Influence of Desulfurization Methods on the Phenol Content and

3024

Energy & Fuels 2009, 23, 3024–3031

Influence of Desulfurization Methods on the Phenol Content and Pattern in Gas Oil and Diesel Fuel Nina Kolbe,† Oliver van Rheinberg,‡ and Jan T. Andersson*,† Institute of Inorganic and Analytical Chemistry, UniVersity of Mu¨nster, Corrensstrasse 30, 48149 Mu¨nster, Germany, and Oel-Waerme Institut GmbH, Kaiserstrasse 100, 52134 Herzogenrath, Germany ReceiVed January 23, 2009. ReVised Manuscript ReceiVed April 30, 2009

Performance characteristics of fuels, such as storage stability and lubricity, are determined mainly by the polar species of the fuel, among them the alkylated phenols. Here, we analyze gas oil and diesel samples, which have been successively desulfurized with different methods, and compare the phenol content and the phenol alkylation pattern of these materials. Because phenols occur in the low parts per million range in the fuel, very sensitive and selective analytical methods are needed for their determination. A derivatization of phenols to ferrocene esters followed by analysis by gas chromatography with both atomic emission detection (iron emission) and mass spectrometric detection is used in this work. Individual concentrations of smaller phenols with up to three substituent carbon atoms and sum parameters were determined. The classical hydrodesulfurization was found to lower the phenol content more drastically than the adsorptive desulfurization. Especially the larger alkylphenols are removed nearly completely during hydrodesulfurization, while they are almost unaffected by adsorptive desulfurization on a Ni/NiO sorbent.

Introduction The lubricity of a fuel and its antioxidant properties depend upon factors such as its phenol content.1,2 Naturally occurring antioxidants, such as phenols, were observed to act synergistically with the synthetic antioxidant 2,6-di-tert-butyl-p-cresol (BHT).2 On the other hand, phenols without the steric hindrance present in BHT are known to promote deposit formation during storage and therefore have a deleterious influence on fuel quality.3,4 In a low-sulfur light cycle oil, the tendency of gum formation was related to the phenols that were shown to couple oxidatively to form sediments.5 It is known that the phenol content may be reduced in a side reaction during the hydrodesulfurization (HDS) process1,6 that is applied to reduce the sulfur content of a fuel to conform with legal demands. Detailed information about the structures of the phenolic species remaining in the fuel after HDS is not available in the literature; * To whom correspondence should be addressed. E-mail: anderss@ uni-muenster.de. † University of Mu ¨ nster. ‡ Oel-Waerme Institut GmbH. (1) Hughes, J. M.; Mushrush, G. W.; Hardy, D. R. The relationship between the base extractable species found in middle distillate fuel and lubricity. Energy Fuels 2003, 17 (2), 444–449. (2) Jones, E. G.; Balster, L. M. Interaction of a synthetic hindered-phenol with natural fuel antioxidants in the autoxidation of paraffins. Energy Fuels 2000, 14 (3), 640–645. (3) Balster, L. M.; Zabarnick, S.; Striebich, R. C.; Shafer, L. M.; West, Z. J. Analysis of polar species in jet fuel and determination of their role in autoxidative deposit formation. Energy Fuels 2006, 20 (6), 2564–2571. (4) Laespada, M. E. F.; Pavon, J. L. P.; Cordero, B. M. Automated online membrane extraction liquid chromatographic determination of phenols in crude oils, gasolines and diesel fuels. J. Chromatogr., A 1999, 852 (2), 395–406. (5) Hazlett, R. N.; Power, A. J. Phenolic compounds in Bass Strait distillate fuels: Their effect on deposit formation. Fuel 1989, 68 (9), 1112– 1117. (6) Wasinski, F. A. H. Ferrocencarbonsa¨urechlorid als hochselektives Derivatisierungsmittel fu¨r Hydroxyverbindungen zur Analyse mittels GCAED und GC-MS. Dissertation, Universita¨t Mu¨nster, Mu¨nster, Germany, 2004.

