More efficient ethanol synthesis from dimethyl ether and syngas over the combined nano-sized ZSM-35 zeolite with CuZnAl catalyst

Article abstract of DOI:10.1016/j.cattod.2020.03.035

Converting syngas into ethanol (EtOH) is highly attractive but remains challenge. Dimethyl ether (DME) carbonylation with CO to methyl acetate (MA) on zeolite and its further hydrogenation to EtOH on Cu-based catalyst open a new EtOH synthesis route from syngas. In this work, a nano-sized ZSM-35 (NZ35) zeolite, possessing abundant active sites and porosity and short diffusion path, is found to realize much better activity of DME to MA than that of the conventional ZSM-35 zeolite (CZ35). In addition, a simple formic-acid-assisted solid-state method is employed for preparation an auto-reduced CuZnAl (CZA_argon) catalyst under argon atmosphere. The prepared CZA_argon catalyst exhibits an excellent catalytic activity for conversion of produced MA to EtOH. By investigating the effects of different integration manners of NZ35 zeolite and CZA_argon catalyst, we find that EtOH can be synthesized only when the NZ35 zeolite and CZA_argon catalyst pack in a dual-catalyst bed reactor. After optimizing the reaction conditions for EtOH synthesis with the combination of NZ35 zeolite and CZA_argon catalyst, it is found that the DME conversion and MA selectivity are stabilized at 47.0 % and 45.6 % respectively, at 220 °C and 2.5 MPa.

Full text of DOI:10.1016/j.cattod.2020.03.035

Journal Pre-proof
More efficient ethanol synthesis from dimethyl ether and syngas over the
combined nano-sized ZSM-35 zeolite with CuZnAl catalyst
Xiaobo Feng (Conceptualization) (Writing - original draft), Jie Yao
(Formal analysis), Yan Zeng (Software), Yu Cui (Data curation),
Shun Kazumi (Investigation), Reubroycharoen Prasert
(Visualization), Guangbo Liu (Resources), Jinhu Wu (Validation),
Guohui Yang (Methodology) (Supervision), Noritatsu Tsubaki
(Writing - review and editing)
PII:
S0920-5861(20)30152-8
DOI:
https://doi.org/10.1016/j.cattod.2020.03.035
CATTOD 12748
Reference:
To appear in:
Catalysis Today
Received Date:
Revised Date:
Accepted Date:
11 October 2019
5 March 2020
17 March 2020
Please cite this article as: Feng X, Yao J, Zeng Y, Cui Y, Kazumi S, Prasert R, Liu G, Wu J,
Yang G, Tsubaki N, More efficient ethanol synthesis from dimethyl ether and syngas over the
combined nano-sized ZSM-35 zeolite with CuZnAl catalyst, Catalysis Today (2020),
doi: https://doi.org/10.1016/j.cattod.2020.03.035

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More efficient ethanol synthesis from dimethyl ether and syngas over the
combined nano-sized ZSM-35 zeolite with CuZnAl catalyst
Xiaobo Fenga,b, Jie Yaoa, Yan Zenga, Yu Cuia, Shun Kazumia, Reubroycharoen Prasertc,
Guangbo Liud, Jinhu Wud, Guohui Yanga,*, Noritatsu Tsubakia,*
aDepartment of Applied Chemistry, School of Engineering, University of Toyama,
Gofuku 3190, Toyama 930-8555, Japan
bKey Laboratory of Coal Processing and Efficient Utilization (Ministry of Education),
China University of Mining & Technology, Xuzhou 221116, Jiangsu, China
cDepartment of Chemical Technology, Faculty of Science, Chulalongkorn University,
Bangkok 10330, Thailand
dQingdao Institute of Biomass Energy and Bioprocess Technology, Chinese Academy
of Sciences, Qingdao, China
*Corresponding author. Tel. +81 76 445 6846; Fax: +81 76 445 6846.
E-mail address: tsubaki@eng.u-toyama.ac.jp (N. Tsubaki).
E-mail address: thomas@eng.u-toyama.ac.jp (G. Yang).
Graphical abstract
70
50
NZ35/CZA-D
45
40
35
60
50
40
30
20
10
0
CZ35/CZA-D
NZ35/CZA-D 30
25
20
15
CZA
10
ZSM-35
CZ35/CZA-D
5
0
MA + H2
EtOH
DME + CO
2
4
6
8
10 12 14 16 18 20 22 24
Time on stream (h)
Highlights

A nano-sized ZSM-35 zeolite with abundant active sites for dimethyl ether carbonylation
was prepared.

