Ruthenium-Mediated Electrochemical Destruction of Organic Wastes By L. Davidson, Y Quinn and D. E Steele AE-4 Technology plc, Dounreay, Scotland

No industrial process is 100 per cent efficient and the generation of waste, both organic and inorganic, is an unavoidable reality. The recycling of waste is becoming increasingly important as concerns about the environment and the availability of resources come more to the fore, but there remain many waste streams which are not yet suitable for recovery of their reusable content, on the grounds of cost orpracticality Historical13 the disposal of such wastes has been either via incineration or release into the environment after neutralisation or imrnobilisation, in a landfill site or into a drain or sewer, or the like. These options are becoming more restricted as regulations tighten and public perception of any but the most benign discharges worsens. This paper sets out reasons for using electrochemical oxidation with a ruthenium electro- catalyst to provide aneffective and environmentally friendly alternative to other technologies. Electrochemical oxidation can effect mineralisation of toxic organic species with minimal generation of secondary waste and eflicient recovery of the ruthenium mediator and is particularly suited for the treatment of highly chlorinated and aromatic compounds.

Organic wastes, of the types which cannot furans which are highly toxic and limited, by be converted into benign biodegradable species, Regulatory bodies, to low ppb levels in the dis- are often treated by oxidation, usually inciner- charges. Toxic waste incineration has thus tended ation, although other chemical treatments are to become very 'high-tech' in recent years, with sometimes used. Many of the oxidants which only a few highly specialised facilities treating could be used are ruled out by cost or toxicity. most of the waste destined for combustion. The use of air for such oxidations, normally Public perception of incinerators, particularly by high temperature incineration, is the cheap- those treating toxic wastes, is poor or hostile, est in terms of reagents. The current best-avail- and the licensing of new facilities in most coun- able technology is capable of achieving essen- tries in the developed world is becoming very tially complete destruction of even refractory difficult and sometimes almost impossible. materials. However, the combustion products from many organic materials, in particular chlo- rinated species, contain acidic gases (for exam- Table I ple, hydrogen chloride, sulphur oxides and nitro- Comparison of Oxidation Costs* gen oxides) which must be scrubbed to very low levels before discharge. As excess air is always Oxidant Cost/rnole of [O] equivalent, f used, the volumes of gases for treatment tend to be large, leading to large scrubbers and other Electron 0.01 related equipment, and associated increased Hydrogen peroxide 0.058 costs. Incomplete combustion, possibly due to Potassium dichromate (Cr"') 0.1 8 Ceric nitrate (Ce'") 8.68 maloperation, can lead to the production of sec- ondary species, such as dioxins and dibenzo- * The Appendix contains more details of comparative costs

Platinum Metals Rev., 1998, 42, (3), 90-98 90 CO+COl ottgas

catholyte recycle

Fig. 1 The design of the plant used for the ruthenium-mediated oxidation of highly toxic industrial waste. The cell at the centre of the process has ruthenium chloride or hydrated ruthenium oxide added to the anolyte. Platinum coated anodes are used for the flow-cells

However, the use of electrochemical oxidation development by AEA Technology for some time to treat organic industrial waste gives a cheaper, as a possible process for the complete oxidation cleaner and renewable alternative: the ‘electron’. of a number of types of organic waste or the The ‘electron’is also cheaper than chemical oxi- organic waste component in a mixed waste. dants on a per/mole basis, see Table I. However, in the silver-mediated system, the In principle, electrochemical oxidation has a presence of sigdicant amounts of halogen (usu- number of advantages over oxidation using con- ally chlorine) in the organics being oxidised can ventional oxidising agents. In practice, however, result in the formation of substantial amounts direct electrochemical oxidation of organic of insoluble silver halide(s), which are, at best, species is often difficult due to mass transfer and a nuisance. kinetic restraints at the electrode surface and Here an alternative, ruthenium-mediated elec- competing reactions with the electrolyte. Many trochemical system is described, see Figure 1, of the restrictions can be overcome by the use which has many similarities to the SILVER IITM of a mediator which is easily oxidised at the system but which is not affected by the presence anode surface to a species which then reacts of halogen, and in particular by chlorine. in the bulk solution with the organic substrate. The mediator can be re-oxidised at the anode Chemical Considerations for further use and can thus be termed an elec- In order to understand the internal workings trocatalyst as it is not consumed in the overall of the ruthenium-mediated oxidation system, it reaction. is necessary to consider some aspects of the The silver-mediated electrochemical oxida- aqueous chemistry of ruthenium. Ruthenium tion of organics (SILVER IITM)has been under can form compounds with a multiplicity of

