CA1176647A - Production of isocyanates from esters of aromatic carbamic acids (urethanes) - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A process for converting aromatic carbamates to their corresponding isocyanates by thermolysis at a pressure of at least atmospheric in the presence of a catalyst selected from Ti, Sn, Sb, and Zr is disclosed.

Description

~1766~L7 1 Background of the Invention

2 The present invention is directed to processes

3 or converting aromatic carbamates and polymeric aromatic

4 carbamates to their corresponding isocyanates by thermo-lysis in the presence of a specifically defined catalyst 6 at atmospheric or super atmospheric pressures.
7 Isocyanates are very useful substances as start-8 ing materials for polyurethanes. Such polyurethanes can g be used in the formation of a variety of products ranging from automative parts to thermal insulation. The pro-11 perties of the final polyurethane end product is to a 12 large extent determined by the number of isocyanate groups 13 i.e.,(-NCO) present on the isocyanate startiny material.
14 For exam~le, difunctional isocyanates do not result in crosslinking and are useful in the production of flexible 16 polyurethane foams. Polyfunctional isocyanates result in 17 crosslinking and conse~uently are useful in the pro-18 duction of rigid polyurethane foams. Within the class of 19 polyfunctional isocyanates is a subclass of isocyanates, namely, polymeric aromatic polyisocyanates, which have 21 gained market recognition and are possessed of unique 22 properties which render them particularly adaptable for 23 specialized end uses such as the manufacture of urethane 24 adhesives. The term, "polymeric isocyanates" as used herein refers to a mixture of compounds containing poly ~6 alkylene or arylene poly aryl isocyanate oligomers such 27 as Poly methylene poly phenyl isocyanate (described 28 hereinafter in more detaill.
29 Non polymeric aromatic isocyanates include such compounds as tolylene diisocyanate, methylene-bis-(4-phenyl 31 isocyanate) and naphthylene diisocyanate.
32 ~ current process for preparing these non-33 polymeric isocyanates, for example, tolylene diisocyanate 34 of the formula:

~76647 ~ ~CO 9r OCN~NCO

NCO
(I) (II~
6 .
8 comprises nitrati~g toluene to ~orm dinl~r~toluene~ xe-9 ducing the latter wi'lh hydrogen t~ form ~he corresponding 0 diami~e and then xeacting the diamine with phosgene.
mus, the aforedescribed process comprises complicated an~ troublesome s~ps, requixlng the use of a large amount of highly toxic phosgene-and permitting the for-mation of hydrogen chloride as by-product.
16 ~n alternative ap~roac~ to preparing non-poly-meric isocyana~es invo~ves ~hé.synthesis o~ carbam~tes 8 fro~ nitro compounds and subsequently~ pyrolyzing car-19 bamates.to form ~he isocyanate:a~d an alcohol co-product.
The reaction for form m g isocyanates by pyroly-21 sis of carbamates may be shown by the following basic 22 equation:
3 RHNC02R'--~ RNCO + R'-OH (l).

2s 26 On ~hermal dissociatlon o~ the carbamate, several unde-27 sirable side~reactions take place;at the same t~me. These 28 si~de~ re~act~ions: are:~ the deca~bo~y~a~iorl reactio~ o~ ~h~
29 ca~bamate ~ accompanying the formation ~f - a primary amine 30~ RNH2 and an olein or o~ a sècond ry ami~e RN~R as a by-31 product; the ~eaction between ~ t~ie produced iso~anate and the startlng c~rbz~ate, ~ermitting the formation of an 34 allophanate as by-product; the reaction between ~he 35 .~roduced isocyana~e and an amine formed as by-proàuct per~
36 mitting the formation of a urea compound as by-produc:t7 37 and the psIymerization of the pxoduced isocyanate, pe~

. .

3 ~766~7 - mitting the formation of an isocyanurate or a pol~mer as 2 by product. The thermal dissociation reaction of equation 3 (l) above is reversible and its equilibrium remains with 4 the le~t-hand side carbamate at low temperature but is shifted to the right-hand side by heating, whereby the 6 dissociation of the carbamate takes place. In this case, 7 the thermal dissociation temperature varies according to 8 the sort of carbamate and the reaction conditions. Ac-9 cordingly, it is important for obtaining isocyanates ad-vantageously ~rom car~amates to perform the pyrolysis 11 reaction of equation (l) selectively while inhibitin~ the 12 above mentioned side and reverse reactions.
3 The probability of certain undesirable side re-14 actions occurring is increased as the reaction temperature is incxeased and as the time during which the isocyanate product remains in contact with the components o~ the re-17 action mixture is increased. As one lowers the reaction 18 temperature, however, the reaction rate decreases, along with the solubility o~ the carbamate in any solvent used in the reaction medium.
; ~ The conventional pyrolysis of carbamates can be 22 roughly classified into reactions carried out in the vapor phase at a high temperature and reactions carried out in the liquid phase at a relatively low temperature. U.S.
Patent No. 3,734,941 discloses a typical vapor phase 26 process wherein a carbamate is pyrolyzed at 400-600C in 27 the presence of a Lewis acid and the resultant v~por is 28 separated by fr~ctional condensation into an isocy~nate and an alcohol. According to this process, for example, - tolylene diisocyanate is ob~ained in a yield of 60% by pyrolysis of diethyl tolylene-2,4-dicarbamate of the 32 formula:

~ HCOOC2Hs (III) 37 NHCOOC2~5 _3_ , ~ 766~7 1 in the pres~nce of ferric chloride. However, this process 2 has the drawbacks of a low yield of the product, decom-3 position of the catalyst, corrosion of the reaction appa-4 ratus at high temperatures, and ~ormation of a consider-able amount of a polymer as by-product. (SPe also Br.
6 Pate~t Spec. No. 1,247,451~.
7 German Patent No. 2,410,505 proposes as an im-8 proved vapor phase method, a process wherein th~ residence 9 tLme of the reactants at 350-550C is controlled within 15 seconds. According to this process, the yield of iso-~1 cyanate is as high as 93%, although the carbamate has to 12 be supplied in the form of powders to the reaction zone.
13 However, a solid pol~mer is also formed by this process 14 as by-product and is gradually deposited in the reactor and in the condenser during the course of sustained oper-16 ation, thus making it difficult to con~uc~ a continuous 17 reaction. In addition, a large quantity of heat required 18 for the endothermic pyrolytic reaction has to be supplied 19 ~O the starting material within a very s~ort period of time. This additional factor causes this process to 21 encounter great difficulty in being adopted into pr2ctice.
22 Liquid phase processes were developed i~ a~
23 attempt to lower the reaction temperature a~d reduce 24 undesirable side reactions.
For example, U.5. Patent No. 2, 409,712 dis-26 ~loses the pryolysis of N-s~stituted carb~mic esters 27 in the liquid phase, in the presence or a~sence o~ a 28 diluen~, at ~emperatures of 15~ to 35~C under a high v~cuum to distill th~ resulting isocyanate overhead~ None o~ the carbamic esters disclosed incIude polymeric aro 32 matic carbamates; Consequen`tly, not drlly does the use of 33 high vacuum add to ~he cost of the process, but the use 34 of hlgh vacuum if applied to the distillation o~ polymeric isocyanat s would be ine~ectiYe due to the very high 36 boiling points of the latter. This patent ~lso does ~ot 37 disclose the US2 of catalysts as described herei~.

