ISSN 1070-4280, Russian Journal of Organic Chemistry, 2016, Vol. 52, No. 3, pp. 307–311. © Pleiades Publishing, Ltd., 2016. Original Russian Text © A.V. Mashkina, L.N. Khairulina, 2016, published in Zhurnal Organicheskoi Khimii, 2016, Vol. 52, No. 3, pp. 327–331.
Catalytic Reaction of Diethyl Disulfide with Methanol A. V. Mashkina and L. N. Khairulina Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, pr. Akad. Lavrent’eva 5, Novosibirsk, 630090 Russia e-mail:
[email protected] Received November 9, 2015
Abstract—Diethyl disulfide reacted with methanol in the presence of solid acid catalysts at 250–350°C to give dimethyl, ethyl methyl, and diethyl sulfides. The most active catalysts were those containing simultaneously moderate basic sites, strong Lewis acid sites, and some amount of strong protonic acid sites. These catalysts ensured a total selectivity of 99% for dialkyl sulfides.
DOI: 10.1134/S1070428016030027 converted into diethyl sulfide (yield 0.5–11 mol %) in the presence of an aluminum boride catalyst at 250– 350°C in helium gas, while the yield of ethanethiol was 9–34 mol%; the reaction was also accompanied by formation of large amounts of ethylene and hydrogen sulfide.
Lower dialkyl sulfides are used as odorants, flotation agents, and starting materials for the synthesis of various important compounds. Dialkyl sulfides are most frequently obtained on a preparative scale by noncatalytic reactions of alkyl halides with alkali metal sulfides or alkanethiols, as well as by reaction of hydrogen sulfide or alkanethiols with alkenes or alcohols in the presence of catalysts [1]. These procedures often require expensive and difficultly accessible reagents and are experimentally complex. Dialkyl sulfides can be synthesized from dialkyl disulfides (R 2 S 2 , R = C1–C4-Alk) that are produced in huge amounts by desulfurization of gases and petroleum and have limited application [2]. For example, it was found previously that the transformation of dimethyl disulfide into dimethyl sulfide by the action of solid catalysts at 150– 350°C in helium gas is characterized by a maximum selectivity of 50–60%; methanethiol, hydrogen sulfide, ethylene, and carbon disulfide are formed as byproducts [3]. The selectivity for dimethyl sulfide in a mixture of dimethyl disulfide with methanol was improved to 95–98%, and only a small amount of byproducts was formed [4]. The S–S bond strength in dimethyl disulfide molecule and its closest homologs is almost the same, but conjugation involving sulfur 3d orbitals in dimethyl disulfide homologs facilitates proton abstraction from the methylene group linked to sulfur [5]. Therefore, one cannot exclude that the transformation of closest homologs of dimethyl disulfide into the corresponding sulfides will be complicated due to cracking processes. Catalytic transformations of lower dialkyl disulfides into dialkyl sulfides have been poorly studied. As shown in [6], diethyl disulfide was
In the present work we studied transformations of diethyl disulfide in a mixture with methanol in the presence of various catalysts. The examined catalysts differed by the nature, strength, and concentrations of acidic and basic sites, which were determined previously [3, 7] (Table 1). With no methanol added, the transformation of diethyl disulfide over different catalysts in helium afforded a small amount of diethyl sulfide, while the major products were ethanethiol, hydrogen sulfide, and ethylene. For instance, the yield of diethyl sulfide over alumina was 3–6 mol %, the conversion of diethyl disulfide being 50–100%; the major product was ethanethiol (22–70 mol %), and hydrogen sulfide and ethylene were also formed. We have found that the reaction of diethyl disulfide with methanol in the presence of different catalysts gives alkanethiols and dialkyl sulfides; hydrogen sulfide and ethylene are formed as by-products via decomposition of diethyl disulfide, and methanol gives rise to dimethyl ether, carbon oxides, and water. The yield of dialkyl sulfides increases as the methanol-todiethyl disulfide ratio increases. For example, in the presence of γ-Al 2 O 3 at 350°C (contact time 0.5 s, molar ratio methanol–diethyl disulfide 0.5 : 1) the major organosulfur products were diethyl sulfide (5 mol %) and ethanethiol (60 mol %). At an equi307
MASHKINA, KHAIRULINA
308 Table 1. Acid–base properties of the catalysts
Strength, kJ/mol (concentration, μmol/m2) of active sites
Catalyst
protonic acid sites
Lewis acid sites
basic sites
1.7 Na/Al2O3
~1200 (0).0
18–20 (2–3)00
830–960 (3.8)
H3PO4/SiO2
~1300 (1.2)
00
<800
1170–1180 (0.33)
33–37 (0.16)0 42–54 (0.013)
800–900 (1.1)
~1170 (0.2)
35.7–59.5 (0.78)00
800–900 (1.2)
0<1200 (0.36)
033–35 (0.12)0 000056 (0.005)
800–900 (1.0)
HSiW/SiO2
<1180 (1.0)
00
800–900 (1.3)
Co/Al2O3
~1170 (0.2)
34–37 (1.1)00
800–900 (1.5)
B/Al2O3
1170–1190 (0.07)
27–35 (1.5)00 40–51 (0.13)0
800–900 (2.0)
γ-Al2O3
1170–1270 (0.03)
32–34 (2.2)00 41–56 (0.2)00
800–900 (2.5)
HZSM-5 Co/HZSM-5 AlSi
molar ratio of methanol and diethyl disulfide, dimethyl, ethyl methyl, and diethyl sulfides were formed in 17, 15, and 3 mol % yields, respectively, and the yield of ethanethiol decreased to 40 mol %. With
Conversion of diethyl disulfide, % or yield, mol %
100
1
80
2.5 mol of methanol per mole of diethyl disulfide, the reaction mixture contained only dimethyl and ethyl methyl sulfides. Further raising the methanol concentration did not affect the reaction outcome. The catalytic activities of different catalysts were compared at 350°C at methanol-to-diethyl disulfide ratios of (2.2–2.5) : 1. The transformation products were dialkyl sulfides, alkanethiols, hydrogen sulfide, and ethylene. In the general case, the process included the reactions shown in Scheme 1. Scheme 1.
