Reactivities of Various Alkyl Dibenzothiophenes in Oxidative

Jun 12, 2017 - (KNPC) with a total sulfur content of 339 ppmw was used as a feedstock to study the oxidation reactivities and kinetics of sulfur compo...
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Reactivities of Various Alkyl Dibenzothiophenes in Oxidative Desulfurization of Middle Distillate with Cumene Hydroperoxide Muhieddine A. Safa,* Tahani Al-Shamary, Rawan Al-Majren, Rashed Bouresli, and Xiaoliang Ma* Petroleum Research Center, Kuwait Institute for Scientific Research, P. O. Box 24885, Safat 13109, Kuwait ABSTRACT: The oxidation reactivities of various alkyl dibenzothiophenes (DBTs) were evaluated and compared by measuring the rate constant of each alkyl DBT in oxidative desulfurization of a hydrotreated middle distillate (HMD) with cumene hydroperoxide at 75 °C over a MoO3/Al2O3 catalyst. It was found that the major sulfur compounds with higher abundance in HMD are the DBTs with alkyl substituents at the 4- and/or 6-positions, and the oxidative reaction of each alkyl DBT follows a pseudo-first-order reaction. The measured rate constants indicate that the oxidation reactivity of these alkyl DBTs decreases in the order of 4-MDBT > 2,4,6-TMDBT ≈ 3,4,6-TMDBT > 1,4,6-TMDBT ≈ 4,6-DMDBT > 4-E,6-MDBT. Comparison of the molecular structures of these sulfur species with their reactivities implies that the oxidation activity of alkyl DBTs with oil-soluble oxidant over a solid catalyst is mainly dependent on the steric hindrance around the central sulfur atom caused by the alkyl substituents at the 4- and/or 6-positions.

1. INTRODUCTION The stringent environmental regulations on sulfur level in transportation fuels have attracted great attention from petroleum refiners worldwide to the production of ultraclean fuels in recent years. Consequently, the refining industry is under massive pressure to produce ultralow-sulfur fuels (S, 4,6-DMDBT. Chica et al. conducted the oxidative desulfurization of model sulfur-containing compounds in the presence of t-BuOOH with a MoO3/Al2O3 or Ti-MCM-41 catalyst in a continuous fixed-bed reactor.16 They found a similar reactivity trend of the sulfur compounds in their oxidation reaction systems irrespective of the catalyst used. Safa and Ma reported recently the oxidation kinetics of DBTs in a model fuel with cumene hydroperoxide as an oxidant over a MoO3/Al2O3 catalyst.12 They found that the oxidation rate constants decreased in the order of DBT > 4-MDBT > 4,6DMDBT, indicating the steric hindrance effect of the methyl groups around the sulfur atom on their oxidative reactivity. Overall, 95% conversion of all DBTs was achieved at 75 °C within 60 min. Furthermore, the oxidation reactivities of DBTs in model fuels were also evaluated comprehensively via various oxidation systems with the water-soluble hydrogen peroxide oxidant, using catalysts such as polyoxometelate,17 polyoxometelate/ H2O2 and formic acid/H2O2,18 and V2O5/TiO2.19 The results showed in general that the oxidation reactivity of sulfur compounds depends on the electron density on the sulfur atom but not on the steric hindrance around the sulfur atom. In addition, plenty of other studies investigated the oxidation reactivities of sulfur compounds in synthetic fuels using different catalytic systems.20,21 It has been shown that the majority of studies in the oxidation reactivity of sulfur compounds have been conducted by using model compounds of BT, DBT, 4-MDBT, and/or 4,6-DBT in solvent,12,15−21 while very limited work has been reported on the oxidation reactivity and kinetics of the alkyl DBTs in real middle distillate feedstock.22,23 Ishihara et al. investigated the oxidative desulfurization of a variety of sulfur species present in a light gas oil (S, 39 ppm) using t-BuOOH with a 16 wt % MoO3/ Al2O3 and an O/S molar ratio of up to 15.22 They found that the reactivity decreased in the order of DBT > 4,6,-DMDBT > C3-DBT, irrespective of the WHSV and the temperature of the oxidation reactions. However, Otsuki et al. studied expansively the reactivities of sulfur compounds present in straight run− light gas oil (S, 1.35 wt %) using a H2O2/formic acid catalytic system.23 The oxidation of each sulfur compound was treated as a pseudo-first-order reaction, and the oxidation rate constants decreased in the order of 4,6-DMDBT > 4-MDBT > DBT > BT. They found that the reactivity of the sulfur species increases with an increase of the electron density on the sulfur atom, indicating that steric hindrance by the methyl group(s) around the central sulfur atom may play a less important role in determining the oxidation activity in this case, which seems contradictory with the finding in the case using a H2O2/formic acid catalytic system by the same group.22 The main objective of this study is to evaluate and compare the oxidation reactivities of various sulfur compounds present in HMD. The kinetic behavior for various sulfur compounds in oxidative desulfurization with cumene hydroperoxide at 75 °C over a MoO3/Al2O3 catalyst was examined to get their rate constants for comparison. This work is expected to offer a better fundamental understanding of the oxidative reactivity of various sulfur compounds in real HMD and thus significantly