66~
. -5-1 In an article in the Journal of the American 2 Chemical Society, Vol. 81, page 2138 et se~. (l959),Dyer 3 et al show that ethyl carbanila~e gi~es phenyl isocyanate 4 (60-75 mole percent baséd on carbinilate degradedj and ethyl alcohol when heatèd ~or 6 hours at 200C under 6 pressuxe sufficiently 7Ow ~60-120~ g~ to v porize the 7 alcohol but high e~ough to retain the isocyanate. At 8 atmospheric pressure no phenylisocyanate is o~tained, 9 although 70 percent of the ethyl carbanilat~ is destroyed.
At 250C and atmospheric pressure alpha-methylbenzyl-11 carbanilate gives major amounts of aniline, alpha-methyl-12 benzyl aniline, styrene and carbon dioxide.
13 U.S. Patent No. 3,054,819 discloses the pryoly-14 sis of an aliphatic mono carbamate and dicarbamate esters in the optional presence of a basic catalyst such as al-16 kali and alkaline earth metal oxides, hydroxides, carbo-17 nates and the like. The pyrolysis is conducted at sub-18 atmospheric pressures and at temperatures of 100C to 19 300C. In accordance with this process the isocyanate pxoduc~ must be separated from the glycol ester co-product 21 preferably by distilling isocyanate alone or i~ combina-2~ tion with the glycol ester and separating the two co-23 products. Either alternative is not available with poly-24 meric aromatic isocyanates. Thus, this patent fails to disclose (1) the use of aromatic carbamates of any kind, 26 and (2) th2 use of the catalysts of the present invention 27 in conjunction with any carbamates.

29 ~.S. Patent No. 3,919j278 is directed to a process for preparing isocya~ates w~ereLn a mononuclear 31 aromatic carbamate is dissolved in a~ inert solvent in an 32 amount such that the total concentr~tion of the carbamate and a proauct obtained by pyro}ysis ~hereof is within a 34 range of about 1-20 mole% nd the pyrolysis of the car-b~mate is carried out at 230-290C in the presence of 36 an inert carrier used ir. an amou~ of at least 3 molar 37 proportion to the carbamate. Po}ymeric aromatic car-. ~~- .

1 bamates are not mentioned in this paten~ nor i5 the use 2 of the catalysts of the present in~entio~ in conjunc~ion 3 with any carbamatesO
g.S. Patent No. 3;glg,279 is directed t~ a 6 process for preparing isocyanates wherei~ a carbamate is 7 dissolved in an inert solv~nt and brough~ into contact at a high temperature (i.e. 175-350C) with a c~talyst g composed of a heavy metal (Mo, ~, Mn. Pe, Co, Cr, Cu or Ni) or a compound thereof to effect the pyrolysis of the 11 carbamate at temperatures of 175 to 350C. The concen-12 tration o~ the car~mate dissolved in the inert solvent is 1~ less than 80%, by weight, e.g. between about 3 and about 14 80~, by weigh~, 3% being the lower limi~ of solubility of the earbamate in the solvent. This patent emphasizes the 16 importance of maintaining the carbamate in a substantially 17 completely dissolved state at reaction temperature duxing 1~ conversion to the isocyanate to minimize the formation of 19 polymerization products such as tars or resins as well as u~desirable by-products. Product alcohol is removed ~rom 21 the reaction mixture in the examples at atmospheric or 22 superatmospheric pressure. The patent fails to disclose - 2~ the catalysts described herein for the present invention 24 or the thermolysis of pol~meric aromatic carbamates.
U.S. Ratent No. 3,962,302 is directed to a 26 proc~ss for producins isocyanates by thermolysis of ~zr~amates while dissolved in a~ inert orga~ic solvent 28 and Ln the abs~nce of a cata~ys`t.. ' Reaction temperatures range from 175 to 350C ~preferably 20D to 3û0C) at carbamate concentrati~ns.of between 3% and 80~; by.
32 weight, of the reaction solutio~. Thi's paten~ fails t~
33~ disclose ~he thermolysis of ~olymeric ~romatic carbamates 34 ~nd the use of ca~a~ysts of the present invention wi~h any carbamates.
36 U.S. Patent No. 4,081,i72 is directed o a pro-37 cess for preparing aromatic isocyanates by the thermoly---6--' ::
. .

9L~76~;47 ~7-1 sis of an arvmatic carbamate at temperatures of 150 to 2 350C (preferably 200 ~o 300C) under substmospheric 3 pressure in the presen~e of a catalyst dissolved in an 4 iner~ solvent. The resulta~ isocyanate and alcohol must

5 be removed in ~apor form during $~e reaction and there-

6 after separately condensed (See Col.5 l~nes 55 et. seq.

7 and Col.9 lines 27 et. seq.). Consequently, the process

8 must be conducted at subatmbspheric pressure. Suitable

9 catalys~s include compounds of Cu, Zn, Al, Sn, Ti, ~, Fe, Co, and Ni. While it is disclosed as being desirable to 11 ........
12 dissolve~ ~he carbamate in a solve~t, the process can be 13 performed with .he carbamate in the suspended or emul-1 sified state (Col 8, lines 20 et. seq.). This patent does not disclose the thermolysi~ of p~lymeric aroma~ic 16 carbamates or the thermolysis o~ aromatic carbamates at 17 atmospheric ox superatmospheric pressurès.
18 ~.S. Patent No. 4,146,727 discloses a method 19 for preparing dicarbamates and polymeric carbamates. In this pa~ent it is suggested (See CoLl lsnes 25 et. seq.
21 znd Col.4 1ines 56 et. seq.) that the polym~ric carbama~`es 22 described therein can be therma~ly decomposed in a soivent 23 to their corresponding polymeric isocyanates in accordance 24 with two of the aforenoted patents, namely, U.S. Patent Nos. 3,919,279 and 3,962,302, notwithstanding ~he lack 26 of detail in either of these tw~ pa~en~s as desc~ibed 27 above,or the 4,146,727 paten~ as to how this can be achieved.
28 U.S. Patent No. 4,1~3,019 discloses a process for 29 preparing 4,4'-alkylidene diphenyl diisocyanate by a two step process involving the condensation of a phenyl alkyl 31 carbamate using an aldehyde or ketone to form a dimer, e.g., 32 dicarbamate, and an exchange reaction wherein a phenyl 33 isocyanate is mixed with the dicarbamate to ~orm a phenyl 34 alkyl carbamate and the corresponding diisocyanate. Certain tin compounds are disclosed as being suitable exchange 36 catalysts. This reference does not disclose a use of these 37 catalysts for the thermolytic cracking of carbamates in 38 the absence of an exchange reaction.

. .

