2.sol−gel route to the tunneled manganese oxide cryptomelane

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Sol-Gel Route to the Tunneled Manganese OxideCryptomelane

S ta nto n C hing * and J ay me L . R o ar k

Department of Chemistry, Connecticut College, New London, Connecticut 06320

N iang ao D ua n a nd S t e ve n L . S uib *

Ch a r l es E . Wa r i n g L a b o r a to r y, Dep a r tm en t of Ch emi str y, U - 6 0, U n i ver si ty of Con n ecti cu t,

Storr s, Connecticut 06269-4060, and I nstit ute of M ateri als Science andDepartment of Chemical Engineering, University of Connecticut, Storrs, Connecticut 06269

Receiv ed September 4, 1996. Revi sed M anu scr i pt Recei ved Decem ber 18, 1996X

The sol-gel reaction between KMnO4 a n d fu ma ric a cid in a 3 :1 mo le ra t io g en e ra t e s aflocculant gel that serves as a precursor to the tunneled manganese oxide, cryptomelane.The elemental composition of sol-gel cryptomelane has been determined to be K 0.12Mn O2.0-(H 2O)0.09. Fu rt h e r ch a ra ct e riza t io n h a s bee n p e rforme d u sin g p o wder X-ra y di ffra ct ion ,scanning electron microscopy, an d Auger electron spectroscopy. The sol-gel process isheavily dependent on reactan t concentra t ion. Solutions tha t a re too concentra ted producethe la yered ma ngan ese oxide birnessite, whereas overly dilute reactions yield mixtures ofcryptomelane and Mn 2O3. The preference for crypt omelane over birnessit e correlat es wit hlow pota ssium content in the gel. The sol-gel procedure for synthesizing cryptomelane is

not easily tr an sferred to the prepara tion of an alogous ma nga nese oxides w ith different t unnelca t ion s. Re a ct ion s t h a t e mp loy p e rma n g a n a t e s ot h e r t h a n KMn O 4genera lly yield Mn 2O3,w ith cryptomelane being a minor product a t best . Therma l ana lyses of cryptomelane gelsindicat e tha t calcinat ion proceeds th rough a series of sta ges tha t involve loss of w at er, lossof residual organics, conversion to cryptomelane, and finally degradation to Mn 3O4. Th eextraction of potassium ions from sol-gel cryptomelane by va rious foreign cations is minima l,with the loss of K+ being on t he ord er of 10%.

Introduction

Cryptomelane is a microporous manganese oxide withthe a pproximat e stoichiometry of K0.125Mn O2(KMn 8O16)an d a str ucture consisting of 4.6 x 4.6 tunnels due

t o a 2 2 ar ra ngement of edge-shar ed MnO 6 octahe-dra , Figure 1.1-5 P ota ssium ions reside in the t unnelsa l on g w i th s ma l l a m ou nt s of w a t e r. Th e a v er a g ema nga nese oxidat ion st a te of 3.88 reflects mixed valencyfrom Mn(IV) and minor a mounts of Mn(III).2,3,6-8 Cryp-t omelane i s rel a t ed t o a b roader cl ass of m a t eria l sknow n as hol landi t es, w hi ch feat ure s im i lar t unnelstructures.1-5,9,10 Manga nese oxides in the h ollanditegroup are isostructural and di ffer only by the tunnelcation. The term hollan dite is used for this generalclassi ficat ion of materials but also defines a specificmanganese oxide with Ba 2+ in the tunnels. In similarfashion, coronadite, ma njiroite, a nd cryptomelane ar edefined as hollandites w hich conta in P b2+, Na +, and K+.

Cryptomelane is the most rea dily synt hesized man-ganese oxide in the hollandite group, reflecting theeffectiveness of K+ i n s t ab i l i z i ng t he 2 2 t unnelstructur e. Cry ptomela ne can be prepared from la yeredmanganese oxides such as birnessi te by thermal 11-14

and hydrothermal5 methods. Other common synthesesinvolve oxidation of Mn 2+ in aqueous acid by reagentss uch a s K Mn O4,8,14 K 2S 2O8,15-17 a n d O2.18 Micro-porous manga nese oxides wit h larger t unnels have a lsob een synt hesi zed. Todoroki t e, for exam pl e, has a

* To w hom correspondence should be sent at the Department ofChemistry.

