Rapid microwave synthesis of sustainable magnetic framework composites of UTSA-16(Zn) with Fe₃O₄ nanoparticles for efficient CO₂ capture

Metal-organic frameworks (MOFs) have shown excellent potential for carbon dioxide capture applications due to their high sorption capacities and selectivities. However, MOFs are typically thermally insulating, and so thermal CO₂ regeneration is challenging, especially on the large scales required in industry. This limitation can be overcome by inclusion of magnetic nanoparticles within the MOF structure, enabling rapid and energy efficient CO₂ regeneration using induction heating. To this end we have developed novel magnetic framework composites (MFCs) comprised of MOF UTSA-16(Zn) (UTSA: University of Texas at San Antonio, a Zn-based MOF with citrate linkers) and Fe₃O₄ nanoparticles. Our work also addresses the sustainability and scalability challenges faced by MFCs required for industrial application, considering the use of inexpensive and widely-available materials. Herein we report a two-step procedure for preparing the MFCs. Firstly, a scalable single-step continuous hydrothermal synthesis method is used to produce highly pure, stable, and crystalline citrate-coated Fe3O₄ nanoparticles (62% yield). The nanoparticles exhibit a uniform particle size (19 ± 11 nm) and a very high saturation magnetisation (78 emu g⁻¹) compared with previously published citrate-coated Fe₃O₄ nanoparticles. Next, various concentrations (2.6-18.7 wt%) of these nanoparticles were incorporated into UTSA-16(Zn) via a rapid microwave-assisted direct-growth strategy (10 min) to form the MFCs (81-83% yield). The MFCs demonstrate high CO₂ adsorption capacities (2.8-3.3 mmol g⁻¹) and recyclability. In addition, the MFCs heat rapidly in an applied magnetic field for CO₂ release, reaching regeneration temperatures in remarkably short times (e.g. 60 °C in 8 seconds). The MFCs developed in this work combine strong CO₂ adsorption profiles and substantial regeneration heating capabilities, whilst being produced in a scalable and sustainable manner. The methods developed to prepare MFCs herein are also applicable to other MOFs, opening routes for a variety of sustainable MFCs to deliver impact for a range of applications across carbon capture and triggered release of other guest molecules.