Methanol conversion to light olefins over nanostructured CeAPSO-34 catalyst: Thermodynamic analysis of overall reactions and effect of template type on catalytic properties and performance

Materials Research Bulletin 50 (2014) 462–475

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Methanol conversion to light olefins over nanostructured CeAPSO-34 catalyst: Thermodynamic analysis of overall reactions and effect of template type on catalytic properties and performance Sogand Aghamohammadi a,b, Mohammad Haghighi a,b,*, Mojtaba Charghand a,b a b

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

A R T I C L E I N F O

A B S T R A C T

Article history: Received 12 June 2013 Received in revised form 19 September 2013 Accepted 5 November 2013 Available online 11 November 2013

TEAOH and morpholine were employed in synthesis of nanostructured CeAPSO-34 molecular sieve and used in methanol to olefins conversion. Prepared samples were characterized by XRD, FESEM, EDX, BET, FTIR and NH3-TPD techniques. XRD patterns reflected the higher crystallinity of the catalyst synthesized with morpholine. The FESEM results indicated that the nature of the template determines the morphology of nanostructured CeAPSO-34 due to different rate of crystal growth. There was a meaningful difference in the strength of both strong and weak acid sites for CeAPSO-34 catalysts synthesized with TEAOH and morpholine templates. The catalyst synthesized with morpholine showed higher desorption temperature of both weak and strong acid sites evidenced by NH3-TPD characterization. The catalyst obtained using morpholine template had the longer lifetime and sustained desired light olefins at higher values. A comprehensive thermodynamic analysis of overall reactions network was carried out to address the major channels of methanol to olefins conversion. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compounds B. Chemical synthesis D. Catalytic properties

1. Introduction Olefins are the basic building blocks for the most of petrochemical industries [1,2]. Remarkable array of light olefins applications creates a sense of excitement among researchers to explore recently accepted processes like methanol to olefins (MTO), thus opening new opportunities for methanol utilization [3–7]. Over the recent decades, the MTO reaction has drawn a remarkable attention due to its industrial interest as it reduces the dependency on petroleum naphtha to produce the highly desired ethylene and propylene derived products [8–10]. Thus, consideration of the recent dramatic increase in crude oil prices and the shortage in the foreseeable future could be some of the reasons for MTO reaction significance [11–14]. Different acidic microporous catalysts have been reported for MTO process [7,15–17]. Reaction over medium-pore zeolites (like ZSM-5) produces extensive amounts of aromatics and paraffins and in the case of large-pore zeolites rapid coke formation results [18]. The silicoaluminophosphate, SAPO-34, was reported to be the most promising catalyst giving maximum light olefins selectivities

* Corresponding author at: Chemical Engineering Faculty, Sahand University of Technology, P.O. Box 51335-1996, Sahand New Town, Tabriz, Iran. Tel.: +98 412 3458096/3459152; fax: +98 412 3444355. E-mail address: [email protected] (M. Haghighi). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.11.014

up to 80% [19–22]. Relatively mild acidity, high thermal stability, pore size of 0.43–0.50 nm and 8-membered ring pores are some of the favorable characteristics of SAPO-34 molecular sieve in MTO process [22–26]. The biggest challenge facing MTO commercialization is about the achievement of the highest lifetime of the catalyst. Rapid deactivation by coke is the major drawback of SAPO-34 utilization in MTO process and resultantly makes the lifetime of catalyst short [18,19,21,27–30]. Metal incorporation into silicoaluminophosphate framework influences properties of resulting samples [19,21,27–32]. MeAPSO-34s synthesis has been attracted due to their obtained longer lifetime compared to conventional SAPO-34 [20,21,33,34]. Several variables including temperature, gel composition, pH, time and template can affect the properties of MeAPSO-34. Organic template organizes tetrahedral oxides into a particular geometric topology around itself and thus provides initial building block for a particular structure type [22,23,35–37]. It is well-recognized that template plays important roles in the synthesis of molecular sieves. The following roles in the formation of specific structure can be expected: (i) space filling; (ii) structure directing agent (SDA); (iii) charge compensating [16,22,23,38]. The space filling role of an organic cation is evident in the example of multiple templates can lead to the synthesis of one structure [10,39,40]. 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

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any other templates. Moreover, template molecular acts as a charge compensator for the negatively charged lattice of SAPOs [10,41,42]. The main objectives of this paper are synthesis of Ceincorporated SAPO-34 and exploring the effect of template on physiochemical properties as well as MTO catalytic performance which have not been investigated yet. In fact, our investigation comprises of three parts: preparation, characterization and catalytic evaluation of the synthesized catalysts in methanol to olefins process. Physiochemical properties of the nanostructured catalysts were identified by XRD, FESEM, EDX, BET, FTIR and NH3TPD techniques. MTO catalytic performance tests were carried out to identify the influence of different temperatures on catalyst activity. Besides experimental section, overall reaction network for the formation of light olefins, dimethyl ether (DME) and methane in MTO process was developed. The Gibbs free energy of each reaction as a function of temperature ranging from 50 to 850 8C governed by genuine thermodynamic variables as enthalpy and entropy was calculated. Obtained results were compared to the experimental data to identify the effect of temperature on product distribution. 2. Thermodynamic analysis of MTO reactions

C3H8 C 4H10

H2 Etha ne formation

H2

C2 H 4

C3H 6 3H 2O

Butane fo rmation

H 2O

H2 Butane formation

C 4H10

ST  ¼ R

R

Z

(1)

C P  dT a2 T a3 T 2 a4 T 3 a5 T 4 a6 þ þ þ þ ¼ a1 þ 2 3 4 5 T RT

(2)

CP  a3 T 2 a4 T 3 a5 T 4 dT ¼ a1 ln T þ a2 T þ þ þ þ a7 RT 2 3 4

(3)

