Energy & Fuels 1996, 10, 91-96
Structural Characteristics and Removal of Visible-Fluorescence Species in Hydrodesulfurized Diesel Oil Xiaoliang Ma, Kinya Sakanishi, Takaaki Isoda, Shinichi Nagao, and Isao Mochida* Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816, Japan Received May 23, 1995. Revised Manuscript Received October 16, 1995X
The fluorescence color of a gas oil and its desulfurized oils was evaluated by using a fluorescence spectrophotometer. The visible-fluorescence species existing in the deeply desulfurized oil were separated by HPLC and identified by GC-MS (mass spectrometry), and their fluorescence emission spectra were examined and compared with those of representative polycyclic aromatic hydrocarbons. Anthracene, fluoranthene, and their alkyl derivatives are concluded to be major fluorescence species existing in the desulfurized oil at the concentration of about 10 ppmw. The color of oil was easily removed by their partial hydrogenation over NiMo catalyst at a relatively lower temperature of 280 °C for 10 min under a low hydrogen pressure of 2.9 MPa. The partial hydrogenation was ascertained to remove the fluorescence color of the model compounds.
Introduction The reduction of sulfur level in diesel fuel to less than 500 ppmw or less has been regulated in some advanced countries1,2 and will be regulated very soon in other advanced countries for the environmental protection.3-5 The Japanese refineries are very sensitive to the specification of diesel fuel in terms of the color, since the color is one of the quality specifications of diesel fuel.6,7 Deep desulfurization under higher hydrogen pressure is of great concern because it raises the refining cost, although it has been recognized as effective in suppressing the color formation on deep desulfurization. A higher reaction temperature is another way to increase the HDS rate without increase in hydrogen pressure; however, it always emphasizes the color formation.7-10 Consequently, it is an urgent task to identify the color Abstract published in Advance ACS Abstracts, November 15, 1995. (1) Parkinson, G. Stricter Standards for California Diesel Fuel. Chem. Eng. 1989, 96, 42. (2) Federal Register. Regulation of Fuels and Fuel Additives: Fuel Quality Regulations for Highway Diesel Fuel Sold in 1993 and Later Calendar Years. Government Printing Office: Washington, DC, Aug 21 1990; Vol. 55, No. 162, p 34120. (3) Wallace, G. M. European Diesel FuelsA Review of Changes in Product Quality 1986-1989. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. Prepr. 1990, 35, 1080. (4) Tsutsumoto, N. Improvement in Diesel Engine for Automobile. Petrotech 1990, 13, 998. (5) Froment, G. F.; Depauw, G. A.; Vanrysselberghe, V. Kinetic Modeling and Reactor Simulation in Hydrodesulfurization of Oil Fractions. Ind. Eng. Chem. Res. 1994, 33, 2975. (6) Takatsuka, T.; Wada, Y. Deep Desulfurization of Diesel Fuel. Shokubai 1991, 33, 306. (7) Takatsuka, T.; Wada, Y.; Suzuki, H.; Komatsu, S.; Morimura, Y. Deep Desulfurization of Diesel Fuel and Its Color Degradation. J. Jpn. Pet. Inst. 1992, 35, 179-184. (8) Wada, Y.; Takatsuka, T. The Main Point in Deep Desulfurization of Diesel Fuel. Petrotech 1990, 13, 418. (9) Sakanishi, K.; Ando, M.; Abe, S.; Mochida, I. Extensive Desulfurization of Diesel Fuel through Catalytic Two-stage Hydrotreatment. Sekiyu Gakkaishi 1991, 34, 553. (10) Sakanishi, K.; Ando, M.; Mochida, I. Extensive Desulfurization of Diesel Fuel through Catalytic Two-stage Hydrotreatment (Part 2): Influence of Reaction Pressure on Desulfurization and Reactivities of Refractory Sulfur Compounds. Sekiyu Gakkaishi 1992, 35, 402. X
