Dual-template synthesis of nanostructured CoAPSO-34 used in methanol to olefins: Effect of template combinations on catalytic performance and coke formation

Accepted Manuscript Dual-Template Synthesis of Nanostructured CoAPSO-34 Used in Methanol to Olefins: Effect of Template Combinations on Catalytic Performance and Coke Formation Sogand Aghamohammadi, Mohammad Haghighi PII: DOI: Reference:

S1385-8947(14)01570-8 http://dx.doi.org/10.1016/j.cej.2014.11.102 CEJ 12957

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

28 April 2014 6 November 2014 23 November 2014

Please cite this article as: S. Aghamohammadi, M. Haghighi, Dual-Template Synthesis of Nanostructured CoAPSO-34 Used in Methanol to Olefins: Effect of Template Combinations on Catalytic Performance and Coke Formation, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.11.102

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Dual-Template Synthesis of Nanostructured CoAPSO-34 Used in Methanol to Olefins: Effect of Template Combinations on Catalytic Performance and Coke Formation

Sogand Aghamohammadi1,2, Mohammad Haghighi*,1,2

1. Chemical Engineering Faculty, Sahand University of Technology, P.O.Box 51335-1996, Sahand New Town, Tabriz, Iran.

2. Reactor and Catalysis Research Center (RCRC), Sahand University of Technology, P.O.Box 51335-1996, Sahand New Town, Tabriz, Iran.

*

Corresponding author: Reactor and Catalysis Research Center, Sahand University of Technology, P.O.Box 51335-1996, Sahand New Town, Tabriz, Iran. Email: [email protected], Tel: +98-41-33458096 & +98-41-33459152, Fax: +98-41-33444355, web: http://rcrc.sut.ac.ir

Abstract Methanol conversion to light olefins was investigated over SAPO-34 catalysts with Co introduction exploring the effect of different mixed templates. Applying mixed templates resulted in enhancement of catalyst life time and causing the reduction in catalyst preparation costs compared to those prepared with single template. Current research was focused on the synthesis of CoAPSO-34 catalysts with various mixtures of templates including TEAOH template. Three sets of mixed templates are taken in to account for CoAPSO-34 synthesis, being TEAOH/TEA, TEAOH/DEA and TEAOH/morpholine with constant composition of 50%:50%. The catalysts were prepared via hydrothermal method and characterized with XRD, FESEM, PSD, TEM, EDX, BET and FTIR techniques. Nanostructured catalysts synthesized with TEAOH/DEA and TEAOH/morpholine mixtures exhibit high crystallinity evidenced by XRD analysis. TEAOH/TEA mixed template has the smallest particle size and the most

uniform

particle

size

distribution.

Application of

TEAOH/DEA

and

TEAOH/morpholine binary templates resulted in higher dispersion of Si and Co metals. Methanol to light olefins reaction was carried at different reaction temperatures. The stability test was conducted maintaining at constant temperature, molar feed ratio and GHSV to distinguish the effect of time on stream. In comparison, later emergence of DME in the products stream for the catalyst prepared with TEAOH/morpholine can be due to its higher crystallinity, better Si distribution and higher structural OH bands evidenced by XRD, EDX and FTIR techniques, respectively. Moreover, TGA, BET and FTIR techniques were implemented to characterize the coke deposit.

Keywords: CoAPSO-34, Hydrothermal, Dual template, MTO, Deactivation.

1

1 Introduction Searching for future energy sources as alternatives to crude oil is one of the most important topics among current researchers [1-3]. Among several alternatives, the conversion of methanol to short chain olefins (MTO) has long been of great interest in the petrochemical industry as a potential means of providing demanded olefins in the future when crude oil becomes more expensive [4-10]. Methanol is one of the largest chemical commodities that can be produced from natural gas, biomass, or from coal via synthesis gas [3, 11-13]. In addition, MTO process has a remarkable advantage in terms of product selectivity stemming from its use of zeolite or zeolite-type catalysts [14-17]. Among the leading candidate catalysts, silicoaluminophosphate SAPO-34 molecular sieve exhibits higher selectivity toward light olefin [18-21]. However, it suffers from rapid deactivation by the blockage of pore mouth arisen from the coke formation [10]. Hence, the MTO process will need frequent regeneration of the catalyst. The formation of coke on the catalyst is an important costbearing factor in petrochemical processes using zeolite catalysts. Therefore, it is very important to investigate the structural factors of zeolite that can affect coke deposition. The variables in the hydrothermal synthesis of zeolites are temperature, time, alkalinity (pH), and chemical composition of the reactant mixtures including template. Organic template plays an important role in the hydrothermal synthesis procedure [22, 23]. Template plays the following roles in the formation of specific structure: i) Space filling; ii) Structure directing agent (SDA); iii) Charge compensating [14, 23]. The space filling role of an organic cation is evident in the example of multiple templates can lead to the synthesis of one structure. The structure-directing role of an organic cation implies that a specific structure can be directed only by a specific organic species and cannot be synthesized by the use of any other templates [24]. Moreover, template molecular acts as a charge compensator for the negatively charged lattice of SAPOs [24]. Such amines as tetraethyl ammonium hydroxide (TEAOH), 2

diethylamine (DEA), triethylamine (TEA), morpholine, dipropylamine (DPA), isopropyl amine (IPA) and piperidine have been used as template in SAPO-34 synthesis [4, 7, 25-30]. Furthermore, TEAOH was commonly used in lab and showed excellent catalytic performance for MTO reaction [25]. However, its relative higher cost would be a significant obstacle to commercial production. Consequently, applying mixed templates compared to those prepared with single template resulted in the reduction in catalyst preparation costs. Different mixed templates were taken in to account in the SAPO-34 synthesis with the aim of reducing the portion of TEAOH in the synthetic gel to cope with the high preparation costs [4, 7, 25, 31, 32]. Among studied references, it has been found out that mixed templates affect physiochemical properties which can fall in to three groups: i) structure of the final product and crystal growth; ii) morphology and particle size; iii) Si distribution and acidity which are going to be explained. About the structure of the final product and crystal growth it can be implied that SAPO-11, SAPO-5 and SAPO-34 can be produced from the same gel composition but the formation of SAPO-34 is favoured by greater amount of template [4, 27]. Consequently, it is proposed that the value of SDA/Al2O3 ratio should be higher than 2 in the synthetic gel in order to inhibit contamination with other phases. In comparison, TEAOH/DEA mixtures have the capability of achieving pure SAPO-34 phase in wide range of combinations but it can be met for the samples synthesized with mixed template of TEAOH/ TEA when the mentioned ratio is in the range of 0.05–0.1 [4, 7]. In addition, it has been observed that SAPO-34 and SAPO-5 are competing phases at higher concentrations of TEA [4]. Discussing about the morphology and particle size, it is correct to say that the final product morphology is dependent on the nature and composition of the template mixture. Particle size reduction to sub-micrometer size has been the challengeable subject in the fields of academic research considering the superior performance of the small particles of SAPO-34 in MTO process [33-36]. For instance, 3

