Methanol-to-olefin over gallosilicate analogues of chabazite zeolite

Microporous and Mesoporous Materials 116 (2008) 253–257

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Methanol-to-olefin over gallosilicate analogues of chabazite zeolite Qingjun Zhu a, Mayumi Hinode b, Toshiyuki Yokoi a, Junko N. Kondo a, Yoshihiro Kubota b, Takashi Tatsumi a,* a b

Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Division of Materials Science and Chemical Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan

a r t i c l e

i n f o

Article history: Received 18 January 2008 Received in revised form 1 April 2008 Accepted 3 April 2008 Available online 4 June 2008 Keywords: Methanol-to-olefin Gallosilicate CHA topology

a b s t r a c t Gallosilicate analogues of chabazite zeolite with CHA topology have been synthesized, characterized and employed as catalysts for the methanol-to-olefin (MTO) process. 71Ga NMR spectra indicate that Ga is incorporated into the framework of these zeolitic materials during the hydrothermal synthesis. IR measurements show that stretching vibration of the hydroxyl groups associated with the framework Ga appears at 3600 cm1. The gallosilicates with high Ga contents show Lewis acidity, which is possibly caused by degalliation during calcination, whereas only Brönsted acid sites are observed on the sample with relatively low Ga content. The catalysts show promising catalytic performance in the MTO reaction, particularly from the stability point of view, compared with H[Al]CHA zeolites. This is attributed to the weak Brönsted acidity associated with the framework Ga in the gallosilicates. Ó 2008 Elsevier Inc. All rights reserved.

1. Introduction Zeolites and zeotype materials are widely used as acidic catalysts in the heterogeneous catalytic reactions [1]. The acidities of these solid catalysts are related with the acid amount and the acid strength. Brönsted acid strength is determined by the nature of the hydroxyl group associated with the heteroatoms in the zeolite framework. Accordingly, one of the most effective methods for adjusting the acid strength is the isomorphous substitution of trivalent heteroatoms (Al, B, Ga, Fe, etc.) for Si in the zeolite frameworks. Ga incorporation into the zeolite is of particularly interest because it has ionic radius of 0.62 Å, which is close to 0.54 Å for Al. Note that if Al is not present in the synthesized crystalline materials, they are termed gallosilicates. Great efforts have been devoted to the incorporation of Ga into the frameworks of microporous and mesoporous materials, and a variety of gallosilicates with different framework topologies were synthesized and tested on different catalytic reactions [2]. One of the most interesting catalytic applications over acidic solids is the methanol-to-olefin (MTO) process because this reaction provides a practical alternative to the current lower olefin providing processes to meet the increasing demands for ethene and propene, which are very basic chemicals for the polymer industry. The most promising catalysts by far are silicoaluminophosphate SAPO-34, which is a kind of zeotype material with CHA topology, and HZSM-5 zeolite [3]. It is believed that the Brönsted acidities of these solid acids, together with their unique microporous structures, play vital roles in the catalytic transformations of methanol * Corresponding author. Tel.: +81 45 924 5238; fax: +81 45 924 5282. E-mail address: [email protected] (T. Tatsumi). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.04.017

to hydrocarbons. The need for high selectivity has incited scientists to investigate novel microporous materials with different heteroatoms in the zeolite framework [4,5]. It is well established that the Brönsted acid induced by the framework Ga is weaker than its counterpart related to the framework Al [6]. Therefore, Ga-induced Brönsted acidity might be beneficial for the MTO reaction since one of the main obstacles to the MTO application is the quick coke formation on the zeolite. However, the researches on MTO using gallosilicates are very limited, possibly owing to the difficulty in the synthesis of gallosilicates for most zeolite topologies [2,3]. In our previous study [7], we synthesized H[Al]CHA zeolites with different Al concentrations and employed those materials for the MTO reaction. In the present paper, we report on the synthesis and characterization of gallosilicate analogues of chabazite zeolite. The objective is to modulate the catalyst acidities while retaining their shape selectivity on MTO. To our knowledge, there is no report of the synthesis of gallosilicate analogues of chabazite zeolite thus far. 2. Experimental Gallosilicates with CHA topology were synthesized with the structure directing agent of TMAdaOH (1-trimethylammonio adamantane hydroxide) [8]. Typically, NaOH and TMAdaOH were dissolved in water, and then Ga(NO3)3 and SiO2 (Cab–O–Sil M5, Cabot) were dissolved in TMAdaOH and NaOH solution. The hydrothermal synthesis was carried out at 423 K for 5 days. To speed up the crystallization process, a tiny amount of CHA zeolite seed (2 wt.% of the SiO2 amount) was used. The obtained samples were washed with deionized water and calcined to remove the template. The proton type materials were obtained by the ion-exchange with NH4NO3

