Method for manufacture of diphenylmethane diisocyanates

Manufacture of a diphenylmethane diisocyanate for an N-phenylcarbamate is economically accomplished in high yield with high selectivity by a method which comprises (A) a process of methylenation for the formation of a condensation product containing at lest 80 mol % of a dinuclear diphenylmethane dicarbamate by the steps of (1) causing a methylenating agent to react upon at least 2 moles of an N-phenylcarbamate, based on 1 mole of the methylene group of said methylenating agent, in a liquid phase in the presence of an aqueous inorganic acid solution, (2) separating the resultant reaction mixture into the aqueous inorganic acid solution and an organic phase reaction mixture containing substantially none of said inorganic acid, and (3) subsequently treating said organic phase reaction mixture in the presence of an N-phenylcarbamate and a carboxylic acid having a pKa of not more than 4 in an aqueous solution at a temperature of 25.degree. C. or a solid acid or an acid consisting of said two acids thereby converting a reaction intermediate possessing a methylene-amino bond and contained in said organic phase reaction mixture to a diphenylmethane dicarbamate, and (B) a process of thermal decomposition described herein.

FIELD OF THE INVENTION 
This invention relates to a method for economically manufacturing 
diphenylmethane diisocyanates in high yields from N-phenylcabamates. 
BACKGROUND OF THE INVENTION 
The mixture consisting of diphenylmethane diisocyanate (hereinafter 
referred to as "MDI") and polymethylene polyphenyl isocyanate (hereinafter 
referred to as "PMPPI"), a higher homolog of MDI, is popularly called 
"crude MDI" and is mass-produced on a commercial scale as an important raw 
material for polyurethanes. 
The aforementioned dinuclear substance, MDI, is mainly composed of 
4,4'-diphenylmethane diisocyanate (namely, pure MDI)and is generally 
isolated from crude MDI by distillation. In recent years, the demand for 
this dinuclear MDI as the raw material such as for polyurethane elastomer, 
Spandex, synthetic leather coating agent, and reaction injection molding 
polyurethane is rapidly increasing. Accordingly, the desirability of 
developing a method capable of economically producing the crude MDI 
containing the dinuclear MDI in a high concentration and entrainning other 
isomers than the 4,4'-isomer in a relatively low concentration has been 
finding growing recognition. 
For the production of this crude MDI, for example, there has been adopted 
the method which obtains the desired crude MDI by causing condensation of 
aniline with formaldehyde in the presence of an acid catalyst thereby 
forming a mixture consisting of diphenylmethane diamine and polymethylene 
polyphenylamine (hereinafter referred to as "crude MDA"), subsequently 
allowing phosgene to react upon the crude MDA in a solvent thereby 
deriving a corresponding carbamic acid choride, then converting the 
carbamic acid chloride through thermal decomposition into crude MDI and 
hydrogen chloride, and expelling from the resultant reaction mixture both 
hydrogen chloride and the reaction solvent. 
For the production of crude MDA containing dinuclear diphenylmethane 
diamine (hereinafter referred to as "MDA") in a high concentration, this 
conventional method must rely on the use of aniline and an acid catalyst 
such as hydrochloric acid both in large excesses relative to formaldehyde. 
Consequently, the amount of base which must be used for the purpose of 
neutralization is inevitably large. Besides, the recovery of the unaltered 
aniline is costly. Thus, the method proves to be uneconomical. Moreover, 
this method has the disadvantage that in spite of an increase in the 
proportion of the dinuclear component in the crude MDA, the proportion of 
the 4,4'-isomer in the dinuclear component decreases and the proportions 
of the rather unwanted isomers, i.e. 2,4'-MDA and 2,2'-MDA, are increased. 
Further, this conventional method is disadvantageous in that the process 
involved necessitates the use of violently poisonous phosgene, that the 
use of phosgene entails generation of corrosive hydrogen chloride in a 
large volume, that the final product entrains hydrolyzable chlorine 
compounds, and that these by-productss are extremely difficult to separate 
away. For the purpose of eliminating all these drawbacks, research is 
under way in search of new processes capable of producing MDI without the 
use of phosgene. 
As one solution for the elimination of the use of phosgene, for example, 
the method which comprises causing condensation of N-phenylcarbamate with 
formaldehyde thereby giving rise to a mixture consisting of 
diphenylmethane dicarbamate and polymethylene polyphenylcarbamate, a 
higher homolog of diphenylmethane dicarbamate,, ad subsequently subjecting 
this mixture to thermal decomposition has been proposed (U.S. Pat. Nos. 
4,349,484 and 4,307,029 and European Patent Nos. 28,337 and 30,039). 
These methods under development for the aforementioned purpose, however, 
are such that the proportions of the dinuclear MDI in the produced crude 
MDI fall roughly in a low range of 40 to 78%, the range virtually 
comparable with the range usual with the process using phosgene. Thus, 
these methods are not satisfactory. 
Various other methods have been proposed which produce condensation 
mixtures consisting of diphenylmethane dicarbamates and their higher 
homologs by the condensation of N-phenylcarbamates with formaldehydes. For 
example, the method which resorts to the reaction of N-phenylcarbamates 
with condensing agents such as formaldehyde, paraformaldehyde, methylal, 
and trioxane in the presence of various acids such as mineral acids and 
organic sulfonic acids. If relatively severe conditions are used in this 
reaction, for example, if a strong acid is used in a large amount, the 
reaction temperature is high or the reaction period is extended, not only 
is the desired diphenylmethane dicarbamate produced but also polynuclear 
polymethylene polypenylcarbamates having the following formula are 
produced in a significant amount: 
##STR1## 
(wherein R is an alkyl group, aromatic group or an alicyclic group; z is 
an integer of 1 or more). Furthermore, if a strong liquid acid is used, 
much difficulty and hence a lot of cost is entailed in separating the acid 
from the reaction mixture and recovering the same in a reusable form. 
In order to eliminate this defect with the recovery of acids, a method was 
proposed for using an aqueous acid solution having a concentration of 10% 
or higher (British Patent No. 2,044,252, Japanese Patent (OPI) Nos. 
81850/80 and 81851/80 and Chemical Abstracts 93 169057e). This method is 
effective for acid recovery because as shown in the working examples, if 
aqueous acid solutions having a concentration of not more than 50% are 
used, the acid can fairly easily be separated from the organic phase in 
the form of layers. However, the presence of a great amount of water 
renders it difficult to complete the reaction without leaving a 
significant amount of compounds having a methylene-amino bond(--CH.sub.2 
--N&lt;) wherein the methylene group is bonded to the nitrogen atom in the 
carbamate group. In order to complete the reaction without these 
compounds, less water must be used to increase the acid concentration to, 
for example, 80% or higher. However, this causes the hydrolysis of the 
starting compound or the reaction product, or leaves them to dissolve in 
the concentrated aqueous acid solution in a large quantity, and as a 
result, the separation of the product from the acid solution becomes 
difficult. 
In any event, it is not industrially advantageous to carry out a one-step 
condensation of N-phenylcarbamates with an aqueous solution of acid and to 
use the resulting product in the preparation of isocyanates. More 
specifically, dinuclear, trinuclear or other polynuclear compounds having 
the methylene-amino bond cannot be easily separated from the condensation 
product containing diphenylmethane dicarbamates and polymethylene 
polyphenylcarbamates. If the condensation product containing these 
compounds with the methylene-amino bond is decomposed thermally, these 
compounds do not provide the desired isocyanates. Furthermore, they enter 
into various side reactions with the isocyanates derived from the 
carbamates such as diphenylmethane dicarbamates, and in consequence, the 
yields of the desired isocyanates are reduced. In addition, the resulting 
by-products cannot be easily separated from the desired isocyanates, 
particularly, the polynuclear polymethylene polyphenyl isocyanates, and 
they are in all cases present in the final product generally referred to 
as a polymeric isocyanate, and properties of the product are impaired. 
It is therefore necessary to perform the condensation of N-phenylcarbamates 
in such a manner that a minimum amount of the compounds with the 
methylene-amino bond is left in the condensation product. One method that 
has been proposed for attaining this object is described in U.S. Pat. No. 
4,146,727, wherein these compounds with the methylene-amino bond are 
subjected to a rearrangement reaction, under substantially anhydrous 
conditions, with a protonic acid catalyst having a strength of at least 
the magnitude of a 75% sulfuric acid, or a Lewis acid at a temperature of 
50.degree. to 170.degree. C. so as to rearrange the methylene group, which 
was bonded to the nitrogen atom, to bond to the benzene ring. However, 
this method must use a large amount of concentrated sulfuric acid or 
paratoluenesulfonic acid and again requires complicated procedures and 
great cost for separating and recovering these acids from the reaction 
mixture. 
Japanese Patent (OPI) No. 7749/81 and Chemical Abstracts, 94 209480s 
propose a method for producing polymethylene polyphenylcarbamate by 
heating only bis(N-carboalkoxyanilino)methane, which is a compound having 
the methylene-amino bond, in the presence of an acid catalyst. However, 
this method is not ideal for selective production of the diphenylmethane 
dicarbamate because it causes not only the desired rearrangement reaction 
but also the undesired condensation reaction, and trinuclear and other 
polynuclear polymethylene polyphenylcarbamates are formed as by-products 
in addition to the desired diphenylmethane dicarbamate. Furthermore, the 
reaction is slow and the rearrangement reaction is not completed without 
leaving the residual bis(N-carboalkoxyanilino)methane in the reaction 
product. 
U.S. Pat. No. 4,319,018, British Patent No. 2,054,584, Japanese Patent 
(OPI) No. 12357/81 and Chemical Abstracts, 94, 124715t propose a method 
for producing diphenylmethane dicarbamates and polymethylene 
polyphenylcarbamates by reacting N-phenylcarbamates with formaldehyde or 
its precursor in the presence of both an acid catalyst and the compounds 
having the methylene-amino bond. However, this method is unable to reduce 
the content of the compounds with the methylene-amino bond, and the 
compounds are unavoidably left in the condensation product in an amount as 
much as ten-odd percent by weight. 
