Efficient Synthesis, Calorimetric and Rheological Studies of

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Efficient Synthesis, Calorimetric and Rheological Studies of Symmetrical Bio-Based Fatty Ethers Remi Nguyen, Jérémie Castello, Isabelle Pezron, Denis Luart, Elisabeth van Hecke, and Christophe Len Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01860 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on August 1, 2017

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Industrial & Engineering Chemistry Research

Efficient Synthesis, Calorimetric and Rheological Studies of Symmetrical Bio-Based Fatty Ethers Remi Nguyen1, Jérémie Castello1°, Isabelle Pezron1, Denis Luart2, Elisabeth Van Hecke1* and Christophe Len1

1

Sorbonne Universités, Université de Technologie Compiègne (UTC), EA TIMR 4297

UTC/ESCOM, Centre de Recherches Royallieu, CS 60319, 60203 Compiègne Cedex, France

2

Ecole Supérieure de Chimie Organique et Minérale (ESCOM), EA TIMR 4297 UTC/ESCOM,

1 allée du Réseau Jean-Marie Buckmaster, 60200 Compiègne, France

KEYWORDS. Etherification, Fatty alcohol, Melting, Viscosity

ABSTRACT Four linear bio-based fatty ethers (di-n-dodecyl, di-n-tetradecyl, di-n-hexadecyl and di-noctadecyl ethers) have been synthetized from free fatty alcohols according to a green and simple efficient method. Their melting properties, Tm, were studied by using DSC technique. Ethers showed higher Tm (28 – 62°C) than their corresponding alcohols, but the temperature difference between one ether and its alcohol rapidly decreases as the carbon number in the molecules 1

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increases. Melting enthalpy, DHm, of an ether was approximately twice the DHm of its corresponding alkanol. Flow properties were also studied between Tm and 100°C. Ethers behaved as Newtonian liquids with viscosities from 1.8 to 20 mPa.s. Below 80 and 65°C respectively, din-dodecyl and di-n-tetradecyl ethers showed lower viscosity than their corresponding alcohol. The temperature-viscosity curves were fitted with an Arrhenius-type relationship. Flow activation energies were significantly lower for the ethers than for the alcohols: around 20 kJ/mol and 28 kJ/mol respectively.

INTRODUCTION Symmetrical fatty ethers appear in many application fields depending on their structure: cyclic, linear and branched. When the number of carbon atoms is lower than sixteen (e.g. di-n-butyl ether, di-n-hexylether, di-n-octyl ether) most of them are used as cosmetics and personal care, surface treatment products, washing and cleaning products, coating products, lubricants and greases, inks and toners, fuels… For the dialkyl ethers with a higher molecular weight (e.g. di-ndodecyl ether, di-n-dodecylethyl ether) they are generally employed as additives to ethanol-based diesel fuel or as wax. Depending on the nature of the alkyl chain, the production of fatty alkyl ethers could be envisaged starting from bio-based derivatives. In comparison with fatty esters, fatty alkyl ethers have the advantage to be uncleavable to hydrolysis and can be used in harder conditions. The conventional Williamson etherification is the most popular way for the synthesis of both symmetrical and unsymmetrical ethers in lab-scale as well as in industry. Among the other methods applied for the production of symmetrical fatty ethers, different catalysts such as iridium, stain based catalysts1,2 or heteropoly acids catalyst3 in liquid phase are well reported. It is noteworthy that some work has also been done in gas phase4 and in alternative conditions 2


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(supercritical solvents, microwave irradiations)5,6. Most of these synthesis processes concern small symmetrical ethers and are not transposable at the industrial scale. Moreover, these methods avoid the drawbacks of employing dangerous reagents, harsh reaction conditions or generating excessive inorganic wastes. Concerning the physico-chemical data, complete analysis of symmetrical fatty ethers was missing. If their crystallization behavior and their phase transition have been studied1,7, no rheological data is available. From a structural point of view, the presence of another bond (RCH2-O-CH2R) has important influence on the crystalline arrangement and should also affect the flow at high temperature. As a consequence, it appears crucial to have rheological data for the concerned industrial applications. In order to develop a greener method for the production of symmetrical fatty ethers (C12 – C18) (Fig. 1) without strong basic nucleophiles and good leaving groups and with no additional purification step, a simple efficient synthesis is proposed. Calorimetric (from -10°C to 120°C) and rheological properties (from melting point to 100 °C) are then discussed for the produced fatty ethers and the corresponding alcohols.

