Journal of Analytical and Applied Pyrolysis, 16 (1989) 117-126
Elsevier Science Publishers B.V., Amsterdam
- Printed in The Netherlands
INFLUENCE OF METAL IONS ON VOLATILE
OF PYROLYSIS OF WOOD
WEI-PING
PAN * and GEOFFREY
N. RICHARDS
117
PRODUCTS
*
Wood Chemistry Laboratory University of Montana, Missoula, MT 59812 (U.S.A.)
(Received
October
2&h, 1988; accepted
February
7th, 1989)
ABSTRACT
Volatile products from the pyrolysis of wood have been determined by gas-phase Fourier
transform infrared spectrometry,
coupled to a thermal balance. Progressive yields of water,
carbon dioxide, carbon monoxide,
acetic and formic acids and methanol
have been determined isothermally at 523 K and at 373-823 K. The influence of the removal of inorganic
salts and of ion-exchanged
cations on these yields has been studied, as also has the effect of
adding back potassium ions and calcium ions by ion exchange. Potassium, but not calcium,
acts as a catalyst in pyrolytic reactions resulting in formation of carbon dioxide and carbon
monoxide (especially from polysaccharides),
acetic acid (from hemicelluloses),
formic acid
(from polysaccharides)
and methanol (from lignin).
Metal ions; pyrolysis;
wood.
INTRODUCTION
Recently [l] we described an investigation of the primary chemical events
in the pyrolysis of wood. The approach
was to determine
by Fourier
transform
infrared spectrometry
(FTIR) the first volatile products
from
cottonwood
held at 523 K in nitrogen in a thermal balance. These results
were related to the progressive changes in the chemical content of the wood
to conclude, inter alia, that uranic acid components
and lignin begin to
degrade first, followed by acetyl and xylose groupings in the hemicelluloses.
The cellulose was relatively stable. It was already known that exchangeable
metal ions in wood have a major influence on pyrolytic behavior [2] and on
char formation. We have now extended this study to determine the influence
* Present address:
KY 42101, U.S.A.
01652370/89/$03.50
Department
of Chemistry,
0 1989 Elsevier
Western
Kentucky
Science Publishers
B.V.
University,
Bowling Green,
118
of specific individual ion exchange bound metal ions on the
of formation of the major evolved gases from the first stages
of wood. The experimental
approach has been to relate
weight loss in the wood to the evolved gases as determined
by FTIR.
nature and rate
in the pyrolysis
the progressive
simultaneously
EXPERIMENTAL
The thermal and FTIR analytical equipment
was exactly as described
earlier [l]. All pyrolyses were carried out in flowing nitrogen at 30 ml/mm.
The wood samples were prepared as described earlier [l]. For acid washing,
the wood (20 g) was immersed in 0.05 M hydrochloric
acid, degassed in
vacuum and eluted in a column overnight with more acid (2 1) before
washing with water (2 1) in the same column and then air-drying
in a
dust-free environment.
All water was deionized to a resistivity greater than
18 MQ cm-‘. To prepare wood samples containing specific metal ions the
acid-washed
wood was treated exactly as above with a solution of the
appropriate
metal acetate (0.05 M) replacing the acid. All samples were
analyzed by inductively coupled argon plasma spectrometry
(ICAPS) and
values for the dominant metal ion contents are shown in Table 1.
RESULTS
AND DISCUSSION
Table 1 shows that the indigenous metals ions are almost completely
removed from wood by washing with 0.05 M hydrochloric
acid at room
temperature.
The subsequent
ion-exchange
treatments
with calcium and
potassium acetate solutions then yield wood samples in which calcium and
potassium, respectively,
are the only significant metal ions, the metal ions
being bound to uranic acid units in the hemicellulose
[3].
The effects of the metal ion exchange procedures
on isothermal weight
loss of wood in nitrogen at 523 K are shown in Fig. 1. The concurrent
analyses of volatile products by FTIR are shown in Figs. 2-6. The effects of
TABLE 1
Metal ion contents
Original wood
Acid-washed
Calcium-exchanged
Potassium-exchanged
(ppm, dry weight basis)
Ca
Mg
P
K
Na
Zn
1500
12
1399
12
200
1
4
2
140
8
11
8
1000
5
45
2122
30
7
43
0
9
0
11
0
119
105
I-Y
and
j ~~~~,
0
30
60
90
120
150
TIME
180
210
240
270
(min)
Fig. 1. Weight loss of ion-exchanged woods at 523 K (303-523 K at 10 K/min).
