The petroleum oil price and the environmental pollution generated from fossil fuels have made biofuel production a developing area. Other alternative non-polluting technologies such as hydrogen-fueled vehicles, hybrids, plug-in electrics, compressed natural gas, and solar enable devices are possible solutions to energy and environmental problems (Kumar and Sharma, 2011). However, biofuels are renewable energy sources with vast potentials to replace fossil fuels in the present and future energy mix (Deora et al., 2021). Biomass sources from which biofuels are naturally abundant and can ensure the stability of the ecosystem. Biodiesel, a form of biofuel, is a renewable source of energy with several environmental benefits.
The methods involved in producing biodiesel depend on the catalyst used in the process. Different catalysts that have deployed in producing biodiesel are base catalysts (e.g., KOH or NaOH), acid catalysts (e.g., H2SO4), and enzymes (e.g., lipases) (Marchetti et al., 2007). Currently, investment in biodiesel production is adversely affected by several factors including, production technology type, nature of raw materials, and market feedstocks price (Gebremariam and Marchetti, 2018). Biodiesel price doubles that of mineral diesel in the market, due to its production cost. Biodiesel production cost involves two parts, viz., raw materials (feedstocks) cost and processing cost (Huang et al., 2010).
It is argued that the biodiesel production cost depends on the technological routes; however, the cost of feedstock, regardless of the technology employed, accounts for the major production costs (Akella et al., 2009; Ahmad et al., 2011). To produce economical biodiesel would require a cheap source of feedstock such as inedible oil, used cooking oil, and animal fats (Gebremariam and Marchetti, 2018). Generally, non-edible oils are not consumed by humans due to the presence of toxic compounds. Also, their availability and the low cost of plantation of non-edible oil crops position them as economically viable feedstocks. However, many non-edible oils are not suitable for biodiesel synthesis and require pretreatment (Gebremariam and Marchetti, 2018; Deora et al., 2021). Several developed countries, due to climate change threats, are embracing alternative energy sources. The increasing interest in biofuel is due to its intrinsic advantages over fossil fuels. To this effect, the growth projection of the biodiesel industry may place huge pressure on nature and biodiversity in emergent nations (Kumar and Sharma, 2011). One way to address this pending crisis is to adopt total biomass-dependent fuel. Waste cooking oil is a good raw material for biodiesel development since it is cheap and abundant in several countries. Its usage in biodiesel synthesis will prevent competition from using edible oils (Soji-Adekunle et al., 2018; Sivarethinamohan et al., 2022). Apart from the converting waste cooking oil (WCO) to fuel, the problems associated with its disposal are equally solved (Predojević, 2008). It is estimated that a minimum of 16.54 million tons of WCO is produced every year, covering major countries in Europe, Asia, and North America (Loizides et al. (2019).
Conventionally, biodiesel is industrially produced using homogeneous catalysts. The use of homogeneous catalysts for the transesterification of triglycerides is not friendly to the environment and equipment, while the enzymatic transesterification process is expensive (Balajii and Niju, 2019; Bhatia et al., 2020). The use of solid catalysts for biodiesel production has several advantages: minimum energy consumption, lower material costs, and minimum water usage. This class of catalysts can easily be separated from the product and recycled many times (Betiku et al., 2016; Oladipo et al., 2018). Green catalysts are mostly derived from biomass resources such as plants or animals (Qiu et al., 2011; Betiku et al., 2017). Catalysts from natural biological sources contain calcium and potassium compounds dominantly. These elements are potential solid base catalysts for the transesterification of triglycerides (Betiku et al., 2016; Abdullah et al., 2017). The production of green catalysts involves a sequence of easy methods with minimal energy requirements (Odude et al., 2017). Previous studies have reported different plant biomass in producing solid catalysts employed in the transesterification reaction (Sharma et al., 2012; Oladipo et al., 2018). The rationale is to discover abundant plant biomass of high catalytic properties which can be efficiently used sustainably. Previous reports have demonstrated the use of banana and plantain wastes. These include Musa Balbisiana colla trunk (Deka and Basumatary, 2011), plantain peels (Betiku and Ajala, 2014), banana peels (Betiku et al., 2016), Musa Balbisiana Colla peels (Gohain et al., 2017), Musa acuminate (Balajii and Niju, 2019) peduncle, and banana peduncle (Balajii and Niju, 2020). Also, a mixture of plant biomass has been used to develop solid catalysts; cocoa pod husk-plantain peel (Olatundun et al., 2020) and cocoa pod husks-plantain peel-kola nut husk (Falowo et al., 2020).
This study reports the synthesis of a green solid catalyst from agricultural wastes and its subsequent application in the transesterification reaction. The focus was to produce biodiesel from WCO via transesterification using the base catalyst developed from ripe and unripe plantain peels mixture. The heterogeneous base catalyst was synthesized from a mixture containing an equal proportion of ripe and unripe plantain peels ashes. The biodiesel yield and the process parameters (reaction time, molar ratio of oil to methanol, reaction temperature, and catalyst loading) influencing the process were investigated using the Taguchi approach. The optimization of the transesterification of WCO was carried out to maximize the utilization of raw materials, and hence, lessen the production cost.