Journal of Analytical
and Applied Pyrolysis, 3 (1982) 283-305
Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
283
REVIEW
INTRODUCTION
TO PYROLYSIS
OF BIOMASS
*
F. SHAFIZADEH
Wood Chemistry Laboratory,
MT 59812 (U.S.A.)
(Received
Department
July 16th, 1981; accepted
of
October
Chemistry,
University
of
Montana,
Missoula,
19th, 1981)
SUMMARY
The pyrolytic
properties
of biomass are controlled
by the chemical composition
of its
major components,
namely cellulose, hemicelluloses,
and lignin and their minor components including
extractives and inorganic materials. Pyrolysis of these materials proceeds
through a series of complex, concurrent
and consecutive reactions and provides a variety
of products.
Pyrolysis
of cellulose at lower temperatures
(below 300” C) involves reduction
in
molecular
weight, evolution
of water, carbon dioxide and carbon monoxide
and formation of char. On heating at higher temperatures
(30~5OO”C),
the molecule is rapidly
depolymerized
to anhydroglucose
units that further react to provide a tarry pyrolyzate.
At still higher temperatures,
the anhydrosugar
compounds
undergo fission, dehydration, disproportionation
and decarboxylation
reactions to provide a mixture of low molecular weight gaseous and volatile products. The composition
of these products and
mechanism and kinetics of their production
are reported.
COMPOSITION
OF BIOMASS
Pyrolysis of biomass involves different
materials and methods and provides a variety of products. An analytical relationship
between these factors
provides the basic knowledge which is needed for new development.
Biomass consists of different types of dead and living plant cells, the structure and composition
of which varies for different parts and species of the
plant. The living cells as in the green leaves contain some proteins in their
protoplasm,
considerable water in the vacuoles which maintains their turgidity and less cellulosic cell wall materials. The woody tissues contain some
living (parenchyma)
cells, but are largely composed of dead (prosenchyma)
cells which consist of several layers and an intercellular
material (middle
lamella). Fig. 1 shows a diagrammatic presentation of a wood fiber (tracheid).
Cellulose microfibrils
embedded in a matrix of hemicelluloses
and lignin
form the main components
of the cell walls [l] and biomass in general In
* Presented at the Specialists’ Workshop
1980, Copper Mountain,
CO, U.S.A.
on Fast Pyrolysis
of Biomass,
October
19-22,
284
addition to these materials, there are some lipids and hydrocarbons
(terpenes) which are soluble in ether and various types of phenolic compounds,
carbohydrates
and proteins, which may be soluble in benzene, alcohol or
Fig. 1. Diagrammatic
presentation
of wood fiber (tracheid).
water. The leaves and bark generally contain more extractives and less cell
wall materials than wood and woody tissues. Table 1 shows the approximate
analysis of several types of biomass.
The cellulose component,
which is a macromolecule
composed of linearly
linked p-(1 + 4)-D-glucopyranose
units, is the same in all types of biomass
except for the degree of polymerization
(DP). However, the nature of the
hemicelluloses and lignin could vary. Aggregation of the linear chains of the
cellulose macromolecules
within the microfibrils provides a crystalline structure which is highly inert and inaccessible to the chemical reagents. Conse-
TABLE
1
Approximate
Species
Softwood
Hardwood
Wheat straw
Rice straw
Bagasse
analysis of some biomass species
Water
sol.
(%)
Lignin
6)
Hemicellulose
(%I
Cellulose
(%I
Gh)
Solvent
sol.
(“ro)
0.4
0.3
6.6
16.1
1.6
2.0
3.1
3.7
4.6
0.3
-
27.8
19.5
16.7
11.9
20.2
24.0
35.0
28.2
24.5
38.5
41.0
39.0
39.9
30.2
38.1
Total
7.4
13.3
-
285
quently, it is very difficult to hydrolyze the cellulose molecules by acids or
enzymes.
In contrast to cellulose, the hemicelluloses are amorphous, have a lower
DP and could be preferentially
hydrolyzed
under relatively mild conditions,
known as prehydrolysis.
Acetyl-4-0-methylglucuronoxylan
(xylan) forms
the main hemicellulose
of the hardwoods and glucomannans (mannan) form
the principal hemicellulose of the softwoods.
Lignin is a randomly linked, amorphous, high-molecular-weight
phenolic
compound
which is more abundant
and polymeric
in softwoods than in
hardwoods.
Furthermore,
the softwood
lignin contains guaiacyl propane
units (phenolic
groups having one methoxy
group). Hardwood
lignin, in
addition to this, contains syringyl propane units (with two methoxy groups).
The higher content of acetyl and methoxy groups in hardwoods explains
why this material has been used in destructive distillation processes to obtain
acetic acid and methanol.
The soluble components
could be removed by stepwise solvent and water
extraction, but isolation of the other cell wal components in the native form
is very difficult,
if not impossible. After the solvent extraction, lignin could
be removed by selective oxidation
to leave a combination
of cellulose and
hemicelluloses
called holocellulose.
