The Role of the Heterogeneous Catalyst to Produce Solketal from Biodiesel Waste: The Key to Achieve Efficiency

1. Introduction

Global societal development has progressively increased energetic demands through history. In order to answer these needs, fossil fuel production and consumption soared throughout the last century, with consistently rising demand for coal, oil and natural gas as the most predominant energy sources. However, a shift towards production and consumption of other energy sources is a major societal challenge due to the negative environmental impact of fossil fuels and their unrenewable supply. [1,2] Fossil fuels are responsible for most carbon dioxide emissions, but are also composed by various sulfur and nitrogen containing compounds which directly contribute to greenhouse gases (GHG) emissions [3,4]. Growing understanding and awareness of climate change has led to the implementation of legislation to regulate fuel production and environmental policies to drive the development and adoption of new energy alternatives for a more sustainable development.[5] Still, fossil fuel sources relate to approximatly 81% of today’s global energy consumption and global demand is projected to peak in 2030. Even though oil demand in advanced economies peaked back in 2005 and its expected to decline further in the coming decade, it is projected to continually grow in other regions until 2050 [6,7]. Fossil fuels are constituted by various sulfur and nitrogen constituents, that when burned, originate oxides (SO_x and NO_x), releasing harmful emissions (Figure 1). Furthermore, fossil fuels are also one of the main direct contributors of greenhouse gases (GHG), responsible for most carbon dioxide emissions [3,8]. As such, the European Union has planned to reduce 55% of GHG emissions by 2030, and completely eliminate them until 2050 [9].

Biodiesel is considered a non-toxic, biodegradable and renewable fuel, allowing for safer handling and reduced carbon, sulfur and particulate matter emissions. Further, when compared to normal fuels, it exhibits a higher cetane number and better lubrication, contributing to a better engine performance and fuel consumption efficiency [3,5]. However, pure biodiesel also demonstrates some drawbacks, related to its higher viscosity and less satisfying cold properties, which could lead to serious problems in engine longevity. As such, biodiesel is normally blended with fuel additives, whose application helps solve these problems and, therefore, benefit biodiesel commercialization [2,10]. Biodiesel is obtained through the transesterification reaction of triglycerides (a type of fat found in vegetable oils) with a short-chain alcohol as methanol (Figure 1), in the presence of an appropriate catalyst and under acidic or basic conditions [1,2,10]. Biodiesel is also known as fatty acid alkyl esters (FAAEs) or fatty acid methyl esters (FAMEs), depending on the alcohol used in the reaction (ethanol or methanol, respectively) [1,11].

Glycerol is the by-product of this reaction, originated as 10% in weight in relation to biodiesel. As such, there is an excess in crude glycerol in the fuel industry, owing to the increased interest in biodiesel production [12,13,14]. As in 2022, global biodiesel production amouted to aproximatly 53000 million liters and OECD’s latest projection gives an increase of 25% in global production until 2032 (Figure 2) [15,16]. In order to increase biodiesel’s own sustainability, it is extremely important to develop strategies capable of reutilizing crude glycerol obtained as waste.

2. Glycerol Valorisation

Propane-1,2,3-triol (C₃H₈O₃), also known as glycerol or glycerin, is a compound with high boiling point, low volatility and low toxicity, constituted by three hydroxyl substituents which allow for the formation of hydrogen bonds. These bonds are responsible for its solubility in water and its high viscosity (Figure 3) [11,12]. Glycerol finds application in various industries, from cosmetics and pharmaceutics to the food industry, but is also a compound of insterst as a chemical platform for convertion into value-added products [11,13].

After transesterification, it is necessary to follow several steps in order to efficiently obtain the produced biodiesel: i) neutralization of the reaction mixture, owing to the acidic or basic nature of the catalyst, ii) removal of the unreacted methanol, that was used in excess, through distillation and iii) separation of biodiesel from glycerol and other substances [2,5,17]. As such, the final glycerol, i.e. crude glycerol, possesses impurities related to these stages, which can vary according to the raw source used, the efficiency of the washing and biodiesel separation procedures, between others. The most common impurities found in crude glycerol are water, salts, ash and methanol [17,18].

As mentioned previously, glycerol has many different uses, but it is important to take into consideration that most are effective using pure glycerol. The conversion of glycerol in added-valuable products can only be proceeded performing a previous purification of crude glycerol, otherwise, the effcieciency of the process can be compromised. [3,14,19] On the other hand, the isolate process of glycerol purification is not an atractive economic method, since this is extremely costly, and therefore, not economically viable [3,18]. As such, the most commonly adopted strategy is glycerol valorisation, where a variety of catalytic reactions were found to be able to transform glycerol. Figure 4 illustrates the most utilized pathways.

