Solvent-free synthesis of SAPO-34 nanocrystals with reduced template consumption for methanol-to-olefins process

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Solvent-free synthesis of SAPO-34 nanocrystals with reduced template consumption for methanol-to-olefins process Meng Li a , Yihui Wang a,b , Lu Bai a,c , Na Chang a,c , Guizhen Nan a,b , Deng Hu a , Yanfeng Zhang a,b,∗ , Wei Wei a,b,∗ a CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China b School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China c School of Material Science, Shanghai University, 99 Shangda Rd, Baoshan District, Shanghai 200444, China

a r t i c l e

i n f o

Article history: Received 6 July 2016 Received in revised form 2 October 2016 Accepted 4 November 2016 Available online xxx Keywords: Zeolite synthesis SAPO-34 Solvent-free Template Methanol-to-olefins

a b s t r a c t SAPO-34 nanocrystals were synthesized by solvent-free method with significantly reduced template consumption. The obtained SAPO-34 samples were characterized by XRD, SEM, nitrogen adsorption, NMR, NH3 -TPD, XPS and XRF. The combination of various Si and Al sources had big impact on the physical-chemical properties of the obtained SAPO-34 crystals, including crystal size, surface area and pore volume, acid strength and distribution and Si environment in the zeolite framework. The nano-sized SAPO-34 crystals exhibited excellent performance in MTO reaction with comparable selectivities to light olefins(combined ∼80% ethylene and propylene selectivity with 50:50 split) and substantially extended catalytic lifetime (from 200 min to 840 min), compared with micron-sized SAPO-34 catalyst synthesized by conventional hydrothermal method. The extended lifetime could be attributed to the presence of abundant mesopores generated by the stacking of nanocrystals, which alleviated the diffusion constrain and allowed more coke deposition. The unique acid strength and distribution as well as Si environment of the SAPO-34 samples may also contribute to long catalytic lifetime. The solvent-free synthesis method provides a cheap and sustainable way of preparing excellent MTO catalyst with significantly reduced template consumption&waste water emission and improved autoclave efficiency. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Zeolites/molecular sieves have been widely used in various industries as catalysts and sorbents due to their uniform molecular sized pores, tuneable acidity and unique adsorption property [1–7]. Zeolite SAPO-34, a silicoaluminophosphate analogue of CHA structure has attracted tremendous interest due to its commercial application as catalyst for methanol-to-olefin (MTO) process, which is an alternative route to produce light olefins with methanol as feedstock [6–11]. However, the rapid deactivation of SAPO34 catalyst (coking caused by slow mass transport of reaction intermediate) significantly affects the process economics. Previous studies suggested that coking can be substantially alleviated by using smaller SAPO-34 crystals (<200 nm), which enhanced

∗ Corresponding authors at: CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China. E-mail addresses: [email protected] (Y. Zhang), [email protected] (W. Wei).

mass transfer [12–16]. Another approach is to synthesize hierarchical SAPO-34 crystals that have not only micropores but also meso or macro pores, which are equivalent to nanocrystals [17–23]. From 1990s, many researchers have studied the synthesis of small SAPO-34 crystals (<1000 nm), including regular hydrothermal synthesis [24–29], microwave synthesis [30–34], ultrasonic-assisted synthesis [35,36], dry gel conversion synthesis [37,38] and fast heating in tubular reactor [37]. Small SAPO-34 crystals, ranging from 20 nm to several hundred nanometres, can be obtained easily as long as excessive amount of template (tetraethylammonium hydroxide:TEAOH, TEAOH/Al2 O3 molar ratio usually >2) was used to enhance nucleation [28,29]. Most of time, product yield (<20%) has to be sacrificed to get small crystals. Therefore, it is still challenging to make SAPO-34 nanocrystals at low cost. Traditionally, zeolites are made by solvothermal (or hydrothermal) synthesis in the presence of expensive organic templates and large amounts of solvents such as water and/or alcohols under autogenous pressure [1–7]. The use of organic template and solvent has many negative impacts on the zeolite’s cost, including raw material cost, autoclave investment, product yield/autoclave, waste disposal