GT  H T  S T  ¼  RT RT R ¼ a1 ð1  ln TÞ þ

a2 T a3 T 2 a4 T 3 a5 T 4 a6    þ  a7 2 6 12 20 T

HT  ¼ DH f  ðTÞ þ

Z

C2 H 4

H 2O

C2 H 4

2CH3OH

2C3H6 3H 2O

C 4 H 8 4 H 2O

C3H 6

Olefin formation

CH3OH

CH3OH

Butene formation

DME formation

Propylene formation

C2 H 4

C3H6 H2O Propan e formation

(4)

It should be noted that the HT  is different to the HT   HT  ðre f Þ function, but it is defined as: T

C P  ðTÞdT

(5)

298

C3H 6 CH3OH H 2O

Olefin CH 3OH formation

C 4H 8

CP  ¼ a1 þ a2 T þ a3 T 2 þ a4 T 3 þ a5 T 4 R

2 H 2O

Propa ne for mation

H2

All the reaction sets were classified in Table 1 resulted in elucidation on reaction thermodynamics. Methanol and DME conversions to light olefins are considered to be favorable reactions in MTO process. Moreover, DME can follow the reforming reaction in the presence of water producing syngas which is going to enhance methane formation reactions. Produced light olefins can react to produce saturated hydrocarbons via hydrogen transfer reactions. Gibbs free energy as a function of temperature ranging from 50 to 850 8C was calculated for each reaction group with an emphasis on the expected industrial temperature range 300– 500 8C. For each reaction involved in the reaction network, the thermodynamic functions for specific heat, enthalpy, entropy and Gibbs free energy as a function of temperature are given in the form of least-square coefficients [43]:

So that HT  =RT, ST  =R and GT  =RT values calculated from the polynomial equations can be directly used to compute standard properties of species and reactions.

 Group A: Methanol to Light Olefins.  Group B: DME to Light Olefins.  Group C: DME Reforming Reactions.

C2 H 6

 Group D: Methane Formation Reactions.  Group E: Water-gas Shift, Carbon Formation and Consumption Reactions.  Group F: Saturated Hydrocarbons Formation Reactions.

HT  ¼ RT

Thermodynamic analysis which is of great importance can be the first step to realize the reaction network. It describes systems at equilibrium and provides a framework for determining which reactions will precede. The inhibition or promotion of reactions such as methane formation, DME reforming and methanol to light olefins, involved in MTO process, plays a central role in the ultimate performance of the reaction system. Whether the former or the latter is desired, it becomes necessary to understand the thermodynamics of the reaction network. Fig. 1 illustrates the proposed reaction network for the formation of light olefins, DME and methane in MTO process. In this reaction network, 6 groups of overall reactions are considered:

463

H2

C3H8

CO 2 WGS reaction

CH3OCH3

CH 3OCH3

H 2O

H 2O DME refor ming

2CO2 6H2

C(S)

4H 2

2 H 2O

carbon gasification

CO 3H 2 2CO 2H 2

CH4 H 2O CH4 CO2

Methane formation

Boudoua rd reaction

CH 4

H 2O

2CO 4H 2

DME refo rming 3H 2O

Methane formation

H2

CO CO2

H 2O

CO H2

Fig. 1. Reaction network for the formation of light olefins, DME and methane over nanostructured CeAPSO-T and CeAPSO-M catalysts.

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Table 1 Thermodynamic propertiesa of overall reactions in MTO process. NO Group (R1) (R2) (R3) (R4) (R5) Group (R6) (R7) (R8) (R9) (R10) Group (R11) (R12) Group (R13) (R14) (R15) Group (R16) (R17) (R18) Group (R19) (R20) (R21) a

Reaction

Description

A: methanol to light olefins 2CH3OH = C2H4 + 2H2O Methanol to ethylene 3CH3OH = C3H6 + 3H2O Methanol to propylene 4CH3OH = C4H8 + 4H2O Methanol to butylene CH3OH + C2H4 = C3H6 + H2O Methanol to propylene CH3OH + C3H6 = C4H8 + H2O Methanol to butylene B: DME to light olefins 2CH3OH = CH3OCH3 + H2O Methanol to DME CH3OCH3 = C2H4 + H2O DME to ethylene 2CH3OCH3 = C2H4 + 2CH3OH DME to ethylene 2CH3OHCH3 = C3H6 + CH3OH +H2O DME to propylene 3CH3OHCH3 = 2 C3H6 + 3H2O DME to propylene C: DME reforming reactions CH3OHCH3 + H2O = 2CO + 4H2 DME reforming to syn-gas CH3OHCH3 + 3H2O = 2CO2 + 6H2 DME reforming to CO2 and H2 D: methane formation reactions CO + 3H2 = CH4 + H2O Methane formation from syn-gas 2CO + 2H2 = CH4 + CO2 Methane formation from syn-gas CO2 + 4H2 = CH4 + 2H2O Methane formation from CO2 and H2 E: water-gas shift, carbon formation and consumption reactions CO + H2O = CO2 + H2 Water-gas shift 2CO = C(S) + CO2 Boudouard reaction H2O + C(S) = CO + H2 Decoking F: saturated hydrocarbons formation reactions C2H4 + H2 = C2H6 Ethane formation C3H6 + H2 = C3H8 Propane formation C4H8 + H2 = C4H10 Butane formation

DH8 (kJ mol1)

DG8 (kJ mol1)

DS8 (kJ mol1 K1)

28.98 101.72 163.44 72.73 61.72

64.22 133.16 194.75 68.94 61.59

0.12 0.10 0.10 201.25 0.00

22.57 6.42 16.15 56.59 135.74

16.19 48.02 31.83 100.77 217.73

0.02 0.14 0.16 0.15 0.27

204.97 688.65

66.41 584.98

0.46 0.35

206.29 247.45 165.14

141.84 170.42 113.26

0.21 0.26 0.17

41.15 172.49 131.34

28.58 119.78 91.20

0.04 0.18 0.13

136.36 124.35 132.71

100.09 88.97 91.29

0.12 0.12 0.14

The polynomial equations were employed to estimate thermodynamic properties using reliable thermodynamic data base from literature (T = 298 K) [43].

