Quantification of hexenuronic acids in E. globulus kraft pulps by chromatographic and

spectroscopic techniques

Regis Teixeira Mendonça Facultad de Ciencias Forestales y Centro de Biotecnología

Universidad de Concepción

Kraft pulping

Main chemical process to convert wood in cellulosic pulp.

Kraft pulping and HexA

The degradation of some hemicelluloses during kraft pulping led

to the formation of the hexenuronic acids (HexA).

• Arabino-4-O-methylglucuronoxylan (softwoods).

• Acetyl-4-O-methylglucuronoxylan (hardwoods).

Methylglucuronic acid (MeGlcA)

Hexenuronic acids

Formation of HexA from MeGlcA in alkaline medium.

• Not a major problem in kraft pulping of softwoods (SW).

• Most SW have low amount of MeGlcA.

• More important when pulping hardwoods (HW).

• Main hemicellulose in HW is acetyl-4-O-

methylglucuronoxylan (~ 15-30% of wood weight).

Hexenuronic acids

Importance of HexA

• HexA reacts with KMnO4 during kappa number determination overestimating residual lignin in pulp.

• 10 mmol HexA/kg pulp = 0.84-0.86 units in kappa (Li and Gellerstedt, 1997).

• HexA increase consumption of bleaching chemicals (Cl2, ClO2 and O3).

• HexA decreases brightness stability.

There are several techniques for HexA quantification

Vuourinen et al. 1999 UV Spectroscopy

Chai et al. 2001 HgCl2/Na-acetate UV Spectroscopy

Teleman et al. 1996, Tenkanen et al. 1995 y Dahlman et al. 2000

Enzymatic hydrolysis Spectroscopy

Electrophoresis

Formic acid/Na-formate

Gellersted and Li. 1996 Hg acetate/acetic acid

Chromatography

Jiang et al. 2001 Sulfuric acid Chromatography

HYDROLYSIS QUANTIFICATION

NIRS SPECTROSCOPY

• It is also a powerful technique used to estimate wood and pulp properties.

Longitud de onda (nm)

Lo

g (

1/

R)

Longitud de onda (nm)

Lo

g (

1/

R)

• Prediction models were developed by using chemometrics tools such as PCA, PCR and PLS.

Class 1

Class 2

SIMC A-P 10.5 - 19/04/2005 11:29:21

E. globulus

E. nitens

Class 1

Class 2

SIMC A-P 10.5 - 19/04/2005 11:29:21

E. globulus

E. nitens

E. globulus

E. nitens

E. globulus

E. nitens

446.5 500 550 597.2

440.8

500

600

651.4

SpecifiedEstim

ate

d

xx

xx

xxx

xxx

xx

x

xx

xx

x

xx xx

xx

xxxxxx

x

x

xx

x

x

xxxxx

x

xxxxxx

x

x

x

xx

xx

Densidad básican= 61

R2= 0,90

SEP = 17,2

Densid

ad e

stim

ada

Densidad experimental446.5 500 550 597.2

440.8

500

600

651.4

SpecifiedEstim

ate

d

xx

xx

xxx

xxx

xx

x

xx

xx

x

xx xx

xx

xxxxxx

x

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Densidad básican= 61

R2= 0,90

SEP = 17,2

Densid

ad e

stim

ada

Densidad experimental

EXAMPLES OF NIRS APPLICATION

Separation of species Estimation of wood density

OBJECTIVES

1. Produce kraft pulps of E. globulus with varied amount of HexA.

2. Evaluate the effect of pulping conditions on HexA formation/degradation.

3. Compare two methodologies for quantification of HexA content in E. globulus kraft pulps:

• UV spectrophotometry (Chai et al., 2001).

• Anion Exchange Chromatography (Jiang et al., 2001).

4. Develop FT-NIR models to estimate HexA composition in E. globulus kraft pulps.

EXPERIMENTAL

Wood

• Wood chips from 10 years old E. globulus.

• Chemical characterization:

• Extractives (ethanol/toluene)

• Carbohydrates (acid hydrolysis and HPLC)

• Lignin (Klason and soluble)

• Uronic acids (UV: Blumenkrantz and Asboe-Hansen, 1973)

• 50 g wood chips (odb) and 250 mL cooking liquor (1-L Parr

reactor heated at 1.6°C/min).

• Cooking liquor:

• 15% and 20% active alkali (AA)

• 25% sulfidity

• Na2O basis

• Temperature: 155°C and 165°C.

• Cooking at H-factors from 200 to 2500.

• 130 pulps were produced.

Kraft Pulping

Kraft Pulping

• Pulp yield

• Kappa number

• Carbohydrates

• Lignin

• Uronic acids

• Hexenuronic acids (UV, AEC and FT-NIR)

HexA by UV

• Chai et al., J. Pulp Paper Sci., 27(5):165-170, 2001.

• Pulp hydrolysis with HgCl2/Na-acetate.

