Biomass valorization with metal-free catalysts: innovations in thermocatalytic, photocatalytic, and electrocatalytic approaches

The catalytic valorization of biomass into high-value chemicals and sustainable fuels is critical for addressing global environmental challenges and advancing a bio-based circular economy. Traditional metal-based catalysts, though effective, face major limitations, including resource scarcity, toxicity, leaching, and cost, underscoring the need for alternative catalytic paradigms. Metal-free catalytic systems have emerged as promising sustainable solutions due to their environmental compatibility, cost-effectiveness, and material abundance. This review comprehensively evaluates recent progress in metal-free catalysis for biomass valorization, uniquely integrating and comparing thermal, photocatalytic, and electrocatalytic methodologies. We systematically discuss diverse classes of metal-free catalysts, including carbon-only materials, heteroatom-doped carbons, and emerging non-carbon frameworks, while highlighting advanced synthesis strategies, tailored active site engineering, mechanistic insights, and catalyst recyclability under varying operational conditions. The comparative analysis reveals distinct advantages and limitations inherent to each catalytic route, emphasizing the tunability and versatility of metal-free systems. Importantly, future proposed directions are rooted in the synergistic integration of photothermal and photoelectrochemical pathways, paving the way for next-generation multifunctional catalytic systems. By identifying persistent challenges such as active site localization, long-term stability, reaction selectivity, and scalability, the review advocates for interdisciplinary efforts incorporating advanced heterostructure design and AI-driven catalyst optimization to realize the full potential of metal-free catalysis in sustainable biomass valorization.