~L3L7i~6~7 1 An article in Chemical Week, November 9, 1977, 2 ppu 57-58 discloses a process which comprises the steps of 3 reacting nitrobenzene, carbon monoxide and an-alcohol to 4 form corresponding urethanes (alkyl phenyl carbamates).
~ The reac ion product ;s reac~ed with formaldehyde to pro-6 duce a condensate which contains p,p'-methylene diphenyl 7 dialkylcarbamate and higher oligomers. This product is, in 8 turn, thermally split into the corresponding "polymeric 9 diisocyanates" and ~lcohol, which is recycled. The set o~
reactions is reported to involve the use of high tempera-11 tures in the range between l00 and 200C in the first 12 reaction step and between 200 and 300~C in the decomposi-13 tion step and the reaction leads to a mixture of polymeric 14 diisocyanates. This article does not disclose the use of the catalysts described herein for the present invention.
16 There are several ~ifficulties which one en-counters 'n attempting to conduct the~molysis of poly-meric axomatic carbamates. Such polymeric materials are g much less soluble in omm~n solvents ~han no~-polymerics.
ConsequentIy, even slight side reactions such as between ` isocyanate and carbamate reduce the solubility of ~he 23 polymeric reaction product even further than would other-24: wise result from similar rèactions using ~on-polymeric reactants. Once khe polymeric material starts to i~-26 solubilize, the formation of tars, gums and other undesir-27 ed by-pxoducts begins to accelerate. Low reaction temper-28 atures also decrease the decomposition reaction rate re-29 quiring longer reaction times. Longer reac~ion times can provide more opportunity for undesirable side reaction~
31 to take ~lace, although at a slower rate. If the reaction 32 temperature is raised to increase the reaction rate and 33 the solubility of the polymeric reactan~s and/or products 34 and by-products, ather u~desirable side reactions begin to take place at an accelerated pace at these elevated 36 temperatures. ~urthermore, if the concentration of poly-37 meric carbamate is too high in solution, the polymeric ~_ ~L76 6i4~
_9~
.

1 isocyanate product (which is non-volatile at- reaction con-2 ditions and cannot be economically removed from the re-3 action medium by vaporiæation) will xeact more readily 4 wit,h the polymeric carbamate to form an allophonate which 5 is even more insoluble than either the polymeric reactants 6 or product isocyanate thereby destroying the reaction 7 sequenceO Dilu~ing the reactants with solvent reduces 8 , the economic efficiency of the process and requires 9 greater capital investment in plant equipment.
Consequently, a balance must bP established 2 between reaction tempera~ure,'and polymeric carbamate 13~ concentra~ion ,and solu~i~ity to permit-~he process to be run economically. Accordingly and in view of ~he ~bove there has been a continuing search for ways to reduce 16 the decom~osition reactio~ tempera~urb of polymeric car-bamates wit~out sacrificing the ~eaction rate to any 18 great extent or alternati~ely to increase .he reactio~
19 rate at similar tempera~ures empl~yed in ~he absence of a: catalyst. A reduction in reaction' tempera,ture would 21 decrease undesirable si~e reactions induced by mor~
22 elevated temperatures.'`Increasing the reaction rate pro-23 ~i~es less time ~or undesirable side:reactions to ta~e 24' place untiI produc~ ~emoval.
2~ Regarding-non-polymeric'aromatic carbamates, 26 the aforedescribed prior art clearly.indicates that con~
27 ~entional disclosed reaction temperatures-~or the ther-28 molysis o~ the c~rbamates to f'orm ~he correspo~ding iso-29 cyana~es ~aries from about 175_ o 350C at atmospheric or supra atmospheric pressures. Accordingly, there has also been a continuing search for ways to either reduce 32 reaction pyrolysis temperatures 'of non-polymeric aroma.ic carb~,mates below 175C to reduce ùndesired condensation ' reactions which occur at eleva~ed temperatures and there-36 by increase selectivity to the isocyanate, or alternative-ly to i~creas~ non-polymeric aromatic carbamate decom-position reaction ra e at conventional pyrolysis ~erab~
_9_ :

-10- ~7~6~'7 1 to reduce the average reactant residence time in a re-2 actor thereby permitting a reduction in capital invest-3 ment in plant equipment (e.g. by reducing reactor size).
4 The present invention was developed in response to the aforedescribed searches.
Summary of the Invention ' In one aspect of the present invention, there is 8 provided a process for producing at least one ~romatic i30cy~
anate from at least one aromatic carbamate. These carbamat~s ~ are described herein and include polymeric as well as non-

11 polymeric aroTnatic carbamates. This process iq conducted

12 by heating a mixture or solution of at least one of said 3 carbamates in the presence of a metal containing catalyst 4 said metal being selected fronl the group con~isting of Ti, Sn, Sb, Zr and mixtures thereof, under conditions and in a manner sufficient-to convert said carbamate to at least one isocyanate, and-at least one alcoholl said heat-8 in~ being conducted at a pressure of at least atmospheric;
and separating said alcohol from said isocyanate and recovering the isccyanate from the solutionO

22 ~
23 In accordance with the present inventio~ aro-24 matic carbamates are thermally deco~posed into their corresponding isocyanate and alcohol in the prese~ce of 26 at least one ca~alys 2~ The ar~matic carbamate~ emp~oyed in the present 2~; in~ention can be catégorized int~ non-polymeric aromatic 29 carbamates and polymeri~ aromatic carbamates.
Non-polymeric aromatic carbamates (iOe. esters 3i of a carbamic acid) which can be employed in the process 32 of the present in~ention are repre~ented by the s~ructura 33 formula:
3 Rl-~NHcooR2)n (IV) 3 wherein Rl is a mono, di- or trivalent ~refPrably mono-37 or divalent) aromatic hydrocarbyl group containing typ-38 ically from about 6 to abou~ 32 (e.g. 6 to 22), preferably , ~766~

1 from about 6 to about 18 (e.g. 6 to 14) and most prefer-2 ably ~rom about 6 to about 10 (e.g. 6) carbon atoms.
3 Rl may contain an isocyanato group or a mono or divalent 4 substituent not reactive with an isocyanato group. R2 in structural formula IV is selected from monovalent:
6 saturated-a~iphatic, saturated-alicyclic, or aromatic 7 hydrocarbyI group having typically not greater than 10 8 (e.g. 8) carbon atoms, preferably not grçater than 6 cas-g bon atoms and most pref rably not greater than 4 (e.g. 2) carbon atoms, and may contain an isocyanato group or a 11 monovalent substituent not reactive with an isocyanato 12 group. Also in structural formula IV, n is a number of typically from 1 to 3, preferably 1 to 2, and most pre-14 ferably 1, and corresponds to the valenc~ of the Rl group.
Illustrati~e of the substituent Rl are aryl 16 groups such as phenyl, tolyl, xylyl, naph~h~l, biphenylyl, 17 anthryl, phenanthryl, terphenyl, naphthacenyl, pentacenyl 18 and methylene biphenyl groups and the divalent or tri 19 valen~ groups formed by removing on~ or two hydrogen a~oms respecti~e~y from these aromatic grQUpS . , These aromatic 2? groups may contai~ an isocyanato group; a substituent not 22 reactive therewith, such as an ~al~yl,typicai~y Cl to C5 23 alkyl group, a halogen atom, nitr~ group, cyano groupr an 24 aIXoxy group typically Ci ~ C5 al~ox~, an acyl group, an acyloxy group or an acylamido group; or à divale~ sub-26 stituent of similar nature, such as a methylene group, an 27 e~her group, a thioether group, a carbonyl grou~ cr a car-28 boxyl group.

