Chemicals from wood instead of petroleum

Petroleum can be used to produce more than just fuel. Petrochemicals are also important raw materials for the chemical industry. Without petroleum, there would be no plastics and few medicines or fertilizers. However, the decline in oil production is foreseeable. To meet the resulting challenge, we need renewable resources as alternatives to petroleum.

Two research projects of the National Research Program "Resource Wood" (NRP 66) have now come an important step closer to replacing petroleum with plant biomass - especially wood. They are focusing complementarily on one of the two main components of wood each: cellulose and lignin. These two renewable substances are the world's most abundant organic compounds.

At EFPL, Sviatlana Siankevich has developed new, powerful catalytic processes that extract hydroxymethylfurfural (HMF) from cellulose, an important feedstock for the production of plastics, fertilizers and biofuels.(*) The team at FHNW in Muttenz (BL), led by Philippe Corvini, drew inspiration from fungi that break down rotting wood to find enzymes that break down lignin into aromatic compounds. These aromatics serve as starting materials for the production of solvents, pesticides, medicines and plastics such as polystyrene.

Chemicals instead of paper
Cellulose is a long-chain sugar molecule (carbohydrate) and makes up about two-thirds of the weight of wood. "Cellulose is mainly used to make paper today. The residues from it could be put to good use in the production of chemicals that are in demand," says Sviatlana Siankevich of EPFL's Institute of Chemical Sciences and Engineering. Together with scientists from Queen's University in Canada and the National University of Singapore, the EPFL team led by Paul Dyson synthesized several types of ionic liquids (liquid salts) to produce HMF from cellulose. Using their process, the scientists achieved a record 62 percent yield in one step.

"Our process works under mild reaction conditions and does not require very high temperatures and pressure or strong acids," Siankevich says. "We were also able to reduce the amount of unwanted byproducts. This is an important aspect for the industrial use of the process. Our process works with wood, but it's often easier to use cellulose derived from herbaceous plants."

Green chemistry
At the University of Applied Sciences Northwestern Switzerland (FHNW) in Muttenz, Philippe Corvini and his doctoral student Christoph Gasser are developing processes for recycling lignin. This long-chain molecule is part of the cell walls and gives trees their stiffness. Wood consists of about 15-40 percent lignin. "This has hardly ever been recycled, but often just burned," says Corvini. "In the process, it can be broken down into aromatic compounds: molecules based on the hexagonal carbon rings that are ubiquitous in organic chemistry. Industry turns over a lot of these compounds, which are almost exclusively derived from petroleum. Currently, lignin is the most promising alternative to petroleum."

There are fungi that secrete a cocktail of enzymes to break down and degrade lignin. Led by Corvini, the FHNW team investigated combinations of dozens of these enzymes to determine the most efficient. With an additional catalytic step, they succeeded in breaking down 40 percent of the lignin into very small molecules - such as vanillin. The process is of interest to the chemical industry: the team is already working with a lignin producer. "Lignin today is mainly extracted from wheat or rice straw," Corvini says. "But softwood - such as that of spruce - would also be well suited, since its lignin is easy to break down."

The FHNW team has also developed a process to reuse the enzymes. "We bound the enzymes to iron nanoparticles coated with silicon dioxide," he explains. "After the reaction, we simply remove the iron particles using a magnet to recover the enzymes." These can be reused up to ten times, significantly reducing the energy and resources needed to produce them. The process thus fits in very well with the concept of "green chemistry".

Utilize all the wood
The wood must be recycled as extensively as possible so that it can be an economical alternative to petrochemicals. "Extracting small amounts of a single component is not enough," says Sviatlana Siankevich. "We therefore need to find complementary processes so that we can use all the wood." There are other aspects to consider when assessing whether wood is an economically viable substitute for petroleum. A third NRP 66 project recently examined the sustainability of producing succinic acid, another important chemical, from wood residues. The study, conducted jointly by ETH Zurich and EPFL, shows that intelligent process design can enable energy savings and also otherwise protect the environment - key factors for competitive biorefineries.

Source: Swiss National Science Foundation (SNSF)