Synthesis of nano-sized SAPO-34 with morpholine-treated micrometer-seeds and their catalytic performance in methanol-to-olefin reactions

Journal Pre-proof Synthesis of nano-sized SAPO-34 with morpholine-treated micrometer-seeds and their catalytic performance in methanol-to-olefin reactions Chao Sun, Yaquan Wang, Aijuan Zhao, Xiao Wang, Cui Wang, Xu Zhang, Ziyang Wang, Jingjing Zhao, Taotao Zhao

PII:

S0926-860X(19)30469-7

DOI:

https://doi.org/10.1016/j.apcata.2019.117314

Reference:

APCATA 117314

To appear in:

Applied Catalysis A, General

Received Date:

27 June 2019

Revised Date:

28 September 2019

Accepted Date:

21 October 2019

Please cite this article as: Sun C, Wang Y, Zhao A, Wang X, Wang C, Zhang X, Wang Z, Zhao J, Zhao T, Synthesis of nano-sized SAPO-34 with morpholine-treated micrometer-seeds and their catalytic performance in methanol-to-olefin reactions, Applied Catalysis A, General (2019), doi: https://doi.org/10.1016/j.apcata.2019.117314

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Synthesis

of

nano-sized

micrometer-seeds

and

SAPO-34 their

with

catalytic

morpholine-treated performance

in

methanol-to-olefin reactions

Chao Sun, Yaquan Wang*, Aijuan Zhao, Xiao Wang, Cui Wang, Xu Zhang, Ziyang

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Wang, Jingjing Zhao and Taotao Zhao

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

Graphical Abstract

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Uniform nano-sized SAPO-34 zeolite was synthesized with the morpholine-treated micrometer-sized SAPO-34 as seeds, during which the morpholine was also used as the template.

1

Highlights 

Uniformly

nano-sized

SAPO-34

zeolites

were

synthesized

using

morpholine-treated micrometer-sized SAPO-34 seeds and morpholine as the template. 

Micrometer-sized seeds were broken into nanoparticles after morpholine treatment for 30 h. Morpholine-treatment of the micrometer-sized seeds was indispensible for the

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

synthesis of nano-sized SAPO-34.

The nano-sized SAPO-34 showed superior performance in MTO reactions.

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

Abstract

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Uniformly nano-sized SAPO-34 zeotypes were synthesized using morpholine

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(MOR)-treated micrometer-sized SAPO-34 (MS-SAPO-34) seeds and MOR as the template. The samples were prepared with different amounts of MOR-treated MS-SAPO-34

seeds

and

characterized 29

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adsorption-desorption, NH3-TPD,

by

XRD,

SEM,

ICP-OES,

N2

Si NMR, TGA and GC-MS. Using MOR-treated

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MS-SAPO-34 seeds decreased the crystal size of the SAPO-34 to 200–500 nm. In addition, the samples prepared with MOR-treated MS-SAPO-34 seeds had larger

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external surface areas, lower Si contents, and lower concentration and weaker strength of the strong acid sites than conventional SAPO-34. The effects of MOR treatment time on the seed properties and on the subsequent SAPO-34 fabrication were also investigated. After MOR-treatment of the MS-SAPO-34 seeds for 30 h, micrometer-sized seeds were broken into nanoparticles, which was indispensable for successful synthesis of nano-sized SAPO-34. When studied as a catalyst for the 2

methanol to olefin reaction, the nano-sized SAPO-34 exhibited excellent catalytic performance. Keywords: Seeds; MOR-treatment; Nano-sized SAPO-34; Methanol to olefin. 1. Introduction Light olefins such as ethylene and propylene are the most important feedstocks in the petrochemical industry [1]. Due to increasing prices for crude oil and growing

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demands for light olefins, non-petroleum based routes for producing light olefins are becoming increasingly desirable [2, 3]. The catalytic conversion of methanol-to-olefin (MTO) has proven to be an important route for the production of light olefins from

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non-petroleum resources [4-7]. SAPO-34 is a small-pore zeotype with 8-membered

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ring pore openings (3.8Å x 3.8Å) and CHA supercages. It has moderate acidity and has been shown to exhibit excellent catalytic performance for MTO processes [2, 8, 9].

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However, the microporous structure of SAPO-34 limits diffusion for bulky components, which results in rapid coke formation during the MTO process [10, 11].

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Consequently, SAPO-34 catalysts have short lifetimes and require continuous catalyst

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regeneration [12].