Alternatively,
the hemicellulose component could be removed by controlled hydrolysis to leave a combination
of
cellulose and lignin called lignocellulose.
Complete hydrolysis of the carbohydrate leaves the lignin as a dark, partially decomposed
residue called
Klason lignin. It is also possible to break the lignin bonds (presumably with
the carbohydrates)
so that it could be removed with solvents at high temperatures as organosolv lignin.
COMPETING
PYROLYTIC
PATHWAYS
Different components
of biomass display different thermal properties and
the thermal properties of the aggregate represents the sum of its major
organic components [ 21.
As discussed later on rapid heating, the carbohydrates
break down at the
glycosidic link to provide lower-molecular-weight
volatile pyrolysis products,
whereas lignin is mainly charred to a carbonaceous residue. This phenomenon is observed on thermogravimetry
of wood and its components shown in
Fig. 2 [2].
Furthermore,
thermal properties of the components
and aggregate are
highly influenced by the presence of inorganic materials and various treatments. As these materials are heated, the availability of sufficient energy for
alternative pathways at higher temperatures
results in a series of complex,
concurrent and consecutive reactions which give a variety of products.
The Arrhenius plot for the rates of weight loss of cellulose in air and nitrogen is shown in Fig. 3 [3]. These data indicate a transition
at ca. 300°C
which reflects the existence of two different pathways. As shown in Fig. 4,
the rate of pyrolysis followed by weight loss under isothermal conditions,
shows an initial period of acceleration and proceeds much faster in air than in
286
TEMPERATURE (‘C)
Fig. 2. Thermogravimetry
of cottonwood
and its components.
I
1
1.70
l-75
;
Fig. 3. Arrhenius
lose in air ( A---A)
plot
1
I
1.80
x103
for the first order reaction
and nitrogen (m- - 1).
in the isothermal
degradation
of cellu-
an inert atmosphere. As the pyrolysis temperature is increased, the initiation
period and the difference between pyrolysis under nitrogen and air gradually
diminish and disappear at 310°C when pyrolysis by the second pathway
takes over. The general pathways for pyrolysis of cellulose [4,5] are shown
in Fig. 5.
LOWER
TEMPERATURE
REACTIONS
The reactions in the first pathway, which dominates at lower temperatures, involve reduction in molecular weight or DP by bond scission, appear-
287
100
200
Tim? (min)
Fig. 4. First order plots for the residual cellulose weight
310°C and 325°C for air and nitrogen are similar.
Gases,iow-molecular-weight
Cellulose
/
‘A
t
Anhydrosugars,
Water,
Fig. 5. Competing
4
char,
pathways
300
(normalized)
versus time. Plots at
volatiles
tar
C02,
CO
for cellulose
pyrolysis.
ante of free radicals, elimination
of water, formation
of carbonyl, carboxyl
and hydroperoxide
groups (especially in air), evolution of carbon monoxide
and carbon dioxide, and finally production
of a charred residue. The mechanism and kinetics of these reactions which contribute
to the overall rates of
pyrolysis of cellulosic materials have been individually
investigated [ 31. Reduction in the degree of polymerization
of cellulose on isothermal heating in
air or nitrogen at a temperature
within the range of 150-190°C
has been
measured by the viscosity method as shown in Fig. 6. These data have been
correlated with rates of bond scission given in Table 2 and used for calculating the Arrhenius plot shown in Fig. 7.
Fig. 8 shows the rate of production
of carbon monoxide
and carbon
dioxide at 170°C in air and in nitrogen. The rate of evolution of these gases
is much faster in air than in nitrogen and, furthermore,
accelerates on continued heating. It is instructive to compare the initial linear rates for the
emission of these gases with the rates of bond scission obtained for depoly-
.:... .
;&?zY
-e.
L .
.
170°C
l
1600
1200
800
400
150°C
160°C
.
180°C
.
190°C
.
10
20
30
40
Time (hours)
Fig. 6. Viscosity
190” c.
average degrees of polymerization
(PV) of cellulose
heated in air at 150-
merization at 17O”C, discussed before. As can be seen in Table 3, for air the
rate of bond scission approximately
equals the rate of production
of carbon
dioxide plus carbon monoxide in moles per glucose unit. In nitrogen, however, the rate of bond scission is greater than the combined rates of carbon
monoxide and carbon dioxide evolution.
Ed
FatAir!
-14
-
-15
-
- 16
I
i
\
\o
= 27 kcal/male
= 21 kcal/mole
\\\\\
\
0.
2.15
2.2
2.25
f
Fig. 7. Arrhenius
(o- - -0).
