Polydopamine-coated open cell polyurethane foams as an inexpensive, flexible yet robust catalyst support: A proof of concept

Article abstract of DOI:10.1039/c6cc00847j

Commercially available polyurethane open cell foams are readily coated with mussel-inspired polydopamine. The polydopamine film allows robust immobilisation of TiO₂ nanoparticles at the surface of the three-dimensional material. The resulting catalyst is efficient for the photo-degradation of an azo dye, reusable and highly resistant to mechanical stress. A novel type of robust structured catalytic support, easily accessible via an inexpensive and green process, is thus described.

Full text of DOI:10.1039/c6cc00847j

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Chau, T. Dintzer, T. ROMERO, D. Favier, T. Roland, D. Edouard, L. Jierry and V. Ritleng, Chem. Commun.,
2016, DOI: 10.1039/C6CC00847J.
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Polydopamine-coated open cell polyurethane foams as
inexpensive, flexible yet robust catalyst support: proof of concept
Received 00th January 20xx,
Accepted 00th January 20xx
Elodie Pardieu,†a,b Nguyet Trang Thanh Chau,†a,b,c Thierry Dintzer,a Thierry Romero,a Damien
Favier, b Thierry Roland,b David Edouard,*d,e Loïc Jierry*b,e and Vincent Ritleng*c,e,f
DOI: 10.1039/x0xx00000x
www.rsc.org/
Commercially available polyurethane open cell foams are readily highly corrosive media.5 Thus, in spite of their promising
coated with mussel-inspired polydopamine. The polydopamine architecture, metallic and ceramic open cell foams have up to
film allows robust immobilisation of TiO2 nanoparticles at the now rarely been used in industrial processes.
surface of the three-dimensional material. The resulting catalyst is
Polyurethane foams are well known materials, and are
efficient for the photo-degradation of an azo dye, reusable and commercially available at very low cost for a large variety of
highly resistant to mechanical stress. A novel class of robust applications involving their physical and mechanical
structured catalytic support, easily accessible via an inexpensive properties.6 When constituted of open cells (Fig. 1a), they are
and green process, is thus described.
notably used for fabricating cushions, mattresses or as filters in
cooker hoods, aquariums or vacuum cleaners. Moreover, they
are also used as templates for the fabrication of ceramic and
metallic open cell foams, and thus present similar
morphological aspects and transport properties.3 Despite
these physical similarities, their non-toxicity (polyurethane is
commonly used as a biomaterial7), and their high mechanical
resistance and elastic properties,8 open cell polyurethane
foams (OCPUF) have never been used as SCS.
The use of OCPUF as catalytic support is hampered both by
the fact that the edges’ and bridges’ surface is smooth and
devoid of microporosity and thus does not present sufficient
adherence to deposit a catalytic phase, and by the fact that
traditional washcoating is incompatible with its limited stabili-
Continuous processes based on Structured Catalytic Supports
(SCS) are widely used in industry. Indeed this type of supports
allows an important surface over volume ratio, a small
pressure loss, efficient mass transfers, an intimate mixing of
the reagents, and an easy separation of the products from the
catalyst.1 Among the variety of SCS, ceramic or metallic open
cell foams are prime candidates, which fulfill all these
features.2-4 The preparation of these foams however requires
several steps, including the physisorption and, when
necessary, the activation of the catalytic phase (usually
metallic or metal oxide particles) via expensive and energy
consuming thermic treatments.3 Moreover these foams
present several drawbacks inherent to their structure: (i) due
to their high rigidity, micro-cracks readily appear and render
them breakable, (ii) due to the presence of many randomly
distributed closed cells, the reproducibility is often
unpredictable, and (iii) the recovery of the catalyst adsorbed
on the foam necessitates numerous chemical treatments in
a
b
3.12 mm
3.55 mm
1 mm
100 !m
d
c
100 !m
1 !m
Fig.1
Optical microscopy (a) and SEM (b) images of OCPUF (1), and SEM (c,d)
images of PDA@OCPUF (2) with different magnifications.
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ty in temperature.9 Therefore, finding a way of grafting
catalysts on the whole surface of OCPUF that is efficient,
environmentally friendly, and compatible with its organic
nature constitutes a true challenge.
