Continuous precipitation of Cu/ZnO/Al2O3 catalysts for methanol synthesis in microstructured reactors with alternative precipitating agents

Article abstract of DOI:10.1016/j.apcata.2012.06.040

Ternary Cu/ZnO/Al₂O₃ catalyst systems were systematically prepared by innovative synthesis routes in microstructured synthesis setups, allowing to study different types of micromixers. The coprecipitation in the slit plate and valve-assisted mixers was operated continuously under exact control of pH, temperature, concentration and ageing time. Due to the enhanced surface to volume ratio in microstructured reactors, a precise temperature control and efficient mixing of the reactants are enabled. The precipitation was performed with sodium, ammonium and potassium carbonate as well as sodium hydroxide. To evaluate the potential of the novel synthesis routes, reference samples in a conventional batch process were prepared. The catalysts were synthesized according to the constant pH method with a molar ratio of 60:30:10 for copper, zinc and aluminum. The synthesis routes applied have a significant influence on the structures of hydroxycarbonate precursors and on the catalytic activity in methanol synthesis. XRD patterns of hydroxycarbonate precursors from the synthesis in micromixers, especially using ammonium carbonate as precipitating agent, display high crystallinity and sharp reflections of malachite and rosasite. Cu/ZnO/Al₂O₃ catalysts prepared in continuously operated micromixers in general show higher specific copper surface areas than catalysts prepared in conventional batch processes. The highest methanol productivity of all prepared catalyst systems was observed with the catalyst precipitated in the slit plate mixer with ammonium carbonate.

Full text of DOI:10.1016/j.apcata.2012.06.040

Applied Catalysis A: General 450 (2013) 1–12
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Continuous precipitation of Cu/ZnO/Al O catalysts for methanol synthesis in
2
3
microstructured reactors with alternative precipitating agents
∗
Georg Simson, Eko Prasetyo, Stefanie Reiner, Olaf Hinrichsen
Catalysis Research Center and Chemistry Department, Technische Universität München, Lichtenbergstraße 4, D-85748 Garching b. München, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Ternary Cu/ZnO/Al O catalyst systems were systematically prepared by innovative synthesis routes in
2
3
Received 15 February 2012
Received in revised form 21 June 2012
Accepted 22 June 2012
microstructured synthesis setups, allowing to study different types of micromixers. The coprecipitation
in the slit plate and valve-assisted mixers was operated continuously under exact control of pH, temper-
ature, concentration and ageing time. Due to the enhanced surface to volume ratio in microstructured
reactors, a precise temperature control and efficient mixing of the reactants are enabled. The precipita-
tion was performed with sodium, ammonium and potassium carbonate as well as sodium hydroxide. To
evaluate the potential of the novel synthesis routes, reference samples in a conventional batch process
were prepared. The catalysts were synthesized according to the constant pH method with a molar ratio
of 60:30:10 for copper, zinc and aluminum. The synthesis routes applied have a significant influence on
the structures of hydroxycarbonate precursors and on the catalytic activity in methanol synthesis. XRD
patterns of hydroxycarbonate precursors from the synthesis in micromixers, especially using ammonium
carbonate as precipitating agent, display high crystallinity and sharp reflections of malachite and rosasite.
Cu/ZnO/Al2O3 catalysts prepared in continuously operated micromixers in general show higher specific
copper surface areas than catalysts prepared in conventional batch processes. The highest methanol pro-
ductivity of all prepared catalyst systems was observed with the catalyst precipitated in the slit plate
mixer with ammonium carbonate.
Available online 3 July 2012
Keywords:
Continuous precipitation
Microstructured mixers
Hydroxycarbonate precursors
Cu/ZnO/Al2O3 catalysts
Methanol synthesis
©
2012 Published by Elsevier B.V.
1
. Introduction
carbonate. Formed hydroxycarbonate precursors are subsequently
calcined at 573–773 K and mixed metal oxides are produced. Mild
reduction of CuO to Cu0 in diluted H2 prior to reaction, results
in the active catalyst species. In industrial batch catalyst produc-
tion, it is challenging to control the synthesis parameters during
precipitation, however a considerable number of studies proved
the crucial influence of pH value [13,14], temperature [13,15],
ageing conditions [10,16] and additionally temperature and con-
centration gradients [10] on precursor structures and thus later
on the catalytic activity. Depending on each synthesis parameter,
the resulting hydroxycarbonate precursors show varying proper-
ties, particularly in crystal size and structure and hence in optical,
electronic and particularly catalytic properties. Therefore, the exact
control of process parameters during the synthesis of catalysts is
decisive for the catalytic activity and stability.
With respect to the control of process parameters, microreaction
engineering offers significant advantages. Thus, huge develop-
ments and clear progress have been made over the last few years.
Because of their outstanding characteristics in heat and mass
transfer and high degree of automation, microstructured reac-
tors provide a promising alternative to common batch synthesis
routes. Micromixers are already widely used in pharmacy, biotech-
nology and production of fine chemicals [17–19]. Especially their
The ternary Cu/ZnO/Al O3 catalyst system is industrially
2
employed in low-pressure methanol synthesis from a CO/CO /H -
containing synthesis gas. The adding of Al O to the previously used
binary Cu/ZnO system leads to a remarkable increase in catalyst
activity and, in particular, stability. Al O is believed to inhibit ther-
2
2
2
3
2
3
mal sintering of copper particles [1–3]. Crystal phase structures of
the ternary hydroxycarbonate precursors such as malachite, ros-
asite and aurichalcite strongly affect the activity of the calcined
catalyst [4–8]. The complex ternary catalyst system is believed to be
subject to a so called ‘chemical memory’. That means that calcined
precursors which show similar structures have different activities
in methanol synthesis, depending on the crystal phase struc-
tures of the preceding hydroxycarbonate precursors [9–12]. Thus,
precipitation parameters are crucial for the activity of the later
catalyst. Conventionally, catalyst synthesis is performed by batch
precipitation with aqueous solutions of metal nitrates and sodium
∗ Corresponding author. Tel.: +49 89 289 13232; fax: +49 89 289 13513.
E-mail address: Olaf.Hinrichsen@ch.tum.de (O. Hinrichsen).
URL: http://www.tc1.ch.tum.de (O. Hinrichsen).
0
926-860X/$ – see front matter © 2012 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.apcata.2012.06.040

