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

Abstract

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, 17O solid-state NMR (SSNMR) should be an excellent tool for characterizing various oxygen species. However, the major obstacles that prevent applying 17O SSNMR to MOF characterization are the synthetic effort needed for 17O isotopic enrichment and the associated high cost. In this work, we successfully prepared several prototypical 17O-enriched MOFs, including Zr-UiO-66, MIL-53(Al), CPO-27-Mg (or Mg-MOF-74), and microporous α-Mg3(HCOO)6. Depending on the target MOF, different isotopic enrichment methods were used to effectively incorporate 17O from 17O-enriched H2O. Using these 17O-enriched MOFs, we were able to acquire 17O 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 17O 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 17O-enriched sample described in this paper are fairly simple and cost-effective.