The time is now for ionic liquids

On Finnish Independence Day in December 2018, Finland’s first lady, Jenni Haukio, wore a dress made from birch trees.

No, she wasn’t covered in bark and leaves. The garment was woven from fibers made using a process, called Ioncell, developed by Aalto University and the University of Helsinki.

The process uses ionic liquids such as 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH][OAc]) and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-enium acetate ([mTBDH][OAc]) to dissolve cellulose and extract it from wood pulp. The natural polymer is spun into fibers.

Making fibers out of cellulose isn’t new. Viscose cellulose has been made since the 19th century in a cumbersome process requiring the harsh chemicals sodium hydroxide, carbon disulfide, and sulfuric acid. Ioncell, according to Herbert Sixta, a professor in Aalto’s Department of Bioproducts and Biosystems, is a simpler, closed-loop process with no chemical discharges.

Aalto is building a pilot plant that will produce 10 kg of fiber per day when it opens later this year. Sixta anticipates 2 years in the pilot phase and then 3 years of scaling up for commercial production. His hope is to establish cellulose as an alternative to fossil fuel–based fibers such as polyester. “At the moment, we are looking for industrial-scale partners,” he says.

Sixta has big dreams for the ionic liquid–based fiber process. So too have many scientists fascinated by ionic liquids over the years. But elsewhere, such as at a Chevron refinery in Salt Lake City, ionic liquids are already moving past the dream stage and into real-world applications that take advantage of the unique properties of the low-temperature molten salts.

Chemists are developing roles for ionic liquids in safer batteries, cleaner solvents, rare-earth metal recycling, and more efficient chemical processes. Meanwhile, the focus of scientists and engineers is shifting from what is technically possible with ionic liquids to what is commercially feasible. And companies that make ionic liquids are scaling up, which should result in lower costs and more applications.

Yet even with all the attention, ionic liquids are far from being go-to tools in the chemical industry toolbox. “Hype” is a word observers tend to utter when describing the excitement that can infect ionic liquids. All too often, they say, ionic liquids have proved to be more expensive to use than the traditional materials they are meant to replace.

Ionic liquids are salts with an organic cation and either an organic or inorganic anion. They have irregular structures, which delocalize their charges.

Because of these irregular shapes and low charges, the molecules don’t pack together as neatly as other salts do, explains Gosia Swadźba-Kwaśny, director of the Queen’s University Ionic Liquid Laboratories (QUILL). “Therefore, they don’t crystallize as easily, and their melting points are lower,” she says. Ionic liquids have melting points below 100 °C, and many are liquid at room temperature. In contrast, table salt melts at 800 °C.

The period from about 2000 to roughly 2012 is often cited as the peak of chemists’ interest in the materials. “There was huge hype around ionic liquids, as scientists were excited about their properties—so different from standard solvents in chemistry,” Swadźba-Kwaśny recalls.

Foremost among these unusual properties are high boiling and low freezing points, along with negligible vapor pressure. “When it comes to working with solvents, when they freeze and when they boil is what gives us the operational range,” Swadźba-Kwaśny says. “Ionic liquids’ range is really quite impressive.”

Additionally, ionic liquids aren’t nearly as flammable as organic solvents. “If you want, you can have a flammable ionic liquid, but you really have to try,” Swadźba-Kwaśny says. They are also conductive. Combined, these properties make the liquids a natural fit to replace battery electrolytes, which have been known to ignite.

Scientists and engineers have many ionic liquids to choose from. Roland Kalb, who founded the Austria-based supplier Proionic in 2004, estimates that 500 ionic liquids are commercially available in quantities from 1 g to 10 kg. Dozens are available for delivery between 10 and 1,000 kg. And about 10 can be sourced in quantities greater than 1 metric ton (t).

Commercial applications are growing. Kalb came up with 57 in a count he conducted while working on a chapter of a 2019 book on ionic liquids. A 2008 survey by Kenneth R. Seddon and Natalia V. Plechkova, both working at QUILL at the time, pinpointed 13.

