4530
Ind. Eng. Chem. Res. 2006, 45, 4530-4536
Electrochemical Oxidation of p-Xylene in Methanol Solvent Catalyzed by the Monometal and Multimetal Compounds J. Zhao, B. Wang,* and H. Zh. Ma Institute of Energy-Chemistry, College of Chemistry and Materials Science, Shaanxi Normal UniVersity, 710062 Xi’an, People’s Republic of China
J. T. Zhang Huaze Limited Corporation, People’s Republic of China
The electrochemical oxidation of p-xylene in a methanol solvent catalyzed by monometal and multimetal compounds to methoxyl p-xylene derivatives on a pair of porous graphite plane electrodes with a chemical conversion of >80% was described and investigated. From the results of ultraviolet-visible (UV-Vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), powder X-ray diffractometry (XRD), gas chromatography-mass spectrometry (GC-MS), and fluorescence spectroscopy, a possible free-radical reaction mechanism was proposed. 1. Introduction Oxidation of organic compounds in the liquid phase has been the basis of several industrial processes that have been developed in the past 50 years.1 As a clean and convenient method for the generation, on a preparative scale, of many reactive intermediates (radical ions, radicals, carbanions, carbocations, quinodimethanes), electrochemistry has become one of the highlights in organic synthesis and industrial chemistry, because of its potential applications in the synthesis of pharmaceutical drugs, amino acids, dyestuffs, pesticides, spicery, and organic reagents.2-4 p-Xylene is an important primary chemical material. It mostly is used in the synthesis of terephthalic acid and p-methylbenzaldehyde, and its derivatives usually are obtained from chemical synthesis processes. Its production not only requires higher temperature (100-250 °C), but it also needs acid and a metal catalyst, which causes concern in regard to long-term environmental pollution.1,5-7 Oxidation that is conducted in the liquid phase provides good yield and selectivity, because of the mild conditions afforded by the use of transition metals as catalysts. The mechanism of the oxidation was generally categorized as three types: reaction with free radicals, reaction with metal ions, and reaction with free-radical cations. 1.1. Reaction with Free Radicals. Generally, an oxidation of hydrocarbons occurs, according to a radical chain process in which an initiator extracts a H atom from the hydrocarbon to form a radical that reacts with dioxygen:8-10
In• + RH f InH + R• •
•
(1)
R + O2 f RO2
(2)
RO2• + RH f R• + ROOH
(3)
1.2. Reaction with Metal Ions. Chemical processes that involve activated reactions by metals between alkyl aromatics and molecular oxygen are of great industrial importance, and extensive studies have been performed in this area. Oxidation * To whom correspondence should be addressed. Tel.: 86 29 85308442. Fax: 86 29 85307774. E-mail address:
[email protected].
proceeds via a free-radical-chain mechanism, which is initiated by a change in the oxidation state of the metal. The benzyl radical reacts rapidly with triplet dioxygen. In the presence of a catalyst, the process is ended by acid formation:7,11
ArCH3 + Co3+ f ArCH2• + H+ + Co2+
(4)
ArCH2• + O2 f ArCH2O2•
(5)
ArCH2O2• + Co2+ f ArCHO + Co3+ + OH-
(6)
ArCHO + 0.5O2 f ArCOOH
(7)
1.3. Reaction with Free-Radical Cations. As a general rule, the oxidation of alkyl aromatics by oxygen is realized in acetic acid in the presence of a catalytic redox couple, such as Co3+/ Co2+:12
ArCH3 + Co3+ h [ArCH3] + + Co2+
(8)
[ArCH3] +• f ArCH2• + H+
(9)
ArCH2• + O2 f ArCH2O2•
(10)
ArCH2O2• + Co2+ f ArCHO + [HOCo]2+
(11)
ArCHO + 0.5O2 f ArCOOH
(12)
The trivalent cobalt (Co3+) regenerated by reaction 11 pursues the catalytic cycle of reactions 8-11. However, too much attention has been focused on the oxidation of alkyl aromatics by oxygen in acetic acid. Our interest in the field of oxidation is to study the liquid-phase reaction in methanol, using the electrochemical technique combined with monometal and multimetal catalysts at different pH. 2. Experiment 2.1. Materials and General Methods. All chemicals used in the experiment were analytically pure reagents and used without any further purification. The kaolin used in this study (provided by Shanghai Reagent Co., China) was composed of Al4[Si4O10](OH)8 and had a surface area of 20 m2/g and a pore
10.1021/ie060272w CCC: $33.50 © 2006 American Chemical Society Published on Web 05/18/2006
Ind. Eng. Chem. Res., Vol. 45, No. 13, 2006 4531
Figure 1. Experimental setup for the electrochemical catalytic oxidation.
