ARTICLE pubs.acs.org/IECR
Environmentally Friendly Efficient One-Pot Electrochemical Synthesis of 2,4-Dimethylanisole by Selective Oxidation of m-Xylene in Methanol with Metals Promoted Sulfated Zirconia Fengtao Chen,* Sanchuan Yu, Xiaoping Dong, and Shishen Zhang Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, People's Republic of China ABSTRACT: Herein, we reported the efficiently catalytic synthesis of 2,4-dimethylanisole with electrochemically selective oxidation of m-xylene in methanol under mild conditions. The catalysts of SO42 /M ZrO2 (M = Mo, Cu, Fe, Co, and Cr), prepared by coprecipitation and postsynthesized sulfation, were characterized by means of Fourier transform infrared (FT-IR) spectroscopy and powder X-ray diffraction (XRD), which indicated that the modification of sulfates and the incorporation of metal ions would not influence the structure of zirconia and the doped metal ions were well dispersed. The electrochemical reaction process was monitored by UV vis spectroscopy, and the distribution of the products was analyzed by gas chromatography mass spectrometry (GC MS). All catalysts exhibited excellent activity for the electrochemical reaction assisted by a pair of porous graphite plane electrodes. X-ray photoelectron spectroscopy (XPS) showed a mixture of Zr2+ and Zr4+ in the catalysts after the reaction, demonstrating the participation of catalyst in this redox reaction. The results revealed that a maximum yield of about 62.1% was achieved for 2,4-dimethylanisole in the SO42 /Mo ZrO2 reaction system in 3 h. This method provides a new environmentally benign and simple way to synthesize 2,4-dimethylanisole.
1. INTRODUCTION The introduction of toxic and pollutant products in industries has seriously threatened the public health and the environment, and has become a worldwide problem for human beings. Consequently, green chemical processes, which are much more environmentally friendly than the traditional processes using hazardous chemicals, have received great attention. For instance, the liquid acid catalysts widely used in organic synthesis are being replaced by new solid acids, due to their toxicity and corrosivity. In recent decades, numerous efforts have been devoted to SO42 /MxOy-type solid superacids, which exhibit better properties over traditional liquid acids and liquid superacids.1 5 As one kind of green catalyst, they have been reported in theoretical research6 and synthetic application, such as isomerization,7 9 cracking,10,11 dehydration, 12,13 alkylation, 3 acylation, 14,15 dehydration and dealkylation,13,16 Friedel Crafts,17,18 and other new applications. Sulfated zirconia (SO42 /ZrO2) is well-known as a solid superacid and is often used as a catalyst for applications in the various solid acid reactions.19 21 Especially the SO42 /ZrO2 catalyst shows a high catalytic activity in the isomerization reaction of low molecular weight paraffins such as n-pentane and n-hexane. Much higher activities have been achieved with metal modified sulfated zirconia, which catalyzed the isomerization reaction even at room temperature; such promotion in activity of the catalyst has been confirmed by several other research groups.13,22 Dimethylanisoles are useful intermediates in the synthesis of a variety of organic compounds, particularly in the preparation of polymers. In addition, they are applied widely in the food processing industry as spices. These dimethylanisole compounds are commonly synthesized from xylene by several routes. For instance, the sulfonation of xylene and subsequent alcoholysis r 2011 American Chemical Society
have been carried out. However, the sulfonation is less selective and the alcoholysis needs a higher temperature. A nitration method has also encountered such problems, which lead to considerable waste and byproduct.23 Herein, we report a clean, one-step approach to synthesize 2,4dimethylanisole from electrochemical catalytic oxidation of m-xylene in methanol assisted by metals promoted sulfated zirconia catalysts under mild conditions. The electrochemical reaction, which offers several advantages such as usage of environmental friendly oxidant, simple workup procedure, no-solvent conditions, short reaction times, and easy recovery and reusability of the catalyst, is necessary for the chemosynthesis industry from the environmental standpoint. In order to improve the acid strength of SO42 /ZrO2 which is regarded as one significant factor for aromatic oxidation, SO42 /ZrO2 promoted with M doping (M = Mo, Cu, Fe, Co, and Cr) is prepared, expecting that the m-xylene oxidation can be improved by this catalyst under mild conditions.
