Synthesis of Modern Synthetic Oils Based on Dialkyl Carbonates

synthesis was carried out under atmospheric pressure at temperatures not higher than 120 °C reaching a relatively high yield of 65-70%. The synthesiz...
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Ind. Eng. Chem. Res. 2003, 42, 5007-5010

5007

APPLIED CHEMISTRY Synthesis of Modern Synthetic Oils Based on Dialkyl Carbonates S. Gryglewicz,* F. A. Oko, and G. Gryglewicz Wrocław University of Technology, Department of Chemistry, 50-344 Wrocław, ul. Gdan´ ska 7/9, Poland

A new two-stage process is elaborated to produce ester oils with dialkyl carbonate structure which show a very good characteristic in view of their use as lubricants. Di-2-ethylhexyl carbonate and di-3,5,5-trimethylhexyl carbonate were synthesized by transesterification of dimethyl carbonate with 2-ethylhexanol and 3,5,5-trimethylhexanol, respectively. Some oxides, hydroxides, and methoxides of sodium, magnesium, calcium, barium, and aluminum were studied as catalysts for this reaction. Calcium methoxide appeared to be the most suitable, due to its high activity and ease of separation from the reaction mixture. The reaction conditions were mild. The synthesis was carried out under atmospheric pressure at temperatures not higher than 120 °C reaching a relatively high yield of 65-70%. The synthesized oils were characterized by high purity (above 98%) and excellent physicochemical properties. They show a low pour point, a good miscibility with hydrocarbon oils, and a high thermal-oxidative stability. Introduction Hydrocarbon oils and petroleum-derived fluids are commonly used in lubrication. However, in many applications the requirements of the tasks exceed the performance capabilities of classical fluids, and synthesis of new products to meet increasingly severe demands is necessary. Fluids with an ester structure, which are similar to natural triglycerides, are a very important class of synthetic lubricants. Currently, synthetics account for up to 10% of the total global production of oils used. In some specific areas of application, ester oils have proven to be in a great demand.1-5 They are required as components for metal working, and dielectric and hydraulic fluids. Nontoxic and biodegradable ester oils are commonly used in the textile and food industries. Thermoplastic synthetic polymers with excellent and highly desirable characteristics such as polycarbonates have found a wide range of commercial applications for many years. However, synthetic oils with the carbonate moiety in their chemical structure are relatively unknown. Most known methods for production of dialkyl carbonates are based on cheap and available raw materials, including phosgene, dimethyl carbonate, ethylene carbonate, and urea. It is also possible to obtain dialkyl carbonates directly from carbon dioxide and alcohol:6,7

CO2 + 2ROH T CO(OR)2 + H2O

(1)

The reaction proceeds with a hydrophilic system using ethylene oxide or zeolites as a catalyst. Phosgene (COCl2) is a very reactive donor of the carbonate group. The carbonyl carbon atom becomes polarized by induction as a result of the strong elec* To whom correspondence should be addressed. Fax: 4871-3221580. E-mail: [email protected] .

Figure 1. Resonance effects in urea.

tronegative character of chlorine atoms. This facilitates the substitution of chlorine by nucleophilic agents such as alcohols. However, this method involves the use of toxic and costly materials. Therefore, there has been a drive to develop alternative methods. In this regard, the use of urea as a carbonate moiety donor has been frequently studied. Unfortunately, the substitution of amine groups in amides, especially diamides by alkoxyl group proceeds with much difficulty due to resonance effects.8 (see Figure 1.) The transfer of electron pairs from the nitrogen atom to the carbon atom of the carbonyl group neutralizes its positive polarity, resulting in its deactivation. The reaction is not thermodynamically favorable. The free energy charge (∆G) for this reaction is positive.9,10 This method suffers in practicality because carbonate formation proceeds under harsh reaction conditions, i.e., at high temperatures. Moreover, peculiar catalysts, for example, dialkyltin compounds are required. Recently, however, it has been observed that interest in organic synthesis using dimethyl carbonate (DMC) has increased.11,12 This is connected with the improvement of economically profitable methods of DMC production. These modern methods are based on the oxidative carbonylation of methanol over CuCl as a catalyst or using a nitric oxide catalyst system.13-16 It is possible to synthesize carbonates of other alcohols in an alcoholysis reaction with dimethyl carbonates.17

CO(OCH3)2 + 2ROH T CO(OR)2 + 2CH3OH (2)

