3748
Energy & Fuels 2007, 21, 3748–3749
Photocatalytic Activity of SO42-/SnO2–TiO2 Catalysts: Direct Oxidation of n-Heptane to Ester under Mild Conditions Bo Wang,* Xue Cui, Hongzhu Ma, and Juan Li Institute of Energy–Chemistry, College of Chemistry and Materials Science, Shaanxi Normal UniVersity, 710062, Xi’an, People’s Republic of China ReceiVed July 13, 2007. ReVised Manuscript ReceiVed September 9, 2007 Photochemical reactions on the surface of light-activated semiconductors are attracting considerable academic and industrial interest.1 The semiconductor photocatalyst generates an electron and hole pair (e-/h+) upon irradiation of light energy that could be used in initiating oxidation and reduction reactions, respectively. Of all of the photocatalysts, TiO2 emerges to be an effectual, easily available, relatively inexpensive, and chemically stable one.2,3 This knowledge, together with the novel approach to material synthesis, is contributing to the design and development of new TiO2-based photocatalysts with high activity and stability for commercial applications. Solid superacids are of the greatest interest in solid catalysis and green catalysts. It is well-known that the solid superacid SO42-/MxOy (H0 < -11.93) have been used in isomerization, alkylation, acylation, polymerization, oligomerization, esterification, and milder temperatures and is far more easily separated from the reaction products stream than the liquid ones.4,5 In recent years, a large variety of solid acids have been reported, as well as their acidic and catalytic properties. Sulfated tin and titanium oxides show superacid strength; the acid strength is lower than -18 and -14.5 on the Hammett function scale, respectively.6 In addition, sulfated titanium (SO42-/TiO2) has been known to possess photocatalytic activity; therefore, Ti was selected to modify sulfated tin oxide. SO42-/SnO2–TiO2 is relatively easy to prepare, with a high acid strength, and the reactivity of a photocatalyst is also strong. Research octane numbers (RONs) provide a sensitive indication of the antiknocking behavior of the fuel. The higher the octane number, the better the gasoline resists detonation and the smoother the engine runs. Different additives were applied to achieve the required specifications of the highly knock-resisting super-grade gasolines used today. The so-called oxygenates, such as methyl tert-butyl ether (MTBE), di-isopropyl ether (DIPE), and dimethoxymethane (DMM), are often used as octane boosters.7,8 Oxalates have attracted much attention as octane boosters, because of their pollution reduction and excellent blending capabilities. In its traditional synthesized method, sulfuric acid was used as the catalyst. These conventional industrial acid catalysts have unavoidable drawbacks because of their severe corrosivity and high susceptibility to water. Thus, the search for environmentally benign heterogeneous catalysts has driven the worldwide research of * To whom correspondence should be addressed. Telephone: +86-2985308442. Fax: +86-29-85307774. E-mail:
[email protected]. (1) Zhang, Q.; Gao, L.; Guo, J. Appl. Catal., B 2000, 26, 207–215. (2) Zuo, G. M.; Cheng, Z. X.; Chen, H.; Li, G. W.; Miao, T. J. Hazard. Mater. 2006, 128, 158–163. (3) López, T.; Bosch, P.; Tzompantzi, F.; Gómez, R.; Navarrete, J.; López-Salinas, E.; Llanos, M. E. Appl. Catal., A 2000, 197, 107–110. (4) Hinoa, M.; Aratab, K. Appl. Catal., A 1998, 173, 121–124. (5) Matsuhashi, H.; Miyazaki, H.; Kawamura., Y.; Nakamura, H.; Arata, K. Chem. Mater. 2001, 13, 3038–3042. (6) Arata, K.; Matsuhashi, H.; Hino, M.; Nakamura, H. Catal. Today 2003, 81, 17–30. (7) Pasadakis, N.; Gaganis, V.; Foteinopoulos, C. Fuel Process. Technol. 2006, 87, 505–509. (8) Moljord, K.; Hellenes, H. G.; Hoff, A.; Tanem, I. Ind. Eng. Chem. Res. 1996, 35, 99–105.
Table 1. Measurement of the BET Surface Area catalyst
surface area (m2 g-1)
SnO2 SO42-/SnO2 SO42-/SnO2–TiO2
29.953 90.235 156.156
new materials as a substitute for current liquid acids and halogenbased solid acids. Oxidation and isomerization are a general process in refineries worldwide. Catalytic oxidations of hydrocarbons using oxygen or air as the oxidant are significant and economical to the chemical industry. Their function is to upgrade low-octane straight run naphthas to higher octane motor-fuel-blending components by catalytically promoting specific groups of chemical reactions, typically those leading to aromatic groups, cycloalkanes, and oxygenous compounds. Binary oxides prepared by the chemical coprecipitation method was investigated.9,10 The concentration of Ti was 10.0 wt %. The direct oxidation of n-heptane at atmospheric pressure and room temperature was selected as the target reaction to investigate the catalytic activity of the prepared catalysts. The measurement of photocatalytic activity was carried out at room temperature in a microreactor connected to a gas chromatograph. The reactor was surrounded with four 8 W UV radiation (365 nm). The catalyst was pretreated in a nitrogen atmosphere at 573 K for 1 h. Typically, 5 g of catalyst powder was placed in the reactor, and 100 mL of n-heptane was introduced into the reaction system, unless stated otherwise. The finely ground catalyst particles were placed on a porous frit or quartz wool. The total volumetric flow rate of air was 1 m3 min-1. The phase-transfer catalysts (PTCs) were introduced to enhance the oxidation rate.11,12 After evacuation, samples were injected directly into an online gas chromatograph–mass spectrometry. Several main products, 2-isopropylcyclohexanol, cyclohexylmethyl tridecyl oxalate, and polymer, are all octane boosters. All of the samples were calcined at 823 K for 3 h to obtain comparable Brunauer–Emmett–Teller (BET) surfaces. The surface areas are reported in Table 1. The areas of the sulfated oxides are much larger compared to those of the oxides without the sulfate treatment. It is evident that the SO42- anions on the surface of SO42-/ SnO2 can effectively impede its crystallization in calcination, thus hindering the loss of its surface area. This high surface area of SO42-/ SnO2–TiO2 is due to the doping effect of Ti. It was also reported that a small amount of metal oxides modified in tin powder can stabilize the tetragonal and cubic phases. The role of Ti in the catalysts is to form a thermally stable solid solution with tin and, consequently, to give their high surface area. A higher activity is usually attributed to its larger specific surface area.13 Alternatively, gas chromatography in conjunction with mass spectrometry (GC–MS) has been known for its superior separation (9) Arata, K.; Nakamura, H.; Miyuki, S. Appl. Catal., A 2000, 197, 213– 219. (10) Lu, G. Appl. Catal., A 1995, 133, 11–18. (11) Seol, Y.; Schwartz, F. W. J. Contam. Hydrol. 2000, 44, 185–201. (12) Hasegawa, K.; Arai, S.; Nishida, A. Tetrahedron 2006, 62, 1390– 1401.
