Synthesis of highly selective and stable mesoporous

G Model PARTIC-1097; No. of Pages 10

ARTICLE IN PRESS Particuology xxx (2018) xxx–xxx

Contents lists available at ScienceDirect

Particuology journal homepage: www.elsevier.com/locate/partic

Synthesis of highly selective and stable mesoporous Ni–Ce/SAPO-34 nanocatalyst for methanol-to-olefin reaction: Role of polar aprotic N,N-dimethylformamide solvent Hossein Akhoundzadeh a , Majid Taghizadeh a,∗ , Hassan Sharifi Pajaie b a b

Chemical Engineering Department, Babol Noshirvani University of Technology, P.O. Box 484, Babol 4714871167, Iran Wood and Paper Science Department, Faculty of Natural Resources, Sari Agricultural Sciences and Natural Resource University, P.O. Box 737, Sari, Iran

a r t i c l e

i n f o

Article history: Received 4 March 2017 Received in revised form 23 October 2017 Accepted 15 November 2017 Available online xxx Keywords: Ni–Ce/SAPO-34 MTO reaction Impregnation Aprotic solvent Protic solvent

a b s t r a c t A series of mesoporous nanocrystalline silicoaluminophosphate (SAPO) zeolites (SAPO-34) were synthesized via an ultrasonic and microwave-assisted hydrothermal method in the presence of [3(trimethoxysilyl)propyl]octadecyldimethylammonium chloride and cetyltrimethylammonium bromide surfactants as soft templates. Nickel and cerium were then doped on SAPO-34 using incorporation and impregnation methods, and all the catalysts were applied to the methanol-to-olefin (MTO) reaction. The catalysts were characterized using X-ray diffraction, field-emission scanning electron microcopy, inductively coupled plasma–atomic emission spectroscopy, transmission electron microscopy, Fouriertransform infrared spectroscopy, Brunauer–Emmett–Teller analysis, NH3 temperature-programmed desorption analysis, and thermogravimetric analysis. For the impregnation method, the effect of using protic or aprotic solvents as impregnation media on the physico-chemical properties of the metal-based SAPO-34 was investigated. Water and N,N-dimethylformamide (DMF) were employed as the protic and aprotic solvents, respectively. The catalyst prepared using the aprotic DMF solvent exhibited higher dispersion and lower aggregation of metal species compared with that prepared using the protic water solvent. Furthermore, the sample synthesized using the incorporation method exhibited good catalytic performance; however, the Ni–Ce/SAPO-34 sample prepared using the impregnation method and aprotic DMF solvent exhibited superior catalytic performance in the MTO reaction. © 2018 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Introduction The increase in oil costs has drawn the attention of researchers to obtaining methanol from syngas as well as to the catalytic transformation process of this oxygenate compound. Moreover, public awareness of alternative energy sources such as coal, natural gas, and biomass that are used to provide fuels and raw materials is another issue associated with this enhanced attention (Aguayo, Gayubo, Vivanco, Olazar, & Bilbao, 2005). Since the 1970s, many researchers have been interested in methanol-toolefin (MTO) conversion over microporous solid acid catalysts as an alternative process for olefin production (White, 2011). Various molecular sieves have been assessed for the MTO reaction in recent decades (Djieugoue, Prakash, & Kevan, 2000). Addition-

∗ Corresponding author. E-mail address: [email protected] (M. Taghizadeh).

ally, the roles of both the framework and acidity in controlling the performances of molecular sieves in the MTO reaction have been acknowledged (Chen, Bozzano, Glover, Fuglerud, & Kvisle, 2005). The aluminosilicate zeolite ZSM-5 and silicoaluminophosphate (SAPO) molecular sieve SAPO-34 are generally considered efficient catalysts for the MTO reaction. The use of SAPO-34 with chabazite cages and an eight-ring pore opening significantly improved the selectivity toward light olefins compared with the use of ZSM-5 (Liang et al., 1990; Wu, Guo, Xiao, & Luo, 2013). Aluminophosphate (AlPO4 )-based molecular sieves, however, are considered microporous materials and cannot be directly applied as acidic catalysts because of their neutral frameworks, which contrast with the negatively charged frameworks of zeolites. Nevertheless, a negatively charged framework can be produced by introducing Si into the AlPO4 framework. Hence, the resulting SAPO-34 silicoaluminophosphates can be used as MTO reaction catalysts (Zhang, Bates, Chen, Nie, & Huang, 2011). However, the deposition of high-molecular-weight hydrocarbons at the pore entrances

https://doi.org/10.1016/j.partic.2017.11.004 1674-2001/© 2018 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Please cite this article in press as: Akhoundzadeh, H., et al. Synthesis of highly selective and stable mesoporous Ni–Ce/SAPO34 nanocatalyst for methanol-to-olefin reaction: Role of polar aprotic N,N-dimethylformamide solvent. Particuology (2018), https://doi.org/10.1016/j.partic.2017.11.004

