U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2007–5013

Development of Acid Functional Groups and 

Lactones During the Thermal Degradation 

of Wood and Wood Components

Development of Acid Functional Groups and 

Lactones During the Thermal Degradation 

of Wood and Wood Components

By David W. Rutherford, Robert L. Wershaw, and James B. Reeves III

Scientific Investigations Report 2007–5013

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia 2008

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Suggested citation: 

Rutherford, D.W., Wershaw, R.L., and Reeves III, J.B., 2008, Development of acid functional groups and lactones 

during the thermal degradation of wood and wood components: U.S. Geological Survey Scientific Investigations 

Report 2007–5013, 43 p.
iii

Contents

Abstract ...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Methods and Materials.................................................................................................................................2

Materials ................................................................................................................................................2

Char Preparation ..................................................................................................................................2

Elemental Analysis ...............................................................................................................................3

Boehm Titration ....................................................................................................................................3

Diffuse Reflectance Mid-Range Infrared Spectroscopy ...............................................................3

Development of Acid Functional Groups and Lactones ..........................................................................3

Compositional Changes in Chars .......................................................................................................3

Cellulose .......................................................................................................................................5

Lignin ...........................................................................................................................................8

Pine Wood ...................................................................................................................................10

Pine Bark ....................................................................................................................................11

Trends in Compositional Change and Implications for Development of 

Acid Functional Groups ........................................................................................................12

Summary........................................................................................................................................................16

References Cited..........................................................................................................................................20

Appendix 1. Results of Boehm Titrations ................................................................................................22

Appendix 2. Diffuse Reflectance Mid-Range Infrared Spectra ...........................................................26
iv

Figures

1. Illustrations of the calculations used in the Boehm titration method..................................3

2. The mass loss of carbon and oxygen during the charring of cellulose 

 at various charring times and temperatures..........................................................................14

3. The carbon-to-oxygen atomic ratio of cellulose chars at various charring 

 times and temperatures.............................................................................................................14

4. The total acidity of cellulose chars at various charring times and temperatures...........14

5. The strong organic acid fraction of cellulose chars at various charring times 

 and temperatures .......................................................................................................................14

6. The sum of the moderate acid and lactone fraction and weak organic 

 acid fraction of cellulose chars at various charring times and temperatures.................15

7. The total acidity of cellulose, lignin, pine wood, and pine bark chars after 

 72 hours of charring at various temperatures .......................................................................16

Appendix Figures 

2-1. Diffuse reflectance mid-range infrared spectra of cellulose heated at 

 200oC for various times...............................................................................................................26

2-2. Diffuse reflectance mid-range infrared spectra of cellulose heated at 

 250oC for various times...............................................................................................................27

2-3. Diffuse reflectance mid-range infrared spectra of cellulose heated at 

 300oC for various times...............................................................................................................28

2-4. Diffuse reflectance mid-range infrared spectra of cellulose heated at 

 350oC for various times...............................................................................................................29

2-5. Diffuse reflectance mid-range infrared spectra of cellulose heated at 

 400oC for various times...............................................................................................................30

2-6. Diffuse reflectance mid-range infrared spectra of lignin heated at 

 250oC for various times...............................................................................................................31

2-7. Diffuse reflectance mid-range infrared spectra of lignin heated at 

 300oC for various times...............................................................................................................32

2-8. Diffuse reflectance mid-range infrared spectra of lignin heated at 

 350oC for various times...............................................................................................................33

2-9. Diffuse reflectance mid-range infrared spectra of lignin heated at 

 400oC for various times...............................................................................................................34

2-10. Diffuse reflectance mid-range infrared spectra of lignin heated at 

 450oC and 500oC for various times............................................................................................35

2-11. Diffuse reflectance mid-range infrared spectra of pine wood heated 

 at 250oC for various times ..........................................................................................................36

2-12. Diffuse reflectance mid-range infrared spectra of pine wood heated at 

 300oC for various times...............................................................................................................37

2-13. Diffuse reflectance mid-range infrared spectra of pine wood heated at 

 350oC for various times...............................................................................................................38
v

2-14. Diffuse reflectance mid-range infrared spectra of pine wood heated at 

 400oC and 500oC for various times............................................................................................39

2-15. Diffuse reflectance mid-range infrared spectra of pine bark heated at 

 250oC for various times...............................................................................................................40

2-16. Diffuse reflectance mid-range infrared spectra of pine bark heated at 

 300oC for various times...............................................................................................................41

2-17. Diffuse reflectance mid-range infrared spectra of pine bark heated at 

 350oC for various times...............................................................................................................42

2-18. Diffuse reflectance mid-range infrared spectra of pine bark heated at 

 400oC for various times...............................................................................................................43

Tables

1. Carbon and oxygen content of cellulose and cellulose char................................................4

2. Carbon and oxygen content of lignin and lignin char.............................................................5

3. Carbon and oxygen content of pine wood and pine wood char...........................................6

4. Carbon and oxygen content of pine bark and pine bark char...............................................7

5. The results of the Boehm titration of cellulose and cellulose char 

 shown as the milliequivalents of total acidity and its three fractions 

 consisting of the strong organic acid fraction, the moderate organic acid 

 and lactone fraction, and the weak organic acid fraction ....................................................9

6. The results of the Boehm titration of lignin and lignin char shown as 

 the milliequivalents of total acidity and its three fractions consisting of 

 the strong organic acid fraction, the moderate organic acid and lactone 

 fraction, and the weak organic acid fraction.........................................................................10

7. The results of the Boehm titration of pine wood and pine wood char 

 shown as the milliequivalents of total acidity and its three fractions 

 consisting of the strong organic acid fraction, the moderate organic 

 acid and lactone fraction, and the weak organic acid fraction..........................................11

8. The results of the Boehm titration of pine bark and pine bark char 

 shown as the milliequivalents of total acidity and its three fractions 

 consisting of the strong organic acid fraction, the moderate organic 

 acid and lactone fraction, and the weak organic acid fraction..........................................12

9. The pseudo-absorbance of selected absorbance bands for diffuse 

 reflectance infrared spectroscopy of cellulose and cellulose char..................................13

10. The pseudo-absorbance of selected absorbance bands for diffuse 

 reflectance infrared spectroscopy of lignin and lignin char...............................................17

11. The pseudo-absorbance of selected absorbance bands for diffuse 

 reflectance infrared spectroscopy of pine wood and pine wood char.............................18

12. The pseudo-absorbance of selected absorbance bands for diffuse 

 reflectance infrared spectroscopy of pine bark and pine bark char.................................19
vi

Appendix Tables

1-1. Results of Boehm titrations for cellulose and cellulose chars ...........................................23

1-2. Results of Boehm titrations for lignin and lignin chars ........................................................24

1-3. Results of Boehm titrations for pine wood and pine wood chars ......................................25

1-4. Results of Boehm titrations for pine bark and pine bark chars ..........................................25
Abstract

Black carbon (pyrogenic materials including chars) 

in soils has been recognized as a substantial portion of soil 

organic matter, and has been shown to play a vital role in 

nutrient cycling; however, little is known concerning the 

properties of this material. Previous studies have largely been 

concerned with the creation of high-surface-area materials for 

use as sorbents. These materials have been manufactured at 

high temperature and have often been activated. Chars occurring in the environment can be formed over a wide range of 

temperature. Because it is extremely difficult to isolate black 

carbon once it has been incorporated in soils, chars produced 

in the laboratory under controlled conditions can be used to 

investigate the range of properties possible for natural chars.

This report shows that charring conditions (temperature 

and time) have substantial impact on the acid functional group 

and lactone content of chars. Low temperatures (250°C) and 

long charring times (greater than 72 hours) produce chars with 

the highest acid functional group and lactone content. The 

charring of cellulose appears to be responsible for the creation 

of the acid functional group and lactones.

The significance of this study is that low-temperature 

chars can have acid functional group contents comparable to 

humic materials (as high as 8.8 milliequivalents per gram). 

Acid functional group and lactone content decreases as 

charring temperature increases. The variation in formation 

conditions expected under natural fire conditions will result in 

a wide range of sorption properties for natural chars which are 

an important component of soil organic matter. By controlling 

the temperature and duration of charring, it is possible to tailor 

the sorption properties of chars, which may be used as soil 

amendments.

Introduction

The ability of soils to retain nutrients is a function of the 

exchange capacities of their mineral and organic components. 

The most active mineral components of soils with regard to 

ion-exchange reactions are the clay minerals and the hydrous 

metal oxides. Sposito and others (1999) have reviewed the 

mechanisms of metal-ion exchange reactions with clay minerals. Hydrous metal oxides undergo similar types of exchange 

reactions to clay minerals; however, the hydrous metal oxides 

do not have interlayer exchange positions like the clay minerals (Sahai, 2002; Sahai and Sverjensky, 1997). Hydrous metal 

oxide surfaces are generally coated with anionic natural 

organic matter (NOM) molecules that interact with dissolved 

metal ions in soil solutions (Wershaw and others, 1995; Zhou 

and others, 2001). 

In addition to the anionic NOM species, pyrogenic 

carbon derived from the burning of biomass also is present in 

most soils (Glaser and others, 1998). Pyrogenic carbon (the 

so-called black carbon or charcoal fraction) is one of the least 

studied components of soil organic matter (SOM). Charcoal 

can markedly alter the physical and chemical properties of 

soils. Lehmann and others (2003) demonstrated that charcoal 

is crucial in maintaining fertility of the so-called ‘terra preta’ 

black soils of the Amazon Basin of Brazil. These soils occur 

in relatively small, isolated sites averaging about 20 hectares 

(ha) in area. The presence of charcoal in the terra preta 

soils differentiates them from the surrounding oxisols. This 

charcoal apparently was incorporated into the terra preta soils 

by native Indian farmers prior to the coming of Europeans 

to the New World and has persisted until the present. The 

presence of the charcoal greatly increases the capacity of these 

soils to bind nutrients such as nitrogen, phosphorus, calcium, 

and potassium; pH values and water-holding capacities also 

are higher. Charcoal is not only important in terra preta soils; 

it also constitutes a large fraction of the NOM in many of the 

Development of Acid Functional Groups and Lactones 

During the Thermal Degradation of Wood and Wood 

Components 

By David W. Rutherford1, Robert L. Wershaw1, and James B. Reeves III2

1

U.S. Geological Survey, Denver, Colorado

2

U.S. Department of Agriculture, Beltsville, Maryland
2 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

most fertile agricultural soils in the world (Skjemstad and 

others, 2002). Liang and others (2006) have shown that black 

carbon increases the cation exchange capacity in soils.

Elucidation of the physical and chemical properties of 

charcoal in soils has been hampered by the difficulty of isolating natural charcoals from the soil components. Chun and 

others (2004a) evaluated the techniques that have been used 

to isolate charcoals from soils and concluded that the surface 

properties of the charcoal isolates are substantially altered 

by the isolation methods that are currently used. Therefore, 

Chun and others (2004b) studied the properties of the charcoal 

derived from pyrolysis of crop residues. They reasoned that 

these charcoals would be similar to the charcoals produced 

during field burning of vegetation and crop residues. 

To better understand the range of properties that could 

be observed in natural char samples, several studies have used 

laboratory-produced chars that were produced under carefully 

controlled conditions. Rutherford and others (2004) produced 

charcoals from pine and poplar wood, pine bark, and purified 

cellulose and lignin, at temperatures from 250oC to 500oC 

and for times ranging from 1 hour to 168 hours. Changes in 

the chemical structures of the precursors were examined by 

Fourier transform infrared (FTIR) and 13C nuclear magnetic 

resonance (NMR) spectrometry, and elemental composition. 

