Evaluation of MOFs for air purification and air quality control applications: Ammonia removal from air

• UiO-66 and 6 variants were synthesized and studied for adsorption of ammonia from air.
• Functional groups were chosen for high interaction with ammonia.
• Hydroxyl groups interact the most effectively with ammonia at low concentration.
• Breakthrough curves were measured on all materials using a microbreakthrough system.
• Large-pore MOFs (>10 Å) are necessary for efficient use of complex functionalization.

UiO-66 is a Zr-based MOF that is being highly investigated for a wide variety of small molecule gas separations since it possess unprecedented thermal, chemical, and mechanical stability. In this work, we have investigated the performance of various functionalized variations of UiO-66 (UiO-66-OH, UiO-66-(OH)2, UiO-66-NO2, UiO-66-NH2, UiO-66-SO3H, and UiO-66-(COOH)2) towards ammonia removal from air. Functionalized UiO-66 analogs have been synthesized solvothermally and characterized using ammonia breakthrough measurements under dry and humid (80% RH) air conditions along with powder X-ray diffraction (PXRD) patterns and results from BET modeling of N2 adsorption isotherms. Counter to chemical intuition, our study demonstrates that the ammonia capacities of UiO-66-SO3H and UiO-66-(COOH)2 are lower than UiO-66-OH and UiO-66-NH2. This is due to significant reduction in the framework porosity (surface area and pore volume) upon functionalization with bulky functional groups such as –COOH and –SO3H. The –OH group is the least bulky functional group considered in the work and interacts favorably with ammonia. UiO-66-OH has a capacity of ~5.7 mmol/g for ammonia under dry conditions which is very close to the ammonia removal goal of 0.1 g/g MOF (or ~6 mmol/g). However, we observed a decrease in the ammonia capacities of functionalized UiO-66 variations under humid conditions due to competition between water and ammonia molecules for adsorption on the active sites. Overall, balancing the water adsorption behavior and high selectivity and high capacity for ammonia is crucial to developing new adsorbents for ammonia removal from air.

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