activated carbons from yellow poplar and

8/12/2019 Activated Carbons From Yellow Poplar And

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PergamonPII: 80008-6223 98)00082-7

Carbon Vol. 36, No. 7-8, pp. 1085-1097, 19980 1998 ElsevierScienceLtd

Printed n Great Britain.All rights eserved0008-6223/98 19.00 + 0.00

ACTIVATED CARBONS FROM YELLOW POPLAR ANDWHITE OAK BY H,PO, ACTIVATION

MARIT J AGTOYEN,* and FRANK DERBYSHIRECenter for Applied Energy Research, 2540 Research Park Drive, Lexington, KY 4051 I-8410, U.S.A.Received 28 October 1997; accepted in revise form 4 December 1997)

Abstract-Results are presented from continuing investigations of the phosphoric acid activation ofhardwoods. Earlier work with white oak has been extended to include yellow poplar. It is found thatthe same general chemical and physical changes occur with both precursors. A discussion is presentedon the possible mechanisms of phosphoric acid activation, drawing upon extensive research on the useof phosphorous compounds as fire retardants for wood and cellulose. Phosphoric acid appears tofunction both as an acid catalyst to promote bond cleavage reactions and the formation of crosslinksvia processes such as cyclization, and condensation, and to combine with organic species to formphosphate and polyphosphate bridges that connect and crosslink biopolymer fragments. The additionor insertion of phosphate groups drives a process of dilation that, after removal of the acid, leaves thematrix in an expanded state with an accessible pore structure. It is considered that activation of theamorphous polymers produces mostly micropores, while activation of crystalline cellulose produces amixture of pore sizes. The different response of crystalline cellulose is attributed to a much greaterpotential for structural expansion than is possible with the amorphous polymers due, among otherfactors, to its higher density and its chemical structure that allows for a more extensive degree ofcombination with phosphoric acid, and hence bulking of the cell walls. The pore size distributionobtained from crystalline cellulose can be altered by increasing the HTT and/or the ratio of acid toprecursor such that, eventually, the structure is dominantly mesoporous. At temperatures above 45OC,a secondary contraction of the structure occurs when the phosphate linkages become thermally unstable.The reduction in crosslink density allows the growth and alignment of polyaromatic clusters, producinga more densely packed and less porous structure. 0 1998 Elsevier Science Ltd. All rights reserved.Key Words-A. Activated carbon, D. reaction mechanisms.

1. INTRODUCTIONIn earlier publications, we have described the resultsof investigations to elucidate the chemical, physicaland morphological changes that occur during thesynthesis of activated carbons by the phosphoric acidactivation of white oak (Quercus al&) [ 1,2]. Toexamine the general applicability of the findings tothe activation of other hardwoods, the research hasbeen considerably extended to obtain comparativeinformation using yellow poplar Li r iodendron uli pif-era) as an activated carbon precursor. Both whiteoak and yellow poplar are hardwoods that grow inabundance in Kentucky and the southeastern U.S.[3]. However, they differ in several respects, such asin their wood cellular structure, biopolymer composi-tion, density and hardness. This paper describes thepoints of correspondence and difference in the beha-vior of the two hardwoods, and adds new informationabout the mechanisms of phosphoric acid activation.

2. EXPERIMENTAL2.1 Materials

Samples of white oak and yellow poplar were usedas activated carbon precursors, and were in the formof wood blocks 2.2 cm x 5 cm x 10 cm) and powder

*Corresponding author.

( 100 mesh (< 150 pm)). In order to follow the dimen-sional changes during reaction, samples of the precur-sors were prepared as thin sections that were cutfrom the blocks. By using parallel sections, separatespecimens of each hardwood contain the same dis-tinctive morphological features, allowing dimensionalchanges during activated carbon synthesis to beaccurately followed and measured: features such asgrowth rings survive the process of chemical activa-tion and can be readily discerned in the carbonproducts. Sections were cut in a plane perpendicularto the axial direction of the tree growth to providespecimens measuring 2.2 cm x 5.0 cm x 0.15 cm forthe white oak. The yellow poplar was treated in asimilar manner to provide specimens of2.3 cm x 7.1 cm x 0.16 cm. The white oak sectionscontained 6.5% moisture while the yellow poplarcontained 5.7%. For other experiments, the woodsamples were used in powder form. The samples wereall stored in sealed containers. Representative analy-ses of the two precursors are shown in Table 1 [4].Notable differences are the higher H/C and O/Cratios for yellow poplar, the higher content of ligninand lower content of hemicellulose, and the muchlower density compared to white oak.2.2 Chemical activation

Samples of wood in the form of sections (about1 l g) or powder (24 g) were used without drying

1085


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1086 M. JAGI oyrh and I:. DKRRYSHINTable I. Typical composition of hardwood precursors [4]

White oak Yellow poplarElemental analysis (wt%)t1 6.1 7.00 42.0 47.3c 51.2 51.3Biopolymer content (~1%)Cellulose i 3-3 52Hemicellulose 24.1 I4Lignin 24.6 34.2Atomic ratiosO/C 0.61 0.7H/C I .42 I .6iDensity (g cm-) 0.68 0.42

and were soaked in phosphoric acid solution for 1hour at room temperature to allow penetration ofacid into the wood structure: the proportion ofH,PO, to wood normally used (as-received basis)was 1.45 g H,PO, or 1.5 x lo- mol of H,PO, pergram of wood. At this ratio of reagent to precursor,the amount of reagent is considered to be in excess[5]. The phosphoric acid was obtained as an 85%solution and was diluted to a concentration of 28%before adding to the wood samples. in order tofacilitate wetting and impregnation through increas-ing the solution volume. In one series of studies, theratio of acid to precursor was varied.