species and to remove them during the deep desulfurization process. Takatsuka and co-workers7 have correlated the color of the product oil, measured by Saybolt colorimeter, with the desulfurization extent. They suspected that the color species existing in the deeply desulfurized oil were polycyclic aromatic hydrocarbons (PAH) newly formed as a part of the desulfurization products, but they did not identify their chemical structures. The analysis of color species in desulfurized oil is a difficult task because their content is in trace amounts. Recently, Wakita and co-workers11 identified color species by means of highperformance liquid chromatography (HPLC) and FDMS (mass spectrometry). They assumed alkyldibenzophenanthrenes with 5-6 alkyl carbon atoms and alkylbenzoperylenes with 1-7 alkyl carbon atoms to be major fluorescence color species in their hydrodesulfurized gas oil. However, they did not show any spectroscopic evidence for such species. Considerable attention has been paid to the analysis of PAH in diesel fuel, because they are suggested to be associated with particulate emissions and mutagenic compounds in the exhaust from diesel engines.12-17 (11) Wakita, M.; Yanazawa, K.; Matunaga, A. Structural Determination of Fluorescent Compounds in Low-sulfur Gas Oil Derived from Deep Desulfurization. Sekiyu Gakkaishi 1994, 37, 561-568. (12) Miller, C.; Weaver, C. S.; Johnson, W. Diesel Fuel Quality Effects on Emissions, Durability and Performance. Final Report EPA Contract 68-01-65443, Sept. 30, 1985. (13) Williams, P. T.; Bartle, K. D.; Andrews, G. E. The Relation between Polycyclic Aromatic Compounds in Diesel Fuel and Exhaust Particulates. Fuel 1986, 65, 1150. (14) Davies, I. L.; Raynor, M. W.; Williams, P. T.; Andrews, G. E.; Bartle, K. D. Application of Automated On-Line Microbore HighPerformance Liquid Chromatography/Capillary Gas Chromatography to Diesel Exhaust Particulates. Anal. Chem. 1987, 59, 2579. (15) Davies, I. L.; Bartle, K. D. On-Line Fractionation and Identification of Diesel Fuel Polycyclic Aromatic Compounds by TwoDimensional Microbore High-Performance Liquid Chromatography/ Capillary Gas Chromatography. Anal. Chem. 1988, 60, 204. (16) Ullman, T. L. Investigation of the Effects of Fuel Composition on Heavy-Duty Diesel Engine Emissions. SAE Paper 892072, International Fuels and Lubricants Meeting and Exposition, Baltimore, Maryland, September 26-28, 1989.
© 1996 American Chemical Society
Energy & Fuels, Vol. 10, No. 1, 1996
Ma et al.
Table 1. Properties and Composition of Gas Oil total sulfur (wt %) density (15 °C) (g/mL) pour point (°C) boiling range (ASTM) (°C) IBP 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% 95% FBP composition (FIA, vol %) aromatics olefins saturates
0.706 0.8422 -10 232.0 257.5 266.5 276.0 281.5 286.5 291.5 297.0 303.0 310.5 321.0 329.5 338.5
Table 2. Chemical Composition and Physical Properties of Catalysts KF-742a (CoMo) chemical composition (wt %) MoO3 CoO NiO SiO2 SO4 P physical properties surface area (m2/g) pore volume (mL/g) shape average diameter average length a
17 5 78
In previous studies,18,19 the present authors reported that the fluorescence color formation during desulfurization process depended on reaction temperature and hydrogen pressure. High reaction temperature (>360 °C) deepened the color of desulfurized oil, whereas high hydrogen pressure suppressed the color formation. In another study,20 the authors found that heavier fractions (>340 °C) of gas oil and their desulfurized product oils exhibited the deep fluorescence color, while no fluorescence color was observed in the lighter fractions (95%), 2,3-benzophenanthrene (GC purity >99%), fluoranthene (GC purity >98%), 1-phenylnaphthalene (GC purity >95%), 2-phenylnaphthalene (GC purity >95%), and cyclohexane were obtained from the Tokyo Chemical Industry Co. Biphenyl (GC purity >98%), phenanthrene (GC purity >95%), (17) Dzidic, I.; Petersen, H. A.; Wadsworth, P. A.; Hart, H. V. Townsend Discharge Nitric Oxide Chemical Ionization Gas Chromatography/Mass Spectrometry for Hydrocarbon Analysis of the Middle Distillates. Anal. Chem. 1992, 64, 2227. (18) Sakanishi, K.; Ma, X.; Mochida, I. Three-stage Deep Hydrodesulfurization of Diesel Fuel under 30 kg/cm2 H2 Pressure without Color Development. J. Jpn. Pet. Inst. 1994, 37, 368. (19) Ma, X.; Sakanishi, K.; Mochida, I. Three-stage Deep Hydrodesulfurization and Decolorization of Diesel Fuel with CoMo and NiMo Catalysts at Relatively Low Pressure. Fuel 1994, 73, 1667. (20) Ma, X.; Sakanishi, K.; Isoda, T.; Mochida, I. Hydrodesulfurization Reactivities of Narrow-Cut Fractions in a Gas Oil. Ind. Eng. Chem. Res. 1995, 34, 748.