G Model PARTIC-1097; No. of Pages 10 2

ARTICLE IN PRESS H. Akhoundzadeh et al. / Particuology xxx (2018) xxx–xxx

causes SAPO-34 catalysts to experience rapid deactivation and likely obstructs the internal cages of the SAPO-34 crystals (Chen, Moljord, Fuglerud, & Holmen, 1999; Qi et al., 2007). Numerous researchers have proposed that the properties and applications of these catalysts would be greatly affected by the size and morphology of the crystallites. The diffusion limitations of the guest molecules in the micropores yield a strong correlation between SAPO-34 catalyst performance and its particle size (Chen et al., 1999; Hirota et al., 2010). Despite weakening the catalyst deactivation with the blockage of pore openings, nanozeolite crystals are still considered when a short inner diffusion path is adopted in MTO reactions. In addition, to improve the mass transfer property of the catalyst and increase the number of catalyst pore openings, mesopores with various forms obtained through HF etching (Xi et al., 2014), hard templating (Jacobsen, Madsen, Houzvicka, Schmidt, & Carlsson, 2000), and soft templating (Choi, Srivastava, & Ryoo, 2006) have been introduced into the zeolite crystal. These two features have positive effects on catalyst anti-deactivation. However, various approaches such as synthesis with mixed templates (Sadeghpour & Haghighi, 2015) or microwave-assisted and ultrasonic synthesis of SAPO-34 (Pajaie & Taghizadeh, 2015) have been employed to synthesize SAPO-34 with a smaller crystal size. Another method involves modifying the zeolite with metal cations, leading to modification of the density and strength of the acid sites and, thus, increasing the selectivity toward light olefins and/or catalyst lifetime. Furthermore, the use of metal ions reduces the formation of methane. The type of modification method (incorporation, impregnation, or co-precipitation) and metal used thus appear to yield very different results. Salmasi, Fatemi, and Najafabadi (2011) studied the incorporation of metal ions such as nickel and magnesium in the structure of SAPO-34 synthesized by mixing tetraethylammonium hydroxide (TEAOH) and morpholine templates. The introduction of metal ions affected the acidity and improved the catalytic activity of the SAPO34 catalyst in the MTO process. The catalyst modified with nickel exhibited the best performance. Sun et al. (2012) investigated the catalytic performance of nano-Au/ZSM-5 in the methanol-topropylene process. They observed that gold nanoparticles reduced the dehydrogenation reaction rate, leading to retardation of coke formation, thereby increasing the lifetime of the catalyst. The catalytic performance of ZSM-5 and SAPO-34 in the catalytic cracking of naphtha was also investigated by Varzaneh, Towfighi, and Mohamadalizadeh (2014). In their research, cerium and zirconium were used to modify the zeolite using the impregnation method. The catalytic performance of zeolite was also enhanced by introducing metal ions. Ce–Zr/ZSM-5 and Ce–Zr/SAPO-34 exhibited better catalytic performance than the other samples. Moreover, the effect of the metal chloride on the catalytic performance of SAPO-34 in the MTO reaction has been studied by Jiang et al. (2015). In their study, SAPO-34 was modified with metal cations using the impregnation method. They reported that different metal ions had completely different effects on the acidity of the catalysts. In addition, the sample modified with Fe exhibited lower acidity, and the catalyst had a longer lifetime than the other samples. Sedighi, Ghasemi, Sadeqzadeh, and Hadi (2016) studied the incorporation of Ni, Ce, Fe, and La into the framework of SAPO-34. They observed that introducing the metals led to improvement of the catalytic performance of SAPO-34. The sample modified with Ce exhibited better catalytic performance than the other samples. In this study, the mesoporous SAPO-34 catalyst was synthesized by employing an ultrasonic and microwave-assisted hydrothermal method and [3(trimethoxysilyl)propyl]octadecyldimethylammonium chloride ([(CH3 O)3 SiC3 H6 N(CH3 )2 C18 H37 ]Cl (TPOAC)) and cetyltrimethylammonium bromide (CTAB) surfactants as mesoporogen agents. The nickel and cerium oxides were loaded on mesoporous SAPO-34

using incorporation and impregnation methods. The impregnation process was performed using a protic and an aprotic solvent. Moreover, the effects of water and N,N-dimethylformamide (DMF) as the protic and aprotic solvents, respectively, on the physico-chemical properties of metal-based mesoporous SAPO-34 were studied. The synthesized catalysts were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), inductively coupled plasma–atomic emission spectroscopy (ICPAES), transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR) spectroscopy, Brunauer–Emmett–Teller (BET) analysis, NH3 temperature-programmed desorption (NH3 -TPD) analysis, and thermogravimetric analysis (TGA). Finally, the catalytic performance of these catalysts in the MTO reaction was evaluated under the same operating conditions in a fixed bed reactor. Experimental Chemicals and regents Tetraethylorthosilicate (TEOS, 98 wt%), orthophosphoric acid (H3 PO4 , 85 wt%), aluminum isopropoxide (Al(i-C3 H7 O)3 , 99 wt%), tetraethylammoniumhydroxide (TEAOH, AIP, 25 wt%), cetyltrimethylammonium bromide (CTAB, 99 wt %), Ce(NO3 )3 ·6H2 O, Ni(NO3 )2 ·6H2 O, and N,N-dimethylformamide (DMF, 99 wt%) were purchased from Merck. In addition, [3(trimethoxysilyl)propyl]octadecyldimethylammonium chloride ([(CH3 O)3 SiC3 H6 N(CH3 )2 C18 H37 ]Cl, TPOAC, 72 wt%) was purchased from Sigma–Aldrich. All the chemicals were used as received without further purification. Catalyst preparation SAPO-34 synthesis Mesoporous SAPO-34 was synthesized hydrothermally with a molar composition of 1 Al2 O3 :1 P2 O5 :0.4 SiO2 :2 TEAOH:0.042 TPOAC:0.032 CTAB:70 H2 O. Briefly, certain amounts of AIP, TEAOH, and H2 O were mixed. After complete dissolution, the surfactants (including CTAB and TPOAC) were added to the suspension. Afterwards, TEOS and phosphoric acid were gradually added. Next, a microwave oven was used to heat the suspension at 200 W for approximately 1 h, followed by sonication for 15 min at room temperature. The gel obtained was transferred to a 200-mL Teflon-lined stainless-steel autoclave and then heated in an oven at 200 ◦ C for 18 h. The resultant product was centrifuged and washed several times with deionized water. Finally, the product was dried at 110 ◦ C for 12 h and calcined at 550 ◦ C for 6 h. Ni–Ce/SAPO-34 synthesis using incorporation method Ni–Ce/SAPO-34 was synthesized using the incorporation method with a molar composition of 1 Al2 O3 :1 P2 O5 :0.4 SiO2 :2 TEAOH:0.042 TPOAC:0.032 CTAB:0.006 NiO:0.012 CeO2 :70 H2 O. The Ni–Ce/SAPO-34 sample was synthesized using a procedure similar to that used to the prepare SAPO-34. An aqueous salt solution was added to the gel only after complete preparation of the final gel of SAPO-34, and the suspension was stirred for 1 h. The subsequent steps were analogous to those described above. The catalyst synthesized using the incorporation method was called Ni–Ce/SAPO-34(IN). Ni–Ce/SAPO-34 synthesis using impregnation method Ni–Ce/SAPO-34 was prepared by sequential impregnation of cerium nitrate hexahydrate (Ce(NO)3 ·6H2 O) and nickel nitrate hexahydrate (Ni(NO)3 ·6H2 O). First, a calculated amount of calcined SAPO-34 was immersed in an aqueous solution of cerium nitrate. The concentration of metal nitrate in the aqueous solution was


G Model

ARTICLE IN PRESS

PARTIC-1097; No. of Pages 10

H. Akhoundzadeh et al. / Particuology xxx (2018) xxx–xxx

0.05 M. The solution was then stirred for 24 h. The suspension was centrifuged, and the resultant solid was dried at 110 ◦ C for 12 h. The product was calcined in an air atmosphere at 550 ◦ C for 5 h. The aforementioned procedure was repeated for nickel nitrate. The amounts of nickel and cerium were 0.5% and 1% (w/w), respectively, and the final sample was called Ni–Ce/SAPO-34(IM-water). DMF was also used as the solvent instead of water, and the final sample was called Ni–Ce/SAPO-34(IM-DMF).

