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Synthesis of mesoporous SAPO-34 catalysts in the presence of MWCNT, CNF, and GO as hard templates in MTO process
Accepted Manuscript Synthesis of mesoporous SAPO-34 catalysts in the presence of MWCNT, CNF, and GO as hard templates in MTO process
Saeed Soltanali, Jafar Towfighi Darian PII: DOI: Reference:
S0032-5910(19)30500-5 https://doi.org/10.1016/j.powtec.2019.07.008 PTEC 14461
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
21 February 2019 28 June 2019 1 July 2019
Please cite this article as: S. Soltanali and J.T. Darian, Synthesis of mesoporous SAPO-34 catalysts in the presence of MWCNT, CNF, and GO as hard templates in MTO process, Powder Technology, https://doi.org/10.1016/j.powtec.2019.07.008
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ACCEPTED MANUSCRIPT Synthesis of mesoporous SAPO-34 catalysts in the presence of MWCNT, CNF, and GO as hard templates in MTO process Saeed Soltanalia,b , Jafar Towfighi Darianb,* [email protected] a
Catalysis Research Division, Research Institute of Petroleum Industry (RIPI), Tehran, Iran. Department of Chemical Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran.
b
Corresponding author.
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*
order
to
study
the
effect
of
carbon
nanostructures
as
hard
templates,
three
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In
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ABSTRACT
silicoaluminophosphate samples using carbon nanotubes (SAPO-34/CNT), carbon nanofiber
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(SAPO-34/CNF) and graphene oxide (SAPO-34/GO) as well as one sample in the absence of
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nanocarbon structures were successfully synthesized. The effect of each of the hard templates on the physicochemical properties of the synthetic samples was studied using XRD, FESEM, BET,
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FTIR and NH3 -TPD analyses. Furthermore, the catalytic activity of the synthetic catalysts was
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compared in the methanol to olefin conversion (MTO) at atmospheric pressure, temperature of 400o C and WHSV of 4.0 h-1 . The results of the investigation of physical characteristics of the
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catalysts indicate that varying the structure type changes such parameters as crystallinity and
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mesoporosity. This difference also results in different catalytic behavior in MTO reaction. Of the three carbon nanostructures used, graphene oxide had the greatest effect on the mesoporosity and
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pore size of SAPO-34 catalyst.
Keywords: SAPO-34; Carbon nanotube; Graphene; MTO; Hard templates 1. Introduction
*
Corresponding author. E-mail address: [email protected] (J. Towfighi Darian)
ACCEPTED MANUSCRIPT Light olefins (ethylene and propylene) play a major role in the petrochemical industry. Thus, the preparation methods of each of these olefins are also of particular importance. Cracking of naphtha and light alkanes are conventional methods for the production of light olefins. Methanol to light olefin conversion (MTO) is also an attractive method in both research and industrial
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areas [1-6].
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Appropriate catalysts with proper activity, high selectivity and long lifetime play a major part in
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MTO process. Much research has been carried out on the production of light olefins using zeolites with medium and small pores. Most attention has been focused on ZSM-5 and SAPO-34
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catalysts due to their proper catalytic performance in MTO process [7-10].
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The reason for the superiority of SAPO-34 over ZSM-5 in the production of ethylene is the cage structure, pore size and moderate acidity of the former. However, the fast deactivation due to the
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limitation of mass transfer in the small pore sizes is one of the disadvantages of SAPO-34
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catalyst. Numerous solutions have been developed to overcome this problem including the adjustment of acidity, appropriate reduction of catalyst particle sizes and modifications using
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cationic additives such as nickel. Creation of mesoporosity is one of the approaches for
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decreasing the length of penetration and increasing catalyst lifetime. Research results show that the hierarchical SAPO-34 catalyst has a very suitable performance in methanol to olefin
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conversion reaction compared with other common catalysts [10-12]. Dealumination, desilication and application of binary templates, surfactants and hard templates are some of the methods to create and increase the porosity of zeolites. Each of these methods has drawbacks limiting its application. For example, dealumination and desilication are performed by acids and bases, respectively, both of which can decompose SAPO-34 structure.
ACCEPTED MANUSCRIPT Furthermore, the application of binary templates increases the synthesis costs in addition to the undesirable environmental impacts [13-18]. Considering the availability of some hard templates, such as graphite, carbon black, and carbon nanotubes, the application of these compounds to create porosity in zeolites has attracted many
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researchers.
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Hierarchical MCM‐22 zeolite has been prepared by Yang et al. [19] via one pot hydrothermal
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synthesis with the help of carbon particles. Zeolite crystal size and porosity can be tuned using
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carbon black particles of different average particle diameters, as reported by Kustova et al. [20]. Although mesoporosity was reportedly introduced within zeolite crystals using carbon black
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particles, in most cases, the mesopores formed were cavity and not interconnected pores, seriously limiting the availability and mass transfer [21].
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Soltanali et al. [22, 23] have investigated the effect of carbon nanostructures on beta zeolite and
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ZSM-5. They studied the effect of these hard templates on the physicochemical properties of both types of zeolite and found that these templates affected beta zeolites more compared with
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ZSM-5 zeolite.
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Kaskel et al. successfully synthesized hierarchical SAPO-34 using two nano compounds (nano particle and nanotube) as hard templates. They observed that the creation of cave like mesopres
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using nanoparticles causes the formation of mesopores, which are not connected to the external surface of the catalyst and are merely formed inside the crystals. Therefore, no remarkable improvement in the catalytic performance was observed using this method. However, using nanotubes, they managed to obtain remarkable results via the establishment of a relationship between the catalyst surface and mesopores [24-26]. Jhung et al. synthesized hierarchical SAPO34 under microwave irradiation using carbon black as the hard template. They observed
ACCEPTED MANUSCRIPT increased stability and lifetime during dehydration of ethanol and butanol by creating secondary mesopores in the catalyst. Other researchers have also created mesopores in SAPO-34 using CNTs and achieved improvement of the catalytic activity of SAPO-34 in MTO reaction [27-29]. In this work, to improve the catalytic performance of SAPO-34 in MTO reaction, the effect of
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the application of different carbon nanostructures (multi-wall carbon nanotube, MWCNT, carbon
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nanofiber, CNF, and graphene oxide, GO) on the catalytic performance of SAPO-34 in MTO
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reaction has been investigated. The effect of each of the hard templates used on the physicochemical properties of the synthetic samples has been studied using XRD, FESEM, BET,
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FTIR and NH3 -TPD analyses.
