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The beneficial use of ultrasound in rapid-synthesis of SAPO34/ ZSM-5 nanocomposite and its catalytic performances on MTO reaction Eshagh Moradiyan, Rouein Halladj, and Sima Askari Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03772 • Publication Date (Web): 15 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018

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Industrial & Engineering Chemistry Research

The beneficial use of ultrasound in rapid-synthesis of SAPO34/ZSM-5 nanocomposite and its catalytic performances on MTO reaction Eshagh Moradiyan1, Rouein Halladj∗1, and Sima Askari2 1

Faculty of Chemical Engineering, Amirkabir University of Technology, Tehran Polytechnic,

P.O. Box 15875-4413, Hafez Ave., Tehran, Iran. Fax: +98 2166405847; Tel: +98 2164543151 2

Department of Chemical Engineering, Science and research branch, Islamic azad university,

Tehran, Iran. Corresponding author: * Rouein Halladj Professor, Faculty of Chemical Engineering Amirkabir University of Technology e-mail: [email protected] Phone: +98 2164543151 Fax: +98 2166405847

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: SAPO-34 is a high selective and hydrothermal stable for MTO reaction but is rapidly deactivated by the formation of coke through its micropores. The promising SAPO-34/ZSM-5 nanocomposite catalyst with different ratio was synthesized by the ultrasonic-assisted hydrothermal method and obtained products characterized to further investigate their catalytic performance. Utilizing of ultrasound method lead to faster synthesis time, decrease of crystallite size, increase of surface area and dominant mesopore structure. The physicochemical properties of the product were extensively investigated by XRD, FESEM, TEM, FT-IR, N2 adsorptiondesorption techniques, and NH3-TPD. Results indicate composite had high conversion and selectivity compared to the original ZSM-5 and SAPO-34. Composite structures due to synergic effect between ZSM-5 and SAPO-34 where acted as a promoted catalyst improved catalytic properties of composite catalysts. Moreover, SAPO-34/ZSM-5 nanocomposite catalyst with 50% ratio synthesized by ultrasonic-assisted hydrothermal method (U-S/Z (50%)) showed 100% conversion and 90% selectivity to light olefin at 450˚C. Key-words: Methanol, Light olefins, SAPO-34/ZSM-5, nanocomposite catalysts, ultrasonicassisted hydrothermal method


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1. Introduction Over the last couple of years, thanks to the instability of crude oil price in international market, methanol to olefins (MTO) process has been attracted a great deal of attention as an alternative naphtha cracking process to produce light olefins such as ethylene, propylene and butene

1-3

.

Generally, the molecular sieves of 8 to 12 membered ring are utilized as the catalysts in this process. Among various zeolitic structures, SAPO-34 as well as ZSM-5 offers prominent advantages over other choices. Attractive characteristics such as a micro-pores and suitable acidic strength distinguish SAPO-34 from its counterparts 4-6. Possessing high hydrothermal stability along with high selectivity has made SAPO-34 the most widely used catalyst in the MTO process. Nevertheless, it is noteworthy to mention that soon after the formation of coke in its micro pores, it is deactivated and lost its efficiency as a consequence 7, 8. Therefore, their application is strictly limited due in particular to the short lifetime in industry. ZSM-5, on the other hand, demonstrates much better coking resistant properties, resulting in a longer catalyst life compared to SAPO-34. In addition, just like SAPO34, it benefits from excellent hydrothermal stability. Despite its unique properties, it suffers from the lack of a good selectivity to light olefin 7. Operating temperatures and namely crystal size were two most important parameters in terms of the design of SAPO-34 to achieve high activity and selectivity. Several studies investigated kinetic model to predict the behavior of SAPO-34 and ZSM-5 catalyst with respect to reaction mechanism. X. Yuan et al. were experimentally investigated MTO kinetics over SAPO-34 in fixed bed reactor and fluidized bed reactor 9-12. Many researchers have attempted to improved selectivity and activity for each SAPO-34 and ZSM-5 catalysts by physical-chemical modifications to produce ethylene and propylene. For this purpose structure directing agent, initial components, and synthesis conditions are changed 3 ACS Paragon Plus Environment

13-15

.


