2-Ethylhexanol from butyraldehyde

Figure 8-6. The Hoechst AG process for producing 2-ethylhexanol from n-butyraldehyde (1) Aldol condensation reactor, (2) separation (organic phase from liquid phase), (3) hydrogenation reactor, (4) distillation column.

Zeolite catalysts in many forms are used for important commercial processes. The studies were extended to L zeolites, mordenite, erionite, and dealuminated faujasites and mordenites. More attention is paid now to zeolites with univalent and multivalent cations and to multicomponent catalysts. Among these some important examples are the tellurium-containing catalyst for hydrocarbon dehydrocyclization (42), the difunctional Ni- and Pd-zeolite catalysts for benzene hydrodimerization to phenylcyclohexane (42), the catalyst for the hydrogenation of phenol cyclohexanol (44), the 4% Ni/NaY which forms butanol, 2-ethylhexanol, 2-ethylhexanal, and 2-ethylhexanol from a mixture of n-butyraldehyde and hydrogen. 

The production of 2-ethylhexanol from propylene by the rhodium catalyzed, low pressure oxo process is accomplished in three chemical steps. The first step of the process (described in section on n-butanol manufacture) converts propylene to normal butyraldehyde by hydroformylation in the presence of a rhodium catalyst. In a second step, the normal aldehyde is aldoled to form 2-ethylhexena1. 2-Ethylhexenal is then hydrogenated to 2-ethylhexanol and refined in the third and final step(see Figure 3). 

Oxo chemicals include butyraldehyde (normal- and iso-) and the corresponding alcohols, 2-ethylhexanol (from n-butyraldehyde), propionaldehyde, and n-propyl alcohol, and lesser amounts of higher aldehydes and alcohols derived from C5 through C]7 olefins. The total volume of products derived from oxo chemistry exceeds a billion pounds a year. Volumes and applications are given later in this chapter for the most important products. 

Many oxo plants contain an additional aldolization section, e. g. for the manufacture of 2-ethylhexanol from n-butyraldehyde [893, 894]. 

Rhodium Ca.ta.lysts. Rhodium carbonyl catalysts for olefin hydroformylation are more active than cobalt carbonyls and can be appHed at lower temperatures and pressures (14). Rhodium hydrocarbonyl [75506-18-2] HRh(CO)4, results in lower -butyraldehyde [123-72-8] to isobutyraldehyde [78-84-2] ratios from propylene [115-07-17, C H, than does cobalt hydrocarbonyl, ie, 50/50 vs 80/20. Ligand-modified rhodium catalysts, HRh(CO)2L2 or HRh(CO)L2, afford /iso-ratios as high as 92/8 the ligand is generally a tertiary phosphine. The rhodium catalyst process was developed joindy by Union Carbide Chemicals, Johnson-Matthey, and Davy Powergas and has been Hcensed to several companies. It is particulady suited to propylene conversion to -butyraldehyde for 2-ethylhexanol production in that by-product isobutyraldehyde is minimized. 

The important solvent and plasticizer intermediate, 2-ethylhexanol, is manufactured from -butyraldehyde by aldol addition in an alkaline medium at 80-130°C and 300-1010 kPa (3-10 atm). 

Aldehydes fiad the most widespread use as chemical iatermediates. The production of acetaldehyde, propionaldehyde, and butyraldehyde as precursors of the corresponding alcohols and acids are examples. The aldehydes of low molecular weight are also condensed in an aldol reaction to form derivatives which are important intermediates for the plasticizer industry (see Plasticizers). As mentioned earlier, 2-ethylhexanol, produced from butyraldehyde, is used in the manufacture of di(2-ethylhexyl) phthalate [117-87-7]. Aldehydes are also used as intermediates for the manufacture of solvents (alcohols and ethers), resins, and dyes. Isobutyraldehyde is used as an intermediate for production of primary solvents and mbber antioxidants (see Antioxidaisits). Fatty aldehydes Cg—used in nearly all perfume types and aromas (see Perfumes). Polymers and copolymers of aldehydes exist and are of commercial significance. 