therefore, it is difficult to evaluate whether this removal of phenols has negative consequences for the performance characteristics of the fuel. Information about the changes of the phenol distribution during desulfurization of fuels might help in understanding the reaction mechanism on the catalyst in more detail, which is important for the design of optimized catalysts. Phenols in crude oil or fossil fuels are usually analyzed by gas chromatography (GC) with flame ionization or mass spectrometric detection.1,3,7-10 Liquid chromatographic analysis is limited to the analysis of small phenols up to dimethylphenols or to the determination of sum parameters because of the lower resolution of this chromatographic technique.3 Sample preparation usually involves enrichment of polar compounds through liquid-liquid extraction,1 solid-phase extraction, and/or preparative liquid chromatography,3,7,8,10 sometimes followed by derivatization to improve chromatographic performance.8,10 Those sample preparation methods are labor-intensive and involve the risk of analyte loss. They rely on a difference in polarity between the nonpolar matrix and the phenols, but phenols with larger alkyl groups are less polar than smaller phenols and are therefore not necessarily recovered quantitatively. Balster,3 for example, discovered 2- and 6-substituted C3- and C5-phenols in the aliphatic/aromatic fraction; therefore, they were not accounted for in the phenol determination. Other methods do not involve a separation of the phenols from the matrix but rely on selective detection. Thus, derivatization with pentafluorobenzyl bromide (7) Link, D. D.; Baltrus, J. P.; Zandhuis, P.; Hreha, D. C. Extraction, separation, and identification of polar oxygen species in jet fuel. Energy Fuels 2005, 19 (4), 1693–1698. (8) Bennett, B.; Noke, K. J.; Bowler, B. F. J.; Larter, S. R. The accurate determination of C0-C3 alkylphenol concentrations in crude oils. Int. J. EnViron. Anal. Chem. 2007, 87 (5), 307–320. (9) Galimberti, R.; Ghiselli, C.; Chiaramonte, M. A. Acidic polar compounds in petroleum: A new analytical methodology and applications as molecular migration indices. Org. Geochem. 2000, 31 (12), 1375–1386. (10) Green, J. B.; Yu, S. K. T.; Vrana, R. P. GC-MS analysis of phenolic compounds in fuels after conversion to trifluoroacetate esters. J. High Resolut. Chromatogr. 1994, 17 (6), 439–451.

10.1021/ef9000693 CCC: $40.75  2009 American Chemical Society Published on Web 05/20/2009

Desulfurization Methods on the Phenol Content and Pattern Scheme 1. Derivatization Reaction for Phenols with Ferrocenecarboxylic Acid, Using 4-(Dimethylamino)pyridine as a Catalyst

