Solid Residues from Supercritical Extraction of Wood - ACS Publications

Chapter 14

Solid Residues from Supercritical Extraction of Wood

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Characterization of Their Constituents J.

L.

Grandmaison, A. Ahmed, and S. Kaliaguine

Department of Chemical Engineering, Université Laval, Quebec G1K 7P4, Canada The systematic analysis of the solid residues of the supercritical methanol extraction of Populus tremuloides was performed for samples prepared at temperatures varying from 250 to 350°C and pressures from 3.4 to 17.2 MPa, using such analytical techniques as wet chemistry, chromatography, thermogravimetric analysis, diffuse reflectance FTIR spectrometry and photoelectron spectroscopy. The results allow monitoring of the continuous changes in chemical composition of samples ranging from partly extracted wood samples to highly recondensed polyaromatic structures. In the recent studies o f thermal and thermochemical processes o f wood liquefaction, considerable progress has been reported i n the analysis o f gaseous and l i q u i d products . For the last twenty years much work has been done i n the study of the thermal s t a b i l i t y o f l i g n o c e l l u l o s i c materials by thermal a n a l y t i c a l methods. Since these materials are complex mixtures o f organic polymers, thermogravimetric (TG) analysis causes a v a r i e t y of chemical and physical changes depending on the nature of the sample and i t s treatment p r i o r to analysis. These problems have been reviewed recently . L i g n o c e l l u l o s i c material can also be analyzed by IR spectrometry. This a n a l y t i c a l method was used f o r characterization o f modified l i g n i n and c e l l u l o s e i n various ways ~ . Quantificat i o n by infrared spectrometry has been reported, f o r example, i n analysis of the three basic constituents i n sweetgum and white oak chips pretreated at temperatures ranging from 140 to 280°C £> using the d i f f u s e reflectance FTIR spectrometry (DRIFT). The technique i s simple and applicable to powdered s o l i d s and dark samples -Li and (2

3 )

( 5

1 3 )

C

(

0097-6156/88/0376-0139$06.00/0 « 1988 American Chemical Society

Soltes and Milne; Pyrolysis Oils from Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

)

140

PYROLYSIS OILS F R O M

BIOMASS

can be used f o r the characterization of the chemical bonds and t h e i r modifications by thermal processes. In t h i s paper we report our e f f o r t s to characterize the s o l i d residues produced in a s e r i e s of experiments with the semicontinuous extraction of Populus tremuloides (aspen wood) in s u p e r c r i t i c a l methanoH-LS.) , at temperatures ranging from 250 to 350°C ( S u p e r c r i t i c a l Extraction residues or SCE residues), by using wet chemistry and chromâtographic JJËL , thermogravimetric and spectral methods such as DRIFT* 12 and ESCA . (

)

)


Experimental

Procedure

The s o l i d residues analyzed here were produced by s u p e r c r i t i c a l ext r a c t i o n with methanol of Populus tremuloides in a tubular reactor JJi.> . The a n a l y t i c a l procedures used f o r these residues were described previously as elemental analysis, Klason l i g n i n test, thioglycolic acid lignin test, recondensed material and carbohydrates*i£>, thermogravimebric (TG/DTG) and FTIR and ESCA JL£ . Table 1 reports r e s u l t s obtained using these procedures as well as conditions of extraction for each SCE residue. (

(

)

