Catalyst for the fluorination of halogenated organic compounds

Details Catalyst for the fluorination of halogenated organic compounds Catalyst for the fluorination of halogenated organic compounds

1999-05-05

2001-10-09

Siegel, Alan (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Halogen containing

C570S167000, C570S168000, C570S169000

Reexamination Certificate

active

06300530

ABSTRACT:

The present invention relates to the preparation of an improved catalyst for the fluorination of halogenated organic compounds with anhydrous gaseous HF.
More specifically, the present invention relates to a catalyst for the fluorination of HCFC hydrohalocompounds of the 120 and 130 series having higher selectivity and conversion. Specifically those of the 120 series have the general formula C
2
HX
5
(monohydropentahaloethanes) wherein x equal or different from each other can be either fluorine or chlorine or bromine, provided that there is at least a fluorine atom and an halogen different from fluorine. These compounds are commercially known as HFC/HCFC “120 series”.
In particular for the 130 series the general formula is CF
3
CH
2
X wherein X has the above meaning. These products are commercially well known as products of the 130 series, in particular 133a wherein X=Cl.
Generally the present invention relates to a catalyst for the fluorination of HCFC hydrohalocompounds of the C
2
H
n
X
6-n
series wherein x equal or different from each other can be either fluorine or chlorine or bromine, provided that there are at least a fluorine atom and at least an halogen atom different from fluorine, and n is an integer from 1 to 4, preferably it is equal to 1 or 2.
Specifically the catalyst according to the invention is particularly suitable to the synthesis of hydrohalocompounds having 4 or 5 fluorine atoms starting from the corresponding precursors having 3 or 4 fluorine atoms. For the fluorination of the 120 series one specifically refers to the fluorination of 123 having the formula CHCl
2
—CF
3
to obtain 124 having the formula CHClF—CF
3
and/or 125 of formula CHF
2
—CF
3
. For the 130 series the invention preferably relates to the fluorination of 133a having the formula CH
2
Cl—CF
3
to obtain 134a having the formula CH
2
F—CF
3
. It is well known the industrial utility to have available efficient catalysts for said reactions, for example for preparing HCFC 123, 124 and 133a and HFC 125 and 134a, not dangerous for the ozone, which replace the chlorofluorocarbons (CFC) banned by the Montreal Protocol: see for instance U.S. Pat. Nos. 4,967,023, 5,008,475, EP 408,005, WO 90/08755.
Most of these processes use a catalyst in heterogeneous phase formed by a compound of the trivalent chromium, sometimes supported on a suitable support such as alumina, aluminum fluoride or carbon.
The fluorination of hydrohalocompounds undergoes kinetic and/or thermodynamic limitations which make it necessary the use of efficient and selective catalysts.
It has been unexpectedly and surprisngly found a catalyst as defined below, its preparation process and a process for the fluorination of halogenated organic compounds with anhydrous gaseous HF capable to operate with high selectivity and conversion.
An object of the present invention is a catalyst comprising a supported Cr(III) amorphous compound, characterized in that the support is formed by an aluminum trifluoride (AlF
3
) having an high surface area obtainable by alumina fluorination with gaseous HF having a surface area of at least 150 m
2
/g.
An object of the present invention is therefore a catalyst comprising a supported chromium (III) amorphous compound, characterized in that the support is formed by an aluminum trifluoride AlF
3
or fluorinated alumina having an high surface area obtainable by alumina fluorination with gaseous HF having a surface area of at least 150 m
2
/g, characterized in that the alumina is fluorinated with HF at an initial temperature lower than 300° C., preferably in the range 100°-280° C., still more preferably in the range 150°-200° C., the temperature is rised with a temperature gradient≦100° C./hour up to the final temperature>320° C. and<450° C., preferably in the range 350°-400° C.; then the fluorination is continued at the final temperature until feeding a HF molar amount at least equal to the stoichiometric with respect to the alumina, preferably 1.3 times higher than the stoichiometric, the fluorination being continued until a fluorinated alumina having a fluorine content not lower than 95% of the stoichiometric is obtained.
Preferably the HF flow is diluted with air or inert gas, more preferably air, in volume ratios HF/diluent 0.1:1 to 1:1.
Preferably the thermal gradient is 10°-90° C./hour, more preferably 20°-50° C./hour.
The AlF
3
or the fluorinated alumina obtainable with the above process has a surface area higher than that obtainable by direct fluorination of the precursor at the final temperature. According to another aspect of the invention, the alumina to be fluorinated is brought to the initial fluorination temperature and it can be partially fluorinated at this temperature before starting the gradient up to the final temperature, it is then allowed to fluorinate at the final temperature until a fluorinated alumina with a fluorine content not lower than 95% of the stoichiometric is obtained.
The total pressure has no important effects and one generally operates at atmospheric or at slightly higher pressure, generally of some atmospheres.
It is on the contrary advantageous that the HF partial pressure is low, especially at the beginning of the fluorination, to moderate the heat development which could locally increase the temperature over the above limits. Indeed two highly exothermic phenomena contemporaneously occur: the reaction between HF and alumina with formation of AlF
3
and water; and the unreacted HF hydration by water. To moderate this exothermic process it is sufficient to use HF diluted with an inert gas in the fluorination conditions, for example, air or nitrogen, with the indicated HF/diluent ratio by volume.
A better control of the temperature is achieved also by carrying out the reaction in a fluidized bed and this is the preferred way to carry out the fluorination. In this case the aluminas to be fluorinated have a particle size distribution compatible with the use of fluidized beds.
When the aluminas are in hydrated form, it is preferable to precede the fluorination with a calcination phase in air or nitrogen, at a temperatures between 300° C. and 400° C. This limits the water development during the reaction, which is undesirable especially as it favours the plants corrosion.
The preferred aluminas for the fluorination have pseudobohemite crystalline structure, surface area of about 300 m
2
/g.
The aluminas and the aluminum fluorides are characterized by techniques well known to the skilled in the art of the solid characterization: the surface area (SA) is measured by nitrogen adsorption according to the BET method. The analytical composition is obtained by wet way according to known methods.
The aluminas to be fluorinated can optionally comprise up to 15% by weight of silicon oxide, preferably from 1 to 5%.
The chromium amount is in the range from 1 to 20% by weight, preferably from 5 to 15% by weight.
The catalyst preparation can be made with known methods in the art, among which the Applicant has found particularly suitable the one defined by the English term “incipient wetness”, hereinafter described. However any other suitable method can of course be used, as known to the skilled in the art of the catalyst preparation.
The preferred general procedure for preparing the catalyst according to the above method comprises the step of impregnating a determined amount of support obtainable with the above described method with a concentrated solution of a Cr(III) soluble salt, for instance chloride. The volume of the impregnating solution is equal to or lower than the volume of the support pores, in order to avoid the adhesion among the granules of the same.
A first drying treatment is then carried out at a moderate temperature—for instance 120° C.—to evaporate water and allow the salt deposition. When necessary, this procedure is repeated many times until the desired amount of metals on the support is reached.
After the final drying the catalyst is transferred into a tubular reactor and calcined for some hours at 300-400° C. in inert gas flow, for example, nitrogen.

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