Editorial for Special Issue “Carbon-Based Catalysts to Address Environmental Challenges”

Carbon-based materials have attracted increasing attention in recent years due to their structural versatility, high surface area, and chemical tunability, which make them excellent candidates for catalytic applications, either as catalysts [1,2,3,4] or as supports [5,6,7]. The Special Issue “Carbon-Based Catalysts to Address Environmental Challenges” includes two review articles and four original research papers, each contributing to a deeper understanding of how carbon-based materials can help to address environmental challenges.

One of the reviews offers a comprehensive account of biochar-derived catalysts for key CO₂ conversion reactions, focusing on cycloaddition to epoxides, dry reforming of methane and catalytic biomass upgrading (Contribution 1). Emphasis is given to the role of biochar’s origin, preparation methods or tailored surface modifications—such as doping with heteroatoms or supporting metal nanoparticles—which critically influence its structure, surface functionality and, consequently, the catalytic performance towards the envisaged CO₂ conversion reactions.

The second review focuses on catalysis promoted by metal-free carbon-based materials (i.e., “carbocatalysis”), and presents several case studies, including the oxidative dehydrogenation of hydrocarbons, the oxidation of organic pollutants by advanced oxidation processes, acid catalysis (namely, esterification and etherification reactions) and electrocatalysis, particularly the oxygen reduction reaction (Contribution 2). Strategies to improve catalytic activity in selected reactions by tailoring the heteroatom doping and surface modifications are described, and the mechanistic roles played by some oxygen-, nitrogen-, and sulfur-functionalities are discussed.

Regarding the original research articles published in this Special Issue, Eblagon et al. reported a simple and low-cost microwave-assisted method to produce 5-hydroxymethylfurfural (HMF) from sucrose, using N, S-doped hydrochars as carbocatalysts (Contribution 3). The hydrochars, prepared by hydrothermal carbonization of glucose with urea and thiourea, provided both acidic and basic sites. Catalytic tests showed that the HMF yield (up to 37%) depended on the balance of these sites and the nature of the S and N species. A kinetic model confirmed good agreement with experimental data, indicating that higher sucrose loading enhanced fructose dehydration and reduced side reactions. Glucose isomerisation to fructose was minimal in air but slightly improved in oxygen. Enhancing this isomerisation step was reported as the key to increasing HMF selectivity and reducing humin by-products.

Rodríguez-Ramos et al. presented a study based on the Guerbet reaction as a sustainable route to produce 1-butanol from bioethanol, using multifunctional catalysts supported on high-surface-area graphite (HSAG). Catalysts combining Cu and Ni nanoparticles with MgO were tested under moderate conditions (440–580 K, 50 bar) (Contribution 4). The synergy between metal sites, which promote dehydrogenation/hydrogenation reactions, and well-balanced acid/base sites, responsible for aldol condensation and dehydration, improves the selectivity towards 1-butanol. Results showed that medium–high strength basic sites with moderate acidity favor butanol selectivity. An optimized formulation containing 4 wt.% Cu and 1 wt.% Ni minimized byproducts, achieving a butanol selectivity of 44% and a yield of ~9% with the 4Cu1Ni-Mg/HSAG catalyst pretreated in helium at 723 K. The role of the graphite support, promoting nanoparticle formation and maximizing the exposure of active phases, together with its hydrophobicity, was emphasized.

Baldo et al. reported an original study in a different field (Contribution 5). They explored the use of hybrid magnetic catalysts for pollutant degradation. They synthesized a multi-core magnetic catalyst, based on cobalt ferrite (CoFe₂O₄) and magnetite (Fe₃O₄) coated with a thin carbon layer from the carbonization of a phloroglucinol–glyoxal-derived resin, for the catalytic wet peroxide oxidation (CWPO) of nitrophenols, a class of persistent organic pollutants. The carbon coating improved stability, preventing iron leaching, while maintaining high catalytic activity. TEM confirmed multi-core structures with nanoscale carbon layers. In degradation tests, CoFe@C achieved up to 96–99% removal of 2- and 4-nitrophenol, with superior activity in multi-component systems, while Fe₃O₄@C showed comparable performance. By combining the catalytic activity of the hybrid core with the stabilizing carbon coating, these catalysts showed high catalytic efficiency and facile recovery due to their magnetic properties.

Formic acid dehydrogenation has recently shown great promise in the chemical H₂ storage context. In this field, Bernal-Vela investigated Pd nanoparticles supported on C₃N₄-modified activated carbon, derived from biomass residues, as a catalyst in the decomposition of formic acid in the liquid phase (Contribution 6). The work highlighted the beneficial effect of incorporating C₃N₄ for the formation of small, uniformly dispersed Pd nanoparticles, with the catalysts showing outstanding activity and stability; in particular, Pd/AC_C₃N₄ with 19 wt.% of C₃N₄ reached an impressive initial TOF of 2893 h⁻¹ and retained most of its activity over six consecutive cycles, underscoring the robustness of this catalytic system.

When considered together, the contributions to this Special Issue emphasize several key messages. Sustainable catalyst design based on renewable precursors such as biomass emerges as a central theme, aligning the development of novel materials with circular economy principles. Doping with heteroatoms and/or incorporation of metals or other active sites within carbon materials is shown to be a powerful strategy to boost catalytic activity and the potential of carbon-based catalysts to address real environmental problems. Looking ahead, further advances can be expected from the refinement of synthesis strategies, the exploration of new dopants and surface modifications, and the application of advanced characterization tools to identify the active sites and clarify the reaction pathways.

We are deeply grateful to all the authors who contributed to this Special Issue. We trust that this collection will inspire further research in the field of carbon-based catalysts, and contribute to the development of sustainable solutions for new environmental challenges.