Usually, the mixture was then subjected to lou-temperature heat treatment by heating slowly to170C in a stainless steel reactor, under flowingnitrogen at atmospheric pressure It was held at thistemperature for 30 minutes, and then taken to a finalheat treatment temperature (HTT) up to 650C andheld for 1 hour. In some cases, the final HTT wasbelow 17OC, in which case the mixture was subjectedonly to low-temperature heat treatment with a 3 hourhold time at temperature. Carbons were obtainedat HTT between 75565OC for white oak, and150&65OC for yellow poplar. The solid productswere leached with distilled water to pH 6 and vacuumdried at 110C before further characterization. Forcomparison, thermally treated samples were preparedby following the same heat-treatment procedure. butwithout phosphoric acid addition. A limited set ofexperiments was also made with white oak in whichthe ratio of phosphoric acid to precursor was variedfrom 0.17 1.45 g acid per gram of wood with aconstant HTT of 500C.2.3 Analysis and characterization

In selected experiments, an on-line quadrupolemass spectrometer (VG/Fisons Quadrupole MassSpectrometer, Sensorlab 300D-850 with Thermosoftsoftware) was used to measure the composition ofthe gases evolved while heating a sample of wood ora phosphoric acid-wood mixture (heating rate2C min- to 600C in an atmosphere of argon). Thegases were analyzed for four constituents that were

present in high concentration: CO (mass 28) CO2(mass 44), CH, (mass 16. but followed as mass 15.the CH, radical, to avoid interference from 0) andllL (mass 2). Water (mass 18) was present in the gasphase in an approximately constant and relativelyhigh concentration over the whole temperature rangeunder study. It was not possible to distinguishbetween the water added to the reaction mixture andthat liberated by reaction.

The solid carbon products were characterized byseveral techniques: chemical analysis for elementalcomposition; optical and scanning electron micro-scopy; and N, (77 K) adsorption. The morphologicalchanges during synthesis were examined by micro-scopic methods and were also quantified by themeasurement of dimensional changes using amicrometer to determine the distance between growthrings in the radial direction.

Information on the carbon pore structure wasderived from nitrogen adsorption isotherms obtainedat 77 K on a Coulter Omnisorb 610 automatedsurface area analysis system. Surface areas werecalculated from the adsorption isotherms using themethod of Brunauer, Emmet and Teller (BETmethod) [6]. The micropore volume W,, (pores lessthan 2 nm diameter) was determined using theDubinin Raduskevich equation [7]. Mesopore sur-face areas and volumes were calculated using theBJH method [8].

3. RESULTS3.1 Porosity development

In order to select an appropriate ratio of acid toprecursor, a survey was made of the effect of varyingthis ratio on porosity development for the white oakprecursor (powder) at a constant heat treatmenttemperature (HTT) of 500C Fig. 1. As the ratio ofchemical reagent to wood is increased the microporevolume increases to a maximum at a ratio of about0.80 g H,PO,/g wood. At higher ratios, the micropore

0.7 -*y---*

J--.---_..____10.6 -. ,I~,

G 0.5 *. , /;3 , I> Ig 0.4 -- .o /, I, 1,, /F , ./0..g 0.2 --___: .....(0.1 -- .H. ...-,f0, _*.- ,

0 0.5 1Ratio HBPO, : wood (g/g)

1 5

Fig. 1. Effect of acid:wood ratio on porosity developmentm white oak (HTT =SOOC, H,PO, strength 28%, precur-

sor: sections of wood).


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Activated carbons from yellow poplar and white oak 1087volume is approximately constant or may evenslightly decrease. On the other hand, the mesoporevolume is small at low ratios and then increases moreappreciably as the ratio is raised above about 0.8:there is no indication that the mesopore volume hasattained a maximum at the highest ratio used here.Consequently, it can be seen that the ratio of reagentto precursor is an important variable, and it can beused to change the pore size distribution in theactivated carbon. The nature of this dependence alsochanges with HTT [ 11. In all subsequent experiments,phosphoric acid was used at a ratio of 1.45 gH3P0,/g of dry precursor. This ratio was selected asit gives high micropore and mesopore volumes, andit is also a value that is typical of the ratios used incommercial processes [5,9].

A comparison of porosity development in activatedcarbons produced from white oak and yellow poplarat a ratio of 1.45 g H,PO,/g is shown as a functionof HTT in Fig. 2. Within the scatter of data it canbe seen that there is a very similar correspondenceof the results for the two precursors. The bandsdrawn on the figure enclose the two sets of data.Micropore development begins at about 150C andrises to a maximum at about 350C. Mesopores begin

b0 100 200 300 400 500 600 700

HTT FC)Fig. 2. Pore volume of activated carbons from white oakand yellow poplar (H,POjwood = 1.45g g-l, acid strength288, precursor: sections of wood).

0 200 400 600 fHeat treatment temperature (C)

to develop at the higher temperature of about 250Cand the mesopore volume attains a maximum at500-550C. The different temperature dependenciesof the two ranges of pore volume indicate thatdifferent processes may be involved in the formationof micropores and mesopores. At the same time, itshould be recalled that the division into pore sizeranges is a convenient convention, and as the porestructure actually consists of a continuous distribu-tion of pore sizes, there will clearly be some interde-pendence of the size ranges. It is apparent from Fig. 2that most of the increase in the mesopore volume(above about 350C) corresponds to a decrease inmicroporosity, suggesting that pore widening contrib-utes to the increase in mesopore volume. In conjunc-tion with the data shown in Fig. 1, it is also apparentthat the pore size distribution can be modified byselection of the HTT as well as the reagent toprecursor ratio.3.2 Chemical change

Elemental analyses of the white oak and yellowpoplar carbons produced by phosphoric acid treat-ment and thermal treatment confirm other reportedresults [ 1,2] showing that acid treatment acceleratesthe removal of oxygen and hydrogen, at temperaturesas low as 75C, Fig. 3(a)Fig. 3(b). There is littleeffect upon thermal reaction until the HTT is at least150C and a sharp change then occurs between250-350C. By plotting the atomic ratios H/C versusO/C (after van Krevelen [lo]), it is found that thethermal and acid data fall on a straight line with agradient of 2.0, showing that the net change in thetransformation of the precursors to carbon productsis the elimination of water, irrespective of whetherphosphoric acid is present or not, Fig. 4. (As will bediscussed below, dehydration is not the only reactionthat can occur.) The effect of phosphoric acid is topromote dehydration at 150 to 200C lower thanupon thermal reaction.

Some differences are apparent between the twoprecursors. While the H and 0 contents of thecarbons follow a similar curve with HTT for the two

8

266 406 600 IHeat treatment temperature C)Fig. 3. Change in oxygen content (a) and hydrogen content (b) of carbons from yellow poplar (YP) and white oak (WO) asa function of HTT (H,POI activation; H,PO,:wood = 1.45 g g-l, acid strength 28%, precursor: sections of wood).