X Abstra ct published inA dvance ACS Abstracts,Februa ry 1, 1997.(1) Bystrom, A.; Bystrom, A. M. Acta Crystallogr. 1950, 3, 146.(2) P ost , J . E. ; Von D reele, R. B . ; Buseck, P. R. Acta Crystallogr.

1982, B 38, 1056.(3) Vicat , J . ; Fa nchon, E.; S trobel, P.; Qui, D . T. Acta Crystallogr.

1986, B 42, 162.(4) Post , J . E. ; Burnha m, C. W.A m . M i n er . 1986, 71, 1178.(5) Giovanoli, R.; Faller, M. C h i m i a 1989, 43, 54.

(6) Post , J . E. ; Bish, D. L . A m . M i n er . 1989, 74, 177.

(7) Turner, S.; Buseck, P. R. Science1981, 212, 1024.(8) Feng, Q.; Kanoh, H.; Miyai, Y.; Ooi, K. Chem. M ater. 1995, 7,148.

(9) Clear field, A. Chem. Rev. 1988, 88, 125.(10) Pot ter, R. M.; Rossman, G . R. A m . M i n er . 1979, 64, 1199.(11) Giovanoli, R.; Balmer, B. C h i m i a 1981, 35, 53.(12) Golden, D. C.; Dixon, J . B.; Ch en, C. C. Clays Clay Min er. 1986,

34, 511.(13) Chen, C . C.; Golden, D. C.; Dixon, J . B. Clays Clay Min er. 1986,

34, 565.(14) DeGuzma n, R. N.; Sh en, Y. F.; Neth, E. J . ; Suib, S. L.; OYoung,

C. L . ; Levine, S . ; Newsam, J . M. Chem. Mater. 1994, 6, 815.(15) Ambrose, J . ; Covington, A. K.; Thirsk, H. R. Power Sources

1970, 2, 303.(1 6) P a r i da , K . M . ; K a n u n g o, S . B . ; S a n t , B . R . Electrochim. Acta

1981, 26, 435.(17) S trobel, P.; Cha renton, J . C. Rev. Chim. Miner. 1986, 2 3, 125.(18) Hypolito, R.; Valarelli, J . V.; Giovanoli, R.; Netto, S. M. C h i m i a

1984, 38, 427.

Figure1. I llustrat ion of the cryptomelane structure showingthe 4.6 x 4.6 tun nels of edge-sha red MnO 6 octahedra withK+ inside the tunnels.

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3 3 (6.9 6 .9 ) a rray of edge-shared MnO6octahedra.19-22 Romanechite is similarly formed withan asymmetric 2 3 tunnel.23,24 Both these materialsare prepar ed hydrothermally.

Ma nga nese oxides with tunnel str uctures exhibit ion-excha nge a nd molecular sieving properties9,19 t hat m akethem interesting a s potential zeolite an alogues. P ro-posed applications of hollandite-type manganese oxidesinclude heterogeneous cat a lysis an d meta l ion sorption.The ma terials a lso have potential use in batt ery tech-

nol ogy. We use t he t erm octahedr al molecul ar sieve(OMS) to identify tunn eled ma nga nese oxides tha t ha veedge-shared MnO6 octahedra as their basic structuralunit .19 Todorokite an d cryptomelane ha ve been desig-nated OMS-1 and OMS-2, respectively.

We are explori ng sol-gel routes to microporousmanganese oxides,25 with particular interest in realizingthe a dvanta ges of sol-gel processing compar ed to othersynthetic methods.26-28 S ol-gel reactions are knownfor ma ngan ese oxides w ith lay ered structures, such a sbirnessite,25,29-35 but analogous procedures for tunneledmat erials such a s cryptomelane are relat ively unknown.Ba ch a nd co-workers were t he first t o discover a sol-gel route to birnessi te-type manganese oxides, using

reactions of concentra ted KMn O4wi th fumaric acid (E-butenedioic acid).29 We similarly developed a sol-gelsynthesis of birnessite from reactions between KMnO 4and organi c reduc i ng agent s suc h as gl uc ose. 25 Welear ned from our reactions t ha t concentra t ion plays amajor role in defining the types of gels and manganeseoxides produced. This observa tion led us to exam inerelated chemistry in other sol-gel systems.36 Here w ereport the study of a di lute reaction between KMnO 4a n d f um a r i c a c id , t h e f ir s t s ol-g el s y n t he si s of atunneled manganese oxide, cryptomelane.37

Experimental Section

Chemicals. P ot assiu m p e r mang ana t e (J ansse n) and fu -maric acid (Aldrich) were obtained as reagent grade chemicalsand u sed a s r e c eived . O t he r c hemicals we r e l ikewise u se d

with out further purificat ion. All reactions w ere carr ied outu sing d is t i l le d d e ionize d wat e r (18 M) f r o m a B a r n s t e dNanopure II purifying system.

