ABSTRACT
This work focused on the chemical synthesis and characterization of palm kernel oil (PKO) for bio-lubricant production using transesterification of palm kernel methyl ester (PKME) with trimethylolpropane (TMP) and epoxidation-esterification methods. The PKO was extracted using solvent extraction method. The physicochemical characteristics of the PKO and produced bio-lubricant samples were determined using standard methods. Fourier Transform Infrared (FTIR) spectrometry and Gas Chromatographic analyses, were respectively, used to determine the predominant functional groups and fatty acids of PKO and the produced bio-lubricant samples. At 55 °C, 150 min and 0.5 mm particle size, kernel oil yield was 49.82 % (by weight). The viscosities at 40 °C, 100 °C, viscosity index, pour and flash points of the bio-lubricants produced by transesterification of TMP (PKBLT) and epoxidation-esterification (PKBLE) methods, were [42.53 cSt, 10.65 cSt, 139, - 11 °C, 235 °C] and [44.69 cSt, 11.42 cSt, 132, - 12 °C, 240 °C], respectively. Time, mole ratio and temperature effects were the main factors that significantly influenced the transesterification and epoxidation processes. The obtained physicochemical properties of PKBLE and PKBLT samples showed conformity with ISO VG 32 standard, hence, their possible application as bio-lubricant basestock.
Keywords: Bio-lubricant; palm kernel; Transesterification; Trimethylolpropane; Epoxidation-esterification.
Introduction
Globally, the crude oil reserves have significantly drooped, due to its high consumption, as well as the slow natural mineral deposit formation. There is therefore possible impeding energy crisis if not quickly addressed [1]. In other to avert this, scientist and researchers have therefore sought for alternative energy sources using bio-based oil materials. As such, alternative oil sources for chemical process industries, which are renewable, biodegradable, as well as eco-friendly, are currently being researched. Mostly, vegetable oils obtained from seeds and nuts are often used as a suitable substitute to mineral oil obtained from petroleum [2]. There is therefore need for the development/modification of the vegetable oils into industrial products like biodiesel [3], biolubricants [4], transformer fluid [2], as well as other vital fuels that could be used as replacement for conventional mineral base fuels/fluids.
In Nigeria, due to the availability of huge crude oil reserves, as well as the production, exploration, and refining of petroleum into products like, petrol, kerosene, diesel, our interest is on the production of petrochemical products like bio-lubricants. This is because of the environmental friendliness of the bio-lubricants, when compared to its petroleum base counterpart [4].
In the recent past, researchers have successfully synthesized bio-lubricants from different vegetable oil sources for use as environmentally friendly lubricant. Some of these include but not limited to Jatropha curcas oil [4], palm kernel oil [1], palm oil [5], Fluted Pumpkin Seed oil [6], and castor bean biodiesel [7]. It is on this premises that this project seeks to synthesize bio-lubricant from palm kernel oil using different process routes, in other to evaluate the method with better lubricant properties.
Palm kernel oil (PKO) is edible oil that is obtained from the kernel of palm fruit, otherwise scientifically known as Elaeis guinueensis[8-9]. It is yellowish in color and the tree plant (palm tree) originates from tropical Africa and some part of Asia [8]. Palm kernels are essential by-products obtained during the palm oil milling and processing. It is about 45 to 48 % (by weight) of the palm nut of the Elaeis guinueensis fruit. The oil yield of palm kernel is about 47 to 59 % by weight [9-10]. The predominant fatty acid in palm kernel oil is lauric acid, with about 48 % of the fatty acid composition of the oil. It also contains muriatic acid and oleic acid at 16 % and 15 % compositions, respectively [9,11]. Due to the relatively high oil yield of PKO, a number of studies be carried out on its utilization for the production of biodiesel [12], as well as in bio-lubricant production [1].
It is as a result of the foregoing, that the authors extend this present study to the use of PKO (because of its high oil yield, availability, as well as bio-degradability), in bio-lubricant production, using a modified process routes, for the purpose of comparison, since limited or no studies by authors, have been channeled towards this direction.