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1088 M. JAG~OYI:Nnd F. DI:RBYSHIRE

I

0 0.2 0.4 0.6 0.8O/C Atomic RatioFig. 4. Van Krevelcn diagram for carbons producedfrom yellow poplar (YP) and white oak ( WO) (H,PO,activation; H,PO,:wood = I .45 g g-, acid strength 28%.precursor: sections of wood).hardwoods for thermal treatment, they do not corre-spond for the acid treated products, Fig. 3(a and b).Specifically, the dehydration of white oak appears tohave a greater temperature dependence than that ofyellow poplar. One possible cause may be the higherH and 0 content of the parent yellow poplar.

The composition of the gases evolved during ther-mal and acid treatment is indicated by plots of themass spectrometer response for CO/CO, andCH,/H, versus HTT in Figs 5 and 6, respectively. Inthe presence of H,PO,, CO2 and CO begin to beliberated just below about 100C and their productionincreases sharply to a maximum at about 200C.Thereafter, the generation of both gases declinescontinuously with HTT. In the thermal case, CO,and CO evolution starts at higher temperatures (inthe region of 150~200C) and the amounts of these

lE-09^h.Y lE-10Elc?L;I lE-11

Fig. 5. CO and CO, evolution on thermal treatment andH,P04 activation of white oak (H,PO,:white oak=1.45 g-i, acid strength 28%, precursor: white oak powder).

lE-110.0 200.0 400.0 600.0

HIT(C)

Fig. 6. CH, and H, evolution profiles during thermal treat-ment and H,PO, activation of white oak (H,PO,:whiteoak=1.45ggm, acid strength 28%, precursor: white oakpowder).

gases increase to maxima between 250&35OC. At alltimes the concentrations of CO and COZ in the gasphase during thermal pyrolysis appear to be lowerthan in the presence of phosphoric acid. The originof the CO,, and water, is believed to be from boththe lignin and cellulose part of the wood. However,previous work by Shafizadeh showed that the pres-ence of 5% H,PO, during the pyrolysis of celluloseincreased water yield from 11 to 21% while theamount of CO, was unchanged compared to pyrolysisin the absence of H,PO, [ 111. It is therefore believedthat the increased amount of CO, released onH,PO, treatment originates from lignin.

Upon acid treatment, there is a very significantrelease of methane beginning at around 100C andrising to a sharp maximum around 250C. Thegeneration of CH, falls to a minimum between35OG4OOC and then rises to a second, lower maxi-mum between 45OG55OC. The release of CH, in thethermal case is quite different. Here, methane isproduced in much lower amounts and only begins tobecome evident at temperatures above about 250C.The production of H, starts at around 400C in thepresence of H,PO, and also rises to a sharp maximumat about 550C. Upon thermal pyrolysis the gasphase hydrogen content is always low and onlyslightly increases at HTT above 450C.

Thus it is apparent that dehydration is not theonly means of oxygen and hydrogen removal. In thepresence of phosphoric acid, the former is alsoreleased as oxides of carbon, with maximum pro-duction at about 200C and the latter as lighthydrocarbon gases, principally methane, rising to amaximum at about 250C; hydrogen is also liberated


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Activated carbons from yellow poplar and white oak 1089at higher HTT as molecular hydrogen and by second-ary methane formation.3.3 Morphology

The changes in the morphology of carbons pro-duced from white oak relative to the parent woodhave been described earlier [ 11. Similar behavior wasobserved with yellow poplar. Observations of fibrouscells with distinct primary (lignin-rich) and secondary(high content of crystalline cellulose) cell walls in theparent wood showed that, after phosphoric acidtreatment to 125C there is evidence of the flow andredistribution of cellular material. Upon heat treat-ment to 200C this redistribution causes the formerlyempty cell lumens to be filled. However, the bound-aries between the primary and secondary cell wallsare still clearly distinguished and the original cellstructure is still recognizable even after heat treatmentto 350C and above. Thus, it appears that there isrelatively little interaction between the crystalline andamorphous regions. In contrast, thermal treatmenteffects a massive disruption to the structural integrityof the wood. By about 350C the secondary cell wallshave almost disappeared and are no longer distin-guishable from the primary walls.

From the results of this and the earlier study, itseems probable that when acid is present the amor-phous biopolymers (lignin and hemicellulose) are theprimary sources of the redistributed material. Studiesby i3C NMR have shown that, at temperatures aslow as 50C there are detectable changes to thecomposition of these biopolymers, and by 150Cthere is a measurable increase in the ratio of crystal-line to amorphous cellulose [2]. The partial depolym-erization of amorphous biopolymers due to the actionof the acid could account for a limited degree ofmobility and migration. Related studies have shownthat the reflectance of the material that fills the celllumens is similar to that of the primary cell wall,which consists mainly of lignin, whereas the reflec-tance of the secondary cell wall (cellulosic) is appreci-ably lower [12]. In other work, it seems that thereaction of lignin with phosphoric acid can lead tothe formation of a plastic phase during low temper-ature reaction [ 131, also suggesting that this biopoly-mer is readily degraded into lower molecular weightunits.3.4 Di mensional change

The transformation of hardwood to an activatedcarbon is accompanied by significant dimensional aswell as morphological change. As has already beendescribed for white oak, it was found that, upon acidtreatment, there is a contraction at low HTT, fol-lowed by structural dilation at intermediate HTTand secondary contraction at high HTT [ 11. Thedilation and secondary contraction have been directlyrelated to the development and subsequent declineof porosity. In particular, the onset of dilation mea-sured on a relatively coarse (millimeter) scale, and

therefore representing a range of structural composi-tion, and the appearance of microporosity occur atthe same HTT (15OC), and a similar correspondenceis found at a HTT of 250C between the expansionof the secondary cell walls (in microns), which com-prise mostly crystalline cellulose, and the develop-ment of mesoporosity.

The dimensional changes in the radial directionare compared in Figs 7 and 8 for the thermal andacid treatment of white oak and yellow poplar as afunction of HTT. Upon thermal treatment, measur-able contraction commences above about 150 to200C for both hardwoods, and then progressesmonotonically with increasing temperature. At aHTT of 450C the overall contraction in the radialdirection is about 20% for white oak and 30% foryellow poplar.

