• Inorganic Materials for Catalyst Innovation


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    • Abstract: Noted for its refractory nature and. oxide lability, ceria is frequently a part of ... The contrasting surface nature of these materials is. demonstrated by their behavior in ...

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Inorganic Materials
for Catalyst Innovation
AEROSIL®, AEROXIDE® and SIPERNAT®
Metal Oxides and Silica Based Materials
Industry Information
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. 2.3 Fumed Zirconium Dioxide 5
2. 2.4 Cerium Oxide 6
2.3 Mixed and Doped Fumed Metal Oxides (MOX & DOX) 6
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. 1.2.3 Fumed Zirconia 8
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 Roughness / Fractality 12
4. 4 Surface Chemistry & Surface Acidity 12
5. Material Handling Options 13
5.1 Evonik – 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
Technology Platform
1. Pure Materials: The Basis for Catalyst Design
1.1 Preface 1.2 Synthetic Silica and Metal Oxides Overview
Since the beginning of systematic research into Synthetic silica products and metal oxides, such as
the action of heterogeneous catalysts for chemical alumina and titanium dioxide, have been produced
processes it has become ever more apparent that a on a large scale for many decades and are widely
proper carrier plays nearly as important a role as the used in industry. By means of special production
active centers themselves. If the carrier is imagined processes, as well as by corresponding variations in
as the stage in a play, it doesn’t serve the drama if the reaction parameters and a er-treatment meth-
actors have to negotiate cramped, ill-considered ods, these products can be optimally “tailored” for
sets and certainly the negligent banana peel could industrial applications that run the gamut of experi-
turn the night’s efforts into comedy. Likewise, start- ence, from food, feed, agriculture, throughout the
ing your catalyst design with carefully chosen car- extensive world of coatings, to high technology
rier materials, such as AEROSIL®, AEROXIDE®, and industries such as electronics, pharmaceuticals, and
SIPERNAT®, assures a clean and consistent surface aerospace where materials of the highest purity are
for the real drama: your catalysis. critical. Catalyst manufacturers found early that the
high chemical purity and reliability of AEROSIL®
Because with heterogeneous catalysts, the carrier fumed silica, AEROXIDE® fumed metal oxides and
o en plays a direct role in generating or stabilizing SIPERNAT® precipitated silica proved especially
the catalytic active site, it is o en a mistake to treat useful as carrier materials or as a source of silica for
the carrier as simply “inert”. Even recognizing the molecular sieve preparation.
importance of the carrier to the definition of a “cata-
lytic system”, one can also mistakenly assume that all All silica products produced by Evonik Degussa are
chemically-like carriers are “interchangeable”. For derived synthetically under controlled conditions.
this reason starting with the most chemically pure These products are X-ray amorphous [1] and as such
and carefully engineered materials is o en the surest belong to the class of “synthetic amorphous silica” or
way of building precisely the catalyst that will get the “SAS” – a designation commonly found in regulatory
job done time and time again. That’s why we hope discussions to distinguish amorphous silica from crys-
you come to believe: talline 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 2.2 AEROXIDE® Fumed Metal Oxides
The idea and technical development of the original Degussa scientists found that the flame hydrolysis
AEROSIL® process (also known as flame hydrolysis process developed for AEROSIL® had great versatil-
or high-temperature hydrolysis) can be traced back ity for the manufacture of other oxides such as pure
to the Degussa chemist Harry Kloepfer in early 1940 fumed alumina and fumed titania. More recently
(Degussa is one of Evonik’s predecessor companies). fumed zirconia and ceria have been added to the
product portfolio. These metal oxides are marketed
To produce AEROSIL®, a volatile silicon compound, under the AEROXIDE® trade name.
most commonly silicon tetrachloride, is injected into
a flame composed of hydrogen and air. Under these Similar to the AEROSIL® process, the hydrolysis of
conditions, the silicon tetrachloride is hydrolyzed vaporizable metallic precursors in an oxyhydrogen
to silicon dioxide in a highly aggregated, nano- flame provides the basis for AEROXIDE® manu-
structured form. This finely divided structure is what facture. Mixed oxides are also accessible by flame
gives AEROSIL® its unique function and capabilities. hydrolysis; however Evonik employs proprietary
For further detail on the manufacture and properties technology to result unique particle structures
of AEROSIL®, please refer to [1]. 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
CeO2
Doping SiO2
Figure 1
Experimental products demonstrating the core-shell and the doping concept
The upper sequence shows a TiO2-particle com- Table 1
pletely covered with SiO2, while on the bottom one Typical Properties of AEROXIDE® Aluminum oxides
dots of CeO2 can be observed on a SiO2-surface. AEROXIDE®
Virtually no CeO2 can be found in the bulk of the Parameter
SiO2 particle. (test method) Unit Alu 65 Alu C Alu 130
BET Surface m /g
2
65 100 130
The three metal oxides: Al2O3, TiO2, and ZrO2 are Area
produced on a multi-ton basis and as they are repeat- X-Ray Form θ and δ, 33 % δ, γ
edly cited in catalysis research will be featured here. (approx.) little γ 66 % γ
Specific g / cm3 approx. 3.2
2. 2.1 Fumed Aluminum Oxide Gravity (depending on Ignition loss)
Three grades of AEROXIDE® fumed alumina, based pH 4.5 – 6 4.5 – 5.5 4.4 – 5.4
on specific surface area, are available from Evonik (4 % aq. Slurry)
and unlike AEROSIL® all three are distinctly crystal- Loss on Ignition wt. % < 3.0
line in nature [see Table 1]. Aluminum oxide occurs @ 1000 °C
mainly in two modifications: the thermodynamically
stable α-form and the metastable γ-form. The lat-
γ
ter can be subdivided crystallographically into the
γ-group and the δ-group. If AEROXIDE® Alu C is
γ
heated to a temperature above 1200 °C, a conver- 2. 2.2 Fumed Titanium Dioxide
sion into the α-form takes place, which is associated Three grades of titanium dioxide are available from
with a decrease in the specific surface area and an Evonik distinguished by their specific surface areas
enlargement of the primary particles. As expected, and particle morphologies: AEROXIDE® TiO P 25,
hardness and abrasiveness are increased as a result of high surface area AEROXIDE® TiO2 P 90 and granu-
this tempering. lated VP AEROPERL® P 25 / 20.
4
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 VP AEROPERL® P 25 / 20
Specific surface area m2 / g 50 ± 15 90 ± 20 50 ± 15
(BET)
pH 3.5 – 4.5 3.2 – 4.5 3.0 – 4.5
(4 % dispersion in water)
Tamped density g/l approx. 130 approx. 120 approx. 700
(acc. to DIN EN ISO 787 / 11, August 1993)
Moisture wt.-% ≤ 1.5 ≤ 4.0 ≤ 2.5
(2 hours at 105 °C)
Ignition loss wt.-% ≤ 2.0 ≤ 2.0 ≤ 2.0
(2 hours at 1000 °C based on material dried
for 2 hours at 105 °C)
TiO2 content wt.-% > 99.5 > 99.5 > 99.5
(based on ignited material)
Average particle size µm 20
(SEM)
Developmental products are labeled with the pre-
fix VP. The commercialization depends on market
response. Even though they are produced in com-
mercial quantities, future availability should be
verified.
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
20 nm 20 nm
consists of a majority phase anatase with the bal-
ance rutile. This has important implications for the Figure 3
photocatalytic oxidation of organic molecules via Micrographs of AEROXIDE® TiO2 P 25 (left) and AEROXIDE®
TiO2 P 90. The smaller particle size results in a higher surface area
AEROXIDE® TiO2 P 25 and AEROXIDE® TiO2 P 90,
which is detailed elsewhere. [2, 3]
2. 2.3 Fumed Zirconium Dioxide
VP Zirconium Oxide PH and VP Zirconium Oxide
3-YSZ, both developmental products indicated by
the prefix VP, are produced by flame pyrolysis and
differ not by specific surface area so much as by
their crystallographic phase make-up. It should be
noted that because of the isomorphism of zirco-
nium and hafnium oxides, VP Zirconium Dioxide
contains about 2 % Hafnium oxide as accompanying
substance.
20 nm
TEM HRTEM
Figure 2
Micrograph of AEROXIDE® TiO2 P 25 depicting the primary
crystals (right) and their aggregates and agglomerates (left)
5
VP Zirconium Oxide PH is predominantly monoclinic 2. 2.4 Cerium Oxide
with the balance tetragonal. In general, zirconium A recent addition to the AEROXIDE® family is
oxide exists in three crystallographic structures, Cerium Oxide. Noted for its refractory nature and
depending on the temperature: monoclinic, tetrago- oxide lability, ceria is frequently a part of oxidation
nal or cubic. Starting at the melting point at about catalysts of NOx and carbon monoxide. Fumed Ceria
2680 °C, cubic crystals are formed. These take on a VP Ceria 60 is available with a high BET surface
tetragonal structure at approx. 2370 °C and change area: 60 m2 / g. Like other AEROXIDE® materials, VP
to monoclinic at around 1170 °C. This latter change Ceria 60 consists of aggregates of crystalline (cubic)
is accompanied by an increase in volume of 3 – 5 %. primary particles.
The volume increase creates complications for high
temperature applications: If for instance a zirco- 2.3 Mixed and Doped Fumed Metal Oxides
nium oxide ceramic is heated to temperatures above (MOX & DOX)
1170 °C and cooled down a erwards the ceramic
will be destroyed by the volume change. For this rea- 2. 3.1 SiO2 / Al2O3
son the fully tetragonal grade VP Zirconium Oxide Early in the development of AEROSIL® fumed silica
3-YSZ was developed by addition of 3 mol % Y2O3 and AEROXIDE® fumed metal oxides it was found
as a stabilization phase [Table 3]. 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
Table 3 series of low surface area, mixed SiO2 / Al2O3
Typical properties of Zirconium Oxides powders and water dispersions. These materials are
Typical values predominantly silica that has been homogeneously
Parameter VP Zirconium VP Zirconium doped with alumina (≤ 1.3 %). In contrast, through
(test method) Unit Oxide PH Oxide 3YSZ a recent technical break-through, chemical doping
BET Surface Area m2 / g 40 ± 15 40 ± 15 can be manipulated such that the dopant is directed
toward the surface. This has significant effect on the
Tamped density g/l approx. 140 approx. 140
(DIN ISO 787 / 11) stability of the particles against sintering. The advan-
tages of surface doping for thermal stability against
Drying loss wt. % ≤3 ≤3
(2 h at 105 °C) sintering will be described in greater detail in
ZrO2 + HfO2-content wt. % ≥ 98 ≥ 90
Section 3. 3.
(based on ignited material)
Y2O3-content wt. % n. a. 5.0 – 6.0
2. 3.2 SiO2 / TiO2 (SiTi)
(based on ignited material) 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
Developmental products are labeled with the pre- under the high temperature conditions of many
fix VP. The commercialization depends on market catalytic systems by the addition of silica. The details
response. Even though they are produced in com- of this thermal stability enhancement are described
mercial quantities, future availability should be later while Figure 4 shows electron micrographs of
verified. some of these innovative materials that have found
diverse application.
5nm 5 nm 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® pre- As shown in Figure 5, the silanol group density of
cipitated silica are characterized by large specific AEROSIL® fumed silica is to a first approximation
surface area, but differ in the nature of their surface independent of the specific surface. Only in the
structure. The AEROSIL® surface should be seen as case of AEROSIL® OX50 lower silanol densities are
an external surface, arising from the very fineness found (about 2.2 SiOH / nm2), attributed to the fact
of the primary particle size. SIPERNAT® silica on that it is produced at a higher flame temperature. As
the other hand, is composed of tightly aggregated expected, the absolute concentration of the silanol
nanometer sized particles built around a true porous groups rises linearly with the specific surface. This
structure and as a result precipitated silica surface relationship is at the basis of the thickening effect for
area contains both external and internal components. which AEROSIL® is widely used in liquid and poly-
The variability of the wet-process allows control of mer based formulations.
this balance of surface area components.
When heated, the silanol groups are converted to
3. 1.1 Details of the Silica Surface siloxane groups by the splitting off of water. Up to
Two functional moieties, namely the silanol and about 400 °C this reaction is reversible. At higher
siloxane groups, comprise the silica surface. The temperatures this reaction becomes increasingly
hydrophilic character and Brønsted acidity of silica irreversible and at temperatures of 800 °C and higher
is the result of the silanol component. The silox- the surface is completely annealed and the conver-
ane groups in contrast are hydrophobic and largely sion of siloxane groups back to silanol groups is no
chemically inert. As would be expected from their longer possible, even if the substance is boiled in
contrasting production processes, AEROSIL® fumed water.[1]
silica and SIPERNAT® precipitated silica differ in
silanol group density. Because of the origin of 3. 1.2 The Surface of Fumed Metal Oxides
AEROSIL® in a flame process, its silanol group con-
centration is notably lower than that for wet-process 3. 1.2.1 Fumed Alumina
SIPERNAT®. Knowledge about silanol group dynam- Fumed alumina, such as AEROXIDE® Alu C, has
ics and concentration is essential when designing hydroxyl groups on the surface but the material is
catalytic systems based on silica and so a great deal a weak Lewis base. In contrast to AEROSIL® fumed
of research has been devoted to detailing the pre- silica, particles of AEROXIDE® Alu C dispersed in
cise silanol character of AEROSIL® and SIPERNAT® water (pH = 7) have a positive charge (the Al2O3
grades. [4, 5, 6] isoelectric point lies at pH = 9.5; that for SiO2 lies at
pH = 2.5).
Figure 5 3. 1.2.2 Fumed Titanium Dioxide
AEROSIL® fumed silica silanol group density and concentration The surface of titanium dioxide also possesses
hydroxyl groups; however the surface is more aptly
2.0 3.0
characterized as amphoteric. [6]. This dual Lewis
1.8
acid / base character is reflected in the isoelectric
Silanol Group Concentration
2.5
1.6
point for the dispersed particles of TiO2 P 25 which
Silanol Group Density
(Si-OH/nm2)
1.4
2.0 lies at pH = 6. 5.
[Si-OH mmol/g]
[Si-OH/nm2]
1.2
1.0 1.5
0.8 (Si-OH mmol/g)
1.0
0.6
0.4
0.5
0.2
0.0 0.0
0 50 100 150 200 250 300 350 400
BET Surface Area [m /g]
2
7
3. 1.2.3 Fumed Zirconia Table 4
Zirconium oxide has surface hydroxyl groups and is Trace element impurities in AEROSIL® 200 and AEROSIL® OX 50.
Data given are typical values, but do not represent any
more basic than aluminum oxide and titanium diox- specification
ide; however its solubility in acids is negligible. Gen-
Element content
erally, zirconium oxide proves to be extraordinarily
≤ 0.01 ppm ≤ 0.1 ppm ≤ 1 ppm ≤ 10 ppm
inert chemically. In addition, it has a lower thermal
expansion and thermal conductivity, as well as high As Cd Cr Al
thermal stability. Au Co Cu Ba
Sc Mo Hg Ca
The contrasting surface nature of these materials is
Th Pb In Fe
demonstrated by their behavior in water through
measurement of zeta potential (Figure 6) [1, 7]. U Sb K Na
Mg Ni
Mn Sn
Figure 6
Zeta potentials of fumed metal oxides produced by Evonik Indus- Zn
tries as a function of pH value (0.02 m KNO3)
80
60
Similar purity characteristics apply for AEROXIDE®
Zeta potential mV →
40
fumed metal oxides such as AEROXIDE® Alu C and
20
AEROXIDE® TiO2 P 25. These materials have purities
0
exceeding 99.5 % and heavy metal impurities gener-
–20
ally fall beneath common detection limits.
–40
–60
3.3 Thermal Stability through Surface Doping
–80
2 3 4 5 6 7 8 9 10 The ability to direct the doping of one fumed metal
ph value → oxide onto the surface of another fumed metal oxide
was briefly described earlier. One distinct advantage
AEROXIDE® Alu C that materials with such heterogeneous primary par-
AEROXIDE® TiO2 P 25 ticle structure possess is an enhanced thermal stabil-
AEROSIL® OX 50 ity. Evonik has developed two systems that demon-
strate this enhancement and has an active research
program exploring the many possible extensions of
3.2 Catalyst Support Purity the design concept and its application as materials for
With the many different raw material source options catalytic supports.
available for catalyst manufacture, there is a simple
answer to the question of why Evonik products An early example of this particle structure control
should be chosen: Chemical Purity. were materials that combined a silica core with an
alumina rich shell. Evonik has offered particle sys-
A starting material for AEROSIL® is e. g. silicon tems that combined the two oxides for quite some
tetrachloride, which can be distilled and purified time as the so-called MOX grades. The newer tech-
relatively easily. Due to the chemical simplicity of nology however directs the alumina phase to the
the AEROSIL® process, hydrochloric acid is the only outer region of the primary particles. An example is
by-product. As was mentioned, a er-treatment of the developmental product VP DOX 110. In Figure 7
the fumed silica with hot steam reduces the residual the sintering resistance of VP DOX 110 is compared
hydrochloric acid content to less than 0.025 %. to fumed silica with similar surface area (AEROSIL®
Another outcome of the process is that impurities OX 50). This shows that directed doping prolongs
can be maintained at a very low level. Among the the onset of sintering by approximately 100 °C.
other residuals (which together make up a maxi-
mum of 0.2 %), Al2O3, Fe2O3, and TiO2 are the most
prominent. Additional foreign elements occur only in
traces as shown in Table 4.
8
Figure 7 In another example, it was previously described that
Improved sintering resistance of VP DOX 110 compared to the chemical nature of silica starkly differs from that
AEROSIL® OX 50
of titania; by directed doping, the rich chemistry of
the silanol group can be gra ed onto a titanium diox-
1
ide core without disturbing its crystalline nature and
0.9
basic physical properties. This design also has impact
relative density
0.8
on the behavior of the titania core with respect to
0.7
thermal treatments as can be seen in Figure 8 which
0.6
compares the thermal stability of a standard fumed
0.5
titania, AEROXIDE® TiO2 P 25 [8, 9, 10] to titania
0.4
materials that have been co-fumed with silica. In this
0.3
0 200 400 600 800 1000 1200 1400 1600 figure it can be seen that by 800 °C, the phase transi-
temperature [˚C] tion of anatase to rutile has taken place in the pure
1h, 1200 ˚C titanium dioxide (AEROXIDE® TiO2 P 25) powder
thus resulting in a 96 % reduction in surface area. By
VP DOX 110 contrast the addition of silica to the titanium dioxide
AEROSIL® OX 50 particles nearly eliminates any loss of surface area at
temperatures up to 800 °C. Importantly, the directed
doping does not significantly reduce the photocata-
lytic behavior of pyrogenic titania.
Photoactivity Index
AEROXIDE® TiO2 P 25 AEROXIDE® TiO2 P 25 stabilized (AEROXIDE® TiO2 P 25 = 1)
50 50 48 1.0
0.9
BET surface area
BET surface area
0.8


Use: 0.0453