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Ind. Eng. Chem. Res. 2005, 44, 250-253
Study of the Reasons for Discoloration of Hydrotreated Naphthenic Lube Base Oil under Ultraviolet Radiation Sheng Han,† Chao Qiu,† Xingguo Cheng,‡ Shujie Ma,§ and Tianhui Ren*,† School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China, Research and Development Center of PetroChina Lubricating Oil Company, Lanzhou 730060, People’s Republic of China, and Research and Development Center of PetroChina Lubricating Oil Company, Kelemayi 934003, People’s Republic of China
In this paper, the reasons for discoloration of hydrotreated naphthenic lube base oil were studied in detail. To define compositional factors associated with the color deterioration, first liquidsolid chromatography was used to separate hydrotreated naphthenic base oil into five different fractions, i.e., saturates, monoaromatics, diaromatics, polyaromatics, and polars. Then the sensitivity of each fraction to ultraviolet radiation was evaluated through ultraviolet radiation testing. The results of the tests indicate that polyaromatics and polars are the main precursor of color and deposits, respectively, and the compositions of two fractions were characterized by a series of instrument technology including elemental analysis, gas chromatography-mass spectrometry, and X-ray photoelectron spectroscopy. 1. Introduction In comparison with the conventional solvent refining lube base oil, hydrotreated naphthenic lube base oil has many excellent advantages such as high viscosity index, low volatility loss, low aromatic content, and the like.1-4 The demand for this kind of oil has been increased rapidly in recent years, Unfortunately, hydrotreated naphthenic lube base oil suffers from the shortcoming that it is discolored intensely when exposed to ultraviolet radiation or sunlight,5-8 so it cannot meet stringent color requirements of advanced white industrial products, which consequently lessens its commercial value. Therefore it is very necessary to study the reasons for discoloration when exposed to ultraviolet radiation so as to find effective means to overcome such a disadvantage. Many papers have reported the influence of base oil composition on oxidative stability,9-22 while previous studies have generally focused on the influence of base oil composition on thermo-oxidative stability. There has been very little work to address the influence of base oil composition on light stability. In this paper, we define light stability in terms of discoloration after exposure to ultraviolet radiation, and the reasons for discoloration of hydrotreated naphthenic lube base oil were studied in detail. Since base oil has a complex composition, it is impossible in practice to analyze all conceivable chemicals that might be discoloration hazards. Consequently, it is more practical to measure the constituent components by groups. In this paper, liquid-solid chromatography was first used to separate hydrotreated naphthenic base oil into five different fractions, i.e., saturates, monoaromatics, diaromatics, polyaromatics, and polars. Then the * To whom correspondence should be addressed. Tel.: 8621-54747118. Fax: 86-21-54741297). E-mail:
[email protected]. † Shanghai Jiao Tong University. ‡ Research and Development Center of PetroChina Lubricating Oil Company, Lanzhou. § Research and Development Center of PetroChina Lubricating Oil Company, Kelemayi.
