Journal of Analytical and Applied Pyrolysis, 13 (1988) 221-231
Elsevier Science Publishers B.V., Amsterdam
- Printed in The Netherlands
FIRST CHEMICAL
EVENTS IN PYROLYSIS
WILLIAM F. DeGROOT, WEI-PING
and GEOFFREY
N. RICHARDS
*
PAN
l, M.
221
OF WOOD
DALILUR
RAHMAN
Wood Chemistry Laboratory University of Montana, Missoula, MT 59812 (U.S.A.)
(Received
July 13th, 1987; accepted
September
21st, 1987)
ABSTRACT
Wood has been heated at 250 o C on a thermal balance and the evolved gases have been
analyzed by Fourier transform infrared spectroscopy
(FTIR). The heated wood has been
analyzed for glycoses, uranic acids and by nitrobenzene
oxidation to vanillin and syringaldehyde. About 60% of the weight loss is accounted for by five compounds,
which are the only
products detected in the gases by FTIR. These products are water, carbon dioxide, methanol,
acetic acid and formic acid. By relating the rates of formation of these compounds
to weight
loss, the following major conclusions
are reached regarding the first chemical events in
pyrolysis of wood. Uranic acids in the hemicelluloses
and pectic substances decompose very
readily to yield carbon dioxide, water, char (or char precursors) and perhaps some methanol.
This decomposition
may lead to further pyrolysis of the xylose units to which the uranic acids
are attached in the hemicelluloses.
Acetyl ester groups in the hemicelluloses
are much more
resistant to pyrolysis, but are released slowly as acetic acid. A small proportion
of the
potential methanol product is released very readily and at least part of this product is derived
from lignin. Formic acid is released at a slow and continuing rate at 250 o C by unknown
mechanisms and is probably derived from degradation
of hemicelluloses.
Cellulose;
Fourier
transform
infrared
spectroscopy;
hemicellulose;
lignin; pyrolysis;
wood.
INTRODUCTION
The major recent trends in studies of pyrolysis of lignocellulosic materials,
usually with the object of maximizing yields of pyrolytic oils, are towards
more rapid pyrolysis and to some extent towards higher temperatures
[l].
We have been concerned with mechanisms of pyrolysis at low temperature
as part of an on-going study of smoldering combustion
of lignocellulosics,
which involves conditions
under which both pyrolysis
and combustion
* Present address: Department
KY 42101, U.S.A.
0165-2370/88/$03.50
of Chemistry,
0 1988 Elsevier
Western
Kentucky
Science Publishers
University,
B.V.
Bowling Green,
222
reactions occur [2]. This paper describes the investigation of the first volatile
products of pyrolysis of a wood under conditions which we believe are of
relevance in smoldering combustion,
and it is anticipated that the pyrolysis
pathways which may be deduced from such products will in some instances
also be relevant to the first stages of rapid and high-temperature
pyrolysis.
The major experimental
technique has been to couple thermogravimetry
(TG) to vapor phase Fourier transform
infrared spectrometry
(FTIR) in
order to relate the qualitative and quantitative yields of volatile products to
weight loss in the pyrolyzing wood.
EXPERIMENTAL
The pyrolysis was studied by combined TG and FTIR (TG-FTIR).
The
TG system was based on a Cahn R-1000 electrobalance
with a resistively
coated temperature-programmed
furnace surrounding the sample pan. Temperature was measured by a thermocouple
positioned 1 mm below the pan.
The TG system was interfaced to a laboratory
microcomputer
system for
data acquisition and for control of the temperature programme. This system
consisted of a Tektronix
4051 microcomputer
with a ROM-based
A/D
converter and a real time clock (Trans-Era)
and a Hewlett-Packard
Model
6002A programmable
power supply for TG furnace control. The gas outlet
of the TG system was coupled to the FTIR system by a 1 m x 1 mm I.D.