When phosphoric acid is present, there is measur-able contraction at temperatures as low as 50Cconsistent with the action of the acid in lowering thetemperature threshold for dehydration and weightloss. The contraction sharply increases to about 18%for white oak, and 28% for yellow poplar at

0 100 200 300 400 500 600 700HTT (C)Fig. 7. Changes in radial dimension of carbons producedfrom white oak by H,PO, activation and thermal treatment(H,PO,:wood= 1.45 g g-r, acid strength 28%, precursor:sections of wood).-

65~ :0 100 200 300 400 500 600 7H7T C)

Fig. 8. Changes in radial dimension of carbons producedfrom yellow poplar by H,PO, activation and thermaltreatment (H,PO,:wood= 1.45 g g-, acid strength 28%.precursor: sections of wood).


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1090 M. J AG OYEN nd F. DI~RRYSHIRI:100-I 50C. At higher temperatures, the structurebegins to dilate. This process continues with increas-ing HTT to a maximum between 3OOG45OC. whenthe net contraction relative to the original woodstructure is reduced to about 3% for white oak and13% for yellow poplar. With further increase in HTT.a second process of contraction then begins. At aHTT of 650C the overall contraction is 14% forwhite oak and 20% for yellow poplar. which meansthat the structures are still dilated relative to themaximum contraction experienced at 100~~ 50 C. Itis to be noted that with both thermal and acidtreatment, the relative dimensional contraction forthe yellow poplar carbons is always about 10% morethan for the white oak series. A possible reason forthis difference may relate to the much higher lignincontent of yellow poplar, Table 1. If, as proposed.the lignin is readily depolymerized. and can migrateinto empty structural features, then the contractionof yellow poplar should be appreciably more thanfor white oak.

FIN. 9. A combination of two models describing the associa-tion ot cellulose, polyoses (hemicellulose) and lignin in thewood cell wall (adapted from Fengel and Wegner [ 141).

With reference to Fig. 2, it can be clearly seen thatthe onset of dilation corresponds directly to thedevelopment of microporosity for both hardwoods.Moreover, despite the differences in the extent of theinitial contraction of white oak and yellow polar, theoverall dilation in both cases is about 15%. and themicropore and mesopore volumes in the activatedcarbons follow the same pathway with HTT. Above-350C the micropore volume begins to decreasewhile the mesopore volume continues to increase toa maximum between 500~-550C before it also startsto decline.

Cellulose

3.5 Mechanisms ofphosphork acid uct ivat ionThe addition of the present data to the information

already published on phosphoric acid activationallows a reasonably comprehensive account of thisprocess to be assembled, although there still remainmany facets of this complex system that are poorlyunderstood. For convenient reference. the salientchanges and phenomena that have been observed inour own research are summarized in Table 2. 10further facilitate discussion, it is useful to provide abrief description of some pertinent aspects of woodstructure. Essentially, wood is a complex fibers matrixcomposite material formed of natural polymers inwhich the fiber framework consists of crystallinecellulose microfibrils of 225 nm diameter. The matrixbetween the microfibrils is composed mostly of hcmi-cellulose, and lignin provides the strengthening mate-rial that solidifies the surrounding cell wall. Fig. 9[14]. Typical ranges for the biopolymer content ofhardwoods are: 42250% cellulose, 19 25% hemicellu-lose and 16625% lignin [ 151. The molecular structuresof lignin, hemicellulose and cellulose are shown inFig. 10. The microfibrils in the ccl1 walls are formedfrom cellulose chains that are aligned and heldtogether by hydrogen bonding between the hydroxylgroups on the repeating glucose units. Groups of

1 WggHemicellulose

H&OHII;ncOC ,0H

20If

0Hl,ig. IO. Chemical structures of cellulose, lignin and

hemicellulosc.

Lignin

microlibrils are connected by amorphous cellulose(about 10 20% of the total cellulose) and hemicellu-lose, and the array is surrounded by lignin and somehemicellulose. The space between the microfibrils inan array is of the order of a few nm, and in the samesize range as larger micropores and mesopores.

3.5.1 Reaction at low t emperat ure. The reac-tion of wood with phosphoric acid probably beginsas soon as the components are mixed. Certainly.after heat treatment to 50C there is evidence of


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Activated carbons from yellow poplar and white oak 1091Table 2. Summary of phenomena relating to reaction of hardwoods with H&PO, (sources: present study and refs. [1,2])

Conditions oranalysis method H,PG, ThermalBelow 150CElemental

13C NMR

FTIR

Gas analysisAbove 150CMicroscopy

Gas adsorption

Gas analysis

Yield determination

r3C NMR

FTIR

Early elimination of H, 0 that continues to - SOOC-net effect dehydration leading to lower carbon yieldbelow 300CLoss of carboxyl and methyl groups from hemicellu-lose even at 50C and changes to lignin composition;by lOOC, substantial reaction of cellulose and forma-tion of ketones--intense peak by 150C; increase inratio of crystalline to amorphous cellulose; increase inaromaticity and loss of aliphatic, carboxyl and car-bony1 groups; onset of crosslinkingIncreasing aromaticity, loss of aliphatic character; lossof CO-C, C-O-H, C=O groups; appearance ofketone groups, evidence for formation of phosphateesters (cellulose phosphates)Evolution of CO, CO,, CH, begins at - 100C

Evidence for partial depolymerization of biopolymersand redistribution of material-emphasis on amor-phous polymers (lignin)Continuous contraction in radial direction up to- lWC, and in thickness of secondary cell walls(primary cellulose) up to -250C: even at highHTT, morphological features of wood structureretainedDilation in radial direction from 150C to maximumbetween 300-45OC, and of secondary cell wall thick-ness from 250C to maximum at -450CSecondary contraction > -450C for radial dim. andsecondary cell wallCorrespondence between porosity development andstructural dilation: micropore volume increases from150C to max. at -350C relating to overall dimen-sional expansion and reflecting an average effect ofconstituent biopolymers; mesopore volume increasefrom 250C to max. at -500&55oC inferring link withconversion of celluloseLoss of porosity at higher HTT corresponds to second-ary structural contraction