2.1. Methanol/DME conversion to light olefins Gibbs free energy of desired reactions in MTO process, methanol to light olefins (Group A in Table 1) and DME to light olefins (Group B in Table 1) were calculated as a function of temperature and shown in Fig. 2(a) and (b), respectively. It should be noted that reactants and the products are considered in their standard states. Calculation of Gibbs free energy change of the reactions is a valid criterion for determining the spontaneity. It has been found out that all the reactions can be thermodynamically feasible in all temperature range since their DG is negative. The more negative the values for Gibbs free energy, the further to the right the reaction will precede. Furthermore, it is obvious that formation of light olefins from DME, obtained from the dehydration of methanol, is more likely than direct conversion of the methanol. This can be addressed explained by the fact that reactions preferably proceed in the direction that lowers their free energy more. The mentioned result obtained from the thermodynamics analysis is in a good consistency with the reported kinetic results of the MTO process [8]. As mentioned earlier, the silicoaluminophosphate SAPO-34 was reported to be the most promising catalyst giving maximum light olefins selectivity of up to 80%. The average particle size of the typical synthesized SAPO-34 catalysts is usually more than one micro meter which provides a long intra crystalline diffusion length enough for the intermediate DME to react into light olefins [36,44]. Consequently, the most accepted procedure for MTO process using silicoaluminophosphate molecular sieves includes the following two steps. The first step is the dehydration of methanol to DME and water. The equilibrium mixture of methanol, DME and water is then converted to light olefins. Dehydration of methanol to DME can, in principle, occur spontaneously (reaction (R6) in Table 1). However, DME formation which is of great importance to proceed the MTO reaction is relatively beyond the bounds of possibility to occur in the absence of proper catalyst owing to the smaller magnitude of the change in the Gibbs free energy of this reaction.

Gibbs free energy calculations indicated that formation of propylene is more likely than ethylene comparing the spontaneity feature of the reactions in each group of the desired reactions (Groups A and B in Table 1). Thus, thermodynamics analysis shows that in the absence of proper catalyst, the propylene selectivity is going to be higher than the ethylene selectivity. However, relative value of the ethylene to propylene ratio is going to be determined by the structure, acidity and pore sizes of the applied catalyst. 2.2. DME reforming reactions Influence of temperature on the Gibbs free energy of side reactions in MTO process is depicted in Fig. 3 (Groups C–F in Table 1). At expected industrial range, DME reforming can take place to produce syngas rather than producing the mixture of H2 and CO2 gas considering their Gibbs free energy sign as shown in Fig. 3(a) (Group C in Table 1). Syngas production from DME reforming reaction can promote with increasing the temperature with reference to the fact that reactions proceed in the direction that lowers their Gibbs free energy more. 2.3. Methane formation reactions Fig. 3(b) presents the dependence of the Gibbs free energy of the methane formation reactions in MTO process on temperature (Group D in Table 1). Methane is one of the undesired products of the MTO reaction which can be obtained from the reaction between H2 and CO. It indicated that higher temperature is favor to decrease the amount of methane in the product stream from thermodynamic points of view which is in contrast with the reported experimental results of the MTO reaction [31]. This can be addressed by kinetic considerations. 2.4. Water-gas shift, carbon formation and consumption reactions Gibbs free energy of water-gas shift, carbon formation and consumption reactions in MTO process at the mentioned

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465

Fig. 2. Gibbs free energy of desired reactions in MTO: (a) methanol to light olefins and (b) DME to light olefins.

temperature range was studied as illustrated in Fig. 3(c) (Group E in Table 1). Decoking reaction can be beneficial for avoiding the dramatic effect of coke on catalysts in MTO process. However, processes characterized by the positive Gibbs free energy cannot occur practically. Consequently, occurrence of decoking reaction is not possible at expected industrial range. 2.5. Saturated hydrocarbons formation reactions Effect of temperature on Gibbs free energy of the saturated hydrocarbons formation was investigated in a distinct group as shown in Fig. 3(d) (Group F in Table 1). Thermodynamic study demonstrates that saturated hydrocarbons can produce from

hydrogen transfer reactions at expected industrial range. However, it does not tell us how fast the reactions will precede. 3. Materials and methods 3.1. Materials In a typical synthesis to get pure CeAPSO-34 using TEAOH template (denoted as CeAPSO-T), aluminum isopropoxide (Aldrich, 98+%), fumed silica (Aldrich, 99.9%), cerium nitrate hex hydrate (Merck) and phosphoric acid (Merck, 85%) were used as the sources of aluminum, silicon, cerium and phosphorus, respectively. For the synthesis of CeAPSO-34 with morpholine (denoted as CeAPSO-M)

466

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Fig. 3. Gibbs free energy of side reactions in MTO: (a) DME reforming reactions, (b) methane formation reactions, (c) water-gas shift, carbon formation and consumption reactions and (d) saturated hydrocarbons formation reactions.

different sources of silicon and aluminum were applied. Silicic acid (Merck, 98%) and g-Al2O3 (Merck) were used as starting materials for Si and Al respectively. All of the materials were used as received without any further treatments.

for 24 h, followed by calcination at 500 8C for 6 h. The obtained powder was formed into cylindrical species diminishing pressure drop in a fixed-bed reactor. 3.3. Catalyst characterization techniques