2
G. Simson et al. / Applied Catalysis A: General 450 (2013) 1–12
superior heat transfer properties are applied in highly exother-
mic reactions, such as the fluoridation of organic compounds using
elemental fluorine [20], to give a prominent example. Besides gas
phase and liquid phase reactions, first attempts in the application of
micromixers in synthesis of solid particles have been made. Schur
et al. [21] performed principle experiments on the binary Cu/ZnO
system. They showed that catalysts, synthesized in continuously
operated micromixers, can even provide superior properties com-
pared to batch synthesis routes. In solid particle synthesis, low
temperature and concentration gradients are determining for cat-
alyst structure and thus for catalytic activity. Recently, Kaluza and
Muhler [22,23] showed that a continuous precipitation method
using a micromixer is a powerful tool to investigate precipitation
reactions, also in combination with the subsequent steps such as
ageing and spray drying [24].
Besides the Cu/ZnO system, only few solid phase syntheses have
been performed in microstructured continuously operated systems
with respect to catalyst manufacturing and formulation. Commer-
cially available micromixers, for example from Ehrfeld Microtechnik
BTS, use a multi lamination principle, which is based on the split-
ting of the reactants into thin liquid layers, which are successively
recombined in layers. Splitting into layers is achieved by fine slits
in the central microstructure. An aperture plate realizes the down-
stream in the mixer comb and is attached vertically to the slit
plate in the central microstructure. For solid particle synthesis an
additional back pressure valve, located before the mixing zone, pre-
vents back streaming of the reactants and thus a blockage of the
microstructures.
Process parameters such as temperature or pH value as well as
the choice of the precipitation agent itself can have an enormous
influence on structural and catalytic properties of the final catalyst.
Commercially, sodium carbonate is used to precipitate the metal
nitrates, however residual sodium is known to poison the catalyst
and to enhance sintering of the metallic particles in methanol syn-
thesis. It is proposed that a high sodium content leads to a lowering
of the metal-support interaction of copper and ZnO [25,26], thus
promoting deactivation. Moreover, sodium in hydroxycarbonate
precursors causes more pronounced growth of ZnO particles during
calcination and reduction, decreasing the catalytically active cop-
per surface [27]. Studies using alternative precipitation agents such
as ammonium carbonate, potassium carbonate or sodium hydrox-
ide can provide deeper insight into deactivation processes and
activity–structure relationship.
Fig. 1. Schematic drawing of piping and instrumentation of the precipitation set-up
with the valve-assisted mixer.
60:30:10 for Cu:Zn:Al and 300 mL of a precipitating agent solution
(0.18 mol/L) were pumped (HPLC pump, Smartline Pump 100, Fa.
Knauer) into the microstructured reaction setups (Fig. 1) with a
constant flow rate of 35 mL/min. The solutions were preheated at
338 K, whereas ammonium carbonate was kept at 303 K to prevent
thermal decomposition. Before entering the mixing chamber, fur-
ther heating to 338 K took place in two heating modules, followed
by the precipitating reaction in the micromixers. The produced pre-
cipitates were collected in a stirred and heated glass tank reactor
with a starting volume of 400 mL demineralized water. The pro-
cess in either the slit plate mixer or the valve-assisted mixer was
operated continuously under exact control of pH value (pH 6 and
pH 7), temperature (338 K), concentration and ageing time (0 h, 1 h,
2 h). During the precipitation and the ageing process the solution
of metal nitrates was added drop wise to the slurry to maintain a
constant pH (± 0.05 unit), controlled by an automatic titrator (DMS-
Titrino 716, Metrohm AG, dosing volume is about 10 mL). The total
amount of the solution added for pH maintenance was below 1%
compared to the amount of solution fed to the micromixer. After
ageing, the precipitates were recovered by filtration under vacuum
and washed three times with 400 mL ultra pure water each and then
dried in air at 383 K for 20 h. The calcination of the milled hydroxy-
carbonate precursors was performed at 623 K for 4 h (heated up
at a rate of 5 K/min) under synthetic air (20% O2/80% N2) with a
flow rate of 100 mL/min. Moreover, ternary Cu/ZnO/Al2O3 cata-
lysts were prepared in a conventional batch process as reported
elsewhere [10,13] under the same reaction conditions and used as
reference material.
The purpose of this study is to evaluate the feasibility of
microstructured continuously operated synthesis routes for the
precipitation of ternary Cu/ZnO/Al O3 precursors and to compare
2
their activity in methanol synthesis to both an industrial reference
catalyst and batch synthesis routes. Varying the precipitation agent
provides information on catalyst poisoning and especially on the
formation of materials with high surface areas of the active compo-
nent. Application of various synthesis conditions with varying pH
values and ageing periods gives insight into the processes during
solid particle synthesis in micromixers.
2
. Experimental
2.2. Characterization
2.1. Sample preparation
The catalyst samples were characterized by inductively cou-
Ternary Cu/ZnO/Al O3 catalyst systems were prepared in
pled plasma optical emission spectrometry (ICP-OES), elemental
analysis, X-ray diffraction (XRD), nitrogen physisorption mea-
surements, transmission electron microscopy (TEM) and thermal
analysis (TG/DTG). The metal contents of the calcined precur-
sors were determined by ICP-OES, carried out on a SpectroFlame
type FTMOA81A (Spectro Analytical Instruments). Trace element
analysis of sodium and potassium in the calcined precursors was
2
microstructured mixers by coprecipitation of a solution of metal
nitrates and an alkaline solution analogous to the coprecipitation
method described by various research groups [10,13,14]. The pre-
cipitation was performed with sodium, ammonium and potassium
carbonate as well as sodium hydroxide. 300 mL of a solution of cop-
per, zinc and aluminum nitrates (0.15 mol/L) with a molar ratio of

G. Simson et al. / Applied Catalysis A: General 450 (2013) 1–12
3
performed on a Varian AA280FS Fast Sequential Atomic Absorp-
tion Spectrometer. The elemental analysis of carbon, hydrogen
and nitrogen in the calcined precursors was performed on a vario
EL CHNOS element analyzer. XRD patterns of the hydroxycarbon-
ate precursors and the calcined precursors were performed on a
Phillips X ˇı PERT (type 3040/60 MPD Pro DY 1467) diffractometer
with Cu K␣ radiation (1.54056 A˚ ) operating at 45 kV and 40 mA.
◦
The XRD patterns were recorded in the range of 3.9–71.0 (2ꢀ).
Nitrogen physisorption measurements were realized in a NOVA
4
000e Surface Area & Pore Size Analyzer (Quantachrome Instru-
ments). Before analysis, all samples were activated under vacuum
at 523 K for 2 h. The 10 points nitrogen adsorption measurements
were recorded at 78 K. The BET surface area was calculated from
the adsorption data in the relative pressure interval from 0.05 to
0.3. TEM images were taken with a JEOL JEM 100CX transmission
electron microscope operated at 100 kV, equipped with a tung-
sten cathode. The samples were dispersed ultrasonically in ethanol
and then deposited onto a continuous carbon film supported on a
copper mesh grid. The images were processed with ImageJ image
processing software to obtain the particle size distribution of the
samples. In general, 100 particles per sample were counted. Ther-
mogravimetric analyses (TG/DTG) were carried out in a STA 409C
Netzsch thermobalance coupled to a Pfeiffer Vacuum OmniStar GSD
Fig. 2. XRD patterns of hydroxycarbonate precursors prepared with K2CO3 with the
slit plate mixer at pH value of 6.
3
01 O quadrupole mass spectrometer for quantitative data moni-
catalyst. The benchmark catalyst achieved a specific copper surface
2
2
toring. Measurements were performed with approximately 60 mg
area SCu = 22.4 m /gcat before and SCu = 21.6 m /gcat after standard
test conditions. The methanol productivity of the benchmark cat-
alyst at 483 K was determined to PMeOH = 5.5 mmolMeOH/(gcat s) at
of the samples in open Al O3 crucibles in argon flow (50 mL/min)
2
with a heating rate of 5 K/min from 298 to 1073 K.
0
.5 MPa and PMeOH = 12.5 mmolMeOH/(gcat s) at 1 MPa.
2.3. Specific copper surface area and catalytic measurements
3. Results and discussion
The specific copper surface area and the catalytic activity mea-
surements were carried out in a single-pass fixed-bed reactor
hosted in a flow setup equivalent to that described in refer-
ences [28,29]. The following gases and mixtures (purities) were
used: He (99.9999%), H2 (99.9999%), CO (99.999%), CO2 (99.997%),
H /He (2.2% H , 99.9999%), CO /He (6% CO 99.9995%), MeOH/He
3.1. Characterization of hydroxycarbonate precursors
The phase composition of the precursors was qualitatively
investigated by powder diffraction. XRD patterns of hydroxycar-
bonate precursors prepared in the synthesis route using a slit plate
2
2
2
2
(
2941 ppm MeOH), N O/He (1% N O, 99.999%) and a mixture of
mixer with K CO3 as precipitating agent are illustrated in Fig. 2.
2
2
2
5
9.5% H , 8% CO , 6% CO and 1.5% Ar in residual He used as feed
Ageing duration leads to negligible crystalline improvements in
precursors. Reflections representing malachite (Cu (OH) CO ) or
2
2
gas/synthesis gas for methanol synthesis. The analysis of the prod-
uct gas mixture was performed by a calibrated quadrupole mass
spectrometer (Pfeiffer Vacuum OmniStar GSD 301 O). 200 mg of
the 250–355 ␮m sieve fraction of the samples were filled into the
reactor and secured with quartz wool. The tubular reactor is a glass
lined U-tube (GLT) with an inner diameter of 4 mm, which results in
a bed height of the catalyst of about 24 mm. The catalyst samples
were reduced in two steps under mild conditions to prevent the
copper particles from sintering [30–32]. In the first step the sam-
2
2
3
rosasite ((Cu;Zn) (OH) CO ) in the non-aged precursor are even
2
2
3
sharper than those in aged precursors. In the production of non-
aged precursors, water could not be fully quenched from the system
and the following drying step at 383 K may have promoted fur-
ther ageing of the wet precursors at even higher temperatures. On
the other hand, precursors aged for 1 or 2 h show similar diffrac-
tograms. This evidence is supported by the results of Bems et al.
[10] who also found that the crystal growth ended after 30 min
ageing. Moreover, the aurichalcite phase ((Cu;Zn)5(OH) (CO ) ),
ples were treated with a flow of 10 N mL/min of a H /He mixture
2
6
3 2
◦
◦
at 448 K for 15 h, applying a heating rate of 1 K/min. To ensure full
reduction, the catalyst is treated in a pure flow of H2 for 100 min
at 513 K. The reduction of the catalyst is followed by purging the
samples with He for 30 min to remove any impurities.
represented by reflections at 2ꢀ = 13 and 24 , is less observed in
precursors aged for at least 1 h, while the malachite and rosasite
bands retain most of their intensity. This finding is in accordance
with the studies of Waller et al. [35], who proposed the dissolution
of aurichalcite during ageing in mother liquor.
The specific copper surface area of the Cu/ZnO/Al O3 catalyst
2
was measured, using reactive frontal chromatography with nitrous
oxide (N O RFC). In general, the N O RFC measurement was per-
Fig. 3 displays the XRD patterns of hydroxycarbonate precur-
sors prepared with Na CO3 in various synthesis routes performed.
2
2
2
formed corresponding to the method introduced by Chinchen et al.
33] under slightly more moderate reaction conditions [34]. At the
Precursors generated with micromixers exhibit bolder copper zinc
hydroxycarbonate phases, especially aurichalcite, malachite and
rosasite along with a less amorphous structure. These effects are
observed particularly in precursors prepared with the slit plate
mixer based on its split and recombine principle, enabling a high
degree of micromixing. These trends are observed in all precur-
sors prepared with the micromixers, regardless of the precipitating
agent applied. Small aurichalcite reflections at characteristic angles
[
end of the N O RFC the gas flow is switched to He for 30 min in order
2
to flush the catalyst bed. Then, the catalyst was heated to 483 K with
a heating rate of 1 K/min in a flow of 78 N mL/min synthesis gas. In
a typical run, methanol synthesis measurements were performed
under steady-state conditions at 0.5 MPa and 1 MPa for 6 h. The Cu
surface area was measured for freshly reduced and used samples.
An industrial benchmark catalyst, which is produced batchwise,
was used as reference material. The catalytic activity and the cop-
per surface area of all measured samples were compared to this
◦
◦
2ꢀ = 13 and 24 are observed in precursors prepared with the slit
plate mixer with Na CO3 and (NH ) CO as precipitating agents.
2
4
2
3
Since the restructuring processes, including the dissolution of