Results and Discussion Wet Chemistry and Chromatography. For the analysis of wood, the Klason l i g n i n test, performed in concentrated s u l f u r i c acid, i s the accepted method f o r the determination of l i g n i n content. We performed s i m i l a r tests using also t r i f l u o r o a c e t i c acid (TFA), the r e s u l t s of which are almost i d e n t i c a l to those of the Klason tests. TFA has the advantage of allowing further analysis of the saccharides i n the acid soluble f r a c t i o n , as i t can e a s i l y be evaporated from the s o l u t i o n . The a c i d insoluble f r a c t i o n , usually designated as "Klason l i g n i n " , i s referred to i n t h i s work as Klason residue. Figure L shows that f o r the most severe extraction conditions the SCE residue i s almost e n t i r e l y constituted of Klason residue. The fact that t h i s Klason residue cannot be considered as l i g n i n has been established through elemental analysis and IR spectroscopy i n KBr p e l l e t s * 1L> . In order to determine i f the so Lid residues s t i l l contain l i g n i n , the o l d method of forming a soluble l i g n i n derivative with t h i o g l y c o l i c acid was used. This reagent reacts by displacement of α-hydroxy1 and α -alkoxyl groups in l i g n i n , and the derivative so produced can be s o l u b i l i z e d by a l k a l i and recovered. Results are reported i n Table 1 and Figure 1 showing that t h i o g l y c o l i c acid l i g n i n (TGAL) decreases from 15.6% i n wood to 3.3-5.9% in the sam ples prepared at 350°C. It was shown by IR spectrometry that the TGAL keeps the c h a r a c t e r i s t i c features of l i g n i n even f o r SCE tem peratures of 350 C. This confirms our b e l i e f that the t h i o g l y c o l i c acid test i s a s u i t a b l e method f o r the determination of uncondensed l i g n i n i n SCE residues. In s p i t e of the fact that: I) the Klason test induces some condensation reactions, and 2) the t h i o g l y c o l i c acid test may only extract those l i g n i n fragments containing benzyl alcohol groups or a r y l ether groups*.Li> , we would l i k e to suggest that: 1) the o


Soltes and Milne; Pyrolysis Oils from Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 1988. 47.4 44.2 41.2 48.6 48.9

29.2 19.0 16.8 15.0 14.8 29.7 21.6

19.8

20.8 11.6 3.33 18.8

6. 50 5. 84 5.,29 6. 4 9 6.,97

5. 82 5.,12 4.,96 4.,62 5.,35 5..47 5.,17

6..08

4,.34 4 .89 3 .84 5 .00

46.1 50.0 53.5 45.0 44.1

64.9 75.9 78.2 80.4 79.8 64.8 73.3

27.,2 18.,0 12.,8 19.,2 12.,3

7.,9 0.,0 0.,0 0..0 0.,0 3..3 0.,0

47.,6 45.,7 38.,1 38. 0 26..9

25. 9 0.,0 0.,0 0.,6 0.,0 10.,3 0.,7

78.5 70.8 64.1 89.9 90.6

46.4

74.8 83.5 92.8 76.3

1..5 0 .0 0 .0 0 .0 4,.8 0,.0 0 .0 0 .0 11.1 0.8 1.2 3.1 85 .1 88 .5 89 .2 77 .0

5,.2 3 .8 5 .9 3 .3

90.0 92.3 95.1 80.3

26.9 18.2 10.2 6.1

1.5 0.5 2.5 1.5

3.4 10.3 10.3 17.2

350 350 350 350

MP-14 MP-11 MP-8 MP-24

D r y wood b a s i s . D r y SCE r e s i d u e b a s i s . C a l c u l a t e d by d i f f e r e n c e .

74.1 0,.0 7,.1 19.8

66,.7

4..8

71.4

8.4

1.2

17.2

330

MP--27

-

5.0 1.6 2.4 27.2 4.0

30.,1 65.,8 87.,8 88..7 84..7 59..8 81,.3

12.,1 12..3 4.,3 3..8 4..5 6.,0 5..3

42.2 78.1 92.1 92.5 89.2 65.8 86.7

40.5 31.0 16.4 15.9 15.7 18.4 15.0

0.5 2.5 1.5 1.5 1.5 0.5 2.5

(a) (b) (c)

%0 (b,c)

Wt%

(b)

(b) Wt&< b>

%H

%c

xyl

and a n a l y s i s

Glu

conditions

TFA soluble Wt%(b >

extraction

3.4 3.4 10.3 10.3 10.3 17.2 17.2

11..8 8.,3 4.,1 2.,0

15.,6 14.,3 22.,2 6.,5 5..4

Recond. material Wt%(b>

17.6 26.1 30.5 10.6 7.6

Wt%(b>

TGAL

100.0 74.8 55.4 69.7 68.4

Κlason residue wtS»< b )

from Populus tremuloides:

Residue yield Wt*(a)

(SCE) r e s i d u e s

1.5 0.5 2.5 1.5

SCE Flowrate L/h

extraction

3.4 10.3 10.3 17.2

SCK Pressure MPa

Supercritical

300 300 300 300 300 300 300

250 250 250 250

SCE Temperature °C

I.

