Control of Hierarchical Structure and Framework-Al Distribution of

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Control of Hierarchical Structure and FrameworkAl Distribution of ZSM-5 via Adjusting Crystallization Temperature and Their Effects on Methanol Conversion Sungtak Kim, Gyungah Park, Min Hee Woo, Geunjae Kwak, and Seok Ki Kim ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04493 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Catalysis

Control of Hierarchical Structure and Framework-Al Distribution of ZSM5 via Adjusting Crystallization Temperature and Their Effects on Methanol Conversion Sungtak Kima, Gyungah Parkb, Min Hee Woob, Geunjae Kwakb,c, Seok Ki Kimb,c*

aEnergy

& Environmental Research Team, Institute for Advanced Engineering (IAE),

Yongin-Si, Gyeonggi-do, 17180, Republic of Korea bCarbon

Resources Conversion Catalytic Research Center, Korea Research Institute of

Chemical Technology (KRICT), Daejeon 34114, Republic of Korea cAdvanced

Materials and Chemical Engineering, University of Science & Technology,

Daejeon 34113, Republic of Korea

* Corresponding author. Tel.: 82-42-860-7530 Postal Address: 141 Gajeongro, Yuseong, Daejeon 34114, South Korea Email address: [email protected]

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ABSTRACT Incorporating mesoporosity into zeolite catalysts has been regarded as an innovative technology that improves diffusivity and catalytic life time. Here, we propose a facile synthesis of the hierarchically structured ZSM-5 with accompanying intracrystalline mesopores, which was achieved by controlling the growth rate of the zeolite nanocrystals without using extra additives. As the crystallization temperature is strongly related to the formation of primary nanocrystals and their further growth, which fills gaps between those nanocrystals, the hierarchically structured ZSM-5 zeolite was synthesized at low crystallization temperatures (< 140 °C). 27Al MAS NMR and UV-Vis-DRS analyses revealed that the hierarchically structured ZSM-5 prepared in the present study contained Al located in the straight channel at a higher proportion than the conventional microporous ZSM-5. The substitution of Al was calculated to be more difficult at the channel intersection than at other T-sites, supporting the experimental results. The hierarchically structured ZSM-5 exhibited excellent stability as well as selectivity for a methanol-to-olefin reaction. Reaction free energies calculated along the hydrocarbon pool mechanism pathway revealed that the Al located in the straight channel drives the reaction through the alkene-based cycle, which is responsible for the high olefin selectivity of the hierarchically structured ZSM-5.

Keywords: zeolite, hierarchical structure, Al distribution, methanol-to-olefin, hydrocarbon pool


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Introduction Zeolites are crystalline microporous aluminosilicates consisting of tetrahedra coordinated in a framework that results in a unique pore system and strong acidity. Due to their unique physico-chemical properties, they have been widely used as catalytic materials in a wide range of chemical industries including petrochemistry and renewable energy fields.1 For the utilization of zeolites as a heterogeneous catalyst for a target reaction, many studies have been performed to improve their activity, stability and selectivity. In this regard, recent studies achieved significant advances in two major directions: one direction is the development of a hierarchically structured zeolite that exhibited an improved catalytic performance because of its enhanced diffusivity, especially for bulky molecules,2-5 and the other direction is the understanding of the effect of the framework Al (AlF) site on reaction pathways to eventually modulate product selectivity.6-9 When a zeolite is used for reactions with hydrocarbons, its microporosity often limits the diffusion of reactants or products, resulting in deterioration of catalyst performance. To mitigate such limitations of zeolite crystals, mesoporosity is introduced to form a hierarchically structured zeolite through various methods, including the use of a structural template in the synthesis process and treatment of the zeolite crystal afterward using alkaline solution.10-17 The hierarchically structured zeolite catalysts showed much better activity and stability in reactions converting or producing hydrocarbons than conventional zeolite catalysts.18-20 However, the high cost of some organic templates, troublesome preparations, or significant dissipation of the zeolite body by the alkaline treatment is still inevitable in most cases. In this context, relatively facile synthesis methods such as adding a nucleation promotor to the precursor solution, varying the crystallization temperature, and conducting a hydrothermal treatment after zeolite synthesis, have recently drawn attention for use in preparing the mesoporous zeolite


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catalysts.21-24 Recent studies have shown that the position and distribution of AlF atoms of zeolite materials should be considered a very important factor in both acidic and redox catalytic reactions. For instance, Wang and coworkers revealed that the location of the AlF can vary the reaction pathway dominating the methanol-to-olefin (MTO) reaction between aromatic- and alkene-based reaction cycles based on comparisons of catalytic performances and energetics between ZSM-5 and ZSM-11.25 A dependence of the reaction pathway on the location of the AlF and the pore structure was also observed for carbonylation in modernite, ferrierite, and ZSM-22.26-28 Thus, understanding the effect of AlF location on a reaction and engineering its location have a great impact on the design of selective catalysts. To control the location of the AlF, various methods such as the incorporation of heteroatoms,7, 29 the addition of structuredirecting agents (SDA),30-31 and de-alimination32-33 were reported elsewhere. Although both the simple synthesis of mesopore zeolites and the control of AlF have been regarded as a major issue in the zeolite field as described above, the relation between the synthesis parameters and arrangement of AlF has not been investigated. In the first part of this study, we investigate the effect of hydrothermal synthesis temperature and time for the ZSM5 zeolite on its crystal growth and the formation of a hierarchical structure. In addition, spectroscopic analysis and density functional theory (DFT) calculations reveal for the first time that the synthesis temperature is an important factor in determining the locations of the aluminum sites in the structure. In the second part, the prepared ZSM-5 catalysts are evaluated in an MTO reaction, and their crystal structure and the effect of the aluminum site on the catalytic stability and the product selectivity are also investigated. We couple a DFT analysis with experimental results to provide an in-depth understanding of reaction mechanisms, such as hydrocarbon pool species and confinement effect of the zeolite cavity.