2. EXPERIMENTAL SECTION 2.1. Materials. All the reagents employed in this study, including nhexadecane (99%), toluene (>99%), 4,6-dimethyldibenzothiophene (4,6-DMDBT; 97%), 4-methyldibenzothiophene (4-MDBT; 96%), dibenzothiophene sulfone (98%), and cumene hydroperoxide (80%), were bought from Sigma-Aldrich, Europe and used without further purification unless indicated in the experimental procedure. A hydrotreated middle distillate from Kuwait National Petroleum Co. (KNPC) with a total sulfur content of 339 ppmw was used as a feedstock to study the oxidation reactivities and kinetics of sulfur compounds in this study. The major composition and properties of HMD are summarized in Table 1.

Table 1. Major Properties of HMD property

HMD

density (15 °C) (g/cm3) API sulfur (ppmw) nitrogen (ppmw) total aromatics (wt %) saturates (wt %) range of boiling point (°C)

0.834 37.9 339 40 13 87 208−373

2.2. Preparation and Characterization of Catalyst. A MoO3/ Al2O3 catalyst with 12 wt % MoO3 was prepared via an incipient wetness impregnation (IWI) of a commercial γ-Al2O3 support with an aqueous solution of ammonium heptamolybdate.24 The detailed preparation of this catalyst has been reported in our previous work.12 The surface area, pore volume, and pore diameter of MoO3/γ-Al2O3 catalyst were determined with a Micrometrics TriStar surface area and porosity analyzer. These physical properties were measured by lowtemperature N2 adsorption−desorption isotherms. The measured BET surface area, pore volume, and average pore diameter of the prepared MoO3/γ-Al2O3 catalyst are given in Table 2.

Table 2. Surface Properties and Metal Content of the Prepared Catalyst material

BET surface area (m2/g)

pore vol (cm3/g)

av pore diam (Å)

γ-Al2O3 MoO3/γ-Al2O3

252 225

0.850 0.769

124 128

2.3. Oxidation Experiments. The oxidation of the sulfur compounds in HMD was studied using an S1025 STEM Omni batch reaction station. This reaction station with 10 glass tube reactors (100 mL each) in a heating block was used to run the oxidation at the same desired temperature and stirring rate. The 100 mL glass reactors are regularly placed in the heating block and then connected to reflex heads that are connected to an external cooling system. In a typical run, HMD sample (10.0 g) was dispensed into the reactor along with 0.50 g of catalyst (5.0 wt %). Once the temperature of the mixture was stabilized at the desired temperature, the oxidation was started by adding an excess amount of the oxidizing agent, cumene hydroperoxide, with an O/S molar ratio of 20 into the reaction systems. After reaching the desired reaction time, the tube reactor with the mixture was cooled by using an ice bath. Finally, the treated fuel was filtered to remove the catalyst inside, and then the sample was kept in a refrigerator and analyzed soon. 2.4. Analysis of Feedstock and Products. The total sulfur concentrations in HMD and the oxidized HMD were determined B

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Figure 1. GC-PFPD chromatogram of HMD with identification.

