A Comprehensive Review and Experimental Study on Biodiesel Upgrade Through Selective Partial Catalytic Hydrogenation ^†

Abstract

Biodiesel is a promising alternative to conventional diesel, but its widespread use is inhibited by oxidative stability issues. To address this problem, various strategies have been tested, and among them, the partial hydrogenation of biodiesel FAMEs has shown promising results. Within the framework of the present study, a comprehensive review and an experimental study on biodiesel upgrading through selective partial catalytic hydrogenation have been conducted. The literature indicates that biphasic aqueous/organic catalytic systems have great potential for biodiesel catalytic upgrade, offering high reaction rates, good selectivity and convenient catalyst separation. In this context, an aqueous/organic biphasic system with a Ru/TPPTS catalyst was tested for the partial hydrogenation of biodiesel derived from WCOs. The results were comparable to those reported in the literature, indicating the potential of this process and contributing to the scarce body of research on these systems.

1. Introduction

As energy demand, oil prices, population growth and greenhouse gases (GHG) emissions have shown a continuous increase over recent years, biofuels are considered as an alternative solution to meet the demand for fossil fuels and energy with respect to the environment and human health [1]. In this context, biodiesel has emerged as a promising renewable alternative to conventional diesel fuel. It is primarily composed of fatty acid methyl esters (FAMEs) and is produced through the transesterification process [2]. During transesterification, biomass feedstock of lipid nature (e.g., vegetable oils, animal fats, algal oils, waste cooking oils, etc.) reacts with methanol in the presence of a homogeneous alkali catalyst (usually NaOH or KOH) under mild reaction conditions (temperature ≈ 60 °C and atmospheric pressure) [3]. For the time being, the most common feedstock for commercial-scale biodiesel production is edible vegetable oils [4]. Yet, as the demand for edible oils continues to rise and concerns persist over diverting food resources for fuel, non-edible oils, like non-edible vegetable oils from energy crops, waste cooking oils (WCOs) and algal oils, are increasingly considered a more suitable and sustainable option for biodiesel production [5]. Specifically, WCOs are an interesting alternative feedstock, as their utilization for biodiesel production simultaneously offers an environmentally friendly solution for the management and disposal of these wastes, which aligns with the principles of the circular economy [6]. However, their widespread application is limited due to poor quality, as far as oxidative stability is concerned. Biodiesel produced from WCOs has a high content of poly-unsaturated FAMEs, which makes it much more prone to oxidation compared to what is required by the international biodiesel quality standards [2,5,7]. Poor oxidation stability refers to the chemical degradation of biodiesel after a short period, leading to multiple mechanical problems, such as solid deposition on engine components, filter blockages, and even early engine failures [8,9,10].

To address this issue, various strategies have been proposed, including blending, adding antioxidants, and partial hydrogenation. Among these, partial hydrogenation is particularly interesting because it targets the core of the problem, which is the chemical composition of biodiesel. Specifically, partial hydrogenation is a catalytic process that reduces the degree of unsaturation in biodiesel, resulting in a product with enhanced oxidative stability [8,11,12]. However, non-selective hydrogenation may degrade biodiesel’s cold flow properties due to the high saturated and trans-monounsaturated FAME content. According to the literature, cis-monounsaturated FAMEs offer the best compromise between acceptable oxidative stability and satisfying cold flow performance [8], and as a result, the research must focus on studying more selective catalytic systems. Conventional partial hydrogenation processes, even if they effectively increase oxidative stability, usually are not sufficiently selective to prevent the degradation of cold flow properties. As a result, there is an increased academic interest in the development of unconventional partial hydrogenation catalytic processes. Such processes include the catalytic transfer hydrogenation, the simultaneous transesterification and partial hydrogenation, and the partial hydrogenation via a biphasic catalytic system [13,14,15].