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etc. Recently, the idea of green zeolite synthesis was proposed by Xiao group [40–45], which combined template-free, solvent-free as well as other green features in every aspects of zeolite synthesis. Green synthesis not only reduces the zeolite cost (by reducing template consumption&waste disposal and increasing product yield), but also makes the synthesis green and sustainable [40]. Following the green strategy for zeolite synthesis, we propose the solvent-free synthesis of SAPO-34 nanocrystals with significantly reduced template consumption. The effect of various Al and Si sources on the synthesis was investigated. The obtained SAPO-34 samples were characterized by XRD, SEM, NMR, nitrogen adsorption, NH3 -TPD, XPS, XRF and methanol-to-olefins test. 2. Experimental methods 2.1. Chemicals Al(i-C3 H7 O)3 (98%), Silica sol (Ludox AS-40, 40 wt% SiO2 ), fumed silica (Evonic aerosol 200) and Al(OH)3 (50–57% Al2 O3 ) were obtained from Sigma-Aldrich. Tetraethylammonium hydroxide (35 wt% aqueous solution), tetraethyl orthosilicate(TEOS, 99%) and phosphoric acid (85 wt% aqueous solution) were provided by Shanghai Richjoint Chemical Reagents. All chemicals were used without further purification.

TGA Q500 unit in air at a heating rate of 10 ◦ C min−1 from room temperature to 800 ◦ C in air. Chemical compositions were determined with an X-ray fluorescence (XRF) spectrometer (PANalytical, AXIOS). The temperature programmed desorption of ammonia (NH3 -TPD) experiments were performed using a Micromeritics AutoChem II 2920 automated chemisorption analysis unit with a thermal conductivity detector (TCD) under helium flow.

2.4. Catalytic tests Methanol conversion was tested in a quartz tubular fixed-bed reactor at atmospheric pressure. The catalyst (∼2 g, 40–60 mesh) was loaded in the quartz reactor (6 mm inner diameter) and activated at 773 K in a N2 flow of 30 mL/min for 1 h before decreasing the temperature to reaction temperature of 673 K. The feed (50:50 wt% methanol-water mixture) was pumped into the reactor with a fixed WHSV of 1.0 h−1 . The reaction products were analyzed every 30 min by a gas chromatograph (Agilent GC 7890N), equipped with a flame ionization detector (FID) and Plot-Q column. The conversion and selectivity were calculated on CH2 basis and dimethyl ether (DME) was considered as reactant in the calculation. Catalyst is considered as deactivated when methanol conversion is lower than 99.5%.

2.2. Synthesis of zeolite SAPO-34 The starting gel has molar ratio of 1.0 Al2 O3 :1.0 P2 O5 :0.3 SiO2 :1.0 TEAOH:52 H2 O. In a typical synthesis, Al source (Al(iC3 H7 O)3 or Al(OH)3 ), H3 PO4 (85 wt% aqueous solution) and DI H2 O were stirred for 3 h to form an homogeneous solution. Then silica source (Ludox AS–40, TEOS or fumed silica) was added and the resulting solution was stirred for another 3 h. Then tetraethylammonium hydroxide (35 wt% aqueous solution) was added, and the solution was stirred for 1 h. Finally, the obtained precursor was put in a 353 K oven to evaporate the solvents (water and iso-propanol/ethanol when using Al-iso/TEOS). After removing all solvents (monitor the weight change to make sure complete solvent removal), certain amount of water was added into the dry precursor to reach desirable H2 O/Al2 O3 ratio. Then, the final precursor was loaded into an autoclave and held at 493 K for 24 h. After synthesis, the autoclave was quenched with tap water. The obtained products were washed with DI water thoroughly and centrifuged to get solid products. The obtained products were dried at 383 K. For template removal, the powder samples were calcined in a muffle furnace at 823 K for 6 h in air. 2.3. Characterization Powder XRD patterns were collected by a Rigaku Ultima IV Xray diffractometer with Cu K˛ X-radiation (tube voltage: 40 kV and tube current: 40 mA). Scanning electron micrographs (SEM) were taken on a ZEISS SUPRA55 SAPPHIRE field emission scanning electron microscope at 2 kV. Samples for SEM analysis were coated using a Shanghai Fudi sputter coater with a gold-palladium target. Transmission electron microscopy (TEM) images were collected on a Tecnai F20 electron microscope. 29 Si MAS NMR measurements were performed on a Brucker Advance-400 spectrometer operating at 99 MHz. The NMR spectra with high-power proton decoupling were recorded by the use of a sample-rotation rate of 5 kHz and a 4 mm MAS probe head. Infrared spectra were collected on a Thermo Scientific Nicolet 6700 Fourier-transform infrared spectrometer (FT-IR). Nitrogen adsorption measurements were carried out on a Micromeritics TriStarII3020 surface area analyzer. The analyses of the calcined samples were acquired after outgassing at 300 ◦ C. Thermogravimetric (TG) analysis was performed on a TA company

