Journal of Industrial and Engineering Chemistry 20 (2014) 222–227

Catalytic dehydration of methanol to dimethyl ether (DME) overAl-HMS catalysts

Behrouz Sabour a, Mohammad Hassan Peyrovi a,*, Touba Hamoule a, Mehdi Rashidzadeh b

a Department of Chemistry, Faculty of Science, Shahid Beheshti University, G.C., P.O. Box 19396-4716, Tehran, Iranb Research Institute of Petroleum Industry (RIPI), Tehran 1485733111, Iran

A R T I C L E I N F O

Article history:

Received 16 January 2013

Accepted 30 March 2013

Available online 18 April 2013

Keywords:

Al-HMS

Acid catalyst

Methanol dehydration

Dimethyl ether

A B S T R A C T

A series of Al-HMS with different Si/Al ratio was used as a solid acid catalyst for methanol dehydration to

dimethyl ether (DME). The effect of temperature, feed composition, space velocity, and the catalyst Si/Al

ratio on the catalytic dehydration of methanol was investigated. By decreasing Si/Al, the temperature

required to reach equilibrium conversion of methanol decreased due to the increased number of acidic

sites. Compared to commercial g-Al2O3, Al-HMS-5 and Al-HMS-10, catalysts exhibited a high yield of

DME. Among all Al-HMS catalysts, Al-HMS-10 exhibited an optimum yield of 89% with 100% selectivity

and excellent stability for methanol dehydration to DME.

� 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at SciVerse ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

Dimethyl ether (DME) has received considerable attention dueto its potential uses as an alternative to diesel oil and LPG becauseof similarity of its thermal efficiency to that of traditional fuels,low NOx emission, negligible smoke amounts, and less engine noise[1–4]. It has already been used as an ozone-friendly aerosolpropellant to replace ozone destructive CFCs [5]. In addition, it is animportant intermediate for producing highly valuable chemicalssuch as lower olefins, dimethyl sulfate and methyl acetate [6,7].

DME can be produced directly from syngas over bifunctionalCu-based catalysts or synthesized by methanol dehydration oversolid acid catalysts at 200–450 8C in a pressure of up to 18 atm [8–12]. Among different kinds of solid acids used for methanoldehydration, H-ZSM-5 and g-Al2O3 have been investigatedintensively both in laboratory and commercial scales. H-ZSM-5has been reported to be more active than g-Al2O3, but rapiddeactivation occurs on its strong acidic sites due to the generationof undesirable hydrocarbons [13,14]. On the other hand, g-Al2O3 ismore selective to DME, but it has some disadvantages includinglow activity and rapid deactivation in the presence of water.

Water is both produced in significant amounts through directsynthesis of DME from synthesis gas, and it is also present in largequantities (20–30%) in the crude methanol. It is reported thatreplacing pure methanol with crude methanol would bring about

* Corresponding author. Tel.: +98 21 29902892; fax: +98 21 22431663.

E-mail address: m-peyrovi@sbu.ac.ir (M.H. Peyrovi).

1226-086X/$ – see front matter � 2013 The Korean Society of Industrial and Engineer

http://dx.doi.org/10.1016/j.jiec.2013.03.044

great industrial benefits. Therefore, researchers are trying todevelop effective catalysts to optimize the DME production fromcrud methanol and improve the catalyst stability [14,15].

Recently, ordered mesoporous materials have been the subjectof a large number of studies because of their high surface areas,regular frameworks, and large pore size with narrow distribution[16]. Pure mesoporous silica does not have enough acidity, butacidity can be improved through the insertion of foreign metal ionsinto its structure during the synthesis [17–19]. Among suchmaterials, Al-incorporated mesoporous molecular sieves whichpossess the acidic sites and good hydrothermal stability are morefavorable. HMS is a hexagonal mesoporous silica with a particularwormlike pore structure. It has a simple preparation method usingcheap primary alkyl amines which can be extracted withoutpollution [20]. Surprisingly, despite its wide uses for catalyticreactions, to the best of our knowledge, aluminated hexagonalmesoporous material (Al-HMS) has not been employed as an acidiccatalyst to the dehydration of methanol up to now.

In continuation of previous studies on the application of Al-HMS mesoporous material as a solid acid catalyst for variousreactions [21,22], here we report catalytic behavior of thismaterial in the reaction of methanol dehydration to DME. Themain objective of this study is to investigate the effects of theincorporation of Al into the HMS framework on the activity,selectivity, and durability in the methanol dehydration. Also, theeffects of temperature, and feed composition on catalyticperformance were studied. BET, XRD, XRF, NH3-TPD, FT-IRpyridine, and TG/DTA techniques were employed for the materialcharacterization.

ing Chemistry. Published by Elsevier B.V. All rights reserved.

