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Structural and Textural Properties of Pyrolytic CarbonFormed within a Microporous Zeolite Template

J . Rodriguez-Mirasol,*, T. Cord ero, L. R. Ra dovic, a nd J . J . Rodriguez

D epartm ent of C hem i cal Engin eeri ng, U niv ers i ty of M a laga, Cam pus de Teatinos,s/ n 29071 M a laga, Spai n, and Fuel Science Program , Depart ment of M ateri al Science &

Engin eeri ng, T he Penns ylv ani a State U n iv ers i ty , U niv ers i ty Park , Penns ylv ani a 16802

Received A ugust 8, 1997. Revised M anu scr ip t Received N ovember 25, 1997

A new class of pyrolytic carbon materials has been prepared by chemical vapor infiltrationof a microporous zeolite pow der followed by remova l of the zeolitic subst ra te. A w ide-poreY zeolite was used as the substrate for the pyrolytic carbon, and propylene was used as thecarbon precursor. The structure a nd porous texture of the resulting carbons were examinedby X-ra y diffra ction, scann ing a nd t ra nsmission electron m icroscopy, a nd by a dsorption ofN 2 a t 77 K a nd C O2 a t 273 K . C a r b o n r e a ct i v it y s t u d ies w e r e p er f or m ed b y b o t hn on isoth e rm al an d isoth e rm al th e rm og ravim e tric an a lysis . Un d e r th e p re sen t con d it ion sof chemical vapor infiltration (800-850 C, 2.5 vol %C 3H 6, 1 atm of N 2), high-surfa ce-a reamicroporous carbons, wit h wide microporosity, well-developed mesoporosity a nd highad so rption cap aci ty w e re ob ta in ed . Th e carb on yie ld an d th e a p par e n t su rface are a of th e

carbon increa sed wit h increasing propylene pyrolysis t empera ture. The morphology of thecarbons wa s very simila r to tha t of th e zeolite templa te. Their O2-reactivity profiles exhibiteda tw o-sta ge beha vior, w ith t he inflection point betw een t he tw o sta ges occurring a t differentconversion levels depending on the deposition tempera tu re. Such oxida tion behavior suggeststha t t he ca rbons consist of tw o different st ructures wit h different rea ctivities, the oxida tionof the more reactive ca rbon ta king place first a nd t he rema ining more ordered carbon beingconsumed in the second sta ge. These ca rbons do not exhibit molecular sieving propertiesfor sma ll a dsorbate/reacta nt species.

1. Introduction

Zeolites a re highly crystalline a luminosilicates w itha uniform pore structure; Their channel a pertures a re

i n t h e r a n g e 0 . 3-1. 0 n m, de p e n din g o n t h e t y p e o fzeolite a nd its prepar at ion history (e.g. , calcinat ion,leaching, or various chemical treat ments). They ha vebeen of considerable commercial importance not only asca t a ly s t s bu t a ls o a s molecu la r s iev es , du e t o t h e irability to separa te linear from bra nched hydr ocarbons.This separation ability is associated with the extremelyfine pore structure, which permits only certain mol-ecules to penetra te into th e interior cages of the zeoliteparticles.1 T h e s t a bi l i t y o f c a rbo n ma t e ria ls a t h ig ht e mp er a t u r es a n d i n a c id i c m e di a a n d t h ei r l ow e raffinity for wa ter offer unique adva nta ges over inorgan icmolecular sieves and make the development of suchma terials very at t ra ct ive. For exam ple, car bon molec-

ular sieves (CMS) for the separation of O 2 from air hasbecome of great commercial interest . More generally,the design of microporous carbons with a predeterminedpore size distribution (e.g. , for metha ne storage) con-t i n u e s t o b e o f g r e a t f u n d a m e n t a l i n t e r e s t a n d h a sincreasing pract ical importa nce.

St u dies of molecu la r s iev e p rop ert ie s of ca rbo n sderived from polyvinylidene chloride (P VDC)2,3 an d fromSa ra n (cop oly mer of v iny l ide n e c h loride a n d v in y lchloride in a mole rat io of a bout 9:1)4,5 h a v e s h own t h a theat treatment of these polymers results in amorphouscarbons w hich do not ha ve a regular crysta lline struc-ture, but they do present a syst em of ca vities connectedby slit-sha ped pore constr ictions a nd ad sorption capa ci-ties comparable to those of the zeolites.6

More recently, Foley and co-workers,7-9 u s in g asimilar approach, prepa red CMS by contr olled pyrolysisof poly(furfuryl a lcohol). Their results show tha t thechemical a nd physical properties of the CMS , especiallyporosity and pore size, can be systematically modifiedb y v a r y in g t h e t e mp er a t u r e a n d t i m e o f p yr ol y si s.Higher synthesis temperatures and longer soak t imesleaded to CMS w ith na rrower pores, when compared toCMS prepared at lower temperatures and soak t imes.The decrease of micropore diameter with heating tem-

* To w h o m cor r es p on d en ce s h ou ld b e a d d r es s ed . E -m a i l:[email protected].

Un iversity of Mala ga. The Pe nnsy l vani a Sta te U ni ve rsi ty .(1) Br eck, D. W. Zeoli te M olecular Sieves: St ru cture, Chemistry and

U se; Wiley: New York, 1974.

(2) Da cey, J . R.; Thomas, D . G. Tr ans. Faraday Soc. 1954, 50, 647.(3) Kipling, J . J .; Wilson, R. B . Tr ans. Faraday Soc. 1960, 56, 557.(4) Culver, R. V.; Heat h, N. S. Trans Faraday Soc. 1955, 51, 1569.(5) Heat h, N. S.; Culver, R. V. Tr ans. Faraday Soc. 1955, 51, 1575.(6) Lam ond, T. G .; Metcalfe, J . E.; Walker, P . L., J r. Carbon 1965,

3, 59.(7) Lafya tis, D. S ., Tung, J .; Foley, H. C . In d. En g. Chem. Res. 1991,

30, 865.(8 ) Mar i wal a, R. K. ; Fol ey , H. C . Ind. Eng. Chem. Res. 1994, 33,

607.(9 ) Kane , M. S . ; G oel l ner, J . F. ; F ole y , H. C . ; D i Franc e sco, R. ;

Billinge, S. J . L.; Allard, L. F. Chem. Mater. 1996, 8, 2159.

