Hydrogen peroxide, acetic acid benefit from R&D efforts

16 March 1998 00:00  [Source: ICB]

In the second of ECN's Process monitors, Art Brownstein* looks at the production of hydrogen peroxide from the oxidation of carbon dioxide, and a route to acetic acid via a vapour-phase oxidation of ethane.

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Enichem has developed a unique process for hydrogen peroxide (H₂O₂) production based on the oxidation of carbon monoxide (CO) in aqueous solution (European Patent Application 808796).

Although the reaction mechanism is obscure, it would appear to involve the transient production of hydrogen, and to proceed by the following overall equation:

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A catalyst system of palladium acetate and 2,9-dimethyl-4,7-diphenylphenanthroline (1:4 mol ratio, respectively) is contained in a mixture of trichlorobenzene/n-butanol/water (3/4/5 v/v), and pentafluoroctanoic acid. The system is held at 70¼C for one hour under carbon monoxide (7 bar) and oxygen (70 bar) pressure after which aqueous hydrogen peroxide is obtained in 2.8% (wt) concentration.

This corresponds to an hourly rate of productivity of hydrogen peroxide of 0.028 kg/litre of aqueous phase, which is quite comparable to commercial processes. Enichem also reports operating its system on a semi-continuous basis wherein the average hourly productivity of hydrogen peroxide 0.04kg/litre of aqueous solution after four cycles.

Hydrogen peroxide is almost universally produced by the alternate hydrogenation and liquid phase oxidation of an alkylanthraquinone. Although the process is efficient from a yield standpoint, alkylanthraquinones are expensive, reportedly about $20/kg, and the cyclic systems have inevitable losses of this material. Most reported efforts in recent years to develop a better hydrogen peroxide process have focused on direct combination of hydrogen and oxygen.

Global demand for hydrogen peroxide is about 2m tonne/year. The product has enjoyed a 7%/year growth rate which has been largely driven by its use in pulp and paper production.

Most recent efforts to produce hydrogen peroxide by direct combination of hydrogen and oxygen have been coupled with simultaneous or successive production of propylene oxide. Sumitomo Chemical discloses the production of hydrogen peroxide by passing hydrogen and oxygen (1/10 v/v) into a methanol solution of NaBr and sulphuric acid and powdered palladium metal at 20°C (European Patent Application 812836).

The system is pressurised to 9 bar with nitrogen and held for two hours, after which a 0.35% hydrogen peroxide solution is obtained. Addition of propylene and a titanosilicate catalyst results in 95% propylene oxide selectivity at 65% hydrogen peroxide conversion within one hour at 40°C. The net result is the production of propylene oxide from hydrogen and oxygen:

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BASF has been engaged in an analogous effort (World Patents 97/47613, 97/47614 and 97/47386).

Acetic acid

A highly selective route to acetic acid by vapour-phase oxidation of ethane has been reported by Hoechst (World Patent 97/44299).

Passage of a feed gas comprising

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ethane/O2/N2/H₂O (40/8/32/20 v/v) over a promoted molybdenum-rhenium catalyst at 280°C under 15 bar pressure produces acetic acid in 91% selectivity at 4% conversion. The only reported by-products are CO and CO₂. Increasing the residence time of the reactants from 30 to 60 seconds leads to a doubling in conversion to a more acceptable 8%/pass with a slight decline in selectivity to 90%. A small amount of ethylene (2%) is obtained at the higher conversion. Conceivably, the ethylene is recyclable as a feed to the reactor.

The patent is silent on the configuration of the reactor, which is ostensibly fixed bed. The catalyst has the following composition: MoRe0.67V0.7Nb0.19Sb0.08Ca0.05Pd0.02.

With ethane currently priced at about $157/tonne, an ethane-based process for acetic acid could have substantial appeal. At a selectivity of 90%, potential ethane raw material costs could be about $87/tonne of acetic acid produced.

The current list price of acetic acid is $792/tonne. Of some concern is the relatively low ethane conversion which suggests a possibly high fixed investment cost for the process.

The most recent commercial innovation in acetic acid production is that of Showa Denko which started up a 100 000 tonne/year plant based on vapour phase oxidation of ethylene in late 1997.

The commonly used process for acetic acid is carbonylation of methanol. Two proposed efforts, one by Haldor Tops¿e and the other by Canadian Government research laboratories, to streamline this system were described in the January issue of Process monitor (ECN 26 January 1998).

Mitsui Toatsu has defined a highly active catalyst capable of producing nitrous oxide by vapour phase oxidation of ammonia (European Patent Application 799792).

A feed stream of NH3/O2/H2O (3.8/3.9/92.3 v/v)

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is passed over a copper-manganese oxide catalyst at 330°C under 0.5 bar pressure. Nitrous oxide is obtained in 86.5% selectivity at 99% ammonia conversion. Nitrogen (13.5%) is the only reported by-product. The catalyst has a reportedly long life.

