Identification of Nonequivalent Framework Oxygen Species in Metal-Organic Frameworks by 17O Solid-State NMR

Metal-organic frameworks (MOFs) are a class of novel nanoporous materials with many potential applications. Structural characterization is important because understanding the relationship between the properties of these industrially relevant materials and their structures allows one to develop new applications and improve current performance. Oxygen is one of the most important elements in many MOFs and exists in various forms. Ideally, O-17 solid-state NMR (SSNMR) should be an excellent tool for characterizing various oxygen species. However, the major obstacles that prevent applying O-17 SSNMR to MOF characterization are the synthetic effort needed for O-17 isotopic enrichment and the associated high cost. In this work, we successfully prepared several prototypical O-17-enriched MOFs, including Zr-UiO-66, MIL-53(Al), CPO-27-Mg (or Mg-MOF-74), and microporous alpha-Mg-3(HCOO)(6). Depending on the target MOF, different isotopic enrichment methods were used to effectively incorporate O-17 from O-17-enriched H2O. Using these O-17-enriched MOFs, we were able to acquire O-17 SSNMR spectra at a magnetic field of 21.1 T. They provide distinct spectral signatures of various key oxygen species commonly seen in representative MOFs. We demonstrate that O-17 SSNMR can be used to differentiate chemically and, under favorite circumstances, crystallographically nonequivalent oxygens and to follow the phase transitions. The synthetic approaches for preparation of O-17-enriched sample described in this paper are fairly simple and cost-effective.