Rapid Separation and API Grades Identification of Base Oil in Low Viscosity Gasoline Engine Oil SN 0W-16

Abstract

Using low viscosity engine oil is one of the most economical and easily achievable ways to improve fuel economy. Base oil is a main component in low viscosity engine oils, and therefore, the separation and identification of its are of great significance for oil product developers to prepare high-performance lubricants. However, the extraction methods reported for base oils mainly adopt membrane dialysis, which not only fails to completely separate the base oil but also wastes a large amount of solvent. The reason for this result is that the concentration of substances inside and outside the membrane cannot always be in an imbalanced state of permeation resulting from manual operation. Additionally, most studies primarily focus on the characterization of base oil components, while there are few reports on grade identification. For the above reasons, an economically effective separation technique of base oil from low viscosity gasoline engine oil SN 0W-16 is successfully established by combining improved Soxhlet extraction and a column chromatography separation method. By applying this method, the yield of extracting base oil generally exceeds 96%, and the solvent can also save more than 3 times. Besides, an exclusion method is built through several simple characterization steps including viscosity index (VI), FT-IR, size-exclusion chromatography (SEC), and hydrocarbon composition, which can quickly identify the American Petroleum Institute (API) grade and brand of the base oils.

Introduction

With the significant threat posed by global climate change to human society in recent years, CO₂ reduction and improvement of vehicle fuel efficiency are needed to respond to global warming. As a way to improve fuel efficiency, reduction of engine oil viscosity has been proven effective.¹⁻⁵ Consequently, the trend of development for lubricants is a remarkable reduction in the viscosity. As is well-known, lubricants are usually composed of additives and base oil, and the mass fraction of base oil exceeds 80%, which leads to the results that the quality and performance of lubricants are directly determined by the base oil to a large extent. Lube base oils are divided into I to V grades based on the amount of saturates, sulfur content, and viscosity index, which is provided by the American Petroleum Institute (API)⁶ and shown in Table 1. Group I, II, and III base oils are mineral oils, and group IV base oils are polyalphaolefins (PAOs). Group V base oils include all other base oils not included in I to IV. It is worth mentioning that the lube base oil can be a single group I, II, III, IV, and V or can be any blend of two or more groups having different molecular weights and viscosities, where the blend is processed in any suitable manner to create a base oil having suitable properties for use, such as the viscosity value. Due to the requirements of low viscosity and evaporation loss, the low viscosity gasoline engine oil (LVGEO) must choose high-quality base oil with higher viscosity index (VI), and lower viscosity and lower evaporation loss. Thereby, group III base oils produced by hydrotreating/cracking and hydroisomerization and fully synthetic base oil including group IV and V are the best choices for low viscosity lube base oils, which are considered high quality lube base oils owing to their high VI, high saturated hydrocarbon contents, and low impurity levels.⁷

Table 1. Classification of Lube Base Oil Cited from Ref (6).

Group	Saturates (wt %)	Sulfur (wt %)	Viscosity index (VI)
I	<90	>0.03	80 ≤ VI < 120
II	≥90	≤0.03	80 ≤ VI < 120
III	≥90	≤0.03	≥120
IV	Polyalphaolefins (PAOs)
V	All other base stocks not included in groups I to IV

Group III base oils obtained during hydrocracking-hydroisomerization have a series of advantages in comparison with group I and II base oils, such as good chemical stability, improved low-temperature properties, narrow group composition, good additives acceptance, high viscosity index, etc.⁸ SK lubricant is a world-renowned supplier of group III base oils. The YUBASE base oils produced through patented technology UCO are famous both domestically and internationally. The technical specifications are presented in Table 2 for common marketized YUBASE products that have low viscosity. Poly-α-olefin (PAOs), group IV base oils, are synthetic saturated hydrocarbon oils with well-defined molecular weight. The research results which have been reported indicate that PAOs exhibit certain inherent and desirable characteristics including a wide operational temperature range, a high viscosity index, thermal stability, oxidative stability, hydrolytic stability, shear stability, low corrosivity, compatibility with mineral oils, compatibility with common materials, additive response, low toxicity, and biodegradable low viscosity grades.^9,10 Since the advantages of PAOs in reducing wear and tear of engine parts, as well as improving fuel economy, the modern automotive market favors fully synthetic base oil based on PAOs, which leads to the continuous rise of the PAOs market. PAOs are divided into low viscosity, i.e. 2–10 mm²/s, and high viscosity, i.e., 40–100 mm²/s, according to their kinematic viscosity (KV) at 100 °C. Table 3 lists the typical viscosity grades of what are commonly used commercialized PAOs.⁹ By the blending of PAOs with different viscosities, the expected viscosity level of PAOs can be modulated to satisfy end-use application requirements. As far as LVGEO, the PAOs having high VI and low viscosity, together with high performance, have the most reception. Group V base oils, a wide variety of high-performance synthetic base oils, have been developed that contain synthetic esters and others. In LVGEO, the synthetic esters mainly include diesters, polyol esters, and polyesters, while the others are mainly phosphate esters and polyether.

