Effect of Γ-alumina nanorods on CO hydrogenation to higher alcohols over lithium-promoted CuZn-based catalysts

Article abstract of DOI:10.1016/j.apcata.2017.10.001

To achieve high catalytic activities and long-term stability to produce higher alcohols via CO hydrogenation, the catalytic activities were tuned by controlling the loading amounts of γ-alumina nanorods and Al³⁺ ions added to modify Cu-Zn catalysts promoted with Li. The selectivity of higher alcohols and the CO conversion to higher alcohols over a Li-modified Cu_0.45Zn_0.45Al_0.1 catalyst supported on 10% nanorods were 1.8 and 2.7 times higher than those with a Cu-Zn catalyst without nanorods and Al³⁺ ions, respectively. The introduction of the thermally and chemically stable γ-Al₂O₃ nanorod support and of Al³⁺ to the modified catalysts improves the catalytic activities by decreasing the crystalline size of CuO and increasing the total basicity. Along with the nanorods, a refractory CuAl₂O₄ formed by the thermal reaction of CuO and Al³⁺ enhances the long-term stability by increasing the resistance to sintering of the catalyst.

Full text of DOI:10.1016/j.apcata.2017.10.001

Accepted Manuscript
Title: Effect of ␥-Alumina Nanorods on CO Hydrogenation to
Higher Alcohols over Lithium-Promoted CuZn-based
Catalysts
Authors: SuMin Choi, YoungJong Kang, SangWoo Kim
PII:
S0926-860X(17)30479-9
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2017.10.001
APCATA 16432
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Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
29-6-2017
21-9-2017
4-10-2017
Please cite this article as: SuMin Choi, YoungJong Kang, SangWoo
Kim, Effect of ␥-Alumina Nanorods on CO Hydrogenation to Higher
Alcohols over Lithium-Promoted CuZn-based Catalysts, Applied Catalysis A,
General https://doi.org/10.1016/j.apcata.2017.10.001
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Effect of γ-Alumina Nanorods on CO Hydrogenation to Higher Alcohols
over Lithium-Promoted CuZn-based Catalysts
SuMin Choi1,2, YoungJong Kang2, SangWoo Kim1,3,*
1Clean Energy Research Center, Korea Institute of Science and Technology, Hwarangno 14-gil
5, Sungbuk-Gu, Seoul 02792, Republic of Korea
2Department of Chemistry, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul,
Republic of Korea
3Division of Energy & Environment Technology, KIST school, University of Science and
Technology (UST), Seoul 02792, Republic of Korea.
* Corresponding author.
E-mail *swkim@kist.re.kr
Tel: +82-958-5526
1

Graphical abstract
Highlights
•
•
•
•
The introduction of γ-Al2O3 nanorods and Al3+ ions to a Li-promoted Cu-Zn-based catalyst promotes
CO hydrogenation to higher alcohols.
The enhanced catalytic performance of the catalyst is due to the reduction in CuO crystallite size,
incurred by the thermal degradation of hydrotalcite precursors, and due to the increase in basicity.
In particular, a Li-Cu0.45Zn0.45Al0.1 catalyst supported on γ-Al2O3 nanorods is the most optimized for the
synthesis of 2-methyl branched alcohols among the higher alcohol products.
The modified catalyst exhibits excellent thermal stability over long-term reactions due to the sintering
resistance of nanorods and a refractory CuAl2O4.
ABSTRACT
To achieve high catalytic activities and long-term stability to produce higher alcohols via CO hydrogenation,
the catalytic activities were tuned by controlling the loading amounts of -alumina nanorods and Al3+ ions added
to modify Cu-Zn catalysts promoted with Li. The selectivity of higher alcohols and the CO conversion to higher
alcohols over a Li-modified Cu0.45Zn0.45Al0.1 catalyst supported on 10% nanorods were 1.8 and 2.7 times higher
than those with a Cu-Zn catalyst without nanorods and Al3+ ions, respectively. The introduction of the thermally
and chemically stable γ-Al2O3 nanorod support and of Al3+ to the modified catalysts improves the catalytic
2

