cr9700989-super critical water

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Organic Chemical Reactions in Supercritical Water

Phillip E. Savage

University of Michigan, Chemical Engineering Department, Ann Arbor, Michigan 48109-2136

Received June 26, 1998 (Revised Manuscript Received September 28, 1998)

Contents

I. Introduction 603II. Chemical Synthesis 604

A. Hydrogenation/Dehydrogenation 604B. CC Bond Formation 605C. Rearrangements 605D. Hydration/Dehydration 605E. Elimination 606F. Hydrolysis 606G. Partial Oxidation 606H. HD Exchange 607

III. Chemical Conversion 607A. Decomposition 607

1. Complex Materials 6082. Individual Organic Compounds 609

B. Oxidation 6121. Technology Developments 6132. Homogeneous Reactions 6133. Heterogeneous Reactions 618

IV. Summary 618V. Acknowledgments 619

VI. References 619

I. IntroductionWater near or above its critical point (374 C, 218

a t m ) is a t t r a ct in g in cr e a se d a t t e n t ion a s a m e d iu mfor organic chemistry. Most of this new a ttent ion isdriven by the search for more green or environ-m e n t a l ly ben ig n ch e m ica l p r ocesses. U sin g n e a r -cr i t ica l or s u pe rcr i t ica l w a t e r (S C W) i n st e a d oforgan ic solvents in chemical processes offers envi-r o n m e n t a l a d v a n t a g e s a n d m a y l e a d t o p o l l u t i o nprevention. Interest in doing chemistry in SC W is notentirely new , however. There ha s been much previousresearch in this area with applications in syntheticfuels production, biomass processing, waste treat-ment, materials synthesis, and geochemistry.

Wat er nea r its crit ical point possesses propertiesvery different from those of ambient liquid water. Thedielectric constant is much lower, and the numberand persistence of hydrogen bonds are both dimin-ished. As a result , high-temperature w at er behaveslike many organic solvents in that organic compoundse n joy h ig h solu bili t ies in n e a r -cr i t ica l w a t e r a n dcomplete miscibility with SCW. Moreover, gases arealso miscible in SC W so employing a SC W reactione n vir on m e n t p r ovid es a n op por t u n it y t o con d u ctchemistry in a single fluid phase that would other-

w i s e o ccu r i n a m u lt i ph a s e s y s t em u n d er m or econventiona l conditions. The a dvant ages of a singlesupercrit ical phase reaction medium are t ha t h igherconcentrations of reactants can often be attained andt h a t t h e r e a r e n o in t e r p h a se m a ss t r a n sp o r t p r o c-esses t o hinder reaction ra tes.

The ion product, or dissociation constant (Kw) forwater as it approaches the crit ical point , is about 3o r d e r s o f m a g n it u d e h ig h e r t h a n i t is f o r a m bie n tliquid w a ter. Accordingly, in a ddition to near -criticalan d supercritical w at er being an excellent solvent fororganic compounds, it can a lso boa st a higher H+ a n dOH - i on con cen t r a t i on t h a n l iq u id w a t e r u n d er

certain conditions. As such, dense high-temperaturew a t e r is a n e f f e ct ive m e d iu m f o r a cid - a n d ba se -cata lyzed reactions of organic compounds. I n fa ct, thed isso cia t io n o f w a t e r i t se l f n e a r t h e cr i t ica l p o in tgenerates a sufficiently high H + concentration thatsome acid-catalyzed organic reactions proceed with-o u t a n y a d d e d a cid (se e se ct io n s I I .B a n d I I .D f o rexamples). As one exceeds t he critica l point, how ever,Kwdecreases dra ma tically. For exam ple, Kw is about9 orders of magnitude lower at 600 C and about 250a t m t h a n i t is a t a m bie n t co n d it io n s. SCW in t h is

Phillip Savage is currently Professor of Chemical Engineering and ArthurF. Thurnau Professor at the University of Michigan. He received his B.S.from Penn State in 1982 and his M.Ch.E. (1983) and Ph.D. (1986) degreesfrom the University of Delaware. All of his degrees are in ChemicalEngineering. Phils research focuses on the rates and mechanisms oforganic chemical reactions that are or may be of industrial significance.His research group uses experiments, mechanistic modeling, molecularsimulation, and computational chemistry to explore different reactionsystems. Most of the groups recent work has dealt with oxidation chemistryin near-critical and supercritical water. In addition to his research activitiesand classroom teaching at Michigan, Phil is also a co-lecturer in continuingeducation courses on the topics of kinetics, catalysis, and reactionengineering.

603Chem. Rev. 1999, 99, 603621

10.1021/cr9700989 CCC: $35.00 1999 American Chemical SocietyPublished on Web 01/05/1999


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high-temperature, low-density region is a poor me-dium for ionic chemistry.

Another difference between the properties (e.g.,viscosity, d ielectric const a nt, Kw) of SCW a n d a m bien tliquid wa ter is tha t t hey vary continuously over muchlarger ra nges in the supercritical sta te. This va riat ionoffers the possibility of using pressure a nd temper-at ure to t une th e properties of the r eaction m ediumto optimal values for a given chemical transforma-

tion.As its title indicat es, this r eview focuses on organ ic

chemical reactions in supercrit ical water. The pri-mary emphasis is on chemical transformations andreaction chemistry. Accordingly, rea ction pathw ays,p r od u ct s, k in et ics, a n d m e ch a n ism s cla im cen t e rstage. This review does not cover molecular modelingst u d ie s o r s im u la t io n s o f r e a ct io n s in SCW . Su chcom p u t a t ion a l w o r k1-8 u su a lly con sid er s a w e ll-ch a r a c t er iz ed r ea c t ion , a n d t h e a i m i s t o b et t e rcharacterize local solvation effects in SCW. As such,one learn s much about th e physics of SCW solutions,but gains lit t le new insight into reaction chemistryin S CW. Furt her, we a lso exclude studies9-15 of acid-

ba se r e a ct ion s a n d e q u il ibr ia in SCW. Re a ct ion sin volvin g e n t ire ly in or g a n ic com p ou n d s a r e a lsoexcluded, which means t ha t t he field of hydrotherma lmaterials synthesis and processing is not covered.Re a ct ion s a t su bcr i t ica l con d it ion s a r e g e n er a l lyexcluded except for some part icularly significantwork on reactions in near-crit ical water, especiallya s i t r el a t es t o ch em i ca l s y n t h es is . F i n a ll y , t h i sreview is based entirely on a search of the archival,peer-reviewed chemical litera ture. We ma de no at -t e m pt s t o in clu d e w or k a p p ea r in g e xclu sive ly innonrefereed conference proceedings, dis serta tions,government reports, or pat ents.

Our earlier review16 on reactions at supercrit icalcondit ions (a ll fluids, not only w a ter) covered th e fieldup to J une 1994, so the present review begins withwork published around this date. This decision tolimit t he present review to w ork published in 1994or later meant that many important earlier investi-g a t ion s h a ve bee n e xclu d ed . Th e r ef or e , t o g et acomplete a ccount of resear ch in t his field, one needst o r e f e r t o t h e e a r l ie r r e vie w 16 and the referencestherein along with the present review.

Although only four years have elapsed since thatcomprehensive review wa s submit ted, t he field ofr ea c t ion s i n S C W h a s g r ow n s u bs t a n t i a l ly . N ew articles dealing with chemistry in near-crit ical and

supercritical water appear at an ever increasing rate.Evidence to this end was obtained from the ScienceC i t a t i on I n d ex . A g en er a l s ea r c h w a s p er f or m edu sin g su pe r cr i t ica l a n d w a t e r a s t h e k e y w or d s,and then w e examined each a rt icle to assess wh etherit dealt with chemical reactions. Figure 1 summarizest h e r e su lt s by sh o w in g t h e n u m be r o f a r t icle s o nchemical r eactions in SC W per year since 1987. Ofall the art icles published in this f ield in the last 11years, over ha lf ha ve a ppeared since 1995.

Th is r ev iew i s or ga n i zed a r ou n d t w o m a jorthemes: chemical synth esis and chemical conversion.The former topic deals with deliberate at tempts touse SCW as a medium for forming specific chemical

bonds or introducing specific functional groups intom olecules. Th e la t t e r t o pic f ocuse s on t h e m o renumerous studies revealing the kinetics, products,a n d p a t h w a y s a s s oci a t ed w i t h a g iv en ch em i ca lsyst em in SC W. The ar ticles ca tegorized as chemicalsynthesis tend to ask and answer the question howcan one accomplish specific chemical transformationsin S CW? a n d t h e a r t icle s ca t e g or iz ed a s ch em ica lco n ve r sio n t e n d t o a sk a n d a n sw e r w h a t h a p p e n sto this compound in SCW?. The chemical synthesis

a r t icle s t e n d t o be p r od u ct -or ien t e d , w h e r ea s t h ech em i ca l con v er s ion a r t i cl es a r e m or e r ea c t a n t -oriented.

II. Chemical Synthesis

Wa t e r n ea r i t s cr it ica l poi nt ca n s er v e a s a ne n vir o n m e n t a l ly be n ig n so lve n t , a r e a ct a n t , a n d acat aly st in organic chemical reactions. These differenta sp e ct s o f a SCW r e a ct io n m e d iu m h a ve be e n e x -ploited for the purpose of conducting chemical syn-theses. Parsons,17 K a t r i t z k y et a l .,18 An et al.,19 a n dSimoneit 20 provide very good overviews of the typesof synthetic organic chemistry that have been dem-onstra ted in near -critical a nd supercritical w at er. Wesummarize key results from previous studies here,and we take the liberty of including several accountsof organic syntheses in subcrit ical water instead offocusing exclusively on the more limited literaturedealing with supercrit ical water.

A. Hydrogenation/Dehydrogenation

Cr it t e n d on a n d P a r so n s21 sh ow e d t h a t la t e t r a n si-t ion m e t a l com p le xe s a r e a ct ive d e h y dr og e n a t ioncata lysts in SC W. In t he presence of a P tO 2ca t a ly st ,cy cloh e xa n o l w a s d e h yd r og en a t e d via t w o p a r a l lelp a t h s. O n e p a t h in volved d e h yd r og en a t ion of t h er in g , a n d t h e o t h e r p a t h in vo lve d o x id a t io n o f t h e

Figure 1. N um b er of a r t i cl es on r ea c t ion s i n S C Wpublished each year since 1987.