Fig. 1. XRD patterns of Al-HMS catalysts with different Si/Al ratio.

Fig. 2. NH3-TPD on Al-HMS catalysts with different Si/Al ratio.

B. Sabour et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 222–227 223

2. Experimental

2.1. Materials and methods

Al-HMS mesoporous materials with four different Si/Al ratios of5, 10, 20, and 35 were synthesized via neutral templating pathwaysimilar to the procedure reported in previous studies [23] usingtetraethylorthosilicate (TEOS) as the silica source, aluminumisopropoxide (Al(OPri)3) as the aluminum source and dodecyla-mine (DDA) as the surfactant. The molar composition of final gelwas 1.0 SiO2:x Al(OPri)3:0.25 DDA:10 isopropyl alcohol:100 H2O,where the value of x is dependent on various Si/Al ratios. Each solidproduct was separated by filtration and dried at 110 8C overnightand calcined at 540 8C for 6 h in the flowing air. The preparedsamples were named Al-HMS(x), where x is the Si/Al ratio.

2.2. Characterization of catalysts

An X-ray diffraction (XRD) analysis of the calcined samples wasperformed in the 2u range of 1–108, by using an X-PERTdiffractometer employing Ni-filtered Cu Ka radiation at 45 kVand 50 mA.

The specific surface area and pore volume of the samples weremeasured in an ASAP-2010 Micromeritics (USA) using lowtemperature N2 physisorption isotherms. Before the analysis,the sample was evacuated at 350 8C under vacuum conditions.

The acidity of Al-HMS was measured by TPD of ammonia in aTPD/TPR analyzer (2900 Micromeritics) with a thermal conduc-tivity detector. To determine and analyze the type of acidic sites,pyridine adsorption on the samples was performed on a Fourier-transform infrared spectrometer (170-SX). The Si/Al ratio of Al-HMS was determined by XRF (XRF-8410 Rh 60 kV). The content ofcoke laid down on the surface of used catalysts was measured bythermogravimetric analyzer using a STA503M TG/DTA instrument.

2.3. Catalytic evaluation

Vapor phase dehydration of methanol was carried out at thetemperature range of 250–400 8C and atmospheric pressure in acontinuous fixed-bed micro-reactor packed with 0.5 g of thecatalyst. Prior to each experiment, the catalyst was pretreated for1 h at 300 8C in an N2 flow. Nitrogen saturated by pure methanol(11% MeOH in N2) was used as feed with the space velocity (WHSV)1.0 h�1. Moreover, methanol–water mixture (methanol80 mol% + water 20 mol%) was introduced to the reactor underaforementioned conditions to investigate the capability of Al-HMSas a dehydration catalyst for the crude methanol dehydration. Theperformance of the catalysts was measured after 0.5 h time onstream (TOS) at noted temperatures for each experiment. Theanalysis of the reaction products was carried out by on-line gaschromatography using a gas chromatograph (Shimadzu-8A)equipped with a thermal detector.

3. Results and discussion

3.1. Characterization

Fig. 1 shows that the XRD patterns of the Al-HMS materials aresimilar to those reported in literature [20,24]. There is a singlebroad reflection that can be assigned to a lattice with the short-range hexagonal symmetry. The increase of Al content in thesamples results in the broadening of the peak, indicating thatthe incorporation of Al is associated with increasing the latticedisorder.

The chemical compositions, BET surface area, and pore volumeof calcined Al-HMS materials are given in Table 1. It can be seen

that the actual Si/Al ratios are very similar to added metal amountsin the gel compositions, suggesting that most of the addedaluminum heteroatoms are embedded into the HMS bulk. It can beinferred from Table 1 that the surface area and pore volumedecrease with the increase of Al amounts. This may be attributed todecrease in the structural order of the samples as a result of Alincorporation as proved by XRD.

Fig. 2 shows NH3-TPD profiles of all Al-HMS materials withdifferent Si/Al ratios. The asymmetric shapes of the desorptionprofiles indicate the presence of different surface acid sites in therange of 150–500 8C, corresponding to the distribution of acid sitesfrom weak to strong. The maximum of desorption peak is in therange of 250–300 8C corresponding to the medium acid sitesresponsible for the selective formation of DME [25,26]. The TPDprofile of pure HMS shows no evident peak due to the absence ofacidic sites on the HMS (not shown here). Table 1 shows that theconcentration of acidic sites of Al-HMS increases with the decreaseof Si/Al ratios. It can be seen that the maximum of the TPD diagramshifts to the high temperatures with the decreasing Si/Al ratios. It is

Table 1Physicochemical characteristics of the Al-HMS catalysts.