3.2. Catalyst preparation and procedures Nanostructured CeAPSO-34 catalysts were synthesized by a hydrothermal method using TEAOH and morpholine as organic templates. The chemical composition of synthetic gel was 2SDA: 0.3SiO2:1P2O5:0.006CeO2:50H2O. For the successful synthesis of MeAPSO-34, appropriate order of mixing and suitable precursors for each of samples was employed. In detail, weighted aluminum isopropoxide and TEAOH were dissolved in distilled water under stirring for 1.5 h, and fumed silica and cerium nitrate were added in turn. Phosphoric acid aqueous solution was added to the solution under stirring by a drop-wise addition for 3 h. The resulting gel was transferred into autoclave and heated at 200 8C for 56 h. The solid product was recovered by filtration, washed several times with distilled water and dried overnight at 110 8C. Finally, the catalyst sample was calcined at 500 8C for 12 h to remove organic template and trapped water within the micro pores of the as-synthesized solid. Moreover, respective gel for the synthesis of catalyst with morpholine template was prepared by mixing g-Al2O3, phosphoric acid, silicic acid, cerium nitrate and morpholine. The precursor gel was transferred to a stainless-steel autoclave and heated for 72 h at 200 8C under autogenous pressure. The product was filtered, washed and dried at 110 8C

X-ray diffraction patterns (XRD) were recorded on a Bruker D8 Advance diffractometer with Cu Ka 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 crystallite size was calculated using the half-width at half-height of most intense peaks of diffraction pattern and well-known Debye–Scherrer equation. The morphology of the catalyst was investigated using Field Emission Scanning Electron Microscopy (FESEM), HITACHI S-4160. The samples studied were covered with a thin film of gold (ion sputtering) to improve conductivity. The FESEM is equipped with an Energy Dispersive X-ray (EDX) analyser for dot maps were conducted for dispersion analysis. Catalysts acidity was measured by ammonia temperature programmed desorption using BELCAT analyser with a TCD detector. Before analysis, 0.1 g of calcined sample was preheated at 550 8C for 60 min under a 50 cm3/min helium gas flow. Ammonia adsorption was made from a mixture of 5% (molar basis) of ammonia in helium under total flow rate of 50 cm3/min at 100 8C. After adsorption of ammonia, the samples were kept under a helium gas flow at 100 8C to remove physically adsorbed gases. Finally, the helium flow (50 cm3/min) was passed through the sample with increasing temperature up to 800 8C at a

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Fig. 4. Schematic flow chart of experimental setup for activity test of CeAPSO-T and CeAPSO-M catalysts toward methanol to olefins.

3.4. Experimental setup for catalytic performance test The schematic flow diagram of experimental setup consists of a gas feeding section, a fixed bed reactor, and an analytical section (Fig. 4). For producing the gas stream containing N2, CH3OH and H2O a saturator was utilized. The saturator container has an inlet pathway for carrier gas N2. For producing a reaction gas stream with a desired concentration of methanol, N2 with a constant flow rate of 70 ml/min entered the saturator which was composed of 10 mol% methanol and 90 mol% H2O. It was allowed to flow with a gaseous space velocity of 4200 cm3/g h. The reaction was carried out in a U-shape Pyrex fixed bed reactor heated by a temperature controlled electric furnace. Experiments were performed using 1 g of CeAPSO-34 at 300–500 8C and atmospheric pressure. Reactions were carried out to identify the effect of reaction temperature on the catalyst activity. In addition, stability test was carried out at constant feed composition, gas hourly space velocity (GHSV) and temperature. The gas compositions of the reactants and products were analyzed using a gas chromatograph (GC Chrom, Teif Gostar Faraz, Iran) equipped with Plot-U column (Agilent) and FID detector. High purity hydrogen and compressed air were supplied as fuel gases. Argon was used as a carrier gas for gas chromatograph.

pattern corresponding to CeO2 phase was observed in XRD record of catalysts due to the low amount of Ce-loading. Thermal stability under calcination conditions can be confirmed by the observation of all characteristic peaks of SAPO-34 phase. The XRD diffraction peaks of CeAPSO-34 shift to lower angles which can be ascribed to an increase in the interplanar spacing (considering Bragg’s law) and thus lattice parameter as a result of Ce incorporation obtaining from the comparison of the XRD pattern of each catalyst with the reference SAPO-34. Table 2 presents the structural properties of nanostructured CeAPSO-T and CeAPSO-M catalysts. Qualitatively, the diffraction peaks were very sharp and intense for CeAPSO-M but were weak for CeAPSO-T. Quantitatively, relative crystallinity SAPO-34

(a) CeAPSO-T (b) CeAPSO-M

(a)

Intensity (a.u.)

rate of 10 8C/min. Infrared analysis of the catalyst was carried out by a UNICAM 4600 FTIR spectroscopy addressing surface functional groups.

(1 0 1) (1 2 -1) (4 0 1) (1 1 0) (1 0 4) (0 0 3) (0 2 1) (2 2 0)

(b)

(0 1 8)

(1 4 0)

4. Results and discussions Reference patterns

4.1. Catalyst characterizations 4.1.1. XRD analysis Fig. 5 shows the X-ray diffraction patterns of CeAPSO-34 samples prepared by two kinds of templates. A detailed examination of diffraction patterns reveals the formation of SAPO-34 rhombohedra structure (JCPDS 01-087-1527) as indicated by diffraction peak at 2u = 9.4, 12.9, 20.5 and 30.5 without any presence of impurity phase for both of the catalysts. No clear

SAPO-34 (Rhombohedral, 01-087-1527)

5

10

15

20

25

30

35

40

45

50

55

60

65

70

2 (degree) Fig. 5. XRD patterns of nanostructured CeAPSO catalysts: (a) CeAPSO-T and (b) CeAPSO-M.

