Replacement of Hazardous Chemicals the Case of DMC

Dimethyl carbonate (DMC) is a versatile compound that is an attractive alternative to phosgene [68, 69] and which could be synthesized in a eco-friendly process by catalytic oxidative carbonylation of methanol with oxygen (Enichem, Italy [70] and

ch3nh2

CH3CNO

CH3CNO

Phosgene Production Process

Bhopal process

OCONHCH

Alternative route

Figure 1.9 Bhopal and alternative routes to N-methyl 2-naphthyl carbamate [67].

UBE, Japan [71] processes). DMC can act as a substitute for phosgene and toxic methylating agents like dimethyl sulfate and methyl chloride, and can also find use as a safe solvent and as a possible emission-reducing, high-oxygen-containing additive for fuels [69, 72]. The topic of phosgene substitution is of high relevance because its high reactivity has been known since the beginning of the chemical industry. However, owing to the high reactivity and toxicity its utilization is increasingly burdened by growing safety measures to be adopted during the production, transportation, storage and use and by growing waste disposal costs to be faced. The reasons for phosgene substitution stem not only from considerations relating to its high toxicity, but also to the fact that its production and use involves chlorine as a raw material and results in the generation of large amounts of halogenated by-products. The formation of HCl and chlorine salts as by-products gives rise to contaminated aqueous streams that are difficult to dispose of, or the credited value of the by-produced HCl may render uneconomical its recovery, purification and re-use. Moreover, reactions involving phosgene often require the use of halogenated solvents, like CH2Cl2 and chloro- or o-dichlorobenzene, which are likely to raise environmental problems.

Phosgene ranks highly among the industrially produced chemicals. Although its production output is most exclusively captive and, therefore, only approximate production statistics are available, a yearly worldwide production of about 5-6tyr_1 can be estimated. The main uses of phosgene are in the production of isocyanates and polycarbonates. The production of isocyanates represents the major output. Of exceeding importance is the production of di- and polyisocyanates, both as commodities, like TDI (toluene diisocyanate) and MDI (diphenylethane diisocya-nate), and as specialties, like the aliphatic isocyanates (HDI - 1,6-hexane diisocyanate, IPDI - isophorone diisocyanate and HMDI - dicyclohexane diisocyanate), for the production of polyurethanes. Monofunctional isocyanates like methyl isocyanate, cyclohexyl isocyanate and aryl or phenyl isocyanate are used in lower amounts for agrochemicals and pharmaceutical products.

The production of polycarbonates, mostly the aromatic polycarbonates derived from bisphenol A, is the second largest area of phosgene usage and is probably the most important growing area. It accounts for about 1.5 tyr-1 of polycarbonates, corresponding to a yearly phosgene consumption over 0.6 ty-1.

DMC is classified as a non-toxic and environmentally compatible chemical [69]. In addition, the photochemical ozone creation potential of DMC is the lowest among common VOCs (2.5; ethylene = 100). The areas in which DMC acts, or can act, as a potential phosgene substitute correspond to the main areas of phosgene industrial exploitation, that is, production of aromatic polycarbonates and isocyanates, leading the production of these important chemicals out of the chlorine cycle.

The traditional production of DMC involves phosgene:

Therefore, any possible use of DMC as substitute of phosgene should be based on a different synthesis of DMC, not involving phosgene. Non-phosgene alternative routes for DMC production, basically, have relied on the reaction of methanol with carbon monoxide (oxidative carbonylation) or with carbon dioxide (direct carboxyl-ation with CO2, or indirect carboxylation, using urea or alkylene carbonates as CO2 carriers) (Figure 1.10) [72].