activities by decreasing the crystalline size of CuO and increasing the total basicity. Along with the nanorods, a
refractory CuAl2O4 formed by the thermal reaction of CuO and Al3+ enhances the long-term stability by increasing
the resistance to sintering of the catalyst.
Keywords: Cu-Zn-Al Catalyst, Alumina nanorods, CO hydrogenation, Higher alcohols, Li-Promoter, Syngas.
1. Introduction
Global warming and environmental pollution have led researchers to devote much attention to the synthesis of
higher alcohols. To mitigate the strong dependence on natural oil, alternative fuels such as coal, natural gas,
biomass, and shale gas have attracted enormous interest [2,3]. Higher alcohols (C2+OH), which are converted to
carbon-based feedstocks (biomass, coal, natural gas) [3–5], have been used as alternative fuels and chemical
feedstocks [6–10]. To synthesize higher alcohols, many researchers have experimented using different types of
catalysts, which are classified as Rh-based catalysts [9,11,12], Mo-based catalysts [9,13,14], and Cu-based
catalysts such as modified Fischer-Tropsch (FT) catalysts [9,15] and alkali-promoted methanol catalysts [9,16–
18].
Catalysts containing Rh have a high selectivity to synthesize higher alcohols using syngas, but they are too
expensive for industrial application [11,19]. Mo-based catalysts (MoS2, Mo2C) [20,21] are resistant to sulfur and
less sensitive to CO2, but the catalytic process requires high pressure and high temperature; they also require a
long reaction time to reach a stable state [14].
Cu-based catalysts are relatively inexpensive compared to Rh- or Mo-based catalysts and are of interest because
of their ability to synthesize higher alcohols by a simple method that involves doping alkali into the Cu-based
catalysts [16,17]. Modified FT catalysts (Cu-Co, Cu-Fe, and Cu-Ni) are promising with respect to higher alcohol
synthesis using syngas [22,23]. However, since the Cu-Co catalyst has a low catalytic stability in long-term
operation and a low total alcohol selectivity, it is unsuitable for use in industrial processes [15,24]. The Cu-Fe
catalyst has a phase change between Cu and Fe, which weakens or interrupts the interaction between the two
3

transition metals, which adversely affects the CO hydrogenation reaction [25]. The Cu-Ni catalyst is more suitable
for methanol synthesis than higher alcohol synthesis due to the formation of the binary metal alloy [26]. All the
modified FT catalysts are inappropriate because hydrocarbons are produced in large amounts, which decrease the
content of higher alcohols during FT reactions.
Cu-Zn-based methanol catalysts promoted with alkali metals have been widely used because they are highly
effective in the synthesis of higher alcohols due to their high activity in mild environments. However, since the
catalysts tend to be sintered at high temperature and high pressure, the catalytic activities tend to drop sharply
[16,17]. Thus, to prevent deactivation from sintering of the catalysts, metal additives such as Al, Cr, Ce, and Mn
are added to the Cu-Zn catalysts [16,26,27]. Addition of these metals to the catalysts increases the specific surface
area to prevent aggregation, allowing the activity and selectivity of the catalyst to be improved. Cu-Zn-Cr and Cu-
Zn-Ce catalysts operate at high pressure (up to 7.6 and 20 MPa, respectively) to synthesize alcohols [16,27]. Cu-
Zn-Mn shows an increase in the production of methanol and higher alcohol selectivity with lower overall activity
[26]. Cu-Zn-Al catalysts, which can be used under milder conditions, show an optimum catalytic performance in
the synthesis of higher alcohols [26].
To increase the catalytic activity of Cu-based catalysts, it is important to choose an appropriate alkali promoter
for higher conversion and selectivity. The role of the alkali enhancer is to neutralize the surface acidity of the
catalyst and/or support and to facilitate the adsorption of CO molecules on the catalyst surface [28]. However, if
too much enhancer is used, the active sites are blocked, which lowers the catalytic activity [29]. In general, the
alkali promoters used are mainly Na, K, Rb, Cs, and Li in Cu-based catalysts [30]. K mainly increases the
selectivity to methanol and ethanol [23], and Na has the highest selectivity of higher alcohol at the beginning only
because of its low stability [23]. Cs predominantly produces methanol [23]. Cu-based catalysts promoted with Li
have lower activity than those with other promoters but have increased selectivity for higher alcohols around
300°C and are more stable during the reaction [23,31].
On the other hand, the catalytic activity and selectivity for higher alcohols are not solely dependent on the
addition of alkaline ions but also depend on the counter anions of the salt and the support. The counter anion of
the alkali salt chosen for alcohol selectivity should be easily removed by the alkali, and the alkali salt should be
evenly distributed over the catalyst. For even dispersion of the catalyst, the alkali promoter with a relatively low
pKa value was used such as LiNO3; this is readily soluble in water [32].
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In general, the support enhances the stabilization of the active species and promoters for the synthesis of higher
alcohols; promotes the donation or exchange of hydrogen or oxygen; donates and receives electrons of metal
particles; and controls the degree of reduction and dispersion [33]. Particularly in CO hydrogenation, strong
interactions between supports and active metals have a significant influence on the synthesis of higher alcohols
[34]. A strong metal-support interaction (SMSI) leads to changes of phase and morphology triggered by partial
reduction of the active oxide or oxide supports. The formation of oxygen vacancy defects due to the reduction can
cause strong bonding between the active metal and the surface of supports [35,36]. The shaping and sizing of
active metals and supports can also exhibit strong SMSI due to their large surface-to-volume ratio [37]. In addition,
doping small amounts of specific cations such as Li+, Na+, and Sr2+ to the catalyst affects the mutual solid-solid
interactions between the components [38,39]. It indicates the necessity of a strong SMSIs catalyst, which provides
a significant effect on the catalytic reaction.
Thus, we investigate the effect of alumina nanorods (ARs) as a support with a large surface-to-volume ratio on
the activity of Cu-Zn-based catalysts promoted by Li and on the long-term thermal stability for effective
conversion of syngas to higher alcohols.
2. Experimental section
2.1. Synthesis of γ-Al2O3 Nanorods
All chemicals were analytical grade reagents and were used as received without further purification. All
experiments were conducted in air. The γ-Al2O3 nanorods as a support were synthesized using a boehmite
precursor (see Supplementary Fig. S1 and S2) by a hydrothermal route [40]. A 1.00 M solution of aluminum
chloride hexahydrate (AlCl3·6H2O, Sigma Aldrich, 99%) was prepared with 100 ml distilled water under vigorous
stirring at room temperature. Then, 1.00 M of 50 ml ammonia solution (28% NH3 in water, Junsei) was added to
1.00 M of 50 ml sodium hydroxide (NaOH, Yukari, 96%) solution to prepare a mixed precipitant. The precipitant
solution was added dropwise to the aluminum chloride solution, and the pH value was adjusted to 5 with ammonia
solution. The mixture was transferred to a 330 ml Teflon-lined autoclave and kept at 200◦C for 24 h. After the
reaction, the resulting solution was centrifuged and washed three times with distilled water and ethanol then dried
5

