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Editorial ARTICLE Provisionally accepted The full-text will be published soon. Notify me

Front. Chem. | doi: 10.3389/fchem.2018.00577

Editorial: “Nano-(bio)catalysis in lignocellulosic biomass valorization”

  • 1Universidad de Córdoba, Spain
  • 2UMR8247 Institut de Recherche de Chimie Paris (IRCP), France
  • 3Department of Chemistry, Aristotle University of Thessaloniki, Greece

The valorization of lignocellulosic biomass, in the form of forest and agricultural wastes, industrial processing side-streams and dedicated energy crops, towards chemicals, fuels and added-value products has become a major research area with increasing exploitation potential. The efficient and tailored depolymerization of biomass or its primary structural components (hemicellulose, cellulose and lignin) to platform chemicals with varying functionalities, i.e. sugars, phenolics, furans, ketones, organic acids, etc., is highly dependent on the development of novel or modified chemo- and bio-catalytic processes that take into account the peculiarities and recalcitrance of biomass as feedstock, compared for example to petroleum fractions. In many cases, in order to reach the final product, a series of catalytic reactions/process should be applied in a cascade or “one-pot” mode. A representative example is the bio-based polymer PEF (polyethylene furanoate) which can replace petroleum-derived PET (polyethylene terephthalate) for the production of plastics, where the following sequence of reactions should occur: hydrolysis of cellulose to glucose, isomerization of glucose to fructose, dehydration of fructose to hydroxymethylfurfural (HMF), oxidation of HMF to 2,5-furandicarboxylic acid (FDCA), poly-condensation with ethylene glycol.
The most important reactions in biomass conversion include hydrolysis, isomerization, dehydration, hydrogenation, hydrodeoxygenation, hydrogenolysis, oxidation, esterification, ketonization, condensation, and others. Different types of homogeneous and heterogeneous catalysts, as well as biocatalysts (enzymes), with single or dual functionalities (i.e. acidic and hydrogenating, as for example in the acidic zeolite supported Pd, Pt, Ni, etc. catalysts) have been reported so far in the literature, ranging from fundamental catalyst design, to combined optimization of catalyst properties and reaction conditions, to pilot scale validation. Photocatalysis, as well as alternative energy sources (plasma, microwave and ultrasound) have also been explored for the catalytic conversion of biomass and its derivatives.
Based on these premises, the present Special Issue in Frontiers in Chemistry, Section of Green and Sustainable Chemistry, entitled “Nano-(bio)catalysis in lignocellulosic biomass valorization” aims to further contribute to the momentum of research and development in the (bio)catalytic conversion of biomass, by featuring original research papers as well as two review papers, authored and reviewed by experts in the field.
The first review paper provided a comprehensive overview of the recent research on chemical oxidative techniques for the pretreatment of lignocellulosics with the explicit aim to rationalize the objectives of the biomass pretreatment step and the problems associated with the conventional processes. The mechanisms of reaction pathways, selectivity and efficiency of end-products obtained using greener processes such as ozonolysis, photocatalysis, oxidative catalysis, electrochemical oxidation, and Fenton or Fenton-like reactions, as applied to depolymerization of lignocellulosic biomass were summarized with deliberation on future prospects of biorefineries with greener pretreatment processes in the context of the life cycle assessment.
The second review paper discussed the hydrolysis of hemicellulose and the recent advances in the production of furfural. More specifically, the review discussed advances obtained in major production pathways recently explored, splitting them in the following categories: (i) non-catalytic routes, like use of critical solvents or hot water pretreatment, (ii) use of various homogeneous catalysts like mineral or organic acids, metal salts or ionic liquids, (iii) feedstock dehydration making use of various solid acid catalysts; (iv) feedstock dehydration making use of supported catalysts, (v) other heterogeneous catalytic routes. The paper also briefly overviewed current understanding of furfural chemical synthesis and its underpinning mechanism.
The topic of efficient and controlled cellulose hydrolysis, not using enzymes or strong inorganic acids, has attracted considerable interest. To this end, one of the original research papers of this Special Issue focused on the mechanocatalytic depolymerization of cellulose with perfluorinated sulfonic acid ionomers. Very high yields of water soluble sugars (90–97%), mostly as oligosaccharides with a DP up to 11, were obtained under optimized conditions using Aquivion PW98 and PW66, respectively, as solid acid catalysts.
Although, homogeneous and heterogeneous chemo-catalysis can offer high reaction rates as well as other benefits, enzymatic catalysis provides exceptionally high selectivities to the desired products. In a related paper of this issue, it was shown how the enzymatic hydrolysis of organosolv pretreated forest materials can be fine-tuned towards the efficient production of cellobiose, a non-digestible oligosaccharide (NDO), which together with other cello-oligosacharides (COS), are considered as prebiotic candidates that have been related to the prevention of intestinal infections and other disorders for both humans and animals. In this work, the heterologous expression and characterization of two Cellobiohydrolases (CBHs) from the filamentous fungus Thermothelomyces thermophila, and their synergism with endoglucanases (EGs) for cellobiose release from organosolv pretreated spruce and birch, was systematically studied and discussed.
Levulinic acid, being produced from HMF, has been recognized as a very important platform chemical that can be catalytically converted further to a series of functional chemicals. In the present special issue, two papers have focused on the hydrogenation of levulinic acid towards γ-Valerolactone, another very important chemical with a wide range of uses. The first paper deals with the development of silylated zeolitic catalysts (3 wt.% Pt on zeolite Y) with enhanced hydrothermal stability for the aqueous-phase hydrogenation of levulinic acid to γ-Valerolactone (GVL). It was shown that by the use of trichlorosilanes as silylating agents, the hydrothermal stability of zeolite Y can be improved significantly, although an inhibition in the yield of GVL was observed due to blockage of the pores by the silane. The second relevant paper presents a study of nickel modified zeolite optimization as bi-functional catalyst (Ni/HZSM-5) for the vapor-phase hydrogenation of levulinic acid to GVL. The content and state of nickel and its interaction with the zeolite were critical parameters affecting both the acidity of the zeolite as well as the reducibility of Ni. At the best case, the authors reported 99% conversion of levulinic acid and 100% selectivity to GVL at 320oC.
Aldol condensation and C-C coupling reactions in general are also very important in biomass valorization, especially when the targeted products are hydrocarbon based transportation fuels, such as gasoline and diesel. In a relevant paper of this issue, the physico-chemical properties of MgGa mixed oxides derived from the corresponding Layered Double Hydroxides (LDHs), as well as of their reconstructed layered analogues, were studied and correlated to their performance in aldol condensation of furfural and acetone. It was shown that the basicity and the textural properties of the MgGa materials determined their catalytic activity and selectivity while their properties resembled those of MgAl hydrotalcite-based materials.
Fast pyrolysis is one of the most promising thermochemical processes for the direct conversion of biomass into a liquid product, the so called pyrolysis oil or biooil, with gases and char being formed to a lesser extent. Biooil contains water and its organic fraction consists of phenolics, ketones, aldehydes, acids, sugars and other compounds in minor amounts. It is relatively acidic, unstable and not miscible with petroleum fractions, and needs to be upgraded, usually via (hydro)deoxygenation. One of the papers in this issue, describes the continuous hydrodeoxygenation (HDO) of biooil with an in situ sulfided metal oxide catalyst and the effect of reaction temperature (350, 375 and 400◦C) on the deoxygenation activity.
Lignin has gained increased interest within the biorefinery concept as it can act as source of high added value phenolics and aromatics. It can be depolymerized via various types of hydrothermal processes, such as reductive or oxidative hydrogenolysis, or by fast pyrolysis which produces a biooil that contains essentially various alkoxy-phenols. In a relevant paper, the fast pyrolysis of a kraft lignin (byproduct of the production of cellulose pulp from biomass via the Kraft processs) was compared to the catalytic fast pyrolysis using conventional, mesoporous and nanosized ZSM-5 zeolite, aiming at the production of a partially deoxygenated biooil, enriched in alkyl-phenols and aromatics (BTX and naphthalenes). The effect of ZSM-5’s acidity and hierarchical porosity on product yields, selectivity of various compounds and resistance to coking was studied and discussed. In another paper of this issue, monolignols such as sinapyl (SA) and coniferyl (CA) alcohols were linked together with caffeic acid (CafAc) via enzymatic catalysis affording a polymeric network similar with natural lignin. The production of the valuable compound vanillin from isoeugenol under mild conditions was investigated in another paper included in this special issue, using bifunctional nanocatalysts (iron on sulphonated SBA-15) which were prepared by an alternative mechanochemical method.
In summary, the present special issue addresses various representative reactions and processes in biomass valorization, highlighting the importance of developing novel, efficient and stable nano-(bio)catalysts with tailored properties according to the nature of the reactant/feedstock and the targeted products.

