Inorganic Materials
for Catalyst Innovation
AEROSIL®, AEROXIDE® and SIPERNAT®
Metal Oxides and Silica Based Materials
Industry Information 2242
Contents
1.
Pure Materials: The Basis for Catalyst Design
3
1.1
Preface
3
1.2
Synthetic Silica and Metal Oxides Overview
3
2.
AEROSIL® Fumed Silica and AEROXIDE® Fumed Metal Oxides
3
2.1
Flame Hydrolysis – The AEROSIL® Process
3
2.2
AEROXIDE® Fumed Metal Oxides
3
2. 2.1 Fumed Aluminum Oxide
4
2. 2.2 Fumed Titanium Dioxide
4
2.3
6
Mixed and Doped Fumed Metal Oxides (MOX & DOX)
2. 3.1 SiO2 / Al2O3
6
2. 3.2 SiO2 / TiO2 (SiTi)
6
3.
Characterization and Selected Basic Function
7
3.1
Surface Characteristics
7
3. 1.1 Details of the Silica Surface
7
3. 1.2 The Surface of Fumed Metal Oxides
7
3. 1.2.1 Fumed Alumina
7
3. 1.2.2 Fumed Titanium Dioxide
7
3.2
Catalyst Support Purity
8
3.3
Thermal Stability through Surface Doping
8
4.
SIPERNAT® Precipitated Amorphous Silica – Designing Porous Particles
10
4. 1
Mesoporous Silica Grades
10
4. 2
Porosity and Surface Properties
11
4. 3
Surface Chemistry & Surface Acidity
12
5.
Material Handling Options
13
5.1
Evonik Industries – Over 60 years of Powder Handling Experience
13
5.2
AEROPERL® Granulated Fumed Metal Oxides
13
5.3
AERODISP® Fumed Silica and Metal Oxide Dispersions
13
6
Evonik Industries: Part & Partner in Catalyst Innovation
14
6.1
Automotive Emission Control Catalysts
14
6.2
Catalysts for Chemical Manufacture
14
6.3
Energy Catalysts
14
6.4
Exclusive Raw Materials for New Synthesis Routes in Catalysis
15
6.5
Zeolite Catalysts – SIPERNAT® and AEROPERL®
15
6.6
AEROXIDE® TiO2 P 25-Photocatalysis
15
7
Product Overview
16
References
19
2
1. Pure Materials: The Basis for Catalyst Design
1.1 Preface
Since the beginning of systematic research into
the action of heterogeneous catalysts for chemical
processes it has become ever more apparent that a
proper carrier plays nearly as important a role as the
active centers themselves. If the carrier is imagined
as the stage in a play, it doesn’t serve the drama if
actors have to negotiate cramped, ill-considered
sets and certainly the negligent banana peel could
turn the night’s efforts into comedy. Likewise, starting your catalyst design with carefully chosen carrier materials, such as AEROSIL®, AEROXIDE®, and
SIPERNAT®, assures a clean and consistent surface
for the real drama: your catalysis.
Because with heterogeneous catalysts, the carrier
often plays a direct role in generating or stabilizing
the catalytic active site, it is often a mistake to treat
the carrier as simply “inert”. Even recognizing the
importance of the carrier to the definition of a “catalytic system”, one can also mistakenly assume that all
chemically-like carriers are “interchangeable”. For
this reason starting with the most chemically pure
and carefully engineered materials is often the surest
way of building precisely the catalyst that will get the
job done time and time again. That’s why we hope
you come to believe:
1.2 Synthetic Silica and Metal Oxides Overview
Synthetic silica products and metal oxides, such as
alumina and titania, have been produced on a large
scale for many decades and are widely used in industry. By means of special production processes, as well
as by corresponding variations in the reaction parameters and after-treatment methods, these products
can be optimally “tailored” for industrial applications
that run the gamut of experience, from food, feed,
agriculture, throughout the extensive world of coatings, to high technology industries such as electronics, pharmaceuticals, and aerospace where materials
of the highest purity are critical. Catalyst manufacturers found early that the high chemical purity and
reliability of AEROSIL® fumed silica, AEROXIDE®
fumed metal oxides and SIPERNAT® precipitated
silica proved especially useful as carrier materials or
as a source of silica for molecular sieve preparation.
All silica products produced by Evonik are derived
synthetically under controlled conditions. These
products are X-ray amorphous [1] and as such belong
to the class of “synthetic amorphous silica” or “SAS”
– a designation commonly found in regulatory discussions to distinguish amorphous silica from crystalline silica and its association with silicosis.
Evonik Industries, Part and Partner in
Catalyst Innovation.
2. AEROSIL® Fumed Silica and AEROXIDE® Fumed Metal Oxides
2.1 Flame Hydrolysis – The AEROSIL® Process
The idea and technical development of the original
AEROSIL® process (also known as flame hydrolysis
or high-temperature hydrolysis) can be traced back
to the Degussa chemist Harry Kloepfer in early 1940
(Degussa is one of Evonik’s predecessor companies).
2.2 AEROXIDE® Fumed Metal Oxides
Evonik scientists found that the flame hydrolysis
process developed for AEROSIL® had great versatility for the manufacture of other oxides such as pure
fumed alumina and fumed titania. These metal oxides
are marketed under the AEROXIDE® trade name.
To produce AEROSIL®, a volatile silicon compound,
most commonly silicon tetrachloride, is injected into
a flame composed of hydrogen and air. Under these
conditions, the silicon tetrachloride is hydrolyzed
to silicon dioxide in a highly aggregated, nanostructured form. This finely divided structure is what
gives AEROSIL® its unique function and capabilities.
For further detail on the manufacture and properties
of AEROSIL®, please refer to [1].
Similar to the AEROSIL® process, the hydrolysis of
vaporizable metallic precursors in an oxyhydrogen
flame provides the basis for AEROXIDE® manufacture. Mixed oxides are also accessible by flame
hydrolysis; however Evonik employs proprietary
technology to result unique particle structures
and / or combinations. These techniques allow true
particle design resulting in an amazing spectrum of
possibilities from homogeneously doped systems, to
isolated or island-type heterogeneous structures, to
layered sheath-core particles [See Figure 1].
