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Catalyst design and synthesis are among the most important aspects of catalysis, especially for hydrogenation reactions. This work, through a systematic review of relevant literature, seeks to identify key parameters for the application of homogeneous cobalt, nickel, and palladium catalysts in hydrogenation reactions, particularly in the partial hydrogenation of biodiesel. The hydrogenation of biodiesel using the Co, Ni, and Pd catalysts stabilized by different ligands is dependent on the activity of the catalyst and its interaction with a ligand that can stabilize it at low oxidation states during the catalytic cycle. A Pd catalyst coordinated to a monodentate nitrogen ligand shows the highest activity, giving 100% conversion of methyl linoleate (C18:2) to methyl stearate (C18:0). The Pd (II) catalyst with a bidentate ligand, however, shows high activity but low stability as it reduces to Pd(0) species. The product selectivity depends on the time; hence, the desired product or composition mixture of the biodiesel is achieved by kinetic control of the reaction process. The optimized composition of the partially hydrogenated biodiesel sample oil sample was found to be 18.2% C18:0, 49.2% C18:1, 8.1% C18:2, and 23.4% C16:0, which showed improvement of the oxidative stability by 3 h. The use of homogeneous catalysts in biodiesel studies remains a challenge due to the difficulty in separating the catalysts from the hydrogenated material. Highly active catalysts are unsuitable for partial biodiesel hydrogenation as they produce excessive saturated substrates, which are undesirable.
Department of Chemistry, University of Botswana, Gaborone, Botswana
Department of Biochemistry and Industrial Chemistry, Caleb University, Lagos, Nigeria
Fortunate Phenyo Sejie
Department of Chemistry, University of Botswana, Gaborone, Botswana
*Address all correspondence to: segun.olaoye@yahoo.com
1. Introduction
Hydrogenation is the addition of hydrogen atoms across the unsaturated bonds. The strong covalent bonds of the hydrogen atoms make the splitting of the molecule have large energy barriers [1]. To overcome this, a catalyst is necessary for the addition of hydrogen atoms to the olefinic bonds. This reaction plays a crucial role in various industries, including petrochemicals, pharmaceuticals, and biodiesel production [1, 2]. In the context of renewable energy, catalytic hydrogenation is particularly significant for improving the properties of biodiesel, a leading alternative to conventional diesel. Biodiesel, derived from vegetable oils or animal fats, contains polyunsaturated fatty acids that are prone to oxidative degradation. This instability affects its storage, performance, and overall suitability as a transportation fuel. Catalytic hydrogenation addresses these issues by selectively converting polyunsaturated fatty acids into monounsaturated or partially saturated forms, reducing the biodiesel’s oxidative stability [2].
The use of a catalyst, however, subdivides the addition of the two hydrogen atoms to the unsaturated moiety into a few catalytic steps that are not restricted by any symmetry [1]. These include the oxidative addition of hydrogen to the metal center, insertion of the coordinated unsaturated system into a metal-hydrogen bond, and reductive elimination of the hydrogenation product. Metal hydrides are well-known in catalytic hydrogenation reactions. Examples of the classical hydride complexes include HMn(CO)5, H2Fe(CO)4, and HCoCO4, and they are prepared by the protonation of the metal complex or reaction of the metal complex with hydrogen gas. Non-classical dihydride complexes have also been reported, inclusive of H2M(Pr3)2CO3 [3]. Metal hydrides can be either weak bases or acidic, examples include H(CO)5Co, which is acidic, and the η5-(C5H5)2ReH which is a weak base. The source of hydrogen atoms during a hydrogenation reaction can be from molecular hydrogen gas or hydrogen donors (transfer hydrogenation) that produce the gas in situ [2]. The hydrogenation reactions carried out using hydrogen donors instead of molecular hydrogen gas have gained popularity due to their affordability and simple laboratory setup [4].
