Review

# Valorization of Biomass: Deriving More Value from Waste

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Science  10 Aug 2012:
Vol. 337, Issue 6095, pp. 695-699
DOI: 10.1126/science.1218930

## Abstract

Most of the carbon-based compounds currently manufactured by the chemical industry are derived from petroleum. The rising cost and dwindling supply of oil have been focusing attention on possible routes to making chemicals, fuels, and solvents from biomass instead. In this context, many recent studies have assessed the relative merits of applying different dedicated crops to chemical production. Here, we highlight the opportunities for diverting existing residual biomass—the by-products of present agricultural and food-processing streams—to this end.

Times are rapidly changing. Who could have imagined that in 2012 a commercially viable venture would involve shipping ~200,000 tonnes (t)/year of household waste from Italy to Rotterdam for use as a feedstock for electricity generation in Dutch power plants with overcapacity (1)? Waste is lucrative business or, as they say in northern England: “Where there’s muck there’s brass.” Since the early 1990s, attention has been diverted from waste remediation to waste prevention, with the emphasis on applying the principles of “green chemistry” (prevention is better than cure) (2). Now the focus is moving toward exploiting those wastes that are largely unavoidable.

In its most general sense, the term “waste” covers any organic material apart from the primary material for which the plants were originally grown (e.g., corn stover from maize or lignin from paper pulping). Nearly all wastes currently have some value—for instance, stover for improving the soil in the fields, or lignin as a fuel to power paper mills. Here, we concentrate on ways of getting higher value from the waste, particularly via conversion to chemicals. However, making a commercial case for such a process must necessarily include the cost of replacing the original function of the waste—for example, powering the mills with hydroelectricity. Indeed, one can quantify the value of different “waste valorization” strategies (Table 1).

Table 1

Approximate valorization of biomass waste for different uses* (48, 58).

View this table:

Because the sources of waste are so diverse, it is convenient to consider the chemistry in terms of four source-independent categories: polysaccharides, lignin, triglycerides (from fats and oils), and proteins. As explained later, lignin is challenging to break down into chemically useful fragments. By contrast, pretreatment of polysaccharides, triglycerides, and proteins can lead to their constituent building blocks: monosaccharides, fatty acids plus glycerol, and amino acids, respectively. There are several recent specialized reviews on the conversion of biomass to chemicals (36). However, exploiting waste in a profitable way is a highly multidisciplinary problem; therefore, we outline here recent developments for a wider audience with the emphasis on optimizing the valorization of the various components of residual biomass.

Waste is perhaps a concept even broader than the definition above, because it applies to any biomass-derived by-product for which supply greatly exceeds demand. For example, glycerol can be a valuable chemical, but it is being generated in increasing quantities by the biodiesel industry and could become a “waste.” By applying even a crude valorization analysis, one finds that conversion of glycerol to the chemical epichlorohydrin is economically attractive compared to the alternatives, because the value of this conversion is 3 times that of conversion to transportation fuel and 10 times that of burning to generate electricity—hence Solvay’s recent commissioning of a new 100,000 t/year epichlorohydrin plant based on glycerol in Thailand (7). In the longer term, glycerol could become a platform molecule leading to many different fine chemicals, but the establishment of such platforms will require a much more mature bio-based chemical industry.

Most biomass waste is a complex and variable mixture of molecules, and separation becomes a key issue. An added complication is that some of both the bio-waste and the materials to be separated are solid; therefore, separation frequently involves organic solvents. If bio-based chemical production is to become self-sustaining, those solvents must also be bio-based and cannot, in the long term, be derived from crude oil. In addition, bio-based solvents would be highly useful materials in their own right. If such solvents can also function as fuel additives and platform chemicals, one would have the basis for a genuinely robust technology (8).

Some of the processing of petrochemical hydrocarbon feedstocks involves the introduction of oxygen-containing functional groups by, for example, aerobic oxidation. In biomass, oxygen- and nitrogen-containing functional groups are already present in the feedstock. Therefore, direct conversion of biomass to oxygen- and nitrogen-containing platform chemicals should be more attractive than reductive defunctionalization of the biomass followed by conventional refunctionalization of hydrocarbons. Therefore, it is generally preferable to perform redox-neutral conversions of biomass.

