A novel class of bio-lubricants are synthesized by epoxidation of 10-undecylenic acid-based esters

1. INTRODUCTION

Lubricants have a tremendous application in day-to-day life, machine-driven and automobile industries. Most of the existing lubricant oils are non-degradable, causing a tremendous effect on the environment. There has been considerable interest in biodegradable lubricants from renewable resources as substitutes for conventional non-renewable mineral-based lubricants. Environmental concerns and the decrease of fossil fuel reserves have triggered great interest in developing bio-based lubricants from renewable resources.

The performance of an engine can be improved by using various edible and non-edible oil-based lubricants. Vegetable oils are renewable resources and are eco-friendly. Synthesized or modified vegetable oil lubricants have been proven to possess excellent lubricant properties for various applications (Anjana and Preeti, 2013Anjana S, Preeti S. 2013. Vegetable oils as lube base stocks: A review, African J. Biotechnol. 12 (9), 880-891. https://doi.org/10.5897/AJB12.2823 ). Polyol esters from palm and palm kernel oils have excellent oxidative stability (Yunus et al., 2004Yunus R, Fakhru’l-Razi A, TianLye O, Sunny EI, Joseph MP. 2004. Lubrication properties of trimethylolpropane esters based on palm oil and palm kernel oils. Eur. J. Lipid Sci. Technol. 106, 52-60. https://doi.org/10.1002/ejlt.200300862 ). Fatty esters from rubber seed oil and thumba seed oil synthesized from 2-ethyl hexanol, neopentylglycol (NPG), trimethylolpropane (TMP) and pentaerythritol (PE) are good bio-lubricant base-stocks (Kamalakar et al., 2013Kamalakar K, AmitKumar R, Prasad RBN, Karuna MSL. 2013. Rubber seed oil-based biolubricant base stocks: A potential source for hydraulic oils, Indus. Crops Prod.. 51, 249-257. https://doi.org/10.1016/j.indcrop.2013.08.058 ; Kamalakar et al., 2015Kamalakar K, SaiManoj GNVT, Prasad RBN, Karuna MSL. 2015. Thumba (CitrulluscolocynthisL) seed oil: a potential bio-lubricant base-stock. Grasas Aceites 66 (1), e055. https://doi.org/10.3989/gya.0576141 ). High-performance polyol ester lubricants were synthesized from10-Undecenoic acid (Padmaja et al., 2012Padmaja KV, Bhamidipati VSK, Reddy RK, Bhaskar PS, Arun KS, Prasad RBN. 2012. 10-Undecenoic acid-based polyol esters as potential lubricant base stocks. Ind. Crops Prod. 35, 237-240. https://doi.org/10.1016/j.indcrop.2011.07.005 ). The novel class of eco-friendly lubricants was synthesized from undecylenic acid (Cavalcante et al., 2019Cavalcante IM, Rocha NRDC, de Brito DHA, Schuller APD, Câmara Neto JF, de Morais SM, Ricardo NMPS. 2019. Synthesis and characterization of novel polyol esters of undecylenic acid as ecofriendly lubricants. J. Am. Oil Chem. Soc. 96 (1), 75-82. https://doi.org/10.1002/aocs.12160 ). The polyol esters of Calophyllum inophyllum oils showed good tribological properties. (Srinivas et al., 2020Srinivas V, Chebattina KRR, Pranay GVS, Lakkoju B, Vandana V. 2020. Tribological properties of polyol ester-commercial motorbike engine oil blends. J. King Saud Univ. Eng. Sci. https://doi.org/10.1016/j.jksues.2020.07.016 ). Trimethylol triesters from waste cooking oils showed good lubrication properties (Lakkoju and Vemulapalli, 2020Lakkoju B, Vemulapalli V. 2020. Synthesis and characterization of triol based bio-lubricant from waste cooking oil. In AIP Conference Proceedings. Vol. 2297, No. 1, p. 020002. AIP Publishing LLC.). Polyol ester-based bio-lubricants and their blends showed tribological properties (Kotturu et al., 2020Kotturu CM, Srinivas V, Vandana V, Chebattina KR, Seetha Rama Rao Y. 2020. Investigation of tribological properties and engine performance of polyol ester-based bio-lubricant: Commercial motorbike engine oil blends. Proceedings of the Institution of Mechanical Engineers, Part D: J. Autom. Eng. 234 (5), 1304-17.).

The double bonds present in synthesized esters undergo auto-oxidation which is susceptible to degradation on prolonged use, thus decreasing the lubricant efficiency. To improve the lubricant efficiency, double bonds present in esters can be modified chemically by epoxidation reaction.

