Moldings based on pyrogenic silicon dioxide

Details Moldings based on pyrogenic silicon dioxide Moldings based on pyrogenic silicon dioxide

1998-11-06

2003-07-15

Dunn, Tom (Department: 1725)

Catalyst, solid sorbent, or support therefor: product or process

Catalyst or precursor therefor

Metal, metal oxide or metal hydroxide

C502S330000, C502S339000, C423S335000

Reexamination Certificate

active

06593270

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
The present application claims priority under 35 U.S.C. §119 to application 19750238.5, filed in Germany on Nov. 13, 1997.
FIELD OF THE INVENTION
The invention relates to moldings based on pyrogenic silicon dioxide, a process for their preparation and their use as catalyst supports or catalysts.
BACKGROUND OF THE INVENTION
Pyrogenic silicon dioxides are characterised by extreme fineness and a correspondingly high specific surface area, very high purity, spherical particle shapes and the lack of pores. Due to these properties, pyrogenic oxides are becoming increasingly important as supports for catalysts (D. Koth, H. Ferch, Chem. Ing. Techn. 52, 628 (1980)).
Since pyrogenic oxides are particularly finely divided, there are some problems involved when shaping them to provide catalyst supports or catalysts.
DE-A 31 32 674 discloses a process for preparing moldings from pyrogenic oxides in which silica sol is used as a binder.
DE-A 34 06 185 discloses a process for preparing moldings in which a sintered glass powder is used as binder and glycerine is used as lubricant.
DE-B 21 00 778 discloses granules based on pyrogenic silicon dioxides which can be used as catalyst supports for preparing vinyl acetate monomer.
DE-A 39 12 504 discloses a process for preparing moldings in which aluminium stearate, magnesium stearate and/or graphite is used as lubricant and urea and methyl cellulose are used as pore producers.
The known moldings prepared using magnesium stearate are sold as Aerosil tablets No. 350 by Degussa AG, Germany. They contain about 0.4 wt. % of Mg.
EP-B 0 519 435 discloses compressing SiO
2
using binders to provide a support, calcining the support obtained and washing the calcined support particles with acid until no further cations from the binder are liberated.
The known processes have the disadvantage that the moldings obtained do not have the optimum properties required, such as, for example, high purity, high activity, high selectivity, high product yield and high stability, for use in specific catalytic reactions such as, for example, the production of vinyl acetate from ethylene, acetic acid and oxygen, the hydration of ethylene to give ethanol or the hydration of propylene to give isopropanol.
The earlier, but not previously published, patent application DE 196 19 961.1-41 describes moldings based on pyrogenic silicon dioxide with the following physico-chemical characteristics:
External diameter
0.8-20 mm
BET surface area
30-400 m
2
/g
Pore volume
0.5-1.3 ml/g
Breaking strength
10-250 N
Composition
>99.8 wt. % SiO
2
Other constituents
<0.2 wt. %
Abrasion
<5 wt. %
Bulk density
350-750 g/l
SUMMARY OF THE INVENTION
The present invention provides moldings based on pyrogenic silicon dioxide which are characterised by having a pore volume of 0.5 to 1.8 ml/g, preferably 1.31 to 1.8 ml/g.
The moldings according to the invention may have an external diameter of 0.8 to 20 mm, a BET surface area of 30 to 400 m
2
/g and a fracture strength of 7 to 250 N.
The SiO
2
fraction in the moldings according to the invention is preferably greater than 99.0 wt. %. The proportion of other constituents may be less than 0.2 wt. %. Moldings according to the invention may therefore be regarded as containing no binder. The abrasion may be less than 5 wt. %. The compacted density may be 300 to 750 g/l.
The invention also provides a process for preparing moldings based on pyrogenic silicon dioxide with a pore volume of 0.5 to 1.8 ml/g, which is characterised in that pyrogenic silicon dioxide is homogenised with methylhydroxyethyl cellulose, wax and polyethylene glycol with the addition of water, subjected to a compounding and shaping process, extruded, optionally cutting the extrudate to the desired length using a cutting device, drying at a temperature of 70° to 150° C. and conditioning for a period of 30 minutes to 10 hours at a temperature of 400° to 1200° C.
Any compounders, mixers or mills which enable efficient homogenisation and compacting of the material being mixed such as, for example, blade, fluidised bed, propeller or air-flow mixers are suitable for performing the process according to the invention. Mixers with which additional compaction of the material being mixed is possible such as, for example, plough-bar mixers, pan mills or ball mills are particularly suitable. Mixing and compounding may also take place, however, directly in an extruder. The extrudates may also be prepared in single or twin-screw extruders, compression extruders or in compactors. The moldings according to the invention are preferably prepared using extruders.
In a particular embodiment of the invention, the mixture may have the following composition prior to compression:
 50-90 wt. %
silicon dioxide, preferably 65-85 wt. %;
0.1-20 wt. %
methylhydroxyethyl cellulose,
preferably 5-15 wt. %;
0.1-15%
wax, preferably 5-12 wt. %,
0.1-15%
polyethylene glycol, preferably
5-10 wt. %.
The moldings may have a variety of shapes such as, for example, cylinders, spheres or rings with an external diameter of 0.8 to 20 mm. The moldings are conditioned at 400° to 1200° C. for 30 minutes to 10 hours. The fracture strength, the specific total surface area and the pore volume can be adjusted to a certain extent by varying the amounts of starting compounds and the pressure applied.
Moldings according to the invention may be used either directly as a catalyst or as a catalyst support. In the latter case the moldings are placed in contact with a catalytically active substance after their preparation and optionally activated by means of appropriate post-treatment.
In particular, the moldings made from pyrogenic silicon dioxide may be used as supports for the catalyst during the preparation of vinyl acetate monomer from ethylene, acetic acid and oxygen and as a catalyst support in olefin hydration processes, for example during the preparation of ethanol and isopropanol.
Moldings according to the invention have the following advantages:
In comparison with moldings in accordance with document DE-A 39 12 504, moldings according to the invention have no other inorganic constituents apart from silicon dioxide. The known moldings have the disadvantage that they contain about 0.4 wt. % of Mg which is leached out during the process for hydration of olefins.
Moldings according to the invention on the other hand have an improved hydrothermal stability in this type of hydration reaction. In addition, they are very pure and have a high pore volume.
A further advantage is produced from the fact that a larger space/time yield is produced during hydration. During hydration of olefins, the pore volume of the catalyst support plays a very important part. The relatively large pore volume, surprisingly, enables the absorption of more active phase. This results in an increase in the space/time yield.
The invention also provides a supported catalyst for the production of vinyl acetate monomer (VAM), which contains, on a support (molding), as catalytically active components, palladium and/or its compounds and alkali metal compounds, and also gold and/or its compounds (Pd/alkali metal/Au system) or cadmium and/or its compounds (Pd/alkali metal/Cd system) or barium and/or its compounds (Pd/alkali metal/Ba system) or palladium, alkali metal compounds and mixtures of gold and/or cadmium and/or barium, which is characterised in that the support is a molding in accordance with the invention. Potassium compounds such as e.g. potassium acetate are preferred as alkali metal compounds.
The catalytically active components may be present in the following systems:
Pd/Au/alkali metal compounds
Pd/cd/alkali metal compounds
Pd/Ba/alkali metal compounds
The supported catalysts according to the invention are used for the production of vinyl acetate monomer. For this, ethylene, acetic acid and molecular oxygen or air are reacted in the gas phase, optionally with the addition of inert gases, at temperatures between 100° and 250° C. and usually under elevated pressure in the presence of the supported catalyst according

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