SUMMARY OF THE INVENTION 
It has been found that the various methods heretofore proposed for 
producing crude MDI of diphenylmethane dicarbamate and polymethylene 
polyphenylcarbamates, intermediates for crude MDI, from N-phenylcarbamates 
without using phosgene have numerous drawbacks which hinder their 
reduction to commercialization. 
Particularly in the case of the methods which have heretofore been proposed 
for economic production of dinuclear MDI in high yields exceeding 80%, 
including those involving use of phosgene, none of them has proved 
completely satisfactory. 
The inventors, therefore, continued a diligent study with the object of 
developing a method capable of economic production of dinuclear MDI in 
high yields by a novel procedure without using phosgene. They have 
consequently acquired a knowledge that this object is attained by 
combining a specific process of methylenation using N-phenylcarbamate as 
the raw material with a specific process of thermal decomposition. The 
present invention has been completed based on this knowledge. 
To be specific, this invention provides a method for the manufacture of a 
diphenylmethane diisocyanate from a N-phenylcarbamate, comprising: 
(A) a process of methylenation for the formation of a condensation product 
containing at least 80 mol % of a dinuclear diphenylmethane dicarbamate by 
the steps of 
(1) causing a methylenating agent to react upon at least 2 moles of a 
N-phenylcarbamate, based on 1 mole of the methylene group of the 
methylenating agent, in a liquid phase in the presence of an aqueous 
inorganic acid solution. 
(2) separating the resultant reaction mixture into the aqueous inorganic 
acid solution and an organic phase reaction mixture containing 
substantially none of the aforementioned inorganic acid, and 
(3) subsequently treating the aforementioned organic phase reaction mixture 
in the presence of a N-phenylcarbamate and a carboxylic acid having the 
Pka value of not more than 4 in an aqueous solution at a temperature of 
25.degree. C. and/or a solid acid thereby converting a reaction 
intermediate possessing a methylene-amino bond contained in the 
aforementioned organic phase reaction mixture to a diphenylmethane 
dicarbamate, and 
(B) a process of thermal decomposition by the steps of allowing a mixture 
comprising 1 to 50% by weight of the condensation product obtained in the 
preceding process of (A) and 99 to 50% by weight of a thermal 
decomposition solvent having a boiling point under atmospheric pressure in 
the range of 120.degree. to 350.degree. C. and being inactive to 
isocyanates to flow down into a reactor maintained at temperature in the 
range of 180.degree. to 380.degree. C. through the upper part thereof, 
causing the mixture to come into counterflow contact with a carrier 
introduced into the reactor upwardly via the lower part thereof thereby 
producing an organic hydroxyl compound, allowing the organic hydroxyl 
compound to be discharged from the reactor in the form of vapor in 
conjunction with the carrier through the upper part thereof, and 
withdrawing the resultant isocyanate solution from the reactor through the 
lower part thereof. 
As described above, by the method of this invention, the dinuclear 
diphenylmethane dicarbamate is produced from the N-phenylcarbamate in 
yields of at least 80% and then MDI is produced in high yields with high 
selectivity by allowing the condensation product obtained as described 
above to flow down into a thermal decomposition reactor through the upper 
part thereof in the presence of a solvent and causing the introduced 
condensation product to come into counterflow contact with the carrier 
introduced upwardly into the reactor through the lower part thereof 
thereby causing thermal decomposition of the condensation product. 
Further, since the thermal decomposition is carried out in such a specific 
manner as described above and since the carbamate subjected to the thermal 
decomposition is composed mainly of the diphenylmethane dicarbamate, a 
dinuclear substance, the method of this invention is characterized by 
suffering the formation of any by-products only minimally during the 
process of thermal decomposition, enabling the reaction of thermal 
decomposition reaction to proceed at a high rate, and permitting the 
decomposition to be carried out easily in an industrially advantageous 
continuous operation. 
Moreover, the method of this invention is characterized by the fact that 
the process of methylenation is commercially advantageous because it not 
only produces the dinuclear diphenylmethane dicarbamate in high yields but 
also permits the catalyst used therein to be easily separated from the 
reaction mixture and put to reuse. 
DETAILED DESCRIPTION OF THE INVENTION 
The N-phenylcarbamates used in the process of the present invention are the 
compounds represented by formula (I): 
##STR2## 
wherein R is an alkyl group having from 1 to 20 carbon atoms, preferably 
from 1 to 10 carbon atoms, aromatic group or an alicyclic group having 3 
to 30 carbon atoms, preferably from 5 to 18 carbon atoms; R' is hydrogen 
or a substituent such as an alkyl group having from 1 to 20 carbon atoms, 
halogen atom, nitro group, cyano group, alkoxy group having from 1 to 20 
carbon atoms or alicyclic group having from 3 to 20 carbon atoms, provided 
that these substituents are bonded at the ortho- or meta-position to the 
urethane group; r is an integer of 0 to 4; when r is 2 or more, R' may 
represent the same or different substituents; and at least one hydrogen in 
R may be substituted by any of the substituents listed above. 
Preferred examples of R include alkyl groups such as methyl, ethyl, 
2,2,2-trichloroethyl, 2,2,2-trifluoroethyl, propyl (n- or iso-), butyl (n- 
and various isomers), pentyl (n- and various isomers) and hexyl (n- and 
various isomers), alicyclic groups such as cyclopentyl and cyclohexyl and 
aromatic groups such as phenyl and naphthyl; and preferred examples of R' 
include hydrogen, the alkyl groups and alicyclic groups listed above, 
halogens such as fluorine, chlorine, bromine and iodine, nitro groups, 
cyano groups and alkoxy groups having the alkyl moieties listed above. 
Preferred examples of the N-phenylcarbamates represented by formula (I) 
include methyl N-phenylcarbamate, ethyl N-phenylcarbamate, propyl 
N-phenylcarbamate (its isomers), butyl N-phenylcarbamate (its isomers), 
pentyl N-phenylcarbamate (its isomers), hexyl N-phenylcarbamate (its 
isomers), cyclohexyl N-phenylcarbamate, 2,2,2-trichloroethyl 
N-phenylcarbamate, 2,2,2-trifluoroethyl N-phenylcarbamate, methyl N-o- (or 
m-)tolylcarbamate, ethyl N-o- (or m-)tolylcarbamate, 2,2,2-trifluoroethyl 
N-o- (or m-)tolylcarbamate, propyl N-o- (or m-)tolylcarbamate (its 
isomers), butyl N-o- (or m-)tolylcarbamate (its isomers), methyl N-o- (or 
m-)chlorophenylcarbamate, ethyl N-o- (or m-)chlorophenylcarbamate, propyl 
N-o- (or m-)chlorophenylcarbamate (its isomers), butyl N-o- (or 
m-)chlorophenylcarbamate (its isomers), 2,2,2-trifluoroethyl N-o- (or 
m-)chlorophenylcarbamate, methyl N-2,6-dimethylphenylcarbamate, ethyl 
N-2,6-dimethylphenylcarbamate, propyl N-2,6-dimethylphenylcarbamate (its 
isomers), butyl N-2,6-dimethylphenylcarbamate (its isomers), 
2,2,2-trifluoroethyl N-2,6-dimethylphenylcarbamate, methyl 
N-2,6-dibromophenylcarbamate, ethyl N-2,6-dibromophenylcarbamate, propyl 
N-2,6-dibromophenylcarbamate (its isomers), butyl 
N-2,6-dibromophenylcarbamate (its isomers), and 2,2,2-trifluoroethyl 
N-2,6-dibromophenylcarbamate. 
The present invention does not discriminate with respect to the 
N-phenylcarbamate to be used as the raw material by the method by which it 
is produced. Examples of the method by which this raw material is produced 
include a method which resorts to reductive alkoxycarbonylation of a 
suitable aromatic nitro compound with carbon monoxide and an alcohol, a 
method which resorts to reaction of a suitable aromatic amine with a 
chloroformic ester, a method which resorts to interamination of a suitable 
aromatic amine and an N-unsubstituted carbamate, a method which resorts to 
reaction of a suitable aromatic amine with an alcohol and a urea, and a 
method which resorts to oxidative alkoxycarbonylation of a suitable 
aromtic amine and/or N,N'-diaryl urea with carbon monoxide and an alcohol 
in the presence of an oxidizing agent. 
From the standpoint of commercialization, the method resorting to the 
oxidative alkoxycarbonylation of an aromatic amine and/or N, N'-diaryl 
urea proves particularly advantageous over any other of the methods cited. 
Concerning the production of N-phenylcarbamates by this particular method, 
the inventors have already developed a catalyst system which assists in 
producing the N-phenylcarbamate in high yields with high selectivity, 
permits the reaction to proceed quickly, enables itself to be easily 
recovered from the reaction mixture and put to recurrent use, and enables 
the production of the N-phenylcarbamate to be carried out quite 
economically (European Patent No. 83,096). 
This catalyst for the oxidative alkoxycaronylation comprises: 
(a) at least one member selected from the group consisting of platinum 
group metals and compounds containing at least one platinum group element; 
and (b) at least one halogen-containing compound selected from the group 
consisting of alkali or alkaline earch metal halides, onium halides, 
compounds capable of forming onium halides in the reaction, oxo acids of 
halogen atoms and their salts, complex compounds containing halogen ions, 
organic halides and halogen molecules. 
One particularly preferred embodiment of the invention relating to this 
catalyst resides in the use of a catalyst system which comprises metallic 
palladium and an iodine compound (such as an alkali metal iodide or 
quaternary ammonium iodide). 