Previously, a good number of studies have been reported on bio-lubricant production using vegetable oils, for potential use as alternative to mineral base lubricant, using the two stage transesterification and epoxidation-esterification methods. In case of two stage transesterification using methanol (using alkali catalyst) and subsequently trimethylolpropane, have been extensively reported in the literature. For instance, Heikal et al. [13], Menkiti et al. [4], Yunus et al. [14], da Silva et al. [7] and Shote et al. [15], successfully used this approach for the production of bio-lubricants using palm oil, jatropha curcas oil, palm-based methyl ester, castor bean biodiesel, and palm kernel oil, respectively. In this method, the main drawback of straight alkali catalyzed transesterification stage, without initial esterification pretreatment stage, is its low alkyl ester yield, as well as product separation difficulty, due to formation of soap [2]. As such, in this work, successive two-step conversion process: an acid-catalyzed esterification (aimed at lowering the FFA content), followed by the alkali-catalyzed transesterification (aimed at improving the alky ester yield and purity), prior to the final stage of transesterification with trimethylolpropane, was chosen. The essence is to eliminate these short-coming and ensure higher high grade bio-lubricant with high purity level and yield [4].
For the epoxidation-esterification procedure, the method is often considered because of its advantage of better thermo-oxidative stability, as well as the associated low temperature properties of the final product [2]. Epoxidation reaction is the reaction of the double bonds and peracetic acid to produce epoxy ring [2,16]. In this work, it is worth mentioning that instead of using the PKO, the obtained alkyl ester form the alkali catalyzed transesterification, was reacted with the peracetic acid to get the desired epoxy methyl ester. This decision is based on the improved thermo-oxidative stability and lower pour point of the final desired product [2,16]. Within the authors’ knowledge disposal, there is no existing published work that compares bio-lubricants produced from PKO using these two approaches. Though, several works have reported the production of bio-lubricant from PKO using transesterification with trimethylolpropane [1,14,17].
Therefore, this study focuses on bio-lubricant production from PKO, using alkali-catalyzed transesterification, followed by transesterification with trimethylolpropane and transesterification-epoxidation-esterification methods. The qualities of the produced bio-lubricants were evaluated on the basis of the two methods used, and on the referenced standards. Furthermore, the physicochemical properties of the PKO and the PKO bio-lubricants samples produced were evaluated using standard procedures. Also, the prevalent functional groups, and the fatty acid compositions of the PKO and the PKO bio-lubricant samples, were determined using Fourier Transform Infrared (FTIR), and Gas Chromatography, respectively.
  1. Materials and methods
  2. Materials
Palm kernels were obtained from Aguneze, Ahiazu-Mbaise, in Imo State. Trimethylolpropane was purchased from Sigma Aldrich, Germany. Methanol, sulphuric acid (H2SO4), orto-phosphoric (H3PO4) and analytical grade n-hexane were purchased from Conraws, Presidental Road, Enugu. The reagents had purity levels of above 99 %, and they were used without further purification.
Extraction of PKO
Oil extraction from the milled palm kernel oil sample was carried out according to Association of Official Analytical Chemists (AOAC) 963.15 method [18], using soxhlet extractor unit for the five different average particle sizes (0.5, 1.0, 1.5, 2.0, and 2.5 mm). Soxhlet extractor was chosen in order to enhance the solubility of the palm kernel sample in the chosen solvents used. Milled kernels of a particular average particle size (15 g) were packed in a thimble of the soxhlet extractor and the extractor was filled with 150 ml of n-hexane. Oil extraction was performed at temperatures of 35, 40, 45, 50, and 55 °C using n-hexane. For each temperature, extraction was performed for five average particle size (0.5, 1.0, 1.5, 2.0 and 2.5 mm). At a specific temperature and particle size, extraction was carried out for 30, 60, 90, 120, and 150 min. The oil yield obtained at the end of every extraction time for every extraction condition was calculated and recorded. The extraction temperature was measured using an electronic thermometer (Hanna HI-9063), while the time was measured using a stop watch. The oil yield was calculated (see equation 3.1) using AOAC method no. 920.85 at the end of each extraction cycle (30, 60, 90, 120, and 150 min). After each extraction batch/cycle, the solvent was removed at 60 °C using rotary evaporator (model N- 1000S-W, EYELA, Tokyo, Japan). The solute to solvent ratio used for the entire extraction was 1:5 (15 g: 150 ml). Similarly, the extraction times were 30, 60, 90, 120 and 150 min. The entire extraction process carried out under each set of conditions was performed three times and the average values reported, while the total extraction yield was obtained using AOAC 920.85 standard method.
