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Account / Revue Carbohydrates as green raw materials for the chemical industry

Account / Revue Carbohydrates as green raw materials for the chemical industry Frieder W. Lichtenthaler *, Siegfried Peters Clemens-Schöpf-Institut für Organische Chemie und Biochemie, Technische Universität Darmstadt, Petersenstraße 22, 64287 Darmstadt, Germany Received 6 August 2003; accepted 4 February 2004 This review is dedicated to Professor Gérard Descotes in recognition of his basic contributions towards utilization of carbohydrates as organic raw materials Abstract In view of the impending transition of chemical industry from depleting fossil raw materials to renewable feedstocks −the end of cheap oil is predicted for 2040 at the latest −this account gives an overview on chemically transforming low-molecular weight carbohydrates into products with versatile industrial application proﬁles and the potential to replace those presently derived from petrochemical sources. To cite this article: F.W. Lichtenthaler, S. Peters, C. R. Chimie 7 (2004). © 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved. Résumé Du fait de l’imminence de la mutation de l’industrie chimique vers l’utilisation de matières premières renouvelables à la place des ressources fossiles – l’épuisement de ces ressources bon marché est prévu pour 2040 au plus tard –, cet article propose une vue générale de la transformation chimique des sucres, à faible poids moléculaire, vers des applications industrielles poly- valentes et de leur potentiel à remplacer les produits préparés actuellement à partir de ressources pétrochimiques. Pour citer cet article : F.W. Lichtenthaler, S. Peters, C. R. Chimie 7 (2004). © 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved. Keywords: Carbohydrates; Biofeedstock; Industrial utilization; Organic chemicals Mots clés : Sucres ; Biomasse ; Utilisation industrielle ; Produits chimiques organiques 1. Introduction As our fossil raw materials are irrevocably decreas- ing and as the pressure on our environment is building up, the progressive changeover of chemical industry to renewable feedstocks for their raw materials emerges as an inevitable necessity [1–4], i.e. it will have to proceed increasingly to the raw materials basis that prevailed before natural gas and oil outpaced all other sources. Historically, the raw materials basis was substan- tially renewable, as depicted in Fig. 1, with the utiliza- tion of biomass and coal being in equal about 100 years ago [5]. In the 1920s, coal tar-based materials had * Corresponding author. E-mail address: lichtenthaler@chemie.tu-darmstadt.de (F.W. Lichtenthaler). C. R. Chimie 7 (2004) 65–90 © 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved. doi:10.1016/j.crci.2004.02.002 taken the lead, reaching a maximum around 1930; thereafter, fossil gas and oil irresistibly took over, eliminating coal nearly completely and reducing re- newable feedstocks to very modest levels. This over-reliance of chemical industry on fossil raw materials has its foreseeable limits as they are depleting and are irreplaceable, the only question be- ing: when will affordable fossil fuels be exhausted? Or, stated more appropriately: when will fossil raw mate- rials have become so expensive that biofeedstocks are an economically competitive alternative? Experts real- istically predict the end of cheap oil for 2040 at the latest [6,7], a development that we can witness by now already, as chemical industry combats the rising costs of natural oil and gas [8]. Thus, taking the prognosti- cation for the end of cheap oil [7], the curve for the utilization of biofeedstocks in Fig. 1 will have to rise such that it meets that of fossil raw materials some- where around 2030–2040. The transition to a more bio-based production sys- tem is pressing, yet hampered by a variety of obstacles: fossil raw materials are more economic at present, and the process technology for their into organic chemicals is exceedingly well developed and basically different from that required for transforming carbohydrates into products with industrial application proﬁles. This situ- ation originates from the inherently different chemical structures of the two types of raw materials, as terres- trial biomass is considerably more complex, constitut- ing a multifaceted array of low and high molecular weight products, exempliﬁed by sugars, hydroxy and amino acids, lipids, and biopolymers such as cellulose, hemicelluloses, chitin, starch, lignin, and proteins. By far, the most important class of organic compounds in terms of volume produced are carbohydrates, as they represent roughly 75% of the annually renewable bio- mass of about 200 billion tons. Of these, only a minor fraction (ca. 4%) is used by man, the rest decays and recycles along natural pathways. The bulk of the annually renewable carbohydrate biomass are polysaccharides, yet their non-food utili- zation is conﬁned to textile, paper, and coating indus- tries, either as such or in the form of simple esters and ethers. Organic commodity