Green Oxidation Catalyst

Background

Oxidation reactions involve the loss of electrons or the addition of oxygen and reduction is the opposite. These reactions are coupled, occur together and are referred to as redox reactions. Hydrogen peroxide is an example of an oxidizing agent that will remove hydrogen atoms. The decomposition of hydrogen peroxide (H₂O₂) is shown in Equation 1.

Equation 1: Decomposition of Hydrogen Peroxide

\[\ce{H2O2 -> 1/2 O2 +H2O}\label{1}\]

Hydrogen peroxide is a cleaner bleaching alternative to chemicals more commonly used like chlorine (Cl₂), chlorine dioxide (ClO₂) and sodium hypochlorite (NaOCl). These chemicals result in the formation of hazardous byproducts like dioxins¹ (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): 2,3,7,8-Tetrachlorodibenzo-p-dioxin

Acquired from: http://webbook.nist.gov/cgi/cbook.cgi?ID=1746-01-6

The problems with using H₂O₂ as a bleaching agent are that it requires high temperatures and pressures as well as longer reaction time.¹ Dr. Terry Collins (Thomas Lord Professor and Director, Institute for Green Science, Carnegie Melon University, Pittsburgh Pa) developed a catalyst to solve these problems. This oxidative catalyst, Fe-TAML^®(Figure 2), allows H₂O₂ to react at lower temperatures and reduces the time required for the reaction to take place. This molecule also allows the peroxides to be more selective leading to the desired results more efficiently.

Figure \(\PageIndex{2}\): Fe-TAML^®

Modified from www.chem.cmu.edu/groups/colli...out/about.html

Fe-TAMLs are made with an iron (Fe³⁺) at the center surrounded with a ring of nitrogen, which is surrounded by other compounds to increase its ability to undergo reactions without being broken apart. In addition to having four nitrogens attached to the Fe³⁺ there is also a water molecule attached to the iron (figure 2). When the Fe-TAMLs are dissolved in water, another water molecule will attach to the opposite side of the iron. In the case of a solution containing H₂O₂, the H₂O₂ will attach instead of a water molecule. Then it will oxidize Fe³⁺ to [Fe⁴⁺]-OH and finally [Fe⁵⁺]=O. The oxygen will pull electrons away from the Fe causing it to become more positive. The more positive the iron the more reactive it is.⁵ Fe-TAMLs were designed to act like a biological catalyst and speed up the oxidation-reduction reactions that need to occur to eliminate various types of pollutants. As stated before H₂O₂ is highly oxidizing, but the Fe-TAML helps the oxidation reactions occur more quickly and more often by helping to take electrons from the oxidizable molecules.⁵ =====

Equation 2: Oxidation of Fe-TAML by Hydrogen Peroxide

\[\ce{Fe(3+) + H2O2 -> [Fe(5+)]=O + H2O}\]

Reducing Agent=Iron Oxidizing Agent= Hydrogen Peroxide =====

Equation 3: Redox Reaction between Oxidized Iron and Ruthenium Dye⁶

\[\ce{[Fe(5+)]=O + Ru(2+) -> Fe(3+) + Ru(3+)}\]

Reducing Agent= Ruthenium Oxidizing Agent= Iron

Use as a Bleaching Agent

The pulp and paper industry uses wood pulp to make paper. Wood pulp contains cellulose and lignin. Cellulose is used in making white paper and lignin is colored and must be removed. In order to remove the lignin, bleaching is done using mainly chlorine dioxide. Microorganisms then break down small lignin fragments and large ones remain. These large fragments are released with the digested, called effluent, and can stain waterways thus altering the light absorbed.² According to research done by PAPRO New Zealand, Ltd (an industrial collaborator of Collins), 56% of the color is removed from the effluent at room temperature using only small amounts of peroxide and very small amounts of the TAML activator (ca. 500nM). Collins’s team has had higher color reductions in North America² and a 70% reduction in color has been reported¹. (Figure 3)

Figure \(\PageIndex{3}\): Showing a 70% Color Reduction of Effluent

Modified from: portal.acs.org/preview/fileFetch/C/CTP_005623/pdf/CTP_005623.pdf

Collins has also proposed that Fe-TAML^® could be used in the laundry business. It is capable of bonding free dyes and preventing them from bonding to fabric. This has applications in the prevention of colors running, in other words you could wash a red and white shirt together without fear of ending up with a pink shirt. In an interview⁴ he stated that this would be especially important in areas where water may be scarce and clothes need to be washed with little water⁵. This is because dyes will reattach to fabrics more readily with less water. Click the link below to view a video showing Fe-TAML^® removing dye.

Watch a Video of Fe^III-TAML^® breaking down Orange^II

From ChemPRIME: 11.14: Redox Reactions

References:

1. Parent, Kathryn E. Bleaching with Green Oxidation Chemistry. Green Chemistry Institute American Chemical Society. Available from: portal.acs.org/preview/fileFetch/C/CTP_005623/pdf/CTP_005623.pdf

2. www.chem.cmu.edu/groups/Collins/res/dyes.html Copyright 2001-2010 Institute for Green Science.

3. www.chem.cmu.edu/groups/colli...bleaching.html

4. Interview of Dr. Terry Collins with Dr. Moira Gunn. Tech Nation: Endocrine Disruptors. Available from: http://itc.conversationsnetwork.org/shows/detail3585.html#

5. Collins, Terrance J. and Chip Walter. 2006. Little Green Molecules. Scientific American. Available from: www.glatfelter.com/Files/environment/Article_Little_Green_Molecules.pdf

6. Environmental Protection Agency. 2008. Final Report: Understanding Mechanisms of Green Oxidation Catalysis by Iron-TAML Peroxide Activators. Available from: http://cfpub.epa.gov/ncer_abstracts/INDEX.cfm/fuseaction/display.abstractDetail/abstract/7492/report/F

7. www.chem.cmu.edu/groups/Colli...bleaching.html

Contributors and Attributions

• Ed Vitz (Kutztown University), John W. Moore (UW-Madison), Justin Shorb (Hope College), Xavier Prat-Resina (University of Minnesota Rochester), Tim Wendorff, and Adam Hahn.