Table 3. Qualitative and Quantitative Analyses of the Sulfur Species peak no. S compound 1 2 3 4 5 6 7 8 9 10 group 1 2 3 4

compd/group

RT (min)

HMD (ppmw)

4-MDBT 4-EDBT 4,6-DMDBT 3,6-DMDBT/2,6-DMDBT 1,4-DMDBT/1,6-DMDBT 4-E,6-MDBT 2,4,6-TMDBT 1,4,6-TMDBT 3,4,6-TMDBT C4DBT-1

19.73 20.64 20.73 20.93 21.14 21.57 21.82 22.10 22.21 22.32

5.8 2.5 62.7 4.2 3.9 33.9 29.5 29.3 14.2 17.0

C1DBT C2DBT C3DBT C4DBT

∼18−20.5 ∼20.6−21.6 ∼21.8−22.2 ∼>22.3

9.2 105.9 97.0 127.5

using a Mitsubishi Chemical TS-100 V trace sulfur analyzer according to the standard method ASTM D-5453. The instrument was calibrated at a range from 1 to 1000 ppmw. The relative error for analyses of sulfur in fuels is less than 2.5%. In addition, the concentrations of various sulfur compounds in HMD and the products were analyzed by using an Agilent 7890B chromatography with a pulsed flame photometric detector (GCPFPD). A CP-Sil 5CP capillary column (30 m length, 0.32 mm internal diameter, and 0.25 μm film thickness) was used. The fuel samples were injected into the column by using a 7693A automatic liquid sampler (Agilent) with a split injector (Agilent) containing a 4 mm i.d. single tapper liner with glass wool under a split mode at 325 °C. The temperature of the detector was set at 300 °C. The oven temperature was initially held at 35 °C for 3 min, then increased immediately at a rate of 10 °C/min to 300 °C, and held at 300 °C for 1 min. Helium gas was used as a gas carrier at a flow rate of 3 mL/min. In this study, the peaks for 4-MDBT, and 4,6-DMDBT were identified by adding their standards into the samples. Identification of other major DBTs was conducted by comparing the relative retention times of the peaks with those reported in the literature.25,26 The quantification of the major sulfur compounds and groups was carried out by a normalization method, where the concentration of each sulfur

compound or each sulfur group was determined by the following equation:

Cs, i = Cs,total

Ai A total

(1)

where CS,total is the total sulfur concentration in the sample, Ai is the peak area corresponding to the sulfur compound (or sulfur group) i, and Atotal is the total areas of the peaks in the GC-PFPD chromatograms. The relative error for analyses of the sulfur compounds is less than 3%.

3. RESULTS AND DISCUSSION 3.1. Major Sulfur Compounds in Hydrotreated Middle Distillate. The major physical and chemical properties of HMD with a boiling point range from 208 to 373 °C were determined, and the results are summarized in Table 1. Since HMD has been hydrotreated, the sulfur and nitrogen concentrations are relatively low, being 339 and 40 ppmw, respectively. The GC-PFPD chromatography was used to identify and quantify the various sulfur compounds in HMD. Figure 1 C

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Figure 2. GC-PFPD chromatograms of oxidation-treated HMD at different reaction times.

which were present originally in the un-hydrodesulfurized middle distillate and had been removed completely in the hydrodesulfurization process at medium conditions due to their higher HDS reactivities. When carefully analyzing the structures of the sulfur compounds that remained in HMD, it is found interestingly that the major identified sulfur compounds with highest abundance in HDM are those DBTs with alkyl substituents at both the 4- and 6-positions, such as 4,6DMDBT, 4-E,6-MDBT, 2,4,6-TMDBT, 1,4,6-TMDBT, and 3,4,6-TMDBT, which account for about 50% of the total sulfur in HMD, and are classified as the most refractory sulfur compounds in the middle distillate to be removed. Further, the identified sulfur compounds with less abundance in HDM, which account for less than 5% of the total sulfur, are those DBTs with only one alkyl substituents at the 4- or 6-position, such as 4-MDBT, 4-EDBT, 2,6-DMDBT/3,6-DMDBT, and 1,4-DMDBT/1,6-DMDBT, indicating that they are easier to remove than the DBTs with alkyl substituents at both the 4and 6-positions, but more difficult than BTs, and the DBTs without any alkyl substituent at the 4- and 6-positions, as no