volume of 0.5 cm3/g. CH3OH, KF, KBr, H2SO4 (98%), Cu(NO3)2, NaOH, Co(NO3)2, H3PO3, and Na3PO4 were obtained from Xi’an Reagent Co., and p-xylene was obtained from Beijing Reagent Co. The effect of the medium was determined using ultraviolet-visible (UV-Vis) spectroscopy (model UVVis 7504 spectrum apparatus, Shanghai Xinmao Co.). The component and distribution of the products were checked via analysis using a combination of gas chromatography (GC) (Agilent, model 6890) and mass spectroscopy (MS) (HewlettPackard, model 5973) and via UV fluorescence spectroscopy (Japan Shimadzu, model RF-540). The catalyst was monitored using X-ray diffractometry (XRD) (Rigaku, Japan) and X-ray photoelectron spectroscopy (XPS) (Perkin-Elmer, model PHI5400). 2.2. Preparation of the Catalyst-Multimetal Compounds. The catalyst was prepared as follows. First, 50 g of Cu(NO3)2, 20 g of Co(NO3)2, and 50 g of Na3PO4 were added into 250 mL of distilled water (pH 7.1), and then 10 mL of H3PO4 were added to dissolve the metal nitrate (where the metal was copper or cobalt); the solution pH then was adjusted with NaOH solution to neutral conditions. Two hundred grams of kaolin then was added into the solution, with stirring, in a water bath at 50 °C for 4 h. The solution then was aged at room temperature for 48 h, filtrated, and washed; the deposit was dried at 100 °C for 4 h and then calcined at 600 °C for 4 h. Finally, the presence of catalyst was determined via XRD. 2.3. Experimental Setup. Figure 1 shows the schematic diagram of the experimental setup for electrochemical catalytic oxidation. The experiments were conducted via the batch process, using a single cell with a capacity of 0.5 L that we designed at a current intensity of 2.0 A. The reaction cell was cooled by the cooling water in a trough to form the roomtemperature conditions for reaction. A condenser was inserted in the reaction cell to prevent the volatilization of the methanol. The anode and cathode were positioned vertically and aligned parallel to each other, with a constant intergap of 1.0 cm. The material used for both the anode and the cathode was a porous graphite plate (dimensions of 110 mm × 60 mm × 6 mm; supplied by Spring Chemical Industrial Company, Limited, Shaanxi, PRC). Twenty grams of the monometal or multimetal catalyst compounds and 3 g of the KBr assisted catalyst were packed around the working electrode, forming a multiphase electrochemical oxidation packed bed. The solution was constantly stirred at 200 rpm, using a magnetic stirrer to maintain uniform concentration of the electrolyte solution. The electric power was supplied with a regulated DC power supply (model WYK302b, Xi’an, PRC). The current and voltage each were adjustable, in the range of 0-2.5 A and 0-35 V, respectively.
