Enhanced Stability of Spatially Confined Copper Nanoparticles in an Ordered Mesoporous Alumina for Dimethyl Ether Synthesis from Syngas

Article abstract of DOI:10.1021/acscatal.6b00882

A spatial confinement effect of copper nanoparticles in an ordered mesoporous γ-Al₂O₃, which is synthesized by an evaporation induced self-assembly (EISA) method, was investigated to verify the enhanced catalytic activity and stability with less aggregation of copper crystallites during direct synthesis of dimethyl ether (DME) from syngas. The surface acidity of the mesoporous Al₂O₃ and the metallic copper surface area significantly altered catalytic activity and stability. The ordered mesopore structures of Al₂O₃ were effective to suppress the aggregation of copper nanoparticles even under reductive CO hydrogenation conditions through the spatial confinement effect of the ordered mesopores of Al₂O₃ as well as the formation of strongly interacted copper nanoparticles with the mesoporous Al₂O₃ surfaces by partial formation of the interfacial CuAl₂O₄ species. The aggregation of copper nanoparticles on the bifunctional Cu/meso-Al₂O₃ having an ordered mesoporous structure was effectively suppressed due to the partial formation of the thermally stable spinel copper aluminate phases, which can further generate new acid sites for dehydration of methanol intermediate to DME.

Full text of DOI:10.1021/acscatal.6b00882

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Article
Enhanced stability of spatially confined copper nanoparticles in an
ordered mesoporous alumina for dimethyl ether synthesis from syngas
Hyungwon Ham, Jihyeon Kim, Sung June Cho, Joon-Hwan Choi, Dong Ju Moon, and Jong Wook Bae
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00882 • Publication Date (Web): 19 Jul 2016
Downloaded from http://pubs.acs.org on July 20, 2016
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Enhanced stability of spatially confined copper nanoparticles in an ordered
mesoporous alumina for dimethyl ether synthesis from syngas
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Hyungwon Ham1, Jihyeon Kim1, Sung June Cho2, Joon-Hwan Choi3, Dong Ju Moon4,
Jong Wook Bae1,*
1School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon, Gyeonggiꢀdo,
440ꢀ746, Republic of Korea
2Department of Chemical Engineering, Chonnam National University, Gwangju, 500ꢀ757,
Republic of Korea
3 Functional Ceramics Department, Korea Institute of Materials Science (KIMS), Changwon,
Gyeongnam, 642ꢀ831, Republic of Korea
4Clean Energy Research Center, Korea Institute of Science and Technology (KIST), 136ꢀ791
Seoul, Republic of Korea
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*Corresponding author (J.W. Bae): Tel.: +82ꢀ31ꢀ290ꢀ7347; Fax: +82ꢀ31ꢀ290ꢀ7272; Eꢀmail
address: finejw@skku.edu
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Abstract
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A spatial confinement effect of copper nanoparticles in an ordered mesoporous gammaꢀ
Al2O3, which is synthesized by an evaporation induced selfꢀassembly (EISA) method, was
investigated to verify the enhanced catalytic activity and stability with a less aggregation of
copper crystallites during direct synthesis of dimethyl ether (DME) from syngas. The surface
acidity of the mesoporous Al2O3 and the metallic copper surface area significantly altered
catalytic activity and stability. The ordered mesopore structures of Al2O3 were effective to
suppress the aggregation of copper nanoparticles even under reductive CO hydrogenation
condition through the spatial confinement effect of the ordered mesopores of Al2O3 as well as
the formation of strongly interacted copper nanoparticles with the mesoporous Al2O3 surfaces
by partially forming the interfacial CuAl2O4 species. The aggregation of copper nanoparticles
on the bifunctional Cu/mesoꢀAl2O3 having the ordered mesoporous structures was effectively
suppressed due to the partial formation of the thermal stable spinel aluminate phases, which
can further generate the new acid sites for dehydration of methanol intermediate to DME.
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Keywords: Dimethyl ether (DME); ordered mesoporous Al2O3; copper nanoparticle; spatial
confinement effect; syngas; copper aluminate; bifunctional catalyst.
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1. Introduction
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Syntheses and applications of mesoporous materials having an ordered mesopore structure
with a large specific surface area have been largely reported since the ordered mesoporous
materials have superior characteristics such an easy mass transfer and a high dispersion of
active metals.1,2 Therefore, a lot of bifunctional (or hybrid) catalytic systems containing
metallic sites for CO (or CO2) hydrogenation as well as acid sites for a successive
dehydration of alcohols to ethers have been extensively investigated by using the mesoporous
materials,3-6 especially for a direct synthesis of dimethyl ether (DME) from syngas. The
active sites of solid acid sites as well as the metallic copper sites haven been well known to
be related with a catalytic activity,3,6-9 and a catalyst deactivation was strongly attributed to an
aggregation of copper nanoparticles with a successive blockage of the acid sites through a
wellꢀknown sintering process of copper nanoparticles on the bifunctional catalysts.6,10-15
In addition, DME having a similar chemical property with liquefied petroleum gas (LPG)
has been attracted as an alternative clean fuel due to its lower emission of NOx and SOx
pollutants than conventional diesel fuel.16,17 Compared to the commercialized twoꢀstep
synthesis of DME via methanol synthesis by CO (or CO2) hydrogenation, one step synthesis
of DME form syngas on the bifunctional catalysts has some advantages by effectively
operating even at a low H2/CO molar ratio and a high CO2 concentration. These characters
are mainly attributed to the intrinsically high activity for a waterꢀgas shift reaction by
overcoming a thermodynamic equilibrium limitation on the hybridized CuꢀZnOꢀAl2O3/solid
acid catalysts with solidꢀacid components of zeolites, γꢀAl2O3 or silicaꢀalumina and so on.3-
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5,8,18,19
Among the various solid acid catalysts, Al2O3 has been widely reported for a
dehydration reaction of methanol to DME due to its proper acid site strength and amount with
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a less hydrocarbon formation compared to zeolites.10,11,20,21 However, some disadvantages of
the soliꢀacid Al2O3 have been known to possess a relatively small amount of acid sites and a
hydrophilic character, which is prone to deactivate the bifunctional catalysts by a facile water
adsorption and a significant aggregation of the supported copper nanoparticles.3,6,12,13,15,22
Even though the preparation methods of the ordered mesoporous materials and their
applications for various catalytic reactions have been largely reported,23-25 the bifunctional
catalyst possessing copper nanoparticles supported on a regular mesoporous Al2O3 having a
narrow pore size distribution has not been well investigated till now as far as we know. In the
present study, some positive effects of ordered mesoporous γꢀAl2O3 prepared by evaporation
induced selfꢀassembly (EISA) method are tentatively verified with respect to spatial
confinement effect of a small amount of copper nanoparticles in an ordered mesoporous
Al2O3 with the help of newly formed spinel copper aluminate phases. Finally, the enhanced
catalytic stability for a direct synthesis of DME from syngas is mainly explained through a
spatial confinement effect with a less aggregation of copper nanoparticles.
