ONLINE EXCLUSIVE: A Novel Approach to the Synthesis of Aromatic and Aliphatic Isocyanates

July 1, 2004
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Aromatic, as well as aliphatic di- and poly-, isocyanates are used in the production of polyurethane foams, elastomers, fibres, adhesives and coatings. The most widespread application outlets are found in industries such as construction, furniture, automotive, refrigeration and footwear.

A very good insight on polyurethane chemistry and technology is given in References 1-2; Reference 3 explores the chemistry of isocyanates. Additional publications not cited in this article can be traced in the relevant literature.

The global plant capacity of di- and poly- (methylene diphenyl isocyanates) is estimated at around 2.3 million tons, including new investments in the Far East. Major producers are BASF, Bayer, Dow and Huntsman.

Figure 1. Synthesis of Polymeric Aromatic Isocyanates
Starting from benzene, the current synthesis of isocyanates of the MDI type are represented by the four steps in Figure 1.

The phosgenation reaction, which is expressed in an ideal form in Step 4, has very often been a source of research programs aimed at the substitution of phosgene gas by easier-to-handle chemicals.

A few examples of amine (either aliphatic or aromatic) conversion into a mono-isocyanate through the use of phosgene alternatives, along with some references, are given in Figure 2.

Figure 2. Conversion of Amines into Isocyanates without Phosgene
Most reactions in this figure involve the formation of an intermediate component (carbamate, anilide, urea derivatives), which must be thermally dissociated to the corresponding isocyanate. This means an additional manufacturing step is added to the existing process, thus increasing its costs. On the other hand, the interesting aspect of such a process is the attractive price of urea (or dimethyl carbonate) compared to that of phosgene. In addition, both urea and dimethyl carbonate are considered to be more environmentally friendly than phosgene.

Other patented, synthetic approaches that avoid the passage through the amine are shown in Figure 3.

Figure 3. Synthetic Paths to Aromatic Isocyanates
In this article, we would like to propose a different synthetic method, which offers the following advantages.
  • The reduction of the steps involved in the current industrial production processes (Figure 1) - a welcome advantage considering the ever-increasing prices of the raw materials and the subsequent pressures they exercise on the profit margins of companies.
  • Avoiding the hazardous phosgenation step, part of a continuing search for safer production methods.

Such a synthetic route is represented in a simplified form in Figure 4.

Figure 4. New Route to Isocyanate Synthesis
When the reaction shown in Figure 4 is carried out on a laboratory scale (in an alcoholic medium and in the presence of a catalyst), a carbamate is formed.

This is shown on the infrared spectra, sections of which are shown in Figures 5a-5b.

When the same reaction is carried out with a different set of catalysts - and in an aprotic solvent, thus eliminating the possibility of carbamate formation - the only product is an isocyanate.

The latter is identified by the NCO group absorption at 2250 cm-1 (see Figures 6a-6b).

Future work will involve the following.
  • Improvement of the reaction yield.
  • Synthesis of methylene diphenyl isocyanate.
  • Synthesis of linear aliphatic diisocyanates.
  • Eventual scaleup of the process.

For more information, contact Demosthenes (Deny) Kyriacos, GEM-Chem, Avenue G. Mullie 25, 1200 Brussels- Belgium, phone 011-32-2-7710649; or e-mail .

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