Al-MCM-41 as methanol dehydration catalyst

Article abstract of DOI:10.1016/j.apcata.2010.04.007

Al-MCM-41, [SiO₂]/[Al₂O₃] = 30, mesoporous catalysts were synthesized using tetraethylorthosilicate (TEOS) as a source of silica and hexadecyltrimethylammonium bromide (CTAB) as a structure-directing agent (SDA) in the presence of tetrapropylammonium hydroxide (TPAOH) as a co-surfactant additive. The catalytic activities of the calcined samples were evaluated for the dehydration of methanol to dimethyl ether (DME) at 250-450 °C and a gas hourly space velocity (GHSV) of 14,000-80,000 mL g^-1_cat h^-1. Al-MCM-41 samples synthesized in the presence of TPAOH showed higher activity than those synthesized in its absence. The improvement in catalytic activity correlates with the increase in acidity brought about by the enhanced condensation of aluminosilicate framework in the Al-MCM-41 structure. Al-MCM-41 catalysts were also combined with methanol synthesis catalysts to form bifunctional catalysts for the synthesis DME from CO₂/H₂ at temperature, 260 °C; pressure, 735 psi; GHSV, 2000 mL g^-1_cat h^-1 in a fixed-bed reactor.

Full text of DOI:10.1016/j.apcata.2010.04.007

Applied Catalysis A: General 381 (2010) 183–190
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Al-MCM-41 as methanol dehydration catalyst
Sajo P. Naik∗∗, Vy Bui, Taegong Ryu, Jan D. Miller, Wlodzimierz Zmierczak∗
Department of Metallurgical Engineering, College of Mines and Earth Science, University of Utah, 135 S 1460 East, Salt Lake City, UT 84112-0114, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 March 2010
Received in revised form 27 March 2010
Accepted 3 April 2010
Available online 13 April 2010
Al-MCM-41, [SiO2]/[Al2O3] = 30, mesoporous catalysts were synthesized using tetraethylorthosilicate
(TEOS) as a source of silica and hexadecyltrimethylammonium bromide (CTAB) as a structure-directing
agent (SDA) in the presence of tetrapropylammonium hydroxide (TPAOH) as a co-surfactant additive. The
catalytic activities of the calcined samples were evaluated for the dehydration of methanol to dimethyl
−1
ether (DME) at 250–450 ◦C and a gas hourly space velocity (GHSV) of 14,000–80,000 mL g−1
h
. Al-
cat
MCM-41 samples synthesized in the presence of TPAOH showed higher activity than those synthesized
in its absence. The improvement in catalytic activity correlates with the increase in acidity brought about
by the enhanced condensation of aluminosilicate framework in the Al-MCM-41 structure. Al-MCM-41
catalysts were also combined with methanol synthesis catalysts to form bifunctional catalysts for the
Keywords:
Al-MCM-41
Tetrapropylammonium hydroxide
Methanol dehydration catalyst
Dimethyl ether synthesis catalyst
−1
in a
synthesis DME from CO2/H2 at temperature, 260 ◦C; pressure, 735 psi; GHSV, 2000 mL g−1
fixed-bed reactor.
h
cat
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
functionalization of the hydroxyl groups in mesoporous silica with
silanes containing acidic groups provides an alternate method to
introduce acidity in mesoporous silica [7]. The acidity generated by
the incorporation of metals is usually associated with the nature
of the chemical linkages produced by the metal ions in the silica
framework. When a trivalent cation such as Al3+ substitutes for
Si4+, the framework possesses a negative charge that can be com-
pensated by a proton to form a Bronsted acid site (Fig. 1). The extra
framework Al3+ species can also be a source of Lewis acidity in
the structure. Such metal–ion incorporated mesoporous silica is a
candidate acid catalyst for several organic reactions [8–14].
Although Al-MCM-41 is a useful catalyst in organic reactions,
the aluminosilicate framework in the walls of its structure does
not have short-range order similar to that in zeolites, and hence
the structure is amorphous in nature. The degree of condensation
of aluminosilicate in the framework of Al-MCM-41 proceeds only to
a limited extent during synthesis and calcination, which decreases
both the acidity and the hydrothermal stability of Al-MCM-41. Sev-
eral efforts are on-going to improve the acidity and hydrothermal
stability of mesoporous silica for applications in catalysis.
