2964
Ind. Eng. Chem. Res. 1996,34, 2964-2970
Kinetics of Catalytic Reaction between Methanol and Hydrogen Sulfide Viktor Yu. Mashkin,* Valerii M. Kudenkov, and Anna V. Mashkina Boreskov Institute of Catalysis, Pr.Akademika Lavrentieva 5, Novosibirsk 630090, Russia
Methanol reaction with H2S on tungsten- and potassium-promoted alumina was studied in the absence of any diffusional restrictions. We have found that on both W/Al203 and KW/Al203 catalysts methanol reacts with H2S to produce methyl mercaptan, which may convert to dimethyl sulfide through either interaction with the second molecule of methanol or disproportionation. Methanol dehydration yields dimethyl ether. For KW/Al203 catalyst, the generation of methyl mercaptan dominates. Adsorption data permitted us to suggest a mechanism of methanol reaction with H2S. According to this mechanism reaction proceeds via the rapid methoxylation of catalyst surface and further interaction of CH30 groups with the activated H2S, CH3SH, and CH30H. The kinetic equations obtained describe fairly well the reaction on the inhomogeneous catalyst surface.
Introduction Methanol interaction with H2S on heterogeneous catalysts is of prime interest for the commercial production of methyl mercaptan and dimethyl sulfide. Methyl mercaptan is needed t o produce methionine (feed additive, medicine). Dimethyl sulfide is used for the commercial synthesis of dimethyl sulfoxide (medicine, solvent, and extractant). Methyl mercaptan and dimethyl sulfide can be obtained from methanol and H2S on various catalysts. Reaction direction and rate depend on the catalyst nature. Alumina promoted with alkali and transition metal oxides appears to be the most active catalyst for methyl mercaptan production. According to Folkins and Miller (1962a,b),the selectivity toward methyl mercaptan reaches 80-90% at 350-420 "C, H2S:CH30H = 1.5 in the presence of the above catalysts. Mashkina et al. (1988a,b) have shown that support ( A l 2 0 3 , NSi, Si021 promotion with hydroxide, carbonate, or tungstates of potassium or sodium increases the surface basicity, thus increasing the selectivity toward methyl mercaptan generation. Dimethyl sulfide forms in the methanol reaction with H2S on the surface and supported on A l 2 0 3 , AlSi, and oxides and sulfides of transition metals. This fact is proved by Pankratova and Pinegina (1971, 1980) and Miki et al. (1966a-c). Mashkina et al. (1989) have found that acidic additives introduced into the catalyst increase the selectivity toward dimethyl sulfide. Aluminas with acid-base pair sites (strong Lewis acid sites and moderate base sites) are the most efficient catalysts for dimethyl sulfide synthesis. Temperature, rate of reagent feeding, and H2S:CH3OH ratio affect the parameters of methyl mercaptan and dimethyl sulfide synthesis. Folkins and Miller (1962a,b) proved this fact for a 5% K&o3/f%03 catalyst (2' = 373-427 "C); Pankratova and Pinegina (1971)and Miki et al. (1966b,c)proved it for Al203; Kushelev et al. (1978) proved it for A1203 and a 5%NaOWAl203catalyst. Miki et al. (1966~)proposed the ways of reaction product formation on Al2O3. They believed that dimethyl ether, the primary reaction product, may convert to methyl mercaptan and dimethyl sulfide and then decompose to carbon oxides and methane under certain conditions. Analyzing the selectivity toward products as a function of conversion, Mashkina et al. (1989) came to the conclusion that dimethyl ether is the primary product on the aluminum oxide catalysts. This produces methyl
mercaptan in the interaction with H2S. Dimethyl sulfide yields ether because of methyl mercaptan disproportionation or due to its reaction with methanol. For the alkali metal promoted Al2O3, methyl mercaptan is the primary product of methanol and H2S interaction. Miki (1966) as well as Mashkina et al. (1988a,b) have proposed that reaction proceeds through the surface methoxylation and the subsequent interaction of CH30 groups with methanol and H2S. Methanol and H2S reaction has not been thoroughly studied yet. Miki et al. (1966a-c) experimented with A1203 in a flow reactor, under diffusional (2 mm catalyst grains) restrictions, and a t a high reagent conversion. Assuming that adsorption follows the Langmuir mechanism, they have derived a kinetic equation for the reagent concentration dependencies on the rate of methanol reaction with H2S. In the present work we have studied the kinetics of methanol interaction with H2S on alumina modified with tungsten and potassium additives under gradientless conditions.
