Chemicals From The Garden

Sound scientific ideas sometimes lie in limbo for decades until some event, discovery, or urgent need triggers a wave of research. That's exactly what's going on in the field of biomass conversion. Driven by growing concerns about the scarcity of petroleum resources and the environmental price for using them, many scientists have jumped into, or ramped up their efforts in, this area of "green" chemistry. Their primary aim has been to convert plant matter into fuels, but now researchers are also buzzing with ideas for transforming biomass into the basic chemical ingredients that go into many everyday products.

"We're seeing tremendous expansion now in the number of research groups—especially in catalysis—coming into the field," says University of Wisconsin, Madison, chemical engineering professor James A. Dumesic. Along with all those scientists comes a rich assortment of ideas. Some researchers are developing low-temperature methods for hydrolyzing biomass into the sugar molecules contained within plant cell walls and then converting those compounds into transportation fuels and other products.

Other researchers are working on ways to pyrolyze plant matter at moderate temperatures (roughly 500 °C) to form bio-oil, an acidic, viscous, and oxygen-rich mixture of hundreds of complex compounds. Scientists taking that approach are searching for the right combination of reaction conditions and catalysts to upgrade bio-oil into valuable products. Still other scientists are exploring higher temperature methods for gasifying biomass into a mixture of CO and hydrogen, which can be converted into various versatile carbon-based building blocks.

"A lot of strategies for converting biomass are being investigated now, and it's just too early to say which ones will end up being the most useful," says Brent H. Shanks, a professor of chemical engineering at Iowa State University. Shanks, who serves as director of the National Science Foundation Engineering Research Center for Biorenewable Chemicals, points out that a few biomass-based chemical production processes date back many decades. In the 1920s, for one, Quaker Oats was converting oat hulls to furfural, a solvent and component of some resins, on a commercial scale. Nonetheless, he says, "this field is still in its infancy" from a technical standpoint.

Ideally, scientists would like to devise a simple, one-step or one-pot process that directly converts agricultural and municipal plant waste and other forms of raw biomass to valuable products. One reason that type of conversion isn't yet available is that plant cell walls are built up from a complex material that's tough to break down. That material, lignocellulose, consists of three main components: cellulose (35–50%), hemicellulose (23–32%), and lignin (15–30%).

Cellulose is made up of crystalline bundles of polysaccharide chains, each of which consists of hundreds to thousands of glucose molecules linked by β-1,4 bonds. Hemicellulose is also built up from chains of sugar molecules, but these chains, unlike the ones in cellulose, are amorphous. They typically consist of a random combination of various sugar molecules, including xylose, mannose, and arabinose. Sandwiched between cellulose and hemicellulose is lignin, a macromolecule made up of substituted phenols. It binds together the other components of the lignocellulose matrix, thereby strengthening cell walls (C&EN, Dec. 8, 2008, page 10).

Although scientists have not yet come up with an effective one-step method for converting raw lignocellulose to finished products, they are making progress. For example, at Pacific Northwest National Laboratory, a team headed by Z. Conrad Zhang, now at the bio-oil firm KiOR, in Pasadena, Texas, recently demonstrated a one-step procedure for transforming cellulose to 5-hydroxymethylfurfural (HMF). That compound is widely touted as a versatile biomass "platform" chemical because it can be used to synthesize an assortment of compounds that presently are derived from petroleum, including solvents, fuels, and monomers for polymer production.

Three years ago, Dumesic's group reported a way to prepare HMF from fructose, which is several steps downstream from untreated biomass (C&EN, July 3, 2006, page 9). Then in 2007, the PNNL team showed that HMF can be prepared from glucose, the building block of cellulose (C&EN, June 25, 2007, page 8). Now, the PNNL group has come up with a single-step method to make HMF directly from cellulose (Appl. Catal. A 2009, 361, 117).

"Until now, one of the major bottlenecks to using cellulose has been the depolymerization step," Zhang says. That process requires breaking down the cellulose crystal structure and then hydrolytically cleaving the chains into glucose molecules. By using high-throughput methods, the group discovered that a combination of copper chloride and chromium chloride dissolved in an imidazolium ionic liquid catalyzes cellulose depolymerization under mild conditions—about 100 °C. The team reports that their catalytic system depolymerizes cellulose about 10 times faster than conventional hot-acid treatments.

PNNL scientist David L. King points out that on the basis of nuclear magnetic resonance studies, the team has identified fructose as a reaction intermediate but notes that other questions remain unanswered. For example, catalysts with a large mole fraction of copper chloride (roughly 0.9) provide high catalytic activity and large product yields. But it is unclear why pure CuCl₂ hardly exhibits any catalytic activity.

