1. Field of the Invention
This invention relates to a novel extrusion process for preparing silica rich extrudates having a high crush strength. In particular, this invention is concerned with an extrusion process for extruding either silica by itself, a zeolite by itself, or more preferably silica and a crystalline aluminosilicate zeolite such as ZSM-5 in order to produce a material which has substantially the same strength as or greater strength than a corresponding extrudate made from alumina. This invention also relates to the novel extrudates produced by the novel process.
2. Description of the Prior Art
It has long been known in the art that extrusion is one way of obtaining a material which has a high degree of strength for various applications, both catalytic and noncatalytic. Some aluminosilicate zeolites have long been used as catalysts for a wide variety of organic conversion processes and, in general, crystalline aluminosilicate zeolites are incorporated with a matrix or binder material in order to impart strength thereto. The most commonly used matrix materials have included alumina and mixtures of the same with clays for the reason that these materials were very easy to extrude and resulted in the production of an extrudate which had desirable physical strength.
It had long been recognized in the art that silica is a desirable matrix and that it possessed advantages over alumina with regard to certain catalytic reactions. In this connection, U.S. Pat. No. 4,013,732 specifically discloses ZSM-5 with a silica matrix, i.e., see Column 7. U.S. Pat. No. 3,843,741 and U.S. Pat. No. 3,702,886 would broadly teach the use of ZSM-5 with a silica matrix. Thus, although the concept of having a material such as ZSM-5 with a silica matrix is not novel, the fact remains that it was not possible to prepare said composition via an extrusion technique because silica and zeolites will not extrude in conventional extruding equipment to give reasonably strong products and the only way that the prior art was able to obtain mixtures of ZSM-5 and silica was by a filling or pelleting which involved mixing silica and the appropriate zeolite and squeezing it together to form a shaped structure having minimum physical strength.
There also exists a wide body of art for producing catalysts from silica gels, such as U.S. Pat. No. 3,969,274, the entire disclosure of which is herein incorporated by reference. The said patent teaches the advantages of having silica as a support for catalysts but utilizes a silica gel which has been subjected to steam treating in order to enhance its crush strength. It is obvious that the said patent requires a steaming step in order to make its process operable (i.e., see Example 4) whereas steaming per se is not an essential step to produce the novel extrudate of this invention.
There also exists a body of art involving post treatment of a silica-zeolite pellet or extrudate with various materials in order to increase the strength thereof. Thus, for example, U.S. Pat. No. 3,846,337 discloses silica-bound silicate particles of improved crush strength and can be prepared by admixing reactive silica sols with siliceous particles thereafter contacting the resulting combination with ammonium phosphate and/or acid phosphate.
U.S. Pat. No. 4,111,843 teaches the preparation of porous silica particles by adding excess alkali to a body of water glass and then precipitating a hydrogel with acid. The excess alkali is stated to be responsible for increasing pore diameter. The patent specifically teaches at Column 2, line 15 and following, that the extrudability of microporous silica-alumina catalyst carriers of exceptional strength can be achieved by following the process of the patent. However, the patent does not teach anything with regard to extrudates of silica particles, i.e., see Column 6, line 66 and following.