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Conversion of ethanol in aqueous solution over ZSM-5 zeolites
Applied Catalysis, 58 (1990) 119-129
119
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Conversion of Ethanol in Aqueous Solution over ZSM-5 Zeolites Study of the Reaction N e t w o r k T.M. NGUYEN and R. LE VAN MA0*
Catalysis Research Laboratory and Laboratories for Inorganic Materials, Department of Chemistry, Concordia University, 1455 de Maisonneuve Blvd. W., Montreal, Que. H3G 1M8 (Canada) (Received 17 July 1989, revised manuscript received 6 October 1989)
ABSTRACT Diethyl ether has been confirmed as a reaction intermediate in the conversion of aqueous ethanol over steam-treated ZSM-5 and the asbestos-derived ZSM-5 zeolites at reaction temperatures below 260-270 ° C. However, a pathway for the direct-conversion-to-ethylene is also present under these conditions. At higher reaction temperatures, the one-step path predominates. Both zeolites exhibit near-identical values of the apparent activation energy.
INTRODUCTION
The Bioethanol-to-Ethylene (B.E.T.E.) process is a novel route to convert biomass to ethylene [1]. Several catalysts were developed [1-7] to convert ethanol, which is present in very low concentration (2-15 wt.-% ) in the fermentation broth. The steam-treated ZSM-5 [6 ] and the asbestos-derived ZSM5 zeolites [6,7] are among those catalysts which can withstand the massive presence of water in the feed. Earlier, we showed that these two catalysts are fairly stable [6,8]. The present study was undertaken to examine the reaction network of ethanol over these catalysts with a particular emphasis on the effects of water in the feed. EXPERIMENTAL
The preparation and the characterization of the catalysts used in this work were fully described elsewhere [6,8,9]. Two catalysts were used: the steamtreated ZSM-5 (21, St) and the Asb-ZSM (26). (The numbers (21) and (26) refer to the Si/A1 atomic ratio.) The magnesium leaching degree (MLD) of 0166-9834/90/$03.50
© 1990 Elsevier Science Publishers B.V.
120
the ZSM (26) material as defined in previous works [8,9], was 99%. The experimental set-sup and in particular the configuration and size of the quartz reactor were identical to those used in ref. 10. The weight of catalyst used in this work was 4 g. Both the product analysis and catalytic data (total conversion, conversion to hydrocarbons or HC, and selectivity to reaction product "i") were reported in previous papers [6,8]. We define the reaction contact time t as the ratio of the weight of catalyst to the molar flow-rate of ethanol (expressed in g h/mol). In some cases, the weight contact time [reciprocal of the weight hourly space velocity (WHSV), expressed in h] is used. Runs were performed by varying primarily the reaction temperature and the WHSV. The resulting data were used to study the reaction network. Three types of runs were carried out: aqueous ethanol to ethylene, aqueous ethanol to diethyl ether and aqueous diethyl ether to ethylene and ethanol. Hereafter, data reported are average values of data from (at least) two runs carried out under the same reaction conditions and over the same catalyst. Results are estimated to be within + 0.05% based on C-atoms. The apparent activation energies were calculated for each reaction step, using the Arrhenius equation ro=k' .exp (-E/RT), where ro is the initial rate of the reaction, k' is the rate constant, E is the apparent activation energy, and R and T (in K) have their usual significance. The initial rate ro was obtained by the following method: The experimental data for total conversion were fit to a function f(t) of the contact time t using a non-linear regression Simplex algorithm [6,11 ]. The functions used for the "aqueous ethanol to ethylene and aqueous diethyl ether to ethylene reactions" and the "aqueous ethanol to diethyl ether reaction" were, respectively, f(t)=co [ 1 - e x p ( - a t ) ] and f(t)=co+at/(b+t), such that the criteria f(t)-~Co as t-~0 (where co=feed concentration) was obeyed. Subsequently, the initial rates of the reaction were determined by taking the derivative of the function and extrapolating to zero contact time; thus, limd(C)
limd[f(t) ]
ro=t o--gi-=t o
Once the intial reaction rates at various temperatures obtained, the apparent activation energies were calculated from an Arrhenius plot of ro vs. 103/T. RESULTS AND DISCUSSION
In our previous investigations [6,8 ] of the effects of temperature and contact time on the conversion of aqueous ethanol over the ZSM (21, St) and AsbZSM (26) catalysts, we discovered that both catalysts exhibited the same trends in activity and product selectivities. There was a clear change in the product distribution for both materials at a reaction temperature Ti of 260-270 ° C [8 ]. Above Ti, ethylene and some other hydrocarbons (with less than 1 C atom-% )
121
were the only products detected while the ethanol conversion was very high. Below T/, ethanol conversion was incomplete and diethyl ether was produced in substantial amounts [6,8]. Fig. 1 illustrates the product selectivities in the conversion of aqueous diethyl ether over both catalysts at different reaction temperatures. The selectivities are rather similar. However, when the product selectivities are compared to weight contact time (Fig. 2), the steam-treated ZSM-5 zeolite yields an equimolar mixture of ethylene and ethanol, while the Asb-ZSM-5 zeolite produces much smaller quantities of ethanol at ca. 40% diethyl ether conversion. Although the reaction mechanism of ethanol dehydration is not fully understood, there is good experimental evidence that the process may occur via two reaction pathways [12]. The probable pathway involves diethyl ether as a reaction intermediate [13,14], which is subsequently converted to ethylene by dehydration (E 1 mechanism) or to ethanol by dehydration. However, according to Chang [15] and Figueras et al. [16], an E 2 mechanism that leads directly to ethylene is possible if there are coexisting basic and acidic sites on the catalysts surface. The Asb-ZSM zeolite, which also possesses basic sites (MgO),
,oo 50-
g 0 ¢,1
•r"
O" 100-
2 50-
175
225
275
Temperature , OC Fig. 1. Conversion of aqueous diethyl ether (6.5 wt.-% ) to ethylene over ZSM (21, St) (top) and Asb-ZSM (26) (bottom) at various reaction temperatures (WHSV = 3.2 h - 1). ( o ) Ethylene, ( ~ ) diethyl ether. ( • ) ethanol.