3
SiO2
Coating
TiO2
10 nm
CeO2
Doping
SiO2
20 nm
Figure 1
Experimental products demonstrating the core-shell and the doping concept
The upper sequence shows a TiO2-particle completely covered with SiO2, while on the bottom one
dots of CeO2 can be observed on a SiO2-surface.
Virtually no CeO2 can be found in the bulk of the
SiO2 particle.
The metal oxides Al2O3 and TiO2 are produced on
a multi-ton basis and as they are repeatedly cited
in catalysis research will be featured here. Besides
the powdery form of AEROSIL® fumed silica and
AEROXIDE® fumed metal oxides Evonik offers a
broad range of granulated products under the brand
name AEROPERL® and dispersions and the brand
name AERODISP®. Details can be found in chapter
5.2 and 5.3.
2. 2.1 Fumed Aluminum Oxide
Three grades of AEROXIDE® fumed alumina, based
on specific surface area, are available from Evonik
and unlike AEROSIL® all three are distinctly crystalline in nature [see Table 1]. Aluminum oxide occurs
mainly in two modifications: the thermodynamically
stable α-form and the metastable γ-form. The latter can be subdivided crystallographically into the
γ-group and the δ-group. If AEROXIDE® Alu C is
heated to a temperature above 1200 °C, a conversion into the α-form takes place, which is associated
with a decrease in the specific surface area and an
enlargement of the primary particles. As expected,
hardness and abrasiveness are increased as a result of
this tempering.
4
Table 1
Typical Properties of AEROXIDE® Aluminum oxides
AEROXIDE®
Parameter
(test method)
Unit
Alu 65
Alu C
Alu 130
m /g
65
100
130
θ and δ,
little γ
some δ,
predominantly γ
γ
(approx.)
pH
4.5 – 6
4.5 – 5.5
4.4 – 5.4
BET specific
Surface Area
X-Ray Form
2
(4 % aq. Slurry)
Loss on Ignition
@ 1000 °C
wt. % < 3.0
The data represent typical values and are not part of the
specification.
2. 2.2 Fumed Titanium Dioxide
Two grades of titanium dioxide are available from
Evonik distinguished by their specific surface areas
and particle morphologies: AEROXIDE® TiO2 P 25
and high surface area AEROXIDE® TiO2 P 90.
Table 2
Typical data of AEROXIDE® TiO2 P 25, AEROXIDE® TiO2 P 90, and VP AEROPERL® P 25 / 20
Parameter (test method)
Unit
AEROXIDE® TiO2 P 25
AEROXIDE® TiO2 P 90
Specific surface area
m2 / g
50 ± 15
90 ± 20
3.5 – 4.5
3.2 – 4.5
g/l
approx. 130
approx. 120
wt.-%
≤ 1.5
≤ 4.0
wt.-%
≤ 2.0
≤ 3.0
TiO2 content
wt.-%
> 99.5
> 99.5
Average particle size
µm
(BET)
pH
(4 % dispersion in water)
Tamped density
(acc. to DIN EN ISO 787 -11
Moisture
(2 hours at 105 °C)
Ignition loss
(2 hours at 1000 °C based on material
dried for 2 hours at 105 °C)
(based on ignited material)
(SEM)
Developmental products are labeled with the prefix VP. The commercialization depends on market response. Even though they are
produced in commercial quantities, future availability should be verified.
The data represent typical values and are not part of the specification.
The flame process for the production of titania, like
alumina, results in a highly crystalline material. In
the case of fumed titania the crystalline make-up
consists of a majority phase anatase with the balance rutile. This has important implications for the
photocatalytic oxidation of organic molecules via
AEROXIDE® TiO2 P 25 and AEROXIDE® TiO2 P 90,
which is detailed elsewhere. [2]
20 nm
20 nm
Figure 3
Micrographs of AEROXIDE® TiO2 P 25 (left) and AEROXIDE®
TiO2 P 90. The smaller particle size results in a higher surface area.
20 nm
200 nm
TEM
HRTEM
Figure 2
Micrograph of AEROXIDE® TiO2 P 25 depicting the primary
crystals (right) and their aggregates and agglomerates (left)
5
2.3 Mixed Fumed Metal Oxides (MOX & DOX)
2. 3.1 SiO2 / Al2O3
Early in the development of AEROSIL® fumed silica
and AEROXIDE® fumed metal oxides it was found
that the versatility of the flame hydrolysis process
could be extended to the production of mixed metal
oxide systems. The first products offered were a
series of low surface area, mixed SiO2 / Al2O3
powders and water dispersions. These materials are
predominantly silica with a small addition level of
alumina (≤ 1.3 %). In contrast, through a recent technical break-through, mixed oxides can be prepared
such that the minor component is directed towards
the surface of the primary particle. This has significant effect on the stability of the particles against
sintering. The advantages of surface modification for
thermal stability against sintering will be described in
greater detail in Section 3. 3.
5nm
5 nm
2. 3.2 SiO2 / TiO2 (SiTi)
More recently, a series of co-fumed silicon-titanium
mixed oxides with various titania-silica ratios has
been made available. The impetus for the design of
these materials was to see if the unique crystalline
morphology of pure fumed titania could be protected
under the high temperature conditions of many
catalytic systems by the addition of silica. The details
of this thermal stability enhancement are described
later while Figure 4 shows electron micrographs of
some of these innovative materials that have found
diverse application.
5 nm
5 nm
Figure 4
Electron micrographs of SiO2 / TiO2 mixed oxides.