Synthesis of Cr yptomelane. A 0.78-g (6.7 mmol) sa mpleof solid fumaric acid was added to a stirred solution containing

3.16 g (20 mmol) of KMnO 4dissolved in 200 mL of w at er. Thereaction proceeded with mild effervescence followed by theappear an ce of a brown sol within a few m inutes. After 30 min,the sol evolved into a floccula nt brow n-bla ck gel. The gel w a sal lowe d t o se t t le and a f t e r 1 h wa s isolat e d b y f i l t r a t ion a ndwa she d fou r t ime s w it h 100 mL of w at e r . A b lac k xe r og e lfor me d af t e r d r y ing a t 110 C ove r nig ht . C alc inat ion of t hexerogel at 450 C for 2 h yields cryptomelane as a ha rd, blacksolid. The product wa s pulverized, washed th ree t imes wit h0.1 M H C l, a nd t he n w ashe d t hr e e t ime s ag ain w it h d is t i l le ddeionized wa ter. After drying at 110 C, a t ypical isolated yieldwa s 1. 8 g.

Potassium-Ion Extraction. In a typical procedure, 0.5 gof cryptomelane wa s combined wit h 100 mL of a 1 M solutionconta ining the nitra te salt of the foreign cat ion. The mixturewa s s t ir r e d for a t least 24 h b e for e t he p r od u ct wa s isolat e dby filtrat ion, washed four t imes with water, and dried at 110 C .

Characterization. Powder X-ray diffraction (XRD) datawere obtained with a Scintag 2000 PDS X-ray diffractometeru sing C u KR r ad iat ion. Sa mp les we r e spr e ad ou t on g lassslides and scan ned at 5 2/min. The beam volta ge and beamcu r r en t w e r e 4 5 k V a n d 40 m A, r es pe ct i v el y . E l ect r onmicroscopy w as carried out using a n AMRAY Model 1810 Dscan ning electron microscope. Auger electron spectroscopywa s performed with a P erkin-Elmer P HI 610 scan ning Augermicroprobe. The thermogravimetic ana lytical dat a w ere ob-ta ined using a P erkin-Elmer TGA-7 system under a nitrogenat m osp her e . The r mally p r og r amme d d e sor pt ion me asu r e-m e nt s w e r e c a r r i ed ou t a s d es cr i be d i n t h e l it e ra t u r e .38

Elemental ana lysis for meta ls wa s obtained using flame a tomica bsorption spectroscopy w ith a P erkin-Elm er Model 2380 AA.The elemental analysis for carbon was performed by GalbraithLabora tories. The average oxidation stat e of ma nga nese wa sdetermined by iodometric t itra t ion.39

Results and Discussion

Cryptomelane Synthesis. The sol-gel synthesis ofcryptomelan e is shown schemat ica lly in Figure 2. Theexother mic reaction betw een aq ueous 0.1 M KMnO4a ndsolid fumaric acid (E-butenedioic acid) in a 3:1 moleratio generates a brown sol that congeals into a floccu-

(1 9) S h e n , Y . F . ; Z e r ge r , R . P . ; D e G u z m a n n , R . N . ; S u i b, S . L . ;McCurdy, L . ; Pot ter, D . ; OYoung, C. L . Science1993, 26 0, 511.

(20) Shen , Y. F.; Suib, S. L.; OYoung, C . L.J . Am. Ch em. Soc. 1994,116, 11020.

(21) Golden, D. C.; Chen, C . C.; Dixon, J . B. Clays Clay Min er. 1987,35, 271.

(22) Golden, D. C.; Chen, C . C.; Dixon, J . B. Science1986, 2 31, 717.(23) Wad sley, A. D. A m . M i n er . 1950, 35, 485.(24) Giovanoli , R. ; Ba lmer, B . C h i m i a 1983, 37, 424.(25) Ching, S .; Land rigan , J . A.; J orgensen, M. L.; Dua n, N.; Suib,

S. L.; OYoung, C. L. Chem. M ater. 1995, 7, 1604.(26) Hench, L. L.; West, J . K. Chem. Rev. 1990, 90, 33.(27) Cha ndler, C. D.; Roger, C.; Ham pden-Smith , M. J . Chem. R ev.

