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Heterogeneous catalysts for biodiesel production

earth metal oxides are more efficient catalysts for However, methanol dehydration during zeolite-
biodiesel production of biodiesel using feedstocks with catalyzed transesterification may result in the
low FFA. 50 production of unwanted by-products, such as dimethyl
It was recently proposed that doping these catalysts ether, as reported in several studies. The relationship
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using metal oxides will increase their catalytic potential between acid strength and hydrophobicity, which is
and stability. Such doping can increase the pore size, controlled by the SiO /Al O ratio, significantly affects
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durability, and surface area of the catalyst material. 51-53 their efficiency. The ratio decreases as the acidity
As an example, it has been demonstrated that doping increases; weaker acid corresponds to a higher ratio.
CaO with lithium greatly increases yield, with a Nonetheless, other studies have found that zeolites with
23% w/w lithium-doped CaO catalyst achieving over high aluminum concentrations perform poorly in these
100% of the total yield within 20 min of reaction time. 54 reactions. 70

2.3. Solid acid-base heterogeneous catalysts 2.4. Green catalysts (biocatalysts and bio-waste
Lewis acidity makes heterogeneous solid acid-base heterogeneous catalysts)
catalysts highly effective. In addition to being The role that biocatalysts play in producing biodiesel has
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recyclable, these catalysts exhibit minimal deactivation grown as the demand for biodiesel and biofuels continues
during transesterification. Examples include mixed to develop globally at an estimated 5% annual growth
oxides, such as silica-alumina, ion exchange resins, rate. Industrial waste materials provide a sustainable
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sulfonated polystyrene, heteropolyacids, sulfated way to make inexpensive heterogeneous solid base
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zirconia, tungstated zirconia, zeolites, and zeotype catalysts that are environmentally friendly. 72,73 Egg
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materials. Zeolites and zeotype materials are naturally shells, animal bones, fish scales, oysters, mussels, and
occurring crystalline aluminosilicates in which oxygen cockle shells, as well as plant waste ash, are examples
atoms link to produce uniformly sized pores arranged in of calcium-rich materials that have been discovered as
three dimensions. 63 possible catalyst materials. The amount of calcium
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Zeolites act as sieves, allowing molecules of the that is recovered using waste products to create CaO, a
same size to enter their pores while excluding larger versatile catalyst for heterogeneous processes, has been
ones. In addition, this pore structure facilitates ion highlighted by research. 75,76
exchange, enhancing catalytic activity by supplying Despite the fact that enzymes have many advantages,
negative ions. Furthermore, the high electric fields their efficacy may be constrained by their water
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generated by cations at active sites within the zeolite’s sensitivity. Enzymatic activity during transesterification
pores improve adsorption. 65 can be inhibited by the presence of polar molecules,
Recent advancements in biodiesel production have such as phospholipids, glycerol, methanol, and
made it significantly more cost-effective through the use water. Researchers have implemented tactics, such
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of solid acid catalysts, increasing their competitiveness as the gradual injection of methanol and other organic
as an alternative to petroleum-based fuels. Compared solvents to help alleviate this problem. The enzymes
to conventional base catalysts and mineral acids, that act as biocatalysts when oils generated from
heterogeneous solid catalysts containing both Lewis- inedible feedstocks are transesterified are listed in
type (like sulfated mixed oxides) and Brønsted-type (like Table 2, together with information about their reaction
sulfonic acids) sites have several advantages. These circumstances and relevant references.
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catalysts allow simultaneous transesterification and Table 2 summarizes the use of various enzymes in the
esterification reactions. Furthermore, heterogeneous solid transesterification of inedible oils to produce biodiesel,
acids are well-suited for low-grade feedstocks because highlighting their reaction conditions and yields.
they are less susceptible to moisture and have high levels Lipozyme Thermomyces lanuginosus immobilized,
of FFAs. Using less expensive feedstocks reduces the need an immobilized lipase, was employed to transesterify
for acid pre-treatment, thereby lowering manufacturing castor oil with methanol at a 3:1 molar ratio, 45°C for
costs. In addition, eliminating the acid pre-treatment 24 h, achieving a 60% yield. Burkholderia cepacia
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phase reduces the risk of corrosion from acid feedstocks. lipase catalyzed the transesterification of Jatropha
Other benefits of heterogeneous solid acid catalysts curcas oil with ethanol at a 10:1 molar ratio, 35°C for
include ease of regeneration, high efficiency, recyclability, 24 h, resulting in a 100% yield. Candida parapsilosis
decreased deactivation, lower contamination, and lipase was used to transesterify J. curcas oil with
simplified product separation. 38,68 methanol at a 2:1 molar ratio, 30°C for 8 h, achieving an

Volume 22 Issue 5 (2025) 5 doi: 10.36922/AJWEP025130095