Cleaner Cleaners: Creating More Sustainable Surfactants

Widely recognised for the part they play in keeping things clean, surfactants act as a key component in detergents, personal care, and healthcare products. Amanda Jasi spoke to innovators working to devise and establish novel green production routes

SURFACTANTS (or surface active agents) are versatile chemicals most commonly associated with cleaning detergents. With a basic structure comprising a hydrophobic and a hydrophilic component, their properties allow them to mix the immiscible, create foams, and act as a wetting agent, serving use in areas including personal care and healthcare products, and in industrial processes.

Demand for the chemicals totalled around 16.8m t/y in 2018.¹ Traditionally, that has been met with reliance on petrochemical feedstocks, but manufacturers are increasingly turning their attention to sustainable alternatives, as The Chemical Engineer found out.

An extra life

At the University of California, Santa Barbara (UCSB) in the US, researchers are developing a process to make surfactants from plastic. “We can make the carbon that’s in the plastics useful another time and displace some of that carbon which is used from fossil fuels,” said Susannah Scott, distinguished professor of chemistry and chemical engineering at the university.

In a 2020 research paper,² Scott’s research team debuted a catalytic process that converts polyethylene, commonly used in single-use plastic packaging, into alkylaromatic molecules, high-value chemicals that are a precursor to anionic surfactants. The process involves adding a catalyst and the polymer into a pressure reactor – without requiring solvent or hydrogen – and heating the mixture to around 300°C.

Scott said: “The process is very simple, and that’s important because [waste plastics] are high-volume, low-value products…so you need very simple processes that don’t cost a lot to implement if you want to make these work at scale.”

The group was initially successful with a platinum/alumina catalyst, but Scott said the reaction needed to be faster and more selective to make the process suitable for scaling.

“The key breakthrough was when we realised that the hydrogen redistribution that needs to happen in the long polyethylene molecules to make these shorter alkylaromatics would be promoted by catalysts more closely resembling what the industry uses in hydrocracking…platinum catalysts that are dispersed on halogenated aluminas. Industry usually uses chlorinated alumina.”

In work published in June,³ the group used modified catalysts containing a chlorine or fluorine group. This made the reaction ten times faster and increased aromatic production five-fold. The modifications also centred the products at around 20 carbons in size (C20), more in line with conventional surfactants (C18–22) when compared to products in their previous work. Those had an average carbon number of about C30.

Scott said the team is continuing to make the process faster and more selective, adding that “every step-change that we make gets us closer to a process which has some commercial viability and some scalability”. Scott also noted that she is collaborating with a colleague at UCSB to validate the use of their product in surfactant production. She added that the group is in talks with industrial partners that could scale the technology.

Cleaners from nature

Sironix Renewables has already taken its work for more sustainable surfactants beyond academia, spinning out from the University of Minnesota, US in 2016.

CEO and co-founder Christoph Krumm said the company’s core mission is to create ingredients for consumer and industrial cleaning products that are not only environmentally friendly, but also offer performance advantages over petroleum-based ingredients, while being safe for human use. “While that sounds simple and straightforward, it’s actually something that the industry has really struggled with,” he said.

The company initially focused on producing one of the most widely used surfactants in the world, a linear alkylbenzene sulfonate, using plants rather than petroleum. After looking at a variety of production routes, the team discovered a five-step process for converting bio-based and renewable feedstocks, but at a financial cost. The process would have amounted to more than its petrochemical-reliant alternative, so they decided to simplify.

“It turned out a two-step process gave you something that looked a lot like an alkylbenzene sulfonate,” Krumm said. The process the company settled on operates as a platform that can use different renewable and sustainable feedstocks. Currently, Sironix uses fatty acids originated from coconut, or domestically available soya, and furan sourced from the existing furfural supply chain. Furfural is produced from xylose, a large component of agricultural residues such as sugar cane bagasse and corn stover.

Krumm said there are a number of companies that convert furfural to furan derivatives, while derivatising natural oil to produce fatty acids is really common and used in processes such as soap-making. The company is still establishing its supply chain.

For its process, Sironix takes the sourced components and performs an acylation reaction to combine the fatty acid and the furan, creating a carbon-carbon bond between the molecules. The product from the reaction, called an alkylfuran, is then sulfonated.

Krumm explained that sulfonation is the process of adding an SO₃- group to the surfactant, a strongly anionic hydrophilic head group. He said this process is responsible for the water solubility of the product. “In the case of anionic surfactants, the sulfonation is what makes them perform very well in cleaning and cleansing applications,” Krumm added.

Sironix’s resulting surfactants can offer a reduction of up to 65% in greenhouse gas emissions compared to sodium laureth ether sulfate (SLES), an established sustainable surfactant. The company also says they are milder and offer better cleaning than other eco-friendly options, as well as conventional surfactants.