In related work, Emiel Hensen, a professor at Eindhoven University of Technology, in the Netherlands, sought to develop a method for immobilizing and recycling solution-phase catalysts of the type used by the PNNL group. Immobilized catalysts are easily separated from liquid products and reused, which makes them attractive and potentially cost-effective for commercial use.

At last month's North American Catalysis Society meeting in San Francisco, Hensen presented data showing that his group had succeeded in grafting ionic liquid-metal chloride complexes onto porous silica and that the immobilized form of the catalyst is more active in test studies than the solution-phase form.

Also at that conference, Thomas S. Hansen of Technical University of Denmark (DTU), Lyngby, reported that HMF can be made from fructose quickly and simply by irradiating an acidified fructose solution with microwaves. In studies conducted with researchers at Novozymes, in Bagsvaerd, Denmark, Hansen found that at a solution temperature of 200°C, more than 50% of the fructose is converted to HMF in just 60 seconds. He acknowledged that the process is not yet well understood but pointed out that a timescale of hours would be needed to reach a comparable conversion level with conventional heating.

Lactic acid and related lactates, which manufacturers use to make biodegradable polymers and other products, also figure prominently in biomass research programs. Currently, lactic acid is made on a commercial scale by way of fermentation of glucose and sucrose. But that process requires energy-intensive treatment of the fermentation broth, among other processing steps.

So Claus H. Christensen of catalyst manufacturer Haldor Topsøe in Denmark, together with DTU chemist Esben Taarning and coworkers, searched for an alternate route to lactates. The group found that by using a tin-loaded beta zeolite catalyst, they could make the compounds from C₃ sugars selectively and at low temperature. In water, the catalyst isomerizes triose sugars, forming lactic acid in 90% yield at 125 °C. Running the reaction in methanol converts the sugars to methyl lactate quantitatively at just 80 °C (ChemSusChem, DOI: 10.1002/cssc.200900099).

Another strategy for transforming biomass efficiently into chemicals and fuels calls for integrating flow reactors such that the effluent of one reactor is fed into the next. That so-called cascading reactor design, which can be engineered to produce more or less highly branched fuel compounds, fine chemicals intermediates, monomers, and other products, was demonstrated last year by Edward L. Kunkes, Juan-Carlos Serrano-Ruiz, and coworkers in Dumesic's group (Science 2008, 322, 417).

In an ongoing study, the Madison team has shown that sugar and polyol solutions can be converted by a Pt-Re catalyst to hydrophobic alcohols, ketones, carboxylic acids, and heterocyclic compounds. That catalytic step, which strips some 80% of the oxygen from the carbohydrate feed, can then be followed by aldol condensations, self-coupling between ketones, and other types of C–C coupling reactions, to upgrade the product stream selectively to more valuable compounds. For example, in tests of ketone-coupling reactions, the group found that a Pd-Ce-zirconia catalyst can convert 2-hexanone to C₁₂ products with 83% selectivity and 2-butanone to C₈ compounds with 93% selectivity.

While some researchers investigate low-temperature routes to producing chemicals and fuels from biomass derivatives, others continue to study ways of improving the bio-oil produced via higher temperature pyrolysis methods. Converting bio-oil's acids to esters should help alleviate the acidity problem. But that strategy has not worked well in the past for unknown reasons. So Shanks's group at Iowa State conducted systematic studies of reaction conditions. They found that the biggest impediment to esterification of acids in bio-oil is the large concentration of water, which solvates the catalyst and shuts down its activity. They also determined that the high concentration of aldehydes consumes the alcohols that would otherwise be available for esterification.

Meanwhile, at Technical University of Munich, chemistry professor Johannes A. Lercher and coworkers developed a simple catalytic reaction in which phosphoric acid and carbon-supported palladium work in concert to convert phenolic bio-oil components selectively to cycloalkanes and methanol. The two-component catalyst mediates hydrogenation, hydrolysis, and dehydration in a single pot, producing a mixture of alkanes that can be separated easily from the aqueous-phase starting material (Angew. Chem. Int. Ed. 2009, 48, 3987).

As the fast-paced world of biomass research continues to generate reams of data and notebooks full of chemistry ideas, the best methods for turning plant matter into fuels and chemicals have not yet stood up and clearly identified themselves. "It's a chaotic time now in biomass," Dumesic says, noting, however, that "that's a good thing." As happens in new, hot, chaotic areas of research, novel concepts and understanding will begin to emerge in biomass research, and the field will start advancing rapidly. For biomass conversion, that time is just around the corner, he says.