A mass spectrometry study of thermal dissociation of cork

191

International Journal of Mass Spectrometry and Zon Processes, 112 (1992) 191-204

Elsevier Science Publishers B.V., Amsterdam

A mass spectrometry

study of thermal dissociation of cork

M. Filomena Bento’, M. Aurea Cunha’, A.M.C. Moutinho3, H. Pereira4 and M.A. Fortes’ Centro de Fisica Molecular das Universidadesde Lisboa (INIC). Complex0 I, IST, Av. Rovisco Pais, 1000 Lisboa (Portugal)

(First received 13 May 1991; in final form 1 October 1991)

ABSTRACT Mass spectrometry has proved a convenient technique for the analysis work a comparative which

samples

were continuously

fragmentation, temperatures mentation. Analysis

study is made of chemically

are analysed were obtained From

heated

in vacuum.

by a quadrupole for each sample,

such sequences

of spectra

which originates

Keywords: cork; thermal

dissociation;

mass

of plant cell-wall material.

cork and untreated The outgassing

spectrometer.

which allowed

cork, applying

products, Sequences

identification

upon

to different

treatments

proved

electron-induced

in

electron-induced

of spectra

at increasing of frag-

curves for each peak.

useful to identify

the species and even to derive activation

In this

a method

of the main groups

one may derive intensity-temperature

of these curves for samples subjected

to assign the component

treated

the peaks,

energies.

fragmentation.

INTRODUCTION

Cork is a biological tissue with thin-walled cells, produced by a cork cambium in the outer bark of trees. In the cork oak, Quercus suber L., this cambium originates a continuous thick layer of cork and if destroyed, e.g. by removal of the cork layer, it has the ability to regenerate and resume its activity. These characteristics allow the industrial utilization of cork and a sustained production by periodically removing the cork layer. The first cork layer is removed from the tree after 20-30 years (virgin cork) and the successive cork layers (reproduction cork) are removed every 9 years. It is after the I Institute Superior de Engenharia de Lisboa, R. Emidio Navarro, 1900 Lisboa, Portugal. 2 Institute Superior TCcnico, Universidade TCcnica de Lisboa, Av. Rovisco Pais, 1000 Lisboa, Portugal. 3 Faculdade de Ciincias e Tecnologia da Universidade Nova de Lisboa, Qt. Torre, 2825 Mte Caparica, Portugal. 4 Institute Superior de Agronomia, Universidade Tkcnica de Lisboa, Tapada da Ajuda, 1300 Lisboa, Portugal. 0168-l 176/92/$05.00

0

1992 Elsevier

Science Publishers

B.V. All rights reserved.

192

second extraction that cork has a quality suitable for most of its industrial applications, particularly in stoppers for bottles. Cork shows a remarkable set of properties, such as low density, very low permeability to liquids, good thermal and sound insulation, unusual mechanical properties, chemical stability and fire resistance. These are the result of its cellular structure and chemical composition. The cells in cork are closed and hollow, tilled with a gas which is presumably air. The average chemical composition of cork is as follows [l]: suberin, 39%; lignin, 22%; polysaccharides, 18%; extractives, 15%; ash 1%. Suberin is a macromolecule of polyester-linked fatty acids, largely C,, [2]. Lignin is an aromatic polymer of phenylpropane units characteristic of lignocellulosics. Polysaccharides include cellulose and hemicelluloses, mainly xylans, which yield the following monosaccharides by hydrolysis [l]: glucose, xylose, arabinose, galactose, mannose and rhamnose. Finally, extractives include mainly tanins and waxes. The chemical characterization of cork components has been hindered by the need for depolymerization prior to their removal from the cell wall, a process that may result in significant chemical changes. Mass spectrometry, eventually combined with other techniques, has proved an efticient method to study complex macromolecular materials, since it allows the characterization, in a short time interval, of samples without any previous treatment. It is common practice to apply pyrolysis in order to obtain a mass spectrometric “fingerprint” of biological materials [3]. In pyrolysis-mass spectrometry (Py-MS) one in general chooses to heat the sample rapidly to a well-defined temperature, and subsequently record the mass spectrum at a constant temperature. This method has been used for example for wood, coal and tobacco [4-61. In the present work a different method was used which bears relation to the linear temperature program and desorption (LTPD) method. This method, which has been applied to investigate the kinetics of desorption from solid surfaces, was pioneered by Redhead [7]. Cork samples were continuously heated in vacuum, at a given rate, and the outgassing products, upon electroninduced fragmentation, were analysed using a quadrupole mass spectrometer to obtain the mass selection. When cork is progressively heated in air or in vacuum, it first loses water without thermal degradation. This starts at temperatures between 150 and 200°C as observed by thermogravimetry [8]. The mass decreases continuously until, at about 500°C, cork reduces to ash. The experiments reported in this work are complementary to those by thermogravimetry in that they give information on the evolution of thermal degradation products in a vacuum environment.