122
1001
U
50"
0 .Jo
'13
"13 0
0 100-
50-
o
4
lOOx 1/WHSV, h Fig. 2. Conversion of aqueous diethyl ether (6.5 wt.-% ) to ethylene over ZSM (21, St) (top) and Asb-ZSM (26) (bottom) at various (weight) contact times (T=300°C). Symbols as in Fig. 1.
should therefore be capable of producing ethylene directly at low temperature. In our previous work [6,8], no direct "ethylene to diethyl ether interconversion" was observed. Thus, under such conditions, the direct formation of ethylene should also take place, in addition to the classical two-step conversion. By contrast, aqueous diethyl ether produced more ethylene than ethanol over the Asb-ZSM, at low conversion values (Fig. 2). This could be due to the presence of the magnesium component which would not favor the adsorption of water molecules and thereby lead to the dehydration of diethyl ether. However, at higher contact times (conversions greater than 80 C % ), both catalysts show the same product distribution. At temperatures above Ti, both catalysts produced ethylene directly, probably because the E 2 pathway is the more favored route, and is in keeping with the hypothesis of Poutsma [ 17 ]. Tables 1-6 report the conversion data at various contact times and temperatures for aqueous ethanol to ethylene (temperature range: 523-598 K), aqueous ethanol to diethyl ether (temperature range: 448-498 K) and aqueous diethyl ether to ethylene and ethanol (temperature range: 523-573 K), re-
123 TABLE 1 Conversion of aqueous ethanol (10 wt.-% ) to ethylene at vatious temperatures and contact times over steamed zeolite ZSM-5, ZSM (21, St) Temperature (K)
t (g h / m o l )
Conversion ( % ) Total
Ether
Selectivity ( % ) HC
Ethylene
Other hydrocar.
598
0.00 3.59 6.17 8.00 17.97 46.00 71.88
0.00 49.47 75.56 80.05 93.68 97.67 99.75
0.00 0.78 0.34 0.18 0.00 0.00 0.00
0.00 48.69 75.22 79.87 93.68 97.67 99.75
0.00 99.99 99.95 99.93 99.90 97.77 99.70
0.00 0.01 0.05 0.07 0.10 0.23 0.30
573
0.00 6.20 11.95 17.97 35.94 71.88 143.75
0.00 34.69 69.32 79.36 90.32 99.36 99.55
0.00 10.05 19.21 8.98 3.74 3.79 0.00
0.00 24.64 50.11 70.38 86.58 95.57 99.55
0.00 99.93 99.82 99.81 99.78 99.75 99.26
0.00 0.07 0.18 0.19 0.22 0.26 0.64
548
0.00 11.95 17.97 35.94 71.88 143.75
0.00 35.44 57.42 70.42 92.05 99.69
0.00 13.53 25.30 8.58 3.50 0.00
0.00 20.14 32.12 61.84 88.55 99.69
0.00 99.99 99.99 99.78 99.66 99.71
0.00 0.01 0.01 0.22 0.34 0.29
0.00 23.96 23.54 33.56 45.19 55.21
0.00 18.13 12.05 11.37 6.63 3.65
0.00 5.83 11.49 22.19 38.53 51.56
0.00 99.87 99.94 99.97 99.92 99.84
0.00 0.13 0.06 0.03 0.08 0.16
523
0.00 11.95 17.97 35.94 71.88 143.75
spectively. These data were used to calculate the initial rates (Tables 7-9). The corresponding apparent activation energies were determined (Table 10) from the Arrhenius plot of initial rates against reciprocal reaction temperature T (Fig. 3). These results are in good agreement with those obtained by Van Hooft et al. (18) in their work on pure ethanol and diethyl ether. The conditions used by Van Hooft et al. were: Catalyst, H-ZSM-5 sample with a Si/A1 ratio of 25; reaction temperature range, 177 °C to 327 ° C; WHSV, 3.6 h-1 (die_ thyl ether conversion) and 2.2 h - 1 (ethanol conversion). Finally, we wish to note that water in the feed acts primarily as a diluent. However, when used for long periods, steam should have some detrimental
124 TABLE2 Conversion of aqueous ethanol (10 wt.-% ) to ethylene at various temperatures and contact times over Asb-zeolite, Asb-ZSM (26). Temperature (K)
t (g h/tool)
Conversion (%) Total
Ether
Selectivity (%) HC
Ethylene
Other hydroc.