MP-16 MP-20 MP-9 MP-12 MP-21 MP-13 MP-18

MP-15

Wood MP-22 MP--6 MP-17

Sample

Table



PYROLYSIS OILS FROM BIOMASS

Figure 1. Klason residue and t h i o g l y c o l i c a c i d l i g n i n (TGAL) in SCE s o l i d residues (expressed on dry SCE residue b a s i s ) .



14. GRANDMAISON ET AL.

ResiduesfromSupercritical Extraction of Wood

t h i o g l y c o l i c acid l i g n i n represents a good estimate of unconverted l i g n i n , and 2)the Klason residue represents the summation of unconverted l i g n i n and of condensation products formed by p y r o l y s i s reactions during the SCE process. As a consequence, we suggest that the difference between the Klason residue and the t h i o g l y c o l i c acid l i g n i n i s representative of recondensed material (RM) in SCE residues. The calculated values for recondensed material in these residues are reported in Table I. Figure 2 gives the values for the percent recondensed material expressed on a dry wood basis. Figure 3 shows the percentage of recondensed m a t e r i a l , expressed on dry wood basis, plotted as a function of l i g n i n conversion. This graph suggests different condensation reactions at 250°C and at 3 0 0 - 3 5 0 ° C . At 250°C in p a r t i c u l a r , the condensation seems to be a secondary reaction of l i g n i n conversion. As also shown on the figure, for several experiments the percents of recondensed material are higher than the value which would be calculated assuming that a l l converted l i g n i n i s transformed to recondensed material ( l i n e A). It i s believed that this indicates that the condensation reaction involves not only products of degradation of l i g n i n but also some of carbohydrates. The glucose and xylose contents were determined i n the soluble TFA acid hydrolysis f r a c t i o n by l i q u i d chromatography using a cation exchanged r e s i n ( C a form) column. The results are reported i n Table 1. Most of the samples prepared at 300-350°C show only minor amounts of hydrolyzed material, except for samples MP-16, MP-13 and MP-14 which were prepared at low pressure or low flow rate. The percents of glucose and xylose for these samples as well as those for the samples prepared at 250°C, expressed on dry wood b a s i s , are plotted on Figure 4. The rather well defined curve indicates that c e l l u l o s e and hemicellulose are simultaneously degraded at or below 250°C. ++

Thermogravimetric Analysis. Thermogravimetric analysis (TG and DTG) under nitrogen atmosphere was performed for aspen wood and the 16 p a r t i a l l y converted wood residues. The TG and DTG curves are r e produced in Figure 5 for untreated wood and for 4 selected representative SCE residues. The examination of TG and DTG curves, shows that: a) aspen wood loses weight s t a r t i n g near 230°C (pyrolysis of hemicellulose < 2 i ) ) j between 350 and 4 2 0 ° C , with a maximum rate of weight loss at 385°C (cellulose and l i g n i n p y r o l y s i s ^ i ) ) ; the weight lost at 700°C i s 89.4%. b) the SCE residues can be c l a s s i f i e d according to t h e i r temperature of extraction. For the residues of type I prepared at 250°C (e.g. sample MP6), the weight loss takes place between 350 and 4 2 0 ° C , with a maximum rate at 3 7 5 - 3 9 0 ° C . The weight lost at 700°C i s between 82.5 and 94.6%. For the type II residues produced at 300°C (e.g. sample MP12), a continuous weight loss i s observed from 300 to 6 0 0 - 7 0 0 ° C , with a maximum rate at temperatures ranging from 380°C to 5 1 0 ° C . The t o t a l weight loss at 700°C is less important than for samples of the previous type, ranging from 27.2 to 57%. )

a n (

c


143

144



2«ι

Ttmperoturc, *C

Figure 2. Effect of SCE conditions on (expressed on dry wood b a s i s ) .

recondensed material




Residuesfrom Supercritical Extraction of Wood

Figure 3 . Recondensed material (expressed on dry wood basis) as a function of l i g n i n conversion (experiment number shown i n parentheses).