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Table 1 Chemical compositions in the gel and calcined H[Ga]CHA gallosilicates Si/Gaa (In the gel)

H[Ga]CHA(13) H[Ga]CHA(23) H[Ga]CHA (62)

10 20 50

In the calcined samples Si/Gab

Si/Alb

13 23 62

>500 393 410

a Gel compositions: 1.00 SiO2  x Ga(NO3)3  0.20 NaOH  0.20 TMAdaOH  44 H2O, (0.02  x  0.1). b ICP analysis.

and subsequent calcination at 873 K. The catalysts are designated as H[Ga]CHA(x) gallosilicate, where x stands for the Si to Ga ratio in the material. Table 1 lists the chemical compositions in the synthesis gel and the studied catalysts. The Al contents, induced by the CHA zeolite seeds and the impurities in the silica source, are nearly negligible. We take this as proof that the Brönsted acidities discussed in this report are mainly caused by the proton compensating the negative charge due to the Ga incorporation into the structure framework. Powder X-ray diffraction (XRD) was measured using a Rigaku diffractometer. XRD patterns were recorded in the range of 5o < 2h < 40o using Cu Ka radiation with a scanning speed of 0.04o min1. A Hitachi S-5200 microscope was used to record SEM images. NH3 temperature programmed desorption (TPD) was carried out on a Multitrack TPD equipment (Japan BEL), and the ammonia adsorption was performed at 373 K. For the calcination process, thermogravimetric analysis (TGA) measurement was taken on a Rigaku TG8120 instrument. Solid-state 27Al and 29Si magic-angle spinning (MAS) NMR spectra were obtained on a JEOL ECA-400 spectrometer. All the infrared spectra were recorded on a JASCO FT/IR 7300 spectrometer with an MCT detector at a resolution of 4 cm1 and 64 scans. The MTO reactions on the prepared catalysts were carried out in a fixed bed reactor. It is noteworthy that dimethyl ether (DME) is not taken as a reaction product. The details of the experiments were elaborated in our previous report [7].

in the air to remove the template before the measurements, and it is clear that their structures remain almost intact during such severe treatments. Moreover, formation of gallium oxide is not observed. The SEM images of the samples with different Ga loadings are shown in Fig. 2. For the H[Ga]CHA(13) gallosilicate with the highest Ga content, the primary particle size is ca. 50 nm. The primary particles agglomerate to form large aggregates. When the Ga content decreases to the Si to Ga ratio of 23 (Fig. 2B), the particle size increases to 200 nm. Further decrease in the Ga content to the Si/ Ga ratio of 62 leads to particles of cuboid-like crystallites of a particle size of ca. 1 lm. H[Ga]CHA(62) resembles pure silica chabazite zeolite in the shape and the particle size [9].

3. Results and discussion The XRD patterns of the synthesized gallosilicate analogues with different Ga contents are displayed in Fig. 1. No reflections other than those due to CHA topology are visible and all the samples exhibit high crystallinities. The samples are calcined at 873 K

20000

A

Intensity (a.u.)

15000

10000

B 5000

C 0 10

20

30

40

2 theta (degrees) Fig. 1. Powder X-ray diffraction patterns for H[Ga]CHA gallosilicates. (A) H[Ga]CHA(13), (B) H[Ga]CHA(23), (C) H[Ga]CHA(62).

