Full text of DOI:10.1557/JMR.2000.0208

Microporous semicrystalline silica materials
Man Park and Sridhar Komarnenia)
Materials Research Laboratory, The Pennsylvania State University,
University Park, Pennsylvania 16802
W.T. Lim, N.H. Heo, and J. Choi
Departments of Industrial Chemistry and Agricultural Chemistry, Kyungpook National University,
Teagu, Korea 701-702
(Received 15 October 1999; accepted 25 April 2000)
A new family of microporous semicrystalline silica materials (MSSMs) were
developed at room temperature from acidic mixtures of alkyl-substituted silane and
tetramethylalkoxysilane. Hydrolyzed alkyl-substituted silica precursors, having
hydrophilic silanol groups and hydrophobic alkyl groups, presumably act not only as
templates but also as sol stabilizers for continuous pore engineering of silica materials
in the micropore region. Depending on the substituted alkyl (SUA) groups in initial
sols, MSSMs have distinct broad x-ray diffraction peaks in low 2␪ range of 2° to 12°,
distinguishable thermal behavior of SUA groups, highly flexible processability, and
discrete micropore size with good thermal stability of micropores after the removal of
SUA groups. These designer microporous silicas are expected to be useful for
molecular sieving applications.
Precise pore-size control in processed silicas has been
a great challenge. Although various silica materials were
developed with a wide range of morphologies, crystal-
linities, and pore-size properties, there is still a great need
for the development of new and cost-effective silica ma-
terials. Especially important is the development of mi-
croporous processed materials that can be used for
molecular sieving in various technologies such as gas
separation, catalysis, membrane reactors, sensors, and
adsorbents.1–4 Recently, precise pore-size control was
achieved in the mesoporous range (2–50 nm) by the syn-
theses of the M41S family of porous materials through
the sol-gel technique.5–8 However, precise control is still
not accomplished for silica gels or films in the micro-
porous range (<2.0 nm), although a few microporous sili-
cas with narrow pore-size distribution and zeolites have
been investigated for use as membranes.2,3,9–11 Here, we
report the synthesis of microporous semicrystalline silica
materials (MSSM) with different pore sizes in the mi-
croporous regime.
by interacting with frameworks.1–7 The combination of
the template approach with a sol-gel method allow one to
design and fabricate the exact pore size and shape in
processed silicas. There are a few synthesis studies of
amorphous silicas that used the template approach;2,3 the
exact role of templates is unclear. Unfortunately, liquid-
crystal templating,5–8,14 which was used for mesoporous
materials, could not be applied to tailor pore size
(<1.5 nm) in the microporous range.
It is interesting that hydrolyzed alkyl-substituted silica
precursors have hydrophilic silanol groups as well as
hydrophobic alkyl groups. This unique nature led to the
recent synthesis of layered inorganic–organic hybrid
silica materials from long-chain alkyl-substituted silica
precursors.15 These hydrophilic and hydrophobic prop-
erties suggest that substitute alkyl (SUA) silica precursor
can act not only as a template but also as a sol stabilizer,
and that SUA groups can form micelles of microporous
dimension under certain conditions. Using this approach,
we have developed a new family of MSSM with highly
flexible processability and these materials may be useful
in molecular sieving.
A series of MSSMs were synthesized at room tem-
perature by the sol-gel method from acidic mixtures
of tetramethylorthosilicate (TMOS) and an alkyl-
substituted silane. The alkyl-substituted silanes used
were propyltrimethoxysilane, phenyltrimethoxysilane,
hexyltrichlorosilane, and octyltriethoxysilane. (Here-
after the resultant sols and xerogels are referred to as
PRO-, PHE-, HEX-, and OCT-sol or xerogel, respec-
tively.) A molar ratio of 1.0 silica precursor: 3.0 metha-
nol: 32 H2O : 1.4 × 10−2 HNO3 was used for PRO-,
Common strategies for the pore-size control of silica
materials are the manipulation of various process param-
eters such as sol composition, pH, temperature, aging
time and drying method during preparation, and post-
treatment.1,12,13 However, complex origins of porosity
and structure evolution greatly limit the efficiency of
pore-size control. Another recent strategy is the template
approach in which templates direct pore sizes and shapes
a)Address all correspondence to this author.