## The Size of the Opportunity

Waste constitutes an enormous potential resource: hundreds of megatonnes (Mt)/year [i.e., >108 t) across the world. Therefore, a bio-based economy must be established on a corresponding scale. The starting point has to be large-volume chemicals—e.g., lubricants, surfactants, monomers for plastics and fibers, and industrial solvents—because they have the potential to make a substantial impact. Merely targeting fine chemicals, although economically attractive and important, would have a negligible impact on sustainability of chemical production because the demand for such chemicals is small. By contrast, platform and bulk chemicals could satisfy a sizable proportion of industrial output.

The largest waste source for carbohydrates and lignin is from lignocellulosic biomass residues, which are estimated to exceed 2 × 1011 t/year worldwide (9). These residues can be separated into two categories: (i) residues left in the field directly after harvest of crops and (ii) residues separated from the product as it is processed. Although the field residues cannot really be described as waste—because soil quality and crop yield are decreased by their removal—the process residues are waste products that are normally burned and could be converted to small molecules.

The two highest-volume process residues are rice husk and sugarcane bagasse. For every 4 t of rice harvested, 1 t of husk is produced, amounting to 120 Mt of rice husk per year. Of this, only 20 Mt is currently used, leaving 100 Mt that could be converted into fuels or chemicals (10). Another high-volume process residue is bagasse, with each tonne of sugarcane yielding 135 kg of sugar and 130 kg of bagasse, resulting in 220 Mt of bagasse per year (11, 12). Most sugar mills burn the bagasse to recover energy but, with improvements to energy efficiency, there could be a large excess available for conversion to platform chemicals (13).

Solid municipal waste is another potential waste stream that could be converted to chemicals, as a large proportion of it is made up of paper and organics (24% and 38% in the United States) (14). Globally, enough waste paper is generated annually to produce 65 Mt of cellulosic-based ethanol (15). Unsold or unused food in developed countries constitutes another biomass waste stream, because liability issues prohibit donations. Solid municipal waste also contains other potential sources of chemicals such as polystyrene and other plastics, but these will not be considered here, as they are ultimately derived from crude oil and are therefore not sustainable feedstocks in the long term.

## Lignin and Aromatics

Many key commercial chemicals are aromatic compounds, ultimately derived from petrochemical feedstocks. Lignin is the only large-volume renewable feedstock that comprises aromatics. It is an amorphous cross-linked polymer that gives structural integrity to plants, making up 25 to 35% of woody biomass (16), and can be viewed as a disordered polymer of phenyl-propanolic units linked by ether and C-C bonds. Thus, depolymerization of lignin is an alluring route to an important functionality class.

Unfortunately, this route is deceptive. Despite extensive research, there are very few reports of efficient ways of recovering such aromatic products. The only notable commercial process has been the production of vanillin from lignosulfonates, a by-product of the sulfite pulping industry. The maximum yield of vanillin from the optimized industrial process is only about 7.5% by mass (17), and the process struggles to compete with the petrochemical routes to vanillin (18). Currently most lignin is used as an energy source in the pulping industry, and there is no obvious route to valorization beyond the energy route, although new catalysts for lignin conversion are being discovered (19, 20) and the situation could change soon. There is clearly an opportunity for new thinking.

Although there is still no convincing route to single aromatic feedstocks from lignin, there have been important developments in the production of aromatics from nonaromatic biomass sources. It is now possible to obtain totally bio-based styrene from butadiene produced from bio-ethanol or bio-butanol (21) or even directly by fermentation (22). Particularly interesting is recent work by the companies Gevo (23), which has been producing para-xylene from bio-isobutanol, itself produced by fermentation of sugars derived from cellulose, and Virent, which has been using a three-step chemocatalytic process for paraxylene production (24, 25). Paraxylene is an important aromatic feedstock that is currently oxidized to terephthalic acid (TA) via a surprisingly green aerobic oxidation (26). TA is then condensed with ethylene glycol to make polyethylene terephthalate (PET), the polyester used for clothing and beverage bottles on a huge scale, ~40 Mt/year.