Platinum Metals Rev., 1998, 42, (3) 91 oxidation states, ranging from -2 to +8. Its (iii) Partial oxidation of Ru(1V) in neutral or chemistry in aqueous solution mainly relates to acid solution, followed by disproportionation, the +3, +4, +6, +7 and +8 oxidation states, with forms ruthenium tetroxide. the +3 and +4 states being the most stable. Ruthenium is normally supplied as the trichlo- Ruthenium Tetroxide ride, "RuCl,.(H,O)," (n = 2 or 3), a very solu- Volatile RuO, @.p. - 130°C), like isostructural ble dark red-brown solid which contains a com- OsO,, is a powerful and unstable oxidising agent. plex mixture of chloro and chlorohydroxo It may be prepared from an acidic solution or complexes, many of which are polymeric and suspension of any one of a number of ruthe- contain mainly Ru(IV) (1). nium salts or hydrated RuO, by treatment with Although "ruthenium(II1) trichloride" is the a powerful oxidant, such as MnO;, Cl,, IO;, form in which ruthenium is initially added to Celt or NaOCl (bleach). Pure RuO, is not par- the ruthenium-mediated electrolytic system, ticularly stable in either the liquid or vapour Ru(IV), in the form of hydrated RuOzis of much phase and tends to decompose, eventually form- greater significance. The great insolubility of ing RuO,. It is freely soluble in organic solvents, this oxide in water ensures that its production although its reactivity limits the solvents which is favoured when ruthenium 0x0-anions in high can be used (typically CCl, or CHCl,, plus some valent states are reduced (in a similar manner polar solvents with little nucleophilic charac- to the easy formation of insoluble MnO, when ter). These solutions of RuO, are reasonably MnO; is used as an oxidant). stable if kept in the dark and with a slight excess The hydrated oxide can be dissolved oxida- of the oxidant which formed the tetroxide. It tively (for example, using oxygen, persulfate is not normally prepared in bulk quantities, but or permanganate) in alkaline solution (pH > is generated for immediate use. 12) to yield a solution of the orange ruthenate ion, Ru0,Z- (Rum)): Organic Oxidation by Ruthenium 0x0-Species RU'~O,+ 40H- + RuwO:- + 2Hz0+ 2e Ruthenate, perruthenate and, in particular, Ruthenate is only stable at high pH and in the ruthenium tetroxide have all been studied as absence of reducing agents. If the pH is low- oxidants for organic synthesis. RuO, has been ered, Ru0,Z- disproportionates to give hydrated studied fairly comprehensively and prepara- Ru(N)O, and green perruthenate, Ru(VII)O,-: tive oxidations in both the aqueous and organic phases (mainly in unreactive solvents like CHC1, 3Ru"O:- + 4H' + RuIVO2+ 2RuV"0L + 2H20 and CCl,) have been described (2, 3), with the Perruthenate solutions are unstable and exact conditions being tailored to suit the par- decompose, either with the formation of 0, and ticular synthesis. RuO,: One of the first investigations of organic oxi- dations using RuO, included the following 4Ruw0; + 4H' + 4Ru"02 + 30, + 2Hz0 description using only 10 mg of tetroxide with or, if the pH is below - 7.5, the decomposition some common solvents: can produce RuO,: "Benzene - vigorous explosion; pyridine - no 4Rum0, + 4H' + RuNOz+ 3RuW"0, + 2H20 explosion, only flame; anhydrous ether - small explo- sion, followed by yellow jlame" (4). Important points to note from the above are: (i) The favoured reduced form of uncomplexed Of particular interest in the context of destruc- ruthenium in near-neutral aqueous solution is tive, rather than synthetic, oxidation by RuO, Ru(IV)oz. are the following properties: (ii) Uncomplexed ruthenium exists in aqueous The solubility of RuO, in both polar and non- solution as neutral or anionic species. polar organic solvents, allows oxidation to take