2.4. Electrolysis Procedures. The electrolysis was performed in cells without compartments. The solvent-supporting electrolyte system was formed using the following procedures: (i) KF (30 g) and p-xylene (100 mL) were added to 200 mL of anhydrous methanol (pH 7.0, multimetal catalyst); (ii) KF (30 g), sulfuric acid (5 mL, 98%), and p-xylene (100 mL) were added to 200 mL of anhydrous methanol (pH 5.0, multimetal catalyst); (iii) KF (30 g), sulfuric acid (10.7 mL, 98%), and p-xylene (100 mL) were added to 200 mL of anhydrous methanol (pH 3.0, multimetal catalyst); (iv) KF (30 g) and p-xylene (100 mL) were added to 200 mL of anhydrous methanol (pH 7.0, cobaltous nitrate catalyst); (v) KF (30 g), sulfuric acid (5 mL, 98%) and p-xylene (100 mL) were added to 200 mL of anhydrous methanol (pH 5.0, cobaltous nitrate catalyst); (vi) KF (30 g), sulfuric acid (10.7 mL, 98%), and p-xylene (100 mL) were added to 200 mL of anhydrous methanol (pH 3.0, cobaltous nitrate catalyst); (vii) KF (30 g) and p-xylene (100 mL) were added to 200 mL of anhydrous methanol (pH 7.0, cupric nitrate catalyst); (viii) KF (30 g), sulfuric acid (5 mL, 98%), and p-xylene (100 mL) were added to 200 mL of anhydrous methanol (pH 5.0, cupric nitrate catalyst); and (ix) KF (30 g), sulfuric acid (10.7 mL, 98%), and p-xylene (100 mL) were added to 200 mL of anhydrous methanol (pH 3.0, cupric nitrate catalyst). The resulting solutions were placed in the cells, with stirring, and then were electrolyzed at a current intensity of 2.0 A, with an anodic current density of 13.1 mA/cm2 at room temperature, as detected via UV-Vis spectroscopy. The electronic spectra of the reaction system were monitored during each electrolysis, and the conversion of starting material was investigated by UV-Vis spectroscopy every 10 min as follows: 0.01 mL solution was transferred accurately via transfer-pipet and then was diluted to 20 mL in a volumetric flask, and then the electronic spectra were observed at the range of 200-400 nm using methanol as a blank. The current was interrupted at the moment of higher yield in the products. The methanol used for the research of UV-Vis spectrum was reclaimed for the next experiment without pollutions and waste. 2.5. Characterization of the Products and the Catalyst. After the reaction finished, the solution was distilled under the air pressure. The distillates were analyzed by the GC/MS system, using a capillary column (0.25 cm × 30 m) and UV fluorescence spectroscopy. The catalyst was washed with water several times, dried in vacuo, then analyzed using XRD and XPS. 3. Results and Discussions 3.1. UV Adsorption and Fluorescence Spectrum. Figure 2 shows the effect of time on the coupling reaction with the
4532
Ind. Eng. Chem. Res., Vol. 45, No. 13, 2006
Figure 4. Ultraviolet (UV) fluorescence spectrum (multimetal catalyst, pH 5.0, reaction time of 90 min).
Figure 2. Effect of time on the coupling reaction with the multimetal catalyst (pH 5.0).
Figure 3. Effect of the medium and pH on the reaction system (for a reaction time of 90 min).
multimetal catalyst (pH 5.0) that is detected by the UV-Vis adsorption spectrophotometer. With increasing time (0-90 min), the K adsorption band (222 nm) of the benzene ring changed unconspicuously; the B adsorption band of the benzene ring hypsochromic shifted from 267 nm to 252 nm (arrowhead area) and the shoulder appeared from 70 min between 238 nm and 248 nm; the R adsorption band (the rectangular area to the right) presented itself from 50 min at 320 nm and the height of the adsorbed peak enlarged. The B adsorption band of the benzene ring changed and the shoulder appeared, maybe because of the interactions of the solvent and the components of the reaction system: the interaction between the methyl of the p-xylene and hydroxyl group enlarged the energy band of π f π*, in contrast to that of the benzene ring; when the H atom in the methyl of the p-xylene was substituted by a methoxyl group or a hydroxyl group, the conjugation between the two methyl groups in the
p-xylene and the benzene ring was weakened, the energy needed by the π f π* transition increased, the B absorption shifted to hypsochromic, and the shoulder appeared. The R adsorption band presented was likely the n f π* transition, which was caused by the isolated electron in the O atom while the H atom in the methyl of the p-xylene was substituted by a methoxyl group or a hydroxyl group; the adsorbed peak height increased, which indicated that the reaction degree was proportioned directly to the reaction time and the content of the oxygenous aromatic products. The electronic spectra of the reaction system in the different media and pH under the same reaction time (90 min) and identical current intensity (2.0 A) are shown in Figure 3. It can be found that, for the B adsorption band, the shoulder appeared at pH 5.0; others changed unconspicuously. The R adsorbed peak height was decreased as follows:
multimetal catalyst (pH 5.0) > multimetal catalyst (pH 7.0) > cobaltous nitrate (pH 5.0) > cobaltous nitrate (pH 7.0) > multimetal catalyst (pH 3.0) > cupric nitrate (pH 5.0) > cobaltous nitrate (pH 3.0) > cupric nitrate (pH 7.0) > cupric nitrate (pH 3.0). They were related to the content of the oxygenous aromatic products. The result was further proved via GC/MS. It could be found that the effect of the coupling reaction catalyzed by the multimetal catalyst was better than that of the monometal catalyst. This observation may be due to the fact that two metals participated in the oxidation. One also could conclude from the result that the pH greatly impacted the reaction. Using the multimetal catalyst as an example, the R adsorbed peak of the pH 5.0 was the highest and the pH 3.0 peak was the lowest. It was assumed that, because of the fact that a certain quantity of H+ could add to the electromotive force of the Co3+/Co2+ and the Cu2+/Cu+, thus increasing the oxidation and the contrast, the greater amount of H+ restrained the creation of the OHspecies, which slowed the reaction.
