TAML, or tetra amido macrocyclic ligand, catalysts ‘represent a breakthrough technology in sustainability,’ says Paul Anastas, director of the Center for Green Chemistry and Green Engineering at Yale University, US. ‘When we’re looking at the challenges, whether it’s cleaner water, cleaner and more efficient manufacturing processes, efficient use of resources, we’re going to need breakthrough technologies to change where we are.’
Developed by Terrence Collins and colleagues at the Institute for Green Oxidation Chemistry at Carnegie Mellon University in Pittsburgh, US, TAML catalysts are able to destroy a range of harmful compounds, and do so without leaving a trace of themselves. They work by reacting with hydrogen peroxide, explains Colin Horwitz, a research professor at Carnegie Mellon, to produce other reactive intermediates that go on to do the oxidation chemistry to destroy the pollutant. Each catalyst comprises an iron atom surrounded by a square of nitrogen atoms within an array of carbon, onto which other groups can be designed to deal with the specific pollutant to be degraded.
In the 9 February 2007 issue of Science, Collins reports new results showing that one of these reactive intermediates is an iron(V)-oxo complex. ‘It helps to corroborate our thinking into how iron-TAMLs work their catalytic magic. And it is an anchor point and helps us think about the furtherance of our design programme,’ he says.
This design of catalysts not only provides a means to oxidise pollutants; it also offers a way to overcome an inherent problem with oxidation chemistry.
‘The big problem in designing catalysts that do oxidation chemistry is they tend to oxidise themselves. They have suicidal tendencies,’ says Horwitz. Collins, however, has created a design that allows the construction of a ‘firewall’ around the iron and nitrogen atoms, protecting the TAML catalysts from self-destructing until the pollutant is destroyed. ‘It’ll do its work and kill itself. For want of a better term, I’ve called it “dial a lifetime catalysis”,’ says Collins.
In 2006 Collins formed a company, GreenOx Catalysts, to market TAML activators. The catalyst that is on the road to the market, FeB*, can degrade a range of pollutants, including thiophosphate pesticides, dyes from the textile industry, waste products from the pulp and paper industry and chlorinated phenols.
The US Environmental Protection Agency is reviewing toxicity and related data on FeB*, and the compound has been successfully tested on a biocide called PCMC (para-chloro meta-cresol). Peter Hug, founder of Ohio, US-based firm Recombinant Innovations, explains that PCMC is used in machining fluids to kill bacteria and fungi that grow in them. While the biocide effectively destroys these microorganisms, the problem is how to get rid of the chemical, which can also destroy the microbes used in sewage treatment plants.
TAML solves that problem. ‘TAML is very effective at degrading it [PCMC],’ Hug says. ‘This stuff is not only effective; it is cost effective. Nothing goes out the other side.’ Hug’s trials used FeB* at a concentration of between 1.5 and two parts per million. ‘Our intent is to buy TAML from GreenOx, formulate it into a product and sell that,’ says Hug. He estimates the market for FeB* for this ‘niche’ application at between $10m and $15m/year.
Pulp and paper uses
A much larger market for TAMLs exists in the pulp and paper and textile industries,
both of which have field-tested the catalysts. Tests at a paper mill in New
Zealand, found that FeB* removed 36% of the adsorbable organic halogen from
a softwood bleach plant effluent, according to Collins. And pilot work at paper
mills conducted by SCION, a New Zealand government research institute that provides
scientific help for the forest products industry, shows that FeB* can effectively
deal with other environmental problems in the pulp and paper industry.
One major concern is colour in the wastewater from mills. The particular colours depend on the type of wood used in paper-making, and result from the breakdown of the lignin ‘glue’ holding the wood fibres together and which is removed during the pulping process. This colour can damage ecosystems downstream from discharge pipes by reducing the amount of light needed by aquatic plants. Conventional treatments don’t deal adequately with this problem, according to Trevor Stuthridge, a unit leader at SCION, who helped supervise the research on FeB*.