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Experimental Preparation of mesoporous HZSM-5 To synthesize mesoporous ZSM-5 zeolite samples, tetraethyl orthosilicate (TEOS, 98 %, Samchun), sodium aluminate (AlNaO2, Junsei), and tetrapropylammonium hydroxide (TPAOH, 25 % aqueous solution, TCI) were used as silicon source, aluminum source, and organic template, respectively. Homogeneous synthesis gel was first prepared by adding the aluminum source dissolved in deionized water to a mixture of silicon source, organic template, and deionized water and then stirring at 60 °C for 3 h. The molar composition and pH of the gel were Al:25Si:10TPAOH:1100H2O and 11.5, respectively. The gel mixture was transported into a Teflon-lined stainless steel autoclave, and crystallization proceeded at 120, 130, 140, 160, 200, and 230 °C for 1 day to obtain ZSM-5 zeolites. The crystallization time was also varied from 1 to 21 days for the sample synthesized at 130 °C. The white solid product obtained after the crystallization under the hydrothermal conditions was centrifuged, washed with deionized water several times, dried at 110 °C overnight and then calcined in a muffle furnace at 600 °C for 6 h under airflow to remove the organic template trapped in the ZSM-5 pores. After calcination, the resulting Na form of the ZSM-5 zeolite was ion-exchanged for 5 h under stirring with a 1 M solution of ammonium nitrate (NH4NO3, > 99.0 %, Sigma-Aldrich) at a zeolite-to-solution ratio of 1 g of zeolite/50 mL of solution at 80 °C to form NH4-ZSM-5, which was then followed by washing and drying steps. Finally, the resulting product was dried and calcined at 600 °C for 6 h to obtain HZSM-5 zeolite after the ion exchange processes had been repeated three times. The samples were denoted as, for instance, Z5-120-1d, where the numbers following Z5 (ZSM-5) refer to the crystallization temperature (120 °C) and time (1 day), respectively.


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Co ion-exchanged ZSM-5 was prepared via the method introduced by Dědeček and coworkers.34-36 Briefly, the synthesized ZSM-5 sample was first ion-exchanged with aqueous sodium nitrate solution (1 M) at 80 °C for 12 h to obtain the Na form of the ZSM-5 sample, and the obtained sample was ion-exchanged again with aqueous cobalt nitrate solution (0.1 M) at 80 °C for 12 h. The resulting sample was then washed in deionized water multiple times and dried in air at 110 °C for 24 h. The ion exchange was carried out three times to obtain overexchanged Co-ZSM-5.

Characterization The Brunauer-Emmett-Teller (BET) surface area, pore volume, and pore diameter were calculated from a N2 adsorption-desorption isotherm measured at −196 °C using a TriStar 3000 (Micromeritics). Prior to the adsorption-desorption measurements, all of the samples were degassed at 300 °C under 1.0×10−6 Torr for 4 h. Powder X-ray diffraction (XRD) was used to identify peak shape and crystallinity for the fresh zeolite samples. A Rigaku D/MAX IIIB X-ray diffractometer with Cu-Kα radiation was used for bulk phase analysis. All spectra were collected at 2θ values between 3° and 80° using a step size of 0.02° and 2 s per step. The Si/Al ratios of the synthesized ZSM-5 zeolites were determined by means of an X-ray fluorescence (XRF) spectrometer (Thermo/ARL QUANT’X) equipped with a Peltiercooled Si(Li) detector. Temperature programmed desorption of ammonia (NH3-TPD) was carried out to determine the acidity change in the fresh ZSM-5 zeolite samples using an Autochem II 2920 instrument of Micromeritics. Prior to the NH3-TPD experiment, all the samples were pretreated


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at 600 °C under 50 mL/min flow of pure He for 1 h in order to remove adsorbed water and were then saturated with 15 % NH3 (balance He, flow rate: 50 mL/min) for 30 min at 100 °C. After the saturation step, the samples were purged with He for 30 min to eliminate weakly adsorbed ammonia from the surface of the catalysts. Finally, NH3 desorption was performed at a ramping rate of 10 °C/min up to 700 °C under 50 mL/min flow of pure He. Scanning electron microscopy (SEM) images of the ZSM-5 zeolite samples were obtained using a Zeiss Ultraplus Thermal Field Emission Scanning Electron Microscope. 27Al

solid-state magic angle spinning (MAS) NMR experiments were performed on a

Varian Unity INOVA 600 MHz (14.09 T) spectrometer with a 2.5 mm low aluminum-zirconia MAS probe at a rotation rate of 22 kHz. The NMR spectra were obtained at Larmor frequencies of 156.32 MHz using a short radio frequency pulse length of 1.8 µs and a repetition delay time of 2 s. ZSM-5 zeolite samples used to catalyze the MTO reaction were prepared as selfsupporting pellets with a diameter of 12 mm, and their IR spectra were recorded using a Fourier transform infrared (FT-IR) spectrometer equipped with a mercury-cadmium-telluride B detector (Nicolet Nexus 6700). The spectra were collected in single-beam absorbance mode with a spectral resolution of 4 cm-1 at room temperature. The quantities of Brønsted and Lewis acid sites present on the catalysts were evaluated by pyridine (Py) adsorption experiments (Py-IR) carried out in a gas-flowable transmission cell using an FT-IR spectrometer equipped with a mercury-cadmium-telluride-B detector (Nicolet Nexus 4700). An aliquot of catalyst powder (30 mg) was pressed to form a self-supporting disk with a diameter of 12 mm, and the disk was placed in the middle of a custom-made cell. After pretreatment at 300 °C for 3 h to remove adsorbed water and other contaminants, pyridinevapor-saturated He was introduced into the cell, which was then incubated at 150 °C for 30


