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Ind. Eng. Chem. Res. 2009, 48, 6256–6261
Artificial Neural Network (ANN)-Aided Optimization of ZSM-5 Catalyst for the Dimethyl Ether to Olefin (DTO) Reaction from Neat Dimethyl Ether (DME) Kohji Omata,* Yuichiro Yamazaki, Yuhsuke Watanabe, Kyosuke Kodama, and Muneyoshi Yamada Department of Applied Chemistry, Graduate School of Engineering, Tohoku UniVersity, Aoba 6-6-07, Aramaki, Aoba-ku, Sendai 980-8579, Japan
A ZSM-5 catalyst for light olefin synthesis from dimethyl ether (the reaction of dimethyl ether (DME) to olefin, abbreviated as DTO) was developed. In comparison with the reaction of DTO from diluted dimethyl ether (abbreviated as Diluted-DTO), lower light olefin selectivity and shorter catalyst life are the drawbacks of the reaction of DTO from neat (90 vol %) DME (abbreviated as Neat-DTO). After the effects of Si/Al ratio of zeolite, DME concentration, gas hourly space velocity (GHSV) of gas feed, and Si/Al ratio on the activity of calcium-incorporated ZSM-5 were examined, additives were screened using the physicochemical properties of the additive elements and an artificial neural network. The addition of boron or phosphorus to ZSM-5 improved the catalyst life. The catalyst composition such as Si/Al, Si/Ca, Si/P, and Si/B was then optimized for longer catalyst life, using an L9 orthogonal array and an artificial neural network (ANN). Grid search was applied to find the maximum catalyst life. The catalyst life of H-Ca-ZSM-5 (Si/Al ) 250, Si/Ca ) 20, Si/P ) 400, Si/B ) 200) was 146 h when Neat-DTO was performed at 803 K and GHSV ) 1000 h-1. The life is comparable to that of the catalyst supplied to Diluted-DTO. Introduction The industrial production of dimethyl ether (DME) as a fuel has started. An inexpensive compound such as DME attracts attention as an alternative liquified petroleum gas (LPG) fuel and as feedstock. Light olefins, especially propylene, are one of the most valuable chemicals. Recently, the production of light olefins from dimethyl ether (the reaction of dimethyl ether to olefin, or DTO) is feasible as an olefin synthesis process. DTO reaction is the second step of the methanol to olefin (MTO) reaction, because methanol is initially converted to DME in MTO.1 Demonstration plants of the DTO/MTO reaction have been constructed by JGC Corp., UOP/Hydro, and others.2,3 In these plants, DME/methanol is diluted by steam and the effluent gas is usually recycled to feed gas to produce light olefins. In the MTO process using SAPO-34, however, the olefin cracking process (OCP) is necessary, instead of the recycling, to improve the total yield of ethylene and propylene, because the activity of SAPO-34 for decomposition of higher olefins is very low. The dilution is effective to stabilize the catalyst activity and to enhance the olefin selectivity. In the case of MTO reaction, reactant gas was diluted more than the case of DTO reaction, because water is produced by the dehydration of methanol. To increase the productivity of the plant, however, such dilution should be eliminated, and a noble catalyst that is stable under a neat DME (over 90 vol %) supply should be developed. Moreover, the exothermic heat value of DTO is less than that of MTO, because the dehydration of methanol is an exothermal reaction, and DME has higher reactivity to product light olefins, compared to methanol.4 Therefore, considering the operability, DME is more preferable as a reactant than methanol. Although there are some differences between MTO and DTO, as mentioned previously, effective catalysts for MTO basically will be also durable for DTO. Among the effective catalysts for MTO reaction, ZSM-5,5 SAPO-34,6 and Fe-silicate7 are the most promising catalysts. Especially, the selectivity to propylene * To whom correspondence should be addressed. E-mail: omata@ erec.che.tohoku.ac.jp.