\
\
I
plot
2.3
2.35
x 103
for
the rate
of bond
scission
in air (o-
0) and nitrogen
289
TABLE
2
Rate constants
for the depolymerization
of cellulose
in air and nitrogen
-
Temperature,
‘C
150
160
170
180
190
* 162 represents
ke X 10’ in nitrogen,
mole/162 g min l
k. X 10’ in air,
mole/162 g min *
1.1
2.8
4.4
9.8
17.0
6.0
8.1
15.0
29.8
48.9
1 mole of monomer
Tim
unit.
(h)
Fig. 8. Yields of CO and CO* from heating cellulose
(0) CO in nitrogen; (0) CO* in air; (=) CO in air.
at 170°C:
(0) CO*
in nitrogen;
It is assumed that carbon dioxide and carbon monoxide may be formed
by decarboxylation
and decarbonylation,
respectively,
as well as by other
competing
reactions. The significance of the former reactions was determined by measuring the net rate of accumulation
of carboxyl and carbonyl
groups in cellulose on heating in air at 190°C. The results shown in Fig. 9
indicate an almost linear rate of formation
on heating for 50 h. On heating
TABLE
3
Initial rates of glycosidic
tion at 170” C
bond scission and carbon
monoxide
and carbon
Reaction
Rate x lo5 in nitrogen,
mole/162 g h
Rate X lo5 in air,
mole/162 g h
Bond scission
CO evolution
CO2 evolution
2.7
0.6
0.4
9.0
2.1
6.4
dioxide
forma-
290
8 CARBONYL
/
.
s-
6-
__//
male/
162 g
CARBOXYL
LACTONt
Tim
(hr)
Fig. 9. Incorporation
of oxidized groups in cellulose at 190” C. ‘J- - -v
ber of lactone and carboxyl groups formed in nitrogen at 190” C.
represents
num-
for longer periods, the rate of accumulation
of carboxyl groups falls off, and
the rate of accumulation
of aldehyde groups is increased. The latter effect,
however, could be due to the sodium chlorite method employed for the oxidation and measurement of the aldehyde groups. These data show the pro-
a
,
10
20
30
40
50
60
Time (min)
Fig. 10. Effect of heating on carboxyl content of carboxyl celluloses in nitrogen at 190” C.
(0) 1-Carboxyl
cellulose; (B) 2,3carboxyl
cellulose; (0) 6-carboxyl
cellulose; (0) prepared
by heating cellulose in air at 190” C.
291
duction of carboxylic and aldehyde groups on oxidation of cellulose in air.
There was also a very small increase in the number of oxidized functions on
heating in nitrogen. In order to check the extent of decarboxylation
at the
relatively lower pyrolysis temperatures,
samples of low degree of substitution carboxylcellulose
with carboxyl groups at Cl, C2, C3 and C6 were prepared and pyrolyzed. The results shown in Fig. 10 are, by and large, inconclusive, except for the sample oxidized at C2 and C3, which showed a definite reduction
in carboxyl-group
content.
Thus, although both carboxyl
and carbonyl groups, as well as carbon dioxide and carbon monoxide, are
formed during low-temperature
pyrolysis, the production
of the latter compounds could not be directly related to the formation
of the former functions.
The thermal degradation
of cellulose in air, similar to that of synthetic
polymers, may involve a free-radical mechanism. We were not able to observe
these radicals, but were able to monitor the formation
of hydroperoxide
groups on heating cellulose in air. The hydroperoxide
functions are simultaneously formed and decomposed,
and their concentration
rapidly climbs
until a steady state is reached. Fig. 11 shows the development of the steadystate concentration
in air at 170°C during a period of about 100 min. It also
shows the rate of decay in nitrogen over a similar period. This decay of the
hydroperoxide
function
appeared to follow first-order
kinetics with a rate
constant of 2.5 * 10m2 min-’ at 170°C. From the steady-state concentration
of 3.0 . lo-’ mole/162
g, the rate of hydroperoxide
decomposition
is thus
7.5 - lo-’ mole/162 g min. When compared with the initial rate of bond scission in air of 1.5 * 10e6 mole/162 g min at 170°C (Table 2), it is apparent
that the hydroperoxide
formation
could make a significant contribution
to
bond scission.
Based on the above considerations
and in analogy with studies on radiation of carbohydrates,
it is proposed that three stages are involved in the low
temperature
pathway:
initiation
of pyrolysis, propagation
and product formation. As shown in Fig. 12, the initiation
period apparently involves the
formation
of free radicals facilitated by the presence of oxygen or inorganic
impurities. Subsequent reactions of the free radicals could lead to bond scis-
1
6!
Time (min)
Fig. 11. Rate of formation
and decay of hydroperoxide
mation is in air and decay is in nitrogen.
groups in cellulose
at 170°C.
For-
292
Iniation
Iniator
Cell-H
I.
Heat
(I)
>
I*
ten
+ O2 -
+ o2
>
102'
+ Hz)*'
Cell-H
+ I.
