Catalytic dehydration of methanol over synthetic zeolite W

Microporous and Mesoporous Materials 128 (2010) 108–114

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Catalytic dehydration of methanol over synthetic zeolite W Yeong-Hui Seo a, Eko Adi Prasetyanto a, Nanzhe Jiang a, Soon-Moon Oh b,*, Sang-Eon Park a,* a

Laboratory of Nano-Green Catalysis and Nano Center for Fine Chemicals Fusion Technology, Department of Chemistry, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, South Korea b Department of Sanitary and Environmental System Engineering, Gachon University of Medicine and Science, 534-2 Yeonsu-dong, Yeonsu-gu, Incheon 406-799, South Korea

a r t i c l e

i n f o

Article history: Received 21 May 2009 Received in revised form 20 July 2009 Accepted 10 August 2009 Available online 14 August 2009 Keywords: Zeolite W MER Microwave synthesis 8 MR Methanol to dimethyl ether

a b s t r a c t High silica containing zeolite W having silica to alumina ratio of 6.4 was synthesized in solely KOH system using both conventional hydrothermal and microwave methods. The synthetic zeolite W with twinball type morphology was obtained from conventional hydrothermal synthesis and microwave synthesis gave prismatic morphology. Addition of ethylene glycol (EG) in the precursor gel affected to get more uniform morphologies of zeolite W in both hydrothermal and microwave synthesis. The obtained zeolite W was applied to the catalytic dehydration of methanol to dimethyl ether (DME) with 100% selectivity as well as high stability due to the mild acidity of zeolite W. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction Recently, small-pored zeolites (8 MR) have revoked in the research area of nanoporous materials due to their significant applications such as H2 storage, CO2 and H2O removal, membrane separation, water purification as well as the selective catalytic reactions of methanol to olefins or methanol to DME [1–6]. Merlinoite (MER topology, crystal phase of zeolite W) is one of the small-pored zeolites characterized by a low Si/Al ratio and has three-dimensional interconnected 8-membered ring channel system [7]. Such low Si/Al ratio zeolites which are used to be synthesized from aluminosilicate gels in alkaline condition are highly dependent on the cationic species in the precursor gels [8]. For example, the precursor gel containing Na and K cations, which normally gives zeolite A, crystallizes to zeolite RHO if Cs ions are added [9]. Similarly, incorporation of Sr in the precursor gel leads to zeolite ZK-5 instead of zeolite L or W [10]. Lillerud’s group reported that a high potassium concentration was the key factor to facilitate the formation of pure MER [11]. Zeolites such as phillipsite, rhodesite, analcime and nepheline were formed with increasing NaOH concentration, while MER and sanidine grew in KOH solution [12]. It was also reported that zeolite W could be prepared by the excess alkalinity which OH/SiO2 should be greater than 1.4, otherwise LTL zeolite was obtained [13]. Other method was suggested by Quirin et al. who was able to crystallize MER phase combining the use of an organocation ‘template’ molecule and zeolite * Corresponding authors. Tel.: +82 32 860 7675; fax: +82 32 872 8670 (S.-E. Park). E-mail addresses: [email protected] (S.-M. Oh), [email protected] (S.-E. Park). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.08.011