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Table 2 Structural properties of nanostructured CeAPSO-T and CeAPSO-M catalysts. Catalyst

Template

SBET (m2/g)

Relative crystallinitya APSO

Crystallite phaseb APSO

CeAPSO-T CeAPSO-M

TEAOH Morpholine

493 434

54.31 100

Rhombohedral Rhombohedral

a b

Relative crystallinity: peak area of XRD pattern. JCPDS reference number: 01-087-1527.

of CeAPSO-T and CeAPSO-M sample was calculated to be 50.3 and 100%, respectively. This means that the CeAPSO-M was found to be more crystalline and therefore its relative crystallinity was set to 100%. 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. 4.1.2. FESEM analysis Fig. 6 illustrates FESEM images of nanostructured CeAPSO-34 catalysts prepared with TEAOH and morpholine as template. In the case of sample CeAPSO-M, synthesized with morpholine, the typical cubic like rhombohedra morphology can be simply identified, which is quite similar to natural chabazite. However, sample CeAPSO-T, synthesized with TEAOH presents a quite different particle shape and morphology. In comparison, particles with larger sizes obtained by applying morpholine. SEM study confirms that morphology of the final product is dependent on the nature of template used during synthesis. The existence of large grains of CeAPSO-M sample may be the reason for its sharp XRDline in Fig. 5. Particle size distribution histogram of nanostructured CeAPSO34 synthesized with two types of templates is shown in Fig. 7. Quantitative investigation of FESEM images was done by ImageJ software [45]. The catalyst particle size for CeAPSO-T is in the range of 0.18–0.96 mm with an average particle size of about 0.37 mm. CeAPSO-M exhibits lager particle sizes in the range of 8.27–

39.36 mm with an average particle size of about 19.14 mm. As can be seen, by employing different templates, distinct difference in particle sizes was observed. Thus, the effect of particle size on their catalytic performance can be explored. 4.1.3. EDX dot-mapping analysis Fig. 8 shows EDX dot-mappings of Al, P, Si, O and Ce for the CeAPSO-M catalyst. Positions and concentrations of different elements can be located with multi-elemental X-ray dot mapping analysis. It acquires information about all elements in a specimen. Presence of Ce heteroatoms in the corresponding dot-mapping proves the successful synthesis of SAPO-34 with metal addition. 4.1.4. BET analysis The total specific surface area was measured on apparatus using N2 adsorption–desorption at 77 K and the results are shown in Table 2. Small amount of the synthesized catalyst was placed in the sample cell. After degassing step, N2 physisorption was carried out for measuring surface area. The Brunauer–Emmett–Teller (BET) equation was used to calculate specific surface area, SBET. Surface area was measured 434 and 493 m2/g for CeAPSO-M and CeAPSO-T samples, respectively. High specific surface area is one the significant factors which can influence the catalyst activity. Using TEAOH seems to be responsible for the obtained relative high specific surface area. This observation is in excellent agreement with FESEM analysis which is previously presented, confirming

Fig. 6. FESEM images of nanostructured CeAPSO catalysts: (a) CeAPSO-T and (b) CeAPSO-M.

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Fig. 7. Particle size histogram of nanostructured CeAPSO catalysts: (a) CeAPSO-T and (b) CeAPSO-M.

that smaller particles of the catalyst would result in higher specific surface area. Higher specific surface area and appropriate particle size of SAPO-34 must be balanced together in MTO reaction. 4.1.5. FTIR analysis Fig. 9 presents FTIR spectrums of samples. Framework vibrations were characteristically similar to relevant reports. In the initial gel, the Si, P, and Al species are in hydration states, which caused hydroxyl vibrations that could be observed in the IR spectra. Thus, the spectrums of the synthesized catalysts showed intense peaks of stretching vibration of structural O–H at 3450 cm1 which can be the integration of three different types of structural hydroxyl groups capable of existing in zeolites: (i) bridging hydroxyl groups which can act as Brønsted acid sites, protons associated with negatively charged framework oxygen linked to adjacent Al and Si atoms, i.e. Si–OH–Al bridges, (ii) internal Si–OH and P–OH defects generated by hydrolysis of Si–O– Si and P–O–Al linkages, missing framework Si and Al, (iii) terminal OH groups that are always present at the external surface of the zeolite crystals like Si–OH and P–OH [46]. It was stated that various hydroxyl groups exhibit different acid strength [47]. Moreover, bridging hydroxyl groups, i.e. Si–OH–Al, were reported to play active sites role for the MTO process [48]. It is worth noting that the IR technique is limited by the drawback in only capable of providing weak and qualitative information about hydroxyl groups playing active sites role in the catalytic processes. Thus, we are going to continue the study of hydroxyl groups with NH3-TPD which can provide additional quantitative information in the

following section. Peaks at wave numbers about 1100, 840, 640, and 470 cm1 are assigned to the T–O–T symmetric stretching, protonated template, T–O bending in D-6 rings and T–O bending of Si tetrahedral [18,20,30]. Peak at wave number about 2450 cm1 can be related to absorb CO2 from the atmosphere. Stretching vibration around 1650 and 1400 cm1 can be attributed to physically adsorbed water. Generally, the protonated template for charge balance is more difficult to be removed than neutral template occulted in the channel [32]. Thus, presence of protonated template peak in the FTIR spectrums of samples can be explained by the strong template-framework interaction which can be the result of metal substitution (Ce) into SAPO-34 framework [32]. In order to investigate the effect of template on framework functional groups, a detailed comparison between the peaks at wave number 840 cm1 and 3450 cm1 of the synthesized samples was made. Neutral template removal is associated with OH generation shown by the 3450 cm1 peak is more intense for CeAPSO-T sample while 840 cm1 peak ascribed to protonated template is more intense for the CeAPSO-M sample. 4.1.6. NH3-TPD analysis Acidity plays a central role in the catalytic performance of solid acid catalysts. The TPD profiles of ammonia desorption on CeAPSO34 catalysts are presented in Fig. 10 to determine the concentration and strength of acid sites. Two desorption peaks are clearly observed for both of the synthesized samples. The stronger acid sites needs a higher desorption temperature resulting in higher values of temperature maxima in TPD patterns. In addition, the

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470

Fig. 8. EDX dot-mapping analysis of nanostructured CeAPSO catalysts: CeAPSO-M.