>
Ce11'
+ IH
Cell-H
+ 102'
>
Cdl'
+ I02H
>
cell
Ox'
Cell
02H + Cell.
o.,
Propagaticm
ccl>
cell
Fomtion
of
+ o2
O*'
+ Cell-H
p>
Produds
Fig. 12. Thermal
Cell
02H
>
cell
Cdl
02H
>
acts
cell
oz.,
autoxidation
cell
Products
o* -
of cellulose
cel102*
in air.
sion, oxidation
and decomposition
of the molecule, producing char, water,
carbon monoxide and carbon dioxide. Fig. 13 shows the nature of the initiation, propagation
and decomposition
reactions involved in the thermal
decomposition
of cellulose by this pathway.
OH
”
-OH.
OH
Fig. 13. Possible
formed thermally
mechanism
in air.
of formation
and decomposition
of cellulose
hydroperoxide
293
TRANSGLYCOSYLATION
At temperatures
above 3OO”C, cellulose is decomposed by an alternative
pathway which provides a tarry pyrolyzate
containing levoglucosan (1,6anhydro-/3-D-glucopyranose),
other anhydroglucose
compounds,
randomly
linked oligosaccharides,
and glucose decomposition
products [6]. Fig. 14
shows the gas-liquid
chromatographic
(GLC) analysis of the tarry pyrolyzate after trimethylsilylation
of the free hydroxyl
groups [6,7]. The
mechanism for the formation
of these compounds has been established by
extensive investigation
of the pyrolytic reactions of phenyl glucosides and
other related model compounds [ 5,8- lo].
It has been shown that pyrolysis of a glycosidic compound as shown in
Fig. 15 starts with cleavage of the glycosidic group and condensation of the
sugar moiety, which is decomposed
on further heating. The sequence of
these reactions as a function of temperature could be determined by thermogravimetry (TG) [5] and differential
thermal analysis (DTA) as shown for
phenyl /3-D-glucopyranoside
in Fig. 16.
The individual reactions which take place within each thermal event, characterized by changes in mass (TG) and/or energy (DTA), are then followed
by various scanning or isothermal analytical methods. In Fig. 15, cleavage of
the phenolic group
is established by quantitative
recovery of the free phenol
25
Fig. 14. GLC of tar from pyrolysis of trimethylsilylated
cellulose at 350°C in vacuum.
Peaks : A, 5-hydroxymethyl-2-furaldehyde;
B, 1,4:3,6-dianhydro-wD-glucopyranose;
C, 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one;
D, unknown;
E, 3,5-dihydroxy2-methyl-4H-pyran-4-one;
F, unknown;
G, 1,5-anhydro-4-deoxy-D-glycero-hex-1-en-3ulose; H and I, unknown;
J, levoglucosan;
K, 1,6anhdro-p-D-glucofuranose;
L, a-Dglucose; M, P-D-glucose; N, 3-deoxy-D-erythro-hexosulose;
0, 0-D-glucosyl-levoglucosans.
294
HZOH
Evaporation
0
-CL
HO OH
0
Evaporation
T
-0
A
HO
+ D-G~ucos~~s
-+
-0
Decomposition
Products
OH
Fig. 15. Pyrolytic
reactions
of phenyl
p-D-glucopyranoside.
and condensation
of the sugar unit is established by recovery and chemical
analysis of intermediate
pyrolysis products [S] .
It has been shown that the reaction proceeds through a heterolytic
mechanism because it is influenced
by variation of the electron density produced through the introduction
of various substituents
on the aglycone
[ 531. This conclusion eliminated the possibility of a homolytic
mechanism
(free radical cleavage of the glycosidic bond), although further breakdown
and charring of the sugar units involve free radical formation.
It also indicated that the thermolysis,
as a heterolytic
process, must be sensitive to the
effect of acidic and alkaline reagents.
Investigation
of a variety of glycosides containing deoxy, amino and acetamido groups and the corresponding
fully acetylated compounds
[8,9]
showed that the substituent on the sugar moiety, in addition to the inductive
effect on the glycosidic bond, changes the availability and reactivity of the
transglycosylation
sites, and the acetylated compounds
are considerably
more stable than the parent compounds
with free hydroxyl
groups. These
data and production
of randomly linked condensation products [8,11] confirmed that the heterolytic
cleavage of existing glycosidic bonds proceeds
Temperature,
Fig. 16. Thermal
analysis of phenyl
p-D-glucopyranoside.
‘C
150
100
Fig. 17. Thermal
200
analysis of untreated
250
300
Temperature (‘C)
cellulose
350
400
450
powder.
through a transglycosylation
process involving nucleophilic
displacement of
the glycosidic groups by one of the free hydroxyl or amino groups of the
sugar molecule. Thermolysis
of levoglucosan (1,6-anhydro-fl-D-glucopyranose) [6], reducing and non-reducing
disaccharides
[lo] (cellobiose and
CELLULOSE
43
3
sSt4
-
I,4-AM,YD?.lDE
I
\\
I.6-ANHYDRIDE
( PYRANOSE)
I,S-ANHYDCIDE
‘r’-R”=L’
OH
I,&AI;:<YD::IDE
IFUIXNOSE)
Fig. 18. Pyrolysis
tion reactions.