146


4β

bod

44

40

36 Downloaded by CORNELL UNIV on June 20, 2017 | http://pubs.acs.org Publication Date: September 30, 1988 | doi: 10.1021/bk-1988-0376.ch014

. The band at 1740 cm is due to uronic acid and acetyl groups i n hemicellulose . The bands from 1235 to 1605 cm , s p e c i a l l y the one at 1505 cm , are representative of l i g n i n

-1

-1

-1

Spectral region 2870-3050 cm . A band at 3050 cm (aromatic and/or alkene C-H stretching) becomes evident at 300°C (MP-12, Figure 7-III) and dominates this region at 350°C (MP-8, Figure 7-V). The band in the 2900 cm" region ( a l i p h a t i c C-H stretching) which i s broad in the i n i t i a l wood sample, i s progressively resolved in three separate bands (2870, 2930 and 2955 cm ) as the SCE temperature i s 1

-1


GRANDMAISON ET AL.

ResiduesfrontSupercritical Extraction of Wood


ο

SCE TEMPERATURE CO

Figure 6. DTG peak temperature as a function o f ture and pressure.

4000

ι

4000

3000

2000

SCE tempera

1000

1 1 1 1 1 1

3000

2000

1000

WAVENUMBERS (cm" ) 1

Figure 7. DRIFT spectra ( i n KC1) o f Populus tremuloides and of four SCE residues.


149


150

increased (MP-12, MP-11, and MP-8, Figures 7JII, IV and V). The o v e r a l l pattern i n Figure 7V corresponding to the most carbonized sample i s s i m i l a r to the ones reported for higher rank bituminuous coal and f o r v i t r i n i t e , with the 3050 cnr even more i n tense i n our MP-8 sample. As i t was shown e a r l i e r that t h i s sample contains 89.2% of recondensed material, i t may be concluded that t h i s material has a c o a l - l i k e polyaromatic nature. This i s supported by the changes i n the next spectral region. 1

1

-1

Spectral Region 700-950 c n r . The band at 895 cm is discernible in wood and MP-6 (SCE temperature 250°C) but disappears from spectra of samples treated at higher temperatures where saccharides analysis has also shown the absence of c e l l u l o s e . As carbonization proceeds, the out-of-plane bending below 900 cm" (at 870-881, 800-833 and 752-766 cm" ) increases i n intensity; the position for some of the bands ( 800 and 750 cm ) can suggest condensed ring systems. The band at 950 cm* , which i s v i s i b l e i n MP-8, i s probably due to elimination reaction giving t - a l k e n e s .


1

1

-1

1

( 1 9 )

1

Spectral Region 1450-1595 cm" . The c h a r a c t e r i s t i c aromatic r i n g v i b r a t i o n at 1505 cm , c l e a r l y v i s i b l e in the spectrum of wood, i s gradually hidden with an increase i n the SCE temperature. This corresponds to the progressive decrease in l i g n i n content of the residue. Inversely, two bands near 1450 and 1595 cm" become very intense and dominate i n the spectra of residues produced at 300 and 350°C (Figures 7-III, IV and V). These changes p a r a l l e l the modifi cations i n the 2870-3050 cm" region. The band at 1450-1460 cm" can be attributed to methyl and methylene bonding and also to aroma t i c r i n g modes . The band at 1595 cm i s also assigned to aromatic r i n g stretching. Its high i n t e n s i t y i n spectra 7-III, IV and V, could possibly be given the three following explanations 1) aromatic r i n g stretching i n combination with a chelated conjugated carbonyl structure, 2) aromatic r i n g stretching mode, with possible i n t e n s i t y enhancement due to phenolic groups, 3) aro matic r i n g s t r e t c h i n g of aromatic e n t i t i e s linked by methylene and possibly ether linkages. -1

