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IR studies of carbons—IV The vacuum pyrolysis of oxidized cellulose and the characterization of the chars†
WX-6223185 $3.WC.C4l 0 1985Pergamon Press Ltd.
CarbonVol.23,No.3,301-310. 1985 Prmted ,ntheU.S.A.
IR STUDIES OF CARBONS-IV THE VACUUM PYROLYSIS OF OXIDIZED CELLULOSE AND THE CHARACTERIZATION OF THE CHARS? CLALIDIOMORTERRA and M. J. D. Low* Department of Chemistry, New York University, New York, NY 10003, U.S.A. (Received
20 October
1983; in
revisedform
7 February
1984)
Abstract-The pyrolysis of an NO,-oxydized cellulose (NOC) in vacuum from 22 to 700°C was followed by IR photothermal beam deflection spectroscopy. Series of spectra were recorded at various stages of pyrolysis and compared with and contrasted to similar data previously obtained with cellulose. Also, NOC-derived chars were oxidized, and the effects of treating NOC and NOC char with KOH were observed. The detailed IR spectroscopic data indicate that NOC degrades more easily than cellulose and yields chars that possess different functional erouns than cellulose chars, up to charring temperatures of near 500°C. Above that temperature the chars have-similar chemical and physicai properties.-
1. INTRODUCTION
The vacuum pyrolysis of pure cellulose was recently studied with the aim of contributing to the understanding of the charring mechanism of long-chain polysaccaridic materials as well as to the nature of some of the IRactive species often observed on carbonaceous materials of various nature and origin[l]. The IR spectroscopic data indicated that the earliest stages of cellulose carbonization are mainly characterized by the oxidative dehydration of the polysaccaridic network, leading to the formation of carbonylic structures, most likely of a ketone-like nature, characterized by a strong and relatively sharp band at - 1700 cm-‘. Bands at higher wavenumbers could not be observed during the pyrolysis if a vacuum better than lo-’ was constantly maintained, whereas various oxidized species absorbing above 1700 cm ’ could be produced during the controlled oxidation of low temperature chars[2]. The present study deals with the vacuum pyrolysis of an oxidized cellulose and is intended to be a complementary contribution to the earlier study. The vacuum pyrolysis of oxidized cellulose at various temperatures up to 400°C was previously investigated from chemical and kinetic points of view[3] and was found to be rather different from that of a nonoxidized cellulose in that a very fast onset of the degradation phenomenon was found to occur at the lowest charring temperature. Analogous and complementary observations were made in the present study.
2. EXPERIMENTAL
IR spectra were recorded using photothermal deflection spectroscopy (PBDS); the spectroscopic techniques and the experimental procedures for the vacuum production of carbons and their characterization have been --~ +Part III, Ref.[Z]. *Author to whom correspondence should be addressed.
described in detail elsewhere[4]. The starting material for this work was an NO?-oxidized cellulose manufactured for surgical use by Johnson and Johnson and commercially termed Surgicel[S]. It is a sterile, high purity product in which virtually all of the primary alcohol functionality of the C, position, i.e. on the carbon atom off the pyranose ring, has been selectively converted into a carboxyl group, without any appreciable retention of nitrogen oxide(s) in the cellulosic structure. All samples were washed with distilled water before undergoing any treatment in the IR cell. When certain samples were oxidized, pure 0, was introduced to the cell to a final pressure of 150 Ton: at the stated temperature, thus simulating with pure O2 the oxidizing conditions of the atmosphere. IR spectra were obtained using 1000 scans[4] at 8 cm-’ resolution. As the PBDS instrument functions in a single-beam mode, all spectra shown except those of Fig. 2 were “compensated” and/or “corrected for background” by digital computation[4] so that pseudo-double-beam spectra resulted. The ordinate of a spectrum S which was compensated by ratioing against the spectrum So of a standard carbon or against pIatinum black is indicated by S/S,. The computation procedure also makes it possible to generate differential spectra which are useful in making spectral changes stand out clearly; such spectra are marked S/S,,, where S,, is the “background” spectrum as indicated in the figures and captions. To achieve brevity, especially in captions, descriptive abbreviations are needed. The untreated starting material is termed NOC. For chars produced by the vacuum pyrolysis of NOC at T%, the symbol NOCT is used. For chars produced by vacuum pyrolysis which are then treated with oxygen at T”C, the symbol is NOCT ox.T. These and similar obvious abbreviations are used to label spectra, e.g. S/S,, labelled [NOC270 ox.4OO]i]NOC270] is a differential spectrum S of NOC char produced by the vacuum pyrolysis at 270°C followed by oxidation at 400°C vs the spectrum S, of NOC char produced by vacuum pyrolysis at 270°C. The ordinate for most spectra are arbitrary and are displaced to avoid overlapping of traces.
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3.1 Some spectral features of NOC The IR spectra of a pure cellulose specimen and of NOC are shown in Fig. 1. Comparison shows that in the high frequency region (Fig. la) the OH band, which is strong and sharply centered at -3390-l with cellulose, is weaker, ill-defined, much broader and centered at lower wavenumber on NOC; the band extends on the low wavenumber side down to -2200 cm-‘, in agreement with the decreased number of alcoholic OH groups per pyranose unit and with the presence of a COOH group in the C, position, bound with strong hydrogen bonds and possibly assuming a five-membered ring structure. Also, the CH, stretching band at -2900 cm -I is drastically reduced in intensity, as expected and previously observed[6]. The low frequency region (Fig. lb) shows dramatic differences between NOC and cellulose which, on the basis of the literature[6], suggest that the reaction with NO, led to an extensive oxidation of the polysaccaridic network. In fact, the NOC spectrum shows that the strong and complex band system between 900 and 1200 cm-’ is strongly decreased and the band structure in the 13001500 cm-’ region is lost, as previously reported for highly oxidised celluloses[6]. It is difficult to determine if the oxidation proceeded at all beyond the average stage of so-called monocarboxycellulose and also introduced COOH functionalities at the CZand/or C, positions. Certainly the strong band centered at - 1750 cm-’ ascribable to the COOH functionality is a dominant feature of the NOC spectrum with a peak center intensity comparable to that of the strong band at - 1050 cm- ’ , as is sometimes observed in very highly oxidized celluloses of the socalled di- and tricarboxycellulos type[6]. The differential spectrum S,/S,, of Fig. l(c) shows that the carboxyl band of NOC, which may seem to be a single strong peak at 1750 cm-’ with a well resolved shoulder at -1600 cm-’ (Fig. 1b), is, on the contrary, made up of at least two components centered near 1760 and 1720 cm-‘, while there is no evidence for any component near 1600 cm-’ beyond the intensity level normally observed with the purest celluloses; this component is usually ascribed to traces of lignin-like structures. This indicates, on the one hand, that the carboxyl group population is heterogeneous in nature, and on the other hand excludes the occurrence of any appreciable amount of nitrate groups which would yield two strong bands of equal intensities near 1650 and 1300 cm- ‘[7].
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‘NO, GEL,.
RESULTS AND DISCUSSION
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cm” Fig. 1. (a) Spectra of NOC and cellulose. The spectrum S of NOC or of cellulose was compensated by ratioing vs the spectrum S, of a carbon standard. (b) Spectra of NOC and cellulose. The spectrum S of NOC or of cellulose was compensated by ratioing vs the spectrum Soof a carbon standard. (c) Differential spectra of NOC and cellulose. Differential spectra were pro. duced by ratiomg the spectrum S, of NOC vs the spectrum SCELL ot cellulose, and me versa.
