Covalent triazine framework as catalytic support for liquid phase reaction

Article abstract of DOI:10.1021/nl904082k

An important goal in the preparation of highly active supported metal particles is the enhancement of the metal support Interaction, providing a more stable catalyst, especially for liquid phase reactions as the leaching and reconstruction of the active p

Full text of DOI:10.1021/nl904082k

pubs.acs.org/NanoLett
Covalent Triazine Framework as Catalytic
Support for Liquid Phase Reaction
†
‡
†
§
,†
Carine E. Chan-Thaw, Alberto Villa, Phisan Katekomol, Dangsheng Su, Arne Thomas,*
‡
and Laura Prati
†
‡
Technische Universit a¨ t Berlin, Englische Strasse 20, 10587 Berlin, Germany, Dipartimento di Chimica Inorganica
§
Metallorganica e Analitica, Universit a` degli Studi di Milano, via Venezian 21, 20133 Milano, Italy, and Inorganic
Chemistry department, Fritz Haber Institute der Max Planck Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
ABSTRACT An important goal in the preparation of highly active supported metal particles is the enhancement of the metal support
interaction, providing a more stable catalyst, especially for liquid phase reactions as the leaching and reconstruction of the active
phase causes deactivation. In this work, a covalent triazine framework (CTF) as support for Pd nanoparticles is compared to activated
carbon (AC), the typical support used in liquid phase reactions. The results indicate that the presence of the N-heterocyclic moieties
on the surface of the frameworks is beneficial for improving the stability of Pd nanoparticles during the liquid phase glycerol oxidation.
Pd/CTF showed better activity and in particular better stability when compared to Pd supported on activated carbon (AC).
KEYWORDS Porous polymer framework, palladium, liquid phase oxidation, glycerol
9
he development of new supports for the immobiliza-
tion of metal nanoparticles as catalyst for liquid phase
anoacetate and ethyl acetoacetate, Aza-Michael condensa-
10
tion and ethyldecanoate transesterification. Some groups
investigated metal incorporation into MOF. Hermes et al.
successfully loaded metals into MOFs by chemical vapor
deposition. Hwang et al. proposed the incorporation of
palladium by ionic exchange after amine grafting onto
coordinatively unsaturated metal sites of MOF. The genera-
tion of silver and gold nanoparticles in situ was tried by
Moon et al. within a flexible nickel-containing MOF. Metal-
containing MOFs have also been extensively considered as
1
1
T
reactions has been recently the subject of growing
1
interest. A solution for enhancing the catalytic performance
and the stability of these systems is the introduction of
functional, for example nitrogen groups, on the surface of
the support. Jiang et al. observed that the introduction of
12
2,3
nitrogen functionalities on the surface of carbon nanotubes
CNTs) increases the dispersion and the stability of gold
13
(
nanoparticles. The homogeneous distribution of gold nano-
particles was enabled by the strong interaction with the
nitrogen-doped carbon nanotubes. Nitrogen-doped CNTs
supported metal nanoparticles show better activity than
pristine CNTs when used as catalyst for the Heck reaction
14-17
support for catalysis in liquid phase reactions.
Pal-
ladium particles supported on MOF-5 catalyzed the hydro-
genation of ethyl cinnamate without any observation of
palladium leaching, as well as the hydrogenation of styrene,
1-octene and cis-cyclooctene. We recently developed a
new class of covalent organic frameworks that are formed
4
18
of iodobenzene and styrene and the cinnamaldehyde hy-
5
drogenation. The cinnamaldehyde hydrogenation into hy-
19-22
drocinnamaldehyde was successful also on nitrogen-doped
by the trimerization of aromatic nitriles in molten ZnCl2..
6
CNTs using palladium as the active phase. More recently,
CTFs exhibit very high surface areas and high amounts of
nitrogen functionalities in the networks. Because of the fully
covalent structure, they possess an increased thermal and
chemical stability, making them interesting candidates as
new catalyst supports in liquid phase reactions.
mesoporous nitrogen containing carbon nanoparticles were
reported highly active in the base-catalyzed transesterifica-
tion of ꢀ-keto esters of aryl, aliphatic, and cyclic primary
7
alcohols.
Another class of materials, which have been recently
Indeed, CTFs have recently shown to be highly stable
23
investigated as support, are metal organic frameworks
even at harsh conditions. Palkovits et al. applied a CTF
loaded with a platinum salt as catalyst for methane oxidation
in concentrated sulfuric acid at 215 °C. The catalyst and the
CTF support showed no detectable degradation even after
several reaction cycles. We report herein, the application of
CTFs as support for Pd nanoparticles for the liquid phase
oxidation of glycerol. This reaction is typically carried out
in the presence of dioxygen as the oxidant and different
authors report the use of Au, Pd, and bimetallic systems on
8
(
MOFs) exhibiting high surface area and tunable pore size.
