Photo-Cross-Linked Polydimethylacrylamide Hydrogels as Porogens for Mesoporous Alumina

Article abstract of DOI:10.1002/ejic.201601364

Dimethylacrylamide-based hydrogels were utilized as porogenic matrices in the synthesis of mesoporous aluminum oxide (γ-Al₂O₃) with specific BET surface areas up to 360 m² g^–1. Polymers with molecular mass in the range 12000–35000 g mol^–1 were synthesized from dimethylacrylamide and various comonomers by free-radical polymerization. Photo-cross-linking of the polymers and impregnation with aluminum nitrate [Al(NO₃)₃] was carried out in a single step, followed by formation of Al(OH)₃/AlO(OH) and subsequent calcination. Calcination led to the formation of mesoporous Al₂O₃ and simultaneous combustion of the hydrogel. The structural properties of the products were characterized by powder XRD, N₂ physisorption analysis, Hg intrusion porosimetry, and thermogravimetric analysis.

Full text of DOI:10.1002/ejic.201601364

DOI: 10.1002/ejic.201601364
Full Paper
Mesoporous Alumina
Photo-Cross-Linked Polydimethylacrylamide Hydrogels as
Porogens for Mesoporous Alumina
[a]
[a,b]
[b]
[b]
Christian Weinberger, Zimei Chen,
Michael Tiemann*[
Wolfgang Birnbaum, Dirk Kuckling,* and
a]
Abstract: Dimethylacrylamide-based hydrogels were utilized nitrate [Al(NO ) ] was carried out in a single step, followed by
3
3
as porogenic matrices in the synthesis of mesoporous alumi- formation of Al(OH) /AlO(OH) and subsequent calcination. Cal-
3
num oxide (γ-Al O ) with specific BET surface areas up to cination led to the formation of mesoporous Al O and simulta-
2
3
2 3
2
–1
3
3
60 m g . Polymers with molecular mass in the range 12000– neous combustion of the hydrogel. The structural properties of
5000 g mol were synthesized from dimethylacrylamide and the products were characterized by powder XRD, N physisorp-
–
1
2
various comonomers by free-radical polymerization. Photo- tion analysis, Hg intrusion porosimetry, and thermogravimetric
cross-linking of the polymers and impregnation with aluminum analysis.
Introduction
poly(dimethylacrylamide)-based hydrogels as porogenic matri-
[
19]
ces for mesoporous alumina.
Here we extend this approach
Porous alumina (Al O ) plays an important role in industry,
mostly as a catalyst or catalyst support, due to its high specific
surface area and pore volume.
by varying the synthesis conditions and by choice of the poro-
genic structure director. For example, rigid, porous structural
matrices are frequently used for the synthesis of porous metal
2
3
to using a variety of different photo-cross-linked hydrogels pre-
pared by utilizing various building blocks in various ratios. By
introducing the aluminum source (aluminum nitrate) prior to
photo-cross-linking of the polymers, aluminum oxide can be
formed conveniently in a one-pot synthesis approach.
[
1–3]
The pore size can be tuned
[
4–6]
oxides by the so-called nanocasting method.
This approach
comprises the infiltration of precursor compounds (e.g., metal
salts) into the pores of the matrix (e.g., porous silica or carbon)
and conversion to the respective product (metal oxide). Re-
moval of the matrix yields the porous product as a replica of
the matrix pore system. For amphoteric metal oxides, such as
Results and Discussion
Cross-linkable polymers were synthesized by free-radical
polymerization of dimethylacrylamide (DMAAm) monomer and
two comonomers (Scheme 1). The first comonomer was N-[2-
[
7,8]
[9]
[10,11]
alumina,
magnesia,
and zinc oxide,
mesoporous
(dimethylmaleimido)ethyl]acrylamide (DMIAAm; 5 or 10 %); it
carbon materials have proved to be suitable matrices, as they
can be removed without chemical etching under harsh pH con-
serves as the cross-linking unit in the subsequent photoinduced
linking reaction (see below). The second comonomer was tert-
butylacrylamide (tBuAAm) or N-[2-(dimethylamino)ethyl]acryl-
amide (DMAEAAm); its content varied between 5 and 15 %. The
role of the second comonomer is to vary the hydrophilicity of
the polymer by means of the hydrophobic tert-butyl group (in
tBuAAm) and the more hydrophilic tertiary amino group (in
DMAEAAm). The synthesis of DMIAAm has recently been de-
[
12]
ditions.
another versatile opportunity for structuring mesoporous metal
As an alternative to rigid matrices, hydrogels offer
[
13–15]
oxides.
