Carbon-based solid acid catalysts for energy-efficient biodiesel production: Integrating biomass valorization, process intensification, and circular bioeconomy

The transition toward low-carbon energy systems has increased interest in biodiesel production routes capable of upgrading low-grade lipid feedstocks with lower energy, material, and separation demands. Conventional homogeneous acid–base catalysis is still constrained by high alcohol consumption, wastewater generation, catalyst neutralization, and limited tolerance toward feedstocks containing high free fatty acid and water contents. Biomass-derived carbon-based solid acid catalysts (CSAC) have therefore emerged as promising catalytic platforms for esterification–transesterification reactions. Their practical value, however, cannot be judged only from catalytic activity, because industrial relevance also depends on controllable catalyst synthesis, acid-site distribution, pore structure, hydrothermal stability, regeneration capacity, and compatibility with intensified process configurations. This review examines biomass-derived CSAC for biodiesel production by linking catalyst design with process intensification, stability assessment, uncertainty analysis, techno-economic evaluation, and circular bioeconomy implementation. Different carbon precursors, functionalization routes, and sulfonation strategies are discussed in relation to structure–acidity–activity relationships, impurity tolerance, catalyst deactivation, and regeneration behavior. Particular attention is given to oil palm empty fruit bunch (OPEFB)-derived CSAC as representative waste-biomass-based catalysts with potential relevance for regions where palm biomass residues are abundant. Rather than treating biomass valorization and biodiesel catalysis as separate topics, this review frames OPEFB-derived CSAC as a possible interface between agricultural residue utilization, low-quality lipid upgrading, and renewable fuel production. Process intensification strategies, including reactive distillation, microwave-assisted processing, ultrasound-assisted transesterification, and magnetically recoverable catalytic systems, are evaluated based on their potential to shorten reaction time, reduce alcohol demand, simplify catalyst separation, and lower energy intensity. Optimization approaches such as Design of Experiments, Response Surface Methodology, machine-learning-assisted modeling, and uncertainty analysis are also reviewed as tools for defining reliable operating windows and supporting catalyst–process co-design. The main contribution of this review is the integration of catalyst classification, structure–performance relationships, stability and regeneration behavior, intensified processing, uncertainty-aware optimization, and techno-economic considerations into a unified assessment framework. OPEFB-derived CSAC are not presented as immediately commercial catalysts; instead, this review identifies the material, kinetic, operational, and scale-up requirements that must be validated before industrial deployment. Their feasibility will depend on reproducible synthesis, long-term catalytic stability, effective regeneration, process integration, and favorable life-cycle and techno-economic performance under realistic operating conditions.