K–Y as the aluminum source [14]. And recently, Thoma reported the use of organometallic agents for synthesizing zeolite W [15]. Generally to obtain zeolite W, aluminosilicate solution with certain Na and K concentration ratio was utilized as a precursor [16]. But introduction of two or more kinds of metal cations may cause several drawbacks in synthesis such as inhomogenity of active site, different accessible pore size and broadening in product distribution [17,18]. To overcome this limitation, we tried to synthesize zeolite W by using potassium only rather than mixture with sodium, calcium and other alkali atoms without organic or organometallic templates. So far, there is no report on the microwave synthesis of zeolite W crystallization both with and without template. Here, we are reporting successful preparation method of zeolite W in solely KOH system by using both conventional hydrothermal and microwave methods in the presence of ethylene glycol (EG). EG was used as heat dissipater during zeolite synthesis in order to get uniform morphology of zeolite crystals. Likewise typical advantages of microwave synthesis, zeolite W was also synthesized in a short period of time with uniform morphology due to homogeneous and selective heating throughout a reaction vessel [19–21]. MER-type zeolites typically had Si/Al ratios of less than 2.5 [22,23]. According to Flanigen’s notation, MER is included in intermediate Si/Al zeolites defined as having 2 < Si/Al 6 5 [24]. Recent studies on the synthesis of MER-type zeolites were shown that the use of an additional organic cation yields a product with an enhanced Si/Al ratio as high as 3.8 and possibly larger [7,14,25]. Mild acidity was expected by getting higher Si/Al ratio of 6.4 than previously reported zeolite W and applied in the selective dehydration of methanol to DME. And the small-pore and its intrinsic

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acidity were ascribed to the high selectivity on DME with high stability. DME is a useful chemical intermediate for the preparation of many important chemicals, such as dimethyl sulfate and high-value oxygenated compounds as well as renewable fuel as a substitute for diesel [3]. The commercial importance of DME has increased considerably in recent years and more attention has been focused on deriving fuels and chemicals from synthesis gas. DME is extremely clean-burning, does not form peroxides as do higher ethers, low NO emission, near-zero smoke and less engine noise compared with those of traditional diesel fuels [26]. Wang et al. reported methanol dehydration in the LPDMEä (liquid-phase dimethyl ether) process over molecular sieves with weaker Brønsted acidity such as SAPOs, ALPOs, and boron-substituted zeolites. However these zeolites were not active enough and/or deactivated rapidly due to wide pore and strong acidity [6]. In this paper, we propose that zeolite W could be applied to the DME production through the selective dehydration of methanol. To the best of our knowledge, this is the first example of zeolite W utilized in gas-phase selective catalytic reaction for the DME production from methanol.

the amount of Si was added for further 5 min after solution A is added to solution B. This precursor gel solution was applied into two different synthesizing methods; one-half was put into the stainless steel autoclave with Teflon liner vessel then kept at 165 °C for 72 h with static condition (HT synthesis) and another half was transferred to the liner vessel and heated under microwave irradiation at 165 °C for 4 h (MW synthesis). MW synthesis was performed by using CEM MARS5 microwave equipment with OMNI/XI1500 vessel. Each product was filtered, washed and dried at 120 °C. Calcination was done at 550 °C for 6 h as the last step for getting synthetic zeolite W. The materials were then ion-exchanged to be H form zeolite W. For 1 g sample, we used 100 mL of 0.1 M NH4NO3 solution at 70 °C for 6 h with vigorous stirring. The resulting material was washed with deionized water then calcined at 350 °C to remove NH3 and remain H+ on the surface. The above step was repeated three times to ensure that all potassium was exchange by H+. The absence of remaining potassium after ion exchange was confirmed by EDX analysis. 2.3. Characterization of synthetic zeolite W

2. Experimental 2.1. Materials Ludox HS-40 (Aldrich, 40 wt.% suspension in water SiO2) was used as silica source. Potassium hydroxide (Daejung, 93% extra pure grade KOH) and aluminum hydroxide (Duksan, Practical grade Al(OH)3) were used as potassium and aluminum source, respectively. To examine the role of ethylene glycol (EG) from Aldrich with 99% purity was used. Ammonium nitrate (Daejung, 99% extra pure grade NH4NO3) was used in the ion exchange process. Any other chemicals used for catalytic application were purchased either from Aldrich or TCI Tokyo Kasei with high purity. All the chemicals were used as received without further purification. 2.2. Synthesis of zeolite W We adapted the reported synthetic procedure of the precursor gel solution [27] with changing of gel composition, and compared two different heating sources; conventional hydrothermal and microwave. The precursor gel composition of zeolite W is shown in Table 1. Hydrothermally synthesized zeolite W was referred to as W-HT-A without EG and W-HT-B with EG. And zeolite W synthesized by microwave was denoted as W-MW-A and W-MW-B. To prepare precursor gel solution, 11.2 mol of potassium hydroxide was dissolved in deionized water with 2 mol of aluminum hydroxide (solution A). After refluxing for 3 h at 120 °C, the solution A was cooled down to the room temperature. Simultaneously, 6.4 mol of Ludox HS-40 was dissolved in deionized water with mechanical stirring for preparation of solution B. Solution B was vigorously stirred for 10 min then solution A is slowly added to solution B for 6 min. To study the effect of EG, 10% of EG compared with