has been used successfully in combination with GC and negative chemical ionization mass spectrometry.9 However, the injection of the “dirty” sample easily leads to a fast degradation of the chromatographic system, necessitating frequent maintenance. Another problem in quantification may be that phenols can show individual response factors, which means that individual response factors need to be determined for each phenol to be quantified.8 We have developed a fast and simple method, in which the phenols are derivatized to ferrocene carboxylic acid esters in the presence of the matrix,11 illustrated in Scheme 1. The reaction is quantitative for all short- and long-chain phenols, such as 4-n-nonylphenol, and only sterically hindered phenols with bulky substituents, such as t-butyl in the ortho position do not react quantitatively. Their concentrations may therefore be underestimated by this method, but on the other hand, such phenols do not seem to occur frequently in fossil materials. Sample cleanup is reduced to a minimum and merely consists of passing the solution through two aluminum oxide columns for removal of excess reagents and the bulk of aliphatic and aromatic compounds before gas chromatographic analysis. The resulting ferrocene esters are detected by either atomic emission detection in the iron-selective mode or mass spectrometric detection. These two detectors complement one another. The atomic emission detector11 (AED) shows a very high selectivity (4.6 × 106 versus carbon13) as well as an excellent sensitivity for iron (detection limit of 3 nmol of Fe14). This, together with its molar response for different iron compounds makes it highly useful for quantification and analysis of such low phenol contents as about 10 ng/g.15 In mass spectrometric detection, ferrocene esters yield a prominent molecular ion and a characteristic fragmentation ion of m/z 213.16 The mass spectrometer thus has advantages for the qualitative phenol analysis and for detection of possible interferences, for example by ferrocene esters of alcohols, which can be detected by their different fragmentation,16 or other hetero compounds. However, interfering ferrocene compounds were not found in the relevant retention time range in the samples presented here. It is especially fruitful to use the molecular ion mass traces as a second dimension for the analysis of larger alkylphenols, which (11) Andersson, J. T. Some unique properties of gas chromatography coupled with atomic-emission detection. Anal. Bioanal. Chem. 2002, 373, 344–355. (12) Rolfes, J.; Andersson, J. T. Determination of alkylphenols after derivatization to ferrocenecarboxylic acid esters with gas chromatographyatomic emission detection. Anal. Chem. 2001, 73 (13), 3073–3082. (13) Quimby, B. D.; Larson, P. A.; Dryden, P. C. A comparison of the HP G2350A AED vs. HP 5921A AED for average values of MDL and selectivity for selected elements. HP Appl. Note 1996, 228–363. (14) Kolbe, N.; Andersson, J. T. Simple and sensitive determination of o-phenylphenol in citrus fruits using gas chromatography with atomic emission or mass spectrometric detection. J. Agric. Food Chem. 2006, 54 (16), 5736–5741. (15) Kolbe, N. Analysis of phenols in complex matrices after derivatization to ferrocene esters. Dissertation, University of Muenster, Muenster, Germany, 2008. (16) Wasinski, F. A. H.; Andersson, J. T. Qualitative analysis of phenols and alcohols in complex samples after derivatization to esters of ferrocene carboxylic acid by gas chromatography with mass spectrometric detection. J. Chromatogr., A 2007, 1157, 376–385.

Energy & Fuels, Vol. 23, 2009 3025

cannot be separated chromatographically because of the large number of isomers. In this work, we undertake a detailed analysis of both the phenol content and the relative abundance of different phenolic species using the derivatization with ferrocene carboxylic acid chloride. The molar masses of many phenols are used to obtain evidence for the kind of substituents in the phenols. We compare phenol concentrations and patterns in fuels before and after classical HDS with fuels desulfurized using an adsorptive desulfurization method on a Ni/NiO sorbent,17 with the background that data on the phenols are important for the estimation of the stability of a fuel.5 A better understanding of the parameters influencing phenol removal would enable the industry to optimize the desulfurization conditions with respect to the phenols and thereby obtain the desired lubricity and storage stability. Experimental Section Reagents and Chemicals. Dichloromethane (DCM) and pentane for analysis (Acros, Geel, Belgium) were purified by percolation through activated aluminum oxide before use. Cyclohexane and toluene (Acros) were of residue analysis grade. The ferrocene esters of 2-fluorophenol (2FPE) and 4-fluoro-2-methylphenol (4F2MPE) (synthesized in our laboratory from the fluorophenols) were employed as standards. Further chemicals used were 4-(dimethylamino)pyridine (99%, Acros) and ferrocene carboxylic acid chloride (synthesized following the procedure described in ref 15; it may be stored at -18 °C under argon for at least 1 year). Aluminum oxide (Fluka, St. Gallen, Switzerland) was activated at 450 °C, deactivated with water, and stored at 160 °C for at least 24 h for a water content of 1.2% water. Samples. A commercial gas oil (that had been lightly hydrodesulfurized commercially to 850 ppm sulfur) and a low-sulfur gas oil for heating purposes (that had been hydrodesulfurized commercially to 50 ppm sulfur) were further desulfurized to various sulfur concentrations at the Oel-Waerme-Institut (OWI), Aachen, Germany, using adsorption of sulfur compounds on a Ni/NiO catalyst.17 The adsorption desulfurization in a fixed bed column was conducted at 200 °C, at a pressure of 5 bar and a volume flow of liquid hour space velocity (LHSV) ) 0.5 h-1. In ref 17, further details are given about this process. The commercial Ni adsorber on alumina offers a Brunauer-Emmett-Teller (BET) surface of 300 m2/g and is provided as cylindrical particles of 4 mm in size. Samples were taken at different times to show the breakthrough curve of the sulfur adsorption. The density of the gas oil used was 0.849 g/cm3, and that of the low-sulfur gas oil was 0.826 g/cm3. The boiling range of the gas oil was 165-369 °C, and that of the low-sulfur gas oil was 166-349 °C. The diesel samples were obtained from different stages of HDS from a refinery. The first sample with 18 600 ppm sulfur is a blend of distillates before HDS, consisting of 30% light cycle oil, 27% light atmospheric gas oil, 25% light coker gas oil, and 18% heavy atmospheric gas oil. This blend was desulfurized to similar sulfur levels as the gas oils for heating purposes (see above). A list of the samples including their sulfur contents can be found in Table 1. Apparatus. The Agilent GC-AED system consists of a 6890N GC and a G2350A AED, equipped with a 30 m × 0.25 mm inner diameter × 0.25 µm SLB-5 ms column (Supelco, Munich, Germany), a Gerstel MPS2 autosampler, and a Gerstel CISInjector (Gerstel, Mu¨lheim, Germany). The oven temperature was programmed as follows: 60 °C starting temperature, kept for 1.5 min, temperature ramp 45 °C/min up to 250 °C (kept for 1 min), then 4 °C/min to 270 °C, and then 10 °C/min to 300 °C, kept for 5 min. Other GC-AED conditions are injector initial (17) van Rheinberg, O.; Lucka, K.; Ko¨hne, H.; Schade, T.; Andersson, J. T. Selective removal of sulphur in liquid fuels for fuel cell applications. Fuel 2008, 87, 2988–2996.

