The prior art has recognized that adsorbents composed of zeolites X or Y exchanged by means of ions such as barium, potassium or strontium, alone or as a mixture, are effective in selectively adsorbing para-xylene in a mixture comprising at least one other C8 aromatic isomer. Patents U.S. Pat. No. 3,558,730, U.S. Pat. No. 3,558,732, U.S. Pat. No. 3,626,020 and U.S. Pat. No. 3,663,638 disclose adsorbents comprising aluminosilicates exchanged by barium and potassium which efficiently separate para-xylene from a mixture of C8 aromatic isomers.
U.S. Pat. No. 3,878,127 discloses a method for the preparation of adsorbents intended for the separation of xylenes, which consists in treating, in sodium hydroxide solution under hot conditions, agglomerates (zeolite X+binder) with an Na2O/Al2O3 ratio of rigorously less than 0.7, in order to replace the exchangeable cations of the zeolite (such as protons or cations from Group IIA) with sodium prior to a barium or barium+potassium exchange, the preliminary exchange with sodium allowing a larger amount of barium or barium+potassium ions to be added to the zeolitic structure.
These adsorbents may be used as adsorption agents in liquid-phase processes, preferably of simulated countercurrent type, similar to those disclosed in U.S. Pat. No. 2,985,589, which apply, inter alia, to C8 aromatic fractions resulting, for example, from processes for the dialkylation of benzene in gas-phase processes.
Barium-exchanged zeolites X have numerous other applications as adsorption agents, among which may be mentioned:                the separation of sugars, see, for example, EP 115,631 or EP 115,068,        the separation of polyhydric alcohols (EP 137,063),        the separation of substituted toluene isomers, U.S. Pat. No. 4,642,397 (nitrotoluene), U.S. Pat. No. 4,940,548 (diethyltoluene) or U.S. Pat. No. 4,633,018 (toluenediamine),        the separation of cresols (U.S. Pat. No. 5,149,887).        
In the above-listed references, the zeolitic adsorbents are provided in the form of a powder or in the form of agglomerates predominantly composed of zeolite having at least 15 to 20% by weight of inert binder and their Dubinin volume measured by nitrogen adsorption at 77° K after degassing under vacuum at 300° C. for 16 h, is inferior to 0.230 cm3/g.
As the synthesis of zeolites X is predominantly carried out by nucleation and crystallization of silicoaluminate gels, powders are obtained which are particularly difficult to employ on an industrial scale (significant pressure drops when the powders are handled) and granular agglomerated forms are preferred. These agglomerates, whether in the form of blocks, balls or extrudates, are usually composed of a zeolite powder, which constitutes the active component, and of a binder intended to ensure the cohesion of the crystals in the form of grains. This binder has no adsorbent property, its function being to confer on the grain sufficient mechanical strength to withstand the vibrations and movements to which it is subjected during its various uses. The agglomerates are prepared by thickening zeolite powder with a clay paste, in proportions of the order of 80 to 85% of zeolite powder per 20 to 15% of binder, then shaping as balle, blocks or extrudates and heat treating at high temperature in order to bake the clay and reactivate the zeolite, it being possible for exchange with barium to be carried out either before or after the agglomeration of the pulverulent zeolite with the binder. This results in zeolitic bodies with a particle size of a few millimetres which, if the binder is chosen and the granulation is carried out according to the rules of the art, exhibit an array of satisfactory properties, in particular of porosity, of mechanical strength and of resistance to abrasion. However, the adsorption properties are obviously reduced in the ratio of active powder to the powder and its inert agglomeration binder.
Various means have been proposed for overcoming this disadvantage of the binder being inert with regard to the adsorbent performance, including the conversion of the binder, in all or part, to zeolite. This operation is usually carried out when binders of the kaolinite family, precalcined at temperatures of between 500° C. and 700° C., are used. An alternative form consists in moulding kaolin grains and in converting them to zeolite: its principle is set out in “Zeolite Molecular Sieves” by D. W. Breck, John Wiley and Sons, New York. This technology has been applied with success to the production of grains of zeolite A or X which are composed of up to 95% by weight of the zeolite itself and of a residue of unconverted binder (see, to this end, U.S. Pat. No. 3,119,660), the addition of a source of silica being recommended when it is desired to obtain a zeolite X (“Zeolite Molecule Sieves”, Breck, p. 320).
Flank et al. show, in U.S. Pat. No. 4,818,508, that it is possible to prepare agglomerates based on zeolite A, X or Y by digestion of reactive clay preforms (obtained by heat treatment: of unreactive clay, such as halloysite or kaolinite, at least 50% by weight of which exists in the form of particles with a particle size of between 1.5 and 15 μm, preferably in the presence of a pore-forming agent) with an alkali metal oxide. The examples relating to the synthesis of agglomerates based on zeolite X show that it is necessary to add a source of silica, which is not the case when preparing agglomerates based on zeolite A.
JP-05163015 (Tosoh Corp.) teaches that it is possible to form grains of zeolite X with a low Si/Al ratio of less than 1.25 by mixing a zeolite LSX powder, the zeolite LSX having an Si/Al ratio of 1.25, with kaolin, potassium hydroxide, sodium hydroxide, carboxymethylcellulose. Shaping is carried out by extrusion. The grains thus obtained are dried, calcined at 600° C. for 2 hours and then immersed in a sodium hydroxide and potassium hydroxide solution at 40° C. for 2 days.
These two documents teach that it is possible to prepare mechanically strong solids. Nevertheless, the associated processes are cumbersome and suffer either from the excessive reaction time or from the number of stages involved. Furthermore, it may be feared that the heat treatment as claimed in JP-05-163015, after the shaping stage, does not contribute to the amorphization of the grain and that the object of the caustic digestion which follows is to recrystallize it, which would explain the slowness of the process.
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