The advances in the routes to synthesize MDI are reviewed, putting emphasis upon discussing the clean synthesis methods of diphenylmethane diisocyanates by the catalytic cleavage of diphenylmethane dicarbamic acid esters (MDC).

The synthetic methods of diphenylmethane dicarbamic acid esters(MDC ) were reviewed with 31 references.

Then the catalytic activity of the ILs for the synthesis of methylene diphenyl dimethylcarbamate (MDC) from methyl phenyl carbamate (MPC) and formaldehyde (HCHO) was evaluated.

The synthesis of methylene diphenyl dimethylcarbamate(4,4’-MDC) from methyl N-phenyl carbamate(MPC) and formaldehyde was investigated in the presence of [emim]BF4 ionic liquid modified with H+ as both catalyst and reaction solvent.

Synthesis of methylene diphenyl dicarbamate in water-solvent system over mixed acid catalyst;

The synthesis of methylene diphenyl dicarbamate from N-phenyl carbamate(MPC) and formaldenhyde or trioxane with mixed acid as catalyst and water as solvent was studied.

The synthesis of methylene diphenyl dicarbamate(MDC) by the polycondensation reaction of methyl N-phenyl carbamate (MPC)with formaldehyde (HCHO) with phosphotungstic acid as catalyst,was investigated in this paper.

4,4 -Diphenylmethane diisocyanate was derivatized with methanol into 4,4 -methylene diphenyl dicarbamate so as to protect the active NCO group and avoid the usage of 4,4 -diphenylmethane diisocyanate standard which is difficult to transportation and storage.

HPLC analysis of the raw material and the product obtained during decomposition of 4,4\'-methylene di(phenylene carbamate)(4,4\'-MDC)for preparing 4,4\'-methylene di(phenylene isocyanate)(4,4\'-MDI)was studied in this paper.