Many chemical reactions involved the use of one or more catalysts for reaction to reach completion. The type of catalyst used in a particular reaction depends solely on the reaction conditions and the nature of reactants involved. Although catalysts are not to be consumed in the reaction, its presence speeds up or limits reaction rate, and itself recover at the end of product formation. Nowadays, industries such as pharmaceuticals, polymers, petroleum, electronic, environmental treatment, chemical, and agrochemical industries employ the use of catalysts to achieve the end products. Also, the use of catalysts occupies an important place in academic research. A recent report revealed that the worldwide market value of catalysts was estimated in 2019 at USD 26.1 billion, and is expected to reach USD 27.2 billion in 2020, and USD 34.0 billion in 2025 at growth rate 4.5% according to Compound Annual Growth Rate (CAGR). This makes it a value-added income for the financial sustainability of all nations if biomass waste can be employed. However, catalysts are primarily divided into four categories; homogeneous catalyst, heterogeneous catalyst, heterogenized-homogeneous catalysis, and biocatalysts.
Homogeneous catalysis involved the operation of the mixture in the same phase, the possible reactant and the catalyst exhibit a high uniform phase due to high reactivity and selectivity. Most oxidation, carbonylation, hydrogenation, esterification, and hydrocyanation are homogeneous catalysis in reaction. However, this nature of catalyst usage comes with its shortcoming, this includes recoverability problem, highly toxic, and high cost, especially in esterification (bio-fuel production) case.
Heterogeneous catalysis, on the other hand, involved the mixture that exists in different phases; the catalyst usually solid support or bulk form does not dissolve in the reactant, yet exhibit high reactivity. The advantages include ease of recoverability and reused, non-toxic, and of low cost. These have made many industries such as hydrocarbon produced company (Fischer process), ammonia synthesis company (Haber-Bocsch process), sulphuric acid company (Contact process), soap making company (Saponification process), and petroleum company (transesterification process) to adhere to the use of this catalysts, apart from these advantages, heterogeneous catalysts produce in smaller particle size increase its activity due to surface phenomenon. The smaller the particles size the larger the surface area of catalysts during the reaction.
Heterogenized-homogeneous catalysis is the mixture of the heterogeneous and homogeneous catalysts together. The homogeneous catalyst is embedded onto the solid supports to prepare the heterogenic analogy. However, these types of catalysts are difficult to produce due to complexity, less selectivity, reactivity, and covalent bonding between the polymer chain and the surface atoms (grafting).
Biocatalysts are usually referred to as enzymes or ribozymes catalysts obtained from plants, microbes, or Goat tissue, which are used to catalyst reaction that takes place outside the living cells. This type of catalyst is on high industrial usage and has been considered an alternative to most industrial conventional catalysts due to the advantages such as mild reaction conditions, high selectivity, high efficiency, and non-toxic. Companies such as, dairy, baking, detergent, leather, textile, and biofuel utilized this catalyst for their production. Its major drawbacks are in-ability to convert a cellular catalyst into a bioprocess, difficulty in recoverability (brewing process) sustainability in harsh environmental conditions during culture (high temperature, extreme pH, high salt concentrations, organic solvent), instability in aqueous media (protein), cofactor dependability (non-protein chemical compound), possibility of an allergic reaction, and inactivation through inhibition (Dizge et al., 2009).
Among these catalysts, a heterogeneous catalyst is a solid catalyst of calcium-based compound usually prepared from solid wastes and employed as a catalyst for transesterification of oil to biodiesel due to the above-mentioned advantages (Milan et al., 2016). Biodiesel is a renewable, eco-friendly, non-toxic, and sustainable alternative fuel derived from the synthesis of vegetable oil or fat. In the past and also present, reports on solid based developed from solid wastes as heterogeneous catalysts and applied it to the synthesis of biodiesel from vegetable oil. Vadery et al. (2014) synthesized biodiesel from Jatropha oil through methanolysis of a developed based catalyst from coconut husk ash, while Bazargan et al. (2015)  adopted the used of palm kernel shell gasification as a heterogeneous catalyst for biodiesel production. Chouhan et al. (2013) converted Jatropha curcus oil to biodiesel through the help of Lemna perpusila Torrey ash, but, Razaei et al. (2013) used waste mussel shell as a catalyst to synthesized biodiesel. The work of Ikbal et al. (2018)  reported the use of waste snail shells as a heterogeneous catalyst for the production of biodiesel from soybean oil, while in 2019, Subramaniapillai  and co used Donax delltoides shell as heterogeneous catalyst. The use of pearl spar as a heterogeneous catalyst was employed in the study reported by Adepoju et al. (2018), while in another work, Betiku et al. (2015) developed heterogeneous catalyst from plantain peel, applied it to biodiesel synthesis from yellow oleander. The study conducted in another work by Betiku et al. (2017) reported the use of heterogeneous catalyst developed from cocoa pod husks, while Nath et al. (2019) developed heterogeneous catalyst from waste Brassica nigra plant for biodiesel production. In the work of Balajii et al. (2020), the heterogeneous catalyst was made from the Banana peduncle, meanwhile, Minakshi et al. (2020) used Carica papaya stem as a bio-based catalyst. Further study by Hadiyanto et al. (2016) utilized Anadara granosa as a heterogeneous CaO-based catalyst, but, Trisupakitti et al. (2019)  used golden apple cherry snail as a heterogeneous catalyst to synthesized biodiesel from the vegetable. Falowo et al. (2020) developed a novel mesoporous base catalyst synthesis from a mixture of three agro-wastes. All these reports developed catalysts from mixture or single solid wastes, and applied it to the synthesis of biodiesel from vegetable oil or its blend, except in the work of Adepoju, (2020)  where two derived base catalysts were tested on the synthesized biodiesel from the blended oil.
Therefore, to cover the gap between the efficiency of the developed catalysts derived from single or the mixture of solid wastes, and to introduce a novel blend ratio through the BTO5, BTO10, BTO15,………., BTO95 in an interval of 5 between the Beef Tallow Oil (BTO) and Waste Used Vegetable Oil (WUVO) for process industry (bio-fuel or margarine), this study developed three CaO-based catalysts from a burnt Theobroma cacao pod husk (BTCPH), calcined Theobroma cacao pod husk (CTCPH), and submerged fermented calcined Theobroma cacao pod husk (SFCTCPH), applied each for the synthesis of biodiesel from the blended of the oil obtained from Beef tallow-vegetable used oil. The catalysts prepared were characterized using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-ray diffraction analysis (XRD), Fourier transforms infrared spectroscopy (FTIR), and BET adsorption analysis. Process modeling and optimization of biodiesel synthesis was carried out by considering five levels-four factors (reaction time, reaction temperature, catalyst amount, and ethanol to oil molar ratio (EtOH/OMR)) via Box Behnken Experimental Design (BBED). Catalysts regeneration and reusability were carried out, and the suitability of the biodiesel in an internal combustion engine was established by determining the physicochemical properties.