Platinum Metals Rev., 1998,42, (3) 92 place within an immiscible organic phase rather and is the basis for a worldwide industry with than only at the aqueoudorganic interface. a capacity of millions of tonnedyear (8). RuO, cleaves many C=C bonds, usually The principal process at the anode surface yielding carbonyl compounds in the first instance. is oxidation of chloride ions (with very minor RuO, is able to oxidise saturated hydrocar- oxidation of water to form oxygen): bons under mild conditions (5). 2C1- C12 2e The complete oxidation of the highly toxic + + and relatively unreactive polychlorodibenzo-p- At the low pH normally encountered in dioxins (6) and polychlorinated biphenyls (7) chlorine cells, the product is gaseous chlorine. by RuO, has been reported. As the pH is raised, the hydrolysis of the chlo- Aromatic nuclei are attacked and are rine to form HOCl begins to predominate and completely degraded. at pH 5.5 HOCl is the sole product, from hydrol- ysis of the C1, formed at the anode: Destruction of Organics by Mediated Oxidation Clz + HzO 4 HOCl + HCl Direct electrochemical oxidation is in princi- The hypochlorite (HOCl or OCl-, depend- ple easily capable of oxidising any organic species ing on the pH of the solution) generated elec- to carbon dioxide, and other inorganic prod- trochemically can be used to drive mediated ucts, based on thermodynamic, electrode poten- organic oxidations and replace the continuous tial considerations alone. However, in practice additions of, for example, NaOCl solution. As this is usually ruled out by a combination of slow a mediating couple, the chloride/hypochlorite electrode kinetics, difficult mass transfer of sub- system has a number of advantages: strate to the electrode and competition from NaCl is soluble up to - 5M in water, so there side reactions, such as oxygen evolution, so that is little mass transfer limitation on the anode mediated oxidation must be used. A suitable reaction. electro-active mediator must: The production of chlorine at a suitable be soluble in the electrolyte to an extent anode is a fast reaction and the overpotential, which minimises any restrictions on mass even at high current density, is only a few tens transfer to the anode; of mV. have fast electrode kinetics to minimise The reaction of the chlorine either directly activation overpotential at the anode; with Ru(1V) or with water to form HOCl is a react rapidly in the bulk solution with a wide fast reaction and the reduced product (Cl-) is range of organic substrates, forming a reduced easily re-oxidised. species which is easy to re-oxidise at the anode; The complexation or reaction of C1- to form form no insoluble or complex species which species which cannot be re-oxidised is unlikely. remove the mediator irreversibly from solution. The anodic production of chlorine in a NaCl Based on electrode potential considerations anolyte containing a small amount of dissolved alone, a number of possible electrochemical cou- ruthenium or hydrated RuO, thus produces ples may be suitable, but most are ruled out highly reactive RuO, as long as current is passed. by one or more of the above criteria. However, The RuO, in turn attacks organic material fed the simple oxidation of chloride can be used to the anolyte, ultimately oxidising it to car- to regenerate RuO, in a double-mediated, con- bon dioxide, carbon monoxide and water (plus tinuous system. sulfate, phosphate or nitrate from heteroatoms The electrolysis of sodium chloride, NaCl, such as S, P and N, for example). solution to produce chlorine, Cl,, and sodium It is this continuous coupling of electric power hydroxide, NaOH, is one of the best understood with oxidation which makes the ruthenium oxi- electrochemical processes. This electrolysis is dation system so useful for the treatment of carried out industrially in a variety of cell types organic wastes, in particular wastes with a high