( )n O ( )n 1 (n 2 (n 3 (n 4 (n

= 1) = 3) = 5) = 7)

Figure 1. Symmetrical bio-based fatty ethers

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EXPERIMENTAL Materials Fatty n-alcohols, dodecan-1-ol (5), tetradecan-1-ol (6), hexadecan-1-ol (7) and octadecan-1-ol (8), were purchased from Acros and were used without any further purification. A commercially available catalyst, sulfuric acid from Acros, was used.

Methods General Procedure for Etherification A 200 mL opened glass reactor equipped with a magnetical stirrer was charged with 0.2 mol of corresponding alcohol 5-8 (dodecan-1-ol (5), tetradecan-1-ol (6), hexadecan-1-ol (7) and octadecan-1-ol (8)) and heated at 145°C. After dropwise addition of 0.05 mol (0.25 eq) of H2SO4 the reaction mixture was stirred at 145°C for 1.25 hours. After cooling the reaction mixture to 60°C, 150 mL of acetone was added. Then precipitation of the corresponding ethers 1-4 (room temperature for the di-n-hexadecyl ether (3) and di-n-octadecyl ether (4) and 4°C for the di-ndodecyl ether (1) and di-n-tetradecyl ether (2)) was done. After filtration of the solid, recristallisation in 150 mL of acetone furnished the corresponding di-n-dodecyl ether (1), di-ntetradecyl ether (2), di-n-hexadecyl ether (3) and di-n-octadecyl ether (4) in 73%, 76%, 80% and 85%, respectively. Bio-based ethers 1-4 were obtained in purity higher than 99%.

Analysis of Etherification Products by NMR 1

H NMR and 13C NMR analyses were performed on a 400 Hz instrument (Brucker) in deuteriated

chloroform CDCl3. Chemical shifts (δ) are quoted in ppm and are referenced to TMS (tetramethylsilane) as an internal standard. Coupling constants (J) are quoted in Hz, common

4


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splitting patterns and their abbreviations were s (singulet), d (doublet), t (triplet), q (quartet), quin (quintet), sex (sextet), m (multiplet). All fatty synthesized ethers (di-n-dodecyl ether (1), di-n-tetradecyl ether (2), di-n-hexadecyl ether (3) and di-n-octadecyl ether (4)) present similar chemical shifts in 1H NMR and 13C NMR. 1

H NMR (400 MHz, CDCl3): d 0.88 (3H, t, J = 6.8 Hz), 1.25-1.43 (18-30H, m), 1.55 (2H, q),

3.39 (2H, t, J = 6.8 Hz); 13C NMR (100 MHz, CDCl3): 14.15, 22.72, 26.22, 29.39 – 29.81 (7-13 C), 31.95, 70.99.

Analysis of Etherification Products by Gas Chromatography Gas chromatography analyses were performed on a Perkin-Elmer gas chromatography (Autosystem XL GC), using an Altech AT HT column, with a detector at 300 °C, an injector at 340 °C, and a constant flow of nitrogen of 1 mL min-1. The column was heated at 300 °C. Using these parameters, the retention time for the bio-based ethers 1-4 were 1.9, 2.3, 3.3 min and 5.5 min, respectively.

