Advancements in solid acid catalysts for biodiesel production
Biodiesel has emerged as one of the best potential renewable energy sources to replace current petroleum-based diesel. It is a sustainable, biodegradable and non-toxic diesel fuel substitute that can be easily produced through base- or acid-catalyzed esterification and transesterification reactions. The conventional base catalysts, although effective, are limited to use of refined vegetable oils, leading to impractical and uneconomical processes due to high feedstock cost and priority as food resources. Biodiesel production processes based on the use of acid catalysts are good alternatives to conventional processes because of their simplicity and the simultaneous promotion of esterification and transesterification reactions from low-grade, highly-acidic and water-containing oils without soap formation. Highly reactive homogeneous Brønsted acid catalysts are efficient for this process, but they suffer from serious contamination and corrosion problems that require the implementation of good separation and purification steps. More recently, a “green” approach to biodiesel production has stimulated the application of sustainable solid acid catalysts as replacements for such liquid acid catalysts so that the use of harmful substances and generation of toxic wastes are avoided; meanwhile, the ease of catalyst separation after the reactions can be realized. Recent studies have proven the technical feasibility and the environmental and economical benefits of biodiesel production via heterogeneous acid-catalyzed esterification and transesterification. In this perspective, various solid acids including sulfated metal oxides, H-form zeolites, sulfonic ion-exchange resins, sulfonic modified mesostructured silica materials, sulfonated carbon-based catalysts, heteropolyacids and acidic ionic liquids are reviewed as heterogeneous catalysts in esterification and transesterification. Meanwhile, for the purpose of facilitating mass-transport of solid acid-catalyzed biodiesel production processes and improving the catalytic stability of the solid acid catalysts in esterification and transesterification reactions, novel and robust organic–inorganic hybrid acid catalysts with unique advantages including strong Brønsted as well as Lewis acid properties, well-defined mesostructure and enhanced surface hydrophobicity are successfully designed, which have been highlighted in this review.