production and the durability is excellent with ZSM-5 catalysts. Tsubaki8 reported that a H3PO4/ZrO2/H-ZSM-5 catalyst showed high selectivity (up to 45%) of propylene on DTO. Phosphorus impregnation on ZSM-5 was reported to increase light olefin selectivity at low space velocity.9-11 A calcium-incorporated ZSM-5 was reported to show high selectivity and durability at high temperature.12 These modifications would result in a change of acid site and shrinkage of the micropores, to improve olefin selectivity. The influence of such modification is more serious in the Neat-DTO reaction than in the case of a diluted DTO/MTO reaction (as discussed later); this reaction is abbreviated as Diluted-DTO. In the present study, modified ZSM-5 was prepared, and the activity tests were conducted using neat DME to determine and optimize the composition of the zeolite for superior stability. Experimental Section Zeolite Synthesis. The modified H-ZSM-5 catalysts were synthesized hydrothermally at 433 K for 60 h in the presence of calcium salt,13 as well as other metal salts (such as phosphorus, boron, zinc, iron, scandium, tellurium, silver, barium, rhenium, manganese, and neodymium salts). The silicon source was colloidal silica (Snotex S, from Nissan Chemical) or fumed silica (AEROSIL200, from Japan Aerosil). Silicalite that was prepared from colloidal silica contained a small amount of aluminum (SiO2/Al2O3 ) 700). In the optimization experiments, the composition was designed by orthogonal array. After hydrothermal synthesis, the obtained products were washed with deionized water until the concentration of Na+ ions in the wash water was 95%. Longer life than that of the mother H-Ca-ZSM-5 was attained via the addition of phosphorus, boron, manganese, or neodymium with high olefin yield. The effect of the additives on the zeolite acidity was investigated to clarify the reason for their long life. Acid strength was represented by the peak temperature of NH3 desorption from strong acid sites. As shown in Figure 7, large circles (denoting long life) appear between H-ZSM-5 with the strongest acidity and H-Ca-ZSM-5. A clear trend between the strength and the density of acid sites was observed. If the acidity of the zeolite is strong, the density of acid sites should be small, to show the long life, and vice versa. Another factor for long life was the morphology of pores indexed by the ratio of intensities of the (020) plane and the (200) plane of the X-ray diffractogram. The morphology index
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Table 2. Effect of Additives on H-Ca-ZSM-5 Performance for Neat-DTOa element
C2-C4 olefin yield (%)
catalyst life (h)
P B Mn Nd Ag Sc Ba Fe Re Tl Zn H-Ca-ZSM-5 H-ZSM-5
56 58 53 51 62 51 47 32 32 0 18 56 43
27 24 24 23 13 9 0 0 0 0 0 14 1
a Conditions: SiO2/Al2O3 ) 400 SiO2/Ca ) 40, SiO2/additive ) 200, silicon source ) fumed silica, reaction temperature ) 823 K, GHSV ) 2200 h-1.
Figure 7. Effect of additives on the acidity of H-Ca-ZSM-5. The size of the circles indicates the life of the zeolites.
Figure 8. Effect of additives on the acidity and XRD peak ratio of H-CaZSM-5. The size of circles indicates the life of the zeolites.
represents the length of the straight channels of the zeolite pores, relative to that of the zigzag channels. Figure 8 clearly shows that the length of the straight channel should be shorter with the stronger acidity, such as H-Ca-P-ZSM-5, to show the long life. The characteristics of zeolites with long life are located on a linear trend, as shown in the figure. The ANNs were trained using physicochemical properties as input data, and either the catalyst life, acidity, or morphology
Figure 9. Predicted and experimentally verified character of Y-, Tb-, Dyincorporated H-Ca-ZSM-5. Open circles represent the predicted values, and the closed red circles represent the experimental values. Gray circles show the training data in Figure 8.