3026

Energy & Fuels, Vol. 23, 2009

Kolbe et al.

Table 1. List of Samples Analyzed for Alkylphenols, Desulfurization Method(s), and Their Sulfur Content (Reported in ppm ) µg/g) gas oil, adsorptive desulfurization on Ni/NiO 850 ppm S before desulfurization, commercial sample 605 ppm S 410 ppm S 16 ppm S

low-sulfur gas oil, adsorptive desulfurization 50 ppm S before adsorptive desulfurization, commercial hydrodesulfurized sample 18 ppm S 8 ppm S 2.4 ppm S

temperature of 60 °C, heated at 12 °C/s to 300 °C, helium carrier gas with 40 cm/s constant velocity, and transfer line and cavity temperatures of 300 °C. Helium makeup flow for the AED is 240 mL/min, and hydrogen and oxygen plasma gas pressures are 15 and 20 psi, respectively. The GC-MS consists of a Finnigan MAT ion-trap GCQ, fitted with a 30 m × 0.25 mm inner diameter × 0.25 µm SLB-5 ms column (Supelco), operated with EI ionization at 70 eV in full-scan mode from 100 to 600 amu. The oven temperature was programmed as above or with a longer program for the analysis of large phenol esters (60 °C for 1 min, 20 °C/min to 220 °C, then 4 °C/min to 270 °C, and then 10 °C/min to 300 °C, held for 10 min). Other conditions are a split/splitless injector used in the splitless mode at 270 °C (1 min), helium carrier gas at 40 cm/s constant velocity, transfer line of 275 °C, ion source of 200 °C, and filament offset for 7 min. A total of 1 µL was injected using an autoinjector. Sample Preparation. About 200 mg of the sample is spiked with 5 nmol of 4F2MPE as an internal standard (50 µL of a 100 µM solution in toluene), diluted with 1 mL of DCM, and then derivatized by adding 11 mg of ferrocenecarboxylic acid chloride (FCC) and 15 mg of 4-(dimethylamino)pyridine (DMAP) as catalyst. After a reaction time of 10 min at room temperature, the excess reagents are removed on an aluminum oxide minicolumn (1.7 g of aluminum oxide in a 3 mL SPE glass cartridge, packed under DCM). Ferrocene esters are eluted with 5 mL of DCM, and the solution is concentrated almost to dryness using a gentle flow of nitrogen at 40 °C. It is then transferred to a second aluminum oxide column (1 cm inner diameter, 2.5 g of Al2O3), and most of

