An effective catalyst for syngas-to-dimethyl ether process with steamed zeolite HMCM-49 as dehydration component

Article abstract of DOI:10.1246/cl.2004.1456

The HMCM-49 zeolites before and after steam treatment were commingled with methanol synthesis catalyst to perform the single-step syngas-to-dimethyl ether process. The high temperature steam treatment meliorated the acidity of HMCM-49 zeolite, and consequ

Full text of DOI:10.1246/cl.2004.1456

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Chemistry Letters Vol.33, No.11 (2004)
An Effective Catalyst for Syngas-to-Dimethyl Ether Process
with Steamed Zeolite HMCM-49 as Dehydration Component
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Jianchao Xia,
Dongsen Mao, Ning Xu, Qingling Chen, Yahong Zhang, and Yi Tang
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Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials,
Fudan University, Shanghai 200433, P. R. China
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Shanghai Research Institute of Petrochemical Technology, SINOPEC, Shanghai 201208, P. R. China
(Received August 11, 2004; CL-040951)
The HMCM-49 zeolites before and after steam treatment
HZSM-5 zeolite6,8 have been used for DME synthesis. However,
the reaction temperature of the former two does not match that of
methanol synthesis while strong acidity of the latter often dete-
riorates the selectivity to DME because of the water reforming
reaction of methanol/dimethyl ether producing carbon dioxide9
and the subsequent reaction of the produced dimethyl ether pro-
were commingled with methanol synthesis catalyst to perform
the single-step syngas-to-dimethyl ether process. The high tem-
perature steam treatment meliorated the acidity of HMCM-49
zeolite, and consequently enhanced the selectivity to dimethyl
ether significantly by reducing the formation of by-products like
carbon dioxide and hydrocarbons.
7
,10
ducing hydrocarbons.
Here, we adopted H-form MCM-49 (HMCM-49) as the sol-
id acid component in STD catalyst, which exhibited an excellent
performance after deliberately adjusting the acidity of this solid
acid by high-temperature steam treatment. To the best of our
knowledge, the use of MCM-49 zeolite in this reaction has not
been reported so far.
1
Zeolite MCM-49, first synthesized by Bennett et al. in
993, has the same framework topology as calcined MCM-22,
1
which contains two nonintersecting pore systems. One of them
is constituted of two-dimensional sinusoidal, 10-membered ring
channels and the other consists of 12-membered ring supercages
that are accessible through 10-membered rings. As an alumi-
num-rich zeolite, MCM-49 is supposed to be a potential catalyst
in many acid-catalyzed reactions because of its abundant acidic
The zeolite HMCM-49 applied in our study was prepared as
1
1
described by Xu et al. The steam treatments of HMCM-49
were carried out in a quartz tube heated by electric furnace.
ꢁ
ꢁ
ꢂ1
The samples were heated to 400–600 C at 10 C min and
maintained at these temperatures for 4 h in a flow of steam–air
2
,3
sites and unique structure.
Dimethyl ether (DME) was disclosed to have better combus-
ꢂ1 ꢂ1
mixture which was composed of 12 mL min
ꢂ1 ꢂ1
g
of air. Samples that were untreated and
of water
tion performance than diesel fuel, e.g., lower NOx emission, less
4
and 100 mL min
g
ꢁ
smoke and engine noise. Recently, a process called single-step
treated at 400, 500, and 600 C were named as S0, S1, S2, and
S3, respectively. Then the zeolites of S0–3 were mechanically
mixed with an industrial methanol synthesis catalyst (Cu:Zn:Al
= 60:30:10 atomic ratio) at the weight ratio of 1:2, followed by
reducing in a dilute hydrogen flow (5% in N2) at 240 C for 6 h.
After that the activity tests were carried out in a fixed-bed reactor
syngas to DME (STD) has attracted increasing attention for its
high conversion of carbon monoxide and simplified proce-
dures.5 Although no agreement has been reached concerning
the carbon source of methanol synthesis at present, the reactions
involved in the STD process are generally supposed to be meth-
–8
ꢁ
ꢁ
anol synthesis (1), methanol dehydration (2), and water gas shift
,7,8
at 260 C under 4 MPa, which ensured the good performances on
7,11
(
WGS) reaction (3),5
conversion of syngas.
(H2/CO = 2, CO2 = 2.2%) was 1500 mL g
The space velocity of synthesis gas
ꢂ1 ꢂ1
CO þ 2H2 ꢀ CH3OH
CH3OH ꢀ CH3OCH3 þ H2O
H2O þ CO ꢀ H2 þ CO2
CO þ 3H2 ꢀ CH3OCH3 þ CO2
(1)
(2)
(3)
h . The effluent
2
products were heated electrically to avoid the condensation,
and analyzed by an on-line gas chromatograph equipped with
a TCD for CO2 and CO, and a FID for methanol, DME and hy-
drocarbons (HCs).
3
(4)
The combination of reactions (1)–(3) results in a synergistic ef-
fect which relieves the unfavorable thermodynamics for metha-
nol synthesis (1): methanol, the product of reaction (1), is con-
sumed for the formation of DME and water in reaction (2).
The water generated from reaction (2) is shifted by the WGS re-
action (3). The hydrogen produced in reaction (3) is, in return,
the reactant for methanol synthesis in reaction (1). Thus, one
of the products of each step is a reactant for another. This creates
a strong driving force for the overall reaction, allowing very high
CO conversion and selectivity to DME in one pass.
Table 1 lists the CO conversions and the selectivities of var-
