Carbon Dioxide Capture by Metal-Organic Frameworks with Flexible Building Units

Abstract

PART I. SELECTIVE ADSORPTION OF CARBON DIOXIDE USING A POROUS METAL-ORGANIC FRAMEWORK CONSTRUCTED WITH A FLEXIBLE LINEAR LIGAND We have synthesized a porous metal-organic framework {[Zn2(mpm-PBODB)2bpy]⋅3DMF}n (SNU-110) by using an organic ligand containing flexible joints. SNU-110 has a 3D pillared-layer structure generating 1D channels with window size of 4.4 × 3.7 Å2. The solvent accessible volume is 30% of the whole structure. On activating SNU-110 with supercritical CO2 fluid, desolvated solid [Zn2(mpm-PBODB)2bpy]n (SNU-110) resulted, which has a unit cell volume reduced by 8.8%. SNU-110 hardly adsorbs N2 and H2 gases at 77 K, indicating its smaller window size than the kinetic diameter (2.89 Å) of H2. However, it uptakes 97 cm3 g-1 of CO2 at 195 K despite that CO2 has much larger kinetic diameter than H2, indicating that CO2 can open up the gate of the flexible MOF due to its much higher polarizability and quadrupole moment than N2 and H2. SNU-110 shows a two-step CO2 adsorption curve related with structural transformation on CO2 adsorption, together with a big desorption-hysteresis. Expansion of the framework by CO2 adsorption is evidenced by powder X-ray diffraction data measured under CO2 stream at 248 K. The selectivities for CO2 adsorption over N2, H2, and CH4 at 195 K are 35:1, 61:1, and 15:1, respectively.

PART II. ENHANCED CARBON DIOXIDE CAPTURE BY POST-SYNTHETIC MODIFICATION OF METAL-ORGAIC FRAMEWORKS WITH FLEXIBLE ALKANEDIOIC ACIDS A series of porous metal-organic frameworks having flexible carboxylic acid pendants in their pores (UiO-66-ADn: n = 4, 6, 8, and 10, where n de notes the number of carbons in a pendant) have been synthesized by the post-synthetic ligand exchange of terephthalate in UiO-66 with a series of alkanedioic acids (HO2C(CH2)n-2CO2H). The NMR, IR, PXRD, TEM, and Mass spectral data suggested that a terephthalate linker in UiO-66 was substituted with two alkanedioates to result in free carboxyl pendants in the pores. When post-synthetically modified UiO-66 was partially digested by adjusting the amount of added HF/sample, the NMR spectra indicated that the ratio of alkanedioic acid/terephthalic acid was increased with the lesser amount of acid, implying that the ligand substitution proceeded from the outer layer of the particle. Gas sorption studies indicated that the surface area and the pore volume of all UiO-66-ADns were decreased compared to those of UiO-66 while the CO2 adsorption capacities of UiO-66-ADn (n = 4, 8) were similar to that of UiO-66. In case of UiO-66-AD6, the CO2 uptake capacity was greater by 34% at 298 K and by 58% at 323 K compared to those of UiO-66. It was elucidated by the thermodynamic calculations that the introduction of the flexible carboxyl pendants with appropriate length has two effects: 1) Increasing interaction enthalpy between the host framework and CO2 molecules. 2) Mitigating the entropy loss upon CO2 adsorption due to the formation of multiple configurations for the interactions between carboxyl groups and the CO2 molecules. The IAST selectivity of CO2 adsorption over CH4 was enhanced for all UiO-66-ADns compared to that of UiO-66 at 298 K. In particular, UiO-66-AD6 showed the most highly enhanced CO2 uptake capacities and significantly increased selectivities of CO2 adsorption over CH4 at ambient temperature, suggesting that it is a promising CO2 separation material from landfill gas.