Transfer dehydrogenation reactions, often referred to today as transfer hydrogenation (TH) processes, involve donor and acceptor molecules that are typically different from one another. The donor compound needs to have a low oxidation potential to effectively participate in the reaction [1, 5]. Several compounds that can give up hydrogen include alcohols, glycols, aldehydes, amides, ethers, amines, aromatic hydrocarbons, and formate salts (Figure 1). An excellent donor dehydrogenates to give environmentally friendly by-products and should enhance selectivity toward desired products [1]. Transfer hydrogenation has become the center of research because it uses molecules that are safe to handle compared to pressurized H2 molecule and it is easy to handle. Also, the use of hydrogen donors influences interesting selectivity in the reduction of substrates. The major milestone in the development of transfer hydrogenation (TH) reactions was the discovery of their involvement with late transition metals from groups 8, 9, 10, and 11, spanning the first to third rows of the periodic table. Additionally, metals like ruthenium and rhodium emerged as highly active catalysts for these reactions [1, 5]. However, their toxicity and high costs posed significant environmental and economic concerns. This challenge has sparked renewed interest in organometallic chemistry, particularly in the pursuit of more affordable and abundant first-row metal catalysts for transfer hydrogenation processes [6, 7].
Figure 1.
Decomposition of formic acid to give hydrogen gas and carbon dioxide [5].
Hydrogenation reactions play a crucial role in the production of fine chemicals used in industries such as pharmaceuticals, agrochemicals, fragrances, cosmetics, and petrochemicals [6, 7]. These reactions enable the formation of single bonds from alkenes or alkynes, carbon-oxygen bonds from aldehydes, ketones, or esters, and carbon-nitrogen bonds (amines) from nitriles or imines [8]. Transition metals or organometallic compounds have been used as catalysts for the hydrogenation of substrates to the desired products. For instance, nickel, palladium, or platinum catalysts are required to transform alkenes and alkynes into alkanes. Chlorotris(triphenylphosphine)rhodium(I) [RhCl(PPh3)3] Wilkinson’s catalyst is among the most widely used catalysts for the selective hydrogenation of a variety of unsaturated substrates, especially alkene/olefins or alkynes, without affecting functional groups such as C〓O, C–N, and NO2, and its applications have been extensively reported [9]. The hydrogenation mechanism with Wilkinson’s catalyst involves steps such as oxidative addition, alkene insertion, migratory insertion of hydrogen (the rate-determining step), and reductive elimination, making it a highly effective tool in catalytic applications [9].
Following the discovery of Wilkinson’s catalyst, which is pivotal for alkene hydrogenation, researchers focused on rhodium-based complexes to advance homogeneous olefin hydrogenation [10, 11]. Osborn et al. [9] developed cationic rhodium complexes, specifically [Rh(diene)(PR3)2]+ or [Rh(cod)(PR3)2]+ (where R represents methyl or phenyl groups and cod stands for 1,5-cyclooctadiene). These complexes facilitate olefin hydrogenation, yielding a solvated species, [RhH2(PR3)2(S)2]+ (where S denotes the solvent), upon exposure to hydrogen in solvents such as tetrahydrofuran (THF), acetone, or ethanol [10]. Under mild experimental conditions, these complexes produce an active catalyst that is essential for homogeneous hydrogenation. For this catalytic action to occur, the solvent ligand must dissociate before olefin insertion, following Wilkinson’s catalytic mechanism [11]. Additionally, the cationic iridium complex [PCy3(cod)Ir(py)]+PF6− (where Cy is cyclohexyl and py is pyridine), known as Crabtree’s catalyst, exhibits exceptional efficacy in olefin hydrogenation, surpassing previous systems by a factor of 100 [12]. Unlike Wilkinson’s system, which requires ligand dissociation to expose the active site to the substrate, Crabtree’s catalyst allows the formation of [Ir(olefin)2(py)(PCy3)]+, which subsequently reacts with H2 to form a dihydride complex [13].
Apart from the early emergence of iridium, rhodium, and/or ruthenium complexes for the catalytic hydrogenation of unsaturated compounds, nickel and palladium complexes have also been employed to hydrogenate unsaturated/olefinic compounds. Chatterjee et al. [14] investigated the catalytic performance of [Ni(saloph)] (1) [15] (Figure 2) in the hydrogenation of cyclohexene and cyclooctene, converting them into cyclohexane and cyclooctane, respectively [14]. The reactions were carried out at a hydrogen pressure (H2) of 60 bar and a temperature of 50°C. The catalyst (using ethanol as solvent, t = 5 h, 1% catalytic loading) displayed greater reactivity with cyclohexene, achieving a turnover frequency (TOF) of 13 h−1 compared to 7 h−1 for cyclooctene. The proposed reaction mechanism involves the formation of a high-valent [NiIV(saloph)(H)2] species, generated through the oxidative addition of hydrogen to the [Ni(saloph)] complex [14].
