Note: Descriptions are shown in the official language in which they were submitted.
CA 02274099 1999-06-04
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CONTROLLED-PORE AMORPHOUS SILICAS AND PROCESS FOR
MANUFACTURING THE SAME
Field of the Invention
The present invention relates to controlled-pore amorphous silicas and to a
process for manufacturing the same. The present invention more specifically
relates
to amorphous silicas having pore diameters of at least 1,000 Angstroms which
are
particularly useful as enzyme support in biocatalysis.
Background of the Invention
Enzyme supports play an essential role in biocatalysis. The effect of pore
diameter and surface on enzyme efficiency is well known (see for example Use
of
Hydrophobic Controlled - pore Glasses as Model Systems, John A Bosley and John
C Clayton, in Biotechnology and Bioengineering, vol. 43, pp.934-938 - 1994
John
Wiley & Sons). in this paper is stressed the importance of having supports
presenting pores with diameter above 1,000 Angstroms.
Available products, which exhibit this porosity, such as XWP Gels can be
obtained from Grace Davison. They are described as extra wide silica gels for
biotechnological and gene technological applications. They present a pore
diameter
of 300 to 1,500 Angstroms, a surface area of 170 r~/g (for a pore diameter of
300
Angstroms) to 20 mZ/g (for a Pore diameter of 1,500 Angstroms).
Up to now, it has not been possible to propose amorphous silicas having both a
high surface area and a high mean pore diameter and there is a need for such a
stru ctu re.
Another problem is that the production of such high porosity structure is a
very
difficult and expensive process which renders the product unaffordable for
most of
its potential applications. There is therefore a need for a process which
enables the
production of controlled-pore silicas exhibiting high mean pore diameters.
The formation of silica gel at both high and low pH is widely recognised and
gelation at low pH has been practised for many years. However, nobody uses
high
pH systems for the formation of gels) this pH regime is that used for
precipitate
production. The main reason for this is the fact that the gel times under
alkaline
conditions are highly sensitive to small variations in acid flow rates and are
thus
difficult to control at reasonable concentrations. The formation of alkaline
gels is
recognised in the literature.
li has now been discovered that it is possible to prepare a series of gets
with
varying silica concentration and partial neutralisation such that a gel forms
in a
reasonable time. To these gels an electrolyte is added after the gel has set,
the
neutralisation process being then completed.
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Adding salt to precipitated silicas which are made under alkaline conditions
would normally be expected to lead to the formation of dense low porosity
structures. It is therefore surprising that adding salt actually increases the
porosity.
Tests and Definitions
i Surface Area
Surface area is determined by standard nitrogen adsorption methods of
Brunauer, Emmett and Teller (BET) using a multi point method with an ASAP
2400 apparatus supplied by Micrometrics of the USA. The samples are
outgassed under vacuum at 120°C for at least 1 hour before measurement.
Surface area is calculated from the adsorption data measured in the P/Po
region from 0.05 to 0.3. The calculation is restricted to the linear region of
the
BET plot within this pressure range.
ii Pore Volume and Pore Diameter
The porosity of materials with pore sizes greater than about 500 Angstroms (50
nm) cannot be analysed using nitrogen sorption analysis because of severe
theoretical and practical limitations of the method. The maximum pare
diameter that can be measured by nitrogen is about 1000 Angstroms and this
would be insufficient to allow complete characterisation of a 500 Angstrom
pore size material if it had a normal pore size distribution. The best
alternative method for the characterisation of such macroporous materials is
mercury porosimetry which has an analytical range from about 300 microns to
35 Angstroms.
In the present invention, calculations are based on the procedure of Ritter
and
Drake using a contact angle of 140 degrees and an interracial tension for
mercury of 485 dyneslcm.
Porous materials have a number of regions of porosity when measured using
mercury intrusion analysis. The two main identifiable regions are the inter-
and infra-particle regions which can be identified from the cumulative
intrusion
curves. The inter-particle porosity is dependant on the particle size of the
material, the particle shape and the packing geometry. The intra-particle
porosity is the porosity of interest in the present invention and this too can
have more than one component depending on the nature of the material. Such
porosity can exist in micro, meso, or macro pore sizes, as defined by the
IUPAC convention.
In the present invention) the Intra-particle Pore Volume (IPV) is defined as
the
volume of pores in the region less than 5 microns as measured by mercury
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porosimetry and the Macropore Volume (MPV) is defined as the contribution to
the infra-particle pore volume in the region between 5 microns and 0.05
microns (500 Angstroms) as measured by mercury porosimetry.