The porosities of the samples were measured by nitrogen 

adsorption. This work has provided a well-characterized suite 

of charcoals for ion-exchange studies. Brown and others 

(2006) produced chars that were characterized by heliumbased solid density, electrical resistance, elemental composition, and surface-area measurement.

The ion-exchange sites in charcoal consist mainly of 

oxygen-containing functional groups. The acid-base properties 

of activated carbons have been extensively studied (Contescu 

and others 1997; Lopez-Ramon and others, 1999; Salame and 

Bandosz, 2001); however, the exchange properties of soil charcoals should be substantially different from those of activated 

carbons because the conditions of formation are different. 

Activated carbons are generally prepared at high temperatures 

in the presence of activating agents such as steam (Patrick, 

1995). Soil charcoals, however, are generally produced at 

much lower temperatures by forest or grass fires. 

To improve understanding of char development, the U.S. 

Geological Survey and the U. S. Department of Agriculture 

collaborated on a study to determine the conditions favorable 

to the formation of acid functional groups during charring. 

This will allow the estimation of the range of organic acid 

functional groups and lactones that could be expected in natural chars and also will provide insight into ways to produce 

chars with specific properties that could be tailored to specific 

uses.

This report describes development of acid functional 

groups and lactones during thermal degradation of wood 

and wood components. Data on the exchange properties of 

charcoals produced from cellulose, lignin, and pine wood 

and bark under controlled conditions of temperature and 

heating duration in the laboratory are provided. The Boehm 

titration method was used to measure the content of the acidic 

functional groups and lactones (total acidity) in the charcoals 

(Boehm and others, 1964). This method measures the amount 

of NaOH, NaHCO3, and Na2CO3 neutralized by the char. The 

equivalents of NaOH neutralized by the char was used as the 

measure of total acid functional group and lactone content 

of the char. The difference in equivalents of base neutralized 

using the NaOH, NaHCO3, and Na2CO3 treatments was used 

to subdivide the total acidity into three fractions consisting of 

the strong organic acid fraction, the moderate organic acid and 

lactone fraction, and the weak organic acid fraction. This is 

discussed in more detail in the next section.

Diffuse reflectance mid-range infrared (IR) spectroscopy 

and elemental composition will be compared to the titration 

results to elucidate processes taking place during charring 

which produce the acid functional groups and lactones.

Methods and Materials

Materials

Cellulose, purified from wood pulp, was obtained 

commercially as a microcrystalline powder (approximately 

20 micrometer (μm) particle size). The lignin was obtained 

commercially as a powder that had been extracted from mixed 

hardwoods by the Organosolv process (Allan, 1971, 

p. 540-541). The pine wood and bark samples were obtained 

in the Rocky Mountain foothills, north west of Denver, 

Colorado, from a cross-section of a ponderosa pine log (Pinus 

ponderosa). Pine bark material was collected from the same 

log and treated separately from the pine wood. The wood was 

shredded with a hand-held electric plane. The shavings were 

1 millimeter (mm) or less in thickness, 2 to 4 mm in width and 

1 centimeter (cm) or less in length. Wood samples were airdried prior to use. The bark was air-dried and ground by using 

a hand mill. 

Char Preparation 

Char was prepared by weighing the shredded or powdered sample into 250-milliliter (mL) porcelain crucibles. The 

crucibles were heated without tops, with between 10 and 20 

grams (g) of material per crucible. The crucibles were placed 

in a muffle furnace preheated to the desired temperature, with 

nitrogen flow sufficient to produce three air changes per minute within the furnace. The nitrogen used was ultra high purity 

(UHP) grade with less than 1 part per million (ppm) oxygen. After heating, crucibles were placed in a nitrogen-filled 

desiccator to cool to room temperature. The char was then 

pulverized with a mortar and pestle. The percentage yield was 

determined by weighing samples before and after heating and 

has been previously reported (Rutherford and others, 2004). 
Development of Acid Functional Groups and Lactones 3

Elemental Analysis 

Carbon and hydrogen were determined by combustion 

in oxygen at l000°C; the combustion products were passed 

through catalysts to ensure complete oxidation. Water was 

trapped, and carbon dioxide (CO2) was measured with a CO2

coulometer. Moisture was determined by loss on drying at 

105°C overnight. Ash content was determined by combustion in air at 750°C. The results for carbon, hydrogen, and ash 

have been previously reported (Rutherford and others. 2004). 

Cellulose and lignin did not contain any ash. The moisture 

content for all samples and the ash content for wood and bark 

samples were used to adjust the carbon and hydrogen percentage to a dry and ash-free basis. Oxygen was then determined 

as the difference between the sum of the carbon and hydrogen 

percentage and 100 percent. The results for carbon and oxygen 

are summarized in tables 1 through 4. 

Boehm Titration 

Approximately 0.25 gram (g) of char was weighed 

into each of three 60-mL polypropylene bottles. One bottle 

each was filled with 50 mL of 0.05 molar NaOH, NaHCO3, 

or Na2CO3. Each solution was made with a constant ionic 

strength background of 0.1 molar NaNO3. A control without 

char for each solution also was prepared. Samples and controls 

were equilibrated on a tube rotator for approximately 16 hours. 

The solutions with char were filtered through 0.1-micrometer 

glass fiber filters. An aliquot of each solution was titrated with 

0.05 molar HCl. The amount of base neutralized by the char 

was determined by the difference between the solution containing char and the control solution. Each char sample was 

analyzed in duplicate. 

The total acidity is considered to be the amount of NaOH 

neutralized and will consist of any organic acids with an acid 

dissociation constant (pKa) less than 12, and those acids 

formed when lactones hydrolyze. The amount of NaHCO3

neutralized is considered to be due to the strongest of the 

organic acids and is assumed to be primarily carboxylic acids, 

referred to as strong acid fraction. The amount of Na2CO3

neutralized is considered to be due to the strong organic acid 

fraction, and the moderate strength organic acids (low pKa 

phenols) and lactones fraction. When lactones hydrolyze into 

carboxylic acids and phenols, the carboxylic acids and some 

or all of the phenols are expected to be neutralized by the 

Na2CO3. The difference between the milliequivalents (meq) of 

NaHCO3 and Na2CO3 neutralized will be referred to as moderate organic acid and lactone fraction with the understanding 

that this category can include the neutralization of the carboxylic acids and phenol components of lactones after hydrolysis. The difference between the milliequivalents of NaOH 

and Na2CO3 neutralized is composed of the weakest organic 

acids, assumed to be the high pKa phenols, and is referred to 

as weak acid fraction. Some of the phenols could come from 

the hydroysis of lactones. These calculations are illustrated in 

figure 1.

EQUIVALENTS OF BASE NEUTRALIZED

(Mainly carboxylic acids)

(Low pKa phenols and hydroloysis of lactones)

(mainly high pKa phenols)

NaOH NaHCO3 Na2CO3

Weak Acid Fraction

Moderate Acid and Lactone Fraction

Strong Acid Fraction

TREATMENT

Total Acidity 

Figure 1. Illustrations of the calculations used in the Boehm 

titration method.

Diffuse Reflectance Mid-Range Infrared 

Spectroscopy

The diffuse reflectance of all samples was scanned 

in the mid-range infrared from 4000 to 400 cm–1 at 4 cm–1

resolution on a Digilab (Varian, Inc., Palo Alto, Calif.) 

FTS 7000 Fourier transform spectrometer equipped with 

a DTGS (deuterated triglycine sulfate) detector and a 

KBr beam splitter. Each spectrum consists of 64 co-added 

scans. Spectra were obtained in diffuse reflectance mode 

by using a Pike (Pike Technologies, Madison, Wis.) AutoDiff auto-sampler with KBr as the background sample and 

data computed as pseudo-absorbance, log10 1/R, where R 

is the relative reflectance at a given wave length. The term 

pseudo-absorbance is used to differentiate reflectance spectra (pseudo-absorbance) from transmittance spectra (absorbance). The samples were ground and analyzed neat (not 

diluted with KBr). The use of neat samples allows for the 

relative comparison between reflectance spectra of pseudoabsorbance for specific wave lengths.

Development of Acid Functional 

Groups and Lactones

Compositional Changes in Chars

Elemental analyses (carbon and oxygen) are shown in 

tables 1 through 4, as percentage of the char on dry-weight 

and ash-free basis. The percentage mass lost during charring 

is shown, as well as the calculated percentage of carbon and 

oxygen lost.

The results of the neutralization of the NaOH, NaHCO3, 

and Na2CO3 solutions are shown in Appendix 1, along with 

the standard error of the mean. Each titration was performed at 
4 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Table 1. Carbon and oxygen content of cellulose and cellulose char. 

[The mass of carbon and oxygen lost is shown as a percentage of the starting mass of carbon or oxygen; oC, degrees Celsius]

Charring

conditions

Percent1 carbon 

in char

Percent1 oxygen 

in char

Carbon to Oxygen 

atomic ratio

Percent mass 

lost

Percent carbon 

lost

Percent oxygen 

lost

Unaltered

Cellulose 44.4 49.4 1.2 0 0 0

Cellulose Char

200oC

72 hours 46.2 47.6 1.3 7.9 4.1 11.3

168 hours 50.2 44.4 1.5 22.2 12.0 30.0

250oC

1 hour 44.1 49.6 1.2 6.4 7.0 6.0

8 hours 65.1 31.2 2.8 55.9 35.3 72.2

24 hours 63.7 33.9 2.5 64.5 49.0 75.6

48 hours 64.5 33.0 2.6 65.3 49.6 76.8

72 hours 63.3 34.5 2.4 68.6 55.2 78.1

168 hours 65.1 33.2 2.6 71.1 57.7 80.6

300oC

1 hour 73.0 23.0 4.2 66.7 45.2 84.5

8 hours 71.0 26.0 3.6 69.3 50.9 83.8

24 hours 69.3 29.3 3.2 72.8 57.6 83.9

48 hours 65.4 32.8 2.7 76.1 64.8 84.2

72 hours 64.4 33.7 2.5 77.8 67.8 84.9

350oC

1 hour 75.4 20.9 4.8 72.2 52.8 88.3

8 hours 74.3 23.1 4.3 76.5 60.7 89.0

24 hours 69.2 28.9 3.2 81.4 71.0 89.1

48 hours 66.2 32.0 2.8 84.8 77.3 90.2

72 hours 65.3 32.9 2.6 88.7 83.4 92.5

400oC

1 hour 78.5 18.1 5.8 77.7 60.6 91.8

8 hours 77.6 20.2 5.1 81.0 66.8 92.2

24 hours 76.2 22.8 4.5 87.2 79.2 93.2

48 hours 71.5 27.4 3.5 95.0 91.9 97.2

72 hours 71.5 27.2 3.5 97.6 96.1 98.7

1

Dry weight and ash-free basis

least twice for all samples and additionally as needed so that 

the standard error of the mean was less than 10 percent. The 

total acidity and the three fractions calculated according to the 

Boehm method are shown in tables 5 through 8. 

Previous work (Rutherford and others, 2004) reported 

infrared transmittance spectra for wood and wood component 

chars. In this study, IR-reflectance spectrometry has been 

used to further quantify the results that were observed for 

the IR-transmittance spectrometry. The infrared reflectance 

spectra were obtained from neat samples and have been used 

on a semi-quantitative basis to examine trends with time and 

temperature of char formation. These spectra are provided in 

Appendix 2. Four regions have been tabulated because they 

show the greatest response with charring. The pseudoabsorbance for the major bands in the regions of 3500 to 

3300, 3000 to 2800, 1750 to 1700, and 1650 to 1610 cm-1

are provided in tables 9 through 12. The broad band between 

3500 and 3300 cm-1 is characteristic of the OH stretching 

mode (Colthup and others, 1990). Decreases in this region are 

associated with the loss of oxygen during charring. The bands 

between 3000 and 2800 cm-1 are characteristic of the stretching modes of CH3

, CH2, and CH groups. Decreases in this 

region are associated with the loss of aliphatic carbon during 

charring. The sharp bands between about 1750 and 1700 cm-1
Development of Acid Functional Groups and Lactones 5

Table 2. Carbon and oxygen content of lignin and lignin char. 

