TITLE:
Synthesis of meso-porous silica materials having pore size of 3 - 5 nanometer
FIELD OF THE INVENTION:
The present invention relates to a precursor system for synthesizing meso-
porous silica material having pore size in the range of 3 - 5 nanometer. More
specifically, the invention describes the specific formulation/s of the precursor
system to synthesize meso-porous silica and without using any conventional tri-
block co-polymer surfactant material.The present invention also relates to an
aqueous solution to be prepared first, by mixing desired amounts of polyethylene
glycol (PEG) and polypropylene glycol (PPG) with defined molecular weight
range of the two reactants, which is then to be treated with hydrochloric acid with
continuous stirring. A silica source i.e., aqueous solution of tetraethyl
orthosilicate solution (about 95 w%) is then to be added drop-wise to the
resultant solution and the stirring process is to be continued for a period of 6-8
hours by maintaining solution temperature in the range of 40° - 60°C. The thus-
obtained solution is then to be transferred to a Teflon-lined stainless steel

autoclave reactor, in which solution is to be hydrothermally treated for a period of
12-24 hours by maintaining a solution temperature in the range of 80° - 120°C.
A white precipitate mass was resulted in the autoclave reactor after the
hydrothermal treatment and the white mass was filtered and thoroughly washed
with distilled water and then dried at room temperature in which a dry powder
results. The dry powder is then heat treated (calcined) in air at any set
temperature in the range of 550° - 600°C for a period of 6 - 8 hours in order to
get a calcined powder. The resultant calcined powder has a specific surface area
in the range of 700 - 920 m2/g and pores in the range of 3 - 5 nanometer
BACKGROUND OF THE INVENTION:
Porous materials are classified into several kinds by their size. According to
IUPAC notation [1], materials having pore diameters of less than 2 nm are
microporous materials, pore diameters of greater than 50 nm are macro-porous
materials and the meso-porous category lies in the middle of microporous and
macro-porous. A meso-porous material is a material that contains pores with
diameters between 2 and 50 nm. Also, according to the IUPAC, a meso-porous
material can be 'disordered' or 'ordered' in its meso-structure.

Commonly known meso-porous materials are of silica and alumina origin with
similar profiles of meso-pores. Other meso-porous materials include oxides of
niobium, tantalum, titanium, zirconium, cerium and tin etc.
Mesoporous silica was one material among the meso-porous category which was
first patented around 1970 [2, 3, 4]. The former innovation went almost
unnoticed [5] and was reproduced in 1997 [6]. Later on, meso-porous silica
nanoparticles (MSNs) were synthesized in 1990 by Yanagisawa et al [7]. The
same material was later produced at Mobil Corporation, Japan [8] and was
named as 'Mobil Crystalline Materials', or MCM-41 [9]. Recent development in
nanotechnology has lead to the synthesis of silica nanoparticles at the University
of California, Santa Barbara, which an example of another form of meso-porous
silica, SBA-15 having much larger pores (4.6 - 30 nanometer) produced [10].
The material was named 'Santa Barbara Amorphous' type material, or 'SBA-15'
that is also associated with a hexagonal array of pores.
Since then, research in this field has grown up dramatically over the last several
years, not because of the pore structure of these materials, but solely due to the

availability of extremely large surface area within a relatively small volume of
material, which makes several innovative applications possible. Some of these
prospective industrial applications include catalysis, sorption, chemical sensors,
biosensors, gas sensing, ion exchange, optics, imaging, drug delivery,
photovoltaics and several other allied fields.
Several types of ordered meso-porous silica materials were synthesized in the
past using strongly acidic (pH<2) or basic (pH>9) reaction conditions. The use of
surfactants and amphiphilic polymers as structure directing agents of ordered
meso-porous silica materials is known in the literature. Kresge et al. (Nature
1992, 359, 710-712) reported the synthesis of MCM-41 materials showing
hexagonal arrangements of tubular meso-pores. MCM-41 synthesis is
performed under basic conditions using cationic surfactants.
On the other hand, other type of meso-porous silica material, i.e., SBA-15
typically shows surface areas ranging from 600 to 1000 m2/g depending on
reaction temperature and duration. Pore diameters are on the level of 40 A to 80
A, without the use of co-solvents or swelling agents such as trimethyl benzene to

