acid strength control in mfi zeolite for the methanol-to...

Acid Strength Control in MFI Zeolite for the Methanol-to-Hydrocarbons (MTH) ReactionKi-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 propyleneselectivity and P/E (propylene/ethylene) ratio in the methanol-to-hydrocarbons (MTH) reaction. The acid strength of MFIzeolite is controlled by the incorporation of Al3+ and/or Fe3+ into the framework with the same acid site concentration. ThreeMFI 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 physicochemicalproperties 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 withincreasing temperature more easily than that of H-[Al]-ZSM-5, where the decrease was highest for H-[Fe]-ZSM-5. With thelowest acid strength, H-[Fe]-ZSM-5 showed a higher propylene selectivity and P/E ratio at 400 °C, where it exhibited lowmethanol conversion and high DME formation. However, its propylene selectivity was significantly lower than those of the othertwo zeolites at higher reaction temperatures (above 450 °C). The best catalytic performance was obtained with H-[Al,Fe]-ZSM-5with a broad acid strength distribution because of the coexistence of strong Al-based Brønsted acid sites and weaker Fe-basedBrønsted acid sites. Its propylene selectivity was much higher than those of the others at 450 °C, and a maximum propyleneselectivity of 49.3% was achieved at 500 °C. It is demonstrated that the acid strength of MFI zeolite can be optimized by theincorporation 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 applicabletechnology have led to an increase in the availability of naturalgas reserves. The abundance of natural gas has led many industrialresearchers to search for feasible methods for converting this rawmaterial into marketable products. This gas can be converted intomethanol through steam reforming, and methanol can, in turn, beconverted into various chemicals. The catalytic conversion ofmethanol to hydrocarbons is an interesting and promising methodof 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 anattractive alternative method for propylene production. Zeotypes oracidic zeolites, such as HSAPO-34 and HZSM-5, have been used ascatalysts in the MTO and/or MTP processes, and many effortshave been made to understand the effects of acidity together withreaction conditions on the catalytic performance.1−22

The well-known methanol conversion consists of three mainreaction steps: Methanol is dehydrated to dimethyl ether(DME), and the equilibrium mixture formed, consisting ofmethanol, DME, and water, undergoes further dehydration toproduce light olefins. The subsequent conversion of lightolefins to paraffins, aromatics, naphthenes, and higher olefinsoccurs. Equation 1 describes this reaction pathway.

⎯ →⎯⎯⎯⎯

→ ‐

+

− −X Yooooo

n

2CH OH CH OCH light olefins

higher olefins, / isoparaffins, aromatics, naphthenes

3H O

H O3 3

H O

2

2 2

(1)

Our main interest is to increase the propylene selectivity andpropylene/ethylene (P/E) ratio in the MTP process. It wasreported that the propylene selectivity can be enhanced by acooperative effect of increased reaction temperature andincreased SiO2/Al2O3 ratio of HZSM-56 and improved withdecreasing HZSM-5 crystal size.7,8 Modifying the reactionconditions by decreasing the methanol partial pressure orcofeeding water leads to a higher yield of light olefins.1,9 Thepropylene selectivity and P/E ratio can be improved bymodifying HZSM-5 catalyst through ZrO2/H3PO4 addition,

10

alkaline treatment,11 phosphorus treatment,12,13 and aluminumphosphate addition.14−16 However, post-treatment of the catalystmight lead to changes in the physical properties and acid sitedensity, as well as the acid strength of catalyst. Changes in thephysicochemical properties of the catalyst, especially the Brønstedacid sites, strongly affect the olefin selectivity in methanolconversion.The effects of the acidic properties of isomorphously substituted

ZSM-5 zeolites on light olefin selectivity in methanol conversionhave also been reported. Chu and Chang reported that thestrengths of the Brønsted acid sites of isomorphously framework-substituted 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 avery low acid strength was found to be less active than [Al]-ZSM-5in the MTO process.18,19 Iron silicate with an MFI structure

Received: March 2, 2014Revised: May 15, 2014Accepted: May 16, 2014Published: May 16, 2014

Article

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© 2014 American Chemical Society 10072 dx.doi.org/10.1021/ie5009037 | Ind. Eng. Chem. Res. 2014, 53, 10072−10079

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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 isresponsible for the aromatization of olefins in methanolconversion.22

The aim of this work was to optimize the acid strength ofMFI zeolite for the maximization of the propylene selectivityand P/E ratio in the MTH reaction. For this purpose, the acidstrength of MFI zeolite was controlled systematically throughthe incorporation of Al3+ and/or Fe3+ into the framework withthe same acid site concentration to obtain maximized propyleneselectivity 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 hydrothermalsynthesis, and their physicochemical properties were charac-terized by NH3 temperature-programmed desorption (TPD),pyridine IR sepctroscopy, N2 adsorption, X-ray diffraction (XRD),scanning electron microscopy (SEM), and X-ray absorption near-edge structure (XANES) spectroscopy. The differences in thecatalytic performances of the three MFI zeolites were investigatedin terms of acid strength.