Hydrogels are three-dimensional polymer net-
works that can absorb water and are well known for their appli-
cation as super-absorbents, but they have also attracted great
attention for tissue engineering, drug delivery, sensors, actua-
[
16–18]
tors, and catalysis.
We have recently reported on using
[19]
scribed.
All synthesized polymers and their characterization are listed
in Table 1. The molecular weights, dispersities, and yields of the
polymers are typical for free-radical polymerization. Since the
[
[
a] Department of Chemistry - Inorganic Functional Materials,
University of Paderborn,
[18]
polymers are subsequently cross-linked to form hydrogels, the
molecular weights and dispersities are of secondary impor-
tance. Polymer compositions calculated from NMR spectro-
scopic data (see Figure S1 in the Supporting Information) differ
slightly from the feed compositions of the monomers. DMIAAm
Warburger Str. 100, 33098 Paderborn, Germany
E-mail: michael.tiemann@upb.de
www.chemie.uni-paderborn.de
b] Department of Chemistry - Organic and Macromolecular Chemistry,
University of Paderborn,
Warburger Str. 100, 33098 Paderborn, Germany
E-mail: dirk.kuckling@upb.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejic.201601364.
[
20,21]
and DMAEAAm are less incorporated into the polymer.
Specific reactivity ratios for those systems cannot be found in
the literature, but at least a similar behavior was reported for
Eur. J. Inorg. Chem. 2017, 1026–1031
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Scheme 1. Synthesis of polymers by free-radical polymerization of DMAAm monomer and two comonomers (DMIAAm/tBuAAm or DMIAAm/DMAEAAm) in
various ratios (see Table 1).
DMIAAm with reactivity ratios determined by the extended
The thus-obtained aluminum-salt-containing hydrogels were
Kelen–Tüdös method for DMIAAm (r = 1.33) and a very similar then dried and exposed to ammonia steam at elevated temper-
[
22]
monomer, namely, N-isopropylacrylamide (r = 1.45).
ature (60 °C) to form aluminum hydroxide/oxyhydroxide
Al(OH) /AlO(OH)] precursor species for alumina. This technique
[
3
Table 1. Summary and characterization of the synthesized polymers.
is well established in the nanostructure-directed synthesis of
[
7,8]
Al O .
Constant contact times with ammonia steam are nec-
2
3
essary for reproducible results, as this influences the crystallinity
[
8]
of the final product. The samples were finally dried and cal-
cined at 500 °C. Calcination leads to the formation of Al O3
2
from Al(OH) /AlO(OH); simultaneously, the porogenic hydrogel
3
matrix is removed by thermal combustion.
All thus-prepared alumina materials show a rather low de-
gree of crystallinity, comparable to that of mesoporous alumina
[
7,8]
prepared at similar temperature with other porogens.
ure 1 shows the powder XRD pattern of Al O prepared by us-
Fig-
2
3
ing hydrogel B1 [poly(DMAAm-co-DMIAAm-co-tBuAAm);
0.6:4.3:5.1 mol-%] as an example. Only two diffraction peaks
9
are resolved; they can be indexed according to the defect spinel
structure of γ-Al O (JCPDS card No. 29-0063), which is the ex-
2
3
[
7,8]
pected phase under these synthesis conditions.
The XRD
patterns of all other Al O materials prepared with different
2
3
hydrogels are very similar, and revealed no clear trend in crys-
tallinity (Figure S2 in the Supporting Information).
1
[
a] Determined by H NMR spectroscopy. [b] M
n
: number-average molar mass;
D: dispersity; both determined by gel permeation chromatography (GPC) in
CHCl , calibrated with poly(methyl methacrylate) (PMMA) standards.
3
In the next step the as-prepared polymers were cross-linked
by irradiation with UV light to build a cross-linked hydrogel
matrix. For this purpose, the polymers were dispersed in a satu-
rated aqueous solution of aluminum nitrate. The aluminum salt
does not have any impact on the photo-cross-linking, and is
used to prepare alumina in the hydrogel matrix afterwards; it is
added at this stage for convenience, in order to avoid drying
and re-swelling of the cross-linked hydrogel at a later stage.
The DMIAAm in the polymer serves as the photo-cross-linker;
after UV irradiation a three-dimensional polymer network is
formed. The reaction mechanism is primarily a [2+2] cycloaddi-
2 3
Figure 1. Powder XRD pattern of γ-Al O prepared by using hydrogel B1 as
the porogenic structure director, after annealing.
[
23]
tion, but other possible pathways have also been identified.