Illustrative methylenating agents that can be used in the present invention 
include formaldehyde, paraformaldehyde, trioxane, tetraoxane, 
dialkoxymethane, diacyloxymethane, 1,3-dioxolane, 1,3-dioxane, 
1,3-dithiane, 1,3-oxathiane, and hexamethylenetetramine. Preferred 
compounds are formaldehyde, paraformaldehyde, trioxane and 
dialkoxymethanes having the lower alkyl groups of 1 to 6 carbon atoms such 
as dimethoxymethane, diethoxymethane, dipropoxymethane, dipentanoxymethane 
and dihexyloxymethane, as well as diacyloxymethanes having the lower 
acyloxy groups such as diacetoxymethane and dipropioxymethane. These 
methylenating agents may be used either alone or in combination. A 
particularly preferred methylenating agent is an aqueous solution of 
formaldehyde. One feature of the present invention is its ability to 
produce diphenylmethane dicarbamates in high selectivity using the least 
expensive methylenating agent such as an aqueous formaldehyde. 
This process of methylenation is started by the first step which causes a 
N-phenylcarbamate to react with the aforementioned methylenating agent at 
a temperature in the range of 40.degree. to 150.degree. C. by using an 
aqueous inorganic acid solution as a catalyst. Suitable inorganic acids 
include hydrochloric acid, sulfuric acid, phosphoric acid, polyphosphoric 
acid, heteropolyacid and boric acid. Sulfuric acid is particularly 
preferred. The concentration of the inorganic acid in its aqueous solution 
preferably ranges from 20 to 70 wt %, and the range of 30 to 60 wt % is 
particularly preferred. The most preferred is an aqueous solution 
containing 40 to 60 wt % of sulfuric acid. If the concentration of the 
inorgnic acid exceeds 70 wt %, the N-phenylcarbamate and the condensation 
products are hydrolyzed to form the corresponding amino compounds. These 
amino compounds are not desirable since it causes various bad side 
reactions when the diphenylmethane dicarbamate produced is subsequently 
converted to an isocyanate by thermal decomposition. Furthermore, such a 
highly concentrated acid solution dissolves a significant amount of the 
starting materials and the reaction product therein, so that the 
separation of the organic phase from the mixture is performed only with 
complicated procedures. On the other hand, if the concentration of the 
inorganic acid is less than 20 wt %, the reaction is too slow to suit 
practical purposes. 
In the first reaction step of the methylenation process, at least 2 mols, 
preferably 2.2 to 10 mols, more preferably 2.5 to 6 mols, of the 
N-phenylcarbamate is used per mol equivalent of the methylene group of the 
methylenating agent. The aqueous solution of inorganic acid is used in 
such an amount that it preferably contains 0.01 to 20 mol equivalents, 
more preferably 0.05 to 15 mol equivalents, most preferably 0.1 to 10 mol 
equivalents, of the inorganic acid per mol equivalent of the 
N-phenylcarbamate. 
The first reaction step of the methylenation process may be performed in a 
two-component dispersion made of organic and aqueous phases using water as 
the reaction medium. Alternatively, the reaction may be performed in a 
two-component dispersion made of an aqueous phase and an organic phase 
using an organic solvent. In either case, it is preferred that the most 
finely dispersed liquid droplets be formed throughout the reaction. 
Preferred organic solvents are those which have boiling points of not 
higher than 300.degree. C. at atmospheric pressure and which have a mutual 
solubility with water of not more than 10% at room temperature. If organic 
solvents having a mutual solubility with water of not more than 10% are 
used, the organic phase containing the diphenylmethane dicarbamate and 
other condensates can be readily separated from the aqueous phase 
containing the inorganic acid by simple means such as phase separation 
after the first reaction. If, on the other hand, organic solvents having 
boiling points of not higher than 300.degree. C. at atmospheric pressure 
are used, these solvents can be separated from the organic-phase reaction 
mixture by simple means such as distillation. 
Preferred organic solvents include aromatic compounds having electron 
attracting substituents or halogen atoms. Suitable electron attracting 
substituents include nitro, cyano, alkoxycarbonyl, sulfonate, 
trifluoromethyl and trichloromethyl groups. These aromatic compounds are 
substantially inert to the electrophilic substitution of the methylene 
group under the conditions used for the first reaction step of the 
methylenation process. Furthermore, these aromatic compounds have great 
ability to dissolve not only the N-phenylcarbamates (used as one of the 
starting materials) but also the diphenylmethane dicarbamates finally 
produced. 
A particularly preferred electron attracting group is a nitro group. 
Preferred examples of the aromatic compounds having a nitro group or a 
halogen atom or both include nitrobenzene and lower alkyl substituted 
nitrobenzenes such as nitrotoluene (its isomers), nitroxylene (its 
isomers), nitromesitylene and nitroethylbenzene (its isomers); halogen 
substituted nitrobenzenes such as chloronitrobenzene (its isomers) and 
bromonitrobenzene (its isomers); halogenated benzenes such as 
chlorobenzene, dichlorobenzene (its isomers), trichlorobenzene (its 
isomers), bromobenzene, dibromobenzene (its isomers) and tribromobenzene 
(its isomers); halogenated naphthalenes such as chloronaphthalene (its 
isomers), dichloronaphthalene (its isomers) and bromonaphthalene (its 
isomers); and lower alkyl substituted halogenated benzenes such as 
chlorotoluene (its isomers), dichlorotoluene (its isomers), ethyl 
chlorobenzene (its isomers), chloroxylene (its isomers), bromotoluene (its 
isomers) and bromoxylene (its isomers). Particularly preferred organic 
solvents are nitrobenzene, chlorobenzene, and dichlorobenzene (its 
isomers). 
In the first reaction step of the methylenation process, the reaction is 
carried out at a temperature in the range of 40.degree. to 150.degree. C., 
preferably 60.degree. to 130.degree. C., more preferably 70.degree. to 
110.degree. C. The pressure used herein is in the range of 0.5 to 20 
kg/cm.sup.2, preferably 0.8 to 10 kg/cm.sup.2. Generally, the reaction is 
carried out under atmospheric pressure or under a low pressure. The 
reaction period varies with the type, the concentration and the amount of 
the aqueous solution of inorganic acid and the reaction temperature. The 
reaction period also depends on whether any organic solvent is used, or on 
the type of the reactor used. Since it is preferred that the smallest 
possible amount of the methylenating agent is left in the reaction mixture 
coming out of the first reaction step of the methylenation process, the 
duration of the first reaction generally ranges from several minutes to 
several hours, preferably 10 minutes to 6 hours, more preferably 0.5 to 3 
hours. The reaction may be performed either batchwise or continuously. 
The reaction mixture obtained in the first reaction step of the 
methylenation process is separated into the aqueous solution of inorganic 
acid and an organic-phase reaction mixture substantially free from the 
inorganic acid, and it is preferable that resulting aqueous solution of 
inorganic acid is returned to the first reaction step either immediately 
or after the adjustment of the aqueous solution of inorganic acid to the 
predetermined concentration if necessary. 
While there is no particular limitation on the method of separating the 
aqueous solution of inorganic acid from the organic-phase reaction 
mixture, the simple phase-separation technique can be used for the purpose 
under the conditions specified for the present invention. The following 
phase-separation methods may be used: according to one method, the 
reaction mixture is cooled, without using an organic solvent, to a 
temperature close to or lower than room temperature, and in this case, the 
organic-phase reaction mixture forms a solid phase and can be readily 
separated from the aqueous solution of inorganic acid by simple means such 
as filtration. According to the other method, the reaction mixture is 
dissolved in the organic solvent described above or heated to a 
temperature over 50.degree.-60.degree. C., and in this case, two 
immiscible liquid phases (organic phase and aqueous phase) form and can 
readily be separated from each other. 
The organic-phase reaction mixture thus separated from the aqueous solution 
of inorganic acid may sometimes contain a small amount of the inorganic 
acid, which is preferably removed by a suitable method such as washing 
with water. If the inorganic acid remains unremoved from the final 
condensation product, it causes undesirable side reactions or corrodes the 
reactor during the subsequent thermal decomposition of the condensation 
product for producing the isocyanates. 
The concentration of the inorganic acid in its aqueous solution that has 
been separated from the organic-phase reaction mixture in the separation 
step is generally lower than the initial value because water is produced 
in the first reaction step of the methylenation process if a formaldehyde 
is used as the methylenating agent, and if an aqueous solution of 
formaldehyde is used, there is also a corresponding increase in the water 
content. Therefore, if one wants to perform the first reaction under 
constant conditions, the concentration of the inorganic acid must be 
increased to a predetermined level for re-use. For the purposes of the 
present invention, the preferred concentration of the inorganic acid 
solution used in the first reaction step of the methylenation process 
ranges from 20 to 70 wt %, and a particularly preferred range is from 30 
to 60 wt %. As the concentration of this acid is relatively low, the 
concentration can be readily attained by dehydration with less efforts 
than are required for concentrating a diluted acid solution to a highly 
concentrated acid. Needless to say, the aqueous solution of inorganic acid 
separated may be immediately returned to the first reaction step if the 
concentration of the inorganic acid is within the range described above. 
Then, the second reaction step of the methylenation process is carried out. 
This step comprises subjecting the resultant organic phase reaction 
mixture containing substantially no inorganic acid to a treatment at a 
temperature in the range of 40.degree. to 200.degree. C. in the presence 
of a N-phenylcarbamate and a carboxylic acid having a pka of not more than 
4 in an aqueous solution at a temperature of 25.degree. C. and/or a solid 
acid. 