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However, in order to compensate the deficiencies of each aforementioned cases, the other approach was the combination of these two Zeolitic structures. It was shown that zeolite composite with binary structure has none of the disadvantages of its components while it exhibits special catalytic properties due to different functionalities and pore structures of each individual component and leads to a synergistic behavior in most cases

7, 16-18

. In this regard, the

nanocomposites based on ZSM-5 and SAPO-34 have been synthesized through hydrothermal method and studied in propane dehydrogenation process (ZSM-5/SAPO-34 and SAPO-34/ZSM5)

19

, benzene oxidation (Pd/ZSM-5/MCM-48)

20

, ethanol conversion to propylene (HZSM-

5/SAPO-34) 21, 22, methanol to aromatics 23, and methanol conversion to olefins (ZSM-5/SAPO34)

7, 17, 24, 25

. However, the major deficiency of conventional hydrothermal method in

crystallization of zeolites is difficulty in the control of the nucleation process. It is to say, lack of proper mixing in hydrothermal method usually cause an unstable supersaturation which in turn leads to uncontrollable nucleation

26

. Although using agitators which leads to the motion of

macroscopic layers could alleviate the issue in some extent, mixing inside the layers totally depends on the diffusion rate of reactant molecules and finally results in larger particle size. Moreover, another notable issue with this method is the long crystallization time which needs at least 1 day to complete 27. For the preparation of nanocomposite materials additional component are widely use as the substrate for phase growth. The main role of this matrix is the enhancement of reactants and products intraparticle transportation 28, 29. Impacts of nanocatalysts dispersions and particle size on the performance have been studied. It was shown that sonochemical technique results in better performance with fewer expenses 30, 31

13, 27,

. The slow chemical reaction is a major obstacle for promoting the optimization of the

fabrication parameters in materials processing technology. In order to overcome long reaction


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time, the ultrasonic-assisted hydrothermal method is proposed. Moreover, the use of sonochemical technique causes homogenous dispersion of the components in the main matrix. This study aimed at utilizing ultrasonic radiation along with hydrothermal reaction for promoting the synthesis of SAPO34/ZSM-5 composite. Ultrasonic irradiation leads to the acceleration of liquid phase reactions and creates a special reaction field for the preparation of various materials 27, 32

. These accelerating effects are mainly because of the acoustic cavitation phenomenon27. As

a rough estimation, formation, growth, and collapse of cavities give rise to extremely high local temperatures (5000 K) and pressures (100 MPa) 32. In this contribution, a matrix heterostructures of the SAPO-34/ZSM-5 composite formed by the growth of SAPO-34 crystalline overlayer on ZSM-5 nanocrystals, and the mesoporous SAPO-34 layer was synthesized using the ultrasonic assisted hydrothermal procedure. Combining SAPO34 with Chabazite (CHA), and ZSM-5 with (Mordenite Framework Inverted) MFI structure codes into a hierarchical composite ascertains useful physicochemical properties of mentioned types of zeolites alongside the improvement in surface properties and modified distribution of active centers. Although there are several reports on the fabrication of SAPO-34/ZSM-5 composite catalyst applied to MTO reaction, the impact of ultrasonic assisted hydrothermal procedure and different SAPO-34 to ZSM-5 ratios used in a composite has not been reported yet. Present work implemented the ultrasound assisted hydrothermal method for synthesis of SAPO34/ZSM-5. SAPO-34/ZSM-5 nanocomposite catalysts were prepared by physical mixture (named as PM (m)), ultrasonic-assisted hydrothermal synthesis (named as U-S/Z (m)) and hydrothermal synthesis (named as H-S/Z (m)), m expressed SAPO-34/ZSM-5 weight ratio in the catalyst. Furthermore, ultrasound assisted method result in smaller grain size and as a consequence surface area increases. Also, at this work different percent ratio of S/Z composites



are synthesized and analyzed. Physiochemical properties of the composite catalysts and physical mixture of SAPO-34 and ZSM-5 were identified by XRD, FESEM, TEM, BET, NH3-TPD and FTIR techniques. In addition, their catalytic performance in the MTO reaction was evaluated.

2. Experimental 2.1. Catalyst preparation 2.1.1. Synthesis of U-SAPO-34 The SAPO-34 catalysts were prepared by using ultrasonic assisted-hydrothermal synthesis method (U-SAPO-34) from a gel with the molar composition of 1.0Al2O3: 1.0P2O5: 0.6SiO2: 70.0H2O. The source of the framework elements were phosphoric acid (85 wt.%, Merck), aluminum isopropoxide Al(OPri)3 (Merck), Tetraethylorthosilicate (TEOS, Merck) and distilled water. Diethylamine (DEA, 20 wt.%, Merck) was used as a structure-directing agent. The proper amount of aluminum isopropoxide was initially mixed with the template (DEA) and deionized water at room temperature, and stirred for an hour. TEOS as a silica source was then added and stirred for two hours. Finally, phosphoric acid was added dropwise to the above solution with continuous stirring. The achieved gel at a frequency of 24 kHz was irradiated with ultrasound waves. The ultrasonic process was carried out by means of Ultrasonic Processor UP200H (Hielscher) with the titanium sonotrode S7 having a tip diameter of 7 mm, and 300 W/cm2 power intensity (related to 100% amplitude setting). The sonication temperature control assisted by means of the water bath at 50˚C. The process continued until the solution had nucleated and resulted in the slurry. In order to accelerate the crystal’s nuclei growth, the slurry was placed in a 6 ACS Paragon Plus Environment