opposite trends of TEAOH and DEA can be tailored to a certain point by using the mixtures of both SDAs [7]. Thus, implementation of mixed templates can be the first solution to obtain small particles. And finally about the acidity, it has been found out that Si species dispersed finely in the siliceous islands in the samples using mixed templates resulting in optimized strength and concentration of the acid sites. DEA and morpholine can promote high silicon incorporation into the framework of SAPO-34 resulting in higher acidity [37]. As mentioned, the biggest challenge facing MTO commercialization is related to achieving the highest life time of the used catalyst. In recent years, introduction of transition metal ions (TMI) in to SAPO-34 framework has drawn a remarkable attention due to the observed higher resistance to coke. In the present work, cobalt was chosen to be used for the SAPO-34 synthesis due to its proven excellent performance in MTO reaction [9, 15, 31, 38-43]. The main objective of this work is the synthesis of CoAPSO-34s with mixed templates including TEAOH. To the best of our knowledge, SAPO-34s modified by Co addition have not been synthesized by mixed templates. Up to now, investigations were mainly focused on achieving the optimized ratio of the binary template mixtures. The catalytic evaluation of SAPO-34s synthesized with different mixed templates and under the similar conditions can be the named the major novelty of the present work. Three samples were synthesized with TEAOH/TEA, TEAOH/DEA and TEAOH/morpholine mixtures with simultaneous Co metal addition. Physiochemical properties of the catalysts were identified by XRD, FESEM, PSD, TEM, EDX, BET and FTIR techniques. Moreover, TGA, BET and FTIR techniques were implemented to characterize the coke deposit. Performance tests were carried out to investigate the influence of different temperatures on the catalyst activity. The stability test was conducted to investigate the effect of time on stream.

4

2 Materials and Methods 2.1 Materials The reagents used for the preparation of CoAPSO-34 zeotype are aluminium isopropoxide (Aldrich, 98+ %), fumed silica (Aldrich, 99.9%), phosphoric acid (Merck, 85%), cobalt nitrate hex hydrate (Merck) as the sources of Al, Si, P and Co, respectively. TEAOH (Aldrich, 20%), TEA (Aldrich, 99.9%), DEA (Aldrich, 99.9%) and morpholine (Aldrich, 99.9%) were used as the organic templates. All chemicals were used as received without any purification.

The

synthesized

catalysts

with

TEAOH/TEA,

TEAOH/DEA

and

TEAOH/morpholine mixtures were denoted as CoS/TEA-TE, CoS/TEA-DE and CoS/TEAMO, respectively.

2.2 CoAPSO-34 Preparation and Procedures CoAPSO-34 molecular sieves were hydrothermally synthesized with mixed templates as shown in Figure 1. The molar composition of synthesis solution was 1SDA1: 1SDA2: 1Al2O3: 0.6SiO2: 0.05Co 2O3: 1P2O5: 70H2O. In detail, weighted aluminium isopropoxide was dissolved in distilled water under vigorous stirring for 90 min at room temperature. Phosphoric acid aqueous solution was added to the solution under stirring by a drop-wise addition for 60 min. Fumed silica, cobalt nitrate and template mixture were added in turn and it was continually stirred for 24 h. The resulting gel was transferred in to autoclave and heated at 200 ºC for 48 h. The solid product was recovered by filtration, washed several times with distilled water and dried overnight at 110 ºC. Finally, the catalyst sample was calcined at 550 ºC for 6 h to remove organic template and trapped water within the micro pores of the assynthesized solid.

(Figure 1) 5

2.3 CoAPSO-34 Characterization Techniques XRD patterns of the SAPO-34 catalysts were recorded on a Bruker D8 Advance diffractometer operated at 40 kV and 40 mA with Cu Kα radiation (1.54178˚A) to identify crystal phases. The phase identification was made by comparison to the Joint Committee on Powder Diffraction Standards (JCPDSs). The average crystal size was calculated using the half-width at half-height of most intense peaks of diffraction pattern and well-known DebyeScherrer equation. Diffraction peaks recorded in a 2θ range between 5˚ and 50˚. It is worthy to note that the calculated crystallinity is relative crystallinity. It is estimated by the intensity of the major peaks. The relative crystallinity of each sample can be obtained by the sum of major peaks’ intensity of the corresponding sample divided by the sum of the same peaks for the sample which owns the much value among all. The morphology of the samples was observed by the Field Emission scanning electron microscope (FESEM) on a HITACHI S4160 (Japan). The FESEM is equipped with an Energy Dispersive X-ray (EDX) analyser for dot maps were conducted for dispersion analysis. Transmission electron microscopy (TEM) was carried out on a Philips CM-200. Samples for TEM measurements were ultrasonically dispersed in ethanol. The specific surface area of the sample was determined by N2 adsorption–desorption method at 196 ºC on a surface area analyser (Quantachrome ChemBET-3000). FT-IR spectra of the samples were recorded on a UNICAM 4600 FT-IR spectroscopy using KBr pellet. Deposition of coke is significant in SAPO-34 catalysts. Coke analysis was done after the stability test. FTIR, BET and TGA analyses were carried out to investigate the coke deposits. A portion of the sample was taken for thermo gravimetric analysis (TGA). The temperature was increased to 800 ºC under flowing air at a constant ramping rate of 10 ºC/min (NETZSCH 209 f1, Thermal Analysis Instruments Inc.). The sample weight loss resulting from the reaction of deposited species by the oxygen in the atmosphere was taken account. 6

2.4 Experimental Setup for Catalytic Performance Test Experimental setup used for this study is presented in Figure 2 .Catalytic reaction studies were performed in electrically heated fixed bed reactor (ID 8 mm, length 320 mm) under atmospheric pressure. Gases coming from the regulators pass through lines then introduced to the Mass Flow Controllers (MFC). The gases are mixed and passed to the reaction section. Catalyst was loaded and previous to the reaction preheated in 70 ml/min Argon at 550 ºC for 60 min to remove the adsorbed water. After the temperature of reactor was decreased to 300 ºC, methanol and H2O was fed to the reactor using a saturator. The products were analysed on gas chromatograph (GC Chrom, Teif Gostar Faraz, Iran) with flame ionization detector (FID) using Plot-U column (Agilent) column by programming the oven temperature between 40 ºC and 180 ºC. Argon was used as a carrier for gas chromatograph and hydrogen was used for FID. To ensure reproducibility of the results, the experiments were repeated under similar conditions several times during the course of this work. Deviation between tests was not so much. Typical material balance was carried out for some of the experiments and it was found that the value is within the error limit (±3%).