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2. Experimental Details
2.1. Catalyst preparation and activity measurement
A ordered mesoporous Al2O3 was synthesized by EISA method as precisely reported by
other investigators23-25 by mixing Pluronic P123 (MW = 5800, EO20PO70EO20, SigmaꢀAldrich)
dissolved in 60 mL of ethanol solvent with aluminum isopropoxide (Al(OC3H7)3, Junsei)
solution dissolved in another 60 mL of ethanol with HNO3 after a vigorous stirring of two
solutions for 2 h separately. The above solution mixtures were stirred completely for 6 h at an
ambient temperature followed by drying at 60 oC in an oven for 2 days to evaporate a residual
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solvent completely. The asꢀprepared lightꢀyellow powder was further annealed at 400 oC for 4
h and an additional thermal treatment was carried out to prepare solidꢀacid Al2O3 catalyst at
different calcination temperatures. The asꢀprepared solidꢀacid Al2O3 are denoted as mesoAl
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and mesoAl(HT) at the calcination temperatures of 800 and 900 C for 1 h, respectively. For
comparison, a commercial Al2O3 (Puralox γꢀAl2O3 with a purity over 99.9% with a main
impurity of SiO2 (below 150 ppm) from Sasol) was also used as solidꢀacid catalyst after
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thermal annealing at 800 C for 1 h, and it is denoted as Al. On the asꢀprepared solidꢀacid
Al2O3, copper nitrate precursor was subsequently impregnated by incipient wetness
impregnation method with a fixed 10wt% copper based on the total weight of a bifunctional
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Cu/Al2O3 catalyst. The catalyst was dried again at 70 C for overnight, and it was
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subsequently calcined at 350 C for 3 h under air environment. The final asꢀprepared
bifunctional catalysts are denoted as the Cu/mesoAl, and Cu/mesoAl(HT), and the Cu/Al
represents the bifunctional catalyst containing 10 wt% copper on the commercial Al2O3. In
order to get rid of an internal mass transport problem in the mesopores, the solid acid Al2O3
having a particle size of 5 – 30 ꢀm was used, where the size is known to show a trivial effect
for an internal mass transport limitation.26,27 The morphologies and particle size distributions
of the Al2O3 supports with magnified images are displayed in supporting Figure S1
characterized by scanning electron microscopy (SEM) analysis, and the Al2O3 particles are
found to be irregular sphere shapes especially on the commercial Al2O3.
A catalytic activity was measured using a fixedꢀbed tubular flow reactor having an outer
diameter of 9.5 mm and an inner diameter of 7 mm. Before the activity test, the bifunctional
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Cu/Al2O3 catalyst was previously reduced at 300 C for 5 h under a flow of 5vol% H2
balanced with N2. A direct DME synthesis from syngas was performed at the following
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reaction conditions of P = 5.0 MPa, T = 250 oC, and space velocity of 2000 L/(kgcat·h) using
the syngas having a molar ratio of H2/CO/N2 = 63:31.5:5.5 with 0.4 g catalyst. To get rid of
an initial activity loss by a fast ramping of temperature at a high pressure, reaction pressure
was previously adjusted by filling it with syngas at room temperature and the temperature
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was raised up to reaction temperature with a ramping rate of 0.5 C/min. The products were
inꢀsitu analyzed by gas chromatography (YL6100 GC, Younglin) equipped with flame
ionization detector (FID) connected with PlotꢀQ capillary column to analyze methanol, DME,
and hydrocarbons and the GC equipped with thermal conductivity detector (TCD) connected
with Carboxenꢀ1000 packed column for measuring the effluent gases of CO, H2, N2, CO2.
The CO conversion (XCO) and selectivity of products (Si) was calculated based on the
following equations:
COꢄꢅꢆ
ꢂ
ꢃ
Xꢀꢁ mol % = 1 −
× 100
× 100
COꢇꢈ
Cꢇ
COꢇꢈ − COꢄꢅꢆ
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ꢃ
Sꢇ mol % =
where, COout, COin and Ci stand for moles of the inlet and outlet CO and moles of the product
i component (i.e. i = CO2, CH3OH, DME, and C1 ꢀ C2 hydrocarbons). The conversion of CO
and selectivity of products were calculated using the data at steadyꢀstate after reaction of 25 h
on stream on the bifunctional Cu/Al2O3 catalysts.
2.2. Catalyst characterization
N2 adsorptionꢀdesorption analysis was applied to measure BrunauerꢀEmmettꢀTeller (BET)
surface area, pore volume, and BarrettꢀJoynerꢀHalenda (BJH) pore size distribution from the
desorption branch on the asꢀprepared Al2O3 supports and bifunctional Cu/Al2O3 catalysts
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using TRISTARꢀ3000 (Micromeritics) working at ꢀ196 C. BET surface area was estimated
using the data in a relative pressure (P/P0) of 0.03 to 0.2, and pore volume was calculated at a
relative pressure of 0.99 from the desorption branch.
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Temperature programmed reduction analysis with H2 (TPR) was carried out using BELCATꢀ
M instrument to characterize the reducibility of the fresh bifunctional Cu/Al2O3 catalyst in
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the temperature range of 100 ꢀ 500 C at a heating rate of 10 C/min. Before TPR analysis,
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the fresh Cu/Al2O3 catalyst was pretreated at 300 C for 1 h at a ramping rate of 10 C/min
under Ar environment with a flow rate of 30 mL/min to remove any contaminant and
adsorbed water. After the pretreatment, a reduction gas of 5vol%H2 balanced with Ar was
introduced at a flow rate of 30 mL/min, and the hydrogen consumption was measured by
TCD after passing through water trap. In addition, the TPR patterns of the fresh bifunctional
Cu/Al2O3 catalysts after a stepwise reductionꢀoxidation treatment at the respective 300 and
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350 C for 5 h were also measured to verify the aggregation of supported copper crystallites
and the their interaction with the support during reduction step. Furthermore, additional TPR
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experiment with a different ramping rate of 2 C/min on the Cu/mesoAl was carried out to
confirm a homogeneous distribution of copper crystallites as well. Temperature programmed
desorption of NH3 (NH3ꢀTPD) analysis was carried out in the temperature range of 100 ꢀ 600
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oC at a ramping rate of 10 C/min to characterize the strengths and amounts of acid sites on
the fresh Al2O3 supports and Cu/Al2O3 catalysts before and after reduction using BELCATꢀM
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instrument. Before NH3ꢀTPD analysis, the sample was pretreated at 300 C for 1 h under He
flow for the Al2O3 supports and 5vol%H2 balanced with Ar for Cu/Al2O3 catalysts. After the
pretreatment, pure NH3 gas was adsorbed for 1 h at a flow rate of 30 mL/min. After the
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adsorption of NH3, the sample was flushed under He flow for 1 h at 100 C to remove the
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physisorbed NH3. The NH3ꢀTPD pattern was successively measured by TCD and mass
spectrometer (YL6900 MS, Younglin) for the fragment species of m/z = 16, 18 and 44.