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30 ml Teflon-lined stainless steel autoclave and heated in an oven at 200˚C for 4 h. The final solid product was recovered, and washed three times by centrifuging with deionized water, followed by drying at 120 ˚C for 12 h. The final product was calcinated at 500˚C in the air for 5 h.

2.1.2. Synthesis of ZSM-5 ZSM-5 catalysts were synthesized by a hydrothermal crystallization method with the chemical composition of 1.0Al2O3: 80.0SiO2: 8.0Na2O: 16.0TPABr: 3000.0H2O. The sources of the framework elements for Si and Al were colloidal silica (40 wt.% SiO2, Aldrich), and aluminum isopropoxide [98 wt.% Al (OPri)3, Merck], respectively. Tetrapropylammonium bromide (TPABr, Merck), and Sodium hydroxide (98 wt.%, Merck), were also used for the formation of ZSM-5 structure. The ZSM-5 catalyst preparation performed through two main steps. First, at a solution (I) mixture of TPABr (70 wt.%) and colloidal silica were dissolved in distilled water (50 wt.%). In second step namely solution (II) TPABr (30 wt.%), NaOH pellets and aluminum isopropoxide were dissolved in distilled water (50 wt.%). To achieve homogeneous gel each solution was stirred for 3 hours. Then, solution (I) and (II) were mixed dropwise under vigorous stirring and then the resulting gel was stirred for 12 h at room temperature to ensure homogeneity. In order to crystallization the prepared gel was placed into a 40 ml Teflon-lined stainless steel autoclave, and heated in an oven at 190 ˚C for 24 h. The final solid product was recovered and washed with deionized water until the pH value of the washing water reached 7–8,



and then it was dried at 120 ˚C for 12 h. The final product was calcinated at 500 ˚C in the air for 5 h as before.

2.1.3. Preparation of U-S/Z(m) composite and PM (m) In order to synthesis of U-S/Z(m) composite by one-step ultrasonic assisted hydrothermal crystallization method, ZSM-5 powder as a support, and SAPO-34 synthetic gel with the same composition described above were synthesized. Following that, ZSM-5 was mixed with SAPO34 synthetic gel, and irradiated with ultrasound at a frequency of 24 kHz was performed during the mixing. In order to crystallization the prepared gel was then placed in a stainless steel autoclave and heated in an oven at 200 ˚C for different heating times. In this method, autoclave time was reduced to about 2-6 hr, while the time for conventional synthesis method is about 24-48 hr. Finally, the solid product was removed, and washed three times by Centrifuging with deionized water, and dried overnight at 120 ˚C. Afterwards, the catalyst composite sample was calcined at 500 ˚C in the air for 5 h. As-synthesized ZSM-5 and as-synthesized SAPO-34 with equal mass percent were blended in order to prepare the physical mixture (PM).

2.2. Catalyst characterization


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X-ray diffractometer (Equinox 3000) was used to study about the phase and crystallinity of the samples were analyzed by operated at 30 mA and 40 kV with a Cu KαX-ray source (λ=1.54056 ˚A) at ambient temperature. Field Emission Scanning Electron Microscope (FESEM) utilized to the indication of particle morphology of catalysts. Structure of catalysts was evaluated with Transmission Electron Microscopy (TEM) (Philips CM200 at 200 KV). Acidity of samples was determined with Ammonia temperature programmed desorption (NH3–TPD) carried out in the temperature range of 50–800 ˚C using a 0.2 g of catalyst under 2% NH3/He gas mixture stream (60 N ml/min) from ambient temperature up to 800 ˚C at a heating rate of 10 ˚C/min. Furthermore, synthesized catalysts were treated in a He flow rate of 60 N ml/min for 1 h and during the test temperature was increased from room temperature up to 800 ˚C at a heating rate of 10 ˚C/min. Brunauer, Emmett and Teller (BET), t-plot, and Barrett–Joyner–Halenda (BJH) methods were used to study about the textural properties of calcinated samples counting total surface area, total pore volume, micropore volume, and average pore diameter were calculated via. Quanta chrome Autosorb-1 analyzer implemented to prepare isotherm data of nitrogen adsorption in the relative pressure (P/P0) ranges from 0.05 to 0.30 obtained at 77.35 K. Formation of the chemical bonds studied with FT-IR spectrum using KBr-diluted pellet on an IR spectrophotometer (PerkinElmer). Product distributions and methanol conversion are determined by Eqs.(1)–(4), respectively 1: Ethylene (wt%) =