Figure 2.
Ni(saloph) complex explored for the hydrogenation of cyclohexene and cyclooctene [14].
Shelvin et al. [16] reported active and enantio-selective phosphine nickel catalysts (2–7) for the asymmetric hydrogenation of olefins in α,β-unsaturated ester (β-methyl cinnamate). Various diphosphino ligands (Figure 3) combined with [Ni(OAc)]2 were employed for the hydrogenation of olefins in β-methyl cinnamate. From their study, over 77% conversion and 89% enantioselectivity were obtained for the catalytic hydrogenation of β-methyl cinnamate with bis(phosphino)-nickel(II) complexes [16].
Figure 3.
Hydrogenation of 𝛽-methyl cinnamate catalyzed by bis(phosphino) nickel(II) complexes [16].
Similarly, palladium(II) complex of the form [Pd(PNP)Cl]Cl, containing N,N′-bis(diphenylphosphine)-2,6-diamopyridine, was reported by Schirme and his co-worker [17]. The palladium complexes were employed as a catalyst for the hydrogenation of styrene. The pincer-palladium catalysts, using ethanol at solvent at 60°C and 10 atm H2 pressure, were efficient in the hydrogenation of styrene to the corresponding product [17]. Furthermore, Reddy investigated the kinetics of the hydrogenation of cyclohexene catalyzed by [Pd(P^N^P)Cl]Cl complexes (where PNP = N,N’-bis(diphenylphosphine) ethylbenzylamine) under mild experimental conditions (ethanol, 10–40°C and 1 atm H2 pressure) [18]. Palladium(II) complexes (8) derived from protic HPNO acyl hydrazone, [Pd(PNO)X] (X=OAc, Cl, I) (Figure 4) were efficient in the hydrogenation of styrene under mild experimental conditions (MeOH, 40°C, 1 atm H2 pressure, 1% catalyst loading) [19]. The catalyst bearing acetate group showed better catalytic activities, as compared to the one bearing chlorine and iodine with 100% conversion observed after 48 h.
Figure 4.
Palladium(II) complexes bearing acyl hydrazone for hydrogenation reactions [19].
Gawron et al. [20] explored the effect of counterions on the cobalt complex [Co(η4-cod)2]− (cod = 1,5-cyclooctadiene) (9) (Figure 5) for alkene hydrogenation. Using 3 mol% catalysts, they achieved 99% conversion of 1-octene under mild conditions (2 bar H2, 30°C, 3 h). The study demonstrated that counterions significantly enhance the catalytic activity for 1-octene hydrogenation [20].
Figure 5.
A cobalt complex bearing 1,5 -cyclooctadiene ligand explored for the hydrogenation of 1-octene [20].
The efficiency of hydrogenation catalysts is determined by their activity, selectivity, and operational conditions. Wilkinson’s catalyst (RhCl(PPh3)3) has long been a standard in olefin hydrogenation due to its exceptional activity and selectivity [9, 10]. However, the high cost and scarcity of rhodium have spurred interest in alternative catalysts, particularly cobalt (Co), nickel (Ni), and palladium (Pd) complexes. Cobalt complexes have shown promise for hydrogen activation through two-electron oxidative addition, closely mimicking the performance of noble metals like rhodium. These complexes are cost-effective and capable of operating under mild conditions, but their moderate or low turnover frequencies (TOFs) and occasional need for ligand modifications limit their overall efficiency [20]. Nickel complexes, valued for their abundance and affordability, are effective in hydrogenation when combined with pincer ligands that enhance electron donation. Despite these advantages, nickel catalysts typically exhibit lower TOFs and slower reaction rates compared to Wilkinson’s catalyst [16]. Their requirement for higher temperatures and pressures further constrains their scalability. Palladium complexes provide a compelling alternative, balancing cost and performance. These catalysts achieve TOFs and selectivity levels comparable to Wilkinson’s catalyst while operating under milder conditions. Palladium’s resistance to catalyst poisoning and its ability to selectively hydrogenate alkenes without over-reduction make it particularly suitable for industrial applications [8]. Table 1 shows a summary of the comparative analysis of the efficiency of cobalt, nickel, and palladium complexes versus Wilkinson’s catalyst in olefin hydrogenation.