Several methods have been developed to alleviate the diffusion limitations and

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prolong the lifetimes of SAPO-34 catalysts for MTO reactions. Previous studies have shown that introducing hierarchical pore structures into SAPO-34 crystals enhances the mass transfer of molecules and prevents the agglomeration of methanol-derived intermediates, which prolongs the lifetime of SAPO-34 [11, 13-19]. Hierarchical SAPO-34 is typically prepared via one of two strategies, i.e., “bottom-up” or 3

“top-down” methods. The bottom-up method involves the use of secondary templates during SAPO-34 synthesis [19-22]. The secondary templates are classified as either hard- or soft-templates and the hierarchical SAPO-34 are obtained after combustion of the secondary templates. Top-down methods include the generation of hierarchical SAPO-34 by post-synthesis treatments such as dealumination or desilication techniques [23]. Another method is the synthesis of nano-sized SAPO-34 crystals to

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shorten the diffusion distances and improve the accessibility of the reactant molecules

to the active sites [2, 24-27]. This increases the lifetime of SAPO-34 for the MTO

reaction. The effects of the crystallite size on the MTO process have been investigated

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and SAPO-34 with crystal sizes smaller than 500 nm had the best catalytic

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performance owing to their short diffusion distances [28, 29]. Yang and co-workers [30] prepared SAPO-34 nanocrystals using a post-synthesis milling and

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recrystallization method. This catalyst exhibited a prolonged lifetime with improved

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selectivity for light olefins in the MTO reaction. Recently, nanosized SAPO-34 has been used as a seed for the synthesis of

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nanosized SAPO-34 with superior performance in MTO reactions [4, 31]. For example, Chen and co-workers [31] synthesized nano-sized SAPO-34 catalysts using

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morpholine (MOR) as the sole template with the assistance of calcined nanosheet-like seeds. The resulting material exhibited improved catalytic performances in the MTO reaction. They also found that the addition of micrometer-sized seeds led to SAPO-34 with extremely non-uniform morphology with large amounts of micrometer-sized cubic SAPO-34 crystals. These results indicate that size variations in the seeds 4

influence the properties of the resultant zeotypes and oversize seeds can result in products with inferior properties compared to those synthesized with smaller seeds. However, the synthesis of nanosized SAPO-34 crystals is complicated and requires large amounts of the expensive tetraethylammonium hydroxide (TEAOH) [2, 29, 32-34]. Up to now, there has been no report about the synthesis of nano-sized SAPO-34

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using the micrometer-sized SAPO-34 (MS-SAPO-34) seeds. In this work, the

MS-SAPO-34 seeds are first broken into nanoparticles using MOR treatment for 30 h and these seeds are used for the synthesis of nano-sized SAPO-34 zeotypes. The

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physical properties of the samples are characterized and the catalytic performance of

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samples for the MTO process is investigated. In addition, the influence of MOR treatment time on the seed properties as well as on the subsequent SAPO-34 synthesis

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is further investigated. The resultant nanosized SAPO-34 shows a significantly

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prolonged lifetime and improved selectivity for ethylene and propylene (C2=-C3=) in the MTO reaction.

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2. Experiment 2.1 Materials

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Pseudoboehmite (Al2O3, 68%) was supplied from Shandong Aluminum Industry

Co., Ltd., China. Quartz particles (20-40 mesh) and aluminum isopropoxide [Al(OPri)3, 99.5%] were purchased from Tianjin GuangFu Fine Chemical Research Institute Co., Ltd., China. Triethylamine (TEA, 99 wt%) and methanol (AR) were purchased from Tianjin Fuyu Fine Chemical Co., Ltd., China. Phosphoric acid 5

(H3PO4:85 wt %, aqueous solution) was purchased from Tianjin JiangTian Fine Chemical Research Institute Co., Ltd., China. Ludox AS-40 silica gel (SiO2, 40 wt % suspension in water) was obtained from Qingdao Haiyang Chemical Co., Ltd., China. Tetraethyl orthosilicate (TEOS, 98 wt %) and MOR (98.5 wt %) were obtained from Tianjin Damao Chemical Reagent Factory (Tianjin, China). 2.2 Synthesis of MS-SAPO-34 seeds.

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The gel ratio for the synthesis of MS-SAPO-34 zeotypes was 3TEA: 0.6SiO2:1Al2O3:1P2O5:50H2O. The typical hydrothermal synthesis was conducted as

follows: pseudoboehmite and deionized water were first mixed and then phosphoric

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acid was added to the mixture under stirring. After continuous stirring for 1 h, silica

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gel was slowly added to the solution and vigorous stirring was continued for another 1 h. Finally, TEA was added to the solution and the mixture was stirred at ambient

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temperature for 2 h. The solution was then transferred to an autoclave and aged at

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120 °C for 24 h, followed by crystallization at 200 °C for 48 h under static conditions. After crystallization, the product was centrifuged, washed with distilled water and

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dried overnight at 120 °C. The organic template was removed by calcination at 550 °C for 5 h with a ramping rate of 5 °C min-1 in air.