increase pore size. As would be expected for silica with cylindrical geometry, the
pore size and surface area are inversely proportional to each other; as the pore
size decreases, the surface area increases. Pore volumes are typically 0.8 to 1.2
ml/g. However, the most identifiable attribute of SBA-15 is the well-ordered,
parallel pore structure, consisting of non-intersecting pores, hexagonally
oriented, with a pore width to length aspect ratio of 1:1000.
Zhao et al. (Science, 1998, 279, 548-552) reported the synthesis of SBA type
materials under strongly acidic conditions. SBA-15 with uniform pores of 4.6 to
10 nm is synthesized. Conditions for avoiding the formation of silica gel or
amorphous silica have been investigated in detail with various poly (alkylene
oxide) tri-block copolymers (e.g. PEO-PPO-PEO and the reverse PPO-PEO-
PPO) and with TMOS as a source of silica. The article teaches that suitable
conditions include (a) tri-block copolymer concentrations between 0.5 and 6% by
weight in the reaction mixture, (b) temperatures between 35° and 80°C and (c) a
pH below the iso-electric point of silica. In a publication by Zhao et al. (J. Am.
Chem. Spc. 1998, 120, 6024-6036) the use of alkyl poly (ethylene oxide)
oligomeric surfactants and poly (alkylene oxide) tri-block co-polymers in strong

acidic media has been reported for the synthesis of cubic and hexagonal meso-
porous silica structures with pore sizes from 1.6 to 10 nanometer. Pore sizes
from 1.6 to 3.1 nm were obtained with alkyl poly (ethylene oxide) oligomeric
surfactants already at room temperature. Ordered meso-porous materials with
pores from 3 to 10 nm were obtained with poly (alkylene oxide) triblock
copolymers at temperatures from 35° to 80°C.
There is continuing interest for synthesizing meso-porous silica materials by
designing new precursor system suitable to hydrothermal method.
In this context, this invention describes the design and formulation/s of anew
precursor system for synthesizing meso-porous silica material by following a
hydrothermal reaction method and by avoiding the usage of the conventional tri-
block co-polymer surfactant material, for synthesizing such silica materials
having meso-porous structure with pore size in the range of 30 A0 - 50 A0 and
specific surface area in the range of 700-920 m2/g.

OBJECTS OF THE INVENTION:
An object of this invention is to propose a method for synthesizing meso-porous
silica material.
Another object of this invention is to propose a precursor formulation without
having any usual or reported surfactant, tri-block co-polymer for the synthesis of
meso-porous silica material.
Further object of this invention is to define starting raw materials, chemicals,
precursor formulation and specific reaction conditions thereof for the synthesis of
meso-porous silica material having pore size in the range of 3-5 nanometer with
specific surface area in the range of 700 - 920 m2/g.
BRIEF DESCRIPTION OF THE INVENTION:
According to the invention there is provided a method for synthesizing meso-
porous silica-material comprising preparing aqueous solution of PEG
(polyethylene glycol) and PPG (polypropylene glycol), mixing an aqueous

solution of hydrochloric acid and tetraethyl-orthosilicate to the said aqueous
solution under constant stirring, subjecting the resultant homogeneous solution to
the step of hydrothermal treatment, filtering the precipitate, washing the
precipitate with distilled/de-ionized water, at room temperature and subjecting the
dried precipitate to the step of calcinations.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig. 1: shows the Isotherm linear plot for mesoporous silica material
Fig. 2: shows the surface area (BET) for mesoporous silica material
DETAILED DESCRIPTION OF THE INVENTION:
In the present invention, an aqueous solution is to be prepared by dissolving
solid polyethylene glycol having molecular weight about 4000 (PEG) and liquid
polypropylene glycol with molecular weight about 425 (PPG) in distilled water by
maintaining specific weight ratio and concentration. The solution could be turbid
and hence to be stirred by maintaining a solution temperature in the range of 40°
- 60°C until a clear solution was obtained.
To the above solution, an aqueous solution of hydrochloric acid is to be added
first with continuous stirring and then an aqueous solution of tetraethyl-
orthosilicate with specified concentration is added and stirring was continued for
a period of 6 - 8 hours by maintaining a solution temperature in the range of 40°
- 60°C in order to get a homogeneous solution.

The resultant homogeneous solution is then transferred to a Teflon-lined
stainless steel autoclave, wherein the solution was hydrothermally treated for a
period of 12 - 24 hours by maintaining a temperature in the range of 80° -
120°C.
It is to be noted that the reacting ratio of tetraethyl-orthosilicate, hydrochloric acid
and water was kept the same and the reacting ratio of PEG and PPG were varied
in certain cases, during the hydrothermal treatment, besides certain variations in
heat treatment temperature (calcinations) as well.
After the hydrothermal treatment, a white precipitate was formed, which was
filtered and washed with de-ionized/distilled water and then dried at room
temperature that resulted a dried precipitate. The dried precipitate is then
calcined at a temperature in the range of 550° - 600°C in air for a period of 6 - 8
hours by maintaining a heating rate of 1°C/min that resulted a calcined powder.
The resultant calcined powder is meso-porous silica that has a specific surface
area in the range of 720 - 920 m2/g and showed crystalline structure in the X-ray
powder diffraction (XRD) analysis. The corresponding isotherm linear plot of the

