Energy & Fuels 2000, 14, 539-544
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Identification and Reactivity of Nitrogen Molecular Species in Gas Oils Seunghan Shin, Kinya Sakanishi, and Isao Mochida* Institute of Advanced Material Study, Kyushu University, Kasugakoen, Kasugasi, Fukuoka 816, Japan
David A. Grudoski and John H. Shinn Chevron Research and Technology Company, Chevron Way 100, Richmond, California Received June 23, 1999. Revised Manuscript Received February 3, 2000
Nitrogen molecular species in several gas oils were analyzed by gas chromatograph with an atomic emission detector (GC-AED) and mass spectrometer (GC-MS). Nitrogen species in gas oils were divided through acidic extraction into basic species (such as aniline, quinoline, benzoquinoline, and their derivatives) and nonbasic species (such as indole, carbazole, and their derivatives). To be mostly identified, their distribution depended on the cutting point and origins of gas oils. Denitrogenation reactivities of nitrogen species in gas oils were followed in the hydrotreating reactions at 340 °C under 5MPa of H2 to quantify by GC-AED their respective reactivities in GOs. The reactivities orders and reactivity dependence on their chemical structures and matrix compositions are discussed on molecular bases. The reactivity order was found as indole > methylated aniline > methylated indole > quinoline > benzoquinoline > methylated benzoquinoline > carbazole > methylated carbazoles. The number and position of methyl groups were very influential on the reactivities of carbazole derivatives. Methyl groups neighboring the N atom inhibited remarkably the HDN reactions. Comparison of the reactivities of the same species in the GOs must be discussed by taking account of all components and products in the respective oil.
1. Introduction Heteroatoms in the fossil fuel have been a concern in three aspects: environmental concern to reduce the pollutants of NO and SO2 in the exhaust gas, process efficiency to reduce the catalyst poison and plugging, and product stability.1-4 Hence, better process by better catalyst has been sought extensively. For this purpose, molecular identification of heteroatom species and their reactivities in the variable gas oils strongly need to be clarified. Molecular level analyses of sulfur species and their reactivity have been documented.5 The analyses of nitrogen substances in fossil resources are quite challenging. This is because nitrogen substances exist at low concentration level and the matrix of even gas oil is complex mixture of hydrocarbons. Recently, gas chromatography with atomic emission detector (GC-AED) has been developed for this purpose. GC-AED is a technique that can detect the element of interest by using the element selective detector.6,7 * Corresponding author. (1) Dorbon, M.; Ignatiadis, I.; Schmitter, J. M.; Arpino, P.; Guiochon, G.; Toulhoat, H.; Huc, A. Fuel 1984, 63, 565-570. (2) Fathoni, A. Z.; Batts, B. D. Energy Fuels 1992, 6, 681-693. (3) Schmitter, J. M.; Vajta, Z.; Arpino, P. J. Advances in Organic Geochemistry 1979; Pergamon Press: Oxford, 1980; pp 67-76. (4) Dong, M. W.; Locke, D. C.; Hoffman, D. Environ. Sci. Technol. 1977, 11, 773-775. (5) Whitehurst, D. D.; Isoda, T.; Mochida, I. Adv. Catal. 1998, 42, 345-471. (6) Albro, T. G.; Dreifuss, P. A.; Wormsbecher, R. F. HRC&CC 1993, 16, 13-17.
