Research Article - (2014) Volume 5, Issue 8

Microwave Assisted Degradation of Lignin to Monolignols

Shaveta, Harshpinder Kaur and Palwinder Singh*
Department of Chemistry, Guru Nanak Dev University, Amritsar-143005, India
*Corresponding Author: Palwinder Singh, Department of Chemistry, Guru Nanak Dev University, Amritsar-143005, India, Tel: +91-183-2258802 Exn. 3495, Fax: +91-183-2258819 Email:


Objective: To carry out lignin degradation under very mild conditions using microwave technique.
Methods: Lignin salt purchased from HiMedia laboratories was dissolved in distilled water and the solution was
introduced to Biotage Initiator microwave. The reaction temperature was varied between 100-150°C with average
power 85-90 W. Mass spectra were recorded in ACN-H2O (3:7) solution on BrukermicroTOF QII mass spectrometer
in +ve ESI mode. Acetonitrile and water were HPLC grade and were bought from Sigma Aldrich while Sodium
fluoride was purchased from Spectrochem. SEM images were recorded on ZEISS EVO LS10 scanning electron
microscope with prior coating of silver using Quorum Q150R ES coating machine.
Results: 1 g of lignin dissolved in 10 ml distilled water was irradiated under microwaves. Aliquots of samples
were withdrawn and mass spectra were recorded after every 10 min. Maximum conversion of lignin to monolignols
was observed after 90 min. Effect of NaF was also observed when catalytic amount of NaF (10 mg) was used in
each reaction. Surprisingly, the results obtained in the previous conditions after 90 min were achieved only after 30
min in the presence of NaF.
Conclusion: Lignin was degraded to its monomeric units when its aqueous solution in presence of catalytic
amount of NaF was irradiated under microwaves at 150°C for 30 min. Hence, this technique could be used to
degrade lignin into various lower lignols along with monolignols. Using this technique, revolutionary change could
be made in paper, pulp and leather industry. Above all, several pharmaceutical raw materials could be synthesized
efficiently from important monolignols produced from lignin degradation.


ACN: Acetonitrile; ESI: Electrospray ionization; MWI: Microwave irradiation; NaF: Sodium fluoride; SEM: Scanning electron microscope


Succeeded by cellulose, lignin holds a major part of the biomass. It is the key constituent of secondary cell wall of plants and is present in some algae too. The worldwide production of lignin comprises of 1.1 million metric tons per year [1]. The lignocellulosic material includes around 30% by weight of lignin; [2] out of which, it is primarily used in combustion processes and only <5% is utilized for other purposes [3].

Chemically, lignin is a copolymer of three different phenylpropane monomer units, viz., p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. They are linked through β-O-4 aryl ether bonds within the compact network system of lignin (Figure 1). Depolymerization of lignin could be a source of value based molecules that can be used as efficient fuels and for other basic applications [4]. It has a major application in food industry and perfumery along with the sources of various pharmaceutical products. For example, monolignols like vanillin have a broader scope in the synthesis of numerous pharmaceutical ingredients [5].


Figure 1: A) Monomeric units present in lignin, B) polymeric network of lignin sulphonate sodium salt.

As per the available reports, [6] the depolymerisation of lignin has been achieved by (i) acid catalyzed, [7,8] (ii) base catalyzed, [9-11] (iii) metal catalyzed, [12-15] (iv) ionic liquid aided [16-18] and (v) supercritical fluid assisted chemical reactions [19-21]. Acid and base catalyzed depolymerisation involves the use of large amounts of acids/ bases (2-10 weight %) keeping the reaction temperature >250°C. Also, very strong acids/bases viz., HCl, H2SO4, formic acid/ NaOH, KOH, LiOH, etc. are being employed in this type of lignin degradation. The metal catalyzed degradation involves the use of different heavy metal catalysts like, Pt, Pd, Ni, Ru, etc. along with quite high temperature of the reaction vessel. The use of ionic liquids and supercritical fluids is also practiced at high temperature and pressure. In continuation to our previous work on the degradation of biomass resources for preparing ample value added products where cellulose was successfully converted to glucose; [22] here, lignin is made to undergo degradation under the similar reaction conditions.

Materials and Methods

Reaction Procedure

Lignin sulphonic acid sodium salt (1 g) was dissolved in 10 ml distilled water and the solution was introduced to Biotage Initiator microwave. The reaction temperature was varied between 100-150°C with average power 85-90 W. The reaction was monitored by taking out the aliquots after every 10 min and recording the mass spectrum. NaF catalyzed reaction was also performed under similar conditions. Lignin sulphonic acid sodium salt (1 g) and 10 mg NaF were dissolved in 10 ml distilled water and the reaction mixture was subjected to microwave irradiations (Figure 2). The reaction was monitored by recording mass spectra after regular intervals of 10 min. When no further change in the mass spectra was visible, the reaction was stopped and the reaction mixture was distilled off in vacuo on rotary evaporator and a dark brown solid residue was obtained. Solid product was further dried in vacuo and SEM images were recorded.


Figure 2: Chemical reaction and conditions involved in lignin degradation under present investigation.

Recording of mass spectra

Mass spectra were recorded in ACN-H2O (3:7) solution on Bruker Micro TOFQII mass spectrometer in +ve ESI mode and the species formed were identified from their respective m/z values.