material shows a combination of pores having major pore width about 36A and
48A respectively in the BJH isotherm evaluation.
As per the process, the following are the steps for synthesizing the said meso-
porous silica material without using conventional tri-block co-polymer surfactant
material:
• to prepare an aqueous solution of PEG and PPG using distilled/de-ionized
water
• To the resultant solution, mixing an aqueous solution of hydrochloric acid
and tetraethyl-orthosilicate under constant stirring for a period of 6-8
hours by maintaining a solution temperature in the range of 40° - 60°C, to
result a homogeneous solution
• Transferring the resultant homogeneous solution to a Teflon-lined stainless
steel autoclave reactor and allowed to undergo a hydrothermal treatment
for a period of 12 - 24 hours in the temperature range of 80° - 120°C to
result a white precipitate.

• Filtering the resultant precipitate and washing the precipitate using
distilled/de-ionized water and then drying off the washed precipitate at
room temperature to get a dried precipitate.
• Calcining the dried precipitate in air at the temperature range of 550° -
600°C to get calcined powder which is regarded as meso-porous silica
• Characterizing the calcined powder by measuring specific surface area in
the BET, crystallinity by XRD and absorption isotherms by BJH methods
and confirming its meso-porous structure
The resultant calcined powder has a specific surface area in the range of
700 - 920 m2/g depending on the experimental conditions followed in the
synthesis process and showed crystalline structure in the X-ray powder
diffraction analysis (XRD). The corresponding BJH isotherm linear plot of
the calcined powder shows combination of pores in the material having
major pores in the range of 3 - 5 nanometer (pore width of which is about
36A and 48A respectively) confirms meso-porosity in the structure of
derived silica material.

The process could be more realized by citing more examples under
variable experimental parameters and conditions so as to get the same
meso-porous silica material under different experimental conditions.
Example 1
In this example, 2.66 g of PEG and 1.34 g of PPG were dissolved in 130
ml of distilled water to prepare a clear solution by constant stirring at a
solution temperature of 40°C. 20 ml of 2M HCI was added in it and then to
which 9.14 ml of TEOS (about 95 w% aqueous solution) was added drop-
wise to the resultant solution and the stirring process was continued. The
solution was then transferred to a Teflon-lined autoclave reactor and
hydrothermally treated for a period of 12 hours at a temperature of 120°C.
A white precipitate resulted after the hydrothermal treatment. The typical
batch composition is tabulated in the Table 1.
The derived precipitate was filtered and washed with de-ionized water and
dried under ambient conditions after which a dried powder results. The
dried powder was calcined at 550°C in air for a period of 6 hours with the
heating rate of 1°C/min after which a white powder was obtained. The
resultant white powder had a surface area of 920 m2/g in the BET. The

BJH isotherm of the said powder showed two types of pores having
diameters of 3.6 nm and 4.8 nm respectively. The powder further showed
crystalline structure in the XRD, which is analogous to ordered structure
typical to meso-porous silica material.
Table 1: Batch Composition for the synthesis of meso-porous silica
material

Example 2
In this example, the amount of PEG and PPG were varied in reference to
the previous example 1, which were 2.22 g of PEG and 1.78 g of PPG
respectively and were dissolved in 130 ml of distilled water to prepare a
clear solution by constant stirring at a solution temperature of 40°C. 20 ml
of 2M HCI was added to the resultant solution and then to which 9.14 ml of

TEOS (95 w% solution) was added drop-wise to the resultant solution and
the stirring was continued. Then the solution was transferred to a Teflon-
lined autoclave reactor and hydrothermally treated for a period of 12 hours
at a temperature of 120°C. A white precipitate resulted after the
hydrothermal treatment. The typical batch composition in this example is
furnished in the Table 2.
The derived precipitate was filtered and washed with de-ionized water and
dried under ambient conditions after which a dried precipitate results. The
dried precipitate was calcined at 550°C in air for a period of 6 hours with
the heating rate of 1°C/min after which a white powder results. The
resultant white powder recorded a surface area of 864 m2/g in the BET.
The powder further showed crystalline structure in the XRD, which is
analogous to ordered structure typical to meso-porous silica material.


Example 3
In this example, all the procedure remains the same as that of example 1,
except the reaction temperature was increased to 60°C.
In this example, 2.66 g of PEG and 1.34 g of PPG were dissolved in 130
ml of distilled water to prepare a clear solution by constant stirring at a
solution temperature of 60°C, 20 ml of 2M HCI was added in it and then to
which 9.14 ml of TEOS (95 w% solution) was added drop-wise to the
resultant solution and the stirring was continued. Then the solution was
transferred to a Teflon-lined autoclave reactor and hydrothermally treated
for a period of 12 hours at a temperature of 120°C. A white precipitate
resulted after the hydrothermal treatment.