604 Chemical Reviews, 1999, Vol. 99, No. 2 Savage


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alcohol group to the corresponding ketone. Metallicp la t in u m w a s sh ow n t o be t h e ca t a ly t ic sp ecie s f ora lcoh ol ox id a t io n , so m e t a l l ic p la t in u m m u st h a vef or m e d in si t u f r o m t h e P t O 2 init ially loaded in thereactor. Aromatization of the cyclohexyl r ing alsooccu r r ed in t h e se e x pe r im e n t s. B e n ze n e w a s t h ema jor product from the reactions of cyclohexane,cyclohexene, and cyclohexanol in SCW in the pres-ence of a P tO 2cata lyst. Addition of HCl t o the reactor

e sse n t ia l ly close d t h e a r o m a t iz a t io n p a t h , w h e r ea sad dition of NH 4OH simply suppressed aromatizationb ut s t i ll a l l ow e d i t t o p roce ed . I n a d d i t ion , t h epresence of NH 4OH in t he system shifted t he productdistribution from cyclohexanol such tha t cyclohex-anone w as the m ajor product. Low yields of phenol,apparently from dehydrogenation of cyclohexanone,were also observed. These important experimentssh o w e d t h a t o n e ca n co n t r o l t h e f u n ct io n a l g r o u ptransformations and the extent of dehydrogenationby judicious selection of catalysts and pH in SCW.

Adschiri et al.22 used a S CW reaction medium anda conv ent iona l NiMo/Al2O3h y d r ot r e a t in g ca t a ly st t oh y d r og en a t e a n d r em ov e s u lf u r f r om d ib en z o-

thiophene. They obtained higher conversions in CO-SCW a n d HCO O H-SCW m ed ia t h a n in a H 2-S C Wr e a ct ion m e d iu m . Th e a u t h o r s con clu d ed t h a t t h ew a t e r-gas shift reaction was occurring in the CO-a n d H C O OH-S C W m e di a , a n d t h a t i t p rod u ce dchemical species th at hydrogena te dibenzothiophenemore effectively tha n H 2 gas. This r esear ch showedthat hydrodesulfurization can be achieved withoutt h e a d d it ion of e xp en sive H 2 g a s , a s i s c u r r e n t l ypracticed indust rially .

B. CC Bond Formation

Mu ch of t h e w o r k o n u sin g n e a r -cr i t ica l a n d su -

percritical water as a medium for chemical synthesishas investigated ways to form (and break) selectedcarbon-carbon bonds. For example, Friedel-C r a f t salkylation reactions have been accomplished 18,23 inh ig h - t e m p e r a t u r e w a t e r . B o t h p h e n o l a n d p-cresolwere successfully alkylated with tert-butyl alcoholan d 2-propanol at 275 C in the a bsence of a ny a ddedacid cat alyst to produce sterically hindered phenols.23

Th u s , w a t e r s e r ve d a s b ot h t h e s o lv en t a n d t h ecata lyst for these alkylat ion reactions. Reaction timesra nged from a bout 1 h to over 100 h for the differentr e a ct a n t sy st e m s.

Parsons and co-workers24,25 ha ve reported extensivework on palladium-catalyzed alkene-arene coupling

r e a ct ion s in n e a r -cri t ica l a n d su pe r cr i t ica l w a t e r .These reactions can be classified as Heck ar ylat ionr e a ct ion s. F ig u r e 2 sh ow s t h e ca t a ly t ic cy cle f o rcoupling iodobenzene with an alkene. The reactionsin h ig h -t e m pe r a t u r e w a t e r w e r e sim ila r t o t h o seobser ved in t r a d it ion a l or g a n ic so lven t s, bu t t h e ywere more sensit ive to steric effects and the natureo f t h e a lk e n e . O n e ca n a lso t a k e a d va n t a g e o f t h era pid elimina tion reactions (discussed in more deta ilin se ct ion I I .E) t h a t som e com p ou n d s u n d er g o inS C W t o g en er a t e i n s it u t h e a l k en e n ee de d f orcoupling. 25 Alcohols, halides, and carboxyl groups areall rapidly eliminated to generate compounds withdouble bonds. For exam ple, 2-phenylethan ol dehy-

drates to form styrene, which can then couple withan arene. Thus, a larger suite of starting compounds

can be used in Heck coupling reactions in SCW.Diels-Alder cycloaddition reactions have also beenconducted in SCW.26 Severa l different diene/dieno-phile combinations have been explored. Aldol con-densat ion rea ctions ca n a lso be accomplished in high-temperat ure wa ter. 2,5-Hexan edione is unrea ctive inpure wa ter, but in t he presence of a sma ll amount ofbase (NaOH ) this dione underw ent a n intra molecularaldol condensation to form 3-methylcyclopent-2-enonein 81% yield.19 Pa r so n s 17 a n d A n e t a l .19 a lso g iveother examples of cyclization reactions.

Carbon-carbon bond cleavage reactions can alsooccur in nea r-crit ical a nd supercrit ical w at er. Kuhl-m a n n e t a l .27 obtained rapid and irreversible r ing

opening of 2,5-dimethylfuran to produce 2,5-hex-an edione quan tita t ively at 250 C . The reaction w asacid-catalyzed.

C. Rearrangements

K u h lm a n n e t a l .28 r ep or t t h a t p in a c ol a n d t w od iff er e n t bicy clic d io ls u n d e r w en t q u a n t i t a t ive r e -arrangement to the corresponding ketones in purew a t e r a t 275 C . Th e p ot e n t ia l s id e r ea c t ion ofeliminat ion led to n egligible a lkene format ion. Crit-t e n d o n a n d P a r s o n s21 report tha t cyclohexene r e-arranged reversibly to methylcyclopentene in SCWin the presence of a mineral a cid or acidic meta l sa lts.This ring contraction was attributed to acid catalysisand the rearrangement of a carbocation. An et al. 19

sh ow e d t h a t Cla ise n , Ru p e, a n d Mey e r -Sch u st err e a r r a n g e m en t s a r e a lso op er a t ive in p u r e w a t e r a televat ed temperatures.

D. Hydration/Dehydration

Anta l a nd co-workers ha ve been pioneers in study-in g t h e d e h yd r a t io n o f a lcoh ols t o o lef in s in n e a r -crit ical and supercrit ical water. Their most recentwork29,30 in this area centers around tert-buty l a lco-h ol . Th e y f ou n d t h a t t h e con ver sion of tert-butylalcohol to isobutylene was rapid and selective in near-cr i t ica l w a t e r a t t e m pe r a t u r e s a r o u n d 250 C. Th e

Figure 2. Catalytic cycle for Heck coupling reaction (inorganic media). (Reprinted from ref 24. Copyright 1995American Chemical Society.)

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reaction occurs even in the absence of added acid orbase, but the addition of H 2S O4or NaOH did a cceler-a t e t h e r e a ct ion r a t e . I n t h e a bsen ce o f a d d e d a cid ,hydronium ions formed by the dissociation of w at erare the primary catalytic agents. A reaction mecha-nism a nd kinetics for each elementa ry st ep were alsoreported. Figure 3 shows the main reaction steps forthis dehydration reaction.

The dehydration of other alcohols, such as cyclo-

hexanol, 21,27 2-methylcyclohexanol, 27 and 2-phenyl-ethanol25 have also been reported, but there is muchless qua ntita t ive and mechanistic information ava il-able for these compounds. Several different mineralacids a nd a cidic metal complexes a s w ell as NH 4OHare effective catalysts for dehydration.

If a lcohols undergo dehydr a tion to form a lkenes inwater near its crit ical point , then the reverse reac-tion, addit ion of water across a double bond shoulda lso be op er a t ive , a l t h o u gh ch e m ica l e q u il ibr iu mwould be a limiting factor. Crit tendon and P ar sons21

obtained low yields of cyclohexanol from cyclohexenehydra tion in SCW. The reaction required added P tO 2and a cid or base cat alyst . An et a l.19 report hydrationo f so m e o le f in s w it h o u t a n a d d e d ca t a ly st , bu t a tlower temperatures around 250 C. They also con-firmed earlier work that alkynes can be convertedto the corresponding ketones by the addition of water.

E. Elimination

One type of elimination reaction, dehydration, hasa lr e a d y be e n d iscu sse d . I n t h is se ct io n w e br ie f lyconsider other elimination reactions documented inSCW. Carboxylic acids undergo facile decarboxylationin p u r e , h ig h - t e m p e r a t u r e w a t e r , a s e vid e n ce d byreports regarding formic acid,31 citric and itaconicacids,32 a nd cinnam ic an d ind ole-2-carboxylic acid.19

For example, Carlsson et al. 32 sh ow t h a t ci t r ic a cidcan be converted in high yields to methacrylic acidin high-temperat ure wa ter t hrough sequential dehy-d r a t i on a n d d eca r b ox y la t i on r ea c t ion s . F i g ur e 4shows the reaction pathways they postulate for thesevarious chemical transformations.

Dehydrohalogenation reactions are also rapid, asevidenced by the reported behavior of poly(vinylchloride)18 and methylene chloride. 33 Trichloroacetic

a cid d e com p osit ion p r ovide s a n e xa m p le of bot hdecarboxylation and dehydrochlorination. This com-pound d ecomposed completely to H Cl, C O, CO 2, a n dH 2 in S C W a t 600 C a n d 65 s r ea c ti on t im e.34

H y d rol ys is w a s t h ou gh t t o b e a t l ea s t pa r t ia l lyresponsible for t he conversion. More deta ils a nd m oree xa m p les of e lim in a t ion r e a ct ion s in SCW a p pe a rlater in t his review in section I II .A.2.

F. Hydrolysis

As summ a rized by An et a l.19 a n d K a t r i t z ky e t a l .18

several different compounds readily hydrolyze inSC W. Est ers, for example, can undergo an aut ocata -

lytic hydrolysis to form car boxylic acids an d a lcohols.The acid produced catalyzes the hydrolysis, and thisfeature is th e source of the a utocat alysis. Nitriles alsoundergo hydrolysis to form the corresponding amideand then further hydrolysis to the corresponding acid.I y e r a n d K le in 35 provide a thorough kinetics studyof butyronitrile hydrolysis. They observed autocata-lytic kinetics, consistent with the a cid product cata -ly z in g f u r t h er h y d r oly sis r e a ct ion s. Th e y a lso r e -ported on the effect of pressure on the hydrolysis ratean d used their dat a to calculate th e effective activa-tion volumes. Hy drolysis in high-temperature w at eris an effective and efficient means of decomposingsome synthetic polymers. For example, poly(ethylene

t e r e p h t h a la t e ) a n d p o ly u r e t h a n e f o a m s ca n be h y -drolyzed to reusable diacids and glycols a nd dia minesan d glycols, respectively.18 Activated diaryl ethersundergo hydrolysis to form the corresponding hy-droxyarenes. 18

G. Partial Oxidation

Economically converting metha ne to oxygena tes orhigher hydrocarbons that are less expensive to liq-u e f y a n d t r a n sp o r t h a s be e n a n e lu sive g o a l in t h ech e m ica l co m m u n it y f o r se ve r a l y e a r s. Ch e m ist r ytha t gives high yields of the t arget compound w ouldallow use of vast methane reserves in remote loca-

t io n s. I t is in t h is co n t e x t t h a t p a r t ia l o x id a t io n o fmetha ne in SC W around 400 C has been examinedas a potentia l route to metha nol. These investiga tionsha ve explored both homogeneous36,37 free-radicalr e a ct ion s a n d h e t er og en e ou s ca t a ly t ic38 reactions.High selectivit ies t o oxygenates were obtained, butonly at very low methane conversions. As a result ,the highest methanol yields observed were only about1%. Our ana lysis revealed th a t a la rge-scale chemicalprocess based on this chemistry and this highest yieldwould not be profitable. Unless much higher metha -nol yields can be obtained, part ial oxidat ion in S CWdoes not appear to be an economically viable routefor converting metha ne to metha nol.

