34 intergrowth and SAPO-34 zeolites and their catalytic performance for the methanol-to-olefin reaction

Accepted Manuscript Title: Seed-assisted synthesis of hierarchical SAPO-18/34 intergrowth and SAPO-34 zeolites and their catalytic performance for the methanol-to-olefin reaction Author: Chao Sun Yaquan Wang Hengbao Chen Xiao Wang Cui Wang Xu Zhang PII: DOI: Reference:

S0920-5861(19)30181-6 https://doi.org/doi:10.1016/j.cattod.2019.04.038 CATTOD 12133

To appear in:

Catalysis Today

Received date: Revised date: Accepted date:

15 December 2018 16 March 2019 10 April 2019

Please cite this article as: C. Sun, Y. Wang, H. Chen, X. Wang, C. Wang, X. Zhang, Seed-assisted synthesis of hierarchical SAPO-18/34 intergrowth and SAPO-34 zeolites and their catalytic performance for the methanol-to-olefin reaction, Catalysis Today (2019), https://doi.org/10.1016/j.cattod.2019.04.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Seed-assisted synthesis of hierarchical SAPO-18/34 intergrowth and SAPO-34 zeolites and their catalytic performance for the

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methanol-to-olefin reaction Chao Sun, Yaquan Wang*, Hengbao Chen, Xiao Wang, Cui Wang and Xu Zhang

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Key Laboratory for Green Chemical Technology of Ministry of Education, School of

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Chemical Engineering & Technology, Tianjin University, Tianjin 300072, P.R. China Abstract:

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Hierarchical silicoaluminophosphate SAPO-18/34 intergrowth and SAPO-34 zeolites were synthesized by adding SAPO-34 seed crystals to a SAPO-18 zeolite

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precursor in an aminothermal system, in which triethylamine was used as both the

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solvent and the template. The prepared samples were characterized by XRD, SEM,

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TEM, ICP-OES, N2 adsorption-desorption, NMR, TPD, TGA and GC-MS and a possible formation mechanism was proposed. The XRD and NMR results indicated

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that as the amount of SAPO-34 seed crystals increased, the phase of the products changed from SAPO-18 to SAPO-18/34 intergrowth and then to pure-phase SAPO-34. SEM and TEM results showed that the SAPO samples are the aggregates of many nanosized plate-like nanoparticles containing mesopores and/or macropores. These samples had higher external surface areas and mesopore volumes than the single-phase SAPO-18. The NH3-TPD results showed that the amount and strength of the strong acid sites in the SAPO-18/34 intergrowth and SAPO-34 samples were influenced by the amount of added SAPO-34 seed crystals, but all values were higher

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than those of pure SAPO-18. The SAPO-18/34 intergrowth and SAPO-34 zeolites with the hierarchical structure and appropriate acidic properties exhibted excellent

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catalytic properties for the methanol to olefin reaction. Moreover, the selectivities of light olefins varied with the cage type, the acidic properties and hierarchical structure

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of SAPO-18/34 intergrowth and SAPO-34 zeolites.

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Keywords: Seed; Aminothermal synthesis; SAPO-18/34 intergrowth; SAPO-34;

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Hierarchical zeolite; Methanol to olefin.

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1. Introduction Zeolites, with microporous structures, have been extensively utilized in a wide range of catalytic processes due to their unique shape selectivities, high surface areas

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and excellent thermal/hydrothermal stabilities [1-5]. The silicoaluminophosphate molecular sieve SAPO-34 exhibits excellent catalytic performance for the methanol to

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olefins (MTO) process due to its small 8-ring pores (3.8Å x 3.8Å), moderate acidity

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and excellent thermal/hydrothermal stability [6-9]. Since the MTO process is an alternative way to transform non-petroleum based methanol to light olefins, the

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synthesis of SAPO-34 catalysts has received great attention in recent years [10, 11]. However, conventional SAOP-34 only contains micropores and suffers from severely

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restricted intracrystalline diffusion, which slows down the transportation of heavy hydrocarbons [12, 13]. Consequently, SAPO-34 rapidly deactivates during MTO

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catalyst.