Characterization techniques XRD patterns of the zeolites were obtained on an X’Pert Pro diffractometer (PANalytical, Netherlands) using Cu K˛ radiation ( = 1.5406 Å) to detect the crystal phases. The intensity summation method (Eq. (1)) was used to calculate the relative crystallinity of the samples. This method is fundamentally based on the five characteristic peaks at Bragg angles of 9.58◦ , 13.8◦ , 16.18◦ , 20.5◦ , and 30.78◦ (Utchariyajit & Wongkasemjit, 2010). Among the synthesized samples, the catalyst with the highest summation intensity was selected as the reference with maximum crystallinity.

3

Catalytic test A laboratory catalytic evaluation system was used to conduct the MTO reactions over SAPO-34 catalysts at atmospheric pressure (Pajaie & Taghizadeh, 2015). First, 1 g of the calcined catalyst (18–25 mesh) was placed in a tubular stainless-steel reactor (length: 70 cm, internal diameter: 0.8 cm). The upper and lower portions of the catalyst were filled with quartz. Before the reaction, the catalyst was preheated at 550 ◦ C for 1 h under nitrogen flow. Afterwards, the temperature was reduced to 450 ◦ C (reaction temperature). The feed flow (methanol:water = 1:1, wt%) with weight hourly space velocity (WHSV) of 4 h−1 was introduced to the reactor. The product temperature was set to 175 ◦ C. To analyze the product, an on-line gas chromatograph (Varian 3800, Varian, USA) equipped with a flame ionization detector and a 50-m HPPONA capillary column was utilized. The methanol conversion was defined as the percentage of methanol consumed during the MTO reaction; dimethyl ether (DME) was not considered a product. The carbon balance between the inlet and outlet of the reactor was used to calculate the selectivities of products. The following equations were used to determine the selectivity toward the products and methanol conversion: MeOH conversion(%)

5

Crystallinity(%) =

i=1 5

=

Ii,sample × 100,

(1)

Ii,reference

×100,

(2)

Selectivity of ethylene(%)

i=1

= The N2 adsorption–desorption analysis was performed on a Chem-BET-3000 (Quantachrome, USA). Before the analysis, the sample was degassed at 300 ◦ C for 5 h. This analysis was performed to determine the specific area, external surface, and pore volume of the samples. FE-SEM analysis was performed on a Mira 3-XMU (Tescan, Czech Republic) to evaluate the morphology of the catalysts synthesized. TEM analysis was performed using a Zeiss-EM10C instrument (Zeiss, Germany) to determine the dispersion and morphology of the metal particles. Before the measurements, the samples were dispersed ultrasonically in ethanol. Elemental analysis was performed using ICP–AES on an Optima 2000 DV (Perkin Elmer, USA). The solution for the ICP–AES analysis was prepared by digesting 0.1 g of SAPO-34 zeolite powder in a solution prepared by mixing 0.5 mL of 50 wt% HF, 1 mL of 63 wt% HNO3 , 1 mL of 36 wt% HCl, and 4 mL of H2 O. After digestion, the HF was neutralized with 2 g of boric acid. The KBr pellet method was used for FT-IR spectroscopy analysis. The spectra were collected on a FT-IR Vertex80 spectrometer (Bruker, USA) equipped with an MCT cryodetector working at 2 cm−1 resolution. The acidity of the samples was measured using NH3 -TPD analysis on a PulseChemiSorb 2705 instrument (Micromeritics, USA). Before the test, 0.1 g of each sample was degassed at 500 ◦ C for 120 min under a He flow rate of 50 mL/min at a heating rate of 10 ◦ C/min. Subsequently, the catalyst was saturated in a stream of ammonia for 2 h. The sequential introduction of the flow of He was used to remove physisorbed ammonia molecules. Finally, desorption was performed at a heating rate of 10 ◦ C/min to 700 ◦ C, and the amount of desorbed ammonia was determined. To measure the amount of coke deposited, TGA of the used samples was performed in the temperature range of 30–800 ◦ C with a heating rate of 10 ◦ C/min in an air atmosphere using a Netzsch-TGA 209 F1 instrument (Netzsch, Germany).

(mole of MeOH)i − (mole of MeOH)o − 2(mole of DME)o (mole of MeOH)i

2 [(mole of ethylene)o − (mole of ethylene)i ] (mole of MeOH)i − (mole of MeOH)o − 2(mole of DME)o

×100,

(3)

Selectivity of propylene(%) =

3 [(mole of propylene)o − (mole of propylene)i ] (mole of MeOH)i − (mole of MeOH)o − 2(mole of DME)o

×100.

(4)

For the other products (such as light alkanes, butane), the selectivities were also defined using equations similar to Eqs. (2)–(4). The selectivities and conversions were expressed after approximately 30 min of time on stream (TOS) once quasi steady-state was reached. Results and discussion Catalyst characterization X-ray diffraction Fig. 1 presents XRD patterns of the SAPO-34 and modified SAPO34 samples. All the samples had a typical chabazite structure with main peaks at 9.5◦ , 13.8◦ , 16.18◦ , 20.5◦ , and 30.78◦ , which corresponds to results reported in the literature (Yang, Kim et al., 2012). Moreover, no additional peaks were detected in the XRD patterns of any of the samples. It is apparent from the XRD analysis that Ni–Ce/SAPO-34(IN) was successfully synthesized; however, the presence of cerium and nickel ions in the initial gel composition led to a reduction of the relative crystallinity (Table 1) (Zhang et al., 2008). It is clear that the impregnation of metals did not affect the original structure of the SAPO-34; that is, the introduction of the metals did not lead to the destruction of the structure of SAPO-34. However, the intensity of the main peaks decreased, possibly induced by minor damage


G Model

ARTICLE IN PRESS



4

Table 1 Relative crystallinity and elemental analysis of the prepared catalysts. Sample

Relative crystalinity (%)

Nickel contenta (wt%)

Cerium contenta (wt%)

Si/(Al + P)a

SAPO-34 Ni–Ce/SAPO-34(IN) Ni–Ce/SAPO-34(IM-water) Ni–Ce/SAPO-34(IM-DMF)

100 45 63 62

– 0.52 0.36 0.39

– 1.1 0.83 0.88

0.115 0.111 0.104 0.107

a

Obtained by ICP–AES.

for the Ni–Ce/SAPO-34(IM-water) sample than for the Ni–Ce/SAPO34 (IM-DMF) sample.