The surface area (m2/gcat) of metallic copper crystallites was measured by N2O titration
using AutoChem II 2920 instrument. Before the N2O analysis, the sample was reduced at 300
oC for 5 h using a reducing gas of 10vol%H2 balanced with N2, and it was flushed under He
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flow at 100 C for 1 h. The consumption of N2O on the reduced sample was analyzed by
measuring the formed amount of N2 and that of unreacted N2O with TCD equipped with a
Porapak Q packed column. The surface area of metallic copper crystallites was further
calculated based on the value of the generated amount of N2 using an equation of N2O + 2Cu
= Cu2O + N2. A metallic copper surface area was also calculated by assuming 1.46 x 1019
copper atoms/m2 with a molar stoichiometry of 2 for number of copper atom exposed on the
surface per N2O molecule.28
Powder Xꢀray diffraction (XRD) analysis was used to identify the crystalline phases and
crystallite sizes of the supported copper nanoparticles on the bifunctional Cu/Al2O3 catalysts
using D8 ADVANCE instrument equipped with CuꢀKα radiation having a wavenumber of
0.15406 nm. The characteristic XRD patterns of copper species and alumina were collected in
the range of 10 ꢀ 80o at a scanning rate of 4o/min. The crystallite size of CuO and metallic
copper nanoparticles were calculated by using Scherrer equation with the most intense
characteristic peak of copper species at 2θ = 35.4o for CuO on the fresh Cu/Al2O3 catalysts
and 2θ = 43.3o for metallic copper species on the used ones. The ordered mesoporous
structures of the asꢀsynthesized Al2O3 were also confirmed by small angle Xꢀray scattering
(SAXS) analysis using ANTON PAAR instrument with CuꢀKα radiation. For the SAXS
analysis, the collected scattering vector (q, nmꢀ1) was converted to 2θ value using an equation
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of q = 4πsin(θ/λ), where λ is the wavelength of Xꢀray of 0.15406 nm. The Al2O3 sample was
exposed to Xꢀray for 1 min and the scattered Xꢀray was gathered on a detecting film.
Transmission electron microscopy (TEM) analysis was further carried out to clarify the
changes of the surface morphology and local crystallite size distribution of the supported
copper nanoparticles on the solidꢀacid Al2O3 surfaces for the fresh and used Cu/Al and
Cu/mesoAl(HT) catalysts using TECNAI G2 instrument operating at 200 kV.
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The oxidation states and surface concentrations of copper and aluminum species on the
fresh and used Cu/Al2O3 catalyst were characterized using Xꢀray photoelectron spectroscopy
(XPS) analysis with a VG Multilab 2000 (ThermoVG Scientific) instrument. The Al Kα
monochromatized line (1486.8eV) with a resolution of 0.05 eV was adopted for Xꢀray source
to analyze the Al 2p and Cu 2p3/2. The binding energy (BE) of copper and aluminum was
adjusted with a reference BE of C 1s (284.6 eV). The relative oxidation states of copper
nanoparticles were measured using the intensity ratio of Cu and CuO species by comparing
the value of ICu/ICuO (i.e., the integrated area ratio of the deconvoluted Cu and CuO peak). In
addition, the outermost surface concentrations of the copper species were characterized using
the intensity ratio of ICu/IAl of the integrated area of Cu 2p3/2 and Al 2p peak, respectively.
To verify the copper phases on the Cu/Al2O3 catalysts before and after reaction after air
exposure, Xꢀray absorption fine structure (XAFS) analysis of Cu Kꢀedge (8979 eV) was
performed at the 7D1 beam line in Pohang Light Source (PLS). The beam line was equipped
with a Si(111) monochromator and ionization chambers for measuring transmitted beam
intensities. The powder sample was attached on the tape and positioned between two Ohken
ionization chambers filled with nitrogen gas for measuring the incident and transmitted Xꢀray
intensity. A simultaneous measurement of the absorption spectra of the metallic Cu foil, CuO,
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Cu2O and CuAl2O4 were also carried out, and normalization and XAFS analysis were
performed using an Athena graphical interface program.29 XANES (Xꢀray absorption nearꢀ
edge structure) originating from an excitation of core electron to a vacant dꢀorbital of the
copper metal, which reflects the dꢀorbital electron vacancy (denoted as white line area (AWL))
of the Cu/Al2O3 catalysts and EXAFS (extended Xꢀray absorption fine structure) data were
also analyzed to calculate coordination number of CuꢀCu (denoted as CN(CuꢀCu)) and CuꢀO
(denoted as CN(CuꢀO)) as well as to calculate radius (R/Å) of CuꢀO on the fresh and CuꢀCu
on the used Cu/Al2O3 catalysts.
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3. Results and discussion
3.1. Physicochemical and reduction properties of the Cu/Al2O3 catalysts
The ordered mesoporous structures of the solidꢀacid Al2O3 and Cu/Al2O3 catalysts were
confirmed by N2 sorption and SAXS analysis. The pore size distribution patterns and N2
adsorptionꢀdesorption isotherms are displayed in Figure 1 and supporting Figure S2,
respectively. The average ordered mesopore sizes on the mesoAl supports were found to be
6.0 nm as summarized in Table 1. The mesopore size was decreased to 4.0 nm on the
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mesoAl(HT) with an increase of calcination temperature up to 900 C due to a possible
mesopore shrinkage of main Al2O3 frameworks at a high temperature thermal treatment
condition.30,31 The formation of the regular Al2O3 mesopores was also supported by observing
a typical type IV adsorptionꢀdesorption isotherms on the supports as well as bifunctional
catalysts as shown in supporting Figure S2. The type IV isotherm with H1 type hysteresis can
be generally observed on the mesoporous materials, and the hysteresis loop at a lower relative
pressure on the Cu/mesoAl than that of the Cu/Al suggests the formation of relatively smaller
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pores with slightly disordered cylindrical mesopore channels.1,2 On the contrary, the average
pore size on the commercial Al2O3 was found to be a larger size of 9.5 nm with a broad pore
size distribution. As shown in supporting Figure S3, the wellꢀformation of the ordered
mesopores on the Al2O3 supports was further verified by SAXS analysis which showed the
characteristic diffraction peaks of the 2D hexagonal mesopore structures assigned to the plane
of (100) reflection.23-25 Therefore, an observed lower surface area of 150 m2/g and larger pore
volume of 0.47 cm3/g on the commercial Al2O3 compared with those of 204 ꢀ 253 m2/g and
0.37 ꢀ 0.43 cm3/g on the ordered mesoporous Al2O3 seem to be attributed to an irregular pore
size distribution. Interestingly, the ordered mesopore structures of the mesoporous Al2O3 was
preserved even after 10wt% copper impregnation by revealing a decreased surface area in the
range of 173 ꢀ 180 m2/g with a decreased average pore size of 3.8 nm due to a preferential
deposition of copper nanoparticles inside of mesoporous Al2O3 surfaces having an average
pore diameter of ~6 nm by stretching them inside of Al2O3 mesopores.30 These advantageous
physicochemical properties such as large surface area with pore volume and ordered
mesoporous Al2O3 structures were clearly observed on the Cu/mesoAl and Cu/mesoAl(HT).
These characteristics seem to be responsible for an enhanced catalytic activity and stability
on the Cu/mesoAl and Cu/mesoAl(HT) compared with the Cu/Al. It is also attributed to the
formation of active crystalline planes by forming active copper defect sites32,33 through a
spatial confinement effect and a newly formed strong metalꢀsupport interaction possibly.