550 Chem. Mater. 1998, 10 , 550-558

S0897-4756(97)00552-8 CCC : $15.00 1998 American C hemical SocietyP ubl ish ed on Web 01/30/1998

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perature affected the effective diffusivities of differentgases tested, modifying the molecular sieving propertiesof t h e c a r b on s . M a r i w a l a a n d F ol ey 8 a t t r ibu t e d t h eeffect of t ime an d t emperat ure on the CMS propert iesto an annealing of the glassy microstructure, leadingto larger a romatic microdomains with better a lignmentand order a nd less free volume for ga s a dsorption a nddiffusion. Addit ion of high molecular weight (MW >2000 amu) poly(ethylene glycol) to the PFA prior to

pyrolysis developed a meso- and macropore structure(i .e ., t ra n s p ort p ore s) in t h e re su lt a n t CM S,7 whichprovides for more efficient internal ma ss-tra nsportp roce ss es in t h e s orp t ion of g a s e s in t o t h e n a rro wmicropores.

Activa ted car bons (ACs) ha ve been used as pr ecursorsin th e production of ca rbon molecula r sieves. Their pores t ru ct u re , w i t h a wide microp ore s ize dis t r ibu t ion ,provides h igh surfa ce area an d extensive adsorptioncapacity. The a ccess to the na rrow m icropores of theAC ca n be modified to genera te constr ictions simila r ins ize t o a n a ds o rbin g s p ecies , a l lowin g i t s s elect iv ead sorption. Thus, CMS can be produced by altering thesize of pore constriction of ACs to ma tch t he size of the

desired a dsorbate. Wa lker et al.10 reported that coatinga gran ular AC w ith a thermosett ing polymer, followedby curing and carbonizat ion of the polymer layer cancreate molecular-size constrictions in its pore structure,thus genera ting CMS. Using a similar approach, prepa-ration of CMS was carried out by carbon deposition frompyrolysis of propylene on microporous ca rbons.11,12 Thistechnique has been shown to be only partial successfulfor separa t ion of gases. B ecause of their typically widemicrop ore s ize dis t r ibu t ion , ACs s h owe d n o u s ef u ldevelopment of molecular sieving char a cteristics. Onlycarbon precursors possessing na rrow micropore sizedistribution achieved considerable adsorption selectivityin the separat ion of gases whose molecular size falls

wit h in a n a rro w ra n g e . Co n dit ion s of c h emis orp t ionand pyrolysis of pyrolyt ic carbon precursor ha ve to beselected very car efully to sa tisfy the conflicting require-ments of maximum adsorption selectivity a nd minimumpore blocking (i.e. , uniform deposition over the entirep ore s u rf a ce ), wi t h i t s lea s t de t r ime n t a l e ff ect onadsorption capacity.13

Verma and Walker 14 reported that the preparat ionof CMS by d epositing ca rbon from pyrolysis of propylenei n a n AC i s m a d e v ia b l e b y a d d in g a h y d roca r b oncracking cat alyst t o the AC. In this study, Ni, used asthe hydr ocar bon cra cking cata lyst , w as included in ACby t he incipient-wetness technique. The preferentia lcracking of propylene a t the Ni sites with in t he pores

of t h e AC p a rt icles a n d/or a t t h e p a rt icle e x t ern a lsurfa ce, generat ing constr ictions of th e size required forg a s s e p a r a t i o n , w a s s u g g e s t e d b y t h e a u t h o r s a s apossible explanation for the formation of CMS.

A new approach in the preparation of carbon molec-u la r s iev es f rom a c t iva t e d c a rbon s wa s p res en t e d byNguyen and Do.15 Coke deposition via benzene thermal

c ra c k in g o n a n A C wa s a l lo we d t o p ro c e e d u n t i l a l lmicropores became nearly closed, followed by an activa-t ion step necessary to refine the opening of the poremouth to provide to the carbon molecular sieve char-acteristics.

Toda e t a l .16 s t u died t h e ch a n g e p rodu ce d in t h emicroporous structure and the micropore size distribu-tion of an AC m ade from coal by steam act ivat ion a fterh e a t t re a t me n t a t h ig h t e mpe ra t u re . Th e y s u g ge st e d

that preparat ion of CMSs from ACs may be achievedby heat treat ment only. However, the micropore vol-umes of the heat-trea ted carbons decreased ra pidly withtemperature, without producing appreciable molecularsieve propert ies to ma ke them useful a s C MS.

In similar studies to that of Toda et al . ,16 Verma,11

and Verma and Walker 17 came t o the conclusion tha tt h e t h e rma l t re a t me n t o f a n A C p ro du c e d n o u s e f u lmolecular sieving properties for t he separa tion of gasesof close molecular size. They suggested tha t the f inesieving pores generated init ially by heat treatment atmode ra t e t e mpe ra t u re , du e t o e limin a t io n of s omesurface oxygen functional groups, are reduced in sizedue to pore sintering a t h igher temperat ures.

In recent yea rs there ha s been quite a bit of interestin designing novel mat erials by exploiting the va riat ionsof both zeolites (or oth er inorga nic solids) and carbons.For example, Leboda 18,19 studied the chemica l structurea nd m orphology of carbon deposits conta ined in complexcarbon/mineral adsorbents prepar ed by pyrolysis ofdifferent car bona ceous substances on the surface ofminera l sorbents such a s silica gels, Al2O3, a luminosili-ca t e s , e t c. F o le y20 prepared composite structures ofCM S a n d in org a n ic o xides a n d s u pp ort e d me t a ls inorder to combine th e surfa ce chemical properties of theinorganic oxides with the molecular sieving propertiesof the carbon (i .e. , adding functionality to the carbonmolecular sieves). Model results suggested th at these

carbon m olecular sieve composites could be t a ilored t oproduce catalysts with improved selectivity for synthesisga s conversion in th e Fischer-Tropsch reaction.21 Morerecently, the group of Foley 22 prepared noncrysta llinecomposite catalysts, consisting of silica-a l u m in a a n damorphous carbon molecular sieves, which were ableto produce mono- and dimethylamines from metha noland ammonia with higher select ivity than the silica -a lu min a ca t a ly s t s u s e d in t h e comme rcia l p ra ct ice .Schwa rz a nd co-workers23-26 ha ve investiga ted the porestru cture of ca rbon/minera l na nocomposites preparedby using th e intra crystalline space of layered mineralsfor in situ carbonizat ion. Kyotani et al .27,28 found thathigh-surface-area carbons can be obtained by carboniza-

(10) Walker, P . L., J r.; La mond, T. G .; Metcafe, J . E., III Proc. 2ndC on f . o n I n d u s t r i a l C a r bon a n d G r a ph i t e ; Soci ety for C he mi calIndust ry: London, 1966; p 7.