Keywords: Lignocellulosic biomass, (Bio)Catalysis, biofuels, platform chemicals, added value products

Acknowledgements
The conception of this Special Issue and its realization with regard to the three guest editors and most of the contributing authors have their bases on the European COST Action FP1306 “Valorisation of lignocellulosic biomass side streams for sustainable production of chemicals, materials & fuels using low environmental impact technologies (LIGNOVAL)” which was implemented in 2014-2018 under the frame of Horizon 2020. The main objective of LIGNOVAL was to develop a solid European multidisciplinary network of researchers able to provide a range of innovative alternatives to the valorisation of lignocellulosic residues to chemicals, fuels and materials making use of environmentally sound protocols from pretreatment/fractionation to conversion to valuable end products. The administrative and editorial support of the Frontiers team has been excellent from the point of designing the issue and throughout its implementation and completion. The time and effort devoted by the reviewers of the articles and their constructive comments are also highly appreciated.

Keywords: lignocellulosic biomass, (Bio)Catalysis, Biofuels, platform chemicals, Added value products

Received: 23 Sep 2018; Accepted: 06 Nov 2018.

Edited by:

Steve Suib, University of Connecticut, United States

Reviewed by:

Renato Grillo, Departamento de Física e Química, Faculdade de Engenharia, Universidade Estadual Paulista “Júlio de Mesqui, Brazil  

Copyright: © 2018 Luque, Len and Triantafyllidis. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Prof. Konstantinos Triantafyllidis, Aristotle University of Thessaloniki, Department of Chemistry, Thessaloniki, 54124, Greece, ktrianta@chem.auth.gr