With sustainability becoming an increasingly important point for the chemical industry, the exploration of new so-called “green” solvents is increasing. While initially competing with food streams the tendency is increasingly towards the utilization of (cellulosic)-waste streams. The utilization of biomass is not only important for the chemical industry to become more sustainable, but also to create independence from the often volatile oil prices.
In general two types of biobased solvents can be distinguished. First are the solvents which are already produced by the petrochemical industry and of which a biobased analogue is available.
Examples of this are C1-C4 alcohols such as ethanol and butanol, acetone, butanediol and acetic acid amongst others. From these fermentation streams, downstream products such as diethyl ether, ethyl acetate and tetrahydrofuran can be obtained. For most of these compounds, with the exception of bio-ethanol, the bio-based variants are often still more costly than they are from petrochemical sources. This is due to the fact that over the last 100 years, the petrochemical routes have been optimized and the large volumes allow for high efficiency. Instead, bio-based products have a high startup cost, with processes that need to be further optimized and plants which often newly build.
The second class of biobased solvents are different from their petrochemical brothers and sisters due to the different composition of the feedstock. Examples of this are lactylate esters such as ethyl lactate and butyl lactate, 2-methyl tetrahydrofuran (MeTHF), Cyrene and dimethyl isosorbide. Each previous example could theoretically be prepared from petrochemical sources, but synthesis would be tedious and not worth it. Especially the valorization of cellulose waste streams opened a whole new range of solvents. For example, MeTHF is obtained via the reduction of furfural and while this can be obtained from petrochemical sources, synthesis is costly and as such not economically viable. Even more novel is the levoglucosenone derivative Cyrene, which has been pitched as an alternative to replacing the reprotoxic NMP and DMAc. New molecules have to find their way into the chemical industry and it might take some time before they are used on a wider scale, but it is promising to see new solvents enter the market.
The recently developed Genomatica process to produce bio-BDO has opened further avenues to produce biobased solvents as it can be further transformed to bio-THF, bio-GBL and bio-NMP. Currently these products not yet on the market, but with increasing volumes and decreasing costs of bio-based molecules, a more economically viable situation is coming closer. However, with the relatively low oil prices of the last few years, the urgency for real change has slowed down. Going back to oil prices of the level of 2011-2014 would speed up this process, as it is still hard for biobased processes to compete at these price levels.
Additionally, it is to be expected that policymakers and governments will steer towards a society that becomes increasingly circular and the utilization of bio-mass certainly would be beneficial in this regard. For a real change to occur in the coming years, the introduction of stronger regulations would be the quickest way forward towards an increasingly biobased world.