Natural solvents enable fully bio-based membranes

Porous membranes are used to filter and separate materials in the chemical industry, wastewater treatment, and food production. Most are manufactured using chemicals derived from fossil sources, such as oil and natural gas, which contribute to greenhouse gas emissions.

KAUST researchers have now developed a method that relies entirely on renewable, bio-based materials for membrane production, offering an environmentally friendly alternative^[1].

More than 400 million tonnes of oil are used each year to manufacture polymers, accounting for about six percent of global oil production. Some of these polymers are turned into membranes — porous films that can separate solids, liquids or gases — which typically consume less energy than traditional heat-based separation methods. Only a small fraction of polymers come from sustainable plant-based materials, and these still require fossil-derived solvents to convert them into membranes.

Membranes are typically produced by dissolving the polymer in a solvent, coating the solution onto a fabric support, and immersing the coated film in a non-solvent liquid. This causes the polymer solution to split into two phases, forming a solid polymer network with tiny pores.

One of the most promising biopolymers is poly(ethylene furanoate) (PEF), made from chemical building blocks derived from woody biomass. PEF has similar properties to poly(ethylene terephthalate) (PET), commonly used in food and beverage packaging. “PEF is a highly stable polymer, which means it is very difficult to dissolve,” says Malinalli Ramírez-Martínez, formerly a Ph.D. student in the group of Suzana Nunes, who led the research.

Until now, PEF membranes have relied on toxic fossil-based solvents such as trifluoroacetic acid and dichloromethane. “That motivated us to look for greener alternatives,” adds Ramírez-Martínez.

The team identified vanillin and thymol as promising candidates. Vanillin can be produced from the same kind of forestry waste as PEF, while thymol comes from cultivated thyme plants. Both are solids at room temperature, but together they form a mixture known as a deep eutectic solvent that has a melting point of just 26°C. This thymol-vanillin mixture has not previously been applied to membrane production.

The researchers used the thymol-vanillin solvent, along with ethanol as the non-solvent, to prepare PEF membranes with pores 50–150 nanometers wide. These ultrafiltration membranes removed fine sediment from apple, orange, and pomegranate juices, producing clear juices with longer shelf lives.

The membrane reduced the juice’s cloudiness by more than 98 percent, slightly outperforming commercial ultrafiltration membranes made from fossil-based materials. “This is very encouraging. Even without extensive optimization, the membranes show comparable performance using a biopolymer and bio-based solvents,” says Nunes.

A life-cycle analysis showed that manufacturing the PEF membrane in this way generated 42 percent lower greenhouse gas emissions than an analogous fossil-based ultrafiltration membrane. Further emissions reductions could be achieved by using ethanol derived from fermentation processes rather than fossil sources.

“As bio-based chemicals come into wider use, their costs should fall and accelerate their adoption,” explains Nunes. “More importantly, stricter regulations are being introduced in many countries. In Europe, it is a matter of when, not if, the transition to green solvents and biopolymers will occur.”