Influence of Catalytic Functionalities of Zeolites on Product

VOLUME 23

FEBRUARY 2009 Copyright 2009 by the American Chemical Society

Articles Influence of Catalytic Functionalities of Zeolites on Product Selectivities in Methanol Conversion Seung-Chan Baek,† Yun-Jo Lee,† Ki-Won Jun,*,† and Suk Bong Hong‡ AlternatiVe Chemicals/Fuel Research Center, Korea Research Institute of Chemical Technology (KRICT), Post Office Box 107 Yuseong, Daejeon 305-343, South Korea, and Department of Chemical Engineering and School of EnVironmental Science and Engineering, POSTECH, Pohang, Kyungbuk 790-784, South Korea ReceiVed September 2, 2008. ReVised Manuscript ReceiVed NoVember 26, 2008

In an attempt to understand the textural and acidic properties of zeolites on the methanol conversion and product selectivities, three zeolites, ferrierite, clinoptilolite, and SAPO-34, are studied. Temperature-programmed desorption of ammonia (NH3-TPD) indicated the presence of more weak acidity on ferrierite and clinoptilolite compared to the considerable strong acidity on SAPO-34. At 250 °C, all three samples exhibited comparable methanol conversions, but relatively lower dimethyl ether (DME) and higher light olefins are observed on SAPO-34. Ferrierite and clinoptilolite samples exhibited high selectivity for the formation of DME even at higher reaction temperatures (400 °C), while the product shifted from DME to light olefins with an increasing reaction temperature on SAPO-34. Especially, ferrierite, owing to its mild acidity and small pore size, is found to be a good catalyst for the methanol conversion to DME at 250 °C without significant deactivation for 100 h time on stream.

1. Introduction Dimethyl ether (DME) is an important chemical constituent for the production of a wide spectrum of chemicals, such as light olefins, aromatics, gasoline, and dimethyl sulfate.1 Moreover, by virtue of its qualities, such as thermal efficiency equivalent to traditional diesel fuel, lower NOx emission, near zero smoke, lesser carbon particulates, and lesser engine noise, DME has been widely accepted as an environmentally friendly fuel that is an alternative to diesel.2,3 Commercially pure or phosphoric-acid-modified γ-alumina is applied for the production of DME from methanol. An advent * To whom correspondence should be addressed. E-mail: kwjun@ krict.re.kr. † Korea Research Institute of Chemical Technology (KRICT). ‡ POSTECH. (1) Xu, M.; Lunford, J. H.; Goodman, D. W.; Bhattacharyya, A. Appl. Catal., A 1997, 149, 289. (2) Yaripour, F.; Baghaei, F.; Schmidt, I.; Perregaard, J. Catal. Commun. 2005, 6, 147. (3) Kumar, V. S.; Padmasri, A. H.; Satyanarayana, C. V. V.; Reddy, I. A. K.; Raju, B. D.; Rao, K. S. R. Catal. Commun. 2006, 7, 745.

of zeolites has led to the development of zeolite-based catalysts and processes for several hydrocarbon conversions.4-10 Zeolites are also employed for the conversion of methanol to olefins, where the formation of olefins is observed to progress through a DME intermediate. The methanol-to-olefins (MTO) process employing SAPO-34 or ZSM-type catalysts has been investigated for more than 30 years. SAPO-34 is considered as a promising catalyst, which gives high selectivity of ethylene and propylene, because of its suitable acidity and pore size restriction (4) Kim, J. H.; Park, M. J.; Kim, S. J.; Joo, O. S.; Jung, K. D. Appl. Catal., A 2004, 264, 37. (5) Kim, S. D.; Baek, S. C.; Lee, Y.-J.; Jun, K.-W.; Kim, M. Y.; Yoo, I. S. Appl. Catal., A 2006, 309, 139. (6) Vishwananthan, V.; Jun, K.-W.; Kim, J.-W.; Roh, H.-S. Appl. Catal., A 2004, 276, 251. (7) Mao, D.; Yang, W.; Xia, J.; Zhang, B.; Song, Q.; Chen, Q. J. Catal. 2005, 230, 140. (8) Fei, J.; Hou, Z.; Zhu, B.; Lou, H.; Zheng, X. Appl. Catal., A 2006, 304, 49. (9) Fei, J.-H.; Tang, X.-J.; Huo, Z.-Y.; Lou, H.; Zheng, X.-M. Catal. Commun. 2006, 7, 827. (10) Jin, D.; Zhu, B.; Hou, Z.; Fei, J.; Lou, H.; Zheng, X. Fuel 2007, 86, 2707.

