Results of the performed LCA are shown in **Figure 11**. Here, a comparative LCA was conducted, emphasizing the differences of the environmental impacts between the aerobic and the anaerobic process. The absolute eco-indicator point values were not taken as basis for evaluation in this study. On the ordinate in **Figure 11**, the results are plotted as eco-indicator points, which are dimensionless (see above). The higher the number of eco-indicator points, the stronger is the effect to the environment, human health, or resource consumption. On the abscissa, only those impact categories are shown which are characterized by eco-indicator points larger than 1. The last pair of bars (total/total) shows the environmental effects of all impact categories in total [54].

Most eco-indicator points are gained from resources, subgroup fossil fuels. Fossil fuels are needed for the production of the raw materials like sugar and for the production of the used electricity, which is a mix from German electricity production. Only for the biogas process, electricity from the wind and sun was used. The environmental impacts of human health and ecosystem quality result from the production of the agricultural raw materials which are necessary for the processing of the culture media.

**63**

checked.

**5. Conclusions**

*Integrating Whole Cell Biotransformation of Aroma Compounds into a Novel Biorefinery Concept*

Both, the aerobic and the anaerobic whole cell biotransformation of E3HB, produce CO2 as output. This CO2 is renewable; its origin is the carbon source (glucose). However, within the anaerobic production of E3HB, the biogas process leads to a saving of environmental impacts, since CO2 is converted to bio-methane, which replaces natural gas of fossil origin. In the LCA this leads to eco-indicator point credits lowering the environmental impact of the anaerobic process. In order to model the replacement of fossil resources, a separate process was used in which the same amount of natural gas was produced which could be replaced by bio-methane. The natural gas was then converted into CO2 in a simplified combustion step. The results are the negative values shown in **Figure 11**. As can be seen from **Figure 11**, this effect is small (about 2%) as compared to the total eco-indicator points (total, total) for the whole process.

All indicated impact indicators show that the anaerobic process has lower ecoindicator points than the aerobic process. For total, total, the difference is about 17%. This is within the range of the allocation of the additional products of the

The application of the eco-indicator 99 (E,E) method within a LCIA provides the basis for the comparison of the two biorefinery concepts. A credit for the substitution of natural gas from fossil resources by bio-methane was accounted for in the evaluation of the anaerobic process. The comparative LCA shows that the anaerobic production of E3HB has about 17% less of the environmental impact than the aerobic process. The exhaust gas from the anaerobic process, which mainly consists of CO2, is processed in a biogas process to form bio-methane substituting natural gas. In the aerobic process, the formed CO2 is directly released into the atmosphere. The main contribution to the advantage of the anaerobic process in comparison to the aerobic biotransformation results from the by-product formation. Due to the formation of ethanol and bio-methane in the anaerobic process, the total eco-indicator points were reduced by 15%. Only further 2% eco-indicator point reduction resulted from

All material and energy flows, except for the biogas process, the downstream process, and the natural gas combustion (for crediting the bio-methane production), were fully recorded in previous laboratory tests. Therefore, the data used were considered to be reliable. With regard to the biogas process, the downstream process, and the credit note (natural gas combustion), the data for procedures, implemented in the LCA software package, should be experimentally cross

Various methods have been used for the sensitivity analysis of the aerobic and anaerobic LCA models by the authors. Different weightings were made in the economic allocation, and different allocation methods were applied. Various data sets from ecoinvent were used and compared for the production of E3HB. In the case of the credit, bio-methane was also burned instead of natural gas in the separate process. However, the analysis did only show slight fluctuations in LCIA results.

The presented data from the fed-batch biotransformation processes show that anaerobic production of the aroma compound E3HB from EAA is possible. The production efficiency of the anaerobic process is similar as compared to the aerobic production. This makes it possible to integrate both, the aerobic and the anaerobic

anaerobic process which is 15% less as compared to the aerobic process.

*DOI: http://dx.doi.org/10.5772/intechopen.88158*

**4.4 Interpretation of the LCA**

the minimized CO2 release of the anaerobic process.

processes, into two process-specific biorefinery concepts.

*Integrating Whole Cell Biotransformation of Aroma Compounds into a Novel Biorefinery Concept DOI: http://dx.doi.org/10.5772/intechopen.88158*

Both, the aerobic and the anaerobic whole cell biotransformation of E3HB, produce CO2 as output. This CO2 is renewable; its origin is the carbon source (glucose). However, within the anaerobic production of E3HB, the biogas process leads to a saving of environmental impacts, since CO2 is converted to bio-methane, which replaces natural gas of fossil origin. In the LCA this leads to eco-indicator point credits lowering the environmental impact of the anaerobic process. In order to model the replacement of fossil resources, a separate process was used in which the same amount of natural gas was produced which could be replaced by bio-methane. The natural gas was then converted into CO2 in a simplified combustion step. The results are the negative values shown in **Figure 11**. As can be seen from **Figure 11**, this effect is small (about 2%) as compared to the total eco-indicator points (total, total) for the whole process.

All indicated impact indicators show that the anaerobic process has lower ecoindicator points than the aerobic process. For total, total, the difference is about 17%. This is within the range of the allocation of the additional products of the anaerobic process which is 15% less as compared to the aerobic process.

#### **4.4 Interpretation of the LCA**

*Biorefinery Concepts, Energy and Products*

*Impact categories within eco-indicator 99.*

integrated in the *Umberto NXT LCA* software [54, 57–59]. The individual damage categories have different physical units. For example, climate change is calculated in kg CO2 equivalents and acidification in SO2 equivalents. In order to enable a comparison between the aforementioned categories, the respective balance results are converted into a comparable, aggregated, numerical, and unitless impact indicator (eco-indicator points) for each environmental area. This approach allows to com-

*Comparative life cycle assessment for aerobic and anaerobic production of Ethyl 3-hydroxybutyrate with*

Results of the performed LCA are shown in **Figure 11**. Here, a comparative LCA was conducted, emphasizing the differences of the environmental impacts between the aerobic and the anaerobic process. The absolute eco-indicator point values were not taken as basis for evaluation in this study. On the ordinate in **Figure 11**, the results are plotted as eco-indicator points, which are dimensionless (see above). The higher the number of eco-indicator points, the stronger is the effect to the environment, human health, or resource consumption. On the abscissa, only those impact categories are shown which are characterized by eco-indicator points larger than 1. The last pair of bars (total/total) shows the environmental effects of all impact

Most eco-indicator points are gained from resources, subgroup fossil fuels. Fossil fuels are needed for the production of the raw materials like sugar and for the production of the used electricity, which is a mix from German electricity production. Only for the biogas process, electricity from the wind and sun was used. The environmental impacts of human health and ecosystem quality result from the production of the agricultural

raw materials which are necessary for the processing of the culture media.

pare the different categories directly [59].

categories in total [54].

**Figure 11.**

**Table 3.**

*eco-indicator 99.*

**62**

The application of the eco-indicator 99 (E,E) method within a LCIA provides the basis for the comparison of the two biorefinery concepts. A credit for the substitution of natural gas from fossil resources by bio-methane was accounted for in the evaluation of the anaerobic process. The comparative LCA shows that the anaerobic production of E3HB has about 17% less of the environmental impact than the aerobic process.

The exhaust gas from the anaerobic process, which mainly consists of CO2, is processed in a biogas process to form bio-methane substituting natural gas. In the aerobic process, the formed CO2 is directly released into the atmosphere. The main contribution to the advantage of the anaerobic process in comparison to the aerobic biotransformation results from the by-product formation. Due to the formation of ethanol and bio-methane in the anaerobic process, the total eco-indicator points were reduced by 15%. Only further 2% eco-indicator point reduction resulted from the minimized CO2 release of the anaerobic process.

All material and energy flows, except for the biogas process, the downstream process, and the natural gas combustion (for crediting the bio-methane production), were fully recorded in previous laboratory tests. Therefore, the data used were considered to be reliable. With regard to the biogas process, the downstream process, and the credit note (natural gas combustion), the data for procedures, implemented in the LCA software package, should be experimentally cross checked.

Various methods have been used for the sensitivity analysis of the aerobic and anaerobic LCA models by the authors. Different weightings were made in the economic allocation, and different allocation methods were applied. Various data sets from ecoinvent were used and compared for the production of E3HB. In the case of the credit, bio-methane was also burned instead of natural gas in the separate process. However, the analysis did only show slight fluctuations in LCIA results.

#### **5. Conclusions**

The presented data from the fed-batch biotransformation processes show that anaerobic production of the aroma compound E3HB from EAA is possible. The production efficiency of the anaerobic process is similar as compared to the aerobic production. This makes it possible to integrate both, the aerobic and the anaerobic processes, into two process-specific biorefinery concepts.

A LCA showed that the integration of the anaerobic process into a biorefinery results in a production process with lower environmental impact than a biorefinery with the aerobic process. In both alternatives environmentally friendly ambient conditions are applied. In particular, the anaerobic process produces ethanol as an additional byproduct, replaces natural gas by the production of bio-methane, and does not produce CO2 as a waste product. The knowledge gained during this work can probably be transferred to similar bioreductions, which can be carried out with the yeast *S. cerevisiae*.

Taken together the goal of integrating the whole cell biotransformation of the aroma compound E3HB into an anaerobic biorefinery process was achieved. The present study shows that anaerobic biotransformation can be used as a production process for E3HB, resulting in less environmental impact compared to the aerobic process as part of a biorefinery.

#### **Acknowledgements**

We kindly acknowledge the financial support of this work by the German Federal Ministry of Education and Research (BioIndustrie2021—BioKatalyse2021— ProTool2—FKZ 031A124B), Furtwangen University and University College London.

#### **Author details**

Roland Hirschmann1 , Waldemar Reule1 , Thomas Oppenländer1 , Frank Baganz2 and Volker C. Hass1,2\*

1 Hochschule Furtwangen University, Furtwangen, Germany

2 University College London, London, UK

\*Address all correspondence to<EMAIL_ADDRESS>
© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**65**

*Integrating Whole Cell Biotransformation of Aroma Compounds into a Novel Biorefinery Concept*

1987

pp. 382-389

10.1021/ja00253a051

anie.201205537

[9] Seebach D, Roggo S,

Zimmermann J. Biological-chemical preparation of 3-hydroxycarboxylic acids and their use in EPC-syntheses. In: Stereochemistry of Organic and Bioorganic Transformations: Proceedings of the Seventeenth Workshop Conference Hoechst, Schloss Reisensburg; 8-11 October 1986; Weinheim: Federal Republic of Germany. New York, NY, USA: VCH;

[10] Shang G, Li W, Zhang J. Transition

asymmetric hydrogenation. In: Ojima I, editor. Catalytic Asymmetric Synthesis. 3rd ed. Hoboken, NJ: John Wiley; 2010.

[11] Noyori R, Ohkuma T, Kitamura M, Takaya H, Sayo N, Kumobayashi H, et al. Asymmetric hydrogenation of beta-keto carboxylic esters. A practical, purely chemical access to beta-hydroxy esters in high enantiomeric purity. Journal of the American Chemical Society. 1987;**109**:5856-5858. DOI:

[12] Noyori R. Facts are the enemy of truth-reflections on serendipitous discovery and unforeseen developments in asymmetric catalysis. Angewandte Chemie. 2013;**52**:79-92. DOI: 10.1002/

[13] Juelin S, Ratovelomanana-Vidal V, Genet J-P. Synphos and difluorphos as ligands for ruthenium-catalyzed hydrogenation of alkenes and ketones. In: Roberts SM, Kozhevnikov IV, Derouane EG, editors. Regio- and Stereo- Controlled Oxidations and Reductions. Chichester, England: John Wiley & Sons Ltd; 2007. pp. 87-159. DOI: 10.1002/9780470090244.ch3

[14] Kitamura M, Ohkuma T, Inoue S, Sayo N, Kumobayashi H, Akutagawa S, et al. Homogeneous asymmetric

metal-catalyzed homogenous

*DOI: http://dx.doi.org/10.5772/intechopen.88158*

[1] Carlquist M, Gibson B, Karagul Yuceer Y, Paraskevopoulou A, Sandell M, Angelov AI, et al. Process engineering for bioflavour production with metabolically active yeasts—A mini-review. Yeast. 2015;**32**:123-143.

DOI: 10.1002/yea.3058

**References**

s12934-017-0724-7

Springer; 1997. pp. 1-49

2016]

jf020297f

[2] Lin B, Tao Y. Whole-cell

biocatalysts by design. Microbial Cell Factories. 2017;**16**:1871. DOI: 10.1186/

[3] Cheetham PSJ. Combining the technical push and the business pull for natural flavours. In: Biotechnology of Aroma Compounds. Berlin, New York:

[4] The International Fragrance Association. Ingredients—IFRA

International Fragrance Association—in Every Sense [Internet]. 2016. Available from: http://www.ifraorg.org/en-us/ ingredients [Accessed: 16 September

[5] Jordán MJ, Margaría CA, Shaw PE, Goodner KL. Aroma active components in aqueous kiwi fruit essence and kiwi fruit puree by GC-MS and multidimensional GC/GC-O. Journal of Agricultural and Food Chemistry. 2002;**50**:5386-5390. DOI: 10.1021/

[6] Burdock GA, Fenaroli G. Fenaroli's Handbook of Flavor Ingredients. 6th ed.

Boca Raton: CRC Press; 2010

[7] Seebach D, Züger M. Über die depolymerisierung von poly-(R)- 3-hydroxy-buttersäureester (PHB). Helvetica Chimica Acta. 1982;**65**:495- 503. DOI: 10.1002/hlca.19820650208

[8] Züger M. Mikrobiologische Herstellung von optisch aktiven ß-hydroxy-Carbonsäureestern im präparativen Massstab [dissertation

7514]. Zürich: ETH; 1984

*Integrating Whole Cell Biotransformation of Aroma Compounds into a Novel Biorefinery Concept DOI: http://dx.doi.org/10.5772/intechopen.88158*

#### **References**

*Biorefinery Concepts, Energy and Products*

process as part of a biorefinery.