CT
60 -
0
5
50-
%
8
40-
P
C
E
2
30-
5
lo-
20-
0:.
I
20
.
I
30
.
I
40
I
*
50
’
60
70
Fig. 2. Formation of water from ion-exchanged
+ = potassium-exchanged; o = calcium-exchanged;
0:
20
.
I
30
.
I
.
40
,
50
.
I
woods at 523 K. 0 = Original
A = acid-washed.
wood;
.
60
rme(M)
Fig. 3. Formation of carbon dioxide from ion-exchanged
2.
woods at 523 K. Symbols as in Fig.
cations on the behavior of the wood at 523 K may be summarized
as
follows:
1. The removal of all metal ions (acid-washed wood) resulted in a reduced
rate of weight loss and reduced yields of all volatile products compared with
the original wood. This effect was particularly
dramatic with regard to
carbon dioxide and formic acid yields in nitrogen, which were reduced to
120
c1
Q
6-
5
G
E
6-
4-
2-
=
0
20
30
40
50
60
70
Tlme(min)
Fig. 4. Formation
of acetic acid from ion-exchanged
woods at 523 K. Symbols
as in Fig. 2.
woods at 523 K. Symbols
as in Fig. 2.
5-
4-
3-
2i.p
1
20
30
50
40
60
7
Iime(min)
Fig. 5. Formation
20
30
Fig. 6. Formation
of formic acid from ion-exchanged
40
of methanol
50
60
70
from ion-exchanged
woods at 523 K. Symbols
as in Fig. 2.
one-third at peak value. In comparison, the effects on yields of other
volatiles were much smaller. It is therefore concluded that, rather surprisingly, the pyrolytic decarboxylation of uranic acid salts to produce carbon
dioxide is facilitated compared with the free acid form. It is not possible at
121
this stage however, to speculate on the effects influencing
the formic acid
yields.
2. The behavior of the calcium-exchanged
wood was in general very
similar to the acid-washed wood. The rates of weight loss at 523 K were
almost identical (Fig. 1) and the profiles of yields of individual volatiles
versus time were very similar for these two wood samples.
3. The profiles of water yields versus time were remarkably insensitive to
cations (Fig. 2) at 523 K. Since the acid-washed
and calcium-exchanged
samples lose weight more slowly, however, this means that in these woods
other degradation mechanisms are inhibited compared with dehydration
(i.e.
char-forming)
reactions. Accordingly,
it has previously
been observed [2]
that potassium-exchanged
wood gives a much higher char yield than
calcium-exchanged
wood.
4. In general, the potassium-exchanged
wood behaved similarly to the
original wood and rates of weight loss were almost identical. The thermal
behavior
of the wood therefore
may be dominated
by the indigenous
potassium ions in the wood. Several types of degradation
occur even more
rapidly in the potassium-exchanged
wood in approximate
proportion
to the
potassium present, viz. 1000 ppm in the original wood and 1990 ppm in the
potassium-exchanged
wood. Thus, the formation
of carbon dioxide is increased by 50% at peak and methanol by 70% at peak compared
with
original wood. This is evidence of the specific catalysis by potassium
of
pyrolysis of lignin to produce methanol and of pyrolytic decarboxylation
of
uronates to produce carbon dioxide.
5. The effects of cations observed in this study are essentially the same as
those reported previously
[2] for pyrolysis of similar samples based on
differential thermogravimetric
curves, but the addition of FTIR analysis of
the product gases provides additional information
which may subsequently
lead to better understanding
of the mechanisms responsible for catalysis of
pyrolysis reactions.
The study was next extended to progressive heating of the same wood
samples in nitrogen at 3 K/mm from 373 to 823 K. The results are shown in
Figs. 7-13.
Fig. 7 shows the volatile products determined during the pyrolysis. From
the earlier isothermal results [l] at 523 K, we assume that the first products
(e.g. up to 623 K) are derived mainly from initial and partial decomposition
of lignin and hemicelluloses while the later products are derived mainly from
cellulose and the more resistant portions of lignin and hemicelluloses.