at 80◦C in a vacuum oven. The dried boehmite powder was calcinated in a box furnace and heated to 500◦C
(3◦C/min) for 2 h in air.
2.2. Fabrication of Catalysts
For the catalyst preparation by co-precipitation (see Supplementary Fig. S3), solutions of nitrate and AR were
prepared separately according to the molar ratio as shown in Table 1.
In order to investigate the effect of the introduction of ARs on Cu-Zn based catalysts, the catalysts containing
with molar ratio of Cu:Zn = 5:5 and the amount of ARs was varied through co-precipitation as shown in Table 1
and Supplementary Table S1. The prepared ARs of 0−15 mol% were added to 125 ml of nitrate solutions according
to the compositions of Table 1 and dispersed, and the catalysts were prepared by co-precipitation method. The
nitrate precursors used were a 1.00 M solution of copper (II) nitrate trihydrate (Cu(NO3)2·3H2O, Junsei, 99%) and
zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Sigma Aldrich, 98%) (Cu:Zn = 5:5). 1.2 M Na2CO3 solution as a
precipitant and the prepared solution added dropwise to hot distilled water (100 ml) at 65◦C. During precipitation,
the pH was kept at 6.5−7.5. After aging for 6 h, the precipitates were filtered and washed several times with
distilled water and then dried at 80◦C for 24 h in a vacuum oven. After grinding, the dried catalyst was calcinated
at 350◦C (3◦C/min) under air for 4 h. The lithium promoter (0.5 wt%, Li) was doped to the resulting solids by
vacuum impregnation with an aqueous solution of lithium nitrate (LiNO3, Junsei, 98%) followed by drying and
final calcination in air for 4 h at 350◦C. To investigate the synergistic effect of the Al3+ ions and the ARs, the
catalyst was prepared with a Cu:Zn ratio of 5:5 with 5−15 mol% of ARs, and the amount of Al3+ was varied
through co-precipitation.
The catalysts were synthesized as described above except that the ratio of Cu:Zn:Al was changed with
aluminum nitrate nonahydrate (Al(NO3)3, Junsei, 99%).
2.3 Catalyst characterization
The X-ray diffraction (XRD) patterns of the prepared catalysts were obtained using a diffractometer (M/S,
Shimadzu Instruments, Japan) with Ni-filtered Cu Kα radiation at 40 kV and 30 mA. The Debye-Scherrer equation
6

