By Saren Johnston
She studies chemical reactions -- that's her job.
The reactions she's investigating may one day lead to new chemicals that can be converted into plastics and other kinds of materials, but Andreja Bakac is focused on the fundamental chemistry that must first take place. Because, as she says, nothing happens without it.
The Ames Lab chemist's special interest is photochemistry -- studying the effects of light on chemical reactions. While many photochemical reactions are initiated by ultraviolet light, Bakac is using visible light to do her experiments. Her research centers on the photooxidation of hydrocarbons to industrially important chemicals. But it's not just doing the chemistry that's her concern -- it's also doing it in a simple, inexpensive and environmentally safe way.
Hydrocarbons are chemical compounds that are composed mostly of hydrogen and carbon, and there are lots of them. If you think these compounds may not be relevant to you, think again. Each time you fill your car with gas or adjust the heat in your home, you might remember that petroleum and natural gas are mixtures of hydrocarbons. In fact, petroleum crude oil is the largest source of hydrocarbons.
Bakac's work focuses on saturated hydrocarbons, those that contain only single carbon-carbon bonds, making it impossible to attach additional elements or compounds.
"Were it not for the chemical inertness of saturated hydrocarbons, this class could become an important feedstock for the chemical industry," says Bakac. "They're so plentiful, yet so dead and useless." However, partial oxidation is one way to boot them out of their comfort zone and make them more reactive.
Chemical reactions have to be driven. Any reactions done in the lab must be activated in some way, and the standard method is by heat. This is known as a thermal reaction. But light can also drive chemical reactions and is sometimes a better choice.
Bakac explains that photochemical reactions have some major advantages. "Light is typically a lot cheaper than heat, and it's perfectly clean environmentally. You simply turn on a lamp or move to the window, and there's your energy source."
Of course, oxidation experiments require oxidants, chemicals that give up oxygen easily or remove hydrogen from another compound. Strong oxidants often oxidize things indiscriminately, pollute ground water and are expensive and difficult to handle. "Then you have to consider waste treatment and all the problems associated with it," Bakac reminds us.
Simple air is the oxidant in Bakac's hydrocarbon oxidation experiments. "Air or oxygen -- that's as innocuous as oxidants get," she says with a smile. "That's what's so wonderful about these experiments and what makes them so easy."
In addition to light and air, Bakac's experiments need a photosensitizer -- something to absorb the light so it can be used in the oxidation reactions. She uses a brilliant yellow aqueous solution of uranyl ions. Its yellow color is what allows the solution to absorb the visible light necessary to drive the reaction. As it absorbs light, the solution turns a neon green. The electronic structure of the uranyl changes, and the uranium becomes more reactive -- a condition known as the excited state. The light makes a more energetic uranium that can photocatalyze, or set off, the oxidation of hydrocarbons. Left on their own, Bakac says the hydrocarbons would likely never react, and if they did, the reaction would be so out of control that it would produce many different products.
"Uranyl both increases the reaction rate at which hydrocarbons are oxidized and channels the reaction, leading to improved selectivity in the final product. We've gotten one major product in each of the reactions we've looked at," says a pleased Bakac.
"The way we conduct these experiments is so simple, inexpensive and safe," she continues. "We take a hydrocarbon and uranyl, dissolve them in water, bubble some oxygen or air through the solution, and turn on the lamp. During the reaction, we withdraw samples to monitor product formation."
Bakac notes that the word "uranium" is the only disadvantage of the hydrocarbon photooxidation reactions. "People are afraid of the word because they associate it with radiation pollution," she says. "The fact is, there is no reason to worry because in industrial applications of our experiment, depleted uranium would be used."
In depleted uranium, the amount of the fissile uranium -- the isotope that splits apart -- has been reduced to well below natural levels. And the United States has a lot of depleted uranium just waiting to be put to use. In Bakac's words, "It's there, it's available and it's safe."