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2. Experimental section
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2.1. Catalyst preparation
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All SAPO-34 samples were synthesized hydrothermally from colloidal synthetic mixtures. Aluminum isopropoxide (AIP, Merck Chemical Co.), colloidal silica (40 wt. % SiO 2 , Aldrich
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Chemical Co.), tetraethylammonium hydroxide (35 wt. % aqueous solution of TEAOH, Aldrich Chemical Co.), morpholine (Merck Chemical Co.), multi-wall carbon nanotube (MWCNT)
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(RIPI, Iran), carbon nanofiber (CNF) (RIPI, Iran), graphene oxide (GO) (US Research
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Nanomaterials, Inc.), orthophosphoric acid (85 wt. % H3 PO 4 , Merck Chemical Co.) and deionized water were used as starting materials. 2.1.1. Synthesis of SAPO-34 catalysts with different hard templates A sol-gel medium with the chemical composition of Al2 O 3 : 0.6 SiO2 : 0.5 TEAOH: 1.5 Mor: P2 O 5 : 60 H2 O was used to synthesize SAPO-34 samples (SAPO-34, SAPO-34/CNT, SAPO34/CNF, and SAPO-34/GO) according to the following procedure. A solution containing colloidal silica, aluminum isopropoxide, and specified amounts of the templates (TEAOH,
ACCEPTED MANUSCRIPT morpholine) was prepared in distilled water (solution A). Orthophosphoric acid and specified amounts (with a C/AlPr mass ratio of 0.5) of hard templates (MWCNT, CNF, and GO) were used to prepare another solution (solution B). Table 1 presents the specification of the carbon nanostructures used in this study. Both solutions were homogenized by stirring for 1 h. Solution
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(B) was subsequently added dropwise to solution (A) under vigorous stirring. The obtained gel
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was aged at room temperature for 24 h with agitation. The prepared gel was statically
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crystallized at 483 K for 48 h. The solid obtained was filtered, followed by washing with deionized water until the pH of the washing water was in the 7–8 range. Next, the solid was dried
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at 383 K for 12 h and calcined in air at 873 K for 5 h.
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Table 1 Specification of carbon nanostructures used in this study. 2.2 Characterization
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Crystal structures were determined by X-ray Diffraction (XRD) analysis using a model D5000
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Siemens instrument (scan speed 0.04 s-1 , 2 range between 5 to 60° degree with Cu-K-alpha
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radiation and 0.154056 wavelength of 30 kV and 40 mA, respectively). Product characterization was performed by comparison of XRD images with reference XRD diagrams. Morphology and
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particle sizes were determined using FE-SEM images obtained on a Field Emission electronic
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microscope (FE-SEM), TESCAN, MIRA3 Series. The FTIR spectra were recorded on KBr diluted palletized catalysts using a Bruker Tensor-27 spectrophotometer. BET method was used to determine the specific surface areas. Nitrogen adsorption/desorption isotherms were applied in the determination of the Brunauer-Emmett-Teller surface area at 77 K using a Micromeritics ASAP 2010 analyzer. In order to identify the total acidic strength and weak and strong acidic sites, temperature programmed desorption (TPD) method was applied using ammonia gas. An American Micrometrics TPD/TPR 2900 was used for this purpose.
ACCEPTED MANUSCRIPT 2.3. Catalytic performance The catalytic conversion of methanol over SAPO-34 catalysts was conducted using a fixed bed reactor at a reaction temperature of 400°C and atmospheric pressure. In each run, the catalysts were placed in a quartz tube reactor (with inner diameter of 8 mm) and the SAPO-34 catalyst
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powder was diluted with inert quartz sand and loaded to the reactor. Silicon carbide beads and
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quartz glass were placed on the top and bottom of the catalysts. A thermocouple was positioned
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in the center of the catalyst bed in order to monitor the temperature. The maximum temperature variation along the bed was ±2°C. Before each run, the catalyst was activated in flowing N 2 at
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500°C and maintained at that temperature for 4 h. A methanol/ water liquid mixture with a 30 wt.
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% concentration was injected into the reactor using a syringe pump at a weight hourly space velocity (WHSV) of 4 h−1 . The products were analyzed by a three channel gas chromatograph. A
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Varian CP 3800 equipped with three detectors, including
two thermal conductivity detectors
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(TCD) was used for analyzing H2 ,CO, CO 2 , CH4 and other non-condensable gases and a flame ionization detector (FID) was used to analyze organic liquid products.
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3.1. Characterization
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3. Results and discussion
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Fig. 1 shows the X-ray diffraction patterns of SAPO-34 samples, which have been prepared by various carbon nanostructures as hard templates. All the catalysts clearly show crystallized materials, which are associated with the CHA structure prior to and following the calcination of templates (soft and hard) [27-29]. Synthetic SAPO-34 zeotypes exhibit XRD patterns in 2 radiation angle range, 5-60° angle change at a rate of 1.5 min-1 . The index peaks for SAPO-34 in 2 are 9.6°, 13.0° and 20.6°, observed in all the samples (JCPDS data with reference code 00-
ACCEPTED MANUSCRIPT 037-0359) [30] In addition, the synthetic samples are crystalline, as indicated by the sharpness of the peaks. XRD results show that the presence of carbon nanostructure decreases the intensity of the characteristic peaks. This reduction in intensity is more pronounced in the case of CNT and CNF
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hard templates compared with GO and indicates defects in the crystalline structure of SAPO-34
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particles due to the formation of disordered mesopores.
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Fig. 1 XRD patterns of synthesized SAPO-34 particles before and after calcination: (a) SAPO-
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34, (b) SAPO-34/CNT, (c) SAPO-34/CNF and (d) SAPO-34/GO
The FE-SEM images of the synthesized particles of SAPO-34 zeotypes are illustrated in Fig. 2.
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The size, particle morphology and agglomeration of the synthetic SAPO-34 samples are shown by the FE-SEM images.
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The images show that SAOP-34 particle sizes are more non-uniform when CNT, CNF and GO
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hard templates are used. However, morphologically speaking, the particles are all cubic and the
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application of carbon structures does not seem to have caused much variation in SAPO -34 particle morphology. However, it must be noted that the type of morphology may also be related
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to the synthesis method. Among the hard templates, the application of GO has caused the surface
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of SAPO-34 particles to be unpolished, suggesting that there are grooves on the surface of catalyst particles (Figs 2g, h). In fact, the particle surfaces seem to be made up of a large number of small sheets stuck together. Since GO morphology is of sheet type, the effect of this morphology on SAPO-34 particles is observed. Nevertheless, the application of CNT, CNF and GO seems to cause the formation of hierarchical SAPO-34 (Figs. 2c, d, e, f, g, h). Fig. 2 FE-SEM images of synthesized SAPO-34 particles before and after calcinations: (a, b) SAPO-34, (c, d) SAPO-34/CNT, (e, f) SAPO-34/CNF and (g, h) SAPO-34/GO.
ACCEPTED MANUSCRIPT The FTIR spectra of the samples in the region of 400–4000 cm-1 are shown in Fig. 3. No peaks associated with the amorphous phase (785, 618 and 520 cm−1 ) are observed in the spectra [31]. Thus, typical SAPO-34 phase peaks, which appear at 480, 635, 730, 1100 and 1225 cm−1 , confirm the formation of the SAPO-34 phase. The bands at 485 cm–1 and those at 700 and 1100
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cm–1 correspond to T–O bending of Si tetrahedrons and T–O–T (T= Si, Al and P) symmetric and
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asymmetric stretching vibration modes, respectively [32,33]. The band at 635 cm–1 is due to the
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structurally sensitive double six membered ring vibrations, which is characteristic of the CHA
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framework [32, 33].
Fig. 3 FTIR spectra of catalysts: SAPO-34, SAPO-34/CNT, SAPO-34/CNF and SAPO-34/GO.
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The profiles corresponding to the acidity and amounts of weak and strong acids, based on the thermal desorption (NH3 -TPD), are shown in Fig. 4. As the TPD profiles indicate, three peaks
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appear in the temperature ranges of 180-300o C, 310-450o C and >480o C, which are due to the
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weak, moderate and strong acidic sites, respectively. The weakly bonded ammonia on the defect
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sites such as POH, SiOH, and AlOH is responsible for the low temperature peak and the peak at high temperature is ascribed to strong ammonia adsorption of on the strong Si(OH)Al acidic
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groups [34-38].
