No abstract

Covalent organic frameworks (COFs) are porous crystalline organic polymers which have been the subject of immense research interest in the past 10 years. COF materials are synthesized by the covalent linkage of organic molecules bonded in a repeating fashion to form a porous crystal that is ideal for gas adsorption and storage. Chemists have strategically designed COFs for the purpose of heterogeneous catalysis of gaseous reactants. Presented in this critical review are efforts toward developing COFs for the sequestration of CO 2 from the atmosphere. Researchers have determined the CO 2 adsorption capabilities of several COFs is competitive with the highest surface area materials. Engineering the pore environment of COFs with chemical moieties that interact with CO 2 have increased the CO 2 adsorption performance. The installation of CO 2 binding moieties in the COF has made possible the selective adsorption of CO 2 over other gases such as N 2 . The high degree of control of internal pore composition in COFs is coupled with high CO 2 adsorption to develop heterogeneous catalysts for the conversion of CO 2 to value added products. Two notable examples of this catalysis are the fixation of CO 2 to epoxides for the synthesis of cyclic carbonates and the reduction of CO 2 to CO. Recent examples of COFs for the capture of CO 2 will be discussed followed by COF catalysts which use CO 2 as a feedstock for the production of value-added products.

Bimetallic iron–nickel-based nanocatalysts are perhaps the most active for the oxygen evolution reaction (OER) in alkaline electrolytes. Recent developments in literature have suggested that the ratio of iron and nickel in Fe–Ni thin films plays an essential role in the performance and stability of the catalysts. In this work, the metallic ratio of iron to nickel was tested in alloy bimetallic nanoparticles. Similar to thin films, nanoparticles with iron–nickel atomic compositions where the atomic iron percentage is ≤50% outperformed nanoparticles with iron–nickel ratios of >50%. Nanoparticles of Fe20Ni80, Fe50Ni50, and Fe80Ni20 compositions were evaluated and demonstrated to have overpotentials of 313, 327,, and 364 mV, respectively, at a current density of 10 mA/cm2. While the Fe20Ni80 composition might be considered to have the best OER performance at low current densities, Fe50Ni50 was found to have the best current density performance at higher current densities, making this composition particularly relevant for electrolysis conditions. However, when stability was evaluated through chronoamperometry and chronopotentiometry, the Fe80Ni20 composition resulted in the lowest degradation rates of 2.9 μA/h and 17.2 μV/h, respectively. These results suggest that nanoparticles with higher iron and lower nickel content, such as the Fe80Ni20 composition, should be still taken into consideration while optimizing these bimetallic OER catalysts for overall electrocatalytic performance. Characterization by electron microscopy, diffraction, and X-ray spectroscopy provides detailed chemical and structural information on as-synthesized nanoparticle materials.

The drive toward sustainable phosphorus (P) recovery from agricultural and municipal wastewater streams has intensified. However, combining P recovery with energy conservation is perhaps one of the greatest challenges of this century. In this study, we report for the first time the simultaneous electroless production of struvite and dihydrogen from aqueous ammonium dihydrogen phosphate (NH4H2PO4) solutions in contact with either a pure magnesium (Mg) or a Mg alloy as the anode and 316 stainless steel (SS) as the cathode placed in a bench-scale electrochemical reactor. During the electroless process (i.e., in the absence of external electrical power), the open circuit potential (OCP), the formation of struvite on the anode, and the generation of dihydrogen at the cathode were monitored. We found that struvite is formed, and that struvite crystal structure/morphology and precipitate film thickness are affected by the concentration of the H n PO4 n–3/NH4 + in solution and the composition of the anode. The pure Mg anode produced a porous 0.6–4.1 μm thick film, while the AZ31 Mg alloy produced a more compact 1.7–9.9 μm thick struvite film. Kinetic analyses revealed that Mg dissolution to Mg2+ followed mostly a zero-order kinetic rate law for both Mg anode materials, and the rate constants (k) depended upon the struvite layer morphology. Fourier-transform infrared spectrometry, X-ray diffraction, and scanning electron microscopy indicated that the synthesized struvite was of high quality. The dihydrogen and Mg2+ in solution were detected by a gas chromatography–thermal conductivity detector and ion chromatography, respectively. Furthermore, we fully demonstrate that the reactor was able to remove ∼73% of the H n PO4 n–3 present in a natural poultry wastewater as mainly struvite. This study highlights the feasibility of simultaneously producing struvite and dihydrogen from wastewater effluents with no energy input in a green and sustainable approach.

Chitosan (CS)-graphene oxide (GO) composite films were fabricated, characterized, and evaluated as pressure-driven water filtration membranes. GO particles were incorporated into a chitosan polymer solution to form a suspension that was cast as a membrane via evaporative phase inversion allowing for scale-up for cross-flow testing conditions. Morphology and composition results for nano and granular GO in the CS matrix indicate that the particle size of GO impacts the internal membrane morphology as well as the structural order and the chemical composition. Performance of the membranes was evaluated with cationic and anionic organic probe molecules and revealed charge-dependent mechanisms of dye removal. The CSGO membranes had rejections of at least 95% for cationic methylene blue with mass balances obtained from measurements of the feed, concentrate, and permeate. This result suggests the dominant mechanism of removal is physical rejection for both GO particle sizes. For anionic methyl orange, the results indicate sorption as the dominant mechanism of removal, and performance is dependent on both GO particle size and time, with micrometer-scale GO removing 68–99% and nanometer-scale GO showing modest removal of 29–64%. The pure water flux for CSGO composite membranes ranged from 2–4.5 L/m 2 h at a transmembrane pressure of 344 kPa (3.44 bar), with pure water permeance ranging from 5.8 × 10 –3 to 0.01 L/m 2 h kPa (0.58–1.3 L/m 2 h bar). Based on the 41 μm membrane thickness obtained from microscopy, the hydraulic permeability ranged from 0.24–0.54 L μm/m 2 h kPa (24.4–54.1 L μm/m 2 h bar).