Amino-acid functionalised MOFs for metal recovery: a contribution to the circular economy

The transition from a linear to a circular economy is an integral aspect of transforming the EU into a sustainable, climate-neutral society. Through the circular concept of reduce-recover-recycle-reuse, a number of global challenges are tackled, including climate change, waste, pollution and biodiversity loss. Metals are at the heart of a circular economy due to their almost indefinite recyclability. The development of a greener and more efficient capture and separation technology for the recovery of metals from aqueous solutions has been advanced by a successful collaboration between the Institut Laue-Langevin (ILL), the Basque Center for Materials, Applications & Nanostructures and the University of the Basque Country.

The selective adsorption of hazardous and/or valuable metal ions from aqueous solutions is an important process within the circular economy to enable the recovery of critical raw materials from, for example, radioactive or metal-rich mining wastewaters. Metal-chelating agents and adsorbents are two of the most simple, cost-efficient, and flexible technologies currently used. Chelation involves the bonding of chelating agents to metal ions. The chemical tailorability of the chelator provides efficiency and selectivity to trap specific ions but the recovery of the metal-chelator complexes is time and energy-consuming. Classic sorbents, on the other hand, though easier to recover, have low selectivity and adsorption capacity. 

  Metal-organic frameworks (MOFs) – advanced sorbents that are composed of metal ions or clusters interconnected by organic linkers – are one of the most widely investigated materials of the 21st century. With exceptional porosity and a chemical versatility that enables their properties to be tuned to meet the requirements of a wide range of applications, MOFs are considered a promising emerging adsorbent. “There is huge flexibility possible in their design and specific functionalities that can be achieved by chemical alteration of different parts of the MOF,” explains Mónica Jiménez-Ruiz, ILL scientist and first responsible of the IN1-Lagrange instrument.

For this project, the researchers decorated the pore space of zirconium trimesate (MOF-808) with eight different amino (histidine, cysteine and asparagine) and natural acids (malic acid, mercaptosuccinic acid, succinic acid, fumaric acid and citric acid) in order to explore the influence of each organic chelator on the metal-ion adsorption capacity and specificity of the parent MOF-808 material. “The idea is that water contaminated with metals passes through this specially-designed metal-Chelator-like filter formed by decorating a MOF with amino or natural acids,” explains Jiménez-Ruiz. “The metal ions in the water form a weak interaction with the amino acid thus cleaning the water. The metal and MOF can then be separated enabling the recovered metal to be recycled and the MOF reused.”

The MOF-808 scaffold was synthesised and functionalised with the amino and natural acids, then characterised using X-ray diffraction (XRD), infrared (IR) spectroscopy, thermogravimetry (TGA) and N2 adsorption measurements. The chemical stability of each MOF-808@(amino)acid sample was assessed to establish applicability, while the adsorption capacity, specificity and efficiency was determined in both batch and continuous flow conditions, performed using single and multielement aqueous solutions involving soft, intermediate and hard metal ions.

In order to identify the most promising MOF-808@(amino)acid variants, the adsorption sites were characterised by combining advanced inelastic neutron spectroscopy (INS) with the more conventional spectroscopic techniques of IR and Raman spectroscopy. “By combining these spectroscopic techniques with density functional theory (DFT), it is possible to identify the chemical species that are responsible for the vibrations in the amino acid-decorated MOF,” explains Jiménez-Ruiz. “The change, or even extinction, of one of these vibrational bands after metal adsorption provides information about the absorption site of the metal ion”. Though these spectroscopic techniques study the atom vibrations in the same energy range (up to 500 meV), the crucial contribution of neutrons is their ability to reveal the spectroscopic signals associated with hydrogen atoms. “There are many hydrogen atoms in MOFs and amino acids; the unique ability of neutrons to highlight the vibrations of hydrogen atoms helps you to know which vibration is affected after adsorption and thus where the metal has been adsorbed,” explains Jiménez-Ruiz.

The neutron technique employed – neutron vibrational spectroscopy – requires very high energy neutrons in order to measure a large energy transfer range and access the intramolecular vibrations. Though a number of neutron spectrometers suitable for these measurements are available at different facilities, only three can compete in terms of flux, energy range and energy resolution and it was the IN1-Lagrange neutron spectrometer at the ILL that was used to acquire the data for this study. IN1-Lagrange is the only spectrometer at the ILL installed on the hot source and thus with access to such high energy neutrons. In addition to the very high energy neutrons needed to measure atomic vibrations, the ILL also provides a very high flux of neutrons, enabling the measurement of the very small samples used in this study. Indeed, the large number of samples studied was made possible by the increased efficiency provided by a specially-designed sample changer for IN1-Lagrange, which this study was the first to benefit from.

The results, published in Chemistry of Materials in 2022, demonstrate that the installation of natural and amino acids within a MOF can significantly alter the adsorption capacity and affinity toward metal ions with varied characteristics. In particular, outstanding adsorption capacities were measured for heavy metals even in continuous flow conditions, with the results obtained for MOF-808(amino)acids rivalling the figures of the best MOFs reported to date for cadmium, lead and mercury. This first study represents the beginning of a successful collaboration, formalised from September within the framework of a PhD project, that will use the capabilities of IN1-Lagrange at the ILL under the supervision of Jiménez-Ruiz to investigate the atmospheric greenhouse gas (e.g., CH4 and CO2) adsorption capacity of MOFs, with complementary laboratory measurements to be carried out in Bilbao, supervised by Dr. Roberto Fernández de Luis and Dr. José María Porro.

ILL instrument used: IN1-Lagrange neutron spectrometer

Reference: Designing Metal-Chelator-like Traps by Encoding Amino Acids in Zirconium-Based Metal−Organic Frameworks, Chem. Mater. 2022, 34, 21, 9666–9684, October 2022

Research team: 

Ainara Valverde, Gabriel I. Tovar, Natalia A. Rio-López, Dimas Torres, Maibelin Rosales, Stefan WuttkeArkaitz Fidalgo-Marijuan, José María Porro, Mónica Jiménez-Ruiz, Victoria García Sakai, Andreina García, José Manuel Laza, José Luis Vilas-Vilela, Luis Lezama, María I. Arriortua, Guillermo J. Copello, Roberto Fernández de Luis.