[The mass of carbon and oxygen lost is shown as a percentage of the starting mass of carbon or oxygen; oC, degrees Celsius]

Charring

conditions

Percent1 carbon 

inchar

Percent1 oxygen 

in char

Carbon to Oxygen 

atomic ratio

Percent mass 

lost

Percent carbon 

lost

Percent oxygen 

lost

Unaltered

Lignin 67.5 26.6 3.4 0 0 0

Lignin Char

250°C

24 hours 71.6 23.2 4.1 15.3 10.3 26.4

72 hours 70.3 24.7 3.8 19.4 16.2 25.3

168 hours 72.2 23.3 4.1 23.3 18.0 33.1

300°C

1 hour 71.2 23.6 4.0 23.2 19.1 32.0

8 hours 72.2 23.1 4.2 30.0 25.2 39.2

24 hours 73.8 22.2 4.4 32.2 26.0 43.4

48 hours 71.2 25.1 3.8 36.3 33.0 40.1

72 hours 69.9 27.5 3.4 40.7 38.7 39.0

350°C

1 hour 74.3 21.6 4.6 35.7 29.4 47.9

8 hours 73.5 23.5 4.2 44.7 39.9 51.2

24 hours 69.3 28.9 3.2 47.2 45.8 42.7

48 hours 67.6 30.8 2.9 67.2 67.2 62.1

72 hours 66.3 32.1 2.8 66.1 66.8 59.3

400°C

1 hour 76.7 19.8 5.2 48.2 41.2 61.4

8 hours 73.9 23.8 4.1 61.8 58.3 65.9

24 hours 73.2 25.8 3.8 70.6 68.2 71.6

48 hours 72.2 26.5 3.6 88.2 87.4 88.3

72 hours 71.5 27.4 3.5 96.8 96.7 96.8

450°C

24 hours 79.3 19.6 5.5 86.2 83.8 90.1

1

Dry weight and ash-free basis

most likely represent carbonyl-stretching bands of lactones 

and carboxylic acids. Increases in this region are associated 

with the creation of lactones and carboxylic acids during charring. The band between 1650 and 1610 cm-1 represents skeletal 

vibrations of fused-ring aromatic structures and conjugated 

ketones. The growth of this band during charring reflects the 

increase in aromaticity and the possible presence of hydrogen 

bonded quinones.

Cellulose 

Cellulose was the only material charred at 200°C. After 

72 and 168 hours of heating, 7.9 and 22.2 percent of the 

original mass had been lost, which shows that little thermal 

degradation occurs at this temperature. The percentage oxygen 

in the char had decreased by 1.8 and 5 percent, respectively. 

The carbon-to-oxygen atomic ratio did not change appreciably 

with charring at this temperature (table1).

Unaltered cellulose has less than 0.1 milliequivalents 

per gram (meq/g) of total acidity. When heated at 200°C for 

72 and 168 hours, the total acidity of cellulose was 0.66 and 

1.96 meq/g, respectively. The increase in total acidity after 

72 hours was due to increases of 0.15, 0.36, and 0.15 meq/g 

in the strong acid fraction, moderate acid and lactone fraction 

and weak acid fraction, respectively (table 5). The increase in 

total acidity after 168 hours was due to increases of 0.41, 0.61, 

and 0.94 meq/g in the strong acid fraction, moderate acid and 

lactone fraction, and weak acid fraction, respectively (table 5).

The infrared reflectance spectra for cellulose charred at 

200°C are shown in Appendix 2, figure 2-1. The bands at 3449 

and 2901 cm-1 showed no substantial change after heating for 

72 and 168 hours at 200°C. The band at 1722 cm-1 increased 
6 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Table 3. Carbon and oxygen content of pine wood and pine wood char. 

[The mass of carbon and oxygen lost is shown as a percentage of the starting mass of carbon or oxygen; oC, degrees Celsius]

Charring

conditions

Percent1 carbon 

in char

Percent1 oxygen 

in char

Carbon to Oxygen 

atomic ratio

Percent mass 

lost

Percent carbon 

lost

Percent oxygen 

lost

Unaltered

Pine Wood 51.2 42.5 1.6 0 0 0

Pine Wood Char

250°C

1 hour 53.7 40.4 1.8 20.8 16.8 24.6

8 hours 63.0 32.5 2.6 44.2 31.4 57.3

24 hours 65.4 31.7 2.8 56.7 44.8 67.8

48 hours 65.5 32.0 2.7 63.8 53.8 72.8

72 hours 65.9 32.0 2.7 63.9 53.6 72.9

168 hours 62.8 35.0 2.4 69.8 63.1 75.3

300°C

1 hour 69.1 26.6 3.5 57.6 42.8 73.5

8 hours 71.2 25.7 3.7 67.1 54.4 80.2

24 hours 67.1 30.6 2.9 74.9 67.4 82.1

48 hours 67.3 30.9 2.9 80.1 74.1 85.7

72 hours 65.9 32.0 2.7 82.8 78.1 87.2

350°C

1 hour 76.2 20.5 4.9 73.2 60.3 87.1

8 hours 69.8 28.0 3.3 78.9 71.7 86.3

24 hours 68.9 28.5 3.2 94.9 93.8 96.9

400°C

1 hour 77.4 20.0 5.2 77.5 66.2 89.5

8 hours 79.4 18.1 5.8 86.1 78.7 94.2

1

Dry weight and ash-free basis

by 0.50 and 0.93 pseudo-absorbance units (PAU), respectively. The band at 1626 cm-1 increased by 0.15 and 0.45 PAU, 

respectively. In the region between 1400 and 900 cm-1, the 

individual cellulose bands remained distinct and showed no 

substantial change. The change in total acidity was reflected 

by changes in the 1722 cm-1 band, which is indicative of carbonyl formation. The change in aromaticity of the sample was 

reflected by changes in the1626 cm-1 band (table 9).

After heating at 250oC for 168 hours, cellulose lost 

71.1 percent of the original mass, which shows that considerable degradation has occurred. While little mass is lost in the 

first hour (6.4 percent), 55.9 percent was lost in the first 8 

hours. The percentage oxygen in the char remains constant 

for the first hour, decreased from 49.4 percent for unaltered 

cellulose to 31.2 percent after 8 hours, and then increased 

slightly to 33.2 percent after 168 hours. The carbon-to-oxygen 

ratio increases from 1.2 percent for unaltered cellulose to 2.8 

at 8 hours and remains constant for longer charring times 

(table 1). The losses in oxygen and carbon mass are shown in 

figure 2. The changes in carbon-to-oxygen ratio are shown in 

figure 3.

For cellulose char produced at 250oC, total acidity 

increased to 3.93 meq/g at 8 hours and to 8.83 meq/g at 

168 hours. After 168 hours, the total acidity was composed of 

3.84, 2.54, and 2.46 meq/g in the strong acid fraction, moderate acid and lactone fraction and weak acid fraction, respectively. The percentages of strong acid fraction, moderate acid 

and lactone fraction and weak acid fraction after 8 hours were 

21, 29, and 50 percent. After 168 hours, the percentages were 

43, 29, and 28 percent, respectively. Although total acidity 

increased with charring time, strong acid fraction increased 

the most, and continued to increase with long duration of 

charring. The weak acid fraction appears to have maximized 

in the first 8 hours of charring (table 5). The increases in total 

acidity by time and temperature are shown in figure 4. The 

changes in strong organic acid fraction by time and temperature are shown in figure 5. The combined changes in the 

moderate acid and lactone fraction and weak acid fraction 
Development of Acid Functional Groups and Lactones 7

Table 4. Carbon and oxygen content of pine bark and pine bark char. 

[The mass of carbon and oxygen lost is shown as a percentage of the starting mass of carbon or oxygen; oC, degrees Celsius]

Charring

conditions

Percent1 carbon 

in char

Percent1 oxygen 

in char

Carbon to Oxygen 

atomic ratio

Percent mass 

lost

Percent carbon 

lost

Percent oxygen 

lost

Unaltered

Pine Bark 51.3 42.7 1.6 0 0 0

Pine Bark Char

250°C

1 hour 57.4 37.3 2.0 26.7 18.6 36.3

8 hours 62.4 33.7 2.5 39.8 28.1 53.4

24 hours 62.6 34.2 2.4 48.1 38.2 59.4

72 hours 60.1 37.5 2.1 57.3 52.2 64.1

168 hours 59.2 38.5 2.1 65.7 62.6 70.8

300°C

1 hour 65.6 30.1 2.9 46.3 32.8 62.8

8 hours 64.6 32.1 2.7 52.5 42.1 65.4

24 hours 63.0 34.1 2.5 64.5 58.4 72.9

48 hours 60.2 37.4 2.1 75.7 73.6 80.2

72 hours 57.2 40.3 1.9 81.9 82.6 85.2

350°C

1 hour 68.3 28.0 3.3 55.5 42.7 71.8

8 hours 68.5 28.2 3.2 66.7 58.4 79.4

24 hours 65.0 32.1 2.7 80.5 78.3 87.1

48 hours 52.0 46.0 1.5 90.9 93.5 93.1

400°C

8 hours 80.8 15.8 6.8 71.3 61.0 90.9

1

Dry weight and ash-free basis

content of cellulose chars by time and temperature are shown 

in figure 6.

The infrared reflectance spectra for cellulose charred at 

250°C are shown in Appendix 2, figure 2-2. The maximum 

absorbance between 3500 and 3300 cm-1 shows little change in 

the first hour but decreases by 0.76 PAU by 8 hours, which is 

consistent with the loss of oxygen from the char. By 24 hours, 

the absorbance band has broadened and is showing a shoulder 

at 3070 cm-1, which could indicate the C-H stretching of aromatic compounds. The maximum absorbance between 3000 

and 2800 cm-1 shows no substantial change for one hour of 

charring. There was a reduction of 0.54 PAU after 8 hours and 

after 24 hours there was no absorbance band remaining. This 

is consistent with the loss of aliphatic carbon as determined 

by 13C NMR in previous work (Rutherford and others, 2004). 

By 8 hours, the maximum absorbance between 1750 and 

1700 cm-1 has increased by 1.33 PAU and remains constant 

for longer charring times. There is a consistent shift in band 

location from 1728 to 1755 cm-1 with prolonged charring. 

The maximum absorbance between 1650 and 1610 cm-1

increases by 0.91 PAU in the first 8 hours and then remains 

relatively constant for longer charring times (table 9). In the 

region between 1400 and 900 cm-1, the individual cellulose 

bands coalesce into one broad band by 8 hours.

For cellulose charred at 300oC for 8 hours, 69 percent 

total mass, 51 percent of the carbon mass, and 84 percent of 

the oxygen mass is lost. After 72 hours of charring at 300oC, 

78 percent total mass, 68 percent of the carbon mass, and 

85 percent of the oxygen mass is lost. The carbon-to-oxygen 

ratio increased to 3.6 after 8 hours and decreases to 2.5 after 

72 hours. For cellulose charred at 350oC for 8 hours, 

77 percent total mass, 61 percent of the carbon mass, and 

89 percent of the oxygen mass was lost. After 72 hours of 

charring at 350oC, 89 percent total mass, 83 percent of the 

carbon mass, and 93 percent of the oxygen mass has been lost. 