3.2 General aspects of the vacuum pyrolysis of NOC The pyrolysis of NOC in vacua begins at relatively low temperatures and yields a carbonaceous residue; abundant gaseous decomposition products are released but very little tarry material is formed. The gaseous decomposition products of cellulose were analysed and described elsewhere[3]. A temperature as low as 110°C is sufficient to turn the initially pure white NOC into a homogeneously yellow-brownish mass, while at - 150°C the color of the residue is a verv dark brown. and at
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IR studies of carbons-IV 180°C, i e. at the temperature at which pure cellulose just begins to turn yellow, it is fully black to the eye and has all the appearance of a char. However, the appearence is misleading. Figure 2 shows a series of single-beam, uncompensated spectra. with the overall intensities of the spectra normalized to mat of a Pt black absorbed. (The negative excursions near 2300 cm-’ are due to atmospheric CO,; the “noise” in the 1800-1200 cm-’ region is caused by atmospheric water vapor; see Part 1141.) Each spectrum is the summation of a portion containing spectral structure (the latter is clearly brought out in the compensated spectra of Fig. 2) and a portion which is unstructured, i.e. a continuum absorption. The series of spectra shows that the structured portion of the spectrum begins to change even at very low temperatures, indicating that the NOC is undergoing significant chemical changes, but that the structureless portion of the spectrum, as monitored by the over-all photothermal response, does not increase appreciably until the temperature approaches 500°C. The increase of the continuum absorption parallels the progressive reduction of the electronic band gap of the organic semiconductor char, brought about by the increase of the average size of the polyaromatic domains formed during charring[l,S]. The IR “blackening” and growth of polyaromatic domains of NOC is thus the same as that of pure cellulose, see Fig. 7, Part II[ 11. It is worth noting that, despite the observation that at early stages of pyrolysis the absorption continuum is not intense, so that low tem~mture NOC chars are not highly absorbing in the IR range (although opaque in the Visible), such chars as well as NOC itself cannot conveniently be studied by IR transmission methods because the materials scatter so extensively that the transmission coefficient is virtually reduced to zero with most samples.
3.3 ~etui~s of the IR Spectra Fig. 3A shows the high frequency range of spectra of NOC samples charred in vucuo at temperatures up to -600°C. It is noted that, despite the copious release of water vapor during the early stages of carbonization[3], the beginning of the NOC pyrolysis is characterized by a strong intensification of the absorption due to H-bonded OH stretching. That band reaches its maximum intensity after a vacuum treatment at 180°C; the peak maximum becomes sharper and centered near 3400 cm --‘, i.e,, some 100 cm- ’ higher than with the starting material. At higher temperatures the band declines in intensity and becomes more diffuse, and eventually disappears at T Z 550°C. There are accompanying changes of the CH, absorption. Originally it appears sharp and centered at 2900 cm-‘, but then becomes broader on the high wavenumber side. At 180°C a new component of intensity comperable to the 2900 cm-’ one is resolved near 29.50 cm-‘, and at higher temperatures that high wavenumber component becomes predominant and eventually remains the only resolved peak in the aliphatic CH, stretching region. Evidence for unsaturated CH stretching bands is first clearly observed at 7’ 2 320°C either two bands being
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Fig. 2. NOC pyrolysis. The number by each spectrum indicates the pyrolysis temperature in “C. The top trace is the spectrum S, of Pt black used for compensating some spectra. A11spectra arc single-beam. uncom~nsated. The ordinates are displaced and no~alized to the Pt spectrum.
resolved at 3040 and 3100 cm- ‘, or just one broad band at -3060 cm-‘. Evidence for unsaturated CH stretching bands of aliphatic nature and for CH bands of aromatic nature at T = 6OO*C Figure 3b shows the spectral changes in the 2OtXl-550 cm- ’ range undergone by NOC samples charred at increasing temperatures up to 700°C. It is seen that up to a charring temperature of -250°C the most evident spectra1 changes are: (i) a gradual decrease of the broad and ill-defined band centered near 1070 cm-’ which monitors the progressive destruction of the polysaccaridic network; (ii) an increase of the band complex above 1500 cm-‘. For example, at 180°C the band at 1640 cm-’ is broad and no longer describable as a shoufder, whereas the prominent band at 1740 cm-’ is still centered at that frequency but is much broader on the high frequency side (shoulder at 1800 cm-‘). At 230°C the intensity ratio of the high wavenumber bands is further modified in the manner described. After charring at temperatures as high as 300°C the highest frequency components seem to dectine and only a major sharp band remains centered at 1700 cm-’ which also declines as the carbonization proceeds and eventually disappears at T Z 600°C. When a charring temperature of -350°C is reached, a band centered at 1600 cm- ’ becomes the prominent spectral feature in the 15002000 cm-’ range.
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M. J. D. Low The spectral changes outlined are quite complex and, to help figure out the mechanism of NOC carbonization, differential spectra of the materials at the various charring stages have been computed; some are shown in Fig. 4. It is observed that, beginning at the lowest charring temperatures, a fairly sharp and symmetrical band builds up at 1800 cm-‘, i.e. where a mere broadening of the existing bands could be observed in the spectra of Fig. 3(b). Also, a broad and complex band centered near 1580 cm-’ is formed. The two bands cannot be thought of as being partner modes of the same system because the former band grows up to a charring temperature of -250°C and then declines (see, in particular, the 2301180 and 270/230 differential plots 5 and 7 of Fig. 4), while the latter band keeps growing. Beginning at T 2 2OO”C, a relatively sharp component is resolvable at 1680 cm-’ (curves 4-8, Fig. 4) which seems to decline at T 2 400°C (curves 9 and 10, Fig. 4). The broad and complex band at 1580 cm-’ ~USI include a component at 1600 cm-’ growing in intensity as the carbonization proceeds up to the stage at which the surface oxidized layer declines. This band, which is typical of all carbonaceous materials and which monitors the formation of a polyaromatic network in a superficially oxidized char was discussed elsewhere[ 1,9], is seen in Fig. 3(b) to be virtually the sole remaining component in the 1500-1700 cm-’ region after charring at T 2 400°C. The 2301180 differential trace 5 shows that, when the 1600 cm-’ band is almost absent (in fact, very little increase of it would be expected in the small temperature range; it is possibly observable in trace 5 as a small shoulder at -1600 cm-‘) another component is resolved; it is broad and asymmetric on the low wavenumber side, centered near 1540 cm-’ and extends down to -1420 cm-‘. Other interesting features which were not so clearly seen in the spectra of Fig. 3 but are more readily observable in the differentials of Fig. 4 are: (i) a polysubstituted aromatic network system is being built up on charring NOC at temperatures as low as 150-18O”C, and can be clearly revealed by the out-of-plane deformation modes in the 700-1000 cm-’ region much before any evidence for the aromatic CH stretching bands is seen in the 3000 cm-’ region; (ii) it is difficult to say when a thermal decomposition of the NOC carboxyl groups actually begins, but certainly by the time a charring temperature as high as 420°C has been reached, the carboxy1 band at -1760 cm-’ is severely decreased in intensity as shown by traces 9 and 10 of Fig. 4. Charring temperatures between 300 and 410°C were previously reported to bring about an abundant release of COZ[3]; (iii) trace 10, Fig. 4 shows that at 420°C the broad band centered near 1540 cm-’ is also appreciably decreased. 3.4 An interpretation of the formation of NOC chars The creation of a strong and complex band system above 1500 cm-’ during the early stages of NOC pyrolysis indicates that, much as with cellulose, auto-oxidations dominate the initial phase of carbonization. The earlier onset of pyrolysis with NOC than with cellulose confirms the previous observation that “. the whole
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structures. It is thought that, as in the case of the chemical action of some oxidizing agents such as periodic acid upon cellulose, the cleavage of the C& bond is the dominant reaction path of the auto-oxidation process, leading to C2 and/or C, oxidized species. The 1800 cm ’ band, first observed as a shoulder to the - 1760 cm-’ carboxyl band, is thought to be one more component of the same complex band assigned to the heterogeneous carboxylic acid functionality, the relevant species being even more strongly affected by the electronic and inductive effects that also lead the other components to have rather high frequencies. The oxidative production of an increased amount of COOH groups is certainly consistent with the observed increase of the hydroxyl band as well as with Zhbankov’s observation[ 121 that the insertion into the cellulosic structure of a multiple carboxylic functionality produces a broader and very diffuse CO band in the 1650-1850 cm-’ region as well as a shift of the CH stretching band from 2900 up to 2940 cm“, a shift similar to the one reported above. A further and more definitive argument in favor of the proposed assignment comes from the downward shifts of the 1800 cm-’ band and of all other carbonyl bands above 1700 cm-’ after NOC was treated with alkali solutions. Figures 5 and 6 show the spectral changes produced
Fig. 4. NOC pyrolysis: differential spectra. Each differential spectrum S/S, is defined by the ratio at the right of the spectrum, e.g. spectrum 10 was obtained by ratioing the spectrum S of the char produced at 420°C (NOC420) against the spectrum S, of the char produced at 330°C (NOC330). cellulose ring structure is weakened by the oxidation of the primary alcohol[3] “. However, the over-all mechanism of the NOC auto-oxidation must be different from that of cellulose, for which an oxidative dehydration leading to mostly ketone-like structures was invokedl I]. In fact, a dehydration would not be at ail consistent with the increase of the hydroxyl band. Also, an oxidative attack on the secondary alcohol groups in the Cz and C, positions of the pyranose ring leading to either simple ketones possibly having an a-hydroxy configuration (single oxidation), or to tw-diketones (double oxidation), would not explain the early formation of a sharp and dominant band at 1800 cm.- ‘. In fact, isolated ketone CO groups do not absorb at such high frequencies{ IO,1 l]. This kind of functionality was associated with a band at -1700 cm’-’ observed with cellulose chars[ 11. The possible formation of a-diketones could also not explain the 1800 cm-’ band because carbonyls conjugated with other carbonyls are hardly different in frequency than comparable ketones without the CO-CO conjugation[ IO]. Such reasoning does not exclude the fo~ation of ketonic structures in any of the phases of NOC pyrolysis (and such structures must indeed be formed if a prominent ketone-like CO band at -1700 cm-’ is clearly resolved after charring at T 2 350°C). However, the 1800 cm ’ band must be assigned to other carbonyl
cm-’ Fig. 5. KtXI-treatment of NOC. Compensated or differential spectra. The ratios S is, of the spectra are as follows. (1) [NOC] / [Pt]. (2) [NOC,lst KOH treatment]/[Pt]. (3) [NOC,Sth KOH treatmentJ/[Pt]. (4) [NOC, 1st KOH treatment]/[NOC]. (5) [NOC,3rd KOH treatment]/[NOC]. (6) [NOC,Sth KOH treatment]i[NOC].