Nevertheless, metal-incorporated mesoporous materials and
zeolites are increasingly replacing conventional acid catalysts,
usually mineral acids, in organic reactions of industrial impor-
tance [8–14]. Use of such catalysts has been demonstrated in
Friedel–Crafts, aldol condensation, epoxidation, and Diels–Alder
reactions. Al-MCM-41, due its larger pore diameter than that of
zeolites, is useful in the catalytic transformation of bulky organic
molecules. The use of solid acid catalysts is also environmentally
beneficial as these catalysts are easy to handle, can be separated
from the reaction medium without difficulty, are reusable, and can
Mesoporous silica [1,2] belongs to the M41S class of materi-
als and is an ordered porous material with a long-range structural
order. It comprises interconnected or straight pore channels of
diameter 2–50 nm (mesopore range) in the bulk of its structure.
Mesoporous silica has a high surface area and is thermally stable. Its
pore dimensions can be tailored to specific requirements. Various
metal ions or metal oxides can also be introduced to have a uni-
form dispersion of active components in its structure. As a result
there is a significant interest in the synthesis and applications of
mesoporous silica in catalysis, adsorption, etc.
Ordered mesoporous silica can exist in the form of hexagonal,
cubic, or lamellar structures which may be prepared using a source
of silica together with a structure-directing agent (SDA) usually a
surfactant, through a self-assembly process under a range of syn-
thesis conditions [2].
Hexagonal MCM-41 mesoporous silica has been the subject of
a large number of studies in recent years. Purely siliceous MCM-
41 does not have appreciable acidity, but acidity can be introduced
through the incorporation of metal ions such as aluminum, tita-
nium, and zirconium into its framework structure [3–6]. Surface
∗
Corresponding author. Tel.: +1 801 581 3060; fax: +1 801 587 8329.
Corresponding author at: 135 S1460 E, Room 412, Department of Metallurgical
∗∗
Engineering, College of Mines and Earth Science, University of Utah, 135 S 1460 East,
Salt Lake City, UT 84112-0114, USA. Tel.: +1 801 581 3191; fax: +1 801 587 8329.
E-mail addresses: s.naik@utah.edu, spnaik@gmail.com (S.P. Naik),
w.zmierczak@utah.edu (W. Zmierczak).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.04.007

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S.P. Naik et al. / Applied Catalysis A: General 381 (2010) 183–190
Fig. 1. A schematic illustration of Al-MCM-41 structure and the aluminosilicate framework in its walls.
be further functionalized to introduce multiple catalytic sites in the
structure.
formation and reduction of the yield of DME, and ultimately shorten
the life of the catalyst.
Dehydration of alcohols to ethers or hydrocarbons is an indus-
trially important acid catalyzed reaction. Dimethyl ether (DME) is
gaining prominence as a next-generation synthetic fuel [15–19].
It is produced by the direct dehydration of methanol or by a two-
step process from CO/CO2 and H2, via intermediate formation of
methanol, on a bifunctional catalyst comprising methanol synthe-
sis and methanol dehydration components.
In fact, it is necessary to develop new improved methods for the
efficient reductive conversion of CO2 to methanol and/or DME in
order to manage the growing problem of excess CO2, a green house
gas, in the environment [18].
DME is a clean fuel for diesel engines and has a very high ther-
mal efficiency compared to other synthetic fuels. Vehicular engines
running on DME have considerably lower NOx emissions and near-
zero production of solid particulates. By using DME as a starting
material, several useful chemicals or other fuels such as ethanol,
hydrocarbons, and higher oxygenates can be produced. Further-
more, DME is a useful carrier of hydrogen for applications such as
in fuel cells, etc.
In this paper, studies on the synthesis of active Al-MCM-41 cata-
lysts for the selective dehydration of methanol to DME are reported.
The activity and selectivity of Al-MCM-41 catalysts are enhanced
by modification of the conventional synthesis process. Specifically,
small amounts of tetrapropylammonium hydroxide (TPAOH) were
added during synthesis, which enhanced the activity of Al-MCM-41
catalysts. The catalytic activity of the calcined material was evalu-
ated for the direct synthesis of DME from methanol in a fixed-bed
reactor. The most-effective Al-MCM-41 catalyst was then admixed
with a commercial methanol synthesis catalyst to form a bifunc-
tional (mechanical mixture) catalyst [15,18–20] for the synthesis
of DME from CO2/H2 in a high-pressure fixed-bed reactor.
