Analytical pyrolysis of alcohol bisulfite lignin

Journal of Analytical and Applied Pyrolysis 31 ( 1995) 239. 247

ELSEVIER

JOURNAL 01 ANALYTICAL and APPLIED PYROLYSIS

Analytical pyrolysis of alcohol bisulfite lignin Ken-ichi Kuroda ‘,*, Sung-Phil Mun b, Kokki Sakai ’

Received I6 March 1994; accepted 26 August 1994

Abstract A high molecular weight lignin fraction, alcohol bisulfite lignin, was isolated from the spent liquor of alcohol bisulfite pulping of Japanese cedar (Cryptomeriu juponiu D. Don) wood and subjected to analytical pyrolysis. The resulting products were compared with those from the original wood, its milled wood lignin and a synthetic lignin. The alcohol bisulfite lignin yielded several characteristic pyrolysis products compared with the other samples: a large yield of 4-propylguaiacol and small amounts of methylated compounds. Furthermore. analytical pyrolysis of the alcohol bisulfite lignin gave a much smaller yield of coniferyl alcohol than that from Japanese cedar wood. These differences are due to the chemical change in the lignin structure as a result of the alcohol bisulfite treatment. Keywords:

Pyrolysissgas

Alcohol bisulfite lignin; Japanese cedar (Crytomeriu juponica D. Don); chromatography; Pryolysis gas chromatography/mass spectrometry

Pyrolysis;

1. Introduction The

use of the analytical

in a number

of reports

* Corresponding

author

Ol65-2370/95/$09.50 SSDI Ol65-2370(

pyrolysis

[l-7].

$3 1995 ~ Elsevier 94)00839-6

technique

Because

to study

of the high

lignin

sensitivity

Science B.V. All rights

reserved

has been described and speed of the

240

K. Kurodu cural. /J.

And.

Appl. Pyrol.vsis 31 (1995) 239-247

analysis, analytical pyrolysis is recognized as a powerful analytical tool for natural and synthetic polymers [8]. A characteristic of the analytical pyrolysis of lignin is that it gives rise to many products with different side chains. This information is attractive to lignin researchers because it provides insight to the nature of side chains in the lignin macromolecule. This paper describes analytical pyrolysis characterization of a high molecular weight lignin fraction (HML), dissolved in the spent liquor during alcohol bisulfite pulping. Alcohol bisulfite pulping at elevated temperature cleaves most b-aryl ether linkages in the phenolic lignin units and produces several low molecular weight phenols such as eugenols and 4-propenylsyringols in relatively high yields from softwoods and hardwoods [9,10]. An earlier study compared the characteristics of HML with those of a milled wood lignin (MWL) using chemical degradation methods such as alkaline nitrobenzene and potassium permanganate oxidations [ 111. However, these degradation methods showed no differences in the side chain structures between HML and MWL, despite the fact that HML was subjected to a drastic treatment compared with MWL. The main reason was felt to be that conventional lignin degradation methods provide little information about side chains because of their degradation to aldehydes or carboxylic acids. The objective of this work was to detect any modifications in lignin side chains during alcohol bisulfite pulping of wood. For this purpose, we compared product profiles from analytical pyrolyses of wood meal, HML, MWL and a synthetic lignin (DHP).

2. Materials

and methods

2.1. Materials After treating extractive-free Japanese cedar (Cryptomeria japonica D. Don) wood meals with 50% aqueous isopropyl alcohol containing a small amount of Mg( HSO,), at 165 “C for 160 min, HML dissolved in the spent liquor was isolated as the barium salt through several purification steps [ 111. The yield of HML was 49% of dissolved lignin, i.e. about 40% based on wood lignin. MWL was prepared from the same wood species by Bjorkman’s method [ 121. Its yield was about 25% of wood lignin. DHP was prepared from coniferyl alcohol by the “Zulauf” method [ 131. 4-Propylguaiacol was prepared by catalytic hydrogenation of isoeugenol (Aldrich) with palladium-carbon. Homovanillin was prepared from dl-metanephrine (Sigma) according to the method of Robbins [14]. 2.2. Pyrolysis -gas

chromatography

(Py - GC)

