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Branching Structure of Diesel and Lubricant Base Oils Prepared by Isomerization/Hydrocracking of Fischer-Tropsch Waxes and r-Olefins Manabu Kobayashi,* Masayuki Saitoh, Seiji Togawa, and Katsuaki Ishida Petroleum Refining Research & Technology Center, Japan Energy Corp., 3-17-35 Niizo-Minami, Toda, Saitama 335-8502, Japan ReceiVed July 2, 2008. ReVised Manuscript ReceiVed October 1, 2008
Lubricant base oils were prepared from two Fischer-Tropsch (FT) waxes and two R-olefins with different carbon number distributions under several isomerization/hydrocracking conditions. The molecular structures of the resulting oils were investigated using 13C NMR analysis to determine the location and length of branches. Peak areas assigned to the CH carbons were divided into eight groups and correlated with the progress of the isomerization reaction. Each group showed good correlation with the density of branching, which was expressed as the ratio of the average branching number (ABN) to the average carbon number (ACN). This trend was independent of the feedstock used and the reaction conditions. The probability of methyl branching at a carbon atom depended on its location from the terminal carbon; that is, in order of decreasing probability, the carbon location is second > third > fourth, and so forth, and the probability of the seventh and eighth or inner carbon atoms was almost equal. A trend of increasing proportion of branches located at the second carbon was observed. Diesel oils were also obtained by isomerization/hydrocracking of FT waxes, and the most likely position of methyl branching was the second carbon from the terminal carbon. Branching at the second carbon showed a decreasing trend with increasing density of branching in diesel oil, whereas that in lubricant base oil showed an increasing trend. The present work demonstrated that the position and length of the branches in lubricant base oils, and diesel oils prepared by isomerization/hydrocracking of FT waxes and R-olefins, are determined by the density of branching, thus supporting previous findings that viscosity properties of lubricant oils, such as the kinematic viscosity and viscosity index, can be expressed using only the ACN and ABN.
1. Introduction As fuel and lubricant oils produced by gas-to-liquid (GTL) technology contain virtually no heteroatoms such as sulfur and nitrogen, they are expected to be the ultraclean oils of the next generation, which are difficult to produce from petroleum sources.1,2 Because they mainly consist of noncyclic paraffins, very high performance, such as a high cetane number of diesel oils or a high viscosity index of lubricant base oils, can be expected.3-6 To maximize the viscosity, combustion, and cold flow properties, isomerization/hydrocracking reaction of FischerTropsch (FT) waxes and molecular structures of oils formed have been widely studied. * To whom correspondence should be addressed. Telephone: +81 48 431-1952. Fax: +81 48 431-1949. E-mail:
[email protected]. (1) Ondrey, G. Gas-to-Liquid Projects Get the Green Light. Chem. Eng. 2004, 111, 23–27. (2) Skrebowski, C. Rising Gas Investments Herald LNG/GTL Boom. Petroleum ReView 2004, 691, 18–22. (3) Shah P. P.; Sturtevant G. C.; Gregor J. H.; Humbach M. J.; Padrta F. G.; Steigleder K. Z. Fischer-Tropsch Wax Characterization and Upgrading: Final Report; Report DE88014638; UOP, Inc., 1988. (4) Myburgh, I.; Schnell, M.; Oyama, K.; Sugano, H.; Yokota, H.; Tahara, S. The Emission Performance of a GTL Diesel Fuel - A Japanese Market Study; Prepr. SAE2003-01-1946, 2003. (5) Abu-Jrai, A.; Tsolakis, A.; Theinnoi, K.; Cracknell, R.; Megaritis, A.; Wyszynski, M. L.; Golunski, S. E. Effect of Gas-to-Liquid Diesel Fuels on Combustion Characteristics, Engine Emissions, and Exhaust Gas Fuel Reforming. Comparative Study. Energy Fuels 2006, 20, 2377–2384. (6) Kobayashi, M.; Saitoh, M.; Ishida, K.; Yachi, H. Viscosity Properties and Molecular Structure of Lube Base Oil Prepared from Fischer-Tropsch Waxes. J. Jpn. Petrol. Inst. 2005, 48, 365–372.