In the subsequent second reaction step of the methylenation process, the 
reaction is preferably carried out in the presence of a minimum amount of 
water because water has an undesirable influence on the reactivity of the 
reactants and the reaction rate. Water is particularly undesirable if a 
carboxylic acid is used as the catalyst because this must be finally 
separated from water. Therefore, it is desired that as much water as 
possible be removed from the organic-phase reaction mixture obtained in 
the separation step. One method for attaining this object is by azeotropic 
distillation in the presence of an azeotropic agent. If an organic solvent 
is used in the first reaction step of the methylenation process, the 
distillation of the water can be achieved simultaneously with the 
distillation of a portion of or all of this organic solvent. 
In the second reaction step of the methylenation process, the reaction is 
preferably performed in the substantial absence of a methylenating agent. 
If the organic-phase reaction mixture that has been subjected to phase 
separation and optional washing with water still contains a methylenating 
agent, the methylenating agent is preferably removed simultaneously with 
the removal of the water from the mixture. However, if formaldehyde or its 
precursor which generates formaldehyde in the reaction system is used as a 
methylenating agent, it seldom occurs that such a methylenating agent is 
left in the organic-phase reaction mixture because formaldehyde or its 
precursor is in most cases water-soluble. 
The organic-phase reaction mixture thus obtained is substantially free from 
the methylenating agent, but it does contain the intermediate compounds 
with the methyleneamino bond (--CH.sub.2 --N&lt;), for example, 
bis(N-carboalkoxyanilino)methane and 
(N-carboalkoxyanilinomethyl)phenylcarbamate. The purpose of the second 
reaction step is to convert these compounds to diphenylmethane 
dicarbamates by an easy and simple method, and it is essential that in 
this second reaction step, the reaction must be carried out in the 
presence of N-phenylcarbamates. This object can be achieved by carrying 
out the intermolecular transfer reaction of the intermediate compounds 
with an N-phenylcarbamate. 
As described before, one conventional method to convert these intermediate 
compounds having the methyleneamino bond to diphenylmethane dicarbamates 
and polymethylene polyphenylcarbamates has been proposed (see U.S. Pat. 
No. 4,146,727). This method, however, consists of the intramolecular 
rearrangement and condensation reactions of the intermediate compounds, so 
that it requires a very strong protonic acid having a strength equal to or 
greater than a 75% concentrated sulfuric acid or a very strong Lewis acid 
such as antimony pentafluoride, and it also requires a considerable length 
of the reaction time, in order to complete the reaction. On the other 
hand, according to the process of the present invention, the compounds 
having the methylene-amino bond are subjected to an intramolecular 
transfer reaction with an N-phenylcarbamate which may be the same as or 
different from the N-phenylcarbamate used as the starting material. 
Therefore, the process of the present invention does not require the use 
of an acid as strong as what is used in the conventional method that 
depends on intramolecular rearrangement reaction of the compounds having 
the methylene-amino bond. Instead, the present invention used a much 
weaker carboxylic acid having a pKa of not more than 4, preferably from 3 
to -4, more preferably from 2.5 to -4, in an aqueous solution at 
25.degree. C. or solid acid. Even in the presence of this weak acid, the 
process of the present invention permits the intended reaction to proceed 
quantitatively at a fast rate, and the desired diphenylmethane 
dicarbamates can be obtained with high selectivity. 
For the sake of clarity, the process of the intermolecular transfer 
reaction carried out in the second reaction step of the methylenation 
process is illustrated below with reference to the case where an 
unsubstituted N-phenylcarbamate is reacted with the compound having the 
methylene-amino bond: 
##STR3## 
(wherein R" may be the same or different from R). 
As shown above, in the reaction between the di-nuclear compound having the 
methylene-amino bond with the N-phenylcarbamate, the N-phenylcarbamate as 
one of the reactants is regenerated and a compound wherein R" is replaced 
by R is also formed. But in any event, one of the reaction products 
obtained is a dinuclear diphenylmethane dicarbamate which can be used as a 
starting material for the production of diphenylmethane diisocyanates. In 
commercial operation, R and R" are usually the same and the production of 
the above mentioned by-product can be avoided. 
Further, trinuclear and other polynuclear compounds having the 
methylene-amino bond can be converted to diphenylmethane dicarbamates as 
illustrated below. 
##STR4## 
As will be understood from the reaction schemes for the reaction of the 
compounds having the methylene-amino bond, even if the compounds having 
the methylene-amino bond are reacted with an N-phenylcarbamate used in an 
amount less than one equivalent of the methylene-amino bond, the desired 
diphenylmethane dicarbamate can be produced, because an N-phenylcarbamate 
is also formed as a by-product in the course of the intermolecular 
transfer reaction. However, in this case the reaction is slow. Therefore, 
in order to increase the reaction rate and enhance the selectivity for the 
diphenylmethane dicarbamates, the N-phenylcarbamate is preferably present 
during the intermolecular transfer reaction in an amount greater than one 
equivalent of the methylene-amino bond in the intermediate. If the amount 
of the N-phenylcarbamate remaining unreacted in the organic-phase reaction 
mixture is not sufficient for this purpose, an additional amount of the 
N-phenylcarbamate is preferably incorporated in the second reaction step 
of the methylenation process. For the purpose, N-phenylcarbamate is 
preferably present in an amount of from 1 to 200 mol equivalents, more 
preferably from 5 to 100 mol equivalents, per equivalent of the 
methylene-amino bond. 
As described above, the greatest feature of the second reaction step of the 
methylenation process is to use at least one catalyst selected from the 
group consisting of a carboxylic acid which has a pKa or not more than 4 
in an aqueous solution at 25.degree. C., and a solid acid. Suitable 
carboxylic acids meeting this requirement include formic acid; halogenated 
acetic acids such as fluoroacetic acid, difluoroacetic acid, 
trifluoroacetic acid, chloroacetic acid, dichloroacetic acid, 
trichloroacetic acid, bromoacetic acid, dibromoacetic acid, tribromoacetic 
acid, iodoacetic acid, diiodoacetic acid and triiodoacetic acid; 
.alpha.-halogenated and .alpha.,.alpha.-dihalogenated aliphatic carboxylic 
acids such as .alpha.-fluoropropionic acid, 
.alpha.,.alpha.-difluoropropionic acid, .alpha.-chloropropionic acid, 
.alpha.,.alpha.-dichloropropionic acid, .alpha.-fluorobutyric acid and 
.alpha.-chlorobutyric acid; .alpha.-cyano aliphatic carboxylic acids such 
as cyanoacetic acid, .alpha.-cyanopropionic acid and .alpha.-cyanobutyric 
acid; acylacetic acids such as acetoacetic acid, dichloroacetyl acetic 
acid and fluoroacetyl acetic acid; alkoxy acetic acids and phenoxy acetic 
acids such as methoxy acetic acid, ethoxy acetic acid, chlorophenoxy 
acetic acid (its isomers) and cyanophenoxy acetic acid (its isomers); 
halogenated benzoic acids such as chlorobenzoic acid (its isomers), 
fluorobenzoic acid (its isomers), difluorobenzoic acid (its isomers), 
bromobenzoic acid (its isomers) and trichlorobenzoic acid (its isomers); 
hydroxy benzoic acids such as salicyclic acid, dihydroxy benzoic acid (its 
isomers) and trihydroxy benzoic acid (its isomers); nitrated benzoic acids 
such as nitrobenzoic acid and dinitrobenzoic acid; glycolic acid; lactic 
acid; malic acids such as malic acid, dimethyl malic acid and dihydroxy 
malic acid; tartaric acids such as tartatic acid, dimethyl tartaric acid 
and dihydroxy tartaric acid; citric acid; malonic acids such as malonic 
acid and dimethyl malonic acid; oxalic acid; maleic acid; fumaric acid; 
mandelic acid; phthalic acids such as phthalic acid (its isomers) and 
halogenated phthalic acid (its isomers); furancarboxylic acids; 
thiophencarboxylic acids; thioacetic acid; cyclopropane-1,1-dicarbaoxylic 
acids; sulfoacetic acids such as sulfoacetic acid and difluorosulfoacetic 
acid; halogenated malonic acids such as difluoromalonic acid and 
dichloromalonic acid; and halogenated succinic acids such as 
1,2-difluorosuccinic acid, perfluorosuccinic acid and perchlorosuccinic 
acid. Among these carboxylic acids, halogenated acetic acids, 
.alpha.-halogenated and .alpha.,.alpha.-dihalogenated aliphatic carboxylic 
acids are preferred, with halogenated carboxylic acids wherein the halogen 
is chlorine or fluorine being particularly preferred. Fluorinated 
carboxylic acids are more preferred, and trifluoroacetic acid is most 
preferred. 
Examples of the solid acid that can be used in the second reaction step of 
the methylenation process are listed below: acidic clay minerals and 
inorganic cation exchangers such as acid clay, bentonite, kaolin, zeolite 
and montmorillonite; these acidic clay minerals and inorganic cation 
exchangers that have been treated with inorganic acids such as 
hydrofluoric acid, hydrochloric acid, perchloric acid and sulfuric acid, 
or ammonium salts of these acidic clay minerals and inorganic cation 
exchangers which have been subjected to protonation treatment by 
calcination; the solidified acids that are prepared by supporting liquid 
acids such as sulfuric acid, phosphoric acid, organic carboxylic acids and 
organic sulfonic acids or the heteropoly-acids such as 
dodecamolybdophosphoric acid, dodecamolybdosilicic acid, 
dodecatungstophosphoric acid, dodecatungstosilicic acid and 
tungstomolybdophosphoric acid on carriers such as alumina, silica, 
silica-alumina, silica-alumina-zirconia, zirconia, titania, boria, 
zeolite, silica-titania, barium sulfate, calcium carbonate, asbestos, 
bentonite, diatomaceous earth, activated carbon, graphite, activated clay 
and acidic clay minerals, followed by heat treatment; solid sulfuric acid 
products that are prepared by first gelling water-soluble sols (e.g., 
alumina sol, silica-alumina sol and silica sol) in the presence of 
sulfuric acid, adding a large amount of sulfuric acid to dissolve the gel, 
and then cooling the solution to solidify, or precipitating a crystal from 
the solution, or heating the solid obtained to a temperature between 
100.degree. and 600.degree. C.; metal oxides and mixed metal oxides such 
as silica, alumina, zinc oxide, titania, antimony oxide, silica-alumina, 
silica-titania, titania-alumina, and silica-zirconia; acidic solid 
sulfates, nitrates and phosphates such as nickel sulfate, aluminum 
sulfate, iron sulfate, chromium nitrate, bismuth nitrate, zirconium 
phosphate, aluminum phosphate, and these sulfates, nitrates and phosphates 
that are supported on the carriers listed above; organic cation exchange 
resins having at least one acidic group such as fluoroalkyl sulfonic acid 
group, fluoroalkyl carboxyl group or alkyl phosphoric acid group; and 
inorganic oxides having either --R"'--SO.sub.3 H or --R"'--COOH or both 
bound thereto. 
