Homogeneous Catalysis in Biomass Conversion

Over 40 million tonnes of biomass are produced annually worldwide – a large percentage of which is burned or discarded. Plant biomass consists of inedible carbon-based byproducts of plant material initially used for agriculture, food, or other purposes: lignin, for example, is a primary source of organic waste from paper mills. One goal of biomass conversion is to transform this nonfood plant matter into higher value chemicals, ultimately in the form of liquid “second-generation” biofuels, as a means of valorization or recycling of organic waste.

Most inedible plant material consists of cellulose, hemicellulose, and lignin – collectively named lignocellulosic biomass. The process of converting lignocellulosic biomass into second-generation biofuels is challenging in no small part due to the diversity of biomass waste.

 

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Lignocellulosic biomass consists of polymers of lignin, hemicellulose, and cellulose. Image from Nature Education.

Cellulose, hemicellulose, and lignin are polymers. The simplest, cellulose, consists of a linear chain of thousands of monomers (i.e., single units of polymers) of the sugar β-D-glucose. Hemicellulose is a heteropolymer comprised of shorter branched chains of a variety of sugar monomers, including galactose, xylose, mannose, and arabinose. Complex, randomly cross-linked chains of phenolic compounds (molecules that consist of an aromatic phenyl group bound to a hydroxyl group) make up the polymer lignin, giving lignin its rigidity and strength.

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Cellulose, hemicellulose, and lignin are themselves polymers that can be broken down into small molecules through chemical conversion. Image from Catalysis Science & Technology.

Converting lignocellulosic polymers to second-generation biofuels requires the breakdown of these polymers into small molecules, in order to access their constituent chemical building blocks. Two realms of research are biochemical conversion and thermochemical conversion. Biochemical conversion can utilize microorganisms to ferment biomass or enzymes such as hemicellulase and cellulase to hydrolyze carbohydrates, while thermochemical and chemical pathways use heat, pressure, or specially designed artificial catalysts – substances that lower the activation barrier of a chemical reaction – to facilitate depolymerization and defunctionalization. The resulting small molecules can then be converted to biofuels or functionalized to be used as other higher value chemicals.

 

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“Top-down” production/deconstruction of biomass to produce second-generation biofuels and higher value chemicals versus “bottom-up” functionalization of fossil fuels. Image from Catalysis Science & Technology.

While research in heterogeneous catalysis has made significant strides in the production of biofuels, homogeneous catalysis – in which the catalyst is in the same phase as the reactants – has its own benefits. Due to their solubility, homogeneous catalysts can efficiently target specific chemical bonds for depolymerization; this minimizes the loss of product to competing pathways. Homogeneous catalysis can also be undertaken at lower temperatures and in milder conditions than many heterogeneous catalytic processes during defunctionalization or depolymerization. Finally, high bond selectivity is a characteristic of homogeneous catalysts.

The advantages of homogeneous catalysis are exemplified in the depolymerization of lignin. As lignin is composed of aromatic small molecules, it is a highly promising fossil fuel alternative. However, selective depolymerization of lignin is particularly challenging due to the structural diversity and bulkiness of the lignin polymer. In addition to being amorphous, lignin is comprised of a variety of C-O and C-C cross-linking bonds. Lignin also consists of different ratios of “monomer” phenolic compounds, depending on the plant source, which results in different ratios of types of C-C or C-O bonds. Studies have indicated that β-O-4 bonds in lignin are not only the most common but are also relatively weak, providing a reasonable target for bond cleavage. Homogeneous catalysts, in the form of transition-metal complexes, are well-suited to the depolymerization of lignin.

 

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Lignin depolymerization is complicated by the variety of C-C and C-O bonds that can be cleaved to produce small molecules. Image from Catalysis Science & Technology.

Toste and coworkers have used a vanadium catalyst to effectively cleave β-O-4 bonds in not only a lignin model compound but also in real lignin samples that had undergone organosolv pulping, forming aromatic molecules including vanillic acid, 4-hydroxybenzaldehyde, and syringic acid.

 

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Vanadium catalysis to cleave β-O-4 bonds in a lignin model compound. Image from Catalysis Science & Technology.

Other researchers such as Corvini and coworkers have sought to mimic Nature’s solution to lignin breakdown using oxidative homogeneous transition-metal catalysts. White-rot fungi break down lignin and cellulose, resulting in rotten wood that appears white and soft; manganese-based enzymes in the fungi achieve this through oxidative mechanisms.

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Nature’s solution: White-rot fungi breaks down lignin and cellulose through oxidative mechanisms initiated by manganese-based enzymes. Image from University of Wisconsin-Madison’s Department of Botany.

Valorization of biomass is a promising field of research that can potentially reduce CO2 production and develop cost-effective sources of renewable energy. Homogeneous catalysis has the potential to give greater access to lignocellulosic biomass as a chemical feedstock for second-generation biofuels from biomass conversion.

 

References:

Deuss, P.J.; Barta, K.; de Vries, J.G. Catal. Sci. Technol. 2014, 4, 1174.

Potters, G.; Van Goethem, D.; Schutte, F. Nature Education. 2010, 3(9), 14.