1. Field of the Invention
This invention relates to an improved method for preparation of certain zeolites having a high silica/alumina mole ratio, i.e. greater than about 10. The improvement resides in adding a source of aluminum ions to a silica-rich amorphous reaction medium at a slow and controlled rate whereby the concentration of aluminum ions in the amorphous phase of the reaction mixture is maintained at steady state during crystallization.
More particularly, this invention relates to the above improved method of preparation of certain zeolites whereby crystallization time is substantially reduced from that required when conventional prior art methods of preparation are utilized and the resulting zeolite exhibits somewhat improved steam stability, i.e. stability toward steam deactivation and dealuminization.
Even more particularly, this invention relates to organic compound conversion, such as, for example, catalytic conversion of oxygenates, such as methanol, and syn-gas conversion where water is always present during reaction, with the improved zeolite product of the present improved method as a catalyst.
2. Discussion of Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversions. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as (Ca/2), (Sr/2), Na, K or Li is equal to unity. One type of cation may be exchanged either entirely or partially by another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic aluminosilicates. These aluminosilicates have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882-243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. 3,247,195), zeolite ZK-4 (U.S. Pat. No. 3,314,752) and zeolite ZSM-5 (U.S. Pat. No. 3,702,886) merely to name a few.
Applicants know of no prior art methods of zeolite preparation utilizing the present improvement. In fact, the present improved method of zeolite preparation is distinctly different from the current practice in the synthesis of zeolites having a silica/alumina mole ratio greater than 10.