470

640

840

1100

1400

1650

3450

2450

area underlying the curves indicates the amount of ammonia desorbed which can be a representative of acid sites concentration. The first peak, assigning to weak acid sites, are related to the P–OH groups not fully linked to AlO4 tetrahedra. It is reported that weak acid sites are useless for the conversion of methanol to light olefins,

Transmittance (a.u.)

(a) CeAPSO-T (b) CeAPSO-M

4000 400

(a)

(b)

3500 900

3000 1400

25 000 190

2000 240 0

150 0 2900

1000 3400

5000 390

Wavenumber (cm -1) Fig. 9. FTIR spectrum of nanostructured CeAPSO catalysts: (a) CeAPSO-T and (b) CeAPSO-M.

but they are useful for the dehydration of methanol to DME which can be catalyzed over catalysts with lower acidity, such as g-Al2O3 [37]. The second desorption peak is attributed to the bridging hydroxyl group as a strong Brønsted acid site generated by the incorporation of silicon into the framework of the SAPO molecular sieve [49]. There is a meaningful difference in the strength of both strong and weak acid sites for CeAPSO-34 catalysts synthesized with TEAOH and morpholine template. CeAPSO-M sample showed higher desorption temperature of both weak and strong acid sites. In the case of CeAPSO-T sample, the second desorption peak temperature is about 295 8C which can be ascribed to moderately strong acid sites. The area under the low and high temperature desorption peak is calculated to determine the concentration of weak and strong acid sites respectively. It indicated that the catalyst synthesized with TEAOH as a template owns more weak acid sites and higher concentration of strong acid sites is achieved for the CeAPSO-M. Total acidity is calculated to be 1.3 and 1.13 mmol/g for the CeAPSO-M and CeAPSO-T respectively. The mentioned values for the total acidity provide additional support for the obtained result from XRD analysis discussed before. It was found that acidity increases with increasing crystallinity due to proton-donor centers [50]. Thus, higher concentration of acid sites for CeAPSO-M is predictable from its higher crystallinity. In conclusion, catalyst prepared with morpholine template seems to be a favorable MTO catalyst in terms of acidity but it suffers from its larger particle sizes bringing longer diffusion length. Thus, changing the structure directing agent during the synthesis

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3000 Zone-I

Zone-II

Zone-II

2500

TPD signal (uv)

0.44 mmolNH3/g 2000

0.70 mmolNH3/g

1500

150ºC 295ºC

1000

500 (a) CeAPSO-T 0 50

100

150

200

250

300

350

400

450

500

550

600

650

Temperature (°C) 7000 Zone-II

Zone-I

6000

(b) CeAPSO-M

Zone-II

1.17 mmolNH3/g

5000

TPD signal (uv)

397ºC

4000 3000 2000 1000

190ºC

0.13 mmolNH3/g

0 50

100

150

200

250 300

350

400 450

500

550

600 650

Temperature (°C) Fig. 10. NH3-TPD analysis of nanostructured CeAPSO catalysts: (a) CeAPSO-T and (b) CeAPSO-M.

procedure can be an effective way to investigate the effect of both acidity and particle size of the catalyst used in the MTO reaction.

4.2.1. CH3OH conversion The catalytic activity of the synthesized catalysts was studied at temperatures ranging from 300 to 500 8C and shown in Fig. 11. In this section all reactions were carried out at constant feed composition and gas hourly space velocity (GHSV = 4200 cm3/ g h). It indicates that higher temperature is favor to methanol conversion. The highest conversion of CH3OH obtained at 350 and 400 8C for sample CeAPSO-T and CeAPSO-M respectively. Catalyst activity based on CH3OH conversion is an acceptable value for CeAPSO-T sample even at lower temperatures which can be attributed to the result given from FESEM analysis that CeAPSO-T sample has smaller particle size providing shorter diffusion length. Gaseous molecules diffusivity coefficient is considered a function

100

CH3OH Conversion (%)

4.2. Catalytic performance study toward methanol conversion to light olefins

CeAPSO-T

CeAPSO-M 100

94.8

100

100

100

100

100

100

94.6

80 60 49.6

40 20 0

300

350

400

450

500

Temperature (°C) Fig. 11. CH3OH conversion vs. temperature over nanostructured CeAPSO-T and CeAPSO-M catalysts.

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(a) CeAPSO-T CH4

C2H4

C2H6

C3H6

C3H8

DME

C4+

Temperature (°C)

500

450

400

350

300 0

10

20

30

40

50

60

70

80

90

100

Selectivity (%)

(b) CeAPSO-M CH4

C2H4

C2H6

C3H6

C3H8

DME

C4+

Temperature (°C)

500

450

400

350

300 0

10

20

30

40

50

60

70

80

90

100

Selectivity (%) Fig. 12. Product selectivities vs. temperature over nanostructured CeAPSO catalysts: (a) CeAPSO-T and (b) CeAPSO-M.

of temperature. Therefore, in the case of the CeAPSO-M catalyst, methanol gaseous molecules are not capable of reaching the internal acid sites situated inside the micro pores at such lower temperatures and they convert to hydrocarbons at the external surface acid sites. The achieved results of methanol conversion are in good consistency with the thermodynamic analysis in which it is concluded that higher temperature favor methanol conversion considering the variation of DG of reactions (R1)–(R10) (Groups A and B in Table 1).

