ring-opening polymerization (absence of water), which produces two lactoyl units in the growing chain. This process suffers from racemization. Consequently, when commercial L-lactic acid was used, a 5% to 12% yield of undesired product (meso-lactide) was produced. The properties of polylactic acid depend mainly on the stereo-composition of the feed used. Therefore, stereo-pure L,L-lactide is used to obtain stereo-pure poly-L-lactic acid. As a result, improvements in catalysts, process parameters, and configuration have been reported.
Catalytic dehydration of lactic acid leads to acrylic acid, while propanoic and pyruvic acids were obtained by lactic acid reduction and dehydrogenation, respectively. 2,3-pentanedione can be produced by condensation and acetaldehyde by either decarbonylation or decarboxylation. The lactic acid conversion to acetaldehyde was investigated on silica-supported heteropolyacid with an 83% yield.
Hydrogenation of lactic acid to 1,2-propanediol was performed in both the liquid as well as the vapor phase using Ru/C and Cu/SiO2 catalysts, respectively. The direct hydrogenolysis of lactic acid seems an attractive option, but deactivation of catalyst makes the process undesirable. The catalyst deactivation occurs as a result of the polymerization of lactic acid and formation of the side-products, propionic acid. Therefore, to avoid this problem, carboxylic acids are usually converted into more readily reducible esters.
In summary, the catalytic conversions of sugars to commodity chemicals are widely discussed, but the industrial applications are limited. Therefore, further research for the improvements of the catalytic conversion and selectivity are still required for achieving the goal of integrated biorefineries. The areas that need attention are the search for novel reaction media to use efficient catalysts for the biomass conversion processes and the extraction/purification steps to isolate the chemicals with high yield and purity.
See also: Biochemicals, Carbohydrates, Coniferyl Alcohol, p-Coumaryl Alcohol, Lignin, Sinapyl Alcohol, Sugars and Starch.
Bioconversion
Bioconversion is the use of biological agents to carry out a structured deconstruction of lignocellulose components. This platform combines process elements of pretreatment with enzymatic hydrolysis to release carbohydrates and lignin from wood.
The first step is a pretreatment stage which is based on existing pulping processes; however, traditional pulping parameters are defined by resulting paper properties and desired yields, while optimum bioconversion pretreatment is defined by the accessibility of the resulting pulp to enzymatic hydrolysis. This function of this step is to optimize the biomass feedstock for further processing and is designed to expose cellulose and hemicellulose for subsequent enzymatic hydrolysis, increasing the surface area of the substrate for enzymatic action to take place. The lignin is either softened or removed, and individual cellulosic fibers are released creating pulp.
In order to improve the ability of the pretreatment stage to optimize biomass for enzymatic hydrolysis, a number of non-traditional pulping techniques have been suggested and include (i) water-based systems, such as steam-explosion pulping, (ii) acid treatment using concentrated or dilute sulfuric acid, (iii) alkali treatment using recirculated ammonia, and (iv) organic solvent pulping systems using acetic acid or ethanol. As with traditional pulping, pretreatment tends to work best with a homogenous batch of wood chips, but the pretreatment option may have to be selected according to the type of lignocellulosic feedstock.
Once pretreated, the cellulose and hemicellulose components of wood can be hydrolyzed (in this option) using enzymes to facilitate bioconversion of the wood. Enzymatic hydrolysis of lignocellulose materials uses cellulase enzymes to break down the cellulosic microfibril structure into the various carbohydrate components.
The enzymatic hydrolysis step may be completely separate from the other stages of the bioconversion process, or it may be combined with the fermentation of carbohydrate intermediates to end-products. Separate hydrolysis and fermentation (SHF) stages may offer this option more flexibility insofar as process adaptation to feedstock type and product slate is available. Simultaneous saccharification and fermentation (SSF) have been found to be highly effective in the production of specific end products, such as bioethanol.
The benefit of bioconversion is that it provides a range of intermediate products, including glucose, galactose, mannose, xylose, and arabinose, which can be relatively easily processed into value-added bioproducts. The process also generates a quantity of lignin or lignin components; depending upon the pretreatment, lignin components may be found in the hydrolysate after enzymatic hydrolysis, or in the wash from the pretreatment stage. The chemical characteristics of the lignin are therefore heavily influenced by the type of pretreatment that is employed. Finally, a relatively small amount of extractives may be retrieved from the process. These extractives are highly variable depending upon the feedstock employed, but may include resins, terpenes, or fatty acids.
Once hydrolyzed, six-carbon sugars can be fermented to ethanol using yeast-based processes. Five-carbon sugars, however, are more difficult to ferment and lack the efficiency of six-carbon sugar conversion. Bacterial fermentation under aerobic and anaerobic conditions is also an option to expand the variety of other products.
A large number of options on the various aspects of bioconversion are available. The environmental performance of bioethanol, including air quality (NOx, PM, SOx, etc.) is also well documented as are the mass-balance and energy-balance of the bioconversion process and economic analyses.
See also: Biochemical Conversion.
Bioconversion Platform
The bioconversion platform is an industrial option (as might be used in a biorefinery) for producing fuels from biomass using biochemical reactions and/or biochemical agents. For example, fermentation or anaerobic digestion to produce fuels and chemicals from organic sources is a bioconversion platform. The bioconversion platform therefore has the ability to serve as the basis for wood-based biorefining operations, generating value-added bioproducts as well as fuel and energy for the forest sector.
The bioconversion platform typically uses a combination of physical or chemical pretreatment and enzymatic hydrolysis to convert lignocellulose into its component monomers. Once liberated, the carbohydrate components of wood may be processed into a number of chemical and fuel products.
Bioconversion technology is leading the way to new chemical products from the lignocellulose-based biorefinery, including bioethanol, lactic acid and polylactide, propanediol, and succinic acid. Other chemical products can be used to create consumer products such as bioplastics, or as platform chemicals in a number of industrial applications. The development of better ways to separate lignin from the lignocelluloses matrix during bioconversion has created the possibility of developing value-added lignin-based products as well.
The bioconversion platform uses biological agents to carry out a structured deconstruction of lignocellulose components and combines process elements of pretreatment with enzymatic hydrolysis to release carbohydrates and lignin from the wood. The first step is a pretreatment stage which must optimize the biomass feedstock for further processing. In the bioconversion platform, this step must be designed expose cellulose and hemicellulose for subsequent enzymatic hydrolysis, increasing the surface area of the substrate for enzymatic action to take place.
Bioconversion proceeds at lower temperatures and lower reaction rates and can offer high selectivity for products. Bioethanol production is a biochemical conversion technology used to produce energy from biomass. For ethanol production, biochemical conversion researchers have focused on a process model of dilute acid hydrolysis of hemicelluloses followed by enzymatic hydrolysis of cellulose. Biodiesel production is a biochemical conversion technology used to produce energy