Fig. 1. XRD patterns of catalyst samples.

resulting from the introduction of the metals into the structure of the catalysts (Zhang et al., 2008). No peaks related to nickel and cerium species were detected in the modified samples; therefore, the metal species may have been uniformly dispersed on the surface of SAPO-34. The Si/(Al + P) molar ratio and the nickel and cerium contents of the synthesized catalysts measured by ICP–OES analysis are listed in Table 1. The amounts of these metals were similar to the theoretical values. Morphological analysis Fig. 2 presents FE-SEM images of the SAPO-34 and modified SAPO-34 samples. All the samples exhibited a cubic morphology related to the typical SAPO-34 structure. The use of microwave and ultrasonic irradiation in the synthesis procedure resulted in a nanosized SAPO-34 catalyst. The average sizes of the particles of SAPO-34, Ni–Ce/SAPO-34(IN), Ni–Ce/SAPO-34(IM-water), and Ni–Ce/SAPO-34(IM-DMF) were 126, 98, 125, and 122 nm, respectively. The presence of metal cations in the initial gel of Ni–Ce/SAPO-34(IN) as impure components led to a reduction of the crystallization rate. Thus, the crystals did not have sufficient time to grow, and their size consequently decreased. In addition, the reduction in the size of the crystals may have been due to an increase in the nucleation rate (Jhung, Lee, & Chang, 2008). After the sequential impregnation of the metals, the size of crystals did not change, which may be related to the trace of metals. TEM images of the two samples prepared using the impregnation method are presented in Fig. 3. This analysis focused on the morphology and size of the metal particles. The average metal particles of the Ni–Ce/SAPO-34(IM-water) and Ni–Ce/SAPO-34(IMDMF) catalysts were approximately 11 and 9 nm, respectively. The Ni–Ce/SAPO-34(IM-water) sample exhibited severe aggregation and a low dispersion of metal species; in contrast, the Ni–Ce/SAPO-34(IM-DMF) sample exhibited better dispersion and lower aggregation of metal species on the catalyst surface. Metal species more easily migrated and aggregated during heat treatment

FT-IR spectra Fig. 4 presents the FT-IR spectra of the samples. The peak at 490 cm−1 is attributed to silicon tetrahedral bending, and the peak at 640 cm−1 is attributed to T O bending in D-6 rings (Rahmani & Haghighi, 2015). The existence of this latter peak verifies the formation of the chabazite structure assigned to SAPO-34, which is consistent with the XRD and FE-SEM results. The peaks at 710, 835, and 1100 cm−1 are assigned to the asymmetric stretching of O P O, protonated template, and T O T symmetric stretching, respectively. The peaks at 1630 and 2360 cm−1 are attributed to ambient adsorption of water and CO2 from the atmosphere, respectively (Aghaei, Haghighi, Pazhohniya, & Aghamohammadi, 2016). The peak at approximately 3450 cm−1 is assigned to bridging hydroxyl groups, including Si–OH–Al (Brønsted acid sites) and internal P–OH and Si–OH (Lewis acid sites) (Askari, Halladj, & Sohrabi, 2012). FT-IR analysis only provides information about the amount of acid sites and not their strength or type. The peaks at 3692 and 3782 cm−1 are assigned to P–OH and Si–OH located on ˜ the external surface of the catalyst, respectively (Álvaro-Munoz, Márquez-Álvarez, & Sastre, 2012). Textural properties Fig. 5(a) presents the nitrogen adsorption–desorption isotherms of the samples. All the calcined catalysts exhibited I- and IV-type isotherms at relative pressures of P/P0 < 0.1 and 0.50 < P/P0 < 0.98, respectively, indicating the existence of micropores and mesopores in all the samples. The pore size distribution of the samples verified the existence of a mesoporous structure in all of the samples (Fig. 5(b)). The decline in the area of the nitrogen adsorption–desorption curves of modified SAPO-34 was mainly associated with the introduction of metals into the structure of SAPO-34. It can be concluded that the introduction of the metal species caused blockage of certain channels and pores of the catalysts. The SAPO-34 sample had the highest specific area among the samples (Table 2). Generally, after the introduction of metal species, the specific area significantly decreased due to the channels and pores of the catalyst being blocked by metal species. Ni–Ce/SAPO-34(IN) had a higher surface area than the other modified samples, which may be related to the existence of smaller particles in this sample (Rezaei, Halladj, Askari, Tarjomannejad, & Rezaei, 2016). Ni–Ce/SAPO-34(IM-DMF) exhibited a higher surface area, external surface, and pore volume than Ni–Ce/SAPO-34(IM-water). This is attributed to the metal species in the Ni–Ce/SAPO-34(IM-DMF) sample being less aggregated than those of Ni–Ce/SAPO-34(IM-water); therefore, the surface area of this catalyst decreased less than that of the other sample (Bang et al., 2016). NH3 -TPD Fig. A1 (Appendix A) and Table 3 present the data obtained from the NH3 -TPD analysis for the SAPO-34 and modified SAPO-34 samples. The strength of acid sites is indicated by the desorption temperature. Moreover, the amount of desorbed ammonia was




5

Fig. 2. FE-SEM images of catalysts: (a) SAPO-34, (b) Ni–Ce/SAPO-34(IN), (c) Ni–Ce/SAPO-34(IM-water), and (d) Ni–Ce/SAPO-34(IM-DMF).

Fig. 3. TEM images of catalysts prepared by impregnation method: (a) Ni–Ce/SAPO-34(IM-water) and (b) Ni–Ce/SAPO-34(IM-DMF).

determined from the area under the curves, and this value is associated with the density of acid sites. Two ammonia desorption peaks were observed in the NH3 -TPD profiles for all the samples; additionally, the desorption temperature ranges for the weak and strong acid sites were Td1 of 180–210 ◦ C and Td2 of 390–410 ◦ C, respectively. Logically, defective structural OH groups (including weak acid Si–OH, P–OH, and Al–OH) were responsible for the first peak sites. In addition, the second desorption peak corresponded to the

strong acid sites, which were related to Si–OH–Al groups (Brønsted acid sites). Brønsted acids are the active sites involved in the conversion of methanol to light olefins on zeolites; however, this is not true for Lewis acids according to the report by Anderson, Mole, and Christov (1980). However, the importance of Lewis acid sites for the MTO reaction is not negligible as DME formation occurs at weak acid sites (Sánchez del Campo, Gayubo, Aguayo, Tarrío, & Bilbao, 1998).