illustrates the GC-PFPD chromatograms of HMD. The identification of the major sulfur compounds present in HMD was also shown in the figure. The identified sulfur compounds with their retention times and concentrations are summarized in Table 3. The dominant sulfur compounds are the di- and trialkyl-substituted DBTs. The major sulfur compounds identified in HMD are 4,6-DMDBT, 4-E,6MDBT, 2,4,6-TMDBT, 1,4,6-TMDBT, 3,4,6-TMDBT, and C4DBT-1 (DBT with four carbon atoms in its alkyl substituents) with sulfur contents of 63, 34, 30, 29, 14, and 17 ppmw, respectively. In addition, trace amounts of some sulfur compounds have also been identified and quantified, such as 4-MDBT, 4-EDBT, 2,6-DMDBT/3,6-DMDBT, and 1,4-DMDBT/1,6-DMDBT. Some peaks were also found at retention times higher than 22.3 min, which are the alkylated DBTs (C4DBT + C5DBT) with four or five carbon atoms in their alkyl substituents, and known as the higher boiling point species. It is apparent that some special alkyl DBTs have significantly lower HDS reactivity than sulfides, disulfides, alkyl thiophenes, alkyl benzothiophenes, and even other alkyl DBTs, D

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1,4,6-TMDBT > 4-E,6-MDBT, being 100, 94, 93, 90, 91, and 85%, respectively, at 75 °C for 90 min. There are new peaks found in the GC-PFPD chromatograms of the oxidized product samples at relatively higher retention times (>22.3 min; see Figure 2). These peaks of the sulfur compounds should correspond to the formed analogue sulfones. According to their retention times and change pattern of the peaks, the new peaks of O3, O6, O7, and O8 should correspond to the sulfones of 4,6-DMDBT, 4-E,6-MDBT, 2,4,6-TMDBT, and 1,4,6-TMDBT, respectively. 3.3. Oxidation Kinetics of Alkyl DBTs. In order to compare the oxidative reactivity of the various alkyl DBTs quantitatively, the kinetics analysis was conducted on the basis of the measured concentration of each sulfur compound in the product oil. Plots of ln Ct vs the oxidation reaction time from 5 to 90 min are shown in Figure 4. A good linear relationship

alkyl BTs and the DBTs without any alkyl at the 4- or 6positions were detected in HMD. The results imply that the differences in the HDS reactivities of the sulfur compounds can be attributed mainly to their different steric hindrance extent caused by the alkyl substituents at the 4- and/or 6-positions. The DBTs with alkyl substituents at both the 4- and 6-positions have more steric hindrance than the DBTs with only one alkyl substituent at the 4- or 6-position, while the latter have more steric hindrance than DBTs without any alkyl substituent at the 4- or 6-position. Such steric hindrance blocks the approach way of the sulfur atom in the sulfur compounds to the active sites on the HDS catalysts. It can also be expected that major sulfur compounds in C4DBTs detected in HMD should also contain the alkyl substituents at the 4- and/or 6-positions, respectively, as they have low reactivity, and thus remained in HMD. The results further confirm that the conventional HDS process is highly effective in the removal of the sulfur compounds without the steric hindrance, leaving behind only the DBTs with alkyl substituents at the 4- and/or 6-positions. Consequently, special attention should be paid to understanding of the oxidation reactivity of the DBTs with alkyl substituents at the 4- and/or 6-positions in development of an integration process of HDS and ODS for the cost-efficient production of ultralow-sulfur diesel. 3.2. Oxidation of Alkyl DBTs. In order to investigate oxidation reactivity of different sulfur compounds in HMD, the oxidation reaction of the sulfur species in HMD was carried out at 75 °C over the MoO3/Al2O3 catalyst with an O/S molar ratio of 20. The oxidized fuel samples at different reaction times were collected and analyzed by GC-PFPD, and the obtained GC-PFPD chromatograms are shown in Figure 2. The determined conversions for the major sulfur compounds as a function of the reaction time are shown in Figure 3. It is

