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Ind. Eng. Chem. Res. 2003, 42, 6034-6039
Vanadosilicate Molecular Sieve as a Catalyst for Oxidative Desulfurization of Light Oil Yasuhiro Shiraishi,* Tomoko Naito, and Takayuki Hirai Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan
An oxidative desulfurization process for light oil has been investigated using a vanadosilicate molecular sieve as the catalyst and H2O2 as the oxidizing agent. The catalytic activities for three kinds of vanadosilicates, having different structures and pore-size distributions, were compared with those for the corresponding titanosilicates. The oxidation of dibenzothiophene and benzothiophene in acetonitrile was catalyzed more effectively by vanadosilicates than titanosilicates, where the mesoporous vanadosilicate showed the highest activity. The vanadosilicate also accelerated the desulfurization of the actual light oil in an oil/acetonitrile two-phase system: the sulfur content of the oil was decreased successfully from 425 ppm to less than 50 ppm (ultradeep desulfurization level). During the process, nitrogen-containing compounds were also removed successfully from the light oil. However, the vanadosilicate, recovered following the reaction, could not be reused for further treatment of light oil. This is because the catalytic activity of the vanadosilicate decreases significantly during the reaction, owing to the dissolution of vanadium species in the silica framework into the acetonitrile solution. Introduction
Table 1. Properties and Composition of Feed Light Oil
There has been much recent interest in the deep desulfurization of light oil. In our previous work,1 an oxidative desulfurization process for light oil was investigated based on the oxidation of sulfur-containing compounds by H2O2 using titanosilicates (silica-based molecular sieves containing highly dispersed Ti species in the framework) as the catalyst. When light oil and acetonitrile containing H2O2 are stirred in the presence of titanosilicate (Ti-HMS: Ti-containing hexagonal mesoporous silica) at 373 K, dibenzothiophenes (DBTs) and benzothiophenes (BTs) in light oil are distributed into the polar acetonitrile phase and oxidized there into the corresponding sulfoxides and sulfones. These products are highly polarized, such that they do not distribute into the nonpolar light oil, thus providing a successive removal of sulfur compounds from the light oil to the acetonitrile phase. By use of this process, the sulfur content of light oil (1800 ppm) is decreased successfully to less than 500 ppm, which is below the regulatory value that presently applies in Japan and Europe. In this process, nitrogen-containing compounds are also removed from the light oil, and the Ti-HMS recovered can be reused for further desulfurization and denitrogenation of light oil. However, the process requires a relatively long reaction time (>12 h) for deep desulfurization of light oil. A more effective catalyst is therefore required for the development of a more efficient oxidative desulfurization process. Several researchers have reported that vanadosilicate molecular sieves (containing highly dispersed V species in the silica framework) catalyze the oxidation of aromatics, alkanes, olefins, and alkyl sulfides more effectively than titanosilicates.2-7 In the present work, vanadosilicates were used as the catalyst for the de* To whom correspondence should be addressed. Tel.: +81-6-6850-6271. Fax: +81-6-6850-6273. E-mail: shiraish@ cheng.es.osaka-u.ac.jp.
density at 288 K (g/mL) sulfur (ppm) nitrogen (ppm) saturated fraction (vol %)a aromatics (vol %)a,b one-ring two-ring
0.8378 425 74.9 78.4 19.4 2.2
a By the JPI-5S-49-97 normal-phase HPLC method. b Threeand greater-than-three-ring aromatic compounds are present only in trace quantities ( VS-2 > VS-1. The order agrees reasonably well with the order of the average pore diameter for the catalyst, as shown in Table 2. This suggests that it is difficult for the bulky DBT and BT molecules to penetrate the small micropores of VS-1 and VS-2, but they can easily penetrate the large mesopores of V-HMS, as is also the case for titanosilicates.1,15 V-HMS is therefore shown to be the most effective catalyst for the oxidation of DBT and BT with H2O2, among the three vanadosilicates used in the present study. Figure 2 shows the conversion of DBT and the percent consumption of H2O2, during the reaction in acetonitrile using V-HMS or Ti-HMS catalyst in the presence of a 3-fold molar excess of H2O2 based on the initial DBT concentration. Using Ti-HMS, the conversion of DBT increases with an increase in the reaction temperature, accompanied by an increase in the quantity of H2O2 consumed. The conversion of DBT, obtained using V-HMS, is two times higher than that using Ti-HMS at aniline > indole > BT > carbazole, suggesting that the carbazole is the most difficult compound to oxidize among the nitrogen compounds. The order of the reactivity is the same as that obtained using Ti-HMS, as shown in Figure 4 and as described.1 The oxidation products of the respective nitrogen compounds were then identified. GC/MS analysis revealed that the reaction of aniline with H2O2 in the presence of V-HMS produces nitrobenzene as the sole product (Supporting Information available: data 1), whereas the use of Ti-HMS produces azoxybenzene as the sole product.1,17 This is probably due to the difference of the active species on the catalysts. The nitrobenzene is reported to be also produced from aniline by the reaction with tert-butyl hydroperoxide in the presence of VS-1.18 For carbazole, carbazole-1,4-dione is the sole