Examples of the substituent R2 include aliphatic 31 groups, such as methyl, ~thyl; pxopyl, butyl, pen yl, hexyl, heptyl, octyl and methoxyethyl groups and alicycli~
33 groups, such as a cyclohexyl group.
~ epresentative examples of the carbamates utilizable -in the present invention include methyl -N-phenylcarbamate, ethyl phenylcarbamate,~ propyl phenyl-carbamate, butyl phenylcarbamate, octyl ~henylcarbamate, ethyl naphthyl-1-carbamat~ r ethyl anthryl-l-carbamate, - 1 1 ~

~76647 1 ethyl anthry1-9-carbamate, diethyl.anthryl~ne-9,10 di-2 carbamate, ethyl p-biphe~ylyl carbamate, die~hyl m~
3 phenylenedicarbamate, diethyl 'naphthylene-1,5-dicarbamate, 4 methyl p-tol'ylcarbamate, ethyl p-~rifluoromethylphenyl-carbamate, isopropyl-m-chlorophenylcarbamat~, ethyl 2-me~hyl-S-nitrophenylcarbamate, e'~yi ~-methyl-3-nitro-phenylcar~amate, ethyl 4-me'thyl-3-is'ocyanatophenylcar~a-mate, methyIene-~is(ph~nyl-4-methylcarbamate), dime~hyl toIylene-2,4-dlcarbamat`e, dieth~ tolylene 2.,4-diczr bamate, diethyl tolylene-2,~-dicarbamate, diisopropyl 2 t~lylene-2,4-dicarbamate, dibutyl tolylene-2,4-d~oar-~amate, diDhenyl'tolyle~e-2;4-dicarbamate, diphenyl tolylene-2,6-dicarbama~e,'di(ethoxyethyl) tolylene-2,4-dicarbamate, diethyl 4-chlorophenylene-1,3 dicarbamate, 16 methyl p-butoxylphenylcarbamate;, ethyl'p-acetylphenylcar-7 bamate, ethyl o-nitrophenylcarbama~e,lsopropyl m-tri-18 f1uoromethylphenylcarbamate, andt~ hyl-N-p~yl.tsi~amate.
Of these carbamate .compounds, the most practical examples are the toIylenedicarbamates~ naphthyle~edicar~amates,' 21 methylene-bis-(phenylcarbamates) and mixtures thereof.
22 The polymeric aroma~ic carbama~es which can be 23 employed'in'the process of the present inventi~n comprise 24 a mixture o~ carbamates, the componen~s of said mix~ure being represen~ed by ~he structural formula:
(X ~ Ar~- R ~ Ir~--R ~ Ar~--(X)n~ (V) 28 ~X)n~ ~ n~
29~ wherein: X represents the monovalent group -NHCO2R2, R2 being as defined in connection with structural formula IV
31 above; R3 is independently selected from (a) a divalent 32 straight or branched chain saturated aliphatic group having 33 typically from about 1 to about 10, preferably from about 1 34: to about 5, and most preferably from about 1 to about 2 car-bons, (b) a divalent saturated alicyclic group having typi-3~ cally from about 4 to about 10, preferably from about 5 to .

~L76647

-13-1 about 8, and mos~ preferably from about 6 to.abo~t 8 car-2 bons, and (c) a divalent aromatic-group having typically 3 from about 6 to about18, preferably from about 6 to about 4 14, and most preferably from about 6 to about 10 carbons;
n' is ~ number of from about 1 to about 4, preferably 6 from about 1 to about 3, and most preferably from about 7 1 to about 2 (e.g. 1~; Ar is a substituted or unsubsti-8 tuted aromatic hydrocarbyl group, typically an aromatic 9 hydrocarbyl group having from 6 to 14, preferably 6 to 10, and most preferably 6 carbons,exclusive o subs~ituents, 11 said substituents being selected from halogen (i.e., F
12 Cl, Br and I),-NH2, and mixtures thereof; and n" is a 13~ number which can vary from 0 to about 5 or higher on any

14 individual carbamate'in the mixture and the number averaqe value or n" for all the carbamates in the mixture typically 16 ~ill vary from about 2.0 to about 3.5, preferably 17 from about 2.2 to about 3.0, and most preferably from 18 , about 2~5 to abo~t 2.8. All of the aforenoted polymeric 19 aromatic carbamates are believed to be conventional in the art.
21 Representative examples of suitable R2 and R3 22 groups in structural formula V associated together in a 23 single carbamate include the following:

'26 27 m~thyl me~hylene 28: methyl dimethylene 29 methyl trime~hylene methyl methyethylene 31 methyl 'ethylethylene 32 methyl 2,2-dimethyltrime~hyIene 33 methyl 2-methyltrimethylene 34 ' methyl 1,3-cyclope~tylene methyl 1,4-cyclohexylene 36 methyl 1~4-phenylene 37 ~ ethyl ' methylene' , -13-.. ..

3L~7~647 ethyl dimethylene ethyl 2,2-dimethyltrimethylene isopropyl methyl isopropyl trimethylene isopropyl 1,4-phenylene cyclopentyl methylene phenyl methylene The most preferred R2 group is methyl since it will form an alcohol co-product having the lowest boiling point.
The most preferred polymeric aromatic carbamate is a mixture of poly-N-lower alkyl (e.g., Cl to C4) -poly-methylene polyphenyl carbamates.
In structural formula V, the identity of the substituents on the aromatic hydrocarbyl group can be con-trolled to be halogen in a manner effective to impart fire retardancy to the ultimate polyurethane into which the iso-cyanate derived from the carbamate is incorporated. More-over, some of these substituents in structural formula V can be residual -NH2 groups depending on and left over from, the method used to prepare the polymeric carbamate.
Methods for preparing non-polymeric aromatic carbamates are well known in the art and need not be comment-ed on further. The preparation of polymeric aromatic carbamates can be conducted in accordance with U.S. Patent Nos. 4,146,727; 4,172,948; and 4,202,986.
The catalyst which is employed to faciliate the thermolysis reaction comprises at least one metal, preferably utilized in the form of at least one metal containing polar compound, preferably polar organo compound, said metal being selected from the group consisting of Ti, Sn, Sb, Zr and mixtures. For homogeneous reactions ~.

