Single-step synthesis of dimethyl ether from syngas on Al2O 3-modified CuO-ZnO-Al2O3/ferrierite catalysts: Effects of Al2O3 content

Article abstract of DOI:10.1016/j.cattod.2013.11.015

Single step synthesis of dimethyl ether (DME) from syngas on the bifunctional catalysts of CuO-ZnO-Al₂O₃ deposited on Al₂O₃-modified H-ferrierite was investigated to elucidate effects of Al₂O₃ modification of H-ferrierite in terms of CO conversion and DME yield. The enhanced catalytic activity was observed on the bifunctional catalysts at an optimum content of Al₂O₃ on H-ferrierite around 2.5-5 wt%. It is attributed to a higher dispersion of Al₂O₃ on H-ferrierite with an enhanced interaction of copper particles with Al₂O₃ modified H-ferrierite with a lower aggregation property by adjusting acidic sites and by increasing the dispersion of copper particles. The main role of Al₂O₃ on H-ferrierite is found to reduce strong acidic sites by depositing on the sites selectively, which resulted in showing a lower formation of byproducts. Both of copper surface area and amount of acidic sites are well correlated with an intrinsic activity (turn-over frequency; TOF), which revealed a structure-insensitive reaction character of a single-step synthesis of DME. The measurements of the dispersion of Al₂O₃ and copper particles with the number of acidic sites concomitantly were employed to verify the effects of Al₂O₃ modification of H-ferrierite though X-ray photoelectron spectroscopy, temperature-programmed methods and surface morphology characterizations.

Full text of DOI:10.1016/j.cattod.2013.11.015

Catalysis Today 228 (2014) 175–182
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Single-step synthesis of dimethyl ether from syngas on
Al2O3-modified CuO–ZnO–Al2O3/ferrierite catalysts: Effects of
Al2O3 content
Yeong Jun Leea, Min Hye Junga, Jong-Bae Leeb, Kwang-Eun Jeongb, Hyun-Seog Rohc,
Young-Woong Suhd, Jong Wook Baea,∗
a School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon, Kyeonggi-do 440-746, Republic of Korea
b Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107, Yuseong, Daejeon 305-600, Republic of Korea
c Department of Environmental Engineering, Yonsei University, 1 Yonseidae-gil, Wonju, Gangwon 220-710, Republic of Korea
d Department of Chemical Engineering, Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 June 2013
Received in revised form 3 November 2013
Accepted 11 November 2013
Available online 17 December 2013
Single step synthesis of dimethyl ether (DME) from syngas on the bifunctional catalysts of
CuO–ZnO–Al2O3 deposited on Al2O3-modified H-ferrierite was investigated to elucidate effects of Al2O3
modification of H-ferrierite in terms of CO conversion and DME yield. The enhanced catalytic activity was
observed on the bifunctional catalysts at an optimum content of Al2O3 on H-ferrierite around 2.5–5 wt%.
It is attributed to a higher dispersion of Al2O3 on H-ferrierite with an enhanced interaction of copper
particles with Al2O3 modified H-ferrierite with a lower aggregation property by adjusting acidic sites
and by increasing the dispersion of copper particles. The main role of Al2O3 on H-ferrierite is found to
reduce strong acidic sites by depositing on the sites selectively, which resulted in showing a lower for-
mation of byproducts. Both of copper surface area and amount of acidic sites are well correlated with an
intrinsic activity (turn-over frequency; TOF), which revealed a structure-insensitive reaction character of
a single-step synthesis of DME. The measurements of the dispersion of Al2O3 and copper particles with
the number of acidic sites concomitantly were employed to verify the effects of Al2O3 modification of
H-ferrierite though X-ray photoelectron spectroscopy, temperature-programmed methods and surface
morphology characterizations.
Keywords:
Single-step synthesis of DME
Bifunctional catalyst
CuO–ZnO–Al2O3
Al2O3-modified H-ferrierite zeolite
Syngas
Intrinsic activity
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
The general reactions for synthesizing DME involve two steps;
the first step is for COx hydrogenation to methanol on CuO–ZnO
Dimethyl ether (DME) synthesis through a single-step directly
using syngas, which can be derived from a reforming reaction
of hydrocarbons or gasification of biomass, has been largely
investigated as an alternative renewable energy production. A
biomass-derived CO2-abundant syngas having a low H2/CO ratio
below one has been getting more attractive for the single-step syn-
thesis of DME [1,2], since a bifunctional catalyst for a single-step
synthesis of DME from biomass-derived syngas can be efficiently
utilized even at low H2/CO feed ratio due to its high activity for
water–gas shift reaction. The utilization of DME as a renewable
energy source has been also reported by replacing it with a liquefied
petroleum gas, which has a high potential to minimize the emis-
sion of CO2 and NOx pollutants into the atmosphere by decreasing
a global warming problem [3].
based catalyst, and the subsequent second step is for dehydra-
tion of methanol to DME on solid-acid catalysts such as alumina
or modified-zeolites [4–8]. The higher equilibrium conversion
through a single-step reaction of DME synthesis from syngas on
bifunctional catalyst can be obtained thermodynamically, since
the second step of DME synthesis from methanol is much faster
with a higher equilibrium conversion compared with the first
step of methanol synthesis, which is characterized as a lower
equilibrium conversion especially at a lower H2/CO feed ratio. To
overcome the thermodynamic conversion limitation, an applica-
tion of bifunctional catalyst composed of Cu–ZnO and solid–acid
component as well is beneficial for obtaining a higher conversion
of reactants by producing surplus hydrogen through simultaneous
water–gas shift (WGS) reaction during a single-step reaction as
shown in following equations;
CO hydrogenation to methanol :
∗
Corresponding author. Tel.: +82 31 290 7347; fax: +82 31 290 7272.
E-mail address: finejw@skku.edu (J.W. Bae).
CO + 2H2 ↔ CH3OH ꢀHo = −90.6 kJ/mol
(1)
0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.11.015

176
Y.J. Lee et al. / Catalysis Today 228 (2014) 175–182
kept at 110 ◦C, followed by calcination at 500 ◦C for 3 h under air
environment.