3. Results and discussion 3.1. The effect of H2 O/Al2 O3 ratio Fig. 1 shows powder XRD patterns and SEM images of SAPO34 samples prepared with different H2 O/Al2 O3 ratios (gel molar recipe: 1.0 Al2 O3 :1.0 P2 O5 :0.3 SiO2 :1.0 TEAOH:X H2 O, at 493 K for 24 h, Ludox AS40 and Al-iso as default Si and Al sources, respectively). As shown in Fig. 1, highly crystalline materials were obtained when the H2 O/Al2 O3 ratio was in the range of 1–15. Pure SAPO-34 was obtained when H2 O/Al2 O3 ratio was greater than 3. Further reducing water content, SAPO-5 started to form and became dominant at H2 O/Al2 O3 ratio of 1. Without addition of any water in the precursor, the product was nearly amorphous, but tiny SAPO-5 peaks can still be observed. When H2 O/Al2 O3 ratio was greater than 10, SAPO-34 crystals with rectangular plate morphology was obtained with diameter of 200–800 nm and thickness of 50–200 nm. Tiny cubic crystals (∼100 nm) along with some big rectangular plates (400 nm in diameter) were obtained at H2 O/Al2 O3 ratio of 5. At H2 O/Al2 O3 ratio of 3, uniform cubic SAPO-34 crystals with diameter ∼100 nm were obtained with high yield (>90%). TEM images in Fig. 1 clearly show the cubic morphology of the SAPO-34 sample with size 100–150 nm. The lattice fringe of SAPO-34 crystal (Fig. 1) and selected area electron diffraction pattern (SAED) suggests its single crystalline nature.

3.2. The effect of TEAOH/Al2 O3 ratio Fig. 2 shows the effect of TEAOH/Al2 O3 ratio on the product. XRD patterns in Fig. 2 indicates that pure SAPO-34 was obtained when TEAOH/Al2 O3 ratio ≥0.5. Lower TEAOH content led to the formation of SAPO-5 with an un-identified dense phase. SEM image shows cubic SAPO-34 crystals with diameter 200–300 nm were obtained at TEAOH/Al2 O3 ratio of 0.5. Apparently, the decrease of TEAOH content led to bigger crystals since TEAOH has strong positive impact on the nucleation process. Higher TEAOH content was not tried, since our focus is low template synthesis. But even smaller crystals are expected at higher TEAOH content.

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Fig. 2. Powder XRD patterns and SEM images of SAPO-34 samples prepared with different TEAOH/Al2 O3 ratio (1Al2 O3 :1P2 O5 :0.3SiO2 :XTEAOH:3H2 O, at 493 K for 1 d).

Fig. 1. Powder XRD patterns and SEM/TEM images of SAPO-34 samples prepared with different H2 O/Al2 O3 ratio (1Al2 O3 :1P2 O5 :0.3SiO2 :1TEAOH:XH2 O, at 493 K for 1 d).

