Oxidative conversion of thiols to disulphides by Indian ocean manganese nodules.

T.S.R. Prasada Rao and G. Murali Dhar (Editors)

Recent Advances in Basic and Applied Aspects of Industrial Catalysis

975

Studies in Surface Science and Catalysis, Vol. l l3 9 1998 Elsevier Science B.V. All rights reserved

O x i d a t i v e c o n v e r s i o n o f thiols to disulphides b y Indian o c e a n m a n g a n e s e nodules. K M Parida*, A Samal & N N Das. Regional Research Laboratory, Bhubaneswar- 751 013, India.

ABSTRACT The oxidation of thiols to corresponding disulphides by Indian ocean ferromanganese nodules has been studied under varying experimental conditions. More than 90% conversion of thiols (2.5 x 10-3mol.) was achieved at 35~ using 0.1g nodules. The oxides of Mn, Fe, Ca, Mg, A1 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 < 1-hexanethiol < 1,4-butanedithiol < c~-toluenethiol.

Key words: Oxidation, thiol, disulphide and manganese nodules. 1. INTRODUCTION The oxidative conversion of thiols to disulphides has been studied using a variety of reagents including NOx, transition metal oxides/complexes [1,2-10]. Highly specific first step oxidation products i.e. disulphides was obtained in presence of mild oxidising agents while polysulphides and other side products were also obtained in presence of strong oxidant. Also it is well known that oxidation of thiols is base catalysed due to higher reactivity of RScompared to RSH [ 10]. Marine manganese nodules is a naturally occurring material which possesses reasonably high surface area and contains the oxides/oxyhydroxides of Mn, Fe, Si and A1 as major components while that of Cu, Zn, Co, Ni etc. as minor components [11,12]. The presence of basic oxides such as MgO and CaO is an added quality to use this material as oxidant for the title reaction. The catalytic activity of manganese nodules for oxidation of CO and HC, reduction of NOx, decomposition of H202, hydrodemetallation and hydrodesulphurisation of petroleum crude, methanation etc. have been extensively studied and the results are well documented in literature [12]. However, their oxidising ability and catalytic activity for organic reactions have been sparsely studied [13,14]. With these in view, the oxidation of thiols to disulphides by different Indian Ocean manganese nodules has been studied and the results are reported.

* To whom all the correspondence should be made.

976 2. EXPERIMENTAL The collection of manganese nodules and their detailed physico-chemical characterisation were described previously [ 11 ]. Partial chemical analysis and various surface properties of 110~ dried samples (-75 + 45~tm) used in the present study are collected in Table 1. Synthetic 8-MnOz was also prepared [ 15] and used for comparison. The oxidation of 1-dodecanethiol, 1-Hexanethiol, c~-toluenethiol and 1,4-butanedithiol (Fluka) were 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 equilibriated 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 content in the vessel was stirred mechanically with the help of a magnetic stirrer. At the end of the reaction, the reaction mixture was filtered quantitatively and the unreacted thiol was estimated by titrating against standard alcoholic iodine solution using pyridine as the base [16]. In a few experiments the oxidation product (disulphides) was precipitated by addition of ethanol followed by cooling to 0~ The purity of the product as disulphide was checked by melting point, FT-IR and UV-visible spectra and compared with that of authentic disulphide prepared from the corresponding thiols [ 17]. 3. R E S U L T S AND D I S C U S S I O N

The chemical composition of different nodules used in the present study is widely varied (See Table 1). XRD patterns showed a few diffused peaks characteristic of 8-MnO2, c~-FeOOH and o t - S i O 2. As expected the samples with high silica contents showed more surface area than the samples with lower silica contents. The surface oxygen, however, does not vary widely. Table 1 Chemical analysis (in part) and surface properties of manganese nodules. Sample wt. percent of element/oxide Surface Surface code area oxygen Mn Fe Cu Ni Co SiO2 A1203 (m2/g) (meq/g) Mn-1 24.91 8.23 1 . 1 5 0.98 0.12 19.9 4.52 97.1 0.284 Mn-2 22.72 10.0 0.87 0.84 0.14 25.0 2.56 106.6 0.295 Mn-3 25.10 9.27 1 . 1 3 1 . 0 1 0.13 21.0 2.67 107.5 0.295 Mn-4 22.73 12.0 0.92 0.86 0.13 22.1 4.40 118.4 0.256 Mn-5 23.71 9.18 1 . 2 1 1.10 0.09 22.8 4.17 103.2 0.303 Mn-6 22.76 9.18 1 . 4 3 1.17 0.11 2 4 . 1 4.66 87.4 0.272 Mn-7 10.90 12.3 0.26 0.27 0.10 33.3 3.64 130.7 0.139 Mn-8 18.5 11.9 0.82 0 . 7 1 0.89 27.5 3.81 123.0 0.226 8MnO z 56.83 . . . . . . 94.2 1.20 Before use