Hydrogenolysis of glycerol occurs in a catalytic system combining dehydration and hydrogenation processes [21,22,23,24]. In general, glycerol suffers dehydration, in the presence of an acidic catalyst, followed by the addition of a hydrogen source, commonly using transition metals. The most commercially interesting products originated by this reaction are 1,2-propanediol and 1,3-propanediol, whose applications range from pharmaceuticals, cosmetics and most commonly, polymer formulation [13,21]. Oxidation of glycerol can originate a wide variety of products, depending on the nature of the catalyst used and the reaction environment [25,26,27,28]. The most known products are glyceric acid and dihydroxyacetone, obtained when the oxidation occurs in a primary or secondary hydroxyl group, respectively. Applications range from pharmaceutical and cosmetics, to use as protective agents in coatings [22,25]. Dehydration of glycerol in the presence of catalysts with acidic nature, such as Brönsted or Lewis acids, originates acrolein. This compound is used as an intermediate for many other products, such as acrylic acid, mostly for polymer formulation [29,30]. The etherification reaction of glycerol originates fuel additives, such as di-ethers and tri-ethers, in the presence of acid or basic catalysts [31,32,33]. Other pathways can be reduction, carboxylation, oligomerization and pyrolisis [34].

This review is focused in one pathway for glycerol valorisation, namely via the acetalization reaction. This reaction is used for the preparation of acetal structures, applied as oxygenated fuel additives [12]. Further, acetalization can be considered a green reaction, allowing the use of reactants from renewable sources, in a fast occuring reaction (even at room temperature), while simultaneously demonstrating high product selectivity [10,35].

3. Acetalization Reaction

The glycerol acetalization reaction takes place in the presence of aldehydes or ketones, originating a five-membered cyclic compound and a six-membered cyclic compound [11,18]. Finally, water is obtained as a by-product of the acetalization reaction When in the presence of acetone (Figure 5), the reaction product obtained with highest selectivity is 2,2-dimethyl-1,3-dioxolane-4-methanol (C₆H₁₂O₃), commonly known as solketal. This compound is considered environmentally friendly, combining low toxicity with high miscibility in most solvents, which favours its application in various industries [3,10]. However, the use of solketal as an oxygenated fuel additive is extremely interesting, especially when applied to biodiesel blends. As mentioned previously, biodiesel cannot be used in its pure form, since its high viscosity and under-performing cold flow properties can become a very serious problem for engine functioning, and the high NO_x emissions raise an environmental concern. However, when biodiesel is blended, i.e. combined with fuel additives, these problems are eliminated, since these substances have the ability to improve fuel characteristics [13,19]. Between many, additives can decrease the viscosity of the fuel, act as cleanliness agents and provide a shorter ignition delay, which prevents unecessary particulate matter and NO_x emissions [10,12,18]. Further, the use of solketal as a fuel additive for biodiesel is economically advantageous, allowing the application of a Circular Economy perspective [11], since i) the biodiesel formation reaction originates glycerol as a by-product, creating an overplus, ii) through acetalization, glycerol can be reporpused as solketal, whose interest as a fuel additive has been peaked and iii) biodiesel requires the use of fuel additives to be commercialized.

The acetalization of glycerol is a reversible reaction, hindered by the existence of a large thermodynamic setback owing to its low equilibrium constant [10,19]. Further, this reaction originates water as a by-product, whose presence has been proven to largely decrease the solketal yield obtained [3,11,18]. As such, it is essential to adopt strategies that guarantee that the reaction is shifted in favour of the products, while assuring optimal conditions for solketal formation. This reaction efficiency is linked to the correct choice of the substrate, solvent and catalyst.

3.1. Substrate

One of the most adopted strategies to increase glycerol conversion is to use a substrate in excess, increasing the glycerol/substrate ratio. In acetalization, substrates are oxygen-containing compounds, such as aldehydes and ketones (Figure 6). Many different substrates have been used in acetalization reactions before, with the most reported ones being butanal [36], furfural [37], citral [38], benzaldehyde [39], formaldehyde [40] and acetone. This review will be focused on acetone, as it is by far the most studied substrate, and its application in glycerol conversion has proved incredibly effectiveness [41,42,43]. Further, excess acetone has been reported to increase glycerol conversion to solketal, while also acting as an entrainer, helping the removal of water from the reactional system, and increasing its miscibility with the viscous glycerol [10,12]. When the reaction is finalized, the unreacted acetone can be recuperated through distillation and be continuously reutilized [12], which helps ensure the sustainability of the acetalization reaction.