1993, 93, 1205.

(28) Schwarz, J . A.; Contescu, C.; Contescu, A. Chem. Rev. 1995,95, 477.

(29) B ach, S.; Henry, M.; B affier, N.; Livage, J .J. Solid StateChem.1990, 88, 325.

(30) Ba ff ier , N. ; Ba ch, S . A n n . C h i m . F r . 1991, 16, 467.(31) Ba ch, S . ; P ereira-Ramos, J . P. ; B aff ier , N. , Messina, R. Elec-

trochim. Acta1991, 36, 1595.(32) Pereira-Ramos, J . P . ; Ba ddour, R. ; Bach, S . ; B aff ier , N.So l i d

State Ionics1992, 53-56, 701.(33) Le Goff, P.; B affier, N.; Bach, S.; P ereira-Ramos, J . P.; Messina,

R. Solid State Ionics1993, 61, 309.(34) Ba ch, S . ; P ereira-Ramos, J . P . ; Ba ff ier , N. Electrochim. Acta

1993, 38, 1695.(35) Le Goff, P.; B affier, N.; Ba ch, S.; Pereira-Ramos, J . P. J . M a t er .

Chem. 1994, 4, 875.(36) C hing, S.; P etrovay, D . J . ; J orgensen, M. L.; Suib, S. L. Inorg.

Chem. in press.(37) Duan, N.; Suib, S . L . ; OYoung, C. L . J. Chem. Soc. Chem.

C o m m u n .1995, 1367.

(38) Yin, Y. G .; Xu, W. Q.; Shen, Y. F.; Suib, S . L.; O Young, C. L.Chem. M ater. 1994, 6, 1803.

(39) Mu rra y, J . W.; Ba listieri , L. S.; Pa ul, B. Geochim. Cosmochim.Acta1984, 48, 1237.

Figure 2. Sc he mat ic d iag r am of t he sol-g e l sy nt he sis ofcryptomelane.

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lant gel within 30 min. The fragmented nat ure of thegel a l l ow s i t t o b e w ashed w i t h w at er b efore dryi ng.Calcination of the result ing xerogel at 450 C for 2 hyields a crude cryptomelane sample which is purifiedb y sequent i a l w ashi ngs w i t h 0.1 M HC l and di st i ll eddeionized wa ter. A similar reaction occurs betweenKMnO4 and maleic acid (Z-but enedioic a cid).

The powder X-ray diffraction (XRD) pattern of sol-gel cryptomelane shows sha rp peaks and no evidenceof o t her phases ( Figure 3). The X RD pat t ern i s i nexcellent agreement with XRD results obtained for non-sol-gel cryptomelane materials.14 Scanning electronmicroscopy r eveals w ell-defined pa rticles wit h smoothfaces and overall dimensions that are dependent on theext ent of gr indi ng (Fi gure 4). The m orphol ogy i ssignificantly different from the finer needlelike a ppear -ance of cryptomelane prepared by reflux methods. 14

An empirical formula of K 0.12Mn O2.0(H2O)0.09ha s beendetermined for sol-gel cryptomelan e from elementa l

a n a l y s es a n d m a n g a n es e ox id a t i on s t a t e d a t a . Th epota ssiu m (4.9%) a nd m a ng a nes e (59.7%) composition swere obta ined using at omic absorption spectroscopy,while the percent oxygen was calculated by differenceon t he assum pt ion of negl igib le hydrogen cont ent .C arb on w as ana l yzed a t l ess t ha n 0. 5%. An a v eragem anganese oxi dat i on s t a t e of 3 . 87 w as m easured b yi odom et ric t i t ra t i on. C rypt om elane i s general ly ex-pressed with a stoichiometr y of K0.125Mn O2(KMn 8O16),which compares favorably with the sol-gel material .Thorough wa shing w ith dilute a cid, as d escribed in thesynthesis, was necessary to remove surface-adsorbedpota ssium ions. Analytical data for unwa shed samplesand samples washed only with wa ter revealed %K th at

were t ypical ly tw ice a s h igh compared t o acid-wa shedcryptomela ne. Auger electron spectra of cryptomela nebefore and after acidic washing shows the removal ofsurfa ce pota ssium (Figure 5). Adsorption of alka li-meta lcat ions has been observed previously in syntheses oflayered ma ngan ese oxides.33,36

