The pyrolysis chemistry of a β-O-4 type oligomeric lignin model compound

Article abstract of DOI:10.1039/c2gc36332a

The pyrolysis behavior of a β-O-4 type oligomeric lignin model compound is studied at a temperature range from 250 °C to 550 °C. Thermogravimetric analysis (TGA) indicates the model compound has three distinct thermal decompositions peaks when a slow temp

Full text of DOI:10.1039/c2gc36332a

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The pyrolysis chemistry of a β-O-4 type oligomeric
lignin model compound
Cite this: Green Chem., 2013, 15, 125
Sheng Chu, Ayyagari V. Subrahmanyam and George W. Huber†*
The pyrolysis behavior of a β-O-4 type oligomeric lignin model compound is studied at a temperature
range from 250 °C to 550 °C. Thermogravimetric analysis (TGA) indicates the model compound has three
distinct thermal decompositions peaks when a slow temperature ramp is applied while only one
decomposition peak is observed when a higher temperature ramp is used. The pyrolysis behavior of this
model compound is similar to that of lignin prepared by enzymatic hydrolysis of Maplewood. 1H-NMR
shows that the β-O-4 linkage thermally decomposes at a temperature between 250 °C and 350 °C with
the formation of solid products at 350 °C. This solid product undergoes transformation to a polyaromatic
from 350 °C to 550 °C. Around twenty five volatile compounds are quantified by pyroprobe-GC-MS with
vanillin and 2-methoxy-4-methyl phenol being the most abundant monomeric products. A free radical
reaction pathway is proposed to explain the product chemistry for pyrolysis of the lignin model
compound. Char is most likely formed by random repolymerization of the radicals.
Received 22nd August 2012,
Accepted 8th October 2012
DOI: 10.1039/c2gc36332a
www.rsc.org/greenchem
phenolic compounds yield was 17.2 wt% for Alcell lignin and
individual yield of most of the compounds were less than 1 wt
Introduction
The pyrolysis of biomass is receiving tremendous interest as a %. The thermal decomposition and weight loss of various
potential method for converting solid biomass into liquid lignin sources were studied by D. J. Nowakowski.20 He found
transportation fuels.1–9 Lignocellulosic biomass is one of the the major decomposition of lignin occurred at a temperature
most promising renewable resources because it is cheap and range of 350 °C to 450 °C and that the heating rate affected the
abundant.10–12 Lignin is the second most abundant com- amount of volatile products. Most research on lignin pyrolysis
ponent of biomass and occupies about 15%–30% in biomass has focused on the product identification. However, to further
by dry weight.13,14 Lignin is a three dimensional amorphous gain the insight into the behaviour of pyrolyzing lignin, it is
polymer containing methoxylated phenyl propane structure imperative that we study its kinetic parameters. Unlike under-
which is polymerized by monolignols (p-coumaryl, coniferyl standing the chemistry and kinetics of the pyrolysis of cellu-
and sinapyl alcohols).15 Over eight types of linkages have been lose and hemicelluloses,21–28 the complexity of the lignin
identified in lignin structure.16 The β-O-4 bond is the major structure and its high molecular weight present make the
type of linkage which occupies 46%–60% of the total linkages study of lignin pyrolysis more complex. Lumped kinetic par-
depending on the type of wood.17
ameters and apparent activation energies for lignin pyrolysis
Pyrolysis of lignin has been studied by a handful of people have been estimated by others.9–31 However the detailed mech-
over the decades. In 1970s, Iatridis et al. pyrolyzed lignin in a anism and kinetics are unknown.
“captive sample” reactor at temperature of 400 °C–700 °C and
Several so called “lignin model compounds” have been
only identified a few compounds by gas chromatography studied. These lignin model compounds have simple struc-
including hydrocarbons, methanol, acetone, phenol and guaia- tures and product distributions compared to real lignin.
col due to the limited analytical technology.18 Recently, Guaiacol is the simplest monomeric model compound and its
Guozhan Jiang et al. identified about 50 compounds from pyrolysis behaviour was studied by Bredenberg in 1987. Cate-
lignin pyrolysis at a temperature range of 400 °C–800 °C.19 The chol and phenol were the dominant products at 400 °C. A free
radical reaction and a concerted reaction mechanism were
suggested to explain guaiacol pyrolysis.32 Other substituted
Department of Chemical Engineering, University of Massachusetts, Amherst, 686
monomeric phenolic compounds such as syringol, isoeugenol,
North Pleasant Street, 159 Goessmann Lab, Amherst, MA 01003, USA.
