Superior dehydration of CH3OH over double layer bed of solid acid catalysts - A novel approach for dimethyl ether (DME) synthesis

Article abstract of DOI:10.1246/cl.2004.598

A novel catalyst bed packed with two layers of γ-Al₂O ₃ and Na-H-ZSM-5 showed an outstanding CH₃OH dehydration activity, total selectivity to DME and longer stability during the reaction time. The TPD of NH₃ confirmed that appropriate amounts of Na-impregnation on H-ZSM-5 drastically reduced the strong acid-sites resulting in 100% selectivity to DME.

Full text of DOI:10.1246/cl.2004.598

598
Chemistry Letters Vol.33, No.5 (2004)
Superior Dehydration of CH3OH over Double Layer Bed of Solid Acid Catalysts
—A Novel Approach for Dimethyl Ether (DME) Synthesis
Hyun-Seog Roh, Ki-Won Jun,ꢀ Jae-Woo Kim, and Venkataraman Vishwanathan
Chemical Technology Division, Korea Research Institute of Chemical Technology,
P. O. Box 107, Yuseong, Daejeon 305-600, Korea
(Received January 15, 2004; CL-040059)
A novel catalyst bed packed with two layers of ꢀ-Al2O3 and
sistent with the increase in the activation energy for DME syn-
thesis.10 Also, the presence of water has the tendency to shift
the reaction equilibrium backward. This suggests that the pres-
ence of large amount of water will retard the dehydration activity
of methanol to DME formation. As a result, hydrophilic ꢀ-Al2O3
catalyst located in the bottom bed is not so effective in methanol
dehydration. Therefore, it is expected that the efficiency of the
catalyst bed could be improved by substituting hydrophobic sol-
id acid catalysts for hydrophilic ꢀ-Al2O3 catalyst in the bottom
bed of the catalysts.
Na-H-ZSM-5 showed an outstanding CH3OH dehydration activ-
ity, total selectivity to DME and longer stability during the reac-
tion time. The TPD of NH3 confirmed that appropriate amounts
of Na-impregnation on H-ZSM-5 drastically reduced the strong
acid-sites resulting in 100% selectivity to DME.
Air pollution is a serious environmental concern in all over
the world. A large portion of the transport vehicles use diesel
fuel. Unfortunately, it emits a significant amount of solid pollu-
tants like unburnt hydrocarbons, NOx and carbon particulates.
With stringent environmental regulations, a clean alternate fuel
has become the global necessity. Dimethyl ether (DME) has re-
ceived a world-wide attention since it has a great potential as a
clean alternative fuel for diesel engines because of its thermal ef-
ficiencies equivalent to traditional diesel fuel, lower NOx emis-
sion, lesser carbon particulates, near-zero smoke and less engine
noise.1 In addition to its usage as fuel, DME is also useful inter-
mediate for the preparation of many important value-added
chemicals such as lower olefins, methyl acetate and dimethyl
sulphate.2,3 In view of this growing demand, there is a need to
produce a larger amount of DME in the future.