diesel fuel, HDS 18 600 ppm S before HDS, blend 550 ppm S 145 ppm S 26 ppm S

the aliphatic and aromatic compounds are removed using 7 mL of 7:3 pentane/DCM. Then, the analytes are eluted using 11 mL of DCM and spiked with 5 nmol of 2FPE as a second internal standard (50 µL of a 100 µM solution in toluene) to check for losses during sample preparation. The sample is concentrated again almost to dryness and finally taken up in 1 mL of cyclohexane for analysis. The derivatized samples are stable for several months at 5 °C. Data Evaluation. The C0-C3-phenols in the gas oil and diesel samples were identified by a comparison of retention times to authentic standards or by co-injection of oil sample and standards and by their mass spectra in GC-MS. An exemplary iron-selective GC-AED chromatogram with assigned phenol ferrocene ester peaks is shown in Figure 1. The internal standard 4F2MPE is used for quantification. Integration is performed automatically after defining a baseline at 8 min retention time (shortly before the elution of the first phenol ferrocene esters) and checked manually for partly resolved peaks. Even though not completely resolved chromatographically, all available C0-C3-phenols were quantified (as a sum parameter if co-eluting) to obtain a complete picture of the sample. Peak areas were compared to the peak area of the internal standard 4F2MPE, and individual phenols were quantified without the need for response factors. This is possible because of the molar response of the AED for iron. Calibration curves for phenylphenol ferrocene ester were previously found to be linear between 0.006 and 12 nmol of Fe/mL of sample.14 Phenol contents in the analyzed samples were all within this linear range. Also, the sum of all integrated peaks was used to calculate the total phenol content. To be able to determine the ratio of the

Figure 1. Iron-selective GC-AED chromatogram of a hydrodesulfurized gas oil containing 50 ppm sulfur. The labeled peaks were identified through comparison to authentic alkylphenol ferrocene esters. Abbreviations: DMPE, dimethylphenol ester; TMPE, trimethylphenol ester; EPE, ethylphenol ester; nPPE, n-propylphenol ester. Numbers show substituent position(s) in the benzene ring. 2FPE and 4F2MPE are the internal standards.

Desulfurization Methods on the Phenol Content and Pattern


Figure 2. Iron-selective GC-AED chromatograms showing the phenol distribution in gas oil after adsorptive desulfurization (left) and diesel fuel after HDS (right), with successively declining sulfur content from top to bottom. Internal standards are present at 8.4 and 8.9 min (see Figure 1).

Figure 3. Iron-selective GC-AED chromatograms showing the phenol distribution in commercially hydrodesulfurized gas oil (top trace) after subsequent adsorptive desulfurization, with successively declining sulfur content from top to bottom. 2FME and 4F2MPE are the added internal standards 2-fluorophenol and 4-fluoro-2-methylphenol, respectively.

3028


Kolbe et al.

Table 2. Phenol Content (µg/g) in a Gas Oil Desulfurized by Adsorption to Different Sulfur Levels (Given in ppm) sulfur content (ppm)

850

605

410

16

sum C0-C3 phenols (RSD, n ) 3) total phenols (RSD, n ) 3) mean weighted retention time (min) (C0-C3 phenols)/total phenols

48 (15%) 929 (9%) 15.61 0.05

29 (5%) 988 (9%) 15.64 0.03

30 (3%) 941 (5%) 15.58 0.03

25 (2%) 903 (1%) 15.61 0.03

Table 3. Phenol Content in Gas Oil Desulfurized Using Adsorptive Desulfurization after HDS to 50 ppm S (Content in µg/g of Oil)a sulfur content (ppm)

50

18

8

2.4


20 (17%) 51 (33%) 12.18 0.39

15 (15%) 53 (8%) 11.86 0.28

8.9 33 11.90 0.27

3.7 16 11.62 0.23

a RSD was not calculated for samples 8 and 2.4 ppm sulfur because of only two complete analyses each.

identified C0-C3-phenols to all phenols in the sample, the total phenol content was calculated in µg/g of fuel, which made it necessary to determine a mean molecular mass of the phenols. As a first step, a weighted mean retention time was calculated using eq 1:6