The surface area of metallic copper species plays an important role for CO hydrogenation to
methanol, and a large surface area of metallic copper by an easy reducibility of CuO is
responsible for a high catalytic activity for methanol synthesis.12,34,35 In addition, the strong
interactions of copper crystallites with solidꢀacid Al2O3 sites are also important to suppress
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the aggregation of copper nanoparticles during the reaction.6,10,14,31 The reduction behavior
and proper metalꢀsupport interaction between copper nanoparticles and solidꢀacid Al2O3 were
measured by TPR experiment, and the reduction patterns of the fresh Cu/Al2O3 catalysts
(solid line) are shown in Figure 2 with the reduction patterns on the Cu/Al2O3 catalysts after
successive reductionꢀcalcination treatment (dashed line) as well. Two distinctive reduction
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peaks were clearly observed on the fresh Cu/Al at around 145 C and 221 C, which can be
attributed to the reduction of heterogeneously dispersed copper oxides on the Al2O3 surfaces
and the reduction of bulk copper oxides, respectively.19,36 However, only one large reduction
peak with a Gaussian pattern was observed on the Cu/mesoAl and Cu/mesoAl(HT) at around
216 and 185 oC, respectively. This observation suggests the homogeneous size distribution of
copper nanoparticles inside of the mesoporous Al2O3. The observed Gaussian pattern of the
TPR peak on the Cu/mesoAl and Cu/mesoAl(HT) compared to the Cu/Al suggests the
homogeneous distribution of the supported cupper nanoparticles. In addition, the observed
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Gaussian reduction patterns at a different ramping rate of 2 C/min during TPR experiment
strongly suggest a homogeneous distribution of copper nanoparticles as shown in supporting
Figure S4. The reduction peak shift to a lower temperature on the Cu/mesoAl(HT) compared
to the Cu/mesoAl revealed the decreased interaction between copper nanoparticles and solid
acid Al2O3 since a thermal treatment at high temperature of the Al2O3 can induce a
crystallization as well as dehydration of Al2O3 surfaces, which can largely decrease an
interaction of copper nanoparticles with the acidic Al2O3 surfaces.37
As summarized in supporting Table S1, a larger amount of H2 consumption during TPR
experiment, which was related with the well dispersion of copper nanoparticles, was also
found on the Cu/mesoAl and Cu/mesoAl(HT) with the values around 0.87 ꢀ 0.95 mmolH2/g
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than that of Cu/Al with the value of 0.80 mmolH2/g. The roles of the acidic Al2O3 surfaces
for a different interaction of the supported copper nanoparticles were further verified by
measuring the crystallite size of the copper precursor of Cu2(OH)3NO3 before calcination.
The XRD patterns and crystallite sizes of copper precursor are shown in supporting Figure
S5. The size of the copper precursor before calcination was found to be somewhat larger on
the Cu/mesoAl(HT) with a value of 52 nm than that of the Cu/mesoAl with 46 nm due to the
decreased metalꢀsupport interaction. However, the copper precursor size is much larger on the
Cu/Al with a value of 76 nm. It suggests that the relatively smaller copper crystallites seems
to be effectively confined on the Cu/mesoAl and Cu/mesoAl(HT) through a strong metalꢀ
support interaction (SMSI) between the copper nanoparticles and acidic surface of Al2O3. In
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addition, the TPR patterns of the reduced Cu/Al2O3 catalysts at 300 C with subsequently
reoxidized Cu/Al2O3 catalysts were obtained (dashed line) as shown in Figure 2. The second
reduction patterns on the Cu/Al after reductionꢀoxidation treatment showed a similar two
characteristic reduction peaks due to its intrinsic nature of the heterogeneous size distribution
of the supported copper nanoparticles by easily forming aggregated copper
crystallites.6,10,14,15,31 However, similar reduction patterns on the Cu/mesoAl and
Cu/mesoAl(HT) even after reductionꢀoxidation treatment were observed by approaching the
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maximum reduction temperature at around 185 C from the respective temperature of 216
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and 165 C on the fresh ones. This observation suggests that the thermally stable copper
nanoparticles can be preserved by forming a new strong interaction in the ordered mesopores
of Al2O3 with the help of a spatial confinement effect. This seems to be possibly attributed to
the partial formation of spinel CuAl2O4 species, which can generate the much stronger
interaction between the copper nanoparticles and Al2O3.
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Interestingly, a partial formation of the spinel copper aluminate species seems to be more
favorable on the Cu/meosAl catalysts due to an observed lower H2 consumption and a larger
amount of acid sites on the mesoporous Al2O3. The SMSI effect was possibly enhanced
through the spatial confinement effect of the supported copper nanoparticles in the ordered
mesopores of Al2O3 having a size of 4 – 6 nm, which probably resulted in changing the
electronic property and crystalline planes of the supported copper nanoparticles.32,33
Therefore, the degree of reduction of the Cu/Al2O3 catalysts was calculated by using H2
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consumption below 300 C divided by a total theoretical amount of H2 consumption and the
results are summarized in Table 1. The degree of reduction was found to be around 29.1, 32.2,
and 34.5 % on the Cu/Al, Cu/mesoAl, and Cu/mesoAl(HT), respectively. The lower reduction
degree on the Cu/Al seems to be attributed to a bimodal size distribution of the strongly
interacted copper nanoparticles on the commercial Al2O3. However, the larger degree of
reduction and the Gaussian reduction pattern even after the reductionꢀoxidation treatment on
the Cu/mesoAl and Cu/mesoAl(HT) seem to be attributed to a strong interaction of copper
nanoparticles with the acid sites of the ordered Al2O3 structures. In addition, the SMSI effect
on the Cu/mesoAl and Cu/mesoAl(HT) can be attributed to the partial surface decoration of
the metallic copper with aluminum species,38 which was also supported by TPR patterns by
shifting to a high temperature on the Cu/mesoAl(HT) than that of the Cu/Al. A higher
reducibility of the copper nanoparticles and partial formation of thermally stable copper
aluminates on the Cu/mesoAl and Cu/mesoAl(HT) can be beneficial for suppressing active
copper aggregation during the reaction with the help of the SMSI and spatial confinement
effect, which was clearly verified by XAFS analysis as well.33
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3.2. Surface acidity of the Cu/Al2O3 catalysts to the dispersion of copper nanoparticles
The amounts and strengths of the acid sites on the Cu/Al2O3 catalysts are important for the
high DME selectivity through a successive dehydration reaction of methanol formed by CO
hydrogenation.3,8,39 A large number of the weak acid sites on the solid acid γꢀAl2O3 or zeolites
have been known to be closely related with a high dispersion of metal crystallites as well. In
addition, a high byproduct formation like light hydrocarbons can be also attributed to the
presence of strong acid sites through an subsequent dehydration or decomposition of DME or
methanol.7,9,10 As shown in supporting Figure S6, the desorption peak below 300 oC in NH3ꢀ
TPD can be assigned to the only desorption of NH3 (m/z =16) and others are assigned to
water desorption from the bifunctional Cu/Al2O3 surfaces. This behavior was confirmed by a
simultaneous analysis with mass spectrometer, and the adsorbed amount of NH3 was only
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assigned to the deconvoluted area with a Gaussian pattern below 300 C of the NH3ꢀTPD
peaks. The summaries of NH3ꢀTPD analysis on the solid acid Al2O3 and fresh Cu/Al2O3
catalysts are presented in Table 1. The amount of acid sites on the solid acid Al2O3
themselves were found to be 0.11, 0.29 and 0.24 mmolNH3/g on the commercial Al2O3,
mesoAl and mesoAl(HT), respectively. In general, large acid sites of the supporting materials
are responsible for a high dispersion of the supported metals through the selective adsorption
of metal ions.30,40-42 Interestingly, total acid sites on the fresh Cu/Al2O3 catalysts were
significantly increased to 0.13, 0.45 and 0.42 on the Cu/Al, Cu/mesoAl and Cu/mesoAl(HT),
respectively as summarized in Table 1. These increases seem to be attributed to a newly
formed copper aluminate species, especially on the Cu/mesoAl and Cu/mesoAl(HT). Even
though the acidic sites on the reduced Co/Al2O3 catalysts were decreased as summarized in
supporting Table S1, the acid sites on the reduced Cu/mesoAl and Cu/mesoAl(HT) still
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showed a larger amount than that of the Cu/Al.
