Evolution of carbon structure in chemically activated wood

Carbon Vol. 33, No. 9, pp. 1247-1254.1995 Copyright 0 1995ElsevierScienceLtd Printed in Great Britain.All rights reserved 0008-6223/95$9.50+ 0.00

Pergamon

0008~6223( 95)00067-4

EVOLUTION

OF CARBON STRUCTURE ACTIVATED WOOD

IN CHEMICALLY

M. S. SOLUM,’ R. J. PUGMIRE,~ M. JAGTOYEN~ and F. DERBYSHIRE~ ‘Departments of Chemistry and %hemical and Fuels Engineering, University of Utah, Salt Lake City, UT 84112, U.S.A. %enter for Applied Energy Research, University of Kentucky, Lexington, KY 4051 l-8433, U.S.A. (Received 15 August 1994; accepted in revised form 2 March 1995) Abstract-‘% NMR and FTIR analyses have been employed to follow the evolution of chemical structure in relation to porosity development, as a function of heat treatment temperature (HTT), for activated carbons produced from white oak by phosphoric acid activation. The chemical changes effected by acid treatment at low HTT are: by 50°C there is significant alteration of the lignin structure; by 100°C a significant portion of the cellulose has reacted, with the formation of ketones and esters; the formation of phosphate esters becomes apparent around 150°C; crosslinking reactions are initiated below 15O”C, consistent with the higher carbon yield obtained in chemical activation; and generally there is an increase in aromaticity and loss of aliphatic, carboxyl, and carbonyl groups, The low temperature phenomena precede, and relate to, the development of porosity and structural dilation that commences around 250°C and attains a maximum between 350 and 450°C. Up to 450°C pore volume is found to correlate with crosslink density. Above 450°C there is a dimensional contraction and a reduction in porosity. Among the accompanying phenomena are: the elimination of cellulose phosphates and oxygen functionahties; and a dramatic increase in the estimated aromatic cluster size. The latter would require a reduction in crosslink density to facilitate cluster growth, and the resulting structural rearrangement and increased alignment of clusters would produce a more densely packed structure with reduced porosity. Key Words-Activated

carbon,

phosphoric

acid, porosity,

1. INTRODUCTION

The high adsorptive capacities and low cost of activated carbons have provided incentives for their large-scale manufacture for several decades. The same characteristics have led to a progressive diversification of their use in processes that require purification, separation, chemical recovery and catalysis. Many new applications can be envisaged if their adsorptive properties can be tailored to meet specific requirements. Appreciable differences exist in the adsorptive behaviors of activated carbons. It is known that the precursor structure, and the method and conditions employed for carbon synthesis, exert a strong, inlluence, and that the selection of these variables determines the pore structure and surface chemistry of the adsorptive surface. However, for the most part, only tenuous relationships have been established between measurable properties and adsorption kinetics or capacities. For example, correlations with BET surface area rarely prove successful. A reassessment of the relevant properties, and of the analytical techniques used to characterize activated carbons, could improve this situation. Our own approach has been to try to develop an understanding of the mechanisms of activated carbon synthesis and the origins and development of porosity, in the belief that this can provide fresh insights into the factors that affect adsorption. Further, we have focused on chemical activation, rather than thermal processing. There have been relatively few published studies in

mechanisms

this area and little information is publicly available. The use of chemical reagents allows another degree of freedom in the choice of process conditions, and the incorporation of the reagent into the carbon structure may advantageously alter the surface chemistry. Previously we have examined the activation of coals with phosphoric acid and potassium hydroxide[ l-31. Latterly, we have concentrated on the phosphoric acid activation of hardwoods. The reasons for this choice are that this is an important commercial process, that the research is germane to the improved utilization of biomass resources and wastes, and that the regular morphology of the wood structure allows dimensional and other changes to be readily followed. The processes that occur in the conversion of wood, either to a low porosity carbonized product by thermal treatment, or in the presence of phosphoric acid treatment to a highly porous (activated) carbon, directly concern the reactions and interactions of the different constituent biopolymers. These are, principally, cellulose, hemicellulose and lignin. The ultimate aims of this research are to attempt to understand the mechanisms and processes by which the phosphoric acid-promoted chemical and structural alterations of the biopolymers can lead to the development of an extensive pore structure. To this end, a more detailed fundamental study of the reactions of lignin and cellulose with phosphoric acid, and their contributions to the behavior of wood is the subject of on-going studies. In this context, the

1247

1248

M.