Platinum Metals Rev., 1998, 42, (3) 93 chlorine content, which are often toxic and insoluble hydrated RuOJ forming CO and difficult to dispose of. The integration of the (mainly) CO,, plus inorganic anions from, for ruthenium-mediated oxidation into a complete example, S and P, system is described next. (e) Ru02 re-oxidation to RuO, by further HOCI. Process Description The overall process thus consists of two linked At the heart of the ruthenium-mediated oxi- cycles, (a)-(c) and (c)-(e). The oxygen which dation process, see Figure 1, is an electro- ultimately appears as CO+CO, is derived from chemical cell, very similar to the type used for water molecules in the anolyte, via the forma- chlorine manufacture. The anode, where chlo- tion and reduction of HOCl and RuO,. The ride is oxidised, has to favour the production of overall anode/anolyte reaction thus becomes: chlorine over oxygen and in practice this restricts Organics + H,O 4 CO + COZ+ nH' + ne the material (or the coating) to platinum or plat- inumhridium (or RuO,, but not the hydrated with the electrons passing to the external cell form). Modern cell designs have a titanium anode circuit. which is coated with a few microns of the noble Current through the cell is carried by Na' ions metal or oxide (9). Development trials of the ruthe- which cross the membrane to the catholyte, usu- nium process have employed anodes consisting ally in association with 1 to 3 molecules of water of either platinum mesh (lab-cell scale) or 2.5 pm per Na' ion. The Na' ions do not react at the platinum coated on titanium (flow-cell scale). cathode, but merely transport charge to the As the anolyte and catholyte chemistries are catholyte (NaOH solution) while the cathode very different, the cell is divided with a NafionTM reaction is hydrogen evolution: cation exchange membrane which permits the 2Hz0+ 2e- + 20H- + HL passage of cations to carry the cell current but prevents bulk mixing of the two solutions. The transfer of Na' ions and water to the NafionTMis a sulfonated perfluoropolymer which catholyte and the formation of hydroxyl ions combines extreme chemical resistance with selec- produces NaOH solution, exactly as in a chlor- tive permeability to cations and is widely used alkali cell. The catholyte thus becomes more industrially in cells for chlorine production. The alkaline while the production of hydrogen ions selective permeability has a further advantage in the anolyte causes a shift to a more acid pH. in that it prevents migration of soluble ruthe- These changes are controlled by returning nium species (which tend to be anionic under NaOH solution from the catholyte to the anolyte operating conditions) from anolyte to catholyte. at the same rate as Na' transfers from the A number of materials can be used for the anolyte. The overall reaction across the cell then cathode - stainless steel has been used during becomes: development trials, but nickel, or one of the pro- Organics + HZO4 CO, + CO + H, prietary types of chlor-alkali cathode, would be a better choice for a full-scale plant. The process is thus a net consumer of water, rather than a producer as is the case with most Electrolyte Chemistry organic oxidations, and the organic hydrogen The following processes occur in the anolyte: fed to the anolyte appears from the cathode as (a) C1, production at the anode, hydrogen, rather than as water. A water feed is (b) C1, hydrolysis to HOCl, thus needed to replace that lost as H2and CO+ (c) RuO, production (with reduction of the CO, or, alternatively, dilute organic streams can HOCl to C1-, which is available for further oxi- be treated with equal ease. Combustion of the dation to C1, at the anode), hydrogen can be used to recover about 30 per (d) The reaction of the RuO, with the organic cent of the power fed to the cell as useful heat, wastes fed to the anolyte (with the formation of if this is deemed cost-effective andor necessary.

Platinum Metals Rev., 1998, 42, (3) 94 Table II Summary of Substrates Oxidised and Offgas Compositions

Substrate Temperature, Carbon dioxide (2). Carbon monoxide (2). OC maximum per cent maximum per cent

(2-Chloroethy1)ethylsulphide (1) 50 73-95 2.6-6.7 Tributyl phosphate 50 93 2.6 Dodecane ao 93 2.9 Chlorobenzene 80 85.5 11.8 Trichlorobenzene (1) 95 68-83 0.8-1.2 PCBs (Aroclor 54) 90 67.6 1.2 Pentachlorophenol a5 91.7 7.7 2.4-Dinitrotoluene a5 76.5 2.3 Hexachlorobutadiene 95 84.8 16.9 Tetrachloroethylene 60 96.1 21.6

(1) Results from repeat runs (2) The maximum percentages of CO, and CO observed during run(s) are not necessarily observed simultaneously