623
0.00 1.61 2.42 3.52 6.17 11.95 17.97 35.94
0.00 52.47 59.42 74.51 88.75 94.70 97.40 99.80
0.00 1.14 0.49 0.27 0.06 0.00 0.00 0.00
0.00 51.33 58.93 74.24 88.15 94.70 97.40 99.80
0.00 99.97 99.97 99.86 99.85 99.85 99.78 99.71
0.00 0.03 0.03 0.14 0.15 0.15 0.22 0.29
598
0.00 2.82 4.95 8.73 17.97 35.94 46.00 71.88 143.75
0.00 39.19 58.87 72.08 90.30 97.52 99.05 99.45 99.99
0.00 8.39 13.91 8.47 0.04 0.02 0.00 0.00 0.00
0.00 30.80 44.96 63.61 90.26 97.50 99.05 99.45 99.99
0.00 99.85 99.83 99.83 99.83 99.80 99.78 99.74 99.42
0.00 0.15 0.17 0.17 0.17 0.20 0.22 0.26 0.58
573
0.00 4.82 7.28 14.20 21.70 35.94 71.88 143.75
0.00 34.38 48.10 69.75 79.06 96.17 99.05 99.80
0.00 9.94 12.56 7.61 1.36 0.00 0.00 0.00
0.00 24.44 35.54 62.14 77.70 96.17 99.05 99.80
0.00 99.99 99.92 99.83 99.69 99.68 99.60 99.50
0.00 0.01 0.08 0.17 0.31 0.32 0.40 0.50
548
0.00 8.04 17.97 35.94 71.88 143.75
0.00 20.55 39.46 52.27 78.18 97.88
0.00 10.90 16.72 9.43 3.04 1.31
0.00 9.65 22.74 43.84 75.14 96.57
0.00 99.95 99.98 99.84 99.73 99.63
0.00 0.05 0.02 0.16 0.27 0.37
523
0.00 17.97 35.94 71.88 143.75
0.00 26.67 29.12 40.13 62.63
0.00 19.99 18.74 20.49 19.98
0.00 6.68 10.38 19.64 42.65
0.00 99.87 99.93 99.98 99.86
0.00 0.13 0.07 0.02 0.14
125 TABLE 3 Conversion of aqueous ethanol (10 wt.-%) to diethyl ether at various temperatures and contact times over steamed zeolite ZSM-5, ZSM (21, St) Temperature (K)
t (g h/tool)
Cony. HC (%)
Selectivity ( % ) Ether
Ethylene
498
0.00 17.97 35.94 71.88
0.00 14.90 20.04 29.52
0.00 13.83 18.53 26.92
0.00 1.07 1.51 2.60
473
0.00 17.97 35.94 71.88 143.75
0.00 8.04 12.90 24.78 28.75
0.00 7.92 12.74 24.59 28.02
0.00 0.12 0.16 0.19 0.73
448
0.00 35.94 71.88 143.75 184.00
0.00 7.70 10.96 18.56 20.85
0.00 7.68 10.92 18.53 20.83
0.00 0.02 0.04 0.01 0.02
TABLE 4 Conversion of aqueous ethanol (10 wt.-%) to diethyl ether at various temperatures and contact times over Asb-zeolite, Asb-ZSM (26) Temperature (K)
t (g h/mol)
Conv. HC (%)
Selectivity (%) Ether
Ethylene
498
0.00 11.95 21.49 35.94 71.88
0.00 10.14 14.91 28.94 33.27
0.00 9.94 13.75 27.04 30.46
0.00 0.20 1.16 1.90 2.81
473
0.00 21.48 35.94 71.88 143.75 184.00
0.00 8.08 12.11 19.42 27.01 30.93
0.00 7.97 11.89 18.93 26.48 30.15
0.00 0.11 0.22 0.49 0.53 0.78
448
0.00 35.94 71.88 143.75 184.00
0.00 6.94 10.35 14.38 16.86
0.00 6.92 10.32 14.37 16.81
0.00 0.02 0.03 0.01 0.05
effect on the steam-treated zeolite, even though the reaction temperature may be low enough to age the material slowly. The relatively high hydrophobicity of the Asb-zeolite matrix [ 10 ] should protect this material from a significant
126 TABLE 5 Conversion of aqueous diethyl ether (6.5 wt.-%) at various temperature and contact times over steamed zeolite ZSM-5, ZSM (21, St) Temperature (K)
t (g h/tool)
Conversion (%)
Selectivity (%)
Total
Ethanol
HC
Ethylene
Other hydrocarbons
573
0.00 8.17 24.22 41.25 82.45
0.00 60.09 80.68 93.94 99.86
0.00 28.29 21.24 10.06 5.43
0.00 31.80 59.04 83.28 94.43
0.00 99.77 99.79 99.86 99.82
0.00 0.23 0.21 0.14 0.18
548
0.00 20.51 41.25 82.45 164.99
0.00 66.59 80.67 94.24 98.30
0.00 32.74 22.77 18.59 7.80
0.00 33.85 57.97 75.65 90.50
0.00 99.88 99.89 99.86 99.86
0.00 0.12 0.11 0.14 0.14
523
0.00 41.25 82.45 123.73 164.99
0.00 61.88 79.23 88.82 93.54
0.00 33.76 29.67 23.57 20.34
0.00 28.12 49.56 65.25 73.20
0.00 99.63 99.78 99.83 99.82
0.00 0.37 0.22 0.17 0.18
TABLE 6 Conversion of aqueous diethyl ether (10 wt.-% ) at various temperatures and contact times over Asb-zeolite, Asb-ZSM (26) Temperature (K)
t (g h/tool)
Conversion (%)
Selectivity (%)
Total
Ethanol
HC
Ethylene
Other hydrocarbons
573
0.00 8.24 20.47 41.47 82.63 165.75
0.00 68.84 86.84 97.99 99.95 99.98
0.00 26.86 19.58 12.86 4.26 0.93
0.00 41.98 67.26 85.13 95.69 99.05
0.00 99.84 99.79 99.77 99.64 99.63
0.00 0.16 0.21 0.23 0.36 0.37
548
0.00 11.38 20.47 41.47 82.93 124.35 165.74
0.00 37.04 79.24 95.29 99.65 99.95 99.99
0.00 14.50 30.66 26.26 9.13 6.93 3.96
0.00 22.54 48.58 69.03 90.52 93.02 96.03
0.00 99.66 99.83 99.71 99.82 99.76 99.49
0.00 0.34 0.17 0.29 0.18 0.24 0.51
423
0.00 41.47 82.93 124.35 165.74
0.00 74.99 92.03 98.99 98.99
0.00 39.89 26.56 23.40 18.86
0.00 35.10 65.47 75.59 80.13
0.00 99.77 99.89 99.90 99.88
0.00 0.23 0.11 0.10 0.12
12'l TABLE 7 Determination of the initial rate of the aqueous ethanol (10 wt.-% )-to-ethylene reaction Catalysts
Temperature (K)
f(t) = Co [1 - e x p ( - a t ) ] a
Deviation
ro (mol/g h)
ZSM (21, St)
598 573 548 523
0.20184 0.05837 0.02518 0.00582
2.48 3.26 3.82 3.14
20.184 5.837 2.518 0.582
Asb-ZSM(26)
623 598 573 548 523
0.38819 0.12151 0.06714 0.01744 0.00357
2.17 2.92 3.12 3.45 2.84