1

1

1

-1

1

Spectral Regions 1035-1370 and 1705-1740 cm" . The bands from 1035 to 1171 cm" (C-0 bonds in polysaccharides, l i g n i n alcohols and l i g n i n ethers), present i n wood and samples obtained at 250°C, drop in the spectra of SCE residues produced at and above 300°C. The aromatic ethers bands (up to 1330 cm* ), and the phenolic s t r e t c h i n g near 1250 cm" , decrease also. The hemicellulose band, at 1740 cm* , present on untreated wood almost disappears i n residues prepared at 250°C. The unconjugated carbonyl and/or carboxyl and/or ester of conjugated acids at 1710•1720 cm from o r i g i n a l l i g n i n i s s t i l l v i s i b l e at 250°C when hemi c e l l u l o s e i s p a r t l y removed, but at higher SCE temperatures i t i s hidden by the highly intense 1705 cm* band. This last band can be a t t r i b u t e d to a conjugated carbonyl or carboxyl structure, but i t would be s u r p r i s i n g that carboxyl could r e s i s t at severe SCE con d i t i o n s . Further study i s necessary f o r d e f i n i t e assignment of t h i s band. 1

1

1

1

-1

1


14.

ResiduesfromSupercritical Extraction of Wood

GRANDMAISON ET AL.

151

Quantification. Schultz et alLL? reported correlations of FTIH absorbance r a t i o s with such variables as percent glucose, xylose and Klason l i g n i n for wood chips pretreated by the RASH process at temperature ranging from 140 to 280°C. These correlations do not f i t c o r r e c t l y our data, so we developed our own equations by non-linear least squares regression. For quantitative evaluation of absorbances, baseline was defined as shown on spectra on Figure 7. These equations are described i n Table I I . In equations 1-5, the i n t e n s i t i e s (I) are expressed in Kubelka-Munk units. Selection of the independent variables in equations 1-5 was done by f i r s t s e l e c t ing 16 s i g n i f i c a n t bands i n the 17 DRIFT spectra. A l l l i n e a r regression equations describing the f i v e dependent variables, as function of a l l possible combinations of i n t e n s i t y r a t i o s , were obtained using two to s i x of these variables. A preliminary s e l e c tion was made on the basis of R values and F test s i g n i f i c a n c e l e v e l s . A f i n a l s e l e c t i o n was performed, among the s t a t i s t i c a l l y equivalent regression equations, on the basis of the a n a l y t i c a l s i g n i f i c a n c e of the bands selected, y i e l d i n g equations 1-5.


2

KSÇA ESCA i s a surface s e n s i t i v e technique, based on the measurement of k i n e t i c energies of photoelectrons ejected from a given atomic energy l e v e l under the action of a monoenergetic X-ray beam. I t provides quantitative information on the elemental composition as well as on the chemical environment of each atom (bonding and oxidation s t a t e ) . The k i n e t i c energy o f photoelectrons ( E k ) , as measured with respect to the vacuum l e v e l , i s expressed as: Ek

=

Ex

- (EB +

Φ

+

E

C

)

(6)

where E x i s the energy of the incident photon, EB i s the binding energy of the electron on i t s o r i g i n a l l e v e l , φ i s the work function of the spectrometer, and E c i s the energy lost i n counteracting the p o t e n t i a l associated with the steady charging o f the surface. Φ and E c are e s s e n t i a l l y corrections, φ depends on the spectrometer and i s not l i a b l e to be modified between experiments. E c i s high on low conductivity samples and can be made lower by the use o f a flood gun. ESCA spectra corresponding to carbon Is peaks of Populus tremuloides and 3 samples i s o l a t e d at three different SCE temperatures are i l l u s t r a t e d i n Figure 8. There i s a general agreement i n the l i t e r a t u r e on the assigment of components C i , C 2 and C 3 in wood derived materials: C i corresponds to carbon linked to Η or C, C 2 has one l i n k to oxygen, whereas C 3 has two. In the s o l i d phase, C i i s referenced at 285.0 eV and C 2 and C 3 are usually close to 287.0 and 289.5 eV C2s>. In a l l SCE residues, a fourth C i s component i s found on the low binding energy side of the spectrum, s h i f t e d from the C i component by 1.4 +0.5 eV. This i s thereafter designated as the Co component. As the temperature of extraction i s increased from 250 to 300°C, the C o component increases continuously whereas the general trend o f C i , C 2 and C 3 components in a continuous decrease.