2. EXPERIMENTAL SECTION 2.1. Materials and General Methods. All chemicals in the experiment were of analytical grade and used without any purification. The influence of the reaction time on the electrochemical synthesis process was monitored by UV vis (UV vis 759, Shanghai Precision and Scientific Instrument Co.). The products were isolated by distillation and were analyzed by gas chromatography mass Received: April 7, 2011 Accepted: November 12, 2011 Revised: October 3, 2011 Published: November 12, 2011 13650
dx.doi.org/10.1021/ie200740p | Ind. Eng. Chem. Res. 2011, 50, 13650–13654
Industrial & Engineering Chemistry Research
Figure 1. Representative infrared spectrum of SO42 /M ZrO2 catalysts. (a) SO42 /Cu ZrO2; (b) SO42 /Fe ZrO2; (c) SO42 /Cr ZrO2; (d) SO42 /Co ZrO2; (e) SO42 /ZrO2.
spectrometry (GC MS; QP2010, Japan). The catalyst was detected by Fourier transform infrared (FT-IR) spectroscopy (Nicolet Avatar 370, America), powder X-ray diffraction (XRD; Rigaluc, Japan), and X-ray photoelectron spectroscopy (XPS; Axis Ultra DLD, U.K.). 2.2. Preparation of SO42 /M ZrO2 catalysts. The SO42 / M ZrO2 catalysts were prepared by a two-step route. In the first stage, the metal zirconium hydroxide gel [n(M):n(Zr) = 1:9] was prepared by the sol gel process at room temperature,24 and then the obtained gel adsorbed SO42 over its surface by a impregnation method in the second stage. In a typical route for SO42 /Cu ZrO2 catalyst, 24 g of ZrOCl2 3 8H2O and 2 g of Cu(NO3)2 3 3H2O were dissolved in 350 mL of alcohol water (3:1 v/v) solution. By adjusting the pH value to 9 with ammonia solution (25 wt %) under continuously vigorous stirring, the precursors were spontaneously hydrolyzed to form a hydroxide gel, which was collected by filtration and washed until free from chloride ions. After being dried at 383 K for 12 h, and then being ground carefully, the hydroxide powder was immersed in a 0.5 M H2SO4 solution overnight under ambient temperature, dried at 383 K for another 12 h, and calcined at 773 K for 3 h in air. Finally, Cu-doped SO42 /ZrO2 catalyst was obtained. In addition, SO42 /ZrO2 and other catalysts were also prepared using the same procedure. 2.3. Experimental Setup. The experiments were conducted in a single cell of 0.25 L capacity at 30.0 V voltage and 2.2 A current intensity. The reaction cell was cooled by cooling water in a trough to form the room-temperature condition. The reaction cell was air-proofed to prevent the volatilization of the methanol. The anode and cathode (porous graphite plate) were positioned vertically and parallel to each other with a constant intergap of 1.0 cm. A 3.0 g sample of catalyst of metals promoted sulfated zirconia and 1.0 g of assisted catalyst of KF were packed around the working electrode, forming a multiphase electrochemical oxidation packed bed. The solution was constantly stirred at 300 rpm using a magnetic stirrer to maintain a uniform concentration of the electrolyte solution. The current and voltage were adjustable in the ranges 0 3.0 A and 0 35.0 V, respectively. 2.4. Electrolysis Procedures. The electrolysis was carried out in cells without compartments. The anode and cathode were
ARTICLE
Figure 2. XRD patterns of SO42 /M ZrO2 catalysts. (a) SO42 /ZrO2; (b) SO42 /Cu ZrO2; (c) SO42 /Fe ZrO2; (d) SO42 /Co ZrO2; (e) SO42 /Cr ZrO2.
activated by methanol solution before use. The solvent-supporting electrolyte system was formed as follows: 1.0 g of KF and 50 mL of m-xylene were added in 80 mL of anhydrous methanol with 3.0 g of SO42 /M ZrO2 (M = Mo, Cu, Fe, Co, and Cr, respectively). The resulting solutions were placed in the cells and electrolyzed at a current intensity of 2.2 A (with the time prolonged and the current intensity decreased to 1.0 A gradually) with stirring at room temperature. The conversion of the starting material was investigated by UV vis spectrometry. The electronic spectra of the reaction system were detected during each electrolysis, and the conversion of starting material was investigated by a UV vis spectrum every 30 min as follows: 0.01 mL of solution was transferred by transfer pipet accurately and diluted to 20 mL in a volumetric flask; then the electronic spectra were observed in the range 200 400 nm with methanol used as blank. The methanol used for the research of the UV vis spectrum was reclaimed for the next experiment without pollution and waste. 2.5. Characterization of Products and Catalyst. After the reaction finished, the solution was distilled under air pressure. The distillates were analyzed by a GC MS system using a capillary column (0.25 cm 30.0 m). The catalyst was washed with water for several times, dried in vacuo, and then detected by XPS.
