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Comparatively Speaking: Natural- vs. Oxo- vs. Ziegler-derived Alcohols
By: Anthony J. O'Lenick, Jr., Siltech LLC
Posted: March 23, 2010
Natural fatty alcohols are produced through the reduction of methyl esters with high-pressure hydrogen. This process (see Figure 1) has been known for almost 70 years and has been refined a number of times. The catalyst used for the reaction determines the product prepared. For instance, if copper chromite is used as a catalyst with an unsaturated methyl ester, the hydrogenation of any C=C unsaturation also occurs, resulting in a saturated species alcohol. However, if a group-specific catalyst such as certain aluminum oxide is used, the unsaturation is not affected, resulting in an unsaturated alcohol. This is the route used to make oleyl alcohol.
The process for preparing high purity alcohol requires a clean triglyceride from which a methyl ester is derived. Certain materials poison the catalyst used to make the fatty alcohol. These include sulfur compounds and chlorine-containing materials. The resultant methanol is distilled away. The methyl ester used as the raw material could be a wide cut product or fractionated into narrow cut products. This allows for the preparation of a variety of fatty alcohol products tailored for specific surfactant synthesis.
Oxo alcohols are prepared by the reaction of alpha olefin with hydrogen and carbon monoxide using a catalyst, typically cobalt compounds. The reaction, shown in Figure 2, occurs in two parts—the preparation of the aldehyde followed by a reduction of the aldehyde to the alcohol. It is important that two different aldehyde compounds, one linear and the other with a methyl branch, form in the first part of the reaction. Both compounds then rearrange into alcohols. The raw material used in the reaction is alpha olefin, which is made from ethylene. The key factor among alcohol properties is the exact “R” distribution of the alpha olefin. Typically, the alpha olefin can either be a narrow range or a wide range.
Ziegler alcohols are produced through the oxidation of trialkyl aluminum alkoxylates, chain growth and subsequent hydrolysis, as seen in Figure 3. Continental Oil Co., Ethyl Corp. and Condea developed the technology in the mid-1960s. Since ethylene is the material used in the chain growth portion of the reaction, the molecules are linear, unlike oxo alcohols. Due to their lack of branching, Ziegler alcohols are more similar to natural alcohols than to oxo alcohols. A comparison of natural fatty alcohol, oxo alcohol and Ziegler alcohol can be seen in Table 1.