However, there are few reports on the detail analyses of nitrogen species in petroleum products by GC-AED. In this study, nitrogen species in gas oils (GOs) were identified with GC-AED and GC-MS to the molecular level. The denitrogenation reactivities of respective nitrogen compounds in GOs were also followed by using GC-AED to find structure/reactivity correlation in HDN. Type of nitrogen species, and number and location of methyl groups are concerned to establish the correlation. The competitive and inhibiting effects of the GO matrix on the HDN reactivities of nitrogen species also need to be clarified. GOs were extracted with sulfuric acid to separate basic and nonbasic nitrogen species groups in order to ensure the gas-chromatographic separation of the components in the respective groups. 2. Experimental Section 2.1. Materials. Three kinds of gas oils, such as light cycle oil (LCO), medium cycle oil (MCO), and a straight gas oil from Furrial crude prepared in Facilities Separation Laboratory, Chevron (FSL), were used in this study. Some representative properties of gas oils are summarized in Table 1. LCO and MCO carried different cutting points. 2.2. Measurement. Hewlett-Packard GC-AED (HP6890/ G2350A) and GC-MS (HP6890/5973) were used for the molecular characterization of nitrogen species in gas oils. In both (7) Quimby, V.; Giarrocco, J.; McCleary, K. A. HRC&CC 1993, 15, 705-709.
10.1021/ef990136m CCC: $19.00 © 2000 American Chemical Society Published on Web 03/30/2000
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Table 1. Composition of Various Gas Oils boiling range, °C density (15 °C) carbon, wt %b hydrogen, wt %b sulfur, wt %c nitrogen, ppmb a
LCO
MCO
FSL
180-343 0.89 89.90 9.56 0.42 345
NDAa 0.89 90.19 9.06 0.47 695
208-275 NDA 85.97 13.05 0.91 295
NDA: no data available. b Elemental analysis. c GC-AED. Table 2. Chromatographic Conditions for Nitrogen Analysis GC conditions inlet volume, µL inlet mode split ratio inlet temperature, °C inlet pressure, psi oven temperature, °C AED conditions transfer line temperature, °C cavity temperature, °C hydrogen reagent, psi oxygen reagent, psi methane reagent, psi makeup gas, mL/min
1 split 10 280 11.8 40-320 by 10 °C/min nitrogen by 388 nm 350 350 40 80 50 230
Table 3. HDN Reaction Condition of Gas Oils catalyst catalyst/oil by weight temperature, °C H2 pressure, MPa time, min
NiMo/Al2O3 0.1 340 5 10,20,30,60,90
instruments, a HP-1MS column was used in the temperature range 40-320 °C by 10 °C/min of heating rate. As for GCAED, N388 emission line was selected for the measurement of nitrogen species in gas oil due to its high selectivity for nitrogen.8 Chromatographic conditions for N388 analysis are described in Table 2. Neat gas oil was injected for GC-AED analysis. Identification of nitrogen species in gas oils was performed basically by comparing the retention times of target molecule and standard material. In this study, the assignment of carbazole derivatives was carried out by comparison of retention time. Prof. Kuroki supplied some carbazole derivatives and retention time data of other species for this assignment. Other nitrogen species were identified with GC-MS by analyzing their fragmentation patterns. Basic nitrogen was extracted from gas oil by separation funnel with sulfuric acid solution according to ref 9. Basic species were also identified with GC-MS. Such a separation eliminated the humping background in N chromatograms of both basic and nonbasic groups. 2.3. Hydrotreatment. Hydrotreatment of gas oils was performed by 100 mL of an autoclave equipped with auto sampling apparatus. A commercially available NiMo/alumina catalyst was used for hydrotreatment. The catalyst was presulfided before use by H2S (10 wt %)/H2 atmosphere. The hydrotreatment conditions are summarized in Table 3.
3. Results 3.1. Identification of Nitrogen Species in Gas Oils. Figure 1 shows nitrogen chromatograms of gas oils measured by GC-AED. Nitrogen chromatograms of gas (8) Quimby, B. D.; Grudoski, D.; Giarrocco, V. J. Chrom. Sci. 1998, 36, 435-445. (9) Mao, J.; Pacheco, C. R.; Traficante, D. D.; Rosen, W. Fuel 1995, 74, 880-887.
