Vol.9 No.3 2017

Research paper : Development of HASClay® as a high-performance adsorption material (M. Suzuki et al.)−156−Synthesiology - English edition Vol.9 No.3 (2017) the adsorption isothermal curves can be roughly divided into two. One is the S-shaped curve that shows rapid start-up in a certain relative humidity range as in the low-temperature reused zeolite and mesoporous silica, and the linear type in which the relative humidity and water vapor adsorption volume show a linear relationship without rapid start, as in the polymer adsorbents and allophane. With adsorption heat pump, the operating environment temperature, waste heat temperature, and cold water temperature in the system are controlled, and because the operating humidity range is determined by these three temperatures, an adsorbent with an S-shape with rapid start-up in the operating humidity range is suitable. On the other hand, because the desiccant air conditioning takes in air with varying humidity, an adsorbent with a linear property where adsorption is possible at a wide humidity range is more suitable.In this context, (1) no merit was seen in developing adsorption heat pump because the device size would be twice as large as the absorption heat pump that was already widely used, and (2) the adsorption isothermal curves of allophane and imogolite that we had been studying showed a linear characteristic in the medium humidity range. Therefore, we decided to target desiccant air conditioning, and develop a linear inorganic water vapor adsorbent that has performance equivalent to polymer adsorbents. 2 History of the development (1990s–2005)2.1 Nanoparticles that exist in the soilThe National Industrial Research Institute of Nagoya (NIRIN), Agency of Industrial Science and Technology to which the authors belonged had been researching ceramics and metals in the Chubu region since its inception in 1952. The work of the Imperial Ceramic Experimental Institute, which was one of the institutes integrated when NIRIN was established, was carried over to Unit 6 (later, Department of Ceramic Technology), and the ceramics research was continued there. The authors were at the Raw Materials Technology Laboratory, Department of Ceramic Technology, and engaged in the research on clay, the raw material of ceramics, as well as utilization of pore that was a major clay characteristics. Allophane and imogolite were clays that had interesting forms.When one hears of nano-capsules and nanotubes, one immediately thinks of C60 fullerene or carbon nanotubes, but nano-capsules and nanotubes exist in the weathered soil derived from volcanic ash. These are substances known as allophane and imogolite. Allophane has the shape of nano-capsules with a diameter of 3.5–5.0 nm, while imogolite is in the shape of nanotubes with an exterior diameter of 1.8–2.2 nm and length of several ten nm–several μm. The schematic diagrams of allophane and imogolite structures are shown in Fig. 3,[5][6] and the transmission electron microscope (TEM) photographs are shown in Fig. 4.[7] The basic structure of allophane and imogolite is the SiO4 tetrahedron in which the pinnacle is the hydroxyl group bond, and the bottom three oxygen atoms are shared with aluminum, inside the gibbsite sheet composed of aluminum hydroxide. This structure does not include the Si-O-Si bond, and the distance between oxygen atoms of the gibbsite sheet that is bonded to the silicone is quite short. The gibbsite sheet is curved due to this mist, and this gives the characteristic structure to allophane and imogolite.Since allophane and imogolite can be seen occasionally in the soil derived from volcanic products such as pumice or volcanic ash, they were mainly studied in the field of soil science until about 1990, but later, they were investigated for their functions as catalysts or gas storage materials. They not only have special nano-size shapes and high specic surface areas, but also have excellent hydrophilicity and adsorption capacity. Therefore, they are studied for application to adsorbents of hazardous pollutants, humidity controlling agents that autonomously control the humidity in the living area, polymer nano-composites, and for application in medical elds.[8]2.2 Development of humidity controlling building materials using allophaneJapanese buildings were traditionally constructed with wood and mud, and therefore, had good humidity control, Fig. 2 Water vapor adsorption isothermal curve of the adsorbents that can be reused by low-temperature treatment(AQSOA®-Z01[2], AQSOA®-Z02[2], TMPS[3], TAFTIC®-HU[4])Water vapor adsorption volume (wt%)Relative humidity (%): Zeolite (AQSOA®-Z01): Zeolite (AQSOA®-Z02): Mesoporous silica (TMPS): Silica gel A type: Silica gel B type: Allophane (natural, from Kitakami): Polymer adsorbent (TAFTIC®-HU)0010202030404050606070808090100: Imogolite (natural, from Kitakami)

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