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Observation of the acidities shown in table 1 indicates that the addition of each of the hard templates has significantly decreased the acidity of the samples. One reason for this may be the reduction of Si entering the network and the formation of silicon islands in part of the structure. TPD analysis shows that the concentration of acidic sites in samples prepared with hard templates has decreased. In addition, the concentration of acidic sites in all three SAPO-34/CNF, SAPO-34/CNT and SAPO-34/GO samples has remarkably decreased compared with SAPO-34 sample. What is interesting is that the desorption peak shifts to higher temperatures
ACCEPTED MANUSCRIPT corresponding to strong acidic sites. For example, the desorption peak of SAPO-34 sample is about 500o C whereas the corresponding value for the samples with hard templates is in the range of 500-600o C. According to this observation, although the concentration of the acidic sites of the three samples with hard templates has decreased a lot, the acidic strength of this low
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concentration of acidic sites can be considerable.
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Fig. 4 NH3 -TPD profiles of SAPO-34 catalysts with different carbon nanostructure templates. Among the samples with hard templates, the order of increase in the concentration of weak
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acidic sites is as follows: SAPO-34/CNT>SAPO-34/CNF> SAPO-34/GO. In addition, the order of increase in the concentration of medium and strong acidic sites is as follows: SAPO-34/GO
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>SAPO-34/CNF> SAPO-34/CNT.
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Table 2 NH3 -TPD concentration of SAPO-34 catalysts with different carbon nanostructure
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templates.
The characterization of SAPO-34 catalysts has been performed by N 2 adsorption/desorption.
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The corresponding BET surface areas, external surface areas, micropore volumes, and pore
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diameters are summarized in table 3.
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As observed in table 3, the surface area of the samples with carbon nanostructure templates has considerably decreased (about 34%) while the mesopore volume of these samples has increased by about 27% compared with the SAPO-34 sample. This is the greatest for samples with GO as the hard template. The considerable increase in pore diameter of SAPO-34/CNT, SAPO-34/CNF and SAPO-34/GO compared with the SAPO-34 sample indicates that the application of carbon nanostructure hard templates increases mesoprosity. The sample with GO has the highest pore diameter. As observed in table 1, GO has the highest mesopore volume and pore diameters as
ACCEPTED MANUSCRIPT well. Therefore, pore volumes and diameters of hard templates are very important parameters in the synthesis and engineering of the physical properties of SAPO-34. In fact, in addition to surface area, volume and diameter parameters of SAPO-34 are directly related to the volume and pore of the hard template selected.
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Table 3 Textural properties of the synthesized samples.
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Fig. 5 shows pore sizes of all four catalysts. As observed in the figure, the SAPO-34/GO sample has the most mesopores compared with other samples while sample SAPO-34, with a pore
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diameter of 4.8 nm, has the least number of mesopores.
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Fig. 5 Distribution of catalyst pore sizes.
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Absorption/desorption isotherms of the products can be categorized as type IV isotherms. This
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isotherm is characteristic of mesoporous inorganic compounds. Fig. 6 shows that these isotherms are similar in SAPO-34, SAPO-34/CNT, SAPO-34/CNF, and SAPO-34/GO samples and the
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presence of hysteresis loops at high pressures indicates mesopores. Given the y intercept of the
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four diagrams, the number of micropores for each sample in increasing order is as follows:
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SAPO-34>SAPO-34-CNT>SAPO-34/CNF> SAPO-34/GO Fig. 6 Nitrogen absorption-desorption isotherms for SAPO-34 catalysts with various hard templates.
3.2. Catalytic performance in MTO reaction In order to investigate the performance of the synthetic catalysts, each of the four catalysts were evaluated under identical conditions in methanol to olefin conversion reaction. Fig. 7 shows methanol conversions during different reaction times. Percent conversions are the same for all
ACCEPTED MANUSCRIPT the catalysts in the beginning of the reaction. Usually, the differences in the lifetime and percent conversions are functions of catalyst acidity, mesoporosity and particle size. Given the approximate uniformity of the particle sizes of the synthetic catalysts, mesoporosity and effective acidity factors seem to be responsible for the better catalytic performance of SAPO-34/GO
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catalyst compared with the other three catalysts.
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According to the hydrocarbon pool mechanism by which MTO occurs, polyaromatic compounds accumulate in cages and function as reaction centers in breaking of C-C bonds and formation of
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olefin products. C₆H(CH₃)₅ and C₆(CH₃)₆ are the most significant and reactive reaction centers
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of polymethylbenzene intermediates, which are formed in SAPO-34 cavities [39-49]. Upon reacting with DME or methanol, benzenoids in the hydrocarbon pool form higher
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homologues such as xylenes and ethylbenzenes, which can further take part in such reactions as The catalytic cycle is then
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elimination to form light olefins, leaving the catalyst pores.
completed by re-alkylation of the original compounds. Light olefins are coke generators due to
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their high tendency for oligomerization. When they are trapped in micropores, they form coke
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deposits rather than being discharged into the gas phase. When aromatics and heavy branched compounds are formed within large SAPO-34 cavities, they cannot exit through the pores and
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thus remain inside to form carbon deposits, clogging pores and blocking the path of the molecules into active sites. Therefore, catalysts with high mesoporosity have a small extent of deactivation because precursors forming coke exit through mesopores associated with the external surface when the length of penetration decreases. Thus, as observed in Fig. 7, SAPO-34 catalysts have a faster deactivation rate given their high acidity because they have fewer pores and smaller pore sizes compared with the other catalysts synthesized. High mesoporosity and consequently increased mass transfer and pore size cause SAPO-34 catalyst to show more
ACCEPTED MANUSCRIPT appropriate catalytic activity in comparison with the other catalysts. In addition, coke formation can be postponed and the catalyst stability may be improved by the reduced acidic concentration of carbon templated SAPO-34 catalysts. Given the stronger basicity of light olefins in comparison with other hydrocarbons, the former turn into major proton acceptors by increasing
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strong acidic sites. Therefore, the conversion of olefins to alkanes and aromatics via secondary
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reactions can be facilitated by the increased concentration of strong acidic sites [50]. Fig. 7 Methanol conversion of light olefins versus time on stream at T = 400ºC and WHSV = 4.0
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h-1 over samples with various hard templates
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Fig. 8 shows the selectivity of each of the synthetic catalysts for light olefins and other hydrocarbons. The distribution of favorable products (ethylene and propylene) is almost similar
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for all the catalysts. This similarity indicates the identical reaction centers in all the catalysts. The
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absence of aromatic molecules shows that the reaction centers are inside the cavities and the presence of mesopores has not caused any changes in the nature of the products.
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In all the samples, the favorable olefin products are on the rise in relation to the time on stream.
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This trend is clearer for SAPO-34 and SAPO-34/CNT catalysts. After some time through the reaction, this increase will stop and olefin selectivity will start decreasing due to coke formation.
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As observed in Fig. 8, towards the end of the reaction, ethylene to propylene ratio increases because as the reaction time proceeds, pore sizes become too small for heavy hydrocarbons to pass through due to the formation of oligomers inside the pores and blocking of the pathways for reactants and consequent catalyst deactivation. Therefore, mostly light compounds leave the pores. This ratio is more obvious for SAPO-34/CNF catalyst. The better performance of carbon templated SAPO-34 catalysts in the production of butane can be related to the mild acidity of these catalysts compared with SAPO-34 catalyst, which reduces
ACCEPTED MANUSCRIPT hydrogen transfer to the hydrocarbon and aromatic compounds. Moreover, since the penetration length for carbon templated SAPO-34 catalysts is shorter than that of SAPO-34 catalyst, butane penetration out of the catalyst takes place faster in the latter catalyst and thus the selectivity of carbon templated SAPO-34 catalysts to butane will be higher. On the other hand, among
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catalysts with carbon hard templates, SAPO-34/CNT has the highest selectivity because of its
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milder acidity.
