Method of producing rod-shaped ZSM-5 zeolite

This disclosure is concerned with the crystallization of ZSM-5 zeolites and, more particularly, is concerned with the preparation of high silica ZSM-5 zeolites having a silica-to-alumina ratio of greater than 70, up to and approaching infinity, by control of the pH so as to obtain rod-shaped crystals.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is concerned with the crystallization of high silica ZSM-5 
zeolite and, more particularly, is concerned with the preparation of high 
silica ZSM-5 zeolites having a silica-to-alumina ratio of greater than 70, 
up to and approaching infinity, by control of the pH so as to obtain 
zeolitic compositions having a rod-shaped morphology by controlling the 
final pH of the reaction medium to a range of 9.5-10.5. 
2. Description of Prior Art 
High-silica zeolites are extremely well known in the art and have been the 
subject of much attention in both the patent and technical literature. It 
has now been discovered that the morphology of the crystals produced from 
a forming solution vary depending upon the final pH of the reaction 
medium. As is well known in the art of the synthesis of zeolites, it is 
extremely difficult to control the pH and, in fact, it is extremely 
difficult to even measure the pH during the crystallizaton since 
crystallization is usually carried out in closed vessels under autogeneous 
pressure such that exact measurements are not practical. It is known in 
the art that there is a certain pH range over which ZSM-5 type zeolites 
can be prepared and, in this connection, U.S. Pat. No. 4,061,724 discloses 
at column 3 the pH range of 10-14. However, this patent does not teach 
that the morphology of the resulting crystals is dependent on the pH of 
the reaction medium. It is also known that the pH is difficult to control 
and that it varies during the course of crystallization. 
Rod-shaped silica polymorphs are disclosed in U.S. Pat. No. 4,073,865. 
These materials are stated to be a novel silica polymorph and they 
require, for their preparation, the presence of a fluoride ion in the 
reaction media. The resulting rod-shaped products are stated to be 
extremely hydrophobic thereby making it particularly advantageous in 
absorption separations requiring minimum water absorption or at least 
interference from water vapor and the absorption of less polar molecules. 
Preparation of the materials involving the use of a fluoride ion has many 
disadvantages including disposal problems in connection with the waste 
liquids. Similarly, the off gas resulting from the calcination of the 
silica materials contains hydrogen fluoride and silica tetrafluoride 
thereby preventing releasing to the atmosphere until suitable purification 
techniques are employed. 
Accordingly, it is the object of this invention to produce high silica 
ZSM-5 in the absence of a fluoride from a reaction medium which does not 
contain a fluoride ion said ZSM-5 being rod-shaped and having a high 
degree of hydrophobicity. It is noted that the production of rod-shaped 
ZSM-5 from a fluoride free media is not novel per se and, in fact, Example 
27 of U.S. Pat. No. 3,702,886 specifically teaches the same. However, 
instant invention represents an improvement over the process toward said 
Example in that the products obtained by the process of this invention are 
more uniformly rod-shaped than those in said Example 27. 
SUMMARY OF THE INVENTION 
This invention is concerned with carrying out the crystallization of ZSM-5 
type zeolites using the same reactants as have previously been described 
in various patents and technical articles; but carrying out the 
crystallization in the presence of a buffer such that the pH is maintained 
within the range of 9.5-10.5 to produce products which are rod-shaped and 
characterized by having a length-to-width (L/W) ratio of at least 3:1 and 
a water sorption capacity of less than 2.0 grams/100 grams of solid. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
It has long been recognized in the synthesis of various crystalline 
aluminosilicates of the low-silica type that complexing agents such as 
phosphates, arsenates, tartrates, citrates, ethylenediaminetetraacetate, 
etc. also can act as a buffer. The use of these materials has been 
primarily directed to those situations where it was desired to increase 
the silica-to-alumina ratio of the zeolite by complexing the alumina. 
Thus, procedures of this type are disclosed in U.S. Pat. Nos. 3,886,801; 
4,203,869; as well as an article entitled "Influence of Phosphate and 
Other Complexing Agents on the Crystallizaton of Zeolites," appearing in 
Molecular Sieves, Soc. of Chem. Industry, London, pp 85 and following 
(1967). 
All of the above three publications have for a common goal the use of a 
material which complexes the aluminum such that a zeolitic product is 
obtained which has a higher silica-to-alumina ratio than that which would 
normally be obtained from the same reaction medium in the absence of such 
complexing agents. As indicated earlier, certain of the complexing agents 
are also buffers, but they are used in amounts such that their primary 
objective is to complex all or a portion of the aluminum thereby raising 
the silica-to-alumina ratio. In the above referred to article, as well as 
U.S. Pat. No. 3,386,801, low-silica zeolites are dealt with such that the 
complexing agent added does not complex all the aluminum due to 
equilibrium considerations. On the other hand, U.S. Pat. No. 4,088,605 
which is directed towards high silica-to-alumina ratio zeolites, discloses 
that the function of the complexing agent is indeed to complex 
substantially all the available aluminum which, of course, also raises the 
silica-to-alumina ratio of the final crystalline product. Patents such as 
U.S. Pat. No. 3,949,059 teach the use of buffers in the crystallization of 
a low-silica zeolite. The novel process of this invention is not concerned 
with the use of complexing agents which may additionally be buffers in 
amounts such that they complex with aluminum and raise the 
silica-to-alumina ratio of the zeolitic product nor is it concerned with 
low-silica zeolites. In other words, in the novel process of this 
invention a buffer is used in an amount such that the silica-to-alumina 
ratio of the product is substantially unaffected, i.e., it would have the 
same high silica-to-alumina ratio whether or not a buffer were used. 
