WO2012025630A1

WO2012025630A1 - Method for the high-temperature catalytic decomposition of n2o

Info

Publication number: WO2012025630A1
Application number: PCT/EP2011/064769
Authority: WO
Inventors: Christian Hamon
Original assignee: Institut Regional Des Materiaux Avances
Priority date: 2010-08-26
Filing date: 2011-08-26
Publication date: 2012-03-01
Also published as: FR2964043A1; FR2964043B1; EP2616164A1

Abstract

The present invention relates to a method for the catalytic decomposition of N₂O, in a gaseous effluent containing N₂O, by passing said gaseous effluent over a zeolite catalyst bed, said zeolite being ferrierite that has been partially or totally exchanged with a transition metal and having an Si/Al ratio of between 20 and 40, at a temperature of between 800 and 900°C.

Description

PROCESS FOR CATALYTIC DECOMPOSITION OF N ₂ 0 A HAUTE

TEMPERATURE

The present invention relates to a high temperature process for the catalytic decomposition of N ₂ 0 in a gaseous effluent containing N ₂ 0 in the presence of a ferriérite zeolite type catalyst exchanged with a transition metal and having a Si / Si ratio. High al.

Nitrous oxide is a gas that contributes to the greenhouse effect. In particular, it has a radiative power 310 times greater than that of C0 ₂ and therefore contributes significantly to global warming.

It is emitted by certain industrial installations and in particular by nitric acid units where N ₂ 0 is formed on platinum webs (a by-product of the oxidation of NH ₃ to NO) and is found in tail with NOx not converted to nitric acid. N ₂ 0 is also formed in chemical industries aiming to synthesize caprolactam or hydrocyanic acid, by secondary reaction during a catalytic reaction with ammonia (NH ₃ + O ₂ for caprolactam or NH ₃ + CH ₄ for hydrocyanic acid) on platinum canvas. This reaction is very exothermic and the temperature under the cloths is between 800 and 900 ° C. The content of N ₂ 0 is generally between 300 and 2000 ppm and is a function of the age of the fabrics.

In the nitric acid units, N ₂ O is found in the tail gases and can be treated (decomposition or reduction to produce N ₂ N ₂ ) catalytically, especially on zeolites exchanged with iron ( WO 99/34901 and WO 2008/049557).

The other way to decompose the N ₂ 0 in N ₂ and O ₂ , without however interacting with the NO, is to have under the canvases of platinum a stable catalyst at this temperature level and active for the intended reaction. This has already been the subject of various patents and industrial applications. There may be mentioned, for example, patent application FR 2 936 718 using mixed refractory oxides (solid solution) based on ceria and lanthanum oxide or patent EP 1 301 271 using a catalyst based on ceria oxide. cobalt doped.

The inventors have been able to demonstrate that it is possible to use ferrierite zeolites (FER) exchanged, partially or totally, with transition metal cations and having a high Si / Al ratio for the treatment of gaseous effluents containing N ₂ 0 to reduce the N ₂ 0 content at elevated temperatures, such as those observed after platinum webs in a nitric acid unit.

Zeolites are tetrahedral silicoaluminates of the family of crystallized microporous tectosilicates with a high specific surface area, with pore sizes varying according to the structure of the zeolite from 3 to 13 Å.

There are zeolites in their natural state that have been known for a long time, the main ones being clinoptilolite, chabazite and mordenite, and synthetic zeolites known since the 1950s with numerous industrial applications implementing their adsorption properties. ions or catalysis.

Synthetic zeolites, unlike natural zeolites, are pure products. To date, more than 200 different structures are known, the main ones being A, faujasite (X and Y), mordenite, ferrierite, pentasil and beta.

These zeolites are obtained by hydrothermal synthesis. The structure of zeolites is described as a sequence of Si0 ₄ and A10 ₄ ^" tetrahedra (carrying a negative charge), with pooling of oxygen in the three directions of space: these are the sequences that generate channels and cavities specific to each structure.

