3D printing of a heterogeneous copper-based catalyst

Article abstract of DOI:10.1016/j.jcat.2015.11.019

One of the most important environmental challenges of modern society is to develop new catalysts that make possible chemical processes with reduced environmental impact. Catalyst immobilization is an appealing strategy that, in addition to facilitating catalyst recovery, has proved to give higher catalytic performance, since the solid support usually provides chemical, thermal, and mechanical stabilization to the catalytic species. In this work Cu/Al₂O₃ catalytic system with a woodpile porous structure is synthesized by 3D printing and then sintered at high temperature to generate a copper-supported rigid structure with high mechanical strength, a high surface-to-volume ratio, and controlled porosity. The catalytic species (Cu) are immobilized in the Al₂O₃ matrix to avoid the leaching of the metal into the reaction medium. Al₂O₃ was selected because it is a good material to obtain a structure with high mechanical stability after high-temperature treatment. The resulting device shows high catalytic efficacy and good recyclability and did not produce leaching of copper to the reaction medium in different Ullmann reactions. Ease of preparation, excellent reactivity, recyclability, and negligible metal contamination all make the 3D printing technique a good strategy for fabricating other types of metal/oxide heterogeneous catalytic systems.

Full text of DOI:10.1016/j.jcat.2015.11.019

Journal of Catalysis 334 (2016) 110–115
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
3D printing of a heterogeneous copper-based catalyst
a
b
b
b
a
b,
⇑
Carmen R. Tubío , Jhonny Azuaje , Luz Escalante , Alberto Coelho , Francisco Guitián , Eddy Sotelo ,
Alvaro Gil
a
a,
⇑
Instituto de Cerámica, Universidade de Santiago de Compostela, Santiago de Compostela 15782, Spain
Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CIQUS), Universidade de Santiago de Compostela, Santiago de Compostela 15782, Spain
b
a r t i c l e i n f o
a b s t r a c t
Article history:
One of the most important environmental challenges of modern society is to develop new catalysts that
make possible chemical processes with reduced environmental impact. Catalyst immobilization is an
appealing strategy that, in addition to facilitating catalyst recovery, has proved to give higher catalytic
performance, since the solid support usually provides chemical, thermal, and mechanical stabilization
Received 28 September 2015
Revised 26 November 2015
Accepted 27 November 2015
2 3
to the catalytic species. In this work Cu/Al O catalytic system with a woodpile porous structure is syn-
thesized by 3D printing and then sintered at high temperature to generate a copper-supported rigid
structure with high mechanical strength, a high surface-to-volume ratio, and controlled porosity. The cat-
Keywords:
3
D printing
alytic species (Cu) are immobilized in the Al
2 3
O matrix to avoid the leaching of the metal into the reaction
Catalysis
medium. Al was selected because it is a good material to obtain a structure with high mechanical sta-
2 3
O
Ullmann reaction
Heterogeneous catalyst
Copper
bility after high-temperature treatment. The resulting device shows high catalytic efficacy and good recy-
clability and did not produce leaching of copper to the reaction medium in different Ullmann reactions.
Ease of preparation, excellent reactivity, recyclability, and negligible metal contamination all make the
3D printing technique a good strategy for fabricating other types of metal/oxide heterogeneous catalytic
systems.
Ó 2015 Elsevier Inc. All rights reserved.
1
. Introduction
The increasing environmental consciousness of modern society
impregnation [4,5], co-precipitation [6], electrophoretic deposition
[
7], and sol–gel [8] and vapor deposition techniques, the latter of
which includes chemical vapor deposition (CVD) [9] and atomic
layer deposition (ALD) [10]. Liquid phase procedures have proven
to be useful, but they do suffer from some drawbacks, which are
generally associated with low loading levels, arduous experimental
procedures, nonuniform catalytic species distribution [7,11,12],
ineffective catalyst immobilization, and insufficient mechanical
strength.
