Acid Strength Control in MFI Zeolite for the Methanol-to-Hydrocarbons

May 16, 2014 - selectivity and P/E (propylene/ethylene) ratio in the methanol-to-hydrocarbons (MTH) reaction. The acid strength of MFI zeolite is cont...
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Acid Strength Control in MFI Zeolite for the Methanol-toHydrocarbons (MTH) Reaction Ki-Yong Lee, Seung-Woo Lee, and Son-Ki Ihm* Department of Chemical and Biomolecular Engineering, KAIST, 335 Gwahak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ABSTRACT: This article considers the optimization of the acid strength of MFI zeolite for the maximization of propylene selectivity and P/E (propylene/ethylene) ratio in the methanol-to-hydrocarbons (MTH) reaction. The acid strength of MFI zeolite is controlled by the incorporation of Al3+ and/or Fe3+ into the framework with the same acid site concentration. Three MFI zeolites, namely, H-[Al]-ZSM-5, H-[Fe]-ZSM-5, and H-[Al,Fe]-ZSM-5, with the same amount of acid sites [SiO2/(Al2O3 + Fe2O3) = 400] were prepared by hydrothermal synthesis and used for the MTH at different temperatures. Their physicochemical properties were characterized by NH3 TPD, FT-IR spectroscopy of adsorbed pyridine, N2 adsorption, XRD, SEM, and XANES. The acid strengths of the prepared MFI zeolites followed the sequence of H-[Fe]-ZSM-5 < H-[Al,Fe]-ZSM-5 < H-[Al]-ZSM-5. The Brønsted acid densities of H-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5, obtained from pyridine IR spectra, decreased with increasing temperature more easily than that of H-[Al]-ZSM-5, where the decrease was highest for H-[Fe]-ZSM-5. With the lowest acid strength, H-[Fe]-ZSM-5 showed a higher propylene selectivity and P/E ratio at 400 °C, where it exhibited low methanol conversion and high DME formation. However, its propylene selectivity was significantly lower than those of the other two zeolites at higher reaction temperatures (above 450 °C). The best catalytic performance was obtained with H-[Al,Fe]-ZSM-5 with a broad acid strength distribution because of the coexistence of strong Al-based Brønsted acid sites and weaker Fe-based Brønsted acid sites. Its propylene selectivity was much higher than those of the others at 450 °C, and a maximum propylene selectivity of 49.3% was achieved at 500 °C. It is demonstrated that the acid strength of MFI zeolite can be optimized by the incorporation of Al3+ and Fe3+ into the framework for the maximization of the propylene selectivity in the MTH reaction.

1. INTRODUCTION The discovery of new gas fields and development of applicable technology have led to an increase in the availability of natural gas reserves. The abundance of natural gas has led many industrial researchers to search for feasible methods for converting this raw material into marketable products. This gas can be converted into methanol through steam reforming, and methanol can, in turn, be converted into various chemicals. The catalytic conversion of methanol to hydrocarbons is an interesting and promising method of converting natural gas into chemicals. Methanol-to-hydrocarbons (MTH) processes, especially methanol-to-propylene (MTP) and methanol-to-olefins (MTO), have attracted a good deal of interest in the past few years as an attractive alternative method for propylene production. Zeotypes or acidic zeolites, such as HSAPO-34 and HZSM-5, have been used as catalysts in the MTO and/or MTP processes, and many efforts have been made to understand the effects of acidity together with reaction conditions on the catalytic performance.1−22 The well-known methanol conversion consists of three main reaction steps: Methanol is dehydrated to dimethyl ether (DME), and the equilibrium mixture formed, consisting of methanol, DME, and water, undergoes further dehydration to produce light olefins. The subsequent conversion of light olefins to paraffins, aromatics, naphthenes, and higher olefins occurs. Equation 1 describes this reaction pathway. −H 2 O

