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October 1, 2010

Jekyll and Hyde material

A porous polymer network that researchers can make reactive at will can store gases and hasten chemical reactions

Figure 1: A schematic representation of a polymer network after irradiation by ultraviolet light. Orange balls, light green balls and pink panels represent the nitrogen atoms of nitrene, zinc ions, and azide molecules, respectively. The three-dimensional porous structure is constructed by mutual interdigitation of two-dimensional layers.  

Reproduced from Ref. 1 © 2010 Hiroshi Sato et al.

A team of researchers in Japan has developed a porous material, decorated with a highly reactive ‘species’ of molecules that can be activated remotely using a technique called photoactiviation1. Since porous materials have large surface areas for a given volume, they can be used for gas storage, and for the acceleration of chemical reactions. The ability to turn these molecular species ‘on’ or ‘off’ increases their utility. The novel porous material is also unique for its high degree of reactivity, which traditionally has been difficult to achieve while maintaining material stability.

Ryotaro Matsuda, Susumu Kitagawa of Kyoto University, the Japan Science and Technology Agency and the RIKEN SPring-8 Center and their colleagues, from these institutes and Japan Synchrotron Radiation Research Institute, made their porous material from a polymer network with an interlinked structure (Fig. 1) constructed from aryl azide molecules, which are relatively inactive but produce the highly reactive molecule aryl nitrene when irradiated with ultraviolet light. 

The researchers exposed a single crystal or crystalline powder of their novel polymer to ultraviolet light and then measured the result with infrared spectroscopy, spin resonance and x-ray diffraction. Their results indicated that the irradiation converted a significant fraction of the dormant azides into reactive nitrenes, without disrupting the underlying porous network. The product was therefore a set of dense, highly reactive nanoscale pores. 

The reactivity of these pores imparted new functionality to the polymer network, explains Matsuda. For example, the polymer’s oxygen storage capacity increased by a factor of 29 after irradiation. The researchers also observed nitrenes reacting with carbon monoxide, suggesting that the polymer could be used to detect or filter this dangerous gas. Furthermore, because reactive species besides nitrenes can also be activated in this way, the technique has the potential to allow the capture and conversion of a variety of gases. The ability to increase the storage capacity or the speed of a chemical reaction remotely, and at a particular time, also significantly increases the range of available applications, he notes. 

The approach represents the confluence of well-understood photochemistry and the materials science behind porous networks, with potential implications for devices such as sensors and purifiers. However, while the initial results are promising, several critical features need to developed, according to Matsuda. “In addition to demonstrating the trapping of gases besides oxygen and carbon monoxide, we need to make our material reusable,” he says. “Currently, it cannot desorb gas molecules after the photoreaction, and therefore cannot be reused.”

References

  1. Sato, H., Matsuda, R., Sugimoto, K., Takata, M. & Kitagawa, S. Photoactivation of a nanoporous crystal for on-demand guest trapping and conversion. Nature Materials 9, 661–666 (2010). (Link)