Simple Method for the Conversion of Light Cracked Naphtha into Efficient Lubricity Improvers for Ultra-Low Sulfur Diesel.

Light cracked naphtha (LCN) is one of the olefin streams obtained from oil refinery in the petrochemical fluidized catalytic cracking unit. In this communication, we report a new method for the conversion of LCN into lubricity improvers for ultra-low sulfur diesel (ULSD) through a feasible two-step synthetic procedure. In the first step, olefins of LCN were subjected to the hydroboration reaction using BH3 to get the hydroxy LCN derivative which was then subjected to the esterification reaction with different organic acids to get the final LCN esters (6a-j). The lubricating property of the LCN esters was studied at two blending concentrations (300 and 150 ppm, wt/vol) with ULSD. Interestingly, ester (6a) derived from stearic acid showed the tiniest wear scar diameter in both dosage levels. The mechanism of lubricity action of LCN esters on metallic surfaces was studied by analyzing the worn surfaces using scanning electron microscopy and energy-dispersive X-ray spectroscopy techniques. The studies reveal that the lubricity additives derived from cracked naphtha through a simple chemical reaction strategy are promising precursors in enhancing the lubricity of ULSD.

Introduction

The global fossil fuel demand is escalating day by day. As per the latest reports, the utilization of energy derived from fossil fuel is comparatively higher than energy derived from other sources.[1−3] Accordingly, air pollution occurring from combustion of fossil fuel has been rising. It contributes to the formation of smog and acid rain, which drives the gradual changes in the climate.[4] There are several efforts being made to reduce the emission of polluting gases particularly from the automobiles. Therefore, it was thought of adopting stringent fuel standards for diesel in order to reduce allowed sulfur and aromatics content in the fuel. Accordingly, the national regulators in the countries such as the US, Europe, China, and India took active steps to reduce diesel emissions.[5−8] As per the European fuel standard (Euro 1, introduced in the year 1992), the maximum allowed sulfur content in the diesel (MASCD) fuel was 0.2%. Thereafter, due to increased environmental awareness, the regulations have been made stricter to reduce the sulfur content in the fuel. Accordingly, Euro 2 fuel standard (in 1996, the MASCD fuel was 500 ppm), Euro 3 fuel standard (in 2000, the MASCD fuel was 350 ppm), Euro 4 fuel standard (in 2005, the MASCD fuel was 50 ppm), and Euro 5 fuel standard (in 2009, the MASCD fuel was 10 ppm) were introduced. As per the current European standard (Euro 6, since 2014 to present), the MASCD fuel is 10 ppm.[9] Such a diesel fuel with very low sulfur content, that is, less than 10 ppm, is termed as ultra-low sulfur diesel (ULSD). ULSD is produced by a catalytic chemical technique called hydrodesulfurization process, which removes mainly the sulfur of the fuel along with nitrogen and oxygen-based polar trace compounds.[10−12] However, the direct usage of ULSD in motor vehicles causes friction between the metal surfaces of fuel injection system which leads to the engine damage.[13,14] In the absence of lubricity imparting components such as sulfur and other polar components, the diesel fuel must get its lubricity property by an alternative source. The lubricity of ULSD can be improved by adding lubricity additives.[13] However, there is a challenge to prepare cost effective lubricity improvers. According to the literature, ester and acid groups are favorable to enhance the fuel lubricity rather than ether and aldehyde groups in the additives.[10] However, direct use of an acid-based additive is not recommended due to its corrosive nature toward metal surfaces. Gerhard et al.[15] reported that polar heteroatoms in the additive increase the lubricity of ULSD. Baek et al. synthesized succinic acid alkyl half-esters by a one-step reaction of succinic anhydride with various fatty alcohols and these esters enhanced the lubricity of ULSD.[16] Recently, Liu et al. synthesized tung oil-based fatty acid esters through chemical modification of the eleostearic acid methyl ester derived from the oil, and the additives imparted excellent lubricity property to ULSD at low dosage levels (500–1000 ppm).[17] Kenar et al.[18] found that the lubricity of ULSD increased with an increase in the number of carbon atoms in the alkyl chain of a neat carbonate blended with it. Based on these reports, it has been planned to synthesize a series of esters derived from light cracked naphtha (LCN). LCN is one of the olefin streams obtained from the petrochemical fluidized catalytic cracking (PFCC) unit of the oil refinery. The olefin content in the LCN is in the range of 40–45 vol % and the detailed composition analysis of the LCN employed in the present study is represented in Figure . The main objective of the present study was to convert the olefins into a mixture of esters through hydroboration reaction, followed by the esterification of the LCN alcohol with different carboxylic acids. The mixture of LCN esters comprising polar functional groups is expected to improve the lubricating property of ULSD when dosed with it in small quantities. Accordingly, we have synthesized LCN-based esters (6a–j) and evaluated their lubricity properties. To the best of our knowledge, this is the first report on LCN-based lubricity improvers for ULSD. The synthetic procedure followed in the present study could be a feasible protocol for the oil refineries to develop ULSD additives in a commercial scale by employing the refinery raw material LCN.

