Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Halogen or compound containing same
Reexamination Certificate
1998-11-18
2001-02-06
Wood, Elizabeth D. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Halogen or compound containing same
C502S224000, C502S229000, C502S305000, C502S313000, C502S314000, C502S315000, C502S319000, C502S320000, C502S231000
Reexamination Certificate
active
06184172
ABSTRACT:
FIELD OF THE INVENTION
The subject-matter of the present invention is a mixed fluorination catalyst based on nickel and chromium. It also relates to a process for the fluorination of halogenated hydrocarbons in the gas phase by means of hydrofluoric acid (HF) employing the said catalyst.
BACKGROUND OF THE INVENTION
The catalytic fluorination of halogenated hydrocarbons in the gas phase by HF is a well-known method of access to fluorinated hydrocarbons and in particular to hydrofluorocarbons (HFC). The latter are currently used as substitutes for chlorofluorocarbons (CFC), which are suspected of contributing to the weakening of the stratospheric ozone layer.
French Patent Application FR 2,669,022 thus makes known a mixed catalyst composed of nickel and chromium oxides, halides and/or oxyhalides deposited on a support composed of aluminium fluoride or of a mixture of aluminium fluoride and alumina.
The content by weight of nickel and chromium mentioned for the said catalyst is between 0.5 and 20% for each metal, the nickel/chromium atomic ratio being between 0.5 and 5. This catalyst is used for the preparation of 1,1,1,2-tetrafluoroethane (also known as F134a) from 1-chloro-2,2,2-trifluoroethane (F133a). F134a is an HFC, used in particular for refrigeration and freezing, which has no harmful effect on the stratospheric ozone layer.
Application FR 2,669,022 mentions maintenance of the effectiveness over time of the catalyst for a duration ranging up to approximately 400 hours of operation.
However, it is highly desirable to have available a catalyst which, under the conditions for the industrial production of fluorinated hydrocarbons, retains its activity over an operating period of several thousand hours.
The increase in the lifetime of the catalyst for such operating periods is all the more desirable since the decrease in activity, when it occurs, cannot be put right by an in situ regeneration treatment but then requires the shutdown of the production unit in order to replace the charge of the catalyst.
A catalyst which makes it possible to decrease the number of shutdowns of the production unit and which makes it possible, for a given charge of the said catalyst, to produce a greater amount of desired final product is particularly advantageous with respect to the running of the production unit and the cost price of the desired final product.
DESCRIPTION OF THE INVENTION
One aim of the invention is to provide a mixed fluorination catalyst based on nickel and on chromium which retains its activity over an operating period of several thousand hours.
Another aim of the invention is to provide a catalyst which makes it possible to carry out fluorination reactions at a higher temperature.
Another aim of the invention is to provide a fluorination catalyst for which the charge, in the industrial reactor, has to be replaced less frequently.
Another aim of the invention is to provide a more economical process for the fluorination of halogenated hydrocarbons.
Another aim of the invention is to provide a process for the manufacture of 1,1,1,2-tetra-fluoroethane (F134a) which exhibits a high degree of conversion and a high selectivity.
It has now been found that these aims can be achieved, in all or in part, by means of the fluorination catalyst and process described below.
The subject-matter of the present invention is thus, firstly, a mixed fluorination catalyst comprising one or more nickel and chromium oxides, halides and/or oxyhalides deposited on a support composed of aluminium fluoride or of a mixture of aluminium fluoride and alumina, characterized in that the weight of nickel/weight of chromium ratio is between 0.08 and 0.25, preferably between 0.1 and 0.2. The said fluorination catalyst has a lifetime which, surprisingly, is very much improved with respect to that of the catalyst taught by Patent FR 2,669,022.
The catalyst according to the invention can contain, by weight, from 0.1 to 6% of nickel salts and from 1 to 20% of chromium salts, preferably from 0.35 to 4.5% of nickel salts and from 3 to 16% of chromium salts and, more preferably still, from 1 to 2% and from 6 to 12% respectively.
The percentages indicated above are percentages by weight, expressed in the form of the equivalent weight of metal. Unless otherwise mentioned, all the percentages relating to the composition of the catalyst according to the invention are percentages by weight.
Use is preferably made, as chromium and nickel salts, of chlorides but it is also possible to employ other salts, such as, for example, oxalates, formates, acetates, nitrates and sulphates or nickel dichromate, provided that these salts are soluble in the amount of water which can be absorbed by the support.
The catalyst according to the invention can be prepared in a way known per se from an alumina. The latter can, in a first stage, be converted into aluminium fluoride or into a mixture of aluminium fluoride and alumina by fluorination using air and hydrofluoric acid, the degree of conversion of the alumina to aluminium fluoride depending essentially on the temperature at which the fluorination of the alumina is carried out (generally between 200 and 4500° C., preferably between 250 and 400° C.) . The support is subsequently impregnated using aqueous solutions of chromium trioxide, of nickel salt and of an agent which reduces the chromium, such as methanol.
When chromium trioxide (CrO
3
) is used as precursor of the chromium, this chromium can be reduced by any means known to the person skilled in the art (chemical reducing agent, thermal reduction, and the like), provided that the technique used does not harm the properties of the catalyst and thus its activity. The preferred chemical reducing agent is methanol.
The catalyst according to the invention can also be prepared by direct impregnation of the alumina using solutions of the abovementioned chromium and nickel compounds. In this case, the conversion of at least a portion (70% or more) of the alumina to aluminium fluoride is carried out during the stage of activation of the catalyst, generally carried out before it is employed.
The aluminas which can be used and which are defined below for the preparation of the catalyst according to the present invention are well-known products which are commercially available. They are generally prepared by calcination of hydrated aluminas at a temperature of between 300 and 800° C. and can comprise high contents (up to 1000 ppm) of sodium, without this harming the catalytic performance.
Before it is employed, the catalyst according to the invention has to be conditioned, that is to say converted into constituents which are active and stable (to the reaction conditions) by a prior so-called activation operation.
This treatment can be carried out either “in situ” (in the fluorination reactor) or else in appropriate equipment designed to withstand the activation conditions. The activation generally comprises one or more of the following stages:
low-temperature drying (100 to 250° C., preferably 110 to 200° C.) in the presence of air or of nitrogen,
high-temperature drying (250 to 450° C., preferably 300 to 350° C.) under nitrogen or under air,
low-temperature fluorination (180 to 300° C., preferably at approximately 200° C.) by means of a mixture of hydrofluoric acid and of nitrogen, the content of HF being controlled so that the temperature does not exceed 350° C., and
finishing under a stream of pure hydrofluoric acid or of hydrofluoric acid diluted with nitrogen at a temperature which can range up to 450° C.
During this operation, the catalytic precursors (nickel and chromium halides, chromate, nickel dichromate, chromium oxide) are converted into corresponding fluorides and/or oxyfluorides, which results in water and/or hydrochloric acid being given off.
This activation also contributes to increasing the fluorination of the alumina, when the impregnation has been carried out on an already partially fluorinated support, or, when the alumina is directly impregnated, to the fluorination of the latter. In the latter case, it is necessary to exe
Bonnet Philippe
Jorda Eric
Lacroix Eric
Elf Atochem S.A.
Smith , Gambrell & Russell, LLP
Wood Elizabeth D.
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