Fluorination catalyst and process

Details Fluorination catalyst and process Fluorination catalyst and process

1999-03-08

2002-06-11

Wood, Elizabeth D. (Department: 1755)

Catalyst, solid sorbent, or support therefor: product or process

Catalyst or precursor therefor

Metal, metal oxide or metal hydroxide

C502S305000, C502S306000, C502S319000, C570S164000, C570S165000, C570S169000, C570S170000

Reexamination Certificate

active

06403524

ABSTRACT:

This invention relates to a fluorination catalyst and the production and use thereof and particularly to an improved fluorination catalyst based on chromia, a process for producing the catalyst and a fluorination process using the catalyst.
Fluorination processes comprising reaction of a starting material with hydrogen fluoride to introduce one or more fluorine atoms into the starting material are well known and are used extensively in industry. Vapour phase processes in which the starting material and hydrogen fluoride are reacted in the vapour phase at elevated temperature are common and such processes usually employ a fluorination catalyst which often is a catalyst comprising or based on chromia which has been subjected to a pretreatment with hydrogen fluoride to provide the working catalyst. It is generally accepted that chromium oxide catalysts of high surface area and wherein the chromium is present as chromium (III) have high initial activity and that such active chromia catalysts are in an amorphous or essentially amorphous state. A recent development in chromia catalysts is a catalyst of enhanced activity produced by incorporating an activity-promoting amount of a divalent metal oxide such as an oxide of zinc, nickel or cobalt, especially zinc, in the catalyst, the oxide or at least the chromia remaining in the essentially amorphous state and having a large surface area. Catalysts containing other divalent metal oxides such as magnesia have also been proposed.
When used in the production of hydrofluorocarbons [HFCs], the known chromia catalysts and especially those promoted by a divalent metal such as zinc have a high initial activity and can result in high conversions and high selectivities. They suffer from a progressive reduction in activity due to deposition of coke on the catalyst but they can be regenerated a number of times by heating in an oxygen-containing atmosphere such as air or a mixture of air with hydrogen fluoride and have a reasonable and generally acceptable lifetime. However, the catalysts suffer the disadvantage that they are not particularly robust, especially in respect of chemical robustness and are deteriorated under the conditions of use and especially when subjected to high temperatures in the presence of hydrogen fluoride so that their lifetime leaves something to be desired.
The present invention is based on the discovery that the robustness of chromia—based catalysts and hence their useful working lifetimes is increased by inducing or introducing crystallinity and preferably a controlled degree of crystallinity into the chromia. Moreover, the initial activity of the catalysts can be slightly but significantly enhanced, without a reduction in selectivity, by introducing an activity-promoting amount of zinc or a compound of zinc into the catalyst.
According to the first aspect of the invention there is provided an improved chromia-based fluorination catalyst wherein the chromia is at least partially crystalline.
Preferably, the chromia exhibits an apparent degree of crystallinity as represented by alpha chromia type crystals greater than 8%, preferably greater than 20%, and less than 50% by weight.
Introducing crystallinity into the chromia results in a decrease in the surface area of the catalyst and too high a degree of crystallinity results in an unacceptably low surface area, for example below 20 m
2
/gm. The degree of crystallinity in the catalyst can be controlled so as to result in a catalyst having a surface area greater than about 20 m
2
/gm, preferably from about 30 to about 70 m
2
/gm.
According to a further aspect of the invention, there is provided an improved zinc-promoted chromia fluorination catalyst wherein the chromia is at least partially crystalline and the catalyst comprises zinc or a compound of zinc in an amount of less than about 3% by weight of the catalyst.
In a further aspect of the invention there is provided an improved zinc-promoted chromia-based fluorination catalyst wherein the chromia is at least partially crystalline produced by inducing crystallinity in chromia and subsequently introducing zinc or a compound of zinc into the crystallised chromia by impregnation with a solution of a soluble zinc salt. The catalyst preferably contains from 0.1% to about 2% by weight of zinc or a compound of zinc depending upon the degree of crystallinity induced in the chromia.
Inducing crystallinity in the chromia results in a decrease in the surface area of the catalyst and a very high a degree of crystallinity results in a very low surface area, for example below 10 m
2
/gm. The degree of crystallinity in the catalyst of the invention can be controlled such that the catalyst has a surface area greater than about 20 m
2
/gm, preferably from about 30 to about 70 m
2
/gm.
Suitably, the catalyst according to the first aspect of the invention contains zinc or a compound of zinc. A catalyst according to the invention may contain an activity-promoting amount of a divalent metal such as cobalt, magnesium or nickel or a compound thereof in addition to or instead of zinc or a zinc compound. Nevertheless, the preferred metal is zinc and in this case the amount of the zinc is important since it is known that zinc can act as a catalyst poison if present in too large an amount. We have found that whilst the activity-promoting amount of zinc in catalysts wherein the chromia is amorphous is generally greater than about 2% by weight and usually greater than about 5% by weight depending upon the method of production of the catalyst, the activity promoting amount of zinc in the partially crystallised catalysts of the invention should generally be less than about 2% by weight, preferably no greater than about 1% by weight.
According to a preferred embodiment of the invention there is provided a chromium-based fluorination catalyst comprising from 0.1 to 2% by weight of zinc or a compound of zinc wherein the chromia is at least partially crystalline. The catalyst preferably has an apparent degree of crystallinity as represented by alpha chromia type crystals of from about 8% to about 50% and has a surface area greater than about 20 m
2
/gm.
If present, the amount of divalent metal other than zinc in the catalyst, whether the divalent metal be an activity promotor or not, is not critical since such metals are not generally regarded as catalyst poisons even if present in large amounts. The amount of such metals may vary over a wide range up to 50% by weight or even higher of the catalyst, although the amount will usually be in the range from about 5% to about 25% by weight.
The apparent degree of crystallinity or the degree of crystallinity induced in the chromia is determined by X-ray diffraction analysis using the standard NIST [National Institute of Standards and Technology] technique and comparing the result with that obtained by analysis of a pure alpha chromia standard prepared by sintering chromia at 1223 k in air for 24 hours (100% crystallinity). The catalysts do not have a true alpha chromia structure so that the % degree of crystallinity determined by comparison with the results for pure alpha chromia is not a true % degree of crystallinity and therefor is referred to herein as the “apparent degree of crystallinity”. Morover, since the catalyst structure is not true alpha chromia so that the X-ray diffraction peak tends to be slightly distorted, the apparent degree of crystallinity is expressed herein as being represented by “alpha chromia type crystals”.
The apparent degree of crystallinity as represented by alpha chromia type crystals is determined by measuring the integrated area of the 104 peak of both the catalyst sample and the pure alpha chromia standard (at ca. 33.6 °20 for Cu K radiation) between 32.5 and 35.0°20, subtracting the background to provide corrected integrated areas and then ratioing the corrected area for the catalyst sample to the corrected area for the standard sample.
The catalyst exhibits an X-ray diffraction peak at a spacing of lattice planes from 2.65 to 2.7 of half maximum peak width

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