FR2740995A1 - Preparation of chromium oxide based catalyst used for fluorinating halohydrocarbons - Google Patents

Abstract

Chromium oxide or chromium oxides based catalysts and possibly one other catalytically active metal, of surface area greater than 100 m<2>/g are claimed. Manufacture of the catalyst is also claimed and consists of: a) forming an aqueous gelatinous precipitate of chromium hydroxide or chromium hydroxides and at least one other active metal; b) substituting water of the precipitate by a supercritical fluid gas in a continuous mode to form a porous solid and; c) calcining the resulting porous solid. Also claimed is the fluoronation of halogeno saturated or unsaturated hydrocarbons chosen from fluoro compounds in C1-C5, in the gas phase by HF, using the above catalyst.

Description

MASS CATALYZERS BASED ON CHROME OXIDE. THEIR PROCESS
OF PREPARATION AND THEIR APPLICATION TO FLUORATION
HALOGENOUS HYDROCARBONS
The present invention relates to the field of the fluorination of halogenated hydrocarbons by gas phase catalysis. It relates more particularly to bulk catalysts, fluorinated or not, based on chromium oxide or chromium oxides and optionally on at least one other catalytically active metal, their preparation process and their application to the synthesis of hydrohaloalkanes. .

Metal compounds (for example chromium, iron, cobalt, nickel, vanadium, zinc, copper, manganese, magnesium, etc.) are known for their catalytic effect in fluorination reactions of halogenated hydrocarbons. In this respect, the chromium-based derivatives, in particular of hydrated chromium trifluoride or of hydrated chromium oxide, are particularly recommended (see patents US Pat. No. 2,745,886 and US Pat. No. 3,426,009).

The fluorination catalysts proposed in the literature are either supported (mainly on carbon or alumina), or in mass.

Chromium-based mass catalysts are produced from various raw materials.

Thus, in EP 313061, a catalyst based on chromium oxide is prepared by pyrolysis of ammonium dichromate at a temperature between 500 and 700 ° C.

Patent FR 2 501 062 proposes amorphous chromium oxide microbilies obtained by gelation of a chromium hydroxide sol in a solvent which is immiscible or partially miscible with water and drying.

In EP 514 932, the preparation of an amorphous chromium oxide-based catalyst is described from a chromium hydroxide precipitate, which precipitate is obtained by mixing a chromium salt and ammonia. The product obtained after drying (70-200 "C) and calcination (380-460 C) has a specific surface of at least 170 m 2 / g.

US Pat. No. 4,912,270 proposes an airgel based on chromium oxide or hydroxide by reduction of chromium trioxide by means of an alcohol and elimination of said alcohol under supercritical conditions. The airgel obtained has a specific surface area greater than 400 m2 / g, a pore volume at least equal to 2 cm3 / g and a homogeneous distribution of the pores (mercury porosimetry).

The addition of "dopants" in chromium catalysts has been recommended to improve catalytic performance and in particular to limit side reactions such as the oxidation of hydrochloric acid -susceptible of generating chlorine which reacts with hydrogenated hydrocarbons.

Thus, in EP 546 883, there is proposed a catalyst based on chromium and nickel oxides obtained by gelation of a sol of chromium and nickel hydroxides, drying and calcination at a temperature between 250 and 450 ° C. The product obtained has an Ni / Cr atomic ratio of between 0.05 and 5.

In US 4,547,483, a chromium and magnesium catalyst is prepared by drying (20-500 "C) a paste resulting from the mixture in aqueous medium of a chromium salt and oxide or hydroxide. of magnesium.

Finally, in EP 502 605, a chromium and zinc catalyst is described obtained either by impregnation of chromium oxide with a zinc compound (halide, nitrate or carbonate), or by co-precipitation of chromium and zinc hydroxides or again by mixing and grinding insoluble zinc and chromium catalyst.

Oxygenated chromium derivatives of amorphous structure react with fluorinating agents, and more particularly with hydrofluoric acid. The rate of fluorination of these compounds decreases exponentially as a function of the rate of fluorination of the catalyst.

Under the conditions generally used in the gas phase (T <450 "C), fluorination breaks down into two stages:
- a first rapid fluorination, which makes it possible to achieve an F / Cr atomic ratio of the order of 1.5 in a few hours (fluorination rate: 50%),
- and a second step, much longer (several hundred hours), which leads to an F / Cr atomic ratio close to 3.