2. Experimental
2.1. Synthesis of Al-MCM-41
Al-MCM-41 samples were prepared by modifying the con-
ventional synthesis process. The SiO2-to-Al2O3 molar ratio
Synthesis of DME requires a dehydration catalyst component
of high specificity and durability. The catalyst should have an
optimum acidity to selectively produce DME [18,19]. Usually the
presence of strong acid sites in the catalyst will promote side reac-
tions to generate undesired hydrocarbons, which may lead to coke
was kept equal to 30 in all samples.
A clear solution (A)
was first prepared by dissolving aluminum isopropoxide (Alfa
Aesar, 99%) in tetraethylorthosilicate (Sigma–Aldrich, 99.9%) in a
polypropylene (PP) bottle and stirred at 25 ◦C for 2 h. Another
solution (B) was prepared by dissolving hexadecyltrimethy-
Table 1
List of the synthesized Al-free and Al-MCM-41 samples, their acidity values, and their catalytic activity toward dehydration of methanol, [CTAB/TPAOH] is the molar ratio.
Catalyst code
Catalyst type
[CTAB/TPAOH]
Acidity, ␮m/g (250–500 ◦C)
Methanol conversion, %
Selectivity, %
DME
C-1–C-3a
MC-1
MC-2
MC-3
MC-4
Al-free MCM-41
Al-MCM-41
Al-MCM-41
Al-MCM-41
–
–
6
100
98
98
0
2
2
3
∞
140
260
460
45
72
73
0.75
0.20
97
−1
; time on-stream, 5 h; methanol vapor pressure, 97.48 mm of Hg.
Reaction conditions: temperature, 400 ◦C; GHSV, 14,000 mLg−1
h
cat
a
Hydrocarbons.

S.P. Naik et al. / Applied Catalysis A: General 381 (2010) 183–190
185
lammonium bromide (CTAB, Sigma–Aldrich, 99%) in
a 30%
aqueous solution of NH4OH (Sigma–Aldrich), tetrapropylammo-
nium hydroxide (TPAOH, 40% aqueous, Sachem) and de-ionized
water in
a
PP bottle, under stirring at 25 ◦C. The solution
(A) was then added in a drop-wise manner to the solution
(B) which was kept under stirring at 25 ◦C. This procedure
resulted in the formation of a precursor gel of molar composition
of 1SiO2:0.033Al2O3:0.12CTAB:7.37NH4OH:0–0.62TPAOH:75H2O.
The gel was stirred for 30 min, followed by hydrothermal treatment
at 100 ◦C for 3 days. The white precipitate obtained was filtered and
dried at room temperature for 24 h, followed by further drying at
110 ◦C for 1 h.
A series of samples as MC-1 to MC-4, as shown in Table 1, were
prepared by varying the amount of TPAOH in the synthesis mixture.
The variation in amounts of TPAOH added to each of the samples
during their preparation is expressed as a molar ratio of CTAB to
TPAOH, as shown in Table 1. An aluminum-free MCM-41 sample
was also prepared by the same method but without aluminum and
TPAOH in the gel. The dried samples were calcined at 550 ◦C in air
for 10 h in a muffle furnace at a heating rate of 5 ◦C/min.
Fig. 2. Small-angle XRD patterns of the calcined Al-free and Al-MCM-41 samples.
2.2. Characterization
2.3.2. Dimethyl ether synthesis from CO2/H2
X-ray diffraction (XRD) analysis of the calcined samples was
performed in the 2-Â range of 1.5–10◦, by using a Philips XRD instru-
ment employing Ni-filtered Cu K␣ radiation at 45 kV and 50 mA.
The N2 adsorption/desorption isotherms were obtained using a
Micromeritics ASAP 2000 instrument. Prior to the measurements,
the catalysts were degassed at 250 ◦C for 1.5 h. Infrared spectra
were collected on a Spectrum RX-1 (PerkinElmer) instrument using
the KBr pellet technique and a resolution of 2 cm−1. The composi-
tion of 95% KBr and 5% calcined sample was used for FTIR analysis
and maintained for all measurements.