~7~64~

-15-1 these metal containing compounds are preferably selected 2 in conjunction with a suitable inert organic solvent such 3 that the metal moiety (With which the catalytic activity 4 is associated) is soluble therein. Accordingly, the non-metal ~oiety of the catalyst compound preferably possesses 6 at least one p~lar functional group sufficient to solu-7 bilize, in the liquid carbamate (i.e. non-solvent embodi-8 ment) and/or solYent (i;e. solvent embodiment), a cataly-' g tic amount of metal as defined hereinafter. Accordingly while the preferred method for solubilizing the metal 11 catalyst is a metal pslar organic compound, any other 12 method for solubilizing the catalyst in an inert solvent can be employed.
Included within the scope of metal organic com-1~ pounds are metal salts with aliphatic, alicyclic and

16 aromatic carboxylic acids such as formic acid, acetic

17 acid, lauric acid, stearic acid, oxalic acid, azelaic '18 acid,naphthenic acid, tetrahydrophthalic acid, benzoic 19 acid, phthalic acid and pyromeilitic ac'id; metal ?lcoh~lates 20~ wi~h aliphatic a~d alicyclic alcohols such as methanol, 22 ethanol, propanol, buta~ol'; oc~anol,:dodecyl alcohol, 23 benzyl alcohol, e'thy~ene 'glycol; propylene glycol, poly-ethylene glycol, glycerol j pentaerythritol and cyclohexyl alcohol as ~eil as the correspondi~g metal thioalcohola~es;
metal phenolates with mo~ohydric or polyhyar'ic phenol 7 deri~ati~e5 such as phenol, cresol, nonylphe~ol, catechol -''2`8~--'~'an~ hydr'o~uinone as well as the corresponding metal thio-29 phenola~es; metal salts with sulfonic~acids such as meth-anesulfonic acid, éthanesul'fonic-aci'd, dodecanesulfonic 3j acid, cyclohexanesulfonic a~id, benzenesul~onic acid, 32 toluenesulfonic acid and dodecylbenzenesulfonic acid;
33 metal chelates with chelating agents, for example,beta 34 dike.bnes such a acetylacetone and:benzoylacetone, ketoesters such as ethyl acetoacetzte and ethyl benz-36 oaeetate; metal carbamates with the carbamates defined as 37 the starting material for the present invention as well as .. . .... . .

~L~766~7 1 the corresponding metal thiocarbamates and dithiocarbamate5;
2 metal salts with compounds having anionic li~ands such as 3 nitric acid group, phosphoric acid group, boric acid group 4 and cyanato group; and metal complexes of the above men-tioned variou-- metal salts with ligands having a non-6 covalent electron pair such as amines, phosphines, phos 7 phites, nitriles and amides.
8 Representative examples of suitable catalysts 9 include zirconium tetra 2,4-pentanedionate, tributoxy antimony, tetrabutoxy titanium, tetrapropoxy zirconium, 11 tetraoctyloxy titanium and mixtures thereof.
12 A preferred class of catalysts contain tin.
13 Such tin compounds preferably are organo-tin compounds 14 represented by the structural formula:
(R4)4-aSn(3)a (VI) wherein R4 is a hydrocarbyl group independently selected from alkyl, typically alkyl having ~rom about 1 to about

18, preferably from about 1 to about 10, and most pre-ferably from about 1 to about 5 carbons, and aryl, typi-21 cally aryl having from about 6 to about 14, preferably 2~2 6 carbons; B is independently selected from the group con-23 sisting of halogen ti.e. F, Cl, Br, I) preferably Cl, alkoxy, 24 (i.e., -OR), typically alkoxy having from about 1 to about 8, preferably from àbout i to about 6l and most preferably from about l to about 4 carbons; alkanoyloxy 27 (i.e., l ), typically alkanoyloxy having from about -O-C-R
28 1 to about 8, preferably from about 1 to about 6, and 29 most preferably from about 1 to about 4 carbons, oxo, and hydroxy; and "a" is an integer of from 1 to 3.
31 The group (R4) is preferably alkyl to enhance the vola-32 tility of the catalyst where desired.
33 Representative examples of suitable tin ca~alysts 3$ represented by structural formula VI include butyl-Sn (O)OHJ dipropyl dimethoxytin, dibutyloxotin, tributyl 36 methoxytin, triphenyl hydroxytin, trichloromethyltin, 37 dibutyldimethoxytin,-tributylmethoxytin, trimethylhydroxytin, -17- ~766~7 1 dichlorodimethyltin, trimethylchlorotin, triphenylethanoyl-oxytin, diphenyldichlorotin and mixtures thereof. Inorganic 3 halogenated tin compounds, such as tin tetrachloride, and 4 tin dichloride, can also be employed.
Preferred catalysts include butyl-Sn(O)OH, dibutyl-6 oxotin, tributylmethoxytin, triphenylhydroxytin, trichloro-7 methyltin, dibutyldimethoxytin, tributylmethoxyti~, 8 tin tetrachloride, and mixtures thereof.
g ~etal compounds which have been found to possess ineffective catalytic activity include tetraethyltin 11 (characterized by its lack of a polar functional group) 12 dichlorotriphenyl antimony (characterized by the +5 valence state of antimony), and titanium dichlorodi~2,4-pentane-dionate (characterized by extreme steric hinderance around the titanium metal moiety). Accordingly, in selecting a 16 suitable catalyst, the immediately aforenoted characteris-17 tics preferably should be avoided to obtain a catalyst 18 exhibiting effective activity.
9 The catalysts described herein, particularly the tin catalysts, have been found to substantially accelerate 21 the initial thermal decomposition of aromatic carbamates 22 to their corresponding alcohols up to a conversion of about 23 90%. This is extremely beneficial in terms of providing the option of conducting the reaction below conventional pyrolysis temperatures or of operatins at conventional temperatures but producing product at a much faster rate 27 thereby reducing the size of the reactor needed to produce similar quantities of isocyanate product obtained in accordance with conventional techniques. It is also an -advantage of the present invention that the process usinq 32 the aforedescribed catalysts is run at atmospheric 33 (i.e., 14 psia) or super atmospheric pressures (e.g., 34 14 to 200 psia). The use of atmospheric pressure permits the use of more desirable solvents which have relatively 38 low boiIing points and which would otherwise be recovered 37 with product at subatmospheric pressures necessitating additional separation steps. Tt also eliminates the need .

~7~6~7 1 for expensive vacuum e~uipment.
2 The process of the present invention is conducted 3 in the liquid phase by heating the carbamate, preferably 4 a solution of the carbamate, in the presence of the afore described catalyst. If no solvent is empl~yed, the 6 carbamate must be in the liquid state during the thermolysis 7 reaction. This is achieved by selecting the reaction 8 temperature to be above the carbamate melting point. To g dissolve the carbamate, an inert organic solvent is pre-ferably used. Any solvent which is stable at reaction 11 temperature, i.e., will not decompose or react with any 12 of the reactants, can solubilize the carbamate at reaction 13 temperature, and which has a boiling point above, pre-14 ferably at least 25 & above, the reaction temperature ~5 at the reaction pressure can be employed.
16 Thus, the inert organic solvent functions to 17 dissolve the carbamate as well as the resulting iSocyanate 18~ at reaction temperature, and optionally, to dissolve the

19 catalyst, and other by-products, if any. The inert org~nic solvent also functions to evenly disperse heat throughout 21 the reaction mixture, and to dilute the carbamate and 22 reaction products to the extent that undesirable side 23 reactions are kept to a minimum subject to economic con-24 siderations. Preferably,the solvent will also solubilize the catalyst although the catalyst can be employed in a 26 heterogeneous state, e.gO, in supported form.
Suitable inert organic solvents incl~de hydro-carbons, ethers, thioethers, ketones, thioketones, sulfones, esters, organo silane compounds, haloge~ated 3~
31 aromatic compounds and mixt~res thereof.
Representative examples of suitable solvents in-clude chlorobenzene, o-dichlorobenzene; diethylene glycol-34 dimethylether, triethylene glycoldimethylether, tetra-ethylene glycoldimethylether (also referred to as tetra-glyme), 1,6-dichioronaPhthalene, methoxy naphthalene;
aliphatic hydrocarbons such as the higher alkanes~
:
:, ....