The fluorination of the catalyst being exothermic, it is necessary to carry out the reaction very gradually (dilution in an inert, slow and controlled increase in temperature). In addition, the reaction produces water which can, on the one hand, inhibit the catalytic activity and, on the other hand, cause, in the presence of hydracids, corrosion of the industrial tool.

For these reasons, it is generally preferred to carry out the rapid fluorination step in the absence of an organic product capable of reacting with the fluorinating agent.

The partially fluorinated solid obtained after this so-called “catalyst activation” step already catalyzes the fluorination reactions of halogenated hydrocarbons.

Due to the too long duration, the slow fluorination step often takes place, industrially, in the unit for fluorinating the organic product in the presence of the reaction mixture (organic, HF, HCl, etc.).

The perfluorination of the catalyst in the industrial reactor has drawbacks:
- it requires starting the industrial unit gradually in order to spread over time the production of water and calories due to the fluorination of the catalyst,
- it presents risks of temperature runaway with degradation of the industrial tool and the mechanical properties of the catalyst,
- finally, it can occasionally lead to large quantities of water, undesirable from the point of view of corrosion.

In order to limit the production of water and calories in the slow phase of the fluorination reaction, several solutions are proposed.

A first solution consists in using chromium catalysts having a very low reactivity with respect to the fluorinating agent, which thus makes it possible to significantly reduce, or even eliminate, the step of activating the catalyst. Such highly crystallized catalysts are for example described in patent application EP 657408 in the name of the Applicant.

A second solution recommends the use of amorphous chromium catalysts exhibiting high reactivity with regard to the fluorinating agent.

This type of catalyst is in particular proposed by GRANIER et al. (Bulletin de la Société Chimique de France, No. 34, pp. 1.91-1.96, 1984). The authors have shown that pores with a diameter of between 2 and 10 nm play a preponderant role in the fluorination reaction whereas pores smaller are only the site of irreversible reactions between the chromium and fluorine atoms (this type of fluorine atom does not intervene directly during the fluorination reaction of halogenated hydrocarbons).

It has now been found that the drying of catalysts based on amorphous chromium oxide by means of a supercritical fluid makes it possible to obtain catalysts with improved reactivity. Such catalysts, while retaining the advantages of a bulk catalyst, have a significantly improved fluorination rate during the activation step and thereby make it possible to limit the formation of water in the industrial reactor.

A subject of the invention is therefore mass catalysts based on chromium oxide or on chromium oxides and optionally on at least one oxide of another catalytically active metal, obtained by a process which consists essentially of:
a) forming an aqueous gelatinous precipitate of chromium hydroxide or chromium hydroxides and optionally of at least one other catalytically active metal, and
b) substituting the water in this precipitate with a gas in the state of a supercritical fluid to form a porous solid, and
c) calcining said solid.

A subject of the invention is also the application of these catalysts to the fluorination with HF in the gas phase of halogenated, saturated or olefinic hydrocarbons.

In the catalysts according to the invention, the specific surface is greater than 100 m2 / g and preferably 150 m2 / g and the median diameter of the pore volume distribution measured according to the nitrogen adsorption method is greater than 5 nm .

Among these catalysts, the invention relates more particularly to those whose pore volume is greater than 0.15 g / cm3 and preferably 0.18 g / cm3 (nitrogen adsorption method; pore diameter between 2 and 200 nm ).

In the catalysts according to the invention, the atomic ratio: other active metal / chromium is generally less than 1.2 and preferably between 0.1 and 1.

The gelatinous precipitate of the process according to the invention can be obtained in a manner known per se.

The precipitate can, for example, be formed by dissolving a chromium salt and a salt of at least one other metal in water, and gelling using a base.

As chromium salt, mention may be made of chromium chlorides, nitrates, sulphates, fluorides, perchlorates, acetates and acetylacetonates. Preferred are sulfates, chlorides, nitrates and acetates.

The salt of a metal other than chromium is generally chosen from salts of the same nature as those mentioned above, the metal of which belongs to the group formed by Mg,
Ni, Zn, Fe, Co, V, Mn, Cu and Al. Preferably, nitrates or chlorides of Ni are used.

As a base, mention may be made of NaOH, KOH, Ca (OH) 2, Na2CO3, NaHCO3, (NH4) 2CO3, NH4HCO3 and amines.