Oxidative carbonylation of methanol to DMC, which takes place in the presence of suitable catalysts, has been developed industrially by EniChem (later Polimeri Europa). Carbonylation/transesterification of ethylene oxide to DMC via ethylene carbonate is also an attractive route. However, this route is burdened by the complexity of the two-step process, the co-production of ethylene glycol (even if it

Oligomer Phosphorus Reaction Isocyanate
Figure 1.10 DMC synthesis routes.

could be recycled) and the use of toxic and risky ethylene oxide, which is presented as carbon-friendly because it allows the use of CO2 instead of CO.

The UBE process was also developed on a commercial scale, in Japan, and uses methyl nitrite as intermediate for the gas-phase palladium-catalyzed (PdCl2/CuCl2 on active carbon) carbonylation to DMC.

Copper compounds, besides being the most widely used co-catalysts for palladium re-oxidation, are themselves active in DMC formation. Exploiting the catalytic properties of CuCl, EniChem developed its DMC production process of one-step oxy-carbonylation of methanol. This process has operated industrially since 1983. The single step is carried out in the liquid phase in a continuous reactor fed with CH3OH, CO and O2. Reaction conditions are in the range of 120-140 °C and 2-4 MPa. The CO:O2 ratio is kept outside the explosion limits by the use of a large excess of CO and the adopted high oxygen conversion per pass. As depicted in Figure 1.11, the reactor-evaporator concept is adopted: the catalyst is kept inside the reactor, where the products are vaporized, mainly taking advantage of the heat of reaction (DHr = — 74kcalmol_1), and removed from the reaction system together with the excess gas leaving the reactor [73]. This design allows the use of high catalyst concentrations and simplifies catalyst separation from the products. Quite high DMC productivity (up to 250gL—1h—1) is achieved under optimized reaction conditions.

The use of CuCl as a catalyst affords minimization of by-products, high purity of the product and practically endless catalyst life. The only co-products are water and CO2, which are produced in substantial amounts. By adopting a suitable process, the co-produced CO2 can be re-utilized as a carbon source in the CO generation. All these features characterize the presented DMC production process as a clean technology.

Since a halide free, non-corrosive catalyst for DMC production would be a further process improvement, alternative catalytic systems have been investigated. Cobalt(II) complexes with N,O ligands, such as carboxylates, acetylacetonates and Schiff bases, have been shown to produce DMC with a good reaction rate and selectivity [74].

Production Phosgene
Figure 1.11 Conceptual scheme of the EniChem one-step DMC production process. Source: Rivetti [72].
Table 1.5 Comparison between DMC- and phosgene- or dimethyl sulfate (DMS)-based reactions. Source: Tundo [75].

Phosgene or DMS

DMC

Dangerous reagent

Harmless reagent

Use of solvent

No solvent

Waste water treatment

No waste water

NaOH consumption

The base is catalytic

By-products: NaCl, Na2SO4

By-products: MeOH, CO2

Exothermic

Slightly or not exothermic

DMC is thus considered a prototype example of a green reagent, since it is nontoxic, made by a clean process, is biodegradable and it reacts in the presence of a catalytic amount of base, thereby avoiding the formation of undesirable inorganic salts as byproducts [75-77]. Table 1.5 shows the major environmental benefits of DMC-based procedures. DMC Enichem/Polimeri Europa technology has been licensed to (i) General Electric Japan for a DMC/DPC unit at Chiba (DMC unit was 11.7 ktyr-1 capacity and started up in 1993) and (ii) General Electric España for a DMC/DPC unit at Cartagena (DMC unit was 48.3 ktyr-1 capacity and started up in 1998; in 2004 the total capacity has been increased to 96.6 ktyr-1 with the start up of a second unit).

DMC has been proven to perform advantageously as a substitute for phosgene in several reactions. A non-phosgene process for the melt polymerization production of aromatic polycarbonates has been established commercially [69, 72]:

Phosgene Production Process

This process also avoids the use of methylene chloride as a solvent and the co-production of NaCl salt. Another well-established application of DMC in the field of polycarbonates relates to the production of poly[diethyleneglycol bis(allylcarbo-nate)], a thermosetting resin used in the production of optical glasses and lenses. The non-phosgene process involves the intermediate formation of diallyl carbonate from DMC - whereas the traditional process was based on the use of diethylene-glycol bis(chloroformate) that in turn was obtained from phosgene - and allows high flexibility in terms of customer-tailored products.