Maxima in CO, CO2 evolution at -200C; maximumin CH, evolution at -250C second maximum at- 500C; -4OOC, onset of H, evolution rising tomaximum at 550CHigher carbon yield > 300C-attributed to crosslink-ing reactions and retention of altered cellulose>45OC, dramatic increase in aromatic cluster size,indicative of reduction in crosslinking and structuralrarrangement>45OC, significant reduction in intensity of cellulosephosphate bands, and disappearance of bands due toketones and estersMaximum retention of phosphorus in char at -450C

Negligible change

No equivalent changes until - 250C

No equivalent changes until > 250C

CO, CO, evolution begins at - 150C

Contraction of secondary cell wall and in radialdirection starts -150C and continues mono-tonically with HTT: -350C most of second-ary cell walls have disappeared with major lossof wood structure morphology

Negligible porosity in thermal chars

Maxima in CO, CO, evolution at 250-350C;CHI evolution low and only evident >3OOC;H, evolution very low, slight increase > 450C

Equivalent change starts - 550C


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1092 M JAG I oym and F DERHYSHWchemical and physical change. It appears that theacid first attacks hemicellulose and lignin, possiblybecause of easier access to these amorphous biopoly-mers than to the crystalline cellulose. Elsewhere,cellulose has been reported to be more resistant toacid hydrolysis than other polysaccharides [ 161, andthe results obtained in this and previous work furtherindicate that cellulose does not experience the samedegree of degradation, since the general integrity ofthe cell structure is retained after reaction withH,P04, even to elevated HTT [ I]. The primary effectsof acid attack are to hydrolyze glycosidic linkages inpolysaccharides (hemicelhtlose and cellulose) and tocleave aryl ether bonds in lignin. These reactions arcaccompanied by further chemical transformationsthat include dehydration, degradation and condensa-tion [ 161. The primary reactions can lead to a reduc-tion in molecular weight, principally of hemicelluloseand lignin, which is consistent with the observedredistribution of woody material and with the abilityto form extrudable mixtures of wood and phosphoricacid [5]. Further. it is known that the acid-catalyzedhydrolysis of ether linkages in lignin can lead to theformation of ketones [ 161, whose appearance wasobserved by FTIR and 13C NMR [2].

These and other bond cleavage reactions thatproceed through ionic mechanisms can explain therelease of CO, CO, and CH,. The low temperatureevolution of CO and CO, is consistent with theobserved reduction in C-O functionality that isaccounted in part by esters and carboxylic acidgroups in the parent wood (mostly present in hemicel-lulose and lignin). The substantial release of CH, atsimilar temperatures suggests that the cleavage 01aliphatic side chains is relatively facile, and is consis-tent with a loss of aliphatic character and a corrc-sponding increase in aromaticity.

3.5.2 Reuction ut intermediutr trmperuturesThe liberation of gases and volatile products duringreaction, and the removal upon leaching of anywater-soluble products that are formed by depolym-erization, each contributes to the accelerated weightloss and volumetric contraction that is observed forthe water-insoluble char up to about 15OC. Upongoing to higher temperatures, the rate of weight lossslows appreciably and the structure begins to dilate.The stabilization and dilation of the wood structurecorrespond to the development of porosity. In thisregime. crosslinking reactions, that first becomeapparent at lower HTT [2], begin to dominate overbond cleavage and depolymerization reactions, asindicated by the findings that CO and CO, evolutionpeak at 200C. and the first maximum in CH,evolution occurs at 250C. The greater yield ofcarbon obtained by acid treatment above about3OO.C is attributed to a high incidence of structuralcrosslinking that serves to retain relatively low molec-ular weight species in the solid phase

There is evidently a direct connection betweenporosity development and the process of structural

dilation. In attempting to interpret the mechanismsof dilation, a wealth of relevant information existson the use of phosphorus compounds as flame retar-dants for wood and cellulose [ 17-191 and in otherwork on the chemical modification of wood [20].The origins of flame retardant properties that areintroduced by phosphorylation are the same as thosefound in phosphoric acid activationvolumetric dila-tion or intumescence (providing a thermal barrier)and reduced loss of volatiles due to crosslinking(raising the ignition temperature). Research on theuse of phosphoric acid as a flame retardant forcellulose has shown that phosphorus compounds canform ester linkages with -OH groups on cellulose attemperatures below 200C [21], helping to crosslinkthe polymer chains. Reaction with phosphoric acidalso stabilizes the cellulose structure by inhibiting theformation of levoglucosan which otherwise offers aroute to the substantial degradation of cellulosethrough its decomposition to volatile products,Fig. I I [ 19,221. Other work has shown that phos-phates can function as effective flame retardants forsynthetic polymers [23,24]. In studies of the fireretardance of the system ammonium polyphosphateand pentaerythriol. it was found that a swollen orintumescent char is produced between 280 and 350C.A particular distinction of this system from thepresent work is that the experiments were performedin a Row of air. At 28OC, the structure is consideredto consist of relatively small polyaromatic units thatare connected mostly by phosphate and polyphos-phate bridges that can include polyethylene linkages[ (CH,), 1. As the temperature is increased, cycliza-tion and condensation reactions lead to an increasein aromaticity and in the size of the polyaromaticunits, enabled by the scission of PPO- C bonds,Between 350 to 500C the char is considered to bestable, but at 430C and above, the continued cleav-age of crosslinks leads to a very extensive growth inthe size of the aromatic units. It was proposed that.at lower temperatures, the formation and persistenceof organic phosphocarbonaceous esters limits depo-lymerization and inhibits the development of con-densed polyaromatic structures. In the presentresearch. the appearance of an absorption band thatis attributed to phosphate esters is first observed byFTIR at about 150C. It persists with increasingHTT, although its intensity is reduced above about450 c.

Studies of the modification of wood by reactionwith various other chemicals have also shown thatthere is often a volumetric expansion that accompa-nies chemical reaction [20]. The major types ofreaction involve bonding chemicals to biopolymerhydroxyl groups to form esters, acetals and ethers.One of the criteria used to indicate successful reactionwithin the cell wall and chemical bonding to thebiopolymers is a volume expansion of the wood thatIS equivalent to the volume of chemical added; thevolume increase may be of the order of 10%.


