Verified Syntheses of Zeolitic Materials

2nd Revised Edition

Source materials for zeolite synthesis

Gunter Kuhl
Department of Chemical Engineering, University of Pennsylvania, Philadelphia, PA, USA

1. Introduction

The chemistry of zeolite synthesis is subject to perturbations caused by impurities present in the source materials. Such contaminants may remain insoluble during the crystallization and cause undesired species to nucleate. They may be soluble and result in formation of different silicate or metallosiicate species in solution, or they may cause an insoluble silicate species to precipitate. Therefore, it is desirable to apply pure chemicals as starting materials. Depending on the zeolite to be synthesized and the application intended for the product, less pure source materials are frequently employed, in order to reduce the cost. If the materials are not pure, they may vary from batch to batch and from different suppliers. It is of utmost importance to know the source materials for zeolite synthesis, and technical grade materials need to be assayed and analyzed for impurities. A few frequently used chemicals are discussed below.

2. Water Content

Clearly, when commercially available solutions of chemicals are employed, such as aqueous NaOH, H₂F₂, or silicate, the water content of the solution has to be taken into account when the composition of the reaction mixture is established. Most other source materials contain more or less water, while the water content of fume silica may be only 3 wt.%, that of aluminum nitrate, Al(NO₃)₃⋅ 9 H2O, is about 43 wt.%. Moreover, aluminum nitrate is deliquescent, and the water content will change once the jar has been opened and the chemical exposed to moist air. On the other hand, aluminum sulfate, Al₂(SO₄)₃⋅ 18H₂O (48.6 wt.% H₂O), weathers by losing water upon exposure to the atmosphere. Technical grade aluminum sulfate usually contains about 14 H₂O.

Since the composition of a reaction mixture is given as ratio of oxides, any hydroxides employed are to be considered as oxides plus water, for example, NaOH = 1/2Na₂O + 1/2H₂O (22.5 wt.% H₂O). Additionally, a small percentage of free water may be present, for example, sodium hydroxide pellets may contain about 97 or 98% NaOH. Similarly, an 85% H₃PO₄ contains 61.6 wt.% P₂O₅ and 38.4 wt.% H₂O. The water content of source materials may or may not constitute an important fraction of the total water content, and it is recommended that the water contained in these chemicals always be considered when the amount of water to be added in the preparation of the reaction mixture is calculated.

3. Sources of Aluminum

Some aluminum sources have been mentioned above. A disadvantage of using salts is that, after pH adjustment or addition of alkali silicate solutions, alkali salts are formed which have a strong electrolytic effect on gel formation. For example, such salts may cause sodalite to be crystallized instead of zeolite A type materials. For this reason, it is advantageous, particularly for reaction mixtures of low SiO₂/Al₂O₃ ratios, to introduce aluminum in the anionic form, that is, as sodium aluminate.

Sodium aluminate is subject to formation of aluminum oxide hydrates upon exposure to atmospheric carbon dioxide or just upon storage. It is not a widely used chemical, and, when obtained from a chemical supplier, is frequently aged to an extent that makes it unsuitable for zeolite synthesis. Sodium aluminate should dissolve completely with stirring in water at ambient temperature within a few minutes. If it does not, the precipitate or cloudiness usually can not be dissolved by adding small quantities of alkali hydroxide, and the chemical is not suitable. Fresh sodium aluminate can be obtained in a technical form in larger quantities. such as 50-lb. bags. It is advisable to transfer the chemical with as little exposure to moisture and CO₂ as possible into small jars which should be sealed tightly and stored at or below room temperature. The composition of such technical sodium aluminate varies widely with Na/Al ratios from near 1.0 to 1.2 or higher so that a reliable assay is required. A small iron content manifests itself by the brownish discoloration of the otherwise clear sodium aluminate solution. This iron hydroxide can be removed by filtration or, for less critical preparations, just ignored.

In preparations containing phosphate, aluminum phosphate, AlPO₄ 2H₂O, is a viable alternative as it dissolves completely in alkaline phosphate or hydroxide solutions upon mild heating with stirring.

4. Silica Sources

A widely used silica source is aqueous sodium silicate, such as waterglass from PQ Corporation, Philadelphia, PA, USA. The PQ N-Brand product contains about 8.9 wt.% Na₂O and 28.7 wt.% SiO₂. It usually is slightly cloudy, and it is recommended that filtered N-Brand be purchased. The precipitate can also be filtered off in the laboratory, but the vacuum filtration is slow and the ifitrate has a reduced water content and needs to be reassayed. A small contamination with aluminum usually prevents zeolites with SiO₂/Al₂O₃ ratios above ~600 to be crystallized.

When a lower aluminum content in the product is desired, colloidal silica sol, such as Ludox from E. L DuPont de Nemours, Wilmington, DE, USA, can be used. This material is available in concentrations of 30 and 40 wt.% SiO₂ with different stabilizers employed. Without added aluminum, SiO₂/Al₂O₃ ratios in the range of 3000 to about 3500 can be obtained.

Zeolites of still lower aluminum content can be prepared with fume silica as the silica source. Such materials are on the market as Cab-O-Sil (Cabot) or Aerosil (Degussa). The water content of fume silica is very low, ~3 wt.%. Products obtained when using fume silica as the silica source, without adding an alumina source, have SiO₂/Al₂O₃ ratios above 20,000.

Precipitated silica is available in different qualities. Hi-Sil (PPG Industries, Pittsburgh, PA, USA) contains about 90 wt.% of SiO₂ as well as 1 wt.% of NaCl (it is recommended that the percentage of SiO₂ be determined; alternatively, the ash content minus 1% NaCl should approximate the SiO₂ content). The aluminum contamination prevents zeolites with SiO₂/Al₂O₃ ratios above ~220. A somewhat purer material is Ultrasil (Degussa). The particle size of precipitated silica may need to be considered. Whereas finely divided fume silica may yield a thick paste of a reaction mixture, which is difficult to homogenize, a large-size precipitated silica, for example, Ultrasil VN3SP, may react too slowly to provide the desired aluminosiicate precursors in solution.

Tetramethyl- and tetraethylorthosilicate are available in high purity and yield the highest SiO₂/Al₂O₃ ratios. Any noticeable aluminum contamination is likely brought in from other sources. The compounds are usually hydrolyzed, for example, in a stainless-steel beaker, prior to incorporation in a reaction mixture. It is recommended that the alcohol generated be removed by heating, although its effect on the crystallization of high-silica materials is generally slight.

In order to prevent contamination, plastic containers such as polypropylene or Teflon, are recommended for the preparation of all solutions, for the reaction mixture, and for the crystallization. Glass vessels should be avoided, as glass participates in the reaction, and silica, alumina, and boron are known to be leached out of glass. For example, the catalytic activity of a borosilicate or a ferrosilicate may be influenced by contamination with traces of aluminum. When pressure vessels are used for the crystallization, removable Teflon or stainless-steel liners are recommended. The reaction vessels should be thoroughly cleaned prior to use by heating with aqueous sodium hydroxide, if seeding is to be avoided. Teflon may also be cleaned with hydrofluoric acid.