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in MTO conversion.11 Very recently, we have observed that the SAPO-34 catalyst prepared by using the mixed template of morpholine and tetraethylammonium hydroxide can exhibit high activity and selectivity in the MTO reaction.12 A proton type of ferrierite is also proven to be a promising catalyst for its break through performance in skeletal isomerization of 1-butene to isobutene.13 A series of studies conducted by Aramendia et al. revealed the importance of structure and topology of zeolites on the hydrocarbon product distribution in methanol conversion, where zeolite ferririte is observed to be selective for the production of alkenes, especially pentene and 2-methyl propene from methanol.14 Clinoptilolite is another small pore zeolite that is widely used because of its selective cation-exchange properties and its widespread availability in nature.15 Recent studies of Royaee et al. revealed the interesting aspects of Iranian clinoptilolite that has exhibited excellent performance in the MTD reaction after suitable catalyst modifications.16,17 The properties of clinoptilolite also may vary with the place of its origin, Further, the information available for understanding the structure and properties of the abovementioned three types of zeolites related to the production of olefins and DME in methanol conversion is not clearly documented in the literature. The present study is thus aimed to explore the possibility of using ferrierite and clinoptilolite zeolites as catalysts for methanol conversion, where the properties and catalytic activities of these samples are compared to that of SAPO-34, which is known for its superior activity toward the MTO reaction. The main purpose of the study is to understand the catalytic factors governing the product selectivities toward olefins or DME. Acidity and porosity are two important parameters that can influence the catalytic activity of zeolites. Clinoptilolite and ferrierite possess elliptical pores with 10- and 8-membered ring pore channels (pore size of 2.8-7.5), whereas ferrierite possesses near circular of 8-membered ring pore openings with a size of ∼3.8 Å. In methanol conversion to DME and light olefins, the molecules involved are of smaller magnitude and thus may not be influenced by porosity but by the acidity of zeolites. Focus is given in this study to optimize the catalytic as well as reaction parameters to obtain an active and stable catalyst for the production of DME. Interesting aspects of acidity and reaction temperature and their influence on product selectivity toward DME or light olefins has been realized through the systematic characterization and reaction studies. 2. Experimental Section 2.1. Catalysts Preparation. The SAPO-34 with sub-micrometer particle size was prepared by the method described by Mertens and Strohmeier.18 Synthesis was carried out in a 500 cm3 stainlesssteel reactor lined with Teflon at autogenous pressure with vigorous stirring. The molar composition of the final gel was Al2O3/P2O5/ 0.30 SiO2/2.0 tetraethylammonium hydroxide/1.59 dipropylamine/ 52 H2O. (11) Sto¨cker, M. Microporous Mesoporous Mater. 1999, 29, 3. (12) Lee, Y.-J.; Baek, S.-C.; Jun, K.-W. Appl. Catal., A 2007, 329, 130. (13) Lee, S.-H.; Shin, C.-H.; Hong, S. B. J. Catal. 2004, 223, 200. (14) Aramendia, M. A.; Borau, V.; Jimenez, C.; Marinas, J. M.; Roldan, R. Chem. Lett. 2002, 673. (15) Concepcion-Rosabal, B.; Rodriguez-Fuentes, G.; Bogdanchikova, N.; Bosch, P.; Avalos, M.; Lara, V. H. Microporous Mesoporous Mater. 2005, 86, 249. (16) Royee, S. J.; Sohrabi, M.; Falamaki, C. Mater. Sci. (Poland) 2007, 25, 1149. (17) Royee, S. J.; Falamaki, C.; Sohrabi, M.; Talesh, S. S. A. Appl. Catal., A 2008, 338, 114. (18) Mertens, M.; Strohmaier, K. G. U.S. Patent 6,773,688, 2004.