PTFE tube which was heated to 100°C. The effluent from the connecting
tube entered a 100 mm X 10 mm I.D. heated gas cell in a Nicolet MX-1
FTIR system.
Black cottonwood
(Populus trichocarpa) sapwood, air-dried, was Wileymilled repeatedly to - 80 mesh and stored at - 20 o C until required. In the
thermal regime giving the results shown in Fig. 4, an 1%mg sample of wood
was heated under nitrogen (30 ml/mm)
from 30 to 120 o C at 10 o C/mm,
held at 120°C for 10 min, then heated to 250°C at 10” C/mm and held at
250” C. The times in Fig. 4 are given from the start of this regime. FTIR
spectra were taken at 3-min intervals.
Calibration
of FTIR spectra was carried out using wavelengths
and
compounds shown in Table 1. For the four cases where the product was a
liquid at room temperature
the calibration
was carried out by placing a
sample of the pure liquid in a covered aluminum pan on the thermal balance
in air at 30 ml/mm at several appropriate
constant temperatures,
and IR
spectroscopy
on the evolved vapor normally showed constant reading after
about 10 min. When constancy in rate of weight loss and spectrum were
achieved, the IR absorbance was related to the concentration
of the vapor in
the gas stream, and in all cases this relationship was linear over the range of
temperatures
used (including zero values). The IR bands and temperatures
used in each case are detailed in Table 1. Where a range of wavenumbers
is
223
TABLE 1
Infrared absorbances used to quantify
conditions used for calibration
Carbon
dioxide
Acetic acid
Water
Formic acid
Methanol
from pyrolysis
of wood and
Temperature
Rate of weight loss
@S/mm)
2240-2400
(“C)
-
1140-1230
1653
1105
1032
23,30
23,30
23,26
23,30
16, 32
17,61
12.6, 30.0
42.6, 137
Wavenumber
(cm-‘)
Compound
the first volatile products
specified, the concentration
was related to the integrated peak area between
these values; where a single wavenumber
is specified, this represents
a
Q-band and the peak height was used for calibration.
In the case of water,
the height of a major peak at 1653 cm-i in the spectrum fine structure was
used. For calibration of carbon dioxide the natural atmospheric
concentration of 0.033% (v/v) was used, and for carbon monoxide
a commercial
reference mixture of 0.930% carbon dioxide and 0.953% carbon monoxide in
nitrogen was then used to derive a relative IR response with respect to
carbon dioxide.
RESULTS
AND DISCUSSION
Cottonwood
was selected for this study because it has been the subject of
previous chemical and thermal studies in this laboratory [3]. In selecting the
pyrolysis temperature
it was necessary to compromise between the need to
keep the temperature
as low as possible, so as to limit reactions to the first
steps in pyrolysis, and the need to produce measurable
concentrations
of
pyrolysis products in the gas stream. The latter parameter is related to the
flow-rate of the inert gas (nitrogen) over the sample, but it was evident that
too great a reduction in this flow could have two deleterious effects. Firstly
there would be a danger of secondary pyrolysis due to slow removal of
primary pyrolysis products from the heated zone and secondly the greater
the time lag between pyrolysis and FTIR measurement,
the greater the
possibility of secondary reaction of primary products in the gas phase. A gas
flow-rate of 30 ml/min
was set as reasonable
compromise
of the above
events after preliminary
experiments
had shown that the TG curves were
insensitive to flow-rates down to this value.