The general formula of a zeolite is written as follows:

A10 xSi0 M ₂ ₂ 0 wH ₂

where x represents the atomic ratio Si / Al, w represents the amount of water present and M represents the compensation cation which is most often Na ⁺ (sodium form) and / or optionally K, 1 M compensating for the negative charge related to aluminum (lNa ⁺ / l Al) due to the tetrahedral structure (AIO ₄ ^" ) and the pooling of oxygen.

These zeolites are also commercially available in acid or ammonium form, that is to say in a form in which the compensation cations have been exchanged respectively with H ⁺ or Η ₄ ⁺ ions.

In the normal state, there is water (capillary condensation) in the pores of a zeolite (wH ₂ 0). However, this water can be removed by raising the temperature.

Synthetic zeolites, that is to say more than 99% pure, are crystallized microporous silicates whose channel and cavity sizes vary according to the structure between 3 and 13 Å. They are in the form of powdery powder, the size of the crystals being on average a few microns, advantageously between 1 and 2 microns.

Finally, for a defined structure, it is possible, by known technologies, namely dealumination (treatment under steam at high temperature followed by acid treatment), or replacement of Al with Si (for example by treatment with S1CI ₄ ), increase the ratio Si / Al while maintaining the same structure.

In the context of the present invention, a particular zeolite is used, namely ferrierite, which has two channel arrays whose pore openings are 4.3 × 5.5 Å and 3.4 × 4.8 Å.

The zeolites can be exchanged, partially or totally, with a metal cation, that is to say that the compensation cation is replaced partly or entirely by a metal cation, for example Fe ²⁺ (2Na ⁺ → 1 Fe ²⁺ ).

The compensation cation, most often Na ⁺ , can indeed be partially or totally replaced by the ion exchange mechanism with other cations, with different degrees of valence (m), for example Fe ²⁺ (m = 2), the degree of valence representing the number of positive charges carried by the exchange cation.

After exchange, the formula of the zeolite exchanged is then written:

where x is always greater than 1 and represents the atomic ratio Si / Al, M ⁿ represents the compensation cation, C ^{m +} represents the ion exchanged with M ^{n +} (ex: Fe with m = 2), m represents the valence degree of C ^m , n represents the valence degree of the compensation cation M ^{n +} and y represents the exchange rate.

The subject of the present invention is thus a process for the catalytic decomposition of N ₂ 0 in a gaseous effluent containing N ₂ 0 by passing said gaseous effluent over a ferrierite-type zeolite catalyst bed exchanged, partially or totally, with a transition metal and having an Si / Al ratio of 20 to 40, at a temperature of 800 to 900 ° C, and especially of about 850 ° C.

The inventors have indeed discovered that zeolites having such an Si / Al ratio were more stable under the process conditions, in particular under the conditions of high temperature, compared to a zeolite having a lower Si / Al ratio.

Such a process thus makes it possible, by decomposition of N ₂ 0 in the form of nitrogen

(N ₂ ) and oxygen (0 ₂ ), to reduce the N ₂ 0 content of the gaseous effluent, in particular by at least 90%, advantageously by at least 95%, and even more advantageously by at least 99% %.

Such a process may be used more particularly in a nitric acid unit, behind the platinum webs.

In the context of the invention, the terms "gaseous effluent containing N ₂ 0" and "gaseous effluent" are used interchangeably. The process of the invention can be carried out at a volumetric rate in h ^-1

(WH) between 30,000 and 120,000 h ^-1 , advantageously between 50,000 and 100,000 h ^-1 , in particular between 70,000 and 100,000 h ^-1 .

By "VVH" is meant, in the sense of the present invention, the ratio between the flow of gaseous effluent (in Nl / h) and the volume of catalyst used (in liters).

The pressure used has little influence on the process of the present invention.