The crystallinity of synthesized materials was measured using powder X-ray diffraction (XRD) which were obtained on a Rigaku diffractometer using CuKa radiation (k = 0.1547 nm). Effectiveness of EG and thermal stability of material were also revealed through the XRD patterns. Surface area analysis was measured using Micromeritics 2020 porosimetry analyzer at liquid N2 temperature. The SEM images were collected with a Hitachi S-4200 microscope equipped with energy-dispersive X-ray (EDX) spectroscopy. Thermogravimetric analysis (TG) were performed on a Netzsch STA 429 simultaneous thermal analysis apparatus in the range of 25– 800 °C with ca. 25 mg of sample, a heating rate 10 °C/min and under N2 flow of 40 ml/min. Raman spectra were acquired using a Jobin–Yvon Raman spectroscopy with laser of 354 nm. The strength and distribution of acid sites were measured by temperature programmed desorption (TPD) technique with ammonia, which were performed on a Micromeritics Autochem II 2920 TPD analyzer. Typically, 0.28 g of sample was placed in the sample holder and heated up to 450 °C with heating rate of 5 °C/min under helium flow of 50 ml/min, then the sample was heated for further 3 h at isothermal condition as pretreatment. The sample was saturated with ammonia at 40 °C for 1 h prior to desorption via temperature ramping to 450 °C under helium flow. The desorbed ammonia was detected by thermal conductivity detector (TCD) detector. 27Al MAS NMR experiments were carried out on a Bruker AVANCE400 spectrometer operating at 104.3 MHz. 2.4. Catalytic application for dehydration of methanol to dimethyl ether 2.4.1. Fixed-bed reactor system The home-made fixed-bed reactor system was designed to evaluate the activity and stability of the catalyst for the dehydration of

Table 1 Precursor gel compositions and experimental conditions. Sample name

W-MW-A W-MW-B W-HT-A W-HT-B A B C

Starting gel composition (mol)

Crystallization

K2O

Al2O3

SiO2

H2O

EG

t/h

T/°C

Condition

5.6 5.6 5.6 5.6

1 1 1 1

6.4 6.4 6.4 6.4

164.6 164.6 164.6 164.6

– 0.64 – 0.64

4 4 72 72

165 165 165 165

MWa MWa HTb HTb

Si/Al mol ratioc (final product)

Silica source

4.3 4.4 3.9 3.8

Ludox Ludox Ludox Ludox

Microwave irradiation. Conventional hydrothermal heating. Determined by EDX analysis. Sample name with ‘‘A” means zeolite W synthesized without EG. Sample name with ‘‘B” means zeolite W synthesized with EG.

