The Biological Route to Forest-Based Bioproducts

There are 4 to 5 steps in a bioconversion process of lignocellulose to bio-based chemicals including bioethanol. They include (i) pretreatment, (ii) enzyme production, (iii) enzymatic saccharification, (iv) fermentation, and (v) further refining into desired chemicals.

Cellulose in wood is largely inaccessible to microbial and enzymatic attack protected by the presence of a lignin barrier, and therefore wood needs some form of pretreatment to become susceptible to biodegradation. Pulping is a process in which wood is delignified using chemical processes such as Kraft and Sulfite procedures to produce cellulose pulp for paper manufacture. For bioconversion purposes, a cost-effective and efficient pretreatment procedure that makes cellulose amenable to enzymatic hydrolysis is steam-explosion, and much of this technology was developed in Canada during the past 35 years.

Steam-explosion besides pretreating lignocellulosic residues like forest biomass also fractionates its components; the hemicelluloses are partially degraded in water-soluble form and appear in the liquor fraction, cellulose and lignin appear in the pulp. Lignin can easily be extracted from the pulp under mild conditions to produce a functionally-reactive lignin that can serve as phenol substitutes in chemicals and polymers manufacture. The presence of lignin in the pulp, however, does not impede or hinder enzymatic degradation of the cellulose component, and so it is feasible to extract the lignin from the hydrolysis-spent residue following the enzymatic saccharification step. Thus three product streams arise after the steam-explosion step: partially-degraded hemicelluloses, cellulose and lignin, as well as a spent residue fraction that can best be incinerated to produce process steam.

Steam-explosion is a physicochemical process in which wood is subjected to high temperatures (200-250 ºC) for short durations (0.5-5 mins) under high pressure (steam injected heating) in a special reactor vessel. At the end of the heating cycle, the contents of the reactor are released suddenly creating an explosive effect that is effective in defibrillating the fibres comprising the chemical components in the cell wall layer of plant material. The fraction constituting the pulp then serves as a source of fermentable sugars through the action of enzymes (cellulases) hydrolyzing the cellulose component without the need to remove the lignin.

The steam-exploded pulp fraction can be hydrolyzed (process referred to as saccharification) by a employing mixtures of enzymes that constitute the cellulase complex (exo- and endo- β-1,4-glucanases (also called cellulases) and cellobiases, or β-1,4-glucosidases) of suitable microorganisms such as the bacterium Clostridium thermocellulum, the fungus Trichoderma reesei, or from cloned microorganisms (bacteria, yeasts or fungi) in which the gene(s) constituting the cellulase complex has been genetically engineered. Under optimized conditions this can lead to ~80 % conversion of cellulose-to-glucose within a 12-24 h period. This is then followed by recovery of the liquid fraction containing the fermentable sugars, and its separation from the hydrolysis-spent residue. Glucose then serves as a chemical feedstock for the production of bio-chemicals (e.g., bioethanol through yeast fermentation). The spent residue following hydrolysis can be extracted under mild alkaline conditions to remove the lignin component. The spent extracted and hydrolyzed residue fraction remaining is burnt to generate process steam at the factory.

The final stage in bio-based chemicals from cellulose involves microbial fermentation of glucose. Glucose therefore serves as the C-6 starting feedstock from which many different fermentation products can be produced. Presently, the chief chemical of interest from cellulose is bioethanol as a biofuel, and this is produced by fermentation employing yeasts such as Saccharomyces cerevisiae. The production of bioethanol by fermentation is a well-established industrial process. Glucose can, however, serve as the feedstock for other bio/chemicals, some of higher value. Examples include organic acids such as acetic, citric and lactic acids; essential amino acids (lysine) by Corynobacteria spp.; antibiotics and antifungal compounds; and enzymes. The global market for enzymes continues to increase yearly, which in 2008 was ~$2.3 billion. Most enzymes find technical applications as ingredients in laundry detergents, in animal feeds as a digestive aid, and as a processing agent in the manufacture of foods and beverages.

There is therefore a market for enzymes in bioconversion processes, chiefly cellulases, xylanases and laccases. Hemicelluloses arising from steam-explosion pretreatment appear in the liquor fraction and can be further treated by either (a) acid or enzymatic hydrolysis to the pentose sugars, xylose and arabinose, and these fractions serve as chemical feedstocks for the production of bio/chemicals (xylitol, arabinitol or bioethanol by fermentation), or (b) the partially-degraded and solubilised hemicellulose fraction can be used per-se as a source of oligosaccharides for food applications such as prebiotics.

Finally, a further step can be added to complement the bioconversion of cellulose into chemicals, and this is the further refining of bioethanol by chemical reactions. In this case, ethanol now serves as the starting material from which other chemical products can be produced including “green” chemicals such as plastics (polyethylene and polypropylene).

Propylene is the world’s second largest petrochemical commodity. It is used commercially for the production of polypropylene, as well as for other chemicals (includes acrylonitrile, acrylic acid, acrolein, propylene oxide and glycols). The growth in propylene production is driven primarily by the manufacturing industry demand for polypropylene used in a variety of products: including automotive parts and as construction materials. The current practice of making propylene is from ethylene and 2-butene (meta-synthesis reaction based upon Lewis acid/base chemistry), but can also be made by catalytic dehydrogenation of propane (OleflexTM Process). Both feedstocks are presently derived from petrochemicals. Polymerisation of propylene results in polypropylene, and this is performed industrially using metallocene/methylaluminoxane-based catalyst technology. The technology of propylene production is well-developed based on petrochemical catalytic conversion processes. It is of interest at the BRI to explore and develop technologies in which propylene (and ethylene) can be produced from forest biomass (cellulose to ethanol conversion steps), thereby not displacing a food crop for chemicals production.

Thus cellulose from forest biomass is an excellent renewable resource base for producing chemicals. The cellulose conversion technology has been well-researched during the last 35 years, and although its interest as a chemical feedstock has waxed-and-waned over the past 20 years, the high petroleum prices reached in 2008 ($US 140/barrel) prompts the re-investigation of lignocellulosic materials to produce chemicals. The biological conversion route therefore can add value to the raw forest materials; the basis of a biorefining process.