193

Fig. 1. Layout of the experimental apparatus: C, cylindrical cell sample; S, sample; R, resistance; T, thermocouple; P, pipe for gas transfer to the ionizer; I, ionizer with variable electron energy; Q, quadrupole; M, electron multiplier.

EXPERIMENTAL

The experimental set-up is shown schematically in Fig. 1. The oven and the mass spectrometer are assembled in two separately-pumped vacuum chambers connected by a short pipe. Each chamber is pumped by an oil diffusion pump provided with a liquid nitrogen trap. In the oven chamber the ultimate pressure is about 2 x low5 Pa, which increases to 5 x lop5 Pa under working conditions. In the mass spectrometer chamber the pressure remains at about lop7 Pa. The oven is resistively heated and the temperature of the cell where the cork sample is placed is controlled by a chromel-alumel thermocouple. The cylindrical sample cell, with an internal volume of 3 cm3, is made of stainless steel and metal-sealed. The cork outgassing products leave the cell via a thin-wall capillary tube, which leads to the entrance of the mass spectrometer ionizer in the second vacuum chamber. The sample volume is pumped at a constant speed of about 2 x low3 1SK’,which corresponds to an average residence time in the cell of 1.5 s. Consequently only stable molecules will reach the detector. The normal procedure is to pump the system to high vacuum after introduction of the sample in the cell. Then the oven is heated to 100°C and subsequently the temperature is increased at a rate close to 20°C h-’ up to about 260°C. Around 100°C the outgassing is already measurable. With this low heating rate it is possible to obtain complete mass spectra every 2°C. The mass spectrum of the gas leaving the cell is obtained with an Extranuclear quadrupole mass spectrometer with a mass range up to 300~. Since there are large differences in the peak intensities, the spectra were recorded on a logarithmic scale. In order to discriminate the signal from the background

194

Fig. 2. Schematic diagram showing the conversion of untreated reproduction cork (sample A) to the other samples E, S, L, P used in this study: 1, solvent extraction; 2, desuberinization by methanolysis; 3, delignification by nitric acid/acetic acid; 4, polysaccharide acid hydrolysis.

of residual gases, detection can be performed with the lock-in technique after modulation of the gas effusing from the cell. However, the preliminary runs without sample proved that it was not necessary to take these precautions since the background was negligible. Complete mass spectra were recorded in 3min. Considering the temperature rise during this period the temperature assigned is correct within + 1°C. All cork samples were heated in the apparatus described above from room temperature to 260°C and mass spectra were recorded for 70 eV and 15 eV electron bombardment energies. The samples were in the form of square plates with dimensions 1.7 cm x 1.7 cm x 0.4cm, and with a mass of approximately 200mg. They included untreated reproduction cork (sample A) and samples that were submitted to various chemical treatments for the selective removal of particular chemical components (samples E, S, L and P) as shown in Fig. 2. The chemical treatments were as follows. Sample E. Successive solvent extractions, in soxhlet, with petroleum ether, ethanol and water (10 h for each solvent). Sample S. Desuberinisation of the pre-extracted cork (sample E) with 3% NaOCH, in methanol under reflux for 6 h. Sample L. Treatment of the pre-extracted cork with a solution of HNO, and CH,COOH under reflux for 1 h to remove lignin. Sample P. Treatment of the pre-extracted cork with 72% H,S04 at room temperature for 2 h, followed by 2 h at 110°C with 2% H,SO, to remove polysaccharides. The effect of these chemical treatments on the cellular structure of cork was investigated by Pereira and Marques [9].

