James G. Speight - Encyclopedia of Renewable Energy

In the manometric method, the sample is kept in a sealed container fitted with a pressure sensor. A substance that absorbs carbon dioxide (typically lithium hydroxide) is added in the container above the sample level. The sample is stored in conditions identical to the dilution method. Oxygen is consumed, and dioxide is released. The total amount of gas, and thus the pressure, decreases because carbon dioxide is absorbed. From the drop of pressure, the sensor electronics computes and displays the consumed quantity of oxygen.

Biochemicals, as opposed to petrochemicals, are in the context of this encyclopedia, chemicals produced from biomass.

The production of chemicals from biomass, a renewable feedstock, is highly desirable in replacing petrochemicals to make biorefineries more economical. The best approach to compete with fossil-based refineries is the upgradation of biomass in integrated biorefineries. The integrated biorefineries employed various biomass feedstocks and conversion technologies to produce biofuels and bio-based chemicals. Bio-based chemicals can help to replace a large fraction of industrial chemicals and materials from fossil resources. Biomass-derived chemicals, such as 5-hydroxymethylfurfural (5-HMF), levulinic acid, furfurals, sugar alcohols, lactic acid, succinic acid, and phenols, are considered platform chemicals. These platform chemicals can be further used for the production of a variety of important chemicals on an industrial scale. However, current industrial production relies on relatively old and inefficient strategies and low production yields, which have decreased their competitiveness with fossil-based alternatives.

Biomass feedstocks, such as agricultural residues and wood chips, constitute an inexpensive renewable resource for commercial large-scale biorefineries, as these waste products are widely available and can sequester carbon. The target chemicals include alcohol derivatives, organic acid derivatives such as formic acid and levulinic acid, and furan derivatives such as 5-hydroxymethylfurfural (5-HMF) and furfural derivatives. These chemicals can further be converted to a range of derivatives that have potential applications in biofuels, polymers, and solvent industries. Due to these differences in their chemical composition and structure, cellulose, hemicellulose, and lignin have different chemical reactivities. In addition to the complex nature of bio-resources, the inert chemical structure and compositional ratio of carbon, hydrogen, and oxygen in molecules in biomass present difficulties in the chemo-catalytic conversion of biomass to fuels and chemicals. Therefore, besides using the natural lignocellulosic biomass as a reactant, researchers often use model compounds for conversion process studies. In addition, the development of highly active and selective catalysts for the chemo-selective catalytic conversion of lignocellulosic biomass to desired products remains a daunting challenge.

The development of processes and technologies to convert lignocellulosic biomass to fuels and value-added chemicals remains a significant challenge. In this context, the major difficulty in producing a high yield of target chemicals and fuels is the complex chemical composition of lignocellulosic biomass feedstocks. Structurally, cellulose contains anhydrous glucose units and hemicellulose consists of different C5 sugar monomers. On the other hand, lignin is a complex, three-dimensional, and cross-linked biopolymer having phenylpropane units with relatively hydrophobic and aromatic properties. Due to these differences in their chemical composition and structure, cellulose, hemicellulose, and lignin have different chemical reactivities. In addition to the complex nature of bio-resources, the inert chemical structure and compositional ratio of carbon, hydrogen, and oxygen in molecules in biomass present difficulties in the chemo-catalytic conversion of biomass to fuels and chemicals.

A variety of methods can be employed to obtain different product portfolios of bulk chemicals, fuels, and materials. Biotechnology-based conversion processes can be used to ferment the biomass carbohydrate content into sugars that can then be further processed. For instance, the fermentation path to lactic acid shows promise as a route to bio-degradable plastics and has been demonstrated commercially. An alternative is to employ thermochemical conversion processes which use pyrolysis or gasification of biomass to produce a hydrogen-rich synthesis gas. This synthesis gas can then be used in a wide range of chemical processes.

While the concept of exploiting the wide range of chemicals from plants may appear novel, the published literature shows that large numbers of metabolites have already been identified and characterized from a wide variety of plant species. For example, over 37,000 different potential and unexploited materials can be identified. These have a wide range of chemical, physical, and biological properties and include phenolics, nitrogen containing compounds, and terpenes (terpenoids). The variety of molecular compounds is vast. For example, in the terpene group, there are six sub-groupings of molecules with a large number of applications including use in anti-cancer drugs.

Extraction procedures can have a major impact on the availability of these chemicals, and, to ensure optimal exploitation, some of the well-established extraction procedures may need to be revised. For example, in winter rapeseed, the harvested seed is crushed and rapeseed oil extracted mechanically. The residual meal is then treated with hexane to extract the remaining oil, before being used as feed, primarily for ruminants. Rapeseed oil components have numerous applications including use in bio-diesel, and specialty chemicals.

However, innovative oil-extraction procedures could allow greater exploitation of protein-based metabolites in the rapeseed, which can comprise 25% or more of the rapeseed mass. Research from studies, such as the EC-funded Enhance project, has demonstrated that this separation would allow products to be produced for numerous applications (see diagram) with base cellulose material and some other metabolites remaining in the residual meal.

See also: Biofuels, Petrochemicals.

Basic knowledge of the mechanisms of common reactions such as dehydration, hydrogenation, and hydrodeoxygenation involved in biomass upgradation processes is discussed in the following section.

Dehydration

Dehydration is a reaction in which a water molecule is removed from the substrate, mainly alcohol, forming an alkene or other unsaturated product depending on the substrate. The reaction is commonly catalyzed by Lewis or Brønsted acids, as the hydroxyl group is a poor leaving group. Dehydration in the presence of a Brønsted acid catalyst occurs by first protonating the hydroxyl group, as the protonated hydroxyl group (R-H 2O+) is a better leaving group than the hydroxyl group. As a result, the catalyst is eliminated as water. Simultaneously, a carbon-carbon double bond (C=C) is formed in the carbon skeleton of the substrate, as per Zaitsev’s rule.

In Lewis acid-catalyzed reactions, however, the reaction proceeds through the bonding of Lewis acid to the lone electron pair of hydrogen-oxygen. The electrophile nature of the Lewis acid lowers the electron density in the alcohol carbon-oxygen bond, which results in the cleavage of the alcohol carbon-oxygen (C-O) bond and the formation of alkene and Lewis acid hydroxide specie. The Lewis acid hydroxide reacts with the released β-proton, forming water and the original catalyst species.

Due to the abundances of hydroxyl groups in a wide variety of natural resources, dehydration reactions are the most common and important ways to valorize biomass. As a result, different dehydration products can be obtained from biomass, and are used as high-value chemicals.