The present study has a twofold approach. First, a theoretical scale was developed, and a comprehensive literature review was conducted to investigate both conventional and emerging catalytic approaches for the selective partial hydrogenation of biodiesel FAMEs. Secondly, an experimental study on upgrading biodiesel derived from WCOs using a Ru-TPPTS biphasic catalytic system was conducted to contribute to the limited existing literature on applications of biphasic catalytic systems for biodiesel upgrading.

2. Materials and Methods

2.1. Literature Review

According to the recent literature data, upgrading biodiesel to optimize its properties has been proposed by selectively hydrogenating polyunsaturated FAMEs to cis-monounsaturated FAMEs [8,16,17]. In a recent literature review [8], it was found that conventional hydrogenation catalytic systems used for biodiesel upgrading, although they achieve their primary goal of improving oxidative stability, often fail to preserve the fuel’s cold flow properties due to their insufficient selectivity toward cis-monounsaturated FAMEs. As a result, the development of non-conventional hydrogenation methods is of utmost importance for identifying an effective catalytic system with high selectivity.

For this reason, over the last few years, research on testing new approaches has increased. There are many studies in which the researchers tried a new approach in terms of an unusual or modified catalyst support [18,19,20], contributing to the understanding of how the support can manipulate selectivity and reactivity. Another approach is the use of a bimetallic catalytic system [21,22,23,24]. In these systems, two metals commonly used as catalysts (such as Ni, Pd, Cu, etc.) are combined into one catalytic system [21,22,23,24], and by acting synergistically, they have the potential to be even more effective than conventional catalysts [21]. Additionally, there is a growing amount of research on the unconventional hydrogenation method called Catalytic Transfer Hydrogenation (CTH) [23,24,25], in which the necessary hydrogen is supplied via a compound that acts as a hydrogen donor, thereby avoiding the use of explosive and expensive H₂ gas [14,24,25]. Moreover, there are studies on homogeneous catalysis for the partial hydrogenation of biodiesel [25,26], which was not favored in the past, because, even though they offer great selectivity and reactivity, the recovery of the catalyst cannot be facilitated easily [27]. Furthermore, of great interest are advanced catalytic systems such as solvent-stabilized colloid catalysts [28], which have the potential to increase both selectivity and reactivity like a homogeneous catalyst and can be recovered like a heterogeneous catalyst.

Finally, one of the least studied unconventional methods with high potential is the partial hydrogenation via an aqueous/organic biphasic catalytic system (PHBC). According to the scarce body of research on the PHBC [15,28,29,30], it combines the valorization of organometallic catalysts, achieving much higher reaction rates with high selectivity, and facilitates the convenient catalyst separation and reuse.

By employing a biphasic aqueous/organic system, exceptionally high reaction rates can be achieved, while efficient catalyst recovery and the environmentally friendly nature of such systems make them a highly attractive area of research [28,29,31,32].

2.2. Experimental Section

Consequently, in the experimental part of the present study, biodiesel upgrading was carried out using an aqueous/organic biphasic catalytic system. The aqueous phase of the studied biphasic catalytic system consisted of ultrapure water in which the Ru-TPPTS organometallic catalyst was contained, while the organic phase consisted solely of biodiesel derived from WCO. A flow diagram of the process steps is illustrated in Figure 1. The Ru-TPPTS catalyst was generated in situ by adding TPPTS (triphenylphosphine trisulfonate) to the organic biodiesel phase, while the RuCl₃·xH₂O (ruthenium(III) chloride hydrate) precursor was dissolved in deoxygenated water and subsequently combined with the biodiesel. The hydrogenation reaction was conducted in a 100 mL batch reactor (Autoclave Engineers, Erie, Pennsylvania, USA). Before pressurization and heating, the reactor was purged with hydrogen, and the reaction was carried out under the following conditions: 90 °C, 50 bar, 400 rpm, for 60 min.