∑tA

i i

weighted mean retention time: r )

i

∑A

(1)

i

i

where i is the number of the peak, t is the retention time, and A is the peak area. The assignment of the weighted mean molecular mass has to be performed from a GC-MS chromatogram (using the GC-AED temperature program) by looking at the mass spectrum at the weighted mean retention time. This means that the molecular mass has to be regarded as a semiquantitative figure ((1 CH2 group) because retention times of GC-AED and GC-MS measurements differ slightly and because at a given retention time several phenol esters with different molecular masses elute. All samples were analyzed at least 3 times (including sample preparation), except gas oil samples desulfurized with both methods to 8 and 2.4 ppm sulfur, which were analyzed twice.

Results In Figures 1-3, iron-selective GC-AED chromatograms of the derivatized samples are shown. All samples exhibit a very complex distribution of alkylphenols. The smaller phenols (those with less than four carbon atoms in the side chains, with a retention time up to ca. 11 min) can be distinguished as separated ferrocene ester peaks, as illustrated in Figure 1, in which the peaks have been identified. In some samples, however, larger, chromatographically unresolved alkylphenol derivatives dominate the chromatograms. In Figure 2, the influence of the HDS on the one hand and the adsorptive desulfurization on the other hand on the phenol pattern is compared. In the left panel, the phenol pattern in a gas oil that has first been lightly desulfurized by HDS (to 850 ppm S; top left panel in Figure 2) and then deeply desulfurized by the adsorption process17 to different sulfur contents appears similar before and after the adsorption desulfurization (left panels in Figure 2). The unresolved phenols appear to represent a higher fraction of the phenols the further the desulfurization has proceeded, as indicated by the big hump in the lowest left panel in Figure 2. All resolved phenols are removed in a fairly similar fashion, as shown by the persistent pattern of phenols. In contrast to that, smaller phenols with a less complex pattern dominate the alkylphenol distribution in hydrodesulfurized fuels

(right panels in Figure 2). The chromatograms in the figure, right-hand panels, show a diesel fuel that has been successively hydrodesulfurized with a concomitant loss of preferentially the longer chain substituted phenols. Mainly C1- and C2-phenols remain after this treatment. Quantitative Analysis by GC-AED. The sum of all identified C0-C3-phenols and the determined total phenol concentrations for the different fuel samples are summarized in Tables 2-4, together with relative standard deviations (RSD) that, in most cases, were below 10%. Even though phenol concentrations can be determined reproducibly with the described analytical method, some standard deviations exceed 30%. The uncertainty of sum parameters is larger than that of single analyte concentrations because of the uncertainty of baseline definition and overlapping peaks in this complex mixture. Moreover, the diesel samples were already 6 years old at the time of analysis and probably subject to some oxidative degradation, with unknown degradation products causing an elevated baseline in the chromatogram. However, the precision of the data is sufficient to analyze the effects of the different desulfurization procedures as intended because the differences between the samples are much larger than the uncertainty of the data. A discussion of individual phenol concentrations is outside the scope of this work, but the data are provided in the Supporting Information. Qualitative Analysis of Higher Alkylated Phenols by GC-MS. Alkylphenols with several or longer alkyl chains cannot be resolved chromatographically because of the large number of compounds and are therefore rarely investigated in the literature. Our first results showed a different effect of desulfurization on these large phenols than on small phenols; therefore, structural elucidation of this class of compounds was one focus of this study. The unresolved complex mixture (UCM; top trace in Figure 4) of phenol ferrocene esters can partly be resolved and identified in the second dimension of molecular mass in GC-MS, using the prominent molecular ion of the ferrocene esters. As demonstrated in Figure 4, derivatives of up to C15-alkylated phenols could thus be detected in gas oil after partial desulfurization by adsorptive desulfurization. While separated peaks can be distinguished in the mass selective chromatograms of C4- and C5-alkylated phenol esters, derivatives of larger phenols with more than eight alkyl carbons show a complex mixture of isomers. Besides phenols containing only saturated chain substituents, a large number of derivatives with rings and/or double bonds in the substituents can be found. They are indicated by a molar mass that shows a hydrogen deficiency of one molecule of hydrogen for each ring or double bond. This is expressed as the additional double-bond equivalents (DBEs), i.e., those DBE that do not arise from the phenol ring. For higher molecular masses (>C7), phenol derivatives with one or two additional DBEs dominate the chromatogram. In Figure 5, the distribution of the C7-substituted phenol esters with different numbers of (additional) DBEs is demonstrated. The peak shown with additional DBE 3 may derive from an ester of trimethylnaphthol, whereas compounds with additional DBE 4 are probably derivatized from methylphenylphenols or benzylphenols. In contrast to this situation, only derivatives of up to C6phenols can be detected in the hydrodesulfurized gas oil samples.



