Base catalytic activity of alkaline earth MOFs: a (micro)spectroscopic study of active site formation by the controlled transformation of structural anions

Abstract

A new method has been developed for generating highly dispersed base sites on metal–organic framework (MOF) lattices. The base catalytic activity of two alkaline earth MOFs, M₂(BTC)(NO₃)(DMF) (M = Ba or Sr, H₃BTC = 1,3,5-benzenetricarboxylic acid, DMF = N,N-dimethylformamide) was studied as a function of their activation procedures. The catalytic activity in Knoevenagel condensation and Michael addition reactions was found to increase strongly with activation temperature. Physicochemical characterization using FTIR, ¹³C CP MAS NMR, PXRD, XPS, TGA-MS, SEM, EPR, N₂ physisorption and nitrate content analysis shows that during activation, up to 85% of the nitrate anions are selectively removed from the structure and replaced with other charge compensating anions such as O²⁻. The defect sites generated via this activation act as new strong basic sites within the catalyst structure. A fluorescence microscopic visualization of the activity convincingly proves that it is exclusively associated with the hexagonal crystals, and that reaction proceeds inside the crystal's interior. Theoretical analysis of the Ba-material shows that the basicity of the proposed Ba²⁺–O²⁻–Ba²⁺ motifs is close to that of the edge sites in BaO.