Solar-Driven Ammonia Production through Engineering of the Electronic Structure of a Zr-Based MOF.

As a hydrogen carrier and a vital component in fertilizer production, ammonia (NH3) is set to play a crucial role in the planet's future. While its industrial production feeds half of the global population, it uses fossil fuels and emits greenhouse gases. To tackle this issue, photocatalytic nitrogen fixation using visible light is emerging as an effective alternative method. This strategy avoids carbon dioxide (CO2) emissions and harnesses the largest share of sunlight. In this work, we successfully incorporated a 5-nitro isophthalic acid linker into MOF-808 to introduce structural defects and open metal sites. This has allowed modulation of the electronic structure of the MOF and effectively reduced the band gap energy from 3.8 to 2.6 eV. Combination with g-C3N4 enhanced further NH3 production, as these two materials possess similar band gap energies, and g-C3N4 has shown excellent performance for this reaction. The nitro groups serve as acceptors, and their integration into the MOF structure allowed effective interaction with the free electron pairs on N-(C)3 in the g-C3N4 network nodes. Based on DFT calculations, it was concluded that the adsorption of N2 molecules on open metal sites caused a decrease in their triple bond energy. The modified MOF-808 showed superior performance compared with the other MOFs studied in terms of N2 photoreduction under visible light. This design concept offers valuable information about how to engineer band gap energy in MOF structures and their combination with appropriate semiconductors for solar-powered photocatalytic reactions, such as N2 or CO2 photoreduction.

Improving the capability of UiO-66 for Cr(vi) adsorption from aqueous solutions by introducing isonicotinate N-oxide as the functional group.

Considering the widespread production of Cr(vi) from various industrial processes and its damaging toxicity as a carcinogenic agent, it is imperative to investigate stable and efficient adsorbents with a rapid performance towards Cr(vi) adsorption. Zirconium-containing metal-organic frameworks (Zr-MOFs) have been recently used as environmentally friendly adsorbents for the reduction of metallic contaminants in aqueous media. Preparation from nontoxic metal sources, remarkable stabilities and distinguished physicochemical features, such as the high tendency of Zr-clusters to adsorb oxo-anions, beneficial structural defects, and modification of their properties via modulator synthesis, can be enumerated as the advantages of Zr-MOFs. In this study, we improved the adsorption capacity of UiO-66 as the most stable Zr-MOF for Cr(vi) adsorption from aqueous solutions through the gradual addition of an N-O functional group. This strategy ultimately led us to afford a new Zr-MOF structure (TMU-66) with a maximum Cr(vi) adsorption capacity of 60.241 mg g-1, which is 4 times the absorption capacity of UiO-66, and very fast kinetics (<3 min) that followed the pseudo-second-order kinetics. This study demonstrates that a conceptual design can be helpful in synthesizing safe and stable adsorbents with appropriate capacity and kinetics for adsorption.