Figure 3. Acid-catalyzed reactions of tert-buty l a lcohol inhigh-temperature water. (Reprinted from ref 30. Copyright1997 American Chemical Society.)



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The ca ta lytic partia l oxidat ion of a lkylaroma tics innear-critical water (around 300-350 C) to producea l d eh y d es , k et on es , a n d a c id s h a s a l s o b e en ex -plored.39 The precise products and their yields are

strong functions of the reaction conditions and thespecific catalyst employed. For example, p-xylene canbe converted to terephtha lic acid in 64%yield w ith aM n B r 2 ca t a ly st , bu t t h e m a jor p r od u ct s a r e p-tolu-a ld e h y de a n d p-toluic acid, both in 24%yield whenCo(OAc)2 is t h e ca t a ly st . Sim ila r ly , t o lu e n e ca n beconverted t o a 63% yield of benzoic acid under oneset of reaction conditions, but to benzaldehyde (30%y ield ) a n d less ben z oic a cid (10% y ield ) u sin g adifferent cat a lyst, less oxygen, and a shorter reactiontime. All of these experiments were done in noniso-therma l, bat ch reactors so no kinetics informat ion isava ilable. Nevertheless, the r esults clearly show thepotential utility of near-crit ical water as a mediumfor pa rtia l oxida tion reactions.

H. HD Exchange

Some compounds undergo hydr ogen a tom excha ngereactions with w at er at high temperatures and pres-s u re s. K u h lm a n n e t a l .27,28 h a v e i n ves t i ga t e d t h ebehavior of several different organic compounds insubcrit ical D 2O t o d e t e r m in e t h e k in e t ics o f H-De xch a n g e . Sim p le a lcoh ols d o n ot p a r t icipa t e inhydrogen exchan ge reactions, w hereas t he Rpositionsof ketone carbonyl groups (e.g., CH 3 groups in ace-tone) undergo rapid an d nearly complete exchange.Evilia and co-workers 40-43 a n d J u n k e t a l .44 showed

tha t supercritical D 2O with added ba se is an excellentmedium for H-D excha nge in amino acids, very w eakor g a n ic a cid s, a n d a n il in es. Re a ct ion w it h su pe r -critical D 2O provides a convenient alternate route tot h e sy n t h e sis of d e ut e r a t e d or g a n ic com p ou n d s.Deuterium w as also present in th e products formedfrom tert-butylbenzene pyrolysis45 and polymer crack-in g 46 in supercritical D 2O. These experiments clearlyshow that near-critical and supercritical water is notmerely a solvent, but ra ther also a reacta nt , a t leastin h ydrogen exchange r eactions.

III. Chemical Conversion

This section is apportioned into tw o subsections,decomposit ion an d oxidat ion. The decomposit ionsubsection discusses t he chemica l beha vior of differ-

e n t cla sses of com p ou n d s in SCW, a n d a t t im e s inthe presence of catalysts or other addit ives such asNH 3, d ih y d r o a n t h r a ce n e , f o r m ic a cid , a n d f o r m a t eion. The oxidat ion subsection describes recent re-search into tota l oxidat ion rea ctions in SC W.

A. Decomposition

This section focuses on the decomposition of organiccompounds a nd ma terials in SC W in the a bsence ofoxygen. We first describe work dea ling w ith complexmaterials such as tires, polymers, vegetable oils, andbioma ss. Next w e outline recent w ork on t he reactiv-ity of different types of individual compounds and

Figure 4. Reaction netw ork for decomposition of citric acid in h igh-temperat ure w at er. (Reprinted from ref 32. Copyright1994 American Chemical Society.)



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f un ct i on a l g r ou ps i n S C W, b ot h p ur e a n d w i t hadditives.

1. Complex Materials

Supercritical w at er has been explored as a mediumfor t he degrada tion of w ast e synthetic polymers.46-51

Rubber t ires w ere converted t o a 44% oil yield byreaction in S CW at 400 C. 50 Polystyrene-based ion-excha nge resins were subjected to SC W a t 380 C for

1 h .49 Less than 5%of the polymer decomposed, andthe products included styr ene and several oxygenat edarenes such a s a cetophenone a nd benzaldehyde.

Wa t e r a bo ve i t s cr it ica l t e m pe r a t u r e w a s u sed t oextract oil and oil precursors from oil shale.52 Al-though nominally termed extra ction, this t reatm entcertainly involved cleavage of chemical bonds in theo il sh a le . Pr o ce ssin g in SCW , a n d e sp e cia l ly w it ha d d e d CO , le d t o h y d r o ca r bo n y ie ld s h ig h e r t h a nt h o se obt a in e d f r om m or e con ven t ion a l p y r oly t ict r e a t m e n t .

Holliday et al.53 r e por t t h a t w a t e r n e a r i t s cri t ica lpoint is a good medium for the hydrolysis of triglyc-eride-based vegetable oils into their fat ty acid con-stituents. A process based on this chemistry mightbe faster than conventional processes and also givea product free of catalyst residues. Conditions belowt h e cr i t ica l p oi n t a p pe a r ed t o b e m os t f a v or a b l ebecause undesired degrada tion, pyrolysis, an d poly-merization reactions occurred at the higher super-crit ical temperatures.

The allure of using biomass as a renewable sourceo f f u e ls o r ch e m ica ls a n d t h e k n o w le d g e t h a t ce l-lulose, a ma jor component of bioma ss, can be hydro-ly z ed t o p r od u ce g lu cose a n d ot h e r p r od u ct s h a sprompted several r ecent s tud ies into th e decomposi-tion of cellulose,54,55 glucose, and related compounds

in near-crit ical and supercrit ical water. Holgate eta l. 56 examined glucose hydrolysis at temperaturesbe t w e e n 425 a n d 600 C a n d f o r r e sid e n ce t im e saround 6 s. These reaction conditions led to glucoseconversions of 97% or higher in all experiments.Thus, H olgat e et a l. were not a ble to determine ther e a ct ion k in et ics. Th e y d id f in d t h a t t h e p r od u ctd ist r ibu t ion w a s se n sit ive t o t e m p er a t u r e . At t h elower temperatures t he gas yield wa s low, a nd mostof the decomposit ion products were liquid pha se,lower molecular weight compounds such as aceticacid, acetonylacetone, and aceta ldehyde. At t he highertempera tures, t he ga s yield wa s essentially 100%,and the ma in components w ere H2a n d C O2in 1.7 to

1 molar ra t io. This result w as consistent w ith previ-ous work showing tha t glucose can be completelygasified by reactions in supercrit ical w at er.

Kabyemela et al. 57-60 performed a more extensivecomplementa ry st udy a t much milder conditions (T) 300-400 C, ) 0.02-2.0 s) where the kineticsan d prima ry r eaction networks w ere accessible. Theyalso examined the effect of pressure on the reactionrates. Their experiments included glucose, cellobiose(a disaccharide of glucose), and the glucose decom-position products dihydroxyacetone and glyceralde-hyde, as reactants. By investigating the reactions ofa v a r i et y of s t a r t i ng m a t e r ia l s t h ey w e r e a b le t oa sse m ble t h e r e a ct ion n e t w or k in F ig u r e 5, w h ich

summa rizes the va rious chemical conversions occur-r i ng i n w a t e r n ea r i t s cr i t ica l p oi n t . C e ll ob ios edecomposes both thermally (k1 a n d k2) a n d h y d r o -lytically (kH) t o f or m t h e p r od u ct s sh ow n . Glu coseinterconverts to fructose (kG F) or it decomposes to t heproducts shown. Int erestingly, high yields of eryth-rose can be obtained from glucose in supercrit icalw a t e r .59 The authors reported a selectivity of nearly80% a t a glucose conversion of 50%. E ryt hrose is au sef u l ch e m ica l in t h e f ood , f in e ch em ica ls , a n dpharmaceutical industries.

Xu et a l.61

explored the activity of various charcoalsa n d a ct iva t e d ca r bon s a s biom a ss g a si f ica t ion ca t a -l ys t s i n S C W. C a t a l y st s a r e r eq u ir ed t o a ch i ev ecomplete gasification of concentrated feedstocks (>20w t % or g a n ics in w a t e r ). Co m plet e g a si f ica t ion ofdifferent feedstocks w as achieved a t 600 C, a nd H 2,CO , CO2, a n d CH 4were th e ma jor components of thegases. Hydrogen production is the goal of gasification,so t h e a bu n d a n ce of CO a n d CH 4, w hich could ha vereacted further to produce more H 2 b y t h e w a t e r-gas shift reaction and re-forming, was not desired.

In a follow-up study, Mat sumura et al. 62 gasified ag r a n u la r a ct iva t e d ca r bo n in SCW a r o u n d 600 C.T h e g a s w a s p r im a r i ly H 2 a n d C O2 in a 2:1 m o la r

r a t io , w it h m u ch sm a lle r a m o u n t s o f m e t h a n e a n dCO . Th e y f o un d t h a t t h e k in e t ics a t su pe r cr i t ica lconditions were in agreement with rates measuredfor carbon gasification in steam at am bient pressures,which indicated that the kinetics were largely insen-sit ive t o pressure.

A t ea m a t t h e P a cific N or t h w e st N a t ion a l L a bo r a -tory 63-67 has done extensive research and develop-ment work on chemical processing in near-crit icalwater. They report a process whereby a wide varietyof organic compounds and lignocellulosic m at erialsca n be ca t a ly t ica l ly con ve rt e d t o a m e t h a n e -r ich ,medium-BTU fuel gas at temperatures around 350 C for reaction times a round 10 min. P rocessing high-

Figure 5. G lu cose decomposit ion p a t hw a ys in SCW.

(Reprinted fr om ref 60. Copyrigh t 1998 American Chem icalSociety.)



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moisture biomass in an aqueous environment meansthere is no need for t he costly dew at ering or dryingpretreatment steps that accompany other conversionschemes. This process can also be used to destroyor g a n ic w a st e s.