Propylene (wt%) =

× ()


× ()


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DME (wt%) =

× ()

MeOH conversion (wt%) =

( ! )

× (")

For other products, (Butene, light alkanes,...) it is also determined similar to Eqs.(1)–(3).

3. Results and discussions 3.1 XRD analysis. XRD spectrograms of prepared molecular sieves including the U-S/Z (m) composite catalyst, physical mixture, along with reference components i.e. U-SAPO-34 and ZSM-5 are presented in Fig. 1. The XRD pattern in Fig. 1e reveals the formation of pure ZSM-5 crystal with tetragonal structure. The diffraction peaks emerged at 2θ = 7.8, 8.8, 23.3, 23.4 and 24.1˚ are similar to those of ZSM-5 reported in the literature

17, 33

. Based on the Fig. 1f, the U-SAPO-34 molecular sieve

possesses characteristic peaks at 2θ = 9.6, 12.9, 20.8 and 31˚. Compared to XRD patterns of USAPO-34 and ZSM-5, the composite and physical mixture samples show all characteristic XRD peaks of both phases. Consequently, binary structure of U-S/Z(m) nanocomposite could be prepared via ultrasonic-assisted hydrothermal synthesis methods. In the various percent of ZSM5 in the composite preparation procedure, XRD patterns showed instable intensities for the USAPO-34. Also ZSM-5 could severe as a seed for the composite. However, it seems that ZSM-5 was partly solved in the basic solution of U-SAPO-34 synthetic gel and provides a silica and aluminum source for the crystallization of U-SAPO-34. Moreover, the CHA structural pattern intensities are related to the quantity of employed ZSM-5 as a substrate. Based on XRD results, 10 ACS Paragon Plus Environment

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using ZSM-5 seeds in the range of 30% to 70% in the crystallization step of U-SAPO-34 considerably increased U-SAPO-34 diffraction peaks and leads to the obvious binary structure of ZSM-5 and U-SAPO-34 in the composite. It found that using ultrasonic waves during the crystallization step can facilitate the chemical interaction between ZSM-5 and U-SAPO-34. Liu et al. 17 reported the same results using the microwave-assisted hydrothermal method. Moreover, the use of sonochemical technique causes homogenous dispersion of the ZSM-5 nanoparticles as a catalytic binder in the synthesis of U-S/Z(m) composite. The XRD analyses in Fig. 2 are from the nanocomposite samples before (fresh U-S/Z (50 wt. %)) and after (used U-S/Z (50 wt. %)) MTO reaction. These obtained observations here proves the hydrothermal stability of nanocomposite as the structure of sample preserved during the process.



Figure 1. XRD patterns of (a) U-S/Z(10 wt. %), (b) U-S/Z(30 wt. %), (c) U-S/Z(50 wt. %),(d) PM (50%), (e) ZSM-5, (f) and U-SAPO-34.


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Figure 2. XRD of fresh U-S/Z (50 wt. %) and used U-S/Z (50 wt. %) in MTO reaction.

3.2. FESEM analysis. FESEM applied in order to analyze the particle size and morphology of the composite and individual components (fig. 3). U-SAPO-34 characterized in the form of cubic particles (fig. 3a and c), whereas ZSM-5 was spherical-like morphology along with some agglomerates (fig. 3b). In spite of coarse H-S/Z (50%) nanocomposite crystals with the diameter of about 2 µm which are observed in synthesized structure prepared via hydrothermal method (Figs. 3d), grain size obtained by ultrasound assisted hydrothermal method is about 100 nm (Figs. 3e, f, g and h). Physical mixture shows two kinds of crystals, namely large crystals of ZSM-5 and submicron crystals of U-SAPO-34 (fig. 3i). It seems that overgrowth of cubic U-SAPO-34 covered spherical particle of ZSM-5 crystals. It could be expected that there are two nucleation centers for the growth of U-SAPO-34 zeolite crystal: one in the synthesis gel, and the other on the



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external surface of the ZSM-5 crystal, which is the main field responsible for the formation of US/Z(m) composite. Although the external surface area of ZSM-5 zeolite crystal is dominant in the competition, the two nucleation centers act competitively 34. According to Zheng et al.