(Figure 2)

3 Results and Discussions 3.1 CoAPSO-34 Characterizations 3.1.1 XRD Analysis The powder XRD patterns of the synthesized catalysts with various template mixtures are illustrated in Figure 3. It is obvious that the corresponding diffractive peaks of all three samples agree well with previously reported patterns of SAPO-34 rhombohedra structure (JCPDS 01-087-1527) [7, 9, 12, 25, 27, 42, 43]. The mentioned structure is isomorphs to the 7

natural zeolite chabazite with the most intense peaks at 2θ = 9.4, 12.9, 20.5 and 30.5. All three samples obtained without presence of any impurity phase like Co2O3 due to the low amount of Co-loading during the synthesis procedure. Moreover, the introduction of metal heteroatoms prior to the addition of template can inhibit the presence of any impurity phase such as SAPO-5 [40]. Qualitatively, it can be identified that the intensity of the peaks varied according to the applied mixed template. The diffraction peaks of SAPO-34 were very sharp and intense for the catalysts synthesized with TEAOH/DEA and TEAOH/morpholine mixtures but were weak for the sample prepared with TEAOH/TEA mixed template. Thermal stability of the samples can be confirmed by the observation of all characteristic peaks of SAPO-34. Thermal stability of the

as-synthesized catalysts was predictable

by

implementation of calcination at proper temperature of the 550 ºC. A detailed examination of diffraction patterns was done by calculating the crystallite size and also relative crystallinity depicted in Figure 4. Despite the obvious variation of the diffractive peaks intensity, it indicated that there is not any noticeable difference in peaks width. Thus, the calculated crystallite size of the samples is going to be relatively the same. Crystallite size of SAPO-34 was calculated to be 21.5, 20.3 and 22.1 nm using Scherrer equation for the samples CoS/TEA-TE, CoS/TEA-DE and CoS/TEA-MO, respectively confirming the nano structure framework of SAPO-34 for the investigated samples. Relative crystallinity based on the peak intensity at 2θ = 9.5 was calculated to be 60.2, 100 and 96.4% for the samples CoS/TEA-TE, CoS/TEA-DE and CoS/TEA-MO, respectively. In all three samples synthesized with binary mixtures of template, TEAOH is the repeating agent. Therefore, the differences arise from the other agent of the mixtures applied during the synthesis. The samples synthesized with DEA and morpholine exhibit much more intense reflections comparing to the sample prepared with TEA which is in accordance to reported references [5, 25, 27, 31]. The relatively ill-defined pattern of the CoS/TEA-TE catalyst can be due to the 8

incapability of small crystallites to grow and/or amorphous phase formation. It can be concluded from XRD analysis that the nature of structure directing agent can have influential effect on both nucleation and crystal growth rates.

(Figure 3) (Figure 4)

3.1.2 FESEM Analysis FESEM technique was applied to investigate the morphology of the synthesized samples with different mixed templates as shown in Figure 5. The typical cubic like rhombohedra structure can be identified for all three samples confirming the corresponding XRD analysis. As indicated, the catalysts synthesized with binary template mixtures of TEAOH/DEA and TEAOH/morpholine resulted in bigger particles. Qualitative analysis of FESEM images demonstrated that the catalyst prepared with TEAOH/TEA mixed template has the smallest particle size and the most uniform particle size distribution in comparison with the other two samples. With simultaneous consideration of XRD and FESEM results, it can be implied that the production of smaller particles and not capability of them to grow inhibited the complete crystal growth. As a result, XRD reflection peaks of the CoS/TEA-TE sample exhibited with the least intense peaks. Furthermore, existence of some non-cubic particles can evidence the formation amorphous phase alongside crystalline phase for the CoS/TEA-TE sample. Different morphological structure has been obtained for the CoS/TEA-MO sample in which cubic like and spherical particles exist in the corresponding FESEM images altogether. Each of the applied templates affected the formation of special type of morphology which can afford additional evidence for the implementation of mixed template in the synthesis procedure. Surface of the catalytic particles of the CoS/TEA-DE and CoS/TEA-MO samples 9

are smoother than the surface of CoS/TEA-TE particles which can be attributed to their crystallinity. In other words, it seems that the samples with higher crystallinity appeared with much more smoother particles in FESEM micrographs.

(Figure 5)

3.1.3 PSD Analysis Particle size distribution (PSD) of the samples has been calculated using ImageJ software [44] and illustrated in Figure 6. The average particle size was calculated to be 470 nm, 1.1 and 1.4 μm for the CoS/TEA-TE, CoS/TEA-DE and CoS/TEA-MO samples, respectively. Particle size and its distribution have great influences on the performance of SAPO-34 catalysts applied for the MTO reaction. Results of the reported SAPO-34 catalysts used for the MTO process showed that particles in a range of about 500 nm had the best activity and stability [26, 27, 34]. Thus, it can be predicted that the CoS/TEA-TE catalyst is going to have excellent performance.

(Figure 6) 3.1.4 TEM Analysis TEM images of the nanostructured catalyst synthesized with binary template of the TEAOH/TEA were illustrated in Figure 6. The production of some nanometer scale particles alongside bigger ones can be clearly observed from the corresponding TEM images. Nanocatalysis research can be explained as the preparation of heterogeneous catalysts in the nanometer length scale. They are very promising and it can be expected that use of them can decrease the energy usage in the chemical processes results in a greener chemical industry.

10

(Figure 6)

3.1.5 EDX Analysis EDX is a widely used technique to analyse the chemical components in a material under SEM. Figure 8 shows the EDX of Al, Si, O, P and Co for the synthesized catalysts. Presence of all used elements in the corresponding EDX spectrums and their dot-mappings prove the successful synthesis. The determination of metal amount was carried out with EDX area analysis and compared with the initial gel composition as depicted in Figure 9. The obtained materials and the initial gel compositions are relatively close to each other. Al and P of the CoAPSO-34s were the most scattered elements which is simply observable by the EDX spectrum peaks and their dot-mappings. It is known that the Si distribution in the SAPO-34 framework might have great impact on the catalytic behaviour of the catalysts [37]. Si dotmapping of the CoS/TEA-TE demonstrated the lowest dispersion compared to the Si distribution of the other samples. Another detectable difference may be attributed to the Co heteroatoms distribution on the surface which is responsible for enhancing the stability of the SAPO-34. It has the same trend as mentioned for the Si dispersion on the surface of the catalysts. Thus, applying TEAOH/DEA and TEAOH/morpholine mixed templates resulted in higher dispersion of Si and Co.

(Figure 8) (Figure 9)

3.1.6 BET Analysis One of the most influential parameters of the heterogeneous catalysts is their specific surface area. In the presented research, total specific surface area (SSA) was measured on apparatus 11

using N2 adsorption-desorption at 77 K. The Brunauer-Emmett-Teller (BET) equation was used to calculate specific surface area, SBET as reported in Figure 10 for the synthesized samples. It is obvious that the use of TEAOH/DEA and TEAOH/morpholine mixtures caused the increase of SSA comparing to the one prepared with TEAOH/TEA mixed template. The SSA reduction for the sample synthesized with TEAOH/TEA mixed template can be related to its crystallinity loss and/or filling of the pores.