Table 1. Electrolysis of p-Xylene in Various Media and Different pHa Product Concentration (mass%) catalyst
pH
a
b
c
d
e
f
g
h
l
m
n
total concentration (mass %)
self-regulating catalyst self-regulating catalyst self-regulating catalyst
3.0 5.0 7.0
27.83 15.97 29.04
13.60 18.92 21.61
0.91 10.29 3.54
8.87 5.22 2.84
15.14 0.11 0.72
1.60 1.54 3.76
15.61 21.19 25.68
0.34 0.43 1.19
0.22 21.05 5.21
0.45 3.66 0.21
0.00 0.00 0.00
84.57 98.38 93.80
cobaltous nitrate cobaltous nitrate cobaltous nitrate
3.0 5.0 7.0
19.80 8.30 27.39
5.89 3.22 12.59
21.73 6.82 1.54
0.83 4.82 0.63
4.61 0.57 0.80
13.15 23.48 2.47
12.34 20.96 35.21
0.58 1.24 0.51
0.76 1.51 8.04
0.85 4.54 0.32
1.73 15.12 0.00
82.27 90.58 89.50
cupric nitrate cupric nitrate cupric nitrate
3.0 5.0 7.0
24.18 16.79 29.36
18.17 6.88 10.52
1.21 0.93 2.32
2.06 0.24 0.57
3.11 1.76 0.77
7.48 28.72 15.89
20.33 13.14 17.63
1.37 2.77 1.07
1.11 3.08 0.28
0.32 8.67 0.82
0.78 0.91 1.41
80.12 83.89 80.64
a Reaction conditions: current intensity, 2.0 A; reaction temperature, 298 K; reaction time, 90 min. See ref 13 for an explanation of the abbreviations legend.
Ind. Eng. Chem. Res., Vol. 45, No. 13, 2006 4533 Table 2. Components of the Multimetal Catalyst after the Coupling Reaction element
area counts eV/s)
sensitivity factor
concentration (%)
54327 34864 12728 11948 1768 9798 7462 3006
0.296 0.711 1.833 1.000 0.339 5.321 3.590 1.685
69.95 18.69 2.65 4.55 1.99 0.70 0.79 0.68
C 1s O 1s Ca 2p F 1s Si 2p Cu 2p Co 2p Na 1s
Table 3. Data for the Value Simulation Curve of the Cobalt
Figure 5. Gas chromatography (GC) of the products (with cobaltous nitrate, pH 5.0): (a) 4-methyl methoxymethyl benzene, (b) 1,4-methoxymethyl benzene or 1-dimethoxymethyl-4-methyl benzene, (c) 4-methoxymethyl benzyl alcohol, (d) 4-methoxymethyl benzaldehyde, (e) 1-dimethoxymethyl4-methoxymethyl benzene, (f) 4-methyl benzyl alcohol, (g) 4-methyl benzaldehyde, (h) 4-dimethoxymethyl benzyl alcohol, (l) 4-dimethoxymethyl benzaldehyde, (m) 1,4-dimethoxymethyl benzene, and (n) 2,5-dimethoxymethyl phenol. (See ref 13 for an explanation of the abbreviations legend.)
Figure 6. X-ray diffraction (XRD) patterns of the multimetal catalyst before and after the reaction.
