Plastic and fuel that grows on trees

2009Biofuels continue to steal the spotlight when it comes to the search for a renewable, environmentally friendly replacement for crude oil. While that’s understandable when considering the transport industry, but crude oil is also used in the production of conventional plastics and chemical products such as fertilizers and solvents. Now chemists have learned how to convert plant biomass directly into a chemical building block that can not only be used to produce fuel, but also plastics, polyester and industrial chemicals cheaply and efficiently.

Earlier work by researchers at the Department of Energy's Pacific Northwest National Laboratory (PNNL) saw the development of a process to convert glucose and fructose derived from cellulose, into a primary building block for fuel and polyesters known as hydroxymethylfurfural, or HMF. Although it is a fairly simple process to convert HMF into plastics or biofuel, it is seldom used because HMF is costly to make. Other researchers had previously converted fructose into HMF, but the PNNL research group made a series of improvements that raised the HMF output, and also made the HMF easier to extract.

Using a chemical and solvent known as an ionic liquid, the PNNL team was able to convert the simple sugars into HMF. The chemical, a metal chloride known as chromium chloride, converted sugar into highly pure HMF, but the team still needed to break down cellulose into simple sugars – a step they wanted to skip.

By trying different metal chloride catalysts in the ionic solvent to speed up the conversion of cellulose to HMF, the team has now managed to convert cellulose directly into HMF in one step. They found that a combination of copper chloride and chromium chloride under 120 degrees Celsius broke down the cellulose without creating a lot of unwanted byproducts and allowed them to bypass the sugar-forming step. According to the researchers the simplified process generates a high yield of HMF and allows the use of raw cellulose as feed material.

In additional experiments, the team tested how well their method compared to acid, a common way to break down cellulose. The metal chlorides-ionic liquid system worked ten times faster than the acid and at much lower temperatures.

Optimizing their method, the team found that they could consistently achieve a high yield of HMF. The team’s method converted about 57 percent of the sugar content in the cellulose feedstock to HMF through this single step process. The team recovered more than 90% of the HMF formed, and the final product from the process was 96% pure. In addition, the metal chlorides and ionic liquid could be reused multiple times without losing their effectiveness, thereby lowering the cost of HMF production by being able to recycle the materials.

The new process means that it may soon be viable to harness the energy from carbon dioxide and solar energy that crop plants pick up, instead of relying on the ancient solar energy locked up in fossil fuels.

The single step process developed by the PNNL team is detailed in a paper available at ScienceDirect.

Darren Quick