2. EXPERIMENTAL SECTION2.1. Preparation of Catalysts. Na-[Al]-ZSM-5 was

prepared by hydrothermal synthesis according to the methodof 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 whitecrystalline powder having the MFI structure as shown by X-raydiffraction.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 gof Ludox HS-40 (Aldrich) and 40 g of distilled water. Next, asolution of 13.5 g of tetrapropylammonium bromide (TPABr,Aldrich) dissolved in 30 g of distilled water was added to themixture with stirring. A NaOH solution was then added withvigorous stirring. A strong gel mixture was formed at first, andthen it gradually became a transparent solution. Finally, themixture solution was transferred to a Teflon-lined stainless steelautoclave and heated at 170 °C for 3 days. The product was awhite crystalline powder, indicated by XRD to be of the MFIstructure type.All of the as-synthesized samples were filtered, washed, dried,

and calcined overnight at 550 °C. The calcined samples wereconverted to the NH4

+ forms by ion exchange with 1 M NH4Clsolution. The samples in NH4

+ form were calcined again inflowing 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 sameamount of acid sites in each case. (H-[Al,Fe]-ZSM-5 containsSi, Al, and Fe atoms in T positions, whereas H-[Fe]-ZSM-5contains only Si and Fe atoms.)2.2. Catalyst Characterization. Temperature-programmed

desorption (TPD) of NH3 for the three MFI zeolites wascarried out in a conventional flow apparatus (Pulsechemisorb2705, Micromeritics Inc.). A 0.05-g of sample was loaded in aU-type tube. Before adsorption, the samples were pretreated at500 °C with a He flow of 20 mL/min for 2 h. After the sampleshad cooled to 100 °C, ammonia was adsorbed by pulseinjection. The temperature of the samples was increased at a

rate of 5 °C/min from 100 to 600 °C, and the signal fordesorption 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 ofadsorbed pyridine. IR spectra of pyridine adsorbed on thesamples 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 werepressed into self-supported thin wafers (5 mg/cm2) and placedinto a stainless steel cell with CaF2 windows. The sample diskswere preheated at 500 °C for 3 h and then allowed to cool to150 °C. Then, adsorption was carried out at 150 °C by injecting5 mL of pyridine. Physically adsorbed pyridine in the sample wasevacuated for 1 h at 150 °C, and IR spectra were recorded atdifferent 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 modefrom 5° to 50° (2θ) with a 0.01° sampling interval and a 3°/minscan 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 atPohang Light Source. The energy was calibrated by the distinctpeak at the Fe K-edge of an Fe foil at 7112 eV. FePO4 powderand Fe2O3 powder from Aldrich were used as the modelcompounds 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 MFIzeolites were obtained with a field-emission-type scanningelectron microscope (Hitachi S4800) operating at an accelerationvoltage of 1.0−2.0 kV. The samples were prepared by sprinklingthe powder materials onto one face of double-sided sticky carbontape whose other face was pasted on a microscope stub andsputtering them with gold.The Brunauer−Emmett−Teller (BET) surface areas and

pore volumes of the three MFI zeolites were measured by N2adsorption using an ASAP2010 instrument (Micromeritics Inc.).The samples were degassed at 250 °C for 6 h, and N2 adsorptionwas carried out at −196 °C.

2.3. Catalytic Conversion of Methanol. Methanolconversion was carried out at atmospheric pressure in a fixed-bed quartz reactor at 400, 450, and 500 °C. Prior to eachreaction, the samples (0.5 g) were pretreated in flowing He at550 °C for 2 h and cooled to the reaction temperature.Methanol (Sigma-Aldrich, ≥99.9%) was fed into the reactor bya liquid mass flow controller (Bronkhorst High-Tech, LIQUI-FLOW 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 tovaporize the methanol. All products were passed through aheated transfer line to a gas chromatograph with a thermalconductivity detector and a flame ionization detector (HP-PLOT Qcolumn, Agilent) in series.

3. RESULTS AND DISCUSSIONIn 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 confirmthat all of the prepared zeolites have the MFI structure.