of 11 types of zeolites as output data. The three trained ANNs predicted that Y-, Tb-, or Dy-incorporated H-Ca-ZSM-5 show adequate character for long life, as shown by the open circle in Figure 9. If the predictions of acidity and morphology index of these zeolites are correct, they would show long life, because their characters are on the same line in Figure 8. As shown by the arrows in the figure, however, the difference between the prediction data and the experimental data was large in the case of dysprosium and yttrium incorporation. The precision of acidity prediction of Dy-incorporated H-Ca-ZSM-5, and that of the predicted morphology of Y-incorporated H-Ca-ZSM-5, was not sufficient; therefore, the catalyst life was quite low (5 and 3 h, respectively). On the other hand, the predicted characters of Tb-incorporated H-Ca-ZSM-5 were similar to those observed, and the predicted life (25 h) was almost the same as the experimental result (23 h). Unfortunately, the catalyst life of Tb-incorporated H-CaZSM-5 was not longer than P- or B-incorporated ZSM-5. As a result, the catalyst that was used for the training data was the best. Such phenomena were often observed, because training elements are selected orthogonally20 for high dispersion in the search space with featured physicochemical properties. From the characterization of acidity and morphology, the origin of the positive effect of boron or phosphorus is different, and the synergistic effect of boron and phosphorus incorporation will be expected. In the next step, the Al/Si, Ca/Si, B/Si, and P/Si ratios were optimized for better performance. Optimization of Composition of H-Ca-B-P-ZSM-5. Data for ANN training to optimize the composition of H-Ca-B-P-ZSM-5 was designed by L9 orthogonal array for four factors with three levels. Table 3 shows the catalyst composition, the life, and C2-C4 olefin yield. Light olefin yield is almost the same (61-65 C-mol %), whereas the catalyst lives are not constant. This light olefin yield is almost the same as that in the case of Diluted-DTO,8 and the life of 42 h with Cat.7 is much longer than ever reported. After the ANN was trained by the results of Cat.1-Cat.9 in Table 3, grid search was conducted using the ANN. All the response surfaces were overlapped and illustrated as functions of the Si/Ca ratio and the Si/Al ratio in Figure 10. In these ranges, the optimum values of zeolite compositions are given as follows: Si/Al ) 200, Si/Ca ) 20, Si/P ) 400, and Si/B ) 182. These values were quite similar to those of
Ind. Eng. Chem. Res., Vol. 48, No. 13, 2009 Table 3. Design of H-Ca-ZSM-5 Composition by L9 Orthogonal Array for Neat-DTOa
Cat.1 Cat.2 Cat.3 Cat.4 Cat.5 Cat.6 Cat.7 Cat.8 Cat.9
Si/Al
Si/Ca
Si/P
Si/B
life (h)
C2-C4 olefin yield (%)
50 50 50 100 100 100 200 200 200
20 40 60 20 40 60 20 40 60
100 200 400 200 400 100 400 100 200
100 200 400 400 100 200 200 400 100
14.5 11.5 3.0 12.0 19.0 3.5 42.0 9.0 5.5
63.7 64.8 64.4 64.2 63.0 62.1 64.5 65.0 61.7
a Silicon source ) fumed silica, reaction temperature ) 823 K, GHSV ) 2200 h-1.
Figure 10. Overlapped response surface of catalyst life as a function of the Si/Ca ratio and the Si/Al ratio. Table 4. Catalyst Life of Cat.7 and Cat.10 for Neat-DTO GHSV (h-1) temperature (K) life (h) C2-C4 olefin yield (%) Cat.7 Cat.7 Cat.10 Cat.10
2200 1000 1000 1000
823 823 823 803
42 40 82 146
64.5 64.6 64.7 63.9
Cat.7, and at the edges of the ranges of each level of the orthogonal array. Therefore, we extended the ratios as Cat.10 to Si/Al ) 250, Si/Ca ) 20, Si/P ) 400, and Si/B ) 200. The life of Cat.10 was compared to that of Cat.7 under a variety of reaction conditions, as shown in Table 4. Despite a slight change in the SiO2/Al2O3 ratio, the life of Cat.10 is much longer than that of Cat.7 (more than double). This result indicated that lower acid site density critically affected catalyst life around this. The catalyst life of Cat.10 (Si/Al ) 250, Si/Ca ) 20, Si/P ) 400, Si/B ) 200) reached 146 h when the GHSV was 1000 h-1 at 803 K. This is almost the same as that of the catalyst supplied to the reaction of diluted DME, and there is no report to our knowledge that the complete conversion was maintained for such a long time using the high-partial-pressure DME. Conclusion Zeolite catalysts for light olefin synthesis from neat dimethyl ether (DME) have been developed. Light olefin synthesis from neat DME suffered from poor light olefin selectivity and severe