**Acknowledgements**

A LCA showed that the integration of the anaerobic process into a biorefinery results in a production process with lower environmental impact than a biorefinery with the aerobic process. In both alternatives environmentally friendly ambient conditions are applied. In particular, the anaerobic process produces ethanol as an additional byproduct, replaces natural gas by the production of bio-methane, and does not produce CO2 as a waste product. The knowledge gained during this work can probably be transferred to similar bioreductions, which can be carried out with the yeast *S. cerevisiae*. Taken together the goal of integrating the whole cell biotransformation of the aroma compound E3HB into an anaerobic biorefinery process was achieved. The present study shows that anaerobic biotransformation can be used as a production process for E3HB, resulting in less environmental impact compared to the aerobic

We kindly acknowledge the financial support of this work by the German Federal Ministry of Education and Research (BioIndustrie2021—BioKatalyse2021— ProTool2—FKZ 031A124B), Furtwangen University and University College London.

**64**

**Author details**

Roland Hirschmann1

and Volker C. Hass1,2\*

, Waldemar Reule1

1 Hochschule Furtwangen University, Furtwangen, Germany

\*Address all correspondence to<EMAIL_ADDRESS>
© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

2 University College London, London, UK

provided the original work is properly cited.

, Thomas Oppenländer1

, Frank Baganz2

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[2] Lin B, Tao Y. Whole-cell biocatalysts by design. Microbial Cell Factories. 2017;**16**:1871. DOI: 10.1186/ s12934-017-0724-7

[3] Cheetham PSJ. Combining the technical push and the business pull for natural flavours. In: Biotechnology of Aroma Compounds. Berlin, New York: Springer; 1997. pp. 1-49

[4] The International Fragrance Association. Ingredients—IFRA International Fragrance Association—in Every Sense [Internet]. 2016. Available from: http://www.ifraorg.org/en-us/ ingredients [Accessed: 16 September 2016]

[5] Jordán MJ, Margaría CA, Shaw PE, Goodner KL. Aroma active components in aqueous kiwi fruit essence and kiwi fruit puree by GC-MS and multidimensional GC/GC-O. Journal of Agricultural and Food Chemistry. 2002;**50**:5386-5390. DOI: 10.1021/ jf020297f

[6] Burdock GA, Fenaroli G. Fenaroli's Handbook of Flavor Ingredients. 6th ed. Boca Raton: CRC Press; 2010

[7] Seebach D, Züger M. Über die depolymerisierung von poly-(R)- 3-hydroxy-buttersäureester (PHB). Helvetica Chimica Acta. 1982;**65**:495- 503. DOI: 10.1002/hlca.19820650208

[8] Züger M. Mikrobiologische Herstellung von optisch aktiven ß-hydroxy-Carbonsäureestern im präparativen Massstab [dissertation 7514]. Zürich: ETH; 1984

[9] Seebach D, Roggo S,

Zimmermann J. Biological-chemical preparation of 3-hydroxycarboxylic acids and their use in EPC-syntheses. In: Stereochemistry of Organic and Bioorganic Transformations: Proceedings of the Seventeenth Workshop Conference Hoechst, Schloss Reisensburg; 8-11 October 1986; Weinheim: Federal Republic of Germany. New York, NY, USA: VCH; 1987

[10] Shang G, Li W, Zhang J. Transition metal-catalyzed homogenous asymmetric hydrogenation. In: Ojima I, editor. Catalytic Asymmetric Synthesis. 3rd ed. Hoboken, NJ: John Wiley; 2010. pp. 382-389

[11] Noyori R, Ohkuma T, Kitamura M, Takaya H, Sayo N, Kumobayashi H, et al. Asymmetric hydrogenation of beta-keto carboxylic esters. A practical, purely chemical access to beta-hydroxy esters in high enantiomeric purity. Journal of the American Chemical Society. 1987;**109**:5856-5858. DOI: 10.1021/ja00253a051

[12] Noyori R. Facts are the enemy of truth-reflections on serendipitous discovery and unforeseen developments in asymmetric catalysis. Angewandte Chemie. 2013;**52**:79-92. DOI: 10.1002/ anie.201205537

[13] Juelin S, Ratovelomanana-Vidal V, Genet J-P. Synphos and difluorphos as ligands for ruthenium-catalyzed hydrogenation of alkenes and ketones. In: Roberts SM, Kozhevnikov IV, Derouane EG, editors. Regio- and Stereo- Controlled Oxidations and Reductions. Chichester, England: John Wiley & Sons Ltd; 2007. pp. 87-159. DOI: 10.1002/9780470090244.ch3

[14] Kitamura M, Ohkuma T, Inoue S, Sayo N, Kumobayashi H, Akutagawa S, et al. Homogeneous asymmetric

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[15] Faber K, Patel R. Chemical biotechnology: A happy marriage between chemistry and biotechnology: Asymmetric synthesis via green chemistry. Current Opinion in Biotechnology. 2000;**11**:517-519. DOI: 10.1016/S0958-1669(00)00157-9

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[17] Gröger H, Borchert S, Kraußer M, Hummel W. Enzyme-catalyzed asymmetric reduction of ketones. In: Flickinger M, editor. Encyclopedia of Industrial Biotechnology, Bioprocess, Bioseparation, and Cell Technology. Hoboken, NJ: Wiley; 2010

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hydrogenation of functionalized ketones. Journal of the American Chemical Society. 1988;**110**:629-631. of the American Chemical Society. 2004;**126**:12827-12832. DOI: 10.1021/

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[23] Friedmann E. Acetessigsäure und Hefe. Biochemische Zeitschrift.

of the ustilaginales: IV. The

DOI: 10.1071/CH9762459

[24] Lemieux RU, Giguere J. Biochemistry

configurations of some β-hydroxyacids and the bioreductions of β-ketoacids. Canadian Journal of Chemistry.

[25] Deol BS, Ridley DD, Simpson GW. Asymmetric reduction of carbonyl compounds by yeast. II. Preparation of optically active α- and β-hydroxy carboxylic acid derivatives. Australian Journal of Chemistry. 1976;**29**:2459.

[26] Seuring B, Seebach D. Synthese von vier chiralen, elektrophilen C 3- und C 4-synthesebausteinen aus hydroxycarbonsäuren. Helvetica Chimica Acta. 1977;**60**:1175-1181

[27] Simon H, Bader J, Günther H, Neumann S, Thanos J. Chiral

(51)]. Angewandte Chemie International Edition in English. 1985;**24**:539-553. DOI: 10.1002/

[28] Leuenberger HGW. Biotransformations applied to the synthesis of vitamins and

anie.198505391

compounds synthesized by biocatalytic reductions [new synthetic methods

pharmaceuticals. In: Microbial Reagents in Organic Synthesis. Proceedings of the NATO Advanced Research Workshop on Microbial Reagents in

1931;**243**:125-144

1951;**29**:678-690

ja0469479

DOI: 10.1021/ja00210a070

[15] Faber K, Patel R. Chemical biotechnology: A happy marriage between chemistry and biotechnology: Asymmetric synthesis via green chemistry. Current Opinion in Biotechnology. 2000;**11**:517-519. DOI: 10.1016/S0958-1669(00)00157-9

[16] Strauss UT, Felfer U, Faber K. Biocatalytic transformation of racemates into chiral building blocks in 100% chemical yield and 100% enantiomeric excess. Tetrahedron: Asymmetry. 1999;**10**:107-117. DOI: 10.1016/S0957-4166(98)00490-X

[17] Gröger H, Borchert S, Kraußer M, Hummel W. Enzyme-catalyzed asymmetric reduction of ketones. In: Flickinger M, editor. Encyclopedia of Industrial Biotechnology, Bioprocess, Bioseparation, and Cell Technology.

Hoboken, NJ: Wiley; 2010

[19] Huisman GW, Liang J,

DOI: 10.1021/ar700167a

Krebber A. Practical chiral alcohol manufacture using ketoreductases. Biocatalysis and Biotransformation Bioinorganic Chemistry. 2010;**14**:122-129. DOI: 10.1016/j.cbpa.2009.12.003

[20] Moore JC, Pollard DJ, Kosjek B, Devine PN. Advances in the enzymatic reduction of ketones. Accounts of Chemical Research. 2007;**40**:1412-1419.

[21] Kaluzna IA, Matsuda T, Sewell AK, Stewart JD. Systematic investigation of *Saccharomyces cerevisiae* enzymes catalyzing carbonyl reductions. Journal

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[29] Sybesma WFH, StraathofA JJ, Jongejan JA, Pronk JT, Heijnen JJ. Reductions of 3-oxo esters by Baker's yeast: Current status. Biocatalysis and Biotransformation. 1998;**16**:95-134

[30] Perles CE, Moran PJS, Volpe PLO. Bioreduction of ethyl 3-oxobutyrate by Saccharomyces cerevisiae: A metabolic in vivo study. In: Selected Papers from the Eighth International Symposium on Biocatalysis and Biotransformations. Vol. 52-53. 2008. pp. 82-87. DOI: 10.1016/j.molcatb.2007.12.002

[31] Kometani T, Kitatsuji E, Matsuno R. Bioreduction of ketones mediated by Baker's yeast with acetate as ultimate reducing agent. Agricultural and Biological Chemistry. 1991;**55**(3):867-868. DOI: 10.1080/00021369.1991.10870651

[32] Wipf B, Kupfer E, Beratazzi R, Leuenberger HGW. Production of (+)-(S)-ethyl 3-hydroxybutyrate and (−)-(R)-ethyl 3-hydroxybutyrate by microbial reduction of ethyl acetoacetate. Helvetica Chimica Acta. 1983;**66**:485-488

[33] Borup B, Daussmann T, Collier S, Flickinger MC. Chiral alcohols by enzymatic preparation. In: Flickinger MC, editor. Encyclopedia of Industrial Biotechnology. New Jersey, USA: John Wiley & Sons, Inc; 2009. DOI: 10.1002/9780470054581.eib215

[34] Zeror S, Collin J, Fiaud J-C, Zouioueche LA. Enantioselective ketoester reductions in water: A comparison between microorganismand ruthenium-catalyzed reactions. Henri Kagan: An 80th birthday Celebration special issue—Part 1. Tetrahedron: Asymmetry. 2010;**21**:1211-1215. DOI: 10.1016/j. tetasy.2010.05.014

[35] IEA. Task 42: Biorefinery [Internet]. 2013. Available from: http://www. iea-bioenergy.task42-biorefineries.com/ upload\_mm/a/9/a/801acd78-e830- 42ba-8ad3-019d4d3f927f\_Poster%20 Hans%20Langeveld%20LR%20 nieuw%20.pdf [Accessed: 09 July 2015]

[36] Kamm B, Gruber PR, editors. Biorefineries—Industrial Processes and Products: Status Quo and Future Directions. Weinheim: Wiley-VCH; 2010

[37] Waldron K, editor. Advances in Biorefineries. Cambridge, UK: Woodhead Publishing; 2014

[38] Hong K-K, Nielsen J. Metabolic engineering of *Saccharomyces cerevisiae*: A key cell factory platform for future biorefineries. Cellular and Molecular Life Sciences. 2012;**69**:2671-2690. DOI: 10.1007/s00018-012-0945-1

[39] Hass VC, Oppenländer T, Hirschmann R, Reule W. Bioraffinerie-Verfahren, DE, Patent DE 10 2015 002 830 B4; Hochschule Furtwangen University. 2019

[40] Reule W. Bioverfahren Und Anlage Zur Erzeugung von Methan, DE, Patent, DE 10 2017 000 576 A. 2018

[41] Hirschmann R, Borodkin N, Baganz F, Hass VC. Towards the integration of the ethyl (S)-3 hydroxybutyrate production process into a biorefinery concept. Chemical Engineering Transactions. 2018;**70**:559-564. DOI: 10.3303/ CET1870094

[42] Csuk R, Glaenzer BI. Baker's yeast mediated transformations in organic chemistry. Chemical Reviews. 1991;**91**:49-97

[43] Dahl AC, Madsen JØ. Baker's yeast: Production of d- and l-3-hydroxy esters. Tetrahedron: Asymmetry. 1998;**9**:4395-4417. DOI: 10.1016/ S0957-4166(98)00471-6

[44] Seebach D, Sutter MA, Weber RH, Züger MF. Yeast reduction of ethyl acetoacetate: (S)-(+)-ethyl 3-hydroxybutanoate. In: Hassner A, Stumer C, editors. Organic Syntheses Based on Name Reactions. 2nd ed. Burlington: Elsevier; 2002. p. 1. DOI: 10.1002/0471264180.os063.01

[45] Engelking H. Reaktionstechnische Untersuchungen zur Asymmetrischen Reduktion von β-Ketoestern mit Rekombinanten Hefen [Internet]. 2004. Available from: https://mediatum. ub.tum.de/doc/601895/601895.pdf [Accessed: 07 July 2015]

[46] Matsuda T, Yamanaka R, Nakamura K. Recent progress in biocatalysis for asymmetric oxidation and reduction. Tetrahedron: Asymmetry. 2009;**20**:513-557. DOI: 10.1016/j.tetasy.2008.12.035

[47] Ebert C, Boccu E, Gardossi L, Linda P, Gianferrara T. Chemometric optimization of an asymmetric reduction catalyzed by Baker's yeast. Biotechnology and Bioengineering. 1990;**35**:928-934

[48] Giovannini F. Chirale synthesebausteine durch enantioselektive reduktion mit mesophilen und thermophilen mikriorganismen [dissertation 8004]. Zürich: ETH; 1986