The pore size of particular relevance in the present invention is in the
macropore region, namely greater than 500 Angstroms (or 50 nm). The
Macropore Diameter (MPD) is determined as the pore diameter position of the
maximum of the first differential of the cumulative intrusion curve plotted
between 5 microns and 500 Angstroms.
iii) Degree of Neutralisation (D of N)
Sodium silicate can be regarded as a mixture of sodium oxide (NalO) and
silica (Si02) and is available with various molar ratios of silica and sodium
oxide. When reacted with acid the sodium oxide is converted to a sodium salt
and water as illustrated by the reaction:
NaZO + 2HX = 2NaX + I-iz0
The silica is deposited from solution as an amorphous solid. Knowing the
quantity of sodium silicate which is to be neutralised, and the silica to
sodium
oxide ratio of the silicate, it is possible to calculate the amount of acid
needed
to complete that neutralisation. tf this value is taken as 100% then the
Degree
of Neutralisation (D of N) is the fraction of that amount of acid which is
added
in the initial mixing stage expressed as a percentage. Thus at 50% D of N only
half of the available alkali is reacted during the initial stage of the
process.
Summary of the Invention
According to one aspect of the present invention there is provided a
controlled-pore amorphous silica having a surface area of between 10 mz/g and
900
mZlg, a Macropore Diameter of between 1,000 and 10,000 Angstroms) the Surface
Area (SA) and the Macropore Diameter (MPD) satisfying the following equation:
MPD > 2300 -(25 x SA).
Preferably, the controlled-pore amorphous silica of the invention has a
surface
area of between 52 mZ/g and 900 mZ/g.
Preferably the controlled-pore amorphous silica of the invention has an
Infra-particle Pore Volume of between 1 and 3.5 cc/g.
Preferably also) the controlled-pore amorphous silica of the invention has a
Macropore Volume of between 1 and 3.0 cc/g. Most preferably, the Macropore
volume represents at least 80% of the Infra-particle Pore Volume.
It has been found that the amorphous silica of the invention can be used as a
metal support in catalysis. When metals are deposited on a support, due to the
high
price of the metals used, great attention is paid to the fact that only a
limited
amount of metal is deposited on sites which are not going to be accessible
during
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the reaction. Therefore, it is important to avoid, as much as possible, the
deposition
of metal in micropores. It has been found that it was possible to calcine the
amorphous silica of the invention, thence eliminating the micropores while
keeping
the macropores intact. A product is then obtained with almost unchanged pore
volume and macropore diameter but in which, the micropores having disappeared,
the surface area has considerably decreased. In a preferred embodiment of the
invention, the amorphous silica of the invention presents a surface area of
between
52 mz/g and 200 mzlg, preferably below 150 m2/g.
According to a second aspect of the present invention there is provided a
process for manufacturing controlled-pore amorphous silicas wherein:
~ sodium silicate and sulphuric acid are mixed together, the resultant sol
being
allowed to set as a gel,
~ the gel is subsequently broken up and mixed with a solution of electrolyte,
~ the neutralisation of sodium silicate is completed later by adding further
sulphuric
t 5 acid,
~ the resulting product is then filtered, washed and optionally dried.
Preferably the electrolyte is sodium chloride but sodium sulphate, potassium
chloride and potassium sulphate have also been used.
If it is necessary to decrease the microporosity, the washed and dried product
is subsequently calcined by heating in air at a temperature between 70tp and
1000°C, preferably between 700° and 800°C.
A hydrothermal ageing step can be optionally introduced between the gelation
and electrolyte addition steps, the electrolyte addition and the second acid
addition
steps, between the second acid addition and filtration and washing steps
and/or any
combination of these ageing steps.
The most interesting products for the proposed applications are those made at
relatively high silica concentration and low D of N. Typically, the sol
presents)
before the electrolyte addition, a Degree of Neutralisation of 25% to 30% and
a
silica content of 12% to 15% by weight. These materials set within a few hours
to
form an easily processable solid. This is then slurried with an equal volume
of
electrolyte solution (eg 200g/1 NaCI) and stirred at room temperature for a
few
minutes.
It is the electrolyte addition that is the key to this process. The
electrolyte is
added such that the concentration of cations is between 1 and 3.5 Molar with
respect
to the cation. It should be noted that the electrolyte concentration referred
to in the
examples is expressed as the concentration in the liquid which is added to the
gel in
order to form a processable slurry and not the concentration in the final
slurry. The
electrolyte may be added either as a solution to the milled gel or as a dry
solid to
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the gel after slurrying. The electrolyse causes the gel to form localised
clusters
which create the macropore matrix.