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2.3 Synthesis of conventional SAPO-34 zeotypes The typical gel ratio for the synthesis of conventional SAPO-34 was

4MOR:0.6SiO2:1.0Al2O3:1P2O5:70H2O. The synthesis was carried out as follows: Al(OPri)3 (9.52 g) was mixed with water (28.56 g) and stirred for 15 min, and then phosphoric acid (5.37 g) was added dropwise and the solution was vigorously stirred 6

for 1 h. Then TEOS (2.91 g) was slowly added to the mixture. After vigorous stirring for 1 h, MOR (8.20 g) was added to the solution. Finally, the resulting mixture was stirred for 2 h and then transferred to a 50-mL autoclave. Crystallization was conducted at 200 °C for 48 h and the solid products were collected by filtration, washed several times with distilled water and then dried overnight at 120 °C. The organic template was removed by calcination at 550 °C for 5 h with a ramping rate of

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5 °C min-1 in air. The obtained sample is denoted as C-SAPO-34.

2.4 Synthesis of nano-sized SAPO-34 zeotypes with MOR-treated MS-SAPO-34 seeds

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The synthesis of nano-sized SAPO-34 zeotypes with the MOR-treated

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MS-SAPO-34 seeds was hydrothermally synthesized with a synthesis gel consisting of a molar composition of 4MOR:0.6SiO2:1.0Al2O3:1P2O5:70H2O. MS-SAPO-34

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seeds were firstly treated with MOR solution under stirring and reflux for 30 h:

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solution A was prepared by mixing MOR (8.20 g) and distilled water (10 g) with stirring for 15 min. Then the desired amount of MS-SAPO-34 seeds was added to the

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solution and this mixture was treated at 80 °C for 30 h under stirring and reflux. Next, solution B was prepared by first dissolving Al(OPri)3 (9.52 g) in distilled water (18.56

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g) and stirring for 15 min. Then phosphoric acid (5.37 g) was added dropwise, followed by vigorous stirring for 1 h. Finally, TEOS (2.91 g) was added and this solution was vigorously stirred for 1 h. Subsequently, solution A was slowly added into solution B and the final mixture was stirred at room temperature for another 2 h to obtain a homogenous solution. The resulting gel was then transferred into a 50-mL 7

Teflon-lined autoclave and hydrothermally treated at 200 °C for 48 h. The collection and treatment of the final product was the same as that for C-SAPO-34. The obtained samples are denoted as TS-SAPO-34-x, where TS and x refer to MOR-treated seeds and the weight ratio of the SAPO-34 seeds added to the inorganic oxides in the initial synthesis gel, respectively. 2.5 Synthesis of SAPO-34 zeotypes with MOR-treated MS-SAPO-34 seeds with

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different treatment times

SAPO-34 zeotypes with 4% MS-SAPO-34 seeds after MOR treatment for 5, 15 or 45 h were prepared by following the above synthesis procedure for TS-SAPO-34-4,

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except that the SAPO-34 seeds were MOR-treated for 5, 15 or 45 h instead of 30 h. In

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addition, SAPO-34 zeotypes prepared with 4% MS-SAPO-34 seeds without any MOR treatment was conducted by adding 4% MS-SAPO-34 seeds into the synthesis

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gel for C-SAPO-34, using the same synthesis procedure as that for of C-SAPO-34.

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This sample is denoted as S-SAPO-34-4. 2.6 Characterization

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X-ray diffraction (XRD) patterns of the solid products were acquired on an X-ray diffractometer (model: Bruker D8 Focus) with Cu Kα radiation (𝜆 = 0.15418 nm)

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from 5‒40° at a scanning rate of 8° min-1. The crystal size and morphology of the samples were determined using scanning

electron microscopy (SEM, Hitachi S-4800) at 3 kV and 5 μA. The chemical compositions of the samples after calcinations were determined with inductively coupled plasma optical emission spectroscope (ICP-OES, 8

VISTA-MPX). The textural data of the calcined samples were determined using a Micrometermeritics TriStar 3000 automated physisorption apparatus. Before measuring the N2 adsorption-desorption isotherms at liquid N2 temperature (–196 °C), the samples were first pretreated at 300 °C for at least 4 h under a flow of N2. The

acid

properties

of

the

samples

were

determined

using

the

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temperature-programmed desorption of ammonia (NH3-TPD, Xianquan TP-5076 TPD analyzer). A 0.10 g sample (40–60 mesh) was outgassed under a He flow at 400 °C

for 60 min and then cooled to 100 °C. Subsequently, the ammonia was injected and

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adsorbed for 10 min. To remove the physically adsorbed ammonia, the samples were

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purged in a He flow for 2 h. Measurement of chemisorbed ammonia was carried out from 100‒700 °C with a heating rate of 10 °C per minute and detected by thermal

lP

conductivity detector (TCD).

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All solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) experiments were carried out on a Varian infinity plus 300 MHz NMR spectrometer at 29

Si MAS NMR spectra were measured at 59.6

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a magnetic field strength of 7.1 T.