G Model

ARTICLE IN PRESS



6 Table 2 Textural properties of the catalyst samples.

Surface area (m2 /g)

Sample

SBET SAPO-34 Ni–Ce/SAPO-34(IN) Ni–Ce/SAPO-34(IM-water) Ni–Ce/SAPO-34(IM-DMF) a b c

a

598 577 559 574

Pore volume (cm3 /g) SMicro

b

SExt

425 407 422 417

b

173 170 137 157

VTotal

VMicro b

VMeso c

0.493 0.447 0.396 0.429

0.182 0.152 0.171 0.164

0.311 0.295 0.225 0.265

BET surface area. The external surface area (SExt ), micropore area (SMicro ), and micropore volume (VMicro ) were calculated using the t-plot method (Du & Wu, 2007). The mesopore volume (VMeso ) was calculated by subtracting the micropore volume from the total pore volume.

Table 3 Results of NH3 -TPD analysis of the catalyst samples. Catalyst

Peak temperature (◦ C)

Distribution and concentration of acid sites (mmol NH3 /g)

SAPO-34 Ni–Ce/SAPO-34(IN) Ni–Ce/SAPO-34(IM-water) Ni–Ce/SAPO-34(IM-DMF)

Region I (weak)

Region II (strong)

Total

Td1

Td2

0.42 0.45 0.44 0.43

0.48 0.38 0.41 0.33

0.9 0.83 0.85 0.76

186 195 192 190

408 398 396 400

Fig. 4. FT-IR spectra of catalyst samples.

Furthermore, another important factor in the MTO reaction is the variation in the Lewis acidity. In reactions such as alkylation and methylation, which are fundamental agents in olefin formation, Lewis acidity is used as a catalyst (Xu, Ma, Zhang, Weiyong, & Fang, 2013). The main reason for the improved anti-coking capability of weak acid sites compared with that of strong acid sites is

the efficient avoidance of hydrogen-transfer reactions that produce alkanes (Yang, Sun et al., 2012). The beginning of the MTO reaction (formation of the C C bond) and conversion of methanol are attributed to the Brønsted acid sites as major active sites. Another feature that should be noted is the acceleration of the coke formation rate through side reactions by ˜ et al., 2012). This phenomenon Brønsted acid sites (Álvaro-Munoz is considered a main reason for catalyst deactivation. Stronger Brønsted acid sites promote more coke formation reactions. Thus, moderate acid density and strength are preferred. The effective ionic radii of cerium and nickel ions are approximately less than 1.48 and 0.83 Å, respectively (Shannon, 1976), meaning that metal species are capable of entering SAPO-34 channels inside (diameter of 3.8 Å). The SAPO-34 sample exhibited the greatest strength and density of Brønsted acid sites. For the Ni–Ce/SAPO-34(IN) sample, the incorporation of metals resulted in a reduced density and strength of the Brønsted acid sites as well as an increased strength and density of the Lewis acid sites. In general, the substitution of phosphorus with silica leads to the generation of Brønsted acid sites (Ashtekar, Chilukuri, & Chakrabarty, 1994). The substitution of phosphorus with other metals (Ni and Ce) does not produce Brønsted acid sites, resulting in a decrease in the density of Brønsted acid sites (Salmasi et al., 2011).

Fig. 5. (a) Nitrogen adsorption–desorption and (b) Barrett–Joyner–Halenda (BJH) pore size distributions of catalyst samples.




7

Impregnation of metal ions led to a reduction in the density and strength of the Brønsted acid sites. The neutralization of the Brønsted acid sites and the pore blockage by metal species were responsible for this decrease in density (Rostamizadeh & Yaripour, 2016). The density of Brønsted acid sites of the Ni–Ce/SAPO-34(IMwater) sample did not sufficiently decrease; however, the density of Brønsted acid sites of the Ni–Ce/SAPO-34(IM-DMF) sample significantly decreased because the impregnation conditions of both catalysts differed. According to the BET analysis, the surface area of Ni–Ce/SAPO-34(IM-DMF) decreased less than that of Ni–Ce/SAPO34(IM-water). Thus, it appears that the reduction of the density of Brønsted acid sites in Ni–Ce/SAPO-34(IM-DMF) is more relevant to neutralizing acidic sites than blocking pores. Effect of impregnation solvent Tao, Meng, Lv, Bian, and Xin (2016) investigated the effect of the impregnation solvent on the dispersion and catalytic activity of nickel for Ni/SBA-15 in the CO methanation reaction. They used ethanol and water as solvents. The ethanol-synthesized Ni/SBA-15 catalyst exhibited better catalytic performance than the water-synthesized catalyst, which may have been due to the better dispersion and smaller size of metal species in this catalyst. The effect of the solvent may be related to its polarity. The decreased polarity of the solvent caused an increase in the interaction between the nickel hydrate and silica surface, resulting in the formation of more species. Thus, the silanol groups on the surface of the catalyst could directly form bonds with nickel ligands and, thereby, a higher density of nickel atoms was dispersed on the zeolite surface. Protic solvents such as water are widely used as impregnation solvents for doping metal ions on zeolites. Protic solvents contain H+ , and the molecules of these solvents easily give protons to the reagent. The zeolites have a negatively charged surface. For the Ni–Ce/SAPO-34(IM-water) sample, water molecules formed hydrogen bonds with the zeolite surfaces (Tao et al., 2016), which led to disorder in the access of the metal species to the zeolite surfaces during the impregnation process. This phenomenon caused a relatively weak interaction between the SAPO-34 support and metal precursor in the SAPO-34 impregnation process. The metal species could be redistributed during calcination and drying, resulting in aggregation of metal oxides on the SAPO-34 support (Zhu et al., 2013). Aprotic solvents lack acidic hydrogen; therefore, they are not hydrogen bond donors, and their molecules cannot impress zeolite surfaces. When using an aprotic DMF solvent as the impregnation medium in the Ni–Ce/SAPO-34(IM-DMF) sample, the metal species could form bonds with a negative SAPO-34 zeolite surface without interference by the solvent molecules. This formation led to an increase in the interaction between the SAPO-34 surface and metal species, and therefore, the metal species would be more stable against heat treatment. This phenomenon could lead to lower aggregation of the metal species on the surface of the zeolite relative to the use of water as the impregnating solvent, which is consistent with the TEM results. Catalytic performance Fig. 6 shows the conversion of methanol as a function of time on stream (TOS) for different samples. The reaction was performed at 450 ◦ C and WHSV 4 h−1 in a fixed bed reactor. Each test was repeated three times, and the average value was reported. All of the catalysts exhibited almost maximum methanol conversion within the first 200 min of the reaction, with the catalyst conversion then gradually decreasing due to coke formation. Finally, the catalysts were deactivated at different rates. The parent SAPO-34 catalyst was deactivated more rapidly than the