The versatile coordination chemistry and polytopic linkers
allow for the design of unlimited structures of MOFs. When
functionalized with amino groups, MOFs are active in the
Knoevenagel condensation of benzaldehyde with ethyl cy-
*
To whom correspondence should be addressed. E-mail: arne.thomas@
tu-berlin.de.
Received for review: 10/9/2009
Published on Web: 01/19/2010
24-27
activated carbon (AC) as the catalytic system.
Under
such conditions, Pd supported on AC suffers from severe
©
2010 American Chemical Society
537
DOI: 10.1021/nl904082k | Nano Lett. 2010, 10, 537-541

FIGURE 1. Reaction scheme of the dynamic trimerization of terephthalonitrile leading to the formation of a 2D framework. At 400 °C, an
organized triazine network is formed whereas at 600 °C an amorphous network is obtained.
deactivation, which is ascribed to the following factors: metal
sintering and/or segregation, leaching, irreversible adsorp-
tion, and overoxidation.
The use of CTFs as support for Pd-NPs, could represent
an advantage in terms of metal-support interactions that
in turn could modify and possibly improve the resistance of
the catalyst.
TABLE 1. Characterization of the Samples (Activated Carbon
X40S, CTF, and Pd/CTF) by Nitrogen Sorption Measurements and
Combustion Elemental Analysis
25
elemental analysis
(
wt %)
surface area pore volume
2
-1
3
-1
(
m g
)
(cm g
)
C
N
H
CTF
X40S
2814
1100
2490
1.79
1.50
1.56
71
80
67
9.0
0.3
9.0
0.9
0.3
0.9
The CTF support was synthesized using 1,4-dicyanoben-
1
wt % Pd/CTF
zene and ZnCl
transferred to a sealed quartz ampule and heated to 400 and
00 °C. During the first temperature step (400 °C) an
2
in a molar ratio of 1:5. This mixture is
CTF and Pd/CTF were characterized by X-ray diffraction
6
(XRD), nitrogen sorption measurements, and combustion
elemental analysis. As expected, the CTF and Pd/CTF are
fully amorphous. The diffractograms of the Pd/CTF before
and after the catalytic reaction show a broad peak at 2θ )
26° corresponding to an average distance of 3,4 Å showing
a partial stacking of aromatic units and a peak at 2θ ) 38.6°
for Pd/CTF confirming the presence of Pd(111). The
nitrogen sorption results are consistent with former re-
organized triazine network is formed, which is reorganized
at the second higher temperature step (600 °C), involving
CN eliminations and irreversible condensation reactions into
an amorphous network (Figure 1). Indeed, at 600 °C open-
ing triazine cross-links is leading to local expansions of the
20-22
network.
This reorganization yields on the one hand a
decrease of the overall nitrogen content, from 20 wt % for
the ideal CTF structure to 10 wt % for the CTF heated to 600
1
9-22
sults reported by Kuhn et al.
CTF and Pd/CTF are
°
C. On the other, a hierarchical pore structure is formed
micro- and mesoporous and have high surface areas, over
2400 m g (Table 1).
2
-1
exhibiting mesopores beside micropores and surface area
and pore volume increase to a large extend in these amor-
phous networks.
The transmission electron microscopy (TEM) (Figure 2)
and scanning electron microscopy (SEM) (Figure 3) overview
images of the Pd/CTF show that the Pd nanoparticles are well
dispersed on the support. In Table 2, the median size of Pd
nanoparticles before immobilization and immobilized on AC
and CTF are shown. In both cases, a slight growing of the
metal nanoparticles size during the immobilization has been
observed, mainly due to aggregation of the nanoparticles.
However, the Pd particles on CTF have a smaller average
size and also exhibit a more homogeneous distribution when
compared to Pd/AC. The catalysts were tested in the liquid
phase oxidation of glycerol (glycerol solution 0.3 M and
NaOH/glycerol ratio ) 4 mol/mol, substrate/metal 1000 mol/
mol were mixed in distilled water with a total volume of 10
2
After the reaction, the ZnCl is removed by extensive wash-
ing with water (0.3 wt % Zn remains checked by inductive
coupled plasma, ICP). The CTF was dried and carefully grinded
before the application of the metal nanoparticles.