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cm” Fig. 6. KOH treatment of NOC230. Com~nsat~d or differential spectra. The ratios S/S, are as foIlows. (I) [NOC230]1 [Pt]. (2) [NOC230,5th treatment]l[Pt]. (3) [NOC230,2nd KOH treatment]/[NOC230]. (4) [NOC230,3rd KOH treatment]/ [NOC230]. (5) [NOC230,4th KOH treatment]/ [NOC230]. (6) [NOC230,5thKOHtreatment]/[NOC230]. (7) [NOC23O]/[NOC]. (8) [NOC230,4th KOH treatment]/[NOC]. (9) [NOC230,5th KOH treatment]/ [NOC].
when NOC and NOC230 (with the latter, the 1800 cm-’ band has nearly its maximum intensity) were treated successively with small doses of aqueous 1O-4 M KOH solution. With NOC, the carboxyl band centered at - 1750 cm-’ is progressively consumed (and in doing so, clearly reveals its hete~geneous nature) while a strong band grows at 1580 cm-’ and a much weaker and broad band develops at -1400 cm-’ which is hardly resolved even in the differential spectra. With NOC230, the reaction with KOH brings about the progressive elimination of all carboxylic components absorbing above 1700 cm -I including the 1800 cm-’ band formed upon carbonization; the latter seems to undergo an even faster decline than the other bands. The asymme~c stretching mode of the ionized carboxyl group formed on ~eatment with KOH absorbs very strongly at - 1520 cm-’ with NOC230, while the symmetric mode is hardly seen at all in the differential spectra and can be found at -1400 cm-’ in ordinary spectra (see trace 2, Fig. 6). The data of Figs. 5 and 6 confirm the assignment of the 1800 cm-’ band to a carboxyl group, and make easier the assignment of the broad and strong band near 1540 cm-’ which is formed together with the 1800 cm-’ band during NOC carbonization: the 1540 cm-’ band is in fact ascribable to some carboxylic groups, either to those initially present on NOC or to those newly formed during the pyrolysis, which assume an ionized configuration, This may be caused by the presence in the material of some cationic impurities. However, with the starting material, on which such impurities should have been present as well, vitally none of the carboxyl groups were in an ionized form. An ahemative explanation could be that there was some reaction of carboxyls with organic cations (or dye-like cations) formed as byproducts of the pyrolytic degradation. In fact, the pyrolysis of oxidized cellulose is known to yield very little tar[3], so that most of the fragments with the size of a monomer glucosidic unit (and giving, in the case of cellulose, levoglucosane and tar) must undergo different chemical reactions and remain within the carbonaceous residue in the case of NOC. The assignment of the band at -1680 cm-‘, which forms during somewhat later stages of the pyrolysis and also undergoes thermal decomposition somewhat later than the carboxyl species, is not as straight forward. One possibility is that, as the aromatization brought about by the car~nization at higher and higher temperatures proceeds, some of the carboxyls might get involved in a resonant (conjugated) system, so that the CO stretching mode shifts to lower frequencies, as it does in aromatic acids[lO], and the stability of the functional group increases. Such an assignment is hardly consistent with the virtually unchanged intensity of the 1680 cm-’ band under the effect of alkali solutions. Alternativeiy, the 1680 cm-’ band could be the first spectroscopic evidence of the ketone carbonyl band which becomes clearly resolved at -1700 cm-’ after the thermal elimination of most of the carboxyl groups. The rather different frequency of the band maximum found with the two materials might either be due to the varying superposition effect of adjoining bands or to the varying extent of
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conjugation of ketone carbonyls in a progressively dehydroxylating system. Finally, it is interesting to note that, after all of the carboxyl groups have been removed by heating to 45O”C, i.e. both those originally present and those formed during pyrolysis, the spectral behavior of chars formed from cellulose or from NOC becomes very much alike at higher temperatures. It is thus deduced, at least from a spectroscopic point of view, that the chemical differences of the two char families are strictly due to the carboxyl functionality. 3.5 The oxidation of NOC chars: general remarks Carbonaceous materials of different origin and corresponding to different carbonization stages are known to respond differently to the action of oxidizing agents and to yield different oxidized species. A thorough investigation of the oxidation of cellulose chars was described elsewherel21; a similar comparative study has been done with NOC chars. However, as high temperature cellulose and NOC chars are similar, as mentioned in the previous section, emphasis was placed on low temperature chars and low temperature oxidations, i.e. reactions leading to the formation of the so-called acidic carbons. When NOC is heated in OZ, the spectral changes observed up to -250°C are very much the same as those found when heating NOC in vacuum. As the earliest stages of NOC carbonization correspond to auto-oxidation, this indicates that up to about 250°C the decomposing part of NOC is a more powerful oxidizer than molecular oxygen toward the charring polysaccaridic chains. At temperatures above 25O”C, e.g. Fig. 7, the thermal treatment of NOC in the presence of O2 results in the simultaneous carbonization of the polysaccaride and in the oxidation of the char: (i) the OH band becomes broader and more intense, and all CH, groups are eliminated (Fig, 7a); new carbonylic components absorbing above I700 cm- ’ are produced, as shown by the differential spectra of Fig. 7(b). These spectral features, which are typical of the oxidation of high temperature NOC chars, i.e. NOC charred up to -4OO”C, are better observed when high temperature chars become oxidised and will be discussed later. Consequently, the carbonization and oxidation steps of NOC charred at T z 250°C have been kept separated, and the oxidation of NOC320, NOC540 and NOC700 have been studied. NOC320 corresponds to a charring stage at which the 1800 cm- ’ carboxyl band still exhibits high intensity, as do all the other carboxyl species already present on the starting material, and the carbonization has proceeded far enough so that spectroscopic evidence for the presence of the saccaridic structure has d~sap~~ed. NOC540 corresponds to a charring stage characterized by the complete elimination of all carboxyl species, and the NOC char is spectroscopically like a cellulose-derived char. NOC700 is produced at the lowest temperature at which spectra of NOC chars loose virtually all spectral structure. 3.5.1 The oxidation cf NOC320. The spectra of Part A of Fig. 8 indicate that, as was mentioned for NOC270,
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Fig. 7. (a) Comparison of NOC pyrolysis in vacuum and in 0, (1) [NOC ox:27O]/[Pt]. (2) [NOC270]/[Pt]. (b) Comparison of NOC wrolvsis in vacuum and in 0,. The ratios S/S. of the spectra a& as~follows. (1) [NOC ox.i7O]/[Pt]. (2) [I& ox.27O]/[N~J. (3) [NOC 0~.27O]/[~OC270]. low temperature NOC chars undergo a quick oxidation leading to an increase of the OH population and to a destruction of the still abundant CH, network, while the aromatic hydrocarbon residues seem to resist oxidation at least up to the temperatures at which the char was produced in vacua. Figure 8(b) shows that the oxidation, beginning near 2OO*C, leads to several changes in the
C. MCIRTERRA and M. J. D. Low
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Fig. 8. (a) Oxidation of NOC320. Compensated spectra. (1) [NOC320]/[Pt]. (2) [NOC320 ox.210]/[Pt]. (3) [NOC320 ox.3OO]/[Pt]. (b) Oxidation of NOC320. Compensated spectra. (I) [NOC320]/[Pt]. (2) [NOC320
ox.21O]/[Pt]. (3) [NOCSZOox.300]f[F’t]. (4) [NOC320 ox.4OO]/[Pt]. (c) Oxidation of NOC320. Differential spectra. Each of the ratios S/S. is indicated by the ratios at the right of the spectra.
