Zeolite Catalysts and Sieves

Microscopic Architecture

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We have seen that even with common SiO2 silica (sand) there are many different ways of connecting the SiO4 tetrahedrae via their common oxygen atoms. Large framework mineral structures called ¶zeolites can be produced, containing holes and channels within the structure: the example is an artificial zeolite of pure silica called UTD-1. Zeolite is derived from a Greek word meaning "boiling stones", since zeolites swell and lose water from their porous structure when heated. They have many important industrial applications due to their unique architecture; they can be used as sieves to purify water, to separate out molecules of different sizes, to make detergents for the kitchen, to remove radio-active elements from spent nuclear fuel, and as catalysts for many chemical reactions, especially in the petro-chemical industry. Over 600 zeolites are known, and new synthetic zeolites are developed and patented every year by large chemical companies.

In the 1970s, fears of a decline in available supplies spurred a search in the U.S. for alternative energy sources. Mobil invented a process for converting methanol into high-quality gasoline through the use of the company's versatile ¶ZSM-5 catalyst, an Al2O3-SiO2 artificial zeolite. When Al+++ is substituted for Si++++, an extra postive charge is required in the vicinity to maintain charge balance with the O--. The most obvious source of this charge is hydrogen H+ or H3O+ from water, which makes the Al+++ site very acidic (able to provide H+ and oxidise materials). The catalytic activity of ZSM-5 is in part due to this acidity and in part to the size and shape of the channels which hold the intercalated molecules.

¶Linde Zeolite-A (LZA) is one of the most important zeolites; it is harmless to the environment and is used in washing powders to remove calcium and magnesium ions, which would otherwise be precipitated from 'hard' water. Over 700 million tons, worth hundreds of millions of dollars, are manufactured each year. In this zeolite, some of the tetrahedral silicon sites (green) are again occupied by aluminium (purple); aluminium oxide is another common constituant of sand.

Instead of drawing the SiO4 and AlO4 tetrahedrae, we can understand the architecture of zeolites simply by connecting up the Si and Al atoms; this shows the ¶Zeolite-A frame-work structure, and emphasises the different cavities and channels. You can learn more about zeolites from the Zeolite Atlas at ETH-Zurich, which provided this frame-work drawing, and you can generate your own 3D VRML drawings of other structures from the ILL Zeolite page.

The mineral world is full of beautiful naturally occuring structures, many of which man has only recently learned to synthesize. Lets look at some more examples from nature: gemstones & minerals.

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