Influence of template content on selective synthesis of SAPO-18, SAPO-18/34 intergrowth and SAPO-34 molecular sieves used for methanol-to-olefins process

Abstract

SAPO-18, SAPO-18/34 intergrowth and SAPO-34 molecular sieves have been selective synthesized using different amount of template (tetraethyl ammonium hydroxide, TEAOH). The samples were characterized by XRD, XRF, TG-DSC, ¹³C (²⁹Si) CP/MAS NMR and NH₃-TPD. The synthesized products switched from SAPO-5/18, to SAPO-18, SAPO-18/34, and finally to SAPO-34 with the increasing of TEAOH amount. The TEAOH in SAPO-18 cages mainly acted the role of space filling. While in SAPO-34, the TEAOH was mainly used to compensate the negative charge of the framework. SAPO-34 possessed much more Si(4Al) structure compared to SAPO-18 and SAPO-18/34. For methanol to olefins reaction, SAPO-18 yielded the highest propene and C₄ than that over SAPO-18/34 and SAPO-34, SAPO-34 had the highest ethene yield among the three catalysts. The working lifetimes of SAPO-18/34 and SAPO-34 were significantly longer than that of SAPO-18.

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Introduction

Light olefins such as ethene and propene are important raw materials for the petrochemical industry. Conventionally, these are produced by steam-thermal cracking and catalytic cracking of oil feed stocks.¹ Nowadays, with the growing demand for light olefins, researchers endeavor to produce them from non-oil feed stocks. The methanol-to-olefins (MTO) process is a route to selectively produce light olefins from methanol, which can be produced from non-oil feed stocks, such as coal, natural gas or biomass.^2,3 Silicon-substituted aluminophosphate molecular sieves, such as SAPO-34,^4–11 SAPO-18/34 intergrowth^12–14 and SAPO-18,^15–19 are excellent shape-selective catalysts for the MTO reaction.

The framework topology of SAPO-34 (IZA code is CHA), is comprised of cylinder-like cages (6.7 × 6.7 × 10.0 Å) with 8-ring openings (3.8 × 3.8 Å), and the small pore opening only allows linear olefins and small molecules to diffuse through.^13,20 The microporous framework structure of SAPO-18 (IZA code is AEI) is similar to that of SAPO-34 (see Fig. 1). Both SAPO-34 and SAPO-18 include sheets of connected double six-membered rings (D6R). The sheets are connected along the z-axis (via O-bridges) to form a stacked structure. The lateral shift between the sheets is a rotation of 180° around the z-axis of AEI structure, while it is 0° rotation for the CHA structure. The connection creates a three-dimensional channel system for SAPO-18, and its eight-membered pore openings is the same as SAPO-34.^8,15,17 SAPO-18/34 is a type of composite molecular sieve with an intergrowth structure of CHA and AEI.^12–14 The stacking faults of SAPO-18/34 intergrowth crystal structures were described by Sławiński et al.^21,22 and were displayed in Fig. 2. The stacking layers of SAPO-34 can be described as sequences of AAAA… or BBBB…, and the stacking layers sequence of SAPO-18 is ABAB…, while SAPO-18/34 intergrowth is between SAPO-34 and SAPO-18, due to the stacking faults. The SAPO-18, SAPO-18/34 and SAPO-34 molecular sieves are usually synthesized by hydrothermal method. The accurate control of conditions for selective synthesis of these samples is very important, because of similarity in the structures of SAPO-18, SAPO-18/34 intergrowth and SAPO-34 molecular sieves.

Fig. 1 Cage structures of SAPO-34 and SAPO-18.

Fig. 2 Crystal structure of SAPO-18, SAPO-18/34 and SAPO-34.

Chen et al.^15,17 reported the synthesis of SAPO-18 and SAPO-34 with C₈H₁₉N as a template. The relative amount of template was crucial to form the pure SAPO-18. For the ratio of C₈H₁₉N/(Si + Al + P) smaller than 0.4, SAPO-5 was very likely to appear, and when the ratio is larger than 0.6, the product includes none or very low crystalline-form content. Unlike the high Si/(Si + Al + P) ratio (typically higher than 0.10) for SAPO-34, the SAPO-18 can be prepared with very low amounts amount of silicon. Liu and co-workers⁵ reported that pure SAPO-34 could be synthesized if n(DEA)/n(Al₂O₃) ≥ 1.5 and n(SiO₂)/n(Al₂O₃) > 0.1. When n(DEA)/n(Al₂O₃) was between 0.4 and 0.8, pure SAPO-11 can be obtained. Smith et al.¹³ prepared a series of SAPO-18/34 intergrowth molecular sieves with a range of SiO₂/Al₂O₃ ratio from 0 to 0.3 in the presence of TEAOH as template. It was found that low levels of silicon content resulted in the AEI structure and high levels of silicon content resulted in an AEI/CHA intergrowth structure. Wendelbo et al.¹⁸ found that less water and less Mg and Zn were used for preparing MgAPO-18 and ZnAPO-18 relative to MgAPO-34 and ZnAPO-34. Fan et al.²³ reported that SAPO-18 can be synthesized at higher crystallization temperatures (200 °C) than SAPO-34.