2. Individual Organic Compounds

Th e r e a ct ivi t ie s of a la r g e n u m ber of d i ff er e n t

organ ic compounds in S CW ha ve been determined.Most of these studies were motivat ed by a desire tounderstand the effect of SCW processing on someother more complex material, such as biomass, coal,o i l sh a le , s lu d g e s, m il i t a r y w a st e s, o r w a st e w a t e rcomponents. Katritzky et al.18 provide an overviewof largely their own work in this field through 1995,a n d S a v a g e et a l .16 provide a more comprehensivereview of th e pre-1994 resear ch.

a. Hydrocarbons. B iphenyl, 1,1-binaphthyl, di-phenylmethan e, 1-benzylnapht halene, naphth alene,a n d p h en a n t h r e n e a r e r e sist a n t t o sig n if ica n t r e a c-tion in pure SC W a t 460 C for up to 1 h. 47,68,69 Otherpolycyclic arenes70 likewise exhibited conversions of

o n ly a f e w p e r ce n t , w it h t h e m a in p r o d u ct s be in ghydroarenes. Hydrocarbons with weaker bonds (e.g.,tert-buty lbenzene, benzylcyclohexan e, benzyl tetr a lin,hexylbenzene, 1-decylnapht ha lene, cyclohexylben-zene, and cyclohexylnaphthalene) did undergo de-composition in SC W a t T g 460 C , an d the rea ctionpaths in SC W ar e similar to those in a hydrocarbon

solvent.45,69 Free-radical chemistry is almost certainlyresponsible. Water does not appear to be either areactant (i.e., a means of incorporating oxygen func-tionalities into the hydr ocarbons) or cat alyst for t hesecompounds. As noted ea rlier in th is review, however,proton exchange reactions between hydrocarbons andwater can certainly occur. These reports are fullyconsistent wit h th e previous w ork in the field relat edto hydrocarbon pyrolysis in SCW.16

b. Nitrogen-Containing Compounds. I y e r a n dKlein have conducted detailed studies of the reactionsof 1-nitrobutane71 and butyronitrile35 in pure high-t e m p e r a t u r e w a t e r . A s s h o w n i n F i g u r e 6 , n i t r o -buta ne undergoes para llel tran sforma tions to buta nala n d bu t y r on it r i le . B u t y r on it r i le is h y d r oly z ed t obuta na mide, which then hydrolyzes further and losesNH 3 to form but yric acid. But an al ca n be convertedto buta nol, wh ich can undergo a condensat ion reac-tion with but yric acid to form butyl but yra te. Kineticsa n d m e ch a n ism s f or seve ra l of t h e ch em ica l t r a n s-formations are proposed.

W a n g e t a l .72 provide a thorough examination ofthe kinetics, reaction netw orks, a nd possible mech-

an isms of the reactions of six different nitroanilinesin w at er a t 300 C. 2-Nitroaniline formed benzofur-azan as the major primary product. 4-Nitroanilineformed 4-aminoaniline as the major product. Ther ea c ti on s i n w a t e r w e re 4-8 t im es f a st er t h a ndecomposit ion via neat pyrolysis under otherwiseidentical conditions. Additiona lly, the presence of

Figure 6. Reaction network for the conversion of 1-nitrobutane in high-temperature water. (Reprinted with permissionfrom ref 71. Copyright 1996 PRA Press.)



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high-tempera ture wa ter also influenced t he productdistribution.

Lee and co-workers examined the decomposition ofnitrobenzene73 an d 4-nitroaniline74 in supercrit icalwa ter. Nit robenzene decomposed even in t he a bsenceof oxygen, and a global rate law was reported. Thema in products were benzene and nitrite, along withsom e CO a n d CO 2. Decomposition of 4-nitroaniline74

in the absence of added oxygen followed first-order

kinetics. NH 3, C O , C O2, a n d N 2 w e r e a m o n g t h eproducts formed.

4-Nitrotoluene decomposed in SCW to form smalla m o u n t s of a n il in e, 4-t o lu id in e , a n d t o lu e n e, bu tmuch higher yields of ta r/char. 75 The add it ion ofdihydroanthra cene (a hydrogen donor) or NH 3 en -h a n ced t h e y ield s of a n il in e a n d 4-t o lu id in e . Th eaddit ion of ZnCl2 promoted the formation of phenolan d 4-cresol, a t t he expense of a niline a nd 4-toluidine.Sim ila r e f f e ct s o f a d d it ive s w e r e o bse r ve d f o r t h edecomposition of 4-toluidine in SCW.

All o f t h e ot h e r r e ce n t w o r k o n t h e beh a vior ofnitrogen-conta ining compounds in pure S CW ha s

focused less on t he kinetics a nd more on productidentit ies a nd differences therein for reactions withdifferent addit ives. Quinuclidine [CH(CH 2C H 2)3N]formed alkylpyridines in SCW at tempera tures around425 C.75 3-P h e n y lp y r id in e w a s less r e a ct ive t h a nq u in u clid in e, a n d a lk y lbe n ze n es a n d t a r s w e r e t h emajor products. The addition of ZnCl2led t o completeremoval of the organic nitrogen as NH 3.

K a t r it z ky et a l .76,77 e xa m in e d t h e r e a ct ivi t y ofseveral heterocyclic nitr ogen-cont a ining compoundsi n S C W a t 4 6 0 C f o r u p t o 6 0 m i n o f e x p o s u r e .P yridine, indole, and carbazole are essentially unre-a c t iv e i n p ur e S C W a t 460 C f or u p t o 6 0 m i n .76

Acridine and phenanth ridine are a lso largely sta bleunder these conditions, with only small amounts ofdihydroacridine and dihydrophenanthridine, respec-tively, being formed. B oth 1,2,3,4- a nd 5,6,7,8-tetr a -hydroquinoline underwent dehydrogenation to formquinoline in 5.1%a nd 6.1%yields, respectively. 2,3-Dimethylindole and 1-and 2-methylindole underwentdemethylation and methylation reactions, with con-versions of only a few percent at 460 C and 60 min.Sim ila r beh a vior w a s obser ved f or p y r r ole, 2,5-dimethylpyrrole, a nd 2,4,6-trim ethylpyridine. 4-P ro-pylpyridine produced prima rily sm aller a lkylpyri-dines. 1,2,3,4-Tetra hydr oca rba zole wa s m ore reactive.Th e m a in r e a ct ion p a t h s a r e d e hy d r og en a t io n t o

carbazole and ring opening to produce methylindoles.2-Aminobiphenyl70 underwent 15% conversion inpure SCW aft er a 1-h reaction. 9H-Carbazole, formedvia r in g closu r e, w a s t h e m a jor r e a ct ion p r od u ct .B iaryl nitrogen-conta ining compounds (2-phenyl-pyridine, 2-(1-naphthyl)pyridine, 2-phenylquinoline,2-(1-naphthyl)quinoline, and 2-phenylindole) are es-s en t i a l ly s t a b l e in S C W a t 460 C f or u p t o 1 h .68

W a t e r i t se l f d o e s n o t a f f e ct t h e r e a ct io n p a t h s f o rthese heterocyclic nit rogen-conta ining compounds.Th e a d d i t ion of f or m i c a c id or s od iu m f or m a t e ,h o w e ve r , r e su lt s in t h e r e d u ct io n o f t h e a r o m a t icr in g s a n d cle a va g e o f bia r y l bo n d s in m a n y o f t h ecompounds. These hydr ogena tion rea ctions were n ot

a s p r eva le n t f or n on -n it r og e n -con t a in in g h e t er o-cycles.

Olobunmi a nd Berkowitz 78 reexa mined t he decom-posit ion of quinoline and isoquinoline in SCW. Inpure wat er at 400 C , both compounds are sta ble foru p t o 48 h . K a t r i t z k y e t a l .76 a lso f o u n d t h a t t h e secompounds, along with 2-methylquinoline, un der-went less than 3%conversion at 460 C for 60 min.In the presence of added Fe2O3, however, quinoline

produced o-xylene, benzeneamines, and aniline in thehighest yields.78 Iso-quinoline was less reactive, andit produced ethylbenzene, xylene, and other dialkyl-substituted benzenes. CO2 w a s t h e m os t a b un d a n tga seous product from both compounds. The forma tionof t h e se pr od u ct s in d ica t e s t h a t SCW se r ve d a s a noxidant (to form CO 2) a n d p er h a p s a s a h y d r og endonor. The authors at tr ibuted at least a portion ofthe chemistry to ionic rea ctions involving H + or OH-.

c. Sulfur-Containing Compounds. There a p-pears to have been no kinetics studies of the decom-posit ion of sulfur-conta ining compounds in SC Ww it h in t h e la st f ou r y e a r s, a l t h o u gh st u d ies o f t h istype have been reported in the more distant past . 16

The r ecent work 75,78,79 t h a t h a s be e n co m p le t e d islargely descriptive. The behavior of selected com-pounds is examined in SCW an d often compar ed withthe beha vior observed in the a bsence of wa ter, or inwater with addit ives, or in a hydrocarbon solvent.

B enzyl sulfide reacts completely w ithin 30 min inSCW a t 400 C t o f or m be n z en e a n d t o lu e n e a s t h ema jor volat ile products. 75 Th ia n a p h t h e n e is m or estable in pure SCW,79 a n d a Z n C l2 ca t a ly st o r N H 3were required to a chieve a ppreciable desulfuriza-tion.75 An Fe2O3 catalyst was also effective, 78 bu t t oa lesse r d e gr e e. Th ia n t h r e n e p r od u ce s d iben z o-thiophene as the exclusive product. 78 K a t r i t z k y e t

a l.79

r e por t on t h e beh a vior of n in e o t h er su lfu r -conta ining compounds in S CW a t 460 C. Thiophene,benzothiophene, and diphenyl sulfide were all es-sentially unreactive in pure SCW under the condi-tions investiga ted. Thiophenol reacted to give near lyqua ntita tive yields of diphenyl sulfide, apparently viaa reversible reaction tha t rea ched equilibrium w ithina few minut es. 1-Napht ha lenethiol reactions w ereanalogous to those of thiophenol. Cyclohexyl phenylsulfide produced 1-methy lcyclopentene a nd thiophe-nol in n early equa l yields. The 1-methy lcyclopentenew a s a t t r i b ut e d t o a c id -ca t a l yz ed cl ea v a g e of t h esu lf id e a n d r e a r r a n g e m e n t o f t h e cy clo h e x y l f r a g -ment. Results for 1-na phth yl phenyl s ulfide, 1-phen-

y lt h iot e t r a l in , d ioct y l su lfid e, a n d t e t r a h y d r o t h io-p h en e a r e a lso a va i la ble. S iskin e t a l . 68 report that2-a rylt hiophenes a nd 2-a ryl-benzoth iophenes (2-phen-ylt hiophene, 2-(1-na pht hyl)th iophene, 2-pheny lbenzo-[b]thiophene, and 2-(1-naphthyl)benzo[b]thiophene)exhibit low reactivity in pure SCW at 460 C for 1 h.C l ea v a g e of t h e b ia r y l b on d ca n b e a ch i ev ed b yreaction in the presence of added sodium formate,however, which produces basic reaction conditions.K a t r i t z k y e t a l .79 a lso r e p or t on h ow t h e p r od u ctdistribut ions shift as different a dditives (formic acid,sodium formate, sodium carbonate, phosphoric acid)were included in t he rea ctors. Adding formic acid, forexample, enhances acid-catalyzed reactions and pro-



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duces reducing reaction conditions. Formic acid isalso thought to be an intermediate in the wa ter-ga sshift r eaction, so its ad dition simula tes t he CO/H 2Otreatment of hydrocarbons, which is often used in fuelprocessing studies.

d. Oxygen-Containing Compounds. Ma r t in oa n d S a v a g e80 report kinetics a nd products from th edecomposition of cresols, hy droxybenzaldehy des, ni-trophenols, and benzenediols in supercrit ical wa ter

a t 460 C f or t i m es on t h e o r de r o f 1 0 s . Th es econditions a re representa tive of t hose encounteredin t h e su pe r cr i t ica l w a t e r ox id a t io n p r ocess (d is-cussed later in section III .B in this review). Theyfound th at the cresols are la rgely stable under thesecon d it ion s a n d t h a t h y d r ox y be n z a ld eh y d es a n d n i-trophenols are reactive. Phenol was formed in highyields and wit h nea rly 100%selectivity from all th reehydroxybenzaldehyde isomers. Phenol was also theonly product consistently present in high yields fromd e com p osit ion of n i t r op h en ols. Th e a u t h o r s a lsoobserved that for a given substituent position, nitro-phenols were more reactive than hydroxybenzalde-hydes, wh ich w ere more reactive tha n cresols. For a

given substituent, t he ortho isomer of the substitut edphenol was the most reactive in SCW.