Present elements:
a
View Article Online
C(6), O(D8)O, TI:i(1202.)1039/C6CC00847J
The seminal work of Messersmith et al., placed
polydopamine (PDA) into the spotlight as a mussel-inspired
coating material that can bind to virtually all kinds of inorganic
and organic support.10 In addition to these strong adherence
properties, another valuable feature lies in the presence of
catechol and amine functional groups that can serve both for
further covalent modification with chosen molecules and for
the loading of metal ions or metal oxide particles.11 PDA thus a
priori possesses all the required features for allowing the use
OCPUF as versatile SCS, and beyond this, for allowing
functional coatings of these three-dimensional (3D) materials.
Herein, we show that OCPUF can indeed be efficiently
coated with a layer of PDA12 and further functionalized with
metal oxide particles. The catalytic potential of the novel
PDA@OCPUF structured catalytic support is illustrated by the
efficiency of immobilized TiO2 nanoparticules for the photo-
degradation of an azo dye, its reusability, and its high
resistance to mechanical stress.
1 !m
Present elements:
C(6), O(8)
b
1 !m
Fig. 3
SEM images (COMPO) of TiO2@PDA@OCPUF (3) (left) and EDX spectra
of the darkened areas (right)
content of 607 ± 80 mg/kg (i.e. 12.7 ± 1.7 mmol/kg). Finally,
compression tests performed on 1, 2 and 3 showed similar
stress/strain responses (characterized by a typical hysteresis
loop8) for all three foams (Fig. 4A), thereof demonstrating that
neither the PDA coating nor the further functionalization with
TiO2 NPs affected the elastic properties of OCPUF 1.
The catalytic activity of the TiO2-loaded OCPUF 3 was next
investigated for the photo-degradation of the noxious azo dye,
acid orange 7 (AO7 – Fig. S3 - ESI),15 as a model reaction.16 For
that purpose, three pieces of 3 of 0.5 × 2 × 2 cm each (ca. 150
mg, i.e. ca. 1.91 μmol of TiO2) were immerged in an aqueous
solution of AO7 (40 mL, 28.5 μM), and the reaction medium
was placed under UV irradiation (125 W) without stir (Fig. S4 -
ESI). The degradation of AO7 was then followed by measuring
Cubic samples (8 cm3) of OCPUF (1) (20 PPI; Fig. 1a,b) were
coated with PDA by simple immersion for 12 h at room
temperature in an aqueous solution of dopamine buffered to a
pH typical of marine environments (Fig. 2), followed by
thorough washings with water. Adsorption of PDA at the
surface of the resulting dark brown material (PDA@OCPUF, 2)
was confirmed by X-ray photoelectron spectroscopy (XPS) (see
Table S1 - ESI). In addition, low magnification scanning electron
microscopy (SEM) image of 2 (Fig. 1c) revealed a coating of
PDA on the whole surface of the 3D material at a macroscopic
level. Higher magnification SEM image (Fig. 1d) highlighted a
rough film with the presence of dispersed aggregates as
typically observed on flat surfaces.13
In agreement with the known high binding abilities of the
catechol groups of the PDA layer for Ti,14 the new SCS 2 was
then functionalized with TiO2 nanoparticules (NPs) (anatase,
10 nm) by immersion in a well-dispersed suspension (2 % w/v)
for 12 h at 40 °C (Fig. 2). The presence of titanium on the
obtained TiO2@PDA@OCPUF (3) material was confirmed by
XPS (Fig. S1 - ESI). SEM micrographs combined to energy
dispersive X-ray spectroscopy (EDX) (Figs. 3 & S2 - ESI) showed
that the TiO2 NPs were not uniformly distributed but were
rather present as micrometric clusters randomly dispersed all
over the surface of the modified 3D material. Inductively
coupled plasma-optical emission spectrometry (ICP-OES)
measurements on several samples of 3 revealed a mean Ti
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Fig. 4
(A) Stress/strain responses of 1 (black), 2 (green), as-synthesized 3 (red) and
Dopamine
(2 mg/mL)
TiO2
(2% w/v)
fatigue-tested 3 (blue). (B) UV-visible spectra of the AO7 solution vs. UV irradiation
time (min) in the presence of 3. (C) Degradation of AO7 (%) vs. UV irradiation time
(min) for neat AO7 (black horizontal squares), AO7 with 2 (green squares), and AO7
with 3 (red dots); at least two runs have been performed in each case. (D) Degradation
of AO7 (%) after 200 min UV irradiation for runs 1 to 5 with as-synthesized 3 (grey
histograms) and fatigue-tested 3 (red histograms); the average value and standard
deviation of at least three experiments are represented.
