Mixed-Component Sulfone-Sulfoxide Tagged Zinc IRMOFs: In Situ Ligand Oxidation, Carbon Dioxide, and Water Sorption Studies

Reported here are the syntheses and adsorption properties of a series of single- and mixed-component zinc IRMOFs derived from controlled ratios of sulfide and sulfone functionalized linear biphenyldicarboxylate (bpdc) ligands. During MOF synthesis the sulfide moieties undergo in situ oxidation, giving rise to sulfoxide functionalized ligands, which are incorporated to give mixed-component sulfoxide-sulfone functionalized MOFs. The single- and mixed-component systems all share the IRMOF-9 structure type as determined by a combination of single crystal and powder X-ray diffraction analyses. The functionalized IRMOF-9 series was investigated by N2, CO2, and water adsorption measurements. MOFs containing higher proportions of sulfoxide have slightly larger accessible surface areas and pore volumes, whereas MOFs containing a greater proportion of the sulfone functionality demonstrated higher CO2 adsorption capacities, enthalpies of CO2 adsorption, and CO2/N2 selectivities. Water adsorption studies at 298 K showed the MOFs to have pore-filling steps starting around 0.4 P/P0. In general, only small changes in water adsorption were observed with regards to ligand ratios in the mixed-component MOFs, suggesting that the location of the step is primarily determined by the pore size. A ligand-directed fine-tuning approach of changing alkyl chain length was demonstrated to give smaller more hydrophobic pores with better adsorption characteristics.