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As the production of light olefins decreases, the production of dimethyl ether (DME) increases. This increase is the highest for SAPO-34 catalyst. Since methanol to DME conversion takes
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place on weak sites, this trend does not seem unexpected considering the high density of the
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weak sites of SAPO-34 catalyst. After the catalyst deactivation by coke, the weak acidic sites are still active and thus DME production increases as time goes by. Among the samples with hard
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templates, DME production growth is the highest by SAPO-34/CNT.
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Fig. 8 Mass fraction of light olefins versus time on stream at T = 400ºC and WHSV = 4.0 h-1 over
samples with various hard templates: (a) SAPO-34; (b) SAPO-34/CNT; (c) SAPO-34/CNF; and
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4. Conclusions
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(d) SAPO-34/GO
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To find out the effects of types of carbon nanostructures used as hard templates, four catalysts were successfully prepared, three of which containing CNT, CNF and GO carbon nanostructures. The effect of application of carbon hard templates on increasing mesoporosity and decreasing the number of acidic sites is completely observed. Despite the approximately 34% reduction in the
surface area of the catalysts with hard templates, mesopores and pore sizes of these catalysts showed a high increase compared with common SAPO-34 catalyst. In addition, the density of these samples became much milder compared with the common catalyst. In methanol to olefin
ACCEPTED MANUSCRIPT conversion (MTO), increasing mesoporosity, pore sizes and mild acidity caused the lifetime of carbon templated SAPO-34 catalysts to increase, led to remarkable selectivity to olefin products and reduced the production of undesirable DME product compared with common SAPO-34 catalyst. Among the catalysts with carbon nanostructures as hard templates, the catalyst
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templated with graphene oxide (GO) showed a very appropriate performance in terms of
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respectively, of the catalyst have given it remarkable properties.
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catalytic activity and selectivity. The surface area and mespore volume of 920 m2 /g and 4.8 cc/g,
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ACCEPTED MANUSCRIPT [28] A. Zeinali Varzaneh, J. Towfighi, S. Sahebdelfar, H. Bahrami, Carbon nanotube templated synthesis of metal containing hierarchical SAPO-34 catalysts: Impact of the preparation method and metal avidities in the MTO reaction, Microporous Mesoporous Mater. 236 (2016) 1-12. [29] S. Rimaz, R. Halladj, S. Askari, Synthesis of hierarchal SAPO-34 nano catalyst with dry gel conversion method in the presence of carbon nanotubes as a hard template, J. Colloid Interf. Sci. 464
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ACCEPTED MANUSCRIPT [38] A. Zeinali Varzaneh, J. Towfighi, A. Mohamadalizadeh, Comparative study of naphtha cracking over SAPO-34 and HZSM-5: Effects of cerium and zirconium on the catalytic performance, J. Anal. Appl. Pyrolysis 107(2014) 165-173. [39] I.M. Dahl, S. Kolboe, On the reaction mechanism for hydrocarbon formation from methanol over SAPO-34: 2. Isotopic labeling studies of the co-reaction of propene and methanol, J. Catal. 161(1996)
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[41] J.F. Haw, J.B. Nicholas, W. Song, F. Deng, Z. Wang, T. Xu, C.S. Heneghan, Roles for cyclopentenyl cations in the synthesis of hydrocarbons from methanol on zeolite catalyst HZSM-5, J. Am. Chem. Soc.
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[42] O. Mikkelsen, P.O. Ronning, S. Kolboe, Use of isotopic labeling for mechanistic studies of the
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methanol-to-hydrocarbons reaction. Methylation of toluene with methanol over H-ZSM-5, H-mordenite
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and H-beta, Microporous Mesoporous Mater. 40(2000) 95-113. [43] B. Arstad, S. Kolboe, Methanol-to-hydrocarbons reaction over SAPO-34 molecules confined in the
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[44] B. Arstad, S. Kolboe, The reactivity of molecules trapped within the SAPO-34 cavities in the methanol-to-hydrocarbons reaction, J. Am. Chem. Soc. 123 (2001) 8137-8138.
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[45] A. Sassi, M.A. Wildman, H.J. Ahn, P. Prasad, J.B. Nicholas, J.F. Haw, Methylbenzene chemistry on zeolite HBeta: Multiple insights into methanol-to olefins catalysis, J. Phys. Chem. B. 106 (2002) 22942303.
[46] W. Song, H. Fu, J.F. Haw, Supramolecular origins of product selectivity for methanol-to-olefin catalysis on HSAPO-34, J. Am. Chem. Soc. 123 (2001) 4749-4754. [47] W. Song, H. Fu, J.F. Haw, Selective synthesis of methylnaphthalenes in HSAPO-34 cages and their function as reaction centers in methanol-to-olefin catalysis, J. Phys. Chem. B. 105(2001)12839-12843.
ACCEPTED MANUSCRIPT [48] W. Song, J.B. Nicholas, J.F. Haw, A persistent carbenium ion on the methanolto- olefin catalyst HSAPO-34: Acetone shows the way, J. Phys. Chem. B. 105(2001) 4317-4323. [49] H. Fu, W. Song, D.M. Marcus, J.F. Haw, Ship-in-a-bottle synthesis of methylphenols in HSAPO-34 cages from methanol and air, J. Phys. Chem. B 106(2002) 5648-5652. [50] S. Kotrel, H. Knozinger, B.C. Gates, The Haag–Dessau mechanism of protolytic cracking of alkanes,
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Figure Captions Fig. 1 XRD patterns of synthesized SAPO-34 particles before and after calcination: (a) SAPO-34, (b) SAPO-34/CNT, (c) SAPO-34/CNF and (d) SAPO-34/GO
ACCEPTED MANUSCRIPT Fig. 2 FE-SEM images of synthesized SAPO-34 particles before and after calcinations: (a, b) SAPO-34, (c, d) SAPO-34/CNT, (e, f) SAPO-34/CNF and (g, h) SAPO-34/GO Fig. 3 FTIR spectra of catalysts: SAPO-34, SAPO-34/CNT, SAPO-34/CNF and SAPO-34/GO Fig. 4 NH3 -TPD profiles of SAPO-34 catalysts with different carbon nanostructure templates Fig. 5 Distribution of catalyst pore sizes.
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Fig. 6 Nitrogen absorption-desorption isotherms for SAPO-34 catalysts of various hard templates
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Fig. 7 Methanol conversion of light olefins versus time on stream at T = 400ºC and WHSV = 4.0 h-1 over samples with various hard templates.
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Fig. 8 Mass fraction of light olefins versus time on stream at T = 400ºC and WHSV = 4.0 h-1 over samples with various hard templates: (a) SAPO-34; (b) SAPO-34/CNT; (c) SAPO-34/CNF; and (d)
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S B ET (m2 g -1 )a S ext (m2 g -1 )b Vtotal (cm3 g -1 )c Vmicro (cm3 g -1 )b D (nm)d Hard template MWCNT 245 236 0.38 0.004 6.3 CNF 273 254 0.51 0.01 7.5 GO 920 875 4.6 0.03 19.8 a Surface areas were obtained by the BET method using adsorption data in the p/p 0 range from 0.05 to 0.25. b Measured by the t-plot method. c Total pore volumes were estimated from the adsorbed amount at p/p 0 = 0.995. d Average pore diameters were derived from the adsorption branches of the isotherms by using the BJH method.