Transition metal-catalyzed reactions (TMCR) occupy a promi-
nent position among modern synthetic methods in industrial and
academic laboratories [13]. The incorporation of these reactions
into the synthetic armory provided conceptually novel approaches
for the formation of C–C and C–heteroatom bonds, as well as for
C–H functionalization [14]. The excellent selectivity and functional
group compatibility of TMCR offer robust and mild synthetic alter-
natives for the preparation of complex natural products, pharma-
ceuticals, heterocyclic compounds, agrochemicals and polymers,
etc. [13,14]. The abundance of copper, along with its diverse chem-
istry and low cost, make it an attractive alternative to precious
metals (Pd, Pt, Au, Rh, Ir) for use in organic synthesis [15]. In fact,
copper-mediated reactions have a longer history than palladium-
catalyzed reactions and, indeed, Ullmann [15,16] and Glaser
challenges the chemical community to develop catalytic systems
that enable chemical processes with a reduced environmental
impact (E-factor) [1]. In this context, catalyst immobilization is
an appealing strategy that, in addition to facilitating catalyst recov-
ery and operation in a flow-continuous format, has proven to give
higher catalytic performance, since the solid support usually pro-
vides chemical, thermal, and mechanical stabilization to the cat-
alytic species [2]. In addition to the intrinsic features defined by
the catalytic species and the solid support, the key properties of
the heterogeneous catalytic material are highly dependent on the
fabrication method (e.g., catalytic performance, loading levels of
the catalyst, morphology and dimensions of the system). Leaching
of the metal into the reaction medium is a critical problem for
applications in the pharmaceutical industry, since regulatory agen-
cies have established acceptable levels for transition metals within
drugs in the low ppb range [3]. The synthesis of heterogeneous cat-
alysts is currently dominated by wet chemical methods such as
⇑
Corresponding authors. Fax: +34 981564242.
E-mail addresses: e.sotelo@usc.es (E. Sotelo), alvaro.gil@usc.es (A. Gil).
http://dx.doi.org/10.1016/j.jcat.2015.11.019
021-9517/Ó 2015 Elsevier Inc. All rights reserved.
0

C.R. Tubío et al. / Journal of Catalysis 334 (2016) 110–115
111
[
15,17] reactions have been employed for more than 100 years.
motion was controlled by the software Robocad 3.4 (3D Inks, USA).
A Performus VII air-powered fluid dispenser attached to an HP7x
(Nordson EFD, USA) was used to control the ink flow rate (pressure
21 bar and print velocity of 2 mm s ). For a cylindrical woodpile
structure (10 mm diameter and 12 mm height) with a body-
centered tetragonal (bct) symmetry, the rod spacing was
Although it is less active than its noble metal analogues (e.g., Pd
or Rh), copper oxide (CuO) has been extensively employed in the
Ullmann reaction for the formation of C–N, C–O, and C–S bonds,
which are prevalent linkages in compounds of biological and phar-
maceutical interest. The results of recent studies have shown that
the use of CuO nanoparticles enables the conditions for the Ull-
mann reaction [18] to be far less harsh and also extends its scope
to nonactivated substrates. We describe here, for the first time,
the direct fabrication of a highly robust, efficient, and reusable cop-
per heterogeneous catalyst system using 3D printing [19,20]. This
is a layer-by-layer technique in which concentrated ink with the
desired rheological behavior is extruded through a nozzle. The
most common inks are based on ceramics, polymers, colloidal sys-
tems, or semiconductor materials. These inks can be assembled
into complex 3D structures with high surface-to-volume ratios
with precise control of porosity, size, and shape. 3D printing has
recently been employed for the fabrication of structures with dif-
ferent applications, such as photonic band gap materials [21], tis-
sue engineering [22], catalysts [23,24], integrated electronic
devices [25], and vascularized tissues [26].
ꢁ1
d = 960 lm and the rod diameter was w = 410 lm. This corre-
sponds to an open porosity of 57%. This structure was formed in
air at a relative humidity of 60–70% and a temperature of
23–25 °C. Finally, the woodpile structure was dried at room
temperature for 24 h and subsequently sintered at 1400 °C for
ꢁ1
2 h in air in a conventional furnace at a heating rate of 10 °C min
.
2.2. Catalyst characterization
The copper loading of the sample was determined by induc-
tively coupled plasma-optical emission spectroscopy (ICP-OES,
Varian Liberty 200). The sample was prepared in a Claisse Fluxer
automatic fusion device (Corporation Scientifique Claisse, Quebec,
Canada) and was fitted with one 20 mL platinum 5 wt.% gold alloy
crucible and with 200 mL quartz beakers for solution preparation.