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min. Next, the cell was purged with pure He at 240 °C for 3 h to remove any physically adsorbed pyridine molecules prior to the collection of the IR spectra. The quantity of Brønsted and Lewis acid sites was calculated from the integrated absorbances of the IR bands at 1545 cm-1 and 1455 cm-1, respectively, using the extinction coefficients given by Guisnet et al.37: 1.13 cm/μmol and 1.28 cm/μmol for Brønsted and Lewis acidities. The concentrations of the different acid sites were calculated from the formula presented below (Beer–Lambert’s law):

C =

𝐴 𝑆 ∗ ∗ 1000 𝜀 𝑚

(1)

where C is the concentration of acid sites (μmol/g), A is the area of band (cm−1), S is the surface area of the wafer (2 cm2), ε is the molar extinction coefficient (cm/μmol) and m is the mass of the sample (mg). Element analysis (EA) was carried out to determine the carbon content and H/C ratio of carbon deposits formed on the spent Z5-120-1d and Z5-230-1d zeolites using a Thermo Scientific Flash 2000 instrument equipped with a thermal conductivity detector (TCD) after the samples had been pretreated with He at 200 °C to remove weakly chemisorbed carbon species. Ultraviolet-visible diffuse reflectance spectra (UV-Vis-DRS) were obtained using a Scinco-4100 spectrometer equipped with a photo diode array detector and a diffuse reflectance attachment. Prior to the measurement, all the samples were dehydrated in air at 400 °C for 7 h. The thermal decomposition behavior of the spent ZSM-5 samples collected after the MTO reaction was investigated by a thermal gravimetric analyzer (TGA, TA lnstruments, SDT Q600) under 100 ml/min flow of air. Before each measurement, the samples were heated from 30 °C to 1000 °C with a ramping rate of 10 °C/min.


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ACS Catalysis

Catalytic evaluation: methanol-to-olefin reaction An MTO reaction was conducted in a 1/2’’ stainless steel reactor of a fixed-bed reactor system. The prepared ZSM-5 zeolite samples were used as a catalyst, and 1 g of each catalyst was placed in the center of the reactor. Prior to each reaction, the catalyst was pretreated in He at a rate of 100 mL/min at 400 °C for 1 h to remove any contaminants. After pretreatment, the temperature was maintained, and the flow of He was reduced to 50 mL/min. Then, a pure methanol feed was introduced at a flow rate of 0.2 mL/min by a Younglin YL 9200 pump into a preheater placed prior to the reactor tube. The product gas was directly transferred into an online gas chromatograph (Younglin 6100GC) equipped with an Rtx-DHA column (0.25 mm ID x 100 m, Restek) and a flame ionization detector for analysis. The MTO reaction was performed at 1 bar and 400 °C for 24 h. The methanol conversion and carbon distribution were defined by the equations (2) and (3), respectively.

Methanol conversion (%) =

Methanol in ― Methanol out ∗ 100 Methanol in

Carbon disribution (%) =

Ci ∗ 100 ∑product

(2)

(3)

Computational details Electronic structure calculations were performed using DFT with a plane-wave basis set implemented in the Vienna ab initio simulation package (VASP).38-39 The kinetic cutoff energy was set to 450 eV. Inner electrons were treated using projector augmented wave (PAW) potentials40 with the revised Perdew-Burke-Ernzerhof (rPBE) exchange correlation functional41 within a generalized gradient approximation (GGA). The van der Waals


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interactions were taken into account in these calculations using the vdW-DF functional.42-43 The convergence criteria for the electron density between the electronic steps was 1 x 10-4 eV for all calculations. A periodic 96T model of an MFI-structured zeolite was used in the present study. The unit cell was optimized using a cutoff energy of 800 eV, resulting in the lattice constants of a = 20.36 Å, b = 20.01 Å, and c = 13.44 Å, which are similar to the experimental values (a = 20.08 Å, b = 19.89 Å, c = 13.37 Å).44 It is very complex to simulate the whole process of inserting an Al atom into the MFI structure, but for simple comparisons of relative energies for the formation of ZSM-5 from a thermodynamic perspective, the insertion of an Al atom into a tetrahedral Si site was assumed to proceed via following reaction: Al(OH)4– + Z5 → Si(OH)4 + Z5Al–

(4)

where Z5 and Z5Al– denote a ZSM-5 composed of pure Si-O and one in which a Si atom is substituted with an Al atom, respectively. Thereby, the reaction energy of the formation of Alsubstituted ZSM-5, ΔE(Z5Al–), was written as ΔE(Z5Al–) = E(Z5Al–) + E(Si(OH)4) – E(Al(OH)4–) – E(Z5)

(5)

where E(Z5Al–), E(Si(OH)4), E(Al(OH)4–), and E(Z5) denote the electronic energies of Alsubstituted ZSM-5, Si(OH)4, Al(OH4)–, and ZSM-5, respectively. The free energies of the MTO reaction at two distinctive Al sites, located in the straight channel and the intersection between the straight and the sinusoidal channel, were calculated based on the hydrocarbon pool mechanism. Two hydrocarbon pool species, 1,2,3,5tetramethylbenzene (TMB) and 2,3-dimethyl-2-butene (iso-C6), were adopted to simulate aromatic- and alkene-based cycles according to previous studies.45-47


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ACS Catalysis

Results and discussion Structural and textural properties First, we investigated the effect of crystallization temperature on structural and textural properties of ZSM-5 using XRD, SEM, and N2 adsorption-desorption isotherms. XRD patterns indicated a typical MFI structure for all the synthesized ZSM-5 zeolite samples, with high crystallinity and an orthorhombic phase (Figure S1). Relative crystallinities calculated by integrating the peak area in the range of 22.5° – 25° increased with the crystallization temperature:48 taking the crystallinity of Z5-230-1d as 100 %, the relative crystallinities of Z5120-1d, Z5-130-1d, Z5-140-1d, Z5-160-1d, and Z5-200-1d were 68 %, 87 %, 92 %, 92 %, and 96 %, respectively (Figure 1).

