6.2 N EW LUBRICANT ESTER STRUCTURES – S YNTHESIS AND
New estolide structures
However, the hydroformylation setup requires a lot of expertise and expensive hardware.
Hence, an alternative route that could be conducted in lab scale had to be identified.
First, the esterification of oleic acid had to be optimized. In contrast to the conventional esterification that requires an excess of alcohol and Brønsted acid catalysis under reflux conditions, we decided to utilize the commercially available, immobilized lipase CAL-B (Novozym 435) for the esterification. In contrast to the conventional approach, only stoichiometric amounts of alcohol are required and the process can be run at lower temperatures (here 50 °C) and under neat conditions. To drive the reaction equilibrium towards complete conversion, we used molecular sieves (4 Å pore size) to bind the one equivalent of water that is formed during the esterification. Regarding the alcohol of choice for the esterification, we decided to use Guerbet alcohols. These alcohols are known for their excellent softener qualities and low viscosity levels and have already been proven to be the most promising alcohols to give estolides excellent viscosity properties (see Table 20). Furthermore, some earlier research had already been conducted for the synthesis of esters of fatty acids with Guerbet alcohol, including by means of biocatalytic esterification in up to 3000 L scale.[183–188] With this method, we were able to obtain the synthesized esters with 97-99% purity (according to GC analysis) on gram scale (Scheme 53). These esters were highly pure because of the mild reaction conditions of the lipase catalyzed esterification.
Scheme 53: Biocatalytic esterification of oleic acid with several Guerbet alcohols on gram scale.
Since the 2-ethylhexyl esters are privileged for low temperature applications due to their low melting points, we scaled up the synthesis for the 2-ethylhexyl oleate to 100 mmol scale and obtained 37.3 g (95% isolated yield) 2-ethylhexyl oleate.[167,178]
Due to the high cost of the biocatalyst, recycling of it for several production cycles is a requisite for its economic viability. As a consequence, we decided to conduct the biocatalytic esterification with an equipped catalyst container for heterogeneous catalysts (SpinChem-reactor), in which we deposited the immobilized biocatalyst and the molecular sieves (
Figure 31). Fortunately, we could not observe any deterioration of the biocatalyst activity after three production cycles. This proves the high practicability of the Novozym 435. More precisely, 96-98% conversion towards 2-ethylhexyl oleate was observed in all production batches (Figure 32). For better mixing of the components, we decided to use cyclohexane as a solvent in these experiments since it can easily be removed after esterification in vacuo. The high conversion values correlate very well with the batch production in a stirred flask.
Figure31: Left: Reaction setup for the biocatalytic esterification of oleic acid with 2-ethylhexanol in a SpinChem-reactor. Top right: Rotating reactor during a biotransformation. Bottom right: Catalyst container filled with Novozym 435 and molecular sieves 4 Å.
0 5 10 15 20 25
0 20 40 60 80 100
Conversion [%]
time [h]
neat, flask
cyclohexane, Flask cyclohexane, Spin, 1st cyclohexane, Spin, 2nd cyclohexane, Spin, 3rd
New estolide structures
For further upscaling of this esterification, in vacuo removal of the formed H2O in the reaction should be considered since molecular sieves are a major cost factor. The in situ removal of H2O for biocatalyzed estolide synthesis could already been shown by Martin-Arjol et al.[189] in 2015.
As an alternative to the hydroformylation of oleic acid esters exists a literature known approach via ene reaction with paraformaldehyde to introduce the required C1 fragment in one step as an hydroxy group. For this, either free oleic acid or its esters are treated with paraformaldehyde in dichloromethane in presence of aluminium Lewis acids (Scheme 54).[163,165,195–197] After several optimization experiments, it was decided to conduct the ene reaction only for the 2-ethylhexyl oleate instead of pure oleic acid since the overall yields were significantly higher. Under optimized conditions, the unsaturated, hydroxymethylated Guerbet ester could be obtained in up to 69% yield (17.6 g) after vacuum distillation. One of the advantages of this access route is the clearly defined position of the fatty acid chain modification. While the HClO4-catalyzed approach by Cermak, Isbell et al.[169] (see Scheme 49) leads to mixtures of C5-C13 adducts, this Lewis acid catalyzed ene reaction always leads to a 1:1 mixture of C9/C10 adducts. These positions have been proven to be the best ones for optimal properties of the estolides. A drawback of this synthesis is the required overstochiometric amount (3.3 eq.) of aluminium Lewis acid and paraformaldehyde due to high waste generation. An alternative, catalytic approach would enhance the viability of this promising modification method for oleic acid and its derivatives. Additionally, Friedel-Crafts acylation of oleic acid derivates with acid chlorides followed up by hydrogenation of the obtained carbonyl moiety would open up the path to structures with higher branching and probably even better properties for use as lubricants.[165,198]
Scheme 54: Ene reaction of oleic acid or 2-ethylhexyl oleate.
Once the hydroxymethylated oleic acid derivatives were obtained, hydrogenation of the C-C double bond was conducted with molecular hydrogen (H2) under atmospheric pressure, catalyzed by palladium immobilized on carbon (Pd/C) at room temperature. The hydrogenated, saturated alcohols could be obtained in 25% yield for the free acid derivative and 62% yield for the 2-ethylhexyl ester, respectively (Scheme 55). However, tedious work-up via column chromatography was necessary since hydrogenation of unprotected alcohols by Pd/C is accompanied by formation of side products by de-/hydrogenation of the OH-group. To avoid this drawback, the hydrogenation was conducted after esterification of the unsaturated, hydroxymethylated alcohol. In this case, hydrogenation was highly selective and yielded the saturated new dimer in high purity with
up to 81% yield (1.62 g). This hydrogenation could be conducted on bigger scale with similar yields.