The non-phosgene production of isocyanates takes place through thermolysis of the corresponding carbamate. The carbamate synthesis may involve several alternative possible ways, such as the reaction of a nitro-compound with CO, or the reaction of an amine with CO and O2, with urea and alcohol, or with a carbonic ester. Among these routes, the reaction of DMC, or DPC (diphenyl carbonate), with aliphatic amines is a very efficient way to produce carbamates.

A non-phosgene process for the production of methyl isocyanate, starting from methylamine and diphenyl carbonate as raw materials, has been established by EniChem/Polimeri Europa, resulting in the commercialization of two production units in the USA (1988) and China (1994) [78].

Recently, a comparative evaluation of dimethyl carbonate versus methyl iodide, dimethyl sulfate and methanol as methylating agents has been made in terms of green chemistry metrics [79]. These provide a quantitative comparisons based on measurable metrics able to account for several aspects of a given chemical transformation, including: (i) the economic viability, (ii) the global mass flow and the waste products and (iii) the toxicological and eco-toxicological profiles of all the chemical species involved (reagents, solvents, catalysts and products) [80-83].

Figure 1.12 summarizes the result of this comparative evaluation of DMC as green methylating agent. The assessment was based on atom economy (AE) and mass index (MI) for three model transformations: O-methylation of phenol, the mono-C-methylation of phenylacetonitrile and the mono-N-methylation of aniline. In terms of chemical and toxicological properties, DMC shows the lower toxicity index (LD50, e.g., lethal dose for 50% of rats in toxicology experiments) and irritating properties, but costs about twice as much as methanol.

The atom economy (AE, as percentage) was calculated considering the massbalance of a process related to its stoichiometric equation, that is, the percentage of atoms of the reagent that end up in the product:

MW product

SMW of all reagents used where MW is the mass weight in gmol-1. To include the chemical yield and the selectivity towards the desired product, as well as the mass of all reagents, solvents, catalysts, and so on, used in the examined reactions a more all-encompassing metric, the mass index (MI), could be used. All values are expressed by weight (kg).

Mi S reagents + catalysts + solvents + etc.

Desired product

The atom economy generally follows the trend (Figure 1.12a): MeOH » DMC > DMS>MeI

Two factors account for this behavior: (i) for methanol, 47% of its mass is incorporated in the final products, more than twice as much as the other reagents (DMC 16%, DMS 12%, or 24% when both methyl groups are incorporated, MeI 11%) and (ii) methanol and DMC require catalytic base or zeolites, as opposed to DMS and MeI. MeOH and DMC offer similar low values of MI (on average, in the range 3-5.5), which are better than those achievable with DMS and MeI (Figure 1.18b).

Zeolites Sustainable Chemistry
DMS Mel DMC MeOH
Zeolites Sustainable Chemistry

DMS Mel DMC MeOH

Figure 1.12 Atom economy (a) and mass index (b) for the reaction of phenol, phenylacetonitrile and aniline with different methylating agents. Source: Selva and Perosa [79].

DMS Mel DMC MeOH

Figure 1.12 Atom economy (a) and mass index (b) for the reaction of phenol, phenylacetonitrile and aniline with different methylating agents. Source: Selva and Perosa [79].

DMC thus yields very favorable mass indexes (in the range 3-6), indicating a significant decrease of the overall flow of materials (reagents, catalysts, solvents, etc.) and thereby providing safer greener catalytic reactions with no waste.

One conclusion that may be derived from these studies is that DMC is an ideal reactant and that no reasons could exist to still use phosgene. As mentioned above, the process of clean DMC production has been commercialized for over 15 years, an apparently long time and long enough for a substantial substitution of phosgene by DMC. It is thus interesting to look at the market for phosgene in comparison with DMC.