3.2. Solvent

As seen previously, when glycerol suffers acetalization, it originates two other products besides solketal: acetal and water. Removal of water can be ensured by the use of entrainers, desiccants and membranes, between others [10,44]. The removal of acetal helps shift the reaction in order to obtain higher glycerol conversions, while simultaneously allowing its recuperation. Further, while solketal is a product of higher commercial interest, acetal also demonstrates fuel additive qualities, and therefore should not be wasted. Traditionally, the removal of acetal from the reactional system was possible through the use of solvents. Some examples that have been previously reported in the literature include toluene [45], ethanol [46] and acetonitrile [47]. The evolution of research in the last years allowed the development of highly efficient catalysts, that can assure a favourable reaction equilibrium by themselves. As such, acetalization reactions have evolved into solvent-free environments [12,41,48,49,50].

3.3. Catalyst

In glycerol acetalization, the correct choice of the catalysts is one of the most important reaction parameters since, without the presence of a catalyst, the reaction pratically does not occur and no glycerol conversion can be observed [51,52,53,54]. The importance and the role of catalysts in this reaction becomes clear when observing the reaction mechanism behind glycerol acetalization. Such as been previously speculated, and a proposed mechanism can be seen in Figure 7, in this case specifically for a Brönsted acid catalyst [18]. In general, the reaction is kickstarted when the catalyst interacts with the carbonyl of the substrate, either by protonation or coordination with a metal site (for Brönsted and Lewis acids, respectively) [35,41,42,55]. This interaction forms a protonated intermediate structure that, when interacting with the hydroxyl groups in glycerol, originates a hemiketal/hemicetal. Once the water molecules are removed from the reaction, the formation of a tertiary carbenium ion occurs [12,18]. Finally, this structure suffers an attack from the hydroxyl groups from glycerol and solketal is originated from the interaction of the ion with a secondary −OH, and acetal occurs from the interaction with a primary −OH [51,52,56]. As such, product selectivity for solketal is much higher, as a consequence that the attack of the secondary hydroxyl is more facilitated, since the primary −OH suffers steric hindrance [3,10,50].

Essentially, the role of the catalyst is to assure the activation of the substrate and initiate the acetalization of glycerol. Further, it has been extensively reported that the efficiency of this initial activation, and thus the efficiency of glycerol conversion, is highly dependent on catalyst acidity [35,52,56,57].

4. Heterogeneous Catalysts for Glycerol Conversion

Conventionally, acetalization reactions required the use of homogeneous catalysts, such as sulfuric acid, hydrochloric acid and p-toluenesulfonic acid (pTSA) [10,12]. However, the use of these catalysts lead to various reaction drawbacks requiring long reaction times and exhibiting difficult recuperation from the reaction medium, which increased the cost [12]. Further, and most importantly, these catalysts are known for their environmental problems, raising attention for their toxicity [3,10]. The awareness for reaction sustainability and its alignment with the Principles of Green Chemistry raised interest in the search for alternative catalysts that allowed high catalytic efficiency and recyclability, while facilitating handling/recovery and being environmentally friendly [3,19,58]. As such, heterogeneous catalysts appeared as potential candidates for the acetalization reaction of glycerol [10,12]. In the last years, many different catalysts and their application in glycerol conversion have been reported, with some examples being heteropolyacids (HPAs), mesoporous silicas, metal-organic frameworks (MOFs), resins, carbon-based materials and polymers. Table 1, Table 2, Table 3 and Table 4 presents various reported glycerol conversion and solketal selectivity results, using different types of heterogeneous catalysts, in the acetalization of glycerol using acetone under solvent-free systems.