Table 2
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Catalyst
Weak (180300ºC)
Moderate (310450ºC)
SAPO-34
2.0(218ºC)
SAPO34/CNT
Strong
Moderate + Strong
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2.10(498ºC)
2.1
1.64(284ºC)
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0.60(640ºC)
0.6
SAPO34/CNF
1.54(221ºC)
0.76(409ºC)
0.33(660ºC)
1.09
SAPO34/GO
1.40(235ºC)
0.90(412ºC)
0.40(643ºC)
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SB ET (m2 g -1 )a
Sext (m2 g -1 )b
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Acidity concentration (mmol NH 3 /g)
Vtotal (cm3 g -1 )c
Vmicro (cm3 g -1 )b
D (nm)d
528 365 334 348
135 176 167 166
0.64 0.68 0.64 0.69
0.18 0.09 0.08 0.09
4.8 7.4 7.7 8.0
SAPO-34 SAPO-34/CNT SAPO-34/CNF SAPO-34/GO
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Catalyst
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Table 3
a
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Surface areas were obtained by the BET method using adsorption data in the p/p 0 range from 0.05 to 0.25. Measured by the t-plot method. c Total pore volumes were estimated from the adsorbed amount at p/p 0 = 0.995. d Average pore diameter were derived from the adsorption branches of the isotherms by using the BJH method.
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Graphical abstract
Highlights
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Hierarchical SAPO‐34s were synthesized using carbon nanostructures as hard template. SAPO‐34s were synthesized using morpholine and TEAOH, simultaneously. Hard templates had the significant effects in the specification of SAPO‐34. Performances of SAPO-34s in conversion of methanol to light olefin were compared.
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Saeed Soltanali, Jafar Towfighi Darian PII: DOI: Reference:
S0032-5910(19)30500-5 https://doi.org/10.1016/j.powtec.2019.07.008 PTEC 14461
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
21 February 2019 28 June 2019 1 July 2019
Please cite this article as: S. Soltanali and J.T. Darian, Synthesis of mesoporous SAPO-34 catalysts in the presence of MWCNT, CNF, and GO as hard templates in MTO process, Powder Technology, https://doi.org/10.1016/j.powtec.2019.07.008
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ACCEPTED MANUSCRIPT Synthesis of mesoporous SAPO-34 catalysts in the presence of MWCNT, CNF, and GO as hard templates in MTO process Saeed Soltanalia,b , Jafar Towfighi Darianb,* [email protected] a
Catalysis Research Division, Research Institute of Petroleum Industry (RIPI), Tehran, Iran. Department of Chemical Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran.
b
Corresponding author.
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*
order
to
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effect
of
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nanostructures
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three
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In
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ABSTRACT
silicoaluminophosphate samples using carbon nanotubes (SAPO-34/CNT), carbon nanofiber
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(SAPO-34/CNF) and graphene oxide (SAPO-34/GO) as well as one sample in the absence of
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nanocarbon structures were successfully synthesized. The effect of each of the hard templates on the physicochemical properties of the synthetic samples was studied using XRD, FESEM, BET,
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FTIR and NH3 -TPD analyses. Furthermore, the catalytic activity of the synthetic catalysts was
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compared in the methanol to olefin conversion (MTO) at atmospheric pressure, temperature of 400o C and WHSV of 4.0 h-1 . The results of the investigation of physical characteristics of the
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catalysts indicate that varying the structure type changes such parameters as crystallinity and
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mesoporosity. This difference also results in different catalytic behavior in MTO reaction. Of the three carbon nanostructures used, graphene oxide had the greatest effect on the mesoporosity and
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pore size of SAPO-34 catalyst.
Keywords: SAPO-34; Carbon nanotube; Graphene; MTO; Hard templates 1. Introduction
*
Corresponding author. E-mail address: [email protected] (J. Towfighi Darian)
ACCEPTED MANUSCRIPT Light olefins (ethylene and propylene) play a major role in the petrochemical industry. Thus, the preparation methods of each of these olefins are also of particular importance. Cracking of naphtha and light alkanes are conventional methods for the production of light olefins. Methanol to light olefin conversion (MTO) is also an attractive method in both research and industrial
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areas [1-6].
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Appropriate catalysts with proper activity, high selectivity and long lifetime play a major part in
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MTO process. Much research has been carried out on the production of light olefins using zeolites with medium and small pores. Most attention has been focused on ZSM-5 and SAPO-34
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catalysts due to their proper catalytic performance in MTO process [7-10].
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The reason for the superiority of SAPO-34 over ZSM-5 in the production of ethylene is the cage structure, pore size and moderate acidity of the former. However, the fast deactivation due to the
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limitation of mass transfer in the small pore sizes is one of the disadvantages of SAPO-34
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catalyst. Numerous solutions have been developed to overcome this problem including the adjustment of acidity, appropriate reduction of catalyst particle sizes and modifications using
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cationic additives such as nickel. Creation of mesoporosity is one of the approaches for
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decreasing the length of penetration and increasing catalyst lifetime. Research results show that the hierarchical SAPO-34 catalyst has a very suitable performance in methanol to olefin
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conversion reaction compared with other common catalysts [10-12]. Dealumination, desilication and application of binary templates, surfactants and hard templates are some of the methods to create and increase the porosity of zeolites. Each of these methods has drawbacks limiting its application. For example, dealumination and desilication are performed by acids and bases, respectively, both of which can decompose SAPO-34 structure.
ACCEPTED MANUSCRIPT Furthermore, the application of binary templates increases the synthesis costs in addition to the undesirable environmental impacts [13-18]. Considering the availability of some hard templates, such as graphite, carbon black, and carbon nanotubes, the application of these compounds to create porosity in zeolites has attracted many
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researchers.
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Hierarchical MCM‐22 zeolite has been prepared by Yang et al. [19] via one pot hydrothermal
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synthesis with the help of carbon particles. Zeolite crystal size and porosity can be tuned using
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carbon black particles of different average particle diameters, as reported by Kustova et al. [20]. Although mesoporosity was reportedly introduced within zeolite crystals using carbon black
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particles, in most cases, the mesopores formed were cavity and not interconnected pores, seriously limiting the availability and mass transfer [21].
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Soltanali et al. [22, 23] have investigated the effect of carbon nanostructures on beta zeolite and
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ZSM-5. They studied the effect of these hard templates on the physicochemical properties of both types of zeolite and found that these templates affected beta zeolites more compared with
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ZSM-5 zeolite.
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Kaskel et al. successfully synthesized hierarchical SAPO-34 using two nano compounds (nano particle and nanotube) as hard templates. They observed that the creation of cave like mesopres
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using nanoparticles causes the formation of mesopores, which are not connected to the external surface of the catalyst and are merely formed inside the crystals. Therefore, no remarkable improvement in the catalytic performance was observed using this method. However, using nanotubes, they managed to obtain remarkable results via the establishment of a relationship between the catalyst surface and mesopores [24-26]. Jhung et al. synthesized hierarchical SAPO34 under microwave irradiation using carbon black as the hard template. They observed
ACCEPTED MANUSCRIPT increased stability and lifetime during dehydration of ethanol and butanol by creating secondary mesopores in the catalyst. Other researchers have also created mesopores in SAPO-34 using CNTs and achieved improvement of the catalytic activity of SAPO-34 in MTO reaction [27-29]. In this work, to improve the catalytic performance of SAPO-34 in MTO reaction, the effect of
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the application of different carbon nanostructures (multi-wall carbon nanotube, MWCNT, carbon
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nanofiber, CNF, and graphene oxide, GO) on the catalytic performance of SAPO-34 in MTO
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reaction has been investigated. The effect of each of the hard templates used on the physicochemical properties of the synthetic samples has been studied using XRD, FESEM, BET,
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FTIR and NH3 -TPD analyses.
