Article pubs.acs.org/IECR
Synthesis of Succinic Acid Alkyl Half-Ester Derivatives with Improved Lubricity Characteristics Seung-Yeob Baek,† Young-Wun Kim,*,† Keunwoo Chung,† Seung-Hyun Yoo,†,‡ Nam Kyun Kim,† and Yeong-Joon Kim*,‡ †
Green Chemistry Research Division, Surfactant & Lubricant Research Team, KRICT, Daejeon City 305-600, Republic of Korea Department of Chemistry, Chungnam National University, Daejeon City 305-764, Republic of Korea
‡
S Supporting Information *
ABSTRACT: A series of succinic acid alkyl half-esters were synthesized with higher than 97% yields by a one-step reaction of succinic anhydride with various fatty alcohols. The synthesized esters were soluble in 100N base oil at 1 wt %, and the lubricating properties of the esters in ultralow-sulfur diesel fuel were measured using a high-frequency reciprocating test rig and a four-ball wear machine. Wear scar diameter results showed that lubricity depended on the presence of a carboxylic acid group and that enhancements to lubricity depended strongly on the chain lengths of the alkyl group.
1. INTRODUCTION Diesel fuel quality has improved through the use of refining processes such as hydrotreating and hydrocracking. These processes reduce the amounts of sulfur and aromatics and generally improve other fuel-quality parameters such as ignition time and thermal stability. However, because desulfurization removes polyaromatics and polar compounds that enhance fuel lubricity,1−5 these same processes tend to reduce the lubricating properties of the fuel. Lubricity significantly affects fuel quality and can be enhanced with various fuel additives. Longer hydrocarbons and sulfur compounds are known to enhance the lubricity of fuel oil. Lubricity additives comprise a range of surface-active chemicals with an affinity for metal surfaces. These materials form boundary films that prevent metal-to-metal contact that can lead to wear under moderate loads between 50 and 200 ppm.6−9 A moderate dose of a chemically suitable additive is beneficial in most cases. However, higher doses of some common diesel fuel additives can induce the formation of fuel injection deposits. In recent years, fatty acid methyl esters (FAMEs), commonly known as biodiesel, have been used successfully to improve the lubricity of diesel fuel. Fatty acids themselves, including linoleic acid, have also been used to enhance lubricity, which means that these types of compounds can help prevent wear on contacting metal surfaces.10−16 However, when the acidic functional groups of these molecules are fully blocked by alkyl ester groups, the metal surfaces cannot interact with the fuel additives. Therefore, it is important that the additive not be completely esterified. This implies a delicate balance of carboxylic acid and ester groups in order to have proper physical and chemical properties. Some alkyl acetoacetates and dicarboxylic esters have been used as lubricating additives in low-sulfur diesel fuels. However, for alkyl acetoacetates, concentrations below 750 ppm are ineffective. Small improvements in fuel lubricity have been observed with dicarboxylic ester additives at concentrations between 500 and 750 ppm.17 Specific types of fatty acid derivatives, such as some amide and ester derivatives of lauric © 2012 American Chemical Society
acid and palmitic acid, have also been used as lubricating additives in low-sulfur fuels.19 At high concentrations (0.13− 0.50 vol %), wear scar diameter (WSD) values for these mixtures significantly decrease to around 200 μm in some cases. Succinic acid monoesters have been used as lubricating additives in some fuels, and it has been shown that WSDs values from high-frequency reciprocating rig (HFRR) tests are significantly lower than or near 300 μm.18 However, the exact structure of the ester was not reported. In the current report, we present a systematic study of the lubricating properties of succinic acid derivatives based not only on various functional groups but also complete molecular structures.
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. 2-Dodecen-1-ylsuccinic anhydride (TCI, 95%), n-octylsuccinic anhydride (TCI, 95%), n-dodecylsuccinic anhydride (TCI, 95%), n-hexadecyl succinic anhydride (TCI, 95%), n-hexadecenyl succinic anhydride (TCI, 94%), 1-hexanol (aldrich, 98%), 1-octanol (Aldrich, 99%), 2ethylhexano1 (Aldrich, 99.6%), 1-decanol (Aldrich, 98%), 1dodecanol (Aldrich, 98%), 1-tetradecanol (Aldrich, 97%), and oleyl alcohol (Aldrich, 85%) were used in the syntheses of half alkyl ester derivatives. p-Toluenesufonic acid monohydrate (Junsei, 99%), toluene (SK Chemical, LP grade), ethyl acetate (SK chemical, LP grade), and anhydrous magnesium sulfate (Daejung, 99%) were used without further purification. 1H NMR spectra were obtained with a 300 MHz Bruker DPX-300 spectrometer using CDCl3 as an internal standard. FT-IR spectra were measured on a Bio-RAD FTS165 spectrometer. Ring-opening reactions of alkyl succinic anhydride by the fatty alcohols were monitored by gas chromatography (Agilent Technology 7890A model) after silylation. An HP-1 capillary Received: Revised: Accepted: Published: 3564
September 18, 2011 February 7, 2012 February 9, 2012 February 9, 2012 dx.doi.org/10.1021/ie202137r | Ind. Eng. Chem. Res. 2012, 51, 3564−3568
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column with dimensions of 0.25 mm × 30 m i.d. × 0.25 μm film thickness (Agilent Technologies, Palo Alto, CA) was used for the separation of products. The initial temperature of 50 °C was raised to 320 °C at a rate of 10 °C/min. The split ratio was 1:50, and helium was used as the carrier gas at a flow rate of 1 mL/min. The temperatures of the injector and detector were 250 and 300 °C, respectively. 2.2. General Synthetic Procedure. Alkyl succinic half alkyl ester derivatives were synthesized from several alkyl succinic anhydrides (1) and fatty alcohols (2) through the ringopening reaction shown in Figure 1.20 Final target products
AISI52100 steel. WSD values were obtained in accordance with ASTM D4172.22 The morphology of the worn surface was observed on a TESCAN Vega II LSU scanning electron microscope (SEM). Samples for morphology measurements were coated with platinum using a Cressington sputter coater 108 auto at 10 mA for 120 s. Energy dispersive X-ray spectrometry (EDX; Bruker, QUANTAX 200 model) was used to measure the elemental distribution of the worn surfaces (energy resolution < 127 V, peak shift 5−300 kcps, HSE-C12-R′ > HSE-C16R′, which may be due to the increasing the number of methylene units in the alkyl group (R) at the 2-position of the succinic acid derivatives, HSE-R-R′. These results suggest that carboxylic acid groups in the half-esters coordinate to the metal surface to form lubricated films, which improve the wear properties under mild conditions. Note also that the 4-ball WSD depended on the length of the alkyl group (R) at the 2position and not on that of the alkyl group (R′) on the esters. These results differ from those of the HFRR experiments due to differences in the mode of measurement; the 4-ball wear is in a circular motion while the HFRR wear is a reciprocating motion. To understand these differences, SEM images and EDX spectra were obtained on the worn surfaces. Because the SEM images and EDX spectra of wear scars lubricated with ULSD containing the various half-esters were similar to each other, only the morphologies of worn surfaces lubricated with ULSD, DSE-C16-C12, and HSE-C12-oleyl are shown in Figure 5. The WSD of HSE-C12-oleyl and HSE-C16-C12 are relatively
Figure 2. HFRR lubricity data of ULSD containing HSE-R-R′ ester derivatives and various control materials.
Figure 3. Dependence of WSD on the concentration of HSE-C12oleyl and HSE-C16-C12.
reduction in WSD from 606 μm with the blank to 366 μm with HSE-C8-R′, 321 μm with HSE-C12-R′, and 323 μm with HSEC16-R′. Thus, lubricity depended on the length of the alkyl (R′) group of the half-esters (HSE-R-R′). Maximum reductions were observed with a C12 alkyl group on HSE-C8-R′ and HSE-C16R′ and an oleyl group on HSE-C12-R′. These results demonstrate the effects of molecular structure on lubricity. The wear scars with HSE-C8-C12, HSE-C12-C12, and HSEC16-C12 were 366, 376, and 323 μm, respectively. This shows a decrease in wear with increasing length of R. This is in good agreement with the general observation that shorter chain lengths reduce molecular interactions and decrease the temperature stability of a protective lubricant film. The WSD values of half-esters with carboxylic acid groups were significantly lower (321−435 μm) than that (590 μm) of
Figure 4. Schematic explanation of enhanced HFRR lubricity of HSE-R-R′ (right) esters compared to that of DSE-R-R′ (left). 3566
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distributed on surfaces lubricated with ULSD containing HSEC12-oleyl and HSE-C16-C12. However, oxygen was not present on worn surfaces lubricated with ULSD containing DSE-C16-C12. This suggests that the oxygen atoms contained in HSE-R-R′ provided stable coordination sites for metal atoms. This, in turn, allowed the even distribution of oxygen atoms contained in HSE-R-R′ over the metal surface. The interactions between the carboxylic acid group and the metal surface lead to the formation of coordination films thereby improving the tribology at the interface. This indicates that the worn surfaces lubricated with ULSD containing HSE-C12-oleyl and HSEC16-C12 were coated with more tribochemical films made through the coordination of carboxylic acids with the metal surfaces.
4. CONCLUSIONS A series of succinic acid alkyl half-ester derivatives, to be used as lubricity additives in ULSD, were synthesized in excellent yield by the simple reaction of succinic anhydrides and fatty alcohols. No purification steps were required. Enhancements in lubricity were measured using two independent methods, HFRR and 4ball wear. HFRR measurements yielded relatively low WSD values of ∼300 μm for some HSE-C12-R′ and HSE-C16-R′ compounds. The performance of these compounds was equally excellent in 4-ball wear measurements with WSD values of 300−400 μm. The SEM images were in good agreement with the WSD values, and the EDX spectra confirmed the interaction between the metal surfaces and the oxygen atom of the carboxylic acid group in the additives.
Figure 5. SEM images of the wear scars lubricated with ULSD and ULSD containing HSE-C12-oleyl, HSE-C16-C12, and DSE-C16-C12 additives.
smaller, and the worn surfaces are smoother compared to those of ULSD and DSE-C16-C12. The EDX spectra of the worn surfaces are shown in Figure 6. Carbon and oxygen are well-
Figure 6. EDX spectra of worn surfaced lubricated with ULSD containing HSE-C12-oleyl, HSE-C16-C12, and DSE-C16-C12. 3567
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(16) Knothe, G.; Steidley, K. R. Lubricity of Components of Biodiesel and Petrodiesel. The Origin of Biodiesel Lubricity. Energy Fuels 2005, 19, 1192. (17) Anastopoulos, G.; Lois, E.; Zannikos, F.; Kalligeros, S.; Teas, C. Influence of Acetoacetic Esters and Di-carboxylic Acid Esters on Diesel Fuel Lubricity. Tribol. Int. 2001, 34, 749. (18) Anastopoulos, G.; Lois, E.; Serdari, A.; Zanikos, F.; Stournas, S.; Kalligeros, S. Lubrication Properties of Low-Sulfur Diesel Fuels in the Presence of Specific Types of Fatty Acid Derivatives. Energy Fuels 2001, 15, 106. (19) Brewer, M. L.; Armstrong, P.; Reid, J. Succinic acid derivatives as lubricity increasing fuel additives. PCT Int. Appl. WO 2002002720 A2 20020110, 2002. (20) Baek, S.-Y.; Kim, Y.-W.; Chung, K.-W.; Yoo, S.-H. Synthesis and Anti-corrosion Properties of Succinic Acid Alkyl Half-ester Derivatives. Appl. Chem. Eng. 2011, 22 (4), 367. (21) ASTM D6079-04: Standard Test Method for Evaluating Lubricity of Diesel Fuels by the High-Frequency Reciprocating Rig (HFRR). In 2006 Annual Book of ASTM Standards; American Society for Testing and Materials (ASTM): West Conshohocken, PA, 2004. (22) ASTM D4172: Standard Test Method for Wear Preventive Characteristics of Lubricating Fluid (Four-Ball Method). In 2004 Annual Book of ASTM Standards; American Society for Testing and Materials (ASTM): West Conshohocken, PA, 1994.
ASSOCIATED CONTENT
S Supporting Information *
Physical properties, 1H NMR data, and 13C NMR spectra of HSE-R-R′ esters. 4-Ball WSD results of ULSD containing HSER-R′ esters. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Y.-W. Kim: phone, +82 42 860 7605; fax, +82 42 860 7669; email,
[email protected]. Y.-J. Kim: phone, +82 42 821 5476; fax, +82 42 821 8896; e-mail,
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by Grant No. 10033530 funded by the Ministry of Knowledge Economy, Republic of Korea.
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REFERENCES
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