KF-842a (NiMo) 14.9
3.1 4.8 0.65
268 0.53 four leaves 1.4 × 1.2 2.8
273 0.52 four leaves 1.3 × 1.1 3.5
anthracene (GC purity >98%), 9,10-dihydroanthracene (GC purity >95%), and pyrene (GC purity >98%) were obtained from Wako Chemicals. Acenaphthene (purity >99%) and naphthacene (GC purity >97%) were obtained from Aldrich Chemical Co. These reagents were used without further purification. 1,2,3,10b-Tetrahydrofluoranthene was prepared by hydrogenation of fluoranthene over a NiMo catalyst and was purified by recrystallization in n-hexane,21 its GC purity being 93%. 2. Catalysts. Three commercial catalysts were used in the present study. KF-742 (CoMo/Al2O3) and KF-842 (NiMo/Al2O3) were obtained from Nippon Ketjen Co. Their properties and composition are listed in Table 2. Both catalysts were presulfided before use as described in a previous paper.19 Palladium catalyst (Pd/Al2O3, Pd: 5 wt %) obtained from N. E. Kemukyatto Co. was used without further treatment. 3. Hydrodesulfurization (HDS) and Hydrogenation. Both hydrodesulfurization and color removal were performed in a 50-mL magnetically stirred (1000 rpm) batch autoclave with the feed oil charge of 10 g at a catalyst-to-oil weight ratio of 0.1. The heating rate from 100 °C to the reaction temperature was ca. 25 °C/min, and the cooling rate was ca. 30 °C/ min. The reaction time was counted from the moment when the temperature of the reactor reached the prescribed level. The total reaction pressure was controlled at 2.9 MPa throughout the reaction by adding gaseous hydrogen to compensate for its consumption. 4. Separation and Identification. The fluorescence color of the oils was analyzed by RF-5000S spectrofluorophotometer (Shimadzu Co.). The concentration of the feed and desulfurized oils for the fluorescence measurement was fixed at 5.0 wt % in cyclohexane, and the concentration of standard aromatic compounds was 10 ppmw in cyclohexane or benzene. The fluorescence spectra were corrected for the wavelength dependence of the source intensity and detector sensitivity. The separation and identification schemes of hydrodesulfurized oil, which was derived from the feed by HDS reaction at 380 °C and 2.9 MPa over CoMo catalyst, are shown in Figure 1. The oil was first separated into seven fractions by HPLC equipped with Unisil Q100-10 column (6.0 Å, ∼250 mm) and UV-vis detector (L4200, Hitachi Co.), using 254 nm wavelength. The mobile phase was HPLC grade n-hexane and its flow rate was fixed at 2 mL/min. The seven fractions were collected and separated from the solvent by a rotary vacuum evaporator. Each fraction was weighed to determine its yield as shown in Figure 1. The quantitative analysis of each fraction was carried out using gas chromatography (Yanaco G6800) with a flame ionization detector (FID) and Shimadzu GC-Mass (GC-17A, QP-5000, quadrupole mass spectrometer, (21) Mochida, I.; Otani, K.; Korai, Y. Efficiency of Hydrogen Transferring Liquefaction of a Subbituminous Coal Using Hydrogenated Fluoranthene for Short Contact Time at High Temperature and Low Pressure. Fuel 1985, 64, 906-910.
Visible Fluorescence Species in Diesel Oil
Figure 1. Separation and identification scheme of hydrodesulfurization gas oil.