Kalb has found them used as solvents in chemical processes and biomass refining. Ionic liquids are also employed as auxiliaries and catalysts in chemical synthesis. They are used in analytical equipment. They make up electrolytes in lithium-ion batteries, supercapacitors, and metal plating baths. They can be found as lubricants and coolants. Even polymers are starting to incorporate ionic liquids as additives.

But while a quadrupling of applications in a decade sounds great, Kalb says, ionic liquids haven’t caught on as quickly as optimists had once hoped. And none of them are what Kalb would call “mass applications” that generate hundreds of millions of dollars in value.

When asked about the number of new commercial projects that use ionic liquids, Jeffrey Kolpa, global marketing director for phosphorus at the big Belgian chemical maker Solvay, replies, “Not as many as we would like and never as big as we would like.”

The holdup, Kolpa says, is economics. Solvay collaborates with academia, start-ups, and large companies on many projects that attempt to replace an incumbent chemistry with ionic liquids. “At the end of the day, what kills them isn’t that the chemistry doesn’t work,” he says. “It’s whether they can make the chemistry work better than the alternative at a certain price.”

And ionic liquids can be very expensive. Kalb says some of Proionic’s products go for more than $800 per kilogram. Prices can decline with greater economies of scale. An order of a metric ton of the same material might cost as little as $300 per kilogram, and that price might get down to $150 per kilogram for a 5 t order.

But some ionic liquids don’t scale so easily. Kolpa says Solvay’s quotes aren’t going to drop an order of magnitude from a 10 t order to a 1 t order, because the firm’s phosphine chemistry requires careful handling. “The chemistry is very dangerous, and there is a lot that goes into the chemistry,” he says.

Similarly, many ionic liquids used in batteries and other electrochemical applications are made from bistriflimide anions. These are fluorinated molecules that require 37 synthesis steps, according to Swadźba-Kwaśny. “So even with economies of scale, it is not possible to make them cheap,” she says.

But some important classes of ionic liquids, like those based on imidazolium cations, do scale up, according to Thomas Schubert, managing director of IoLiTec Ionic Liquids Technologies, an ionic liquid supplier in Heilbronn, Germany. “Prices will drop as soon as there is higher demand,” he says.

Betting that this is starting to happen, some ionic liquid suppliers are expanding to increase economies of scale and meet higher demand. Proionic is planning to invest significantly to expand its plant to be able to make thousands of metric tons per year, up from hundreds today. Kalb anticipates strong demand from biomass processing and electrolyte applications.

In May, IoLiTec will open a $2 million plant at Dow’s ValuePark in Schkopau, Germany, that will initially have a 1 t reactor, a big step up from the firm’s pilot-scale production in Heilbronn.

Schubert says the plant will help IoLiTec reduce production costs. It will also help the firm meet demand as customer projects graduate from the laboratory to industrial scale. “We already have the demand. That is why we are forced to go to the next step,” he says.

Getting to this point hasn’t been easy. An obstacle for ionic liquids is the sheer number of potential applications. Kalb estimates that researchers in the field have filed 20,000 patents worldwide, and the surfeit of options tends to dilute the spending needed to choose the best candidates for commercialization. “If you think about the money which is available in the world for developing new technologies, it is not unlimited,” he says.

Swadźba-Kwaśny argues that ionic liquid research is starting to be honed for practical success instead of mere academic curiosity. “The first interest of the scientific community was in what can ionic liquids do and how far can we push the boundaries of what’s possible rather than worrying about the price,” she says. “Now, cost feasibility is an important part of any good study.”

Reducing costs can sometimes be a matter of finding ionic liquids for a commercial process that aren’t as expensive as the ones that were originally used in the laboratory.

BASF, a supplier of workhorse imidazolium-based ionic liquids, says a lot of customer projects are now focused on costs. “It usually takes an effort to adapt and change over processes to accommodate the use of ionic liquids,” the company tells C&EN. “In the short term, established and incumbent technology would seem to be advantageous; however, longer-term vision would certainly suggest otherwise.”

With the focus on costs, experts say, industry is doing a better job of picking the winners, and commercialization of ionic liquids will turn the corner soon. “We are at the point where the first big applications are being commercialized,” IoLiTec’s Schubert says.

