The Art of the Possible

I’ve been intending to start a lay-person’s version of my weekly postings on the American Chemical Society website. Several of my Twitter followers have mentioned that they are impressed, but confused, by these tech-heavy synopses, written for an audience of professional chemists. I thought I might wait for a week with an “accessible” topic, like medieval bones or counterfeit currency, but I decided that I might as well just jump in and start today.

I really picked a doozy of a week to start. This week’s post deals with an energy minimization study of silica- and Germania-based zeolites. Say what?

OK, let’s start with “zeolites”. These are inorganic materials, some found in nature, some made in a lab or a factory. If you’re going to get really picky about it, you can call them “microporous solids”. At the atomic scale, these materials form frameworks with a lot of open space, so you can use them as “molecular sieves”, or you can make them into containers for other molecules. Zeolites are great for catalytic converters in cars, for making all kinds of chemicals from petroleum, or for absorbing huge amounts of water in disposable diapers, among other uses.

Scientists have spent decades assembling databases of zeolite framework structures. Some of these structures already exist in the real world. Others “ought to” exist, from geometrical considerations, but they’ve never been made before. Every now and then, someone runs across one of these hypothetical structures, and they want to know whether it’s worth their time and effort to try and make the actual material. Maybe it has cavities of just the right shape, or it has channels of the right size, for some application that this person has in mind.

In addition to structural databases, scientists have access to databases that contain information on how stable certain chemical compositions and arrangements are. These databases are the results of years of research gained from chemical reactions, melting, compressing, and otherwise poking and prodding various materials. As a result, scientists can examine a series of framework structures to see which ones are the most stable.

A research group from Arizona State University recently published the results of their structural stability calculations, using hypothetical zeolite structures made from silica (silicon dioxide, the same stuff that beach sand is made of) and Germania (germanium dioxide, which is chemically very similar to silica). (Chemistry of Materials 2014, 26, 1523–1527). They looked at framework structures based on building blocks containing one silicon (or germanium) atom surrounded by four oxygen atoms, each oxygen atom acting as a bridge between one building block and the next. These frameworks are referred to as “tetrahedral networks”. The group was especially interested in the angles formed by two silicon (or germanium) atoms and the oxygen atom bridging them, or T–O–T angles.

The research group did calculations for a series of known structures, distorting the structures over a wide range of T–O–T angles and watching to see whether the stability increased or decreased. For all of the silica structures, the structures became more stable as the angles increased from 120º to 140º. From 140º to 180º, the stability stayed about the same. Germania structures, however, were most stable in a narrow angular range between 128º and 130º.

Several commercially important zeolites have structures with T–O–T angles in a range that would make silica very happy but would be an uncomfortable contortion for Germania. In fact, of the 5824 stable frameworks described in the Atlas of Prospective Zeolite Structures, 994 have all of their T–O–T angles in the optimal range for silica (135º–180º), but only 48 are in the most favorable range for Germania (117º–145º). Mixing germanium and silicon in the framework might extend the range of stable bond angles and increase the number of stable structures available for synthesis.

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