Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2002-02-28
2004-03-23
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
Reexamination Certificate
active
06710010
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods of producing catalysts comprising zirconia and their use in nitrous oxide abatement. The catalysts have relatively high crush strength and relatively low density when compared to those catalysts made by previously known methods.
TECHNICAL BACKGROUND
Zirconia is not an easy support material to fabricate. Unlike alumina- or silica-containing supports, pure zirconia extrudates have traditionally been difficult to make strong without resorting to very high sintering temperatures, which dramatically reduces the surface area of the support. Thus, zirconia extrudates usually include alumina or silica to strengthen them, or the zirconia is tableted instead of extruded. Extrudates have several advantages over tablets: they are cheaper to produce in bulk, they have a wider choice of cross sections, and they typically have higher porosity and lower tap densities. Depending on the reaction chemistry, impurities in the zirconia support, such as alumina, silica, or iron, may not be tolerable. It is known that small amounts of hafnium are a normal impurity in zirconium compounds and are generally not a concern, since the chemistries of hafnium and zirconium are very similar.
The production of zirconia-based catalysts and their use in nitrous oxide abatement is known. However, in general, the starting material is ZrO
2
, not Zr(OH)
4
as disclosed herein, nor is it a mixture of particle sizes, an acidic solution, or a combination thereof employed.
Japanese Patent Application 05168921A (Jul. 2, 1993) discloses the use of Zr(OH)
4
and a zirconium salt. No mention is made of a mixture of particle sizes or of the beneficial effect of acid dilution of the salt solution.
Commonly owned U.S. patent application, Ser. No. 515,006 (filed Feb. 29, 2000) discloses zirconia catalysts, including iron, and their subsequent use in nitrous oxide abatement. However, iron is an intrinsic part of these catalysts, and thus such preparations are not directly comparable to the pure zirconia supports disclosed herein.
Nitrous oxide is a greenhouse and ozone-depleting gas, and is a by-product of adipic and nitric acid manufacturing. There is a need for catalysts which can decompose N
2
O into N
2
and O
2
, have long lifetimes, can survive high-temperature excursions, are inexpensive, and are strong enough to resist breakage in handling and use.
SUMMARY OF THE INVENTION
Disclosed is a process for making a zirconia catalyst, comprising the steps of:
(a) preparing a paste comprising a step selected from the group consisting of:
(i) mixing zirconium hydroxide with a solution of zirconyl nitrate, water, and nitric acid;
(ii) mixing relatively fine particle size zirconium hydroxide and relatively coarse particle size zirconium hydroxide with a solution of zirconyl nitrate and water; and
(iii) mixing relatively fine particle size zirconium hydroxide and relatively coarse particle size zirconium hydroxide with a solution of zirconyl nitrate, water, and nitric acid;
wherein, in (i), (ii), and (iii) there may be one or more additional solvents added in addition to water;
(b) forming a shaped particle from the step (a) paste;
(c) drying the step (b) shaped particle; and
(d) calcining the dried step (c) shaped particle at a temperature of at least 400° C.
In the above process one can optionally add at least one metal selected from the group consisting of cobalt, nickel, rhodium, palladium, iridium, platinum, manganese, lanthanum, and cerium to step (a) or to the calcined step (d) shaped particle.
In the above process one can also optionally add at step (a) one or more additives selected from the group consisting of binders, lubricants, rheology control agents, and pore forming agents.
Further disclosed is a catalyst comprising zirconia, prepared by the steps of:
(a) preparing a paste comprising a step selected from the group consisting of:
(i) mixing zirconium hydroxide with a solution of zirconyl nitrate, water, and nitric acid;
(ii) mixing relatively fine particle size zirconium hydroxide and relatively coarse particle size zirconium hydroxide with a solution of zirconyl nitrate and water; and
(iii) mixing relatively fine particle size zirconium hydroxide and relatively coarse particle size zirconium hydroxide with a solution of zirconyl nitrate, water, and nitric acid;
wherein, in (i), (ii), and (iii) there may be one or more additional solvents added in addition to water;
(b) forming a shaped particle from the step (a) paste;
(c) drying the step (b) shaped particle; and
(d) calcining the dried step (c) shaped particle at a temperature of at least 400° C.
In making the above composition one can optionally add at least one metal selected from the group consisting of cobalt, nickel, rhodium, palladium, iridium, platinum, manganese, lanthanum, and cerium to step (a) or to the calcined step (d) shaped particle.
The above composition can optionally be made wherein one or more additives selected from the group consisting of binders, lubricants, rheology control agents, and pore forming agents are added at step (a).
DETAILS OF THE INVENTION
The present invention relates to the preparation of high strength, low density, zirconia catalyst supports, and their subsequent use in nitrous oxide abatement. A zirconia extrudate is conventionally made by mixing zirconium hydroxide with a solution of water and zirconyl nitrate. Generally speaking, there are herein described three embodiments of the invention which entail changes to this conventional process: 1) mixing relatively fine (on the order of 1 &mgr;m) Zr(OH)
4
and relatively coarse (on the order of 15 &mgr;m) Zr(OH)
4
with subsequent extrusion; 2) diluting a zirconyl nitrate solution with about 10-16% nitric acid to achieve a “15% ZrO
2
” solution; and 3) a combination of 1) and 2) as described above.
The process of this invention also includes the use of one or more solvents selected from conventional liquid solvents that are inert in the context of the process of the present invention and easily removed by drying (evaporation) and/or by combustion during calcination. These solvents include water; alcohols, such as methanol, ethanol and propanol; ketones, such as acetone and 2-butanone; aldehydes, such as propanal and butanal; and aromatic solvents such as toluene and benzene. Water is the preferred solvent.
The amount of solvent used in preparing the paste of step (a) is an amount that provides a consistency which allows for a shaped particle to be mechanically formed out of the paste, but the amount of solvent in the paste should not make it so fluid as to fail to hold its form or shape or become sticky and agglomerate with other particles. Typically, the total amount of solvent in the paste is from about 10% to about 40% by weight of the paste.
The paste of the present process may also contain rheology control agents and pore forming agents. Rheology control agents include starches, sugars, glycols, polyols, powdered organic polymers, graphite, stearic acid and its esters. Pore forming agents include graphite, polypropylene or other organic polymer powders, activated carbon, charcoal, starches, and cellulose flour. The rheology control agents and pore forming agents (some materials may perform both functions) are well known to those of ordinary skill in the art and are used as necessary to obtain the desired viscosity of the paste or porosity of the formed particle, as the case may be. Typically, any of these may be present in the amount of from about 0.5% to about 20% by weight, preferably, from about 1% to about 10% by weight of the paste. The rheology control agents and pore forming agents incorporated in the paste are removed from the finished shaped particle by a combination of volatilization and combustion during the final steps of drying and calcination of the shaped particle.
A formed or shaped particle is then prepared from the paste. Extrusion is the preferred forming technique. The formed particle may have a variety of cross sections such as cylinders, trilobes, or star shaped. The formed particles are
E. I. du Pont de Nemours and Company
Johnson Edward M.
Siegell Barbara C.
Silverman Stanley S.
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