An auto-reduced CuZnAl catalyst with excellent hydrogenation ability was prepared by a
simple solid-state method.
1



The proximity effect for both nano-sized ZSM-35 and CuZnAl catalysts on ethanol
synthesis was clarified.
Combined nano-sized ZSM-35 and CuZnAl catalysts realized more efficient ethanol
synthesis.
Abstract
Converting syngas into ethanol (EtOH) is highly attractive but remains challenge.
Dimethyl ether (DME) carbonylation with CO to methyl acetate (MA) on zeolite and
its further hydrogenation to EtOH on Cu-based catalyst open a new EtOH synthesis
route from syngas. In this work, a nano-sized ZSM-35 (NZ35) zeolite, possessing
abundant active sites and porosity and short diffusion path, is found to realize much
better activity of DME to MA than that of the conventional ZSM-35 zeolite (CZ35).
In addition, a simple formic-acid-assisted solid-state method is employed for
preparation an auto-reduced CuZnAl (CZAargon) catalyst under argon atmosphere.
The prepared CZAargon catalyst exhibits an excellent catalytic activity for conversion
of produced MA to EtOH. By investigating the effects of different integration
manners of NZ35 zeolite and CZAargon catalyst, we find that EtOH can be synthesized
only when the NZ35 zeolite and CZAargon catalyst pack in a dual-catalyst bed reactor.
After optimizing the reaction conditions for EtOH synthesis with the combination of
NZ35 zeolite and CZAargon catalyst, it is found that the DME conversion and MA
selectivity are stabilized at 47.0% and 45.6% respectively, at 220 oC and 2.5 MPa.
Keywords: Ethanol synthesis, syngas, DME, nano-sized ZSM-35 zeolite, CuZnAl
catalyst
1. Introduction
Ethanol (EtOH), as a renewable energy source, has attracted increasing attentions
because of its environmental and long-term economic advantages [1-3]. As an
2

alternative route, syngas (CO+H2) convertion to EtOH is regarded as one of the most
promising EtOH synthesis methods. This production process of EtOH is highly
competitive compared with fermentation of sugars or corns, and hydration of ethylene
because syngas can be efficiently produced from natural/shale gas, coal, biomass, and
even CO2 [4-6]. At present, direct synthesis of EtOH from syngas is limited because
of the usage of the expensive Rh-based catalyst and the low yield of EtOH [2, 7].
Moreover, some indirect methods for EtOH synthesis from syngas have been
developed. The typical alternative processes are using methanol (MeOH) or dimethyl
oxalate (DMO) as raw materials [8, 9]. However, their usage of noble-metal or
corrosive catalysts and the complicated processes usually limit their application.
Dimethyl ether (DME) can be easily synthesized by one-step from syngas through
H-ZSM-5/CuZnAl capsule catalyst, which makes it an attractive candidate as clean
and high efficiency energy [10]. Recently, it is reported that the zeolite catalyst such
as MOR, FER, and EU-12 are discovered their carbonylation ability for converting
DME to methyl acetate (MA), which provides a halide-free and noble metal-free
method to produce MA [11, 12]. Considering that the DME carbonylation to MA and
ester hydrogenation to alcohols occur at a similar temperature, a new route for direct
synthesis of EtOH from DME and syngas was proposed in our previous research [13].
In this method, a dual-catalyst bed reactor with combined zeolite and Cu-based
catalyst was designed. MA is firstly produced by DME carbonylation over the upper
zeolite catalyst, and then the formed MA is further hydrogenation over the low
Cu-based catalyst bed to generate EtOH and MeOH [13, 14]. It should be noted that
the formed main byproduct of MeOH can be recycled easily through dehydration
reaction to produce the feedstock of DME over solid acid catalysts, significantly
improving the utilization efficiency of raw materials. For DME carbonylation on
zeolite, it is still difficult to realize excellent catalytic stability and activity
simultaneously though many post-synthesis modification approaches, such as
ion-exchange with metals, selective dealumination, selective poisoning with pyridine,
are adopted for improving their carbonylation ability [15-17]. Therefore, preparation
of zeolite catalyst with both high activity and stability in DME carbonylation is very
3

attractive but challenging. Nano-sized zeolites attract considerable attention because
their larger accessible surface area and fewer diffusion limitations than conventional
ones [18-20]. In contrast to the enormous investigations on DME carbonylation over
nano-sized H-MOR zeolite [20, 21], the study of DME carbonylation over nano-sized
ZSM-35 zeolite has not been widely studied. Therefore, it is highly desirable to
develop a nano-sized ZSM-35 zeolite for achieving more efficient DME
carbonylation.
Moreover, the MA hydrogenation to MeOH and EtOH is also an important step in
this new EtOH synthesis route. The employed CuZnAl (CZA) catalyst prepared by
conventional co-precipitation method is considered as highly active for converting
MA to EtOH [22]. However, this method creates large amounts of concentrated nitrate
wastewater and consumes extra cost of reduction energy, which makes the whole
reaction process inefficient. In our previous research, Cu-based catalyst, prepared by a
novel formic acid-assisted solid-state method, can be directly used for MeOH
synthesis from syngas [23]. As compared with the co-precipitation method, this novel
method is easily operated. Moreover, the prepared Cu-based catalyst can be
automatically reduced in situ by the released H2 and CO during the metal-formic acid
precursor calcination in argon atmosphere. Therefore, this novel and simple means
will be suitable for preparing CZA catalyst, which is employed for MA hydrogenation
to EtOH and MeOH.
In the present study, a nano-sized ZSM-35 zeolite (NZ35) is directly synthesized
by hydrothermal method. The influences of temperature on DME carbonylation
performance over NZ35 zeolite are investigated. The NZ35 zeolite shows remarkably
high activity and stability with respect to the conversional ZSM-35 (CZ35) zeolite for
DME carbonylation reaction. The CZA catalyst is designed and prepared by a novel
formic-acid-assisted solid-state method. In this method, the auto-reduced CZA
catalyst is calcined in argon atmosphere (CZAargon).This method is simple, waste
water free and without further reduction. The prepared CZAargon catalyst exhibits
excellent hydrogenation ability for converting MA to EtOH. The combined catalysts
in a dual-catalyst bed reactor with tandem of NZ35 zeolite and CZAargon catalyst
4

