Miniaturized catalysis: monolithic, highly porous, large surface area capillary flow reactors constructed in situ from polyhedral oligomeric silsesquioxanes (POSS)

Article abstract of DOI:10.1039/c5cy00510h

A single-step molding process utilizing free-radical cross-linking reaction of vinyl POSS in microliter-sized dimensions leads to hierarchically-structured, mechanically robust, porous hybrid structures. Functional variants show excellent performance in Suzuki-type coupling reactions. Due to their small volume, long-term operational robustness, and potential chemical diversity, these materials are promising candidates for catalyst screening applications.

Full text of DOI:10.1039/c5cy00510h

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Miniaturized catalysis: monolithic, highly porous,
large surface area capillary flow reactors
constructed in situ from polyhedral oligomeric
silsesquioxanes (POSS)†
Cite this: DOI: 10.1039/c5cy00510h
Received 7th April 2015,
Accepted 2nd June 2015
DOI: 10.1039/c5cy00510h
*
P. Scholder and I. Nischang
www.rsc.org/catalysis
A single-step molding process utilizing free-radical cross-linking
reaction of vinyl POSS in microliter-sized dimensions leads to hier-
archically-structured, mechanically robust, porous hybrid struc-
tures. Functional variants show excellent performance in Suzuki-
type coupling reactions. Due to their small volume, long-term
operational robustness, and potential chemical diversity, these
materials are promising candidates for catalyst screening
applications.
Polyhedral oligomeric silsesquioxanes (POSS) with the for-
mula IJRSiO1.5 n are nano-building blocks have already been
)
used for the creation of advanced porous materials.11 We
have demonstrated that vinyl POSS cages can be woven into
(bulk) porous monolithic networks directly via a free-radical
initiated in situ reaction in suitable porogenic diluents lead-
ing to large specific surface area sorbents.11c Such hybrid
porous materials have not yet entered the micro-engineering
arena but possess the option for scaling due to the in situ
preparation. An important asset of these porous materials is
the tailorable amount of tightly tethered vinyl functionality
on their internal structure.11c
Palladium-based catalytic systems for cross-coupling have
been pioneered by Richard F. Heck, Ei-ichi Negishi and Akira
Suzuki.1 Due to their recognized importance and the incep-
tion of a multitude of possibilities emerging therefrom, they
have stimulated a great deal of research in the academic and
industrial communities.2 A recent outreach for related car-
bon–carbon coupling reactions can be found in the develop-
ment of advanced materials.3
While homogeneous catalytic systems provide high prod-
uct yields,4 their inherent disadvantage is in the necessity to
separate products from the catalyst.5 This situation has
spurred developments in heterogeneous catalytic systems
with examples reporting packed bed reactors,6 monolithic
flow-through formats with silica-based materials7 and organic
(monolithic) polymers.8 Notwithstanding, we note that the
catalytic reaction is often quasi-homogeneous in nature.9
The recent developments in micro-engineering demand
the utilization of miniaturized formats allowing the reduction
in solvent consumption, fast heat and mass transfer, as well
as continuous operation with the option of massive para-
llelization.10 Such systems allow the use of minute catalyst
amounts for screening experiments before up-scaling and
producing fine chemicals on a larger scale.
We herein communicate our first results in a single-step
creation of such large surface area hybrid materials in 100
μm I.D. fused-silica capillaries covalently anchored to the
confining capillary wall. This is followed by straightforward
functionalization of their internal porous structure to chelate
palladium utilized for Suzuki-type coupling reactions.
The internal surface of the 100 μm I.D. fused-silica capil-
laries was first decorated with pendant methacrylate func-
tionality (for detail see the ESI†).12 The porous hybrid mono-
lith precursor mixture was prepared by weighing 20% vinyl
POSS as monomer, 51% tetrahydrofuran, and 29% poly(ethyl-
ene glycol) 200 as porogenic solvents (all w/w). The precursor
mixture additionally contained azobisisobutyronitrile (16
wt% with respect to the vinyl POSS). The homogeneous solu-
tion was filled in the capillary mold by means of a syringe.
Subsequently, both ends of the capillary were sealed with
rubber stoppers and immersed in a water bath with tempera-
ture at 60 °C. After monolith formation, the capillary was
flushed with tetrahydrofuran to remove the porogenic sol-
vents and initiator.