The device fuses the sample in the platinum–gold alloy crucible
In this work, we designed a heterogeneous catalytic system
consisting of a sintered Al
species are immobilized, thus generating a catalytic structure with
high mechanical and chemical stability. Cu/Al woodpile struc-
2
O
3
support on which catalytic copper
over an air–propane flame with lithium metaborate (Li
quantity of 0.5 g of finely powdered sample and 1.5 g of Li
2 4 7
B 0 ). A
2 4 7
B 0
2
O
3
were mixed in the platinum crucible. Previously, a nonwetting
agent (0.1 g of LiI) was added to the fusion mixture to prevent
the molten flux from adhering to the walls of the crucible, as well
as to guarantee the complete transfer of the fused sample from the
crucible into the solvent. When the fusion is over, the molten glass
containing the sample is cast into a solution of 5 wt.% NO H and
3
dissolved. After the solution is diluted to 100 mL, it is analyzed
by ICP-OES.
X-ray diffraction (XRD) patterns of the dried and calcined ink
powder samples were obtained with a Siemens D5000 diffrac-
tometer using Cu Ka radiation at k = 1.5418 Å. Diffraction data
were scanned in a 2h range of 10–80° with a step size of 0.05°. A
quantitative phase analysis was carried out using Rietveld refine-
ment with the FullProf program.
tures were synthesized and then sintered at high temperature,
thus generating a copper-supported rigid structure with excep-
tional mechanical strength, a high surface-to-volume ratio, and
controlled porosity. The resulting device was tested in a model Ull-
mann reaction, validating its high catalytic efficacy and good recy-
clability, and did not produce leaching of copper species to the
reaction medium. The experimental protocol was performed in
four stages: (1) Preparation of an aqueous concentrated colloidal
gel ink [20,27] in which Al
and Cu(NO O were mixed to obtain a good dispersion
ꢀ2.5 H
and achieve quantitative control of the copper level in the Al
2 3
O ceramic powder, polymer binders,
)
3 2
2
2
O
3
network (5 wt.% Cu in the ink). (2) Extrusion to build a 3D wood-
pile structure. (3) Sintering of the structure at high temperature.
(
4) Validation of the catalytic efficacy of the Cu/Al
2
O
3
in a model
Optical microscopy images were obtained on an Olympus
SZX12 stereomicroscope (Olympus, Japan). The microstructural
surface of the sintered woodpile structure was analyzed by scan-
ning electron microscopy (SEM) (Model JEOL 6400, Japan).
Ullmann reaction.
2
. Experimental
2.3. Catalytic activity evaluation
2.1. Preparation of the catalyst system
To equimolar amounts (0.1 mmol) of the halide and the nitro-
The ink was prepared as follows: 92.31 g of Al
2
O
3
powder with a
genated nucleophile in DMF were added KOH and the 3D heteroge-
neous Cu/Al catalytic system. The reaction mixture was heated
2
ꢁ1
mean particle size of 0.5
and a density of 3.96 g cm (Almatis GmbH, Germany) was added
l
m, a specific surface area of 7.85 m g
,
2 3
O
ꢁ
3
at 80 °C under orbital stirring in a Kimble vial (7 mL) for 2–4 h.
Once the reaction had finished (TLC control), the solvent was sep-
arated from the catalyst and this was successively washed with
methanol and dichloromethane (5 mL). The organic solvents were
removed under vacuum and the residue was purified by column
chromatography to afford the desired compound (2–4).
to a 2.56 M aqueous solution of copper(II) nitrate hemi(pentahy-
drate), Cu(NO
modifier hydroxypropyl methylcellulose (HPMC, viscosity 2600–
600 cP) (Sigma–Aldrich) was added at 5 mg per mL of the Al
)
3 2
2
ꢀ2.5H O, P98% (Sigma–Aldrich). The viscosity
5
2 3
O
powder. The resulting suspension was mixed in a planetary cen-
trifugal mixer (ARE-250, Thinky Corp., Tokyo, Japan) at 2000 rpm
for 2 min and was allowed to stand at room temperature for 1 h.
