Recent Advances and Challenges in Photoreforming of Biomass-Derived Feedstocks into Hydrogen, Biofuels, or Chemicals by Using Functional Carbon Nitride Photocatalysts

Selective C(aryl)-O bond cleavage in biorenewable phenolics

Biorefining of lignocellulosic biomass via a lignin first approach delivers a range of products with high oxygen content. Besides pulp, a lignin oil rich in guaiacols and syringols is obtained bearing multiple C(aryl)-OH and C(aryl)-OMe groups, typically named phenolics. Similarly, technical lignin can be used but is generally more difficult to process providing lower yields of monomers. Removal of the hydroxy and methoxy groups in these oxygenated arenes is challenging due to the inherently strong C-O bonds, in addition to the steric and electronic deactivation by adjacent -OH or -OMe groups. Moreover, chemoselective removal of a specific group in the presence of other similar functionalities is non-trivial. Other side-reactions such as ring saturation and transalkylation further complicate the desired reduction process. In this overview, three different selective reduction reactions are considered. Complete hydrodeoxygenation removes both hydroxy and methoxy groups resulting in benzene and alkylated derivatives (BTX type products) which is often complicated by overreduction of the arene ring. Hydrodemethoxylation selectively removes methoxy groups in the presence of hydroxy groups leading to phenol products, while hydrodehydroxylation only removes hydroxy groups without cleavage of methoxy groups giving anisole products. Instead of defunctionalization via reduction transformation of C(aryl)-OH, albeit via an initial derivatization into C(aryl)-OX, into other functionalities is possible and also discussed. In addition to methods applying guaiacols and syringols present in lignin oil as model substrates, special attention is given to methods using mixtures of these compounds obtained from wood/technical lignin. Finally, other important aspects of C-O bond activation with respect to green chemistry are discussed.

Solar-Driven Biomass Reforming for Hydrogen Generation: Principles, Advances, and Challenges

Hydrogen (H2) has emerged as a clean and versatile energy carrier to power a carbon-neutral economy for the post-fossil era. Hydrogen generation from low-cost and renewable biomass by virtually inexhaustible solar energy presents an innovative strategy to process organic solid waste, combat the energy crisis, and achieve carbon neutrality. Herein, the progress and breakthroughs in solar-powered H2 production from biomass are reviewed. The basic principles of solar-driven H2 generation from biomass are first introduced for a better understanding of the reaction mechanism. Next, the merits and shortcomings of various semiconductors and cocatalysts are summarized, and the strategies for addressing the related issues are also elaborated. Then, various bio-based feedstocks for solar-driven H2 production are reviewed with an emphasis on the effect of photocatalysts and catalytic systems on performance. Of note, the concurrent generation of value-added chemicals from biomass reforming is emphasized as well. Meanwhile, the emerging photo-thermal coupling strategy that shows a grand prospect for maximally utilizing the entire solar energy spectrum is also discussed. Further, the direct utilization of hydrogen from biomass as a green reductant for producing value-added chemicals via organic reactions is also highlighted. Finally, the challenges and perspectives of photoreforming biomass toward hydrogen are envisioned.

Pt-TiO2 catalysts for glycerol photoreforming: comparison of anatase, brookite and rutile polymorphs

We compared the H2 production from glycerol photoreforming for different TiO2 polymorphs, highlighting an increase of activity in the order Pt-rutile < Pt-P25 ≈ Pt-anatase < Pt-brookite with a different distribution of the reaction intermediates. We show that the highest ability to adsorb water and the different distribution of Pt active sites in brookite can positively influence its photoactivity.

Designing Metasurfaces for Efficient Solar Energy Conversion

Metasurfaces have recently emerged as a promising technological platform, offering unprecedented control over light by structuring materials at the nanoscale using two-dimensional arrays of subwavelength nanoresonators. These metasurfaces possess exceptional optical properties, enabling a wide variety of applications in imaging, sensing, telecommunication, and energy-related fields. One significant advantage of metasurfaces lies in their ability to manipulate the optical spectrum by precisely engineering the geometry and material composition of the nanoresonators' array. Consequently, they hold tremendous potential for efficient solar light harvesting and conversion. In this Review, we delve into the current state-of-the-art in solar energy conversion devices based on metasurfaces. First, we provide an overview of the fundamental processes involved in solar energy conversion, alongside an introduction to the primary classes of metasurfaces, namely, plasmonic and dielectric metasurfaces. Subsequently, we explore the numerical tools used that guide the design of metasurfaces, focusing particularly on inverse design methods that facilitate an optimized optical response. To showcase the practical applications of metasurfaces, we present selected examples across various domains such as photovoltaics, photoelectrochemistry, photocatalysis, solar-thermal and photothermal routes, and radiative cooling. These examples highlight the ways in which metasurfaces can be leveraged to harness solar energy effectively. By tailoring the optical properties of metasurfaces, significant advancements can be expected in solar energy harvesting technologies, offering new practical solutions to support an emerging sustainable society.

Sustainable Production of Bioactive Molecules from C-Lignin-Derived Propenylcatechol

Catechyl lignin (C-lignin) is a naturally occurring linear homogeneous biopolymer composed solely of caffeyl alcohol subunits with cleavable benzodioxane linkages. The inherent structural features of propenylcatechol, a direct depolymerized product of castor seed coats C-lignin, render it a sustainable and promising platform for the synthesis of bioactive molecules. Herein, diversified transformations of propenylcatechol, including C=C bond difunctionalization, β-modification, β,γ-rearrangement, and γ-methyl derivatization, were reported based on known or developed methods. A series of functional molecular skeletons involved in the current synthetic routes for the preparation of pharmaceuticals and bioactive molecules were obtained. Starting from castor seed coats, annuloline (natural product) and CC-5079 (antitumor) were synthesized using facile and inexpensive reagents in only four- and five-sequence reactions, respectively, thereby demonstrating a superior step-efficiency to that of reported synthetic routes. Almost all atoms in the C-lignin biopolymer were incorporated into the final products owing to the intrinsic structures of naturally occurring C-lignin. Bioactive molecules produced from C-lignin integrate a low-carbon footprint with high-quality and economical manufacture of pharmaceuticals.

Two-Dimensional Dirac Nodal Line Carbon Nitride to Anchor Single-Atom Catalyst for Oxygen Reduction Reaction

Two-dimensional carbon nitride (2DCN) materials have emerged as an important class of 2D materials beyond graphene. However, 2DCN materials with nodal-line semimetal characteristic are rarely reported. In this work, a new nodal-line semimetal 2DCN with the stoichiometry C₄ N₄ is designed by using density functional theory (DFT) calculations and its application to anchor single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) is investigated. C₄ N₄ is a planar covalent network (sp² hybridization) with regular holes formed by the four N atoms, which is dynamically, thermodynamically, and mechanically stable. The nodal line is contributed by the p_z orbitals of C and p_x/y orbitals of N atoms. C₄ N₄ shows an anisotropic Fermi velocity and high electron mobility. Because of its porous structure, C₄ N₄ can anchor heteroatoms as SACs for electrocatalysis. C₄ N₄ anchored with Fe or Co is shown to be highly active for the ORR with a rather high half-wave potential of around 0.90 V, which is higher than those of SACs on other carbon nitrides. These findings may provide a new strategy to design novel substrates for SACs.