As simple as the photooxidation experiments are, Bakac says she would still like to make the reactions more efficient. In all of her reactions, the uranium, oxygen and hydrocarbons get mixed together in a homogeneous solution, and nothing is saved when the reaction is over. "It would be ideal if we could take the uranium out, separate it from the final product and then reuse it," she says.
Before her photooxidation experiment can move from the lab to the real world, Bakac says she needs additional funding to develop a heterogeneous system that would make it possible to immobilize the uranium so that when the reaction was over, the catalyst could be removed.
"I want to take a solid support and deposit the uranium onto it so that when it's put into solution the uranium will still stick to that support -- it won't wash into the solution," she explains. "There would be a layer of uranium on some inert surface, which would need to have a lot of specific properties, such as certain pore and channel sizes. The molecules would have to be able to go in and come out and react with each other. The support would also have to be thin enough to let the light in."
Bakac says there are some tremendous advantages to the heterogeneous system and the ability to recover the uranium. "You have to be able to give assurances to the community you work in that you are not disposing of uranium in the ground water, soil or anywhere else," she says. "Also, recovering the uranium would make the whole process more cost-efficient. If you can pull the catalyst out and reuse it, that has to be a better way to go."
Bakac says that she and her co-investigators, Ames Lab physicist Marek Pruski and Iowa State University chemical engineer Brent Shanks, haven't yet done any experiments on developing a heterogeneous system, but adds that they're looking forward to working on it as funds become available.
In the meantime, Bakac has had some noteworthy success with the photooxidation of a number of different hydrocarbons in homogeneous solutions. She and Yun Mao, a former Ames Lab postdoctoral fellow, were able to oxidize benzene to phenol, a process that is industrially very significant because phenol is an important starting chemical in the manufacture of such products as resins, paints, adhesives, phenolic plastics, synthetic fibers, herbicides and insecticides.
"Phenol is currently produced in a very cumbersome, multistep reaction," says Bakac. "Yet, we do it very nicely with our uranium and light system, slightly modified for this purpose."
The benzene-to-phenol reaction works great, according to Bakac. "It's a very clean reaction, and we get fantastic yield, which means light is being used very efficiently."
Another notable reaction that Bakac and her colleagues have performed is the photooxidation of the hydrocarbon toluene to benzaldehyde, a very important intermediate in the chemical industry. "The yield is not great," she says, "but this may change in the heterogeneous system.
"Excited uranium has been around for a very long time, yet people haven't tried to use it as a photocatalyst because some fundamental chemistry has not been understood. And you can't develop the application unless you understand the basics," Bakac states.
Although she has photooxidized several different kinds of hydrocarbons, Bakac is still hoping to conquer the most difficult reaction. "That would be the oxidation of methane to methanol," she says. "There are huge sources of methane available, but it presents some problems. It's a gas, so the problem of transport is a serious one. Also, it's so unreactive that it really doesn't do anything other than burn. And yet it would be an excellent source of methanol and all of the chemicals derived from it. So this first stage, the oxidation of methane to methanol, is an extremely active area of research right now."
Bakac says the reason they've been unable to oxidize methane to methanol is because the solubility of methane in water is very low, and the fundamental research experiments are done in homogeneous solution. She notes that they might be able to oxidize the methane in a high-pressure reactor equipped with the uranium support they plan to design. "The higher the pressure, the greater the solubility of gas in solvents," she explains. "Eventually we would reach a concentration where the methane would have to react. It's just that the reaction is slow right now, and the high-pressure reactor could speed it up by increasing the concentrations."
Bakac says she's excited about taking on the methane to methanol challenge. She hopes to obtain the necessary funding and get together with Pruski and Shanks to start working on the project.
"The initial effort is just to get the right support for the uranium and see if we can do any of the reactions we've already done in homogeneous solutions," says an enthusiastic Bakac. "It would be a major success if we could get anything to work in the heterogeneous phase at this time. If we could, hey, then anything is possible."
For more information:
Andreja Bakac, (515) 294-3544
bakac@ameslab.gov
Research funded by:
DOE Office of Basic Energy Sciences
Last revision: 9/15/00 sd
Home | Disclaimer