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Activated carbons from yellow poplar and white oak 1093Formation of levoglucosan

A\H3PO4

H OH levoglucosan(volatlle)

Esterification would block formation of cyclic levoglucosan

6CH,O-P-O,H,S ----Q

Fig. Il. Reaction mechanism for flame retardance of phosphoric acid impregnated cellulose (adapted from Barker and Hendrix[191 and Lyons [22]).

Dimensional stability is achieved in cases where thebulking chemical crosslinks the cell wall polymers.

Thus phosphoric acid appears to be able to func-tion in two ways: as an acid catalyst in promotingbond cleavage reactions and the formation of cros-slinks via such processes as cyclization, and condensa-tion; and by being able to combine with organicspecies to form phosphate linkages, such as phos-phate and polyphosphate esters, that can serve toconnect and crosslink biopolymer fragments t24].The formation of phosphate esters by reaction ofcellulose with phosphoric acid is shown in Fig. 12.The addition or insertion of phosphate groups sepa-rates the organic species and effects a dilation of thestructure. An important distinction between phos-phoric acid activation and the chemical modificationof wood by phosphoric acid or other chemicals isthat, in the latter case, the added reagent is retainedin the expanded wood structure. In the synthesis ofactivated carbons, the phosphoric acid is removedafter reaction by leaching to recover the reagent. Forthis reason, the phosphorus content of the recoveredcarbons only indicates the amount of residual mate-rial that is present, either combined with the organicstructure, in the form of insoluble metal phosphates,or physically entrapped.

To examine the stability of the structure afterremoval of the acid, a short series of additionalexperiments was performed. Samples of white oakwere activated with H3P0, to a given HTT, leached,and then reheated to a HTT that was 50C higherthan the first. The yield and porosity of the productswere compared with those of carbons that had beenheat treated in the presence of acid to the same uppertemperature. It is found that the additional 50C ofthermal treatment reduces the yield of carbon,Fig. 13, and that the biggest reduction in yield occursbetween about 250-400C; that is, over the temper-ature range where there is the greatest dilation, andwhere it is expected that a high concentration of

phosphate linkages will have been formed. There arealso significant accompanying reductions in micro-pore volume, and increases in mesopore volume,Table 3. These observations are consistent with theproposition that phosphate linkages make a consider-able contribution to the total crosslink density. Uponleaching, due to their strong affinity for water, thesecrosslinks are readily broken by hydrolysis whichallows almost complete recovery of the acid (seeFig. 12, eqn (2)). When condensed phosphoric acidsare dissolved in water, hydrolysis to orthophosphoricacid takes place in a matter of minutes at 100C [25].As a consequence of the removal of the phosphatelinkages and a reduction in crosslink density, thestructure is metastable. Further heat treatment thenincreases the loss of volatile constituents that are nolonger strongly bonded into the solid phase, reducesthe carbon yield, and may lead to pore widening.

The overall structural dilation begins at about150C and that of the secondary cell walls at about250C. Up to 250C the dilation can be ascribed to

Table 3. Effect of acid removal for last 50CMicropore volume Mesopore volume(cm g-l) (cm3 g-r)

no H,PO, no H,PO,HTT (C) H,P0, last 50C H,P0, last 50C250 0.11 0 0.03 0.01300 0.42 0.20 0.06 0.05350 0.68 0.30 0.10 0.05450 0.67 0.59 0.36 0.74500 0.64 0.53 0.55 0.70550 0.61 _ 0.37600 _ 0.46 0.72

Micropore and mesopore volume for carbons producedat a certain HTT compared to runs where the acid wasremoved before the last 50C of the heat treatment(H,PO,:white oak = 1.45 g g-l, acid solution strength =28%, precursor: white oak sections and powder).


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1094 M. JAC;TOYI:N and F. ~kKHYSHIKIT< 450C: Formation of phosphate esters on cellulose side chains and crosslinking

CH20H0

lclr

OHI

OH OH-p=O T==tOH 0 IOH OH

CH20H0

0OH 1- H20

OHPOH -P=O

OH ?

Q

OH

H20HEsters can be derivatives of ortho- pyro- and meta-phosphoric acids:

OH-r=OOH orthophosphoric acidt

0 0II IIOH-P-O-p -OH +H20IOH OHpyrophosphoric acid

T>450C: Elimination of H3P04CH20H

t--s

0OH--P=0

bOH-P=0

0OH

OQ

+ -P=OHOH-&OIOH

Fig. 12. Mechanisnr of phosphate cstcr formation by phosphorylation of cellulose.

changes in the structure of the amorphous biopoly-mers, including amorphous cellulose. Microporedevelopment also begins at IWC, which suggeststhat a proportion of the micropores that are ulti-mately developed arises from a combination of acid-promoted reactions and the formation of phosphateesters with the amorphous biopolymers. The insertionof the phosphate esters drives a process of expansionthat, after removal of the acid, leaves the matrix inan expanded state with an accessible pore structure.The simultaneous removal of carbon in the form of

CO. CO, and light hydrocarbon gases must alsocontribute to the pore structure.

At 250C a thickening of the secondary cell wallsbecomes apparent, adding to the total dilation of thestructure, and also marking the beginning of meso-pore development. The expansion of crystalline cellu-lose is attributed to essentially the same causes asthat of the amorphous biopolymers. Namely, reactionwith phosphoric acid promotes chemical change thatincludes the stabilization of the cellulose chains,crosslinking through the formation of etheric and


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0 200 400 600 600H-IT (C)

Activated carbons from yellow poplar and white oak 1095ing total micropore volume, and the volume occupiedby mesopores also increases steadily. By about 350Cthe extent of dilation of the crystalline cellulose issuch that the existing micropores are widened toapproach mesopore dimensions, and the latterbegin to increase at the expense of the former. Hence,while the micropore volume passes through a maxi-mum, the mesopore volume continues to rise.Interestingly, over the temperature range 350-550Cthe total pore volume remains constant, indicatingthat new pores are not created to any significantextent, and that the principal change is in the poresize distribution. By the time that the mesoporevolume attains a maximum at 550C the microporevolume has fallen to about the same value as it wasat 250C. These observations support the propositionthat activation of the amorphous polymers producesmostly micropores, while activation of crystallinecellulose produces a mixture of pore sizes. The poresize distribution obtained from crystalline cellulosecan be altered by increasing the HTT and/or theratio of acid to precursor such that, eventually, thestructure is dominantly mesoporous.

