Making chemical separation greener with self-assembled nanoscale membranes

April 30, 2022

(News from Nanowerk) Chemical separation processes are essential in the manufacture of many products, from gasoline to whiskey. These processes are energy-intensive, accounting for around 10-15% of global energy consumption.

In particular, the use of so-called “thermal separation” processes, such as distillation to separate petroleum hydrocarbons, is deeply rooted in the chemical industry and has a very large associated energy footprint. Membrane-based separation processes have the potential to significantly reduce this energy consumption.

Membrane filtration processes that separate contaminants from the air we breathe and the water we drink have become commonplace. However, membrane technologies for separating hydrocarbons and other organic materials are much less developed.

Penn engineers are developing new membranes for energy-efficient organic separations by redesigning their physical structure at the nanoscale.

Nanostructured membranes can be used in filtration applications (water purification for example). Nanostructured membranes provide highly selective separation without compromising permeability.

Nanofiltration using self-assembled membranes has been a major research area for Chinedum Osuji, Presidential Professor Eduardo D. Glandt in the Department of Chemical and Biomolecular Engineering, and his lab. The performance of these membranes was highlighted in a previous study (ACS Nano“Rapid Fabrication by Lyotropic Self-Assembly of Thin Nanofiltration Membranes with Uniform 1 Nanometer Pores”) describing how the structure of the membrane itself helped to minimize the limiting trade-off between selectivity and permeability found in traditional nanofiltration membranes.

This technology was also included in last year’s Y-Prize competition, and winners put forward a case for its use to produce alcohol-free beer and wine at a startup called LiberTech.

Now, Osuji’s latest study adapts the membrane for filtration in organic solutions such as ethanol and isopropyl alcohol, and its self-assembled molecules make it more efficient than organic solvent nanofiltration (OSN) traditional.

The researchers demonstrated how the methods used to create their membranes can fine-tune the spacing of the nanostructures in the resulting filter.

The study, published in Scientists progress (“Tunable Organic Solvent Nanofiltration in Sub-1 nm Scale Self-Assembled Membranes”), describes how the uniform pores of this membrane can be refined by changing the size or concentration of the self-assembled molecules that ultimately form equipment.

This adaptability now opens doors for using this membrane technology to solve more diverse organic filtration problems in the real world. Researchers from Osuji’s lab, including first author and former postdoctoral researcher, Yizhou Zhang, postdoctoral researcher, Dahin Kim, and graduate student, Ruiqi Dong, as well as Xunda Feng from Donghua University, contributed to this work.

One of the challenges the team faced was the difficulty of maintaining membrane stability in organic solvents of different polarities. They selected molecular species, surfactants, that exhibited low solubility in organic fluids and that could be efficiently bonded together chemically to provide the required stability.

Surfactants self-assemble in water when they exceed a certain concentration and form a soft gel. Such self-assembly – the formation of an ordered state – as a function of concentration is called lyotropic behavior: “lyo-” referring to solution and “-tropic” referring to order. The gels thus formed are called lyotropic mesophases.

The membranes developed in this study were created by first forming lyotropic mesophases of the surfactant in water, spreading the soft gel as a thin film, and then using a chemical reaction to bind the surfactants together to form a polymer. nanoporous. The pore size in the polymer is fixed by the self-assembled structure of the lyotropic mesophase.

“At a certain concentration in an aqueous solution, surfactant molecules aggregate and form cylindrical rods, and then these rods self-assemble into a hexagonal structure, resulting in a gel-like material,” Osuji explains. “One of the ways we can manipulate the permeability, or pore size, of our membranes is to alter the concentration and size of the surfactant molecules used to create the membrane itself. In this study, we manipulated these two variables to tune our pore sizes from 1.2 nanometers to 0.6 nanometers.

These membranes are compatible with organic solvents and can be adapted to meet different separation challenges. Nanofiltration of organic solvents can reduce the footprint of traditional thermal separation processes. The uniform pore size of the membranes developed here offers compelling advantages in terms of membrane selectivity and ultimately energy efficiency.

“A specific application of this technology is the production of biofuels,” says Osuji. “The isolation of water-miscible alcohols from bioreactors is a key step in the manufacture of ethanol and butanol biofuels. Membrane separations can reduce the energy used in the separation of product alcohols or fuels from the aqueous medium in the reactor. The use of membranes is particularly advantageous in smaller scale operations such as this, where distillation is not economical.

“Additionally, the manufacturing of many pharmaceuticals often involves multiple synthetic steps in different solvent environments. These steps require the transfer of a chemical intermediate from one solvent to another miscible solvent, making this new membrane a perfect solution to the filtration needs of drug development.

The next steps in their research involve both theory and practice. The team plans to develop new membrane rejection and permeability models that address the unique flow pattern of solutions through their membranes, as well as identify other future applications for their tunable technology.

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