The gelatinous precipitate can also be formed from a sol of chromium hydroxide and hydroxide of at least one other metal, according to methods known to those skilled in the art, for example by peptization or by evaporation of part of the solvent.

As hydroxide of a metal other than chromium, there may be mentioned the hydroxides of Mg, Ni, Zn, Fe, Co, V, Mn, Cu and Al and preferably of nickel.

Various additives can also be added during the preparation of the gelatinous precipitate to improve their physicochemical and catalytic properties of the final catalyst. It is thus possible to add (% relative to the weight of the final catalyst)
- 1 to 80% of A1203, xH2O to improve the mechanical strength of the final catalyst,
- 0.1 to 5% graphite to facilitate shaping by pelletizing or extrusion,
- 0.1 to 20% of flocculant such as polyacrylates or polyacrylamides to facilitate filtration of the cake recovered after neutralization.

From the gelatinous precipitate of chromium hydroxide or chromium hydroxides and optionally from at least one other catalytically active metal, the catalyst according to the invention is prepared by drying the precipitate in a device as shown in Figure 1. .

This device consists of a drying chamber [1] heated by an oven [2]. The enclosure is supplied with supercritical fluid by means of the tube [3] connected to the reservoir [4] and equipped with the discharge tube [5] for the percolation fluid.

Under operating conditions, the aqueous gelatinous precipitate to be dried is introduced into the chamber [1]. The liquefied gas contained in the tank [4] equipped with the dip tube [6] is withdrawn by means of the pump [8]. In general, the liquefied gas is cooled beforehand before pumping by contact with the exchanger [9]. The liquefied gas from the pump is heated by means of the exchanger [10] and introduced into the chamber [1] via the tube [3]. In the enclosure [1] pressurized by means of the valve [7] and heated by the furnace [2], the pressure and the temperature are such that the fluid circulating continuously is in the supercritical state. The fluid containing the water extracted from the gelatinous precipitate is continuously discharged through the tube [5].

After an operating time which can vary from the order of one hour to several tens of hours, the gas supply is stopped and the chamber is depressurized [1]. A dried porous solid is recovered.

During drying, the gelatinous precipitate is advantageously shaped according to known techniques, for example by extrusion.

The gas used in the device is chosen from gases exhibiting a critical pressure of less than 20 MPa, and preferably 8 MPa, and a critical temperature not exceeding 200 ° C., and preferably 1600 C. CO2 is advantageously used, SO2, CH3Cl, C2H5CI and FORANESe, in particular chlorodifluoromethane, trifluoromethane and tetrafluoroethane, as well as mixtures of these gases.

The temperature of the enclosure is higher than the critical temperature of the gas used, which temperature obviously depends on the nature of this gas. By way of illustration, when carbon dioxide is used, the oven is heated to a temperature above 30 "C and preferably above 80" C.

The temperature of the exchanger [9] is generally between -5 and +15 "C.

The temperature of the exchanger [10] is generally greater than 31 "C and preferably between 80 and 1200C.

The pressure applied in the enclosure of the device is greater than the critical pressure of the gas used, which pressure depends on the nature of this gas. Generally, the pressure is greater than 8 MPa and preferably between 20 and 40
MPa.

After an operating time which can vary from the order of one hour to several tens of hours, a porous solid based on chromium oxide or chromium oxides and optionally on at least one other metal catalytically is recovered. active.

The porous solid can be shaped in a manner known per se, for example by pelletizing, extrusion or granulation. In order to facilitate shaping by pelletizing or extrusion, it is advantageous to add a quantity of graphite such that the content varies from 0.1 to 5% relative to the weight of the final catalyst.

Step c) of calcining the porous solid is generally carried out under an inert atmosphere, for example under nitrogen, at a temperature of between 200 and 500 ° C.

The mass catalysts according to the invention can be used for the fluorination with HF in the gas phase of halogenated, saturated or olefinic hydrocarbons.

They are particularly suitable for the fluorination of halogenated hydrocarbons leading to fluorinated C1 to C5 compounds containing one or more hydrogen atoms. As examples of starting halogenated hydrocarbons, mention may be made, without limitation, of the following compounds: CHCl3, CH2C12, CH2CIF, CC12 = CHCl,
CH3-CHC12, CH2CI-CH2Cl, CH2 = CHCI, CHC12-CCIF2, CHC12-CF3, CH2F-CCIF2,
CHCIF-CF3, CHF2-CCIF2, CH2CI-CF3, CC13-CH3, CC12F-CH3, CCIF2-CH3, CH3 CCl2-CH3, CC13-CF2-CH3, CC13-CF2-CHC12, CC13-CF2-CH2CI, CHC12-CHCI -CH3,
CH2Cl-CHCl-CH3 as well as CCl2 = CCl2; the latter compound does not contain hydrogen, but the addition of HF leads to hydrohalogen compounds.

