Studies on CO2 sorption behavior and structural flexibilities of various metal-organic frameworks

Due to its flexibility, the structure of 1 was changed to 1 with narrow and closed pores by elimination of guest molecules. Interestingly, the structural transition of 1 upon CO uptake at 195 K occurred in two distinct steps, gating and respiration, independently and was confirmed by in situ X-ray powder diffraction (XRPD) experiment and isotherms of gas absorption. It was revealed by single-crystal X-ray diffraction and XRPD results that 2-as and 3-as have 2D layered structures, the layers stacked endlessly with guest molecules intercalated between the layers.

On the other hand, the dried compounds of 3 (3) had the same XRPD pattern of 3. The similar but not identical XRPD patterns suggested that the dried structure 2 had the same interlayer distance and the same intralayer structure as 3 and 3 , while its layer stacking mode remained the same as that of 2 . Single-crystal X-ray diffraction and XRPD results revealed that 2-as and 3-as have identical structures.

Schematic representation of light-responsive and guest-induced breathing of [Zn2(bdc)2(dabco)]⊃AB and the change in its gas sorption behavior before/after UV light irradiation.

Introduction

Flexible metal-organic frameworks

• Breathing
• Gate-opening

Kitagawa's group focused on why the adsorption only occurred after a certain pressure, so they studied the kinetics of the gate opening process. The hypothesis suggested that the adsorption of gas occurs by the diffusion process with a double exponential (DE) model (Equation 1) and that the gas molecules are not adsorbed before the gate opening. Thus, the adsorption process occurred in two steps near or after the gate pressure, and this is called the gate opening (GO) model (Equation 2).

In addition to finding a kinetic model of gating, this research revealed the dependence of the gating process on the boiling point of guest molecules due to increased intermolecular interaction. However, the rotational parts should change the void space of the framework from non-porous to porous reversibly after changing their conformation to demonstrate the gate-opening process. Thus, the choice of functional groups on the rotation groups is very important for the design of gate-opening MOFs.

The framework, [Cd2(pzdc)2L], showed the gate opening behavior on water vapor uptake in a three-step adsorption isotherm and a two-step desorption profile.

Figure 1.2. Schematic representation of structural transformation of MIL-53 (Cr).

Amine-functionalized metal-organic frameworks …

Therefore, many researchers have focused on solid adsorbents, whose heat capacity is obviously smaller than solution.1 Solid adsorbents are classified as physisorbents and chemisorbents. Considering the increased density of mmen-Mg2(dobpdc) due to mmen functionalization, it showed comparable capacity to Mg2(dobpdc) (Table 1.1). 𝐶 (Eq. 5) Using the Clausius-Clapeyron equation (Eq. 5) and the isotherms fitted via the previous equations, the isosteric heat of mmen-Mg2(dobpdc) was calculated.

The reaction between amine and CO2 is the same as the usual mechanism, but the product after taking up the first CO2 molecules is not the most stable form. The strong interaction between CO2 and amines calculated from the proposed mechanism showed that mmen-Mg2(dobpdc) adsorbs CO2 with high selectivity over N2, and experimental data from Long's group proved this by calculating the selectivity of CO2.30 Since the isotherm mmen- Mg2 (dobpdc) could not be modeled by a single equation over the entire pressure range, ideal adsorbed soln. theory (IAST) could not calculate the mmen-Mg2(dobpdc) selectivity. Despite the ultradilute CO2 concentration, en-Mg2(dobpdc) adsorbed a significant amount of CO2. 2.83 mmol/g), which exceeds the values ​​of well-performing adsorbents such as the PEI/Zr-SBA-15 series (PEI.

Adsorption-desorption cycle of CO2 for en-Mg2(dobpdc) with (a) simulated air, (b) simulated flue gas. c) Time-dependent CO2 adsorption for porous materials.

Figure 1.20. Trend graph in the level of atmospheric CO 2 and in annual average temperature

Experimental Section

A yellow solid was then formed, which was filtered off, washed with MeOH and dried under vacuum. An orange-yellow crystalline product is then obtained, which is filtered, washed with MeOH and dried under vacuum. The product obtained as a yellow precipitate was filtered, washed with MeOH and dried under vacuum.

The synthesized compounds 2-as were heated in round bottom flasks at 100 oC under vacuum for 7 hours. The synthesized compounds 3-as were heated in round bottom flasks at 100 oC under vacuum for 7 hours. The synthesized compounds 4-as were heated in round bottom flasks at 90 oC under vacuum for 7 hours.

The as-synthesized compounds 5-as were heated under vacuum in round-bottom flasks at 120 oC for 6 hours. For 2 and 3, crystals of 2-as and 3-as were loaded into gas sorption equipment after filtration and the degassing process was carried out under vacuum at 100 oC until all guest molecules were removed (ca. 7 h). For 4 and 5, the filtered crystals were added to the gas sorption apparatus and the samples were heated under vacuum at 90 oC for 7 hours.

