2372
Ind. Eng. Chem. Res. 1991,30,2372-2378
Conversion of Synthesis Gas to Dimethyl Ether over Bifunctional Catalytic Systems' Alkeos C. Sofianos* and Mike S. Scurrell Catalysis Programme, Division of Energy Technology, CSIR, P.O. Box 395, Pretoria 0001, Republic of South Africa
The conversion of syngas to hydrocarbon synfuels via methanol (MeOH) involves dimethyl ether (DME) as a key intermediate stage. An attractive option would involve direct production of a mixture of oxygenates (MeOH DME). Microreactor studies have been used to investigate the oxygenate synthesis over bifunctional multicomposite catalysts comprising a methanol synthesis component and a dehydration partner such as alumina, amorphous silica-alumina, Y zeolite, mordenite, or ZSM-5 zeolites. By directing the syngas conversion toward DME in a single-step reaction, the overall chemical equilibrium was changed in favor of the production of DME/MeOH. The synergistic combination of MeOH synthesis, hydrocondensation to DME, and the concurrent water gas shift (WGS)reaction, which provides the necessary COz, gives high CO conversions and oxygenate yields far greater than the conventional MeOH synthesis. Of all the dehydration components tested, y-alumina provided the best conversions of CO and the highest DME/MeOH yields.
+
Introduction Significant energy savings in thermodynamically controlled equilibrium reactions can be achieved either by the development of catalysts exhibiting an improved activity compared with conventional systems, or by the modification of the process in order to obtain higher conversions in the reactor so that the recycling of the unreacted gases becomes unnecessary. In the case of conventional methanol synthesis, where the development of low-pressure Cu-Zn-Al-based catalysts has reached a certain maturity, special attention must be given to improving the process in order to avoid the high recycle ratio of the syngas, which is required to achieve sufficiently high carbon monoxide conversions. Since the overall path for the conversion of syngas to synfuels via methanol (MeOH), according to the Mobil process (Chang, 1983), includes the formation of dimethyl ether (DME) as a key intermediate, one should aim at producing a mixture of oxygenates (MeOH + DME) rather than methanol in the first step of the syngas conversion. With the use of a bifunctional catalytic system (i.e., a combination of a methanol synthesis component with an acidic partner) for the direct conversion of syngas to DME, the overall chemical equilibrium would be expected to change in favor of the production of oxygenates, far beyond the thermodynamic limit of methanol yield. In other words, the equilibrium constraints of methanol formation are avoided by its continuous removal from the gas phase through its conversion to dimethyl ether and other downstream products (hydrocarbons). These consecutive reactions essentially suppress the reverse reaction of the methanol equilibrium, thus allowing carbon monoxide conversion to proceed further than dictated by the above-mentioned equilibrium. Dimethyl ether is a useful chemical intermediate for the preparation of many important chemicals, including dimethyl sulfate. More recently it has been increasingly used as an aerosol propellant to replace chlorofluorocarbons, which were found to destroy the ozone layer of the atmosphere. Finally, it may be used directly as a transportation fuel in admixture with methanol or as a fuel additive. 'Presented a t the "Advances in Ether Process Technology" Symposium on Alternative Fuels and Energy Technology, AIChE
