Microwave synthesis of mesoporous SAPO-34 with a hierarchical pore structure

Materials Research Bulletin 47 (2012) 3888–3892

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Microwave synthesis of mesoporous SAPO-34 with a hierarchical pore structure Seung-Tae Yang a, Ji-Ye Kim a, Ho-Jeong Chae b, Min Kim b, Soon-Yong Jeong b, Wha-Seung Ahn a,* a

Department of Chemical Engineering, Inha University, Incheon 402-751, Republic of Korea Research Center for Green Catalysis, Division of Green Chemistry & Engineering Research, Korea Research Institute of Chemical Technology, P.O. Box 107, 141 Gajeong-ro, Yuseong, Daejeon 305-600, Republic of Korea b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 27 April 2012 Received in revised form 28 July 2012 Accepted 16 August 2012 Available online 23 August 2012

SAPO-34 possessing a unique microporous–mesoporous hierarchical pore structure (Meso-SAPO34(MW)) with a reduced particle size was prepared using a microwave heating method and applied as a catalyst for methanol-to-olefin (MTO) conversion reaction. All synthesized SAPO-34 catalysts were characterized via XRD, N2 adsorption isotherm and SEM. The Meso-SAPO-34(MW) demonstrated clearly improved MTO performance compared with those synthesized using the conventional SAPO-34 catalysts due to the catalyst deactivation alleviated by the introduced modifications in textural property. ß 2012 Elsevier Ltd. All rights reserved.

Keywords: A. Microporous materials B. Chemical synthesis D. Catalytic properties

1. Introduction Zeolites are crystalline microporous aluminosilicates that are widely used as catalysts for crude oil refining and petrochemical conversion. Over the past decade, much effort has been devoted to prepare zeolites with enhanced accessibility of the reactant molecules to the active sites in order to achieve higher product yield/selectivity and to meet the undergoing commercial oil composition changes. Diverse attempts ranging from the synthesis of nano-sized zeolites [1], zeolitically-ordered mesoporous materials [2], zeolites with a secondary porosity created via steaming [3], desilication [4] or based on the confined space synthesis protocol [5] and delaminated zeolites [6] have been reported. In relation to these efforts, hierarchical zeolite is designated as zeolites with a bimodal distribution of porosity, which consequently exhibit reduced steric and diffusional restrictions [7]. It is noteworthy that such microporous–mesoporous hierarchical structures have been successfully prepared through a simple one-step hydrothermal crystallization process by introducing a rationally designed organosilane surfactant as a mesopore modifier into the conventional zeolite substrate composition [8– 11]. Apparently, enhancements in accessibility in hierarchical zeolites can be further improved via shortening the diffusion path if the material can be prepared with smaller particle sizes. In this regard, a microwave (MW) heating process that provides accelerated nucleation through rapid and uniform heating of the substrate mixture can be useful, which usually produces small

* Corresponding author. Tel.: +82 32 860 7466; fax: +82 32 872 0959. E-mail address: [email protected] (W.-S. Ahn). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.08.041

particles with a uniform size distribution in inorganic syntheses [12–14]. SAPO-34 is a zeolitic molecular sieve material with a CHA framework structure [15] with average micropores of ca. 0.38 nm, which has been intensively investigated for the separation of CO2/ CH4 [16] or CO2/N2 [17] and as a catalyst for methanol-to-olefin (MTO) reactions [18–21]; high selectivity to ethene and propene has been achieved due to its moderate acid strength and relatively small pore opening [22]. However, the strong exothermicity of the reaction and rapid deactivation of the SAPO-34 catalyst due to coke formation have been problematic in the MTO process [23], and a fluidized bed unit with advantages of fast heat transfer and continuous regeneration of the catalyst has been proven effective [24,25]. For the MTO process, a catalyst design that promotes diffusion rates in catalyst pores is still strongly desired in order to reduce the deactivation rate and to speed up the regeneration of the deactivated catalyst [26]. In this work, the SAPO-34 with a smaller particle size in a unique microporous–mesoporous hierarchical pore structure (Meso-SAPO-34) was prepared by microwave heating and applied as a catalyst for the MTO reaction. Comparisons were made with the performances of the SAPO-34 samples prepared using the conventional hydrothermal method. 2. Experimental 2.1. Synthesis A series of mesoporous SAPO-34 with a hierarchical pore structure (Meso-SAPO-34) were synthesized as follows. Initially, a solution of 15.37 g of phosphoric acid (85%, Merck) in 18.0 g of