Basically, Py-GC analyses of lignocellulosic materials were the same as those described previously [5]. Each sample (about 200 pg), wrapped with a 500°C ferromagnetic pyrofoil, was pyrolyzed for 4 s in a Curie-point pyrolyzer (JHP-3

model, Japan Analytical Industry) coupled directly with a gas chromatograph (GC) (Shimadzu GC-14A). The resulting volatile compounds were separated on a stainless steel capillary column (Ultra Alloy-l, Frontier Lab., 30 m x 0.80 mm i.d.. 2.0 Ltrn thickness). The pyrolysis products were identified by comparing their retention times with those of authentic samples, and by comparing their Py-GC/ mass spectrometry (Py-GC/MS) data with published data [7,15].

About 200 pg of sample placed on a platinum boat was pyrolyzed in a furnace-type pyrolyzer (Shimadzu PYR-2A. in a He atmosphere) at 500 C for 10 s. The volatile pyrolysis products were carried onto a 25 m CBP-1 fused silica capillary column ( 0.2 mm i.d.; 0.25 /lrn thickness) fitted in a Shimadzu GCMSQPlOOOEX mass spectrometer set in the split mode. The GC oven was kept at 50 C during pyrolysis and subsequently programmed to 250’ C at a rate of IO C min ‘. Products were ionized at 70 eV. Helium was used as the carrier gas. 2.4. Lignirl rlrtermincition Lignin content method [ 16).

in wood

meal

was determined

by the standard

Klason

lignin

3. Results and discussion Fig. 1 shows the Py-GC trace of HML at 500’C for 4 s. For comparison. the Py-GC traces of wood, MWL and DHP are shown in Figs. 2(a)-(c). respectively. Fig. 3 shows the total ion chromatogram (TIC) of HML. Peaks identified are summarized in Table 1. The peak numbers correspond to the elution sequence of the compounds on the TIC. Most of the peaks on the TIC are displayed on the Py-GC trace of HML, although for some compounds the elution sequence on the Py-GC trace of HML is different, e.g. peaks 25 and 30. The main peaks 3,5 and 11 on the Py-GC trace of HML were identified as guaiacol, 4-methylguaiacol and 4-vinylguaiacol, respectively. Other prominent compounds were 4-ethylguaiacol (peak 9), vanillin (peak 15), truns-isoeugenol ( peak 17). acetoguaiacone (peak 19) and coniferaldehyde (peak 30). The peak intensity of trrms-coniferyl alcohol (peak 25t) was low. These products also appeared on the Py-GC traces of other samples. Therefore, the profile of the pyrolysis product composition of HML was basically the same as those of MWL and DHP. However. note the height of peak 14 on the Py-GC trace of HML. Peak 14 was present in trace amounts in the pyrolysates of the other samples. Interestingly, even on the Py-GC trace of Japanese cedar wood. peak 14 was small. This strongly suggests that original wood lignin has very small quantities of lignin building units that yield peak 14 on pyrolysis. Fig. 4 shows the electron impact ionization (EI) mass spectrum of peak 14 at 70 eV. A parent ion of 166 and the mass fragmentation pattern of peak 14 suggest two

F

3

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FID

RESPONSE

0.

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w

2 71 2p

FID

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K. Kurodu et cd. / J. And. Appl. PJwl~sis 31 (19951 239

247

243

15

5

b) MWL

17

IS-21 30

I

13

4 I4

;

s

‘Uy

,,I

retention

time

(min)

5

c) DHP

it

9

retention

time

(min)

Fig. 2. Pyy CC traces at 500 “C for 4 s of (a) Japanese cedar (Crypvpromrriajuponicu D. Don) wood. (b) MWL and (c) DHP. Note: (I) Peak numbers and names refer to those in Table I. (2) Peaks 25~ and 25t mean cis- and trans-coniferyl alcohol, respectively.

244

K. Kuroda rr ui. /J.

And.

Appl. Pyrolysis

Fig. 3. Pyrogram (total ion chromatogram) of alcohol Peak numbers and names refer to those in Table’ I.