4
G. Simson et al. / Applied Catalysis A: General 450 (2013) 1–12
Fig. 3. XRD patterns of hydroxycarbonate precursors prepared with Na2CO3 at a pH
value of 6, aged for 2 h.
Fig. 5. XRD patterns of hydroxycarbonate precursors prepared in a slit plate mixer
at a pH value of 6, aged for 2 h. Comparison of precipitation with different carbonate
species is shown.
aurichalcite phases, take place mainly during the ageing process,
slightly lower aurichalcite bands in all precursors precipitated
with the slit plate mixer cannot be directly correlated with the
micromixing effects during the coprecipitation in the mixer, but
rather with ageing processes in the mother liquor.
displays slightly more pronounced aurichalcite bands, particularly
the band at 2ꢀ = 13 . Moreover, the malachite and rosasite bands
◦
for these two precipitating agents K CO and Na CO show a strong
2
3
2
3
resemblance. Similar to Na CO3 or (NH ) CO samples, non-aged
The effect of the precipitating agent applied regarding crys-
tallinity and exposing of phases in hydroxycarbonate precursors
2
4
2
3
precursors prepared with K CO3 exhibit a less amorphous struc-
2
ture. Non-aged precursors are still susceptible to ageing since water
cannot be fully extracted during the filtration process. The aurichal-
cite bands are also observed weaker in all catalysts aged for 2 h
regardless of the precipitating agents used. This evidence confirms
the dissolution of aurichalcite as already discussed in the previous
section.
is illustrated in Figs. 4 and 5. Synthesis with (NH ) CO gener-
4
2
3
ates precursors with high crystallinity and distinctive reflections
Fig. 4). Furthermore, the sample prepared with (NH ) CO dis-
(
4
2
3
plays intense reflections of both highly active phases malachite and
rosasite as well as slight reflections of the less active aurichalcite
phase. The precipitation with Na CO results in precursors display-
2
3
◦
◦
◦
The pH value in precipitation reaction and ageing has an
important impact on the structure of the hydroxycarbonate pre-
ing reflections of aurichalcite at 2ꢀ = 13 and 24 more intensely
compared to reflections of malachite at 2ꢀ = 32 and rosasite at
ꢀ = 34–38 and 55 . In general, (NH ) CO produces catalysts
◦
◦
cursors. XRD patterns for precursors prepared with (NH ) CO in
2
4 2 3
4
2
3
as precipitating agents using a valve-assisted mixer are illustrated
in Fig. 6. Higher pH values ensure a high degree of crystallinity
along with sharper, bolder malachite and rosasite bands. The
with the least amorphous structures and high malachite and rosa-
site contents (Fig. 5). On the other hand, both K CO3 and Na CO3
2
2
generate catalysts with similar crystallinity, independent of the
synthesis route applied in precipitation reaction. However, K CO3
2
Fig. 4. XRD patterns of hydroxycarbonate precursors prepared in a batch process at
pH value of 7, aged for 2 h. Comparison of results using precipitating agents Na2CO3
and (NH4)2CO3 is shown.
Fig. 6. XRD patterns of hydroxycarbonate precursors prepared in a valve-assisted
mixer with (NH4)2CO3, aged for 2 h. Influence of pH value in precipitation is shown.

