This invention relates to the production of aromatic polycarbamates and polyisocyanates. More particularly, it relates to the preparation of aromatic polycarbamates and polyisocyanates from alkylated aromatic compounds such as toluene via a multireaction step process.
Polyurethanes fill a very important commercial need in both the flexible and rigid plastic fields. For both the flexible and rigid types, the urethane is the product of the reaction of an alcohol and an isocyanate. Much effort and time is being spent on developing a means for producing these isocyanates in a less expensive and/or less toxic manner. Desirable isocyanates for flexible and rigid plastic applications include methylene bis-(phenyl isocyanate) (hereafter designated as MDI) and polymeric polyisocyanates such as polymethylene polyphenyl isocyanates (hereafter designated as PMPPI).
There are two conventional processes for the manufacture of isocyanates such as MDI or PMPPI, namely, phosgene technology and carbonylation technology.
(1) Phosgene technology which may be illustrated as follows: ##STR1## wherein n is zero or an integer ranging from 1 to 5 for the product is generally a mixture of various molecular weight oligomers.
Thus, for the desired isocyanate compounds n is zero for MDI and n is 1 to 5 for PMPPI. The reaction product providing the latter usually includes some MDI.
A variation of the phosgene technology employs the ammoxidation of xylene to the corresponding dinitrile, i.e. terephthalonitrile C.sub.6 H.sub.4 (CN).sub.2. However, the resulting dinitrile is converted to xylene diamine [C.sub.6 H.sub.4 (CH.sub.2 NH.sub.2).sub.2 [ which in turn is converted to 1,4-xylylene diisocyanate via phosgenation. This overall process is different from the present invention.
The phosgene approach is highly unsuitable since the reactant phosgene gas is highly toxic and the process highly wasteful of material and energy since a reactant HNO.sub.3 is manufactured by oxidation of ammonia (NH.sub.3) and after nitration of benzene to nitrobenzene it is reduced. One approach to overcoming the wastefulness of the process was to directly aminate the benzene with ammonia as is taught in U.S. Pat. No. 4,031,106. However, this approach is not yet commercially useful because of the inefficiency of the amination, even under severe reaction conditions.
(2) Carbonylation technology
In order to overcome some of the deficiencies of the phosgene technology, a carbonylation approach has been developed which may be expressed as follows: ##STR2## wherein n is as earlier defined and R represents a hydrocarbyl radical, usually a C.sub.1 to C.sub.5 lower alkyl such as methyl. (See for example: Chemical Week, Nov. 9, 1977, pp 57-58).
This carbonylation technology overcomes some of the defects of phosgene technology by eliminating use of the toxic phosgene gas, HCl and NaOH. However, there are defects in carbonylation technology which include:
(1) use of more expensive CO in place of hydrogen; PA1 (2) high pressure equipment is required for carbonylation of nitrobenzene to carbamate; PA1 (3) use of expensive metal compounds or toxic material, e.g. selenium as a catalyst; and, PA1 (4) recycle of unconverted CO to the high pressure reactor requires energy. PA1 1. ammoxidation of alkylated aromatics (preferably toluene) to form a nitrile containing compound; PA1 2. hydrolysis of aromatic nitrile (preferably benzonitrile) to form an amide; PA1 3. conversion of the amide to a carbamate; PA1 4. condensation of the carbamate with a carbonyl containing compound; and, optionally PA1 5. conversion of poly(carbamate) to polyisocyanate.
Because of these defects which are apparent in the teachings of U.S. Pat. No. 4,038,377 and German DOS No. 2,635,490, the carbonylation technology has little advantage over the phosgene technology.
It is an object of this invention to overcome many of the defects of the process technology currently used to produce MDI and PMPPI.