product when using Ti-HMS.1 However, when using V-HMS, no peak was detected by GC analysis. When the product was concentrated by evaporation of acetonitrile and treated with diazomethane, still no peak was observed by GC, thus suggesting that the product of carbazole has a polymeric structure. The IR spectrum for the product showed new absorption bands, attributable to CdO and -OH groups, and the 1H NMR spectrum exhibited new resonances, attributable to protons for the -OH group and a large number of aromatic protons (Supporting Information available: data 2 and 3). These findings suggest that the product of carbazole has a polymeric structure containing several CdO and -OH groups. Also, for indole, the product showed no peak on the GC chart and was inactive against treatment with diazomethane. IR and 1H NMR spectra also demonstrated the presence of CdO and -OH groups (Supporting Information available: data 4 and 5), thus suggesting that the product of indole also has a complicated polymeric structure. The oxidation of indole on the Ti-HMS catalyst also gives rise to a polymerized material.1 The products of aniline, indole, and carbazole were confirmed to be insoluble in nonpolar solvents, such as n-hexane, but to be soluble in acetonitrile. Therefore, the nitrogen compounds in the actual light oil as well as the sulfur compounds should therefore be removed into the acetonitrile phase during the reaction using V-HMS. The nitrogen content of the actual light oil, which was obtained following reaction (6 h) with V-HMS (the sample is indicated by closed square symbols in Figure 3), was found to be decreased successfully from 75 to 8 ppm. Using Ti-HMS at the same reaction conditions (open square symbols in Figure 3), the nitrogen content of the oil was decreased to 9 ppm. The results suggest that V-HMS has potential for the denitrogenation as well as the desulfurization of light oil. 4. Reuse of Vanadosilicate Catalyst. It is necessary to clarify the reusability of the V-HMS catalyst for further desulfurization and denitrogenation of light oil. It was examined by repeating the batch reaction procedure, as used for closed square symbols in Figure 3, three times over. In each stage, V-HMS, following the reaction, was recovered by filtration, washed with water, dried under vacuum, and then treated again with fresh light oil and acetonitrile containing H2O2. The variations in the sulfur content of the light oil at each stage are shown in Figure 5b, where the results obtained using Ti-HMS are also shown for comparison. Using Ti-HMS, as shown by open symbols and as described previously,1 the decreases in the sulfur contents of the oil at the second and third stages are almost the same as those obtained at the first stage, thus suggesting that Ti-HMS can be reused without the loss of catalytic activity. Using V-HMS, the decrease in the sulfur content of the oil is, on the contrary, decreased with the number of repeated times, and at the third stage, the catalytic activity of V-HMS becomes significantly lower than that of Ti-HMS. This suggests that the catalytic activity of V-HMS is decreased during the reaction. When using V-HMS, the color of the acetonitrile solution was seen to become yellow, whereas the use of Ti-HMS did not cause the color to change. ICP analysis confirmed the presence of V species in the acetonitrile solution, whereas no V species were detected in the light oil phase. Figure 5a shows the variation in the amount of V or Ti on the catalyst at each stage. The amount of V
6038 Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003
Acknowledgment The authors are grateful for the financial support of Grants-in-Aid for Scientific Research (12555215) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and to the Division of Chemical Engineering, Osaka University, for the Lend-Lease Laboratory System. Y.S. acknowledges the financial support by Showa Shell Sekiyu Foundation for Promotion of Environmental Research. Supporting Information Available: GC/MS spectrum for the oxidation product of aniline (data 1), IR (data 2) and 1H NMR spectra (data 3) for the oxidation product of carbazole, and IR (data 4) and 1H NMR spectra (data 5) for the oxidation product of indole. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited Figure 5. Time-course variation in (a) the amount of V and Ti on the V-HMS or Ti-HMS catalyst and (b) the sulfur content of light oil, when the desulfurization is carried out three times sequentially using the recovered catalyst at each stage. Reaction conditions at each stage are the same as those used for open or closed square symbols in Figure 3.
on V-HMS is clearly shown to be decreased gradually with the number of repeated times. The results therefore suggest that the decrease in the catalytic activity of V-HMS results because the V species in the silica framework dissolve into the acetonitrile solution during the reaction. Several literatures have also reported the leaching of V species from vanadosilicate during oxidation.2,18 The above findings suggest that V-HMS exhibits high catalytic activity for the desulfurization and denitrogenation of light oil but cannot be reused for the further treatment of light oil. For the application of vanadosilicate to the practical refining of light oil, more stable vanadosilicate must be synthesized. Conclusion The oxidative desulfurization and denitrogenation of light oil have been investigated using H2O2 as the oxidizing agent and a vanadosilicate molecular sieve as the catalyst, and the following results have been obtained. (1) The vanadosilicates catalyze the oxidation of DBT and BT in acetonitrile more effectively than the titanosilicates, where the mesoporous vanadosilicate (V-HMS) shows the highest activity. V-HMS oxidizes the sulfur compound very effectively even in the presence of aromatic hydrocarbons. (2) V-HMS accelerates the desulfurization of the actual light oil in an oil/acetonitrile containing H2O2 two-phase system more effectively than Ti-HMS: the sulfur content of the oil is decreased successfully to less than 50 ppm (ultradeep desulfurization level). V-HMS is also effective for the denitrogenation of light oil. (3) V-HMS, recovered following the reaction, cannot be reused for further desulfurization and denitrogenation of light oil. This is because the V species on V-HMS dissolve into the acetonitrile solution during the reaction, thus decreasing the catalytic activity of V-HMS significantly.
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Received for review April 15, 2003 Revised manuscript received September 15, 2003 Accepted September 17, 2003 IE030328B