realize more efficient EtOH synthesis using DME and syngas as starting feedstocks.
2. Experimental
2.1. Catalysts preparation
The NZ35 zeolite was prepared via a direct hydrothermal synthesis route and the
CZAargon catalyst was prepared by a novel solid-state method with metal nitrates and
formic acid. The detailed preparation processes and catalyst characterization were
described in Supplementary data.
2.2. Catalytic performance evaluation
All the catalytic performance evaluation reactions were carried out in a fixed-bed
steel reactor with inner diameter of 9.5 mm. For the single DME carbonylation
reaction, only 0.5 g zeolite (40-60 mesh) was packed in the reactor. After being
o
pre-treated at 400 C under N2 atmosphere for 2 h, the reactor was cooled to the
designed reaction temperatures, and then
a
reactant gas (2.1% DME
/46.4%CO/50.0%H2/1.5%Ar, in volume ratio) with pressure of 1.5 MPa was
introduced into the reactor at a gas hourly space velocity (GHSV) of 4800 mL/(g·h).
For EtOH synthesis, different integration manners of NZ35 zeolite and CZAargon
catalyst were designed to clarify the proximity effect in the two composites. Firstly,
the NZ35 zeolite and the CZAargon catalyst were pressed, crushed and sieved to
granules in the range of 40-60 mesh, respectively. The reaction was carried out in a
dual-catalyst bed reactor consisting of upper NZ35 (0.5 g) zeolite and bottom
CZAargon (0.5 g) catalyst. In this case, the two components (denoted as NZ35/CZA-D)
were separated in a millimeter distance. For comparison, the physically mixed catalyst
(1.0 g) with granules of the NZ35 /CZAargon=1:1 (40-60 mesh) in weight ratio was
also evaluated. The use of the mixture of NZ35 granules and CZAargon granules
(denoted as NZ35/CZA-M) increased the proximity of the two components to
micrometer scale. For further increasing the proximity between the two components
to nano-scale, a bi-functional catalyst (denoted as NZ35/CZA-G) was prepared by
grinding of NZ35 and CZAargon catalyst. The powder of NZ35 zeolite and the CZAargon
5

catalyst with weight ratio of 1:1 were mixed well in a mortar grinding for 5 min. Then,
the mixed sample was pressed, crushed and sieved to particles in the range of 40-60
mesh. Before the reaction, the reactant gas containing DME/CO/H2/Ar
(2.1/46.4/50.0/1.5) was introduced.
The effluent gas was continuously analyzed by an on-line gas chromatograph
(GC) equipped with a TCD and a connected dual columns (Porapak Q column for
DME and Activated Carbon column for Ar, CO, CH4 and CO2, Ar was used as the
internal stand gas). The liquid products (MA, MeOH and EtOH) in off-gas were
collected by ice trap using 1-butanol as solvent and then detected by GC-8A with a
flame-ionization detector (FID, Pora-PLOT Q column). For quantitative analysis of
the liquid products, 1-propanol was employed as the internal standard. The calculated
methods of DME conversion and products selectivity were according to our previous
studies [24].
3. Results and discussion
3.1. Catalyst characterization
3.1.1 Structure and texture properties of the NZ35 and CZ35 zeolite
Fig. 1(a) presents the XRD patterns of NZ35 and CZ35 zeolite. All the
characteristic peaks in the 2θ region from 5o to 50o can be indexed to the typical FER
zeolite structure [25]. From the images displayed in Fig. 1 (b, c), it is evident that the
NZ35 zeolite consists of nanoparticles about 80 nm while the CZ35 zeolite has a
cuboid-shaped morphology in size of nearly 250 nm. The textural properties are
summarized in Table 1. The bulk SiO2/Al2O3 ratio, measured by XRF, is 15.3 for
NZ35 and 16.2 for CZ35 sample. NZ35 zeolite has higher BET surface area and
larger pore volume than those of CZ35, which is attributed to the existing of
nanocrystal and intercrystal mesopores. The excellent textural properties of NZ35
zeolite are beneficial for improving the migration ability of reactants and intermediate
products [20, 21], and thus will help to enhance catalytic performance for DME
27
carbonylation. In the Al MAS NMR spectra of the NZ35 (Fig. S1), there is only an
obvious resonance at 58 ppm attributed to tetrahedrally coordinated framework
6