2CH3 OCH3 ¼ C2 H4 þ 2CH3 OH

(R8)

2CH3 OHCH3 ¼ C3 H6 þ CH3 OH þ H2 O

(R9)

3CH3 OHCH3 ¼ 2 C3 H6 þ 3H2 O

(R10)

4.2.2. Product selectivity Fig. 12 presents the product distribution over nanostructured CeAPSO-T (a) and CeAPSO-M (b) catalysts as function of reaction temperature. The measurements of reaction products correspond to an acceptable time on stream at each temperature to achieve steady state conditions. The disadvantageous side-effect of increasing temperature is the dramatic increase in the amount of methane formed in the products stream. A possible reaction mechanism leading to methane is as follows according to Table 1:

2CH3 OH ¼ C2 H4 þ 2H2 O

(R1)

3CH3 OH ¼ C3 H6 þ 3H2 O

(R2)

4CH3 OH ¼ C4 H8 þ 4H2 O

(R3)

CH3 OH þ C2 H4 ¼ C3 H6 þ H2 O

(R4)

2CH3 OH ! CH3 OCH3 þ H2 O

CH3 OH þ C3 H6 ¼ C4 H8 þ H2 O

(R5)

CH3 OCH3 þ H2 O ! 2CO þ 4H2

(R11)

2CH3 OH ¼ CH3 OCH3 þ H2 O

(R6)

2CO þ 2H2 ! CO2 þ CH4

(R14)

CH3 OCH3 ¼ C2 H4 þ H2 O

(R7)

As presented, thermal decomposition of DME to methane and carbon dioxide via reactions R11 and R14 is thermodynamically

(R6)

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100

(a) CeAPSO-T

Conversion/Selectivity (%)

80

60

40 CH3OH C2H4

20

C3H6 DME Olefines

0 0

200

400 600 800 Time on stream (min)

1000

1200

100

(b) CeAPSO-M

Conversion/Selectivity (%)

80

60

40 CH3OH C2H4

20

C3H6 DME Olefines

0 0

200

400

600

800

1000

1200

1400

Time on stream (min) Fig. 13. Stability test of nanostructured CeSAPO-34 catalysts: (a) CEAPSO-T and (b) CEAPSO-M.

possible. From thermodynamic points of view, it was previously noted that methane formation reactions will suppress by increasing temperature which is in contrast with the obtained experimental results. Methanol reacts with the surface to form surface methoxy (ZOCH3), and this surface methoxy reacts with MeOH to form higher hydrocarbons such as DME as summarized below [51]:

CH3 OH þ ZOH ! ZOCH3 þ H2 O ZOCH3 þ CH3 OH ! ZOH þ CH3 OCH3 At such higher temperatures, the formation of bulkier hydrocarbons would be enhanced which provides hydrogen donor sites to obtain methane from methoxy groups. Resultantly, higher percentage of methane in the products stream is achieved. Furthermore, at such higher temperatures reactant molecules can efficiently diffuse into the micro pores of zeolite and higher capacity of the catalyst would be used to convert methanol molecules to methoxy groups. Consequently, at constant residence time and feed composition, increasing temperature would result in lack of methanol molecules to convert methoxy groups to higher hydrocarbons and transformation of mentioned groups to

methane will occur more. From the obtained results, it is obvious that methane selectivity is higher for the catalyst synthesized with TEAOH as SDA. For instance, the difference in methane selectivity at 450 8C between the two samples is more than 50%. One of the main contributors of methane formation is the presence of extra framework metal oxides which promotes the reactions mentioned above [31]. As discussed before, organic template has a clear effect on degree of metal incorporation. Ethylene selectivity reaches a maximum at 400 8C for sample CeAPSO-T and CeAPSO-M and then decreases at higher temperatures due to increasing methane formation. For all the examined temperatures except 350 8C, the value of C2H4 selectivity for the catalyst synthesized with morpholine is higher than the mentioned value for the catalyst templated with TEAOH expected due to the result obtained from TPD characterization having higher amount of strong acid sites. Specifically, as the temperature increases from 300 to 500 8C the C3H6 selectivity decreases which can be the result of simultaneous occurrence of methane formation and propylene conversion to ethylene reactions at higher temperatures. It indicated that lower temperature is favor to propylene production. In summary, as the temperature increases from 300 to 500 8C, light olefins selectivity decreases for both of the

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Table 3 Different SAPO-34 catalysts evaluated in MTO reaction. Catalyst

Template

Feed

T (8C)

Methanol conversion (%)

Olefins selectivity (%)

Life time (min)

Ref.

SAPO-34 SAPO-34 SAPO-34 SAPO-34 SAPO-34 CeAPSO-34 CeAPSO-34

TEAOH TEAOH TEAOH TEAOH TEAOH Morpholine TEAOH

CH3OH/N2 = 1/10 (molar ratio) CH3OH/He = 1/5 (molar ratio) CH3OH/N2 = 15/85 (molar ratio) – CH3OH/N2 = 1/1 (molar ratio) CH3OH/H2O = 3/7 (molar ratio) CH3OH/H2O = 3/7 (molar ratio)