3.3. The effect of silica and alumina sources It is well known that silica and alumina sources have big impact on zeolite synthesis, including phase selectivity, crystal size and morphology, nucleation and crystal growth, etc [1,2,4]. Here, three silica sources (TEOS, silica sol and fumed silica) and two alumina sources (Al-iso and Al(OH)3 ) were chosen and four combinations were tried, including, AS40 + Al-iso, AS40 + Al(OH)3 , TEOS + Al-iso and fumed silica + Al-iso. Based on the XRD patterns in Fig. 3, SAPO34 crystals with high crystallinity were obtained for all four cases. There are some minor impurity (peak at 2␪ = 16.9 ◦ ) in the samples made with Al-iso + AS40 and Al-iso + fumed silica, which could be assigned as zeolite SAPO-18, a common competing phase in the synthesis of zeolite SAPO-34. SEM images in Fig. 4 indicate that SAPO-34 crystals made with TEOS + Al-iso are the smallest ( < 100 nm, morphology not clear under SEM), while the other three combinations led to cubic crystals with diameter around 100 nm.

Fig. 3. Powder XRD patterns of SAPO-34 samples prepared with different Al and Si sources (1Al2 O3 :1P2 O5 :0.3SiO2 :1TEAOH:3H2 O, at 493 K for 1 d).

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Fig. 4. SEM and TEM images of SAPO-34 samples prepared with different Al and Si sources (1Al2 O3 :1P2 O5 :0.3SiO2 :1TEAOH:3H2 O, at 493 K for 1 d).

TEM images in Fig. 4 shows that SAPO-34 crystals made with TEOS + Al-iso are only 20–40 nm in diameter, with irregular morphology. For the other three cases, cubic crystals with diameter 80–150 nm were obtained. The electron diffraction patterns indicate the single crystalline nature of these nanocrystals. 3.4. Characterizations Various characterization techniques were applied to these four samples, including nitrogen adsorption, NMR, NH3 -TPD, elemental analysis by XRF and XPS. The N2 adsorption isotherm of regular SAPO-34 sample should be type I isotherm, which is characteristic for microporous materials [20,22]. However, the four N2 adsorption isotherms in Fig. 5 are either type IV(Al-iso + fumed silica) or a mixture of type I and II(the other three samples). Hysteresis (0.7 < P/P0 < 0.95) was observed for all four samples, which indicates the presence of mesopores and macropores. Pore size distribution (calculated by BJH desorption branch) shows the presence of mesopores from 10–50 nm, which should come from the stacking of these nanocrystals. Table 1

Fig. 5. Nitrogen adsorption isotherms and pore size distribution analysis (BJH desorption branch) of SAPO-34 samples prepared with different Al and Si sources (1Al2 O3 :1P2 O5 :0.3SiO2 :1TEAOH:3H2 O, at 493 K for 1 d).

lists the detailed BET surface area and pore volume data calculated from the isotherms. All four samples have decent BET surface area (576–845 m2 /g) and micropore volume (0.2–0.3 cm3 /g, determined by t-plot method), which are consistent with previous publications. The sample made with AS40 + Al-iso has the highest BET surface area of 845 m2 /g and micropore volume of 0.30 cm3 /g, which are substantially higher than literature result [24–39]. Usually, SAPO34 crystals have BET surface area between 400–600 m2 /g and micropore volume ∼0.20 cm3 /g. Only a few publications reported BET surface area higher than 600 m2 /g [18,32,33,39] and two publications reported BET > 700 m2 /g [46,47]. One publication listed