From left to right: 0 wt-% (AEROXIDE® TiO2 P 25), 0.54 wt-%, 9.71 wt-% and 24.84 wt-% SiO2-content
6
3. Characterization and Selected Basic Function
3.1 Surface Characteristics
Both AEROSIL® fumed silica and SIPERNAT® precipitated silica are characterized by large specific
surface area, but differ in the nature of their surface
structure. The AEROSIL® surface should be seen as
an external surface, arising from the very fineness
of the primary particle size. In the AEROSIL® structure the primary particles are linked together in an
open branched structure to larger aggregates and
agglomerates. SIPERNAT® silica on the other hand,
is composed of tightly aggregated primary particles
built around a true porous structure and as a result
precipitated silica surface area contains both external
and internal components. The variability of the wetprocess allows control of this balance of surface area
components.
3. 1.1 Details of the Silica Surface
Two functional moieties, namely the silanol and
siloxane groups, comprise the silica surface. The
hydrophilic character and Brønsted acidity of silica
is the result of the silanol component. The siloxane groups in contrast are hydrophobic and largely
chemically inert. As would be expected from their
contrasting production processes, AEROSIL® fumed
silica and SIPERNAT® precipitated silica differ in
silanol group density. Because of the origin of
AEROSIL® in a flame process, its silanol group concentration is notably lower than that for wet-process
SIPERNAT®. Knowledge about silanol group dynamics and concentration is essential when designing
catalytic systems based on silica and so a great deal
of research has been devoted to detailing the precise silanol character of AEROSIL® and SIPERNAT®
grades. [3, 4, 5]
Figure 5
AEROSIL® fumed silica silanol group density and concentration
3. 1.2 The Surface of Fumed Metal Oxides
3. 1.2.1 Fumed Alumina
Fumed alumina, such as AEROXIDE® Alu C, has
hydroxyl groups on the surface but the material is
a weak Lewis base. In contrast to AEROSIL® fumed
silica, particles of AEROXIDE® Alu C dispersed in
water (pH = 7) have a positive charge (the Al2O3
isoelectric point lies at pH = 9.5; that for SiO2 lies at
pH = 2.5).
3. 1.2.2 Fumed Titanium Dioxide
The surface of titanium dioxide also possesses
hydroxyl groups; however the surface is more aptly
characterized as amphoteric. [5]. This dual Lewis
acid / base character is reflected in the isoelectric
point for the dispersed particles of AEROXIDE®
TiO2 P 25 which lies at pH = 6. 5.
3.0
1.8
2.5
1.4
2.0
1.2
1.5
1.0
0.8
1.0
0.6
0.4
[Si-OH/nm2]
1.6
0.5
0.2
0.0
When heated, the silanol groups are converted to
siloxane groups by the splitting off of water. Up to
about 400 °C this reaction is reversible. At higher
temperatures this reaction becomes increasingly
irreversible and at temperatures of 800 °C and higher
the surface is completely annealed and the conversion of siloxane groups back to silanol groups is no
longer possible, even if the substance is boiled in
water.[1]
Silanol Group Density
[Si-OH mmol/g]
Silanol Group Concentration
2.0
As shown in Figure 5, the silanol group density of
AEROSIL® fumed silica is to a first approximation
independent of the specific surface. As expected,
the absolute concentration of the silanol groups
rises linearly with the specific surface. This relationship is at the basis of the thickening effect for which
AEROSIL® is widely used in liquid and polymer based
formulations.
0
50
100 150 200 250 300 350
BET Surface Area [m /g]
400
0.0
2
7
The contrasting surface nature of these materials is
demonstrated by their behavior in water through
measurement of zeta potential (Figure 6) [1, 6].
Figure 6
Zeta potentials of fumed metal oxides produced by Evonik as a
function of pH value (0.02 m KNO3)
80
Zeta potential mV →
60
40
20
0
–20
–40
–60
–80
2
3
4
5
6
7
ph value →
8
9
10
AEROXIDE® Alu C
AEROXIDE® TiO2 P 25
AEROSIL® OX 50
3.2 Catalyst Support Purity
With the many different raw material source options
available for catalyst manufacture, there is a simple
answer to the question of why Evonik products
should be chosen: Chemical Purity.
A starting material for AEROSIL® fumed silica is e. g.
silicon tetrachloride, which can be distilled and purified relatively easily. Due to the chemical simplicity
of the AEROSIL® process, hydrochloric acid is the
only by-product. As was mentioned, after-treatment
of the fumed silica with hot steam reduces the residual hydrochloric acid content to less than 0.025 %.
Another outcome of the process is that impurities
can be maintained at a very low level. Among the
other residuals, Al2O3, Fe2O3, and TiO2 are the most
prominent. Additional foreign elements occur only
in traces.
8
Similar purity characteristics apply for AEROXIDE®
fumed metal oxides such as AEROXIDE® Alu C and
AEROXIDE® TiO2 P 25. These materials have purities
exceeding 99.5 % and heavy metal impurities generally fall beneath common detection limits.
3.3 Thermal Stability through
Surface Modification
The ability to direct one fumed metal oxide onto the
surface of another fumed metal oxide was briefly
described earlier. One distinct advantage that materials with such heterogeneous primary particle structure possess is an enhanced thermal stability. Evonik
has developed two systems that demonstrate this
enhancement and has an active research program
exploring the many possible extensions of the design
concept and its application as materials for catalytic
supports.
An early example of this particle structure control
were materials that combined silica and alumina.
Evonik has offered these particle systems for quite
some time as the so-called MOX grades. The newer
technology however directs the alumina phase to the
outer region of the primary particles. An example is
the developmental product VP DOX 110, which consists mainly of silica with a small portion of alumina.
In Figure 7 the sintering resistance of VP DOX 110
is compared to pure fumed silica with similar surface
area (AEROSIL® OX 50). This shows that directed
doping prolongs the onset of sintering by approximately 100 °C.