10.1021/ef700402u CCC: $37.00 2007 American Chemical Society Published on Web 09/26/2007
Communications
Energy & Fuels, Vol. 21, No. 6, 2007 3749 Table 2. Main Product Distribution (SO42-/SnO2 as the Catalyst)
Table 3. n-Heptane Oxidation by the Different Catalystsa product percentage (%) catalysts
alcoholize
ester
polymer
conversion of n-heptane
SO42-/SnO2 SO42-/SnO2–TiO2 SO42-/SnO2–TiO2 (UV)
2.094 2.056 9.344
14.148 14.433 42.470
83.785 83.511 48.186
8.53 14.41 13.39
a
Reaction conditions: reaction temperature, 298 K; reaction time, 120 min.
Figure 1. GC of the products. (a) SO42-/SnO2 as the catalyst without UV irradiation. (b) SO42-/SnO2–TiO2 as the catalyst, without UV irradiation. (c) SO42-/SnO2–TiO2 as the catalyst, operated in the presence of UV.
of complex organic compounds, greater sensitivity, and shorter measuring time and, hence, is better suitable for the detection and identification of volatile organic compounds. The aromatic hydrocarbon and oxide fraction was analyzed using GC–MS. The GC of the products was shown in Figure 1 (the peak of the solvent was taken off). Table 2 shows the product distribution formed on SO42-/SnO2. Other catalysts also gave the same products as those, although their selectivities were different. The selectivity to these target products is shown in Table 3. Comparisons were made between the SO42-/SnO2 and SO42-/ SnO2–TiO2 catalysts at the same reaction time. The conversion of n-heptane and the yield of the products with SO42-/SnO2–TiO2 as the catalyst were higher than that of the SO42-/SnO2 catalyst, indicating that the addition of metal oxide to tin oxide promoted the catalytic activity. When UV radiation was applied, the catalytic activity of SO42-/SnO2–TiO2 seems to be more promising; a very high selectivity
to the oxidation products was obtained. Among the various products, 2-isopropylcyclohexanol oxalate was also obtained with high selectivity (45%). Undesirable side reactions, such as polymerization, refer to the formation of C12, C14, or more carbon numbers, which were also sufficiently produced. The result of the GC–MS coincided with the BET analysis. Although the conversion of n-heptane is low (only 13.39%), fortunately, a 51.814% oxidation product (alcohol and ester) in onepot oxidation of n-heptane has been successively accomplished with the modified sulfated oxide at room temperature, an air atmosphere, and in the presence of UV light (this result has not been reported). Oxidations of hydrocarbons in air are most likely to involve classical free-radical autoxidation pathways with the intermediate formation of RO2H.14 The latter may subsequently undergo heterolytic oxygen transfer processes. Similar mechanisms are usually reported.15–17 The mechanism of these reactions is not clear at this stage; therefore, it still demands further research. Metal-promoted solid superacids, which are highly photocatalytic active for the n-heptane oxidation, have been prepared by the addition of Ti salts to SO42-/SnO2. The introduction of Ti into SO42-/SnO2 can increase its surface area greatly. SO42-/SnO2–TiO2 shows certain photocatalytic activities in n-heptane oxidation using air as the oxidant in the presence of UV light. Some main products, such as 2-isopropyl cyclohexanol, cyclohexylmethyl tridecyl oxalate, and polymer, that have not been reported in the direct oxidation of alkane by the traditional method have been obtained. It was inferred that this method of modification resulted in an increase in the acid strength as well as acidity of the parent oxide catalyst. In comparison to the traditional oxidation methods, this method achieved less waste, a simple workup procedure, milder reaction conditions, and shorter reaction times. This new application of solid superacid also sets light on its special characteristics and its application field. Acknowledgment. We are grateful to be supported in part by a graduated innovation fund from Shaanxi Normal University. EF700402U (13) Li, B.; Gonzalez, R. D. Ind. Eng. Chem. Res. 1996, 35, 3141– 3148. (14) Sheldon, R. A.; Arends, I. W. C. E.; Lempers, H. E. B. Catal. Today 1998, 41, 387–407. (15) Gugumus, F. Polym. Degrad. Stab. 2002, 76, 381–391. (16) Marquez, A.; Raul, J.; Lvarez, A. EnViron. Sci. Technol. 2005, 39, 8797–8802. (17) Arata, K.; Matsuhashi, H.; Hino, M.; Nakamura, H. Catal. Today 2003, 81, 17–30.
