Heteropoly acids as supported solid acid catalysts for sustainable biodiesel production using vegetable oils: A review

Biodiesel is a promising renewable energy source with environmentally friendly properties such as non-toxicity, sulfur-free, and low emission of carbon dioxide. However, the higher cost of biodiesel production relative to petroleum diesel is the major challenge for the commercialization of biodiesel. The utilization of suitable feedstocks and an appropriate catalyst are two dominant factors, which play a crucial role in the determination of reaction conditions, biodiesel yield, and cost of biodiesel production. Heterogeneous catalysts were found to be promising relative to homogeneous catalysts owing to their high selectivity, reusability, and stability. Among several heterogeneous catalysts, heteropoly acids (HPAs) are efficient catalysts that have recently stimulated many scientists to investigate their application as solid acid catalysts using different support materials and functionalization techniques during transesterification. This review presents a comprehensive overview of the employment of various types of plant oils (edible and non-edible) as feedstocks, several heterogeneous catalysts, their properties, benefits, and drawbacks for biodiesel production. The primary objective of this review is to indicate the advances of supported heteropoly acid catalysts as efficient heterogeneous solid acid catalysts over other types of catalysts. This goal is thoroughly addressed by discussing the performance of different HPA catalysts by providing catalyst features. Further, the influence of critical factors on the properties of solid acid catalysts, different structures of HPAs, their advantages and challenges, suitable support materials for HPAs, modification of supported HPAs through the post, and direct functionalization and their mechanism is highlighted. This review is also addressed overcoming the leaching problem of HPAs to improve the activity and reusability of the HPA catalysts; further, economic challenges are discussed in detail to produce high-quality bio-diesel via HPA catalysts.