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Quantum Dots: A New Nanohighway to Renewable Fuels

The hunt is on for technologies that use carbon dioxide and non-potable water since they are available in such tantalizing abundance. So, lets dig into one of the most interesting ways to make renewable fuels to appear in a month of Sundays.

The National Science Foundation’s division of Emerging Frontiers in Research and Innovation, widely known as EFRI, made a $2 million grant to a group of researchers led by Lehigh chemical and bioengineering professors Steve McIntosh and Bryan Berger — in a project that aims to make methanol using only carbon dioxide, sunlight and water.

The project utilizes a new low-cost technology that McIntosh and Berger developed to produce low-cost quantum dots from bacteria.

Er, Quantum Dots?

OK, let’s do a little science backgrounder here. A quantum dot is a really tiny crystal — so small (in the 5 to 50 nanometer range, at the low end roughly equivalent to the smallest transistor ever made) that the crystal begins to exhibit properties associated with quantum mechanics.

Specific to fuels, when a photon, arriving on planet Earth after an eight minute journey from the Sun, happens to strike a quantum dot (instead of say, a plant’s light harvesting mechanism) — it produces an excited electron (which is to say, an electron in a higher energy state than the garden variety electrons that power your computer).

Two things make this effect important for the production of fuels. First, there’s no limitation imposed by photovoltaic or photosynthetic efficiency — you get one electron for every photon. Second, the Lehigh team has theorized that these excited electrons catalyze the removal of hydrogen from water and carbon from CO2, and produce methanol in a continuous flow process.

That theory is what is being tested under the EFRI grant.

What Makes It Low Cost?

Quantum dots were discovered several decades ago, and are today employed in industrial applications. But the use of them is severely constrained by the high cost of producing them, using rare-metal catalysts, toxic solvents, and high temperature process conditions.

In a foundational bit of work that preceded this EFRI grant, McIntosh and Berger demonstrated their novel method for producing quantum dots in bacteria. In the EFRI project, an expanded research team will use quantum dots in tandem with a series of yeast-synthesized enzymes — providing the energy to a process that will create methanol from the aforementioned carbon dioxide and water.

There are a variety of outcomes worth noting — should the technology prove out.

First, one has a perfectly good, densified means of transporting captured energy — which is to say, more economically feasible than transporting gases, and more energy-efficient than transporting electrons.

Second, one has an energy system based on abundant materials — which is to to say, CO2, sunlight and water — instead of comparatively scarce resources such as sugars or fossil fuels.

Third, the high photon efficiency could prove to surpass not only current biological processes for making fuels; it could prove more efficient in utilizing solar energy than solar PV technology.

Fourth — and this is looking well down the line — here’s a pretty decent theoretical system for generating liquid fuels on neighboring planets like Mars, to fuel return trips.

OK, It’s a Renewable Fuel, but is It a Biofuel?

As the Digesterati point out from time to time, because biofuels are usually renewable fuels, many people equate the two terms. But not all renewable fuels are biofuels, and not all biofuels are renewable fuels.

As we see it, a biofuel is a fuel made using a biologically based process — so, for example, Calysta Energy makes biofuels by using biocatalysts to make fuels from fossil methane. But, in this example, Calysta is not making renewable fuels.

In the case of companies like Joule — and with this technology — though they are not using biomass (rather, using the same sources that plants use: sunlight, water and CO2) they are using biological processes. In this case, yeast-based enzymes and quantum dots made in bacteria. Voila, biofuels.

#rewpage#

Speaking of Joule, how is this different from other fuels made directly from sunlight, CO2, and water?

There are the electrofuels, and the Joule technology — these also make fuels directly from CO2, sunlight and water — the same material that plants use to make biomass. With the Lehigh technology, however, the light harvesting mechanism is not the same, and thereby is not limited by photosynthetic efficiency.

Is Methanol a Viable Fuel?

There have been methanol vehicles developed over the years, but a combination of short range, the toxicity of methanol itself, and the cost of building out the infrastructure — have bedevilled deployment of the technology. Mostly, we’ve seen methanol vehicles in China.

But hang on — you can make ethanol, for example, from methanol. Enerkem does so, as a matter of fact, as part of their process.

Celanese TCX is another process. Now, Celanese has owned a core technology for years that reacts methanol and carbon monoxide with catalyst, to make acetic acid. It’s newer TCX process converts acetic acid to ethanol. ZeaChem also has a process for this.

Not to mention, the Mobil MTG process, which converts methanol to gasoline.

In this process — which ExxonMobil has been generally touting in recent years as a means of converting coal to gasoline — “methanol is first dehydrated to dimethyl- ether (DME). Then an equilibrium mixture of methanol, DME and water is converted to light olefins (C2-C4). A final reaction step leads to the synthesis of higher olefins, n/iso-paraffins, aroma- tics and naphthenes. The shape selective MTG catalyst limits the hydrocarbon synthesis to C10 and lighter.”

Which is to say, lots of ways to use methanol as a renewable fuel. There’s also this duel fuel system system, which was developed a few years back and marketed under the “Itz-a-Gaz” brand.

In this system, its developers explain that “the primary fuel used to propel the vehicle is either gasoline or diesel. The secondary liquid fuel is an alcohol optimally methanol. Methanol readily dissociates into hydrogen and carbon monoxide at a temperature as low as 300 degrees C. This dissociation reaction can be driven by the heat of the exhaust gases. In fact slightly more than 20% additional energy is gained in the products of the disassociation reaction and this additional energy is essentially recuperated from the hot exhaust.

“The mixture of hydrogen, carbon monoxide, and gasoline or diesel will burn in a highly lean (excess air added) mixture of fuel and oxidant. This allows for lean combustion of the fuel in an internal combustion engine that can be turbocharged and operates under a high compression ratio.”

The Bottom Line

This Lehigh technology has a long ways to go towards commercial scale — but it’s a rare bird amongst biofuels technologies — a real disruptor, if it proves possible to industrialize, scale and the yields pan out as hoped for.

One thing we sure hope pans out: the use of waste or saline water. Far more sustainable. Should the technology prove to be able to use CO2 extracted from flue gas — as well as power plant coolant — why, that would be an excellent integrated project for coal-fired power plants that could revive their economics while mitigating their emissions problems.

“Currently, there is no commercial route to directly and photocatalytically produce liquid fuels,” McIntosh told Lehigh technology writer William Johnson in an extended review of the technology, here. “Certainly, using sunlight to create liquid fuel is a high-risk, high-reward proposition, but that is what is so exciting. The implications for our nation’s economy are significant.”

This article was originally published on Biofuels Digest and was republished with permission.

Read more bioenergy news here.

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