Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Silicon containing or process of making
Reexamination Certificate
1997-05-16
2001-03-27
Bell, Mark L. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Silicon containing or process of making
C502S243000, C502S245000, C502S258000, C502S261000, C502S262000, C501S133000, C501S154000, C423S335000, C423S337000, C106S481000, C106S482000
Reexamination Certificate
active
06207610
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to compacts based on pyrogenically produced silicon dioxide, a method of producing them and use thereof as catalyst carriers or catalysts.
2. The Prior Art
Pyrogenically produced silicon dioxide is characterized by extremely fine division and a consequently high specific surface area, very high purity, spherical shape of particles and absence of pores. As a result of these properties, pyrogenically produced oxides are becoming increasingly important as carriers for catalysts (D. Koth, H. Ferch, Chem. Ing. Techn. 52, 628 (1980).
Since pyrogenically produced oxides are very finely-divided, there is some difficulty in shaping them to form catalyst carriers or catalysts.
DE-A 31 32 674 discloses a method of producing compacts from pyrogenically produced oxides, using silica sol as the binder.
DE-A 34 06 185 discloses a method of producing compacts using glaze frit powder as a binder and glycerol as a mold release agent.
DE-B 21 00 778 discloses use of granulates based on pyrogenically produced silicon dioxide for production of catalyst carriers in the form of vinyl acetate monomers.
DE-A 39 12 504 discloses a method of producing compacts wherein aluminum stearate, magnesium stearate and/or graphite are used as mold release agents and urea and methyl cellulose are used as pore-forming agents.
These known compacts are commercially available as AEROSIL silicon dioxide pellets number 350, by Degussa A. G. They contain about 0.4 wt. % Mg.
It is known from EP-B 0 519 435 to compress SiO
2
into carriers, using binders, to anneal the resulting carriers and to wash the annealed carrier particles in acid until no further binder actions are given off.
The known methods have the disadvantage that in some catalytic reactions such as vinyl acetate production from ethylene, acetic acid and oxygen, or hydration of ethylene to ethanol, the compacts obtained do not have the required optimum properties such as high purity, high activity, high selectivity, high yield of product and high stability.
SUMMARY OF THE INVENTION
The invention relates to compacts based on pyrogenically produced silicon dioxide and having the following physical and chemical characteristics:
Outer diameter
0.8-20 mm
BET surface area
30-400 m
2
/g
Pore volume
0.5-1.3 ml/g
Breaking strength
10 to 250N
Composition
>99.8 wt. % SiO
2
Other constituents
<0.2 wt. %
Abrasion
<5 wt. %
Apparent weight
350-750 g/l.
The invention also provides a method of producing compacts based on pyrogenically produced silicon dioxide and having the following physical and chemical characteristics:
Outer diameter
0.8-20 mm
BET surface area
30-400 m
2
/g
Pore volume
0.5-1.3 ml/g
Breaking strength
10 to 250N
Composition
>99.8 wt. % SiO
2
Other constituents
<0.2 wt. %
Abrasion
<5 wt. %
Apparent weight
350-750 g/l
characterized in that pyrogenically produced silicon dioxide is homogenized with methyl cellulose, microwax and/or polyethylene glycol with addition of water, dried at a temperature of 80-150° C. and optionally comminuted to a powder, and the powder is compressed into compacts and heat-treated at a temperature of 400 to 1200° C. for a time of 0.5 to 8 hours.
The method according to the invention can be worked on all mixers or mills which ensure good homogenization, e.g., paddle mixers, fluidized-bed, gyratory or air-flow mixers. Mixers of use for additional compaction of the mixed material are particularly suitable, e.g. ploughshare mixers, pan grinders or ball mills. Homogenization can be followed by thorough drying at 80-150° C., followed optionally by comminution so as to obtain a free-flowing powder. The compacts can be produced in hand presses, eccentric presses, isostatic presses, extruders or rotary presses or on compactors.
