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

Abstract

Metal-organic frameworks (MOFs) have been studied in a wide range of fields such as gas storage, molecular separation, catalysis, and sensing. Recently, flexible MOFs which exhibit structural transformations by external stimuli attracted considerable attention due to those distinctive properties. Here, well-designed MOFs were constructed via self-assembly of macrocyclic complexes and organic ligands, and those CO2 sorption properties as well as structural flexibilities upon various stimuli were examined. A MOF, {[(NiLpropyl)2(BPTC)]∙4DMF∙2H2O} (1-as), was synthesized via self-assembly of [NiLpropyl](ClO4)2 and H4BPTC in DMF/H2O mixture solution ([NiLpropyl](ClO4)2 = [Ni(C14H34N6)](ClO4)2, H4BPTC = 2,2',5,5'–biphenyltetracarboxylic acid, and DMF = N,N-dimethylformamide). Single crystal X-ray diffraction data revealed that 1-as has a three-dimensional (3D) structure with accessible open pore occupied by guest molecules. Due to its structural flexibility, the structure of 1-as was changed into 1 with closed pores and contracted cell volume upon elimination of guest molecules. Gas sorption experiments with N2 and H2 confirmed that 1 has the closed structure as evidenced by negligible amount of uptake. However, CO2 isotherm indicated two-step adsorption profile with significantly large uptake amount at 195 K, indicating that CO2 gas triggered a structural transformation of 1 into open and expanded porous structure. Interestingly, the structural transition of 1 upon CO2 uptake at 195 K occurred in two distinct steps, gate-opening and breathing, independently, which were studied by in-situ X-ray powder diffraction (XRPD) experiment and gas sorption isotherms. Gate-opening is caused by the rotation of Ni (II) macrocycles accompanied with breaking and reconstructing hydrogen bonds between secondary amines on macrocycles and carboxylates. This brought about the abrupt increase in CO2 adsorption without significant changes on XRPD patterns. On the other hand, breathing resulted from the rotation of single bonds in BPTC4- ligands, and consequently CO2 adsorption amount gradually increased with drastic changes on XRPD patterns. In addition, two 2D MOFs, {[(NiLallyl)2(BuTC)]∙2DEF∙2H2O} (2-as) and {[(NiLallyl)2(BuTC)]∙3H2O} (3-as), were obtained from the reaction between [NiLallyl](ClO4)2 and H4BuTC in DEF/H2O and MeCN/H2O, respectively ([NiLallyl](ClO4)2 = [Ni(C14H30N6)](ClO4)2, H4BuTC = 1,2,3,4–butanetetracarboxylic acid, DEF = N,N-diethylformamide, MeCN = acetonitrile). It was revealed by single crystal X-ray diffraction and XRPD results that 2-as and 3-as have 2D layered structures, and the layers were stacked infinitely with intercalated guest molecules in between the layers. Since same building blocks were used to construct both MOFs, the structure of each layer was identical. However, layer packing and the layer-layer distances in each MOF were different depending on the size and the nature of guest molecules. Interestingly, DEF guest molecules intercalated in 2-as could be easily liberated from the host due to those weak interaction with host MOF 2-as, resulting in dried compound 2. XRPD pattern of 2 was same as that of 3-as with slightly different relative intensities. On the other hand, dried compounds of 3-as (3) showed the same XRPD pattern of 3-as. The similar but non-identical XRPD patterns suggested that the dried structure 2 had the same interlayer distance and the same intra-layer structure as 3-as and 3, whereas its layer stacking manner retained, same as 2-as. This subtle structural difference of 2 and 3 caused the difference in the reversibility of structural flexibility and in CO2 uptake behaviors at 195 K. Lastly, {[(NiLamine)2(BPDC)2]∙6H2O} (4-as) and {[(NiLpropyl)2(BPDC)2]∙5H2O} (5-as) were synthesized via self-assembly of H2BPDC with [NiLamine](ClO4)2 in DEF/MeCN/H2O mixture solution and with [NiLpropyl](ClO4)2 in MeCN/H2O mixture solution, respectively ([NiLamine]ClO4)2 = [Ni(C12H32N8)](ClO4)2, H2BPDC = 4,4-biphenyldicarboxylic acid). Single crystal X-ray diffraction and XRPD measurement revealed that 2-as and 3-as have identical structure. The infinite coordination between BPDC2- and Ni (II) macrocycles yielded the 1D chains and they are extended in three different directions to construct a double network of threefold braids. The three-directional packing generates honeycomb-like 1D channels, which are occupied by pendants with different terminal functional groups such as ethylamine and propyl as well as the guest water molecules. After removing guest water molecules, the activated compounds 4 and 5 were obtained, respectively. Since ethylamine and propyl groups exposed to pore surface have different affinity toward CO2 molecules, MOFs 4 and 5 showed different CO2 sorption behavior. While CO2 molecules were chemically bonded to amine groups incorporated in the framework 4, 5 adsorbed CO2 molecules through physical interaction. It was confirmed by temperature dependent CO2 gas sorption isotherms and adsorption-desorption cycling experiments.