2 7
60
EtSSEt + 2 MeOH
EtSSEt
2 EtSSEt
40
EtSSEt
20
0 0.0
3 4 5 6 0.1
0.2 Contact time, s
0.3
0.4
Fig. 1. Effect of contact time on the (1) conversion of diethyl disulfide and yields of (2) dimethyl sulfide, (3), ethyl methyl sulfide, (4) diethyl sulfide, (5), ethanethiol, (6) methanethiol, (7) ethylene, and (8) hydrogen sulfide; HZSM-5, 350°C, molar ratio methanol–diethyl disulfide (2.2–2.5) : 1.
H+
2 EtSH
Et2S + H 2S 2 H 2C=CH 2 + H 2S + S
H 2 S + MeOH
MeSH + H 2 O
H 2S + 2 MeOH
Me 2S + 2 H 2O
Me 2 S
8
2 MeSEt + 2 H 2 O
EtSSEt
2 MeSH + H 2S
EtSH + S
In all cases, the conversion of diethyl disulfide and the product yields increased with contact time (Fig. 1). The contact time at which the conversion of diethyl disulfide reached 70%, and the rate of its transformation and the yields were calculated from the kinetic curves (Table 2).
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309
Table 2. Activity of different catalysts in the reaction of diethyl disulfide with methanol at 350°C; molar ratio methanol– diethyl disulfide (2.2–2.5) : 1; initial concentration of diethyl disulfide 0.8 vol % Yield, mol %
Rate of Et2S2 transformation, mmol g–1 h–1
Me2S
MeEtS
Et2S
MeSH
EtSH
H 2S
0.9
00
00
0
19
42
10
H3PO4/SiO2
00.05
00
00
0
00
00
00
HSiW/SiO2
3.6
07
08
0
12
40
02
HZSM-5
5.0
40
11
8
03
07
02
Co/HZSM-5
5.1
23
10
0
00
06
10
AlSi
1.8
21
20
0
00
15
12
Co/Al2O3
9.2
20
06
0
10
25
08
B/Al2O3
2.6
04
64
0
Traces
07
02
γ-Al2O3
5.2
14
56
0
00
00
00
Na/Al2O3
With account taken of the adsorption data and the results of studying catalytic transformations of dimethyl sulfide in helium gas and its reactions with hydrogen sulfide and methanol [1, 4, 5], it may be presumed that the reaction of diethyl disulfide with methanol is catalyzed by acid–base sites on the catalyst surface. Methanol is adsorbed on acid catalysts with formation of surface-bound methoxy groups [8, 9]. When contacted with the catalyst, diethyl disulfide readily dissociates at the weak S–S bond to give two EtS groups [5]. The latter abstracts a proton from the catalyst surface to form ethanethiol, and the reaction of EtS with surface methoxy group yields ethyl methyl sulfide. Some EtS groups undergo profound decomposition to produce hydrogen sulfide and ethylene. The reaction of hydrogen sulfide with MeO groups yields methanethiol and dimethyl sulfide. An additional amount of dimethyl sulfide is likely to be formed by condensation of methanethiol. The reaction of diethyl sulfide with methanol over Na/Al2O3 which possesses no protonic acid sites but weak Lewis acid sites and strong basic sites was slow, and dialkyl sulfides were not formed. The other examined catalysts contained medium-strength basic sites and acidic sites. Silica-supported phosphoric acid (H 3 PO 4 /SiO 2 ) possessing weak protonic acid sites almost did not catalyze the reaction of diethyl disulfide with methanol. High catalytic activity was demonstrated by catalysts containing strong protonic acid sites and strong Lewis acid sites (HSiW/SiO2, HZSM-5, Co/HZSM-5, AlSi, Al2O3, Co/Al2O3, B/Al2O3). These catalysts favored formation of alkanethiols and dialkyl sulfides. The reaction in the presence of γ-Al2O3 (very
small number of strong protonic acid sites and many strong Lewis acid sites) afforded only dimethyl and ethyl methyl sulfides, while alkanethiols and hydrogen sulfide were virtually absent among the products. Presumably, the presence of strong protonic acid sites on the catalyst surface is a necessary condition for the formation of alkanethiols. The conversion of diethyl disulfide over γ-Al2O3 and the yield of dialkyl sulfides increased as the temperature rose and the time of contact increased; some amount of hydrogen sulfide was formed on prolonged contact time (Table 3). The selectivity for dimethyl sulfide and ethyl methyl sulfide did not change as the conversion of diethyl disulfide increased (Fig. 2), which indicated independent paths of their formation. The overall selectivity for dialkyl sulfides in the reaction catalyzed by γ-Al2O3 attained 99%. 100 Selectivity, %
Catalyst
2 60
1
20 20
40 60 80 Conversion of diethyl disulfide, %
100
Fig. 2. Selectivities for (1) dimethyl sulfide and (2) ethyl methyl sulfide at different conversions of diethyl disulfide; γ-Al 2O 3, 350°C, molar ratio methanol–diethyl disulfide (2.2–2.5) : 1.