195 RESULTS AND DISCUSSION

The mass spectra obtained for the outgassing products from the heating of cork samples showed significant contributions in the mass range below 200 u. The results shown here are a small fraction of the total and were chosen to highlight the main effects observed. Examples of spectra are shown in Figs. 3 and 4. The spectra contain mainly C,H,O, groups as expected from the fragmentation of an organic material. The low mass region of the spectra up to approximately 9Ou corresponds to a highly volatile fraction, consisting mainly of non-specific products such as H,O, CO, CO,, formaldehyde, acetic acid, acetone and low molecular weight alcohols. The higher mass peaks should correspond to larger molecular fragments which have resisted fragmentation, retaining approximately the structure they had in cork. The effect of the electronic energy may be observed by comparing two typical mass spectra from the same sample at 230°C obtained at 15 and 70 eV, as shown in Fig. 3. It is apparent that at 15 eV the region of higher mass has a greater abundance. This is expected, since at low collision energies the internal energy accumulated in the vibrational modes of these macromolecules is lower than at high energies. Since the more interesting fragments for characterization of the cork thermal dissociation products are those of higher mass, most spectra were obtained for an electron energy of 15 eV. For this electron energy, mass spectra at 230°C for all cork samples are shown in Fig. 4. There are clear differences among these spectra, both in the most abundant peaks as well as in their relative intensities. For example, the mass peaks 94,110, 124,138 and 164 are well pronounced in sample S and relatively weak in sample L, when compared with the same peaks in sample A. These mass peaks have been identified as lignin markers in wood [lo] which suggests that they are due to thermal degradation of lignin, since they significantly decrease in the delignitied cork sample. However it is difficult to conclude on the origin of the various peaks, because the intensity of a given peak in a spectrum can be the result of the contribution of different species with the same m/z value and also the same species may originate from different components in cork. An analysis of the evolution of the peaks with temperature, exemplified by the intensity-temperature curves of Fig. 5, proved useful in deciding from which components the species may originate. In addition, assuming the process rate to be determined by a Boltzmann factor, one can derive dissociation energies [7,1 l] from these temperature-dependence curves. The temperature dependence can be studied in two limiting cases: with a high rate of temperature increase so that the desorption rate is much higher than the pumping speed and vice versa. In our experiments the conditions

196

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198

correspond to the second case. One ensures that the gas evolved is rapidly removed as the temperature rises. Therefore the partial pressure rise is proportional to the desorption rate and has a maximum at a temperature T,,, which leads to a maximum in the intensity-temperature curve. From the relationship derived by Redhead [7] for a first-order reaction: EIW,,,

= ln (v, LX/P) - 3.64 (1) where /I is the heating rate and v, is the rate constant in the Arrhenius equation, which we take as lOI SC’, one can calculate the activation energy, E, of a well-resolved peak due to a lirst-order process. From the shape of the peak it is, in principle, possible to determine the order of the reaction. An asymmetric peak is characteristic of a first-order process and a symmetric peak of a second-order process. In the case of first-order processes one can also derive the activation energy from the high temperature halfwidth, 6, using the equation [l 11: E = xRT;,,/6

where x is the order of the reaction.

P

‘2

40-

2 xl-

“a-

. . .. . ,., oL,.~. ...y....g...an....’ 103

150

200

250 300 Temp (0 C)

Fig. 5. Intensity-temperature curves for peak 96 in samples A and E. Electron For sample A, 6 is the high temperature halfwidth (see text).

energy

15 eV.

