Chemical simulation of high-performance CaO/La2O3 catalysts on its anti-leaching property during biodiesel production

Abstract

Biodiesel has garnered significant attention as a renewable liquid fuel owing to its biodegradability and potential to achieve net-zero carbon emissions. However, conventional homogeneous catalysts such as KOH or NaOH face challenges in saponification control and product purification due to their high solubility. Although CaO-based heterogeneous catalysts help mitigate these issues, their susceptibility to leaching during transesterification remains a critical limitation, hindering their industrial application. Due to the insufficient understanding of the underlying mechanisms, progress in enhancing the anti-leaching properties of CaO has been slow. This study introduces a novel competitive-adsorption strategy to enhance the anti-leaching properties of CaO active sites by incorporating La₂O₃ as a protective phase. Based on this strategy, a CaO/La₂O₃ catalyst with enhanced anti-leaching properties was successfully constructed through a citric acid-assisted wet impregnation method. The resulting CaO/La₂O₃ catalyst, with a Ca/La molar ratio of 0.12 and CaO nanoparticles dispersed on the La₂O₃ support, demonstrated exceptional stability, retaining 96.7% of its initial catalytic activity after 10 reuse cycles. The competitive-adsorption theory was proved by investigating the relationship between the Ca/La ratio and the activity retention rate in a reusability test. Because of the larger exposed La₂O₃ area, which competitively adsorbs glycerol, CaO/La₂O₃ catalysts with lower Ca/La ratios remain more stable in reaction. Density functional theory (DFT) calculations employing a four-glycerol molecular model demonstrated that glycerol-induced Ca leaching originates from the synergistic effects of intermolecular hydrogen bonding, intramolecular hydrogen bonding, and 1′-OH group interactions at CaO(100) sites. Ca leaching behavior was compared between CaO and CaO/La₂O₃. It was found that the incorporation of La₂O₃ was shown to substantially mitigate glycerol-induced leaching of the supported CaO compared to pure CaO, as confirmed by the metal content analysis of the reaction products. This work provides a mechanistic framework for designing leaching-resistant heterogeneous catalysts for sustainable biodiesel production.