Future of Glycerol : New Usages for a Versatile Raw Materialby Mario Pagliaro
By-products of global biodiesel manufacturing are a global fact and the immense amount of glycerol by-product stacking unsold until mid 2005 gave a visual image of the huge loss of energy and material resources. This was due to the lack of suitable conversion processes for this, the oldest organic molecule known to man, despite various experiments by some biodiesel
By-products of global biodiesel manufacturing are a global fact and the immense amount of glycerol by-product stacking unsold until mid 2005 gave a visual image of the huge loss of energy and material resources. This was due to the lack of suitable conversion processes for this, the oldest organic molecule known to man, despite various experiments by some biodiesel producers. The large surplus of glycerol by-product which entered the chemical market has caused closure of existing glycerol plants and the discovery of processes that use glycerol as a raw material for the production of value-added chemicals and even of energy. This was followed by 3-4 years of intense research activity worldwide, where human chemical ingenuity opened up a number of practical avenues to convert glycerol into value added products of mass consumption. For instance, the batteries of your laptop and iPod, as well as your car's antifreeze will soon be based on glycerol, the same sweet viscous substance currently present in soaps. Reporting and commenting on such achievements this book aims to inform chemistry professionals, including managers and technologists, on the large potential of glycerol as versatile biofeedstock for the production of a variety of chemicals, polymers and fuels. Whilst filling a gap in the current literature, this nicely illustrated book is written in a clear, concise style and presents the numerous uses of glycerol as a new raw material which are starting to have an impact on industry worldwide. Elucidation of the principles governing the new chemistry of glycerol goes along with updated industrial information that is generally difficult to retrieve. Through its 10 chapters, the monograph tells the story of a chemical success that of converting glycerol into value added products and highlight the principles that made it possible. Whether as solvent, antifreeze, detergent, monomer for textiles or drug, new catalytic conversions of glycerol have been discovered that are finding application for the synthesis of products whose use range from everyday life to the fine chemical industry. Readers are also shown how a number of practical limitations posed by glycerol chemistry, such as the low selectivity encountered employing traditional stoichiometric and older catalytic conversions, were actually solved based on the understanding of the fundamental chemistry of glycerol and by application of catalysis science and technology. Readers also find a thorough discussion on the sustainability issues of bioglycerol production covering societal, environmental and economic dimensions to reflect the needs of politicians and citizens of today who require cross border research. By explaining the advantages and problems as well as offering solutions the book aids understanding as to whether biodiesel and glycerol refineries are convenient and economically sound. Chemical research on glycerol has shown that given a strong economic input, chemists are able to rapidly devise a whole set of new upgrading processes for the biorefinery and that the latter integrated unity for production energy and chemicals is not just dream of environmentally-minded scientists but an inevitable reality of today. Due to the ever decreasing energy return on energy invested, global society is being forced to switch from fossil to renewable fuels until cheap and abundant energy of solar origin becomes a reality. In this evolution, biofuels, particularly biodiesel, will certainly play a role and therefore, glycerol will be a key raw material for the biorefinery for many years to come. The book's users include industry's top managers and management consultants and also R&D and marketing managers. Along with technical content of a high quality, this is also a strategic book for top managers of the chemical, biofuel, oleochemical and detergent industries.
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The Future of Glycerol
New Uses of a Versatile Raw Material
By Mario Pagliaro, Michele Rossi
The Royal Society of ChemistryCopyright © 2008 Mario Pagliaro and Michele Rossi
All rights reserved.
Glycerol: Properties and Production
1.1 Properties of Glycerol
Glycerol (1,2,3-propanetriol, Figure 1.1) is a colorless, odorless, viscous liquid with a sweet taste, derived from both natural and petrochemical feedstocks. The name glycerol is derived from the Greek word for "sweet," glykys, and the terms glycerin, glycerine, and glycerol tend to be used interchangeably in the literature. On the other hand, the expressions glycerin or glycerine generally refer to a commercial solution of glycerol in water of which the principal component is glycerol. Crude glycerol is 70–80% pure and is often concentrated and purified prior to commercial sale to 95.5–99% purity.