17.2

-42.8

* Klason residue =

% Glucose =

+

+

35.3

20.0

(

(

x

114.1

(

31.1

(

97.7

( χ

x

=

.879

=

.967

Il370/

]

J

=

-

+

.961

IlSQs/

Ιΐ2β0\

= .940

11595/ 2

-

l30S0\ J

=

J

I l 505 ν

Η

χ

2

-

.962

IlS95/

R

χ

2

l29SS\

H

Iisos'

J

I30S0\

-

*

V

V

141.8

/

^

\

(9.1

/

χ

χ

ί 69.5

/

162.2

/

Î77.7 χ

/

+

Il595 '

1

IlS9S/

Ιΐ2β0\

*

-

j -

Il370'

/

V

*

>

(33.2

/

χ I l 5 9 5 '

J

IlS0S\

I i s o s /

J

I l 370 \

+

/ \

[12.5

*

χ Il595

\

'

J

J

\

Ιΐ595'

l89S

Ιβ95

χ

I i 3 7 0 '

/ IlSOS \ ί 111. 1 x I -

χ

ί 54.9

/

122.8

/

ί51.1

1 -

12930 \

Ιΐ21θ\

χ

χ

IlS95'

)

l2870\

Iisos'

1 +

12870 \

IlS9s/

)

I l l 6 0 \

*

Il370/

/ Ιβ95 \ 1181.2 χ J

χ

Table I I . Regression equations' to c a l c u l a t e f i v e parameters i n SCE residues


(5)

(4)

(3)

(2)

Cl)

δ

en Ο

ΝΗ

^

Ο £Η



GRANDMAISON ET AL.

Figure 8. ESCA spectra o f and for 3 SCE residues.

Cie peak

f o r Populus tremuloides


153


154


It i s i n t e r e s t i n g to note that the uncorrected experimental C i s binding energy f o r dibenz (a,h) anthracene and for a sample of high rank commercial coal i s very close to the binding energy for t h i s C o peak. As polyaromatics are e l e c t r i c a l conductors, the charging i s expected to be low and E c close to 0. On this basis, C o component i s assigned to carbon in polyaromatics. This complementes the i n frared conclusion about the polyaromatic nature of recondensed material. The r a t i o CRM/CSR (where CSR i s the carbon content of the whole s o l i d residue as determined by elemental analysis, whereas CRM i s the calculated mass of carbon i n the recondensed material contained in a given sample) was calculated for 6 samples. Figure 9 shows that t h i s r a t i o i s correlated to the C o f r a c t i o n of the C i s peak f o r ground chars, but also that both values are almost the same f o r each sample; t h i s represents the case where residues are well ground (black dots on the f i g u r e ) . For two samples, MP-22 and MP-16, the ESCA analysis were ran on p a r t i a l l y ground samples. When the grind ing i s less severe (MP-22-a and MP-16-a), the f i b e r s are only separated and not broken. The C o area value i s the same as f o r nonground samples. Otherwise, by increasing the e f f i c i e n c y of the grinding (MP-22-b and MP-16-b), the Co% value decreases and, ultimately i s equal to the bulk % carbon in recondensed material. It can be concluded that i n p a r t i a l l y converted material, the recon densed material i s located at the external surface of the f i b e r s but uniformally d i s t r i b u t e d within the raw samples. 100 MP-1K 80

-

MP-160

MP-20

60 MP-22Q

σ ω σ

MP-16b

MP-22b o °

4

0

20 0 0

20

40 C

RM

/ C

SR ·

60

80

100

0/ο

Figure 9. Relation between carbon i n recondensed material of some SCE residues and ESCA C o peak. Therefore a simple means reactions by a nent i n the C i s

i t may be concluded that the ESCA technique provides for the determination of the extent of recondensation mere determination of the proportion of the C o compo band of the s o l i d residue.

Ackn ow1edgement s We thank A. Adnot f o r performing ESCA spectra; P. Chantai (CANMET, Ottawa) f o r running thermogravimetric analysis; G. Chauvette (DREV,




V a l c a r t i e r ) f o r having allowed the use of his FTS-40 spectrometer for running some infrared spectra and J . Thibault for c a l c u l a t i n g the regression equations.

Literature Cited 1. 2. 3.


4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

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Received March 31, 1988


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