WO2006010904A1

WO2006010904A1 - Oxidation process

Info

Publication number: WO2006010904A1
Application number: PCT/GB2005/002882
Authority: WO
Inventors: Sean Alexander Axon; James Richard Foy
Original assignee: Johnson Matthey Plc
Priority date: 2004-07-30
Filing date: 2005-07-22
Publication date: 2006-02-02
Also published as: GB0416982D0

Abstract

An oxidation process, e.g. an ammonia oxidation process is described in which a reactant is combined with an oxidant and passed over a perovskite oxidation catalyst of formula ABO3 in which A comprises bismuth and/or one or more lanthanide metal cations excluding Lanthanum and B comprises one or more transition metal cations, characterised in that A and B in the presence of sulphur compounds form metal sulphates that have decomposition temperatures below the operating temperature of the process.

Description

Oxidation Process

This invention relates to oxidation processes an in particular to oxidation processes catalysed by perovskite catalysts.

In oxidation processes operated on an industrial scale, the reactant, usually in the gas phase, is combined with an oxidant, often air, oxygen or an oxygen-enriched gas, and passed at elevated temperature over an oxidation catalyst, which may be in the form of a particulate fixed bed, filamentary material or monolith, disposed in an oxidation reactor. Perovskite catalysts are known to be active oxidation catalysts for the oxidation of hydrocarbons at about 300⁰C to form carbon oxides and for the oxidation of ammonia at temperatures greater than about

800⁰C, particularly about 900⁰C to form nitric oxide (the Ostwald Process) or, in the presence of methane, to form hydrogen cyanide (the Andrussow process).

Side reactions, such as the formation of nitrogen or nitrous oxide in ammonia oxidation processes, are undesirable. Consequently, in addition to good activity, the catalyst is required to have a good selectivity.

Perovskite oxidation catalysts are generally represented to be of formula ABO₃ wherein A represents metal cations selected from Group I, Group II, the lanthanide and actinide metais and B represents metal cations selected from the transition metals, particularly Groups IVB, VB, VIB, VIIB, VIII and IX of the periodic table.

Perovskite catalysts, for example where A is lanthanum (La), La and strontium (Sr), or La and cerium (Ce), and B is cobalt (Co), manganese (Mn), iron (Fe), chromium (Cr), nickel (Ni) and copper (Cu), or mixtures thereof, are active for hydrocarbon or ammonia oxidation reactions. For example, Kaliaguine et al in Applied Catalysis B: Environmental, 2003 Vol. 43, pages 81-92 describe lanthanum-containing cobalto-ferrate perovskites of formula LaCoi_-xFe_xO₃ for methane oxidation reactions at 300-500⁰C. US 4812300 describes lanthanum-containing perovskite catalysts LaMnO₃, LaCoO₃, LaNiO₃, Lao.₇₅Sr₀.₂₅Mn0₃, La₀₇₅Sr₀.₂₅CoO₃, LaCo₀.₅Cu₀.₅O₃ and LaMn₀.₅Cu₀.₅θ₃ and their activity in ammonia oxidation reactions operated at about 900⁰C.

EP0946290 describes cobalt perovskite catalysts where the A cation comprises a mixture of at least one variable valency element Vv selected from cerium and praseodymium and at least one non-variable valency element Vn selected from yttrium and the non-variable valency rare earth elements such as lanthanum or neodymium. The examples therein describe the preparation of La₀,₈Ce₀.₂CoO₃ and La₀.₇₅Ce₀.₂₅CoO₃ catalysts and their use in ammonia oxidation reactions operated at 900-1100⁰C.

However we have found that these catalysts are, under normal operating temperatures, susceptible to poisoning, particularly sulphur poisoning, which both reduces the catalyst activity and selectivity. Sulphur poisoning may occur when the catalyst is exposed to sulphur compounds such as hydrogen sulphide, thiols, disulphides, thiophenes, sulphur oxides and the like. The sulphur may be present in the reactant or oxidant, for example levels in air fed to ammonia oxidation processes may be ca 50ppb or higher. In oxidation processes operated at elevated temperatures, it is believed that the sulphur compounds exist primarily as sulphur oxides. Sulphur oxides can react with metal oxides to form metal sulphates.

