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Studies on Indian Ocean Manganese Nodules
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
197, 236–241 (1998)
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Studies on Indian Ocean Manganese Nodules 9. Catalytic Oxidation of Thiols to Disulphides by Central Indian Ocean Ferromanganese Nodules K. M. Parida, A. Samal, and N. N. Das Regional Research Laboratory, Bhubaneswar 751 013, India Received April 29, 1997; accepted October 24, 1997
The oxidation of thiols to corresponding disulfides by Indian Ocean ferromanganese nodules has been studied under varying experimental conditions. More than 90% conversion of thiols (2.5 1 10 03 mol) was achieved at 357C using 0.1 g nodules. The oxides of Mn, Fe, Ca, Mg, and Al and surface oxygen in the nodules are most likely responsible for the oxidation of thiols. Under identical conditions the oxidative conversion of thiols decreases in the order 1-dodecanethiol õ 1-hexanethiol õ 1,4-butanedithiol õ a-toluenethiol. q 1998 Academic Press Key Words: oxidation; thiols; disulfides and manganese nodules.
INTRODUCTION
The first step products of oxidation of thiols to disulfides have many industrial and biological applications (1–3). The oxidative conversion of thiols in gasoline is important to reduce the bad smell in its cracking components (4). Further, it is essential to oxidize the thiols emitted from the paper and pulp industry, in order to reduce air pollution (5). The oxidative conversion of thiols to disulfides has been extensively studied using a variety of reagents including NOx , and transition metal oxides/complexes (4, 6–14). Highly specific first step oxidation products, i.e., disulfides, were obtained in the presence of mild oxidizing agents, while polysulfides and other byproducts were also obtained in the presence of a strong oxidant. Also, it is well known that oxidation of thiols is base catalysed due to the higher reactivity of RS 0 compared to RSH (4). Marine manganese nodules are a naturally occurring material which possesses reasonably high surface area and contains the oxides/oxyhydroxides of Mn, Fe, Si, and Al as major components, while those of Cu, Zn, Co, Ni, etc., occur as minor components (15, 16). The presence of basic oxides such as MgO and CaO in manganese nodules may enhance the oxidation activity. The catalytic activity of manganese nodules for oxidation of CO and HC, reduction of NOx , decomposition of H2O2 , hydrodemetallation and hydrodesulfurization of petroleum crude, methanation, etc., have been
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EXPERIMENTAL
The collection of manganese nodules and their detailed physicochemical characterization were described previously (15). However, for ready reference, the outline of the methods is as follows: Total manganese content was estimated by titrating with EDTA, total iron by dichromate and, insoluble silica by HF method (19). The minor components were analyzed by AAS. Surface oxygen (20), surface acidity (21), and surface hydroxyl groups (22) were determined as given in the references. Surface area was determined by nitrogen adsorption–desorption (BET method) at liquid nitrogen temperature using a Quantasorb (Quantachrome, USA). Partial chemical analysis and various surface properties of 1107C dried samples ( 075 / 45 mm) used in the present study are collected in Table 1. Synthetic d-MnO2 was also prepared (23) and used for comparison. To prove that thiol oxidation is promoted in the presence of basic sites, nodule samples were intimately mixed with varying amounts of MgO in the range 5–20 wt% and used in the reaction. The oxidation of 1-dodecanethiol, 1-hexanethiol, a-toluenethiol, and 1,4-butanedithiol (Fluka) was carried out in a specially designed double-walled glass vessel. At the start of the experiment, a weighed quantity of manganese nodule suspended in dry xylene was thermally equilibrated to a desired temperature by circulating water through the vessel. When the desired temperature was attained a known amount of thiol in dry xylene was injected into the vessel. The total volume of the reactant mixture was kept to 40 ml. The contents in the vessel were stirred mechanically with a magnetic stirrer. At the end of the reaction, the reaction mixture
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extensively studied and the results are well documented in the literature (16). However, their oxidizing ability and catalytic activity for organic reactions have been sparsely studied (17, 18). With these in view, the oxidation of thiols to disulfides by different Indian Ocean manganese nodules has been studied and the results are reported.