Fig. 6. Methanol conversion versus reaction time of SAPO-34 and modified SAPO-34 catalysts.

other catalysts. Thus, after 350 min of TOS, the methanol conversion declined to less than 90%, and the catalyst was deactivated. The Ni–Ce/SAPO-34(IM-water), Ni–Ce/SAPO-34(IN), and finally the Ni–Ce/SAPO-34(IM-DMF) catalysts were deactivated, sequentially. It is important to note that the catalytic performance of the catalysts synthesized using the impregnation method was quite different. The lifetime of the Ni–Ce/SAPO-34(IM-DMF) catalyst remarkably improved compared with that of the parent SAPO-34, whereas that of the Ni–Ce/SAPO-34 (IM-water) sample was not significantly increased. The main reason for the difference in catalyst performance between Ni–Ce/SAPO-34(IM-DMF) and Ni–Ce/SAPO34(IM-water) appeared to result from the different impregnation conditions of the catalysts. Fig. 7 presents the selectivities of ethylene, propylene, light olefins, C2 /C3 , and methane as a function of TOS. The ethylene selectivity of all the catalysts was low at the beginning of the reaction; however, with increasing TOS, it gradually increased, reached a maximum, and then decreased. At the beginning of the reaction, all the samples exhibited high propylene selectivity, which then decreased at different rates. The increasing ethylene selectivity and decreasing propylene selectivity with increasing time could be due to coke formation, leading to a reduction of the pore diameter of the catalysts. Hence, depending on the shape selectivity, the smaller components would be more easily released, resulting in increasing ethylene selectivity. In addition, this finding may be related to the ability of propylene to oligomerize to a larger compound, which consequently makes it possible to crack and form ethylene (Chen et al., 1999; Lee, Baek, & Jun, 2007). In general, a smaller increase in the C2 /C3 ratio results in a lower rate of catalyst deactivation. The increase in the C2 /C3 ratio of the Ni–Ce/SAPO-34(IM-DMF) catalyst was lower than that of the other catalysts. This finding could indicate that the reduction in the pore diameter of this catalyst due to the coke formation was lower than that for other samples. Accordingly, the catalyst deactivation rate was lower than that for the other catalysts. Generally, the introduction of metals into the SAPO-34 framework led to increasing light olefin selectivity. The Ni–Ce/SAPO34(IM-DMF) sample exhibited the greatest olefin selectivity (98%) among the samples. The Ni–Ce/SAPO-34(IN) and Ni–Ce/SAPO34(IM-water) catalysts exhibited 96% and 94% selectivity toward light olefins, respectively. The selectivity of SAPO-34 was 91%. The methane selectivity for all the catalysts increased with increasing TOS. Generally, the introduction of metal species led to reduced methane selectivity, and the Ni–Ce/SAPO-34(IM-DMF)




Fig. 7. Variations of selectivities of SAPO-34 and modified SAPO-34 catalysts with reaction time for various products: (a) ethylene, (b) propylene, (c) light olefins, (d) C2 /C3 ratio and (e) methane.

sample exhibited the lowest methane selectivity. DME could be decomposed into methane; however, the presence of metal species could have led to the adsorption of DME, and thus, methane formation was inhibited (Obrzut et al., 2003). In general, SAPO-34 with the lower density and strength of Brønsted acid sites, exhibited a slower rate of catalyst deactivation and/or higher olefin selectivity in the MTO process. The promotion of side reactions constituting hydrogen transfer was available via stronger Brønsted acid sites. The coke formation produced from these side reactions would cause blockage of the cages and entrance of pores and poison the acid sites (Teketel, Olsbye, Lillerud, Beato, & Svelle, 2010). However, a catalyst with a low density of Brønsted acid sites does not have a long lifetime (Gao et al., 2016). The

main purpose of metal doping is to adjust the acidity of the catalysts, which leads to a reduction of the rate of side reactions. The Ni–Ce/SAPO-34(IM-DMF) catalyst exhibited the lowest density of Brønsted acid sites among all the catalysts. It appears that the density of Brønsted acid sites of this catalyst was just sufficient for the MTO reaction, thereby leading to a decrease in the rate of the side reactions and a delay in the coke formation and, thus, improved catalytic performance. Coke formation To measure the amount of coke deposited on the spent samples, TGA was performed, and the TGA curves of all samples




are presented in Fig. A2 (Appendix A). These curves consist of two different weight loss steps. The first weight loss before 300 ◦ C is attributed to physically adsorbed water, and the second weight loss is associated with the combustion of coke species. The amount of coke deposited for the SAPO-34, Ni–Ce/SAPO34(IM-water), Ni–Ce/SAPO-34(IN), and Ni–Ce/SAPO-34(IM-DMF) samples was 9.9%, 8.8%, 7.3%, and 6.4%, respectively. In addition, the rates of coke formation in the SAPO-34, Ni–Ce/SAPO-34(IM-water), Ni–Ce/SAPO-34(IN), and Ni–Ce/SAPO-34(IM-DMF) samples were approximately 0.027, 0.024, 0.02, and 0.017 weight loss (%)/min, respectively. Thus, the TGA analysis verified the catalytic performance of the samples. The use of Ni–Ce/SAPO-34(IM-DMF) resulted in the lowest rate of coke formation, and therefore, this catalyst had a longer lifetime than the other samples. The formation of amorphous coke on the external surface and within the pores of the catalyst resulted in blockage of the external surface and pore entrances and eventually in deactivation of the catalyst. The role of Brønsted acid sites on the external surface of a catalyst is negligible for the MTO reaction, whereas the internal acid sites are active sites for this reaction (Kim & Ryoo, 2014). However, external acid sites can catalyze side reactions that are not selective for the production of olefins, simply leading to coke formation; this coke results in a blocked external surface of the catalyst (Kang & Inui, 1998). Therefore, it appears that the catalyst deactivation results from the formation of coke inside the pores and on the external surface of the catalyst. Stronger Brønsted acid sites promote more coke formation reactions. Thus, the neutralization of Brønsted acid sites can reduce the rate of coke formation. In the Ni–Ce/SAPO34(IM-water) sample, the use of water as an impregnating solvent did not result in sufficient neutralization of Brønsted acid sites and therefore did not lead to a significant improvement in the lifetime of the catalyst. The metal species with lower aggregation in the Ni–Ce/SAPO-34(IM-DMF) sample than in the Ni–Ce/SAPO-34(IMwater) sample reduced the density of the Brønsted acid catalyst, thus considerably increasing the lifetime of the catalyst.

9

Acknowledgement This study was financially supported by the Iranian Nanotechnology Initiative Council. Appendix A.

