Understanding Mechanisms of Green Oxidation Catalysis by Iron-TAML Peroxide ActivatorsEPA Grant Number: R832245
Title: Understanding Mechanisms of Green Oxidation Catalysis by Iron-TAML Peroxide Activators
Investigators: Collins, Terrence J. , Ryabov, Alexander D.
Institution: Carnegie Mellon University
EPA Project Officer: Nolt-Helms, Cynthia
Project Period: September 1, 2004 through August 31, 2007
Project Amount: $242,300
RFA: Targeted Research Grant (2004) Recipients Lists
Research Category: Targeted Research
The project is aimed at understanding the basic mechanistic behavior of the oxidation catalysis exhibited by iron (III)-Tetra-Amidato Macrocyclic Ligand (TAML) catalysts that are important green chemistry catalysts. TAML catalysts, also called TAML oxidant activators, utilize hydrogen peroxide as their most important source of oxidizing equivalents. The primary objective will be to learn the underlying chemical reasons for their superior performance in a variety of environmentally important processes such as the bleaching of dyes, wood pulp, and colored effluents, the degradation of persistent organic pollutants, and the decontamination of chemical and biological warfare agents. Preliminary studies reveal that iron-TAML catalysis reaction rates compare with what is typical for related enzymatic reactions and they are capable of at least 103–104 turnovers. We will explore the hypothesis that TAML activators function in a mechanistically parallel fashion to peroxidase and catalase enzymes. Once the chemical origin of the unique reactivity for synthetic peroxide activators has been identified, the information will be used for further catalyst improvement.
The techniques of synthetic chemistry, analytical chemistry and mechanistic chemistry will be used throughout the project. The speciation of the iron-TAML complexes in water and other solvents as well as all the different intermediates that form on exposure to hydrogen peroxide will be determined by various spectroscopic techniques. The study will include analysis of the behavior and stability of the catalysts over a wide range of conditions including high temperature and extreme pH. A detailed mechanistic description will be obtained for each of the many processes that are believed to participate in or impact on the catalytic cycle—this will be built upon extensive preliminary results. This will be further achieved through the investigation of the kinetics and characterization (and possibly isolation) of key catalytic intermediates, especially the reactive intermediate (RI) via electronic, EPR and Mössbauer spectroscopies after its generation from the starting complex and H2O2 in water in the absence of added electron donors. Additionally, there will be a direct investigation of the kinetics and mechanism of RI formation and behavior by stopped-flow techniques. Transient kinetic studies will be performed of the reactivity of RI toward various electron donors of different nature allowing evaluation of the rate laws and determination of activation parameters—results will be correlated with steady state data.
By building on preliminary results, a detailed understanding of the mechanistic properties of TAML activators will be achieved. This work will give deep scientific credence to the facts that TAML activators, which mostly employ biochemically common elements, can replace polluting oxidation technologies based upon chlorine chemistry and metals. A case study will evolve showing how to practice risk reduction via a design approach to the reduction of the hazard component of oxidation chemistry supported by a detailed mechanistic foundation that will be convincing to all chemists.
Understanding the mechanisms of oxidation catalysis by Fe(III)-TAML activators is important for both academic and industrial reasons. Synthetic oxidation catalysts that function in water, utilize environmentally friendly oxidants, and display unprecedented selectivities and high reactivity can be major actors in the development of the field of green chemistry. Developing an in-depth understanding of the mechanisms will positively impact the further development of green oxidation catalysis aimed at risk elimination (as in the many developed cases of green processes as replacements for polluting processes) and risk management (as in the many cases of the detoxification of effluent streams) and will allow for the further improvement of TAML systems for various new green applications. Risk is a function of both hazard and exposure. TAML activators reduce the hazard component when compared with incumbent technologies. There are many examples of this now so that when the full story emerges, we will have shown how TAML activators have been used to eliminate hazard over a wide range of technologies. This TAML risk reduction story will be backed by mechanistic understanding consolidated by the studies proposed herein that show how the processes work and how to more effective catalysts can be designed. Therefore, the work will provide a case study of how to practice risk reduction.