TRIS
(10 mM, pH 8.5)
RT, 12 h
TRIS
(10 mM, pH 8.5)
40 °C, 12 h
OCPUF
(1)
PDA@OCPUF
(2)
TiO2@PDA@OCPUF
(3)
Fig. 2
Polyurethane open cell foam (OCPUF) coating with polydopamine (PDA) and
functionalization with TiO2 nanoparticles
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the decrease of its absorbance peak at 485 nm in the UV- least five times, and did not leach even under rough
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DOI: 10.1039/C6CC00847J
visible spectra of the reaction solution in function of the time conditions.
of irradiation (Fig. 4B). Complete degradation of AO7 was
This pioneering work opens the way to other uses of
observed after 200 min (Figs. 4B and 4C - red dots) and the PDA@OCPUF as structured catalytic support. In particular, the
resulting solution had turned colorless (Fig. S5 - ESI). In presence of catechol and amine functional groups opens the
contrast, in the presence of the non-TiO2 functionalized possibility to covalently modify it with well-chosen
PDA@OCPUF (2) or in the absence of foam, only ca. 35% molecules,10,11 and therefore perhaps to use it as support for
degradation of AO7 was observed after 200 min (Fig. 4C - black organo- and/or organo-metallic catalysts. Moreover, the ability
and green squares) and the solutions remained orange (Fig. S5 of polydopamine to reduce some metal ions18 may allow to
- ESI). Furthermore, when 3 is removed from the reaction circumvent the impossibility to activate immobilized metallic
medium, the catalytic process stops, but restarts when 3 is re- salts or metal oxides by classical reduction methods at high
immersed in the solution (Fig. S6 - ESI). These results temperature9,19 (in reason of the organic nature of OCPUF),
unambiguously demonstrate that at least some PDA- and therefore allow its use as support for metal(0)-based
embedded TiO2 NPs of 3 are accessible to the substrate, active phase. These attractive perspectives are currently under
catalytically efficient, and are not released in the reaction study in our laboratories.
medium. Moreover, the similar rates of AO7’s degradation
observed in the presence of 2 or in its absence strongly
suggest that no significant adsorption of AO7 onto PDA occurs,
Acknowledgements
and thus that there is no interference of such an adsorption
process with the photo-degradation catalysis.
We are grateful to the University of Strasbourg Institute for
Advanced Study (USIAS) for financial support (2012 fellowship
to DE, LJ and VR).
This being established, we then assessed the mechanical
resistance of 3 by carrying out a fatigue test that consisted in
compressing it to a strain of 75%, then 5000 times to a strain
of 25%, and once again to a strain of 75% (Fig. S7 - ESI).
Gratifyingly the stress/strain response recorded after this
fatigue test was found to be similar to that recorded before
(Fig 4A – red and blue curves), which shows that the flexible
material 3 is highly resistant to mechanical stress.
Notes and references
1
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The reusability of the mechanically stressed 3 was next
examined and compared to that of as-synthesized 3. For that
purpose, the PDA@OCPUF-supported catalyst was removed
from the AO7 solution after each run of 200 min, thoroughly
washed with water, and then reused under the same
conditions. As shown in figure 4D, the catalytic activity of
mechanically stressed 3 is remarkably similar to that of as-
synthesized 3 and remains almost constant after five runs.
Moreover, negligible amounts of titanium metal were
detected from the filtrates after each run by ICP-OES (1.2×10-5
% < Ti < 4×10-6 %). Thus, the TiO2 NPs remain catalytically
active and robustly anchored at the surface of the
PDA@OCPUF support even when the latter is submitted to an
important mechanical stress and/or to repeated UV
irradiations. This later observation is in perfect agreement with
the well-known free radical scavenging properties of PDA that
make it useful as a UV protective layer on polymer materials.17
In summary, polydopamine has been used to coat
macroscopic 3D structures in the form of flexible polyurethane
open cell foams. Thanks to its remarkable adherence
properties, the mussel-inspired coating allows to consider
polyurethane open cell foams as a new structured catalytic
support as illustrated by the robust immobilization of TiO2 NPs,
and its successful use for the photo-degradation of AO7. Both,
the obtained PDA@OCPUF support and TiO2@PDA@OCPUF
catalytic material remarkably conserve the flexibility, high
mechanical resistance, and morphological characteristics (and
thus the transport properties) of the non-coated OCPUF.
Moreover, this new catalytic tool proved to be reusable at
6
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DOI: 10.1039/C6CC00847J
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