vanillin, anisole and dimethoxy-phenols were tested by Klein
E-mail: huber@engr.wisc.edu; Tel: +1 608-283-0346
†Current address of G. W. Huber, University of Wisconsin-Madison, Chemical
in 1981 to study the effects of functional groups on reactivity.33
A free radical mechanism has been proposed by Schlosberg34
and Biological Engineering, 1415 Engineering Hall, Madison, WI 53706.
and Masuku35 to explain the pyrolyzing monomeric lignin
Email: huber@engr.wisc.edu; Tel: +1 608-283-0346.
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model compound. Since β-O-4 is the most common linkage diisopropylamide solution by the nucleophilic addition of car-
existing in lignin, the pyrolysis behavior of β-O-4 type dimers banion to an aldehyde group (Step 2 in Fig. 1). In the third
has been widely studied. Britt et al.36 studied the fast vacuum step, the t-butyl group was reduced to a hydroxyl group by
pyrolysis of phenethyl phenyl ether (PPE) and proposed a lithium aluminium hydride. The oligomeric lignin model com-
complex reaction pathway which was dominated by free-radical pound synthesis was completed after acetylation.
reactions, molecular rearrangements, and concerted elimin-
Lignin residue from maple wood
ation reactions. A comprehensive computational study of PPE
including oxygen–carbon bond dissociation enthalpies, Pyrolysis of lignin residue from maple wood was used to
phenyl-shift, hydrogen abstraction and substituent effects have compare with lignin model compound. Maple wood was
been done by Beste et al.37–39 Kawamoto and co-workers have treated with hot water to extract hemicelluloses. Solid residues
proposed that β-ether bond homolysis initiated the radical containing lignin and cellulose were separated by filtration.
reaction. However, with different side chain or substituted After washing several times, the solid was hydrolysed by
groups, the mechanism may change.40 The products distri- enzymes (Spezyme and Novozyme) at pH 4.8 and 50 °C. Lignin
bution of pyrolyzing dimer is more complex than that of residue was obtained after extracting cellulose. The detailed
monomers because secondary reactions occur.41 Recently, preparation method and analysis are described by Jae et al.14
Jarvis et al. studied the pyrolysis of PPE in a wide temperature The major impurities in the lignin residue sample were cellu-
range of 300 °C to 1350 °C. They suggest that concerted reac- lose (11.6 wt%) and hemicellulose (3.3 wt%).
tions dominate over free radical reaction under typical pyroly-
Thermogravimetric analysis
tic condition.42 Clearly more research is needed to understand
the complicated pyrolysis chemistry. Although pyrolysis of Pyrolysis experiments were conducted using TA instruments
monomers and dimmers has provided us insight into the SDT Q600 system attached to a quadrupole mass spectrometer
lignin pyrolysis chemistry, the polymeric linkages in lignin (Extorr XT 300 with an electron ionization voltage at 27 eV).
bring much complexity to pyrolysis chemistry. Oligomeric A flow rate of 100 ml min−1 high purity helium was used as
lignin model compounds are more similar to lignin than carrier gas in the chamber to sweep gas and volatile products.
monomeric and dimeric model compounds. However, little The behavior of pyrolyzing lignin model compounds was
research has been done on the pyrolysis of oligomeric lignin tested using different temperature ramps. Before the tempera-
model compounds.43
ture program started, samples were put into the alumina pan
The objective of this paper is to study the pyrolysis chem- to dry for one hour at 110 °C in the TGA chamber. In order to
istry of an oligomeric lignin model compound that contains analyze the solid char at different temperatures, the samples
β-O-4 linkage by using both TGA and pyroprobe at a relative were heated at a temperature ramp of 150 °C min−1, the
slow heating ramp. We propose a free radical pathway to desired temperature was maintained for three minutes before
explain the products we observe. This paper strives to provide cooling down to room temperature.
a scientific basis to understand the chemistry of the pyrolysis
of lignin.