Nitrous oxide is produced by many companies by thermal decomposition of ammonium nitrate at 170-250°C. Its primary market is as a general anaesthetic where it is commonly known as laughing gas.

A potential step change in its uses was announced in 1997 by Solutia (formerly Monsanto Chemical) which contemplates employing nitrous oxide as a reactant with benzene to produce phenol. Although Solutia intends to employ by-produced nitrous oxide from adipic acid production, its primary production, as in the Mitsui Toatsu development, is another option.

Polycarbonate

In a technique disclosed by Idemitsu Petrochemical, polycarbonates can be produced in shorter reaction times and lower temperatures during solid phase polymerisation (European Patent Application 807656). The technique entails the use of a swelling solvent such as toluene or paraxylene in the conversion of a polycarbonate oligomer to high polymer. In addition, crystallisation of the oligomer prior to its polymerisation can be eliminated.

In an example, a bisphenol-A oligomer obtained from diphenyl carbonate and bisphenol-A is treated with toluene at 220°C and at a space velocity of 0.0049 l/hr-g for three hours. A polycarbonate of 16 523 viscosity average molecular weight is obtained. In the presence of NaOH as a catalyst, the molecular weight increases to 24 655, which is 3.9 times that of the oligomer.

Compression moulding yields a colourless and transparent product. The diphenyl carbonate employed in oligomer production can presumably be derived from dimethylcarbonate, thereby enabling a phosgene-free system to be used. The volume increase of the oligomer using toluene or paraxylene in comparison to nitrogen is illustrated in the figure at the top of the second column of this page.

Polycarbonate demand is about 900 000 tonne/year worldwide, and has been increasing at an annual rate of about 5%.

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Polyaspartic acid

Polyaspartic acid with low colour can be produced by thermal condensation of L-aspartic acid according to a disclosure by Solutia (European Patent Application 813562). The process comprises the use of a high boiling solvent such as hexadecane in which L-aspartic acid is held at 200-220°C for 210 minutes. The product is ostensibly obtained in 100% conversion and in about 10 000 molecular weight.

The system is actually a two-step process in which L-aspartic acid is first converted to a polysuccinimide as described, and this is subsequently hydrolysed to the sodium salt of polyaspartic acid by heating with 15% aqueous NaOH at 50-70°C for 30 minutes. The polyaspartic acid salt forms an aqueous solution which can be easily separated from the hexadecane. The probable reaction sequence is as follows:

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In related work, Solutia also discloses that the reaction time for the polysuccinimide can be decreased, and its molecular weight can be increased by using a methylenephosphonic acid as a catalyst (European Patent Application 809668). In other work, Donlar Corp describes the production of D,L-aspartic acid from maleic anhydride and ammonia in good yield and minimum by-product formation (World Patent 97/47587).

Water soluble polyaspartic acid has been receiving considerable attention recently for a number of applications, such as in detergents, water treating and as a nutrient absorption enhancer in agriculture. Donlar has had a multi-million pound plant under construction which was scheduled for completion in 1997, meanwhile Rohm and Haas has more than 1m lb (450 tonne)/year of capacity.

Demand has been forecast to grow to $700m over the next two decades, and should have a substantial impact on the aspartic acid market whose primary end-use has been production of aspartame, the artificial sweetener. Aspartic acid sells for $2.00-$2.75/kg.

Syndiotactic PS

A semi-crystalline syndiotactic polystyrene (75% styrene/25% vinyltoluene) has been described by Dow Chemical (World Patent 97/06221). The system comprises the solution polymerisation in toluene at 70°C of styrene and p-vinyltoluene via pentamethylcyclopentadienyl trimethoxide catalysis. The polymer (Mn=191 000; Mw/Mn=4.1) remains soluble in the toluene and can be precipitated with methanol. A film of the polymer has 0% crystallinity which on stretching and heat setting increases to 28%. Such a technique enables the production of a crystalline polymer with the convenience of a solution process.

It is well-known to those engaged in catalyst development that small changes in structure can result in large changes in performance. Accordingly, Nippon Shokubai finds that slight changes in catalyst crystal structure can lead to enhanced production of acrylic acid (European Patent Application 811597).

The system under study is the two-stage vapour phase oxidation of propylene to acrylic acid.

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The second stage, copper-promoted molybdenum vanadate catalysed oxidation of acrolein, can be enhanced to a yield of 93.3%/pass compared to 88.5-89.8% by the use of cuprous oxide in place of either cupric oxide or nitrate in catalyst preparation.

The performance appears to be directly related to greater peak intensity at d=4.00 A° of the x-ray diffraction pattern of the catalyst.

This correlation is also directly related to longer catalyst life.

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