Figure 7
Improved sintering resistance of VP DOX 110 compared to
AEROSIL® OX 50
relative density
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0
200
400
600
800
1000
temperature [˚C]
1200
1400
1600
1 h heating time
VP DOX 110
AEROSIL® OX 50
AEROXIDE® TiO2 P 25
In another example, it was previously described that
the chemical nature of silica starkly differs from that
of titania; by directed doping, the rich chemistry of
the silanol group can be grafted onto a titanium dioxide core without disturbing its crystalline nature and
basic physical properties. This design also has impact
on the behavior of the titania core with respect to
thermal treatments as can be seen in Figure 8 which
compares the thermal stability of a standard fumed
titania, AEROXIDE® TiO2 P 25 [7, 8, 9] to titania
materials that have been co-fumed with silica. In this
figure it can be seen that by 800 °C, the phase transition of anatase to rutile has taken place in the pure
titanium dioxide (AEROXIDE® TiO2 P 25) powder
thus resulting in a 96 % reduction in surface area. By
contrast the addition of silica to the titanium dioxide
particles nearly eliminates any loss of surface area at
temperatures up to 800 °C. Even though part of the
silica in these products can be found at the surface of
the primary titania particles, they are not completely
coated, and the material still shows the characteristic
properties of titania. This can be seen e. g. by the
photocatalytic behaviour, which is not significantly
reduced compared to pure fumed tianaia.
Photoactivity Index
(AEROXIDE® TiO2 P 25 = 1)
VP TiO2 545 S
50
48
1.0
0.9
BET surface area
BET surface area
50
0.8
0.7
0.6
0.5
0.4
2
20 ˚C
800 ˚C, 3h
20 ˚C
≈15 %
800 ˚C, 3h
≈15 %
≈15%
0.3
0.2
0.1
0
AEROXIDE®
TiO2 P 25
≈85 %
Rutile
≈85 %
≈85 %
VP TiO2 545 S
TiO2 is still catalytically active in
stabilized AEROXIDE® TiO2 P 25
Anatase
Figure 8
Fumed titania loses 96 % of its surface area when heated to 800 °C while titania that has been co-fumed with silica displays excellent thermal stability.
Stabilization influences the catalytic activity only to small extent as illustrated by the Photoactivity Index.
9
4. SIPERNAT® Precipitated Amorphous Silica – Designing Porous Particles
SIPERNAT® precipitated amorphous silica and silicates are produced by acidification of an aqueous
alkali silicate solution. Aggregation and agglomeration take place in parallel to particle growth and precipitation, so that the mean diameter of the resulting
synthetic amorphous silicon dioxide and / or silicate
particles are, as delivered, typically in the micron
range and far above 100 nm.
SIPERNAT® precipitated silica grades are suitable in
various areas of catalyst manufacturing such as:
• A silica source for zeolite synthesis.
• A process aid, e. g. as a binder or rheology control
agent during the forming process.
• The main constituent of carrier materials providing
controlled surface area and porosity for the final
catalytic system.
While for zeolite synthesis typically SIPERNAT®
grades 22, 320, 2200 are being applied, for rheology control finer particle size materials like
SIPERNAT® 22 S or an AEROSIL® grade can be used.
If controlled surface area and porosity play a major
role SIPERNAT® 50 or one of our mesoporous materials described below are recommended.
4. 1 Mesoporous Silica Grades
Silica materials are well known as catalyst carriers in
the chemical industry. Advantages offered are:
• High surface area.
• A controlled pore size distribution already in the
raw material prior to the forming process at the
customer.
• A surface chemistry that can easily be modified
through the functionalization of surface silanol
groups.
The range of commonly available granular silica
materials is quite limited. The beaded gel-type
grades available are mechanically stable and to a
certain degree attrition resistant, but have the disadvantage of providing only limited combinations of
surface area and pore volume.
Non-spherical granular silica gels offer a wider
porosity range but due to inherent sharp edges suffer from poor attrition resistance. Hence it is useful
to have a wide range of fine sized silica materials
available, that can be shaped to appropriate particle
size and geometry by the catalyst manufacturer.
Examples for silica-based extrudates can be found in
the patent literature [10]. Choice of appropriate recipes and extrusion conditions help to tailor bimodal
pore size distributions thus allowing for better percolation of reaction media within the final catalyst
extrudate.
10
Thanks to decades of experience in wet-process
silica production and various production technologies
available in numerous plants over the world, Evonik
can provide a wide range of fine powdered silica
with excellent processability during extrusion and
thereby highly suitable for catalyst manufacturing.
When choosing among properties known in the
industry to be relevant, e. g. BET surface area, pore
volume, and purity, it is helpful that the SIPERNAT®
catalog offers an extensive set of property
combinations.
Table 3
Basic physical-chemical data for SIPERNAT® precipitated silica products
Properties
Unit
Range Available
BET Surface Area
m2 / g
50 – 750
g/l
nm
0.4 – 1.7
4 – 35
ml / 100 g
100 – 300
SiO2
%
97 – 99
Na
%
<1
ppm
< 1000
ppm
< 400
µm
3 – 350
g/l
50 – 550
(ISO 9277)
N2-Pore Volume
Average Pore Diameter
(Mesoporous grades)
DOA-absorption
(ISO CD 19246)
Purity
(ISO 3262-19)
Internal method
Al
Internal method
Fe
Internal method
Agglomerate size
(Laser diffraction
following ISO 13320)
Tamped density
(DIN ISO 787 / 11)
DOA = Dioctyladipate
One important distinction between wet-processed
silica and fumed or flame processed silica is that in
the precipitation process particle porosity develops
and can be controlled. While fumed silica show typically a linear or branched structure, wet-processed
silica products have a sponge-like structure as schematically shown in figure 9.
SIPERNAT® Precipitated Silica and Silicates
AEROSIL® Fumed Silica
Sponge-like Structure
Chain-like Structure
Figure 9
Morphological Differences between SIPERNAT® precipitated silica and AEROSIL® fumed silica materials
The following micrographs shall illustrate the range
of porosity types available. Figure 10 shows precipitated silica having networks of small silica clusters
forming basically meso- and some macropores. The
resulting surface area is in the range of > 700 m²/g,
the material having a wide pore size distribution.
