Silica-alumina gel humidity control beads with bimodal pore structure produced by phase separation during the sol–gel process

• Spherical silica-alumina beads material with bimodal porous structure was prepared.
• Al atoms' adding increases BET surface area, total mesopore volume and improves heat resisting property of the material.
• The material has high desorption ability and small equilibrium humidity control range.
• The material has industrial mass production perspective.

Silica-alumina gel with micrometer-sized macropores and nanometer-sized mesopores has excellent advantages in humidity control properties. The macropores provide pathways for rapid H2O molecular transportation while the mesopores present a large surface area. 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 poly(ethyleneoxide) (PEO). It features high moisture adsorption capacity, super fast response to humidity changes, small equilibrium humidity control range and macroscopic spherical morphology. The appearance and structural properties of the material were investigated by digital camera, scanning electronic microscope (SEM) and thermal gravimetry and differential thermal analyses (TGA/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 27Al magic angle spinning nuclear magnetic resonance (27Al MAS NMR). The humidity control performance of the material at relative humidity was measured. The results revealed that 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. Therefore, the material could maintain a suitable relative humidity of 40–60% in enclosed spaces such as showcases in museums and galleries.

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