Homogeneity of flexible metal-organic frameworks containing mixed linkers

Very sophisticated porous materials known as multivariate functional MOFs (also known as MixMOFs) can be designed using a synthesis method that starts from solutions composed of two or more different linkers. For this procedure to be successful, one must have access to techniques that characterize the homogeneity of MOF crystallites containing two different linkers. This is of particular relevance for MOFs made of 2-aminobenzene-1,4-dicarboxylate (abdc), which are excellent platforms for the introduction of additional functions by post-modification. In this paper, we show that adsorption/desorption isotherms and thermodiffraction studies on flexible structures can indirectly characterize the homogeneity of MOFs made from a mixture of linkers. Breathing pressures and temperatures for a series of MIL-53(Al) functionalized with amino tags, i.e. Al(OH)(bdc)(1-)n(abdc)(n), were measured as a function of the amino content. The linear relationship between the CO2 breathing pressure and the amine content in the MIL-53(Al) structure clearly illustrates the homogeneity of the crystallite composition; in other words, the crystallites have the same abdc : bdc ratio. On the other hand, the functionalization of MIL-53(Al) with low amine content (10% abdc) results in a profound modification of the breathing properties triggered by the temperature. Much higher temperatures are required for full conversion of the np (narrow pore) to the lp (large pore) phase. We also suggest an interplay between coexisting np and lp microcrystalline domains that may smooth the breathing properties at the macroscopic level.