SOLUM

results presented here, and in the previous publication[4], allow some discussion of possible reaction mechanisms. As previously reported[4], it seems that the increase in carbon yield which occurs due to phosphoric acid activation can be attributed primarily to the increased retention of modified carbohydrates, although our own studies and other published information have shown that lignin also contributes to the pore structure. Activated carbons with similar and high adsorptive capacities have been synthesized from both lignin and cellulose by chemical activation with ZnCl, [ 51. Jagtoyen and A recent publication by Derbyshire[4] describes the H,PO, activation of white oak (QLWCUSalba) in terms of the changes in chemical composition, and morphology, in relation to porosity development. In order to further understand the significance of these findings, we have investigated the accompanying changes in chemical structure by analyzing the same series of carbon samples by i3C NMR and FTIR. In this paper, the results of these analytical studies are used to describe the evolution of chemical structure in relation to porosity development for carbons produced by heat treatment in the presence and absence of phosphoric acid.

2. EXPERIMENTAL

2.1 Synthesis Sections of white oak (Quercus alba) (about 1.1 g) were soaked in a 30% phosphoric acid solution for 1 hour at room temperature to allow penetration of the structure: the weight ratio of H,PO, to wood on an as-received basis was 1.45. The mixture was heated for 2 hours to 170°C in a stainless steel reactor, under flowing nitrogen at atmospheric pressure, held for 30 minutes, and then taken to a final heat treatment temperature (HTT) between 200 and 650°C and held for 1 hour. For experiments at HTT 50, 100 and 150°C the mixture was heated directly to temperature and held for 3 hours. The solid products were leached with distilled water to pH=6 and vacuum dried at 110°C before further characterization. For comparison, thermally treated samples were prepared by following the same heat-treatment procedure, but without phosphoric acid addition. 2.2

Characterization

The leached and dried, heat treated solids were routinely analyzed for elemental composition, ash and moisture contents. The hydrogen and carbon contents were determined by a Leco CHN-600 Elemental analyzer. Oxygen content was determined directly by a Perkin Elmer 240 Elemental Analyzer. Ash and moisture contents were determined using a DuPont TGA 2950 thermogravimetric analyzer. Phosphorus content was determined by Plasma Emission Spectroscopy or calorimetric determination by the Molybdenum Blue Method.

et d.

Information on the carbon pore structure was derived from nitrogen adsorption isotherms obtained at 77 K on a Coulter Omnisorb 1OOCX apparatus. Specific surface areas were obtained using the BET equation. Mesopore surface areas were obtained using the CI,method [ 61, standard isotherm data were taken from Rodriguez-Reinoso et nl.[7]. The micropore volume W, was determined using the DubininRaduskevich equation[8]. Fourier transform infra-red spectroscopy (FTIR) spectra were acquired using a Nicolet 20 SX spectrometer at 4 cm-’ resolution. Pressed KBr pellets at a sample/KBr weight ratio of 1: 200 were scanned and then normalized to 0.5 mg. Spectral subtractions were conducted on an equivalent weight basis. All 13C NMR spectra were obtained on a Chemagnetics CMX-100 solid state spectrometer using a 7.5 mm pencil rotor system with a carbon frequency of 25.152 MHz. All pulse sequences included background suppression during crosspolarization[9]. The spinning speed was regulated at 4100+ 2 Hz. The decoupling power was approximately 54 kHz. For the higher temperature samples (550 and 65O’C with H,PO, activation and the 650 thermal only sample) the proton 90” pulse length increased from a range of 4-5 ~LSin the lower temperature samples to 7-8 vs. This increase in pulse length is caused by a lowering of the probe Q due to sample conductivity even when the probe could be impedance matched. The variable contact time (VCT) experiments used 21 contact time values ranging from 5 ps to 25 ms. The VCT data was fit separately for the aromatic and aliphatic regions of the spectra and the results used to calculate the aromaticity as has been previously described [ 10,111. Several different models (as described in Ref. [ 111) were required to fit the VCT data depending on the number of time constants associated with magnetization buildup (Ten) and decay parameters (T$ for any particular data set. The dipolar dephasing experiments were run with 23 delay values ranging from 2 to 200 p.s. For the lower temperature samples a 2 ms contact time was used but for the 350 and 450°C acid treated and 650°C thermal samples a 10 ms contact time was used. For the 550 and 650°C acid treated samples a 20 ms contact time was used to achieve optimum crosspolarization results. These data were fit to the standard model[ lo] and used for the aromatic region only to determine the fraction of aromatic carbons that were protonated. Approximately 3000 scans were acquired for each spectrum in both VCT and DD experiments. An additional spectrum of 10000 scans with a 2 ms contact time was taken on each sample from which the sub aromatic and aliphatic integrals were taken. For the three conductive samples described above, no sub integrals were taken due to excessive line broadening from very fast relaxing spins which would obscure any small amount of aliphatic material still present in the sample and also make aromatic