Pyroprobe-GC-MS system
Pyrolysis experiments were also conducted in a model 2000
pyroprobe analytical pyrolizer (CDS Analytical Inc.). The pyrop-
robe was connected to a 5890 model GC attached to a Hewlett
Experimental
Packard model 5972A mass spectrometer to measure the
Oligomeric lignin model compound
product composition. A capillary column (Restek Rtx-5sil MS)
was used with helium as the sweep gas to perform the
separation.
The oligomeric lignin model compound was synthesized
according to the method of Katahira et al.44 as shown in Fig. 1.
The first step was synthesizing t-butoxycarbonlymethyl vanillin
by reacting vanillin with t-butyl-2-bromoacetate and K2CO3/KI
(Step 1 in Fig. 1). Polymerization of t-butoxycarbonlymethyl
vanillin was conducted in the presence of lithium
1H-NMR and 13C-NMR
Samples were dissolved in CDCl3 and scanned by nuclear mag-
netic resonance spectrometer (Bruker 400, AV400). 1H scan was
carried out with a transmitter frequency at 400 MHz with a
receiver gain at 362 and dwell time at 60 μs. 13C signal was col-
lected at a frequency of 100 MHz with a receiver gain at 32 768
and dwell time at 20 μs.
Gel permeation chromatography
Gel permeation chromatography was used to measure the mol-
ecular weight distribution. Samples were dissolved in THF and
injected into Shimadzu HPLC system (SIL-20ACHT Auto
sampler, LC-20AD Solvent Delivery Module, DGU-20A5 Degas-
ser, CTO-20A Column Oven, SPD-M20A UV-Vis detector) with
Fig. 1 Synthesis of the oligomeric lignin model compound.
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mesopore column from Agilent at a flow rate of 0.5 ml min−1
.
Polystyrene was used as calibration standard and the signal
wave length for UV-Vis was 254 nm.
Fourier transform infrared spectroscopy (FTIR)
Bruker Equinox 55 infrared spectrometer was used to charac-
terize the structure change of the solid products during the
lignin model compound pyrolysis. The number of scans was
set at 256 with a resolution of 4 cm−1 over the range
4000–400 cm−1. KBr was mixed with the model compound in
the FTIR measurement to lower the viscosity, while solid char
powder was measured without KBr.
Total organic carbon analysis (TOC)
The carbon content of the solid products from pyrolysis was
quantified using TOC analyzer. Solids were combusted at
900 °C in an oxygen stream in a Shimadzu Solid Sample
Module SSM-5000 A. Carbon dioxide was quantified with a Shi-
madzu TOC-V CPH. Potassium hydrogen phthalate was used
as calibration standard.
Results and discussion
Characterization of oligomeric lignin model compound
Fig. 3 13C-NMR of lignin model compound.
1
Fig. 2 shows the H-NMR spectrum of the lignin model com-
pound. The acetyl group peak is at 2.0 ppm. The methoxyl
group peak is around 3.8 ppm which shows the same chemical
shift as in lignin.45 The peaks at 4.6 ppm and 6.0 ppm demon-
strate the existence of Hβ and Hα, which proves the presence of
β-O-4 linkage in the model compound. The chemical shift of
side chain protons are seen from 4.0 ppm to 5.0 ppm and aro-
matic peaks seen around 7.0 ppm.
The peak of each carbon from 13C-NMR in lignin model
compound is labeled in Fig. 3. The peaks at 80 ppm and
Fig. 4 GPC of Lignin model compound (Mn = 1264 Mw = 1755 PD = 1.38 DPn
= 4.51).
74 ppm indicate a β-O-4 structure in the compound. All the
peaks have the same chemical shift as the work of Katahira
et al. with the exception of an extra peak at 1.3 ppm44 in Fig. 2
indicative of tert-butyl group which was not reduced to a
hydroxyl group probably due to steric hindrance effects.
Fig. 4 shows the GPC data of the lignin model compound.
The peak has a molecular weight range from 200 Da to
10 000 Da. The average number molecular weight (Mn) is 1264
Da and the average weight molecular weight (Mw) is 1755 Da.
Fig. 2 1H NMR of lignin model compound.