[49] Katz M, Frejd T, Hahn-Hägerdal B, Gorwa-Grauslund MF. Efficient anaerobic whole cell stereoselective bioreduction with recombinant saccharomyces cerevisiae. Biotechnology and Bioengineering. 2003;**84**:573-582. DOI: 10.1002/ bit.10824

[50] Dahl AC, Fjeldberg M, Madsen JØ. Baker's yeast: Improving the d-stereoselectivity in reduction of 3-oxo esters. Tetrahedron: Asymmetry. 1999;**10**:551-559. DOI: 10.1016/ S0957-4166(99)00025-7

[51] Chin-Joe I, Nelisse P, Straathof MA, Jaap JJ, Jongejan A, Pronk JT, et al. Hydrolytic activity in Baker's yeast limits the yield of asymmetric 3-oxo ester reduction. Biotechnology and Bioengineering. 2000;**69**:370-376

[52] DIN Deutsches Institut für Normung e.V. Umweltmanagement— Ökobilanz—Grundsätze und Rahmenbedingungen [Internet]. 2012. Available from: http:// perinorm-fr.redi-bw.de/volltexte/ CD21DE04/1555059/1555059.pdf? [Accessed: 20 October 2014]

[53] Klöpffer W, Grahl B. Ökobilanz (LCA): Ein Leitfaden für Ausbildung Und Beruf. 1st ed. S.L: Wiley-VCH; 2012

[54] Schmidt M, Häuslein A. Ökobilanzierung mit Computerunterstützung: Produktbilanzen und Betriebliche Bilanzen mit dem Programm Umberto®. Berlin Heidelberg: Springer; 1997. DOI: 10.1007/978-3-642-80236-2

[55] Frischknecht R, Rebitzer G. The ecoinvent database system: A comprehensive web-based LCA database. Journal of Cleaner Production. 2005;**13**:1337-1343. DOI: 10.1016/j. jclepro.2005.05.002

[56] DIN Deutsches Institut für Normung e.V. Umweltmanagement— Ökobilanz—Anforderungen und Anleitungen (ISO 14044:2006) [Internet]. 2006 [Accessed: 27 July 2017]

[57] Goedkoop M. The Eco-Indicator 99 LCIA Methodology—An Introduction. Zürich: Eidgenössische Technische Hochschule; 2000

[58] Goedkoop M, Demmers M, Collignon M. The Eco Indicator 95: Weighting Method for Environmental Effects that Damage Ecosystems or Human Health on a European Scale Contains 100 Indicators for Important

**69**

*Integrating Whole Cell Biotransformation of Aroma Compounds into a Novel Biorefinery Concept*

*DOI: http://dx.doi.org/10.5772/intechopen.88158*

Materials and Processes Manual for

[59] Goedkoop M, Spriensma R. The Eco-Indicator 99: A Damage Oriented

Designers [Internet]. 1995

Method for Life Cycle Impact Assessment: Methodology Annex [Internet]. 2001. Available from: https:// www.pre-sustainability.com/download/

EI99\_annexe\_v3.pdf

*Integrating Whole Cell Biotransformation of Aroma Compounds into a Novel Biorefinery Concept DOI: http://dx.doi.org/10.5772/intechopen.88158*

Materials and Processes Manual for Designers [Internet]. 1995

*Biorefinery Concepts, Energy and Products*

Weber RH, Züger MF. Yeast reduction of ethyl acetoacetate: (S)-(+)-ethyl 3-hydroxybutanoate. In: Hassner A, Stumer C, editors. Organic Syntheses Based on Name Reactions. 2nd ed. Burlington: Elsevier; 2002. p. 1. DOI:

[51] Chin-Joe I, Nelisse P, Straathof MA, Jaap JJ, Jongejan A, Pronk JT, et al. Hydrolytic activity in Baker's yeast limits the yield of asymmetric 3-oxo ester reduction. Biotechnology and Bioengineering. 2000;**69**:370-376

[52] DIN Deutsches Institut für Normung e.V. Umweltmanagement—

Ökobilanz—Grundsätze und Rahmenbedingungen [Internet]. 2012. Available from: http:// perinorm-fr.redi-bw.de/volltexte/ CD21DE04/1555059/1555059.pdf? [Accessed: 20 October 2014]

[53] Klöpffer W, Grahl B. Ökobilanz (LCA): Ein Leitfaden für Ausbildung Und Beruf. 1st ed. S.L: Wiley-VCH; 2012

Produktbilanzen und Betriebliche Bilanzen mit dem Programm

[55] Frischknecht R, Rebitzer G. The ecoinvent database system: A comprehensive web-based LCA

[56] DIN Deutsches Institut für Normung e.V. Umweltmanagement— Ökobilanz—Anforderungen und Anleitungen (ISO 14044:2006) [Internet]. 2006 [Accessed: 27 July

jclepro.2005.05.002

Hochschule; 2000

2017]

Umberto®. Berlin Heidelberg: Springer; 1997. DOI: 10.1007/978-3-642-80236-2

database. Journal of Cleaner Production. 2005;**13**:1337-1343. DOI: 10.1016/j.

[57] Goedkoop M. The Eco-Indicator 99 LCIA Methodology—An Introduction. Zürich: Eidgenössische Technische

[58] Goedkoop M, Demmers M, Collignon M. The Eco Indicator 95: Weighting Method for Environmental Effects that Damage Ecosystems or Human Health on a European Scale Contains 100 Indicators for Important

[54] Schmidt M, Häuslein A. Ökobilanzierung mit Computerunterstützung:

[45] Engelking H. Reaktionstechnische Untersuchungen zur Asymmetrischen Reduktion von β-Ketoestern mit

Rekombinanten Hefen [Internet]. 2004. Available from: https://mediatum. ub.tum.de/doc/601895/601895.pdf

[44] Seebach D, Sutter MA,

10.1002/0471264180.os063.01

[Accessed: 07 July 2015]

[46] Matsuda T, Yamanaka R, Nakamura K. Recent progress in biocatalysis for asymmetric oxidation

and reduction. Tetrahedron: Asymmetry. 2009;**20**:513-557. DOI:

10.1016/j.tetasy.2008.12.035

[48] Giovannini F. Chirale synthesebausteine durch enantioselektive reduktion mit mesophilen und thermophilen

1990;**35**:928-934

Zürich: ETH; 1986

[47] Ebert C, Boccu E, Gardossi L, Linda P, Gianferrara T. Chemometric optimization of an asymmetric reduction catalyzed by Baker's yeast. Biotechnology and Bioengineering.

mikriorganismen [dissertation 8004].

[49] Katz M, Frejd T, Hahn-Hägerdal B, Gorwa-Grauslund MF. Efficient anaerobic whole cell stereoselective bioreduction with recombinant saccharomyces cerevisiae.

Biotechnology and Bioengineering. 2003;**84**:573-582. DOI: 10.1002/

Madsen JØ. Baker's yeast: Improving the d-stereoselectivity in reduction of 3-oxo esters. Tetrahedron: Asymmetry.

[50] Dahl AC, Fjeldberg M,

1999;**10**:551-559. DOI: 10.1016/ S0957-4166(99)00025-7

**68**

bit.10824

[59] Goedkoop M, Spriensma R. The Eco-Indicator 99: A Damage Oriented Method for Life Cycle Impact Assessment: Methodology Annex [Internet]. 2001. Available from: https:// www.pre-sustainability.com/download/ EI99\_annexe\_v3.pdf

**71**

**Chapter 5**

**Abstract**

**1. Introduction**

emerging markets in the industry.

Concise Review

Lignin Hydrothermal Liquefaction

Lignin, the second largest biomass after cellulose is underutilized. Yet, it remains the only natural source of aromatic, and phenolic compounds. It is imperative to, amidst the expanding interest on biomass conversion, to accord the necessary attention towards lignin degradation into value added chemicals. Specifically, its phenyl, guaiacyl, and syringyl derivatives. Understanding lignin degradation chemistry, goes a long way in its selective valorization into fuels and chemicals via thermochemical routes such as hydrothermal liquefaction (HTL). Therefore, development of technologies targeting value addition of products and by-products from lignin, would undoubtedly give way to emerging markets in the industry. Previous review papers focused on the general HTL of biomass, food waste, algae, and their model compounds. However, review on HTL of lignin is scarcely available. This paper presents the detailed literature analyses of the current trend in lignin degradation via HTL. Effect of HTL conditions including temperature, heating rate and catalyst has been reviewed. In-depth discussion on use of ionic liquids as catalyst for HTL of lignin has also been compiled. Other lignin degradation techniques such as pyrolysis and hydrolysis were also discussed. This is aimed at bringing together an up-to-date

into Bifunctional Chemicals: A

*Yahaya Alhassan, Ursel Hornung and Idris M. Bugaje*

information on lignin degradation into selected chemical intermediates.

Nowadays, it is evident that, processing lignocelluloses biomass, specifically, lignin into different industrial chemicals via the biorefinery approach; has become economically attractive. Biorefinery, is a pinnacle of simple and advanced technologies, put together; in converting biomass such as lignin into sustainable fuels, intermediates and chemical products [1]. Therefore, lignin processing, including hydrothermal liquefaction (HTL) route is expected to ensure only useful products are produced. Pollutants, waste, and losses are limited, in what is referred to as an atom economy model [2]. Lignin, the second largest biomass after cellulose is largely underutilized. It is largely available in woody biomass, agricultural waste, and as an industrial process by-product from paper milling, and pulping industries [3], and waste stream in organosolv process [4]. It is also the only natural source of aromatic and phenolic compounds. Therefore, development of well-defined technologies targeting value addition of products and by-products from lignin, would undoubtedly give way to

**Keywords:** aromatics, catechol, lignin, guaiacol, phenol

#### **Chapter 5**

## Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review

*Yahaya Alhassan, Ursel Hornung and Idris M. Bugaje*

#### **Abstract**

Lignin, the second largest biomass after cellulose is underutilized. Yet, it remains the only natural source of aromatic, and phenolic compounds. It is imperative to, amidst the expanding interest on biomass conversion, to accord the necessary attention towards lignin degradation into value added chemicals. Specifically, its phenyl, guaiacyl, and syringyl derivatives. Understanding lignin degradation chemistry, goes a long way in its selective valorization into fuels and chemicals via thermochemical routes such as hydrothermal liquefaction (HTL). Therefore, development of technologies targeting value addition of products and by-products from lignin, would undoubtedly give way to emerging markets in the industry. Previous review papers focused on the general HTL of biomass, food waste, algae, and their model compounds. However, review on HTL of lignin is scarcely available. This paper presents the detailed literature analyses of the current trend in lignin degradation via HTL. Effect of HTL conditions including temperature, heating rate and catalyst has been reviewed. In-depth discussion on use of ionic liquids as catalyst for HTL of lignin has also been compiled. Other lignin degradation techniques such as pyrolysis and hydrolysis were also discussed. This is aimed at bringing together an up-to-date information on lignin degradation into selected chemical intermediates.

**Keywords:** aromatics, catechol, lignin, guaiacol, phenol

### **1. Introduction**

Nowadays, it is evident that, processing lignocelluloses biomass, specifically, lignin into different industrial chemicals via the biorefinery approach; has become economically attractive. Biorefinery, is a pinnacle of simple and advanced technologies, put together; in converting biomass such as lignin into sustainable fuels, intermediates and chemical products [1]. Therefore, lignin processing, including hydrothermal liquefaction (HTL) route is expected to ensure only useful products are produced. Pollutants, waste, and losses are limited, in what is referred to as an atom economy model [2].

Lignin, the second largest biomass after cellulose is largely underutilized. It is largely available in woody biomass, agricultural waste, and as an industrial process by-product from paper milling, and pulping industries [3], and waste stream in organosolv process [4]. It is also the only natural source of aromatic and phenolic compounds. Therefore, development of well-defined technologies targeting value addition of products and by-products from lignin, would undoubtedly give way to emerging markets in the industry.

Detailed literature review papers focused either on the general HTL processing of biomass and algae [5, 6]; HTL of food waste and model compounds [7]. Other areas covered included; the techno-economic, and life cycle assessment of lignocelluloses biomass via thermo-chemical conversion technologies [1]. A few case studies, such as the environmental profile of algae HTL have been reported [8]. However, review on HTL of specific feedstock other than algae and whole biomass are scarcely available; just as the detailed review on HTL of feedstock into specific value-added products, is equally rare. Except for the detailed review on catalytic valorization of lignin for the production of fuels and chemicals [9], such reviews are rare until recently.

It is therefore imperative to, amid the expanding research into biomass conversion, to accord necessary attention towards lignin degradation into value added chemicals, specifically, its hydroxylphenyl derivatives (H-type), guaiacyl derivatives (G-type), and Syringyl derivatives (S-type). In so doing, economically viable, and environmental friendly technology like HTL needs to be explored. This review paper, is aimed at appraising the literature information available on lignin degradation into valuable chemicals, with emphasis on HTL route.

#### **2. Lignin composition and chemistry**

Lignin is largely made up of polymerized monolignols (G-type, H-type and S-type), which are interconnected by well-defined benzodioxane linkages in a radical recombination coupling reaction [10]. The chemical structure of lignin according to different authors [11, 12]; suggested that it consisted of complex 4-phenylpropanol macromolecular units. **Figure 1** depicts the monomeric units of lignin. Principally; it consists of phenylpropane monomer units of phenyl, guaiacyl and syringyl geometry (**Figure 1**), linked together via ether bonds (α–O–4; β–O–4; 5–O–4) and carbon–carbon bonds (β–1; β–5; β–β; 5–5). The β–O–4 is the most pointing ether bond in lignin, where different reactions take place.

