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Ind. Eng. Chem. Res. 2010, 49, 6674–6677
Covalently Anchored Polymer Immobilized Co(II) Phthalocyanine as Efficient Catalyst for Oxidation of Mercaptans using Molecular Oxygen as Oxidant Jomy K. Joseph, Suman L. Jain,* and Bir Sain Chemical Sciences DiVision, Indian Institute of Petroleum (Council of Scientific and Industrial Research), Dehradun-248005, India
The present paper describes the covalent immobilization of cobalt phthalocyanine “MEROX catalyst” to polystyrene resin via sulfonamide linkage and its catalytic activity for the oxidation of mercaptans to disulfides using molecular oxygen as an oxidant under alkaline conditions. Polymer-supported cobalt phthalocyanine, wherein the catalyst is chemically bounded to the support which prevents the leaching of the catalyst/ligand, can readily be recovered from the reaction mixture and reused for several runs without significant decrease in catalytic activity. 1. Introduction Oxidation of mercaptans to disulfides is a tremendously important reaction from the industrial, environmental, and biological points of view.1,2 The mercaptans present in the petroleum products like LPG, naphtha, gasoline, kerosene, etc. are highly undesirable since they have foul odor, highly corrosive nature, and lead to the catalyst poisoning; thus, it is necessary to remove them from the petroleum products.3,4 Although there are a number of processes known for the removal of mercaptans from petroleum products,5-8 the most commonly practiced is the sweetening process, which involves the oxidation of mercaptans to disulfides with air in the presence of a catalyst particularly cobalt phthalocyanine and their derivatives.9,10 Generally, the lower mercaptans present in LPG, and light petroleum fractions, are first extracted with alkali solution and then oxidized to disulfides with air in the presence of cobalt phthalocyanine based catalysts.11 However, the higher molecular weight mercaptans present in petroleum fractions like heavy naphtha, FCC gasoline, and ATF are oxidized to disulfide with air in a fixed bed reactor containing cobalt phthalocyanines impregnated on a suitable support, such as the cobalt phthalocyanines distributed on charcoal have been used for the autoxidation of thiols present in the petroleum fractions.12 However, impregnation of the catalyst on activated carbon/other similar supports is amenable to leaching as it is only physically adsorbed.13,14 Therefore, immobilization of the metal complexes via covalent attachment to the functionalized polymers is an interesting approach to facilitate catalyst recovery, recycling, and to reduce effluent contamination.15-18 In addition, these immobilized catalysts resistant to leaching due to the chemical bonding between the support and the metal complex. Covalent grafting of the cobalt phthalocyanines onto the functionalized polymeric supports such as cobalt phthalocyanine grafted to maleic anhydride-styrene copolymer chemically linked with γ-aminopropylated silochrome for the aerobic oxidation of sulfides has been reported.19 Chen et al.20 reported the synthesis of soluble polymer, poly(N-isopropylacrylamide) supported cobalt phthalocyanine for the oxidation of thiols. Sanchez et al.21 reported the covalent immobilization of metallo(chlorosulfonyl)phthalocyanines of Fe, Co, and Mn to the amino-function* E-mail:
[email protected]. Tel.: 91-135-2525901. Fax: 91-1352660202.