chemicals, however, are usually of low molecular weight, so they are more expediently obtained from low-molecular-weight car- bohydrates than from polysaccharides. Accordingly, the constituent repeating units of these polysaccha- rides −glucose (cellulose, starch), fructose (inulin), xylose (xylan), etc., inexpensive and available on multi-ton scale −are the actual carbohydrate raw mate- rials for basic organic chemicals. Intense efforts within the last decade [9–17] to ad- vance the use of inexpensive, large-scale-accessible mono- and di-saccharides as raw materials for chemi- cal industry have so far not been able to basically bridge the conceptual, technological and economical gap between fossil hydrocarbons and renewable carbo- hydrates; so, this account reviews the present utiliza- tion of low-molecular-weight carbohydrates as feed- stock for organic chemicals and −that being rather modest −accentuates existing methodologies that ap- pear practical enough to be developed into industrially viable processes and products of presumed industrial relevance, i.e. bulk, intermediate, and ﬁne chemicals, pharmaceuticals, agrochemicals, high-value-added speciality chemicals, or simply enantiopure building blocks for organic synthesis. 2. Sugars as biofeedstocks The following overview concentrates on transfor- mations of large-scale accessible mono- and disaccha- rides that either can be performed in one-pot proce- dures or in a few simple practicability-oriented steps, in which the carbon chain of the sugar is fully retained. As will become clear along the way, only a few of the 1850 1900 1950 2000 2050 renewable feedstocks coal natural gas, oil year Fig. 1. Raw materials basis of chemical industry in historical pers- pective. 66 F.W. Lichtenthaler, S. Peters / C. R. Chimie 7 (2004) 65–90 products described have reached industrial status, yet this setting that is going to change with the rising costs for petrochemical raw materials. 2.1. Non-food valorisation of sucrose 2.1.1. Structure and conformation Sucrose is a non-reducing disaccharide, because its component sugars, D-glucose and D-fructose, are gly- cosidically linked through their anomeric carbon at- oms. Hence, it constitutes a b-D-fructofuranosyl a-D- glucopyranoside (Fig. 2). It is widely distributed throughout the plant kingdom, is the main carbohy- drate reserve and energy source and an indispensable dietary material for humans. For centuries, sucrose has been the world’s most plentiful produced organic com- pound of low molecular mass, the present annual pro- duction from sugar-cane and sugar-beet being an im- pressive 130 × 106 t. Due to the usual overproduction, and the potential to be producible on an even higher scale if required, it is, together with starch-derived glucose, the major carbohydrate feedstock of low mo- lecular weight from which to elaborate chemicals. Due to its eight hydroxyl groups, chemical reactions of unprotected sucrose at a single hydroxyl group are difﬁcult to achieve, i.e. getting useful regioselectivities is a basic issue [23–25]. The subtle reactivity differ- ences between primary and secondary hydroxyl groups have been generalized such that the three pri- mary ones are preferentially alkylated, acylated, oxi- dized and displaced by halogen in the order 6g-OH ≈ 6f-OH >> 1f-OH [23] −an over-generalization as this order of reactivity mainly covers comparatively bulky reagents which necessarily favour reaction at the 6g- and 6f-OH groups. (For ready differentiation of the Fig. 2. Common structural representations of sucrose (top entries). The molecular geometry realized in the crystal is characterized by two intramolecular hydrogen bonds between the glucose and fructose portion [18–20] (centre left). In aqueous solution, the two sugar units are similarly disposed towards each other, caused by insertion of a water molecule between the glucosyl-2-OH and fructosyl-1-OH [21,22], this ‘water-bridge’ being ﬁxed by hydrogen bonding (centre right). 67 F.W. Lichtenthaler, S. Peters / C. R. Chimie 7 (2004) 65–90 oxygens in the fructose (primed numbers usually) and the glucose portions, they are denoted with ‘f’ and ‘g’ superscripts, respectively.) Molecular modelling of the electrostatic potential at the solvent accessible surface of sucrose clearly revealed the highest electropositivity being at the glucosyl-2-OH [20–22], entailing that it is the one most readily deprotonated. Accordingly, under basic conditions in ensuing reactions the 2g-OH is preferentially alkylated or acylated if the moiety to be attached to the oxygen is not too bulky (cf. § 2.1.2 and 2.1.3). In addition, the regioselectivity attainable is also depending on the nature of the electrophilic re- agent, on the catalyst used for promoting the reaction and, not the least, on the solvent or solvent mixtures used. Sucrose is only soluble in DMF and DMSO aside water, and differently solvated in each one [10,22]. In the sequel, only those ‘entry reactions’ into su- crose derivatives are covered, which either are of in- dustrial relevance or have uploads/Finance/ articulo-3-quimica-agro-pdf.pdf