(11) Verma, S. K. Carbon 1991, 29, 793.(12) Verma, S. K.; Walker, P. L., J r. Carbon 1992, 30, 829.(13) Moore, S. V.; Trimm, D . L. Carbon 1977, 15, 177.(14) Verma, S. K.; Walker, P. L., J r. Carbon 1993, 31, 1203.

(15) Nguyen, C.; Do, D. D. Carbon 1995, 33, 1717.(16) Toda, Y., Yuki, N.; Toyoda, S. Carbon 1972, 10, 13.(17) Verma, S . K.; Walker, P . L., J r. Carbon 1990, 28, 175.(18) Leboda, R. M ater. Chem. Phys. 1992, 31, 243.(19) Leboda, R. M ater. Chem. Phys. 1993, 34, 123.(20) Foley, H. C. ACS Symp. Ser. 1988, 36 8, 355.(2 1) L afy at i s, D. S . ; Fol ey , H. C . Chem. Eng. Sci. 1990, 45, 2567.(2 2) Fole y , H. C . ; L a fy at i s, D. S. ; Mari wa l a, R. K . ; Sonni chse n, G.

D . ; B r a k e , L . D . Chem. E ng. Sci. 1994, 49, 4771.(2 3) B a n d o s z, T. J . ; P u t y e r a , K . ; J a g i el lo , J . ; S c h w a r z , J . A.

Microporous Mater. 1993, 1, 73.(24) Ba ndosz, T. J .; Gomez-Sa laza r, S.; P utyera , K.; Schw ar z, J . A.

Microporous Mater. 1994, 3, 177.(25) Ba ndosz, T. J .; J agiello, J .; Put yera, K .; Schwa rz, J . A. Carbon

1994, 32, 659.(26) Ba ndosz, T. J .; J agiello, J .; P utyera , K.; S chwa rz, J . A. Chem.

M a t e r . 1996, 8, 2023.

Pyr ol yt i c Car bon Ch em . M at er ., V ol . 10, N o. 2, 1998 551

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tion of poly(furfuryl a lcohol) (P FA) in a zeolite templa te,thu s avoiding the conventiona l a ctivat ion process. Theyalso reported that carbons obtained in this way retaint h e ir h ig h s u rf a c e a re a e v e n a f t e r t re a t me n t a t h ig htemperature. Hernadi et al .29 ha ve even been successfulin preparing carbon nan otubes by decomposing a cety-le n e o n t h e s u rf a c e o f a Y ze o l i t e imp re g n a t e d wit hcobalt.

I n t h e p re s e n t wo rk , a n a l t e rn a t iv e a p p ro a c h wa s

pursued. Microporous ca rbons prepar ed by infiltra tionof a w idepore zeolite w ith pyr olytic carbon from pyroly-sis of propylene has been studied. Zeolite, a moleculars iev e, is u s ed a s t e mpla t e f or t h e p y roly t ic c a rbo ndeposition. If the carbon deposition is assumed to takeplace on the surface of the internal channel of the zeoliteduring the pyrolysis of propylene, once the zeolite isremoved, i .e. , by acid washing, the remaining carbonshould ha ve acquired a porous structure similar t o thatof the zeolite, i.e., a porous structure of a CMS. Thisa p p ro a c h , is u s e d in t h is s t u dy t o t a i lo r t h e p h y s ic a lsurface propert ies of the carbon ma terials obtained bythis technique.

2. Experimental Section

2.1. Materials and Carbons Preparation. Zeolite/carbon composites h ave been prepared by chemical vaporinfi l tration of zeoli te powder with pyrolytic carbon at 800-850 C. A wide-pore microporous Y zeolite (Z-14 US , Da visonChemical Co.), widely used as a hydrocarbon cracking ca ta lyst,was used as the substrate for pyrolytic carbon deposition. Thee xper i m ent s w e r e c ar r i e d out i n a conve nt i onal hor izont a ltubula r furn a ce (5 cm i.d.). P ropylene (2.5 vol %), in a str eamof nitrogen at at mospheric pressure, wa s used as the pyrolyticcarbon precursor. In a ty pical experiment, the zeolite powderwa s placed in a ceram ic boat inside the isothermal zone of thefurnace. The temperature wa s increased from room temper-at ur e t o t he r e ac t i on t e m per at ur e a t a he at i ng r at e of 10 C /m i n i n N 2 at m ospher e . O nce t he r e ac t i on t e m pe r at ur e w a s

reached, the inlet gas was changed from N 2 to the propylene/N2 mixture, and the rea ction w as carried out for ca. 24 h. Thet e m pe r at ur e w a s m a i nt ai ne d w i t hi n (1 C . The compositeswere denoted by the n am e of the zeoli te followed by CVD a ndthe deposition tempera tur e in C (e.g., Z14US/CVD800). Theamount of pyrolytic carbon accumulated on the zeoli te wascalculated by weighting the zeoli te sample before and afterthe deposition experiments.