If heteroatoms, such as C1, S or P, are present this manner, the oxidising power of both the C1, in the organic feed additional NaOH has to be and the RuO, is returned to the anolyte and the fed to the system (usually to the recycle stream escape of valuable ruthenium is prevented. from the catholyte) to balance the additional formation of H’ in the anolyte, for instance: Operating Conditions The process is typically operated with an elec- Organic [S] + 4H20+ SO:- + 8H’ + 6e trolyte temperature of 50 to 80°C. The anolyte The net product from organic heteroatoms consists of a NaCl solution held near neutral is the appropriate sodium salt (sulfate or phos- pH and usually buffered to limit pH changes. phate, for instance). Note that organic chlorine The catholyte consists of NaOH solution, typ- produces chloride, which is one of the process ically 35 wt.% NaOH or less (the limit being set reagents. When heteroatoms are present the by the properties of the membrane - the process overall reaction becomes: will operate chemically at up to 50 wt.%). Conditions in the anolyte are highly aggres- Organic [C,H,N,O,S,P,CI] + H,O + NaOH + sive to materials like stainless steel, so titanium CO, + CO + H, + Na salts or polymers, such as PTFE or PVDF, must be used there. The catholyte is much less Offgas Treatment aggressive and cheaper materials, such as Electrolysis of chloride-containing solutions, polypropylene can be used. like the ruthenium process anolyte, can lead to chlorine evolution ifthe pH is allowed to fall. Organic Substrates Also, RuO, is volatile and could be flushed fi-om The ruthenium-mediated system is surpris- the anolyte along with the CO, and CO formed ingly reactive compared to literature reports kom organic oxidation. In order to prevent either of organic oxidations using RuO,. This may species from escaping, some of the recycle be because the oxidation is carried out in the stream of NaOH is used to maintain alkaline aqueous phase rather than in an inert solvent, conditions in a scrubber through which the off- and thus the formation of reactive radicals from gases from the anolyte pass. Any C1, is absorbed water, for example, OH, is possible. as NaOCl while the RuO, is absorbed as “hyper- In Table I1 is shown some of the compounds ruthenic acid” (H2RuO5)or its sodium salt. In which have been successfully oxidised during

Platinum Metals Rev., 1998, 42, (3) 95 NaOH Product Fig. 2 RuCl3 (and also Catholyte recycle possibly NaCI) is added to stream the bulk dilute waste I as RuCI) nyO which then acts the additions anolyte in the oxidation Anolyte Oxidation cell where the organic NaCl waste additions solution circuit process content is catalytirally ( it required) I I I development trials, together with the compo- supplied to the cell. Electrochemical cells and sition of the gases leaving the anolyte. The per- DC power supplies are also expensive items of centage of CO+CO, in the anolyte offgas gives equipment and cells suffer from a relatively poor a good indication of the electrochemical effi- economy of scale compared to many process ciency (that is the percentage of the current flow- plant items (doubling the plant throughput ing which is driving useful oxidation, rather than requires twice as many cathode-anode pairs, oxygen evolution, at the anode). while pipework and tankage would only have to The amount of current which has to flow per be increased by a factor of- 2”*).The treatment mole of organic material oxidised is very depen- of bulk quantities of relatively benign organic dent on the chemical name of the organic mate- wastes is therefore not likely to be very economic rial and the composition of the offgases. For unless driven by environmental and regulatory instance, in the slightly extreme case of tetra- pressures which eliminates incineration, for chloroethylene, C,Cl,, oxidation to CO? requires instance. 4 faradaydmole: When organic wastes arise in conjunction with salts and water, as is the case with many indus- C,C1, + 4H,O + 2C02+ 8H’ + 4C1- + 4e trial processes, particularly the manufacture while production of CO is actually hydrolysis of pharmaceuticals, dyestuffs and explosives, rather than oxidation: ruthenium-mediated oxidation becomes a much more attractive prospect. The incineration of C2C14+ 2H,O --t 2CO + 4H’ + 4C1 salty wastes is often not very practical and the Oxidation to CO always requires less current toxicity/salinity of the waste may rule out more to flow than oxidation to CO,, with a conse- ‘aqueous’ treatments such as reed beds and bio- quent saving in the power consumed by the cell. degradation. The dilution of the organic in the The ruthenium process is thus able to oxidise water is not a particularly limiting factor. highly chlorinated substrates, such as arise as The ‘ideal’ feed for the process shown in Figure waste from pesticide and herbicide manufac- 1 would consist of a mixture containing the cor- ture, organophosphates (insecticides) and aromatic rect amount of water to replace that consumed nitro-compounds (explosives manufacture) and to form the CO+CO, and H,. However, more can do so when these are in association with dilute feeds can be treated on a batch recycle chloride ion or other salts in an aqueous stream. basis, and the waste stream itself functions as The ruthenium process is therefore seen as com- the anolyte with additions of ruthenium (and plementary to the related SILVER IIT” electro- possibly NaCl if it is not already present) being chemical oxidation process in terms of feeds. made, see Figure 2. If Na’ ions are already present in the waste liquor, a ‘product’ of Economic and Practical Aspects catholyte NaOH solution can be withdrawn for Like most electrochemical processes, the recycleheuse, if desired. economics of ruthenium-mediated oxidation The ruthenium is only required at low con- are closely tied to the cost of the electric power centrations as its role is principally catalytic, but