Fig. 1 shows the sample SEM images of the hybrid pristine
monolith. Its pore space possesses a hierarchical structure
and the structure is covalently attached to the fused-silica
capillary inner wall. A dry-state Brunauer–Emmett–Teller
(BET) surface area of 898 m2 g−1 together with an appreciable
Institute of Polymer Chemistry, Johannes Kepler University Linz, A-4060,
Leonding, Austria. E-mail: ivo.nischang@jku.at
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy00510h
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Fig. 1 The scanning electron microscopy (SEM) images of the large
surface area vinyl POSS hybrid pristine polymer. Cross-section of
monolithic hybrid polymer incorporated in 100 μm I.D. fused silica
capillaries clearly showing the macroporous structure and indication of
an appreciable amount of nanopores (<50 nm) further supported by
nitrogen adsorption/desorption measurements of the bulk material
(Fig. S1†). Seen as well is the anchorage to the fused-silica capillary wall.
Fig. 2 Energy-dispersive X-ray spectroscopy (EDX) analysis of cross-
sections of porous materials with the pristine and reactor variants
(Scheme 1).
substitution under secondary amine formation (Scheme 1,
route (i)). For catalyst 2, the hybrid monolith was modified
dry-state Barrett–Joyner–Halenda (BJH) mesopore volume of
0.4 cm3 g−1 (Fig. S1†) of the prepared bulk material clearly
support the existence of a large surface area, hierarchical
porous structure.
In flow operation, the materials showed a linear increase
in back pressure at increased flow rates (Fig. S2†), indicating
their maintained mechanical integrity. A total porosity of
83% was determined with flow experiments of a small hydro-
philic tracer (for detail see the ESI†). The free-radical initi-
ated linking of vinyl POSS as well results in a multiplicity of
pendant vinyl groups (Scheme 1).11b Two in situ modification
strategies were explored for immobilizing the desirable
phenanthroline ligand to the internal surface of the scaffold,
both employing thiol-ene addition as the primary step
(Scheme 1).
For catalyst 1, the hybrid monolith was functionalized
with 3-chloro-1-propanethiol via thiol-ene addition. Then,
5-amino-1,10-phenanthroline was bound via nucleophilic
Fig.
3 (a) Impact of fluid contact time in the reactors on yield
obtained for the reaction of 0.1 mol L−1 iodobenzene with 0.125 mol
L−1 p-tolylboronic acid (entry 1, Table 1). (b) Long term stability of the
reactors in steady-state at a fluid contact time of 49 min. (c) Yield
achieved from flow-through catalysis of the reaction of iodobenzene
with p-tolylboronic acid keeping a stoichiometric ratio of 1 : 1.25 but
varying concentration at a fluid contact time of 32.6 min. The reactors
had the same length of 30 cm. Other conditions: fluid phase of 75/25
acetonitrile/water (%, v/v) and 2 equiv. triethylamine (to that of the
iodobenzene) as the base. Reactions were performed at a constant
reactor temperature of T = 80 °C.
Scheme 1 In situ functionalization of hybrid porous polymer via thiol-
ene addition as the primary step with either (i) 3-chloro-1-propane-
thiol, or (ii) thioglycolic acid. The following nucleophilic substitution
leads to linkage of the 5-amino-1,10-phenanthroline as the chelating
functionality for palladium. Further experimental details are provided in
the ESI.†
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Table 1 Catalytic reactions performed in the 100 μm I.D. fused silica capillaries containing catalyst 1 operated with 75/25 acetonitrile/water (%, v/v).
Concentration of each aryl halide was 0.1 mol L−1 and of each boronic acid 0.125 mol L−1 with 2 equiv. of triethylamine (to that of the aryl halide) as the
base. The reactions were performed at T = 80 °C with a reactor fluid contact time of 32.6 min
Aryl halide
Boronic acid
Main product
%Yielda
85
95
95
97
17
10
58
a
Conversion of the aryl halide was found equivalent to the yield of main product; yield of homocoupling of the boronic acid was <1% in all
cases.
with thioglycolic acid via thiol-ene addition. The acid was
The reaction of iodobenzene with p-tolylboronic acid in
75/25 acetonitrile/water (%, v/v) was used as a model to judge
the reactor performance under typical conditions, i.e. at a
temperature of T = 80 °C and continuous flow. Fig. 3a com-
pares the respective yields of the flow reactor systems origi-
nating from the same pristine material. The materials with
chelated palladium functionality (catalysts 1 and 2, Scheme
1) approach greater than 85% quantitative yield after a mere
30 min fluid contact time (Fig. 3a). The shape of the yield–
fluid contact time curves of catalysts 1 and 2 is similar. The
performance of both reactors with chelated palladium is bet-
ter than the homogeneous “control” variants in batch experi-
ments and an open tube implementation both with 1 mol%
pristine palladium complex PdIJMeCN)2Cl2 as a catalyst (with
respect to the iodobenzene) in the same fluid phase (Fig.