Figure 1

Graphical representation of (a) LCN composition and (b) composition of olefin in LCN.

Experimental Part

Materials

1-Hexene, borane tetrahydrofuran complex solution (1.0 M in THF), organic/fatty acids, solvents, and all the other chemicals were purchased from Sigma-Aldrich. LCN was collected from the PFCC unit and ULSD was collected from the hydrocracker unit of Mangalore refinery and petrochemical limited (MRPL), Mangalore.

Instruments

The component analysis of the LCN was carried out using a PAC Reformulyzer-M4 analyzer. The gas chromatogram and the mass spectra of the compounds were recorded using a Leco, Pegasus GCXGC time of flight mass spectrometer with liquid-N2 modulation. The evaluation of lubricity property of the fuel was carried out using a high-frequency reciprocating rig (HFRR, PCS instruments, USA). A Bruker (Alpha) KBr/ATR Fourier transform infrared (FTIR) spectrophotometer was used to confirm the presence of different functional groups in the compounds. The worn surface of the ball was analyzed using a JSM-7610F plus field emission scanning electron microscope (Japan electron optics laboratory Co. Ltd). The composition and chemical states of worn surface of the balls were analyzed using a X maxN (80) energy-dispersive spectrometer (Oxford EDS, Aztec 4.0 UK).

Synthesis of LCN Esters (6a–j)

In this synthetic route, the LCN from the PFCC unit was subjected to the hydroboration reaction to get the hydroxy LCN derivative which was then esterified with different acids to get the mixture of esters. Initially, the overall process was optimized using a neat olefin where in 1-hexene was subjected to hydroboration reaction, followed by the esterification of the 1-hexanol with different organic acids. The detailed synthetic procedure for the process is given below.

Synthesis of Esters from 1-Hexene

The conversion of 1-hexene into corresponding esters is depicted in Scheme .

Scheme 1

Synthetic Route for 1-Hexene-Based Esters (3a–c)

Hydroboration of 1-Hexene

A mixture of 1-hexene (8.4 g, 0.10 mol) and THF (20 mL) was taken in a three-necked (500 mL) RB flask equipped with a dropping funnel, condenser, and N2 purging wall. The mixture was cooled for 5 min at 0–5 °C with continuous stirring. To the reaction mixture, BH3/THF (40 mL, 0.42 mol) was added drop wise (1 drop/sec) while maintaining the temperature at 0–5 °C for 1 h. After the completion of addition, the reaction mixture was stirred for 1 h at room temperature. It was then quenched with 1 N of NaOH (20 mL) and 50% of H2O2 (20 mL, 0.85 mol) mixture at 0–5 °C with continuous stirring. The mixture was washed with methanol (15 mL) and the product was extracted using ethyl acetate and washed with excess of water. The organic phase was dried over sodium sulfate and the solvent was removed under reduced pressure to yield the final product 1-hexanol as white thick liquid (2) (yield 90–95%).

Esterification of 1-Hexanol

Synthesis of ester (3a) from octanoic acid.

