Improving the mechanical stability of zirconium-based metal–organic frameworks by incorporation of acidic modulators†
The ability to retain structural integrity under processing conditions which involve mechanical stress, is essential if metal–organic frameworks (MOFs) are to fulfil their potential as serious candidates for use in gas sorption, separation, catalysis and energy conversion applications. A series of zirconium dicarboxylates, predicted to be amongst the more mechanically robust MOFs, have been found to undergo rapid collapse upon ball-milling, resulting in catastrophic losses of porosity. An inverse relationship between collapse time and framework porosity has been found. Addition of acidic modulator ligands (e.g. trifluoroacetic acid) to UiO-66 provided a striking increase in mechanical robustness, the degree of which is inversely related to modulator pKa. This effect, caused by an increased strength of the zirconium–carboxylate bond, provides an important concept to design microporous hybrid frameworks capable of retaining their structure under harsh processing conditions.