Platinum Metals Rev., 1998, 42, (3) 96 (air) Fig. 3 One method for recovering the ruthenium after catalytic oxidation of the various organic wastes in the cell is to allow the pH to fall, thereby forming chlorine along with the RuO,. Anolyte scrubber The chlorine gas and volatile RuO, are swept from the anolyte by an air sparge and absorbed (as hypochlorite and perruthenate) in the offgas scrubber which is fed with catholyte NaOH. The Ru-and hypochlorite-containingliquor is then used to prepare the anolyte for treatment of the next batch of waste

RU Oxidatipn process

anolyte catholyte

Recycle to process

Air sparge (optional) Water additions as required

its cost probably precludes its disposal, albeit as batch of waste liquor requiring treatment, see insoluble and inert Ru02,with the treated waste. Figure 3. However, the low solubility of the RuO,, which The incoming waste liquor may contain metal is the form the ruthenium takes when the cell ions which would precipitate in the membrane is switched off and the electrolyte is allowed to and these could therefore be usefully removed stand, means that simple filtration can be used from the waste before the oxidative step, for sep- for recovery. aration and disposal. The continuous produc- Alternatively, the ruthenium can be swept from tion of NaOH at the cell cathode provides a con- the anolyte by allowing the pH to fall so that venient source of caustic with which many metals some chlorine is evolved. This (possibly assisted can be precipitated as their insoluble hydrox- by an air sparge) sweeps the ruthenium from the ides. An alternative use for the catholyte NaOH anolyte as RuO, which is then absorbed into is that refractory materials (such as highly chlo- NaOH (formed simultaneously at the cathode) rinated, hydrophobic solids, for example hexa- using the anolyte offgas scrubber. In this way a chlorobenzene) may be partially hydrolysed fresh batch of anolyte or reagent is effectively using the catholyte recycle stream (10). Figure reformed, and can be used for dosing the next 4 shows how the additional NaOH that must

Concentrated Fig. 4 The NaOH used to organic - NaOH balance organic CI, S, P, feed Hydmlysisl pretreatment - = NaOH Feed unit , etc., can be added to the Catholyte recycle catholyte reeycle stream to stream give a highly concentrated solution for pretreatment of the waste feed, for Water example to precipitate feed Oxidat ion metals or hydrolyserefrac- process tory organics in the feed