Related Sol-Gel Reactions.Our sy nt hesis of cryp-tomelane from KMnO4and fuma ric acid is an outgrowt hof sol-gel chemistry t ha t w e and others ha ve developedfor layered birnessi te-type manga nese oxides.25,29-36

Bach and co-workers reported a similar reaction be-t w een perm anganat e and fum ari c ac i d i n a 3: 1 m ol era tio tha t yields birnessite instea d of cryptomelan e.29,33,35

The critical difference betw een these tw o systems is the

reactant concentra t ion. B ach a nd co-workers used a0.25 M KMn O4solution, wh ich produced unfra gmentedgels. Our reactions use 0.10 M KMnO 4 and generat e

flocculan t gels. We have ma de simila r observa tions ona rel a t ed sol-gel reaction between KMnO4 a n d g lu -cose.36 In that system, highly concentrated solutionsof K MnO4 and glucose produce unfragm ent ed gel sleading t o birnessite w hile more dilute solutions gener-at e f loc cul ant gels t hat y i el d crypt om elane and/orMn 2O3. O ur s ol-gel synthesis of cryptomelane pre-sented here u nderscores the concentra tion dependenceof the reaction between KMnO4and fum ari c ac id. Ouruse of di lute KMnO4 solutions while maintaining the3: 1 m ol e ra t i o w i t h fum ari c ac i d has enab l ed us t omodify the reaction of B a ch and co-workers to genera teflocculant gels t ha t ul t imately lead t o cryptomelaneinstea d of birnessite. However, if the reactions become

too di lute, th en mixtures of Mn2O3 and cryptomelanebegin to form. Our results from rea ctions with va riousoverall concentrations of KMnO4 and fum ari c ac id a resummarized in Table 1.

Ba ch a nd co-workers a lso reported a non-sol-gelroute to cryptomelane from KMnO 4 and fum ari c aci dwhich cal ls for an increase in t he KMnO4: fumaric acidratio from 3:1 to 4:1 as well as an increase in acidity. 40

I n our experi ence, t he ra t i o of K MnO4: fumaric acidexerts a relat ively sma ll influence on th e synth esis ofcryptomelane. Reactions with 0.1 M KMnO4and v ary-

(40) Ba ch, S . ; P ereira-Ramos, J . P. ; Ba ff ier , N. So l i d St a t e I o n i cs 1995, 80, 151.

Figure 3. Powder X-ray diffraction pattern of sol-gel cryp-tomelane.

Figure 4. Scanning electron microscopy photographs of sol-gel cryptomelane.

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ing amounts of sol id fumaric acid show that KMnO4:fuma ric acid ra tios of 4:1 or less give floccula nt g els tha tresult in cryptomelane formation. Ra tios above 5:1 givem i xt ures t hat do not progress b eyond t he sol s t age.Changes in acidi ty or basici ty, however, significantlyal t er t he sol-gel products. Reactions of KMnO4 a ndfumaric acid in 0.1 M HNO3 produce precipitates thatyield Mn 2O3 while the same reactions in 0.1 M KOHgenerat e flocculant gels t hat lead to birnessite.

We believe the sol-gel pat hs t o c rypt om elane and

birnessite are determined by the potassium content ofthe ma nga nese oxide gels. This hypothesis is supportedb y experi m ent s i n w hi ch m i xt ures of K MnO4 a ndfum ari c ac i d w ere sub jec t ed t o a v ari et y of reac t i oncondit ions and w orkup procedur es (Ta ble 2). Ana lysesof the result ing xerogels show a clear preference forcryptomela ne a t low pota ssium levels a nd sm all K /Mnra tios. Cryptomelan e is obta ined from xerogels withpota ssium content s on th e order of 7%an d K /Mn ra tiosof 0.2 or less. B y contr a st, birnessite is observed whenthe xerogels ha ve pota ssium contents of 14% or moreand K /Mn ra t i os great er t han 0. 4. Our f i ndi ngs a reconsistent with the fact that cryptomelane inherentlyhas a much lower potassium content than birnessite.41,42

I t i s part i cul arl y i ll umi nat i ng t ha t t he unfragm ent edgels reported by B a ch an d co-workers can be convertedfrom birnessite precursors t o cryptomela ne precursorsaf t er b ei ng di spersed and w ashed w i t h w at er t o ex-tract K+ (Ta ble 2, last tw o entr ies). It is a lso noteworth ythat while flocculant gels generally produce cryptom-elane aft er w ashing, t he flocculant gels obtained fromreac t i ons i n K OH sol ut i on are ab l e t o ret a i n a hi ghpota ssium content a nd yield birnessite (Ta ble 2, fourthentry).