Fig. 1 shows the rate of weight loss at 250 “C. Pyrolysis occurs at
significant rates at much lower temperatures,
but the rate of production
of
volatiles was too low for adequate accuracy in the FTIR. The general form
of the curve suggests the occurrence of initial rapid weight loss followed by a
224
O”C/min
--
25O’C
7
120
60
180 min
Fig. 1. Weight loss of cottonwood
by pyrolysis
at 250 ’ C under nitrogen.
continuing “steady state” rate of weight loss. In order to reveal the first
chemical events in the pyrolysis our investigation was concentrated on the
first rapid weight loss phase, i.e. up to about 1 h at 250° C. The FTIR
spectra given by the effluent gas after 20 min at 250 OC are shown in Figs. 2
and 3, together with spectra of methanol, acetic acid and formic acid. The
latter three compounds, together with water and carbon dioxide, were the
only compounds identified in the spectra from wood under these conditions
and it was concluded that no other JR-absorbing materials were present in
100.0
WOOD
2~“~/150mln
925
I
I
METHANOL
30
I
4400
3250
2000
1400
WAVENUMBER
Fig. 2. Spectra of gases evolved from cottonw~
at 250 * C.
000
225
97.5-0
-e-
w
m
0
_
I
ii
2
FORMIC
ACID
F
5
2950.
z
2
I<
a
METHANOL
92.5-
90.0
I
1300
I
1200
1100
1000
900
800
700
WAVENUMBER
Fig. 3. Spectra of gases evolved from cottonwood
methanol.
at 250 o C against authentic
formic acid and
significant
amount in the pyrolysis gases reaching the IR cell. The system
was calibrated for the five volatile pyrolysis products, as indicated in the
experimental
section and shown in Table 1 using the height of the Q-bands
for methanol and formic acid, the height of a selected single frequency for
water and integrated areas under major peaks for carbon dioxide and acetic
acid.
Using the above calibrations
and pyrolysis conditions
the yields of the
five volatile products were followed by spectra taken at 3-n-A intervals from
wood heated at 250” C in nitrogen,
and results are shown in Fig. 4
(mol/min).
In this figure the wood reaches 250 o C at 32 min (see Experimental section) and evidently much pyrolysis is already occurring by that
time. Evidently the dominant product on both a weight and molar basis is
water, peaking soon after the sample reaches 250’ C. It is significant,
however, that both carbon dioxide and methanol peak at about the time the
sample reaches 250” C and both of these products appear to be formed in
the first pyrolysis reactions of the wood. In Table 2 the yields of the five
volatile products detected by FTIR are compared with the weight loss in the
wood and it is evident that 40-50s
of the total volatile material from the
wood was condensed before reaching the FTIR cell.
Fig. 4. Volatile products
from cottonwood
at 250 ’ C under nitrogen.
The most likely source of carbon dioxide at low temperature
is the
decarboxylation
of uranic acids in the hemicelluloses
and pectins. The
amount of carbon dioxide generated in the first 80 min of heating is ca. 0.37
mmol/g
of wood, calculated from Fig. 4. The uranic acid content of the
wood, presumed to be predominantly
4-0-methylglucuronic
acid (MGA)
and galacturonic acid, is ca. 0.26 mmol/g of wood, calculated from Table 3.
The form of the carbon dioxide curve in Fig. 4 shows continuing production
of carbon dioxide at a lower level into the “steady-state”
pyrolysis region. It
is evident therefore that the initial wave of rapidly produced carbon dioxide
corresponds
quite well to the uranic acid content. It is concluded therefore
that the initial low-temperature
peak in carbon dioxide formation in Fig. 4
corresponds
to decarboxylation
of the uranic acid components,
and in
confirmation
of this, the uranic acid analysis of the wood heated for 50 rnin
was found to have fallen to 1.7% from the original 4.9% (Table 3).
