Making Plastics Production More Energy Efficient | News
Northwestern engineering researchers have demonstrated a new approach to chemical catalysis that results in high propylene yields with less energy. The results could support more energy-efficient production processes for many plastics.
One of the highest volume chemical products, over $ 100 billion worth of propylene, is manufactured each year and is primarily used to make polypropylene for a variety of materials, from injection molding in auto parts to consumer products. The production of propylene is also energy-intensive and requires temperatures of around 800 degrees Celsius to convert propane gas into propylene.
A technique known as oxidative dehydrogenation has long been studied as an alternative method of making propylene from propane without the high temperature constraints. This approach reacts propane and oxygen on a catalyst to form propylene and water. However, because propylene is more reactive with oxygen than propane, the reaction typically yields only a small amount of propylene.
“The reaction works, but similar to when you turn on your gas grill to cook at home, you don’t produce propylene, you just burn the propane,” said Justin Notestein, a professor of chemical and biological engineering at the McCormick School of Engineering and a co-author of the research. “Instead of looking for the right catalyst, we split the oxidative dehydrogenation reaction into two components – dehydrogenation and selective hydrogen combustion – and then developed a tandem material that performs both reactions in a specific order. This resulted in the highest propylene yields ever reported. “
An article entitled “Tandem In2O3 Pt / Al2O3 Catalyst for Coupling Propane Dehydrogenation to Selective H2 Combustion” was published on March 19 in the journal Science. Peter Stair, Professor of Chemistry at Weinberg College of Arts and Sciences, was the other co-corresponding author on the paper.
In the new approach, researchers developed two catalysts with nanoscale proximity: a platinum-based catalyst that selectively removes hydrogen from propane to produce propylene, and an indium oxide-based catalyst that selectively burns the hydrogen, but not propane or propylene.
“We found that the nanostructure is really important,” said Notestein. “Indium oxide on platinum works great. Platinum on indium oxide does not. Platinum is not physically combined with indium oxide. This nanostructure can separate and sequence the reactions, although both catalysts can perform both reactions. This organization is widespread in biology, but very rare in man-made materials. “
The team’s tests showed remarkable improvements in propane yield for making propylene. At 450 degrees Celsius, tests showed 30 percent yield from a single pass through the reactor, while ensuring that more than 75 percent of the carbon atoms in propane were propylene. In comparison, when propane is heated in the absence of oxygen, it is impossible to achieve yields greater than 24 percent and the catalysts required are often unstable.
“Nobody has ever demonstrated yields that exceed these thermodynamic limits,” said Notestein. “From a utility point of view, our results are some of the first to truly warrant trying to conduct this reaction oxidatively rather than just performing dehydration.”
The simple design of the system could be further optimized by adjusting the reactor conditions and changing the two catalyst components. Current processes for generating higher yields require more complex and expensive technical solutions.
“Because we rely on proven design-build-test cycles from engineering, additional improvements can be made,” said Notestein, director of the Center for Catalysis and Surface Science, which is part of the Institute for Sustainability and Energy in the Northwest. “These results give us new compositions and rational strategies for the search for high-performance catalyst systems. This could particularly benefit smaller chemical plants where energy consumption is very important and current engineering strategies may not be feasible. He added that the team’s approach reflects the greater efforts of the National Science Foundation’s Center for Innovative and Strategic Transformation of Alkane Resources (CISTAR), which funded the work.
The results could also further improve energy efficiency in the manufacture of many plastics for structural and material applications. Plastic parts in cars, for example, make vehicles lighter and more energy efficient, while polymer house packaging and siding are durable and help keep houses warm and dry.
“While plastics are highly malignant, they are essential to modern society, including efforts to make society more energy efficient,” Notestein said. “Making propylene and materials like polypropylene using this new approach could be a lot less energy-intensive, which would be good news for everyone.”