Perfecting the phosphorous process
The Cummins Group investigates the efficiency and environmental impact of industrial phosphorus processing
Lab of the Week
The Cummins Group
Course 5: Chemistry
Phosphorus, with the symbol P and the atomic number 15, is one of the six biogenic elements that are essential to living organisms. Phosphorus is in our bones and teeth as hydroxylapatite. It makes up the DNA backbone, and in adenosine triphosphate (ATP), it serves as the primary energy carrier in living cells.
Professor Christopher ‘Kit’ Cummins, Henry Dreyfus Professor of Chemistry, has been interested in phosphorus and its processing for industrial use for many years. When I met with him and his graduate student Michael B. Geeson, he pulled out a wide, laminated flow-chart explaining the pathways of phosphorus processing.
“This is a graphic that I obtained from a company called ThermPhos,” he began.
Pure phosphorus P4 does not occur naturally because it is highly reactive. Phosphorus is mined from phosphate rock, also referred to as “mineral apatite,” Cummins said. “Basically animal bones and teeth, it’s sedimentary and can have varying degrees of purity and wide ranging quality of ore.”
How is the phosphate rock processed and made available for commercial use?
The phosphorus from phosphate rock is processed in two main pathways: in order to make phosphate-containing fertilizers for global agricultural, most of the phosphate (~95%) is processed by the “wet process,” where phosphate is treated with sulfuric acid to produce phosphoric acid.
On the other hand, the production of phosphorus containing chemicals requires the “thermal process,” an energy-intensive process that reduces the phosphate to pure white phosphorus. The white phosphorus is then treated with environmentally hazardous chlorine to create phosphorus trichloride PCl3. “The chlorides can be replaced with formation of things like carbon phosphorus bonds or others,” Cummins explained, while many of the phosphorus chemical products do not end up containing any chlorine.
“White phosphorus becomes the starting point, the first pure substance,” Cummins said. It is waxy, “like sticks of butter,” and used to be shipped in plastic containers under water, with which it does not react. The end products are important to our economy, used in flame retardants, lithium batteries that power our laptops, smartphones and cars, as well as pharmaceuticals, household cleaning products and even the Coke we drink. “Coca Cola is one example of food-grade phosphoric acid coming from white phosphorus.”
“I think we have taken for granted, traditionally, the notion that white phosphorus is the starting point,” Cummins said. According to Cummins, the thermal process is “a legacy process.” Although many people had wondered whether one could cross-link the “wet process” to make chemical compounds and circumvent the production of white phosphorus and chlorine waste, it had never been done before. “There has been this kind of wall between the fertilizer chemicals on the one hand and all these other phosphorus containing chemicals on the other hand.”
The pair published a paper in Science on Feb. 9, in which they described a novel method to make chemical compounds that are normally produced via the thermal process from phosphoric acid, a product of the wet process.
Cummins and Geeson discovered a previously unknown intermediate, bis(trichlorosilyl)phosphide, an “isolatable, free-flowing, white powder” which can be stored and transported under controlled conditions. Geeson was able to make several different phosphorus chemicals including hexafluorophosphate (contained in lithium batteries) and (4-phenylbutyl)phosphine (used in pharmaceuticals) from the intermediate.
Phosphoric acid, produced in the wet process, can be dehydrated with sodium chloride, producing trimetaphosphate. Trimetaphosphate can have different forms, according to Cummins, the cyclic trimer is the easiest one to make.
“In order to produce bis(trichlorosilyl)phosphide, the trimetaphosphate has to be reduced using trichlorosilane,” Cummins said, pointing at the arrow between the molecules in the reaction. Trichlorosilane is produced in commercial scales to make silicone in the semiconductor industry. It is purified from sand. “Trichlorosilane is quite volatile with a boiling point of 35°C,” Cummins said, raising his eyebrows. “We actually do this reaction at 110°C in a steel pressure reactor, ” added Geeson. The reactor was made by the MIT Museum and fits into the palm of a hand.
If the method became widely available, scientists could potentially avoid the challenges of obtaining white phosphorus to make phosphorus chemicals and instead make it themselves with commercially available products.
Is the method more energy efficient and environmentally friendly than the traditional process? “A detailed analysis as to how this compares to making white phosphorus would be of value. We don’t really know just yet. It may be better because of the economics of scale,” Cummins said.
Is the new method likely to replace the traditional processes in phosphorus chemical production? Cummins’ answer to that question is twofold: The goal is to disrupt the way that people make phosphorus containing chemicals. We would like to have a new way that is better ... There are many solutions to the problem that we have framed. And we have put forward one solution in this paper.”
Cummins’ lab is exploring alternative ways and options of optimizing phosphate processing and welcomes UROPs to join his projects. In 2010, Cummins and his graduate student Daniel Tofan published a paper in Angewandte Chemie describing a method to eliminate environmentally hazardous chlorine in the production of phosphorus chemicals from white phosphorus by using UV light.
Earth’s phosphate resources are depleting; the main phosphate rock reserves today are located in Morocco and Western Sahara. Phosphorus is not a renewable resource. Separately, the phosphorus used in agriculture, providing nutrients to the soil for growing plants, runs off into groundwater, rivers, and ultimately oceans, causing eutrophication and the disruption of marine ecosystems. “It is turning a nutrient into a pollutant,” Cummins said, and proposed finding solutions to these problems with chemistry.