Figure 2. Fluorescence emission and excitation spectra of desulfurized gas oil. Desulfurization conditions: 360 °C, 2.9 MPa, 20 min, KF-742 catalyst. ion source EI mode). Both were equipped with a methyl silicone capillary column (0.25 mm, 50 m, liquid phase 007 series methyl silicone, Quadrex Co.). The oven temperature was programmed from 90 to 300 °C at 7 °C/min and then kept at 300 °C. Some peaks were identified by comparing mass spectra and retention time with those of authentic standards. Most of other peaks were tentatively identified by their ionic molecular peaks (m/z). Some species of interest were further verified by comparing the fluorescence spectra of authentic samples with those of HPLC fractions derived from the desulfurized oil. The concentration of HPLC fractions for fluorescence measurement was 0.040 wt % in cyclohexane.
Results 1. Fluorescence Color Characteristics of Desulfurized Gas Oil. Fluorescence spectra of a product oil desulfurized under commercial reaction conditions (360 °C, 2.9 MPa over KF-742 catalyst) are illustrated in Figure 2A, using different excitation wavelengths of 320, 340, 360, 380, and 400 nm. The different exciting wavelengths gave different fluorescence emission spectra. Among five different excitation wavelengths, the wavelength of 360 nm excited the fluorescence emission most strongly in the visible range from 400 to 550 nm.
Energy & Fuels, Vol. 10, No. 1, 1996 93
Figure 3. Fluorescence emission spectra of initial feed and product oils. Exciting wavelength: 364 nm. (1) Feed. (2, 3, 4, and 5) Product oils desulfurized over KF-742 catalyst under 2.9 MPa by 20 min at 380, 360, 340, and 320 °C, respectively. (6) Deeply desulfurized oil at 360 °C, 2.9 MPa with sulfur content of 0.035 wt %. (7) Decolorized oil derived from deeply desulfurized oil (6) by hydrotreatment at 280 °C, 2.9 MPa, 10 min over KF-824 catalyst.
Figure 2B illustrates an excitation spectrum (6) of the desulfurized oil at an emission wavelength of 438 nm and a fluorescence emission spectrum (7) at an excitation wavelength of 364 nm. Thus, the excitation at 364 nm allowed the desulfurized oil to emit the strongest fluorescence spectrum with a peak at 438 nm. The fluorescence emission spectra of the initial feed and oils desulfurized at different reaction temperatures are illustrated in Figure 3, where an exciting wavelength of 364 nm was applied. All oils showed fluorescence in the visible range from 400 to 550 nm with a peak around 438 nm being bluish. The original feed exhibited fluorescence in this region, although the highest peak was located at 410 nm. Hydrodesulfurization reduced the fluorescence intensity when the reaction was performed at lower temperatures (for example 320 °C, spectrum 5 in Figure 3). On the contrary, it sharply increased the fluorescence intensity at higher reaction temperatures. Since the emission spectra of desulfurized oils usually exhibited a peak around 438 nm by the excitation wavelength at 364 nm, their fluorescent intensities at 438 nm were defined as RFI (relative fluorescence intensity) for their comparison. 2. Separation and Identification of the VisibleFluorescence Species in the Desulfurized Oil. The oil desulfurized at 380 °C (chromatogram 2) with RFI of 45 was further separated into seven fractions according to the retention times. Figure 4 shows the fluorescence emission spectra of the fractions. Fractions A, B, and C showed essentially no emission in the visible range. Fraction D showed a weak emission with a maximum peak at about 405 nm. Fraction E exhibited a significant emission with two peaks at 375 and 415 nm, respectively. Fractions F and G emitted very strong fluorescence, both having similar peaks at about 440 nm. Their RFI values were about 3 and 4.5 times larger, respectively, than that of fraction E. Major componds in each fraction identified by GC and GC-MS are listed as follows: fraction A, saturates with major constituents of C8-C24 n-alkanes and branched/ cyclic alkanes; fraction B, monocyclic aromatic hydrocarbons; fraction C, naphthalene and its alkyl derivatives with 1-3 alkyl carbons atoms; fraction D, alkylnaphthalenes with 2-8 alkyl carbon atoms, biphenyl and its alkyl derivatives with 1-2 alkyl carbon atoms,
Energy & Fuels, Vol. 10, No. 1, 1996
Figure 4. Fluorescence emission spectra of HPLC fractions. Sample concentration: 0.04 wt %.