The oil yield of sample was calculated using equation 3.1.
( 1)
The extracted oil sample was palm kernel oil (PKO).
Physicochemical properties of PKO and PKO bio-lubricant samples
The oil yield (AOAC 920.85) was determined according to AOAC approved techniques [18]. On the other hand, viscosity index (ASTM D2270), viscosity (ASTM D445) and specific gravity (ASTM D1217 – 15), were determined using ASTM standard methods. Also, the pour and flash points were determined using ASTM D97 and ASTM D93 standard methods, respectively. Each physicochemical property was measured three times, and the average values of the properties were determined and noted.
Fatty acid composition of PKO and PKO bio-lubricants samples
Fatty acid profile was evaluated in line with the AOAC 996.06 (1990). In this procedure, a gas chromatograph (GC) (Shimadzu GC–14B, Model 910), was used to determine quantitatively, the prevalent fatty acids in the PKO and PKBL samples. According to the equipment, a HP 88 capillary column (0.25 mm i.d. × 100 m, film thickness 0.25 µm – Shimadzu Corporation, Tokyo, Japan), was used to equip the GC’s flame ionization detector and integrator. This was achieved by using 250 °C temperatures for both the injector and detector. However, the oven temperature was retained at 190 °C for a period of 15 min. Thereafter, this temperature was then increased intermittently, up to 230 °C, at the rate of 5 °C per min. Afterwards it was maintained at this temperature for the same time interval as the initial step. The carrier gas used was nitrogen, and was maintained at a pressure of 500 kPa. Finally, the prevalent fatty acids were identified and compared with standard compounds; while the quantity of each fatty acid was calculated from the percentage area of the individual fatty acid [19]. The analysis was carried out three times.
  1. Transesterification experiment
  2. Synthesis of Palm kernel methyl ester
25 ml of each of the palm kernel oil (Triglyceride) sample was poured into 250 ml conical flasks and heated to 60 °C using a water bath. Solution batches of potassium methoxide were prepared by dissolving 5.1 g (30 wt. %) of KOH pellets in an agitated 250 ml beaker containing 150 ml anhydrous methanol. The potassium methoxide solution was then transferred into warm 25 ml palm kernel oil (triglyceride) sample at a methanol to oil ratio of 6:1. The solution was then stirred strongly, using magnetic stirrer at 500 revolutions per minute (RPM) for 120 min. In other to ensure proper settling, the mixture was left undisturbed in a separating funnel for 24 hrs. At ending of the settling process, the upper layer (methyl ester sample) was poured into a beaker and afterwards properly washed with distilled water. This was aimed at removing unreacted methanol, catalyst, glycerin, soap and other impurities. The demoisturization of the fatty acid methyl ester (FAME) sample was then carried out by heating slowly to constant temperature of 100 °C. Finally, the lower layer that consists of glycerol and soap was collected via the bottom of the funnel.
The percentage methyl ester yield of the palm kernel oil sample was calculated using the relationship in equation 2:
(2)
The product of the transesterification of the extracted palm kernel oil is the palm kernel methyl ester (PKME), or generally referred to as fatty acid methyl ester (FAME); see equation 3.
Triglyceride Methanol Glycerol Methyl Esters
(Extracted oil) (FAME) (3)
On the other hand, in the present method, FAME of PKO was also obtained using this procedure. Here, a mixture comprising of 300 g of palm kernel oil (Triglyceride), 100 g of methanol, as well as 1% wt/wt orthophosphoric acid catalyst were decanted into continuously stirred reactor. This reactor was that was equipped with a water-cooled reflux condenser to ensure complete reaction. Thereafter, the mixture was then heated to 65 °C and maintained at this temperature for 1hr 30min. After the reaction, the mixture was dosed with 0.2 molar solution of sodium trioxocarbonate IV. This was to ensure complete neutralization of the acid, and eventually stop the reaction. The mixture was then poured into a separating funnel and subsequently allowed to stand for 24 hrs. This was to ensure complete separation of methyl esters and glycerol phases. Glycerol phase at the bottom was emptied into a clean container and allowed to stand. The PKME (FAME) was then heated to 65 °C, so as to ensure the removal of the residual methanol. Finally, the remaining catalyst in the PKME was removed by successively rinsing with hot distilled water at 80 °C. Thereafter, the remaining water in the PKME was eliminated by oven-heating at 100 °C [4].