Fig. 2. SEM images of H[Ga]CHA gallosilicates. (A) H[Ga]CHA(13), (B) H[Ga]CHA(23), (C) H[Ga]CHA(62).

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Q. Zhu et al. / Microporous and Mesoporous Materials 116 (2008) 253–257 1458

160

90 80

80

40

3635

0.4

1625 3600

Absorbance (a.u.)

Weight Loss (%)

120

DTA signal (microvoltage)

100

0.4

A 0.2

B

0.2

0.0

C

0.0

-0.2

0

3800

3700

3600

70

3500 1700

1600

1500

1400

-1

300

400

500

600

700

800

900

Wavenumbers (cm )

1000

Temp (K)

Fig. 5. Differential IR spectra of NH3 adsorption on H[Ga]CHA gallosilicates. (A) H[Ga]CHA(13), (B) H[Ga]CHA(23), (C) H[Ga]CHA(62).

Fig. 3. TGA profile of H[Ga]CHA(23).

The representative TGA/DTA curve of the as-made H[Ga]CHA(23) gallosilicate is presented in Fig. 3. The removal of the TMAdaOH template by calcination in the air starts at around 673 K, as evidenced by the sharp decrease in weight of the sample and the simultaneous heat generation. Two steps in the heat generation indicate the burning of different carbonaceous species. The template constitutes about 20% of the sample weight, and complete combustion of the carbonaceous species can only be achieved at temperatures higher than 900 K. Fig. 4 shows the infrared spectra of calcined H[Ga]CHA gallosilicates in the range of 4000–1300 cm1. All the samples show typical overtones of framework vibrations at around 1850 cm1. In the hydroxyl stretching vibration region, two bands are visible at 3740 cm1, which is ascribed to SiOH groups, and 3600 cm1, generally assigned to the Brönsted acid sites. Because the Al contents in the gallosilicates are very low, this Brönsted acidity reflects the hydroxyl groups associated with the framework Ga [6]. The C–H stretching mode is observed between 3000 and 2800 cm1 due to the incomplete removal of the template used in the synthesis. This is corroborated by the slight band at around 1535 cm1, which is attributed to the hydrogen-deficient hydrocarbon species [10]. Here, we note that the calcination at temperatures higher than 873 K results in significant degalliation and thus is not applied in our research.

Ammonia is used as a probe base molecule to detect the acidities of synthesized gallosilicates, and the differential spectra after evacuation of the gas phase NH3 at 373 K are exhibited in Fig. 5. Two negative bands at 3743 and 3737 cm1 can be distinguished in the region of silanol stretching vibration, indicating the presence of different silanol groups. A sharp negative band centered at 3600 cm1 points to the consumption of the proton due to the interaction of NH3 with Ga-induced Brönsted acid sites. A slight, but distinct negative band is visible at 3635 cm1; we tentatively assign this band to the hydroxyl groups associated with the extra-framework Ga species, which possibly act as Lewis acid sites. Two bands appear in the region of the NHþ 4 bending mode; the positive band at 1458 cm1 is attributed to the ammonia adsorbed on Brönsted acidity, and the band at 1625 cm1 is related to the Lewis acid sites [11]. Those positive signals decrease with increasing temperature for all the samples. Moreover, no peak relating to the Lewis acid sites (3635 and 1625 cm1) is observed on the lowest Ga content of H[Ga]CHA(62) gallosilicate. One possibility is that extra-framework Ga species are formed by the removal of framework Ga during the calcinations; degalliation is more pronounced on gallosilicates with high Ga contents, in line with the report that degalliation is less extensive on H[Ga]MFI with low Ga contents [12]. NH3-TPD is conducted to study the acidities of the prepared gallosilicates and the profiles are displayed in Fig. 6 [13]. There are debates concerning the relationship between the acid strength and

3740

-12

8.0x10

3600

Absorbance (a.u.)

1535

A 1.5

B 1.0

C 0.5 4000

Desorbed NH3 signal (a.u.)

