Hydrothermal carbonization (HTC) is a promising method for converting wet biomass into carbon materials; however, the mechanisms underlying pore formation during HTC remain poorly understood. This study examined how reaction temperature affects pore development in cypress hydrochar. Hydrochars were prepared at 170–240 °C and characterized by nitrogen adsorption and thermogravimetric analysis. While samples prepared at 170 and 200 °C showed almost no porosity, the hydrochar produced at 240 °C exhibited a significant increase in mesopore volume despite a low surface area, indicating the formation of relatively large pores (~50 nm). Thermogravimetric analysis revealed extensive hydrolysis of hemicellulose and cellulose at 240 °C, yielding a thermally stable structure enriched in lignin-derived components. To further develop the pore structure, the hydrochar was carbonized at 900 °C. This carbonization greatly increased microporosity while partially maintaining larger pores, creating a hierarchical pore structure. In contrast, raw cypress char exhibited mainly microporosity, with little mesoporosity. These findings suggest that higher HTC temperatures play a key role in mesopore formation, likely through enhanced hydrolysis and heterogeneous solid formation, offering insights into the design of biomass-derived porous carbon materials.

Carbon materials containing pyridinic nitrogen have been reported to exhibit high catalytic and electrode performance. Especially, one sp²C-H group adjacent to pyridinic nitrogen on one pyridine ring (denoted as N-SOLO) has been reported to act as an active site for oxygen reduction reactions in fuel cells. However, for the mass production of carbon materials with such structures, precise structural control must be achieved without using metal catalysts because the difficulty in removing metals afterwards increases costs. In this study, brominated compounds with two fused rings were utilized as precursors to increase the percentages of both pyridinic nitrogen and N-SOLO. Heat treatment of 3,7-dibromo-1,5-naphthyridine at 623 K yielded 74% pyridinic nitrogen (N content: 11 at.%) with 74% of N-SOLO/one sp²C-H group on one aromatic ring (SOLO), which is the highest percentage of N-SOLO/SOLO in carbon materials ever obtained without using catalysts. Screening of precursors by density functional theory calculations and molecular dynamics simulations with a reactive force field (ReaxFF) revealed that 3,7-dibromo-1,5-naphthyridine favored the formation of pyridinic nitrogen due to the suppression of tertiary nitrogen formation. Furthermore, N-SOLO exhibited the highest occupied orbital energy among various sp²C-H groups adjacent to pyridinic nitrogen, suggesting that N-SOLO is the most active site.

Developing efficient carbon-based adsorbents for CO₂ and H₂O capture is critical for mitigating climate change. Although functionalization of carbon materials is essential for efficient adsorption, the structural control of carbon materials remains challenging. While oxygen functional groups can enhance surface polarity, conventional oxidation methods lack control over group arrangement and often degrade the carbon framework. This study successfully synthesized “Vicioxite”, a carbon material with controlled structures and adjacent oxygen sites (especially OH groups) via catalysts-free synthesis to elucidate the effective configurations for CO₂ and H₂O capture. We synthesized model materials using coronene-coated activated carbon fibers (ACFs) via a regioselective two-step bromination and hydrolysis process. This method successfully introduced adjacent OH groups while preserving the π-conjugated basal plane. Computational analyses revealed that the specific edge topology of coronene energetically favors the formation of adjacent OH pairs, whereas steric hindrance restricts their formation on armchair edges. Experimental characterization, including ¹³C nuclear magnetic resonance, confirmed these structures. Crucially, CO₂ temperature-programmed desorption demonstrated that samples rich in adjacent OH groups exhibited superior CO₂ adsorption capacity compared to untreated ACF. These results suggest that adjacent OH groups induce interactions, demonstrating that controlling the local spatial arrangement of functional groups is essential for designing high-performance adsorbents.