2000-500 cm-l range which can be summarized and interpreted as follows. (i) There is a rapid build-up of carbonyl species absorbing above 1700 cm-‘, the absorptions being much stronger and extending to higher frequencies than was found in the absence of oxygen {plots 5,6, Fig. 8b). The differential spectra (traces 8, 9; oxidized char vs unox-
idized char) show that a two-band complex absorption is mainly produced, absorbing strongly at 1850 cm ’ but less strongly at -1800 cm-‘. These frequencies match quite nicely the spectral features reported in the literature for conjugated acid anhydrides[lO,l I]. Also, the observed intensity ratio of the absorptions indicates that the anhydride-like species should have a non-cyclic
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(ii) The ketone CO band at - 1700 cm-’ is progressively eliminated upon oxidation, e.g. see the negative peak at that frequency in the differential spectra 8 and 9, and ncr trace of it remains after an oxidation at 400°C (trace 4, Fig. 8b). The rapid elimination of ketone StNctures upon oxidation is common ta ceIiu]ose and NOC chars, and is consistent with the parallel eIimina~on of aliphatic hydroc~~nac~us residues, as mentioned pre-
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cm” Fig. 9. Cfxidationof NoC54.0. ~o~~nsarEd or differentid spectra. The ratios S/S,+are as f&lows. (Ij ~~U~~4~]/~~]. (2) [~~5~ox.~~]/[~], (3) ~N~5~ox.4~]/~~]~ (4) [NOCM? ox.SOO]/[Pt].(5) /N06540 ox.300]i[NOC540].(6) [NO040 0~.400]/[NOC540]. (7) [NOC540 0~.500]i[NOCS40]. (8) [NOC540oxSQ@]/[NOC5400x.400]. structure, on the basis of and within the limits of the applicability to heterogeneous systems of data reported for hom~3geneous systems. There is thus a marked difference between NQC320 oxidation and the oxidation of cellulose charf2J. The interpretation seems consistent with the previous postulate that the early beginning of the auto-oxidatian of NOC led to the copious rupture of pyranose rings,
(iii) When oxidation is carried out up to -400°C i.e. at a temperature somewhat higher than the original charring temperature, the charring continues and this process can be monitored by the increase in intensity of the 1600 cm-’ band. Also, the newly formed oxidized species absorbing above 1750 cm-‘, as we11as possibly some of the original carboxyl species, decline (trace 7). At higher temperatures NOC320 undergoes a quick burnoff. 3.5.2 The oxidation of‘NOC540. NQC540 was observed to oxidize only at T 2 300°C; some observable spectral features are shown in Fig. 9, Two consecutive and partly overfapping steps lead to the furmation of the carbonylic species abso~~ng at high wavenumbers. (ij Up to -4OVC the oxidation mainfy leads to the formation of strong bands absorbing at I850 and - t 790 cm-‘, i.e. much at the same frequencies of bands reported fur the oxidation of NOC320. However, the band intensity ratio observed is reversed. As similar bands are also observed during the oxidation of low temperature cellulose chars[2], the assignment of the bands is the same, namely, to anhydride-hike structures of the conjugated cyclic type. It is thus inferred that, as the carbonization proceeds, i.e. with the development of a more organized and larger polyaromatic network, oxidized heteroatumic cyclic structures are also formed on NOC chars. (ii) a strong and complex band system with apparent maxima at -1770 and -1720 cm-” is formed upon oxidation at @C-500°C. The same bands are also characteristic of the oxjdation of cellulose chars formed at medium to high temperatures, and are usually ascribed to lactone-like heterocyclic structures[2]. While those bands are formed, the anhydride-like structures formed in the earlier stages of NOC.540 oxidation are progressively destroyed, as is clearly shown by the differential spectrum 8. The oxidation of NOC540 also brings about the progressive elimination of the aromatic hydrocarbonaceous residues, so that also in this respect NUCS40 behaves in a manner that is indistinguishable from that of a corresponding cellulose char. A final comment remains to be made concerning spectra l-4 of Fig, 9. During oxidation, and especially at the highest temperatures, the overall intensities of the spectra increase appreciably. This increase is ascribable to an overall increase of the photothermal response brought about by the burn-off of the char. Bum-off is known to produce a substantial increase in the surface area and porosity of carbonaceous materials; effectively, the material behaves as if the particle size were decreased and scattering increased, so that the photothermal response is increased[ 131. These effects, while being explainable
C. MORTERRA and M. J. D. Low
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spectra that (i) NOC700 starts to develop an oxidized layer at temperatures as high as 45O’C; (ii) the oxidized layer does not contain any spectroscopically detectable species absorbing above 1800 cm-‘, i.e. no anhydridelike structures are detectably formed; (iii) the lactonelike species absorbing at 1770 cm-‘, with a weak shoulder on the low wavenumber side, are formed and grow in number up to about 500°C and then decline; and (iv) at oxidation temperatures higher than 6OO”C, i.e. when an easily detectable bum-off of NOC700 takes place, no carbonyl species absorbing above 1700 cm-’ are left, in agreement with the observation that CO rather than CO, is released during the high temperature oxidation (gasification) of relatively highly charred carbons. Acknowledgemenr-Support by NSF grant CPE-792100 and AR0 contract DAAG29-83-K-0063 is gratefully acknowledged.
REFERENCES
cm-l Fig. 10. Oxidation of NOC700. Differential spectra. The ratios S/S. are as follows. (1) [NOC700 0~.450]/[NOC700]. (2) [NOC700 ox.5OO]/[NOC7OO]. (3) [NOC700 oxS70]/[NOC700]. (4) [NOC700 0~.660]/[NOC700].
in qualitative terms, unfortunately make quantitative reasoning very difficult. 3.5.3 The oxidation of NOC700. The oxidation of NOC700 in 0, does not differ appreciably from that of a comparable cellulose char. Some spectra are shown in Fig. 10, mainly because the previous study of the oxidation of cellulose chars[2] did not consider some details of the oxidation above 500°C. It is apparent from the
1. C. Morterra and M. J. D. Low, Carbon 21, 283 (1983). 2. C. Mortem, M. J. D. Low and A. G. Severdia, Carbon, 21, 5 (1984). 3. S. L. Madorsky, V. E. Hart and S. Strauss, J. Res. Natl. Eur. Standards 60, 343 (1958). 4. M. J. D. Low and C. Morterra, Carbon 21, 275 (1983). 5. E. C. Yackel and W. 0. Kenyon, J. Am. Chem. Sot 64, 121 (1942). 6. R. G. Zhbankov, Infrared Spectra of Cellulose and its Derivatives. Consultants Bureau, New York (1966). 7. Ref.[6], p. 262. 8. E. A. Kmetko, Phys. Rev. 82, 456 (1951). 9. C. Morterra and M. J. D. Low, Spectrosc. Left. 15, 689 (1982). 10. N. B. Kolthup, L. H. Daly and S. E. Wiberley, Introduction to Infrared and Raman Spectroscopy. Academic Press, New York, (1975). 11. K. Nakanishi, Infrared Absorption Spectroscopy. HoldenDay, New York (1962). 12. Ref.[6], pp. 123-124. 13. C. Morterra, M. J. D. Low and A. G. Severdia, Infrared Phys. 22, 221 (1982).
CarbonVol.23,No.3,301-310. 1985 Prmted ,ntheU.S.A.