3. RESULT AND DISCUSSION 3.1. Characterization of the Catalysts. The different catalyst samples were dried in vacuo at 373 K for 1 h first and then were diluted in KBr. They showed the infrared absorption spectra, at room temperature, displayed in Figure 1. In all cases, the strong absorption peak at 3430 cm 1 and the mild peak at 1631 cm 1 may be assigned to the dissociative hydroxyl of H2O absorbed on the solid catalysts. The peaks between 880 and 1490 cm 1 assigned to the SdO or S O bond were characteristic peaks, and the structure of the solid catalysts remained stable even when many other components existed. These results are in accordance with those in the published literature of SO42 /MxOy-type solid superacids.25,26 The influences of dopants and the sulfation on the crystal structure of catalysts were examined by XRD technology (Figure 2). All diffraction peaks in the zirconia sample without metal ion doping and sulfating treatment can be indexed as a tetragonal phase.27 XRD patterns of SO42 /M ZrO2 catalysts do not show 13651
dx.doi.org/10.1021/ie200740p |Ind. Eng. Chem. Res. 2011, 50, 13650–13654
Industrial & Engineering Chemistry Research
ARTICLE
Figure 3. Gas chromatogram of the products (SO42 /Cr ZrO2 as catalyst).31
Table 1. Electrolysis of m-Xylene in Methanol Solvent by the Different Catalystsa 31
Scheme 1. Possible Mechanism for the Main Reactionsa,b
selectivity conversion of
yield of 2,
of objective
catalysts
m-xylene (%)
4-dimethylanisole (%)
product (%)
SO4 /ZrO2 SO42 /Mo ZrO2
55.1 71.5
41.6 62.1
75.5 86.8
SO42 /Cu ZrO2
74.9
58.3
77.8
SO42 /Fe ZrO2
74.7
54.6
73.1
SO42 /Co ZrO2
75.2
54.2
72.1
SO42 /Cr ZrO2
74.6
48.5
65.0
2
a
Reaction conditions: current intensity, 1.0 A; reaction temperature, 298 K; reaction time, 180 min.
apparently different diffraction peaks from tetragonal zirconia, indicating that the doping and sulfating treatments have no influence on crystal structure. It is noteworthy that there were no diffraction peaks corresponding to any metallic oxide phase, demonstrating the good dispersion of dopants in catalysts. 3.2. Electrochemically Catalytic Synthesis of 2,4-Dimethylanisole. The influence of reaction time on the synthesis of 2,4dimethylanisole catalyzed by SO42 /M ZrO2 catalyst was detected by UV vis measurement. The absorbance of the K absorption band at 215 nm, ascribed to benzene ring, increased but changed inconspicuously with the increment of reaction time. However, the absorbance of the B absorption band at 265 and 272 nm, attributed to oxygenous aromatic products, drastically increased as the reaction time increased from 0 to 150 min, and then was nearly unchanged from 150 to 180 min. On the basis of the above results, we chose a time of 180 min as the reaction time. The reaction products were determined by the GC MS method because of its short measuring time and high sensitivity for the separation and identification of complex organic compounds.28 A typical GC MS spectrum of the products catalyzed by SO42 /Cr ZrO2 is shown in Figure 3. There are five main compounds being produced by the electrochemical oxidation of m-xylene in methanol, including the objective product 2, 4-dimethylanisole and the byproducts of 1-(4-methoxyphenyl)ethanol, 2-(3-methoxy-2-methylphenyl)acetic acid, 4-methoxy3-methylbenzaldehyde, and 1-methyl-3-methoxy-4-methylbenzoate.
a
Anode reaction: 2OH 2e f H2O + [O] (occurred at the thin hydroxyl anion layer near the anode). b Cathode reaction: 2CH3OH + 2e f 2CH3O + H2 (occurred near the cathode).