Experimental Section
+
Catalysts. ( y % ) - A 2 0 3 ($BET = 330 m2/g) were prepared by thermal decomposition of aluminum hydroxide. The WAl2O3 sample was obtained by the incipient wetness impregnation of ( y %)-A1203with a aqueous solution of potassium hydroxide, followed by subsequent drying at 110 "C for 5 h and calcination in a flow of dry air a t 400 "C for 2 h. Aluminum-tungsten (W/Al2O3) and aluminum-potassium-tungsten (KW/ A12031 catalysts were prepared by the incipient wetness impregnation of ( y + x)-Al2O3 and WA1203 with an aqueous solution of ammonium tungstate, followed by subsequent drying at 110 "C for 5 h and calcination in a flow of dry air at 500 "C for 4 h. Catalyst properties are presented in Table 1. The acid-base properties of catalysts were obtained with IR spectroscopy using the adsorbed probe molecules (Paukshtis and Yurchenko, 1983). Reagents. Hydrogen sulfide was obtained by the interaction of sulfur with hydrogen in the presence of an NiW/Al203 catalyst (H2S grade 99.5%). Methyl mercaptan was prepared by the decomposition of methylisothiourea sulfate using alkali, and dimethyl ether was synthesized by the dehydration of methanol
0888-5885/95/2634-296~~~9.00l00 1995 American Chemical Society
+
Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995 2966 Table 1. Catalyst Parameters acid-base properties
catalyst
content in catalyst (mass %) W K
WlAl203 m/&o3
8.5 8.5
0 3.6
H+
310 290
LA). We have determined initial reagent conversions (%), product yields (mol %) and selectivities (%) (equal to the ratio of theoretical product yields and conversions). Reaction rate was expressed either in mmol of substrate converted or product formed on 1 g of catalyst per 1 h. Conventional contact time (z, s) is the ratio of a catalyst volume loaded (cm3)and a gas rate (cm3/s)at 20 "C under atmospheric pressure.
Results and Discussion Routes of Product Formation. Methanol reaction with H2S was studied on W/Al203 and Kw/&o3 catalysts at 280-500 "C, different contact times, and various initial concentrations of reagents. Methyl mercaptan, dimethyl sulfide, water, and dimethyl ether were the main reaction products on both catalysts. At the elevated temperature and high contact time, methane and carbon oxides formed as well. To elucidate the ways of product formation, we have analyzed the selectivity as a function of initial reagent conversions, which change at the contact time variations. Additionally, we added products to reaction mixtures and studied the conversion of presumed intermediates. At constant temperature, the reagent conversion increases with the increasing contact time (Tables 2 and 3, Figure 1). If conversion is constant, the increasing temperature raises the reaction rate (Table 4). H2S is consumed only for methyl mercaptan and dimethyl sulfide production, as its conversion equals the total yield of both products. As XH~S increases, the selectivity toward methyl mercaptan decreases, and that toward sulfide increases. This means that methyl mercaptan forms directly from methanol and H2S:
+ H2S = CH3SH + H20
Sulfide is the secondary product.