S4†). Yet, we calculated overall catalyst concentrations of 10
mmol L−1 (catalyst 1) and 4 mmol L−1 (catalyst 2) in the
then activated to an acyl chloride and 5-amino-1,10-
phenanthroline was immobilized via amide bond formation
(Scheme 1, route (ii)). Both phenanthroline pendant scaffolds
were subsequently provided with chelated palladium species
by flushing solutions of PdIJMeCN)2Cl2 complex in acetoni-
trile through the monoliths. Further details are provided in
the ESI.†
To indicate the existence of chelated palladium on the
internal structure of the highly porous material according to
Scheme 1, the as-prepared and washed reactors were ana-
lyzed by energy-dispersive X-ray spectroscopy (EDX) on their
cross-sections. The results in Fig. 2 clearly confirm the exis-
tence of sulfur, indicating successful thiol-ene addition, as
well as Pd and Cl functionalities indicating successful Pd
chelation for catalysts 1 and 2 (Scheme 1). These signals are
totally absent for the pristine material.
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monolithic reactors (see the ESI†). This qualitatively indicates
that at a reactant concentration of 0.1 mol L−1 in the fluid
phase of one reactor volume, a 10 mol% of the palladium
sites is present for catalyst 1. Eventually, not all of the palla-
dium sites in the material are catalytically active, which may
originate from the limited accessibility of the very small
pores providing diffusional resistance and significantly con-
tributing to the large dry-state surface areas (Fig. S1†).
other carbon–carbon cross-coupling reactions is only one
implementation these materials may find use in. Further tai-
loring of porous, hydrodynamic, and in particular chemical
properties of the described pristine porous hybrid polymer in
this article may enable further exploration of diversity and
associated performance. The materials may pave the way for
a family of catalyst systems which allow independent tailor-
ing of porous and hydrodynamic properties as well as inter-
nal chemistry, both building a pivotal role for catalytic
implementations.
Notwithstanding, the performance of the reactors could
be deduced from a good distribution of flow and significant
interactive contact area with chelated palladium functional-
ity. We further found stability and robustness of the reactors
over prolonged periods of time demonstrated for this exam-
ple reaction using catalyst 1 for more than 122 reactor vol-
umes and more than 100 hours of continuous operation
(Fig. 3b). In contrast, catalyst 2 lost performance at longer
times of operation, though initial steady-state reactor perfor-
mance under these conditions is readily similar to both.
These results strongly indicate the lower stability of catalyst 2
associated with the ligand linking strategy (Scheme 1). It is
worth noting that the demonstrated fluid contact times in
this experiment are smaller and the obtained yields larger
than those reported in the literature.8b,c Stability of the
microscale reactor containing catalyst 1 as well is demon-
strated for a timescale twice larger than that of a recently
reported larger scale packed bed reactor.6d Varying reactant
concentrations over more than an order of magnitude for
both reactor systems showed an expected, though decent,
loss of quantitative yield at increased reactant concentration
until the reactant solubility limit in the fluid phase is
reached (Fig. 3c).
The catalytic system worked for other reactants as well
and was studied for more stable catalyst 1. The determined
conversion and yields are summarized in Table 1. High selec-
tivity for the catalyzed reaction with an exclusive absence of
homocoupling of the aryl halide together with small concen-
trations of the boronic acid homocoupling product is
observed (Table 1). The coupling reactions with that of
electron-donating methoxy groups on the aryl halide
improved yields, except for the methoxy group in ortho posi-
tion. These observations may be explained by steric effects.
We found that electron-withdrawing groups such as esters to
decrease the respective yields. In comparison, the amine sub-
stituent in para position shows a good yield under otherwise
similar reactions conditions. This brief study indicates a sys-
tematic insight into catalytic implementations involving vary-
ing reaction partners, made possible with the microscale
reactors.
Acknowledgements
This work was supported by the Austrian Science Fund (FWF)
under project no. [P24557-N19]. The authors acknowledge
Günter Hesser at the Center for Surface and Nanoanalytics,
Johannes Kepler University Linz for experimental support
with the scanning electron microscopy and energy-dispersive
X-ray spectroscopy measurements.
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