Our main interest is to increase the propylene selectivity and propylene/ethylene (P/E) ratio in the MTP process. It was reported that the propylene selectivity can be enhanced by a cooperative effect of increased reaction temperature and increased SiO2/Al2O3 ratio of HZSM-56 and improved with decreasing HZSM-5 crystal size.7,8 Modifying the reaction conditions by decreasing the methanol partial pressure or cofeeding water leads to a higher yield of light olefins.1,9 The propylene selectivity and P/E ratio can be improved by modifying HZSM-5 catalyst through ZrO2/H3PO4 addition,10 alkaline treatment,11 phosphorus treatment,12,13 and aluminum phosphate addition.14−16 However, post-treatment of the catalyst might lead to changes in the physical properties and acid site density, as well as the acid strength of catalyst. Changes in the physicochemical properties of the catalyst, especially the Brønsted acid sites, strongly affect the olefin selectivity in methanol conversion. The effects of the acidic properties of isomorphously substituted ZSM-5 zeolites on light olefin selectivity in methanol conversion have also been reported. Chu and Chang reported that the strengths of the Brønsted acid sites of isomorphously frameworksubstituted ZSM-5 increase in the order B(OH)Si ≪ Fe(OH)Si < Ga(OH)Si < Al(OH)Si.17 The boron-substituted ZSM-5 with a very low acid strength was found to be less active than [Al]-ZSM-5 in the MTO process.18,19 Iron silicate with an MFI structure

−H 2 O

2CH3OH XoooooY CH3OCH3 ⎯⎯⎯⎯⎯→ light olefins

Received: Revised: Accepted: Published:

+H 2 O

→ higher olefins, n ‐/ isoparaffins, aromatics, naphthenes (1) © 2014 American Chemical Society

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rate of 5 °C/min from 100 to 600 °C, and the signal for desorption was recorded with a thermal conductivity detector (TCD). The surface acid densities of the three MFI zeolites were studied by Fourier transform infrared (FT-IR) spectroscopy of adsorbed pyridine. IR spectra of pyridine adsorbed on the samples were recorded on a NEXUS FT-IR spectrometer (Nicolet) equipped with a mercury cadmium telluride (MCT) detector with a resolution of 4 cm−1 and 200 scans per spectrum. Before the measurement of pyridine adsorption, all samples were pressed into self-supported thin wafers (5 mg/cm2) and placed into a stainless steel cell with CaF2 windows. The sample disks were preheated at 500 °C for 3 h and then allowed to cool to 150 °C. Then, adsorption was carried out at 150 °C by injecting 5 mL of pyridine. Physically adsorbed pyridine in the sample was evacuated for 1 h at 150 °C, and IR spectra were recorded at different temperatures in the range of 150−500 °C. Powder X-ray diffraction (XRD) patterns were obtained on a Rigaku D/MAX-III diffractometer using Cu Kα radiation (λ = 1.54173 Å). Data were collected in continuous scan mode from 5° to 50° (2θ) with a 0.01° sampling interval and a 3°/min scan rate for the confirmation of MFI structure. X-ray absorption near-edge structure (XANES) experiments were performed for the Fe K-edges of H-[Fe]-ZSM-5 and H[Al,Fe]-ZSM-5 at room temperature using Beamline 7C1 at Pohang Light Source. The energy was calibrated by the distinct peak at the Fe K-edge of an Fe foil at 7112 eV. FePO4 powder and Fe2O3 powder from Aldrich were used as the model compounds for tetrahedrally and octahedrally coordinated Fe3+ species, respectively.25 For analysis of the surface morphology and crystallite size, scanning electron microscopy (SEM) images of the three MFI zeolites were obtained with a field-emission-type scanning electron microscope (Hitachi S4800) operating at an acceleration voltage of 1.0−2.0 kV. The samples were prepared by sprinkling the powder materials onto one face of double-sided sticky carbon tape whose other face was pasted on a microscope stub and sputtering them with gold. The Brunauer−Emmett−Teller (BET) surface areas and pore volumes of the three MFI zeolites were measured by N2 adsorption using an ASAP2010 instrument (Micromeritics Inc.). The samples were degassed at 250 °C for 6 h, and N2 adsorption was carried out at −196 °C. 2.3. Catalytic Conversion of Methanol. Methanol conversion was carried out at atmospheric pressure in a fixedbed quartz reactor at 400, 450, and 500 °C. Prior to each reaction, the samples (0.5 g) were pretreated in flowing He at 550 °C for 2 h and cooled to the reaction temperature. Methanol (Sigma-Aldrich, ≥99.9%) was fed into the reactor by a liquid mass flow controller (Bronkhorst High-Tech, LIQUIFLOW series L1) and the weight hourly space velocity (WHSV) was 2.55 h−1. A homogeneous mixture of MeOH (10%) and He (90%) was achieved by using a preheater to vaporize the methanol. All products were passed through a heated transfer line to a gas chromatograph with a thermal conductivity detector and a flame ionization detector (HP-PLOT Q column, Agilent) in series.

