Humins are carbon-based macromolecular substances, that can be found in soil chemistry or as a by-product from saccharide-based biorefinery processes.

Humins in soil chemistry

Soil consists of both mineral (inorganic) and organic components. The organic components can be subdivided into fractions that are soluble, largely humic acids, and insoluble, the humins. Humins make up about 50% of the organic matter in soil.[1]

Due to their very complex molecular structure, humic substances, including humin, do not correspond to pure substances but consist of a mixture of many compounds that remain very difficult to characterize even using modern analytical techniques.[2]

Humins from biomass sources

Humins also produced during the dehydration of sugars, as occurs during the conversion of lignocellulosic biomass to smaller, higher value organic compounds such as 5-hydroxymethylfurfural (HMF). These humins can be in the form of either viscous liquids or solids depending on the process conditions used.

Humin structure and mechanism of formation

Both the structure of humins and the mechanism by which they are synthesized is at present not well defined as the formation and chemical properties of humins will change depending on the process conditions used. Generally, humins have a polymeric furanic-type structure, with hydroxyl, aldehyde and ketone functionalities.[3] However, the structure is dependent on feedstock type (e.g. xylose or glucose) or concentration, reaction time, temperature, catalysts and many other parameters involved in the process.[4] These parameters also influence the mechanism of formation which is still a matter of debate. Different pathways have been considered, including ring-opening hydrolysis of HMF (believed to be the key intermediate for the formation of humins),[5] nucleophilic additions,[6] or via the formation of an aromatic intermediate.[7] While there is no clear evidence to substantiate or exclude the mechanisms, general consensus is on a series of condensation reactions that reduce the efficiency of biomass conversion strategies.

Safety aspects

Humins are not considered to be a dangerous substance according to officially recognized hazardous material classification systems based on physical-chemical properties such as flammability,[8] explosiveness, susceptibility to oxidation, corrosiveness or eco-toxicity.[9] Heating of humins forms a macroporous material known as humins foams[10] and also these materials did not present critical fire behaviour despite their highly porous structure.[8]

Potential applications of humins

In the past, humins from biomass sources have been mostly considered as combustible materials to supply heat for biorefinery processes. However, high value applications have started to receive more attention, notably the use of humins in the preparation of catalytic materials[11] and in material applications (e.g. plastic reinforcement and construction materials).[12][13][14] Humins can also be subjected to thermal treatments in order to form interesting solid materials, such as lightweight and porous humin foams.[15][16] Overall, humins appear to improve the final properties of the materials although research is mainly at the proof-of-principle stage (early).

See also

References

  1. ↑ Rice, James A. "Humin" Soil Science 2001, vol. 166(11), pp. 848-857. doi:10.1097/00010694-200111000-00002
  2. ↑ Lehmann, J.; Kleber, M. (2015-12-03), "The contentious nature of soil organic matter", Nature, 528 (7580): 60–68, doi:10.1038/nature16069, PMID 26595271
  3. ↑ van Zandvoort, I., "Towards the Valorisation of Humin By-products: Characterisation, Solubilisation and Catalysis", 2015
  4. ↑ Heltzel, Jacob; Patil, Sushil K. R.; Lund, Carl R. F. (2016), Schlaf, Marcel; Zhang, Z. Conrad (eds.), "Humin Formation Pathways", Reaction Pathways and Mechanisms in Thermocatalytic Biomass Conversion II: Homogeneously Catalyzed Transformations, Acrylics from Biomass, Theoretical Aspects, Lignin Valorization and Pyrolysis Pathways, Green Chemistry and Sustainable Technology, Springer Singapore, pp. 105–118, doi:10.1007/978-981-287-769-7_5, ISBN 9789812877697
  5. ↑ Horvat, Jaroslav; Klaić, Branimir; Metelko, Biserka; Šunjić, Vitomir (1985-01-01). "Mechanism of levulinic acid formation". Tetrahedron Letters. 26 (17): 2111–2114. doi:10.1016/S0040-4039(00)94793-2. ISSN 0040-4039.
  6. ↑ Sumerskii, I. V.; Krutov, S. M.; Zarubin, M. Ya. (2010-02-01). "Humin-like substances formed under the conditions of industrial hydrolysis of wood". Russian Journal of Applied Chemistry. 83 (2): 320–327. doi:10.1134/S1070427210020266. ISSN 1608-3296. S2CID 84984623.
  7. ↑ Luijkx, Gerard C. A.; van Rantwijk, Fred; van Bekkum, Herman (1993-04-07). "Hydrothermal formation of 1,2,4-benzenetriol from 5-hydroxymethyl-2-furaldehyde and d-fructose". Carbohydrate Research. 242: 131–139. doi:10.1016/0008-6215(93)80027-C. ISSN 0008-6215.
  8. 1 2 Muralidhara, A., Tosi, P., Mija, A., Sbirrazzuoli, N., Len, C., Engelen, V., de Jong, E., Marlair, G., ACS Sustainable Chem. Eng., 2018, 6, 16692-16701
  9. ↑ Muralidhara, A., Bado-Nilles, A., Marlair, G., Engelen, V., Len, C., Pandard, P., Biofuels, Bioproducts and Biorefining, 2018, 1-7
  10. ↑ Tosi, Pierluigi; van Klink, Gerard P. M.; Celzard, Alain; Fierro, Vanessa; Vincent, Luc; de Jong, Ed; Mija, Alice (2018). "Auto-Crosslinked Rigid Foams Derived from Biorefinery Byproducts". ChemSusChem. 11 (16): 2797–2809. doi:10.1002/cssc.201800778. ISSN 1864-564X. PMC 6392144. PMID 29956889.
  11. ↑ Filiciotto, L., Balu, A.M., Romero, A.A, Rodriguez-Castellon, E., van der Waal, J.C., Luque, R., Green Chemistry, 2017, 19, 4423-4434
  12. ↑ Mija, A., van der Waal, J.C., Pin, J-M., Guigo, N., de Jong, E., "Humins as promising material for producing sustainable carbohydrate-derived building materials", Construction and Building Materials, 2017, 139, 594 doi:10.1016/j.conbuildmat.2016.11.019
  13. ↑ Sangregorio, A., Guigo, N., van der Waal, J.C., Sbirrazzuoli, N., "All 'green' composites comprising flax fibres and humins' resins", Composites Science and Technology, 2019, 171, 70. doi:10.1016/j.compscitech.2018.12.008
  14. ↑ Pin, J.M., Guigo, N., Mija, A., Vincent, L., Sbirrazzuoli, N., van der Waal, J.C., de Jong, E., ACS Sustain. Chem. Eng., 2014, 2, 2182-2190
  15. ↑ Mija, A., van der Waal, J.C., van Klink, G., de Jong, E., Humins-containing foam, 2016, WO2017074183A8
  16. ↑ Tosi, P., van Klink, G.P., Celzard, A., Fierro V., Vincent, L., de Jong, E., Mija, A., ChemSusChem, 2018, 11, 2797-2809

Further reading

Singer, Michael J., and Donald N. Munns (2005). Soils: An Introduction (6th Edition). Upper Saddle River: Prentice Hall. ISBN 978-0-13-119019-1.

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