Fig. 13. Yield of carbon as a function of HTT for carbonsproduced with phosphoric acid compared to runs where theacid was removed before the last 50C of the heat treatment(H,PO,:white oak = 1.45 g g-l, acid solution strength =28%, precursor: white oak sections and powder).

other linkages, and crosslinking through the forma-tion of phosphate and polyphosphate esters. Thepotential for the dilation of crystalline cellulose isbelieved to be much greater than for the amorphouspolymers due, among other factors, to its higherdensity and its chemical structure that allows for amore extensive degree of combination with phospho-ric acid, and hence bulking of the cell walls. Theaverage density for wood is 1.53 g cmm3, for lignin1.40gcm-3 and for cellulose 1.60 g cme3 [26].

From Fig. 12, it is easy to visualize how a chemicalsuch as phosphoric acid can insert between thecellulose chains, disrupt the existing hydrogen bonds,replace them with other chemical linkages, and simul-taneously separate the chains and dilate the structure.Dilation can continue as more acid is progressivelyincorporated, and the phosphate crosslinks maybecome more bulky at higher HTT due to polymer-ization of the acid to form polyphosphates. The factthat the mesopore volume is found to increase withthe ratio of phosphoric acid to wood is also consistentwith this proposition, since at higher ratios, more ofthe acid can be incorporated. Although the swellingof the secondary cell walls was measured directly, itis difficult to estimate the extent of swelling of theamorphous regions by subtraction from the overalldimensional change, since there are many large openvessels that could accommodate some of the expan-sion. However, as a rough indication of the totaldilation that is experienced between 150 and 450Capproximately one third occurs up to 250C and twothirds above this temperature, which is when thesecondary cell wall dilates. Moreover, the extent ofdilation expressed as a percentage of the originaldimensions is about 15% for the overall change whileit is about 25% for the secondary cell walls.

The above findings and inferences are consistentwith some of those reported in the published litera-ture. Cameron and MacDowall [27] proposed that,upon phosphoric acid activation, the acid serves toswell the wood structure allowing the cellulosemicrofibrils to separate. Upon carbonization, thealtered microfibrils form an open mesoporous struc-ture of loosely packed rods of carbonized cellulosewith pores of lo-30 nm in diameter. These authorsdid not attempt to account for the mechanism ofmicropore formation. However, their proposal isgenerally consistent with the present results. Otherstudies have shown that the reaction of phosphoricacid with cellulose, even at low temperatures, pro-duces considerable intercrystalline and intracrystal-line swelling [28,29]. Pandey and Nair [281 examinedthe reaction of phosphoric acid with cotton at temper-atures of 10, 29 and 40C for 0.5 hours using varyingconcentrations of phosphoric acid (O-81% acid con-centration with a constant cotton to liquor ratio of1 : 100). They found that the degree of cellulosepolymerization decreased with an increase in eithervariable:acid concentration or temperature. This andthe other published research [29] shows that phos-phoric acid begins to interact with cellulose at temper-atures as low as 10C producing swelling in bothcrystalline and intercrystalline regions and causing abreakdown of the cellulose polymer chains.

Around 250C it is considered that the porositycontributed by the modified secondary cell walls ismostly microporous, with some wider pores present.As dilation progresses with increasing HTT, themicropores from this source contribute to the grow-

3 5 3 Reaction at high temperature At HTTabove about 450C the structure begins to contract,as determined both by the overall dimensions and bythe thickness of the secondary cell walls. However,while the micropore volume decreases steadily withHTT above 350C there is no apparent effect on themesopore volume until about 550C when it startsto decrease quite steeply. It is possible that between450 and 550C the structural contraction is accom-


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1096 M. _iAGTOY1:Nnd F. hUJYSHlKlimodated by a narrowing of pore diameters withoutcausing an appreciable shift between pore size ranges.Above 550C a sharp decline in mesopore volume isparalleled by the secondary contraction of the cellwalls. Between 450.-650C the cell wall thickness isreduced by about 30% of the original dimension.while the overall contraction in the radial directionis only about 11%. The extent of the collapse of thesecondary cell walls is such that their thickness at650C is less than at any lower HTT. while theoverall structure is still dilated relative to the extentof contraction experienced at 15OC.

The contraction of the altered crystalline cellulosemirrors its previous extensive dilation. Just as dilationhas been explained by the formation of bulky phos-phate linkages. its reversal is consistent with theirbreakdown, due to their attaining the limit of thermalstability at about 450C [ 19,23,24]. Since the phos-phate bridges are considered to be more highlyconcentrated in the altered cellulose, it is logical thattheir decomposition would exert the greatest effecton this structure, Relatedly. we reported earlier thatFTIR analysis showed a reduction in the intensity ofbands attributed to phosphate esters above 450 C.There is also a dramatic increase in the aromaticcluster size above 450C (as estimated by C NMR).indicative of a substantial structural rearrangementthat would require a reduction in crosslink density[2]. These findings are in close agreement with theresults of research on flame retardance [23,24]. Thepeak in hydrogen evolution at 550C is an expectedconsequence of aromatic condensation. The corrc-sponding decrease in porosity is also consistent withthe occurrence of major structural reordering. whcrcthe increased size and alignment of clusters wouldresult in a more densely packed (and possibly moreanisotropic) structure. Over this same range of tem-perature, the elimination of residual aliphatic andoxygen-containing groups is consistent with theobserved secondary evolution of CH, and the disap-pearance of infrared absorption bands due to ketonesand esters.