31 (1995) 239

bisulfite lignin (HML)

247

at 500 “C for IO s. Note:

plausible compounds; 4-propylguaiacol and homovanillin [ 7,151. To identify and estimate peak 14, the authentic samples were injected from the septum of the sample holder at the time of the Py-GC analysis of HML. 4-Propylguaiacol overlapped peak 14 and homovanillin overlapped a peak appearing at a retention time of 23.5 min (indicated by an asterisk). Accordingly, peak 14 was identified as 4-propylguaiacol. Table 2 compares the yield of 4-propylguaiacol (peak 14) from Japanese cedar wood, its lignin preparations and DHP. The yield of 4-propylguaiacol (peak 14) from HML was about five to six times that of the other samples. On the wood lignin basis, the yield of 4-propylguaiacol from HML was twice that from wood. The yield of HML was about 40% of original wood lignin. It is not possible, therefore, that HML is the fraction of the original lignin rich in 4-propylguaiacol precursor. The production of 4-propylguaiacol in a large yield may thus be diagnostic for the alcohol bisulfite pulping process. However, it is very difficult to explain the production of large yields of such a compound based on the conventional pyrolysis mechanisms since the pyrolysis of lignin model compounds showed that the formation of products with a saturated side chain is due to a secondary [ 171. The large yield of 4-propylguaiacol suggests that relatively large reaction amounts of the precursor of 4-propylguaiacol had been produced by modification of side chains in lignin during alcohol bisulfite pulping of wood, although the exact nature of the precursor is not clear. The observation of signals assignable to isopropyl group in “C- and ‘H-nuclear magnetic resonance spectra of HML [ 1 I] is noteworthy in this connection. The Py-GC/MS data of HML showed the presence of several methylated compounds, 4_methylveratrole, 4-propylanisole, veratraldehyde, vanillic acid methyl ester and acetoveratrone (peaks 6, 12, 18, 22, and 24, respectively), with small peak intensities. To our knowledge, however, methylation is rare in analytical pyrolysis of lignocellulosic materials. These compounds were observed only on the TIC of

245

Table I Py -CC/MS

data

Peak no.

4 6

lh

X Y

IO II 12 13 14 15 16 17 18 I9 20 h 21 h 22 73 74 25 26 ” 27 28 29 30 “ Bold figures:

of an alcohol

bisulfite

lignin (HMLI

Assignment

MW

Marker

Phenol p-Methylphenol Guaiacol 2.4-Dimethylphenol 4-Methylguaiacol 4-Methylveratrole 2-Methyl-4-ethylphenol 4-Propylphenol 4-Ethylguaiacol 4-Propenylphenol 4-Vinylguaiacol 4-Propylanisole Eugenol 4-Propylguaiacol Vanillin ci.\-Isoeugenol trcm.s-Isoeugenol Veratraldehyde Acetoguaiacone Guaiacylpropyne Guaiacylallene Vanillic acid methyl ester Guaiacylacetone Acetoveratrone Coniferyl alcohol Guaiacyl vinyl ketone Propioguaiacone Vanillic acid Dihydroconiferyl alcohol Coniferaldehyde

94 108 124 123 I38 I52 I36 136 I52 134 150 150 164 166 I52 164 164 166 166 162 162 182 1x0 180 1x0 I78 1x0 I68 1x2 I78

94. 66 108, 107, 124. 109, 122. 121, 138. 123. 152. 137. 136. 121 107 152. 137 134. 133 150. 135. 121 164. 149. 137 152. 151 164, 149. 164. 149, 166. 165. 166. 151. 162. 147. 162. 147, 18’. 151. 137 180. 165 1x0. 137. 151 I.51 168. 153 1x2. 137 178. 147.

base ion peak; ’ No authentic

sample

~7~: > 30% of base peak ,’

79. 77 RI 107, 77 95 109

107. 77 131. 103. 77

139. 137, Y5 123 130, 130. 121

77 77

119. 91 119, 91

124

135. 107, 77

was available.

Fig. 4. El (70 eV) mass spectrum

of peak

14.

