Mechanical and Hydrothermal Stabilities of Aged Periodic Mesoporous Organosilicas

Article abstract of DOI:10.1021/jp035189q

Methylene-, ethylene-, and phenylene-bridged periodic mesoporous organosilicas (PMOs) synthesized with nonionic alkylethylene oxide templates exhibited significantly better mechanical and hydrothermal stabilities than periodic mesoporous silica. Synthesis

Full text of DOI:10.1021/jp035189q

12628
J. Phys. Chem. B 2003, 107, 12628-12634
Mechanical and Hydrothermal Stabilities of Aged Periodic Mesoporous Organosilicas
Mark C. Burleigh,† Michael A. Markowitz,* Shalini Jayasundera, Mark S. Spector,
Chris W. Thomas, and Bruce P. Gaber
Laboratory for Molecular Interfacial Interactions, Code 6930, Center for Bio/Molecular Science and
Engineering, NaVal Research Laboratory, Washington, DC 20375
ReceiVed: May 2, 2003; In Final Form: August 20, 2003
Methylene-, ethylene-, and phenylene-bridged periodic mesoporous organosilicas (PMOs) synthesized with
nonionic alkylethylene oxide templates exhibited significantly better mechanical and hydrothermal stabilities
than periodic mesoporous silica. Synthesis of PMOs utilized the acid-catalyzed hydrolysis and condensation
of bis(triethoxysilyl) precursors around supramolecular polyoxyethylene(10) stearyl ether (Brij 76) templates.
Nitrogen gas sorption, thermogravimetry, and X-ray diffraction have been used to characterize the effects of
aging, mechanical compression, and hydrothermal treatment on these materials. Both as-synthesized composites
containing surfactant templates and extracted PMOs showed no degradation after 10 months. The enhanced
stability of these nanoporous organosilicas relative to meoporous silica makes them potential candidates for
use in advanced catalysis and adsorption applications.
Introduction
able damage upon mechanical compression.9,13,14 This damage
can be reduced significantly by the careful exclusion of
moisture,15 increasing the hydrophobicity of the silica surface
by trimethylsilylation,13 or increasing the pore wall thickness.9
The discovery of the M41S family of mesoporous molecular
sieves1,2 has led to great scientific interest in the field of porous
materials. One member of this family, MCM-41, with its
hexagonal arrays of uniform pore channels has been of particular
interest because of its potential for catalytic and adsorbent
applications. The ordered microstructure and relatively large
pore size of MCM-41 allows for fast diffusion of molecules
leading to quick access to its large internal surface areas. The
surfactant template approach used to synthesize these materials
allows one to control the pore size (∼2-10 nm) by the
appropriate choice of surfactant and synthetic conditions.3,4 Pore
size control gives one the ability to design catalysts and
adsorbents that allow or deny access to specific molecules to
the reaction events that take place within the pores. This size
selectivity is particularly advantageous in the mesoporous range
because this allows for the “sieving” of moderately sized
molecules, all of which are excluded from microporous zeolites.
Unfortunately, these materials lack the hydrothermal and
mechanical stability commonly required for catalysis and
adsorption. This could limit their potential industrial applica-
tions. Ryoo and Kim5 reported that the ordered microstructures
of hexagonal MCM-41 and cubic MCM-48 disintegrate readily
in boiling water. Characterization by X-ray powder diffraction
and 29Si solid-state NMR indicates that the mechanism for this
disintegration is silicate hydrolysis. It has since been shown that
the hydrothermal stability can be improved in a number of ways.
These include the addition of large concentrations of various
salts during synthesis,6,7 the synthesis of ordered microstructures
with thicker pore walls,8,9 and the substitution of silicon with
aluminum.10,11 Exposure to water, even under mild conditions,
has been shown to degrade ordered mesoporous silica. Broyer
et al. have reported a 54% decrease in the pore volume of freshly
calcined MCM-41 after 3 months under ambient conditions.12
When it was stored under dry nitrogen for the same time period,
no significant change was noted. The microstructures of ordered
mesoporous silicas have also been shown to undergo consider-
Because of their vulnerability to silicate hydrolysis, the lack
of structural integrity in mesoporous silicas can be overcome
by eliminating water. Because evacuating materials and storing
them under dry nitrogen can be expensive, tailoring the surfaces
of ordered porous materials with nonpolar functional groups
may be a more practical approach. The incorporation of organic
functional groups into nanoporous materials has been ac-
complished by employing a number of different synthetic
strategies.16-21 The most effective method for functionalizing
ordered nanoporous materials involves the use of bis(alkoxysilyl)
precursors containing organic bridging groups. This method
incorporates the organic functionality directly into the pore walls
simply by the selection of the appropriate precursor22 and was
first reported by Inagaki and co-workers.23 This synthetic
approach eliminates the multistep protocols required in postsyn-
thetic grafting techniques.16-18 It also avoids the problems
associated with different precursor hydrolysis rates and reduced
order incurred during co-condensation polymerizations.19-21 The
hydrophobicity imparted on periodic mesoporous organosilicas
(PMOs) due to the presence of organic bridging groups should
increase their aging, hydrothermal, and mechanical stabilities.