PA 5 1300 0.1 0
S,, (m2/g)
(pure grade 98-99%). Other substances were manufactured chemical "pure" grade. Kinetics. Kinetic runs were carried out in a flowcirculation reactor at atmospheric pressure. Liquid components were fed through a saturator while gases were fed from vessels. Flows were mixed and introduced into the reactor filled with the catalyst heated by a gradientless furnace. The whole system was thermostated at 150 "C to prevent reaction mixture condensation. The reaction mixture was stirred by a circulating pump with a 400 LA circulating factor, which is sufficient for leveling concentrations on the input and output the of catalyst bed. We used 0.160.5 mm catalyst grains providing no diffusion restrictions. The composition of initial mixture and reaction products was determined by means of a chromatograph with a thermal conductivity detector. The fxed phase was Porapak R and Q (l:l), the column length 2 m, inside diameter 3 mm, thermostat temperature 175 "C, detector current 130 mA, and gas carrier helium (3.6
CH,OH
L
(CH+) at CL 14 0.5
B Qco 36 31
CB
PA"
1.3 2.9 0.7
800-900 800-900 925
100 60
20 0.1
0.2
Figure 1. Effect of contact time on methanol conversion (l), on yield of methyl mercaptan (2), on yield of dimethyl sulfide (3), and = on yield of dimethyl ether (4) on W/Al203 a t 360 "C, [CH~OHIO 3.5-3.9 mmoVL, [HzSIo = 7.1-7.5 mmoVL. Table 2. Methanol Interaction with H2S on W/Al203; [CHsOHIo = 3.5-3.9 mmoVL, [HzS]0 = 7.1-7.5 mmol/L selectivity (%) with respect to conversion (%) H2S CH30H T ( S ) CH30H HzS CH3SH (CH&S CH3SH (CH3)zS (CH3)20 T = 280 "C 100 0 25 0 75 0.02 16 2 100 0 29 0 71 0.07 34 5 92 8 41 7 50 0.16 56 13 90 10 49 9 38 0.34 68 19 75 24 60 33 2 2.0 87 36 66 33 52 46 0.3 2.3 95 38 T = 360 "C 100 0 29 0 71 0.01 31 5 83 17 48 14 36 0.02 44 12 85 12 60 22 14 0.03 70 26 73 24 58 39 3 0.04 77 33 0 71 28 58 42 0.1 90 35 66 34 54 46 0 0.15 93 37 53 47 36 64 0 0.24 94 34 34 60 20 82 0 0.30 98 38 Table 3. Methanol Interaction with HzS on K W / A l 2 0 3 ; [CHsOHlo = 2.9-3.7 mmoVL, [H~SIO = 6.6-7.5 mmoVL ~~
~
product yield with respect to CH30H (mol %)
X M ~ ( %CHBSH ) (CH&S (CH3)zO T = 360 "C 0 1.9 0.2 12 10 0 3.6 0.4 26 22 1.0 41 36 0 2.9 0.3 2.8 51 49 1.1 1.9 0 9.6 69 66 T = 450 "C 0 0 0.006 6 5 0.03 15 14 0.2 1.1 0.5 1.4 0.08 24 22 0.18 39 33 1.2 2.0 2.2 2.2 0.78 54 47 5.0 0.5 3.0 81 74 T(S)
CHI
COt-COz
0 0 0 0 0
0 0 0 0 0
0.1 0.3 0.4 0.7 0.8 1.0
0.1 0.2 0.3 0.4 0.6 1.0
Dimethyl ether found among reaction products is the primary product of methanol conversion. This follows from the fact that the selectivity toward ether is largest at a low methanol conversion (Table 2, Figure 2). Dimethyl ether results from the methanol dehydration: 2CH30H = (CH,),O
+ H20
Chang suggested (1983) that this reaction occurs on different catalysts. Our runs have evidenced that
2966 Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995 Table 4. Temperature Effect on Parameters of Methanol Interaction with HzS on KW/&&; [CHsOHIo = 7.5 mmoliL, [HzSlo = 12.9 "ow, XM, = 70% yield (mol %) selectivity (%)
T ("C)
W M (mmoI4g.h)) ~
CH3SH
(CH3)zS
(CH3h0
CH4
CO + COz
CH3SH
(CH3)zS
(CH3)zO
360 400 450 500
46 62 148 225
51 70 65 69
0.6 1.8 3.9 5.5
1.4 0.8 0.6 0
0 0.5 1.4 8.2
0 0.2 0.7 3.5
96 95 90 78
1.1 2.4 5.4 6.2
2.6 1.1 1.1 0
S(%I
60
i
10 30 50 70 90 X(%I Figure 2. Selectivity toward methyl mercaptan (11, dimethyl sulfide (2), dimethyl ether (31, and methane carbon oxides (4) at various methanol conversions on KW/Al203 at 2' = 450 "C.
