Recent advancement in deoxygenation of fatty acids via homogeneous catalysis for biofuel production

Highlights

• Biofuel is produced from renewable resources of fatty acids.

• Homogeneous catalysis is highly efficient for producing biofuel from fatty acids.

• Radical-based reactions are active for decarboxylation of fatty acids.

• Transition metal catalysts are selective for decarbonylation of fatty acids.

Abstract

Fuel-like hydrocarbons (also known as biofuel) isolated from the deoxygenation of fatty acids present different advantages as compared with fossil fuels. In particular, the homogeneous and heterogeneous catalytic deoxygenation methods have been the center of attention during recent years. Although catalytic deoxygenation of fatty acids via heterogeneous catalysis has been widely investigated, there is a high demand to review the progress in using the homogeneous catalysis pathways. Among the various homogeneous pathways, radical-based reactions and transition metal catalysis demonstrate the most promising results in the decarboxylation and decarbonylation processes. It is shown that radical-based reactions are more active in decarboxylation meanwhile the transition metal catalysts are rather selective to decarbonylation of fatty acids. Besides, the reaction conditions and type of catalysts are capable of enhancing biofuel production. Homogenous catalysis provides the huge potential for commercializing viability of biofuel via deoxygenation of fatty acids.

Introduction

Nowadays, the replacement of new energy sources that are greener than fossil fuels, particularly for transportation fuels is inevitably demanded. Of particular interests are renewable biomass feedstock and their derivatives [[1], [2], [3]]. Biomass is the most commonly utilized current renewable energy source that typically is provided from forestry residues, industrial and/or household waste, and agricultural productions [2].

The advantage of providing carbon backbones from a renewable resource has potentially caused the increment of the exploitation of biomass feedstock for the fuel and energy industry [2,4,5]. They are environment-friendly and form a closed loop that balance out between emission due to the human activities and CO₂ consumption via photosynthesis [6]. In addition, their production prices are reasonable; based on an economic assessment that was previously published, the production cost of biomass-based biofuels hovers between 0.13 and 0.99 USD/L [2,7]. Moreover, the utilization of biomass residual (i.e. triglycerides as the byproducts of food streams, oils, and fats) can ensure better use of waste [8].

According to the energy content of various biomass products, terpenes are top of the list, followed by vegetable oils, lignin, and sugars. Nonetheless, terpenes do not meet the requirement for transport biofuels due to the low production quantity [9]. Consequently, vegetable oil is always a preferred candidate. Vegetable oils and animal fats, as well as their derivatives (i.e. fatty acids), have raised their profiles as biofuels of choice in recent years [[10], [11], [12], [13]]. It was estimated that the production of the vegetable oils reach 187 million metric tons (MMT) in 2016/2017 [14], and has exceeded 200 MMT worldwide in 2019/2020 [15]. Among naturally existing fatty acids, which are well over 1000 varieties, only 20–25 types are found extensively in nature and are important in the commercial markets [16]. These fatty acids, typically 10–22 carbons in length, are present in large amounts from vegetable/tree oils and animal fats [17]. The most common plant oils are obtained from sunflower, olive, cottonseed, peanut, rapeseed (canola), soybean, coconut, palm, and palm kernel oils. The corresponding fatty acids that are derived from these plant resources are lauric, myristic, palmitic, oleic, linoleic, α-linoleic, stearic, and erucic fatty acids [18,19]. On the other hand, animal fats mainly provide myristic, palmitic, palmitoleic, oleic, stearic, eicosenoic, arachidonic, eicosapentaenoic, docosahexaenoic, and docosenoic acids [20]. However, the viscosity of fatty acids derived from plant oils and animal fats is 11–17 times higher than the common fuel like diesel fuel [21]; therefore, their direct usage in fuel engines is infeasible. Furthermore, a substantial increment in the cloud point (around 10−15 ℃) makes the fatty acids less effective as a normal fuel in colder climates [22].

In recent years, several approaches have been considered in order to enhance the properties of fatty acids to meet the biofuel requirements. For example, fatty acids are esterified with alcohol (e.g. methanol) to become fatty acid methyl esters (FAMEs). The FAMEs are known as biodiesel and commonly blended with diesel ranging from 5 to 20 wt% [23]. However, the use of biodiesel also suffers from chronic issues such as oxygen content, fluid density, flash point and viscosity. These have limited its direct application in diesel engines [23]. In order to improve the biofuel quality, conversion-based techniques that produce biofuels with simple hydrocarbon chains (e.g. alkanes or alkenes) are favorable [[24], [25], [26]]. There is an initiative by a number of researchers [27,28] to improve the quality of biofuel. They were researching into the oxygen atoms elimination from fatty acids in order to obtain simple hydrocarbon chains that are more compatible with diesel. The more recent conversion technologies such as hydrodeoxygenation (HDO) and deoxygenation (DO) are gaining popularity [[29], [30], [31]]. In the HDO process, hydrocarbon components are generated from biomass derivatives with lower oxygen content via a catalytic reaction in the presence of H₂ [30,32]. Whereas in the DO procedure, modification of hydrocarbons skeleton is carried out by the removal of oxygen through decarboxylation, decarbonylation, and dehydration without hydrogen gas; and produced byproducts are CO₂, CO, and H₂O [27,33].

