Influence of self-organization of Al₂O₃ and TiO₂/Al₂O₃ nanofilaments into nanotubes caused by high temperature hydrogen treatment on propane cracking

Introduction: New nanomaterials based on porous amorphous aerogels composed of Al₂O₃ nanofilaments with a diameter of 5 nm were obtained by oxidation of metallic aluminum. Al₂O₃ nanofilaments were covered by a TiO₂ monolayer to prepare a TiO₂/Al₂O₃ composite aerogel. It has been shown that the hydrogen treatment of inactive aerogel formed by alumina nanofilaments and its composite TiO₂/Al₂O₃ at temperatures of 923-1173 K gave rise to a catalytic activity in the cracking of propane. It was found that the hydrogen treatment at high temperatures resulted in the self-organization of Al₂O₃ intertwined nanofilaments (5 nm in diameter) and TiO₂/Al₂O₃ composite into the η-Al₂O₃ nanocrystalline hollow nanotubes with a diameter of 33 nm and 6 nm-thick walls followed by packing these nanotubes into bundles. This transformation was evidenced by electron microscopy, transmission electron microscopy, low-temperature adsorption of nitrogen vapors, X-ray fluorescence and X-ray diffraction. Method: The high-temperature hydrogen activation of aerogels prepared from Al₂O₃ and Al₂O₃/TiO₂ nanofilaments significantly added to their efficiency in the cracking of propane compared both to the commercial platinum catalyst supported by alumina and the thermal cracking, as well. Result & Conclusion: The mesoporous crystalline structure of alumina nanotubes ensured a high selectivity (63% ethylene) and a catalytic activity (0.3 mmol/g×s) in the cracking of propane over the temperature range of 923-1123 K. In the case of TiO₂/Al₂O₃ nanotubes, the reaction products changed from ethylene mainly to propylene with the selectivity of 68% and the catalytic activity of 0.1 mmol/g×s in the same temperature range.