It is clear that synthetic conditions are crucial for selective synthesis of SAPO-18, SAPO-18/34 and SAPO-34. For the synthesis of these molecular sieves, there are many different chemicals can be used as template.^5,15,24–31 The templates play an important role in the synthesis process. For example, templates are required for space filling, structure directing and charge compensated compensation.^5,32,33 It is worth mentioning that one template can be used to direct the syntheses of different type of molecular sieves under the different synthetic conditions, and synthesis using different templates may also yield the same molecular sieve. In this work, preferential formation of SAPO-34, SAPO-18, and SAPO-18/34 intergrowth molecular sieves was studied by adjusting the amount of template (TEAOH). Samples obtained were characterized and their catalytic performances in the MTO process were investigated.

Experimental

Synthesis of SAPO molecular sieves

SAPO molecular sieves were synthesized from a gel with different molar compositions by the hydrothermal crystallization method. The molar composition of the reaction mixture and the synthesis conditions are given in Table 1.

Table 1 Gel composition^a, pH value, yield, particle size and product phase of SAPO molecular sieves^b

Sample	x (TEAOH)	pH of gel	Yield (%)	D50 (μm)	Product phase
a Gel molar composition is xTEAOH : 1.0Al2O3 : 1.0P2O5 : 0.1SiO2 : 40H2O.b Crystallization condition is 100 °C – 24 h, then 180 °C – 48 h.
1.0T	1.0	2.98	65	2.962	SAPO-5/18
1.2T	1.2	3.42	59	0.500	SAPO-5/18
1.4T	1.4	3.60	69	0.366	SAPO-18
1.6T	1.6	4.30	60	0.215	SAPO-18
1.8T	1.8	5.11	47	0.243	SAPO-18
2.0T	2.0	6.77	41	0.515	SAPO-18/34
2.2T	2.2	7.06	39	1.931	SAPO-18/34
2.4T	2.4	7.54	33	2.941	SAPO-34

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The sources of the framework elements were pseudoboehmite (76.4 wt% Al₂O₃, XinNian Petrochemical Additives Company), orthophosphoric acid (85 wt%, Tianjin Guangfu Fine Chemical Research Institute) and silica sol (30 wt% SiO₂, Yinfeng Silicon Products Co., Ltd). The structure directing agent was TEAOH (35 wt%, Shanghai Cainorise Chemicals Co., Ltd). The preparation procedures are as follows. First, the silica sol and pseudoboehmite were added to TEAOH solution under continuous stirring at room temperature. Then dilute phosphoric acid solution was added to the SiO₂–Al₂O₃–TEAOH mixture, and a resultant homogeneous synthesis gel (SiO₂–Al₂O₃–TEAOH–H₃PO₄–H₂O mixture) was prepared through intensive stirring. After that, the synthesized gel was sealed in a Telfon-lined stainless-steel autoclave. The autoclave temperature was kept at 100 °C for 24 h and then at 180 °C for 48 h under rotation. During the hydrothermal treating process, the gel crystallized to SAPO series molecular sieves. The molecular sieves obtained were treated with centrifuging, rinsing and drying. Then calcined at 600 °C for 6 h to remove the organic template inside the molecular sieves. The yield of molecular sieves was the weight ratio of the recovered solid to the Al₂O₃, P₂O₅ and SiO₂ in the synthesis gel.

Characterization

Powder X-ray diffraction (XRD) patterns of as-synthesized samples were recorded on X-ray diffractometer (Bruker D8 Advance, Germany), using CuKα radiation at room temperature and instrumental settings of 40 kV and 50 mA. The scanning rate was 1° min⁻¹ and scanning range was from 5° to 50°. The D₅₀ quantiles of the volumetric distribution of the sample, before treatment of centrifugation, was measured by laser particle size analysis (Bettersize 2000, China), using distilled water to prepare the molecular sieve suspension. The crystal morphology was observed by scanning electron microscopy (SEM, FEI Quanta200F, USA) operating at 20 kV. The pore structure data was obtained with a ASAP2020M instrument (Micrometritics Instrument Corporation, USA) by N₂ adsorption at −196 °C. The total surface area was calculated based on the BET equation. The micropore volume and micropore surface area were evaluated by the t-plot method. Elemental analysis of Al, P and Si in calcined samples was performed by X-Ray Fluorescence (XRF) on AxiosmAX (PANalytical B.V., Netherlands). Thermo-gravimetric (TG) analysis and differential scanning calorimetry (DSC) were performed using STA 409 PC/PG instrument (Netzsch, Germany) under an oxygen flow (20 mL min⁻¹) at a heating rate of 10 °C min⁻¹ from 25 °C up to 800 °C. Samples were dried carefully before TG analysis. The sample's carbon content were determined on a HIR-944B infrared carbon analyzer (Wuxi High-speed analyzer Co., China), and the samples were dried before analysis. ¹³C cross polarization/magic angle spinning nuclear magnetic resonance (¹³C CP/MAS NMR) experiments were performed on Bruker AVANCE III 600 (Germany) spectrometer operating at 150.9 MHz, with a 4 mm MAS probe, a contact time of 2 ms, a recycle delay of 5 s, and a spinning rate of 11 kHz. ²⁹Si CP/MAS NMR spectra were recorded at room temperature using a spectrometer operating at 79.5 MHz, with a 4 mm probe spinning at 12 kHz, a π/4 pulse length of 2.6 μs, and a recycle delay of 80 s. The chemical shifts of ¹³C and ²⁹Si were externally referenced to tetramethylsilane. The surface acidity of the catalysts was measured by temperature programmed desorption of ammonia (NH₃-TPD), using a gas chromatograph (SP2100, Beifen, China) equipped with a thermal conductivity detector (TCD). In a typical analysis, 0.3 g of the calcined sample was pretreated to remove adsorbed water and impurities at 600 °C for 0.5 h. Then the sample was cooled down to 110 °C and saturated with ammonia for 0.5 h. After saturation the sample was purged with nitrogen for 2 h to remove physically adsorbed ammonia. The temperature of the sample was then raised from 110 to 600 °C at a heating rate of 10 °C min⁻¹. TCD was used to monitor the desorbed ammonia.