Th e r ea c t ion s of d ib en z yl e t he r i n S C W w e r erecently reexamined 81 i n a n e ff or t t o e xp a n d t h ekinetics da ta base for the competing para llel hydroly-sis and pyrolysis paths. The hydrolysis pat h leads t obenzyl alcohol and the pyrolysis pa th leads t o benz-a l d eh y d e a n d t ol ue ne a s p ri m a r y p rod u ct s . Th eaut hors determined the kinetics for th ese pat hs a ndothers in the rea ction netw ork.

Formic acid decomposes rapidly in SCW via twop a r a l lel p a t h s, d e ca r bo xy la t ion a n d d eh y d r a t io n .31

The major products are CO 2 a n d H 2 from t he decar-

b ox yl a t i on p a t h , b u t s m a l l a m ou n t s of C O (a n dpresumably H 2O) a re also formed, from t he dehydra -tion path. First-order kinetics provides a good de-scription of the transformations, and rate constantsfor t he tw o paths are a vaila ble for decomposit ion a tdifferent temperat ures and pressures. Wat er appearsto be a cat a lyst in formic acid decomposition becauset h e r a t e s in SCW a r e f a st e r t h a n in t h e g a s p h a se .Additionally, water shifts the product spectrum sot h a t d eca r b ox yl a t i on i s t h e f a s t er p a t h , w h e re a sdehydrat ion is fast er in the gas phase. This cata lyticrole of water molecules was confirmed through abinitio quant um chemical calculat ions,82 which showedt h a t t h e p res en ce of w a t e r m ol ecu l es p er m it t e d

formation of lower energy transit ion states.K a t r it z ky et a l .69,70 r e por t on t h e r e a ct ion s of

several oxygen-containing compounds in pure SCWa t 460 C a n d f o r e i t h e r 7 o r 60 m in o f e x p o su r e .1-Na phth ol underwent 16%conversion a fter 60 min,a n d 1, 1-b in a p h t h yl e t he r w a s t h e s ol e p rod u ct .1-Octan ol underwent 5%conversion a fter 7 min, a ndthe major products were octanal, octene, and heptene.B enzophenone, 9-fluorenone, an thra quinone, acri-done, 3,4-benzocouma rin, a nd 2-phenylphenol exhib-i t ed con v er s ion s of a t m os t a f ew p er cen t u pont r e a t m e n t i n p u r e S C W f o r u p t o 1 h . 70 P h e n a n -thr enequinone wa s completely converted a fter 1 h in460 C SCW. Th e m a in p r od u ct is 9-f luor e n on e ,

apparently formed via decar bonylat ion. 1,4-Naph-thoquinone is also very reactive under t hese condi-tions. 1-Napht hyl phenyl ether 69 hydrolyzed after 1h in SCW to form phenol and 1-naphthol in about10% yield. Diphenyl ether and dibenzofura n wereboth sta ble under these condit ions. Cyclohexyl phenylether, on the other ha nd, w as completely converteda f t e r 7 m i n o f r e a c t i o n t i m e . F i g u r e 7 s h o w s t h ereaction network. The major products are phenol and

1-methylcyclopentene, presumably formed from acid-ca t a l y ze d cl ea v a g e of t h e C-O b on d a n d s u bs e-quent rearr a ngement of the cyclohexyl ca tion to formm e t h y lcyclop en t e n e. I n t h is ca se, w a t e r (or m or eprecisely H+ d er i ve d f r om w a t e r) s e r ve s a s t h eca t a l ys t . P h e n ol a n d cy cl oh ex en e a r e t h e m a j orproducts obta ined in the a bsence of wa ter, an d th esep r esu m a bly a r ise f r om t h e f r ee -r a d ica l r e a ct ion sillustra ted in Figure 7. This competit ion betweenfree-rad ical an d ionic reaction path s in SC W occursfrequently for heteroatom containing organic com-pounds. This competition and the tunability of theproperties of SCW a fford t he opportu nity to engineerthe rea ction medium to fa vor w hichever reaction is

desired. Kat ritzky et al.69,70

a lso provide informa tiona bout th e effect of added formic a cid (to promote acid-cata lyzed reactions) a nd sodium format e (to promotebase-cata lyzed reactions) on the decomposit ion ofthese compounds in SCW.

Although diphenylmetha ne itself is sta ble in SCW,t h e a r y l-a lk y l C-C linkage in dihydroxydiphenyl-m et h a n e s ca n b e c l ea v e d i n S C W.47 The ma jorproducts are phenol an d cresol in about 40% yieldfrom reaction at 430 C for 1 h.

Eth yl benzilate (Ph 2C O H C O2C 2H 5) was completelyconverted t o oth er products by react ion in pure S CWa t 400 C f or 30 m i n .75 Th e m a i n pa t h w a y w a shydrolysis of the ester functionality to ethanol and

benzylic a cid. The acid underwent decar boxylationand eventually formed benzophenone and diphenyl-methane as stable products. The addit ion of ZnCl 2to the reactor led to cracking reactions and higheryields of single-ring products such as toluene andbenzene.

e. Compounds with Two Heteroatoms. Only afew compounds recently studied un der S CW condi-t ion s con t a in m or e t h a n on e h e t er oa t om . Re sult sfrom some of these have already been reviewed inone of the preceding sections. For example, most ofthe compounds with nitro (NO2) su bst i t u en t s w e r eincluded with the nitrogen-containing compounds.The one exception is nitr ophenol, wh ich is discussedin t h e con t e xt of ot h e r p h en ols in t h e se ct ion onoxygen-conta ining compounds.

O n e c om pou n d w i t h b ot h s u lf u r a n d n i t rog enfunctiona lit ies has been exam ined.78 2-(methylthio)-benzothiazole formed primarily aniline and benzo-thia zole during decomposit ion in S CW at 400 C for5 h, but it formed prima rily benzothiophene duringneat pyrolysis under otherwise identical conditions.These a uth ors78 a lso report results for the decomposi-tion of a sulfur- and oxygen-conta ining compound,thiochroman-4-ol.

K a t r i t z ky e t a l .70 examined the decomposition ofoximes a nd N-oxides in SC W at 460 C. B enzophe-



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known a s supercritical w at er oxida tion (SCWO). Thetechnology t a kes adva nta ge of the complete miscibil-ity of organic compounds and oxygen with SCW sothat there is a single fluid phase at reaction condi-tions. Moreover, the temperatures are sufficientlyhigh (400-600 C) t h a t in t r in sic r ea ct ion r a t e s a r erapid and essentially complete conversion of organicc a r b o n t o C O2 occu r s on t h e t i m e s ca l e of a f ew minutes. Several news reports 88-91 a p p ea r e d in t h e

past few years t ha t describe applications and devel-opments of SCWO technology.

1. Technology Developments

Th e r e ce n t l i t er a t u r e con t a in s m a n y r e por t s ofprocess development work r elated to S CWO t echnol-ogy. These studies have provided new informationabout the treatability of different wastes by SCWO,and they ha ve made a dvances in several engineeringaspects of the technology. The process technologyr a t h e r t h a n t h e c h e m i s t r y w a s t h e f o c u s i n t h e s ereports, so w e mention t hem only briefly h ere. Theinterested reader should consult the overview articlesby Gloy n a a n d L i92,93 an d Levec94 for more details on

the engineering a spects of S CWO.Much of the financial support for SCWO research

and development in the U nited Sta tes ha s been fromthe U .S. Depart ment of Defense, which is interestedin technologies for safely treat ing haza rdous milita ryw a s t e s s u c h a s e n e r g e t i c m a t e r i a l s a n d c h e m i c a lw e a p on s. Ge n e r a l At o m ics is on e of t h e D e f en seDepartment contractors, and they have reported 95 onthe efficacy of SCWO for treating different chemicalw e a p on s a g e nt s . Th ey h a v e a l s o d on e ex t en s iv ecorrosion testing and found that plat inum and t ita-n iu m p e r f o r m e d t h e be st u n d e r t h e h a r sh SCW Oconditions in the presence of halides. Other groupsh a ve a lso e x a m in ed t h e issue o f cor r osion in t h eSCWO environment.96-101

A t e a m a t L os Al a m os N a t i on a l L a b or a t o ry h a sdemonstrated34 the treatability of concentrated chlo-r in a t e d h y d roca r b on s b y S C WO . To com b a t t h ecorrosive conditions that would exist , they used at i t a n iu m r e a ct o r a n d a d d e d so d iu m bica r bo n a t e t oneutralize the HCl generated during processing. Theyalso operated a t high pressures (650 bar ) so tha t theN a Cl p r o d u ce d w o u ld r e m a in in a f lu id p h a se a n dnot deposit on the reactor wa lls. Casa l and Schmidt 102

took a different approach for destroying chlorinatedhydr oca rbons. They used a ceramic vessel to confinet h e r e a ct ion m e d iu m a n d a sep a r a t e st a in le ss st e el

pressure vessel to withs ta nd th e high pressures. Thegap between the reactor and the pressure vessel wa sf il led w i t h w a t e r, w h i ch s er v ed a s t h e cou pl in gmedium.

Th e r e h a ve bee n r e ce n t in vest ig a t ion s in t o t h etrea tment of ion-excha nge resins from n uclea r powerplants,49 solid pa rticulates,103 dioxin-contaminated flya s h ,104 contaminated soils,105 and different types ofsludges106-111 b y S C WO . I n deed i t a p pea r s t h a tslu d g e s a r e o n e o f t h e co m p le x w a st e s f o r w h ichSCWO might be best suited. The organic content isusually high enough tha t a uxiliar y fuel is not needed,and existing t reatm ent processes such a s incinerat ionor landfilling require expensive an d a t t imes difficult

d ew a t e r i ng s t ep s. S C WO h a s b ee n s h o w n t o b eeffective for pulp and paper mill sludges, 106,107 a n dt h e e con om ics a p p ea r t o be a t t r a ct ive r e la t ive t oi n ci n er a t i on b u t n ot a l w a y s s o i n com p a r is on t olandfilling. Sludges from municipal w ast ewat er treat-ment facilit ies have also been treated.108-111 SCWOproduces a clear and odorless aqueous effluent anda settla ble ash. Thus it offers a tremendous reductionin t h e m a ss o r vo lu m e o f t h e o r ig in a l s lu d g e in a

single unit operation. Oxidation at subcritical condi-tions is also used, but the reactor effluent containsorganic compounds tha t must undergo a subsequentbiological treat ment.