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dx.doi.org/10.1021/ie5009037 | Ind. Eng. Chem. Res. 2014, 53, 10072−1007910073

SEM images of the three MFI zeolites prepared by hydro-thermal synthesis are shown in Figure 2. The three MFI zeolitesare composed of many small crystals, with estimated crystalsizes 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 in-corporation of Fe are similar to that of H-[Al]-ZSM-5 (Figure 2a).The crystal size and shape of zeolites are important factorsinfluencing the methanol conversion to light olefins because longdiffusion paths increase the probability of further conversion oflight olefins produced from methanol and the diffusionresistance.8,9 As confirmed by SEM, all of the prepared MFIzeolites 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-edgeregion) of H-[Al,Fe]-ZSM-5, H-[Fe]-ZSM-5, and modelcompounds with tetrahedral (FePO4) and octahedral (Fe2O3)coordination are shown in Figure 3. The pre-edge spectra ofH-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5 in Figure 3a showed acharacteristic single peak at 7114 eV, similar to that of the FePO4reference compound in Figure 3b. The peaks of H-[Al,Fe]-ZSM-5and H-[Fe]-ZSM-5, however, are decreased in intensity comparedto that of the reference compound. This result indicates that mostof the Fe3+ species in H-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5 are inthe framework with tetrahedral and distorted tetrahedralsymmetries.19 Nevertheless, H-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5were considered to contain traces of extraframework Fe3+ speciesinduced by partial breaking of some bonds connecting iron to theoxygen of the framework through template burning.25−28

The NH3 TPD results for the three MFI zeolites are shownin Figure 4. Generally, the NH3 TPD profile of H-ZSM-5zeolite shows two peaks: a low-temperature peak at around150 °C (weak acid sites) and a high-temperature peak ataround 350 °C (strong acid sites). The acid strength can beevaluated 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 resultsindicate that the acid strength decreased with the incorporationof Fe3+ into the framework because of the weaker electron-acceptor properties of Fe3+ ion compared to Al3+.30 H-[Al,Fe]-ZSM-5 shows a broad high-temperature peak at 315 °C andspans the desorption range of the other two zeolites. This result

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 zeolitesremain the same, whereas the acid strength follows the sequenceH-[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 zeoliteswere 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 MFIzeolites, H-[Al]-ZSM-5, H-[Al,Fe]-ZSM-5, and H-[Fe]-ZSM-5,recorded after pyridine desorption at 150, 200, 300, and 400 °Care presented in panels a−c, respectively, of Figure 5. Thebands at 1546 cm−1 are attributed to Brønsted acid sites,whereas the bands at 1450 cm−1 are attributed to pyridineadsorbed 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

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

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.



due to pyridine adsorbed on Lewis acid sites.30 Comparison ofthe three MFI zeolites shows that the concentrations ofBrønsted and Lewis acid sites occupied by pyridine decreasedwith increasing temperature and that pyridine desorbed moreeasily from Lewis acid sites than from Brønsted acid sites. Theconcentrations of Brønsted and Lewis acid sites were determinedfrom the areas of the bands at 1546 and 1450 cm−1, respectively, byemploying the corresponding extinction coefficients εB = 1.67 ±0.12 cm/μmol and εL = 2.22 ± 0.21 cm/μmol, as reported in ref32. The results are summarized in Table 1 and shown in Figure 6.According to the pyridine IR spectra obtained at 150 °C, the threeMFI zeolites showed almost the same Brønsted acid site densities.However, the concentrations of Brønsted acid sites occupied bypyridine on H-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5 with the Feincorporation decreased as the temperature was increased from200 to 400 °C more easily than did that on H-[Al]-ZSM-5, so theacid sites on H-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5 can thereforebe considered weaker. Comparison of the changes in theconcentrations of Brønsted acid sites on the three MFI zeolitesshows 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 havesimilar physicochemical properties except for acid strength.