Oils with unsaturated fatty acids were epoxidized using different catalysts. H₂SO₄ was an effective catalyst for forming an oxirane ring in the presence of H₂O₂ across the double bonds in vegetable oils (Abdullah and Salimon, 2010Abdullah MB, Salimon J. 2010. Epoxidation of vegetable oils and fatty acids: catalysts methods and advantages. J. Applied Sci. 10, 1545-1553. https://doi.org/10.3923/jas.2010.1545.1553 ). Cottonseed oil was epoxidized using sulphuric acid (Dinda et al., 2008Dinda S, Patwardhan AV, Goud VV, Pradhan NC. 2008. Epoxidation of cotton seed oil by aqueous hydrogen peroxide catalysed by liquid inorganic acids, Bior. Technol. 99, 3737-3744. https://doi.org/10.1016/j.biortech.2007.07.015 ), peracetic acid (Sinadinović-Fišer et al., 2012Sinadinović-Fišer S, Janković M, Borota O. 2012. Epoxidation of castor oil with peracetic acid formed in situ in the presence of an ion exchange resin. Chemical Engineering and Processing: Process Intensification 62, 106-113. https://doi.org/10.1016/j.cep.2012.08.005 ), and soybean fatty methyl esters by performic acid (Campanella et al., 2008Campanella A, Fontanini C, Baltanás MA. 2008. High yield epoxidation of fatty acid methyl esters with performic acid generated in situ, Chem. Eng. J. 144 (3), 466-475. https://doi.org/10.1016/j.cej.2008.07.016 ), jatropha and castor oils by performic acid and peroxyacetic acid in acid ion exchange resin (Goud et al., 2007Goud VV, Patwardhan AV, Dinda S, Pradhan NC. 2007. Kinetics of epoxidation of jatropha oil with peroxyacetic and peroxyformic acid catalysed by acidic ion exchange resin, Chemical Engineering Science, 62 4065-4076. https://doi.org/10.1016/j.ces.2007.04.038 ). The epoxidation reaction of mahua oil with hydrogen peroxide (H₂O₂) and glacial acetic acid in the presence of an acidic medium was reported (Goud et al., 2006aGoud VV, Patwardhan AV, Pradhan NC. 2006a. Studies on the Epoxidation of Mahua oil (Madhumicaindica) by hydrogen peroxide, Bior. Technol. 971365-1371. https://doi.org/10.1016/j.biortech.2005.07.004 ). Okieimen reported the epoxidation of rubber seed oil using peroxyacetic acid (Okieimen et al., 2002Okieimen FE, Bakare OI, Okieime CO. 2002. Studies on the epoxidation of rubber seed oil, Ind. Crops Prod. 15 (2), 139-144. https://doi.org/10.1016/S0926-6690(01)00104-2 ). Vegetable oils such as oil from the Karanja seed (Goud et al., 2006bGoud VV, Pradhan NC, Patwardhan AV. 2006b. Epoxidation of karanja (Pongamiaglabra) oil by H₂O_2, J. Am. Oil Chem. Soc. 83, 635-640. https://doi.org/10.1016/j.biortech.2005.07.004 ), soybean oil (Petrovic et al., 2002Petrovic ZS, Zlatanic A, Lava CC, Snezˇana S. 2002. Epoxidation of soybean oil in toluene with peroxoacetic and peroxoformic acids - kinetics and side reactions, Eur. J. Lipid Sci Technol. 104, 293-299. https://doi.org/10.1002/1438-9312(200205)104:5<293::AID-EJLT293>3.0.CO;2-W ; Geethanjali et al., 2013Geethanjali G, Sony MK, Padmaja KV, Karuna MSL, Prasad RBN. 2013. Preparation and properties of lubricant base stocks from epoxidized karanja oil and its alkyl esters. Ind. Eng. Chem. Res. 52 (47), 16598-16605. https://doi.org/10.1021/ie4024325 ), canola oil, canola oil derivatives (Omonov et al., 2016Omonov TS, Kharraz E, Curtis JM. 2016. The Epoxidation of Canola oil and its derivatives, RSC Adv. 6, 92874-92886. https://doi.org/10.1039/C6RA17732H ), jatropha seed oil (Goud et al., 2007Goud VV, Patwardhan AV, Dinda S, Pradhan NC. 2007. Kinetics of epoxidation of jatropha oil with peroxyacetic and peroxyformic acid catalysed by acidic ion exchange resin, Chemical Engineering Science, 62 4065-4076. https://doi.org/10.1016/j.ces.2007.04.038 ) and Parkia biglobosa seed methyl esters (Ikhuoria et al., 2007Ikhuoria EU, Obuleke RO, Okieimen FE. 2007. Studies on the kinetics of epoxidation of the methyl esters of parkiabiglobosa seed oil, Journal of Macromolecular Science Part A: Pure Appl. Chem. 44, 235-238. https://doi.org/10.1080/10601320601031424 ) were also converted to respective epoxidized oils. Epoxidized vegetable oils have various applications as valuable intermediate or end products due to their biodegradability and non-toxicity (Armylisas et al., 2017Armylisas AHN, Hazirah MFS, Yeong SK, Hazimah AH. 2017. Modification of olefinic double bonds of unsaturated fatty acids and other vegetable oil derivatives via epoxidation: A review, Grasas Aceites 68 (1), e174. https://doi.org/10.3989/gya.0684161 ).

