Hierarchical zeolite for biomass conversion to biofuel: A review

The development of biofuel production from biomass has become a promising breakthrough and could tremendously enhance the potential of industrial technology. In this case, zeolites have emerged as suitable materials for catalyzing biomass conversion due to their outstanding catalytic properties, including the presence of the intrinsic acid sites, shape-selectivity properties, and high thermal stability. However, the sole micropores in zeolite cause diffusional limitation issues, especially for the bulky molecules involved in biomass feedstocks. For instance, many oxygenate molecules are too large to enter the micropores, so they cannot be converted into the product. In this case, hierarchical porosity could facilitate these molecules to access the acid site within the zeolite crystals. Research on synthetic strategies, modifications, and evaluations of their catalytic properties has consistently grown every year. This article reviews the recent development of hierarchical zeolite catalysts for biomass conversion to biofuel. Numerous strategies of hierarchical zeolite fabrication (bottom-up, top-down methods, and green synthesis approaches) and its modification (metals and functionalization with organic materials) and their characteristics are comprehensively reviewed. The key point in the fabrication of hierarchical zeolites is the development of notable mesopores while preserving the intrinsic micropores. Moreover, the role of hierarchical zeolites in various biofuel and bio-based chemicals for biofuel and biofuel additives production reactions, i.e., pyrolysis, hydrolysis, esterification and transesterification, isomerization, condensation, upgrading of bio-oil, and catalytic cracking reactions, are discussed in detail. Finally, the remaining challenges and insight that can be considered for further improvement are provided.