34, 35

,

the key factors in the formation of a matrix structure are compatible framework compositions, and close crystallization conditions. Moreover, the rapidity of shell formation and hydrothermal stability of the core material give rise to aforementioned phenomenon. Subsequently, the incompatibility of the core and the shell layer with the different zeolite structure types, chemical composition and crystallization conditions can be treated by using ZSM-5 zeolite which is the source of Al and Si. It could be expected that during this process, ZSM-5 cores are dissolved and act as a nutrient for the growth of U-SAPO-34 zeolite in the shell. Suitable nucleation sites and controlled crystal growth in ultrasonic assisted hydrothermal procedure facilitate the formation of the U-SAPO-34 in small size ranges

36-38

. Using ultrasonic wave at this stage leads to rapid

homogeneity in the dispersion of the initial nuclei. Moreover, sonochemical cavitation during the ultrasonic process assists the composite growth by local increase of temperature and pressure in the gel. An energy disperse spectroscopy (EDS) analysis was conducted to investigate the elemental composition of resultant materials to accredit further the presence of both parent zeolite types. The results are listed in Table 1. Obviously, composites showed different P/Al and Si/Al ratios from individual SAPO-34 and ZSM-5 indicating a strong interaction between two zeolitic structures. Compared with U-S/Z(10 wt. %), U-S/Z(30 wt. %) and U-S/Z(50 wt. %), U-S/Z(30 wt. %) has a higher content of Si, which is relevant to the higher amount of ZSM-5 in the synthesis precursor.


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Table 1. Elemental compositions of SAPO-34, ZSM-5 and nanocomposites molecular sieves obtained by EDS analysis.

Sample SAPO-34 ZSM-5 U-S/Z(10 wt. %) U-S/Z(30 wt. %) U-S/Z(50 wt. %)

Composition (wt. %) Si Al P 4.7 16.3 17.2 73 2.9 -37 9.3 3.2 42 9.2 3.1 49 9.8 3.2




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Figure 3. FESEM images of (a) U-SAPO-34, (b) ZSM-5, (c) H-SAPO-34, (d) H-S/Z(50 wt. %), (e) U-S/Z(50 wt. %), (f) U-S/Z(30 wt. %) and (g) U-S/Z(50 wt. %), (h) U-S/Z(10 wt. %), (i)PM (50%).



3.3. TEM analysis. Fig. 4 displays TEM images for U-S/Z (m) structures. According to fig. 4 ZSM-5 is a good substrate to grow U-SAPO-34 around it and obviously there is a matrix structure for synthesized catalysts. Also, fig. (4b and 4c) indicate a hierarchical pore size distribution for U-S/Z (m) catalysts where obviously Fig. 4a indicate ZSM-5 with dominant mesopore pore size distribution. Furthermore, TEM images has good agreement with BET and FESEM results. Additionally, TEM images shows lattice fringes interconnected in U-S/Z(m) between matrix (fig. 4d ). From the TEM image (fig 4e), we can observe hierarchically structured composites based on porous that ZSM-5 with mesopore interconnected to SAPO-34 with micropore together.


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Figure 4. TEM images of (a) ZSM-5, (b) U-S/Z(30 wt. %), (c) and (d) U-S/Z(50 wt. %) and (e) U-S/Z(10 wt. %).

3.4. FTIR analysis. Figure 5 represents the FTIR spectra of as-prepared samples. The results confirm the formation of aluminosilicate MFI structure and the silicoaluminophosphate CHA framework

24

. Based on

FTIR spectra, band around 3450 cm-1 belong to structural O-H that Si (OH) Al attached to the 22 ACS Paragon Plus Environment

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hydroxyl group

39, 40

. Meanwhile, at the U-SAPO-34 and composite of U-S/Z(m), there is high

peak intensity of hydroxyl group. Moreover, peaks variation around 1650 cm-1 can be attributed to the physically adsorbed water 41, 42. Moreover, T-O-Si bending of the catalyst at the tetrahedral framework is at 460 cm-1 wavenumber 43, 44. Absorption asymmetric stretching band at 560 cm-1 can be related to five membered rings in both ZSM-5 and U-S/Z(m) composite. The peaks at the range of 1400 cm-1 to 680 cm-1 can be assigned to the asymmetric stretching of TO4 tetrahedral and T–O–T symmetric stretching

45-47

. Note that, compering the FTIR spectra of SAPO-34, and

ZSM-5 in physical mixture and composite, it seems that IR bands of the T\O binding and TO4 unit vibrations for composites of U-S/Z(m) slightly shifted toward the low wavenumber relative to those for PM(m). These shifts are mainly due to the chemical interactions in the composite of U-S/Z (m). Based on above discussion, these results are in agreement with those of XRD pattern.