Figure 2 shows the N2 physisorption isotherm and corre-
In addition, the DMIAAm content in the polymer proved to sponding BJH mesopore size distribution of the same Al O3
2
be critical to prepare hydrogels and should be greater than sample as in Figure 1 (prepared with hydrogel B1). The isotherm
[
21]
[24]
2–3 %.
exhibits a type IV(a) shape with H2 hysteresis, which confirms
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that the material exhibits mesopores of a somewhat uniform
size of about 4 nm (although the BJH method regularly under-
estimates the size of small mesopores by up to 20 %). Thus, the
hydrogel matrix has a porogenic impact, as previously dis-
Table 2. Specific surface areas ABET and specific mesopore volumes Vpore of
the alumina materials prepared by using hydrogels of various cross-linked
polymers as porogens.
Polymer porogen[a]
A
BET/m g
2
–1
Vpore/cm g
3
–1
[
19]
cussed. Al O is formed between bundles of polymer strands
2
3
A1
A2
A3
A4
A5
A6
B1
B2
B3
B4
B5
B6
C1[
312
336
345
367
306
332
341
273
314
342
356
253
362
0.45
0.36
0.36
0.51
0.41
0.47
0.46
0.39
0.50
0.49
0.50
0.37
0.44
in the continuous hydrogel network; removal of the hydrogel
by combustion leaves behind disordered, tubular mesopores.
Comparison of the physisorption data of all prepared Al O ma-
2
3
terials shows that the choice of porogenic hydrogel matrix has
only little impact on the porosity, that is, on the mesopore size
distribution, specific mesopore volume, and specific BET surface
area. Table 2 shows the respective values for all samples (for
data, see Figure S3 in the Supporting Information). On average,
the pore volumes and BET surface areas tend to be slightly
larger in the samples prepared with hydrogels containing the
hydrophilic DMAEAAm comonomer than in those prepared
with the hydrophobic tBuAAm comonomer. Likewise, the width
of the mesopore size distribution peak tends to be slightly nar-
rower. Both effects are very weak, and more data will be neces-
sary to confirm that the observed trends are statistically signifi-
cant. Such findings would suggest that the more hydrophilic
hydrogels have a slightly more favorable structure-directing,
porogenic impact on alumina synthesis. Stronger attractive in-
teraction between the aluminum salt and the polymer strands
may result in more uniform mesopores. No clear trend was ob-
served concerning the impact of the ratio of the comonomers
in the hydrogel networks.
18]
[
a] For designation of polymers, see Table 1.
regime at about 5 nm, which is consistent with the above-pre-
sented findings from N physisorption analysis. In addition, two
2
more, very broad peaks are observed in the macropore regime
around 1.4 and 100 μm. The specific volumes of the three types
3
–1
of pores are 0.32, 1.94, and 4.60 cm g , corresponding to the
respective peak areas in the pore size distribution curve. The
first value is consistent with the results obtained by N physi-
2
3
–1
sorption analysis (0.46 cm g ; see Figure S4 in the Supporting
Information). The macropores may originate from an inhomo-
geneous distribution of the polymer during cross-linking (clus-
tering of polymer bundles) and from inhomogeneous shrinking
of the solid Al O material on calcination. The macropores may
2
3
therefore be considered to be “textural pores”, that is, basically
voids between adjacent (intrinsically mesoporous) alumina par-
ticles. This is consistent with the observation that grinding of
the porous alumina sample in a mortar drastically reduces the
Figure 2. N physisorption isotherm and mesopore distribution (inset) of the
2 3
same Al O sample as in Figure 1.
2
In addition to the uniform mesopores, the materials also
contain macropores, as evidenced by mercury intrusion poro-
simetry. Figure 3 shows the pore size distribution of an alumina
sample prepared by the same procedure with a poly(DMAAm-
co-DMIAAm) containing 5.7 mol-% cross-linker as the porogenic
[
19]
matrix.
The distribution curve of the as-synthesized material
Figure 3. Meso- and macropore size distribution of Al
2 3
O before and after
exhibits three maxima: a narrow peak occurs in the mesopore
grinding in a mortar, measured by Hg intrusion porosimetry.