Catalytic performance testing

The catalytic performance of SAPO molecular sieves for MTO reaction was tested in a tubular stainless steel reactor (9 mm in internal diameter) under atmospheric pressure. The template-free catalyst sample (2.5 g) was pelletized, and then crushed to particles of 0.45–0.90 mm. The reaction temperature was measured using a thermocouple inserted in the reactor; this was 470 °C. Methanol solution (5 wt% water content in the feedstock) was fed with a tranquil flow pump, the weight hourly space velocity (WHSV) was 2.5 h⁻¹ for methanol. The hydrocarbon products were analyzed by a gas chromatograph (SP3420, Beifen, China) with a capillary HP-PLOT Q column (30 m, 0.53 mm i.d., Agilent) and a flame ionization detector (FID). The carbon content found in the effluent was about 96–97 wt% of that in the feed. Methanol conversion and hydrocarbon product yields of gas-phase were measured on a carbon basis. Both methanol and dimethyl ether were considered as reactant for the calculation of conversion, as previous researchers did.^34,35 The methanol conversion was defined as the conversion of oxygenates (methanol and dimethyl ether). Catalyst lifetime was defined as the time when 98% of methanol conversion could be achieved.

Results and discussion

Crystalline, morphological and pore structure analysis

The effect of template concentration on the structure of SAPO samples was investigated with the gel composition of 1.0Al₂O₃ : 1.0P₂O₅ : 0.1SiO₂ : 40H₂O : xTEAOH. The peaks in the X-ray diffraction patterns of as-synthesised samples were compared with standard CHA³⁶ (SAPO-34), AEI¹⁵ (SAPO-18) and AFI³⁷ (SAPO-5) structures (Fig. 3). When the TEAOH/Al₂O₃ molar ratio was lower than 1.4, the peaks indicated the existence of AEI and AFI structure in sample 1.0T and 1.2T. Moreover, the absence of the peaks at 10.4° implied that sample 1.0T and 1.2T were not just the mixture of AFI and AEI, they were AFI/AEI (SAPO-5/18) intergrowth molecular sieves. Futhermore, the change of intensity of peaks at 7.4, 19.7, 22.4° indicated that there were more AEI structure in sample 1.2T than in sample 1.0T.

Fig. 3 X-ray diffraction patterns of as-synthesised samples (gel composition is xTEAOH : 1.0Al₂O₃ : 1.0P₂O₅ : 0.1SiO₂ : 40H₂O).

When the TEAOH/Al₂O₃ molar ratio was 1.4, 1.6 and 1.8 (4 < pH < 6), the diffraction peaks of sample 1.4T, 1.6T and 1.8T matched well with the standard AEI structure, this demonstrated these samples were SAPO-18 molecular sieves. In addition, sizes of SAPO-18 are smallest among synthesized samples (see Table 1). Increasing the TEAOH/Al₂O₃ molar ratio of the gel to 2.0 and 2.2, the peaks indicated that there were both AEI and CHA structure in sample 2.0T and 2.2T, however, the absence of the peaks at 10.4° implied that, they were not physical mixture of AEI and CHA, and instead it was the SAPO-18/34 (AEI/CHA) intergrowth molecular sieves. The peak intensity at 16.9, 20.2 and 21.0° revealed that, the relative content of SAPO-34 structure was higher in sample 2.2 than in sample 2.0. The SAPO-34 s (sample 2.4T) were obtained from the gel with the TEAOH/Al₂O₃ molar ratio was 2.4 (pH = 7.54).

Combined with the results of pH value of gel and XRD of as-synthesized samples, it can be concluded that, when TEAOH/Al₂O₃ molar ratio x of the gel was 1.0 < x < 1.4, the pH value was 3–3.5, and the synthesized samples were SAPO-5/18. When 1.4 < x < 1.8, the pH value was 3.5–5.5, the obtained samples were SAPO-18. SAPO-18/34 intergrowth molecular sieves can be synthesized when 2.0 < x < 2.2, 5.5 < pH < 7. When x = 2.4, pH was 7.54, SAPO-34 sieves were obtained. With the increase of template amount x from 1.0 to 2.4, the pH value of gel changed notably, this affect the combination structure of Si, Al and P ions, resulting in different crystallization products. The yield of the molecular sieves decreased with increasing template amount. The size of samples increased with the amount of TEAOH when n(TEAOH/Al₂O₃) > 1.6.

The SEM images of calcined samples were shown in Fig. 4. From the SEM images, it can be seen that SAPO-5/18 (sample 1.0T, 1.2T) formed in the plate-like crystals. SAPO-18 s (sample 1.4T, 1.6T and 1.8T) formed in the disc-like or strip-like crystals. The particle size of SAPO-18s obtained from SEM images was obviously smallest among these samples, agreeing with the laser particle size analysis results (Table 1). SAPO-18/34 intergrowth phase formed in the cubic crystals (sample 2.0T and 2.2T). SAPO-34 formed in the cubic crystals too (sample 2.4T).

Fig. 4 SEM images of calcined samples.

