Method for producing catalysts containing metal...

Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters

Reexamination Certificate

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C502S325000, C502S330000

Reexamination Certificate

active

06603038

ABSTRACT:

Process for producing catalysts comprising nanosize metal particles on a porous support, in particular for the gas-phase oxidation of ethylene and acetic acid to give vinyl acetate.
DESCRIPTION
The invention relates to a process for producing a catalyst comprising one or more metals selected from the group of metals encompassing transition groups Ib and VIIIb of the Periodic Table of the Elements on porous support particles. Here, the metals are present as nanosize particles in the finished catalyst. In particular, the invention relates to the production of “coated” catalysts on porous, preferably nanoporous, supports by this method.
Catalysts, preferably coated catalysts, can be used for many heterogeneously catalyzed reactions such as hydrogenations and oxidations. Among other things, Pd/Au coated catalysts are extremely well suited to the catalysis of the gas-phase oxidation of ethylene and acetic acid to give vinyl acetate. Here, the catalytically active metals are deposited in the form of a shell on or in the outermost layer of the support. They are often produced by penetration of the support with metal salts into a surface region and subsequent precipitation by alkalis to form water-insoluble Pd/Au compounds.
GB-A-1 283 737 discloses the production of a coated noble metal catalyst by preimpregnation of the support with an alkaline solution and saturation with 25-90% water or alcohol. Subsequent impregnation with Pd salts and reduction of the precipitated salts to the metal gives coated catalysts in which the penetration depth of the noble metal is said to be up to 50% of the pellet radius.
According to U.S. Pat. No. 3,775,342 and U.S. Pat. No. 3,822,308, coated catalysts are produced by impregnating the support with a solution of Pd/Au salts and with an aqueous base, preferably NaOH, which results in precipitation of insoluble palladium hydroxide and gold hydroxide in a shell-like surface zone of the pellets. The hydroxides which have been fixed in the shell in this way are then reduced to the metals.
GB-A-1 521 652 obtains coated catalysts of the egg white type, i.e. only an inner ring of the spherical SiO
2
support contains the noble metals while the inner core and a thin outer shell remain virtually free of noble metal, by a comparable procedure (preimpregnation with Pd, Au salts, drying, base precipitation, reduction).
U.S. Pat. No. 4,048,096 teaches the precipitation of water-insoluble Pd and Au compounds on the support preimpregnated with Pd/Au salts using sodium silicates in place of NaOH. The thickness of the shell is less than 0.5 mm.
U.S. Pat. No. 5,567,839 precipitates water-insoluble Pd and Au compounds on the support preimpregnated with Pd/Au salts using barium hydroxide in place of NaOH. The thickness of the shell is 1 mm. The catalyst can also be doped with barium acetate.
EP-A-0 519 435 discloses the production of a Pd/Au/K or Pd/Cd/K coated catalyst in which a specific support material is washed with an acid prior to impregnation and is treated with a base after impregnation.
U.S. Pat. No. 5,314,858 concerns double fixing of the noble metals in an outer shell by means of two separate precipitation steps using NaOH.
WO-A-94/08714 achieves particularly uniform shells by rotational motion of the support impregnated with Pd, Au salts during the fixing step, i.e. immersed in the alkaline fixing solution (NaOH).
EP-A-0 723 810 employs pretreatment (impregnation) of the support with metal salt solutions to produce a support which is preferably doped with Al, Zr, Ti and is subsequently used for the above-described base precipitation to form a Pd/Au/K coated catalyst.
U.S. Pat. No. 5,347,046 describes the use of Cu, Ni, Co, Fe, Mn, Pb, Ag as promoters in Pd/Au systems on SiO
2
supports pretreated with alkali metal hydroxide and alkali metal silicate.
Another method of producing coated catalysts is prenucleation with metals and subsequent deposition of the intended amount of noble metals.
The published Japanese patent application 48-10135/1973 describes the production of a Pd/Au coated catalyst. Here, a small amount of reduced metal (gold) is deposited on the porous support in a pretreatment step. Subsequent impregnation results in deposition of Pd in a surface zone having a thickness of about 15% of the particle radius.
U.S. Pat. No. 4,087,622 teaches the production of coated catalysts by prenucleation with (reduced) Pd/Au metal nuclei in a low concentration, by impregnating the porous SiO
2
or Al
2
O
3
support with a Pd/Au salt solution, drying it and then reducing the Pd/Au salt to the metal. This prenucleation step is followed by deposition of the catalytically necessary amount of noble metal, i.e. the main amount which is then concentrated in a shell near the surface.
The use of different variants of “deficiency techniques” likewise enables coated catalysts to be obtained.
These include, inter alia:
deficiency of precipitants, e.g. NaOH, in combination with multiple precipitation;
deficiency of impregnation solution (less than the pore volume of the support);
limitation of the contact time during absorption of the noble metals;
insufficient noble metal concentration (per impregnation step) combined with multiple impregnation; and
combinations of the abovementioned variants.
EP-A-0 565 952 describes the formation of shell-like Pd/K/Au, Pd/K/Ba and Pd/K/Cd catalysts by atomizing a solution of appropriate metal salts by means of ultrasound and then applying this to the support particles in such a limited a mount and within such a restricted time and commencing drying in such a way that the catalytically active metal salts cannot penetrate to the core of the support particles, but only into an outer part of varying thickness, namely the shell.
According to EP-A-0 634 214, coated catalysts are obtained by spraying a viscous solution of appropriate metal salts in the form of droplets or liquid jets onto the support particles, where the solution volume in each spraying step is 5-80% of the pore volume of the support particles and drying is commenced immediately after spraying.
EP-A-0 634 209 obtains coated catalysts by impregnating the support particles with a viscous solution of appropriate metal salts, where the solution volume in each impregnation step is 5-80% of the pore volume of the support particles and drying is commenced immediately after each impregnation step.
According to EP-A-0 634 208, coated catalysts are obtained by impregnating the support particles with a viscous solution of salts of the appropriate elements and then drying them, where the solution volume in the impregnation is more than 80% of the pore volume of the support particles and the duration of impregnation and the time to commencement of drying are made so short that the metal salts specified are present in a shell of 5-80% of the pore volume of the support particles after the end of drying.
U.S. Pat. No. 5,576,457 concerns Pd/Cd/K coated catalysts which are doped with Zr and/or Re, where the shell can be produced as described in EP 0634208, EP 0634209 or EP 0634214.
U.S. Pat. No. 5,591,688 describes fluidized-bed VAM catalysts (VAM=vinyl acetate (monomer)) comprising Pd—Ba, Au, La, Nb, Ce, Zn, Pb, Ca, Sr, Sb on silica, alumina or zirconia, using halide-free precursors.
U.S. Pat. No. 5,536,693 describes fluidized-bed VAM catalysts comprising Pd—Au, Cd, Bi, Cu. Mn, Fe, Co, Ce, U which are produced by milling a fixed-bed catalyst precursor preimpregnated with Pd—M and compounding with a binder comprising silica, alumina, zirconia or titania.
The production and stabilization of nanosize noble metal particles in solution is prior art. Other customary terms for such solutions are sols or colloids. A summary overview may be found in G. Schmid, Cluster and Colloids, From Theory to Applications, VCH Weinheim 1994.
Stable sols are produced by reduction of metal salt solutions with a reducing agent in the presence of a stabilizer which envelops the nanosize particles and prevents further agglomeration of the nanosize particles.
With a suitable choice of reducing agent and stabilizer,

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