P ur e c ar b on sam pl es w e r e ob t ai ne d b y r e cove r ing t hepyrolytic carbon a fter d eminera lizing th e zeolite/carbon com-posite. A sta nda rd demineralizat ion procedure, involving HCland H F, w a s used .30 The carbons thus obtained were denotedb y Z C VD f ol lo w ed b y t h e d e po si t io n t e m pe r a t u r e (e .g . ,Z C VD800). C ar b ons Z C VD850L T and Z C VD850H T w e r eprepared by ca rboniza tion of car bon ZCVD850 (10 C /min) inN2 at 1000 C for 2 h (LT) an d by further hea t tr eatment in a

graphite induction furnace (

100 C/min) at 2900 C in Arfor 5 min (HT).2.2. Carbon C haracterization. The st r uct ur e of t he

carbons w as stu died by XRD (40 kV, 20 mA, Cu KR r ad i at i on,Rigaku), scan ning a nd tr a nsmission electron microscopy (SEMand TEM), and N 2 (77 K, Omnisorp 100 CX, Coulter) and CO 2(273 K , Aut osor b , Q uant ac hr om e) ad sor pt i on. R e act i vit ystudies (1 a tm of O 2, 60 cm 3(STP )/min ) were perf ormed b y bot h

nonisotherma l (5 C/min) and isotherma l t hermogravimetrican alysis (Cah n m icrobalance, Model 113X). The rea ctivityprofiles, 1 - X vs temperature and R vs X (where X representscarbon conversion or burnoff (BO) a nd R ) [1/(1 - X)]dX/d t)were obtained from the noisothermal and isothermal experi-ments, respectively.

3. Results and Discussion

3.1. Porous Structure and Morphology. Figure

1 s h o ws t h e X - ra y di f f ra c t io n ( X R D ) p a t t e rn s o f t h esubstr a te, Z14US, the composite, Z14US /CVD800, a ndthe recovered matr ix materia l, ZCVD800. Clearly, thedisappearance of all zeolite peaks and the appearanceof broad C(002) and C(10) peaks for ZCVD800 indicat et h a t t h e p roce dure of re mov in g t h e zeol it e by a c idleaching wa s successful. Selected ar ea electron diffrac-t ion a n d e le men t a l a n a ly s es of t h e c a rbon s a mp lesfurther supported th e complete removal of the zeolitefrom the carbons. The average crystallographic para m-eters d002 a nd L c for ZCVD800 w ere 0.363 and 1.7 nm,respectively. Simila r results were found for ZCVD850carbon, w ith corresponding va lues for d002 a n d L c being0.356 and 1.3 nm, respectively. These values indica te

tha t a n increase in the propylene pyrolysis tempera turetends to result in carbon with lower interplana r dista ncean d lower avera ge thickness of para llel pseudogra phiticlayer.

F ig u re 2 s h o ws t h e N 2 adsorption isotherms of thezeolite and th e car bons. Table 1 summarizes the weightpercentages of carbon in the composites, the effectivemicropore volume values, obtained from the applicationof the t me t h o d t o t h e N 2 adsorption isotherm data 31

an d from the a pplication of the Dubinin-Radushkevich(DR) equation 32 t o b ot h t h e C O 2 a n d N2 adsorptionis ot h e rm da t a , a n d t h e a p p a re n t B E T a n d D R s u rf a c e

(27) Kyota ni, T.; Na tt y, T.; Tomita, T. Extended Abstracts of Car bon92, Essen, 1992; p 437.

(28) Kyota ni, T.; Inoue, S.; Naga i, T.; Tomita, A. Chem. Mater. 1997,9, 609.

(2 9) He rnadi , K. ; Fonsec a, A.; Nag y , J . B . ; Be rna e rts, D. ; Fuda l a,A.; Lucas, A. A. Zeolites1996, 17, 416.

(30) Radovic, L. R.; Walker, P. L., J r.; J enkins, R. G. Fuel 1983,62, 849. (31) L ippen s, B . C .; de B oer , J . H . J. Ca t a l . 1965, 4, 319.

Figure1. X-ray diffraction patterns for the zeolitic substrate,Z14U S (a), the composite, Z14US/CVD800 (b), a nd the recov-ered ma trix mat erial , ZCVD800 (c).

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area values (derived from the data of the N 2 a n d C O2isotherms, r espectively) for th e zeolite, composites a ndcarbons. The chara cterist ic type I isotherm of the Yzeolite (Z14US) confirms its r elatively high surfa ce areaand n arr ow microporosity. The fact that its SB E T valuei s l o w e r t h a n SD R c a n b e a t t r i b u t e d t o t h e l a c k o fachievement of true N2 adsorption equilibrium due torestricted diffusion through the pores at the low tem-peratur e of 77 K;33 it a lso furth er supports the existenceof very narrow micropores in the structure of the zeolite.Infiltrat ion of the zeolite w ith pyrolyt ic car bon at 800 C (Z14US /CVD800) yielded 45%of car bon ma trix a ndreduced its BETsurface area from 720 to 4 m 2/g, mostp ro ba bly du e t o bo t h p a rt ia l f i l l in g o f t h e p o re s a n dblockage of the pore entra nce by the pyrolyt ic carbon.When the infiltrat ion temperature was 850 C, a 57%ma trix yield wa s achieved (Table 1); pyrolyt ic carbondeposition also resulted in a decrease of the surface areaof the zeolite. Thus, under t he experimenta l conditionsused in this study, th e higher the deposit ion t empera -ture, the higher the pyrolyt ic carbon yield. Once thezeolit ic material was leached out , the isolated carbonproducts exhibited a relat ively high B ET surface area ,590 a nd 1380 m2/g for ZC VD800 an d ZC VD850, respec-tively. Further hea t trea tment of ZCVD850 at 1000 C(ZCVD850LT) and 2900 C (ZCVD850HT) reduced theapparent surface area to 963 and 39 m 2/g, r especti vely .