Much research has been conducted on the molecular structure of isomerized or hydrocracked products from n-paraffins using the gas chromatography analysis. However, most studies involve paraffins with carbon numbers smaller than 15.7-9 Paraffins with larger carbon numbers have a large number of isomers, many of which are difficult to identify by chromatography. Miki et al. attempted to observe the molecular structures of diesel oil prepared via FT synthesis and analyzed the products obtained from the isomerization/hydrocracking process by gas chromatography (GC); however, many isomers remain unidentified.10 Calemma et al. assessed the effects of operating conditions on hydroconversion of FT waxes on reactivity and selectivity,11 and the authors also investigated the reaction pathway assuming two lumps in isoparaffins, that is, iso-Cn lube and iso-Cn (7) Steijns, M.; Froment, G. Hydroisomerization and Hydrocracking. 2. Product Distributions from n-Decane and n-Dodecane. Ind. Eng. Chem. Prod. Res. DeV. 1981, 20, 654–660. (8) Martens, J. A.; Jacobs, P. A.; Weitkamp, J. Attempts to Rationalize the Distribution of Hydrocracked Products. I Qualitative Description of The Primary Hydrocracking Modes of Long Chain Paraffins in Open Zeolites. Appl. Catal. 1986, 20, 239–281. (9) Martens, J. A.; Tielen, M.; Jacobs, P. A. Attempts to Rationalize the Distribution of Hydrocracked Products. III. Mechanistic Aspects of Isomerization and Hydrocracking of Branched Alkanes on Ideal Bifunctional Large-Pore Zeolite Catalysts. Catal. Today 1987, 1, 435–453. (10) Miki, Y.; Toba, M.; Yoshimura, Y.; Murata, K. Compositional Analysis of GTL and BTL Diesel Oils. J. Jpn. Petrol. Inst. 2007, 50, 108– 116. (11) Calemma, V.; Correra, S.; Perego, C.; Pollesel, P.; Pellegrini, L. Hydroconversion of Fischer-Tropsch Waxes: Assessment of the Operating Conditions Effect by Factorial Design Experiments. Catal. Today 2005, 106, 282–287.
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nolube.12 Leckel evaluated the branching structure of diesel oil prepared from FT waxes by GC-mass spectrometry and obtained the location of methyl branching.13 The 13C NMR method was applied to determine the molecular structures of base oils,14-16 for example Montanari et al. evaluated the branching nature of several petroleum-based and synthesized lubricant base oils and found a relationship between their branching nature and performance.16 All of these works evaluated the molecular structures of product oils formed or the effects of operation conditions on reactivity and selectivity, but the effects of isomerization/ hydrocracking reaction conditions on the branching nature of product oils from FT waxes have been seldom reported. In our previous studies,6,17 fuel and lubricant base oils were prepared from FT-synthesized waxes and R-olefins with different carbon number distributions. The oils obtained were analyzed by 13C NMR, and the average branching number (ABN) of isoparaffins was derived. The ABN and the average carbon number (ACN) were used as the key parameters for investigating the molecular structures of the fuel and lubricant base oils formed, and the effects of the severity of isomerization/ hydrocracking reactions on the ABN and ACN were observed. The relationship between the molecular structural parameters such as the ABN and ACN and viscosity properties of lubricant oils, such as, kinematic viscosities and viscosity index, was also investigated; it was confirmed that these properties could be characterized using the ABN and ACN alone;6 however, the location and length of branching in the isoparaffins were still unknown. It seemed unusual that viscosity properties could be represented only by the ACN and ABN because viscosity properties are significantly affected by the number, position, and length of branches.18 In the present work, 13C NMR chemical shifts of the CH carbons were investigated in detail in diesel and lubricant base oils prepared from two FT waxes and two R-olefins to determine the position and length of branching. These diesel and lubricant base oils were prepared under several isomerization/hydrocracking conditions, and the effect of the degree of branching on the position and length of branching in isoparaffins was investigated. 