Table 1 compares the conversion of m-xylene, the yield of 2,4dimethylanisole, and the selectivity of 2,4-dimethylanisole with various catalysts in the same reaction conditions. With the undoped catalyst of SO42 /ZrO2, only half of m-xylene was converted in this electrochemical reaction, although the selectivity of 2,4-dimethylanisole is at a relatively higher level (75.5%). As this reaction was catalyzed by doped samples of SO42 / M ZrO2, the conversion of m-xylene was obviously enhanced to above 70%, whereas these catalysts with different dopants exhibited remarkably different selectivities. The Mo-doped catalyst has the highest selectivity of 86.8%, and the Cr-doped catalyst has the lowest of 65.0%. A possible mechanism of the reaction for the objective product was proposed and is shown in Scheme 1. The m-xylene oxidation is initiated by an anodic electron transfer, which results in the formation of the radical cation and the reduction of partial Zr4+ ions to Zr2+ ions (step 1). This radical cation is stabilized in alcoholic solvents by the formation of m-xylyl radical (step 2). Because of the lower oxidation potential of m-xylyl radical, the 13652
dx.doi.org/10.1021/ie200740p |Ind. Eng. Chem. Res. 2011, 50, 13650–13654
Industrial & Engineering Chemistry Research
ARTICLE
’ REFERENCES
Figure 4. Zr 3d3/2 binding energy of SO42 /Cr ZrO2 catalyst after reaction.
formation of m-xylyl cation occurs quickly at the anode (step 3). Subsequently, dimethylanisole was produced by the reaction of m-xylyl cation with methoxyl anion, which was produced by the cathode reaction (step 4).29,30 Zr2+ ions in the catalysts can be successively oxidized to tetravalence by atomic oxygen, which was obtained by the anode reaction. For verifying the existence of divalent zirconium in this reaction, we analyzed by XPS technology the catalyst dried in vacuo after the reaction. Figure 4 depicts the XPS Zr 3d3/2 spectrum of SO42 /Cr ZrO2 sample. Two peaks centered at 180.9 and 183.1 eV in binding energy were observed, which are respectively responding to divalent and tetravalent zirconium. According to the area of deconvoluted peaks, the molar ratio of Zr4+ and Zr2+ was calculated as 62.2:37.8. This result indicated the reduction of tetravalent zirconium to divalent in this reaction process.
4. CONCLUSION We reported here a novel method for the room-temperature synthesis of 2,4-dimethylanisole by electrochemical oxidation of m-xylene with methanol, catalyzed by metals promoted sulfated zirconia. It needs to be noted that this one-pot process was much better in safety and convenience and more effective than the traditional methods, especially in the given conditions. A series of SO42 /M ZrO2 (M = Mo, Cu, Fe, Co, and Cr) prepared by coprecipitating and sulfating were investigated, among which the sample of SO42 /Mo ZrO2 showed the highest catalytic activity, a yield of 62.1%, and a selectivity of 86.8%. In addition, the major advantages of this one-pot process are the utilization of electron as oxidizing agent and making good use of the low-cost methanol, thus avoiding hazardous waste disposal and reducing environmental pollution, which makes the process environmentally friendly. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: +86 571 86843228. Fax: +86 571 86843217. E-mail: cft0923@ 163.com.
’ ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the Qianjiang talent project of Zhejiang Province of China (2011R10048).