Sample Si/Al Surface area (m2/g) Pore volume

(cm3/g)

Peak temperature

of TPD (8C)

Acidity

(mmol NH3/g)

B/L BrOnsted acidity

(mmol NH3/g)

Lewis acidity

(mmol NH3/g)

Al-HMS-35 36.3 1370 1.7 258.9 0.426 1.8 0.277 0.153

Al-HMS-20 18.7 1289 2.1 270.9 0.845 1.37 0.489 0.355

Al-HMS-10 9.6 1109 2.0 278.4 1.398 1.06 0.720 0.677

Al-HMS-5 4.5 870 0.79 287.1 1.556 0.86 0.719 0.837

Fig. 3. Catalytic performance of Al-HMS catalysts versus temperature (0.5 g of

catalyst and WHSV = 1 h�1). (a) Methanol conversion profiles; (b) activity per

surface area profiles; (c) DME selectivity profiles; (d) yield of DME profiles.

B. Sabour et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 222–227224

obvious that the strength of the acidic sites enhances in high Alcontent [21].

In order to investigate the Bronsted and Lewis acid sites, the FT-IR spectra of pyridine adsorption were recorded at roomtemperature (Table 1). As it can be seen, the B/L acid site ratiodeclines with increasing Al content. It is in good agreement withthe results reported previously; i.e. the tetrahedrally coordinatedframework aluminum (potential Bronsted acid site) decreases withincreasing Al content while octahedrally extra framework alumi-num (potential Lewis acid site) increases [27]. The number ofBronsted and Lewis acid sites was calculated from the NH3-TPDresults by using the B/L ratios (Table 1).

3.2. Catalytic performance

3.2.1. Activity

According to the literature, methanol dehydration is an acidcatalyzed reaction and both Lewis and Bronsted acid sites areactive in this reaction [28–31]. Fig. 3 illustrates the catalyticperformance of a series of Al-HMS on methanol vapor phasedehydration at atmospheric pressure in the temperature range of250–400 8C and WHSV = 1 h�1. From Fig. 3a it can be seen thatmethanol conversion increases with the increase in reactiontemperatures, and after reaching maximum conversion, it shows aslight decrease due to thermodynamic limitations as thedehydration of methanol is slightly an exothermic reaction[14,29]. With the decrease in Si/Al ratios, the temperaturerequired to achieve equilibrium conversion decreases due tothe increased number of acidic sites which are favorable formethanol dehydration. It is apparent from Fig. 3a that theconversion over Al-HMS-20 and Al-HMS-35 did not reachequilibrium conversion at reaction conditions since thesecatalysts possessed low number of acid sites as confirmed byNH3-TPD. However, the conversion of methanol on Al-HMS-5 andAl-HMS-10 exceeded the equilibrium values predicted bythermodynamics for dehydration reaction in temperature rangeof 275–325 8C. Similar results were reported by other authorspreviously using molecular sieves, e.g. HZSM5 and H-Mordeniteas an acid catalyst for methanol dehydration [26,32]. In the case ofour catalysts, easily separation of product molecules over Al-HMSmolecular sieve may explains the reason why conversion ofmethanol exceeded the equilibrium values predicted by thermo-dynamics for reaction in this temperature range. However, theconversions at higher temperature follow the values predicted bythermodynamics since dehydration of methanol is an exothermicreaction and increasing in temperature results in a shift of theequilibrium composition toward the left direction.

Fig. 3b shows the calculated reaction rates per surface area as afunction of reaction temperature. The rate of the methanoldehydration reaction depends strongly on the acidity of catalystemployed. It can be seen that the order of reaction rates is Al-HMS-5 > Al-HMS-10 > Al-HMS-20 > Al-HMS-35 which is in the sameorder as the increase in acidic sites density.

Despite the fact that the mesoporosity of Al-HMS samplesdecreased slightly with increasing Al content, these catalystsshowed higher surface area and pore volume compared tocommercial acid catalysts applied for methanol dehydration [9,14].

Table 2Catalytic performance of synthesized catalysts as compared to commercial g-Al2O3

catalyst at 300 8C.

Sample Methanol conversion (%) DME selectivity (%) DME yield (%)

g-Al2O3 76.2 100 76.2

Al-HMS-10 89 100 89

Al-HMS-5 91.2 94.1 85.8

0

10

20

30

40

50

60

70

80

90

100

Al-HMS-20 Al-HMS-10 Al-HMS-5

Met

hano

l con

vers

ion

(%)

Pure Methanol crude methanol

Fig. 5. Methanol conversion variations for pure methanol and crude methanol

(reaction conditions were 300 8C, 0.5 g of catalyst, reaction time = 72 h, and

WHSV = 1 h�1).