S.-T. Yang et al. / Materials Research Bulletin 47 (2012) 3888–3892

2.3. MTO reaction The MTO reactions were carried out in a packed-bed reactor using 200 mg of catalyst under the following conditions: methanol (MeOH) concentration of 10 vol.% balanced by N2, reaction temperature of 673 K and WHSV of 1.6 h 1. The reaction products were analyzed using an online gas chromatograph (Varian GC3800) equipped with a FID linked with a CP-Volamine capillary column and a TCD linked with a Porapak Q packed column. 3. Results and discussion Fig. 1A shows the XRD pattern of the Meso-SAPO-34(MW) samples obtained after different synthesis times. All samples exhibited identical characteristic peaks corresponding to the CHA structure of the SAPO-34 [28]; a major peak at 2u  9.58 corresponding to a (1 1 0) reflection with minor peaks at 138, 16.18, 20.78, and 30.78. As shown in the inset, additional shape peak around 1.18 was clearly detected in the diffraction pattern of MesoSAPO-34(MW)_2, indicating the presence of mesopores in the sample [8]. The mesopores did not influence the characteristic XRD patterns of SAPO-34 [29]. Furthermore, no SAPO-5 phase impurities at 2u  78 were detected. The XRD peaks of the samples grew in intensity as the synthesis time increased and leveled off after 2 h of synthesis. Interestingly, the characteristic XRD pattern of the SAPO-34 began to emerge after only 10 min of synthesis. The SAPO-34(MW)_2 also exhibited slightly diminished peaks, which reflected its smaller particle size compared with Meso-SAPO-34(C) or SAPO-34(C) (see later). Fig. 1B shows the relative crystallinity and surface area of the Meso-SAPO-34(MW) measured as a function of the synthesis time. The relative crystallinity was estimated by summing up the peak intensities of the characteristic peaks at 2u = 9.58, 138, 16.18, 20.78 and 30.78 and normalized against the maximum value attained. As

Intensity

(A) Intensity (A.U.)

distilled water was prepared and 9.2 g of pseudoboehmite (70% Al2O3, Vista Catapal-B) was added slowly to the solution, which was then stirred for 2 h. Next, 10.0 g of distilled water was added and stirring was maintained another 7 h (Solution A). Concurrently, Solution B, which was composed of 1.23 g of fumed silica (99 + % SiO2, Degussa Aerosil-200) and 53.0 g of tetraethylammonium hydroxide (TEAOH; 35% in water; Aldrich), was prepared and 1.40 g of [3-(trimethoxysilyl)propyl]octadecyldimethyl-ammonium chloride (TPOAC; 72%, Aldrich) was added afterwards. Solution B was then added dropwise to solution A and stirred for 7 h. The resulting synthesis gel composition was 2.0 TEAOH:0.3 SiO2:1.0 Al2O3:1.0 P2O5:50 H2O:0.03 TPOAC. 7 ml of the synthesis precursor solution was transferred into a 35 ml teflon vessel and placed inside a microwave heating unit (Discover S-Class, CEM Corporation, USA). The vessel was heated to 463 K in approximately 2 min and maintained at that temperature for a predetermined time of 10 min to 5 h. The microwave power was 200 W throughout the synthesis including the heating stage. The substrate mixture was stirred using a magnetic stirrer during the microwave heating. For the conventional hydrothermal synthesis of Meso-SAPO-34, the synthesis mixture was transferred to a 150 mL teflon-lined autoclave. The sealed autoclave was heated to 448 K under mechanical stirring (250 rpm) [27] and maintained at that temperature for 84 h. The white crystalline product was centrifuged for separation and washed with distilled water several times. The obtained product was dried at 373 K for 6 h, and then calcined at 473 K for 1 h in air and at 823 K for 5 h in air (heating rate: 1 K/min). The synthesis gel composition of the microporous SAPO-34 was identical to that used for Meso-SAPO-34, except TPOAC was not used in the preparation of the substrate mixture. The resulting synthesis gel was 2.0 TEAOH:0.3 SiO2:1.0 Al2O3:1.0 P2O5:50 H2O. The mixture was transferred to the same teflon-lined autoclave and prepared similarly: heated to 448 K under stirring (250 rpm) and maintained at that temperature for 48 h. The drying and calcination treatments are identical. In this work, the Meso-SAPO-34 prepared via microwave heating is designated as Meso-SAPO-34(MW) and that prepared via the conventional hydrothermal heating method is designated as Meso-SAPO-34(C). Microwave heating time (x) for the MesoSAPO-34 is specified as Meso-SAPO-34(MW)_x. The microporous SAPO-34 prepared via conventional hydrothermal heating method is designated as SAPO-34(C).