The cationic polyelectrolyte poly(ethylenimine) (PEI, Mw = 2000,
Sigma–Aldrich) was added in a ratio of 6.3 ꢂ 10 mL per mL of
the ceramic powder, and the suspension was again mixed four
times at 2000 rpm for 2 min. Finally, the ink was concentrated by
the removal of water by evaporation until suitable for extrusion.
2.4. Evaluation of the copper leaching with Cu/Al
2 3
O : Three-phase test
ꢁ
3
To a 50 mL glass fritted reaction vessel was added Wang resin
ꢁ1
(4.0 g, 1.25 mmol g , 5 mmol). The resin was first washed with
dry dichloromethane (2 ꢂ 30 mL) and then suspended in dry
dichloromethane (30 mL). To the resin slurry was added diethyliso-
propylamine (30 mmol) and the vessel was submitted to orbital
stirring for 5 min before 4-iodobenzoyl chloride (25 mmol) was
added in one portion. The resulting slurry was stirred for 24 h at
room temperature. The resin was filtered, successively washed
with dichloromethane (2 ꢂ 40 mL), methanol (2 ꢂ 30 mL), and
A concentrated Cu/Al
viscosity of = 323.7 Pa s) was loaded into a syringe (3 mL, Nord-
son EFD, USA) attached by a nozzle with a diameter of 410
Nordson EFD, USA). A robotic deposition A3200 system (Aerotech
Inc., USA) was used to create the woodpile structures. The robotic
2 3
O aqueous ink (51 vol.% solids, with a
g
l
m
(

1
12
C.R. Tubío et al. / Journal of Catalysis 334 (2016) 110–115
tetrahydrofuran (3 ꢂ 40 mL), and dried under vacuum for 8 h at
room temperature. To the supported phenyl iodide 7 (2.0 g, 2.5
mmol) in (1:4) dichloromethane-DMF (20 mL) were added imida-
respectively. One of the aims of this study was to obtain a reusable
woodpile structure, and for this reason, the system needed to have
sufficient mechanical strength to withstand continuous contact
with the reactor. In order to generate a sintered woodpile structure
with high mechanical strength, to immobilize the copper on the
substrate, and to remove the binders, the structure was heated at
1400 °C (which is the typical temperature range for the sintering
2 3
zole (11 mmol), KOH (6 mmol), and the Cu/Al O woodpile catalyst
(
Cu 2.3 wt.%). The resulting slurry was submitted to orbital stirring
at 80 °C for 24 h. The resin was filtered off, successively washed
with dichloromethane (2 ꢂ 40 mL), methanol (2 ꢂ 30 mL), and
dichloromethane (3 ꢂ 40 mL), and dried under vacuum for 8 h at
room temperature. To a suspension of the previously obtained resin
in dichloromethane (15 mL) was added trifluoroacetic acid
2 3
stage of Al O ) for 2 h in air. The 3D structures after the heat treat-
ment are shown in Fig. 1c and d. The 3D woodpile structure was
free of cracks and retained its morphology after sintering, but there
was visible shrinkage. This shrinkage was 29% for rod diameter and
rod spacing, which resulted in an open porosity of 57%. The
shrinkage was accompanied by a color change in the material from
blue for the dried structure to dark brown for the sintered
structure. The color change is a consequence of the transformation
(
3.0 mL), and the resulting slurry was stirred at room temperature
for 2 h, filtered, and washed with dichloromethane (3 ꢂ 15 mL).
The combined filtrates were concentrated and the residue was
examined by analytical and spectroscopic methods, the results of
which confirmed the isolation of 4-iodobenzoic acid.
of Cu(NO
treatment of the woodpile structure.