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The polydispersity is 1.38 and the degree of polymerization is weight loss peak at 320 °C and a very slow decomposition
4.51. The lignin model compound has a lower molecular above 400 °C. In the case of a higher temperature ramp
weight than pyrolytic lignin produced by D. Meier et al. from (150 °C min−1), the major decomposition peak for the lignin
pyrolysis of beech wood which ranges from 162 to 50 000 Da.45 model compound shifts to 350 °C–380 °C with a shoulder
From our previous study, the soluble part of lignin residue around 300 °C (Fig. 5(e) and (f)). The major weight loss for the
from Maplewood has a similar molecular weight to the lignin lignin residue is around 400 °C. Compared with lignin residue,
model compound.46 There is a small peak from 200 to 300 Da the lignin model compound decomposes at a lower tempera-
which indicates that a small part of the vanillin-based ture and faster than lignin residue (Fig. 5(a), (c) and (e)). Less
monomer did not polymerize correctly.
char forms in the pyrolysis of the model compound than the
actual lignin residue. This is due the relatively simpler struc-
ture and smaller molecular weight of the model compound.
However, the lignin model compound displays similar pyrol-
ysis behavior to that of the lignin residue (Fig. 5(d) and (f))
with the major decomposition occurring in the same tempera-
ture region when applying a temperature ramp of 150 °C
Thermogravimetric analysis
The lignin model compound and the lignin residues from
enzymatic hydrolysis of maple wood were pyrolyzed in a TGA
at various heating ramps as shown in Fig. 5. The pyrolysis of
the lignin residue has previously been characterized in detail
by Cho et al.46 At a temperature ramp of 1 °C min−1 both the
lignin model compound and the lignin residue show several
decomposition peaks (Fig. 5(a) and (b)). The lignin model
compound decomposes at a lower temperature than the lignin
residue. Three major weight losses for the lignin model com-
pound are at temperatures of 190 °C, 260 °C and 550 °C
whereas lignin residue has two major weight losses at peaks of
260 °C and 470 °C, respectively. However, the peak at 260 °C
from lignin residue is believed to be the decomposition of
impurities (cellulose and hemicelluloses).At the temperature
ramp of 15 °C min−1, the weight loss peaks shift to higher
temperature (Fig. 5(c) and (d)).The lignin model compound
starts to decompose at 230 °C followed by the decomposition
at 300 °C and 650 °C. The lignin residue only has one major
min−1
.
The temperature programming in the TGA was set to four
different final temperatures (250 °C, 350 °C, 450 °C and
550 °C) with a temperature ramp of 150 °C min−1, after which
the final temperature was maintained for three minutes fol-
lowed by cooling down to room temperature. All the products
in TGA alumina pan were transferred into an organic solvent
such as THF or chloroform for separation and analysis. The
products which can be extracted by the organic solvent are
hereby referred to as soluble products. The soluble products
were analyzed by GPC and 1H-NMR. A black solid was left after
organic solvent extraction and we claim it to be solid char. All
pyrolysis products at a final temperature of 250 °C were com-
pletely soluble in the organic solvent. Char formation was first
Fig. 5 Thermogravimetric (a, c and e) and differential thermal (b, d, and f ) curves for the pyrolysis of lignin model compound (dash line) and lignin residue after
enzymatic hydrolysis (solid line) at temperature ramps of 1 (a and b) 15 (c and d) and 150 °C min−1 (e and f).
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Fig. 6 GPC data of lignin model compound (solid) and samples taken at
different temperatures in TGA at a temperature ramp of 150 °C min−1: 250 °C
(dash), 350 °C (dot).
observed at the temperature of 350 °C. The products obtained
from 450 °C and 550 °C were not soluble in organic solvents.
Fig. 6 shows the GPC results for the soluble products. The
black line is the unpyrolyzed model compound. The peak
around 200 Da to 300 Da in the model compound disappears
in the products produced at 250 °C and 350 °C. This indicates
Fig. 7 1H NMR data of lignin model compound (a) and samples pyrolyzed at
different temperatures: 250 °C (b), 350 °C (c).
unpolymerized monomers are volatile below 250 °C. Based on compound as shown in Fig. 6. This indicates the repolymeriza-
this observation, we can conclude that the peaks at 190 °C, tion of the small species which are formed via bond cleavage.