(Figure 10)

3.1.7 FTIR Analysis Figure 11 illustrates the FTIR spectra of the non-calcined catalysts synthesized with different template mixtures. The infrared spectra were obtained using spectrometer in the wavenumber 400-4000 cm-1. Major peaks were identified at wave numbers 500, 650, 740, 885, 1110, 1540, 3000 and 3450 cm-1. The stretching vibration of the structural OH groups appeared at 3450 cm-1 with lower intensity for all the samples [45-47]. Template removal is carried out under calcination and associated with OH generation. Thus the lower intensity of the OH bands for the non-calcined samples might be due to the occulted template molecules within the pores of the molecular sieves and incomplete formation of Bronsted acid sites. Furthermore, template molecules within the pores of resulted in C-H bending vibrations at 1400, 1490, 1540 and 3000 cm-1. The bands at 650, 740 and 1110 cm-1 were assigned to the characteristic vibrations of the SAPO-34 phase [30, 48]. Changes in the peak positions and their intensities after calcination are going to be discussed. FTIR spectra of the as-synthesized samples after calcination are shown in Figure 12. All samples show bands at about 500, 650, 725, 900, 1100, 1650 and 3450 cm-1 which are the typical vibrations in SAPO-34 framework. The peak at 500 cm-1 is assignable to the bending 12

vibration of PO4, AlO4 and SiO4 [30, 32, 49]. The peak at wavenumber 3450 cm-1 can be attributed to bridging hydroxyl groups in D-6 rings. Si-OH-Al bridges position and intensity of them in the IR spectra can provide information to judge about the products acidity [7, 31, 48]. Sample CoS/TEA-MO appeared with the most intense peak of OH bands in comparison to the other catalysts. Therefore, it can be expected that the acidic properties of the CoS/TEAMO sample and its catalytic performance are going to be enhanced by the increment of the structural OH groups’ intensity. The peak at wavenumber 1650 cm-1 is assigned to physical adsorption of the H2O molecules on the products surface [50-52]. Peaks at wave number 640, 725, 1100 cm-1 were assignable to the T-O bending in D-6 rings, asymmetric and symmetric vibrations of O-P-O, respectively. It is worthy to note that the asymmetric bands appear at higher wavenumbers compared to symmetric ones [48]. In order to investigate the effect of calcination on framework vibrations of the functional groups, a detailed comparison between the intensity of peaks was made. Calcination is an essential pre-treatment for the template removal. SAPO-34 molecular sieves have porous structure which cannot be obtained without calcination. In addition, template decomposition is associated with OH generation to play its role as a charge compensating agent. The intensity of the OH groups has been increased after the calcination. Elimination in the intensity of the characteristic peaks of SAPO-34 at 650, 720 and 1100 cm-1 is noticeable after calcination which can be explained by the sharp increase of OH bands.

(Figure 11) (Figure 12)

13

3.2 Catalytic Performance Study toward Methanol to Light Olefins Conversion 3.2.1 Effect of Different Dual Templates on Methanol Conversion The catalytic activity of the synthesized nano-structured catalysts in terms of CH3OH conversions was studied at temperatures ranging from 300 to 500°C as shown in Figure 13. In this section all the reactions were carried out at constant molar feed ratio and gas hourly space velocity (GHSV = 4200 cm3/gcat.h). The measurements of reaction products belong to an acceptable time on stream at each temperature to achieve steady state conditions. Conversion of methanol increased with the increasing temperature which is in accordance to the thermodynamic studies of MTO process [31]. Therefore, higher temperature is favour to light olefins production from methanol. Average particle size of the synthesized samples estimated by FESEM technique and showed that it is around 0.5 to 1.5 µm. Obtained smaller particles could provide shorter diffusion length enhancing the diffusivity of methanol molecules to the acidic sites of the SAPO-34. Thus, methanol conversion gains higher values even at lower temperatures.

(Figure 13)

3.2.2 Effect of Different Dual Templates on Products Selectivity Figure 14 shows the dependence of product distribution on temperature for the synthesized catalysts. At higher temperatures, the effect of methane formation side reactions becomes more. The highest value of methane selectivity obtained at 500°C for all the examined samples. The catalyst prepared with TEAOH/DEA mixed template showed lower methane formation at constant temperatures compared to the other samples. Light olefins selectivity, ethylene and propylene, reached a maximum value at 350, 450 and 400°C for the CoS/TEATE, CoS/TEA-DE and CoS/TEA-MO samples, respectively. 14

Ethylene selectivity reaches a maximum value and then decreases at higher temperatures. Moreover, as the temperature increases from 300 to 500°C, the C3H6 selectivity decreases which can be the result of propylene conversion to ethylene at higher temperatures. It indicated that lower temperature is favour to propylene production. At higher temperatures, transformation of propylene molecules to ethylene occurs dominantly resulting in higher selectivity of C2H4. However, at higher temperatures, methane formation becomes dominant and ethylene selectivity is about to decrease. As a result, operational temperature can be changed to achieve desired selectivity of light olefins. In short, optimum temperature was selected 400°C to study time on stream performance.

(Figure 14)

3.2.3 Effect of Different Dual Templates on Time on Stream Performance High activity alongside excellent stability can be the properties of a promising catalyst. The time-dependant products selectivity and methanol conversion at constant temperature, feed ratio and GHSV for the nanostructured catalysts synthesized with different mixed templates are plotted in Figure 15. Stability test was conducted at 400°C. Effluent gas analysis was done at time intervals of 30 minutes. Figure 15 (a) shows the variation of the major products distribution and methanol conversion with time on stream for the sample prepared with TEAOH/TEA mixed template. Average methanol conversion during the 11 h of the stability test is about 90% for the CoS/TEA-TE sample. As indicated, its elimination is not so much and can be neglected. It can be attributed to the conversion of methanol to dimethyl ether (DME) on weakened acid centres by the coke deposition. DME can be easily obtained from methanol dehydration by using catalysts with lower acidity, such as γ-Al2O3. Thus, DME emergence in the products stream can be a good 15

criterion to investigate catalyst stability. For the discussed catalyst, DME exists after 3 h and reaches at about 2% after 6 h. As observed, the DME increase for the CoS/TEA-TE sample occurred with a gentle slope and reached at about 20% at the end of stability test. At this time, production of ethylene increased gradually with TOS, giving a maximum value and then decreased at TOS = 9 h. Ethylene selectivity increase at earlier times could be related to the coking effect, which reduced the pore size of SAPO-34 and thus increased the ethylene selectivity [4, 15, 35]. For more explanation, at higher temperature (400°C), propene and butene might oligomerize to bigger oligomers, and subsequently formed oligomers which could be cracked to ethylene. In addition, propylene selectivity had a constant decreasing trend. Totally, light olefins selectivity (C2H4 + C3H6) starts decreasing after 6 h and drops to 72% at the time reaction of 11 h. MTO reaction consists of three steps listed below: 1) Methanol dehydration to DME and water over an acidic catalyst 2) Light olefins formation from the equilibrium mixture of DME and H2O 3) Light olefins reaction forming paraffins, aromatics, naphthenic and heavier olefins.