To work at optimum activity, the catalyst requires treatment with hydrofluoric acid, diluted or not with nitrogen. It is generally recommended to control the exothermicity of the activation by varying the addition of an HF diluent and starting this treatment at low temperature (150-250 "C). After the adsorption peaks of l 'HF are passed, we can gradually increase the temperature to reach a maximum of 350 450 C at the end of activation.

Such partially or totally fluorinated catalysts also constitute an object of the present invention. They are characterized in that they have a specific surface area greater than 25 m2 / g, preferably 30 m2 / g, and a median pore diameter greater than 9 nm, preferably 10 nm.

In addition, they have a pore volume greater than 0.08 cm3 / g and preferably greater than 0.1 cm3 / g.

The temperature of the fluorination depends on the reaction studied and of course on the reaction products desired. Thus, for an addition of HF to a double bond or a partial substitution of the chlorine atoms by fluorine, one works at temperatures of between 50 and 350 ° C. The total substitution of the chlorine atoms generally requires temperatures of between 300 and 500 "C.

The contact time also depends on the reaction studied and the products sought. It generally varies within wide limits between 0.1 and 100 seconds. Industrially, a good compromise between conversion rate and productivity is obtained for contact times of less than 30 seconds.

The HF / organic compound molar ratio is also linked to the reaction studied. It depends among other things on the stoichiometry of the reaction. In the majority of cases, they can vary between 1/1 and 20/1, but, again, in order to obtain high productivities, it is often less than 10.

The working pressure is preferably between 1 and 20 bars absolute (0.1 to 2 MPa).

The catalysts according to the invention can, depending on their mechanical strength, work in a fixed bed or in a fluidized bed.

Catalysts whose activity has fallen as a result of fouling can be regenerated by flushing the catalyst with a compound capable of oxidizing and transforming the products (organic, coke, etc.) deposited on the catalyst into volatile products. . As such, oxygen or a mixture containing oxygen (for example air) is perfectly suitable and makes it possible to restore the initial activity of the catalyst.

It is of course necessary to control the exothermicity of this "combustion" by controlling the flow of oxygen (at the start of regeneration, significant dilution in an inert) to prevent this regeneration from running away and to avoid exceeding temperatures above 600. "VS.

To maintain the activity of the catalyst, it is also possible to carry out the fluorination reaction in the presence of oxygen and / or chlorine introduced in an oxidant / organic compound molar ratio which can range from 0.001 to 0.05 and , preferably between 0.005 and 0.03.

The following examples illustrate the invention without, however, limiting it.

In the examples, the porous mineral solid is characterized according to the following methods:
The mass loss is calculated from the weighings carried out before and after drying.

The SBET specific surface is measured by the conventional method according to Brunauer Emmet Teller.

The pore volume VpN2 is measured by determining the amount of nitrogen condensed in the pores according to the BJH method (J. Am. Chem. Soc. Vol. 73, p. 373, 1951).

The pore volume VPH9 is measured by intrusion of pressurized mercury according to the Washburn method.

The specific surface area SPH9 is calculated from the experimental data of intrusion of mercury under pressure by assuming cylindrical pores, according to the formula
SpHg = 4x103 Ej Vi / Di
in which
SpHg is the total pore surface area expressed in m2 / g
Vi is the volume of pore i expressed in cm3 / g
Di is the diameter of pore i expressed in nm.

PREPARATION OF CATALYSTS
EXAMPLE 1 Catalysts A and B
Two identical batches of catalysts A and B are prepared under the following conditions.

100 g (0.25 mol) of Cr (NO3) 3.9H2O and 59.5 g of NiCl2.6H2O are dissolved in 500 ml of water at 80 ° C. with stirring (300 RPM) for 2 hours.

After cooling, 90 ml of an ammoniacal solution (28% by weight of NH 3) are added to the solution thus obtained at a rate of 2 ml / min.

After decantation and removal of the supernatant, a gelatinous precipitate is recovered which is washed four times with 250 ml of water.