IR STUDIES OF CARBONS-IV THE VACUUM PYROLYSIS OF OXIDIZED CELLULOSE AND THE CHARACTERIZATION OF THE CHARS? CLALIDIOMORTERRA and M. J. D. Low* Department of Chemistry, New York University, New York, NY 10003, U.S.A. (Received
20 October
1983; in
revisedform
7 February
1984)
Abstract-The pyrolysis of an NO,-oxydized cellulose (NOC) in vacuum from 22 to 700°C was followed by IR photothermal beam deflection spectroscopy. Series of spectra were recorded at various stages of pyrolysis and compared with and contrasted to similar data previously obtained with cellulose. Also, NOC-derived chars were oxidized, and the effects of treating NOC and NOC char with KOH were observed. The detailed IR spectroscopic data indicate that NOC degrades more easily than cellulose and yields chars that possess different functional erouns than cellulose chars, up to charring temperatures of near 500°C. Above that temperature the chars have-similar chemical and physicai properties.-
1. INTRODUCTION
The vacuum pyrolysis of pure cellulose was recently studied with the aim of contributing to the understanding of the charring mechanism of long-chain polysaccaridic materials as well as to the nature of some of the IRactive species often observed on carbonaceous materials of various nature and origin[l]. The IR spectroscopic data indicated that the earliest stages of cellulose carbonization are mainly characterized by the oxidative dehydration of the polysaccaridic network, leading to the formation of carbonylic structures, most likely of a ketone-like nature, characterized by a strong and relatively sharp band at - 1700 cm-‘. Bands at higher wavenumbers could not be observed during the pyrolysis if a vacuum better than lo-’ was constantly maintained, whereas various oxidized species absorbing above 1700 cm ’ could be produced during the controlled oxidation of low temperature chars[2]. The present study deals with the vacuum pyrolysis of an oxidized cellulose and is intended to be a complementary contribution to the earlier study. The vacuum pyrolysis of oxidized cellulose at various temperatures up to 400°C was previously investigated from chemical and kinetic points of view[3] and was found to be rather different from that of a nonoxidized cellulose in that a very fast onset of the degradation phenomenon was found to occur at the lowest charring temperature. Analogous and complementary observations were made in the present study.
2. EXPERIMENTAL
IR spectra were recorded using photothermal deflection spectroscopy (PBDS); the spectroscopic techniques and the experimental procedures for the vacuum production of carbons and their characterization have been --~ +Part III, Ref.[Z]. *Author to whom correspondence should be addressed.
described in detail elsewhere[4]. The starting material for this work was an NO?-oxidized cellulose manufactured for surgical use by Johnson and Johnson and commercially termed Surgicel[S]. It is a sterile, high purity product in which virtually all of the primary alcohol functionality of the C, position, i.e. on the carbon atom off the pyranose ring, has been selectively converted into a carboxyl group, without any appreciable retention of nitrogen oxide(s) in the cellulosic structure. All samples were washed with distilled water before undergoing any treatment in the IR cell. When certain samples were oxidized, pure 0, was introduced to the cell to a final pressure of 150 Ton: at the stated temperature, thus simulating with pure O2 the oxidizing conditions of the atmosphere. IR spectra were obtained using 1000 scans[4] at 8 cm-’ resolution. As the PBDS instrument functions in a single-beam mode, all spectra shown except those of Fig. 2 were “compensated” and/or “corrected for background” by digital computation[4] so that pseudo-double-beam spectra resulted. The ordinate of a spectrum S which was compensated by ratioing against the spectrum So of a standard carbon or against pIatinum black is indicated by S/S,. The computation procedure also makes it possible to generate differential spectra which are useful in making spectral changes stand out clearly; such spectra are marked S/S,,, where S,, is the “background” spectrum as indicated in the figures and captions. To achieve brevity, especially in captions, descriptive abbreviations are needed. The untreated starting material is termed NOC. For chars produced by the vacuum pyrolysis of NOC at T%, the symbol NOCT is used. For chars produced by vacuum pyrolysis which are then treated with oxygen at T”C, the symbol is NOCT ox.T. These and similar obvious abbreviations are used to label spectra, e.g. S/S,, labelled [NOC270 ox.4OO]i]NOC270] is a differential spectrum S of NOC char produced by the vacuum pyrolysis at 270°C followed by oxidation at 400°C vs the spectrum S, of NOC char produced by vacuum pyrolysis at 270°C. The ordinate for most spectra are arbitrary and are displaced to avoid overlapping of traces.
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3.1 Some spectral features of NOC The IR spectra of a pure cellulose specimen and of NOC are shown in Fig. 1. Comparison shows that in the high frequency region (Fig. la) the OH band, which is strong and sharply centered at -3390-l with cellulose, is weaker, ill-defined, much broader and centered at lower wavenumber on NOC; the band extends on the low wavenumber side down to -2200 cm-‘, in agreement with the decreased number of alcoholic OH groups per pyranose unit and with the presence of a COOH group in the C, position, bound with strong hydrogen bonds and possibly assuming a five-membered ring structure. Also, the CH, stretching band at -2900 cm -I is drastically reduced in intensity, as expected and previously observed[6]. The low frequency region (Fig. lb) shows dramatic differences between NOC and cellulose which, on the basis of the literature[6], suggest that the reaction with NO, led to an extensive oxidation of the polysaccaridic network. In fact, the NOC spectrum shows that the strong and complex band system between 900 and 1200 cm-’ is strongly decreased and the band structure in the 13001500 cm-’ region is lost, as previously reported for highly oxidised celluloses[6]. It is difficult to determine if the oxidation proceeded at all beyond the average stage of so-called monocarboxycellulose and also introduced COOH functionalities at the CZand/or C, positions. Certainly the strong band centered at - 1750 cm-’ ascribable to the COOH functionality is a dominant feature of the NOC spectrum with a peak center intensity comparable to that of the strong band at - 1050 cm- ’ , as is sometimes observed in very highly oxidized celluloses of the socalled di- and tricarboxycellulos type[6]. The differential spectrum S,/S,, of Fig. l(c) shows that the carboxyl band of NOC, which may seem to be a single strong peak at 1750 cm-’ with a well resolved shoulder at -1600 cm-’ (Fig. 1b), is, on the contrary, made up of at least two components centered near 1760 and 1720 cm-‘, while there is no evidence for any component near 1600 cm-’ beyond the intensity level normally observed with the purest celluloses; this component is usually ascribed to traces of lignin-like structures. This indicates, on the one hand, that the carboxyl group population is heterogeneous in nature, and on the other hand excludes the occurrence of any appreciable amount of nitrate groups which would yield two strong bands of equal intensities near 1650 and 1300 cm- ‘[7].
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‘NO, GEL,.
RESULTS AND DISCUSSION
&
-SELL ‘NO, L
2000
1000
5
cm” Fig. 1. (a) Spectra of NOC and cellulose. The spectrum S of NOC or of cellulose was compensated by ratioing vs the spectrum S, of a carbon standard. (b) Spectra of NOC and cellulose. The spectrum S of NOC or of cellulose was compensated by ratioing vs the spectrum Soof a carbon standard. (c) Differential spectra of NOC and cellulose. Differential spectra were pro. duced by ratiomg the spectrum S, of NOC vs the spectrum SCELL ot cellulose, and me versa.
3.2 General aspects of the vacuum pyrolysis of NOC The pyrolysis of NOC in vacua begins at relatively low temperatures and yields a carbonaceous residue; abundant gaseous decomposition products are released but very little tarry material is formed. The gaseous decomposition products of cellulose were analysed and described elsewhere[3]. A temperature as low as 110°C is sufficient to turn the initially pure white NOC into a homogeneously yellow-brownish mass, while at - 150°C the color of the residue is a verv dark brown. and at
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IR studies of carbons-IV 180°C, i e. at the temperature at which pure cellulose just begins to turn yellow, it is fully black to the eye and has all the appearance of a char. However, the appearence is misleading. Figure 2 shows a series of single-beam, uncompensated spectra. with the overall intensities of the spectra normalized to mat of a Pt black absorbed. (The negative excursions near 2300 cm-’ are due to atmospheric CO,; the “noise” in the 1800-1200 cm-’ region is caused by atmospheric water vapor; see Part 1141.) Each spectrum is the summation of a portion containing spectral structure (the latter is clearly brought out in the compensated spectra of Fig. 2) and a portion which is unstructured, i.e. a continuum absorption. The series of spectra shows that the structured portion of the spectrum begins to change even at very low temperatures, indicating that the NOC is undergoing significant chemical changes, but that the structureless portion of the spectrum, as monitored by the over-all photothermal response, does not increase appreciably until the temperature approaches 500°C. The increase of the continuum absorption parallels the progressive reduction of the electronic band gap of the organic semiconductor char, brought about by the increase of the average size of the polyaromatic domains formed during charring[l,S]. The IR “blackening” and growth of polyaromatic domains of NOC is thus the same as that of pure cellulose, see Fig. 7, Part II[ 11. It is worth noting that, despite the observation that at early stages of pyrolysis the absorption continuum is not intense, so that low tem~mture NOC chars are not highly absorbing in the IR range (although opaque in the Visible), such chars as well as NOC itself cannot conveniently be studied by IR transmission methods because the materials scatter so extensively that the transmission coefficient is virtually reduced to zero with most samples.