ious products on the prepared composite catalysts containing
HMCM-49 zeolites as the dehydration components. It was found
that besides methanol and DME, a large amount of by-products
Table 1. Catalytic performance of the HMCM-49-containing
a
composite catalysts for STD reaction
Selectivity/%
Methanol DME
Solid
Acid
CO Conversion
/%
HCs
CO2
S0
S1
S2
S3
87.81
92.19
93.99
93.85
9.44
5.43
0.88
0.78
2.96
3.22
3.06
3.12
34.60
56.55
62.03
62.34
53.01
35.13
34.04
33.76
The catalysts applied in STD process are composed of two
essential components: a copper-based catalyst for methanol syn-
thesis and a solid acid catalyst for methanol dehydration. Com-
pared with that of methanol synthesis, the research on the com-
ponent for methanol dehydration has received less attention. To
a260 C, 4 MPa, H2/CO = 2, CO2 = 2.2%, 1500 mL h
ꢁ
ꢂ1 ꢂ1
g , and
5
7
the data were taken at the stable period of ca. 4 h time on stream.
Selectivity = mole of C in certain product/total converted C mole.
date, a few solid acids such as Al2O3, silica–alumina, and
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.11 (2004)
1457
like hydrocarbons and CO2 was found on the unmodified
HMCM-49 zeolite. After steam treatment, especially at the tem-
signed to the octahedral (Oh) Al species, i.e., the extraframework
Al (EFAL). The two NMR signals remained almost unchanged
ꢁ
ꢁ
peratures ꢃ500 C, the formation of CO2 and hydrocarbons de-
when the samples were steamed at ꢄ400 C, but peak at
creased remarkably, so the selectivity to DME increased signifi-
cantly from 34.60% for the parent HMCM-49 to >62%. Further-
more, higher CO conversions were also achieved on the modi-
fied zeolites for the increase of the ‘‘driving force’’ for the
overall reaction (4). The highest DME yield (58.5%) was achiev-
55 ppm became lower and peak at 0 ppm was broadened when
ꢁ
the treating temperatures were ꢃ500 C. The weakened signal
at 55 ppm showed that part of Td Al was removed from frame-
work of HMCM-49 and changed to EFAL (Oh Al). Hence, it
is the removal of FAL that leads to the decrease of acidic sites
on HMCM-49. The broadened peak at 0 ppm indicated the ag-
ꢁ
ed with HMCM-49 zeolite treated at 600 C. This value was
close to the equilibrium yield of DME in our experimental con-
ditions (ca. 65%).
1
2
gregation of EFAL. The cooperation of Td and Oh Al species
was believed to have strong ability to produce hydrocarbons in
1
3
the methanol conversion process. So, besides the decrease of
strong acidic sites, the aggregation of EFAL of the steamed
HMCM-49 should be another important reason for the decrease
of hydrocarbons.
D
C
B
A
D
C
200
300
400
500
600
o
Temperature /
C
55
B
Figure 1. TPD curves for ammonia of the parent HMCM-49
ꢁ
0
ꢁ
A
zeolite (A) and steam treated at 400 C (B), 500 C (C), and
ꢁ
600 C (D).
100
50
0
-50
-100
3+
2 6
ppm for Al(H O)
Figure 1 presents the NH3-TPD profiles of the HMCM-49
Figure 2. 27Al MAS NMR spectra of the parent HMCM-49
zeolites. Although there are some debates on assigning the peaks
in the NH3-TPD profile, it is widely accepted that the peak at
higher temperature is referred to the strong acidic sites while
the peak at lower temperature is corresponding to weak acidic
sites. Accordingly, two types of acidic sites exist on the surface
of the parent HMCM-49 zeolite, which are represented by the
ꢁ
ꢁ
zeolite (A) and steam treated at 400 C (B), 500 C (C), and
ꢁ
6
00 C (D).
In conclusion, zeolite HMCM-49 can be used as the metha-
nol dehydration component in STD catalyst. Steam treatment
was a good method to ameliorate the acidity of zeolite by remov-
ing FAL from the framework of HMCM-49. After high temper-
ature steam treatment, the conversion of CO increased about 6%,
while the selectivity of DME was enhanced almost two times.
The modified HMCM-49 zeolite was a potential replacer of
the methanol dehydration component in STD catalyst.
ꢁ
peaks of ammonia desorbed at about 300 and 500 C, respective-
ly (pointed out by arrows in Figure 1). After steam treatment, the
peak at high temperature was weakened and almost disappeared
ꢁ
when the treating temperature was ꢃ500 C, implying that the
steam treatment can remove the strong acidic sites on HMCM-
49. Strong acidic sites have been believed to catalytically pro-
7
,10
duce hydrocarbons and water. Furthermore, the produced wa-
ter promoted the water reforming reaction of methanol/dimethyl
The authors thank the National Basic Research Program of
China (2003CB615801, 2003CB615807, 2000077500) and the
NSFC (20273016, 20233030, 20303003, 20325313) and the
SNPC (0249nm028, 03DJ14004) for financial support.
9
ether, resulting in the formation of larger amount of CO2. The
elimination of strong acid sites of HMCM-49 through steam
treatment would minimize the side-reactions, increasing the se-
lectivity of dehydration reaction. Meanwhile, the equilibrium
conversion of reaction (4) would shift toward the right-hand side
owing to the decrease in the formation of CO2. Therefore, the
CO conversion also increased evidently by steam modification.
On the other hand, although the acidity of zeolite was decreased
apparently by steam treatment, the selectivity to methanol of all
catalysts were very low, indicating that acidity of each catalyst
was strong enough to dehydrate the methanol. This result is in
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Published on the web (Advance View) October 9, 2004; DOI 10.1246/cl.2004.1456