Figure 1. Effect of crystallization temperature on the ratio of mesopore volume to total pore volume (Vmeso/Vtotal) and the crystallinity relative to Z5-230-1d. SEM images of the Z5-1201d and Z5-230-1d samples are shown on the right-hand side.

The SEM images shown in Figure 1 show the difference between Z5-120-1d and Z5230-1d morphologies. The Z5-120-1d exhibited sphere-like particles with a cluster size of 200


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– 500 nm that is made of smaller crystals with sizes of 10 – 30 nm. As a result, a rugged surface was formed for the clusters. In contrast, hexagonal or cubic crystals with sizes of 300 – 800 nm were observed for Z5-230-1d. Agglomeration of the small crystals became significant at the elevated crystallization temperature, resulting in the formation of dense and even surfaces of Z5-230-1d. Gradual changes in the morphology of ZSM-5 samples synthesized at different crystallization temperatures are shown in Figure S2 in the Supporting Information. The increase in the crystallization temperature also induced changes in the pore structure of ZSM-5, as indicated by the N2 adsorption-desorption isotherms shown in Figure 2A. The Z5-120-1d and Z5-130-1d showed the H3 type of hysteresis loops, which result from slit-shaped pores,49 while Z5-140-1d and Z5-160-1d exhibited the H4 type of hysteresis loops, which indicates narrow slit-like pores with the microporosity.49 For Z5-200-1d and Z5-230-1d, the loop almost disappeared, indicating that the zeolite crystals rapidly grew at a high crystallization temperature, resulting in the dissipation of the margin for developing mesopores and the formation of dense crystals with micropores. The ratios of mesopore volume to total pore volume (Vmeso/Vtotal) of the ZSM-5 samples as a function of the crystallization temperature are also illustrated in Figure 1. Other textural properties are summarized in Table S1. Decreases in external surface area and mesopore volume were observed with increases in crystallization temperature. This trend became more distinct at relatively low crystallization temperatures, that is, the external surface area (Sext) and mesopore volume (Vmeso) of Z5-120-1d were 122.2 m2/g and 0.213 cm3/g, respectively, both of which were three times larger than those of Z5140-1d (Sext 41.9 m2/g; Vmeso 0.073 cm3/g), while Z5-200-1d and Z5-230-1d exhibited similar values of Sext and Vmeso. The Barrett-Joyner-Halenda (BJH) pore size distribution (Figure 2B) showed a gradual increase in pore diameter with increasing crystallization temperatures up to 160 °C, and no mesopores were found for those synthesized at temperatures above that. The enlargement of the pore diameter in the series from Z5-120-1d to Z5-160-1d is attributed to an


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aggregation of small crystals sized from 10 – 30 nm and thus the filling of the space between the crystals as shown in Figure 1 and Figure S2. To summarize, hierarchically structured ZSM-

(A)

0

dv/dlog (D) pore volume (cm3/g)

5 was obtained by lowering the crystallization temperature during hydrothermal synthesis.

Z5-120-1d Z5-130-1d Z5-140-1d Z5-160-1d Z5-200-1d Z5-230-1d

Quantity adsorbed (a.u.)

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

ACS Catalysis

0.2

0.4

0.6

0.8

Relative pressure (P/P0)

1

0.3

(B)

Z5-120-1d Z5-130-1d Z5-140-1d Z5-160-1d Z5-200-1d Z5-230-1d

0.2

0.1

0 0

20

40

60

80

100 120 140 160

Pore diameter (nm)

Figure 2. (A) N2 adsorption-desorption isotherm curves and (B) corresponding pore size distributions of the synthesized ZSM-5 zeolites under different crystallization temperature.

The acidic characteristics of the synthesized ZSM-5 catalysts were investigated using NH3-TPD and Py-IR. The NH3-TPD curves of the ZSM-5 zeolites given in Figure S3 show two distinctive desorption peaks, centered at ~190 °C and ~370 °C and attributed to weakly and strongly adsorbed NH3, respectively. An additional peak centered at ~280 °C was included for the quantitative analysis performed using peak deconvolution (Table 1). The acid content, regardless of the adsorption strength, increased with crystallization temperature. The number of Brønsted acid sites also a showed similar proportional correlation with the crystallization temperature. The low number of strong Brønsted acid sites of the samples synthesized at low temperature indicates its slow formation rate of bridged –Al(OH)Si– in the framework.


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Table 1. Si/Al ratio and acid content of the synthesized ZSM-5 zeolites under different crystallization temperatures.