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TABLE 1 Chemical Analyses (in Part), Surface Area, and Surface Oxygen of Manganese Nodules Wt. percent of element/oxides Sample code
Mn
Fe
Cu
Ni
Co
Mg
Ca
SiO2
Al2O3
Surface area (m2/g)
Surface oxygen (meq/g)
Mn-1 Mn-2 Mn-3 Mn-4 Mn-5 Mn-6 Mn-7 Mn-8
24.91 22.72 25.10 22.73 23.71 22.76 10.90 18.50
8.23 10.02 9.27 12.04 9.18 9.18 12.29 11.90
1.15 0.87 1.13 0.92 1.21 1.43 0.26 0.82
0.98 0.84 1.01 0.86 1.10 1.17 0.27 0.71
0.126 0.149 0.137 0.134 0.091 0.111 0.101 0.890
10.19 — 9.67 8.42 9.39 9.92 5.96 —
1.27 — 1.28 0.72 1.07 1.18 — —
19.9 25.0 21.0 22.1 22.8 24.1 33.3 27.5
4.52 2.56 2.67 4.40 4.17 4.66 3.64 3.81
97.1 106.6 107.5 118.4 103.2 87.4 130.7 123.0
0.284 0.295 0.295 0.256 0.303 0.272 0.139 0.226
was filtered quantitatively into a conical flask. The unreacted thiol was estimated by titrating against standard alcoholic iodine solution using pyridine as the base (24). Similar experiments were carried out in the absence of manganese nodules to see the effect of oxidation in air.
In a few experiments the oxidation product (disulfides) was precipitated by addition of ethanol followed by cooling to 07C. The purity of the product as disulfide was checked by comparing the melting point and FT-IR spectra with those of authentic disulfide. All the authentic disulfides were pre-
TABLE 2 Oxidation of Thiols on Manganese Nodules with Varying Experimental Conditions % of conversions Sample code
Thiol 1 1003 (mole)
Wt. of nodule (g)
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
Reaction temp. (7C)
Reaction time (h)
(1)
(2)
(3)
(4)
64 65 71 54 73 79 34 45
— 53 77 61 — — 37 —
— 79 85 73 89 87 53 —
— — 70 a 64a — 78a 53a —
— — 62b 86b 95b
35 52 91 100 —
69 82 91 97 —
79 98 — — —
29 — 98
40 87 94
60 86 100
95 100 —
88 87 90
88 — 93
98 — 100
— 100 —
48 32 29 78 40 29
— — — — — —
— — — — — —
— — — — — —
(a) On different grades of manganese nodules Mn-1 Mn-2 Mn-3 Mn-4 Mn-5 Mn-6 Mn-7 Mn-8
35 35 35 35 35 35 35 35
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
(b) Variation in temperature Mn-3 Mn-3 Mn-3 Mn-3 Mn-3
2.5 2.5 2.5 2.5 2.5
0.1 0.1 0.1 0.1 0.1
Mn-3 Mn-3 Mn-3
2.5 2.5 2.5
0.05 0.15 0.2
Mn-3 Mn-3 Mn-3
2.5 2.5 2.5
0.1 0.1 0.1
25 30 40 45 50
0.5 0.5 0.5 0.5 0.5
(c) Variation in concentration of nodules 35 35 35
0.5 0.5 0.5
(d) Variation in time of reaction 35 35 35
1.0 1.5 2.0
(e) Variation in concentration of thiol Mn-4 Mn-4 Mn-4 d-MnO2 d-MnO2 d-MnO2
5.0 7.5 10.0 2.5 5.0 7.5
0.1 0.1 0.1 0.05 0.05 0.05
35 35 35 35 35 35
0.5 0.5 0.5 0.5 0.5 0.5
Note. (1) 1-dodecanethiol; (2) 1-hexanethiol; (3) 1,4-butanedithiol; (4) a-toluenethiol. a [Thiol] Å 5.0 1 1003 mole. b Mn-4 is used in these cases.
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FIG. 1. Percentage of thiol conversion versus surface oxygen/manganese content of various manganese nodules. Condition, thiol Å 2.5 1 10 03 mol, nodule Å 0.1 g, temp. Å 357C.