1990 Summer National Meeting, San Diego, CA, Aug 19-22,1990.
The most common bifunctional catalysts used in the literature for syngas conversions would involve a carbon monoxide hydrogenation component (i.e., a methanol-active metal or a mixed metal oxide) directly impregnated or ion exchanged on a shape-selectivezeolite (Chang et al., 1979; Huang and Haag, 1981; Fujimoto et al., 1984; Rao and Gormley, 1990). Other systems consist of a physical admixture of methanol synthesis catalysts and zeolites or other acidic components such as alumina (Saima et al., 1985; Fujimoto et al., 1986, 1989; Gogate et al., 1990). In a parallel development bifunctional catalytic systems composed of a Fischer-Tropsch component and a zeolite component have been investigated (Shamsi et al., 1986; Gormley et al., 1988; Nair et al., 1988). In this case ZSM-5 has been the preferred zeolite component and the main aim was to change the product distribution of the Fischer-Tropsch reaction from the one dictated by the Anderson-Schulz-Flory polymerization kinetics. Microreactor studies have been used to investigate the direct synthesis of dimethyl ether from carbon monoxide/hydrogen with or without additional carbon dioxide in admixture. The synthesis was activated by using bifunctional catalytic systems comprising a methanol catalyst and a second dehydration component. As dehydration components alumina, amorphous silica-alumina, zeolite Y, mordenite, and various samples of the ZSM-5 class of zeolites were utilized. The aim of the present study of the various catalytic systems was to investigate the complexity of the overall conversion, to show the importance of the choice of catalyt Components, and to identify potential problems associated with the bifunctional character of the catalyst (for instance, deactivation and regeneration problems). Experimental Section Figure 1 presents a schematic diagram of the experimental system used for testing the catalysts, which essentially consisted of a microreactor described previously (Snel, 1985). The reactant gases (CO, COz,and hydrogen) as well as the nitrogen used for catalyst activation were fed into the microreactor after being pursed. Brooks mass flow measuring and control systems were used for controlling the individual flow rate of each gas and for establishing the gas mixtures required. The product stream from the reactor was passed through heated lines and, after
0888-5885191/2630-2372$02.50/0 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991 2373
MASS R O W CONTROLLERS
REACTION TEMPERATURE ("C) Figure 2. Reaction of syngas over bifunctional catalysts (P= 4 MPa, GHSV = 16000 h-l, H2:C0 = 2:l).
Figure 1. Fixed-bed microreactor rig used for DME/methanol synthesis.
the pressure was reduced by a Tescom back-pressure regulator, the stream was sampled by a Carlo Erba gas chromatograph equipped with a multiport injection valve. The mixture of carbon monoxide, carbon dioxide, methane, ethane, water, and methanol was separated on a Porapak Q column while C1Xl0hydrocarbons, methanol, dimethyl ether, and higher alcohols were analyzed simultaneously on a bonded OV-1 capillary column, using a temperature program. Bifunctional catalytic systems were prepared by intimate mixing of finely milled samples of the two components in a 1:l ratio (by mass). The resultant powder was molded under pressure to tablets, which were then crushed into granules and sieved to size (300-500 pm). The procedure adopted for catalyst activation was similar to the one used in commercial methanol plants. The catalyst was slowly heated up to 130 "C (50 "C/h) under a flow of nitrogen (4 L/(kg.h)). At this temperature, hydrogen was added to the nitrogen stream to a level of 2.5% and the temperature was further raised to 200 "C (at a rate of 20 "C/h). The activation was continued for 16 h at this temperature. Finally, the hydrogen concentration of the inert gas was increased to 12% and the temperature was raised to 240 "C. After 2 h at this temperature, the catalyst was ready for operation and the temperature was adjusted to 200 "C for the first test. Several types of different methanol catalysts were prepared and tested. The first type comprised commercial samples of methanol synthesis catalysts suitably sized (300-500 pm) for charging to the microreactor. Second, in-house-prepared, coprecipitated Cu-Zn-A1 systems containing various promoters were tested and their activity was compared with that of the above. As measures of the catalytic activity, and for purposes of quantitative comparison with commercial results, the conversion of the feed carbon monoxide and the space-time
yield (STY) of the products (MeOH + DME) were used. These were defined as follows: CO conversion = COz total hydrocarbons MeOH BDME X CO + COP total hydrocarbons + MeOH + BDME 100 (1) space-time yield = conversion X selectivity X CO feed rate (mo1/(kgcat.h)) (2) In addition, the selectivity of oxygenates (DME + MeOH) was calculated in terms of MeOH: oxygenate selectivity = MeOH BDME x 100 C02 + total hydrocarbons + MeOH + 2DME (3) All values were calculated on a molar basis. In most cases a mixture of H2:C0 = 2:l with a gas hourly space velocity (GHSV) of 8000 h-' was used for the DME synthesis tests. However, in order to establish the activity of the in-house-prepared methanol synthesis catalysts and to make a comparison with commercial samples, a synthesis gas containing 31.9% CO, 4.9% C02, and 63.2% H2 was contacted with the catalyst at a temperature in the range from 180 to 325 "C. Further, the H2:C0 ratio and GHSV were varied over a wide range in order to study their influence on CO conversion and oxygenate selectivity.