As for the inorganic oxides having --R"'--SO.sub.3 H or --R"'--COOH bound 
thereto, those having a divalent organic residual group or organometallic 
compound residue as R"' and having not more than 30, especially not more 
than 20, carbon atoms are preferred. Suitable examples of the organic 
residual group include aliphatic hydrocarbon groups, aromatic hydrocarbon 
groups, aralkyl hydrocarbon groups, and fluoroalkyl groups, as well as 
those which have an ether bond, thioether bond, sulfone bond, carbonyl 
bond, ester bond, amido bond, imido bond or heterocyclic portion at 
terminal or in the backbone of these hydrocarbon groups. Suitable examples 
of the organometallic compound residues include those which have a 
metallic element bound to the terminal or backbone of the organic residual 
groups listed above. Organosilicon compound residues having a silicon atom 
at terminal, for example, those having a halosilyl or alkoxysilyl group 
bound to the terminal are advantageous because they are easy to prepare 
and form a stable bond with inorganic oxides. 
The organic residual groups or organometallic compound residues listed 
above may have part of the hydrogen atoms present replaced by a halogen 
atom such as fluorine, chlorine or bromine, or substituents such as alkyl, 
alkoxy, aryl, aryloxy, hydroxyl, nitrile, alkoxycarbonyl, carboxyl, and 
sulfonic acid groups. Advantageous inorganic oxides include those having a 
hydroxyl group on the surface such as silica, silica-alumina, alumina, 
titania, zirconia, magnesia, zeolite, diatomaceous earth, clay materials, 
glass, titania-alumina, silica-titania and silica-zirconia. Silica, porous 
glass and silica-alumina are particularly preferred. 
Preferred examples of the solid acids include acidic clay minerals and 
inorganic cation exchangers, or these acidic solid materials that have 
been subjected to acid or protonation treatment; acidic metal oxides and 
mixed metal oxides, or these acidic solid materials that have been 
subjected to acid or protonation treatment; organic cation exchange resins 
having either fluoroalkyl sulfonic acid groups or fluoroalkyl carboxyl 
groups or both; and inorganic oxides having an organic group bound thereto 
having either a sulfonic acid group or a carboxyl group or both. 
Particularly preferred solid acids are cation exchange resins having 
fluoroalkyl sulfonic acid groups and zeolite. It is not preferred to use 
the well-known sulfonated polyaromatic ion exchange resins having the 
framework made by copolymerization of styrene and divinylbenzene in the 
second reaction step of the methylenation process, because the 
deterioration of the activities of those resins occurs in a short length 
of the reaction time. The reasons seen to be that the condensation 
products such as diphenylmethane dicarbamates and polymethylene 
polyphenylcarbamates are easily adsorbed on the resins and cover the 
acidic points of the resins, since those resins have a lot of benzene 
rings which have a strong affinity for polar aromatic compounds such as 
these condensation products. On the other hand, this problem is not likely 
to occur with the cation exchange resins having fluroalkyl chains which 
are used in the present invention. 
In the process of the present invention, these carboxylic acids and solid 
acids may be used either alone or in combination. There is no particular 
limitation on the amount in which these carboxylic acids and solid acids 
are used. If the reaction is carried out batchwise or if carboxylic acids 
are used in the flow process, the acids are preferably used in an amount 
of 10.sup.-3 to 10.sup.4 equivalents, more preferably 10.sup.-2 to 
10.sup.2 equivalents, per equivalent of the methylene-amino group in the 
compounds having the methylene-amino bond. If the reaction is carried out 
in a flow reactor retaining a solid acid, the flow rate of the compound 
having the methylene-amino group preferably ranges from 10.sup.-3 to 
10.sup.4 equivalents, more preferably from 10.sup.-2 to 10.sup.3 
equivalents, per hour per liter of the solid acid. The carboxylic acid may 
be used in an excess amount so that it may also serve as a solvent. 
The reaction temperature for the second reaction step of the methylenation 
process generally ranges from 40.degree. to 200.degree. C., preferably 
from 60.degree. to 180.degree. C., and more preferably from 70.degree. to 
160.degree. C. The reaction pressure generally ranges from 0.1 to 20 
kg/cm.sup.2, preferably from 0.5 to 10 kg/cm.sup.2 and more preferably 
from 0.8 to 5 kg/cm.sup.2. The reaction period varies with the type and 
amount of the acid catalyst used, the reaction temperature, the amount of 
the compound present having the methylene-amino bond, the amount of the 
N-phenylcarbamate present, and the nature of the specific reaction process 
(whether batchwise, continuous or flow process). Usually, the reaction 
continues for a period of several minutes to several hours, preferably 3 
minutes to 5 hours, more preferably 5 minutes to 1 hour, but in almost all 
cases, the reaction in the second reaction step of the methylenation 
process can be completed within one hour. The reaction may be performed 
batchwise or in a continuous manner. If the acid catalyst consists of only 
a carboxylic acid, the reaction liquor may simply be passed through a flow 
reactor held at a predetermined temperature. If the acid catalyst consists 
of a solid acid, either the batchwise or flow process may be employed, and 
in either case, the solid acid is preferably retained within the reactor, 
or the solid acid is separated by a solid-liquid separator that 
immediately follows the reactor and is then returned to the reactor. The 
solid acid may be retained within the reactor by either fluidizing the 
acid within the reaction liquor or by fixing a catalyst bed of the acid in 
the reactor. Whichever reaction process is used, the solid acid permits a 
very easy separation from the reaction solution. Therefore, if the solid 
acid is used alone, the desired diphenylmethane dicarbamate can be 
directly obtained from the second reaction step. 
The reaction in the second reaction step of the methylenation process may 
be performed without solvents, but if desired, it may be carried out in 
the presence of a suitable solvent. Illustrative solvents include 
aliphatic or alicyclic hydrocarbons such as pentane, hexane, heptane, 
octane, nonane, decane, n-hexadecane, cyclopentane and cyclohexane; 
halogenated hydrocarbons such as chloroform, ethylene chloride, carbon 
tetrachloride, dichloroethane, trichloroethane and tetrachloroethane; 
alcohols such as methanol, ethanol, propanol and butanol; aromatic 
compounds such as benzene, toluene, xylene, ethylbenzene, 
monochlorobenzene, dichlorobenzene, bromonaphthalene, nitrobenzene, and 
o-, m- or p-nitrotoluene; ethers such as diethyl ether, 1,4-dioxane and 
tetrahydrofuran; esters such as methyl acetate, ethyl acetate and methyl 
formate; and sulfolanes such as sulfolane, 3-methylsulfolane and 
2,4-dimethylsulfolane. Also usable are aliphatic carboxylic acids such as 
acetic acid and propionic acid, and halogenated aliphatic carboxylic acids 
such as monochloroacetic acid, dichloroacetic acid, trichloroacetic acid 
and trifluoroacetic acid. Acid anhydrides of these carboxylic acids may 
also be used. If an organic solvent is used in the first reaction step of 
the methylenation process, the same solvent is preferably used in the 
second reaction step of that process. Particularly preferred organic 
solvents are nitrobenzene, chlorobenzene, and dichlorobenzenes. 
If a carboxylic acid is used in the second reaction step of the 
methylenation process, it is separated from the reaction mixture, and a 
condensation product containing the desired diphenylmethane dicarbamate 
and sometimes a small amount of its higher homolog (i.e., polymethylene 
polyphenylcarbamate) is obtained. As already mentioned, 
.alpha.-halogenated carboxylic acids are preferred carboxylic acids, and 
of the .alpha.-halogenated carboxylic acids, trichloroacetic acid and 
trifluoroacetic acid are particularly preferred. These acids have boiling 
points lower than the N-phenylcarbamate used as the starting material and 
the diphenylmethane dicarbamate formed as the reaction product, and 
therefore they can be easily separated from the reaction mixture. The 
carboxylic acid thus separated is preferably returned for further use in 
the second reaction step of the methylenation process either immediately 
or after being properly adjusted for its composition. 
When the solid acid is used, the separation of the solid acid and the 
reaction solution can be effected by a simple treatment such as 
filtration. When the solid acid is used by a more desirable method, namely 
the flow reaction method involving the use of a fixed bed or fluidized bed 
requiring the solid acid to be retained inside the reactor, the reaction 
of this second step can be carried out without entailing any treatment for 
the separation of the solid acid from the reaction solution. 
Therefore, the acid catalyst used in the second reaction step of the 
methylenation process, whether it is a solid acid or carboxylic acid, can 
be separated very easily from the reaction solution. 
If the solvents other than the carboxylic acids listed above are used in 
the second reaction step of the methylenation process, they may optionally 
be separated by distillation, preferably under 200.degree. C., so as to 
obtain the desired condensation product containing at least 80 mol % of 
diphenylmethane dicarbamate. 
When this solvent is the same solvent that is used in the subsequent 
process of thermal decomposition, the solvent is not required to be 
separated from the resultant reaction mixture. 