oils were successfully obtained by GC-AED. Three different patterns of nitrogen chromatograms were observed, depending on the cutting point and origins of gas oils. LCO and MCO showed similar distributions of nitrogen species although relative abundances of the species were different from each other. Nitrogen species in LCO were separated into basic and nonbasic fractions by sulfuric acid extraction. The chromatogram of GC-MS showed that the basic fraction of LCO mainly consisted of mono- and di-methylated anilines, and quinoline and its methylated derivatives. Among them, dimethylanilines constituted the most abundant component in the basic fraction of LCO. It is difficult to identify nonbasic nitrogen compounds with GC-MS because of their less amount covered by the matrix hydrocarbons. Therefore, the nonbasic nitrogen species were identified on the basis of retention times of model compounds in the AED chromatogram. In this study, indole and its monomethylated derivatives and carbazole and its derivatives were confirmed by model compounds. However, some peaks, which were suspected as indole derivatives, were left unidentified. In summary, LCO carried aniline derivatives, quinoline, and indole derivatives as major nitrogen compounds. In contrast to LCO, MCO was found to carry carbazole and its derivatives as main nitrogen species. The detailed identification of carbazole derivatives was carried out by comparison of the retention times of model and target compounds. Consistent with results of previous paper,10 elution order of monomethylated carbazole derivatives was 1-methyl, 3-methyl, 2-methyl, and 4-methyl. A peak appearing after that of 4-methylcarbazole was assigned to 1,8-dimethylcarbazole, which eluted fastest among di-methylated carbazoles. This is probably because two methyl groups neighboring a N atom may sterically hinder its adsorption. Carbazole, monomethylated carbazoles, and 1,8-dimethylcarbazole were representative nitrogen species in MCO. It is interesting to note that nitrogen species in LCO were mainly basic while those in MCO were nonbasic. FSL showed another chromatogram from those of LCO and MCO, showing a broad hump of background. Figure 2 shows the GC-AED chromatograms of the original FSL, basic nitrogen-free FSL and basic nitrogen species in FSL, respectively. The hump of FSL was removed successfully by acidic extraction, indicating that hump of background consists mainly of basic nitrogen species in FSL. The further identification of this hump was performed by aid of GC-MS. GC-MS showed that the hump of background in FSL mainly consists of quinoline, benzoquinoline, and their derivatives. In contrast, the chromatogram of nonbasic components in FSL was very similar to that of MCO, carrying carbazole and its derivatives as the main nitrogen compounds. 3.2. Reactivity of Nitrogen Species in Gas Oils. Figure 3 shows the reactivity of nitrogen species in LCO in the hydrotreating reaction over NiMo/alumina catalyst at 340 °C under 5MPa of H2 atmosphere. Nitrogen species in LCO showed different reactivities, depending on their skeletal and substituent structures. Indole, which is the representative nonbasic nitrogen species, (10) Ignatiadis, I.; Kuroki, M.; Arpino, P. J. J. Chromatogr. 1986, 366, 251-260.
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Figure 1. Nitrogen species in LCO, MCO, and FSL measured by GC-AED. Assignment was performed.