pared from the corresponding thiols according to the method described in Vogel (25). Formation of disulfide as the sole product was also confirmed by taking the UV–visible spectra of the filtrate (after oxidation of thiol) and comparing them with those of authentic disulfides in the presence and absence of thiols. A Chemito 2500 recording UV–visible spectrophotometer was used to record the spectra, while IR spectra were recorded on a JASCO FTIR spectrometer. RESULTS AND DISCUSSION
The chemical composition of different nodules used in the present study varied widely (see Table 1). XRD patterns showed a few diffuse peaks characteristic of d-MnO2 , aFeOOH, and a-SiO2 . As expected, the samples with high silica content showed more surface area than the samples with lower silica content. The surface oxygen, however, did not vary widely. Preliminary observation indicates that oxidation of thiols in air in the absence of manganese nodules is negligible for a long period ( ú3 h) under the experimental conditions. The results of oxidative conversion of different thiols to disulfides under varying experimental conditions are presented in Table 2. It is evident from Fig. 1 that the percentage of conversion
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is almost linearly dependent on the manganese content (corr.coeff. ú 0.98) and surface oxygen (corr. coeff. ú 0.96) of manganese nodules. It is also interesting to note that the lines due to Mn content and surface oxygen in the conversion of thiols to disulfides are parallel to each other. This indicates that both parameters are equally effective for this reaction and also interrelated. It is also noticed that there is little differences in oxidizing capability between the samples having equimolar quantities of MnO2 but with different molar concentrations of other oxides. This indicates that other oxides present in the manganese nodules are less active than MnO2 . The percentage of conversion was directly proportional to the MnO2 content and decreased with increase in surface area of the nodules (see Fig. 2). This is not unusual, as the samples with high surface area contains less MnO2 but more SiO2 and Al2O3 , which largely contribute toward the higher surface area. As expected, increased reaction temperature or amount of nodules also enhanced the percentage of conversion. As the mole ratio of [thiol]/[metal oxides] was varied between 4 and 16, 1 a gradual decrease in the 1 To calculate the moles of metal oxides in manganese nodules only oxides/oxyhydroxides of Mn and Fe as MnO2 and FeOOH were taken into consideration. The amount of other oxides having oxidizing capability, such as oxides of Co, Cu, and Ni, is very small.
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FIG. 2. Percentage of thiol conversion versus surface areas of various manganese nodules. Condition: thiol Å 2.5 1 10 03 mol, nodule Å 0.1 g, temp. Å 357C.
disulfide conversion was observed. Within this [thiol]/[metal oxide] ratio, the oxidation products of thiols were identified as their corresponding disulfides. The formation of disulfides as the only product was evident by comparison of the melting points and IR spectra of the isolated disulfides from the reaction mixture with the authentic disulfides prepared as described in the experimental section. UV–visible spectra of disulfides in xylene, alone or along with varying amount of thiols, showed absorption maxima ( lmax ) at 295 nm. The reaction mixture containing disulfide and thiol in the filtrate also shows a similar spectra and absorption maxima at same wave length. This indicates that no other product than disulfide is formed in the oxidation of thiols. Under identical conditions, the percentage of thiols that undergo disulfide conversion decreases in the order a-toluenethiol ú 1,4-butanedithiol ú 1-hexanethiol ú 1-dodecanethiol. A similar trend was also observed by Wallace in the oxidation of thiols by transition metal oxides (26). The higher reactivity of a-toluenethiol compared to 1-dodecanethiol is
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attributed to the lower p Ka of the former (10.5) than of the latter (14.0) (26). The results in Table 2 also indicate that the activity of d-MnO2 is comparable with that of manganese nodules under identical experimental conditions. A previous study (26) of the oxidation of thiols by transition metal oxide(s) in the presence of olefins resulted in the formation of corresponding sulfides indicating a free radical addition reaction in which metal oxide acts as an initiator for the production of thiyl radicals (RSr). The disulfide is formed by the dimerization of thiyl radicals (RSr). Based on this, a mechanism for thiol oxidation on manganese nodules (only oxides of Mn, Fe, Co, and Cu in manganese nodules are responsible for the oxidation of thiols) is delineated as follows: RSH / Mn IV O2 r RSr / HOMn IIIO
[1]
RSH / HOMn O r RSr/ Mn O / H2O
[2]
III
2 RSr r RSSR.
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II
[3]
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FIG. 3. Percentage of thiol conversion versus Ca and Mg content of manganese nodules. Thiol Å 2.5 1 10 03 mol, nodule Å 0.1 g, temp. Å 357C.
Similar reaction steps can also be presented for other oxides. Obviously the above oxidation reactions involve a corresponding reduction of the metal oxides. Interestingly, our results did not agree fully with the above reaction scheme (Eqs. [1] to [3]) as the only mechanism operating in the oxidation of thiols, because in each case the amount (mole) of thiol oxidized is not stoichiometric with respect to manganese nodules and it is 1.1 to 2.2 times greater than the moles of MnO2 and FeOOH present in the manganese nodules. The other factors which account for this higher activity could be (a) the presence of basic oxides such as CaO, MgO, and Al2O3 , which facilitate the cleavage of the S–H bond; (b) the provision active sites for the adsorption of oxygen and thiol in proximity; and (c) the use of surface oxygen (adsorbed) in the oxidation reaction (27). Figure 3 shows the enhancement in thiol conversion with increased Mg and Ca content in the manganese nodules. To account for this effect, the reaction was carried out in the presence of MgO (added in excess) in the range 5–20 wt%. An increasing trend in
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the conversion was observed by addition of MgO up to 10 wt% in Mn-3 (76% conversion) and thereafter remained constant. A decrease of surface oxygen in the nodule samples is also noticed after the reaction. Also, the oxidizing capacity of manganese nodules is significantly reduced to 46% in the second run and 31% in the third run compared to 64% conversion in the first run (Sl. No. 1 of Table 2). In conclusion, naturally occuring manganese nodules can be efficiently used as an oxidant for the conversion of thiols to disulfides under mild experimental conditions. The method is simple and economical with high product selectivity. Further, the extraction of metals from the deactivated nodules containing metal oxides in reduced oxidation states will be easier than extraction from unused nodules (28). ACKNOWLEDGMENTS We are thankful to Professor H. S. Ray, Director, Regional Research Laboratory, for his permission to publish this paper and Dr. S. B. Rao for
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STUDIES ON INDIAN OCEAN MANGANESE NODULES his constant encouragement throughout the work. The financial assistance of the Department of Ocean Development, New Delhi, is gratefully acknowledged.