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The large amount of acid sites on the Cu/mesoAl and Cu/meaoAl(HT) can increase the CO
conversion though a consecutive dehydration of methanol to DME, which seems to be crucial
for a lower aggregation of the supported copper nanoparticles through a strong metalꢀsupport
interaction on the acidic sites as well. To confirm the acid site strengths, the acid sites were
deconvoluted to weak and medium acid sites as summarized in supporting Table S1. The
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amount of weak acid sites below the desorption temperature of 200 C (assigned to W as
shown in Table S1) was found to be larger on the mesoAl compared to the medium acid sites
(assigned to M as shown in Table S1) with the respective values of 0.23 and 0.06
mmolNH3/g on the mesoAl. Interestingly, the weak and medium acid sites on the Cu/mesoAl
were dramatically changed to the respective values of 0.20 and 0.25 mmolNH3/g. This
observation can be attributed to the formation of an interfacial copper aluminate species by
generating new acid sites with a medium strength. A large amount of acid sites with a
medium strength was found on the Cu/mesoAl(HT). Therefore, the acid sites on the ordered
mesoporous Al2O3 are prone to interact with the copper nanoparticles strongly by forming
much smaller nanoparticles than that of the Cu/Al. This newly formed strong interaction is
probably attributed to the interfacial formation of copper aluminate on the acid sites of the
ordered mesoporous Al2O3, which is responsible for a less aggregation due to a simultaneous
spatial confinement effect in the ordered mesopores of Al2O3. However, on the commercial
Al2O3, the surface acidity was decreased due to the blockage of Al2O3 pores by forming
larger copper nanoparticles during the reduction of reaction. The enhanced thermal stability
of copper nanoparticles on the Cu/mesoAl and Cu/mesoAl(HT) seems to be attributed to the
preserved acid sites by simultaneously generating strongly interacted copper nanoparticles
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with the partial formation of spinel copper aluminate.
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From the results of N2O titration as summarized in Table 1, the observed somewhat larger
surface area of metallic copper with the values of 5.9 – 7.1 m2/gcat on the fresh Cu/mesoAl
and Cu/mesoAl(HT) than that of 5.5 m2/gcat on the Cu/Al can be possibly attributed to the
wellꢀdispersed copper nanoparticles on the acidic sites of the ordered mesoporous Al2O3 due
to the presence of a larger amount of acid sites.40-43 These properties can largely enhance the
catalytic activity and stability by increasing CO hydrogenation activity to methanol and a
successive dehydration activity to DME.11 This observation was also supported by XRD
analysis as shown in Figure 3(A) and supporting Table S1. The fresh Cu/Al showed a larger
crystallite size of CuO of ~50 nm, which was calculated from the most intense characteristic
diffraction peak of CuO. However, the Cu/mesoAl and Cu/mesoAl(HT) showed the much
smaller sizes not to be detected from XRD analysis. In general, the surface area of metallic
copper has been reported to be inversely correlated with the copper crystalline size.43
Compared to a significantly different crystallite size, the observed insignificant surface area
variation of metallic copper seems to be attributed to the heterogeneous crystallite size
distribution on the Cu/Al. The thermal stability of copper nanoparticles in the ordered
mesoporous Al2O3 can be largely enhanced even under reaction due to the strong interaction
of copper nanoparticles with acid sites as reported in our previous researches6,10,30,40-42 and
the spatial confinement effect.
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3.3. Catalytic performance of the Cu/Al2O3 catalysts
The CO conversion and product distribution for direct DME synthesis from syngas on the
bifunctional Cu/Al2O3 are summarized in Table 2 and the CO conversion with time on stream
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for 40 h is shown in Figure 4. A similar CO conversion of 11.5% and DME selectivity of
76.1% (based on the total hydrocarbons and oxygenates formed without CO2) were observed
on the Cu/mesoAl(HT) and the respective values of 9.6 and 78.5% on the Cu/mesoAl. These
values are much larger than the Cu/Al with the respective values of 5.7 and 67.9%. The
observed relatively low CO conversion on the Cu/mesoAl with a high stability was attributed
to a lower content of copper with 10 wt% on the ordered mesoporous alumina compared with
a high concentration of copper (above 70 wt%Cu) on hybridized CuꢀZnOꢀAl2O3/solid acid
catalysts with CO conversion above 40% and a fast deactivation nature as reported from our
previous works.6,11,14 The extent of CO2 formation on the Cu/Al2O3 catalysts was proportional
to CO conversion due to the proportional waterꢀgas shift reaction activity with water formed
during methanol dehydration to DME. The observed high catalytic activity on the Cu/mesoAl
and Cu/mesoAl(HT) was attributed to the ordered mesoporous structures of the Al2O3 with
the highly dispersed copper nanoparticles on a large amount of acid sites. As shown in Table
1, a fast methanol dehydration rate to DME,33,44,45 which was formed by CO hydrogenation
on the active metallic copper sites, seems to be responsible for a high CO conversion on the
Cu/mesoAl and Cu/meaoAl(HT) which has large metallic copper sites and acid sites. The
observed higher DME selectivity of 78.5% on the Cu/mesoAl than the Cu/mesoAl(HT) of
76.1 % seems to be also attributed to a larger amount of newly formed mediumꢀstrength acid
sites (M), which can be possibly originated from copper aluminate formation as summarized
in supporting Table S1 and section 3.2. The high selectivity of byproducts (mainly methane)
on the Cu/Al of 17.8% compared with 6.3% on the Cu/mesoAl can be attributed to the large
crystallite sizes of metallic copper, which are active sites for methanation of CO or
decomposition of intermediates such as methanol or DME possibly.6,14,31,46,47 The catalytic
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stability in terms of CO conversion and reaction rate for 40 h on stream was found to be
much higher on the Cu/mesoAl and Cu/mesoAl(HT) than the Cu/Al as shown in Figure 4
and supporting Figure S7. The observed long induction period to approach steadyꢀstate
conversion for ~12 h on all Cu/Al2O3 catalysts was mainly attributed to a slow ramping rate
of reaction temperature for minimizing an initial deactivation. However, selectivity to DME
with time on stream as shown in supporting Figure S8 revealed a fast approach to steadyꢀ
state value. In addition, the reference catalyst of Cu/mesoAl(w/o P123) which was prepared
through a similar method of the Cu/mesoAl without using P123 (shown in supporting Table
S2) also showed a fast deactivation like the Cu/Al due to an irregular pore structure formation
on the Cu/mesoAl(w/o P123). CO conversion was decreased from 7.8% at 12 h on stream to
5.6% at steadyꢀstate for 40 h reaction on the Cu/mesoAl(w/o P123), however, CO conversion
was slightly increased from 8.0% at 12 h on stream to 9.6% at steadyꢀstate for 40 h reaction
on the Cu/mesoAl. Therefore, it seems to be mainly attributed to the stabilization of the
active sites of metallic copper nanoparticles with a suppressed aggregation through the
formation of strongly interacted copper crystallites by possibly forming copper aluminate on
the acid sites.