+
Table 5. Methanol Dehydration at 360 "C, [CH~OHIO = 4.2 mmoyL, X,. = 85% WIAlzO3 T(S)
KWI~ZO~
xMe(%)
SE(%)
T(S)
XMe(%)
100 100 100 100 99Q 94"
3.3 4.0 6.7 8.2 13.0 18.0
13 17 23 25 30 38
20 40 68 80 84 85
0.01 0.03 0.07 0.1 0.7 3.3
sE(%)
96 96 95 94 92" 88"
Table 6. Dimethyl Ether Interaction with HzS; T = 360 "C, [(CH3)20]0 = 5.0-5.8 "on, [HzS]o = 7.5-7.9 mmoyL WIAlzO3
KW/Al203 selectivity (%I
selectivity (%) T(s)
0.04 0.05 0.07 0.7
XE(%)
74 80 90 100
SM
SDMS
SMe
94 85 71 21
0 8 22 78
7 7 6 2
T ( S ) xE(%)
SM
SDMS
SMe
3.3 6.7 10.0 13.0
66 74 75 77
30 26 25 26
3 0 0 0
25 35 38 42
methanol dehydration also proceeds on W/Al2O3 and KW/Al203 catalysts (Table 5). Dimethyl ether yield increases with the contact time rise. The selectivity toward ether remains 100% up to the equilibrium methanol conversion (Xeqis 85%at T = 360 "C). At high contact times, the selectivity toward dimethyl ether decreases presumably owing t o its decomposition,yielding methane and carbon oxides. Vladyko et al. (1984) believed that these products can result from methanol decomposition. Dimethyl ether, formed during methanol dehydration, can interact with H2S to produce methyl mercaptan (Table 6): (CH3),0
+ H2S = CH3SH + CH30H
Under similar reaction conditions in the H2S medium, the overall rate of methanol and dimethyl ether conversion and the selectivity toward methyl mercaptan are commensurable. For W/Al203 catalyst a t 360 "C and X = 80%, the rate of methanol and dimethyl ether conversion is 406 and 460 mmol/(g h), respectively; the selectivity toward mercaptan is 75% in both cases. For KWlAl2O3catalyst at X = 45%,the rate of methanol and
+ COZ
CO
0.7 2.0 9.3
0 0.0 1.0 4.0
dimethyl ether conversion is 4 and 3 mmol/(g h), respectively; the selectivity toward methyl mercaptan is 96 and 74%, respectively. Thus, we should bear in mind that methyl mercaptan may be also obtained from dimethyl ether as it was in H2S and methanol runs. With increasing ether conversion, the selectivity toward methyl mercaptan decreases and that toward dimethyl sulfide increases (Table 6). This allows the suggestion that at first methyl mercaptan forms and then it interacts with ether to produce dimethyl sulfide: (CH3),0
+ CH3SH = (CH3),S + CH30H
Methanol and H2S runs have shown that, if methanol conversion increases, the selectivity toward methyl mercaptan first rises to some level and then it drops while the selectivity toward dimethyl sulfide starts to grow (Table 2, Figure 2). Methyl mercaptan presumably interacts with methanol: CH3SH
Besides dimethyl ether and water, reaction products contain methane and carbon oxides.