REFERENCES 1. Karmarz, J., and Chmura, M., Schmierangstechnik 15, 209 (1984). 2. Matsuoka, S., Amano, M., Yoshuneri, K., Yamanoto, S., and Kida, Y., Jpn. Kokai Tokkyo Koho. [ Japanese patent 01, 158 009, (89,158,009) ] 3. Schulze, A. W., Short, G. H., and Crouch, W. W., Ind. Eng. Chem. 42, 916 (1950). 4. Eliyas, A. E., Ivanova, V. V., Prahov, L. T., and Petrov, L. A., Appl. Catal. 107, 181 (1994). 5. Dharmadhikari, D. M., and Agarwal, A. L., Ind. J. Environ. Prot. 19, 699 (1990). 6. Capozzi, G., and Modena, G., in ‘‘Chemistry of Thiols Group’’ Patai S Edn, (S. Patai, Ed.), Chap. 17, p. 785. Wiley, New York, 1974, and references cited therein. 7. Pryor, W. A., Church, D. F., Glovindan, C. K., and Crank, G., J. Org. Chem. 47, 156 (1982). 8. Endo, T., Hashimoto, M., Orii, T., and Ito, M. M., Bull. Chem. Soc. Jpn. 57, 1562 (1984). 9. Srinivastava, R. G., and Venkataramani, P. S., Ind. J. Chem. B 20, 996 (1981). 10. Dhar, D. N., and Bagn, A. K., Ind. J. Chem. B 23, 974 (1984). 11. Leite, S. L. S., Pardini, V. L., and Viertler, H., Synth. Commun. 20, 393 (1990).
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12. Firouzahudi, H., Naderi, M., Sardarian, A., and Vessal, B., Synth. Commun. 13, 611 (1983). 13. Nakayama, J., Mizota, A., Nomoto, F., and Hoshino, M., Sulfur Lett. 1, 25 (1982). 14. Corma, A., Forne´s, V., Rey, F., Cervilla, A., Llopis, E., and Ribera, A., J. Catal. 152, 237 (1995). 15. Parida, K. M., Satapathy, P. K., Sahoo, A. K., and Das, N. N., J. Colloid Interface Sci. 173, 112 (1995). 16. Parida, K. M., Satapathy, P. K., Das, N. N., and Rao, S. B., J. Sci. Ind. Res. 55, 234 (1996). 17. Parida, K. M., Samal, A., and Das, N. N., submitted for publication. 18. Dodet, C., Noville, F., Crine, M., Marchot, Germain, A., and Pirard, J. P., Acta Chim. Hungar. 124, 65 (1987). 19. Vogel, A. I., ‘‘Quantitative Inorganic Chemical Analysis,’’ p. 579. Longmans, Green, London, 1961. 20. Uchijima, T., Takahashi, M., and Yoneda, Y., J. Catal. 9, 102 (1967). 21. Pitchard, J. G., and Long, F. A., J. Am. Chem. Soc. 79, 2365 (1957). 22. Kanungo, S. B., Parida, K. M., and Sant, B. R., Electrochim. Acta 26, 1157 (1981). 23. Parida, K. M., Kanungo, S. B., and Sant, B. R., Electrochim. Acta 26, 435 (1981). 24. Donald, P. H., and Tarbell, D. S., Anal. Chem. 21, 968 (1949). 25. Vogel, A. I., ‘‘A Textbook of Practical Organic Chemistry.’’ Longman Green, London, 1959. 26. Wallace, T. J., J. Org. Chem. 31, 1217 (1966). 27. Liu, K. T., and Tong, Y. C., Synthesis 699 (1978). 28. Das, R. P., Anand, S., Das, S. C., and Jena, P. K., Hydrometallurgy 16, 335 (1986).