One final class of engineering investigations thatha ve been published recently a re relat ed to chemicalreactor operation and performance. The feed char-acteristics required for a utotherma l reactor opera tionhave been delineated. 112 It ha s been determined tha trapid preheating of the SCWO reactor feed streamis advantageous for some wastes.113 This rapid pre-h e a t in g a p pa r e n t ly a vo id s t h e f o r m a t ion of m or edifficult to treat ma terial (cha r) via t herma l reactionsin t he preheater line. A numerical model of a pilot-

scale concentric tube reactor has been reported.114The model accounts for heat transfer and reactionk in e t ics. T h e f lo w f ie ld w a s t a k e n t o be t h a t o f adispersed plug-flow reactor. The m ost deta iled reactormodels have been those of Oh and co-workers, 115,116

w h o u sed com p u t a t ion a l f lu id d y n a m ics cod e s t odescribe the flow fields in a pilot-sca le MODAR vesselreactor. This coupling of results from kinetics studieswith computational f luid dynamics descriptions ofreal rea ctors is likely to yield importa nt insigh ts int ot h e e n gin ee r in g of p r a ct ica l r e a ct ion sy st e m s f orSCWO processes.

2. Homogeneous Reactions

Previous studies of oxidation reactions in SCWcanbe ca t e gor iz ed a ccor d in g t o t h e r e a ct ion m e d iu mbeing either homogeneous or het erogeneous. H omo-g en e ou s sy st em s a r e t h o se w h e r e in t h e r e a ct ionoccurs in a single fluid phase. Research dealing withthese single-phase systems is reviewed in this sec-tion. P revious work on solid-fluid oxidation reactionsin S CW is reviewed in section I II .B .3.

This section on homogeneous reactions is furtherd ivid e d in t o t w o su bsect ion s. Th e f irst d e scr ibesrecent research into the reactions of single organiccompounds und er S CWO condit ions. The compound swere typically selected because they represent com-

pound classes thought to be important for differentw a s t e s t rea m s t h a t m ig h t b e t r ea t e d b y S C WO .These compounds tend to be sufficiently la rge t hatm e ch a n ist ic d et a i l is n o t a va i la ble. As a r e sult t h ein for m a t ion obt a in e d is g en e r a l ly g loba l k in et ics,r e a ct ion n e t w or k s, a n d t h e id en t i t ies a n d y ield s o fthe products of incomplete oxidat ion. The secondsubsection describes research into the reaction mech-an isms opera tive during S CWO. Simpler compoundssu ch a s H 2 a n d C H 4 are used in these studies.

a.Model Compounds. Different classes of organiccompounds ha ve recently been subjected to SC WOconditions. These include alkanes,110 aromatics, 110

phenols, 117-124 other oxygenates, 110,117,125-130 chloro-



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carbons,33,34,117,131,132 an d nitrogen-conta ining com-pounds.73,74,110,133-135 This section reviews t hese stud -ies and highlights the more thorough reaction inves-tigations.

i. Phenols.P henol and substituted phenols are themodel pollutants that have received the most scru-tiny in the SCWO reaction environment. This atten-tion is warranted because wastewaters from diverseindustries often contain large amounts of phenolic

compounds.

The global oxida tion kinetics an d rea ction net workf or p he nol h a s b een e xa m i n ed b y a t l ea s t f ou rdifferent groups.118,119,122,124 All four groups agree thatthe ra te of phenol disappeara nce is essentia lly firstorder in phenol and that the rate is sensit ive to theconcentra tion of oxygen. Additiona lly, Kr ajnc a ndLevec,119 Go pa la n a n d Sa va g e,118 and Oshima et al.124

concur that the global reaction order for oxygen isabout 0.4-0.5, a n d Go p a la n a n d Sa va g e f o u n d t h ereaction order for wa ter t o be a bout 0.4. Koo et a l.,122

on the other hand, reported that the effect of oxygenon the rate wa s better modeled by sat ura tion kinetics

an d tha t t he wa ter rea ction order wa s 1.38. The mostn ot e w o rt h y a sp e ct of t h e w or k o f K o o et a l . is t h a tthey w ere the first t o dist inguish between the effectof pr es su r e a n d w a t e r d en s it y on t h e a p pa r en toxidation rate. They found that the rate was insensi-tive to pressure increases a t consta nt w at er density,but very sensit ive to increases in the w at er concen-tra tion at consta nt pressure. All previous investiga-t i o n s c h a n g e d t h e w a t e r d e n s i t y b y c h a n g i n g t h esystem pressure. Koo et a l., on th e other ha nd, a ddeda presumably inert component (He) to the reactor toeffect changes in water density without changes inthe total pressure. Rice and Steeper 117 also providesome limited da ta on SC WO of phenol.

P r od u ct s of p h en ol ox id a t i on i n S C W i n cl u dedimers (e.g., phenoxyphenols, biphenols, dibenzo-furan), single-ring compounds (e.g., hydroquinone),ring-opening products (e.g., ma leic acid, glyoxylicacid, acetic acid, a nd other organic acids), a nd ga ses(e.g., CO, CO 2). Figure 8 shows a reaction networkfor phenol oxidation that illustrates the formationand destruction paths for these different compoundclasses.

Ma r t in o a n d Sa va g e120,121 reported on the oxidationof -C H 3 a n d -CHO substituted phenols in super-critical water. They developed a global rate equationfor o-cresol oxidation under diverse conditions, but

their kinetics ana lyses for th e other compounds w ereconfined to 460 C . The focal point of the w ork w asin comparing the reactivity of different compoundsr a t h e r t h a n i n s t u d y in g t h e r e a ct i vi t y o f a s in g lecom p ou n d in d e t a i l. Th e y f o u n d t h a t t h e or d e r o fr e a ct ivi t y f o r bo t h su bst i t u t e d p h e n o ls w a s in t h eorder ortho > p a r a > meta. Moreover, the -C H Osu bst i t u t e d p h e n o l w a s a lw a y s m o r e r e a ct ive t h a nt h e ot h e r w ise id en t ica l -C H 3-substituted phenol.B oth of the substitut ed phenols w ere more reactivetha n phenol itself, however. They proposed t he rea c-t ion n et w or k i n F ig ur e 9 a s a s um m a r y o f t h eim por t a n t r e a ct ion p a t h s d u r in g t h e ox id a t io n o fthese compounds in SCW. F

igure8.

Reactionpathwayspropo

sedforphenoloxidationinSCW(Fromref119).(ReproducedwithpermissionoftheAmericanInstituteofChemicalEngineers.

Copyright

1996AIChE.

Allrightsreserved.)



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The oxidat ion of hydroquinone (forma lly a n -OH -

su bst i t u t e d p h en ol) h a s r e ce n t ly bee n st u d ied inSCW.123 The authors found that p-benzoquinone isthe principal int ermediat e product formed. They a lsoobser ved t h a t t h e p-benzoquinone yield was highera n d t h e CO2yield lower for identical residence timesat supercrit ical conditions a s opposed t o subcrit icalcon d it i on s . Th e w a t e r d en s it y a n d t h e r ea c t a n tconcentrations were lower at supercritical conditions,h ow e ver , a n d t h e e ff ect of con ce n t r a t ion a n d t h ew a t e r d e n sit y o n t h e o x id a t io n r a t e m ig h t e x p la inthis observation.

i i . Other Oxygenates. Ethanol, 1- and 2-propanol,and 2-butanol have all been oxidized in SCW.110,125,127

One r eport 125 provides ra te da ta for t he oxidat ion of

these alcohols in mixtures, a t t imes also with aceticacid. These experiments provide an opportunity tod isce r n k in et ic in t e r a ct ion s t h a t ca n occu r d u r in gSCWO a n d t o va l id a t e m e ch a n ism -ba se d k in et icsm od el s. Th es e a u t h or s u s ed a l um p in g s t r a t e gyw h e r eby e a ch e lem e n t a r y r e a ct ion w a s a ssig n ed t oone of eight u nique r eaction fa milies. The kinetics ofe a ch f a m ily w a s t h e n a ssu m ed t o f ol low a n Eva n s-P ola nyi relat ion. The mixture dat a w ere used to tunea few m odel par am eters, but most of the para metersw e r e d e t er m in ed f r om e xp er im en t s w it h sin g le -component oxida tion.

2- Pr o p a n o l o x id a t io n in SCW w a s su bse q u e n t lystudied127 in much more detail. The principal inter-

mediat e product wa s acetone, which formed in yieldsa pproaching 50%. Of course, t he a cetone product w a salso subject to oxidation. The authors found that afirst-order reaction network of 2-propanol facetonef oxidized products provided a good description oft h e e x pe r im e n t a l d a t a .

Acetic acid oxidat ion in SC W ha s been th e subjectof several different kinetics studies during the lastfour years. Meyer et al. 126 exam ined the kinetics a ndproducts at high temperatures (426-600 C), Krajnca n d L ev ec128 explored intermediate temperatures(420-470 C ), a n d S m i t h a n d S a v a g e129 performedexperiments at lower temperatures (380-440 C).Each of these three studies found that the global rate

of a cet ic a cid d isa p p ea r a n ce w a s n e a r ly f ir st or d e ri n a c et i c a c id a n d b et w e en 0. 3 a n d 0. 6 or d er i noxygen. Experiments126 at 550 C and different wa terdensities revealed a modest increase in the rate within cr e a sin g w a t e r d e n sit y . G iven t h e u n cer t a in t y int h e d a t a a n d t h e s m a l l r a n g e o f w a t er d en s it iese xp lor e d ( le ss t h a n a f a ct or of 2), h ow e ver , i t isd iff icult t o d r a w d e fin it ive con clu sion s. A m u chstronger effect of water density was evident in the

low-temperature experiments.129

H e r e, t h e g lob a lr e a ct io n r a t e w a s f o u n d t o be se co n d o r d e r in t h ewa ter concentra tion. The ma jor products from aceticacid oxidation were CO, CO 2, C H 4, a n d H 2. F in a l ly,w e n o t e t h a t L e e130 found the kinetics of acetic acidoxidat ion in SC W to be sensit ive to the nat ure of thereactor surfa ce. More specifically, he reported th atthe addit ion of stainless steel chips to his stainlesssteel batch reactor accelerated the reaction rate.