The surface area and pore volume of some synthesized SAPO samples were measured by the nitrogen adsorption–desorption method and the results are shown in Table 2. As shown in Table 2, the micropore volume and micropore surface area of these samples were very similar to each other, indicating the crystal-lattice volume and structure of SAPO-34 and SAPO-18, which is the source of the micropore volume and micro-pore surface area, is almost identical. Of all the three samples measured, 1.6T sample had the largest mesopore volume (0.19 cm³ g⁻¹) and external surface area (40 m² g⁻¹), resulting from the fact that the 1.6T sample had the smallest crystal size. The smaller the molecular crystals, the larger their external specific surface area. The SAPO-34 (sample 2.4T), which had the largest crystal size, had the smallest mesopore volume and external specific surface area.

Table 2 The surface area and pore volume of synthesized samples

Sample	Surface area (m^2 g^−1)			Pore volume (cm^3 g^−1)
	Smicro^a	Sext^b	Stotal^c	Vmicro^d	Vmeso	Vtotal
a t-Plot micropore surface area.b t-Plot external surface area.c BET surface area.d t-Plot micropore volume.
1.6T (SAPO-18)	402	40	442	0.19	0.19	0.38
2.0T (SAPO-18/34)	412	12	424	0.20	0.08	0.28
2.4T (SAPO-34)	421	4	425	0.21	0.04	0.25

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Elemental composition and Si coordination structure

Elemental composition of samples 1.6T (SAPO-18), 2.0T (SAPO-18/34) and 2.4T (SAPO-34) were obtained for analyzing the contents of Al, P and Si of calcined samples. It can be seen from Table 3 that, the silicon content and its incorporation in the framework of SAPO samples rose with TEAOH amount. This may be due to higher concentration of the basic template being favorable for the dissolution of silicon. As reported in literature,^17,38–42 Si was incorporated into aluminophosphate framework by two different substitution mechanisms: one is SM2 substitution mechanism, the substitution of phosphorus by silicon, forming Si(4Al) environment in the AlPOs framework, with the chemical shift of −87 ppm. The other is the SM3 substitution mechanism, the simultaneous substitution of a pair of neighbouring Al and P atoms by two Si atoms, forming Si(3Al), Si(2Al), Si(1Al) and Si(0Al) structures with their corresponding chemical shifts at −94, −99, −104 and −110 ppm respectively. ²⁹Si CP/MAS NMR spectra of the SAPO-18, SAPO-18/34 and SAPO-34 are shown in Fig. 5.

Table 3 Elemental composition analyses of calcined samples

Sample	n (TEAOH/Al2O3)	Product composition	(Si/Al2)gel	(Si/Al2)solid	Si incorporation^a
a Defined as the molar ratio of [Si/(Si + Al + P)solid]/[Si/(Si + Al + P)gel].
1.6T (SAPO-18)	1.6	Al0.521P0.453Si0.044O2	0.10	0.17	1.76
2.0T (SAPO-18/34)	2.0	Al0.513P0.449Si0.054O2	0.10	0.21	2.18
2.4T (SAPO-34)	2.4	Al0.505P0.439Si0.059O2	0.10	0.23	2.40

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Fig. 5 The calculated (dotted lines) and measured (solid lines) ²⁹Si CP/MAS NMR spectra of calcined samples. 1 Si(4Al), 2 Si(3Al), 3 Si(2Al), 4 Si(1Al), 5 Si(0Al).

It can be seen from Fig. 5 that SAPO-18, SAPO-18/34 and SAPO-34 all exhibited broad band centred at −87 ppm and −110 ppm, attributed to Si(4Al) and Si(0Al) environments respectively. The band had a certain asymmetry suggesting the presence of Si(3Al), Si(2Al), Si(1Al) structures. Therefore, the silicon substitution of SM2 and SM3 occurred simultaneously for all the three samples. The proportional distributions of Si(nAl) (n = 0–4) of the SAPO molecular sieves are displayed in Table 4. It is evident from Fig. 5 and Table 4 that higher intensity of Si(4Al) existed in SAPO-34 than in SAPO-18 and SAPO-18/34. This indicates a more pronounced effect of substitution mechanism SM2 in SAPO-34, and this mechanism creates a negative charge per Si atom in the framework. This is usually balanced by the positive charge of the organic molecules occluded within the microporous structure. However, SM3 leads to the formation of neutral silicon islands.⁴⁰ According to the results of elemental composition and Si coordination structure analysis, the increasing of TEAOH amounts results in more silicon incorporated into framework in SAPO-34 than in SAPO-18/34 and SAPO-18 molecular sieves. Furthermore, SAPO-34 possesses more Si(4Al) structure. Therefore, SAPO-34 requires more template molecules to balance the negative charges of its framework.

Table 4 The proportional distribution of ²⁹Si CP/MAS NMR of the SAPO molecular sieves

Sample	Si(4Al)	Si(3Al)	Si(2Al)	Si(1Al)	Si(0Al)
1.6T (SAPO-18)	0.43	0.15	0.02	0.19	0.23
2.0T (SAPO-18/34)	0.50	0.08	0.03	0.10	0.29
2.4T (SAPO-34)	0.56	0.02	0.11	0.10	0.20

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Thermogravimetric analysis and ¹³C CP/MAS NMR investigation of as-synthesized samples

In order to investigate the incorporation of the template in the as-synthesized samples, TG and ¹³C CP/MAS NMR analysis of samples 1.6T (SAPO-18), 2.0T (SAPO-18/34) and 2.4T (SAPO-34) were performed. The moles of template per cage were calculated based on the elemental composition (Table 3) and topological structure of the samples. There are three cages and 36T sites for each unit cell in SAPO-34.^5,43,44 The mole mass of template were calculated based on (CH₃CH₂)₄N–OH or (CH₃CH₂)₄N⁺ respectively.

