Silica-alumina Gel Beads With Bimodal Pore Structure And Their Humidity Control Properities

The environment comfort inside building has directly affected people’s health, the parameters of the environment have been the focus of attention and the relative humidity in the air is one of important factor affecting human health, all kinds of items stored and decreasing energy consumption. It’s the optimal choice for both people and goods when the relative humidity remains the range from 40% to 60%. Developing and applying the humidity control materials with perfect properties to all aspects of society, for example, improving the residential environment, promoting the healthy growth of plant, enhancing the quality of food preservation, medicine, heritage, ensuring the normal operation of the instrument and maintaining the ecological environment for sustainable development, undoubtedly have important application value and social significance. According to the special requirements of textile cultural relics, making full use of the properties of bimodal structures of silica-alumina gel, the spherical shape humidity control beads were prepared, and its properties were measured. The relationship between pretreatment humidity and target humidity was concluded.In this study, an amorphous spherical silica-alumina beads material with a distinct bimodal pore structure was prepared by sol-gel reactions of tetraethoxysilane（TEOS） and aluminum nitrate in the presence of polyethylene glycol（PEG）. Liquid Paraffin and 1,2-Dichloropropane severed as a co-solvent preparing a silica-alumina gel beads in the process of shaping spherical morphology. Silica-alumina gel with micrometer-sized macropores and nanometer-sized mesopores has excellent advantages in humidity control properties. The macropores provide pathways for rapid H2 O molecular transportation while the mesopores present a large surface area. The appearance and structural properties of the material were investigated by digital camera, scanning electronic microscope（SEM） and thermal gravimetry and differential thermal analysis（TG-DTA）. The porosity features are further established by N2 sorption experiments and pore size distribution analysis. The bonding characteristics and coordination features around aluminum have been studied using fourier transform infrared spectroscopy（FTIR） and 27 Al magic angle spinning nuclear magnetic resonance（27Al MAS NMR）. The humidity control performance of the material at relative humidity was measured. The conclusions are as following.（1） The conclusions of humidity control measurement of silica-alumina gel beads indicated that: the Al atoms are homogeneously dispersed without local aggregation in the silica framework. It is considered that Al cations are embedded in the silica network and coordinated by organic addictives, which results in increasing thermal stability. When the reaction conditions meet all the requirements: the molecular weight of PEG is 10000, the calcination temperature of 500 oC, Si/Al = 9.99. The best humidity control results as follows: the material kept a relative humidity in the range of 39.3%-40.3%, 47.6%-50.7% and 57.2%-60.7%, respectively, under three different pre-treating conditions. And the humidity controlling equilibrium levels were all reached within 3 h.（2） The aim of using three kinds of salt solution containing NH⁴⁺（NH₄NO₃, CH₃COONH₄, NH₃·H₂O） for modifying silica-alumina gel beads is to enlarge the size of mesopore. The results indicated that the size of mesopore increased to 9.068 nm, yet the specific surface area decreased from 620.257 m2·g-1 to 253.558 m2·g-1, which resulted in the wide humidity control range（45.5%-52.3%）. Compared with the unmodified silica-alumina gel, the size and specific surface area of silica-alumina with ammonium nitrate modification increased to 2.847 nm and 718.401 m2·g-1 respectively, but the absorption properties decreased in some degree. The specific surface area of the modified silica-alumina with ammonium acetate increased to 826.767 m2·g-1 and the group of [Al O4]-1 occupied the largest proportion of aluminum. In conclusion, the target humidity can be controlled between 48.5%-50.4%（the extremely small equilibrium）.In this paper, the humidity control material has the characters of structural stability, simple preparation technology, good humidity control properties, circulate and repeatability of humidity control properties.