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macroporosity (Figure 3). The larger mesopores (ca. 100 μm) attributed to the loss of physisorbed water from the polymer
completely collapse under the mechanical stress; they no network. A further mass loss of 93 % is observed between 170
longer exist after 2 min of grinding. The peak of the smaller and 630 °C. (TGA data for all other prepared polymers show
macropores (ca. 1.4 μm) drastically decreases in intensity by very similar results; see Figure S6 in the Supporting Informa-
5
2
6 % after grinding for 2 min and by a total of 67 % after tion). Hence, a higher temperature is necessary to decompose
0 min. [Grinding also has a slight impact on the mesopores, the polymer completely when Al O is absent. The presence of
2
3
which is in line with the observations from N physisorption Al O , finely dispersed in the hydrogel, reduces the combustion
2
2 3
analysis (see Figure S4 in the Supporting Information)].
temperature, which indicates that the decomposition of the
. Such a catalytic effect has been
To confirm that the hydrogel is quantitatively removed from polymer is catalyzed by Al O
2 3
the alumina during the calcination procedure in the final part observed before for the combustion of porous carbon serving
of the synthesis, thermogravimetric analysis (TGA) was carried as a structure matrix for various mesoporous metal oxides, in-
[
12b]
out. Figure 4 shows the results for the calcined Al O sample cluding Al O .
2
3
2 3
from Figure 1 (prepared by using hydrogel B1). A weight loss
of about 19 % is observed in the entire temperature range from
25 to 1000 °C, most of which takes place below about 250 °C.
Since the calcination temperature was 500 °C (for 4 h) the mass
loss is apparently attributable to desorption of water taken up
by the calcined porous sample from air during storage. This
water may be physisorbed or chemically bound to coordina-
tively unsaturated Al sites in the (poorly crystallized) material
–
(
either as H O or as OH with protonation of adjacent Al–O–Al
2
bridges, that is, reverse oxolation). TGA of all other prepared
alumina samples gave very similar results (see Figure S5 in the
Supporting Information). The fact that no considerable mass
change occurs above 500 °C (i.e., above the calcination temper-
ature) confirms that the calcined materials do not contain any
residual organic moieties.
3
Figure 5. DTA (TGA; solid lines) of the Al(OH) /AlO(OH)–hydrogel composite
and of the pure hydrogel [poly(DMAAm-co-DMIAAm-co-tBuAAm), polymer
B1]. The DTG curves (dashed lines) are the first derivatives of the TGA curves.
Conclusions
Poly(dimethylacrylamide)-based hydrogels are suitable as poro-
genic structure matrices for the synthesis of mesoporous Al O .
2
3
The products exhibit mesopore sizes in the range from 3 to
2
–1
8
nm, large specific BET surface areas up to 360 m g , and
3
–1
specific pore volumes up to 0.50 cm g . Variation of the poly-
mer composition has only little impact on the structural proper-
ties of the products. The calcination of the hydrogel/alumina
composite materials (i.e., the final step of the synthesis) com-
prises the annealing of Al O and simultaneous combustion of
2 3
Figure 4. TGA (black line) of the same Al O sample as in Figure 1. The differ-
2
3
ential thermogravimetric (DTG) curve (gray line) is the first derivative of the
TGA curve.
the hydrogel. This procedure benefits from the fact that hydro-
gel decomposition is catalyzed by the freshly formed Al O .
2
3
It is interesting to compare the TGA data of the Al(OH)3/
hydrogel composites (after ammonia treatment, before anneal-
ing) with those of the pure (aluminum-free) hydrogels. Figure 5
Experimental Section
shows the TGA curve of poly(DMAAm-co-DMIAAm-co-tBuAAm) Chemicals: Acryloyl chloride (Alfa Aesar, 96 %), aluminum nitrate
nonahydrate [Al(NO ) ·9H O, Sigma-Aldrich, ≥ 98.0 %], 25 % ammo-
(
(
before cross-linking) with composition of 90.6/4.3/5.1 mol-%
3 3
2
nia solution (Stockmeier), N,N-dimethylethylenediamine (Aldrich,
98.0 %), 2,6-di-tert-butyl-4-methylphenol (BHT, Fluka, 99 %), so-
dium carbonate (99.5 %), and tert-butylacrylamide (tBuAAm, Acros,
7 %), were used as received. N,N-Dimethylacrylamide (DMAAm,
sample B1) with and without the Al(OH) /AlO(OH) species. A
3
≥
steep mass loss of about 40 % occurs in the composite material
at approximately 220 °C, attributable to the conversion (“de-
hydration”) of Al(OH) /AlO(OH) to Al O . This is immediately fol-
lowed by further a mass loss of about 20 % in the temperature
range from 220 °C to 550 °C, corresponding to the decomposi-
9
3
2 3
TCI, 99 %) was distilled under reduced pressure. Azobisisobutyro-
nitrile (AIBN, Fluka, > 98 %) was recrystallized from methanol. Di-
ethyl ether (Hanke+Seidel), ethyl acetate (Stockmeier), and meth-
tion of the polymer. For the pure polymer, by contrast, a first anol (Stockmeier) were of technical grade and used as received. 1,4-
weight loss of about 7 % occurs below 170 °C, which can be Dioxane (Carl Roth, ≥ 99.5 %) and THF (BASF) were distilled under
Eur. J. Inorg. Chem. 2017, 1026–1031
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reduced pressure. Dichloromethane (Stockmeier) was distilled and
dried with CaCl2.
polymers were dried under high vacuum. All polymers were charac-
terized by NMR spectroscopy and GPC (see Table 1).