Differential scanning calorimetry The melting temperature and enthalpy of alcohols 5-8 and ethers 1-4 were measured by a differential scanning calorimeter (DSC Q100, TA Instruments). 10 to 20 mg of sample 1-8 were placed in aluminum pans and sealed hermetically. To ensure a good thermal contact between the solid samples and the pan, the samples were first melted by heating to a temperature of 120°C and allowed to solidify by cooling to -10°C after 5 min held at 120°C. After 5 min at -10°C, they were heated again to 120°C, held for 5 min and finally cooled to ambient temperature. The heating and cooling rates were respectively 2°C/min and -2°C/min. Melting properties were considered on the second heating step. The onset temperature of endothermic heat flow peaks 5



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was taken as the melting point. Melting enthalpy was calculated by integrating the area of the endothermic peak on a time basis. Measurements were carried out in duplicate. Uncertainties were ± 1°C for the melting temperatures, and ± 5 J/g for the enthalpy. Rheology Viscosity measurements were carried out using a Physica MCR301 (Anton Paar) rotational rheometer equipped with the double gap (DG 26.7) Couette measuring system suited for lowviscosity samples (sample volume: 3.8 mL). The rheometer is equipped with a Peltier temperature control system that can regulate the temperature in the − 40 to 200°C range. Measurements were monitored by the Rheoplus software. All experiments were started at 100°C after the complete melting of the products inside the Couette measuring system. Newtonian behavior was firstly checked at 100°C for shear rates ranging from 0.1 to 1000 s-1. The temperature-dependency of the viscosity was then studied by linearly decreasing the temperature (-1°C/min) at constant shear rate (10 s-1) until the solidification point was reached. Two replicates were performed. Uncertainties were ± 0.3 mPa.s for the viscosity, and ± 0.5 °C for the temperature.

RESULTS AND DISCUSSION The four fatty ethers: di-n-dodecyl ether (1), di-n-tetradecyl ether (2), di-n-hexadecyl ether (3) and di-n-octadecyl ether (4) were obtained through a solvent-free process in a batch reactor starting from the corresponding fatty alcohols 5-8 in presence of sulfuric acid (0.25 mol%) as catalyst at 145°C for 1.25 hours (Scheme 1). Ethers 1-4 were precipitated in acetone then purified by crystallization in acetone and no further purification was needed. Yields of the solid ethers 1-4 were 73%, 76%, 80% and 85%, respectively. It was notable that yields of the target ethers 1-4

6


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increased with the length of the alkyl chain. During the O-alkylation, some sulfated supposed byproducts are formed but unfortunately no intermediates could be isolated.

OH ( )n 5 (n 6 (n 7 (n 8 (n

( )n

H 2SO 4 O

145°C ( )n

= 1) = 3) = 5) = 7)

1 (n 2 (n 3 (n 4 (n

= 1), 73% = 3), 76% = 5), 80% = 7), 85%

Scheme 1. Etherification of fatty alcohol in solvent-free process

Melting properties Fig. 2 shows the DSC curves obtained during the second heating step of the four fatty ethers 1-4 and the corresponding fatty alcohols 5-8. As shown on Fig. 2, a single phase transition peak was observed for ethers 1, 2 and 4 as well as for alcohols 6 and 8, whereas two peaks were detectable on alcohol 5 curve but also clearly observed for di-n-hexadecyl ether (3) and its corresponding alcohol 7. Tyagi et al.7 observed two transitions peaks for all those four ethers 1-4: a major peak and a premelt peak appearing as a shoulder band to the main peak, one or two degree Celsius below. A double peak in heat flow rate-temperature curves reveals the existence of polymorphism. The polymorphism of long-chain alcohols (C > 12) has been demonstrated for many decades by using various methods8. Below its melting point, each fatty alkanol 5-8 has a specific temperature at which it changes from a crystalline structure to another form, a phenomenon referred to as transition, and the temperature being called the transition temperature or transition point. We only observed a clear polymorphism for hexadecan-1-ol (7) probably because the temperature difference between melting and transition for this alcohol is larger 7



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(around 5°C) than for other alkanols 5, 6 and 8. Particularly, when cooled from a temperature above its melting point, alcohol 7 is known to solidify at that temperature, adopting a hexagonal structure, and about five degrees below, the structure changes to monoclinic8. According to our study, the corresponding ether 3 also exhibits a clear solid-solid transition during heating (confirmed during cooling) with a temperature difference between transition and melting similar to that of alcohol 7 but slightly larger (9°C and 5 °C respectively). The temperature difference we observed for ether 3 is considerably larger than the one mentioned in Tyagi et al.’s study (< 2°C) performed at higher heating/cooling rates (10°C/min).

