Designing New Lewis and Brønsted Solid Acid Catalysts: Incorporation of Nb and Sb into High-Surface Aluminium Fluoride
Clara Patricia Marshall1,2, Thomas Braun1 and Erhard Kemnitz1
1Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489, Berlin, GER
2School of Analytical Sciences Adlershof (SALSA), Humboldt-Universität zu Berlin, Unter den Linden 6, 10099, Berlin, GER
Solid acids play an important role in the field of catalysis: they can activate numerous substrates and can be used as heterogeneous catalysts, as green alternatives in industrial processes. Therefore, research has focused on the preparation of such catalysts, in particular on the control of the acid sites in type -Lewis and Brønsted-, number, and strength. 1 AlF3 as a metal halide is a good candidate, but its crystalline modifications exhibit typically a low surface area and a low catalytic activity. This fact changed when the fluorolytic sol-gel synthesis was explored giving access to amorphous AlF3 which is characterized by a high surface area (HS-AlF3).2 This catalyst exhibits very strong Lewis acid sites similar in strength to SbF5, the strongest Lewis acid known, and can be used in heterogeneously catalyzed reactions.3
Herein, we present an approach to adjust the strength and number of Lewis acid sites and to introduce in a controlled way Brønsted acid sites in HS-AlF3. Considering the interesting properties of Nb-containing materials 4, and the strong acidity of SbF5, we modified the original fluorolytic synthesis by using several metal sources containing Nb and Sb (salts, alkoxides, and oxides) and different types of fluorinating agent. The catalysts prepared that way were characterized by several techniques. They exhibit high surface areas (160 to 290 m2/g), high number of acid sites (up to 1.4 mmol/g), and carry both, Brønsted (for the Nb-doped catalysts) 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-AlF3.