exhibited a higher selectivity in the formation of light olefins.20,21 It was also reported that the gallium in the [Si,Ga]-ZSM-5 is responsible for the aromatization of olefins in methanol conversion.22 The aim of this work was to optimize the acid strength of MFI zeolite for the maximization of the propylene selectivity and P/E ratio in the MTH reaction. For this purpose, the acid strength of MFI zeolite was controlled systematically through the incorporation of Al3+ and/or Fe3+ into the framework with the same acid site concentration to obtain maximized propylene selectivity in methanol conversion. Herein, three MFI zeolites, namely, H-[Al]-ZSM-5, H-[Fe]-ZSM-5, and H-[Al,Fe]-ZSM-5, with different acid strengths were prepared by hydrothermal synthesis, and their physicochemical properties were characterized by NH3 temperature-programmed desorption (TPD), pyridine IR sepctroscopy, N2 adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray absorption nearedge structure (XANES) spectroscopy. The differences in the catalytic performances of the three MFI zeolites were investigated in terms of acid strength.

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. Na-[Al]-ZSM-5 was prepared by hydrothermal synthesis according to the method of a U.S. patent.23 Na-[Fe]-ZSM-5 was synthesized as described elsewhere,24 using Ludox HS-40 as the silica source and Fe(NO3)3·9H2O (Merck) as the iron source. The final product was a white crystalline powder having the MFI structure as shown by X-ray diffraction. Na-[Al,Fe]-ZSM-5 was synthesized as follows: Separately, 0.26 g of Fe(NO3)3·9H2O (Merck) and 0.25 g of Al(NO3)3· 9H2O (Junsei) were dissolved in 23.3 g of distilled water. Then, these salt solutions were added dropwise to a mixture of 38.3 g of Ludox HS-40 (Aldrich) and 40 g of distilled water. Next, a solution of 13.5 g of tetrapropylammonium bromide (TPABr, Aldrich) dissolved in 30 g of distilled water was added to the mixture with stirring. A NaOH solution was then added with vigorous stirring. A strong gel mixture was formed at first, and then it gradually became a transparent solution. Finally, the mixture solution was transferred to a Teflon-lined stainless steel autoclave and heated at 170 °C for 3 days. The product was a white crystalline powder, indicated by XRD to be of the MFI structure type. All of the as-synthesized samples were filtered, washed, dried, and calcined overnight at 550 °C. The calcined samples were converted to the NH4+ forms by ion exchange with 1 M NH4Cl solution. The samples in NH4+ form were calcined again in flowing air at 550 °C for 3 h to obtain their acidic form. H-[Al]-ZSM-5 (SiO2/Al2O3 = 400), H-[Fe]-ZSM-5 (SiO2/ Fe2O3 = 400), and H-[Al,Fe]-ZSM-5 [SiO2/(Al2O3 + Fe2O3) = 400, Al/Fe = 1] were synthesized so as to form the same amount of acid sites in each case. (H-[Al,Fe]-ZSM-5 contains Si, Al, and Fe atoms in T positions, whereas H-[Fe]-ZSM-5 contains only Si and Fe atoms.) 2.2. Catalyst Characterization. Temperature-programmed desorption (TPD) of NH3 for the three MFI zeolites was carried out in a conventional flow apparatus (Pulsechemisorb 2705, Micromeritics Inc.). A 0.05-g of sample was loaded in a U-type tube. Before adsorption, the samples were pretreated at 500 °C with a He flow of 20 mL/min for 2 h. After the samples had cooled to 100 °C, ammonia was adsorbed by pulse injection. The temperature of the samples was increased at a

3. RESULTS AND DISCUSSION In Figure 1, the XRD patterns of the prepared H-[Al]-ZSM-5, H-[Al,Fe]-ZSM-5, and H-[Fe]-ZSM-5 catalysts are compared. The XRD patterns of all three zeolites are the same and confirm that all of the prepared zeolites have the MFI structure. 10073

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Figure 1. XRD patterns of H-[Al]-ZSM-5, H-[Al,Fe]-ZSM-5, and H-[Fe]-ZSM-5.