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The positive confinement effect of the homogeneously distributed copper crystallites in the
ordered mesoporous Al2O3 with the possible generation of new acid sites was also verified by
comparing the catalytic activity of methanol dehydration to DME on the Cu/mesoAl and
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Cu/mesoAl(HT) at the reaction condition of T = 250 C and P = 0.1 MPa. As summarized in
supporting Table S3, a slightly higher methanol conversion and DME selectivity were
observed on the Cu/mesoAl with the respective values of 27.9 and 67.3% compared to the
Cu/mesoAl(HT) with the respective values of 24.9 and 64.8%. The observed higher methanol
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conversion and DME selectivity with a lower formation of methyl formate and methyl acetate
on the Cu/mesoAl than Cu/mesoAl(HT) can be attributed to a fast dehydration of methanol to
DME on the largely exposed acid sites including newly formed acid sites and metallic copper
crystallites.6,10,11 This observation also suggests that the spatially confined copper
nanoparticles with smaller crystallite sizes on the Cu/mesoAl increases the exposed acid sites
compared with Cu/mesoAl(HT). In addition, the strongly interacted copper crystallites with a
high metal surface area on the acidic Al2O3 surfaces (confirmed by TPR and XRD) can be
responsible for a further dehydrogenation to methyl formate.48 The catalytic stability of the
copper nanoparticles on the bifunctional Cu/Al2O3 catalysts has been known to be largely
affected by the extent of sintering phenomena of the supported copper nanoparticles and the
simultaneous changes of the acid site density during the reaction.6,10,11,14,31,46,47
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To further verify the intrinsic activity on the Cu/Al2O3 catalysts, the reaction rate defined as
the reacted CO mmol/(gcat·h) and turnover frequency (TOF) defined as the reacted CO
molecules/(surface copper metal atom·s) using the metallic copper surface area of the used
catalysts are summarized in Table 2. The reaction rates of 2.69, and 3.24 mmol/(gcat·h) were
found on the Cu/mesoAl and Cu/meaoAl(HT) and these values are two times larger than the
Cu/Al of 1.60 mmol/(gcat·h). However, TOF values on all Cu/Al2O3 catalysts showed a small
variation in the range of 0.0049 ꢀ 0.0063 molecule/(surface copper metal atom·s) due to the
structural sensitive characters in the range of a lower value of (surface area of copper) x
(amount of acid site) for the direct DME synthesis reaction from syngas.6,10,11 In general, the
reaction rates on the Cu/Al2O3 catalysts have been known to be strongly affected by the total
number of the defect sites and types of metallic copper crystallite facets32,38 as well as by the
interfaces between the copper crystallites and solid acid Al2O3 surfaces.49 The thermal
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stability of copper nanoparticles inside of the mesoporous Al2O3 seems to change the
catalytic activity significantly, and the spatial confinement effect of the copper nanoparticles
inside of the ordered Al2O3 mesopores can largely enhance the thermal stability by partially
forming stable copper aluminate during the oxidation and reaction period.
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3.4. Thermal stability of copper nanoparticle on the mesoporous Cu/Al2O3 catalysts
To further verify the thermal stability of the copper nanoparticles in the ordered mesoporous
Al2O3, the XRD, N2O titration, XPS and TEM analysis on the fresh and used catalysts were
carried out. XRD patterns of the fresh Cu/Al2O3 catalysts showed characteristic diffraction
peaks of CuO at 2θ = 35.4, 38.6, 48.6, 53.4, 58.2, and 61.3 (JCPDS No: 048ꢀ1548) and γꢀ
Al2O3 at 2θ = 19.6, 32.2, 37.1, 39.7, 46.0, 60.8, and 66.9o (JCPDS No: 00ꢀ010ꢀ0425). Even
though the CuO phase was not detected on the Cu/mesoAl and Cu/mesoAl(HT) due to highly
dispersed CuO nanoparticles inside of ordered Al2O3 mesopores possibly, the crystallite size
of CuO on the fresh Cu/Al was found to be around 48.2 nm as calculated by the FWHM
value from Figure 3(A). The impregnated copper nanoparticles on the ordered mesoporous
Al2O3 with a pore size of 4 ꢀ 6 nm seem to be well dispersed with a smaller size than that of
mesopores on the Cu/mesoAl and Cu/mesoAl(HT). However, new characteristic diffraction
peaks of the metallic copper at 2θ = 43.3, 50.3, and 74.1 (JCPDS No: 04ꢀ0836) were clearly
observed on the used Cu/Al2O3 catalysts as shown in Figure 3(B). The calculated crystallite
sizes of the metallic copper nanoparticles with the help of Scherrer equation using the most
intense diffraction peak at 2θ = 43.3o were found to be around 88.7 and 5.2 nm on the used
Cu/Al and Cu/mesoAl(HT), respectively. This observation also supports a less aggregation of
copper nanoparticles inside of the ordered mesoporous Al2O3, especially on the Cu/mesoAl
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without showing any characteristic diffraction peaks of metallic copper. As summarized in
Table 1, the surface area variation on the fresh and used catalysts were found to be from 7.1
to 5.3 m2/gcat on the Cu/mesoAl(HT) and 5.9 to 4.9 m2/gcat on the Cu/mesoAl and from 5.5
to 3.8 m2/gcat on the Cu/Al. The small variation of the surface area of metallic copper
measured by N2O titration on the fresh and used Cu/mesoAl strongly suggests the suppressed
aggregation of copper nanoparticles on the ordered mesoporous Al2O3. We believe that a less
aggregation of copper nanoparticles inside of the ordered mesoporous Al2O3 can be attributed
to the spatial confinement effect as well as the formation of the strongly interacted copper
nanoparticles by forming copper aluminate species, which are preferential on the Cu/mesoAl.