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Langmuir surface area of 818 m2 /g, which should translate to BET area ∼700 m2 /g [48]. This result indicates that the obtained SAPO34 sample has great crystallinity, which might be the result of high super-saturation of solvent-free synthesis [40]. Besides micropores, these four samples all have decent mesopores, with mesopore volume ∼0.2 cm3 /g, comparable with their corresponding micropore volume (0.2–0.3 cm3 /g). Apparently, different combination of Si and Al source has big impact on the surface area (internal and external) and pore volume (micropore and mesopore), which might affect their catalytic performance. It is generally believed that the presence of abundant mesopores is beneficial to diffusion of reactants, intermediates and products in MTO process, which might lead to longer lifetime for the catalyst [24,26]. Compared with literature result, our solvent-free synthesis clearly has several advantages. Besides high conversion, high product yield/autoclave and reduced waste water emission, we obtained uniform SAPO-34 nanocrystals with low consumption of TEAOH, which was never achieved with traditional methods. The obtained SAPO-34 crystal has hierarchical pore structure, which usually can only be obtained by special treatment, such as the addition of porogen, post treatment, etc [17–23]. In SAPO zeolites, Si atoms incorporate into the AlPO4 framework by two substitution mechanisms, SM2 and SM3. In SM2 mechanism, one Si atom substitutes for one P atom to form Si(4Al) entities, which leads to negatively charged framework and one Brönsted acid site. In SM3 mechanism, the simultaneous substitution of two neighbouring Al and P atoms by two Si atoms would generate multiple chemical environments for Si atom, Si(nAl) (n = 0–3) structures, which leads to the formation of complex acidic bridge hydroxyl groups [49]. Fig. 6 shows the 29 Si, 27 Al and 31 P NMR spectra of these four calcined SAPO-34 samples. The 29 Si NMR spectrum of sample made with Al-iso + AS-40 shows a broad signal composed of components at −90, −94, −99, −106 ppm, which correspond to Si(4Al), Si(3Al), Si(2Al) and Si(1Al) respectively. Peaks at −90 and −94 ppm are much stronger which suggests the presence of more Si(4Al) and Si(3Al) components. No peak was observed at −110 ppm, which indicates the absence of Si(0Al), or the Si-island. The incorporation of silicon through SM2 generates Si(4Al) groups while SM3 mechanism generates the others. It appears that SM2 mechanism contributed more than SM3. The weak band around −78 to −85 ppm can be assigned to Q1, Q2, and Q3 Si species ((Si(OT)n )OH)4-n ) due to hydration at room temperature and breaking of Si-OH-Al bonds, disordered Si(4Al) or partially hydrated Si species located at the edge of a Si domain [49]. The 29 Si NMR spectra of the other three samples are similar to this one, but with two differences: no peak at −106 ppm (Si(1Al)) and higher concentration of Si(4Al) and Si(3Al) than Si(2Al). This result indicates that silica source and alumina source have big impact on the Si insertion of zeolite SAPO-34, which might affect its catalytic properties. The 27 Al NMR spectrum of calcined nano-SAPO-34 samples shows an intense peak at 33 ppm which is due to tetrahedral aluminum in the SAPO-34 framework. The very weak signals at −11 and 14 ppm could be attributed to octa and penta coordinated aluminum. The strong resonance peak at −30 ppm in the 31 P NMR spectrum suggests the predominant P (4Al) environment in the framework. The weak should peak at −26 ppm can be attributed to P atoms coordinated with water molecules in the form of P(OAl)x (H2 O)y species [33]. The 27 Al and 31 P NMR spectra are similar to literature result [49]. TPD of ammonia was applied to characterize the acidic properties of SAPO-34 samples. The results are presented in Table 2 and Fig. 7. NH3 -TPD analyses of these four samples reveal a typical twopeak profile, with an intense peak at ∼150 ◦ C and a weak shoulder peak between 320 and 450 ◦ C. The low temperature peak at ∼150 ◦ C could be assigned to weak Brönsted or Lewis-acid sites and the

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Fig. 6. 29 Si, 27 Al and 31 P solid state NMR spectra of calcined SAPO-34 samples prepared with different Al and Si sources (1Al2 O3 :1P2 O5 :0.3SiO2 :1TEAOH:3H2 O, at 493 K for 1 d).

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Table 1 N2 adsorption results of the SAPO-34 samples prepared with different Al and Si sources (1Al2 O3 :1P2 O5 :0.3SiO2 :1TEAOH:3H2 O, at 493 K for 1 d). Sample

BET surface area (m2 /g)

Micropore volume (cm3 /g)

Mesopore volume (cm3 /g)

Micropore surface area (m2 /g)

External surface area (m2 /g)

Catalyst lifetimea (min)

Al-iso + AS40 Al(OH)3 + AS40 Al-iso + TEOS Al-iso + Fumed silica

845 660 753 576

0.30 0.23 0.29 0.20

0.098 0.14 0.11 0.18

809 627 694 522

36 33 59 54

330 600 600 840

SBET (total surface area) calculated by applying the BET equation using the linear part (0.05 < P/Po < 0.30) of the adsorption isotherm. Smicro (micropore area), Sext (external surface area) and Vmicro (micropore volume) calculated using the t-plot method. Vmeso (mesopore volume) calculated using the BJH method (from desorption branch). a Catalyst lifetime: catalyst considered as deactivated when methanol conversion ≤99.5%.