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reactions due to coke deposition which blocks the internal channels in the SAPO-34

Previous studies have shown that the introduction of hierarchical pore structures

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into zeolites could alleviate the diffusion limitations of these zeolites [14, 15]. In general, two related synthetic routes which are classified as bottom-up and top-down have been developed to fabricate hierarchical SAPO-34 [10, 16]. The bottom-up methods, also known as multiple templating methods, includes hard- and soft-template methods. In these methods, secondary templates are introduced to the gel during the synthesis of the SAPO-34 zeolites. The hierarchical SAPO-34 zeolites are formed after the removal of the secondary templates [17,18]. However, the bottom-up routes suffer from problems such as the high cost of the secondary templates, complicated synthesis procedures and low crystallinity of the obtained 3

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zeolites [19-21]. Recently, a simple strategy that does not employ extra templates was developed to construct hierarchical SAPO-34. Nano-sized hierarchical SAPO-34 zeolites were prepared through a seed-assisted method and no mesoporogen templates

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were used. This zeolite had excellent catalytic performance for the MTO reaction [22]. The creation of mesoporosity through the top-down route is based on post-synthesis

cr

treatments, which preferentially extract the framework cations of SAPO-34 [8, 12, 23].

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Decreasing the crystal size to shorten the diffusion path of the SAPO-34 is also a facile method to reduce diffusion limitations. For example, Yang et al. [24]

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synthesized nano-size SAPO-34 using a post-synthesis milling and recrystallization method. This significantly improved the catalytic performance in the MTO reaction.

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Currently, SAPOs are predominantly synthesized by hydrothermal methods in the presence of large amounts of water and organic amines [25,26]. SAPOs can also

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be prepared by other methods, such as solvothermal [27-29], solvent-free [30],

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ionothermal [31-33] and dry-gel conversion methods [34-36]. Recently, a novel aminothermal SAPO zeolite synthesis route was reported by Fan et al. [37], in which

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organic amines were used as both the solvent and the template. SAPO-18, SAPO-34, SAPO-44, DNL-6 and SAPO-56 have all been obtained using the aminothermal synthesis route [37-39]. Several zeolites with special properties have been obtained using the aminothermal method [39, 40]. For example, Fan et al. [37] synthesized plate-like SAPO-18 with an AEI-framework topology using triethylamine (TEA) as the template in aminothermal environment. This method avoided the use of the more expensive

template

agents

tetraethylammonium

hydroxide

(TEAOH)

and

N,N-diisopropylethylamine. TEA has often been used as a template for the synthesis of SAPO-34 [41]. SAPO-18 has an AEI-type topological structure whereas SAPO-34 has a CHA-type topological structure, but both contain double six-rings (D6Rs) and 4

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have the same channel sizes [42, 43]. CHA cages are formed by the parallel arrangement of D6Rs, namely, via AAAA··· stacking whereas AEI cages are the result of the cross arrangement of D6Rs, in other words, the rings are stacked in an

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ABAB··· pattern. Given the structural similarities between SAPO-34 and SAPO-18, it is not surprising that SAPO-18 can also be synthesized by the aminothermal synthesis

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route with TEA as the template as for SAPO-34.

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The similar pore geometries between SAPO-18 and SAPO-34 make it easy to form an intergrowth material [44-46]. Recently, there have been many reports on the

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synthesis of CHA/AEI intergrowth materials and their application as catalysts for MTO reactions [44-48]. However, none of these works have used the seed-assisted

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method to synthesize SAPO-18/34 intergrowth. So, in this work, SAPO-34 seed crystals were introduced into the synthesis gel of SAPO-18 in an aminothermal

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system while using TEA both as the solvent and the template. As a result, for the first

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time, hierarchical SAPO-18/34 intergrowth and SAPO-34 zeolites were synthesized in an aminothermal environment, which avoided the use of mesoporogen templates. The

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effect of different amounts of SAPO-34 seed crystals on the structure, morphology and acidity of the synthesized samples were studied in detail and their catalytic performances were investigated in the MTO process. In addition, a possible formation mechanism is proposed.

2. Experiment 2.1 Materials

Pseudoboehmite (Al2O3, 68%) was purchased from Shandong Aluminium Industry CO., Ltd., China. Quartz sand (20-40 mesh) was obtained from Tianjin GuangFu Fine Chemical Research Institute Co., Ltd., China. Triethylamine (TEA, 99 5

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wt %), methanol (AR) and phosphoric acid (H3PO4, 85 wt %) were purchased from Tianjin

JiangTian

Fine

Chemical

Research

Institute

Co.,

Ltd.,

China.

Tetraethylammonium hydroxide solution (TEAOH, 35 wt % in water) was purchased

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from Zhenjiang Runjing High Purity Chemical Co., Ltd., China. Colloidal silica (SiO2, 40 wt %) was obtained from Qingdao Haiyang Chemical Co., Ltd., China.