Comparative analysis of the efficiency of Co, Ni-, and Pd- complexes versus Wilkinson’s catalyst in olefin hydrogenation [9, 14, 15, 16].
n.d = not determined.
2.1 Homogeneous divalent nickel, palladium, and cobalt with nitrogen ligands and their applications in hydrogenation reactions
From the literature findings, nitrogen ligands have been proven to be valuable in stabilizing several homogeneous metal complexes that are widely used in hydrogenation reactions [21]. Nickel, an abundant and eco-friendly metal, is commonly used in hydrogenation reactions [22]. In a study comparing nickel-amide and nickel-imine catalysts for the transfer hydrogenation of 4-bromoacetophenone, the nickel-imine catalyst outperformed, achieving a 72% conversion under identical conditions, while the nickel-amide showed lower activities. Chiral diamine could also be used as an efficient ligand for palladium-catalyzed asymmetric hydrogenation of (E)-α-phenylcinnamic acid [23]. Palladium(II) complexes (10 and 11) with bidentate amine ligands, for instance, (S)-2-aminomethyl-1-ethyl pyrrolidine and (R,R)-1,2-diphenylethylenediamine, are reported to be active and efficient in the hydrogenation of olefin moiety in the cinnamic acid. Interestingly, 100% of the substrate is converted, yielding 76% ee conformer in the asymmetric hydrogenation of (E)-α-phenylcinnamic acid using (S)-2-aminomethyl-1-ethyl pyrrolidine as ligands. However, the use of (R,R)-1,2-diphenylethylenediamine led to a slight increase in enantioselectivity (Figure 6) [23].
Figure 6.
Catalytic hydrogenation of olefin in cinnamic acid with complexes 10 and 11 [23].
The palladium(II) complex, bearing the N^O group, dates back to 1770 and has shown versatility as both homogeneous and heterogeneous catalysts, depending on the solvent [15, 24]. NHC ligand systems have demonstrated strong catalytic efficiency in the semi-hydrogenation of alkynes to produce Z-alkenes [15]. Additionally, the palladium complex with an imidazole moiety was among the first zero-valent catalysts known for its chemoselectivity and stereoselectivity in the semi-hydrogenation of both aliphatic internal alkynes and aromatic C〓C bonds using formic acid as the hydrogen source [22]. Notably, product selectivity varies considerably with the choice of solvent, with MeCN and THF yielding different products under identical conditions [15]. Schiff base palladium complexes remain promising catalysts for biodiesel transfer hydrogenation. Similarly, pyridyl imine complexes of palladium, synthesized for various catalytic and biological applications, have been recognized for their selectivity in homogeneous catalysis, despite concerns over Pd(II) complex instability under molecular hydrogen pressure in the recent years. For instance, palladium complex bearing triaryl phosphine ligand effectively catalyzed styrene hydrogenation under mild conditions (40°C, 1 atm, and 1% catalyst load) [15]. Cobalt(II) complexes stabilized with nitrogen ligands present numerous catalytic opportunities, as nitrogen ligands help maintain lower oxidation states throughout catalytic cycles [25]. Unsaturated nitrogen ligands, such as imines, also serve as efficient π-acceptors and facilitate a deeper examination of cobalt chemistry compared to their saturated counterparts [25]. The catalytic use of cobalt complexes has been documented since the 1980s. A cobalt complex, [Co(py)6][(BPh4)], for example, demonstrated high selectivity and activity for carbonyl hydrogenation at room temperature and atmospheric pressure [15]. Cobalt(II) complexes with nitrogen-based pincer ligands have also emerged as effective catalysts for the hydrogenation of CO2, olefins, alkynes, ketones, aldehydes, and esters [15]. Although first-row transition metals such as cobalt are increasingly favored, the stability of their metal hydride or dihydride forms remains challenging. Notably, Lapointe et al. [26] investigated the catalytic activities of cobalt complexes (12–14) with bulky P^N^P ligands for alkene and alkyne hydrogenation (Figure 7). Their findings highlight the efficacy of these ligands in stabilizing cobalt centers while inhibiting undesirable side reactions. Furthermore, investigation of ligand effects revealed a two-electron process involving oxidative addition and reductive elimination, similar to noble metal reactivity. The best results were obtained at 100°C with 5 bar H2 and 5 mol% catalyst loading [26].