Commercially, DME is produced by the catalytic dehydra-
tion of methanol at around 290 ꢁC and 10 atm over solid acid cat-
alysts such as ꢀ-Al2O3.4–6 In our previous study on the title re-
action using ꢀ-Al2O3 and H-ZSM-5 independently, we have
noticed that ꢀ-Al2O3 was found to be active and stable only in
the absence of water; while the H-ZSM-5 was highly active in
the initial stage of the reaction but slowly deactivated with reac-
tion time due to the formation of lower hydrocarbons.4–6 Howev-
er, the remarkable observation was the longer stability of H-
ZSM-5 in the presence of water for a period of 100 h.4 It was also
found that modification of H-ZSM-5 with suitable metal cations
to remove strong acid sites improved further the dehydration ac-
tivity as well as the selectivity to DME by preventing the forma-
tion of hydrocarbons. Recently, we have reported that when
crude methanol containing water was used as a starting material,
the formation of hydrocarbons on the hydrophobic surface can
be avoided.4 However, when anhydrous or pure methanol was
used as a raw material, the catalyst deactivation was severe with
coke formation.4
In this paper, based on our previous studies, a strategy was
adopted to design a double layer catalyst bed, having the charac-
teristics resulting from two separate materials viz., ꢀ-Al2O3 and
Na-H-ZSM-5, and use them as one entity for the dehydration of
methanol to DME reaction. According to this scheme, when the
reactant methanol contacts the upper layer (ꢀ-Al2O3) first, the
down-stream products viz., DME, water and the unreacted meth-
anol, would be allowed to contact the bottom layer, Na-H-ZSM-
5. By this procedure, the deactivation of Na-H-ZSM-5 can be re-
tarded owing to the presence of water produced over ꢀ-Al2O3. In
this communication, a double layer bed (ꢀ-Al2O3 + Na-H-ZSM-
5) showing a remarkable catalytic performance in terms of supe-
rior methanol dehydration activity, total selectivity to DME and
longer catalyst life is reported. It is the first report of this kind
studied on the title reaction at lower temperatures.
Commercial supports, ꢀ-Al2O3 (SASOL, SBET = 220 m2/g)
and H-ZSM-5 (ZEOBUILDER, SiO2/Al2O3 = 40, SBET
=
403 m2/g) were employed in this study. The Na-modified H-
ZSM-5 catalysts were prepared by the conventional impregna-
tion method using an aqueous NaNO3 solution. All the samples
were calcined at 823 K for 4 h in air. Methanol dehydration was
carried out in a fixed-bed down-flow reactor at a gauge pressure
of 10 atm. The double layer bed was arranged in such a manner
that 20 vol % of Nax-H-ZSM-5 was loaded into the bottom bed of
the reactor while 80 vol % of ꢀ-Al2O3 was loaded into the top
bed of the reactor and the total amount of catalyst was 5.0 mL
by volume. The symbol (x) in Nax-H-ZSM-5 refers to the mole
percentage of Naþ based on the mole of Hþ in H-ZSM-5. Before
the start of the reaction, the catalyst was activated in the stream
of N2 at 623 K for 1 h under normal atmospheric pressure. Meth-
anol was introduced by a HPLC pump. The products were ana-
lyzed by a GC equipped with thermal conductivity and flame
ionization detectors.
It is known that in the catalytic dehydration of methanol, a
significant amount of water is present in the product stream.5,7–10
Since water adsorbs more strongly on the surface as compared to
methanol, the blocking of active sites by water decreases the de-
hydration activity of methanol more drastically and hence the
DME formation. At high water coverage, the heat of adsorption
of water on ꢀ-Al2O3 is approximately 16 kcal/mol which is con-
Figure 1 shows the catalytic dehydration of methanol with
reaction time. The ꢀ-Al2O3 catalyst showed rather low methanol
conversion (<25%). In the industrial process, DME is produced
at 290 ꢁC and 10 atm over ꢀ-Al2O3 catalyst. In the case of dou-
ble layer bed (ꢀ-Al2O3 + Nax-H-ZSM-5), the methanol conver-
sions were higher than 80% except ꢀ-Al2O3 + Na-ZSM-5.
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.5 (2004)
599
Because Na-ZSM-5 is totally inactive in methanol dehydration,
only ꢀ-Al2O3 contributed to the activity in the system composed
of ꢀ-Al2O3 + Na-ZSM-5. Actually, Na-ZSM-5 did not show
any activity in a separate experiment.
selectivity (¼ 40% at 290 ꢁC and 10 atm) resulting from severe
hydrocarbon formation in the initial stage. This strongly indi-
cates that the location of the hydrophilic and hydrophobic cata-
lyst is very significant.