Authors have reported different simple and complex reactions taking place during lignin degradation. Even though, multitude of reactions are taking place, at this point (β–O–4), it was proposed that at supercritical point of water, for example; less energy intense reactions are a preferentially favored. Greater scheme of reactions take place at the bonds, forming different products depending on the relative thermodynamic stability of the bond, determined by the strength of double bond equivalence.

According to Berstis et al. [10], during lignin HTL, the reactivity of particular bond is depending on its reaction enthalpy, bond strength, and geometry. It was observed that, α–bonds are slightly weaker than their corresponding β–bond counterparts, and similar in strength to that of the conventional β–O–4 linkage. According to Beauchet et al. [13], the α- and β-aryl-ether-bond followed by the

**73**

**Table 1.**

*Types of bonds existing in lignin structure.*

*Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review*

*DOI: http://dx.doi.org/10.5772/intechopen.90860*

**Figure 1.** *Different monomeric units of lignin.*

*Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review DOI: http://dx.doi.org/10.5772/intechopen.90860*

**Table 1.** *Types of bonds existing in lignin structure.*

*Biorefinery Concepts, Energy and Products*

Detailed literature review papers focused either on the general HTL processing of biomass and algae [5, 6]; HTL of food waste and model compounds [7]. Other areas covered included; the techno-economic, and life cycle assessment of lignocelluloses biomass via thermo-chemical conversion technologies [1]. A few case studies, such as the environmental profile of algae HTL have been reported [8]. However, review on HTL of specific feedstock other than algae and whole biomass are scarcely available; just as the detailed review on HTL of feedstock into specific value-added products, is equally rare. Except for the detailed review on catalytic valorization of lignin for the

It is therefore imperative to, amid the expanding research into biomass conversion, to accord necessary attention towards lignin degradation into value added chemicals, specifically, its hydroxylphenyl derivatives (H-type), guaiacyl derivatives (G-type), and Syringyl derivatives (S-type). In so doing, economically viable, and environmental friendly technology like HTL needs to be explored. This review paper, is aimed at appraising the literature information available on lignin degrada-

Lignin is largely made up of polymerized monolignols (G-type, H-type and S-type), which are interconnected by well-defined benzodioxane linkages in a radical recombination coupling reaction [10]. The chemical structure of lignin according to different authors [11, 12]; suggested that it consisted of complex 4-phenylpropanol macromolecular units. **Figure 1** depicts the monomeric units of lignin. Principally; it consists of phenylpropane monomer units of phenyl, guaiacyl and syringyl geometry (**Figure 1**), linked together via ether bonds (α–O–4; β–O–4; 5–O–4) and carbon–carbon bonds (β–1; β–5; β–β; 5–5). The β–O–4 is the most

Authors have reported different simple and complex reactions taking place during lignin degradation. Even though, multitude of reactions are taking place, at this point (β–O–4), it was proposed that at supercritical point of water, for example; less energy intense reactions are a preferentially favored. Greater scheme of reactions take place at the bonds, forming different products depending on the relative thermodynamic stability of the bond, determined by the strength of double bond equivalence.

According to Berstis et al. [10], during lignin HTL, the reactivity of particular bond is depending on its reaction enthalpy, bond strength, and geometry. It was observed that, α–bonds are slightly weaker than their corresponding β–bond counterparts, and similar in strength to that of the conventional β–O–4 linkage. According to Beauchet et al. [13], the α- and β-aryl-ether-bond followed by the

production of fuels and chemicals [9], such reviews are rare until recently.

tion into valuable chemicals, with emphasis on HTL route.

pointing ether bond in lignin, where different reactions take place.

**2. Lignin composition and chemistry**

**72**

**Figure 1.**

*Different monomeric units of lignin.*

aryl–aryl bond, are the weakest bonds in the lignin polymers, thus; hydrocracking, and thermal hydro-deoxygenation have frequently been employed for depolymerisation, targeting these weak bonds. **Table 1** presents the predominant bonds in lignin. According to Forchheim et al. [14], reactions including mild alkylation, hydro deoxygenation, reploymerization, and depolymerisation are predominant in lignin HTL. Liguori and Barth [15], reported reactions like hydro-deoxygenated, demethylated and demethoxylated at the ether bond positions. In addition, radical recombination and decomposition chemistry; are dimensional factors in the reaction rate.

#### **2.1 Lignin as a source of chemicals**

Lignin, is the only natural source of phenolic compounds, thus; its liquefaction is increasingly been investigated. Although it was envisaged that the complex nature of lignin makes it valorization a difficult task; recent discoveries in selective bond cleaving of the predominant β–O–4 bond [16], is encouraging further research. The increasing research interest in C3 and C5-ortho reactivity and production of genetically modified lignin, have instigated lignin valorization work. The HTL of lignin into substituted Phenol, and aromatic ethers was conducted by Singh et al. [17]. These authors attributed the presence of Phenol, and aromatic ethers from the selective cleaving of β–O–4 and α–O–4 bonds, forming hydroxyl and alkyl groups, respectively. Similarly, Zhou [18] successfully converted Kraft lignin into value-added chemicals, specifically, guaiacol (2-methoxy Phenol), was formed by the degradation of β–O–4 bond. The cleavages of other functional groups including hydroxyl, aromatic, and carbonyl resulted in increased phenolic –OH presence.

#### **3. Lignin degradation technologies**

Literature has availed different proposed schemes for lignin degradation. For example, According to Kruse and Dahmen [19], during biomass liquefaction, steam explosion occurs within 140–240°C leading to its structural disruption, just as the hydrothermal carbonization begin to manifest at around 200°C. Finally, liquefaction sets-in, between 300 and 350°C, and gasification reaction completes the liquefaction cycle around 450–600°C; depending on the feedstock. This temperature range was found to be perfect for the thermal degradation range of lignin which is 200–600°C [20]. Apart from HTL route, other biomass conversion techniques found suitable for lignin degradation, including pyrolysis [21, 22], hydrolysis [23], and gasification [24] have been reported.

#### **4. Hydrothermal liquefaction technique**

Hydrothermal liquefaction (HTL), is one of the emerging biomass conversion technologies gaining desirable attention. This thermo-chemical processing method, has superior advantages in comparison to other processes. **Figure 2** presents the modified vapor pressure curves for HTL processes.

Water, as the solvent in HTL makes the route environmental friendly; in addition to its bifunctional role as catalyst and solvent for multiple reactions taking place [19]. This is because, water, on approaching its supercritical point (above 375°C) exhibits good solvent ability, resulting from its improved properties like polarity, solubility and transportation properties [6, 7].

In economic terms, HTL reduces the energy consumption required during biomass drying, as it is suitable for conversion of high moisture content feedstock into good quality and stable products [7]. For these reasons, HTL has

**75**

**Figure 3.**

**Figure 2.**

Phenol formaldehyde adhesive.

*A semi-continuous process biomass liquefaction flow chart [34].*

*Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review*

turned out to be a super-specialty method, found to be appropriate for converting different feedstock. These including cyanobacteria [25]; low-lipid and high protein algae [26]; sewage sludge [27]; bio-cakes [28]; and lignocelluloses model systems [29] etc.; into different targeted products such as protein, lipids, heterocyclic and their derivatives (nitrogenous and sulfur) [30]; phenolic and aromatic compounds [17]; fatty acids and light gases [31]; and nitrogenated compounds [32] etc. Yang et al. [33] identified Phenol and benzene derivatives from Corncob, with the two compounds showing improved active sites than the parent lignin. Thus, making the former suitable substituents in the synthesis of

*DOI: http://dx.doi.org/10.5772/intechopen.90860*

*Modified vapor pressure curves for hydrothermal conversion process.*

*Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review DOI: http://dx.doi.org/10.5772/intechopen.90860*

#### **Figure 3.** *A semi-continuous process biomass liquefaction flow chart [34].*

turned out to be a super-specialty method, found to be appropriate for converting different feedstock. These including cyanobacteria [25]; low-lipid and high protein algae [26]; sewage sludge [27]; bio-cakes [28]; and lignocelluloses model systems [29] etc.; into different targeted products such as protein, lipids, heterocyclic and their derivatives (nitrogenous and sulfur) [30]; phenolic and aromatic compounds [17]; fatty acids and light gases [31]; and nitrogenated compounds [32] etc. Yang et al. [33] identified Phenol and benzene derivatives from Corncob, with the two compounds showing improved active sites than the parent lignin. Thus, making the former suitable substituents in the synthesis of Phenol formaldehyde adhesive.

*Biorefinery Concepts, Energy and Products*

**2.1 Lignin as a source of chemicals**

**3. Lignin degradation technologies**

and gasification [24] have been reported.

**4. Hydrothermal liquefaction technique**

modified vapor pressure curves for HTL processes.

polarity, solubility and transportation properties [6, 7].

aryl–aryl bond, are the weakest bonds in the lignin polymers, thus; hydrocracking, and thermal hydro-deoxygenation have frequently been employed for depolymerisation, targeting these weak bonds. **Table 1** presents the predominant bonds in lignin. According to Forchheim et al. [14], reactions including mild alkylation, hydro deoxygenation, reploymerization, and depolymerisation are predominant in lignin HTL. Liguori and Barth [15], reported reactions like hydro-deoxygenated, demethylated and demethoxylated at the ether bond positions. In addition, radical recombination and decomposition chemistry; are dimensional factors in the reaction rate.

Lignin, is the only natural source of phenolic compounds, thus; its liquefaction is increasingly been investigated. Although it was envisaged that the complex nature of lignin makes it valorization a difficult task; recent discoveries in selective bond cleaving of the predominant β–O–4 bond [16], is encouraging further research. The increasing research interest in C3 and C5-ortho reactivity and production of genetically modified lignin, have instigated lignin valorization work. The HTL of lignin into substituted Phenol, and aromatic ethers was conducted by Singh et al. [17]. These authors attributed the presence of Phenol, and aromatic ethers from the selective cleaving of β–O–4 and α–O–4 bonds, forming hydroxyl and alkyl groups, respectively. Similarly, Zhou [18] successfully converted Kraft lignin into value-added chemicals, specifically, guaiacol (2-methoxy Phenol), was formed by the degradation of β–O–4 bond. The cleavages of other functional groups including hydroxyl, aromatic, and carbonyl resulted in increased phenolic –OH presence.

Literature has availed different proposed schemes for lignin degradation. For example, According to Kruse and Dahmen [19], during biomass liquefaction, steam explosion occurs within 140–240°C leading to its structural disruption, just as the hydrothermal carbonization begin to manifest at around 200°C. Finally, liquefaction sets-in, between 300 and 350°C, and gasification reaction completes the liquefaction cycle around 450–600°C; depending on the feedstock. This temperature range was found to be perfect for the thermal degradation range of lignin which is 200–600°C [20]. Apart from HTL route, other biomass conversion techniques found suitable for lignin degradation, including pyrolysis [21, 22], hydrolysis [23],

Hydrothermal liquefaction (HTL), is one of the emerging biomass conversion technologies gaining desirable attention. This thermo-chemical processing method, has superior advantages in comparison to other processes. **Figure 2** presents the

Water, as the solvent in HTL makes the route environmental friendly; in addition to its bifunctional role as catalyst and solvent for multiple reactions taking place [19]. This is because, water, on approaching its supercritical point (above 375°C) exhibits good solvent ability, resulting from its improved properties like

In economic terms, HTL reduces the energy consumption required during biomass drying, as it is suitable for conversion of high moisture content feedstock into good quality and stable products [7]. For these reasons, HTL has

**74**

**77**

**Lignin source** Kraft lignin

Kraft lignin, heterogeneous catalyst (K2CO3), phenol and deionized water, is uninterruptedly pumped by high pressure diaphragm pump at a flow rate of 1 kg/h. This was a continuous reactor set-up. The reaction temperature was 350°C and 25 MPa.

Organosolv lignin Wheat straw biomass

Kraft lignin Kraft lignin

HTL tests were completed in batch autoclave. The reactor was heated with an electric heating jacket. Temperature and pressure were checked online. A 700 mL chilled condenser was connected to the autoclave exit port. Lignin, catalyst, and deionized water with and without catalysts.

Sequential biomass pretreatment and followed by room temperature extraction with an activated resin. The biorefinery approach also included supercritical CO2 extraction of phenolic compounds. The treated Kraft lignin, ionic liquid catalyst and, and Pd/C catalyst were placed in a 75 mL autoclave reactor. The reactor was sealed and purged with H2. The reaction was conducted at 200°C at a stirring speed of 800 rpm.

Kraft lignin and deionized water were reassigned into the reactor. The system

was vented with nitrogen three times. For each experiment the reactor was

heated in 30 min from ambient to a working temperature of 130°C or 180°C or

230°C, after which the experiment was continued for 15 or 60 min.

**Table 2.** *Reviewed HTL of lignin degradation into different bio-chemicals.*

Maximum liquids products yield was 50 wt%, while based on Kraft lignin was 13 wt% yield.

Phenolic oil content ranges from 5.4 to 10.6

Guaiacol

[19]

wt%, with 78 wt% guaiacol.

**Liquefaction preview**

**Yield** With respect to dry lignin fed into the reactor, approximately 70 wt% of lignin oil was obtained.

Approximately 25 wt% of monomeric phenol.

Not reported.

**Reported biomolecules**

Phenol, alkylphhenols, phenolic dimers, catechol and guaiacol.

[11]

**Ref.**

*Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review*

*DOI: http://dx.doi.org/10.5772/intechopen.90860*

[4]

A complex mixtures of monomeric catechol and methoxylated and alkylated phenol.

High molecular weight phenolic compounds, targeting Tricin.

Phenol, catechol and guaiacol.