8


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Figure 2. Heat flow versus temperature during heating (DSC) The melting temperature, Tm, of ethers (Table 1) rises when the carbon chain length is extended. Adding two carbon atoms to one ether increases Tm by 10°C on average. The melting temperatures found for ethers 1-4 in this study are slightly lower than those found by Tyagi et al.7. Those authors compared the melting temperature of ethers to that of n-alkanes with a total carbon number of 25, 29, 33 and 37, and observed that the incorporation of a central functionality like the ether bond in an n-alkane chain, by substituting the central carbon atom by one oxygen 9



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atom, decreases its melting temperature of about 20°C. In our study, we compare the melting temperature of ethers 1-4 to that of the corresponding alkanols 5-8 (Table 1). Ethers 1-4 have higher melting temperatures than their corresponding alcohols 5-8, but the temperature difference between one ether and its corresponding alcohol rapidly decreases as the carbon number in the molecules increases: 13 to 3°C from ether 1 / alcohol 5 to ether 4 / alcohol 8. Table 1. Melting point (peak onset) and melting enthalpy of compounds 1-8 obtained by DSC Compound

a

Melting point (°C)

Melting enthalpy (kJ/mol)

This study

Literature

This study

Literature

1

28

30.6a-31.7b7

66

937

2

41

42.4a-43.6b7

90

1137

3

48a-57b

50.0a-51.5b7

94*

1177

4

62

60.0a-62.1b7

122

1067

5

15a -20b

23 to 259

35

409

6

30

36 to 399

45

22 to 499

7

43a-48b

45 to 509

54*

33 to 579

8

59

55 to 589

61

40 to 709

Transition. b Melting.* Transition and Melting

The melting enthalpy, DHm, of ethers 1-4 is in the range 66-122 kJ/mol. It gradually increases when lengthening the carbon chains (Table 1). Tyagi et al. 7 found higher enthalpy values but the trend observed with increasing carbon chain length is the same, except for the longest ether (ether 4) that they found out of line with other values. From Wide Angle X-ray Scattering studies at ambient temperature, they attributed that anomaly of ether 4 to a possible loosening of monoclinic crystal packing in favor of orthorhombic structure, similar to that of long n-alkanes. 10


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Our DSC study does not clearly confirm that hypothesis. Compared to alkanols, our study shows that the melting enthalpy of an ether with 2x2n carbon atoms is about twice the melting enthalpy of the alkanol with 2n carbon atoms (Table 1). The comparison to alkanols with 24 to 36 carbon atoms would be interesting but the melting enthalpies of long-chain alkanols (n > 14) are still scarce and not reliable enough until now in the literature, probably due to polymorphism phenomena.

Rheological properties Viscosity values always depend on the measuring temperature. Fig. 3 shows the temperatureviscosity dependency for the four fatty ethers 1-4 and their corresponding alcohols 5-8 above their melting point. As expected, the viscosity exponentially increases as the temperature is decreased (Fig. 3). The curves are stopped at the temperature of solidification of the products that was clearly identified experimentally by a sudden and sharp increase in viscosity (not drawn on Fig. 3). Below 80°C, our results for bio-based alkanols 5-7 having 12, 14 and 16 carbon atoms are close to those obtained with Ostwald or Ubbelohde viscometers as reported in the literature10,11 (respectively Í and Æ symbols on Fig. 3). As shown on Fig.3, the viscosity at a given temperature increases with incrementing carbon chain length within both alkanol family 58 and ether family 1-4. Moreover, the viscosities at temperatures close to 100°C are higher for ethers 1-4 than for their corresponding alkanols 5-8. Viscosities at 100°C are reported in Table 2. However, when decreasing the temperature, the viscosity curves of ethers 1 and 2 cross over the viscosity curves of their corresponding alcohols 5 and 6. The cross over temperature is shifted towards lower values as the carbon chain length is increased: around 85°C for ether 1/alcohol 5 cross over and 65°C for ether 2/alcohol 6 cross over. Therefore, at a given temperature below