Dehydration to DME :
2CH3OH ↔ CH3OCH3 + H2O ꢀHo = −23.4 kJ/mol
(2)
Subsequently, the bifunctional catalysts were prepared by
co-precipitation method in a slurry solution of Al2O3-modified H-
ferrierite kept at 70 ◦C using the metal precursors of copper nitrate,
zinc acetate, and alumina nitrate with (NH4)2CO3 precipitant with
an aging time of 2 h at 70 ◦C. The sample was dried for 24 h in an
oven at kept at 110 ◦C. Finally, the bifunctional catalysts were cal-
cined at 300 ◦C for 3 h. The molar ratio of CuO, ZnO, and Al2O3 was
fixed at 3.0:1.0:0.5, and the weight of CuO on Al2O3-modified H-
ferrierite was kept at a value of 1.75, to prepare CuO–ZnO–Al2O3
methanol synthesis catalyst combined with solid-acid component
of Al2O3-modified H-ferrierite. The as-prepared bifunctional cat-
alysts are denoted as CZA/Al(x)-FER, where Al(x)-FER, C, Z, and A
indicate Al2O3-modified H-ferrierite, CuO, ZnO, and Al2O3 respec-
tively, and x digit represents the weight ratio of Al2O3 to H-ferrierite
such as 0, 2.5, 5, 10 wt%.
The catalytic activity was tested in a fixed bed tubular reactor
with an outer diameter of 12.7 mm using catalyst of 0.4 g. Prior to
reaction, the bifunctional CZA/Al-FER catalysts were reduced in a
flow of 5 vol% H2 balanced with N2 at 300 ◦C for 5 h to obtain an
active metallic copper particles. Subsequently, syngas was fed to
the reactor, which has a molar ratio of H2/CO of 2.0 with an internal
standard gas of N2. The reaction was carried out for around 20 h on
stream at the following reaction conditions; T = 250 ◦C, P = 3.5 MPa
and space velocity (SV) = 2000 ml/gcat/h. The CO conversion and
products distribution on the bifunctional catalysts were obtained
from the steady-state average values for 5 h after 15 h on stream.
The products were analyzed using an on-line gas chromatograph
(Younglin GC, YL6100) using a thermal conductivity detector (TCD)
to analyze N2, H2, CO and CO2 and a flame-ionized detector (FID)
to analyze methanol, DME, and other byproducts.
Water–gas shift reaction :
CO + H2O ↔ CO2 + H2 ꢀHo = −41.2 kJ/mol
(3)
In general, solid acid catalysts such as alumina or zeolites reveal
two sorts of acidic sites, which is known to be active sites for
methanol dehydration to DME by shifts to higher conversions of
COx and H2. However, the weak acidic sites act preferentially for
methanol dehydration to DME, and the strong acidic sites are
known to produce the byproducts such as light hydrocarbons by
the possible decomposition of DME [9–12]. The strong acidic sites
also interfere to form DME through the possible methanol to gaso-
line (MTG) process on strong acidic sites of zeolites. Therefore, it
is necessary to remove strong acidic sites for the sake of obtaining
a higher DME selectivity by adopting some modifiers on zeolites.
From our previous works [10,11], Zr modifier on H-ferrierite zeo-
lite was reported as one of the proper species for enhancing the
DME selectivity by suppressing strong acid sites. In general, acidic
sites of zeolites can be adjusted by employing alkali metals or rare
earth metals such as K, Mg, Ca, and La, Ce, Fe and so on and by
varying Si/Al ratios of zeolites [9,13–16,8]. In general, the catalytic
activity for DME synthesis by methanol dehydration is much higher
on zeolite-type catalysts compared with gamma-Al2O3, due to the
presence of a larger number of acidic sites on zeolites [13,14,17–20].
Therefore, Al2O3 component, which is also known to be active cat-
alyst for dehydration of methanol on the weak acidic sites, can be
also adopted to eliminate strong acidic sites on zeolites, by simul-
taneously controlling strong acidic sites and enhancing the number
of acidic sites on Al2O3-modified H-ferrierite zeolite. However, the
researches for investigating the effects of Al2O3 modifier on zeolites
for methanol dehydration are not much carried out based on our
knowledge, especially for Al2O3-modified H-ferrierite as solid–acid
catalyst for single step synthesis of DME from syngas.
2.2. Characterization of the bifunctional catalysts
BET surface area of bifunctional CZA/Al-FER catalysts was
measured by nitrogen adsorption method at −196 ◦C by using
a constant-volume adsorption apparatus (Micromeritics, ASAP-
2020). The pore volumes were determined at a relative pressure
(P/Po) of 0.99 and the pore size distribution of bifunctional cata-
lysts was determined by BJH (Barett–Joyner–Halenda) model from
the data of desorption branch of nitrogen isotherm.
The crystalline phases and particle sizes of copper species were
characterized using powder X-ray diffraction (XRD) analysis using
a Rigaku diffractometer with CuK␣ radiation in order to identify
the metallic Cu, CuO, ZnO, ␥-Al2O3, and ferrierite before and after
reaction. The average particle size of copper species was calculated
suing the values of full width at half maximum (FWHM) of the most
intensive XRD diffraction peaks at 2ꢁ = 35.8◦ for CuO on the fresh
bifunctional catalyst, and 2ꢁ = 43.3◦ for metallic copper (Cu0) on the
used catalysts.
In the present investigation, we studied the effects of the Al2O3
modification of acid sites on H-ferrierite to compare with that of
Zr-modified ferrierite in our previous works [10,11], which sig-
nificantly altered the catalytic activity of methanol dehydration to
DME. The different catalytic performances by varying a weight ratio
of Al2O3 to H-ferrierite on bifunctional catalysts was explained by
mainly characterizing the copper surface area and the number of
acidic sites on CuO–ZnO–Al2O3 incorporated with Al2O3-modified
H-ferrierite for a single-step synthesis of DME.