Also with the Department of Agronomy.
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PHE-, and OCT-sol and 1.0 silica precursor: 12.0 metha-
nol: 10 H2O for HEX-sol. Silica precursors in all sols
except OCT-sol consisted of a molar ratio of 3 TMOS to
1 alkyltrialkoxysilane (for OCT-sol, a ratio of 4 to 1 was
used). Also tripropylamine (T1) and tripentylamine (T2)
were tested as template molecules for PHE- and HEX-
sols, respectively. (For PHE-sol, acid amount equimolar
to tripropylamine was also added.) All sols except OCT-
sol (biphase) were transparent and homogeneous for a
certain duration before they became opaque. Sol stability
based on visual transparency depended on the type of
SUA group and solvent ratio. PRO-sol showed precipi-
tation after 30 h and PHE-sol after 12 h, whereas HEX-
sol became opaque within 1 h. Sols were cast either as thin
sheets in a petri dish to quickly obtain xerogels at various
time intervals or continued to be stirred until precipitates
formed. Xerogels, transparent films, and white soft lumps
were prepared by drying sols or precipitates at room tem-
perature (hereafter referred to as as-synthesized xero-
gels). The processing of these silicas into films is useful
for membrane applications. The SUA groups were removed
by calcination in air (3 h at 250 °C for PRO-xerogels,
10 h at 400 °C for PHE-, HEX-, and OCT-xerogels with
a heating rate of 2.5 °C/min). As-synthesized xerogels
from HEX-sol and the HEX-sol with the addition of
guest template were washed with an aqueous ethanol
solution containing 70% ethanol on a volume basis and
dried at room temperature before calcination.
All the as-synthesized xerogels showed a distinct
broad x-ray diffraction (XRD) (hump) in low 2␪ range of
2° to 12° [Fig. 1 (a)] unlike amorphous silicas that show
a hump around 20° two theta. To the best of the authors’
knowledge, such semicrystalline nature was not previ-
ously reported for silicas. The d spacings (based on the
apex of the hump) and intensities were systematically
affected by SUA groups. Intensity and d values increased
with an increase in the bulk size of SUA groups, although
they were also slightly affected by sol-aging time. Slight
increases in d value (about 0.1 nm) and intensity of the
XRD hump were observed by extended sol aging time.
The as-prepared PRO-, PHE-, and HEX-xerogels showed
d values of 1.1, 1.2, and 1.5 nm, respectively. The addi-
tion of guest templates (T1 and T2) caused an increase in
d values. Distinct XRD humps persisted after the re-
moval of SUA groups by calcination [Fig. 1 (b)]. The
XRD humps became broad and d values increased in all
the PRO-, PHE- and HEX-xerogels [Fig. 1 (b)] after cal-
cination. In the PRO-xerogel, XRD peak intensity de-
creased upon calcination. However, these changes were
not observed in OCT-xerogels (Fig. 1), which exhibited
somewhat stronger XRD peaks after calcination.
FIG.1. Representative XRD of (a) as-synthesized and (b) calcined
xerogels. A: PRO-, B: PHE-, C: HEX-, D: OCT-xerogel
caused by oxidation of SUA groups in Fig. 2). This re-
port is the first to show a distinguishable thermal behav-
ior of SUA groups in inorganic–organic silica hybrid
materials. It has been shown in normal ORMOSILs
that SUA groups are mainly distributed over the sur-
face of primary pure silica particles.16,17 However,
Fig. 2 strongly suggests that SUA groups are encapsu-
lated by silica matrix in these as-synthesized xerogels as
indicated by a very high temperature ( 600 °C) of their
decomposition.