G. Simson et al. / Applied Catalysis A: General 450 (2013) 1–12
5
pH value of precipitation affects all precursors prepared with
Na CO , (NH ) CO , K CO and NaOH regardless of the synthesis
2
3
4
2
3
2
3
route applied. A basic pH value amplifies the crystallinity of the
hydroxycarbonate precursors, especially of those prepared with
micromixers. Wassermann et al. [36] and Li und Inui [15] studied
the effect of the pH value onto the Cu/ZnO/Al O3 system in batch
2
synthesis and correlated the precipitation pH value with the
phase composition of the precursors. When carbonate salts are
added to an aqueous solution of metal nitrates Cu(NO ), Zn(NO )
3
3 2
and Al(NO3)3 under pH control, the solid formation starts with
the initial precipitation of amorphous copper hydroxide (Eq.
(
1)). The copper hydroxide then reacts with zinc and carbonate
ions, forming a mixed amorphous hydroxycarbonate precursor
Eq. (2)). Concurrently, the more slowly solidifying zinc forms a
(
hydroxyl-rich material together with the residual copper ions.
Cu2 + 2OH− → [Cu(OH) ]s
+
(1)
(2)
(3)
(4)
2
Fig. 7. Thermal analysis of hydroxycarbonate precursors prepared in a slit plate
mixer at a pH value of 6, aged for 2 h. Comparison of precipitating agents is shown.
Cu(OH) ]s + Zn + CO32− → [(Cu, Zn) (OH) (CO )]s
M(OH)x]s + NO3− → [M(OH)x−1(NO )]s + OH−
2+
[
[
[
2
2
2
3
3
due to the release of H O and CO2 is similar to that of Na CO .
2
2
3
NO ]s + HCO3− → [M(OH)x−1(HCO )]s + NO3
−
M(OH)
x−1
3
3
The second mass loss of about 21% occurs as a continuous mass
+
loss of H O, CO, CO2 and NO between 473 and 623 K. The third
The concentration of H ions in the mother liquor increases with
2
and fourth mass losses at 792 and 843 K are caused primarily by
the decrease of the pH value applied during precipitation and age-
ing process, whereby the concentration of carbonate ions is directly
affected. Due to the shift of equilibrium, the concentration of car-
bonate ions is reduced by protonation to hydrogen carbonate (Eq.
5)) followed by further protonation to carbonic acid and decom-
position to CO2 and H O.
high-temperature carbonates (HT-CO ). Different from the single
3
step mass loss of HT-CO3 in precursors prepared with Na CO , the
2
3
double step mass loss indicates two different carbonates with two
different decomposition temperatures present in the precursors.
The results of thermogravimetric analysis of precursors prepared
(
2
with K CO3 (Fig. 7) follow the trend of precursors prepared with
2
+
2−
−
ꢀ HCO3
H
+ CO3
(5)
the conventional precipitating agent Na CO . The first mass loss
2
3
of 2.5% is correlated with H O and CO2 present in hydroxycarbon-
ate precursors. The sharp mass loss of 20% at 649 K comprises of
2
Therefore, the reorganization of the precipitate (Eq. (3) and Eq.
4)) is hindered at acid pH values. The reorganization of the solid
(
mainly the evolution of H O, CO, CO2 and NO. The decomposi-
2
eventually leads to the formation of zincian malachite and aurichal-
cite from either georgeite or the amorphous hydroxyl-rich material.
As a result, high concentration of carbonate ions at a basic pH value
facilitates the formation of more zincian malachite, rosasite and
aurichalcite, resulting in less amorphous products.
In thermogravimetric analysis the thermal decomposition reac-
tion of hydroxycarbonate precursors elapses in three main steps
analogous to the results presented by Melian-Cabrera et al. [37].
The first step shows the release of crystallization water present
in the hydroxycarbonate structure as well as weakly adsorbed
CO2 at a temperature around 400–420 K followed by the elimi-
tion of HT-CO3 occurs in two steps at 794 and 844 K. Similar to
the (NH ) CO samples, the decomposition of precursors prepared
4
2
3
with K CO3 indicates two different carbonates present in the pre-
2
cursors. The thermal decomposition of precursors prepared with
NaOH elapses in two main steps according to the different chemical
system compared to precipitation with carbonate species. This evi-
dence is in accordance with the results presented in Ref. [38]. In the
temperature range of 383–413 K crystallization water is released
from the sample. The decomposition of hydroxide compounds and
gerhardtite associated with the release of H O, NO, NO2 and O2
2
occur at 503–533 K. Both mass losses can be attributed with a sharp
nation of structural hydroxyl groups with the release of H O and
2
band at 384 K (2.4%) to the release of H O and at 524 K (22.4%) to the
2
decomposition of rosasite, malachite, aurichalcite and other car-
bonate containing phases, forming CO2 at about 620 K. Finally,
the decomposition of high-temperature carbonates occurs in the
range of 700–900 K. High-temperature carbonates are attributed to
amplify the interaction of active metal species and support material
decomposition of hydroxides and gerhardtite. Further temperature
increase above 1073 K reveals another mass loss of the catalyst sam-
ples, which released mainly O , CO and CO2 from the system. The
2
composition of the evolved gases points to the reducing reaction of
copper (II) oxide (CuO) to copper (I) oxide (Cu O) or even elemen-
2
(
Cu–ZnO) [10]. As a result, the sintering process of copper particles
tal copper with formation of brass [39], as indicated by the reddish
brown color of the sample after thermogravimetric analysis.
In general, the decomposition behavior of catalysts prepared
with carbonate species as precipitating agents is similar. Due to
the different chemical composition, the decomposition behavior of
NaOH samples deviates according to expectations from the behav-
ior of carbonate species. The precipitation reaction in the presented
different synthesis routes results in precursors with a comparable
decomposition behavior.
is reduced and thus more active catalysts are obtained. The decom-
position of precursors prepared with Na CO3 occurs in these three
2
steps with the first mass loss of 2.5% associated with the release
of crystallization water and weakly adsorbed CO2 at a tempera-
ture of 416 K (Fig. 7). The second mass loss of 20% over a broad
temperature range of about 523–623 K is mainly due to the decom-
position of hydroxycarbonate structures and nitrates which are
still left after the washing step, releasing mainly H O, CO, CO2 and
2
NO. The decomposition reaction of the so-called high-temperature
carbonates is responsible for the third mass loss of 5% at 847 K,
as indicated by CO and CO2 detected in the mass spectrometer.
Thermogravimetric analysis results of precursors prepared with
3.2. Characterization of calcined precursors
(
NH ) CO in a slit plate mixer (Fig. 7) display a similarity to the
The metal contents of the ternary Cu/ZnO/Al O3 catalysts pre-
4
2
3
2
precursors prepared with Na CO3 except a broader mass loss in
temperature range of about 473–623 K. The first mass loss of 3%
pared at different pH values were determined. Table 1, for example,
summarizes the results for the samples prepared at pH 6. For the
2

6
G. Simson et al. / Applied Catalysis A: General 450 (2013) 1–12
Table 1
Metal contents of Cu/ZnO/Al2O3 catalysts prepared in synthesis route “slit plate
mixer” at pH 6 depending on duration of ageing period.
Precipitating agent
Na2CO3
Ageing duration
0 h
Cu [wt.%]
Zn [wt.%]
Al [wt.%]
63.3
62.5
62.6
24.7
25.9
25.9
12.0
11.6
11.5
1
2
h
h
(
NH4)2CO3
0 h
63.3
62.6
62.6
24.7
25.9
25.9
12.0
11.5
11.5
1
2
h
h
K2CO3
NaOH
0 h
65.0
63.4
63.5
24.0
26.0
25.8
11.0
10.6
10.7
1
2
h
h
0 h
70.9
70.4
70.4
16.9
18.0
18.1
12.2
11.6
11.5
1
2
h
h
precipitation with carbonate species the copper contents were
measured in the range of 62.5–64.0 mol%, while zinc and aluminum
was detected in the range of at 24.5–26.0 mol% and 11–12 mol%,
respectively. Hence, different synthesis routes do not affect the
metal contents in the produced catalysts significantly. The molar
ratio of Cu:Zn:Al has been determined to 63:25:12, clearly differ-
ent from the deployed molar ratio in the synthesis process. Samples
prepared at pH 7 (results are not shown here) attain the deployed
ratio of 60:30:10 also very close. Precursors prepared with ammo-
nium carbonate at pH 7 showed a slightly lower copper content
than the other precursors. The molar ratio of 59 mol% copper in the
Fig. 8. BET surface areas of calcined precursors prepared at a pH value of 6 depending
on synthesis route and precipitating agent applied.
carbonate ions. Regarding the molar ratios of copper, zinc and alu-
minum a minor influence on the ageing duration can be observed.
Samples without ageing (0 h) display a slightly higher copper con-
tent and a slightly lower zinc content than catalysts after ageing for
one or 2 h. These findings confirm the reaction model for the precip-
itation of hydroxycarbonate precursors explained earlier. The metal
contents of the catalysts are barely affected by the applied synthe-
sis route (batch-process, slit plate mixer and valve-assisted mixer)
and the duration of ageing time. The choice of the precipitating
agent and pH value in contrast exhibits a big influence on the reor-
ganization of the precipitate and the incorporation of zinc in the
primary precipitation product. A neutral pH value and the presence
of carbonate ions in the mother liquor promote the precipitation of
zinc.
(
NH ) CO samples can be explained by a “leaching” effect, caused
4 2 3
by the formation of copper ammine complexes during the precipi-
tation reaction. At pH 7 the release of NH3 from (NH ) CO cannot
4
2
3
be suppressed. The characteristic blue complex compound is solu-
ble in the mother liquor and thus removed from the system during
the washing step.
The effect of the pH value during precipitation reaction on the
final metal composition of precipitated catalysts is reported by
many authors [15,36]. The extended reaction model by Baltes et al.
The results of trace element analysis as well do not show any
influence of the applied synthesis routes (batch-process, slit plate
mixer and valve-assisted mixer), pH value or ageing time on residual
contents of carbon, nitrogen and hydrogen in the calcined precur-
sors. On an average the residual contents of carbon are in the range
of 0.6–1.0 wt.%. Hydrogen and nitrogen residues are detected in the
range of 0.3–0.5 wt.% and 0.1–0.3 wt.% or even below, respectively.
The degree of removal of reagents and byproducts is substantially
affected by the processes of washing and calcination rather than
by the synthesis route. Using a longer ageing period however leads
to a decreasing amount of residues of sodium and potassium in
catalyst samples. During the ageing process in mother liquor the
reorganization of the precipitate with ion exchange, dissolution and
re-crystallization takes place. Enclosures out of place like sodium
and potassium ions can be removed from the precipitate during this
[
(
13] is further modified for the used precipitating agents (Eqs.
1)–(5)). The reduced incorporation of zinc into the precipitate
can be correlated directly to the pH value and the precipitation
process. By adding a carbonates-containing basic solution to an
aqueous nitrate solution of copper, zinc and aluminum, the precip-
itation starts with the formation of amorphous copper hydroxide.
The shaped copper precipitate reacts with Zn and carbonate ions
to form mixed amorphous hydroxycarbonate precursors. Parallel
slowly crystallizing Zn ions form a precipitate rich in hydroxides
with residual copper. Using an acidic pH value in the precipitation
process the equilibrium of carbonate ions is relocated to hydrogen
carbonate and thus hinders the formation of a Zinc-rich precipi-
tate. The effect of reduced incorporation of Zn into the precipitate
is increasingly observed using sodium hydroxide as precipitating
agent. As displayed in Table 1, the molar ratio of copper, zinc and
aluminum in calcined precursors is determined to be 70:18:12 and
therefore far-off the deployed ratios. That is due to the fact, that
in precipitation with NaOH a completely different chemical system
exists compared to the precipitation reaction with the carbonate
species Na CO , (NH ) CO and K CO . In synthesis with NaOH
process. The separation of Na+ and K+ ions from the Cu/ZnO/Al O3
2
catalyst is important since both alkali metals act as catalyst poi-
son. Sodium and potassium cause an increasing sintering process
of the specific copper surface area associated with a decrease of
catalytic activity in methanol synthesis [10,41]. The critical limit of
0.15 wt.% for sodium was achieved in the majority of catalyst sam-
ples [42]. The driblet of residues is a good evidence for the sufficient
removal of metal nitrates, carbonates and particularly sodium and
potassium.
2
3
4
2
3
2
3
only the hydroxides of copper, zinc and aluminum are obtained.
Based on the absence of carbonate species, the incorporation of Zn
ions with carbonate ions into the initially formed copper hydrox-
ides is hindered. Another reason for the decreased precipitation
of Zn is the higher solubility of zinc hydroxide in acidic solutions
compared to hydroxides of copper and aluminum. The ratio of
aluminum in the precipitate is constant, since the precipitation
of aluminum hydroxide is independent of the concentration of
Fig. 8 summarizes the data of BET surface areas calculated from
nitrogen adsorption measurements for the calcined precursors.
Using ammonium and potassium carbonate-containing precipi-
2
tating agents, BET surface areas of 71–81 m /gcat are obtained,
which are equal to the surface areas achieved with conventional