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2. Experimental section
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2.1. Catalyst preparation
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All SAPO-34 samples were synthesized hydrothermally from colloidal synthetic mixtures. Aluminum isopropoxide (AIP, Merck Chemical Co.), colloidal silica (40 wt. % SiO 2 , Aldrich
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Chemical Co.), tetraethylammonium hydroxide (35 wt. % aqueous solution of TEAOH, Aldrich Chemical Co.), morpholine (Merck Chemical Co.), multi-wall carbon nanotube (MWCNT)
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(RIPI, Iran), carbon nanofiber (CNF) (RIPI, Iran), graphene oxide (GO) (US Research
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Nanomaterials, Inc.), orthophosphoric acid (85 wt. % H3 PO 4 , Merck Chemical Co.) and deionized water were used as starting materials. 2.1.1. Synthesis of SAPO-34 catalysts with different hard templates A sol-gel medium with the chemical composition of Al2 O 3 : 0.6 SiO2 : 0.5 TEAOH: 1.5 Mor: P2 O 5 : 60 H2 O was used to synthesize SAPO-34 samples (SAPO-34, SAPO-34/CNT, SAPO34/CNF, and SAPO-34/GO) according to the following procedure. A solution containing colloidal silica, aluminum isopropoxide, and specified amounts of the templates (TEAOH,
ACCEPTED MANUSCRIPT morpholine) was prepared in distilled water (solution A). Orthophosphoric acid and specified amounts (with a C/AlPr mass ratio of 0.5) of hard templates (MWCNT, CNF, and GO) were used to prepare another solution (solution B). Table 1 presents the specification of the carbon nanostructures used in this study. Both solutions were homogenized by stirring for 1 h. Solution
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(B) was subsequently added dropwise to solution (A) under vigorous stirring. The obtained gel
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was aged at room temperature for 24 h with agitation. The prepared gel was statically
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crystallized at 483 K for 48 h. The solid obtained was filtered, followed by washing with deionized water until the pH of the washing water was in the 7–8 range. Next, the solid was dried
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at 383 K for 12 h and calcined in air at 873 K for 5 h.
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Table 1 Specification of carbon nanostructures used in this study. 2.2 Characterization
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Crystal structures were determined by X-ray Diffraction (XRD) analysis using a model D5000
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Siemens instrument (scan speed 0.04 s-1 , 2 range between 5 to 60° degree with Cu-K-alpha
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radiation and 0.154056 wavelength of 30 kV and 40 mA, respectively). Product characterization was performed by comparison of XRD images with reference XRD diagrams. Morphology and
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particle sizes were determined using FE-SEM images obtained on a Field Emission electronic
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microscope (FE-SEM), TESCAN, MIRA3 Series. The FTIR spectra were recorded on KBr diluted palletized catalysts using a Bruker Tensor-27 spectrophotometer. BET method was used to determine the specific surface areas. Nitrogen adsorption/desorption isotherms were applied in the determination of the Brunauer-Emmett-Teller surface area at 77 K using a Micromeritics ASAP 2010 analyzer. In order to identify the total acidic strength and weak and strong acidic sites, temperature programmed desorption (TPD) method was applied using ammonia gas. An American Micrometrics TPD/TPR 2900 was used for this purpose.
ACCEPTED MANUSCRIPT 2.3. Catalytic performance The catalytic conversion of methanol over SAPO-34 catalysts was conducted using a fixed bed reactor at a reaction temperature of 400°C and atmospheric pressure. In each run, the catalysts were placed in a quartz tube reactor (with inner diameter of 8 mm) and the SAPO-34 catalyst
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powder was diluted with inert quartz sand and loaded to the reactor. Silicon carbide beads and
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quartz glass were placed on the top and bottom of the catalysts. A thermocouple was positioned
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in the center of the catalyst bed in order to monitor the temperature. The maximum temperature variation along the bed was ±2°C. Before each run, the catalyst was activated in flowing N 2 at
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500°C and maintained at that temperature for 4 h. A methanol/ water liquid mixture with a 30 wt.
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% concentration was injected into the reactor using a syringe pump at a weight hourly space velocity (WHSV) of 4 h−1 . The products were analyzed by a three channel gas chromatograph. A
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Varian CP 3800 equipped with three detectors, including
two thermal conductivity detectors
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(TCD) was used for analyzing H2 ,CO, CO 2 , CH4 and other non-condensable gases and a flame ionization detector (FID) was used to analyze organic liquid products.
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3.1. Characterization
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3. Results and discussion
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Fig. 1 shows the X-ray diffraction patterns of SAPO-34 samples, which have been prepared by various carbon nanostructures as hard templates. All the catalysts clearly show crystallized materials, which are associated with the CHA structure prior to and following the calcination of templates (soft and hard) [27-29]. Synthetic SAPO-34 zeotypes exhibit XRD patterns in 2 radiation angle range, 5-60° angle change at a rate of 1.5 min-1 . The index peaks for SAPO-34 in 2 are 9.6°, 13.0° and 20.6°, observed in all the samples (JCPDS data with reference code 00-
ACCEPTED MANUSCRIPT 037-0359) [30] In addition, the synthetic samples are crystalline, as indicated by the sharpness of the peaks. XRD results show that the presence of carbon nanostructure decreases the intensity of the characteristic peaks. This reduction in intensity is more pronounced in the case of CNT and CNF
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hard templates compared with GO and indicates defects in the crystalline structure of SAPO-34
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particles due to the formation of disordered mesopores.
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Fig. 1 XRD patterns of synthesized SAPO-34 particles before and after calcination: (a) SAPO-
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34, (b) SAPO-34/CNT, (c) SAPO-34/CNF and (d) SAPO-34/GO
The FE-SEM images of the synthesized particles of SAPO-34 zeotypes are illustrated in Fig. 2.
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The size, particle morphology and agglomeration of the synthetic SAPO-34 samples are shown by the FE-SEM images.
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The images show that SAOP-34 particle sizes are more non-uniform when CNT, CNF and GO
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hard templates are used. However, morphologically speaking, the particles are all cubic and the
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application of carbon structures does not seem to have caused much variation in SAPO -34 particle morphology. However, it must be noted that the type of morphology may also be related
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to the synthesis method. Among the hard templates, the application of GO has caused the surface
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of SAPO-34 particles to be unpolished, suggesting that there are grooves on the surface of catalyst particles (Figs 2g, h). In fact, the particle surfaces seem to be made up of a large number of small sheets stuck together. Since GO morphology is of sheet type, the effect of this morphology on SAPO-34 particles is observed. Nevertheless, the application of CNT, CNF and GO seems to cause the formation of hierarchical SAPO-34 (Figs. 2c, d, e, f, g, h). Fig. 2 FE-SEM images of synthesized SAPO-34 particles before and after calcinations: (a, b) SAPO-34, (c, d) SAPO-34/CNT, (e, f) SAPO-34/CNF and (g, h) SAPO-34/GO.
