""this book presents a short and nearly comprehensive overview of the established and more recent developments in the chemistry of glycerol.""
The Future of Glycerolby Mario Pagliaro, Michele Rossi
By-products of global biodiesel manufacturing are a modern day global fact responsible for igniting a number of year’s worldwide intense research activity into human chemical ingenuity. This fully updated and revised 2nd edition depicts how practical limitations posed by glycerol chemistry are solved based on the understanding of the fundamental chemistry of
By-products of global biodiesel manufacturing are a modern day global fact responsible for igniting a number of year’s worldwide intense research activity into human chemical ingenuity. This fully updated and revised 2nd edition depicts how practical limitations posed by glycerol chemistry are solved based on the understanding of the fundamental chemistry of glycerol and by application of catalysis science and technology. The authors report and comment on employable, practical avenues applicable to convert glycerol into value added products of mass consumption. This book is the best-selling reference book in the field. The highly anticipated 2nd Edition is essential reading for anyone interested in understanding whether biodiesel and glycerol refineries are convenient and economically sound.
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The Future of Glycerol
By Mario Pagliaro, Michele Rossi
The Royal Society of ChemistryCopyright © 2010 M. Pagliaro and M. 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 originates 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 and 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, with more than a thousand uses and applications. It is used in a wide range of products on account of its distinctive characteristics. Its physical and chemical properties make it a versatile organic compound. Food products, cosmetics, toiletries, and drugs are just a few of the products in which glycerol is employed either as a constituent or forming part of the processing. It serves as a humectant, sweetener, solvent or preservative. Its hygroscopic properties are of particular value in the food and beverage industries. In prepared or manufactured foods containing low amounts of fat, glycerol is used as a filler.
In the modern era, glycerol was identified in 1779 by Swedish chemist Carl W. Scheele, who obtained a novel transparent, syrupy liquid by heating olive oil with litharge (PbO, used in lead glazes for ceramics). It is completely soluble in water and alcohols, 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, 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 many 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 adverse 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 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 the left conformation 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 with structures without intramolecular hydrogen bonds.
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 only 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 being 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 transition at about 187 K to a glassy state, whose nature has been the object 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, 190 K) comprizing 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 intermolecular hydrogen bonds, however, do not significantly stabilize energetically unfavored gas phase structures. In the gas phase within the range 300–400 K glycerol in practice exhibits only three backbone conformations, namely αα, αγ and γγ (Figure 1.3).
Intermolecular interactions slightly stabilize the αα conformation and destabilize the γγ conformation as the temperature decreases (Table 1.2). As the liquid approaches 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, which means that 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 timescales, 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 effect of its polyhydric alcohol functionality, 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 dye was squeezed on to a cylinder of glycerol. When the cylinder rotated (Figure 1.4), the dye 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 dye regained 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 going 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 are 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). Also available is Food Grade Kosher glycerol, which has been prepared and maintained in compliance with the customs of the Jewish religion. However, technical grades of glycerol that are not certified as USP or FCC quality are also available.
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 various materials, such as sheets 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 of paramount 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, and pharmaceutical and industrial applications. Characteristics of polyols are a reduction in calories, a pleasant sweetness, the ability to retain moisture, and improved processing. The most widely used polyols are sorbitol, mannitol, 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 and providing lubrication, and also acts as a humectant, a hygroscopic substance which keeps the preparation moist. Glycerol also helps to maintain texture, it 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 over 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 unable to absorb 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 US following 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 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, a number of poisoning cases being reported in Italy and southern Europe.
The glycerol market has undergone radical changes, driven by very large supplies of glycerol arising from booming biodiesel production, the intrinsic volatility of which, due to changing taxation regulations in the richest European countries and in the USA, is adding to the historic volatility of the glycerol market. In any case, researchers and industry have been investigating new outlets for glycerol to replace petrochemicals as a source of chemical raw materials, and in a relatively few years there have been an impressive series of achievements. After the lengthy period of increasing oil prices from the early 2000s, glycerol is establishing itself as a major platform for the production of chemicals and fuels. These topics are discussed in the chapters which follow.
Excerpted from The Future of Glycerol by Mario Pagliaro, Michele Rossi. Copyright © 2010 M. Pagliaro and M. Rossi. Excerpted by permission of The Royal Society of Chemistry.
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Meet the Author
Professor Pagliaro is a research chemist and science methodology professor at Italy's National Research Council based in Palermo where he leads a research group collaborating with researchers of 10 countries and the new Institute for Scientific Methodology. His research interests lie at the interface of materials science, chemistry and biology. Reported in 60 scientific papers, book chapters and several patents, his achievements include chemical technologies now on the market.
Professor Rossi is a chemistry professor at the University of Milan. His scientific activity - documented by some 160 scientific papers and several patents - currently deals with catalysis for fine chemicals synthesis and recently resulted in the discovery of the surprising catalytic activity of gold nanoparticles in liquid phase oxidation of organic compounds.
James H Clark is Professor of Chemistry and Director of the Green Chemistry Centre of Excellence, The University of York, UK. He has led the green chemistry movement in Europe for the last 15 years and was the first scientific editor of the journal Green Chemistry and is Editor-in-chief of the RSC Green Chemistry book series.
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