(137c) Biodiesel Synthesis from Oil Having High Free Fatty Acid Content Using Low-Cost Heterogeneous Catalysts

Environmental pollution due to carbon emissions is increasing worldwide due to increased crude oil consumption to meet energy demands. Energy production from renewable energy sources and its proper use is essential to reduce environmental pollution, increase energy security and lower fossil fuel consumption. Some renewable energy resources are solar, wind, geothermal, and biofuels. Biodiesel is one type of biofuel that can be produced locally. Biodiesel is a mixture of different fatty acid alkyl esters (FAAE), which can be used as a fuel-blend in diesel engine vehicles and reduces CO₂ emissions [1]. The chemical reactions involved in biodiesel synthesis include the transesterification of triglycerides (TGs) and esterification of free fatty acids (FFA) with alcohol (methanol or ethanol). Both are homogeneous or heterogeneous catalytic reactions; a base catalyst for transesterification and an acid catalyst for esterification are required. In comparison, the kinetics of transesterification is faster than the esterification at low reaction temperatures (T = 65 °C) when a base catalyst is used. So, transesterification of TGs is preferred over esterification of FFA to prepare the biodiesel commercially. Nowadays, edible oil containing 100 % TGs is used for commercial biodiesel synthesis – however, this leads to cooking oil scarcity and a food-versus-fuel debate [2].

The production cost of biodiesel is higher than petroleum diesel (about 3 times higher) if edible vegetable oil is used as a feedstock. The production cost can be significantly reduced using low-cost feedstocks like non-edible vegetable oil and waste cooking oil. It would solve the environmental problems arising from waste cooking oil disposal [3]. However, these feedstocks have higher FFA content (>2 wt.%), resulting in soap formation with the base catalyst during the transesterification of TGs in the oil. So, the TG to biodiesel conversion is adversely affected due to the consumption of the base catalyst. Also, the presence of soap reduces the biodiesel quality [4]. The alternative way of avoiding the saponification of FFAs is to reduce the FFA amount in the oil by performing the esterification first. After that, transesterification is performed to convert the remaining TGs to biodiesel. This two-step method of biodiesel synthesis increases the overall production costs. If a homogeneous catalyst (base or acid) is used, then water washing of biodiesel is recommended after the synthesis. Water washing of biodiesel is required to remove homogeneous catalysts and soap from the biodiesel, which causes a lot of wastewater generation [5].

To overcome these problems, synthesizing a heterogeneous solid catalyst containing both acidic and basic sites (bi-functional catalyst) is recommended, which can be separated after reaction easily through simple filtration. The bifunctional catalyst can help both the transesterification and esterification of TGs and FFAs simultaneously in a single pot at the same reaction conditions. The bifunctional catalysts can be synthesized by impregnation, sol-gel and co-precipitation methods [6]. It is noted that there are only a few works reported on using a physical mixture of acid and base catalysts for the synthesis of biodiesel from oil containing higher FFA.

A physical mixture of two low-cost heterogeneous catalysts is used to synthesize biodiesel from high FFA (5 wt.%, 10 wt.% and 20 wt.%) containing oil in this work. The catalysts used are an acid catalyst in the form of a silica-alumina-based compound (SiAl) from an industry and base catalysts CaO and calcium diglyceroxide (CaD).The CaD catalyst is synthesized by refluxing the CaO, glycerol and methanol in a mass ratio of 1:14.667:3.33 at 60 °C [7]. The CaO is synthesized by calcining the waste eggshell powder (particle size <500 µm) at 900 °C for 3 hours. The catalyst SiAl is also calcined at 900 °C to remove the unwanted impurities from the surface. The catalysts used are derived from waste materials and help in the efforts to move towards a circular economy for biodiesel synthesis. The proportions of CaO and SiAl mixing used were CaO + 5 % SiAl, CaO + 10 % SiAl, and CaO + 20 % SiAl. The biodiesel synthesis was carried out at reaction conditions of 65 °C, 3 wt.% of catalyst loading, 6:1 methanol-to-oil molar ratio and 200 rpm for the stirring. The reaction setup includes a round bottom flask (RBF) connected with a condenser; mixing and heating the reactants in the RBF were carried out by a mechanical stirrer and an oil bath, respectively. The structural and physio-chemical characterizations of the catalysts were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform-infrared spectroscopy (FT-IR) techniques. The biodiesel conversion and ester content present in the biodiesel are analyzed using nuclear magnetic resonance spectroscopy (¹H-NMR) and gas chromatography coupled with a flame ionization detector (GC-FID), respectively. A thermogravimetric method was developed to quantify the soap formation from FFA. The solid residue (SR) of the reaction was collected through filtration. The SR contains the catalyst and soap. The amount of soap in the SR was quantified by the thermogravimetric method. It helped us to understand the performance of the SiAl acid catalyst. The calcium amount in the biodiesel due to the leaching of Ca²⁺ ions was determined by the inductively coupled plasma atomic emission spectroscopy (ICP-AES).