and alkylacenaphthenes with 2-3 alkyl carbon atoms; fraction E, alkyl biphenyls, fluorene, and its alkyl derivatives, phenanthrene and its alkyl derivatives with 1-3 alkyl carbon atoms, and anthracenes (ca. 60 ppm); fraction F, alkyl biphenyls with 3 alkyl carbon atoms, alkylfluorenes with 2-4 alkyl carbon atoms, alkylphenathrenes/alkylanthracenes with 2-4 alkyl carbon atoms, and alkyl phenylnaphthalenes, and fluoranthene (ca. 9 ppm) and methylfluoranthenes/methylpyrenes; fraction G, alkyl fluorenes with 3-4 alkyl carbon atoms, alkyl phenylnaphthalenes with 2-4 alkyl carbon atoms, alkylfluoranthenes with 2-3 alkyl carbon atoms, and alkylbiphenyls with 3-6 alkyl carbon atoms. It is noted that fluorescence color species are principally contained in the fractions E to G, which exhibited the strong fluorescence peaks in the wavelength range from 400 to 500 nm, suggestting that tri- and tetracyclic aromatic hydrocarbons should be the fluorescence color species in the desulfurized oil. 3. RFI of Representative PAH Models. RFI values of representative PAH models measured are listed in Table 3. The major fluorescence emission peaks in the range of >300 nm, and the absorption peaks in the visible range of the models obtained from refs 22 and 23 are also listed in the table for comparison. Most of di- or tricyclic aromatic compounds, including indene, diphenylnaphthalene, acenaphthene, fluorene, phenanthrene, 1-phenylnaphthalene, and 2-phenylnaphthalene, scarcely displayed an emission in the visible range (400-780 nm). Only anthracene exhibited a strong fluorescence in this range with two major peaks at 420 and 444 nm, respectively, the largest one being at 397 nm in the ultraviolet range. Fluoranthene also exhibited a very strong emission with the peaks at the wavelengths of 440 and 460 nm, their relative intensities being 22.4 and 25.6, respectively. 9,10-Dihydroanthracene, a main hydrogenation derivative from anthracene, emitted no fluorescence in the visible range. The partial hydrogenation of anthracene to 9,10-dihydroanthracene completely removed the fluorescence emission in the visible range. No fluorescence emission was observed with 1,2,3,10b-tetrahydrofluoranthene which is a major hydrogenated product from (22) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules; Academic Press: New York, 1971. (23) Grasselli, J. G.; Ritchey, W. M. Atlas of Spectral Data and Physical Constants for Organic Compounds; CRC Press: Boca Raton, FL, 1975.
Ma et al.
fluoranthene. These results indicate that the partial hydrogenation of tri- and tetracyclic aromatics may be enough to remove their fluorescence color. 4. Removal of Fluorescence Color in Deeply Desulfurized Oil. Removal of fluorescent color in deeply desulfurized oil was performed by hydrogenation at 2.9 MPa over Pd and KF-842 catalysts. The deeply desulfurized oil (no. 6 in Figure 3) carried sulfur content of 0.035 wt % and RFI of 32.0. Pd catalyst was very effective at the temperature below 300 °C to remove the fluorescence as shown in Figure 5, where RFI was reduced from 32.0 in the feed to less than 3 at 300 °C and 0.3 at 140-180 °C both for 10 min. The NiMo catalyst was also effective at a temperature range of 260-280 °C, RFI being reduced to a minimum of 4.0. The corresponding fluorescence spectrum as shown in Figure 3 (spectrum 7) indicates that such hydrotreatment removed the most of fluorescence color of deeply desulfurized oil in the visible range. Removal of the fluorescence became inefficient below 240 °C over the NiMo catalyst. Figure 6 shows that the first 10 min reaction sharply reduced RFI from 32.0 of the feed to 5.0 of the hydrogenated oil at 260 °C, 2.9 MPa over KF 842 catalyst. Longer reaction time slightly reduced the RFI, being 1.4 at 30 min. Discussion By identifying the PAH in the desulfurized oil and measuring their fluorescence emission spectra, the present study revealed that anthracene, fluoranthene, and their alkylated derivatives are major origins of fluorescence color in the oil. Anthracene and its alkyl derivatives dominantly show the violet fluorescence in the wavelength range of 400-440 nm, while fluoranthene and its alkyl derivatives do the blue fluorescence in the range of 420-500 nm. The starting oil carried some fluorescence color which appears to originate also from the same species, particularly