Imogolite is a naturally occurring mineral in clay. It consists of nano-scale hollow tubes of aluminum silicate. However, only minute quantities of imogolite can be obtained in natural clays, and obtaining it is quite difficult. This method synthesizes imogolite in concentrations 20 to 30 times higher than the levels achieved by conventional methods. This large-scale synthesis produces quantities of imogolite that can now be applied to industrial needs such as absorbents, binders, and desiccants. The material presents physical properties that are different from, and differently useful than, other absorbents and desiccants such as silica gel, zeolite, and allophone (also a natural clay mineral).

 

In the synthesis of imogolite, the mixing of aqueous solution of monosilicate and aluminum aqueous solution forms imogolite precursor. By removing the coexisting anion as the formation inhibiting factor using a centrifuge and continuously adding the precursor into acid solution, it becomes possible to synthesize imogolite at 20 to 30 times higher concentrations than the levels achieved by the conventional method.

 

The raw materials for imogolite synthesis are inexpensive and inorganic.

Imogolite's physical properties can be compared to other absorbents such as zeolite, silica gel, and allophone (also a natural clay mineral). However, imogolite absorbs and desorbs better than all three. At high humidity – 90%+ -- Imogolite absorbs and desorbs at extremely high rates, rates that are much higher than competing absorbents.

 

As a desiccating agent, Imogolite absorbs much faster than allophone, zeolite, or silica gel, and absorbs for a much longer period of time.

 

For example, zeolite is used as an absorbent in a PSA gas separation system. Since zeolite does not release CO₂under small pressure changes, lots of energy is required in order to make it a vacuum. Imogolite, whose length is controlled appropriately, can release CO₂very easily under only a small pressure change. AIST is now investigating the possibility of imogolite as CO₂absorbent.

 

The properties of imogolite vary with the length of the tube, which can be controlled during synthesis.

 

As an example, Figure 1 shows water absorption of imogolite with tube lengths of 10-100nm (outer diameter is 2.5nm and inner diameter is 1nm). This variety of imogolite shows very high water absorption, even when the relative humidity is high (more than 90%). This is a unique property compared to other absorbents such as zeolite, silica gel, and allophone (a natural clay material). This suggests that this variety of imogolite offers a promising application as an anti-dewing agent and as the heat-exchange medium for a heat pump.

 

 

Figure 1

 

Figure 2 shows that a variety of imogolite whose length is 10-100nm has a very high adsorption rate of water vapor. This result suggests that the imogolite can be applied as a fast-drying desiccating agent, and as the desiccating medium of an air-conditioning system. AIST is now developing an air-conditioning desiccating system that uses imogolite.

 

Figure 2

 

Other promising applications based on the physical properties of imogolite include:

·         Fast-drying desiccating agent

·         Anti-dewing agent

·         Heat-exchange medium for heat pump

·         Self-regulating humidity conditioning material (such as wall tile or wallpaper)

·         Desiccating medium for an air-conditioning system

·         CO₂absorbent for a PSA gas-separation system

·         Controlled release agents

 

 

Technology

In the synthesis of imogolite, the mixing of aqueous solution of monosilicate and aluminum aqueous solution forms the imogolite precursor. By removing the coexisting anion as the formation inhibiting factor using a centrifuge and continuously adding the precursor into acid solution, it became possible to synthesize imogolite at 20 to 30 times higher concentrations than the levels achieved by the conventional method.

 

In the use of imogolite as a heat exchange material for heat pump, the most appropriate characteristics for using low-temperature exhaust heat have already been made known.  It became apparent that even in low-temperature condition around a dehydration temperature of 40 degrees C a considerable amount of heat exchange could be achieved.

 

As anti-dewing agent, imogolite can adsorb about twice its weight if humidity exceeds 90 percent. If humidity is below 90 percent, however, it releases almost the entire moisture.  It also has a characteristic that it can be used repeatedly, unlike the conventional anti-dewing agents. Like carbon nanotube, imogolite is expected to draw attention as a new nanotube material in the future. Besides the application areas described above, it is hoped that imogolite will be applied to various other areas.