Long-chain alcohols

1.2.1.1 Applications for long-chain alcohols

Long-chain or fatty alcohols (C12 and higher) are indispensable intermediates for the production of surfactants, but are also employed as free alcohols in cosmetics and some other applications (intermediates to amines and other chemicals). Surfactants account for 70 – 75 % of fatty alcohol production (Brackmann & Hager, 2004). The hydroxyl group may undergo a large number of chemical reactions making fatty alcohols versatile intermediates. Their amphiphilic character, which results from the combination of a non-polar, lipophilic carbon chain with a polar, hydro-philic hydroxyl group, confers surface activity upon these compounds (Presents et al., 2000).

Surfactants are used in a wide range of fields. By far, the most important field of application is the washing and cleansing sector as well as textile treatment and cosmetics. These use more than 50

% of the total amount of surfactants. Surfactants are also used in the food sector, in crop protec-tion, mining, and the production of paints, coatings, inks, and adhesives (Hill, 2007).

Global fatty alcohol production was predicted to be more than 2 million metric tons in 2010 with a 3.8% increase in annual demand until 2020 (Colin A. Houston & Associates, 2006). Market value was estimated to be about 3 billion $US, with an approximate price of 1,500 $US/metric ton (Steen et al., 2010). Fatty alcohols manufacture is dominated by tropical-oil-based production (oleochemical), but around 35 % (~700 k metric tons) are being still produced from petrochemical feed stocks (Figure 1).

Oleochemical- and petrochemical-based surfactants have traded market dominance over the years based on factors such as consumer preference, capacity availability, and especially increas-ing crude oil price (McCoy, 2005).

Figure 1.1 - Global fatty alcohol production. Sources: CESIO, 2001 (1990-2000); Colin A. Houston

& Associates, 2006 (2004-2010*). * Predicted.

1.2.1.2 Industrial production processes

Fatty alcohols derived from natural fats and oils are normally produced by the hydrogenation of the corresponding fatty acid methyl-esters. Most of the methanol is recovered in this process and is recycled for use in the ester exchange step. Another route of manufacturing is the hydrolyzation of fats and oils to the corresponding fatty acids followed by a subsequent catalytical reduction to the alcohols.

A number of synthetic routes have been developed for producing detergent-range alcohols from petroleum-derived raw materials. Ethylene, olefins, or n-paraffins are the basic chemical starting materials and the Ziegler chemistry (ELPAL®, ALFOL®) and OXO process are the most important routes (Figure 1.2).

In the Ziegler process ethylene is added to triethyl aluminum to build a mixture of high-molecular-weight trialkyl aluminums known as the ethylene growth product. After the oxidation with air the corresponding aluminum alkoxides are formed. The subsequent hydrolysis of these alkoxides leads to a mixture of linear primary alcohols having the same number of carbon atoms as the alkyl groups in the trialkyl aluminum growth product. Ziegler alcohols have even-numbered carbon chain lengths just like natural oil-based alcohols.

The OXO reaction as applied to the synthesis of detergent-range alcohols is currently employed commercially in a variety of modifications. Although each of these processes represents unique technology, they all involve the reaction of olefins with synthesis gas (CO/H₂) in the presence of an OXO catalyst to yield higher alcohols. The major differences among the processes are the type and source of the olefin, catalyst and process conditions. Most of the OXO plants in the world use processes in which first the intermediate aldehydes are isolated, purified and then hydrogenated in a second reactor. The Shell SHOP process with a cobalt type catalyst allows the

hydroformylation and hydrogenation of the intermediate aldehyde in the same reactor (Brackmann & Hager, 2004).

OXO-alcohols, which contain 20-40% branching of the alkyl chain, consist of both even- and odd-numbered carbon chain lengths. These alcohols also compete directly in some markets with natu-ral oil-based alcohols (Brackmann & Hager, 2004). Balanced mixed surfactant systems based on branched and linear alcohols are important for cleaning performance depending on application.

Branched alcohols are only industrially available from petrochemical processes.

Figure 1.2 - Commercial routes to obtain fatty alcohols from petrochemical and oleochemical feed stocks.

A series of studies in the mid-1990s examined the production of surfactants from a life-cycle in-ventory (LCI) point of view (Hirsinger et al., 1995a; Hirsinger et al., 1995b; Hirsinger, 1998;

Stalmans et al., 1995). The LCI work has resulted in calculations that compare the total energy used and waste generation (and emitted to air, water, and soil) in the processing of surfactants based on oleochemicals and petrochemicals. In general, there are environmental trade-offs for both sources. For example, while oleochemical surfactants are derived from a renewable re-source, they typically produce more air emissions and solid wastes. On the other hand, petro-chemical surfactants consume more total energy, as the raw materials are produced from energy resources. Whether the feedstock source is animal fat, plant oil, or crude oil, there are energy requirements and environmental wastes that are a part of the feedstock and production stages of turning raw materials into surfactants.

Reducing significantly energy consumption in the production of fatty alcohols from petrochemical feed stocks would make these products more competitive in a market that in the last decade has shifted production to oleo based alcohols. A higher variety of fatty alcohols (even and odd carbon chain lengths, linear and branched, and di-alcohols) could be available for a wider range of appli-cations overcoming inherent limitations of natural base products or those limitations imposed by the use of costly petrochemical based alcohols.

For instance, long-chain linear or branched >C14 paraffins (up to 7% in some crude oil sources, Espada et al., 2010) could be directly oxidized to alcohols by selective catalysis (e.g. biocatalysis) reducing significantly energy consumption used to produce building blocks employed in synthesis of petrochemical long-chain alcohols.

Photosynthesis

1.2.1.3 Long-chain alcohols through biocatalysis

There are no commercial production processes of fatty alcohols through biocatalysis. However, research efforts have been made to investigate the possibility to produce these alcohols either from natural oils and fats by enzymatic reduction of fatty acids or by the conversion of n-alkanes to the corresponding fatty alcohols through terminal hydroxylation (see 1.3).

1.2.1.3.1 Long-chain alcohols by enzymatic reduction of fatty acids

Fatty acid biosynthesis is the preferred pathway to accumulate energy storage compounds in many organisms. During biosynthesis, fatty acids are activated as thioesters with coenzyme A (i.e.

fatty acyl-CoAs) or acyl carrier protein (i.e. fatty acyl-ACPs). Fatty alcohols can be synthesized by enzymes reducing these fatty acyl-thioester substrates. These enzymes are referred to as fatty alcohol-forming fatty acyl-CoA reductases (FAR). Several of these enzymes such as that encoded by acr1 from Acinetobacter calcoaceticus BD413 have been described, but the best-studied fatty alcohol-generating enzymes (FARs) are eukaryotic (Schirmer et al., 2009). Naturally, fatty alco-hols produced by FARs are often incorporated as esters to waxes, cuticles and other structures, serving as hydrophobic/protection barriers and typically non-esterified fatty alcohols are only found in very limited amounts. Yields and productivities in the range of 1 g/L and 0,05 g/L·h of fatty alcohols by this route have been found with recombinant microorganisms engineered to express heterologous FAR enzymes (McDaniel et al., 2011; Steen et al., 2010; Schirmer et al., 2009).