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, [Zn5(OH)4(bbta)3], bbta2– = benzo-1,2,4,5-bistriazolate) and MFU-4l (2a/b-OH, [Zn5(OH)4(btdd)3], btdd2– = 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.