The oxidation sates of the copper nanoparticles and Al2O3 with their surface concentrations
were measured by XPS analysis on the fresh and used Cu/Al2O3 catalysts. The summarized
surface concentrations of ICu/ICuO and ICu/IAl and BEs of Cu and Al species are presented in
Table 1, supporting Table S1 and Figure S9. The intensity ratios of the ICu/ICuO for
comparing the oxidation states of copper nanoparticles was calculated by deconvoluting Cu
2p3/2 peak appearing at 932 ꢀ 935 eV on the used Cu/Al2O3 catalysts. In addition, the surface
ratios of ICu/IAl were also calculated using the integrated area of Cu 2p3/2 divided by that of Al
2p on the fresh and used Cu/Al2O3 catalysts after correcting the area using atomic sensitivity
factors of Cu and Al species.50 The BEs of copper species were found in the range of 934.7 –
935.4 eV on the fresh Cu/Al2O3 catalysts without significant deviations, which corresponds to
the CuO phase of the supported copper crystallites mainly. However, BEs of copper species
on the used Cu/Al2O3 catalysts shifted to a lower BE in the range of 932.8 – 932.9 eV, which
suggests the presence of partially reduced CuO species as summarized in supporting Table
S1 and Figure S9. Interestingly, somewhat higher binding energy of aluminum species was
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observed at the BE of 76.18 eV on the fresh Cu/Al and that of 74.59 eV on the used Cu/Al
compared with those of the Cu/mseoAl of the respective values of 74.36 and 74.44 eV, which
corresponds to gammaꢀphase Al2O3.50 The observed higher BE of Al 2p on the Cu/Al may be
attributed to a large number of defect sites on the commercial Al2O3 surfaces by strongly
interacting with CuO species. As shown in supporting Figure S9, the observed similar BEs of
aluminum species with a large peak intensity on the used Cu/mesoAl seem to be attributed to
a preservation of the stable gammaꢀphase Al2O3 before and after reaction. We believe that the
higher BE of aluminum species on the Cu/Al can be also responsible for the higher byproduct
formation through the further reaction of methanol (or DME) to hydrocarbons. As shown in
Table 1, the ICu/ICuO ratios can be also related with a strong metalꢀsupport interaction between
copper nanoparticles and Al2O3. The ICu/ICuO ratios were found to be smaller on the
Cu/mesoAl(HT) with the value of 1.97 and larger values between 2.46 and 2.52 on the Cu/Al
and Cu/mesoAl, respectively. A higher oxidation state and lower ICu/ICuO ratio of copper
nanoparticles on the Cu/mesoAl(HT) than those of the Cu/mesoAl seems to be attributed to
the preferential aggregation of copper nanoparticles as confirmed by XRD analysis. In
addition, the strong interaction of copper nanoparticles with the acidic Al2O3 sites can form a
thermally stable copper aluminate species as well, which was confirmed by TPR experiments.
In addition, it was also attributed to the abundant formation of partially reduced Cu2O
surfaces confirmed by XAFS analysis during the reduction and reaction period possibly.
Furthermore, the variation of surface concentrations of the copper species was verified by
comparing the ratio of ICu/IAl on the fresh and used Cu/Al2O3 catalysts as summarized in
Table 1. Although the surface concentration of copper species on the fresh Cu/mesoAl(HT)
was found to be a higher than other Cu/Al2O3 catalysts from the value of 0.087, the copper
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nanoparticles seem to be redispersed in the inside of mesoporous Al2O3 judged from the
decreased intensity ratio of 0.056 after reaction on the Cu/mesoAl(HT). This observation can
be surely attributed to a spatial confinement effect of copper nanoparticles with a suppressed
aggregation by forming strong metalꢀsupport interaction, and the phenomena were similar on
the Cu/mesoAl as well by changing from 0.066 to 0.077 of the ratio of ICu/IAl during the
reaction. However, the more significant aggregation of the copper nanoparticles was observed
on the Cu/Al by showing the increased intensity ratio of ICu/IAl from 0.068 to 0.077 during the
direct synthesis reaction of DME from syngas without less effects of the spatial confinement.
The proposed spatial confinement effect inside of the mesoporous Al2O3 were also verified
by TEM analyses on the Cu/mesoAl(HT) and Cu/Al before and after reaction for 40 h. The
TEM images are displayed in Figure 5 with the large scale images in the inset figure of
Cu/mesoAl(HT) and supporting Figure S10 for the Cu/Al. The copper nanoparticles with the
crystallite sizes of ~ 5 nm were well dispersed in the ordered mesoporous Al2O3 surfaces on
the fresh Cu/mesoAl(HT), and the size of copper crystallites was not significantly altered
even after the reaction for 40 h as shown in Figure 5. However, the larger copper crystallites
above the average crystallite size of ~50 nm were observed on the fresh Cu/Al, and the size
of copper particles significantly increased to the much larger crystallite size above ~80 nm as
shown in supporting Figure S10. The significant aggregation of the supported copper
nanoparticles on the Cu/Al2O3 catalytic systems has been generally observed on an irregular
pore size distribution having a larger pores.6,10,11 Therefore, the suppressed aggregation of the
copper nanoparticles inside of the ordered mesoporous Al2O3, especially on the Cu/mesoAl
compared to the Cu/Al, can be attributed to a spatial confinement effect of the ordered
mesoporous Al2O3, which are strongly interacted with the acid sites on the Al2O3 by partially
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generating the copper aluminate species (confirmed by TPR and XPS experiments).
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3.5. Enhanced catalytic stability through an interfacial copper aluminate formation
The verification of a newly formed SMSI and a thermally stable copper aluminate formation
between the wellꢀdispersed copper nanoparticles and the ordered mesoporous Al2O3 surfaces
were further verified by XAFS analysis of the fresh and used Cu/Al2O3 catalysts. As shown in
Figure 6 and supporting Figure S11, XANES spectra and their first derivatives showed that
the main phase of copper species on the fresh Cu/Al2O3 was found to be the CuO phase
confirmed by the characteristic absorption energy at around 8985 and 8997 eV without
significant differences. As summarized in Table 1 and supporting Table S4, the CNs of the
CuꢀO on the fresh Cu/Al2O3 catalysts were found to be around 3.4 ꢀ 3.6 and the white line
areas (AWL) were found to be similar on the Cu/mesoAl and Cu/mesoAl(HT) with the
respective values of 7.29 and 7.74 compared to the Cu/mesoAl with the value of 6.51. As
shown in supporting Figure S12 and Table S4, the linear combination fittings (LCF) of the
fresh and used Cu/Al2O3 catalysts were also performed to verify the possible compositions of
copper species. These fitting results showed that the fresh Cu/Al2O3 catalysts mainly have the
phases of the CuO and CuAl2O4 with the respective compositions of 0.41 and 0.59 on the
Cu/Al, 0.10 and 0.90 on the Cu/mesoAl and 0.09 and 0.91 on the Cu/mesoAl(HT). This result
suggests that the Cu/mesoAl and Cu/mesoAl(HT) possess much stronger interactions
between copper crystallites and the Al2O3 surfaces by preferentially forming the copper
aluminate compared to the fresh Cu/Al. However, the compositions of the copper aluminate
after reaction were dramatically decreased from 0.59 to 0.40, 0.90 to 0.70, and 0.91 to 0.60
on the used Cu/Al, Cu/mesoAl, and Cu/mesoAl(HT), respectively. This result strongly
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suggests the partial reduction of copper aluminate to metallic copper during the reaction.
Nevertheless, the Cu/mesoAl and Cu/mesoAl(HT) still had an abundant copper aluminate
species even after reaction by simultaneously containing CuO, Cu2O, and CuAl2O4 species.