The N2 adsorption isothermal profiles for ZCVD800,ZCVD850, a nd ZCVD850LT ar e q uite similar to ea chot h e r ( F ig u re 2). Th e y a re a l l t y p e I is ot h e rms a n dexhibit a more rounded knee a t low rela tive pressuresand more pronunced slope at higher relat ive pressuret h a n t h e ze ol it e . Th e a p p a re n t s u rfa c e a re a a n d t h emicropore volume of these carbons, derived from the N 2is o t h e rms , a re la rg e r t h a n t h o s e o bt a in e d f ro m CO 2isotherms (Ta ble 1). These tw o observa tions indicat e

that these carbons possess a much wider microporosityt h a n t h e ze ol it ic s u bst ra t e . Th e re la t iv ely h ig h C O2a p p a re n t s u rf a c e a re a a n d microp ore v o lu me o f t h ezeolite-derived carbons (Table 1) indicate that , eventhough t hese ca rbons ha ve developed a wide m icroporesize distribution, they maint ain a considerable volumeof narr ow micropores, which represents m ore tha n 50%of the tota l micropore volume for all the carbons. Thisfeature ma y be important for t he sieving propert ies of

the carbons that, under certain conditions, could be usedt o s e p a ra t e s ma ll g a s mo le c u le s . 26 The presence ofna rrow m icropores in th ese car bons is supported by th ere s u lt s s h o wn in F ig u re 3, wh ic h re p re s e n t s t h e D Rp lot s f or t h e z eol it e a n d ca r b on s , b a s e d on t h e N2adsorption isotherm dat a. The DR plots exhibit devia-t ion from linearity at low relat ive pressure values, asit is frequently observed for coa ls, ca rbonized mat erialsa n d ca r b on s w i t h v er y n a r r ow m i cr op or es .34 This

behavior has been at tributed to act ivated diffusion inthe narrow micropores of the carbons.35 Such deviationscan be linearized to some extent by using th e Dubinin-Astakhov equation 36 wit h v a lu e s o f t h e n p a ra me t e rbeing 3, 4 , a n d 5 for ZCVD800, ZCVD850, andZCVD850LT, respectively. A value of n h ig h er t h a n 2has been a ssociat ed to molecular sieve carbons with asystem of relatively narrow micropores.37

Th e p res en ce of a t y p e H 3 h y s t e res is lo op in t h edesorption branch of the isotherms, including that ofZ14US, at relative pressures above 0.4, which exhibitsno limit ing adsorption a t P/P0 1, is indicative of thepresence of slit-sha ped pores.38

The m orphology of th e Z14U S/CVD 800 composite a nd

ZCVD800 carbon is not different from tha t of the pa rentzeolite, as can be concluded from th e scanning electronmicrographs shown in Figure 4. Similar results wereobtained for sa mples Z14U S/CVD850 an d ZCVD850.This suggests t ha t chemical va por infiltra tion of zeoliteby pyrolysis of propylene, followed by removal of thezeolit e p rodu ce s a p y roly t ic ca rbo n t h a t a c q u ire s amorphology q uite similar to tha t of th e zeolite, with afine microporosity and a relat ively high surface area.

3.2. Oxidation Behavior. F i gu r e 5 s h ow s t h eweight loss vs temperature curve obtained upon gasifi-cat ion of the Z14US/CVD850 composite in O2. Th epyrolytic ca rbon sta rts t o oxidize at 500 C (no w eightlo s s wa s o bs e rv e d in bla n k e x p e rime n t s , wh e n o n ly

zeolite wa s used). The ra te of weight loss is very lowwithin t he tempera ture ra nge 500-600 C an d increasess ign ifica n t ly a t 600 C. Abov e 700 C t h e re ma in in gwe ig h t s t a y s c o n s t a n t a t a v a lu e t h a t c o rre s p o n ds t o44%of the initial weight of the composite, indicatingt h a t t h e ca r b on m a t r i x o f t h e com pos it e h a s b ee ncompletely consumed. (This w eight loss agr ees verywe ll wi t h t h e p erce n t a g e o f c a rbon ma t r ix in Z 14U S /CV D 850 g ive n in Ta ble 1. ) F ig u re 6 comp a re s t h enonisotherma l rea ctivity profiles of the pyrolytic ca rbonin t he pres ence (Z14U S/CVD 800) an d a bsence (ZCVD800)of the zeolite substra te. The free-sta nding pyrolit iccarbon (ZCVD800) is seen to be more rea ctive tha n t hecorrespond ing ca rbon in t he composite (Z14US /CVD 800),

in a g re e me n t wit h i t s h ig h e r a p p a re n t a n d re a c t iv esurface area.39 It is important to note tha t it a lso clearlyexhibits a tw o-sta ge reactivity beha vior, w ith t he inflec-t i on p oi n t b et w e en t h e t w o s t a g es occu r r in g a t aconversion level of 25%. This beha vior suggest s th e

(32) Dubinin, M. M.; Radushkevich, L. V. Proc. Acad. Sci., SSSR1947, 55, 331.

(33) Rodrguez-Reinoso, F., In Ca r b on a n d Coa l G a sif ica t io n ;Figueiredo, J . L., Moulijn, J . A., Eds.; Martinus Nijhoff Publishers:Dordrecht, 1986; pp 601-642.

(3 4) Marsh, H.; Ra nd, B. J. Colloid Interface Sci. 1970, 33, 101.(3 5) Mc E nany , B . ; Mast e r, K. J . Thermochim. Acta 1984, 82, 81.(36) Dubinin, M. M., Surface and M embran e Science; C ade nhe ad,

D. A., Ed.; Academic Pr ess: New York, 1975; Vol. 9, pp 1-70.(37) Kra ehenbuehl, F.; Stoeckli, H. F.; Addoun, A.; Ehr burger, P .;

Donne t , J . B . Carbon 1986, 24, 483.(38) Linares-Solano, A. In Carbon and Coal Gasif i cation; Figueire-

do, J . L., Moulijn, J . A., Eds.; Mar tinus Nijhoff P ublishers: Dordrecht,1986; pp 137-178.

(39) Lizzio, A. A.; J ian g, H .; Ra dovic, L. R. Carbon 1990, 28, 7.

Figure2. N2 adsorption-desorption isotherm s for th e zeoliteand c ar b ons.