200 nm
Figure 10
Precipitated silica network of small silica
clusters forming meso- and macropores;
resulting in high surface area (> 700 m²/g),
but wide pore size distribution
In figure 11 an example closer to a gel type material is shown, with clusters forming a mesoporous
sponge-like structure with medium surface area,
and narrow pore size distribution. Figure 12 shows a
precipitated silica macroporous structure and a very
smooth surface. The BET surface area is low (approx.
50 m²/g), and a narrow pore size distribution with
virtually no micro- or mesopores can be observed.
200 nm
Figure 11
200 nm
Precipitated silica clusters forming a
mesoporous sponge-like structure with,
medium surface area, and narrow pore size
distribution
4. 2 Porosity and Surface Properties
Quite often precipitated silica materials are chosen by
simply comparing typical data like BET surface area
and total pore volume. This can be quite misleading,
as this does not consider the distribution of pores,
surface roughness and so on, which are typically not
specified.
Control of reaction conditions allows one to tune
pore size distributions to some extent. Figures 13
and 14 show the pore characteristics of different experimental precipitated silica materials (EXP
4215-1, SIPERNAT® 50). Both have a surface area
in the range of 350 – 500 m²/g, but quite differ-
Figure 12
Macroporous structure of a precipitated
silica with smooth surface; low surface
area (50 m²/g), narrow pore size distribution with virtually no micro- or mesopores
ent pore characteristics. EXP 4215-1, for example,
has a narrower pore size distribution curve than
SIPERNAT® 50. Comparing both materials by means
of IGC-ID*, it is confirmed, that SIPERNAT® 50
shows more pronounced size exclusion effects.
The impact of this can be seen also during extrusion.
Typically very small pores can be clogged by process
additives and binders and as a consequence a major
portion of the surface area is not available for the
active component of the catalyst. Hence it is important
to design catalysts using silica with an optimum pore
size combined with highly accessible surface area.
*IGC-ID = Inverse Gas Chromatography at
Infinite Dilution. Measured at Adscientis SARL,
Wittelsheim
11
Figure 13
Cumulative pore surface area (N2-desorption) of the mesoporous
experimental precipitated silica grade EXP 4215-1 compared
to SIPERNAT® 50, which has a similar surface area but different
pore characteristics
450
Desoption Surface Area
[m2/g]
400
350
300
250
200
150
100
50
0
1
EXP 4215-1
10
Pore Diameter [nm]
100
SIPERNAT® 50
Figure 14
Differential pore area characteristic of the same materials as in
Fig. 13. SIPERNAT® 50 exhibits much more pores in the low
pore diameter region than EXP 4215-1, resulting in a smaller
accessible surface area
Differential Desoption
Surface Area
6
5
4
3
2
1
1
EXP 4215-1
10
Pore Diameter [nm]
SIPERNAT® 50
*Measured at Adscientis SARL, Wittelsheim
12
4. 3 Surface Chemistry & Surface Acidity
The most obvious relevant surface characteristic of
silica materials is the density of silanol groups. While
several methods to measure this characteristic exist,
titration with LiAlH4 is quite commonly employed. By
this technique, Evonik’s mesoporous silica materials
show a silanol density of about 3.5 OH / nm².
100
While highly purified silica exhibits very weak
acidic behavior, trace contaminants and certain
manufacturing conditions will modify this. This can
be important in cases where the active catalytic compound to be deposited on the carrier does not contribute to the overall acidity of the system. Inverse
Gas Chromatography has been used to characterize
the surface acidity [13, 14] of precipitated silica samples EXP 4215-1, SIPERNAT® 50 from section 4.2*.
Although the purity of the materials was basically the same, due to the different manufacturing
routes different surface acidities could be detected:
SIPERNAT® 50 exhibits a higher acidic character
ISP(Ether)~19.7 kJ/mol) than does EXP 4215-1
(ISP(Ether)~18.0 kJ / mol). The basic character
remains quite similar for both samples
(ISP(CHCl3)~10.3 – 10.6 kJ / mol). (ISP = Specific
Interaction Potential).
5. Material Handling Options
5.1 Evonik Industries – Over 70 years
of Powder Handling Experience
Because of Evonik’s and its predecessor company
Degussa’s many years of experience with fumed
metal oxides, we can offer the most diverse portfolio
of material handling options. Fumed metal oxides, by
their nature, are very fine, low bulk density materials
and their hydrophilic nature requires the utmost in
care in order to deliver quality products for the most
sensitive and technically demanding application.
Details covering the handling of both AEROSIL® and
AEROXIDE® as well as SIPERNAT® silica, including
highly efficient technologies unique to Evonik can
be found in our brochures [15] or by contacting your
local Evonik representative.
Evonik offers two lines of products that reduce the
complications sometimes involved with working with
fumed metal oxide powders: AEROPERL® granulated
1 µm
fumed metal oxide powders and AERODISP® water
dispersions of fumed metal oxides.
Figure 16
5.2 AEROPERL® Granulated
Fumed Metal Oxides
Offered as a dust-free alternative to standard fumed
metal oxide powders, AEROPERL® granulated
fumed metal oxides dramatically change the way
fumed metal oxides can be handled. AEROPERL® is
manufactured from either AEROSIL® or AEROXIDE®
using a proprietary granulation process developed by
Evonik that does not employ any binders. The result
is a granulate of pure fumed metal oxide that is highly porous and of a bulk density and flowability that
are sufficient to allow its use in fixed and fluidized
bed reactors. Examples are shown in figures 15 and
16. More information on AEROPERL® can be found
in our Technical Information 1341 [23].
