Anomalous Infrared Intensity Behavior of Acetonitrile Diffused into UiO-67

UiO-67 metal-organic frameworks (MOFs) show promise for use in a variety of areas, especially in industrial chemistry, as stable and customizable catalyst materials often driven by catalytically active defects (coordinatively unsaturated metal sites) present within the MOF crystallite. Thermal activation, or postsynthetic thermal treatment, of MOFs is a seldom used method to induce catalytically active defects. To investigate the effect of thermal activation on defect concentration in UiO-67, we performed Fourier transform infrared (FT-IR) spectroscopy studies of adsorbed CD3CN, a versatile infrared active probe molecule. Our results suggest that, under cryogenic, ultrahigh-vacuum conditions, CD3CN must be thermally diffused into UiO-67 to successfully detect binding sites and defects. Below dehydroxylation temperatures, blueshifted nu(CN) modes of diffused CD3CN indicate multiple avenues of hydrogen bonding within UiO-67, as well as binding to Lewis acid sites, assigned to be coordinately undersaturated Zr4+, consistent with in situ FT-IR of adsorbed CO. FT-IR of CD3CN diffused into UiO-67 activated to 623 K shows that while thermal activation eliminates hydrogen-bonding moieties, it also induces stronger Lewis acid sites, identified by a blueshifted nu(CN) doublet. Through density functional theory (DFT) calculations, we demonstrate that the nu(CN) doublet is a result of CD3CN interacting with two distinct nodal defect sites present in dehydroxylated UiO-67. Additionally, the infrared cross sections of CD3CN's nu(CN), nu(CD)(s) and nu(CD)(as) modes change, primarily due to hydrogen bonding, when diffused into UiO-67, as confirmed through DFT calculations. The nonlinear IR cross section behavior suggests that the Beer-Lambert law cannot trivially extrapolate the concentration of an analyte diffused into a MOF. These studies reveal the impact of postsynthetic thermal treatment on the concentration and type of Lewis acid defects in UiO-67 and caution the simple use of integrated infrared absorbance as a metric of analyte concentration within a MOF.

Automated summary

UiO-67 metal-organic frameworks (MOFs) have potential applications in various fields, and thermal activation can induce catalytically active defects. In this study, the effect of thermal activation on defect concentration in UiO-67 was investigated using Fourier transform infrared spectroscopy. The results show that thermal activation affects the concentration and type of Lewis acid defects in UiO-67.