Stein and co-workers reported that an ethylene-bridged PMO
synthesized with alkyltrimethylammonium templates in basic
media had better hydrothermal stability than MCM-41.24 We
have recently reported the synthesis of a new family of PMOs
using neutral alkylethylene oxide templates in acidic media.25,26
These PMOs have large surface areas, ordered microstructures,
and thick pore walls. The hydrophobic surface groups and thick
pore walls may enhance the hydrothermal and mechanical
stability of these materials.
In this paper, we examine the effects of aging, mechanical,
and hydrothermal treatments on the microstructure of our PMOs.
Specifically, we have used nitrogen gas sorption, thermogravi-
metric analysis, and powder X-ray diffraction to characterize
methylene, ethylene, and phenylene-bridged nanoporous orga-
† Deceased.
10.1021/jp035189q CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/29/2003

Mechanical and Hydrothermal Stabilities
J. Phys. Chem. B, Vol. 107, No. 46, 2003 12629
vacuum distillation to remove excess TEOS (65 °C, 0.5 Torr),
followed by product (130 °C, 0.2 Torr), which appeared as a
colorless oil (30 g, recovered yield 37%). 1H NMR (400 MHz,
CDCl3/TMS) δ 7.68 (s, 4H), 3.87 (quartet, 12H), 1.24 (t, 18H).
13C NMR (100.7 MHz, CDCl3/TMS) δ 134.0, 133.2, 58.7, 18.1.
PMO Synthesis. The method previously described for the
synthesis of periodic nanoporous ethane silica25 was modified
by the use of a specific amount of HCl and the appropriate
organosilane precursor (Figure 1). Concentrated HCl (13.1 mL,
12.2 M) was added to deionized water (186.9 mL) to make the
initial aqueous solutions. The Brij 76 (C18H37(OCH2OCH2)10-
OH, 4.0 g) surfactant template was added to the 200 mL
solutions upon stirring, and the mixtures were covered and
heated at 50 °C for 12h. The precursors were then added to the
resulting clear solutions. The reactant molar ratios were as
follows: 0.11 Brij 56/222 H2O/3.20 HCl/0.56 organosilane.
The synthesis mixtures were covered and stirred at 50 °C
for 12 h, followed by heating at 90 °C under static conditions
for an additional 24 h. The precipitates were recovered by
suction filtration and air-dried. A portion of these as-synthesized
nanoscopic composites were characterized, while the remaining
amount was extracted with acidified ethanol to produce nano-
porous organosilicas. A pure silica analogue was prepared by
substituting an equimolar amount of tetramethyl orthosilicate
(TMOS) for the bis(alkoxysilyl) precursors.
Postsynthetic Treatment. As-synthesized nanoscopic com-
posites were placed in excess (350 mL/g) acidified ethanol (1
M HCl) and refluxed for 6 h to extract the surfactant template.
The products were recovered by filtration, washed with absolute
ethanol, and dried under vacuum at 60 °C for 10 h. The
extraction procedure was repeated twice.
Aging. Following postsynthetic treatment, a portion of each
nanoporous material was aged for 10 months under ambient
conditions. These samples were simply placed in open vials on
a shelf. No special attempts were made to control either the
temperature or humidity.
Figure 1. Structures of the PMO precursors: (1) bis(triethoxysilyl)-
methylene; (2) bis(triethoxysilyl)ethylene; (3) 1,4-bis(triethoxysilyl)-
benzene.
nosilicas before and after such treatments. Solid-state NMR was
used to characterize the nanoporous materials before and after
surfactant template extraction. The stabilities of these materials
have been compared to that of nanoporous silica synthesized
under similar conditions. The mechanical stabilities of the as-
synthesized composite materials containing surfactant templates
have also been compared to those of the surfactant template
extracted nanoporous materials derived from them.
