Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2000-02-29
2002-04-30
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S235000, C502S338000
Reexamination Certificate
active
06379640
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for the conversion of nitrous oxide (N
2
O) to nitrogen and oxygen in the presence of a supported metal-containing catalyst. The invention also includes a novel catalyst composition and a method for making the catalyst composition.
BACKGROUND OF THE INVENTION
Nitrous oxide is a greenhouse and ozone-depleting gas, and is a by-product of adipic and nitric acid manufacturing.
U.S. Pat. No. 5,705,136 discloses a process for the decomposition of nitrogen oxides to nitrogen and oxygen comprising contacting the nitrogen oxides with a mixed oxide catalyst wherein the catalyst comprises a first metal oxide selected from the oxides of Mn, Fe, Co, Ni, Cu, Zn and mixtures thereof on a metal oxide support consisting essentially of MgO, CaO, ZnO, TiO
2
, MoO
3
—CoO—Al
2
O
3
, ZnO—Al
2
O
3
, TiO
2
—MgO, TiO
2
—Al
2
O
3
, TiO
2
—ZnO, MgO—CuO and MgO—NiO or mixtures thereof.
U.S. Pat. No. 5,314,673 discloses a process for the conversion of N
2
O to nitrogen and oxygen which comprises contacting the N
2
O with a catalyst consisting essentially of nickel oxide and cobalt oxide on a zirconia substrate.
There is a need for catalysts which can decompose N
2
O into N
2
and O
2
, and have a minimal environmental impact of their own. That is, they should contain readily-available and non-toxic materials, be simple to make, have a long lifetime, and not pose disposal problems. The catalysts should also be hard and porous.
SUMMARY OF THE INVENTION
This invention provides a process for the conversion of nitrous oxide (N
2
O)) into nitrogen (N
2
) and oxygen (O
2
) comprising contacting N
2
O with a metal-containing catalyst supported on zirconia under conditions effective to decompose the N
2
O to N
2
and O
2
, wherein the catalyst comprises iron and optionally at least one metal selected from the group consisting of cobalt, nickel, rhodium, palladium, iridium, platinum, manganese, lanthanum and cerium and the catalyst is prepared by the steps of:
(a) preparing a paste comprising contacting zirconium hydroxide with a solution of an iron salt and a zirconium salt, optionally in the presence of binders and lubricants;
(b) forming a shaped particle from the step (a) paste;
(c) drying the step (b) shaped particle;
(d) calcining the dried step (c) shaped particle at a temperature of at least 400° C.; and
(e) optionally adding 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 another embodiment, this invention provides a catalyst composition useful in a process for the decomposition of nitrous oxide, wherein the composition comprises a metal-containing catalyst supported on a zirconia shaped particle, wherein the metal comprises iron and optionally at least one metal selected from the group consisting of cobalt, nickel, rhodium, palladium, iridium, platinum, manganese, lanthanum and cerium, wherein the catalyst is prepared by the steps of:
(a) preparing a paste comprising contacting zirconium hydroxide with a solution of an iron salt and a zirconium salt, optionally in the presence of binders and lubricants;
(b) forming a shaped particle from the step (a) paste;
(c) drying the step (b) shaped particle;
(d) calcining the dried step (c) shaped particle at a temperature of at least 400° C.; and
(e) optionally adding 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; and wherein the crush strength of the calcined shaped particle is at least 22.2 newtons.
DETAILED DESCRIPTION
Zirconium hydroxide (i.e., “Zr(OH)
4
”, sometimes referred to as zirconium oxyhydroxide or hydrated zirconia) powder is dried before use at about 50° C. to 150° C., preferably at about 100° C. The zirconium hydroxide can be doped with various elements such as Ca, Mg, Si, and La to help maintain a high surface area upon calcination.
The iron and zirconium salts can be chosen from a wide variety of salts, which readily decompose upon calcination to produce iron and zirconium oxides, such as acetates, carbonates, citrates, nitrates, oxalates and chlorides. Surprisingly, even chlorides may be used, although the other salts are preferred. Sulfates and phosphates can also be included in small amounts, as these anions help maintain a high surface area upon calcination. In addition, other components, such as binders and lubricants, can be added to the paste to aid in the shaping process, e.g., extrusion., and provide green strength. The iron in the iron salts can be in either the +2 or +3 oxidation states, with the +3 oxidation state being preferred. The minimum iron content is 0.5% Fe or a minimum iron nitrate content of the pepping solution is 5%. The preferred iron concentration in the catalyst is 1.5% to 7%, with a most preferred iron concentration of about 3% to 4%.
The process of this invention also includes the use of one or more solvents selected from conventional liquid solvents which 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 propanol 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 not 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 30% 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.
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, and star shaped. The formed particles are air dried under conditions sufficient to form a particle that is not malleable (or soft) or friable. The dried formed particles are then calcined in air or in inert gases such as nitrogen or argon or mixtures thereof at a temperature of from about 400° C. to about 650° C. The result is a surprisingly hard and porous iron-zirconia formed particle. The crush strength of the shaped particles is at least about 22.2 newtons (5 pounds).
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.
In one embodiment of this invention, catalytic metals for the decomposition of nitrous oxide may be incorporated into the step (a) paste or preferably, impregnated on the calcined step (d) shaped particle. At least one metal is selected from the group consisting of cobalt, nickel, rhodium, palladium, iridium, platinum, manganese, lanthanum and cerium. Suitable sources of catalytically active components include both
Griffin Steven P.
Johnson Edward M.
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