2. Experimental
2.1. Catalyst preparation and activity test
The surface area of metallic copper on the bifunctional cata-
lysts was further measured by N2O titration method. Prior to N2O
titration, the sample was reduced at 300 ◦C for 5 h under a flow
of 5 vol% H2/N2, and the consumption of N2O with a concomi-
tant release of N2 on metallic copper sites through the reaction,
N2O + 2Cu = Cu2O + N2, was analyzed by TCD equipped on BELCAT
instrument. The surface area of metallic copper on the bifunctional
catalysts before and after reaction for 20 h was calculated by assum-
ing the concentration of 1.46 × 1019 Cu atoms/m2 with a molar ratio
of 0.5 for N2O/Cus (Cu atom on surface) [17]. The elemental analysis
of Ferrierite was analyzed using X-ray fluorescence (XRF; SEA5120).
To elucidate the reducibility of copper oxides on the bifunctional
catalyst, temperature-programmed reduction (TPR) experiment
was carried out. The sample was pretreated by flowing He up
to 200 ◦C for 1 h to remove the adsorbed water, and followed by
The bifunctional catalysts were prepared by co-precipitation
method using CuO–ZnO–Al2O3 component for CO hydrogenation
on a solid-acid catalyst of Al2O3-modified H-ferrierite zeolite. The
Al2O3-modified H-ferrierite was previously prepared by precipi-
tation method using aluminum nitrate precursor (Al(NO3)26H2O)
with (NH4)2CO3 precipitant at a specific weight ratio of Al2O3
to H-ferrierite in an aqueous solution at 70 ◦C, which possesses
a SiO2/Al2O3 molar ratio of around 20 supplied by Zeolyst with
the surface area of around 366 m2/g, and it was further confirmed
by X-ray fluorescence analysis (supplementary Table S1). The spe-
cific weight ratio of Al2O3 to H-ferrierite varies from 0 to 10 wt%,
and the solution was kept for aging for 2 h at the same tem-
perature. After aging, the sample was dried for 24 h in an oven

Y.J. Lee et al. / Catalysis Today 228 (2014) 175–182
177
cooling to 50 ◦C. A 5 vol% H2/He mixture was introduced in BEL-
CAT instrument at a flow rate of 30 ml/min with a heating rate of
10 ◦C/min up to 600 ◦C. The effluent gas from reactor was passed
through a molecular sieve to remove water formed during TPR
experiment, and the amount of H2 consumption was analyzed by
TCD.
Temperature programmed desorption of ammonia (NH3-TPD)
was carried out using the H-ferrierite and fresh bifunctional cata-
lyst to determine the number of acidic sites and its acidic strength.
Around 0.1 g of sample was initially flushed with a He flow at 250 ◦C
for 2 h, and subsequently cooled down to 100 ◦C and then the sam-
ple was saturated with NH3. After NH3 exposure, the sample was
purged under a He flow until it reached an equilibrium point, then
it was allowed TPD experiment using the BELCAT instrument in the
temperature range of 100–600 ◦C with a heating rate of 10 ◦C/min
and kept at that temperature for 10 min.
The Fourier-transformed infrared-spectroscopy (FT-IR) was
adopted to characterize the structures of metal oxides on the fresh
and used catalysts. The bifunctional CZA/Al-FER catalysts are com-
posed of several kinds of bonds of metal oxides such as CuO, ZnO
and Al2O3 and so on. By observing the different intensities of metal
oxide bonds, the dispersion of CuO between copper and/or alu-
minum and oxygen bonds assigned the characteristic peaks of 697
and 1730 cm−1 was compared on the bifunctional catalysts [21,22].
The electronic states of surface copper species were character-
ized by using X-ray photoelectron spectroscopy (XPS; (ESCALAB
MK-II)) analysis. During the experiment, Al-K␣ monochromatized
line (1486.6 eV) was adopted and the vacuum level was kept around
10−7 Pa. The fresh CZA/Al-FER was previously pressed to a thin pel-
let and the binding energy (BE) was corrected with the reference
BE of C1s (284.6 eV).
Fig. 1. Pore size distribution of the fresh CZA/Al-FFER catalysts.
catalysts is found to be in the range of 0.27–0.34 cm3/g, and the
trend is in line with the surface area variation. The average pore
diameter was found to be above 6.7 nm, and the pore size was
increased by increasing Al2O3 content, and it showed a maximum
value of 10.0 nm on CZA/Al(5)-FER catalyst. The pore size distribu-
tion on the CZA/Al-FER catalysts was also shown in Fig. 1, and the
bimodal pore size distribution with the distinctive peaks at around
4 and 10 nm in diameter was observed on all CZA/Al-FER catalysts.
The first narrow small pore can be possibly attributed to the intra-
crystalline pores of CuO–ZnO–Al2O3 particles and Al2O3-modified
H-ferrierite powders itself, and the second broad large pores could
be ascribed to the inter-crystalline pores between CuO–ZnO–Al2O3
particles and Al2O3-modified H-ferrierite, which was previously
observed on CuO–ZnO–Al2O3–based catalysts [11,23]. With the
increase of Al2O3 content on H-ferrierite, the large pores at around
10 nm shifted to larger pore diameters by increasing the intensities,
except for CZA/Al(10)-FER catalyst. This observation suggests the
abundant formation of inter-crystalline pores of CuO–ZnO–Al2O3
particles on Al2O3-modified H-ferrrieite by changing the surface
properties of H-ferrierite, and it was contributed to the increase of
average pore size on CZA/Al(5)-FER catalyst as shown in Table 1.
The co-presence of bimodal-size pores is reported to enhance cat-
alytic performances by increasing the transport rates of reactants
and products to outer surfaces of inter-crystalline pores [11,19].
The XRD patterns of CZA/Al-FER catalysts before reaction were
displayed in Fig. 2. The characteristic multiple diffraction peaks of
The particle sizes of copper oxides and the variation of surface
morphologies after reaction for 20 h were further characterized
by using the transmission electron microscopy (TEM; TECNAI G2
instrument) to support the results of XRD analyses and N2O titra-
tion.