38.819 12.151 6.714 1.744 0.357
TABLE 8 Determination of the initial rate of the aqueous ethanol (10 wt.-% )-to-diethyl ether reaction Catalysts
Temperature (K)
f(t) =at / ( b + t ) +Co
ro (mol/g h)
a
b
Deviation
ZSM(21, St)
498 473 448
40.667 44.359 40.075
38.404 74.944 170.700
0.74 1.59 0.61
1.059 0.592 0.235
Asb-ZSM(26)
498 473 448
52.497 47.119 25.619
46.755 107.278 103.819
2.37 0.26 0.34
1.123 0.439 0.247
TABLE 9 Determination of the initial rate of the aqueous diethyl ether (6.5 wt.-% )-to-ethylene reaction Catalysts
Temperature (K)
f(t) = Co[ 1 - exp ( - at) ] a
Deviation
ro (mol/g h)
ZSM(21, St)
573 548 523
0.04051 0.01890 0.00824
2.59 3.28 0.84
4.501 1.890 0.824
Asb-ZSM(26)
573 548 523
0.05555 0.02811 0.01124
3.25 3.12 3.07
5.555 2.801 1.124
128 TABLE 10 Values of apparent activation energies Catalyst
Reaction
Temperature Apparent activation range (K) energy(kJ/mol) Calculated From ref. 18a
ZSM (21, St)
Aq. Ethanol-to-diethylether 448-498 Aq. Diethyl ether-to-ethylene/ethanol 523-573 Aq. Ethanol-to-ethylene 523-598
56 79 119
92 122
Asb-ZSM (26) Aq. Ethanol-to-diethylether 448-498 Aq. Diethyl ether-to-ethylene/ethanol 523-573 Aq. Ethanol-to-ethylene 523-623
56 80 123
92 122
aPure reagents; catalyst ZSM-5 (Si/Al:25).
3
m o
E
:Ol c .J
-1
i
1.6
i
i
i
1.8
1
2.0
1000//Temp., K-1
Fig. 3. Arrhenius plot of the initial rate for the conversion of aqueous ethanol (10 wt.-% ) over ZSM (21, St) (o) and Asb-ZSM (26) (e). decay in activity even for very long time runs. However, some attention must be directed at the formation of bulky oligomers at the zeolite pore apertures. These compounds are believed to be the main cause for the gradual decrease in ethanol dehydration activity of the triflic acid loaded ZSM-5 zeolite (blockage of zeolite pore openings). T r e a t m e n t s which eliminate the strong acid sites from the zeolite pore apertures lead to enhanced catalyst activity [19].
129 CONCLUSION T h e p r e s e n t s t u d y o f t h e r e a c t i o n n e t w o r k for t h e c o n v e r s i o n of a q u e o u s e t h a n o l o v e r s t e a m - t r e a t e d Z S M - 5 a n d A s b - Z S M - 5 zeolites s h o w s t h a t s e v e r a l p a t h w a y s m a y exist. A t a r e a c t i o n t e m p e r a t u r e b e l o w Ti, t h e t w o - s t e p p a t h w a y w i t h d i e t h y l e t h e r as t h e i n t e r m e d i a t e coexists w i t h t h e d i r e c t c o n v e r s i o n - t o e t h y l e n e route. T h e l a t t e r a p p e a r s to be f a v o r e d a t t e m p e r a t u r e s a b o v e Ti. ACKNOWLEDGEMENT T h e a u t h o r s t h a n k t h e N a t u r a l Science a n d E n g i n e e r i n g R e s e a r c h Council of C a n a d a ( N S E R C ) a n d t h e Q u e b e c A c t i o n s S t r u c t u r a n t e s P r o g r a m for fin a n c i a l s u p p o r t , a n d P r o f . N, S e r p o n e ( D e p a r t m e n t of C h e m i s t r y , C o n c o r d i a U n i v e r s i t y , M o n t r e a l , C a n a d a ) for helpful discussions.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
19
R. Le Van Mao, T.M. Nguyen and G.P. McLaughlin, Appl. Catal., 48 (1989) 265. R. Le Van Mao and L.H. Dao, U.S. Patent 4 698 452 (Oct. 6, 1987). R. Le Van Mao, P. Levesque, G.P. McLaughlin and L.H. Dao, Appl. Catal., 34 (1987) 163. R. Le Van Mao and T.M. Nguyen, U.S. Patent 4 847 223 (July 11, 1989). R. Le Van Mao, U.S. Patent Appl. (Apr. 25, 1989), granted (May 1989). T.M. Nguyen, M. Sc. Thesis, Concordia University, Montreal, July 1988. R. Le Van Mao, U.S. Patent Appl. (June 20, 1989). R. Le Van Mao and T.M. Nguyen, Appl. Catal., in press. R. Le Van Mao. P. Levesque, B. Sjiariel and P.H. Bird, Can. J. Chem., 63 (1985) 3464. R. Le Van Mao, P. Levesque, B. Sjiariel and T.N. Do, Can. J. Chem. Eng., 64 (1986) 462. M.S. Caceci and W.P. Cacheris, Fitting curves to data. Simplex Algorithm, Bytes, (May 1984) 340. H. Knozinger and R. Kohne, J. Catal., 5 (1966) 264. M.T. Aronson, R.J. Gorte and W.E. Farneth, J. Catal., 98 (1986) 434. M.T. Aronson, R.J. Gorte and W.E. Farneth, J. Catal., 105 (1987) 455. C.D. Chang, Hydrocarbons from Methanol, Marcel Dekker, New York, p. 198. F. Figueras, A. Nohl, L. De Mourgues, and Y. Trambouze, J. Chem. Soc., Faraday Trans. 1, 67 {1971) 1155. M.L. Poustma, in J.A. Rabo (Editor), Zeolite Chemistry and Catalysis, A.C.S. Monograph, Chapter 9, 1976, p. 171. J.H.C. van Hooft, J.P. van den Berg, J.P. Wolthuisen and A. Volmer, in Olson and Bisio (Editors), Proceedings 6th International Zeolite Conference, Butterworth, London 1984, p. 489. R. Le Van Mao, H. Li, B. Sjiariel and Y. Yu, Catal. Lett., submitted for publication.