HS-40 HS-40 HS-40 HS-40

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methanol. The reactor and pre-heater temperatures were set by an electric jacket equipped with a PID controller to the predetermined temperature with the accuracy of ±0.5 °C. Helium was used as a carrier gas. The flow rate of helium gas was controlled by a mass flow controller (TSC-110, MKP, USA). Liquid methanol was injected into the system by a dozing pump (KD Scientific, USA with Hamilton 25 mL syringe, USA). The methanol dehydration products were analyzed by on-line gas chromatograph (Agilent model 7890 N) using a thermal conductivity detector (TCD) and a Porapak Q (80–100 mesh) packed column (2.4 m  3.2 mm O. D.). 2.4.2. Reactor operating conditions The vapor-phase methanol dehydration reaction was carried out in a fixed-bed reactor as described above. In a typical experiment, each catalyst (0.2 g, 30–50 mesh) was loaded in the reactor. Before the reaction, the catalyst was activated in a stream of pure helium at 350 °C for 3 h under atmospheric pressure. After stabilization to the reaction temperature, liquid methanol was injected with a flow rate of 0.5 ml/h. Before entering the reactor, liquid methanol was consequently evaporated in the pre-heater and mixed in gaseous state with helium. The WHSV in the reaction was 2 h1. 3. Results and discussion Two kinds of heating methods were adapted to synthesize zeolite W. HT synthesis was expected to give hydrophilic zeolite whereas MW synthesis could provide several advantages such as rapid and homogeneous heating of the entire sample so that it could enhance the reaction rates, facilitate the formation of uniform nucleation centers and rapid crystallization with energy efficiency. Also, MW synthesis dramatically reduced the synthesis time from days to hours. There was a report on high enhancement in reaction rate because of direct coupling from microwave to molecular species [28]. In a typical HT synthesis method, zeolite W can be obtained in 3 days (72 h). In this study, zeolite W was successfully synthesized in only 4 h using MW synthesis. The XRD patterns from obtained zeolite W are shown in Fig. 1. The XRD pattern was confirmed using JADE program (PDF 5 version) and found to be MER topology which is the crystalline structure of zeolite W. There was no big difference between the samples obtained by HT and MW synthesis methods except that the W-HT-A showed slightly higher intensity compared with W-MW-A. SEM photographs of obtained zeolite W (Fig. 2) show an unusual habit for the crystallites of MER topology. Natural MER typi-

Intensity (a. u.)

W-HT-B

W-HT-A

W-MW-B

W-MW-A 10

20

2θ

30

40

Fig. 1. Powder XRD patterns of synthetic zeolite W obtained by MW and HT synthesis methods.

cally appears as pseudo-tetragonal prismatic crystals elongated along the c-axis and terminated by orthorhombic prisms [29] and synthetic MER-type crystals have also an elongated prismatic shape [11,13,14,30]. In our research, different types of morphologies were obtained depending on the heating sources as well as the presence of EG. MW synthesis in the presence of EG gave prismatic shape of zeolite W same as the previously reported work. However, twin-ball shaped morphology was obtained by MW synthesis without EG as well as by HT synthesis with and without EG. It was caused by the assembly of elongated prismatic crystals. Further, well organized surface of W-HT-B was confirmed from the inset in Fig. 2d compared in Fig. 2a and c. From the nitrogen adsorption analysis, the BET surface area, Langmuir surface area of W-HT-B and W-MW-B were 32.7, 24.1 and 68.2, 68.8 m2/g, respectively. It was reported that during MW synthesis the morphology control of synthesized material could be observed in the presence of alcoholic medium such as EG because EG has most high-value of loss tangent, tan (d) = 1.350 [31]. Loss tangent, tan (d), determines the ability of a specific substance to convert electromagnetic energy into heat at a given frequency and temperature. Furthermore, MW synthesis with EG can be applied more efficiently to get higher crystallization by taking the benefit of highest loss tangent. In order to enhance the quality of zeolite morphology, EG was added to the initial synthetic gel mixture. By addition of EG, the same of microwave power will be converted to more heat energy. As a result, the zeolite W obtained from this technique gave more uniform and fine morphologies with both prismatic and twin-ball shapes as shown in SEM images of Fig. 2b and d, respectively. XRD patterns from the sample with EG also confirmed the effect of EG by giving higher intensity. XRD patterns of W-MW-B and W-HT-B in Fig. 1 have higher intensities than peaks in W-MW-A and W-HT-A. It means the addition of EG enhanced better crystallized morphologies. The uniform and clear morphologies of the zeolite W in the presence of EG are probably resulted from the fast and homogeneous condensation during microwave heating. This behavior is seemed to be due to the rapid heating from dielectric dissipation of microwave energy through ionic conduction and dipole rotation. Interestingly, the addition of EG to the precursor gel with hydrothermal treatment also gave similar effect. In both synthetic methods, addition of EG resulted in the well-defined crystals with less of impurities. To examine thermal stability of zeolite W, thermal treatment by calcination was carried out. Each sample was heated at 350, 450, 550, and 650 °C for 6 h. Calcined samples were then analyzed by X-ray diffraction and the result is shown as Fig. 3. The synthetic zeolite W shows a very good thermal stability. No significant decrease of the peak intensity was observed. The high thermal stability observed here is consistent with the previous report of zeolite W synthesized using organic template [7]. This fact could be ascribed to the high silica contents of zeolite W. Because of this high thermal stability, zeolite W would be a promising catalyst for the gas-phase catalytic reaction. The thermogravimetric analysis (TGA) could help us to understand the interactions between the absorbed water and cation as well as the thermal behavior of zeolites. In the zeolite system which consist silica alumina framework, SiO4 and AlO4 tetrahedral linked together by common oxygen atoms [16]. The isomorphic substitution of Si by Al causes a negative charge density in the zeolite lattice and it is neutralized by introducing cations in the structural sites of the zeolite [32]. These counter cations in the zeolite are usually coordinated with the water molecules within the channel [33]. Since zeolite W obtained in this research was synthesized without any template, the weight loss observed from TG analysis is only due to the absorbed water. The TG profile of the samples showed a water loss which varied as the different heating sources.