Thus,
in Fig. 7, acetic acid from the hemicelluloses
reaches a peak at about 570 K
and carbon dioxide (at least partly from decarboxylation
of uranic acids)
shows a marked inflection at a similar temperature,
as do water and formic
acid, which are both believed to arise predominantly
from hemicellulose
at
Methanol
(predominantly
from lignin) shows three
lower temperatures.
peaks at about 510, 570 and 630 K which appear to represent three distinct
122
60
60
40
30
20
IO
3
573
673
773
Temperature(K)
Fig. 7. Volatile products from wood heated in nitrogen
C = acetic acid; D = CO; E = formic acid; F = methanol.
0
I
500
600
700
at 3 K/min.
A = H,O;
B = CO,;
I
800
Fig. 8. Formation of water from ion-exchanged
woods heated in nitrogen from 373 K at 3
= Original
. . . . . = calciumwood;
= potassium-exchanged,
K/ min. _._._
exchanged; - - - = acid-washed.
Ol
500
I
600
700
Fig. 9. Formation of carbon dioxide from ion-exchanged
K at 3 K/min. Symbols as in Fig. 8.
woods heated in nitrogen
from 373
123
-I
500
550
600
650
700
750
600
Temperature(K)
Fig. 10. Formation
373 K at 3 K/min.
of carbon monoxide from ion-exchanged
Symbols as in Fig. 8.
0 !
500
woods heated
in nitrogen
from
I
700
600
Temperature(K)
Fig. 11. Formation of acetic acid from ion-exchanged
K/min. Symbols as in Fig. 8.
500
600
700
woods heated in nitrogen
from 373 at 3
800
Temperature(K)
Fig. 12. Formation of formic acid from ion-exchanged
at 3 K/min. Symbols as in Fig. 8.
woods heated
events in the pyrolysis of lignin. Carbon monoxide,
only trace amounts on pyrolysis at 523 K, shows a
550 K and a major peak at 620 K and hence may
and/or
hemicelluloses
as well as from cellulose. The
in nitrogen
from 373 K
which was produced in
significant inflection at
be derived from lignin
major volatile product
124
0 I
500
600
700
800
Temperature(K)
Fig. 13. Formation of methanol from ion-exchanged
3 K/min. Symbols as in Fig. 8.
woods heated in nitrogen
from 373 K at
of the pyrolysis detected in these experiments,
peaking at about 630 K, is
water and this must be produced from both hemicelluloses
and cellulose by
elimination
reactions,
e.g. from glycol groups, which yield unsaturated
systems and ultimately
chars. Formic (but probably
not acetic) acid is
evidently also a significant product of pyrolysis of cellulose. The mechanism
of formic acid production
is not known. There is an analogy in the
formation of high yields of the same acid in anaerobic alkaline degradation
of cellulose, also by unknown mechanisms [4].
The influences of cations on yields of carbon dioxide, carbon monoxide
(formed above 523 K), methanol
and formic acid during pyrolysis
at
temperatures up to 700 K are shown in Figs. 8-13. The yields of water (Fig.
8) and acetic acid (Fig. 11) were relatively insensitive to cation variation
except in the zone of hemicellulose pyrolysis, where potassium is evidently a
more effective catalyst than calcium. Fig. 9 shows that the potassium ions
also favor increased formation of carbon dioxide and lower the temperature
of peak production
compared with the acid-washed or calcium forms. The
total yield of this catalyzed formation
of carbon dioxide is much greater
than could be explained by decarboxylation
of uranic acids alone. The
mechanism of formation of this carbon dioxide is not known. Since the bulk
of the carbon dioxide is formed above 570 K it must be derived at least
partly from cellulose.
Carbon monoxide was not formed in significant quantities at 523 K, but
was produced in similar molar amount to carbon dioxide from original and
from potassium-exchanged
wood at higher temperatures,
peaking at about
620 K (Fig. 10). It seems probable that this product also is produced largely
from cellulose. The yield of CO from acid-washed
and from calciumexchanged wood was only about one-tenth of the other wood samples. We
consider it unlikely therefore that the CO is formed by secondary gasification reaction of CO, with char, because we have previously shown [5] that
calcium is a particularly
effective catalyst of this gasification
reaction.
125
Furthermore,
the gasification reaction is extremely slow at the temperature
of maximum CO formation. We conclude therefore that the CO is a primary
pyrolysis product derived from wood by unknown mechanism(s)
catalyzed
by potassium.