A bifunctional DME synthesis catalyst was prepared by mechan-
ical mixing of 1.8 g of commercial methanol synthesis catalyst
(MK-121, Haldor Topsoe) and 0.12 g Al-MCM-41, the methanol
dehydration catalyst prepared in this work. The reaction was per-
formed in a high-pressure tubular stainless steel fixed-bed reactor
operating at 260 ◦C, 735 psi, and GHSV of 2000 mL g−1cat h−1, using
the reactant gas of composition 25% CO2:75% H2.
2.3.3. Analysis of reaction products
The reaction products exiting the reactors were analyzed using
an online GC (Agilent 7890) equipped with a thermal conductiv-
ity detector and an HP-Plot Q (bonded polystyrenedivinylbenzene)
capillary column (15 m length). The tail gas lines in the reactors
were kept heated at 150 ◦C all the time to avoid condensation of
the products. The identity of the products was further confirmed
on a gas chromatograph coupled with mass spectroscopy (GC-MS)
on a Hewlett Packard (5897 series) instrument equipped with a
single quadruple detector.
Temperature-programmed desorption (TPD) of ammonia was
performed on
a Micromeritics Chemisorb 2720 instrument
equipped with a thermal conductivity detector. Typically 0.1 g of
the catalyst was charged in a U-shaped quartz cell and pretreated
in helium at 300 ◦C for 1 h. The catalyst was then saturated with
NH3 at room temperature using a gas mixture of 5% NH3 and 95%
He. The catalyst was heated to 700 ◦C at a rate of 5 ◦C/min for des-
orption of NH3. The amount of desorbed NH3 was quantitatively
analyzed by a thermal conductivity detector and recorded as a
function of temperature. Scanning electron microscopy (SEM) mea-
surements on the samples were conducted on a Topcon SM-300
instrument equipped with an energy dispersive X-ray spectrome-
ter (EDS) allowing visualization of the elemental distribution and
their quantification.
3. Results and discussions
The XRD patterns of the prepared mesoporous catalysts shown
in Fig. 2 confirm their hexagonal mesoporous structure. A very
intense (1 0 0) diffraction peak with additional higher order peaks of
lower intensity which could be indexed to the p6mm structure [2].
In Fig. 3, the SEM images of the samples captured under the same
magnification are shown. All the samples showed almost identi-
cal morphology of aggregated spheroid particles. The size of the
particles was 1–2 ␮m in samples MC-1, MC-2, and MC-3, however,
MC-4 had slightly larger particles of size from 3 to 4 ␮m. MC-4
was prepared at a much higher concentration of TPAOH in the gel,
and therefore the increase in the size of particles could be a direct
effect of the extra TPAOH in the gel. The EDAX analyses of all the
samples confirmed their SiO2-to-Al2O3 ratio to be ∼30. Further-
more, the uniform distribution of Si and Al in the material was also
confirmed by the mapping of Si and Al obtained from EDAX. The
mapping results obtained on MC-4 are shown as an example in
Fig. 4.
2.3. Catalytic property measurements
2.3.1. Methanol dehydration reaction
The activity of the different catalysts for methanol dehydra-
tion was evaluated in a tubular fixed-bed stainless steel reactor
operating at 250–400 ◦C and gas per hourly velocity (GHSV) of
14,000–80,000 mL g−1cat h−1. Prior to the catalytic activity mea-
surements, the calcined samples were finely powdered in an
agate mortar, pelletized (50 MPa pressure), crushed, and sieved to
25–40 mesh size using the standard US sieve series. Methanol was
introduced to the reaction zone by bubbling ultrapure helium gas
through a thermostated glass saturator filled with methanol and
maintained at 20 ◦C. A fresh catalyst was used for each run. The
catalyst was activated at 400 ◦C in flowing air for 3 h, after which
the temperature was brought down under flowing He and the flow
of methanol was started. The reaction time on-stream was 5 h.
Table 1 shows the catalytic activities of the calcined materials f−o1r
the dehydration of methanol at 400 ◦C, GHSV 14,000 mL g−1cat h
and time on-stream of 5 h. The molar ratio of CTAB and TPAOH,

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S.P. Naik et al. / Applied Catalysis A: General 381 (2010) 183–190
Fig. 3. SEM images of the Al-free and Al-MCM-41 samples.
Fig. 4. EDS maps of MC-04 sample, light and dark spots in the two images correspond to the distribution of Si, and Al, respectively, in the sample.