1,4:3P-C:f.I:::YO..,DE
of cellulose
-
to anhydrosugars
and other
compounds
by transglycosyla-
296
trehalose), cellulose [6] and xylan [12] and the isolation and structural
investigation
of the products showed that the transglycosylation
reactions
give mixtures of anhydrosugars and randomly linked oligosaccharides.
There is a close similarity between the thermolysis
of phenyl glucoside
(Figs. 15 and 16) and the thermolysis of cellulose (Fig, 17) which starts at
300°C and proceeds rapidly until most of the substrate is evaporated. Also
chromatographic
analysis of the tarry pyrolyzate (Fig. 14) shows the presence of similar products. These products, as shown in Fig. 18, are formed by
the intramolecular
substitution
of the glycosidic linkage in cellulose by one
of the free hydroxyl
groups (transglycosylation).
Subsequent
inter- and
intra-molecular
transglycosylations
provide several anhydrosugars
and randomly linked oligosaccharides,
which could dehydrate and decompose on
further heating. The initial substitution
requires changes in the conformation
of the sugar units and increased flexibility
of the molecule. This could be
achieved at the elevated temperatures
by reduction of molecular weight,
breaking of hydrogen bonds and glass transition which are expected to activate the molecule.
KINETICS
OF CELLULOSE
PYROLYSIS
Kinetic studies have shown that at higher temperatures
the tar-forming
reactions accelerate rapidly and overshadow the production
of char and
gases. The data in Table 4 show the production
of diminishing
amounts of
char and increasing amounts of tar, the anhydrosugars and other compounds
(that could be hydrolyzed
to reducing sugar) as the oven temperature
is
raised from 300°C to 500°C [13]. At this time, it should be pointed out that
evaporation
of levoglucosan and the volatile pyrolysis products is highly
endothermic
(see Fig. 17 and ref. 13). Thus, the increased oven temperature
could raise the rate of heat transfer but not necessarily the temperature
of
TABLE
4
Effect of temperature
Oven
temp.,
“C
Pyrolysis
time,
min
on the products
from pyrolysis
of cellulose
powder
under vacuum
Percent yield from cellulose
Char
Tar
Levoglucosan
1,6-Anhydro-
Reducing
@D-&Co-
sugar
furanose
300
325
350
375
400
425
450
475
500
180
60
30
10
5
4
3
3
3
21
10
8
6
5
4
4
3
3
60
70
70
70
77
78
78
80
81
34
38
38
38
39
40
39
38
38
4
4
4
4
4
4
4
47
54
52
59
60
59
57
58
57
297
400
300
1 “C
200
100
0
L
I
I
0
I
2
3
Time in oven (min)
I
I
4
5
6
Fig. 19. Temperature
of pyrolysis as measured by a thermocouple
Final temperatures
are indicated.
(---)
Temperature
measured
cooling curve follows removal of a sample from the oven at 335°C.
in cellulose sample.
in empty boat. The
the ablating
substrate. As shown in Fig. 19 [14], the oven temperature
is
reached when the rapid evaporation and the accompanied change in entropy
is over. In other words, at the higher temperatures
the pyrolysis process is
controlled by the rate of heat transfer rather than the kinetics of the chemical reaction. Material transport presents another major obstacle to the investigation of chemical kinetics, because if the products of primary reactions
are not removed, they could undergo further decomposition
reactions.
Table 5 shows the difference
between the yield of different
pyrolysis
products in vacuum, which removes the primary volatile products, and in nitrogen at atmospheric pressure, which allows more decomposition
of the anhydrosugars [ 61. It also shows the effect of inorganic catalysts in changing the
TABLE
5
Analysis
of the pyrolysis
The percentages
products
of cellulose
are based on the original
at 300” C under nitrogen
amount
of cellulose.
Condition
Char (%)
Tar (%)
levoglucosan 1%)
1,6-anhydro$-D-glucofuranose
D-glucose (%)
hydrolyzable
materials (%)
(%)
Atm. pressure
1.5 mm Hg
1.5 mm Hg,
5% SbCls
34.2
19.1
3.57
0.38
trace
6.08
17.8
55.8
28.1
5.6
trace
20.9
25.8
32.5
6.68
0.91
2.68
11.8
298
Volatiles
kv
ki
Cellulose -“Active
Cellulose”
Wcell
WA
WV
/
3”
\
Char + Gases
where
dt
d(WA)
- dt
=ki[wcenl -(k, +kc)[WA]
d(wc )
=o.%k,[WA]
dt
Fig. 20. Pyrolysis model for cellulose.
nature of the reactions and products. In view of these considerations,
the
chemical kinetics of cellulose pyrolysis has been investigated within the limited temperature
range of 250-340°C
and under vacuum in order to obtain
chemically meaningful data. Under these conditions, the chemical kinetics of
cellulose pyrolysis could be represented by the three-reaction
model shown
in Fig. 20 [14]. In this model, it is assumed that the initiation reactions discussed before lead to the formation
of an active cellulose, which subsequently decomposes by two competitive
fir&order
reactions, one yielding
anhydrosugars
(transglycosylation
products) and the other char and a gaseous fraction.