10.1021/ie030322m CCC: $25.00 © 2003 American Chemical Society Published on Web 09/18/2003

5008 Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003

Lewis acids or strong bases usually catalyze this reaction. The enzymatic exchange of carbonates has been also reported.18-20 Dialkyl carbonates are clear liquids or oils, and most have been noted as possessing a pleasant odor. These compounds are vulnerable to hydrolysis while being more resistant to saponification than other esters. Moreover, during the process of hydrolysis, dialkyl carbonates do not yield free carboxyl acids which are responsible for corrosion. This in turn means that autocatalytic hydrolysis is not encouraged.21 Oils with an organic carbonate structure have excellent lubricating properties. Their strong polar ester group enables them to adhere strongly to metallic surfaces. In addition, they are generally highly compatible with polymers. Products of thermo-oxidative disintegration of dialkyl carbonates form insignificant amounts of sediments. Organic carbonates exhibit a low toxicity to human beings and the natural environment and are easily biodegradable. The aim of this work was the synthesis of di-2ethylhexyl carbonate and di-3,5,5-trimethylhexyl carbonate. This was conducted through the alcoholysis of dimethyl carbonate by 2-ethylhexanol and 3,5,5-trimethylhexanol. A series of catalysts was tested for this reaction, i.e., oxides, hydroxides, and methoxides of sodium, magnesium, calcium, barium, and aluminum. The selected catalyst was then used to synthesize a few hundred grams of di-2-ethylhexyl carbonate and di3,5,5-trimethylhexyl carbonate samples.

Figure 2. Schematic representation of the synthesis of di-2ethylhexyl carbonate and di-3,5,5-trimethylhexyl carbonate (in brackets).

Experimental Section

Table 1. Activity of Catalysts in Alcoholysis Reaction of Dimethyl Carbonate by 2-Ethylhexanol

Materials. Dimethyl carbonate (DMC), 2-ethylhexanol (EH), and 3,5,5-trimethylhexanol (TMH) were purchased from Aldrich Steinheim, Germany. Magnesium oxide, calcium oxide, and calcium hydroxide were obtained from POCH Gliwice, Poland. Sodium methoxide, calcium methoxide, magnesium methoxide, and aluminum methoxide were synthesized by direct reaction of an appropriate metal with methanol. In the case of the synthesis of magnesium and aluminum methoxides, it was necessary to carry out the reactions, respectively, in the presence of minute quantities of iodine and mercury (II) chloride as catalysts. Gas Chromatographic Analysis. GC analysis was performed using an Agilent 6850 Series gas chromatograph with a capillary column (HP INNOWAX, 25 m × 0.32 mm) and flame ionization detector (FID). Nitrogen was used as the carrier gas. The oven temperature was programmed from 70 to 200 °C at 10 °C/min after an initial one minute isothermal period. The split ratio was 10:1. The inlet and detector were set at 240 °C and 270 °C, respectively. The retention time of signals was as follows: C8H17OH, 6.19 min; C9H19OH, 6.31 min; CO(C8H17O)CH3O, 6.74 min; CO(C9H19O)CH3O, 6.95 min; CO(C8H17O)2, 12.20 min; and CO(C9H19O)2, 13.11 min. Testing of Catalyst Activity. The catalytic activity of selected sodium, magnesium, calcium, barium, and aluminum compounds was tested in a transesterification reaction of dimethyl carbonate and 2-ethylhexanol. The test reaction was conducted in a 20-mL glass flask fitted with a reflux condenser. Dimethyl carbonate (0.01 mol), ethylhexanol (0.02 mol), and 50 mg of catalyst, in powder form, were placed in the flask. The reaction was run for 5 h under refluxing conditions (about 100 °C). At the end of the test, the mixture was cooled to room temperature and its composition was analyzed by GC.