3.3 ~etui~s of the IR Spectra Fig. 3A shows the high frequency range of spectra of NOC samples charred in vucuo at temperatures up to -600°C. It is noted that, despite the copious release of water vapor during the early stages of carbonization[3], the beginning of the NOC pyrolysis is characterized by a strong intensification of the absorption due to H-bonded OH stretching. That band reaches its maximum intensity after a vacuum treatment at 180°C; the peak maximum becomes sharper and centered near 3400 cm --‘, i.e,, some 100 cm- ’ higher than with the starting material. At higher temperatures the band declines in intensity and becomes more diffuse, and eventually disappears at T Z 550°C. There are accompanying changes of the CH, absorption. Originally it appears sharp and centered at 2900 cm-‘, but then becomes broader on the high wavenumber side. At 180°C a new component of intensity comperable to the 2900 cm-’ one is resolved near 29.50 cm-‘, and at higher temperatures that high wavenumber component becomes predominant and eventually remains the only resolved peak in the aliphatic CH, stretching region. Evidence for unsaturated CH stretching bands is first clearly observed at 7’ 2 320°C either two bands being
S
cm
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Fig. 2. NOC pyrolysis. The number by each spectrum indicates the pyrolysis temperature in “C. The top trace is the spectrum S, of Pt black used for compensating some spectra. A11spectra arc single-beam. uncom~nsated. The ordinates are displaced and no~alized to the Pt spectrum.
resolved at 3040 and 3100 cm- ‘, or just one broad band at -3060 cm-‘. Evidence for unsaturated CH stretching bands of aliphatic nature and for CH bands of aromatic nature at T = 6OO*C Figure 3b shows the spectral changes in the 2OtXl-550 cm- ’ range undergone by NOC samples charred at increasing temperatures up to 700°C. It is seen that up to a charring temperature of -250°C the most evident spectra1 changes are: (i) a gradual decrease of the broad and ill-defined band centered near 1070 cm-’ which monitors the progressive destruction of the polysaccaridic network; (ii) an increase of the band complex above 1500 cm-‘. For example, at 180°C the band at 1640 cm-’ is broad and no longer describable as a shoufder, whereas the prominent band at 1740 cm-’ is still centered at that frequency but is much broader on the high frequency side (shoulder at 1800 cm-‘). At 230°C the intensity ratio of the high wavenumber bands is further modified in the manner described. After charring at temperatures as high as 300°C the highest frequency components seem to dectine and only a major sharp band remains centered at 1700 cm-’ which also declines as the carbonization proceeds and eventually disappears at T Z 600°C. When a charring temperature of -350°C is reached, a band centered at 1600 cm- ’ becomes the prominent spectral feature in the 15002000 cm-’ range.
C. MORTERRA and
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580 500 420 370 270 u? \ v)
230 200 180 150 110 20 3500
2500 cm-’
1000
2000 cm-’
Fig. 3. NOC pyrolysis. The number beside each spectrum indicates the pyrolysis temperature in “C. The spectra are compensated, with the ordinates displaced.
M. J. D. Low The spectral changes outlined are quite complex and, to help figure out the mechanism of NOC carbonization, differential spectra of the materials at the various charring stages have been computed; some are shown in Fig. 4. It is observed that, beginning at the lowest charring temperatures, a fairly sharp and symmetrical band builds up at 1800 cm-‘, i.e. where a mere broadening of the existing bands could be observed in the spectra of Fig. 3(b). Also, a broad and complex band centered near 1580 cm-’ is formed. The two bands cannot be thought of as being partner modes of the same system because the former band grows up to a charring temperature of -250°C and then declines (see, in particular, the 2301180 and 270/230 differential plots 5 and 7 of Fig. 4), while the latter band keeps growing. Beginning at T 2 2OO”C, a relatively sharp component is resolvable at 1680 cm-’ (curves 4-8, Fig. 4) which seems to decline at T 2 400°C (curves 9 and 10, Fig. 4). The broad and complex band at 1580 cm-’ ~USI include a component at 1600 cm-’ growing in intensity as the carbonization proceeds up to the stage at which the surface oxidized layer declines. This band, which is typical of all carbonaceous materials and which monitors the formation of a polyaromatic network in a superficially oxidized char was discussed elsewhere[ 1,9], is seen in Fig. 3(b) to be virtually the sole remaining component in the 1500-1700 cm-’ region after charring at T 2 400°C. The 2301180 differential trace 5 shows that, when the 1600 cm-’ band is almost absent (in fact, very little increase of it would be expected in the small temperature range; it is possibly observable in trace 5 as a small shoulder at -1600 cm-‘) another component is resolved; it is broad and asymmetric on the low wavenumber side, centered near 1540 cm-’ and extends down to -1420 cm-‘. Other interesting features which were not so clearly seen in the spectra of Fig. 3 but are more readily observable in the differentials of Fig. 4 are: (i) a polysubstituted aromatic network system is being built up on charring NOC at temperatures as low as 150-18O”C, and can be clearly revealed by the out-of-plane deformation modes in the 700-1000 cm-’ region much before any evidence for the aromatic CH stretching bands is seen in the 3000 cm-’ region; (ii) it is difficult to say when a thermal decomposition of the NOC carboxyl groups actually begins, but certainly by the time a charring temperature as high as 420°C has been reached, the carboxy1 band at -1760 cm-’ is severely decreased in intensity as shown by traces 9 and 10 of Fig. 4. Charring temperatures between 300 and 410°C were previously reported to bring about an abundant release of COZ[3]; (iii) trace 10, Fig. 4 shows that at 420°C the broad band centered near 1540 cm-’ is also appreciably decreased. 3.4 An interpretation of the formation of NOC chars The creation of a strong and complex band system above 1500 cm-’ during the early stages of NOC pyrolysis indicates that, much as with cellulose, auto-oxidations dominate the initial phase of carbonization. The earlier onset of pyrolysis with NOC than with cellulose confirms the previous observation that “. the whole
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IR studies of carbons-IV
NOC 270 NOC 230 NOC 230 NOC I80
cnc \ v)
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2000
1500
1000
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structures. It is thought that, as in the case of the chemical action of some oxidizing agents such as periodic acid upon cellulose, the cleavage of the C& bond is the dominant reaction path of the auto-oxidation process, leading to C2 and/or C, oxidized species. The 1800 cm ’ band, first observed as a shoulder to the - 1760 cm-’ carboxyl band, is thought to be one more component of the same complex band assigned to the heterogeneous carboxylic acid functionality, the relevant species being even more strongly affected by the electronic and inductive effects that also lead the other components to have rather high frequencies. The oxidative production of an increased amount of COOH groups is certainly consistent with the observed increase of the hydroxyl band as well as with Zhbankov’s observation[ 121 that the insertion into the cellulosic structure of a multiple carboxylic functionality produces a broader and very diffuse CO band in the 1650-1850 cm-’ region as well as a shift of the CH stretching band from 2900 up to 2940 cm“, a shift similar to the one reported above. A further and more definitive argument in favor of the proposed assignment comes from the downward shifts of the 1800 cm-’ band and of all other carbonyl bands above 1700 cm-’ after NOC was treated with alkali solutions. Figures 5 and 6 show the spectral changes produced
Fig. 4. NOC pyrolysis: differential spectra. Each differential spectrum S/S, is defined by the ratio at the right of the spectrum, e.g. spectrum 10 was obtained by ratioing the spectrum S of the char produced at 420°C (NOC420) against the spectrum S, of the char produced at 330°C (NOC330). cellulose ring structure is weakened by the oxidation of the primary alcohol[3] “. However, the over-all mechanism of the NOC auto-oxidation must be different from that of cellulose, for which an oxidative dehydration leading to mostly ketone-like structures was invokedl I]. In fact, a dehydration would not be at ail consistent with the increase of the hydroxyl band. Also, an oxidative attack on the secondary alcohol groups in the Cz and C, positions of the pyranose ring leading to either simple ketones possibly having an a-hydroxy configuration (single oxidation), or to tw-diketones (double oxidation), would not explain the early formation of a sharp and dominant band at 1800 cm.- ‘. In fact, isolated ketone CO groups do not absorb at such high frequencies{ IO,1 l]. This kind of functionality was associated with a band at -1700 cm’-’ observed with cellulose chars[ 11. The possible formation of a-diketones could also not explain the 1800 cm-’ band because carbonyls conjugated with other carbonyls are hardly different in frequency than comparable ketones without the CO-CO conjugation[ IO]. Such reasoning does not exclude the fo~ation of ketonic structures in any of the phases of NOC pyrolysis (and such structures must indeed be formed if a prominent ketone-like CO band at -1700 cm-’ is clearly resolved after charring at T 2 350°C). However, the 1800 cm ’ band must be assigned to other carbonyl
cm-’ Fig. 5. KtXI-treatment of NOC. Compensated or differential spectra. The ratios S is, of the spectra are as follows. (1) [NOC] / [Pt]. (2) [NOC,lst KOH treatment]/[Pt]. (3) [NOC,Sth KOH treatmentJ/[Pt]. (4) [NOC, 1st KOH treatment]/[NOC]. (5) [NOC,3rd KOH treatment]/[NOC]. (6) [NOC,Sth KOH treatment]i[NOC].