DEHYDRATION
REACTIONS
As discussed before, detailed analysis of the pyrolysis tar (see Figs. 14 and
18) shows the presence of levoglucosan, its furanose isomer (1,6-anhydro-flD-glucofuranose)
and their transglycosylation
products as the main components. In addition
to these compounds,
the pyrolyzate
contains minor
amounts of a variety of products formed from dehydration
of the glucose
units. The dehydration
products detected include 3-deoxy-D-erythro-hexosulose, 5-(hydroxymethyl)-2-furaldehyde,
2-furaldehyde
(furfural),
other
furan derivatives, levoglucosenone
(1,6-anhydro-3,4dideoxy-/3-D-glycero-hex3-enopyranose,
1,5-anhydro-4deoxy-D-glycero-hex-l-en-3-ulose
and other
pyran derivatives. The dehydration
products are important
as intermediate
compounds in char formation.
The 3-deoxy-D-hexosulose,
which is the initial dehydration
product of
hexoses, plays a significant role as an intermediate in the production
of furan
derivatives. Isomers of this compound have been identified among the pyrolysis products of cellulose [6,7], D-glUCOSe,
D-fructose and D-xylose [5].
299
Moreover, the oligosaccharides formed from the pyrolytic transglycosylation
of carbohydrate
derivatives or the condensation of the reducing sugars often
show UV and IR absorption
bands that could be due to the presence of
3deoxy-hexosulose
units or its tautomeric forms [lo].
In view of the above considerations,
pyrolytic
reactions of 3-deoxy-Derythro-hexosulose
(3-deoxy-glucosulose)
were investigated
with dynamic
thermal analysis and parallel chemical methods in order to gain better understanding of the dehydration,
rearrangement,
decarboxylation
and charring,
which could ensue after the initial formation
of this compound. The resulting data showed that 3deoxy-D-erythro-hexosulose
[ 151 decomposes
rapidly within the range of 100-200°C
as compared to D-glucose [16], levoglucosan and glycosides [8] which decompose at the much higher temperatures of 250-350°C.
Within this range, a small fraction of the compound
gives 5-(hydroxymethyl)-2-furaldehyde,
pyruvaldehyde,
glycolaldehyde,
glyceraldehyde
and glucometasaccharinic
acid as monomeric
products of
dehydration,
fission and rearrangement,
while most of the compound undergoes condensation and chars.
As in aqueous reactions [ 141, the dehydration
reactions are strongly catalyzed by presence of acidic reagents. GLC analysis (see Fig. 21) has shown
that acid catalyzed pyrolysis of cellulose at ca. 350” C produces a pyrolyzate
containing levoglucosenone
(instead of levoglucosan)
as the major component and 1,4:3,6dianhydro-c-D-glucopyranose,
2-furaldehyde
and 5-(hydroxymethyl)-2-furaldehyde
as minor components
[ 171. Levoglucosenone,
formed by dehydration
reactions shown in Fig. 22, could se separated by
fractional distillation,
and is a highly reactive compound
that could be ob-
Fig. 21. GLC analysis of the pyrolyzate from cellulose + 2% phosphoric acid at 350°C.
Peaks: A, furfural; B, levoglucosenone; C, 1,4:3,6-dianhydroiu-n-glucopyranose;
D, 5-hydroxymethyl-2-furaldehyde.
300
1
Cellulose
N
Ho#-q - &OH
0
OH
Levoglucosan
.1,4: 3,6- Dianhydro
-D-glucopyranose
Levoglucosenone
Fig. 22. Dehydration
of cellulose
and glucose derivatives
-IX -
to levoglucosenone.
tamed by pyrolysis of waste paper, treated with mineral acids. It could be
readily converted to a variety of vinyl-substituted
and addition compounds.
Some of these reactions are already investigated, but the industrial applications of this compound still remain to be explored.