Figure 7.
Cobalt catalysts (12–14) bearing P^N^P group for the hydrogenation of olefins [26].
Additionally, PNP-stabilized cobalt hydrides have also been reported in the literature, though the cobalt hydrides are expected to be highly unstable and not detectable using spectroscopic techniques [27]. Cobalt hydride complex 15 with P^N^P group (Figure 8) has been extensively studied as an ethylene hydrogenation catalyst, though it showed limited activity with other alkenes [27]. Complex 16 was isolated as an ionic species stabilized with BAr4F, which was used to hydrogenate methyl cyclohexanecarboxylate (MCHC) and ethyl cyclohexanecarboxylate (ECHC) [28]. The observed selectivities toward hydrogenation of the C〓C were 66% and 2%, respectively (reaction conditions: substrate (0.5 mmol), catalyst loading (2 mol%), THF (1.0 mL), H2 (55 bar), 120°C, 20 h).
Figure 8.
Cobalt catalysts bearing P^N^P group for the hydrogenation of olefinic moiety in organic substrates [27, 28].
Biodiesel can be produced from plant sources (such as Jatropha and sunflower) through base or acid-catalyzed transesterification of fatty acid methyl esters (Figure 9) [29]. In our lab, we recently reported the biodiesel produced from indigenous seed oils, such as Jatropha [29]. In our study, we detailed a standard transesterification process for Jatropha curcas oil where 50 g of the seed oil is reacted with methanol at 60°C using 0.5 g of potassium hydroxide potassium hydroxide as a catalyst [29]. The synthesized biodiesel was further purified by washing the product twice with warm distilled water to remove residual glycerol (Figure 10). A typical representation of a transesterification reaction is shown in Figure 9.
Figure 9.
The general equation for a transesterification reaction [29].
Figure 10.
A typical representation of biodiesel production.
3.1 Hydrogenation of methyl linoleate using the homogeneous Ni, Co, and Pd divalent metal complexes stabilized with nitrogen ligands.
Many researchers have investigated the partial hydrogenation of fatty acid methyl esters (FAMEs) to improve the oxidative stability of biodiesel [30, 31, 32]. Biodiesel is composed of different amounts of FAMEs, of which some are unsaturated, while others are saturated. When a biodiesel sample contains high amounts of polyunsaturated components, the sample becomes prone to autooxidation reactions [29, 30]. Liu et al. [30] recently established 4 nm palladium nanoparticles dispersed in polyethylene glycol (PEG) to facilitate the selective/partial hydrogenation of methyl linoleate (Figure 11) [30]. Their study highlights the use of in situ-generated 4 nm palladium nanoparticles dispersed in PEG. The catalyst gave high selectivity toward methyl oleate (MO) without producing the saturated methyl stearate (MS) in the hydrogenation of methyl linoleate and sunflower oil [30], and the reported selectivities were attributed to the preferred coordination complex that is formed between the substrate and the palladium nanoparticles. This can either be coordinated through both C〓C in the FAMEs or only one C〓C. Furthermore, Carvalho and co-workers developed a novel catalytic system to address the low oxidative stability of biodiesel derived from soybean oil [32]. They utilized in situ generated palladium nanoparticles that are dispersed in imidazolium-based ionic liquid (BMI·BF4) to enhance catalytic activity and reduce agglomeration. These catalysts also gave selectivity toward MO, and no MS was detected [32]. These two catalysts outperformed traditional heterogeneous catalysts like Pd/C in the partly hydrogenation of methyl linoleate with selectivity toward MO. In addition, the catalysts are recyclable up to three times without significant activity loss. Carvalho et al. also investigated reaction parameters such as temperature, hydrogen pressure, and catalyst concentration, highlighting the influence of the ionic liquid in stabilizing active species in different intermediates that are formed during the catalytic cycle, as well as enhancing selectivity [31]. In 2013, Cheng et al. [32] investigated the effects of different metal types (Ni and Pd) supported on SiO2 or carbon as catalysts for the partial and/or full hydrogenation of cottonseed oil-derived fatty acid methyl esters (FAME) to improve oxidative stability. Their goal was to enhance the oxidative stability of biodiesel while maintaining favorable cold flow properties. Among the tested catalysts, Pd showed the highest catalytic activity and selectivity for producing mono-unsaturated FAME, which improves oxidative stability [32].