Glycerol is one of the most versatile and valuable chemical substances known to man. In the modern era, it was identified in 1779, by Swedish chemist Carl W Scheele, who discovered a new transparent, syrupy liquid by heating olive oil with litharge (PbO, used in lead glazes on ceramics). It is completely soluble in water and alcohols, is slightly soluble in many common solvents such as ether and dioxane, but is insoluble in hydrocarbons.
In its pure anhydrous condition, glycerol has a specific gravity of 1.261 gmL-1, a melting point of 18.2 °C and a boiling point of 290 °C under normal atmospheric pressure, accompanied by decomposition. At low temperatures, glycerol may form crystals which melt at 17.9 °C. Overall, it possesses a unique combination of physical and chemical properties (Table 1.1), which are utilized in many thousands of commercial products. Indeed, glycerol has over 1500 known end uses, including applications as an ingredient or processing aid in cosmetics, toiletries, personal care products, pharmaceutical formulations and foodstuffs. In addition, glycerol is highly stable under normal storage conditions, compatible with many other chemical materials, virtually non-irritating in its various uses, and has no known negative environmental effects.
Glycerol contains three hydrophilic alcoholic hydroxyl groups, which are responsible for its solubility in water and its hygroscopic nature. It is a highly flexible molecule forming both intra- and intermolecular hydrogen bonds. There are 126 possible conformers of glycerol, all of which have been characterized in a recent study using density functional theory (DFT) methods. In particular, a systematic series of ab initio molecular orbital and density functional theory optimizations of all the possible staggered conformers of glycerol (including calculation of the Boltzmann distributions in the gas and aqueous phases) has indicated that the enthalpic and entropic contributions to the Gibbs free energy are important for an accurate determination of the conformational and energetic preferences of glycerol. In the lowest energy conformer at the CCSD–(T)/6-31 + G(d,p)//HF/6–31G(d) and CBS–QB3 levels, the hydroxyl groups form a cyclic structure with three internal hydrogen bonds. This conformer, termed gG'g,g'Gg (γγ), has the structure with the internal hydrogen bond lengths displayed in Figure 1.2 and provides the starting geometry for the mechanism of many reactions with practical applications. This conformer (left in Figure 1.2) is the only form of glycerol which possesses three intramolecular hydrogen bonds, a structure offering considerable energetic benefit.
In the aqueous phase, glycerol is stabilized by a combination of intramolecular hydrogen bonds and intermolecular solvation of the hydroxyl groups. Indeed, taking into account solvation, the conformer with two intramolecular hydrogen bonds shown on the right in Figure 1.2 is the most energetically stable. This is due to the fact that in aqueous solution conformation 1 is now of higher relative energy, because all three hydroxyl groups are involved in intramolecular hydrogen bonding and are therefore unavailable to interact with the solvent. Many structures that possess intramolecular hydrogen bonding arrays still provide low energy conformations in aqueous solution, even when compared to structures without intramolecular hydrogen bonding.
In condensed phases, glycerol is characterized by a high degree of association due to hydrogen bonding. A first molecular dynamics simulation suggests that on average 95% of molecules in the liquid are connected. This network is very stable and very rarely, especially at high temperature, releases a few short living (less than 0.5 ps) monomers, dimers or trimers. In the glassy state, a single hydrogen-bonded network is observed, involving 100% of the molecules present.