[24]

[39]

#### **Table 2.**

*Reviewed HTL of lignin degradation into different bio-chemicals.*

#### *Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review DOI: http://dx.doi.org/10.5772/intechopen.90860*

*Biorefinery Concepts, Energy and Products*

[15]

**76**

**Lignin**

**Liquefaction preview**

**Yield** 10–12% monomeric aromatic compounds

Phenolic oil was obtained

[35]

consisting monomeric phenol

and oligomeric aromatic

compounds.

Liquid products yield (85%) was reported.

Substituted phenol and

aromatic ethers.

Major products included

[36]

phenol, guaiacol and syringol.

Isolated products include

[37]

benzenediols, monophenolic

hydroxyl products, weak-polar

products, and water-soluble

products (low-molecularweight organic acids, alcohols,

etc.).

Maximum yield obtained was 41–57 wt%.

Not reported.

[38]

**Reported biomolecules**

**Ref.**

**source**

Organosolv

The liquefaction involved the preheating of lignin and formic acid in a

supercritical fluid consisted of CO2/acetone/water in molar ratios of 2.7/1/1.

The reaction was completed within 3.5 h at 300°C and 10 bar pressure.

The reactor was loaded with lignin and ethanol/methanol (1:10 by weight)

hardwood

and wheat

straw lignin

Agricultural

fibrous

within 30 min and 200°C.

feedstocks

Sugarcane

The biomass loading was fixed at a solid:liquid ratio of 1:10 (w/w) including

HTL was conducted in a 250 mL autoclave with heating power of 1.5 kW. An

alkaline lignin (8.0 g) and 100 mL of water are added and reactor was purged

with nitrogen and agitated using a stirrer (200 rpm). The temperature was

varied for 30 min.

Grass silage

Lignin is extracted from the solid lignin residue from the mechanically

supported enzymatic hydrolysis and subjected to ethanolic organosolv

processing, at a reaction temperature and time of 195°C and 80 min,

A mixture of lignin and deionized water was added and reactions were carried

Methoxyphenol (12 wt%), catechols

Phenol, catechols and methoxy

[21]

phenol.

(8 wt%) and phenol (15 wt%).

Varying yields ranging from 3 to 15 mg/g

Phenol, catechol and guaiacol.

[13]

lignin.

out at different temperatures (593–653 K). The residence time varied between

pressed

cake

respectively.

Lignin-rich

residue from

ethanol

15 and 480 min.

production

Enzymatic

The lignin, water and RANEY Nickel (dry matter) were placed in a 5mL

autoclave. The autoclave was purged with nitrogen. The reaction mixture was

heated at different temperatures for different reaction times.

hydrolysis

lignin

Not reported. Not reported.

bagasse

Alkaline

lignin

NaOH.

The use of co-solvent system in biomass liquefaction and by extension, lignin degradation has been investigated. A co-solvent hydrothermal degradation of alkali lignin into bi-phenolic compounds was investigated by Cheng et al. [12], suggesting an improved yield from the water-ethanol co-solvent system for sub/ supercritical conditions. Whereas, time had negligible effect on the yield and quality of liquefaction products, temperature and co-solvent ratio had significant effects on yield and quality of bio-crude oil. Singh et al. [17], employed ethanol and methanol (1:10) co-solvent system for the synthesis of substituted Phenol and aromatic ethers. **Figure 3** presents a semi-continuous process biomass liquefaction flow chart.

Lignin HTL is therefore suitable technology for effectively converting it into useful products with high selectivity, and quality. **Table 2** reviewed the literature works on the HTL of lignin into reported products, highlighting the recounted yields and the major bifunctional chemicals obtained.

#### **5. Factors affecting lignin hydrothermal liquefaction**

#### **5.1 Effect of temperature/heating rate**

The reaction temperature is the predominant factor in thermo-chemical processes like liquefaction. Lignin HTL is a temperature dependent process, since the degree of liquefaction increases with increasing reaction temperature. A careful selection of the heating rate is desired in reducing the rate, at which, condensation of unsaturated oligomeric phenol occur [38]. Depending on the temperature, HTL is sub-divided into subcritical liquefaction and supercritical liquefaction. **Figure 4** presents the effect of reaction temperature on yield of different feedstock reported in the literature.

From the reported result, degradation temperature for lignin feedstock is between 200 and 300°C. Higher temperature beyond 300°C tends to reduce biocrude oil yield. According to Hu et al. [40], lignin degradation is temperature dependent, in a manner that, increasing temperature in degradation of black liquor lignin was found to have favored bond cleavage, and elimination of functional groups, and carbonization. To add to this, Yang et al. [33] found that, temperature affected products distribution much more than reaction time.

#### **5.2 Effect of catalyst type/concentration**

Different classes of catalysts have already shown positive impact during lignin degradation. From **Table 3**, it could be argued that homogenous catalyst like NaOH, were found suitable [37, 41]. Also, mixed oxide catalysts [12] are increasingly been investigated. However, recent literature trend, suggested a paradigm shift towards ionic liquids.

Ionic liquids [ILs] application in biomass refining has received increasing attention, recently. ILs catalysis in lignin degradation, specifically, have received appreciable attention. Different classes of ILs including biocompatible, bio-renewable, protonic ILs have been investigated [42]. The main factor affecting lignin HTL using ILs as catalysts, and/or co-solvents is their selectivity. This selectivity of ILs revolves around the cationic alkyl chain length, anionic hydrophobicity, temperature and solvent type among others. The catalytic activity of acidic ILs, for example, is largely associated with the Lewis/Bronsted acidity of the alkyl chain length.

Significant acidity of ILs catalysts is derived from inter-molecular bonding interactions. This is in contrast to the acidity of conventional acid catalysts, whose

**79**

RANEY Nickel

**Figure 4.**

Raney Nickel 2400, 4200

Dual catalyst (cholinederived ionic liquids and Pd/C).

*Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review*

**Catalyst Chemical composition Feedstock used Efficiency/findings Ref.**

*Effect of reaction temperature on lignin liquefaction for different feedstock. Data obtained from feedstock reported*

*including; sugarcane bagasse [38, 42–44]; agricultural waste [15]; Loblolly pine [45]; Pinewood [25].*

The efficiency was achieved using 4% NaOH caused a decrease in ester linked *p*-coumarates in the residues, while at 9% NaOH both *p*-coumarates and ferulates decreased. The last was symbolic for the degradation of particular

The catalyst produced more carbon dioxide in the gaseous phase at the expense of low C2–C4 and much phenol in the organic phase in comparison to catechol and guaiacol. Low char formation was also observed.

The catalyst favored the production of phenol in all cases. However, the efficiency depends on the Ni/Al ratio and other promoter metallic composition of

effective in selective fractionation of cellulose, hemicellulose and lignin into relatively high purity fractions. The catalyst was also effective in the valorization of the

the catalysts.

phenolic fraction.

anions combinations

production was virtually the same, since the catalytic activity was dependent on the cation and

Kraft lignin The phenol and catechol

Wheat straw The use of the ionic liquid was

[36]

[13]

[4]

[39]

[24]

β–O–4 linkages.

bagasse.

Enzymatic hydrolysis lignin.

Organosolv lignin.

*DOI: http://dx.doi.org/10.5772/intechopen.90860*

NaOH - Sugarcane

Nickel (88 wt%) and aluminum (12 wt%).

Nickel (81 wt%), aluminum (6 wt%), iron (2 wt%) and chrome (2

([emim][OAc]).

methylimidazolium acetate

[Ch][Ace]; [Ch][CF3CO2]; [Ch][H2PO4]; [Ch][Lev], and [Ch]-[MeSO3] with

wt%).

Pd/C.

Ionic liquids 1-ethyl-3-

#### *Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review DOI: http://dx.doi.org/10.5772/intechopen.90860*

#### **Figure 4.**

*Biorefinery Concepts, Energy and Products*

the major bifunctional chemicals obtained.

**5.1 Effect of temperature/heating rate**

**5. Factors affecting lignin hydrothermal liquefaction**

affected products distribution much more than reaction time.

**5.2 Effect of catalyst type/concentration**

flow chart.

in the literature.

ionic liquids.

The use of co-solvent system in biomass liquefaction and by extension, lignin degradation has been investigated. A co-solvent hydrothermal degradation of alkali lignin into bi-phenolic compounds was investigated by Cheng et al. [12], suggesting an improved yield from the water-ethanol co-solvent system for sub/ supercritical conditions. Whereas, time had negligible effect on the yield and quality of liquefaction products, temperature and co-solvent ratio had significant effects on yield and quality of bio-crude oil. Singh et al. [17], employed ethanol and methanol (1:10) co-solvent system for the synthesis of substituted Phenol and aromatic ethers. **Figure 3** presents a semi-continuous process biomass liquefaction

Lignin HTL is therefore suitable technology for effectively converting it into useful products with high selectivity, and quality. **Table 2** reviewed the literature works on the HTL of lignin into reported products, highlighting the recounted yields and

The reaction temperature is the predominant factor in thermo-chemical processes like liquefaction. Lignin HTL is a temperature dependent process, since the degree of liquefaction increases with increasing reaction temperature. A careful selection of the heating rate is desired in reducing the rate, at which, condensation of unsaturated oligomeric phenol occur [38]. Depending on the temperature, HTL is sub-divided into subcritical liquefaction and supercritical liquefaction. **Figure 4** presents the effect of reaction temperature on yield of different feedstock reported

From the reported result, degradation temperature for lignin feedstock is between 200 and 300°C. Higher temperature beyond 300°C tends to reduce biocrude oil yield. According to Hu et al. [40], lignin degradation is temperature dependent, in a manner that, increasing temperature in degradation of black liquor lignin was found to have favored bond cleavage, and elimination of functional groups, and carbonization. To add to this, Yang et al. [33] found that, temperature

Different classes of catalysts have already shown positive impact during lignin degradation. From **Table 3**, it could be argued that homogenous catalyst like NaOH, were found suitable [37, 41]. Also, mixed oxide catalysts [12] are increasingly been investigated. However, recent literature trend, suggested a paradigm shift towards

Ionic liquids [ILs] application in biomass refining has received increasing attention, recently. ILs catalysis in lignin degradation, specifically, have received appreciable attention. Different classes of ILs including biocompatible, bio-renewable, protonic ILs have been investigated [42]. The main factor affecting lignin HTL using ILs as catalysts, and/or co-solvents is their selectivity. This selectivity of ILs revolves around the cationic alkyl chain length, anionic hydrophobicity, temperature and solvent type among others. The catalytic activity of acidic ILs, for example, is largely associated with the Lewis/Bronsted acidity of the alkyl chain length. Significant acidity of ILs catalysts is derived from inter-molecular bonding interactions. This is in contrast to the acidity of conventional acid catalysts, whose

**78**

*Effect of reaction temperature on lignin liquefaction for different feedstock. Data obtained from feedstock reported including; sugarcane bagasse [38, 42–44]; agricultural waste [15]; Loblolly pine [45]; Pinewood [25].*

#### **Table 3.**

*Effect of catalysts on lignin degradation for different feedstock.*

acidity is derived from their protonation. Therefore, the former exhibited the tendency towards eliminating reactor corrosion; a major reaction engineering problem that has been posing serious challenges. It was postulated that, lignin dissolution was aided via a π–π interaction between an alkyl imidazolium chloride catalyst and

**81**

products yield.

*Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review*

the π–bond in the aromatic rings structure of lignin, suggesting the additional dissolution potentials of alkyl based ILs [9]. According to Zhuo et al. [44]; the acidity of 2-phenyl-2-Imidazoline based ILs with shorter-side chain length at C-1, was found to be higher than same ILs having longer-side chain length; and were both

Dual-functionalized ILs have even greater acidity. Products distribution is also associated with the acidity of ILs as catalysts/co-solvent medium employed. Highly acidic medium preferentially favored hydrolysis reaction, yielding water-soluble products. While basic medium promotes liquefaction reactions, yielding organic

However, the most attractive ILs in this field, nowadays, are the deep eutectic solvents (DESs) [45, 47]. Additional properties of DESs including; low volatility, thermal and chemical stability, high selectivity, green characteristic; ease of preparation [28, 48] are among their added advantages of DESs as catalysts. The chemistry of these catalysts suggests that their delignification efficiency was dependent on the acidic amount, its strength, chemical composition of the quaternary salt and the nature of the hydrogen bond acceptor or both [45, 49]. Liu et al. [23], conducted the selective hydrogenolysis of Kraft lignin into mono-phenol, catalyzed by using Choline-derived DESs, in which, [Ch][MeSO3] showed excellent lignin dissolution,

Accordingly, Wu et al. [47] investigated among others, the lignin extractability of monocarboxylic acids, dicarboxylic acids and polycarboxylic acids based DESs. The authors reported monocarboxylic acids with high acidic strength showed improved lignin extractability, as compared to low acidic monocarboxylic acids and dicarboxylic acids, due to carbon dioxide liberation in case of using dicaroxylic acids. Controversially, the low viscosities of polycarboxylic acids makes their hydroxyl group available for interaction with the etherified hydroxyl components

Apart from their general applications in biomass refining including lignin degradation, ILs have been reportedly used in selective production and isolation of biorefinery products such as furfural [48], total reducing sugars [44, 50], and glycerin separation [51]. For example, in place of microwave assisted methylation, the selective oxidation of these benzylic alcohols in lignin into benzylic ketones,

Technically, two-step lignin depolymerization involving the selective oxidation of primary benzylic alcohols into benzylic ketones on one side and its succeeding β–O–4 cleavage; would have been the most favorable conversion route. However, challenges associated with the low products yields and poor selectivity remained unresolved. Unlike the methylation route, the benzylic ketone (selective oxidation) route, showed low products yield and selectivity as well. In addition, products

The effect of reaction time during lignin degradation was studied by Yuan et al. [53]. The authors discovered that; long reaction time was needed to ensure complete degradation of all ether bonds in lignin, and gradual degradation of the stable C–C bonds, afterwards. Long reaction induction period of the intermediates enabled secondary reactions including re-polymerization, cross-linking and rearrangement, thereby forming marginally higher products yields. Reaction time has significantly influenced the properties of individual lignin liquefaction products. According to Chen et al. [12], reaction time had no significant effect on

resulting from its strong acidity and better thermal stability.

of the lignin, thus; resulting in high lignin extraction [47].

separation becomes challenging on a large scale [41].