~7~i6~7 1 dodecane, hexadecane, octadecane, and liquid paraffin;
2 the corresponding alkenes; petroleum fractions of 3 paraffin eries such as those usually employed as lubri-4 cating oils or cutting oils; alicyclic hydrocarbons such as petroleum fractions of the naphthene series; aromatic 6 hydrocarbons such as dodecylbenzene, dibutylbenzene, 7 methylnaphthalene, phenylnaphthalene, benzylnaphthalene, 8 biphenyl, diphenylmethane, terphenyl and aromatic petro-~ leum fractions usually employed as rubber-treatin~ oils;
and substituted aromatic compounds having no reactivity 11 with the isocyanate such as chloronaphthalene, nitrobi-12 phenyl and cyanonaphthalene; ethers and thioethers such as diphenyl ether, methylnaphthyl ether, diphenyl thio-14 ether and the like aromatic ethers and thioethers;
ketones and thioketones such as benzophenone, phenyl 16 tolyl ketone, phenyl benzyl ketone, phenyl naphthylketone 17 and the like aromatic ketones or thioketones; sulfones 18 such as diphenyl sulfone and the like, aromatic sulfones;
19 esters such as animal and vegetable oils, dibutyl phthalate, dio~tyl phthalate, phenyl benzoate and the 21 like aliphatic and aromatic esters; organosilane com-22 pounds such as conventional silicone oils and materials 23 thereof.
24 The preferred solvents include hexadecane, chlorobenzene, o-dichlorobenzene, diethyleneglycoldimethyl-26 ether, triethyleneglycoldimethylether, tetraethylene-27 qlycoldimethylether, dichloronaphthalene, methoxYnaphthalene 28 and-mixtures thereof.
29 While any amount of solvent effective to perform the aforedescribed functions can be employed~ such 31 effective amounts typically will constitute from about 32 50 to a~out 98, preferably from about 50 to about 90, 33 and most preferably from about 50 to about 80%, by weight, 34 based on the combined weight of solvent and carbamate.
35 - The amount of catalyst which is present during 36 reaction, preferably dissolved in the solvent, is any 37 amount effective to accelerate the pyrolysis reaction in ~ 76~

-20-1 relation to the uncatalyzed reaction. Thus, while any 2 effective amount of catalyst can be employed, such effec-3 tive amounts typically will constitute from about .001 4 to about O . 3 moles, preferably from about .01 to about 5 0:2 moies, and most pre~erably from about oOl to-about 6 0.1 moles of catalyst metal, per mole of carbamate ester 7 group on the carbamateO
The pyrolysis of non-polymeric aromatic carba-g mates is conducted at temperatures of from about 50 to about 200 &, (e.g., R0 to 200 C), and preferably from about 11 80 to about 150 &,(e.g., 100 to 125 &).
12 The pyrolysis of polymeric aromatic carbamates 13 is conducted at a temperature of from about 50 to about 14 300 C, preferably from about 80 to about 250 C, (e.g., 80 to 180c),and most preferably from about 80 to about 16 150C.
17 It is critical to the present invention that thP
18 pressure at which the pyrolysis reaction is run for either 19 polymeric or non-polymeric aromatic carbamates be at leas~
atmospheric. Supra atmospheric pressure~ can also be

21 employed.

22 The reaction time for the pyrolysis of non-poly-

23 meric aromatic carbamates will vary depending on the part-

24 icular carbamate selec~ed, the reaction temperature em~loy-ed, the type and amount of catalyst employed, an~ the part-2~ icular mode of reaction. However, the reaction ~ime is 27 shortened by the catalyst of the present invention relative 28 ,o the reaction time using no catalyst under similar react-ion conditions up to a conversion of about 90~. Accord-i~sly, for batch reactions conducted within the reaction 31 conditions recited above,reaction times will typically vary 32 from about 1 to about 120 minutes, preferably from a~out 1 to about 60 minutes, and most preferably from about 1 to about 15 minutes for non-polymeric aromatic isocyanates.
Reaction times under similar conditions recited above for 36 batch reactions employing polymeric aromatic isocyanates typically will vary ~rom about 0.1 to about 120 minutes, ~7~i4~7 1 preferably from about 0.5 to about 60 minutes, and most 2 preferably from about 1 to about 15 minutes.
3 Reaction times for a continuous process will vary depending on the concentration of carbamate at various steps within the reaction sequence (e.g. if multiple 6 reactions are employed).
7 As implied above, the process of the present 8 invention can be conducted in either a batchwise or g continuous manner. In a continuous process, for example, the carbamate, in powdery or molten~form or as a mixture 11 with inert solvent is supplied to at least one reactor 12 which has previously been charged with a given catalyst 13 and optionally additi-onal inert solvent and has optionally 14 been preheated to a selected reaction temperature and pressure. In the absence of a solvent, the reaction 16 temperature must be sufficient to permit the thermolysis 17 reaction to be conducted in the liquid phase, i.e., above 18 the melting point of the carbamate feed and isocyanate 1~ product. Thus, a liquid phase can be achieved by dis-solving the carbamate in a solvent as described herein 21 or by melting the carbamate in the absence of a solvent.
, ~
22 If the alcohol co-product is lower boiling than the 23 isocyanate, as is preferably the case, then the alcohol 24 can either be distilled from the solvent as formed or be removed by the assistance of an inert gas carrier (such 26 as nitrogen, argon, carbon dioxide, methane, ethane, 27 propane and mixtures thereof), being passed through the 28 solution, such as through a fitted disc or similar means 29 for dispersion or by the use of a solvent having a boiling point between the isocyanate and alcohol and distilling 31 between the boiling points of the isocyanate and alcohol.
32 By this means recombination of the isocyanates is minimized.
33 The use of a carrier gas is particularly preferred in the 34 absence of a solvent to facilitate alcohol product removal.
Alternatively, if the alcohol is higher boiling 36 than the generated isocyanate the isocyanate can be re-3~ covered in a manner similar to that described above for ., _ , ~76647 1 the alcohol co-product. Where a polymeric aromatic iso-2 cyanate is formed this option is unavailable since the 3 polymeric isocyanate is not volati~e at reaction tempera-4 tures. In this instance, catalyst and solvent can be removed from the reaction mixture by any means capable 6 of achieving this effect. For example, catalyst and 7 solvent can be removed overhead by distillation. Thus, 8 for this embodiment a catalyst is selected which is 9 sufficiently volatile to vaporize at distillation tempera-tures. Alternatively, the solvent is removed o~erhead, 11 and the catalyst removed from the reaction mixture by 12 contacting the polymeric isocyanate with a suitable ex-13 traction solvent, e.g. hexadecane, in which the catalyst 14 is preferentially soluble or which contains a complexing agent. It may even be commercially desirable to avoid 16 removal of the catalyst from the polymeric isocyanate 17 since these catalysts can also be employed as catalysts 18 in the formation of a polyurethane end product.
19 Desirably, the reaction conditions are controlled to achieve as high a degree of conversion as possible to 21 avoid the need to separate unreacted polymeric carbamate 22 from the poIymeric aromatic isocyanate. This can be 23 achieved by increasing the amount of catalyst and/or the 24 degree of alcohol co-product removal. However, since the polymeric aromatic carbamate is less soluble in the sol-26 vents described herein than the polymeric aromatic iso-27 cyanate product, separation of polymeric carbamate from 2~- polymeric isocyanate product can be achieved by solvent 29 extraction techniques which make use of these different solubilities when the reaction is conducted at low con-31 version. The virtual elimination of identifiable by-32 products make this technique extremely simple.
33 Unreacted carbamate can be collected and fed to a 34 second reactor or recycled.
It has been further found that the process of the 36 present invention performs increasingly better as the purity 3 of the carbamate employed therein increases. In some in-.