+
+
+
+
+
Results and Discussion Influence of Temperature and Acidic Component. Comparative results for the reaction of syngas to DME over various bifunctional catalysts as a function of the reaction temperature are given in Figures 2 and 3. The catalysts, prepared as described above, were composed of one part methanol synthesis catalyst and one part of several dehydration partners. The latter were in this case y-alumina, H-ZSM-5 zeolite of high Si02:A1203ratio (Si02:A1203= go), amorphous silica-alumina (%A1 = 3.1), and Y zeolite in the H-form. For comparison purposes the performance of the original commercial methanol catalyst is given as well. Figure 2 presents a comparison of the oxygenate production (expressed as the STY of methanol) over the
2374 Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991 Table I. Results of Oxygenate Synthesis from Syngas over Bifunctional Catalystsa catalyst COconv, % COZb CH,b C,+ C2=* C,+ C8=b DMEb 1 AS1 (commercial) 12.50 6.92 0.57 0.18 0.14 0.18 2 A (own) 12.01 5.80 0.23 0.11 0.05 5.42 3 ASl/y-A1203 41.02 13.19 0.012 2.03 4 ASl/H-ZSM-5 28.27 9.58 0.46 0.52 0.20 5 ASl/H-Y zeolite 37.47 17.63 2.31 0.21 0.14 6 ASl/AMSiAI 47.27 27.84 0.13 0.06 0.05 1.28 7 ASl/ZM760 (mordenite) 32.30 16.88 1.85 0.57 0.35
MeOHb 4.45 5.42 2.03
C4+ C p b 0.05 0.03
i-C4*
0.44 0.71
+Cbb 0.01 0.01 0.03
1.28
0.02 0.14
0.03
'MeOH component/acidic component (l:l),temperature = 275 "C; pressure = 4 MPa; Hz:CO ratio = 2:l; GHSV = 16000 h-l.
C mol
0.59
%.
I FH-y I 90
I
I Y/"-zsM-5 I
I
I
I
2 b
275
300
I
325
350
REACTION TEMPERATURE ("C)
Figure 3. Conversion of CO on the same systems. Same reaction conditions as in Figure 2.
various bifunctional catalysts, whereas Figure 3 displays the overall catalytic activity of the systems in terms of the molar conversion of the feed carbon monoxide obtained under the same reaction conditions (pressure 4.0 MPa, GHSV 16000 h-l, and H2:C0 ratio 2:l). The catalyst containing 50% active y-alumina exhibited the highest catalytic activity, as regards both CO conversion and the production of oxygenates. For example, around 300 "C,the yield of DME/methanol was nearly 155 mol/(kgcat.h) (i.e., 5 kg/(kgcat-h)), which is many times more than the corresponding STY of the commerical methanol catalyst alone, the conversion being 5 times as high as with the latter. As shown in Figure 4, the selectivity toward oxygenates over this system is similarly high and stable over the temperature range of the test, whereas the water gas shift (WGS) activity &e., the formation of C02)is comparatively low (Figure 5 ) . The system having amorphous silica-alumina as its acid component initially displays a behavior similar to the one based on y-alumina. A t lower temperatures, perhaps due to its higher acidity, the yield of oxygenates produced via silica-alumina as well as the carbon monoxide conversion is slightly higher than over the system containing active alumina. An increase of the reaction temperature to above 250 OC,however, effects a drastic increase in the yield of oxygenates obtained over the y-alumina-containing system which retains its high selectivity. At higher temperatures, a major by-product of the catalyst based on silica-alumina is carbon dioxide. The amount of carbon dioxide produced over this system is very high, and over 275 OC it becomes
I
I
I
I
I
260
2j5
360
325
350
REACTION TEMPERATURE ("C) Figure 4. Selectivity of oxygenate formation vs temperature over bifunctionalcatalysta (P= 4 m a , GHSV = 16000 h-l, H2:C0 = 21).