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111

Fig. 2. SEM images of W-MW-A (a), W-MW-B (b), W-HT-A (c) and W-HT-B (d). Inserted figures in figure a, c, and d are closer look of the twin-ball surface. An inserted figure in figure b is enlargement of prismatic crystal.

From Fig. 4, we can see that W-MW-B has less weight loss compared with W-HT-B. In detail, the characteristic of the TG curves could be divided into the two parts. The TG profile showed a water loss in 30–110 °C, 30–160 °C range in case of W-MW-B and W-HTB, respectively due to the weakly-bound water. Then it showed another weight loss in 110–250 °C, 160–260 °C range in case of WMW-B and W-HT-B, respectively due to the water located in zeolite cavities and bound to the non-framework cations [34]. Furthermore, the profile of W-HT-B is shifted to the higher temperature which means stronger interaction between the adsorbed water and cation. Also, the destruction of zeolite structure resulted from both MW and HT synthesis could not be observed in temperature up to 800 °C. This result showed the high thermal stability of zeolite W in agreement with the XRD patterns discussed before (Fig. 3). Raman spectra of the solid-phase zeolite W are shown in Fig. 5. This technique is found to be a powerful tool for characterization of the zeolite framework. The band at 416 cm1 was assigned to x1 vibration mode of [SiO4] tetrahedral in the network with four silicon atom neighbor. The band at 480 cm1 can be attributed to the symmetric stretching mode of T–O–T bond (T refers to Si or Al atom) due to the motion of an oxygen atom in a plane perpendicular to the T–O–T bonds [35]. This band also could be interpreted as D1 vibration mode of [SiO4] tetrahedral in the network with the one oxygen atom not bonded to another silicon atom [36]. The D1 vibration mode is responsible to the incorporation of aluminum in the framework. A broad band at 700–750 cm1 could be assigned to Al–O stretches. The bands at 975, 1065 and 1100 cm1 are associated with different lattice oxygen atoms in the framework of zeolite [37]. In the perfect lattice of zeolite with small Si/Al ratio (2), there would be four bands present between 954 and 1105 cm1 in the Raman spectrum [38]. To know the state of the aluminum species, 27Al MAS NMR spectroscopy was carried out. The 27Al MAS NMR spectra (Fig. 6) of the W-MW-B and W-HT-

