JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY

Volume 41, Issue 9, Sep 2013

Online English edition of the Chinese language journal

Received: 11-Feb-2013; Revised: 27-May-2013 * Corresponding author. Tel: +202 22757840; Fax: +202 22581243; E-mail: samehaboulfotouh@yahoo.com. Copyright 2013, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

RESEARCH PAPERCite this article as: J Fuel Chem Technol, 2013, 41(9), 10771084

Effect of ultrasonic irradiation and/or halogenation on the catalytic performance of -Al2O3 for methanol dehydration to dimethyl ether Sameh M. K. Aboul-Fotouh

Ain Shams University, Faculty of Education, Chemistry Department, Roxy, Cairo 11566, Egypt

Abstract: Dimethyl ether (DME) is amongst one of the most promising alternative, renewable and clean fuels being considered as a

future energy carrier. In this study, the comparative catalytic performance of the halogenated γ-Al2O3 prepared from two halogen

precursors (ammonium chloride and ammonium fluoride) is presented. The impact of ultrasonic irradiation was evaluated in order to

optimize both the halogen precursor for the production of DME from methanol in a fixed bed reactor. The catalysts were characterized

by SEM, XRD, BET and NH3-TPD. Under reaction conditions where the temperature ranged from 200 to 400C with a WHSV =15.9

h−1 was found that the halogenated catalysts showed higher activity at all reaction temperatures. However, the halogenated alumina

catalysts prepared under the effect of ultrasonic irradiation showed higher performance of γ-Al2O3 for DME formation. The chlorinated

γ-Al2O3 catalysts showed a higher activity and selectivity for DME production than fluorinated versions.

Keywords: ultrasonication; methanol; DME; γ-Al2O3; Cl; F

Dimethyl ether (DME) has been found to be an alternative diesel fuel because it has low NOx emission, near-zero smoke amounts and less engine noise compared with traditional diesel fuels[1,2]. It can also be used to replace chlorofluorocarbons (CFCs) which destroy ozone layer of the atmosphere and used as an intermediate for producing many valuable chemicals such as lower olefins, methyl acetate, dimethyl sulfate and liquefied petroleum gas (LPG) alternative. It is also used in power generation and as an aerosol propellant, such as in hair spray and shaving cream, due to its liquefaction property[3–7]. Hence, there is a growing demand to produce a large amount of DME to meet the global need.

Dimethyl ether can be produced by methanol dehydration over a solid-acid catalyst or direct synthesis from syngas by employing a hybrid catalyst, comprising a methanol synthesis component and a solid-acid catalyst[8]. Methanol dehydration to dimethyl ether is a potentially important process and more favorable in the views of thermodynamics and economy[9]. Commercially, γ-Al2O3 is used as the catalyst for this reaction. It has high surface area, excellent thermal stability, high mechanical resistance and catalytic activity for DME formation due to its surface acidity. Recently, many methods have been applied to synthesize alumina with a higher specific surface area and activity for DME synthesis[10].

Methanol to DME (MTD) dehydration over a solid acid catalyst in a fixed bed reactor was first reported by Mobil in 1965. Since then, many methanol dehydration catalysts have been examined[11] including γ-Al2O3

[12–17], crystalline aluminosilicates[17,18], zeolites (ZSM-5)[19], clays[20] and phosphates such as aluminum phosphate[21,22]. However, the most common catalysts used are γ-Al2O3 and zeolites.

Chlorination and fluorination of Al2O3 and zeolites have been carried out by the author[23–25] to promote the catalytic acidity. The results of hydroconversion of cyclohexene over metal/Al2O3 with or without Cl– and F– ions have been studied by Aboul-Fotouh et al[24]. Introducing Cl– or F– ions into aluminate aluminas in different ways causes Brönsted acid sites to appear and a drastic increase of both skeletal isomerization and total conversion. Also, Aboul-Fotouh et al[23] studied the effect of chlorination and fluorination modified H-mordenite on the dehydration of methanol to DME and found that the fluorination enhances the acidity and catalytic activity of H-MOR towards DME production.

Ultrasound irradiation can enhance and improving the catalytic performance of the catalyst. Application of power ultrasound has been shown to be instrumental in improving the rates, yields and product properties of a variety of processes in synthetic chemistry[26,27].