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these critical values, the shortest ethers 1 and 2 having 24 and 28 carbon atoms respectively, are less viscous than their corresponding alcohols 5 and 6 (12 or 14 carbon atoms). In addition, at temperatures up to approximately 100°C, they remain less viscous than alcohols 7 and 8 having 16 and 18 carbon atoms respectively. Viscosities of the longest ethers 3 and 4 are higher than viscosities of corresponding alkanols 7 and 8 over the entire operating temperature range. Another remarkable point from Fig.3 is that viscosity at a temperature close to solidification (ie maximal viscosity of the liquid state) increases with incrementing carbon chain length within ether family 1-4 whereas it decreases within alkanol family 5-8. The increase from ether 1 to ether 4 is from 8 to 13 mPa.s and the decrease from alkanol 5 to alkanol 8 is from 20 to 11 mPa.s (Table 2).

Fig. 3: Viscosity vs temperature for alkanols and ethers (× and + symbols: literature data) Table 2 Viscosity at 100°C, solidification temperature and viscosity close to solidification temperature deduced from the viscosity-temperature curves 12


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Compound

Viscosity at 100°C (mPa.s)

Solidification temperature (°C)

Viscosity close to solidification temperature (mPa.s)

1

2.0

30

8

2

2.8

42

9

3

4.2

51

12

4

6.7

59

13

5

1.8

20

20

6

2.2

35

14

7

2.8

47

12

8

3.3

56

11

The temperature-dependent viscosity can be fitted with an Arrhenius-type relationship:

h (T) = C1 . exp ([Efa/R)/T] with h (T), the viscosity (Pa.s) at temperature T (K), C1, a pre-exponential constant (Pa.s), Efa, the flow activation energy and R, the gas constant (8.314 J.mol-1.K-1). The flow activation energy, Efa, generally depends on the size of the molecule and on the intermolecular forces. It characterizes the energy needed by a molecule to be set in motion against the frictional forces of neighboring molecules. Higher Efa values indicate a more rapid change in viscosity with temperature. In accordance with the relationship described above, the Arrhenius curves (ln h – 1/T) are plotted on Fig. 4. The lines obtained for the ethers (continuous lines) are parallel to each other meaning comparable flow activation energies. The slope of the lines obtained for ethers 1-4 is smaller than the slope of alcohols lines (dotted lines) meaning a lower flow activation energy, Efa, of ethers 1-4 compared to alcohols 5-8. The fitting parameter, Efa, is reported in Table 3. The flow 13



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activation energies of ethers 1-4 are around 20 kJ/mol with a slight augmentation with increasing carbon atoms number from ether 1 to ether 3 (19.4 to 21.2 kJ/mol respectively). It seems to be slightly lower for ether 4 (18.7 kJ/mol). The flow activation energies of alkanols are higher: around 28 kJ/mol. That indicates a more rapid change in viscosity of alkanols with temperature compared to ethers. Again a slight increase of Efa with increasing the number of carbon atoms was observed in alkanols series (27.5 to 28.9 kJ/mol from alcohol 5 to alcohol 8 respectively). Significant increase of the flow activation energy with increasing the size of the molecule within a homologous series has already been demonstrated for hydrocarbons from 4 to 30 carbon atom12. However, it was mentioned that for a hydrocarbon chain length of 20 to 30 carbons, Efa reaches a steady value of 20 to 25 kJ/mol. Compared to that series, ethers 1-4 and hydrocarbons with approximately the same number of carbon atoms have similar flow activation energies. That means that the incorporation of a central functionality like the ether bond (RCH2-O-CH2R) in an n-alkyl chain by substituting the central carbon atom by one oxygen atom does not greatly affect the van der Waals interactions between molecules in the liquid state. The significantly higher Efa found for the alcohols 5-8 may be attributed to the polar hydroxyl group which introduces additional intermolecular hydrogen forces to van der Waals ones.

