Evolution of acid and basic sites in UiO-66 and UiO-66-NH2 metal-organic frameworks: FTIR study by probe molecules

Abstract Metal-organic frameworks (MOFs) and MOF-based materials find increasing and diverse applicability. The performance of MOFs in adsorption and catalysis is strongly related to their acid-basic properties. Here we report on the development of acidity (protonic and Lewis) and basicity during thermo-vacuum treatment of two MOFs of practical importance, UiO-66 and UiO-66-NH 2 . Residual DMF is more strongly bound to UiO-66-NH 2 but is practically removed from both samples at 473 K. The structural μ 3 -OH groups (belonging to the Zr 6 cluster) for both samples are observed at 3678-3672 cm −1 . Their intrinsic frequency was estimated to be at 3682 cm −1 and the lower values detected are due to a very weak H-bonding to the MOF walls. Both samples are practically dehydroxylated at 523 K but easily re-hydroxylated at ambient temperature in presence of water. Three probe molecules (CO, N 2 and CD 3 CN) were utilized to assess the acidity and basicity of the samples. Low-temperature CO adsorption experiments revealed a weak protonic acidity of the UiO-66 sample evacuated at 298 K: the CO induced shift of the O H modes (Δ t ν OH, calculated on the basis of the intrinsic frequency) was −83 cm −1 with a small fraction of more acidic groups (Δ t ν OH  = −93 cm −1 ). Evacuation at 473 K leads to a strong (and reversible) decrease in the population of the structural OH groups in UiO-66 and creation of a fraction of more acidic hydroxyls (Δ t ν OH  = −108 cm −1 ). Similar results were obtained with the DMF-free UiO-66-NH 2 : the Δ t ν OH was −91 cm −1 for a sample evacuated at 298 K while for a sample evacuated at 473 K two shifts were observed, −98 and −117 cm −1 . These results were fully confirmed by adsorption of N 2 . In this case an additional N N band was detected at 2324 cm −1 (2246 cm −1 after adsorption of 15 N 2 ) and attributed to N 2 polarized by O 2− basic sites. The band developed with the pre-evacuation temperature evidencing creation of basic sites. No Lewis acidity was established by CO and N 2 probes on samples evacuated up to 573 K. However, with samples evacuated at 473 K or higher temperature, Zr 4+ Lewis acid sites were unambiguously monitored by CD 3 CN through a ν (CN) band at 2299 cm −1 . The existence of this “hidden” Lewis acidity is explained by structural re-arrangement of the Zr 4+ environment induced by relatively strong bases as CD 3 CN. Subsequent re-hydroxylation of the sample provokes almost full disappearance of the Lewis acid sites at the expense of OH groups formed.