Baek et al. Hydrothermal crystallization was performed at 175 °C for 48 h. After crystallization, the solid product was separated from the mother liquor by centrifugation and washed several times with water. The product was then dried overnight at 120 °C. Assynthesized product was slowly heated to 550 °C and allowed to calcine for 24 h at this temperature before using it for the catalytic reaction. The ammonium form of ferrierite (Tosoh chemical, Si/Al ) 10) was calcined in air at 600 °C for 6 h to obtain the proton form. The potassium form of clinoptilolite (Tosoh chemical, Si/Al ) 6) was also converted into its proton form by refluxing twice in 1.0 M of ammonium nitrate solution overnight, followed by its calcination in air at 600 °C for 6 h. The proton forms of zeolites are employed for the reaction. 2.2. Characterization of the Catalysts. The X-ray diffraction (XRD) patterns of the synthesized samples were recorded on a Rigaku D/MAX IIIB X-ray diffractometer with Cu KR radiations. The morphology of the SAPO-34 catalyst was observed with a scanning electron microscope (Philips XL 30S FEG microscope). The surface acidity of the samples were conducted by temperatureprogrammed desorption of ammonia (NH3-TPD) using a BELCAT PCI 3135 with a thermal conductivity detector (TCD). In a typical analysis, 0.2 g of the calcined sample was pretreated to remove adsorbed water at 500 °C for 3 h and then saturated with ammonia at 100 °C for 1 h. After saturation, the sample was purged with helium for 30 min to remove the weakly adsorbed ammonia on the surface of the catalyst. The temperature of the sample was then raised at a heating rate of 10 °C/min from 100 to 700 °C. The amount of ammonia desorbed from the catalyst was measured by comparing peak areas of the standard sample. The Brunauer-Emmett-Teller (BET) surface area, pore volume, and pore diameter were measured by N2 adsorption-desorption isotherm at -196 °C using TriStar 3000 (Micromeritics). Prior to the adsorption-desorption measurements, all of the samples were degassed at 300 °C in a vacuum for 4 h. The elemental compositions of catalysts were obtained from X-ray fluorescence (XRF) analyses conducted on SEA 5120 (Seiko Instruments, Inc.). 2.3. Catalytic Activity. Catalytic activities of all samples for the dehydration of methanol were carried out in a fixed bed microreactor (316 stainless-steel tubing, inner diameter ) 1 cm, and length ) 30 cm) and at atmospheric pressure in the temperature range of 250-450 °C. In a typical experiment, prior to each experiment, the catalyst was activated for 1 h at 500 °C in a He flow. A methanol with a liquid flow rate of 0.01 mL/min was fed by a syringe pump into a preheater at 100 °C. He as an inert diluent gas was co-fed with methanol into the reactor at a flow rate of 60 mL/min (MeOH/He ) 1:11, v/v), and the reaction was carried out under weight hourly space velocity (WHSV) ) 1 h-1. Effluent gas from the reactor was analyzed by an online gas chromatograph (Donam GC 6100) employing GS-Q capillary columns for hydrocarbons and oxygenates, respectively, with FID detection and Porapak Q-packed columns for carbon oxides with TCD detection. Product compositions were calculated on the basis of a standard gas mixture.