MHz at a spinning frequency of 4 kHz and chemical shifts were referenced to

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tetramethylsilane (TMS) for 29Si. The amount of coke deposition on the samples after the MTO reaction was

determined using thermogravimetric analysis (TGA, Shimadzu TGA-50) under an oxygen flow. The samples were heated from room temperature to 750 oC with a ramp rate of 10 °C per minute. The average coke deposition rate was calculated using the 9

equation: Rcoke= coke content (%, g g−1)/reaction time (h). The composition of the coke from the deactivated SAPO-34 samples was determined by gas chromatography-mass spectrometry (GC-MS, Agilent 6890/5975). Before GC-MS measurements, the deactivated SAPO-34 samples were finely ground and then dissolved in HF, followed by extraction with dichloromethane. The organic coke species were identified using the NIST11 database.

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2.7 Catalytic tests

The catalytic performance of the SAPO-34 samples for the MTO reaction was

determined at 425 °C in a fixed-bed reactor (with the length of 500 mm and inner

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diameter of 10 mm) operating at atmospheric pressure. In a typical run, the calcined

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catalyst (1.0 g; 20–40 mesh) and quartz sand (1.0 g; 20–40 mesh) were mixed, and then loaded into the reactor. Prior to each reaction, the catalyst was pretreated under a

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N2 atmosphere of 60 mL min−1 at 425 °C for 1 h. A mixture of methanol and water with a molar ratio of 1:1 was then fed into the reactor using a HPLC pump. The

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weight hourly space velocity (WHSV) of the methanol was 2 h−1. The products were

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analyzed using an on-line BEIFEN SP-2100 gas chromatograph, with a flame ionization detector and a KB-PLOT-QP capillary column (50 m × 0.32 mm × 10 μm).

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The methanol conversion and selectivity of products were calculated on the basis of carbon. The dimethyl ether in the product was treated as an unreacted species rather than as a product. 3. Results and discussion 3.1 Catalyst characterization 10

Figure 1 shows the XRD patterns of the prepared SAPO-34 samples. All of them match well with the typical diffraction patterns of the CHA-type structure, without the presence of impurity [32, 33]. The peak intensities of TS-SAPO-34-x are lower than those in C-SAPO-34 and the broadening of the diffraction peaks can be discernible, suggesting that the particle size of TS-SAPO-34-x is smaller than those in C-SAPO-34 [2, 24, 30]. The yields for the SAPO-34 samples are shown in Table 1

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and in all cases the addition of MOR-treated MS-SAPO-34 seeds result in higher yields compared to that for C-SAPO-34.

SEM images of the MS-SAPO-34 seeds and the as-prepared SAPO-34 samples

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are shown in Figure 2. The MS-SAPO-34 seeds (Figure 2(a)) have a cubic-like

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morphology with crystal sizes of 3‒5 μm. All of the SAPO-34 samples also have cubic-like morphologies but the crystal size of the cubes varies significantly.

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C-SAPO-34, synthesized without the addition of the MOR-treated MS-SAPO-34

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seeds, has crystal sizes ranging from 5 to 10 μm (Figure 2(b)). When MOR-treated MS-SAPO-34 seeds were added, the crystal sizes gradually decreased as the amount

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of MOR-treated SAPO-34 seeds increased from 0 to 4%. With 4% MOR-treated SAPO-34

seeds,

the

large

SAPO-34

crystals

completely

disappear

and

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TS-SAPO-34-4 exhibits the smallest crystals ranging from 200 to 500 nm (Figure 2(e)). However, when the seed amount is increased to 8%, some large SAPO-34 crystals appear in TS-SAPO-34-8 (Figure 2(f)). In addition, SEM images of the different amount (0-8%) of the MS-SAPO-34 seeds after MOR-treatment for 30 h are shown in Figure S1. After MOR-treatment for 30 h, 0–4% of the MS-SAPO-34 seeds 11

is broken into small fragments. However, completed cubic SAPO-34 crystals could be observed among the nanoparticles when the amount of the MS-SAPO-34 seeds is 8%. This is probably because the synthesis gel has insufficient alkalinity to dissolve this higher concentration of micrometer-sized seeds during the MOR-treatment process, which led to the formation of the large SAPO-34 crystals. The chemical composition of the SAPOs samples were obtained by ICP-OES

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and the results are listed in Table 1. As the amount of MOR-treated MS-SAPO-34

seeds increased, the silicon contents of the TS-SAPO-34 samples gradually decrease. This suggests that the addition of the MOR-treated MS-SAPO-34 seeds hinders the

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incorporation of silicon into the SAPO-34 framework and this is consistent with

The

texture

parameters

of

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previous reports [31, 35]. the

samples

were

measured

by

N2

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adsorption-desorption analyses and the results are shown in Figure 3 and Table 2. As

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shown in Figure 3, C-SAPO-34 has a type I N2 adsorption-desorption isotherm, which is typical for microporous materials [6, 25]. The TS-SAPO-34-x samples also exhibit

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type I N2 adsorption-desorption isotherms. However, the TS-SAPO-34-x samples have a much higher nitrogen uptake at relative pressures above 0.85. This can be attributed

