Synthesis of three crystalline forms of Al2O3 featuring rod-like fibers and their effect on the gaseous degradation of 1-chloronaphthalene

Abstract

Three crystalline forms of Al₂O₃ featuring rod-like fibers are synthesized, to perform the gaseous degradation of 1-chloronaphthalene (CN-1), a model compound. The degradation efficiency (DE) decreases as follows: micro/nano-γ-Al₂O₃ > micro/nano-η-Al₂O₃ > regular α-Al₂O₃. The high DE of 92.2% attained over γ-Al₂O₃ is attributed to its specific defective spinel structure, resulting in its large surface area and pore volume, large amounts of surface-adsorbed oxygen species, and Brønsted and Lewis acid sites. The degradation products, naphthalene and 1,4/1,6-dichloronaphthalene (DiCN), with yield ratios of 7.3–13.3, are detected, indicating the occurrence of hydrodechlorination and side chlorination reactions. 1,6-DiCN is believed to be the most favored DiCN isomer product based on the calculated ΔH_r and ΔG_r. However, the percentage values of intermediates formed via these two reactions, in the degradation of CN-1, are 9.1%, 4.5% and 4.4%, over α-, η- and γ-Al₂O₃, respectively. The oxidative degradation pathway possibly dominantly occurs especially over micro/nano-γ-Al₂O₃ with oxidative species such as O₂⁻, O⁻ and O²⁻, following the Mars–van Krevelen mechanism. Oxidation intermediates containing –CH₂–, –CH₃, and C–O groups are produced, which can be subsequently oxidized to low-molecular-weight products, such as formic, acetic and propanoic acids, and eventually completely oxidized to CO₂.