3889

1

2

3 2 Theta

4

2.2. Characterization 10

20

30

40

50

100

(B)

700 600

80

500 60

400 300

40

200 20

2

Relative Crystallinity (%)

2 theta (degree)

BET surface area (m /g)

The XRD patterns of the SAPO-34 samples were obtained on a Rigaku diffractometer using CuKa (l = 1.54 A˚) radiation. The N2 adsorption–desorption isotherms were measured on a BELSORPmini (BEL, Japan) at liquid nitrogen temperature. Prior to the isotherm measurements, the samples were activated at 523 K for 5 h under vacuum conditions. The specific surface areas of the samples were calculated using the Brunauer–Emmett– Teller (BET) method and the micropore volumes and external surface areas were determined using the t-plot method. The mesopore volumes were determined via the Barrett–Joiner– Halenda (BJH) method. The sample morphologies were examined via a scanning electron microscopy (SEM) using a Hitachi S-4300 electron microscope. High-resolution transmission electron microscope (TEM) images of the SAPO-34 samples were obtained on a JEM-2100F model operated at 200 kV. The surface acidity of the catalysts was measured by temperature programmed desorption of ammonia (NH3-TPD) using a BEL-CAT TPD analyzer (BEL, Japan). NH3-TPD profiles were obtained from 100 to 600 8C with a temperature ramping rate of 10 8C/min.

5

(f) (e) (d) (c) (b) (a)

100 0

0

0

1

2 3 4 Synthesis time (h)

5

Fig. 1. (A) XRD patterns of the prepared Meso-SAPO-34(MW) as a function of the synthesis time (inset is the small angle XRD pattern of Meso-SAPO-34(MW)_2): (a) 10 min, (b) 0.5 h, (c) 2 h, (d) 5 h, (e) M-SAPO-34(C) and (f) SAPO-34(C) and (B) relative crystallinity () and BET surface area (&) vs. synthesis time for the MesoSAPO-34(MW).

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300

0.05 dVp/ddp (cm3/g.nm)

400

3

Volume adsorbed (cm /g, STP)

500

0.04 0.03 0.02 0.01 0.00

5 10 15 Pore diameter (nm)

200

10 min 0.5 h 2h 5h

100

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0) Fig. 2. N2 adsorption–desorption isotherm of (a) Meso-SAPO-34(MW) as a function of synthesis time and BJH pore size distribution (inset).

expected, both the BET surface area and relative crystallinity of the synthesized Meso-SAPO-34(MW) developed similar patterns as the synthesis time elapsed. Because further increases in the synthesis time of longer than 2 h did not improve the surface area or crystallinity. Meso-SAPO-34(MW)_2 was selected as the standard sample for the subsequent catalytic evaluation. Fig. 2 shows the N2 adsorption–desorption isotherms of the calcined Meso-SAPO-34(MW) samples as a function of the synthesis time. The isotherms of the Meso-SAPO-34 exhibited a mixture of type I and IV in the IUPAC classification, which indicated

the presence of both microporosity and mesoporosity in the samples. The corresponding BET surface areas, external surface areas, micropore volumes and mesopore volumes are summarized in Table 1. The BET surface areas of Meso-SAPO-34(MW)_2, MesoSAPO-34(C), and SAPO-34(C) were similar: 686, 638, and 665 m2/g, respectively. However, the external surface areas were more distinctive at 152, 110 and 69 m2/g, respectively, implying a particle size reduction in that sequence. The isotherms of MesoSAPO-34 exhibited a narrow hysteresis at relative pressures higher than P/P0 = 0.4, and micropores steadily developed at the expense of mesopores in the Meso-SAPO-34(MW) samples as the synthesis time elapsed (as shown in Table 1 for pore volume data). When synthesis time was not sufficient (<1 h), a significantly smaller micropore volume than the conventional SAPO-34(C) (0.25 cm3/g) was obtained in the Meso-SAPO-34(MW), indicating that the fundamental microporous SAPO-34 structure was not formed yet. The average mesopore size of the Meso-SAPO-34(MW) remained almost constant at ca. 3.3 nm irrespective of the synthesis time, which became sharper with synthesis time (Fig. 2, inset). Fig. 3 shows the TEM micrographs of the Meso-SAPO-34(MW)_2, which clearly show the mesoporous regions in a hierarchical pore structure when compared with the TEM image of SAPO-34(C). Fig. 4 shows the scanning electron microscope (SEM) images of the calcined Meso-SAPO-34(MW) samples as a function of the synthesis time. As reported, the crystal structure observed is orthorhombic when the TEAOH was used as a template for the SAPO-34 [30]. The particles grew steadily in size as the synthesis time increased and became almost constant after 2 h (Figs. 3(a)– (d)). However, Meso-SAPO-34(MW)_2 (Fig. 3(c)) was smaller than Meso-SAPO-34(C) in Fig. 3(e), which in turn was smaller than SAPO-34(C) with ca. 200 nm in size. The size difference is in agreement with the external surface area measurements; the

Table 1 Synthesis conditions and physicochemical properties of the prepared SAPO-34 samples. Samples

Synthesis temperature (K)