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2.2 Synthesis of SAPO-34 seed

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SAPO-34 seed crystals were prepared by hydrothermal synthesis with a molar composition of 1.0 Al2O3:1.0 P2O5:0.3 SiO2:0.94 TEAOH:1.56 TEA:50 H2O. In a

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typical synthesis, the pseudoboehmite, deionized water and phosphoric acid were mixed in sequence and vigorously stirred for 1 h. Then the colloidal silica was added

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dropwise and the mixture was stirred for 1 h. Subsequently, TEAOH was added dropwisely followed by a continuous stirring for 15 min. Finally, TEA was slowly

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added into the mixture. The resultant mixture was then stirred for 2 h to form a

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homogeneous solution. The synthesis solution was then transferred into a 50-mL Teflon-lined autoclave and aged at 120 oC for 24 h. Finally the products were

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crystallized at 200 oC for 48 h under static condition. The crystallized products were collected after centrifugation and washed with deionized water. Then the products were dried at 120 oC overnight and the templates were removed by calcination at 550 o

C for 6 h.

2.3 Synthesis of hierarchical SAPO-18/34 intergrowth and SAPO-34 zeolites The molar composition for the aminothermally syntheses of hierarchical

SAPO-18/34 intergrowth and SAPO-34 zeolites were 7.0 TEA:1.0 Al2O3:1 P2O5:0.3 SiO2:15.7 H2O. Typically, TEA, water and pseudoboehmite were mixed in sequence followed by stirring for 5 min. Then colloidal silica was slowly added to the mixture. This mixture was then vigorously stirred for 5 min and then phosphoric acid was 6

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added dropwisely followed by 30 min of stirring. Next, the desired amount of SAPO-34 seed crystals was added and the synthesis gel was stirred at ambient temperature for 2 h. This solution was then transferred into a 50-mL Teflon-lined

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autoclave. Crystallization was conducted at 200 oC with rotation (30 rpm) for 48 h. The zeolite products were obtained after centrifugation, washed with deionized water

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and dried at 120 oC overnight. The samples were then calcined at 550 oC for 6 h to

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remove the organic templates. The resultant samples are designated as S-x, where x refers to weight ratio of the SAPO-34 seed crystals added to the inorganic oxides in

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the initial synthesis gel. S-0 refers to the sample with no added SAPO-34 seed crystals.

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The yield of the product is defined as: yield (%) = (Wsample ‒Wseed)/Wgel, where Wsample, Wseed and Wgel are the weights of the calcined zeolite, the calcined SAPO-34

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2.4 Characterization

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seed crystals and the inorganic oxides in the initial synthesis gel, respectively.

X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Focus

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diffractometer with Cu Kα radiation ( = 0.15418 nm) at 40 kV and 40 mA, from 5 o to 40o with a scanning rate of 8o min-1. The morphology and crystal size of the samples were obtained on an S-4800 field

emission scanning electron microscope, operating at 3 kV and 5 μA. Transmission electron microscopy (TEM) observations were performed on a

JEM-2100F instrument with an accelerating voltage of 200 kV. The chemical compositions of samples were measured using inductively coupled plasma optical emission spectroscopy (ICP-OES). The N2 adsorption-desorption isotherms were performed on a Micromeritics TriStar 3000 automated physisorption apparatus at liquid-N2 temperature (77 K). 7

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Samples were pretreated at 300 oC in an N2 flow for 4 h prior to measurements. The total specific surface areas (SBET) of the samples were calculated applying the Brunauer-Emmett-Teller (BET) equation. The t-plot method was used to evaluate the

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external surface areas (Sext) and the micropore volumes (Vmicro) of the samples. The total pore volume (Vtotal) was obtained from the amount of nitrogen adsorbed at p/p0 =

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0.99, and the mesopore volume (VMeso) was calculated from the difference between

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Vtotal and Vmicro. The micropore surface area (S micro) was calculated by subtracting Sext from SBET. The pore size distribution was derived from the desorption branch of the

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N2 isotherms by applying the Barrett–Joyner–Halenda (BJH) method.

Temperature-programmed ammonia desorption (NH3-TPD) profiles were

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collected using a Xianquan TP-5076 TPD analyzer. The samples (ca. 100 mg) were first pretreated under a continuous He flow at 400 °C for 1 h. After cooling to 100 oC,

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the samples were saturated with pure NH3 for 10 min. Then a He flow was passed

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over the sample for 2 h to remove the physisorbed ammonia. Once a stable base line was attained, the sample was heated to 700 oC at a ramping rate of 10 oC min-1. The

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chemisorbed ammonia in the effluent He stream was monitored with a thermal conductivity detector.

Solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) was

used to investigate the coordination of Si and P atoms in SAPO-34 framework. The measurement was performed on a Varian infinityplus 300 MHz NMR spectrometer at a field strength of 7.1 T. The resonance frequencies in this field strength were 79.4 and 121.372 MHz for

29

Si and

31

P, respectively. Chemical shifts were referenced to

tetramethylsilane (TMS) for 29Si and 85% H3PO4 for 31P.

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Thermogravimetric analysis (TGA) was carried out using a Shimadzu TGA-50 instrument under a flow of oxygen from 30 to 750 oC with a heating rate of 10 oC min-1.

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2.5 Catalytic tests The MTO catalytic tests were performed at 425 oC in a quartz tube fixed-bed

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reactor at atmospheric pressure. The weight hourly space velocity (WHSV) of the

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methanol was 2 h-1. Typically, 1 g calcined catalyst (20-40 mesh) was mixed with 1 g quartz sand of the same size, and this mixture was loaded in the middle zone of the

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reactor. Prior to each reaction run, the catalyst was pretreated at 425 oC for 1 h under a N2 flow of 60 mL min-1. The mixture of CH3OH/H2O (1:1 mol ratio) was introduced

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using an HPLC pump and it was immediately vaporized after entering the reactor. The analysis of the products was performed using an on-line gas chromatograph

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(SP-2100), equipped with a flame ionization detector and a KB-PLOT-QP capillary

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column (50 m × 0.32 mm × 10 μm). The effluent line of the system was constantly heated to avoid the condensation of bulky hydrocarbons. CH2 was used as the basis to

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calculate the methanol conversion and selectivity of the products. Dimethyl ether was regarded as an unreacted species in the calculations because it is the result of methanol condensation. The regeneration process for the catalyst was calcined in air at 550 oC for 6 h.

The coke species in the deactivated catalysts were analyzed by gas

chromatography-mass spectrometry (GC-MS). The deactivated catalysts were ground for 5 min and etched by HF solution for 24 h. Next, the organic species were extracted by dichloromethane (CH2Cl2) solution for 1 h and analyzed by GC-MS (Agilent 6890/5975) equipped with HP-5 capillary column. The organic compounds were identified using the NIST11 database. 9

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3. Results and discussion 3.1 Catalyst characterization Figure 1 shows the X-ray power diffraction patterns of the as-synthesized

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samples. The S-0 sample which was prepared without addition of SAPO-34 seed crystals has a typical AEI structure that is consistent with that of pure SAPO-18 [42,

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45]. This result agrees with the results reported by Fan et al. [37]. As the amount of added SAPO-34 seed crystals increased, the SAPO-34 peak intensities (2θ =16.0o,

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17.9o, 20.6o, 25.2 o and 30.5 o) gradually increased, while the SAPO-18 peak intensities

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(2θ = 10.6o, 17.0 o, 21.3o and 23.9 o) decreased. The samples with less than 7% SAPO-34 seed crystals addition (sample S-1, S-3 and S-5) contain both AEI and CHA

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structures, suggesting that these samples are SAPO-18/34 intergrowth zeolites. For S-7, the peaks belonging to SAPO-18 have disappeared. Consequently, the XRD

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pattern of S-7 is characteristic of a pure CHA-structure [49], confirming that the use

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of 7% SAPO-34 seed crystals produces pure SAPO-34. These results suggest that the presence of SAPO-34 seed crystals is favorable for the formation of SAPO-34 and

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inhibit the formation of SAPO-18 [50]. The scanning electron microscopy (SEM) images of the SAPO-34 seed crystals

are presented in Figure S1. The crystals are cubic with diameters of 200‒800 nm. Figure 2 shows the SEM images of the SAPO samples. S-0 exhibits a plate-like morphology which is characteristic of AEI-type zeolites [42, 46]. With the addition of SAPO-34 seed crystals, the SAPO samples all have “thin” cubic-like morphologies. Furthermore, these samples are discerned to be aggregates of many nanosized plate-like nanoparticles. The surface of the samples became rougher when the amount of SAPO-34 seed crystals added increased. The formation of hierarchical

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structure of these samples could decrease the transportation distances of the reactant and product molecules in catalyzed reactions [19]. The TEM micrographs of the samples synthesized with different amounts of

added, more meso- and macropores formed in the samples.

