A smaller carbon footprint for petrochemicals? New ultra-thin membranes lead the way

Global warming, energy shortages and dwindling resources have stressed the importance of creating more efficient, sustainable and environmentally friendly technologies. Such improvements are especially vital in energy-intensive industries like the chemicals and petrochemicals sector. Scientists of the EU-funded research project ENACT have made great strides in addressing these challenges through the development of sustainable chemical technologies.

Most production processes in the petrochemical industry take place at extreme temperatures, which use large amounts of energy. Propylene is one of the products derived from such a process, and is used in adhesives, fibres, paints, and many other consumer and industrial items. When propylene is purified, it’s separated from propane through cryogenic distillation, an energy-consuming process that involves cooling the gases to ultra-low temperatures.

The promise of MOFs

A more energy-efficient alternative is found in a class of porous polymers called metal-organic frameworks, or MOFs. These crystalline compounds consist of metal ions that are bound to organic ligands to form 3D structures. The unique features of MOFs, such as their high porosity, large surface areas and diversity of structures, have made them suitable for a wide range of industrial processes, including gas storage, purification and separation, as well catalysis and sensing applications. They are also promising materials for carbon capture applications because of their high carbon adsorption capacities and the fact that their properties can be finely tuned.

MOF-based membranes perform especially well when it comes to separating gases. Their nanosized pores are ideal for trapping molecules, while allowing other substances to pass through. An exceptionally high performer in the separation of propylene and propane mixtures is a class of MOFs called ZIF-8 (zeolitic imidazolate framework-8). This ultra-thin film allows propylene to diffuse through its pores 125 times more efficiently than other materials. In addition, the separation process is carried out at ambient temperatures of roughly 30 °C and therefore consumes less energy.

A new twist to the MOF

Up to now, complex modifications have had to be made to the porous supports of ZIF-8 membranes for them to perform well in propylene-propane separation. With this challenge in mind, the ENACT team developed a method for synthesising ZIF-8 without modifying the support. Their method, called electrophoretic nuclei assembly for crystallisation of highly intergrown thin-films, is described in the paper published in the ‘Wiley Online Library’.

Using this new method, the scientists synthesised defect-free, 0.5-μm-thick ZIF-8 membranes on a wide range of unmodified supports such as porous polyacrylonitrile, anodised aluminium oxide, metal foil, porous carbon and graphene. The results showed one of the best propylene-propane separation performances for MOF membranes to date. As the authors explain, their novel approach is “straightforward, reproducible and can be extended to a wide range of nanoporous crystals.”

Using computer simulations, materials synthesis and experimental characterisation, the ENACT (Enhancing sustainable chemical technologies through the synergy of computer simulation and experiment) project strives to optimise the design of liquid-phase systems for chemical technologies. Its ultimate aim is to use the advances it has made in various fields, such as porous liquids and biomimetic membranes, to develop efficient and sustainable processes with a low environmental impact. This will help to tackle air pollution, energy shortages and global warming.

For more information, please see:
ENACT project website