Materials and Methods
Chemicals. The PMO precursors used include bis(triethoxy-
silyl)methylene, bis(triethoxysilyl)ethylene, and 1,4-bis(triethoxy-
silyl)benzene (Figure 1). The synthesis of the 1,4-bis(triethoxy-
silyl)benzene monomer was carried out in house (see monomer
synthesis), while the other organosilanes were obtained from
Gelest, Inc. Ligroin (Fisher) was used without further purifica-
tion. All other chemicals were obtained from Aldrich and used
as received. Water used in all synthetic procedures was
deionized to 18 MΩ cm.
Characterization. Solid-state cross-polarization magic angle
spinning (CP MAS) 13C NMR experiments were performed at
room temperature on a Bruker DMX-500 MHz at a resonance
frequency of 125.77 with a 4.0 mm MAS probe. The samples
were packed in 4 mm zirconia Bruker rotors fitted with Kel-F
end caps for magic angle spinning at 10-12.5 kHz. A two-
pulse phase modulation (TPPM) decoupling was used for proton
Mechanical Treatment. The aged powders were placed in
a 13 mm die and compressed for 30 min at various pressures.
The samples were not placed in a vacuum oven prior to pressing,
and no vacuum was applied to the die to remove any moisture
adsorbed on their surfaces.
decoupling.27
A
13C CP contact time of 2 ms was used for all
of the samples with up to 1000 scans and 10 s delays. The 13
C
reference was set to external tetrakis(trimethylsilyl) silane at
3.5 ppm with respect to TMS at 0 ppm. X-ray diffraction
measurements were made on an Enraf-Nonius FR591 rotating
anode operating at 13 kW. A singly bent graphite monochro-
mator selected Cu KR radiation and provided in-plane resolution
of 0.014 Å-1 full width at half-maximum. Powder samples were
placed in 1.0 mm quartz capillary tubes. Gas sorption experi-
ments were performed using a Micromeritics ASAP 2010.
Nitrogen gas was used as the adsorbate at 77 K. Thermogravi-
metric analyses were performed with a TA Instruments TGA
2950 thermogravimetric analyzer. All measurements were made
in high-resolution dynamic mode.
Monomer Synthesis: 1,4-Bis(triethoxysilyl)benzene. A N2-
purged three-neck round-bottom flask with stir bar and mag-
nesium turnings (15 g, 0.62 mol) was flame-dried before adding
TEOS (450 mL, 2 mol), dry THF (250 mL), and a few crystals
of iodine. The solution was brought to reflux (65 °C) and
p-dibromobenzene (48 g, 0.20 mol) in THF (125 mL) was added
dropwise to the flask during 2 h. This solution was allowed to
reflux during another 1.5 h, becoming dark yellow. After the
solution cooled to room temperature, the THF was removed by
rotary evaporation, and ca. 200 mL of ligroin was added to
precipitate MgBr; it was removed by vacuum filtration. The
liquor was condensed by rotary evaporation and subjected to
Hydrothermal Treatment. The aged nanoporous powders
were boiled in deionized water for 18 h, isolated by suction
filtration, and dried under vacuum at 60 °C for 10 h.
Results and Discussion
Aging Stability. Powder X-ray diffraction and nitrogen gas
sorption analyses were performed on all of the extracted
nanoporous materials before and after aging. Solid-state 13C CP
MAS NMR spectroscopy was used to show the successful
removal of the surfactant templates during the postsynthetic
extraction and the stable Si-C bonding throughout the synthesis.
The 13C CP MAS NMR spectra of the as-synthesized and
extracted methylene (A), ethylene (B), phenylene (C), and silica
(D) nanoporous materials are shown in Figure 2. All four sets
of spectra show the disappearance of the primary surfactant
resonances located at δ ≈ 30 and 70 ppm after extraction
indicating near-complete removal of the surfactant using the
acidified ethanol extraction method. The small resonances at δ
≈ 18 (methyl) and 58 (methylene) ppm in the extracted spectra
are associated with trace amounts of ethanol used in the
extraction process.