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SAPO-34 seed crystals are shown in Figure 3. As more SAPO-34 seed crystals were

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The chemical compositions of the samples determined from ICP-OES and the

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relative yields are presented in Table 1. The samples prepared with SAPO-34 seed crystals all contain higher amounts of Si than SAPO-18 (sample S-0). For the samples

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prepared with the addition of SAPO-34 seed crystals, the amount of Si in the samples decreased in the order S-1 > S-3 > S-5  S-7. The variation of the Si contents in the

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resulted SAPO samples may be due to the synergistic effect between the increase of SAPO-34 phase in the samples and the increasing addition of the SAPO-34 seed

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crystals. On the one hand, the silicon resides preferentially in the CHA cages instead

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of in the AEI cage and high level of silicon favors the formation of SAPO-34 over SAPO-18, meaning that SAPO-34 has higher amount of Si contents than the

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SAPO-18 [47, 51]. So the increasing SAPO-34 fractions in the samples would lead to higher Si contents. On the other hand, the introduction of the SAPO-34 seed crystals would hinder the incorporation of Si into the matrix [50, 52]. As a result, the change of the Si contents in the SAPO samples is a result of a combination of these two factors. The Si content directly affects the acidic properties of the samples, which will be discussed later.

The product yields for all the samples (Table 1) are comparable implying that the addition of the SAPO-34 seed crystals does not affect the yield of the resultant products. 11

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The N2 adsorption-desorption isotherms and the corresponding BJH pore size distributions of the calcined SAPO samples are shown in Figure 4. Sample S-0 has a type I isotherm (Figure 4(a)). The N2 uptake that occurs at relative high pressures

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(P/P0 > 0.9) can be attributed to intercrystalline porosity, which may stem from intercrystalline pores caused by the stacking of the plate-like SAPO-18 [53]. The

cr

isotherms for the samples prepared with the addition of SAPO-34 seed crystals at 3%

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or more exhibit a combination of type I and type IV isotherms with obvious hysteresis loops. The hysteresis loops are indicative of mesopores in the samples [54, 55].

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The BJH pore size distributions of the samples are shown in Figure 4(b) and the textural properties are given in Table 2. The samples prepared with addition of 3% or

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more seed crystals have broad pore size distributions ranging from 10‒120 nm. These pore size distributions are in good agreement with the TEM results (Figure 3). As the

d

SAPO-34 crystal seeds increased from 1% to 5%, the specific surface areas, the total

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pore volumes, the external surface areas and the mesopore volumes of the samples all gradually increased (Table 2). However, further increasing the amount of SAPO-34

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seed crystals to 7% did not greatly affect any of these parameters and the results for S-5 and S-7 are quite similar. Compared to SAPO-18 (sample S-0), the micropore surface areas and micropore volumes increased slightly with the addition of any amount of SAPO-34 seed crystals. The

29

Si and

31

P MAS NMR NMR spectra are measured to investigate the

coordinations of Si and P in these samples and the results are shown in Figure 5. As shown in Figure 5(a), 29Si NMR spectra show that all the samples exhibit a dominant resonance peak at ‒91 ppm, which stems from the contribution of the Si(4Al) coordination [26]. Several resonance peaks at ‒95, ‒100, –105 and ‒110 ppm could 12

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also be observed, which are attributed to Si(3Al), Si(2Al), Si(1Al) and Si(0Al) species, respectively [56, 57]. Moreover, the pure-phase SAPO-18 (S-0) has more Si(0Al)

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species, i.e., Si islands, suggesting that SAPO-18 prompts the formation of Si islands [37]. In the 31P MAS NMR spectra (Figure 5(b)), S-0 exhibits two resonance peaks at

cr

‒29 and ‒12 ppm, which is typical for SAPO-18 [47, 57]. After the addition of

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SAPO-34 seed crystals, the resonance peak at ‒12 ppm gradually decreases and then vanishes for S-7 with the increasing addition of SAPO-34 seed crystals. It is reported

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that the existence of the resonance peak at ‒12 ppm is attributed to the occurrence of the SAPO-18 or SAPO-34/18 intergrowth [57]. As a result, the decrease of the

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resonance peak at ‒12 ppm is owing to the reduction of the SAPO-18 or SAPO-34/18 intergrowth fractions. Moreover, S-7 exhibits one single resonance peaks at ‒29 ppm,

the XRD results.

