Catalyst – solid sorbent – or support therefor: product or process – Zeolite or clay – including gallium analogs – And additional al or si containing component
Patent
1991-06-20
1992-07-21
Dees, Carl F.
Catalyst, solid sorbent, or support therefor: product or process
Zeolite or clay, including gallium analogs
And additional al or si containing component
B01J 2906, B01J 3700
Patent
active
051322609
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention pertains to a new procedure for obtaining granular zeolitic molecular sieves comprised uniquely of zeolitic compounds.
In the present context, the zeolitic molecular sieves are crystalline aluminosilicates of the general formula case of zeolites A and has values between 2 and 6 in the case of zeolites X or Y, in which y characterizes the hydration state of the zeolite, the value 0 corresponding to anhydrous zeolites (calcined).
The crystalline structure of these zeolites consists essentially of a three-dimensional framework of SiO.sub.4 and AlO.sub.4 tetrahedrons linked to each other by sharing an oxygen atoms. In the tetrahedral structure, the ratio of the number of oxygens to the total sum of the Si an Al atoms is thus 2. A structure formed entirely of SiO.sub.4 tetrahedrons has a zero electric charge. Replacing SiO.sub.4 tetrahedrons with AlO.sub.4 tetrahedrons results in a negative charge which is compensated for by the inclusion of cations in the lattice. These cations can be exchanged with cations from aqueous solutions and the exchange techniques based on this property are well known by the expert in the field.
The geometry of the assembly of SiO.sub.4 and AlO.sub.4 tetrahedrons is such that the structure has cavities, refereed to as cages, separated by well-calibrated opening windows. At room temperature, these cages are ordinarily occupied by water. A suitable thermal treatment eliminates this water and releases a porous system.
Such zeolites have remarkable adsorption properties which originate, on the one hand, from the strength of the affinity that they develop for certain types of molecules and which make them an adsorption agent of choice, notably when it is desired to reach very low equilibrium pressures of the adsorbate and, on the other hand, a high selectivity associated with the narrow distribution of the micropores providing access to the cavities of the crystalline structure. These structures function as veritable molecular sieves, which is the origin of their common name. Because of these qualities, they are often preferred over older adsorbent agents such as silica gel, alumina and activated carbon.
Zeolites are conventionally obtained in the form of fine powders, whether they originate from natural deposits or industrial synthesis operations. For their employment as adsorption agents, they have to be put in the form of balls or granules by agglomeration, with the conventionally used binder being a clay mineral such as bentonite, kaolin or attapulgite. Other binders which have been proposed include aluminum hydroxide, silicic acid salts and siliceous earth.
The problem created by agglomeration results from the introduction into the granular adsorbent of a non-negligible percentage of a phase which does not contribute anything to its adsorbent properties and which may even induce disastrous performances in certain applications (for example, bentonite catalyzes acid reactions such as, for example, the polymerization of olefins). Thus, for a long time it has been proposed to employ zeolitizable binders which would be activated after agglomeration is carried out.
PRIOR ART
The most common methods are based on the possibility of producing a zeolite A from metakaolin and sodium hydroxide under practically stoichiometric conditions.
Thus, U.S. Pat. No. 3,119,659 (Taggart et al.) sets forth a procedure for obtaining molecular sieve particles by agglomeration of zeolite A from kaolin and a sodium hydroxide solution, followed by calcination of the agglomerate, then crystallization in a sodium hydroxide solution. The method has the drawback of requiring double treatment of the agglomerate, first a high-temperature treatment required to transform the kaolin into metakaolin, which is the reactive form, then a recrystallization in sodium hydroxide.
Attempts have been made to replace the kaolin with metakaolin in the preparation of the extrudate, as in the procedures described in U.S. Pat. No. 4,381,255 (Nozemack et al.) and U.S. Pat. No. 4,381,26 (Hildebra
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patent: 2865867 (1958-12-01), Van Dyke et al.
patent: 3377295 (1968-04-01), Pryor
patent: 3764563 (1973-10-01), Minachev et al.
patent: 4528276 (1985-07-01), Cambell et al.
patent: 4601992 (1986-07-01), Derleth et al.
CECA S.A.
Dees Carl F.
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