However, the metallic copper species was the main phase on the used Cu/Al due to a severe
aggregation of copper nanoparticles, which was more preferential species due to the larger
copper crystallite formation on the used Cu/Al. These characterization results strongly
suggests that the ordered mesoporous Al2O3 can sufficiently inhibit a complete reduction of
the confined copper nanoparticles to the metallic copper due to a newly formed strong
interaction with ordered mesoporous Al2O3 surfaces by preferentially forming the copper
aluminate species which still having acidic properties. Therefore, the enhanced catalytic
stability seems to be attributed to the less aggregation of copper nanoparticles during the
reaction on the Cu/mesoAl and Cu/mesoAl(HT) compared with the Cu/Al. In addition, the
larger white line area (AWL) on the used Cu/mesoAl and Cu/mesoAl(HT) with the respective
values of 6.92 and 6.24 compared to the Cu/Al with the value of 5.85 supports the thermally
stability of the newly formed copper aluminate. The k3ꢀweighted χ(k) EXAFS spectra of the
Cu/Al2O3 catalysts before and after reaction are displayed in Figure 7 and the results are
summarized in Table 1 with supporting Table S4. Two main peaks at around 1.0 ꢀ 1.5 and 2.0
ꢀ 2.6 Å were originated from first oxygen coordination shell (CuꢀO) around the absorbing
atoms and the second CuꢀCu contribution through the twoꢀstep diffusionꢀlimited reduction of
Cu2+ ꢀ Cu+ ꢀ Cu0, respectively.51,52 The observed different radial distances of the Cu/Al
with Cu/mesoAl and Cu/mesoAl(HT) as shown in Figure 7 were mainly caused by the
difficult reduction degree of the copper oxides due to a strong confinement effect. The second
peak of CuꢀCu on the Cu/Al was the main peak originated from the metal copper phase.
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However, the second peak of CuꢀCu on the Cu/mesoAl and Cu/mesoAl(HT) was mainly
attributed to the copper oxides such as CuAl2O4 species, since the peak position was similar
with the CuAl2O4 reference. These results strongly support that the degrees of SMSI are in
the order of Cu/mesoAl > Cu/mesoAl(HT) > Cu/Al, which are well corresponding with the
results of TPR, XRD, XPS and TEM analysis. Therefore, the first derivatives of the copper
Kꢀedge XANES spectra and the Fourier transform of k3ꢀweighted χ(k) EXAFS spectra as
shown in Figure 7 and Figure S11 clearly revealed the formation of the CuAl2O4 phase on
the used Cu/mesoAl and Cu/mesoAl(HT). Furthermore, the CNs of CuꢀO on the used
Cu/mesoAl and Cu/mesoAl(HT) were found to be around 3.0 ꢀ 3.3 without significant
differences from fresh ones, except for the Cu/Al with a large variation from 3.6 to 2.0 due to
a severe aggregation of copper nanoparticles instead of the thermally stable copper aluminate
formation. The CNs of the CuꢀCu on the used Cu/Al2O3 catalysts also showed a similar trend,
i.e., the CN of CuꢀCu on the used Cu/Al showed a larger value of 3.5, however, smaller CNs
of the CuꢀCu with the values of 0.9 – 1.2 were observed on the used Cu/mesoAl and
Cu/mesoAl(HT), which strongly suggest the stable maintenance of copper aluminate species
even after reaction. From the above XAFS analyses, the aggregation of copper nanoparticles
during the reaction seems to be less favorable than the formation of copper aluminate on the
Cu/mesoAl and Cu/mesoAl(HT) compared to the Cu/Al due to a spatial confinement effect of
copper nanoparticles in the ordered mesoporous Al2O3, which seem be mainly responsible for
the largely enhanced catalytic stability and activity. Therefore, we believe that the formation
of the interfacial CuAl2O4 layers between copper oxide (mainly Cu2O) crystallites and acid
sites of the ordered mesoporous Al2O3 as schematically proposed in Figure 5 (TEM analysis)
seems to be the main reason for the enhanced thermal stability of copper crystallites and
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catalytic activity by suppressing a thermal aggregation of copper nanoparticles inside of the
ordered mesopores, especially on the bifunctional Cu/mesoAl and Cu/mesoAl(HT).
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In summary, the supported copper nanoparticles on the bifunctional Cu/Al2O3 having an
ordered mesopore structures with an average pore size of ~ 4 nm can effectively suppress the
catalyst deactivation by forming the thermally stable copper nanoparticles inside of the
ordered mesopores through an efficient spatial confinement effect of copper crystallites in the
ordered mesoporous Al2O3 frameworks. The strongly interacted copper nanoparticles with the
solid acid mesoporous Al2O3 surfaces can also form interfacial CuAl2O4 layers partially, and
it can stabilize the copper nanoparticles through SMSI even under the reaction. These
positive characteristics of the ordered mesoporous Al2O3 can enhance the catalyst stability
and DME selectivity as well. Therefore, the proposed novel catalyst preparation method by
forming thermally stable and spatially confined copper nanoparticles inside of an ordered
mesoporous Al2O3 can be applied for preparing an efficient stable hybridized catalytic system
of a direct DME synthesis from syngas without significant catalyst deactivation.
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4. Conclusions
The direct synthesis of DME from syngas using the bifunctional Cu/Al2O3 catalyst, where
the copper nanoparticles were impregnated on the ordered mesoporous solid acid Al2O3, was
investigated to elucidate the spatial confinement effect of the ordered mesoporous Al2O3 for a
significant suppression of the copper nanoparticle aggregation during the reaction. The
supported copper nanoparticles in the ordered mesoporous Al2O3 were wellꢀdispersed by
partially forming the interfacial CuAl2O4 species, which is thermally stable species by
providing new acid sites as well. The bifunctional Cu/mesoAl catalyst having a regular pore
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size of ~ 4 nm showed the homogeneous dispersion of the copper nanoparticles by forming
thermally stable copper nanoparticles inside of the ordered mesoporous Al2O3 structures
through the spatial confinement effect. The strongly interacted copper nanoparticles with the
acid sites of the ordered mesoporous Al2O3 surfaces was newly formed through the formation
of the interfacial CuAl2O4 layers, and it also provides some efficient acid sites for the
methanol dehydration to DME. The highly dispersed copper nanoparticles with strong
interactions on the abundant acid sites of the ordered mesoporous Al2O3 largely enhanced the
catalyst stability and DME selectivity as well.
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Acknowledgments
The authors acknowledge the financial support from the National Research Foundation of
Korea (NRF) grant funded by the Korea government (NRFꢀ2014R1A1A2A16055557, NRFꢀ
2015M3D3A1A01064898 and NRFꢀ2016M3D3A1A01913253). This work was supported by
the Korea Institute of Energy Technology Evaluation and Planning (KETEP) with Project
numbers of 20132010201750 of the Ministry of Knowledge Economy (MKE) of Korea. This
work was also financially supported by an institutional program grant (2E26570ꢀ16ꢀ037)
from the Korean Institute of Science and Technology (KIST) and by Fundamental Research
Program of the Korea Institute of Materials Science (PNK4310).
Supporting information
The supporting information contains additional characterization results of the SEM images of
Al and mesoAl (Figure S1), N2 adsorptionꢀdesorption patterns (Figure S2), small angle Xꢀ
ray scattering (SAXS) patterns (Figure S3), H2ꢀTPR profiles at a different ramping rate of 2
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