The carbon-to-oxygen ratio has increased to 4.3 after 8 hours 

and then decreases to 2.6 after 72 hours. For cellulose charred 

at 400oC for 8 hours, 81 percent of the total mass, 81 percent 

of the carbon mass, and 93 percent of the oxygen mass was 

lost. After 72 hours of charring at 400oC, 98 percent of the 

total mass, 96 percent of the carbon mass, and 99 percent of 

the oxygen mass has been lost. The carbon-to-oxygen ratio has 
8 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

increased to 5.1 after 8 hours and then decreased to 3.5 after 

72 hours (table 1).

For cellulose chars produced at 300oC, 350oC, and 

400oC, for 8 hours charring time, total acidity was 4.04, 3.04 

and 2.35 meq/g, respectively. For cellulose chars produced 

at 300oC, 350oC, and 400oC, for 72 hours charring time, total 

acidities were 7.38, 5.62 and 4.04 meq/g respectively. When 

charred for 8 hours at 300oC, 350oC and 400oC, the percentages of the strong acid fraction were 24, 29, and 16 percent 

respectively. When charred for 72 hours at 300oC, 350oC and 

400oC, the percentage of the strong acid fraction was 34, 30, 

and 30 percent respectively. The strong acid fraction and moderate acid and lactone fraction increased with longer duration 

of charring. All three fractions showed decreases in quantity 

with increasing temperature (table 5).

The infrared reflectance spectra for cellulose charred at 

300, 350, and 400oC are shown in Appendix 2, figures 2-3, -4, 

and -5. The maximum absorbances for bands between 3500 

and 3300 cm-1 for chars produced at 300, 350, and 400oC for 

8 hours, are 0.56, 0.42, and 0.24 PAU, respectively. After 

charring for 72 hours, the maximum absorbances are 0.76, 

0.59, and 0.28 PAU, respectively. The decrease in absorbance 

at higher charring temperature is consistent with the lower 

percentage of oxygen at higher charring temperature. The 

absorbance band between 3000 and 2800 cm-1 disappears for 

chars heated for greater than 8 hours at 300oC, and 1 hour at 

350 and 400oC. For chars produced at 300, 350, and 400oC 

for 8 hours, the maximum absorbances between 1750 and 

1700 cm-1 are 1.23, 1.07, and 0.88 PAU, respectively. After 

72 hours, the maximum absorbances are 1.48, 1.37, and 1.14 

PAU, respectively. There is a shift in absorbance band location 

from about 1710 to 1760 cm-1 with increased charring time 

at all temperatures. The maximum absorbance between 1650 

and 1610 cm-1 for chars produced at 300, 350, and 400oC for 8 

hours, are 1.17, 1.14, and 1.23 PAU, respectively. After charring for 72 hours, the maximum absorbances are 1.34, 1.31, 

and 1.19 PAU, respectively (table 9).

For cellulose chars produced at temperatures above 

250oC, the change in maximum absorbance between 3500 and 

3300 cm-1 is consistent with the change in percentage oxygen 

in the char. This trend shows an initial decrease in percentage 

oxygen and maximum absorbance at between 3500 and 3300 

cm-1 from the unaltered state, followed by a slight increase in 

both percentage oxygen and absorbance between 3500 and 

3300 cm-1 with prolonged heating. At fixed times, there is a 

decrease in both percentage oxygen and maximum absorbance 

between 3500 and 3300 cm-1 with increased charring temperature. The change in maximum absorbance between 1750 

and 1700 cm-1 is consistent with the change in acid functional 

groups in the char. This trend shows an initial increase in percentage of acid functional groups and maximum absorbance 

between 1750 and 1700 cm-1 from the unaltered state, followed by a slight increase in both acid functional groups and 

absorbance between 1750 and 1700 cm-1 with prolonged heating. At fixed times, there is a decrease in both acid functional 

groups and absorbance between 1750 and 1700 cm-1 with 

increased charring temperature. The maximum absorbance 

between 1650 and 1610 cm-1 increased with the observed 

increase in aromatic structure previously reported (Rutherford 

and others, 2004). 

Lignin 

After heating at 250oC for 168 hours, lignin has lost 

23.3 percent of the original mass. The percentage oxygen in 

the char has decreased from 26.6 percent in unaltered lignin 

to 23.3 percent after 168 hours. The carbon-to-oxygen ratio 

has increased slightly from 3.4 in unaltered lignin to 4.1 after 

168 hours of charring (table 2).

Unaltered lignin has 1.23 meq/g of total acidity. When 

lignin was charred at 250oC for 168 hours, the total acidity of 

lignin char was 1.45 meq/g. This slight increase in total acidity 

was due to an increase in the strong acid fraction (table 6).

The infrared reflectance spectra for lignin chars produced 

at 250oC are shown in Appendix 2, figure 2-6. The maximum 

absorbance between 3500 and 3300 cm-1 shows little change 

in the first 24 hours but decreases by 0.51 PAU by 168 hours. 

The maximum absorbance between 3000 and 2800 cm-1 shows 

similar decreases. The maximum absorbance between 1750 

and 1700 cm-1 and between 1650 and 1610 cm-1 also show 

decreases in PAU with increased charring time which is in 

contrast to cellulose which shows increases in PAU between 

1750 and 1700 cm-1 and between 1650 and 1610 cm-1 (table 9 

and 10).

For lignin charred at 300oC for 8 hours, 30 percent of 

the total mass, 25 percent of the carbon, and 39 percent of the 

oxygen is lost. After 72 hours at 300oC, 41 percent of total 

mass, 39 percent of the carbon, and 39 percent of the oxygen 

is lost. The carbon-to-oxygen ratio is 4.2 after 8 hours and 3.4 

after 72 hours. This mass loss shows that there is thermal degradation occurring at 300o

C, but that it is progressing slowly 

(table 2).

For lignin char produced at 300oC, total acidity after 

72 hours was 2.55 meq/g. This increase in total acidity, 

compared to the unaltered lignin, is caused by an increase in 

the strong acid fraction and the moderate acid and lactone 

fraction. The strong acid fraction increased from 0.15 meq/g 

for unaltered lignin to 1.18 meq/g after 72 hours, while the 

moderate acid and lactone fraction increased from 0.41 meq/g 

for unaltered lignin to 0.85 meq/g (table 6).

The infrared reflectance spectra for lignin chars produced 

at 300oC are shown in Appendix 2, figure 2-7. The maximum 

absorbance between 3500 and 3300 cm-1 shows a steady 

decrease from 0.81 PAU (unaltered) to 0.33 PAU after 72 

hours. The maximum absorbance between 3000 and 2800 cm-1

shows similar decreases. The maximum absorbance between 

1750 and 1700 cm-1 and between 1650 and 1610 cm-1 show 

decreases in PAU with increased charring time (table 10).

Charring of lignin at higher temperatures (350oC and 

400oC) causes increased loss for total mass, percentage carbon 

and percentage oxygen for any given charring time. Charring 

at 350oC for 8 hours caused the loss of 45 percent total mass, 
Development of Acid Functional Groups and Lactones 9

Table 5. The results of the Boehm titration of cellulose and cellulose char shown as the milliequivalents of total acidity 

and its three fractions consisting of the strong organic acid fraction, the moderate organic acid and lactone fraction, and 

the weak organic acid fraction.

[

o

C, degrees Celsius ; meq/g, milliequivalents per gram]**

Charring

conditions

Total Acidity 

(meq/g)

Strong Acid 

Fraction 

(meq/g)

Moderate Acid and 

Lactone Fraction

(meq/g)

Weak Acid

 Fraction

 (meq/g)

Unaltered Cellulose 0.09 0.00 0.07 0.02

Cellulose Char

200°C

72 hours 0.66 0.15 0.36 0.15

168 hours 1.96 0.41 0.61 0.94

250°C

8 hours 3.93 0.82 1.14 1.98

24 hours 6.59 2.06 2.17 2.37

72 hours 8.72 3.52 2.60 2.60

168 hours 8.83 3.84 2.54 2.46

300°C

8 hours 4.04 0.98 1.72 1.35

24 hours 6.21 1.81 2.37 2.04

48 hours 7.16 2.35 2.66 2.16

72 hours 7.38 2.55 2.44 2.39

350°C

8 hours 3.04 0.88 1.03 1.14

24 hours 5.12 1.57 1.68 1.87

48 hours 5.02 1.58 1.84 1.60

72 hours 5.62 1.67 2.11 1.84

400°C

8 hours 2.35 0.38 1.25 0.72

24 hours 3.25 0.91 1.18 1.16

48 hours 4.21 1.33 1.32 1.59

72 hours 4.04 1.21 1.40 1.44

40 percent of the carbon mass, and 51 percent of the oxygen 

mass, while after 72 hours, 66 percent of the total mass, 67 

percent of the carbon mass, and 59 percent of the oxygen mass 

was lost. The carbon-to-oxygen ratio is 4.2 after 8 hours and 

2.8 after 72 hours. Charring at 400oC for 8 hours caused the 

loss of 62 percent total mass, 58 percent of the carbon mass, 

and 66 percent of the oxygen mass, while after 72 hours, 

97 percent of the total mass, 97 percent of the carbon mass, 

and 97 percent of the oxygen mass was lost. The 

carbon-to-oxygen ratio was 4.1 after 8 hours and 3.5 after 72 

hours (table 2). 

When charred at 350oC and 400oC for 72 hours, the total 

acidity of lignin char was 2.58 meq/g and 3.47 meq/g. When 

charred at 350oC for 72 hours, the strong acid fraction was 

1.27 meq/g and the moderate acid and lactone fraction was 

1.03 meq/g. Both the strong acid fraction and the moderate 

acid and lactone fraction have increased compared to unaltered 

lignin, while the weak acid fraction has decreased to 0.28 

meq/g compared to 0.68 meq/g for unaltered lignin (table 6). 

The infrared reflectance spectra are shown in Appendix 2, 

figure 2-8, -9, and -10. The maximum absorbance between 

3500 and 3300 cm-1 shows a sharp decrease from 0.81 PAU 

(unaltered) to 0.28 and 0.12 PAU after 1 hour when charred 

at 350oC and 400oC. The absorbance increases slightly with 

longer charring times at both temperatures. The maximum 

absorbance between 3000 and 2800 cm-1, between 1750 and 

1700 cm-1, and between 1650 and 1610 cm-1 shows similar 

trends (table 10). 
10 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Table 6. The results of the Boehm titration of lignin and lignin char shown as the milliequivalents of total acidity 

and its three fractions consisting of the strong organic acid fraction, the moderate organic acid and lactone 

fraction, and the weak organic acid fraction.

[

o

C, degrees Celsius ; meq/g, milliequivalents per gram]

Charring 

conditions

Total Acidity 

(meq/g)

Strong Acid 

Fraction (meq/g)

Moderate Acid and 

Lactone Fraction 

(meq/g)

Weak Acid 

Fraction 

(meq/g)

Unaltered Lignin 1.23 0.15 0.41 0.68

Lignin Char

250°C

72 hours 0.97 0.18 0.20 0.60

168 hours 1.45 0.49 0.37 0.59

300°C

24 hours 1.18 0.17 0.19 0.83

48 hours 1.51 0.65 0.66 0.20

72 hours 2.55 1.18 0.85 0.52

350°C

24 hours 1.23 0.62 0.32 0.29

48 hours 3.64 1.65 1.10 0.89

72 hours 2.58 1.27 1.03 0.28

400°C

24 hours 2.55 0.82 0.66 1.08

48 hours 2.42 1.25 1.17 0

72 hours 3.47 1.97 1.20 1.31

450°C

24 hours 2.24 0.62 0.86 0.76

Pine Wood

Pine wood charred at 250oC for 8 hours has lost 

44 percent total mass, 31 percent carbon, and 57 percent 

oxygen. After 168 hours of charring at 250oC, pine wood has 

lost 70 percent total mass, 63 percent of carbon mass, and 

75 percent of oxygen mass. The oxygen to carbon ratio for 

unaltered pine wood is 1.6. The carbon-to-oxygen ratios for 

chars produced at 250oC for 8 and 168 hours are 2.6 and 2.4, 

respectively (table 3).