A highly branched network of molecules connected by hydrogen bonds exists in all phases and at all temperatures. The average number of hydrogen bonds per molecule ranges from about 2.1 in the glassy state to 1.2 in the liquid state at high temperature, with an average activation energy of 6.3 kJ mol-1 required to break the hydrogen bond. Crystallization, which occurs at 291 K, cannot be directly achieved from the liquid state but requires special procedures. Due to the existence of such an extended hydrogen bonded network, the viscosity and the boiling point of glycerol are unusually high. Glycerol readily forms a supercooled liquid which, by lowering the temperature undergoes at about 187 K transition to a glassy state whose nature has been the subject of a number of investigations. Remarkably, a recent single-molecule analysis has revealed a foam-like structure for glycerol at temperatures above the glass transition point (Tg, 190K) comprising pockets of fluid isolated from one another by glass-like regions, which retain their distinct dynamics over surprisingly long timescales.
The intramolecular energy is the prevailing factor in determining the average molecular structure in the condensed phases. The inter-molecular hydrogen bonds, however, do not significantly stabilize energetically unfavored gas phase structures. In the gas phase in the range 300–400 K glycerol in practice exhibits only three backbone conformations, namely αα, αγ and γγ (Figure 1.3).
Intermolecular interactions stabilize slightly the α α-conformation and destabilize the γγ-conformation as the temperature decreases (Table 1.2). As the liquid approaches the freezing point about half of the molecules are present in the αα-conformation.
The activation energy for a conformational transition is around 20 kJ mol-1 in other words it is much higher at room temperature than at freezing temperature. The high activation energy, and the fact that nearly half of the molecules must undergo at least one conformational transition to reach the αα-structure typical of the crystalline state, is responsible for the remarkable stability of supercooled glycerol. The network dynamics involved spans three distinct and increasingly longer time scales, due to vibrational motion, to neighbor exchange and to translational diffusion, respectively.
In biochemistry, glycerol plays a major role in stabilizing enzymes due to the action of polyhydric alcohol functions, a fact which is generally attributed to the enhancement of the structural stability of the entire protein by a large alteration in the hydrophilic–lipophilic balance (HLB) upon clustering around the protein. This results in practical implications of dramatic importance, as glycerol also protects biologicals during sol–gel entrapment in a silica-based matrix, either by formation of poly(glyceryl silicate) as sol–gel precursors or by direct addition to bacteria prior to the sol–gel polycondensation. This enables reproducible and efficient confinement of proteins, cells and bacteria inside hybrid bio-doped glasses. These materials display activity approaching or even exceeding those of the free biologicals, together with the high stability and robustness that characterizes sol–gel bioceramics, and accordingly are finding application over a wide field, ranging from biocatalysis to biosensing and biodiagnostics. Finally, it is worth recalling at this point that theoretical physicist David Bohm formulated the implicate order theory, inspired by a simple experiment in which a drop of ink was squeezed on to a cylinder of glycerol. When the cylinder rotated (Figure 1.4), the ink diffused through the glycerol in an apparently irreversible fashion; its order seemed to have disintegrated. When the inner cylinder was rotated in the opposite direction, the ink recovered its initial profile.
1.2 Traditional Commercial Applications
Traditional applications of glycerol, either directly as an additive or as a raw material, range from its use as a food, tobacco and drugs additive to the synthesis of trinitroglycerine, alkyd resins and polyurethanes (Figure 1.5).
Currently, the amount of glycerol that goes annually into technical applications is around 160 000 tonnes and is expected to grow at an annual rate of 2.8%. Of the glycerol market, pharmaceuticals, toothpaste and cosmetics account for around 28%, tobacco 15%, foodstuffs 13% and the manufacture of urethanes 11%, the remainder being used in the manufacture of lacquers, varnishes, inks, adhesives, synthetic plastics, regenerated cellulose, explosives and other miscellaneous industrial uses. Glycerol is also increasingly used as a substitute for propylene glycol.
As one of the major raw materials for the manufacture of polyols for flexible foams, and to a lesser extent rigid polyurethane foams, glycerol is the initiator to which propylene oxide and ethylene oxide is added. Glycerol is widely used in alkyd resins and regenerated cellulose as a softener and plasticizer to impart flexibility, pliability and toughness in surface coatings and paints.