To a mixture of 1-hexanol (2 g, 0.02 mol), p-toluene sulfonic acid (PTSA) (0.1 g, 0.0006 mol), and toluene (40 mL), octanoic acid (2.8 g, 0.02 mol) was added in a 100 mL RB flask. The flask was connected to a dean-stark apparatus and kept in a heating mantle with a magnetic stirrer and temperature controller. The reaction mixture was refluxed at 110 °C with constant stirring. The water liberated during the reaction was removed using the dean-stark apparatus. The progress of the reaction was monitored using thin-layer chromatography. After completion of the reaction, the whole reaction mixture was cooled to room temperature. The product was extracted using ethyl acetate and the organic phase was washed with excess of water followed by 5% sodium bicarbonate (200 mL) solution. The organic layer was filtered through sodium sulfate and the solvent was evaporated using rotary evaporator to get the product (3a) as dark yellow liquid (yield 88.4%).

Similarly, esters 3b (yield 86%) and 3c (yield 89%) were synthesized using the reactants hexanoic acid and butyric acid, respectively.

Synthesis of Esters from LCN

The conversion of LCN into mixture of esters is schematically depicted in Scheme , considering a general structure (4) for the olefins present in the LCN. The hydroboration of LCN (200 g batch) was carried out following the same procedure described above for 1-hexene. The hydroxy LCN was subjected to the esterification reaction with long chain (C4–C18 alkyl groups) organic acids following the procedure described above for the esterification of hexanol to get the final esters (6a–j). The chemical structures of all the esters are given in Table .

Scheme 2

Synthetic Route for Mixtures of Esters (6a–j) from LCN through the Hydroboration Reaction

Table 1

Chemical Structures of LCN Esters (6a–j)a

where, R, R1 = H, C1, and C2 alkyl or aryl groups.

Tribological Tests

Evaluation Method of HFRR

The lubricating properties of newly synthesized compounds on metal surfaces was studied using a HFRR (Figure ) on the basis of the ASTM D 6079-18 standard method (a vibrator with 50 Hz frequency, 1 mm stroke length, 200 g weight load, 2 mL sample, temperature 60 °C, and test duration of 75 min). The wear scar diameter (WSD) on the surface of the ball was measured using the equation WSD = (X + Y)/2 μm, where X and Y are the lengths (as μm) of the scar formed on the ball’s surface in X and Y directions, respectively.[19−22] The maximum allowed limit for the lubricity value of BS VI ULSD is 460 μm.[5] Therefore, the WSD less than or equal to 460 μm at 60 °C was considered as acceptable for ester-ULSD blends. The size of the wear scar is directly related to the lubrication property of the sample. The smaller WSD value represents better lubricating property.

Figure 2

Schematic image of HFRR.

Sample Preparation for HFRR Analysis

The samples (6a–j/3a–c) (0.003 g) were weighed into a 10 mL standard flask. 5 mL of ULSD was added to it and the mixture was sonicated for 3 min. Then, the solution was made up to the mark with ULSD and the solution was shaken well for uniform concentration. This stock sample with blend concentration of 300 ppm was used for HFRR analysis. The same procedure was used to prepare a sample with a blend concentration of 150 ppm by weighing 0.0015 g of the ester into a 10 mL standard flask. After the completion of analysis, the ball was removed from HFRR and stains were washed using acetone. Then, WSD was measured using an optical microscope with a magnification of 100.

Surface Analysis of Wear Scar

The mechanism of lubricity action on metal surface was studied by analyzing the worn surface of the ball using a scanning electron microscope operated at an accelerating voltage of 20 kV. The micrographs of worn surface of the balls were obtained at 100× magnifications. Energy-dispersive X-ray spectroscopy (EDS) was used to estimate the composition of chemical states of the worn surface of the ball.

Test Methods of Physical and Chemical Parameters of Diesel

The synthesized LCN ester should not affect other properties of diesel fuel such as oxidation stability, viscosity, and so forth. Hence, physical and chemical parameters of the ULSD-ester blend were determined to confirm their specification limit according to Indian standards.[5]

Results and Discussion

The prepared n-hexane esters were purified through column chromatography using ethyl acetate: hexane (40:60) as the mobile phase and silica 100–200 mesh as the stationary phase. The purified samples were characterized through 2D-GCMS and IR spectroscopic techniques to confirm their chemical structure.