Hydroformylation is an important industrial process carried out using rhodium phosphine or cobalt carbonyl catalysts. The major industrial process using the rhodium catalyst is hydroformylation of propene with synthesis gas (potentially obtainable from a renewable resource, see Chapter 6). The product, butyraldehyde, is formed as a mixture of n- and iso- isomers the n-isomer is the most desired product, being used for conversion to butanol via hydrogenation) and 2-ethylhexanol via aldol condensation and hydrogenation). Butanol is a valuable solvent in many surface coating formulations whilst 2-ethylhexanol is widely used in the production of phthalate plasticizers. 

The first stage of the process is a hydroformylation (oxo) reaction from which the main product is n-butyraldehyde. The feeds to this reactor are synthesis gas (CO/H2 mixture) and propylene in the molar ratio 2 1, and the recycled products of isobutyraldehyde cracking. The reactor operates at 130°C and 350 bar, using cobalt carbonyl as catalyst in solution. The main reaction products are n- and isobutyraldehyde in the ratio of 4 1, the former being the required product for subsequent conversion to 2-ethylhexanol. In addition, 3 per cent of the propylene feed is converted to propane whilst some does not react. 

Ethylhexanol is produced by aldol condensation of butyraldehyde followed by reduction. It can also be made in one step from propylene and synthesis gas converted to butanols and 2-ethylhexanol without isolating the butyraldehydes. See Chapter 10, Section 6. 

Another oxo plant, now being constructed, will make butyl compounds (88). These may be the source of butyl alcohol, butyl acetate, butyric acid for the manufacture of cellulose acetate butyrate and other products, butyraldehyde for polyvinyl butyral, and the eight-carbon compounds including 2-ethylhexanol. All these will add to the present production of the same compounds made by the older methods from acetaldehyde via aldol condensation. 

The overall growth for -butyraldehyde depends primarily on -butanol and 2-ethylhexanol. 2-Ethylhexanol is expected to face competition from other alcohols, eg, isodecyl alcohol [25339-17-7/, as well as from newer production sources. 

Di-2-EthylhexylPhthalate. In Western Europe, di-2-ethylhexyl phthalate [117-81-7] (DEHP), also known as dioctyl phthalate (DOP), accounts for about 50% of all plasticizer usage and as such is generally considered as the industry standard. The reason for this is that it is in the mid-range of plasticizer properties. DEHP (or DOP) is the phthalate ester of 2-ethylhexanol, which is normally manufactured by the dimerization of butyraldehyde (eq. 2), the butyraldehyde itself being synthesized from propylene (see Butyraldehydes). 

Homogeneous rhodium-catalyzed hydroformylation (135,136) of propene to w-butyraldehyde (qv) was commercialized in 1976. tf-Butyraldehyde is a key intermediate in the synthesis of 2-ethylhexanol, an important plasticizer alcohol. Hydroformylation is carried out at <2 MPa (<290 psi) at 100°C. A large excess of triphenyl phosphine contributes to catalyst life and high selectivity for -butyraldehyde (>10 1) yielding few side products (137). Normally, product separation from the catalyst [Rh(P(C6HB)3)3(CO)H] [17185-29-4] is achieved by distillation. 

Ethanol s use as a chemical intermediate (Table 8) suffered considerably from its replacement in the production of acetaldehyde, butyraldehyde, acetic acid, and ethylhexanol. The switch from the ethanol route to those products has depressed demand for ethanol by more than 300 x 106 L (80 x 106 gal) since 1970. This decrease reflects newer technologies for the manufacture of acetaldehyde and acetic acid, which is the largest use for acetaldehyde, by direct routes using ethylene, butane (173), and methanol. Oxo processes (qv) such as Union Carbide s Low Pressure Oxo process for the production of butanol and ethylhexanol have totally replaced the processes based on acetaldehyde. For example, U.S. consumption of ethanol for acetaldehyde manufacture declined steadily from 50% in 1962 to 37% in 1964 and none in 1990. Butadiene was made from ethanol on a large scale during Wodd War II, but this route is no longer competitive with butadiene derived from petroleum operations. 

