Transformable periodic mesoporous organosilica materials

Article abstract of DOI:10.1039/b924570g

The synthesis of novel periodic mesoporous organosilica (PMO) materials incorporating a mixture of functional monomers (cis- and trans-stilbene-based) and structural monomers has been accomplished. EDX analysis of these materials in combination with HAADF

Full text of DOI:10.1039/b924570g

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Transformable periodic mesoporous organosilica materialsw
Steven E. Dickson and Cathleen M. Crudden*
Received (in Berkeley, CA, USA) 23rd November 2009, Accepted 25th January 2010
First published as an Advance Article on the web 16th February 2010
DOI: 10.1039/b924570g
The synthesis of novel periodic mesoporous organosilica (PMO)
materials incorporating a mixture of functional monomers
(cis- and trans-stilbene-based) and structural monomers has been
accomplished. EDX analysis of these materials in combination
with HAADF imaging of OsO4-stained samples reveals a
homogeneous distribution of trans-olefin within the co-condensed
aromatic organosilica matrix. Ozonolysis of the functional
component demonstrates the significant potential of this material
for further modification.
benzene- or biphenyl-bridged monomers (3 and 4) as the
structural elements (Scheme 1). The stilbene unit was chosen
because of the large number of transformations that this
substructure is known to undergo, as well as the possibility
of accessing different conformations or morphologies from the
cis and trans isomers.
To prepare the trans-stilbene functional monomer (1), a
Grubbs olefination reaction was employed which results in the
dimerization of 4-bromostyrene with loss of ethylene in
quantitative yield.5 Conversion of the bromo substituents to
iodides6 is then followed by a Masuda coupling7 to introduce
the polymerizable siloxane units (Scheme 2). For the more
synthetically challenging cis isomer (2), a lithium-free Wittig
reagent was prepared by reaction between triphenylphosphine
and 4-bromobenzylbromide followed by deprotonation with
KOH.8 Treatment with 4-bromobenzaldehyde affords cis-4,40-
dibromostilbene in 72% yield with 80% stereochemical
control. Following halogen exchange, the siloxane units were
installed using the Masuda reaction as described above (see ESIw).
PMO materials were then prepared by mixing the functional
monomer (1 or 2) along with either 1,4-bis(triethoxysilyl)-
benzene (3) or 4,40-bis(triethoxysilyl)biphenyl (4) as the structural
component, in the presence of a structure directing template
(small or large pore-producing) under acidic or basic conditions
(see ESIw). The synthetic conditions and resulting nitrogen
adsorption data are shown in Tables 1 and 2 respectively.
The PMOs prepared exhibited Type IV isotherms with
moderate to high surface areas and narrow pore size distributions.
Periodic mesoporous organosilicas (PMOs)1 have received
considerable attention since they were first reported in 1999,
due in large part to the potential for introducing a wide variety
of structures into the backbone of the material via organic
spacers of the monomer unit [(R0O)3Si–R-Si-(OR0)3].2 Unlike
conventional modifications of mesoporous silica phases that
rely on grafting of siloxanes onto the surface, PMOs can be
prepared with high loadings of the functional group, distributed
homogeneously throughout the bulk of the material. In addition
to imparting interesting properties such as increased hydro-
phobicity or enhanced hydrothermal stability, incorporating
the organic fragment into the wall of the silica matrix circumvents
the commonly associated problem of pore diameter reduction
or blockage following post synthetic modifications, which may
limit the interactions of guest molecules. Finally, as we will
demonstrate, significant modifications can be made to the
organic core in the case of the PMO materials that result in
uniformly distributed organic functionality.
+
+
PMO-1/4H and PMO-2/4H made with Brijs 76 surfactant
Despite the number of reports addressing the design of
PMOs, the majority of these are based on aromatic or
precursors having short, rigid structures. Unfortunately, these
linkers offer limited reactivity and therefore, with few
exceptions,3 have seen limited use in further modifications.
