-X) by incorporating extra open metal sites of Fe 3+ and Al 3+ . 150 The addition of the extra sites led to 22.1% and 112.2% increases of CO 2 adsorption compared to the pristine PCN-X at 1 bar and 298 K. Although PCN-X-100%-Fe had lower surface area compared to the pristine PCN-X (S BET = 441 vs 955 m 2 /g), higher CO 2 adsorption was observed, indicating that higher surface area did not guarantee higher adsorption capacity. Extra open metal sites provided additional interactions between host and guest molecules and increased the CO 2 adsorption capacity in PCN-X-100%-Al to ∼1.75 mmol/g @ 298 K and 1 bar (see Table 14).…”

“…The CO 2 adsorption capacity of VPI-100­(Cu) at 296 K and 1 atm was 3.76 wt% (19.15 cm 3 /g at STP, 0.85 mmol/g), while that for VPI-100­(Ni) was 2.70 wt% (13.79 cm 3 /g at STP, 0.62 mmol/g). To increase CO 2 uptake on Zr-MOFs under post-combustion conditions, Li et al (2018) studied the structure and performance of porphyrinic Zr-MOFs (known as PCN-X) by incorporating extra open metal sites of Fe 3+ and Al 3+ . The addition of the extra sites led to 22.1% and 112.2% increases of CO 2 adsorption compared to the pristine PCN-X at 1 bar and 298 K. Although PCN-X-100%-Fe had lower surface area compared to the pristine PCN-X ( S BET = 441 vs 955 m 2 /g), higher CO 2 adsorption was observed, indicating that higher surface area did not guarantee higher adsorption capacity.…”

The Role of Water in Carbon Dioxide Adsorption in Porphyrinic Metal-Organic Frameworks

“…The observed CO2 adsorption capacities of 0.6 mmol g −1 and 1.0 mmol g −1 at p/p0(CO2) = 0.75 for PCN-222 and PCN-223, respectively, were in line with values reported in literature (Figure 2b, see ESI for further explanation) and the fact that PCN-222 hosts half the microporous CO2 sorption sites compared to PCN-223. 5,[23][24][25] For the non-porous, free linker, we retrieved a sorption capacity of 0.1 mmol g −1 at p/p0(CO2) = 0.75, indicating that porosity and/or interaction with Zr clusters is necessary to adsorb CO2 in the studied PCN-MOFs. Accordingly, a 30 cm −1 red-shift of the Zr-OH band at 3673 cm −1 (Figure 2c) was observed during exposure to CO2, which we attribute to CO2•••HO-Zr interactions.…”