Figure 4 a and b shows the respective emission (315 nm) and excitation (478 nm) wavelength of the reaction product previously mentioned. The result indicated that the elemental cobalt had not formed the metal complex with the reaction product for the emission (315 nm) and excitation (478 nm) wavelength, relative to the normal emission and the excitation wavelength of the aromatic compounds. 3.2. GC/MS Spectrum.13 A GC analysis of the products is shown in Figure 5 (the peak of the solvent was taken off). Table
cation
peak position
height
area
percentage of total area
Co2+ Co3+
780.5 779.25
329 171
1364 599
69.49 30.51
Table 4. Data for the Value Simulation Curve of the Copper cation
peak position
height
area
percentage of total area
Cu+ Cu2+
932.51 933.70
1161 243
3254 828
79.72 20.28
1 compared the concentration of the products in different media and pH at the same reaction time (90 min) and the identical current intensity (2.0 A). It can be found that the content of the oxygenous aromatic products obtained using a multimetal catalyst as the medium was greater than that of the monometal catalyst at the same pH, which may be due to the strong adsorption of the kaolin and the two metals, all of which participated in the oxidation process. According to the strong adsorption of the kaolin, more reactant was adsorbed at the surface of the multimetal catalyst to form the center of the oxidation. In addition, this observation was consistent with the results of the UV-Vis spectroscopy:
multimetal catalyst (pH 7.0) > cobaltous nitrate (pH 7.0) > cupric nitrate (pH 7.0) multimetal catalyst (pH 5.0) > cobaltous nitrate (pH 5.0) > cupric nitrate (pH 5.0) multimetal catalyst (pH 3.0) > cobaltous nitrate (pH 3.0) > cupric nitrate (pH 3.0) The result also indicated that the various products can be
Figure 7. Binding energy of all the elements of the multimetal catalyst after the coupling reaction.
4534
Ind. Eng. Chem. Res., Vol. 45, No. 13, 2006
and tested via XPS. The binding energy and its composition of the catalyst are shown in Figure 7 and Table 2. Because of the good porosity and adsorbability of the kaolin in the catalyst, a mass of the organic compounds and the electrolyte were adsorbed at the surface of the catalyst.
Figure 8. Binding energy of the cobalt compounds in the multimetal catalyst after the coupling reaction.
acquired by controlling the reaction conditions, namely controlling the degree of oxidation. For example, 4-methyl benzaldehyde could obtained from the reaction catalyzed by cobaltous nitrate at pH 7.0 with a yield of 35%, which is lower than the chemical oxidation with a yield of 80%,5-7 but it was cleaner and produced more simply in the oxidation process. 3.3. X-ray Diffractometry (XRD). The multimetal catalyst that is detected via XRD before and after the reaction is shown in Figure 6. Through comparison to the stander cards, it can be shown that, in regard to the catalyst before the reaction, the elemental cobalt presented itself in the form of Co2O3 and the elemental copper was all in the form of CuO. After the reaction, the elemental cobalt presented itself as a mixture of Co2O3 and CoO; the elemental copper existed in the form of CuO and Cu2O. 3.4. X-ray Photoelectron Spectroscopy (XPS). After the coupling reaction, the multimetal catalyst was dried in vacuo
Figures 8 and 9 show the binding energy and the valence simulation curve of the cobalt, respectively. The normal binding energy of the elemental cobalt, CoO, and Co2O3 were 777.9, 780.6, and 779.4 eV, respectively, and the testing result was 781.0 eV; therefore, it can be inferred that the valence of the cobalt was a mixture in the catalyst. Table 3 and Figure 9 indicate the concentration of the cobalt, relative to different valences. The divalent cobalt form (Co2+) accounted for 69.49% of the total elemental cobalt and the trivalent cobalt form (Co3+) occupied 30.51%. In comparison to the Co3+, which occupied 100% of the total cobalt element before the coupling reaction, this finding illuminates the fact that the Co2O3 was involved with the coupling reaction and was reduced to CoO. Figures 10 and 11 show the binding energy and the valence simulation curve of the copper, respectively. The normal binding energy of the elemental copper, CuO, and Cu2O were 933.1, 933.8, and 932.6 eV, respectively, and the testing result was 932.8 eV, which is possibly due to the fact that the Cu2O accounted for the greatest proportion of the total copper in the catalyst. Table 4, which was obtained from Figure 11, indicated the concentration of the copper, relative to different valences. The monovalent copper form (Cu+) accounted for 79.72% of the total elemental copper and the divalent copper form (Cu2+) occupied 20.28%. This observation shows that the CuO was involved in the coupling reaction and was reduced to Cu2O,
Figure 9. Valence simulation curve of the cobalt in the multimetal catalyst after the coupling reaction.
Figure 10. Binding energy of the copper compounds in the multimetal catalyst after the coupling reaction.