It can be deduced that the rate of steps 1 and 3 get increased in prolonging the reaction time. On the other site, light olefins production (step 2) decreased with time on stream. The increase in reaction rates of steps 1 and 3 might be attributed to the DME production over mildly deactivated sites and secondary reactions of the coke precursors within the pores or near the pore mouths to form higher hydrocarbons, respectively. SAPO-34 cages have enough space to accommodate coke precursors to progress polymerization reactions to form paraffins, aromatics, naphthenic and higher hydrocarbons. A mild increase in methane selectivity for longer reaction times may stem from the pore blockage by coke formation, enhancing methane formation [53].

16

Figure 15 (b) gives the methanol conversion and selectivity of final products with time on sample CoS/TEA-DE. Intermediate product DME exists at TOS = 1.5 h for this catalyst and gives a maximum value of 50% at the end of stability test. As observed, the DME increase was happened more rapidly for the CoS/TEA-DE comparing to the CoS/TEA-TE sample. Figure 15 (c) shows the selectivity of the products with time on stream (TOS) on CoS/TEAMO. It is clear that the slowest increase of DME occurred for the catalyst synthesized with TEAOH/morpholine, reported 13% at reaction time of 11 h. Short chain olefins selectivity remains constant until 7 h and then decreased to 73% in the following reaction period. There can be different criterions to evaluate the catalytic behaviour during the process; some of them were listed below:
DME emergence time in the effluent gas stream: CoS/TEA-MO (t = 4.5h) > CoS/TEA-TE (t= 3h) > CoS/TEA-DE (t= 1.5h)
Reaction time attaining of DME selectivity equals to 2%: CoS/TEA-MO (t = 7.5h) > CoS/TEA-TE (t= 6h) > CoS/TEA-DE (t= 3.5h)
DME selectivity at TOS = 11 h: CoS/TEA-DE (34%) > CoS/TEA-TE (20%) > CoS/TEA-MO (14%)
Light olefins selectivity at TOS = 11 h: CoS/TEA-MO (73%) > CoS/TEA-TE (72%) > CoS/TEA-DE (50%)

Samples templated with TEAOH/TEA and TEAOH/morpholine showed outstanding results considering their light olefins selectivity as the representative of catalyst activity. Excellent stability of the CoS/TEA-TE can be ascribed to its proper average particle size of 500 nm for the MTO reaction. It has been reported that the 0.4-0.5µm particles of SAPO-34 gave the largest capacity of olefin formation [26, 29, 35]. The smaller particles allow the rapid diffusion of the products to the reaction media, avoiding subsequent transformations of them 17

to heavier products which cause the deactivation of the catalyst. On the other side, higher crystallinity, specific surface area and surface metal enrichment might be the reasons for the stable methanol conversion over CoS/TEA-MO. Distinct MTO results belonging to the synthesized samples arise from differences in their physiochemical properties including crystallinity, specific surface area, particle size, metal dispersion and acidity. All mentioned factors influence overall results, but the effect of one parameter can be dominant. For instance, the catalyst prepared with TEAOH/DEA mixture owning higher specific surface area and crystallinity compared to the CoS/TEA-TE sample shows faster deactivation by the time passing. Thus, in the case of CoS/TEA-TE and CoS/TEA-DE samples, the particle size impact seems to be determining. Furthermore, it can be implied that CoS/TEA-TE and CoS/TEA-MO catalysts have similar results in terms of final amount of light olefins. However, selective deactivation of the acid sites happens at different rates. In comparison, later emergence of DME in the products stream for CoS/TEA-MO catalyst can be due to its higher crystallinity, better Si distribution and higher structural OH bands evidenced by XRD, EDX and FTIR techniques, respectively. In a word, it seems that lower density of acid centres for the CoS/TEA-TE leads to earlier deactivation and emergence of intermediate product (DME).

(Figure 15)

3.3 CoAPSO-34 Deactivation Study 3.3.4 TGA Analysis Figure 16 shows the TGA curves for the spent catalysts. After the stability test and progress of MTO reaction, CoAPSO-34s were collected to identify the nature and amount of the coke residues. Coking is a kind of condensation-polymerization on the surface resulting in 18

macromolecules having an empirical formula approaching CHx, in which x may vary between 0.5 and 1 [54-57]. For catalytic reactions involving hydrocarbons side reactions occur on the catalyst surface leading to the formation of carbonaceous residues (usually referred to as coke) which tend to physically cover the active surface. Coke deposits may amount to 15% or even 20% (w/w) of the catalyst and accordingly they may deactivate the catalyst either by covering of the active sites or by pore blocking. MTO process involves the production of olefins. It has been reported that coke formation occurs more rapidly when there is a hydrogen acceptor, such as an olefin [56, 57]. It may be controlled to a certain extent by using an optimal catalyst composition and an appropriate process conditions. By increasing the temperature to the 800°C, deposited hydrocarbons over the catalyst reacts with oxygen and enters to the atmosphere. It gives two stages of weight loss so the TGA curve divided in to two parts: 0–200 and 200–800°C. The first weight loss corresponds to the release of moisture and physically adsorbed water. The second weight loss belongs to the oxidation of hydrocarbon deposits over the catalyst. We note here that the overall coke content in CoS/TEA-TE, CoS/TEA-DE and CoS/TEA-MO is approximately 5.7, 8.7 and 9.7%, respectively. The obtained result shows meaningful relation with the respective physiochemical characterizations and catalytic performance. The average particle size and final coke content was in the below order:
CoS/TEA-MO (1.4 µm) > CoS/TEA-DE (1.1 µm) > CoS/TEA-TE (0.47 µm)
CoS/TEA-MO (9.7%) > CoS/TEA-DE (8.7%) > CoS/TEA-TE (5.7%)

As reported in literature, larger particles of SAPO-34 would provide longer diffusion pass enhancing secondary reactions to form higher hydrocarbons [26, 35]. Resultantly, coke content expected to be increased for larger particles. Nevertheless, the CoS/TEA-MO catalyst maintains activity until the end of stability test considering its slow DME increment trend and 19

final percentage of DME in the products stream. It can be attributed to the higher density of the OH active sites evidenced by FTIR analysis of the calcined catalysts.

