GB744467A - Improvements in and relating to the isomerization of dialkyl aromatic hydrocarbons - Google Patents

Abstract

Dialkyl aromatic hydrocarbons are isomerized by a continuous process which comprises subjecting the hydrocarbon to a temperature of 350-550 DEG C. in the vapour phase under a total pressure of 10 to 80 atmospheres and a hydrogen partial pressure which is preferably at least 20 atmospheres, the hydrogen: hydrocarbon molar ratio being at least 2 : 1, and the degree of attainment of equilibrium being maintained at 50 to 80 per cent by controlling the liquid space velocity at or above 2 volumes of hydrocarbon per bulk catalyst volume per hour, in the presence of a hydrogenation catalyst comprising from 1 to 10 per cent by weight of nickel or from 0.05 to 4.0 per cent by weight of platinum or palladium supported on a composite, preferably a gel, having catalytic activity for cracking hydrocarbons and low alkali content, selected from silica and alumina or silica and an oxide or oxides selected from, magnesia, titania, zirconia, chromia, and activated aluminas comprising gamma-alumina, the weight of catalytic metal being calculated on the dry weight of the composite. A suitable alumina is one which has been treated with hydrofluoric acid. Suitable starting materials are xylenes, and diethyl, di-n-propyl, di-isopropyl, dibutyl and di-isobutyl benzenes. The process is particularly useful for producing p-xylene from mixtures of xylenes containing up to 30 per cent by weight of ethyl benzene. The preferred proportion of platinum or palladium in the catalyst is 0.1 to 1.0 per cent by weight of the composite. The preferred composite contains alumina and silica at a weight ratio in the range 5 : 95 to 50 : 50, and with a surface area of 150 to 500 sq. metres per gram. The catalytic metal may be applied to the composite by impregnation or precipitation (see Group III). The platinum catalyst may be ignited at 300 DEG to 600 DEG C., and the nickel catalyst may be reduced with hydrogen at 450-550 DEG C., prior to use. Preferably the process is operated at a temperature of 410-480 DEG C., a hydrogen partial pressure of 20 to 40 atmospheres gauge and a xylene pressure of 1 to 5 atmospheres, the hydrogen : hydrocarbon ratio not exceeding 20 : 1. The amount of hydrogenation of aromatics in the process is generally less than 2 per cent. As the catalyst activity decreases, it is preferred to raise the temperature to maintain constant conversion. For the production of p-xylene from a xylene mixture, fresh feed is preferably mixed with a xylene fraction separated from the isomerization products, the mixture is fed to a freezing unit to remove p-xylene, and the residue is compressed, mixed with recycle hydrogen, then heated to the desired temperature and fed as vapour to the isomerization reactor. In examples: (1) to (3) mixtures of xylenes predominating in o- and m-xylene, and containing ethyl benzene, are isomerized over nickel catalysts to produce mixtures with increased p-xylene content; (4) p-xylene is isomerized over a platinum catalyst, showing the decrease in activity of the catalyst with time. Specification 695,947 is referred to. According to the Provisional Specification, the catalyst comprises 0.05-20 per cent by weight of a metal of Groups VIII and 1B, additional specified metals being iron, cobalt and copper. The treatment of alumina-silica gels with silicon tetrafluoride is also mentioned.ALSO:A catalyst for use in isomerization of dialkyl aromatic hydrocarbons comprises from 1 to 10 per cent by weight of nickel or from 0.05 to 4.0 per cent by weight of platinum or palladium supported on a composite, preferably a gel, having catalytic activity for cracking hydrocarbons, and low alkali content, selected from silica and alumina, or silica and an oxide or oxider selected from magnesia, titania, zirconia, chromia and activated aluminas comprising gamma-alumina, the weight of catalytic metal being calculated on the dry weight of the composite. A suitable alumina is one which has been treated with hydrofluoric acid. The preferred proportion of platinum or palladium is 0.1 to 1.0 per cent by weight of the composite. The preferred composite contains alumina and silica at a weight ratio in the range 5 : 95 to 50 : 50 and with a surface area of 150 to 500 sq. metres per gram. The catalytic metal may be applied to the composite by mixing an aqueous metal salt solution with freshly precipitated alumina-silica gel and precipitating metal ions thereon; e.g., by treating with aqueous nickelous nitrate and precipitating basic nickel carbonate by adding a small excess of aqueous ammonium carbonate, or by treating with hydrochloraplatinc acid and precipitating platinum by adding a mixture of formaldehyde and caustic soda. The catalytic metal may also be applied by impregnating a wet gel with a salt of the catalytic metal. The platinum catalysts may be ignited at 300 DEG to 600 DEG C., and the nickel catalysts may be reduced with hydrogen at 450 DEG to 500 DEG C., prior to use. The latter procedures may be performed in the reactor. According to the Provisional Specification, the catalyst comprises 0.05-20 per cent by weight of a metal of Groups VIII and IB, additional specified metals being iron, cobalt and copper. Treatment of the aluminasilica gels with silicon tetrafluoride is also mentioned.