Unfortunately, the sol-gel synthesis of cryptomelan efrom KMnO4 and fumaric acid does not easi ly extendto other hollandite-type man ga nese oxides. Reactionsof LiMnO4, NaMnO4, Mg(MnO4)2, and Ca (MnO4)2w i t h

fuma ric acid all result in the format ion of Mn2O3eitherexclusively or as the major product in mixtures withcryptomelane. Of the permanga nat es we studied, onlyKMnO4 a n d C s M n O4 generated hollandites withoutconta mination from other phases.

Thermal Measurements. A t herm ograv i m et ri can alysis of the cryptomela ne xerogel is shown in F igure6. The initia l ma ss cha nge betw een room temperat urean d 200 C is due to loss of wa ter. A subsequent weightloss from 200 to 500 C is a result of the decomposition

(41) McKenzie, R. M. M i n e r . M a g . 1971, 38, 493.(42) Strobel P. ; Cha renton, J . C . ; Lenglet M. Rev. Chi m. M iner.

1987, 24, 199.

Figure 5. Auger electron spectra of sol-gel cryptomelane:( a ) b e f o r e w a s h i n g w i t h 0 . 1 M H C l a n d w a t e r ; ( b ) a f t e rwa shin g. The unit s of the vertical a xis ar e dN(E)/dE. The peaksrepresent pota ssium (LMM: 252 eV), oxygen (KLL : 503 eV),an d ma nga nese (LMM: 542, 589, and 635 eV).

Table 1. Effects of Overall Reactant Concentration onSol-Gel Reactions between K MnO4 and Fumaric Acid

[KMnO4](M)

[fumaric acid](M)

ge ldescription

calcined product(450 C, 2 h)

0.250 0.083 u nf ra g men t ed K -bir nes sit e0.100 0.033 floccula nt cr ypt om ela ne0.050 0.017 floccula nt cr ypt om ela ne0.020 0.007 floccula nt cr ypt om ela ne + Mn 2O3

Figure 6. The r mog r avime t r ic ana ly sis of a cr y p t ome lanexerogel.

Table 2. Xerogel Analyses and Manganese OxideProducts from Various Sol-Gel Reactions between

KMnO4 and Fumaric Acid

xerogel analysisreaction conditions(gel t rea tment ) %K K/Mn product

0.100 M KMnO4/0.033 Mfuma ric acid (filtered,

w a s hed )

7% 0. 13 cr yp tom el a ne

0.100 M KMnO4/0.033 Mfuma ric acid (filtered,unw a s hed )

18% 0. 46 b ir n es sit e

0.100 M KMnO4/0.033 Mfuma ric acid (unfiltered,unw a s hed )


0.100 M KMnO4/0.033 Mfuma ric acid/0.1 M KOH(filtered, washed)


0.250 M KMnO4/0.083 Mfuma ric acid (unfiltered,unw a s hed )


0.250 M KMnO4/0.083 Mfumaric acid (mechanicallydispersed, filtered, washed)

7% 0. 21 cr yp tom el a ne+ Mn 2O3

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of residual organic fragments originating from fumaricacid. The sma ll decrease from 500 to 550 C representsthe forma tion of cryptomelane. The fina l ma ss cha ngefrom 800 to 900 C is due to the conversion of crypto-m el ane i nt o Mn 3O4. Th e l os s o f w a t e r a n d o r ga n i cspecies was detected by FT-IR spectroscopy.

Temperat ure-programm ed desorption (TP D) results

for O2 evolution on a calcined sample of cryptomelaneare show n i n Fi gure 7. I nt egrat i on of t he TP D dat areveals that between 273 and 873 K there is a loss ofonly 3%of the lat tice oxygen at oms. The indica tion hereis that the ma terial is very sta ble over this temperat urerange.