Now Gevo has teamed up with the Japanese chemical giant Toray to demonstrate the production of totally bio-based PET from bio-based paraxylene and ethylene glycol (27). Most notably, Coca Cola, Pepsico, and Heinz, some of the largest users of PET bottles, have expressed a commitment to switch to bio-based bottles as soon as practicable (28). Such companies have the necessary market share to drive real innovation in bio-based commodities.

## Bulk Chemicals from Carbohydrate Residue

The most abundant prospective feedstocks for sustainable liquid production are carbohydrates, which, together with lignocellulosic biomass, make up most biomass residue. Such feedstocks could provide the basis of biorenewable chemical production without the need to allocate land for specific crops dedicated to this aim, although this conflict is small when compared to the food-versus-fuel problem because the scale of production of chemicals is dwarfed by that of fuels (Fig. 1).

In the first stage of any conversion, the cellulose and hemicellulose must be broken down into their constituent hexose and pentose sugars. The chemistry of this conversion involving hydrolysis with mineral acids has been known for over 100 years and was implemented for production of ethanol from wood during World War II, when cost was not the major constraint. In peacetime, the products were too expensive compared with their low-cost petroleum analogs, and some of the by-products were not marketable (29). Interest in conversion of biomass to sugars was renewed in the mid-1970s by concerns over security of fuel supplies, and now the conversion is high on the scientific agenda. The key barrier is the efficient recycling of the acid. The current field appears to be led by three companies: BlueFire Renewables, Virdia, and Renmatix. The BlueFire and Virdia processes are similar, the main difference being the recovery step of the mineral acid. Renmatix, by contrast, uses a two-step hydrolysis in high-temperature water for the selective recovery of pentoses from hemicellulose and hexoses from cellulose. All three processes claim to be economically competitive with production of sugars from traditional agricultural sources such as sugar cane, For example, the Virdia process has been quoted (30) as producing sugars at 0.25 $/kg compared to 0.45$/kg for cane sugar.

Licella has developed a catalytic hydrothermal technology to convert corn stover, sugar cane trash, grasses, and farm waste to BioCrude, which can be readily converted to biofuels (31). The combination of catalytic hydrolysis, dehydration, and oxidation results in a very stable crude oil substitute with less than 10 weight % (wt %) oxygen and more than 8 wt % hydrogen. Both ethanol and butanol can be produced by fermentation of the sugars obtained from residual biomass with fermentations similar to those used for first-generation bioethanol today. Such biofuels will be needed where energy needs cannot be met by electrical means, as in the case of aviation fuels (e.g., the 77.2 Mt of jet fuel used in the United States in 2008) or remote sites with low population, and biomass-based liquid fuels could be the only sustainable option. By contrast, the volume of gasoline is so enormous that only a small portion of it could be replaced by biofuels even with a third generation of bioethanol. We believe that current agricultural waste streams could easily be used for smaller-scale applications, without requiring discrete energy crops and simultaneously reducing problems with waste disposal. The utilization of biomass for the production of carbon-based consumer products will become a reality at the latest when all of the economic fossil fuel resources become exhausted, so that most of the research money already spent on biofuels will still benefit later generations. For example, the production of ethylene and propylene—two of the most important seven basic industrial chemicals—from bioethanol would require about 30% of the currently farmed land, which perhaps could be dropped to a sustainable level (less than 2%) with second- and third-generation bioethanol technologies. The recent rise of shale gas (32, 33) gives us a welcome reprieve to develop these technologies, but this gas is also a finite resource and not a long-term solution to sustainable chemical production.

Beyond application as a fuel, fuel additive, or solvent, ethanol can be dehydrated to produce the versatile commodity compound ethylene (34), which in turn can be oxidized to ethylene oxide and ethylene glycol or polymerized. For example, the Brazilian company Braskem (35) is already producing renewable polyethylene from ethanol on a commercial scale. A key point is that these are major chemicals in the petrochemical economy and, therefore, production from ethanol can easily be incorporated into existing supply chains. Ethanol does, however, have drawbacks as a transportation fuel; it is hygroscopic and expensive to purify after fermentation, because fractional distillation is limited by the low-boiling azeotropic mixture incorporating 5% water. These problems are focusing interest on butanol as an alternative fuel. The ready availability of butanol would have important implications for chemical production because it would make the conversion of butanol to butenes and other chemicals commercially attractive.