3. Results and discussion
The physicochemical properties of copper surface area and the
number of acidic sites can largely alter the catalytic performances
by changing the reaction rates of CO hydrogenation and dehy-
dration of methanol to DME simultaneously. The roles of acidic
sites on solid–acid zeolite are more dominant factors for varying
the total reaction rates on the bifunctional CZA/Zr-FER catalysts as
reported in our previous works [10,11]. Therefore, Al2O3-modified
H-ferrierite can also act as a proper dehydration catalyst by sup-
pressing the strong acidic sites, and it can also affect the dispersion
of CuO–ZnO–Al2O3 particles by changing the surface properties of
H-ferrierite. The correlation of copper surface area and the number
of acidic sites with catalyst performances was further investigated
on the CZA/Al-FER catalysts based on our previous works [10,11].
3.1. Physicochemical properties of the CZA/Al-FER catalysts
The surface area, pore volume, and average pore diameter of
CZA/Al-FER catalysts were significantly altered by the amount of
Al2O3 modification of H-ferrierite, and the results are summarized
in Table 1 and Fig. 1. For comparison, the surface area and pore size
distribution of H-Ferrierite are summarized in Supplementary Fig.
S1, and its surface area was found to be 366.2 m2/g. The surface area
is found to be in the range of 120.7–175.1 m2/g, and it is maximized
on CZA/Al(2.5)-FER catalyst with the value of 175.1 m2/g. However,
the surface area gradually decreased to 120.7 m2/g by increasing
Al2O3 content on H-ferrierite. The pore volume of CZA/Al-FER
Fig. 2. XRD patterns of the CZA/Al-FER catalysts before reaction.

178
Y.J. Lee et al. / Catalysis Today 228 (2014) 175–182
Table 1
Physicochemical properties of the bifunctional CZA/Al-FER catalysts.
Notationa
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore diameter (nm)
CZA/Al(0)-FER
CZA/Al(2.5)-FER
CZA/Al(5)-FER
CZA/Al(10)-FER
160.0
175.1
155.4
120.7
0.27
0.34
0.30
0.29
6.7
7.8
10.0
7.3
a
The bifunctional CZA/Al-FER catalysts are denoted as CZA/Al(x)-FER, where Al(x)-FER, C, Z, and A indicates Al2O3-modified H-ferrierite, CuO, ZnO, and Al2O3, respectively.
The x digit represents the weight ratio of Al2O3 to H-ferrierite zeolite.
Table 2
Acidity, particle sizes and metallic copper surface areas on the bifunctional CZA/Al-FER catalysts.
Notation
Acidic sites
(mmolNH3/g) from
NH3-TPD
Particle size of CuO and
Cu0 (nm) from XRD
analysis
Cu surface area
(m2/gCu) from N2O
titration
I2/I1 raio from XPS
[Cu surface area
(after)] × [acidic
site (total)]
analysisa
T1 (<400 ◦C)
T2 (<600 ◦C)
Total
CuO (before)
Cu0 (after)
Fresh
Used
CZA/Al(0)-FER
CZA/Al(2.5)-FER
CZA/Al(5)-FER
CZA/Al(10)-FER
10.6
13.1
12.2
11.7
3.2
3.6
4.1
2.7
13.8
16.7
16.3
14.4
7.8
4.9
5.2
7.2
16.8
7.2
10.3
24.1
3.8
10.1
8.1
5.40
6.95
7.02
4.27
1.19
1.58
1.80
1.36
57.0
90.7
85.6
50.1
3.1
a
The ratio (I2/I1) is defined as the intensity of Cu+ at 930 eV (assigned to I1) to shoulder peak at around 932 eV (assigned to I2).
H-ferrierite were observed below 2ꢁ = 30◦, and the larger diffrac-
tion peak intensity of CuO particles were observed at 2ꢁ = 35.8
and 38.0◦. All CZA/Al-FER catalysts showed the characteristic H-
ferrierite structures even after coprecipitation of CuO–ZnO–Al2O3
components on AlO3-modified H-ferrierite without a significant
variation of diffraction peak intensities. The particle sizes of CuO on
the fresh catalysts were summarized in Table 2. It was calculated
from the values of FWHM using Scherrer equation at 2ꢁ = 35.6◦ for
CuO species. The particle size of CuO was found to be in the range
of 4.9–7.8 nm, and it was decreased by modifying H-ferrierite with
Al2O3 compared with unmodified CZA/Al(0)-FER catalyst. The aver-
age particle size of CuO was minimized on CZA/Al(2.5)-FER catalyst
with a value of 4.86 nm, and it can be attributed to the variation of
surface acidity induced from Al2O3 modification of H-ferrierite by
changing CuO-support interaction. The larger amount of Al2O3 dis-
tribution on H-ferrierite surfaces can be the reason for the enhanced
dispersion of CuO–ZnO–Al2O3 with a formation of smaller CuO par-
ticles, especially on CZA/Al(2.5)-FER, which was explained by the
following results of acidity measurement and N2O titration.
characteristic peaks as shown in Supplementary Fig. S2. Further-
more, the larger peak intensity at 1090 cm−1 can also support the
increase of Cu–O–Si population on the H-ferrierite surface due to
the phase segregation of Al–O–Si linkages [26]. The high disper-
sion of CuO species interacting with H-ferrierite was obtained on
3.2. Surface properties of CZA/Al-FER catalysts (FT-IR, TPR, XPS,
and NH3-TPD)
To verify the types of metal oxides and the degree of forma-
tion on CZA/Al-FER catalysts, FT-IR experiments were carried out
using the fresh and used bifunctional catalysts and they are shown
in Fig. 3. The characteristic peaks of CuO bonds were confirmed by
FT-IR at the wavenumbers of around 697 and 1730 cm−1, and other
peaks assigned to the wavenumbers of 1090, 1207 and 3300 cm−1
can be assigned to SiO, AlO and OH stretching, respectively [24,25].