te

d

indicative of pure-phase SAPO-34 [47]. The above phenomena are in agreement with

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The acidic properties of the samples were determined by NH3-TPD. The desorption profiles are shown in Figure 6 and the acid parameters are listed in Table 3. Figure 6 indicates that all the samples exhibit two distinct desorption peaks centered at approximately 150‒250 and 350‒450 oC, which correspond to weak and strong acid sites, respectively. The MTO reaction mainly occurs at strong acid sites [58, 59], so they will be the focus of this discussion. The strong acid peaks shifts to higher temperatures when SAPO-34 seed crystals were added, indicating that the strong acid sites in SAPO-18/34 intergrowth (samples S-1, S-3 and S-5) and in SAPO-34 (sample S-7) are stronger than those in the SAPO-18 (sample S-0). As shown in Table 3, SAPO-18/34 intergrowth (samples S-1, S-3 and S-5) and SAPO-34 (sample S-7) also 13

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possess more strong acid sites than the parent SAPO-18 (sample S-0). As the SAPO-34 crystal seeds increased from 1% to 5%, the strength and the amount of the strong acid sites in the samples gradually decreased. Increasing the amount of seed

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crystals to 7% did not appreciably change these parameters for S-7 compared with S-5. The acid properties of the SAPO samples are influenced by the changes in the Si

cr

content [41], and the NH3-TPD results are in accordance with the variation of Si

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content seen in Table 1.

To study the formation process of the hierarchical SAPO-18/34 intergrowth and

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SAPO-34 zeolites, a series of time-dependent experiments were conducted for the sample S-7. Figures 7 and 8 show the XRD patterns and the SEM images of S-7

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crystallized at different times, respectively. When the synthesis time was 0.5 h, the gel exhibits weak CHA diffraction peaks at 2θ = 9.5o, 16.0o and 20.6 o (Fig. 7), which is

d

due to the presence of the SAPO-34 crystal seeds. At longer times (1 to 2 h), the

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SAPO-34 peak intensities increase as the SAPO-34 grows with significant increases in intensities between 1.5 and 2 h. After 8 h, the peak intensities do not increase

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appreciably further.

The SEM images (Figure 8) show that at the beginning of the crystallization

process, the synthesis gel contains cube-like SAPO-34 crystal seeds. At longer times (1 h), the seeds gradually dissolve into small species and large amounts of nanoplate-like particles begin to form. At 1.5 h, the nanoparticles have begun to coalesce to form “thin” cubic crystals with rough surfaces and as the crystallization proceeds, nanoplate-like particles are attached to the surfaces of the crystals. Thus the nano-pieces gradually disappear and hierarchical “thin” cubic-like SAPO-34 is formed. At crystallization times of 8 h or longer, the morphology of the samples remains basically unchanged. 14

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Based on these results, a possible growth mechanism for the hierarchical SAPO-18/34 intergrowth and SAPO-34 is proposed and illustrated in Figure 9. At the early stage of the crystallization process, the SAPO-34 seed crystals begin to dissolve

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into small species to provide a large number of nucleuses. Thanks to the rapid formation of the large amounts of primary particles of SAPO-34, the raw materials

cr

could directly grow on the external surface of the nucleuses and SAPO-34 is formed

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[22]. Moreover, the introduction of SAPO-34 seeds could decrease the stacking faults and suppress the formation of SAPO-18, thus more SAPO-34 could be produced [22,

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50]. Then a large amount of nanoplate-like particles are formed. Finally, hierarchical “thin” cubic crystals begin to form due to the stacking of nanoplate-like particles,

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which is the key process for the formation of mesopores and macropores. 3.2 Catalytic performance in the MTO reaction

d

The catalytic properties of the samples were evaluated for the MTO reaction.

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The conversions of methanol versus time-on-stream (TOS) over the samples are depicted in Figure 10. The catalyst life is defined as the reaction duration from the

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beginning to the time when the methanol conversion drops below 99%. The SAPO-18/34 intergrowth (samples S-1, S-3 and S-5) and SAPO-34 (sample S-7) all exhibited remarkably longer catalytic lifetimes than the parent SAPO-18 (3.1 h). It is reported that SAPO-18 possesses relative weak acidity than that of SAPO-34, and thus SAPO-18 always has longer lifetime in MTO than SAPO-34 [46, 48]. Here, considering that SAPO-18/34 intergrowth and SAPO-34 have increased acid strength and higher acid concentration of the strong acid sites than SAPO-18, the prolonged lifetime of SAPO-18/34 intergrowth and SAPO-34 can be attributed to the shortened diffusion paths owing to the formation of the hierarchical structures. As the SAPO-34 crystal seeds increased from 1% to 5%, the lifetime of the samples S-1, S-3 and S-5 15

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gradually increased to 4.1 h, 5.6 h and 6.6 h, respectively. The lifetime for S-7 was similar to that of S-5 (6.6 h). The longest lifetime of S-5 and S-7 may be further attributed to their largest external surface and appropriate acidities for the MTO

