Aluminium fluoride-based catalysts as Lewis and Brønsted solid superacids
Clara Patricia Marshall1,2, Thomas Braun1 and Erhard Kemnitz1
1Institut für Chemie, Humboldt-Universität zu Berlin, Unter den Linden 6, 10099, Berlin, GER
2School of Analytical Sciences Adlershof – SALSA, Humboldt-Universität zu Berlin, Unter den Linden 6, 10099, Berlin, GER
Solid acids are the main catalysts in the petrochemical and hydrocarbon industry. More than 127 different types of industrial processes use solid acid-base catalysts, most of them being zeolites and metal oxides. Extensive research is done on the preparation of such catalysts, in particular on the control of the acid sites in type -Lewis and Brønsted-, number, and strength.
On the other side, metal halides have always been awarded a niche role in the field of catalysis. Analogous to metal oxides, metal fluorides should exhibit higher Lewis acidity because of the higher electronegativity of fluorine; however they have usually a low surface area and a low catalytic activity. This changed in 2003, when Kemnitz et al. introduced the fluorolytic sol-gel synthesis to access amorphous metal fluorides. AlF prepared by this way is characterized by a high surface area (HS-AlF) and exhibits very strong Lewis acid sites, similar in strength to SbF, the strongest Lewis acid known.
Herein, we present an approach to modify the strength and number of Lewis acid sites and to introduce in a controlled way Brønsted acid sites in HS-AlF. For this, the interesting properties of Nb-containing materials were considered, in addition to the possibility to prepare aluminium hydroxide fluorides. Thus, we prepared a series of novel catalysts by modifying the original fluorolytic synthesis using various dopants (Nb and Sb metal sources) and different types of fluorinating agent.
The catalysts prepared this way were characterized by several techniques such as MAS NMR spectroscopy, adsorption of basic probe molecules (e.g. CO, pyridine, CDCN), and nitrogen physisorption. The catalysts exhibit high surface areas (up to 290 m2/g), high number of acid sites (up to 1.4 mmol/g), and carry both, Brønsted and Lewis acid sites. Their catalytic activity was tested in a number of reactions, where improved conversion values (e.g. 99% in the cyclization of citronellal) were obtained for selected catalysts when compared to the original HS-AlF or other solid superacids such as aluminium chlorofluoride (ACF). ,