from anthracene and its alkyl derivatives. The present study also revealed that the fluorescence emission of PAH in the visible range may depend mainly on their aromatic skeletons, while their alkyl substituents have a minor influence on the shift of their spectra. The partial hydrogenation of PAH, which shortens the conjugation of polyaromatic rings, removed the fluorescence in the visible range of both model PAH and the desulfurized oil. Hydrodesulfurization at lower reaction temperatures, where the condensation reaction is minimized and the hydrogenation of polyaromatic rings is thermodynamically favorable, suppresses the formation of the fluorescence color species even under low hydrogen pressure, although a satisfactory level of deep desulfurization is not achieved under such conditions. Higher reaction temperature under low hydrogen pressure desulfurized even the refractory sulfur compounds such as 4-methyl- and 4,6-dimethyldibenzothiophene, achieving the required level (S < 500 ppm) of desulfurization; however, a strong color developed in the product oil, exceeding the fluorescence intensity of the starting oil. The hydrogenation of polyaromatic rings hardly proceeds under the high-temperature and low-pressure conditions. What is worse, color species are newly produced during the desulfurization reaction
Visible Fluorescence Species in Diesel Oil
Energy & Fuels, Vol. 10, No. 1, 1996 95 Table 3. RFI of Representative PAHs
aromatic compounds name
absorptiona in visible range (), nm
Dicyclic Aromatics 116
Tricyclic Aromatics 154
400, 380, 422
512, 480, 550
Tetracyclic Aromatics 216
Obtained from ref 22. b Wavelength at the maximum intensity. c Relative fluorescence intensity at 438 nm.
Energy & Fuels, Vol. 10, No. 1, 1996
Figure 5. Effect of reaction temperature on removal of fluorescence in desulfurized oil. Reaction conditions: 2.9 MPa, 10 min. Feed: deeply desulfurized oil at 360 °C, 2.9 MPa with RFI of 30.2 and sulfur content of 0.035 wt %.
Ma et al.
temperature is thermodynamically favorable for the hydrogenation of polyaromatic compounds.24-26 The efficiency of the two- or three-stage processes for deep desulfurization with color removal proposed in our previous papers18,19 is thus confirmed. The Pd catalyst appears very effective for the color removal of desulfurized oil at the lower temperatures, below 200 °C. However, H2S produced in the preceding desulfurization stage has to be excluded because of its poisoning by H2S. The NiMo has a significant activity for the color removal at the temperature of 260-280 °C, and it is tolerant against H2S. Thus, the same NiMo catalyst may be applicable to the first and the second stages at different reaction temperatures. A combined use of CoMo and NiMo catalysts is also proposed for the higher efficiencies of the desulfurization in the first stage and the hydrogenation in the second stage, respectively.18,19 Conclusions
Figure 6. Effect of reaction time on removal of fluorescence in desulfurized oil. Reaction conditions: 260 °C, 2.9 MPa, NiMo catalyst. The feed is the same as described in Figure 5.
under such conditions, increasing the fluorescence emission in the region of 440-550 nm. Based on the present study, a post-hydrogenation at a rather low temperature can be proposed, which can effectively reduce the color in the deeply desulfurized oil under the relatively low hydrogen pressure. The low
1. The desulfurized oil exhibits strong fluorescence color in the visible range with a peak at 438 nm, which corresponds to the excitation at the wavelength of 364 nm. 2. Anthracene, fluoranthene, and their alkyl derivatives are concluded to be the major fluorescence species which exist in desulfurized oil with about 10 ppmw concentration. 3. The color species can be removed by the partial hydrogenation of such tri- and tetracyclic aromatics in the desulfurized oil using NiMo catalyst at relatively lower temperatures below 300 °C under the low hydrogen pressure of around 3 MPa. EF950097O (24) Frye, C. G. Equilibria in the Hydrogenation of Polycyclic Aromatics. J. Chem. Eng. Data 1962, 7, 592. (25) Assim, M. Y.; Yoes, J. R. Confronting New Challenges in Distillate Hydrotreating. AM-8-59, NPRA AM, March 1987. (26) Stanislaus, A.; Cooper, B. H. Aromatic Hydrogenation Catalysis: A Review. Catal. Rev. Sci. Eng. 1994, 36, 75.