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reaction. The product distributions of the samples are presented in Figure 11. All the

cr

samples prepared with SAPO-34 seed crystals have significantly improved

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selectivities to ethylene plus propylene (C2=-C3=) and ethylene compared to those for the parent SAPO-18 (sample S-0). Conversely, the selectivities to propylene and

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butylene are lower over the seed-crystal samples than those over sample S-0. The decreased selectivity for ethylene and increased selectivities for propylene and

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butylene can be attributed to the fact that the internal cage size of SAPO-18 (AEI) is larger than that of SAPO-34 (CHA), and thus more propylene and butylene are

d

produced over SAPO-18 than over SAPO-34 in the MTO reaction [44, 46, 60].

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Moreover, with increasing amounts of SAPO-34 seed crystals added, both the C2=-C3= and ethylene selectivities gradually increased. As mentioned above, the considerably

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enhanced ethylene fraction may stem from two reasons: firstly, the increasing ratio of CHA cages in the resultant products, which is favorable for the formation of ethylene. Secondly, the relatively mild acidity also is beneficial for higher ethylene production [16]. It is expected that increasing the ratio of SAPO-34 in the resultant products should lead to a decrease in propylene and butylene selectivities. However, as shown in Figures 10 (a) and (c), the propylene and butylene selectivities are comparable for SAPO-18/34 intergrowth (samples S-1, S-3 and S-5) and SAPO-34 (sample S-7). This may be because propylene and butylene can easily escape from the hierarchical structures of SAPO-18/34 intergrowth and SAPO-34, which is attributed to the improved mass transfer of these samples [21]. 16

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Regeneration process of sample S-7 was conducted for three times to investigate the catalyst hydrothermal stability, which is important for the MTO process in

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fluidized bed reactor. As shown in Figure S2, the lifetime of sample S-7 after the three cycles of regeneration process are quiet similar with the parent catalyst and the

cr

C2=-C3= selectivities after these regenerations are stable. This result proves the

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excellent hydrothermal stability of the resultant catalyst during the harsh regeneration process.

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The amount of coke on the spent catalysts was characterized after the MTO reaction by TGA and the results are shown in Figure 12 (a). The weight losses of the

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deactivated samples are between 300-750oC and these are all related to the combustion of the deposited coke. The amounts of coke deposition on S-1, S-3, S-5

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and S-7 are all much higher than that on S-0 and more coke was deposited on the

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samples with the addition of larger amounts of SAPO-34 crystal seeds in the synthesis. This is in agreement with the catalyst lifetime results and stems from the fact that

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more coke can deposit on the samples that have higher external surface areas [55, 61]. The average coke deposition rates of the samples are shown in Figure 12 (b). The rates for the hierarchical SAPO-18/34 intergrowth and SAPO-34 zeolites are all lower than that of the parent SAPO-18. This is due to the longer lifetimes of these samples. The deposition rates trend is in good agreement with the lifetime trends where the samples with the longest lifetimes (S-5 and S-7) have the lowest average coke deposition rates. Clearly the addition of 5% or 7% SAPO-34 seed crystals is optimal in the synthesis for anti-coking properties.

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After the reactions, the composition of the coke in spent catalysts was analyzed by GC-MS and the results are shown in Figure S3. All samples exhibit the similar

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composition of organic species, such as the benzenes, methyl-substituted benzenes and the polycyclic aromatics. According to the hydrocarbon pool mechanism, the

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polymethyl-substituted benzenes and the corresponding carbenium ions are the active

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species in MTO reactions [62-66]. However, compared with the parent SAPO-18, the hierarchical SAPO samples possess more phenanthrene, pyrene and their derivatives.

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It is supposed that the hierarchical samples could accommodate more bulky coke

lifetime of the catalysts [13,52, 67]. 4. Conclusions

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species due to the improved diffusion of the reactants and products, thus prolong the

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With the assistance of SAPO-34 seed crystals, hierarchical SAPO-18/34

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intergrowth and SAPO-34 zeolites were synthesized using SAPO-18 zeolite

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precursors in an aminothermal system, which used the TEA as both the solvent and the template. Adding SAPO-34 seed crystals resulted in the formation of hierarchical SAPO-18/34 intergrowth zeolites (for 1% to 5% seed crystals) and pure SAPO-34 zeolite (for 7% seed crystals). The SAPO-18/34 intergrowths and SAPO-34 zeolites had larger external surface areas, higher mesopore volumes, and more numerous and stronger strong acid sites than the single phase SAPO-18. The amount and strength of the strong acid sites decreased with increasing amounts of SAPO-34 seed crystals up to 5%. Further increasing the amount of seed crystal did not decrease these parameters further. The hierarchical SAPO-18/34 intergrowths and SAPO-34 zeolites had 18