Evolution

of carbon

structure

sub integrals inaccurate, Either of two sets of integration regions were used depending on whether there were still unreacted cellulose peaks in the spectra. If the cellulose peak at 105 ppm was visible, the aromatic integration region was taken from 110 to 240 ppm. This neglects the slight overlap of lignin aromatic resonances with the anomeric carbon peak of cellulose[ 121. In the absence of unreacted cellulose spectral lines the standard aromatic integration range of 90-240 ppm was used. If the 105 ppm cellulose peak was visible, it was assumed that only single ring aromatic carbons were present in the sample, and hence, an absence of bridgehead carbons. Using the data from the above experiments, elemental analysis and the cluster size model[ lo], twelve structural and eight lattice parameters were estimated. This procedure has been described in great detail elsewhere[10,11,13]. The parameters found particularly useful for studying the changes occurring in these samples are: the aromaticity f,, the fraction of total carbons that are sp’; C, the average number of aromatic carbons in an aromatic cluster; B.L., the number of bridges between aromatic clusters (also included in this parameter are loops, i.e. hydroaromatic carbons that cannot be distinguished from bridges). An upper limit for the estimated average cluster size of the three conductive samples was based only upon the dipolar dephasing data. 3. RESULTS

3.1 Porosity

AND DISCUSSION

development

in acid-treated

in chemically activated wood

1249

volume begins to decrease. The mesopore volume development is also significant and follows a similar pattern, although it is shifted to a higher temperature. Mesoporosity evolves slowly up to about 35O”C, then increases and passes through a sharp maximum of over 0.55 cc/g at 500°C. In comparison, thermally treated samples possess negligible porosity. Treatment with phosphoric acid was found to promote an expansion of the wood structure over the temperature range 250-45O”C, corresponding directly to the development of porosity and the creation of an extensive surface area. At higher HTT, a secondary dimensional contraction paralleled the reduction in porosity. A further effect of the phosphoric acid was to increase the carbon yield. It was proposed that an important function of the acid is to promote bond cleavage in the biopolymers at low temperatures. These reactions are followed by extensive crosslinking that can account for the high carbon yields, due to the bonding of otherwise volatile materials into the carbon product. Evidence from other work[ 147 suggests that the increased yield is due primarily to the retention of cellulose and hemicellulose, although biopolymers such as lignin can contribute similarly to porosity. No explanation has yet been advanced to account for the dilation of the wood structure. It is against this background that the present work was conducted, with the aims of verifying some of these hypotheses, and to attempt to elucidate the reasons for the observed phenomena.

white

oak

3.2 Chemical composition

As described in an earlier publication[4], phosphoric acid-treated carbons from white oak begin to develop porosity at low heat treatment temperatures (HTT), Fig. 1. Micropore volume development starts at about 25O”C, and increases rapidly with HTT to a broad maximum of almost 0.7ml/g at 350°C: the maximum BET surface area is about 1800m’ g-l. At temperatures above about 350°C the micropore