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197, 236–241 (1998)
CS975260
Studies on Indian Ocean Manganese Nodules 9. Catalytic Oxidation of Thiols to Disulphides by Central Indian Ocean Ferromanganese Nodules K. M. Parida, A. Samal, and N. N. Das Regional Research Laboratory, Bhubaneswar 751 013, India Received April 29, 1997; accepted October 24, 1997
The oxidation of thiols to corresponding disulfides by Indian Ocean ferromanganese nodules has been studied under varying experimental conditions. More than 90% conversion of thiols (2.5 1 10 03 mol) was achieved at 357C using 0.1 g nodules. The oxides of Mn, Fe, Ca, Mg, and Al and surface oxygen in the nodules are most likely responsible for the oxidation of thiols. Under identical conditions the oxidative conversion of thiols decreases in the order 1-dodecanethiol õ 1-hexanethiol õ 1,4-butanedithiol õ a-toluenethiol. q 1998 Academic Press Key Words: oxidation; thiols; disulfides and manganese nodules.
INTRODUCTION
The first step products of oxidation of thiols to disulfides have many industrial and biological applications (1–3). The oxidative conversion of thiols in gasoline is important to reduce the bad smell in its cracking components (4). Further, it is essential to oxidize the thiols emitted from the paper and pulp industry, in order to reduce air pollution (5). The oxidative conversion of thiols to disulfides has been extensively studied using a variety of reagents including NOx , and transition metal oxides/complexes (4, 6–14). Highly specific first step oxidation products, i.e., disulfides, were obtained in the presence of mild oxidizing agents, while polysulfides and other byproducts were also obtained in the presence of a strong oxidant. Also, it is well known that oxidation of thiols is base catalysed due to the higher reactivity of RS 0 compared to RSH (4). Marine manganese nodules are a naturally occurring material which possesses reasonably high surface area and contains the oxides/oxyhydroxides of Mn, Fe, Si, and Al as major components, while those of Cu, Zn, Co, Ni, etc., occur as minor components (15, 16). The presence of basic oxides such as MgO and CaO in manganese nodules may enhance the oxidation activity. The catalytic activity of manganese nodules for oxidation of CO and HC, reduction of NOx , decomposition of H2O2 , hydrodemetallation and hydrodesulfurization of petroleum crude, methanation, etc., have been
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EXPERIMENTAL
The collection of manganese nodules and their detailed physicochemical characterization were described previously (15). However, for ready reference, the outline of the methods is as follows: Total manganese content was estimated by titrating with EDTA, total iron by dichromate and, insoluble silica by HF method (19). The minor components were analyzed by AAS. Surface oxygen (20), surface acidity (21), and surface hydroxyl groups (22) were determined as given in the references. Surface area was determined by nitrogen adsorption–desorption (BET method) at liquid nitrogen temperature using a Quantasorb (Quantachrome, USA). Partial chemical analysis and various surface properties of 1107C dried samples ( 075 / 45 mm) used in the present study are collected in Table 1. Synthetic d-MnO2 was also prepared (23) and used for comparison. To prove that thiol oxidation is promoted in the presence of basic sites, nodule samples were intimately mixed with varying amounts of MgO in the range 5–20 wt% and used in the reaction. The oxidation of 1-dodecanethiol, 1-hexanethiol, a-toluenethiol, and 1,4-butanedithiol (Fluka) was carried out in a specially designed double-walled glass vessel. At the start of the experiment, a weighed quantity of manganese nodule suspended in dry xylene was thermally equilibrated to a desired temperature by circulating water through the vessel. When the desired temperature was attained a known amount of thiol in dry xylene was injected into the vessel. The total volume of the reactant mixture was kept to 40 ml. The contents in the vessel were stirred mechanically with a magnetic stirrer. At the end of the reaction, the reaction mixture
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extensively studied and the results are well documented in the literature (16). However, their oxidizing ability and catalytic activity for organic reactions have been sparsely studied (17, 18). With these in view, the oxidation of thiols to disulfides by different Indian Ocean manganese nodules has been studied and the results are reported.
