Next-generation biodiesel production: catalytic innovations and scalable microalgal biorefinery systems

The transition toward low-carbon liquid fuels is increasingly urgent due to fossil fuel depletion, carbon budget overshoot, and the escalating environmental and public health impacts associated with combustion-based energy systems. Biodiesel has emerged as a strategically important renewable fuel owing to its compatibility with existing diesel infrastructure and its potential to deliver meaningful life-cycle greenhouse gas reductions. This review provides a comprehensive and critical assessment of next-generation biodiesel production, integrating catalytic science, feedstock development, process engineering, sustainability metrics, and industrial implementation perspectives within a unified framework. Fundamental aspects of biodiesel chemistry, fuel properties, and quality standards are first outlined to establish a rigorous basis for subsequent analysis. The evolution of biodiesel feedstocks from first-generation edible oils to second-generation waste-derived lipids and third-generation microalgae is critically examined, highlighting sustainability trade-offs, resource constraints, and scalability considerations associated with each feedstock class. Particular attention is devoted to microalgal biorefinery systems, which offer exceptional lipid productivity, non-arable land utilization, carbon dioxide capture potential, and opportunities for resource recovery, while simultaneously facing significant cultivation, harvesting, and downstream processing challenges. The review further provides a mechanistic and comparative evaluation of homogeneous, heterogeneous, bifunctional, and enzymatic catalytic systems, together with quantitative assessment of catalytic performance, feedstock tolerance, operational stability, and industrial applicability. Recent advances in process-intensification technologies, including supercritical processing, continuous-flow reactors, reactive distillation, and non-thermal enhancement techniques, are critically evaluated in the context of biodiesel commercialization. Furthermore, techno-economic analysis (TEA), life-cycle assessment (LCA), technology-readiness considerations, and industrial scalability challenges are integrated to identify key bottlenecks and future development priorities. By integrating catalytic innovation, feedstock evolution, process-intensification technologies, sustainability assessment, and commercialization perspectives, this review provides a distinctive roadmap for translating laboratory-scale advances into commercially competitive, low-carbon biodiesel technologies capable of supporting future energy-transition objectives.