The reaction of wood and coals with phosphoric acid promotes chemical modifications at lower temperatures than thermal treatment alone, the net effect of the principal reactions being dehydration [4,14]. Elemental analyses also show that between 0.2 and 0.9% wt% phosphorus is retained after heat treatment and extensive leaching. The phosphorus may be physically bound, or some may be chemically incorporated into the carbon. Hitherto unpublished data show the change in the volatile matter content of the white oak carbons with heat treatment temperature, Fig. 2. (This is the vola-

O.*1

300

"0

400

600

Heat Treatment Temperature Fig. 1. Porosity

development in H,PO, from white oak.

activated

800

(“C) carbons

01 0

600 200 400 Heat Treatment Temperature (“C)

IO

Fig. 2. Change in volatile matter content with HTT for carbons from white oak by acid and thermal treatment.

M.

1250

SOLUM

as shown by: significant reductions in the C-O-C and C-OH groups that absorb at 1069, 1118 and 1273 cm-‘; loss of aliphaticity, as shown by the reduction in intensity of the C-H stretching bands that are observed in the parent wood at 2965, 2924 and 2873 cm-‘; increasing aromaticity that becomes apparent by the appearance of absorption bands in the out of plane bending region at 753, 815 and 865 cm ’ [ 15,161. Equivalent reactions do not occur in the thermally treated samples until HTT=250 C. Phosphoric acid treatment also affects the C-O absorption bands. There is a very intense band at 17 14 cm _ ’ in the white oak precursor, due to the C=O stretch in esters and carboxylic acids. After heat treatment to 150°C this carbonyl band is reduced in intensity, but a new one is introduced at 1694 cm-‘. This band is attributed to aliphatic ketones: the possibility of aldehydes can be eliminated since there is no peak at 2700 cm-‘[ 171. The ketone and ester groups are clearly present up to a heat treatment temperature of 45O’C, although the intensities of the bands decrease slightly with increasing HTT. In the thermally treated samples, there is no change in the intensity of the carbonyl band until 25O’C. At higher temperatures the intensity of the band is weaker than in the acid treated samples, suggesting the decomposition of carbonyl groups without the formation of new ones. Examination of the FTIR data further suggests that reaction with phosphoric acid may lead to the formation of cellulose phosphate esters, indicated by the appearance of a band at 1203 cm- ’ after heat treatment to 150’ C. Earlier work on the phosphoric

tile loss, calculated on a moisture-free basis, that is produced by heating the volatiles in an inert atmosphere to 950°C in a standard test.) With acidtreatment there is an apparent initial increase in volatile matter content at HTT 50°C (when there is minimal yield loss), which may represent a loosening of the structure due to the cleavage of connecting bonds. Thereafter, there is a rapid decrease in volatile matter content with increasing temperature up to 200-25O”C, followed then by a more gradual reduction. The apparent increase in volatile matter content at 500°C with H,PO, is considered to be a valid result. It corresponds to the secondary contraction phase, and it may be generally indicative of the structural rearrangement that must occur in this temperature region. An implication is that, at around SOO”C, bonds that were stable at lower temperatures are broken, producing components that could become volatile under the conditions used to make the volatile matter determination. For thermal treatment, little change takes place until about 250-C. when there is a steep fall in volatile matter content up to 350-450-C. The differences between the two curves in the Figure, and the temperature ranges over which the changes occur, are characteristic of the differences between acid and thermal treatments: the latter typically lags the former by 200-250 ‘C. 3.3 FTIR The FTlR spectra samples produced at in Fig. 3(a) and (b) 15O”C, the influence

et 01.

of H,PO, and thermally treated different temperatures are shown respectively. At HTT as low as of the acid is already apparent,

Thermal

H3PO4 C-OH c-p-c

650%

650°C 550%

550%

250°C

White Oak 4000 3590 3190 2770 2360 lh150 1540 1130

720

310

Wavenumber

Fig. 3. FTIR

spectra

for carbons

1

4000 3600 3200 2800 2400 2000 1600 1200 I.,-..__ .-L_. Y"a"enumDer

from white oak at different

HTT.