Table 4. Phenol Content in Hydrodesulfurized Diesel Fuels (Content in µg/g of Fuel) sulfur content (ppm)

18600

550

145

26


174 (11%) 2243 (2%) 14.03 0.078

3.5 (36%) 41 (26%) 13.12 0.09

2.3 (39%) 7 (27%) 11.67 0.33

1.6 (38%) 3 (15%) 11.18 0.53

The GC-MS chromatograms of these samples are dominated by C2-C4-phenol derivatives without additional DBEs (data not shown). These samples have a different origin and can therefore not be compared directly, but coherence with the desulfurization method is probable. Discussion High-Sulfur Gas Oil with Nickel Adsorption. Gas oils that were desulfurized mainly by the use of the Ni/NiO adsorbers all had high phenol contents, even if their residual sulfur content was decreased by 98% from 850 to 16 ppm. These relatively high phenol concentrations could be determined reproducibly with relative standard deviations of 1-15%. As shown in Table 2, the total phenol content was 900-1000 µg/g and was independent of the sulfur content. Highly alkylated phenols dominate in these samples (left panels in Figure 2), and the identified C0-C3-phenols comprise only about 3% of the total phenols. These refractory phenols could be partly characterized by GC-MS to be a complex mixture of isomers with up to C15-alkyl groups and abundant additional rings in the substituents. A reason for their high abundance in desulfurized fuel may be suggested to be the steric hindrance of these phenols, which prevents an adsorption at the Ni/NiO catalyst. Small phenols are affected by adsorptive desulfurization to some

extent: the sum of C0-C3-phenols is decreased from 48 to 25 µg/g in the desulfurization process. The calculation of the weighted mean retention time shows the retention time where half of the molar amount of the phenols has eluted. It is therefore a measure for the ratio between lower and higher alkylated phenols. The data are given in Tables 2-4 for the three series of samples (Table 1). For the high-sulfur gas oil desulfurized lightly by HDS and then further by adsorption on Ni/NiO adsorbents, no change in mean weighted retention time can be recorded (Table 2), showing that this procedure has the same effect on all phenols regardless of the size of the alkyl substitution. Gas Oil Desulfurized by HDS. In contrast to the situation discussed above when the nickel adsorbent was used, HDS was found to preferentially remove highly alkylated phenols (right panels in Figure 2 and top trace in Figure 3). Such fuels (see 50 ppm S sample in Table 3 and desulfurized samples with 550, 145, and 26 ppm in Table 4) contain significantly lower total phenol contents of 3-51 µg/g. The identified smaller phenols contribute 9-53% to the total phenol content and partly remain even in deeply desulfurized fuel. Highly alkylated phenols are absent after deep HDS, as shown by GC-AED and confirmed by GC-MS analysis (see above). The reason for this reverse reactivity with a preferential removal of sterically

Figure 4. GC-MS chromatograms with selected ion traces of molecular ions of derivatized C4-C15-phenols in gas oil partly desulfurized to 605 ppm S using an adsorptive desulfurization method.15

3030


Kolbe et al.

