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 mu(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 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 O-H modes (Delta(t)nu(OH), calculated on the basis of the intrinsic frequency) was -83 cm(-1) with a small fraction of more acidic groups (Delta(t)nu(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 (Delta(t)nu(OH) = -108 cm(-1)). Similar results were obtained with the DMF-free UiO-66-NH2: the Delta(t)nu(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 N-15(2)) and attributed to N-2 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 N-2 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 nu(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.