The gelatinous precipitate obtained (144.4 g) is put in the form of extrudates (diameter: 3 mm; length: 4-8 mm) which are placed in the mesh basket of the drying chamber (diameter: 35 mm; length : 120 mm) of the device of figure 1.

The drying is carried out by means of CO 2 (flow rate: 2.8 kg / h) at 110 ° C., under pressure (26 MPa) for 5.5 hours.

The extrudates are calcined under permanent nitrogen flushing under the following conditions:
- heating up to 200 C (100 "C / h),
- level at 200 C for 2 h,
-heating up to 350 C (100 C / h),
- level at 350 C for 4 h,
- cooling.

The physicochemical characteristics of the extrudates obtained after drying and of catalysts A and B (after calcination) are presented in Table 1.

The cumulative curves of the pore volumes measured by adsorption / desorption of nitrogen are presented in figure 2.

EXAMPLE 2 tcomoaraUfl catalyst C
The non-dried extrudates obtained according to Example 1 are dried in a vacuum oven at 50 ° C. under a pressure of 20 kPa for 12 hours, then calcined under the conditions of Example 1.

The physicochemical characteristics of the extrudates obtained after drying and of catalyst C (after calcination) are presented in Table 1.

The cumulative curve of the pore volumes measured by adsorption / desorption of nitrogen is presented in figure 2.

EXAMPLES OF FLUORATION
In the following examples:
- the percentages indicated are molar percentages,
- the hydrofluoric acid used is a commercial product containing only traces of water,
- the starting 1-chloro-2,2,2-trifluoroethane (F133a) is a 99.9% pure product,
- The reactor used is a 20 cm3 Inconel tube heated by means of a tube furnace.

The activation of the catalyst by HF is carried out in this reactor on a sample of 10 cm3. After drying under nitrogen (5 l / h) for 2 hours at 250 ° C., hydrofluoric acid diluted with nitrogen (molar ratio N2 / HF = 2/1) is added gradually at this same temperature.

After the peaks of exothermicity have passed, the temperature is brought to 350 ° C. and the HF flow rate to 0.2 mol / hour for 10 hours.

Before their introduction into the reactor, the reactants are mixed and heated to the reaction temperature in an Inconel preheater.

After washing with water - in order to remove the hydracids - and drying over CaCl 2, the reaction products are analyzed online, by gas chromatography.

The characteristics of catalysts A, B and C after the activation step (activated catalyst) are shown in Table 1.

EXAMPLES 3 AND 4
The fluorination tests of F133a are carried out under atmospheric pressure, in the absence of oxygen, with catalysts A, B and C. The fluorination results are summarized in Table 2.

In Example 3, Catalyst B was operated for 28 hours at 350 ° C then 24 hours at 320 ° C.

For each catalyst A, B and C recovered at the end of the fluorination tests (used catalyst), the fluorination rate (table 1) and the distribution curve of the cumulative pore volume as a function of the diameter of the pores are measured (Figure 3). ). TABLE 1

Porosity <SEP> Hg <SEP> Porosity <SEP> N2 <SEP> Rate <SEP> of
<tb> 8 <SEP> nm <D <126 <SEP> m <SEP> 8 <SEP> nm <D <2 <SEP> m <SEP> 2 <SEP> nm <D <200 <SEP> nm <SEP > fluoridation
<tb> EXAMPLE <SEP> Status <SEP> Vp <SEP> Sp <SEP> Vp <SEP> Sp <SEP> S (BET) <SEP> Sp # <SEP> VpN2 <SEP> (%) ##
<tb> (cm3 / g) <SEP> (m2 / g) <SEP> (cm3 / g) <SEP> (m2 / g) <SEP> (m2 / g) <SEP> (m2 / g) <SEP > (m2 / g)
<tb> A <SEP> Dried <SEP> extrudates <SEP> 0.288 <SEP> 18.8 <SEP> 0.222 <SEP> 18.7 <SEP> 75 <SEP> 0.115
<tb> Catalyst <SEP> 0.434 <SEP> 45.6 <SEP> 0.353 <SEP> 45.5 <SEP> 106 <SEP> 139 <SEP> 0.212
<tb> 1 <SEP> Catalyst <SEP> activated <SEP> 71.9 <SEP> 101.6 <SEP> 0.202 <SEP> 38.9
<tb> Catalyst <SEP> used <SEP> 45.2 <SEP> 62.1 <SEP> 0.143 <SEP> 89.8
<tb> B <SEP> Dried <SEP> extrudates <SEP> 0.299 <SEP> 18.5 <SEP> 0.231 <SEP> 18.4 <SEP> 82 <SEP> 60 <SEP> 0.099
<tb> Catalyst <SEP> 0.384 <SEP> 41 <SEP> 0.324 <SEP> 41 <SEP> 106 <SEP> 134 <SEP> 0.226
<tb> Catalyst <SEP> activated <SEP> 67.8 <SEP> 94.6 <SEP> 0.202 <SEP> 40.7
<tb> Catalyst <SEP> used <SEP> 40 <SEP> 55.6 <SEP> 0.142 <SEP> 82.3
<tb> 2 <SEP> C <SEP> Dried <SEP> extrudates <SEP><0.09<SEP><4<SEP><0.019<SEP><4
<tb> (compa- <SEP> Catalyst <SEP> 0.06 <SEP> 9.2 <SEP> 0.038 <SEP> 9.2 <SEP> 107 <SEP> 140 <SEP> 0.133
<tb> ratif) <SEP> Catalyst <SEP> activated <SEP> 31.6
<tb> Catalyst <SEP> used <SEP> 25.7 <SEP> 43.7 <SEP> 0.066 <SEP> 58.1
<tb> Sp *: Pore surface of diameter D calculated by the BJH method from nitrogen desorption data.