Most of the glycerol marketed today meets the stringent requirements of the United States Pharmacopeia (USP) and the Food Chemicals Codex (FCC). However, technical grades of glycerol that are not certified as USP or FCC quality are also found. Also available is Food Grade Kosher glycerol, which has been prepared and maintained in compliance with the customs of the Jewish religion.
The primary function of glycerol in many cases is as a humectant, a substance for retaining moisture and in turn giving softness. Glycerol draws water from its surroundings and the heat produced by the absorption makes it feel warm. Due to this property, glycerol is added to adhesives and glues to keep them from drying too rapidly. Many specialized lubrication problems have been solved by using glycerol or glycerol mixtures. Many thousand tonnes of glycerol are used each year to plasticize a variety of materials such as sheeting and gaskets. The flexibility and toughness of regenerated cellulose films, meat casings and special quality papers can be attributed to the presence of glycerol. It also acts as a solvent, sweetener, and preservative in food and beverages, and as a carrier and emollient in cosmetics. The effectiveness of glycerol as a plasticizer and lubricant gives it wide applicability, particularly in food processing, because it is nontoxic. Glycerol is also used in alkyd resin manufacture to impart flexibility. Alkyd resins are used as binders in products such as paints and inks, where brittleness is undesirable. Glycerol is used in specialist lubricants where oxidation stability is required, for example in air compressors. In all applications, whether as a reactant or as an additive, the non-toxicity and overall safety of glycerol is a significant benefit.
Due to the rapid decline in its price, glycerol is rapidly substituting other polyols which are used on a large scale as sugar-free sweeteners. Polyols are used mostly in confectionery, food, oral care, pharmaceutical, and industrial applications. Some characteristics of polyols are reduced calories, a pleasant sweetness, the ability to retain moisture, and improved processing. The most widely used polyols are sorbitol, man-nitol, and maltitol. Sorbitol is facing particularly stiff competition from glycerol. Glycerol contains approximately 27 calories per teaspoonful and is 60% as sweet as sucrose; it has about the same food energy as table sugar. However, it does not raise blood sugar levels, nor does it feed the bacteria that cause plaque and dental cavities. As a food additive, glycerol is coded E422. Baked goods lose their appeal when they become dry and hard during storage. Being hygroscopic, glycerol reduces water loss and prolongs shelf life.
Glycerol is used in medical and pharmaceutical preparations, mainly as a means of improving smoothness, providing lubrication and as a humectant, that is as a hygroscopic substance which keeps the preparation moist. Glycerol helps to maintain texture and adds humectancy, controls water activity and prolongs shelf life in a host of applications. It is also widely used as a laxative and, based on the same induced hyperosmotic effect, in cough syrups (elixirs) and expectorants.
In personal care products glycerol serves as an emollient, humectant, solvent, and lubricant in an enormous variety of products, including toothpaste, where its good solubility and taste give it the edge on sorbitol. Toothpastes are estimated to make up almost one-third of the personal care market for glycerol. Related applications include mouthwashes, skin care products, shaving cream, hair care products and soaps. It is for example a component of "glycerin soap" (Figure 1.6), which is used by people with sensitive, easily irritated skin because its moisturizing properties prevent skin dryness. In general, however, very low concentrations (0.05–1%) are employed, which are not able to significantly reduce the large surplus of biodiesel-generated glycerol on the market.