(Figure 16)

3.3.5 BET Analysis Specific surface area of the deactivated CoAPSO-34 catalysts is shown in Figure 17. The results demonstrate that the SSA of the coked catalysts decreases sharply after 11 h on stream. It is worthy to note that BET surface areas of the fresh and spent catalysts were taken in to account and then the amount SSA reduction was calculated. The reduction of BET surface area can be attributed to the blockage of pores by the coke deposits and calculated to be 73, 77 and 61% for the CoS/TEA-TE, CoS/TEA-DE and CoS/TEA-MO catalysts, respectively. The smaller SSA decline belongs to CoS/TEA-MO which can be the reason for its relative higher activity. BET surface area of the fresh CoS/TEA-DE and CoS/TEA-MO are similar. But, it is reported to be 121.6 and 208.9 m2/g respectively, after the process. This is consistent with the reactor test results that higher SSA of the CoS/TEA-MO was helpful for its superior activity.

(Figure 17)

3.3.6 FTIR Analysis The nature of the coke can be extensively carried out by using FTIR spectroscopy. FTIR spectra of the deactivated CoAPSO-34 catalysts after 660 min time of reaction at 400°C are shown in Figure 18. Deactivated catalysts contain adsorbed water which is observable by the peak at 1650 cm-1 confirming the TGA results. Coke formed over the sample can be 20

identified by the characteristic peaks of IR spectra. Peak at wave number 1360 cm-1 is assignable to the CH3 bending vibration indicating paraffinic nature of the hydrocarbon residues. C-C stretching of the aromatic coke band can be distinguished by the peak at 1520 cm-1. Too weak peaks at wave numbers 2800 and 2900 cm-1 can be seen which are representative of C-H vibrations of aliphatic coke. Typical vibrations of the coke species are similar for all three samples. Thus, cokes deposited over all three samples are of the same nature. Characteristic peaks of coke residues of the CoS/TEA-MO exhibit with more intense peaks. This result is in agreement with the results obtained from TGA analysis.

(Figure 18)

4 Conclusions Cost-effective synthesis of SAPO-34 catalyst is one of the most significant challenges to MTO industrialization which was improved by mixed templates application. CoAPSO-34 molecular sieves synthesized with three different template mixtures being TEAOH/TEA, TEAOH/DEA and TEAOH/morpholine are shown to exhibit different physicochemical properties as well as catalytic performance. It indicated that the nature of the template determines the morphology of final product due to different rate of crystal growth shown by XRD and FESEM analyses. It appears from results that the catalyst prepared with TEAOH/morpholine mixture is the best catalyst among synthesized samples in terms of life time in the MTO process sustaining light olefins selectivity at higher values (about 73% after 11 h TOS). Cokes deposited over all three samples are of the same nature according to FTIR technique. Characteristic FTIR peaks of coke residues over the CoS/TEA-MO exhibit with more intense peaks confirming the TGA analysis.

21

Acknowledgements The authors gratefully acknowledge Sahand University of Technology for the financial support of the research as well as Iran Nanotechnology Initiative Council for complementary financial supports.

22

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28

Figures Caption Figure 1. Schematic flow chart for the preparation steps of nanostructured CoAPSO-34 using dual template combinations of TEAOH/TEA, TEAOH/DEA and TEAOH/MOR via hydrothermal method. ..................................................... 31 Figure 2. Experimental setup for activity test of nanostructured CoAPSO-34 used in methanol to light olefins conversion. ............................................................ 32 Figure 3. XRD patterns of nanostructured CoAPSO-34 synthesized using dual templates: (a) CoS/TEA-TE, (b) CoS/TEA-DE and (c) CoS/TEA-MO. ......................... 33 Figure 4. Crystallite size and relative crystallinity of nanostructured CoAPSO-34 synthesized using different dual templates (TEAOH/TEA, TEAOH/DEA and TEAOH/MOR). ........................................................................................... 33 Figure 5. FESEM images of nanostructured CoAPSO-34 synthesized using dual templates: (a) CoS/TEA-TE, (b) CoS/TEA-DE and (c) CoS/TEA-MO. ........ 34 Figure 6. Particle size histogram of nanostructured CoAPSO-34 synthesized using dual templates: (a) CoS/TEA-TE, (b) CoS/TEA-DE and (c) CoS/TEA-MO. ........ 35 Figure 7. TEM images of nanostructured catalyst synthesized with the TEAOH/TEA mixed template. ............................................................................................ 36 Figure 8. EDX analysis of nanostructured CoAPSO-34 synthesized using dual templates: (a) CoS/TEA-TE, (b) CoS/TEA-DE and (c) CoS/TEA-MO. ......................... 37 Figure 9. Gel and surface composition of nanostructured CoAPSO-34 synthesized using dual templates (TEAOH/TEA, TEAOH/DEA and TEAOH/MOR). .............. 38 Figure 10. BET surface analysis of nanostructured CoAPSO-34 synthesized using dual templates (TEAOH/TEA, TEAOH/DEA and TEAOH/MOR). ..................... 38 Figure 11. FTIR spectra of nanostructured CoAPSO-34 synthesized using dual templates (before calcination): (a) CoS/TEA-TE, (b) CoS/TEA-DE and (c) CoS/TEAMO. ............................................................................................................. 39 Figure 12. FTIR spectra of nanostructured CoAPSO-34 synthesized using dual templates (after calcination): (a) CoS/TEA-TE, (b) CoS/TEA-DE and (c) CoS/TEA-MO. .................................................................................................................... 40 Figure 13. Effect of different dual templates (TEAOH/TEA, TEAOH/DEA and TEAOH/MOR) on methanol conversion over nanostructured CoAPSO-34. . 40 Figure 14. Effect of different dual templates (TEAOH/TEA, TEAOH/DEA and TEAOH/MOR) on product selectivities over nanostructured CoAPSO-34. ... 41 29

Figure 15. Effect of different dual templates (TEAOH/TEA, TEAOH/DEA and TEAOH/MOR) on performance of nanostructured CoAPSO-34 toward methanol conversion to light olefins. ............................................................ 42 Figure 16. TGA analysis of used CoAPSO-34 synthesized using dual templates: (a) CoS/TEA-TE, (b) CoS/TEA-DE and (c) CoS/TEA-MO. .............................. 43 Figure 17. BET surface analysis of used CoAPSO-34 synthesized using dual templates (TEAOH/TEA, TEAOH/DEA and TEAOH/MOR). ..................................... 43 Figure 18. FTIR spectra of used CoAPSO-34 synthesized using dual templates (after process): (a) CoS/TEA-TE, (b) CoS/TEA-DE and (c) CoS/TEA-MO. .......... 44

30

Figures

(a) Precursor preparation Co-Precursor Si-Precursor Fumed silica Cobalt nitrate hexa hydrate

Reaction Medium De-ionized water

Template =1/1(molar ratio)

SDA1/SDA2

Al-Precursor

P-Precursor AIP: Aluminium isopropoxide Phosphoric acid: 85 wt% (aq.)