2. Experimental Section 2.1. Preparation of Diesel and Lubricant Base Oils. Two FT waxes and two long-chain R-olefins with different carbon number distributions (Figure 1), which were obtained from a commercial FT plant and an R-olefin plant, respectively, were used as feedstocks (12) Calemma, V.; Peratello, S.; Stroppa, F.; Giardino, R.; Perego, C. Hydrocracking and Hydroisomerization of Long-Chain n-Paraffins. Reactivity and Reaction Pathway for Base Oil Formation. Ind. Eng. Chem. Res. 2004, 43, 934–949. (13) Leckel, D. Low-Pressure Hydrocracking of Coal-Derived FischerTropsch Waxes to Diesel. Energy Fuels 2007, 21, 1425–1431. (14) Cook, B. R.; Berlowitz;, P. J.; Silbernagel, B. G.; Sysyn, D. A. Use of 13C NMR Spectroscopy to Produce Optimum Fischer-Tropsch Diesel Fuels and Blend Stocks. U.S. Patent 6,210,559, 2001. (15) Sarpal, A. S.; Kapur, G. S.; Chopra, A.; Jain, S. K.; Srivastava, S. P.; Bhatnagar, A. K. Hydrocarbon Characterization of Hydrocracked Base Stocks by One- and Two-Dimensional N.M.R. Spectroscopy. Fuel 1996, 75, 483–490. (16) Montanari, L.; Montani, E.; Corno, C.; Fattori, S. NMR Molecular Characterization of Lubricating Base Oils: Correlation with Their Performance. Appl. Magn. Reson. 1998, 14, 345–356. (17) Kobayashi, M.; Togawa, S.; Ishida, K. Properties and Molecular Structures of Fuel Fractions Obtained from Fischer-Tropsch Waxes. J. Jpn. Petrol. Inst. 2006, 49, 194–201. (18) Properties of Hydrocarbons of High Molecular Weight; API Research Project 42 Report, 1967.
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Figure 1. Carbon number distribution of feedstock FT waxes and R-olefins.
for the preparation of diesel and lubricant base oils. R-Olefins with carbon numbers of 20 or more are separated from ones with carbon numbers of 18 or less and were used in this study. The carbon number distribution in AO-1 is consequently asymmetric. A commercial isomerization catalyst, HOP-302 (Japan Energy Corporation) and a hydrocracking (HC) catalyst were used for preparation of the diesel and lubricant base oils. HOP-302 was prepared by impregnating ammonium heptamolybdate solution and nickel nitrate solution separately on extrudates of alumina, silica, and mordenite mixture. HC catalyst was prepared by impregnating ammonium tungstate solution and nickel nitrate solution separately on extrudates of alumina, silica–alumina, and ultrastable Y zeolite mixture. One hundred milliliters of catalyst was loaded in a fixed-bed flow reactor with R-alumina (100 mL) as diluent. The isomerization/ hydrocracking conditions are summarized in Table 1. Product oils were separated with a true boiling point (TBP) distillation apparatus. Fractions with boiling points in the range of 250-360 °C were categorized as diesel oils. When obtaining bottom oil fractions (TBP > 360 °C), except when the n-paraffin content was below 1 wt %, bottom oils were dewaxed with a mixed solvent (50% 2-butanone/50% toluene). Dewaxed oils were then distilled with a TBP distillation apparatus, and the bottom oils with boiling points above 360 °C were taken as the lubricant base oil samples. 2.2. Analysis of Product Oils. The DEPT method was used to assign the peaks of the 13C NMR spectrum of product oils to CH, CH2, and CH3 carbons assuming that no quaternary carbon atoms were present. Analysis was performed with a GSX-270 NMR spectrometer (JEOL Ltd.) with a 10 mm sample tube filled with approximately 50% of sample oil diluted with CDCl3. Each peak was quantified by the analysis of 1H gated decoupling without nuclear Overhauser effect because the peak area obtained by the DEPT method is not quantitative. Average molecular weights were calculated from the kinematic viscosities at 37.8 and 98.9 °C by the ASTM D2502-92 method. ACN was calculated using eq 1, assuming that base oil samples in this study consisted mostly of noncyclic paraffins with the molecular formula CnH2n+2.
