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

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-NH2. Residual DMF is more strongly bound to UiO-66-NH2 but is practically removed from both samples at 473 K. The structural μ3-OH groups (belonging to the Zr6 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, N2 and CD3CN) 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 OH modes (ΔtνOH, calculated on the basis of the intrinsic frequency) was −83 cm−1 with a small fraction of more acidic groups (ΔtνOH = −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 (ΔtνOH = −108 cm−1). Similar results were obtained with the DMF-free UiO-66-NH2: the ΔtνOH 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 N2. In this case an additional NN band was detected at 2324 cm−1 (2246 cm−1 after adsorption of 15N2) and attributed to N2 polarized by O2− basic sites. The band developed with the pre-evacuation temperature evidencing creation of basic sites. No Lewis acidity was established by CO and N2 probes on samples evacuated up to 573 K. However, with samples evacuated at 473 K or higher temperature, Zr4+ Lewis acid sites were unambiguously monitored by CD3CN through a ν(CN) band at 2299 cm−1. The existence of this “hidden” Lewis acidity is explained by structural re-arrangement of the Zr4+ environment induced by relatively strong bases as CD3CN. Subsequent re-hydroxylation of the sample provokes almost full disappearance of the Lewis acid sites at the expense of OH groups formed.