20

22

24

26

28

30

0 10 20 30 40 50 60 70

Al-HMS-5 (P) Al-HMS-10 (P) Al-HMS-10 (C) Al-HMS-20 (P)

Rea

ctio

nra

te *

10-3

Time on stream (h)

Fig. 4. Long-term test of catalyst samples at 300 8C; (P): tested for pure methanol

feed; (C): tested for crude methanol feed.

B. Sabour et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 222–227 225

As compared to a commercial g-Al2O3 with BET surface area of202 m2/g and surface acidity of 0.728 mmol NH3/g, the catalyticactivity of our synthesized catalysts was much higher (Table 2).The lower activity of g-Al2O3 can be attributed to the lowernumber of acid sites as well as lower surface area.

It seems that a combination of high porosity and high surfacearea increases the accessibility of methanol to active sites andconsequently enhances the catalyst activity. It is noteworthy thatpure HMS showed no noticeable methanol conversion even athigher temperatures. Obviously, this incompetence of HMS for thisprocess is due to its lower acidity according to NH3-TPD results.

3.2.2. Selectivity and yield

The effects of temperature and Si/Al ratios of the catalysts onthe selectivity and yield of DME were also investigated. Aspresented in Fig. 3c, while the selectivity of DME is almost 100% onAl-HMS-35 and Al-HMS-20 in the temperature range of 250–400 8C, Al-HMS-10 and Al-HMS-5 follow different trends. Theselectivity of Al-HMS-10 decreases slightly from 100% to 92% withthe increase in temperature up to 400 8C, whereas the increase intemperatures causes drastic change in Al-HMS-5 selectivity,decreasing from 100% to 45% toward DME as the temperaturerises from 250 8C to 400 8C. Illustrated in Fig. 3d, although arelatively similar yield of DME was obtained over Al-HMS-5 andAl-HMS-10, keeping approximately 100% selectivity of DME up tohigher temperatures in the case of Al-HMS-10 can be considered itsmain advantage over Al-HMS-5. The results revealed that higheryields (>80%) have been preserved in a wide temperature range of275–350 8C over Al-HMS-10, which is included the operatingtemperature range for the direct synthesis of DME from synthesisgas [33]. The by-products were CH4 and light olefins mostly. Thiscan be interpreted by the fact that acid sites with relatively weakand intermediate strengths are favorable in methanol dehydrationreaction while strong acid sites catalyze methanol to hydrocarbonreaction, especially at higher temperatures [9,14,34]. This trend isin good accord with the increase in the acidity strength shown inTable 1.

3.2.3. Stability

Because dehydration catalysts cannot be regenerated in situ,keeping high stability during a long-term reaction is veryimportant; therefore, based on the experimental results, Al-HMS-5, Al-HMS-10 and Al-HMS-20, which showed a high activity,were chosen to be surveyed for durability tests. As it is indicated inFig. 4, Al-HMS-10 and Al-HMS-20 virtually keep their activity withtime on stream and show no remarkable changes during the 72 htime on stream. However, the activity of Al-HMS-5 showed a slightdecrease with time-on-stream, which might be due to cokeformation on its strong acid sites and/or the blocking of strong acidsites by water produced during the reaction [8,33].

Generally speaking, even though Al-HMS-10 showed a slightlylower activity (89%) in comparison with Al-HMS-5 (91.2%) at300 8C, combining its high activity, excellent selectivity (100%) toDME in the wide range of operating temperature, and highresistance to deactivation makes it the best catalyst for methanoldehydration.

3.2.4. Effect of water

Replacing pure methanol by crude methanol (containing20 mol% water) as a feed for the methanol dehydration to DMEwould reduce the process cost [14]. Thus, crude methanol wasintroduced to the reactor in the same conditions to investigate thecapability of our catalysts for crude methanol dehydration. Fig. 5shows that the changes in methanol conversion for pure methanolfeed and crude methanol feed after 72 h time on stream. As seen,the less Si/Al ratio catalysts have, the more decline in activity theyshow for crude methanol dehydration.