Structural Characterization of Hexanol and Corresponding Esters (3a–c)

The single sharp peak observed in the two-dimensional gas chromatogram (Figure a) of hexanol (HL) confirms the purity of the sample. Also, there is no unwanted signal in the density distribution spectrum which further confirms its purity. The IR spectrum of HL displayed a broad OH band at 3370 cm–1. The one-dimensional chromatogram (Supporting Information, Figure S3), two-dimensional chromatogram (Figure b–d), mass spectra, density distribution spectra, and IR spectra (Supporting Information, Figures S4–S8) confirm the formation of the esters (3a–c). The purity of the samples was established by the single sharp peak detected in both one and two-dimensional gas chromatogram. The mass spectra of the esters (Supporting Information, Figure S4) displayed characteristic peak corresponding to [M + Na]+ at (m/z) 228.15 for 3a, 200.18 for 3b, and 172.13 for 3c. With an increase in the molecular weight of the product from 3c to 3a, the corresponding density distribution spectra (Supporting Information, Figure S5) shifts from lower to higher period (from 1500 to 2500 s) which further confirms the efficient conversion of HL into corresponding esters (3a–c). The FTIR spectra of samples (3a–c) displayed a strong band in the range of 1733–1737 cm–1 corresponding to −C=O stretching of the ester group along with other characteristic peaks. The broad OH band and band corresponding to C=O of the carboxylic acid group were absent in the spectra which further confirms the completion of the reaction and the purity of the final products.

Figure 3

Two-dimensional chromatogram of (a) HL, (b) 3a, (c) 3b, and (d) 3c (t1 is the first dimensional retention, and t2 is the second dimensional retention).

The FTIR spectral data of HL and (3a–c) are given below.

HL: IR (KBr, cm–1): 3370 (OH), 2952, 2871, 1720, 1457, 1370, 1053 and 670. 3a: IR (KBr, cm–1): 2926, 2881, 1737 (C=O from ester), 1461 and 1170. 3b: IR (KBr, cm–1): 2931, 2865, 1737 (C=O from ester), 1461 and 1172. 3c: IR (KBr, cm–1): 2931, 2885, 1733 (C=O from ester), 1357 and 1178.

Structural Characterization of Hydroxy LCN (HLCN) and LCN Esters (ELCN)

The 1D and 2D gas chromatograms (Figure a and Supporting Information, Figure S9) as well as the 2D density distribution spectrum (Supporting Information, Figure S10) of HLCN displayed multiple peaks, indicating the presence of a mixture of LCN-based hydroxy compounds. The compounds are identified tentatively based on the GC traces and the details are given in Table . The 2D gas chromatogram of HLCN displayed peaks in the range of 415–1330 s, whereas for the LCN esters, peaks were observed in the range of 2200–3700 s (Figure b), which confirms the successful conversion of HLCN into ELCN. The FTIR spectrum of HLCN displayed a broad OH band at 3348 cm–1, whereas in the case of ELCN (6e), a strong band appeared at 1734 cm–1 corresponding to −C=O stretching of the ester group (Supporting Information, Figures S11–S13). The mass spectrum of major component of 6e confirms the presence of the [M + Na]+ peak at m/z 257.26 (Figure ).

Figure 4

Two-dimensional chromatogram of (a) HLCN and (b) ELCN (6e) (t1 is the first dimensional retention, and t2 is the second dimensional retention).

