Tuning porosity and activity of microporous polymer network organocatalysts by co-polymerisation

Article abstract of DOI:10.1039/c3cc49220f

Microporous polymer networks based on binaphthyl phosphoric acids are suitable heterogeneous asymmetric organocatalysts. Herein we show that the porous characteristics of such networks can be fine-tuned by co-polymerisation. This enables us to investigate the influence of the surface area and porosity in microporous networks on their catalytic performance. In this case, the activity of the polymers in an asymmetric hydrogenation reaction is increased by the use of polymers with higher surface areas. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cc49220f

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Tuning porosity and activity of microporous
polymer network organocatalysts by
co-polymerisation†
Johannes Schmidt,*a Dipti Sankar Kundu,b Siegfried Blechert*b and Arne Thomasa
Cite this: Chem. Commun., 2014,
50, 3347
Received 4th December 2013,
Accepted 9th February 2014
DOI: 10.1039/c3cc49220f
www.rsc.org/chemcomm
Microporous polymer networks based on binaphthyl phosphoric we report that the surface area and pore volume of these micro-
acids are suitable heterogeneous asymmetric organocatalysts. porous polymer networks can be tuned by co-polymerisation. This
Herein we show that the porous characteristics of such networks enables us to compare the catalytic performance of the networks in
can be fine-tuned by co-polymerisation. This enables us to inves- transfer hydrogenations as a function of their surface area and
tigate the influence of the surface area and porosity in microporous porosity. It is known that co-polymerisation with structure directing
networks on their catalytic performance. In this case, the activity of motifs of different sizes/lengths can result in networks differing in
the polymers in an asymmetric hydrogenation reaction is increased surface areas and pore volumes.17,18 These porous characteristics
by the use of polymers with higher surface areas.
are some of the key aspects in catalysis as all reactions take place on
interfaces, and therefore an increase in the surface area and pore
Catalysis, one of the main fields of application of porous materials, volume should in principle increase the accessibility of the active
has moved into the focus of microporous polymer research.2–6 centres and thus the reaction rate.
In recent years several examples of microporous networks
To obtain a catalytically active network we polymerised (R)-
based on organic molecules have been shown to be active in 3,30-bis(10-(thiophen-3-yl)anthracen-9-yl)-1,10-binaphthalen-2,20-
various catalytic reactions,7–10 while especially organocatalysis diyl-phosphoric acid (BNPPA) with 1,3,5-tris(2-thienyl)benzene
is an emerging topic.1,7,11–16 As microporous polymer networks (TTB) in a random oxidative co-polymerisation in monomer
can be formed by polymerisation of molecular catalysts, no ratios of 1 : 1 and 1 : 5 of BNPPA to TTB (Scheme 1).
further post-immobilisation process is required. In addition, as
the catalytic active sites are a major part of the polymer back- area of 386 m2 gꢀ1 while networks based on TTB have a surface
bone, the often observed low loading of immobilised molecular area of 1060 m2 gꢀ1 19 By mixing these tectons we could obtain
BNPPA is known to build porous networks with a surface
.
catalysts on preformed supports can be avoided. Therefore, due networks with increased surface areas compared to the one
to their high surface area and insoluble nature, these networks solely composed of BNPPA (see Table 1). Thus for monomer
combine the benefits of both homogeneous and heterogeneous ratios of 1 : 1 and 1 : 5, networks with BET surface areas of
catalysis. The insolubility enables easy separation after the 577 m2 gꢀ1 and 668 m2 gꢀ1 and pore volumes of 0.49 cm3 gꢀ1
reaction while the high porosity allows good accessibility of
catalytic centres, important to achieve high conversion rates.
Recently we showed that microporous polymer networks based on
binaphthyl phosphoric acids can be used in asymmetric hetero-
geneous organocatalysis.1,7 Using oxidative polymerisation we
obtained microporous networks with surface areas up to
386 m2 gꢀ1 that showed very high selectivity in the transfer hydro-
genation reaction and C–C bond formation. In this communication
a
¨
Technische Universitat Berlin, Hardenbergstraße 40, 10623 Berlin, Germany.
E-mail: johannes.schmidt@tu-berlin.de
b
¨
Technische Universitat Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany.
E-mail: blechert@chem.tu-berlin.de
Scheme 1 Monomers used for the co-polymerisation: BNPPA (1) and
TTB (2).
† Electronic supplementary information (ESI) available: General procedures,
synthetic details, and NMR spectra. See DOI: 10.1039/c3cc49220f
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 3347--3349 | 3347