Sample

Si/Ala

Z5-120-1d

Acidity by NH3-TPD (mmol/g)b

Acidity by Py-IR (mmol/g)c

Total

weak

medium

strong

Total

Lewis

Brønsted

20.6

0.58

0.21

0.16

0.21

0.369

0.093

0.276

Z5-130-1d

20.8

0.56

0.22

0.14

0.22

0.413

0.103

0.311

Z5-140-1d

20.5

0.61

0.25

0.12

0.24

0.428

0.092

0.336

Z5-160-1d

20.1

0.67

0.27

0.11

0.29

0.434

0.082

0.352

Z5-200-1d

20.6

0.68

0.27

0.13

0.28

0.452

0.078

0.374

Z5-230-1d

21.1

0.71

0.29

0.14

0.28

0.461

0.073

0.388

aCalculated

using XRF analysis by deconvolution of the NH3-TPD curves into Gaussian peaks (R2>0.99) cCalculated using equation (1) bCalculated

To investigate the effect of crystallization time on the morphology of the zeolite, we extended the synthesis time by 21 days at a fixed temperature of 130 °C, where the hierarchical structure was successfully formed in 1 day of crystallization. The relative crystallinities slightly increased with synthesis time, but their differences were not significant. The SEM images of the Z5-130-1d and Z5-130-21d also show similar morphologies (see Figure 3). However, N2 adsorption-desorption isotherms and textural properties, shown in Figure S5 and Table S2, respectively, indicate that the hierarchical structure of the zeolite crystal gradually disappeared by filling vacancies between the nanocrystals, as increases in the synthesis time caused no dramatic change in the physical properties of the zeolite crystal.


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Figure 3. Effect of crystallization time on the ratio of mesopore volume to total pore volume (Vmeso/Vtotal) and the crystallinity relative to Z5-230-1d. SEM images of the Z5-130-1d and Z5130-21d are shown on the right-hand side.

Generally, a mechanism of the hydrothermal synthesis of a zeolite includes three stages:(1) an initial formation of primary nanoparticles (NPs), whose size is smaller than 3 nm, from a disordered silica-core/TPA shell (referring to a nucleus or primary gel precursor),50 (2) the growth of the primary NPs into 10 – 50 nm-sized secondary NPs, and (3) the development of the secondary NPs into tertiary crystals (> 200 nm) via further aggregation.1, 50-51. However, the formation of primary NPs by nucleation and crystal growth are strongly influenced by many factors: the silicon and aluminum source52, aluminum content53, template/silicon ratio, nature of the cations present in the synthesis medium52,

54-55,

alkalinity56-57, temperature of the

crystallization53, presence of seeds55, water content58, etc. Among the variations, the crystallization temperature significantly affects nucleation and crystal growth; although the second and third stages during the hydrothermal synthesis of zeolite crystals are slower than the first stage (the formation of primary NPs), lower temperatures are favorable for nucleation, and higher temperatures are favorable for crystal growth.1, 59 Therefore, not only the crystal


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size but also the formation of hierarchical structure can theoretically be controlled by temperature control during the hydrothermal synthesis. Wang et al. indeed reported the direct synthesis of mesopore zeolite material by adjusting the hydrothermal temperature in this context.60 The effects of crystallization temperature and time on the morphology of ZSM-5 are illustrated in Scheme 1. At high crystallization temperatures, the nucleation of the precursor into primary NPs (stage 1) was quickly followed by their simultaneous growth and aggregation (stage 2 and 3), resulting in the formation of large ZSM-5 crystals (> 500 nm) that exhibit an even surface resulting from complete crystallization among the secondary NPs. At low crystallization temperatures, on the other hand, most of the Si and Al sources are consumed by stage 2 while each NP slowly grows from their nucleic center. Accordingly, when the hydrothermal synthesis is terminated at low temperature prior to the complete growth of the crystal, spaces between the secondary NPs are retained as mesopores, and a hierarchically structured ZSM-5 zeolite is obtained.


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Scheme 1. Illustration of crystal growth of ZSM-5 zeolites from primary gel precursor under different hydrothermal synthesis conditions

Determination of Al sites in the ZSM-5 framework The location of the Al site is a key factor that determines the chemical nature of the zeolite because the AlF site serves as an active site, including acting as a proton or electron donor and forming metal-oxo complexes, in various catalytic and adsorption processes. In particular, the Al distribution in the zeolite framework is known to influence the catalytic activity when the Al is applied to a chemical reaction as a solid acid catalyst.61-62 To study the influence of the crystallization temperature on the AlF distribution, the location of Al in the Z5120-1d and Z5-230-1d were analyzed using 27Al MAS NMR, ICP-AES, and UV-Vis-DRS, and the results are summarized in Table 2.


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The 27Al MAS NMR was used to analyze the coordination environments of Al. As shown in Figure 4, both zeolite samples exhibited two major peaks. Peaks centered at 0 and 55 ppm are associated with octahedrally and tetrahedrally coordinated Al atoms, respectively. 64

63-

The former originated from extra-framework Al sites, while the latter is attributed to Al sites

in the zeolite framework.63-64 However, we note that the ZSM-5 crystal used in the present study were assumed to be ideal. Thus possible interferences on the isotropic chemical shift that arise from the presence of extra-framework Al sites and mesopores were not considered here.6566 Apparently, the tetrahedrally coordinated Al sites (Al T-sites) were dominant for both zeolite

samples. When the peak intensities at 0 ppm were compared, the Z5-120-1d showed a higher fraction (0.22) of the octahedrally coordinated Al site than the Z5-230-1d (0.15), which is in accordance with XRD results that showed a low crystallinity for the zeolite synthesized at a low temperature. Furthermore, the peak corresponding to the tetrahedral Al can be deconvoluted with five peaks, centered at 52, 53, 54, 56, and 58 ppm.65-66 The peak centered at 54 ppm corresponds to an Al site located at the intersection of the straight and sinusoidal channels, whereas the peak centered at 56 ppm corresponds to an Al site located within either straight or sinusoidal channels.65-66 The Z5-120-1d and Z5-230-1d samples exhibited similar fractions of the peak centered at 56 ppm (14.8 and 15.7 %, respectively), suggesting consistent formation of Al in the channels, regardless of the crystallization temperature. Meanwhile, Z5230-1d showed a higher fraction of the peak centered at 54 ppm (47.3 %) than Z5-120-1d (26.0 %), suggesting that the formation of Al sites at the channel intersections are facilitated at high crystallization temperatures.