The soluble products at 350 °C still contains aromatic based
230 °C and 300 °C in Fig. 5(b), (d) and (f) respectively are
caused by the volatilization of monomers. Since all the pro- structure containing the methoxyl group (3.8 ppm) and ali-
ducts produced from 250 °C were soluble in organic solvent, phatic side chains.
Solid products were taken from the 350 °C to 550 °C reac-
tions and analyzed by FTIR. The results are shown in Fig. 8.
this indicates the lignin oligomeric compound does not
decompose under 250 °C.
The soluble products produced at 250 °C and 350 °C were Table 1 shows the characterization of peaks in the spectrum.
The band at 3471 cm−1 represents the –OH stretch and it
1
dissolved in d-chloroform and analyzed by H-NMR as shown
in Fig. 7. The original lignin model compound and the soluble decreases in size from 350 °C to 550 °C (Fig. 8(b)–(d)).
products obtained at 250 °C have a very similar structure However, a small proportion of this band still exists in the
1
sample collected at higher temperature suggesting it still con-
based on the H-NMR spectra. The peak of Hβ at 4.6 ppm and
Hα at 6 ppm are still observed. This indicates the existence of tains hydroxyl groups in the product structure. The bands of
Cα–Cβ bond and β-O-4 linkage in the products formed at –CH stretch in aliphatic chain and methoxyl group are at
2940 cm−1 and 2840 cm−1, respectively. These bonds also
250 °C. These results are in an agreement with GPC results
which confirm the lignin model compound does not decom- decrease with increasing temperature. A small amount of ali-
pose below 250 °C. The changes at lower chemical shifts can phatic stretching exists in the sample collected at higher temp-
erature. The carbonyl group in the lignin model compound
be explained by the reaction of unreduced tert-butyl group or
monomeric volatilization. The spectrum of sample obtained at shows at the band of 1740 cm−1. This peak decreases and
350 °C is shown in Fig. 7(c). We do not observe the peak at almost disappears in the sample pyrolyzed at higher tempera-
ture. The aromatic ring vibration bands appear at a range from
4.6 ppm or 6.0 ppm which means β-O-4 linkage was cleaved
and lignin model compound begins to decompose before 1400 cm−1 to 1600 cm−1. In the lignin model compound
350 °C. This is consistent with the major weight loss observed (Fig. 8(a)), the aromatic ring vibration peaks are separate.
However, the peaks merge to a broad band in the sample col-
in thermal curve at 350 °C (Fig. 5(f)), which is the major reac-
tion stage after the monomer volatilization. Since the breaking lected at higher temperature which implies that a polyaromatic
of β-O-4 linkage occurs between 250 °C and 350 °C, we can structure forms. The typical guaiacol band appears at
1226 cm−1. This peak is in the original compound but disap-
surmise that the structure of the lignin model compound
changes as well in this temperature range. However we observe pears in the solid products. The deformation of C–H bond in
the products at 350 °C have similar molecular weight distri- aromatic ring is at 1032 cm−1. This band also disappears when
the pyrolysis temperature is above 350 °C. This indicates the
bution as both the products at 250 °C and the lignin model
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Fig. 9 Pyrolysis of lignin model compound at temperature ramp of 15 °C
min−1 by TGA. Mass spectroscopy recorded several mass to charge ratios during
the reaction as shown 18 (solid), 28 (dash) and 44 (dot), respectively.
a function of reaction temperature at a temperature ramp of
15 °C min−1. Water is produced primarily at a temperature
range of 350 °C–400 °C. This corresponds with the –OH peak
decreasing in solid char (Fig. 8 from b to c). A small amounts
of water is also produced at higher temperature. A large
amount of carbon dioxide is observed at temperatures from
600 °C to 800 °C. The mass charge ratio of 28 represents
carbon monoxide and it increases with temperature until it
reaches a plateau. This is probably caused by char reduction
and water gas shift reaction described by Suyitno.48 However,
we cannot detect other species in our system due to the low
concentration of products.
Fig. 8 FTIR spectrum of (a) lignin model compound and solid residue obtained
from the temperature of (b) 350 °C, (c) 450 °C and (d) 550 °C.