A bellwether project the industry is watching is the one in Salt Lake City. Later this year at its refinery there, Chevron will complete an alkylation plant that uses new ionic liquid–based technology.

Alkylation facilities react butene and other olefins with isobutane to produce octane and other hydrocarbons that are blended into gasoline. Traditional processes use either sulfuric or hydrofluoric acid (HF) as catalysts. The new Chevron catalysts are chloroaluminate ionic liquids with strong acidic properties but simpler and safer handling than the traditional catalysts.

Chevron has been working on the technology since it kicked off an ionic liquid initiative in 1999. It started up an alkylate demonstration plant in 2010 and in 2016 opted to build a commercial unit to replace an HF-based reactor on the site.

Chevron has picked Honeywell’s UOP engineering unit to license the technology to others under the Isoalky name. UOP has already nabbed its first client, Sinochem Hongrun Petrochemical, which will install the technology in China next year.

Rajesh Gattupalli, senior business leader for alkylation and treating technology at UOP, says the primary benefit of ionic liquids is their ease of handling. HF is extremely toxic; ionic liquids are not. If the ionic liquid “falls on the floor, it just stays as a puddle,” he says. “The ionic liquids are far less corrosive to the skin than other liquid acids.”

Meanwhile, sulfuric acid–based alkylation plants require huge amounts of corrosive acid—at least a tanker truck a week—that is regenerated on- or off-site. Ionic liquids, Gattupalli says, can be easily regenerated in a small, on-site unit.

The technology boasts operational benefits, too, Gattupalli says. The Isoalky process can handle a wide range of olefins in a single reactor. Other technologies must run different olefins through different reactors. Additionally, Isoalky yields higher octane levels than competing processes, he says.

The Chevron project isn’t the first to deploy ionic liquid–based alkylation. Three Sinopec refineries started up alkylation units last year using a technology licensed from the Canadian firm Well Resources. PetroChina has been rolling out its own technology as well.

The potential market for ionic liquid alkylation technology is massive, Gattupalli says. US refiners operate 100 alkylation units, split roughly equally between HF and sulfuric acid technology. Worldwide, about 350 alkylation units are in operation.

Other big firms are rolling out industrial ionic liquid–based technologies. Working with QUILL, the Malaysian oil company Petronas has commercialized a process to extract mercury from natural gas using a chlorocuprate ionic liquid. Mercury can corrode processing equipment, even at the parts-per-billion level present in gas. The low vapor pressure of ionic liquids suits it to the application because they won’t mix with natural gas, Swadźba-Kwaśny says. The chemical maker Clariant has signed on to license the technology to third parties.

BASF is using ionic liquids as additives to enhance the electrical conductivity of polymers. They are an alternative to carbon black, which needs to be loaded into the polymer at high concentrations. Ionic liquids offer easier handling and better polymer properties, BASF says.

And smaller players continue to dream about the next big use for ionic liquids.

Seren Technologies, a company cofounded by Queen’s University Belfast professor Peter Nockemann, plans to commercialize an ionic liquid–based method to recover neodymium and dysprosium from magnets used in electronics and wind turbines. Supplies of rare-earth elements like neodymium are tenuous because they are mined primarily in China.

Proionic’s Kalb predicts that biomass processes, such as Aalto’s Ioncell process, will turn out to be one of the more consequential uses of ionic liquids. “There is no other solvent that is capable of dissolving biomass as gently as ionic liquids,” he says.

For instance, Kalb says, cellulose has a higher molar mass when it is processed with ionic liquids than with any other method. He expects this to open the door not only to fibers but also to applications for biobased and biodegradable plastics. Ionic liquids could even lead to plastics derived from chitin that’s extracted from insects and shrimp shells, he says.

Ionic liquids might also help in developing new fuels. Kalb is working with the US Department of Energy’s Joint BioEnergy Institute on ionic liquids that separate cellulose from lignin so it can be converted enzymatically into sugars.

Even if some of these applications for ionic liquids don’t pan out, many agree that the chemistry tool is on the cusp of making big gains. This is provided, of course, that scientists and engineers will surmount the obstacles that are often in place between the laboratory bench and the plant floor. “The party’s over,” Kalb says. “Now we have to do the real work.”