Ind. Eng. Chem. Res., Vol. 45, No. 13, 2006 4535
Figure 11. Valence simulation curve of the copper in the multimetal catalyst after the coupling reaction.
Figure 12. Schematic of the possible mechanism for the main reactions.
according to the contrastive method of the elemental cobalt mentioned previously. 3.5. Assumed Mechanism. The assumed mechanism of the reaction was proposed. A portion of the hydroxyl anions were adsorbed near the anode, forming a thin layer between the solvent and the anode; the remaining amount of hydroxyl anions were adsorbed at the surface of the catalyst.
The anode reaction is described as follows:
4OH- - 4e f 2H2O + O2 (occurred at the thin hydroxyl anions layer near the anode) The cathode reaction is described as follows:
4536
Ind. Eng. Chem. Res., Vol. 45, No. 13, 2006
2CH3OH + 2e f 2CH3O- + H2 (occurred near the cathode) The possible catalyst reaction mechanism can be described as follows:
Co2O3 + 2CH3ArCH3 f 2[CH3ArCH3]+• + 2CoO (I) CuO + CH3ArCH3 f [CH3ArCH3]+•+ Cu2O
(II)
[CH3ArCH3]+• f CH3ArCH2• + H+ + e-
(III)
CH3ArCH2• + O2 f CH3ArCH2O2•
(IV)
CH3ArCH2O2• + CoO f Co2O3 + CH3ArCHO
(V)
CH3ArCH2O2• + Cu2O f CuO + CH3ArCHO (VI) The catalyst that is regenerated by reactions V and VI pursues the catalytic cycle of reactions I-VI. A p-xylene free-radical cation that was formed in the oxidation of the side chain of the p-xylene by the catalyst was combined with a hydroxyl anion to produce a free radical. All products were obtained via the reaction of the free radical and the anion. It can be assumed that all the useful reactions were accomplished at the surface of the catalyst and the thin hydroxyl anions layer near the anode; the possible mechanism was proposed, as shown in Figure 12. The formation of the p-xylene free-radical cation was the ratedetermining step in the coupling reaction. Therefore, the spectra of the different media, pH, and reaction times varied. 4. Conclusion Methoxy p-xylene derivatives, such as 4-methyl methoxymethyl benzene, 1,4-methoxymethyl benzene, 4-methoxymethyl benzyl alcohol, 4-methoxymethyl benzaldehyde, 1-dimethoxymethyl-4-methoxymethyl benzene, 4-methyl benzyl alcohol, 4-methyl benzaldehyde, 4-dimethoxymethyl benzyl alcohol, 4-dimethoxymethyl benzaldehyde, 1,4-dimethoxymethyl benzene, and 2,5-dimethoxymethyl phenol were obtained via the electrochemical oxidation of p-xylene in a methanol solvent, in conjunction with monometal and multimetal compounds. The multimetal catalyst exhibited more availability than the monometal catalyst on the coupling reaction of the methanol and the p-xylene. The various products can be obtained by controlling the reaction conditions, namely controlling the oxidation degree such as the pH, catalyst, and so on. In addition, the possible mechanism of the reaction was assumed and it fell into the freeradical-cation mechanism. Literature Cited (1) Raghavendrachar, P.; Ramachandran, S. Liquid-Phase Catalytic Oxidation of p-Xylene. Ind. Eng. Chem. Res. 1992, 31, 453.