**5.3 Effect of reaction time**

prior to the β–O–4 hydrogenolysis treatment has been reported [52].

*DOI: http://dx.doi.org/10.5772/intechopen.90860*

lower than –SO3H functionalized ILs.

products [23].

#### *Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review DOI: http://dx.doi.org/10.5772/intechopen.90860*

the π–bond in the aromatic rings structure of lignin, suggesting the additional dissolution potentials of alkyl based ILs [9]. According to Zhuo et al. [44]; the acidity of 2-phenyl-2-Imidazoline based ILs with shorter-side chain length at C-1, was found to be higher than same ILs having longer-side chain length; and were both lower than –SO3H functionalized ILs.

Dual-functionalized ILs have even greater acidity. Products distribution is also associated with the acidity of ILs as catalysts/co-solvent medium employed. Highly acidic medium preferentially favored hydrolysis reaction, yielding water-soluble products. While basic medium promotes liquefaction reactions, yielding organic products [23].

However, the most attractive ILs in this field, nowadays, are the deep eutectic solvents (DESs) [45, 47]. Additional properties of DESs including; low volatility, thermal and chemical stability, high selectivity, green characteristic; ease of preparation [28, 48] are among their added advantages of DESs as catalysts. The chemistry of these catalysts suggests that their delignification efficiency was dependent on the acidic amount, its strength, chemical composition of the quaternary salt and the nature of the hydrogen bond acceptor or both [45, 49]. Liu et al. [23], conducted the selective hydrogenolysis of Kraft lignin into mono-phenol, catalyzed by using Choline-derived DESs, in which, [Ch][MeSO3] showed excellent lignin dissolution, resulting from its strong acidity and better thermal stability.

Accordingly, Wu et al. [47] investigated among others, the lignin extractability of monocarboxylic acids, dicarboxylic acids and polycarboxylic acids based DESs. The authors reported monocarboxylic acids with high acidic strength showed improved lignin extractability, as compared to low acidic monocarboxylic acids and dicarboxylic acids, due to carbon dioxide liberation in case of using dicaroxylic acids. Controversially, the low viscosities of polycarboxylic acids makes their hydroxyl group available for interaction with the etherified hydroxyl components of the lignin, thus; resulting in high lignin extraction [47].

Apart from their general applications in biomass refining including lignin degradation, ILs have been reportedly used in selective production and isolation of biorefinery products such as furfural [48], total reducing sugars [44, 50], and glycerin separation [51]. For example, in place of microwave assisted methylation, the selective oxidation of these benzylic alcohols in lignin into benzylic ketones, prior to the β–O–4 hydrogenolysis treatment has been reported [52].

Technically, two-step lignin depolymerization involving the selective oxidation of primary benzylic alcohols into benzylic ketones on one side and its succeeding β–O–4 cleavage; would have been the most favorable conversion route. However, challenges associated with the low products yields and poor selectivity remained unresolved. Unlike the methylation route, the benzylic ketone (selective oxidation) route, showed low products yield and selectivity as well. In addition, products separation becomes challenging on a large scale [41].

#### **5.3 Effect of reaction time**

The effect of reaction time during lignin degradation was studied by Yuan et al. [53]. The authors discovered that; long reaction time was needed to ensure complete degradation of all ether bonds in lignin, and gradual degradation of the stable C–C bonds, afterwards. Long reaction induction period of the intermediates enabled secondary reactions including re-polymerization, cross-linking and rearrangement, thereby forming marginally higher products yields. Reaction time has significantly influenced the properties of individual lignin liquefaction products. According to Chen et al. [12], reaction time had no significant effect on products yield.

*Biorefinery Concepts, Energy and Products*

Ionic liquids The ILs including

Palladium catalysts

Mixed oxides catalysts

K2CO3 LignoBoost

1-(4-sulfobutyl)-3-methyl imidazolium hydrosulfate ([C4H8-SO3Hmim]HSO4), N-methyl imidazolium hydrosulfate. HSO4), 1-butyl-3-methyl imidazolium hydrosulfate ([bmim] HSO4), and 1-(2-carboxyethyl)- 3-methyl

imidazolium chloride ([C2H4COOHmim]Cl).

Commercially available catalysts, (10%) Pd/C, (30%) Pd/C, Pd(OH)2/C, Pd(OAc)2, Pd-PEPPSI-iPr and Pd/Lindlar.

A γ-Al2O3 and active carbon (AC) supported metallic (Pt, Ru, and Ni).

e.g ([C4mim]MeSO3); ([C4mim]OAc); ([C4mim]

*Effect of catalysts on lignin degradation for different feedstock.*

Ionic liquids Dialkylimidazolium-based

Cl).

**Catalyst Chemical composition Feedstock used Efficiency/findings Ref.**

Kraft lignin

Sugarcane bagasse

Spruce lignin; lignin from acidic hydrolysis; enzymatic hydrolysis; acidic hydrolysis; strong acidic hydrolysis; desulfonated Kraft lignin.

NaOH - Kraft lignin The base-catalyzed lignin

Increasing mass fraction of K2CO3 resulted in remarkable increasing in phenolic oil yield, showing selectivity towards anisole, alkylphenol and catechol

de-polymerization yielded 8.4 wt% monomeric-rich fraction. The catalyst favored deoxygenated aliphatic OH and

All ILs studied were very effective towards total degradation of lignin components, showing excellent recyclability up to five times. However, the results into numerous products which caused characteristic separation

It was reported that the metallic catalysts did not have significant change in the yield, generally, but Ni and Ru showed preferential improved yield than Pt

The pH, IL composition, and IL content were established to significantly affect the degradation and chemical conversion of lignin structure. It was concluded that low pH helped lignin depolymerization nevertheless destroyed the substructure of lignin.

guaiacyl groups.

difficulty.

[11]

[46]

[43]

[14]

[12]

[40]

**80**

**Table 3.**

acidity is derived from their protonation. Therefore, the former exhibited the tendency towards eliminating reactor corrosion; a major reaction engineering problem that has been posing serious challenges. It was postulated that, lignin dissolution was aided via a π–π interaction between an alkyl imidazolium chloride catalyst and

Regenerated lignin

#### **5.4 The effect of reaction time**

The composition of lignin is yet, another important factor in its degradation. Zhou [18] observed that, the yield of water-soluble hydrocarbons during HTL of Kraft lignin was low as compared to the yield of same components in liquefaction of sawdust. They attributed their finding to the composition of Kraft lignin, having low carbohydrate content. Another important factor in lignin degradation is the composition of the solvent. According to Yuan et al. [53], the composition of solvent greatly influenced the products distribution. They observed that; phenol addition in the reaction medium hindered side reactions like re-polymerization of products intermediates, resulting in low residue formation. Using co-solvent system showed improved lignin degradation, specifically, water-ethanol co-solvent degradation was much effective as compared to individual mono-solvent systems [12].

#### **6. Composition of liquefaction products**

As earlier discussed, decomposition of the different classes of lignin components (p-coumaryl, coniferyl and sinapyl alcohols) results in complexing chemical products, intermediates and by-products during liquefaction. Characteristically, p-coumaryl alcohols decomposed into corresponding hydoxyphenyl derivatives (H-type) including Phenol, Phenol-methyl-, Phenol-di-methyl-, Methylphenol and other secondary Benzoic acid hydroxyl- derivatives.

The guaiacyl derivatives (G-type) are largely containing methoxyphenol, methyl methoxyphenol, vinylphenol, methoxy propyl and vanillin. According to Zhou [15], the products distribution from an organosolv liquefaction of Kraft lignin, for example; suggested up to 65 wt% constituted of volatile products, of which 78 wt% was made up of 2-methoxyphenol.

#### **6.1 Phenol**

Phenol and its derivatives are probably the most interesting products of lignin liquefaction. Phenol and its derivatives formed the major composition of HTL of lignin and its compounds. Selective cleavage of the different ether bonds (C–O–C), yield highly phenolic products, showing excellent fuel additive properties, when the oxy-aromatic configuration is retained [11]. Phenolic compounds formed the greater composition in bio-crude oil obtained from Corncob lignin degradation. Generally, phenolic products from lignin degradation are classified as G-phenols (methoxy phenols), S-phenols (dimethoxyphenols) and H-phenols (methylphenols).

Khampuang et al. [54], conducted an alkali catalyzed Corncob liquefaction in supercritical water-ethanol, with phenol and its derivatives (phenol, 4-methoxyacetate; phenol, 2-ethyl-; phenol, 4-ethyl-2-methoxy-; phenol, 2,6-dimethoxy-), constituting major proportion of the bio-crude oil. **Figure 5** presents some selected Phenolic products from HTL of lignin.

Similarly, Riaz et al. [55], confirmed the presence of monomeric phenol as one of the major components of bio-crude oil obtained from the acid hydrolysis lignin degradation in supercritical ethanol. The authors noted that, though, phenol (C6H5OH) was the major product, traces of methyl, methoxy, and ethyl groups were equally observed. Base catalyzed HTL of lignin seems to promote phenol formation. According to Nazari et al. [56], phenol derivatives (mainly 2-methoxy phenol), and aliphatic compounds, constituted significant composition of bio-crude oil yield, resulting from the use of base catalyst (KOH). Yuan et al. [53], reported that, higher temperature, and long reaction time increased the phenol combination rate, leading

**83**

*Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review*

to re-polymerization, and cross-linking among phenol and the side chains of the degraded lignin. Yang et al. [33], found that, compounds in bio-crude oil had extra active sites than the parent lignin, signifying that the bio-crude oil obtained from lignin degradation was a promising feedstock for industrial synthesis of phenol formaldehyde adhesive. Decomposition of p-coumaryl alcohol (H-type), yield

The widely accepted mechanism for the production of guaiacol is via β–O–4 bond cleavage. Recalling that, β–O–4 bond is the predominant in lignin structure, large quantity of guaiacol is expected during HTL. The production of guaiacol depends on the cleavage of C–O and C–C bonds during lignin degradation [14]. Increasing reaction temperature reduces the yield of guaiacol-derivatives at the expense of their benzendiol counterparts (Catechol). This was confirmed by the work of Nguyen et al. [11], who reported that, base catalysts, such as K2CO3 tend to decrease the yield of guaiacol and increasing the yields of catechol. Zhou [18], conducted the HTL conversion of Kraft lignin, indicating the predominance of guaiacol (19–78 wt%), depending on reaction conditions. The bio-crude oil yield was indicated by improved sum of phenolic –OH groups present in Kraft lignin and, diminished amount of β–O–4 linkages, hydroxyl groups, carbonyl groups, aromatic rings. Thus, suggested that, the guaiacol yield obtained via HTL of lignin was much higher than that obtained from its liquefaction. **Figure 6** presents some Guaiacol derivatives reported

from HTL of lignin. **Figure 6** presents the relative distribution of products.

Catechol exists in different monomer forms including pyrocatechol, 3-methylcatechol, 4-methylcatechol, propylcatechol, 4-ethylcatechol and so on. Its production mechanism reported by authors differ. For example, Beauchet et al. [13] proposed the production of pyrocatechol, and methanol via direct hydrolysis of O–CH3 bond, while its decarboxylation at ether bond of 3-hydroxy-4-methoxybenzeneacetic acid, yielded significant proportion of methylcatechol. **Figure 7** presents

Forchheim et al. [14], proposed lignin degradation, and found that, RANEY Nickel catalyst favored ether bond cleavage near critical water condition. Authors also noted the conversion of catechol into phenol as the secondary degradation

significant bio-crude oil with high amount of phenol.

**6.2 Guaiacol**

**Figure 5.**

**6.3 Catechol**

some selected catechol derivatives.

*DOI: http://dx.doi.org/10.5772/intechopen.90860*

*Some phenolic products from HTL of lignin.*

*Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review DOI: http://dx.doi.org/10.5772/intechopen.90860*

#### **Figure 5.**

*Biorefinery Concepts, Energy and Products*

**6. Composition of liquefaction products**

other secondary Benzoic acid hydroxyl- derivatives.

which 78 wt% was made up of 2-methoxyphenol.

Phenolic products from HTL of lignin.

The composition of lignin is yet, another important factor in its degradation. Zhou [18] observed that, the yield of water-soluble hydrocarbons during HTL of Kraft lignin was low as compared to the yield of same components in liquefaction of sawdust. They attributed their finding to the composition of Kraft lignin, having low carbohydrate content. Another important factor in lignin degradation is the composition of the solvent. According to Yuan et al. [53], the composition of solvent greatly influenced the products distribution. They observed that; phenol addition in the reaction medium hindered side reactions like re-polymerization of products intermediates, resulting in low residue formation. Using co-solvent system showed improved lignin degradation, specifically, water-ethanol co-solvent degradation was much effective as compared to individual mono-solvent systems [12].

As earlier discussed, decomposition of the different classes of lignin components (p-coumaryl, coniferyl and sinapyl alcohols) results in complexing chemical products, intermediates and by-products during liquefaction. Characteristically, p-coumaryl alcohols decomposed into corresponding hydoxyphenyl derivatives (H-type) including Phenol, Phenol-methyl-, Phenol-di-methyl-, Methylphenol and

The guaiacyl derivatives (G-type) are largely containing methoxyphenol, methyl methoxyphenol, vinylphenol, methoxy propyl and vanillin. According to Zhou [15], the products distribution from an organosolv liquefaction of Kraft lignin, for example; suggested up to 65 wt% constituted of volatile products, of

Phenol and its derivatives are probably the most interesting products of lignin liquefaction. Phenol and its derivatives formed the major composition of HTL of lignin and its compounds. Selective cleavage of the different ether bonds (C–O–C), yield highly phenolic products, showing excellent fuel additive properties, when the oxy-aromatic configuration is retained [11]. Phenolic compounds formed the greater composition in bio-crude oil obtained from Corncob lignin degradation. Generally, phenolic products from lignin degradation are classified as G-phenols (methoxy phenols), S-phenols (dimethoxyphenols) and H-phenols (methylphenols).