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p res en ce of t wo di ff ere n t s t ru ct u re s wit h di ff ere n treactivities in this carbon.40 The oxidat ion of the m orereactive one (25%of the car bon) ta kes place first w hilethe rema ining less rea ctive carbon (75%) is consumedin the second stage, at higher temperat ures. The facttha t t he car bon in t he Z14U S/CVD800 composite, w hich,in principle, is expected to have similar structure to thatof ZCVD800, shows a lower reactivity than ZCVD800,an d does not exhibit a tw o-sta ge behavior, suggests tha to n ly a s ma ll f ra c t io n o f t h e t o t a l s u rf a c e a re a o f t h isca r b on i s a v a i l a b le t o t h e o xi da n t g a s . M os t of t h es u rf a c e a re a o f t h e c a rbo n in t h e c o mp o s it e mu s t beb lock ed a n d b ecom es a v a i la b l e t o t h e g a s on l y a s

oxidation proceeds.Figure 7 shows the react ivity profiles for ZCVD850carbon. Those for ZCVD850LT a nd ZCVD850HT a rea l s o i n cl ud ed a n d w i ll b e d i s cu s sed l a t er . S a m p leZCVD850, even though prepared at higher temperature,is seen to be more reactive than ZCVD800 (Figure 6),in a g re e men t w it h i t s h ig h er s u rf a c e a re a . O xida t ionof ZCVD850 also shows a tw o-step rea ctivity pr ofile, butt h e in flect ion p oin t f or t h is ca rbo n (50% burnoff)suggests tha t , under the experimental condit ions usedin this study, a s th e deposit ion t empera ture increases,t h e p erce n t a g e o f t h e more re a c t iv e s t ru ct u re in t h ecarbon also increases. However, for both ZCVD800 andZCVD850 the more reactive carbon and the less reactive

carbon are oxidized wit hin the sam e tempera ture ra nge,500 to 550 C a n d 575 to 675 C, respectively.The existence of two distinct structures 40 is also sug-gested by the isothermal reactivity profiles at differenttemperatures, i l lustrated in Figure 8 (with the transi-tion burnoff value for ZCVD850 being 50%).

The infiltrat ion of Y zeolite with pyrolyt ic carbon,followed by removal of the zeolit ic substrate, is thusconcluded t o produce carbons whose surfa ce area ishigher than tha t of the zeolite itself. A striking feat ureof these car bons is t hat they a lso exhibit tw o different

regions in their oxidat ion behavior. This f inding is inagreement with our earlier reports on the behavior ofC/C composites made w ith f iber an d ma trix ma terials

(40) Cordero, T.; Thrower, P . A.; Ra dovic, L. R. Carbon 1992, 30,365.

Table 1. Percentage of the Matrix in the Composite and Porous Structure Characteristics of Zeolite, Composites, andCarbons

sample Vt N2 (cm 3/g) VDR N2 (cm 3/g) SB ET (m2/g) VDR CO 2 (cm 3/g) SDR CO 2 (m2/g) %m a tr ix

Z14U S 0.277 0.282 720 0.379 1052Z14U S/CVD 800 4 45Z14U S/CVD 850 3 57ZC VD 800 0.250 0.206 590 0.137 374ZC VD 850 0.601 0.493 1380 0.322 920ZC VD 850L T 0.436 0.318 963 0.235 641

Figure 3. N2 DR plots for the zeoli te and carbons.

Figure 4. Scanning electron micrographs for Z14US zeolite(top), Z14US/CVD800 composite (middle), an d ZCVD800carbon (bott om).


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with similar structure but possessing different surfa cea re a .40-42 Additional polarized light microscopy studiesre v e a le d t h a t f o rma t io n o f o n ly la min a r s t ru c t u re in

ZCV D 800 a n d ZCV D 850 ca rbo n s, wh ich is in t u rnconsistent with the finding that at relat ively low tem-peratures, intermediate hydrocarbon concentration andlarge bed surface areas, laminar carbon can form on thesubstrate surface.43,44

Figure 9 shows the Arrhenius plots for ZCVD850oxidation a t different car bon burnoff levels. Highercarbon rea ctivity is ag a in clea rly seen below 50%burnoff(s ee a ls o F ig u res 7 a n d 8). N e ve rt h eles s , p a ra l le lstraight lines are observed, indicating similar apparentact ivat ion energy values in the entire range of carbonconversion. Table 2 summarizes the a pparent a ct iva-

tion energies at different conversion levels for oxidationof ZCVD850. The a vera ge va lue of 175 kJ /mol a greesreasonably w ell with th e values reported in the litera-t u re f o r t h e c a rbo n-oxygen reaction.45-48 The large

(41) Rodrguez-Mirasol, J .; Thrower, P . A.; Ra dovic, L. R. Carbon1993, 31, 789.

(42) Rodrguez-Mirasol, J .; Thrower, P . A.; Ra dovic, L. R. Carbon1992, 33, 545.

(43) Bokros, J . C. In Chemi stry and Physics of Carbon; Walker, P.L., J r., E d.; Ma rcel Dekker: New York, 1969; Vol. 5, pp 1-118.

(44) McAllister, P .; H endricks, J . F.; Wolf, E . E . Carbon 1990, 28,579.

(45) Lizzio, A. A.; Piotrow ski, A.; Radovic, L. R. Fuel 1988, 67, 1691.(46) Miura, K.; H ash imoto, K.; Silveston, P. L. Fuel 1989, 68, 1461.(4 7) Mi ura, K. ; Naka mura, K . ; Hashi moto, K. Energy Fuels1991,

5, 47.(48) Rodrguez-Mirasol, J .; Cordero, T.; R odrguez, J . J . Carbon

1996, 34, 43.

Figure 5. Nonisothermal weight-loss vs temperature profilefor the oxidat ion of Z14US/CVD800 composite, a t a heatingra te of 5 C/min.

Figure 6. Nonisotherma l rea ctivity profi les for (0) Z14US /

C VD800 (c al c ul at e d onl y for t he c ar b on m at e r i al ) and (9

)ZCVD800 at a heat ing ra te of 5 C/min.

Figure 7. Nonisotherma l reactivity profiles for (0) ZCVD850,(4) ZCVD850LT and (]) ZCVD850HT carbons, at a heatingra te of 5 C/min.

Figure 8. Isothermal reactivity profiles for ZCVD850 carbonat different temperatures.

Figure 9. Arrhenius plots for ZCVD850 oxida tion a t d ifferent

carbon conversion.


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differences in r eact ivity at low and high conversionlevels may be a t tr ibuted to different surfa ce ar ea (andthus, most probably, different react ive surface area 39)ava ilable to the gas at low and high conversion levels.

