Energy & Fuels 2007, 21, 1859-1862
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Esterification and Polymerization by Catalytic Oxidation of Cyclohexane at Room Temperature over Modified Sulfated TiO2 in the Air Hongzhu Ma,* Juan Li, and Bo Wang Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, Institute of Energy Chemistry, School of Chemistry and Materials Science, Shaanxi Normal UniVersity, Xi’an, 710062, People’s Republic of China ReceiVed March 27, 2007. ReVised Manuscript ReceiVed May 31, 2007
The products of esters and long-chain alkanes were obtained by one step of the catalytic oxidation of cyclohexane at room temperature over modified sulfated TiO2 in the air. It is a clean and convenient way to achieve esterification and polymerization. Such products are very important to improve the quality of the fuel. Several catalysts related to TiO2 were prepared and characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy techniques. From the molecule structure and its atomic valence of the SO42-/TiO2-Co2O3, it can be confirmed that the modified sulfated TiO2 efficiently reacted with the reaction. According to the results of UV and gas chromatography-mass spectrometry, SO42-/ TiO2-Co2O3 exhibited better catalytic activity than the other catalysts. The value and yield of the products were also discussed.
1. Introduction Catalytic oxidation of cyclohexane is one of the attractive research directions in the chemical field. Traditionally, the process for cyclohexane oxidation is carried out at 423-433 K in the presence of a homogeneous catalyst. This process is low in energy efficiency and generates plenty of byproducts and waste.1 It also requires high-energy consumption for the separation of the catalyst and the products, which may cause pollution.2-5 So it is necessary to find an environmentally friendly way to improve the catalytic process of oxidizing cyclohexane under mild conditions. As environmental regulations on motor gasoline became stricter throughout the world, petroleum refiners were compelled to search for new routes to enhance the octane rating. For this objective, refiners have relied on a variety of options, including increasing the aromatics content and the use of oxygenated compounds.6 The so-called oxygenates, such as methyl tertbutyl ether, diisopropyl ether, and dimethoxymethane, are often used as octane boosters.7,8 Oxalates have attracted much attention as octane boosters, due to their pollution reduction and excellent blending capabilities. In its traditional synthesized method, sulfuric acid was used as a catalyst; this conventional * Corresponding author. Tel: +86 29 85308442. Fax: +86 29 85307774. E-mail:
[email protected]. (1) Zhou, L.; Xu, J.; Miao, H.; Wang, F.; Li, X. Appl. Catal. 2005, 292, 223-228. (2) Patcas, F.; Patcas, F. C. Appl. Catal. 2006, 117, 253-258. (3) Huang, G.; Guo, C.; Tang, S. J. Mol. Catal. 2006, 261, 215-230. (4) Hussain, A.; Shukl, R. S.; Thorat, R. B.; Padhiyar, H. J.; Bhatt, K. N. J. Mol. Catal. 2003, 193, 1-12. (5) Zhua, Y.; Li, J.; Xie, X.; Yang, X.; Wu, Y. J. Mol. Catal. 2006, 246, 185-189. (6) Rezgui, Y.; Guemini, M. Energy Fuels 2007, 21, 602-609. (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.
industrial acid catalyst has unavoidable drawbacks because of their severe corrosion and high susceptibility to water. Thus, the search for environmentally benign heterogeneous catalysts has driven the worldwide research of new materials as a substitute for current liquid acids and halogen-based solid acids. SO42-/MxOy is a new type of catalyst; it has some advantages such as its acid strength, not leading to corrosion for the reactor, and being free from pollution. So it has been extensively used in many organic catalytic reactions of esterification, isomerization of n-alkanes, polymerization, acylation, and so on.9 Recently, several other research groups reported that the activity, selectivity, and stability of sulfated metal oxide catalysts are also improved by the addition of noble metals and transition metal oxides such as Cu and Co. The modified catalysts have been shown to be highly active for alkane isomerization, to have high reactivity at low temperatures, and to have a high degree of reusability.10 Esterification and polymerization are useful reactions in the applied chemistry field, but the esters are usually obtained by direct esterfication of the acid or transesterification of natural oils with an alcohol, at a relatively high temperature, and polymerization is caused by unsaturated compounds.11 Now, such products can be gained by only one step of catalytic oxidation of cyclohexanes. This reaction also upgrades lowoctane straight-run naphthas to higher-octane motor fuel blending components by catalytically promoting specific groups of chemical reactions, typically those leading to aromatic groups and oxygenous compounds. Dibutyl phthalate, the consumption of which is over 2 million tons each year worldwide, is used widely in the production of (9) Reddy, B. M.; Sreekanth, P. M.; Reddy, V. R. J. Mol. Catal. 2005, 225, 71-78. (10) Arata, K.; Matsuhashi, H.; Hino, M.; Nakamura, H. Catal. Today 2003, 81, 17-30. (11) Arata, K.; Nakamura, H.; Shouji, M. Appl. Catal., A 2000, 197, 213-219.
