In Situ FTIR Spectroscopy for Scanning Accessible Active Sites in Defect-Engineered UiO-66

Area of Science:

• Materials Science
• Catalysis
• Surface Chemistry

Background:

• UiO-66 is a highly stable metal-organic framework (MOF) with tunable properties.
• Defect engineering is a key strategy to modify MOF characteristics, including acidity.
• Understanding acid site behavior is vital for UiO-66 applications in adsorption and catalysis.

Purpose of the Study:

• To investigate the effect of defect engineering using benzoic acid on the acidic properties of UiO-66.
• To characterize the nature and availability of acid sites in modified UiO-66 samples.
• To assess the utility of FTIR spectroscopy in probing these acid sites.

Main Methods:

• Solvothermal synthesis of UiO-66 with benzoic acid as a modulator.
• Characterization using various techniques, including FTIR spectroscopy.
• Assessment of acidic properties via adsorption of CO and CD3CN probe molecules.

Main Results:

• Bridging μ3-OH groups were present in samples evacuated at room temperature.
• Dehydroxylation at 250 °C removed μ3-OH groups.
• Modulator-free synthesis yielded open Zr sites, while modulator-containing samples had benzoate-saturated sites.
• Dehydroxylation generated new bare Zr sites interacting with acetonitrile, not CO.

Conclusions:

• FTIR spectroscopy is effective for identifying and quantifying acid sites in UiO-66.
• Benzoate saturation in modulated UiO-66 hinders probe molecule interaction.
• Defect engineering significantly influences the type and accessibility of acid sites in UiO-66, impacting its performance.