In the reaction mixture which is obtained as described above, there is 
generally entrained the N-phenylcarbamate. This N-phenylcarbamate may be 
partly or wholly separated from the reaction mixture by a suitable 
treatment such as, for example, distillation (preferably at temperatures 
not exceeding 200.degree. C.) before the reaction mixture is subjected to 
the process of thermal decomposition. Otherwise, the reaction mixture 
still containing this N-phenylcarbamate may be subjected in conjunction 
with the condensation product to the process of thermal decomposition and 
the phenyl isocyanate resulting from the decomposition may be collected in 
the upper part of the decomposition reactor while it is rising from the 
decomposition mixture. This phenyl isocyanate may be seized with an 
alcohol so as to be recovered in the original form of N-phenylcarbamate. 
The so-obtained condensation product of N-phenylcarbamates mainly consists 
of the dinuclear diphenylmethane dicarbamate and contains little or no 
trinuclear dimethylene triphenylcarbamates. The selectivity for the 
desired diphenylmethane dicarbamate is over 80%. 
Now, the resultant condensation product which contains diphenylmethane 
dicarbamate in a concentration of at least 80 mol % is subjected to the 
process of thermal decomposition so as to be decomposed into MDI and an 
organic hydroxyl compound. 
The process of thermal decomposition contemplated by the present invention 
is characterized by the steps of allowing a mixture comprising 1 to 50% by 
weight of the aforementioned condensation product and 99 to 50% by weight 
of a solvent having a boiling point in the range of 150.degree. to 
350.degree. C. and being inactive to isocyanates to flow down into a 
reactor kept at temperatures in the range of 180.degree. to 380.degree. C. 
through the upper part of the reactor, causing the introduced mixture to 
come into counterflow contact with a carrier introduced upwardly into the 
reactor through the lower part thereof, allowing the resultant organic 
hydroxyl compound to be discharged in the form of vapor in conjunction 
with the carrier through the upper part of the reactor, and withdrawing 
the isocyanate solution through the lower end of the reactor. 
In the thermal decomposition carried out in the manner described above, the 
liquid component introduced downwardly into the reactor is particularly 
desired to descend in the shape of a thin film within the reactor. It has 
been found that the thermal decomposition reaction proceeds at a high rate 
without entailing any significant formation of by-products to produce MDI 
in high yields with high selectivity when the liquid component is 
introduced in the desirable shape described above. The reactor to be used 
for this thermal decomposition, therefore, is desired to be a vertical 
cylindrical reactor. Preferably, this reactor is packed with a solid 
packing or a solid catalyst or both. As the solid packing, any of the 
packings which have found popular acceptance for use in distillation 
columns and absorption columns is advantageously used. Naturally, the 
solid packing may be in any of the forms in which such solid packings are 
commercially available at all. A particularly desirable solid packing is 
one made of a material of high thermal conductivity. The solid substance 
thus packed in the reactor not merly produces an effect of increasing the 
surface area of the liquid component flowing down the interior of the 
reactor but also serves as a desirable medium for imparting the heat for 
thermal decomposition to the liquid component. The solid catalyst is used 
in the form of a fixed bed and therefore, unlike any homogeneous catalyst 
system, is not required to be separated from the isocyanate solution. This 
fact constitutes an advantage for the commercialization of the process of 
thermal decomposition. The catalyst of this nature is effective in 
lowering the temperature of thermal decomposition and increasing the 
reaction rate. Particularly when the catalyst is in a metallic form, it 
has high thermal conductivity and, therefore, gives the notable 
convenience for providing the heat of thermal decomposition. 
Examples of the catalyst advantagously used for thermal decomposition 
include rare earth elements, antimony, and bismuth in their respective 
simple form, oxides, sulfates, and salts of such elements; boron in its 
simple form and boron compounds; metals of the copper group, zinc group, 
aluminum group, carbon group, and titanium group in the Periodic Table of 
Elements and oxides and sulfates of such metals; and carbides and nitrides 
of the elements of the carbon group except for carbon, titanium group, 
vanadium group, and chromium group in the Periodic Table of Elements. 
The solvent to be used in the process of thermal decomposition is required 
to be inactive to any isocyanates under the conditions of thermal 
decomposition and to have a boiling point under atmospheric pressure in 
the range of 120.degree. to 350.degree. C., preferably 150.degree. to 
300.degree. C. Only for the sake of the thermal decomposition reaction 
itself, the solvent may possess a boiling point exceeding 350.degree. C. 
The temperature range specified above is significant because this solvent 
must be separated from the resultant isocyanate solution. This separation 
is effected by distillation because this treatment is simple and 
effective. If the boiling point of the solvent is very high, then the 
temperature of distillation is proportionately high and the isocyanate is 
consequently suffers from secondary reactions. For the purpose of 
precluding these secondary reactions, therefore, the boiling point of the 
solvent under atmospheric pressure is desired to be not more than 
350.degree. C., preferably not more than 300.degree. C. 
The solvents satisfying this requirement include aliphatic, alicyclic, and 
aromatic substituted or unsubstituted hydrocarbons and mixtures thereof 
and also include certain types of oxygenated compounds such as ethers, 
ketones, and esters. 
As preferred solvents, there may be cited alkanes such as nonane, decane, 
undecane, dodecane, tridecane, tetradecane, pentadecane, hexadecane, 
heptadecane, and octadecane and their corresponding alkenes; aromatic 
hydrocarbons or alkyl-substituted aromatic hydrocarbons such as cumene, 
diisopropyl benzene, diethyl benzene, ethyl toluene, dibutyl benzene, 
naphthalene, methyl naphthalene, ethyl maphthalene, and dodecyl benzene; 
aromatic compounds substituted with nitro group and halogens such as 
chlorobenzene, dichlorobenzene, trichlorobenzene, bromobenzene, 
dibromobenzene, chlorotoluene, dichlorotoluene, chloronaphthalene, 
bromonaphthalene, nitrobenzene, chloronitrobenzene, nitronaphthalene; 
polycyclic hydrocarbon compounds such as diphenyl, substituted diphenyl, 
diphenyl methane, terphenyl, anthracene, phenanthrene, various 
dibenzyl-toluene isomers, triphenyl methane, and tetrahydronaphthalene; 
ketones such as acetophenone and benzophenone; esters such as dibutyl 
phthalate, dihexyl phthalate, and dioctyl phthalate; ethers and thioethers 
such as diphenyl ether and diphenyl sulfide; sulfoxides such as dimethyl 
sulfoxide and diphenyl sulfoxide; and nitriles such as benzonitrile. 
A preferred embodiment resides in using an organic solvent in the process 
of methylenation and using an organic solvent of the same kind in the 
process of thermal decomposition. In this case, therefore, the mixture 
comprising the condensation product resulting from the process of 
methylenation and containing at least 80 mol % of diphenylmethane 
dicarbamate and an organic solvent is, in its unmodified form, in a form 
diluted with the same organic solvent, or in a form concentrated to a 
prescribed composition, subjected to the process of thermal decomposition. 
The solvent to be used herein is desired to be an aromatic compound 
substituted with a nitro group and/or a halogen. A particularly desirable 
solvent is a halogenated aromatic hydrocarbon such as chlorobenzene or 
dichlorobenzene. 
During the thermal decomposition, the diphenylmethane dicarbamate is 
converted into a corresponding MDI and an organic hydroxyl compound. To 
prevent these products of the conversion from being recombined into the 
carbamate, the organic hydroxyl compound so produced in consequence of the 
reaction is desired to be separated from the reaction system. For the 
purpose of enhancing this separation, the carrier is introduced upwardly 
into the thermal decomposition reactor throuogh the lowerpart thereof. 
Inside the reactor, the carrier runs into counterflow contact with the 
liquid component which is flowing down the reactor interior. Then, the 
carrier is withdrawn from the reactor through the upper part thereof in 
conjunction with the vapor of the organic hydroxyl compound produced by 
the reaction. This carrier is desired to be preheated before introduction 
into the reactor. 
Examples of the carrier which is used advantageously in this case include 
inert gases and hydrocarbon gases such as nitrogen, argon, helium, carbon 
dioxide, methane, ethane, propane, and butane. Other compounds which 
fulfil the function of the carrier described above include low boiling 
organic solvent such as halogenated hydrocarbons represented by 
dichloromethane, chloroform and carbon tetrachloride, lower hydrocarbons 
represented by pentane, hexane, heptane and benzene, and ethers 
represented by tetrahydrofuran and dioxane. 
These carriers may be used either singly or in the form of mixtures of two 
or more members. 
The carrier withdrawn from the reactor through the upper part thereof 
entrains the organic hydroxyl compound and, occasionally, part of the 
thermal decomposition solvent. Desirably, therefore, separation of this 
mixture into the individual components may be effected by passing the 
mixture through a condenser kept at suitable temperatures or by subjecting 
the mixture to distillation, for example. The organic hydroxyl compound is 
withdrawn from the reaction system, while the carrier and the thermal 
decomposition solvent are circulated back to the reactor. 
The organic hydroxyl compound thus withdrawn from the reaction system is 
preferred to be sent back to be used again in the process for the 
production of N-phenylcarbamate. 
This reaction of thermal decomposition is desirably carried out 
continuously at a temperature in the range of 180.degree. to 380.degree. 
C., under a vacuum, under atmospheric pressure, or under application of 
pressure. The duration of this reaction is generally in the range of 
several minutes to several hours, preferably 1 minute to 5 hours, more 
preferably 2 minutes to 2 hours, although it is variable with the kinds of 
the thermal decomposition solvent, packing or catalyst, and carrier, the 
shape of the reactor, or the reaction temperature. 
The thermal decomposition, when performed under the conditions described 
above, proceeds quickly without entailing any appreciable formation of 
by-products and affords the isocyanates in high yields. 