I

800

I

400

Evolution

of carbon

structure

in chemically

acid activation of coals[2] showed the introduction of a peak at 1190cm-‘, which was also attributed to phosphates. Research on the use of phosphoric acid as a Aame retardant for cellulose has shown that phosphorus compounds can form ester linkages with cellulose side chains at temperatures below 2OO”C[18], and the FTIR adsorption band of the phosphoryl group in these cellulose phosphates was found to be between 1200 and 1230cm-‘[18]. The P-O-C stretch is generally around 1200 cm-‘[ 191. The generation of cellulose phosphate esters is in agreement with the retention of phosphorus shown by elemental analysis [ 201. The cellulose phosphate peak is present up to 650°C but is significantly reduced in intensity above 450°C.

activated

wood

1251

lignin structures resonate from 160-100 ppm. An overlap of the aromatic region arises from the anomeric carbons in cellulose and hemicellulose at 106 and 103 ppm respectively. There are also unassigned resonances due to aliphatic moieties in the lignin structures that are found in the 15-90 ppm range. Spectra from the phosphoric acid treated and thermally treated samples are shown in Fig. S(a) and (b) respectively for a range of HTT. Once again, it can be clearly seen that phosphoric acid treatment promotes chemical changes at low temperatures. For thermally treated samples, the spectrum is identical to that of the parent up to 150°C whereas at this temperature most of the wood carbohydrates (cellulose and hemicellulose) have reacted in the acidtreated sample. At 250°C some of the carbohydrates have reacted to form ketone groups observed at about 208 ppm and other new aromatic (or alkenic) and aliphatic resonances: these types of structural changes have also been noted from FTIR studies and have been reported by others[18]. It is not until 350°C that the carbohydrate peaks have completely disappeared. As the heat treatment temperature is increased from 350 to 550°C there is loss of aliphatic material in the O-60 ppm range, the aromaticity increases, and the phenolic shoulder on the aromatic band at about 150 ppm decreases. At 650°C the sample conductivity increases as the NMR probe begins to detune, and a low-intensity, broad aromatic resonance appears which may obscure any residual aliphatic signals if still present. In the acid-treated samples, even at 50°C a slight loss of carboxyl and methyl groups from the hemicellulose and some change in the chromatic lignin peaks can be seen. This coincides with earlier observations of the partial digestion of lignin by phosphoric acid

3.4 NMR A “C CPMAS spectrum of the white oak precursor is shown in Fig. 4. The principal constituent biopolymers are cellulose, hemicellulose and lignin, whose resonances have been previously assigned [21-231. The peaks identified by numbers represent the cellulose portion of the structure with the crystalline regions having a C-4 chemical shift of 89 ppm and a C-6 chemical shift of 66 ppm while the chemical shifts of amorphous material are 84 ppm and 63 ppm for C-4 and C-6 respectively. The resonances from C-2,3,5 in the 70-80 ppm range are not completely resolved or individually assigned. The resonances at 172 and 22 ppm are the carboxyl and methyl resonances respectively, from acetate groups on hemicellulose with other hemicellulose resonances overlapping the cellulose resonances. The peak at 56 ppm is from methoxy groups that can arise from syringyl and guaiacyl units of lignin and methoxy groups on hemicellulose. The aromatic carbons from

2.3.5

:*I c, OR

R

i

mm 1

“‘I’

250

Fig. 4. “C

CPMAS

I” 200

spectrum

I 150

’

“1

1 100

’

“1

11 50

of white oak. (Contact time 2 ms, 1 s pulse delay, cellulose resonances numbered.)

11

1, -0

no line broadening,

1252 (4

M.

SOLUM et al.