Table 1 Characterization of FTIR spectra of lignin model compound
Wave number
(cm−1
)
Characteristics
3471
2940
2840
1740
1592
1513
1422
1373
1226
1141
1032
OH group
CH (aliphatic and aromatic)
CH (methoxy group)
CvO group
Aromatic ring vibration
Pyroprobe-GC-MS analysis
Detailed product distributions for pyrolysis of the lignin
model compound were collected by pyroprobe-GC-MS system.
Table 2 shows the mass balance of pyrolyzing lignin model
compounds at a temperature ramp of 150 °C min−1. A fast
temperature ramp of 1000 °C s−1 was also applied for compari-
son. We report our products in four different categories: gas,
liquid, solid and unidentified. Gas mainly consists of carbon
monoxide and carbon dioxide. Liquid products are quantified
by all the species detected by GC-MS except gas products.
These liquid compounds are mainly mono aromatics. After
Aromatic ring vibration
C–H deformation and aromatic ring vibrations
OH in-plane bending and CH bending
Guaiacol unit (G ring and CvO vibrations)
Guaiacol unit (CH in-plane deformation)
Aromatic C–H deformation and C–O, C–C
stretching
formation of a nonproton polyaromatic ring. It is noticed that
obvious changes happen to the solid structure from 350 °C to
450 °C which indicates the major reaction happens in this
temperature range, which corresponds to the huge weight loss
in Fig. 5(f). From the previous work of lignin pyrolysis, a poly-
cyclic aromatic hydrocarbon structure has been identified to
form at higher temperature.47 It is clearly observed that even
with a simple lignin model compound with a single type of
linkage, we observe similar char formation as has been
observed in pyrolyzing lignin.
Table 2 Mass balance of pyrolysis of lignin model compound in pyroprobe at
temperature ramp of 150 °C min−1
Temperature (°C) Gas
Liquid
Solid
Unidentified
250
350
450
550
550b
0.10% (1 : 4.8)a 11.80% 74.10% 14.00%
2.20% (1 : 5.1)
3.40% (1 : 5.4)
4.30% (1 : 8.9)
2.90% (1 : 3.1)
20.40% 66.30% 11.10%
40.40% 50.30%
59.70% 28.30%
5.90%
7.70%
The gas phase species produced from pyrolysis in the TGA
were measured with mass spectroscopy. The major species
observed were water (MW = 18), carbon monoxide (MW = 28)
and carbon dioxide (MW = 44). Fig. 9 shows these products as
48.30% 32.40% 16.40%
a Parenthesis designates weight ratio of carbon monoxide to carbon
dioxide. b Designates heating rate of 1000 °C s−1
.
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Table 3 Carbon balance of pyrolysis of lignin model compound in pyroprobe at temperature ramp of 150 °C min−1
Temperature (°C)
Gas
Liquid
Solid
Unidentified
250
350
450
550
550b
0.42% (25% : 75%)a
1.00% (23% : 77%)
1.49% (22% : 78%)
1.85% (15% : 85%)
1.28% (34% : 66%)
8.91%
16.06%
31.97%
46.34%
32.97%
86.19%
71.13%
64.75%
48.93%
51.82%
4.48%
11.81%
1.79%
2.88%
13.93%
a Parenthesis designates weight ratio of carbon monoxide to carbon dioxide. b Designates heating rate of 1000 °C s−1
.
each reaction we collected the solid in the reactor tube. The monomer used to produce the model compound. Some of the
mass of the solid was found with a balance and the carbon compounds in Table 4 had a low similarity with the GC-MS
content was analyzed by TOC. We also report the unidentified library indicating that there are some uncertainties in whether
products, which are most likely heavier molecular weight com- or not we correctly identified these compounds. The products
pounds that cannot be detected by GC-MS. Only 0.1 wt% to from the pyrolysis at the fast temperature ramp have also been
4.3 wt% gas was observed with carbon dioxide being the major analyzed. The carbon selectivity of acetic acid and 1,4-butane-
gas product. As the temperature increases, the ratio of carbon diol diacetate decreases with an increased temperature ramp.
monoxide to carbon dioxide decreases. The yield of liquid In contrast, the carbon selectivity of the heavier aromatics
product reaches a maximum (about 60 wt%) at 550 °C. The oli- increases at the faster temperature ramp. This indicates that in
gomeric lignin model compound produces more liquid pro- the fast pyrolysis, the carbon tends to remain in heavier
ducts than lignin19 due to its simpler structure, lower heat species such as char, which is also reflected in the weight and
resistance and higher reactivity. The amount of char formation carbon increase in the Tables 2 and 3 for the experiment at
decreases at the higher temperature. At the highest tempera- 1000 °C s−1
ture, it has 30 wt% char formation. In the previous TGA Compared with dimeric model compound, the reaction
.