When the isocyanate solution is withdrawn from the reactor through the 
lower part thereof as described above, it is treated to expel the thermal 
decomposition solvent. This separation of the solvent from the isocyanate 
solution is desired to be effected by distillation. For the purpose of 
curbing possible degradation of the yield and purity of the produced 
isocyanate by secondary reactions, this separation by distillation is 
desired to be carried out at the lowest possible temperature, for example, 
below 180.degree. C., preferably below 150.degree. C., over the shorted 
possible period. In this case, the thermal decomposition solvent is 
preferably completely removed from the product. The isolation of the 
product from the solvent may be advantageously effected by distilling the 
mixture to expel part of the diphenylmethane diisocyanate in conjunction 
with the solvent and returning the distillates back to the treatment for 
separation and purification. 
By carrying out the process of methylenation and the process of thermal 
decomposition of the present invention as described above, MDI can be 
economically produced with a high selectivity of at least 80%, preferably 
at least 90%, from N-phenylcarbamate and a methylenating agent as the raw 
materials. It has also been demonstrated that the method of this invention 
enjoys various outstanding characteristics which render the method 
commercially feasible. 
Now, this invention will be described more specifically below with 
reference to working examples. It should be noted, however, that this 
invention is not limited by these examples. 
In these examples, the reaction products were analyzed as by gas 
chromatography and high-speed liquid chromatography.

EXAMPLE 1 
First, methylenation of ethyl N-phenylcarbamate was carried out by a 
continuous flow method. In the first step of reaction, there was used a 
complete mixture type system consisting of three overflow type glass 
reactors each having an inner volume of 3 liters and provided with a 
stirrer. This system was constructed so that the reaction solution 
overflowing the reactors of the upper stages would be introduced into the 
reactors of the next lower stages. The reactors were each maintained at 
90.degree. C. Into the reactor of the uppermost stage in the system, a 
solution containing ethyl N-phenylcarbamate in a concentration of 33% in 
nitrobenzene and preheated to 90.degree. C. was introduced at a flow rate 
of 15 ml/min. At the same time, an aqueous 37% formaldehyde solution was 
introduced at a flow rate of 0.7 ml/min and an aqueous 55% sulfuric acid 
solution preheated to 90.degree. C. was introduced at a flow rate of 18 
ml/min respectively into the reactor. After the contents of the reactor 
had assumed a steady stage, the reaction solution was led into a two-layer 
separator, there to be continuously separated into a nitrobenzene layer 
and an aqueous sulfuric acid solution layer. The nitrobenzene solution was 
introduced downwardly into a counterflow-contact type multi-stage 
extraction column maintained at 90.degree. C. and hot water at 90.degree. 
C. was delivered upwardly into the extraction column to remove a trace of 
sulfuric acid. Then, the nitrobenzene solution was dehydrated by expelling 
a small amount of water from the solution in conjunction with a part of 
nitrobenzene by distillation under a vacuum. 
The nitrobenzene layer was found, by analysis, to comprise 67.7% by weight 
of ethyl N-phenylcarbamate, 27.5% by weight of diethyl 
4,4'-diphenylmethane dicarbamate, 2.3% by weight of diethyl 
2,4'-diphenylmethane dicarbamate, 0.8% by weight and 1.2% by weight 
respectively of bis-(N-carboethoxyanilino)-methane and ethyl 
(N-carboethoxyanilinomethyl)-phenylcarbamate each possessing a 
methylene-amino bond, and 0.5% by weight of trinuclear triethyl 
dimethylenetriphenylcarbamate. The reaction of the second step was carried 
out by combining the nitrobenzene solution with trifluoroacetic acid of 
the same weight and introducing the resultant mixture upwardly into a 
cylindrical reactor 3 cm in inside diameter maintained at 80.degree. C. 
The residence time was fixed at 20 minutes. After the contents of the 
reactor had assumed a steady state, the reaction solution was found by 
analysis to contain no compound having a methylene-amino bond. The 
reaction mixture was distilled to expel trifluoroacetic acid. 
Subsequently, nitrobenzene and unaltered ethyl N-phenylcarbamate was 
separated by distillation. The condensation product obtained as the 
residue of the distillation was composed of 90.55 mol % of diethyl 
4,4'-diphenylmethane dicarbamate, 9.44 mol % of diethyl 
2,4'-diphenylmethane dicarbamate, and 0.01 mol % of triethyl 
dimethylenetriphenylcarbamate. 
Under application of heat, this condensation product was dissolved in a 
concentration of 10% by weight in ortho-dichlorobenzene. The resultant 
solution was preheated to 150.degree. to 160.degree. C., then led to the 
upper part of a decomposition reactor with 5 cm in inside diameter and 3 m 
in length maintained at 280.degree. C., and sprayed downwardly into the 
reactor interior at a flow rate of 25 ml/min. The reactor was packed with 
Raschig rings of stainless steel. Through the lower part of the reactor, 
preheated nitrogen gas was continuously introduced upwardly at a flow rate 
of 3 Nl/min. The reactor was provided in the upper part thereof with a 
partial condenser adapted to condensate ortho-dichlorobenzene. To the tip 
of this condenser was connected an alcohol trap kept cooled at -50.degree. 
C. 
The reaction of decomposition was continuously carried out under a pressure 
of 15 kg/cm.sup.2. Consequently, there was obtained an isocyanate solution 
containing no unaltered carbamate. The ethanol was recovered substantially 
quantitatively by the trap. 
The solution was distilled under a vacuum at a temperature of not more than 
100.degree. C. to expel ortho-dichlorobenzene. Consequently, there was 
obtained an isocyanate mixture consisting of 89.2% of 4,4'-MDI, 9.3% of 
2,4'-MDI, and 1.5% of dimethylene triphenyl isocyanate by weight ratio. 
EXAMPLE 2 
A reaction vessel of glass having an inner volume of 400 ml was charged 
with 230 g of an aqueous 45 wt % sulfuric acid solution, 50 g methyl 
N-phenylcarbamate, 5.5 g of an aqueous 37% formaldehyde solution, and 50 g 
of nitrobenzene as a solvent. The contents of the reaction vessel were 
stirred at 90.degree. C. for two hours to effect reaction. Then, the 
resultant reaction mixture was separated into an organic layer and an 
aqueous layer. The organic layer was washed with hot water to remove a 
small amount of remaining sulfuric acid. Then, a small amount of water 
still contained in the organic layer was removed by azeotropic 
distillation with a part of nitrobenzene. When the resultant organic layer 
was analyzed, it was found that the conversion of methyl N-phenylcarbamate 
was 41%, the yield of dimethyl 4,4'-diphenylmethane dicarbamate was 32%, 
the yield of dimethyl 2,4'-diphenylmethane dicarbamate was 2.8%, and the 
yields of bis-(N-carbomethoxyanilino)-methane and methyl 
(N-carbomethoxyanilinomethyl)-phenylcarbamate each possessing a 
methylene-amino bond were respectively 2.9% and 3.3%. The analysis did not 
detect any trinuclear or higher compound. 
No formaldehyde was detected in the organic layer. 
Then, a reaction tube of stainless steel 10 mm in inside diameter and 30 cm 
in length was packed with beads of fluorinated sulfonic acid resin having 
the repeating units of the following formula: 
##STR5## 
This reaction tube was kept at 120.degree. C. and the aforementioned 
nitrobenzene solution was injected upwardly into the reaction tube through 
the lower part thereof at a flow rate of 0.5 ml/min. The reaction solution 
which was discharged through the upper end of the reaction tube was found 
to contain no compound possessing a methylene-amino bond. The reaction 
solution was distilled under a vacuum to expel nitrobenzene. The resultant 
reaction mixture consisted of 57% of methyl N-phenylcarbamate, 38% of 
dimethyl 4,4'-diphenylmethane dicarbamate, and 5% of dimethyl 
2,4'-diphenylmethane dicarbamate. No trinuclear trimethyl 
dimethylenetriphenylcarbamate was contained in the reaction mixture. 
By subjecting this mixture to vacuum distillation, most methyl 
N-phenylcarbamate distilling at 110.degree. to 112.degree. C./3 mmHg was 
recovered. As the residue of this distillation, there was obtained 23.1 g 
of a mixture consisting of 5.4% of methyl N-phenylcarbamate, 83.6% of 
dimethyl 4,4'-diphenylmethane dicarbamate, and 11% of dimethyl 
2,4'-diphenylmethane dicarbamate by weight ratio. The condensation product 
consisted of 88.4 mol % of dimethyl 4,4'-diphenylmethane dicarbamate and 
11.6 mol % of dimethyl 2,4'-diphenylmethane dicarbamate. 
A solution of 23.1 g of this mixture in 200 g of n-pentadecane was 
introduced downwardly into a reaction tube of stainless steel maintained 
at 260.degree. C. (2 cm in diameter and 2 m in length and packed with 
small particles of silicone carbide) at a flow rate of 3 ml/min and 
nitrogen gas heated to 250.degree. C. was introduced upwardly into the 
reaction tube through the lower part thereof at a flow rate of 1 Nl/min. 
The phenyl isocyanate produced by the decomposition of methyl 
N-phenylcarbamate was discharged in the form of vapor and led into a 
receptacle adapted to condense methanol. It was consequently recovered in 
its original form of methyl N-phenylcarbamate. A part of the n-pentadecane 
used as the solvent was entrained as a distillate. 
The reaction solution produced in the amount of 210 g by the thermal 
decomposition was subjected to vacuum distillation to distill out the 
n-pentadecane at 98.degree. to 100.degree. C./0.5 mmHg. As the residue of 
this distillation, there was obtained 17.4 g of a mixture consisting of 
88.4% of 4,4'-MDI and 11.6% of 2,4'-MDI. 
EXAMPLE 3 
This example represents a case in which the process of methylenation was 
carried out by using no organic solvent. 
