Zeolite oxygen concentrators: chemistry in three dimensions

At the heart of this technology are synthetic frameworks of silica and alumina with nanometer-sized pores that are rigid and inflexible

Updated - October 16, 2021 11:56 pm IST

Published - October 16, 2021 08:55 pm IST

Preferentially adsorbed: Zeolite beads separate oxygen from nitrogen in air by tightly binding to hydrogen, while oxygen remains free.

Preferentially adsorbed: Zeolite beads separate oxygen from nitrogen in air by tightly binding to hydrogen, while oxygen remains free.

Chemists, when they are designing or building new molecules, can be thought of as architects and builders. An organic chemist can plan a blueprint for a new molecule, and synthesize it with precision out of atoms of carbon, oxygen, hydrogen and so on. After centuries of fine-tuning this skill, chemists in the early 20th century moved up to synthesizing long, thread-like one-dimensional polymers. The polyethylene of plastic bags is made from repeating units of the ethylene molecule, (importantly, the units are linked by the same firm chemical bonds as are seen within an organic molecule. This provides the stability that ensures that a shirt made from polyester-mixed yarn is long lasting). In biological systems, proteins are 1-dimensional polymers of amino acids.

Adding new dimensions

In recent years, this has been taken to a new level by the creation of extended 2- or 3-dimensional structures from linking together molecular units just as was done for polymers, but in two or three dimensions. The basic units go on fitting together to form large networks, like a wire mesh fence. The network is constructed by repeated additions of a molecule with symmetry. A few such networked sheets, when stacked one over another, form a functional 2-D entity. Because words like polymer do not do justice to this complex arrangement of atoms, such molecular networks are called frameworks.

Uses for these Covalent Organic Frameworks (COFs) take advantage of their stability, large surface area, controlled pore sizes, and tunable chemical environments. Just as you choose the size of the ‘pore’/hole in a wire mesh, frameworks can be designed to act as sieves in separating out molecules of a specified size. The smallest whiff of a toxic gas could be sensed - in an industrial environment, or in airline baggage. They are also suitable for both storing energy (as capacitors) and for conducting it (along membranes in fuel cells).

Metal Organic frameworks (MOFs) are structured like COFs but have metals in complexes with organic entities. The choice of metals is wide, from Beryllium to Zinc, though relatively abundant metals are preferred for economic and environmental reasons. They offer great advantages: for gas storage, as in the case of hydrogen storage in fuel cells; in catalysis, where they replace very expensive metals; in sensors; and in drug-delivery – anti-cancer and other drugs with severe side effects can be trapped in the porous confines of MOFs, to be released in small and steady doses.

Use of zeolites

Zeolites are highly porous, 3-D meshes of silica and alumina. In nature, they occur where volcanic outflows have met water. Synthetic zeolites have proven to be a big and low-cost boon. One biomedical device that has entered our lexicon during the pandemic is the oxygen concentrator. This device has brought down the scale of oxygen purification from industrial-size plants to the volumes needed for a single person. At the heart of this technology are synthetic frameworks of silica and alumina with nanometer-size pores that are rigid and inflexible. Beads of one such material, zeolite 13X, about a 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 air. Being highly porous, zeolite beads have a surface area of about 500 square meters per gram. At high pressures in the column, nitrogen is in a tight embrace, chemically speaking, with the zeolite. Interaction between the negatively charged zeolite and the asymmetric nucleus (quadrupole moment) of nitrogen causes it to be preferentially adsorbed on the surface of the zeolite.

Oxygen remains free, and is thus enriched. Air has 78% nitrogen, 20.9% oxygen and smaller quantities of argon, carbon dioxide, etc. Once nitrogen is under arrest, what flows out from the column is 90%-plus oxygen. After this, lowering the pressure in the column releases the nitrogen, which is flushed out, and the cycle is repeated with fresh air.

Global volunteer efforts have made available very detailed instructions on building your own oxygen concentrator, with locally available resources. In India, IISc has transferred the technology of making oxygen concentrators to over 20 companies.

( Co-authored with molecular modeler Dr. Sushil Chandani )

dbala@lvpei.org and sushilchandani@gmail.com

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