View Article Online
Communication
ChemComm
Table 1 Surface area and pore volume of the polymer networks
Surface area/
m2 gꢀ1
Pore volume/
cm3 gꢀ1
TOF/
hꢀ1
Polymer network
BNPPA network
386
577
668
1060
—
0.28
0.49
0.58
0.71
—
5.24
7.45
10.07
—
BNPPA/TTB 1 : 1 co-polymer
BNPPA/TTB 1 : 5 co-polymer
TTB network19
Molecular catalyst
12.38
TOF was calculated after 65 min reaction time.
Fig. 2 Transfer hydrogenation of 2-phenylquinoline monitored by in situ
NMR measurements.
The conversion rate of the co-polymer 1 : 1 is higher than
that obtained for the pure BNPPA catalyst. The reactions with
the co-polymer network 1 : 5 shows even higher conversion than
that obtained with co-polymer 1 : 1, which corresponds to the
difference in surface area and porosity.
This experiment shows that the reaction rate can be
enhanced by increasing the accessible surface area and pore
volume of the microporous polymer network catalysts. This is
most probably due to the more easily accessible active centres
Fig. 1 Nitrogen sorption isotherms of polymer networks.
and 0.58 cm3 gꢀ1 were obtained, respectively. Nitrogen sorption and the higher diffusion rate of substrates within the network
isotherms of the polymer networks are shown in Fig. 1. The ratio which enables the possibility to tune porosity in a way to fit to
of the monomers in the resulting co-polymer networks was particular substrates and their reaction conditions. It should be
determined by ICP-OES and is comparable to the initial ratio. however noted that porous characteristics are measured at low
Additionally no iron residues could be detected via this method. pressures at 77 K, and thus are actually not comparable to the
As shown before, the network solely composed of molecule 1 situation in a reaction mixture, i.e. in solvents at elevated
showed excellent performance as an organocatalyst in the temperatures. For example, some swelling of the polymer net-
transfer hydrogenation on benzoxazines (499% conv. after 2 h, works in the reaction mixture has to be assumed, as this has
98% ee). To study the influence of porosity on the catalytic been even observed to happen during nitrogen sorption mea-
performance we chose a substrate with lower electron density at surements.20–23 This swelling should increase the accessibility
which a lower reaction rate can be expected. The kinetic study was of catalytic centres, and even materials which showed lower
therefore performed using 2-phenylquinoline as a substrate which surface areas in nitrogen sorption measurements can be quite
was converted in good yields and selectivity before (99% conv. after active catalysts under realistic reaction conditions. However,
24 h, 99% ee, 5 mol% catalyst).1 The reaction was performed in the measurements presented here prove that there is an
deuterochloroform at room temperature using 2 mol% of the influence of the intrinsic porosity of microporous polymers
catalyst only, calculated for the catalytically active centres. We used on the catalytic performance. Thus the accessibility of active
such little amounts to be able to easily follow the conversion of the sites is provided not only by swelling of polymer networks, but
heterogeneous reaction by in situ NMR measurements. Blank also by their open channel structure.
experiments without any catalyst and with PTTB showed no
In summary, we synthesised microporous polymer networks
conversion of the substrate. As the catalyst is covalently bonded based on BNPPA that can be used to bridge the gap between
in the network, catalyst leaching is improbable as shown asymmetric homogeneous and heterogeneous catalysis. The
in previous studies.1,7 In Fig. 2, the catalytic performance of the control over the surface area and pore volume of the polymer
pure BNPPA network is compared with the performance of the networks via co-polymerisation enables the possibility to study
co-polymers and the molecular catalyst. As expected, the homo- their influence on the catalytic performance. We could show
geneous catalysis using the molecular catalyst shows a higher that polymer networks with higher surface areas show higher
conversion under the given conditions. For the network composed reaction rates in asymmetric transfer hydrogenation. This
of pure BNPPA a conversion of 50% can be obtained after 6 hours. indicates that the catalytic reaction takes place inside the
The same reaction was performed with the co-polymer networks network and the porosity directly influences the diffusion and
using always the same molar amount of the binaphthyl phospho- accessibility of the active catalytic centres.
ric acid. Fig. 2 shows the progress of the reaction over time, and
the corresponding TOFs are shown in Table 1.
We acknowledge financial support from the European
Research Council within the project 278593_ORGZEO and the
3348 | Chem. Commun., 2014, 50, 3347--3349
This journal is ©The Royal Society of Chemistry 2014