In a glass flask having an inner volume of 1 liter, 190 g of ethyl 
N-phenylcarbamate, 770 g of an aqueous 55 wt % sulfuric acid solution, and 
19 g of an aqueous 37% formaldehyde solution were stirred at 90.degree. C. 
for two hours to effect reaction. Then, the resultant reaction mixture was 
transferred into a separation funnel, to recover an organic layer and an 
aqueous layer separately of each other. The organic layer was washed with 
hot water and then treated with a rotary evaporator to expel water. The 
washings and the separated aqueous layer were combined and the combined 
water was treated with the rotary evaporator to expel a stated amount of 
water. Consequently, there was recovered 770 g of an aqueous 50 wt % 
sulfuric acid solution. 
When the organic layer was analyzed, it was found that the conversion of 
ethyl N-phenylcarbamate was 38.5% and that the reaction produce was 
composed of 30.1% by weight of diethyl 4,4'-diphenylmethane dicarbamate, 
4% by weight of diethyl, 2,4'-diphenylmethane dicarbamate, 1.9% by weight 
and 2.4% by weight respectively of bis-(N-carboethoxyanilino)-methane and 
ethyl (N-carboethoxyanilinomethyl)-phenylcarbamate each possessing a 
methylene-amino bond, and 0.9% by weight of trinuclear and higher 
compounds. No formaldehyde was detected in the organic layer. 
Then this organic layer was combined with 150 g of trifluoroacetic acid and 
the resultant mixture was heated at 75.degree. C. for 20 minutes to effect 
reaction. The resultant reaction mixture was distilled to expel 
trifluoroacetic acid. The reaction mixture thus obtained consisted of 
60.2% of ethyl N-phenylcarbamate, 34.5% of diethyl 4,4'-diphenylmethane 
dicarbamate, 4.2% of diethyl 2,4'-diphenylmethane dicarbamate, and 1.1% of 
trinuclear triethyl dimethylenetriphenylcarbamate. No compound possessing 
a methylene-amino bond was detected. 
By subjecting the condensation product to vacuum distillation, there was 
recovered 116 g of ethyl N-phenylcarbamate distilling at 108.degree. to 
110.degree. C./1 mmHg. As the residue of the distillation, there was 
obtained 76.7 g of a condensation product. By the analysis of this 
condensation product, it was found that the selectivity of diethyl 
4,4'-diphynelmethane dicarbamate was 87.5%, that of diethyl 
2,4'-diphenylmethane dicarbamate was 10.7%, and that of triethyl 
dimethylenetriphenylcarbamate was 1.8% respectively. The combined 
selectivity of dinuclear diethyl diphenylmethane dicarbamate was 98.2%. 
By distillation, 149 g of trifluoroacetic acid was recovered. This 
recovered acid could be put to reuse in its unmodified form. 
A mixture consisting of 15% by weight of the condensation product and 85% 
by weight of ortho-dichlorobenzene was subjected to thermal decomposition 
by following the procedure of Example 1. The reaction tube used herein 
measured 2 cm in inside diameter and 4 m in length and was packed with 
Raschig rings made of aluminum and Dixon packing made of stainless steel. 
The liquid mixture preheated to 150.degree. to 160.degree. C. was sprayed 
downwardly into the reaction tube kept at 270.degree. C. at a flow rate of 
10 ml/min. Preheated nitrogen gas was introduced upwardly into the 
reaction tube at a flow rate of 1.5 Nl/min. 
The reaction of decomposition was continuously carried out under a pressure 
of 8 kg/cm.sup.2. As the result, there was obtained an isocyanate solution 
containing no unaltered carbamate. This solution was subjected to vacuum 
distillation at a temperature of not more than 100.degree. C. to expel 
ortho-dichlorobenzene. Consequently, there was obtained an isocyanate 
mixture consisting of 87.5% of 4,4'-MDI, 10.7% of 2,4'-MDI, and 1.8% of 
dimethylenetriphenyl isocyanate. 
EXAMPLE 4 
The process of methylenation of ethyl N-phenylcarbamate was carried out by 
using the same apparatus as used in Example 1. Into the reactor of the 
uppermost stage in the system consisting of three reactors kept at 
90.degree. C., a solution containing ethyl N-phenylcarbamate in a 
concentration of 28% in ortho-dichlorobenzene preheated to 90.degree. C. 
was introduced at a flow rate of 20 ml/min. At the same time, an aqueous 
37% formaldehyde solution was introduced at a flow rate of 0.6 ml/min. and 
an aqueous 60% sulfuric acid solution was introduced at a flow rate of 15 
ml/min respectively into the reactor. After the contents of the reactor 
had assumed a steady state, the reaction solution was led into a two-layer 
separator, there to be continuously separated into an 
ortho-dichlorobenzene layer and an aqueous sulfuric acid layer. The 
orthodichlorobenzene layer was introduced downwardly into a 
counterflow-contact type multi-stage extraction column kept at 90.degree. 
C. and hot water at 90.degree. C. was delivered upwardly into the 
extraction column to remove a trace of sulfuric acid. Then, a small amount 
of the water contained in the ortho-dichlorobenzene solution was removed 
by vacuum distillation with a part of ortho-dichlorobenzene. 
When the ortho-dichlorobenzene solution was analyzed, it was found that the 
conversion of ethyl N-penylcarbamate was 52%, the selectivity of diethyl 
4,4'-diphenylmethane dicarbamate and that of diethyl 2,4'-diphenylmethane 
dicarbamate were respectively 66.2% and 6.5%, and the selectivity of ethyl 
(N-carboethoxyanilinomethyl)-phenylcarbamate and that of trinuclear 
compounds (inclusive of compounds possessing a methylene-amino bond) were 
respectively 18.3% and 9%. 
To the ortho-dichlorobenzene solution, trifluoroacetic acid was added to a 
concentration of 30% by weight. The resultant mixture was introduced 
upwardly into a cylindrical reaction vessel kept at 80.degree. C. to 
effect the intermolecular transfer reaction of the second step. The 
residence time was fixed at 20 minutes. When the reaction solution was 
analyzed, it was found that no compound possessing a methylene-amino bond 
was present, that the selectivity of diethyl 4,4'-diphenylmethane 
dicarbamate and that of diethyl 2,4'-diphenylmethane dicarbamate were 
increased respectively to 84.5% and 8.2%, and that the selectivity of 
trinuclear triethyl dimethylenetriphenylcarbamate was decreased to 7.3%. 
Then, this reaction solution was distilled to expel trifluoroacetic acid. 
The resultant ortho-dichlorobenzene solution was subjected to thermal 
decomposition by following the procedure of Example 1. In this case, the 
reaction tube had a length of 4 m and it was kept at 260.degree. C. The 
phenyl isocyanate and ethanol which were formed in consequence of the 
thermal decomposition were withdrawn through the upper part of the 
reaction tube. This phenyl isocyanate was recovered substantially wholly 
in the form of ethyl N-phenylcarbamate. 
The solution obtained through the lower part of the reaction tube was 
distilled at a temperature of not more than 100.degree. C. to expel 
ortho-dichlorobenzene. Consequently, there was obtained an isocyanate 
mixture consisting of 83% of 4,4'-MDI, 8% of 2,4'-MDI, 7.3% of 
dimethylenetriphenyl isocyanate, and 1.7% of carbodiimide compounds 
derived from MDI. 
EXAMPLE 5 
The ortho-dichlorobenzene solution obtained at the end of the reaction of 
the first step in the process of methylenation obtained in Example 4 was 
introduced upwardly into a cylindrical reaction vessel kept at 150.degree. 
C. to effect the intermolecular transfer reaction of the second step. This 
reaction tube was packed with the zeolite catalyst with high silica ratio 
to alumina. This zeolite was synthesized by the method of EPC Application 
No. 83113159.4 (produced by heating aluminum sulfate and silica zol in the 
presence of 1,8-diamino-4-aminomethyl octane and water at pH 12 at 
170.degree. C. for 48 hours thereby producing crystals and subsequently 
separating the crystals by filtration, washing, and drying). The residence 
time was fixed at 30 minutes. The condensation product obtained by this 
reaction comprised substantially the same composition as the condensation 
product obtained in Example 4. When the ortho-dichlorobenzene solution was 
subjected to thermal decomposition by following the procedure of Example 
4, there was obtained an isocyanate having substantially the same 
composition as the isocyanate obtained in Example 4. 
EXAMPLE 6 
The nitrobenzene solution obtained at the end of the reaction of the first 
step in the process of methylenation in Example 1 was introduced upwardly 
into a cylindrical reaction vessel kept at 150.degree. C. to effect the 
intermolecular transfer reaction of the second step. This reaction tube 
was packed with powdered anhydrous aluminum sulfate. The residence time 
was fixed at 30 minutes. The reaction which ensued produced a condensation 
product having substantially the same composition as the condensation 
product obtained in Example 1. 
The nitrobenzene solution was distilled under a vacuum to expel 
nitrobenzene. The residue of the distillation was combined with 
ortho-dichlorobenzene of the same weight. The resultant solution was 
preheated to 150.degree. to 160.degree. C. and was introduced downwardly 
into the same thermal decomposition device as in Example 1. The reaction 
tube was kept at 260.degree. C. and it was packed with Raschig rings made 
of copper. As a carrier, gasified n-pentane preheated to 240.degree. C. 
was introduced upwardly into the reaction tube. The decomposition reaction 
was continuously carried out under a pressure of 12 kg/cm.sup.2. 
Consequently, there was obtained an isocyanate having substantially the 
same composition as the isocyanate obtained in Example 1. 
From the working examples cited above, it is noted that the method of the 
present invention permits economic production of MDI containing 
substantially no polymeric MDI or a small amount of polymeric MDI in high 
yields with high selectivity from N-phenylcarbamate. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.