Transmission electron microscope image of a section cut through
AEROPERL® 300 / 30 granulated fumed silica
5.3 AERODISP® Fumed Silica and
Metal Oxide Dispersions
All AERODISP® fumed silica, fumed alumina, fumed
titania and fumed mixed metal oxide dispersions
offer mono-disperse aggregates, sub-micron in size,
of the particular silica or fumed metal oxide formulated only with pure, deionized water and a charge
stabilizing agent of either acid or base. AERODISP®
products are milky white in appearance, stable
against settling, and have low viscosities. More
details may be taken from. The dispersions maintain
the high level of purity expected from the fumed
starting materials as residual metals are generally
lower than 5 ppm. In this way the catalyst manufacturer can dose into any formulation all of the quality
value of fumed oxide simply and without any need
for external shear. And if total efficiencies are considered, the dispersions by many measures outperform
the powders.
100 µm
Figure 15
SEM picture of VP AEROPERL® P 25 / 20 granulated fumed
titanium dioxide
13
6. Evonik Industries: Part & Partner in Catalyst Innovation
The use of oxidic materials such as: Al2O3, SiO2, TiO2
among others, has a rich history of success in the
production of catalysts. While these oxides are widely available in nature, such materials often contain
small to considerable amounts of impurities that can
have significant influence on the catalysis. With this
in mind, there is a preference to use synthetic oxides
and thereby control the entire chemical nature of the
catalytic surface. Starting with precisely synthesized,
highly pure materials gives the catalytic technologist
the broadest flexibility and reliability.
There are three key reasons for this: 1) fumed metal
oxides have extraordinarily high chemical purity; 2)
their particle morphology-aggregates of ball-shaped
primary particles-results in a narrow distribution of
pore sizes allowing the optimization of the microarchitecture of the catalyst; and 3) the fumed metal
oxides are highly heat-resistant in the catalyst, thus
enabling higher working temperatures.
6.2 Catalysts for Chemical Manufacture
Like most catalyst systems, chemical catalysts, whether they are for organic chemical synthesis or polymAlong with the systems already referred to in this
erization reactions, require top quality raw materials
publication, some of the many catalytic systems that
with high surface areas and the lowest possible levels
have successfully used Evonik silica or fumed metal
of catalyst-poisoning impurities. Evonik offers a wide
oxides are briefly described here.
range of oxides suitable for use as catalyst carrier
structures as well as precipitated and fumed oxides
6.1 Automotive Emission Control Catalysts
that are used as raw materials in the synthesis of
Increasingly stringent exhaust gas laws and new motor high performance zeolite catalysts. Silica materials
technologies mandate the ever-improving automobeing used for this purpose comprise AEROSIL® 90,
tive emission control catalysts. One constant in this
AEROSIL® 200, and AEROSIL® OX 50 or
technology evolution is the need for high surface
SIPERNAT® 22, SIPERNAT® 320, SIPERNAT® 2200,
area, chemically pure, and stable base supports, such
and SIPERNAT® 50.
as Evonik’s AEROSIL® fumed silica and AEROXIDE®
fumed metal oxides.
6.3 Energy Catalysts
Gas-to-Liquid catalysis gets a big boost from the
The typical automotive catalytic converter design
unique combination of purity, surface area, and
employs a washcoat for a honeycomb support. The
phase composition that are offered by Evonik’s
washcoat consists of a complex mixture of several
fumed silica, alumina and titania products. Since
fine-grained, highly porous inorganic oxides and
these reactions are surface driven, increasing the
mixed oxides (commonly alumina, ceria, or zirconia). available surface area in a reaction while keeping all
The high porosity is what gives the washcoat grains
other factors constant will generally increase catalyst
the required large surface (up to 400 m² per gram of efficiency, and thus reaction rate, in a given reactor
washcoat). The particle structure and the purity of
– requirements well matched by Evonik fumed metal
the oxidic washcoat components affect cluster foroxides. In addition, the exceptional purity of fumed
mation and stability and, as a result, the performance oxides ensures the maximum catalytic activity of a
of the catalyst. The use of fumed metal oxides helps
system due to the minimization of potential catalyst
to increase the efficiency and the well time of the
poisoning impurities [16, 17, 18].
catalyst.
14
6.4 Exclusive Raw Materials for New Synthesis
Routes in Catalysis
In collaboration with Uhde GmbH, Evonik has
developed a novel catalytic process for synthesizing
propylene oxide. In this HPPO (hydrogen peroxide
propylene oxide) process, a heterogeneously catalyzed reaction between propene and H2O2 yields
propylene oxide. [19, 20]
The developmental product, a fumed silica doped
with titania such as described in chapter 2.3,
proved to be a cost-effective starting material for
synthesizing the required catalyst. Evonik offers
several developmental grades (VP) of fumed silica
and titania, available with a wide range of titania
contents and surface areas to fit the specific properties required for a given catalyst composition.
6.5 Zeolite Catalysts –
SIPERNAT® and AEROPERL®
Synthesis of high performance zeolite catalysts demands high performance raw materials.
SIPERNAT® precipitated silica products are an excellent choice as raw materials for making high-silica
zeolites for a variety of reasons. High purity coupled
with consistent trace metal content is the primary
reason that SIPERNAT® products are the starting
point for reliable zeolite synthesis. In addition to
purity, SIPERNAT® offers very high surface areas and
high bulk densities. The result of this is that the powder does not float on the surface when added to a
reactor, yet is rapidly digested by the alkaline slurry.
to wet into the reaction slurry slowly and increase
viscosity once mixed. As a solution to this behavior,
AEROPERL®, our granulated form of AEROSIL® can
be used. These granules wet into the slurry more
rapidly than standard fumed silica powder and do not
affect the viscosity as significantly. They do, however, maintain the exceptional purity and high surface
area of fumed silica powder and thus digest rapidly
and do not introduce unwanted impurities.
6.6 AEROXIDE® TiO2 P 25-Photocatalysis
A notable characteristic of titanium dioxide is its
ability to filter UV light. Anatase absorbs electromagnetic radiation of wavelengths less than 385 nm, for
rutile the cut-off is 415 nm. The unique combination
of phases in AEROXIDE® fumed titania results in a
photoconductivity and, in conjunction, photocatalysis of organic materials that has been extensively
remarked upon. Further detail on the application of
AEROXIDE® fumed titanium dioxide to photocatalysis is available [2, 21, 22].