(1) Reddy, B. M.; Sreekanth, P. M.; Lakshmanan, P. Sulfated zirconia as an efficient catalyst for organic synthesis and transformation reactions. J. Mol. Catal. A: Chem. 2005, 237, 93–100. (2) Sohn, J. R.; Seo, D. H. Preparation of new solid superacid catalyst, zirconium sulfate supported on γ-alumina and activity for acid catalysis. Catal. Today 2003, 87, 219–226. (3) Yadav, G. D.; Pathre, G .S. Novel mesoporous solid superacids for selective C-alkylation of m-cresol with tert-butanol. Microporous Mesoporous Mater. 2006, 89, 16–24. (4) Kogelbauer, D.; Vassena, R.; Prins, J. N. Solid acids as substitutes for sulfuric acid in the liquid phase nitration of toluene to nitrotoluene and dinitrotoluene. Catal. Today 2000, 55, 151–160. (5) Clark, J. H. Solid acids for green chemistry. Acc. Chem. Res. 2002, 35, 791–797. (6) Zheng, A.; Zhang, H. L.; Chen, L.; et al. Relationship between 1H chemical shifts of deuterated pyridinium ions and Bronsted acid strength of solid acids. J. Phys. Chem. B 2007, 111, 3085–3089. (7) Farcasiu, D.; Lee, K. H. Isomerization of hexane by zeolite HZSM-5 the effect of cyclic hydrocarbons. J. Mol. Catal. A: Chem. 2000, 161, 213–221. (8) Chaabene, S. B.; Bergaoui, L.; Ghorbel, A.; et al. In situ preparation of zirconium sulfate pillared clay: study of acidic properties. Appl. Catal., A 2004, 268, 25–31. (9) Kim, S. Y.; Goodwin, J. G., Jr.; Farcas-iu, D. The effects of reaction conditions and catalyst deactivation on the mechanism of n-butane isomerization on sulfated zirconia. Appl. Catal., A 2001, 207, 281–286. (10) David, J. Z.; Alerasool, S.; Doolin, P. K. Characterization of catalytically active sulfated zirconia. Catal. Today 1999, 53, 419–432. (11) Samantaray, S. K.; Parida, K. Review: effect of anions on the textural and catalytic activity of titania-silica mixed oxide. J. Mater. Sci. 2004, 39, 3549–3562. (12) Salas, P.; Hernandez, J. G.; Montoya, J. A. Effect of tin content on silica mixed oxides: sulfated and unsulfated catalysts. J. Mol. Catal. A: Chem. 1997, 123, 149–154. (13) Sohn, J. R.; Lee, S. H.; Lim, J. S. New solid superacid catalyst prepared by doping ZrO 2 with Ce and modifying with sulfate and its catalytic activity for acid catalysis. Catal. Today 2006, 116, 143–150. (14) Yadav, G. D.; George, G. Friedel-Crafts acylation of anisole with propionic anhydride over mesoporous superacid catalyst UDCaT-5. Microporous Mesoporous Mater. 2006, 96, 36–43. (15) Yadav, G. D.; Pimparkar, K. P. Insight into Friedel-Crafts acylation of 1,4-dimethoxybenzene to 2,5-dimethoxyacetophenone catalysed by solid acids;mechanism, kinetics and remedies for deactivation. J. Mol. Catal. A: Chem. 2007, 264, 179–191. (16) Sohn, J. R.; Dong, C.; Shin, D. C. Environmentally friendly solid acid catalyst prepared by modifying TiO2 with cerium sulfate for the removal of volatile organic chemicals. Appl. Catal., B 2008, 77, 386–394. (17) Zhou, D.-Q.; Yang, J.-H.; Dong, G.-M.; Huang, M.-Y.; Jiang, Y.