Converting shale gas into hydrocarbon fuels
Dr. Neil Canter, Contributing Editor | TLT Tech Beat April 2018
A new catalyst activates carbon-hydrogen bonds under viable operating conditions.
© Can Stock Photo / serrnovik
KEY CONCEPTS
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A single atom alloy prepared by dispersion of platinum atoms in a copper metal surface catalyzed the conversion of light alkanes into their corresponding alkenes.
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No coking and good durability were observed with the single atom alloy catalyst.
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If higher concentration of platinum were added to the copper metal surface, then catalyst performance declined due to coking.
The rapid development of hydraulic fracturing over the past decade has led to the production of larger quantities of shale gas that mainly contain light alkanes such as ethane and propane. One of the challenges facing researchers is finding ways to activate the C-H bonds in these alkanes so that they can be used as precursors to derivatives such as hydrocarbon fuels.
A previous TLT article describes a new approach at activating carbon-hydrogen bonds by inserting an aromatic molecule to produce alkylated aromatic hydrocarbons (
1). This process occurs through the formation of a phenyl carbocation intermediate. Alkylated aromatic hydrocarbons are precursors to aromatic sulfonates utilized in the lubricant industry as detergents, corrosion inhibitors, emulsifiers and extreme-pressure additives.
The main industrial approach for activating carbon-hydrogen bonds in alkanes is steam cracking, but this reaction is inefficient and energy intensive. One reaction with significant industrial interest is converting light alkanes to alkenes such as ethylene, which is a precursor to an important base stock used in lubricants, polyalphaolefins (PAOs).
E. Charles H. Sykes, professor of chemistry at Tufts University in Medford, Mass., indicates that metal catalysts have been used in an effort to improve the conversion of alkanes to alkenes, but yields are limited due to coking problems. He says, “Coking is a buildup of carbon that takes place as the carbon-hydrogen bonds are breaking. Current catalysts such as nickel and platinum are so reactive that they break carbon-hydrogen and carbon-carbon bonds leading to a buildup of carbon on the catalyst surface. This causes major difficulties because it literally blocks the catalyst sites shutting down the reaction.”
Sykes continues, “Platinum has an additional disadvantage due to its high cost. One option is to work with copper, which is just as inexpensive as nickel but is much more resistant to coking. The problem with using copper is the high energy barrier making it difficult to activate carbon-hydrogen bonds.”
If an approach can be found that combines the high reactivity of platinum with the ability of copper to minimize coking, then a catalyst could be prepared that activates carbon-hydrogen bonds under viable operating conditions. Such a catalyst has now been developed.
Single atom alloys
Sykes and his colleagues from Tufts and University College London, UK, developed a new catalyst in which single platinum atoms are dispersed in a copper metal surface to produce a single atom alloy (SAA) that acts to convert light alkanes to their corresponding alkenes under realistic operating conditions. He says, “Based on our work using scanning tunneling microscopy (STM), we have achieved a higher magnification of a metal alloy surface than has been seen before. This enables us to see every single atom on the metal surface. We decided to produce a catalyst with the smallest amount of the precious metal platinum that can alloy in the copper.”
One of the reasons the researchers selected this approach is that past work showed that platinum dispersed in a copper surface acted as a catalyst for selective hydrogenation reactions including the industrially relevant partial hydrogenation of butadiene. The SAA catalyst was prepared by reducing cupric nitrate in an aqueous solution with sodium borohydride followed by deposition on a silica support and calcination in air at 300 C to produce copper nanoparticles (0.2-0.3 micron average aggregate particle size). Platinum atoms were added to form the SAA catalyst through a galvanic replacement procedure.
Sykes says, “Dissolving platinum in copper is a thermodynamically favored process. We found that adding 1-4 atomic % platinum into copper produced the most desirable concentration of single platinum atoms in the copper (
see the left STM image in Figure 2). When we added higher concentrations of platinum, dimers and trimers of platinum atoms started to form and higher levels of coking were observed during reaction of light alkanes.”
Figure 2. A scanning tunneling microscopy image on the left showed a model of the single atom alloy catalyst with three platinum atoms in a copper metal surface. The image on the right showed a simulation of how methane is activated on the surface of a single atom alloy catalyst. (Figure courtesy of Tufts University.)
The researchers conducted catalyst studies under flow conditions evaluating C-H activation in butane and found that the SAA catalyst is effective at 250 C as compared to 550 C for copper. Platinum catalyzed the reaction at 100 C but catalytic activity quickly deteriorated due to coking. The researchers then showed that SAA catalyst were stale for over two days on stream converting butane to butene and hydrogen at 400 C.
Sykes says, “Based on quantum mechanical studies by our colleagues at University College London, we found that the single platinum atoms reduced the barrier to carbon-hydrogen activation without breaking any carbon-carbon bonds that would lead to coking.”
The researchers also examined the addition of methyl groups to the SAA catalyst surface by using methyl iodide. Sykes says, “We used a technique called temperature-programmed reaction combined with STM and initially placed the methyl iodide on the catalyst surface at 5 K. Then we heated the surface to whatever temperature activated the carbon-iodine bond and then cooled the reaction back down to gain an atomic-scale snapshot of the reaction by examining the active sites.”
The right image in Figure 2 shows theoretically how a methane is activated on the surface of the SAA catalyst.
Sykes indicates that future work will include evaluation of metal catalysts in activating other bonds such as oxygen-hydrogen, nitrogen-hydrogen, carbon-oxygen and nitrogen-oxygen in addition to carbon-hydrogen using a combination of experiments under ultra-high vacuum conditions and theoretical studies. He adds, “We also are looking to design and then test catalysts that can be effective in accelerating many other reactions such as the conversion of ethanol to ethylene glycol. Our efforts will include identifying catalysts that will break one specific bond selectively.”
Additional information can be found in a recently published paper (
2) or by contacting Sykes at
charles.sykes@tufts.edu.
REFERENCES
1.
Canter, N. (2017), “New approach to functionalize aromatic hydrocarbons,” TLT,
73 (10), pp. 14-16.
2.
Marcinkowski, M., Darby, M., Liu, J., Wimble, J., Lucci, F., Lee, S., Michaelides, A., Stephanopoulos, M., Stamatakis, M. and Sykes, E. (2018), “PtCu single-atom alloys as coke-resistant catalysts for efficient C-H activation,”
Nature Chemistry,
10, pp. 325-332.
Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at neilcanter@comcast.net.