SEM images of the three MFI zeolites prepared by hydrothermal synthesis are shown in Figure 2. The three MFI zeolites are composed of many small crystals, with estimated crystal sizes of about 2 μm. The shapes of crystals of H-[Al,Fe]-ZSM-5 (Figure 2b) and H-[Fe]-ZSM-5 (Figure 2c) with the incorporation of Fe are similar to that of H-[Al]-ZSM-5 (Figure 2a). The crystal size and shape of zeolites are important factors influencing the methanol conversion to light olefins because long diffusion paths increase the probability of further conversion of light olefins produced from methanol and the diffusion resistance.8,9 As confirmed by SEM, all of the prepared MFI zeolites have almost the same crystal size and shape. XANES spectroscopy was employed to investigate the coordinative state of framework Fe3+ species in H-[Al,Fe]ZSM-5 and H-[Fe]-ZSM-5. The XANES spectra (pre-edge region) of H-[Al,Fe]-ZSM-5, H-[Fe]-ZSM-5, and model compounds with tetrahedral (FePO4) and octahedral (Fe2O3) coordination are shown in Figure 3. The pre-edge spectra of H-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5 in Figure 3a showed a characteristic single peak at 7114 eV, similar to that of the FePO4 reference compound in Figure 3b. The peaks of H-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5, however, are decreased in intensity compared to that of the reference compound. This result indicates that most of the Fe3+ species in H-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5 are in the framework with tetrahedral and distorted tetrahedral symmetries.19 Nevertheless, H-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5 were considered to contain traces of extraframework Fe3+ species induced by partial breaking of some bonds connecting iron to the oxygen of the framework through template burning.25−28 The NH3 TPD results for the three MFI zeolites are shown in Figure 4. Generally, the NH3 TPD profile of H-ZSM-5 zeolite shows two peaks: a low-temperature peak at around 150 °C (weak acid sites) and a high-temperature peak at around 350 °C (strong acid sites). The acid strength can be evaluated from the high-temperature peak of the TPD profile. The high-temperature peaks of H-[Al]-ZSM-5, H-[Al,Fe]ZSM-5, and H-[Fe]-ZSM-5 are at about 350, 315, and 280 °C, respectively, in agreement with an earlier study.29 These results indicate that the acid strength decreased with the incorporation of Fe3+ into the framework because of the weaker electronacceptor properties of Fe3+ ion compared to Al3+.30 H-[Al,Fe]ZSM-5 shows a broad high-temperature peak at 315 °C and spans the desorption range of the other two zeolites. This result

Figure 2. SEM images of the three MFI zeolites: (a) H-[Al]-ZSM-5, (b) H-[Al,Fe]-ZSM-5, and (c) H-[Fe]-ZSM-5.

is consistent with the coexistence of Al- and Fe-based acid sites. The total amounts of acid in H-[Al]-ZSM-5, H-[Al,Fe]-ZSM-5, and H-[Fe]-ZSM-5 were found to be 50.4, 51.0, and 49.1 μmol/g, respectively. The amounts of acid sites in the three MFI zeolites remain the same, whereas the acid strength follows the sequence H-[Fe]-ZSM-5 < H-[Al,Fe]-ZSM-5 < H-[Al]-ZSM-5. To elucidate the nature and amounts of the Brønsted and Lewis acid sites, pyridine IR analyses of the three MFI zeolites were conducted. Generally, bands at around 1545 and 1445 cm−1 in IR spectra are characteristic of Brønsted (PyH+) and Lewis (L-Py) acid sites, respectively.31−33 The FT-IR spectra of the three MFI zeolites, H-[Al]-ZSM-5, H-[Al,Fe]-ZSM-5, and H-[Fe]-ZSM-5, recorded after pyridine desorption at 150, 200, 300, and 400 °C are presented in panels a−c, respectively, of Figure 5. The bands at 1546 cm−1 are attributed to Brønsted acid sites, whereas the bands at 1450 cm−1 are attributed to pyridine adsorbed on Lewis acid sites. The bands at around 1600 cm−1 in the FT-IR spectrum of H-[Fe]-ZSM-5 (Figure 5c) are also 10074

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Figure 3. X-ray absorption spectra, pre-edge region: (a) H-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5, (b) reference compounds.