Pd/AC (1 wt %) and 1 wt % Pd/CTF were synthesized via
28
the sol immobilization technique (NaBH
ladium metal sol was generated by NaBH
Na PdCl salt in the presence of a protective agent (polyvinyl
4
/PVA). The pal-
4
reduction of
2
4
alcohol, PVA) which provides electrostatic as well as sterical
stabilization of the nanoparticles. In principle, the immobi-
lization of preformed Pd nanoparticles assured a similar
particle size distribution on different support. To study a
possible contribution of Zn as catalytic metal in the liquid
phase oxidation, additional Zn was added to the support (1.3
wt % determined by ICP) and on the modified CTF, Pd NPs
were immobilized.
2
mL. The reactor was pressurized at 3 bar of O and thermo-
statted at 50 °C). A first reaction using Pd-free CTF was
performed in order to determine a possible activity of the
Zn; no activity was observed.
©
2010 American Chemical Society
538
DOI: 10.1021/nl904082k | Nano Lett. 2010, 10, 537-541

TABLE 3. Activity and Selectivity of Pd/AC and Pd/CTF for
a
Glycerol Oxidation
c
b
selectivity
initial rate of
consumption of glycerol glyceric tartronic glycolic oxalic
-
1
-1
catalyst
(mol*h *mol
)
acid
acid
acid
acid
Pd/AC
Pd/CTF
948
982
76
81
6
12
14
4
4
3
a
Glycerol solution (0.3 M and NaOH/glycerol ratio ) 4 mol/mol),
and the catalyst (substrate/metal) 1000 mol/mol were mixed in
distilled water (total volume 10 mL). The reactor was thermostated at
b
5
0 °C and pressurized at 3 bar of O
2
.
Mols of glycerol converted
c
divided by time and total mol of metal. Calculated at 50% of
conversion.
FIGURE 2. TEM image of 1 wt % Pd/CTF catalyst before reaction.
FIGURE 4. Supported Pd-based catalysts for glycerol oxidation.
Glycerol solution (0.3 M and NaOH/glycerol ratio ) 4 mol/mol), and
the catalyst (substrate/metal) 1000 mol/mol were mixed in distilled
water (total volume 10 mL). The reactor was pressurized at 3 bar of
2
O and thermostatted at 50 °C.
FIGURE 3. SEM image of 1 wt % Pd/CTF catalyst before reaction.
this result a stability test of Pd/CTF, that is, recycling tests,
has been performed (Figure 6). We observed a decreasing
activity of the catalyst only after the fourth run, and TEM
investigation (Figure 7) revealed an increase of the particles
size (from 3.08 to 4.88 nm) shown in Table 4, which is
accompanied by transport of the noble metal particles to the
external surface of the catalyst.
TABLE 2. Unsupported and Supported Pd Nanoparticles Size
before Reaction
statistical median (nm)
standard deviation σ
Pd colloid
Pd/AC
Pd/CTF
2.76
3.94
3.08
0.59
1.15
0.73
a
a
Data from ref 31.
However, it can be stated that Pd/CTF deactivates much
slower than Pd/AC, which can be addressed to the enhanced
stability of the Pd NPs by increasing the interaction with the
support due to the nitrogen functionalities present in the
structure of CTF. The same behavior was previously ob-
served introducing nitrogen functionalities on the surface of
Pd/CTF showed a slightly better initial activity than Pd/
AC (respectively 982 and 948 mol h-1 mol ) as shown in
Table 3. However the following reaction profile (Figure 4) of
the two catalysts is completely different. For Pd/AC, a strong
deactivation has been observed as already reported in the
-1
4-6
CNTs.
25,29-31
literature.
After only three hours of reaction, glycerol
Strategies for a further improvement, that is, stabilization
of the catalysts can thus be envisaged by introducing more
heteroatoms with even higher coordination ability into the
networks. Such networks could be made for example using
was fully converted on Pd/CTF whereas the full conversion
was not reached on Pd/AC. This loss of activity of Pd/AC can
be addressed here to Pd aggregation as evidenced by TEM
of the used catalyst (Figure 5) but also we cannot exclude
metal structural modification, leaching, irreversible adsorp-
21,23
dicyanopyridine instead of dicyanobenzene monomers,
which is the subject of current studies.
25,29-31
tion, overoxidation of Pd.
On the contrary, Pd/CTF
Furthermore Pd/CTF showed a slightly better selectivity
to glyceric acid than Pd/AC (respectively 81 and 76%) but
did not show any obvious decrease of activity. To confirm
©
2010 American Chemical Society
539
DOI: 10.1021/nl904082k | Nano Lett. 2010, 10, 537-541

FIGURE 7. Electron micrograph of 1 wt % Pd/CTF after 4 cycles.