We have found that the sulphur-poisoning of perovskite oxidation catalysts may be significantly reduced if the A and B metal cations are selected so that at the operating temperature of the process a stable metal sulphate does not form.

Accordingly, the invention provides an oxidation process in which a reactant is combined with an oxidant and passed over a perovskite oxidation catalyst of formula ABO₃ in which A ■ comprises bismuth and/or one or more ianthanide metal cations excluding Lanthanum and B comprises one or more transition metal cations, characterised in that A and B in the presence of sulphur compounds form metal sulphates that have decomposition temperatures below the operating temperature of the process.

Whereas oxidation processes may be operated at temperatures above about 300⁰C, preferably the process of the present invention has an operating temperature greater than about 800⁰C, more preferably 900⁰C. The maximum operating temperature of the oxidation process of the present invention may be about 115O⁰C, preferably 1100⁰C.

By the term Operating temperature' we mean the average measured temperature of the exit (oxidised) gas stream immediately after it has passed over the catalyst.

The perovskite catalysts used in the oxidation processes of the present invention are of formula ABO₃ wherein A represents one or more metal cations selected from bismuth and the Ianthanide metals, excluding Lanthanum, and B represents one or more metal cations selected from the transition metals, particularly Groups IVB, VB, VIB, VIIB, VIII and IX of the periodic table. Both A and B cations form metal sulphates with a decomposition temperature below the operating temperature of the process of the present invention, i.e. preferably having decomposition temperatures of below 115O⁰C. The metal sulphate decomposition temperature may be readily determined using thermogravimetric analysis using techniques known to those skilled in the art.

The A and B cations may be partially replaced with other A and B cations, e.g. upto 50 atom%, to form ternary catalysts of formula A_1-XA'_XBO₃, AB^B'_yOs and quaternary catalysts of formula A^xA'xB^yB'yOs where x and y may independently be = 0.005 to 0.5, preferably 0.01 to 0.3. Furthermore the selection of A and B may, depending upon the oxidation state of the cations, lead to a perovskite catalyst which is non-stoichiometric, i.e. oxygen rich or oxygen depleted, e.g. ABO_3±δ, where δ = 0 to 0.2. All such variations of the basic perovskite catalyst are included within the scope of the catalysts used in the process of the present invention.

Preferred B cations are cations of Co, Fe, Mn, Cu, Cr, Ti or Ni or mixtures thereof, more preferably Co, Mn, Fe and Ni optionally partially replaced by 1-30 atom % Cu. Most preferably B is Mn or Co, particularly Co. Where B comprises cobalt, preferably less than 25% (by atoms) of the cobalt is present as free cobalt oxides, and in particular it is preferred that less than 15% (by atoms) of the cobalt is present as the cobalt monoxide, CoO. The proportion of the various phases may be determined by X-ray diffraction (XRD) or by thermogravimetric analysis (TGA) making use, in the latter case, of the weight loss associated with the characteristic thermal decomposition of Co₃O₄, which occurs at approximately 930⁰C in air. Preferably less than 20%, particularly 15% or less by weight of the composition is free cobalto-cobaltic oxide and less than 2% by weight is free cobalt monoxide. Mn(II) sulphate has a decomposition temperature of 850⁰C and therefore is suited for use in perovskite oxidation catalysts operated at temperatures greater than 850⁰C, preferably 900⁰C. Co(II) sulphate has a decomposition temperature of 735⁰C. Cobalt (II) is therefore is suited for use in perovskite oxidation catalysts operated at temperatures greater than 735⁰C, preferably greater than 75O⁰C and more preferably greater than 80O⁰C. Similarly, anhydrous chromium sulphate has a decomposition temperature above 57O⁰C, ferric sulphate has a decomposition temperature 48O⁰C, nickel sulphate has a decomposition temperature about 76O⁰C and copper sulphate has a ' decomposition temperature about 900⁰C.