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prolonged lifetimes and increased C2=-C3 = selectivities compared with the single phase SAPO-18 in MTO reaction. This can be attributed to the higher external surface areas

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and the appropriate acidic properties of these samples. Moreover, the samples prepared with more SAPO-34 seed crystals had enhanced ethylene fractions owing to

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higher ratios of CHA cages and relatively mild acidity of these samples; however,

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higher ratios of CHA cages did not lead to decreased selectivities for propylene and butylene for the hierarchical SAPO-18/34 intergrowths and SAPO-34 zeolites. The

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similar selectivities for propylene and butylene are due to the hierarchical structure of

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these samples, which resulted in short diffusion distances.

Acknowledgement

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This work was partially supported by the National Natural Science

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Foundation of China (21276183).

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Table 1 Compositions and yields of the as-prepared samples. Sample Chemical composition (mol)a S-0 Al0.506P0.409Si0.085O2 S-1 Al0.484P0.392Si0.124O2 S-3 Al0.489P0.396Si0.115O2 S-5 Al0.494P0.402Si0.104O2 S-7 Al0.496P0.398Si0.106O2 a Measured by ICP-OES.

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Pore volume (cm3 g-1) Vtotal c Vmicrod VMesod 0.270 0.249 0.021 0.300 0.250 0.050 0.350 0.251 0.099 0.373 0.255 0.118 0.379 0.257 0.122

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Table 2 Textural properties of SAPO samples. Surface area (m2 g-1) Sample SBET a Smicrob Sextb S-0 550 531 19 S-1 563 534 29 S-3 578 537 41 S-5 601 540 61 S-7 604 545 59

Yield 79.3% 78.6% 80.0% 80.9% 80.8%

a

SBET was calculated from the BET equation. Sext was determined using the t-plot method, Smicro = SBET – Sext. c Vtotal was calculated using the adsorbed volume at p/p0 = 0.99. d Vmicro was evaluated from the t-plot method, VMeso = Vtotal – Vmicro.

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b

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Table 3 Amount and types of acid sites in the SAPO samples. Acid amounts (mmol g-1)a Sample Total acidity Strong acid S-0 1.227 0.173 S-1 0.967 0.283 S-3 1.026 0.252 S-5 1.115 0.229 S-7 0.897 0.221 a Calculated by NH3-TPD.

Weak acid 1.054 0.684 0.774 0.886 0.676

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Figure captions Figure 1 XRD patterns of SAPO samples. Figure 2 SEM images of sample S-0 (a, b), S-1 (c, d), S-3 (e, f), and S-5 (g, h) and

Figure 3 TEM images of sample S-1 (a), S-3 (b), S-5 (c) and S-7 (d).

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S-7 (i, j).

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Figure 4 N2 adsorption−desorption isotherms (a) and the corresponding BJH pore size distributions (b) of the calcined SAPO samples.

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Figure 5 29Si (a) and 31P (b) MAS NMR spectra of as-synthesized SAPO samples.

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Figure 6 NH3-TPD profiles of calcined SAPO samples.

Figure 7 XRD patterns of sample S-7 prepared for different crystallization times.

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Figure 8 SEM images of sample S-7 prepared for different crystallization times. Figure 9 Schematic illustration of the formation of hierarchical SAPO-18/34

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intergrowth and SAPO-34 zeolites.

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Figure 10 Methanol conversions with time on stream over SAPO samples. Figure 11 Propylene plus ethylene selectivity (a), ethylene selectivity (b), propylene

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selectivity (c) and butene selectivity (d) of MTO reaction with time on stream over SAPO samples.

Figure 12 TGA profiles (a) and carbon deposition rates (b) of deactivated SAPO samples.

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Highlights 

Hierarchical SAPO-18/34 intergrowth and SAPO-34 zeolites were prepared by adding SAPO-34 seeds to SAPO-18 zeolite precursor. The phase of the products varied with the amount of added SAPO-34 seed crystals.



The SAPOs are aggregates of many nanosized plate-like nanoparticles.

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Hierarchical SAPOs showed superior catalytic performance in the MTO reaction.

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Graphical Abstract

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The amount of the added SAPO-34 seed crystals had an important effect on the phase and the catalytic performance of the products.

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