In response to the global demand for cleaner energy, this research presents a resourceful and sustainable route for biodiesel production through the design of a novel catalyst derived from two agro-waste materials: chicken eggshells and coconut shell–pith biomass. The CaO/activated carbon (CaO/AC) composite catalysts were synthesized via calcination, carbonization, and wet impregnation, followed by thermal treatment, without the use of surfactants, binders, or toxic chemicals, ensuring an environmentally benign process. Characterization by XRD confirmed the presence of crystalline CaO phases and amorphous carbon. FTIR and SEM analyses further validated the functional group interactions and porous surface morphology, which are critical for catalytic performance. The catalyst was evaluated by transesterifying waste cooking oil (WCO) at varying methanol-to-oil ratios (8:1–10:1), catalyst loadings (6–8 wt%), and reaction temperatures (60–65 °C). The best performance was observed with the C-CCS catalyst (derived from coconut shell), achieving a maximum biodiesel yield of 92.48% at a 10:1 MeOH: oil ratio, 8 wt% catalyst loading, 65 °C, and 90 min. The resulting biodiesel met ASTM D6751 specifications in all key fuel properties, including viscosity, density, flash point, acid value, and moisture content. This research presents a low-cost and scalable synthesis strategy that valorizes dual biowaste streams while producing engine-grade biodiesel. Its operational simplicity and high efficiency make it a strong candidate for industrial biodiesel applications, particularly in resource-constrained settings.

Dual-waste valorization of eggshell and coconut shell–pith biomass into CaO/activated carbon catalysts: synthesis, characterization, and structure–performance correlations in biodiesel transesterification
    Abstract
    1 Introduction
    2 Materials and methods
        2.1 Materials
        2.2 Catalyst preparation
            2.2.1 CaO synthesis from waste eggshells
            2.2.2 AC synthesis from coconut shell–pith biomass
            2.2.3 CaO/AC composite synthesis
    2.3 Transesterification procedure
        2.3.1 Feedstock pre-treatment
        2.3.2 Determination of acid value and FFA content
        2.3.3 Experimental setup and process design
    2.4 Biodiesel analysis
    3 Results and discussion
        3.1 Catalyst characterization
            3.1.1 X-Ray Diffraction (XRD) analysis
            3.1.2 Scanning electron microscopy
            3.1.3 FTIR spectroscopy
    3.2 Catalytic performance in biodiesel production
        3.2.1 Effect of process parameters on biodiesel yield
        3.2.2 Determination of optimal conditions
    3.3 Fuel property testing of optimized biodiesel
    3.4 Comparative benchmarking with literature
    4 Conclusion and outlook
    References

• Moeen Ali Rashad(Department of Chemical Engineering, COMSATS University Islamabad, Lahore Campus, Lahore, Pakistan)
• Muhammad Haris Hamayun(Department of Chemical Engineering, COMSATS University Islamabad, Lahore Campus, Lahore, Pakistan)
• Ali Ahsan(Department of Chemical Engineering, COMSATS University Islamabad, Lahore Campus, Lahore, Pakistan)
• Murid Hussain(Faculty of Engineering and Technology, Muscat University, Muscat, Oman, New Energies Research Center, Muscat University, Muscat, Oman) Corresponding author
• Farrukh Jamil(Faculty of Engineering and Technology, Muscat University, Muscat, Oman, New Energies Research Center, Muscat University, Muscat, Oman)
• Parveen Akhter(Department of Chemistry, The University of Lahore, 1-km Defense Road, Lahore, Pakistan)
• Young-Kwon Park(School of Environmental Engineering, University of Seoul, Seoul 02504, South Korea) Corresponding author