was used to calculate the average metal particle size of all the catalysts.
Thermogravimetry/Differential Thermal Analysis (TG/DTA) was used to determine the thermal properties
using an SDT Q 600 instrument (TA Instruments, USA). The samples were loaded into an alumina pan and heated
to 900◦C at a heating rate of 10◦C/min with flowing air.
The BET surface areas of the Li-promoted catalysts were measured using an automated gas sorption system
(Moon Sorp II, KIST); N2 adsorption-desorption isotherms were obtained at 77 K. The pore diameters and
volumes were calculated using the Barrett-Joyner-Halenda (BJH) method. The results are shown in Table 1.
The reduction characteristics were examined by temperature programmed reduction (TPR, AutoChem II 2920,
Micromeritics, USA). The Li-promoted and calcined catalyst samples (100 mg) were placed between quartz wool
in a U-type quartz reactor and pretreated under a flowing Ar stream at 300◦C for 2 h to remove physically absorbed
impurities and water, followed by cooling at room temperature under pure Ar gas. After pretreatment, the
temperature was raised to 900◦C at a heating rate of 10◦C/min in a 5% H2/He flow (30 ml/min).
X-ray photoelectron spectroscopy (XPS) was performed with a PHI 5000 Versa Probe (Ulvac-PHI)
spectrometer using monochromated Al Kα (1486.6 eV) radiation. The binding energy was calibrated using the
adventitious carbon peak at 284.6 eV. The background pressure in the analysis chamber was maintained in the
2.0 × 10-7 Pa range. Survey scans were recorded for surface elemental analysis (pass energy 117.4 eV), with high
resolution spectra recorded at a pass energy of 46.95 eV.
Temperature Programmed Desorption (TPD) of CO2 was performed with a Thermal Conductivity Detector
(TCD)-TPD system (AutoChem II 2920, Micromeritics, USA). The Li-promoted and calcined catalyst samples
(100 mg) were charged between quartz wool in a U-type quartz reactor, preheated under flowing He at 300◦C for
2 h, and cooled to room temperature. After pretreatment, 10% CO2/He as the absorption gas was passed at 60◦C
for 1 h and the samples were flushed with flowing He at 60◦C for 1 h. The temperature was raised to 900◦C at a
heating rate of 10◦C/min in a He flow. TCD was used to measure the amount of CO2 absorbed on the catalysts.
To examine the morphology of the catalysts, scanning electron microscopy (SEM) imaging and energy
dispersive analysis of X-rays (EDAX) studies were performed with an Inspect F50 instrument. For the SEM-
EDAX measurements, samples were prepared on a carbon tape and coated with Pt.
7

2.4 Catalytic performance
Carbon monoxide hydrogenation to higher alcohols was measured in a stainless steel fixed-bed flow reactor
(0.25 m height × 0.0102 m I.D., 0.0127 m O.D.) with 0.5 g catalysts. Prior to the reaction, the Li-promoted catalyst
was reduced under pure H2 at a flow rate of 30 ml/min at 300◦C for 3 h. Then, the reactor was cooled to 493 K and
the pressure of the reaction mixture was raised to 4.5 MPa. The reactions were carried out at 300◦C. The molar
ratio of the reactants (H2/CO) was maintained at 1 throughout the studies and the space velocity (GHSV) at 4,000
ml Hg. The products were analyzed with an online gas chromatograph (HP -7890 A) with Porapak-Q and HP-
Inno wax columns connected to a TCD and a Flame Ionization Detector (FID), respectively. The former column
was used for the analysis of H2, CO, CH4, methanol, and CO2, while the latter was used for the analysis of
hydrocarbons and alcohols (C2–C5).
CO conversion was calculated as the mole percentage of carbon monoxide converted to products.
CO conversion (mol%) = 퐶푂푖푛 − 퐶푂표푢푡 × 100
퐶푂표푢푡
The carbon selectivity was defined as the moles of carbon in a given product divided by the total moles of carbon
converted in both the liquid and gas streams.
3. Results and discussion
3.1. Catalyst characterization
The crystalline phases were identified from the XRD patterns of as-prepared precursors and calcined catalysts
shown in Fig. 1(A–D). Fig. 1(A) shows the change in the XRD patterns of the precursors according to the amount
of ARs added to the catalysts. With added ARs, the aurichalcite phase and the malachite phase coexist, while only
the aurichalcite phase exists in CZA(0)/0, which is defined in Table 1. Especially, the malachite phase is the best
developed in CZA(0)/10. Fig. 1(B) shows XRD patterns of the catalysts with 10 mol% ARs and different amounts
8

of Al3+. The CuZnAl hydrotalcite phase with a layered structure is formed in the catalysts with the addition of
Al3+, and the associated physicochemical properties are listed in Table 1. Three phases coexist in all the samples
and the hydrotalcite phase is the most well-developed in CZA(12)/10 among other samples. Fig. 1(C) and (D)
exhibit XRD patterns of catalysts that had calcined precursors and were finally calcined after Li promotion at
350°C. In the patterns, CuO appears at 2θ near 35° and Cu2O near 36°. The crystallite sizes of CuO and Cu2O
decrease as the content of ARs added to the Li-promoted Cu-Zn catalysts in Fig. 1(C) increases. The sizes of CuO
and Cu2O further decrease as a result of addition of Al3+ to the Li-promoted Cu-Zn catalysts containing 10 mol%
ARs, which results in an increased surface area. The increase in the specific surface area is related to the brucite-
2-
type hydrotalcite phase, in which Al3+ is partially substituted for anions (OH- and CO3 ) in the interlayer [41]
during formation and pyrolysis.
Fig. 2(A and B) and (C and D) show DTA and TG curves of the as-prepared precursors of the Cu-Zn catalyst
and the Cu-Zn-Al catalyst, respectively. The endothermal peaks of the CZA(0)/0 catalyst appeared at around
242°C, corresponding to aurichalcite. For all the samples with added ARs (CZA(0)/5, 10, and 15), endothermal
peaks appeared at around 375°C, corresponding to malachite. The results indicate that only aurichalcite exists in
the catalyst CZA(0)/0, but aurichalcite and malachite coexist in the CZA(0) catalysts containing ARs [42]. In Fig.
2(B), the endothermal peaks at 139‒153°C are assigned to the hydrotalcite due to the addition of Al3+.
Thermogravimetry (TG) curves in Fig. 2(C) and (D) show that the weight losses are assigned to the removal of
structural water at 100‒150°C [43], the first emission of CO2 at around 370°C, and the second decarbonation at
about 480°C.
Fig. 3(A) and (B) show the H2 TPR profiles for the L-CZA(0) catalysts containing ARs and L-CZA containing
10 mol% ARs, respectively. In Fig. 3(A), L-CZA(0)/0 has peaks at 300°C with a broad shoulder at 389°C, but only
one peak appeared for the L-CZA(0) catalysts containing ARs. The peaks at 260‒300°C are assigned to the
reduction of CuO and/or Cu2O to Cu with H2. The reduction temperature is shifted to a lower temperature as the
content of ARs increases, which is due to the decrease in the crystallite size of CuO and Cu2O as shown in Table
1. In particular, L-CZA (0)/0, which has a Cu2O size of 8.99 nm, has a large shoulder peak. The addition of ARs
not only reduces the crystallite size of CuO but also results in good dispersion [44]. Therefore, as the amount of
added ARs increases, the reduction temperature shifts to a lower value, and L-CZA(0)/15 is reduced at 261°C,
9

which is approximately 40°C lower than L-CZA(0)/0. The highly dispersed CuO is reduced at a much lower
temperature than bulk CuO, which makes the catalyst active in the CO hydrogenation reaction in the synthesis of
higher alcohols from syngas [26].
In Fig. 3(B), the TPR peaks shift from 281 to 300°C with the addition of Al3+ ions. Although the CuO crystallite
size of L-CZA(0)/10 is nearly twice that of L-CZA(5)/10, the reduction peaks appeared at almost the same
temperature, at 281 and 283°C, respectively. When the amount of Al3+ was increased to above 10 mol%, the peaks
shifted significantly to higher temperatures, even though the crystallite size of CuO slightly increased compared
to that of 5 mol% Al3+. This means that refractory material can be formed in the catalysts containing Al3+.
Fig. 4(A–D) shows the oxidation state of copper and the surface compositions of different catalysts determined
with XPS. The Cu 2p3/2 core-level spectra of the four catalysts (L-CZA(0)/0, L-CZA(0)/10, L-CZA(10)/10, and
L-CZA(12)/10) are displayed in Fig. 4, and the elemental analysis of the surface of the catalysts is shown in Table
2. From Fig. 4, it is clear that the catalysts display three components: a peak at around 932 eV assigned to Cu2O
[44], a peak at around 933 eV assigned to CuO, and a peak at about 935 eV assigned to a CuAl2O4 spinel type
compound [3]. A satellite peak is also observed at about 943 eV, which is the fingerprint of Cu2+ ions. In the
catalysts containing 10 mol% of ARs, listed in Table 2, the peak at around 935 eV is attributed to Cu2+ in CuAl2O4,
and the peak at around 933 eV is assigned to Cu2+ in CuO. This indicates that amorphous CuAl2O4, which is
invisible in the XRD patterns of Fig. 1(C) and (D), is formed due to the strong interaction between CuO and Al3+
ions, resulting in a greater shift in the reduction temperature at the TPR peaks in Fig. 3(B) despite the decrease in
the CuO crystallite size.
Fig. 5 shows the effect of the phase change of the catalyst on the CuO crystallite size and the BET surface area
at different AR contents. In Fig. 5(A), only the aurichalcite phase is observed in the catalyst without ARs, but the
malachite phase is newly observed in the catalysts containing ARs. As the appearance of the malachite phase
decreases the crystallite size of CuO from 11.42 to 6.34 nm, it increases the specific surface area from 20 to 38
m2/g. As the content of ARs increases, the BET surface area, which is related to the CO conversion, also increases
due to formation of the malachite phase. Fig. 5(B) shows the effect of the phase change incurred by addition of
Al3+ to the catalysts, including the effect of the ARs on the CuO crystallite size and BET surface area. In the
catalyst containing 5 mol% of Al3+, the CuO size decreased by half and the BET surface area increased, which
10