From a chemical perspective, a downside to fermentation is that the alcohols have lost much of the chemical complexity of the original sugars, which then has to be rebuilt to make the target chemicals. By contrast, more of the complexity can be retained after acid-catalyzed dehydration of pentose and hexose sugars, which yields furfural and 5-hydroxymethyl-furfural (5-HMF) (36), respectively (19). Both compounds show promise as platform organic molecules. Furfural can be the starting material for the synthesis of a series of derivatives including furfuryl alcohol, furoic acid, furan, tetrahydrofuran, 2-methyl-tetrahydrofuran, and related resins.

5-HMF is an even more attractive platform (37) (Fig. 2). It can easily be converted into dimethylfuran, which has applications as both solvent and transportation fuel (38). It can also be converted to furan dicarboxylic acid (3941), which has the potential to become a major bulk chemical because it can be copolymerized with ethylene glycol to make a renewable polymer with properties similar to those of the PET polyesters used for textiles and packaging. Thus, 5-HMF combines the two key criteria for valorization of biomass: It retains a reasonable proportion of the original chemical complexity, and it can also be converted to high-tonnage chemicals.

5-HMF can also be converted to gamma-valerolactone (GVL) via levulinic acid (Fig. 2). The beauty of this route is that the coproduct of levulinic acid is formic acid, which can act as a source of the hydrogen needed for the subsequent conversion to GVL (42, 43). GVL is a platform molecule in its own right, which can even be converted to adipic acid, the precursor for Nylon, opening up another “bio-route” to a major chemical.

Fermentation of carbohydrates can also lead to lactic acid, used primarily in the production of poly(lactic acid), a biodegradable plastic. The properties of poly(lactic acid) are sufficiently similar to those of petroleum-derived polymers that it can replace them in a wide range of applications (e.g., packaging, fibers, and foams) (44). The relevance in the context of this review is that this technology has already been commercialized on a scale of >105 t, thereby demonstrating that such an approach can be both sustainable and profitable (45).

## Protein Waste to Platform Chemicals

Substantial amounts of protein-containing waste are generated in the production of foods and beverages. Examples include vinasse (from sugar beet or cane), distiller’s grains with solubles (from wheat or maize), press cakes (from oil seeds like palm and rapeseed), fish silage, protein from coffee and tea production, and agricultural residues from various crops. For example, poultry slaughterhouses produce large quantities of feathers with a crude protein content of more than 75% w/w (46), 65% of which consists of nonessential amino acids. Similarly, the production of shrimp meat generates large amounts of protein waste together with the carbohydrate chitin (47). Much of this protein is currently processed as animal feed but, applying Table 1, it would have more value as a feedstock for commodity organic compound production. This is an area that has been discussed rather less than carbohydrates and lignin. In the ideal scenario, the essential amino acids contained in this protein waste could be used as animal feed and the nonessential amino acids, with no real value for food or feed, as chemical feedstocks. This would again circumvent the fuel-versus-food issue.

Another, potentially enormous source of protein waste that could be exploited in the future will be the by-products from the production of biofuels. It is generally agreed that, in the long term, this will largely involve second-generation biofuels from lignocellulosic biomass and nonedible triglycerides. It has been estimated (48) that if 10% of transportation fuels, currently derived from fossil feedstocks, were substituted by biomass-derived fuels, this could generate 100 Mt/year of protein worldwide. This amount is of the same order of magnitude as the protein requirement of the world population. In an integrated biorefinery, these proteins could form a source of bulk chemicals via a three-stage process: (i) isolation and hydrolysis of the proteins to mixtures of amino acids, (ii) separation of the individual amino acids, and (iii) conversion of the amino acids into bulk organic compounds, which will generally comprise large-volume industrial monomers.

A promising technology for the isolation of proteins from biomass residues involves so-called ammonia fiber expansion (AFEX) pretreatment whereby cellulose is separated from proteins that are solubilized. The protein fraction can then be hydrolyzed. Traditionally, this involved prolonged treatment with concentrated mineral acids at elevated temperatures, leading eventually to the formation of copious amounts of inorganic salts as waste. A milder and more environmentally attractive alternative involves the use of proteases, such as the alkaline protease alcalase, which is widely used in laundry detergents. It has been used, for example, in the hydrolysis of proteins in poultry (46) and shrimp waste (47). A possible drawback of this method is the relatively high cost of the enzymes. Immobilization of the protease could provide greater economic viability by enabling recovery and reuse of the enzyme. Furthermore, immobilization would suppress degradation of the protease by autolysis (49).

