Hydrodealkylation of Alkylaromatic Hydrocarbons

The preparation of a catalyst (a) according to the present invention containing 0.5%, by weight, of rhodium deposited on a support consisting of stoichiometric magnesium aluminate is described in this example.

The support of stoichiometric magnesium aluminate was prepared as follows: a solution of aluminum isopropylate and magnesium acetyl acetonate in isopropanol containing 7%, by weight, of aluminum and 2.75%, by weight, of magnesium (corresponding to the stoichiometric ratio in the aluminate MgO-Al.sub.2 O.sub.3) was introduced into an agitated autoclave and then hydrolyzed in the cold by adding an equivalent volume of an aqueous solution of isopropanol which contained an amount of water which was just sufficient to decompose the two organo-metal salts and form the hydroxides of the aluminum and the magnesium. The entire mixture was then heated in the autoclave to a temperature of 250.degree. C. for several hours and was then subjected to a hypercritical drying at this temperature, whereby the pressure was released to normal atmospheric pressure.

By subjecting the resulting powder to an oxidizing treatment at a temperature of 600.degree. C. for 3 hours, a finely crystallized magnesium aluminate was obtained, the crystalline structure of which was visible under X-ray irridiation, and the specific surface of which (determined according to the BET method) was 450 m.sup.2 /g, and the pore volume of which was 0.5 cm.sup.3 /g.

This crystalline aluminate powder was subsequently formed into pellets according to conventional methods and then was impregnated with a rhodium chloride solution.

The impregnation was effected by treating 100 g of said support material with 50 ml of an aqueous solution of rhodium trichloride containing 0.5 g of rhodium. After 3 hours of contact, the impregnated pellets were dried at 120.degree. C. and then calcined under air at a temperature of 600.degree. C. for 3 hours.

In this example, the preparation of two catalysts (B) and (C) according to the present invention comprising 0.5 or 1%, by weight, respectively, of rhodium deposited on a support of alumina balls the surface of which is made of copper aluminate is described.

Said support was prepared by impregnating 100 g of alumina which were obtained according to the process disclosed in U.S. Pat. No. 2,915,365, and which exhibited a specific surface of 250 m.sup.2 /g and a pore volume of 0.60 cm.sup.3 /g, with an aqueous solution of copper nitrate containing 4 g of the metal calculated as copper oxide. After several hours of contact, the impregnated alumina balls were dried at 120.degree. C., then calcined under air at a temperature of 900.degree. C. for 3 hours.

The data of the X-ray diffraction of the thus prepared support, indicated the presence of a spinel phase corresponding to a stoichiometric copper aluminate of the formula CuAl.sub.2 O.sub.4 at the outsides of the alumina.

100 g each of the support are than impregnated with 60 ml of an aqueous solution containing either 0.5 or 0.1% of rhodium in the form of rhodium trichloride. After 3 hours of contact, the balls are dried at 120.degree. C., and then calcined under air for 3 hours.

In this example, the procedure of Example 2 was repeated yet using as a starting support material alumina having a tetragonal structure. This particular alumina was prepared according to the French Pat. No. 1,449,904, by autoclaving agglomerates of active alumina in the presence of acetic acid and subsequent drying and calcination, and exhibited a specific surface of 120 m.sup.2 /g, a pore volume of 0.6 cm.sup.3 /g.

The resulting catalyst (D) contained 0.5%, by weight, of rhodium deposited on a support the surface of which was made of copper aluminate and was distinguished from the catalyst (B) only by the type of alumina which was used to prepare the catalyst support according to the present invention.

In this example, the procedure of Example 2 was repeated. Yet, impregnating the 100 g of active alumina which was obtained according to the method disclosed in the U.S. Pat. No. 2,915,365, by an aqueous solution of nickel nitrate, containing 4 g of the metal calculated as nickel oxide.

The data of the X-ray diffraction of the support which was obtained after impregnating, drying at 120.degree. C. and then calcining at a temperature of 900.degree. C. under air for 3 hours, indicated the presence of a non-stoichiometric nickel aluminate of spinel structure corresponding to the formula 2NiAl.sub.2 O.sub.4.7Al.sub.2 O.sub.3.

By impregnating this nickel aluminate containing support with a solution of rhodium trichloride, a catalyst (E) according to the present invention containing 0.5%, by weight, of rhodium was obtained.

In this comparative example, the preparation of five catalysts (F), (G), (H), (I), and (J) are described which comprised only rhodium as an active element and wherein different alumina supports were used.

These supports corresponded to:

an active alumina obtained according to the process described in the U.S. Pat. No. 2,915,365, for the catalysts (F) and (G).

an active alumina prepared by treatment in an autoclave in the presence of acetic acid according to the process disclosed in the French Pat. No. 1,449,904, for the catalyst (H).

an alumina of alpha structure exhibiting a specific surface of 70 m.sup.2 /g and a pore volume of 0.60 cm.sup.3 /g for the catalyst (I).

an alumina of gamma cubic structure exhibiting a specific surface of 200 m.sup.2 /g and a pore volume of 0.55 cm.sup.3 /g for the catalyst (J).

The latter alumina is conventionally used as a catalyst support for catalysts for catalytic reforming processes.

The catalysts (F), (G), (H), (I), and (J) were prepared by impregnating the above cited supports each with 60 ml of a rhodium trichloride solution containing 0.5 g of rhodium in the form of rhodium trichloride for the catalysts (F), (H), (I), and (J), and 0.1 g of rhodium in the form of rhodium trichloride for the catalyst (G).

They are then dried at a temperature of 120.degree. C. and calcined under air at a temperature of 600.degree. C. for 3 hours.