Epoxidized soybean oil exhibited improved low-temperature stability, thermo-oxidative stability friction and wear properties compared to existing lubricants (Erhan et al., 2008Erhan SZ, Sharma BK, Liu Z, Adhvaryu A. 2008. Lubricant base stock potential of chemically modified vegetable oils, J. Agric. Food Chem. 56 (19), 8919-8925. https://doi.org/10.1021/jf801463d , Adhvaryu et al., 2005Adhvaryu A, Liu Z, Erhan SZ. 2005. Synthesis of novel alkoxylated triacyl glycerols and their lubricant base oil properties. Indust. Crops Prod. 21 (1), 113-119. https://doi.org/10.1016/j.indcrop.2004.02.001 ). Epoxidized jatropha oil showed better antioxidant properties than butylated hydroxyl toluene and zinc dimethyl dithiocarbamate (Sammaiah et al., 2014Sammaiah A, Padmaja KV, Prasad RBN. 2014. Synthesis of epoxy jatropha oil and its evalution of lubricant properties. J. Oleo Sci. 63 (6), 637-643. https://doi.org/10.5650/jos.ess13172 ). Epoxidised Rapeseed oil had superior oxidative stability, better friction reduction and high-pressure abilities (Wu et al., 2000Wu X, Zhang X, Yang S, Chen H, Wang D. 2000. The study of epoxidized rapeseed oil used as a potential biodegradable lubricant, J. Am. Oil Chem. Soc. 77 (5), 561-563. https://doi.org/10.1007/s11746-000-0089-2 ). Epoxidized fatty methyl esters from castor oil have more thermo-oxidative stability than their esters (Venubabu and Goud, 2014Venubabu B, Goud VV. 2014. Epoxidation of castor oil fatty acid methyl esters (COFAME) as a lubricant base stock using heterogeneous ion-exchange resin (IR-120) as a catalyst. Energy Procedia 54, 75-84. https://doi.org/10.1016/j.egypro.2014.07.249 ). The present study focused on synthesizing a novel class of epoxides from undecylenic esters of 2-ethyl hexanol, NPG and TMP. The synthesized products were assessed for lubricant properties and correlated with their corresponding esters.

2. MATERIALS AND METHODS

2.2. Methods

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2.2.1. Spectral analysis

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A 1600 FT-IR Perkin-Elmer spectrometer (Norwalk, CT) was used for recording Infrared (IR) spectra with a liquid film between the NaCl cells. The streching frequencies of the IR spectra were measured in cm^-1. A Brucker AVANCE 400 MHz spectrometer was used to record proton and Carbon - 13 Nuclear Magnetic Resonance Spectra (¹H and ¹³C NMR) with respect to tetra methyl silane (TMS) an internal standard. The CDCl₃ was used as solvent in ¹H, ¹³ C NMR. The δ values were recorded in parts per million (ppm).

2.2.2. Characterization properties

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Physico-chemical, and lubricant properties of synthesized esters and epoxy compounds were determined by standard ASTM (American standard testing method) methods. The density and specific gravity were tested by pycnometer at room temperature. The Kinematic viscosity at various temperatures (40 ^oC, 100 ^oC) was tested in a Kinematic viscosity bath. Flash point, and fire point were tested in a Clevland flash and fire point apparatus. Cloud, and pour point were tested in the automatic cloud and pour point apparatus. Copper strip corrosion was tested in the Copper strip corrosion bath. All the above apparatus were procured from culture instruments India LLP, Bengaluru, India.

2.2.3. Determination of lubricant properties

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Kinematic viscosity and viscosity index. The kinematic viscosity was measured at two different temperatures of 40 and 100 ^oC using an Ostwald direct flow type viscometer tube (type-C, D, E tubes, having definite constant values) in a ‘CI’ kinematic viscosity bath Digital and calculated according to the ASTM D 445 method. The viscosity index was calculated according to the ASTM D 2270 method. Measurements were done thrice and average values were taken.

$\mathrm{K}\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{ }\mathrm{v}\mathrm{i}\mathrm{s}\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}\mathrm{ }=\mathrm{ }\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{t}\mathrm{ }\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}\mathrm{ }\mathrm{o}\mathrm{f}\mathrm{ }\mathrm{v}\mathrm{i}\mathrm{s}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{ }\times \mathrm{ }\mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e}\mathrm{ }\mathrm{i}\mathrm{n}\mathrm{ }\mathrm{s}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{s}$  

Cloud and pour point. The cloud and pour points were determined according to standard ASTM D 2500, ASTM D 97 methods using a ‘CI’ ultra crystat cloud and pour point apparatus. The cloud point of the samples was detected automatically in a cloud and pour point apparatus. The pour point of the samples was checked manually for every 1 ^oC decrease in temperature. Cloud and pour point determinations were made thrice and the average was reported.

Flash and fire points. The flash and fire points were determined according to ASTM D 92 method using a ‘CI’ Cleveland flash and fire point apparatus. The open-cup method for the detection of flash point was used by means of a vessel, which is exposed to the outside air. The sample was placed in the vessel and the temperature was raised gradually until it flashed and ignited at a certain point. After obtaining the flash point, the sample was heated until the fire came. The flash and fire ponts of all samples were determined thrice and the average was reported.

Copper strip corrosion. The copper strip corrosion of the samples was determined by using a ‘CI’ copper strip corrosion test apparatus as per the ASTM 130 method. A polished copper strip was immersed in the sample solution and heated at 100 ^oC for 3 h. After that, the tarnished copper strip was removed and assessed against an ASTM standard.