alized acrylic copolymers for the oxidation of 2,4,6-trichlorophenol using hydrogen peroxide as oxidant. During our research, we have extensively used the cobalt phthalocyanine and its derivatives for various oxidation reactions including oxidation of R-hydroxyketones,22 secondary alcohols,23 and thiols24 using molecular oxygen as oxidant. To improve the cyclic utilization, we have focused our attention toward the immobilization of the cobalt phthalocyanine based catalysts to the surface of functionalized supports via covalent bonding. In the present report, we describe a successful synthesis of covalently anchored cobalt phthalocyanine “MEROX catalyst” to polystyrene resin via sulfonamide linkage by the reaction of aminomethylated polystyrene resin with tetrachlorosulfonated cobalt phthalocyanine in a very simple manner. The prepared catalyst was found to be an efficient catalyst for the oxidation of mercaptans to disulfides using molecular oxygen as oxidant under alkaline conditions. 2. Experimental Section 2.1. Materials. All the substrates were either commercially available or prepared by the following the literature procedures. Aminomethylated polystyrene resin (3.5 mmol/g) was purchased from Aldrich and used as received. ICP-AES analysis was carried out by inductively coupled plasma atomic emission spectrometer (ICP-AES, PS-3000UV) by Leeman Laboratories. Cobalt tetrasulfophthalocyanine (CoPcS) 1 was synthesized as following the literature procedure.25 Refluxing of the mixture containing catalyst 1, with thionyl chloride readily gave cobalt tetrachlorosulfophthalocyanine 2 in almost quantitative yield. 2.2. Synthesis of Cobalt Tetrasulfophthalocyanine 1. Cobalt tetrasulfophthalocyanine (CoPcS) was prepared according to the reported method of Weber and Busch.25 In a typical experiment, the mixture of monosodium salt of 4-sulfophthalic acid (8.64 g, 32.4 mmol), ammonium chloride (0.94 g, 18 mmol), urea (11.6 g, 194 mmol), ammonium molybdate (136 mg, 0.12 mmol), and CoCl2 · 6H2O (2.30 g, 8.6 mmol) in nitrobenzene (25 mL) was heated at 180-190 °C for 8 h with vigorous stirring. The resulting solid cake was separated by filtration, washed thoroughly with methanol, and dried. The purification of prepared CoPcS was performed according to the published method. The blue colored Co(II) tetrasulfophthalocyanine was obtained in 75% yield (5.92 g). IR (KBr cm-1); 1720, 1632, 1502, 1452, 1489, 1409, 1379, 1134, 1049, 935, 770, 618. Anal calcd for C32H12N8O12S4Na4Co, calcd: C,
10.1021/ie100351s 2010 American Chemical Society Published on Web 06/18/2010
Ind. Eng. Chem. Res., Vol. 49, No. 14, 2010 Table 1. Aerobic Oxidation of Mercaptans 5 to Disulfides 6 with Catalyst 4
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a
a Reaction conditions: substrate (2 mmol), 1 N NaOH (5 mL, pH 14), catalyst (0.027 g, 1 mol %) under dioxygen balloon at room temperature, completion of the reaction was indicated by the change of the color of the reaction mixture from greenish black to blue. b Isolated yield. c Determined by GC. d By using homogeneous catalyst 2. e Blank experiment. f At lower pH (8.5). g TOF ) turnover number per hour.
39.42%; H, 1.23%; N, 11.49%. Found: C, 37.82; H, 1.59; N, 11.08. UV-vis: λ max 688.5, 649.0, 354.0, 213.0. 1H NMR: 7.89 (Ar-H); 8.30 (Ar-H); 8.52 Ar-H). 2.3. Synthesis of Cobalt Tetrachlorosulfophthalocyanine 2. A mixture containing catalyst 1 (5.36 g, 5.5 mmol) was treated with excess of thionyl chloride (15 mL), added slowly by dropping funnel and the resulting mixture was heated at 80 °C with stirring for 1 h. The reaction mixture was then poured over the crushed ice and the solid so obtained was separated by filtration, washed thoroughly with ice-cold water and then methonal, and dried under vacuum to give cobalt tetrachlorosulfophthalocyanine 2; yield 6.41 g (97%). IR (KBr cm-1) 3166, 1582, 1472, 1452, 1409, 1380, 1186, 930, 754. Anal calcd for C32H12N8O12S4Cl4Co, calcd: C, 37.17%; H, 1.16%; N, 10.84%. Found: C, 37.21; H, 1.08; N 10.68. 2.4. Immobilization of Cobalt Tetrachlorosulfophthalocyanine 2 to Aminomethylated Polystyrene Support 3. A suspension of the cobalt tetrachlorosulfophthalocyanine 2 (3.94 g, 4.0 mmol) in pyridine (20 mL) and amino-functionalized polystyrene (1 g, 3.5 mmol/g) 3 was refluxed for overnight. The resulting dark green material was separated by filtration and washed thoroughly with distilled water and methanol and dried for 12 h under vacuum at 50 °C. IR (cm-1): 3422, 3105, 2924, 1656, 1512, 1462, 1377, 1282, 1184, 1162, 1096, 967. UV-vis (nm): 724, 682, 275. The loading of the catalyst was calculated by the nitrogen content as determined by elemental analysis: N % found (6.1%, 0.48 mmol/g); in the ICP analysis, Co (%) found (2.1%, 0.37 mmol/g). 2.5. General Experimental Procedure for the Oxidation of Mercaptans 5 to Disulfides 6. A mixture consisting of mercaptan 5 (2 mmol) and catalyst 4 (0.027 g, 1.0 mol %) in 1 N NaOH (5 mL, water), contained in a 50 mL two-necked roundbottomed flask was stirred under an oxygen atomosphere for
