Synthesis of methyl diphenylmethane dicarbamate by methyl phenylcarbamate condensation catalyzed by NKC-9: non-phosgene preparation of key precursors for diphenylmethane diisocyanate

Methyl diphenylmethane dicarbamate (MDC) was synthesized by condensation of methyl phenylcarbamate (MPC) using trioxane as methylenation reagents over sulfonic acid resin catalyst. The effect of Brønsted's acid amount on the condensation reaction was investigated, and the optimal sulfonic acid resin (NKC-9) with an acid amount of 4.57 was determined. 2-MDC, featuring a methylene-bridged biphenyl structure, achieved 91.6% selectivity and 83.2% yield after response surface methodology optimization. In situ FTIR spectroscopy was employed to monitor the condensation process. The spectroscopic results unequivocally demonstrate that trioxane undergoes catalytic depolymerization to formaldehyde over NKC-9 resin, which subsequently reacts with MPC to form MDC. After sulfuric acid regeneration, NKC-9 maintains its catalytic performance after 4 cycles.