199

The intensity-temperature curves observed for several mass peaks of cork correspond to first- and second-order processes (Figs 5 and 6) and also to superimposed processes (Fig. 7). Peaks such as m/z 96 and 114 observed in sample A are clearly resolved. Assuming a first-order process, binding energies of 153 + 1 kJmol-’ are estimated for those two peaks, by using the observed T,,, value in eqn. 1; from eqn. 2, taking x = 1, the value is 152 f 14 kJ mol-’ which is very similar to the previous value. This agreement of the two values is an indication that the hypothesis of the first-order process is correct, which is confirmed by the small asymmetry in the peaks. Such low energies indicate

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200

that they probably correspond to the decomposition of the polymers into their monomeric units. Comparing the intensity-temperature curves of untreated cork (sample A) with those for chemically treated samples, it can be concluded that in sample A the released products are detected at higher temperatures than in the other cases, suggesting a greater stability towards thermal degradation. This means that the chemical manipulation of cork reduces the heat stability of the remaining components, even in the case of mild treatments such as the solvent extraction of cell-wall solubles. This can be seen in Fig. 5 for the m/z 96 peak of samples A and E. The shift of the maximum abundance observed for that mass peak, from 220°C in sample A to 190°C in sample E, suggests that the removal of extractives changes the cell-wall structure of cork, reducing the stability of the structural components. This is in agreement with SEM observation of voids and separation of lamella in cell walls of extracted cork samples [9]. Untreated cork shows a great abundance of mass peaks 96 and 114 with a maximum at about 214°C but these peaks are depressed in samples E and L and practically absent in the other treated samples. This can be observed in Fig. 6 which shows the intensity-temperature curve for peak 114 for all cork samples. From these results, it can be concluded that these peaks are probably related to xylans, which are the main hemicelluloses in cork. In fact, hemicelluloses are known to be the least heat-resistant components in cork [9,12] and also in wood [13]. It has been shown that cork xylans decompose to a major extent at temperatures around 200°C [ 141.The mass peaks at 96 and 114 have also been referred to as polysaccharides markers [15]. This is consistent with the fact that they are absent in sample P, from which polysaccharides have been removed by hydrolysis, and are strongly depressed in sample L and even more in sample S. In fact, delignification with nitric and acetic acids, as in the case of sample L, in addition to lignin depolymerization, also hydrolyses the hemicelluloses [16], and in the case of sample S desuberinization using transesterification of the polyester linkages of suberin is also likely to partially hydrolyse the hemicelluloses. The peaks at m/z 94,96, 98, 110 and 124 of sample S (see Fig. 7) show a common feature in the intensity-temperature curves with a maximum at about 23OOC.Since this sample has been desuberinised, these peaks should be related to thermal dissociation of lignin and some polysaccharides. In sample A this effect is not pronounced, probably because the temperature reached in the experiment is not high enough to originate that dissociation. In fact, it has been shown that in cork, lignin degradation will be significant only for temperatures above 250°C [14]. Curves for the same peaks are shown in Fig. 8 for sample P which, in principle, should be free from polysaccharides. Up to 225”C, the intensities of the peaks under consideration are quite small but at

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Fig. 7. Intensity-temperature curves for peaks 94, 96, 98, 110 and 124 for sample S. The m/z ratio of each curve is indicated. Electron energy 15 eV.

this temperature one observes a significant contribution from lignin and polysaccharides. This seems to imply that the treatment used to obtain sample P has not removed those polysaccharides associated with lignin. Moreover the intensity-temperature curves show maxima at temperatures (w 24OOC)higher than for sample S. This behaviour is consistent with the hypothesis [14] that the increase in temperature induces a condensation reaction in lignin. In conclusion, the released products detected under the experimental conditions used derive from thermal dissociation of low binding energy con-

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Fig. 8. Intensity-temperature curves for peaks 94, 96, 98, 110 and 124 for sample P. The m/z ratio of each curve is indicated. Electron energy 15 eV.

stituents and appear in two temperature ranges. At lower temperature they are due to the degradation of xylans, originating from the thermal dissociation of hemicelluloses, the characteristic peaks being those at 96 and 114. At higher temperatures there is thermal dissociation of lignin and some polysaccharides, and the important peaks are 94, 110 and 124 related to lignin, and 96, 98 and 1IO related to polysaccharides. Suberin does not seem to have a significant contribution, probably because it has a greater stability compared with the other components, requiring higher dissociation temperatures than those