For zeolites requiring extremely high purity raw
materials, AEROSIL® fumed silica products offer high
surface area for rapid digestion and extremely low
trace metal content for tight composition control.
Being comprised of highly structured, high surface
area particles, however, AEROSIL® fumed silica tend
15
7 Product Overview
A selection of recommended grades and their typical properties are given below. Please contact us for
more specific information or for materials with more
specific requirements, which can be made on large
pilot or plant scale on request.
Recommended Commercial Products
AEROSIL® OX 50
AEROSIL® 90
AEROSIL® 200
AEROSIL® 300
AEROSIL® 380
AEROSIL® MOX 80
AEROSIL® MOX 170
AEROXIDE® Alu 65
AEROXIDE® Alu C
AEROXIDE® Alu 130
AEROXIDE® TiO2 P 25
SIPERNAT® 22
SIPERNAT® 22 S
SIPERNAT® 320
SIPERNAT® 2200
SIPERNAT® 50
Recommended Developmental Products
EXP 4210-1
EXP 4215-1
EXP 4230-1
VP DOX 110
VP TiO2 545 S
VP TiO2 1580 S
16
Recommended AEROSIL® Fumed Silica Grades
Properties
Unit
AEROSIL® OX 50
AEROSIL® 90
AEROSIL® 200
AEROSIL® 300
AEROSIL® 380
Specific surface area (BET)
m /g
50 ± 15
90 ± 15
200 ± 25
300 ± 30
380 ± 30
Tamped density*
g/l
approx. 130
approx. 80
approx. 50
approx. 50
approx. 50
wt. %
≤ 1.5
≤ 1.0
≤ 1.5
≤ 1.5
≤ 2.0
wt. %
≤ 1.0
≤ 1.0
≤ 1.0
≤ 2.0
≤ 2.5
3.8 – 4.8
3.7 – 4.7
3.7 – 4.5
3.7 – 4.5
3.7 – 4.5
≥ 99.8
≥ 99.8
≥ 99.8
≥ 99.8
≥ 99.8
2
acc. to DIN EN ISO 787 -11
Loss on drying*
2 hours at 105 °C
Ignition loss
2 hours at 1000 °C based on material dried
for 2 hours at 105 °C
pH
in 4 % dispersion
SiO2 content
wt. %
based on ignited material
* ex plant
The data represent typical values (no product specification)
Recommended Fumed Silicon-Aluminum Mixed Oxides
Properties
Unit
VP DOX 110
AEROSIL® MOX 80
AEROSIL® MOX 170
Specific surface area (BET)
m /g
65 ± 20
80 ± 20
170 ± 30
Tamped density*
g/l
approx. 80
approx. 60
approx. 50
wt. %
≤ 2.0
≤ 1.5
≤ 1.5
wt. %
≤ 2.0
≤ 1.0
≤ 1.0
3.5 – 5.5
3.6 – 4.5
3.6 – 4.5
2
acc. to DIN EN ISO 787 -11
Loss on drying*
2 hours at 105 °C
Ignition loss
2 hours at 1000 °C based on material dried
for 2 hours at 105 °C
pH
in 4 % dispersion
SiO2 content
wt. %
≥ 99
≥ 98.3
≥ 98.3
Al2O3 content
wt. %
0.05 – 0.50
0.3 – 1.3
0.3 – 1.3
based on ignited material
* ex plant
The data represent typical values (no product specification)
Recommended Fumed Titania and Silica Titania Mixed Oxides
Properties
AEROXIDE® TiO2 P 25
VP TiO2 545 S
AEROXIDE® TiO2 P 90
VP TiO2 1580 S
Specific surface area (BET)
50 ± 15 m²/g
45 ± 10 m / g
90 ± 20 m²/g
80 ± 15 m²/g
Tamped density
100 - 180 g / l
approx. 100 g / l
approx. 120 g / l
approx. 60 g / l
3.3 – 4.5
3.3 – 4.5
3.2 – 4.5
3.3 – 4.5
2
acc. to DIN EN ISO 787 -11
pH
in 4 % dispersion
SiO2 content
approx. 5 %
approx. 15 %
based on ignited material
The data represent typical values (no product specification)
Developmental products are labeled with the prefix VP. The commercialization depends on market response.
Even though they are produced in commercial quantities, future availability should be verified.
17
AEROXIDE® Fumed Alumina Grades
Properties
Unit
AEROXIDE® Alu 65
AEROXIDE® Alu C
AEROXIDE® Alu 130
Specific surface area (BET)
m /g
65 ± 10
100 ± 15
130 ± 20
Tamped density*
g/l
approx. 50
approx. 50
50
wt. %
≤ 5.0
≤ 5.0
≤ 5.0
wt. %
≤ 3.0
≤ 3.0
≤ 3.0
4.5 – 6.0
4.5 – 5.5
4.4 – 5.4
≥ 99.8
≥ 99.8
≥ 99.8
2
acc. to DIN EN ISO 787 -11
Loss on drying*
2 hours at 105 °C
Ignition loss
2 hours at 1000 °C based on material dried
for 2 hours at 105 °C
pH
in 4 % dispersion
Al2O3 content
wt. %
based on ignited material
* ex plant
The data represent typical values (no product specification)
Recommended SIPERNAT® Precipitated Silica Products
Properties
Unit
SIPERNAT® 22
SIPERNAT® 22 S
SIPERNAT® 2200
SIPERNAT® 320
SIPERNAT® 50
Specific surface area (N2)
m /g
190
190
190
180
500
Particle size, d50
µm
120
13.5
320
20
50
%
≤7
≤7
≤7
≤7
≤7
%
≤ 4.5
≤ 6.0
≤ 6.0
≤ 6.0
≤ 5.0
6.5
6.5
6
6.2
6
2
Tristar, multipoint
following ISO 9277
Laser diffraction
following ISO 13320
Loss on drying
2 h at 105 °C
following ISO 787-2
Loss on ignition
based on dry substance
2 h at 1000 °C
following ISO 3262-1
pH value
5 % in water
following ISO 787-9
SiO2 content
%
≥ 97
≥ 97
≥ 97
≥ 97
≥ 97
Fe content
ppm
≤ 400
≤ 400
≤ 400
≤ 400
≤ 400
(based on ignited substance)
following ISO 3262-19
(based on ignited substance)
internal method
* ex plant
The data represent typical values (no product specification)
18
Recommended Experimental Precipitated Silica Grades
Properties
Unit
EXP 4210-1
EXP 4215-1
EXP 4230-1
Specific surface area (N2)
m /g
565
355
290
Average pore size
nm
ca. 10
ca. 15
ca. 20
µm
< 20
< 30
< 10
%
≤ 8.0
≤ 8.0
≤ 8.0
%
4
4
4
5
5
7
2
Tristar, multipoint
following ISO 9277
nitrogen adsorption
Particle size, d50
Laser diffraction
following ISO 13320
Loss on drying
2 h at 105 °C
following ISO 787-2
Loss on ignition
based on dry substance
2 h at 1000 °C
following ISO 3262-1
pH value
5 % in water
following ISO 787-9
SiO2 content
%
≥ 98
≥ 98
≥ 98
Fe content
ppm
≤ 400
≤ 400
≤ 400
(based on ignited substance)
following ISO 3262-19
internal method
The data represent typical values (no product specification)
The commercialization of EXP experimental precipitated silica grades depends on market response.
