Calcium-carbide residue: A precursor for the synthesis of CaO–Al₂O₃–SiO₂–CaSO₄ solid acid catalyst for biodiesel production using waste lard

Highlights

This study aims to synthesis solid-acid catalysts from the low-cost calcium-carbide residue (CCR) through thermal modification and sulphonation process. The objectives of the study are:

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Characterisations of the CCR using XRF and TGA, to determine its suitability for the catalyst synthesis.

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Synthesis of the catalyst by calcination and thereafter sulfonation process to produce anhydrite-based solid-acid catalysts.

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The catalysts were characterised for various properties such as functional group, morphology, composition, surface area and porosity.

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The functionality of the catalysts was tested for biodiesel production using waste lard.

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The biodiesel samples produced were tested for their physicochemical properties and FAME profiles.

Abstract

Calcium-carbide residue (CCR) was used as a precursor to synthesise CaO–Al₂O₃–SiO₂–CaSO₄ heterogeneous acid catalyst for biodiesel production. The synthesis was through a two-step process of thermal treatment of the CCR at 500, 700, and 900 °C and sulphonation, to produce anhydrite-based solid-acid catalysts (ASACs) as ASAC₅₀₀, ASAC_700, and ASAC₉₀₀ respectively. The CCR and ASACs were characterised using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Brunauer–Emmet–Teller (BET). The ASACs were tested for biodiesel production using high free fatty acid waste lard. The XRD analyses show that the three catalysts consist of CaSO₄ in addition to each containing Ca₂.62Al₉.8Si₂₆.2O₇₂H₄.₅₆ (ASAC₅₀₀), carbon atom and Ca₂.62Al₉.8Si₂₆.2O₇₂H₄.₅₆ (ASAC₇₀₀), Ca₂.62Al₉.8Si₂₆.2O₇₂H₄.₅₆ and Ca₂Al₄Si14O₃₆•14H₂O (ASAC₉₀₀). The SEM images of the ASACs show the formation of different surface morphology with active sites and improved porosity based on calcination temperatures. The BET analysis presents a surface area of 94.48 (ASAC₅₀₀), 90.28 (ASAC₇₀₀), and 98.22 m²/g (ASAC₉₀₀). The biodiesel yields obtained using 5% (w/w) catalyst, 12:1 of MeOH: Lard molar ratio, 120 min reaction time and 60 °C reaction temperature are 94.8% (ASAC₅₀₀), 89.2% (ASAC₇₀₀) and 98.9% (ASAC₉₀₀). The excellent performance of CaO–Al₂O₃–SiO₂–CaSO₄ catalyst with the high yields of biodiesel and recyclability of seven cycles are attributed to the synergy among the calcium, aluminium, and silicon. Therefore, the CCR is a suitable precursor to synthesise a novel heterogeneous acid catalyst that is highly effective for biodiesel production.