G. Simson et al. / Applied Catalysis A: General 450 (2013) 1–12
7
Fig. 9. BET surface areas of calcined precursors prepared in a valve-assisted mixer
with NaOH at different pH values.
sodium carbonate precipitation within the accuracy of measure-
ment. The ternary catalysts from the synthesis with carbonate
species display an equal BET surface area independent from the
synthesis route applied. Using sodium hydroxide as precipitating
reagent leads to significantly smaller BET surface areas than in pre-
cipitation with carbonate species. Catalyst samples prepared in a
conventional batch process display the lowest BET surface areas
Fig. 10. XRD patterns of calcined precursors prepared with Na2CO3, aged for 2 h.
Comparison of synthesis route and pH value applied.
patterns of catalyst samples prepared in the synthesis route using
the slit plate mixer with Na2CO3, (NH4)2CO3, K2CO3 and NaOH as
precipitating agent. XRD patterns of catalyst samples prepared with
carbonate species do not indicate any differences. NaOH samples
show the same patterns like carbonate samples but display sharper
and more distinctive bands with higher intensities. In total, XRD
patterns of NaOH samples exhibit less amorphous structures than
carbonate samples.
Calcination transforms the hydroxycarbonates present in the
precursors into oxides while removing the nitrates and other
volatile impurities. At a temperature of 623 K almost all nitrates,
hydroxyl groups and carbonates present in the precursors were
successfully removed, as illustrated by the results of elemental
analysis. Another increase of the calcination temperature would
reduce the amount of high-temperature carbonates in the cata-
lyst and promote sintering processes of the copper surface area
2
with 18 m /gcat. The application of the synthesis routes using the
micromixers achieves catalysts with BET surface areas in the range
2
of 40–41 m /gcat.
Independent from precipitating agent and precipitation
method, a higher BET surface area at pH value of 7 can be observed
than in the precipitation process at pH 6. Applying carbonate
species in precipitation the increase of the surface area is with
2
2
–4 m /gcat rather small compared to precipitation with sodium
2
hydroxide with an increase of 16 m /gcat. Fig. 9 exemplarily shows
the detailed study of the effect of pH value on the BET surface area
during synthesis of hydroxycarbonate precursors with sodium
hydroxide in a micromixer. Syntheses were conducted at varying
pH values of 6.0, 6.5, 6.7 and 7.0. The illustration clearly displays
the increase of BET surface area with increasing pH values in
2
catalyst synthesis. Starting from a surface area of 40 m /gcat at pH
value 6.0 in precipitation, an increasing surface area is detected,
2
leading to 66 m /gcat at pH 7.0. Analogous to an acid base titration,
the maximum increase of BET surface area is observed in the step
from a pH value of 6.7 to pH 7.0. The influence of the duration of
ageing on the BET surface area is included in Fig. 9. As a general
trend samples with higher surface area are obtained with ageing
times up to for 2 h in the mother liquor. Moreover, it can be
observed in all synthesis routes that an ageing time of 2 h enlarges
the surface area of the Cu/ZnO/Al O3 catalysts up to 40%.
2
The XRD patterns of the calcined catalysts prepared with sodium
carbonate in all synthesis routes are shown in Fig. 10. The phase
structures of the calcined samples are mainly composed of tenorite
(
CuO), zincite (ZnO) and other copper, zinc and aluminum oxide
composites. The diffractograms of all calcined catalysts show simi-
larities separate from the synthesis route applied. The characteristic
◦
band for CuO at 2ꢀ = 39 displays the presence of tenorite with
more distinctive bands for those catalysts prepared at higher pH
values. The phase of zincite is observed by the characteristic bands
◦
◦
◦
◦
at 2ꢀ = 32 and 57 . Typical bands for alumina (2ꢀ = 19 , 45 ) are
not observed. That is probably due to the fact that alumina is
incorporated into the other phases of Cu/ZnO/Al O3 catalysts as
2
amorphous phase [43]. The introduction of amorphous Al O in the
2
3
Cu/ZnO/Al O catalyst systems leads to a higher surface area than
2
3
Fig. 11. XRD patterns of calcined precursors prepared in a slit plate mixer at a pH
in the binary Cu/ZnO system by up to 50%. Fig. 11 displays the XRD
value of 6, aged for 2 h. Comparison of precipitating agent is shown.