the time as given in Table 1. The completion of the reaction was indicated by the change of reaction mixture color from black to blue. The reaction mixture was stirred further for an additional 5 min, and then, the catalyst was separated by filtration and the filtrate was extracted with dichloromethane (3 × 10 mL). The combined organic phase was dried over anhydrous sodium sulfate. Evaporation of the solvent and distillation at the reduced pressure yielded corresponding disulfides 6 in high to excellent yields. The products were identified by comparing their physical spectral data and GC retention times with those of authentic samples. 3. Result and Discussion 3.1. Catalyst Preparation and Characterization. Cobalt tetrachlorosulfophthalocyanine 2 was synthesized by treating the 1 with thionyl chloride, which on subsequent reaction with aminomethylated polystyrene resin 3 in the presence of pyridine gave a dark green polymer containing cobalt tetrasulfophthalocyanine 4 covalently attached to the support via sulfonamide linkage. Synthetic strategy for the preparation of polymer immobilized CoPcS 4 is shown in Scheme 1. The covalent immobilization of the catalyst to the support was confirmed by IR spectral analysis, as it revealed the characteristic peaks of the phthalocyanine moiety at 1184, 1096, 967 cm-1. In addition, the presence of the typical peaks at 1282 (due to C-N stretching), 1184 (S-O), and 1162 (C-H), also confirmed the covalent attachment of the catalyst 2 to the support 3. Whereas absence of these typical peaks in the IR spectra of the aminomethylated polystyrene support, clearly indicated the immobilization of CoPcS to the support during the synthesis. In the UV-vis spectra of the supported CoPcS 4, the presence of the typical peaks of the phthalocyanine at 724, 682, and 275
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Scheme 1. Synthetic Strategy for Polymer Immobilized Co(II) Phthalocyanine 4
Scheme 2. Heterogeneously Catalyzed Aerobic Oxidation of Mercaptans
further proved the loading of the CoPcS 2 to the support. The loading of the catalyst to the support was calculated on the basis of the nitrogen content as estimated by elemental analysis. The value of the nitrogen content was found to be 6.1%, which corresponds to the 0.48 mmol/g. In the present study, we have considered the attachment of the CoPcS through one end (as shown in Scheme 1, 4); however, the possibility of the covalent linking of the catalyst to the support through more than one ends can not be ruled out. The percentage of the cobalt in the prepared catalyst was determined by ICP-AES analysis and was found to be 2.1% (0.38 mmol/g). 3.2. Catalytic Oxidation of Thiols. The catalytic activity of the prepared polymer immobilized CoPcS 4 was studied for the oxidation of mercaptans 5 to disulfides 6 by using molecular oxygen as oxidant under alkaline conditions (Scheme 2). A wide variety of mercaptans consisting of aliphatic, aromatic, and heterocyclic moieties were selectively oxidized to their corresponding disulfides in near quantitative yields without any evidence for the formation of corresponding sulfonic acids. These results are presented in Table 1. The reaction conditions optimized for the present study consist of simply bubbling of molecular oxygen into a solution of mercaptans in 1 N NaOH containing polymer immobilized cobalt phthalocyanine 4 (1 mol %) under ambient conditions. The completion of the reaction could readily be indicated by change of reaction mixture color from greenish black to blue. Among the various substrates, tertiary mercaptans were found to be less reactive (Table 1, entry 9,15) whereas, in case of aliphatic mercaptans the reactivity decreases with the increase in chain length and accordingly require more reaction time (Table 1, entry 6, 8, 10, 14, and 16-17). Polymer immobilized CoPcS 4 exhibited comparable catalytic activity as corresponding homogeneous catalyst 2 for the oxidation of 1-octane mercaptans under described reaction conditions (Table 1, entry 11). On the other hand, in a controlled blank experiment, there was no oxidation of 1-octanethiol to corresponding disulfide taken place even after prolonged reaction time in the absence of catalyst (Table 1, entry 12). We also
Table 2. Results of Recycling Experimentsa
a
Reaction conditions as mentioned in Table 1. b Isolated yield.