B show a prominent resonance at around d 60 ppm, assigned to tetrahedral Al in the framework. No octahedral aluminum signals are detected, suggesting there is little, if any, dealumination occurring during the calcination. For the utilization of these characteristics of synthetic zeolite W, the formation of DME by the catalytic dehydration of methanol was investigated. Generally, methanol could be dehydrated on the acid sites of catalyst surface to form DME and consecutively to form ethylene [39]. And if the acidity was too strong, ethylene could further react to make coke. So, the appropriate mild acidity for getting high selectivity on DME would be demanded. The properties of acid sites were studied by NH3–TPD and the results are shown in Fig. 7. The TPD profiles show bimodal distribution of desorbed ammonia corresponding to the weak (below 300 °C) and strong (above 300 °C) acid sites. The integrated peak area of weak acid sites on W-MW-B and W-HT-B are 0.7 and 1.4 mmol/g while for the strong acid sites the values are 0.2 and 0.5 mmol/g, respectively. The right-shifted maximum peak on W-HT-B at 110 °C indicated stronger interaction between the adsorbed ammonia and catalyst compared with W-MW-B. From the NH3 TPD we can see that the acidity of W-MW-B sample is smaller than W-HT-B sample. To understand the behaviors, we have examined our samples using EDX analysis method to determine the final Si/Al ratio. It is widely known that number of aluminum in the zeolitic framework is responsible for giving lewis acidity beside counter cations such as H+, Na+, K+ or Ca2+. Because we have same counter cation in each samples, for comparison we could take an account to the number of aluminum and neglected the role of counter cation. As the result, W-MW-B sample has higher Si/Al ratio which means less aluminum incorporated in the framework. So, W-MW-B sample has lower acidity than W-HT-B sample. These acidic natures of catalysts would be the key point of utilization for zeolite W as catalyst. This fact is in good agreement with the activity sequence of zeolite W in the dehydration of methanol as follows.

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W-MW-B-650

W-MW-B-450

W-MW-B-350

Intensity(a. u.)

Intensity (a. u.)

W-MW-B-550

W-HT-B

W-MW-B-AS

W-MW-B 10

20

2θ

30

40 200

400

600

800

1000

1200

1400

1600

wavenumber [cm-1]

W-HT-B-650

Fig. 5. Raman spectra of the solid-phase W-MW-B and W-HT-B catalysts.

Intensity (a. u.)

W-HT-B-550 W-HT-B-450

W-HT-B-350

W-HT-B-AS

10

20

2θ

30

W-HT-B

40

W-MW-B

Fig. 3. Powder XRD patterns of zeolite W before and after thermal treatment. Number behind the name indicated the temperature of the treatment.

150

100

50 27 δ ( Α l)

0

-50

100 Fig. 6.

99

27

Al MAS NMR spectra of a W-MW-B and W-HT-B catalysts.

98 97

W-MW-B

95 94 93 92 91 90

W-HT-B

89 88 200

400

600

800

TCD Signal(a. u.)

TG [%]

96

W-HT-B

Temp [°C] Fig. 4. Thermogravimetric analysis curves of W-MW-B and W-HT-B catalysts.

W-MW-B 100

Firstly, zeolite W catalysts were tested for the methanol dehydration at different temperatures; at 250, 300 and 325 °C using W-MW-B catalyst (Fig. 8a) and W-HT-B catalyst (Fig. 8b). As increasing temperature, the conversion of methanol increased from 0% at 250 °C to 28% at 325 °C and from 3% at 250 °C to 43% at 325 °C over W-MW-B and W-HT-B catalyst, respectively. Interestingly, the selectivity on DME was 100% even at higher temperature in using both catalysts. We did not again increase the

200

300

400

Temp [° C] Fig. 7. NH3 TPD profiles of W-MW-B and W-HT-B catalysts.