A comparison of the physicochemical properties of the
nanoporous organosilicas (Table 1) shows little evidence of any

12630 J. Phys. Chem. B, Vol. 107, No. 46, 2003
Burleigh et al.
Figure 2. 13C CP MAS NMR spectra of as-synthesized (As) and template surfactant extracted (Ext) (A) methylene, (B) ethylene, (C) phenylene,
and (D) silica nanoporous materials.
TABLE 1: Structural Properties of Extracted Nanoporous
Organosilicas before and after Aging
BET
surface
area
total
pore
pore
wall
Da
volume sizeb thicknessc
sample
(Å) (m2/g) (cm3/g) (Å)
(Å)
methylene PMO
methylene PMO (aged)
ethylene PMO
ethylene PMO (aged)
phenylene PMO
phenylene PMO (aged)
nanoporous silica
74
74
61
63
59
57
61
880
872
1070
1080
940
901
990
0.90
0.90
1.10
1.10
0.70
0.60
1.40
1.30
50
50
44
43
35
33
46
44
35
35
26
29
33
33
24
24
nanoporous silica (aged) 59
1010
a d(100) spacing of the extracted products ((2 Å). b Calculated from
adsorption branch. c Estimated from a0-pore size, where a0 ) (2x3/
3)d(100).
microstructural changes as a result of the aging process. The
phenylene-bridged PMO does exhibit slight decreases in the
BET surface area (∼4%) and total pore volume (∼10%). The
substantial decrease in total pore volume reported by Bellat and
co-workers12 for calcined MCM-41 materials after aging under
ambient conditions is not exhibited by any of our nanoporous
materials. The difference in the aging stability of these materials
may be due to the rather large discrepancy in pore wall
thickness. The MCM-41 materials synthesized with alkyl-
ammonium templates in basic media exhibited relatively thin
(7-10 Å) pore walls. All of our nanoporous materials synthe-
sized with alkylethylene oxide templates in acidic media exhibit
much thicker (24-35 Å) walls.26
Figure 3. Powder X-ray diffraction patterns of the template-extracted
(A) methylene, (B) ethylene, (C) phenylene, and (D) silica nanoporous
materials before and after pressure treatment at 268 MPa for 30 min.
powder X-ray analyses. The ethylene-bridged PMO remained
a fluffy powder. The powder X-ray diffraction patterns of the
nanoporous materials before and after compaction at 8000 lb
are shown in Figure 3.
Although the XRD patterns indicate that all of the nanoporous
materials exhibit a decrease in structural ordering upon com-
pression at 8000 lb, the degree of this change differs consider-
ably from one material to another. The methylene-bridged PMO
and the nanoporous silica both exhibit a substantial decrease in
the intensity and broadening of their (100) reflections. The
phenylene-bridged PMO shows a less-pronounced decrease in
intensity but a noticeable increase in the full width at half-
maximum. The ethylene-bridged PMO (Figure 3b) exhibits the
Mechanical Stability. The aged extracted nanoporous pow-
ders were first pressed at 8000 lb (268 MPa) for 30 min. The
methylene- and phenylene-bridged PMOs and the nanoporous
silica formed hard disks during compression and had to be
ground with a mortar and pestle prior to nitrogen sorption and

Mechanical and Hydrothermal Stabilities
J. Phys. Chem. B, Vol. 107, No. 46, 2003 12631
TABLE 2: Physicochemical Properties of Extracted
Nanoporous Organosilicas before and after Mechanical
Treatments
exhibits an 80% decrease in total pore volume, a decrease in
the pore size distribution maximum from 50 to 26 Å, and a
50% decrease in surface area. The ethylene-bridged PMO shows
better mechanical stability but still exhibits a decrease in total
pore volume of ∼18% upon compression at 8000 lb for 30 min.
It is interesting to note that the nitrogen adsorption isotherms
of the ethylene- and phenylene-bridged PMOs (Figure 4b,c) do
not change significantly in the microporous region from those
of the untreated samples. In contrast, the step in the mesoporous
region decreases noticeably. This seems to indicate that the
changes in the microstructure in these materials is due to a
collapse of mesopores, while the micropores contained primarily
in the pore walls remain unchanged. This is in stark contrast to
what is shown by the methylene-bridged PMO, which exhibits
a large decrease in micropore volume upon pressing. The pore
size distributions of the ethylene-bridged PMO show little
change upon compression.
