A Post-Synthetic Modification Strategy for Enhancing Pt Adsorption in MOF/Polymer Composites

Growing polymers inside of porous supports, like metal-organic frameworks (MOFs), allows incoming guests to access the backbone of otherwise non-porous polymers, possibly boosting the number and/or strength of available adsorption sites inside the porous material. Given this, we have focused attention on the design of MOF-polymer composites for the recovery of targeted metals from liquid streams and shown that the separation performance of such composites is significantly higher than the sum of the individual MOF or polymer counterparts. Now, in an effort to further boost performance, we have devised a novel post-synthetic modification (PSM) strategy that allows one to graft metal-chelating functionality onto the polymer backbone, while inside MOF pores. For the proof of concept, polydopamine (PDA) was first grown inside of a MOF, Fe-BTC. Next, a small molecule, 2,3-dimercapto-1-propanol (DIP), was grafted to the PDA via a Michael addition. Notably, the DIP contains thiols that can chelate targeted soft metals, like platinum (Pt). After the PSM of the PDA, the Pt adsorption capacity of the composite is shown to reach a record value of 684 mg/g. More importantly, the Pt uptake is greatly enhanced at low Pt concentrations, owed to the high affinity of Pt towards the thiols. Techniques, including XPS and in-situ XAS, provide insight into the Pt adsorption/reduction process, which is clearly enhanced after the PDA modification with thiols. Last, it is shown that various thiols, beyond DIP, can be grafted to the Fe-BTC/PDA, demonstrating the versatility of the chemistry. It is hoped that this work will open up a pathway for the future design of other novel composites that are fine-tuned for the rapid, selective retrieval of high-value and/or critical metals from complex liquids.