3. Results and Discussion 3.1. Physico-chemical Properties of Samples. The X-ray diffraction patterns of all three zeolites shown in Figure 1 are consistent with the reported structural patterns and confirm the structures of SAPO-34, ferrierite, and clinoptilolite, respectively. Among these, SAPO-34 and clinoptilolite exhibit lower intensities. The line broadening of XRD peaks observed in SAPO-34 suggests the formation of smaller crystals in this sample. The SEM images shown in Figure 2a indeed support the formation of cube-shaped aggregates of nanosized crystals (50-200 nm) in the SAPO-34 sample as the reason for its lower intensity of XRD peaks. However, the line broadening and intensity reduction observed in clinoptilolite may be due its lower crystallinity.

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Figure 1. X-ray powder diffraction patterns of (a) H-SAPO-34, (b) H-ferrierite, and (c) H-clinoptilolite.

Other two samples, ferrierite and clinoptilolite, possessed bigger crystals with micrometer range size particles with different shapes: ferririte with rice-grain-type particles (Figure 2b) and the clinoptilolite with thin-plate-like particles (Figure 2c). The samples also exhibited differences in textural properties (Table 1). SAPO-34 stands superior in terms of its higher surface area and micropore volume that might be originated from the presence of nanosized crystallites in this sample. The clinoptilolite sample exhibits a much lower surface area and micropore volume that are in agreement with its lower crystallinity observed in its XRD patterns.19,20 The acidic properties of the samples are estimated by NH3-TPD, and the ammonia desorption profiles of all of the samples are shown in Figure 3. SAPO-34 exhibits the ammonia desorption at two distinct temperatures; a low-temperature peak at 187 °C and a high-temperature peak at 415 °C correspond to the weak and strong acidity of the sample. The results are in accordance with the reported literature data for the acidity of SAPO materials.21 The other two samples, ferrierite and clinoptilolite, possess similar desorption trends at low temperature; however, the high-temperature desorption of ammonia in this case is much lower, and ammonia desorbs in a broad range of temperatures from 300 to 600 °C. Among the three samples, clinoptilolite exhibits the highest intensity of the lowtemperature band representing the weak acidity. NH3-TPD of ferrierite showed a similar desorption pattern to that of clinoptilolite. However, the peak in the weak acid site region showed a relatively weaker intensity because of a lower acidic density caused by its higher framework Si/Al ratio. A little stronger intensity of the high-temperature band was also observed in ferrierite when compared to the clinoptilolite. The data provided in Table 1 help for the quantitative measure of acidity, where the ammonia desorbed at two temperatures has been estimated for all of the samples. The total acidity of the samples is in the order of SAPO > clinoptilolite > ferrierite. The strong acidity is more in SAPO-34, but the other two samples exhibited much lower values of strong acidity. Although it is difficult to compare the acidity of P containing SAPO-34 with that of the alumino-silicate framework, samples ferrierite and clinoptilolite and the higher Si/Al ratio of ferrierite and clinoptilolite may be responsible for their lower values of the total as well as strong acidity. Potassium is also known to mask (19) Pluth, J. J.; Smith, J. V. J. Phys. Chem. 1989, 93, 6516. (20) Treacy, M. M. J.; Higgins, J. B. Collection of Simulated XRD Powder Patterns for Zeolites; Elsevier: New York, 2007; p 180. (21) Parlitz, B.; Schreier, E.; Zubowa, H.-L.; Eckelt, R.; Lieschke, E.; Fricke, R. J. Catal. 1995, 155, 1.

Figure 2. SEM images of (a) H-SAPO-34, (b) H-ferrierite, and (c) H-clinoptilolite.

the strong acidity in zeolites, such as clinoptilolite.22,23 As is given in Table 1, the XRF analysis indicates the presence of a considerable amount of K ion in ferrierite and clinoptilolite that is equivalent to the masking of 26 and 39% of protons in these samples, respectively, to cause the decrease in strong acidity. Thus, the high-potassium-containing clinoptilolite exhibits the least amount of strong acid sites and most of the weak acid sites. Overall, these factors resulted in lower acidity of ferrierite and clinoptilolite samples. 3.2. Performance of Catalysts in the Methanol Dehydration Reaction. The conversion of methanol is known to produce a variety of products, such as DME, light olefins, polyolefins, (22) Lee, H. C.; Woo, H. C.; Chung, S. H.; Kim, H. J.; Lee, K. H.; Lee, J. S. J. Catal. 2002, 211, 216. (23) Song, Y.; Zhu, X.; Xie, S.; Wang, Q.; Xu, L. Catal. Lett. 2004, 97, 31.