In the second stage, individual amino acids must be isolated from the protein hydrolysate. Standard methods generally involve fractionation on the basis of physicochemical characteristics such as molecular size, charge, and hydrophobicity. Here, there is a clear need for innovation. Certain amino acids could perhaps be selectively converted—by enzyme-catalyzed decarboxylation, for example—to products that can be more easily separated. In one approach, protamylase, an amino acid–rich waste from potato starch processing, was used as a feedstock for the pilot-scale coproduction of ethanol and cyanophycin (CGP: a nitrogen storage polymer produced in vivo by cyanobacteria) by yeast fermentation (Fig. 3) (50). CGP consists of a poly(l-aspartic acid) backbone with equimolar amounts of l-arginine side chains and, because it is insoluble under physiological conditions, it can be easily isolated from the fermentation broth.

In the third stage, individual amino acids can be converted into commodity chemicals. To be competitive with petrochemical-based routes, these conversions should preferably involve as few functional group changes as possible and be close to redox neutral. Obvious candidates would be, for example, a variety of nitrogen-containing compounds that retain the amino group of the amino acid (51). The nonessential amino acid, l-arginine, produced by hydrolysis of cyanophycin can be converted in two enzymatic steps—hydrolysis and decarboxylation—to 1,4-diaminobutane (Fig. 3) (52), the starting material for Nylon-4,6.

There is a great need here for out-of-the-box thinking. Chemists are accustomed to thinking of proteinogenic amino acids as largely chiral molecules that are synthesized from simple starting materials. Now they have to start thinking about how to convert these same amino acids back to relatively simple commodity chemicals in an economically viable manner. An illustrative example is provided by l-phenylalanine, which can be made by enzymatic addition of ammonia to cinnamic acid. However, the fermentative production of l-phenyalanine from biomass has become so efficient that the most economical way to produce cinnamic acid is probably from l-phenylalanine by the reverse reaction. One can also imagine that, at a certain feedstock price and with efficient technology, integration of the deamination with subsequent decarboxylation could afford a bio-based process for styrene (Fig. 4).

Glutamic acid, the most abundant, nonessential amino acid derived from the hydrolysis of many plant and animal proteins, can be converted to a variety of commodity chemicals via initial decarboxylation catalyzed by glutamic acid α-decarboxylase (53). However, not all the routes are sustainable. A life-cycle assessment study (54) compared four bio-based commodity chemicals derived from glutamic acid with their petrochemical equivalents (Fig. 5); N-methyl- and N-vinyl-pyrrolidone had less environmental impact, whereas acrylonitrile and succinonitrile (not illustrated) had more impact than the petrochemical route. The unfavorable enviro-economics of the latter two bio-based products were directly related to the inferior chemistry involved, inter alia, oxidation with hypochlorite.

l-Lysine is another interesting platform chemical that can be converted to a number of industrial monomers, such as 1,5-diaminopentane (55), caprolactam (56), and 5-amino valeric acid (57) (Fig. 5). Although it is an essential amino acid, in the future there may be sufficient overproduction from waste protein to make it an interesting feedstock for organic compound manufacture.

## Outlook

These examples are just a few of the possibilities for valorization of the huge amounts of animal and plant residue that we produce each year. The pace of research in this area is accelerating, and chemical manufacturers are showing increased interest in renewable feedstocks. Here, we have focused on agricultural and food waste but, with rapid urbanization across the world, municipal waste is likely to become an increasingly important source of waste biomass. This is particularly likely in economically developing countries where the demand for chemical products is growing and the waste has a higher organic content than in more developed economies.

We have not considered the question of water availability, which may be a limiting factor in the processing of biomass. Fortunately, most of the world’s population lives near the coast, and promising results are beginning to emerge in the use of seawater and brine for the processing of biomass. This may indeed be the way forward. However, genuine progress will require a close partnership between chemists and engineers so that new ideas in chemistry can be quickly transformed to meet the needs of our planet’s expanding population.

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