Field tests at paper mills found that, at 2ppm, FeB* removed 78% of the colour in bleaching wastewater in 15 minutes. ‘In our opinion, FeB* represents a key option for improved colour control in the pulp and paper industry. This option requires relatively small levels of investment to retrofit the system and will not generate toxic by-products or secondary wastes such as sludges,’ he says.
FeB* is similarly effective against the dreadful odours emanating from paper mills. Stuthridge reports that, in tests, FeB* at concentrations of 5 ppm was far more effective against lipophilic compounds, such as hydrogen sulfide and dimethyl disulfide, than conventional treatments, such as gas scrubbing. These compounds are readily detectable.
And in a trial at pulp and paper mill, 99% of the dimethyl disulfide was removed in four minutes.
Sulfur removal
Another TAML catalyst, meanwhile, has also been tested for its effectiveness
in making diesel fuel cleaner. ‘I think it’s got more potential in
removing sulfur from things like jet fuel,’ says Horwitz. ‘We got
about 75% of the sulfur removed from diesel in very rudimentary kinds of experiments
in the laboratory.’
Using TAMLs to remove sulfur would be another step in refining, probably the last step before the fuel gets delivered to service stations.
In work published during 2006, Collins and coworkers found that one of their TAML catalysts successfully and efficiently destroyed bacterial spores. Using Bacillus atrophaeus spores as surrogates for Bacillus anthracis, the bacteria responsible for deadly anthrax disease, the researchers were able to reduce the number of spores by 107 in 15 minutes. This system used tert–butyl hydroperoxide instead of hydrogen peroxide. Hydrogen peroxide is also effective, but slower.
‘It’s great to have a material that is capable of such a broad spectrum application, but at the same time you don’t want to introduce any new harm in the process, and so this kind of elemental composition – carbon, nitrogen, oxygen, and iron – leads to them not contributing any toxicity. That’s a key attribute,’ says James Hutchison, professor of chemistry at the University of Oregon.
The fact that TAMLs do not have any potentially toxic residue is significant. But Collins remains alert to the possibility of some kind of toxic by-product resulting from the TAML remediation process. ‘There are literally thousands of chemicals that break down in water. So clearly there’s lots of room to make sure that you’re not converting some visible problematic compound into an invisible thing that’s more hazardous,’ he says.
Commercial success?
A crucial question is whether TAMLs will be a commercial success. No matter
how effective TAMLs are in breaking down pollutants, or how brilliant the science,
will industries adopt this new technology? And that to some extent hinges on
cost, Collins acknowledges.
‘We’re working with industries that can tolerate very little cost – pulp and paper industries and textiles, for example. We’re pretty sure it’s going to be commercialiseable in those areas. If you have a chemical or pharmaceutical plant, you often pay a huge amount of money to treat your effluent. My guess is, although we’ve done less work here, that TAMLs will be potentially very valuable because it’s possible they’ll be cost saving,’ he says.
There are other pollutants of growing concern. Endocrine disrupters are one example where studies have shown TAMLs may prove useful. ‘It’s possible in the long run that TAML technology may be a good back up to sewerage treatment plants, a final treatment where you chase out the last resistant compounds,’ he says.
Collins and colleagues are also working on other significant contaminants, such as dioxins and PCBs. These, he acknowledges, do pose challenges. They are hard to break down by oxidation, which explains why they are so persistent in the environment. Unfortunately, TAML catalysts preferentially attack hydrogen peroxide instead of the dioxin or PCB substrate. But Collins is ‘cautiously optimistic’ about the chances for success. ‘We need other approaches. We think we know what they are… and there is a reasonable chance that we will eventually get those.’
In brief
- TAML catalysts can destroy harmful compounds and leave no trace of themselves
- GreenOx Catalysts is set to market FeB*, which can degrade a range of pollutants
- The potential market in biocides for FeB* could be between $10m and £15m/year
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