The electrolyte also promotes the formation of siloxane bonding and increased
strength through sodium ion bridging while the gel is in the alkaline
condition and
thus these wide pores have significant wall strength and do not collapse
easily on
drying.
Macropore Diameters achieved through this process vary between a few
hundred and 10,000 Angstroms (measured by mercury intrusion). The surface
areas
of the products can be much higher than might be predicted on the basis of the
porosity alone using the well known (4,000 ' Pore volume/Surface Area) rule.
In the
examples which have been prepared the surface area covers a wide range. The
surface area is a feature of the smaller pore structures in the watts that
surround the
macropores. The presence of these meso and micro-pores within the structure
therefore makes it possible to effect a significant surface area reduction by
calcination at temperatures between 700° and 800°C without a
significant loss of
either the macropore size or macropore volume and the small reduction in pore
volume that is observed is associated mainly with the loss of the micro and
mesopore volumes.
Specific Description of the Invention
The present invention will be further illustrated in the following examples.
Example 1
Sulphuric acid and sodium silicate (Na~O : 3 SiO~ were mixed together by
pumping the acid and silicate through an in line high speed, high shear mixer
head
(in fine Silverson mixer). The flow rates and solution concentrations were
such that
the resultant so! had a silica concentration of 15 % (wt/wt) and a D of N of
30°~. The
sot was allowed to set end had a gel time of about 15 minutes. The gel time is
defined as the point at which the resultant sot behaves as a single mass
rather than
a viscous suspension/solution. It can be identified using a small sample
collected in
a beaker. When the beaker is tilted the attainment of the gel point is
identified by
the fact that the sot peals away from the side of the beaker rather than
flowing
across the beaker. A clearly defined margin corresponding to the position of
the
meniscus at the beaker wall will be discernible on the inclined gel surface.
The sof was allowed to set and harden up for 48 hours before being coarsely
broken up by forcing it through a 3.5 mm stainless steel mesh.
A weighed amount of the disintegrated gel was added to an equal weight of
demineralised water and solid sodium chloride was added slowly over a period
of
about 10 minutes with continuous stirring. The amount of salt added was
sufficient to
create a 200g/1 solution in the added water. The resultant slurry was stirred
for 10
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minutes at ambient temperature before adding sufficient sulphuric acid
solution to
complete the neutralisation and achieve a pH of 3.
The white particulate solid that results was isolated and washed with
demineralised water using a plate and frame fitter press. The material
obtained was
freeze dried by cooling the sample quickly in liquid nitrogen and placing the
frozen
solid in an SB4 laboratory freeze drier supplied by Chemlab Instruments. The
water
was then removed from the sample by sublimation at reduced pressure over a
period
of 72 hours.
A second sample of the filter cake was dried over night in a laboratory fan
oven at 120°C. The freeze dried material had a total intra-particle
pore volume of
2.38 cclg, a macropore volume of 2.09 cc/g and a macropore diameter of 7000
Angstroms and a BET surface area of 674 rr>z/g. The oven dried material had a
total intra-particle pore volume of 2.17 cclg, a macropore volume of 1.87
cclg, and a
macro pore diameter of 4800 Angstroms and a BET surface area of 536 rc~/g.
Example 2
In a second preparation a sample was prepared as above with a silica
concentration in the gel of 15% and a D of N of 20%. The gel time was 3 hours
15
minutes and the material was allowed to harden for 24 hours. 400 g of the gel
broken into particles was slurried with 400g of a 200g/1 sodium chloride
solution.
After stirring for 10 minutes the excess alkali was neutralised and the pH
adjusted to
3 before filtering and washing using a Buchner filter.
The resultant product was freeze dried and had a macropore diameter of 4200
Angstroms and a total intra-particle pore volume of 3.22 cc/g, a macropore
volume
of 2.64 cc/g, and a BET surface area of 306 r~/g.
Example 3
This preparation was the same as in Example 2 above with a silica
concentration in the gel of 15% and a D of N of 20°~6 but after contact
with the
sodium chloride solution the slurried gel was aged in a stirred vessel under
reflux at
90C for 1 hour. The resulting material after washing and freeze drying had a
total
intra-particle pore volume of 3.28 cclg, a macropore volume of 2.98 cclg, a
macropore diameter of 5000 Angstroms and a BET surface area of 78 rr~/g.
Example 4
This material was prepared as in Example 1 above with a silica concentration
in the gel of 15% and a D of N of 30°.6. The gel was slurried with a
solution of
sodium chloride at 100 g/1 and then aged in a stirred vessel at ambient
temperature
for 1 hour before completing the neutralisation. The product after filtering,
washing
and oven drying had a total intra-particle pore volume of 1.64 cc/g, a
macropore
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volume of 1.40 cc/g, a macropore diameter of 3000 Angstroms and a BET Surface
Area of 704 mZlg.