The nitrogen gas adsorption isotherms and pore size distribu-
tions of the nanoporous silica subjected to compression at
various pressures are shown in Figure 4d. In agreement with
our XRD data, the pure silica sample is less-stable under high-
pressure conditions than any of the three PMOs characterized
in this study. At 8000 lb, the silica exhibits a 90% decrease in
total pore volume, a decrease in pore size from 44 to 23 Å, and
a decrease in BET surface area from 1010 to 390 m2/g (Table
2).
BET
surface
area
total
pore
pore
sizeb
(Å)
Da
(Å)
volume
sample
(m2/g)
(cm3/g)
methylene PMO
methylene PMO (8 K)
ethylene PMO
ethylene PMO (4 K)
ethylene PMO (8 K)
phenylene PMO
phenylene PMO (8 K)
nanoporous silica
74
870
470
1080
1060
1010
901
650
1010
780
0.90
0.20
1.10
1.10
0.90
0.60
0.30
1.30
0.70
0.30
0.10
50
26
43
44
40
33
26
44
39
26
23
63
63
63
57
57
59
55
55
nanoporous silica (2 K)
nanoporous silica (4 K)
nanoporous silica (8 K)
570
390
a d(100) spacing of the extracted products ((2 Å). b Calculated from
adsorption branch.
highest mechanical stability. The best indication of decreased
structural ordering in this material is the rather poor resolution
of the (200) and (110) reflections following compression. It
should be noted that these secondary reflections are no longer
present in the silica sample following mechanical treatment.
Because our results indicated that the ethylene-bridged PMO
had the highest mechanical stability, while a near-complete
collapse in pore structure was exhibited by the nanoporous silica,
these two materials were investigated further by pressing at both
4000 lb (134 MPa) and 2000 lb (67 MPa) for 30 min. The
physicochemical properties of the organosilicas before and after
compaction are given in Table 2.
To determine whether the surfactant templates could improve
the mechanical stability of the nanostructured materials, the as-
synthesized composites were also pressed at 8000 lb for 30 min.
The powder X-ray diffraction patterns are shown in Figure 5.
It is interesting to note that the mechanical stabilities of the
methylene-bridged PMO and the silica, the two materials that
exhibited the lowest mechanical stability in their porous
(template-free) state, are improved significantly if the templates
are present. The X-ray diffraction patterns (Figures 5a,d) show
Nitrogen gas sorption isotherms and Bopp-Jancso-Heinz-
inger (BJH) pore size distributions (insets) shown in Figure 4
further illustrate the effects of mechanical compression on the
nanoporous materials. The methylene-bridged PMO (Figure 4a)
Figure 4. Nitrogen gas adsorption/desorption isotherms and pore size distributions (insets, PSD) of template-extracted (A) methylene, (B) ethylene,
(C) phenylene, and (D) silica nanoporous materials before and after pressure treatment.

12632 J. Phys. Chem. B, Vol. 107, No. 46, 2003
Burleigh et al.
TABLE 3: Thermogravimetric Properties of Extracted and
As-Synthesized Composite Nanoporous Materials
H2O-EtOH
weight
total
weight
H2O-EtOH/total
sample
lossa (%)
lossb (%)
ratio
methylene PMO
methylene PMO (as)
ethylene PMO
ethylene PMO (as)
phenylene PMO
phenylene PMO (as)
nanoporous silica
nanoporous silica (as)
7.0
3.0
4.0
14.0
44.0
16.5
49.0
34.0
38.0
22.0
57.0
0.50
0.07
0.23
0.04
0.35
0.31
0.59
0.03
2.0
12.0
12.0
13.0
2.0
a Percentage weight loss up to 150 °C. b Percentage weight loss up
to 950 °C.
Figure 5. Powder X-ray diffraction patterns of the as-synthesized (A)
methylene, (B) ethylene, (C) phenylene, and (D) silica composite
materials before and after pressure treatment at 268 MPa for 30 min.
Figure 7. Percentage of water-ethanol weight loss versus residual
porosity for nanoporous materials following pressure treatment at 268
MPa.
Figure 6. Thermogravimetric weight loss curves of (A) extracted and
(B) as-synthesized methylene-bridged PMO.
little difference after high-pressure treatment. This stabilizing
effect of surfactant templates has been reported before.4,28
Surprisingly, the (100) reflection of the as-synthesized phe-
nylene-bridged PMO exhibits a significant decrease in intensity.