8
G. Simson et al. / Applied Catalysis A: General 450 (2013) 1–12
Table 2
Specific copper surface area of Cu/ZnO/Al2O3 catalysts prepared at a pH value of 6,
aged for 2 h – before and after methanol synthesis, Batch (batch process), SPM (slit
plate mixer), VAM (valve-assisted mixer).
2
2
2
Precipitating
agent
Batch [m /gcat]
SPM [m /gcat]
VAM [m /gcat]
Before
After
Before
After
Before
After
MeOH synthesis
MeOH synthesis
MeOH synthesis
Na2CO3
17.5
16.8
4.6
16.8
15.5
2.0
20.5
17.2
16.9
4.6
19.3
16.4
10.5
3.8
19.0
18.6
18.0
7.9
18.3
17.8
12.4
2.5
(
NH4)2CO3
K2CO3
NaOH
4.9
2.6
generated catalysts with a mean particle size of about 7.6 nm while
precipitation in the micromixers leads to mean particle sizes of
7
.5 nm and 7.1 nm, respectively. All synthesis routes display similar
Fig. 12. Thermal analysis of calcined precursors prepared in a slit plate mixer at a
particle size distribution for carbonate species. The results obtained
from TEM analysis show that the morphology as well as the parti-
cle size of the catalysts do not correlate directly with the synthesis
routes implemented. This can be regarded as a result of the ion
exchange reactions during the ageing in the mother liquor. Within
the series of precipitating reactions with carbonate agents no sig-
nificant differences in morphology and particle size distribution
can be observed. All samples contain round and oval particles with
mean particle sizes of 7–9 nm. With using NaOH as precipitating
agent, substantially bigger particles were obtained (Fig. 15). In the
batchwise synthesis route particles with a mean size of 54 nm were
produced, whereas in the micromixers slightly smaller particles
with mean sizes of 42 nm (slit plate mixer) and 49 nm (valve-
assisted mixer), respectively, were received. Hence, precipitation
with NaOH results in particles increased by a factor of 6–7 com-
pared to synthesis with carbonate species. A maximum particle
size of about 19 nm is found in catalysts prepared with carbonate
species, whereas a maximum of about 200 nm is observed in NaOH
pH value of 6, aged 2 h. Comparison of precipitating agents is shown.
and consequently lowering its catalytic activity. The calcined cat-
alysts were analyzed in thermogravimetric experiments to verify
the presence of high-temperature carbonates as well as to affirm
a complete decomposition of nitrates during the calcination pro-
cess. In general, calcined Cu/ZnO/Al O catalysts decompose in two
2
3
main steps (Fig. 12). The first mass loss occurring at 403–413 K is
due to the release of the crystallization water as well as the weakly
absorbed CO2 present in the Cu/Zn hydroxycarbonates structure.
This phenomenon can also be observed during the thermal decom-
position process of hydroxycarbonate precursors. The second mass
loss is associated with the decomposition of high-temperature
carbonates. Fig. 12 shows the thermal decomposition of calcined
precursors prepared in the synthesis route using the slit plate mixer
with different reagents. In case of calcined catalysts prepared with
Na CO , this occurs at 843 K with a mass loss of about 1.8%. Volatile
2
3
samples. Therefore particle size distributions in Cu/ZnO/Al O cata-
components detected by mass spectrometry were mainly CO and
2
3
lysts precipitated with NaOH are significantly broader. The particle
morphology of NaOH samples is very inconsistent (Fig. 13). Large
shapeless particles together with small particles with round or oval
shape can be observed.
CO . The presence of more HT-CO3 in calcined catalysts is ben-
eficial since it could delay the sintering process in the activated
2
Cu/ZnO/Al O catalyst. In catalysts prepared with (NH ) CO , the
2
3
4
2
3
decomposition reaction of HT-CO3 took place at a slightly lower
temperature (803 K) with a mass loss of about 1.6%. A slightly lower
HT-CO3 part was released from the system, although the residual
carbon content of the catalyst sample is similar to the Na CO sam-
In summary, the different synthesis routes applied in precipi-
tation leads after calcination to similar Cu/ZnO/Al O3 with respect
2
to elemental and structural composition. Preparation in both types
of micromixers generates particles with analogous properties com-
pared to the conventional batch process. However, the choice of the
precipitating agent and pH value in contrast exhibits a big influence
on the reorganization of the precipitate and therefore on catalyst
properties. Within synthesis with carbonate-containing precipitat-
ing agents no striking differences can be observed. In precipitation
with sodium hydroxide remarkable differences in catalyst proper-
ties are observed due to the different chemical system.
2
3
ple. This effect is probably caused by a strong HT-CO3 copper–zinc
interaction. Thermal analysis of catalysts precipitated with K CO3
2
matches that of Na CO . A slightly higher HT-CO mass loss of about
2
3
3
2
.2% was detected at a temperature of about 845 K. In calcined
samples prepared with NaOH, no further mass loss except the one
for water at 395 K can be observed at defined temperature bands.
Therefore, the thermal decomposition of gerhardtite structures and
hydroxides of copper, zinc and aluminum completely took place
during the calcination process. In synthesis with NaOH no com-
pounds comparable to the HT-CO3 in precipitation with carbonate
species can be obtained. Fig. 12 confirms the strong similarities
in the thermal decomposition of the calcined precursors particu-
larly of the carbonate species separate from the precipitating agent
applied in synthesis. As described before in Section 3.1 a further
temperature increase above 1073 K reveals another mass loss of
catalyst samples, related to the reducing reaction of copper (II)
3.3. Specific copper surface area and methanol synthesis
The specific copper surface areas of the prepared Cu/ZnO/Al O3
2
catalysts were quantitatively measured with reactive frontal
chromatography (RFC) using N O as described elsewhere [33,34].
2
The N O RFC measurement of the activated catalyst (Section 2.3)
2
was performed in a standardized procedure. The results of N O
2
oxide (CuO) to copper (I) oxide (Cu O) [39].
RFC measurements are summarized in Table 2. The specific copper
surface areas of the freshly reduced catalysts are always higher
2
Analysis with transmission electron microscopy demonstrates
no clear relationship between particle size and the synthesis route
applied in the precipitation process with carbonate species. Both
batch process and micromixers produce catalyst particles with sim-
ilar morphology as shown in Fig. 13. The corresponding particle size
distributions are drawn in Fig. 14. Synthesis in the batch process
than those of spent catalysts. Cu/ZnO/Al O3 catalysts prepared
2
with Na CO3 in micromixer achieve 91.5% and 84.8% compared
2
to the industrial reference material (Section 2.3). Using Na CO3
2
and (NH ) CO as precipitating agent generates catalysts with
4
2
3
high copper surface areas, whereas synthesis with NaOH produces

G. Simson et al. / Applied Catalysis A: General 450 (2013) 1–12
9
Fig. 13. TEM images of Cu/ZnO/Al2O3 catalysts prepared in (a) a batch process, (b) a slit plate mixer and (c) a valve-assisted mixer with Na2CO3 at pH 6, aged for 2 h. Image
d) shows a sample prepared with NaOH in a valve-assisted mixer at a pH value of 6, aged for 2 h.
(
Fig. 14. Particle size distribution of catalysts prepared in a batch process, a slit plate
mixer and a valve-assisted mixer with Na2CO3 at a pH value of 6, aged for 2 h.
Fig. 15. Particle size distribution of catalysts prepared in a batch process, a slit plate
mixerand a valve-assisted mixer with NaOH at pH 6, aged for 2 h.

1
0
G. Simson et al. / Applied Catalysis A: General 450 (2013) 1–12
Table 4
Methanol production rate over Cu/ZnO/Al2O3 catalysts prepared at pH value of 6,
aged 2 h, Batch (batch process), SPM (slit plate mixer), VAM (valve-assisted mixer).
Precipitating
agent
Batch
SPM
VAM
5
bar
10 bar
5 bar
10 bar
5 bar
10 bar
[mmolMeOH/gcat s]
[mmolMeOH/gcat s]
[mmolMeOH/gcat s]
Na2CO3
4.0
3.6
0.7
1.9
9.2
8.3
1.4
4.3
4.2
4.7
3.7
3.0
9.9
11.2
8.8
3.9
4.3
3.7
2.9
9.0
10.1
9.4
(
NH4)2CO3
K2CO3
NaOH
6.5
5.5
In Table 3 the influence of the different synthesis routes (batch
process, slit plate mixer and valve-assisted mixer) on the specific
copper surface area of Cu/ZnO/Al O3 catalysts prepared with car-
2
bonate species is shown. From this overview the advantages of
the novel, continuously operated precipitation routes in micromix-
ers compared to the conventional batch process get obvious. The
maximum of specific copper surface area is either obtained in the
synthesis route using a slit plate mixer or valve-assisted mixer,
whereas the minimum for copper surface area in synthesis with
carbonate species is always obtained in the classical batch pro-
cess. Likewise the minimum of copper surface area decrease during
methanol synthesis is achieved throughout in catalyst samples pre-
pared in micromixers.
Fig. 16. Specific copper surface area of Cu/ZnO/Al2O3 catalysts prepared with
Na2CO3 at a pH value of 6, aged for 2 h. Comparison of different synthesis routes
is shown.
catalyst samples with low copper surface areas together with the
highest decrease of copper surface during methanol synthesis
with an average of 44%. Fig. 16 illustrates the copper surface areas
achieved in precipitation with Na CO3 in the different synthesis
The methanol productivity of the prepared ternary catalyst
systems is reported in Table 4. The catalyst samples prepared
in the continuously operated synthesis route using a slit plate
mixer displays the highest methanol production rate of all pro-
2
routes applied. Precipitation in the micromixers generates catalyst
2
samples with specific copper surface areas of S = 20.5 m /gcat
Cu
2
and SCu = 19.0 m /gcat, whereby this value is significantly higher
than in the batchwise precipitation process, yielding a value of
duced samples. Cu/ZnO/Al O3 catalysts prepared in micromixers
2
2
SCu = 17.5 m /gcat. In general catalysts precipitated with Na CO
with the alternative precipitating agents (NH ) CO and K CO3 in
2
3
4 2 3 2
display the lowest decrease of copper surface area during methanol
synthesis with an average decrease of 4.3%. The minimum of copper
surface area lost during MeOH synthesis is shown by the catalyst
prepared in the valve-assisted mixer. With only 3.7% of loss it gets
very close to the industrial reference catalyst with 3.6% loss of
copper surface area. Using K CO3 and (NH ) CO as precipitating
general display higher or at least the same activity than samples
prepared according to the classical batch process with Na CO .
2
3
Samples precipitated with NaOH basically show very slight cat-
alytic activity in methanol synthesis and achieve about 44–55%
of the catalytic performance of the reference material. The mini-
mum of catalytic activity is displayed for the samples obtained by
K CO in a batch process. This is in accordance with the small spe-
2
4
2
3
agents in the different synthesis routes displays similar trends than
precipitation with Na CO . The copper surface areas obtained in
2
3
2
3
cific copper surface area of the catalyst sample (Table 2), which
is related to the high residual content of potassium in the sam-
ple as described before. Fig. 17 illustrates the achieved methanol
productivity for the catalysts prepared with (NH ) CO in the dif-
the continuously operated micromixers are higher than in conven-
tional batch synthesis. The catalyst sample prepared with K CO in
2
3
2
the batch process displays a copper surface area of SCu = 4.6 m /gcat,
which is far below the values for the samples prepared in the
micromixers. This result is associated with a high residual content
of potassium ions of 0.2 wt.% in this catalyst sample, which may
result from an insufficient washing step or a mistake in catalyst
4
2
3
ferent synthesis routes. The catalytic activity correlates with the
+
preparation. Although the role of K in catalysts is not yet fully
+
understood, it is accepted, that residues of K promote sintering
processes of copper crystals and therefore lead to a substantial
decrease of copper surface area [41]. Separate from the synthesis
route applied, catalysts prepared with NaOH show very small
copper surface areas. Related to the industrial reference sample
they merely achieve 20–35% of the specific copper surface areas.
Table 3
Maximum and minimum of specific copper surface area and relative decrease of cop-
per surface area for catalysts prepared with carbonate species in different synthesis
routes, Batch (batch process), SPM (slit plate mixer), VAM (valve-assisted mixer).
Precipitating
agent
Specific Cu surface area
Rel. decreaseofCusurfacearea
Maximum
Minimum
Maximum
Minimum
Na2CO3
SPM
VAM
VAM
Batch
Batch
Batch
SPM
Batch
Batch
VAM
SPM
VAM
(
NH4)2CO3
Fig. 17. Methanol productivity of Cu/ZnO/Al2O3 catalysts prepared with (NH4)2CO3
at pH 6, aged for 2 h. Comparison of different synthesis routes is shown.
K2CO3