CH4 0
+ CH30H = (CH,),S + H 2 0
This reaction is known to go easily and fast on aluminum oxide catalysts with a high selectivity toward dimethyl sulfide (Mashkina et al., 1992). Table 7 proves this reaction to proceed on W/Al203 and K W / A l 2 0 3 catalysts as well. Reaction occurs also when methyl mercaptan is added to a methanol and H2S mixture. For KW/Al2O3 (T= 360 "C, t = 5.3 s, methanol conversion 56%), a 4-10 mmoVL mercaptan, added to methanol converted in H2S, increases the dimethyl sulfide yield from 0.6 to 3 mol %. The selectivity toward sulfide increases by &fold. When the reagent conversion increases, the selectivity toward dimethyl sulfide rises, though it never attains 100% even at the complete substrate conversion, presumably owing to methyl mercaptan disproportionation: 2CH3SH = (CH,),S
+ H,S
This reaction occurs on aluminum oxide catalysts (Mashkina et al., 1991) and on W/Al203 at 360 "C as well. When the methyl mercaptan initial concentration is 4 mmoVL, its conversion begins to increase from 0.02 to 0.12 s with contact time, and approaches equilibrium (Xeq= 69% at 360 "C). The yields of dimethyl sulfide and H2S are quite no different. The selectivity toward the sulfide is 100%. As the contact time increases from 0.6 to 3.4 s, the selectivity toward the sulfide slightly decreases (to 98-95%) because of dimethyl sulfide and methyl mercaptan decomposition,yielding methane. For KW/Al203, the methyl mercaptan disproportionation is hindered. Thus, at T = 360 "C, t = 3-15 s, methyl mercaptan conversion is 6-10% and the selectivity toward dimethyl sulfide and methane is 95-77% and 5-14%, respectively. The so-obtained results may indicate that the products of methanol and H2S reaction appear from the following reactions. Methanol interacts with H2S to produce methyl mercaptan which converts to dimethyl
Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995 2967 Table 7. Methanol Interaction with Methyl Mercaptan at 360 "C, [CHsOHlo = 4.2 mmoVL, [CHsSHIo = 4.2 mmoYL Kw/&o3 yield (mol %)
w/&o3
yield (mol %) t (SI
X M(%) ~
0.083 0.35
60 76
0.67
88 93
3.3
CH4
+ CO + COz
(CH3)zS 45
(CH3)zO 14
70
6
0 0
85 92
3 0.4
0 0.5
sulfide either because of its interaction with the second methanol molecule or because of disproportionation. Methanol dehydration produces dimethyl ether. Ether reaction with H2S can produce methyl mercaptan. The latter interacting with the dimethyl ether gives dimethyl sulfide. Carbon oxides and methane result from the decomposition of dimethyl ether (and probably methanol).
Kinetic Models In order to derive the rate dependence on the reagent concentration, we have carried out kinetic runs on W/Al2O3 and KW/Al203 catalysts: T = 280-400 "C, initial methanol and H2S concentrations were 1-20 mmoVL, and methanol conversion was 1 0 4 0 % . Methyl mercaptan, dimethyl sulfide, dimethyl ether, and water appeared to be the main reaction products. The products of deep decomposition, methane and carbon oxides, were hardly observed. According to thermodynamic calculations, reaction goes far from equilibrium (at 280400 "C, the equilibrium methanol conversion to methyl mercaptan is 98-loo%, and that to dimethyl ether is 97-100%) under experimental conditions. We studied this reaction with one-parameter runs, when the concentration of one component was varied and that of others was constant. The rate of methyl mercaptan formation can be described by the empirical equation
where n = 0.5-0.6 and m = 0.6-0.8; n and m are the functions of T. The rate of dimethyl sulfide formation follows the equation
5
(s)
X M(%) ~
(CH3)zS
3.3
12 24 28 33
6
6.7
10.0 13.0
15 20 22
(CH3)O 4 4 5 5
CH4
+ CO + COz 2 3 4 6
from weakly adsorbed on B sites). The chemisorbed ether dissociation may cause further methoxylation of catalyst surface. Note that the acid and base site strength distribution is rather wide (Paukshtis et al., 1982). Thus the catalyst surface is strongly inhomogeneous. Kinetic fundamentals of heterogeneous catalytic reactions on inhomogeneous surfaces were developed by Temkin (1979). He suggested that catalytic activity changes with respect to coverage when passing from one site on an inhomogeneous surface to another. In most cases, we may consider the catalyst surface as continuously nonuniform, when adsorption heat depends linearly on the covering: q = 40 -
cs