2.0

-12

6.0x10

A -12

4.0x10

B

-12

2.0x10

C 0.0

3500

3000

2500

2000

1500

-1

Wavenumber (cm ) Fig. 4. IR spectra for H[Ga]CHA gallosilicates. (A) H[Ga]CHA(13), (B) H[Ga]CHA(23), (C) H[Ga]CHA(62).

400

500

600

700

800

Temp (K) Fig. 6. NH3 temperature programmed desorption profiles of H[Ga]CHA gallosilicates. (A) H[Ga]CHA(13), (B) H[Ga]CHA(23), (C) H[Ga]CHA(62).

Q. Zhu et al. / Microporous and Mesoporous Materials 116 (2008) 253–257

-111

-103 -101

C -80

300

200

100

0

-90

-100

-110

-120

-130

-140

Chemical Shift (ppm) Fig. 8. 29Si MAS NMR spectra of H[Ga]CHA gallosilicates. (A) H[Ga]CHA(13), (B) H[Ga]CHA(23), (C) H[Ga]CHA(62).

100

80

60

40

20

0 0

50

100

150

200

250

300

Time on stream (min) Fig. 9. Time on stream of methanol-to-olefins at 548 K over H[Ga]CHA(13) (j), H[Ga]CHA(23) (N), H[Ga]CHA(62) (*). Reaction condition: 6.1% CH3OH diluted in N2, WHSV of methanol 0.5 h1, 548 K.

100

80

60

40

A

20

B

0 0

50

100

150

200

250

300

Time on stream (min)

C 400

A B

Conversion (%)

the desorption maximum of adsorbed NH3 although it is widely agreed that the Brönsted acid sites contribute to the strong acidity. As shown in Fig. 6, physisorbed NH3 or NH3 adsorbed on weak acid sites are removed at elevated temperature of 400 K, and it is evident that the amount of such ammonia is related to Ga contents in the samples. The more the Lewis acid sites as confirmed on Fig. 5, the more the physisorbed ammonia and ammonia adsorbed on the weak acid sites. Moreover, there is no detectable adsorbed ammonia on the weak acid sites for H[Ga]CHA(62). These observations might indicate that there is a direct relationship between the Lewis acid sites and the weak acid sites. The NH3 attached to the strong acid sites desorb at around 610 K, being shifted by 20 K to the low temperature region compared with NH3 adsorbed on the strong acid sites of H[Al]CHA zeolites [7]. It is concluded that the acid sites on H[Ga]CHA gallosilicates are weaker than those on H[Al]CHA zeolites. 71 Ga NMR is measured to study the coordination state of the Ga species in the prepared gallosilicates. Fig. 7 exhibits the 71Ga NMR spectra of prepared gallosilicates after the samples are calcined. The spectra are dominated by a resonance peak centered at around 165 ppm, which is commonly assigned to the tetrahedrally coordinated Ga. This chemical shift is higher than the resonance of Ga in the framework of other zeolitic materials (ca. 155 ppm) [14,15]. It is suggested that some tetrahedrally coordinated extra-framework Ga species show the chemical shifts at 174 and 140 ppm [16]. Therefore, we tentatively conclude that the majority of Ga is incorporated into the framework of synthesized gallosilicates, whereas minor extra-framework Ga species, which act as Lewis acid sites as shown in Fig. 5, are also tetrahedrally coordinated. Moreover, the characteristic band at 0 ppm for the octahedrally coordinated Ga species is not observed. However, it is to be noted that such a resonance might be masked by the broadening effects of the surrounding environments. 29 Si NMR is widely employed to characterize the gallosilicate to gain local structural information [17,18]. The 29Si NMR spectra of calcined samples are presented in Fig. 8. The spectra of the studied samples show similar distinguished bands at 111, 103 and 101 ppm. The sharp line at 111 ppm is a characteristic band for the tetrahedral Si (Q4) in the framework of gallosilicate. Additionally, two bands with relatively low intensities, centered at 101 and 103 ppm, can be observed. They could be attributed to the isolated silanol sites of (SiO)3  SiOH (Q3) and the Si(1Ga) unit, respectively. The exact assignments remain unclear. No 29Si resonance due to Si(2Ga) and Si(3Ga) units is observed. The influence of Ga contents on the MTO reaction at 548 and 598 K are presented in Figs. 9 and 10, respectively, in which the conversion of methanol is plotted as a function of time on stream.