Table 1. Concentrations of Brønsted (PyH+) and Lewis (L-Py) Acid Sites in the Three MFI Zeolites Determined by FT-IR Spectroscopy after Pyridine Adsorptiona T (°C)

Brønsted acid sites (μmol/g)

Lewis acid sites (μmol/g)

H-[Al]-ZSM-5 150 200 300 400 500 150 200 300 400 500

Figure 4. NH3 TPD results for the three MFI zeolites.

due to pyridine adsorbed on Lewis acid sites.30 Comparison of the three MFI zeolites shows that the concentrations of Brønsted and Lewis acid sites occupied by pyridine decreased with increasing temperature and that pyridine desorbed more easily from Lewis acid sites than from Brønsted acid sites. The concentrations of Brønsted and Lewis acid sites were determined from the areas of the bands at 1546 and 1450 cm−1, respectively, by employing the corresponding extinction coefficients εB = 1.67 ± 0.12 cm/μmol and εL = 2.22 ± 0.21 cm/μmol, as reported in ref 32. The results are summarized in Table 1 and shown in Figure 6. According to the pyridine IR spectra obtained at 150 °C, the three MFI zeolites showed almost the same Brønsted acid site densities. However, the concentrations of Brønsted acid sites occupied by pyridine on H-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5 with the Fe incorporation decreased as the temperature was increased from 200 to 400 °C more easily than did that on H-[Al]-ZSM-5, so the acid sites on H-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5 can therefore be considered weaker. Comparison of the changes in the concentrations of Brønsted acid sites on the three MFI zeolites shows that the acid strength increases in the order Fe-(OH)-Si < Al,Fe-(OH)-Si < Al-(OH)-Si. Table 2 summarizes the physicochemical properties of the three MFI zeolites. All of the MFI zeolites were found to have similar physicochemical properties except for acid strength.

150 200 300 400 500

41.6 40.1 34.6 28.5 18.6 H-[Al,Fe]-ZSM-5 41.0 39.7 33.6 25.7 9.4 H-[Fe]-ZSM-5 37.1 35.8 31.2 21.1 3.8

8.3 5.2 3.1 1.4 1.1 9.2 5.9 3.4 1.8 1.5 11.6 7.0 4.4 2.9 2.4

a Extinction coefficient values εB = 1.67 ± 0.12 cm/μmol and εL = 2.22 ± 0.21 cm/μmol were used.32

The MTH activities of the three MFI zeolites with almost the same Brønsted acid densities at different reaction temperatures are summarized in Table 3 and shown in Figure 7. Methanol was completely converted into hydrocarbons over H-[Al]ZSM-5 with the highest acid strength, and the selectivities to ethylene and propylene were 4.9% and 26.1%, respectively, where the P/E ratio of 5.3 was the lowest. Compared with H-[Al]-ZSM-5, the propylene selectivity of H-[Fe]-ZSM-5 with the weakest acid strength was dramatically improved, and the P/E ratio was the highest, with a value of 31. The methanol conversion, however, was only 75.6%, and large amount of dimethyl ether (DME) was produced as an intermediate. The low methanol conversion and DME not converted to hydrocarbons might be due to a lower catalytic activity caused by a lower acid strength. In the case of H-[Al,Fe]-ZSM-5, the 10075

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Figure 5. FTIR spectra of pyridine adsorbed on the three MFI zeolites at different temperatures: (a) H-[Al]-ZSM-5, (b) H-[Al,Fe]-ZSM-5, (c) H-[Fe]-ZSM-5.