The A cation is bismuth (Bi) or a lanthanide metal cation and mixtures of these wherein the metal forms a metal sulphate with a decomposition temperature below the operating temperature of the process of the present invention, i.e. preferably one having a decomposition temperature of below 115O⁰C. Bismuth forms a bismuth (III) sulphate with a decomposition temperature of 405⁰C and therefore is suitable for use in catalysts operated at temperatures above 405⁰C. However, bismuth oxide, which may be present in small amounts in the perovskite catalyst, has a low melting point of 83O⁰C and therefore the oxidation process where Bi is present as a catalyst component is preferably operated below 80O⁰C, preferably 75O⁰C. Praseodymium (Pr) and Samarium (Sm) sulphate decomposition temperatures have been measured using thermogravimetric analysis at about 89O⁰C and so may be used in catalysts operated above 89O⁰C, preferably above 95O⁰C. A preferred A cation is gadolinium (Gd). Gd(III) sulphate has a decomposition temperature of 88O⁰C and therefore is suited for use in catalysts operated at temperatures greater than 88O⁰C, preferably greater than 900⁰C. Ce(III) sulphate has a decomposition temperature of 92O⁰C and therefore Ce is suited as an A-site dopant in catalysts operated at temperatures in excess of 92O⁰C. Hence the A-site cation preferably comprises a cation of Bi, Pr, Sm, Ce or Gd or mixtures thereof. Particularly preferred A cations consist of Gd and Ce, i.e. Gd_1-xCe_x, where x is 0.01-0.3.

Lanthanum (III) sulphate has a decomposition temperature greater than 1150⁰C and therefore lanthanum is unsuitable as a component of a perovskite oxidation catalyst used in the process of the present invention.

Hence the preferred perovskite oxidation catalysts are lanthanum-free transition metal perovskite oxidation catalysts, comprising cobalt or manganese, whose A cation metal sulphates decompose below about Vl 5O⁰C, particularly GdCoO₃, optionally with 1-30 atom% of the Gd replaced with Ce. GdCoO₃ and GdCeCoO₃ catalysts are suited for oxidation processes with operating temperatures greater than 88O⁰C, preferably greater than 900⁰C.

The process of the present invention may be a hydrocarbon oxidation process, e.g. methane oxidation, or an ammonia oxidation process or another oxidation process in which a reactant is combined with an oxidant, such as air or other oxygen-containing gas (including pure oxygen) and is oxidised at elevated temperature over a perovskite catalyst as described herein. The process is preferably an ammonia oxidation process. In ammonia oxidation processes, ammonia is combined with air or other oxygen-containing gas at temperatures greater than about 800⁰C, particularly 900-1100⁰C to form nitric oxide (the Ostwald Process) or, in the presence of methane, to form hydrogen cyanide (the Andrussow process). Hence, where the process of the present invention is an ammonia oxidation process, the preferred perovskite oxidation catalyst is GdCoO₃, optionally with 1-30 atom% of the Gd replaced with Ce. In the oxidation of ammonia to nitric oxide for the manufacture of nitric acid, the oxidation process may be operated at pressures of 1 to 15 bar abs., with ammonia in air concentrations of 5-15%, often about 10%, by volume.

The catalyst may be formed by heating a stoichiometric mixture of the A and B component oxides, preferably in air, to a temperature in the range 800-1200⁰C. Where the B-cation is Co, the temperatures are preferably 900-1200⁰C in order to produce a material in which only a small proportion of the cobalt is present as free oxides.

Alternatively, the compositions may be made by precipitation, e.g. by adding a solution of soluble salts in the appropriate proportions of the relevant metals to a solution of a base, e.g. ammonium carbonate or hydroxide, to precipitate the relevant metals as (basic) carbonates, hydroxides, or oxides followed by calcination to convert the precipitated compounds to the oxides. The use of alkali metal compounds as the base to effect precipitation is less preferred as they inevitably cause some contamination of the product with sodium which could act as a catalyst poison. The precipitation may alternatively, but less preferably, be effected by adding the base to the solution of the mixed salts. Alternatively, the composition may be made by forming a solution of thermally decomposable salts, e.g. nitrates or salts of organic acids, e.g. oxalates or citrates, of the metals in the appropriate proportions and evaporating the solution to dryness followed by calcination to effect decomposition to the appropriate oxides.