Unaltered pine wood has 0.89 meq/g total acidity. At 

250oC, the total acidities in pine wood charred for 8 and 

168 hours were 2.93 meq/g and 7.83 meq/g, respectively. After 

8 hours, the percentages of the strong acid fraction, the moderate acid and lactone fraction, and weak acid fraction in unaltered pine wood are 6, 13 and 81 percent, respectively. After 

168 hours, the percentages are 36, 33, and 31 percent respectively. Although all three fractions increase with charring, the 

weak acid fraction reached a maximum in the first 8 hours 

while the strong acid fraction increases steadily with increased 

charring time. The largest increase of total acidity observed 

in pine wood char was for charring at 250oC for 168 hours, to 

approximately seven times the total acidity of unaltered pine 

wood (table 7).

The infrared reflectance spectra for pine wood chars 

produced at 250oC are shown in Appendix 2, figure 2-11. The 

maximum absorbance between 3500 and 3300 and between 

3000 and 2800 cm-1 shows little change in the first hour but 

decreases by 0.37 and 0.25 PAU after 168 hours. After 168 

hours, the maximum absorbance between 1750 and 1700 cm-1

has increased by 0.55 PAU. There is a consistent shift in band 

location from 1740 to 1750 cm-1 with prolonged charring. The 

maximum absorbance between 1650 and 1610 cm-1 increases 

by 0.42 PAU after 168 hours. In the region between 1400 and 

900 cm-1, the individual bands coalesce into one broad band 

and decrease in intensity slightly by 168 hours (table 11). This 

is consistent with the changes observed for cellulose charred 

under these conditions.

When charred at 300oC for 8 hours, pine wood has lost 

67 percent total mass, 54 percent of carbon mass, and 80 

percent of oxygen mass. After 72 hours of charring at 300oC, 

83 percent total mass, 78 percent of carbon mass, and 

87 percent of oxygen mass have been lost. The 
Development of Acid Functional Groups and Lactones 11

Table 7. The results of the Boehm titration of pine wood and pine wood char shown as the 

milliequivalents of total acidity and its three fractions consisting of the strong organic acid fraction, the 

moderate organic acid and lactone fraction, and the weak organic acid fraction.

[

o

C, degrees Celsius ; meq/g, milliequivalents per gram]

Charring

conditions

Total Acidity 

(meq/g)

Strong Acid 

Fraction 

(meq/g)

Moderate Acid and 

Lactone Fraction 

(meq/g)

Weak Acid 

Fraction 

(meq/g)

Unaltered Pine Wood 0.89 0.05 0.12 0.72

Pine Wood Char

250°C

8 hours 2.93 0.32 0.20 2.41

24 hours 5.40 1.09 2.02 2.29

72 hours 7.10 2.16 2.61 2.34

168 hours 7.83 2.80 2.58 2.46

300°C

1 hour 2.47 0.15 0.70 1.62

8 hours 4.39 1.02 1.60 1.77

24 hours 5.49 1.48 2.06 1.94

48 hours 6.09 1.68 2.15 2.26

72 hours 5.88 1.83 1.98 2.08

350°C

8 hours 2.38 0.57 0.89 0.93

400°C

8 hours 1.35 0.18 0.49 0.68

carbon-to-oxygen ratio for chars produced at 300oC for 8 and 

72 hours are 3.5 and 2.7, respectively (table 3). 

For pine wood chars produced at 300oC for 8 and 

72 hours, the total acidities are 4.39 and 5.88 meq/g, respectively. The percentages of strong acid fraction, moderate acid 

and lactone fraction and weak acid fraction after 72 hours, are 

31, 34, and 35 percent, respectively (table 7).

The infrared reflectance spectra for pine wood charred at 

300oC are shown in Appendix 2, figure 2-12. The maximum 

absorbances between 3500 and 3300 cm-1 and between 3000 

and 2800 cm-1 decrease to 0.60 and 0.45 PAU, respectively, 

after 168 hours. After 168 hours, the maximum absorbance 

between 1750 and 1700 cm-1 has increased to 1.18 PAU. The 

maximum absorbance between 1650 and 1610 cm-1 increases 

to 1.09 PAU after 168 hours (table 11). 

Pine Bark 

Pine bark charred at 250oC for 8 hours has lost 40 percent 

total mass, 28 percent carbon, and 53 percent oxygen. After 

168 hours of charring at 250oC, pine bark has lost 66 percent 

total mass, 63 percent of carbon mass, and 71 percent of 

oxygen mass. Unaltered pine bark contains 43 percent oxygen 

by weight. The percentage oxygen in the char decreases to 

34 percent at 8 hours and then rises slightly, to 39 percent 

at 168 hours. For unaltered pine bark, the carbon-to-oxygen 

atomic ratio is 1.6. The carbon-to-oxygen ratio is 2.5 after 

8 hours and 2.1 after 168 hours (table 4).

Unaltered pine bark has 1.57 meq/g of total acidity. 

For pine bark charred at 250oC for 8 and 168 hours, the total 

acidities are 2.70 and 5.96 meq/g, respectively. The maximum 

increase of total acidity observed in pine bark char was for 

250oC for 168 hours and was approximately four times the 

total acidity of unaltered pine bark. The percentages of the 

strong acid fraction, the moderate acid and lactone fraction, 

and weak acid fraction are 9, 19, and 73 percent, respectively, 

for charring for 8 hours. After 168 hours, the percentages 

of the strong acid fraction, the moderate acid and lactone 

fraction, and weak acid fraction are 27, 34, and 39 percent, 

respectively (table 8).

The infrared reflectance spectra for pine bark charred at 

250oC are shown in Appendix 2, figure 2-15. The maximum 

absorbances between 3500 and 3300 and between 3000 and 

2800 cm-1 show little change in the first hour but decreases 

by 0.33 and 0.25 PAU after 168 hours. After 168 hours, 

the maximum absorbance between 1750 and 1700 cm-1 has 
12 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Table 8. The results of the Boehm titration of pine bark and pine bark char shown as the milliequivalents of total 

acidity and its three fractions consisting of the strong organic acid fraction, the moderate organic acid and lactone 

fraction, and the weak organic acid fraction. 

[°C, degrees Celsius ; meq/g, milliequivalents per gram]

Charring

conditions

Total Acidity 

(meq/g)

Strong Acid 

Fraction 

(meq/g)

Moderate Acid and 

Lactone Fraction 

(meq/g)

Weak Acid 

Fraction 

(meq/g)

Unaltered Pine Bark 1.57 0.18 0.67 0.73

Pine Bark Char

250°C

8 hours 2.70 0.24 0.50 1.96

24 hours 3.97 0.63 1.38 1.97

72 hours 5.47 1.29 1.70 2.49

168 hours 5.96 1.60 2.03 2.33

300°C

1 hour 2.24 0.09 0.83 1.33

8 hours 2.38 0.27 0.65 1.46

24 hours 2.86 0.45 0.91 1.51

48 hours 3.62 0.45 1.15 2.03

72 hours 3.52 0.25 1.23 2.05

350°C

8 hours 1.59 0.00 0.41 1.19

400°C

8 hours 0.47 0.00 0.24 0.23

increased by 0.49 PAU, while the maximum absorbance 

between 1650 and 1610 cm-1 increases by 0.11 PAU (table 12). 

When charred at 300oC for 8 hours, pine bark has lost 

53 percent total mass, 42 percent of carbon mass, and 

65 percent of oxygen mass. After 72 hours of charring at 

300oC, 82 percent total mass, 83 percent of carbon mass, and 

85 percent of oxygen mass has been lost. At 300oC, the percentage oxygen in the char reached a minimum of 30 percent 

after 1 hour of charring and then increased to 40 percent 

after 72 hours. At 300oC, the carbon-to-oxygen atomic ratio 

maximized at 2.9 after 1 hour and then decreased to 1.9 after 

72 hours (table 4). 

 For pine bark charred at 300oC for 8 and 72 hours, the 

total acidities are 2.24 and 3.52 meq/g, respectively. The 

percentages of the strong acid fraction, the moderate acid 

and lactone fraction, and weak acid fraction are 11, 27, and 

61 percent, respectively, for charring for 8 hours. After 168 

hours, the percentages of the strong acid fraction, the moderate 

acid and lactone fraction, and weak acid fraction are 7, 35, and 

58 percent, respectively (table 8).

Trends in Compositional Change and 

Implications for Development of 

Acid Functional Groups

Pyrolysis reactions lead to the formation of the so-called 

char/charcoal black carbon fraction in soils formed by the 

charring of biomass mainly of plant origin. This black 

carbon is the least studied component of soil organic carbon. 

Although there have been numerous studies of 

high-surface-area chars, which have a long history as sorbents, 

there has been little study of low temperature charring of 

natural organic matter that is expected to make up black 

carbon in soils.

Simoneit (2002) pointed out that during the heating of 

wood, the sequence of decomposition with increasing temperature is hydrolysis, oxidation, dehydration, and finally pyrolysis. Soil chars are typically highly carboxylated, linearly fused, 

and/or highly condensed aromatic polymers (Kramer and 

others, 2004; Rutherford and others, 2004). Shafizadeh (1984) 

pointed out: “The production of volatiles leaves a solid residue 
Development of Acid Functional Groups and Lactones 13

Table 9. The pseudo-absorbance of selected absorbance bands for diffuse reflectance infrared 

spectroscopy of cellulose and cellulose char. 

[The absorbance is given in pseudo-absorbance units. The number in parenthesis is the wave number for the highest value of the 

absorbance band. oC, degrees Celsius ; --, not determined]

Charring Temperature (degrees Celsius)

Charring Time 

(hours) 200°C 250°C 300°C 350°C 400°C

Region 1 (3500 - 3300 cm-1)

Unaltered Cellulose: 1.31 (3449)

1 -- 1.53 (3435) 0.55 (3422) 0.44 (3443) 0.31 (3518)

8 -- 0.77 (3418) 0.56 (3414) 0.42 (3416) 0.24 (3493)

24 -- 0.82 (3345) 0.72 (3327) 0.53 (3331) 0.25 (3333)

48 -- 0.77 (3343) 0.70 (3329) 0.60 (3321) 0.33 (3354)

72 1.40 (3449) 0.87 (3327) 0.76 (3329) 0.59 (3310) 0.28 (3333)

168 1.44 (3408) 0.83 (3319) -- -- --

Region 2 (3000 - 2800 cm-1)

Unaltered Cellulose: 0.97 (2901)

1 -- 1.09 (2901) 0.51 (2928) 0.42 (2926) 0.27 (2924)

8 -- 0.58 (2901) 0.45 (2930) 0.31 (2926) 0.15 (2924)

24 -- 0.55 (2901) 0.51 (2930) 0.37 (2926) 0.16 (2924)

48 -- 0.52 (2901) 0.50 (2930) 0.43 (2926) 0.24 (2924)

72 1.01 (2901) 0.58 (2901) 0.53 (2930) 0.43 (2926) 0.20 (2924)

168 1.05 (2901) 0.57 (2901) -- -- --

Region 3 (1750 - 1700 cm-1)

Unaltered Cellulose: 0.17 (1722)

1 -- 0.40 (1711) 1.20 (1711) 1.09 (1711) 0.86 (1709)

8 -- 1.50 (1728) 1.23 (1721) 1.07 (1717) 0.88 (1715)

24 -- 1.48 (1749) 1.36 (1753) 1.27 (1753) 0.98 (1759)

48 -- 1.42 (1749) 1.41 (1755) 1.36 (1757) 1.18 (1761)

72 0.67 (1722) 1.50 (1749) 1.48 (1755) 1.37 (1759) 1.14 (1763)

168 1.10 (1726) 1.41 (1755) -- -- --

Region 4 (1650 - 1610 cm-1)

Unaltered Cellulose: 0.37 (1636)

1 -- 0.37 (1626) 1.10 (1605) 1.06 (1605) 0.93 (1603)

8 -- 1.28 (1609) 1.17 (1607) 1.14 (1607) 1.01 (1605)

24 -- 1.29 (1614) 1.23 (1612) 1.23 (1612) 1.04 (1611)

48 -- 1.24 (1612) 1.28 (1614) 1.29 (1614) 1.23 (1614)

72 0.52 (1626) 1.31 (1616) 1.34 (1612) 1.31 (1616) 1.19 (1618)

168 0.82 (1626) 1.23 (1618) -- -- --

that is neither intact substrate nor pure carbon, but a different 

material at various stages of charring and carbonization. The 

intermediate chars are characterized by the functional groups 

present (including aromatic and olefinic structures); a high 

concentration of free spins trapped in a rigid structure or stabilized by aromatic and olefinic structures; a large surface area; 

and high degree of reactivity.” 