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Table 2. Summary of AlF distribution in Z5-120-1d and Z5-230-1d zeolite samples determined by various analytical techniques. 27Al

MAS NMRa (%)

ICP-AESb (%)

UV-Vis-DRSc (%)

Al(54)

Al(56)

Single Al

Al pair

α

β

γ

26.0

15.7

53.3

46.7

20.8

53.7

25.4

Z5-230-1d 47.3 14.8 49.2 50.8 13.6 by a deconvolution of the 27Al MAS NMR spectra. bCalculated by equations (6) and (7) for the Co/ZSM-5 samples. cDetermined by the deconvolution of the Co(Ⅱ) UV-Vis-DRS of the Co/ZSM-5 samples.

60.4

26.0

Z5-120-1d aDetermined

Figure 4. (A) 27Al MAS NMR spectra and individual Gaussian bands of (B) Z5-120-1d and (C) Z5-230-1d in the chemical shift range of 40 to 70 ppm.

In the zeolite framework, the tetrahedral Al site is present in an Al−O−(Si−O)n−Al sequence, and the Al site can be treated as either a single Al atom or an Al pair depending upon the number of (Si−O) groups. If Al atoms are separated by more than two (Si−O) groups, they are denoted single Al atoms due to the long distance between them. Otherwise, only the Al−O−(Si−O)2−Al sequence can be formed, denoted Al pairs.35, 66-67 The ratio between the


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single Al atoms and Al pairs in the zeolite framework could be obtained by quantifying the amount of ion-exchanged [Co(II)(H2O)6]2+, which is selectively deposited on the Al pairs. That is, single Al atoms, in the 5-6-membered rings of the ZSM-5, cannot participate in the ion exchange due to the long distance between the two single Al atoms, while Al pairs, present in a 10-membered ring, are the only species that coordinate the Co(II) cations.34, 66 Hence, the concentrations of single Al atoms and Al pairs can be calculated by the Al content and Co content in the Co-exchanged ZSM-5 (Co/ZSM-5) samples, using equations (6) and (7) below [Single Al atoms] = [Altotal] – [Al pairs]

(6)

[Al pairs] = 2[Comax]

(7)

where [Altotal] and [Comax] are the Al content and Co content, respectively, in the Co/ZSM-5 samples, and both were quantified by using ICP−AES. As summarized in Table 2, 50.8 % of Al atoms in the Z5-230-1d are present in the form of Al pairs, which is slightly more prevalent than in Z5-120-1d (46.7 %). Although the difference was not significant, this result indicates that the location of AlF was affected by the crystallization temperature. A more detailed description of the Al pair location was estimated using the UV-VisDRS of the Co/ZSM-5 samples. The deconvolution of the UV-Vis-DRS of Co(II) cations was performed using Gaussian functions.36 As shown in Figure 5, several peaks were observed in the range of 12,000 to 25,000 cm-1, and these peaks were classified into three types of Co(II) ions, each coordinated by different Al T-sites: alpha-type Co(II) ions coordinated in the straight channel were identified by a single peak at 15,350 ± 250 cm-1; beta-type Co(II) ions coordinated at the intersections between straight and sinusoidal channels were identified by peaks at 16,300 ± 300 and 17,150 ± 150 cm-1; and gamma-type Co(II) ions coordinated in the sinusoidal channels were identified by peaks at 20,100 ± 150 and 22,300 ± 200 cm-1.36 The area fraction of these peaks for Z5-120-1d and Z5-230-1d are summarized in Table 2. As the crystallization


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temperature increased from 120 to 230 °C, the fraction of Al pairs located at the channel intersection slightly increased (beta-type, 46.1 % for Z5-120-1d, 53.2 % for Z5-230-1d), and the fraction in the straight channel decreased to the same extent (alpha-type, 21.0 % for Z5120-1d, 14.0 % for Z5-230-1d). The Al pairs located in the sinusoidal channel (gamma-type) were present at a similar fraction in the two samples. We note that both independent spectroscopic analyses, 27Al MAS NMR and UV-Vis-DRS analyses, indicated that Z5-230-1d possesses more AlF at the channel intersection than Z5-120-1d, regardless of the Al−O−(Si−O)n−Al sequences (Table 2).

Figure 5. UV-Vis-DRS of Co(II) ions for (A) Z5-120-1d and (B) Z5-230-1d.


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To understand how the Al location varied with the crystallization temperature, reaction energies for the formation of the Al-substituted ZSM-5, ΔE(Z5Al-), at the three different types of T-sites classified by the UV-Vis-DRS analyses were compared using DFT calculations. However, one should bear in mind that the SDA molecules, i.e. TPAOH, were not included in this computational model, and therefore, the energies have only a limited validity to explain the Al siting. The locations of these sites were assigned according to previous reports that provided their geometric configurations in the MFI structure.35, 65 As shown in Figure 6A, 8 equivalent T-sites are present for each site type (that is, alpha, beta, and gamma) in the 96T MFI model. The mean values of ΔE(Z5Al-) of these three types are compared in Figure 6B. The ΔE(Z5Al-) at gamma sites were calculated to be the lowest (0.24 eV), suggesting that the substitution of Al in the sinusoidal channels results in the most stable product and may thus occur more easily at the initial stage of the hydrothermal synthesis at these sites than at other sites. The similarity in the fractions of gamma-type Al pairs in Z5-1201d and Z5-230-1d (Table 2) is due to the rapid Al substitution that initially takes place in the sinusoidal channel at all tested synthesis temperatures. With the same logic, it can be assumed that Al substitution at the channel intersection proceeds slower than at other sites, based on the substitution at the beta sites having the highest ΔE(Z5Al-) (0.33 eV). Z5-230-1d exhibited a higher fraction of Al pairs at beta sites than Z5120-1d because the Al substitution at the channel intersection, which competes with the substitution in the straight channel (alpha), become more feasible at elevated synthesis temperatures. However, differences in ΔE(Z5Al-) among these sites were less than 0.1 eV, which leads to a certain distribution of different types of T-sites rather than a completely selective substitution. The different ΔE(Z5Al-) values for the three types of T-sites are attributed to their different angular strains.68