Figure 11.
Selective/partial hydrogenation of methyl linoleate (modeled biodiesel) [29].
In our group, we investigated the homogeneous catalysts (17–28) shown in Figures 12 and 13 for the hydrogenation of methyl linoleate and biodiesel, using hydrogen gas and hydrogen donor (formic acid), respectively. It is noted that the catalyst’s efficiency depends on the active metal used to synthesize the catalyst and the ligand used to stabilize these metal centers [29, 33]. The catalysts in Figure 8 are pyrazole complexes where sp2-hybridized nitrogen from a monodentate ligand coordinates with the metal center, forming a square planar complex. Modifications to the ligand’s R group, from hydrogen to bulky alkyl or ferrocene groups, introduce steric hindrance, potentially restricting bulky substrate access to the catalytic site. In Figure 9, a bidentate ligand with sp2 and/or sp3 hybridized nitrogen atoms coordinate to the metal center forming a five-membered ring between the ligand and the metal center.
Figure 12.
Monomeric pyrazolyl nickel(II) and palladium(II) complexes (17–22) employed for the molecular hydrogenation reaction [29].
Figure 13.
The structure of imine (23–25) and amide (26–28) Co-, Ni-, and Pd-complexes explored for the transfer hydrogenation of biodiesel [33].
As described in Table 2, complexes 17–28 were thoroughly investigated as catalysts for the catalytic performance of the methyl linoleate as modeled biodiesel [29, 33]. All six complexes (17–22) proved capable of catalyzing the molecular hydrogenation of methyl linoleate, resulting in the products depicted in Figure 11. However, complex 22 bearing the ferrocenyl ligand demonstrated superior catalytic activity in terms of selectivity toward methyl oleate (MO) under the same reaction conditions (40°C, 30 min), as shown in Table 2, entry 6 [29].
= 6 h. Reaction mixture components were analyzed using GC-MS and/or 1H NMR spectroscopy. TONinmolsubstratemolcatalyst−1.
The structures of the nickel catalysts 17, 24, and 27 studied using DFT- MO6,6-31G** are shown in Figure 14. Due to the chelate effect, complexes 24 and 27 are expected to have enhanced stability compared to complex 17. The optimized structures of the three metal complexes show expected behavior, with negative minimization energies indicating favored structures. Complexes with nickel coordinated to a chelation ligand exhibit lower energy, while those with a monodentate ligand show higher energy values. However, based on the experimental studies by Olaoye et al. [34] and Sejie et al. [35], the catalysts show to have enhanced catalytic activity with monodendate pyrazolyl and bidentate nitrogen ligands. This behavior could be attributed to the stability of the metal complexes before the activation of the nickel complexes [34, 35].
Figure 14.
The optimized geometry of nickel-pyrazole complex (17), imine complex (24), and amide complex (27) with minimization energies of −7265.03 kJ/mol, −2564.95 kJ/mol, and −1240.49 kJ/mol, respectively [34, 35].
Additionally, Sejie et al. [33] evaluated complexes 23–28 for transfer hydrogenation of methyl linoleate using formic acid as a source of hydrogen [33]. The hydrogenation results reveal that the 23–28 complexes/catalysts produced varying compositions of saturated (MS) and unsaturated (MO) fatty acids. Complex 23 achieved complete hydrogenation (100%) to MS, while complexes 23 and 24 yielded lower percentages of MS (86% and 96%, respectively) (Table 2, entries 7 and 8). Complexes 24–26, based on amide ligands, efficiently converted 100% of ML to MS without side products, outperforming their imine-based counterparts (23–24) [33]. The amide catalysts’ superior activity is attributed to the electron-withdrawing nature of the C〓O group, enhancing the metal center’s electrophilicity. Pd-based complexes exhibit the highest activity overall compared to the Ni- and Co-based complexes. Reducing reaction time to 6 h results in partial hydrogenation, allowing control over product composition, as evidenced by a mix of MS, MO, and ML (Table 2, entry 13).
