Advances in lignocellulosic biomass pyrolysis and catalytic upgrading for sustainable biofuel production: process design strategies and reaction rationales

Biomass pyrolysis offers a promising route toward renewable fuels and chemicals, significantly reducing reliance on fossil-based resources. However, conventional pyrolysis produces bio-oils rich in oxygen, resulting in undesirable properties such as high acidity, poor thermal stability, corrosiveness, and excessive water content, which complicate downstream upgrading. To overcome these challenges, various upstream (e.g., modified fast pyrolysis, catalytic pyrolysis, catalytic hydropyrolysis, etc.) and downstream (e.g., hydrodeoxygenation, catalytic cracking, esterification) strategies have been developed to efficiently remove oxygen and enhance bio-oil quality. Although substantial research has addressed these methods individually, comprehensive reviews that examine their collective impacts on bio-oil properties and catalyst performance remain limited. Furthermore, while common model compounds like guaiacol and phenol are frequently studied, real biocrudes contain complex mixtures of bulky polyaromatic hydrocarbons (PAHs), significantly accelerating catalyst deactivation, a critical yet often overlooked issue. This review critically assesses recent advances in biomass pyrolysis techniques and catalytic upgrading strategies, emphasizing the roles of bifunctional catalysts and hydrogen donors in minimizing coke formation, prolonging catalyst lifetime, and enhancing bio-oil quality. Catalytic hydropyrolysis is highlighted as an effective single-step method for generating high-quality hydrocarbons directly from biomass. Additionally, we underscore the importance of employing realistic mixed-model compounds (phenolics and PAHs) and continuous-flow reactor conditions to accurately represent industrial processes and catalyst deactivation mechanisms. Finally, key research gaps and scale-up challenges, including catalyst stability, product selectivity, hydrogen requirements, scalability, and techno-economic considerations, are identified, providing clear directions for future research. By integrating knowledge across pyrolysis and upgrading strategies, this review aims to guide the development of more efficient, economically viable, and sustainable pathways toward commercial biofuel production.