(2) Nishiguchi, I.; Hirashima, T. Electroorganic Synthesis. 4. Facile Synthesis of Aromatic Aldehydes by Direct Anodic Oxidation of Parasubstituted Toluenes. J. Org. Chem. 1985, 50, 539. (3) Ma, H. Zh.; Wang, B.; Liang, Y. Q. Synthesis of Benzyl Alcohol by Indirect Electrochemical Catalyzed Oxidation of Toluene in Basic Methanol Solvent. Catal. Commun. 2004, 5, 617. (4) Chang, C. J.; Deng, Y.; Shi, C.; Chang, C. K.; Anson, F. C.; Nocera, D. G. Electrocatalytic Four-electron Reduction of Oxygen to Water by a Highly Flexible Cofacial Cobalt Bisporphyrin. Chem. Commun. 2000, 5, 1355. (5) Basudeb, S.; James, H. E. Combined Acid Additives and the MC Catalyst for the Autoxidation of p-Xylene to Terephthalic Acid. J. Mol. Catal. A: Chem. 2005, 241, 33. (6) Alberto, C.; Roberto, O.; Giacomo, C. Kinetics and Related Engineering Aspects of Catalytic Liquid-Phase Oxidation of p-Xylene to Terephthalic Acid. Catal. Today 1999, 52, 331. (7) Partenheimer, W. Methodology and Scope of Metal/Bromide Autoxidation of Hydrocarbons. Catal. Today 1995, 23, 69. (8) Bejan, D.; Savall, A. Electrochemical Oxidation of p-Methoxytoluene in Acetic Acid Saturated by Molecular Oxygen. J. Electroanal. Chem. 2001, 507, 234. (9) Lindermeir, A.; Horst, C.; Hoffmann, U. Ultrasound Assisted Electrochemical Oxidation of Substituted Toluenes. Ultrason. Sonochem. 2003, 10, 223. (10) Schulz, G. D. Oxidation by Metal Salts. J. Org. Chem. 1972, 37, 2564. (11) Guo, C. C.; Liu, Q.; Wang, X. T.; Hu, H. Y. Selective LiquidPhase Oxidation of Toluene With Air. Appl. Catal. 2005, 282, 55. (12) Bejan, D.; Lozar, J.; Falgayrac, G.; Savall, A. Electrochemical Assistance of Catalytic Oxidation in Liquid-Phase Using Molecular Oxygen: Oxidation of Toluenes. Catal. Today 1999, 48, 363. (13) GC/ MS spectrum, 70 eV, M/e (relative intensity). For 4-methyl methoxymethyl benzene (“a” in Table 1 and Figure 5): 136 M+(63), 121(100), 105(99), 91(62), 77(35), 65(14), 51(7), 39(6), 29(4), 15(1). For 1,4methoxymethyl benzene or 1-dimethoxymethyl-4-methyl benzene (“b” in Table 1 and Figure 5): 166 M+(5), 150(1), 135(100), 119(20), 105(11), 91(24), 77(4), 65(7), 51(1), 39(2), 27(1), 15(1). For 4-methoxymethyl benzyl alcohol (“c” in Table 1 and Figure 5): 152 M+(99), 135(100), 121(51), 105(14), 91(24), 77(21), 65(7), 43(12), 39(7). For 4-methoxymethyl benzaldehyde (“d” in Table 1 and Figure 5): 150 M+(100), 133(56), 119(32), 105(52), 91(88), 77(64), 65(32), 51(24), 39(32). For 1-dimethoxymethyl-4-methoxymethyl benzene (“e” in Table 1 and Figure 5): 196 M+(12), 195(100), 179(4), 164(7), 149(18), 135(3), 121(29), 105(3), 91(4), 77(5), 65(2), 51(2), 39(1). For 4-methyl benzyl alcohol (“f” in Table 1 and Figure 5): 122 M+(99), 105(100), 91(56), 77(56), 65(31), 51(13), 39(19). For 4-methyl benzaldehyde (“g” in Table 1 and Figure 5): 120 M+(98), 119(100), 105(7), 91(87), 77(7), 65(27), 51(13), 39(20). For 4-dimethoxymethyl benzyl alcohol (“h” in Table 1 and Figure 5): 182 M+(27), 165(7), 150(7), 135(100), 119(13), 105(27), 91(33), 77(27), 63(13), 51(13), 39(7). For 4-dimethoxymethyl benzaldehyde (“l” in Table 1 and Figure 5): 180 M+(73), 165(100), 150(20), 135(40), 121(20), 105(13), 91(13), 77(13), 65(7), 51(80), 39(33). For 1, 4-dimethoxymethyl benzene (“m” in Table 1 and Figure 5): 226 M+(24), 210(52), 195(100), 179(32), 165(12), 149(28), 135(80), 119(48), 105(24), 91(32), 77(16), 65(12), 51(4), 39(8). For 2,5dimethoxymethyl phenol (“n” in Table 1 and Figure 5): 242 M+(23), 225(71), 210(48), 195(100), 179(54), 165(36), 149(29), 135(62), 119(51), 105(20), 91(32), 77(14), 65(9), 51(11), 39(6).
ReceiVed for reView March 6, 2006 ReVised manuscript receiVed April 14, 2006 Accepted April 21, 2006 IE060272W