TG and DSC profiles are plotted in Fig. 6. Generally, there are three weight loss steps. The first step of weight loss (I) is the removal of adsorbed water at temperatures below 100 °C with an endothermic process. The second step (II) is the decomposition of the template with a strongly exothermic process between 100 °C and 430 °C. The third final step (III), at temperatures higher than 430 °C with an exothermic process too, is combustion of organic residues occluded in the channels and cages of the SAPO molecular sieves.^10,42 The weight losses at different steps are presented in Table 5. At step I, SAPO-34 and SAPO-18/34 lose ∼0.7 wt%, while SAPO-18 lose 1.2 wt% of physically absorbed water. When the temperature was higher than 100 °C, the weight loss II and III of SAPO-18 was much larger than that of SAPO-34 and SAPO-18/34. This indicates more template molecules were incorporated in the cage of SAPO-18 than in the SAPO-34 and SAPO-18/34 samples. The template content for three samples was determined by carbon analyzer, and the results were shown in Table S1.† The template content measured by carbon analyzer for sample 1.6T, 2.0T and 2.4T agreed well with that measured by TG. According to the reports,^5,43 the value of moles of template per cage in SAPO-34 with TEAOH is 1.0, and it was well matched by the calculated result of SAPO-34 (sample 2.4T, Table 5). Due to the large cage size of AEI structure, SAPO-18 and SAPO-18/34 should possess more template in the cage, and the results in Table 5 confirmed that. However, as previously discussed, in compared to SAPO-18/34 and SAPO-18, the SAPO-34 (sample 2.4T) needed much more template to balance the negative charges of the framework. Consequently, the role of the template must be in a different way in selectively directing the formation of SAPO-18, SAPO-18/34 intergrowth and SAPO-34 molecular sieves.

Fig. 6 TG (solid lines) and DSC analysis (dotted lines) of samples (blue lines, sample 1.6T, SAPO-18; red lines, sample 2.0T, SAPO-18/34; black lines, sample 2.4T, SAPO-34).

Table 5 TG analyses of samples

Sample	Weight loss (wt%)				Moles of template per cage
	I < 100 °C	II 100–430 °C	III > 430 °C	II + III
1.6T (SAPO-18)	1.15	14.17	7.97	22.14	1.42–1.60
2.0T (SAPO-18/34)	0.71	7.47	9.81	17.28	1.03–1.17
2.4T (SAPO-34)	0.68	4.95	11.00	15.95	0.95–1.07

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The ¹³C CP/MAS NMR characterization of as-synthesized samples 1.6T, 2.0T and 2.4T was performed, and the spectra were shown in Fig. 7. From Fig. 7 it can be seen that template TEAOH give two types of peaks, at 6–9 and 52–55 ppm respectively. The peak at 6–9 ppm was ascribed to –CH₃ group of template, and –CH₂ group gave signals at 52–55 ppm. We believe resonances in 7.2 and 53.0 ppm are due to the interaction of (CH₃CH₂)₄N⁺ with framework of SAPO molecular sieve, and resonances in 7.7 and 53.7 ppm are due to that of (CH₃CH₂)₄N–OH. In SAPO-34 (sample 2.4T), the template mainly existed as (CH₃CH₂)₄N⁺ cations, they were used to balance the negative charge of the framework. However, for SAPO-18 (sample 1.6T), the peak areas in 7.2 and 53.0 ppm were larger than that in 7.7 and 53.7 ppm. It means that the template was not mainly presented as (CH₃CH₂)₄N⁺ cations, but as (CH₃CH₂)₄N–OH, and the template was mainly for space filling. For SAPO-18/34 intergrowth (sample 2.0T), because of the relatively large proportion of Si content and Si(4Al) structure (see Tables 3 and 4), plus the existence of AEI cage, the template in SAPO-18/34 presented both as (CH₃CH₂)₄N⁺ cations and (CH₃CH₂)₄N–OH, and the content of the two forms showed no obvious difference.

Fig. 7 ¹³C CP/MAS NMR spectra of as-synthesized samples.

Acidity and catalytic activity

Fig. 8 shows the NH₃-TPD results for the three samples, and the quantitative results are shown in Table 6. It can be seen that the curves of the three samples contained two peaks, centred at ∼200 °C and ∼420 °C. This corresponds to the weak and strong acid sites respectively. The areas of the low temperature desorption peak decreased in the order of SAPO-18/34 > SAPO-18 > SAPO-34, while the areas of the high temperature desorption peak decreased in the order of SAPO-34 > SAPO-18/34 > SAPO-18. This indicates the SAPO-34 has highest concentration of strong acid sites and lowest weak acid sites. Moreover, the desorption peak of strong acid sites for SAPO-34 was found at the temperature (450 °C) slightly higher than those of SAPO-18/34 (430 °C) and SAPO-18 (400 °C). Therefore, the strong acid sites of the SAPO-34 were not only larger in number, but also slightly stronger in acidity than those of the other samples. So, it can be concluded that, the total acidity amount increased when TEAOH/Al₂O₃ molar ratio of the gel changed from 1.6 to 2.0, while decreased a little from 2.0 to 2.4. With increase of template amount from sample 1.6T to 2.4T, the strength of weak acid sites changed little, the amount of weak acid sites decreased in the order of sample 2.0T > 1.6T > 2.4T. While, the strength and amount of strong acid sites increased with template amount. That might be due to the increase of silicon content and its incorporation in the framework of SAPO samples.