Thermogravimetric analyses were performed (20-1000 °C)
on all of the as-synthesized and extracted nanostructured
materials prior to mechanical treatment. The amount of residual
water and ethanol can be easily estimated because they come
off at temperatures below 150 °C, before most matrix decom-
position occurs. The thermogravimetric weight loss curves of
the as-synthesized and extracted methylene-bridged PMO shown
in Figure 6 are typical. While the total weight loss for the as-
synthesized composite (Figure 6B) is much larger than that of
the extracted material (Figure 6A), the weight loss due to
residual water and ethanol (∼3%) is less than half that of the
porous sample (∼7%). The larger total weight loss of the as-
synthesized composite can be attributed to the decomposition
of the surfactant (∼34%), which is not present in the extracted
material. The presence of residual ethanol in all of the extracted
material was evident in the 13C MAS NMR spectra at resonances
δ ≈ 18 (methyl) and 58 (methylene) ppm (Figure 2). The
increase in the amount of adsorbed water and ethanol following
extraction is likely due to the significant increase in surface
area and overall decrease in hydrophobicity resulting from
template removal. The percent weight loss due to residual water
Figure 8. Thermogravimetric weight loss curves of as-synthesized and
extracted methylene, ethylene, phenylene, and silica composite materi-
als.
and ethanol and the total percent weight loss for the nanoporous
materials are listed in Table 3.
The collapse of ordered nanoporous silicas during compres-
sion has been explained in terms of a mechanochemical
hydrolysis of siloxane groups.29 A comparison of the residual
water and ethanol present in the nanoporous materials with their
mechanical stabilities is therefore in order. Thermogravimetric
analyses of our organosilicas show that those containing larger
organic bridging groups such as ethylene and phenylene lose
more weight due to matrix decomposition. However, the
relatively smaller bridging group methylene and silica materials
show a higher percent weight loss due to water-ethanol
indicating that these materials contain more water and ethanol
within the pores. Therefore, the ratio of the weight loss due to

Mechanical and Hydrothermal Stabilities
J. Phys. Chem. B, Vol. 107, No. 46, 2003 12633
gravimetric data of the as-synthesized composites with those
of the extracted samples (Figure 8) illustrates this trend once
again. In general, the as-synthesized composites are more
hydrophobic. This explains their enhanced mechanical stability
over the extracted samples, as indicated by XRD data (Figures
3 and 5). The surprising instability of the phenylene as-
synthesized composite (Figure 5c) can now be explained in
terms of its relatively high water content measured by TGA.
The reasons for the relatively high hydrophilic character of the
phenylene PMO are not fully understood at this time.
Hydrothermal Stability. Powder X-ray diffraction and
nitrogen sorption measurements were performed on the aged
nanoporous materials before and after hydrothermal treatments.
The X-ray diffraction results shown in Figure 9 indicate no
structural degradation in any of the PMOs after hydrothermal
treatment. In fact, the ethylene- and phenylene-bridged PMOs
exhibit a moderate hydrothermal restructuring that improves the
order of their microstructures. The ethylene-bridged PMO
(Figure 9b) shows slight increases in the intensities of its (100),
(110), and (200) reflections. The phenylene-bridged PMO
(Figure 9c) shows an increase in its (100) and (200) peaks and
significant enhancement of the (110) reflection at 2θ ) 2.70,
indicating hexagonal ordering (P6mm space group). The
methylene-bridged PMO (Figure 9a) shows little change in its
wormhole microstructure after treatment. Unlike our PMOs,
diffraction on nanoporous silica (Figure 9d) shows a decrease
in the intensity of both the (100) and (110) reflections and the
disappearance of the (200) peak, indicating reduced structural
ordering after hydrothermal treatment. The (100) and (110)
reflections are also shifted to higher values of 2θ.