Sameh M. K. Aboul-Fotouh / Journal of Fuel Chemistry and Technology, 2013, 41(9): 10771084

Fig. 1 XRD diffraction of the current catalysts

a: Al2O3(non); b: Al2O3(Cl); c: Al2O3(Cl-U); d: Al2O3(F); e: Al2O3(F-U)

The effects of ultrasound have been investigated for

different cases, including polymerization reactions and the syntheses of various amorphous and crystalline materials. Significant changes have been commonly observed in the processes and properties of the reaction products in the presence of ultrasound. The main purpose of using ultrasound in different chemical reactions has been to enhance the reaction rates, yields and selectivity to desired product.

The application of ultrasound irradiation in the preparation of halogenated γ-Al2O3 catalyst has not been previously studied. In the present work, the catalytic dehydration of methanol to DME has been studied using chlorinated or fluorinated γ-Al2O3 catalysts prepared by impregnation method with or without ultrasonic irradiation. The effects of ultrasonication and/or halogenation on the textural, acidic properties and catalytic activity of γ-Al2O3 samples have been investigated.

1 Experimental 1.1 Preparation of the catalysts

The two halogenated catalysts were prepared by

impregnation. The powder γ-Al2O3 was impregnated in an

aqueous solution of ammonium chloride or ammonium fluoride containing the requisite quantity for 3.0% NH4Cl or NH4F, respectively. Ultrasonic irradiation was applied using Ultrasonic Processor UP50H (Hielscher) using the titanium sonotrode S3 having a tip diameter of 3 mm, with 50 W/cm2 power intensity (related to 100% amplitude setting). The irradiation with ultrasound lasted for 1 h at 25C, and then centrifuged for 30 min. The catalyst was dried at 110C overnight and calcined at 400C for 2 h in air flow. The catalysts were defined as Al2O3(Cl-U) and Al2O3(F-U). For comparison, a conventional impregnation catalyst was similarly prepared except that the ultrasonic irradiation was replaced by stirring of the catalysts in the impregnation of NH4Cl and NH4F at room temperature. The catalysts were defined as Al2O3(Cl) and Al2O3(F). 1.2 Hydroconversion reactor system and reaction product analysis

A silica glass flow-type tubular reactor system loaded with

0.1 g of the zeolite catalyst was used. The reactor was heated in an insulated wider silica tube jacket, thermostated to ±1C. Argon was used as a carrier gas at a flow rate of 30 cm3·min–1 in all runs. The methanol feed was introduced into the reactor via continuous evaporation applying argon flow passing into a closed jar thermostated at a fixed temperature of 26°C, whereby the quantity of methanol was always 4.9810–2 mol·h–1. The reaction runs were investigated at temperatures ranging between 200–400°C, with 25°C increments. The reaction effluent was analysed using a Perkin-Elmer Autosystem XL gas-chromatograph with a 4 m long column, packed with 10% squalane plus 10% didecyl phthalate supported on Chromosorb W-HP of 80–100 mesh. A flame ionization detector and a Totalchrom Navigator Programme computed were used.

1.3 Temperature programmed desorption (TPD) of Ammonia

Table 1 Surface properties of the current alumina catalysts

Catalyst Surface area ABET/(m2·g–1) Pore volume v/(cm3·g–1) Pore diameter

d/nm Particle size da/nm Crystallite size db/nm

γ-Al2O3 203.7 0.101 1.975 7.9 3.9

Al2O3(Cl) 267.3 0.130 1.948 6.1 4.0

Al2O3(Cl-U) 282.3 0.132 1.831 5.7 5.0

Al2O3(F) 188.5 0.091 1.923 8.6 3.1

Al2O3(F-U) 321.3 0.152 1.890 5.0 5.1

a: determined by BET area; b: determined by XRD results

Sameh M. K. Aboul-Fotouh / Journal of Fuel Chemistry and Technology, 2013, 41(9): 10771084

Fig. 2 SEM image (200) of the current γ-Al2O3 catalysts

(a): -Al2O3; (b): Al2O3(F-U); (c): Al2O3(F); (d): Al2O3(Cl-U); (e): Al2O3(Cl)

Scheme 1 Enhanced acidity of hydroxyls by the electron-drawing force of fluorine

The TPD of presorbed ammonia on the acid sites of the

zeolite supports was carried out in differential scanning calorimeter (DSC) using nitrogen as a purge gas according to the procedure adopted by the author[28]. 1.4 X-ray diffraction patterns of the catalysts

The X-ray diffraction patterns of the catalysts under study

were carried out using a Phillips X, Pert Diffractometer PW 1390 at 40 kV and 30 mA with Ni Filter and Cu K radiation. The XRD runs were carried out up to 2 of 60o. The traditional XRD patterns obtained for the current catalysts show more or less similar 2 of the diffraction peaks. 1.5 Scanning electron microscope (SEM)

The SEM samples were mounted on aluminum slabs and sputter-coater with a thin gold layer of ~15 nm thicknesses using an Edward sputter-coater. The samples were then examined in a scanning electron microscope model JSM-5410 with Electron probe micro-analyzer (JEOL) at 30 kV.

