erythritol.
Succinic Acid
Succinic acid is one of the 12 high-value bio-based chemicals investigated by Werpy and Peterson as a compound that has the potential to improve the profitability and productivity of biorefineries. Conventionally, succinic acid is produced from maleic acid using Pd/C heterogeneous metal catalysts. Other methods reported for succinic acid production are oxidation of 1,4-butanediol with nitric acid; the carbonylation of ethylene glycol, acetylene, and dioxane; hydrogenation of fumaric acid in the presence of Ru catalyst; and the condensation of acetonitrile to produce butanedinitrile, which can be subsequently hydrolyzed to succinic acid.
Lactic Acid
Lactic acid (2-hydroxypropanoic acid) is an important chemical. It is an alternative for producing alkyl lactates, propylene glycol, propylene oxide, acrylic acid, and poly (lactic acid). Lactic acid has applications in food, pharmaceuticals, and cosmetics. In particular, the biopolymers from lactic acid have created a strong interest. Conventionally, lactic acid is produced via fermentation from carbohydrates.
Lactic acid can be made from different reagents such as lignocellulosic materials, cellulose, carbohydrates, sugars, trioses, glycolaldehyde, and glycerol. The production of lactic acid involves complex reactions of several types of transformations such as aldol condensation, retro-aldol condensation, dehydration, and 1,2-hydride shifts.
Several homogeneous and heterogeneous catalysts have been reported for the production of lactic acid from biomass. The catalysts such as alkali metal ions, tin chloride, tin dioxide (SnO2), acidic resins, zeolites, metal-modified zeolites, mesoporous materials, tungstated alumina, mixed-oxides, and carbon-silica hybrid materials have been reported in the literature. The use of Rh/C, Ru/C, Ir/C, Ir/CaCO3, and Pt/C catalysts for the transformation of glycerol to lactic acid was discussed. Typically, the highest yield of lactic acid has been achieved in the presence of an inert gas and alkaline medium (CaCO3). An alkaline platinum-calcium carbonate (Pt/CaCO3) catalyst has been shown to be an efficient catalyst for glycerol transformation to lactic acid, with 54% selectivity for lactic acid at 45% conversion in the presence of borate esters at 200°C (390°F). The Rh/Al2O3 catalyst also gives high selectivity for lactic acid, i.e., 69% in the presence of borate derivatives.
Chemicals such as pyruvic acid, acrylic acid, 2,3-pentanedione, polylactic acid (PLA), lactic acid esters, and 1,2-propanediol (1,2-PDO) can be synthesized from lactic acid. The polylactic acid can be synthesized by two ways: direct polycondensation of LA and ring-opening polymerization of the lactide monomer (cyclic). The direct poly-condensation of lactic acid is a difficult process because of the strong equilibrium between polylactic acid, water, and lactide that limits the synthesis of high molecular weight products. The most commonly used process is using the lactide intermediate. The lactide intermediate was polymerized via a homogeneously catalyzed 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