In another alternative some or all of the A-cation oxide material may be used as a support on to which the B-cation oxide and any remaining A-cation oxide is coated. Thus a finely divided A- cation oxide, e.g. gadolinia, may be impregnated with a solution containing a cobalt salt, and possibly also another A-cation salt, e.g. a cerium salt, followed by decomposition of the cobalt and, if present, the A-cation salt. Alternatively, such a supported material may be made by precipitation by precipitating the cobalt, and optionally some of the A-cation, as heat decomposable compounds on to a finely divided, e.g. precipitated, A-cation oxide or compound decomposable thereto.

Whichever route is used to make the oxides composition, the composition should be calcined, e.g. in air, at a high enough temperature for long enough to form the mixed oxide structure, i.e. the Perovskite structure in which most, if not essentially all, of the free oxides are combined into one or more mixed oxide phases. As indicated above the calcination temperature when cobalt is a B-cation is preferably in the range 900-1200°C. The duration of the heating required will depend on the temperature employed and on the route employed to make the composition. If the heating temperature is below 1100⁰C, heating for at least 6 hours is preferred. On the other hand the duration of heating at a temperature above 1150°Cϊs preferably less than 6 hours in order to minimise the decomposition of the perovskite, e.g. of cobalt oxide containing phases into free cobalt monoxide. However catalysts prepared by evaporating a solution containing a mixture of organic salts, e.g. citrates, of the relevant metals to dryness followed by calcination may require heat treatment for shorter times and/or at temperatures 200-300⁰C below the temperatures required for compositions made for example by precipitation. Alternatively, if the catalyst is made by calcining a mixture of preformed oxides, longer times and/or higher temperatures may be required to produce a material of sufficient phase purity. The perovskite oxidation catalyst may be disposed in the oxidation reactor in the form of a fixed particulate bed. The particles of the perovskite oxidation catalyst may be shaped units such as extrudates or pellets, which may contain holes or have flutes or lobes to desirably increase their geometric surface area. The pellets or extrudates may be formed using methods known to those skilled in the art. The shaped units will typically have a cross-section of 1-15 mm, preferably 3-10 mm and will have an aspect ratio less than 2. In order to minimise pressure drop through the fixed particulate bed, the bed will preferably have a thickness (L) less than the width (D) of the oxidation reactor (i.e. has a L/D ratio <1 ), and more preferably has an L/D ratio of 0.001-0.5. Depending upon the width of the oxidation reactor, the fixed bed may have a thickness of 10-1000 mm, preferably 10-500 mm, most preferably 10-250 mm. Alternatively, the perovskite oxidation catalyst may be disposed in the oxidation reactor in the form of a ceramic honeycomb or foam made from the catalyst itself.

Alternatively, the perovskite oxidation catalyst may be disposed in the oxidation reactor supported on a metal or metal oxide particulate, filamentary or monolithic support. The metal may be a base metal such as a steel alloy or a precious metal or precious metal alloy such as a platinum or palladium alloy thereof. The oxide supports may be inert oxides such as alumina or may be an element A- or element B- cation oxide. The supports may be particulate, e.g. in the form of shaped units or may be filamentary, i.e. in the form of fibres or wire which may be non- woven or woven or knitted into gauzes. Alternatively the support may be in the form of one or more monoliths in the form of a honeycomb or foam of a ceramic material such as alumina or zirconia, or a monolithic structure formed from an iron/aluminium alloy.

The support may be treated with a suitable wash-coat in order to generate adequate catalyst surface area. When using a metal support, it is necessary to give the support a ceramic coating, termed a wash coat, and the active material is then deposited on this wash coat. With conventional high temperature steel supports, there is a risk that material of the wash coat, or impurities remaining therein, e.g. alkali, resulting from the use of alkali aluminate solutions to form the wash coat, may in use gradually diffuse into the active material, upsetting the desired structure, and interfering with the catalytic performance. We have found however that by the use of primary supports made from a high temperature, aluminium containing, ferritic alloy, it is possible to obtain good adhesion of the wash coat to the primary support without the use of alkaline wash coat solutions and so the problem of migration of alkali impurities into the active catalysts may also be avoided.