The United States, Western Europe and Asia are currently the major producing and consuming regions for phosgene - primarily consumed captively to manufacture p,p'-methylene diphenyl diisocyanate (MDI), toluene diisocyanate (TDI) and polycarbonate resins. In 2006, global production/demand was estimated at over 7 million metric tons. Demand for phosgene grew by about 3.25% per year in the period 2001-2006, while it was about 6.4% per year in the period1997-2002. About 75-80% of global phosgene is consumed for isocyanates, 18% for polycarbonates and about 5% for other fine chemicals. Fine chemical applications are further broken down to 50% for intermediates, 25% for agrochemicals, 20% for pharmaceuticals and 5% for monomers and coloring agents.

By contrast, total global dimethyl carbonate capacity was about 170000ty-1 in 2002 and output and consumption were both about 90 000 tonnes. Production was concentrated in Western Europe, the USA and Japan and capacity in these regions accounts for about 70% of the total. Consumption was in polycarbonate synthesis (about 50000tonnes, accounting for 56.1% of the total), pharmaceutical production (about 20 000tonnes, accounting for 22.5%), pesticide production (about 7000 tonnes, accounting for 7.9%) and other sectors (about 12 000 tonnes, accounting for 13.5%).

With respect to the global production of polycarbonate, three companies (GE, Cartagena, Spain; Bayer, Antwerp, Belgium; and Asahi Kasei, Taiwan in 2002) use non-phosgene based manufacturing units, with a market share of 12% of the polycarbonate produced by this phosgene-free technology. This market share increased to ca. 20% in 2007.

We may thus conclude that more than 15 years after the introduction of clean processes of DMC synthesis and the large amount of advertising by the scientific community, which still continues, about the use ofDMC as clean and safer reactant as a replacement for phosgene, the market penetration of DMC is still quite limited. The reasons for this are several, some general, as discussed in the following section, and some more specific. These include, first, the already noted observation that a safer use of phosgene is possible. There are two main options:

• On-demand (or on-site) production. Over 99% of produced phosgene is not transported and is consumed on-site to avoid risk of transport. New legislations limit the amount of phosgene that can be stored on-site, and on-demand production is spreading. Davy Process Technology - DPT (Switzerland) offers modular phosgene generators with production ranging from 3 to 10000kgh-1 [55]. These modular generators produce phosgene from CO and Cl2 over carbon-supported catalysts (carbon itself is active or may by doped with 0.1-2% of active metal, in particular to reduce the formation of CCl4 side product to less that 150 ppm). Figure 1.13 shows the process flow of the phosgene generation section of a phosgene generator from DPT [55]. It consists of two sections, a phosgene generator (Figure 1.13) and a safety absorption module. Note that for safety the reactors are located in a secondary containment and all the lines and systems could be vented with an inert gas. Novartis Crop Protection Inc. (Monthey, Switzerland) has developed an intrinsically safe equipment for the on-demand manufacture of phosgene [84]. Furthermore, confinement in a double envelope of the phosgene

34 1 From Green to Sustainable Industrial Chemistry PURGE AIR r>-

Phosgene synthesis reactors

34 1 From Green to Sustainable Industrial Chemistry PURGE AIR r>-

Phosgene Production Process

SAFETY ABSORPTION OFF GASES TO__.

VACUUM UNIT

Heat

Heat

exchanger

-

r

exchanger

Cooling water Cooling water Figure 1.13 Process flow ofthe phosgene generation section of a phosgene generator of Davy ProcessTechnology. Source: adapted from Cotarca and Eckert [55].

SAFETY ABSORPTION OFF GASES TO__.