HML. Therefore, the pyrolytic production of methylated yields of 4-propylguaiacol were a result of the modification molecule during alcohol bisulfite pulping.

compounds and high of the lignin macro-

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Table 2 Yields (‘X,) of 4-propylguaiacol and coniferyl alcohol bisulfite hgnin (HML) and MWL coniferyl alcohol Products

4-Propylguaiacol Coniferyi alcohol Dihydroconiferyl Coniferaldehyde Coniferyl alcohol

Appl. Pyro1wi.s 31 (1995) 239-247

alcohol isolated

type products from Japanese cedar wood and an from Japanese cedar and DHP prepared from

Sample

type products alcohol (cis- + mm-)

Wood

HML

MWL

DHP

0.11

0.54 (0.22)

0.09 (0.02)

0.09

0.66 1.93 4.58

0.28 (0.1 I) 0.69 (0.28) 0.47 (0.19)

0.46 (0.12) 0.90 (0.23) 0.35 (0.09)

1.71 2.05 1.70

Yields (‘%I) of products from the wood sample are based material weight, respectively. The numbers in parentheses

on Klason lignin and lignin samples on dry show the yields based on Klason lignin

Another large difference was observed in the yield of trans-coniferyl alcohol (compare Fig. 1 with Fig. 2(a)). On the Py-GC trace of the wood sample, trans-coniferyl alcohol was the most prominent peak, but it was a small peak on that of HML. Table 2 shows that the yield of trans-coniferyl alcohol from HML was only a tenth of that from the wood sample. Lignin has some coniferyl alcohol-end groups. These groups probably give rise to coniferyl alcohol type pyrolysis products, cis- + trans-coniferyl alcohol (peak 25), dihydroconiferyl alcohol (peak 29) and coniferaldehyde (peak 30). If so, the production of the large yields of these products from the DHP sample (Table 2) mostly depends on coniferyl alcohol end-groups because DHP prepared by the “Zulauf” method has a high abundance of coniferyl alcohol-end groups [ 18,191. MWL which is an “end-wise” polymer with small amounts of coniferyl alcohol-end groups [ l&20,21] produced trans-coniferyl alcohol in small yields. On the wood lignin basis, MWL showed the yields of coniferyl alcohol type pyrolysis products similar to those of HML. These findings suggest that the alcohol bisulfite treatment has split off considerable amounts of coniferyl alcohol end-groups in Japanese cedar lignin, leading to the low yields of coniferyl alcohol type products on pyrolysis. In addition, guaiacylglycerol-b-guaiacyl ether, a representative model compound for the b-0-4 substructures in the lignin macromolecule, also gives rise to coniferyl alcohol type products by the pyrolytic cleavage of the b-aryl ether linkage [2,22]. Therefore, the low yields of coniferyl alcohol type pyrolysis products from HML may also indicate that the phenolic /?-guaiacyl ether linkages in lignin were extensively cleaved by the alcohol bisulfite treatment. In other words, HML may be a residual fraction of lignin after liberation of the coniferyl alcohol precursors. This view is consistent with our previous observations that relatively large yields of eugenol and isoeugenol were liberated from wood lignin by the reductive cleavage of the phenolic p-guaiacyl ether bonds during the alcohol bisulfite treatment [9-l 1,231.

4. Conclusion A chemically changed lignin, produced by treatment of wood with a mixture of isopropyl alcohol and magnesium bisulfite at elevated temperature, was analyzed by Py-GC and PyyGC/MS. The results are summarized as follows. ( 1) The peculiarity of the structure of HML is indicated by the production in a large yield of 4-propylguaiacol and the production of small amounts of 4-propylanisole, 4methylveratrole, veratraldehyde, vanillic acid methyl ester and acetoveratrone. ( 2) During alcohol bisulfite pulping, lignin loses some lignin building units such as coniferyl alcohol end-groups and the p-O-4 substructures. which reduces the yield of coniferyl alcohol type products on pyrolysis. Analytical pyrolysis was effectively used to show the ditTerences in the side chain structure between HML and other samples (wood, MWL and DHP), the differences which have not been revealed by chemical degradation methods such as alkaline nitrobenzene oxidation and permanganate oxidation. However, the exact interpretation of results is not straightforward.

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