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to the adsorption of nitrogen by the inter-crystal pores in the nanoparticles [30, 36]. These results confirm the existence of nanosized SAPO-34, as demonstrated by the SEM images. All of the samples possess comparable micropore volumes and micropore surface areas (Table 2), indicating that the microporosity of TS-SAPO-34 is well preserved after the addition of the MOR-treated MS-SAPO-34 seeds. In addition, 12

the TS-SAPO-34-x samples have larger external surface areas than C-SAPO-34 due to the smaller particle sizes. The acid properties of C-SAPO-34 and TS-SAPO-34-x were determined by NH3-TPD. The NH3-TPD curves and the amount of strong acid sites are given in Figure 4 and Table 2, respectively. The NH3-TPD curves contain two desorption peaks centered at around 150‒280 and 350‒500 °C, and these are due to weak and strong

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acid sites, respectively [32, 37-39]. It has been reported that the strong acid sites

which stem from the incorporation of Si into the SAPO-34 framework are the primary

active sites for MTO reactions [5, 27, 40]. As seen in Figure 4, the peak areas of the

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high temperature peak gradually decrease and peak positions of the high temperature

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peak shift to a lower temperature as the amount of added morpholine-treated seeds increased. These phenomena indicate that the concentration and strength in the strong

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acid sites both decrease with increasing amounts of morpholine-treated seeds. Thus,

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the TS-SAPO-34-x samples have lower concentration and weaker strength in strong acid sites than C-SAPO-34. These results are consistent with the changes of Si content

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in ICP-OES results (Table 1), since the decreased concentration and strength in strong acid sites are both a function of lower Si content.

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The chemical environment of the Si in the C-SAPO-34 and TS-SAPO-34-x were

investigated by solid-state

29

Si MAS NMR spectroscopy. The results of

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Si MAS

NMR are shown in Figure 5. As seen in Figure 5, all the samples exhibit strong peaks at around −92 ppm, which are attributed to the existence of isolated Si(4Al) species in the SAPO-34 samples [40]. The peaks at around –95, –100 and –110 ppm are 13

assigned to Si(3Al), Si(2Al) and Si(0Al) species, respectively [27]. The percentage of the Si(4Al) species in TS-SAPO-34-x is higher than that of the C-SAPO-34 and gradual increase with the increasing addition of the MOR-treated MS-SAPO-34 seeds, while the proportion of Si(3Al), Si(2Al) and Si(0Al) species of TS-SAPO-34-x present an opposite trend. Those results are consistent with previous report that lower Si content results in more Si(4Al) species in the TS-SAPO-34-x sample, which could

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form moderate acid sites [4, 26, 27, 40]. Considering that the acid strength corresponding to different Si species follows the order of Si(2Al) > Si(3Al) > Si(4Al)

[8], this result demonstrates that the acid strength in TS-SAPO-34-x is weaker than

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that in C-SAPO-34 and decreases with increasing amounts of MOR-treated seeds, in

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agreement with NH3-TPD results.

resultant SAPO-34.

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3.2. Effect of treatment time on the MS-SAPO-34 seeds and the properties of the

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The XRD patterns of MS-SAPO-34 seeds after MOR treatment for different times are shown in Figure 6. The peak intensities of the MOR-treated seeds are much

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lower than those for the MS-SAPO-34 seeds and the intensities decreased greatly with increasing treatment time. After 15 h or longer of treatment, few characteristic

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SAPO-34 peaks are observed, suggesting that the zeotypes structures have been destroyed.

The SEM images of the MS-SAPO-34 seeds after MOR treatment for different times are shown in Figure 7. The MS-SAPO-34 seeds have cubic morphologies with crystal sizes of 3‒5 μm. When the treatment time is 5 h, the MS-SAPO-34 seeds begin 14

to dissolve and the cubic morphologies start to become distorted. When the treatment time is extended to 15 h, the seeds continue to dissolve and many small pieces can be seen among the MS-SAPO-34 crystals. When the SAPO-34 seeds are treated with MOR solution for 30 h or longer, the SAPO-34 seeds are broken into small fragments. These results are in accordance with the destruction of the crystal structure seen in the XRD results (Figure 6).

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The SEM images of SAPO-34 prepared with the MS-SAPO-34 seeds after MOR treatment for different times are shown in Figure 8. For the sample prepared with

untreated SAPO-34 seeds, the SAPO-34 crystals are larger than 1 μm and show a

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broad size distribution. As the treatment time is extended, the prepared SAPO-34

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crystals become smaller and more uniform. The sample prepared with seeds treated for 30 h and 45 h contain nano-sized SAPO-34 with crystal sizes ranging from 200 to

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500 nm. These results confirm that the MS-SAPO-34 seeds after MOR treatment for

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30 h are valid for the synthesis of the nano-sized SAPO-34 and the MOR treatment of micrometer-sized seeds play a key role for the synthesis of nano-sized SAPO-34.

