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Heteropolyanion-Based Ionic Liquid for Deep Desulfurization of Fuels in Ionic Liquids Wangli Huang,† Wenshuai Zhu,† Huaming Li,*,† Hua Shi,† Guopeng Zhu,† Hui Liu,† and Guangying Chen*,‡ School of Chemistry and Chemical Engineering, Jiangsu UniVersity, Zhenjiang 212013, P. R. China, and College of Chemistry and Chemical Engineering, Hainan Normal UniVersity, Haikou 571158, P. R. China
A heteropolyanion-based ionic liquid, [(3-sulfonic acid) propylpyridine]3PW12O40 · 2H2O, [PSPy]3PW12O40 · 2H2O, abbreviated [PSPy]3PW, was synthesized and approved as an effective catalyst for desulfurization of fuels in [omim]PF6 by using aqueous H2O2 as oxidant. The catalysis was fulfilled with advantages of high activity, simplified workup, and flexible recyclability. The catalytic oxidation reactivity of sulfur-containing compounds was in the order dibenzothiophene (DBT) > 4,6-dimethyldibenzothiophene (4,6-DMDBT) > benzothiophene (BT). The effects of the amount of [PSPy]3PW, H2O2, and reaction time and temperature were investigated in detail. Under the optimal conditions, the removal of DBT achieved 99.4%. Especially, we found that the removal of 4,6-DMDBT could be up to 98.8%, and the system could be recycled at least 9 times without significant decrease in activity. The sulfur level of FCC gasoline could be reduced from 360 to 70 ppm in the extraction and catalytic oxidation system. 1. Introduction In recent years, large amounts of fuel oils have been demanded in industries and daily life with the rapid development of society. At the same time, environmental concerns have also attracted more attention. That can be attributed to the main exhaust gas SOx, a major source of acid rain, and poison noble metal catalysts that are irreversibly generated by automobiles. Consequently, many countries legislate more stringent environmental regulations to restrict the S-content of fuels. To meet the growing demand, ultraclean fuels ( 4,6-DMDBT > BT at 30 °C for 1 h under the same reaction conditions in the extraction and catalytic oxidation desulfurization system (ECODS). It was noteworthy that the removal of 4,6-DMDBT (98.8%) was near the removal of DBT (99.4%) after 1 h while the BT removal was 69.9%. As the reaction went on, the DBT and 4,6-DMDBT were removed completely in 90 min, whereas the BT only reached 84.3%. The result could be attributed to the lower electron density of the sulfur atom on BT (5.739). As for DBT and 4,6-DMDBT, the electron density of the former (5.758) was as much as the latter (5.760).15 Therefore, it was clearly demonstrated that the removal of DBT and 4,6-DMDBT were higher than BT. However, DBT and 4,6-DMDBT had slight differences in sulfur electron density, so the difference in reactivity was dominantly
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the apparent consumption of DBT was obtained from the pseudo-first-order equation -dCA /dt ) kCA ln
CA0 ) kt CA
(1)
(2)
where CA0 and CA were the sulfur concentrations at time zero and time t (min) and k was the first-order rate constant (min-1). The plot of ln(CA0/CA) against t, a straight line with slope k was obtained (Figure 10). Half-lives were calculated using eq 3, which was derived from eq 2 by replacing Ct with C0/2. t1/2 ) Figure 9. Removal of sulfur compounds vs the reaction time at 30 °C. Conditions: IL ) [omim]PF6 ) 1 mL, t ) 1 h, model oil ) 5 mL, n([PSPy]3PW) ) 0.65 µmol, n(H2O2) ) 0.312 mmol, T ) 30 °C.
Figure 10. Pseudo-first-order kinetics for oxidation of different substrates. Conditions: IL ) [omim]PF6 ) 1 mL, t ) 1 h, model oil ) 5 mL, n([PSPy]3PW) ) 0.65 µmol, n(H2O2) ) 0.312 mmol, T ) 30 °C.
due to the influence of steric hindrance. 4,6-DMDBT had two methyl groups, as well as the higher steric hindrance. As a result, the reactivity of DBT was higher than that of 4,6-DMDBT. Furthermore, the result could also be evidenced from the plot of Figure 10, which was observed to be a straight line in all the cases. The values of the rate constant k were calculated and summarized in Figure 10. Obviously, the rate constant decreased in the order DBT > 4,6-DMDBT > BT. 3.7. Effect of the Recycle of Ionic Liquid. To account for the advantage of the ECODS better, recycling the system was an important factor as well and was necessary to investigate. After the first reaction run, the upper layer was separated out by decantation as completely as possible. At the same time, the residual model oil and the IL phase were distilled at 40 °C for 10 h, and then fresh H2O2 and model oil were added for the next circle under the same conditions. It was found that the desulfurization system could be recycled at least nine times without significant decrease in activity. 3.8. Kinetics Study of Catalytic Oxidation on S-Compound Oxidation. Reaction kinetics was of great importance in explaining the reaction mechanism. Experiments to obtain kinetics parameters of the oxidation of sulfur compounds were performed under the optimal conditions. The rate constant for
0.693 k
(3)
The apparent rate constants of DBT, BT, and 4,6-DMDBT were 0.0890, 0.0201, and 0.0635 min-1 and the half-lives were 7.79, 34.5, and 10.9 min. 3.9. Investigation of Actual FCC Gasoline. The extraction and catalytic oxidation desulfurization process was applied to the actual FCC gasoline, which was obtained from Jiangdu petrifaction. The FCC parameters were as follows: density (g/ mL), 0.7346; total content of sulfur (ppm), 360; value of bromine (g of Br/100 g), 73.1. The sulfur-containing compounds in the gasoline could be oxidized with 0.0444 g of [PSPy]3PW, 2 mL of H2O2, and 1 mL of [omim]PF6 at 30 °C for 3 h. The upper phase was withdrawn and analyzed by microcoulometry (detection limit: 0.5 ppm). The total sulfur level of FCC gasoline was decreased from 360 to 70 ppm. High desulfurization of real FCC gasoline could be achieved after several runs at the same conditions. This suggested that [PSPy]3PW had high catalytic activity for all kinds of sulfur-containing compounds present in actual FCC gasoline. 4. Conclucions In this work, deep desulfurization of dibenzothiophene (DBT) with high activity, based on a combination of extraction and catalytic oxidation using hydrogen peroxide, the [PSPy]3PW, and the ionic liquid ([omim]PF6), under mild conditions (30 °C, 1 h), was demonstrated and the main findings were summarized as follows: (1) Heteropolyanion-based ionic liquid [PSPy]3PW used as the catalyst in the system showed high selectivity and reactivity toward DBT oxidation. And the catalytic oxidation activity of the sulfur-containing compounds occurred in the following order: DBT > 4,6-DMDBT > BT. (2) The reaction rates of the oxidation of sulfur-containing compounds increased with temperature, the amount of the [PSPy]3PW catalyst, and the molar ratio of H2O2 and sulfurcontaining compounds. (3) The catalytic oxidation system could be recycled at least nine times without a significant decrease in activity. Noticeably, the removal of 4,6-DMDBT also achieved 98.4% under the same conditions. (4) The [PSPy]3PW catalyst showed high catalytic activity in the process of catalytic oxidation of actual FCC gasoline using H2O2 as an oxidant under mild conditions. Acknowledgment This work was financially supported by the National Nature Science Foundation of China (No. 20676057, 20876071),
Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010
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ReceiVed for reView February 1, 2010 ReVised manuscript receiVed July 22, 2010 Accepted July 28, 2010 IE100234D
