Bio-butanol: Fuel of the Future
Written by: Austin Yantes
Edited by: Katherine Hill, Sienna Schaeffer
Ethanol has gained enormous popularity in the U.S. over the past few decades as a renewable alternative to traditional gasoline. Instead of depending on diminishing reserves of petroleum, ethanol makes it possible for us to grow our fuel source, that is, to turn plant material into liquid fuel. The downside is that ethanol substantially lowers your car’s fuel efficiency; Consumer Reports(1) found that E-85 fuel, which uses 85% ethanol and 15% gasoline, caused a 20–30% reduction in gas mileage. Ethanol is also more corrosive to some metals and rubber than traditional gasoline, so it can deteriorate your car’s engine. That means that unless your car has been specially adapted, the gasoline that we use put in our tanks can only contain about 10% ethanol.
Luckily for the future of renewable energy, there is a new biofuel on the block — butanol. You can think of butanol as ethanol’s bigger, more powerful cousin. It packs nearly as much energy as gasoline, so it will not decrease your gas mileage as much as ethanol. Even better, it doesn’t damage your car, which means it can be pumped straight into your tank without having to make any modifications to your engine.
You may be asking yourself — if butanol is such a great fuel, why isn’t it being sold at the pump? To answer that question, we need to understand how biofuels are manufactured. It all begins with a source of sugar, traditionally corn (although other plants such as sugarcane, switchgrass, and sugar beets can also be used). The kernels are ground into a meal and mixed with water to form a starchy “mash”. Enzymes are added to break this starch down even further and turn it into sugar. After cooking the mash, microorganisms, such as yeast, are added to the mixture and a process called fermentation begins. During fermentation, the yeast turn the sugars into alcohol, creating a sort of corn-based beer. Since we can’t just pour beer into our gas tanks, additional refining is required. The yeast is filtered out and then the “beer” is sent to distillation column where the alcohol is separated from the water. The alcohol is further purified by sending it through a molecular sieve (a sort of filter), resulting in a brew that is about 200 proof. This alcohol, called ethanol, is mixed with a little bit of gasoline, rendering it undrinkable, and shipped out to gas stations nationwide. The fuel we ultimately put into our cars is mixed with more gasoline to bring it down to 10% ethanol by volume.
Remember the process of distillation mentioned a few sentences back? Turns out it requires massive amounts of energy, and consequently is quite expensive. The only way this cost could be avoided is if ethanol magically separated out from the water mixture on its own, without having to be distilled. This is where butanol comes into play. While ethanol is very soluble, meaning it mixes well with water, butanol is not. When the “beer” has an 11% alcohol by volume (ABV) or higher, butanol separates out from water and floats to the surface like an oil. That means if vodka was made with butanol instead of ethanol, all of the alcohol would be sitting in the top 40% of the bottle, and the remaining 60% at the bottom would be pure water. While this would be a disaster for vodka distributors, it’s a dream come true for the biofuel industry. Butanol could save us the trouble and expense of distillation by simply separating out of the beer-like mixture by its own devices. Biofuel producers would easily be able to skim pure butanol off the top of the solution, which could then be dropped directly into our fuel tanks without being diluted to a lower percentage. In this way, man-made butanol has the potential to completely replace traditional gasoline.
As with any technology that sounds too good to be true, there is one small catch. Alcohol is known for its ability to sterilize (think rubbing alcohol, alcohol swabs, etc.), in other words, its capacity to kill microorganisms. Butanol is no exception, and therefore is toxic to the very organisms that create it above an “ABV” similar to Bud Light, around 4%. Since butanol only floats to the top of the solution at a concentration of 11% or higher, the microorganisms doing the fermenting die long before the “beer” reaches this threshold. This is the challenge researchers are trying to overcome: how can we keep these little creatures alive long enough for them to generate the volume of butanol that we desire?
In a paper published in January of 2017, Liu et al. describe the advances that have been made in the butanol production process. According to their report, there are actually many different kinds of microorganisms that can be used to ferment sugar into butanol. A yeast called Saccharomyces cerevisiae, which is also used to brew beer and make bread, is an excellent option because scientists have studied its genes for over 40 years. We now have a complete map of yeast’s DNA, making it fairly easy to change its genes to enhance its performance. For example, we can design strains of yeast that yield substantially more butanol than they would naturally. In 2016, scientists were able to construct a strain of yeast that produces around 130 milligrams/liter (mg/L) of butanol(2) — we refer to this quantity as the titer. Unfortunately, yeast is still not able to tolerate high enough levels of butanol to efficiently produce biofuel. Using Synechococcus elongates, a type of cyanobacteria (also known as blue-green algae), a titer of 400 mg/L was achieved, but like yeast, cyanobacteria simply cannot handle the toxicity of butanol(2).
Another microorganism with well-mapped and easily-manipulable genes is Escherichia coli (E. coli). Although you may associate E. coli with food poisoning outbreaks at Chipotle, it is also a useful bacteria for industrial production of butanolbutanol formulation. Through genetic engineering, scientists have been able to develop strains that result in a butanol titer of 1254 mg/L, and can endure butanol concentrations of up to 2%2. A few species of Lactobacillus, better known as the probiotics in your yogurt, have also shown promising butanol outputs of 66–300 mg/L and tolerances of 3–4%(2). So what is the closest we have gotten to the magic number of 11%? The answer is somewhat disappointing — the strongest of all the butanol-tolerant bacteria, Pseudomonas putida, can only withstand concentrations of up to 6%. Even more problematic, scientists have not yet figured out how to get P. putida to actually produce butanol(2).
All things considered, great strides have been made towards butanol becoming the next big biofuel. Scientist and engineers have improved both butanol yields and microorganisms’ ability to withstand high concentrations of butanol. There is great hope that one day we will be able to create a super-microorganism that can not only generate large quantities of butanol, but can also survive long enough to reach an 11% titer. Once this is accomplished, fueling our cars could be as simple as skimming the butanol off the top of some “beer” and pumping it straight into our tanks — less energy, less effort, and less engine damage. Cheers to the fuel of the future!
References
- “Ethanol (E85) Fuel Alternative.” Consumer Reports. Jan. 2011. Web. http://www.consumerreports.org/cro/2011/01/the-great-ethanol-debate/index.htm
- Liu S, Qureshi N, Hughes SR (2017) Progress and perspectives on improving butanol tolerance. World Journal of Microbiology and Biotechnology, 33:51.