Scheme 55: Hydrogenation of the C-C double bonds of the hydroxymethylated oleic acid derivatives by molecular hydrogen (H2).
The selective esterification of the hydroxymethylated 2-ethylhexyl oleic acid esters was the next important step for the synthesis of the estolide dimers. Initially, we esterificated the saturated alcohol with the fatty acids in MTBE at 50-60 °C to obtain the dimer with 85%
or 56% isolated yield (see Scheme 56).
Scheme 56: Biocatalytic esterification of the hydroxymethylated 2-ethylhexyl oleic acid ester with stearic acid or oleic acid.
However, the above mentioned selectivity problems with the hydrogenation of non-protected alcohols prompted us to esterificate the unsaturated product of the ene reaction, 2-ethylhexyl (E)-9+10-(hydroxymethyl)octadec-10+8-enoate (C9/C10 adduct, 1:1 ratio), directly with stearic acid and conduct the hydrogenation with the formed estolide dimer (see Scheme 55 and Scheme 57). After the desired dimer was filtrated over silica, it could be obtained with up to 73% yield (11.9 g) in high purity.
New estolide structures
Scheme 57: Biocatalytic esterification of the unsaturated, hydroxymethylated alcohol with stearic acid.
There are different synthetic routes to obtain the new dimer structures. Based on the above mentioned results, the following one was chosen as the most promising one with respect to selectivity and yield: First, esterification of oleic acid with Guerbet alcohols is conducted.
Second, ene reaction with paraformaldehyde of the Guerbet oleates is conducted. Third, esterification of fatty acids with the allyl alcohol derivate is conducted. Last, palladium catalyzed hydrogenation yields the saturated, new dimer (Scheme 58).
Scheme 58: Synthetic route overview for the synthesis of new lubricant esters. The preferred route is marked in blue. All reactions after the ene reaction include the
C10-New estolide structures
After successfully establishing an access towards the new estolide structures, the author decided to synthesize a reported estolide structure to get a direct comparison between the old and new structures in terms of their biodegradation. Towards this end, it was decided to synthesize a monoestolide 2-ethylhexyl ester derived from 12-hydroxystearic acid that is capped with stearic acid (Scheme 59). 12-Hydroxystearic acid is accessible by hydrogenation of ricinoleic acid from castor oil.[165,167]
Scheme 59: Synthesis of 2-ethylhexyl (stearoyloxy)octadecanoate starting from 12-hydroxystearic acid.
The biocatalytic esterification of 2-ethylhexanol with 12-hydroxystearic acid was conducted with Novozym 435 (30 mg/mmol) at 75 °C for 5 hours and yielded the 2-ethylhexyl 12-hydroxyoctadecanoate with 73% yield after purification via vacuum distillation. Regarding the possible formation of 12-hydroxystearic acid oligomers by self-condensation, no amount of this side-product was detected via 1H-NMR after five hours. This can be explained with the manifold faster catalyzed esterification of the primary hydroxy moiety with the carboxyl group through the Novozym 435. The slow reaction speed for the esterification of secondary esters with Novozym 435 was already shown by Martin-Arjol et al.[189] (see Scheme 51, chapter 6.1).
To avoid slow transesterification of the 2-ethylhexyl 12-hydroxyoctadecanoate with stearic acid, stearoyl chloride (n-C17H35COCl) was used for the selective esterification to yield the monoestolide 2-ethylhexyl 12-(stearoyloxy)octadecanoate with a total yield of 54%
(36.0 g) after column chromatography. For that synthetic step, products from four separate reactions on 10 g scale (referring oleic acid) were combined for the purification via column chromatography.
The most important criteria for the sustainability of a lubricant is its biodegradability, since every year huge amounts of lubricants are leaked into the environment, polluting huge amounts of water and ground. Hence, we decided to test the biodegradability of the newly synthesized estolides according to the OECD guideline 301 F. This guideline describes the biodegrading of a chemical compound in a closed-bottle test. For a successful biodegradation, over 60% of the investigated compound has to be decomposed after a defined time frame (28 days) under aerobic conditions.
To get a valuable comparison in terms of biodegradability, it was decided to test the newly synthesized lubricant ester, harboring a bridging methylene moiety, against the already reported estolide structure by Cermak et al.[176], which is derived from 12-hydroxystearic acid (Figure 33).
Figure 33: Structures of the investigated estolides for the biodegradibility test according to guideline OECD 301 F (closed-bottle test).
Beside the minium amount of 60% which have to be degraded, another criterion is the degradation of 50% of the compound in a time frame of 10 days once the first 10% have been degraded.
The closed-bottle test was conducted by Klüber Lubrication in Munich and the result is excellent.[199] After 28 days, 81.3% of the new lubricant ester have been degraded and the first 60% have been degraded after 8 days (Figure 34). These values proof the ready biodegradability of the new estolide and underline its potential as a sustainable alternative based on renewable resources. In comparison, the known estolide structure was also degraded to a total amount of 83.3% after 28 days, with the first 60% being degraded after 7 days (Figure 35). This result is very similar to the newly synthesized lubricant ester and demonstrates that the additional methylene moiety poses no threat to the biodegradability of a lubricant ester.
New estolide structures
Figure 34: Biodegradability test according to OECD 301 F of the new lubricant ester, performed by and at Klüber Lubrication.[199]
Figure 35: Biodegradability test according to OECD 301 F of the known estolide, performed by and at Klüber Lubrication.[199]