It may especially be between 10 ⁵ and 12.10 ⁵ Pa. The gaseous effluent used in the context of this process may contain from 200 to 2000 ppm of N ₂ O. It will contain mainly nitrogen (N ₂ ) and may also contain nitrogen oxides (NOx), water, or oxygen. It may be in particular the gaseous effluent obtained at the exit of the plates of plates of a nitric acid unit. Such a gas generally has the following composition:

N ₂ 0: 200 to 2000 ppm,

- NO: 10 to 12% by volume,

0 ₂ : 3 to 5% by volume,

H ₂ O: 15 to 18% by volume, and

N ₂ : for the rest.

By "nitrogen oxides" or "NOx" is meant compounds of the formula NO _x where x is a number greater than or equal to 1. In particular, NOx includes nitrogen monoxide (NO) and carbon dioxide. nitrogen (N0 ₂ ).

Advantageously, the ferrierite according to the invention will have an Si / Al ratio of between 22 and 35, preferably of between 25 and 30, and in particular of approximately 27.5.

The transition metal may be iron or cobalt and in particular iron.

Advantageously, the transition metal content in ferrierite will be between 0.1 and 3%, preferably between 0.2 and 1.6%.

The ferrierite used to carry out the partial or total exchange with a transition metal may be in sodium and / or potassium form (that is, the compensation cation that compensates the negative charge of A10 ₄ ^" is Na ⁺ and / or K ⁺ ), in acid form (when the compensation cation is H ⁺ ), or in ammonium form (when the compensation cation is NH ₄ ⁺ ), these different forms being commercially available or easily accessible The ammonium form can be obtained from the sodium form by cation exchange in the presence of ammonium sulphate, and the acid form can be obtained from the ammonium form by heat treatment, especially at a temperature of about 400 ° C. or more In particular, the ferrierite may be in acid form. Under these conditions, the zeolite exchanged with a transition metal according to the invention can meet the following formula:

or :

- C ^{m +} represents a metal cation of a transition metal, preferably selected from Fe, Fe, Co ^ZT and C <T ^" , more preferably selected from Fe ²⁺ and Fe ³⁺ , and preferably being Fe ²⁺ ,

m represents the valence of the metal cation C ^{m +} (that is to say the number of positive charges borne by this metal cation) and may more particularly represent 1, 2 or 3, and preferably 2 or 3,

M ^{n +} (cation of compensation) represents an alkaline or alkaline-earth ion, such as Na ⁺ , K ⁺ , Li ⁺ or Ca ²⁺ , an H ⁺ ion or a mixture thereof (i.e. that the Α10 ₄ ^"groups are compensated by different cations compensation), preferably Na ^+, K ⁺ and / or H ^+, including H ^+,

n represents the valence of the compensation cation M ^{n +} and may more particularly be between 1 and 2, and may be more particularly 1,

y represents the exchange rate of the compensation cation M ^{n +} by the cation C ^{m +} and is advantageously between 0.2 and 1, in particular between 0.2 and 0.8, and

x represents the Si / Al ratio and can be between 20 and 40, advantageously between 22 and 35, preferably between 25 and 30, and especially about 27.5.

The ferrierite according to the invention will in particular be greater than 99% pure (synthetic ferrierite). It will occur in the form of powder powder, the size of the crystals being on average a few microns, preferably between 1 and 5 microns, especially between 1 and 2 microns.

Preferably, the zeolite is exchanged with iron, and preferably with iron (II)

(Fe ²⁺ ).

Ferrierite may be used alone, that is to say without additive, or in admixture with a binder. In the latter case, the binder may be a binder based on silica (for example a gel such as Ludox) or based on alumina (also called aluminous binder). Preferably, the binder will be an aluminous binder, in particular converted into gel by peptization with nitric acid. The binder content, expressed as A1 ₂ 0 ₃ or SiO ₂ , depending on the type of binder, may be between 10 and 40%, and preferably between 15 and 25% by weight, relative to the total weight of the catalyst.