ACCEPTED MANUSCRIPT The FTIR spectra of the samples in the region of 400–4000 cm-1 are shown in Fig. 3. No peaks associated with the amorphous phase (785, 618 and 520 cm−1 ) are observed in the spectra [31]. Thus, typical SAPO-34 phase peaks, which appear at 480, 635, 730, 1100 and 1225 cm−1 , confirm the formation of the SAPO-34 phase. The bands at 485 cm–1 and those at 700 and 1100
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cm–1 correspond to T–O bending of Si tetrahedrons and T–O–T (T= Si, Al and P) symmetric and
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asymmetric stretching vibration modes, respectively [32,33]. The band at 635 cm–1 is due to the
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structurally sensitive double six membered ring vibrations, which is characteristic of the CHA
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framework [32, 33].
Fig. 3 FTIR spectra of catalysts: SAPO-34, SAPO-34/CNT, SAPO-34/CNF and SAPO-34/GO.
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The profiles corresponding to the acidity and amounts of weak and strong acids, based on the thermal desorption (NH3 -TPD), are shown in Fig. 4. As the TPD profiles indicate, three peaks
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appear in the temperature ranges of 180-300o C, 310-450o C and >480o C, which are due to the
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weak, moderate and strong acidic sites, respectively. The weakly bonded ammonia on the defect
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sites such as POH, SiOH, and AlOH is responsible for the low temperature peak and the peak at high temperature is ascribed to strong ammonia adsorption of on the strong Si(OH)Al acidic
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groups [34-38].
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Observation of the acidities shown in table 1 indicates that the addition of each of the hard templates has significantly decreased the acidity of the samples. One reason for this may be the reduction of Si entering the network and the formation of silicon islands in part of the structure. TPD analysis shows that the concentration of acidic sites in samples prepared with hard templates has decreased. In addition, the concentration of acidic sites in all three SAPO-34/CNF, SAPO-34/CNT and SAPO-34/GO samples has remarkably decreased compared with SAPO-34 sample. What is interesting is that the desorption peak shifts to higher temperatures
ACCEPTED MANUSCRIPT corresponding to strong acidic sites. For example, the desorption peak of SAPO-34 sample is about 500o C whereas the corresponding value for the samples with hard templates is in the range of 500-600o C. According to this observation, although the concentration of the acidic sites of the three samples with hard templates has decreased a lot, the acidic strength of this low
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concentration of acidic sites can be considerable.
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Fig. 4 NH3 -TPD profiles of SAPO-34 catalysts with different carbon nanostructure templates. Among the samples with hard templates, the order of increase in the concentration of weak
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acidic sites is as follows: SAPO-34/CNT>SAPO-34/CNF> SAPO-34/GO. In addition, the order of increase in the concentration of medium and strong acidic sites is as follows: SAPO-34/GO
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>SAPO-34/CNF> SAPO-34/CNT.
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Table 2 NH3 -TPD concentration of SAPO-34 catalysts with different carbon nanostructure
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templates.
The characterization of SAPO-34 catalysts has been performed by N 2 adsorption/desorption.
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The corresponding BET surface areas, external surface areas, micropore volumes, and pore
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diameters are summarized in table 3.
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As observed in table 3, the surface area of the samples with carbon nanostructure templates has considerably decreased (about 34%) while the mesopore volume of these samples has increased by about 27% compared with the SAPO-34 sample. This is the greatest for samples with GO as the hard template. The considerable increase in pore diameter of SAPO-34/CNT, SAPO-34/CNF and SAPO-34/GO compared with the SAPO-34 sample indicates that the application of carbon nanostructure hard templates increases mesoprosity. The sample with GO has the highest pore diameter. As observed in table 1, GO has the highest mesopore volume and pore diameters as
ACCEPTED MANUSCRIPT well. Therefore, pore volumes and diameters of hard templates are very important parameters in the synthesis and engineering of the physical properties of SAPO-34. In fact, in addition to surface area, volume and diameter parameters of SAPO-34 are directly related to the volume and pore of the hard template selected.
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Table 3 Textural properties of the synthesized samples.
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Fig. 5 shows pore sizes of all four catalysts. As observed in the figure, the SAPO-34/GO sample has the most mesopores compared with other samples while sample SAPO-34, with a pore
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diameter of 4.8 nm, has the least number of mesopores.
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Fig. 5 Distribution of catalyst pore sizes.
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Absorption/desorption isotherms of the products can be categorized as type IV isotherms. This
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isotherm is characteristic of mesoporous inorganic compounds. Fig. 6 shows that these isotherms are similar in SAPO-34, SAPO-34/CNT, SAPO-34/CNF, and SAPO-34/GO samples and the
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presence of hysteresis loops at high pressures indicates mesopores. Given the y intercept of the
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four diagrams, the number of micropores for each sample in increasing order is as follows:
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SAPO-34>SAPO-34-CNT>SAPO-34/CNF> SAPO-34/GO Fig. 6 Nitrogen absorption-desorption isotherms for SAPO-34 catalysts with various hard templates.
3.2. Catalytic performance in MTO reaction In order to investigate the performance of the synthetic catalysts, each of the four catalysts were evaluated under identical conditions in methanol to olefin conversion reaction. Fig. 7 shows methanol conversions during different reaction times. Percent conversions are the same for all
ACCEPTED MANUSCRIPT the catalysts in the beginning of the reaction. Usually, the differences in the lifetime and percent conversions are functions of catalyst acidity, mesoporosity and particle size. Given the approximate uniformity of the particle sizes of the synthetic catalysts, mesoporosity and effective acidity factors seem to be responsible for the better catalytic performance of SAPO-34/GO
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catalyst compared with the other three catalysts.
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According to the hydrocarbon pool mechanism by which MTO occurs, polyaromatic compounds accumulate in cages and function as reaction centers in breaking of C-C bonds and formation of
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olefin products. C₆H(CH₃)₅ and C₆(CH₃)₆ are the most significant and reactive reaction centers
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of polymethylbenzene intermediates, which are formed in SAPO-34 cavities [39-49]. Upon reacting with DME or methanol, benzenoids in the hydrocarbon pool form higher
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homologues such as xylenes and ethylbenzenes, which can further take part in such reactions as The catalytic cycle is then
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elimination to form light olefins, leaving the catalyst pores.
completed by re-alkylation of the original compounds. Light olefins are coke generators due to
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their high tendency for oligomerization. When they are trapped in micropores, they form coke
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deposits rather than being discharged into the gas phase. When aromatics and heavy branched compounds are formed within large SAPO-34 cavities, they cannot exit through the pores and
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thus remain inside to form carbon deposits, clogging pores and blocking the path of the molecules into active sites. Therefore, catalysts with high mesoporosity have a small extent of deactivation because precursors forming coke exit through mesopores associated with the external surface when the length of penetration decreases. Thus, as observed in Fig. 7, SAPO-34 catalysts have a faster deactivation rate given their high acidity because they have fewer pores and smaller pore sizes compared with the other catalysts synthesized. High mesoporosity and consequently increased mass transfer and pore size cause SAPO-34 catalyst to show more
ACCEPTED MANUSCRIPT appropriate catalytic activity in comparison with the other catalysts. In addition, coke formation can be postponed and the catalyst stability may be improved by the reduced acidic concentration of carbon templated SAPO-34 catalysts. Given the stronger basicity of light olefins in comparison with other hydrocarbons, the former turn into major proton acceptors by increasing
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strong acidic sites. Therefore, the conversion of olefins to alkanes and aromatics via secondary
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reactions can be facilitated by the increased concentration of strong acidic sites [50]. Fig. 7 Methanol conversion of light olefins versus time on stream at T = 400ºC and WHSV = 4.0
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h-1 over samples with various hard templates
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Fig. 8 shows the selectivity of each of the synthetic catalysts for light olefins and other hydrocarbons. The distribution of favorable products (ethylene and propylene) is almost similar
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for all the catalysts. This similarity indicates the identical reaction centers in all the catalysts. The
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absence of aromatic molecules shows that the reaction centers are inside the cavities and the presence of mesopores has not caused any changes in the nature of the products.