3
)
2
ꢀ2.5H
2 2 4
O to CuO and CuAl O during the thermal
3
. Results and discussion
This transformation is in good agreement with the XRD patterns
Fig. 2) of the dried and sintered inks at 1400 °C. The XRD pattern
(
3
.1. Preparation and characterization of the catalyst
of the ink before calcining is shown in Fig. 2a (dried). The diffrac-
tion peaks correspond to the -Al phase and one peak at
2h = 13.05° corresponds to Cu(NO O. Fig. 2b shows the
experimental XRD pattern and the corresponding Rietveld refine-
ment of the ink upon calcination. The XRD data shows the presence
a
2 3
O
3
D periodic structures with cylindrical symmetry were assem-
3
)
2
ꢀ2.5H
2
bled from an Al -based ink. Al was selected because it is a
good material from which to obtain a structure with high mechan-
ical stability after high-temperature treatment. Such treatment
leads to sintering of the Al
in the mechanical strength of the structure. In the approach
reported here, the ink was extruded though a nozzle with diameter
2
O
3
2
O
3
of three species,
and one peak due to copper aluminate spinel (CuAl
2h = 36.80 °. This is in agreement with the Al –CuO phase dia-
a-Al
2
O
3
, tenorite (CuO) at 2h = 35.55° and 38.60°,
O
3
and, as a consequence, an increase
2 4
O ) at
2
2 3
O
gram [28]. A small difference can be observed between the exper-
4
10
lm. The material was patterned layer by layer, with each layer
imental XRD pattern and the calculated profile, which indicates
2
rotated 90° with respect to the previous one. The layers were
high reliability of the Rietveld refinement (
v
= 2.5). The refine-
formed by parallel rods of the material, which are separated at a
ment of the XRD data yielded that sintered ink was a mixture con-
sisting of 96.5 wt.% of -Al , 2.5 wt.% of CuO, and 1 wt.% of
CuAl . According to these results, the concentration of Cu in
distance of 960
lm. The pile sequence is repeated every two layers.
a
2 3
O
The resulting cylindrical woodpile structure with a lattice of
orthogonally interconnected rods is illustrated in Fig. 1a and b.
The design of the woodpile structure (e.g., size and shape) was
adapted to the dimensions of the reaction vials (Kimble vials) in
which the test reactions were carried out (Fig. 1e). After fabrica-
tion, the dried structure experienced some shrinkage and the
2 4
O
the sintered ink was 2.35 wt.%.
Visual inspection of the sintered material provided evidence
that a portion of the metal is lost during the sintering process
(1400 °C for 2 h) because it diffuses through the woodpile structure
and remains absorbed on the plate that supports the woodpile dur-
ing the sintering process. ICP-OES measurements were employed
resulting rod diameter and rod spacing were 354 and 806 lm,
2 3
Fig. 1. Optical images of the Cu/Al O structure: dried (a and b) and sintered (c and d). Schematic illustration and image of the experimental set used for catalytic tests (e).

C.R. Tubío et al. / Journal of Catalysis 334 (2016) 110–115
113
Fig. 2. (a) X-ray diffraction (XRD) patterns of the dried Cu/Al
2
O
3
powders.
(
b) XRD pattern and Rietveld refinement of the sintered Cu/Al O powders.
2 3
⁄
:
Cu(NO
3
2
) ꢀ2.5H
2
O, d:
a-Al
2
O
3
, +: CuO,
2 4
r: CuAl O .
2 3
to determine the copper loading within the sintered Cu/Al O
structure. These experiments showed a copper loading of 2.3 wt.%,
which is an optimum value for catalytic applications. This value
is in very good agreement with the result of 2.35 wt.% obtained
from the Rietveld analysis.
Scanning electron microscopy (SEM) images of the cylindrical
2 3
Cu/Al O woodpile structure (Fig. 3a and b) show how the 3D sin-
tered structure retains a regular morphology and the intercon-
nected rods have a smooth exterior surface and a regular
diameter with minimal deflection. A magnified view (Fig. 3c) of
2 3
the surface filament shows the dense assembly of the Al O crys-
tals sintered at 1400 °C without open meso- or macroporosity. In
addition, a laminar structure of the eutectic phases can be
observed clearly in this image, and this indicates the growth of
the copper oxide phases (CuO, CuAl
2 4 2 3
O ) over the Al O surface.
This result is in excellent agreement with the XRD pattern (Fig. 2b).