The shaping of the catalyst comprising the ferrierite in admixture with a binder may be carried out through a die after having obtained, continuously, in an Aoustin-type mixer, a homogeneous mixture and optimized rheology between the zeolite and the binder. , in particular A1 ₂ 0 ₃ peptized with HN0 ₃ . It is thus possible to obtain extrudates of different sizes (for example between 1.8 and 3.2 mm in diameter and with a length of 5 to 10 mm).

Especially shaped "pellets" can be manufactured to limit pressure drops, for example, trilobal extrusions, "hollows" (hollow extrusions) or extrusions of 6 to 8 mm with 1 mm holes.

The preparation of a ferrierite exchanged according to the present invention is well known to those skilled in the art.

In general, in order to carry out the exchange with a transition metal, ferrierite (commercially available or easily obtained by those skilled in the art according to the form used, as indicated above) is suspended in suspension under stirring. an aqueous solution of a metal salt which it is desired to introduce the cation C ^{m +} (Fe ²⁺ for example in the form of sulfate (m = 2)).

The parameters that will influence the exchange, and thus the final content of C ^{m +} cation after exchange, will include the temperature, the concentration of metal salt in the solution, the solution / volume ratio of ferrierite (V / P), and the reaction time.

The zeolite obtained after exchange will advantageously be heat treated, especially at a temperature of about 400 ° C. or more.

The present invention will be better understood in the light of the nonlimiting examples which follow. FIGURES

FIG. 1 represents the evolution of the conversion rate of N ₂ 0 in N ₂ and O ₂ over time and according to the hourly volumetric velocity VVH for a catalyst according to the invention.

FIG. 2 shows the evolution of the conversion rate of N ₂ O to N ₂ and O ₂ over time for a ferrierite-based catalyst with an Si / Al ratio of 8.8.

EXAMPLES

1. Catalyst preparation

For the preparation of the catalyst, a ferrierite zeolite, commercial, acid form, whose Si / Al (atomic) ratio is 27.5 was used.

This zeolite is exchanged from an iron salt, sulphate FeSO ₄ · 7H ₂ 0, in aqueous solution.

50 g of ferrierite (Si / Al = 27.5) are suspended, with stirring, in 150 cm ³ of an aqueous solution of molar iron sulphate, ie 278 g / l of FeSO ₄ H ₂ O 4, for 4 hours. at 60 ° C.

At the end of the exchange, the zeolite exchanged is recovered by filtration on a filter funnel and is washed by percolation with a liter of demineralised water (conductivity <10 μβίεπιεηβ). The zeolite exchanged with iron is dried at 100 ° C. The iron content measured by ICP (Inductively Coupled Plasma, plasma torch) is 0.7% (on dry product). 2. Catalytic test

The iron ferrierite zeolite whose preparation is described above is pelletized (without binder) in pellets 5 mm in diameter which are then heat-treated at 400 ° C. in air for three hours. This is followed by crushing and sieving these pellets between 0.1 and 1 mm to carry out the catalytic test described below. The test is conducted in a 1 "(2.7 cm) diameter fixed bed reactor surrounded by heating shells.

The reaction mixture is prepared from dry air, nitrogen and a standard 2% N ₂ O bottle in N ₂ . Concentrations are controlled by mass flow meters and saturator adjusted water content.

10 cm ³ of the iron / ferrierite catalyst described above in the form of granules of 0.5 - 1 mm, ie a height of 20 mm, are introduced into the reactor.

The operating conditions are as follows:

■ Composition of the gaseous effluent (% by volume):

- N ₂ 0: 1000 ppm

O ₂ : 3% by volume

H ₂ O: 15% by volume

N ₂ : for the rest

■ Flow rate (standard): 500 Nl / h, ie an hourly volumetric velocity of 50 000 h ^"1, then tests are carried out at higher WH (70 000 to 110 000 h ^{" 1} ). ■ Temperature: 850 ° C.

The analysis of the N ₂ O content at the inlet and the outlet is determined by infrared analysis.

The evolution of the rate of transformation of N ₂ O into N ₂ and O ₂ over time according to the hourly volumetric velocity is shown in FIG.