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In all the samples, the favorable olefin products are on the rise in relation to the time on stream.
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This trend is clearer for SAPO-34 and SAPO-34/CNT catalysts. After some time through the reaction, this increase will stop and olefin selectivity will start decreasing due to coke formation.
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As observed in Fig. 8, towards the end of the reaction, ethylene to propylene ratio increases because as the reaction time proceeds, pore sizes become too small for heavy hydrocarbons to pass through due to the formation of oligomers inside the pores and blocking of the pathways for reactants and consequent catalyst deactivation. Therefore, mostly light compounds leave the pores. This ratio is more obvious for SAPO-34/CNF catalyst. The better performance of carbon templated SAPO-34 catalysts in the production of butane can be related to the mild acidity of these catalysts compared with SAPO-34 catalyst, which reduces
ACCEPTED MANUSCRIPT hydrogen transfer to the hydrocarbon and aromatic compounds. Moreover, since the penetration length for carbon templated SAPO-34 catalysts is shorter than that of SAPO-34 catalyst, butane penetration out of the catalyst takes place faster in the latter catalyst and thus the selectivity of carbon templated SAPO-34 catalysts to butane will be higher. On the other hand, among
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catalysts with carbon hard templates, SAPO-34/CNT has the highest selectivity because of its
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milder acidity.
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As the production of light olefins decreases, the production of dimethyl ether (DME) increases. This increase is the highest for SAPO-34 catalyst. Since methanol to DME conversion takes
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place on weak sites, this trend does not seem unexpected considering the high density of the
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weak sites of SAPO-34 catalyst. After the catalyst deactivation by coke, the weak acidic sites are still active and thus DME production increases as time goes by. Among the samples with hard
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templates, DME production growth is the highest by SAPO-34/CNT.
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Fig. 8 Mass fraction of light olefins versus time on stream at T = 400ºC and WHSV = 4.0 h-1 over
samples with various hard templates: (a) SAPO-34; (b) SAPO-34/CNT; (c) SAPO-34/CNF; and
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4. Conclusions
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(d) SAPO-34/GO
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To find out the effects of types of carbon nanostructures used as hard templates, four catalysts were successfully prepared, three of which containing CNT, CNF and GO carbon nanostructures. The effect of application of carbon hard templates on increasing mesoporosity and decreasing the number of acidic sites is completely observed. Despite the approximately 34% reduction in the
surface area of the catalysts with hard templates, mesopores and pore sizes of these catalysts showed a high increase compared with common SAPO-34 catalyst. In addition, the density of these samples became much milder compared with the common catalyst. In methanol to olefin
ACCEPTED MANUSCRIPT conversion (MTO), increasing mesoporosity, pore sizes and mild acidity caused the lifetime of carbon templated SAPO-34 catalysts to increase, led to remarkable selectivity to olefin products and reduced the production of undesirable DME product compared with common SAPO-34 catalyst. Among the catalysts with carbon nanostructures as hard templates, the catalyst
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templated with graphene oxide (GO) showed a very appropriate performance in terms of
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respectively, of the catalyst have given it remarkable properties.
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catalytic activity and selectivity. The surface area and mespore volume of 920 m2 /g and 4.8 cc/g,
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Figure Captions Fig. 1 XRD patterns of synthesized SAPO-34 particles before and after calcination: (a) SAPO-34, (b) SAPO-34/CNT, (c) SAPO-34/CNF and (d) SAPO-34/GO
ACCEPTED MANUSCRIPT Fig. 2 FE-SEM images of synthesized SAPO-34 particles before and after calcinations: (a, b) SAPO-34, (c, d) SAPO-34/CNT, (e, f) SAPO-34/CNF and (g, h) SAPO-34/GO Fig. 3 FTIR spectra of catalysts: SAPO-34, SAPO-34/CNT, SAPO-34/CNF and SAPO-34/GO Fig. 4 NH3 -TPD profiles of SAPO-34 catalysts with different carbon nanostructure templates Fig. 5 Distribution of catalyst pore sizes.
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Fig. 6 Nitrogen absorption-desorption isotherms for SAPO-34 catalysts of various hard templates
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Fig. 7 Methanol conversion of light olefins versus time on stream at T = 400ºC and WHSV = 4.0 h-1 over samples with various hard templates.
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Fig. 8 Mass fraction of light olefins versus time on stream at T = 400ºC and WHSV = 4.0 h-1 over samples with various hard templates: (a) SAPO-34; (b) SAPO-34/CNT; (c) SAPO-34/CNF; and (d)
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Table 1
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S B ET (m2 g -1 )a S ext (m2 g -1 )b Vtotal (cm3 g -1 )c Vmicro (cm3 g -1 )b D (nm)d Hard template MWCNT 245 236 0.38 0.004 6.3 CNF 273 254 0.51 0.01 7.5 GO 920 875 4.6 0.03 19.8 a Surface areas were obtained by the BET method using adsorption data in the p/p 0 range from 0.05 to 0.25. b Measured by the t-plot method. c Total pore volumes were estimated from the adsorbed amount at p/p 0 = 0.995. d Average pore diameters were derived from the adsorption branches of the isotherms by using the BJH method.
Table 2
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Catalyst
Weak (180300ºC)
Moderate (310450ºC)
SAPO-34
2.0(218ºC)
SAPO34/CNT
Strong
Moderate + Strong
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2.10(498ºC)
2.1
1.64(284ºC)
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0.60(640ºC)
0.6
SAPO34/CNF
1.54(221ºC)
0.76(409ºC)
0.33(660ºC)
1.09
SAPO34/GO
1.40(235ºC)
0.90(412ºC)
0.40(643ºC)
1.3
SB ET (m2 g -1 )a
Sext (m2 g -1 )b
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(>480ºC)
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Acidity concentration (mmol NH 3 /g)
Vtotal (cm3 g -1 )c
Vmicro (cm3 g -1 )b
D (nm)d
528 365 334 348
135 176 167 166
0.64 0.68 0.64 0.69
0.18 0.09 0.08 0.09
4.8 7.4 7.7 8.0
SAPO-34 SAPO-34/CNT SAPO-34/CNF SAPO-34/GO
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Table 3
a
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Surface areas were obtained by the BET method using adsorption data in the p/p 0 range from 0.05 to 0.25. Measured by the t-plot method. c Total pore volumes were estimated from the adsorbed amount at p/p 0 = 0.995. d Average pore diameter were derived from the adsorption branches of the isotherms by using the BJH method.
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Graphical abstract
Highlights
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Hierarchical SAPO‐34s were synthesized using carbon nanostructures as hard template. SAPO‐34s were synthesized using morpholine and TEAOH, simultaneously. Hard templates had the significant effects in the specification of SAPO‐34. Performances of SAPO-34s in conversion of methanol to light olefin were compared.
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