70-60c
x
50-ASVH-Y
n
40-
I
250
275
300
325
350
REACTION TEMPERATURE ("C)
Figure 5. Temperature dependence of the water gaa shift activity of the various bifunctionalcatalysts (P= 4 m a , GHSV = 16000 h-l, H&O = 2:l).
higher than the oxygenate yield (Table I). The conversion of CO was relatively high in the case of all the catalysts containing zeolites (H-Y, mordenite, and H-ZSM-5).The system containing mordenite, however,
Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991 2375 Table 11. Results of the Conversion of Syngas over Bifunctional Catalysts with a Zeolite as the Second Partner AS11 H-ZSM-5 ~.~ 280 5 2: 1 16000 ~~
temperature, OC pressure, MPa H,:CO ratio space velocity, h-' mixing ratio conversion of CO, % results, C mol %
co co2 CH4 c*+ c,=
+ c3= i-C4 c4 + c4= C6 + cg= ce + cs= c3
DME
catalvst AS11 H-Y 280 5 2: 1 16000
Ivv-
1: 10%A1203
h
0-0
AS11 mordenite 280 5 2:l 16000
1:l
1:l
1:l
49.41
68.96
51.72
50.59 17.98 1.58 1.60 0.409 0.917 0.131 0.51 0.06 26.69
31.04 42.03 6.44 3.69 1.82 4.73 0.91 0.33 0.23 8.78
48.28 32.06 4.55 1.02 1.12
07 175
225
250
275
300
325
I
350
REACTION TEMPERATURE (OC)
Figure 6. Influence of the content of acidic component (P= 3 MPa,
GHSV = 16000 h-', H&O
s
2:l).
200 1
1.12
0.51 0.11 0.06 11.18
retained its initially high activity only for a few hours. The oxygenate yields obtained in all three cases, despite increasing proportionally with the temperature up to a value of 85-90 mol/(kgc,,-h) (at 275 "C), dropped dramatically after this maximum had been reached, with the formation of large amounts of by-products of which C02 and the lower alkanes (C,-C,) were the dominant ones (Figures 2-5, Tables I and 11). In the temperature range above 275 OC, the catalysts containing zeolites (H-ZMS-5, H-Y) convert further the originally formed DME to produce mainly low alkanes (C,-C,) as displayed in Table 11. Carbon dioxide is also formed in relatively large quantities, since the WGS reaction is kinetically enhanced by an increase in the reaction temperature. The C02 formation is highest over the catalyst containing the H-Y zeolite, whereas it remains very low over the catalyst based on y-alumina, even at high reaction temperatures. Methanation plays almost no role in all cases, since the experimental conditions adopted are rather moderate. Only over 325 OC did the methane formation slightly exceed 690 (ASl/H-Y). Specifically, the systems containing zeolite Y and mordenite exhibited the highest activity in methane formation whereas over catalysts based on y-alumina and ZSM-5 zeolites, a very low methanation activity was found. Over the bifunctional systems containing zeolites (especially zeolite Y and ZSM-5) relatively high yields (>15%) of lower alkanes (C2-Cd were produced a t temperatures over 300 "C. Isobutane formation was predominant, especially over ASl/H-Y. Temperatures over 275 "C are however too high for stable operation of the Cu-based methanol synthesis component of the systems, and a sintering of the copper would lead to a deactivation of the catalyst. This is one of the reasons why Saima et al. (1985) preferred to use a Pd-Si02 catalyst as their methanol synthesis component despite its much lower activity. In Figures 2 and 3 and in Table I the results of the methanol synthesis over a commercial sample and a homemade Cu-Zn-A1 methanol catalyst are displayed as well. The use of synthesis gas without any C02admixture led, in the case of the Cu-Zn-A1 catalysts, to substantially lower CO conversions and methanol STY, and to a rapid deactivation of the catalyst. Influence of the Acidic Component Content. A very important parameter to be considered in the preparation of the bifunctional catalytic systems is the ratio between
200
1
2
3
4 5 PRESSURE (MPa)
6
Figure 7. Oxygenate space-time yield as a function of pressure (T = 275 "C, GHSV = 16000 h-l, H,:CO = 21).
E 70v)
E
50-
z
0
o
40-
W
SI x
30-
p
20-
z
8
10-
3
01 1
2
3 4 5 REACTION PRESSURE (MPa)
6
I
Figure 8. Carbon monoxide conversion as a function of pressure. Same reaction conditions as in Figure 7.
