Post-synthetic modification of UiO-66 analogue metal-organic framework as potential solid sorbent for direct air capture

Direct air capture (DAC) offers a powerful strategy to counteract climate change by reducing the carbon dioxide levels in the air. Developing sorbent materials capable of sucking carbon dioxide gas continuously emitted into our atmosphere is critical. In this work, we present a functionalization strategy for a metal-organic framework (UiO-66), intending to increase the uptake and selectivity for capturing carbon dioxide at its ppm levels from the multi-component mixture of air. (3-Aminopropyl)triethoxysilane (APTES) was used to chemically modify the hydroxyl analog of UiO-66 (UiO-66-(OH)₂) framework to form UiO-66-APTES. The developed UiO-66-APTES fully characterized using several analytical techniques, such as Fourier Transform Infrared spectroscopy (FTIR), Powder X-ray diffraction (PXRD), and X-ray photoelectron microscopy (XPS). The calculated BET surface areas of UiO-66-(OH)₂ and UiO-66-APTES were 752 m² g⁻¹ and 381 m² g⁻¹, respectively. Although the surface area decreased, adding several aliphatic amine groups enhanced the CO₂ adsorption capacity, at 298 K and 1 bar, from 42.7 cm³ g⁻¹ to 65.6 cm³ g^−1, exhibiting a ∼150 % improvement over the unfunctionalized sample. Not only was the material proven to be selective for CO₂ within a CO₂/N₂ gas mixture with a high selectivity of 160, but it also showed a mere marginal reduction in performance (i.e., CO₂ uptake capacity) after 15 cycles. Moreover, the UiO-66-APTES system was used for the dynamic separation of CO₂ from air mixture, revealing its potential as an attractive adsorbent for direct air capture.