sensitivity of each fraction to ultraviolet radiation was evaluated through ultraviolet radiation testing, and the compositions of polyaromatics and polars were characterized by a series of instrument technology including elemental analysis, gas chromatography-mass spectrometry (GC-MS), and X-ray photoelectron spectroscopy (XPS). 2. Experimental Section 2.1. Reagents and Apparatus. Silica gel (60/200 mesh) and neutral alumina absorbents (100/200 mesh) were purchased from Qingdao Chemical and Engineering Plant (Qingdao, China). The other reagents used in this experiment were all analysis grade. Petroleum ether was purified by percolation through silica gel. Elemental analysis was carried out using Foss Heraeus (CHN-O-Rapid, Germany), REN-1000 (Nitrogen Analyzer, Jiangsu, China), and OS-1 (Sulphur Analyzer, JiangSu, China) apparatuses. All ultraviolet transmittance data were acquired using a Cary50 ultraviolet spectrophotometer (Varian, U.S.); the emission wavelength is 500 nm. All synchronous fluorescence spectra were measured with a RF-5301PC spectrofluorophotometer (Shimadzu), using wavelength separation ∆λ ) 3 nm; the fluorescence excitation and emission wavelengths are 250 and 260 nm, respectively. A QP5050 GC-MS was used for this work. The conditions of GC-MS were as follows: carrier gas (helium), 20 mL/min; the column temperature, injector temperature, and interface temperature were set at 300 °C. The mass spectrometer was operated under electron impact conditions using ionization energy of 70 eV. All X-ray photoelectron spectra were measured with an X-ray photoelectron spectrometer (VG, U.K.). 2.2. Hydrotreated Naphthenic Lube Oil Sample. The oil sample was provided by the Kelemayi refinery. The physical and chemical properties of lube base oil are given in Table 1. 2.3. Separation Procedure. The separation scheme developed is shown in Figure 1. The process was divided
10.1021/ie049550m CCC: $30.25 © 2005 American Chemical Society Published on Web 12/24/2004
Ind. Eng. Chem. Res., Vol. 44, No. 2, 2005 251 Table 2. D-1500 Color Periodic Change against Time (h) during Exposure to Ultraviolet Radiation
Figure 1. Separation scheme of the hydrotreated naphthenic lube base oil into fractions. Table 1. Physical and Chemical Properties of Lube Oil characteristics appearance,15 °C density viscosity, 40 °C viscosity index viscosity, 100 °C flash point pour point neutralization sulfur, µg/g nitrogen, µg/g color, Saybolt aniline point refractive index, 20 °C viscosity-gravity-constant UV absorption, 260 nm molecular mass boil range hydrocarbon type analysis CN CP CA compositional analysis clay gel,% mass saturates aromatics polars
unit
test method
typical data
ASTM D4176 clear bright Kg/dm3 ASTM D4052 895.7 mm2/s ASTM D4052 130.6 14 mm2/s ASTM D4052 9.666 °C ASTM D93 218 °C ASTM D97 -27 koH/mg ASTM D974 0.01 wt % ASTM D2622 diaromatics > polars. Monoaromatics and saturates almost showed no color change during exposure to ultraviolet radiation. The above results indicate that polyaromatics are the main color precursor and polars are the main precursors of deposits. But the detailed mechanism of discoloration still is not clear, so if the compositions of polyaromatics and polars were characterized, it would further our understanding of the reasons for discoloration of hydrotreated naphthenic lube base oil. 3.2. Characterization of Polyaromatics and Polars. 3.2.1. Elemental Analysis. For the purpose of comparison, the elemental compositions of all fractions are given in Table 3. Elemental analysis results show that as the polar fractions increase, the content of sulfur and nitrogen increases. The content of carbon and hydrogen of saturates, monoaromatics, diaromatics, and polyaromatics accounted for about 99.98%, indicating the absence of heteroatom compounds in these fractions.
Table 3. Elemental Composition (in wt %) of the Different Fractions fraction
C%
H%
saturates monoaromatics diaromatics polyaromatics polars
86.31 86.65 88.56 88.97 85.53
13.68 13.35 11.44 10.99 13.64
S(ppm)% N(ppm)% 1.6 2.7 17 95 1400
1 1 2 32 1100
O%
H/C
diaromatics > polars. Monoaromatics and saturates almost show no color change during exposure to ultraviolet radiation. The experimental results indicate that polyaromatics and polars are the main precursors of color and deposits, respectively. (2) Polyaromatics mainly consist of triaromatics (phenanthrenes and naphthenephenanthrenes), tetraaromatics (pyrenes and chrysenes), pentaaromatics (perylenes and dibenzanthracenes), and thiopho aromatics (benzothiophenes, dibenzothiophenes, and naphthobenzothiophenes). (3) Oxygen-containing compounds such as hydroxyl, carbonyl, and carboxyl groups constitute the main body of polars. Nitrogen-containing compounds and sulfurcontaining compounds account for a minor percent of polars. Nitrogen-containing compounds include two types which might be NH2, NH, or NO2 and -NdN-. In the case of S, there also exist two types of sulfurcontaining compounds which are SO42- and PHSSPH. Acknowledgment The authors thank the R&D center of PetroChina Lubricating Oil Company, Lanzhou, for financial support and Dr. Huidong Wang for technical assistance.
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Received for review May 25, 2004 Revised manuscript received September 15, 2004 Accepted November 11, 2004 IE049550M