showed the highest reactivity among the species. The reactivity order of nitrogen species type in LCO was indole > methylated anilines g monomethylated indole > quinoline > carbazole > methylated carbazole. The effects of number and position of methyl substituents on the HDN reactivity of carbazole derivatives were observed with MCO. The reactivity of carbazole decreased with methyl substitution, and the extent of decrease was subject to the position of the substituent. Figure 4 shows that the most refractory species in carbazole derivatives of MCO was 1,8-dimethylcarbazole. Among the monomethylated carbazoles, 1-methylcarbazole showed the lowest reactivity, which was lower than those of some di-methylated carbazoles. The reactivity of carbazole with trimethyl groups was slightly higher than that of 1,8-dimethylcarbazole. However, 1,4,8-trimethylcarbazole is less reactive than 1,8-dimethylcarbazole. Summarizing the above results, the order of reactivity of carbazole and its derivatives was carbazole > monomethylated carbazole > dimethylated carbazole without a 1-position methyl group > 1-methylcarbazole > trimethylated carbazole without 1 and 8-position methyl groups > 1,8-dimethylated carbazole > 1,4,8-trimethylated carbazole. The reactivity of carbazole derivatives in FSL was difficult to quantify accurately due to the hump of background. This hump must be subtracted from the carbazole peak. In this study, the reactivity of carbazoles
was calculated by assuming that the portion in the background represented basic nitrogen species as shown in Figure 5. Strictly speaking, nitrogen species in the hump may not all be basic; however, since the chromatogram of FSL after the extraction of basic nitrogen showed almost no hump of background, the above assumption was not too bad. The reactivity of nitrogen species in FSL determined on above assumption is summarized in Figure 6. As expected, the reactivity of carbazole derivatives in FSL showed the same trend as that of MCO. The conversion of carbazole derivatives in FSL seems to be slightly lower than that of MCO, although the reactivity of carbazole may be underestimated by above calculation method because of hump. The hump was noted to disappear faster than carbazole derivatives in the hydrotreatment over NiMo/alumina catalyst, suggesting the reactivity order of benzoquinoline > carbazoles. 4. Discussion GC-AED proved to be a very powerful instrument to analyze the trace amount of nitrogen species in the gas oil. This performance enabled HDN study of molecular level on models to real feed. The representative nitrogen species in gas oils were divided into two groups: (1) basic species such as aniline, quinoline, benzoquinoline, and their derivatives, and (2) nonbasic species such as indole, carbazole, and their methylated compounds. Low
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Figure 2. Basic fraction of FSL extracted by sulfuric acid. Background was successfully removed and it was analyzed as quniline and benzoquinoline derivatives using GC-MS.
Figure 3. Reactivity of nitrogen species in LCO hydrotreated over NiMO/Al2O3 catalyst at 340 °C under 5 MPA of H2, Cz, an, in, and qu stand for carbazole, aniline, indole, and quinoline, respectively. The number before and after the symbol means the number of methyl groups and the location, respectively.
cutting point gas oil, LCO, carried mainly indole, aniline, and their derivatives. In contrast, high cutting point gas oil, MCO, contained mainly carbazole derivatives. It is interesting to know that acridine was not found in LCO. The acridine is sometimes difficult to
distinguish from the carbazole due to the similarity of their retention times. However, the peak in LCO suspected as acridine was confirmed to carbazole because the latter was not found in the basic fraction of LCO extracted by sulfuric acid.
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Figure 4. Reactivity of nitrogen species in MCO hydrotreated over NiMo/Al2O3 catalyst as a 340 °C under 5 MPa of H2. The number before and after the symbol means the number of methyl groups and the location, respectively.
Figure 5. Suggested baseline of FSL to separate carbazole and basic nitrogen.
The elution order of different type nitrogen species depended basically on its boiling point. Among the same type of nitrogen species, elution order was governed by the polar interaction between packing materials and nitrogen atom. Therefore, nitrogen species having methyl groups at neighboring positions of nitrogen atom such as 1-methyl and 1,8-dimethyl derivatives showed faster elution because of steric hindrance.5 It is worthwhile to note that the overlapping of numbers of different nitrogen species having similar retention times, such as benzoquinoline and carbazole derivatives causes a hump of background in the chromatograms as observed with FSL. Separation by acidic extraction can help remarkably the chromatographic separation also in this case. The reactivity of nitrogen species in gas oil in the hydrotreatment over NiMo/Al2O3 catalyst at 340 °C was in the following order: indole > methylated aniline > methylated indoles > quinoline > carbazole > methylated carbazole. Considering the previous work done with model compounds,11 HDN of nitrogen species undergoes
first the hydrogenation of aromatics or pyrrole rings and then C-N fission of hydrogenated intermediates. The rate-determining step is believed to be the C-N bond fission. However, the bonding order to show unsaturation may give some measure for the reactivity of different type nitrogen species in the hydrogenation stage of which equilibrium is an important factor for the reactivity, especially when the reactivities of coexisting homologues are compared. The bonding order around nitrogen species calculated according to molecular orbital was found to be indole > quinoline > benzoquinoline > carbazole > aniline. The different skeletal structure of nitrogen species also can affect HDN reactivity by controlling the adsorption behavior on the NiMoS/Al2O3.12 Nitrogen species in gas oil can be classified into six-membered or five-membered nitrogen heterocycles and anilines in term of number of hydrogens bonded to N atom and according to the number of rings surrounding the (11) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 20212058.
