Investigation of the Potential of Carica papaya Seeds as a Biomass-based Catalyst for Biodiesel Production
Filbert Paul *
School of Materials, Energy, Water and Environmental Sciences, Nelson Mandela African Institution of Science and Technology, Arusha, Tanzania.
Isack Kandola
School of Materials, Energy, Water and Environmental Sciences, Nelson Mandela African Institution of Science and Technology, Arusha, Tanzania.
Justine Ntalikwa
College of Earth Sciences and Engineering, The University of Dodoma, Dodoma, Tanzania.
Thomas Kivevele
School of Materials, Energy, Water and Environmental Sciences, Nelson Mandela African Institution of Science and Technology, Arusha, Tanzania.
*Author to whom correspondence should be addressed.
Abstract
Biomass-derived catalysts offer a potential route for using agricultural residues in biodiesel production. A cost-effective catalyst synthesis route utilising biomass was explored, whereby Carica papaya seeds were investigated as a sustainable catalyst for biodiesel production. Carica papaya seed ash was produced by calcining dried papaya seed powder for three hours at 500 °C, 550 °C, 600 °C, 650 °C, and 700 °C. The calcination temperature, typically ranging between 500 °C and 700 °C, is strongly influenced by the nature of the precursor material; below this range oxide formation is incomplete, whereas higher temperatures lead to the destruction of active oxide sites. In this study, calcination was carefully controlled for three hours at each temperature. Subsequently, transesterification of methanol with Eucalyptus globulus oil was carried out using the calcined Carica papaya seed ash as a catalyst, and the effects of reaction time, catalyst calcination temperature, and catalyst loading were systematically investigated. Biodiesel properties were evaluated according to EN 14214. Carica papaya seed ash was characterised by SEM-EDX, FTIR, BET and XRD. SEM-EDX confirmed active oxides such as K₂O, MgO, and CaO, which act as catalytic sites in transesterification. Optimised conditions yielded 96.5% biodiesel at 60 °C, 2 h reaction time, 600 °C calcination, a 2 wt% catalyst loading, and a 12:1 methanol-to-oil ratio. Several measured biodiesel properties were within the stated EN 14214 limits, although the reported viscosity was slightly below the specified minimum. Carica papaya seed ash calcined at 600°C is a low-cost and efficient catalyst. Further investigation is required before industrial application can be assessed.
Keywords: Carica papaya seeds, transesterification, biodiesel, Eucalyptus globulus, heterogeneous catalysis, calcination temperature, catalyst reusability