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Figure 9 shows the NH3-TPD curves of the samples prepared with the SAPO-34 seeds after MOR treatment for different times. As the treatment time increased, the

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peak areas of the high temperature peaks gradually decrease and the peak positions of the high temperature peaks gradually shift to lower temperatures. These results suggest that both the concentration and strength of the strong acid sites gradually decrease with longer treatment times. Thus, the treatment of the micrometer-sized seeds with MOR greatly affects the acidic properties of the resultant zeotypes. 15

Previous reports have shown that seeds with different crystal sizes have diverse structure-directing activities and nano-sized or smaller seeds are beneficial for the synthesis of nano-sized SAPO-34 [4, 31]. By breaking the MS-SAPO-34 into small fragments, nanoparticles can be more evenly dispersed into the amorphous precursors due to their significantly smaller crystal sizes and this provides more viable nuclei for the subsequent crystal growth stage. Meanwhile, the generation of uniform

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nanoparticles can result in larger external surface areas than those obtained from

conventional micrometer-sized seeds, which increases the crystal growth rate [41].

These phenomena explain the formation of uniform nano-sized SAPO-34 zeotypes

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that are obtained with the MOR-treated MS-SAPO-34 seeds. When untreated

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MS-SAPO-34 is used as seeds, the relatively large crystals cannot completely dissolve during the synthesis process and thus non-uniform SAPO-34 crystals are obtained

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[31].

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3.3 Catalytic performance in MTO reaction

The MTO catalytic performance of C-SAPO-34 and TS-SAPO-34-x were

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investigated and the results are shown in Figure 10. The catalytic lifetime is defined as the duration of the reaction during which a methanol conversion of > 99% is

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achieved. The TS-SAPO-34-x samples had considerably longer lifetimes than C-SAPO-34. As the amount of seeds increased from 0 to 4%, the lifetimes of the TS-SAPO-34-x samples gradually increased from 1.1 h to 4.6 h. Further increasing the amount of the seeds to 8% resulted in a shortened lifetime of TS-SAPO-34-8 (3.6 h). 16

Crystal size and acid properties are known to greatly influence the catalytic behavior of catalysts for the MTO reaction [5, 37, 42]. Smaller crystal sizes result in shorter transportation paths for the reactants and products which significantly reduces mass transfer restrictions [33]. Accordingly, the TS-SAPO-34 samples with nano-sized crystals have longer catalyst lifetimes for MTO reactions than catalysts with micrometer-size crystals. Mild acidity is also favorable for MTO reaction [27].

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So the lower concentrations and weaker strength in strong acid sites of

TS-SAPO-34-x are beneficial for longer catalyst lifetimes [43]. TS-SAPO-34-8 is the exception; even though it has the lowest concentration and weakest strength in strong

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acid sites, it does not exhibit the longest lifetime. This is due to the presence of large

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crystals and too few strong acid sites [27].

The product distributions of MTO over C-SAPO-34 and TS-SAPO-34-x are

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displayed in Figure 11. The C2=-C3= selectivity of TS-SAPO-34-x are higher than that

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of the C-SAPO-34 and the selectivity gradually increase with increasing amounts of MOR-treated seeds. Figure 11(b) and (c) show that the selectivity for ethylene and the

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selectivity for propylene are also both higher for the TS-SAPO-34-x samples and these values also gradually increase with increasing the amounts of seeds. The

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increasing C2=-C3= selectivity should result from the reduced particle size and decreased acidity of TS-SAPO-34-x. On one hand, owing to short diffusion distance of TS-SAPO-34-x, the formation of coke is depressed and ethylene and propylene can easily escaped from the pore structure of TS-SAPO-34-x before being further transformed into aromatics and paraffins via cyclization and hydrogen transfer of 17

olefins [3,44-47]. On the other hand, the decreased acidity could inhibit the secondary reactions, such as oligomerization and hydrogen-transfer reactions, resulting in lower selectivity for aromatics and alkanes (such as ethane and propane) and higher selectivity for light olefins (C2=-C3=) [27,48-49]. The hydrogen transfer index (HTI, C3H8/C3H6 ratio) for the methanol conversion over all the samples is shown in Figure 11(d). The HTI values of the TS-SAPO-34-x

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samples are all appreciably lower than those for C-SAPO-34 and they gradually decrease with increasing amounts of MOR-treated SAPO-34 seeds. This is the result

of the decreased acidity of the TS-SAPO-34-x samples [37, 50]. Lower HTI values

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imply the suppressing of side reactions and these values are in good agreement with

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the enhanced selectivity for ethylene and propylene for the TS-SAPO-34-x samples. The coke formation over the deactivated samples in the MTO reation was

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characterized by TGA and the results are presented in Figure 12(a). The weight loss of

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the samples between 300-750 °C is related to the combustion of the retained coke species. The amount of coke accumulated on the TS-SAPO-34-x samples is higher

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than that on C-SAPO-34, which is presumbly due to the longer reaction lifetimes. In constrast, the average coke deposition rates are signifacantly lower for the