VACUUM UNIT

Cooling water Cooling water Figure 1.13 Process flow ofthe phosgene generation section of a phosgene generator of Davy ProcessTechnology. Source: adapted from Cotarca and Eckert [55].

production, supply and utilization equipment makes it possible to collect any leakage with ultimate destruction ofthe phosgene in specific installations. Chemical Design, Inc. (US) (http://www.chemicaldesign.com/Phosgene.htm) has also designed and built phosgene plants ranging from 0.5 to over 160 tons per day of high purity phosgene. State-of-the-art bellow seal valves are used to virtually eliminate emissions. They use a compact, skid mounted phosgene reactor design that allows the entire reaction system to be installed inside a controlled building that acts as secondary containment. The complete plant allows phosgene to be safely produced on-site, on demand, thereby eliminating transportation and storage concerns.

• Use of a safer phosgene source. Triphosgene is used as a phosgene source. It may be used in pre-packaged cartridge for on-demand production of phosgene by triphosgene catalytic depolymerization. Laboratory generators for on-demand production of phosgene are available. In cooperation with Buss ChemTech, Sigma-Aldrich offers a safe and reliable phosgene generation kit, giving simple access to small quantities of high purity, gaseous phosgene exactly when needed, while no transport and storage of liquid phosgene is necessary. The generator converts safe triphosgene into phosgene on demand using a patented catalyst [85]. Phosgene generation can be stopped at any time. A total containment approach eliminates the risk that phosgene can reach the environment.

Phosgene substitution is thus an emblematic case for sustainable industrial chemistry and how this question should be considered in view of a rational risk assessment more than on generic principles. Phosgene is still central to the chemistry of pharmaceutical, polyurethanes and polycarbonates. This is a very large market and still about eight million tons of phosgene are used industrially worldwide. New uses have also been discovered, from the synthesis of high purity synthetic diamonds to the production of the nutritive sweetener aspartame; it was also used as fuel in molecular motors. The book Phosgenations - A Handbook [55] discusses in detail novel and old uses of phosgene (see Sections 1.4 and 1.5, in particular).

However, phosgene is clearly highly toxic (threshold limit value - TLV of 0.1 ppm), but acrolein, for example, has the same TLV and is produced in quantities of several millions worldwide. Acrolein is also produced at barbecue parties by roasting foods, without provoking health alarms. Clearly, a low TLV implies the adoption of special safety procedures and limited storage. On-demand production and other safety procedures, such as those discussed above, are the solution to minimizing the risk to a sustainable level.

The substitution of phosgene with other chemicals (DMC, in particular) should be thus weighted between intrinsic (yield, reactivity, handling, work-up) and extrinsic (safety, toxicity, environmental impact) criteria [55]. Modern technologies for industrial chemical production allow proper and safe operation with toxic chemicals. Their sustainability (or green content) in terms of effective impact on environment and safety of workers is not necessarily lower than their substitution. It is only a problem of economy. More toxic feedstocks means higher costs in safety devices and thus alternative, less toxic reactants are implemented when the overall cost is lower. However, in terms of risk assessment, a more toxic reactant could be equally used, when the appropriate measures are adopted. This is why phosgene is still largely used.

The example of Bhopal teaches that the background for the accident is a poor design (actual processes no longer have this problem) but the reason is a poor management. The human factor is a critical element for all industries, but particularly for chemical ones. It is wrong to give the image of a clean and safe ("green") chemistry in opposition to the "red" and/or "black" chemistry cited in Section 1.2.2. All chemical production should be sustainable, but unavoidably they handle risky substances, not only in terms of intrinsic toxicity but also of explosive/flammable nature and so on. There are proper procedures and technologies to operate safely, and their progress is in continuous evolution. However, these need skilled operators, high-quality management, continuous maintenance and training. Under these conditions risk could be sustainable, and therefore exporting chemical production to regions of cheap labor could be a problem, owing to weakness in education and sensibility to risk.

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  • andrea
    What could be used insteda of phosgene for polycarbonate production?
    8 months ago

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