Two catalysts are better than one (and one reactor is better than two).
What makes cough medicines syrupy, couches cushy, paints robust and airplanes de-iced? The answer is a man-made molecule called propylene glycol.
This colorless liquid is widely used in industrial, cosmetic, pharmaceutical and food applications.
Propylene glycol is so popular that around 3.5 billion pounds of it are made worldwide each year – enough to fill more than 600 Olympic-sized pools.
The Center for Environmentally Beneficial Catalysis, or CEBC, is developing a cheaper, safer and greener process for making propylene glycol.
How? CEBC engineers are making propylene glycol from vegetable oil-derived glycerol instead of petroleum. They are also streamlining the procedure by making the hydrogen needed for the reaction inside the reactor, rather than piping it in.
For decades, propylene glycol has been made from petroleum. But growing demand for oil and gas have prompted the search for a replenishable raw material for making value-added chemicals like propylene glycol. Glycerol is a good option because it is economical, abundant, readily synthesized from plants, and is a byproduct of biodiesel production.
One method to convert glycerol into propylene glycol is hydrogenolysis. In this process, a metal-based catalyst breaks off one of the three oxygen atoms in glycerol and replaces it with hydrogen.
This reaction requires hydrogen, which is typically pumped into the reactor from a separate tank. But making hydrogen – by reforming natural gas with steaming hot water at temperatures as high as 850 °C – is costly, in terms of fossil fuels and energy, and emits substantial amounts of carbon dioxide into the atmosphere.
The CEBC scientists wondered if they could use some of the glycerol to make hydrogen and then use this hydrogen to convert the rest of the glycerol into propylene glycol, all in one reactor.
“Based on previous research reports, we know that hydrogen can be made from glycerol by a platinum catalyst,” said R.V. Chaudhari, Distinguished Professor of Chemical Engineering at the University of Kansas. “But until now, no one has coupled this reaction to hydrogenolysis.”
In a study recently published in Catalysis Today, Chaudhari and his colleagues describe their patented approach.
They designed a new catalyst – one that contains two different metals, platinum and ruthenium. The platinum metal converts some of the glycerol into hydrogen. The ruthenium metal uses this hydrogen to convert the rest of the glycerol into propylene glycol, specifically 1,2-propanediol.
This new technique is operated at lower pressure than the conventional glycerol hydrogenolysis method, making it safer and cheaper.
The researchers identified all the possible liquid and gaseous products that form during the reaction. Compared with the conventional method, the new CEBC process generates more propylene glycol and fewer unwanted waste products.
If the results hold up, this bimetallic-catalyst approach could significantly shrink the environmental impact of producing billions of pounds of propylene glycol each year – a much greener route to cushy couches and moisturizing shampoo.
--Story by Claudia Bode
Roy, D., Subramaniam, B., and Chaudhari, RV. "Aqueous phase hydrogenolysis of glycerol to 1,2-propanediol without external hydrogen addition." Catalysis Today, 156 (2010) 31-37.