H o lg a t e et a l .56 exam ined glucose oxidat ion inSCW. They argued that glucose, the main hydrolysisproduct of cellulose, is a good model compound forslu dg e be ca u se slu dg es h a ve a h ig h f iber con t e n t ,most of which is cellulose. Conversions were nearly

co m p le t e in a l l o f t h e e x p e r im e n t s, a n d CO 2 w a sa lw a y s t h e p r od u ct p r ese n t in h ig h est y ield . Th eproduct distribution wa s sensitive to the tempera tureused in the experiment, however. At temperaturesaround 450 C there were compara tively high yieldsof organic acids, aldehydes, and ethylene. At 600 C,however, these products w ere no longer evident, a ndthe most abunda nt carbon-conta ining products w ereC O2 a n d C H 4. Th e a u t h or s con clu d ed t h a t t h e d e -composition of glucose proceeds t hrough th ree kineticregimes: (1) ra pid hydr olysis/oxida tion t o form t wo-or three-carbon atom intermediates, (2) slower de-composition of these interm ediat es to form light ga sessuch as methane and ethylene, and (3) oxidation of

the light gases to ult imately produce CO 2 a n d H 2O.More limited experimental studies of a cetic a cid,

methanol, ethylene glycol, and methyl ethyl ketoneS C WO a t h i gh er con ce nt r a t i on s h a v e a l s o b ee nrecently reported.117

i i i . C h l o ri n a te d H yd ro ca rb o n s. As noted earlier inthis review, chlorinat ed compounds t ypically elimi-na te chlorine a toms under SC WO conditions. Chlo-ride ion in solution leads t o a corrosive environment ,which complicates the effective treatment of chloro-carbons by SC WO.

In the presence of an oxidant (H 2O2), C O2w a s t h eonly ma jor ca rbon-conta ining product detected from

t h e ox id a t io n of t r ich lor oe t h y len e a n d t r ich lor o-e t h a n e .34 All ch lor in e a p pe a r s a s ch lor id e io n insolution. Limited data on methylene chloride, 117 1,1,1-trichloroethane, 117 and 3-chlorobiphenyl131 SC WO arealso available.

A more detailed kinetics study 33,86 ha s been com-pleted for meth ylene chloride in S CW. Both h ydroly-sis a nd oxidat ion occur und er SC WO condit ions, andt h e r a t e s a r e com p a r a ble . Th e ch ief e ff ect of t h eadded oxidant is to shift t he product distribution tom o r e CO a n d CO2 a n d le ss CH 2O , CH 3O H , a n d H 2.The rea ction netw ork proposed involves t hree globalpathw ays: (1) hydrolysis of CH 2C l2 to yield formal-dehyde or methanol plus HCl, (2) oxidation of form-

Figure 9. R ea ct ion net work for oxida t ion of CH 3- a n dCH O-substit uted ph enols in SC W. (Reprinted from ref 121.Copyright 1997 American Chemical Society.)



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ald ehyde or metha nol to CO, and (3) oxida tion of COt o C O2.

The oxidation of 2,4-dichlorophenol has been ex-a m i n ed i n S C W.132 Th e d is a p pe a r a n ce r a t e w a sreported to be nearly first-order in both the chloro-phenol and in oxygen, but none of t he experimenta ldata were reported. The reactions were reportedlya ccom p lish ed in a ba t ch r e a ct o r f o r 5-1 0 m i n a tt e mp er a t u r es b et w e en 673 a n d 873 K . I f t h es e

reported reaction conditions are correct , then thiscompound appears to be surprisingly less reactivetha n r elated phenolic compounds. These a uthors a lsof ou n d t h a t t h e p r e sen ce of F e3+ a n d N a + ions insolution enhanced t he conversion an d suppressedbyproduct formation.

i v. N i tro g e n -C o n ta i n i n g C o mp o u n d s. Lee and co-workers exam ined t he decomposit ion of nitroben-zene73 an d 4-nitroaniline74 in su pe r cr i t ica l w a t e r .Nitrobenzene decomposed even in the absence ofoxygen, but in the presence of oxygen the decomposi-tion ra te is fast er. The products of incomplete oxida -tion included aniline, phenol, 2-(2-pyridinyl)-benzo-

nitrile, and dibenzofura n. The a uthors concluded tha tm ost of t h e n i t r og en a p p ea r s a s N 2 in the reactionproducts. 4-Nitroaniline conta ins both a n NO 2group,w h i ch i s a p ot e nt i a l o xi d a n t , a n d a n N H 2 group,which can be further oxidized. The rate of SCWO wasindependent of the oxygen concentration, but 0.85order in nitroaniline and -0.9 order in wa ter.

B uelow an d co-workers133-135 examined nitrogensp ecia t ion a n d t h e e f fica cy o f n i t r a t e a n d n it r i t e a soxidants in SCWO. They showed 135 t h a t r e a ct io n sbetween NH 3 a n d M N O3, w h e r e M is m o n o va le n tcation (Na, Li, H ), converted nitr ogen la rgely to N 2,with lesser a mounts of N2O and NO. This chemistrycan provide an effective nitrogen control stra tegy for

SCWO. The authors also reported the kinetics fort h e se r e a ct ion s a n d t h e y p ost u la t e d m e ch a n ism s.Moreover, nitrat e and nitrite w ere shown 133,134 to beeffective oxidant s for organic compounds as well a sfor NH 3.

b. Mechanisms. Th e r e h a s bee n con sid er a bleeffort expended on elucidat ing t he rea ction mecha -nisms opera tive during S CWO. One working hypoth-es is , w h ich a p pea r s t o b e l a rg el y v a li d, i s t h a thomogeneous SCWO chemistry is analogous to free-r a d ica l g a s-p h a se o xid a t ion ch em ist r y in t h e sa m etemperat ure regime. If th is hypothesis is correct, oneca n u s e t h e v a s t g a s -p ha s e com b u st i on k in et i csd a t a ba se f o r e le m e n t a r y r e a ct io n st e p s t o d e ve lo pqua ntita t ive, predictive, mechanism-based reactionmodels for SCWO. Such an approach could accelerateo u r u n d e r st a n d in g o f t h e f u n d a m e n t a ls o f SCW Ochemistry a nd at the sam e t ime provide a f irm ba sisfor reliable engineering process models.

Adapting ga s-pha se combust ion models to developquantitat ive, mechanism-based kinetics models forSCWO requires that one account for the effect of highp r e ssu r e s a n d h ig h w a t e r d e n sit ie s o n t h e r a t e s o fu n im olecu la r r e a ct ion s a n d ot h e r r e a ct ion s w it hpressure-dependent rat es. One should also accountfor fluid-phase nonidealities when calculating chemi-cal equilibrium consta nts.

Nearly a ll of the mechanistic studies completed t odate have (understandably) dealt with simple com-pounds such a s H 2, CO , CH 4, a n d C H 3OH, for whichthe ga s-phas e combustion mecha nisms a re relativelysma ll and well esta blished. One exception is t he workof G op a la n a n d S a v a g e136,137 on developing a mech-an ism a nd m odel for phenol oxidation in SC W.

Validat ion of a proposed mecha nism a nd set of rat econ st a n t s a n d t h e r m och e m ica l d a t a r e q u ir e s com -

parison with experimental measurements. To thise n d , t h e r e h a ve be e n r e ce n t e x p e r im e n t a l s t u d ie sof t he S C WO of H 2,138 C O, 138 C H 4,139,140 a n dC H 3OH .141,142 Th e d a t a a va i la ble in clu d e r e a ct a n tconversions an d product y ields as functions of resi-dence time, temperat ure, a nd species initial concen-trations. These experimental investigations are allaccompanied by a complementa ry mecha nistic mod-eling component. There have also been accounts ofexclusively modeling studies143-149 of previously pub-lish e d d a t a .

To d a t e , SCWO m e ch a n ist ic m o de ls h a ve bee nva lid a t e d by co m p a r in g t h e ir p r e d ict io n s w it h e x -p er im en t a l m e a su r em e n t s of t h e y ield s o f s t a ble

molecular products. I mprovements in mechan isticm od e ls ca n be a ch ie ve d by a lso com p a r in g m od e lp r ed ict ion s w it h e xp er im en t a l m e a su r em e n t s f orr e a ct ive in t e r m ed ia t e m olecu la r a n d f r ee -r a d ica lp r od u ct s. Th e se m e a su r e m en t s w o u ld n e ed t o bema de in situ, and to dat e there have been no reportsof measuring radical concentrations during SCWOprocessing. Croiset an d Rice,150 h o w e ve r , h a ve r e -cently reported the direct measurement of H 2O2 byRaman spectroscopy during SCWO of simple alco-hols. They found that the measured H 2O2concentra -t i on w a s l ow e r t h a n t h a t pr ed ict ed b y d et a i ledchemical kinetics models. There ha ve been severalother reports of in situ measurements of chemicalspecies (molecules an d ions) in near-crit ical an dsupercritical w a ter. The techniqu es developed includefiber-optic Ra ma n spectroscopy,151,152 Fourier trans-form infrared (FTIR) spectroscopy,153-156 a n d e m is-sion sp ect r o m et r y , w h ich h a s bee n u se d in h ig h -pressure diffusion flames.157

Th e SCWO m e ch a n ism s p r op ose d r e ce n t ly f orsim p le co m p o u n d s a r e a l l ch e m ica l ly sim ila r , a n dthey typically comprise on the order of 100 individualelementa ry reaction steps. The m ain differences int h e q u a n t i t a t ive p r e d ict io n s o f t h e m o d e ls ca n bet r a ce d t o t h e va lu es o f t h e r a t e con st a n t s f o r a f ew e le m en t a r y r e a ct ion s, t h e st a n d a r d h e a t s of f o r m a -

tion of a few chemical species, and the t reatm ent ofthe effect of pressure on t he ra tes of a few chemica llyactiva ted reactions. Although the quant ita tive deta ilsmay differ , all of the mechanistic modeling work isqualitatively consistent in pointing out essentially thesa m e m a jo r r e a ct io n p a t h s f o r t h e o x id a t io n o f C 1compounds and H 2 in SCW. For example, Figure 10shows the major reaction paths for the oxidation ofC O a n d H 2 in SCW.

Se n sit ivi t y a n a ly se s p e r f o r m e d o n t h e se m e ch a -nism-based models have revealed that the calculatedconversions an d product yields a re very sensit ive toon l y a r em a r k a b ly s m a l l n u m be r of e le me nt a r yr ea c t ion s . S o m e o f t h e se r ea c t ion s a r e r ea c t a n t



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specific (e.g., OH and HO 2a t ta ck on th e hydrocarbonreactan t), but several elementa ry st eps a ppear t o beim p o r t a n t f o r t h e o x id a t io n o f n e a r ly a n y o r g a n iccompound in SCW. These steps are H 2O2 ) 2 O H, 2H O2 ) H 2O2 + O2, OH + H O2 ) H 2O + O2, a n d H 2O2+ OH ) H 2O + H O2. An improved knowledge of thekinetics of these st eps under S CWO conditions a ndthe thermochemistry of the species involved, espe-cially HO2, w ould lead to more reliable mechanism-based models for SCWO.

