Continuous Hollow Skeletal Macroporous Aluminum Hydroxide Monoliths and Their Calcined Alumina Derivatives with High Diffuse Reflectance

Macroporous aluminum hydroxide monoliths with continuous hollow skeletons were fabricated through a scalable, template-free sol-gel synthesis from metal-salt precursors. Tuning the concentration of propylene oxide, which serves as a proton scavenger, yielded crack-free monoliths several centimeters in diameter. Scanning electron microscopy (SEM) revealed a three-dimensional porous network composed of an interconnected nanoplatelet shell. The as-prepared sample exhibited a BET specific surface area of 429 m² g−¹. Calcination at 500 °C converted the framework into γ-alumina while preserving the hollow architecture and a high specific surface area of 314 m² g−¹. Subsequent heating to 1250 °C produced α-alumina while preserving the monolithic shape and continuity of the skeletal framework; however, the hollow structure collapsed, and the surface area decreased significantly. Optical measurements showed that aluminum hydroxide and α-alumina monoliths both exhibited high total diffuse reflectance across the UV–Vis–NIR range (400–1200 nm), comparable to Spectralon. The α-alumina retained over 90% reflectance up to 2500 nm and demonstrated excellent thermal stability. These results suggest that the fabricated monoliths could be used as diffuse reflectance reference materials.