-~669L7 stances commercially available carbamates may possess impuri-ties which insolubilize during reaction in a disadvantageous manner.
The present invention for the pyrolysis of polymeric aromatic carbamates to the corresponding polymeric isocyanates can be incorporated into the multi-step process described in co-pending Canadian Patent Application Serial No. 395,963, filed 10 February 1982 of common assignee for the production of polymeric aromatic carbamates and isocyanates from alkylated aromatic compounds.
The aforesaid multireaction step process has the following sequential steps: ammoxidation of an alkylated aromatic compound to form a nitrile, hydrolysis of the nitrile to an amide, conversion of the amide to a carbamate via e.g., a Hofimann rearrangement, condensation of the carbamate with an aldehyde to form a polycarbamate, and optionally, decomposi~
tion of the polycarbamate to a polyisocyanate.
The invention disclosed and claimed herein has particular utility in the aforesaid multireaction process as an improved means for decomposition of the polymeric aromatic carbamate to the corresponding polymeric aromatic polyisocyanate.
The following examples are given as specific illustrationsof the claimedinvention. It should be understood, however, that the invention is not limited to the specific details set forth in the examples. All parts and percentages in the examples as well as in the remainder of the specifica-tion are by weight unless otherwise specified.
In the examples which follow unless specified otherwise, the equipment used to conduct the reaction con-sists of a lOOml round bottomed indented three-necked flask equipped with a GlasColTM high temperature heating mantle and magnetic stirbar. A thermometer is connected through an open-ended U-shaped tube to allow addition of Jj -24- ~766~
re~ction mixture components which are syringed in through a rubber septum on the other opening of the "U". The center neck is fitted with two, stacked, water-cooled 6" condensers through which the gases exit. The upper co~denser has the ability to collect condensed liquids to prevent contamina-tion of the reactor by potentially reintroducing condensed 7 alcohol. Nitrogen is drie~ through a bed of Linde 4AT~
molecular sieves after passing through a Drierite TM column and dispensed under the reaction liquid level. Gases exiting the reaction equipment also pass through a Drierite column which also has a slight positive nitrogen flow from 12 a bubbler. This is to prevent air from entering the flask ~3. when the nitroqen addition tube is opened for samplingO
14 The nitrogen rate is regulated with a Fisher-Porter FlowraterT~ tube which has been calibrated against a 16 wet-test meter. Temperature is controlied with an I2R thermowatch device.

19 Example 1 The following example is intended to illustrate 21 ~he effect of various catalysts on the first order rate 2~ constant for the pyrolysis reaction. The general procedure 23 for conducting the reaction is as follows. To a dried nit-24 rogen flushed flask as described above is charged 50 ml of solvent. The solvent (i.e~ tetraglyme) is then pre-26 heated under atmosphexic pressure to 200 ~ and maintained 27 thereat during the course of the reaction. To this flask 28 is then added, over a period of 1-2 minutes, 5 9 l31 mmoles) 29 of methylenediphenylene dicarbamate (~DC) and sufficient catalyst to achieve about a 10 mole ~ concentration based 31 on the moles of carbamate. Samples of isocyanate product 32 are removed from the flask over time and quenched by adding 33 them to a solution of dibutylamine (referred to herein as 34 DBA) dissolved in tetraglyine solvent (10~ DBA by weight 36 of the solution) to form a urea derivative fr~m the isocy-: 36 anate present in each sample. This derivative, which is indicative o the moles of isocyanate formed is analyzed by : -24-, . .

~L3L766~L7

-25-1 a Hewlett-Packard 1084B high pressure liquid chromatograph (referred to herein as HPLC) on a C8 reverse phase column 3 using water and acetonitrile mobile phases. The la~t sam 4 ple is taken after about 120 to 180 minutes of reaction.
5 Sample analysis is also confirmed by infrared analysis 6 f the product solution co~taining the isocyanate product 7 and by comparison of the isocyanate in the sample with a 8 control sample.
g A linear pIot is drawn of the log of isocyanate concentration (determined by HLPC analysis) versus time.
11 From the slope of this linear plot the first order rate 12 constant is determined. The first order rate constants for various catalysts employed in conjunction with ~DC
14 determined in accordance with the above procedures are summarized at Table 1. For control pur~oses one run is 16 conducted in the absence of a catalyst.

18 TAELE 1 First Order Rate Run No. Catalyst* Constant K~\min.-l-at-2~Qoc) 1 None .0134 21 2 Bu2Sn(OMe)2 .0247 22 3 Sb(OMe)3 .0276 24 *8u=Butyl Me=Methyl

26 From the above rate constants, it can be seen

27 that the catalysts substantially improve the rate of the

28 pyrolysis reaction at 200 &.

29 Exam~le 2 j To a nitrogen flushed 100 ml flask equipped as 32 described above is charged at atmospheric pressure 4.589 (30 m moles) of methyl-N-phenyl~carbamate, 2.94g of 34 chlorobenzene as an internal standard, 0.899 dibutyltin dimethoxylate as the catalyst and 25 ml of 1,2-dichloro-36 ethane as solvent. The solution is heated to reflux 37 (88C) atmospheric pressure and 5.8 ml of solvent contain-.. ...

-26- 1~766~

1 i~g methanol is slowly distilled overhead ovex a period of 2 75 minutes. A sample of the undistilled product is analyzed 3 by Gas Phase Chromatographic Analysis (hereinafter GPC).
~ The sample analysis shows 5.2 m moles of phenylisocyanate is formed at a selectivity of 99 mole % and conversion 6 of 17.3 mole %.
7 Example 3 8 The procedure in Example 2 is repeated except for 9 replacement of the 1,2-dichloroethane wit~ toluene. The reaction is ~onducted for a period of 40 minutes at a 11 temperature of 87 C. Toluene forms an azeotrope with 12 methanol and facilitates its removal from the reaction 3 mixture. The selectivity to phenylisocyanate is 99 mole 14 ~ and the conversion is 33 mole %.
Example 4 7 A 50 ml 3-necked flask equipped with a thermometer (with Thermowatch~, a magnetic stirrer, a short path dis-ti~lation head, and a nitrogen sparger is charged with15.23g methyl-N-phenyl carbamate and .8137g di-butyl tin dimethoxylate.
21 The mixture is heated at atmospheric pressure within the range 22 of from 100 to 120 & over 3 hours with nitrogen sparging.
23 After 3 hours of reaction GPC analysis shows a selectivity 24 to phenyl isocyanate of 99 mole ~ and a conversion of 2~ 40 mole %. After continued heatin~ ~or an additional ~6 peric~ of 3 hours, the selectivity is ~5 mole ~ and the 27 conversion is 44.3 mole %. On coolin~, the solution 28~ separates into two phases (i.e. solid and liquid). To 29 check for methanol removal, a small amount of methanol is added. The liquid phase became solid white and a 31 lo& temperature exotherm is measured, thus indicating 32 the presence of active isocyanate groups. This example 33 demonstrates that the thermolysis reaction can be conducted 34 in the absence of a solvent and that at initial high con-centrations of feed carbamate,isocyanate product can be 36 generated without the formation of undesirable by-products because of the reduced temperature employed and the sparging . . .