306
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and M. J. D. Low
cm”
cm” Fig. 6. KOH treatment of NOC230. Com~nsat~d or differential spectra. The ratios S/S, are as foIlows. (I) [NOC230]1 [Pt]. (2) [NOC230,5th treatment]l[Pt]. (3) [NOC230,2nd KOH treatment]/[NOC230]. (4) [NOC230,3rd KOH treatment]/ [NOC230]. (5) [NOC230,4th KOH treatment]/ [NOC230]. (6) [NOC230,5thKOHtreatment]/[NOC230]. (7) [NOC23O]/[NOC]. (8) [NOC230,4th KOH treatment]/[NOC]. (9) [NOC230,5th KOH treatment]/ [NOC].
when NOC and NOC230 (with the latter, the 1800 cm-’ band has nearly its maximum intensity) were treated successively with small doses of aqueous 1O-4 M KOH solution. With NOC, the carboxyl band centered at - 1750 cm-’ is progressively consumed (and in doing so, clearly reveals its hete~geneous nature) while a strong band grows at 1580 cm-’ and a much weaker and broad band develops at -1400 cm-’ which is hardly resolved even in the differential spectra. With NOC230, the reaction with KOH brings about the progressive elimination of all carboxylic components absorbing above 1700 cm -I including the 1800 cm-’ band formed upon carbonization; the latter seems to undergo an even faster decline than the other bands. The asymme~c stretching mode of the ionized carboxyl group formed on ~eatment with KOH absorbs very strongly at - 1520 cm-’ with NOC230, while the symmetric mode is hardly seen at all in the differential spectra and can be found at -1400 cm-’ in ordinary spectra (see trace 2, Fig. 6). The data of Figs. 5 and 6 confirm the assignment of the 1800 cm-’ band to a carboxyl group, and make easier the assignment of the broad and strong band near 1540 cm-’ which is formed together with the 1800 cm-’ band during NOC carbonization: the 1540 cm-’ band is in fact ascribable to some carboxylic groups, either to those initially present on NOC or to those newly formed during the pyrolysis, which assume an ionized configuration, This may be caused by the presence in the material of some cationic impurities. However, with the starting material, on which such impurities should have been present as well, vitally none of the carboxyl groups were in an ionized form. An ahemative explanation could be that there was some reaction of carboxyls with organic cations (or dye-like cations) formed as byproducts of the pyrolytic degradation. In fact, the pyrolysis of oxidized cellulose is known to yield very little tar[3], so that most of the fragments with the size of a monomer glucosidic unit (and giving, in the case of cellulose, levoglucosane and tar) must undergo different chemical reactions and remain within the carbonaceous residue in the case of NOC. The assignment of the band at -1680 cm-‘, which forms during somewhat later stages of the pyrolysis and also undergoes thermal decomposition somewhat later than the carboxyl species, is not as straight forward. One possibility is that, as the aromatization brought about by the car~nization at higher and higher temperatures proceeds, some of the carboxyls might get involved in a resonant (conjugated) system, so that the CO stretching mode shifts to lower frequencies, as it does in aromatic acids[lO], and the stability of the functional group increases. Such an assignment is hardly consistent with the virtually unchanged intensity of the 1680 cm-’ band under the effect of alkali solutions. Alternativeiy, the 1680 cm-’ band could be the first spectroscopic evidence of the ketone carbonyl band which becomes clearly resolved at -1700 cm-’ after the thermal elimination of most of the carboxyl groups. The rather different frequency of the band maximum found with the two materials might either be due to the varying superposition effect of adjoining bands or to the varying extent of
307
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conjugation of ketone carbonyls in a progressively dehydroxylating system. Finally, it is interesting to note that, after all of the carboxyl groups have been removed by heating to 45O”C, i.e. both those originally present and those formed during pyrolysis, the spectral behavior of chars formed from cellulose or from NOC becomes very much alike at higher temperatures. It is thus deduced, at least from a spectroscopic point of view, that the chemical differences of the two char families are strictly due to the carboxyl functionality. 3.5 The oxidation of NOC chars: general remarks Carbonaceous materials of different origin and corresponding to different carbonization stages are known to respond differently to the action of oxidizing agents and to yield different oxidized species. A thorough investigation of the oxidation of cellulose chars was described elsewherel21; a similar comparative study has been done with NOC chars. However, as high temperature cellulose and NOC chars are similar, as mentioned in the previous section, emphasis was placed on low temperature chars and low temperature oxidations, i.e. reactions leading to the formation of the so-called acidic carbons. When NOC is heated in OZ, the spectral changes observed up to -250°C are very much the same as those found when heating NOC in vacuum. As the earliest stages of NOC carbonization correspond to auto-oxidation, this indicates that up to about 250°C the decomposing part of NOC is a more powerful oxidizer than molecular oxygen toward the charring polysaccaridic chains. At temperatures above 25O”C, e.g. Fig. 7, the thermal treatment of NOC in the presence of O2 results in the simultaneous carbonization of the polysaccaride and in the oxidation of the char: (i) the OH band becomes broader and more intense, and all CH, groups are eliminated (Fig, 7a); new carbonylic components absorbing above I700 cm- ’ are produced, as shown by the differential spectra of Fig. 7(b). These spectral features, which are typical of the oxidation of high temperature NOC chars, i.e. NOC charred up to -4OO”C, are better observed when high temperature chars become oxidised and will be discussed later. Consequently, the carbonization and oxidation steps of NOC charred at T z 250°C have been kept separated, and the oxidation of NOC320, NOC540 and NOC700 have been studied. NOC320 corresponds to a charring stage at which the 1800 cm- ’ carboxyl band still exhibits high intensity, as do all the other carboxyl species already present on the starting material, and the carbonization has proceeded far enough so that spectroscopic evidence for the presence of the saccaridic structure has d~sap~~ed. NOC540 corresponds to a charring stage characterized by the complete elimination of all carboxyl species, and the NOC char is spectroscopically like a cellulose-derived char. NOC700 is produced at the lowest temperature at which spectra of NOC chars loose virtually all spectral structure. 3.5.1 The oxidation cf NOC320. The spectra of Part A of Fig. 8 indicate that, as was mentioned for NOC270,
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Fig. 7. (a) Comparison of NOC pyrolysis in vacuum and in 0, (1) [NOC ox:27O]/[Pt]. (2) [NOC270]/[Pt]. (b) Comparison of NOC wrolvsis in vacuum and in 0,. The ratios S/S. of the spectra a& as~follows. (1) [NOC ox.i7O]/[Pt]. (2) [I& ox.27O]/[N~J. (3) [NOC 0~.27O]/[~OC270]. low temperature NOC chars undergo a quick oxidation leading to an increase of the OH population and to a destruction of the still abundant CH, network, while the aromatic hydrocarbon residues seem to resist oxidation at least up to the temperatures at which the char was produced in vacua. Figure 8(b) shows that the oxidation, beginning near 2OO*C, leads to several changes in the
C. MCIRTERRA and M. J. D. Low
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Fig. 8. (a) Oxidation of NOC320. Compensated spectra. (1) [NOC320]/[Pt]. (2) [NOC320 ox.210]/[Pt]. (3) [NOC320 ox.3OO]/[Pt]. (b) Oxidation of NOC320. Compensated spectra. (I) [NOC320]/[Pt]. (2) [NOC320
ox.21O]/[Pt]. (3) [NOCSZOox.300]f[F’t]. (4) [NOC320 ox.4OO]/[Pt]. (c) Oxidation of NOC320. Differential spectra. Each of the ratios S/S. is indicated by the ratios at the right of the spectra.