Table 2. Kinematic Viscosity and Viscosity Index of Commercialization YUBASE Products, Cited from the Referenced Material Displayed in the Supporting Information.

	YU2	YU3	YU4	YU5	YU6	YU8
KV 100 °C, mm2/s–1	2.41	3.12	4.23	4.99	6.52	7.6
KV 40 °C, mm2/s–1	8.65	12.43	19.57	25.24	36.82	47.0
VI	96	112	122	126	131	128

Table 3. Kinematic Viscosity and Viscosity Index of Commercialization PAOs Cited from Ref (9).

	PAO2	PAO4	PAO6	PAO8	PAO10	PAO40	PAO100
KV 100 °C, mm2/s–1	1.8	3.9	5.9	7.8	9.6	40	100
KV 40 °C, mm2/s–1	5.54	16.8	31.0	45.8	62.9	395	1250
VI	-	125	135	136	137	151	168

Currently, the membrane dialysis or column chromatography separation technology alone are main methods for separating base oil from lubricants. However, the methods are applicable only for rough separation rather than complete separation. In addition, the methods are time-consuming and costly rather than time-saving and economical. Besides, the products that have been roughly separated are used for component identification rather than API grade and brand identification. Considering that the base oil accounts for nearly 90% of LVGEO, it is difficult to completely separate the base oil and additives, which is caused by the results that small molecule polarity additives are oozed out or some less polarity additives are washed down together with the base oil during the processes of dialysis and column chromatography separation, respectively. For the above reasons, herein we reported a successful separation technology of base oils and additives in LVGEO via the combination of an improved Soxhlet extraction and column chromatography separation method. The advantages of this technology include (1) being able to completely separate the base oil from LVGEO, (2) being suitable for large-scale separation, and (3) being economical and time-saving. Furthermore, an exclusion method is built through several simple characterization steps including FT-IR, VI, size-exclusion chromatography (SEC), and hydrocarbon composition, which can quickly identify the API grade and brand of the base oils and can also provide convenience for formula developers.

Materials and Methods

Materials

Petroleum ether (PE, AR; boiling range 30–60 °C), methanol (CH₃OH, AR), ethanol (CH₃CH₂OH, AR) and dichloromethane (CH₂Cl₂, AR) were purchased from Tianjin Li’anlong Bohua Medicine Co., Ltd. (China) and used directly without further purification.

The analyzed sample is commercial low viscosity gasoline engine oil (LVGEO) SN 0W-16.

Comparison samples: YUBASE base oils produced by SK lubricant applying patented technology UCO, including YUBASE3, YUBASE4 and YUBASE6, abbreviated as YU3, YU4 and YU6; LA4, LA6 and LA10 prepared by LU’AN group using coal to liquid technology; polyalphaolefins (PAOs) including PAO2, PAO3.6+, PAO4, PAO6, PAO8, PAO10, PAO40 and PAO100.

Validation samples: an additive package (PAC) used for concocting low viscosity gasoline engine oil and another low viscosity engine oil 0W-20 that can be used for hybrid vehicles.

Separation of Base Oil in LVGEO

The extraction of the base oil consists of two primary steps (Scheme 1). First, a mixture of base oil and small molecule additives was extracted through improved Soxhlet extraction method. Then, the mixture was separated by column chromatography according to the polarity differences of each substance, and the elution solvents were PE, V_PE:V_CH2Cl2 = 1:1, and CH₃OH in sequence. The base oil was successfully received from the PE eluent and name it 0W16-BO1.