To prepare the sample for high-pressure gas sorption measurements, 1-MeCN crystals were filtered, loaded into a sample holder, and evacuated at 110 oC for 4 hours under vacuum. Therefore, the crystals were finely ground in a mother liquor and filled into a capillary (diameter, 0.3 mm; wall thickness, 0.01 mm), and then the solvent was carefully removed by slow evacuation under vacuum at room temperature. To prepare sample 4, the filtered crystals of 4-as were dried at 90 oC in a vacuum for 7 hours, finely ground and packed in a capillary under an Ar atmosphere in a glove compartment (diameter 0.3 mm; wall thickness 0.01 mm).

Then the capillary was evacuated at 383 K until sample was completely dried under vacuum (approx. 15 min.). After activation, the sample was cooled to the measurement temperature using a cryocurrent, 195 K and 298 K respectively, under vacuum.

Results and Discussion

The change in angle (i) affects b-axis directly, and others, angle (ii) and (iii), give little effect to the extension in b-axis direction. This seemed to contradict the observed phenomenon, but other dihedral angles resulted in the opposite effect to the change of angle (i). Therefore, the combination of the entire change in each dihedral angle resulted in the expansion in b-axis.

Comparing each structure measured at 195 K, the cell volume decreased by 1.55 % mainly due to the reduction of the length in a-axis by 0.48 Å. The change in dihedral angles in BPTC4- was attributed to the decrease in cell volume. Therefore, the increase of angle (ii) & (iii) brought about the contraction in a-axis, and the rotation of carboxylates led to the change in orientation of the pendants due to rotation of macrocycles.

XRPD patterns collected at 298 K during CO2 sorption in the range of 0 to 30 bar showed a similar trend to the result at 195 K, but the change in patterns occurred at a pressure close to gate opening. In addition, the cell volumes at each adsorption point were calculated from the measured XRPD patterns and single crystal structures whose simulated patterns corresponded to the CO2-pressurized patterns. On the other hand, the breathing effect is due to changes in BPTC4-, whose freely rotatable bonds caused a broadening of the b-axis and the ac plane.

In addition, the change in the angles between carboxylate groups and phenyl rings (②,③,④ and ⑤) contributed little to the breathing behavior. Consequently, the rotations of single bonds in BPTC4 ligand led to the breathing behavior of 1 during the CO2 gas sorption, resulting in the increase in cell volume (5628 Å3 to 7168 Å3). However, in the coordination polymer 3-axis, due to the existence of a two-fold helical axis between the layers, two different types of layers are alternatively packed by the c-axis in the ∙∙∙A-B-A-B∙∙∙ manner (Figure 3.22) .

Consequently, they are attributed to the instability of DEF molecules in the 2-as structure, and the DEF molecules were rapidly removed from the interlayer spaces, resulting in the coexistence of the dried 2 structure with the original 2-as structure. It was confirmed that the different guest inclusion behaviors of structure 2 and structures 3-as and 3 were attributed to the different interlayer interaction. H2 sorption supported their non-porous properties, and the uptake amount of 4 was greater than that of 5 due to the difference in void space.

The low uptake of CO2 for 4 at 195 K can be attributed to the small pore and the interaction between amine and CO2.

Figure 3.2. The structure of 1-as measured at 195 K (a) A perspective view on ac plane

Conclusion

To study about the gas sorption properties of MOFs especially for CO2, many researches have been done on amine-functionalized MOFs and have brought much attention to the potentials of MOFs as CO2 adsorbers. However, most of them showed the effect of amine groups without comparison group or with comparison group of different weight, shape and pore size. Thus, we plan to incorporate methyl and amine functional groups for synthesizing two structurally similar MOFs with different functional groups of similar formula weight for direct comparison of the weight percentage of adsorbed CO2 between the two MOFs with and without amine groups.

Therefore, two isostructural MOFs with a small difference in formula mass and similar pores having different functional groups, 4-as and 5-as, were constructed. After the activation of the compounds, 4 and 5 were obtained and the gas sorption isotherms for both frameworks were measured.

Supporting Information

Kitaura, R.; Seki, K.; Akiyama, G.; Kitagawa, S., Porous coordination polymer crystals with gated channels specific for supercritical gases. Ferey, G.; Serre, C., Large breathing effects in three-dimensional porous hybrid materials: facts, analyses, rules and consequences. L.; Daturi, M.; Filinchuk, Y.; Leynaud, O.; Barnes, P.; Férey, G., An explanation for the very large breathing effect of a metal-organic framework during CO2 adsorption.

-Titus, M.; Farrusseng, D., Guest-induced gate opening and breathing phenomena in soft porous crystals: building thermodynamically consistent isotherms. Globe continues its hottest decades on record. http://newswatch.nationalgeographic.com globe-continues-hottest-decade-ever/. accessed on October 14); (b) 6 ways climate change will affect you. Wang, S.; Yan, S.; Ma, X.; Gong, J., Recent developments in carbon dioxide capture using alkali metal-based oxides.

R.; Smith, B.; Gagliardi, L., Mechanism of carbon dioxide adsorption on an alkylamine-functionalized metal-organic framework.