Catalyst Decorated Magnesium Metal-Organic Frameworks for the Capture and Conversion of Carbon Dioxide to Chemical Fuels

Abstract

A series of magnesium-based metal-organic frameworks (Mg-MOFs) were prepared and decorated with metal nanoparticles to develop hybrid sorbent/catalysts for carbon dioxide (CO2) capture and conversion to chemical fuels.

A straightforward thermal treatment approach was first established to construct hybrid sorbent/catalysts. By exploiting the disparity in Ni-O and Mg-O coordination bond strength within mixed-metal NiMg-MOF-74, Ni2+ was selectively reduced to generate dispersed Ni nanoclusters constrained by the initial MOF pore diameter. By modifying the Ni:Mg ratio in the parent MOF, accessible surface area and framework crystallinity were tuned upon thermal treatment, influencing CO2 uptake and hydrogenation selectivity. The dispersed Ni nanoclusters proved to be an active catalyst towards CO2 hydrogenation, while the preserved section of Mg-MOF retained a portion of its CO2 adsorption capacity.

To gain further insight into factors governing CO2 uptake and hydrogenation, the catalyst loading method was expanded to Mg-CUK-1. The optimised NiMg-CUK-1 was deployed within a dual-mode reactor system constructed for temperature swing CO2 capture and conversion to methane (CH4). Upon repeated cycles, the CH4 generated surpassed similar sorbent-based catalysts reliant on the chemical looping of metal oxides and carbonates. The approach highlighted the significant role of physisorption-based materials in facilitating low-temperature CO2 desorption, extending the working capacity of hybrid sorbent/catalysts.

The dual-mode reactor system was extended towards industrially relevant CO2 capture conditions. Low catalyst loadings of Ru and Ni within stable Mg-CUK-1 enhanced overall system performance by preserving additional CO2 uptake. Simulated dry flue gas was deployed to probe the influence of oxygen exposure on catalyst performance. Ru-loaded Mg-CUK-1 sustained CH4 generation over ten cycles, while Ni-loaded Mg-CUK-1 registered a decrease in performance attributed to Ni passivation. When a portion of Ni was replaced with Ru, the resulting dual-metal catalyst emulated the performance of the monometallic Ru hybrid sorbent/catalyst. Ru was found to aid the re-reduction of surface Ni2+ to Ni0 upon oxygen exposure, which resulted in sustained CH4 generation. The strategy developed provided an approach to foster resilient hydrogenation catalysts capable of withstanding reactive species when exposed to point source CO2 emissions.