Figure 4. Pseudo-first-order plots of the sulfur compounds in oxidation reactions at 75 °C.

with a R2 value more than 0.996 was acquired for each sulfur compound, suggesting that the oxidative reaction of each alkyl DBT follows a pseudo-first-order reaction, which is the same as that reported previously in the oxidative reaction of the sulfur compounds in model or real fuels.12,15,18,22 The rate constants obtained by the linear regression are summarized in Table 4. In comparison of these rate constants with those measured in our previous study12 for the same sulfur compounds in the model fuel at the same catalyst and reaction conditions, as also shown in Table 4, it is found that the rate constants for both 4-MDBT and 4,6-DMDBT in the model fuel (20 wt % of toluene in nTable 4. Rate Constants of Various Sulfur Compounds at 75 °C

Figure 3. Oxidation conversions of the alkyl DBTs in HMD as a function of time at 75 °C.

apparent that the oxidation of the alkyl DBTs with cumene hydroperoxide proceeded rapidly. After 90 min reaction, the conversion for the major sulfur compounds was higher than 80% as shown in Figure 3, indicating the effectiveness in the oxidation of the alkyl DBTs at 75 °C over the MoO3/Al2O3 catalyst with an O/S molar ratio of 20. The conversions of the major alkyl DBTs were different, decreasing in the order 4MDBT ≫ 2,4,6-TMDBT ≈ 3,4,6-TMDBT > 4,6-DMDBT ≈

peak no.

compd

rate constants (min−1)

R2 a

1 3 6 7 8 9

DBT 4-MDBT 4,6-DMDBT 4-E,6-MDBT 2,4,6-TMDBT 1,4,6-TMDBT 3,4,6-TMDBT

0.0343 0.0234 0.0192 0.0289 0.0239 0.0280

0.9962 0.9988 0.9978 0.9994 0.9988 0.9971

rate constants in model fuel (min−1)b 0.09197 0.06138 0.04842

a 2

R = correlation factor. bData from ref 12 at the same conditions.

E

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the steric hindrance of the alkyl substituents in determination of the oxidation activity of alkyl DBTs.

