|Ph.D Student||Khoury Christine|
|Subject||Development of Grafted Acid-Base Catalysts for Biomass|
|Department||Department of Energy||Supervisor||ASSOCIATE PROF. Oz Gazit|
|Full Thesis text|
The utilization of renewable resources such as biomass is a promising route for developing sustainable processes. Aldol condensation and the glucose to fructose isomerization are reactions in green chemistry, which allows the upgrade of biomass-derived molecules to fuels and chemicals. Performing these reactions in the liquid phase using heterogeneous catalysts is more sustainable as opposed to homogeneous catalysts. Therefore, the development of reusable and highly active and selective solid catalysts for liquid reactions is drawing increasing interest.
This work aims to develop grafted acid-base catalysts for the aldol condensation and the glucose to fructose isomerization. The first three sections focus on the development of cooperative heterogeneous catalysts for the probe Henry reaction. The fourth section focuses on the development of acid catalysts for the glucose to fructose isomerization.
Cooperative interactions between acid and base sites can produce a more active catalyst for the aldol-type reactions, which can facilitate running the reactions under milder conditions. However, making a synthetic heterogeneous catalyst that utilizes cooperative interactions between active sites is a great experimental challenge. Cooperative interactions are highly dependent on the coexistence of the acid and base functions in sufficient proximity and correct orientation. Without the specific positioning of both functionalities, a significant reduction in catalytic activity is observed. The flexibility of polymers can allow tuning the right conformation at the molecular level required for inducing cooperative interactions.
In the first two parts, a hybrid metal-polymer system is used to study the parameters involved in inducing cooperative catalytic interactions in the Henry reaction. To incorporate acid and base sites, metal sites of Sn and Ti are grafted to the hydroxy groups of chitosan (CS) polymer, which is inherently functionalized with basic amine sites. The grafting of the metal to CS stabilizes the structure and introduces Lewis acid sites. The results show that the synthesis conditions affect the three-dimensional structure of the obtained materials. Combining several spectroscopic techniques with kinetic measurements allowed us to determine the effect of the material structure on the catalytic activity.
In the third section, rigid silica support is used to study new cooperative sites on a solid support. Silica supports are more chemically stable than polymers and do not suffer from fast deactivation under the Henry reaction conditions. Using the steric hindrance effect allows controlling the degree of cooperative interactions between amine and metal sites grafted on silica. Cooperative interactions between amines and Sn-OH or Ti-OH sites grafted on silica are found to enhance both the Henry reaction activity and selectivity toward the unsaturated product (from 59% to 84-92%).
The Sn-OH and Ti-OH sites on silica are also shown to have acidic properties. Hence, the use of these materials as acid catalysts for the glucose the fructose isomerization is suggested in the fourth section. Si-OH, Sn-OH, and Ti-OH sites grafted on silica are found to be active for the glucose to fructose isomerization. Specifically, Si-OH sites grafted on silica can be 100% selective to fructose under specific reaction conditions.