was closely related to the formation of the hydrotalcite phase and its pyrolysis. The BET value increases with the
Al3+ content at more than 5%, with a slight increase in the crystallite size of CuO. This is attributed to the formation
of highly porous bodies by pyrolysis of the hydrotalcite phase, as shown in the XRD results in Fig. 1(B).
3.2. Catalytic performance in CO hydrogenation to higher alcohols
For the synthesis of higher alcohols, Li-promoted Cu-Zn catalysts with varying mol% of ARs and Al3+ were
reacted at 300°C and 4.5 MPa, and the conversion of CO and selectivity to the carbon-based products are shown
in Table 3 and Fig. 6.
Fig. 6(A) and (B) show the selectivity to C2+OH according to the basicity at different contents of ARs and Al3+.
In Fig. 6(A), the selectivity to C2+OH increased with the basicity. Among the L-CZA catalysts with ARs, the
selectivity was the highest at 8.7% with 15 mol% ARs. In Fig. 6(B), as the basicity increased with the addition of
Al3+, the C2+OH selectivity increased up to 10 mol% Al3+, peaking at 17.55%, and then decreased at over 10 mol%
Al3+. The fact that the selectivity to C2+OH at 15 mol% decreases even though the basicity is 1.5 times higher than
that at 10 mol% indicates that an optimal basicity is needed for maximum selectivity.
Fig. 6(C) and (D) show the CO conversion according to the BET surface area at different AR and Al3+ contents.
In Fig. 6(C), the conversion of CO increased with the BET surface area, which is related to the decrease in the
CuO crystallite size due to the addition of ARs. This indicates that the dispersion of Cu directly reacting with CO
tends to be increased by reducing the size of CuO coated on the surface of the ARs [45]. The CO conversion was
the highest at 12.77% at 15 mol% of ARs. In Fig. 6(D), the CO conversion was the highest at approximately 15%
at 10 mol% Al3+ and then decreased to 12.6% at 12 mol% Al3+. The CO conversion also increased with the BET
surface area, which is related to the size of CuO formed by thermal decomposition of the hydrotalcite phase,
except at 12 mol% Al3+. The drop in the CO conversion at 12 mol% Al3+ is attributed to the increase in less-active
amorphous CuAl2O4, as judged by the increase in the ratio of CuAl2O4/CuO shown in Table 2.
Fig. 7(A) and (B) show the distribution of alcohols in the reaction products obtained from L-CZA(0)/0, L-
CZA(0)/10, L-CZA(10)/0, and L-CZA(10)/10 catalysts for a 6 h reaction. The 2-methyl branched alcohols such
11

as tert-pentyl alcohol and isopropyl alcohol were significantly increased and the primary product, methanol, was
much decreased in the Li-promoted catalysts containing ARs, particularly ARs and Al3+. It represents that the
incorporation of nanorods and Al3+, which lower the crystallite size of Cu and increase the basicity, play a crucial
role in increasing the production of higher alcohols. Fig. 8 shows the morphologies of the catalysts before and
after the reaction. Unlike the precursor of L-CZA(0)/0 without ARs, the precursors containing ARs are found to
be rod-shaped, and the calcined and the reacted catalysts are also observed to have this shape. This is attributed
to desintering by the formation of amorphous CuAl2O4 derived from the thermal decomposition of hydrotalcite
precursors.
Fig. 9 shows the results of long-term stability tests on L-CZA(10)/0 and L-CZA(10)/10 at 300°C for 40 h to
investigate the effect of ARs on thermal stability. The conversions of H2 and CO for 1 h and 40 h reactions are
listed in Table 4. L-CZA(10)/10 containing ARs exhibits less catalytic deactivation and is more suitable for the
long-term reaction than L-CZA(10)/0 without ARs. This demonstrates that the highly thermal resistant ARs helps
prevent the catalyst from sintering and deactivating as the reaction proceeds. On the other hand, as shown in the
XPS spectral results in Table 2 and Supplementary Table S2, the ratio of CuAl2O4/CuO in L-CZA(10)/10 is almost
twice as large as that in L-CZA(10)/0 (see the detailed characteristics of L-CZA(10)/0 in Supplementary Fig. S4–
S7, and Supplementary Table S1–S3), indicating that the thermally stable CuAl2O4 phase also improves the
stability of the catalyst. Thus, it is found that the addition of ARs and the formation of CuAl2O4 prevent catalyst
deactivation and enhance long-term catalytic stability.
The XRD patterns of the L-CZA(10)/0 and L-CZA(10)/10 catalysts with or without ARs after long-term
stability tests are shown in Supplementary Fig. S8. Before the reaction, the reduction of CuO to Cu was performed,
and this was sequentially reacted for 40 h. After reaction, Cu and ZnO phases were observed. The crystallite size
of Cu in L-CZA(10)/10 was smaller than in L-CZA(10)/0, as shown in Table 5. Fig. 10 shows the TG-DSC curves
of the reacted catalysts under air. The initial weight losses below 130°C are caused by dehydration and the weight
gains above 130°C are due to oxidation of the catalysts in the order Cu → Cu2O → CuO. Amorphous and graphite
carbon were identified in TG-DSC, while the carbon species were not detected in the XRD patterns in Fig. 10. In
Table 5, very low amounts of carbon below 2.0% were produced, indicating that neither catalyst is affected by
coking. Therefore, it is found that the stability of the L-CZA(10)/10 catalyst is significantly improved by the
12