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TS-SAPO-34-x samples than for C-SAPO-34 (Figure 12(b)). TS-SAPO-34-4 has the lowest coking rate, which is in aggrement with its longest lifetime. The high stabilities of the TS-SAPO-34-x samples can be atrributed to their small crytal sizes and mild acidities, which is consistent with their low HTI values (Figure 11(d)). To further invesitigate the effect of the crystal size on coke deposition, the coke 18

species from the deactivated samples were analyzed by GC-MS and the results are shown in Figure 13. The retained organic coke species for all the samples are similar and correlate well with the hydrocarbon pool mechanism [4, 51]. However, C-SAPO-34 retained more monocyclic and bicyclic aromatic compounds, whereas the main species on TS-SAPO-34-x were phenanthrene, pyrene and their derivatives. Since the TS-SAPO-34-x samples have smaller transportation distances, monocyclic

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and bicyclic aromatic compounds can easily escape from the internal pores and so are not retained as much [13]. It is believed that bulky polycyclic aromatics are the

primary cause of catalyst deactivation [2, 52]. So, the TS-SAPO-34-x samples,

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especially TS-SAPO-34-4, have excellent anti-coking properties since they can

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accormodate more bulky polycyclic aromatics [43].

Finally, the influence of the MOR treatment of the seeds on the MTO catalytic

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performance was investigated by comparing MTO results of S-SAPO-34-4 (prepared

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with 4% untreated seeds) with TS-SAPO-34-4. The results are shown in Table 3. TS-SAPO-34-4 has a longer lifetime and higher C2=-C3= selectivity than

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S-SAPO-34-4, which is due to its smaller particle sizes and fewer concentration and weaker strength in strong acid sites. These results further verify that the

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MOR-treatment of SAPO-34 seeds is beneficial for the synthesis of uniform nano-size SAPO-34, resulting in excellent MTO performance. In addition, TS-SAPO-34-4 has a lower Rcoke than S-SAPO-34-4, which is in accordance with its longer lifetime. 4. Conclusions Nano-sized SAPO-34 was synthesized by the introduction of MOR-treated 19

MS-SAPO-34 seeds, using MOR as the template. The adoption of MOR-treated MS-SAPO-34 seeds significantly decreased the crystal size of the resultant SAPO-34 and the addition of 4% MOR-treated seeds led to the formation of uniform nano-sized SAPO-34 in the range of 200‒500 nm. The SAPO-34 samples prepared with the MOR-treated seeds showed larger external surface areas than the conventional SAPO-34 owing to their significantly smaller particles. Furthermore, the Si content of

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the SAPO-34 prepared with the MOR-treated seeds was lower than that of

conventional SAPO-34, which resulted in a lower concentration and weaker strength of the strong acid sites. It was revealed that the MOR treatment of the

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micrometer-sized seeds for 30 h was critical for the successful synthesis of nano-sized

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SAPO-34. The nano-sized SAPO-34 was obtained after the MS-SAPO-34 seeds were MOR-treated for 30 h, during which the MOR broke large seed crystals into

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nanoparticles before crystallization. Compared to conventional SAPO-34, the

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resultant nano-sized SAPO-34 had prolonged lifetimes and higher C2=-C3= selectivity in the MTO reaction due to their small crystal sizes and appropriate acidities. This

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work develops a cost-effective approach that uses non-nanosized seeds for the synthesis of nanosized SAPO-34 zeotypes with superior MTO performance.

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Acknowledgement

This work was partially supported by the National Natural Science Foundation of

China (21276183). References [1] Y. Hirota, K. Murata, M. Miyamoto, Y. Egashira, N. Nishiyama, Catal. Lett. 140 (2010) 22‒26. 20

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[51] J.F. Haw, W.G. Song, D.M. Marcus, J. B. Nicholas, Acc. Chem. Res. 36 (2003) 317–326. [52] J.R. Chen, J.Z. Li, Y.X. Wei, C.Y. Yuan, B. Li, S.T. Xu, Y. Zhou, J.B. Wang, M.Z. Zhang, Z.M. Liu, Catal. Commun. 46 (2014) 36‒40.

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Figure captions

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Figure 1. XRD patterns of C-SAPO-34 and TS-SAPO-34-x.

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Figure 2. SEM images of MS-SAPO-34 seeds (a), C-SAPO-34 (b), TS-SAPO-34-1

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(c), TS-SAPO-34-2 (d), TS-SAPO-34-4 (e) and TS-SAPO-34-8 (f).

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Figure 3. N2 adsorption-desorption isotherms of the calcined SAPO samples.

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Figure 4. NH3-TPD profiles of C-SAPO-34 and TS-SAPO-34-x.

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Figure 5. 29Si MAS NMR spectrum of C-SAPO-34 and TS-SAPO-34-x.

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Figure 6. XRD patterns of MS-SAPO-34 seeds (a) and MS-SAPO-34 seeds after

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MOR treatment for 5 h (b), 15 h (c), 30 h (d) and 45 h (e).