Mixing for 90 min Drop wise addition of phosphoric acid and mixing for 60 min Gradually addition of fumed silica and mixing for 30 min Addition of cobalt nitrate and mixing for 30 min Drop wise addition of template solution and mixing for 24 h Gel formation in appropriate molar ratio of Al2O3/SiO2/P2O5/Co2O3/SDA1/SDA2/H2O = 1/0.6/1/0.05/1/1/70 (pH= 7-8) (b) Hydrothermal synthesis Hydrothermal synthesis: in a teflon-lined stainless steel autoclave reactor at 200ºC for 48 h (c) Post treatment Filtration and washing with de-ionized water Drying at 110°C for 24 h under air flow Calcination at 550°C for 6 h under air flow Catalyst forming: CoAPSO-34: CoS/TEA-TE, CoS/TEA-DE, CoS/TEA-MO Figure 1. Schematic flow chart for the preparation steps of nanostructured CoAPSO-34 using dual template combinations of TEAOH/TEA, TEAOH/DEA and TEAOH/MOR via hydrothermal method.

31

PI-02 FM (02)

Ar Notations: C: Cylinder GC: Gas Chromatography MFC: Mass Flow Controller PI: Pressure Indicator

PRV: Pressure Regulator Valve R: Reactor TIC: Temp. Indicator & Controller 3WV: 3-Way Valve

PI-01 FM (01)

Ar

Ar

Ar+CH3OH Feed to GC

Ar

Feed to GC

Feed to reactor

Vent Reactor outlet

CH3OH

Saturator

Reactor inlet

PRV-02 PRV-01 Ar H2

Ar

Ar

Vent

Air

Electrical heating furnace with temperature controller

Products to GC

GC Ar

Cooler

Vent

Air

Temperatur e Controller

H2

TIC-01 C-1

C-2

Controlled power supply

C-3 C-4 C-5

R-01 Furnace/Reactor

Figure 2. Experimental setup for activity test of nanostructured CoAPSO-34 used in methanol to light olefins conversion.

32

SAPO-34 pattern

Intensity (a.u.)

(a) CoS/TEA-TE

(b) CoS/TEA-DE

(c) CoS/TEA-MO 5

10

15

20

25

30

35

40

45

50

2θ (degree) Figure 3. XRD patterns of nanostructured CoAPSO-34 synthesized using dual templates: (a) CoS/TEATE, (b) CoS/TEA-DE and (c) CoS/TEA-MO.

30 Crystallite size (nm) 96.4

100

22.1

21.5

20.3

20

15

100

Relative crystallinity

80

60.2

60

10

40

5

20

0

CoS/TEA-TE

CoS/TEA-DE

CoS/TEA-MO

Relative Crystallinity

Crystallite Size (nm)

25

0

CoAPSO-34 Figure 4. Crystallite size and relative crystallinity of nanostructured CoAPSO-34 synthesized using different dual templates (TEAOH/TEA, TEAOH/DEA and TEAOH/MOR). 33

(a) CoS/TEA-TE

10.0µm

5.0µm

(b) CoS/TEA-DE

10.0µm

5.0µm

(c) CoS/TEA-MO

10.0µm

5.0µm

Figure 5. FESEM images of nanostructured CoAPSO-34 synthesized using dual templates: (a) CoS/TEATE, (b) CoS/TEA-DE and (c) CoS/TEA-MO.

34

100

Frequency (%)

80

(a) CoS/TEA-TE

Min: 0.2 µm Max: 1.4 µm Average = 0.47 µm Standard deviation: 0.3

1.3 µm

57.6

60

500nm

40 18.6

20 0

15.3 3.4 1.7 3.4 0.0 0.0 0.0

0.0 0-0.1

0.2-0.3

0.4-0.5

0.6-0.7

2.0µm

0.8-0.9

Particle size (µm) 100

Frequency (%)

80 60

52.5

40

1.1 µm

500nm

31.4

20 0

(b) CoS/TEA-DE

Min: 0.6 µm Max: 4.0 µm Average = 1.1 µm Standard deviation: 0.5

11.9 2.5 0.0 0.8 0.0 0.8 0.0

0.0 0-0.5

1-1.5

2-2.5

3-3.5

5.0µm

4-4.5

Particle size (µm) 100

Frequency (%)

80

(c) CoS/TEA-MO

Min: 0.5 µm Max: 4.1 µm Average = 1.4 µm Standard deviation: 0.7

60

1.5 µm

500nm

36.9

40 27.7

16.9

20

12.3 0.0 3.1 0.0 1.5 0.0

1.5 0 0-0.5

1-1.5

2-2.5

3-3.5

5.0µm

4-4.5

Particle size (µm) Figure 6. Particle size histogram of nanostructured CoAPSO-34 synthesized using dual templates: (a) CoS/TEA-TE, (b) CoS/TEA-DE and (c) CoS/TEA-MO.

35

CoS/TEA-TE

Figure 7. TEM images of nanostructured catalyst synthesized with the TEAOH/TEA mixed template.

36

(a) CoS/TEA-TE

Al

25.0µm

Si

25.0µm

P

25.0µm

Co

25.0µm

O

25.0µm

Al-Si-P-Co-O

25.0µm

Al

25.0µm

Si

25.0µm

P

25.0µm

Co

25.0µm

O

25.0µm

Al-Si-P-Co-O

25.0µm

Al

25.0µm

Si

25.0µm

P

25.0µm

Co

25.0µm

O

25.0µm

Al-Si-P-Co-O

25.0µm

keV

(b) CoS/TEA-DE

keV

(c) CoS/TEA-MO

keV

Figure 8. EDX analysis of nanostructured CoAPSO-34 synthesized using dual templates: (a) CoS/TEATE, (b) CoS/TEA-DE and (c) CoS/TEA-MO.

37

Co

Al

Si

P

Composition (wt %)

100

80

60

40

20

0

Gel Composition

CoS/TEA-TE

CoS/TEA-DE

CoS/TEA-MO

CoAPSO-34 Figure 9. Gel and surface composition of nanostructured CoAPSO-34 synthesized using dual templates

(TEAOH/TEA, TEAOH/DEA and TEAOH/MOR).

Specific Surface Area (m2/g)

600

500

540.9

540.8

CoS/TEA-DE

CoS/TEA-MO

500.7

400

300

200

100

0

CoS/TEA-TE

CoAPSO-34 Figure 10. BET surface analysis of nanostructured CoAPSO-34 synthesized using dual templates (TEAOH/TEA, TEAOH/DEA and TEAOH/MOR).

38

885 740 650 650 580 540 490 460

1640 1540 1560 1490 1400 1360 1220 1110

2350

3000

3450

Transmittance (a.u.)

(a) CoS/TEA-TE

(b) CoS/TEA-DE

(c) CoS/TEA-MO

Before calcination 400 4000

900 3500

1400 3000

1900 2500

2400 2000

Wavenumber

2900 1500

3400 1000

3900 500

(cm-1)

Figure 11. FTIR spectra of nanostructured CoAPSO-34 synthesized using dual templates (before calcination): (a) CoS/TEA-TE, (b) CoS/TEA-DE and (c) CoS/TEA-MO.

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500

725 650

900

1100

1650

2350

3450

Transmittance (a.u.)