Table 2

Composition of HLCN Identified through 2D-GCMS

alcohol	formula	R.T.
3-methyl-2-pentanol	C6H14O	415.007 s, 1.643 s
2-methyl-1-pentanol	C6H14O	505.014 s, 1.858 s
2-methylcyclopentanol	C6H12O	525.016 s, 2.374 s
1-pentanol	C5H12O	375.004 s, 1.631 s
cyclopentanol	C5H10O	405.006 s, 2.180 s
2-methyl-3-pentanol	C6H14O	380.004 s, 1.478 s
3-hexanol	C6H14O	425.008 s, 1.635 s
2-hexanol	C6H14O	435.008 s, 1.666 s
2,3-dimethyl-1-butanol	C6H14O	490.013 s, 1.902 s
3-methylcyclohexanol	C7H14O	645.025 s, 2.351 s
2-methyl-1-hexanol	C7H16O	755.034 s, 2.124 s
1-hexanol	C6H14O	590.021 s, 2.092 s
4-methyl-2-pentanol	C6H14O	360.002 s, 1.383 s
3-methyl-1-pentanol	C6H14O	535.016 s, 1.981 s
2-ethyl-1-butanol	C6H14O	525.016 s, 1.953 s
1,3-dimethylcyclopentanol	C7H14O	610.022 s, 2.470 s
(Z)-3-hepten-2-ol	C7H14O	620.023 s, 2.329 s
cis-2-methylcyclohexanol	C7H14O	760.034 s, 2.920 s
4-heptanol	C7H16O	580.02 s, 1.833 s
3-methyl-2-hexanol	C7H16O	625.024 s, 2.001 s
cyclohexanol	C6H12O	625.024 s, 2.962 s
cycloheptanol	C7H14O	775.036 s, 2.781 s
4-methyl-3-hexanol	C7H16O	610.022 s, 1.949 s
cyclopentanemethanol	C6H12O	675.028 s, 2.910 s
cis-4-methylcyclohexanol	C7H14O	670.027 s, 2.454 s
trans-4-methylcyclohexanol	C7H14O	655.026 s, 2.415 s
5-methyl-3-hexanol	C7H16O	655.026 s, 2.000 s
2-pentanol	C5H12O	670.027 s, 2.013 s
4-heptanol	C7H16O	640.025 s, 1.925 s
2,3-dimethyl-1-pentanol	C7H16O	650.026 s, 1.986 s
1-cyclopropylpentan-1-ol	C8H16O	980.052 s, 2.615 s
4-methyl-3-hexanol	C7H16O	600.022 s, 1.914 s
5-methyl-3-hexanol	C7H16O	550.018 s, 1.787 s
3-(2,2-dimethylpropoxy)-butan-2-ol	C9H20O2	830.04 s, 1.299 s
(Z)-3-penten-1-ol	C5H10O	360.002 s, 1.795 s
4-butoxy-1-butanol	C8H18O2	1330.08 s, 3.089 s
trans-3-methylcyclohexanol	C7H14O	780.036 s, 2.954 s
3-methyl-1-hexanol	C7H16O	765.035 s, 2.215 s
cyclohexanemethanol	C7H14O	830.04 s, 2.871 s
3-ethyl-2-pentanol	C7H16O	620.023 s, 2.116 s
3-methyl-3-pentanol	C6H14O	350.002 s, 1.593 s
4-methylcyclohexanol	C7H14O	800.038 s, 2.834 s
cis-2-ethyl-2-hexen-1-ol	C8H16O	930.048 s, 2.659 s
3-octanol	C8H18O	845.041 s, 2.051 s
2,6-dimethylcyclohexanol	C8H16O	885.044 s, 2.649 s
4-ethylcyclohexanol	C8H16O	905.046 s, 2.622 s
11-methyldodecanol	C13H28O	1445.09 s, 1.811 s
2,4-dimethylcyclohexanol	C8H16O	800.038 s, 2.389 s

Figure 5

Mass spectra of the major peak identified in ELCN (6e). Inset shows the enlarged spectrum in the region m/z 190–260.

The FTIR spectral data of HLCN and ELCN (6e) are given below.

HLCN: IR (KBr, cm–1): 3349 (OH), 2948, 2876, 1455, 1386, 1047 and 669. ELCN (6e): IR (KBr, cm–1): 2926, 2860, 1734 (C=O from ester), 1458, 1372, 1174, 1109 and 725.

Measurement of Lubricity

The lubricity of (3a–c) and (6a–j) samples was analyzed through the HFRR method at 60 °C. Addition of 3a and 6a–f (at both dosage levels, 150 and 300 ppm), 3b and 6g (at a dosage level of 300 ppm) lowered the WSD value of the ULSD signifying their lubricity characteristic and the WSD values for the blends are in the range 459–391 μm which is below the accepted value of 460 μm.[5] The WSD values for neat ULSD, ULSD-(3a–c), and ULSD-(6a–j) blends are given in Table . The optical microscopic images of the wear scars on the balls employed in lubricity measurement of neat ULSD and ULSD-ester blends (3a, 6a–b, 6g, and 6j) are shown in Figure and Supporting Information (Figure S14).