Before compression, in a special embodiment of the invention, the mixture can have the following composition:
50-90 wt. % silicon dioxide, preferably 65-85 wt. %
0.1-20 wt. % methyl cellulose, preferably 5-15 wt. %
0.1-15% microwax, preferably 5-10 wt. %
0.1-15% polyethylene glycol, preferably 5-10 wt. %
The compacts can have various shapes, such as cylindrical, spherical or annular, with an outer diameter of 0.8 to 20 mm. The compacts are heat-treated at 400-1200° C. for 30 minutes to 8 hours. The amounts used and the pressures applied can be varied so as to adjust the breaking strength, the total specific surface area and the pore volume within a specific range.
The compacts according to the invention can be used either directly as a catalyst or as a catalyst carrier. In the latter case the compacts after manufacture will be brought into contact with a catalytically active substance and optionally activated by suitable after-treatment.
More particularly, compacts made of pyrogenically produced silicon dioxide can be used as carriers for the catalyst for producing vinyl acetate monomer from ethylene, acetic acid and oxygen and as the catalyst carrier in the olefin hydration process.
The compacts according to the invention have the following advantages:
Compared with the compacts according to document DE-A 39 12 504, the compacts according to the invention contain no inorganic constituents apart from silicon dioxide. The known compacts have the disadvantage of containing about 0.4 wt. % Mg, which is leached out during the olefin hydration process.
The compacts according to the invention, by contrast, have improved hydrothermal stability during hydration processes of this kind. They also have high purity and a large pore volume.
The invention also provides a carrier catalyst for production of vinyl acetate monomer (VAM) containing catalytically active components in the form of palladium, gold and alkali-metal compounds on the silicon dioxide carrier according to the invention, and a method of production and use thereof.
Carrier catalysts containing gold, palladium and alkali-metal compounds are used for production of vinyl acetate. To this end, ethane, acetic acid and molecular oxygen or air in the gas phase are reacted, optionally with addition of inert gases, at temperatures between 100 and 250° C. and normal or elevated pressure in the presence of the carrier catalyst. A method of production of this kind is known from documents DE 16 68 088, U.S. Pat. No. 4,048,096 and EP 0 519 435 B1.
These patent specifications also disclose a method of producing carrier catalysts containing gold, palladium and alkali-metal compounds. Depending on the embodiment, catalysts are obtained with a homogeneous distribution of noble metal across the carrier cross-section and with a relatively marked shell profile.
These catalysts are usually obtained by impregnating the carriers with a basic solution and a solution containing gold or palladium salts, the impregnation steps occurring simultaneously or successively, with or without intermediate drying. The carrier is then washed in order to remove any chloride components. Before or after washing, the insoluble noble-metal compounds precipitated on the carrier are reduced. The resulting catalyst precursor is dried and, in order to active the catalyst, is impregnated with alkali-metal acetates or alkali-metal compounds which under the reaction conditions are converted partly or completely into alkali-metal acetates in the production of vinyl acetate monomer.
The catalyst can be reduced in the aqueous phase or in the gas phase. Formaldehyde or hydrazine are suitable for reduction in the aqueous phase. Reduction in the gas phase can be brought about with hydrogen or forming gas (95 vol. % N
2
+5 vol. % H
2
) or ethane. According to EP 0 634 209 the reduction is brought about with hydrogen at temperatures between 40 and 260° C., preferably between 70 and 200° C. Frequently, however, the catalyst is first activated with alkali-metal acetate before being directly reduced with ethane in the production reactor.
In the production process the catalyst is first slowly loaded with the reactants. During this starting phase the activity of the catalyst increases, usually taking some days or weeks to reach i
Burmeister Roland
Krause Helmfried
Lansink Rotgerink Hermanus
Panster Peter
Tacke Thomas
Bell Mark L.
Degussa-Huls AG
Hailey Patricia L.
Pillsbury Madison & Sutro LLP
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