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Figure 6. (A) Location of the Al-substituted sites in the MFI structure and (B) ΔE(Z5Al-) for three different substitution geometries. Catalytic performance for the MTO reaction Figure 7A shows the results of the MTO reaction conducted at 300, 350, and 400 °C using Z5-120-1d and Z5-230-1d. Methanol conversion and product selectivity are summarized in more detail in Table S3. The catalytic activity of the Z5-230-1d was higher than that of Z5120-1d based on the reaction results obtained at a temperature of 300 °C, where Z5-230-1d converted 100 % of methanol to hydrocarbons and Z5-120-1d converted 84.2 % of methanol, showing 24.6 % hydrocarbon selectivity. The activity of Z5-230-1d was expected to be higher than that of the Z5-120-1d based on its larger number of acidic sites, as determined by the NH3TPD and Py-IR analyses. For the reaction performed above 350 °C, however, the catalytic activities of the two zeolites were not comparable because both converted all the methanol into hydrocarbons.


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Figure 7. Methanol conversion results obtained using Z5-120-1d and Z5-230-1d. (A) Effects of the reaction temperature on product yields and ratio of ethylene to propylene selectivities (C2=/C3=), measured at a time on stream of 2 h. (B) Decrease in methanol conversion as a function of time on stream for reactions conducted at 400 °C.

The product distributions of the two zeolites differed over the range of reaction temperatures tested in the present study. In particular, at the reaction temperature where hydrocarbons were mainly produced (350 and 400 °C), Z5-120-1d tended to produce more C2C6 olefins (C2=−C6=) than Z5-230-1d. At 350 °C, the Z5-120-1d showed a C2=−C6= selectivity of 63.0 %, which was more than twice that of Z5-230-1d (27.5 %). Instead, Z5-230-1d exhibited higher benzene/toluene/ethylbenzene/xylene (BTEX) selectivities than the Z5-1201d. Although the MTO reaction is very complex, hydrocarbon pool mechanisms, which suggest the presence of hydrocarbon species working as an intermediate promoter inside the micropore of the zeolite, have been widely accepted based on experimental and theoretical evidence.69-77 The ratio of ethylene-to-propylene selectivities (C2=/C3= ratio) can serve as an indicator to identify the hydrocarbon pool species of the MTO reaction because short- and longchain olefins are preferentially produced via aromatic and olefinic hydrocarbon pool species,


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respectively.25,

78

We note that both olefinic and aromatic hydrocarbon pool species work

interdependently, so certain relative proportions can be expected, but neither species works exclusively.78 Figure 7A also shows that the C2=/C3= ratio decreases with reaction temperature for both zeolites. Nevertheless, Z5-120-1d showed lower C2=/C3= ratios than Z5-230-1d at all the reaction temperatures, indicating that Z5-120-1d contained more olefinic hydrocarbon pool species than Z5-230-1d. Similarly, the relative proportion of the aromatic hydrocarbon pool species was greater in Z5-230-1d than in Z5-120-1d. This result was also supported by the high olefin and BTEX selectivities of Z5-120-1d and Z5-230-1d, respectively. To simply verify the effect of acid site density on the MTO selectivity, we conducted reaction test using conventional ZSM-5 samples with different Si/Al ratios as shown in Figure 8A. The C2=/C3= ratio tended to increase with the acid site density, but BTEX and olefin selectivity were not clearly dependent to that. When the C2=/C3= ratio was plotted as a function of strong acidity for conventional and synthesized catalysts, no obvious correlation was found as well (Figure 8B). This implies that the difference in product distribution between the Z5120-1d and Z5-230-1d are not well explained by their acidity. In a later section, the atomicscale understanding of these differences will be discussed using DFT calculations.


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Figure 8. (A) Methanol conversion results obtained using conventional ZSM-5 with different Si/Al ratio. (B) Effect of strong acid site density on C2=/C3= ratio. The acid site density was calculated based on the NH3-TPD analyses. Reaction conditions: WHSV = 4.75 gMeOH/gcath, P = 0.1 MPa, T = 400 °C

The methanol conversion by the MTO reaction by Z5-120-1d and Z5-230-1d as a function of time on stream is presented in Figure 7B. The reaction temperature of 400 °C was chosen, as the MTO reaction dominantly occurs at this temperature.79 An elongated catalytic lifetime such as slow deactivation rate for the MTO reaction can be achieved via implementation of a hierarchical structure into the zeolite catalysts, regardless of their preparation method.11, 18 In the present study as well, the hierarchically structured Z5-120-1d exhibited a much longer catalytic lifetime than the typically structured Z5-230-1d for the MTO reaction. During the initial period of the reaction, the methanol conversion obtained from both zeolite catalysts was nearly 100 %; however, after at a certain point, the deactivation trend was different for these catalysts. Z5-230-1d showed rapid deactivation after 10 h on stream, whereas Z5-120-1d maintained a conversion rate of nearly 100 % for up to 30 h on stream and


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deactivated relatively slowly. We note that a larger amount of heavy coke was deposited on Z5-120-1d than on Z5-230-1d based on the coke analyses using the post-run catalysts (Figure S10 and Table S4). Since the similar conversion levels exhibited by the two catalysts at the end of the reaction, the presence of mesopores serves as a carbon reservoir enhancing coke resistance during the MTO reaction. 80-82