Scheme 1. Extraction of Base Oil from Low Viscosity Gasoline Engine Oils.

Characterization

The FT-IR spectroscopic measurement was conducted on a NEXUS 670 FT-IR spectrometer (Nicolet, WI, USA) and liquid samples were coated on a potassium bromide (KBr) sheet prior to the measurements.

The size-exclusion chromatography (SEC) analysis was used to determine the molecular weight (M_avg) and polydispersity index (PDI) of samples. SEC was carried out using chromatographic-grade THF at 40 °C as the eluant at a flow rate of 1 mL/min.

The standard evaluation method D445 from the American Society for Testing and Materials (ASTM) was used to test the kinematic viscosities (KV) of samples at 40 and 100 °C.

Viscosity index (VI) was calculated according to eq 1, where L and H are constants based on KV100 °C, tabularized in ASTM 2270.¹¹

1

The standard evaluation method ASTM D2786 was employed to provide the hydrocarbon composition of the samples.

Results and Discussion

Extraction of the Mixture of Base Oil and Small Molecule Additives in LVGEO

Soxhlet extraction, also known as continuous extraction, is a method of extracting compounds from solid substances. By improving the extraction device, it can be applied to the separation of liquid mixtures. Specifically, the liquid mixture to be separated is first placed in a polyurethane sanitary membrane and sealed. Then, the membrane containing the mixture inside a porous stainless steel sleeve is packed and finally placed in the Soxhlet extractor. The extraction solvent used here is PE with a boiling range of 30 to 60 °C, and the temperature of extraction is 50 °C. The base oil and small molecule additives in LVGEO can be efficiently extracted utilizing the principles of solvent reflux and siphoning. In the LVGEO studied here, 96.3% of the mixture, i.e., base oil and small molecule additives, is obtained through Soxhlet extraction (Table 4).

Table 4. Results of Soxhlet Extraction of LVGEO and Column Chromatography Separation of Mixture.

	mSN 0W-16 = 5.9300 g
Improved Soxhlet extraction	Base oil and small molecule additives	Macromolecules and other additives
Production (g)	5.7100	0.2200
Proportion (%)	96.3	3.7
Total yield (%)	(5.7100 + 0.2200)/5.9300 × 100% = 100%

Column chromatography	mmixture of base oil and small molecule additives = 1.0472 g
eluant	PE	VPE:VCH2Cl2 = 1:1	CH3OH
Production (g)	0.9914	0.0153	0.0122
Proportion (%)	97.3	1.5	1.2
Total yield (%)	(0.9914 + 0.0153 + 0.0122)/1.0472 × 100% = 97.3%
Total amount of additives (%)	1 – (1 × 96.3% × 97.3%) × 100% = 6.3%

Separation of the Base Oil, 0W16-BO1, in LVGEO

To obtain base oil, column chromatography separation is used, and the results are shown in Table 4. It is obvious that the 0W16-BO1 is a major constituent with 97.3% in the mixture. Moreover, It can be calculated from the results in Table 4, that the 0W16-BO1 accounts for approximately 93.7% in this commercial LVGEO, while additives only account for 6.3%. This result is consistent with the formula ratio of the base oil and additives in low viscosity lubricating oils.

Validation of the Reliability of Separation Technology

To verify the reliability of the separation technology, the same two-step method is further applied to separate the base oil from an additive package (PAC) used for concocting low viscosity gasoline engine oil and from 0W-20 engine oil used for hybrid vehicles. In the PAC composed of base oil (diluted oil) and a mixture of additives, additives account for 60–70%, while the total amount of additives is about 10% in 0W-20. Because the data range of total amount of additives obtained from the two samples (Table S1 and S2) is in accord with the data provided by the supplier, which further validates the effectiveness and reliability of this separation technology.