aluminium ((AlIV). It indicates that the NZ35 samples almost have no
extra-framework aluminium (AlVI) [26]. To explore the distribution of acid sites of
NZ35 and CZ35 zeolite in different channels, FTIR spectra are characterized in the –
OH stretching region (3250-3650 cm-1). As shown in Fig. S2, five asymmetric bands
corresponding to -OH species vibration in different channels can be observed. The
band at 3358 cm-1 is attributed to -OH groups in 5-membered rings (5-MRs) while the
bands at 3434 and 3499 cm-1 are associated with in 8- and 6-MRs, respectively. The
band at 3549 cm-1 corresponds to -OH groups located in 8-MRs channels, while the
band at 3600 cm-1 belongs to OH groups vibrating in the 10-MRs channels [27, 28].
Many studies have demonstrated that the acidic sites in 8-MRs channels are
responsible for the carbonylation reaction [11, 14, 29]. As indicated by the
deconvolution of FTIR and NH3-TPD (Fig. 1(f), Table S1) as well as the Py-IR (Fig.
S3) results, the amount of Brønsted acid sites located in the 8-MRs channels in NZ35
zeolite is obviously higher than that in CZ35 zeolite, implying that the NZ35 zeolite
has strong potential ability to facilitate DME conversion to MA.
(a)
(b)
(C)
CZ35
NZ35
NZ35
CZ35
5
10 15 20 25 30 35 40 45 50 55 60
2 Theater (degree)
14
500
NZ35
CZ35
NZ35
CZ35
(e)
(d)
NZ35
CZ35
(f)
12
10
8
400
300
200
100
0
6
4
2
0
-2
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
P/P0
0
100
200
300
400
500
600
700
0.5
1.0
1.5
2.0
2.5
Pore diameter (nm)
Temperature (oC)
Fig. 1. The characterization of NZ35 and CZ35 zeolite: (a) XRD patterns; (b) and (c)
SEM images; (d) N2 adsorption-desorption isotherms; (e) pore size distribution; (f)
NH3-TPD.
Table 1 Textural and physical properties of NZ35 and CZ35 zeolite and CZAargon
7

catalyst.
b
c
b
c
SBET
(m2 g-1)
Sext
Vmicro
Vmeso
Sample Composition a
(m2 g-1)
154.3
33.3
(cm3 g-1)
0.19
(cm3 g-1)
0.13
NZ35
CZ35
SiO2/Al2O3=15.2 407.3
SiO2/Al2O3=21.3 270.1
0.20
0.08
Cu/Zn/Al=
CZAargon
30.3
26.6
0.25
0.22
0.98/0.95/0.09
a: Detection by the XRF
b: Sext and Vmicro were calculated by the t-plot method.
c: Vmeso was calculated from BJH method.
3.1.2 Structure and texture properties of CZAargon catalyst
As displayed in Fig. S4, the characteristic diffraction peaks of CZAargon catalyst
are attributed to Cu and ZnO phase. This result confirms that the copper-formic acid
precursors are absolutely reduced to metallic Cu during decomposition of the
metal-formic acid complexes under argon. For the SEM image of CZAargon catalyst,
some small nanoparticles about 100 nm are observed. The CZAargon catalyst is
preserving an actual molar of 0.98/0.95/0.09 of Cu/Zn/Al, which is very close to the
initial molar composition (1.0:1.0:0.1) designed in preparation process. The presence
of metal Cu species in CZAargon sample is further checked by H2-TPR. For CZAargon
sample, only a very small peak appears, being attributed to the reduction of a very
small quantity of the surface CuO species generated during passivation by 1% oxygen.
o
For CZAair, the H2-TPR plot mainly consists of a small peak at 200 C and a large
o
peak at 300 C, which are corresponding to the reduction of highly dispersed CuO
species and the bulk CuO phase, respectively. Thermal decomposition behavior of the
formic acid-precursors is detected by TG-DTA under argon. As shown in Fig. S5(a),
o
the DTA curve exhibits a large endothermic peak at about 210 C and the TG plot
appears a sustained weight loss from 100 to 220 oC, which is attributed to pyrolysis of
the metal-formic acid precursors. During this process, the released H2 and CO are
used as the reducing agents to obtain metallic Cu species from Cu2+ in the
8