O/Mn ratio a 1.87 1.95 1.90 1.91 2.01 1.98 2.00 1.69 1.90

a

Preliminary observation indicates that aerial oxidation of thiols in absence of manganese nodule is negligible under the experimental conditions for a long period (> 3h). The results of oxidative conversion of different thiols to disulphides under varying experimental conditions

977 are presented in Table 2. It is evident from Fig. 1 that the percent o f conversion is almost linearly dependent with m a n g a n e s e content (corr.coeff.>0.98) and surface o x y g e n (corr. coeff.> 0.96) o f m a n g a n e s e nodules. It is also noticed that there is little differences in the oxidising capability b e t w e e n the samples having equimolar quantities o f MnO2 but with different m o l a r concentrations o f other oxides. This indicates that other oxides present in the m a n g a n e s e nodules are less active than MnO2. The percentage o f c o n v e r s i o n was, however, decreased with increase in surface area o f the nodules (See Fig.2). This is not unusual as the samples with high surface area contains less amount o f MnO2 but m o r e SiO2 and A1203which largely contribute towards the higher surface area. As expected, increase in reaction temperature or a m o u n t o f nodules, also enhanced the percent o f conversion. Table 2 Sample code

Oxidation o f thiols by different m a n g a n e s e nodules Thiol x Wt. o f Reaction Reaction % o f conversions 10 .3 nodule(g) temp.(~ time(h)

Mn- 1 Mn-2 Mn-3 Mn-4 Mn-5 Mn-6 Mn-7 Mn-8 Mn-3 Mn-3 Mn-3 Mn-3 Mn-3 Mn-3 Mn-3 Mn-3 Mn-4 Mn-3 Mn-3 Mn-3 Mn-3 Mn-3 6-MnO2 8-MnO2 6-MnO2 Mn-4 Mn-4 Mn-4

(mole) 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 5.0 7.5 0.5 7.5 10.0

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.5 0.05 0.15 0.15 0.2 0.1 0.1 0.1 0.1 0.05 0.05 0.05 0.1 0.1 0.1

35 35 35 35 35 35 35 35 25 30 35 40 45 50 35 35 35 35 35 35 35 35 35 35 35 35 35 35

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 1.0 1.5 2.0 2.5 0.5 0.5 0.5 0.5 0.5 0.5

1 64 65 71 54 73 79 34 45 54 b 62 b 86 b 95 b 29 67 98 88 87 90 . 78 40 29 48 32 29

.

2 .... 53 77 61 37 35 52 79 91 100 40 87 94 88 93 . -

3 79 85 73 89 87 53 69 82 85 91 97 60 86 100 98 100

70" 64" 78" 53" 79 98 100 95 100 100 -

-

-

.

(1) 1-dodecanethiol, (2) 1-hexanethiol, (3) 1,4-butane dithiol, (4)ot-toluenethiol. [Thiol] = 5.0 x 10 .3 m o l e b M n - 4 is used in this cases a

978 As the mole ratio of [thiol]/[metal oxides] (see foot note*) was varied within 4 to 16, a gradual decrease in the disulphide conversion was observed. Within this ratio of [thiol] / [metal oxide], the oxidation products of thiols were identified as their corresponding disulphides. The formation of disulphides as the only product was evident by comparing the melting points and IR spectra of the isolated disulphides from the reaction mixture with the authentic disulphides prepared as described in the experimental section. UV-visible spectra of disulphides in xylene, alone or along with varying amount of thiols showed absorption maxima (Z,max) at 295nm. The reaction mixture containing disulphide and thiol in the filtrate also shows a similar spectra and absorption maxima at same wave length. This indicates that no other product than disulphide is formed in the oxidation of thiols. Under identical conditions, the percentage of thiols to disuphide conversion decreases in the order: cx-tolunenethiol > 1,4-butanedithiol > 1-hexanethiol > 1-dodecanethiol. A similar trend was also observed by Wallace in the oxidation of thiols by transition metal oxides[ 18]. The higher reactivity ~x-tolunenethiol compared to 1-dodecanethiol is attributed to lower pKa of former (10.5) than the latter (14.0) [ 18]. The results in Table 1 also indicate that the activity of 8-MNO2 is comparable with manganese nodule under identical experimental conditions. Previous study [18] on oxidation of thiols by transition metal oxide(s) in the presence of olefins resulted in the formation of corresponding sulphides indicating a free radical addition reaction in which metal oxide acts as an initiator for the production of thiyl radicals(RS.). The disulphide is formed by the dimerization of thiyl radicals (RS.). Based on this, a mechanism for thiol oxidation by manganese nodule (Only oxides of Mn, Fe, Co and Cu in manganese nodule are responsible for oxidation of thiols) is delineated as follows: RSH + MnWO2 RSH + HOMnmO 2 RS.

RSSR

RS.