results, we observe around 30 wt% and 20 wt% char formation network of pyrolyzing oligomeric model compounds is much
at 450 °C and 550 °C, respectively (Fig. 5(e)). However, in the more complicated. Free radical with concerted reactions have
pyroprobe the char yield is 50 wt% and 30 wt% at the corre- been proposed in PPE. However, in the presence of oligomeric
sponding temperatures. The pyroprobe has slower rates of structure and acetyl groups, there can be various possibilities
mass transfer than the TGA because the carrier gas does not for the reaction pathways. We propose a free radical reaction
flow through the pyrolysis tube. This most likely increases the dominant pathway (Fig. 10) based on the major products
rate of secondary reactions that form coke compared to the observed in Table 4. Previous studies have indicated that the
TGA. The fast heating rate experiment is labelled with asterisk β-O-4 bond cleavage happens at relatively low temperature
in Table 2. The gas and liquid products decrease at the faster because of its low dissociation energy.37,49 This is in agree-
heating rate with an increase in the solid char formation and ment with our FTIR and NMR results discussed above. Rad-
the unidentified products.
icals are generated after Cβ–O homolysis cleavage. This is
Table 3 shows the carbon balance for pyrolysis in the pyrop- believed to be the initiation step for free radical chain reac-
robe. The solid product contains 30% to 75% of the carbon tion.50 In Fig. 10, β-O-4 linkage breaks (cleavage 1 in Fig. 10)
from the lignin model compound. The gas products have less between the temperature 250 °C and 350 °C in our experiment.
than 2% of total carbon content. The carbon in liquid pro- The radicals can abstract the proton from other species which
ducts increases from 9% at lower temperature to 46% at have weak C–H or O–H bonding (such as C6H5–OH) and form
higher temperature. When applying the fast pyrolysis at products. Vanillin and 2-methoxy-4-methyl phenol being the
550 °C, the carbon content in the liquid decreases while the two most abundant monomeric aromatic products are pro-
carbon content in the unidentified products increases. The duced by β-O-4 bond cleavages and H-abstraction. This indi-
lignin model compound has a similar weight and carbon dis- cates the bond cleavage tend to happen at 1, 3 and 4 positions
tribution as lignin produced from maple wood by hydrolysis.46 in Fig. 10. The radicals are passed to other species for further
However, lignin residue has more char formation.
reaction leading to chain propagation. We observe large
Table 4 shows the carbon selectivity of the liquid products amount of acetic acid and 1,4-butanediol diacetate formation.
detected by Pyroprobe-GC-MS under different temperatures at This implies the C–O bond at 4 and 7 positions can easily
a temperature ramp of 150 °C min−1. We were able to detect break. When two radicals collide with each other, they form
more than 25 distinct peaks in the GC-MS. As shown in products and terminate the chain reaction such as Reaction (a)
Table 4, most detectable liquid products are mono aromatics and (b) in Fig. 10. Some products in Table 4 were not ident-
with different functional groups and side chains. These com- ified with a high similarity by GC-MS. These compounds are
pounds are listed by their abundance. Acetic acid is the most not shown in Fig. 10. Secondary reactions can also happen.
abundant product being produced from the acetyl group. The H-abstraction, double bond formation, rearrangement, isomeri-
most abundant monomeric aromatic is vanillin, which is the zation and concerted reaction would diversify the products
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Table 4 Carbon selectivity of liquid products detected by Pyprobe-GC/MS under different temperatures at a temperature ramp of 150 °C min−1
Compound
250 °C
42.6%
350 °C
32.2%
450 °C
34.5%
550 °C
22.1%
550 °Ca
16.3%
Sb
Fc
91
1,3
8.3%
21.7%
8.3%
12.2%
9.1%
7.8%
7.6%
9.1%
8.2%
90
97
(a)
21.8%
1,4
5.7%
6.1%
0.9%
2.5%
3.0%
7.5%
7.1%
7.2%
6.7%
8.6%
96
72
1,3
0.8%
0.7%
3.3%
3.8%
2.6%
3.1%
6.7%
6.3%
8.1%
4.7%
40
59
2.1%
8.2%
4.9%
4.9%
6.0%
58
4.3%
4.7%
2.5%
7.5%
3.4%
5.8%
1.9%
2.1%
0
35
14
3.3%
1.6%
1.2%
2.4%
2.4%
2.7%
50
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Table 4 (Contd.)