All the as-synthesized xerogels were completely non-
porous; that is, no surface areas were detected by N2
adsorption. On the other hand, all calcined xerogels in-
cluding both films and lumps exhibited a highly micro-
porous property as indicated by the type I N2 isotherm
with no hysteresis loop (Fig. 3). None of these xerogels
showed mesopores and this behavior is similar to that of
zeolites. N2 adsorption patterns at low relative pressure
(below 0.1 P/Po) indicated that the pore size systemati-
cally increased with an increase in bulk size of SUA
Differential thermal analysis clearly showed much
higher thermal stability of SUA groups in the present
as-synthesized PHE xerogels compared to normal or-
ganically modified silicas (ORMOSILs) (see exotherms
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groups as well as the addition of guest template. From a
ment. Larger alkyl group (leading to larger pore size) and
extended sol-aging time enhanced thermal stability. Sur-
face area was also affected by the size of SUA groups.
However, no significant effect of sol-aging time was
comparison of these isotherms to those of zeolites, the
pore size of PRO- and PHE-xerogels appears to be about
0.5 nm (pore size of zeolite 5A is 0.5 nm; this sample
was supplied by Linde division, Union Carbide Com-
pany) and those of PHE + T1, HEX-, HEX + T2, and
OCT-xerogels to be slightly higher (Fig. 4). Transmis-
sion electron microscopy (TEM) revealed uniformly dis-
tributed micropores (Fig. 5).
Thermal stability of micropores depended on the na-
ture of SUA groups (Table I), as indicated by the change
of surface areas and/or N2 isotherms upon thermal treat-
FIG. 4. Pore-size distribution by Horvath-Kawazoe method of cal-
cined (᭺) octyl, (᭿) hexyl xerogels, and (᭹) zeolite 5A.
FIG. 2 Thermal analysis of as-synthesized PHE-xerogel and PHE-
containing ORMOSIL.
FIG. 3. Representative N2 isotherms of calcined xerogels.
FIG. 5. TEM of as-synthesized HEX-MSSM.
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observed on the surface areas of the xerogels. When
tion takes place slowly and irreversibly to lead to weakly
branched polymers. In the presence of unhydrolyzable
alkyl groups, the polymerization is further inhibited by
steric hindrance.12 Quasi-micelle formation seemed to be
reversible and metastable, as indicated by the observa-
tions that initial opaque sols became transparent by in-
creasing solvent ratio, and that ORMOSILs were
obtained by the addition of alkali catalysts and/or by an
increase in preparation temperature. Even in the case
where high solvent ratio was used for preparation of the
transparent film, MSSMs could be synthesized by drying
sols at room temperature because rapid volatilization of
organic solvent led to highly hydrophilic environment.
Quasi-micelle became stable by irreversible formation of
dense wall with further polymerization. Silica matrix of
their walls may lead to distinct XRD hump, although
exact morphology of pore is not known.
PRO-xerogels were treated at high temperatures
(>600 °C), nonporous amorphous silica resulted with no
XRD hump. The intensity of the XRD hump related
closely to surface area, which strongly suggested that
XRD hump resulted from the silica matrix surrounding
regularly packed SUA groups.
Based on the above-observed results, we suggest that
hydrophobic SUA groups of partially hydrolyzed and
poorly polymerized alkylalkoxylsilanes came together
and were loosely bonded by hydrophobic interaction to
form quasi-micelles (seemingly, micelles) under acid and
highly hydrophilic environment (Fig. 6). Under acid en-
vironment below isoelectric point of silica, silica precur-
sors are rapidly hydrolyzed to form monomers or
oligomers. However, polymerization by their condensa-
Like the synthesis of M41S family,5 MSSM family
could be synthesized with various reagents, pH (less than
isoelectric point of silica), and sol compositions. Further-
more, MSSM family could be obtained in various shapes
like films, gels, and opaque lumps because of highly
flexible processability.
TABLE I. Textural property of MSSMs.
Thermal
MSSM
Surface area (m2/g)
stabilitya
PRO
PHE
PHE+T1
HEX
HEX+T2
OCT
400
440
485
500
525
560
230
350
400
400
400
500
ACKNOWLEDGMENT
This research was supported by Gas Research Institute
under Contract No. 5902-260-2546.
aBased on the change in surface area and N2 isotherm after thermal treat-
ment for 1 day at each temperature.
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