Table 3

Lubricity Data of Neat ULSD and Blended ULSD (WSD Values Lower Than 460 μm Are Represented in Bold)

HFRR 60 °C
		WSD (μm)				WSD (μm)
sample ID	concn. ppm	ball X	ball Y	avg. (WSD (X + Y)/2)	concn. ppm	ball X	ball Y	WSD, (X + Y)/2
neat, ULSD	NA	534; 542	480; 476	507; 509	NA	NA	NA	NA
3a	150	468; 462	402; 410	435; 436	300	447; 440	407; 418	427; 429
3b		468; 490	462; 454	465; 472		450; 455	428; 415	439; 435
3c		499; 485	441; 471	470; 478		520; 507	410; 419	465; 463
6a		485; 490	355; 354	420; 422		439; 430	343; 360	391; 395
6b		472; 468	381; 382	427; 425		442; 447	358; 357	400; 402
6c		435; 443	423; 427	429; 435		450; 449	368; 365	409; 407
6d		452; 458	448; 440	450; 449		460; 457	400; 395	430; 426
6e		460; 453	444; 447	452; 450		480; 475	390; 391	435; 433
6f		462; 464	456; 456	459; 460		500; 490	374; 390	437; 440
6g		475; 483	469; 475	472; 479		472; 480	444; 432	458; 456
6h		500; 490	480; 474	490; 482		501; 505	451; 431	476; 468
6i		501; 525	499; 471	500; 498		544; 541	440; 427	492; 484
6j		530; 533	474; 483	502; 508		508; 503	490; 473	499; 488

Figure 6

Optical microscopy images of wear and scar for (a) neat ULSD and ULSD blends (b) 3a—I, (c) 3a—II, (d) 6a—I, (e) 6a—II, (f) 6b—I, (g) 6b—II, (h) 6g—I, (i) 6g—II, (j) 6j—I, and (k) 6j—II. (I) and (II) represent dosage levels of 300 and 150 ppm, respectively.

From the above data it is clear that, the lubricity of ULSD increases by the addition of lubricity improver (6a–j) to ULSD. Comparing 3a to 3c, the least WSD value obtained for 3a is due to the longer carbon chain length in 3a than in 3c. The least WSD value obtained for 6a in both the blended concentrations indicates that the long nonpolar carbon chain influences significantly to enhance the fuel lubricity as compared to the shorter nonpolar carbon chain on the additive[18] (Table ). As the carbon chain length is shortened the lubricity property decreases gradually from 6a to 6f. For efficient lubricity enhancers, the WSD of worn surfaces of the ball becomes shorter with minimum scratch marks on the surface (Figure ). The friction coefficient and film % graphs obtained from HFRR for neat ULSD and ULSD- (6a–b) blends are given in Figure , whereas corresponding graphs for the blends of 3a, 6g, and 6j are given in the Supporting Information (Figure S15). With the increase in the carbon chain length of the additive, the thickness of the protective film increases. The protective film minimizes the direct metal to metal contact and hence friction on the metallic surfaces decreases which improves the lubricity of the fuel.[16] Interestingly, most of the LCN esters imparted good lubricity with ULSD, except (6h–j), with minimum wear and scar on the surfaces. Hence, the LCN esters could be considered as efficient lubricity enhancers for ULSD. Film (%) is the measurement of thin film formed (%) in between the ball and disc containing fuel/blended fuel.

Figure 7

Friction coefficient and film % graph obtained from HFRR for neat ULSD and (6a–b)-ULSD blends (300 ppm).

Effect of New Lubricity Improvers on Fuel Properties of the Diesel

A study of different key parameters of the ester-ULSD blend fuels[23−33] was carried out to verify the influence of these additives on diesel fuel properties, and the results of the studies are presented in Table . The additive effectively increases the lubricity property of fuel by reducing WSD from 507 to 420 μm but do not affect any other key parameters of diesel fuel. Hence, the blended ULSD meets the Euro VI/BS VI fuel specifications.[5]