The highly-dispersed copper oxides on CZA/Al(5)-FER catalyst were
supported by observing the intense vibration band at 697 and
1730 cm−1 which can be assigned to characteristic CuO absorp-
tion band on CZA/Al(5)-FER catalyst. The FT-IR peaks observed on
CZA and Al(5)-FER samples also support the proper assignment
of characteristic peaks of metal oxides as shown in Fig. 3(A). In
addition, the decreased intensity of shoulder peak at 1207 cm−1
and increased intensity at 1090 cm−1 on the CZA/Al(5)-FER can
also be attributed to the deteriorated silica framework, which is
possibly attributed to strong interaction of CuO due to the well
dispersion of CuO on Al2O3-modified H-ferrierite [11]. The opti-
mum calcination temperature of 300 ◦C for CZA/Al-FER catalyst
was also confirmed by observing the higher intensity of these
Fig. 3. Spectra of Fourier-transformed infrared spectroscopy of CZA/Al-FER catalysts
before (A) and after reaction (B).

Y.J. Lee et al. / Catalysis Today 228 (2014) 175–182
179
CZA/Al(5)-FER catalyst calcined at 300 ◦C. As shown in Fig. 3(A) for
CZA/Al-FER before reaction, which were calcined at 300 ◦C with a
different amount of Al2O3 modification, the characteristic peaks of
CuO at the wavenumbers of 697 and 1730 cm−1 were maximized
on CZA/Al(2.5)-FER and CZA/Al(5)-FER catalyst, which suggests a
higher dispersion of CuO on Al2O3-modfied H-ferrierite. In addi-
tion, the high peak intensities at 1090 and 1207 cm−1 revealed the
strong interactions of CuO with Al2O3-modified H-ferrierite. These
characteristic peaks were also verified by characterizing the Al2O3-
modified H-ferrierite and Cu–ZnO–Al2O3 and by observing higher
intensity at 1730 cm−1 for Cu–ZnO–Al2O3 components and higher
intensity in the ranges of 1250–1700 cm−1 on Al2O3-modifed H-
ferrierite as shown in Fig. 3(A). In general, the higher dispersion of
Cu-ZnO-Al2O3 species on H-ferrierite is responsible for the higher
CO conversion and DME selectivity as reported from our previous
researches [10,11,19,20,23,27]. Interestingly, these characteristic
peaks with high intensities as shown in Fig. 3(B) retained even
after reaction of 20 h, and the superior characters on CZA/Al(2.5)-
FER and CZA/Al(5)-FER catalyst can be explained by comparing
with the variation of copper surface area according to the types
of bifunctional catalysts. From the results of FT-IR analysis, the
optimum concentration of Al2O3 modifier on H-ferrierite by form-
ing a proper interaction with CuO is important to obtain a higher
dispersion of CuO particles on Al2O3-modified H-ferrierite. Even
though FT-IR peaks are not significantly altered by modifying with
a different amount of Al2O3 on H-ferrierite, the small variation of
characteristic peak intensities of CuO before and after reaction on
CZA/Al(2.5)-FER and CZA/Al(5)-FER catalyst suggest the high sta-
bility of those catalysts by suppressing the significant aggregation
of Cu–ZnO–Al2O3 particles. The suppressed aggregation of CuO
on the properly Al2O3-modified H-ferrierite is also beneficial for
decreasing the amount of strong acidic sites and reducibility of CuO
species simultaneously. The strong interactions of CuO with Al2O3-
modified H-ferrierite were confirmed by characterizing CZA/Al-FER
catalysts using XPS analysis as shown in Supplementary Fig. S3. The
BE of Cu2p3/2 photoelectron spectra of the fresh CZA/Al-FER cata-
lysts can be assigned to 930 eV for Cu+ and 932 eV for Cu2+ with the
characteristic satellite peaks around 940 eV. As shown in Table 2,
the intensity ratio of Cu+ at 930 eV (assigned to I1) to shoulder peak
of Cu2+ at around 932 eV (assigned to I2) is found to be in the range
of 1.19–1.80, and the ratio is maximized on CZA/Al(5)-FER catalyst
with the value of 1.80. The more detailed results are summarized in
Supplementary Table S2. The increased shoulder peak assigned to
Cu2+ species on CZA/Al(2.5)-FER and CZA/Al(5)-FER catalysts sug-
gest the strong interaction of CuO with Al2O3-modified H-ferrierite,
and it also reveals a small variation of copper particle size with a
well-dispersed two-dimensional Cu0–Cu+ layer even after reaction
[28].
Fig. 4. TPR profiles of the fresh CZA/Al-FER catalysts.
in Fig. 4. The reduction peak around 210 ◦C can be attributed to
CuO reduction on Cu–ZnO–Al2O3 component, and the shoulder
peaks at a higher temperature of above 250 ◦C can be assigned
to the strongly interacted CuO species with Al2O3-modified H-
ferrierite. The observed high intensity at a higher temperature at
293 ◦C on CZA/Al(5)-FER catalyst suggests the heterogeneously dis-
tributed CuO particles with a strong metal-support interaction on
Al2O3-modified H-ferrierite as confirmed by XPS and FT-IR analy-
ses. In general, the formation of homogeneous microstructures of
CuO–ZnO at similar chemical composition is important to obtain
a higher CO conversion due to the easy formation of a smaller
copper particles with a higher surface area [30], which can be
confirmed by observing Gaussian pattern of reduction peak as
assigned in low temperature peaks around 210 ◦C on all CZA/Al-
FER catalysts. However, in terms of catalyst stability, the presence
of shoulder peak at a higher temperature can be assigned to the
strongly interacted CuO with Al2O3-modified H-ferrierite, which
is responsible for the lower aggregation of copper particles during
a single-step DME synthesis reaction. Interestingly, the shoulder
peak at a higher temperature was observed on CZA/Al(5)-FER cat-
alyst with the highest intensity, and the peak was observed on all
CZA/Al-FER catalysts except for CZA/Al(0)-FER catalyst. The peak
split of reduction peaks was observed on CZA/Al(10)-FER cata-
lyst, and it also suggests the heterogeneous distribution of CuO on
Al2O3-modified H-ferrierite due to the uneven distribution of Al2O3
on H-ferrierite surfaces at a higher Al2O3 concentration. TPR pro-
files on CZA/Al(2.5)-FER and CZA/Al(5)-FER catalyst suggest that
CuO particles are well distributed on Al2O3-modified H-ferrierite
surfaces by forming CuO–ZnO matrix. The homogeneous distribu-
tion by forming small particles of CuO and strong interaction with
Al2O3-modified H-ferrierite on CZA/Al(5)-FER catalyst can largely
reduce the aggregation of copper particles during the reaction. The
degree of reduction of CuO–ZnO–Al2O3 can be also strongly affected
by the dispersion of Al2O3 on H-ferrierite, which is further verified
by NH3-TPD and the copper surface area measurement through
N2O titration method.