There has been some significant recent progressi n t h e m ea s u r em en t of t h e r a t e s of e le me nt a r yreactions in SCW. Recognizing the sensitivity of theresults of SCWO mechanistic models to the kinetics

f or H 2O2 d e com p osit ion in t o t w o O H r a d ica ls ledCroiset et al.158 to measure the rate of this reactionin SC W up to 450 C. The a na lysis wa s complicat edby the presence of surface-catalyzed reactions and amultistep decomposition mechanism. Nevertheless,t h e se in ve st ig a t o r s w e r e a ble t o m e a su r e t h e r a t econst an t for this importa nt rea ction. Their measuredvalue is higher tha n th e recommended159 value of thehigh-pressure limit rate constant for this reaction int h e g a s p h a s e . C r o i s e t e t a l . f o u n d t h a t t h e n e w ,higher va lue improved t he predictions of their mecha -n ist ic m o d el f or m e t h a n o l SCWO . I n t e r est in g ly ,h o w e v e r , D a g a u t e t a l .144,145 had previously foundt h a t t h e y n e e d e d t o r e d u c e t h e v a l u e o f t h i s r a t e

const a nt below it s recommended h igh-pressure limitto obtain agreement between their model and pub-lished experimenta l da ta for simple compounds.

Ferry and F ox160 very recently provided kinetics forthe self-reaction of th e hydr oxycyclohexadienyl ra di-ca l i n S C W. Th is r a d ica l w a s f or m ed f rom t h eaddition of OH radical to benzene in a pulse radioly-sis experiment. These investigators also examinedt h e com p et i t ion b et w e en O H a d d i t ion a n d on e-electron oxidat ion in SC W for different substitutedphenolate anions. Additional work with pulse radi-o l y s i s i n S C W i s n e e d e d t o m e a s u r e t h e r a t e s o fimporta nt elementary reactions, such as OH + fuel,for SC WO.

Ea ch o f t h e p a r a m e t e r s (r a t e co n st a n t s , t h e r m o -chemical da ta ) in mechanism-based deta iled chemicalkinetics models for SC WO ha s a n associat ed uncer-tainty. Two recent reports have examined how thispara metric uncertaint y is ma nifested in th e predictedreactant conversions and product yields for SCWOof H 2149 a n d m e t h a n o l .147 These studies show thatt h e r e is con sid er a ble u n ce r t a in t y in t h e se m o d elca l cu la t i on s . F or e xa m p le , F i gu r e 11 s h ow s t h em e d ia n va lu e a n d t h e 95% con f id e n ce r e gion f orm od e l ca lcula t ion s f or SCWO of H 2. M os t of t h eu n ce r t a in t y in t h e se ca lcula t ion s ca n be a t t r ibu t edt o t h e u n cer t a i n t ie s i n ju s t t w o p a r a m et e rs , t h ef or w a r d r a t e con s t a n t f or H 2O2 ) 2OH a n d t h e

sta nda rd heat of forma tion for the HO 2radical. G iventhe uncertainties in the model predictions and in theexperimental da ta used for model validat ion, B rocket al.147 concluded t ha t their m echa nism-based kinet-ics models was consistent with SCWO experimentsfor metha nol. Thus, the w orking hypothesis tha t ga s-phase combustion chemistry can be ada pted to modelhomogeneous SCWO chemistry appears to be reason-able.

Wh e n d oin g t h is t y p e of m e ch a n ist ic m od e lin gw o r k, h ow e ver , on e m u st be ca r ef u l t o con sid erpossible effects of wa ter molecules on the structurea n d e n e r g e t ics o f t r a n si t io n st a t e s f o r e le m e n t a r yreactions. I t is possible that the transit ion state for

Figure 10. M a jor free-ra dica l rea ct ions for CO a nd H 2oxida tion in SC W. Arrow t hicknesses indicate rela tive ra tesof rea ct ion. ( R eprint ed from ref 138. C opyright 1994American Chemical Society.)

Figure 11. Effect of para metric uncerta inty on concentra-t ions of H 2 a n d H 2O2 ca lcu la t ed b y a mecha nis m-b a s edkinetics model for H 2oxidat ion in SC W at 823 K. The solidcu rve is t he media n p redict ed va lu e, a nd t he s ha ded a reaconta ins 95%of the predicted concentra tion va lues. Fromref 149. Reprinted by permission of Elsevier Science fromI ncorp ora t ion of P a ra met ric U ncert a int y int o Comp lexKinetic Mecha nisms: Application to Hydrogen Oxidationin S u p ercrit ica l Wa t er , b y P henix, B . D . ; Dina ro, J . L . ;Tat an g, M. A.; Tester, J . W.; Howa rd, J . B . ; McRae, G . J .C om b u st i o n a n d F l a m e 1998, 11 2, 132-146. Copyright1998 by The C ombustion Inst itute.



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a r e a ct io n in w a t e r is d i f f e r e n t t h a n t h e t r a n sit io nsta te for tha t reaction in the gas phase. Formic aciddecomposition provides one exam ple.82 The water-gas s hift reaction (CO + H 2O ) C O2 + H 2) is a s econdex a m pl e. Th e r a t e of t h i s r ea c t ion i n S C W161 ise x t r e m e ly se n sit ive t o t h e w a t e r d e n sit y . Abo u t a5-fold increase in the density at 450 C resulted inabout a 100-fold increas e in the a pparent first-orderrat e constant . This qua litat ive effect of wat er on t he

ra te wa s previously predicted by a q ua ntum chemicalca l cu la t i on , w h i ch s h ow e d t h a t w a t e r m ol ecu le sp a r t ici pa t e d i n t h e t r a n s i t ion s t a t e , a n d t h er eb yinfluenced the rat e of reaction.

3. Heterogeneous Reactions

The desire t o reduce the severity of the processingconditions of SCWO and thereby improve the eco-n om ics h a s m ot iva t e d m u ch r e ce n t r e se a r ch in t owa ys to enhance the oxidation ra tes. One approach 162

i s t o a d d r a t e e n h a n ce rs s u ch a s H 2O2 o r HN O3,which are more powerful oxidants than O 2 a n d c a nmore rapidly initia te the free-ra dical chain oxidat ionreactions in the fluid phase. A second approach is to

use homogeneous oxidat ion cat alyst s, an d a third ist o e m p loy h e t er og e n eou s ca t a ly st s . I t is t h is t h ir doption that we consider in this portion of the review.

Ca t a ly t ic o x id a t io n in su p e r cr i t ica l w a t e r is t h esubject of a thorough review by Ding et al. 163 Theys u m ma r i z e t h e r el a t i ve ly s m a l l n u m be r of a c t u a lca t a ly t ic o x id a t io n st u d ie s in SCW t h a t h a d be e ncon d u ct e d u p t o t h a t t im e (ea r ly 1996), bu t m or eimportantly, they place catalytic SCWO in the con-text of other catalytic oxidation technologies. Theya l s o d e lv e i n t o t h e i m por t a n t i s su es of ca t a l y sthydrothermal stability, activity, and preparation. Forexample, Cr 2O3 is not a good SC WO cata lyst becauseC r

2O

3 is n o t s t a ble in SCW, a n d t h e ch r om iu m ca n

be leached out of the cata lyst . 164 Loss of cat alyst a ndthe presence of toxic chromium in th e process effluentare both problema tic.

S o me s t u d ie s i n t h i s a r ea a r e s u rv ey s w h e r ei nd iff er e n t p ot e n t ia l ca t a ly st s a n d d iff er e n t or g a n icreacta nts w ere used. For exam ple, Kra jnc an d Levec165

r e por t e d t h a t a cop pe r ox id e/z in c ox id e ca t a ly stprovided substa ntia lly higher conversions a nd higherselectivities to C O2(tha n uncat a lyzed SCWO) for theoxidat ion of alcohols, a cetic a cid, methyl pyrrolidone,benzoic acid, and phenol. Ding et al. 166,167 reportedsimilar observations for the oxidation of benzene,phenol, and dichlorobenzene over Cr 2O3, V2O5, a n d

Mn O2 ca t a ly st s .The more recent reports tend to be more focused

on a single compound and single catalyst combina-tion. These studies have examined several specifics ys t em s i n d e t a il s o t h a t r ea ct i on r a t e l a w s a n dinformation about the catalyst stability and activitym a i n t en a n c e a r e a v a i l a b le. S y s t em s i n ve st i ga t e dinclude pyridine168 a n d N H 3169 over a commercialMn O2/C eO 2cata lyst , phenol170 and acetic acid128 overa proprieta ry copper/zinc/coba lt oxide supported cat a -lyst, a nd phenol171 over a commercial ma nga nese andco p p e r o x id e su p p o r t e d ca t a ly st . I n a l l ca se s, t h epresence of the cat alyst greatly accelerated th e rat eo f d isa p p e a r a n ce o f t h e t a r g e t co m p o u n d a n d t h e

conversion of orga nic carbon t o CO2. The ca ta lyst a lsoreduced the yield of organic byproducts. Figure 12shows r epresenta tive results. This enha nced r eactionr a t e a n d se le ct ivit y a t m ilde r r e a ct ion con d it ion scontinues t o motivate research int o cata lytic SC WO,which appears to offer a route to improved SCWOprocess economics.

IV. Summary

This comprehensive review is the first to be dedi-cated exclusively to the topic of organic chemicalreactions in supercrit ical w at er. The review dea lt

with the three broad areas of research in this f ield,na mely chemica l synt hesis, decomposition of orga nicmaterials and compounds, and complete oxidation.Over ha lf of the a rticles cited in this review dealt w itht h e la st a p p lica t io n , su p e r cr i t ica l w a t e r ox id a t io n .Abou t 25% d e a lt w it h t h e con ver sion of or g a n icmaterials by processing in supercrit ical water, andonly about 15% of the ar t icles cited in the reviewdealt w ith chemical syn thesis. Given the limited workdone to date on chemical synthesis in supercrit icalwa ter an d the current interest in and importance ofenvironmenta lly benign chemical processing, the a reaof chemical synthesis in high-temperature water isthe one tha t w ill grow m ost ra pidly. There are simply

vas t expanses of uncha rted territ ory in this field, butthere ha s a lso been sufficient explora tory w ork tha tsome guidance is available.

This review showed tha t t here is a broad r an ge ofchemical tra nsforma tions tha t can be effected in thenonconventional reaction medium of supercrit icalwa ter. These tra nsformat ions include hydrogena tion,carbon-carbon bond forma tion, dehydrat ion, decar-boxylation, hyd rodeha logenation, pa rtia l oxidation,and hydrolysis. The rates and selectivit ies of thesedifferent reactions can be ma nipulat ed by judiciousse le ct io n o f t e m p e r a t u r e , p H, ca t a ly st , a n d w a t e rdensity, so tha t one can t hereby control the functiona lgroup transformat ions in SC W. Some of the knowl-

Figure 12. C om p a r i son of ca t a l y z ed a n d u n ca t a l y ze doxidat ion of phenol in SC W at 400 C. The conversions ar ecomp a ra b le, b u t t he t ime s ca le for u nca t a lyz ed oxida t ionis 2 orders of magnitude longer. (Reprinted from ref 170.Copyright 1997 American Chemical Society.)



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