WHITE OAK CHARS HTT & ACID

04

WHITE OAK CHARS HTT ONLY

215”C 250”C

--_I_

100” c 50” c PARENT

7’ 450

r-1

_iT-’

--V-TT -50

ZOO

-300

~~~~~-71

I-

450

1

7_ -300

-50

200

PPM from TMS

PPM from TMS

Fig. 5. 13C CPMAS spectra of white oak carbons: (a) H,PO, activated carbons, (b) thermally treated samples (contact time 2 ms, pulse delay 1 s). at low temperatures[4]. At 100°C a significant portion of the cellulose has reacted, with the formation of a new peak at 203 ppm due to ketone groups (possibly by the formation of levoglucosenone [ 141) and new aromatic(or alkenic) and aliphatic resonances. In the temperature range up to 15O”C, the ratio of the crystalline to amorphous cellulose increases both for the 89/84 ppm region and the 66/63 ppm region, indicating that the amorphous cellulose which is mixed with hemicellulose surrounding the crystalline cellulose region of the cellulose microfibril[24] reacts first with acid treatment. In the thermally treated samples, all the carbohydrates appear to react at approximately the same rate. The acetate groups in hemicellulose that reacted early in the acid treated samples are still seen at 250°C in the thermally treated samples. At 150°C all the cellulose resonances have disappeared in the acid treated samples, and the ketone peak has become very intense. As the HTT is increased, there is a general increase in aromaticity and loss of aliphatic, carboxyl and carbonyl groups. At 550°C and above, the samples became conductive and exhibit the broad aromatic resonance that could obscure any residual aliphatic material. Phosphoric acid significantly accelerates the development of aromaticity on heat treatment, Fig. 6. A sharp increase in carbon aromaticity occurs above 5O”C, while corresponding changes in the thermally treated carbons do not take place until about 200°C higher. In both series the aromaticity approaches 1.0 at heat treatment temperatures above 45o”C, although the FTIR data suggest that only the acid treated product at 650°C is completely free of ali-

0.2’ 0

I 200

I 400

600

800

Heat Treatment Temperature(“C) Fig. 6. Aromaticity development in carbons

from white oak.

phatic C-H stretching bands. There are two regions where the slope of the upper curve in Fig. 6 appears to be reduced: between 150-250°C and 350-450°C. These apparent discontinuities in the curve may be indicative of other significant changes that take place over these temperature ranges. The extent of crosslinking, as measured by the number of bridges and loops per cluster estimated from the 13C NMR data, increases significantly at low temperatures in the H,PO, treated samples, whereas in the thermally treated samples there is little change in the extent of crosslinking below about 250°C (Fig. 7). Thereafter, the two curves are essentially parallel, with the number of crosslinks being significantly higher in the acid treated samples at any given HTT. Figure 8 shows Ty’, versus HTT for the acid and thermally treated samples. For both sample sets Tyo is essentially constant up to 250°C. Above 250°C values for the thermal samples decrease to 4.3 ms at

Evolution

of carbon

7 ,

structure

in chemically

activated

wood

1253 1

0.8

1

0.7

350

A450

g 0.6 v E 0.5

275 3 2 g 0 2 I

I

0

I

I

0.4 0.3 0.2

250 A /

I

200 300 400 500 100 Heat Treatment Temperature (“C)

600

Fig. 7. Extent of crosslinking measured by the number of bridges and loops per cluster in carbons from white oak.

3

4 Bridges

Fig. 9. Correlation link density

0

200

400

600

1

I

5

6

7

and Loops per Cluster

between micropore volume and cross(correlation coefficient R,= 0.93).

800

Heat Treatment Temperature 1°C)

Fig. 8. Tyo vs HTT for H,PO, and thermally oak.

350°C as free radicals

treated

white

are created[25]. As charring proceeds and the electrons on growing aromatic structures become more “mobile” due to strong electron exchange interactions [ 261 the Ty,,values increase with a trend similar to pyrolyzed coals[27]. In the instance of the acid-treated samples the usual decrease found in other pyrolysis sets[27] is not observed. The dip observed in T& at 450°C indicates that a significant change has occurred in the evolving structure which may arise from the onset of thermally induced bond breaking of phosphorus structures not found in normal carbonaceous materials. It is interesting to note that this decrease in T$ occurs just before the increase in volatile matter content of the acid treated material. In trying to establish some connection between porosity development and the extent of crosslinking in the phosphoric acid treated samples, we have noted that there is a reasonable correlation between micropore volume and the number of bridges and loops per cluster in the range 200-350°C (Fig. 9). The correlation does not apply at 450°C where the porosity begins to significantly decrease. The correlation coefficient is 0.93 if the 450°C data point is excluded. A similar, but less well-defined relationship is also found for the mesopore volume. At temperatures above 450 and 550°C in the