Synthesis of 2-ethylhexyl undec-10-enoate (2-ethyl hexyl ester) (a). 0.3839 moles of 2-ethyl hexanol (50 g), 0.5759 moles of UDA (106 g), 1% pTSA (total weight of 2-ethyl hexanol and UDA, 1.56 g) and toluene (100 mL) in an RB flask were refluxed at 120-130 ^oC. The reaction proceeded at 120-130 ^oC until 7 mL of wáter were collected in the dean-stark apparatus. The reaction progress was checked by observing thin layer chromatography (TLC) with 9:1 v/v hexane and ethyl acetate as elutent at regular intervals. After reaching the required product temperature of the reaction mixture, it was decreased to 23-25 ^oC. The toluene present in the reaction mixture was removed by a vacuum. The excess or unreacted acid in the reaction mixture was neutralised using a saturated NaHCO­₃ solution, and ethyl acetate was added to the aqeous solution. The organic (ethyl acetate) layer, which contained crude 2-ethyl hexyl ester, was separated using a separating funnel and thoroughly washed with water twice. Anhydrous sodium sulphate was used to remove moisture from the crude (2-ethyl hexyl ester) present in the organic layer. The crude 2-ethyl hexyl ester was purified by using the column chromatography technique. Basic aluminum oxide was used as a stationary phase, 10% ethyl acetate in hexane was used as a mobile phase in column chromatography. 96.8 g of pure 2-ethyl hexyl ester were obtained. The 2-ethyl hexyl ester was assessed by ¹HNMR, ¹³C NMR and IR spectral studies.

¹HNMR spectra: 0.8-9.0 (t, 6H, -CH₃), 1.2-1.6 (m, 18H, -CH₂-), 2.0-2.1 (m, 3H, C=C-CH₂) and -CH(CH₂)₃), 2.3-2.4 (t, 2H, -CO-CH₂-), 3.9-4.0 (m, 2H, -O-CH₂-), 4.9-5.0 (m, 2H, CH=CH₂), 5.7-5.8 (m, 1H, -CH=CH₂).

¹³C NMR spectra: 66.5 (O-CH₂), 115.10 (CH=CH₂), 139.27 (CH=CH₂), 173.44(C=O).

IR spectra: 3020 (str, C=C-H), 2929-2857 (str, -C-H), 1727 (str, C=O), 1217 (str, C-O-C), 771 (C-C).

Synthesis of 2,2-dimethylpropane-1,3-diyl diundec-10-enoate (NPG ester)(b) . 0.678 moles of NPG (70.6 g), 1.356 moles of UDA (250 g), 1% pTSA (total weight of NPG and UDA, 3.2 g) and toluene (100 mL) were refluxed in a RB flask at 120-130 ^oC. The reaction took place at 120-130 ^oC until 24 mL of water were collected in the dean-stark apparatus. The reaction progress was monitored by observing TLC with 9:1 v/v hexane and ethyl acetate as elutent at regular intervals. After the formation of ester, the temperature of the reaction mixture was decreased to 23-25 ^oC. The toluene present in the reaction mixture was removed by vacuum. Ethyl acetate was added to the neutralized excess or unreacted acid present in the reaction mixture with a saturated NaHCO­₃ solution in the aqeous solution. The organic (ethyl acetate) layer, which contains crude NPG ester, was separated using a separating funnel and thoroughly washed with water twice. Anhydrous sodium sulphate was used to remove moisture from the crude (NPG ester) present in the organic layer. The crude NPG ester was purified using the column chromatography technique. Basic aluminum oxide was used as stationary phase, 10% ethyl acetate in hexane was used as a mobile phase in column chromatography. 245 g pure NPG ester were obtained. The NPG ester was assessed by ¹HNMR, ¹³C NMR and IR spectral studies.

¹HNMR spectra: 0.9 (s, 6H, -CH₃), 1.2-1.4(m, 20H, -CH₂-), 1.5-1.6(m, 4H, -CO-CH₂-CH₂-), 2.0-2.1(q, 4H, -CH₂-CH=CH₂), 2.3-2.4(t, 4H, -CO-CH₂-), 3.9(s, 4H, -O-CH₂-), 4.9-5.0(m, 4H, -CH=CH₂), 5.7-5.9 (m, 2H, -CH­=CH₂).

¹³C NMR spectra: 68.76 (O-CH₂), 114.78 (CH=CH₂), 139.09 (CH=CH₂), 172.76 (C=O).

IR spectra: 3021(str, C=CH), 2927 (str, C-H), 1736 (str, C=O), 1214(str, COC), 755 (C-C).

Synthesis of 2-ethyl-2-((undec-10-enoyloxy)methyl)propane-1-3-diyl diundec-10-enolate (TMP ester) (c). 0.378 moles of TMP (52 g), 1.356 moles of UDA (250 g), 1% pTSA (total weight of NPG and UDA, 3.02 g) and toluene (100 mL) in a RB flask were refluxed at 120-130 ^oC. The reaction proceeded at 120-130 ^oC until 21 mL of water were collected in the dean-stark apparatus. The reaction progress was monitored by observing TLC with 9:1 v/v hexane and ethyl acetate as elutent at regular intervals. The temperature of the reaction mixture decreced to 23-25 ^oC. The toluene present in the reaction mixture was removed by vacuum. Ethyl acetate was added to the neutralized excess or unreacted acid in the reaction mixture using a saturated NaHCO­₃ solution, in the aqeous solution. The organic (ethyl acetate) layer, which contains crude TMP ester, was separated using a separating funnel and thoroughly washed with water twice. Anhydrous sodium sulphate was used to remove moisture from the crude (TMP ester) present in the organic layer. The crude TMP ester was purified by using the column chromatography technique. Basic aluminum oxide was used as a stationary phase, 10% ethyl acetate in hexane was used as a mobile phase in column chromatography. 208 g of pure TMP ester were obtained. The TMP ester was assessed by ¹HNMR, ¹³C NMR and IR spectral studies.