Determination of API Grades Range of 0W16-BO1

Viscosity index (VI) representing the degree of viscosity change with temperature is one of the most important characteristics to distinguish and determine the quality grade of a lube base oil, and the VI value strongly depends on the molecular structure of the constituent oil.⁷ According to formula (1), the prerequisite for obtaining the VI value is to measure the KV values at 40 and 100 °C. Hence, the KV 40 and KV 100 °C of 0W16-BO1 are tested by employing ASTM D445 method, and the results are 22.19 mm²/s and 4.618 mm²/s, respectively. Therefore, the VI value obtained is 126, which indicates that 0W16-BO1 belongs to groups III–V of base oils based on API VI classification (Table 1).

Identification of Specific API Grade of 0W16-BO1

The simple exclusion method is used to identify the API grade of base oils in LVGEO. First, compared with group III and IV base oils, the presence or absence of group V base oils can be identified through infrared spectra analysis because they have special functional groups. Figure 1 shows the infrared spectrogram of the 0W16-BO1. From the spectrum, the characteristic wavenumbers of 2955 cm^–1, 2924 cm^–1, and 2854 cm^–1 can be seen, which are caused by the result of overlapping antisymmetric and symmetric stretching vibration of methyl and methylene groups. The absorption band at 1464 cm^–1 is the overlapping of antisymmetric and symmetric bending vibration of methyl and methylene, while the 1377 cm^–1 is the symmetric bending vibration absorption band of the methyl group. Moreover, the weak absorption at 722 cm^–1 is the in-plane rocking vibration of −CH₂–. Besides the absorption peaks of hydrocarbon mentioned above, there are no typical absorption bands of functional groups C=O at 1738 cm^–1, C–O–C at 1171 and 1116 cm^–1, P=O at 1243 cm^–1, and Si–C at 864 cm^–1 on the spectrum.¹² Accordingly, group V base oils are absent in the 0W16-BO1.

Figure 1.

FT-IR spectrum of the 0W16-BO1.

Benefited by the synthesis processes, i.e., alphaolefin polymerization, hydrogenation, and distillation, the low viscosity PAOs embrace predominantly the trimer (C30), tetramer (C40), and pentamers (C50) of 1-decene¹³ and have relatively narrow molecular weight distribution compare to group III base oils, thus, the group IV base oils whether existing in the 0W16-BO1 can be judged by SEC. The SEC analysis of 0W16-BO1 shows an unimodal and symmetric elution peak (Figure 2a), while the PAO3.6+, PAO4 and PAO6 with similar viscosity exhibit multiple elution peaks, as displayed in Figure 2b. Table 5 compares the molecular weight (MW) and molecular weight distribution (polydispersity index, PDI) of 0W16-BO1 to a series of commercially available PAOs. From the data, it is evident that PAOs have a smaller PDI compared to 0W16-BO1. Therefore, it can be demonstrated from SEC results that the 0W16-BO1 belongs to group III base oils.

Figure 2.

SEC elution traces of (a) 0W16-BO1 and (b) PAOs using THF as an eluent.

Table 5. Summary of MW and PDI of Base Oils Determined by SEC.

	Peak limits (min)	M (avg)	Mw/Mn—PDI
0W16-BO1	34.077–39.868	3.914 × 102	1.113
PAO 2	36.338–40.336	2.788 × 102	1.032
PAO 3.6+	34.853–39.266	4.862 × 102	1.023
PAO 4	33.897–39.068	5.531 × 102	1.043
PAO 6	33.451–38.933	5.976 × 102	1.060
PAO 8	31.988–38.834	6.578 × 102	1.065
PAO 10	31.773–38.547	7.402 × 102	1.094
PAO 40	28.234–37.953	1.008 × 103	1.228
PAO 100	24.732–37.778	1.102 × 103	2.088

Determination of the Brand of 0W16-BO1

In addition to the hydrocracking-hydroisomerization, the base oils produced by synthetic techniques, including coal to liquid (CTL) and gas to liquid (GTL) are also classified as API group III. Due to the higher content of alkane, it can be determined that the base oils prepared by CTL and GTL exist through hydrocarbon composition analysis. Table 6 shows the content of saturated hydrocarbon constituents of 0W16-BO1 and LAs obtained through the ASTM D2786 method. It can be seen from the data that alkane contents of 0W16-BO1, LA4, LA6, and LA10 are 52%, 99.1%, 98.6%, and 97.5%, respectively. This result suggests that 0W16-BO1 belongs to group III base oils manufactured by hydrogenation technology.