ACN ) (average molecular weight - 2) ⁄ 14
(1)
Because ABN is equal to the average number of CH carbons in one molecule, it was calculated with eq 2.
ABN ) ACN × [CH carbons ⁄ total carbons]
(2)
2.3. Assignment of 13C NMR Chemical Shift to Branching Structure. An NMR simulator, ACD/CNMR Predictor (Advanced Chemistry Development, Inc.), was used to predict the chemical
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Table 1. Operation Condition of Isomerization/Hydrocracking Reaction and Product Properties Lubricant Base Oils Feedstock
FTW-1
sample number
unit
Catalyst H2 pressure H2/oil ratio operation temperature LHSV n-paraffin (C22+) conversion 360 °C+ conversion ACN ABN
M Pa NL/L °C h-1 wt %
1
2
HOP302 9.0 1500 340 0.44 93.9
wt % 14.3 molecule-1 26.3 molecule-1 2.04
FTW-2
AO-1
3
4
5
6
7
8
9
HOP302 9.0 1500 350 0.44 98.9
HOP302 9.0 1500 350 1.00 78.9
HOP302 4.0 660 370 1.00 99.9
HOP302 9.0 1500 360 0.44 99.5
HOP302 9.0 1500 355 0.44 97.6
HOP302 9.0 1500 355 0.33 99.7
HOP302 9.0 1500 355 1.00 73.0
30.9 26.0 2.27
8.3 26.6 1.90
83.5 24.4 2.41
45.4 28.1 2.64
28.0 29.2 2.64
46.6 28.2 2.72
9.6 30.1 2.40
AO-2
10
11
12
13
14
HOP302 5.0 800 390 2.00 99.6
HOP302 5.0 800 400 2.00 100.0
HOP302 9.0 1500 360 0.50 91.8
HOP302 9.0 1500 370 0.50 98.3
HOP302 9.0 1500 360 0.50 91.2
HOP302 9.0 1500 370 0.50 98.8
67.5 26.0 2.53
91.4 23.7 2.45
53.4 27.2 2.42
70.3 26.7 2.60
22.9 33.4 3.06
49.6 32.0 3.17
Diesel Oils FTW-1
Catalyst H2 pressure H2/oil ratio operation temperature LHSV 360 °C+ conversion diesel oil yield n-paraffin content ACN ABN
M Pa NL/L °C h-1 wt % wt % wt % molecule-1 molecule-1
FTW-2
1
2
3
4
5
6
7
8
9
HC 4.0 660 330 1.00 89.9 41.9 4.3 17.9 1.91
HC 4.0 660 320 1.00 39.9 23.2 7.6 17.9 1.75
HOP-302 4.0 660 370 1.00 83.5 26.9 4.4 17.1 1.88
HOP-302 5.0 800 380 2.00 34.1 20.1 14.0 17.5 1.59
HOP-302 5.0 800 390 2.00 67.5 29.6 6.9 17.2 1.74
HOP-302 5.0 800 400 2.00 91.4 23.4 5.2 16.6 1.76
HOP-302 4.0 660 390 1.00 95.9 12.4 4.6 16.8 1.86
HOP-302 4.0 660 385 1.00 83.6 19.6 5.2 17.0 1.87
HOP-302 4.0 660 388 1.00 89.3 17.5 5.0 16.9 1.84
Table 2. Chemical Shift and CH Carbon Peak Assignment to Branching group
chemical shift, ppm
corresponding branching of isoparafins
CH-a CH-b CH-c CH-d CH-e CH-f CH-g CH-h
28.00-29.00 32.75-32.90 32.90-33.15 33.15-33.25 33.25-33.45 33.45-33.60 34.50-34.80 39.00-39.50
2-methyl branching 4-methyl branching 8- or inner methyl branching 7-methyl branching 3-methyl branching 7- or inner ethyl branching 5-,6-methyl, 4-, 5-, 6-ethyl branching 3-ethyl branching
shifts of the CH carbon atoms in monomethyl, dimethyl, monoethyl, and diethyl paraffins. Paraffins with 30 carbon atoms and different branching positions were studied, and the chemical shifts of the CH carbons were correlated with the position of branching (Table 2 and Figure 2). The peak areas belonging to the CH carbons were divided into 8 groups according to their chemical shifts. Observed peak areas of diesel and lubricant base oils were assigned to these 8 groups, and the normalized peak areas (eq 3) were compared.