Figure 5. GC-MS chromatogram of the gas oil with 605 ppm S (see Figure 4) with selected ion traces of molecular ions of C7-phenol ferrocene esters but with different additional DBEs.

hindered compounds has not been investigated yet, although phenol removal is a known side reaction of HDS.1,10 The higher reactivity of phenols with several alkyl groups compared to phenol and the cresols might be rationalized by a higher electron density in the phenolic ring, which may facilitate hydrogenation at the catalyst surface. The products of the phenol removal are not known. Gas Oil with Nickel Adsorption after HDS to 50 ppm S. When adsorptive desulfurization of a low-sulfur gas oil is performed subsequent to HDS, a different behavior of the phenols was found than discussed above for the high-sulfur gas oil. As demonstrated in Figure 3 and Table 3, both the total phenol content and the smaller phenol concentrations were lowered by the adsorptive desulfurization. The larger phenols were affected somewhat less than the smaller phenols, reflected in a decrease of the small phenols/total phenols ratio from 0.39 to 0.23. The reason for the significant removal of highly alkylated phenols in these samples remains speculative. One possibility is that extensive phenol removal during adsorptive desulfurization only takes place when a fuel is desulfurized to a sulfur content of less than 16 ppm. The hypothesis would be that the active sites are blocked to phenol adsorption because the sulfur species adsorb more strongly and only when the sulfur species concentration is low do the phenols compete efficiently for the active catalyst sites. This reason for the more efficient removal of the larger phenols, which presumably have a higher electron density in the aromatic ring and therefore should adsorb more easily, remains to be proven. Previous work on a different type of Ni adsorbent has shown good desulfurization properties, but the effect on phenols was not investigated.18 Again, the results are confirmed by the mean weighted retention times. For the sample that is adsorption-desulfurized after HDS, a small decrease is observed toward lower sulfur concentrations. In contrast, the lowering is strong for the purely

hydrodesulfurized diesel, showing that in this process the more highly alkylated phenols are depleted more easily. An exact comparison is not possible between the series of desulfurizations because the starting and finishing sulfur concentrations are different for the three sets of samples and the materials are of different origins; a proper comparison should therefore only be performed with the same sample desulfurized to the same sulfur levels using both methods. The identity of sulfur species in the gas oils after adsorptive desulfurization has been discussed recently.17 It was found that primarily sterically hindered alkylated dibenzothiophenes remain in adsorption desulfurized fuels. These are the same compounds that are also known to be recalcitrant to HDS.19 In contrast to the changes in phenol patterns discussed above, no significant difference was found in the patterns of remaining sulfur compounds from the two desulfurization methods. Conclusion The derivatization of phenols to yield ferrocene esters has proven itself to be an excellent method for the characterization of phenols in complex mixtures. The complementary use of the AED and MS detectors facilitates the information gained in the analysis. HDS of middle distillates is shown here to affect the alkylphenol concentrations more severely than the adsorptive desulfurization method.17 In hydrodesulfurized diesel and gas oil, the larger alkylphenols were almost completely removed. In contrast to that, the adsorptive desulfurization lowered the (18) Velu, S.; Ma, X.; Song, C.; Namazian, M.; Sethuraman, S.; Venkataraman, G. Desulfurization of JP-8 jet fuel by selective adsorption over a Ni-based adsorbent for micro solid oxide fuel cells. Energy Fuels 2005, 19, 1116–1125. (19) Schade, T.; Andersson, J. T. Speciation of alkylated dibenzothiophenes in a deeply desulfurized diesel fuel. Energy Fuels 2006, 20 (4), 1614–1620.


concentrations of smaller alkylphenols to a minor extent, whereas large phenols were not depleted down to a sulfur content of 20 ppm. These highly alkylated phenols were characterized here by GC-MS and revealed a complex distribution of at least up to C15-phenols with abundant additional rings in the side chains. The effect of different phenol patterns in fuels on lubricity and storage stability of the desulfurized fuel should be investigated in the future to optimize desulfurization conditions concerning the desired fuel properties. Subsequent research that will be reported separately on the low-sulfur gas


oil has shown that the oxidation stability according to the modified DIN EN 14112 and the thermal stability according to E DIN 51371 are high, as expected for a material with a low phenol content. Supporting Information Available: Phenol concentrations (in µg/g of oil) in the materials investigated. This material is available free of charge via the Internet at http://pubs.acs.org. EF9000693

Influence of Desulfurization Methods on the Phenol Content and

Recommend Documents