Figure 5. FT-IR spectra of (a) U-S/Z (50%), (b) U-S/Z (10%), (c) U-S/Z (30%), (d) PM (50%), (e) U-SAPO-34, (f) ZSM-5.

3.5. BET analysis. The nitrogen adsorption-desorption isotherms of the synthesized samples are represented in Figure 6 and the corresponding textural properties are summarized in Table 2. Interestingly, USAPO-34 synthesized via an ultrasonic-assisted hydrothermal possesses the higher mesoporous surface area in comparison to that prepared via typical hydrothermal, which can be attributed to the high dispersion of reactants and modification of its surface by the sound waves. What is more, as it is expected that U-S/Z (50%) composite also prepared under an ultrasonic-assisted 24 ACS Paragon Plus Environment

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have dramatically texture properties so that showed the higher mesopore volume as a key factor in the catalysts framework for MTO reaction. Furthermore, as shown in table. 1 with increase of ZSM-5 ratio total surface and mesopore surface area decreases. In contrast, there is different trend for pore volumes that with increase of ZSM-5 content total pore volume and mesopore volume increases. Interestingly, micropore volume increases with addition of ZSM-5 too and this can be the main reason for faster deactivation of the catalysts. Thus, due to deactivation of catalysts by formation of coke deposit in micropores, this is favorable to have a hierarchical structure where decrease tendency to deactivation with higher mesoporosity consequently, a new distinct pore system was successfully created in U-S/Z(m) composite mostly contributing to minimized diffusion resistance, enhanced lifetime and reaction activity

48, 49

. It is suggested

increasing mesopore volume may be due to location of crystallite of U-SAPO-34 and ZSM-5 in interface contact in the form of hierarchical.



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Figure 6. The N2 adsorption–desorption isotherms of samples: H-SAPO-34, U-SAPO-34, ZSM-5, U-S/Z (50%), US/Z (30%) and U-S/Z (10%).

Table 2 Textual properties and acidity of SAPO-34, ZSM-5and S/Z. Surface area (m2/g) Nomenclature

Pore volume(ml/g)

Acid amount (mmol/g)

Synthesis Method Smicro

Stotal

Vmicro

Vtotal

Weak acidity

Strong acidity

ZSM-5

Hydrothermal

289

352

0.18

0.57

0.24

0.76

H-SAPO-4

Hydrothermal

286.3

377.8

0.19

0.29

0.88

0.91

U-SAPO-34

Hydrothermal/Ultrasound

173.3

453.7

0.13

0.35

0.78

1.01

U-S/Z (50%)


228

460

0.10

0.49

0.67

0.70


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U-S/Z (30%)


244

425

0.11

0.63

0.51

0.61

U-S/Z (10%)


270

392

0.14

0.69

0.37

0.54

3.6. NH3- TPD analyses. Proper acidity of catalysts is important in MTO reaction where weak acid sites can convert methanol to DME and strong acid sites converting DME to light olefins. However, Surface acidity of synthesized catalysts was evaluated by temperature programmed desorption of ammonia (NH3-TPD). Fig. 7 displays NH3-TPD for synthesized U-SAPO-34, H-SAPO-34, ZSM-5 and U-S/Z(m), accordingly there are two distinct peak for all of samples. Thus, U-SAPO34 and H-SAPO-34 has a peak at 200 ˚C assigned to weak acid site and also there is a peak at 420 ˚C belong to strong acidity. Furthermore, weak acid site for ZSM-5 and U-S/Z(m) catalysts is around 160 while strong acid site for these samples is around 460, indicating stronger acidity of ZSM-5 and U-S/Z(m) catalysts in comparison to U-SAPO-34. Additionally, this can be interpret that presence of weak acid sites assigned to surface hydroxyl groups and on the other hand strong acid sited possibly attributed to structural acidity. Thus, ZSM-5 catalyst has stronger acidity at its strong acid site than U-S and due to fabrication of matrix structure in composite samples acidity of U-S/Z(m) catalyst is between parents of U-SAPO-34 and ZSM-5. On the other hand there is different manner for weak acid site where ZSM-5 has weakest acidity in comparison to U-SAPO-34 and U-S/Z(m) acidity again due to matrix structure is between USAPO-34 and ZSM-5. Table. 1 indicate summary results of NH3-TPD, obviously with increase the ratio of H-Z in U-S/Z(m) there is no change at the position of peaks but concentration of acid site reduced. However, U-S/Z(m) ratio has distinct effect on the concentration of acid sites of composite catalysts.