10.1021/ef0701541 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/30/2007
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Figure 1. SEM images of (a) TiO2 (2500×) and (b) SO42-/TiO2-Co2O3 (2500×).
plastic material and rubber as a plasticizer; the long-chain alkanes are the main components of diesel fuel and play an important role in the improvement of the cetane number and performance of diesel fuel; (E)-3-tert-butyloct-6-en-1-ol and cyclohexylmethyl tridecyl oxalate, which have not been reported before, were obtained in the reaction at high yields. So it is more economic for a wider range of markets than traditional oxidation of cyclohexane to cyclohexanol and cyclohexanone. In the present paper, the modified sulfated titanium oxide (SO42-/TiO2-Co2O3) and other general catalysts (SO42-/TiO2, SO42-/Co2O3, and TiO2) were prepared for catalyzation of the oxidation reaction of cyclohexane at room temperature. The influence of the different catalysts on the reaction was studied and compared; in particular, the results of characterizing the catalysts and the yield of various products were discussed. 2. Experimental Section 2.1. Reagent. All chemicals used in the experiment were analytical pure reagents and used without any further purification. TiCl4, CoCl3, H2SO4 (98%), and C6H12 were obtained from Xi’an Reagent Co. 2.2. Catalyst Preparation. The synthesis of a modified sulfated TiO2 catalyst was presented as follows: 20 g of TiCl4 and 1 g of CoCl3 were dissolved under stirring, and the pH was adjusted to 8-9 with 25% aqueous ammonia. The hydroxide was thoroughly washed in order to eliminate the subsistent chlorine ions, then dried in the air at 373 K for 24 h and immersed in a 0.8 mol/mL sulfuric acid solution for 12 h. The sulfated sample was washed and dried in vacuo at 383 K. Finally, the solid material was calcinated at 753 K for 3 h and ground into powder. For comparison, SO42-/TiO2, SO42-/Co2O3, and TiO2 were prepared by a conventional method adapted from the literature.11 The used SO42-/TiO2-Co2O3 can be regenerated after calcination at 753 K for 3 h again. 2.3. Experimental Setup. Catalytic tests were carried out at room temperature in a microreactor connected to a gas chromatograph. The catalysts were pretreated at 573 K for 1 h in a nitrogen atmosphere before each term. The feed mixture of 120 mL cyclohexane and 9.8 g modified sulfated TiO2 were introduced into the reactor for 120 min. The reaction experiments were carried out at atmospheric pressure and room temperature. The total volumetric flow rate of air was 1000 mL/min. Products were analyzed by online UV spectroscopy and GC/MS using a Hewlett-Packard 5985 B quadrupole mass spectrometer and RTE-IV data system. 2.4. Characterization of the Catalyst and Analysis of the Products. The solid morphology and average crystal size of the catalysts were determined by scanning electron microscopy (SEM; Quanta 200, Holland). The molecular structure of the catalysts was tested by Fourier transform infrared (FT-IR) spectrometry (model
Figure 2. FT-IR spectroscopy of the different catalysts (a) TiO2 before reaction, (b) SO42-/TiO2-Co2O3 before reaction, and (c) SO42-/TiO2Co2O3 during regeneration (753 K).
FT-IR Equinx55, Germany). After the reaction finished, the catalysts were dried in vacuo, then analyzed by X-ray photoelectron spectroscopy (XPS) using a machine equipped with a Mg KR (1253.6 eV) source. Ultra-absorbance of the samples was monitored using a double-beam UV-vis spectrophotometer (UV-7504, China). The component and its distribution of the products were analyzed by gas chromatography-mass spectrometry (GC/MS; QP2010, Japan).
3. Results and Discussions 3.1. Morphology of the Catalyst. SEM was used to determine the particle morphology and particle size of the catalyst. SEM pictures of the TiO2 (a) and SO42-/TiO2-Co2O3 (b) samples (10 µm) before the reaction, given in Figure 1, were typical for the morphology of the block and alveolate mesoporous structures. The surface of TiO2 was smooth, only with a few small holes, while it became rough and loose after being exposed to H2SO4 and Co2O3. 3.2. Molecular Structure of the Catalyst. The different catalyst samples, which were dried in vacuo at 373 K for 1 h first, were diluted in KBr. The infrared absorption spectra of the catalyst, at room temperature, are displayed in Figure 2. In all cases, a wide band appeared at 1631 cm-1 corresponding to the flexion vibration band of the H2O molecule, which was ascribed to the physisorbed water in the catalyst. The peaks between 800 and 1400 cm-1 assigned to SdO bond or S-O bond were the characteristic peaks; their shapes and relative intensities suggested that, in the film, the structure of the solid
Catalytic Oxidation of Cyclohexane
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Figure 3. Binding energy of all the elements in SO42-/TiO2-Co2O3 after the reaction.