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Fig. 4 Viscosity-Temperature Arrhenius plot

Table 3 Flow activation energy of ethers and alcohols Compounds

Flow activation energy, Efa (kJ/mol)

1

19.4

2

20.3

3

21.2

4

18.7

5

27.5

6

28.0

7

28.3

8

28.9

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CONCLUSION This study focused on comparing melting properties and flow properties (from melting to 100°C) of a series of four linear fatty ethers 1-4 (di-n-dodecyl, di-n-tetradecyl, di-n-hexadecyl and di-noctadecyl ethers) to the corresponding alcohols 5-8 from which they have been synthetized. The synthesis was conducted according to a green and simple efficient method. The heat flow versus temperature curves obtained by DSC confirmed a clear polymorphism of din-hexadecyl ether (3) and its alcohol: hexadecan-1-ol (7). As expected, both melting temperatures and enthalpies increase with lengthening the carbon chains within the homologous series. Despite melting temperatures and enthalpies values slightly lower than those mentioned in the literature, it was observed that ethers have higher melting temperatures than their corresponding alcohols, but the temperature difference between one ether and its alcohol rapidly reduces as the carbon number in the molecules increases. This study also shows that the melting enthalpy of an ether is about twice the melting enthalpy of its corresponding alkanol. Concerning flow properties at temperatures higher than melting point, ethers as well as alcohols behave as Newtonian liquids with moderate viscosities between 1.8 and 20 mPa.s. It was shown that the viscosities at temperatures close to 100°C are higher for ethers than for their corresponding alkanols. However, when decreasing the temperature, viscosity-temperature curves of the two shortest ethers cross over the viscosity-temperature curves of their corresponding alcohols. The cross-over temperature decreases with increasing the carbon number: 85°C for din-dodecyl ether (1)/dodecan-1-ol (5), 65°C for di-n-tetradecyl ether (2)/tetradecan-1-ol (6). The temperature-viscosity curves were fitted with an Arrhenius-type relationship. Flow activation energies were found significantly lower for the ethers than for the alcohols: around 20 kJ/mol and 28 kJ/mol respectively, meaning a less rapid change in viscosity with temperature for ethers compared to alcohols. The flow activation energies obtained for ethers were similar to those 16


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mentioned in the literature for hydrocarbons with a same carbon number. Only a very small increase with increasing the molecule length was observed within all three homologous series. Based on their solid state at ambient temperature and their ability to give low-viscous liquids when heated at relatively moderate temperatures, the four bio-based fatty ethers could be advantageously used as alternatives to long chain petrochemical hydrocarbons (paraffin waxes) in many traditional fields. In addition, due to their high melting enthalpy (comparable to that of paraffin waxes), the bio-based fatty ethers could also find applications in some emerging sectors like thermal energy storage as Phase Change Materials.

AUTHORS INFORMATION Corresponding author Elisabeth van Hecke: [email protected]

Present Address ° Ministère de l'éducation nationale, de l'enseignement supérieur et de la recherche, DGESIP, 1 rue Descartes, 75005 Paris, France. AKNOWLEDGMENT This research work was conducted in the framework of the GREENWAX FUI Project (16th AAP) with the financial support from the french Public Investment Bank (BPI) and the Region Picardie (France). The authors thank Michael Lefebvre for carrying out the calorimetric and rheological measurements.

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Academic/Plenum Publishers: New YorK, 2001. (9) Linstrom, P.J.; Mallard, W.G. NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Nat. Inst. Standards and Technol., Gaithersburg. 2016, (http://webbook.nist.gov). (10) Matsuo, S.; Makita, T. Viscosities of six 1-alkanols at temperatures in the range 298-348 K and pressures up to 200 MPa. Int. J. Thermophysics. 1989, 10, 833-843. (11) Liew, K.Y.; Seng, C.E.; Ng, B.H. Viscosities of long-chain n-alcohols from 15 to 80°C. J. Solution Chem. 1993, 22, 1033-1040.

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(12) Tabor, D. The role of surface and intermolecular forces in thin films lubrication. Tribology Series, 1982, 7, 651-682.

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Table of Contents Graphic:

( ) n

OH ( ) n (n=1,3,5,7)

H 2SO 4 145°C

O ( ) n

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Efficient Synthesis, Calorimetric and Rheological Studies of

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