normalized peak area of CH carbons ) CH-x carbons ⁄ total CH carbons (x ) a-h) (3) 3. Results and Discussion
The ratio of CH-c/total CH carbons decreases (part a of Figure 3) with an increase in branching. However, the ratio of ABN/ACN is better correlated with the ratio of CH-c/total CH carbons, regardless of the feedstock used and the experimental conditions (part b of Figure 3). Because the density of branching (ABN/ACN) is also well correlated with the ratio of other groups (Figure 4), the ratio of ABN/ACN can be regarded as the determining factor for ratios of CH carbon groups. This result revealed that, if ABN and ACN were determined, the probability of the position and length of branching would automatically be determined. This finding matches the previous report that product distribution from n-decane and n-dodecane isomerization/hydrocracking is a function of the total conversion alone,9 in spite of the difference of carbon chain length of feedstock and catalyst type. Figure 4 shows the ratio of CH-x/total CH carbons (x ) a-h) in lubricant base oils as a function of the ratio of ABN/ ACN. The CH-c group that corresponds to methyl branches located on the eighth or inner carbon was present in the highest proportion when the density of branching, that is the ratio of ABN/ACN was low, but decreased drastically with an increase in the ratio of ABN/ACN. The CH-c group is present in the highest proportion because the eighth or inner carbons are
3.1. Branching of Lubricant Base Oils. Figure 3 shows the ratio of CH-c/total CH carbons, which corresponds to the CH carbons of branching located on eighth or inner carbon in the main carbon chain (Figure 2) as a function of ABN (a), and the density of branching was expressed as a ratio of ABN/ACN (b).
Figure 2. Analysis of branching nature of isoparaffin by CH carbon.
Figure 3. Ratio of CH-c/total CH carbons: (a) correlation with ABN, (b) correlation with ABN/ACN.
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Figure 4. Correlation between ratio ABN/ACN and ratio CH-x/total carbons in lubricant base oil (x ) a-h).