3.2 Hydrogenation of biodiesel with nitrogen-based Co-, Ni-, and Pd-complexes
Our group investigated the partial/selective transfer hydrogenation of biodiesels produced from Jatropha seed oil using complexes 23–28 (Figure 15). The catalysts (23–28) were used to hydrogenate Jatropha biodiesel, which consists of different fatty acids and methyl esters of different chain lengths (Figure 9). The hydrogenation reactions were carried out under conditions designed for methyl linoleate (ML) hydrogenation. As demonstrated by earlier observations, all C18:1 and C18:2 were efficiently converted to C18:0 by catalysts 23–25. On the other hand, less than 90% conversion of C18:1 and C18:2 to C18:0 was observed when using catalysts 26–28 [33].
Figure 15.
Transfer hydrogenation of biodiesel with formic acid using complexes 21–26 [33].
The composition of the partially hydrogenated oil sample (H-BD1) was 18.2% C18:0, 49.2% C18:1, 8.1% C18:2, and 23.4% C16:0. This sample was tested for volatile water-soluble combustion products by passing oxygen gas over a heated sample of the oil sample (induced oxidation reaction), and the conductivity was determined over time [33]. The infrared (IR) spectrometer was used to study changes in the functional groups of the diesel before and after the oxidation reaction, after long-term storage. The findings showed that biodiesel samples with the higher levels of polyunsaturated fatty acids (C18:2) had shorter induction times, which indicate that they undergo oxidative reactions faster than those with lower levels of C18:2 components. The amount of the monounsaturated fatty acid (C18:1) content was less significant in affecting the oxidation reactions as the hydrogenated sample showed a longer induction period compared to the hydrogenated sample despite the highest C18:1 content (41.54%). When compared to its un-hydrogenated counterpart, the partially hydrogenated sample has a higher induction time of 5 h compared to the un-hydrogenated samples, which had a 3-h induction period. However, the hydrogenation using homogeneous catalysts was shown to be less effective in improving the oxidative stability of the diesel as compared to blending the un-hydrogenated sample with 20% conventional diesel (LP5) with biodiesel. Catalytic activity was higher for the amide-based (26–28) compared to imine complexes (23–25). This was evident in the observed conversion of C18:2 to C18:0 [33].
Biodiesel hydrogenation enhances oxidative stability (in terms of selectivity) by converting unsaturated fatty acid methyl esters (FAME) into more stable mono or partially saturated esters. This process significantly influences biodiesel’s storage and performance properties. A detailed comparison of catalysts, including cobalt (Co), nickel (Ni), and palladium (Pd), reveals advancements in catalytic efficiency, turnover metrics, and selectivity [29, 30, 31, 32, 33]. Cobalt-based complexes (23 and 26) exhibit high selectivity for methyl stearate (MS) formation, achieving complete conversion within extended reaction times (e.g., 12 h) as seen in Table 3. While cobalt is cost-effective and abundant, its lower activity compared to palladium stems from its higher activation energy, necessitating elevated temperatures and pressures [36]. Furthermore, nickel catalysts provide cost-effective alternatives for biodiesel hydrogenation but exhibit slower reaction rates compared to palladium-based systems. Chen et al. [32] observed that nickel catalysts had the lowest activity for the hydrogenation of biodiesel, indicating slower hydrogenation kinetics. Lastly, for palladium complexes, Liu et al. [30] reported the effectiveness of magnetic mesoporous palladium catalysts in selective hydrogenation of sunflower oil. Palladium systems achieved superior catalytic activity with a 92% yield of methyl oleate (MO) with full conversion of methyl linoleate. Pd also exhibited excellent reusability with no significant deactivation after multiple cycles [30]. The advantage of this is the high selectivity for mono-unsaturated products and low undesired products. Pd catalysts operate efficiently under mild conditions, making them ideal for biodiesel optimization. The summary of the performance metrics of the oxidative stability (in terms of selectivity) improvement achieved by cobalt, nickel, and palladium catalysts is given in Table 3.