Platinum Metals Rev., 1998, 42, (3) 97 be added to balance the halogen content of the Appendix organics, may be added to the recycle stream to In a cell (193000Cb, 54 Ah) in which oxidation give the highest practical concentration for the occurs, with cell voltage of 3V, 2 moles of electrons carry out the oxidation process, equivalent to one hydrolysis. mole of [O]and costs - lp. The same amount of oxidation delivered using hydrogen peroxide (one of Conclusions the cheapest chemical oxidants) at a nominal cost of Double mediated electrochemical oxidation L600ite for a 35 wt.% solution (L1700/te H,O,, 2941 1 using the Cl(-I)/Cl(I) and Ru(IV)/Ru(VIII) cou- moles [O])costs - 5.8 p. High-valent metal-based oxi- dants such as Cr”’ and Ce“ are much more expensive. ples in a cell very similar to a chlor-alkali cell Bulk K,Cr20,costs E1835ite (3400 moles, 10200 has been shown to have promise for the treat- moles [O])while Cel” nitrate - Ll OOOOite (2300 moles ment of organic waste streams, particularly halo- Ce”, equivalent to 1150 moles [O])and the equiva- genated or dilute, salty liquors. Currently, stud- lent oxidation, carried out using this reagent costs ies are concentrating on optimising the core L8.68 (all costs assume 100 per cent efficiency for simplicity). Like peroxide, the electron (the oxidant process and examining the application of itself) leaves no residues which are toxic or diffcult the system to a variety of ‘real’ wastes with an to dispose of, while oxidants like Crv’and Gel" leave industrial origin. large quantities of spent oxidant for disposal. References 1 F. A. Cotton and G. Wilkinson, “Advanced 5 U. A. Spitzer and D. G. Lee, J. Org. Chem., 1975, Inorganic Chemistry”, Wiley Interscience, New 40, 2539 York, 1968 6 D. C. Ayres, Nature, 1981,290, 323 2 J. L. Courtney and K. F. Swansborough, Rev. Pure 7 C. S. Creaser, A. R. Fernandes and D. C. Ayres, Appl. Chem., 1972,22,47 Chem. Ind., 1988, 499 3 D. G. Lee and M. van den Engh, “The Oxidation 8 A. M. Couper, W. N. Brooks and D. A. Denton, of Organic Compounds by Ruthenium Tetroxide” “Modern Chlor-alkali Technology”, 1989, 4, 71 in “Oxidation in Organic Chemistry”, ed. W. S. 9 P. C. S. Hayfield, Platinum Metals Rev., 1998,42, Trahanovsky, 1973, Academic Press, New York (2), 46 4 C. Djerassi and R. R. Engle, J. Am. Chem. SOC., 10 J. K. Beattie, R. W. Kaziro and P. A. Lay, World 1953,75, 3838 Appl. 89/05 172 Direct Methane Oxidation by Platinum Catalyst The conversion of natural gases into more use- oped. They found that the addition of a bidiazine ful alcohols is usually achieved by changing the ligand to a platinum(I1) salt gave high stability alkane component of the natural gas into a com- to strong acids and oxidising conditions. One of bination of carbon monoxide and hydrogen (syn- the most effective catalysts was dichloro(q-2- thesis gas) which is then converted into higher { 2,2‘-bipyrimidyl})platinum(II), [(bpym)PtCI,]. order alkanes by the Fischer-Tropsch reaction Three key steps were proposed for the conver- (1). These processes require a lot of energy, so sion of methane: C-H activation, oxidation and it would be less costly if the alkane component, functionalisation. usually methane, could be directly converted The present reaction, which takes place at into alcohols, by selective oxidation; the main 220°C, converts - 72 per cent of the methane difficulty being the low reactivity of the C-H to methyl bisulfate, and is thought to proceed bond. One such method, from Catalytica via the formation of a methyl-platinum species. Advanced Technologies Inc., California, used The authors state that the mechanism of the C- toxic mercury(II) salts, and resulted in methane H reaction needs more investigation as the fac- being converted into a methanol ester in an tors controlling its reactivity could lead to greater approximately 43 per cent yield. control of the reaction selectivity. Now, however, the same group has reported the development of a successful, stable platinum References catalyst (2). Platinum salts were already known 1 R. C. Everson and D. T. Thompson, Platinum to be more efficient for the oxidation of methane Metals Rev., 1981, 25, (2), 50; F. King, E. Shutt and A. I. Thompson, op. ciz., 1985, 29, (4), 14 in strong oxidising acids, but were not stable in 2 R. A. Periana, D. J. Taube, S. Gamble, H. Taube, hot (>lOO°C) acid. By utilising stabilising lig- T. Satoh and H. Fujii, Science, 1998,280, (5363), ands, catalysts of greater stability were devel- 560

Platinum Metals Rev., 1998, 42, (3) 98