metal-formic acid complexes. For the CZAargon catalyst detecting by TG-DTA under
o
air, a gradual weight increasing from 120 to 310 C for TG plot is observed. In this
o
process, metallic Cu undergoes a two-step oxidation from Cu to Cu2O (110-200 C)
o
and from Cu2O to CuO (210-310 C). With continual temperature increasing, the TG
o
curve exhibits a slight weight loss from 520 to 600 C with an exothermic peak
observed in the corresponding DTA plot, which is attributed to the combustion of
organic residues [23]. This phenomenon demonstrates that when formic acid is used
as chelating agent and local reductant, only a small amount of carbonic residues is left
in the CZAargon catalyst.
3.2 The single DME carbonylation over zeolite
Fig. 2 investigates the effect of reaction temperature on DME carbonylation over
single NZ35 zeolite. As shown in Fig. 2 (a1), the initial conversion of DME increases
dramatically with increasing reaction temperature. However, the MA selectivity and
the catalyst stability present a crosscurrent trend. When the reaction temperature is
o
above 220 C, the selectivity of CH4 increases significantly, suggesting that the side
reactions are promoted. At high temperature, although more DME is converted, the
undesired reactions such as formation of CH4 and coke species are preferentially
occurring. In this case, the Brønsted acid sites catalyze the DME reacting with
methoxyl group to coke species instead of carbonylation with CO to MA. In addition,
as proposed by Li et al., the formed MA can be decomposed more readily to generate
light hydrocarbons at high temperature [14]. Therefore, the productivity of MA
displays a volcano-type variation with increasing the reaction temperature from 190 to
o
o
240 C. Based on above findings, the temperature of 220 C is considered to be the
optimized reaction temperature for DME carbonylation over NZ35 zeolite.
9

240 o
220 o
200 o
C
230 o
C
(a1)
(a2)1000
900
100
90
80
70
60
50
40
30
20
10
0
70
MA CH4
C
C
210 oC
190 oC
CO2 MeOH
60
50
40
30
20
10
800
700
600
MA
500
400
300
200
100
0
190
200
210
220
230
240
0
2
4
6
8
10
Temperature (oC)
Time on stream (h)
Fig. 2. The influences of reaction temperature on DME carbonylation performance
over single NZ35 zeolite; reaction condition: P=1.5 MPa, weight (NZ35)=0.5 g, feed
gas: DME/CO/H2/Ar=2.1/46.4/50/1.5, flow rate= 40 mL/min.
The DME carbonylation performance over NZ35 and CZ35 zeolite is compared
in Fig. 3. The MA selectivity of both catalysts is above 90%, only yielding few
byproducts, like MeOH and CH4, reflecting the specificity of ZSM-35 zeolite for
converting DME to MA. However, the DME conversion and catalyst stability of the
two catalysts show remarkable difference. For the CZ35 zeolite, an induction period
of 5 h is observed after which the conversion of DME reaches a maximum value of
17.6%. Thereafter, the conversion of DME slightly decreases to 8.9% after reaction of
24 h, which results from coke deposition in 10-MR channels. On the NZ35 zeolite,
the catalytic activity initially increases with time and reaches steady state with DME
conversion of 42.5% and then keeps stable with further extended reaction time. It is
evident that the DME carbonylation performance on NZ35 zeolite is much better than
that on CZ35 zeolite. The improved activity and anti-deactivation ability should be
attributed to the short diffusion path and the abundant porosity, which facilitates mass
transfer and has a more efficient utilization of the active sites. In addition, the large
amount of acid sites in NZ35 zeolite, especially for acid sites located within 8-MR
channels, also has the strong ability to activate DME to methoxy species, then
reacting with CO to form acetyl groups, which is the key step in DME carbonylation
reaction. Consequently, the excellent DME carbonylation performance on NZ35
zeolite is attributed to the synergistic effect of the abundant of acidic sites, porosity
and the short diffusion path.
10

50
45
40
35
30
25
20
15
10
5
NZ35
CZ35
0
0
2
4
6
8
10 12 14 16 18 20 22 24
Time on stream (h)
Fig. 3. The comparison of DME carbonylation activity and stability performance on
NZ35 and CZ35 zeolite; reaction conditions: weight (zeolite)=0.5 g, T=220 oC, P=1.5
MPa, feed gas: DME/CO/H2/Ar=2.1/46.4/50/1.5, flow rate= 40 mL/min.
Table 2 DME conversion and product selectivity of DME carbonylation over single
zeolite and EtOH synthesis over combination of zeolite and CZAargon catalyst in a
dual-catalyst bed reactor
Catalysts
Conv. (%)
DME
41.3
Sel. (%)
EtOH MeOH
0
0
45.6
42.3
MA CH4 CO2 EA
NZ35a
CZ35a
NZ35 & CZAargon 47.0
CZ35 & CZAargon 10.4
0.8
0.5
46.3
50.0
96.7 2.1
97.5 1.8
0.4
0.2
4.4
5.2
0
0
1.2
0.5
7.6
b
0
0
2.5
2.0
b
Reaction conditions: weight (NZ35)=0.5 g, weight (CZ35)=0.5 g, weight
(CZAargon)=0.5g; feed gas: DME/CO/H2/Ar=2.1/46.4/50/1.5, flow rate= 40 mL/min;
a: T=220 oC, P=1.5 MPa, Time on stream=24 h
b: T=220 oC, P=2.5 MPa, Time on stream=24 h.
3.3 Effect of proximity of NZ35 zeolite and CZAargon catalyst for EtOH synthesis
In this study, our catalyst is composed of two components, NZ35 zeolite for
DME carbonylation to MA and CZAargon catalyst for hydrogenation of the produced
MA to EtOH. However, the proximity of the two components in EtOH synthesis from
DME and syngas has not been clarified. As compared in Fig. 4, when NZ35 zeolite
and CZAargon catalyst are integrated with the closest proximity, this combination
exhibits 10.0% CO conversion and a minus DME conversion along with 90%
selectivity of MeOH. Moreover, a small amount of CO2 is also generated due to the
water-gas shift reaction (WGSR). This mainly contains two steps, which are, CO
hydrogenation to MeOH over the CZAargon catalyst, and MeOH in situ dehydration to
11