In ammonia oxidation processes of the present invention, the catalysts may be used in combination with or as a complete or partial replacement for the conventional platinum alloy meshes or gauzes. Where a partial replacement of the precious metal gauzes is used the process may be affected using 1 or more platinum alloy gauzes in combination with the catalyst of the present invention. For example, an ammonia oxidation process according to the present invention, in an oxidation reactor of 0.5-3.5 metres diameter, may comprise mixing 5-10% vol. ammonia with air, preheating the mixture to between 200-400⁰C and passing the pre-heated gas mixture over a precious metal catalyst comprising between 1 and 5 platinum alloy gauzes, optionally with one or more palladium alloy gauzes, and then passing the partially or fully oxidized gas stream over a 10-50 mm thick particulate fixed bed of GdCoO₃ or an GdCeCoO₃. Where the ammonia is fully oxidized by the precious metal catalyst, the perovskite catalyst acts to decompose the undesirable side reaction product N₂O to N₂.

The invention is illustrated by the following examples. Example 1 : Preparation of GdCoO₃

Approximately 15Og Gd₂O₃ (Alfa Aesar Product 11291) and 10Og Co₃O₄ (Alfa Aesar Product 040184) were weighed and mixed together (by tumbling) in a sample bag. Approx. 42g of the mixture was placed in each of 2 ball mills (mill diameter = 10cm, mill height = 6cm) with -80 balls (ball diameter =1cm) and milled for 15 minutes at a speed setting = 5 (mid range speed). This method was repeated until all the material had been milled. After milling about 0.5wt% graphite (Timcal Timrex T44 grade) was added. The material was compacted to about 20 tons using an Enerpac semi-automated press and a 3cm diameter punch and die set. The compacted material was broken up with a pestle and mortar then sieved to a particle size of 355-850 microns. This material was then pelleted using a Manesty Tableting Machine with a 1/8" diameter punch and die set, providing cylindrical pellets of diameter 3.45 mm and length 3.5 mm. The pellets were fired at 900⁰C for 96 hours (ramp at 2.5°C/min) in air in a muffle furnace to form the perovskite structure. The fired pellet density was ca. 3.7g/cc. X-ray Diffractometry on the fired material indicated GdCoO₃ perovskite phase purity about 85%.

Example 2: Activity & Selectivity of GdCoO₃ in an ammonia oxidation reaction

All experiments were performed in a 25mm bore silica tubular micro-reactor.

General Method: The 25mm diameter reactor was packed with a 20mm deep bed of catalyst pellets. A mixture of 10.5% oxygen/ 1.0% argon/ 88.5% helium gas at a flow of 35.0 l/min was mixed with ammonia gas at a flow of 1.84 l/min resulting in a gas feed containing 5% ammonia. This was passed over the catalyst at a velocity of 1.24m/s. The gas inlet temperature was ramped from 100⁰C at 20°C/min up to 415 - 435°C. The reaction was then monitored under steady state at this temperature for 30 minutes before being cooled at a similar rate until the reaction extinguished.

The exit gas from the reactor was analysed by mass spectrometry. The gas inlet and catalyst temperatures were monitored throughout each run. The data were used to calculate light-off temperature (LO onset) and then the selectivity to NO at gas inlet temperatures of 415 - 435°C. High selectivity to NO and light off <250°C are desired

The following catalysts were evaluated;

1. Comparative A - 3 mm pellets of Lao.₈Ce₀.₂Co0₃.

2. Present Invention (Example 2a) GdCoO₃ made according to method of Example 1. The catalysts were further tested as described above but with sulphur dioxide at ca. 2ppm added to the process for a further 45minutes, followed by a recovery period after the sulphur dioxide was switched off. The sulphur dioxide was supplied to the process as 1.05% SO₂ in N₂. Gas feed at 0.4 - 0.45 litres/hour (1.94 - 2.19 ppm). The results are given below;

The GdCoO₃ catalyst example 2a sample was subjected to a further two cycles under similar conditions and recovered post-SO₂ treatment to give a NO selectivity of 94.8% and 93.0% respectively. The results indicate that the GdCoO₃ catalyst of the present invention has equally effective oxidation and light off characteristics to the LaCeCoO₃ catalyst, but is less effected by, and recovers to a greater extent when poisoned by SO₂ at high levels. The small percentage differences in these lab tests correspond to significant commercial advantage in an industrial scale nitric acid manufacturing unit.