Fig. 8 NH₃-TPD plots of calcined samples.

Table 6 Acid properties of the SAPO catalysts

Sample	Acidity (mmol NH3 g^−1)
	Weak	Strong	Total
1.6T (SAPO-18)	0.62	0.57	1.20
2.0T (SAPO-18/34)	0.66	0.67	1.33
2.4T (SAPO-34)	0.53	0.77	1.30

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The MTO catalytic performances of samples were tested in a fixed bed reactor and experiment results are summarized in Table 7 and Fig. 9. From Table 7, it can be concluded that, among these sample, sample 1.2T, which contained SAPO-5, has the lowest light olefins yield in MTO reaction. SAPO-5 consists of an one-dimensional 12 membered-ring channel system with a pore diameter of 0.73 nm, which falls into the large pore category.⁴⁰ So C₄ and C₅+ product yields were obviously higher for sample 1.2T (SAPO-5/18) than for the others (SAPO-18, SAPO-18/34 and SAPO-34). The yields for propene and C₄ over SAPO-18 (sample 1.4T, 1.6T and 1.8T) are higher than that over SAPO-34 (2.4T) and SAPO-18/34 (2.0T and 2.2T), which might be attributed to the slightly larger size of AEI cage of SAPO-18 than the CHA cages of SAPO-34. According to the hydrocarbon pool mechanism and previous research work on spatial confinement effects imposed by the cage structure of SAPO molecular sieves,^13,45,46 the AEI cage has more space for higher-substituted methylbenzenes to reside, and favors the production of propene and C₄ hydrocarbon. SAPO-18/34 intergrowth molecular sieve possesses the longest catalytic lifetime and comparatively higher yield of ethene and propene, due to its relatively moderate acidity and AEI/CHA intergrowth cages. SAPO-34 has the highest ethene yield because of its smaller CHA cage.

Table 7 Catalysts lifetime^a and hydrocarbon product yields^b in MTO reaction^c over SAPO molecular sieves

Sample	Hydrocarbon product yields (wt%)					Lifetime (min)
	C2H4	C3H6	C4	C5+	C1–C3 alkanes
a Catalysts lifetime was defined as the time when 98% of methanol conversion can be achieved.b Hydrocarbon product yields were considered on a carbon basis, and the value displayed correspond to the data that the highest yield of ethene plus propene achieved when methanol conversion = 100%.c Reaction conditions: 470 °C, WHSV for methanol is 2.5 h^−1, 95 wt% methanol solutions.
1.2T (SAPO-5/18)	37.88	24.85	21.06	9.61	6.60	200
1.4T (SAPO-18)	38.52	41.32	12.04	4.05	4.07	95
1.6T (SAPO-18)	39.43	40.58	12.52	3.83	3.64	95
1.8T (SAPO-18)	40.01	40.14	11.39	3.54	4.92	95
2.0T (SAPO-18/34)	50.65	34.37	7.53	1.55	5.90	185
2.2T (SAPO-18/34)	53.86	33.28	5.54	1.44	5.88	185
2.4T (SAPO-34)	55.36	32.34	5.20	1.37	5.73	170

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Fig. 9 Methanol conversion and yields of hydrocarbons in the MTO reaction to the sample 1.6T (SAPO-18, ), 2.0T (SAPO-18/34, ) and 2.4T (SAPO-34, ). Reaction conditions: 470 °C, WHSV = 2.5 h⁻¹, 95 wt% methanol solutions.

Fig. 9 shows the methanol conversion and yields of hydrocarbons with time on stream at sample 1.6T (SAPO-18), 2.0T (SAPO-18/34) and 2.4T (SAPO-34). The initial methanol conversion of SAPO-18, SAPO-18/34 and SAPO-34 reached 100%. Furthermore, the yield of ethene plus propene was significantly higher in SAPO-18 and SAPO-18/34 than in SAPO-34 in the initial stage (time on stream < 80 min), and the yield of propane was higher in SAPO-34 than the others. Combined with the results of NH₃-TPD (Table 6), this phenomenon can be explained by that the strong acid site favors the hydrogen transfer reaction, then the yield of propane will be increased over catalysts which contained more strong acid sites.^9,47 With time on stream increasing, the methanol conversion and the yields of light olefins (C₂⁼ and C₃⁼) of all the three samples decreased. The working lifetimes of 2.0T and 2.4T were significantly longer than that of 1.6T, and the working lifetime of 2.0T was slightly longer than that of 2.4T.

Rapid deactivation due to coke formation during MTO reaction over these small pore-size molecular sieves, the coke can reduce the mass transport of reactants and products.^48,49 The coke content on deactivated SAPO catalysts was analyzed by carbon analyzer, and the results were shown in Table 8. It can be seen that coke content increased with the sample changed from 1.6T (SAPO-18) to 2.0T (SAPO-18/34) then to 2.4T (SAPO-34). The coke content on SAPO-18 was obviously less than SAPO-18/34 and SAPO-34. This might be due to that the total acidity amount for SAPO-18 were the least among the three samples (see Table 6). However, SAPO-34 contained less acid amount but more coke content than SAPO-18/34. This might be due to the obvious difference of crystal size between sample 2.4T (SAPO-34) and 2.0T (SAPO-18/34). The larger crystal size, the more diffusion limitation for reactants and products. So, the coke content on 2.4T was larger than 2.0T.