2 Results and discussion 2.1 Catalyst characterization

The XRD patterns for the current alumina are given in

Figure 1. All treated sample give similar XRD patterns as the original γ-Al2O3. However, these treated samples acquire lower intensities than the untreated one, which indicate relative crystallinity caused via the applied treatments.

Some physic-chemical properties of the current catalysts are listed in Table 1. The BET surface area, total pore volume and crystallite size of the halogenated and ultrasonicated alumina catalysts are significantly increased except for the fluorinated alumina without ultrasonication for which surface area, total pore volume and crystallite size decreased from 203.7 to 188.5 m2·g–1, 0.101 to 0.091 cm3·g–1 and 3.9 to 3.1 nm, respectively. These may be attributed to the pores of alumina being filled up with aluminum fluoride. According to Rodriguez et al[29], two reactions may exist in the fluorination of alumina by NH4F solution:

Al-OH + NH4F Al-F + H2O + NH3 (1)

Sameh M. K. Aboul-Fotouh / Journal of Fuel Chemistry and Technology, 2013, 41(9): 10771084

Al-O-Al + NH4F Al-OH + F-Al + NH3 (2) Reaction (1) is the replacement of external hydroxyl by F–,

which has little effect on the pore structure of alumina. However, reaction (2) means the destruction of the oxy-bridges in the alumina framework. So it is easy to understand that the sample would obtain some new surface via mild eroding the surface of alumina, while losing some of surface area.

The results in Table 1 indicate that ultrasonic irradiation for the halogenated γ-Al2O3 catalyst increases its surface area and decreases the particle size according to the following order: Al2O3 (F-U) > Al2O3 (Cl-U) > Al2O3(Cl) > -Al2O3 > Al2O3(F)

This order is attributed to the ultrasonication energy that attracted γ-Al2O3 catalyst during its preparation obtains more porous catalyst and more dispersed particles[30]. On the other hand, the halogenated alumina catalysts have lower surface area and higher crystallite size than ultrasonicated catalysts. This is caused by distracted the agglomerated particle when ultrasonic irradiation attacks too much to the catalyst[31]. Chave et al[32] found that the BET surface area of mesoporous alumina increases upon ultrasonication.

Fig. 3 NH3-TPD for current Al2O3 catalysts

Fig. 4 Effect of reaction temperature on the production of DME

using alumina catalysts

Fig. 5 Effect of time on methanol conversion using Al2O3(Cl) and

Al2O3(Cl-U) catalysts

Undoubtedly, this finding reflects the drastic changing in mesoporous alumina chemical composition and also its morphology under the effect of power ultrasound.

Ultrasonication gives very significant effect to the surface morphology of halogenated alumina. The SEM (magnification 200) photographs (Figure 2) exhibited smaller crystals for halogenated alumina catalysts (Figures 2(b), 2(d)) synthesized with ultrasound irradiation than the halogenated alumina catalysts (Figures 2(c), 2(e)) synthesized without ultrasound irradiation. It clearly indicated the influence of ultrasound irradiation on the crystal size of halogenated γ-Al2O3 catalysts. However, ultrasound irradiation did not influence the structure of γ-Al2O3. This was confirmed by XRD patterns of γ-Al2O3(Cl-U) or γ-Al2O3(F-U) catalysts (Figure 1) which was similar to parent γ-Al2O3.

Figure 2(a) shows angular particles with larger sizes of γ-Al2O3, compared to the halogenated catalysts (Figures 2(c), 2(e)) which give more rounded and agglomerated particles. However, the particles of the Al2O3(Cl) catalyst are evidently smaller than the Al2O3(F) catalyst.

The ultrasonicated catalysts give highly dispersed particles compared to non-ultrasonicated catalysts. Figure 2(b), 2(d) show that ultrasonication has distracted the agglomerated particles of Al2O3 both after chlorination or fluorination. However, fluorination gives larger particles than chlorination by ultrasonication since F– is more electronegative than Cl–.

The most effective crystallite fragmentation is obtained using fluorination of the Al2O3 as shown in Table 1 and Figure 2.