3.2. Evaluation of the catalytic performance of the woodpile Cu/Al
2 3
O
catalyst
Once the novel catalytic system had been synthesized and char-
acterized, we focused on the evaluation of its catalytic perfor-
mance. The Ullmann reaction is a highly versatile transformation
employed for the synthesis of diverse organic compounds, and it
was selected as a model reaction. The classical methods for the Ull-
mann reaction generally require harsh experimental conditions
Fig. 3. SEM images of sintered woodpile structure fabricated from a concentrated
Cu/Al O ink deposited through a 410 lm nozzle. (a) Top view, (b) cross-sectional
2 3
view, and (c) high magnification of the surface microstructure.
[
15,16] and usually provide only moderate yields, but they also
systems, ligands, bases, and an inert atmosphere to perform the
reaction. Accordingly, the development of novel, simple, and effi-
cient catalytic systems for this transformation remains a challenge.
The catalytic efficacy of the material reported here was evaluated
in three model Ullmann reactions (see Scheme 1).
generally employ catalytic systems that may contaminate the reac-
tion products. Although substantial improvements in cross-
coupling reactions for C–N bond formation have been documented
[
29], they still require the use of relatively sophisticated catalytic

1
14
C.R. Tubío et al. / Journal of Catalysis 334 (2016) 110–115
Scheme 1. Cu/Al
2
O
3
-catalyzed Ullmann reaction for the synthesis of imidazoles (2), benzimidazoles (3), and N-aryl amides (4).
The studied transformations provided the target N-aryl com-
pounds (2–4) under ligand-free conditions, with excellent yields
78–94%) and relatively short reaction times (2–4 h). The excellent
chemoselectivity of the transformation warrants particular atten-
tion, with the bromo substituent of the starting material (1b)
remaining intact during all transformations. A comparative analy-
sis of the information in Scheme 1 makes it possible to qualita-
tively evaluate the effectiveness of the catalytic system
documented herein in comparison to other heterogeneous cat-
3.3. Three-phase test
(
In addition to its catalytic efficacy and recyclability, the leaching
of metal into the reaction media is a key issue to be considered
during the development of tailored heterogeneous catalytic sys-
tems. To challenge our supported reagents in this regard, and to
obtain evidence as to whether this is a true heterogeneous catalyst
or if migration from the 3D printed woodpile catalytic structure
produces active homogeneous copper species, a three-phase test
[35] was designed (Scheme 2). In such experiments the immobi-
lized catalyst is simultaneously exposed to soluble reagents and
reagents bound to a solid support. If an active catalyst leaches from
the immobilized medium, it will result in conversion of the
resin-bound reagent. With this aim in mind, supported phenyl
2
alytic systems (e.g., CuO [30,31] or CuO/SiO [32,33]). While the
obtained yields are similar, the short reaction times (2–4 h) expe-
rienced during the use of the 3D printed woodpile catalytic system
are noteworthy. It should be noticed that, although the catalytic
system described herein contains a quite low copper loading
(
2.3 wt.%) in comparison with analogous systems (e.g., 5–10%),
complete conversion of the reactants is generally observed after
h, whereas other catalytic systems generally require 8–48 h
30,34]. The recyclability of the catalytic system was examined
iodide 7 was prepared (Scheme 2) and polystyrene-bound
ꢁ
1
4-benzyloxybenzyl alcohol (1.25 mmol g , Wang resin) was chosen
as an alternative support due to its rapid and efficient cleavage
protocol. First, 4-iodobenzoyl chloride (5) was tethered to Wang
resin (6) to generate the immobilized iodide (7), which was then
subjected to reaction with imidazole under previously optimized
Ullmann reaction conditions (Scheme 2). In a parallel experiment,
the supported phenyl iodide 7 and imidazole were reacted in the
presence of a conventional catalytic system (Scheme 2). After
24 h under optimal experimental conditions, both reactions were
worked up and the substrate was cleaved from the solid support.
Analysis of the cleaved material allowed the isolation of the start-
2
[
for selected examples (2a and 4a). It was verified that, once washed
and dried in vacuo, the heterogeneous catalytic systems recovered
from a reaction can be reused at least 10 times in new Ullmann
reactions without dramatic yield loss (the average yields for the
synthesis of compound 2a were 1: 91%, 2: 90%, 3: 90%, 4: 91%, 5:
8
9%, 6: 87%, 7: 89%, 8: 90%, 9: 87%, and 10: 86%). These reactions
also generated products with purities similar to those obtained in
the first run. The excellent catalytic activity and recyclability of the
material were complemented by its remarkable mechanical stabil-
ity. The woodpile structure did not show any signs of deterioration
after successive use in at least 10 experiments with orbital stirring.
