Mineral oils: processes and products – Chemical conversion of hydrocarbons – Cracking
Patent
1998-06-08
1999-10-19
Yildirim, Bekir L.
Mineral oils: processes and products
Chemical conversion of hydrocarbons
Cracking
208120, 208135, 585481, 585533, 585651, 585653, 585739, 502 77, 502 79, 502 85, 423702, 423705, 423714, 423DIG26, 4233282, C10G 4704, C07C 406, C01B 3934, B01J 2970
Patent
active
059683414
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1) Field of the Invention
The subject of the present invention is a silica-enriched activated mazzite obtained by a crystallization gel containing zeolite crystals. The present invention also relates to its process of preparation and to its application as reaction catalyst for the conversion of hydrocarbons, in particular by isomerization, or as molecular sieves.
2) Background Art
Obtained for the first time by a synthetic route in 1966 under the name of zeolite omega by Flanigen and Kellberg (U.S. Pat. No. 4,241,036), mazzite was identified in 1972 in basaltic rocks from Mont Semiol, near Montbrison, Loire, France. Its crystalline structure was resolved by Galli (Cryst. Structure Comm., 3, 339, 1974) and Rinaldi et al. (Acta Cryst., B31, 1603, 1974). From its structure, of hexagonal symmetry, it belongs to the category of highly acidic zeolites with broad pores and unidirectional porosity, which makes it particularly advantageous for applications in catalysis, in particular for the conversion of hydrocarbons.
The omega claimed by Flanigen (U.S. Pat. No. 4,241,036) is characterized by an X-ray spectrum common to all the types of mazzites prepared subsequently, such as ZSM-4, LZ 202 or MZ-34. However, these mazzites, although identical in structure, are distinguished from one another by their specific synthetic route and different physical characteristics, such as the Si/Al ratio, their specific surface and their porosity.
In order to obtain a mazzite, so-called crystallization gels, containing a trivalent aluminium source, a silicon source, at least one alkali metal or alkaline earth metal cation in the hydroxide form, water and optionally an organic structuring agent, can be formed. However, from these constituents, by varying the stoichiometry of the gel or the conditions of the subsequent hydrothermal treatment necessary for the crystallization, it is possible to obtain zeolites which are very different from mazzite, such as offretite (OFF) or zeolite L (LTL) (S. Ernst and J. Weitkamp, Catalysis Today, 19, 1994, 27-60).
In order to improve the physical characteristics of these mazzites, silica gels, colloidal silicas, precipitated silicas, silicates or hydrolyzable silicic esters have been introduced into the synthesis or crystallization gel, as silicon source, and aluminates, alumina hydroxides, alumina in the pure or commercial form or amorphous aluminosilicates have been introduced into the synthesis or crystallization gel as trivalent aluminium sources. These aluminium sources were subsequently replaced by natural or synthetic crystalline aluminosilicates, mainly natural clays. Such a substitution has made it possible to obtain homogeneous growth of the crystals, due to the slow and even dissolution of these aluminosilicates in the synthesis medium (Dwyer, U.S. Pat. No. 4,091,007; Fajula, U.S. Pat. No. 4,891,200).
Other methods have been developed for synthesizing a novel mazzite from seeds of mazzite or of another zeolite in the presence or in the absence of organic structuring agent containing alkylammonium ions (Cannan, U.S. Pat. No. 4,840,779, Di Renzo, FR 2,651,221 and FR 2,698,862).
Thus, by varying the crystallization temperature to between 90.degree. C. and 150.degree. C. and the content of sodium cations and of organic structuring agent, such as tetramethylammonium, choline or p-dioxane, in the crystallization gels, it has been possible to obtain the various known types of mazzite called omega, ZSM-4, LZ 202 or MZ-34 (cf W. M. Meier and D. H. Olson, "Atlas of Zeolite Structure Types", Third Revised Edition, Butterworth, London 1992).
However, whatever the method of synthesis employed above, it has been impossible to synthesize a mazzite exhibiting an Si/Al ratio in the precursors or alternatively crude synthetic products greater than 5, these ratios generally being between 2.5 and 5, which corresponds to a molar aluminium concentration varying between 0.166 and 0.285. Now, the combination of a high aluminium content and of a unidirectional porosity does not favo
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Di Renzo Francesco
Fajula Francois
Fitoussi Fredj
Schulz Philippe
Elf Aquitaine
Yildirim Bekir L.
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