Table 2 NH3 -TPD of SAPO-34 samples. Area ratio Sample

Total acid amount (mmol/g)

Ratio of strong acid/weak acida

Catalyst lifetime (min)

Al-iso + AS40 Al(OH)3 + AS40 Al-iso + TEOS Al-iso + Fumed silica

0.70 0.79 0.73 0.64

0.248 0.212 0.262 0.147

330 600 600 840

a

Split at 523 K.

than that in the bulk, indicating the Si enrichment phenomenon on the surface of crystals. The similar phenomena were also observed for the synthesis of other SAPO molecular sieves [49]. It is worthy to point out that the SAPO-34 sample made with Al-iso + fumed silica has the highest surface Si concentration. Compared the Si content and acid amount data, we found that the SAPO-34 sample made with Al-iso + fumed silica has the highest bulk Si concentration (from XRF), however, the acidity of this sample is not the highest. This result is strange because usually higher Si content in SAPO zeolites means more acid sites. We think the relationship between Si atoms and protons might not be 1:1 ratio. The chemical environment of Si atoms has direct impact on the acid sites. In SM2 insertion mechanism, one Si atom means one proton, while in SM3 mechanism, no proton was generated. The presence of disordered Si(4Al) or partially hydrated Si species located at the edge of a Si domain may also affect the acid sites. The presence of Si island leads to fewer protons (not the case this time) [49]. 3.5. Catalytic test

Fig. 7. NH3 -TPD curves of calcined SAPO-34 samples prepared with different Al and Si sources (1Al2 O3 :1P2 O5 :0.3SiO2 :1TEAOH:3H2 O, at 493 K for 1 d).

high temperature peak at ∼400 ◦ C could be attributed to strong Brönstead acid sites. The strong-acid sites (bridging SiOHAl groups) was generated by the incorporation of silicon into the framework of the SAPO-34 and the weak-acid sites are most probably Brönsted centres originating from POH groups, which can be ascribed to phosphorus atoms that are not fully linked to AlO4 tetrahedra. The presence of these POH indicates imperfections within the structure of the framework [27,49]. This dual-peak profile is similar to literature result for the acidity of different SAPO materials [17,23,27,49]. However, unlike conventional microporous SAPO-34 sample, our SAPO-34 samples have high concentration of weak acid sites and low concentration of strong acid sites, which is similar to Yu’s thin plate SAPO-34 crystals made with oil-bath heating [39]. As shown in Table 2, the total acid amount was between 0.64–0.79 mmol/g, similar to literature result. The ratio of strong acid/weak acid varied from 0.15–0.26. XPS and XRF were used to obtain the information of the surface composition and bulk composition of SAPO-34 samples (shown in Table 3). It was found that the Si content on the surface is higher

Catalytic tests of methanol-to-olefin conversion were performed in a fixed bed reactor at a fixed condition (WHSV = 1 h−1 , T = 673 K, catalyst weight = 2 g). The conversion of methanol and selectivities for various products versus time-on-stream (TOS) over the four SAPO-34 catalysts are shown in Fig. 8, and the detailed MTO results are summarized in Table 4. Although these four catalysts exhibited quite different lifetimes, there are some general trends regarding the catalytic behaviour. For all four catalysts, ethylene and propylene are the main products (70–80% in total), indicating all SAPO-34 catalysts are very selective for the light alkenes. Before catalyst deactivation, the sum of ethene and propene selectivities was ∼80% (with a nearly 50:50 split) and the butylene selectivity was 13–15%. The total selectivity for C2–C4 alkenes reached ∼95%. The formation of other products, whether small molecules like CH4 and C2 H6 , or big molecules (C5+), is not substantial. Ethylene and propylene selectivities increased with time on stream, and the formation of long-chain alkenes, C4–C6 alkenes, exhibited a decreasing trend, which might be explained by the increased diffusion hindrance caused by coke accumulation [31]. Compared with conventional SAPO-34 sample (micron size cubic crystals, lifetime usually <200 min), all four samples exhibited remarkably prolonged lifetimes (ranging from 330–840 min). It is worthwhile to point out that catalyst was considered deactivated when the methanol conversion was <99.5%. The increased lifetime could be simply attributed to small size of the SAPO-34 crystals, which can greatly enhance the mass transfer of reactants and generated products in MTO reaction. Notably, the lifetime of sample made with Al-iso + fumed silica was the longest, ∼840 min, while the sample made with Al-iso + AS40 was the shortest, ∼330 min. The other two samples exhibited intermediate lifetimes ∼600 min. After careful comparison of the nitrogen adsorption data in Table 1,