The novel process of this invention is based on the the discovery that the 
pH of the final reaction mixture is of paramount importance in 
establishing the production of high silica, rod-shaped ZSM-5 type 
crystalline products. It has been discovered that if the final pH is 
controlled in the range of 9.5-10.5 that rod-shaped ZSM-5 is crystallized. 
On the other hand, if the final pH is not controlled within the 9.5-10.5 
pH range, a ZSM-5 havig different morphology is obtained. For example, 
near spherulitic morphology is obtained when final pH values were above 12 
and up to 12.5 and an intermediate type morphology is found in the range 
of 11 to 12. 
As has heretofore been stated, it is well known in the art that the pH of a 
reaction mixture in zeolite synthesis cannot be carefully controlled and 
it does vary over fairly wide ranges during the steps of gel preparation, 
aging and during the course of crystallization. The novel process of this 
invention minimizes the variation in pH by using a buffer which would 
effectively control the pH to the desired 9.5-10.5 value thereby greatly 
facilitating the crystallization of ZSM-5 having rod-shaped morphology. 
The buffer utilized is not narrowly critical and any buffer capable of 
stabilizing the pH in this range at 200.degree. C. in a pressure vessel 
can facilitate crystallization of the desired morphology. Typical buffers 
would include phosphates, tartrates, citrates, oxalates, 
ethylenediaminetetraacetate, and acetate, particularly the ammonium salts, 
alkali metal salts, such as sodium, and the free acids. 
For reasons which are not understood, it has been found that a carbonate 
buffer, simply will not correlate with the final pH to produce the desired 
rod-shaped ZSM-5. Accordingly, the use of a carbonate buffer is outside 
the scope of this invention. 
The amount of buffer which is used is determined by many factors, including 
the particular nature of the buffer itself, as well as the final pH which 
is desired. In general, it can be stated that the buffer has to be used in 
sufficient amounts such that it does act as a buffer in order to stabilize 
the pH. 
In general, it can be stated that the amount of buffer used is such that 
there is present in the reaction medium 0.1 to about 0.35 equivalents of 
buffer per mol of silica. Greater amounts of buffer can be used, but the 
increased salt concentration reduces the rate of crystallization. As has 
heretofore been stated, the novel process of this invention resides in 
controlling the pH during crystallization of a high-silica zeolite. The 
only practical method to control the pH is by the use of a buffer. As has 
also been indicated, it is difficult to measure the pH during 
crystallization so that a very effective correlation has been made by 
measuring the final pH, i.e., the pH after crystallization. It is 
precisely this final pH which has been correlated with a morphology of the 
high-silica zeolites which are produced. 
The novel process of this invention concerned with the synthesis of 
high-silica ZSM-5 zeolites this expression is intended to define a 
crystalline which has a silica-to-alumina ratio greater than 70 and 
preferably, greater than 500, up to and including those siliceous 
materials where the silica-to-alumina ratio infinity or as reasonably 
close to infinity as practically This latter group of highly siliceous 
materials is exemplified by U.S. Pat. Nos. 3,941,871; 4,061,724; 
4,073,865; 4,104,294 (herein incorporated by reference); wherein the 
materials are prepared from reaction solutions which involve no deliberate 
addition of aluminum. However, trace quantities of aluminum are usually 
present due to the impurity of the reactants. It is also to be understood 
that the expression "high-silica zeolite" also specifically includes those 
materials which have other metals besides silica and/or alumina associated 
therewith, such as boron, iron, gallium, etc. U.S. Pat. No. 3,702,886 
describing and claiming ZSM-5 is incorporated herein by reference. 
As is known in the art, zeolites of this invention are prepared from a 
forming solution containing water, a source of quaternary ammonium 
cations, an alkali metal, silica, with or without added alumina or with or 
without the presence of additional metals. As is known in the art, the 
forming solution is held at elevated temperatures and pressures until the 
crystals are formed and thereafter the crystals are removed. 
The novel process of this invention resides in using the exact forming 
solutions which have previously been taught for the preparation of ZSM-5 
and adding therewith a buffering agent so as to have a final pH within the 
range of 9.5-10.5. 
As has heretofore been indicated, the rod-shaped ZSM-5 zeolite produced by 
the novel process of this invention is rod-shaped, has a silica-to-alumina 
ratio greater than 70, has a length-to-width ratio greater than 3, and has 
a water sorption capacity of less than 2.0 grams per 100 grams of solid. 
It has also been discovered that the hydrophobicity of the products 
increases with increasing length-to-width ratios. 
The following examples will illustrate the novel process of this invention 
using various buffering agents. In all cases, colloidal silica sol 
identified as Ludox LS containing 30 weight percent silica was used for 
crystallizations. The molar ratio of silica to tetrapropylammonium bromide 
(TPABr) was held nearly constant at 19.8 to 19.9 and that of sodium 
hydroxide to TPABr was held constant at 3.05 and the crystallization 
temperature was held constant at 200.degree. C. Percent crystallinity is 
based on X-ray comparison with a highly crystalline reference sample. 
Generally, the reaction mixtures were prepared by dissolving 
tetrapropylammonium bromide, alkali hydroxide and a particular salt, 
water, and adding (Ludox) silica sol to this solution. 
All crystallizations were carried out in non-stirred pressure vessels 
equipped with Teflon liners and heated by immersion in a constant 
temperature silicone oil bath. 
It is specifically noted that only Examples 2, 3, 4, 6, 10, 12, 17, 19, and 
21 produce rod-shaped crystalline alminosilicates and are within the scope 
of this invention. The remaining Examples are presented merely to show 
that unless the final pH is controlled to within the 9.5-10.5 final pH 
range that rod-shaped ZSM-5 simply is not obtained. Examples 22 thru 25 
show the unreliability of using a carbonate buffer.