425 450 425 400 400 400 400

100 100 100 100 100 100 100

82 89 40 90 90 82 52

260 140 360 720 300 590 540

[37] [36] [33] [31] [22] Present Study Present Study

catalysts. It reaches a maximum value of 89% for the sample CeAPSO-T at 400 8C and a maximum value of 84% for the sample CeAPSO-M at 400 8C. Methane formation is dominant for both of the synthesized catalysts at 500 8C. After simultaneous consideration of the highest CH3OH conversion and light olefins selectivity for both of the catalysts, 400 8C was selected to investigate the effect of time on steam. 4.2.3. Stability test The time-dependant conversion of CH3OH and products selectivity at constant temperature, feed composition and GHSV are plotted in Fig. 13. Fig. 13(a) shows the distribution of major products with time on stream (TOS) for the catalyst prepared with TEAOH template. Methanol conversion is relatively stable at 100% for the first 9 h TOS while olefins selectivity drops from 87 to 52%. The catalysts lifetime is defined as the reaction time sustaining catalyst activity until intermediate product DME exists in the products. CeAPSO-34 synthesized with TEAOH as SDA exhibits relative longer lifetime (9 h) compared to the reported SAPO-34s in literature [21,23,37,48]. After 9 h TOS, catalyst deactivates and the conversion decreases to 89% at reaction time 13 h. The primary reason for the deactivation of the catalyst is ascribed to coke formation which blocks pores and poisons acid sites. Weak acid sites are useless for olefins production, but conversion of methanol to DME can occur on weak acid sites [37]. Thus, after deactivation when most strong and moderate acid sites were blocked by coke formation, the selectivity of light olefins decreased rapidly to 22% at reaction time 13 h, with simultaneously increasing of DME in the product stream. SAPO-34 has CHA cage (7.5 A˚  8.2 A˚) enough to accommodate coke or coke precursor [49]. Relative high methane formation observed for CeAPSO-T at 400 8C can be another reason for the decreased olefins selectivity. Fig. 13(b) shows the distribution of major products with TOS for the synthesized catalyst with morpholine. According to defined statement for lifetime, catalyst CeAPSO-M shows longer lifetime (9.8 h) compared to sample CeAPSO-T as well as evaluated ones in literature [23,30,48]. The selectivity of light olefins was maintained over 82% until catalytic activity abruptly decreased by deactivation of catalyst. Table 3 illustrates different SAPO-34 catalysts evaluated in MTO reaction. CH3OH conversion and olefins selectivity is a proper criteria to compare the results obtained to that of investigated in literature to ascertain the effect of Ce introduction. Catalyst life time defined as the time the intermediate product DME exists in the effluent gas. As shown, the Ce incorporated catalysts demonstrates excellent performance in comparison to those SAPO-34s reported in literature. Initial selectivity seems to be similar for both catalysts, and it is independent of acid sites density and particle size. However, deactivation rates are critically dependent on the mentioned properties of the molecular sieves. MTO process can define as a driving forward sequence of methanol dehydration and carbon– carbon bond formation reactions. Moreover, formation of the desired products (i.e. light olefins) occurs on the strong acid sites.

Selective deactivation of the corresponding acid sites happens at a slower rate over the CeAPSO-M than that over CeAPSO-T catalyst due to the higher concentration of strong acid sites. There is a significant difference in light olefins selectivity between the two synthesized catalysts at the onset of deactivation, reported 52% for CeAPSO-T and 82% for CeAPSO-M. Lower amounts of main products (i.e. light olefins) observed for sample CeAPSO-T can be addressed by incomplete incorporation of Ce compared to CeAPSO-M catalyst which was claimed by characterization techniques. More Ce remained as the extra framework cation in the case of sample CeAPSO-T promotes methane formation resulting in decrease of light olefins selectivity [22,31,33,36,37]. 5. Conclusions Catalyst lifetime increasing is one of the most significant challenges to MTO industrialization which was improved by introduction of Ce into the SAPO-34 framework. It indicated that the nature of the template determines the morphology of final product due to different rate of crystal growth. FESEM analysis indicated that sample synthesized with TEAOH has smaller particle size providing shorter diffusion length for the reactant molecules. In addition, the catalyst templated with morpholine exhibits more strong acid sites. It appears from results that neither small nor large particle size is favorable for catalysts to prolong lifetime. Optimal particle size is necessary for the MTO reaction. The catalyst obtained by using morpholine template showed longer lifetime as well as sustaining light olefins selectivity at higher values (about 82% after 9.8 h TOS). Acknowledgements The authors gratefully acknowledge Sahand University of Technology for the financial support of the research and Iran Nanotechnology Initiative Council for complementary financial supports. References [1] D. Zhang, X. Li, S. Liu, S. Huang, X. Zhu, F. Chen, S. Xie, L. Xu, Applied Catalysis A: General 439–440 (2012) 171–178. [2] S. Kamijo, S. Yokosaka, M. Inoue, Tetrahedron 68 (2012) 5290–5296. [3] N.A.M. Razali, K.T. Lee, S. Bhatia, A.R. Mohamed, Renewable and Sustainable Energy Reviews 16 (2012) 4951–4964. [4] W. Guo, W. Xiao, M. Luo, Chemical Engineering Journal 207–208 (2012) 734–745. [5] M. Menges, B. Kraushaar-Czarnetzki, Microporous and Mesoporous Materials 164 (2012) 172–181. [6] J. Zhang, H. Zhang, X. Yang, Z. Huang, W. Cao, Journal of Natural Gas Chemistry 20 (2011) 266–270. [7] N. Fatourehchi, M. Sohrabi, S.J. Royaee, S.M. Mirarefin, Chemical Engineering Research and Design 89 (2011) 811–816. [8] D. Chen, K. Moljord, T. Fuglerud, A. Holmen, Microporous and Mesoporous Materials 29 (1999) 191–203. [9] S. Kulprathipanja, Zeolites in Industrial Separation and Catalysis, Wiley-VCH Verlag GmbH & Co, KGaA, 2010. [10] K. Simmance. The design and understanding of the mechanism of formation of nanoporous catalytic materials, University College, London. PhD Thesis, 2011, p. 326.

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