Figure 4. Binding energy of S2p in SO42-/TiO2-Co2O3 after the reaction.
catalysts remained stable even when many other components existed.12-14 Because of the absence of SO42- in the sample of TiO2, it only had one band of the H2O molecule. As Figure 2 shows, the bands at 980 and 1097 cm-1, associated with the symmetric stretching vibration band of the OdSdO, and the band occurring at 1388 cm-1 indicated the asymmetric stretching vibration of the OdSdO. It was noted that a chelate bidentate SO42- coordinated to metal oxides such as Ti4+ because the highest stretching vibration of the SO42- in the samples was above 1200 cm-1. It should be pointed out that the bands of 1240∼1310 and 1352∼1390 cm-1 respectively belonged to the inorganic chelate, in which the SdO was mainly in the shape of an ionic bond and organic sulfate structure, in which it was a covalent double bond. According to Figure 2, the SdO of the solid catalysts was a covalent double bond, so it may have induced the acid and improved the catalytic activity. From Figure 2, it could also be found that the curve of the regenerated catalyst was similar to that of the fresh SO42-/TiO2Co2O3, though the intensity of acid features decreased faintly. Such a result was identical with those reported by other authors.15 3.3. XPS Analysis of the Catalyst. After the reaction, SO42-/ TiO2-Co2O3 was tested via XPS. The binding energy and the (12) Schulz, G. D. J. Org. Chem. 1972, 37, 25-34. (13) Partenheimer, W. Catal. Today 1995, 23, 69. (14) Salas, P.; Heranadez, J. G.; Montoya, J. A.; Navarrete, J.; Salmones, J.; Schifter, I.; Morales, J. J. Mol. Catal. 1997, 123, 149-154. (15) Sohn, J. R.; Lee, S. H.; Lim, J. S. Catal. Today 2006, 116, 143150.
composition of the catalyst are shown in Figure 3. The catalyst was not polluted by any other substance, and the absorbed amounts of sulfur in it were relatively high. The S2p XPS for SO42-/TiO2-Co2O3 is shown in Figure 4. It was observed that the binding energy of sulfur was 168.4 eV, which suggested that the sulfur in the catalyst existed in a six-oxidation state (S6+). It is an important sign to indicate the formation of the acid. The valence simulation curve of the cobalt (Figure 5) shows that the normal binding energies of CoO and Co2O3 were 780.6 and 779.4 eV, respectively, and the testing result was 780.3 eV; therefore, it could be inferred that the valence of the cobalt was a mixture in the catalyst. In Figure 5, the divalent cobalt form (Co2+) accounted for 60.42% of the total elemental cobalt, and the trivalent cobalt form (Co3+) occupied 39.58%. In comparison to Co3+, which occupied 100% of the total cobalt element before the reaction, this finding illuminated the fact that Co2O3 was involved with the reaction and was reduced to CoO. 3.4. Analysis of the Products via UV and GC/MS. The UV-vis absorption spectrophotometer was used to supervise the whole process of the reaction. With increasing time, the curve with three obvious absorbed peaks (211, 228, and 257 nm) ascended gradually, which was similar to the curve of toluene; but the E absorption band of the benzene ring bathochromically shifted from 217 to 228 nm, and the R absorption band presented itself at 211 nm. All of these may be because of the oxidation of cyclohexane and molecular oxygen: when the H atom or methyl of the toluene was
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Figure 5. Binding energy and valence simulation curve of cobalt in SO42-/TiO2-Co2O3 after the reaction.
Figure 7. GC of the products (SO42-/TiO2-Co2O3 as the catalyst). Figure 6. Effect of different catalysts on the reaction system (90 min): (a) SO42-/TiO2-Co2O3, (b) SO42-/TiO2, (c) regeneration of SO42-/ TiO2-Co2O3, (d) TiO2, and (e) SO42-/ Co2O3.
substituted by a carboxyl, the energy needed by the π f π* transition reduced, and the E absorption generated a bathochromic effect. The R absorption band was likely the n f π* transition, which may be due to the alkane being oxidized to the carboxylic acid or aldehyde; the absorption peak’s height increased, which indicated that the reaction degree was proportioned directly to the reaction time and content of the oxygenous aromatic products. At the same reaction time (90 min), the UV results of the effect of different catalysts on the reaction system are shown in Figure 6. The results suggested that SO42-/TiO2-Co2O3 was superior in catalytic activity to the other catalysts, and it can be reused. The cobalt oxide was perfect as a modified element, but it was invalid to catalytically oxidize cyclohexane as a single catalyst. The GC of the products is shown in Figure 7 (the peak of the solvent was taken off). Table 1 lists the main products and compares their percentages in different mediums at the same reaction time of 90 min. Long-chain alkanes (84.733%) and esters (13.293%) with high yields were the main products. The addition of sulfate and cobalt oxide to titanium oxide was found to promote the catalytic activity because the yield of the products was higher when SO42-/TiO2-Co2O3 was used as a catalyst than when the other two catalysts were used. The result of the GC/MS coincided with the UV analysis.
Table 1. Main Products and their Distribution by Different Catalysts
4. Conclusion The effect of the new catalyst (SO42-/TiO2-Co2O3) on the esterification and polymerization of cyclohexane was studied at room temperature and atmospheric pressure. Compared with the oxidation of cyclohexane by traditional methods, the process became rather easy with molecular oxygen over a modified catalyst. The introduction of the modified sulfated TiO2 changed the kinetics of the reaction and made the conditions of the reaction mild. Excellent cyclohexane conversion could be obtained at room temperature. EF0701541