Characterization: 1H and 13C NMR spectra were recorded with a
Photo-Cross-Linking and Alumina Formation/Hydrogel Re-
moval: A solution of the respective polymer (196 mg) in a saturated
Bruker AV 500 spectrometer at 500 and 125 MHz, respectively. Sol-
vent signals at δ = 7.26 and 2.56 ppm were used as reference for
spectra in CDCl and [D ]DMSO. The monomer contents of the poly-
–
1
solution of Al(NO ) ·9H O in water (800 μL, 1.9 mol L ) was irradi-
3 3
2
ated for 4 h with a 100 W high-pressure mercury vapor arc lamp
equipped with an IR filter (FSUV1, HEBO Spezialglas) and water
cooling in the light path. The sample solution was cooled by a
water bath and covered with a glass plate to prevent solvent evapo-
ration. After cross-linking, the samples were dried and treated at
3
6
mers were calculated from NMR spectra by using the ratios of the
methyl proton signal of DMIAAm at about 1.94 ppm, the methyl
proton signals of DMAAm in the range of 2.77–3.22 ppm, the
methyl proton signal of tBuAAm at about 1.33 ppm, and the methyl
proton signal of DMAEAAm at about 2.16 ppm. GPC was performed
with a Jasco 880-PU Liquid Chromatograph with a Waters RI De-
6
3
0 °C with the vapor of an aqueous ammonia solution (12.5 %) for
h to convert Al(NO3)3 to Al(OH) /AlO(OH) and then dried over-
3
–
1
night at 60 °C. The material was calcined in a tube furnace under
tector 2410 in chloroform at 30 °C and a flow rate of 0.75 mL min .
–
1
6
air for 4 h at 500 °C at a rate of 1 °C min to simultaneously form
Al O and degrade the polymer matrix.
The instrument was equipped with a PSS-GRAM 10 Å, a PSS-GRAM
5
3
2
1
0 Å, a PSS-GRAM 10 Å, and a PSS-GRAM 10 Å column. All sam-
2 3
ples were calibrated with PMMA standards. TGA was conducted un-
–
1
der synthetic air at a heating rate of 10 °C min by using a Mettler
Acknowledgments
Toledo TGA/SDTA 851e balance. N physisorption analysis was per-
2
formed at 77 K with a Quantachrome Autosorb 6B instrument; sam- C. W. thanks the Fonds der Chemischen Industrie for a PhD
–
1
ples were degassed at 120 °C for 12 h at a rate of 5 °C min prior Fellowship.
to measurement. Specific surface areas were determined by multi-
point BET analysis[ in the range of 0.1 ≤ p/p ≤ 0.3. Pore volumes
25]
0
Keywords: Mesoporous materials · Nanocasting · Gels ·
Template synthesis · Copolymerization
were calculated at p/p = 0.99. Pore size distributions were calcu-
0
26]
lated by BJH analysis[
from the desorption branches of the iso-
therms. Powder XRD was performed with a Bruker AXS D8 Advance
diffractometer with Cu-Kα (λ = 0.154 nm) radiation (40 kV, 40 mA),
a step size of 0.02°, and a counting time of 3 s per step. Mercury
porosimetry was performed with a Quantachrome Poremaster 60
instrument with a contact angle of 140° for intrusion und extrusion.
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3:1 to 1:1). The pure product was obtained as a slightly yellow oil
1
(4.35 g, 46 %). H NMR (500 MHz, CDCl ): δ = 2.18 [s, 6 H, N(CH ) ],
3
3 2
2
3
2
1
1
1
.39 (t, J = 6.04 Hz, 2 H, NCH ), 3.34 (m, 2 H, NHCH ), 5.55 (dd, J =
.65, J = 10.22 Hz, 1 H, =CH ), 6.08 (dd, J = 10.22, J = 17.04 Hz,
2
2
3
3
3
2
2
3
H, =CH), 6.21 (dd, J = 1.65, J = 17.04 Hz, 1 H, =CH ), 6.43 (br. s,
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Received: November 11, 2016
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© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim