Catalyst Carrier
Newsletter
Issue 09 / 2014
Silica-Alumina Fumed Mixed Oxides –
A Base Material for Mild Acidic Catalysts
Introduction
With decades of experience Evonik Industries has extensive know-how in the production and development of
fumed metal oxides such as silica, alumina and titania. A
broad range of these products are commercially available
under the brand name AEROSIL® and AEROXIDE® for
the range of fumed silica and other fumed metal oxides
respectively.
The unique process of flame hydrolysis also allows the
specific design of a broad range of mixed metal oxides.
Some of these have been highlighted in earlier newsletters. Metal oxides are widely used in heterogeneous
catalysis. They play a role as support material (catalyst
carrier) as well as active catalysts. In some cases they have
both functions in one.
In the present newsletter we introduce our silica rich
silica-alumina mixed oxides and their application as base
material for acidic catalysts.
Evonik Industries AG | Catalyst Carrier | Newsletter | Issue 09 / 2014
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Catalytic activity of silica alumina mixed oxides
It is well known that mixed oxides of alumina and
silica exhibit acidic surface groups and thus are used
as acidic catalyst in industrially relevant reactions such
as aromatization, cracking, dehydrogenation, hydrocracking, hydrogenation, metathesis, and oxidation.
In many of these processes crystalline materials, zeolites in particular, are used. An example is
the production of olefins from methanol- catalyzed
with HZSM-5 zeolites. For this system L. Hong et al.
describe that a high ratio silica to alumina leads to a
high ratio of propene versus ethane. [1]
Amorphous silica-alumina mixed oxides play a
role as well. Various co-precipitated silica-alumina
mixed oxides as support material for a Pt catalyst are
described by E.D. Park et al.[2] for the hydrocracking
of n-paraffins to middle distillates. The authors found
an increase of Lewis acidic sites with increasing alumina content, whereas the density of Brønsted acidic
sites went through a maximum at ca. 30% alumina
while the yield of middle distillates depended on the
Brønsted acidic sites. Similar results were found by I.K.
Song et al. [3, 4]
E.M. Gaigneaux et al [5] worked on the influence
of the alumina content in silica-alumina mixed oxides
as support material for molybdenum oxide used in
metathesis reactions and in the allylic oxidation of
propene. They found that small percentages of alumina improve the metathesis reaction whereas the allylic
oxidation is favorable on pure silica.
Another process that may use silica-alumina mixed
oxides is the decomposition of methyl tert-butyl ether
(MTBE). R. Trotta et al.[6] described the technical
process for preparation and decomposition of MTBE
as a method for purification of isobutylene, which otherwise is difficult to separate from its isomers.
+ MeOH
OMe
Figure 1: Synthesis and cleavage of MTBE
The desired products for the ether cleavage are
methanol and isobutene, however a number of side
reactions can occur leading to products such as isobutane, tert-butanol dimethylether, C8 species or isoprene.
To optimize the ether cleavage reaction a broad
variety of different silica-alumina mixed oxides as acid
catalyst were investigated by A. Nau [7] and Zanthoff
et al.[8]
They describe for a number of different silica-alumina mixed oxides infrared spectra using adsorbed
NH3 as probe molecule – which allowed differentiation between Lewis and Brønsted acidic sites. With
Evonik Industries AG | Catalyst Carrier | Newsletter | Issue 09 / 2014
27Al-NMR spectra the presence of Lewis acidic sites
could be correlated to alumina clusters with octahedral coordinated alumina. Lewis acidic centers were
found predominantly with high alumina contents
> 5%. The best selectivity for the desired ether cleavage was found with catalysts that exhibit a mild Brønsted acidity, whereas Lewis acidic sites predominantly
catalyze the undesired side reactions, in particular the
formation of DME.
AEROSIL® MOX 170 was identified as particularly
mild Brønsted- acidic catalyst- leading to a high selectivity to the desired ether cleavage reaction.
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AEROSIL® MOX grades
The Evonik AEROSIL® process allows the design of silica rich silica-alumina mixed oxides exhibiting a mild
Brønsted acidity. The alumina content and the surface
area can be widely adjusted to the customer’s needs. In
the following table two commercially available silica
rich silica-alumina mixed oxides are listed.
AEROSIL® MOX 80
AEROSIL® MOX 170
Specific surface area [m²/g]
60 – 100
140 – 200
pH value
3,6 – 4,5
3,6 – 4,5
SiO2 content [%]
≥98,3
≥98,3
Al2O3 content [%]
0,30 – 1,30
0,30 – 1,30
The values given in this table are typical values, specification upon request
Literature
[1] H. Liu, L. Peng, N. Xue, X. Guo, W. Ding, W.
Yang, Z. Xie, Applied Catalysis, A: General,
421 – 422, 2012, 108 – 113.
[2] M. Y. Kim, Y.-A. Kim, K.-E. Jeong, H.-J. Chae,
C.-U. Kim, S.-Y. Jeong, J.Han, E. D. Park, Catalysis Communications Volume 26, 2012, 78–82.
[3] J. Lee, S. Hwang, J. G. Seo, U. G. Hong, J. C.
Jung, I. K. Song, Journal of Industrial and Engineering Chemistry Volume17, Issue 2, 2011,
310 – 315.
[4] J. Lee, S. Hwang, J. G. Seo, S.-B. Lee, J. C. Jung,
I. K. Song, Journal of Industrial and Engineering
Chemistry, Volume 16, Issue 5, 2010, 790 – 794.
[5] D. P. Debecker, D. Hauwaert, M. Stoyanova,
A. Barkschat, U. Rodemerck, E. M. Gaigneaux,
Applied Catalysis A: General, Volume 391, Issues
1–2, 2011, 78–85.
[6] R.Trotta, I.Miracca, Catalysis Today 34, 1997,
447 – 455.
[7] A. Nau, PhD Thesis, Technische Universität Dortmund, 2012.
[8] C. Schulze-Isfort, H.-W. Zanthoff, H.-W.
Quandt, T. Böing, Evonik Industries AG, WO
2012 / 123185, 2012.
Evonik Industries AG | Catalyst Carrier | Newsletter | Issue 09 / 2014
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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, NONINFRINGEMENT, 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.
AEROSIL® and AEROXIDE® are registered
trademarks of Evonik Degussa GmbH.
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Evonik Industries AG
Inorganic Materials
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Evonik Corporation
Inorganic Materials
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Inorganic Materials
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Evonik Industries AG | Catalyst Carrier | Newsletter | Issue 09 / 2014
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