G. Simson et al. / Applied Catalysis A: General 450 (2013) 1–12
11
Table 5
of the pH value, temperature, concentration and ageing time.
Reaction conditions were kept constant over the complete reac-
tion time. The catalysts were prepared according to the constant
pH method by coprecipitation of aqueous metal nitrate solu-
tions of copper, zinc and aluminum with aqueous solutions of
sodium, ammonium and potassium carbonate as well as sodium
hydroxide. After precipitation and ageing the precipitate was trans-
ferred to conventional post processing. The characterization of
Maximum and minimum methanol production rate over Cu/ZnO/Al2O3 catalysts in
terms of synthesis route applied.
Preciptating agent
MeOH production rate
Maximum
Minimum
Na2CO3
SPM
SPM
VAM
SPM
VAM
(
NH4)2CO3
Batch
Batch
Batch
K2CO3
NaOH
the Cu/ZnO/Al O3 catalysts illustrates that a completely different
2
chemical system exists in precipitation reaction with carbonate
species than in precipitation with sodium hydroxide. In synthesis
with carbonate species a complex system of hydroxycarbonates is
obtained in addition to hydroxide compounds of copper, zinc and
aluminum.
Table 6
Maximum and minimum methanol production rate over Cu/ZnO/Al2O3 catalysts in
terms of precipitating agent applied.
Synthesis route
MeOH production rate
Maximum
The metal contents of the catalysts are barely affected by the
synthesis route applied and the duration of the ageing process.
However, the choice of precipitating agent and pH value exhibits
a distinct influence upon the reorganization of the precipitate. A
neutral pH value and the presence of carbonate ions facilitate the
incorporation of zinc, leading to the deployed metal ratios in the
catalyst. The operation of synthesis in micromixers amplifies the
specific Cu surface area of samples prepared with NaOH by a factor
of 2, whereas in synthesis with carbonate species no substantial
influence of the synthesis routes applied can be observed. XRD pat-
terns of hydroxycarbonate precursors from synthesis in a slit plate
mixer and a valve-assisted mixer, especially with the alternative
precipitating agent (NH ) CO , display high crystallinity and sharp
Minimum
Batch
SPM
VAM
Na2CO3
(NH4)2CO3
(NH4)2CO3
K2CO3
NaOH
NaOH
trend of specific copper surface areas (Table 2). Catalysts precipi-
tated in micromixers clearly display higher methanol production
rates than the batch sample. The Cu/ZnO/Al O3 sample prepared
with (NH ) CO in a slit plate mixer reaches the industrial ref-
erence catalyst by 86% at 0.5 MPa respectively 89% at 1 MPa. The
catalytic activity of catalysts prepared with K CO3 and NaOH in
the three synthesis routes shows qualitatively the same trend
like (NH ) CO samples. The methanol productivity of the cata-
2
4
2
3
2
4
2
3
reflections of malachite and rosasite. These phases are thought to
be responsible for the high catalytic activity in methanol synthe-
sis [4–8]. The effect of fast and intense mixing as well as a high
degree of micromixing is responsible for this observation. Synthe-
sis in the conventional batch process produces samples with lower
crystallinity and higher intensity of the less active phase aurichal-
cite. For catalysts prepared with NaOH only reflections of the crystal
phases of gerhardtite and hydroxide compounds of copper, zinc and
aluminum are obtained.
4
2
3
lysts from synthesis in micromixers is significantly higher than
of those prepared in classical batch precipitation. Table 5 gives a
summary of the maximum and minimum methanol productivity
of catalysts prepared with Na CO , (NH ) CO , K CO and NaOH in
2
3
4
2
3
2
3
terms of the synthesis route applied. Separate from the precipitat-
ing agent applied, the maximum of catalytic activity in methanol
synthesis is found with a catalyst prepared in the innovative, con-
tinuously operated micromixers. The minimum of catalytic activity
in synthesis with (NH ) CO , K CO and NaOH is found in the con-
ventional batch precipitation. Only in synthesis with Na CO3 the
minimum is obtained in micromixer synthesis, whereby it has to
be pointed out, that the methanol productivity of Na CO3 sam-
ples is within a close range. The results displayed in Table 5 affirm
the potential of the novel microstructured setups with continuous
operational mode to provide an alternative solution to the prepa-
The results of the N O RFC measurements for catalysts prepared
2
4
2
3
2
3
in continuously operated micromixers in general show higher spe-
cific copper surface areas than catalysts prepared by conventional
batch precipitation. Furthermore, the samples obtained from the
micromixers exhibit a less pronounced decrease of copper surface
area during methanol synthesis and therefore present a higher sta-
2
2
bility of the achieved copper surface area. Employing Na CO3 and
2
(
NH ) CO as precipitating agents results in catalysts with high
4 2 3
ration of Cu/ZnO/Al O3 catalyst systems in conventional batch
2
specific copper surface areas, whereas precipitation with K CO3
2
synthesis. Table 6 indicates the possibility to produce highly active
catalysts for methanol synthesis using the alternative precipitating
agent (NH ) CO . Especially in continuously operated micromixer
synthesis (NH ) CO samples display the maximum of methanol
productivity. In synthesis with Na CO3 and K CO3 catalysts with
good catalytic activities are obtained likewise. The low catalytic
activity of the K CO3 sample in batch process is evaluated as aber-
ration due to the content of potassium in the sample. The other
and NaOH provides samples with lower copper surface area, lead-
ing also to a pronounced decrease in copper surface area during
methanol synthesis. The maximum of methanol productivity of all
4
2
3
4
2
3
prepared Cu/ZnO/Al O3 catalysts is observed with samples syn-
2
2
2
thesized in the continuously operated microstructured synthesis
setup regardless of the precipitating agent applied. Particularly cat-
alystsbased on the (NH ) CO route displaya high catalyticactivity,
2
4
2
3
which is comparable to the performance of an industrial refer-
ence catalyst. Therefore, the precipitating agent (NH ) CO has the
K CO samples indeed show methanol productivity comparable to
2
3
4
2
3
the Na CO samples. Finally, it can be noticed that the application
of NaOH as precipitating agent is not suitable for the production of
highly active Cu/ZnO/Al O3 catalyst systems for methanol synthe-
sis.
2
3
potential to replace the conventionally used Na CO as a sodium
2
3
free alternative.
2
In summary, the continuous coprecipitation using micromixers
generates ternary Cu/ZnO/Al O3 catalyst systems with equivalent
2
or even superior physicochemical properties compared to the con-
ventional batch processing. Especially the composition of crystal
phases, the specific copper surface area and the catalytic activity in
methanol synthesis of catalyst samples prepared in micromixers
displayed industrially important capabilities. Hence, the contin-
uously operation in precipitation of methanol synthesis catalysts
possesses the potential to replace the conventional synthesis in
batch reactors.
4
. Conclusions
In this study ternary Cu/ZnO/Al O3 catalyst systems were
2
prepared systematically in innovative synthesis routes using
microstructured synthesis setups with micromixers as a main
component. The precipitation in a slit plate mixer and valve-
assisted mixer was operated continuously under exact control