Subsequently, a whole host of both lower (C3, C4, C5) and higher (C5, Cy, C9, C q, C, etc.) oxo alcohols have been commercialized. Of all these alcohols, the most important by far have turned out to be n-butanol and 2-ethylhexanol - both of which are derived from n-butyraldehyde based on hydroformylation of propylene. In addition to n-butyraldehyde, the lower valued isobutyraldehyde is produced as a by-product. Some of this is converted to isobutanol. 

While n-butanol from the oxo process did not become a major intermediate for PVC plasticizers, the development was nonetheless a breakthrough of far-reaching significance. The precursor of n-butanol in the oxo process is n-butyraldehyde. This made possible the manufacture of 2-ethylhexanol by "single aldol" at lower cost than the "double aldol" route via acetaldehyde. 

The largest volume hydroformylation reaction converts propylene into n-butyraldehyde, from which is made 1-butanol for solvents, or 2-ethylhexanol (the phthalate ester of which has been widely used as a plasticizer for PVC) via an aldol condensation. Estimated world production of butanol is approaching 2 Mt/a. 

Rhodium Catalysts. - The hydroformylation of propene with a Rh/triphenyl-phosphine catalyst is now an established industrial process which will consume over a million tonnes per annum of propene when all licensed plants are operational. Most of the product n-butyraldehyde is converted to 2-ethylhexanol for plasticiser applications. The process is also applicable to the hydroformylation of C2, C4, and C5 alkenes. The process is remarkable for the long lifetime of the Rh catalyst but by-products are formed which deactivate the catalyst and have to be removed. The formation of triphenyl-phosphine oxide, benzaldehyde, and propyldiphenylphosphine under hydroformylation conditions has been investigated where benzaldehyde is produced by or /zo-metallation of triphenylphosphine followed by CO insertion and P-C bond cleavage and propyldiphenylphosphine was assumed to result from reaction of propene with the co-ordinated diphenylphosphine group remaining after benzaldehyde formation. The same authors have also studied the kinetics of the formation of heavy by-products which are dependent on... 

The rate of the reaction changes depending upon the partial pressure of CO it reaches a maximum at ca 3 MPa. The yield of the reaction decreases as the temperature increases. At low temperatures the rate of the hydroformylation is small and therefore higher temperatures are applied. Hydroformylation allows the preparation of various valuable products because the oxo synthesis may utilize different compounds containing a carbon-carbon double bond, for example, dienes, polyenes, and unsaturated aldehydes, ketones, nitriles, alcohols, esters, etc. For example, dienes may afford dialdehydes. A substantial amount of aldehydes is converted to alcohols which find considerable application in the preparation of detergents, plasticizers, and lubricants. The aldol condensation generally is not desired in hydroformylation processes. Nevertheless, via aldol condensation followed by hydrogenation, 2-ethylhexanol is obtained from n-butyraldehyde see equation (13.117). 

For example, in the case of propene hydroformylation from n-butyraldehyde, 2-ethylhexenal, 2-ethylhexanal, 2-ethylhexanol, various trimer aldol, Tishchenko, and ether-type products have been identified (27). 

The operating plants produce aldehydes in the range Cg-Cjg which are either hydrogenated as such to give the corresponding alcohols or subjected to aldol condensation prior to the hydrogenation. In the latter case the resulting alcohols contain double the number of carbon atoms as the aldehyde used for the aldol condensation (e. g. 2-ethylhexanol is made from two moles of n-butyraldehyde via 2-ethylhexenal). 

From 1974 onwards, Rh-based hydroformylation became industrial. The use of a catalyst metal that is about 1000-times more expensive than cobalt was driven by several reasons. First, Rh-hydroformylation is more active and thus requires much lower process pressures (lower energy consumption in compression units) and smaller reactors. Second, Rh-hydroformylation shows a very high selectivity to the aldehyde product with only minimal hydrogenation activity being observed. This is of particular importance for propylene hydroformylation where butyl alcohol is not the principle market use. In contrast, for the desired end-use of w-butyraldehyde in the form of its aldol condensation product 2-ethylhexanol a pure aldehyde feed is required as hemiacetals (formed by reaction of aldehyde and alcohol) complicate product purification and add to operating costs. 

Ethylhexanol is derived as follows in Figure 9.6 from propylene, acetaldehyde, or butyraldehyde. 

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