Balula et al. studied the influence of Keggin-type HPAs, with the use of phosphotungstic acid (PW₁₂), phosphomolybdic acid (PMo₁₂) and silicotungstic acid (SiW₁₂), in the acetalization reaction of glycerol at room temperature (Figure 8). [41] The results reported a catalytic efficiency trend of PW₁₂ (99.2%) > PMo₁₂ (91.4%) > SiW₁₂ (90.7%) after only 10 min, where PW₁₂ is widely reported to be the most acidic out of the three HPAs. [41,59,60] Da Silva et al. developed a cation-exchanged HPA, where the protons of silicotungstic acid were substitued by tin(II) cations. [51] Such change assured HPA salt insolubility, in an effort to solve the recuperation problems associated with this type of catalyst. [59,61] Glycerol conversion reached 99% after 1 h, with high selectivity at room temperature, owing to the characteristic acidic behaviour of Sn₂SiW₁₂O₄₀, with the catalyst possessing both Brönsted and Lewis acid sites. [51] The catalyst was reused for four consecutive cycles, demonstrating catalytic stability; however, catalyst recuperation was very burdensome [51]. Also, cationic exchange was performed by Ali et al. using imidazolium cations, however, the conversion and selectivity of the glycerol acetalization was not increased when compared with the commercial acids of polyoxometalates.{Ali, 2023 #55} Chen et al. investigated another possibility of facilitating HPAs as catalysts in acetalization, through the preparation of a cesium phosphotungstic salt, and its consequent immobilization in KIT-6 silica. [53] The conversion results obtained for the catalyst in its bulk and incorporated form were very similar (94 and 95%, respectively), with Cs_2.5H_0.5PW₁₂O₄₀@KIT-6 reaching higher conversions after only 15 minutes. Stability tests showed no loss of activity after three consecutive cycles, demonstrating its effectiveness. [53]

Mallesham et al. prepared modified SnO₂ catalysts, whose catalytic performance was studied in the acetalization reaction at room temperature [42,64]. After 1 hour, the following results were obtained: SO₄^2-/SnO₂ (98%) > MoO₃/SnO₂ (61%) > WO₃/SnO₂ (55%). All three catalysts exhibited higher conversion results than the non-modified SnO₂ solid acid (15% after 1.5 h), with SO₄^2-/SnO₂ demonstrating superior conversion owing to the presence of super acidic sites in its structure, further confirming the influence of catalyst acidity [64]. In summary, from the metalic oxide basic catalysts, the polyoxometalates showed to conciliate a higher conversion and higher selectivity for solketal productions.

Between the various heterogeneous catalysts based in silica (Table 2) used for the acetalization of glycerol with acetone, the work from Gadamsetti et al. presented one of the best catalytic result. In this case, it is reported the development of a silica-incorporated molybdenum phosphate catalyst, and its consequent study in acetalization of glycerol, at room temperature [50]. The prepared catalyst demonstrated perfect glycerol conversion, combined with high solketal selectivity (98%), after only 1 hour. Through material characterization, it was shown that MoPo@SBA-15 possessed Brönsted acidic sites, responsible for the high glycerol conversion. Catalyst stability was evaluated for four consecutive recycling cycles, demonstrating the existence of acidic sites leaching, corresponding to a decrease in catalytic efficiency [50]. Other important catalytic achievement was obtained by Ammaji et al. incorporating transition metals in the SBA-15 structure, further studying the application of SBA-15 based catalysts in acetalization reactions [52]. At room temperature, it was obtained the follow order of conversion capacity: Nb-SBA-15 (95%) > Zr-SBA-15 (92%) > Ti-SBA-15 (65%) > Al-SBA-15 (60%), with the Nb-SBA-15 catalyst demonstrating the best catalytic results, along with complete solketal selectivity [52]. Similarly to previous reports, the best performing catalysts (Nb-SBA-15 and Zr-SBA-15) were those that exhibited the highest amount of Brönsted acidic sites, highlighting its importance for this particular reaction. The Nb-SBA-15 catalyst was continuously applied in acetalization reactions for four cycles, showing a decrease in glycerol conversion which was related to leaching of acidic sites [52]. Comparing in general the catalytic results obtained with the functional silica catalysts (Table 2) with the previous heterogeneous polyoxometalates (Table 1), it is possible to observe that identical results were obtained for solketal conversion and selectivity, using shorter reaction time (0.08 or 0.25 h) when polyoxotungstates were used and, in the presence of lower ratio glycerol/acetone (1:3), when Nb-SBA-15 or Zr-SBA-15 were used.

