This molecule [ethylene] is a Lego block with two prongs...you can snap them together into four, six, eight, [oligomerize] so on and so forth, or a million, that would be polyethylene. You go a mixture of 8 to 10, that’s gasoline, you go a mixture of 16 to 20, that’s diesel, you go a million it becomes polyethylene, you go 50 mil it becomes Kevlar and so on and so forth. But all of those are fairly robust, existing, in most cases established commercial technology, and in some cases technologies practiced and not practiced anymore, but easy to revive. But once you are at a two-carbon molecule with a double bond, you can go anywhere in the chemical industry.

Once you’ve gotten to [ethylene], you can partner with people in this business that’s a $160-billion business in and of itself. Today this molecule is made from oil, by a process known as steam cracking. That’s the chemical industry’s largest energy consumer and largest CO₂ producer, because that chemistry is endothermic. In order to make ethylene from naphtha, you mix it with 800 Centigrade superheated steam and you break the carbon-carbon bonds basically with physical force, until you end up with smaller stretches that are 2 and 3 carbons. It’s a robust technology, but you have to pay the oil price, and if you have to burn an awful lot of heat in order to enable that endothermic chemistry, if you were to care about the energy footprint of this, it’s the three pounds of CO₂ produced to make one pound of polyethylene.

—Dr. Alex Tkachenko, President, Siluria Technologies

The OCM reaction. The oxidative coupling of methane converts methane into ethane and ethylene (C₂ hydrocarbons). The basic OCM reaction (which is exothermic) is:

2CH₄ + O₂ → C₂H₄ + 2H₂O

In the OCM reaction, methane (CH₄) is activated on the catalyst surface, forming methyl free radicals (CH₃) which then couple in the gas phase to form ethane (C₂H₆). The ethane subsequently undergoes dehydrogenation to form ethylene and water.

Since the discovery of the reaction in the 1980s, researchers have looked for a commercially viable OCM catalyst, but have been largely unsuccessful. Methane is thermodynamically very stable, making activation difficult. In a presentation at the Spring 2006 AIChE national meeting, researchers from Mesoscopic Devices noted that today’s catalysts exhibit either high selectivity (>70%) coupled with low conversion (<5%) or high conversion (>75%) with low selectivity. In addition, the OCM process involves formation of undesirable products such as CO₂ and CO because of the highly exothermic hydrocarbon combustion reactions, noted Choudhary et al. in a 1997 paper in Science.

Siluria. Siluria’s catalyst synthesis process uses proteins on the surface of a genetically modified bacteriophage as nucleation sites for growing nanoscale wires of catalyst material. By growing the catalyst nanowires on an engineered biological template, Siluria is able to access crystal structures and surface morphologies not formed through conventional crystallization of the material. The novel crystal structures, in turn, give rise to catalyst active sites with unique properties that are critical to achieve the selectivity and yield required for an economically viable OCM process. The enabling piece is around the specific active site that is doing the oxidative coupling, noted Dr. Erik Scher, Siluria’s Vice President, R&D.

The catalyst materials are proprietary, doped metal oxides of early transition metals that are designed for compatibility with existing petrochemical industry infrastructure. Siluria has developed a library of compounds with a range of crystal structures, and has tested their behavior in catalyzing the OCM reaction.

The first step is creating the library of organic templates—that’s the phage. Second, you do synthetic prep, so inorganic synthetic chemistry on each of those phages, and you create a diversity of catalyst nanowires: the same composition, but a different active site. Then we take these individual compositions, we combine them with high-throughput screening to screen not some incidental or ancillary property that you then infer; we actually run every single one in the reaction of interest. We get a direct measurement on each one of those little dots [256 per wafer] which is a different catalyst.

—Erik Scher

With Siluria’s high-throughput screening, processing a wafer containing 256 different formulations takes about 48 hours; absent this process, it would take some 8-9 months, Tkachenko said. The company aims to achieve the catalyst performance required for commercialization in 2011.

The chemistry we are pursuing, you could never make economic with a conventional catalyst. You want to activate one and only one of these carbon-hydrogen bonds and you want to do it selectively, because if you activate more than one you will burn, and you want to do it at low enough temperature that the methyl radical sticks around the surface of the catalyst for long enough to bump into another one like that and to couple and to make a C₂ molecule.

It is the ability to influence the surface of the catalyst that gives rise to the selectivity. And it just so happens that for this particular reaction [OCM], conventional methods of making catalysts, which are endearingly referred to in the industry as “cook-and-look” and “shake-and-bake”, have not produced the kind of surface that would run this reaction selectively.

—Alex Tkachenko

Fuels. The products produced via a Fischer-Tropsch process (gasification or steam cracking to produce syngas; FT reaction; and refining) in general follow the Anderson-Schulz-Flory distribution. Methane is the largest single product; total products range from the very light hydrocarbons all the way up to the very long-chained hydrocarbons (waxes), which are solid at room temperature. For production of liquid transportation fuels, it thus becomes necessary to crack some of the Fischer-Tropsch products. By contrast, Scher noted, ethylene-to-fuels is a very selective process, avoiding the Anderson-Schulz-Flory distribution.

The excitement and the promise is where we will be 5 years from now. The non-reducible advantages of OCM versus FT and a syngas-based route is threefold. One, [OCM] is a simpler chemistry, in terms of the number of steps required to get to the end product. With OCM, its two steps: methane to ethylene, oligomerize to liquids. In FT, there are three steps: steam methane reforming to syngas, syngas to a mixture, hydrocracking to clean it up. Non-reducible.

Two, [OCM] is a chemistry that is easier to control. Ethylene is a versatile and flexible molecule that is easy through existing technologies to convert to longer chains: detergents, lubricants, fuels. And three, it has a better energy balance. The first step in FT is endothermic; the first in OCM is exothermic.

—Alex Tkachenko

Resources

• Vasant R. Choudhary, Anil K. Kinage, and Tushar V. Choudhary (1997) Low-Temperature Nonoxidative Activation of Methane over H-Galloaluminosilicate (MFI) Zeolite. Science doi: 10.1126/science.275.5304.1286