tetradecane) are about two times higher than those in the real hydrotreated middle distillate. It indicates that some coexisting species, such as the nitrogen and aromatic compounds in the real feedstock may take the responsibility in reduction of the oxidative activity of the alkyl DBTs, as suggested previously in several studies.27,28 Such effect is significant and has to be considered in ODS of real feedstock. 3.4. Oxidative Reactivities and Structures of Alkyl DBTs. According to the measured rate constants, it is clear that the oxidative reactivity of the alkyl DBTs decreases in the order of 4-MDBT > 2,4,6-TMDBT ≈ 3,4,6-TMDBT > 1,4,6TMDBT ≈ 4,6-DMDBT > 4-E,6-MDBT. The significant reduction in the oxidative reactivity of the alkyl DBTs in the order of DBT > 4-MDBT > 4,6-DMDBT have been found in the previous studies using a model fuel with t-BuOOH as oxidant over a MoO3/Al2O3 catalyst by Wang et al.15 and using a model fuel ODS with cumene hydroperoxide as oxidant over MoO3/Al2O3 catalyst by the present authors.12 The higher oxidative reactivity of 4-MDBT than that of 4,6-DMDBT is further confirmed in the present study using the real HMD. As discussed in our previous work,12 the oxidative reactivity of the sulfur compounds in the electrophilic oxidation depends dominantly on the two major factors: electronic density on the sulfur atom and steric hindrance around the sulfur atom. Since the electron densities on the sulfur atom in DBT, 4MDBT, and 4,6-DMDBT are almost the same, or even slightly increased in the order of DBT (5.758), 4-MDBT (5.759), and 4,6-DMDBT (5.759) according to the calculation by Ma et al.,29 the different reactivities of them depend dominantly on the severity of their alkyl steric hindrance, i.e., the number of alkyl substituents in the 4- and 6-positions. Consequently, the number of alkyl substituents in the 4- and 6-positions determines dominantly oxidative reactivity of alkyl DBTs. Moreover, it is interesting to find that among all C2DBTs and C3DBTs examined in this study, 4-E,6-MDBT shows the least reactivity. The presence of the bulkier ethyl group at the 4position is most likely the reason for reducing further the reactivity of 4-E,6-MDBT, indicating that not only the number of alkyl substituents at the 4- and 6-positions but also the steric size of these alkyl substituents influence the oxidative reactivity of alkyl DBTs. On the basis of this finding, it can be expected that 4,6-diethyldibenzothiophene (4,6-DEDBT) and 4-isopropyl,6-methyldibenzothiophene (4-i-P,6-MDBT), if they present in the fuel, should have lower reactivity than that of 4-E,6MDBT. All 2,4,6-TMDBT, 3,4,6-TMDBT, 1,4,6-TMDBT, and 4,6-DMDBT have methyl substituents at both 4- and 6positions, implying that they have the same steric hindrance around the sulfur atom. Consequently, the difference in their reactivities may be ascribed to their different electronic densities on the sulfur atom, suggesting that the methyl substituent at the 2- or 3-position may enhance the electronic density on the sulfur atom through a π-conjugation effect, and thus increases the oxidative reactivity of the alkyl DBTs. Further studies, such as the quantum chemical calculations, are needed to precisely clarify the effect of the alkyl substituents on the electronic property on the sulfur atom.30 It should be highlighted that such steric hindrance effect on the oxidative reactivity was found significantly in all the cases using oil-soluble oxidants, such as t-BuOOH and cumene hydroperoxide, over the solid catalysts,15,12,22 However, a contrary result was observed in the cases when using H2O2 oxidant over the liquid catalysts, where the higher electron density on the sulfur atom may play a more important role than

4. CONCLUSIONS The major sulfur compounds that remained in a hydrotreated middle distillate were quantified and identified accordingly. The alkyl DBTs with alkyl substituents at both the 4- and 6positions were found to be the major sulfur compounds remaining in the hydrotreated middle distillate. The oxidation reactivities of various sulfur compounds present in the hydrotreated middle distillate were evaluated through the oxidation reaction of the real hydrotreated middle distillate with cumene hydroperoxide at an O/S molar ratio of 20 in a batch reactor system at 75 °C over a MoO3/Al2O3 catalyst. More than 90% conversion for all alkyl DBTs was achieved within 90 min. It was found that the oxidation of each sulfur compound follows a pseudo-first-order reaction. The measured rate constants for the major sulfur compounds indicate that the oxidative reactivity of the various sulfur compounds are quite different, decreasing in the order of 4-MDBT > 2,4,6-TMDBT ≈ 3,4,6-TMDBT > 1,4,6-TMDBT ≈ 4,6-DMDBT > 4-E,6MDBT. Comparison of the molecular structures of these alkyl DBTs with their reactivities implies that the oxidation activity of alkyl DBTs is mainly dependent on the steric hindrance around the central sulfur atom caused by the alkyl substituents at the 4- and/or 6-positions in the heterogeneously catalytic oxidation of alkyl DBTs with oil-soluble oxidant. Consequently, more attention should be paid to treat the alkyl DBTs with the alkyl substituents at both the 4- and 6-positions in developing an efficient ODS process.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +965 24956826. Fax: +965 23980445. E-mail: msafa@ kisr.edu.kw (M.A.S.). *Tel.: +965 24956826. Fax: +965 23980445. E-mail: [email protected] (X.M.). ORCID

Muhieddine A. Safa: 0000-0002-1580-4744 Xiaoliang Ma: 0000-0003-0450-0662 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Kuwait Institute for Scientific Research (KISR) for financial support. Also special thanks are given to KNPC for providing the hydrotreated middle distillate for this study.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.7b01272 Energy Fuels XXXX, XXX, XXX−XXX