Example 3: Preparation of Gdn«Ce_{n 7}CoO₃

Co(NO₃)₂.6H₂O (1164.2g), Ce(NO₃)₃.6H₂O (309.4g) and Gd(NO₃)₃.6H₂O (1291.1g) were weighed out into a stirred tank and dissolved in 10 litres of cold demineralised water giving a solution with pH 2.94 at 20⁰C. Separately, ammonium bicarbonate [(NH₄)HCO₃] (2530.9g) was . dissolved in demineralised water and made up to a total volume of 16 litres at 35⁰C giving a solution with a pH of 7.85 at 37.1⁰C. Separately, oxalic acid [HO₂CCO₂H.2H₂O] (639.9g) was dissolved in demineralised water and the total volume made up to 7.8 litres at 50°C giving a solution with a pH of 1.32 at 67°C.

The three solutions were pumped into the stirred tank using separate peristaltic pumps at flow rates so as to maintain a pH of 6.7. The resulting mixed precipitate suspension was then passed into a stirred collection vessel. The precipitate was aged for 1 hour and then filtered under vacuum on filter paper. The filter cake was washed with demineralised water, re-slurried to 60 litres in demineralised water and filtered again. The filter cake was then washed with demineralised water, until a filtrate with a conductivity of 197μS was obtained. The solid was dried at 150⁰C for 8h. The dried material was then pre-calcined at 450⁰C for 6h.

The pre-calcined material was mixed with 0.5% w/w of graphite powder and compacted to 20 tonnes force, then crushed and sieved to between 355 and 850μm particle size (tapped bulk density 1.49gcm^"3). The compacted material was pelleted on a Manesty Tabletting Machine with a 1/8" diameter punch and die set (tapped bulk density 1.71 gem^"3, pellet density 2.68gcm^"3). The pellets were fired in a muffle furnace at 900⁰C for 24h (loss on firing 10%) giving the perovskite, as confirmed by X-Ray Diffraction.

Example 4: Activity & Selectivity of Gd_{n R}Ce_n 7C0O3 in an ammonia oxidation reaction

The test method of Example 2 was repeated except that the gas inlet temperature was ramped from 100⁰C at 20°C/min up to 415 ⁰C. It was held at this temperature for 30 minutes before commencing the addition of the 2ppm SO₂ at 1 hr 27 mins from the start of the run.

The following catalysts were evaluated;

1. Comparative B - 3 mm pellets of La₀.sCe₀.₂CoO₃ freshly prepared according to the methods described in EP 0946290.

2. Present Invention (Example 4a) Gd₀₈Ce₀₂CoO₃ made according to method of Example 3.

The results are given below;

These results demonstrate the considerably higher tolerance to high SO₂ levels of the catalyst of the present invention.

Claims

Claims.

1. An oxidation process in which a reactant is combined with an oxidant and passed over a perovskite oxidation catalyst of formula ABO₃ in which A comprises bismuth and/or one or more lanthanide metal cations excluding Lanthanum and B comprises one or more transition metal cations, characterised in that A and B in the presence of sulphur compounds form metal sulphates that have decomposition temperatures below the operating temperature of the process.

2. A process according to claim 1 wherein B is a cation of one or more transition metals selected from Groups IVB, VB, VIB, VIIB₁ VIII and IX of the periodic table.

3. A process according to claim 1 or claim 2 wherein B is a cation of Co, Fe, Mn, Cu₁. Cr, Ti or Ni or mixtures thereof.

4. A process according to any one of claims 1 to 3 wherein A is a cation of Bi, Pr, Sm, Ce or Gd or mixtures thereof.

5. A process according to any one of claims 1 to 4 wherein the A site consists of Gd or Gd and Ce cations, Gd_1-xCe_x, where x is 0.01-0.3, and B is Co.

6. A process according to any one of claims 1 to 5 wherein the perovskite catalyst is in the form of a fixed particulate bed.

7. A process according to any one of claims 1 to 6 wherein the perovskite catalyst is supported on a metal or metal oxide particulate, filamentary or monolithic support.

8. A process according to any one of claims 1 to 7 wherein the process is an ammonia oxidation process.

9. A process according to claim 8 wherein the perovskite catalyst is used in combination with a precious metal ammonia oxidation catalyst.

10. A process according to any one of claims 1 to 9 operated at temperature greater than 800⁰C.