Table 2. Physicochemical Properties of the Three MFI Zeolites BET surface area (m2/g) pore volume (cm3/g) (micropore/mesopore volume ratio) crystal sizea (μm) total acid amountb (μmol/g) acid contentc (μmol/g) (Bronsted/Lewis acid site ratio) TPD peak temperature (°C) a

H-[Al]-ZSM-5

H-[Al,Fe]-ZSM-5

H-[Fe]-ZSM-5

451 0.25 (0.16/0.09) 1.5−2 50.4 49.9 (41.6/8.3) 350

467 0.24 (0.16/0.08) 2−3 51.0 50.2 (41.0/9.2) 315

487 0.25 (0.16/0.09) 2−2.5 49.1 48.7 (37.1/11.6) 280

Estimated from SEM images. bFrom NH3 TPD measurements. cFrom pyridine IR analysis at 150 °C.

methanol conversion was 96.9% at 400 °C (with a small amount of DME), and the selectivities to ethylene and propylene were 4.4% and 33.4%, respectively (the P/E ratio was 7.6). An increase in temperature from 400 to 500 °C increased the ethylene and propylene selectivities of H-[Al]ZSM-5 and H-[Al,Fe]-ZSM-5, whereas the selectivities to light paraffins (C2−C4 saturated hydrocarbons) and oligomerized products (C5+ hydrocarbons) decreased. For H-[Al]-ZSM-5, the ethylene and propylene selectivities were 5.1% and 39.2%, respectively, at 450 °C and 8.1% and 46.1%, respectively, at

500 °C. Compared with H-[Al]-ZSM-5, H-[Al,Fe]-ZSM-5, with broad acid strength distribution, provided an improved propylene selectivity above 450 °C but slightly decreased selectivities to ethylene and C2−C4 saturated hydrocarbons. As a result, a maximum propylene selectivity of 49.3% was achieved on H-[Al,Fe]-ZSM-5 at 500 °C. In the case of H-[Fe]-ZSM-5 with the lowest acid strength, the increase in reaction temperature caused an increase in the conversion of oxygenates (MeOH + DME) but a decrease in the propylene selectivity. Moreover, the selectivity to oligomerized C5+ hydrocarbons increased with 10076

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Figure 6. Changes in the concentrations of (a) Brønsted acid sites and (b) Lewis acid sites occupied by pyridine over the three MFI zeolites.

Figure 7. MTH activities of the three MFI zeolites at different reaction temperatures: (a) 400, (b) 450, and (c) 500 °C. Reaction conditions: WHSV = 2.55 h−1, time on stream = 2 h, reaction temperature = 450 °C, MeOH/He = 1:9.

reaction temperature. The propylene selectivity of H-[Fe]-ZSM-5 was 38.1% at 450 °C and was 34.5% at 500 °C. An important step in the MTP process is the controlling the reaction at the olefin formation stage, where the Brønsted acid sites of the catalyst play a crucial role. In the case of H-ZSM-5 without Brønsted acid sites, methanol was not converted to hydrocarbon, and the main product was DME.34 In addition, the selective cracking of long-chain hydrocarbon intermediates to form propylene is a key for the effective conversion of methanol to propylene. Selectivity to light olefins in methanol conversion is known to be favored at high temperatures due to secondary cracking reactions,6,35,36 and the cracking activity of a catalyst is enhanced by an increase in the strength of its Brønsted acid sites.37,38 The lower ethylene and propylene selectivities of H-[Fe]-ZSM-5 at high temperature (above 450 °C) can be attributed to a low cracking activity caused by a lower acid

strength. The results for H-[Fe]-ZSM-5 are consistent with those of a previous study on the cracking of pentenes reported by Bortnovsky et al.39 However, H-[Fe]-ZSM-5 showed a higher P/E ratio than H-[Al,Fe]-ZSM-5 and H-[Al]-ZSM-5 over the whole range of reaction temperatures, and the lower acid strength of H-[Fe]-ZSM-5 played an important role in increasing the P/E ratio in the MTP process. H-[Fe]-ZSM-5 also produced less ethene but more methane, which is undesirable for the MTH process. The P/E ratio of H-[Al]-ZSM-5 with a high propylene selectivity was lower than those of the two other zeolites at high reaction temperature because of its higher ethylene selectivity. It is implied that the high acid strength of H-[Al]-ZSM-5 encourages secondary cracking reactions to produce ethylene as well as propylene. 10077

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Table 3. MTH Reaction over H-[Al]-ZSM-5, H-[Al,Fe]-ZSM-5, and H-[Fe]-ZSM-5 Catalystsa product distribution (C mol %) reaction temperature (°C)