Conversion (%)

256

-100

Fig. 10. Time on stream of methanol-to-olefins at 598 K over H[Ga]CHA(13) (j), H[Ga]CHA(23) (N), H[Ga]CHA(62) (*). Reaction condition: 6.1% CH3OH diluted in N2, WHSV of methanol 0.5 h1, 598 K.

Chemical shift (ppm) Fig. 7. 71Ga MAS NMR spectra of H[Ga]CHA gallosilicates. (A) H[Ga]CHA(13), (B) H[Ga]CHA(23), (C) H[Ga]CHA(62).

We reported the MTO over H[Al]CHA zeolites under the same reaction conditions in our previous paper, so the results are compara-

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might activate alkanes to form alkenes, thus reducing the production of alkanes.

Table 2 Products selectivity of MTO over H[Ga]CHA(23) gallosilicate at 598 K Products

CO + CO2 + CH4 C¼ 2 C2 C¼ 3 C3 Aliphatic C4–C6

257

Time on stream (min) 30

90

150

210

270

1 32 0 44 6 17

0 39 0 52 0 9

0 39 0 51 0 10

0 40 0 52 0 8

0 41 0 55 0 4

ble [7]. There are continual decreases in the methanol conversion for all the reactions, indicating unavoidable accumulation of the coke on the catalysts [19]. At 548 K, the H[Ga]CHA(13) gallosilicate, with the highest Ga concentration, shows the highest activity and a significant decrease in the methanol conversion, whereas the methanol conversion is more or less stable on H[Ga]CHA(62) with the lowest Ga content. Moreover, no induction period as those reported in the literature is observed [20]. It is suggested that the hydrocarbon-pool is already present for the reaction due to the incomplete removal of the template [21]. H[Ga]CHA catalysts are inferior to H[Al]CHA catalysts in the reaction conversion at 548 K, and it is attributed to the relatively weak Ga-induced Brönsted acidity. The MTO catalytic performance is not only related to the Ga contents, but also determined by the reaction temperatures. As shown in Fig. 10, H[Ga]CHA(62), with the lowest Ga content, clearly has an advantage over H[Ga]CHA(13) from the reaction stability point of view. This is in agreement with our previous observation that the less acid amount, in a certain range, is beneficial for the MTO reaction stability [7]. Obviously, suitable acidities, including the acid strength and acid amount, and reaction variables need to be optimized to find practical applications in the MTO reaction. More importantly, the products selectivity has to be taken into consideration. Representative products selectivity of H[Ga]CHA(23) is listed in Table 2. Ethene and propene constitute the majority of the products with their overall selectivity of higher than 80%, and their components increase with the deactivation process. The amount of C2H6 and C3H8 are negligible after 90 min of the reactions, which is in sharp contrast to their presence on H[Al]CHA aluminosilicates [7]. The side reactions due to hydrogen transfer could be inhibited on H[Ga]CHA gallosilicate because of the weaker acidity. Another possible explanation is that the Ga species, either in the framework or extra-framework locations,

4. Conclusions Gallium can be incorporated into the framework of zeolitic materials with CHA topology in the presence of the structure directing agent of TMAdaOH. Significant presence of Lewis acidity is observed on the gallosilicate with a high Ga content, whereas such Lewis acidity is nearly negligible on the gallosilicate with a low Ga content. The Brönsted acidities associated with the framework Ga is weaker than those of H[Al]CHA aluminosilicate. This renders promising MTO catalytic performance over H[Ga]CHA gallosilicates from the stability point of view. Acknowledgments The authors are grateful for the financial support from Mitsubishi Chemical Company (MCC) and scientific discussions with Dr. Satoshi Inagaki. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

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