studied the effect of the pH for the present reaction by using 1-octanethiol as a representative example. The reaction was found to be highly dependent upon the pH required lesser reaction time at higher pH (Table 1, entry 13). Next, we checked the recycling of the catalysts 4, a catalytic experiment was designed using thiophenol as a model substrate under described reaction conditions (Table 2). Upon completion of the reaction, the catalyst could readily be recovered by simple filtration and reused for subsequent experiments without further activation (for 5 runs). In all the experiments, reaction times and yields of the corresponding disulfide remained nearly same, establishing the recycling ability of the catalyst. Furthermore, no metal/ligand leaching was observed, as ascertained by the analysis of the filtrates via ICP-AES spectroscopy. It is important to mention that, under similar reaction conditions, the catalytic efficiency of the polystyrene immobilized CoPcS 4 was found to be comparable to its homogeneous CoPcS 2. In addition the supported catalyst could efficiently be recycled for several runs without significant loss in catalytic activity. These results are indeed contrary to the literature reports demonstrating the lower catalytic activity of the polymer supported catalysts than homogeneous catalysts.26 This is probably due to the poor thermo-oxidative and mechanical stability of the organic polymers which sometimes causes difficulties in the regeneration as well as in recycling of the supported catalysts. 4. Conclusion In conclusion, we have demonstrated an efficient heterogeneous aerobic oxidation of mercaptans to the corresponding
Ind. Eng. Chem. Res., Vol. 49, No. 14, 2010
disulfides by using polymer supported cobalt(II) phthalocyanine as catalyst under alkaline conditions. The simplicity of the system, excellent yields, wide applicability, comparable catalytic activity as homogeneous one, facile recovery, and reusability of the catalyst without loss in activity and leaching of metal/ ligand make this an attractive and environmentally acceptable synthetic tool for the oxidation of mercaptans to disulfides. Acknowledgment We are thankful to the director of IIP for his kind permission to publish these results. J.K.J. and S.L.J. acknowledge the CSIR for their research fellowships. Literature Cited (1) Jocelyn, P. C. Biochemistry of the Thiol Groups.; Academic Press: New York, 1992. (2) Bodanszky, M. Principles of Peptide Synthesis: ReactiVity and Structure Concepts in Organic Chemistry; Springer Verlag: Hiderberg, 1984; p 307. (3) Basu, B.; Satapathy, S.; Bhatnagar, A. K. Merox and Related Metal Phthalocyanine Catalyzed Oxidation Processes. Catal. ReV. Sci. Eng. 1993, 35, 571. (4) Bashkova, S.; Bagreev, A.; Bandosz, T. J. Adsorption of Methyl Mercaptan on Activated Carbons. EnViron. Sci. Technol. 2002, 36, 2777. (5) Chauhan, S. M. S.; Gulati, A.; Sahay, P.; Nizar, N. H. Autooxidation of Alkyl Mercaptans Catalyzed by Cobalt(II) Phthalocyanine Tetrasodium Sulphonate in Reverse Micelles. J. Mol. Catal. A: Chem. 1996, 105, 159. (6) Kastner, J. R.; Das, K. C.; Buquoi, Q.; Melear, N. D. Low Temperature Catalytic Oxidation of Hydrogen Sulfide and Methanethiol using Wood and Coal Fly Ash. EnViron. Sci. Technol. 2003, 37, 2568. (7) Chu, H.; Chu, Y. H.; Chiou, Y. Y.; Horng, K. H.; Tseng, T. K. Catalytic Incineration of C2H5SH and Its Mixture with CH3SH over a Pt/ Al2O3 Catalyst. J. EnViron. Eng. 2001, 127, 438. (8) Cha, J. M.; Cha, W. S.; Lee, J. H. Removal of Organo-sulphur Odour Compounds by Thiobacillus novellus SRM, Sulphur-Oxidizing Microorganisms. Process Biochem. 1999, 34, 659. (9) Chatti, I.; Ghorbel, A.; Grange, P.; Colin, J. M. Oxidation of Mercaptans in Light Oil Sweetening by Cobalt(II) Phthalocyanine-Hydrotalcite Catalysts. Catal. Today 2002, 75, 113. (10) Liu, H.; Min, E. Catalytic Oxidation of Mercaptans by Bifunctional Catalysts Composed of Cobalt Phthalocyanine Supported on Mg-Al Hydrotalcite-Derived Solid Bases: Effects of Basicity. Green Chem. 2006, 657. (11) Das, G.; Sain, B.; Kumar, S.; Garg, M. O.; MuraliDhar, G. Synthesis, Characterization and Catalytic Activity of Cobalt Phthalocyanine Tetrasulphonamide in Sweetening of LPG. Catal. Today 2009, 141, 152.
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ReceiVed for reView February 15, 2010 ReVised manuscript receiVed May 1, 2010 Accepted June 5, 2010 IE100351S