4. CONCLUSIONSPhosphoric acid appears to function both as an

acid catalyst to promote bond cleavage reactions andthe formation of crosslinks via processes such ascyclization, and condensation, and to combine withorganic species to form phosphate and polyphosphatcbridges that connect and crosslink biopolymer frag-ments. The addition or insertion of phosphate groupsdrives a process of dilation that, after removal of theacid, leaves the matrix in an expanded state with anaccessible pore structure. It is considered that activa-tion of the amorphous polymers produces mostlymicropores, while activation of crystalline celluloseproduces a mixture of pore sizes. The differentresponse of crystalline cellulose is attributed to amuch greater potential for structural expansion than

IS possible with the amorphous polymers due, amongother factors, to its higher density and its chemicalstructure that allows for a more extensive degree ofcombination with phosphoric acid, and hence bulk-ing of the cell walls. The pore size distributionobtained from crystalline cellulose can be altered byincreasing the HTT and/or the ratio of acid toprecursor such that, eventually, the structure is domi-nantly mesoporous. At temperatures above 450C asecondary contraction of the structure occurs whenthe phosphate linkages become thermally unstable.The reduction in crosslink density allows the growthand alignment of polyaromatic clusters, producing amore densely packed and less porous structure.

I

2

i

33

6

7xY

IO.I I

I?.

Ii.14.

IS.16.

17.

IX.

REFERENCESJagtoycn, M. and Dcrbyshirc, F.. Some considcrattonaof the origins of porosity in carbons from chemicallyactivated wood, Carbon, 1989, 27, 191.Solum, M. S., Pugmire, R. J., Jagtoycn, M. andDerbyshire, F.. Evolution of carbon structure in chemi-cally activated wood, Curhon, 1995, 33(9). 1247.Badger, P. C. and Stephenson. C. D., An overview 01wood energy in the southeastern United States, in Pro-cwdings of the International Energy AgencylBioenerg ,Agreement Integrated Harvesting (TASK VI, A&vi/)2). Workshop, New Orleans, LA, May 29 June I, 1989,published by Mississippi State University Departmentof Forestry, Starkville, MS, 1989.Wood Clremisrrq, ed. L. E. Wise. Reinhold PublishingCorporation, New York, 1944.Tolles. E. D., Dimitri, M. S. and Matthews, C. C..Emission control device, US Patent No. 5,238,470.August 1993.Brunauer, S., Emmett, R. H. and feller, E., Adsorptionof gases in multimolecular layers. /. Am. Chem. Sot..1038, 60, 309.Dubinin, M. M.. Zavcrina, t. D. and Raduskevich, L.V.. Zh. Fiz. Khimii, 1947, 1351.Barrett, E. P., Joyner, L. G. and Halenda. P. ll., J. An7.c lw?l. sot. 1951. 73, 373.Yan, Z. Q., Preparation of high activity, high densityactivated carbon, US Patent No. 5,250,491, 5 October1993.Van Krevelcn, D. W., in Coul. Vol. V. Elsevier, Amster-dam. 1981, p. 113.Shafizadeh, F. and Chin, P. S., in Wood 72chnology.Chc,micul Aspects, ed. I. S. Goldstein. ACS symposiumseries, 43(5), 1977, p. 57.Jagtoyen. M.. Derbyshire, I:., Rimmer. S. andRathbone, R., Relationship between reflectance andstructure of high surface arca carbons, Fuel, 1995.74(4). 610.Dtmitri, M. S.. Preparation for high activity, high dcn-city carbon, US Patent No. 5.304,527, April 1994.Fengcl, D. and Wegner, G., Wood Chemislry, (iltru-slructure and Reuclions. Walter de Gruyter, New York,1984, p. 236.Browning, B. L.. The Chemisrry of Wood. Robert E.Krieger Publishing Company, Huntington, NY, 1975.Lai, Y. Z., in Wood and Cellulosic Chemistry, Vol. IO.cd. D. N. S. Hon and N. Shirashi. Marcel Dekker, NewYork, 1991, p. 455.tlindersin, R. R. and Witschard, G., in Flume Retur-c mc~~ f Polymeric, Materials, Vol. 4( 1 , cd. H. Kurylaand A. J. Papa. Marcel Dekker. New York, 1979,pp. I 48.Hinderschinn, R. R. and Witschard, G., lntumcscence


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Activated carbons from yellow poplar and white oak 1097and char, in Flame Retardancy of Polym eric M aterials,Vol. 4, ed. W. C. Kuryla and A. J. Papa. Marcel Dekker,New York, 1979, pp. 67-78.19. Barker, R. H. and Hendrix, J. E., Cotton and othercellulosic polymers, in Flame Retardancy of PolymericM aterials, Vol. 5, ed. W. C. Kuryla and A. J. Papa.Marcel Dekker, New York, 1979, pp. 35565.20. Rowell, R. M., in Wood and Cell ulosic Chemistr y,Vol. 15, ed. D. N. S. Hon and N. Shirashi. MarcelDekker, New York, 1991, p. 703.21. Katsuura, K. and Inagaki, N., Thermal degradationof phsophorus-containing polymers. VIII Relationbetween the thermal reaction of lame-retardant celluloseand its flammability, Text i le Res. J., February, 1975,103.22. Lyons, J. W., Mechanism of fire retardation withphosphorus compounds: some speculation, J. Fire andFlammability, 1970, 1, 302.

23. Bourbrigot, S., Le Bras, M., Delobel, R., Breant, P.and Tremillon, J. M., Carbonization mechanisms result-ing from intumescence-part II. Association with anethylene terpolymer and the ammonium polyphosphate-

pentaerythriol fire retardant system, Carbon, 1995,33 3), 283.

24. Delobel, R., Le Bras, M. and Ouassou, N., Fire retar-dance of polypropylene by diammonium pyrophos-phate-pentaerythriol: spectroscopic characterization ofprotective coatings, Polym. Degradation and Stabil i ty,1990, 30,41.

25. Van Wazer, J. R., in Phosphoric Acid and Phosphates,Encyclopedia of Chemical Technology, Vol. 15. JohnWiley and Sons, New York, 1969, p. 273.26. Wood Chemistry, ed. L. E. Wise. Reinhold PublishingCorporation, New York, 1944, p. 500.27. Cameron, A. and MacDowall, J. D., The pore structureof wood based activated carbons, in Pro ceedings,RILEM jCNR Internati onal Symposium. Milan, Italy,26629 April 1983, p. 251.28. Pandey, S. N. and Nair, P., Effect of phosphoric acidtreatment on physical and chemical properties of cottonfiber, Text i l e Res. J., December, 1974, 927.29. Porter, B. L. and Rollins, M. L., Changes in porosityof treated lint cotton fibers. I. Purification and swellingtreatments, J. Appl. Polym. Sci., 1972, 16, 217.

activated carbons from yellow poplar and

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