In order to better evaluate the advantages which are achieved by using aluminate containing supports as supports for hydrodealkylation catalysts, on the one hand, the catalysts (A), (B), (D) and (E) according to the present invention were compared with the catalysts (F), (H), (I), and (J) and, on the other hand, the catalyst (C) according to the present invention was compared with the catalyst (G).

In this example, the preparation of five catalysts designated (K), (L), (M), (N), and (O) which demonstrate the use of bi-metal compositions of rhodium and palladium, which are, according to the present invention, deposited on a support the surface of which is comprised of an aluminate of either magnesium, cobalt, nickel, zinc, or iron.

The supports were prepared according to the following method: portions of 100 g of the active alumina described above in Example 2, each were impregnated with a solution of either magnesium nitrate, cobalt nitrate, nickel nitrate, zinc nitrate or iron nitrate in order to deposit thereon an amount of the salt corresponding to 4%, by weight, of the respective metal oxide. The material was allowed to ripen for several hours, then dried at 120.degree. C., and then the cobalt-, nickel-, zinc-, or iron-containing supports were calcined at 800.degree. C. for 3 hours, and the magnesium-containing support was calcined at 1,000.degree. C. for 3 hours. 100 g of each of the thus prepared supports were subsequently impregnated with an aqueous solution of rhodium trichloride and palladium chloride containing 0.15 g of rhodium and 0.10 g of palladium. The resulting catalyst which was obtained after ripening, drying and calcining contained 0.15%, by weight, of rhodium and 0.10%, by weight, of palladium deposited on the support according to the present invention.

The bi-metal catalyst (P) which is described in this example, is given as a comparative catalyst for comparison with the catalysts (K), (L), (M), (N), and (O). It differs from the latter by the type of the support which was used, but not with regard to the active metal component, which also comprised 0.15%, by weight, of rhodium and 0.10%, by weight, of palladium. Yet, the support was an active alumina having a specific surface of 10 m.sup.2 /g and a pore volume of 0.60 cm.sup.3 /g.

The method which was used to prepare the catalyst, comprised impregnating the alumina with an aqueous solution of rhodium trichloride and palladium chloride, 3 hours of ripening, drying at 120.degree. and calcining at 600.degree. C. for 3 hours.

Here, for comparative purposes, a catalyst (Q) is described which contained 4%, by weight, of nickel oxide deposited on a silica support having a specific surface of 200 m.sup.2 /g, and a total pore volume of 1 cm.sup.3 /g.

The catalyst was prepared by impregnating the silica support with a solution of nickel nitrate, subsequently drying at 120.degree. C. and calcining at 600.degree. C. for 3 hours. After this thermal treatment, the nickel is present in free oxide form which is reducible under hydrogen at 600.degree. C.

Its catalytic behavior which is described further below can be compared with that of the catalyst having a support the surface of which is nickel aluminate as described in Example 5, and wherein the nickel which is completely bound to the alumina of the starting support material after 3 hours of calcination at 800.degree. C. exhibits no activity under the reaction conditions which are used in the text for hydrodealkylating toluene into benzene, which is described below.

The catalyst (R) which is described in this example, represents a conventional prior art catalytical formulation which gives good results only at more elevated reaction temperatures and pressures than those used in the process according to the present invention. This catalyst (R) contained 10%, by weight, of chromium oxide, Cr.sub.2 O.sub.3, and was prepared by impregnating an alumina of a gamma cubic structure having a specific surface of 200 m.sup.2 /g and a pore volume of 0.55 cm.sup.3 /g with a solution of chromic acid. After impregnating, the catalyst is subjected to drying at 120.degree. C. and then calcined under air for 3 hours.

The catalysts (A), (B), (C), (D), (E), (F), (G), (H), (I), (J), (K), (L), (M), (N), (O), (P), (Q), and (R), prepared according to the Examples 1-9, all were subsequently reduced under a hydrogen blanket at 550.degree. C. for 2 hours, and then were tested in the absence of water vapor in a pilot plant assembly for hydrodealkylating toluene into benzene under the following reaction conditions:

Pressure: 10 bar

Temperature: 570.degree. C.

Molar ratio hydrogen/toluene: 5

Liquid space velocity: 4 volumes of liquid per volume of catalyst per hour.

The conversions, selectivities and yields which were obtained after 3, 24 and 76 hours of continuous reaction, are given in Table I.

For simplifying purposes, in the Table I below, the active alumina support material which was obtained according to the process described in the U.S. Pat. No. 2,915,365, will be designated "flash alumina". In the same manner, the alumina having a gamma tetragonal structure which was prepared according to the process which is disclosed in the French Pat. No. 1,449,904 will be designated "autoclaved alumina".

The tests results which are given in the Table I below demonstrate the superior performance of the catalysts (A), (B), (C), (D), and (E) according to the present invention, as compared with that of the catalysts (F), (G), (H), (I), and (J). It is particularly apparent that in the reaction of hydrodealkylating toluene into benzene that catalysts which were prepared by depositing rhodium on a support the surface of which was comprised of a metal aluminate led to much superior yields than those which were obtained by depositing the same amount of rhodium on an alumina support.

In the same way, the comparison of the catalysts (K), (L), (M), (N), and (O) according to the present invention with the catalyst (P) leads to the same conclusions as above, with regard to bi-metal rhodium-palladium formulations using lower contents of noble metals.

Furthermore, under these test conditions, are noted, on the one hand, a relatively strong initial activity and a very poor selectivity of the catalyst (Q) wherein the nickel is not present in the form of an aluminate and, on the other hand, a very poor hydrodealkylating activity of the prior art catalyst (R) comprising chromium oxide.

While the invention has now been described with reference to certain preferred embodiments, the skilled artisan will readily appreciate that various modifications, changes, substitutions, and omissions, may be made without departing from the spirit thereof. Accordingly, it is intended that the scope of the present invention be limited solely by that of the following claims.