Figure 7. NH3-TPD patterns of different samples: H-SAPO-34, U-SAPO-34, ZSM-5, U-S/Z (50%), U-S/Z (30%) and U-S/Z (10%).

4. Catalyst performance 4.1. Methanol conversion Catalytic performance analyses of all prepared samples with different weight percent ratios were carried out in a differential reactor as shown schematically in Fig. 9. Tests were performed at 723 K with a feed WHSV of 4.5 h-1 for methanol conversion to olefins and results were compared with those of individual U-SAPO-34, ZSM-5 and PM (50%)-derived catalysts (Fig. 8). As shown in Fig. 8 for individual catalyst, although the SAPO-34 exhibited relatively high methanol 28 ACS Paragon Plus Environment

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conversion, its catalytic performance rapidly decreased with time on stream, probably due to the trapped coke in cavities which limited the active sites for methanol conversion. On the contrary, favorable stability and lower methanol conversion observed in the case of ZSM-5. This result could be related to minimization of the diffusion restrictions in transfer of reactants and products molecules duo to the MFI structure of ZSM-5

48

. All binary structures performed better than

ZSM-5 and SAPO-34 single structure catalyst. On the other hand nanocomposite catalyst showed higher conversion than physical mixture. In all of them, the methanol conversion increases with time on stream, and after about 60 min the methanol conversion remains at almost 100%. Induction period in the early time on stream (TOS) leads to incomplete conversion of methanol

38, 50, 51

that can be explained through hydrocarbon pool (HP) mechanism. Haw and

Kolboe reported that cyclic organic species such as hexamethylbenzene (HMB) play the role of the reaction centers for light olefins production as MTO reaction proceeds by a HP mechanism 27, 52

. Accordingly, the time for the formation of these cyclic organic species causes the induction

period, i.e. an increase in activity before maximum conversion. However, the small cage size of the SAPO-34 molecular sieve causes the HMB to form in the cages works as a diffusion barrier for methanol and lower olefins. Thus, it is expected that due to growth of the active intermediates, enhancement of HMB concentration increases the methanol conversion

52

.

Aforementioned common feature of the MTO reaction is related to the buildup of the HP, which is essential for catalytic activity

38, 50

. It is well known that, pore structure and topology of a

molecular sieve has significant effect on subsequent catalytic performance. consequently, considerable effects of combined MFI and CHA frameworks in proper ratio and subsequent proper micro- and mesopores ratio in distinct pore structure lead to better performance of US/Z(m) composite than individual SAPO-34, ZSM-5 and physical mixture catalyst. On the other



hand homogeneous disperse of particle and chemical connection via ultrasonic wave help to these well performance.

Figure 8. Methanol conversion over the SAPO-34, ZSM-5, physical mixture and nanocomposite U-S/Z(m) catalysts. Reaction conditions: Catalyst = 0.5 g, 450 °C, WHSV = 4.5 h-1, Feed: methanol in water (20 vol. %).


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Figure 9. Schematic diagram for catalytic performance evaluation (1- Syringe pump, 2- Reactor, 3- Furnace 4condenser 5- Gas chromatograph 6- Monitoring system 7- Gas cylinder).

4.2. Product activity and selectivity Figure 10 illustrate the product distribution over U-SAPO-34, ZSM-5, U-S/Z(m) and PM(50%) nanostructured catalysts with reaction time on stream (TOS). The three-dimensional CHA structure of U-SAPO-34, among the different types of light olefins products assists ethylene production. Moreover, ZSM-5 powder alongside two-dimensional MFI structure promotes propylene production. Subsequently, the C2H4 + C3H6 can be altered applying ZSM-5, U-SAPO34 and different ratios of U-S/Z(m) as nanostructured catalysts for light olefins production. Although ZSM-5 has the longest life time, U-SAPO-34 deactivates very quickly. Moreover, the physical mixture of ZSM-5 and U-SAPO-34 showed slight improvement in the catalytic stability. To obtain the appropriate amount of ZSM-5, the U-S/Z(m) were prepared with differing amounts of ZSM-5. From Fig. 10, it can be shows that when the U-SAPO-34 content in the US/Z(m) composite was low, the catalyst performance showed a relatively low value. However, 31 ACS Paragon Plus Environment


