Verified Syntheses of Zeolitic Materials

2nd Revised Edition

Safety considerations for zeolite synthesis

Harry Robson*
Department of Chemical Engineering
Louisiana Stare University, Baton Rouge, LA 70803, USA

In general, synthesis experiments are neither complex nor hazardous, and normal precautions which apply to any chemistry laboratory are adequate to protect both equipment and the investigator. But certain precautions should be called to the attention of the inexperienced operator.

1. Source Materials

Silica and alumina are inert in most forms, but ingestion of dust should be avoided. The alkoxides are skin irritants and should be handled with gloves and in a hood. Hydrofluoric acid (HF) is corrosive and skin contact or inhalation must be avoided, likewise strong caustic is corrosive to skin and equipment. Gloves and a face mask are indicated, and the experiment should be done in a hood.

In one case an "overflow" occurred when an inexperienced student put all the powdered aluminum required for a synthesis into a caustic solution. The exothermic dissolution heated the solution, accelerated the dissolution autocatalytically, and resulted in "boil over." Obviously, if hydrogen is given off, danger of explosion exists. The use of aluminum wire, which dissolves more slowly, is suggested.

Organic templates cover a wide range of volatility and toxicity. In many cases these materials are quite rare and their toxicity may not be well defined. Some are known to be biologically active; in all cases the investigator should avoid skin contact or inhalation by using gloves and working in a hood.

In some cases, the potential exists for explosive decomposition of the template under crystallization condition. There is no good way to rule out this possibility, but a strong exotherm in the thermal analysis curve of the template indicates it should be treated with caution.

No preparations in this book use beryllium (Be) or thallium (TI), but reports can be found in the literature of zeolite syntheses using these materials. Be and TI are extremely toxic and should be used only by experts who thoroughly understand the necessary precautions.

2. Batch Preparation

The hazards of handling strongly caustic materials are increased when the solutions are hot. Gloves and face mask are still more important. Hydrolysis of the alkoxides produces heat; this may not be a problem in gram quantities but becomes so with scale-up. The alcohol produced is often removed by evaporation which can be a fire hazard. High-speed mixers can be splash hazards unless totally contained.

3. Crystallization

Zeolite crystallization is usually done at autogenous pressure at temperatures up to 200°C. With water present, this can mean pressure up to 15 atm. When organic templates are present, the pressure can be considerably higher. The manufacturerâs specifications for the autoclave must be carefully considered. The liquid fill of the cold autoclave should not exceed 75%; the expansion of liquid water between 25 and 200°C is not negligible. Teflon loses its strength above 200°C. Teflon gaskets and liners must be treated with caution at these temperatures. When working in metal autoclaves without liners, stainless steel exposed to caustic mixtures undergoes caustic embrittlement at temperatures above 200°C, which can quickly ruin an autoclave. To clean autoclave liners between experiments, strong, hot mineral acids are usually used, for example HNO₃. It takes copious amounts of rinse water to remove all the acid.

Polypropylene jars may be used for crystallizations below 100°C. After extended use, they tend to spring a leak, as the bottom begins to separate. Although there is no danger of the jars bursting at this temperature, organic vapors are liberated, so that the oven or bath used for heating should be placed under a hood or have otherwise good ventilation.

Sealed quartz tubes are often used as the crystallization container, especially for small quantities and high throughput. Also one can monitor progress of crystallization visually. Qpartz is a poor conductor of heat, but prolonged heating for sealing can heat the synthesis batch to boiling if the neck is too short. After crystallization, the quartz reactor should be opened behind a shield to protect the operator in case of residual gas pressure.

Heat transfer by air, for example in an oven, is slow, and heating in a bath is preferred. As heating fluid, a polyglycol-type, which does not attack polypropylene, is recommended. A silicon oil leaches plasticizer out of the polypropylene thus making the jar brittle and subject to cracking. Glycerol can be used, but has a relatively high vapor pressure (vapors condense on all surfaces in the lab making them sticky) and should be used under a hood. The temperature controls of a liquid bath should have a high-temperature shut-off to prevent overheating, in case the temperature controller fails.

Combinations of organic templates and strong oxidizing agents such as chromate and permanganate are potential explosives, particularly in low-water conditions.

4. Product Recovery

Opening the reaction vessel after crystallization should be treated with caution. Even thoroughly cooled, residual gas pressure should be expected. Organic templates may degrade during the reaction the products may be toxic and often cause a stench problem. The mother liquor after a templated synthesis may be a disposal problem. In most cases it should be collected for disposal as a hazardous chemical.

The synthesis batch may crust over during crystallization and retain residual gas pressure. When the crust is penetrated, the contents may constitute a splash / missile hazard.

A template not removed by washing is usually expelled by calcination. The template or its decomposition products are usually combustible and it is easy to start a fire. The heating equipment should be properly vented; if necessary nitrogen blanketed.

* With contributions by Kenneth Balkus, Günter Kühl and Robert Thompson