The pyrolysis of cellulose, the most abundant constituent 

biomass, has been studied more extensively than that of 

any other biomass constituent. Shafizadeh (1982, 1984) has 

delineated two different pathways for cellulose pyrolysis. At 

temperatures below about 300oC bond scission, formation 

of free radicals, dehydration, and formation of carbonyl, 

carboxyl, and hydroperoxide groups predominate. These 

reactions are followed by production of carbon monoxide, 

carbon dioxide, and char. The carbon monoxide is apparently 

the result of decarbonylation and the carbon dioxide of 

decarboxylation. At temperatures above 300oC bond cleavage 
14 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Mass loss of oxygen 

0

20

40

60

80

100

0 20 40 60 80 100

Mass loss of carbon

0

20

40

60

80

100

0 20 40 60 80 100

CHARRING TIME, IN HOURS

200°C 250°C 300°C

350°C 400°C

PERCENTAGE OF STARTING MASS LOST

EXPLANATION

CARBON TO OXYGEN ATOMIC RATIO

0

1

2

3

4

5

6

7

0 20 40 60 80 100

CHARRING TIME, IN HOURS

 200°C 250°C 300°C 350°C 400°C

EXPLANATION

Figure 3. The carbon-to-oxygen atomic ratio of cellulose 

chars at various charring times and temperatures.

Figure 2. The mass loss of carbon and oxygen during 

the charring of cellulose at various charring times and 

temperatures.

TOTAL ACIDITY, IN MILLIEQUIVALENTS 

PER GRAM

0

2

4

6

8

10

0 20 40 60 80 100

CHARRING TIME, IN HOURS

200°C 250°C 300°C 350°C 400°C

EXPLANATION

STRONG ACID FRACTION, 

IN MILLIEQUIVALENTS 

PER GRAM

0

2

4

6

0 20 40 60 80 100

CHARRING TIME, IN HOURS

200°C 250°C 300°C 350°C 400°C

EXPLANATION

Figure 5. The strong organic acid fraction of cellulose 

chars at various charring times and temperatures.

Figure 4. The total acidity of cellulose chars at various 

charring times and temperatures.
Development of Acid Functional Groups and Lactones 15

ACID FRACTIONS, IN 

MILLIEQUIVALENTS PER GRAM

0

2

4

6

0 20 40 60 80 100

200°C 250°C 300°C 350°C 400°C

CHARRING TIME, IN HOURS

EXPLANATION

Figure 6. The sum of the moderate acid and lactone fraction 

and weak organic acid fraction of cellulose chars at various 

charring times and temperatures.

by transglycosylation, bond fission, and disproportionation 

reactions take place. This second pathway leads to the 

formation of anhydro sugars such as levoglucosan by 

dehydration. (See Nimlos and others, 2003, for a detailed 

discussion of dehydration of carbohydrates.) Sanders and 

others (2003) have developed a more detailed model of the 

saccharides. They found that at temperatures between 300 and 

600oC low-molecular weight oxygenated compounds such as 

anhydro sugars and simple aromatic compounds are formed. 

At higher temperatures polynuclear aromatic hydrocarbons 

are produced. However, McGrath and others (2003) found 

that polynuclear aromatic hydrocarbons form between 300 

and 650oC. Rutherford and others (2004) have shown that 

polynuclear aromatic hydrocarbons form as low as 250oC. 

Polynuclear aromatic hydrocarbons also are formed from 

pyrolysis of lignin (Sharma and Hajaligol, 2003) and amino 

acids (Sharma and others, 2003).

This study used purified cellulose and lignin because they 

are the most abundant components of plant materials. Wood 

and bark are mixtures of these end-members. Soft wood is 

composed of 64.5 ± 4.6 percent carbohydrates of which 43.7 ± 

2.6 percent is cellulose. Lignin makes up 28.8 ± 2.6 percent of 

a softwood (Pettersen, 1984). Bark also is composed primarily 

of carbohydrates and lignin but with a lower percentage of carbohydrates and a higher percentage of lignin than in the wood.

As was shown in a previous study (Rutherford and others, 

2004), the loss of material during charring is a general indicator of thermal decomposition but does not give a clear picture 

of the decomposition reactions. Aliphatic carbon is lost rapidly 

during early thermal decomposition, while aromatic carbon is 

initially being formed. Prolonged heating results in loss of aromatic carbons at temperatures above 300o

C. The rate of loss 

of carbon and oxygen are not proportional. Oxygen is lost 

preferentially to carbon during initial decomposition for 

all materials. These changes are reflected in the carbon-tooxygen atomic (C/O) ratios, which reach a maximum in the 

first hours of charring, and then decrease with prolonged 

charring. As charring temperatures increase, thermal degradation occurs more rapidly and a greater percentage of total 

and oxygen mass is lost. The discrepancy between the loss 

of carbon and oxygen is magnified, with higher C/O ratios 

being reached at short charring times.

There is a general correlation of rapid oxygen loss 

during early decomposition with loss of aliphatic carbon, 

as determined in the previous study (Rutherford and others, 

2004). If the labile oxygen is associated with the aliphatic 

carbon, then the recalcitrant oxygen lost with prolonged 

heating will be associated with the loss of aromatic carbon. 

In the case of cellulose, there is no aromatic carbon present in the starting material. This aromatic carbon must be 

formed from the remaining aliphatic carbon. The oxygen 

functional groups may be attached to aromatic carbon at 

this time. As the charring temperature is increased, the 

percentage oxygen decreases. It is speculated that the lower 

charring temperature and longer reaction times allow more 

oxygen functional groups to become attached to aromatic 

carbons.

For cellulose at all charring temperatures, total acidity increases with charring time. Total acidity reaches a 

maximum at 250oC with long charring times for cellulose 

and cellulose containing materials (wood and bark). This 

is shown in figure 7. Even though unaltered lignin starts 

out with approximately 10 times more total acidity than 

cellulose, lignin char consistently has less total acidity than 

cellulose char. The total acidity content of the chars for a 

fixed time and temperature decreased in the order cellulose 

char > pine wood char > pine bark char > lignin char. The 

total acidity for lignin char increased steadily to 400oC. 

With charring at 400oC for 72 hours, the difference in total 

acidity due to starting material has essentially disappeared.

The largest increase observed for total acidity in cellulose char was approximately a factor of 80 times the total 

acidity of unaltered cellulose, while the largest increase for 

lignin char was approximately a factor of three times the 

total acidity of unaltered lignin. The maximum increase in 

total acidity observed in pine wood char was approximately 

seven times the total acidity of unaltered pine wood while 

the maximum increase total acidity observed in pine bark 

char was approximately four times the total acidity of unaltered pine bark. This suggests that cellulose is the major 

contributor to acid functional group development in chars 

of wood and bark.

For cellulose, pine wood, and pine bark, the increase 

in total acidity results from an increase in the three 

fractions of acidity (strong acid fraction, moderate acid and 

lactone fraction, and weak acid fraction). The increase in 

total acidity for lignin is a result of an increase in the strong 

acid fraction and the moderate acid and lactone fraction. 
16 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Of the three fractions of acidity, the strong acid fraction 

increased the most but takes longer to develop for cellulosecontaining materials. The weak acid fraction appears to 

develop the fastest and reaches a maximum within the first 

8 hours of charring.

Each of the fractions of acid functional groups in cellulose, pine wood, and pine bark follow the same trend as total 

acidity. For charring at a fixed time, the maximum increase 

occurs at 250oC and decrease with increasing temperature 

above 250oC.

The oxygen content of a char is not a good indicator 

of acid functional groups. The percentage oxygen initially 

decreased at the onset of charring, then increased slightly 

with charring time. The acid functional group contents of the 

chars increased uniformly with increased charring time. This 

indicates that once the initial loss of labile oxygen is complete, 

acid functional groups continue to be formed and would indicate that rearrangement of oxygen within the char structure is 

occurring with prolonged charring. 

The difference in acid functional group formation in 

cellulose and lignin suggests that in cellulose, as aliphatic 

carbon is being converted to aromatic carbon, a percentage 

of the oxygen also is converted to acid functional groups on 

the aromatic carbon when charred at low temperature. Charring at higher temperatures either reduces the number of acid 

functional groups formed or removes them once they are 

formed. In lignin, there is less oxygen relative to carbon in the 

uncharred material, and it is not clear whether additional acid 

functional groups are being formed or if they already existed 

and are being concentrated by the charring process. 

It was observed for all samples that thermal degradation caused reduction of the broad band in the region between 

3500 and 3300 cm-1 and of the sharp bands between 3000 and 

2800 cm-1 and increases in the intensities of the sharp bands 

between about 1750 and 1700 cm-1. The individual sharp 

bands between 1400 and 900 cm-1 increased in width with 

increasing temperature until they coalesced into a single 

broad band. The broad band between 3500 and 3300 cm-1

is characteristic of the OH stretching mode. The stretching modes of CH3

, CH2, and CH groups are represented 

by sharp bands near 2962 and 2872 cm-1, 2926 and 2853 

cm-1, and 2890 cm-1 respectively; the stretching mode of 

aromatic CH groups is located at approximately 3030 

cm-1. The sharp bands between about 1750 and 1700 cm-1

most likely represent carbonyl-stretching bands of lactones 

and carboxylic acids. The carbonyl groups of lactones 

generally have bands between about 1735 and 1800 cm-1; 

saturated aliphatic acids occur in the region between 1725 

and 1700 cm-1, and aryl acids occur between 1700 and 

1680 cm-1. Carbonyl groups of esters and ketones also are 

found in the region between 1750 and 1690 cm-1 and may 

be present in some of the samples. The band between 1650 

and 1610 cm-1 represents skeletal vibrations of fused-ring 

aromatic structures and conjugated ketones. The growth 

of this band during charring reflects the increase in aromaticity and the possible presence of hydrogen bonded quinones. 

Carboxylate groups have two carbonyl stretching modes—an 

asymmetric and a symmetric mode. The asymmetric mode is 

represented by a band between 1610 and 1550 cm-1 and the 

symmetric mode by a band between 1400 and 1300 cm-1. The 

individual sharp bands between 1400 and 900 cm-1 are characteristic of the lignin and cellulose components of wood. These 

bands mostly represent OH deformations and CO stretching 

modes of alcohols and phenols. Faix (1991) has shown that 

lignin has a number of well-resolved bands in this region. For 

example, a prominent band near 1127 cm-1 is indicative of 

methoxy groups of guaiacyl and syringyl units in the lignin 

structure. The bands between 1070 and 1030 cm-1 arise from 

alcoholic groups in carbohydrates (Colthup and others, 1990). 