time (h) catalyst

0.5

1.0

5.0

CH3ONa MgO (CH3O)2Mg Ca(OH)2 CaO (CH3O)2Ca Ba(OH)2 (CH3O)3Al

47.4 0.0 33.5 0.0 0.9 35.6 0.9 40.0

48.3 0.0 40.1 0.0 1.5 45.1 1.4 46.1

49.9 0.0 44.7 0.0 5.9 48.1 4.8 50.2

The conversion rate of 2-ethylhexanol was the measure of catalyst activity. The results of the catalyst activity tests are displayed in Table 1. Method of Dialkyl Carbonate Synthesis. A scheme for the two-stage synthesis of di-2-ethylhexyl carbonate and di-3,5,5-trimethylhexyl carbonate is shown in Figure 2. The first stage is conducted in a 1000-mL glass reactor fitted with a reflux condenser and magnetic stirrer. The substrates of the reaction were the following: 1 mol of alcohol (EH or TMH), 0.5 mol of dimethyl carbonate, and 2 g of calcium methoxide as catalyst. The reaction was carried out for 1 h at a boiling temperature of the reagents. Then the process was interrupted, and the unreacted dimethyl carbonate and the formed methanol were removed by distillation. The second stage was conducted at 120 °C for 4 h with a continuous removal of methanol and dimethyl carbonate from the reaction system. When the reaction was complete, the catalyst was removed from the liquid products by filtration. The unreacted substrates were separated by distillation at a temperature of 170 °C under 3 mmHg pressure. Physicochemical Properties Determination. The physical properties of the prepared esters were deter-

Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 5009 Table 2. Physicochemical Properties of Di-2-ethylhexyl Carbonate and Di-3,5,5-trimethylhexyl Carbonate

a

carbonate

acid value [mgKOH/g]

viscosity, ν 40 °C [cSt]

pour point [°C]

di-2-ethylhexyl di-3,5,5-trimethylhexyl

0.11 0.10

4.34 6.90

-63 -67

thermo-oxidative stability ∆ν/ν[%]a ∆Acb +1.1 +1.8

+0.05 +0.09

∆ν/ν[%], relative viscosity change. b ∆Ac, acid number change.

Figure 3. Basic transesterification of methyl ester catalyzed by methoxide anion.

mined according to the following standard test methods: acid number ASTM D 974, viscosity ASTM D 445, and pour point ASTM D97. Thermo-oxidative stability was evaluated on the base of changes in viscosity and acid number increase. It was determined after treatment under the following conditions: a temperature of 100 °C, bubbling of 12 L/h air in a 45-g sample, and a treatment time of 72 h. The test was carried out in the presence of metallic copper in the form of filings as a catalyst. At the end of the test, the relative increase of viscosity at 40 °C (∆ν/ν %) and the acid number increase (∆ Ac) were determined. The results are given in Table 2. Discussion The method adopted in the synthesis of dialkyl carbonates was based on the reaction mechanism characteristic of alcoholysis of esters. The mechanism of the alcoholysis reaction of methyl esters is given schematically in Figure 3. This reaction is catalyzed by methoxide ion. A methoxide ion can be directly introduced into the reaction system in the form of metal methoxide, for example. Moreover, some metal hydroxides or oxides can form methoxides with methanol22,23 according to the following reactions:

CaO + CH3OH T Ca(CH3O)OH

(3)

Ba(OH)2 + CH3OH T Ba(CH3O)OH + H2O (4) The synthesis of di-2-ethylhexyl carbonate is a twostep process according to the following reactions:

CO(CH3O)2 + C8H17OH T CO(C8H17O)CH3O + CH3OH (5) CO(C8H17O)CH3O + C8H17OH T CO(C8H17O)2 + CH3OH (6) As can be seen in Table 1, the alcoholysis reaction of dimethyl carbonate by 2-ethylhexanol was catalyzed effectively by methoxides of sodium, magnesium, calcium, and aluminum. In the process of alcoholysis carried out in the presence of these catalysts, equilibrium was reached within 1 h, giving a nearly 50% conversion of 2-ethylhexanol. The composition of the reaction mixture changed insignificantly with longer reaction time up to 5 h. When calcium oxide and barium

hydroxide were used as catalysts, the conversion degree of 2-ethylhexanol was not more than 6% after 5 h of reaction time. Magnesium oxide and calcium hydroxide did not catalyze the alcoholysis reaction of dimethyl carbonate by 2-ethylhexanol. For a further study, calcium methoxide was chosen as the most favorable catalyst for the alcoholysis reaction of dimethyl carbonate by 2-ethylhexanol. As already mentioned, all the methoxides used in our work were characterized by comparable catalytic activity in alcoholysis. However, only calcium methoxide did not make difficulties in its separating from the reaction products. It formed a heterogeneous mixture easy to separate by filtration. Sodium methoxide created a homogeneous solution with the reaction mixture, whereas magnesium methoxide and aluminum methoxide formed colloidal solutions. Because of the fact that the reaction system reaches equilibrium after about a 50% conversion of 2-ethylhexanol, it is impossible to achieve a high yield of di-2ethylhexyl carbonate in a one-stage process. Moreover, two very volatile compounds are present in the reaction system, i.e., dimethyl carbonate as a substrate and methanol as a product. This makes it difficult to shift the reaction equilibrium to the right through a continuous removal of methanol from the reaction environment for example. This is why a two-stage process was adopted in the synthesis of di-2-ethylhexyl carbonate and di-3,5,5-trimethyl carbonate. In the first step lasting about an hour, the system is brought to a thermodynamic equilibrium. At this stage, about 75% of the starting dimethyl carbonate is held mainly in the form of CO(C8H17O)CH3O and CO(C8H17)2, in accordance with reactions 5 and 6. These compounds have high boiling temperatures in comparison with those of methanol and dimethyl carbonate. As a result, the latter are easily removed from the reaction environment by distillation, thereby displacing the existing equilibrium for the process to continue as follows:

CO(C8H17O)CH3O + C8H17OH T CO(C8H17O)2 + CH3OHv (7) 2 CO(C8H17O)CH3O T CO(C8H17O)2 + CO(CH3O)2v (8) Reaction 8 is undesirable because it leads to the release of dimethyl carbonate resulting in lowering the final yield of di-2-ethylhexyl carbonate to just 65%. A chromatographic analysis of the final product showed a purity of 98.5%. Di-2-ethylhexyl carbonate was contaminated to a very small degree with unreacted 2-ethylhexanol and methyl-2-ethylhexyl carbonate (CO(C8H17O)CH3O), which is a coproduct of the reaction. The side product of the process (a mixture of 2-ethylhexanol and methyl-2-ethylhexyl carbonate) can be recirculated as shown in Figure 2. A similar result can be achieved through an analogous two-stage synthesis of di-3,5,5-trimethylhexyl carbonate. The yield of the latter reached the value of 70%.

5010 Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003

The main physicochemical properties of oils based on dialkyl carbonates are given in Table 2. Di-2-ethylhexyl carbonate and di-3,5,5-trimethylhexyl carbonate were clear colorless liquids, completely miscible with hydrocarbon oils with the structure of saturated alkanes, such as poly(R-olefins). It is worth noting the very low pour points of both dialkyl carbonates in the region of -65 °C. The oils obtained also showed a very high resistance to long contact with air at high temperatures in the presence of metallic copper as a catalyst. Under these conditions, only a slight change in viscosity and acidity was observed. This proves that the synthesized oils are highly resistant to oxidation and polymerization processes. The main failure of di-2-ethylhexyl and di-3,5,5trimethylhexyl carbonates is their low viscosity. However, in many technological applications high viscosity is not required. Conclusions This study focused on the synthesis of ester oils with dialkyl carbonate structure. These oils were obtained by catalytic transesterification of dimethyl carbonate by 2-ethylhexanol and 3,5,5-trimethylhexanol. As catalysts, oxides, hydroxides, and methoxides of Na, Mg, Ca, Ba, and Al were studied. Taking into account the catalytic activity and ease of catalyst separation from the reaction products, calcium methoxide was found to be the best as catalyst in alcoholysis reaction. Magnesium oxide and calcium hydroxide did not catalyze the alcoholysis reaction at all. The yield of di-2-ethylhexyl carbonate synthesis was significantly improved using a two-stage process compared to a one-stage process (65% vs 45.1%). Di-2-ethylhexyl and di-3,5,5-trimethylhexyl carbonates were characterized by a high purity as much as 98.5% and 98.1%, respectively. Both dialkyl carbonates show a very low pour point and a high resistance to oxidation and polymerization reactions. This is very important in view of their use as lubricant oils. Acknowledgment This work was financed by the Polish State Committee for Scientific Research in Poland (3T09B 04020). Literature Cited (1) Mirci, L. E.; Herdan, J. M.; Boran, S. New Synthetic Ester Type Base Oils with Biodegradability Potential. J. Synth. Lubr. 2001, 17, 295. (2) Gryglewicz, S.; Beran, E.; Janik, R.; Steininger, M. R-134a Compatible Lubricants Based on C4-C6 Esters of Pentaerythritol and Polyalkyleneglycol. J. Synth. Lubr. 1997, 13, 337. (3) Remmele, E.; Widmann, B. Suitability and Environmental Compatibility of Rapeseed Oil Based Hydraulic Fluids for Agricultural Machinery. J. Synth. Lubr. 1999, 16, 129. (4) Mahanti, S.; Datta, N.; Pandey, N.; Barman, S.; Maiti, N.; Nambiar, P. R. Role of Synthetic Esters in the Overall Performance of Cold Rolling Oil for Steel. J. Synth. Lubr. 1996, 13, 3.