The reducibility of CZA/Al-FER catalysts was characterized by
TPR experiment, and the maximum reduction temperatures were
observed below 300 ◦C as shown in Fig. 4. The reduction behav-
iors of CuO on the CZA/Al-FER catalysts largely altered the catalytic
activity and stability by changing the surface area of metallic cop-
per and by influencing the extent of aggregation of active metals.
In general, the fast reaction rate of this consecutive reaction was
attributed to the higher formation rate of methanol from syngas
on metallic copper surfaces due to the lowest activation barrier
for the adsorption pathway of two contiguous methanol molecules
to generate DME molecule [10,11,29]. Since the methanol forma-
tion from syngas, which is a first step for DME production via
dehydration of methanol, occurs on the metallic copper surfaces,
the higher surface area of copper on Al2O3-modified H-ferrierite
is an important factor for a fast consecutive reaction to DME
from syngas [10]. The reducibility of copper oxides on CZA/Al-
FER was verified by TPR experiments, and high reduction degree
on all bifunctional catalysts below 300 ◦C was observed as shown
The different reduction patterns of CZA/Al-FER catalysts can
be also affected by the amount of surface acidic sites, therefore,
their characteristics were analyzed by NH3-TPD experiments as
shown in Fig. 5 and the results were summarized in Table 2.
The different distribution of Al2O3 modifier on H-ferrierite even-
tually alters the acidic properties of the CZA/Al-FER catalysts by
changing the degree of surface blockage of acid sites through the
deposition of CuO–ZnO–Al2O3 components [31]. The characteristic
NH3-TPD peaks and their amount of acidic sites of H-Ferrierite itself
are summarized in Supplementary Fig. S4. The weak and strong

180
Y.J. Lee et al. / Catalysis Today 228 (2014) 175–182
small amount of acidic sites on Al2O3-modified H-ferrierite with
uneven distribution of Cu–ZnO–Al2O3 particles since there is not
sufficient sites to interact with the precipitates of Cu–ZnO–Al2O3
particles [19,20,23,27]. Interestingly, the amount of weak and total
acidic sites on CZA/Al-FER catalysts showed a volcano curve, and
the this observation can explain that the effects of AlO3 modifica-
tion of H-ferrierite is related with the enhancing the weak acidic
sites by adding Al2O3 component and small copper particle forma-
tion, which is beneficial for obtaining higher catalytic performance
of single-step DME synthesis.
3.3. Catalytic activity correlated with acidic sites and surface
area of metallic copper
The CZA/Al-FER catalysts were further characterized using the
used catalysts after 20 h to elucidate the effects of AlO3 modi-
fication of H-ferrierite after the stabilization of catalytic activity
using XRD, FT-IR, N2O titration, and TEM analyses. The particle
size of copper was significantly altered after reaction, especially on
CZA/Al(0)-FER and CZA/Al(10)-FER catalyst, which showed a lower
dispersion (large particle size of CuO) of Cu–ZnO–Al2O3 particles
on H-ferrierite surfaces. From XRD analysis on the used CZA/Al-FER
catalysts as shown in Table 2 and Supplementary Fig. S5, the char-
acteristic metallic copper peak at 2ꢁ = 43.3◦ were observed on all
CZA/Al-FER catalysts without a significant variation of XRD inten-
sities of characteristic peaks of H-ferrierite, which suggests that no
significant changes of the Al2O3-modified H-ferrierite structures.
The peak intensity of metallic copper was found to be larger on
CZA/Al(10)-FER catalyst, and smallest on CZA/Al(2.5)-FER catalyst.
The calculated particle size of metallic copper after reaction, sum-
marized in Table 2, is found to be in the range of 7.2–24.1 nm.
The smallest particle size of metallic copper of 7.2 nm without a
mild aggregation of copper particles was observed on CZA/Al(2.5)-
FER catalyst, which showed a higher dispersion of Cu–ZnO–Al2O3
species on H-ferrierite and largest amount of acidic sites from NH3-
TPD. The significant aggregation of copper particles with a size of
24.1 nm was observed on CZA/Al(10)-FER catalyst which showed a
lower amount of acidic sites. This was also supported by the results
of FT-IR analyses on used catalysts as shown in Fig. 3(B). The larger
characteristic peaks of Cu O stretching bond at the wavenum-
bers of 710 and 1730 cm−1 were observed on CZA/Al(2.5)-FER
catalyst, which supports the high dispersion of CuO on Al2O3-
modfied H-ferrierite even after reaction. The larger peak intensities
at 1207 and 1090 cm−1 were also observed on CZA/Al(2.5)-FER
and CZA/Al(5)-FER which possessed a stronger interaction of CuO
with Al2O3-modified H-ferrierite (results of FT-IR and XPS analy-
sis). To further support the particle size variation after reaction for
20 h, copper surface area measurements were carried on the used
CZA/Al-FER catalysts as summarized in Table 2. The surface areas
were significantly decreased, and the values were found to be in
the range of 4.3–6.95 m2/g. The smallest variation of surface area of
metallic copper were found on CZA/Al(2.5)-FER and CZA/Al(5)-FER
catalyst with the values of 6.95 and 7.02 m2/g, which were charac-
terized as a smaller particle formation of CuO and larger amount of
acidic sites. Therefore, the amount of acidic sites after Al2O3 mod-
ification of H-ferrierite can alters the particle size of CuO as well
as catalytic stability by suppressing aggregation of copper species
during the reaction.