The derived precipitate was filtered and washed with de-ionized water and
dried under ambient conditions after which a dried precipitate results. The
dried precipitate was calcined at 600°C in air for a period of 6 hours with
the heating rate of 1°C/min after which a white powder results. The
resultant white powder recorded a surface area of 714 m2/g in the BET.
The powder further showed crystalline structure in the XRD, which is
analogous to ordered structure typical to meso-porous silica material.


WE CLAIM:
1. A method for synthesizing meso-porous silica-material having pore
size in the range of 2 - 5 nanometer with specific surface area in the
range of 700 - 920 m2/g comprising preparing aqueous solution of
PEG (polyethylene glycol) with molecular-weight about 4000 and PPG
(polypropylene glycol) having molecular weight about 425, mixing an
aqueous solution of hydrochloric acid and tetraethyl-orthosilicate to the
said aqueous solution under constant stirring,
subjecting the resultant homogeneous solution to the step of
hydrothermal treatment, filtering the precipitate,
washing the precipitate with distilled/de-ionized water,
drying the precipitate at room temperature and subjecting the dried
precipitate to the step of calcinations.
2. The method as claimed in claim 1, wherein the said aqueous solution/s
of the starting reactants which are prepared by mixing and dissolving
the required PEG and PPG raw materials in de-ionized/distilled water
by maintaining a weight ratio in the range of 1.0- 2.0

3. The method as claimed in claim 1, wherein the resultant aqueous
solution of the reactants which is mixed together with aqueous
solutions of HCI and tetraethyl orthosilicate is continuously stirred for 6
to 8 hours by maintaining a solution temperature in the range of 40° -
60°C.
4. The method as claimed in claim 1, wherein the step of hydrothermal
treatment is performed in an autoclave for a period of 12 to 24 hours at
a temperature range of 80° -120°C.
5. The process as claimed in claim 1, wherein the said hydrothermal
reaction is also carried out by maintaining the reactor volume empty by
1/4th and by applying autogenously pressure to the Teflon reactor in
order to yield the white precipitate.
6. The process as claimed in claim 1, the filtering the white precipitate
using de-ionized/distilled water was carried out until the filtrate was
free from chloride ion

7. The method as claimed in claim 1, wherein the resultant rneso-porous
silica powder obtained by heat treating the powder at a set
temperature in the range of 550° - 600°C with a heating rate in the
range of 1°/min for a period of 6 to 8 hours at the set temperature
8. The process as claimed in claim 1, the resultant meso-porous silica,
showed crystalline structure in the XRD and with a specific surface
area in the range of 700 - 920 m2/g.
9. The process as claimed in claim 1, the BJH isotherm linear plot of the
said calcined powder showed combination of pores in the material
having pore size in the range of 3 - 5 nanometer that is associated
with major pore width about 36Å and 48 Å .

ABSTRACT

The invention describes a precursor system to synthesize meso-porous silica
material having pore size in the range of 3 - 5 nanometer. More specifically, the
invention describes the specific formulation/s of the precursor system to
synthesize meso-porous silica and without using any conventional tri-block co-
polymer surfactant material. As per the invention, an aqueous solution is to be
prepared first, by mixing desired amounts of polyethylene glycol (PEG) and
polypropylene glycol (PPG) with defined molecular weight range of the two
reactants, which is then to be treated with hydrochloric acid with continuous
stirring. A silica source i.e., aqueous solution of tetraethyl orthosilicate solution
(about 95 w%) is then to be added drop-wise to the resultant solution with
constant stirring process for a fixed duration maintaining solution in a fixed
temperature in the range . The thus-obtained solution is then to be transferred to
a Teflon-lined stainless steel autoclave reactor, in which solution is to be
subjected to hydrothermal treatment. A white precipitate mass was resulted in
the autoclave reactor after the hydrothermal treatment and the white mass was
filtered and thoroughly washed with distilled water and then dried at room
temperature in which a dry powder results. The dry powder is then heat treated
(calcined) in air at set temperature range. The resultant calcined powder has a
specific surface area in the range of 700 - 920 m2/g depending on the
experimental conditions followed in the synthesis process and showed crystalline
structure in the X-ray powder diffraction analysis (XRD). The corresponding BJH
isotherm linear plot of the calcined powder shows combination of pores in the
material having major pores in the range of 3 - 5 nanometer (pore width of which
is about 36Å and 48 Å respectively) confirms meso-porosity in the structure of
the derived silica material.