119
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Conversion of Ethanol in Aqueous Solution over ZSM-5 Zeolites Study of the Reaction N e t w o r k T.M. NGUYEN and R. LE VAN MA0*
Catalysis Research Laboratory and Laboratories for Inorganic Materials, Department of Chemistry, Concordia University, 1455 de Maisonneuve Blvd. W., Montreal, Que. H3G 1M8 (Canada) (Received 17 July 1989, revised manuscript received 6 October 1989)
ABSTRACT Diethyl ether has been confirmed as a reaction intermediate in the conversion of aqueous ethanol over steam-treated ZSM-5 and the asbestos-derived ZSM-5 zeolites at reaction temperatures below 260-270 ° C. However, a pathway for the direct-conversion-to-ethylene is also present under these conditions. At higher reaction temperatures, the one-step path predominates. Both zeolites exhibit near-identical values of the apparent activation energy.
INTRODUCTION
The Bioethanol-to-Ethylene (B.E.T.E.) process is a novel route to convert biomass to ethylene [1]. Several catalysts were developed [1-7] to convert ethanol, which is present in very low concentration (2-15 wt.-% ) in the fermentation broth. The steam-treated ZSM-5 [6 ] and the asbestos-derived ZSM5 zeolites [6,7] are among those catalysts which can withstand the massive presence of water in the feed. Earlier, we showed that these two catalysts are fairly stable [6,8]. The present study was undertaken to examine the reaction network of ethanol over these catalysts with a particular emphasis on the effects of water in the feed. EXPERIMENTAL
The preparation and the characterization of the catalysts used in this work were fully described elsewhere [6,8,9]. Two catalysts were used: the steamtreated ZSM-5 (21, St) and the Asb-ZSM (26). (The numbers (21) and (26) refer to the Si/A1 atomic ratio.) The magnesium leaching degree (MLD) of 0166-9834/90/$03.50
© 1990 Elsevier Science Publishers B.V.
120
the ZSM (26) material as defined in previous works [8,9], was 99%. The experimental set-sup and in particular the configuration and size of the quartz reactor were identical to those used in ref. 10. The weight of catalyst used in this work was 4 g. Both the product analysis and catalytic data (total conversion, conversion to hydrocarbons or HC, and selectivity to reaction product "i") were reported in previous papers [6,8]. We define the reaction contact time t as the ratio of the weight of catalyst to the molar flow-rate of ethanol (expressed in g h/mol). In some cases, the weight contact time [reciprocal of the weight hourly space velocity (WHSV), expressed in h] is used. Runs were performed by varying primarily the reaction temperature and the WHSV. The resulting data were used to study the reaction network. Three types of runs were carried out: aqueous ethanol to ethylene, aqueous ethanol to diethyl ether and aqueous diethyl ether to ethylene and ethanol. Hereafter, data reported are average values of data from (at least) two runs carried out under the same reaction conditions and over the same catalyst. Results are estimated to be within + 0.05% based on C-atoms. The apparent activation energies were calculated for each reaction step, using the Arrhenius equation ro=k' .exp (-E/RT), where ro is the initial rate of the reaction, k' is the rate constant, E is the apparent activation energy, and R and T (in K) have their usual significance. The initial rate ro was obtained by the following method: The experimental data for total conversion were fit to a function f(t) of the contact time t using a non-linear regression Simplex algorithm [6,11 ]. The functions used for the "aqueous ethanol to ethylene and aqueous diethyl ether to ethylene reactions" and the "aqueous ethanol to diethyl ether reaction" were, respectively, f(t)=co [ 1 - e x p ( - a t ) ] and f(t)=co+at/(b+t), such that the criteria f(t)-~Co as t-~0 (where co=feed concentration) was obeyed. Subsequently, the initial rates of the reaction were determined by taking the derivative of the function and extrapolating to zero contact time; thus, limd(C)
limd[f(t) ]
ro=t o--gi-=t o
Once the intial reaction rates at various temperatures obtained, the apparent activation energies were calculated from an Arrhenius plot of ro vs. 103/T. RESULTS AND DISCUSSION
In our previous investigations [6,8 ] of the effects of temperature and contact time on the conversion of aqueous ethanol over the ZSM (21, St) and AsbZSM (26) catalysts, we discovered that both catalysts exhibited the same trends in activity and product selectivities. There was a clear change in the product distribution for both materials at a reaction temperature Ti of 260-270 ° C [8 ]. Above Ti, ethylene and some other hydrocarbons (with less than 1 C atom-% )
121
were the only products detected while the ethanol conversion was very high. Below T/, ethanol conversion was incomplete and diethyl ether was produced in substantial amounts [6,8]. Fig. 1 illustrates the product selectivities in the conversion of aqueous diethyl ether over both catalysts at different reaction temperatures. The selectivities are rather similar. However, when the product selectivities are compared to weight contact time (Fig. 2), the steam-treated ZSM-5 zeolite yields an equimolar mixture of ethylene and ethanol, while the Asb-ZSM-5 zeolite produces much smaller quantities of ethanol at ca. 40% diethyl ether conversion. Although the reaction mechanism of ethanol dehydration is not fully understood, there is good experimental evidence that the process may occur via two reaction pathways [12]. The probable pathway involves diethyl ether as a reaction intermediate [13,14], which is subsequently converted to ethylene by dehydration (E 1 mechanism) or to ethanol by dehydration. However, according to Chang [15] and Figueras et al. [16], an E 2 mechanism that leads directly to ethylene is possible if there are coexisting basic and acidic sites on the catalysts surface. The Asb-ZSM zeolite, which also possesses basic sites (MgO),
,oo 50-
g 0 ¢,1
•r"
O" 100-
2 50-
175
225
275
Temperature , OC Fig. 1. Conversion of aqueous diethyl ether (6.5 wt.-% ) to ethylene over ZSM (21, St) (top) and Asb-ZSM (26) (bottom) at various reaction temperatures (WHSV = 3.2 h - 1). ( o ) Ethylene, ( ~ ) diethyl ether. ( • ) ethanol.