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Table 3. Transformation of diethyl disulfide in a mixture with methanol in the presence of γ-Al2O3; MeOH–Et2S2 ratio (2.2–2.5) : 1 Contact time, s
Conversion of Et2S2, %
Yield, mol % Me2S
MeEtS
H 2S
Selectivity for Me2S + MeSEt, %
22 35 37 48 50
0.0 1.9 2.3 3.7 5.4
98 95 96 95 93
23 37 50 63 66 72 70
0.5 1.0 0.0 0.4 0.0 0.0 0.0
97 98 98 99 99 99 99
250°C 0.50 0.80 1.10 1.30 1.60
42 58 68 84 90
19 20 28 32 34 350°C
0.04 0.06 0.10 0.20 0.31 0.52 0.70
30 48 63 80 86 98 100
6 10 12 16 19 25 29
EXPERIMENTAL Reagents of pure grade were used. The catalysts, γ-Al2O3 (Ssp = 275 m2/g), SiO2 (Ssp = 260 m2/g), amorphous aluminosilicate (AlSi, Ssp = 430 m2/g), and highsilicon zeolite HZSM-5 (H-form, SiO 2 : Al 2 O 3 34; Ssp = 500 m2/g), and supports were commercial products. Samples were calcined for 5 h at 500–530°C in a stream of dry air before use. Supported catalysts were prepared by impregnation of preliminarily calcined supports with aqueous solutions of phosphoric or silicotungstic acid (HSiW) or cobalt or boron oxide. The impregnated samples were dried for 5 h at 110°C in air and were then calcined for 5 h. The catalysts contained 30 wt % of phosphorus (H 3 PO 4 /SiO 2 ), 10 wt % of HSiW (HSiW/SiO2), 1.7 wt % of sodium (Na/ Al 2 O 3 ) , 2 . 5 a n d 1 0 w t % o f c o b al t o x i d e (Co/HZSM-5, Co/Al2O3), and 2 wt % of boron oxide (B/Al2O3). The acid–base properties of the catalysts were determined previously by IR spectroscopy using adsorbed probe molecules [7]. The concentrations of acid–base sites are given in μmol/m2. The strength of protonic acid sites was estimated by the proton affinity for pyridine, of Lewis acid sites, by the heat of CO adsorption, and of basic sites, by the CDCl 3 deuterium affinity. The reactions were carried out under atmospheric pressure in a flow fixed bed reactor (catalyst grain size
0.25–0.5 mm). A fresh portion of the catalyst was used in each run. All system was maintained at a constant temperature. Helium was supplied from a gas cylinder to saturators filled with diethyl disulfide and methanol and placed in thermostats. After mixing, the helium gas saturated with the reactants was fed into the reactor heated to a required temperature. The duration of each run was about 2 h. Samples of gaseous products were withdrawn intermittently for analysis. Mean value from 2–3 samples was taken as the result of analysis; the difference between the results did not exceed 10%. The products were identified by GC/MS on an Agilent 7000 GC/MS Triple Quad instrument (HP-5MS capillary column, 30 m × 0.25 mm). Quantitative analysis of the products was performed on an LKhM-8MD chromatograph equipped with a thermal conductivity detector and a 2-m × 3-mm column packed with Porapak Q+R (1 : 1); carrier gas helium. The time of contact was determined as the ratio of the catalyst volume (cm3) to the gas flow (cm3/s) at room temperature and atmospheric pressure. The conversion (%) was calculated from the analysis results. The yields (mol %) were determined as the ratio of the product concentration to the initial concentration of diethyl disulfide, and the selectivity was determined as the ratio of the yield to the substrate conversion. The rate of the transformation of diethyl disulfide is given in mmol per gram of catalyst per hour.
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