In this communication, we report a novel approach to the
synthesis of PMOs based on the use of two types of mono-
mers, one which is meant to impart function or to provide a
handle for the introduction of further functionality, and one
which is present for structural stability. We recently described
a related material in which the functional component was a
chiral biaryl unit and the structural, an achiral biaryl mono-
mer.4 Herein, we describe a system in which a stilbene-based
monomer (trans or cis) (1 and 2) can be co-incorporated with
displayed type H4 hysteresis loops that do not close at low
Scheme 1 (1) 4,40-Bis(triethoxysilyl)trans-stilbene BTETS; (2) 4,40-
bis(triethoxysilyl)cis-stilbene BTECS; (3) 1,4-bis(triethoxysilyl)-
benzene BTEB; (4) 4,40-bis(triethoxysilyl)biphenyl BTEBP.
Department of Chemistry, Queen’s University, 90 Bader Lane,
Kingston, Ontario, Canada K7L 3N6.
E-mail: cruddenc@chem.queensu.ca; Fax: +1-613533-6669;
Tel: +1-613533-6000
w Electronic supplementary information (ESI) available: Material
preparations and characterizations including N2 isotherms, CP MAS
NMR spectra and XRD patterns are available. See DOI: 10.1039/
b924570g
Scheme 2 (i) Grubbs catalyst (2nd Gen.), CH2Cl2, room temp. 16 h;
(ii) n-BuLi, THF, ꢀ78 1C, 30 min and (iii) I2, ꢀ78 1C to room temp. 16 h.
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Table 1 Synthesis of PMO materials using stilbene and benzene or
biphenyl-based monomersa
and 100% of the functional monomer 1 using P123 as the
surfactant under acidic conditions. It was found that at these
higher concentrations, the stilbene precursor hydrolyzes
readily to the bis(silanetriol), which forms large crystals
instead of condensing as indicated by CP MAS 29Si NMR
and TEM analysis (see ESIw). While at 30% loading, the
material was characterized by a mixture of crystalline and
condensed material, at 100% loading of the stilbene monomer,
only crystals were present, as observed by TEM or inspection
under a regular optical microscope (see ESIw). This has been
observed previously by Corriu and co-workers by hydrolysis
of 1 in a biphasic medium of ether and aqueous HCl.13
Interestingly, despite the fact that our conditions were harsher
Functional
monomer
Structural
monomer
Material
Template
pH
+
PMO-1/4H
1
1
2
4
4
4
3
Brijs 76
C18TMACl
Brijs 76
P123
Acidic
Basic
ꢀ
PMO-1/4OH
+
PMO-2/4H
Acidic
Acidic
+
PMO-1/3H
1
+
Molar ratios: PMO-1/4H (1 : 4 : Brijs 76 : H2O : HCl : NaCl)
a
ꢀ
(0.15 : 0.85 : 0.534 : 601 : 8.05 : 19.0);9 PMO-1/4OH (1 : 4 :
C
18TAMCl : H2O : NaOH) (0.15 : 0.85 : 1.28 : 1320 : 12 100);10
+
PMO-2/4H (2 : 4 : Brijs 76 : H2O : HCl : NaCl) (0.15 : 0.85 :
+
0.534 : 601 : 8.05 : 19.0);9 PMO-1/3H (1 : 3 : P123 : HCl : H2O)
(0.15 : 0.85 : 0.068 : 0.944 : 800).11
than those of Corriu et al., no triol was observed when
+
PMO-1/4H was prepared using 15% loading of 1 relative
to 4. This suggests that at least 15% (but not 30%) of 1 can be
incorporated into a PMO with 3 without forming detectable
amounts of bis(silanetriol).
Table 2 Textural properties of prepared PMOs
BET surface
area/m2 gꢀ1
Pore
diametera/A
Pore
volumea/cm3 gꢀ1
Material
An important question to be addressed with mixed
PMO systems is the degree of homogeneity of the various
+
PMO-1/4H
1009
694
952
282
27
20
26
74
0.75
0.40
0.65
0.85
ꢀ
PMO-1/4OH
+
components within the final material.14 To probe this
PMO-2/4H
+
question, PMO-1/3H containing 15% functional monomer
1 was treated with OsO4 vapour in a closed jar, and allowed to
react for 4 h at room temperature (Scheme 3). As OsO4 reacts
selectively with olefins and (under these conditions) remains
bound to the olefin in the form of an osmate ester, we
employed this as a marker for the stilbene component of the
PMO. TEM imaging under high angle annular dark field
conditions (HAADF) is highly sensitive to atomic number
(known as Z-contrast imaging) and in combination with
energy dispersive X-ray spectroscopy (EDX), the presence
+
PMO-1/3H
a
Calculated from BJH adsorption branch.
