Preparation of monolithic SiO₂-Al₂O₃ cryogels with inter-connected macropores through ice templating

Macroporous monoliths of SiO₂-Al₂O₃ cryogels were prepared. Macropores were generated by using ice crystals as the template, while the walls which surround the macropores were tailored as porous cryogels by freeze drying. Macropores and walls formed honeycomb-like structures, which were confirmed from scanning electron microscopy images of cross-sections of the samples. It was confirmed that the sizes of the macropores and the wall thicknesses were respectively in the ranges of 10-20 m and 200-500 nm. Al mapping analysis by energy dispersive X-ray diffractometry showed that Al atoms were homogeneously dispersed throughout the samples without local aggregation. Moreover, Raman spectroscopy and ²⁷Al NMR spectroscopy indicated that Al atoms were incorporated into the silica framework by forming an Al-O-Si polymeric network. Nitrogen adsorption-desorption measurements indicated that the walls were micro/mesoporous with high BET surface areas (>700 m² g^-1) and large pore volumes (>0.45 cm ³ g^-1). Moreover, NH₃-TPD measurements revealed that the samples had acid sites, which allowed this material to be used as a solid acid catalyst.