The output of the elimination of oxygen from feedstock in DO and HDO are essentially different. The produced hydrocarbons from the HDO process possess the same number of carbon atoms than their precursor, while DO provides the hydrocarbons with one carbon atom less than their mother feedstock [34]. The loss of one carbon atom in the DO process is led to lower atom efficiency in produced hydrocarbons obtained by this pathway; however, the decrement of hydrogen gas consumption compensates for this consequence. Since the industrial production of green fuels using deoxygenation of renewable resources is more convenient, efficient and economical than HDO, there are tremendous efforts for the development of deoxygenation process [35].

The removal of oxygen from the backbone of fatty acids with a deoxygenation process leads to the production of a wide range of hydrocarbon chains [36,37]. The fatty acids with typically long saturated fatty acids (between 9 and 28 carbon) such as lauric acid (C12), myristic acid (C14), palmitic acid (C16) and stearic acid (C18) are normally used for DO. Oleic and linoleic acids (C18) are the main unsaturated fatty acids for conversions due to its abundance in vegetable oils such as palm, olive, sunflower, rapeseed and grape seed oils [[38], [39], [40]]. The distribution percentage of natural resources-derived fatty acids with different chain lengths has been outlined in Table 1 [41], which shows that oleic acid and lauric acid are the two most commonly present compounds.

Based on various advantages such as being environmentally friendly, low sulfur content, acceptable Cetane number and calorific value, the obtained products from these converted precursors have generally met the specification for various modern fuels such as jet fuels [42] and biodiesel [38,39], as well as renewable diesel [36,43]. Thereby, deoxygenation of vegetable oils appears to become a notable technology for biofuel production. Apart from fatty acids, triglycerides are known as the most significant components of vegetable oils and/or animal fats. Triglycerides were used as fuel in old engines; however, their high viscosity and density prevent their direct usage in modern engines [44]. The chemical composition of triglycerides comprises esters of fatty acids and glycerol, where various fatty acid chains may attach to a glycerol backbone with an ester linkage. In order to make triglycerides compatible as fuel, conversion processes based on oxygen elimination, such as HDO and DO, are the most favorable approaches [45]. However, the need for high pressure of H₂ and/or usage of sulfided catalysts (which may react with water and produce sulfur species that can contaminate the final fuel product) disfavors the HDO process [46]. Instead, catalytic deoxygenation of triglycerides via decarboxylation and decarbonylation pathways, similar to the deoxygenation of fatty acids, generates fuel-like hydrocarbons efficiently [47]. The general schematic representation of the deoxygenation reaction in triglyceride molecules is shown in Fig. 1. In spite of generating linear fuel-like hydrocarbons in this approach, other hydrocarbons with a range of different carbon chain lengths may produce via additional cracking of the fatty acid branches [45]

Many researchers have reported their observations during the conversion of triglycerides to linear hydrocarbons via the deoxygenation process which in principle confirms the early splitting of the free fatty acids from the glycerol backbone and subsequent elimination of carbon monoxide and/or carbon dioxide from their molecular structure [45,[48], [49], [50]]. The mechanism for the formation of major intermediate molecules during deoxygenation of triglycerides, particularly free fatty acids, has been proposed by researchers in different conditions [[51], [52], [53], [54], [55]].

As shown in Fig. 2a, three fatty acids and a glycerol molecule can be formed via hydrolysis of ester groups in aqueous media [52,53]. In contrast, two variant mechanisms are suggested which may occur in solvent-free conditions or in organic media. The first pathway follows a β-elimination mechanism to furnish a fatty acid molecule and an unsaturated di-glyceride as shown in Fig. 2b [51]. This approach can subsequently follow by two consecutive hydrogenation/elimination processes to afford three fatty acids and a propane molecule. For the second pathway, a γ-hydrogen transfer mechanism has suggested (Fig. 2c) [51]. A terminal olefin that possesses two carbons less than the mother fatty acid domain may form based on this mechanism. In another proposed mechanism, the selective hydrogenolysis of the CO bond in triglycerides, as represented in Fig. 2d, can lead to the formation of three fatty acids and one propane molecule [55], similar to the β-elimination/hydrogenation pathway.

Moreover, Selective Deoxygenation (SDO) is another promising approach for the upgrading of triglyceride molecules that directly converts triglycerides into linear fuel-like hydrocarbons, without splitting of the fatty acid side branches. This method can accomplish via different conversion pathways including decarbonylation-dehydration, decarboxylation, and/or dehydration (Fig. 3) [56,57].

In order to dissociate the existing strong chemical bonds, such as CO and CO, a high level of activation energy is needed [47]. In pyrolysis of fatty acids and their ester forms, for instance, a high temperature around 400 $℃$ is required [58]. To minimize the required activation energy (reaction temperature) and also to maximize the selectivity, numerous homogeneous and heterogeneous catalysts have been scientifically studied. Several promising heterogeneous catalysts such as Pt/C [53], Pd/C [59], zeolites [27], metal oxide supported catalysts [31], and others [27,60] have been evaluated. Although the catalytic deoxygenation of fatty acids and vegetable oils with heterogeneous catalysts has been critically reviewed previously [[61], [62], [63], [64]], the recent progress of homogeneous catalysis has not yet been reviewed. The homogeneous catalysis of fatty acids and their derivatives can be divided into two classes of reaction, namely radical-based reactions and transition metal catalysis. For the radical-based reaction, silver-based and electro-oxidation reactions have been employed mainly for the decarboxylation process. In contrast, the transition metal catalysis is frequently utilized for the decarbonylation process using palladium, rhodium, iridium, ruthenium, nickel, and iron.

In this review, the recent advancement of the homogeneous catalysis for deoxygenation of fatty acids is highlighted. The effect of homogeneous catalysts on decarboxylation and decarbonylation of fatty acids seems to be critical for the production of green fuels.