Fig. 5. NH3-TPD profiles of the fresh CZA/Al-FFER catalysts.
acidic sites are found to be 37.4 and 15.9 mmolNH3/g, respec-
tively, and the amounts of acidic sites were significantly reduced
after CZA/Al-FER catalyst preparation, especially for the strong
acidic sites. Therefore, the extent of surface modification of acidic
sites can be estimated by comparing the particle size of CuO and
the amount of acidic sites on the CZA/Al-FER catalysts simulta-
neously. As shown in Fig. 5, two distinguishable desorption peaks
can be divided with the peaks appearing below 400 ◦C (T1) and
400–600 ◦C (T2), assigned to weak and strong acidic sites, respec-
tively [19,20,27]. The desorption peak appearing below 400 ◦C is
attributed to the weakly absorb NH3, and the peak above 400 ◦C
is for the strongly adsorbed NH3 molecules or desorption of H2O
from main frameworks of CuO–ZnO–Al2O3 or Al2O3-modifed H-
ferrierite. The weak acidic sites (peak I) plays an important role for
methanol dehydration to DME, and the formation of byproducts
such as hydrocarbons and CO2 can be enhanced with the increase
of strong acidic sites (peak II) by the possible reforming reaction of
DME and methanol [3,9–11]. The activity of single step DME synthe-
sis from syngas significantly depends on the amount of acidic sites
on Al2O3-modified H-ferrierite and that of metallic copper sites.
The summarized quantities of acidic sites on CZA/Al-FER catalysts
are shown in Table 2. By varying the amount of Al2O3 modifier
on H-ferrierite, the weak acidic sites (peak I) were maximized on
CZA/Al(2.5)-FER catalyst with the value of 12.2 mmolNH3/g, and
its values was found to be smallest on the unmodified CZA/Al(0)-
FER catalyst with the value of 10.6 mmolNH3/g. The different acidic
sites can be correlated with the dispersion of Al2O3 species on H-
ferrierite, and the highest dispersion of Al2O3 was observed on
CZA/Al-FER catalysts with Al2O3 concentration of 2.5–5.0 wt%. The
variation of strong acidic sites (peak II) and total acidic sites showed
the almost same trend like weak acidic sites, with the values of
13.8–16.7 mmolNH3/g. The amount of Al2O3 on H-ferrierite can
be essential for enhancing catalytic performances by maximizing
the weak acidic sites, and it also alters Cu–ZnO–Al2O3 dispersion
by forming a small particles of copper of 4.9 and 5.2 nm in size
on the fresh CZA/Al(2.5)-FER and CZA/Al(5)-FER catalyst as con-
firmed by XRD analysis (Fig. 2 and Table 2). The formation of small
copper particles on the fresh catalysts was also verified by mea-
suring the surface area of metallic copper through N2O titration
as shown in Table 2. The surface area of copper was found to be
in the range of 3.1–10.1 m2/g, and its area was maximized on the
fresh CZA/Al(2.5)-FER catalyst with the value of 10.1 m2/g(copper)
which is in line with the results of XRD analysis. The lower value of
copper surface area on CZA/Al(10)-FER catalyst is attributed to the
In addition, the morphologies and size distribution of copper
particles on CZA/Al(0)-FER and CZA/Al(5)-FER catalyst before and
after reaction were characterized by TEM analyses as shown in
Fig. 6. The sphere type particles in the size of 5–10 nm were homo-
geneously distributed on the fresh CZA/Al-FER catalysts, and the
size of CuO is found to be smaller on CZA/Al(5)-FER which is in
line with the results of XRD and N2O titration. However, a sig-
nificant aggregation of copper particles was observed on both

Y.J. Lee et al. / Catalysis Today 228 (2014) 175–182
181
Fig. 6. TEM images of CZA/Al-FER catalysts before and after reaction. (a) After reaction of CZA/Al(0)-FER, (b) before reaction of CZA/Al(0)-FER, (c) before reaction of CZA/Al(5)-
FER, (d) after reaction of CZA/Al(5)-FER
catalysts after reaction by forming larger particles up to 50 nm
in size, and its shapes are mainly a rod-type aggregate on both
catalysts. The aggregation is less significant on CZA/Al(5)-FER cat-
alyst which is also in line with the results of XRD and N2O
titration. Especially, the small copper particles of 10 nm in size are
more abundant on CZA/Al(5)-FER catalyst compared with that of
CZA/Al(0)-FER catalyst. This can be attributed to the strong interac-
tion of Cu–ZnO–Al2O3 particles on the Al2O3-modified H-ferrierite
surfaces as confirmed by TPR experiment by showing a larger shoul-
der peak at higher reduction temperature region, and by NH3-TPD
with a larger number of acidic sites which can be supported by the
observation of FT-IR.
The catalytic performance at steady state after 15 h on stream
was summarized in Table 3. The CO conversion and DME selectivity
were found to be higher on CZA/Al(2.5)-FER and CZA/Al(5)-FER
catalyst compared with the unmodified CZA/Al(0)-FER catalyst.