TABLE 2
Yield of volatile products by infrared
tonwood at 250 ’ C in nitrogen
detection
as percentage
of total
weight
Compound
Percentage
of total weight loss
Methanol
Formic acid
Acetic acid
Carbon dioxide
Water
3.5
5.0
7.5
10.5
21.5
3.7
6.4
15.5
9.2
18.3
3.8
8.2
19.0
12.0
16.0
4.4
7.0
23.0
13.2
13.2
Total
47.5
53.1
59.0
60.8
Time at 250 o C (min)
Total weigth loss (X)
11
5.5
23
8.0
40
11.0
58
12.5
loss; cot-
227
TABLE 3
Analyses of cottonwood after heating at 250 o C under nitrogen
Compound
(anbydroglycoses)
Percentage dry weight
Original dry wood
After heating for 50 min
Rhamnose
Arabinose
Xylose
Mannose
Glucose
Uranic acid
Total carbohydrate
Vanillin
Syringaldehyde
0.2
0.5
15.8
3.0
51.3
4.9
75.7
2.7
5.7
0
Weight loss (W)
0
Trace
12.3
2.4
51.3
1.7
67.7
1.8
2.2
11.9
The amount of methanol produced
in the first waves (up to 60 min
heating) was ca. 0.13 mmol/g
of wood from Fig. 4 and there are two
possible sources, viz. decomposition
of MGA units of the hemicelluloses
or
degradation
of syringyl and guaiacyl units in lignin. The MGA units are
likely to comprise about half of the total uranic acid content, the balance
being due to pectic substances. Earlier studies in this laboratory
[4] of the
total cell wall uranic acid content of cottonwood
sapwood, after removal of
pectic substances
by acid washing, indicate a MGA content of 0.087
mmol/g of wood. On this basis, the maximum yield of methanol possible
from pyrolysis of hemicellulose
MGA units would have been ca. 0.06
mmol/g of wood (Table 3 shows that about two thirds of the uranic acids
are lost at this stage). A very small amount of methanol may also be derived
from methyl ester groups in pectic substances. We conclude therefore that at
least part of the early waves of methanol formation on heating is derived
from lignin. In confirmation
of this conclusion,
wood heated for 50 min
(Table 3) gave much reduced yields of vanillin (from 2.7 to 1.8%) and
syringaldehyde
(from 5.7 to 2.2%) on nitrobenzene
oxidation. The fate of the
lignin units which degrade at 250°C to yield methanol
is not known.
Another recent study of primary pyrolysis products of lignin by direct mass
spectrometry
[5] has shown three waves of methanol formation, with the first
wave coinciding with formation of formaldehyde
and conifer-y1 alcohol. In
this earlier work, however, the pyrolysis was carried out at 800-900°C.
We
have also observed a third wave of methanol production from cottonwood
at
higher temperatures
(350°C) and will report on this study at a later date.
We found no evidence of formaldehyde
formation at 250°C and although
this is an early product of high-temperature
pyrolysis, it is not a significant
product
at lower temperature.
Coniferyl
alcohol would not have been
228
detected in our system because it is not sufficiently volatile, but it should be
noted that this product retains its methoxyl group, and therefore its formation does not represent the source of the methanol from lignin.
The formation of methanol by pyrolysis of lignin is in contrast to some
recent studies of pyrolysis of phenoxymethyl
groups in coals and related
model compounds. Thus Chu et al. [6] concluded that the phenoxymethyl
groups in some methylated coals pyrolyse to produce mainly methane and
carbon monoxide.
Also, Vuori [7] has studied the pyrolysis
of several
hydroxymethoxybenzenes
and observed the same two procucts leading him
to the conclusion that the pyrolyses occur by homolytic
mechanisms.
In
these examples from coal chemistry the phenoxymethyl
groups are very
similar to those in lignin, and similar pyrolysis mechanisms might have been
anticipated. A major difference in the two systems, however, is the formation of much larger amounts of water from the polysaccharides
in the
pyrolysis of wood, and in these circumstances
it is more likely that a
heterolytic hydrolysis mechanism may account for the formation of methanol
from the phenoxymethyl
groups of lignin.
The formation of acetic acid shown in Fig. 4 commenced
considerably
after carbon dioxide and methanol and was still continuing at a significant
level at the end of the experiment.
The obvious source of this acid is the
hydrolysis
of the acetyl ester groups of the 4-0-methylglucuronoxylans,
utilizing water which is abundantly produced in other pyrolytic reactions of
the polysaccharides.