Even though they are produced in commercial quantities, future availability should be verified.
References
1 Evonik Technical Bulletin Fine Particles 11,
Basic Characteristics of AEROSIL® Fumed Silica
2 Evonik Industries, Technical Information 1243
3 Ralph, K. Iler, The Chemistry of Silica, 1979,
John Wiley&Sons
4 G.Ertl, H. Knözinger, J. Weitkamp; Preparation
of Solid Catalysts, 1999, Wiley-VCH
5 Roger Mueller, Hendrik K. Kammler, Karsten Wegner,
and Sotiris E. Pratsinis, Langmuir 2003, 19, 160 – 165
6 Evonik Industry Broschure, AERODISP® Fumed Silica
and Metal Oxide Dispersions
7 N. R.C. Fernandes Machado, V. S. Santana,
Catalysis Today 107 – 108 (2005) 595 – 601
8 Jing Zhang, Meijun Li, Zhaochi Feng, Jun Chen,
and Can Li, J. Phys. Chem. B 2006, 110, 927 – 935
9 Porter et al., J. Mat.Sci.34 (1999), 1523 –1531
10 EP0309048, EP0502301, US4937394
11 B. Sahouli et al. Langmuir 1996, 12, 2872
12 G. M. S. El Shafei et al. J. Colloid Interface
Sci. 2004, 277, 410 – 416
13 E. BRENDLE and E. PAPIRER; Journal of colloid
interface science 194, 217 – 224 (1997).
14 V. GUTMANN; Plenum Press, New York (1978).
The Donor-Acceptor Approach to Molecular Interactions
15 Evonik Industries, Technical Bulletin – No. 28,
The Handling of Synthetic Silicas and Silicates
16 WO 99 / 39825
17 Fernando Morales et al., J. Phys. Chem. B, 2006, 110
(17), 8626 – 8639
18 A. K. Dalai a, B. H. Davis, Applied Catalysis A:
General 348 (2008) 1 – 15
19 J. Jarupatrakorn and T. D. Tilley, J. AM. CHEM. SOC. 9
VOL. 124, NO. 28, 2002, 8380 – 8388
20 Mario F. Borin et al., J. Phys. Chem. B 2006, 110,
15080 – 15084
21 D. C. Hurum et al., J.Phys.Chem. B 2003, 107,
4545 – 4549
22 D. C. Hurim, K. A. Gray, J.Phys.Chem., B2005,
109, 977 – 980
23 Evonik Industries Technical Information 1341,
AEROPERL® – Granulated Fumed Oxides.
19
This information and any recommendations, technical or otherwise, are presented in good faith
and believed to be correct as of the date prepared.
Recipients of this information and recommendations must make their own determination as to
its suitability for their purposes. In no event shall
Evonik assume liability for damages or losses of
any kind or nature that result from the use of or
reliance upon this information and recommendations. EVONIK EXPRESSLY DISCLAIMS ANY
REPRESENTATIONS AND WARRANTIES OF
ANY KIND, WHETHER EXPRESS OR IMPLIED,
AS TO THE ACCURACY, COMPLETENESS,
NON-INFRINGEMENT, MERCHANTABILITY
AND/OR FITNESS FOR A PARTICULAR PURPOSE (EVEN IF EVONIK IS AWARE OF SUCH
PURPOSE) WITH RESPECT TO ANY INFORMATION AND RECOMMENDATIONS PROVIDED.
Reference to any trade names used by other
companies is neither a recommendation nor an
endorsement of the corresponding product, and
does not imply that similar products could not
be used. Evonik reserves the right to make any
changes to the information and/or recommendations at any time, without prior or subsequent
notice.
Europe / Middle-East /
Africa / Latin America
North America
Asia / Pacific
Evonik Resource Efficiency GmbH
Business Line Silica
Rodenbacher Chaussee 4
63457 Hanau
Germany
Evonik Corporation
Business Line Silica
299 Jefferson Road
Parsippany, NJ 07054-0677
USA
Evonik (SEA) Pte. Ltd.
Business Line Silica
3 International Business Park
#07 – 18 Nordic European Centre
Singapore 609927
 +49 6181 59-8118
 +49 6181 59-78118
[email protected]
 +1 800 233-8052
 +1 973 929-8502
[email protected]
 +65 6809-6877
 +65 6809-6677
[email protected]
www.evonik.com
07-2015
AEROSIL®, AEROXIDE® and SIPERNAT® are
registered trademarks of Evonik Industries
or its subsidiaries.