(% F mass) / 19 **: fluorination rate = X 100 (% mass Ni) X 2 / 58.7 + (% mass Cr) X 3/52 TABLE 2

EXAMPLE <SEP> 3 <SEP> EXAMPLE <SEP> 4 <SEP> (comparative)
<tb> CATALYST <SEP> A <SEP> B <SEP> C
<tb> Age <SEP> of <SEP> catalyst <SEP> (hours) <SEP> 28 <SEP> 96 <SEP> 28 <SEP> 52 <SEP> 24 <SEP> 48
<tb> Molar <SEP> ratio <SEP> HF / F <SEP> 133a <SEP> 4.1 <SEP> 4 <SEP> 4 <SEP> 4 <SEP> 3.7 <SEP> 3.8
<tb><SEP> time of <SEP> contact <SEP> (second) <SEP> 0.5 <SEP> 0.5 <SEP> 0.5 <SEP> 0.5 <SEP> 0.5 <SEP > 0.5
<tb> Temperature <SEP> (C) <SEP> 350 <SEP> 350 <SEP> 350 <SEP> 320 <SEP> 350 <SEP> 350
<tb><SEP> rate of <SEP> transformation <SEP> of <SEP> 17.7 <SEP> 16.9 <SEP> 16.8 <SEP> 7.1 <SEP> 10.4 <SEP> 10 , 2
<tb> F133a <SEP> (%)
<tb> Selectivity <SEP> F134a <SEP> (CF3-CH2F) <SEP> 98.1 <SEP> 98.2 <SEP> 98.9 <SEP> 98.1 <SEP> 98.1
<tb> (%)
<tb> Selectivity <SEP> F1122 <SEP> (CF2 = CHCl) <SEP> 1.3 <SEP> 1.4 <SEP> 1.4 <SEP> 1.0 <SEP> 1.5 <SEP> 1 , 7
<tb> (%)
<tb> Selectivity <SEP> series <SEP> F120 <SEP>(#)<SEP> (%) <SEP> 0.3 <SEP> 0.1 <SEP> 0.1 <SEP> 0.1 <SEP > 0.4 <SEP> 0.2
<tb> Selectivity <SEP> F143a <SEP> (CF3-CH3) <SEP> 0.1 <SEP> 0.2 <SEP> 0.2 <SEP> traces <SEP> traces <SEP> traces
<tb> (%)
<tb> Selectivity <SEP> 0.2 <SEP> 0.1 <SEP> 0.1 <SEP> traces <SEP> traces <SEP> traces
<tb> (*) series F 120: all saturated haloethanes having a single hydrogen atom.

Claims (3)

1. Mass catalysts based on chromium oxide or chromium oxides and optionally on at least one other catalytically active metal, partially or totally fluorinated, characterized in that they have a specific surface area greater than 25 m2 / g and a median pore diameter greater than 9 nm. 2. Catalysts according to claim 1 characterized in that the surface is greater than 30 m2 / g. 3. Catalysts according to one of claims 1 or 2 characterized in that they have a pore volume greater than 0.08 cm3 / g.