DME over the NZ35 zeolite acidic sites. When NZ35 and CZAargon are combined by
granule mixing, the distance between NZ35 zeolite and CZAargon catalyst is enlarged
to micrometer scale, resulting in a slight increase in DME conversion. Because the
concentration of CO and H2 is much higher than that of DME, MeOH becomes the
main product even if the conversion of CO is lower than that of DME. Under these
conditions, CO hydrogenation to MeOH, even with low conversion, over the CZAargon
catalyst preferentially occurs. Competitive adsorption between DME and MeOH onto
the active sites of NZ35 zeolite will suppress the carbonylation of DME. MeOH
becomes the major product by CO hydrogenation over the mixture catalyst.
Accordingly, the MA formation is severely suppressed, lowering the EtOH production
rate. All of these findings indicate that the reaction over the mixture catalysts is very
difficult for EtOH synthesis from DME and syngas. The CZAargon catalyst acts here
mainly as the MeOH synthesis catalyst rather than the competitive ester
hydrogenation catalyst. When CZAargon catalyst is packed below NZ35 zeolite and
separated by a thin layer of inert quartz sand in a dual-bed configuration, the distance
between zeolite acidic sites and CZAargon active sites gets longer. Resultantly it is
improved dramatically that the conversion of DME is 43.6% and the selectivity of
EtOH is 44.5%, demonstrating that the formed MA is successfully in situ
hydrogenated on the lower CZAargon catalyst bed, obeying the designed reaction
sequence. Accordingly, MA selectivity in final products is less than 2.0% during the
reaction process, very lower than that in the case of DME carbonylation on single
NZ35 zeolite bed. It is demonstrated that the suitable spatial arrangement of NZ35
zeolite and CZAargon catalyst exerts significant influence on DME and CO conversion
and products distribution for converting DME and syngas. Based on these discussions,
the reaction routes occurring at different active sites with the varied integration
manners of NZ35 zeolite and CZAargon catalyst are compared in Scheme 1.
12

(a1)
(b1)
(c1)
Conv.
Sel.
Conv.
DME
Sel.
MeOH
Conv.
Sel.
100
55
100
90
80
70
60
50
40
30
20
10
0
MeOH
50
45
40
35
30
25
20
15
10
5
90
80
70
60
50
40
30
20
10
0
EtOH
MeOH
CO
CH4
CO2
DME
DME CO
EtOH
CO
2CH4
CO2
CO
MA
CH4
-10
0
NZ35/CZA-G
NZ35/CZA-D
NZ35/CZA-M
(a2)
(b2)
(c2)
Fig. 4. Effect of integration manner on catalytic behaviors of the composite catalysts
containing NZ35 zeolite and CZAargon catalyst; (a) NZ35/CZA-G, (b) NZ35/CZA-M,
(c) NZ35/CZA-D; reaction conditions: weight (zeolite)=0.5 g, weight (CZAargon)=0.5
o
g, T=220 C, P=1.5 MPa, feed gas: DME/CO/H2/Ar=2.1/46.4/50/1.5, flow rate= 40
mL/min.
EtOH
(a)
(b)
H2+CO
MeOH
MeOH
DME
MeOH
H2+CO
MA
DME+CO
DME+CO
NZ35/CZA-G
NZ35/CZA-M
(c)
H2
MA
EtOH
DME+CO
MeOH
NZ35/CZA-D
NZ35
CZA
Scheme 1. Reaction routes controlled by the proximity between two NZ35 and
CZAargon catalysts
3.4 Effects of reaction condition on EtOH synthesis from DME and syngas
The influence of reaction pressure on the tandem reaction for EtOH synthesis in
the dual-catalyst bed reactor is displayed in Fig. 5. In this process, increasing the total
pressure, that is increasing the partial pressure of CO and H2, can obviously promote
the total reaction forward, thus realizing improved conversion of DME and MA. As
13