However, ultrasonication of this Al2O3(F) sample has caused its higher dispersed particles as well as enhanced the surface area from 188.5 to 321.3 m2·g–1. Also, ultrasonication of the chlorinated sample acted similar to that of fluorination, i.e., caused distracted the agglomerated particles to smaller crystals and enhances the surface area from 267.3 to 282.3 m2·g–1.

Sameh M. K. Aboul-Fotouh / Journal of Fuel Chemistry and Technology, 2013, 41(9): 10771084

Fig. 6 Effect of ultrasonication on the d-spacing value of

halogenated alumina catalysts

The theoretical particle sizes are also calculated from surface area, assuming spherical particles, by the following equation:

dBET= 6000/ × A (3) Where dBET is the equivalent particle diameter (nm), is the

density of the material (g·cm–3), and A is the specific surface area (m2·g–1)[33]. It can be observed from Table 1 that the equivalent particle diameter decreased with the ultrasound irradiation. This observation confirmed the positive effect of ultrasound irradiation in decreasing the particle size. As a result, it can be seen that the particle size of catalysts Al2O3(Cl-U) and Al2O3(F-U) are smaller than those of catalysts of Al2O3(Cl) and Al2O3(F). However, the particles of the Al2O3(F-U) catalyst are smaller than the Al2O3(Cl-U) catalyst, which evidently by SEM photographs (Figure 2). Interestingly, applying ultrasound irradiation in the synthesis of halogenated Al2O3 catalyst is found to dramatically alter the morphology of the product. In this case, smaller irregular nanoparticles with size of 5.0 nm are formed instead of rounded and agglomerated particles of both Al2O3(Cl) and Al2O3(F) catalysts with size of 6.1 and 8.6 nm, respectively.

NH3-TPD method was used to investigate the acid properties of the alumina samples and the results are depicted in Figure 3. Two endothermic peaks appeared for all catalysts, a low temperature peak (100250C), which represents NH3 desorption from the weak acid sites and a high temperature peak (275400C) representing NH3 desorption from the strong acid sites. As shown in Figure 3, when the γ-Al2O3 was treated by halogenation (F– or Cl–) the total acidity was increased. However, the ultrasonicated catalysts have an increase of the total acidity of the halogenated alumina. Moreover, the chlorine treatment of alumina enhances the acidity than that using fluorine.

As claimed by Rodriguez et al[29], the bonding of F– to aluminum would enhance the acidity of adjacent hydroxyls (as illustrated in Scheme 1).

Fig. 7 Apparent activation energy of dehydration of methanol to

DME using alumina catalysts

: Al2O3(non), Ea=92.76 kJ/mol;

: Al2O3(Cl), Ea=66.52 kJ/mol;

: Al2O3(Cl-U), Ea=86.90 kJ/mol;

: Al2O3(F), Ea=37.77 kJ/mol;

: Al2O3(F-U), Ea=55.10 kJ/mol

The hydroxyl bond in group (B) is weakened by the electron-drawing force of F–, which brings on the facile ionization of the hydrogen in the hydroxyl group and thereby strengthens its acidity. Otherwise, Al(O4F2) and Al(O3F3), which have stronger acidity than Al(O5F), are also produced during the fluorination[34].

Thereby, we might think that the sample with fluorine have a stronger acidity than with chlorine. However, as shown in Figure 3, the acidity is increased with the chlorination of alumina catalysts. Thus, it can be believed that the framework destruction of the fluorinated alumina plays a dominating role on the acidity, so that fluorination weakens the acidity of the flourinated sample than the chlorinated sample with or without ultrasonication.

Only few data are available for Cl– treated γ-alumina. The result of isomerization of cyclohexene over highly pure aluminas with or without Cl– ions has been studied by Ozimek et al[35,36]. Introducing Cl– ions into aluminate aluminas in different ways causes Brönsted acid sites to appear and both skeletal isomersation and total conversion greatly increase[37]. Arena et al[37] have indicated that Cl– adsorbed on the surface of γ-Al2O3 results in a significant change in the the electronic properties of the outer layer of Al2O3 that decreases the basic Lewis sites, or inducing a stronger “inductive effect” of Cl– on the neighboring hydroxyl groups. This electronic effect weakens the O-H bond, rendering the proton more acidic.

2.2 Catalytic performance

The activity of the mono- and bimetallic supported on

γ-Al2O3 catalysts, either halogenated with Cl or F has been

Sameh M. K. Aboul-Fotouh / Journal of Fuel Chemistry and Technology, 2013, 41(9): 10771084

studied by the author for aromatics hydrogenation[38], cyclohexane dehydrogenation[39] and cyclohexene hydroconverion[24]. It is found that an optimum promotion of the catalytic activities is accomplished using 3.0% of the halogen. Hence, this halogen content is used in preparing the halogenated alumina catalysts under study.