In hardwoods the ratio of acetyl groups to xylose units
is typically about one acetyl to seven xyloses [S]. Using this ratio and the
xylose value from Table 3 we derived a maximum possible acetic acid yield
of 0.84 mmol/g of wood. The observed yield of acetic acid after 80 min
heating derived from Fig. 4 is ca. 0.33 mmol/g
of wood. We conclude
therefore that the acetyl groups of the hemicelluloses
pyrolyze relatively
slowly at 250 o C and that a large proportion
survives prolonged heating at
this temperature.
It may be noted in passing that the partial acetylation
of
cellulose has been utilized to improve thermal stability [9].
The production of formic acid from the heated wood commenced at a low
level simultaneously
with carbon dioxide and methanol from the earliest
stages of pyrolysis and continued
at a significant
level throughout
the
experiment.
The mechanisms
leading to production
of this acid are unknown. We speculate that it is more likely to be derived from pyrolysis of
hemicelluloses than from lignin or (at 250 o C) from cellulose, and there may
be parallels with the formation
of high yields of the same product in
anaerobic alkaline degradation
of cellulose, also by unknown mechanisms.
Water is the major product of pyrolysis. After 80 min of heating the
amount indicated by Fig. 4 is ca. 1.4 mmol/g of wood. At that stage Fig. 1
indicates that the total weight loss of the wood is 127 mg/g of wood. The
results discussed above suggest that most of the products of the pyrolysis
have originated from decomposition
of hemicelluloses
and if we assume an
229
average monomer weight of ca. 150 for these polysaccharides, then 127
mg/g of weight loss corresponds to ca. 0.85 mm01 of hemicellulose monomer degraded per g of wood. On this admittedly very approximate basis, the
pyrolysis of an average monomer unit of hemicellulose yields ca. 1.6 mol of
water, and there is no doubt that the major part of the balance of products
from the pyrolyzed units is converted ultimately to char.
The fate of the glycose units in general is shown in Table 3. The glycose
analyses were carried out by hydrolysis, initially with 72% sulfuric acid,
followed by reduction to alditols, acetylation and gas chromatography as
described earlier, using myo inositol added to the wood as internal standard
before hydrolysis and correcting for acid degradation of glycoses [8]. Uranic
acids were determined on the same hydrolysates using the 3-hydroxydiphenyl reagent [lo]. The heated wood samples were resistant to complete
hydrolysis by the normal procedure, presumably because of “ hornification”
as a result of the heating [ll]. These samples were reactivated by keeping
under a small amount of water (vacuum-degassed) at room temperature for
one day, then freeze-dried before hydrolysis. The accuracy of the glycose
analytical procedure is not high, but several generalizations can be drawn.
The trace components rhamnose (from pectic substances) and arabinose
(from hemicelluloses and pectic substances) are completely decomposed
after 50 mm heating. The arabinose is present predominantly
as Larabinofuranoside units; these are the most acid-sensitive glycosidic linkages
present in the wood and it is not surprising that they are also the most
sensitive to pyrolysis. The mannose is present in the wood as glucomannan
and is apparently relatively resistant to pyrolysis. The greatest change in the
major glycose components occurs with xylose (15.8-12.3%). Although the
observed glucose content is the same in the heated wood as in the original
wood, the 11.9% concurrent weight loss translates to an actual loss of 12% of
the total glucose units originally present, compared with an actual loss of
32% of the total xylose units originally present. This difference is rather
surprising since the xylan and glucan chains are chemically so similar. The
relative rate constants for acid hydrolysis (which may be relevant to relative
ease of pyrolysis) of B-D-xylopyranosides and P-D-glucopyranosides
are
4.8 : 1 [12] and although this difference does suggest a higher rate of
pyrolysis of the former, the observed difference between loss of xylose and
loss of glucose is greater than expected. A possible explanation of this effect,
however, is associated with the facile pyrolysis of the uranic acid units,
many of which are glycosidically attached to the 2-position of xylose units in
the hemicelluloses. We have concluded above that the decarboxylation of
these uranic acids is one of the first chemical events in pyrolysis and it is
likely that after decarboxylation the subsequent decomposition of the uranic
acid residue will influence the reactivity of the xylose unit to which it is
attached. Similar predictions can be made with respect to the effects of the
facile pyrolysis of L-arabinofuranoside units on the reactivity of the xylose
230
Decarboxylation
PECTIC
SUBSTANCES
HEMICELLULOSES
_
Loss
Chain
CELLULOSE
_____+
LIGNIN
_____,
uranic
acid
in
xylose
acetyl
groups
scission
Transglycosylation
Small
25O"C/N2
of
of
arabinose
Decrease
Loss
WOOD
of
Loss
IOther
decrease
of
some
in
glucose
methanol
degradation?