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Baek et al. Table 1. Physicochemical Properties of the Catalyst Samples desorbed ammonia (mmol/g)a

XRF analysis (mol ratio) catalyst

Al

Si

P

K

BET surface area (m2/g)b

micropore volume (cm3/g)

weak (I)c

strong (II)d

total (I + II)

H-SAPO-34 H-FER H-CLI

1.00 1.00 1.00

0.18 10.16 5.47

0.95 0 0

0 0.26 0.39

680 364 214

0.24 0.13 0.07

0.79 0.67 1.05

0.75 0.33 0.31

1.54 1.00 1.36

a

Analyzed by NH3-TPD. b Measured by N2 adsorption at -196 °C. c Temperature range of 100-300 °C. d Temperature range of 300-600 °C.

Figure 3. NH3-TPD profiles for calcined samples: (a) H-SAPO-34, (b) H-ferrierite, and (c) H-clinoptilolite. Table 2. Reaction Results of Methanol Conversiona

catalyst

H-SAPO-34

H-FER

H-CLI

product distribution (%) at time-on-stream ) 1 h MeOH temperature conversion selectivity selectivity selectivity of (°C) (%) of DME of C22--C42- saturated HCs 250 300 350 400 450 250 300 350 400 450 250 300 350 400 450

91 100 100 100 100 91 88 85 85 83 97 90 90 88 96

85.6 28.4 0.0 0.0 0.0 100.0 88.2 86.0 87.3 85.6 99.9 98.7 98.9 93.8 31.9

11.7 61.8 84.7 89.1 91.7 0.0 3.2 3.7 0.9 0.6 0.1 0.8 0.7 4.3 53.9

2.7 9.8 15.3 10.9 8.3 0.0 8.6 10.3 11.8 13.8 0.0 0.5 0.4 1.9 14.2

a Reaction conditions: catalyst amount, 0.49 g; WHSV (MeOH), 1 h-1; MeOH/He, 1:11 (mol/mol).

aromatics, and even the undesirable coke precursors, depending upon the acid strength and reaction severity. The three catalysts with different textural properties are thus studied for their performance in methanol conversion at various reaction temperatures to understand the factors influencing the formation of DME and light olefins, which are two important chemical commodities desired for industrial applications. 3.2.1. Effect of the Reaction Temperature on the Performance of Catalysts. The methanol conversion studies are conducted on all of the catalysts at various reaction temperatures, and the products obtained after 1 h of reaction time are summarized in Table 2. At 250 °C, all of the catalysts exhibited more than 90% conversion to produce DME as a major product. Ferrierite and clinoptilolite catalysts exhibited ∼100 selectivity to DME, whereas about 85% selectivity to DME along with 11% selectivity to light olefins (C22--C42- olefins) is observed over the SAPO-34. The higher acid density and strong acidity in SAPO-34 may be responsible for the production of olefins as byproducts. The reaction temperature greatly influenced the product selectivity on the H-SAPO-34 catalyst. Interestingly, the product