FISSION
AND
DISPROPORTIONATION
On further heating,
tion, disproportionation,
TABLE 6
Pyrolysis products
Pyrolysis
product
fission of the sugar units accompanied by dehydradecarboxylation
and decarbonylation
provides a vari-
of 1,6-anhydro+D-glucopyranose
at 600” C
Yield
Neat
Acetaldehyde
Furan
Acrolein
Methanol
2,3-Butanedione
2-Butenal
1-Hydroxy-2-propanone
Glyoxal
Acetic acid
2-Furaldehyde
5-Methyl-2-furaldehyde
Carbon dioxide
Water
Char
Balance (tar)
REACTIONS
1.1
1.0
1.7
0.3
0.5
0.7
0.8
1.4
1.7
0.9
0.1
2.9
8.7
3.9
74.3
+ZnClz
0.3
1.3
< 0.1
0.4
0.8
0.2
< 0.1
< 0.1
0.7
3.0
0.3
6.8
20.1
29.0
36.8
+NaOH
-
7.3
1.6
2.6
0.7
1.6
2.2
1.1
4.9
1.5
0.4
5.7
14.1
16.0
40.3
301
ety of carbonyl, carboxyl and olefinic compounds as well as water, carbon
dioxide, carbon monoxide and char [ 5,18,19]. The analyses of these products are closely similar to those obtained from pyrolysis of levoglucosan
shown in Table 6 [20]. These compounds
may be divided into three categories. The first category includes furan compounds,
water and char, which
are the expected products of the better understood acid catalyzed dehydration reactions of carbohydrates
under aqueous acid conditions [4]. The second category includes glyoxal, acetaldehyde and other low-molecular-weight
carboxyl compounds, which are similar to the alkaline catalyzed fission products of the sugar molecule formed through the reverse aldol condensation
mechanism. The products formed through these pathways are further randomized by disproportionation,
decarboxylation
and decarbonylation
reactions to provide a third category which is characteristic
for the pyrolytic
CHOH
CHOH
I
HCOH
I
H-OCH
I
HCOH
HCOH
I
H&OH
I
CH20H
I
HCO-I
&i20H
-CH2
CH20H
COH
I
COH
!H
-+
CH20H
-->
HCOH
CH20H
2
CHO
!H
I
c=o
CH20H
CH
CHO
co
I
c=o
I
CH20H
CH2
60
CH20H
CH3
I
2
HCOH
I
;OH
!H
CHO
I
COH
I
HCOH
I
c-
CHO
II
CHOH
+
CHO
I
3
I
CHO
CH3
c=o
I
c=o
CH3
CH
+3
CH20
I
c=o
I
I
c--
+
I
I
i
co
+
co-co
l ---~
'ZH6
Fig. 23. Degradation
I
CH3
CO'
I
I
CH3
of levoglucosan
CH3
to volatile
products.
c-
CH20
+
CHO
I
CH3
2-Furaldehyde
2,3-Butanedione
Pyruvaldehyde
Acetaldehyde
Glyoxal
Carbon dioxide
Carbon monoxide
Compound
TABLE 7
Percentage of the pyrolysis
30.2
16.5
23.3
6.3
25.5
31.2
38.4
103.4
54.6
26.3
30.5
19.2
24.5
18.9
35.8
31.3
19.1
36.0
6.9
6.3
16.8
60.8
24.8
27.3
10.1
15.4
34.3
21.1
l-‘4c
6-14C
2-‘4c
l-‘4C
100.7
31.0
42.0
29.2
48.3
17.7
18.0
2-‘4C
73.0
56.5
30.7
55.1
36.0
8.9
13.7
6-14C
carbons of 1,6-anhydro-p-D-glucopyranose
5% NaOH
traced to the labeled
Neat
products
86.0
64.8
49.7
4.4
29.5
43.7
36.7
1J4C
5% ZnC1.2
95.8
57.7
46.7
7.3
28.2
33.3
27.6
2-‘4c
16.6
26.9
29.6
29.8
28.2
9.5
11.4
6-14C
303
-CH
r-
I
CHOH
I
HCOH
I
HCOH
I
HCOH
!OH
I
I
I
HOCH
CHOH
CHO
-0CH
--+
I.
HCOH
I
HCOH
--j
I
HCOH
I
HCO-
I
-CH2
I
-0CH2
-
co
CH20
+
CHO
CH20H
CH20H
I
c-o
c=o
I
- oc
I
II
CH
<----
I
I
-CH
Fig.
24.
Production
of
2-furaldehyde
under
alkaline
conditions.
reactions, especially at the elevated temperatures.
In the absence of acidic or alkaline catalysts, both fission and dehydration
products are formed. However, as expected, addition of these catalysts promotes the formation of one type of product at the expense of the other.
The chemical reactions involved were further investigated by the synthesis
of levoglucosan labeled at Cl, C2, and C6 and tracing of the major pyrolysis
products to the original positions of the sugar molecule [ 191. The resulting
data shown in Table 7 indicate the major dehydration
and dission pathways
shown in Figs. 23 and 24. These schemes are more complex and less specific
than the single pathways generally postulated for normal reactions, because
at high temperatures,
the energy barrier becomes less significant and the
thermal decomposition
could proceed through different routes [ 41.