MeOH conversion (%) [MeOH + DME conversion (%)]

400 450 500

100 100 100

400

96.9 [96.8] 100 100

450 500 400 450 500 a

75.6 [65.6] 96.6 [96.3] 99.5

C1

H-[Al]-ZSM-5 0.20 0.25 0.47 H-[Al,Fe]-ZSM-5 0.26 (0.26) 0.52 1.31 H-[Fe]-ZSM-5 0.32 (0.37) 1.11 (1.11) 3.54

C2 

C3 

C4

C2−4b

C5+

DME

P/Ec

4.9 5.1 8.1

26.1 39.2 46.1

23.7 28.5 26.0

10.7 8.6 6.8

34.4 18.4 12.5

− − −

5.3 7.7 5.7

4.4 (4.4) 4.5 7.3

33.4 (33.4) 45.1 49.3

26.5 (26.5) 28.8 25.8

9.6 (9.6) 7.7 6.3

25.7 (25.7) 13.4 10.0

0.13 (−) − −

7.6 (7.6) 10.0 6.6

1.2 (1.4) 1.8 (1.8) 3.8

37.8 (43.6) 38.1 (38.2) 34.5

25.2 (29.0) 24.3 (24.4) 19.3

6.9 (8.0) 5.7 (5.7) 4.4

15.3 (17.6) 28.7 (28.8) 34.4

13.2 (−) 0.28 (−) −

31.0 (31.1) 21.2 (21.2) 9.0

Reaction conditions: WHSV = 2.55 h−1, time on stream = 2 h, MeOH/He = 1:9. bC2−C4 saturated hydrocarbons. cPropylene/ethylene ratio.

Compared with H-[Al]-ZSM-5, H-[Al,Fe]-ZSM-5 provided a higher propylene selectivity and P/E ratio at high reaction temperature but a lower ethylene selectivity. Considering these results, the propylene selectivity and P/E ratio are affected by the acid strength of the catalyst in methanol conversion, and there is an optimum acid strength of catalyst for the high propylene selectivity. H-[Al,Fe]-ZSM-5 with a broad acid strength distribution induced by the coexistence of strong Al-based Brønsted acid sites and weaker Fe-based Brønsted acid sites was found to provide the highest propylene selectivity. It is concluded that the incorporation of Al3+ and Fe3+ into the framework can optimize the acid strength in MFI zeolite for the maximization of propylene selectivity in the MTH reaction.

Al3+ and Fe3+ into the framework for the maximization of propylene selectivity in the MTH reaction.

4. CONCLUSIONS H-[Al]-ZSM-5, H-[Fe]-ZSM-5, and H-[Al,Fe]-ZSM-5 with the same amount of acid sites were prepared by hydrothermal synthesis and used for the MTH reaction at different reaction temperatures. The acid strength in MFI zeolite was controlled by the incorporation of Al3+ and/or Fe3+ into the framework with the same acid site concentration. The acid strengths of the prepared MFI zeolites followed the sequence H-[Fe]-ZSM-5 < H-[Al,Fe]-ZSM-5 < H-[Al]-ZSM-5. The Brønsted acid densities of H-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5, obtained from pyridine IR spectra, decreased as the temperature was increased from 200 to 300 °C more easily than that of H-[Al]ZSM-5, where the decrease was highest for H-[Fe]-ZSM-5. H-[Fe]-ZSM-5, with the lowest acid strength, showed a higher propylene selectivity and P/E ratio at 400 °C, where the methanol conversion was low and the DME formation was high. However, its propylene selectivity was significantly lower than those of the other two zeolites at higher reaction temperatures (above 450 °C). The best catalytic performance was obtained with H-[Al,Fe]-ZSM-5, which has a broad acid strength distribution because of the coexistence of strong Albased Brønsted acid sites and weaker Fe-based Brønsted acid sites. Its propylene selectivity was much higher than those of the other zeolites at 450 °C, and a maximum propylene selectivity of 49.3% was achieved at 500 °C. The acid strengths in the MFI zeolites could be optimized by the incorporation of

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Corresponding Author

*Tel.: +82-42-350-3915. Fax: +82-42-350-5955. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2012R1A1A2000922). REFERENCES

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