+
P/P0, which could be indicative of slit-like pores.12 PMO-1/3H
synthesized using P123 as surfactant exhibited a nitrogen
adsorption profile typical of SBA-15 (see ESIw).
+
TEM images show that although PMO-1/4H
and
ꢀ
PMO-1/4OH appear to have less ordered worm-hole type
pore structures, the other two materials have well-organized,
mesoporous structures (Fig. 1). CP MAS 29Si NMR spectra
are characterized by T1, T2 and T3 sites, with no evidence of
Q sites, indicating Si–C bond retention under all synthetic
conditions studied. Materials were also prepared with 30%
and general distribution of Os within the organosilica matrix
+
was determined. Treatment of PMO-1/3H with OsO4
resulted in a material with highly dispersed Os, as determined
by HAADF imaging. Because the Os was so well dispersed, we
carried out a control experiment by mixing two batches of
material, one that was stained and one that was not, on the
same TEM grid. The unstained material was prepared by
condensing purely structural monomer 3, which contained
no olefin, and reacting it with OsO4. This provided an
additional control which was the determination of whether
OsO4 would merely adsorb to the unfunctionalized material.
As can be seen in Fig. 2, particles with uniformly high
contrast were observed in addition to particles with much
lower contrast (Fig. 2a and b). The presence of osmium in the
high-contrast particles and the absence in the lower-contrast
particles was further confirmed by examination of the EDX
spectra of the two types of particles (Fig. 2c and d). It should
be noted that for each of the individual samples, the same
EDX spectrum was observed at various beam locations.
+
Fig. 1 TEM images of (a) PMO-1/4H , (b) PMO-1/4OH , (c)
ꢀ
+
+
PMO-2/4H and (d) PMO-1/3H . Insets of (a) and (b) show close-up
of pore structure and (c) and (d) are electron diffraction patterns.
Scheme 3
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aldehyde peak at 15% 1, a material with 25% 1 does display
an aldehyde peak at 197 ppm. Importantly, nitrogen adsorption
analysis of the material following ozonolysis demonstrated
that the ordered mesopore structure was unaffected by this
treatment (see ESIw). The BET surface area increased slightly
from 282 m2 gꢀ1 to 345 m2 gꢀ1 while the pore dimensions
remain largely unaltered. This method demonstrates an ability
to introduce a uniform reactivity to periodic mesoporous
organosilica without pore reductions.
In summary, novel periodic mesoporous organosilica
precursors have been synthesized containing both a functional
and a structural component. The degree of homogeneity of the
structural component was assessed from HAADF TEM
images in tandem with EDX spectra. Reaction of the materials
with ozone can be carried out without loss of mesoscale order,
thanks to the presence of the structural units, while permitting
the introduction of a well distributed, highly useful aldehyde
functionality throughout the material. Further efforts to
employ this group in subsequent transformations are underway.
The authors would like to thank Dr A. El Kadib for his
contributions.
+
Fig. 2 HAADF images of a mixed sample of (a) PMO-1/3H -Os and
(b) control particles. Brightest areas indicate the presence of Os atoms.
The insets (c) and (d) are EDX spectra showing the presence of Os in
(a), but not in (b).
Distinct Os M lines can be found at 1.96 keV overlapping
slightly with Si K lines. Deconvolution of the peaks confirmed
that the ratio of peak intensity of Os M to Si K was
consistent in all locations tested. This supports a homogeneous
distribution of the stilbene component within the organosilica
material.
Notes and references
z The sampling volume of micro-Raman is sufficient to penetrate well
into the porous mesostructure.
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+
monomers within the material, PMO-1/3H was subjected to
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+
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showing complete consumption of CQC bond at 1632 cmꢀ1
.
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2102 | Chem. Commun., 2010, 46, 2100–2102