the reaction pressure increases from 0.5 to 4.0 MPa, the DME conversion obviously
improves from 37.6% to 54.8%. The selectivity of MA is 2.0% at 0.5 MPa and 0.0%
at 4.0 MPa, indicating that the CZAargon catalyst, prepared by the simple solid-state
method, exhibits excellent ability for converting MA to EtOH and MeOH. The
selectivity of EtOH rises from 41.5% to 45.5% following increasing the reaction
pressure from 0.5 to 2.5 MPa, demonstrating that the high partial pressure of H2
obviously accelerates MA hydrogenation. However, when the reaction pressure
further increases to 4.0 MPa, the selectivity of EtOH slightly decreases to 42.4% and
the selectivity of ethyl acetate (EA) increases to 2.8%. Here, the EA is formed by
transesterification of MA with EtOH [14]. In addition, the selectivity of CO2 increases
gradually from 2.3% up to 5.8% when the reaction pressure increases from 0.5 to 4.0
MPa, indicating that the high CO partial pressure is favorable to proceeding WGSR in
the presence of Cu-based catalyst. These results indicate that the EtOH synthesis from
syngas and DME can be performed in the optimized reaction conditions of 220 oC and
2.5 MPa with the combination of NZ35 zeolite and CZAargon (CZ35/CZA-D) catalyst.
Fig. S6 presents the results of DME conversion and products distribution under
different reaction temperatures after reaction for 6 h. The DME conversion increases
from 24.8 % up to 45.2 % when the reaction temperature rises from 200 oC to 230 oC.
When further boosting up the reaction temperature to 240 oC, the conversion of DME
decreases to 34.4% after reaction for 6 h because the NZ35 catalyst is easily
deactivated at high reaction temperature. Meanwhile, the by-products (MeOH, CO2
and CH4) present an increase trend because the CO hydrogenation and the WGSR
reaction are promoted over the CZAargon catalyst at higher temperature. The
influence of CO/H2 ratios and GHSV (gas hourly space velocity) on catalytic
performances for EtOH synthesis are presented in Fig. S7 and Fig. S8, respectively.
With the decrease of CO/H2 from 3:1 to 1:1, the selectivity of EtOH increases slightly
from 28.8 to 44.3%. Whereas, when the CO/H2 ratios is 1:3, the selectivity of MeOH
rapidly increases to 68.3%, indicating that the CO hydrogenation to MeOH is
promoted at lower CO/H2. Meanwhile, the conversion of DME is proportionally
decreased with decreasing CO/H2 ratio, indicating that DME carbonylation to MA is
14

limited at lower CO/H2. It coincides with the results of Bhan et al. [11] that the rate of
MA synthesis did not rely on the DME pressure, but was increased linearly with the
CO pressure. As observed, the conversion of DME is decreased with increasing the
GHSV because the reactant has a short contact time with NZ35 zeolite.
60
50
50
40
40
30
MeOH EtOH
30
20
10
0
CO2 CH4
20
10
0
EA MA
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Reaction pressure (MPa)
Fig. 5. The influences of reaction pressure on catalytic behaviors for EtOH synthesis
over NZ35/CZA-D catalyst in a dual-catalyst bed reactor; reaction conditions: weight
(zeolite)=0.5 g, weight (CZAargon)=0.5 g, feed gas: DME/CO/H2/Ar=2.1/46.4/50/1.5,
flow rate= 40 mL/min, T=220 oC.
As a comparative study, a tandem CZ35 zeolite and CZAargon catalyst for EtOH
synthesis is performed in a dual-catalyst bed reactor under the optimized reaction
conditions (Fig. 6). For the combination of CZ35 and CZAargon catalyst, a continuous
deactivation occurs after reaction for 12 h and the conversion of DME slightly
decreases to 10.8% at 24 h. However, over the tandem NZ35 and CZAargon catalyst,
the catalytic performance remains a slight deactivation with the conversion of DME
drops to 40.1% at 80 h (Fig. S9). Therefore, more efficient EtOH strategy with
outstanding catalytic activity and stability is realized using the tandem NZ35 zeolite
and CZAargon catalyst.
15