temperature beyond 325 °C due to the thermodynamically limiting regime at 325 °C [40]. Since the reaction of methanol dehydration is exothermic, reaction equilibrium will move to left by further

113

100

100

80

80

80

80

60

60

60

60

40

40

W-MW-B-325

W-HT-B-325 40

40

% Selectivity

100

% Conversion

100

% Selectivity

% Conversion

Y.-H. Seo et al. / Microporous and Mesoporous Materials 128 (2010) 108–114

W-HT-B-300

W-MW-B-300

20

20

20

0

0

20

W-HT-B-250

W-MW-B-250 0 20

40

60

0 20

80

40

60

Time on stream [min]

Time on stream [min]

(a)

(b)

80

Fig. 8. Effect of reaction temperature on the methanol dehydration over W-MW-B catalyst (a) and W-HT-B catalyst (b) on the (N) conversion and (d) selectivity. Number behind the name of catalyst indicated the reaction temperature.

increasing of temperature. For this reason, our next investigation on methanol dehydration to DME was carried out at 325 °C. The methanol dehydration to DME is considered by acid sites controlled reaction. Weak acid sites govern the formation of DME via dehydration whereas strong acid sites further catalyze DME to olefins. Different from reported elsewhere that zeolite catalyst gave several by products depending on various reaction conditions [6,40,41], all of our catalysts gave 100% selectivity towards DME without any olefin products (Figs. 8 and 9). It is due to the weak acidity of synthetic high silica content zeolite W. The less presence of strong Lewis acid keeps DME not to further dehydrate to form olefin. The TPD profiles of W-MW-B and W-HT-B reflected the activity difference depending on the heating source. The hydrothermally prepared zeolite W, W-HT-B has superior activity showing higher conversion of methanol together with larger amount of desorbed NH3 at the lower maximum temperature. The conversion of methanol using W-HT-B was 40% in average and it was stable for 37 h of time-on-stream where as W-MW-B shows less conversion of methanol. But both catalysts showed much higher stability than previously reported catalysts [6,40,42]. Besides the differences of acidity between hydrothermally prepared W-HT-B and microwave

100

100

80

80

60

60

synthesized W-MW-B, TG analysis showed big differences in water sorption, which told the hydrophilicity differences. This hydrophilic nature of hydrothermally prepared catalyst also would affect the reaction performance due to the hydrophilic nature of reactant. Higher hydrophilicity of hydrothermally synthesized catalyst facilitated methanol to reach to the acid active site on the surface. It is also worthy to be mentioned that coke formation could not be observed during the long-lifetime reaction of 37 h. It is because of the less strong acid sites in the zeolite W which is responsible for coke formation. Hence, the hydrothermally synthesized zeolite W with high Si/Al ratio could be an appropriate catalyst in the production of DME from methanol with high stability and selectivity. 4. Conclusion Zeolite W having high silica to alumina ratio was successfully synthesized by using solely KOH as basic solution without templates via both HT and MW synthesis methods which resulted in different morphologies and surface properties. Adding EG in the precursor gel contributed to uniform morphologies of zeolite W. The synthetic zeolite W was illustrated to be used as a potential catalyst for the DME production via methanol dehydration with high selectivity and durability due to the appropriate pore size and acidity.

W-HT-B-325

40

40

% Selectivity

% Conversion

Acknowledgments This work was supported by the Korea Science and Engineering Foundation (KOSEF) and funded by Korean Government (MEST) (Grant number: 36379-1). References

W-MW-B-325 20

20

0

[1] [2] [3] [4]

0 10

15

20

25

30

35

Time on stream [h] Fig. 9. Methanol conversion and DME selectivity versus time-on-stream for longlifetime test. (j) Conversion over W-MW-B catalyst, (h) selectivity over W-MW-B catalyst, (N) conversion over W-HT-B catalyst and (4) selectivity over W-HT-B catalyst. Number behind the name of catalyst indicated the reaction temperature.

[5] [6] [7] [8] [9] [10]

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