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TABLE 1 Chemical Analyses (in Part), Surface Area, and Surface Oxygen of Manganese Nodules Wt. percent of element/oxides Sample code
Mn
Fe
Cu
Ni
Co
Mg
Ca
SiO2
Al2O3
Surface area (m2/g)
Surface oxygen (meq/g)
Mn-1 Mn-2 Mn-3 Mn-4 Mn-5 Mn-6 Mn-7 Mn-8
24.91 22.72 25.10 22.73 23.71 22.76 10.90 18.50
8.23 10.02 9.27 12.04 9.18 9.18 12.29 11.90
1.15 0.87 1.13 0.92 1.21 1.43 0.26 0.82
0.98 0.84 1.01 0.86 1.10 1.17 0.27 0.71
0.126 0.149 0.137 0.134 0.091 0.111 0.101 0.890
10.19 — 9.67 8.42 9.39 9.92 5.96 —
1.27 — 1.28 0.72 1.07 1.18 — —
19.9 25.0 21.0 22.1 22.8 24.1 33.3 27.5
4.52 2.56 2.67 4.40 4.17 4.66 3.64 3.81
97.1 106.6 107.5 118.4 103.2 87.4 130.7 123.0
0.284 0.295 0.295 0.256 0.303 0.272 0.139 0.226
was filtered quantitatively into a conical flask. The unreacted thiol was estimated by titrating against standard alcoholic iodine solution using pyridine as the base (24). Similar experiments were carried out in the absence of manganese nodules to see the effect of oxidation in air.
In a few experiments the oxidation product (disulfides) was precipitated by addition of ethanol followed by cooling to 07C. The purity of the product as disulfide was checked by comparing the melting point and FT-IR spectra with those of authentic disulfide. All the authentic disulfides were pre-
TABLE 2 Oxidation of Thiols on Manganese Nodules with Varying Experimental Conditions % of conversions Sample code
Thiol 1 1003 (mole)
Wt. of nodule (g)
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
Reaction temp. (7C)
Reaction time (h)
(1)
(2)
(3)
(4)
64 65 71 54 73 79 34 45
— 53 77 61 — — 37 —
— 79 85 73 89 87 53 —
— — 70 a 64a — 78a 53a —
— — 62b 86b 95b
35 52 91 100 —
69 82 91 97 —
79 98 — — —
29 — 98
40 87 94
60 86 100
95 100 —
88 87 90
88 — 93
98 — 100
— 100 —
48 32 29 78 40 29
— — — — — —
— — — — — —
— — — — — —
(a) On different grades of manganese nodules Mn-1 Mn-2 Mn-3 Mn-4 Mn-5 Mn-6 Mn-7 Mn-8
35 35 35 35 35 35 35 35
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
(b) Variation in temperature Mn-3 Mn-3 Mn-3 Mn-3 Mn-3
2.5 2.5 2.5 2.5 2.5
0.1 0.1 0.1 0.1 0.1
Mn-3 Mn-3 Mn-3
2.5 2.5 2.5
0.05 0.15 0.2
Mn-3 Mn-3 Mn-3
2.5 2.5 2.5
0.1 0.1 0.1
25 30 40 45 50
0.5 0.5 0.5 0.5 0.5
(c) Variation in concentration of nodules 35 35 35
0.5 0.5 0.5
(d) Variation in time of reaction 35 35 35
1.0 1.5 2.0
(e) Variation in concentration of thiol Mn-4 Mn-4 Mn-4 d-MnO2 d-MnO2 d-MnO2
5.0 7.5 10.0 2.5 5.0 7.5
0.1 0.1 0.1 0.05 0.05 0.05
35 35 35 35 35 35
0.5 0.5 0.5 0.5 0.5 0.5
Note. (1) 1-dodecanethiol; (2) 1-hexanethiol; (3) 1,4-butanedithiol; (4) a-toluenethiol. a [Thiol] Å 5.0 1 1003 mole. b Mn-4 is used in these cases.
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FIG. 1. Percentage of thiol conversion versus surface oxygen/manganese content of various manganese nodules. Condition, thiol Å 2.5 1 10 03 mol, nodule Å 0.1 g, temp. Å 357C.