Summary of performance metrics of the cobalt, nickel, and palladium catalysts in the methyl linoleate and biodiesel hydrogenation [29, 30, 31, 32].
The Fourier molecular orbitals were calculated for the optimized geometries of the catalysts using the theoretical methods (Figures 16–19). This study is important as it helps to understand the catalytic activity and selectivity of the catalysts [37]. Methyl linoleate (HOMO of −9.0 eV and a LUMO of +0.4 eV) (Figure 15) interacts with the catalysts either through their HOMO or LUMO. Catalyst 17 is the most favorable due to its smaller energy gaps. The HOMO of catalyst 17 (−6.0 eV) interacts with the substrate’s LUMO with a gap of 6.4, while its LUMO (−1.5 eV) interacts with the substrate’s HOMO with a gap of 7.5 eV (Figure 13). These are significantly smaller compared to catalyst 24, which have gaps of 8.7 eV and 10.5 eV, and catalyst 27, with gaps of 9.5 eV and 10.8 eV. The smaller energy gaps of catalyst 17 enhance electron transfer in both donor-acceptor interactions, making it the most efficient catalyst for the hydrogenation reaction, as observed in the experimental data, where it takes short reaction times to hydrogenate substrates.
Figure 16.
Fourier molecular orbitals calculated for the optimized geometries catalyst 17 [37].
Figure 17.
Fourier molecular orbitals calculated for the optimized geometries catalyst 24 [37].
Figure 18.
Fourier molecular orbitals calculated for the optimized geometries of catalyst 27 [37].
Figure 19.
Fourier molecular orbitals calculated for the methyl linoleate (modeled biodiesel) [37].
3.3 Conclusion and future perspectives
In this work, we reviewed the design, synthesis, characterization, and application of cobalt, nickel, and palladium complexes as catalysts for the hydrogenation of fatty acid methyl esters (FAMEs). These catalysts convert C18:2 in methyl linoleate and jatropha biodiesel into biodegradable C18:0 waxes Through kinetic control of the reaction time, it was possible to achieve partial hydrogenation after 6 h using formic acid as the hydrogen donor. The catalyst stabilized with monodentate ligands is more stable and catalytically active compared to those with a bidentate nitrogen ligand which is supported by the DFT analysis. The exploration of cobalt, nickel, and palladium complexes for biodiesel hydrogenation highlights their versatility and potential to address the challenges associated with renewable energy and sustainable fuel production. These metal complexes, with their unique catalytic properties, have demonstrated efficacy in the hydrogenation of unsaturated fatty acids to improve the oxidative stability of biodiesel, in a time-dependent process. Though the catalyst converts over 50% of the substrate in a short period, controlled reaction kinetics can lead to the desired partially hydrogenated product. It is, however, difficult to control the isomerization of the C〓C bonds in the C18:2 chains in the presence of the homogeneous Co, Ni, and Pd metal complexes. The catalyst structure proves to be a major contributor to the activity of the catalysts, while the product evolution from the four studies seems to be highly dependent on the reaction time. Palladium-based catalysts stand out for their superior selectivity and activity, offering a promising pathway for optimizing biodiesel’s performance while minimizing the formation of fully saturated by-products. The integration of computational methods like density functional theory (DFT) can aid in unraveling the electronic and structural dynamics of these catalysts, paving the way for the rational design of next-generation systems. Additionally, leveraging renewable hydrogen sources and exploring catalytic systems that combine the strengths of homogeneous and heterogeneous catalysis could revolutionize biodiesel production. As the demand for clean and sustainable energy solutions grows, advancements in catalyst development will play a pivotal role in reducing reliance on fossil fuels and mitigating greenhouse gas emissions. By fostering innovation in catalytic chemistry, the energy sector can move closer to achieving a sustainable future.
The author(s) declare no potential conflicts of interest.
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Written By
Oluwasegun Emmanuel Olaoye and Fortunate Phenyo Sejie
Submitted: 20 January 2025Reviewed: 03 February 2025Published: 29 April 2025
Open access peer-reviewed chapter - ONLINE FIRST