It should be noticed that, as validated by ICP-OES analysis, the cop-
per content present on the woodpile catalyst employed during the
2 3
ing 4-iodobenzoic acid (5) on using the Cu/Al O woodpile catalyst,
whereas the desired 4-(1H-imidazol-1-yl)benzoic acid (9) was
obtained under conventional homogeneous conditions (CuI, DIPEA,
DMF, 80 °C) [36]. These findings unequivocally confirm that leach-
ing of copper species to the reaction medium does not occur when
the 3D printed heterogeneous catalyst developed in this work is
used. These attributes make this new catalyst useful in reactions
aimed at the preparation of pharmaceutical agents, where the fate
of the metal catalyst and ancillary ligands is of particular interest.
1
%
0 replications of the model reaction remained unaltered (2.3 wt.
). A detailed description of the experimental conditions and the
structural characterization of the resulting compounds is provided
in Supplementary Information.

C.R. Tubío et al. / Journal of Catalysis 334 (2016) 110–115
115
2 3
Scheme 2. Three-phase test to evaluate copper leaching with Cu/Al O catalysts.
[
7] R. Chamoun, U.B. Demirci, D. Cornu, Y. Zaatar, A. Khoury, R. Khoury, P. Miele,
Appl. Surf. Sci. 256 (2010) 7684–7691.
4
. Conclusions
[
8] I. Lee, R. Morales, M.A. Albiter, F. Zaera, Synthesis of heterogeneous catalysts
with well shaped platinum particles to control reaction selectivity, Proc. Natl.
Acad. Sci. USA 105 (2008) 15241.
2 3
In summary, the first example of a 3D heterogeneous Cu/Al O
catalytic system fabricated by 3D printing is reported. The macro-
porosity, interconnectivity, and copper loading of the catalyst sys-
tem can be controlled. The 3D printing process enabled the direct
and experimentally simple fabrication of a woodpile porous cata-
´
[
9] M. Becker, R.N. dAlnoncourt, K. Kähler, J. Sekulic, R.A. Fischer, M. Muhler,
Chem. Vapor Depos. 16 (2010) 85–92.
[
10] S. Sun, G. Zhang, N. Gauquelin, N. Chen, J. Zhou, S. Yang, W. Chen, X. Meng, D.
Geng, M.N. Banis, R. Li, S. Ye, S. Knights, G.A. Botton, T.-K. Sham, X. Sun, Sci.
Rep. 3 (2013) 1–9.
11] T. Vergunst, F. Kapteijn, J.A. Moulijn, Appl. Catal. A 213 (2001) 179–187.
12] E.M. Slavinskaya, R.V. Gulyaev, A.V. Zadesenets, O.A. Stonkus, V.I. Zaikovskii, Y.
V. Shubin, S.V. Korenev, A.I. Boronin, Appl. Catal. B 166–167 (2015) 91–103.
13] M.L. Crawley, B.M. Trost, Applications of Transition Metal Catalysis in Drug
Discovery and Development: An Industrial Perspective, John Wiley & Sons,
Hoboken, New Jersey, 2012.
2 3
lyst. The novel Cu/Al O woodpile system exhibits excellent cat-
[
[
alytic performance in different Ullmann reactions without
leaching contamination of the reaction products. The ease of
preparation, reactivity, recyclability, and negligible metal contam-
ination all make the 3D printing technique a good alternative to
fabricate other metal/oxide heterogeneous catalytic systems.
[
[14] A. de Meijere, F. Diederich, Metal-Catalyzed Cross Coupling Reactions, Wiley-
VCH, Weinheim, Germany, 2004.
[
[
[
[
15] G. Evano, N. Blanchard, Copper-Mediated Cross-Coupling Reactions, Wiley,
Acknowledgment
Weinheim, Germany, 2013.
16] T.D. Nelson, R.D. Crouch, Cu, Ni, and Pd Mediated Homocoupling Reactions in
Biaryl Syntheses: The Ullmann Reaction, Wiley, Weinheim, Germany, 2004.