SEM imaging

All the samples were vacuum dried and were precoated with silver using Quorum Q150R ES coating machine. SEM images were recorded on ZEISS EVO LS10 scanning electron microscope.

Results and Discussion

After the successful conversion of cellulose (major part of biomasss) into glucose [22] under very mild reaction conditions (pH-6, NaF/NaCl) utilizing microwave technique by our group; we planned to degrade lignin (another major part of biomass) also under the microwave assisted mild reaction conditions. For the present investigation, sodium salt of lignin sulphonic acid was used (purchased from HiMedia laboratories) as the starting material. 1 g of the lignin salt was dissolved in 10 ml distilled water. The reaction mixture was subjected to microwave irradiation at 100-150°C at an average power of 85W. Aliquots of the samples were withdrawn from the reaction vials after regular intervals of 10 min for recording mass spectra. Formation of degraded species was detected after 90 min of irradiation at 150°C (Chart 1, Table 1, Figure 3).


Chart 1: Various species detected in the mass spectra of depolymerized lignin sulphonic acid sodium salt.

S.No. Species detected Mol.
[M + Na]+
[M + Na]+
1 A C9H12O2 175.0730 175.1133
2 B C9H12O4 185.0808* 185.1238*
3 C C9H10O3 189.0312 189.0323
4 D C10H12O3 203.0679 203.0505
5 E C10H12O3 203.0679 203.0505
6 F C11H14O4 211.0965* 211.0320*
7 G C11H16O3 219.0992 219.0439
8 H C10H12O4 219.0992 219.0439
9 I C12H10O4 241.0471 241.0245
10 J C12H10O7 267.0499* 267.0909*
11 K C11H16O6 267.0839 267.0909
12 L C17H20O4 289.1434* 289.1739*
13 M C16H18O4 297.1097 297.1039
14 N C18H22O5 319.1540* 319.2211*
15 O C20H22O4 327.1591* 327.1185*
16 P C19H22O4 337.1410 337.1771
17 Q C19H20O5 351.1567 351.1912
18 R C20H26O6 385.1622 385.1602
19 S C21H26O6 397.1622 397.2298
20 T C22H30O7 429.1884 429.0588
21 U C17H19O5SO3Na 487.1009 487.0940


Table 1: The species detected in mass spectra of depolymerized lignin.


Figure 3: Mass spectra of lignin-sulphonic acid salt on irradiation at 150oC for 90 min under microwaves; (a) Full spectrum; (b) expansion of m/z 100- 300 region and (c) expansion of m/z 300-450 region.

Intriguingly, to see the effect of NaF on the degradation process, catalytic amount (10 mg) of NaF was added to the reaction mixture and it was irradiated under microwaves at 150°C. The sample aliquots were withdrawn after regular intervals and mass spectra were recorded. The chemical species detected after 30 min of reaction (Figure 4) were similar as observed after 90 min during the absence of sodium fluoride. Complete transformation of polymeric structure of lignin was accomplished in 30 min of microwave irradiation.


Figure 4: Mass spectra of lignin-sulphonic acid salt in presence of cat. amount of sodium fluoride on irradiation at 150 oC for 30 min under microwaves; (a) Full spectrum; (b)expansion of m/z 100-300 region and (c) expansion of m/z 300-500 region.

In general, it was observed that the intensity of chemical species with m/z <300 was low in the reaction mixture without NaF while these species have quite intense peaks when the reaction was performed in presence of NaF (Figure 4). Moreover, the peaks with m/z>600, though present in low intensities in the reaction mixture without NaF, were completely vanished in presence of NaF. Evidently, the microwave assisted degradation of lignin seems to be more effective in presence of NaF. Advantageously, the milder method used here involves only a small amount of an economical catalyst (NaF) as compared to the conventional methods used so far for lignin degradation where much higher temperatures and drastic conditions (acidic/basic) were used along with much expensive catalysts.

In order to spot out the morphological changes during the depolymerisation of lignin, SEM micrographs were recorded and a drastic change was observed between the polymerized and depolymerized states of lignin. From the much fine globular structures in case of polymerized lignin, morphology transformation to irregular and finer particles was taken place in case of depolymerized state (Figure 5).


Figure 5: SEM images of (A) untreated Lignin-sulphonic acid salt; (B) depolymerized lignin.


A moderate technique for degradation of lignin has been demonstrated. The hydrogen bonding network of lignin was broken up and hydrolyzed so as to obtain mono and higher lignols. Microwave degradation technique was employed along with small amount of NaF. Since the presence of NaBr, NaI and NaCl did not affect the depolymerization process, F- may be playing a strong H-acceptor role in breaking the H-bond network of the polymer. Since, this method involves the use of much lower temperature and milder catalyst (in small amount) as compared to the previously reported conventional methods; the method could be of considerable significance from the environmental as well as industrial point of view. Consequently, a number of chemical entities of pharmaceutical importance could be obtained from lignin.


Financial assistance from CSIR, New Delhi and DST, New Delhi are gratefully acknowledged. Shaveta thanks UGC-BSR for fellowship. UGC, New Delhi is also gratefully acknowledged for grant under UPE programme to Guru Nanak Dev University.


Citation: Shaveta, Kaur H, Singh P (2014) Microwave Assisted Degradation of Lignin to Monolignols. Pharm Anal Acta 5:308.

Copyright: © 2014 Shaveta, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.