Khampuang et al. [54], conducted an alkali catalyzed Corncob liquefaction in supercritical water-ethanol, with phenol and its derivatives (phenol, 4-methoxyacetate; phenol, 2-ethyl-; phenol, 4-ethyl-2-methoxy-; phenol, 2,6-dimethoxy-), constituting major proportion of the bio-crude oil. **Figure 5** presents some selected

Similarly, Riaz et al. [55], confirmed the presence of monomeric phenol as one of the major components of bio-crude oil obtained from the acid hydrolysis lignin degradation in supercritical ethanol. The authors noted that, though, phenol (C6H5OH) was the major product, traces of methyl, methoxy, and ethyl groups were equally observed. Base catalyzed HTL of lignin seems to promote phenol formation. According to Nazari et al. [56], phenol derivatives (mainly 2-methoxy phenol), and aliphatic compounds, constituted significant composition of bio-crude oil yield, resulting from the use of base catalyst (KOH). Yuan et al. [53], reported that, higher temperature, and long reaction time increased the phenol combination rate, leading

**5.4 The effect of reaction time**

**82**

**6.1 Phenol**

*Some phenolic products from HTL of lignin.*

to re-polymerization, and cross-linking among phenol and the side chains of the degraded lignin. Yang et al. [33], found that, compounds in bio-crude oil had extra active sites than the parent lignin, signifying that the bio-crude oil obtained from lignin degradation was a promising feedstock for industrial synthesis of phenol formaldehyde adhesive. Decomposition of p-coumaryl alcohol (H-type), yield significant bio-crude oil with high amount of phenol.

#### **6.2 Guaiacol**

The widely accepted mechanism for the production of guaiacol is via β–O–4 bond cleavage. Recalling that, β–O–4 bond is the predominant in lignin structure, large quantity of guaiacol is expected during HTL. The production of guaiacol depends on the cleavage of C–O and C–C bonds during lignin degradation [14]. Increasing reaction temperature reduces the yield of guaiacol-derivatives at the expense of their benzendiol counterparts (Catechol). This was confirmed by the work of Nguyen et al. [11], who reported that, base catalysts, such as K2CO3 tend to decrease the yield of guaiacol and increasing the yields of catechol. Zhou [18], conducted the HTL conversion of Kraft lignin, indicating the predominance of guaiacol (19–78 wt%), depending on reaction conditions. The bio-crude oil yield was indicated by improved sum of phenolic –OH groups present in Kraft lignin and, diminished amount of β–O–4 linkages, hydroxyl groups, carbonyl groups, aromatic rings. Thus, suggested that, the guaiacol yield obtained via HTL of lignin was much higher than that obtained from its liquefaction. **Figure 6** presents some Guaiacol derivatives reported from HTL of lignin. **Figure 6** presents the relative distribution of products.

#### **6.3 Catechol**

Catechol exists in different monomer forms including pyrocatechol, 3-methylcatechol, 4-methylcatechol, propylcatechol, 4-ethylcatechol and so on. Its production mechanism reported by authors differ. For example, Beauchet et al. [13] proposed the production of pyrocatechol, and methanol via direct hydrolysis of O–CH3 bond, while its decarboxylation at ether bond of 3-hydroxy-4-methoxybenzeneacetic acid, yielded significant proportion of methylcatechol. **Figure 7** presents some selected catechol derivatives.

Forchheim et al. [14], proposed lignin degradation, and found that, RANEY Nickel catalyst favored ether bond cleavage near critical water condition. Authors also noted the conversion of catechol into phenol as the secondary degradation

**Figure 7.** *Structures of some catechols.*

product. Guaiacol degradation also yield catechol, whereas, catechol losses one –OH group to form phenol at high temperature [9]. Generally, high-temperature and short time favored the formation of catechol, and subdues the formation of char and gaseous components [14]. Depending on the temperature, HTL is sub-divided into subcritical liquefaction and supercritical liquefaction. **Figure 8** presents the relative products distribution in lignin degradation on yield of different feedstock reported in the literature.

Catechol formation was discussed by Hu et al. [40] suggesting that; it is largely formed by the preferential elimination of methoxyl group depending on the truncated strength of the Aryl–OCH3 (DBE = 273 KJ/mol) to the Aryl–O–CH3 (DBE = 416.7 KJ/mol) bond. Specifically, with increasing reaction temperature,

**85**

**Figure 9.**

**Figure 8.**

mal lignin degradation.

*Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review*

simultaneous pyrolysis and hydrolysis reactions taking place favored catechol formation. Lignin depolymerisation kinetics designated the decomposition of catechol into phenol, preferentially at high temperature and long reaction time [20]. Berstis et al. [10], demonstrated the use of density functional theory in predicting the relative energetics as well as bond dissociation enthalpies of the different benzodioxane

*Relative products distribution in lignin degradation. Palm kernel shell, wheat straw, and pine sawdust [31], cotton exocarp and mushroom substrate of cotton [22], hardwood [35], and sugarcane bagasse [38].*

The index of hydrogen deficiency or double bond equivalence is calculated by

**Figure 9** presents the relative distribution of products. Guaiacol – catechol – phenol interaction was considered as an important factor in lignin degradation kinetic studied by Forchheim et al. [14], because, phenol has better stability than either of guaiacol or catechol. **Figure 9**, proposed reaction pathways for hydrother-

Ω = (2 + 2C − H)/2 (1)

the formula proposed by Pedersen and Rosendahl [29] as given below:

linkages in lignin (C–C, C–S, C–G, C–H).

*Proposed reaction pathways for hydrothermal lignin degradation [14].*

*DOI: http://dx.doi.org/10.5772/intechopen.90860*

*Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review DOI: http://dx.doi.org/10.5772/intechopen.90860*

#### **Figure 8.**

*Biorefinery Concepts, Energy and Products*

*Some guaiacol derivatives from HTL of lignin.*

**84**

**Figure 7.**

**Figure 6.**

reported in the literature.

*Structures of some catechols.*

product. Guaiacol degradation also yield catechol, whereas, catechol losses one –OH group to form phenol at high temperature [9]. Generally, high-temperature and short time favored the formation of catechol, and subdues the formation of char and gaseous components [14]. Depending on the temperature, HTL is sub-divided into subcritical liquefaction and supercritical liquefaction. **Figure 8** presents the relative products distribution in lignin degradation on yield of different feedstock

Catechol formation was discussed by Hu et al. [40] suggesting that; it is largely

formed by the preferential elimination of methoxyl group depending on the truncated strength of the Aryl–OCH3 (DBE = 273 KJ/mol) to the Aryl–O–CH3 (DBE = 416.7 KJ/mol) bond. Specifically, with increasing reaction temperature,

*Relative products distribution in lignin degradation. Palm kernel shell, wheat straw, and pine sawdust [31], cotton exocarp and mushroom substrate of cotton [22], hardwood [35], and sugarcane bagasse [38].*

#### **Figure 9.**

*Proposed reaction pathways for hydrothermal lignin degradation [14].*

simultaneous pyrolysis and hydrolysis reactions taking place favored catechol formation. Lignin depolymerisation kinetics designated the decomposition of catechol into phenol, preferentially at high temperature and long reaction time [20]. Berstis et al. [10], demonstrated the use of density functional theory in predicting the relative energetics as well as bond dissociation enthalpies of the different benzodioxane linkages in lignin (C–C, C–S, C–G, C–H).

The index of hydrogen deficiency or double bond equivalence is calculated by the formula proposed by Pedersen and Rosendahl [29] as given below:

$$
\Delta = (\text{ $\mathcal{Q}$ } + \text{ $\mathcal{Z}$ C} - \text{H})/\text{ $\mathcal{Q}$ } \tag{1}
$$

**Figure 9** presents the relative distribution of products. Guaiacol – catechol – phenol interaction was considered as an important factor in lignin degradation kinetic studied by Forchheim et al. [14], because, phenol has better stability than either of guaiacol or catechol. **Figure 9**, proposed reaction pathways for hydrothermal lignin degradation.

#### **7. Useful compounds and their applications**

#### **7.1 Useful compounds obtainable from HTL process**

A significant number of chemicals compounds are theoretically and practically obtainable from HTL process. According to the broad literature available, these compounds are classified into phenols, guaiacols and catechols. However, derivatives of these principal groups have been widely reported. For example, lignin phenolic compounds have been produced by different research groups.

HTL products are largely rich in *ortho-methoxyphenolic* compounds which are detected by the presence of –OCH3 in their NMR spectra. This high volume product has chemical and biochemical applications including the manufacture of drugs and in clinical diagnosis. For example, the monitoring of wood smoke exposure by urinary assay was conducted by measuring the *ortho-methoxyphenols*. The production of perfumes, disinfectants resulting from its anti-oxidant properties, and as starting material for the synthesis of guaiacol compounds has been reported. Similarly, 3-methoxyphenol have dual functions as catalyst and building block in synthesis of anti-oxidants. Guaiacol is obtained by destructive distillation of phenol fraction of coal tar.

#### **8. Pyrolysis of lignin**

Pyrolysis of lignin has been one of the degradation techniques reported. Wang et al. [57] compared the pyrolytic behavior of different lignin (Manchurian ash and Mangolian Scot pine). The authors concluded that, the differences in pyrolytic behavior of the lignin studied were attributed to variation in their composition, and degradation temperature. High methanol yield was observed from degradation of hardwood, due to its higher methoxy group content, while the secondary methanol yield was attributed to aromatic ring degradation at high temperature.

Although HTL and pyrolysis of lignin are closely related, there exist some fundamental differences in their products. For example, Haarlemmer et al. [7], observed that, bio-crude oil (obtained from HTL) had higher acidity and iodine value as compared to an inverse relationship observed in bio-oil (obtained from pyrolysis) from the same feedstock. According to these authors, HTL oils are very viscous, and had strong black oil smell. It also had higher heating value (35–40 MJ/ kg). HTL bio-crude oil is typically proposed as diesel substitute after catalytic upgrading. On the contrast, Pyrolysis oil is often presumed as substitute for fuel oil, and had smoky odor, and is less viscous, with heating value similar to that of the parent biomass. **Table 4** presents the comparative properties of biomass fuels with standards.

Like HTL of lignin, the products distribution during pyrolysis of lignin also depends on the composition of the feedstock. Chang et al. [24], conducted the Py-GC/MS degradation of palm kernel shell. Accordingly, the primary composition of the feedstock indicated the predominance of p-hydroxyphenyl structural units, resulting in high phenolic products, often its degradation. In contrast, pine sawdust and wheat straw contained largely guaiacyl units, and often degradation, yielded significant proportion of methoxy groups. Similarly, Biswas et al. [58], reported the slow pyrolysis of prot, alkali and dealkaline lignin for the production of chemicals. Among these types, alkali lignin was reported to have highest bio-oil yield (34.1%). Bio-oil products showed the presence of guaiacol, syringol, alkylphenols and catechol; depending on the pyrolysis conditions and nature of lignin used.

**87**

*Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review*

NF EN 14214

Carbon residue (%) <0.3 <0.3 <2.5–20 17–24 11–13

Water content (%) <0.02 <0.05 <0.033 <1 8–20

**heavy fuel oil**

820–845 860–900 920–1010 1.14 1.1

<0.5 <0.5 <0.5 32–67 45–109

NA <120 NA 126–127 147–203

45 ≥35 NA 27–30 21–30

3–4 <4 8.5–690 67,000 13–70

**Hydrothermal oil**

NF ISO 8217 In

**Pyrolysis oil**

progress

**Standard Diesel Biodiesel Marine**

NF EN 228

*Comparative physico-chemical properties of different biomass oils [7].*

Pyrolysis of lignin-rich biomass from cotton by-product was conducted by Chen et al. [21]. Maximum bio-crude oil yield from fast pyrolysis was 58.13 (wt%) and 45.01 (wt%) at 600°C for cotton exocarp and spent mushroom substrate of cotton by-products, respectively. As compared to cellulose and hemicellulose, lignin degradation occur slowly and within wide temperature range (150–780°C).

Raiz et al. [55], conducted the concentrated sulfuric acid hydrolysis of lignin in supercritical ethanol, reporting significant reduction in oxygen content (44 wt%), resulting from improved deoxy-liquefaction. Bio-oil obtained from this work, had

Like liquefaction, hydrolysis bio-oil was rich in Phenol, esters, hydrocarbons,

A number of emerging lignin degradation techniques are coming up by the day. Recently, a two-step lignin depolymerisation was reported, in which, lignin was methylated using micro-wave irradiation in the presence of benzylic alcohols. The first step of the reaction proceeded via catalytic hydrogenolysis of β–O–4 bond [52]. Similarly, in place of microwave assisted methylation, the selective oxidation of these benzylic alcohols in lignin into benzylic ketones, prior to the β–O–4 hydrogenolysis treatment has been reported [60]. Technically, two-step lignin depolymerization involving the selective oxidation of primary Benzylic alcohols into Benzylic ketones on one side, and its succeeding β–O–4 cleavage; would have been the most favorable

higher heating value, improved energy recovery, and energy efficiency.

furan and alcohol. Ji et al. [59], conducted the hydrolysis of wheat straw in a dilute sulfuric acid medium using a continuous reactor. The maximum reducing sugar yield obtained was as high as 60.8 wt%, constituting furfural as the major

*DOI: http://dx.doi.org/10.5772/intechopen.90860*

Density at 15°C (kg m<sup>−</sup><sup>3</sup> )

KOH/g)

of fuel)

(MJkg<sup>−</sup><sup>1</sup> )

**Table 4.**

Total acid number (mg

Iodine value (g12/100

Higher heating value

Viscosity at 40°C (mPas)

**9. Hydrolysis of lignin**

**10. Other technologies**

by-product.

*Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review DOI: http://dx.doi.org/10.5772/intechopen.90860*

#### **Table 4.**

*Biorefinery Concepts, Energy and Products*

fraction of coal tar.

**8. Pyrolysis of lignin**

**7. Useful compounds and their applications**

**7.1 Useful compounds obtainable from HTL process**

A significant number of chemicals compounds are theoretically and practically obtainable from HTL process. According to the broad literature available, these compounds are classified into phenols, guaiacols and catechols. However, derivatives of these principal groups have been widely reported. For example, lignin phenolic compounds have been produced by different research groups.

HTL products are largely rich in *ortho-methoxyphenolic* compounds which are detected by the presence of –OCH3 in their NMR spectra. This high volume product has chemical and biochemical applications including the manufacture of drugs and in clinical diagnosis. For example, the monitoring of wood smoke exposure by urinary assay was conducted by measuring the *ortho-methoxyphenols*. The production of perfumes, disinfectants resulting from its anti-oxidant properties, and as starting material for the synthesis of guaiacol compounds has been reported. Similarly, 3-methoxyphenol have dual functions as catalyst and building block in synthesis of anti-oxidants. Guaiacol is obtained by destructive distillation of phenol

Pyrolysis of lignin has been one of the degradation techniques reported. Wang et al. [57] compared the pyrolytic behavior of different lignin (Manchurian ash and Mangolian Scot pine). The authors concluded that, the differences in pyrolytic behavior of the lignin studied were attributed to variation in their composition, and degradation temperature. High methanol yield was observed from degradation of hardwood, due to its higher methoxy group content, while the secondary methanol

yield was attributed to aromatic ring degradation at high temperature.

Although HTL and pyrolysis of lignin are closely related, there exist some fundamental differences in their products. For example, Haarlemmer et al. [7], observed that, bio-crude oil (obtained from HTL) had higher acidity and iodine value as compared to an inverse relationship observed in bio-oil (obtained from pyrolysis) from the same feedstock. According to these authors, HTL oils are very viscous, and had strong black oil smell. It also had higher heating value (35–40 MJ/ kg). HTL bio-crude oil is typically proposed as diesel substitute after catalytic upgrading. On the contrast, Pyrolysis oil is often presumed as substitute for fuel oil, and had smoky odor, and is less viscous, with heating value similar to that of the parent biomass. **Table 4** presents the comparative properties of biomass fuels with

Like HTL of lignin, the products distribution during pyrolysis of lignin also depends on the composition of the feedstock. Chang et al. [24], conducted the Py-GC/MS degradation of palm kernel shell. Accordingly, the primary composition of the feedstock indicated the predominance of p-hydroxyphenyl structural units, resulting in high phenolic products, often its degradation. In contrast, pine sawdust and wheat straw contained largely guaiacyl units, and often degradation, yielded significant proportion of methoxy groups. Similarly, Biswas et al. [58], reported the slow pyrolysis of prot, alkali and dealkaline lignin for the production of chemicals. Among these types, alkali lignin was reported to have highest bio-oil yield (34.1%). Bio-oil products showed the presence of guaiacol, syringol, alkylphenols and catechol; depending on the pyrolysis conditions and nature of

**86**

lignin used.

standards.

*Comparative physico-chemical properties of different biomass oils [7].*

Pyrolysis of lignin-rich biomass from cotton by-product was conducted by Chen et al. [21]. Maximum bio-crude oil yield from fast pyrolysis was 58.13 (wt%) and 45.01 (wt%) at 600°C for cotton exocarp and spent mushroom substrate of cotton by-products, respectively. As compared to cellulose and hemicellulose, lignin degradation occur slowly and within wide temperature range (150–780°C).

#### **9. Hydrolysis of lignin**

Raiz et al. [55], conducted the concentrated sulfuric acid hydrolysis of lignin in supercritical ethanol, reporting significant reduction in oxygen content (44 wt%), resulting from improved deoxy-liquefaction. Bio-oil obtained from this work, had higher heating value, improved energy recovery, and energy efficiency.

Like liquefaction, hydrolysis bio-oil was rich in Phenol, esters, hydrocarbons, furan and alcohol. Ji et al. [59], conducted the hydrolysis of wheat straw in a dilute sulfuric acid medium using a continuous reactor. The maximum reducing sugar yield obtained was as high as 60.8 wt%, constituting furfural as the major by-product.

#### **10. Other technologies**

A number of emerging lignin degradation techniques are coming up by the day. Recently, a two-step lignin depolymerisation was reported, in which, lignin was methylated using micro-wave irradiation in the presence of benzylic alcohols. The first step of the reaction proceeded via catalytic hydrogenolysis of β–O–4 bond [52]. Similarly, in place of microwave assisted methylation, the selective oxidation of these benzylic alcohols in lignin into benzylic ketones, prior to the β–O–4 hydrogenolysis treatment has been reported [60]. Technically, two-step lignin depolymerization involving the selective oxidation of primary Benzylic alcohols into Benzylic ketones on one side, and its succeeding β–O–4 cleavage; would have been the most favorable

conversion route. However, challenges associated with the low products yields and poor selectivity remained unresolved. Unlike the methylation route, the benzylic ketone (selective oxidation) route, showed low products yield and selectivity as well.

#### **11. Conclusions**

Lignin degradation chemistry revolves around its β–O–4 bond cleavage. Products selectivity during its degradation is largely dependent on the catalysts employed, while the yield partly depends on the feedstock and reaction conditions. Temperature is the predominant factor influencing the reaction. Ionic liquids catalysts showed increasing research interest in biomass, and lignin liquefaction, owing to their interaction with lignin structure. Phenolic products are formed from selective cleavage of the different ether bonds (C–O–C). Increasing reaction temperature reduces the yield of guaiacol-derivatives at the expense of catechol. Catechol is formed via hydrolysis of O–CH3 bond, and decarboxylation at ether bonds. Guaiacol – catechol – phenol interaction was considered as an important factor in lignin degradation kinetic studies. Although HTL and pyrolysis of lignin are closely related, there exist some fundamental differences in the products.

#### **Acknowledgements**

The principal author wishes to sincerely thank the support of all the Prof. Naveen Kumar of Delhi Technological University, New Delhi-India, the management and staff support of the management of the National Research Institute for Chemical Technology, Zaria-Nigeria.

#### **Author details**

Yahaya Alhassan1 \*, Ursel Hornung2 and Idris M. Bugaje3

1 Department of Chemistry, Federal University Gusau, Gusau, Nigeria

2 Institute for Catalysis Research and Technology (IKFT), Karlsruhe Institute of Technology, Hermann-von-Helmholtz, Eggenstein-Leopoldshafen, Germany

3 Kaduna Polytechnic, Kaduna, Nigeria

\*Address all correspondence to<EMAIL_ADDRESS>
© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**89**

*Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review*

algal hydrothermal liquefaction—A country specific case study. Algal Research. 2016;**16**:127-140

[9] Zakzeski J, Bruijnincx PCA, Jongerius AL, Weckhuysen BM. The catalytic valorization of ligning for the production of renewable chemicals. Chemical Reviews. 2010;**110**:3552-3599

[10] Berstis L, Elder T, Crowley M, Beckham GT. Radical nature of C-lignin.

[11] Nguyen TDH, Maschietti M,

Theliander H, Vamling L, et al. Catalytic depolymerisation and conversion of Kraft lignin into liquid products using near-critical water. Journal of Supercritical Fluids. 2014;**86**:67-75

[12] Cheng S, Wilks C, Yuan Z, Leitch M, Xu C. Hydrothermal degradation of alkali lignin to bio-phenolic compounds

ACS Sustainable Chemistry and Engineering. 2016: Acssuschemeng.6b00520

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in sub/supercritical ethanol and water-ethanol co-solvent. Polymer Degradation and Stability.

[13] Beauchet R, Monteil-Rivera F, Lavoie JM. Conversion of lignin to aromatic-based chemicals (L-chems) and biofuels (L-fuels). Bioresource Technology. 2012;**121**:328-334

[14] Forchheim D, Hornung U, Kempe P, Kruse A, Steinbach D. Influence of RANEY nickel on the formation of intermediates in the degradation of lignin. International Journal of Chemical Engineering. 2012;**2012**

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depolymerisation of lignin to phenols in formic acid and water. Journal of Analytical and Applied Pyrolysis.

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*DOI: http://dx.doi.org/10.5772/intechopen.90860*

[1] Patel M, Zhang X, Kumar A. Technoeconomic and life cycle assessment on lignocellulosic biomass thermochemical conversion technologies: A review. Renewable and Sustainable Energy Reviews. 2016;**53**:1486-1489

[2] Thiruvenkadam S, Izhar S, Yoshida H, Danquah MK, Harun R. Process application of subcritical water extraction (SWE) for algal bio-products and biofuels production. Applied

Energy. 2015;**154**:815-828

2016;**148**:117-127

[3] López Barreiro D, Gómez BR,

Ronsse F, Hornung U, Kruse A, Prins W. Heterogeneous catalytic upgrading of biocrude oil produced by hydrothermal liquefaction of microalgae: State of the art and own experiments. Fuel Processing Technology.

[4] Strüven JO, Meier D. Hydrocracking of organosolv lignin in subcritical water to useful phenols employing various Raney nickel catalysts. ACS Sustainable Chemistry and Engineering. 2016;**7**(3):Acssuschemeng.6b00342

[5] Tekin K, Karagöz S, Bektaş S. A review of hydrothermal biomass processing. Renewable and Sustainable Energy Reviews. 2014;**40**:673-687

Rudolf A. Hydrothermal liquefaction of biomass: A review of subcritical water technologies. Energy. 2011;**36**(5):2328-2342

[7] Haarlemmer G, Guizani C, Anouti S, Déniel M, Roubaud A, Valin S. Analysis and comparison of bio-oils obtained by hydrothermal liquefaction and fast pyrolysis of beech wood. Fuel.

Hellgardt K. Environmental profile of

[6] Toor SS, Rosendahl L,

2016;**174**:180-188

[8] Patel B, Guo M, Shah N,

**References**

*Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review DOI: http://dx.doi.org/10.5772/intechopen.90860*

#### **References**

*Biorefinery Concepts, Energy and Products*

**11. Conclusions**

**Acknowledgements**

**Author details**

Yahaya Alhassan1

Chemical Technology, Zaria-Nigeria.

3 Kaduna Polytechnic, Kaduna, Nigeria

provided the original work is properly cited.

\*, Ursel Hornung2

\*Address all correspondence to<EMAIL_ADDRESS>
1 Department of Chemistry, Federal University Gusau, Gusau, Nigeria

2 Institute for Catalysis Research and Technology (IKFT), Karlsruhe Institute of Technology, Hermann-von-Helmholtz, Eggenstein-Leopoldshafen, Germany

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

conversion route. However, challenges associated with the low products yields and poor selectivity remained unresolved. Unlike the methylation route, the benzylic ketone (selective oxidation) route, showed low products yield and selectivity as well.

Lignin degradation chemistry revolves around its β–O–4 bond cleavage. Products selectivity during its degradation is largely dependent on the catalysts employed, while the yield partly depends on the feedstock and reaction conditions. Temperature is the predominant factor influencing the reaction. Ionic liquids catalysts showed increasing research interest in biomass, and lignin liquefaction, owing to their interaction with lignin structure. Phenolic products are formed from selective cleavage of the different ether bonds (C–O–C). Increasing reaction temperature reduces the yield of guaiacol-derivatives at the expense of catechol. Catechol is formed via hydrolysis of O–CH3 bond, and decarboxylation at ether bonds. Guaiacol – catechol – phenol interaction was considered as an important factor in lignin degradation kinetic studies. Although HTL and pyrolysis of lignin are closely related, there exist some fundamental differences in the products.

The principal author wishes to sincerely thank the support of all the Prof. Naveen Kumar of Delhi Technological University, New Delhi-India, the management and staff support of the management of the National Research Institute for

and Idris M. Bugaje3

**88**

[1] Patel M, Zhang X, Kumar A. Technoeconomic and life cycle assessment on lignocellulosic biomass thermochemical conversion technologies: A review. Renewable and Sustainable Energy Reviews. 2016;**53**:1486-1489

[2] Thiruvenkadam S, Izhar S, Yoshida H, Danquah MK, Harun R. Process application of subcritical water extraction (SWE) for algal bio-products and biofuels production. Applied Energy. 2015;**154**:815-828

[3] López Barreiro D, Gómez BR, Ronsse F, Hornung U, Kruse A, Prins W. Heterogeneous catalytic upgrading of biocrude oil produced by hydrothermal liquefaction of microalgae: State of the art and own experiments. Fuel Processing Technology. 2016;**148**:117-127

[4] Strüven JO, Meier D. Hydrocracking of organosolv lignin in subcritical water to useful phenols employing various Raney nickel catalysts. ACS Sustainable Chemistry and Engineering. 2016;**7**(3):Acssuschemeng.6b00342

[5] Tekin K, Karagöz S, Bektaş S. A review of hydrothermal biomass processing. Renewable and Sustainable Energy Reviews. 2014;**40**:673-687

[6] Toor SS, Rosendahl L, Rudolf A. Hydrothermal liquefaction of biomass: A review of subcritical water technologies. Energy. 2011;**36**(5):2328-2342

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