desintering of catalyst by the amorphous CuAl2O4 phase and nanorods, not by the coking resistance.
4. Conclusion
Addition of ARs and Al3+ ions to the Cu-Zn-based catalyst formed malachite and hydrotalcite phases in
precursors that contained only the aurichalcite phase and affected the crystallite size and surface area. This phase
change reduced the crystallite size of CuO and increased the BET surface area and basicity to improve CO
conversion and C2+OH selectivity. In particular, L-CZA(10)/10 showed the highest catalytic activity with a CO
conversion of 15% and a C2+OH selectivity of 17.5%, which was the most optimized for the synthesis of 2-methyl
branched alcohols among the higher alcohol products. An appropriate ratio of CuAl2O4/CuO stabilizes the
modified catalyst containing ARs and maintains the activity of the catalyst even when it is operated for 40 hours,
which is attributed to desintering of the catalyst by the amorphous CuAl2O4 and nanorods.
Acknowledgments
We gratefully acknowledge financial support of this work from the Korea Institute of Science and Technology
(Grant No. 2E27500).
13

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15

List of Figure Captions
Fig. 1. XRD patterns of the Cu-Zn & Cu-Zn-Al catalysts: (A), (B) as-prepared catalyst (precursor); (C), (D) Li-
promoted & calcinated catalyst.
Fig. 2. DTA profiles of precursors to (A) Cu-Zn catalysts; (B) Cu-Zn-Al catalysts. TG curves of precursors (C)
Cu-Zn catalysts; (D) Cu-Zn-Al catalysts (A: aurichalcite, M: malachite, H: hydrotalcite).
Fig. 3. TPR patterns of Li-promoted (A) Cu-Zn catalysts with varying mol% of γ-Al2O3 nanorods; (B) Cu-Zn-Al
catalysts with varying mol% of Al3+.
Fig. 4. XPS spectra of (A) L-CZA(0)/0; (B) L-CZA(0)/10; (C) L-CZA(10)/10; (D) L-CZA(12)/10.
Fig. 5. Effect of crystalline phases on CuO crystallite size and BET surface area.
Fig. 6. (A), (B) effect of basicity, (C), (D) effect of BET surface area on catalytic performance (Reaction conditions:
H2/CO = 1; 4.5 MPa; 300°C, GHSV = 4000 h-1; 6 h). (A) & (C) L-CZA(0)/0–15; (B) & (D) L-CZA(0–12)/10.
Fig. 7. Product distribution of alcohols over Li-promoted Cu-Zn & Cu-Zn-Al catalysts after 6 h reaction (Reaction
conditions: H2/CO = 1; 4.5 MPa; 300°C, GHSV = 4000 h-1).
Fig. 8. SEM images of catalysts: the samples (A) as-prepared, (B) Li-doped, and (C) spent catalysts (Reaction
conditions: H2/CO = 1; 4.5 MPa; 300°C, GHSV = 4000 h-1; 6 h).
Fig. 9. Long-term stability test of (A) L-CZA(10)/0; (B) L-CZA(10)/10 (Reaction conditions: H2/CO = 1; 4.5
MPa; 300°C; GHSV = 4,000 h-1; 40 h).
Fig. 10. TG-DSC of after long-term stability reaction. (A) L-CZA(10)/0; (B) L-CZA(10)/10.
16

Fig. 1. S.M. Choi, et al.
Title: Effect of γ-Alumina Nanorods on CO Hydrogenation to Higher Alcohol over Lithium-Promoted CuZn-
based Catalysts.
17