present in the highest proportion. The decreasing trend of the CH-c group proportion is consequently interpreted as a decrease in the eighth or inner carbons caused by shortening of carbon chain length by the hydrocracking reaction.6 Because the proportions of CH-b (4-methyl branching) and CH-d (7methyl branching) are both almost constant regardless of ratio ABN/ACN, 5- and 6-methyl branchings are also expected to be constant. 4-, 5-, and 6-ethyl branchings are also expected to be constant for the same reason. Therefore, the increasing trend in CH-g that includes 5- and 6-methyl and 4-, 5-, and 6-ethyl branching seems rather unusual. The increasing trend can be attributed to the increase in alkyl-substituted naphthenes, the formation of which was observed by FD-mass analysis.6 The CH-c group is present in the highest proportion because the carbons belonging to the CH-c group are present in the highest proportion. To exclude the influence of existing carbon numbers, the probability of methyl branching formation at a carbon was evaluated with eq4, assuming all of the branches are methyl branches for the calculation of existing carbon numbers of CH-x groups (x ) a-h). probability of methyl branching in a carbon ) normalized peak area of CH-xcarbon ⁄ existing carbon numbers of CH-xgroup (4) where, existing carbon numbers ) 2 (for CH-a, -b, -d, -e, and -h), and existing carbon numbers ) ACN - ABN 14 (for the CH-c group). The CH-a and CH-e groups that correspond to 2-methyl and 3-methyl branches respectively are present in the highest proportion when the ratio of ABN/ACN is low (Figure 5). The proportion of 2-methyl branching increased with the increase in the ratio of ABN/ACN (i.e., increase in density of branching). Schulz and Weitkamp found that predominant isomers from hydrocracking of n-dodecane, i.e., isoparaffins with
Kobayashi et al.
Figure 5. Probability of methyl branching formation at a carbon in lubricant base oil.
carbon numbers of 9 or less were the 2-methyl isomers,20 whereas the 3-methyl isomer was the most easily formed by isomerization of n-dodecane, and the formation of 2-methyl branching is surprisingly low.7,21 Because the ACN of lubricant oils formed decreased in the progress of isomerization, the oils contain a certain amount of hydrocracked molecules. The highest proportion of 2-methyl branching and the increasing trend can be understood based on the contribution of the hydrocracking reaction that preferably forms 2-methyl branching. The proportion of the second carbon that is occupied by methyl branching was calculated with eq 5. proportion of 2nd carbon occupied by methyl branching (%) ) [ABN] × [normalized peak area of CH - a carbons] ⁄ 2 (5)
Figure 6 indicates that, as the ratio of ABN/ACN increased, the proportion of the second carbon occupied by methyl branching increased in all feedstocks. Comparing the proportion of occupation in lubricant oils at the same ratio of ABN/ACN, AO-2 has the highest proportion of the second carbon occupied by methyl branching. Because AO-2 has the highest ACN, the highest 2-methyl branching in AO-2 is considered to be due to the biggest contribution of hydrocracking that forms 2-methyl branching in the lubricant base oil fraction (360 °C+). The probability of 4-methyl branching is approximately half of that of 3-methyl branching. The probabilities of 7-methyl (CH-d) and 8 or inner methyl (CH-c) branching are almost same and are the lowest. Overall, the outer position of the carbon chain is preferable for branching, and the probability of the fourth or inner methyl branching is almost equal. (19) Sie, S. T. Acid-Catalyzed Cracking of Paraffinic Hydrocarbons. 3. Evidence for the Protonated Cyclopropane Mechanism from Hydrocracking/ Hydroisomerization Experiments. Ind. Eng. Chem. Res. 1993, 32, 403– 408. (20) Schulz, H. F.; Weitkamp, J. H. Hydrocracking and Hydroisomerization of n-Dodecane. Ind. Eng. Chem. Prod. Res. DeV. 1972, 11, 46–53. (21) Weitkamp, J. Isomerization of Long-Chain n-Alkanes on a Pt/CaY Zeolite Catalyst. Ind. Eng. Chem. Prod. Res. DeV. 1982, 21, 550–558.
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Figure 6. Probability of the second carbon being occupied by methyl branching.
Figure 8. Correlation between the ratio of ABN/ACN and CH-x/total CH carbons in diesel oil (x ) a-h).
Figure 7. Chemical shifts of dimethyl isoparaffins with close branches.