Summary

This report provides data on acid functional groups and 

lactones in charcoals and on how the content of these groups 

varies during thermals degradation with variation of charring 

time and temperature. Chars were created from purified 

cellulose, purified lignin, pine wood, and pine bark. The 

charring temperatures and charring duration were controlled in 

a laboratory furnace with an inert atmosphere and ranged from 

200 degrees Celsius (oC) to 500oC and for 1 hour to 168 hours 

charring time. 

The acid functional groups and lactones of these chars 

were measured by using the Boehm titration method. This 

method measures the amount of NaOH, NaHCO3, and Na2CO3

neutralized by the char and allows the determination of total 

acidity and the fraction of acidity associated with the strong 

acid fraction, the moderate acid and lactone fraction, and 

weak acid fraction. The total acidity of chars from cellulose 

TOTAL ACIDITY, 

IN MILLIEQUIVALENTS 

PER GRAM

unaltered

0

2

4

6

8

10

0 200 300 400 500

Cellulose Lignin Pine Wood Pine Bark

CHARRING TIME, IN HOURS

EXPLANATION

Figure 7. The total acidity of cellulose, lignin, pine wood, and pine 

bark chars after 72 hours of charring at various temperatures.
Summary 17

Table 10. The pseudo-absorbance of selected absorbance bands for diffuse reflectance infrared spectroscopy of lignin and 

lignin char. 

[The absorbance is given in pseudo-absorbance units. The number in parenthesis is the wave number for the highest value of the absorbance band. °C, 

degrees Celsius ; --, not determined]

Charring Temperature (degrees Celsius)

Charring Time (hours) 250°C 300°C 350°C 400°C

Region 1 (3500 - 3300 cm-1)

Unaltered Lignin: 0.81 (3458)

1 -- 0.70 (3512) 0.28 (3528) 0.12 (3510)

8 -- 0.46 (3516) 0.20 (3316) 0.27 (3092)

24 0.81 (3514) 0.39 (3558) 0.39 (3283) 0.45 (3092)

48 -- 0.31 (3345) 0.45 (3262) 0.35 (3088)

72 0.56 (3512) 0.33 (3248) 0.57 (3250) 0.36 (3092)

168 0.30 (3520) -- -- --

Region 2 (3000 - 2800 cm-1)

Unaltered Lignin: 0.88 (2938)

1 -- 0.69 (2938) 0.27 (2936) 0.09 (2928)

8 -- 0.45 (2938) 0.17 (2938) 0.21 (2928)

24 0.83 (2938) 0.37 (2940) 0.32 (2938) 0.37 (2928)

48 -- 0.28 (2940) 0.39 (2938) 0.26 (2928)

72 0.57 (2940) 0.31 (2959) 0.50 (2938) 0.28 (2928)

168 0.32 (2938) -- -- --

Region 3 (1750 - 1700 cm-1)

Unaltered Lignin: 1.20 (1711)

1 -- 0.80 (1705) 0.40 (1709) 0.31 (1709)

8 -- 0.58 (1711) 0.47 (1755) 0.65 (1765)

24 0.93 (1709) 0.61 (1753) 0.72 (1769) 0.87 (1771)

48 -- 0.57 (1755) 0.80 (1776) 0.73 (1776)

72 0.67 (1709) 0.59 (1767) 0.93 (1776) 0.97 (1765)

168 0.52 (1778) -- -- --

Region 4 (1650 - 1610 cm-1)

Unaltered Lignin: 1.32 (1607)

1 -- 0.93 (1611) 0.45 (1614) 0.33 (1611)

8 -- 0.62 (1612) 0.46 (1626) 0.62 (1628)

24 1.01 (1607) 0.59 (1620) 0.66 (1630) 0.84 (1632)

48 -- 0.55 (1626) 0.74 (1632) 0.69 (1639)

72 0.71 (1612) 0.53 (1630) 0.87 (1632) 1.00 (1632)

168 0.45 (1630) -- -- --

and cellulose-containing materials (wood and bark) was 

maximized at low charring temperatures and long charring 

times. At a charring temperature of 250oC, total acidity from 

cellulose continued to increase for times greater than 72 hours 

and reached 8.8 milliequivalents per gram. For a fixed charring 

time, total acidity from cellulose char decreased as charring 

temperature increased. Total acidity in lignin did not change 

substantially with low temperature charring (250 to 400oC).

The oxygen and carbon content of the chars were 

determined. Upon charring, both oxygen and carbon are lost 

from the starting material, but at unequal rates. Oxygen is 

lost more rapidly than carbon at the beginning of the charring 

process, but for longer durations of heating, carbon continues 

to be lost, while oxygen content remains fairly constant. For 

charring at a given temperature, this loss pattern results in 

the percentage carbon reaching a maximum in from 1 to 8 

hours of charring and then decreasing with longer charring 

times, while percentage oxygen reaches a minimum during 

this period and then increases with charring time. Neither the 

percentage oxygen nor the carbon-to-oxygen atomic ratio 

correlate well with the observed increases in acid functional 

groups.

Diffuse reflectance mid-range infrared spectra were 

obtained for the chars. For all materials, charring caused a 
18 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Table 11. The pseudo-absorbance of selected absorbance bands for diffuse reflectance infrared spectroscopy of pine 

wood and pine wood char. 

[The absorbance is given in pseudo-absorbance units. The number in parenthesis is the wave number for the highest value of the absorbance band. 

o

C, degrees Celsius ; --, not determined]

Charring Temperature (degrees Celsius)

Charring Time (hours) 250°C 300°C 350°C

Region 1 (3500 - 3300 cm-1)

Unaltered Pine Wood: 1.16 (3447)

1 1.20 (3424) 0.71 (3455) 0.45 (3509)

8 0.84 (3453) 0.56 (3426) 0.42 (3395)

24 0.76 (3404) 0.60 (3341) --

48 0.78 (3343) 0.61 (3316) --

72 0.74 (3343) 0.60 (3312) --

168 0.79 (3319) -- --

Region 2 (3000 - 2800 cm-1)

Unaltered Pine Wood: 0.84 (2930)

1 0.84 (2903) 0.57 (2938) 0.37 (2934)

8 0.68 (2938) 0.45 (2938) 0.30 (2934)

24 0.60 (2945) 0.45 (2938) --

48 0.58 (2945) 0.47 (2938) --

72 0.56 (2945) 0.45 (2938) --

168 0.59 (2945) -- --

Region 3 (1750 - 1700 cm-1)

Unaltered Pine Wood: 0.71 (1740)

1 0.82 (1736) 1.01 (1715) 0.91 (1711)

8 1.08 (1730) 1.07 (1728) 1.00 (1749)

24 1.26 (1744) 1.19 (1751) --

48 1.30 (1748) 1.13 (1753) --

72 1.21 (1748) 1.18 (1753) --

168 1.26 (1751) -- --

Region 4 (1650 - 1610 cm-1)

Unaltered Pine Wood: 0.70 (1601)

1 0.82 (1597) 1.03 (1601) 0.98 (1605)

8 1.00 (1601) 1.07 (1607) 1.05 (1614)

24 1.14 (1611) 1.13 (1612) --

48 1.16 (1612) 1.05 (1612) --

72 1.08 (1612) 1.09 (1612) --

168 1.12 (1614) -- --
Summary 19

Table 12. The pseudo-absorbance of selected absorbance bands for diffuse 

reflectance infrared spectroscopy of pine bark and pine bark char. 

[The absorbance is given in pseudo-absorbance units. The number in parenthesis is the wave number 

for the highest value of the absorbance band. oC, degrees Celsius ; --, not determined]

Charring Temperature (degrees Celsius)

Charring Time 

(hours) 250°C 300°C 350°C

Region 1 (3500 - 3300 cm-1)

Unaltered Pine Bark: 1.08 (3439)

1 1.03 (3437) 0.70 (3451) 0.60 (3451)

8 0.78 (3443) 0.64 (3345) 0.52 (3455)

24 0.75 (3426) 0.63 (3333) 0.51 (3339)

48 -- 0.68 (3331) 0.67 (3327)

72 0.77 (3329) 0.71 (3331) --

168 0.75 (3329) -- --

Region 2 (3000 - 2800 cm-1)

Unaltered Pine Bark: 0.84 (2926)

1 0.81 (2926) 0.64 (2930) 0.54 (2930)

8 0.68 (2932) 0.54 (2934) 0.41 (2930)

24 0.61 (2938) 0.50 (2934) 0.39 (2930)

48 -- 0.52 (2934) 0.52 (2930)

72 0.60 (2938) 0.53 (2934) --

168 0.59 (2938) -- --

Region 3 (1750 - 1700 cm-1)

Unaltered Pine Bark: 0.76 (1738)

1 1.05 (1713) 0.99 (1711) 0.93 (1713)

8 1.19 (1732) 1.06 (1722) 0.93 (1713)

24 1.23 (1738) 1.08 (1724) 0.91 (1713)

48 -- 1.20 (1728) 1.10 (1713)

72 1.26 (1744) 1.14 (1726) --

168 1.25 (1746) -- --

Region 4 (1650 - 1610 cm-1)

Unaltered Pine Bark: 1.05 (1614)

1 1.19 (1607) 1.09 (1603) 1.04 (1603)

8 1.20 (1605) 1.10 (1603) 1.10 (1601)

24 1.19 (1622) 1.12 (1601) 1.08 (1595)

48 -- 1.23 (1601) 1.22 (1582)

72 1.16 (1628) 1.21 (1595) --

168 1.16 (1601) -- --
20 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

reduction in absorbance in the broad band between 3500 

reciprocal centimeters (cm-1) and 3300 cm-1. This band is 

associated with oxygen-hydrogen stretching and the decrease 

in absorbance can be related to the overall loss of oxygen. 

Charring caused a reduction in absorbance in the bands 

between approximately 3000 and 2800 cm-1 for all materials. 

These bands are associated with aliphatic carbon-hydrogen 

stretching and the decrease in absorbance can be related to the 

overall loss of aliphatic carbons. For the charring of cellulose 

and cellulose containing materials (wood and bark), there 

were increases in absorbance of the sharp band between 1750 

and 1700 cm-1, which is associated with carbonyl stretching 

and can be related to the creation of lactones and carboxylic 

acids. There are also increases in absorbance of the sharp band 

between 1650 and 1610 cm-1, which is associated with skeletal 

vibrations of fused ring aromatic structures and conjugated 

ketones, and can be related to the formation of fused aromatic 

rings and possibly hydrogen bonded quinones.

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U.S. Geological Survey Scientific Investigations Report 

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References Cited 21
22 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Appendix 1. Results of Boehm Titrations 
Appendix 1. Results of Boehm Titrations 23

Table 1-1. Results of Boehm titrations for cellulose and cellulose chars. 

[The table shows the average base neutralized in milliequivalents per gram of sample (meq/g) for the replicates of 

each treatment (equilibration with NaOH, NaHCO3, and Na2CO3). The number in parentheses is the standard error of 

the mean.]

