Phosgene routes

More recendy, preparation of carbonic esters by nonphosgene routes, such as the reactions of CO or CO2 with appropriate substances in the presence of catalysts, has been preferred. These methods are more economic in many cases and naturally less ha2ardous than phosgene routes. 

Dimethyl carbonate (DMC) is a colorless liquid with a pleasant odor. It is soluble in most organic solvents but insoluble in water. The classical synthesis of DMC is the reaction of methanol with phosgene. Because phosgene is toxic, a non-phosgene-route may be preferred. The new route reacts methanol with urea over a tin catalyst. However, the yield is low. Using electron donor solvents such as trimethylene glycol dimethyl ether and continually distilling off the product increases the yield. ... 

Diphenol carbonate is produced by the reaction of phosgene and phenol. A new approach to diphenol carbonate and non-phosgene route is by the reaction of CO and methyl nitrite using Pd/alumina. Dimethyl carbonate is formed which is further reacted with phenol in presence of tetraphenox titanium catalyst. Decarbonylation in the liquid phase yields diphenyl carbonate. 

There have been many attempts to develop non-phosgenation routes to synthesize diisocyanates and for the synthesis of polyurethanes without the use of isocyanates. None of these has been applied commercially. In the conventional process, the addition of the... 

The above processes are only selected examples of a vast number of process options. In the case of carbonylation, the formation of by-products, primarily isocyanate oligomers, allophanates, and carbodiimides, is difficult to control and is found to greatly reduce the yield of the desired isocyanate. Thus a number of nonphosgene processes have been extensively evaluated in pilot-plant operations, but none have been scaled up to commercial production of diisocyanates primarily due to process economics with respect to the existing amine—phosgene route. Key factors preventing large-scale commercialization include the overall reaction rates and the problems associated with catalyst recovery and recycle. 

Scheme 7.1 The chlorine cycle for DMC synthesis via the phosgene route.

Although only low molecular weight polycarbonates in modest yield have been obtained by this method, it represents an interesting non-phosgene route to aromatic polycarbonates. 

The more volatile isocyanates (such as the monoisocyanates) are inherently hazardous [651] and it makes little sense, in most cases, to invent or evaluate non-phosgene routes to these compounds on the grounds of toxicity considerations alone. However, isocyanate production from phosgene results in the formation of large quantities of hydrogen chloride, and the isocyanate products may be contaminated with chlorine compounds which are difficult to... 

The carbonylation of amines and of nitrocompounds is of interest because it provides a non phosgene route to the synthesis of carbamates, isocyanates and ureas which are products of remarkable commercial value. 

Application The Polimeri/Lummus process is a non-phosgene route using CO, CH3OH (methanol) and 0 to produce dimethyl carbonate (DMC). DMC is a nontoxic intermediate used in the production of polycarbonates, lubricants, solvents, etc., and is also used directly as a solvent or a gasoline/diesel fuel additive. This environmentally safe process can be applied to large capacity plants. 

The feedstock specifications for carbon monoxide used in production of polycarbonates by the phosgenation route are shown in Table 3 [3]. 

Polycarbonates-Non Phosgene Routes, PERP Report, Chem Systems, New York (April 1991). 

This innovative research has resulted in a commercial DuPont process that makes MIC and converts it in-situ to an agrichemical product. Consequently, the potential for exposure is greatly reduced. This trend in in-situ manufacture and derivatization is clearly the way of the future for hazardous chemicals. Non-phosgene routes to isocyanates, and use of solid acids to avoid HP and H2SO4 as alkylation catalysts are other examples of research in progress to minimize fur er the use of hazardous materials. 

This route appears interesting, since it is a single-step synthesis and does not bear the corrosion problems as the phosgenation route. 

However the reaction is rather complexfl233i and besides the use o-f the dangerous phosgene, the presence o-f the corrosive HCl creates several problems. Aliphatic isocyanates can be also obtained -from ole-fins with isocyanate ion in the presence o-f a salt or coordination compound o-f palladium or platinum 024], -from ole-fins with isocyan I c acid in the vapor phase over Pt/Al2O3025], and -from -formamides by oxidation over a silver catalystfl26]. However apparently these methods cannot compete with the phosgene route. 

Isophorone diisocyanate (IPDI) has traditionally been manufactured from isophorone diamine (IPDA) by the conventional phosgenation route as shown below ... 

Recent publications can be divided into two major groups those concerned with the refinement and extension of the conventional phosgenation route to the MDI family of isocyanates (based on 4,4 -di-isocyanato-diphenylmethane, pure MDI ), and those dealing with new routes to isocyanate products. 

A number of studies of the important (non-phosgene) route to aryl isocyanates via carbonylation of nitro compounds have appeared. In a comparative study of Rh(I), Ir(I), Pd(I) and Pd(II) catalysts, Bu,N+[RhX2(CO)2] (X Cl, Br, I) was most effective giving 83-89% PhNCO with 100% PhN02 conversion at 125 C under 80 atm CO pressure . The kinetics of carbonylation by [PdCl2py2l at 170-230 C and 23-94 atm CO pressure are first order in p[CO] and catalyst and zero order in PhNOj . The reductive N-carbonylation of nitroarenes to the carbamates is catalysed by tPtCl2 (PPh3 >2] in ethanol, promoted by Lewis acids (SnCli,... 

Perhaps ideally, the overall target for urethane-based automobile structural panels, is a process based on a non-phosgene route to the isocyanate (currently... 

This route appears to be interesting, since it is a single-step synthesis and does not bear the corrosion problems encountered in the phosgenation route. This reaction requires the presence of a catalyst, most often a compound of a group VIII metal, and proceeds at elevated temperatures and pressures. It has been particularly studied in the case of aromatic nitro compounds, since aliphatic nitro compounds tend to react through their aci form and afford different products. 

Diphenyl carbonate can be made from phenol and phosgene, but if it is made by a non-phosgene route, it can avoid the use of the toxic gases phosgene and chlorine. Diphenyl carbonate can be made from dimethyl carbonate. Dimethyl carbonate is available from the reaction of methanol with carbon monoxide [14] or with carbon dioxide [15]. 

Molecular weights of between 30,000 and 50,000 g/mol can be obtained by the second route, while the phosgenation route results in higher-molecular-weight product. 

Another polyester type, i.e., polycarbonate, is rather seldom solvolytically depolymerized because of losing (at least partially) carbonate groups/bonds obtained via phosgene route synthesis. Troev et al. [29] recently described PC chemical degradation with dialkyl phosphonates (dimethyl or diethyl) or triethyl phosphate. The products, oligomeric carbonates containing phosphorus atoms, can be considered as precursors for the modification of various polymers by improving their flame-retardant properties, thermal stability, and adhesion. 

This reaction has an atom efficiency higher than the phosgene route, and is much safer and cleaner than the ENIChem and UBE processes that feature a comparable use of atoms (Scheme 1.15). The existing limitation to the exploitation of the reaction is the low yield at equilibrium that ranges between 1 and... 

---