2000-500 cm-l range which can be summarized and interpreted as follows. (i) There is a rapid build-up of carbonyl species absorbing above 1700 cm-‘, the absorptions being much stronger and extending to higher frequencies than was found in the absence of oxygen {plots 5,6, Fig. 8b). The differential spectra (traces 8, 9; oxidized char vs unox-
idized char) show that a two-band complex absorption is mainly produced, absorbing strongly at 1850 cm ’ but less strongly at -1800 cm-‘. These frequencies match quite nicely the spectral features reported in the literature for conjugated acid anhydrides[lO,l I]. Also, the observed intensity ratio of the absorptions indicates that the anhydride-like species should have a non-cyclic
IR studies uf carbons--IV
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(ii) The ketone CO band at - 1700 cm-’ is progressively eliminated upon oxidation, e.g. see the negative peak at that frequency in the differential spectra 8 and 9, and ncr trace of it remains after an oxidation at 400°C (trace 4, Fig. 8b). The rapid elimination of ketone StNctures upon oxidation is common ta ceIiu]ose and NOC chars, and is consistent with the parallel eIimina~on of aliphatic hydroc~~nac~us residues, as mentioned pre-
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viously ,
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cm” Fig. 9. Cfxidationof NoC54.0. ~o~~nsarEd or differentid spectra. The ratios S/S,+are as f&lows. (Ij ~~U~~4~]/~~]. (2) [~~5~ox.~~]/[~], (3) ~N~5~ox.4~]/~~]~ (4) [NOCM? ox.SOO]/[Pt].(5) /N06540 ox.300]i[NOC540].(6) [NO040 0~.400]/[NOC540]. (7) [NOC540 0~.500]i[NOCS40]. (8) [NOC540oxSQ@]/[NOC5400x.400]. structure, on the basis of and within the limits of the applicability to heterogeneous systems of data reported for hom~3geneous systems. There is thus a marked difference between NQC320 oxidation and the oxidation of cellulose charf2J. The interpretation seems consistent with the previous postulate that the early beginning of the auto-oxidatian of NOC led to the copious rupture of pyranose rings,
(iii) When oxidation is carried out up to -400°C i.e. at a temperature somewhat higher than the original charring temperature, the charring continues and this process can be monitored by the increase in intensity of the 1600 cm-’ band. Also, the newly formed oxidized species absorbing above 1750 cm-‘, as we11as possibly some of the original carboxyl species, decline (trace 7). At higher temperatures NOC320 undergoes a quick burnoff. 3.5.2 The oxidation of‘NOC540. NQC540 was observed to oxidize only at T 2 300°C; some observable spectral features are shown in Fig. 9, Two consecutive and partly overfapping steps lead to the furmation of the carbonylic species abso~~ng at high wavenumbers. (ij Up to -4OVC the oxidation mainfy leads to the formation of strong bands absorbing at I850 and - t 790 cm-‘, i.e. much at the same frequencies of bands reported fur the oxidation of NOC320. However, the band intensity ratio observed is reversed. As similar bands are also observed during the oxidation of low temperature cellulose chars[2], the assignment of the bands is the same, namely, to anhydride-hike structures of the conjugated cyclic type. It is thus inferred that, as the carbonization proceeds, i.e. with the development of a more organized and larger polyaromatic network, oxidized heteroatumic cyclic structures are also formed on NOC chars. (ii) a strong and complex band system with apparent maxima at -1770 and -1720 cm-” is formed upon oxidation at @C-500°C. The same bands are also characteristic of the oxjdation of cellulose chars formed at medium to high temperatures, and are usually ascribed to lactone-like heterocyclic structures[2]. While those bands are formed, the anhydride-like structures formed in the earlier stages of NOC.540 oxidation are progressively destroyed, as is clearly shown by the differential spectrum 8. The oxidation of NOC540 also brings about the progressive elimination of the aromatic hydrocarbonaceous residues, so that also in this respect NUCS40 behaves in a manner that is indistinguishable from that of a corresponding cellulose char. A final comment remains to be made concerning spectra l-4 of Fig, 9. During oxidation, and especially at the highest temperatures, the overall intensities of the spectra increase appreciably. This increase is ascribable to an overall increase of the photothermal response brought about by the burn-off of the char. Bum-off is known to produce a substantial increase in the surface area and porosity of carbonaceous materials; effectively, the material behaves as if the particle size were decreased and scattering increased, so that the photothermal response is increased[ 131. These effects, while being explainable
C. MORTERRA and M. J. D. Low
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spectra that (i) NOC700 starts to develop an oxidized layer at temperatures as high as 45O’C; (ii) the oxidized layer does not contain any spectroscopically detectable species absorbing above 1800 cm-‘, i.e. no anhydridelike structures are detectably formed; (iii) the lactonelike species absorbing at 1770 cm-‘, with a weak shoulder on the low wavenumber side, are formed and grow in number up to about 500°C and then decline; and (iv) at oxidation temperatures higher than 6OO”C, i.e. when an easily detectable bum-off of NOC700 takes place, no carbonyl species absorbing above 1700 cm-’ are left, in agreement with the observation that CO rather than CO, is released during the high temperature oxidation (gasification) of relatively highly charred carbons. Acknowledgemenr-Support by NSF grant CPE-792100 and AR0 contract DAAG29-83-K-0063 is gratefully acknowledged.
REFERENCES
cm-l Fig. 10. Oxidation of NOC700. Differential spectra. The ratios S/S. are as follows. (1) [NOC700 0~.450]/[NOC700]. (2) [NOC700 ox.5OO]/[NOC7OO]. (3) [NOC700 oxS70]/[NOC700]. (4) [NOC700 0~.660]/[NOC700].
in qualitative terms, unfortunately make quantitative reasoning very difficult. 3.5.3 The oxidation of NOC700. The oxidation of NOC700 in 0, does not differ appreciably from that of a comparable cellulose char. Some spectra are shown in Fig. 10, mainly because the previous study of the oxidation of cellulose chars[2] did not consider some details of the oxidation above 500°C. It is apparent from the
1. C. Morterra and M. J. D. Low, Carbon 21, 283 (1983). 2. C. Mortem, M. J. D. Low and A. G. Severdia, Carbon, 21, 5 (1984). 3. S. L. Madorsky, V. E. Hart and S. Strauss, J. Res. Natl. Eur. Standards 60, 343 (1958). 4. M. J. D. Low and C. Morterra, Carbon 21, 275 (1983). 5. E. C. Yackel and W. 0. Kenyon, J. Am. Chem. Sot 64, 121 (1942). 6. R. G. Zhbankov, Infrared Spectra of Cellulose and its Derivatives. Consultants Bureau, New York (1966). 7. Ref.[6], p. 262. 8. E. A. Kmetko, Phys. Rev. 82, 456 (1951). 9. C. Morterra and M. J. D. Low, Spectrosc. Left. 15, 689 (1982). 10. N. B. Kolthup, L. H. Daly and S. E. Wiberley, Introduction to Infrared and Raman Spectroscopy. Academic Press, New York, (1975). 11. K. Nakanishi, Infrared Absorption Spectroscopy. HoldenDay, New York (1962). 12. Ref.[6], pp. 123-124. 13. C. Morterra, M. J. D. Low and A. G. Severdia, Infrared Phys. 22, 221 (1982).