the catalytic stability of the U-S/Z(m), especially U-S/Z (10%) and U-S/Z (30%), is remarkable enhanced and even close to that of ZSM-5. Note that U-S/Z (50%) shows the highest activity and selectivity to light olefins in comparison to all samples. Although the propylene/ethylene ratio is about one for U-SAPO-34 at the beginning of the time on stream, it decreases when the reaction is terminated. More rapid decrease of Propylene content than ethylene can be attributed to the pore size reduction of U-SAPO-34 duo to the coking effect. In case of the ZSM-5, the selectivity to propylene is higher than that of the U-SAPO-34 and also more stable. The differences between catalytic performances of the U-S/Z(m) with different ratio in comparison to physical mixture suggest that the matrix structure of composite materials effectively enhances the catalytic performance. This can be related to mesoporous structure, small particle size, relatively weak acidity of the U-S/Z(m) composites and homogeneous disperse of particle due to ultrasonic wave, which effectively promotes the product diffusion and alleviates the coke deposition. In addition, the addition of suitable amounts of ZSM-5 in the matrix structure can be beneficial for the formation of olefins. Site accessibility of the catalyst enhanced due to direct connection of hierarchical matrix structure.


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Figure 10. Methanol conversion over the U-SAPO-34, ZSM-5, physical mixture and nanocomposite U-S/Z(m) catalysts. Reaction conditions: Catalyst = 0.5 g, 450 °C, WHSV = 4.5 h-1, Feed: methanol in water (20 vol. %).



Page 34 of 39

4.3. Performance Comparison The catalytic performance of the prepared composites were compared with those of some nanocomposites of U-SAPO-34 catalysts in the MTO reaction. The data in Table 3 show that the nanocomposite of U-S/Z(m) catalyst gives a higher methanol conversion, selectivity, and yields light olefins compared with other nanocomposite of U-SAPO-34 catalysts. Better performance of this catalyst could be related to adequate amount of ZSM-5 and homogeneous disperse of particle in composite and chemical connection via ultrasonic wave.

Table 3 Comparison of performance of prepared nanocomposite catalyst and various reported nanocomposite of SAPO-34 catalysts.

Nanocomposite catalyst

MeOH conv. (%)

Product selectivity and yield (%)

#

%$Reaction conditions

Ref.

#&%

ZSM-5/SAPO-34

97

40.5

___

38

___

MTO reaction, 1.2 g catalyst, WHSV=2.4 h-1, T=400 °C

ZSM-5/SAPO-34

100

26.6

___

29

___


SAPO-34/ZrO2

100

40.2

___

36.1

___

DTO reaction, , 0.2 g catalyst, WHSV=3.54 h-1, T=400 °C

SAPO-34/α-Al2O3

99.5

39.2

___

41.5

___

DTO reaction, , 0.2 g catalyst, WHSV=3.54 h-1, T=400 °C

ZSM-5/SAPO-34

100

10

4

80

31

MTO reaction, 0.5 g catalyst, WHSV=4 h-1, T=600 °C

HZSM-5/SAPO-34

100

___

26.6

___

32.3

ethanol to propylene reaction, 0.5 g catalyst, T=500 °C

U-S/Z (50%)

100

25

___

70

___


5. Conclusions 34 ACS Paragon Plus Environment

17

7

53

53

19

22

This work

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Hydrothermal ultrasound assisted method was used to synthesis of U-SAPO-34 and U-S/Z(m) catalysts. FESEM micrographs indicated matrix structure for U-S/Z(m) catalysts and TEM images revealed hierarchical porosity of synthesized U-S/Z(m) samples. Based on BET results, U-S/Z(50%) showed highest surface area in comparison to other samples, while the increase of ZSM-5 ratio in composite decrease the surface area. Furthermore, BET results approve TEM results that anticipated hierarchical structure. Evaluation of acidity of catalysts through NH3-TPD test showed two acidic sites for all of samples. Catalytic study results demonstrated that US/Z(m) catalysts perform MTO reaction with higher selectivity and conversion in comparison to both U-SAPO-34 and ZSM-5 catalysts. Moreover, U-S/Z(50%) perform MTO reaction with 90% selectivity to light olefins especially with 70% of propylene and 100% conversion of methanol where has best result in MTO reaction in comparison to other catalysts. Thus, U-S/Z(m) catalyst in comparison to U-SAPO-34 and ZSM-5 catalysts has improved lifetime in MTO process.

6. Acknowledgements The authors gratefully acknowledge the financial supports from Iran National Science Foundation and National Petrochemical Company (NPC) of Iran with grant number of LC-WI02.



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