(544i) Conversion of Kraft Lignin to Value Added Aromatic Based Chemicals

Conversion of kraft
lignin to value added aromatic based chemicals

Deepak Raikwar ¹,
Saptarshi Majumdar ¹, and Debaprasad Shee ^1*

¹Department of Chemical Engineering,
Indian Institute of Technology Hyderabad, Kandi, Sangareddy, 502 285,

Telangana, India

Email: ch13m15p000001@iith.ac.in, saptarshi@iith.ac.in, dshee@iith.ac.in

Abstract:

Keywords:
Lignin depolymerization, HZSM5, NaOH, Guaiacol

1.    
Introduction

“Fossil
fuel” is the contemporary non-renewable resources ranging from transportation
fuels to platform chemicals which had played a major role in the modernization
of human society since they recognized. However, depletion of fossil fuel
reserve and its associated harmful effect in environment is triggering a search
towards exploring an eco-friendly alternative which can provide fuels and
chemicals. Among the various availabe alternatives, lignocellulosic biomass
holds the enormous potential to deliver different high-value products including
transportation fuel as well as platform or building-block chemicals. Potential
sources of lignocellulosic biomass include agricultural residues, hardwood,
softwood, corn stover, wheat straw and switchgrass which are inexpensive,
renewable and non-edible. Lignocellulosic biomass is comprised of three major units:
cellulose (40-45%), hemicellulose (25-35%) and lignin (15-30%). Over the
several years various studies have been successfully contributed to converting carbohydrates
(cellulose and hemicellulose to bio-fuel. While, the third unit lignin has been
envisioned as a renewable source of aromatics which are building blocks for
various materials including solvents, detergents and adhesives. However, production
of aromatics from lignin is a big challenge till date as the basic aromatic
alcohol units (coniferyl, sinapyl and p-coumaryl) of lignin are linked through
a variety of bonds including aryl ether (β-O-4, α-O-4, and 4-O-5) and
carbon-carbon bond (β-β, β-5, β-4, and 5-5). The most
predominant and well-known linkage type is the β-O-4 and α-O-4 bonds.
Due to low bond dissociation energy, these bonds are often targeted by lignin
depolymerization methods. Some of the previous strategies such as
base-catalyzed depolymerization (BCD), acid catalyzed, reduction, oxidation and
pyrolysis have been employed for lignin depolymerization [1].
The major shortcomings associated with most of the strategies were: very low lignin
loading, high amount of catalyst employed, long reaction time, difficult to
implement protocols or operations at high temperature and pressure, lower
lignin conversion and complex mixture of compounds in the product stream. Establishing
a methodology for getting value added aromatics from aromatics, it is essential
to convert lignin to a few single ring aromatic entity which can
be further upgraded to the target chemicals.

In
present work, we carried out depolymerization of commercial Kraft lignin in a
batch reactor (Parr Instruments, USA, Model No:4560) with the major objective
of attaining high conversion, low char and high selectivity towards a
particular product. To
the best of our knowledge, for the first time we are performing lignin
depolymerization studies with a high loading of lignin. A combination of HZSM-5
as a catalyst and NaOH as co-catalyst was used in green water-THF (tetrahydrofuran)
co-solvent system at realistic operational conditions using mild reaction
temperature and short reaction time. Various process parameters were optimized
for constructing a realistic and efficient approach. The spectroscopic
techniques such as FT-IR (Fourier Transform Infrared Spectroscopy) and ¹H-NMR
(Nuclear Magnetic Resonance) were utilized as a guiding tool to understand the
bond breakage phenomena during the depolymerization process. A plausible
mechanism for catalytic depolymerization of lignin was also proposed based on
the product distribution under a wide range of process parameters.

2.    
Result and discussion

The
HZSM5 catalyst with different Si/Al ratio was characterized using N₂-physisorption,
Powder X-ray diffraction (XRD), Temperature-programmed desorption of ammonia
(NH₃-TPD), pyridine Fourier transform infrared spectroscopy (py-FTIR).
The liquid product was analyzed qualitatively and quantitatively using
GC-FID/MS. The major compounds identified in the liquid products were single-ring
oxygenated aromatics categorized as (i) Phenol, (ii) Guaiacols
(Phenol, 2-methoxy- (CAS) Guaiacol, Phenol,2-methoxy-3-methyl-(CAS) m-Cresol,
Phenol,4-ethyl-2-methoxy-(CAS), p-Ethylguaiacol, Vanillin, Acetovanillone,
Benzeneaceticacid,4-hydroxy-3-methoxy), (iii) Catechols (1,2-Benzenediol (CAS)
Pyrocatechol, 4-Methyl catechol) and (iv) Others (Benzene, 1,2-dimethoxy)
Veratrol, 3-Eicosene, (E)-, Diethyl Phthalate). The residual lignin was
characterized using FT-IR and ¹H-NMR.