Charring

conditions

meq/g of NaOH 

Neutralized

meq/g of NaHCO3

Neutralized

meq/g of Na2CO3

Neutralized

Number of 

replicates

Unaltered

Cellulose 0.09 (0.005) 0.0 0.007(0.005) 2

Cellulose Char

200°C

72 hours 0.66 (0.015) 0.15 (0.010) 0.51 (0.115) 2

168 hours 1.96 (0.005) 0.41 (0.010) 1.02 (0.060) 2

250°C

8 hours 3.93 (0.160) 0.82 (0.010) 1.96 (0.075) 2

24 hours 6.59 (0.025) 2.06 (0.005) 4.22 (0.040) 2

72 hours 8.72 (0.095) 3.52 (0.020) 6.12 (0.040) 2

168 hours 8.83 (0.140) 3.84 (0.080) 6.38 (0.075) 2

300°C

8 hours 4.04 (0.250) 0.98 (0.090) 2.70 (0.115) 2

24 hours 6.21 (0.130) 1.81 (0.015) 4.17 (0.040) 2

48 hours 7.16 (0.200) 2.35 (0.055) 5.00 (0.080) 2

72 hours 7.38 (0.070) 2.55 (0.004) 4.99 (0.050) 2

350°C

8 hours 3.04 (0.420) 0.88 (0.045) 1.90 (0.010) 2

24 hours 5.12 (0.028) 1.57 (0.031) 3.25 (0.023) 3

48 hours 5.02 (0.025) 1.58 (0.010) 3.42 (0.105) 2

72 hours 5.62 (0.064) 1.67 (0.082) 3.78 (0.090) 3

400°C

8 hours 2.35 (0.020) 0.38 (0.270) 1.63 (0.030) 2

24 hours 3.25 (0.045) 0.91 (0.015) 2.08 (0.005) 2

48 hours 4.21 (0.120) 1.31 (0.010) 2.63 (0.035) 2

72 hours 4.04 (0.060) 1.21 (0.065) 2.60 (0.030) 2
24 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Table 1-2. Results of Boehm titrations for lignin and lignin chars. 

[The table shows the average base neutralized in milliequivalents per gram of sample (meq/g) for the replicates of each treatment (equilibration 

with NaOH, NaHCO3, and Na2CO3). The number in parentheses is the standard error of the mean.]

Charring

conditions

meq/g of

NaOH

Neutralized

meq/g of

NaHCO3

Neutralized

meq/g of

Na2CO3

Neutralized

Number

of

replicates

Unaltered

Lignin 1.23 (0.060) 0.15 (0.005) 0.56(0.175) 2

Lignin Char

250°C

72 hours 0.97 (0.010) 0.18 (0.025) 0.37 (0.030) 2

168 hours 1.45 (0.185) 0.49 (0.025) 0.86 (0.105) 2

300°C

24 hours 1.18 (0.620) 0.17 (0.055) 0.36 (0.035) 2

48 hours 1.51 (0.020) 0.65 (0.000) 1.31 (0.080) 2

72 hours 2.55 (0.195) 1.18 (0.060) 2.03 (0.045) 2

350°C

24 hours 1.23 (0.025) 0.62 (0.015) 0.94 (0.025) 2

48 hours 3.52 (0.142) 1.61 (0.046) 2.72 (0.054) 3

72 hours 2.58 (0.155) 1.27 (0.030) 2.30 (0.195) 2

400°C

24 hours 2.55 (0.000) 0.82 (0.055) 1.47 (0.000) 2

48 hours 2.94 (0.545) 1.23 (0.020) 2.36 (0.065) 2

72 hours 3.47 (0.100) 0.97 (0.065) 2.16 (0.110 2

450°C

24 hours 2.24 (0.005) 0.66 (0.035) 1.44 (0.040) 2
Appendix 1. Results of Boehm Titrations 25

Table 1-3. Results of Boehm titrations for pine wood and pine wood chars. 

[The table shows the average base neutralized in milliequivalents per gram of sample (meq/g) for the replicates of 

each treatment (equilibration with NaOH, NaHCO3, and Na2CO3). The number in parentheses is the standard error of 

the mean.]

Charring

conditions

meq/g of NaOH 

Neutralized

meq/g of NaHCO3

Neutralized

meq/g of Na2CO3

Neutralized

Number of 

replicates

Unaltered

Pine Wood 0.89 (0.035) 0.05 (0.005) 0.17(0.010) 2

Pine Wood Char

250°C

8 hours 2.93 (0.011) 0.32 (0.018) 0.52 (0.111) 2

24 hours 5.40 (0.030) 1.09 (0.050) 3.11 (0.070) 2

72 hours 7.10 (0.170) 2.16 (0.015) 4.77 (0.005) 2

168 hours 7.83 (0.035) 2.80 (0.055) 5.37 (0.010) 2

300°C

1 hour 2.47 (0.428) 0.15 (0.050) 0.85 (0.081) 2

8 hours 4.39 (0.140) 1.02 (0.080) 2.62 (0.070) 2

24 hours 5.49 (0.045) 1.48 (0.062) 3.54 (0.021) 2

48 hours 6.09 (0.005) 1.68 (0.031) 3.83 (0.080) 2

72 hours 5.88 (0.055) 1.83 (0.085) 3.80 (0.180) 2

350°C

8 hours 2.38 (0.010) 0.57 (0.135) 1.46 (0.085) 2

400°C

8 hours 1.35 (0.155) 0.18 (0.015) 0.67 (0.105) 2

Table 1-4. Results of Boehm titrations for pine bark and pine bark chars. 

[The table shows the average base neutralized in milliequivalents per gram of sample (meq/g) for the replicates of 

each treatment (equilibration with NaOH, NaHCO3, and Na2CO3). The number in parentheses is the standard error of 

the mean.]

Charring

conditions

meq/g of NaOH 

Neutralized

meq/g of NaHCO3

Neutralized

meq/g of Na2CO3

Neutralized

Number of 

replicates

Unaltered

Pine Bark 1.57 (0.180) 0.18 (0.025) 0.85(0.035) 2

Pine Bark Char

250°C

8 hours 2.70 (0.215) 0.24 (0.015) 0.74 (0.255) 2

24 hours 3.97 (0.120) 0.63 (0.035) 2.00 (0.030) 2

72 hours 5.47 (0.450) 1.29 (0.155) 2.98 (0.200) 2

168 hours 5.96 (0.245) 1.60 (0.020) 3.63 (0.015) 2

300°C

1 hour 2.24 (0.420) 0.09 (0.005) 0.92 (0.405) 2

8 hours 2.38 (0.165) 0.27 (0.030) 0.92 (0.095) 2

24 hours 2.86 (0.140) 0.45 (0.010) 1.36 (0.035) 2

48 hours 3.62 (0.050) 0.45 (0.030) 1.60 (0.115) 2

72 hours 3.52 (0.040) 0.25 (0.045) 1.47 (0.060) 2

350°C

8 hours 1.59 (0.530) 0.00 (0) 0.41 (0.075) 2

400°C

8 hours 0.47 (0.055) 0.00 (0) 0.24 (0.085 2
26 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Figure 2-1. Diffuse reflectance mid-range infrared spectra of cellulose heated at 200oC for various times.

Appendix 2. Diffuse Reflectance Mid-Range Infrared Spectra

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

168 hours

Unheated

72 hours

PSEUDO-ADSORBANCE
Appendix 2. Diffuse Reflectance Mid-Range Infrared Spectra 27

Figure 2-2. Diffuse reflectance mid-range infrared spectra of cellulose heated at 250oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

24 hours

1 hour

8 hours

Unheated

48 hours

72 hours

168 hours

PSEUDO-ADSORBANCE
28 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Figure 2-3. Diffuse reflectance mid-range infrared spectra of cellulose heated at 300oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

24 hours

1 hour

8 hours

Unheated

48 hours

72 hours

PSEUDO-ADSORBANCE
Appendix 2. Diffuse Reflectance Mid-Range Infrared Spectra 29

Figure 2-4. Diffuse reflectance mid-range infrared spectra of cellulose heated at 350oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

24 hours

1 hour

8 hours

Unheated

48 hours

72 hours

PSEUDO-ADSORBANCE
30 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Figure 2-5. Diffuse reflectance mid-range infrared spectra of cellulose heated at 400oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

8 hours

1 hour

Unheated

24 hours

48 hours

72 hours

PSEUDO-ADSORBANCE
Appendix 2. Diffuse Reflectance Mid-Range Infrared Spectra 31

Figure 2-6. Diffuse reflectance mid-range infrared spectra of lignin heated at 250oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

168 hours

24 hours

72 hours

Unheated

144 hours

PSEUDO-ADSORBANCE
32 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Figure 2-7. Diffuse reflectance mid-range infrared spectra of lignin heated at 300oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

72 hours

24 hours

48 hours

Unheated

8 hours

1 hour

PSEUDO-ADSORBANCE
Appendix 2. Diffuse Reflectance Mid-Range Infrared Spectra 33

Figure 2-8. Diffuse reflectance mid-range infrared spectra of lignin heated at 350oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

24 hours

1 hour

8 hours

Unheated

48 hours

72 hours

PSEUDO-ADSORBANCE
34 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Figure 2-9. Diffuse reflectance mid-range infrared spectra of lignin heated at 400oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

24 hours

1 hour

8 hours

Unheated

48 hours

72 hours

PSEUDO-ADSORBANCE
Appendix 2. Diffuse Reflectance Mid-Range Infrared Spectra 35

Figure 2-10. Diffuse reflectance mid-range infrared spectra of lignin heated at 450oC and 500oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

1 hour, 500°C

Unheated

8 hour, 500°C

24 hour, 450°C

PSEUDO-ADSORBANCE
36 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Figure 2-11. Diffuse reflectance mid-range infrared spectra of pine wood heated at 250oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

Unheated

1 hour

8 hours

24 hours

48 hours

168 hours

72 hours

PSEUDO-ADSORBANCE
Appendix 2. Diffuse Reflectance Mid-Range Infrared Spectra 37

Figure 2-12. Diffuse reflectance mid-range infrared spectra of pine wood heated at 300oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

Unheated

1 hour

8 hours

24 hours

48 hours

72 hours

PSEUDO-ADSORBANCE
38 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Figure 2-13. Diffuse reflectance mid-range infrared spectra of pine wood heated at 350oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

Unheated

1 hour

8 hours

PSEUDO-ADSORBANCE
Appendix 2. Diffuse Reflectance Mid-Range Infrared Spectra 39

Figure 2-14. Diffuse reflectance mid-range infrared spectra of pine wood heated at 400oC and 500oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

Unheated

1 hour, 400°C

8 hour, 400°C

1 hour, 500°C

PSEUDO-ADSORBANCE
40 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Figure 2-15. Diffuse reflectance mid-range infrared spectra of pine bark heated at 250oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

24 hours

1 hour

Unheated

72 hours

168 hours

8 hours

PSEUDO-ADSORBANCE
Appendix 2. Diffuse Reflectance Mid-Range Infrared Spectra 41

Figure 2-16. Diffuse reflectance mid-range infrared spectra of pine bark heated at 300oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

24 hours

1 hour

8 hours

Unheated

48 hours

72 hours

PSEUDO-ADSORBANCE
42 Development of Acid Functional Groups and Lactones During the Thermal Degradation of Wood and Wood Components 

Figure 2-17. Diffuse reflectance mid-range infrared spectra of pine bark heated at 350oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

24 hours

1 hour

8 hours

Unheated

48 hours

PSEUDO-ADSORBANCE
Appendix 2. Diffuse Reflectance Mid-Range Infrared Spectra 43

Figure 2-18. Diffuse reflectance mid-range infrared spectra of pine bark heated at 400oC for various times.

3900 3400 2900 2400 1900 1400 900 400

WAVENUMBER (RECIPROCAL CENTIMETERS)

8 hours

Unheated

PSEUDO-ADSORBANCE
Rutherford and others—Development of Acid Functional Groups of Wood and Wood Components—Scientific Investigations Report 2007–5013