-Y. Solid superacid, silica-supported polytrifluoromethanesulfosiloxane catalyzed Friedel-Crafts benzylation of benzene and substituted benzenes. J. Mol. Catal. A: Chem. 2000, 159, 85–87. (18) Matsuhashi, H.; Tanaka, M.; Nakamura, H.; et al. Formation of acid sites in ordered pores of FSM-16 by modification with sulfated zirconia. Appl. Catal., A: Gen. 2001, 208, 1–5. (19) Reddy, B. M.; Sreekanth, P. M.; Reddy, V. R. Modified zirconia solid acid catalysts for organic synthesis and transformations. J. Mol. Catal. A: Chem. 2005, 225, 71–78. (20) Shimizu, K.; Sunagawa, T.; Vera, C. R.; et al. Catalytic activity for synthesis of isomerized products from benzene over platinumsupported sulfated zirconia. Appl. Catal., A: Gen. 2001, 206, 79–86. (21) Davis, B. H.; Robert, A.; Srinivasan, R. K. Sulfated zirconia as a hydrocarbon conversion. Catal. Today 1994, 20, 219–256. (22) Reddy, B. M.; Sreekanth, P. M.; Lakshmanan, P.; et al. Synthesis, characterization and activity study of SO42 /CexZr1‑xO2 solid superacid catalyst. J. Mol. Catal. A: Chem. 2006, 244, 1–7. 13653
dx.doi.org/10.1021/ie200740p |Ind. Eng. Chem. Res. 2011, 50, 13650–13654
Industrial & Engineering Chemistry Research
ARTICLE
(23) Bates, R. B.; Siahaan, T. J.; Vasey, S. K.; et al. Preparation and Reactions of Trianions from the Dimethylphenols. J. Org. Chem. 1987, 52, 4605–4608. (24) Guevara-Franco, M. L.; Robles-Andrade, S.; Garcia-Alamilla, R.; et al. Study of n-hexane isomerization on mixed Al2O3-ZrO2/ SO42 catalysts. Catal. Taday 2001, 65, 137–141. (25) Kazushi, A.; Hideo, N.; Miyuki, S. Friedel Crafts acylation of toluene catalyzed by solid superacids. Appl. Catal., A: Gen. 2000, 197, 213–219. (26) Salas, P.; Hernandez, J. G.; Montoya, J. A.; et al. Effect of tin content on silica mixed oxides: Sulfated and unsulfated catalysts. J. Mol. Catal. A: Chem. 1997, 123, 149–154. (27) Mercera, P. D. L.; Van Ommen, J. G.; Doesburg, E. B. M.; et al. Zirconia as a support for catalysts: evolution of the texture and structure on calcinations in air. Appl. Catal. 1990, 57, 127–148. (28) Giumanini, A. G.; Verardo, G. Identification of Mononitro and Dinitro Isomers of Diphenylmethane by GC FT IR and GC MS Techniques. Ind. Eng. Chem. Res. 2001, 40, 1449–1453. (29) Bejan, D.; Lozar, J.; Falgayrac, G.; et al. Electrochemical Assistance of Catalytic Oxidation in Liquid Phase Using Molecular Oxygen: Oxidation of Toluenes. Catal. Today 1999, 48, 363–369. (30) Grinstaff, M. W.; Hill, M. G.; Labinger, J. A.; et al. Mechanism of catalytic oxygenation of alkanes by halogenated iron porphyrins. Science 1994, 264, 1311–1313. (31) GC MS spectrum: 70 eV, m/e (relative intensity). 1-Methoxy2,4-dimethylbenzene (a): 136 M•+ (100.0), 121 (89.6), 119 (0.8), 105 (20.8), 103 (12.4), 93 (11.2), 91 (72.8), 77 (37.6), 74 (1.2), 65 (10.4), 63(5.6), 53(4.8), 51(8.8), 41 (4.4), 39 (16.4), 29 (1.2); 1-(4-methoxyphenyl)ethanol (b): 152 M•+ (27.6), 137 (100.0), 135 (10.0), 122 (2.8), 119 (3.2), 109 (58.0), 107 (5.2), 103 (3.2), 94 (40.0), 91 (11.2), 81 (2.4), 77 (43.2), 74 (3.2), 63 (12.0), 53 (5.6), 51 (12.8), 45 (4.8), 43 (47.6), 39 (18.8), 29 (5.6); 2-(3-methoxy-2-methylphenyl)acetic acid (c): 180 M (2), 137 (0.8), 135 (100.0), 120 (3.6), 103 (2.8), 93 (0.8), 91 (20.0), 77 (7.2), 74 (0.8), 65 (6.4), 63 (3.2), 53 (2.0), 51 (5.2), 45 (4.8), 41 (2.0), 39 (6.0), 29 (1.2); 4-methoxy-3-methylbenzaldehyde (d): 149 M•+ (100.0), 147 (0.4), 135 (1.6), 121 (7.6), 119 (0.8), 106 (4.4), 93 (0.8), 91 (36.4), 86 (0.8), 77 (23.2), 74 (2.0), 65 (7.6), 60 (0.4), 53 (3.6), 51 (10.0), 41 (1.2), 39 (8.0), 29 (2.8); 1-methyl-3-methoxy-4methylbenzoate (e): 180 M•+ (48.4), 165 (5.2), 149 (100.0), 137 (2.8), 121 (31.2), 109 (1.6), 106 (11.2), 93 (2.8), 91 (42.0), 86 (0.8), 77 (28.8), 76 (3.6), 63 (12.4), 53 (5.6), 51 (22.0), 41 (1.2), 39 (14.8), 29 (2.8).
13654
dx.doi.org/10.1021/ie200740p |Ind. Eng. Chem. Res. 2011, 50, 13650–13654
