A comparative study of different fluorine-containing compounds in the preparation of novel alumina binders with rich Brönsted acid sites

Alumina is commonly used as a catalyst binder together with aluminum sol in modern fluid catalytic cracking (FCC) catalysts. The surface acidity properties of alumina strongly affect the catalytic performance of FCC catalysts. Lewis acid sites tend to produce coke because of their dehydrogenation activity, while Bronsted ones produce less coke. Thus, it is beneficial to convert the surface Lewis acid sites into Bronsted type. Fluorine-containing modifiers have been demonstrated to be effective to generate Bronsted acid sites on alumina surface. However, different types of fluorine-containing compounds may have different modification effects. In this work, three fluorine-containing compounds, ammonium fluoroborate (NH4BF4), ammonium fluorosilicate [(NH4)2SiF6], and ammonium fluoride (NH4F), were tested and compared in the modification of alumina surface acidity. Results show that NH4BF4 and (NH4)2SiF6 perform equally well in the generation of Bronsted acid sites, while NH4BF4 is more effective in the reduction of Lewis acid sites. In comparison, NH4F is not so effective in the generation of Bronsted acid sites as the other two compounds.