Table 6. Hydrocarbon Types of 0W16-BO1 and LAs.

		0W16-BO1	LA4	LA6	LA10
Concentration (mol %)	Alkanes	52	99.1	98.6	97.5
	1-ring	23.7	0	0	0.2
	2-ring	14.8	0.4	0.5	1.1
	3-ring	5.3	0	0	0
	4-ring	2.3	0	0.1	0.2
	5-ring	1.9	0.2	0.4	0.5
	6-ring	0	0.3	0.4	0.5
	Total cycloparaffins	48	0.9	1.4	2.5

The color of 0W16-BO1 is almost transparent (Figure 3). YUBASE base oils have the same color because of the low content of cycloalkanes and heterocyclic aromatic hydrocarbons attributed to its high refining degree. Therefore, a comparison is made for the hydrocarbon composition and SEC data of low viscosity YUBASE and 0W16-BO1. The results indicate that the hydrocarbon type and content of 0W16-BO1 are near to YU3, YU4, and YU6 (Table 7), while the elution peak and retention time of SEC are closer to YU4 (Figure 4). Ignoring the impact of baseline unevenness, the SEC curves of YU4 and 0W16-BO1 basically overlap; therefore, YU4 is the main constituent of 0W16-BO1 (Figure S3). Based on the above analysis results, it can be concluded that 0W16-BO1 is a high-quality base oil obtained by blending of YUBASE base oils.

Figure 3.

Colors of SN 0W-16, mixture of base oil and small molecule additives, and 0W16-BO1.

Table 7. Hydrocarbon Types of 0W16-BO1 and YUBASE Base Oils.

		0W16-BO1	YU3	YU4	YU6
Concentration (mol %)	Alkanes	52	50.6	61.4	59.6
	1-ring	23.7	25.7	23.2	21.6
	2-ring	14.8	13.9	10.1	12.4
	3-ring	5.3	5.2	3.1	4.1
	4-ring	2.3	3.3	1.3	1.4
	5-ring	1.9	1.3	0.9	0.8
	6-ring	0	0	0	0.1
	Total cycloparaffins	48	49.4	38.6	40.4

Figure 4.

SEC elution traces of 0W16-BO1 and YUBASE base oils using THF as an eluent.

Validation of the Identification Method of API Grade and Brand

The identification process used in this article for API grade and brand of base oil in LVGEO is shown in Scheme 2. The effectiveness of the method is validated by identifying the base oil (PAC-BO) from an additive package (PAC) used for concocting a low-viscosity gasoline engine oil. First, the possibility of PAC-BO being a group V base oils can be ruled out through the FT-IR and elemental analysis (Figure S1a and Table S3). According to SEC (Figure S1b), it is known that PAC-BO does not belong to the group IV base oils. Further analysis of hydrocarbon composition (Table S4) confirms that PAC-BO is not a synthetic technical base oils. Considering that VI of PAC-BO exceeds 120, it is classified as a group III base oils. Through comparative analysis of SEC (Figure S2) finds that PAC-BO is also a blending oil of YUBASE base oils, but YU6 is the main component in PAC-BO. The analytical thinking to obtain this result is similar to that of the 0W-16.

Scheme 2. Identification Process for API Grade and Brand of Base Oil in LVGEO.

Conclusions

In summary, an economical and time-saving separation technology has been established by combining improved Soxhlet extraction and a column chromatography method, which is suitable for large-scale extraction of base oil from low viscosity gasoline engine oils. The separation efficiency of this technology is almost 100%, and the yield exceeds 96%, with a relative error of less than 4%. Furthermore, an exclusion method composed of viscosity index (VI), FT-IR, size-exclusion chromatography (SEC), and hydrocarbon composition is built, which can quickly identify the API grade and brand of the base oils and conveniently provide information for formula developers.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c01291.

• Validation experiment data, the separation results of base oil in PAC and 0W-20, FT-IR, SEC, elemental analysis, and hydrocarbon types data of base oils (PDF)

The authors declare no competing financial interest.