Compound
250 °C
1.0%
350 °C
1.8%
450 °C
1.0%
550 °C
1.4%
550 °Ca
3.2%
Sb
Fc
97
(d)
0
0
0
1.7%
0.8%
1.5%
4.5%
2.4%
0.4%
3.8%
0.4%
0.2%
4.4%
0.6%
0.06%
95
86
94
(c)
0
0
0.7%
0.2%
0.7%
0.6%
0.8%
0.7%
0.8%
0.5%
50
83
(b)
0
0
0.9%
0.1%
0
0
5.9%
4.6%
5.0%
4.2%
83
91
0
0
0
0
1.7%
0
0.07%
4.9%
0.23%
5.7%
81
95
1,2
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Table 4 (Contd.)
Compound
250 °C
0
350 °C
0
450 °C
0
550 °C
0.7%
550 °Ca
0.6%
Sb
Fc
91
0
0
0
0.4%
1.1%
59
1,6,7
a Designating temperature ramp is 1000 °C s−1 b S designates similarity search from MS library. c F designates the bond cleavage or reaction
.
number this compound comes from in Fig. 10. d Designating mass spectrometry has low identity on this compound and we may not observe.
Fig. 10 Proposed reaction mechanism of pyrolysis of lignin model compound.
distribution41 such as reaction (c) and (d) in Fig. 10. From the lignin residue.46 Even though we used relatively simple struc-
mass and carbon balance in Tables 2 and 3, we observe that ture model compound in this work compared with lignin
the solid products contain about 50% of carbon content in residue, char formation is still a dominant process. Char most
original lignin model compound. This is a similar result to likely forms from polymerization of smaller radical species
what we have observed in our previous study on pyrolysis of a such as aromatics, alkanes and alkenes (reaction (e) in
134 | Green Chem., 2013, 15, 125–136
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Fig. 10). The reaction propagates with more radicals causing through the Defense Science Office Cooperative Agreement
further polymerization (reaction (f) in Fig. 10). Polyaromatic W911NF-09-2-0010/09-005334 B 01 (Surf-Cat: Catalysts for pro-
char finally forms after the elimination of functional groups duction of JP-8 range molecules from lignocellulosic biomass).
such as hydroxyl and methoxyl groups.
We thank Prof. Paul Dauenhauer from University of Massachu-
It would be desirable to inhibit radicals chain propagation setts Amherst for his helpful discussion with this paper.
reactions and prevent repolymerization during lignin pyrolysis We want to thank Prof. Charles Wyman’s group in UC-
to decrease the char formation and increase the bio-oil pro- Riverside for preparing and providing lignin residue sample.
duction. This could be done by one of two methods: (1) by con- We also would like to thank Prof. Richard Finke from
verting the lignin products before they undergo free radical Colorado State University for his insight into the free radical
reactions or (2) by the addition of free radical inhibitors. From chemistry.
the proposed reaction chemistry, a hydrogen donor would be
effective to stop the chain reaction after the initial bond break.
Both intermolecular and intramolecular H-abstraction can
achieve this. The weak C–H or O–H bond such as aldehyde
Notes and references
and phenol could be taken into consideration to provide the
proton. Other free radical inhibitors including nitrobenzene,
butylated hydroxyl toluene or diphenyl picryl hydrazyl have
shown the ability to stabilize the resonance of the radicals.
A persistent radical would be another alternative. When the
monomer lacks protons, it can easily abstract them from persist-
ent radical to terminate the reaction. However, these compounds
could introduce unwanted elements into the pyrolysis process.
Moreover, the free radical inhibitors need to be in intimate
contact with the lignin and not degrade at the temperatures of
the lignin pyrolysis. More work is needed before lignin can effec-
tively be decomposed into fungible fuels and chemicals.
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