(a) CoS/TEA-TE

(b) CoS/TEA-DE

(c) CoS/TEA-MO

After calcination 400 4000

900 3500

1400 3000

1900 2500

2400 2000

2900 1500

3400 1000

3900 500

Wavenumber (cm-1) Figure 12. FTIR spectra of nanostructured CoAPSO-34 synthesized using dual templates (after calcination): (a) CoS/TEA-TE, (b) CoS/TEA-DE and (c) CoS/TEA-MO.

CoS/TEA-TE

CoS/TEA-DE

CoS/TEA-MO

CH3OH Conversion (%)

100 80 60 40 20 0

300

350

400

Temperature (°C)

450

500

Figure 13. Effect of different dual templates (TEAOH/TEA, TEAOH/DEA and TEAOH/MOR) on methanol conversion over nanostructured CoAPSO-34.

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CH4

C2H4

C3H6

DME

C4+

(a) CoS/TEA-TE

Temperature (°C)

500 450 400 350 300 0

10

20

30

40

50

60

70

80

90

100

Selectivity (%) CH4

C2H4

C3H6

DME

C4+

(b) CoS/TEA-DE

Temperature (°C)

500 450 400 350 300 0

10

20

30

40

50

60

70

80

90

100

Selectivity (%) CH4

C2H4

C3H6

DME

C4+

(c) CoS/TEA-MO

Temperature (°C)

500 450 400 350 300 0

10

20

30

40

50

60

70

80

90

100

Selectivity (%) Figure 14. Effect of different dual templates (TEAOH/TEA, TEAOH/DEA and TEAOH/MOR) on product selectivities over nanostructured CoAPSO-34.

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100 (a) CoS/TEA-TE

CH3OH C2H4 DME

Conversion/Selectivity (%)

80

CH4 C3H6 Olefines

60

40

20

0 0

100

200

300

400

500

600

700

Time on stream (min) 100 (b) CoS/TEA-DE

Conversion/Selectivity (%)

80

60 CH3OH C2H4 DME

40

CH4 C3H6 Olefines

20

0 0

100

200

300

400

500

600

700

Time on stream (min) 100

Conversion/Selectivity (%)

(c) CoS/TEA-MO

CH3OH C2H4 DME

80

CH4 C3H6 Olefines

60

40

20

0 0

100

200

300

400

500

600

700

Time on stream (min) Figure 15. Effect of different dual templates (TEAOH/TEA, TEAOH/DEA and TEAOH/MOR) on

performance of nanostructured CoAPSO-34 toward methanol conversion to light olefins.

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100

7.3%

20.6%

10.9%

16.5% 16.0%

Weight loss (%)

95

90

9.7%

5.7%

8.7%

(b) CoS/TEA-DE

85

(a) CoS/TEA-TE (c) CoS/TEA-MO

80 Zone-I (0-200 ºC)

Zone-II (200-800 ºC)

75 0

100

200

300

400

500

600

700

800

Temperature (°C) Figure 16. TGA analysis of used CoAPSO-34 synthesized using dual templates: (a) CoS/TEA-TE, (b) CoS/TEA-DE and (c) CoS/TEA-MO.

Specific Surface Area (m2/g)

250

Used catalyst 208.9

200

150

136.6 121.6

100

50

0

CoS/TEA-TE

CoS/TEA-DE

CoS/TEA-MO

CoAPSO-34 Figure 17. BET surface analysis of used CoAPSO-34 synthesized using dual templates (TEAOH/TEA, TEAOH/DEA and TEAOH/MOR).

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500

725 650

900

1100

1650 1510 1380

2350

2900 2800

3450

Transmittance (a.u.)

(a) CoS/TEA-TE

400 4000

(b) CoS/TEA-DE

(c) CoS/TEA-MO

900 3500

1400 3000

1900 2500

2400 2000

2900 1500

3400 1000

3900 500

Wavenumber (cm-1) Figure 18. FTIR spectra of used CoAPSO-34 synthesized using dual templates (after process): (a) CoS/TEA-TE, (b) CoS/TEA-DE and (c) CoS/TEA-MO.

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Dual-Template Synthesis of Nanostructured CoAPSO-34 Used in Methanol to Olefins: Effect of Template Combinations on Catalytic Performance and Coke Formation

Sogand Aghamohammadi1,2, Mohammad Haghighi1,1,2

1. Chemical Engineering Faculty, Sahand University of Technology, P.O.Box 51335-1996, Sahand New Town, Tabriz, Iran. 2. Reactor and Catalysis Research Center (RCRC), Sahand University of Technology, P.O.Box 51335-1996, Sahand New Town, Tabriz, Iran.

Graphical Abstract Applying mixed templates resulted in enhancement of catalyst life time and simultaneously causing the reduction in catalyst preparation costs compared to those prepared with single template. Methanol conversion to light olefins was investigated over SAPO-34 catalysts with Co introduction exploring the effect of different mixed templates. Three sets of mixed templates are taken in to account for CoAPSO-34 synthesis, being TEAOH /TEA, TEAOH /DEA and TEAOH /morpholine with constant composition of 50%:50%. The catalysts were prepared via hydrothermal method and characterized with XRD, FESEM, TEM, PSD, EDX, BET and FTIR techniques. Moreover, TGA, BET and FTIR techniques were implemented to

1

Corresponding author: Reactor and Catalysis Research Center, Sahand University of Technology, P.O.Box 51335-1996, Sahand New Town, Tabriz, Iran. Email: [email protected], Tel: +98-41-33458096 & +98-41-33459152, Fax: +98-41-33444355, web: http://rcrc.sut.ac.ir

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characterize the coke deposit. Application of TEAOH/DEA and TEAOH/morpholine mixed templates resulted in higher dispersion of Si.

46

Dual-Template Synthesis of Nanostructured CoAPSO-34 Used in Methanol to Olefins: Effect of Template Combinations on Catalytic Performance and Coke Formation

Sogand Aghamohammadi1,2, Mohammad Haghighi1,1,2

1. Chemical Engineering Faculty, Sahand University of Technology, P.O.Box 51335-1996, Sahand New Town, Tabriz, Iran. 2. Reactor and Catalysis Research Center (RCRC), Sahand University of Technology, P.O.Box 51335-1996, Sahand New Town, Tabriz, Iran.

Research Highlights • Successful implementation of mixed templates in CoSAPO-34 synthesis • Nature of the mixed templates determines the morphology of final product. • TEAOH/Morpholine mixed templated reflects high performance in MTO process. • Decrease in catalyst synthesis costs which is an obstacle in MTO process development.

1

Corresponding author: Reactor and Catalysis Research Center, Sahand University of Technology, P.O.Box 51335-1996, Sahand New Town, Tabriz, Iran. Email: [email protected], Tel: +98-41-33458096 & +98-41-33459152, Fax: +98-41-33444355, web: http://rcrc.sut.ac.ir

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