(5) Pal, M.; Singhal, S. Environmentally Adapted Lubricants, Part II. Hydraulic Fluids. J. Synth. Lubr. 2000, 17, 219. (6) Fang, S.; Fujimoto, K. Direct Synthesis of Dimethyl Carbonate from Carbon Dioxide and Methanol Catalysed by Base. Appl. Catal. A: Gen. 1996, 142, L1. (7) Bhange, B. M.; Fujita, S.; Ikushima, Y.; Arai, M. Synthesis of Dimethyl Carbonate and Glycols from Carbon Dioxide, and Methanol Using Heterogeneous Basic Metal Oxide Catalysts with High Activity and Selectivity. Appl. Catal. A: Gen. 2001, 219, 259. (8) Alexander, E. R. Principles of Ionic Organic Reactions: Wiley: New York, 1955. (9) Pacheco, M. A.; Marshall, C. L. Review of Dimethyl Carbonate (DMC) Manufacture and Its Characteristics as a Fuel Additive. Energy Fuels 1997, 11, 2. (10) Suciu, E. N.; Kuhlman, B.; Knudsen, G. A.; Michaelson, R. C. Investigation of Dialkylitin Compounds as Catalysts for the Synthesis of Dialkyl Carbonates from Alkyl Carbamates. J. Organomet. Chem. 1998, 556, 41. (11) Ono, Y. Catalysis in the Production and Reactions of Dimethyl Carbonate, an Environmentally Benign Bulding Block. Appl. Catal. A: Gen. 1997, 155, 133. (12) Parrish, J. P.; Salvatore, R. N.; Jung, K. W. Perspectives on Alkyl Carbonates in Organic Synthesis. Tetrahedron 2000, 56, 8207. (13) King S. T. Oxidative Carbonylation of Methanol to Diethyl Carbonate by Solid-State Ion-Exchanged CuY Catalysts. Catal. Today 1997, 33, 173. (14) Knifton, J. F.; Duranleau, R. G. Ethylene Glycol-Dimethyl Carbonate Cogeneration. J. Mol. Catal., 1991, 67, 389. (15) Romano, U.; Tesel, R.; Mauri, M. M.; Rebora, P. Synthesis of Diethyl Carbonate from Methanol, Carbon Monoxide and Oxygen Catalyzed by Copper Compounds. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 396. (16) Tomishige, K.; Sakaihori, T.; Sakai, S.; Fujimoto, K. Diethyl Carbonate by Oxidative Carbonylation on Activated Carbon Supported CuCl2 Catalysts: Catalytic Properties and Structural Change. Appl. Catal. A-Gen. 1999, 181, 95. (17) Shaikh, A. G.; Sivaram, S. Dialkyl and Diaryl Carbonates by Carbonate Interchange Reaction with Diethyl Carbonate. Ind. Eng. Chem. Res. 1992, 31, 1167. (18) Pozo, M.; Gotor, V. Double Enantioselective Enzymic Synthesis of Carbonates and Urethanes. Tetrahedron Assym. 1995, 6, 2797. (19) Bisht, K. S.; Svirkin, Y. Y.; Henderson, L. A.; Gross, R. A.; Kaplan, D. L.; Swift, G. Lipase-Catalyzed Ring-Opening Polymerization of Trimethylene Carbonate. Macromolecules 1997, 30, 7735. (20) Hacking, M. A.; Rantwijk, F.; Sheldon, R. A. Lipase Catalyzed Reactions of Aliphatic and Arylaliphatic Carbonic Acid Esters. J. Mol. Catal. B: Enzymol. 2000, 9, 2001. (21) Boyde, S. Hydrolytic Stability of Synthetic Ester Lubricants. J. Synth. Lubr. 2000, 16, 297. (22) Bradley, D. C.; Mehrotra, R. C.; Gaur, D. P. Metal Alkoxides; Academic Press: London, 1978. (23) Gryglewicz, S. Alkaline-earth Metal Compounds as Alcoholysis Catalysts for Ester Oils Synthesis. Appl. Catal. A: Gen. 2000, 192, 23.

Received for review April 15, 2003 Revised manuscript received July 24, 2003 Accepted July 25, 2003 IE030322M