S.P. Naik et al. / Applied Catalysis A: General 381 (2010) 183–190
187
Fig. 5. Conversion of methanol at different temperatures on MC-2 and MC-4 sam-
Fig. 6. Conversion of methanol for different GHSVs on MC-2 and MC-4 samples.
−1
; time on-stream, 5 h; methanol vapor pressure,
ples. GHSV, 14,000 mL g−1
97.48 mm of Hg.
h
Temperature, 400 ◦C; time on-stream, 5 h; methanol vapor pressure, 97.48 mm of
Hg.
cat
maintained in the synthesis gel during sample preparation, is also
shown in this table. MC-1 showed the lowest methanol conversion
6%, among all the samples; evidently, this lower conversion of Al-
free MCM-41 is due to its lower acidity, as it does not contain Al.
MC-2, synthesized in the absence of TPAOH showed 45% methanol
conversion at a DME selectivity of 98%. Significant increases in cat-
alytic activity were observed from MC-3 and MC-4 samples that
were prepared in the presence of TPAOH in the synthesis gel. MC-
3 prepared at [CTAB]/[TPAOH] = 0.75 showed methanol conversion
of 72%, whereas MC-4 prepared at [CTAB]/[TPAOH] = 0.20 showed
methanol conversion of 73%, at a DME selectivity of 98% and 97%,
respectively. The magnitude of acidity, measured in terms of des-
orbed ammonia on the catalysts in the range of 250–500 ◦C shown
in Table 1, indicated a direct correlation between total acidity and
the activity of the catalysts. The total acidity increased with the
increase of TPAOH to the gel.
The activities of MC-2 and MC-4 prepared in the absence and
presence respectively of TPAOH in the synthesis gel, were fur-
ther evaluated at different temperatures and GHSVs, as shown
in Figs. 5 and 6, respectively. MC-4 performed superiorly to
MC-1 over a range of temperatures and GHSVs. At GHSV of
14,000 mL g−1cat h−1, the conversion of methanol increased with
the increase in temperature ranging from 250 to 400 ◦C for both
catalysts, but the selectivity to DME decreased slightly at higher
temperature. However, there was a substantial difference between
the activities of two catalysts. Similarly at 400 ◦C, with the increase
in GHSV, the conversion of methanol decreased, but the conver-
sion was much lower for MC-2 in the investigated GHSV range. The
selectivity to DME was however not much affected for the inves-
tigated GHSV range. Clearly, MC-4, synthesized in the presence of
TPAOH, showed much higher activity over a range of temperature
and GHSV than MC-2, prepared in the absence of TPAOH.
Fig. 7. N2 adsorption/desorption isotherms measured on Al-MCM-41 samples.
range of 0.2–0.3 (Fig. 7), that is typical of MCM-41 materials [21,22].
It is known that the BJH method does not correctly estimate the
mesopore size. Thus, for capillary condensation in this pressure
range, the mesopore size was estimated according to the corre-
lation obtained from nonlocal density functional theory [21–23]
(NLDFT), and the obtained data are listed in Table 2. The BET sur-
face areas, determined from the linear part of the BET equation
(p/p0 equal to 0.05–0.3), were in the range of 900–1200 m2/g.
MC-4 showed the highest surface area among the samples. The
N2 adsorption/desorption measurements on Al-MCM-41 were
conducted at −195.85 ◦C to obtain the texture properties of the cat-
alysts. All studied samples showed type IV adsorption isotherms,
indicating cylindrical mesopores with hysteresis loops in the p/p0
Table 2
Texture properties of Al-MCM-41 samples.
Catalyst code [CTAB/TPAOH] BET surface
area (m2/g)
External
surface area
(m2/g)
Total pore
Macropore
Mesopore
Pore diameter
a0 (Å) Wall thickness
volume (cm3/g) volume (cm3/g) volume (cm3/g) (Å)
(Å)
MC-2
MC-3
MC-4
∞
900
980
1200
880
900
1000
1
1
0.95
0.40
0.30
0.10
0.60
0.70
0.85
33
30
27
43.34 10.34
41.77 11.77
40.39 13.39
0.75
0.20

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S.P. Naik et al. / Applied Catalysis A: General 381 (2010) 183–190
pore diameter was found to decrease with a decrease of the
[CTAB]/[TPAOH] ratio in the precursor gel. The wall thickness of
the pores, as estimated from the difference between the unit cell
parameter (a0) calculated from XRD and pore diameter, was found
to be in the range of 10–13 Å for the calcined samples and increased
with a decrease in [CTAB]/[TPAOH] ratio. The mesopore volume
was found to increase with a decrease in [CTAB]/[TPAOH] ratio.