Formic acid (Fig. 12) evidently forms at higher temperatures
by potassium-catalyzed
pyrolysis reactions from cellulose. The same acid is a major
product of alkaline degradation
of cellulose in absence of air [4] and it is
probable that the greater effectiveness of potassium compared with calcium
in catalyzing formation of formic acid (and perhaps methanol) is associated
with the greater basicity of the former. The formation of methanol (Fig. 13)
shows a distinct
second peak at about 630 K for original
and for
potassium-exchanged
wood. Presumably
the evolution of methanol above
590 K is catalyzed by potassium and derived by pyrolysis reactions from the
more resistant lignin which survives pyrolysis at lower temperatures.
Varhegyi and coworkers [6,7] have recently studied the effect on pyrolysis
of physical sorption of several inorganic salts on cellulose and on sugar cane
bagasse. In general, they used salts which would be stable at the pyrolysis
temperature, whereas in the present work the ion exchanged cations (like the
‘native’ cations in wood) are converted into oxides, carbonates or free metals
during pyrolysis [8]. Their determination
of volatile products was by connection of a mass spectrometer
to the thermal balance, so that it was not
possible to relate actual yields of any volatile product to weight loss of the
solid, but some relative influences of the salts can be related to the present
work. Thus it was clear that pyrolysis of cellulose to produce water, CO and
CO, is accelerated by addition of sodium chloride (but not by three other
salts) [6]. However the opposite effect appears to operate with bagasse [7]. It
is difficult to reach conclusions
on formation
of other primary pyrolysis
products in this work because generally there are several types of products
which fragment to give the ions detected. These results may be compared
with the present study in which the presence of potassium has little influence
on extent or rate of water formation from wood, but greatly increases the
extent and rate of formation of both CO and CO,.
This present study and many earlier ones, show that traces of inorganic
salts in lignocellulosics
can produce dramatic effects on pyrolysis mechanisms. In the ultimate case, we have shown that 0.01% of sodium chloride
added to pure cellulose can almost halve the pyrolytic yield of levoglucosan
[8]. This represents one formula unit of salt per 3640 anhydroglucose
unit
(AGUs), or about one per 700 AGUs if we consider only the amorphous
regions of the cellulose where pyrolysis commences. It seems most unlikely
that this situation could be achieved by swelling of the solid matrix by the
salt as has been suggested [7]. However, we remain unable at present to
explain such effects beyond the vague suggestion that they may operate via
changes in relative rates of competing
pyrolysis mechanisms
which are
influenced by conductivity
or dielectric constant in the solid. It is tempting
126
to couple such a hypothesis
with the guess that both free-radical
and
heterolytic mechanisms may be operative in the pyrolysis, but despite all of
the studies on this subject, we remain in virtual ignorance of the chemical
mechanisms of pyrolysis of cellulose and of lignocellulosics.
ACKNOWLEDGEMENTS
The authors are grateful to Bill DeGroot for frequent helpful discussions
and experimental
assistance. The work was funded by a grant from the
Center for Fire Research, National Bureau of Standards.
REFERENCES
1 W.F. DeGroot, W.-P. Pan, M.D. Rahman and G.N. Richards, J. Anal. Appl. Pyrolysis, 13
(1988) 221-231.
2 W.F. DeGroot
and F. Shafizadeh,
J. Anal. Appl. Pyrolysis,
6 (1984) 217-232 and
references therein.
3 W.F. DeGroot, Carbohydr. Res., 142 (1985) 172-178.
4 W.M. Corbett and G.N. Richards, Svensk Papperstidning,
60 (1957) 791-794.
5 W.F. DeGroot and G.N. Richards, Fuel, 67 (1988) 352-360.
6 G. Varhegyi, M.J. Antal, T. Szekely, F. Till and E. Jakab, J. Energy Fuels, 2 (1988)
267-272.
7 G. Varhegyi, M.J. Antal, T. Szekely, F. Till, E. Jakab and P. Szabo, J. Energy Fuels, 2
(1988) 273-277.
8 W.F. DeGroot and G.N. Richards, Fuel, 67 (1988) 345-351.
9 M.G. Essig, G.N. Richards and E.M. Schenck, Appl. Polym. Symp., in press.