At the heart of this technology are synthetic scaffolds made of silicon dioxide and aluminum oxide with nanometer-sized pores that are rigid and inflexible
Chemists, when designing or building new molecules, can be viewed as architects and builders. An organic chemist can design a blueprint for a new molecule and precisely synthesize it from carbon, oxygen, hydrogen, and so on. After centuries of fine-tuning this ability, chemists began synthesizing long, thread-like, one-dimensional polymers in the early 20th century. The polyethylene in plastic bags is made up of repeating units of the ethylene molecule (importantly, the units are linked by the same strong chemical bonds as in an organic molecule. This provides the stability that ensures that a shirt made from polyester blend yarn is durable). In biological systems, proteins are one-dimensional polymers of amino acids.
Add new dimensions
In recent years this has been taken to a new level by creating extensive 2 or 3-dimensional structures from the linkage of molecular units, as in polymers, but in two or three dimensions. The basic units keep joining together to form large networks, similar to a chain link fence. The network is built up by repeated additions of a molecule with symmetry. A few such networked sheets stacked on top of one another form a functional 2D unit. Because words like polymer do not do justice to this complex arrangement of atoms, such molecular networks are called scaffolds.
Applications for these covalent organic frameworks (COFs) take advantage of their stability, large surface area, controlled pore size, and tunable chemical environments. Just as you choose the size of the âporesâ / hole in a wire mesh, scaffolds can be designed to act as screens in separating molecules of a certain size. The slightest whiff of a poisonous gas could be felt – in an industrial environment or in flight luggage. They are also suitable for both storing (as capacitors) and conducting energy (along membranes in fuel cells).
Organometallic frameworks (MOFs) are structured like COFs, but have metals in complexes with organic units. The choice of metals is wide, from beryllium to zinc, although relatively common metals are preferred for economic and environmental reasons. They offer great advantages: for gas storage such as hydrogen storage in fuel cells; in catalysis, where they replace very expensive metals; in sensors; and in drug delivery – anticancer drugs and other drugs with severe side effects can be trapped within the porous confines of MOFs and released in small and even doses.
Use of zeolites
Zeolites are highly porous 3-D networks made of silicon dioxide and aluminum oxide. In nature, they occur where volcanic runoff meets water. Synthetic zeolites have proven to be a great and inexpensive boon. One biomedical device that got into our lexicon during the pandemic is the oxygen concentrator. This device has reduced the amount of oxygen cleaning of industrial equipment to the volume required for a single person. At the heart of this technology are synthetic scaffolds made of silicon dioxide and aluminum oxide with nanometer-sized pores that are rigid and inflexible. Beads made of one such material, zeolite 13X, about one millimeter in diameter, are packed into two cylindrical columns in an oxygen concentrator.
The chemistry here is tailored to the task of separating oxygen from nitrogen in the air. Because they are highly porous, zeolite spheres have a surface area of ââaround 500 square meters per gram. At high pressures in the column, nitrogen is chemically closely enclosed with the zeolite. The interaction between the negatively charged zeolite and the asymmetric nucleus (quadrupole moment) of nitrogen means that it is preferentially adsorbed on the surface of the zeolite.
Oxygen remains free and is thereby enriched. Air contains 78% nitrogen, 20.9% oxygen and smaller amounts of argon, carbon dioxide, etc. Once the nitrogen is stopped, more than 90% oxygen flows out of the column. Then, by lowering the pressure in the column, the nitrogen is released, which is purged, and the cycle is repeated with fresh air.
Global Volunteering has provided very detailed instructions on how to build your own oxygen concentrator using locally available resources. In India, IISc has transferred oxygen concentrator manufacturing technology to over 20 companies.
(Co-author with molecular modeler Dr. Sushil Chandani)