The XRD patterns and BET data of the used catalysts for crudemethanol after 72 h reaction at 300 8C were taken (not shown here)and proved that the structural and textural properties of all usedcatalysts had been preserved, and it showed no remarkablechanges in the structural characteristics. As a result, Al-HMScatalysts have a good hydrothermal stability, and a decrease incatalyst activity and durability in the crude methanol dehydrationare not imposed by the changes of catalyst structure. The slightdecrease in Al-HMS conversion in the crude methanol feedindicated the role of different types of acidic sites. It is well-known that water is adsorbed preferentially on Lewis acid sitesrather than Bronsted sites. Based on FT-IR results given in Table 1,lower Si/Al ratio leads to an increase in Lewis acid sites, and thisexplains why Al-HMS-5 activity declines during the crudemethanol dehydration. A similar trend was reported when g-Al2O3 and modified H-ZSM-5 were used for dehydration of crudemethanol. It was proved that water had more negative effects onthe performance of g-Al2O3 than on that of ZSM-5 since ZSM-5 has

-80

-60

-40

-20

0

-14

-12

-10

-8

-6

-4

-2

0

2

0 100 200 300 400 500 600 700 800

Wei

ght

(%)

Temperature (oC)

TGA

DTA

(a)

rel. DT

A

-80

-60

-40

-20

0

20

40

60

-12

-10

-8

-6

-4

-2

0

2

0 100 200 300 400 500 600 700 800

Wei

ght

(%)

Temperature (oC)

TGA

DTA

(b)

rel.D

TA

(%)

Fig. 6. TG/DTA profiles of used Al-HMS-10 for dehydration of (a) pure methanol and

(b) crude methanol at 300 8C after 72 h.

50

60

70

80

90

100

1 2 4

Met

hano

l con

vers

ion

(%)

WHSV h-1

Al-HMS-10

-Al2O3

Fig. 7. Effect of WHSV on methanol conversion over Al-HMS-10 and commercial g-

Al2O3 at 300 8C.

B. Sabour et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 222–227226

higher resistance due to its hydrophobic properties resulting fromits high SiO2/Al2O3 ratio [15]. Based on the above discussion, Al-HMS-10 appears to be a promising catalyst for the crude methanoldehydration. Additionally, coke laid down over Al-HMS-10 catalystused for dehydration of both pure and crude methanol at 300 8Cafter 72 h time on stream, was measured by TG/DTA technique andresults were shown in Fig. 6.

Both used catalysts showed two weight losses in TGA curves.The weight losses at <200 8C corresponding to endothermic peaksin DTA curves, were attributed to desorption of physicallyadsorbed water. The weight losses at >400 8C, corresponding toexothermic peaks in DTA curves, were ascribed to the combustionof carbonaceous material deposited inside used Al-HMS-10catalysts. The amount of coke analyzed by TGA technique is alsolisted in Table 3. It is well-known that catalysts with less stabilitypossess relatively high amount of carbon deposit. As it can be seen,Al-HMS-10 possess high level of coke during dehydration of puremethanol. In other words using crude methanol instead of puremethanol decrease mainly the total amount of coke on the surfaceof the catalyst. As a result relatively more stable catalytic behaviorwas seen using crude methanol owning to attenuation of cokedeposition on Al-HMS-10 (Fig. 4). It is clear from above discussionthat water inhibits coke formation and remove coke easily overmesoporous molecular sieves [35].

3.2.5. Effect of WHSV

The methanol conversion to DME under various WHSV (1–4 h�1) over Al-HMS-10 catalyst, which showed optimum activity,was investigated and compared to commercial g-Al2O3.

Table 3Effect of different feed on coke formation over Al-HMS-10 after 72 h.

Sample Temperature

range (8C)

Coke content

(wt.%)

Al-HMS-10 (used for pure methanol) 400–600 3.14

Al-HMS-10 (used for crude methanol) 445–520 0.9

As it is depicted in Fig. 7, an increase in WHSV causes nosignificant reduction in methanol conversion over Al-HMS-10,while methanol conversion over commercial g-Al2O3 reducessignificantly. This can be interpreted by the fact that Al-HMS-10possesses a high surface area as well as high number of mediumstrength acid sites responsible for selective dehydration ofmethanol to DME. It should be noted that the selectivity of DME is100% under reaction conditions.

4. Conclusion

Several Al-HMS materials with Si/Al ratio of 5–35 weresynthesized and studied as novel acid catalysts for methanoldehydration reaction. According to the NH3-TPD results, Alincorporation into HMS framework enhanced number of surfaceacid sites as well as acidity strength. It was found that increasing Alcontent is followed by increasing conversion of methanol anddecreasing selectivity to DME. The synthesized samples showedgood catalytic performance in the presence of water. Al-HMS-10,as the best catalyst, exhibited optimum activity of 89% with theDME selectivity of 100% and high resistance to deactivation.

Acknowledgments

The authors gratefully acknowledge financial support from theResearch Council of Shahid Beheshti University and Catalyst Centerof Excellence (CCE).

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