Thermal Evolution of 4- and 5-fold Coordinated Al-Sites in Aluminum Hydroxide Fluorides with Low Fluorination Degree

Abstract

A modified approach of the fluorolytic sol–gel synthesis was used to synthesize pure, amorphous aluminum hydroxide fluorides with very low and easily adjustable fluorination degrees, as well as controllable surface areas. The careful investigation of their thermal behavior revealed that these aluminum hydroxide fluorides undergo dehydroxylation reactions already at very low temperatures (i.e., below 80 °C), which is in contrast to all crystalline aluminum hydroxide (fluoride) phases. 27Al and 19F MAS NMR experiments unambiguously demonstrated that this dehydroxylation is clearly linked to a drastic reorganization of local structures and leads to a remarkable evolution of subcoordinated Al-species. A maximum intensity of about 45% for 5-fold (AlV) and 54% for 4-fold (AlIV) coordinated Al-sites was found for sample AlF0.25(OH)2.75, calcined at elevated temperatures. The investigations also revealed that the relative intensity of AlIV- and AlV-species essentially depends on the fluorination degree.