To achieve energy transition, hydrogen and carboxylic acids have attracted much attention due to their cleanliness and renewability. Anaerobic fermentation technology is an effective combination of waste biomass resource utilization and renewable energy development. Therefore, the utilization of anaerobic fermentation technology is expected to achieve efficient co-production of hydrogen and carboxylic acids. However, this process is fundamentally affected by gas–liquid mass transfer kinetics, bubble behaviors, and system partial pressure. Moreover, the related studies are few and unfocused, and no systematic research has been developed yet. This review systematically summarizes and discusses the basic mathematical models used for gas–liquid mass transfer kinetics, the relationship between gas solubility and mass transfer, and the liquid-phase product composition. The review analyzes the roles of the headspace gas composition and partial pressure of the reaction system in regulating co-production. Additionally, we discuss strategies to optimize the metabolic pathways by modulating the gas composition and partial pressure. Finally, the feasibility of and prospects for the realization of hydrogen and carboxylic acid co-production in anaerobic fermentation systems are outlined. By exploring information related to gas mass transfer and system pressure, this review will surely provide an important reference for promoting cleaner production of sustainable energy.

To address the inherent challenge of poor interfacial compatibility in lignin/poly(butylene adipate-co-terephthalate) (PBAT) composites, lignin was extracted from Camellia oleifera shells and subjected to sequential solvent fractionation using ethanol, acetone, and tetrahydrofuran (THF). Two representative fractions—acetone-soluble (ACL) and THF-soluble (THFL)—were selected for composite preparation with PBAT via solvent casting. The influence of lignin fractionation on the structural and performance characteristics of the resulting composites was systematically evaluated through Fourier-transform infrared (FTIR) spectroscopy, the water contact angle (WCA), differential scanning calorimetry (DSC), tensile testing, and scanning electron microscopy (SEM). The results revealed that the abundant hydroxyl groups and benzene rings present in both ACL and THFL facilitated hydrogen bonding and conjugation interactions with the PBAT matrix, significantly improving interfacial adhesion. Notably, the ACL fraction effectively suppressed phase separation and increased the glass transition temperature (T_g) by 1.9 °C, leading to a 13.9% enhancement in tensile strength compared to neat PBAT. More strikingly, the incorporation of only 7 wt% THFL resulted in a remarkable 31% improvement in tensile strength. This substantial enhancement was primarily attributed to the favorable polarity match between THFL and PBAT, as well as the nucleating effect of THFL, which increased the crystallinity of PBAT by 25.3%. This study highlights the effectiveness of sequential lignin fractionation in tailoring the interfacial properties of biodegradable polymer composites. It also provides a promising strategy for the high-value utilization of lignin toward the development of high-performance, environmentally friendly materials.