122
1001
U
50"
0 .Jo
'13
"13 0
0 100-
50-
o
4
lOOx 1/WHSV, h Fig. 2. Conversion of aqueous diethyl ether (6.5 wt.-% ) to ethylene over ZSM (21, St) (top) and Asb-ZSM (26) (bottom) at various (weight) contact times (T=300°C). Symbols as in Fig. 1.
should therefore be capable of producing ethylene directly at low temperature. In our previous work [6,8], no direct "ethylene to diethyl ether interconversion" was observed. Thus, under such conditions, the direct formation of ethylene should also take place, in addition to the classical two-step conversion. By contrast, aqueous diethyl ether produced more ethylene than ethanol over the Asb-ZSM, at low conversion values (Fig. 2). This could be due to the presence of the magnesium component which would not favor the adsorption of water molecules and thereby lead to the dehydration of diethyl ether. However, at higher contact times (conversions greater than 80 C % ), both catalysts show the same product distribution. At temperatures above Ti, both catalysts produced ethylene directly, probably because the E 2 pathway is the more favored route, and is in keeping with the hypothesis of Poutsma [ 17 ]. Tables 1-6 report the conversion data at various contact times and temperatures for aqueous ethanol to ethylene (temperature range: 523-598 K), aqueous ethanol to diethyl ether (temperature range: 448-498 K) and aqueous diethyl ether to ethylene and ethanol (temperature range: 523-573 K), re-
123 TABLE 1 Conversion of aqueous ethanol (10 wt.-% ) to ethylene at vatious temperatures and contact times over steamed zeolite ZSM-5, ZSM (21, St) Temperature (K)
t (g h / m o l )
Conversion ( % ) Total
Ether
Selectivity ( % ) HC
Ethylene
Other hydrocar.
598
0.00 3.59 6.17 8.00 17.97 46.00 71.88
0.00 49.47 75.56 80.05 93.68 97.67 99.75
0.00 0.78 0.34 0.18 0.00 0.00 0.00
0.00 48.69 75.22 79.87 93.68 97.67 99.75
0.00 99.99 99.95 99.93 99.90 97.77 99.70
0.00 0.01 0.05 0.07 0.10 0.23 0.30
573
0.00 6.20 11.95 17.97 35.94 71.88 143.75
0.00 34.69 69.32 79.36 90.32 99.36 99.55
0.00 10.05 19.21 8.98 3.74 3.79 0.00
0.00 24.64 50.11 70.38 86.58 95.57 99.55
0.00 99.93 99.82 99.81 99.78 99.75 99.26
0.00 0.07 0.18 0.19 0.22 0.26 0.64
548
0.00 11.95 17.97 35.94 71.88 143.75
0.00 35.44 57.42 70.42 92.05 99.69
0.00 13.53 25.30 8.58 3.50 0.00
0.00 20.14 32.12 61.84 88.55 99.69
0.00 99.99 99.99 99.78 99.66 99.71
0.00 0.01 0.01 0.22 0.34 0.29
0.00 23.96 23.54 33.56 45.19 55.21
0.00 18.13 12.05 11.37 6.63 3.65
0.00 5.83 11.49 22.19 38.53 51.56
0.00 99.87 99.94 99.97 99.92 99.84
0.00 0.13 0.06 0.03 0.08 0.16
523
0.00 11.95 17.97 35.94 71.88 143.75
spectively. These data were used to calculate the initial rates (Tables 7-9). The corresponding apparent activation energies were determined (Table 10) from the Arrhenius plot of initial rates against reciprocal reaction temperature T (Fig. 3). These results are in good agreement with those obtained by Van Hooft et al. (18) in their work on pure ethanol and diethyl ether. The conditions used by Van Hooft et al. were: Catalyst, H-ZSM-5 sample with a Si/A1 ratio of 25; reaction temperature range, 177 °C to 327 ° C; WHSV, 3.6 h-1 (die_ thyl ether conversion) and 2.2 h - 1 (ethanol conversion). Finally, we wish to note that water in the feed acts primarily as a diluent. However, when used for long periods, steam should have some detrimental
124 TABLE2 Conversion of aqueous ethanol (10 wt.-% ) to ethylene at various temperatures and contact times over Asb-zeolite, Asb-ZSM (26). Temperature (K)
t (g h/tool)
Conversion (%) Total
Ether
Selectivity (%) HC
Ethylene
Other hydroc.