FIGURE 5. Electron micrograph of 1 wt % Pd/AC after reaction.
FIGURE 6. Conversion on recycling of 1% Pd/CTF. Glycerol solution
FIGURE 8. Distribution of products using Pd/CTF for glycerol oxida-
tion. Glycerol solution (0.3 M and NaOH/glycerol ratio ) 4 mol/mol),
and the catalyst (substrate/metal) 1000 mol/mol were mixed in
distilled water (total volume 10 mL). The reactor was pressurized at
(
(
0.3 M and NaOH/glycerol ratio ) 4 mol/mol), and the catalyst
substrate/metal) 1000 mol/mol were mixed in distilled water (total
volume 10 mL). The reactor was pressurized at 3 bar of O
thermostatted at 50 °C.
2
and
3
2
bar of O and thermostatted at 50 °C.
also the product distribution differs (Figure 8). In the case
of the CTF catalyst, the main byproduct is the tartronic acid
from the subsequent oxidation of glycerate, whereas for Pd/
AC the amount of glycolic acid produced due to C-C bond
cleavage increases significantly. As in any case some small
amounts of Zn is remaining in the CTF structures (here 0.3
wt %) we also attempted to exclude any beneficial or
detrimental effect of this residue on the catalyst perfor-
mance. Therefore a second Pd/CTF catalyst was tested
where the Zn content was increased deliberately to 1.3 wt
TABLE 4. Pd/CTF Particles Size before Glycerol Oxidation and
after 4 Times Recycling
statistical median (nm) standard deviation σ
Pd/CTF before reaction
Pd/CTF after 4 cycles
3.08
4.88
0.73
1.12
dinating and thus stabilizing the metal nanoparticles and by
the porous surface of the support that may offer steric
restriction to confine the metal particles and thus to prevent
the growth of metal clusters. Moreover the same constrains
could have a beneficial effect on selectivity for the catalyst.
%
2
by wet impregnation of ZnCl . The two catalysts contain-
ing respectively 0.3 and 1.3 wt % of Zn showed the same
activity; thus we can exclude any contribution of the Zn
present in the support on the catalytic activity.
In summary we could conclude that Pd/CTF appeared
more resistant to deactivation and more selective toward
glycerate than Pd/AC. The longer lifetime of the catalyst Pd/
CTF in the oxidation of glycerol can be explained by the high
amount of nitrogen functionalities in the framework coor-
Acknowledgment. Financial support by the UniCat cluster
of excellence (Unifying Concepts in Catalysis, Berlin) and
Fondazione Cariplo are gratefully acknowledged. We thank
Professor Robert Schl o¨ gl for helpful discussions. The authors
would like to thank Massimo Tagliaferro from Nanovision
and Paolo De Natale from Hitachi High-Technologies for the
SEM images.
©
2010 American Chemical Society
540
DOI: 10.1021/nl904082k | Nano Lett. 2010, 10, 537-541

Supporting Information Available. Experimental part
materials and methods, synthesis), parameters of the glyc-
erol oxidation, BET isotherms and XRD pattern of CTF and
Pd/CTF. This material is available free of charge via the
Internet at http://pubs.acs.org.
(13) (a) Moon, H. R.; Kim, J. H.; Paik Suh, M. Angew. Chem. 2005, 117,
287. (b) Moon, H. R.; Kim, J. H.; Paik Suh, M. Angew. Chem., Int.
1
(
Ed. 2005, 44, 1261.
(14) Ishida, T.; Nagaoka, M.; Akita, T.; Haruta, M. Chem.sEur. J. 2008,
4
, 8456.
(
15) Opelt, S.; Turk, S.; Dietzsch, E.; Henschel, A.; Kaskel, S.; Klemm,
E. Catal. Commun. 2008, 9, 1286.
REFERENCES AND NOTES
(16) Llabr e´ s i Xamena, F. X.; Abad, A.; Corma, A.; Garcia, H. J. Catal.
007, 250, 294.
17) Zhang, X.; Llabr e´ s i Xamena, F. X.; Corma, A. J. Catal. 2009, 265,
55.
(18) Sabo, M.; Henschel, A.; Froede, H.; Klemm, E.; Kaskel, S. J. Mater.
Chem. 2007, 17, 3827.
2
(
1)
(a) Gangwal, V. R.; van der Schaaf, J.; Kuster, B. M. F.; Schouten,
J. C. J. Catal. 2005, 32, 432. (b) Hutchings, G. J.; Carrettin, S.;
Landon, P.; Edwards, J. K.; Enache, D.; Knight, D. W.; Xu, Y. J.;
Carley, A. F. Top. Catal. 2006, 38, 223. (c) Sheldon, R. A.; Arends,
I. W. C. E.; Dijksman, A. Catal. Today 2000, 57, 157.
Jiang, K.; Eitan, A.; Schadler, L. S.; Ajayan, P. M.; Siegel, R. W.
