Production of Aliphatic Isocyanates
HDI The largest volume diisocyanate, HDI, is produced from feedstocks that do not depend on the HDI manufacture for their economic structure. HDI is prepared from the phosgenation of hexane diamine in a manner quite analogous to that of aromatic polyisocyanates. Over 1100 thousand metric tons of hexamethylene diamine was produced in 2010 with an expected annual growth rate of about 2.5%. Most of this volume is prepared for manufacture of nylons (polyamides), and the diamine represents a backward integration into feedstocks . Feedstocks for adiponitrile are butadiene or alternatively acrylonitrile. Once adiponitrile is formed, the diamine is formed by reduction with hydrogen (Fig. 2.76).
From the diamine, the diisocyanate is formed from the acid-catalyzed reaction with phosgene to form the carbamoyl chloride, which converts to diisocyanates upon heating. As usual, the amine is in equilibrium with the hydrochloride salt formed by the evolved HC1, which requires the use of excess phosgene and heat to liberate the free amine (Fig. 2.77) .
IPDI IPDI is prepared by a multistep process beginning with the base-catalyzed aldol condensation of acetone to form isophorone isomers (Fig. 2.78) . Isophorone is subsequently converted to isophorone nitrile by reaction with hydrogen cyanide (Fig. 2.79)  and converted to diamine by catalytic reductive animation, followed
FIGURE 2.78 Trimerization of acetone to form isophone .
FIGURE 2.79 Conversion of isophorone to isophorone mtrile, hydrogenation to isophorone diamine and subsequent phosgenation to isophoronediisocyanate.
FIGURE 2.80 Conversion of MDA to hydrogenated MDA.
by formation of the isocyanate with phosgene . IPDI is formed in cis-/trans-isomers of the reactive centers believed to have equal reactivities. The nonequivalent isocyanate locations make for differential reactivity with the secondary isocyanate being more reactive due to steric factors. This reactivity difference is a valued attribute of IPDI since it allows for controlled and lower viscosity prepolymers (Section 2.17).
4,4'-Diisocyanatodicyclohexylmethane (H12MDI) Unlike HDI and IPDI, H12MDI derives from the urethane industrial base—the aniline/formaldehyde process to MDA. The economics and price sensitivities therefore mirror those of MDI and its feedstocks. The process to make H12MDI follows that of MDI up until the phosgenation step. At that point, a pure 4,4' MDA cut is distilled from the crude pMDA and hydrogenated .
Along with the predicted and desired reaction shown in Figure 2.80, other reactions can occur. One of the most common is deamination resulting in a monoamine. At lower temperatures and pressures, incomplete conversion to aliphatic structures is most common with one ring aliphatic and the other aromatic. This tendency for incomplete hydrogenation is a significant roadblock to expansion of the hydrogenation to the crude pMDI stream as produced. The hydrogenation is done in alcohol or dioxane media using rhodium or ruthenium on alumina catalyst [163, 164]. The use of more common hydrogenation catalysts will often entail the requirement of higher temperatures and the concomitant deamination.