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

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

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

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

T (°C) Brønsted acid sites (μmol/g) Lewis acid sites (μmol/g)

H-[Al]-ZSM-5150 41.6 8.3200 40.1 5.2300 34.6 3.1400 28.5 1.4500 18.6 1.1

H-[Al,Fe]-ZSM-5150 41.0 9.2200 39.7 5.9300 33.6 3.4400 25.7 1.8500 9.4 1.5

H-[Fe]-ZSM-5150 37.1 11.6200 35.8 7.0300 31.2 4.4400 21.1 2.9500 3.8 2.4

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



methanol conversion was 96.9% at 400 °C (with a smallamount of DME), and the selectivities to ethylene andpropylene were 4.4% and 33.4%, respectively (the P/E ratiowas 7.6). An increase in temperature from 400 to 500 °Cincreased the ethylene and propylene selectivities of H-[Al]-ZSM-5 and H-[Al,Fe]-ZSM-5, whereas the selectivities to lightparaffins (C2−C4 saturated hydrocarbons) and oligomerizedproducts (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 improvedpropylene selectivity above 450 °C but slightly decreased selec-tivities to ethylene and C2−C4 saturated hydrocarbons. As a result,a maximum propylene selectivity of 49.3% was achieved onH-[Al,Fe]-ZSM-5 at 500 °C. In the case of H-[Fe]-ZSM-5 withthe lowest acid strength, the increase in reaction temperaturecaused an increase in the conversion of oxygenates (MeOH +DME) but a decrease in the propylene selectivity. Moreover, theselectivity to oligomerized C5+ hydrocarbons increased with

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

H-[Al]-ZSM-5 H-[Al,Fe]-ZSM-5 H-[Fe]-ZSM-5

BET surface area (m2/g) 451 467 487pore volume (cm3/g) 0.25 0.24 0.25(micropore/mesopore volume ratio) (0.16/0.09) (0.16/0.08) (0.16/0.09)crystal sizea (μm) 1.5−2 2−3 2−2.5total acid amountb (μmol/g) 50.4 51.0 49.1acid contentc (μmol/g) 49.9 50.2 48.7(Bronsted/Lewis acid site ratio) (41.6/8.3) (41.0/9.2) (37.1/11.6)TPD peak temperature (°C) 350 315 280

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



reaction temperature. The propylene selectivity of H-[Fe]-ZSM-5was 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 acidsites of the catalyst play a crucial role. In the case of H-ZSM-5without Brønsted acid sites, methanol was not converted tohydrocarbon, and the main product was DME.34 In addition,the selective cracking of long-chain hydrocarbon intermediatesto form propylene is a key for the effective conversion ofmethanol to propylene. Selectivity to light olefins in methanolconversion is known to be favored at high temperatures due tosecondary cracking reactions,6,35,36 and the cracking activity of acatalyst is enhanced by an increase in the strength of itsBrønsted acid sites.37,38 The lower ethylene and propyleneselectivities 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 thoseof a previous study on the cracking of pentenes reported byBortnovsky et al.39 However, H-[Fe]-ZSM-5 showed a higher P/Eratio than H-[Al,Fe]-ZSM-5 and H-[Al]-ZSM-5 over the wholerange of reaction temperatures, and the lower acid strength ofH-[Fe]-ZSM-5 played an important role in increasing the P/E ratioin the MTP process. H-[Fe]-ZSM-5 also produced less ethene butmore methane, which is undesirable for the MTH process. TheP/E ratio of H-[Al]-ZSM-5 with a high propylene selectivity waslower than those of the two other zeolites at high reactiontemperature because of its higher ethylene selectivity. It is impliedthat the high acid strength of H-[Al]-ZSM-5 encourages secondarycracking reactions to produce ethylene as well as propylene.

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.



Compared with H-[Al]-ZSM-5, H-[Al,Fe]-ZSM-5 provided ahigher propylene selectivity and P/E ratio at high reactiontemperature but a lower ethylene selectivity.Considering these results, the propylene selectivity and P/E

ratio are affected by the acid strength of the catalyst inmethanol conversion, and there is an optimum acid strength ofcatalyst for the high propylene selectivity. H-[Al,Fe]-ZSM-5with a broad acid strength distribution induced by thecoexistence of strong Al-based Brønsted acid sites and weakerFe-based Brønsted acid sites was found to provide the highestpropylene selectivity. It is concluded that the incorporation ofAl3+ and Fe3+ into the framework can optimize the acid strengthin MFI zeolite for the maximization of propylene selectivity inthe MTH reaction.

