Synthesis of MDI from Dimethyl Carbonate over Solid Catalysts

Abstract

The catalytic synthesis of methylene diphenyl-4,4′-diisocyanate (MDI) consists of three steps. Starting from the catalytic reaction of aniline and dimethyl carbonate (DMC), methyl phenyl carbamate (MPC) is formed. Then MPC condenses with formaldehyde to produce dimethyl methylene diphenyl-4,4′-dicarbamate (MDC). Last, MDC is decomposed to MDI. For the first step, the properties of supported zinc acetate catalysts on different supports have been examined. Supported zinc acetate catalyst on activated carbon (AC) or R-Al 2 O 3 shows good catalytic properties. Over Zn(OAc) 2 /AC catalyst, MPC yield reaches 78% and the selectivity is 98%. For the second step, when zinc chloride is used as a catalyst and nitrobenzene as a solvent, MDC yield can reach 87.4%. The catalytic activity of AC-supported ZnCl 2 catalyst, which is calculated based on 1 mol of ZnCl 2 , is much higher than that of homogeneous ZnCl 2 . For the third step, zinc powder and its organic salts show higher catalytic activity; MDI yield is 87.3% over zinc powder catalyst.

Important Green Chemistry and Catalysis: Non‐phosgene Syntheses of Isocyanates – Thermal Cracking Way

“…In summary, the results showed that the catalysts with surface Lewis acid sites with higher acid strength favored N-substituted carbamates using alkyl carbamates or aromatic carbamates as carbonyl source. [47] ZnCl 2 Nitro benzene, 100 ℃, 5 h, 9∶1 81 [48] ZnCl 2 /AC Nitro benzene, 140 ℃, 3 h, 4∶1 43 [49] 50% H 2 SO 4 H 2 O, 90 ℃, 2 h, 5∶1 74 [50] Amberlyst 15 CH 3 CN,75 ℃, 4 h, 4∶1 63 [51] Silicotungstic acid Diethylene glycol dimethyl ether, 100 ℃, 4.5 h, 8∶1 63 [52] Phosphotungstic acid Diethylene glycol dimethyl ether, 110 ℃, 4.5 h, 5∶1 65 [53] [ [54] Hβ DMC, 160 ℃, 3 h, 4∶1 76 [55] CO 2 as carbonyl source…”

“…In 2002, Zhang et al [82] employed zinc powder as catalysts for the synthesis of MDI using nitrobenzene-THF and di-n-octyl sebacate (DOS) as the mixed solvent and heat carrier, respectively. The highest MDI yield was 87.3% at ca.…”

Phosgene-free carbamoylation of aniline via dimethyl carbonate

“…Preliminary investigations were conducted employing homogeneous catalysts and comparing their reactivity with the one of zinc acetate, extensively investigated for the carbamoylation of aniline [12][13][14][15]. Scheme 2 shows the mechanism of carbamoylation of aniline via DMC chemistry.…”

“…According to the best results obtained for the carbamoylation of aniline employing homogeneous catalysts (Table 2) and to the bibliographic data available, lead, tin, copper, zinc, and bismuth salts were selected to be supported on silica or alumina [11][12][13][14]. Supported heterogeneous catalysts were investigated with the intent to easily recover the catalyst at the end of the reaction without using any timewasting separation and/or purification needed for the homogeneous catalysts.…”

Highly Selective Phosgene-Free Carbamoylation of Aniline by Dimethyl Carbonate under Continuous-Flow Conditions

“…Recently, several examples of carbamoylation of aniline, 9 4,4′-methylenedibenzenamine 10 (for the synthesis of methylene diphenyl diisocyanate, MDI) and 2,4-diaminotoluene 11 (for the synthesis of toluene diisocyanate TDI) have been reported by DMC chemistry in the presence of either homogeneous or supported catalysts. An example is the efficient carbamoylation of 2,4-diaminotoluene (DAT) by gold-catalyzed phosgene-free synthesis.…”