” 0

”

”

100 200 300 400 500 Heat Treatment Temperature

’ 600

(

’

700

(“C)

Fig, 10. Development of aromatic cluster size and micropore volume with HTT in carbons from white oak by thermal and H,PO, treatment.

thermal and acid cases, there are no data to indicate whether the extent of crosslinking increases further: the 13C NMR spectra are lacking in sufficient definition to allow accurate calculations. However, dramatic increases in the estimated aromatic cluster size are observed (Fig. lo), especially for the acid-treated product. This suggests that there must be a reduction in crosslink density to facilitate this growth, and the accompanying structural rearrangement. These changes are accompanied by a decrease in porosity for the acid-treated carbons, which is also consistent with the occurrence of major structural reordering: the increased alignment of clusters would result in a more densely packed structure and the elimination of microvoids. 4. SYNOPSIS

The thermal treatment of cellulose (and hemicellulose) leads to dehydration reactions, and elimination reactions that result in the liberation of products such as water, furan derivatives and levoglucosan [ 141. There is an accompanying increase in aro-

M. SOLUMetd

1254

maticity, and the onset of crosslinking reactions which become significant above 25O”C, as demonstrated here by the NMR analyses. The crosslinks could be present as ether linkages or carbon-carbon bonds. The effect of phosphoric acid treatment is to significantly reduce the temperature where these reactions take place by 200-250°C. A reflection of this initial bond cleavage is the formation of ketones, as seen from the FTIR and NMR analysis and also by other work[ 141. A significant outcome is that crosslinking now begins below 150°C and in a temperature regime where many of the products of dehydration and elimination reactions are insufficiently volatile to have been driven off. Their retention and incorporation into the final solid structure, through crosslinking reactions, can explain the increased carbon yield. This possibility, which was suggested earlier, is now confirmed by the NMR data. Up to 15O”C, the average number of carbon atoms per cluster increases from six to ten, indicative of an increase from one to two rings. Simultaneously, the B.L. value increases from 1.7 to 3.4 (Fig. 7) indicating that extensive crosslinking has taken place. The promotion of these reactions could be due to the fact that phosphoric acid is a strong dehydrating agent, and being a strong acid it can also induce bond cleavage and crosslinking reactions by ionic mechanisms. The above proposals can begin to account for many of the observed results. However, so far, there are no strong indications why the carbon structure, after an initial sharp contraction, should begin to expand. This phenomenon, which appears to be intrinsically related to the development of porosity, is the key to understanding the process of activation. It is possible that initial bond cleavage reactions allow a relaxation of the biopolymer chains. Any associated expansion could be stabilized by crosslinking reactions that occur around 15O”C, thus providing a larger space-filling conformation than was originally present. However, the chemistry peculiar to phosphoric acid is clearly important and may provide the most significant role in structural dilation. These aspects will be addressed in continuing research. Finally, the loss of porosity at high HTT has now been shown to occur in parallel with a sharp increase in aromatic cluster size. It also corresponds to the elimination of oxygen functionalities present as ketones and esters. At these temperatures it is possible that thermal cleavage of (oxygen-containing) crossfinks allows an increase in the condensation and reorientation of clusters, thus eliminating some of the open

porosity.

The

data

presented

in this

paper

provide

useful

for understanding the processes that occur in activation of white oak. The solid state r3C NMR data has been especially helpful in correlating changes in the evolving chemical structures/properties with the degree of crosslinking, development of aromatic structure and increase in aromatic cluster size. information

Acknowledgements~The NMR studies were supported by the National Science Foundation through the Advanced Combustion Engineering Research Center. The activated studies carbon synthesis were supported by The Commonwealth of Kentucky, through the University of Kentucky. Center for Applied Energy Research.

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