MC-4 showed the lowest average pore diameter and the highest
wall thickness among the three samples. The increase of surface
areas for MC-3 and MC-4 was due to the presence of better qual-
ity mesostructure and a higher degree of aluminum incorporation
in the framework [4,24]. Such Al-MCM-41 samples with superior
mesostructure and high surface areas therefore showed higher
activity toward methanol dehydration, as discussed earlier.
Fig. 8 shows the FTIR spectra for the studied samples. For Si-
MCM-41, two intense bands (␯1 and v2) at 1252 and 1087 cm−1
were assigned to the asymmetric T–O–T (T# Si or Al) stretching
vibration were observed. The band at 817 cm−1 (␯4) is associ-
ated with symmetric T–O–T (T = Si or Al) stretching, and the band
at 458 cm−1 (␯5) is assigned to a TO4 bending mode. The silanol
group stretching vibration (␯3) occurs at 965 cm−1. Generally, the
FTIR spectra of Al-MCM-41 samples show a low intense peak at
∼960 cm−1, assigned to Si–O–H or Si–OH vibrations. The substitu-
tion of silicon by aluminium causes a shift of the lattice vibration
band to a lower wave number. These shifts are due to the increase of
mean T–O distances in the walls of the mesostructure. In this case, it
is caused by the substitution of the small silicon atom by the larger
aluminum atom. The two intense bands (␯1 and v2) at ∼1252 and
1087 cm−1 for MC-1 are assigned to the asymmetric T–O–T (T = Si or
Al) stretching. These vibrations were shifted to lower wave num-
bers in the case of MC-2, MC-3, and MC-4 samples. The shift at
1087 cm−1 was more prominent for MC-3 and MC-4, the Al-MCM-
41 samples prepared in the presence of TPAOH in the synthesis gel.
Fig. 8. FTIR spectra of calcined Al-free and Al-MCM-41 samples in the region of
400–1500 cm−1
.
zeolitic aluminosilicates this peak is observed at ∼550 cm−1 and is
assigned to the asymmetric stretching mode of the five-membered
aluminosilica rings [25–28]. The position of this peak can shift to a
higher wavenumber depending upon the size of the crystallites and
crystallinity of the material [29]. The intensity ratio of the 570 cm−1
to 450 cm−1 peaks for MC-4, MC-3, and MC-2 were 0.14, 0.12, and
0.07, respectively, proving that the addition of TPAOH has clearly
facilitated the formation of catalytically active five-member alumi-
nosilicate rings which promoted the higher acidic activity of MC-3
and MC-4 materials (Fig. 9).
In addition, MC-2 showed a peak of low intensity at ∼570 cm−1
.
The acidity determination data shown in Table 1 also confirm
that Al-MCM-41 synthesized in this work has a moderate acidity
The intensity of this peak increased with the addition of TPAOH. For
Fig. 9. FTIR spectra of calcined Al-free and Al-MCM-41 samples in the region of 400–650 cm−1
.

S.P. Naik et al. / Applied Catalysis A: General 381 (2010) 183–190
189
Fig. 11. DME selectivity on bifunctional DME catalysts prepared using MK-121 as
methanol synthesis components, and MC-4 and MC-2 as methanol dehydration
components. Gas mixture, 25% CO2: 75% H2; temperature, 260 ◦C; pressure 735 psi;
Fig. 10. CO2 conversion on bifunctional DME catalysts prepared using MK-121 as
methanol synthesis components, and MC-4 and MC-2 as methanol dehydration
components. Gas mixture, 25% CO2: 75% H2; temperature, 260 ◦C; pressure 735 psi;
−1
.
GHSV, 2000 mL g−1
h
cat
GHSV, 2000 mL g−1
h
−1
.
cat
shown in Fig. 11, was 76% after 4.8 days of time on-stream whereas
it was only 32% for MC-2-based catalyst for the same period. The
other major product was methanol, and only traces (∼1%) of hydro-
carbons were detected in the products.