where qo is the maximum adsorption heat, C is a constant, and S is a relative number of active surface sites, whose adsorption heat varies from q t o 40. The rate constants of elementary stages are k, = k," exp(fa/W where f = CIRT, a is the inhomogeneous parameters (the sign depends on the stage direction); ki" = ki at S = 0. Using the postulated reaction mechanism, we have simulated kinetic models with regard to the catalyst surface inhomogeneity. The postulated reaction mechanism can be expressed by the following set of reactions: stage 1:
stage 2: When postulating the reaction mechanism, we should consider the catalyst properties and reagents adsorbing on them. W/Al2O3 and KW/Al2O3 catalysts of different compositions possess some common properties. Particularly, the surface of both catalysts has L acid and B sites participating in the reagent activation. Indications are that the reagents under study chemisorb on the aluminum oxide catalyst surface: methanol and dimethyl ether (Chang, 1983; Kazanski, 1988); methyl mercaptan (Saur et al., 1981); hydrogen sulfide (Mastikhin et al., 1989; Desyatov et al., 1990; Okamoto et al., 1986). We may postulate the following mechanism for the methanol reaction with H2S. Methanol adsorbs dissociatively on alumina catalyst and thus causes surface methoxylation. The chemisorbed H2S interacts with the surface methoxy groups and yields methyl mercaptan. Methyl mercaptan chemisorbed on the acid-base sites pair reacts with CH3O groups to produce dimethyl sulfide. Dimethyl ether results from the interaction of the second methanol molecule (from the gas phase or
stage 3:
stage 4: CH30H
+ [X,]
+
[X,] (CH3),0 r4 = k4PMe[[X211 - k-,PE[[Xl1I
stage 5 :
Byproducts (methane, carbon oxides) were neglected, since their amounts were minor under these kinetic conditions. Additionally, the active surface blocking by water was taken into account. Neglecting the reversibility of stages 2, 3, and 4, we obtained the steady state equations for the rates of key
2968 Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995 W, mmol/gh
I
"6
Figure 3. Rate of methyl mercaptan formation on KW/Al203 as a function of water concentration introduced into reaction mixture at 320 (l),360 (2), and 420 "C (3).
[[XzIl = k1P,$(k1pMe
(4)
where [[x211 is the steady state concentration of methoxy groups on the surface; K5 = kdk-5 is the equilibrium constant of stage 5 . For the continuously nonuniform surface, we have derived the following kinetic equations:
VDMs = k 3 " P d
0.6
0.8
1.0
wexp(mmol/gh)
Figure 4. Experimental and calculated rates at 360 "C of H2S conversion (l), dimethyl sulfide formation (21, and dimethyl ether (3) formation a t the reaction of methanol with H2S on W/Al203 (W/WmaX,relative units; W,, = maximum reaction rate in the run).
= 75 [mmol g-l h-l (atm)-0.51;at T = 360 "C, k2"k = 1300, k3"k = 1200, k4"k = 800 [mmol g-l h-l
+ r3 + r4,r5 = 0
+ (1+ KTH20)(k$H2s + k3P+ ~ k4PMe + k-lPH,o)
0.4
k4"k
products formation in the ideal adsorbed layer:
rl = r2
0.2
(atm)-0,51. Special runs, when the "intermediate" methyl mercaptan was used instead of H2S (stage 2 of scheme 3 starts), support the model validity. Dimethyl ether in the gas phase instead of methanol initiates the extra surface methoxylation. This means that stage 4 (scheme 3) plays a significant role in the reverse direction. Experimentally, the rate of dimethyl sulfide and methyl mercaptan formation is approximately 3 times higher than in experiments with methanol (presumably, k-4 > k1). The analysis of the kinetic data, shows that, t o increase the selectivity toward dimethyl sulfide, the starting methanol concentration should be low and H2S concentration should be below its the stoichiometric value (0.51):
=kd"P~$
Potassium, when introduced into W/Al203, weakened the strength of L sites. This hinders the activation of methanol and methyl mercaptan, participating in the formation of dimethyl ether and dimethyl sulfide. Experimentally, the main products of methanol and H2S reaction on K W / A l 2 0 3 are methyl mercaptan and water. These data correlate with the reaction mechanism suggested. For weak L sites, the activation of methyl [(k2"pH2S + k 3 " p + ~ k4"P,, f k-1"pH20)(1 + K~PH,o)I~-" mercaptan and methanol is hindered; i.e., stages 3 and 4 are inhibited and can be neglected. Then, the kinetics (5) on the continuously nonuniform surface looks like With W/Al203, the computer-calculated and experimental data agree most at a = 0.5, assuming that stages 2,3, and 4 are limiting (k2", k3", k4O