Carbon based materials, such as metal-organic frameworks (MOFs), have also been used as heterogeneous catalysts for the acetalyzation of glycerol with acetone (Table 3). From these works, Bakuru et al. presented one of the most active and sustainable catalytic systems based in MOFs. In this case, it was studied the effect of acidity in the structure of UiO-66, and its influence in the acetalization of glycerol, at room temperature [56]. This MOF structure is very interesting for acetalization, since the combination of the oxophilicity behaviour and the existence of defects cause the appearance of more acidic sites in its structure [56]. From the three MOFs studied it was seen that UiO-66 (Hf) (94.5%) > UiO-66 (Ce) (70.9) > UiO-66 (Zr) (1.5%), confirming that UiO-66 (Hf) is the best performing catalyst since it has the highest amount of µ₃-OH groups, acting as Brönsted acidic sites [69]. The higher the oxophilicity of the MOF structure, the higher the acidity, originates a higher glycerol conversion [56]. Mirante et al. compared the catalytic efficiency of another family of MOFs, based in MOF-808 [35]. Similarly, the MOF-808(Hf) exhibited the best catalytic behaviour, reaching 91% after 3 hours at 60 °C, which was expected due to the superior acidity obtained when compared to the MOF-808 (Zr) catalyst (Figure 9). Catalyst recycling was evaluated for ten consecutive cycles, with MOF-808 (Hf) demonstrating high stability [35]. Santos-Vieira et al. reported the preparation of a coordination polymer (UAV-59), constituted by Gd³⁺ cations and nitrilo(trimethylphosphonic acid) [55]. This catalyst was applied to acetalization reactions, at 55 °C, obtaining a glycerol conversion of 94%, with simultaneous high solketal selectivity (97%). The efficiency of this polymer can be explained due to the high concentration of acidic protons in its structure. Catalyst stability studies demonstrated only a minor decrease in activity, after four consecutive recycling cycles [55]. The best catalytic performance between the based materials present in Table 3, is showed by a composite formed by the incorporation of a polyoxotungstate into the MOF-Fe framework [HMIm]₃[PW₁₂O₄₀]@MOF-Fe (Table 3) [70]. This catalyst obtained complete glycerol conversion and complete solketal selectivity, after only one hour using the lowest ration glycerol/acetone reported in the literature. Using [HMIm]₃[PW₁₂O₄₀]@MOF-Fe catalyst during seven recycling cycles, the glycerol conversion and solketal selectivity was mantained, demonstrating the superior acetalization behaviour of this catalyst compared to the isolated polyoxotungstate [70].

Few reported works can also be find in the literature using zeolite based heterogeneous catalysts for acetalization of glycerol with acetone (Table 4). Using these type of catalysts Higher ratio of glycerol/acetone needed to be used to achieve similar results used with MOFs and polyoxotungstate (Table 3). One of the most interesting examples is reported by Saini et al. that developed a metal-free mordenite zeolite catalyst, whose application in acetalization reactions at 60 °C [72]. After 4 hours of reaction, the catalyst obtained 99% of glycerol conversion, while demonstrating high solketal selectivity (99%). Mordenite was recycled for three cycles, showing no loss of activity [72].

6. Conclusions

The implementation of legislations designed to reduce and eventually eliminate completely the use of fossil fuels derivatives has demonstrated the increasingly urgent search for new sustainable energy sources. Biodiesel has been explored as a non-toxic and environmentally friendly alternative, whose formation reaction still needs to be optimized in order to increase its sustainability and economic interest. An obstacle associated with this industry is the large amount of glycerol produced as a by-product, raising importance for the discovery and investigation of glycerol valorisation strategies, such as acetalization. The acetalization reaction of glycerol, in the presence of acetone, originates solketal, a very interesting fuel additive that contributes to the fullfilment of a Circular Economy Model through its use in biodiesel blends. In the last years, heterogeneous catalysts have distinguised themselves in acetalization, allowing high conversion and selectivity, while simultaneously facilitating recuperation and increasing the sustainability od this valorized process. High glycerol conversion results are linked to the catalyst acidity, where it has been extensively reported the preference of Brönsted acidic sites over Lewis sites, owing to their efficient activation of the substrate, and thus increasing its interaction with glycerol and producing higher amounts of solketal. The acetalization of glycerol has been confirmed to be a fast-acting reaction, using mild experimental conditions, since the majority of works published report better conversion results at room temperature. Higher temperatures, such as 55 °C or 60 °C, are also verified, mainly in order to increase acetone/glycerol miscibility and increase diffusion. Several virtuous results have been obtained thus far; however, there is still much room for improvement: i) mesoporous silicas demonstrate high conversion results at fast rates, but many report stability issues linked with leaching of the acidic sites; ii) the application of MOFs has raised interest, owing to the combination of their satisfactory glycerol conversion and recyclability behaviour. Other advantageous of using silica and MOFs based materials is the lower ratio of glycerol/acetone needed to achive near complete conversion and 100% of selectivity for thee solketal after 1 h of reaction. Keggin-type polyoxotunsgstates have showed to achieve the same catalytic results but in shorter reaction times, such as after 10 and 20 min. However, the combination of the most promissing catalysts, i.e. the acid polyoxometalates and MOFs or silicas for the production of solketal is pratically unexplore, and the only example reported is considered the most sustainable and produtive for this acetalization reaction.