Potassium-Ion Extraction. The extra ction a ndinsertion of cations in microporous solids are importantprocesses tha t relat e to potentia l applicat ions in cat aly-sis a nd chemical sorption.9 Alkali-meta l extra ction h asbeen especially well-examined for m a nga nese oxides dueto their promising nature as cathode materials for re-chargeable l i thium bat teries.8,31,34,43-48 Studies of ex-traction and insertion processes on a cation-poor formof Li holla ndite ha ve shown tha t t he level of a lkali-meta lcation insertion is inversely related to ionic radius. 8,49

However, cat ion retention upon acid trea tment showst he opposit e rela t i onshi p a nd i s m ore fav orab l e forlarger cat ions.8

P otassium-ion extraction wa s examined for sol-gelcryptomelane using samples that were st irred in 1 Msolutions of var ious ni tra te sa l ts for 24 h. The degree

of extra ction wa s a ssessed by the K /Mn r at io withoutdistinguishing between ion-exchange and redox mech-anisms.8 The results displayed in Table 3 show poorextraction of potassium from sol-gel cryptomelane. Inmost ca ses, the d ecreases in %K a nd K /Mn ra tio ar e juston the order of 10%. Only NH 4+ gives a significant levelof extra ction a t 42%. The extra ction process w a s notimproved with longer st irring t imes and higher tem-pera tures. Acid t reatment of conventional ly preparedcryptomelane has been shown to extract nearly 50%ofthe potassium ions,8 compar ed t o only 12%for sol-gelcryptomelane.

Conclusions

A sol-gel synthesis of the tunneled ma nga nese oxide,cryptomelan e, has been developed from th e redox reac-tion between KMnO4 and fum ari c ac id. React ant con-centr at ions a re critica l in this process since the la yeredmanganese oxide, birnessite, forms when mixtures aretoo concentra ted a nd substa ntial Mn 2O3 contaminationis observed for mixtur es tha t a re too dilute. Treat mentof the gel prior to drying and calcination is also impor-tant since extraction of potassium ions from the gelfavors cryptomelane over birnessite. The scope of th esol-gel reaction generally does not extend t o other per-manganates besides KMnO4. Thus the procedure has

not been successful in producing a wide va riety of hol-land ite-type ma nga nese oxides. The extra ction of po-ta ssium ions from cryptomelane by excha nge rea ctionswith foreign cat ions a ppea rs t o be rela tively inefficient.

Acknowledgment. We ac know l edge t he D epart -ment of Energy, Office of Basic Energy Sciences, Divi-sion of C hemical S ciences, for support of t his r esearch.

CM960460K

(43) Hunter, J . C. J . So l i d St a t e Ch em . 1981, 39, 142.(44) Ohzuku, T.; Higashimura, H.; Hirai, T.Electrochim. Acta1984,

29, 779.(45) Shen, X. M.; Clearfield, A. J . So l i d St a t e C h e m . 1986, 27 0,

1986.(46) Feng, Q.; Miya i, Y.; Ka noh, H.; Ooi, K.L a n g m u i r 1992,8, 1861.

(47) F eng, Q.; Ka noh, H.; Miya i, Y.; Ooi, K. Chem. M ater. 1995, 7,1226.

(48) F eng, Q.; Ka noh, H.; Miya i, Y.; Ooi, K. Chem. M ater. 1995, 7,1722.

(49) F eng, Q.; Ka noh, H.; Ooi, K.; Tan i, M.; Naka cho, Y. J Electro-chem. Soc. 1994, 141, L135.

Figure 7. Temperatur e-programm ed desorption for O 2evolu-tion of a calcined sa mple of sol-gel cryptomelane.

Table 3. Potassium Ion E xtraction from Sol-GelCryptomelanea

ca t ion %K K /Mn ca t ion/Mn

K+ 4.9 0.12 0.12H+ 4.3 0.10NH 4+ 2.8 0.07Li+ 4.4 0.11Na + 4.3 0.10 0.008C u2+ 4.4 0.10 0.010

a C a t i o n ex t r a ct io n i s ba s ed o n t he d epl et i on of K+. Th eexchan ge cation wa s a nalyzed for selected elements a s shown in

th e cation/Mn column a bove.

754 Ch em . M at er ., V ol . 9, N o. 3, 1997 Ch i n g et al .

2.sol−gel route to the tunneled manganese oxide cryptomelane

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