Fig. 5. First chemical events in pyrolysis of wood.
units to which they are attached. This type of speculation
leads to the
suggestion that the xylose units which are lost in the mild pyrolysis are those
at the branch points of the xylan chain.
The relatively small loss of glucose units in the heated wood does not
mean that most of the cellulose remains unchanged under the above heating
regime. Earlier research in this laboratory
[13] has indicated
that the
molecular weight of the cellulose is likely to fall to less than half after 50
min heating due to pyrolysis scission reactions in the amorphous
regions,
and it is probable that some subsequent reformation
of glucosidic linkages
results in transglycosylation.
The glucose in the heated wood therefore is
probably not entirely present as 1,4-/3-o-glucopyranosyl
units, but conversion of glucose to volatile products under our conditions is certainly very
small.
The general conclusions regarding the predominant
first chemical events
in pyrolysis of wood are summarized in Fig. 5.
ACKNOWLEDGEMENT
The work was partly funded by a grant
Research, National Bureau of Standards.
from
the
Center
for
Fire
REFERENCES
Production, Analysis and Upgrading of Oils from Biomass, Symposium Preprints, 32
(1987) American Chemical Society Division of Fuel Chemistry, Denver, April 1987.
Y. Sekiguchi and F. Shafizadeh, J. Appl. Polym. Sci., 29 (1984) 1267-1286, and earlier
papers.
W.F. DeGroot and F. Shafizadeh, J. Anal. Appl. Pyrolysis, 6 (1984) 217-232.
W.F. DeGroot, Carbohydr. Res., 142 (1985) 172-178.
231
5
6
7
8
9
10
11
12
13
R.J. Evans, T.A. Milne and M.N. Soltys, J. Anal. Appl. Pyrolysis, 9 (1986) 207-236.
C.J. Chu, S.A. Cannon, R.H. Hauge and J.L. Margrave, Fuel, 65 (1986) 1740-1749.
A. Vuori, Fuel, 65 (1986) 1575-1583.
R.C. Pettersen, in R.M. Rowe11 (Editor), The Chemistry of Solid Wood, ACS Advances in
Chemistry Series Vol. 207, American Chemical Society, Washington, DC, 1984, p. 115.
K. Ward and L.J. Bemardin,
in N.M. Bikales and L. Segal (Editors), Cellulose and
Cellulose Derivatives, Vol. 5, Wiley-Interscience,
New York, 1971, p. 1184.
M.J. Neilson and G.N. Richards, Carbohydr. Res., 104 (1982) 121-138.
L. Segal, in N.M. Bikales and L. Segal (Editors), Cellulose and Cellulose Derivatives, Vol.
5, Wiley-Interscience,
New York, 1971, p. 724.
J.N. BeMiller, Adv. Carbohydr. Chem. Biochem., 22 (1967) 25-108.
F. Shafizadeh and A.G.W. Bradbury, J. Appl. Polym. Sci., 23 (1979) 1431-1442.