17] J.J. Li, Name Reactions: A Collection of Detailed Reaction Mechanisms and
Synthetic Applications, Springer, Berlin, Germany, 2009.
18] S. Jammi, S. Sakthivel, L. Rout, T. Mukherjee, S. Mandal, R. Mitra, P. Saha, T.
Punniyamurthy, J. Org. Chem. 74 (2009) 1971–1976.
The authors acknowledge the support of the Galician Govern-
ment under Grant Awards GPC2013/019, GPC2014/003, and
EM2014/022.
[
[
19] J.A. Lewis, Adv. Funct. Mater. 16 (2006) 2193–2204.
20] J.E. Smay, G.M. Gratson, R.F. Shepherd, J. Cesarano III, J.A. Lewis, Adv. Mater. 14
Appendix A. Supplementary material
(
2002) 1279–1283.
[
21] E.B. Duoss, M. Twardowski, J.A. Lewis, Adv. Mater. 19 (2007) 3485–3489.
Supplementary information includes a detailed description of
the experimental details and the structural characterization of
the resulting compounds of the Ullmann reaction. Supplementary
data associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.jcat.2015.11.019.
[22] P. Miranda, E. Saiz, K. Gryn, A.P. Tomsia, Acta Biometer. 2 (2006) 457–466.
[
[
[
[
23] J.N. Stuecker, J.E. Miller, R.E. Ferrizz, J.E. Mudd, J. Cesarano III, Ind. Eng. Chem.
Res. 43 (2004) 51–55.
24] J. Lefevere, M. Gysen, S. Mullens, V. Meynen, J. Van Noyen, Catal. Today 216
(2013) 18–23.
25] B.Y. Ahn, E.B. Duoss, M.J. Motala, X. Guo, S. Park, Y. Xiong, J. Yoon, R.G. Nuzzo, J.
A. Rogers, J.A. Lewis, Science 323 (2009) 1590–1593.
26] Y. Zhang, Y. Yu, A. Akkouch, A. Dababneh, F. Dolati, I.T. Ozbolat, Biomater. Sci. 3
(
2015) 134–143.
References
[
[
[
[
[
[
[
[
[
[
27] R.B. Rao, K.L. Krafcik, A.M. Morales, J.A. Lewis, Adv. Mater. 17 (2005) 289–293.
28] A.M. Gadalla, J. White, J. Brit. Ceram. Soc. 63 (1964) 36–62.
[
1] R.A. Sheldon, Green Chem. 9 (2007) 1273–1283.
29] I.P. Beletskaya, A.V. Cheprakov, Coord. Chem. Rev. 248 (2004) 2337–2364.
30] R.A. Altman, E.D. Koval, S.L. Buchwald, J. Org. Chem. 72 (2007) 6190–6199.
31] L. Rout, S. Jammi, T. Punniyamurthy, Org. Lett. 9 (2007) 3397–3399.
32] C. Chen, Y. Chen, C. Lin, H. Lin, C. Lee, Chem. Commun. 46 (2010) 282–284.
33] F. Monnier, M. Taillefer, Angew. Chem. Int. Ed. 48 (2009) 6954–6971.
34] H. Lin, D. Sun, Org. Prep. Proc. Int. 45 (2013) 341–394.
[
2] M. Benaglia, Recoverable and Recyclable Catalysts, Wiley, Weinheim,
Germany, 2009.
[
[
[
[
3] R. Ciriminna, P.D. Cará, M. Sciortino, M. Pagliaro, Adv. Synth. Catal. 353 (2011)
6
77–687.
4] F.E. López-Suárez, A. Bueno-López, M.J. Illán-Gómez, Appl. Catal. B 84 (2008)
51–658.
5] G.V. Sagar, P.V.R. Rao, C.S. Srikanth, K.V.R. Chary, J. Phys. Chem. B 110 (2006)
3881–13888.
6] M. Behrens, Catal. Today 246 (2015) 46–54.
6
35] J. Rebek, F. Gaviña, J. Am. Chem. Soc. 96 (1974) 7112–7114.
36] H. Huang, X. Yan, W. Zhu, H. Liu, H. Jiang, K. Chen, J. Combinat. Chem. 10 (2008)
1
6
17–619.