It is clear that MC-4 functions as a superior acidic component to
MC-2 in the bifunctional DME synthesis catalyst for the preparation
of DME from CO2/H2 mixtures. Highly porous and ordered active
materials, such as Al-MCM-41 or Al-SBA-15, are of interest for the
preparation of bifunctional DME catalyst as their mesopores could
be loaded uniformly with active components of methanol synthesis
catalysts to form robust and novel catalyst systems.
which was enhanced by the addition of a small amount of TPAOH
to the synthesis gel. It has been reported that only moderate Lewis
acid sites are required for the dehydration of methanol to DME [30].
The TPD of NH3 measures the total acidity in the material, however,
it may be difficult to exactly differentiate between the types of acid
sites, such as Lewis or Bronsted, using this technique. Therefore,
care must be exercised in interpretation of the data obtained using
TPD of NH3 technique.
In this study, well-ordered Al-MCM-41 structures were
obtained up to a CTAB-to-TPAOH molar ratio of 0.15 (data not
shown) in the synthesis gel. Further increase in the amount of
TPAOH leads to the formation of disordered mesoporous zeolitic
silica, as reported by Chiang and co-workers [23]. The addition of
TPAOH during synthesis of Al-MCM-41 favors a higher extent of
condensation in the mesoporous aluminosilicate framework that
facilitates incorporation of Al3+ ions in the framework [31,32]. An
improved condensation of aluminosilicate framework meant an
increased concentration of catalytically active acidic sites in the alu-
minosilicate framework, leading to a higher conversion of methanol
for the MC-3 and MC-4 samples. From the data shown in Table 2,
it is clear that the addition of TPAOH increases the wall thickness
of mesoporous Al-MCM-41. Previous reports [33–35] also suggest
that mesoporous silica of enhanced stability could be synthesized
simply by increasing the pore wall thickness and the extent of sil-
ica condensation. In fact, Mokaya [36] has reported improvement
of silica condensation and an increase in wall thickness of MCM-41
by increasing the period of hydrothermal treatment during syn-
thesis of MCM-41. Therefore, in our approach too, the addition of
TPAOH improved the silica condensation in the walls of Al-MCM-
41, thereby modifying the structural, acidic and catalytic properties
of Al-MCM-41 (Tables 1 and 2).
MC-2 and MC-4 were chosen as dehydration components of a
bifunctional DME synthesis catalyst prepared by mechanical mix-
ing of methanol synthesis catalyst, MK-121, with MC-2 or MC-4 as
methanol dehydration components. The DME synthesis was per-
formed using CO2/H2 as reactants at 260 ◦C, 735 psi, and GHSV
of 2000 mL g−1cat h−1. The MC-4-based bifunctional DME synthe-
sis catalyst showed CO2 conversion of ∼11%, and the MC-2-based
catalyst showed ∼8%, both at the end of 4.8 days of time on-stream,
as shown in Fig. 10. The activity of the MC-4-based catalyst seems
to have stabilized after 2.5 days as compared to that of MC-2-based
catalyst in which the activity was decreasing significantly with
time on-stream. The selectivity to DME for the MC-4-based catalyst,
4. Conclusions
Al-MCM-41 mesoporous catalysts of [SiO2]/[Al2O3] = 30 sam-
ples were prepared using tetraethylorthosilicate (TEOS) as the
source of silica, and hexadecyltrimethylammonium bromide
(CTAB) as the structure-directing agent. The synthesis was
performed under alkaline synthesis conditions at 100 ◦C in the pres-
ence and absence of tetrapropylammonium hydroxide (TPAOH)
in the synthesis gel. MC-3 and MC-4 samples, prepared at
CTAB/TPAOH of 0.75 and 0.20, respectively, showed methanol con-
versions of 72% and 73%, respectively. MC-2 prepared in the absence
of TPAOH showed a methanol conversion of 45%. The improve-
ment in the activities of the catalysts was correlated with the
enhancement of acidity the catalysts generated by better incor-
poration of Al species in the aluminosilicate framework by the
enhanced aluminosilicate condensation effected by TPAOH. MC-4
and MC-2 catalysts were studied as the dehydration components
of bifunctional catalysts for the synthesis of DME from CO2/H2 gas
mixture at temperature of 260 ◦C, pressure of 735 psi and GHSV of
2,000 mL g−1cat h−1. The MC-4-based catalyst showed higher con-
version and stability as compared to MC-2-based catalyst.
Acknowledgements
We thank Ambre Energy Pty Ltd, Brisbane, Australia, for provid-
ing financial support for this work.
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