A freeze dried sample of the material had a total intra-particle pore volume
of
2.43 cc/g, a macropore volume of 2.11 cc/g, a macropore diameter of 3000
Angstroms and a BET Surface Area of 857 r~/g.
Example 5
An alkaline gel was prepared as described in Example 1 above with a silica
concentration of 15°~6 and a D of N of 30°r6. The gel was
allowed to harden for 10
days before contacting with electrolyte solution. 250 g of roughly milled gel
was
slurried with 250 g of demineralised water and 50 g of sodium chloride was
added.
The slurry was stirred at ambient temperature for 1 hour before adjusting to
pH 3
with sulphuric acid. The solid was filtered off using a Buchner filter and
washed by
reslurrying 3 times with demineraiised water and filtering. The solid produced
was
split into two samples and part freeze dried and part oven dried at
120°C.
The freeze dried material had a total intra-particle pore volume of 1.36 cclg,
a
macropore volume of 1.11 cclg) a macropore diameter of 3300 Angstroms and a
BET surface area of 475 rr>zlg. The oven dried material had a total intra-
particle pore
volume of 1.14 cc/g, a macropore volume of 1.03 cc/g) a macropore diameter of
4200 Angstroms and a BET surface area of 99 rr~/g.
Example 6
This sample was prepared as described in Example 5 above but the sodium
chloride was omitted and replaced by 63.758 of potassium chloride.
The freeze dried material had a total intra-particle pore volume of 1.89 cc/g,
a
macropore volume of 1.74 cclg, a macropore diameter of 5000 Angstroms and a
BET surface area of 580 rrtzlg. The oven dried material had a total intra-
particle pore
volume of 1.78 cc/g, a macropore volume of 1.64 cclg, a macropore diameter of
5000 Angstroms and a BET surface area of 99 rr~Jg.
Example 7
This sample was prepared as described in Example 5 above but the sodium
chloride was omitted and replaced by 74.58 of potassium sulphate.
The oven dried material had a total intra-particle pore volume of 1.42 cclg, a
macropore volume of 1.25 cc/g, a macropore diameter of 4200 Angstroms and a
BET surtace area of 249 rr>z/g.
Example 8
This sample was prepared as described in Example 5 above but the sodium
chloride was omitted and replaced by 60.728 of sodium sulphate.
The freeze dried material had a total intra-particle pore volume of 1.83 cclg,
a
macropore volume of 1.63 cclg, a macropore diameter of 5100 Angstroms and a
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BET surface area of 463 mZlg. The oven dried material had a total infra-
particle pore
volume of 1.80 cc/g) a macropore volume of 1.65 cclg, a macropore diameter of
5200 Angstroms and a BET surface area of 487 m~/g.
Example 9
250 kg of alkaline gel, with 30°% degree of neutralisation and 15%
silica, was
made into 210 litres kegs by mixing a 9.5°% (wtlwt) solution of
sulphuric acid, at a
flow rate of 0.172 Iltre/min, with a 19.6% (as SiG2 wt/wt) sodium silicate
(3.3 ratio)
solution, at a flow rate of 0.49 fitre/min. This was left for 24 hours before
an equal
volume of water was added to each keg and the get was broken up with a paddle.
The resulting slurry was added to a 400 litres vessel and the vessel agitator
(straight
six-blade turbine) was used to break the gel down into a slurry.
42 kg of sodium chloride was added to the slurry over approximately 10
minutes, followed by sulphuric acid (over approximately 15 minutes) to lower
the pH
value to 3.
The slurry was dropped into a plate and frame filter-press and washed with
demineraiised water. The resultant filter-cake was oven-dried at l2ffC. The
oven
dried material had a total infra-particle pore volume of 1.49 cclg, a
macropore
volume of 1.32 cc/g, a macropore diameter of 1500 Angstroms and a 8ET surface
area of 318 m2/g.
Samples of this oven dried material were calcined in a static furnace by
heating from room temperature to 700°C and 800°C and holding at
these
temperatures for 1 hour. These materials had the following properties:
After calcination at 700°C the material had a total infra-particle pore
volume of
2.19 cc/g, a macropore volume of 1.96 cc/g, a macropore diameter of 1300
Angstroms and a BET surface area of 163 mzlg.
After calcination at 800°C the material had a total infra-particle pore
volume of
1.86 cc/g, a macropore volume of 1.81 cc/g, a macropore diameter of 1700
Angstroms and a BET surface area of 34 rr>zlg.
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