Figure 7 shows schematically the 3-dimethyl isoparaffins that have branches close to each other. Each of them has distinctive chemical shifts, but the observed spectra do not, or rarely, show such peaks. When two branches are adjoined (Type A), the chemical shift of CH carbons is 36.09 ppm according to the ACD/CNMR predictor, but these peaks were absent or rarely observed in this region. The distinct chemical shift of dimethyl paraffin with two CH2 carbons between branching (Type C, 31.64 ppm) was also not observed. The chemical shift of CH carbons in the Type B structure is 28.03 ppm and is difficult to separate from that of the 2-methyl branches. However, because the CH2 carbons between branches were not observed (44.47 ppm), this structure is also regarded as nonexistent. These results indicated that, even when isoparaffins have two or more branches, the branches are not close to each other, and exist independently in terms of the 13C NMR chemical shift. The absence of close branches can be understood based on the consumption of Type B structure by the β-scission reaction.9 Type A and C structures are also easily converted after an alkyl shift to Type B.7,9
Figure 9. Probability of methyl branching formation at a carbon in diesel oil.
3.2. Branching of Diesel Oil. Because the yield of diesel oils obtained was much greater than that in original feedstock (FTW-1, 5.6 wt %; FTW-2, 2.5 wt %), analyzed diesel oils were formed mostly through the hydrocracking reaction of paraffins with a larger carbon number (Table 1). Diesel oil fractions displayed rather different trends from those in lubricant base oils (Figure 8).
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Except for CH-g that contains 5 types of branching, the CH-a group fraction that corresponds to 2-methyl branching was the highest at lower values of the ratio of ABN/ACN and decreased with an increase in the ratio of ABN/ACN. The slightly decreasing trend in the proportion of 2-methyl branching is due to the formation of branching at other sites, whereas the proportion of the second carbon occupied by methyl branching is almost constant (Figure 6). Comparing the probability of methyl branching formation at a carbon (Figure 9), 2-methyl branching was predominant and the probabilities of other methyl branching are at most 5%. Much higher existence of 2-methyl branching can be explained by the carbenium ion mechanism that proceeds after preisomerization of n-paraffins.7 With the carbenium ion mechanism, scission occurs at the C-C bond in the β position of the carbenium ion (β-scission) that can be formed by hydride transfer from a paraffin molecule. To reduce the energy barrier of β-scission, preisomerization to a dibranched or tribranched isomer is required. The resultant molecules formed by hydrocracking (β-scission) from these branched isomers have methyl branching at the second position. The protonated cyclopropane mechanism proposed by Sie also explains the formation of 2-methyl branching in the products of hydrocracking.19 4. Conclusion Two FT waxes and two R-olefins with different carbon number distributions were converted to diesel and lubricant base oils under several isomerization/hydrocracking reaction conditions, and the molecular structures of these fractions were investigated in detail with the 13C NMR DEPT method.
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The investigation of CH carbons provided an effective way to determine the molecular structures of isoparaffins in oils formed from FT waxes (and R-olefins). The ratio of segmented CH carbon peak areas correlated well with the ratio of ABN/ ACN, irrespective of the different carbon number distributions of feedstocks and reaction conditions. This result suggested that the position and length of branching could be determined only by the density of branching (ABN/ACN) in isoparaffins formed. In other words, when the ABN and ACN are determined, the position and length of the branches are determined. These findings explained the observation in the previous study6 that viscosity properties of prepared lubricant base oils are determined by the ABN and ACN alone and are consistent with the known fact that the position of branching affects the viscosity properties. The lubricant base oils formed mostly through isomerization of waxes showed drastic changes in branching positions with the density of branching. The isoparaffins with 2-methyl and 3-methyl branches are most likely to form at low ratio of ABN/ ACN, but the ratio of 2-methyl branching increased with an increase in the ratio of ABN/ACN, and 2-methyl branching was the most abundant with the increased ratio of ABN/ACN. The increasing trend could be understood based on the contribution of hydrocracking that preferably forms 2-methyl branching. The change in branching is somewhat less in diesel oil fractions that form mostly through hydrocracking of isoparaffins, but it is observed that isomers with 2-methyl branching, which are expected to form by hydrocracking of paraffins, are predominant. EF800530P