¹HNMR spectra: 0.8-1.0 (t, 3H, -CH₃), 1.2-1.3(m, 30H, -CH₂-), 1.3-1.4(m, 2H, CH₃-CH₂-), 1.5-1.7 (m, 6H, -CO-CH₂-CH₂-), 2.0-2.1 (q, 6H, -CH₂-CH=CH₂), 2.3-2.4 (t, 6H, -CO-CH₂-), 3.9 (s, 6H, -O-CH₂-), 4.9-5.0 (m, 6H, -CH=CH₂), 5.7-5.8 (m, 3H, -CH=CH₂).

¹³C NMR spectra: 63.32 (O-CH₂), 114.20 (CH=CH₂), 138.69 (CH=CH₂), 171.97 (C=O).

IR spectra: 3021(str, C=CH), 2927 (str, C-H), 1741 (str, C=O), 1162 (str, COC), 758 (C-C).

Synthesis of 2-ethylhexyl 9-(oxiran-2-yl) nonanoate (2-ethyl hexyl epoxide)(d). 2-ethyl hexyl ester (a) (35 g, 0.1146 moles) 175 mL of DCM (10 vol with respect to ester) were taken in a 1L RB flask and cooled to 0 °C. mCPBA (65% purity, 49 g, 0.1720 moles) was added to the reaction mixture at regular intervals for 3 h to maintain the temperature at 0 °C. The reaction was kept at ambient temperature and stirred overnight. The reaction progress was monitored by observing TLC. The reaction mixture was cooled to 0 °C. After completion of the reaction, the precipitated m-chloro benzoic acid was filtered. Ethyl acetate was added to the filtratewith 10% sodium thiosulphate solution, and then stirred thoroughly. The organic (ethyl acetate) layer was separated and washed twice with distilled water. The organic layer was dried over anhydrous sodium sulphate to remove moisture and concentrated under reduced vacuum to obtain crude 2-ethyl hexyl epoxide. Silica gel column chromatography was used to purify crude 2-ethyl hexyl epoxide with 1:1 ethyl acetate and hexane as an elutent to obtain 26 g of pure 2-ethyl hexyl epoxide. The pure 2-ethyl hexyl epoxide was assessed by ¹HNMR, ¹³C NMR, IR spectral studies, and analyzed for physicochemical and basic lubricant properties.

¹HNMR spectra: 0.9 -1.0 (t, 6H, -CH₃), 1.2-1.4 (m, 19H, -CH₂- and -CH(CH₂-)₃), 1.5-1.7 (m,2H, ), 2.0-2.1 (t, 2H, -CH₂C=O), 2.3-2.5 (m, 2H, ), 2.7 (d, 1H, ), 4.0-4.15 (m, 2H, -CH-O-C=O).

¹³C NMR spectra: 68.17 (O-CH₂), 46.43 (CHOCH₂), 51.85 (CHOCH₂), 173.20 (C=O).

IR spectra: 1724 (str, C=O), 1216 (str, ), 757 (C-C).

Synthesis of 2,2-dimethyl propane-1,3-diyl bis (9-(oxiran-2-yl) nonanoate (NPG epoxide)(e). NPG ester (b) (50 g, 0.1146 moles) 250 mL DCM (10 vol with respect to ester) were taken in a 1L RB flask and cooled to 0 °C. mCPBA (65% purity, 130 g, 0.4535 moles) was added to the reaction mixture at regular intervals for 3 h to maintain the temperature at 0 °C. It was allowed to cool to ambient temperature and stirred overnight. The reaction progress was monitored by observing TLC. The reaction mixture was cooled to 0 °C. After completion of the reaction, the precipitated m-chloro benzoic acid was filtered. A 10% sodium thiosulphate solution was added to the filtrate, along with ethyl acetate and stirred thoroughly. The organic (ethyl acetate) layer was separated and washed twice with distilled water. The organic layer was dried over anhydrous sodium sulphate to remove moisture and concentrated under reduced vacuum to obtain crude NPG epoxide. Silica gel column chromatography was used to purify crude NPG epoxide with 1:1 ethyl acetate and hexane as an elutent to obtain 36 g of pure NPG epoxide. The pure NPG epoxide was assessed by ¹HNMR, ¹³C NMR, IR spectral studies, analyzed for physicochemical and basic lubricant properties.

¹HNMR spectra: 0.98 (s, 6H, -CH₃), 1.3-1.4 (m, 24H, -CH₂-), 1.4-1.5 (m, 4H, , 1.6-1.7 (m, 4H, -CH₂-CH₂C=O), 2.3-2.4 (4H, t, -CH₂C=O), 2.7-2.8 (m, 2H, , 2.9-3.0 (t, 4H, ), 3.9 (s, 4H, -CH₂-O-C=O).

¹³C NMR spectra: 68.56 (O-CH₂), 46.44 (CHOCH₂), 51.86 (CHOCH₂), 172.94 (C=O).

IR spectra: 1727 (str, C=O), 1258 (str, ), 755 (C-C).