The
effect of reaction time on the lignin depolymerization reaction showed that with
in 2h a lignin conversion of 74% was achieved and remained almost constant with
further increase in reaction time (Figure 1). Char formation was observed to
increase from 16.8% at 2h to 20% at 4h. A maximum selectivity of guaiacols
(65%) was obtained at 2h of reaction time which further reduced to 55% at 4h.
The effect of reaction temperature on the conversion, char formation and
product distribution during lignin

depolymerization
was investigated at a range of 493-533K with a reaction time of 2h (Figure 2).
The lignin conversion was observed to increase gradually from 52% to 74.4% with
the increase in temperature from 493K to 533K. There was no significant change
in char formation, however, the selectivity of guaiacols was reduced from 79%
to 51% with the increment in reaction temperature. This reduction in
selectivity of guaiacol was due to their conversion to catechols via
demethylation reaction at elevated temperatures catalyzed by HZSM5. The effect
of water-to-THF ratio was investigated in various ratios of water and THF as a
co-solvent at 533K for 2h (Figure 3). When only alkaline water was used as a
solvent a lower lignin conversion of 50%, and 45% selectivity for guaiacols was
observed. There was also a significant amount of char formation (26%). In a water-THF
mixture of solvent, the conversion of lignin initially increased with an
increasing ratio of water-to-THF. A maximum lignin conversion of 78.2% with
minor char formation (5.7%) was observed at a water-THF ratio of 1:1(v/v) along
with 64.3% selectivity for guaiacols. With the further increase in water-to-THF
ratio the lignin conversion was dropped while selectivity of guaiacols and char
formation increased. These results suggested that water-THF solvent mixture is
more advantageous for lignin depolymerization reactions where alkaline water
promotes the depolymerization of β-O-4 bonds and THF is responsible for guaiacols
stabilization which then reduces the repolymerization reactions. Residual
lignin (RL) obtained after every reaction was investigated with FT-IR to study
the structural changes occurred during the lignin depolymerization reactions
(Figure 4). During parameter analysis, optimal conditions were selected based
on the reaction data and FT-IR spectra simultaneously. For e.g. FTIR spectra of
residual lignin obtained at different water-to-THF ratio is shown in Fig. The
FTIR spectra of residual lignin (RL) revealed that the intensity of bands at
1041, 1084 and 1134 cm^-1 which are related to ether bonds (C-O-C)
decreased considerably to minimal at a water-to-THF ratio of 1:1 (v/v).
Furthermore, the intensity of bands representing aromatic nuclei (1596, 1508,
and 1421 cm^-1) and phenolic -OH (3434cm^-1) also weakened
at a water-to-THF ratio of 1:1 (v/v) indicates the depolymerization of lignin
to phenolic monomers or oligomers which accord well with the GC-MS and ¹H-NMR
results. These results are also in good agreement with the reaction data of
achieving maximum conversion and higher selectivity for guaiacols at a water-to-THF
ratio of 1:1 (v/v). Therefore, 1:1 (v/v) ratio of water-to-THF was selected as
optimum for lignin depolymerization reactions at 533K for 2h respectively.   

            Best
results were obtained with using HZSM280 with a catalyst-to-lignin ratio of
1:10 (w/w) and NaOH-to-lignin (0.083 w/w) at 533K for 2h where 81% of lignin
was converted with minimal char formation (9%) along with 64% selectivity for
guaiacols.

3.    
Conclusion

Depolymerization
of Kraft lignin was efficiently carried out in a high-pressure batch reactor
with higher loading of lignin to obtain a single aromatic entity at a mild
reaction temperature and short reaction time. HZSM5 was used as catalyst and
NaOH as a co-catalyst in the lignin depolymerization reactions. A green
water-THF co-solvent was used as the reaction medium where alkaline water
assist in cleavage of aryl-ether bonds generating monomers (mainly guaiacols) and
oligomers and THF tends to stabilize low molecular weight guaiacols maintaining
their higher selectivity. The guaiacols can then undergo demethylation to
produce catechols which undergoes deoxygenation to generate phenol catalyzed
over HZSM280. Various reaction parameters were studied to accomplish
the best reaction conditions resulting in higher lignin conversion and
selectivity for a single oxygenated aromatic compound (i.e. guaiacols). The
FTIR was utilized as the guiding tool in selecting optimal conditions via
linking the structural changes with the reaction results.

Acknowledgment

The
authors gratefully acknowledge the financial support from Department of Science
and Technology, Govt. of India.

References

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