Silver catalyst for formaldehyde preparation
View Patent ↗The present invention relates to a process for producing coated silver catalysts. The invention further addresses the silver catalysts themselves and the advantageous use thereof in formaldehyde synthesis.
1. A process for producing silver crystals having a thin porous coating of oxidic material of the elements selected from the group consisting of aluminium, silicon, zirconium, titanium and mixtures thereof, which comprises
a) contacting the silver crystals with a sol-gel solution of the elements in question in an organic solvent, and
b) collecting the silver crystals resulting from step a),
c) freeing the silver crystals from the organic solvent, and
d) then heat treating the silver crystals at a temperature between 50° C. and the melting point of silver,
wherein the silver crystals have a mean particle size distribution of 0.15 mm to 2.5 mm.
2. The process according to claim 1 , wherein the sol-gel solution is an alkoxide of the element in question in the corresponding alcohol as the organic solvent.
3. The process according to claim 2 , wherein the alkoxide is selected from the group consisting of methoxides, ethoxides, propoxides, isopropoxides, n-butoxides, sec-butoxides, isobutoxides, tert-butoxides, n-propoxides, and structural isomers thereof.
4. The process according to claim 1 , wherein the resulting silver crystals are freed of the solvent by allowing the solvent to evaporate off under ambient conditions.
5. The process according to claim 1 , wherein the heat treatment is performed at a temperature between 400° C. and 800° C.
6. Silver crystals having a mean particle size distribution of 0.15 mm to 2.5 mm and a thin porous coating of oxidic material of the elements selected from the group consisting of aluminium, silicon, zirconium, tin, titanium, and mixtures thereof made by the process of claim 1 .
7. The silver crystals according to claim 6 , wherein the coating has a thickness of 0.3 μm-10 μm.
8. The silver crystals according to claim 6 , wherein the oxidic material is selected from the group of SiO 2 , various kinds of aluminium oxides, TiO 2 , ZrO 2 , a mixture of oxides or mixed oxides of these compounds, optionally in the form of a ceramic.
9. A catalyst comprising silver crystals according to claim 6 with a bulk density between >1700 g/l and <3800 g/l.
10. A method of preparing formaldehyde which comprises using the silver crystals according to claim 6 as a catalyst.