Synthesis of Bio-lubricant from PKME using trimethylolpropane (TMP)
The synthesis of biolubricant adapted was as described by Surapoj et al. [20] with slight modifications. The obtained palm kernel methyl ester (PKME) or generally Fatty acid methyl ester (FAME) was synthesized with trimethylolpropane (TMP) to obtain trimethylolpropane ester or palm kernel biolubricat (PKBLT) as shown in equation 4.
(4)
Here, trimethylolpropane (TMP) was initially heated using a transesterification experimental set-up. This set-up comprises of 50 mL three necked round-bottom flask that are fitted to a water-cooled reflux condenser, a thermometer, Kipp’s apparatus and a stirrer operated at 1000 rpm. At 1000 rpm, and under the flow of CO2, the TMP in the flask was heated to 110 °C, and maintained at this temperature for 15 min, before allowing cooling. In other to ensure the evolution of moisture from the TMP, the temperature was maintained at 110 °C. Thereafter, a Ca(OH)2 catalyzed batch transesterification reactions between PKME (FAME) and already cooled TMP were conducted at PKME–TMP ratios: 3:1, 4:1, 5:1, 6:1 and 7:1, using the same experimental set-up. At 80, 100, 120, 140 and 160 °C, each of the stated PKME–TMP ratios was subjected to transesterification. At intervals of 1, 2, 3, 4, and 5 h, samples from the respective individual runs of the experiment (at a particular mole ratio and temperature) were monitored, collected and analyzed. After every reaction, the mixture was allowed to cool to room temperature, prior to carrying out filtration process, so as to separate the residual solid catalyst from the liquid mixture, which is the palm kernel biolubricat (PKBLT). The filtered palm kernel bio-based stock was analyzed using the GC to determine the product composition. Furthermore, prior to the characterization of the biobased TMP ester, unreacted methyl ester was not expunged. This was aimed at the improvement of the wear resistance of the biobased TMP ester, as well as to prevent conjugation reaction that occurs at elevated temperatures (180 – 200 °C), which involves poly unsaturated fatty acid (PUFA) [14,21].
Synthesis of Biolubricant from PKME using epoxidation-esterification reaction
Fifty (50) g of the palm kernel methyl ester sample was placed in three-necked flasks. 7 g of acetic acid was placed in the round bottom flask, followed by the addition of 0.02 g (1.5 wt. %) of H2SO4 to the flasks. The mixtures were stirred continuously, followed by the addition of 15 g (1.5 moles) of hydrogen peroxide (15 wt. %). The mixture was added to the flask containing 7 g acetic acid to produce peracetic acid (PAA). The methyl ester sample in the three-necked flask was then heated to 70 ºC, followed by the addition of PAA mixtures. These were allowed to react at 70 °C for 7 hours with continuous stirring at 1,200 rpm. Samples were taken out every 1 hour from each of the reaction set up for FTIR analysis to determine the effect of reaction time on the yields (i.e changes in functional groups as the reaction proceeds) at 70 °C. At the end of the reaction, 3 ml of the epoxy methyl ester sample was then removed two times using diethyl ether (2 x 20 ml) in separating funnels. The organic phases (oily layer) of samples were purified (washed) three times with 5% saturated sodium bicarbonate NaHCO3 (3 x 15 ml) to neutralize the unreacted acid present. Thereafter, organic phase of sample was also washed three times with saturated solutions of sodium chloride NaCl (3 x 15 ml) to obtain epoxy methyl ester of the sample. The sample was dried over anhydrous magnesium sulphate. The solvent (diethyl ether) was then removed using a rotary evaporator. The epoxy oxygen (oxirane) content and iodine value (IV) of the sample was measured according to procedure described by Arumugam et al. [22] and Arumugam and Sriram [23].
The obtained epoxy methyl ester sample was epoxy methyl ester (oxirane) of palm kernel oil (EMPKOo). Fig. 1 shows the reaction scheme for the epoxidation reaction of PKME to obtain EMPKOo.
Fig. 1: Reaction Scheme for Epoxidation Reaction of PKME to obtain EMPKOo [24].