Making membranes

Engineers have developed a method for creating membranes from crystal sieves called zeolites that could increase the energy efficiency of chemical separation processes and enable higher production rates.

Researchers led by Michael Tsapatsis, chemical engineer at the University of Minnesota, developed a rapid-heating treatment to remove structural defects in zeolite membranes that limit their performance, a problem that has plagued the technology for decades.

'Using membranes rather than energy-intensive processes such as distillation and crystallisation could have a major impact on industry,' said Rosemarie Wesson, programme officer at the US National Science Foundation (NSF).

The discovery could increase the energy efficiency of producing important chemical solvents such as xylene and renewable biofuels such as ethanol and butanol.

Researchers create zeolite membranes by growing a film of crystals with small organic ions that are added to help determine which molecules can pass through the material. Then they slowly heat the zeolite film in a process called calcination to decompose the ions and open the pores.

However, Tsapatsis explained: 'This method for creating zeolite films often leaves cracks at the boundaries between grains of zeolite crystals.'

These defects have prevented zeolite films from being used effectively as membranes, as molecules of unwelcome chemicals that are rejected by the zeolite pores can still penetrate through the membrane defects.

'While it may be possible to correct some of these defects, the repair process is difficult and expensive,' said Wesson.

Currently zeolite membranes have found use only in specialised, smaller-scale applications, such as the removal of water from alcohols or other solvents.

In an effort to minimise the formation of cracks and other defects, the heating rate during calcination is very gentle, and the process can take as long as 40 hours. Typically, a material is heated at a rate of 1^oC per minute up to a temperature between 400 and 500^oC, where it is held steadily for several hours before being allowed to slowly cool.

Because conventional calcination is time consuming and energy intensive, it has been difficult and expensive to produce zeolite membranes on a large scale.

Tsapatsis's team has developed a treatment called Rapid Thermal Processing (RTP), a treatment in which zeolite film is heated to 700^oC within one minute and kept at that temperature for no more than two minutes. Acting as an annealing method, RTP refines the granular structure of the zeolite crystal film.

When the researchers examined the RTP-treated films, they found no evidence of cracks at grain boundaries. Although they found other types of defects, these do not seem to affect the membrane properties or performance.

In a comparison to conventionally made zeolite membranes, Tsapatsis said: 'We observed a dramatic improvement in the separation performance of the RTP-treated membranes.'

A second round of RTP treatment improved separation performance even further, to a level on par with current industry separation methods.

The researchers demonstrated the RTP process on relatively thick (several micrometre-sized) zeolite membranes. Tsapatsis and collaborators are now working towards making zeolite membranes 10 to 100 times thinner to allow molecules to pass through more quickly.