The dehydration of methanol to DME as a function of reaction temperature (200400C) using halogenated γ-Al2O3 catalysts has been investigated as shown in Figure 4. It is observed that the halogenation with Cl– or F– with or without ultrasonication enhanced the performance of alumina to DME formation, and the conversion increased with increasing reaction temperature up to 400C. However, the halogenated γ-Al2O3 catalysts with the effect of ultrasonication have significantly enhanced their activities for the dehydration reaction of methanol, producing DME. The selectivity for DME reached 100% at all reaction temperatures. Also γ-Al2O3 is associated with weak to medium acidic sites, and thus is preferable in the methanol to dimethylether reaction[40] providing greater coke stability and lower by-product formation[41]. Figure 4 shows that the most significant enhancement of catalytic activity is observed via ultrasonication of the Al2O3(Cl) catalyst. The order of the activities of the catalysts under study for DME production decreases in the order: Al2O3(Cl-U) > Al2O3(F-U) > Al2O3(Cl) > Al2O3(F) > Al2O3(non). This catalytic behavior agrees with the order of acidity (Figure 3). The catalytic activity improves as the acidity increases. As shown in Figure 4, the Al2O3(Cl-U) catalyst presented a conversion of methanol to DME of approximately 45.0% in methanol at 250C as compared to 11.0% using Al2O3(Cl) catalyst. According to NH3-TPD, Al2O3(Cl-U) presented the greater acidity with predominance of strong acid sites explains its higher activity. On the other hand, using Al2O3(F-U) catalyst showed a high surface area which approximately double the activity of the Al2O3(F) catalyst at all reaction temperatures. This significant enhancement of catalytic activity is attributed to a significant increase of the total acidity and acidity strength as well as surface area via ultrasonication (Figure 3 and Table 1)[42].

Campbell et al[43] concluded from the TPD-CH3OH for characterization of the acid properties of the surface zeolite (strength and nature of acid sites) that the methanol dehydration to DME and the production of light hydrocarbon are related to the type and strength of the acid sites. Brönsted acid sites with moderate to high strength are responsible for the formation of by-products (e.g. ethylene and propylene) during the reaction between methanol molecules, while Lewis acid sites do not have catalytic activity. Concerning the dehydration of methanol to dimethyl ether (DME), Brönsted and Lewis sites cooperate and the conversion is proportional

to the acid strength. The conversion of methanol to DME yield has been

compared as increased time on stream (TOS) to demonstrate the catalytic stability at 300C, where no side reaction took place. Figure 5 shows a better stability of the ultrasonicated Al2O3(Cl) catalyst compared to the unsonicated Cl-version.

Figure 6 shows the change of the unit cell d-spacing obtained from XRD data on halogenated γ-Al2O3 catalysts with or without ultrasonication. The figure shows that the d-spacing value is as high as 0.140409 nm using the flourenated γ-Al2O3 that drops significantly to 0.139128 nm and 0.139158 nm using Al2O3(F-U) and Al2O3(Cl-U) catalysts, respectively. This behavior is in accordance with the performance of DME production (Figure 4) since a true impact of the electronegativity of halogen and ultrasound irradiation on the crystalline structure of the alumina occurs.

The apparent activation energy of catalysts was compared assuming a second order reaction[23,44]. Figure 7 shows the Arrhenius plot for the current alumina catalysts using the DME formation.

The apparent activating energy for DME formation varied between 37.77 and 92.76 kJ·mol–1. These results are in agreement with Xu et al[5] that found 104.6 kJ·mol–1 using γ-alumina. The Ea values obtained for DME formation are compatible with the activities of the catalysts used (Figure 4). The Al2O3(Cl-U) catalyst is the most active and selective for DME formation and its lower Ea (37.77 kJ·mol–1), compared to that for Al2O3(F-U) catalyst (55.10 kJ·mol–1), is compatible with activity sequence. In general, these values are compatible with values carried out using highly active catalysts in the petrochemical industry.

3 Conclusions

Ultrasonic irradiation during catalyst preparation effected the catalyst characters because intensive collision occurs between molecules.

Ultrasonication and/or halogenation enhance the acidity of γ-Al2O3.

Ultrasonic irrdiation enhanced the proformance of the halogenated γ-Al2O3 catalyst to DME production.

The chlorinated catalyst with or without ultrasonic irrdiation show significantly enhanced catalyst selectivity to DME formation.

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