Table 4

Key Parameters of Neat and Blended (150 ppm of 6a) ULSD

				results
SI. no.	parameter	test method	specification	neat ULSD	blended fuel
1	acidity, total, mg ofKOH/g, maxa	ASTM D 974	0.2	0.039 KOH/g	0.041 KOH/g
2	cetane index, mina	IP 380	46	56.1	56.0
3	pour point, °C, maxa	ASTM D 5950	3 for winter, 15 for summer	–33	–30
4	copper strip corrosion test 3 h at 50 °C	ASTM D 130	not worse than no. 1	no. 1	no. 1
5	distillation, 95% recovery, v/v, recovery, °C, maxa	ASTM D 86	360, max	344.5	340.5
6	flash point, °C, mina	IP 170	35, min	>100	>100
7	kinematic viscosity, cSt at 40 °C	ASTM D 445	2.0–4.5	3.051	3.053
8	density@15 °C, kg/m3	ASTM D 4052	810–845	839.7	839.8
9	total sulfur, mg/kg, maxa	ASTM D 5453	10	2.3	3.0
10	lubricity, WSD at 60 °C, microns, maxa	ASTM D 6079-18	460	507	420
11	oxidation stability, g/m3, maxa	ASTM D 2274	25	8.8	9.0
12	cold filter plugging point (CFPP), °C	ASTM D 6371	6 for winter, 18 for summer	–16	–16

max implies maximum allowed value, min implies minimum allowed value.

Lubricity Mechanism

The lubricity action of lubricity improvers on the metal surface was studied by examining the worn surfaces of friction couples through scanning electron microscopy (SEM) and EDS techniques.

SEM Analysis of Wear Scar

The SEM images of the wear scars on balls of the neat ULSD and ULSD-6a blends are shown in Figure . For neat ULSD, the scratch marks on worn surface are thick and the WSD of the worn surface on the ball is comparatively higher. As the concentration of the lubricity additive is increased from 150 to 300 ppm, the WSD of worn surface on the ball becomes smaller and the thickness of the scratch marks becomes thinner and shallower. The existence of polar components in the lubricity improving agent supports easy adsorption on the surface of the metal to form a protective thin layer between the metal surfaces which minimizes the wear and tear.[34−36]

Figure 8

SEM images of the wear scars on balls of the friction couples with (a) neat ULSD, (b) ULSD-6a (300 ppm), and (c) ULSD-6a (150 ppm) blends.

EDS Analysis of Wear Scar

To study the interaction between the additive and the metal surface during the friction process, the worn surface of the balls after the HFRR test was analyzed using the EDS spectroscopic technique. The analysis (Figure ) revealed the presence of elements C (from the diesel), O (from the lubricity improving agent and air), and Fe (from the friction matrix) on the worn surface. The peak positions of elements C, O, Fe, and Cr elements on the surfaces tested with different blended ULSD are compared with those of the surface tested with neat ULSD. EDS spectrum showed higher oxygen content on the worn surfaces lubricated by the blend fuel, while less oxygen content was found on the surfaces lubricated by the neat ULSD which confirms the lubrication action of the lubricity improver. The higher oxygen content observed in the case of blended fuel could be due to the interaction of the metal surface with the oxygen containing functional groups of the ester which promotes in the formation of a protective lubricating film.[15,16,37] The oxygen content on the worn surface increases from 2.2 to 5.7% with an increase in the blend concentration from 150 to 300 ppm which further supports the interaction of the metal surface with the additives through oxygen-containing functional groups.

Figure 9

EDS spectra of the worn surfaces with (a) neat ULSD, (b) blended ULSD (6a 150 ppm), and (c) blended ULSD (6a 300 ppm).

Conclusions

A feasible methodology was developed to convert LCN into efficient lubricity improvers (6a–j) for ULSD. The two-step reaction protocol involved hydroboration of LCN, followed by the esterification of the product with different organic acids to get the lubricity additives in the form of mixture of esters. The studies revealed that all the lubricity improvers possess long-term storage stability. The additives when blended with ULSD at low dosage levels (150/300 ppm) improve the lubricity of the blend fuel but do not alter any other key parameters of the diesel fuel. The ULSD blended with the LCN esters meets the Euro VI/BS VI fuel specifications. From the results of lubricity studies, it can be concluded that the LCN-based esters presents a promising prospect in the improvement of diesel lubricity and reduction of friction and wear in the fuel pump of diesel engines. The methodology followed in the present study could benefit the petroleum refineries to develop in-house low-cost additives using the refinery raw material (LCN).