1
2
G. Simson et al. / Applied Catalysis A: General 450 (2013) 1–12
References
[22] S. Kaluza, M. Muhler, in: E. Gaigneaux, M. Devillers, D.E. De Vos, S. Hermans, P.
Jacobs, J. Martens, P. Ruiz (Eds.), Scientific Bases for the Preparation of Hetero-
geneous Catalysts, Elsevier, 2006, p. 175.
[
[
[
1] R.T. Figueiredo, A.M. Arias, M.L. Granados, J.L.G. Fierro, J. Catal. 178 (1998)
46–152.
2] M. Kurtz, N. Bauer, C. Büscher, H. Wilmer, O. Hinrichsen, R. Becker, S. Rabe, K.
Merz, M. Driess, R.A. Fischer, M. Muhler, Catal. Lett. 92 (2004) 49–52.
3] G. Sengupta, R.K. Sharma, V.B. Sharma, K.K. Mishra, M.L. Kundu, R.M. Sanyal, S.
Dutta, J. Solid State Chem. 115 (1995) 204–207.
[
[
23] S. Kaluza, M. Muhler, J. Mater. Chem. 19 (2009) 3914–3922.
24] S. Kaluza, M. Behrens, N. Schiefenhövel, B. Kniep, R. Fischer, R. Schlögl, M.
Muhler, ChemCatChem 3 (2011) 189–199.
1
[
[
[
25] K.W. Jun, W.J. Shen, K.S.R. Rao, K.W. Lee, Appl. Catal. A: Gen. 174 (1998)
2
31–238.
26] J. Wu, S.C. Luo, J. Toyir, M. Saito, M. Takeuchi, T. Watanabe, Catal. Today 45
1998) 215–220.
27] S.I. Blejoiu, T. Cioroianu, V. Vadeanu, P. Rotaru, O. Bunescu, Rev. Roum. Chim.
8 (1993) 633–641.
[
[
4] J.C.J. Bart, R.P.A. Sneeden, Catal. Today 2 (1987) 1–124.
5] G.C. Chinchen, P.J. Denny, J.R. Jennings, M.S. Spencer, K.C. Waugh, Appl. Catal.
(
3
6 (1988) 1–65.
3
[
6] E.B.M. Doesburg, R.H. Höppener, B. De Koning, X. Xiaoding, J.J.F. Scholten, in: B.
Delmon, P. Grange, P.A. Jacobs, G. Poncelet (Eds.), Preparation of Catalysts IV,
Elsevier, Amsterdam, 1987, p. 767.
[
[
28] T. Genger, O. Hinrichsen, M. Muhler, Catal. Lett. 59 (1999) 137–141.
29] M. Muhler, L.P. Nielsen, E. Törnqvist, B.S. Clausen, H. Topsøe, Catal. Lett. 14
(
1992) 241–249.
[
[
[
7] R.H. Höppener, E.B.M. Doesburg, J.J.F. Scholten, Appl. Catal. 25 (1986) 109–119.
8] M.S. Spencer, Catal. Lett. 66 (2000) 255–266.
9] T. Fujitani, J. Nakamura, Catal. Lett. 56 (1998) 119–124.
[
[
30] R. Naumann d’Alnoncourt, X. Xia, J. Strunk, E. Löffler, O. Hinrichsen, M. Muhler,
Phys. Chem. Chem. Phys. 8 (2006) 1525–1538.
31] A. Behr, U. Onken, in: M. Baerns, A. Behr, A. Brehm, J. Gmehling, H. Hofmann,
U. Onken, A. Renken (Eds.), Technische Chemie, Wiley-VCH, Weinheim, 2006,
p. 559.
[10] B. Bems, M. Schur, A. Dassenoy, H. Junkes, D. Herein, R. Schlögl, Chem. Eur. J. 9
(
2003) 2039–2052.
[
[
11] M. Behrens, J. Catal. 267 (2009) 24–29.
12] M. Behrens, F. Girgsdies, A. Trunschke, R. Schlögel, Eur. J. Inorg. Chem. 10 (2009)
[
32] J.B. Hansen, P.E.H. Nielsen, in: G. Ertl, H. Knözinger, J. Weitkamp (Eds.), Hand-
book of Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2008, p. 2920.
33] G.C. Chinchen, C.M. Hay, H.D. Vanderwell, K.C. Waugh, J. Catal. 103 (1987)
1
347–1357.
[
[
[
13] C. Baltes, S. Vukojevic, F. Schüth, J. Catal. 258 (2008) 334–344.
14] C. Kiener, M. Kurtz, H. Wilmer, C. Hoffmann, H.-W. Schmidt, J.-D. Grundwaldt,
M. Muhler, F. Schüth, J. Catal. 216 (2003) 110–119.
79–86.
[
[
34] O. Hinrichsen, T. Genger, M. Muhler, Chem. Eng. Technol. 11 (2000) 956–959.
35] D. Waller, D. Stirling, F.S. Stone, M.S. Spencer, Faraday Discuss. Chem. Soc. 87
[
[
15] J.-L. Li, T. Inui, Appl. Catal. A: Gen. 137 (1996) 105–117.
16] D.M. Whittle, A.A. Mirazei, J.S.J. Hargreaves, R.W. Joyner, C.J. Kiely, S.H. Taylor,
G.H. Hutchings, Phys. Chem. Chem. Phys. 4 (2002) 5915–5920.
(
1989) 107–120.
[36] I.M. Wassermann, N.I. Silanteva, Russ. J. Inorg. Chem. 13 (1968) 1041.
[37] I. Melian-Cabrera, M.L. Granados, J.L.G. Fierro, Catal. Lett. 84 (2002) 153–161.
[38] I. Schildermanns, J. Mullens, B.J. van der Veken, J. Yperman, D. Franc, L.C.v.
Poucke, Thermochim. Acta 224 (1993) 227–232.
[17] K. Jähnisch, V. Hessel, H. Löwe, M. Baerns, Angew. Chem. Int. Ed. 42 (2003)
2
–42.
[
[
[
18] P. Watts, C. Wiles, Org. Biomol. Chem. 5 (2007) 727–732.
19] G.M. Whitesides, Nature 442 (2006) 368–373.
20] K. Jähnisch, M. Baerns, V. Hessel, W. Ehrfeld, W. Haverkamp, H. Löwe, C. Wille,
A. Gruber, J. Fluorine Chem. 105 (2000) 117–128.
[
39] V. Vagvölgyi, A. Locke, M. Hales, J. Kristof, R.L. Frost, E. Horath, W.N. Martens,
Thermochim. Acta 468 (2008) 81–86.
[41] J.W. Bae, S.H. Kang, Y.J. Lee, K.W. Jun, J. Ind. Eng. Chem. 15 (2009) 566–572.
[42] Y.W. Suh, H.K. Rhee, Korean J. Chem. Eng. 19 (2002) 17–19.
[43] F.S. Stone, D. Waller, Top. Catal. 22 (2003) 305–318.
[21] M. Schur, B. Bems, A. Dassenoy, I. Kassatkine, J. Urban, H. Wilmer, O. Hinrichsen,
M. Muhler, R. Schlögl, Angew. Chem. Int. Ed. 42 (2003) 3815–3817.