Nano Lett. 2003, 3, 275.
(
1
(
2)
(19) Kuhn, P.; Antonietti, M.; Thomas, A. Angew. Chem., Int. Ed. 2008,
47, 3450.
(
(
(
3)
4)
5)
Hou, X.; Wang, L.; Zhou, F.; Wang, F. Carbon 2009, 47, 1209.
Yoon, H.; Ko, S.; Jang, J. Chem. Commun. 2007, 1468.
Lepr o` , X.; Terries, E.; Vega-Cant u` , Y.; Rodriguez-Macias, F. J.;
Muramatsu, H.; Ahm Kim, Y.; Hayahsi, T.; Endo, M.; Torres,
R. M.; Terrones, M. Chem. Phys. Lett. 2008, 463, 124.
Amadou, J.; Chirazi, K.; Houll e´ , M.; Janowska, I.; Ersen, O.; B e´ gin,
D.; Pham-Huu, C. Catal. Today 2008, 138, 62.
(20) Kuhn, P.; Forget, A.; Su, D. S.; Thomas, A.; Antonietti, M. J. Am.
Chem. Soc. 2008, 130, 13333.
(21) Kuhn, P.; Thomas, A.; Antonietti, M. Macromolecules 2009, 42, 319.
(
(
(
22) Kuhn, P.; Forget, A.; Hartmann, J.; Thomas, A.; Antonietti, M. Adv.
Mater. 2009, 21, 897.
23) Palkovits, R.; Antonietti, M.; Kuhn, P.; Thomas, A.; Sch u¨ th, F.
Angew. Chem., Int. Ed. 2009, 48, 1.
24) Carrettin, S.; McMorn, P.; Johnston, P.; Griffin, K.; Hutchings, G. J.
Chem. Comm. 2002, 696.
(
(
(
(
(
(
6)
7) Jin, X.; Balasubramanian, V. V.; Selvan, S. T.; Sawant, D. P.; Chari,
M. A.; Lu, G. Q.; Vinu, A. Angew. Chem., Int. Ed. 2009, 48, 1.
8)
9)
Farrusseng, D.; Aguado, S.; Pinel, C. Angew. Chem., Int. Ed. 2009,
8, 2.
(
(
25) Porta, F.; Prati, L. J. Catal. 2004, 224, 397.
26) Ketchie, W. C.; Fang, Y.; Wong, M. S.; Murayama, M.; Davis, R. J.
J. Catal. 2007, 250, 94.
4
Gascon, J.; Hernandez-Alonso, M. D.; Ameida, A. R.; van Klink,
G. P. M.; Kapteijn, F.; Mul, G. J. Catal. 2009, 261, 75.
(
(
27) Demirel-G u¨ len, S.; Lucas, M.; Claus, P. Catal. Today 2005, 166, 102.
28) Dimitratos, N.; Lopez-Sanchez, J. A.; Lennon, D.; Porta, F.; Prati,
L.; Villa, A. Catal. Lett. 2006, 147.
10) Savonnet, M.; Ravon, U.; Aguado, S.; Bazer-Bachi, D.; Lecocq, V.;
Bats, N.; Pinel, C.; Farrusseng, D. Green Chem. 2009, 11, 1729.
11) Hermes, S.; Schroter, M.-K.; Schmid, R.; Khodeir, L.; Muhler, M.;
Tissler, A.; Fischer, R. W.; Fischer, R. A. Angew. Chem., Int. Ed.
(29) Wang, D.; Villa, A.; Porta, F.; Su, D.; Prati, L. Chemm.Comm. 2006,
2005, 44, 6237.
1956.
(
12) Hwang, Y. K.; Hong, D.-Y.; Chang, J.-S.; Jhung, S. H.; Seo, Y.-K.;
Kim, J.; Vimont, A.; Daturi, M.; Serre, C.; F e´ rey, G. Angew. Chem.,
Int. Ed. 2008, 47, 4144.
(30) Villa, A.; Campione, C.; Prati, L. Catal. Lett. 2007, 115, 133.
(31) Villa, A.; Janjic, N.; Spontoni, P.; Wang, D.; Su, D. S.; Prati, L. Appl.
Catal., A 2009, 221.
©
2010 American Chemical Society
541
DOI: 10.1021/nl904082k | Nano Lett. 2010, 10, 537-541