Synthesis of 2-ethyl-2-((9-(oxiran-2-yl) nonanoxyl) methyl) propane-1, 3-diyl (9-(oxiran-2-yl) nonanoate) (TMP epoxide)(f). TMP ester (c) (50 g, 0.0738 moles) 250 mL DCM (10 vol with respect to ester) were taken in a 1L RB flask and cooled to 0 °C. mCPBA (65% purity, 135 g, 0.4737 moles) was added to the reaction mixture at regular intervals for 3 h to maintain the temperature at 0 °C. The reaction was allowed to cool to ambient temperature and stirred overnight. The reaction progress was monitored by observing TLC. The reaction mixture was cooled to 0°C after completion of the reaction, and the precipitated m-chloro benzoic acid was filtered. A 10% sodium thiosulphate solution and ethyl acetate were added to the filtrate and stirred thoroughly. The organic (ethyl acetate) layer was separated and was washed twice with distilled water. The organic layer was dried over anhydrous sodium sulphate to remove moisture and concentrated under reduced vacuum to obtain crude TMP epoxide. Silica gel column chromatography was used to purify the crude TMP epoxide with 1:1 ethyl acetate and hexane as an elutent to obtain 35 g of pure TMP epoxide. The pure TMP epoxide was assessed by ¹HNMR, ¹³C NMR, IR spectral studies, and then analyzed for physicochemical and basic lubricant properties.

¹HNMR spectra: 0.9 (t, 3H, -CH₃), 1.3-1.4 (m, 32H,-CH₂-), 1.4-1.5 (m, 6H, ), 1.6-1.7 (m, 6H, -CH₂-CH₂C=O), 2.3-2.4 (t, 6H, -CH₂C=O),2.7-2.8 (m, 3H, ), 2.9-3.0 (t, 6H, ),4.0 (s, 3H, -CH-O-C=O).

¹³C NMR spectra: 68.19 (O-CH₂), 46.49 (CHOCH₂), 51.92 (CHOCH₂), 172.89 (C=O).

IR spectra: 1738 (str, C=O), 1256 (str, ), 751 (C-C).

3. RESULTS AND DISCUSSION

Vegetable oils and synthesized polyol esters possess good lubricity, high viscosity index, and flash point along with lower volatility. In vegetable oils, the presence of a high degree of unsaturation lowers thermal and oxidative stability, which can be improved by the chemical modification of unsaturation such as epoxidation. Epoxidized castor oil possesses enhanced lubricity properties such as high density, viscosity and low pour point (Venubabu and Goud, 2014Venubabu B, Goud VV. 2014. Epoxidation of castor oil fatty acid methyl esters (COFAME) as a lubricant base stock using heterogeneous ion-exchange resin (IR-120) as a catalyst. Energy Procedia 54, 75-84. https://doi.org/10.1016/j.egypro.2014.07.249 ). Keeping this in mind, undecylenic acid-based epoxides of 2-ethyl hexanol, NPG and TMP based esters were developed. Undecylenic acid, a second-generation product of castor oil, was a suitable feedstock to prepare many novel products for various applications. Upon pyrolysis, castor oil gives undecylenic acid. Various esters are prepared from undecylenic acid using different alcohols such as 2-ethyl hexanol, NPG and TMP in the presence of pTSA. The yields of synthezied esters were in the range of 75-80%. A peak corresponding to ester O-CH₂ protons in ¹HNMR was observed at 3.9-4.0 ppm in 2-ethyl hexyl ester, 3.9 ppm in NPG ester, and 3.9 ppm in TMP ester. Multiple peaks corresponding to unsaturation were observed at 4.9-5.0 ppm, 5.7-5.8 ppm in 2-ethyl hexyl ester and TMP ester, 4.9-5.0 ppm and 5.7-5.9 ppm in NPG ester was observed (Figure 1). In the ¹³C NMR spectrum (Figure 2), the carbon signal for unsaturation was observed at the 115.10 ppm CH₂ peak, 139.27 ppm CH peak in 2-ethyl hexyl ester, 114.78 ppm CH₂ peak and 139.09 ppm for the CH peak in the NPG ester, 114.20 ppm CH₂ peak and 139.69 ppm or CH peak for TMP ester. A peak was observed at 173.44 ppm for 2-ethyl hexyl ester, 172.76 ppm for NPG ester, 171.97 ppm for NPG ester-assised r C=O indicated ester linkage. C=O stretching was observed in the t IR spectrum at 1727 cm^-1 for 2-ethyl hexyl ester, 1736 cm^-1 for NPG ester and 1741 cm^-1 for TMP ester. C=C-H stretching at 3020 cm^-1 for 2-ethyl hexyl ester, 3021 cm^-1 NPG ester and 3021 cm^-1 TMP ester were observed in the IR spectra, which confirmed the unsaturation group (Figure 3).

These esters were epoxidized using mCPBA in the presence of DCM. TLC is used to monitor the progress of the reaction. After completion, the reaction mixture was quenched with hypo and extracted with ethyl acetate. The organic layer (Ethyl acetate) was concentrated using a high vacuum to afford crude epoxide, and was purified by column chromatography.

The yields of synthesized epoxides were in the range of 70-75%. The epoxidized 2-ethyl hexanol, NPG and TMP based esters (Scheme-1) were characterized using ¹HNMR (Figure 4) ¹³C NMR (Figure 5) and IR (Figure 6) spectral studies. The presence of epoxide ring protons in ¹HNMR was observed at 2.3-2.4 ppm as multiplet, 2.7-2.8 ppm doublet in 2-ethyl hexyl epoxide, 2.7-2.8 ppm as multiplet, 2.9-3.0 ppm as triplet in NPG epoxide, 2.7-2.8 ppm as multiplet, 2.9-3.0 ppm as triplet in TMP epoxide. The multiplet at 5.0-5.5 ppm due to alkene proton in esters (a-c) disappeared in all epoxides, indicating the conversion of the alkene group to oxirane ring (d-f). In the ¹³C NMR spectrum, two new peaks were observed at 46.43 ppm, 51.85 ppm in 2-ethyl hexyl epoxide, 46.44 ppm, 51.86 ppm in NPG epoxide, 46.46 ppm, 51.92 ppm in TMP epoxide, indicating carbon atoms for oxirane. The carbón signals at 114 ppm and 139 ppm for unsaturation in esters was absent, indicating the conversion of a double bond to oxirane.