Fig. A1. NH3 -TPD profiles of catalyst samples.

Conclusions In this study, mesoporous SAPO-34 catalysts were synthesized using an ultrasonic and microwave-assisted hydrothermal method in the presence of TPOAC and CTAB surfactants as mesoporogen agents. Cerium and nickel were then doped on the SAPO-34 using impregnation or incorporation method. Water or DMF were used as the impregnating solvent, and all the catalysts were examined using the MTO process. The aim of metal doping was to control the acidic properties of the catalysts, leading to a reduction in the rate of coke formation and thereby improving the catalytic performance. The introduction of metal species into the SAPO-34 framework resulted in an increase in the selectivity toward light olefins and an increase in the lifetime of all the catalysts. The Ni–Ce/SAPO34(IN) and Ni–Ce/SAPO-34(IM-DMF) samples had significantly longer lifetimes than the parent SAPO-34 catalyst; however, the Ni–Ce/SAPO-34(IM-water) sample did not have a remarkable lifetime improvement. This finding indicated that the catalyst preparation method is an effective parameter in controlling the catalytic performance of Ni–Ce/SAPO-34 catalysts. Moreover, the use of an aprotic solvent DMF as the impregnation solvent resulted in lower aggregation of metal species than the use of a protic solvent (water) possibly because the molecules of aprotic solvents, unlike protic solvents, cannot form hydrogen bonds with the negatively charged surface of the zeolite. The Ni–Ce/SAPO-34(IM-DMF) sample exhibited the best catalytic performance as well as increased olefin selectivity and lifetime and reduced methane yield compared with those of the other catalysts.

Fig. A2. TGA curves of the used catalysts.

References Aghaei, E., Haghighi, M., Pazhohniya, Z., & Aghamohammadi, S. (2016). One-pot hydrothermal synthesis of nanostructured ZrAPSO-34 powder: Effect of Zrloading on physicochemical properties and catalytic performance in conversion of methanol to ethylene and propylene. Microporous and Mesoporous Materials, 226, 331–343. Aguayo, A. T., Gayubo, A. G., Vivanco, R., Olazar, M., & Bilbao, J. (2005). Role of acidity and microporous structure in alternative catalysts for the transformation of methanol into olefins. Applied Catalysis A: General, 283, 197–207. ˜ Álvaro-Munoz, T., Márquez-Álvarez, C., & Sastre, E. (2012). Use of different templates on SAPO-34 synthesis: Effect on the acidity and catalytic activity in the MTO reaction. Catalysis Today, 179, 27–34. Anderson, J., Mole, T., & Christov, V. (1980). Mechanism of some conversions over ZSM-5 catalyst. Journal of Catalysis, 61, 477–484. Ashtekar, S., Chilukuri, S. V., & Chakrabarty, D. K. (1994). Small-pore molecular sieves SAPO-34 and SAPO-44 with chabazite structure: A study of silicon incorporation. The Journal of Physical Chemistry, 98, 4878–4883.




Askari, S., Halladj, R., & Sohrabi, M. (2012). Methanol conversion to light olefins over sonochemically prepared SAPO-34 nanocatalyst. Microporous and Mesoporous Materials, 163, 334–342. Bang, Y., Park, S., Han, S. J., Yoo, J., Song, J. H., Choi, J. H., et al. (2016). Hydrogen production by steam reforming of liquefied natural gas (LNG) over mesoporous Ni/Al2 O3 catalyst prepared by an EDTA-assisted impregnation method. Applied Catalysis B: Environmental, 180, 179–188. Chen, D., Moljord, K., Fuglerud, T., & Holmen, A. (1999). The effect of crystal size of SAPO-34 on the selectivity and deactivation of the MTO reaction. Microporous and Mesoporous Materials, 29, 191–203. Chen, J. Q., Bozzano, A., Glover, B., Fuglerud, T., & Kvisle, S. (2005). Recent advancements in ethylene and propylene production using the UOP/Hydro MTO process. Catalysis Today, 106, 103–107. Choi, M., Srivastava, R., & Ryoo, R. (2006). Organosilane surfactant-directed synthesis of mesoporous aluminophosphates constructed with crystalline microporous frameworks. Chemical Communications, 42, 4380–4382. Djieugoue, M.-A., Prakash, A., & Kevan, L. (2000). Catalytic study of methanol-toolefins conversion in four small-pore silicoaluminophosphate molecular sieves: Influence of the structural type, nickel incorporation, nickel location, and nickel concentration. The Journal of Physical Chemistry B, 104, 6452–6461. Du, X., & Wu, E. (2007). Porosity of microporous zeolites A, X and ZSM-5 studied by small angle X-ray scattering and nitrogen adsorption. Journal of Physics and Chemistry of Solids, 68, 1692–1699. Gao, B., Yang, M., Qiao, Y., Li, J., Xiang, X., Wu, P., et al. (2016). A low-temperature approach to synthesize low-silica SAPO-34 nanocrystals and their application in the methanol-to-olefins (MTO) reaction. Catalysis Science & Technology, 6, 7569–7578. Hirota, Y., Murata, K., Tanaka, S., Nishiyama, N., Egashira, Y., & Ueyama, K. (2010). Dry gel conversion synthesis of SAPO-34 nanocrystals. Materials Chemistry and Physics, 123, 507–509. Jacobsen, C. J., Madsen, C., Houzvicka, J., Schmidt, I., & Carlsson, A. (2000). Mesoporous zeolite single crystals. Journal of the American Chemical Society, 122, 7116–7117. Jhung, S. H., Lee, J. H., & Chang, J.-S. (2008). Crystal size control of transition metal ion-incorporated aluminophosphate molecular sieves: Effect of ramping rate in the syntheses. Microporous and Mesoporous Materials, 112, 178–186. Jiang, Z., Shen, B.-X., Zhao, J.-G., Wang, L., Kong, L.-T., & Xiao, W.-G. (2015). Enhancement of catalytic performances for the conversion of chloromethane to light olefins over SAPO-34 by modification with metal chloride. Industrial & Engineering Chemistry Research, 54, 12293–12302. Kang, M., & Inui, T. (1998). Effects of decrease in number of acid sites located on the external surface of Ni-SAPO-34 crystalline catalyst by the mechanochemical method. Catalysis Letters, 53, 171–176. Kim, W., & Ryoo, R. (2014). Probing the catalytic function of external acid sites located on the MFI nanosheet for conversion of methanol to hydrocarbons. Catalysis Letters, 144, 1164–1169. Lee, Y.-J., Baek, S.-C., & Jun, K.-W. (2007). Methanol conversion on SAPO-34 catalysts prepared by mixed template method. Applied Catalysis A: General, 329, 130–136. Liang, J., Li, H., Zhao, S., Guo, W., Wang, R., & Ying, M. (1990). Characteristics and performance of SAPO-34 catalyst for methanol-to-olefin conversion. Applied Catalysis, 64, 31–40. Obrzut, D. L., Adekkanattu, P. M., Thundimadathil, J., Liu, J., Dubois, D. R., & Guin, J. A. (2003). Reducing methane formation in methanol to olefins reaction on metal impregnated SAPO-34 molecular sieve. Reaction Kinetics and Catalysis Letters, 80, 113–121. Pajaie, H. S., & Taghizadeh, M. (2015). Optimization of nano-sized SAPO-34 synthesis in methanol-to-olefin reaction by response surface methodology. Journal of Industrial and Engineering Chemistry, 24, 59–70. Qi, G., Xie, Z., Yang, W., Zhong, S., Liu, H., Zhang, C., et al. (2007). Behaviors of coke deposition on SAPO-34 catalyst during methanol conversion to light olefins. Fuel Processing Technology, 88, 437–441. Rahmani, F., & Haghighi, M. (2015). Sono-dispersion of Cr over nanostructured LaAPSO-34 used in CO2 assisted dehydrogenation of ethane: Effects of Si/Al ratio and La incorporation. Journal of Natural Gas Science and Engineering, 27, 1684–1701.

