|M.Sc Student||Zaslavski Ekaterina|
|Subject||Effect of Metal Type on the Catalytic Performance of|
Inorganically Grafted Polymers
|Department||Department of Chemical Engineering||Supervisor||Dr. Oz Gazit|
|Full Thesis text|
Recent environmental changes have led researchers to seek ways of replacing fossil fuels with a more sustainable resource for liquid fuels. Waste biomass is an ideal alternative being an abundant and sustainable source of organic compounds. Two key challenges arise in this respect: the first is to decompose the biomass into smaller molecular building blocks and the second is to take these platform molecules and use them to synthesize new fuels and chemicals. Herein we focus on developing catalytic materials for promoting the transformation of platform molecules.
Hydroxymethylfurfural and furfural are platform molecules derived from the hydrolysis and dehydration of biomass. Both molecules can be upgraded to form hydrocarbon fuels (C10-C20) via aldol-condensation reaction. This reaction is traditionally performed using a homogeneous acid or base catalyst. Yet, at large scale these routes suffer from the production of a significant amount of wastewater and the need for high energy consumption together with costly separation processes. Moreover, these catalysts also catalyze the degradation of the desired product, limiting reaction selectivity. Hence, there is a need for the design and synthesis of more efficient catalysts.
The use of a heterogeneous acid or base catalyst instead of the homogeneous one offers a partial solution, giving high reactivity, but lacks in the ability to provide high product selectivity. Our approach for enhancing the performance of the heterogeneous catalyst is to adopt key principles and mechanisms employed by enzymatic systems. Cooperative interaction between organic active sites and metal active centers enable enzymes to achieve high activity and selectivity under mild reaction conditions.
Like enzymes, some polymers can be used in the similar cooperative manner. Herein, we rely on a Chitosan (Cs) polymer which has basic amine sites (NH2) in addition to mildly acidic hydroxyl groups (OH). Cs is grafted by metal oxide sites to open up and rigidify its structure, allowing reactants and products unperturbed diffusion to and from the catalytic active sites.
In this research we study cooperative interaction in our metal-grafted Cs and the effect on the catalytic performance in condensation reaction. We have developed a controlled synthesis procedure for grafting Cs with three metal precursors SiCl4, TiCl4, and SnCl4. These new materials were characterized and tested under batch reaction conditions.
Our results, analyzing the Henry reaction, demonstrate first order kinetic behavior, for both the grafted and un-grafted Cs, with a linear rate increase as a function of the concentration of amine sites. Sn@Cs showed 10 fold higher initial rate over the un-grafted Cs. The activation energy for the Henry reaction is found to decrease in a monotonic fashion for Cs > Si@Cs > Ti@Cs > Sn@Cs, corresponding well with the increase in the acidity of the metal oxides. Selectivity of the Henry reaction is shown to change as a function of amine to metal (NH2 : M) ratio providing more of the unsaturated product as NH2 : M is decreased. However, a full understanding of the mechanism in which this reaction is catalyzed by these materials is still under investigation.