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Table 3 Elemental analyses of calcined SAPO-34 samples prepared with different Al and Si sources. Composition Sample

XRF

XPS

Catalyst lifetime (min)

Al-iso + AS40

Si0.058 Al0.458 P0.402 O2

Si0.094 Al0.491 P0.445 O2

330

Al(OH)3 + AS40 Al-iso + TEOS Al-iso + Fumed silica

Si0.060 Al0.423 P0.374 O2 Si0.057 Al0.493 P0.457 O2 Si0.076 Al0.495 P0.486 O2

Si0.120 Al0.553 P0.563 O2 Si0.091 Al0.590 P0.577 O2 Si0.138 Al0.456 P0.417 O2

600 600 840

Fig. 8. Conversion and Products distribution of SAPO-34 samples prepared with different Al and Si sources in MTO reaction. Experimental conditions: WHSV = 1 h−1 ,T = 673 K, catalyst weight = 2 g.

it is safe to say the catalyst lifetime is positively related to the presence of mesopores (the ratio of mesopore volume/micropore volume or the ratio of external surface area/internal surface area) [20].

The catalytic behaviour of SAPO-34 sample is not only affected by its pore structure, but also by the acidity concentration and strength (distribution) [20,27]. Usually, SAPO-34 samples exhibited a dual-peak profile with more concentration of strong acid

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Table 4 Detailed MTO results of SAPO-34 catalysts at different stages. Catalysts

TOS(min)

Selectivity (%)

90 330 90 600 90 600 90 840

Al-iso+ AS40 Al(OH)3 + AS40 Al-iso+ TEOS Al-iso+ Fumed silica

CH4

C2 H4

C2 H6

C3 H6

C3 H8

C4 H8

C4 H10

C5 +

C2 = +C3 =

0.71 0.95 0.76 1.17 0.77 1.16 0.75 1.33

28.82 38.24 24.91 41.17 23.31 40.32 21.65 38.75

0.39 0.31 0.59 0.43 0.80 0.67 0.81 0.52

37.79 40.78 36.28 41.21 33.70 40.29 32.07 40.25

4.11 0.71 7.79 0.60 10.84 1.10 9.85 0.67

20.63 15.38 22.15 13.05 21.76 13.14 22.72 14.82

0.03 0.04 0.04 0.04 0.18 0.04 0.11 0.01

7.52 3.59 7.48 2.33 8.64 3.28 12.04 3.65

66.61 79.02 61.19 82.38 57.01 80.61 53.72 79.00

Experimental conditions: WHSV = 1 h−1 ,T = 673 K, catalyst weight = 2 g.

Table 5 Coke analysis of SAPO-34 samples. Sample

Coke amount (mg/gcat )

TOS (min)

Rcoke (mg/ gcat /min)