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Figure 7. SEM images of MS-SAPO-34 seeds (a) and MS-SAPO-34 seeds after

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MOR treatment for 5 h (b), 15 h (c), 30 h (d) and 45 h (e).

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Figure 8. SEM images of SAPO-34 prepared with MS-SAPO-34 seeds after MOR

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treatment for 0 h (a), 5 h (b), 15 h (c), 30 h (d) and 45 h (e).

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Figure 9. NH3-TPD curves of SAPO-34 prepared with the MS-SAPO-34 seeds after

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being treated with MOR for 0 h (a), 5 h (b), 15 h (c), 30 h (d) and 45 h (e).

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Figure 10. Methanol conversions with time-on-stream over C-SAPO-34 and TS-SAPO-34-x. Experimental conditions: methanol WHSV= 2 h–1, T = 425 °C,

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catalyst weight = 1 g, molar ratio of methanol and water was 1:1, carrier gas: N2, 60

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ml min−1.

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Figure 11. Selectivity for ethylene plus propylene (a), Selectivity for ethylene (b),

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Selectivity for propylene (c) and Hydrogen transfer index (HTI, C3H8/C3H6) with time-on-stream over C-SAPO-34 and TS-SAPO-34-x (d). Experimental conditions:

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methanol WHSV= 2 h–1, T = 425 °C, catalyst weight = 1 g, molar ratio of methanol

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and water was 1:1, carrier gas: N2, 60 ml min−1.

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samples.

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

Figure 13. GC-MS chromatograms of deactivated SAPO samples. 35

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Table 1 Detailed synthetic conditions of SAPO-34 samples Gel ratio

C-SAPO-3 4 TS-SAPO34-1 TS-SAPO34-2 TS-SAPO34-4 TS-SAPO34-8 S-SAPO-3 4-4

4MOR:0.6SiO2:1.0Al2O3:1P2 O5:70H2O 4MOR:0.6SiO2:1.0Al2O3:1P2 O5:70H2O 4MOR:0.6SiO2:1.0Al2O3:1P2 O5:70H2O 4MOR:0.6SiO2:1.0Al2O3:1P2 O5:70H2O 4MOR:0.6SiO2:1.0Al2O3:1P2 O5:70H2O 4MOR:0.6SiO2:1.0Al2O3:1P2 O5:70H2O

Chemical compositionb (mol)

–

–

1%

30 h

2%

30 h

4%

30 h

8%

30 h

Al0.497P0.324Si0. 179O2 Al0.501P0.326Si0. 173O2 Al0.503P0.332Si0. 165O2 Al0.506P0.335Si0. 159O2 Al0.508P0.350Si0. 142O2

4%

–

a

Prod uct yieldc (%) 66 70 73 80

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Samples

Seed MOR sa treatm (x) ent

–

84 –

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the amount of the added seeds: the weight ratio of the SAPO-34 seeds added to the inorganic oxides in the initial synthesis gel. b Measured by ICP-OES. c product yields are defined as: yield (%) = (Msample ‒ Mseed)/Mgel × 100, where Msample, Mseed and Mgel are the mass of the calcined products, the calcined seeds and the inorganic oxides in the synthesis gel, respectively.

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Table 2 Textural properties of SAPO samples. Surface area Pore volume 2 -1 (m g ) (cm3 g-1) Sample SBETa Smicrob Sextb Vtotalc Vmicrod C-SAPO-34 560 546 14 0.275 0.256 TS-SAPO-34-1 571 542 29 0.284 0.249 TS-SAPO-34-2 584 545 39 0.318 0.251 TS-SAPO-34-4 598 547 51 0.343 0.258 TS-SAPO-34-8 580 534 46 0.314 0.240

Vmesod 0.019 0.035 0.067 0.085 0.074

Strong Acid amountse (mmol g-1) 0.969 0.883 0.770 0.612 0.410

a

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The total specific surface area (SBET) was calculated using the Brunauer–Emmett–Teller (BET) equation. b The external surface area (Sext) were evaluated using the t-plot method. The micropore surface area (Smicro) = SBET – Sext. c The total pore volume (Vtotal) was calculated from the adsorption quantity at p/p0 = 0.99. d The micropore volume (Vmicro) and was calculated using the t-plot method, the mesopore volume

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(Vmeso)=Vtotal – Vmicro. e Calculated by NH3-TPD.

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Table 3 Lifetimea and product distributionb of TS-SAPO-34-4 and S-SAPO-34-4 for the MTO reaction (WHSV=2 h-1, T=425 °C) Sample Lifetime (h) C2= (%) C3= (%) C2= +C3= (%) Rcoke (g g-1 h-1) TS-SAPO-34-4 4.6 h 42.77 40.89 83.66 0.049 S-SAPO-34-4 3.1 h 40.67 41.72 82.39 0.064 Lifetime is expressed as the reaction duration during which the methanol conversion is > 99%.

b

Highest C2= -C3= selectivity with > 99% methanol conversion.

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a

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