ABSTRACT
Isocyanates and diisocyanates are important chemical intermediates for the pesticide, fertilizer, polymer, and pharmaceutical industries. The current process for isocyanate synthesis involves the use of highly toxic phosgene gas as follows:
Isocyanate synthesis from phosgene RNH2 + COC12 -> RNCO + HC1
where R is an alkyl or aryl group. The phosgene toxicity, separation of hydrochloric acid from excess
phosgene, and the use of chlorinated solvents as a reaction medium are the major drawbacks of this reaction process. Extensive studies have suggested that carbamate and dicarbamate can serve as environmentally benign precursors for the synthesis of isocyanate and diisocyanate (1-5). Figure 1 illustrates nonphosgene routes for the synthesis of two important diisocyanates [i.e., 4,4'- diphenylmemane diisocyanate (MDI) and toluene diisocyanate(TDI)] via carbonate and carbamate.