The evolution of the N ₂ O content of the gaseous effluent leaving the catalyst bed is also given in the table below as a function of the VVH.

VVH N ₂ O output N ₂ O conversion

(h ^"1 ) (ppm) (%)

50,000 0 100

70,000 0 100

90,000 3,99.7

110,000 8 99.2 It is clear from the preceding table and from FIG. 1 that the conversion of N ₂ 0 is complete, including at a high hourly volumetric speed of 90,000 h ^-1 and even 110,000 h ^-1 . The experiment was repeated with the same catalyst by adding 2000 ppm NO to the starting reaction mixture (N ₂ 0: 1000 ppm, O ₂ : 3%, H ₂ O: 15%).

The operating conditions are identical, namely a VVH of 50000 h ^-1 and a temperature of 850 ° C. The conversion of N ₂ 0 is then> 99.8% and the output is the 2000 ppm of NO which have not been broken down.

Thus, the presence of NO does not decrease the conversion rate of N ₂ 0 on the catalyst. Such a catalyst could therefore be used in a nitric acid unit, after the platinum webs, to decompose the N ₂ O without modifying the efficiency of the unit in nitric acid production.

3. Comparative test

A catalyst was prepared from a ferrierite having an Si / Al ratio of 8.8 by exchange with ammonium sulphate and then exchanged with 0.5M iron sulphate under the same conditions as for the catalyst according to US Pat. invention.

This catalyst was used under the same catalytic test conditions as for the catalyst according to the invention, except that the VVH was not increased over time.

The results obtained are shown in FIG. 2 and show that the degree of conversion of the N ₂ O decreases sharply over the course of time thus proving that the catalyst is not stable.

Claims

A process for the catalytic decomposition of N ₂ 0 in a gaseous effluent containing N ₂ 0 by passing said gaseous effluent over a zeolite type ferriérite catalyst bed exchanged, partially or totally, with a transition metal and having a Si / Al ratio between 20 and 40, at a temperature between 800 and 900 ° C, and in particular about 850 ° C.

2. Method according to claim 1, characterized in that the N ₂ 0 content of the gaseous effluent is reduced by at least 90%, advantageously by at least 95%, and even more advantageously by at least 99%. .

3. Method according to any one of claims 1 and 2, characterized in that the zeolite has an Si / Al ratio of between 22 and 35, preferably between 25 and 30, and in particular of about 27.5.

4. Method according to any one of claims 1 to 3, characterized in that the transition metal is iron or cobalt.

5. Method according to any one of claims 1 to 4, characterized in that the zeolite is exchanged with iron (II).

6. Method according to any one of claims 1 to 5, characterized in that the transition metal content in the zeolite is between 0, 1 and 3%, especially between 0.2 and 1.6%.

7. Process according to any one of claims 1 to 6, characterized in that the zeolite exchanged with a transition metal has the following formula:

or : C ^m represents a metal cation of a transition metal, advantageously chosen from Fe ²⁺ , Fe ³⁺ , Co ²⁺ and Co ³⁺ ,

m represents 1, 2 or 3,

M ^{n +} represents an alkaline or alkaline earth ion, such as Na ⁺ , K, Li or Ca, an H ⁺ ion or a mixture thereof,

n is between 1 and 2, and in particular is 1,

- is between 0.2 and 1, in particular between 0.2 and 0.8, and

x is between 20 and 40, advantageously between 22 and 35, preferably between 25 and 30, and especially about 27.5.

8. Process according to any one of claims 1 to 7, characterized in that the zeolite is in a sodium, acid or ammonium form.

9. Process according to any one of claims 1 to 8, characterized in that the gaseous effluent containing N ₂ 0 contains 200 to 2000 ppm N ₂ 0.

10. Process according to any one of claims 1 to 9, characterized in that it is carried out at a volumetric rate in h ^-1 (VVH) of between 30 000 and 120 000 h ^-1 , advantageously between 50 000 and 100,000 h ^"1.