• CHexA= 0.287(A260-1.2A290)V/m

Á cido 5 - dehidro - 4 - deoxi - D - glucur ó nico Á cido 5 - dehidro - 4 - deoxi - D - glucur ó nico Á cido 5 - dehidro - 4 - deoxi - D - glucur ó nico

5-dehydro-4-deoxy-D-glucuronic acid

λ1 = 260 nm

λ2 = 290 nm

HexA by AEC

Á cido 5 - dehidro - 4 - deoxi - D - glucur ó nico Á cido 5 - dehidro - 4 - deoxi - D - glucur ó nico Á cido 5 - dehidro - 4 - deoxi - D - glucur ó nico

• Jiang et al., J. Pulp Paper Sci., 27(3):92-97, 2001.

• Pulp hydrolysis with H2SO4.

• AEC analysis: 4 mM NaOH (eluent) at 0.7 mL/min; 25°C;

conductivity detector.

HexA by NIRS

• 110 pulps analyzed for models development.

• 20 pulps used as external validation set.

• Perkin Elmer Identcheck FT-NIR.

• Spectra recorded in diffuse reflectance at 2 nm interval

(1000-2500 nm) using 32 scans.

• Software: Spectrum v. 5.0.1 and QUANT+.

• PCR and PLS analysis on baseline corrected and 2nd

derivative spectra.

RESULTS

Chemical composition of E. globulus

• Glucans: 49.6%

• Xylans: 16.1%

• Lignin: 26.3%

• Extractives: 1.2%

• Uronic acids (MeGlcA): 5.3%*

* Similar amount found for E. globulus grown in Portugal (Simão et al., Ind. Eng. Chem. Res., 2005)

Unscreened Pulp yield

(□) 15% AA, 155°C (■)15% AA, 165°C

(○) 20% AA, 155°C (●) 20% AA, 165°C

Kappa number

(□) 15% AA, 155°C (■)15% AA, 165°C

(○) 20% AA, 155°C (●) 20% AA, 165°C

Model: 51.3 ± 1.7 mmol/kg pulp

45 min, 5.7 pH y 80°C

Experimental: 50.6 ± 0,3 mmol/kg pulp

Model: 35,5 ± 1,5 mmol/kg pulp

3.1 h, 0.7 pH y 97°C

Experimental: 36.5 ± 0.2 mmol/kg pulp

Determination of optimal conditions for HexA hydrolisys

HgCl2/NaAc/UV H2SO4 /AEC

Comparison between UV and AEC quantification methods for HexA

Relationship between UV and AEC techniques for HexA determination

Comparison with other methods used in the determination of HexA

HexA (quantified by UV)

Pulping at 155°C

MeGlcA: (■)15% AA and (●) 20% AA

HexA: (□) 15% AA and (○) 20% AA

HexA (quantified by UV)

Pulping at 165°C

MeGlcA: (■)15% AA and (●) 20% AA

HexA: (□) 15% AA and (○) 20% AA

Kappa number vs. HexA content

Development of FT-NIR models

HexA-UV (mmol/kg)

HexA-AEC (mmol/kg)

Sample number 130 130

Minimal content 10.2 4.7

Maximal content 71.0 59.7

Average content 44.9 34.2

Average SD 0.7 1.2

Minimal SD 0.1 0.1

Maximal SD 2.0 2.0

Sample number and ranges of analytical data for HexA in E. globulus kraft pulps.

2nd derivative espectra of pulps with similar HexA contents but with different kappa numbers.

1000,0 1200 1400 1600 1800 2000 2400 2500,0

0,00

0,0

0,00

nm

K-M

HexA 41.3, Kappa 8.5

HexA 41.0, Kappa 33,0

HexA 41.3, Kappa 8.5

HexA 41.0, Kappa 33.0

K-M

nm

Developed NIR models with statistical values

* A set of 13 samples with kappa number and HexA content lower than 12 and 55 mmol/g pulp, respectively, were excluded from the calibration models.

y = 0,85x + 5,32

R2 = 0,81

Bias= -0,77

SEP y RMSEP= 5,70

20

40

60

80

0 20 40 60 80Valor observado

Va

lor

pre

dic

ho

y = 0,81x + 9,08

R2 = 0,83

Bias= -0,83

SEP y RMSEP= 6,70

20

40

60

80

0 20 40 60 80Valor observado

Valo

r pre

dic

ho

HexA by UV

HexA by AEC

Correlations for PLS-predict model and observed values for HexA

CONCLUSIONS

• Cooking temperature and active alkali have a significant effect in HexA formation and degradation during kraft pulping.

• Good reproducibility were found for both techniques used for HexA quantification (UV and AEC).

• HexA quantification by UV is a fast technique, however chromatography allows direct quantification of degradation products avoiding interference of lignin or products from carbohydrate degradation.

• NIRS models developed for HexA quantification were adequate to estimate HexA in kraft pulps allowing the analysis of several samples in less time when compared with conventional techniques.

Acknowledgements

Co-authors:

• Mariel Monrroy

• Juanita Freer

• José Ruiz

• André Ferraz

• Jaime Rodríguez

• Jaime Baeza

Financial support from:

• FONDECYT grants 1050535 and 7070089.

• CONICYT (M. Monrroy’s PhD Fellowship).

¡Gracias!

Thanks!

Obrigado!