View Article Online
ChemComm
Communication
12 M. Rose, A. Notzon, M. Heitbaum, G. Nickerl, S. Paasch, E. Brunner,
F. Glorius and S. Kaskel, Chem. Commun., 2011, 47, 4814.
13 K. Thiel, R. Zehbe, J. Roeser, P. Strauch, S. Enthaler and A. Thomas,
Polym. Chem., 2013, 4, 1848–1856.
Deutsche Forschungsgemeinschaft (DFG) within the cluster of
excellence ‘‘UNICAT – Unifying Concepts in Catalysis’’.
14 J. Roeser, K. Kailasam and A. Thomas, ChemSusChem, 2012, 5,
1793–1799.
Notes and references
1 D. S. Kundu, J. Schmidt, C. Bleschke, A. Thomas and S. Blechert, 15 C. A. Wang, Z. K. Zhang, T. Yue, Y. L. Sun, L. Wang, W. D. Wang,
Angew. Chem., Int. Ed., 2012, 51, 5456–5459.
2 A. Thomas, Angew. Chem., Int. Ed., 2010, 49, 8328–8344.
3 P. Kaur, J. T. Hupp and S. T. Nguyen, ACS Catal., 2011, 1, 819–835.
Y. Zhang, C. Liu and W. Wang, Chem.–Eur. J., 2012, 18, 6718–6723.
16 E. Merino, E. Verde-Sesto, E. M. Maya, M. Iglesias, F. Sanchez and
A. Corma, Chem. Mater., 2013, 25, 981–988.
4 Y. G. Zhang and S. N. Riduan, Chem. Soc. Rev., 2012, 41, 2083–2094. 17 J. Schmidt, M. Werner and A. Thomas, Macromolecules, 2009, 42,
5 R. Dawson, A. I. Cooper and D. J. Adams, Prog. Polym. Sci., 2012, 37,
530–563.
6 Q. Sun, Y. Jin, L. Zhu, L. Wang, X. Meng and F.-S. Xiao, Nano Today,
2013, 8, 342–350.
7 C. Bleschke, J. Schmidt, D. S. Kundu, S. Blechert and A. Thomas,
Adv. Synth. Catal., 2011, 353, 3101–3106.
8 Z. G. Xie, C. Wang, K. E. deKrafft and W. B. Lin, J. Am. Chem. Soc.,
2011, 133, 2056–2059.
4426–4429.
18 J. X. Jiang, F. Su, A. Trewin, C. D. Wood, H. Niu, J. T. A. Jones,
Y. Z. Khimyak and A. I. Cooper, J. Am. Chem. Soc., 2008, 130, 7710–7720.
19 J. Schmidt, J. Weber, J. D. Epping, M. Antonietti and A. Thomas, Adv.
Mater., 2009, 21, 702–705.
20 D. Z. Tan, W. J. Fan, W. N. Xiong, H. X. Sun, Y. Q. Cheng, X. Y. Liu,
C. G. Meng, A. Li and W. Q. Deng, Macromol. Chem. Phys., 2012, 213,
1435–1440.
9 J. X. Jiang, C. Wang, A. Laybourn, T. Hasell, R. Clowes, 21 J. Weber, J. Schmidt, A. Thomas and W. Bohlmann, Langmuir, 2010,
Y. Z. Khimyak, J. L. Xiao, S. J. Higgins, D. J. Adams and
A. I. Cooper, Angew. Chem., Int. Ed., 2011, 50, 1072–1075.
10 X. Du, Y. L. Sun, B. E. Tan, Q. F. Teng, X. J. Yao, C. Y. Su and
W. Wang, Chem. Commun., 2010, 46, 970–972.
11 Y. Zhang, Y. Zhang, Y. L. Sun, X. Du, J. Y. Shi, W. D. Wang and
W. Wang, Chem.–Eur. J., 2012, 18, 6328–6334.
26, 15650–15656.
22 J. Weber, M. Antonietti and A. Thomas, Macromolecules, 2008, 41,
2880–2885.
23 J. X. Jiang, A. Trewin, F. B. Su, C. D. Wood, H. J. Niu, J. T. A. Jones,
Y. Z. Khimyak and A. I. Cooper, Macromolecules, 2009, 42,
2658–2666.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 3347--3349 | 3349