Encapsulation of urea and caffeine in Cu₃(BTC)₂ metal–organic framework

The Cu₃(BTC)₂ metal–organic framework (MOF) (BTC = 1,3,5-benzenetricarboxylate), also known as HKUST-1 (or MOF-199), is one of the most studied MOFs because it has a large surface area, high pore volume, high chemical stability and the ability to bind water, among other molecules, by coordinating to the unsaturated Cu(II) sites. In this study, Cu₃(BTC)₂ was used for the first time to remove the pharmaceutical products of urea and caffeine. Two strategies for retention of urea and caffeine in Cu₃(BTC)₂ were studied. The first strategy corresponds to in situ encapsulation, where urea and caffeine were added to Cu₃(BTC)₂ synthesis precursors and the porous structure was formed around the urea or caffeine molecules resulting in the encapsulation. The second one corresponds to post-synthesis encapsulation. The adsorption of urea or caffeine in MOF structure was realized by adding the as-synthesized dry Cu₃(BTC)₂ into urea or caffeine aqueous solution. The results show that in situ encapsulation of caffeine and urea in Cu₃(BTC)₂ produced a guest loading of 38 and 25 wt%, respectively, whereas, post-synthesis encapsulation had a guest loading of 52 and 30 wt% for caffeine and urea, respectively.