Synthesis time

SBET (m2/g)a

SEXT (m2/g)b

VMICRO (cm3/g)b

VMESO (cm3/g)c

Meso-SAPO-34(MW) Meso-SAPO-34(MW) Meso-SAPO-34(MW) Meso-SAPO-34(MW) Meso-SAPO-34(MW) Meso-SAPO-34(MW) Meso-SAPO-34(C) SAPO-34(C)

463 463 463 463 463 463 448 448

10 min 0.5 h 1h 2h 3h 5h 84 h 48 h

460 502 582 686 661 656 638 665

441 453 222 152 148 138 110 69

0.10 0.13 0.20 0.24 0.23 0.25 0.21 0.25

0.56 0.54 0.36 0.26 0.27 0.25 0.19 0.13

a b c

SBET (total surface area) calculated using the BET method. SEXT (external surface area)and VMICRO (micropore volume) calculated using the t-plot method. VMESO (mesopore volume) calculated using the BJH method (from desorption).

Fig. 3. TEM images of (a) Meso-SAPO-34(MW)_2 and (b) SAPO-34(C).

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Fig. 4. SEM images of the Meso-SAPO-34(MW) as a function of the synthesis time: (a) 10 min, (b) 0.5 h, (c) 2 h, (d) 5 h, (e) Meso-SAPO-34(C), and (f) SAPO-34(C).

Meso-SAPO-34(MW)_2 had the largest external surface area among the three samples, which indicates a smaller particle size than the others. NH3-TPD was used to investigate the acidic properties of the Meso-SAPO-34(MW), Meso-SAPO-34(C) and SAPO-34(C) catalysts, and the results are shown in Fig. 5 and Table 2. NH3-TPD profiles of the samples show two distinctive NH3 desorption peaks at around 200 and 400 8C, which implies the presence of acid sites in two different strengths. The first peak around 200 8C corresponds to the weak acid sites from surface hydroxyl groups and the second peak around 400 8C is assignable to the moderate to strong acid sites Table 2 NH3-TPD analysis of prepared samples. Sample

Fig. 5. NH3-TPD analyses of (a) Meso-SAPO-34(C), (b) Meso-SAPO-34(MW) and (c) SAPO-34(C).

Meso SAPO-34(C) Meso SAPO-34(MW) SAPO-34(C)

Amount of desorbed ammonia (mmol/g) 200 8C

400 8C

0.031 0.038 0.049

0.076 0.067 0.093

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conventional SAPO-34 and exhibited slower catalytic deactivation rates with the presence of mesoporosity and particle size reduction in the catalyst. Acknowledgments This work was supported by the OASIS Project from Korea Research Institute of Chemical Technology and Technology Innovation Program (project # 10028414, Development of Production Technology of Light Olefins from Methanol/DME) funded by the Ministry of Knowledge Economy (MKE, Korea). References Fig. 6. Catalytic activity of SAPO-34s on MTO reaction: Meso-SAPO-34(MW) (squares; black bars), Meso-SAPO-34(C) (circles; white bars) and SAPO-34(C) (triangles; striped bars).

induced from structural acidity [31,32]. The acidity of SAPO-34(C) was somewhat higher than those of Meso-SAPO-34 samples, while Meso-SAPO-34(MW) and Meso-SAPO-34(C) had similar total acid sites with little differences in acid strength distribution. In MTO reaction, high acidity gives rise to a rapid deactivation and low selectivity to light olefins despite high catalytic activity. Fig. 6 shows the catalytic reactivity of Meso-SAPO-34(MW), Meso-SAPO-34(C) and SAPO-34(C) for the MTO reaction. The Meso-SAPO-34 samples with microporous–mesoporous hierarchical pore structures (Meso-SAPO-34(MW) and Meso-SAPO-34(C)) clearly exhibited lower deactivation rates than the conventional SAPO-34 with micropores only. The difference in catalytic reactivity of Meso-SAPO-34(MW), Meso-SAPO-34(C) and SAPO34(C) catalysts can be explained by the differences in pore structure and acidity. The improved reactivity, especially catalytic durability to deactivation, of Meso-SAPO-34 samples is induced by optimal acidity and mesoporosity. In particular, the mesopores can act as channels for the enhanced transfer of the reaction products from the pore inside to the bulk gas phase. As a consequence, the mesopores can be important in reducing the coke formation caused by a slow diffusion of the products. It is also noteworthy that the Meso-SAPO-34(MW) exhibited an improved MTO activity than Meso-SAPO-34(C), which probably resulted from the smaller particle size of the former reducing the diffusion path of the products. 4. Conclusions In this study, the Meso-SAPO-34(MW) with a unique microporous–mesoporous hierarchical pore structure was successfully prepared using a microwave heating method. The crystallinity and physical properties of the Meso-SAPO-34(MW) were optimized with respect to the microwave heating time. The Meso-SAPO34(MW) was more effective in the MTO reaction than the

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