Table 8 The coke content for deactivated SAPO catalysts

Sample	Coke content on SAPO-34^a (wt%)
a Coke content (wt%) = the mass of carbon of SAPO-34 with coke deposited/the mass of SAPO-34 with coke deposited × 100%, and SAPO-34 with coke deposited was collected when methanol conversion < 50%.
1.6T (SAPO-18)	8.14
2.0T (SAPO-18/34)	13.29
2.4T (SAPO-34)	16.37

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Conclusions

In this study, SAPO-18, SAPO-18/34 intergrowth and SAPO-34 molecular sieves were selectively synthesized through adjusting the usage amount of TEAOH. With the TEAOH/Al₂O₃ molar ratio increased from 1.0 to 2.4 (the gel composition is 1.0Al₂O₃ : 1.0P₂O₅ : 0.1SiO₂ : 40H₂O : xTEAOH), the products synthesized switched from SAPO-5/18, to SAPO-18, then to SAPO-18/34, and finally to SAPO-34. Meanwhile, the yield of the molecular sieves decreased, and the silicon content in SAPO samples increased. ²⁹Si CP/MAS NMR revealed SAPO-34 possessed more Si(4Al) coordination environment than in SAPO-18 and SAPO-18/34 intergrowth. According to results of TG and ¹³C CP/MAS NMR spectra, SAPO-18 possessed more template in the cage than SAPO-18/34 and SAPO-34, the TEAOH of SAPO-18 mainly acted as the role of space filling. While in SAPO-34, the TEAOH were mainly used to compensate the negative charge of the framework. NH₃-TPD results indicated that the amount of strong acid sites increased with increase of template amount used, while the strength of weak acid sites changed little. The amount of weak acid sites decreased in the order of SAPO-18/34 > SAPO-18 > SAPO-34. Among the three catalysts for methanol to olefins reaction, SAPO-18/34 intergrowth molecular sieve possessed the longest catalytic lifetime and higher yield of ethene and propene, due to its relatively moderate acidity and AEI/CHA intergrowth cages; SAPO-18 had higher propene and C₄ yield, SAPO-34 had the highest ethene yield.

Acknowledgements

This research work was supported by the National Natural Science Foundation of China (Grant No: 91534120) and China National Petroleum Corporation (Contract Number: PRIKY 13043).