4. CONCLUSIONS

H-[Al]-ZSM-5, H-[Fe]-ZSM-5, and H-[Al,Fe]-ZSM-5 with thesame amount of acid sites were prepared by hydrothermalsynthesis and used for the MTH reaction at different reactiontemperatures. The acid strength in MFI zeolite was controlledby the incorporation of Al3+ and/or Fe3+ into the frameworkwith the same acid site concentration. The acid strengths of theprepared MFI zeolites followed the sequence H-[Fe]-ZSM-5 <H-[Al,Fe]-ZSM-5 < H-[Al]-ZSM-5. The Brønsted aciddensities of H-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5, obtainedfrom pyridine IR spectra, decreased as the temperature wasincreased 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 higherpropylene selectivity and P/E ratio at 400 °C, where themethanol conversion was low and the DME formation washigh. However, its propylene selectivity was significantly lowerthan those of the other two zeolites at higher reactiontemperatures (above 450 °C). The best catalytic performancewas obtained with H-[Al,Fe]-ZSM-5, which has a broad acidstrength distribution because of the coexistence of strong Al-based Brønsted acid sites and weaker Fe-based Brønsted acidsites. Its propylene selectivity was much higher than those ofthe other zeolites at 450 °C, and a maximum propyleneselectivity of 49.3% was achieved at 500 °C. The acid strengthsin the MFI zeolites could be optimized by the incorporation of

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

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +82-42-350-3915. Fax: +82-42-350-5955. E-mail:[email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research was supported by the Basic Science ResearchProgram through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and FuturePlanning (2012R1A1A2000922).

■ REFERENCES(1) Stocker, M. Methanol-to-hydrocarbons: Catalytic materials andtheir behavior. Microporous Mesoporous Mater. 1999, 29, 3.(2) Soundararajan, S.; Dalai, A. K.; Berruti, F. Modeling of methanolto olefins (MTO) process in a circulating fluidized bed reactor. Fuel2001, 80, 1187.(3) Kumita, Y.; Gascon, J.; Stavitski, E.; Moulijn, J. A.; Kapteijn, F.Shape selective methanol to olefins over highly thermostable DDRcatalysts. Appl. Catal. A: Gen. 2011, 391, 234.(4) Bjorgen, M.; Joensen, F.; Lillerud, K. P.; Olsbye, U.; Svelle, S.The mechanisms of ethene and propene formation from methanolover high silica H-ZSM-5 and H-beta. Catal. Today 2009, 142, 90.(5) Ivanova, S.; Lebrun, C.; Vanhaecke, E.; Pham-Huu, C.; Louis, B.Influence of the zeolite synthesis route on its catalytic properties in themethanol to olefin reaction. J. Catal. 2009, 265, 1.(6) Chang, C. D.; Chu, C. T.-W.; Socha, R. F. Methanol conversionto olefins over ZSM-5: I. Effect of temperature and zeolite SiO2/Al2O3.J. Catal. 1984, 86, 289.(7) Prinz, D.; Riekert, L. Formation of ethene and propene frommethanol on zeolite ZSM-5: I. Investigation of rate and selectivity in abatch reactor. Appl. Catal. 1988, 37, 139.(8) Firoozi, M.; Baghalha, M.; Asadi, M. The effect of micro andnano particle sizes of H-ZSM-5 on the selectivity of MTP reaction.Catal. Commun. 2009, 10, 1582.(9) Marchi, A. J.; Froment, G. F. Catalytic conversion of methanolinto light alkenes on mordenite-like zeolites. Appl. Catal. A: Gen. 1993,94, 91.

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 + DMEconversion (%)] C1 C2 C3 C4 C2−4

b C5+ DME P/Ec

H-[Al]-ZSM-5400 100 0.20 4.9 26.1 23.7 10.7 34.4 − 5.3450 100 0.25 5.1 39.2 28.5 8.6 18.4 − 7.7500 100 0.47 8.1 46.1 26.0 6.8 12.5 − 5.7

H-[Al,Fe]-ZSM-5400 96.9

[96.8]0.26(0.26)

4.4(4.4)

33.4(33.4)

26.5(26.5)

9.6(9.6)

25.7(25.7)

0.13(−)

7.6(7.6)

450 100 0.52 4.5 45.1 28.8 7.7 13.4 − 10.0500 100 1.31 7.3 49.3 25.8 6.3 10.0 − 6.6

H-[Fe]-ZSM-5400 75.6

[65.6]0.32(0.37)

1.2(1.4)

37.8(43.6)

25.2(29.0)

6.9(8.0)

15.3(17.6)

13.2(−)

31.0(31.1)

450 96.6[96.3]

1.11(1.11)

1.8(1.8)

38.1(38.2)

24.3(24.4)

5.7(5.7)

28.7(28.8)

0.28(−)

21.2(21.2)

500 99.5 3.54 3.8 34.5 19.3 4.4 34.4 − 9.0aReaction conditions: WHSV = 2.55 h−1, time on stream = 2 h, MeOH/He = 1:9. bC2−C4 saturated hydrocarbons. cPropylene/ethylene ratio.



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acid strength control in mfi zeolite for the methanol-to...

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