3. Results and Discussion

As illustrated in Figure 2 and Figure 3, the total ion chromatograms (TIC) of WCO biodiesel before and after biphasic catalytic hydrogenation exhibit notable alterations in the FAME profile. Specifically, the content of linoleic acid methyl ester (C18:2) before hydrogenation was 54.00%, while it decreased sharply to 12.67% at the end of the catalytic process, representing a 41.33% reduction. In contrast, the biodiesel’s composition in oleic acid methyl ester (C18:1) remained nearly the same, as it showed only a slight decrease, from 42.61% to 41.99%, indicating its partial conversion. Most notably, the greatest change was observed in the content of fully saturated stearic acid methyl ester (C18:0), which increased substantially from 3.39% to 45.34%.

As reported in the literature, these compositional alterations had a substantial effect on key properties, such as oxidative stability. The measured induction time (in hours) using the Rancimat method before hydrogenation was 0.4 h, while after the biphasic Ru-TPPTS-catalyzed hydrogenation, oxidative stability increased to 0.85 h. A summary of FAME composition and oxidative stability changes is presented in

Table 1. Compositional and oxidative stability changes after hydrogenation.

Biodiesel	Linoleic Acid Methyl Ester (C18:2) (%)	Oleic Acid Methyl Ester (C18:1) (%)	Stearic Acid Methyl Ester (C18:0) (%)	Oxidative Stability (Hours)
WCO biodiesel	54.00	42.61	3.39	0.40
Hydrogenated biodiesel	12.67	41.99	45.34	0.85

. Although oxidative stability remains below the EN 14112 [33] specified limit, there is significant potential for improving crucial properties of waste biomass-derived biofuels.

According to the literature, polyunsaturated FAMEs do not convert automatically to saturated FAMEs; they lose one double bond every time, and gradually they get less unsaturated, until they do not have any double bonds [34,35]. Additionally, it is known that the hydrogenation of C18:2 to C18:1 happens faster than that of C18:1 to C18:0 [36,37,38]. As a result, the nearly unchanged content of C18:1 FAMEs in the experiment’s hydrogenated biodiesel is not due to catalyst inefficiency, but because the hydrogenation reactions of C18:2 FAMEs and C18:1 FAMEs occurred simultaneously, leading to the production of additional C18:1 FAMEs that were subsequently hydrogenated to C18:0 FAMEs. In fact, the higher reaction rate of C18:2 FAME hydrogenation clearly indicates that there was a point when C18:1 FAMEs were the majority in the mixture, but due to continuous hydrogenation, they were converted into C18:0 FAMEs. Consequently, the catalytic system has the potential to produce biodiesel with the desired composition, if the reaction conditions are studied thoroughly in order to achieve higher selectivity towards C18:1 FAMEs. Furthermore, the catalytic system must be engineered in such a way as to promote the hydrogenation of C18:2 to C18:1 much faster than that of C18:1 to C18:0, and consequently lead to the desired composition.

4. Conclusions

Selective partial hydrogenation is a promising route for enhancing biodiesel quality, but for the time being, the conventional catalytic systems cannot ensure the selectivity needed to preserve biodiesel’s acceptable cold flow properties. In this context, unconventional catalytic processes for biodiesel upgrading have emerged and shown great potential. According to the literature, aqueous/organic biphasic catalytic processes are of interest because they combine the high reaction rates of homogeneous organometallic catalysts with the convenience and catalyst recovery of a heterogeneous catalytic system. For this reason, a biphasic aqueous/organic catalytic system was developed and tested in the framework of the present study. Using Ru/TPPTS as a catalyst and WCO biodiesel as feedstock, the experiment’s results show that the biphasic catalytic system was effective, as evidenced by the significant reduction in polyunsaturated FAMEs and the substantial formation of saturated FAMEs without solvent addition. Moreover, the hydrogenation outcomes for biodiesel are comparable to those reported in several studies. Biphasic catalytic systems could be a promising approach for the catalytic upgrading of biodiesel via hydrogenation. Nevertheless, further investigation is required to achieve the optimized FAME composition under mild hydrogenation conditions.