70
60
50
40
30
20
10
0
50
45
40
35
30
25
20
15
10
5
NZ35/CZA-D
CZ35/CZA-D
NZ35/CZA-D
CZ35/CZA-D
0
2
4
6
8
10 12 14 16 18 20 22 24
Time on stream (h)
Fig. 6. The comparison of DME conversion and EtOH selectivity on NZ35/CZA-D
and CZ35/CZA-D catalyst in a dual-catalyst bed reactor; reaction conditions: weight
(zeolite)=0.5 g, weight (CZAargon)=0.5 g, feed gas: DME/CO/H2/Ar=2.1/46.4/50/1.5,
GHSV=2400 mL g-1 h-1, T=220 oC, P=2.5 MPa.
Conclusions
A more efficient and economical EtOH synthesis route, different with the
traditional method of direct conversion syngas to EtOH using Rh-based catalyst, is
realized from syngas and DME in a dual-catalyst bed reactor containing NZ35 zeolite
and CZAargon catalyst. The NZ35 zeolite, which possesses abundant active sites and
porosity and short diffusion path, exhibits much better DME carbonylation
performance than that of the CZ35 zeolite. The CZAargon catalyst is directly prepared
by a simple formic-acid-assisted solid-state method without further hydrogenation,
which exhibits excellent hydrogenation ability for converting MA to EtOH. By
changing integration manners of the NZ35 zeolite and CZAargon catalyst, the proximity
effect in the two catalysts is clarified. In this reaction system, only when the NZ35
zeolite and CZAargon catalyst separately pack in a dual-catalyst bed, EtOH can be
synthesized from DME and syngas as designed. The optimized reaction conditions are
o
found to be 220 C and 2.5 MPa for EtOH synthesis from syngas and DME with the
combination of NZ35 zeolite and CZAargon catalyst. For direct EtOH synthesis in a
dual-catalyst bed reactor with tandem NZ35 zeolite and CZAargon catalyst, we get a
16

stable DME conversion of 47.0% and EtOH selectivity of 45.6% after reaction for 24
h. In this method, EtOH can be synthesized with a stable and high selectivity using
halide-free and noble-free catalysts. Moreover, the produced EtOH can be easily
separated by distillation from other products and then the main byproduct of MeOH
can be recycled to DME by dehydration over solid aid catalyst. Accordingly, DME
can be completely converted to EtOH in theory, thus realizing an economical and
efficient process.
Credit Author Statement:
Feng Xiaobo: Conceptualization, Writing-Original draft preparation. Yao Jie: Formal
analysis. Zeng Yan: Software. Cui Yu: Data curation. Kazumia Shun: Investigation.
Prasertc Reubroycharoen: Visualization. Liu Guangbo: Resources. Wu Jinhu:
Validation. Yang Guohui: Methodology, Supervision. Tsubakia Noritatsu:
Writing-Reviewing and Editing.
Declaration of interests
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
X. F. thanks the China Scholarship Council (CSC) for financial support. This
study was financially supported by the grant from JST-ACT-C of Japan Sci. & Tech.
Agency (JST).
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Table captions:
Table 1 Textural and physical properties of NZ35 and CZ35 zeolite and CZAargon
catalyst.
Table 2 DME conversion and product selectivity of DME carbonylation over single
zeolite and EtOH synthesis over combination of zeolite and CZAargon catalyst in a
dual-catalyst bed reactor.
Table S1 The amount and distribution of acid sites of NZ35 and CZ35 zeolite.
Feng et al.
20

Figure captions:
Scheme 1. Reaction routes controlled by the proximity between two NZ35 and
CZAargon catalysts.
Fig. 1. The characterization of NZ35 and CZ35 zeolite: (a) XRD patterns; (b)
NH3-TPD profiles; (c) SEM images; (d) N2 adsorption-desorption isotherms; (e) pore
size distribution; (f) NH3-TPD.
Fig. 2. The influences of reaction temperature on DME carbonylation performance
over single NZ35 zeolite; reaction condition: P=1.5 MPa, weight (NZ35)=0.5 g, feed
gas: DME/CO/H2/Ar=2.1/46.4/50/1.5, flow rate= 40 mL/min.
Fig. 3. The comparison of DME carbonylation activity and stability performance on
NZ35 and CZ35 zeolite; reaction conditions: weight (zeolite)=0.5 g, T=220 oC, P=1.5
MPa, feed gas: DME/CO/H2/Ar=2.1/46.4/50/1.5, flow rate= 40 mL/min.
Fig. 4. Effect of integration manner on catalytic behaviors of the composite catalysts
containing NZ35 zeolite and CZAargon catalyst; (a) NZ35/CZA-G, (b) NZ35/CZA-M,
(c) NZ35/CZA-D; reaction conditions: weight (zeolite)=0.5 g, weight (CZAargon)=0.5
o
g, T=220 C, P=1.5 MPa, feed gas: DME/CO/H2/Ar=2.1/46.4/50/1.5, flow rate= 40
mL/min.
Fig. 5. The influences of reaction pressure on catalytic behaviors for EtOH synthesis
over NZ35/CZA-D catalyst in a dual-catalyst bed reactor; reaction conditions: weight
(zeolite)=0.5 g, weight (CZAargon)=0.5 g, feed gas: DME/CO/H2/Ar=2.1/46.4/50/1.5,
flow rate= 40 mL/min, T=220 oC.
Fig. 6. The comparison of DME conversion and EtOH selectivity on NZ35/CZA-D
and CZ35/CZA-D catalyst in a dual-catalyst bed reactor; reaction conditions: weight
(zeolite)=0.5 g, weight (CZAargon)=0.5 g, feed gas: DME/CO/H2/Ar=2.1/46.4/50/1.5,
GHSV=2400 mL g-1 h-1, T=220 oC, P=2.5 MPa.
21