is shifted from DME to light olefins (C22--C42- olefins) with an increasing reaction temperature. At 300 °C, the selectivity to DME is decreased to 28%, while that of light olefins is increased to 62%, envisioning the transformation of the reaction mechanism from DME to light olefins. A further increase of the reaction temperature caused a complete disappearance of DME, and the major component observed in this case is light olefins (∼85%), with a small amount of saturated hydrocarbons (∼15%). Overall, the SAPO-34 catalyst exhibited the temperature-dependent product selectivity in methanol conversion. Deviating from the behavior of SAPO-34, ferrierite and clinoptilolite catalysts have exhibited a consistency in product selectivity throughout the temperature range of 250-450 °C, where DME is the main product observed at all of the temperatures on these catalysts. However, a slight decrease in methanol conversion is observed on these catalysts at higher reaction temperatures. This may be due to the fast deactivation rates of highly active sites in the catalyst at higher reaction temperatures. Nevertheless, the selectivity to DME was maintained at more than 85% on these catalysts even at higher reaction temperatures. The observed consistency of DME selectivity in these catalysts can be explained on the basis of the acidity. The acidity data indicates the presence of only a small fraction of strong acid sites, with a major amount of weak acid sites in these catalysts. Thus, the deactivation of relatively a small fraction of strong acid sites can have a negligible influence on the catalytic activity, with weak acid sites being the main source for the formation of DME on these catalysts. This situation strongly suggests that methanol to DME conversion is mainly catalyzed by weak acid sites. Another interesting observation revealed from the above results is the change in behavior of strong and weak acid sites with the reaction temperature. To understand this aspect in a detailed manner, the performances of all of the three catalysts are studied for a 5 h time on stream at two reaction temperatures: 250 and 400 °C. 3.2.2. Low-Temperature Performance of Catalysts. The timeon-stream performance of clinoptilolite, ferrierite, and SAPO34 at 250 °C is given in parts a-c of Figure 4, respectively. Cliniptilolite exhibited consistency in DME selectivity with a slight decrease in MeOH conversion with time on stream. Ferrierite catalyst also exhibited constant MeOH conversion and DME selectivity for 5 h (Figure 4b). SAPO-34 also exhibited comparable MeOH conversion, but the formation of light olefins also facilitated along with DME at the initial time of reaction on this catalyst. However, the catalyst showed a decrease in MeOH conversion and light olefin selectivity with a simultaneous increase of DME selectivity after 2 h of reaction time (Figure 4c) that clearly suggests the phenomenon of the reaction shift from strong-acidity-catalyzed olefin formation to weak-acidity-catalyzed DME formation facilitated after the fast deactivation of strong acid sites in this catalyst. 3.2.3. High-Temperature Performance of Catalysts. The timeon-stream behavior of the catalysts at higher reaction temper-

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Figure 4. Performance of catalysts at two reaction temperatures: (a-c) reaction at 250 °C and (d-f) reaction at 400 °C. (b) Conversion of methanol, (O) selectivity to DME, (9) selectivity to C22--C42-, and (0) selectivity to saturated hydrocarbons. Reaction conditions: feed, MeOH/He ) 1:11 (molar ratio); WHSV (MeOH), 1 h-1.

ature (400 °C) is also given in parts d-f of Figure 4. It is interesting to note that the weak-acid-containing catalysts, clinoptilolite and ferrierite, also exhibited the tendency to form light olefins at higher reaction temperatures. At the early stage of the reaction, the main product formed on these two catalysts is light olefins instead of DME at 400 °C. A relatively high yield of saturated hydrocarbons are also observed at this condition. However, after 2 h on stream, the product is completely shifted from light olefins to DME and the only product observed at this juncture is DME. This observation again suggests the role of highly active sites (strong acidity) to form olefins rather than DME at higher reaction temperatures. After 2 h of time on stream, the deactivation of a small fraction of highly active sites left a situation to progress the reaction only on weak acid sites and, hence, the product is shifted from strongacidity-catalyzed olefin formation to weak-acidity-catalyzed DME formation. However, on SAPO-34, the main product formed is light olefins in the entire reaction period of 5 h. It is important to note that the same catalyst exhibited a decrease in olefin

selectivity with time on stream at 250 °C (Figure 4c). That means that higher reaction temperatures are favorable for the formation of light olefins on the strong acid catalyst, SAPO34. The presence of strong acid sites also facilitated the constant selectivity to light olefins for the entire time period of 5 h. On the other hand, the small fraction of strong acid sites in clinoptilolite and ferrierite make them active for light olefin formation at the initial time of the reaction. However, such as small fraction of strong acid sites are deactivated within a short time of reaction (2 h), and the product selectivity is completely shifted to the DME formation on the weak acid sites. This situation emphasizes the role of acidity as a main factor to determine the nature of the product, namely, DME and light olefins, which is discussed in the following session. 3.2.4. Role of Acidity. Considering the differences in product selectivity over all of the three catalysts and the shifting of the selectivity from one product to the other depending upon the reaction temperature and acidity of