According to NH3-TPD results on Nax-H-ZSM-5 (data
shown as a supplementary data), it is explicit that the surface
density of the strong acid-sites, in the region of 330 to 500 ꢁC,
decreased with increase in sodium level on H-ZSM-5. In addi-
tion, it is noticed that there is a small shift in the position of
Tmax of the third peak towards the region (220 to 330 ꢁC) of me-
dium acid-sites, whose density, though not quantified, has in-
creased. These observations clearly demonstrate that a novel cat-
alyst bed packed with two layers of hydrophilic ꢀ-Al2O3 and a
hydrophobic Na-H-ZSM-5 provides highly active and selective
way in the methanol dehydration reaction to DME. The catalyst,
having 40–80% Na-content, in Na-H-ZSM-5 showed an opti-
mum performance in terms of activity and selectivity (CMeOH
= 89%, SDME = 100%) with an exceptional stability for 400 h
under the same reaction conditions.
100
90
80
(a)
(b)
(c)
(d)
(e)
(f)
30
20
10
0
5
10
15
20
Time on stream /h
In conclusion, the double layer catalyst bed, ꢀ-Al2O3
+
Figure 1. Methanol dehydration to DME over various solid
acid catalysts (T ¼ 250 ꢁC, P ¼ 10 atm, LHSV = 4.8 hꢂ1, sym-
bol: solid = conversion of MeOH and open = selectivity to
DME, ꢀ-Al2O3 (a) and double layer bed [ꢀ-Al2O3 (top, 2.6 g)
+ NaX-H-ZSM-5 (bottom, 0.54 g)] with mol % Na = 10 (b),
= 20 (c), = 40 (d), = 80 (e) and = 100 (f)).
Na-H-ZSM-5, demonstrates a higher catalytic activity as well
as 100% selectivity in the conversion of methanol to DME at
low temperatures. This is possibly ascribed to the synergy effect
shown between the hydrophilicity of ꢀ-Al2O3 and the hydropho-
bicity of Na-H-ZSM-5. The reduction in the surface density of
strong acid-sites by Na-modification of H-ZSM-5 further in-
creases selectivity to DME and makes the catalyst more resistant
towards carbon and/or hydrocarbon formation. Finally, it is our
anticipation that the double layer bed of this combination would
be a suitable candidate for the industrial production of DME
from methanol at low temperatures in the near future.
Unlike methanol conversion, the DME selectivity strongly
depended upon the degree of Na ion impregnated on H-ZSM-
5. When 10% Na was impregnated on H-ZSM-5, the initial
DME selectivity was less than 40%. This indicates that the hy-
drocarbons were formed during the course of the reaction. After
3 h, the selectivity towards DME increased to 99%. This is due to
the fact that the carbon, formed in the initial stage, eliminated the
strong acid-sites responsible for hydrocarbon formation. It is al-
so noticed that as the concentration of Na ions on H-ZSM-5 in-
creased, there was a selectivity of 100% for DME. The acid-sites
on H-ZSM-5 are known to be so strong that catalyst deactivation
is inevitable during the reaction.11 The impregnation of the metal
ions into H-ZSM-5 catalyst eliminates the strong acid-sites and
thereby enhances the resistance towards carbon formation.
Compared with double layer bed composed of ꢀ-Al2O3 and
Na60-H-ZSM-5, mixed bed having the same composition exhib-
ited lower DME yield at low temperatures (Table 1). In addition,
inversely located catalytic system, namely, top bed of Na60-H-
ZSM-5 and bottom bed of ꢀ-Al2O3, showed much lower DME
This research was performed for the Carbon Dioxide
Reduction & Sequestration Center, one of 21st Century Frontier
R&D Programs funded by the Ministry of Science and
Technology of Korea.
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Table 1. Comparison of DME yields over double layer bed,
mixed bed (P ¼ 10 atm, LHSV = 4.8 hꢂ1, catalyst = 2.6 g of
ꢀ-Al2O3 and 0.54 g of Na60-H-ZSM-5) and ꢀ-Al2O3.
DME Yield/%
Temperature
Double
Layer
/ꢁC
Mixed
ꢀ-Al2O3
250
260
270
280
290
86.8
88.0
86.3
86.3
86.7
73.2
81.5
84.2
84.9
83.9
20.7
35.6
61.2
64.5
76.9
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Published on the web (Advance View) May 5, 2004; DOI 10.1246/cl.2004.598