+

HOMn"~O

RS. + MnIIO + H20

(1) (2) (3)

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 (eq. 1 to 3) as the only mechanism operating in the oxidation of thiols. Because in each case the amount (mole) of thiol oxidised is not stoichiometric with respect to manganese nodule and it is 1.1 to 2.2 times greater than the mole of MnO2 and FeOOH present in the manganese nodules.

*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 oxidising capability such as oxides of Co, Cu and Ni are vary small.

979

0 34 i ........

"i

Surface oxygen~meq/g 0.14 0.30 0.26 0.22 0.18 i

,

/

,

,

,

L

.

0.110 _120

1 80 N

,. Ul

90 i

~ I9O

~

~

~

A~~AMn.. , 3

./

""~

/ /

L"--

.

3010

_t

15

I

20 Mn content w t . %

I

25

/

Figure 1. Percentage o f thiol conversion vs surface oxygen/manganese content o f various manganese nodules. Condition" Thiol = 2.5 x 103mol., Nodule = 0.1g, Temp. = 35~

100 80 = o

60

~ 4o o

. 1 , 4 - b u t a n e dithiol 9 Dodecane thiol

r..) 20

75

100

125

150

Surface area (m2/g) Figure 2. Percentage o f thiol conversion vs surface area o f manganese nodules. Condition: Thiol = 2.5 x 10 .3 mol., Nodule = 0.1g, Temp. = 35~

980 The other factors which account for this higher activity could be (1) the presence of basic oxides such as CaO, MgO and A1203 which facilitate the cleavage of S-H bond (2) they provide active sites for the adsorption of oxygen and thiol into proximity and (3) use of surface oxygen (adsorbed) in the oxidation reaction[ 19]. A decrease of surface oxygen in the nodule samples is also noticed after the reaction. Also the oxidising capacity of used manganese nodules is significantly reduced to 46% (S1. No. 1 of Table 2) in the second run compared to 64% conversion in the first run. 4. CONCLUSION In conclusion, the naturally occurring manganese nodules can be efficiently used as an oxidant for the conversion of thiols to disulphides 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 oxide in reduced oxidation states will be easier than those of unused nodules[20]. ACKNOWLEDGEMENT We are thankful to Prof. H. S. Ray, Director, Regional Research Laboratory, for his permission to publish this paper and Dr. S. B. Rao for his constant encouragement throughout the work. The financial assistance by Department of Ocean Development, New Delhi is gratefully acknowledged. REFERENCES

A. E. Eliyas ,V. V. Ivanova, L. T.Prahov and L. A. Petrov, Appl. Catal., 107 (1994) 181. G. Capozzi, G. Modena, "Chemistry of thiols group" Patai S Edn, Wiley, New York, 1974, Chapter 17, p.785 and refs cited therein. .

.

W. A. Pryor ,D. F. Church, C. K.Glovindan and G. Crank, J. Org. Chem., 47 (1982) 156. T. Endo, M. Hashimoto, T. Orii and M. M. Ito, Bull. Chem. Soc. Jpn., 57 (1984) 1562. R. G. Srinivastava and P. S. Venkataramani ; Indian J. Chem., 20B (1981) 996. D. N. Dhar and A. K. Bagn, Indian J. Chem., 23B (1984) 974. S. L. S. Leite, V. L.Pardini and H. Viertler, Synth. Commun., 20 (1990) 393. H. Firouzahudi, M. Naderi, A. Sardarian and B. Vessal, Synth. Commun., 13

.

10.

(1983)611. J. Nakayama, A. Mizota, F. Nomoto and M. Hoshino, Sulfur letter, 1 (1982) 25. A. Corma, Fornes, F. Rey, Cervilla, Llopis and Ribera, J. Catal., 152 (1995) 237.

981 11.

K. M. Parida, P.K.Satapathy, A. K.Sahoo and N. N.Das, J. Colloid Interface Sci., 173(1995) 112.

12.

K.M.Parida, P.K.Satapathy, N N & Rao S B, J. Sci. Ind. Res., 55 (1996) 234.

13.

K. M. Parida, A. Samal, N. N. Das, Indian J. Chem., Sec. B, (submitted).

14.

C. Dodet ,F. Noville, M. Crine, Marchot, A. Germain and J. P. Pirard, Acta Chimica Hungarica, 124(1987) 65.

15.

K..M. Parida, S. B. Kanungo and B. R. Sant, Electrochim. Acta, 26 (1981) 435

16.

P. H. Donald & D.S. Tarbell, Anal. Chem., 21 (1949) 968.

17.

A. I. Vogel," A text book of Practical Organic Chemistry" Longman Green and Co., London, 1959.

18.

T. J. Wallace, J. Org. Chem., 31 (1966) 1217.

19. 20.

K. T. Liu and Y. C.Tong., Synthesis, (1978) 699. R. P. Das, S. Anand, S. C. Das and P. K. Jena., Hydrometallurgy, 16 (1986) 335.