203 TABLE 1 Proposed identification

of mass spectrometry peaks

Identification

Origin

94

Phenol CsHSOH

Lignin

96

Furfural (OCdH,)CHO Dimethylfuran (CH,)2(CqH20)

Polysaccharides Polysaccharides

98

Methylcyclopentanone CH3(C5H70) Hydroxymethylfuran CH3 OH(C,, Hz 0) Dihydromethylfuranone C5H,0,

Polysaccharides Polysaccharides Polysaccharides

110

Hydroxyphenol 0H(C6H,0H) Methylfurfural CHJ(OC4H,)CH0

Lignin Polysaccharides

114

Hydroxypentenolactone

Polysaccharides (Hemicelluloses)

124

Guaiacol (CH30)C6H40H Methylhydroxyphenol (CH3)OH(C,H30H)

ml2

C5H60,

Lignin Lignin

used. In fact, in this experiment the sample mass loss was about 20%, meaning that the detected products correspond to a low degree of sample degradation. A tentative assignment of some peaks (Table 1) was made. It was based upon: (1) the cork chemical composition; (2) the comparison of different sample spectra; (3) the intensity-temperature curves of selected mass peaks; and (4) the tabulated low voltage mass peaks known for the most common biopolymers. The reported results show that the technique used is an adequate method to study the thermal dissociation of cork and may provide information on the fragmentation of its components and on the way they are associated in the cell walls.

ACKNOWLEDGMENTS

This work is part of project 2F of the Instituto National de Investigacao Cientifica (INIC) and was partly supported by Instituto de Ciencia e Tecnologia de Materiais. We thank Laurinda Bhatt for her help with the experimental part of this work and Jo50 Louren9o for technical assistance with the molecular beam apparatus. Helpful discussions with Professor Los are gratefully acknowledged during his stay as a Visiting Professor at the Faculdade de Ciencias e Tecnologia (Universidade Nova de Lisboa) and Gulbenkian Professor in 1990.

204 REFERENCES 1 H. Pereira, Wood Sci. Technol., 22 (1988) 211. 2 P. Holloway, Phytochemistry, 22 (1983) 495. 3 J.J. Boon, Physico-Chemical Characterisation of Plant Residues for Industrial and Feed Use, Elsevier Applied Science, London, 1989. 4 A.D. Pouwels, A. Tom, G.B. Eijkel and J.J. Boon, J. Anal. Appl. Pyrol., 11 (1987) 417. 5 M. Nip, W. Genuit, J.J. Boon, J.W. de Leeuw, P.A. Schenck, M. Blazso and T. Szekeley, J. Anal. Appl. Pyrol., 11 (1987) 125. 6 M.A. Scheijen and J.J. Boon, J. Anal. Appl. Pyrol., 15 (1989) 97. 7 P.A. Redhead, Vacuum, 12 (1962) 203. 8 M. Emilia Rosa and M.A. Fortes, J. Mater. Sci. Lett., 7 (1988) 1064. 9 H. Pereira and A.V. Marques, Int. Assoc. Wood Anat. Bull., 9 (1988) 337. 10 J.J. Boon, A.D. Pouwels and G.B. Eijkel, Biochem. Sot. Trans., 15 (1987) 170. 11 D.P. Woodruff and T.A. Delchar, Modern Techniques of Surface Science, Cambridge University Press, 1986, pp. 285-289. 12 E. Ferreira and H. Pereira, Cortica, 576 (1986) 274. 13 F. Shalizadeh and P.P.S. Chin, in IS. Goldstein (Ed.), Thermal Deterioration of Wood, Wood Technology: Chemical Aspects, 172nd Meeting Am. Chem. Sot., North Carolina University, San Francisco, CA, 1976. 14 H. Pereira, Mater. Sci. Eng., submitted. 15 A.D. Pouwels and J.J. Boon, Holzforschung, submitted. 16 H. Pereira, Wood Fiber Sci., 20 (1988) 82.