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coke deposition on H-ZSM-5. On the other hand, Caincorporated ZSM-5 indicated superior performance. The addition of calcium weakens the acid strength of H-ZSM-5, and increase the amount of acid. By incorporating boron and phosphorus, and by optimization of the content, the catalyst life was prolonged to 146 h at 803 K and GHSV ) 1000 h-1. Literature Cited (1) Dewaele, O.; Geers, V. L.; Froment, G. F.; Marin, G. B. The conversion of methanol to olefins: A transient kinetic study. Chem. Eng. Sci. 1999, 54, 4385–4395. (2) DME Handbook (In Jpn.; edited and published by Japan DME Forum); Toranomon, Minato-ku, Tokyo, Japan, 2006; pp 328-336. (3) Keil, F. J. Methanol-to-hydrocarbons: Process technology. Microporous Mesoporous Mater. 1999, 29, 49–66. (4) Gayubo, A. G.; Aguayo, A. T.; Olazar, M.; Vivanco, R.; Bilbao, J. Kinetics of the irreversible deactivation of the HZSM-5 catalyst in the MTO process. Chem. Eng. Sci. 2003, 58, 5239–5249. (5) Sto¨cker, M. Methanol-to-hydrocarbons: Catalytic materials and their behavior. Microporous Mesoporous Mater. 1999, 29, 3–48. (6) Chen, J. Q.; Bozzano, A.; Glover, B.; Fuglerud, T.; Kvisle, S. Recent advancements in ethylene and propylene production using the UOP/Hydro MTO process. Catal. Today 2005, 106, 103–107. (7) Inui, T.; Matsuda, H.; Yamase, O.; Nagata, H.; Fukuda, K.; Ukawa, T.; Miyamoto, A. Highly Selective Synthesis of Light Olefins from Methanol on a Novel Fe-Silicate. J. Catal. 1986, 98, 491–501. (8) Zhao, T. S.; Takemoto, T.; Tsubaki, N. Direct synthesis of propylene and light olefins from dimethyl ether catalyzed by modified H-ZSM-5. Catal. Commun. 2006, 7, 647–650. (9) Vedrine, J. C.; Auroux, A.; Dejaifce, P.; Ducarme, V.; Hoser, H.; Zhou, S. Catalytic and Physical Properties of Phosphorus-Modified ZSM-5 Zeolite. J. Catal. 1982, 73, 147–160. (10) Tynja¨la¨, P.; Pakkanen, T. T. Modification of ZSM-5 zeolite with trimethyl phosphite. Part 1. Structure and acidity. Microporous Mesoporous Mater. 1998, 20, 363–369. (11) Tynja¨la¨, P.; Pakkanen, T. T.; Mustama¨ki, S. Modification of ZSM-5 zeolite with Trimethyl Phosphite. Part 1. Catalytic Properties in the Conversion of C1-C4 Alcohols. J. Phys. Chem. B 1998, 102, 5280–5286. (12) Kawamura, K.; Noguchi, K.; Murakami, T.; Sano, T.; Takaya, H. Nippon Kagaku Kaishi 1990, 824. (13) Okado, H.; Shoji, H.; Kawamura, K.; Kohtoku, Y.; Yamazaki, Y.; Sano, T.; Takaya, H. Nippon Kagaku Kaishi 1987, 25. (14) Katada, N.; Miyamoto, T.; Begum, H. A.; Naito, N.; Niwa, M. Strong Acidity of MFI-Type Ferrisilicate Determined by TemperatureProgrammed Desorption of Ammonia. J. Phys. Chem. B 2000, 104, 5511– 5518. (15) Barin, I.; Sauert, F.; Schultze-Rhonhof, S.; Sheng, W. Thermochemical Data of Pure Substances, 2nd Edition; VCH: Weinheim, Germany, 1993. (16) Chang, C. D.; Lang, W. H. Process for manufacturing olefins, U.S. Patent 4,025,575, 1977. (17) Chang, C. D. Hydrocarbon from Methanol. Catal. ReV.: Sci. Eng. 1983, 25 (1), 1–118. (18) Chang, C. D. Methanol Conversion to Light Olefins. Catal. ReV.: Sci. Eng. 1984, 26, 323–345. (19) Omata, K.; Sutarto; Hashimoto, M.; Ishiguro, G.; Watanabe, Y.; Umegaki, T.; Yamada, M. Design and development of Cu-Zn oxide catalyst for direct dimethyl ether synthesis using an artificial neural network and physicochemical properties of elements. Ind. Eng. Chem. Res. 2006, 45, 4905–4910. (20) Omata, K.; Kobayashi, Y.; Yamada, M. Artificial neural network aided virtual screening of additives to a Co/SrCO3 catalyst for preferential oxidation of CO in excess hydrogen. Catal. Commun. 2007, 8, 1–5.
ReceiVed for reView November 18, 2008 ReVised manuscript receiVed April 20, 2009 Accepted May 5, 2009 IE801757P
