The Monoterpenes

Limonene and a- and p-pinene can be regarded as natural key molecules for the monoterpene (C10) sector of the fragrance industry. Limonene is a component of orange and lemon peels (different enantiomers present) and is a cheap by-product of the citrus industry. The two pinenes are major components of crude sulfate turpentine (CST), a by-product of wood processing, and are relatively inexpensive starting compounds.

Catalytic conversions in the monoterpene field have recently been reviewed [22]. For instance, limonene is commercially converted to alkoxylated systems by solid-acid-catalyzed addition of lower alcohols [23]. Quite another limonene conversion is dehydrogenation toward a "green" aromatic p-cymene compound that can be converted by oxidation to the hydroperoxide and rearranged to p-cresol.

The networks around a- and p-pinene are versatile. By way of example, Figure 2.3 shows how the important fragrance linalool is approached industrially from both a- and p-pinene. These semisynthetic linalool products are in competition with synthetic linalool, made by building up the C10-system starting from isobutene.

• Semi synthetic routes

• Semi synthetic routes

• Petrochemical route starting from isobutene via methylheptenone

Semi synthetic : petrochemical ~ 1 : 1

FIGURE 2.3 Linalool syntheses.

Another example is the synthesis of campholenic aldehyde by epoxidation of a-pinene followed by isomerization [24]. Campholenic aldehyde is the starting compound for several sandalwood fragrances.

2.6 TRIGLYCERIDES, GLYCEROL

Oils and fats (triglycerides) serve primarily food applications. Some 15% of the annual production of 130 x 106 t (2004 estimate) goes to oleochemicals, such as fatty acids and soaps, fatty amines, long-chain alcohols, and biodiesel (fatty acid methyl esters). In all cases, glycerol is a by-product. Especially because of the growth of biodiesel (2004 estimate 2.5 x 106 t, with Germany as the leading producer [25]), world glycerol production is now approaching 1 million t/yr. Less than 10% of that amount is made by the conventional petrochemical process starting with chlorination of propene. The natural way is a clear winner here in terms of both economy and ecofriendliness.

Glycerol is applied as such in pharmaceutical and cosmetic formulations as well as in food, tobacco, and cellophane. Some chemical conversions are given in Figure 2.4.

A major reaction application of glycerol is as triol in alkyd resin formulations, moreover it is used to grow polyetherpolyols to be used in polyurethanes and in the manufacture of triacetin (glycerol triacetate). New opportunities include selective oxidation over Au-catalysts [26] toward glyceric acid and synthesis of glycidol [27] via the carbonate. In view of the growing glycerol production, further new applications would be welcomed.

2.7 TRIGLYCERIDES, FATTY ACIDS

The fatty acid composition of triglycerides depends strongly on the crop that provides them. Thus, coconut oil and palm-kernel oil contain mostly relatively short (C10, as triol in alkyd resins polyurethanes

as triol in alkyd resins polyurethanes

FIGURE 2.4 Glycerol conversions.

C12, C14, C16) saturated fatty acids, whereas soybean, rapeseed, and sunflower oil contain mainly the C18 unsaturated acids oleic and linoleic acid, with one and two double bonds, respectively. In palm oil, the second in production volume after soybean oil, the dominant fatty acid components are palmitic acid (C16 saturated) and oleic acid (C18 monounsaturated).

Insight into health aspects has been subject to fluctuation. Linolenic acid (C18, three double bonds), belonging to the so-called ra-3 acids, was earlier removed from margarine triglycerides by selective hydrogenation, but nowadays it is treasured. Moreover, it is agreed now that trans double bonds, formed upon partial hydrogenation of natural cis double bonds (catalytic hardening), are unhealthy.

Commercial products obtained from saturated fatty acids include a broad spectrum of esters, linear alcohols, primary and secondary linear amines, amides, and various metal salts. Relatively new are the direct hydrogenation of carboxylic acids to aldehydes over chromia [28] and the coupling of fatty acids (by an amide bond) to amino acids, leading to a new class of surfactants of all natural origin (Ajinomoto). Biocatalytic X- and ra-hydroxylation of fatty acids is a challenge.

In the unsaturated oleic acid molecule, the double bond is an extra reaction site. As shown in Figure 2.5, metathesis [29] especially opens up some interesting conversions, although these are not yet operated industrially. Thus self-metathesis of oleic acid (as methyl ester) gives a C18 dicarboxylic acid (together with a C18 alkene), whereas metathesis with ethene leads to 1-decene and 9-decenoic acid, a precursor of nylon-10. By oxidative ozonization, oleic acid is industrially converted in the C9 dicarboxylic acid azelaic acid and nonanoic acid.

Some recent patents deal with isomerization (branching) of oleic and stearic acid, with the aim of lowering the melting point. For instance, oleic acid is isomerized [30] at 250°C over an acidic zeolite, followed by hydrogenation over Pd. The product obtained is similar to the "isostearic acid" found as a by-product in the dimerization of oleic acid over an acid clay.

dimer-trimer

COOH HOOC-.

COOH HOOC-.

COOH

COOH

epoxide

Nylon - 10

FIGURE 2.5 Oleic acid as a key chemical.

epoxide

COOH

COOH

COOH

dimer-trimer

Civetone

Nylon - 10

FIGURE 2.5 Oleic acid as a key chemical.

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