3.3. Structural Features. Tra nsmission electronmicrographs of samples Z14US and ZCVD850 (Figure10) reveal that the appearance of the ZCVD850 carbonis quite simila r to tha t of the Z14US zeolite, supportingthe results obtained by SE M (Figure 4). Two structuresca n be c le a rly di ff ere n t ia t e d in t h e s k elet o n o f t h e

pyrolytic carbon pa rt icles (Figur e 10, middle a nd bott omp a n e ls ): a de n se a n d comp a ct s t ru ct u re f ormin g t h ee xt e rn a l bo u n da ry a n d a les s de n s e a n d les s o rdere dstructure constitut ing the interior. Qualitat ively simi-lar results were obta ined for th e ZCVD800 car bon. I tappears convincing, th erefore, tha t the interior lessordered st ructure provides a greater contribution ofs u rf a ce a r ea t o t h e ca r b on a n d t h a t t h e m o re d e n seexternal structure represents only a small percentage

of the total surface area of the carbon.The zeolite infiltra tion process by pyrolytic carbon ca n

thus be described as a cata lyt ic cra cking of the propy-lene on t he intern al surfa ce of th e zeolite par ticles, i.e. ,on t h e w a l ls o f t h e microp ore s. O n ce a f i rst la y e r o fpyrolyt ic carbon has covered the zeolite surface, thedeposit ion continues on t he surface of t he pyrolyt iccarbon already deposited, which provides new act ivesites for car bon deposition.49 Simulta neously with thiscatalyzed heterogeneous gas-solid reaction, homoge-neous gas-phase pyrolysis rea ct ions can occur t oo, a ssuggested by McAllister et al . ,44 producing pyrolyticcarbon that grows both on the external surface of thezeolit e a n d a t p ore mo u t h s . As re a ct ion p roce eds ,

accumulation of gas-phase-nucleated pyrolytic carbonreduces pore accessibility and leads to slower diffusionof propylene into the pores. In the second deposit ions t a g e , wh e n t h e e n t ra n c e o f t h e p ore s h a s bee n c om-pletely closed, deposition continues only on the externalsurface of the zeolite, increasing the thickness of thedeposited layer. I f t he pyrolyt ic carbon is depositeduniformly on the su rfa ce of the m icropores a nd d oes notfill them, once the zeolitic substra te ha s been removed,carbon w ith a chan neled structure similar t o that of thez eol it e , b u t e ve n w i t h h i gh er s u rf a ce a r e a , ca n b ef or m ed . O n t h e ot h e r h a n d , t h e p yr ol y t ic c a r b ondeposited on th e externa l surfa ce of the zeolite gives riseto low surface area carbon with a dense and compact

str ucture. Figure 11 shows a scheme of pyrolytic ca rboninfiltrat ion of zeolite, followed by removal of zeolit icsubstra te. These two different structures would explainn ot on ly t h e h ig h a p pa re n t s u rf a ce a re a s f ou n d f o rZ CV D 800 a n d Z CV D 850, bu t a ls o t h e t wo - s t a g e O2-reactivity profiles t ha t these carbons exhibit .

Figure 12 shows the transmission electron micro-graphs of ZCVD800 and ZCVD850 samples and com-pares the structure of these two car bons. I t is clearlyobs erv ed t h a t ZCV D 800 c a rbon p res en t s a t h ick erpart icle external boundary than ZCVD850, in agree-ment with the L c values obtained for these carbons. Thisseems t o indicat e tha t more pyrolyt ic carbon has beendeposited on the external surface of the zeolite when

pyrolysis of propylene wa s carried out a t 800 C inst eadof 850 C. O n t h e o t h er h a n d, t h e f a c t t h a t Z CVD 850presents a higher apparent surface area than ZCVD800(Table 1) suggests tha t at higher deposit ion tempera -tures more surface of the micropores of the zeolite iscovered by pyr olytic car bon before th e pore entr a nce isblock ed. H ig h er p y roly s is t e mpe ra t u re s re s u lt in afaster heterogeneous gas-s ol id re a ct ion , wi t h morepyrolytic carbon being deposited inside the fine micro-str uctur e of the zeolite. The blocka ge of the pore mouthseems to ta ke place in a shorter t ime at lower t emper-

(49) Hoffman, W. P .; Vastola, F . J .; Walker, P . L., J r. Carbon 1985,23, 151.

Table 2. Apparent Activation Energies for ZCVD850Oxidation at Different Conversion Levels

conversionlev el (%)

apparentact ivat ion energy

(kJ /m ol)conversion

lev el (%)

apparentact ivat ion energy

(kJ /m ol)

10 175 60 17420 166 70 18030 186 80 171

Figure 10. Transmission electron micrographs for Z14US(top), ZCVD850 (middle), and ZCVD850 at higher magnifica-tion (bottom).


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a t u re a n d mo re p y ro ly t ic c a rbo n is de p o s i t e d o n t h eexterna l surface of the zeolite at 800 C tha n a t 850 C.

To support experimenta lly our hypothesis on thestructure and oxidation behavior of the zeolite-derived

carbons, ZCVD850 car bon w as part ia lly oxidized up to50%burnoff. This car bon conversion corresponds to theinflection point in the reactivity profile for this carbon.A significant reduction of the surface area of CVD850carbon, from 1380 to 122 m 2/g, a fter pa rtia l oxida tion(50%burn off) wa s observed. This result suggests th a tthe more rea ctive structure of the CVD850 ca rbon, tha tconta ining the higher contribution t o the tota l surfacear ea of the carbon, has been consumed, ha ving the others t ru ct u re , mo re de n se a n d comp a ct , wi t h o ut bein gconsumed. The tra nsmission electron microgra phs ofCVD850 carbon oxidized at 50% burnoff (Figure 13)re v e a ls t h a t t h e in t e rio r o f t h e p a rt ic le s h a s a lmo s tdisappear ed after t he part ia l gasificat ion of the carbon,whereas the structure forming the external boundaryof the part icles rema ins almost inta ct . I t is importa ntto remark t hat the morphology a nd par t icle size of theZCVD850 carbon oxidized at 50% burnoff (Figure 14)a re q u it e s imila r t o t h o s e o f t h e s t a r t in g u n o x idize d