pared from the corresponding thiols according to the method described in Vogel (25). Formation of disulfide as the sole product was also confirmed by taking the UV–visible spectra of the filtrate (after oxidation of thiol) and comparing them with those of authentic disulfides in the presence and absence of thiols. A Chemito 2500 recording UV–visible spectrophotometer was used to record the spectra, while IR spectra were recorded on a JASCO FTIR spectrometer. RESULTS AND DISCUSSION
The chemical composition of different nodules used in the present study varied widely (see Table 1). XRD patterns showed a few diffuse peaks characteristic of d-MnO2 , aFeOOH, and a-SiO2 . As expected, the samples with high silica content showed more surface area than the samples with lower silica content. The surface oxygen, however, did not vary widely. Preliminary observation indicates that oxidation of thiols in air in the absence of manganese nodules is negligible for a long period ( ú3 h) under the experimental conditions. The results of oxidative conversion of different thiols to disulfides under varying experimental conditions are presented in Table 2. It is evident from Fig. 1 that the percentage of conversion
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is almost linearly dependent on the manganese content (corr.coeff. ú 0.98) and surface oxygen (corr. coeff. ú 0.96) of manganese nodules. It is also interesting to note that the lines due to Mn content and surface oxygen in the conversion of thiols to disulfides are parallel to each other. This indicates that both parameters are equally effective for this reaction and also interrelated. It is also noticed that there is little differences in oxidizing capability between the samples having equimolar quantities of MnO2 but with different molar concentrations of other oxides. This indicates that other oxides present in the manganese nodules are less active than MnO2 . The percentage of conversion was directly proportional to the MnO2 content and decreased with increase in surface area of the nodules (see Fig. 2). This is not unusual, as the samples with high surface area contains less MnO2 but more SiO2 and Al2O3 , which largely contribute toward the higher surface area. As expected, increased reaction temperature or amount of nodules also enhanced the percentage of conversion. As the mole ratio of [thiol]/[metal oxides] was varied between 4 and 16, 1 a gradual decrease in the 1 To calculate the moles of metal oxides in manganese nodules only oxides/oxyhydroxides of Mn and Fe as MnO2 and FeOOH were taken into consideration. The amount of other oxides having oxidizing capability, such as oxides of Co, Cu, and Ni, is very small.
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FIG. 2. Percentage of thiol conversion versus surface areas of various manganese nodules. Condition: thiol Å 2.5 1 10 03 mol, nodule Å 0.1 g, temp. Å 357C.
disulfide conversion was observed. Within this [thiol]/[metal oxide] ratio, the oxidation products of thiols were identified as their corresponding disulfides. The formation of disulfides as the only product was evident by comparison of the melting points and IR spectra of the isolated disulfides from the reaction mixture with the authentic disulfides prepared as described in the experimental section. UV–visible spectra of disulfides in xylene, alone or along with varying amount of thiols, showed absorption maxima ( lmax ) at 295 nm. The reaction mixture containing disulfide and thiol in the filtrate also shows a similar spectra and absorption maxima at same wave length. This indicates that no other product than disulfide is formed in the oxidation of thiols. Under identical conditions, the percentage of thiols that undergo disulfide conversion decreases in the order a-toluenethiol ú 1,4-butanedithiol ú 1-hexanethiol ú 1-dodecanethiol. A similar trend was also observed by Wallace in the oxidation of thiols by transition metal oxides (26). The higher reactivity of a-toluenethiol compared to 1-dodecanethiol is
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attributed to the lower p Ka of the former (10.5) than of the latter (14.0) (26). The results in Table 2 also indicate that the activity of d-MnO2 is comparable with that of manganese nodules under identical experimental conditions. A previous study (26) of the oxidation of thiols by transition metal oxide(s) in the presence of olefins resulted in the formation of corresponding sulfides indicating a free radical addition reaction in which metal oxide acts as an initiator for the production of thiyl radicals (RSr). The disulfide is formed by the dimerization of thiyl radicals (RSr). Based on this, a mechanism for thiol oxidation on manganese nodules (only oxides of Mn, Fe, Co, and Cu in manganese nodules are responsible for the oxidation of thiols) is delineated as follows: RSH / Mn IV O2 r RSr / HOMn IIIO
[1]
RSH / HOMn O r RSr/ Mn O / H2O
[2]
III
2 RSr r RSSR.
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II
[3]
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FIG. 3. Percentage of thiol conversion versus Ca and Mg content of manganese nodules. Thiol Å 2.5 1 10 03 mol, nodule Å 0.1 g, temp. Å 357C.
Similar reaction steps can also be presented for other oxides. Obviously the above oxidation reactions involve a corresponding reduction of the metal oxides. Interestingly, our results did not agree fully with the above reaction scheme (Eqs. [1] to [3]) as the only mechanism operating in the oxidation of thiols, because in each case the amount (mole) of thiol oxidized is not stoichiometric with respect to manganese nodules and it is 1.1 to 2.2 times greater than the moles of MnO2 and FeOOH present in the manganese nodules. The other factors which account for this higher activity could be (a) the presence of basic oxides such as CaO, MgO, and Al2O3 , which facilitate the cleavage of the S–H bond; (b) the provision active sites for the adsorption of oxygen and thiol in proximity; and (c) the use of surface oxygen (adsorbed) in the oxidation reaction (27). Figure 3 shows the enhancement in thiol conversion with increased Mg and Ca content in the manganese nodules. To account for this effect, the reaction was carried out in the presence of MgO (added in excess) in the range 5–20 wt%. An increasing trend in
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the conversion was observed by addition of MgO up to 10 wt% in Mn-3 (76% conversion) and thereafter remained constant. A decrease of surface oxygen in the nodule samples is also noticed after the reaction. Also, the oxidizing capacity of manganese nodules is significantly reduced to 46% in the second run and 31% in the third run compared to 64% conversion in the first run (Sl. No. 1 of Table 2). In conclusion, naturally occuring manganese nodules can be efficiently used as an oxidant for the conversion of thiols to disulfides under mild experimental conditions. The method is simple and economical with high product selectivity. Further, the extraction of metals from the deactivated nodules containing metal oxides in reduced oxidation states will be easier than extraction from unused nodules (28). ACKNOWLEDGMENTS We are thankful to Professor H. S. Ray, Director, Regional Research Laboratory, for his permission to publish this paper and Dr. S. B. Rao for
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STUDIES ON INDIAN OCEAN MANGANESE NODULES his constant encouragement throughout the work. The financial assistance of the Department of Ocean Development, New Delhi, is gratefully acknowledged.
