The cornerstone of all clean technologies is the mitigation of the economic, environmental, and political burden of petroleum, which is imported into the US at a rate of 4.1 billion barrels per year (EPA statistics).1 The use of biomass to produce fuels and chemicals has the potential to reduce dependence on foreign oil, cut greenhouse gas emissions, valorize waste, and create new, green industries.
This space is currently full of ideas but short on practical solutions. Many fermentative approaches to biomass processing have been developed, but the expensive nature of such technologies means that many of these will likely be transitional. Methods that are getting increasing attention involve the chemical conversion of biomass into simple organic molecules that can be used as a platform from which to derive a broad portfolio of renewable products.
One such method is the chemical digestion of cellulose into 5-(chloromethyl)furfural, or "CMF" (Figure. 1). This can be accomplished in a biphasic liquid/liquid reactor containing aqueous hydrochloric acid and an organic solvent.2,3 The feedstock is mixed with the acid at around 80°C, which first hydrolyzes the cellulose to glucose and then dehydrates the glucose to 5-(hydroxymethyl)furfural, referred to as "HMF."
Next, the HMF is converted by the HCl into CMF. This is a crucial step in the reaction sequence, since CMF is hydrophobic and is quickly sequestered into the organic phase. The removal of the CMF product from the reaction medium avoids the degradation reactions which have long plagued the acidic processing of biomass. The solvent-CMF mixture exits the reactor and the solvent is evaporated and recycled. The whole process takes 2-3 hours and CMF is isolated in yields of between 80 and 95%, depending on the feedstock.
Figure 1. Process for the conversion of cellulosic biomass into CMF.
In a second reaction module, CMF is processed through one of two product manifolds. One of these, called "furanics" maintains furan ring of CMF intact, and the other, called "levulinics," is based on derivatives of levulinic acid (Figure 2). Both paths provide access to a variety of fuels, renewable polymers, and specialty chemicals.
Figure 2. Furanic or levulinic product classes, as derived from CMF. R1 may represent hydrogen or an alkyl group. R2 and R3 may represent any combination of alkyl, hydroxymethyl, alkoxymethyl, aminomethyl, CHO, or CO2H groups.
One area of particular interest is in the context of processing of oilseed or algae feedstocks for the production of biodiesel. Using the above-described reactor technology, the product is a hybrid lipid/carbohydrate-based fuel consisting of a blend of fatty acid and levulinate esters. Using safflower seeds as a model, we have been able to increase the yield of fuel by 24%. This is accomplished by converting both the lipid and the carbohydrate content of the seeds into fuel. Not only is more biodiesel created in the process, but the blending of fatty acid esters with levulinate esters improves the fuel's cold-flow properties.
Figure 3. Process for the conversion of oil crops into a hybrid biodiesel. The mixture of plant oil and carbohydrate-derived CMF is co-esterified, giving rise to a fuel cocktail containing fatty acid ester (red) and levulinate ester (green).
A general summary of the products in the CMF derivative portfolio is shown in Figure 4. Biofuels include substituted furans, tetrahydrofurans, and levulinate esters. These renewable fuels span the volatility range from diesel to gasoline, and are less toxic, more biodegradable, and cleaner burning than hydrocarbons.
Unlike alcohols, they are hydrophobic, non-corrosive, and non-malodorous (cf. butanol). Bi-functional furans may serve as renewable substitutes for phthalate monomers and their derivatives in commercially important polyesters and polyamides. Finally, specialty chemical products of value may be derived from CMF. For example, 4-aminolevulinic acid and prothrin (Figure 4, bottom row, left and center) are agrochemicals, and levulinic acetals of glycerol (Figure 4, bottom row, right) have a range of uses across the chemical markets.4
Figure 4. Examples of CMF derivatives.
In summary, inexpensive chemical technologies are well positioned to compete in the renewables markets, particularly as petrochemical margins are squeezed by the rising cost and declining availability of oil. The method described here, based on the direct conversion of cellulosic waste into CMF, has the following features:
The process is inexpensive and operates under mild conditions. No complex catalysts, enzymes, microorganisms, or extremes in temperature are involved. Everything is recycled, and there is no waste stream. The only non-organic product is water.
- Mixed-source lignocellulosic feedstock can be directly utilized with minimal pretreatment. Sources may include forestry, agricultural or municipal wastes. Only mechanical reduction to a reasonable particle size is required. Since the process is aqueous, even drying of the substrate is not necessary.
- The derived products are drop-in, non-toxic, environmentally friendly substitutes for petroleum, both in motor fuels and as renewable chemical intermediates. As described above, CMF is a versatile platform chemical from which a diverse portfolio of derivatives can be accessed.
2. M. Mascal and E. B. Nikitin, ChemSusChem 2009, 2, 859.
3. M. Mascal and E. B. Nikitin, Green Chem. 2010, 12, 370
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