ZCVD850 carbon.As a lready seen, ZCVD850 car bon presents a struc-

t u re s imila r t o t h a t of t h e Z 14U S zeol it e w it h ra t h e rhigh apparent surface area (almost twice the value ofthe zeolite, Ta ble 1). This lar ge surfa ce a rea im plies ah ig h a ds orp t ion ca p a c i t y . H o we ve r , t h e s h a p e o f t h eZCVD850 N2 i sot h er m a n d t h e com p a r is on of t h esurface area va lues obta ined by N2 a n d C O2 adsorptionfor this carbon reveal a wide micropore size distribution,questioning the utility of these carbons as molecularsieve for separation of gases whose molecular size fallswit h in a n a rrow ra n g e. I n a n a t t e mpt t o improv e t h em ol ecu l a r s i ev i n g ch a r a ct e r is t i cs of t h i s ca r b o n s,ZCVD850 wa s further heat-treat ed to higher tempera -

Figure 11. Scheme of pyrolytic carbon infiltration of zeolite,followed by r emoval of the zeoli t ic substrate.

Figure 12. Tra nsmission electron micrograph s for ZCVD800(top) a nd ZCVD850 (bott om).

Figure 13. Tra nsmission electron micrograph s for ZCVD850carbon oxidized a t 50%burnoff at different ma gnificat ion.


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t u res . H ea t t r ea t m en t of ZC VD 850 a t 1000 C(ZCVD850LT) reduced its surfa ce ar ea from 1380 to 935m 2/g, increasing its oxidat ion resistance (Figure 7).

ZCVD850LT still exhibits a tw o-sta ge reactivity profile,although now the difference in react ivity between thetw o structures present in the carbon is smaller. Heattreat ment of ZCVD850 a t 2900 C (ZCVD850HT) re-duced drama tically the BET surface area of the carbonto 39 m 2/g. This carbon exhibits a substa ntia lly lowerO2 reactivity than ZCVD850LT and does not exhibit atwo-stage reactivity profile (Figure 7), suggesting thaton ly on e s t ru ct u re (t h e more de n se a n d comp a ct ) isp res en t in i t . Ta ble 1 rev ea ls t h a t h e a t t re a t me n t o fZCVD850 to higher temperat ures not only decreases itsB E T s ur f a ce a r ea a n d N2 micropore volume, but alsoreduces the DR C O2 surface area and micropore volume.Th e S(DR)/S(BE T) ratio for ZCVD850LTshows a constant

value of

0.6, similar to t hat of ZCVD850, indicat ingt h a t bo t h n a rro w a n d wide mic ro p o ro s i t y h a v e be e nreduced to the same extent with temperature and thatuseful molecular sieving propert ies w ere not d evelopedby t h e t h e rma l t re a t me n t .

The process of chemical vapor infiltration of zeolitefrom pyrolysis of propylene, followed by removal of t heze o li t ic s u bs t ra t e h a s de mo n s t ra t e d t o p ro du c e h ig hsurface area pyrolyt ic carbons, which present a struc-

t u re s imila r t o t h a t of t h e ze ol it e . Th e c a rbon s t h u sobta ined ha ve developed a wide microporous a nd me-s op orou s s t ru ct u re , w h ich c on f ers on t h e m a h ig hadsorption capacity, but not useful molecular sievingproperties for sma ll adsorbat e/rea ctan t species. How -ever, it is believed th a t prepar a tion of ca rbon molecularsieves ma y be possible by mean s of the process describedin th is paper, if zeolite with an appropriat e microporesize distribution and no mesoporous structure is used

as substra te. Working condit ions tha t a llow pyrolyt iccarbon growth in the surface of the pores of the zeoliteshould be used. This could be acomplished by selectinga deposit ion temperature a nd a gas pressure as low aspossible in practice in order to favor low rate of pyrolyticcarbon deposition and low hindrance by backdiffusionof the gas eous rea ction products.50 Work is in progresst o f u rt h er in ve st ig a t e t h is is su e u s in g ot h e r ze ol it etypes, tempera tures, propylene part ia l pressures, andtotal pressure.

4. Summary and Conclusions

Microporous carbons with high surface area have been

prepared by pyrolytic carbon infiltration of a Y zeolitefrom pyrolysis of propylene, followed by removal of thezeolit e ma t r ix . Th e p y roly t ic ca rbo n y ie ld a n d t h ea p p a re n t s u rf a c e a re a of t h e c a rbon in cre a s ed wit hpyrolysis tempera ture. The carbons obtained presenta s t ru ct u re v e ry s imila r t o t h a t of t h e zeol it e . Th e irreactivity profiles exhibit a tw o-sta ge behavior, with t heinflect ion point between the two stages occurring atdifferent conversion level depending on the depositiontemperature. Such oxidation behavior suggests tha t thecarbons consist of two different structures with differentreactivit ies, the oxidat ion of the more react ive carbont a k in g p la c e f irs t a n d t h e re ma in in g , mo re o rde redcarbon being consumed in the second stage.

The process of pyrolytic carbon infiltra tion w ithin themicroporous zeolite template produces carbons with awide microporosity, well-developed mesoporosity, andhigh a dsorption capa city. These ca rbons do not exhibitmolecular sieving properties for sma ll a dsorbat e/rea c-ta nt species. I t r emains to be seen whether the pore-size distribution of the resulting carbon can be tailoredby judicius selection of the t emplate a nd carbon deposi-tion conditions.

Acknowledgment. This study wa s m ade possible,i n p a r t , b y a g r a n t (t o J . R. M. ) f rom t h e F u lb r ig h tCommission a nd t he Ministry of Educat ion a nd S ciencesof Spain and a research project QUI97-0872 from theS p a n is h P L a u N a ci on a l d e I + D . We a r e g r a t ef ul t oGr egorio Mar t n Ca ba llero and J ua n J ose Ca nca C uencafor their help with TEM. Discussions with Dr. TakashiKyotani (Tohoku University) are appreciated.

CM970552P

(50) Kotlensk y, W. V. In Chemistry and Physics of Carbon; Walker,P . L., J r., E d.; Ma rcel Dekker: New York, 1973; Vol. 9, pp 173-262.

Figure 14. Scanning electron micrographs for ZCVD850 (top)and ZCVD850 oxidized at 50%burnoff (bottom).


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