623
0.00 1.61 2.42 3.52 6.17 11.95 17.97 35.94
0.00 52.47 59.42 74.51 88.75 94.70 97.40 99.80
0.00 1.14 0.49 0.27 0.06 0.00 0.00 0.00
0.00 51.33 58.93 74.24 88.15 94.70 97.40 99.80
0.00 99.97 99.97 99.86 99.85 99.85 99.78 99.71
0.00 0.03 0.03 0.14 0.15 0.15 0.22 0.29
598
0.00 2.82 4.95 8.73 17.97 35.94 46.00 71.88 143.75
0.00 39.19 58.87 72.08 90.30 97.52 99.05 99.45 99.99
0.00 8.39 13.91 8.47 0.04 0.02 0.00 0.00 0.00
0.00 30.80 44.96 63.61 90.26 97.50 99.05 99.45 99.99
0.00 99.85 99.83 99.83 99.83 99.80 99.78 99.74 99.42
0.00 0.15 0.17 0.17 0.17 0.20 0.22 0.26 0.58
573
0.00 4.82 7.28 14.20 21.70 35.94 71.88 143.75
0.00 34.38 48.10 69.75 79.06 96.17 99.05 99.80
0.00 9.94 12.56 7.61 1.36 0.00 0.00 0.00
0.00 24.44 35.54 62.14 77.70 96.17 99.05 99.80
0.00 99.99 99.92 99.83 99.69 99.68 99.60 99.50
0.00 0.01 0.08 0.17 0.31 0.32 0.40 0.50
548
0.00 8.04 17.97 35.94 71.88 143.75
0.00 20.55 39.46 52.27 78.18 97.88
0.00 10.90 16.72 9.43 3.04 1.31
0.00 9.65 22.74 43.84 75.14 96.57
0.00 99.95 99.98 99.84 99.73 99.63
0.00 0.05 0.02 0.16 0.27 0.37
523
0.00 17.97 35.94 71.88 143.75
0.00 26.67 29.12 40.13 62.63
0.00 19.99 18.74 20.49 19.98
0.00 6.68 10.38 19.64 42.65
0.00 99.87 99.93 99.98 99.86
0.00 0.13 0.07 0.02 0.14
125 TABLE 3 Conversion of aqueous ethanol (10 wt.-%) to diethyl ether at various temperatures and contact times over steamed zeolite ZSM-5, ZSM (21, St) Temperature (K)
t (g h/tool)
Cony. HC (%)
Selectivity ( % ) Ether
Ethylene
498
0.00 17.97 35.94 71.88
0.00 14.90 20.04 29.52
0.00 13.83 18.53 26.92
0.00 1.07 1.51 2.60
473
0.00 17.97 35.94 71.88 143.75
0.00 8.04 12.90 24.78 28.75
0.00 7.92 12.74 24.59 28.02
0.00 0.12 0.16 0.19 0.73
448
0.00 35.94 71.88 143.75 184.00
0.00 7.70 10.96 18.56 20.85
0.00 7.68 10.92 18.53 20.83
0.00 0.02 0.04 0.01 0.02
TABLE 4 Conversion of aqueous ethanol (10 wt.-%) to diethyl ether at various temperatures and contact times over Asb-zeolite, Asb-ZSM (26) Temperature (K)
t (g h/mol)
Conv. HC (%)
Selectivity (%) Ether
Ethylene
498
0.00 11.95 21.49 35.94 71.88
0.00 10.14 14.91 28.94 33.27
0.00 9.94 13.75 27.04 30.46
0.00 0.20 1.16 1.90 2.81
473
0.00 21.48 35.94 71.88 143.75 184.00
0.00 8.08 12.11 19.42 27.01 30.93
0.00 7.97 11.89 18.93 26.48 30.15
0.00 0.11 0.22 0.49 0.53 0.78
448
0.00 35.94 71.88 143.75 184.00
0.00 6.94 10.35 14.38 16.86
0.00 6.92 10.32 14.37 16.81
0.00 0.02 0.03 0.01 0.05
effect on the steam-treated zeolite, even though the reaction temperature may be low enough to age the material slowly. The relatively high hydrophobicity of the Asb-zeolite matrix [ 10 ] should protect this material from a significant
126 TABLE 5 Conversion of aqueous diethyl ether (6.5 wt.-%) at various temperature and contact times over steamed zeolite ZSM-5, ZSM (21, St) Temperature (K)
t (g h/tool)
Conversion (%)
Selectivity (%)
Total
Ethanol
HC
Ethylene
Other hydrocarbons
573
0.00 8.17 24.22 41.25 82.45
0.00 60.09 80.68 93.94 99.86
0.00 28.29 21.24 10.06 5.43
0.00 31.80 59.04 83.28 94.43
0.00 99.77 99.79 99.86 99.82
0.00 0.23 0.21 0.14 0.18
548
0.00 20.51 41.25 82.45 164.99
0.00 66.59 80.67 94.24 98.30
0.00 32.74 22.77 18.59 7.80
0.00 33.85 57.97 75.65 90.50
0.00 99.88 99.89 99.86 99.86
0.00 0.12 0.11 0.14 0.14
523
0.00 41.25 82.45 123.73 164.99
0.00 61.88 79.23 88.82 93.54
0.00 33.76 29.67 23.57 20.34
0.00 28.12 49.56 65.25 73.20
0.00 99.63 99.78 99.83 99.82
0.00 0.37 0.22 0.17 0.18
TABLE 6 Conversion of aqueous diethyl ether (10 wt.-% ) at various temperatures and contact times over Asb-zeolite, Asb-ZSM (26) Temperature (K)
t (g h/tool)
Conversion (%)
Selectivity (%)
Total
Ethanol
HC
Ethylene
Other hydrocarbons
573
0.00 8.24 20.47 41.47 82.63 165.75
0.00 68.84 86.84 97.99 99.95 99.98
0.00 26.86 19.58 12.86 4.26 0.93
0.00 41.98 67.26 85.13 95.69 99.05
0.00 99.84 99.79 99.77 99.64 99.63
0.00 0.16 0.21 0.23 0.36 0.37
548
0.00 11.38 20.47 41.47 82.93 124.35 165.74