Nitrogen gas sorption analyses were performed on all of the
extracted nanoporous samples before and after hydrothermal
treatments. All exhibit type IV isotherms (Figure 10) with a
large step in the mesoporous range and sharp BJH pore size
distributions (insets). The physicochemical properties of the
nanoporous materials are listed in Table 4. In agreement with
the X-ray data, only the nanoporous silica shows any significant
damage. The hydrothermally treated sample exhibits an increase
in pore size, a corresponding decrease (∼35%) in surface area,
Figure 9. Powder X-ray diffraction patterns of template-extracted (A)
methylene, (B) ethylene, (C) phenylene, and (D) silica nanoporous
materials before and after hydrothermal treatment.
residual water and ethanol to the total weight loss can represent
the hydrophilic character of these materials. The hydrophilicity
increased in the order ethylene < phenylene < methylene <
silica. The increase in the carbon chain length and the size of
the nonpolar organic molecule can be contributing factors for
the decreased hydrophilicity in ethylene- and phenylene-bridged
organosilicates.30 Therefore the inclusion of these bridging
structures will impart hydrophobic/hydrophilic characteristics
to the organosilicates. The total pore volumes (from nitrogen
sorption) measured before and after compression at 8 K have
been used to calculate the residual porosity, an accurate
representation of mechanical stability. The water-ethanol
weight loss ratios are plotted against the residual porosity in
Figure 7. The more hydrophobic materials exhibit the best
mechanical stability. This trend supports the hypothesis that the
mechanism of structural collapse in these materials involves
mechanochemical hydrolysis. A comparison of the thermo-
Figure 10. Nitrogen gas adsorption/desorption isotherms and pore size distributions (insets, PSD) of template-extracted (A) methylene, (B) ethylene,
(C) phenylene, and (D) silica nanoporous materials before and after hydrothermal treatment.

12634 J. Phys. Chem. B, Vol. 107, No. 46, 2003
Burleigh et al.
TABLE 4: Structural Properties of Extracted Nanoporous
Organosilicas before and after Hydrothermal Treatment
Dr. Mazyar Zeinali for many helpful discussions. This project
was funded by the Office of Naval Research through a Naval
Research Laboratory Accelerated Research Initative.
BET
surface
area
total
pore
pore
wall
Da
volume sizeb thicknessc
References and Notes
sample
(Å) (m2/g) (cm3/g) (Å)
(Å)
(1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck,
J. S. Nature 1992, 359, 710.
methylene PMO (aged)
methylene PMO (hydro)
ethylene PMO (aged)
ethylene PMO (hydro)
phenylene PMO (aged)
phenylene PMO (hydro)
nanoporous silica (aged)
74
74
63
63
57
57
59
870
930
1080
1120
900
850
1010
660
0.90
1.00
1.10
1.20
0.60
0.70
1.30
1.10
50
50
43
43
33
35
44
52
35
35
29
30
33
31
24
9
(2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge,
C. T.; Schmitt, K. D.; Chu, C. T.; Olson, D. H.; Sheppard, E. W.; McCullen,
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a d(100) spacing of the extracted products ((2 Å). b Calculated from
adsorption branch. c Estimated from a0-pore size, where a0 ) (2x3/
3)d(100).
and a significant decrease in wall thickness from 24 to 9 Å.
Unlike most template-free silicas, which are destroyed after 18
h of boiling, the high hydrothermal stability observed in our
nanoporous materials is most likely due to the thicker pore (24-
35 Å) walls obtained through synthesis with alkylethylene oxide
templates in acidic media. A similar result has been reported
recently for an ethane-bridged PMO synthesized using Pluronic
123 as the structure director.31
Conclusions
We have reported the effects of aging, mechanical, and
hydrothermal treatments on the microstructures of periodic
mesoporous organosilicas (PMOs). Methylene-, ethylene-, and
phenylene-bridged PMOs and ordered mesoporous silica were
synthesized by acid-catalyzed hydrolysis and condensation of
bis(triethoxysilyl) precursors around supramolecular polyoxy-
ethylene(10) stearyl ether (Brij 76) templates. The PMOs were
then generated by extraction of the templates. As-synthesized
composites containing surfactant templates have also been
characterized and compared to the PMOs derived from them.
None of the nanoporous materials in this study exhibited any
significant changes in microstructure following aging for 10
months under ambient conditions. Results obtained from XRD,
nitrogen sorption, and thermogravimetric analyses support the
concept of mechanochemical silicate hydrolysis as the driving
force behind microstructural collapse in these nanoporous
materials. All three PMOs in this study exhibited better
mechanical and hydrothermal stabilities than periodic mesopo-
rous silica. Future experiments will include an investigation of
the mechanical stabilities of PMOs after hydrothermal treatment.
Acknowledgment. Dr. Shalini Jayasundera and Dr. Chris
W. Thomas are NRC/NRL Research Fellows. The authors thank