The existence of stretching at 1216 cm^-1 in the 2-ethyl hexyl epoxide, 1258 cm^-1 in the NPG epoxide, and 1256 cm^-1 in TMP epoxide observed in the IR spectra confirm the presence of the epoxide group. All synthesized epoxides were analyzed for physicochemical and lubricant properties. All properties were triplicated and data correspond to the mean of three determinations plus or minus standard deviation (Table 1).

Density. Density is a crucial property for all lubricant oils. Density was determined by means of a pycnometer at room temperature. The densities of 2-ethyl hexyl, NPG and TMP ester-based epoxides were 1.0351 ± 0.0004, 1.0446 ± 0.0006 and 1.1606 ± 0.0006 g/cc, respectively. These synthesized lubricants were referred to as high-density lubricant oils because of their higher density compared to water. So water floats on those lubricants. High-density lubricants are better to control contamination, because of aiding in the suspension, transport and particulate contamination removal. These contamination partials are held in suspension longer to be removed by filtration (Fitch, 1992Fitch EC. 1992. Proactive maintenance for mechanical systems, Elsevier Science, ISBN-978-1-85617-166-3. https://doi.org/10.1016/C2009-0-11844-5.).

Kinematic viscosity. The kinematic viscosity of 2-ethylhexyl, NPG and TMP-based esters ranges from 1.89 ± 0.01 to 17.52 ± 0.02 cSt at 40 ^oC, which corresponds to low viscosity grade lubricants, which are useful for industrial applications. 2-ethyl hexanol epoxide, NPG epoxide and TMP epoxide viscosities range from 10 ± 0.025 to 260 ± 0.325 cSt at 40 ^oC. The viscosities increased with increasing molecular weight, acyl and epoxy groups. The kinematic viscosity at 100 ^oC of 2-ethyl hexanol, NPG and TMP ester-based epoxides ranges from 2.98 ± 0.269 to 39.3 ± 0.485 cSt (Figure 7). Among all epoxides, the TMP epoxide shows better viscosity, which is suitable for various industrial applications such as an ISO VG 220 gear oil, metal working fluids and high-temperature applications.

Viscosity index. The viscosity indexes of 2-ethyl hexanol ester, NPG ester and TMP ester range from 47-210, indicating that they are suitable for multi-range applications. 2-ethyl hexanol epoxide, NPG epoxide and TMP epoxide exhibit a viscosity index in the range of 126-204. However, all synthesized ester-based epoxides show viscosity indexes of more than 126, which is a better viscosity index than mineral-based oil.

Flash and fire point. The flash and fire points of lubricants are usually known for higher temperature properties, and are used to determine the volatility and fire-resistance required during storage and transportation. The flash points of 2-ethyl hexanol ester, NPG ester and TMP ester were 138 ± 0.82, 254 ± 0.82, 286 ± 0.82 ^oC. The fire points were 147 ± 0.96, 263 ± 0.96 and 294 ± 0.82 ^oC. The flashpoints of 2-ethyl hexanol ester-based epoxide, NPG ester-based epoxide and TMP ester-based epoxide were in the range of 164 ± 2 to 308 ± 2.1 ^oC. Fire points were 173 ± 2.2 to 314 ± 0.072 ^oC. All synthesized epoxides showed ≥164 ^oC flash and fire points, which indicates that they are within the range of standard lubricants. The flash and fire points were increased with increasing molecular weight, carbón chain length and acyl and epoxy groups. So, the synthesized epoxides were stable even at high temperatures and depicted a low tendency to evaporate (Figure 8).

Cloud and pour points. The cloud and pour point are imperative temperature properties at low temperatures for all lubricants. The cloud points of 2-ethyl hexanol ester, NPG ester and TMP ester were - 10 ± 0.19, -8.3 ± 0.10 and -0.6 ± 0.13 ^oC. The pour points were -38 ± 0.29, -35 ± 0.38, -33 ± 0.28 ^oC, respectively. The mono-branched alkyl ester exhibited a much lower pour point, which is suitable for many industrial applications. 2-ethyl hexanol epoxide, NPG epoxide and TMP epoxide showed cloud points ranging from -3.9 ± 0.1 to 3.0 ± 0.152 ^oC and pour points of -35 ± 4 to -12.5 ± 1.75 ^oC. 2-ethyl hexyl ester-epoxide exhibited a lower pour point, due to its low molecular weight and to being less branched. On the other hand, pour points were increased with increasing molecular weights in NPG and TMP ester-based epoxides. The data reveal that all epoxides showed low pour points, suggesting acceptable lubricants (Figure 9).

Copper strip corrosion. Corrosiveness of 2-ethyl hexanol ester, NPG ester and TMP ester and corresponding epoxides were found to be 1a and are less corrosive, indicating that esters and their epoxides are resistant to moisture, as these esters and epoxides were prepared from UDA, a derivative of castor oil, which has less moisture content and absence of non-toxic chemicals like sulfur.

Microbial activity. The microbial activity was tested by the agar well diffusion technique according to the standard procedure. The 2-ethyl hexanol ester, NPG ester and TMP ester and corresponding epoxides were treated against two gram-positive bacterial strains: bacillus subtilis, staphylococcus aureus and two-gram negitive bacterial stains: pseudomonas aeruginosa, klebsiella pneumoniae. In this method, agar plates were inoculated with different strains of bacteria under restraining conditions and borewells (6 mm diameter) and filled with 150 µg/mL of 2-ethyl hexanol ester, NPG ester and TMP ester and corresponding epoxides in a dimethyl sulphoxide (DMSO) solution and incubated at 37 °C for 24 h. After the incubation period, the diameter of the growth inhibition zones was measured. The inhibition zones were reported in millimeters (mm). Vegetable oil was used as standard. All tests were performed in triplicate.

The results of inhibition zone values for 2-ethyl hexanol ester, NPG ester and TMP ester and corresponding epoxides against bacterial strains such as bacillus subtilis, staphylococcus aureus, pseudomonas aeruginosa, Klebsiella pneumoniae are shown in Figure 10. According to Figure 10, all 2-ethyl hexanol ester, NPG ester and TMP ester and their corresponding epoxides did not show a zone of inhibition in gram-positive and gram-negative bacterial strains. This strongly suggests that the synthesized 2-ethyl hexanol ester, NPG ester and TMP ester and their corresponding epoxides are non-toxic.

The synthesized epoxides showed an excessive viscosity index, flash point and lower volatility than the respective esters (Table 2). Among all the three synthesized epoxides, TMP epoxide showed a higher viscosity index, flash point, and lower pour point compared to 2-ethyl hexyl and NPG epoxides. TMP ester-based epoxide has a higher VI compared to various epoxidized vegetable oils such as castor methyl ester (Venubabu and Goud, 2014Venubabu B, Goud VV. 2014. Epoxidation of castor oil fatty acid methyl esters (COFAME) as a lubricant base stock using heterogeneous ion-exchange resin (IR-120) as a catalyst. Energy Procedia 54, 75-84. https://doi.org/10.1016/j.egypro.2014.07.249 ), rapeseed oil (Wu et al., 2000Wu X, Zhang X, Yang S, Chen H, Wang D. 2000. The study of epoxidized rapeseed oil used as a potential biodegradable lubricant, J. Am. Oil Chem. Soc. 77 (5), 561-563. https://doi.org/10.1007/s11746-000-0089-2 ), Karanja oil (Geethanjali et al., 2013Geethanjali G, Sony MK, Padmaja KV, Karuna MSL, Prasad RBN. 2013. Preparation and properties of lubricant base stocks from epoxidized karanja oil and its alkyl esters. Ind. Eng. Chem. Res. 52 (47), 16598-16605. https://doi.org/10.1021/ie4024325 ) and HP Lube parthan SL220 (Table 3). Epoxides, thus synthesized, can be a potential base-stock for lubricants. The standard lubricants widely used in the market for machinery are various ISO VG grades. The 2-ethyl hexanol epoxide, NPG epoxide and synthesized TMP epoxide are comparable to standard ISO VG grades.

Kinematic viscosity at 40 ^oC for NPG epoxide is 37.34 cSt. It is in the range of ISO VG 32 and 46 grades. Standard ISO VG 32 and 46 grades are widely used machinery lubricating oil and turbine oil. Kinematic viscosity at 40 ^oC for TMP epoxide is 260 cSt, which is in the range of standard ISO VG 220 and 320 grades. The Standard ISO VG 220 and 320 grades are widely used in machinery lubricating oil, bearing oils for steel plants and industrial gear oils.

The synthesized 2-ethyl hexanol epoxide, NPG epoxide and TMP epoxide can replace the existing mineral-based lubricating oils of ISO VG 32-46, ISO VG 220-320 grades in the market for machinery, turbine and industrial gear oil applications. The synthesized 2-ethyl hexanol epoxide, NPG epoxide and TMP epoxide is from a renewable source and is biodegradable without polluting the environment. These are recommended for all industrial machinery and other machine parts lubricated by a thin film of oil for anti-friction bearings, plain bearings, drive gears and the pinions of steel mills, where operating conditions are moderate.

4. CONCLUSIONS

The study involved a new class of epoxides synthesized from undecylenic esters as a bio lubricant base-stock. Undecylenic esters of 2-ethyl hexanol, NPG and TMP were epoxidized using mCPBA. The yields of 2-ethyl hexanol epoxide, NPG epoxide and TMP epoxide were in the range of 70-75%.

TMP epoxide showed higher viscosity, higher viscosity indices, higher cloud point, lower pour point, higher flash point, and higher fire point than 2-ethyl hexyl and NPG epoxide. 2-ethyl hexanol epoxide, NPG epoxide and TMP epoxide have superior lubricant properties compared to their corresponding esters.

2-ethyl hexanol epoxide, NPG epoxide and TMP epoxide with high viscosity indices, higher flash point, and lower pour points can be exploited for automotive and hydraulic fluid formulations. Undecylenic esters and their epoxides can be used for numerous applications due to their non-toxicity and biodegradability.