0.00 37.04 79.24 95.29 99.65 99.95 99.99
0.00 14.50 30.66 26.26 9.13 6.93 3.96
0.00 22.54 48.58 69.03 90.52 93.02 96.03
0.00 99.66 99.83 99.71 99.82 99.76 99.49
0.00 0.34 0.17 0.29 0.18 0.24 0.51
423
0.00 41.47 82.93 124.35 165.74
0.00 74.99 92.03 98.99 98.99
0.00 39.89 26.56 23.40 18.86
0.00 35.10 65.47 75.59 80.13
0.00 99.77 99.89 99.90 99.88
0.00 0.23 0.11 0.10 0.12
12'l TABLE 7 Determination of the initial rate of the aqueous ethanol (10 wt.-% )-to-ethylene reaction Catalysts
Temperature (K)
f(t) = Co [1 - e x p ( - a t ) ] a
Deviation
ro (mol/g h)
ZSM (21, St)
598 573 548 523
0.20184 0.05837 0.02518 0.00582
2.48 3.26 3.82 3.14
20.184 5.837 2.518 0.582
Asb-ZSM(26)
623 598 573 548 523
0.38819 0.12151 0.06714 0.01744 0.00357
2.17 2.92 3.12 3.45 2.84
38.819 12.151 6.714 1.744 0.357
TABLE 8 Determination of the initial rate of the aqueous ethanol (10 wt.-% )-to-diethyl ether reaction Catalysts
Temperature (K)
f(t) =at / ( b + t ) +Co
ro (mol/g h)
a
b
Deviation
ZSM(21, St)
498 473 448
40.667 44.359 40.075
38.404 74.944 170.700
0.74 1.59 0.61
1.059 0.592 0.235
Asb-ZSM(26)
498 473 448
52.497 47.119 25.619
46.755 107.278 103.819
2.37 0.26 0.34
1.123 0.439 0.247
TABLE 9 Determination of the initial rate of the aqueous diethyl ether (6.5 wt.-% )-to-ethylene reaction Catalysts
Temperature (K)
f(t) = Co[ 1 - exp ( - at) ] a
Deviation
ro (mol/g h)
ZSM(21, St)
573 548 523
0.04051 0.01890 0.00824
2.59 3.28 0.84
4.501 1.890 0.824
Asb-ZSM(26)
573 548 523
0.05555 0.02811 0.01124
3.25 3.12 3.07
5.555 2.801 1.124
128 TABLE 10 Values of apparent activation energies Catalyst
Reaction
Temperature Apparent activation range (K) energy(kJ/mol) Calculated From ref. 18a
ZSM (21, St)
Aq. Ethanol-to-diethylether 448-498 Aq. Diethyl ether-to-ethylene/ethanol 523-573 Aq. Ethanol-to-ethylene 523-598
56 79 119
92 122
Asb-ZSM (26) Aq. Ethanol-to-diethylether 448-498 Aq. Diethyl ether-to-ethylene/ethanol 523-573 Aq. Ethanol-to-ethylene 523-623
56 80 123
92 122
aPure reagents; catalyst ZSM-5 (Si/Al:25).
3
m o
E
:Ol c .J
-1
i
1.6
i
i
i
1.8
1
2.0
1000//Temp., K-1
Fig. 3. Arrhenius plot of the initial rate for the conversion of aqueous ethanol (10 wt.-% ) over ZSM (21, St) (o) and Asb-ZSM (26) (e). decay in activity even for very long time runs. However, some attention must be directed at the formation of bulky oligomers at the zeolite pore apertures. These compounds are believed to be the main cause for the gradual decrease in ethanol dehydration activity of the triflic acid loaded ZSM-5 zeolite (blockage of zeolite pore openings). T r e a t m e n t s which eliminate the strong acid sites from the zeolite pore apertures lead to enhanced catalyst activity [19].
129 CONCLUSION T h e p r e s e n t s t u d y o f t h e r e a c t i o n n e t w o r k for t h e c o n v e r s i o n of a q u e o u s e t h a n o l o v e r s t e a m - t r e a t e d Z S M - 5 a n d A s b - Z S M - 5 zeolites s h o w s t h a t s e v e r a l p a t h w a y s m a y exist. A t a r e a c t i o n t e m p e r a t u r e b e l o w Ti, t h e t w o - s t e p p a t h w a y w i t h d i e t h y l e t h e r as t h e i n t e r m e d i a t e coexists w i t h t h e d i r e c t c o n v e r s i o n - t o e t h y l e n e route. T h e l a t t e r a p p e a r s to be f a v o r e d a t t e m p e r a t u r e s a b o v e Ti. ACKNOWLEDGEMENT T h e a u t h o r s t h a n k t h e N a t u r a l Science a n d E n g i n e e r i n g R e s e a r c h Council of C a n a d a ( N S E R C ) a n d t h e Q u e b e c A c t i o n s S t r u c t u r a n t e s P r o g r a m for fin a n c i a l s u p p o r t , a n d P r o f . N, S e r p o n e ( D e p a r t m e n t of C h e m i s t r y , C o n c o r d i a U n i v e r s i t y , M o n t r e a l , C a n a d a ) for helpful discussions.
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19
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