Effect of Al position on reaction pathway The main differences in product selectivity between Z5-120-1d and Z5-230-1d samples were the C2=/C3= ratio and BTEX selectivity (Figure 7A), but the origin of these differences could not fully described by their textural properties such as mesoporosity. To understand these differences from an atomistic point of view, energies of MTO reactions at different Al positions, alpha and beta, were calculated using DFT. We note that Z5-120-1d has a relatively higher proportion of Al at the alpha position than Z5-230-1d (Table 2). The energy of reactions at Al at the gamma site was not considered here because both zeolites have similar proportions of that site. According to previous DFT studies of hydrocarbon pool mechanism,45-47 we adopted two hydrocarbon pool species, 2,3-dimethyl-2-butene (iso-C6) and 1,3,4,5-tetramethylbenzene (TMB), for alkene- and aromatic-based cycles, respectively. The proposed reaction mechanisms are shown in Scheme 2. Olefin formation through the hydrocarbon pool mechanism proceeds via a sequential combination of methylation, water-assisted hydrogen transfer, alkyl transition, and olefin liberation. Among these steps, the transition state energy at the methylation step was usually compared because it was suggested to be the most difficult step, thus determining the overall rate.45-47


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Scheme 2. Proposed hydrocarbon pool mechanisms for the methanol-to-olefin reaction via (A) alkene- and (B) aromatic-based cycles. 2,3-Dimethyl-2-butene (iso-C6) and 1,2,3,5tetramethylbenzene (TMB) were used as hydrocarbon pool species for alkene- and aromaticbased cycles, respectively.

Figure 8 shows free energy diagrams of alkene- and aromatic-based MTO reactions at alpha and beta AlF positions. At the alpha position (Figure 9A), the alkene-based cycle was more favorable than the aromatic-based cycle, showing a free energy difference of 2.5 eV at the third methylation step. By contrast, at the beta position (Figure 9B), it was difficult to distinguish which route was favored, although the alkene-based cycle exhibited a slightly lower free energy overall. Previous theoretical studies45 also have shown that the alkene-based cycle are energetically more favorable than the aromatic-based cycle for the MTO reaction in ZSM-5.


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Figure 9. Free energies of alkene- and aromatic-based MTO reactions at (A) alpha and (B) beta Al positions at a reaction temperature of 673 K.

Given that TMB has a larger kinetic diameter (8.6 Å) than iso-C6 (6.0 Å), having sufficient space in the channel intersection (at beta position) may be advantageous in avoiding unnecessary bending or twisting of hydrocarbon pool species (See Figure S11). This hypothesis is supported by the increase in the free energy of the aromatic-based cycle as the methylation proceeds. A recent study dealing with the effect of Al locations in zeolites on the MTO reaction suggested that the acid sites located in the straight channel effectively promote the alkenebased cycle.25 The alkene-based cycle was not as sensitive to the Al position as the aromatic-based cycle. The increases in free energy that occur with methylation progress were not observed. This result suggests that the iso-C6 is capable of working as a hydrocarbon pool species at not only the Al site located at the channel intersection (beta) but also the site located in the straight channel (alpha). Among the three sequential methylation steps in the alkene-based cycle, the third one was calculated to require the least energy, regardless of the Al location.


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The difference in the C2=/C3= ratio between Z5-120-1d and Z5-230-1d can be explained by the difference in the preferred reaction path for each catalyst. In the case of Z5120-1d, where the Al in the straight channel is present in a relatively high proportion, the alkane-based cycle that governs the reaction path promotes propylene production and lowers the C2=/C3= ratio. The alkene-cycle preference of Z5-120-1d is also supported by its high C2=−C6= and low BTEX selectivities relative to the selectivities of Z5-230-1d (Figure 7A). In the case of Z5-230-1d, however, the alkene- and aromatic-based cycles compete with each other, resulting in similar selectivity for ethylene and propylene. Wang et al.78 also reported that the alkene- and aromatic-based cycles are interdependent and promote one another in the MTO process, even though the contribution of the former was more important due to its low free energy barrier.

Conclusions The effects of the crystallization temperature of ZSM-5 on its structural and chemical properties and its catalytic performances in the MTO reaction were investigated. At a low crystallization temperature (~120 °C), the hierarchically structured ZSM-5 was obtained, whereas at an elevated temperature (~230 °C), conventional ZSM-5 without intracrystalline mesopores was synthesized. The formation of mesopores in the zeolite synthesized at a low temperature originates from the absence of silicon and aluminum precursors, which were consumed prior to the slow crystal growth filling gaps between primary nanocrystals. The locations of Al sites in the framework of the synthesized ZSM-5 were identified by both 27Al MAS NMR and UV-Vis-DRS, and the high crystallization temperature fostered the AlF to be located at the intersections between sinusoidal and straight channels. The ZSM-5 synthesized at a low temperature (Z5-120-1d) converted methanol to olefin with high selectivity and


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stability. Reaction free energies calculated along with hydrocarbon pool mechanism revealed that the alkene-based cycle was dominant over the aromatic-based cycle at the AlF located in the straight channel, which is present in Z5-120-1d in a high proportion.

Supporting Information Experimental data, including XRD patterns, SEM images, NH3-TPD curves, N2 isotherms, FTIR of Py, stability tests, TGA curves, and DFT-optimized structures.

Acknowledgements This work was financially supported by the core KRICT project (SI1801-06) from the Korea Research Institute of Chemical Technology and “Next Generation Carbon Upcycling Project” (Project No. 2017M1A2A2043133) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT, Republic of Korea.

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Control of Hierarchical Structure and Framework-Al Distribution of

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