Glycerol is similar in appearance, smell and taste to diethylene glycol (DEG), which has often been used as a fraudulent replacement for glycerol. In the USA, for example, the Food, Drug, and Cosmetic Act was passed in 1938 following the "elixir sulfanilamide" incident, which caused more than 100 deaths due to the contamination of medicines with DEG. The revamped 1938 law provided food standards, addressed the safety of cosmetics, and required that drugs were checked for safety before sale, reinforcing the role of the Food and Drugs Administration (Figure 1.7). As late as 2007 the Food and Drug Administration blocked all shipments of toothpaste from China to the USA after reports of contaminated toothpaste entering via Panama. The toothpaste contained DEG, killing at least 100 people. The poison, falsely labeled as glycerol, had in 2006 been mistakenly mixed into medicines in Panama, resulting in the occurrence of the fatal poisonings. The DEG had originated from a Chinese factory which had deliberately falsified records in order to export DEG in place of the more expensive glycerol. Eventually, a large batch of toothpaste contaminated with DEG also reached the EU market, with a number of poisoning cases being reported in Italy and southern Europe.
The glycerol market is currently undergoing radical changes, driven by very large supplies of glycerol arising from biodiesel production. Researchers and industry have been looking at new uses for glycerin to replace petrochemicals as a source of chemical raw materials, and in a relatively few years there have been an impressive series of achievements. These topics are discussed in the following chapters. After the sustained period of increasing oil prices starting in the early 2000s, glycerol is now becoming established as a major platform for the production of chemicals and fuels.
1.3 Production of Bioglycerol
Glycerol provides the molecular skeleton of all animal and vegetable fats (triglycerides, the energy reservoir for materials in nature). It is also the oldest organic molecule isolated by man, obtained by heating fats in the presence of ash to produce soap as early as 2800 BC. It constitutes on average about 10% by weight of fatty matter. When the body uses fat stores as a source of energy, glycerol and fatty acids are released into the bloodstream. The glycerol component is converted to glucose in the liver and provides energy for cellular metabolism. Natural glycerol is obtained hydrolytically from fats and oils during soap and fatty acid manufacture, and by transesterification (an interchange of fatty acid groups with another alcohol) during the production of biodiesel fuel (Figure 1.8). It therefore comes as no surprise that in these energy intensive days glycerol has become a hot topic in industry at large.
In a certain sense, glycerol was already a national defense priority in the days leading up to World War II, as the supply of glycerol originating from soap making was largely insufficient to meet the wartime demand for nitroglycerine, i.e., for dynamite, the smokeless gunpowder used in all types of munitions, discovered by Swedish industrialist Alfred Nobel. Nobel built bridges and buildings in Stockholm and while researching new methods for blasting rock, he invented in 1863 the detonator for igniting nitroglycerine by means of a strong shock rather than by heat combustion. Nitroglycerine, invented by Italian chemist Ascanio Sobrero in 1846, is very volatile in its natural liquid state. In 1866 Nobel discovered that mixing nitroglycerine with silica (kieselguhr) would convert the liquid into a malleable paste, called dynamite, which could be kneaded and shaped into rods suitable for insertion into drilling holes.
Excerpted from The Future of Glycerol by Mario Pagliaro, Michele Rossi. Copyright © 2008 Mario Pagliaro and Michele Rossi. Excerpted by permission of The Royal Society of Chemistry.
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Meet the Author
Mario Pagliaro is a chemistry scholar based at Palermo's CNR where he leads Sicily's Photovoltaics Research Pole. The achievements of his Laboratory are reported in a large body of research papers spanning many fields of contemporary chemical research. Some discoveries of his Lab are at the origin of new, diverse successful commercial products. Mario has co-authored many bestselling books, including Flexible Solar Cells and Silica-Based Materals. He has a prolonged interest in management and in science methodology and is often cited for his excellence in teaching. His website is qualitas1998.net Michele Rossi holds a chair of inorganic chemistry at the University of Milan. He graduated in industrial chemistry at the University of Milan in 1963 at Professor Malatesta's school. In 1974 he became Professor of inorganic chemistry at the University of Bari and in 1988 he returned to Milan. His research, documented in more than 150 papers and several patents, is focused on metal-based catalysis and has led to important results in the activation of small molecules for catalytic applications and to the discovery of nitrogen fixation.
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