Rezaei, Y., Halladj, R., Askari, S., Tarjomannejad, A., & Rezaei, T. (2016). Hightemperature synthesis of SAPO-34 molecular sieve using a dry gel method. Particuology, 27, 61–65. Rostamizadeh, M., & Yaripour, F. (2016). Bifunctional and bimetallic Fe/ZSM-5 nanocatalysts for methanol to olefin reaction. Fuel, 181, 537–546. Sadeghpour, P., & Haghighi, M. (2015). DEA/TEAOH templated synthesis and characterization of nanostructured NiAPSO-34 particles: Effect of single and mixed templates on catalyst properties and performance in the methanol to olefin reaction. Particuology, 19, 69–81. Salmasi, M., Fatemi, S., & Najafabadi, A. T. (2011). Improvement of light olefins selectivity and catalyst lifetime in MTO reaction; using Ni and Mg-modified SAPO-34 synthesized by combination of two templates. Journal of Industrial and Engineering Chemistry, 17, 755–761. Sánchez del Campo, A. E., Gayubo, A. G., Aguayo, A. T., Tarrío, A., & Bilbao, J. (1998). Acidity, surface species, and mechanism of methanol transformation into olefins on a SAPO-34. Industrial & Engineering Chemistry Research, 37, 2336–2340. Sedighi, M., Ghasemi, M., Sadeqzadeh, M., & Hadi, M. (2016). Thorough study of the effect of metal-incorporated SAPO-34 molecular sieves on catalytic performances in MTO process. Powder Technology, 291, 131–139. Shannon, R. T. (1976). Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallographica Section A, 32, 751–767. Sun, C., Yang, Y., Du, J., Qin, F., Liu, Z., Shen, W., et al. (2012). Dehydrogenation inhibition on nano-Au/ZSM-5 catalyst: A novel route for anti-coking in methanol to propylene reaction. Chemical Communications, 48, 5787–5789. Tao, M., Meng, X., Lv, Y., Bian, Z., & Xin, Z. (2016). Effect of impregnation solvent on Ni dispersion and catalytic properties of Ni/SBA-15 for CO methanation reaction. Fuel, 165, 289–297. Teketel, S., Olsbye, U., Lillerud, K.-P., Beato, P., & Svelle, S. (2010). Selectivity control through fundamental mechanistic insight in the conversion of methanol to hydrocarbons over zeolites. Microporous and Mesoporous Materials, 136, 33–41. Utchariyajit, K., & Wongkasemjit, S. (2010). Effect of synthesis parameters on mesoporous SAPO-5 with AFI-type formation via microwave radiation using alumatrane and silatrane precursors. Microporous and Mesoporous Materials, 135, 116–123. Varzaneh, A. Z., Towfighi, J., & Mohamadalizadeh, A. (2014). Comparative study of naphtha cracking over SAPO-34 and HZSM-5: Effects of cerium and zirconium on the catalytic performance. Journal of Analytical and Applied Pyrolysis, 107, 165–173. White, J. L. (2011). Methanol-to-hydrocarbon chemistry: The carbon pool (r) evolution. Catalysis Science & Technology, 1, 1630–1635. Wu, W., Guo, W., Xiao, W., & Luo, M. (2013). Methanol conversion to olefins (MTO) over H-ZSM-5: Evidence of product distribution governed by methanol conversion. Fuel Processing Technology, 108, 19–24. Xi, D., Sun, Q., Xu, J., Cho, M., Cho, H. S., Asahina, S., et al. (2014). In situ growthetching approach to the preparation of hierarchically macroporous zeolites with high MTO catalytic activity and selectivity. Journal of Materials Chemistry A, 2, 17994–18004. Xu, A., Ma, H., Zhang, H., Weiyong, D., & Fang, D. (2013). Effect of boron on ZSM-5 catalyst for methanol to propylene conversion. Polish Journal of Chemical Technology, 15, 95–101. Yang, S.-T., Kim, J.-Y., Chae, H.-J., Kim, M., Jeong, S.-Y., & Ahn, W.-S. (2012). Microwave synthesis of mesoporous SAPO-34 with a hierarchical pore structure. Materials Research Bulletin, 47, 3888–3892. Yang, Y., Sun, C., Du, J., Yue, Y., Hua, W., Zhang, C., et al. (2012). The synthesis of endurable B–Al–ZSM-5 catalysts with tunable acidity for methanol to propylene reaction. Catalysis Communications, 24, 44–47. Zhang, D., Wei, Y., Xu, L., Chang, F., Liu, Z., Meng, S., et al. (2008). MgAPSO-34 molecular sieves with various Mg stoichiometries: Synthesis, characterization and catalytic behavior in the direct transformation of chloromethane into light olefins. Microporous and Mesoporous Materials, 116, 684–692. Zhang, L., Bates, J., Chen, D., Nie, H.-Y., & Huang, Y. (2011). Investigations of formation of molecular sieve SAPO-34. The Journal of Physical Chemistry C, 115, 22309–22319. Zhu, Z., Lu, G., Zhang, Z., Guo, Y., Guo, Y., & Wang, Y. (2013). Highly active and stable Co3 O4 /ZSM-5 catalyst for propane oxidation: Effect of the preparation method. ACS Catalysis, 3, 1154–1164.