CONCLUSIONS
The above data indicate that pyrolysis at lower temperatures
(below
300°C) favors the production
of char, water, CO* and CO, whereas heating
at higher temperatures (300-500°C)
favors the production
of tar, containing
anhydrosugars, oligosaccharides and some pyran and furan dehydration
products. Flash pyrolysis at still high temperatures
(above 500%) results in the
fission, dehydration, disproportionation,
decarboxylation
and decarbonylation
304
reactions, which provide a mixture of low-molecular-weight
gaseous or volatile products. Indirect evidence shows that these materials are formed from
the secondary reaction of the tar and high temperature
interaction
of char
with water and carbon dioxide (see Fig. 5). However, the possibility of direct
conversion of cellulose molecules to the low-molecular-weight
fission and
disproportionation
products could not be entirely eliminated.
ACKNOWLEDGEMENTS
The author is pleased to acknowledge the contributions
of the postdoctoral fellows, research associates and graduent students who are named in
the following list of references. He is also grateful to the National Science
Foundation
and the National Bureau of Standards for their support of this
program.
REFERENCES
1 F. Shafizadeh and G.D. McGinnis, Morphology
and biogenesis of cellulose and plant
cell walls, Advan. Carbohydr.
Chem., 26 (1971) 297.
2 F. Shafizedeh
and G.D. McGinnis,
Chemical composition
and thermal analysis of
cottonwood,
Carbohyd. Res., 16 (1971) 273.
3 F. Shafizadeh and A.G.W. Bradbury, Thermal degradation
of cellulose in air and nitrogen at low temperatures,
J. Appl. Polym. Sci., 23 (1979) 431.
4 F. Shafizadeh,
Pyrolysis and combustion
of cellulosic materials, Advan. Carbohydr.
Chem., 23 (1968) 419.
5 F. Shafizadeh,
Industrial
pyrolysis of cellulosic
materials, Appl. Polym. Symp., 28
(1975) 153.
6 F. Shafizadeh and Y.L. Fu, Pyrolysis of cellulose, Carbohyd. Res., 29 (1973) 113.
7 F. Shafizadeh,
R.H. Furneaux,
T.T. Stevenson and T.G. Cochran, 1,5-Anhydro-4deoxy-D-glycero-hex-1-en-3-ulose
and other pyrolysis products of cellulose, Carbohyd. Res., 67 (1978) 433.
8 F. Shafizadeh,
R.A. Susott and G.D. McGinnis,
Pyrolysis of substituted
phenyl
p-D-glucopyranosides
and 2-deoxy-or-D-arabino-hexopyranosides,
Carbohyd.
Res.,
22 (1972) 63.
9 F. Shafizadeh,
M.H. Meshriki
and R.A. Susott, Thermolysis
of phenyl glycosides,
J. Org. Chem., 38 (1973) 1190.
10 F. Shafizadeh
and Y.Z. Lai, Thermal
rearrangements
of cellobiose and trehalose,
Carbohyd. Res., 31 (1973) 57.
11 Y.Z. Lai and F. Shafizadeh,
Thermolysis
of phenyl O-D-glucopyranoside
catalyzed
by zinc chloride, Carbohyd. Res., 38 (1974) 177.
12 F. Shafizadeh,
G.D. McGinnis and C.W. Philpot, Thermal degradation
of xylan and
related model compounds,
Carbohyd.
Res., 25 (1972) 23.
13 F. Shafizadeh, R.H. Furneaux, T.G. Cochran, J.P. Scholland and Y. Sakai, Production
of levoglucosan
and glucose from pyrolysis of cellulosic materials, J. Appl. Polym.
Sci., 23 (1979) 3525.
14 A.G.W. Bradbury,
Y. Sakai and F. Shafizadeh,
A kinetic model for pyrolysis of cellulose, J. Appl. Polym. Sci., 23 (1979) 3271.
15 F. Shafizadeh
and Y.Z. Lai, Thermal degradation
of 3-deoxy-D-crythro-hexosulose,
Carbohyd. Res., 40 (1975) 263.
16 F. Shafizadeh
and Y.Z. Lai, Base-catalyzed,
pyrolytic
rearrangement
of some monosaccharides, Carbohyd. Res., 26 (1973) 83.
17 F. Shafizadeh,
R.H. Furneaux and T.T. Stevenson, Some reactions of levoglucosenone, Carbohyd. Res., 71 (1979) 169.
305
13 F. Shafizadeh,
Y.Z. Lai and C.R. McIntyre, Thermal degradation
of Gchlorocellulose
and cellulose-zinc
chloride mixture, J. Appl. Polym. Sci., 22 (1978) 1133.
19 F. Shafizadeh and Y.Z. Lai, Thermal degradation
of 1,6-anhydro-P-D-glucopyranose,
J. Ore. Chem., 37 (1972)
278.
20 F. Shafizadeh,
C.W. Philpot
and N. Ostojic, Thermal analysis of 1,6-anhydro-p-Dglucopyranose,
Carbohyd. Res., 16 (1971) 279.