REFERENCES 1. Karmarz, J., and Chmura, M., Schmierangstechnik 15, 209 (1984). 2. Matsuoka, S., Amano, M., Yoshuneri, K., Yamanoto, S., and Kida, Y., Jpn. Kokai Tokkyo Koho. [ Japanese patent 01, 158 009, (89,158,009) ] 3. Schulze, A. W., Short, G. H., and Crouch, W. W., Ind. Eng. Chem. 42, 916 (1950). 4. Eliyas, A. E., Ivanova, V. V., Prahov, L. T., and Petrov, L. A., Appl. Catal. 107, 181 (1994). 5. Dharmadhikari, D. M., and Agarwal, A. L., Ind. J. Environ. Prot. 19, 699 (1990). 6. Capozzi, G., and Modena, G., in ‘‘Chemistry of Thiols Group’’ Patai S Edn, (S. Patai, Ed.), Chap. 17, p. 785. Wiley, New York, 1974, and references cited therein. 7. Pryor, W. A., Church, D. F., Glovindan, C. K., and Crank, G., J. Org. Chem. 47, 156 (1982). 8. Endo, T., Hashimoto, M., Orii, T., and Ito, M. M., Bull. Chem. Soc. Jpn. 57, 1562 (1984). 9. Srinivastava, R. G., and Venkataramani, P. S., Ind. J. Chem. B 20, 996 (1981). 10. Dhar, D. N., and Bagn, A. K., Ind. J. Chem. B 23, 974 (1984). 11. Leite, S. L. S., Pardini, V. L., and Viertler, H., Synth. Commun. 20, 393 (1990).
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12. Firouzahudi, H., Naderi, M., Sardarian, A., and Vessal, B., Synth. Commun. 13, 611 (1983). 13. Nakayama, J., Mizota, A., Nomoto, F., and Hoshino, M., Sulfur Lett. 1, 25 (1982). 14. Corma, A., Forne´s, V., Rey, F., Cervilla, A., Llopis, E., and Ribera, A., J. Catal. 152, 237 (1995). 15. Parida, K. M., Satapathy, P. K., Sahoo, A. K., and Das, N. N., J. Colloid Interface Sci. 173, 112 (1995). 16. Parida, K. M., Satapathy, P. K., Das, N. N., and Rao, S. B., J. Sci. Ind. Res. 55, 234 (1996). 17. Parida, K. M., Samal, A., and Das, N. N., submitted for publication. 18. Dodet, C., Noville, F., Crine, M., Marchot, Germain, A., and Pirard, J. P., Acta Chim. Hungar. 124, 65 (1987). 19. Vogel, A. I., ‘‘Quantitative Inorganic Chemical Analysis,’’ p. 579. Longmans, Green, London, 1961. 20. Uchijima, T., Takahashi, M., and Yoneda, Y., J. Catal. 9, 102 (1967). 21. Pitchard, J. G., and Long, F. A., J. Am. Chem. Soc. 79, 2365 (1957). 22. Kanungo, S. B., Parida, K. M., and Sant, B. R., Electrochim. Acta 26, 1157 (1981). 23. Parida, K. M., Kanungo, S. B., and Sant, B. R., Electrochim. Acta 26, 435 (1981). 24. Donald, P. H., and Tarbell, D. S., Anal. Chem. 21, 968 (1949). 25. Vogel, A. I., ‘‘A Textbook of Practical Organic Chemistry.’’ Longman Green, London, 1959. 26. Wallace, T. J., J. Org. Chem. 31, 1217 (1966). 27. Liu, K. T., and Tong, Y. C., Synthesis 699 (1978). 28. Das, R. P., Anand, S., Das, S. C., and Jena, P. K., Hydrometallurgy 16, 335 (1986).
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