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catalysts, the role of acidity can be summarized as follows: (1) The high amount of strong acidity in SAPO-34 is responsible for its preferential product selectivity toward light olefins rather than DME, with such behavior being dominant at higher reaction temperatures. (2) The deactivation of strong acidity in SAPO-34 affects the MeOH conversion and light olefin selectivity but not the DME selectivity, suggesting the weak acidity as an active site for DME formation. (3) A small amount of strong acid sites present in clinoptilolite and ferrierite are not active for light olefin formation at lower reaction temperatures but are active at higher reaction temperatures. Such a small amount of active sites capable of producing light olefins deactivates faster at higher reaction temperatures, and then the reaction proceeds on weak acid sites for the preferential formation of DME. (4) The strong acid sites undesirable for DME production can also be controlled by introduction of an appropriate amount of base metals, such as alkali and alkaline earth metals.20 In the present study, the presence of potassium ions in clinoptilolite and ferrierite seems to play a similar role by masking the strong acidity and the resultant catalysts with weak acidic property (Figure 3), exhibiting higher DME selectivity. (5) Both clinoptilolite and ferrierite contain abundant weak acid sites, which are suitable to form DME instead of light olefins during the methanol dehydration reaction, whereas SAPO34 additionally contains a lot of strong acid sites, which are strong enough to convert MeOH to light olefins even at moderate reaction temperatures. Considering the importance of acidity, the lowest strong acidity containing a ferrierite catalyst is selected for its performance toward DME formation for longer reaction times. As shown in Figure 5, the catalyst performed remarkable DME selectivity of more than 99.95% for more than 100 h at 250 °C. At similar reaction conditions, SAPO-34 showed the lifetime of less than 10 h. The fast deactivation of SAPO-34 may be due to the formation of the olefinic product, which can act as a coke precursor. The yield of carbon oxides (CO and CO2) is negligible on all of the catalysts and reaction temperatures that are advantageous for the selective production of DME. 4. Conclusions Acidic property of medium-pore zeolites created remarkable differences in the product selectivities in methanol

Baek et al.

Figure 5. Stability in catalytic activity of H-ferrierite in the methanol to DME reaction: (b) MeOH conversion and (2) DME selectivity. Reaction conditions: feed, MeOH/He ) 1:11 (molar ratio); WHSV (MeOH), 1 h-1; reaction temperature, 250 °C.

conversion. Weak acidity and lower reaction temperatures are favorable for the formation of DME, whereas strong acidity and higher reaction temperatures are favorable for the formation of light olefins in methanol conversion. The SAPO-34 catalyst with considerable strong acidity exhibits an interesting phenomenon of shifting product selectivity from strong-acidity-catalyzed light olefins to weak-aciditycatalyzed DME after the fast deactivation of strong acid sites at 250 °C. The ferrierite and clinoptilolite exhibit preferential formation of DME by virtue of their weak acidic property, originated by partial exchange of potassium ions to the proton form. Especially, the ferrierite catalyst exhibited excellent performance with near 100% selectivity for the DME in the studied period of 100 h. The SAPO-34 catalyst with both weak and moderate strong acid sites exhibited superior activity for the conversion of methanol into light olefins. Acknowledgment. This research was supported by a Grant (CA2-101-1-0-1) from Carbon Dioxide Reduction and Sequestration Center (CDRS), one of 21st Century Frontier Programs funded by the Ministry of Science and Technology, Republic of Korea. EF800736N