Insights into CO2 Adsorption in M-OH Functionalized MOFs

A series of benzotriazolate MOFs containing nucleophilic transition metal hydroxide (M-OH) groups has been synthesized to compare the effects of framework structure, metal composition, and method of postsynthetic ligand exchange (PSLE) on CO2 adsorption. Analogues of MFU-4 (1a/b-OH, [Zn-5(OH)(4)(bbta)(3)], bbta(2-) = benzo-1,2,4,5-bistriazolate) and MFU-4l (2a/b-OH, [Zn-5(OH)(4)(btdd)(3)], btdd(2-) = bis(1,2,3-triazolo)dibenzodioxin) were prepared by direct Cl-/OH- ligand exchange (a) or Cl-/HCO3- ligand exchange followed by thermal activation (b). A Ni/Zn heterobimetallic analogue of MFU-4l (2a/b-NiOH) was also synthesized to investigate the effect of metal identity. The products have been characterized by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). All of the M-OH functionalized MOFs show steep CO2 adsorption at low partial pressures. However, materials synthesized using the direct Cl-/OH- ligand exchange method show greater low-pressure CO2 uptake than those prepared by Cl-/HCO3- PSLE. Notably, the small pore size in 1a/b-OH not only promotes stronger framework-CO2 interactions and higher CO2 uptake than 2a/b-OH but also results in slow adsorption kinetics. The Ni/Zn heterobimetallic analogue 2a-NiOH exhibits the greatest low-pressure CO2 capacity (1.70 mmol g(-1) at 2.6 mbar) among the series. In situ DRIFTS studies reveal that both 2a-OH and 2a-NiOH contain weak Zn-OH binding sites that readily desorb CO2 at room temperature. However, 2a-NiOH also contains strong Ni-OH binding sites that are spectroscopically distinct and only desorb CO2 upon heating.