Towards understanding the role of Lewis and Brønsted acidities in aluminosilicates: A multidisciplinary approach

Abstract

Catalysis is a central concept of green chemistry; it allows to limit waste and carry out reactions under milder conditions (i.e. lower temperature and pressure). A large number of reactions are catalyzed by acid species. Homogeneous acid catalysts are still widely used despite their corrosive nature and their difficult separation from the reaction medium. They can be replaced by heterogeneous catalysts; these are usually solids and are easily recoverable and reusable over several cycles. Among them, mesoporous silica materials modified by the insertion of a heteroelement (i.e. different from Si) are widely used. The insertion of trivalent cations such as aluminum into an SiO2 framework generates a combination of Lewis and Brønsted acid sites. These aluminosilicates are very common in heterogeneous catalysis. However, there is a lack of fundamental understanding on the precise evaluation and of the Lewis/Brønsted (L/B) acid balance of these materials. This work focuses on understanding the L/B acid balance in aluminosilicate catalysts by combining experimental and computational methods. A rapid synthesis at room temperature of aluminosilicate nanospheres has been optimized by varying the nature of the aluminum precursor and the Al loading. All the materials were deeply characterized using various techniques and tested as catalysts for the conversion of glycerol to solketal. This reaction is known to be catalyzed by both Lewis and Brønsted acidities and is particularly valuable in the context of green chemistry. Indeed, glycerol is the main by-product (c.a. 10 wt%) of bio-based fuel production. The catalytic performances of the most promising material were compared to other metallosilicates. In parallel, computational chemistry tools were developed to compute the catalysts properties. A general protocol has been built to generate a reliable model of the synthesized material. The amorphous nature of the silica framework has been generated with molecular dynamics, through a melt-and-quench treatment. The next steps consisted in the production of a realistic reactive surface using both homemade codes and molecular dynamics simulations. Afterwards, tests calculations on a small silica cluster have been performed using the ONIOM method, as a proof-of-context for further investigations on Al-containing bigger systems.

Date of Award	19 Jan 2023
Original language	English
Awarding Institution	University of Namur
Supervisor	Benoit Champagne (Supervisor) & Carmela Aprile (Co-Supervisor)