-27~ 66~7 1 with nitrogen ~as to facilitate alcohol removal. However, 2 in the absence of a solvent, the carbamate m~st be in 3 ~he liquid sta~e at reaction conditions.
4 ~ E~
. A solution of a l.Og of the dimethyl carbamate 6 of methylene diphenylene diisocyanate is dissolved in 50 7 ml of 1,2-dichloroethane and heated at atmospheric 8 pressure to re~lux ~88&)o At reflux, O.O9y of dipropyl g tin dimethoxylate is added. GPC analysis of the product solution after 13 minutes of reaction ~hows a substantial 11 conversion to the diisocyanate methylene diphenylene 12 diisocyanate with no identifiable by-products. This example illustrates that the carbamate groups react independently 14 and that conversion of one carbamate group to its corres-16 ponding isocyanate does not affect convefsion of others 16 in the same molecule. Consequently, the thermolysis 17 reaction can be viewed as a series of first order reactions, 18 with each carbamate functionality acting independently.
19 This applies to any polyfunctional carbamate of ~he type described herein.
21 It also illustrates that the thermolysis reaction 22 can be conducted at low temperatures, e.g. 88 C.

24 The principles, preferred embodiments, and modes of operation of the present invention have been described 26 in the foregoing specification. The invention which is 27 intended to be protected herein, however, is not to be con-strued as limited to the particular forms disclosed, since thes~ are to be regarded as illustrative rather than rest-rictive. Variations and changes may be made by those skill-31 ed in the art without departing from the spirit of the in-32 ~^ntion ~27-

Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for producing at least one aromatic isocyanate from at least one aromatic carbamate repre-sented by the structural formula selected from at least one member of the group consisting of:
(I) and (II) said structural formula II representing a mixture of car-bamates and wherein in structural formula I: R1 is a monovalent, divalent, or trivalent aromatic hydrocarbyl group containing from about 6 to about 32 carbon atoms;
R2 is an monovalent hydrocarbyl group selected from saturated-aliphatic, saturated-alicyclic, or aromatic, said hydrocarbyl group having not greater than about 10 carbon atoms; and n is a number of from 1 to 3 correspond-ing to the valency of R1; and wherein in structural formula II: X is the monovalent group -NHCOOR2 with R2 being as defined in connection with structural formula I; R3 is independently selected from the group consisting of (a) a divalent straight. or branched chain saturated ali-phatic group having from about 1 to about 10 carbon atoms, (b) a divalent saturated alicyclic group having from about 4 to about 10 carbon atoms, and (c) a divalent aromatic group having from about 6 to 14 carbon atoms; Ar is a substituted or unsubstituted aromatic hydrocarbyl group having from 6 to 14 carbon atoms, said substituents being selected from halogen, -NH2, and mixtures thereof; and n" is a number, the number average value of which in said mixture can vary from about 2.0 to about 3.5, and n' is a number of from about 1 to about 4; which comprises.

(1) heating in the liquid phase at least one of said carbamates in the presence of a metal containing catalyst said metal being selected from the group consisting of Ti, Sn, Sb, Zr and mixtures thereof, under conditons and in a manner sufficient to convert by thermolysis said carbamate to at least one isocyanate, and at least one alcohol, said heating being con-ducted at a pressure of a, least atmospheric and said catalyst being effective to accelerate the thermolysis reaction rate relative to the thermolysis reaction rate in the absence of a catalyst; and (2) separating said alcohol from said isocyanate and recovering the isocyanate.

2. The process of Claim 1 wherein said heating is conducted by dissolving said carbamate and a metal catalyst containing polar organic compound into an inert organic solvent to form a solution and heating the solution.

3. The process of Claim 2 wherein: the catalyst is represented by the structural formula (R4)4-aSn(B)a:
wherein R4 is a hydrocarbyl group independently selected from alkyl of from about 1 to about 18 carbons, and aryl of from about 6 to about 14 carbons; B is independently selected from the group consisting of halogen, alkoxy having from about 1 to about 8 carbons, alkanoyloxy having from about 1 to about 8 carbons, oxo, and hydroxy, and "a"
is an integer of from 1 to 3; and the inert organic solvent is selected from the group consisting of hexadecane, chlorobenzene, o-dichlorobenzene, diethyleneglycol dimethyl-ether, triethyleneglycol dimethylether, tetraethyleneglycol dimethylether, dichloronaphthalene, methoxynaphthalene, and mixtures thereof.

4. The process of Claim 3 wherein the carbamate is represented by structural formula 1.

5. The process of Claim 3 wherein the carbamate is represented by structural formula II.

6. The process of Claim 4 wherein the solution which is heated comprises from about 50 to about 98%, by weight, inert organic solvent, based on the combined weight of solvent and carbamate, and from about .001 to about 0.3 moles of metal in the catalyst per mole of carbamate ester group on the carbamate.

7. The process of Claim 5 wherein the solution which is heated comprises from about 50 to about 98% by weight, inert organic solvent, based on the combined weight of solvent and carbamate, and from about .001 to about 0.3 moles of metal in the catalyst per mole of carbamate ester group on the carbamate.

8. The process of Claim 6 wherein the solution is heated to a temperature of from about 50 to about 200°C.

9. The process of Claim 6 wherein the solution is heated to a temperature of from about 80 to about 150°C.

10. The process of Claim 7 wherein the solution is heated to a temperature of from about 50 to about 300°C.

11. The process of Claim 7 wherein the solution is heated to a temperature of from about 80 to about 180°C.

12. The process of Claim 5 wherein in the carbamate of structural formula II, n' is 1, R3 is methylene, and the average value for n" is from about 2.2 to about 3Ø

13. The process of Claim 12 wherein R2 is methyl.

14. The process of any one of Claims 1 and 2 wherein the catalyst is selected from the group consisting of zirconium tetra 2,4-pentanedionate, tributoxyantimony, tetrabutoxyantimony, tetrabutoxytitanium, tetrapropoxy zirconium, tetraoctyloxytitanium, butylhydroxyoxotin, dipropyldimethoxytin, dibutyloxotin, tributylmethoxytin, triphenylhydroxytin, trichloromethyltin, dibutyldimethoxy-tin, tributylmethyloxytin, trimethylhydroxytin, dichloro-dimethyltin, trimethylchlorotin, triphenylethanoyloxytin, diphenyldichlorotin, tin tetrachloride, tin dichloride, and.
mixtures.