Fig. 2. S.M. Choi, et al.
Title: Effect of γ-Alumina Nanorods on CO Hydrogenation to Higher Alcohol over Lithium-Promoted CuZn-
based Catalysts.
18

Fig. 3. S.M. Choi, et al.
Title: Effect of γ-Alumina Nanorods on CO Hydrogenation to Higher Alcohol over Lithium-Promoted CuZn-
based Catalysts.
19

Fig. 4. S.M. Choi, et al.
Title: Effect of γ-Alumina Nanorods on CO Hydrogenation to Higher Alcohol over Lithium-Promoted CuZn-
based Catalysts.
20

Fig. 5. S.M. Choi, et al.
Title: Effect of γ-Alumina Nanorods on CO Hydrogenation to Higher Alcohol over Lithium-Promoted CuZn-
based Catalysts.
21

Fig. 6. S.M. Choi, et al.
Title: Effect of γ-Alumina Nanorods on CO Hydrogenation to Higher Alcohol over Lithium-Promoted CuZn-
based Catalysts.
22

Fig. 7. S.M. Choi, et al.
Title: Effect of γ-Alumina Nanorods on CO Hydrogenation to Higher Alcohol over Lithium-Promoted CuZn-
based Catalysts.
23

Fig. 8. S.M. Choi, et al.
Title: Effect of γ-Alumina Nanorods on CO Hydrogenation to Higher Alcohol over Lithium-Promoted CuZn-
based Catalysts.
24

Fig. 9. S.M. Choi, et al.
Title: Effect of γ-Alumina Nanorods on CO Hydrogenation to Higher Alcohol over Lithium-Promoted CuZn-
based Catalysts.
25

Fig. 10. S.M. Choi, et al.
Title: Effect of γ-Alumina Nanorods on CO Hydrogenation to Higher Alcohol over Lithium-Promoted CuZn-
based Catalysts.
26

List of Table Captions
Table 1 Sample names, compositions of raw materials, and physicochemical properties of Li-promoted Cu-Zn-
based catalysts.
Table 2 Binding energies of Cu 2p3/2 photoemission.
Table 3 Catalytic performance of Li-promoted Cu-Zn catalysts with varying mol% of ARs and Al3+.
Table 4 The effect of ARs in Li-promoted Cu-Zn-Al catalysts on long-term stability test.
Table 5 Crystallite sizes of Cu and quantitative data of carbon deposited on the catalysts after long-term stability
tests.
27

Table 1
Sample names, compositions of raw materials, and physicochemical properties of Li-promoted Cu-Zn-based catalysts.
**Crystallite sizes
(nm)
*BET
Surface
area
γ-Al2O3
Li - Cu : Zn : Al
*Pore
*Pore
***Basicity
(mmol/g)
nanorods/
(CuZnAl)
(mol%)
Catalyst name
**Phase
Diameter Volume
Li
Cu:Zn:Al
(nm)
(cc/g)
Cu2O
(m2/g)
CuO
(wt%)
(mol%)
L-CZA(0)/0
L-CZA(0)/5
L-CZA(0)/10
L-CZA(0)/15
L-CZA(5)/10
L-CZA(10)/10
L-CZA(12)/10
0.5
0.5
0.5
0.5
0.5
0.5
0.5
50 : 50 : 0
0
5
20.54
40.12
33.32
36.57
39.98
42.08
21.81
26.19
0.20
0.28
0.32
0.37
0.28
0.28
0.35
11.42
7.91
6.96
6.34
3.66
4.04
4.74
8.99
6.38
6.12
6.08
6.81
5.85
3.72
0.45
0.461
0.47
0.54
0.60
1.05
1.52
A
50 : 50 : 0
50 : 50 : 0
50 : 50 : 0
A+M
A+M
A+M
33.90
35.32
37.88
10
15
47.5 : 47.5 : 5 10
A+M+H 40.92
A+M+H 52.51
H+A+M 54.87
45 : 45 : 10
44 : 44 : 12
10
10
*Textural properties were studied using low temperature (77 K) N2-physisooprtion using Moonsorp-1.
**Phases were identified by XRD analysis. A: aurichalcite, M: malachite, and H: hydrotalcite.
***Basicity was measured by CO2-TPD method.
28

Table 2
Binding energies of Cu 2p3/2 photoemission.
BE Cu 2p3/2
Area ratio
Cu+
Cu2+
Cu2+
Catalyst name
CuAl2O4/CuO
(Cu2O)
(CuO)
(CuAl2O4)
932.45
932.68
932.15
931.97
933.71
934.23
933.31
933.26
-
-
L-CZA(0)/0
935.80
934.92
935.01
0.17
0.48
0.51
L-CZA(0)/10
L-CZA(10)/10
L-CZA(12)/10
29