Membrane separates hydrocarbon isomers in energy-saving process

A membrane that separates organic solvent molecules based on their shape and size could dramatically cut costs associated with preparing valuable polymer feedstocks, such as p-xylene. It works as part of a reverse-osmosis process, a common method for water desalination that hasn’t been applied to organic solvents until now.

Both the membrane and the organic solvent reverse-osmosis process were ­developed byRyan P. Lively and Dong-Yeun Koh of Georgia Tech, along with Benjamin A. McCool and Harry W. Deckman of ExxonMobil Research & Engineering. The researchers started with a commercially available hollow fiber made of poly(vinylidene fluoride). The fiber consists of a tube of support material full of microscale pores surrounded by a membrane riddled with molecule-sized pores.

This porous material is incompatible with organic solvents, so the researchers converted it to an all-carbon substance, called a carbon molecular sieve, by heating it to 550 °C. But before they did that, they cross-linked the polymer fiber using p-xylylenediamine to prevent all the pores from collapsing when heated.

Using a single treated fiber under high pressure, the team was able to covert a 50:50 mixture of p- and o-xylene into an 85:15 mix (Science 2016, DOI: 10.1126/science.aaf1343). p-Xylene is used to make terephthalic acid, a monomer for the popular plastic polyethylene terephthalate used in water bottles. Typically, engineers separate p-xylene from its isomers via energy-intensive processes, such as distillation.

“If you do a quick calculation on how much energy the world consumes on separation and purification processes, it’s just an enormous number,” Lively says. Reverse osmosis, he notes, can be 10 to 50 times as energy efficient as the methods currently used. The team hopes to scale up the process so it could be used industrially.

Andrew G. Livingston, an expert in membrane separations at Imperial College London, states: “It is entirely feasible that these new membranes and their derivatives will go on to change the way refining and chemical industries approach the separation of organic mixtures.”