If the flow of Middle Eastern oil slows anytime soon, engineers will undoubtedly try to slake our thirst for transportation fuels by turning coal into liquid. That strategy powered Germany's war machine during World War II. But high cost has limited its utility today. Now a team of researchers report that they have discovered a new catalyst that may offer a novel route to liquefying coal.
There are several ways to turn coal into a liquid. One of the oldest is to break its extended network of chemical bonds into shorter chain hydrocarbons, which serve as the precursor for diesel and gasoline. This is typically done with solid iron-oxide-based catalysts. But these only work well with relatively young, so-called "low-rank" coals that are porous enough to allow the catalyst particles to reach most of the coal's hydrocarbon bonds. The catalysts don't work well with other widely abundant and older "high-rank" coals, however, which have more stable ringlike hydrocarbon structures and have had longer to solidify into a more compact network.
In hopes of creating catalysts that can liquefy these coals, chemist Matthias Haenel and colleagues at the Max Plank Institute for Coal Research in Mülheim an der Ruhr, Germany, decided to look for soluble catalysts that could percolate through the tight confines of even the oldest and most dense coal. The team eventually found soluble two-part catalysts made from boron and iodine that break apart high-rank coals in two steps. First, the catalysts add hydrogen atoms to highly stable carbon-carbon double bonds between hydrocarbon rings and the chainlike hydrocarbons that link them, converting these to weaker single carbon-carbon bonds. Next, they break these single carbon-carbon bonds, creating a mixture of hydrocarbon chains of various lengths. Together, these chains can serve as a liquid precursor for making diesel. The team reports its findings in this month's issue of Angewandte Chemie International Edition in English.
"These guys have done some very nice chemistry here," says Harold Schobert, who heads The Energy Institute at Pennsylvania State University in University Park. He adds that because the boron and iodine catalysts are moderately expensive and for now cannot be recovered after they are used, the new catalysts aren't yet ready for commercialization. However, he says, the work opens the door to finding low-cost catalysts that can cheaply convert any type of coal into a liquid. Says Schobert: "It's clearly a step in the right direction."