Metal-organic frameworks cut energy consumption of petrochemicals

In the chemical and the petrochemical industries, separating molecules in an energy-efficient way is one of the most important challenges. Overall, the separation processes account for around 40% of the energy consumed in the petrochemical industry and reducing this can help addressing anthropogenic carbon emissions. One of the most important products in the petrochemical industry is propylene, which is widely used in fibres, foams, plastics etc. Purifying propylene almost always requires separating it from propane. Currently this is done by cryogenic distillation, where the two gases are liquefied by being cooled to sub-zero temperatures. This gives the propylene-propane separation process a very large energy footprint. A solution is to use “metal-organic frameworks” (MOF’s). These are porous, crystalline polymers made of metal nodes that are linked together by organic ligands. The pores in their molecular structure allow MOFs to capture molecules so efficiently that they are now prime candidates in carbon-capture research. In terms of separating molecules, MOF-based membranes are among the highest performers, and can carry out the propylene-propane separation at ambient temperature. One MOF called ZIF-8 (zeolitic imidazolium frameworks-8), allows propylene to diffuse through its pores 125 times more efficiently than propane at 30oC, offering high selectivity without the need for sub-zero temperatures. “The main challenge with this approach is to synthesise high-quality, ultrathin, MOF films on commercial porous substrates without complicated substrate modifications,” says Professor Kumar Varoon Agrawal at EPFL. “Such high-quality films have fewer defects and are necessary for obtaining the highest possible separation selectivity.” His lab at EPFL Sion has now developed a straightforward MOF crystallisation approach called “electrophoretic nuclei assembly for crystallization of highly-intergrown thin-films” (ENACT). The ENACT method allows simple regulation of the heterogeneous nucleation on unmodified (as-obtained) porous and nonporous substrates. This in turn facilitates the synthesis of ultrathin, highly intergrown polycrystalline MOF films. The lab used the ENACT method to synthesise 500-nm-thick MOF membranes. When they tested them, the membranes yielded one of the best separation performances in propylene/propane separation recorded to date. One of the problems for Javier Vela and the chemists in his Iowa State University research group was that a toxic material worked so well in solar cells. And so, any substitute for the lead-containing perovskites used in some solar cells would have to really perform. But what could they find to replace the perovskite semiconductors that have been so promising and so efficient at converting sunlight into electricity? What materials could produce semiconductors that worked just as well, but were safe and abundant and inexpensive to manufacture? “Semiconductors are everywhere, right?” Vela said. “They’re in our computers and our cell phones. They’re usually in high-end, high-value products. While semiconductors may not contain rare materials, many are toxic or very expensive.” Vela, an Iowa State associate professor of chemistry and an associate of the U.S. Department of Energy’s Ames Laboratory, directs a lab that specialises in developing new, nanostructured materials. While thinking about the problem of lead in solar cells, he found a conference presentation by Massachusetts Institute of Technology researchers that suggested possible substitutes for perovskites in semiconductors. Vela and Iowa State graduate students Bryan Rosales and Miles White decided to focus on sodium-based alternatives and started an 18-month search for a new kind of semiconductor. They came up with a compound that features sodium, which is cheap and abundant; bismuth, which is relatively scarce but is overproduced during the mining of other metals and is cheap; and sulfur, the fifth most common element on Earth. The researchers report their discovery in a paper recently published online by the Journal of the American Chemical Society. The paper’s subtitle is a good summary of their work: “Toward Earth-Abundant, Biocompatible Semiconductors.” “Our synthesis unlocks a new class of low-cost and environmentally friendly ternary (three-part) semiconductors that show properties of interest for applications in energy conversion,” the chemists wrote in their paper. In fact, Rosales is working to create solar cells that use the new semiconducting material. Vela said variations in synthesis – changing reaction temperature and time, choice of metal ion precursors, adding certain ligands – allows the chemists to control the material’s structure and the size of its nanocrystals. And that allows researchers to change and fine tune the material’s properties. Several of the material’s properties are already ideal for solar cells: The material’s band gap – the amount of energy required for a light particle to knock an electron loose – is ideal for solar cells. The material, unlike other materials used in solar cells, is also stable when exposed to air and water. So, the chemists think they have a material that will work well in solar cells, but without the toxicity, scarcity or costs. “We believe the experimental and computational results reported here,” they wrote in their paper, “will help advance the fundamental study and exploration of these and similar materials for energy conversion devices.” large propylene permeance (flux normalised with pressure difference), which will help reduce the membrane area needed for industrial applications. The group concludes that the versatile, straightforward ENACT method can be extended to a wide-range of nanoporous crystals.

Phys.org, 15 March 2015 ; http://www.eurekalert.org