methanol catalyst and acidic component. Figure 6 illustrates the dependence of the oxygenate yield on the reaction temperature for bifunctional systems containing varying amounts of active alumina. From Figure 6 it is clear that the highest oxygenate yields are obtained with a catalyst containing 50% yAl2O9 This is also economically of great significance, since half the quantity of the expensive methanol catalyst may be replaced with the much cheaper y-alumina, with an overall beneficial effect as regards the oxygenate yield obtained. However, a further increase of the acidic component to 75% had an adverse effect on the overall performance of the bifunctional system, with both the conversion of the carbon monoxide and the oxygenate space-time-yield dropping significantly. The trend of the curves is otherwise very similar in all four bifunctional systems tested. Influence of Reaction Pressure. The superior performance of the catalytic system containing y-alumina is confirmed in Figures 7 and 8, which demonstrate how the
2376 Ind. Eng. Chem. Res., Vol. 30,No. 11, 1991 GHSV (h-l) 6000 9000
3000
1001
bp
120,OO
15000
r
-- f
SELECTIVITY
--
8 2
5B O," 7800 60-
Yz 8$
50-
$
40-
9$ Y V
50
3020
.'
--
I
/---
--
m'e'O
10-
E
-- 2 Y ._ 2Ln
METHANOL
01
v
0
MeOH YIELD
/
/A-
IL-
I
,
A H A
n o 0
1
2
P O
,
,
,
,
,
,
,
3 4 5 6 7 8 9 1 0 1 1 H 2 : CO RATIO (GHSV CONSTANT AT 10 000)
Figure 11. DME and methanol yields vs COz formation; dependence on H,:CO ratio (250 O C , loo00 h-l, ASl/-pAl2O3). I
100 I
1.(I)
+
co
2H2
AH~OOK = -100,46 kJ/mol (11)
+
co2
3H2
AH~OOK = 61,59 kJ/mol
-
L 4
CH30H
OXYGENATES SELECTIVITY A-A-*
80
/
CO CONVERSION
A G ~ =K +45,36 kJ/mol
+
CH30H
Hi0
$ 2 50
AGsWK = +61,80 kJ/mol
METHANOL SYNTHESIS
+
co
H20
AH~OOK = -38,7 kJ/mol
-
+
COS
H2
AGgOOK = -16,5 kJ/mol
2 CH30H
4
+
4H2
+
CH30CH3
AH~OOK = -35,31 kJ/mol
CH3OCH3 AG6ooK =
3
4
5
6
7
8
9
1011
H20
AG6ooK = -10,71 kJ/mol
4
2
Figure 12. CO conversion, selectivity to oxygenates vs WGSR activity. Same reaction conditions as in Figure 11.
DEHYDRODENSATION OF METHANOL TO DIMETHYL ETHER
2 co
1
H2 : CO RATIO (GHSV CONSTANT AT 10 000)
WATER GAS SHIFT REACTION
A H ~ = K -20,59 kJ/mol
0
+ + 79$7
H20 kJ/mol
DIRECT SYNTHESIS OF DIMETHYL ETHER
Figure 10. Reaction equilibria for methanol and DME syntheses.
operation pressure affects the conversion level of the feed CO and the activity toward the formation of DME/MeOH (STY of oxygenates) at 275 "C. Even at pressures as low as 2.0 MPa, STYs of nearly 55 mol/(kgc,,.h) (corresponding to 1.76kg/(kgcat.h)) can be obtained over this system. Otherwise, the oxygenate yields increase in almost a linear fashion with an increase in the reaction pressure. The performance of the systems containing zeolites is also satisfactory with an increased WGS activity at higher reaction pressures (especially in the case of H-Y). Influence of the Space Velocity. As shown in Figure 9,carbon monoxide conversion and DME yield decrease slightly with an increase in the space velocity. The selectivity of the reaction remains, however, remarkably stable while the overall STY of the oxygenates increases proportionally to the space velocity. At these rather moderate reaction conditions (a temperature of 250 "Cand a pressure of 4 MPa) an oxygenate STY of around 4 kg/(kgh) is obtained if the GHSV is increased to approximately 13 000 h-l. Influence of the H&O Ratio. Synthesis gas conversion over the bifuncional catalysts described above is characterized by a relatively high formation of C02 (Figure
5) which in some cases is higher than the overall oxygenate yield. The reason for the C02 formation is the WGS reaction which is one of the reactions parallel to the methanol synthesis (Figure 10). A possibility for suppressing the excess formation of C02 is to increase the H2:C0 ratio of the reactant syngas. It has already been shown by Bell and Chang (1983)that DME synthesis may be conducted with good results at low H2:C0 ratios (
