A Solar to Chemical Strategy: Green Hydrogen as a Means, Not an End.

Green hydrogen is the key to the chemical industry achieving net zero emissions. The chemical industry is responsible for almost 2% of all CO2 emissions, with half of it coming from the production of simple commodity chemicals, such as NH3, H2O2, methanol, and aniline. Despite electrolysis driven by renewable power sources emerging as the most promising way to supply all the green hydrogen required in the production chain of these chemicals, in this review, it is worth noting that the photocatalytic route may be underestimated and can hold a bright future for this topic. In fact, the production of H2 by photocatalysis still faces important challenges in terms of activity, engineering, and economic feasibility. However, photocatalytic systems can be tailored to directly convert sunlight and water (or other renewable proton sources) directly into chemicals, enabling a solar-to-chemical strategy. Here, a series of recent examples are presented, demonstrating that photocatalysis can be successfully employed to produce the most important commodity chemicals, especially on NH3, H2O2, and chemicals produced by reduction reactions. The replacement of fossil-derived H2 in the synthesis of these chemicals can be disruptive, essentially safeguarding the transition of the chemical industry to a low-carbon economy.

Heterogeneous Fenton-like surface properties of oxygenated graphitic carbon nitride.

The photo-Fenton activity of graphitic carbon nitride (g-C3N4) has been widely studied, nevertheless, its Fenton-like catalytic behavior in the dark has not yet been demonstrated. In the present work, it is shown that oxygenated g-C3N4 obtained at different temperatures (500-600 °C) can degrade indigo carmine with hydrogen peroxide in the dark by a reaction similar to a conventional Fenton's reaction. Based on an extensive characterization of g-C3N4, we conclude that Fenton-like activity is directly related to the oxygenated functional groups on g-C3N4 structure, mainly by -OH functional groups. Oxygenated functional groups (e.g., hydroquinone-like groups) can reduce the H2O2 and generate oxidizing hydroxyl radicals, just like in the Fenton reaction performed by metals. In addition to new information on g-C3N4 surface reactivity revealed by this study, the metal-free oxygenated g-C3N4 catalyst may be an alternative to traditional metal catalysts used in Fenton-like reactions for advanced oxidation.

Purification, Selection, and Partition Coefficient of Highly Oxidized Carbon Dots in Aqueous Two-Phase Systems Based on Polymer-Salt Pairs.

In general, the methodologies for the preparation of carbon dots (CDs) lead to the formation of nanostructures with size and surface chemistry heterogeneity. Because the electronic and optical properties of these nanoparticles are directly associated with these properties, the development of purification and selection strategies is essential. Herein, we report a systematic study of the spontaneous partition and separation of highly oxidized carbon dots (OCDs) prepared by the dehydration and oxidation reactions of cotton cellulose in aqueous two-phase systems (ATPSs) based on polymer-salt pairs. The partition of the CDs was investigated in different ATPSs in which the effects of the cations and anions of the salts, molecular mass and nature of the polymer, tie-line length, initial pH, and surface modification of the nanoparticles on the partition coefficient (K) were evaluated. The results showed that the best separation occurred with a system consisting of PEO1500 + lithium sulfate + water using reduced CDs with hydrazine. Alternatively, the lowest value of K, 0.79, was obtained for a poly(ethylene oxide) PEO1500 + sodium tartrate + water system with pH = 6 using OCDs. The detailed analyses of the top and bottom phases of the systems with fluorescence and ultraviolet-visible spectroscopy showed that ATPSs are capable, in addition to partitioning, of separating the nanoparticles with different optical properties, which are directly associated with the surface properties and particle sizes. We believe that the presented methodology is an alternative, practical, fast, and potentially scalable technique for the separation of carbon nanostructures with different optical properties.

Bio-based nanocomposites obtained through covalent linkage between chitosan and cellulose nanocrystals.

Bio-based nanocomposites were obtained through covalent linkage between cellulose nanocrystals (CNCs) and the natural polymer chitosan (CH). The CNCs were first functionalized with methyl adipoyl chloride (MAC) and the reactive end groups on the surface of the CNCs were reacted with the amino groups of the CH biopolymer in an aqueous medium. The functionalized CNCs and the resulting nanocomposites were characterized using FTIR, TEM, XRD, and elemental analyses. Characterization of the functionalized CNCs showed that up to 8% of the hydroxyl groups in the nanocrystals were substituted by the MAC residue. The covalent linkage between the CNCs and CH was confirmed by FTIR spectroscopy. The nanocomposites demonstrated a significant improvement in the mechanical performance and a considerable decrease in the hydrophilicity relative to the neat chitosan. The approach used in this work can be extended to other natural polymers.

Biobased nanocomposites from layer-by-layer assembly of cellulose nanowhiskers with chitosan.

A new biodegradable nanocomposite was obtained from layer-by-layer (LBL) technique using highly deacetylated chitosan and eucalyptus wood cellulose nanowhiskers (CNWs). Hydrogen bonds and electrostatic interactions between the negatively charged sulfate groups on the whisker surface and the ammonium groups of chitosan were the driving forces for the growth of the multilayered films. The film growth was followed by UV-vis spectroscopy through the maximum value of the absorption band at 194 nm and showed the deposition of 14.7 mg.m(-2) of chitosan polymer in each cycle. Scanning electron microscopy showed high density and homogeneous distribution of CNWs adsorbed on each chitosan layer. Cross-section characterization of the assembled films indicates an average of approximately 7 nm of thickness per bilayer. The results presented in this work indicate that the methodology used can be extended to different biopolymers for the design of new biobased nanocomposites in a wide range of applications such as biomedical and food packaging.