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Figure 6. Reactivity of nitrogen species in FSL hydrotreated over NiMo/Al2O3 catalyst at 340 °C under 5 MPA of H2. Background is removed in calculation. The number before and after the symbol means the number of methyl groups and the location, respectively.
heterocycle. N’s in five-membered rings, in aniline, and in six-membered rings are the reactivity order of the first categories containing the same number of rings. More rings surrounding a heterocycle reduce the reactivity. Among the nitrogen species included in the same class, their reactivity showed consistency with the bonding order of nitrogen species. The reactivity of the same type of nitrogen species depended on the number and the position of substituted methyl groups. As the number of methyl groups increased, the reactivity of nitrogen species decreased. This is because methyl groups substituted on an aromatic ring reduced the adsorption activity of nitrogen species because of steric hindrances around the N atom to be hindered more by the partners. It is interesting to discuss that despite the same number of substituents in the same type of nitrogen compounds, their reactivities are largely affected by the position of methyl groups. As for carbazole derivatives, the effects of substituent location on the HDN seemed to be very similar to that of HDS. That is, 1 and 8 positions neighboring to nitrogen atom led to low reactivity.5 The first step of hydrogenation may be the interactive approach of the N atom in the substrate to the polar site of the catalyst, which appears to be influenced by neighboring substituents. This situation is similar to that with refractory sulfur species such as 4,6-dimethyldibenzothiophene.13 Whether such an explanation is true only in the competitive reaction of the GO will be argued. More detail knowledge of kinetics in competitive and noncompetitive conditions is necessary. Reactivity of nitrogen compounds in the real feed is different from that of the model because the reactivity (12) Ho, T. C. Catal. Rev. Sci. Eng. 1988, 30, 117-160. (13) Ma, X.; Sakanishi, K.; Isoda, T.; Mochida, I. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1994, 39, 622-627.
Table 4. Composition of Heteroaromatics and Aromatics in GOs total S, wt % total N, ppm basic N, ordera PAH, wt % a
LCO
MCO
FSL
0.42 345 1 1.7
0.49 695 3 11.3
0.91 295 2 6.4
Order of quantity determined by basic fraction.
of nitrogen species is affected by other competitive species such as aromatic-, sulfur-, and nitrogen-containing molecules in the feed. Therefore, the influence of the matrix should be considered in interpreting the reactivity of the same nitrogen species in different gas oils. For example, the reactivity order of carbazole was MCO > LCO > FSL as suggested by its initial rate and LCO > MCO > FSL as by the final conversion. Such orders could be explained by the respective competitive poisoning components and products in each gas oils. Table 4 shows competitive components in gas oils. HDN of carbazole seems to be affected also by partners such as sulfur, basic nitrogen species, and aromatics. Sulfur and aromatic substances appear to be major inhibitors in the initial stage, while H2S formed by desulfurization of sulfur species appears to govern the overall conversion of carbazole. LCO showed lower reactivity at the initial stage than that of MCO. The basic nitrogen compounds in LCO and FSL also affect the reactivity of carbazole at the initial stage although they are eliminated rather easily within the initial stage, performing much less inhibition at the later stage. Although it is very complex to quantify the factors affecting HDN of nitrogen species in gas oils, it is very challenging and necessary for design of a better upgraded refinery process and for establishing the molecular level simulation. EF990136M