References

1. Z. Zhu, M. Hartmann and L. Kevan, Chem. Mater., 2000, 12, 2781–2787.
2. M. Stöcker, Microporous Mesoporous Mater., 1999, 29, 3–48.
3. B. Vora, T. Marker, P. Barger, H. Nilsen, S. Kvisle and T. Fuglerud, Stud. Surf. Sci. Catal., 1997, 107, 87–91.
4. B. M. Lok, C. A. Messina, R. L. Patton, R. T. Gajek, T. R. Cannan and E. M. Flanigen, J. Am. Chem. Soc., 1984, 106, 6092–6093.
5. G. Liu, P. Tian, J. Li, D. Zhang, F. Zhou and Z. Liu, Microporous Mesoporous Mater., 2008, 111, 143–149.
6. G. Liu, P. Tian, Y. Zhang, J. Li, L. Xu, S. Meng and Z. Liu, Microporous Mesoporous Mater., 2008, 114, 416–423.
7. H. X. Liu, Z. K. Xie, C. F. Zhang and Q. L. Chen, Chin. J. Catal., 2003, 24, 279–283.
8. H. Van Heyden, S. Mintova and T. Bein, Chem. Mater., 2008, 20, 2956–2963.
9. L. Ye, F. Cao, W. Ying, D. Fang and Q. Sun, J. Porous Mater., 2011, 18, 225–232.
10. T. Álvaro-Muñoz, C. Márquez-Álvarez and E. Sastre, Catal. Today, 2012, 179, 27–34.
11. P. Wang, A. Lv, J. Hu, J. a. Xu and G. Lu, Microporous Mesoporous Mater., 2012, 152, 178–184.
12. Y. Li, Y. Huang, J. Guo, M. Zhang, D. Wang, F. Wei and Y. Wang, Catal. Today, 2014, 233, 2–7.
13. R. L. Smith, S. Svelle, P. del Campo, T. Fuglerud, B. Arstad, A. Lind, S. Chavan, M. P. Attfield, D. Akporiaye and M. W. Anderson, Appl. Catal., A, 2015, 505, 1–7.
14. Y. Hu, H. Chen, Y. Hu, J. Deng, Z. Lv and H. Zhang, Chem. Lett., 2015, 44, 1116–1118.
15. J. Chen, J. M. Thomas, P. A. Wright and R. P. Townsend, Catal. Lett., 1994, 28, 241–248.
16. T. Álvaro-Muñoz, C. Márquez-Álvarez and E. Sastre, Top. Catal., 2015, 1–14.
17. J. Chen, P. A. Wright, J. M. Thomas, S. Natarajan, L. Marchese, S. M. Bradley, G. Sankar, C. R. A. Catlow and P. L. Gai-Boyes, J. Phys. Chem., 1994, 98, 10216–10224.
18. R. Wendelbo, D. Akporiaye, A. Andersen, I. M. Dahl and H. B. Mostad, Appl. Catal., A, 1996, 142, L197–L207.
19. M. Nazari, G. Moradi, R. M. Behbahani, M. Ghavipour and S. Abdollahi, Catal. Lett., 2015, 145, 1893–1903.
20. B. P. Hereijgers, F. Bleken, M. H. Nilsen, S. Svelle, K.-P. Lillerud, M. Bjørgen, B. M. Weckhuysen and U. Olsbye, J. Catal., 2009, 264, 77–87.
21. W. A. Sławiński, D. S. Wragg, D. Akporiaye and H. Fjellvåg, Microporous Mesoporous Mater., 2014, 195, 311–318.
22. R. L. Smith, W. A. Sławiński, A. Lind, D. S. Wragg, J. H. Cavka, B. Arstad, H. Fjellvåg, M. P. Attfield, D. Akporiaye and M. W. Anderson, Chem. Mater., 2015, 27, 4205–4215.
23. D. Fan, P. Tian, S. Xu, Q. Xia, X. Su, L. Zhang, Y. Zhang, Y. He and Z. Liu, J. Mater. Chem., 2012, 22, 6568–6574.
24. J. Tan, Z. Liu, X. Bao, X. Liu, X. Han, C. He and R. Zhai, Microporous Mesoporous Mater., 2002, 53, 97–108.
25. S. Masoumi, J. Towfighi, A. Mohamadalizadeh, Z. Kooshki and K. Rahimi, Appl. Catal., A, 2015, 493, 103–111.
26. S. Rimaz, R. Halladj and S. Askari, J. Colloid Interface Sci., 2016, 464, 137–146.
27. Y. Rezaei, R. Halladj, S. Askari, A. Tarjomannejad and T. Rezaei, Particuology, 2016, 27, 61–65.
28. A. M. Prakash and S. Unnikrishnan, J. Chem. Soc., Faraday Trans., 1994, 90, 2291–2296.
29. B. Han, C.-H. Shin, P. A. Cox and S. B. Hong, J. Phys. Chem. B, 2006, 110, 8188–8193.
30. N. Rajić, D. Stojaković, S. Hoçevar and V. Kaučič, Zeolites, 1993, 13, 384–387.
31. E. Dumitriu, A. Azzouz, V. Hulea, D. Lutic and H. Kessler, Microporous Mater., 1997, 10, 1–12.
32. B. M. Lok, T. R. Cannan and C. A. Messina, Zeolites, 1983, 3, 282–291.
33. R. Vomscheid, M. Briend, M. J. Peltre, P. P. Man and D. Barthomeuf, J. Phys. Chem., 1994, 98, 9614–9618.
34. T. V. Janssens, J. Catal., 2009, 264, 130–137.
35. D. Chen, A. Grønvold, K. Moljord and A. Holmen, Ind. Eng. Chem. Res., 2007, 46, 4116–4123.
36. B. M. Lok, C. A. Messina, R. L. Patton, R. T. Gajek, T. R. Cannan and E. M. Flanigen, US Pat., 4440871, 1984.
37. J. M. Bennett, J. P. Cohen, E. M. Flanigen, J. J. Pluth and J. V. Smith, in Intrazeolite Chemistry, ed. G. D. Stucky and F. G. Dwyer, American Chemical Society, Washington, DC, 1983, vol. 218, ch. 6, pp. 109–118.
38. Z. Yan, B. Chen and Y. Huang, Solid State Nucl. Magn. Reson., 2009, 35, 49–60.
39. L. Xu, A. Du, Y. Wei, Y. Wang, Z. Yu, Y. He, X. Zhang and Z. Liu, Microporous Mesoporous Mater., 2008, 115, 332–337.
40. R. Roldán, M. Sánchez-Sánchez, G. Sankar, F. J. Romero-Salguero and C. Jiménez-Sanchidrián, Microporous Mesoporous Mater., 2007, 99, 288–298.
41. G. Sastre, D. W. Lewis and C. R. A. Catlow, J. Phys. Chem. B, 1997, 101, 5249–5262.
42. T. Álvaro-Muñoz, C. Márquez-Álvarez and E. Sastre, Appl. Catal., A, 2014, 472, 72–79.
43. R. Vomscheid, M. Briend, M. J. Peltre, P. P. Man and D. Barthomeuf, J. Phys. Chem., 1994, 98, 9614–9618.
44. D. Fan, P. Tian, X. Su, Y. Yuan, D. Wang, C. Wang, M. Yang, L. Wang, S. Xu and Z. Liu, J. Mater. Chem. A, 2013, 1, 14206–14213.
45. I. M. Dahl and S. Kolboe, Catal. Lett., 1993, 20, 329–336.
46. J. Chen, J. Li, Y. Wei, C. Yuan, B. Li, S. Xu, Y. Zhou, J. Wang, M. Zhang and Z. Liu, Catal. Commun., 2014, 46, 36–40.
47. U. Olsbye, M. Bjørgen, S. Svelle, K.-P. Lillerud and S. Kolboe, Catal. Today, 2005, 106, 108–111.
48. A. T. Aguayo, A. G. Gayubo, R. Vivanco, M. Olazar and J. Bilbao, Appl. Catal., A, 2005, 283, 197–207.
49. J. F. Haw, W. Song, D. M. Marcus and J. B. Nicholas, Acc. Chem. Res., 2003, 36, 317–326.

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Footnote

† Electronic supplementary information (ESI) available: The carbon content and calculated template weight content of as-synthesized samples. See DOI: 10.1039/c6ra23048b

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