Al-iso + AS40 Al(OH)3 + AS40 Al-iso + TEOS Al-iso + Fumed silica

113 191 149 208

450 750 720 900

0.252 0.255 0.207 0.231

sites. However, our SAPO-34 samples have more weak acid sites than strong acid sites, which is similar to Yu’s result [39]. Previous publications indicated that the weak Brönsted acid sites (desorption peak < 300 ◦ C) cannot catalyze the conversion of methanol at ∼400 ◦ C [27,50]. But our result showed that the catalyst with lowest concentration of strong acid sites exhibited the longest lifetime. This result indicated that too much strong acid sites might lead to coking and reasonable ratio of strong acid/weak acid might result into longer lifetime [39]. Compared with literature result, the life time of our samples is much higher than SAPO-34 samples with tri-level hierarchical porosity(micro–meso–macropore structure, life time 386 min), bilevel hierarchical porosity(micro–macropore structure, life time 186 min) and bi-level hierarchical porosity (micro–mesopore structure, life time: 226 min) [20]. Yu et al. made SAPO-34 crystals with thin plate morphology by oil-bath heating. Their catalysts exhibited similar acid property and catalytic performance(lifetime and selectivity) [39]. After reaction, the SAPO-34 catalysts were discharged and the coke formation over the deactivated catalysts was measured by thermal analysis. As detailed in Table 5, there is a simple correlation between the amount of coke and lifetime of the catalyst, which indicates the hierarchically porous SAPO-34 catalysts can greatly enhance the transfer of the products from the narrow pore to outside space and reduce the coke formation and thus prolong the lifetimes. The coke formation rate was 0.20–0.25 mg/gcat/min (Fig. 9).

Fig. 9. TPO curves of SAPO-34 catalysts after MTO reaction.

3.6. Comparison of solvent-free synthesis and regular hydrothermal synthesis for zeolite SAPO-34 Traditional zeolite synthesis is not a green process, which consumes expensive raw materials and energy, and generates large amount of emissions, including waste water, alkali&salts, NOX , organic template, etc. It is a general trend to reduce the emission and energy cost for zeolite synthesis and make the synthesis green and sustainable. Roger A. Sheldon’s E-factor first provided a general metric that measure aspects of a chemical process relating to the principles of green chemistry. The famous Shelton E-factor is defined by the ratio of the mass of waste per unit of product [51]. Table 6 compared the regular hydrothermal synthesis and our solvent-free synthesis of zeolite SAPO-34 regarding the emission and autoclave efficiency. With solvent-free synthesis, the waste water and template consumption were reduced by 94.5% and 50%

Table 6 Comparison of solvent-free synthesis and regular hydrothermal synthesis for zeolite SAPO-34.

Gel recipe Waste water (kg/kg SAPO-34) Template consumption (kg/kg SAPO-34) Autoclave efficiency (weight of SAPO-34/100 mL autoclave) Shelton E-factor

Regular method

Solvent-free method

Percentage of change

1Al2 O3 :1P2 O5 :0.3SiO2 : 2TEAOH:55H2 O 3.78

1Al2 O3 :1P2 O5 :0.3SiO2 : 1TEAOH:3H2 O 0.206

NA

1.12

0.56

−50%

5.4g

50g

+926%

8.3

2.5

+332%

−94.5%

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respectively, while the autoclave efficiency was increased 926%. The Shelton E-factor was also reduced from 9.3 to 2.5, a 70% reduction. This result indicates that our solvent-free synthesis is much greener than regular hydrothermal synthesis. It is worthy to point out that the obtained catalyst exhibited much longer lifetime in MTO reaction, which is an extra green bonus. 4. Conclusions In summary, nano-sized zeolite SAPO-34 catalysts with high crystallinity have been successfully synthesized by solvent-free method. High quality nano-sized SAPO-34 crystals (<200 nm) were obtained with reduced template consumption. Different combinations of Si and Al sources had big impact on the crystal size, hierarchical pore structure, acid property, Si environment and distribution in the zeolite crystal. The obtained SAPO-34 samples exhibited hierarchical porosity, which comes from the stacking of nanocrystals. Importantly, the nano-sized SAPO-34 zeolites exhibited excellent performance in MTO reaction and comparable catalytic lifetime and selectivity of light olefins, compared with the nano-sized catalyst synthesized by the conventional hydrothermal method. The extended lifetime could be attributed to the presence of abundant mesopores which alleviate the diffusion constrain. The unique acid property of the catalyst might reduce the coke formation. The significantly reduced solvent consumption not only reduced waste water emission but also improved autoclave efficiency. Acknowledgements The authors acknowledge the financial support from National 863 Project (No. 2012AA050104), Advanced Coal Project (No. XDA07040400) and SARI (No. Y426473231). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata.2016.11. 005. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

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