Higher CO conversions of 61.8–62.1% and higher DME selectivity
of 93.4–91.5% were observed on those catalysts, and the lower CO
conversion of 46.6% and DME selectivity of 87.4% were observed
on CZA/Al(0)-FER catalyst. CO2 formation showed a same trend
with CO conversion, i.e., a higher CO conversion is responsible for a
higher CO2 formation rate due to the enhanced WGS reaction. Inter-
estingly, the selectivity of byproducts such as light hydrocarbons
was higher on CZA/Al(0)-FER catalyst, which possessing a relatively
larger strong acidic sites by possibly enhancing the transformation
of DME/methanol to hydrocarbons [3,9–11] compared with the
other catalysts. In addition, reaction rate defined as the reacted CO
mol/gcat/h, and turn over frequency (TOF) defined as the number
of reacted CO molecules/surface metallic copper atom/second,
were also calculated using the values of copper surface area after
reaction and the results are summarized in Table 3. The reaction
rates were found to be in the range of 0.026–0.037 mol/gcat/h
and it is maximized on CZA/Al(5)-FER catalyst. The variation of
reaction rates are similar with the CO conversions, but TOF values
are almost similar on all CZA/Al-FER catalysts with the values of
0.059–0.069 molecules/Cu atom/s by monotonously increasing
with the increase of Al2O3 content on H-ferrierite. This observation
suggests the characteristics of structural insensitive reaction of
single step synthesis reaction of DME from syngas as reported
in our previous work [10]. The Al2O3-modified H-ferrierite on
CZA/Al-FER catalysts showed an enhanced catalytic performance
since the strong acidic sites on H-ferrierite were properly modified
by introducing Al2O3, and Al2O3 modifier also suppressed the
significant aggregation of copper particles by enhancing the inter-
action of copper particles with Al2O3-modifed H-ferrierite. The
results were supported by the FT-IR, XPS, NH3-TPD, XRD, TEM, and
N2O titration. Even though CO2 formation rate is proportional to
CO conversion due to the enhanced WGS reaction with the increase
of CO conversion, the byproduct formation like light hydrocarbons
are significantly suppressed by introducing Al2O3 modifier on
H-ferrierite, which is attributed to the reduced reforming reaction
of DME/methanol on the strong acidic sites [11,18,19]. Therefore,
CO conversion and DME yield on CZA/Al-FER catalysts are strongly
related with two factors such as the surface area of metallic copper
for CO hydrogenation to methanol and the amount of acidic sites
for dehydration of methanol to DME in a single-step reaction.
Finally, we modified our previous results by combining he
present results [10] to correlate the reaction rate and intrinsic activ-
ity (TOF) of CZA/Al-FFER catalysts with respect to the product values
of the amount of acidic sites (mmolNH3/gcat) and surface area of
metallic copper (m2/gCu) after reaction for 20 h using the summa-
rized results in Table 2. The product values of two characteristics
were found to be in the range of 50.1–90.7, and the value was maxi-
mized on CZA/Al(2.5)-FER. The results were summarized in Fig. 7 by
reconsidering the results of previous work [10]. The results showed
a good correlation for structure insensitive characters of a single-
step reaction of DME from syngas above the value of 20, which is a
product value of acidic sites and surface area of metallic copper. As
shown in Fig. 7, TOF values approached a constant value of around
0.07 and reaction rate for around 0.03 above the product valued of
20. This observation can be attributed to the fast dehydration rate
of methanol DME compared with that of methanol formation by CO
hydrogenation on the bifunctional CZA/Al-FER catalysts. Since the
rate determining step is known to be methanol synthesis reaction
on metallic copper surfaces, and the concomitant dehydration of
methanol to DME can be efficiently carried out on weak acidic

182
Y.J. Lee et al. / Catalysis Today 228 (2014) 175–182
Table 3
Catalytic performances of the bifunctional CZA/Al-FER catalysts.a
Notation
CO conversion
(mol%)
CO conversion to
CO2 (mol%)
Selectivity of methanol/
DME/byproductsb (mol%)
Yield of DME
(mol%)
Reaction ratec/10−2
TOFc/10−2
CZA/Al(0)-FER
CZA/Al(2.5)-FER
CZA/Al(5)-FER
CZA/Al(10)-FER
46.6
61.8
62.1
43.0
9.4
13.5
14.5
7.8
5.1/87.4/7.5
4.7/93.4/1.9
5.7/91.5/2.8
/91.6/3.5
32.5
45.2
43.6
32.2
2.77
3.68
3.70
2.56
5.88
6.07
6.03
6.86
a
Catalytic reaction was conducted under the following conditions for 20 h; T = 250 ◦C, P = 3.5 MPa, SV = 2000 ml/gcat/h, feed composition of CO/H2/N2 = 31.6/63.2/5.2. The
CO conversion and product distribution are the averaged values after 15 h at steady-state.
b
+
Byproducts mainly are composed of CH4 and C2 hydrocarbons.
c
Reaction rate was defined as the reacted CO mol/gcat/h and turn over frequency (TOF) is calculated by using the values of copper surface area after reaction with the
equation of the number of reacted CO molecules/surface metallic copper atom/second.
bifunctional CZA/Al-FER catalysts. The optimum concentration of
acidic sites and copper surface area is responsible for a higher
catalytic performance which can be obtained by controlling the par-
ticle size of copper on the acidic site by adjusting Al2O3-modified
H-ferrierite surface on CZA/Al-FER catalysts.
Acknowledgements
The authors would like to acknowledge the financial sup-
port from the National Research Foundation of Korea (NRF) grant
funded by the Korea government (MEST; 2011-0009003 and
2012R1A2A2A02013876).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2013.
11.015.
Fig. 7. Reaction rate and intrinsic activity (TOF) of CZA/Al-FER catalysts with respect
to the product values of the amount of acidic sites (mmolNH3/gcat) and surface area
of metallic copper (m2/gCu) after reaction for 20 h (included with the results of Ref.
[10]).
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Single step synthesis of DME from syngas on the bifunctional
CZA/Al-FER catalysts was investigated to elucidate effects of acidic
sites and copper surface area to the intrinsic activity. The Al2O3
modification of H-ferrierite was found to enhance CO conversion
and DME yield by adjusting acidic sites and dispersion of copper
particles. The optimum concentration of solid–acid Al2O3 modifier
was found to be around 2.5–5 wt% on H-ferrierite, and it altered
the dispersion of Cu–ZnO–Al2O3 particles on H-ferrierite with an
enhanced interaction of copper particles and small copper parti-
cle formation. It also significantly suppressed the aggregation of
copper particles on CZA/Al-FER catalysts with a higher stability.
In addition, Al2O3 modifier of H-ferrierite is also to reduce strong
acidic sites by depositing on the sites selectively and it reduced
the byproduct formation. Both of copper surface area and amount
of acidic sites are well correlated with an intrinsic activity, which
finally shows a structure-insensitive character for a single-step
synthesis of DME from syngas. Finally, TOF values approached a
constant value of around 0.07 and reaction rate for around 0.03
in terms of the product values of acidic sites and surface area
of metallic copper above 20. This intrinsic character can also be
attributed to the fast dehydration rate of methanol to DME com-
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