The thermogravimetric analysis of SR collected after biodiesel synthesis from oil containing 5 % FFA with different catalyst systems (CaO + x % SiAl, x = 5, 10, and 20) shows that the presence of SiAl catalyst significantly reduced saponification of FFA with CaO. It increased the overall conversion of oil into biodiesel. The lowest FFA to soap conversion was obtained for the catalyst mixture of composition CaO + 20 % SiAl; only ~20 wt.% of FFA was converted into soap, and the remaining FFA was converted into biodiesel via esterification. The FFA to soap conversion was ~60 wt.% and ~30 wt.% for CaO + 5 % SiAl and CaO + 10 % SiAl, respectively. The reduction in saponification of FFA increased the availability of CaO to catalyze the transesterification faster. So, the total reaction time taken to get maximum conversion was reduced compared with a transesterification of the same feedstock by CaO catalyst. The total reaction time taken for obtaining >80 % conversion was 14 hours for the reaction catalyzed by CaO without SiAl catalyst. However, the reaction time was reduced to 7 hours when a mixture of CaO and SiAl catalyst was used, and the conversion reached above 99 %. The performance of the CaD + 20 % SiAl catalyst system was studied in the biodiesel synthesis from oil containing 5 wt.%, 10 wt.% and 20 wt.% FFA. The substitution of CaO by CaD in the catalyst mixture further reduces the reaction time to 1 hour with minimum soap formation. The catalyst loading was 3 wt.%, 6 wt.% and 12 wt.%, respectively for 5 wt.% FFA, 10 wt.% FFA and 20 wt.% FFA contained oil. The FFA to soap conversion was ~20 wt% for 20 wt.% FFA case and ~10 wt.% for both 5 wt.% FFA and 10 wt.% FFA cases. The GC-FID analysis of the synthesized biodiesel confirmed the presence of methyl esters of palmitic acid, oleic acid, and linolenic acid in the biodiesel.

This study concludes that biodiesel was produced from a higher FFA-contained oil (5 wt.%, 10 wt.% and 20 wt.% FFA) using a physical mixture of two low-cost heterogeneous solid catalysts, base catalysts CaO/CaD and an acidic SiAl catalyst. The presence of SiAl helped to achieve a higher conversion of biodiesel from higher FFA-contained oil with less soap formation.

[1] Thangaraj B, Solomon PR, Muniyandi B, Ranganathan S, Lin L. Catalysis in biodiesel production—a review. Clean Energy 2019;3:2–23. https://doi.org/10.1093/ce/zky020.

[2] Khan TMY, Atabani AE, Badruddin IA, Badarudin A, Khayoon MS, Triwahyono S. Recent scenario and technologies to utilize non-edible oils for biodiesel production. Renew Sustain Energy Rev 2014;37:840–51. https://doi.org/10.1016/j.rser.2014.05.064.

[3] Pugazhendhi A, Alagumalai A, Mathimani T, Atabani AE. Optimization, kinetic and thermodynamic studies on sustainable biodiesel production from waste cooking oil: An Indian perspective. Fuel 2020;273:117725. https://doi.org/10.1016/j.fuel.2020.117725.

[4] Verma P, Sharma MP. Review of process parameters for biodiesel production from different feedstocks. Renew Sustain Energy Rev 2016;62:1063–71. https://doi.org/10.1016/j.rser.2016.04.054.

[5] Atadashi IM, Aroua MK, Aziz AA. Biodiesel separation and purification: A review. Renew Energy 2011;36:437–43. https://doi.org/10.1016/j.renene.2010.07.019.

[6] Mansir N, Teo SH, Rashid U, Saiman MI, Tan YP, Alsultan GA, et al. Modified waste egg shell derived bifunctional catalyst for biodiesel production from high FFA waste cooking oil. A review. Renew Sustain Energy Rev 2018;82:3645–55. https://doi.org/10.1016/j.rser.2017.10.098.

[7] Hsiao M-C, Chang L-W, Hou S-S. Study of Solid Calcium Diglyceroxide for Biodiesel Production from Waste Cooking Oil Using a High Speed Homogenizer. Energies 2019;12:3205. https://doi.org/10.3390/en12173205