Biomimetic Vanadate and Molybdate Systems for Oxidative Upgrading of Iono- and Organosolv Hard- and Softwood Lignins
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processes Article Biomimetic Vanadate and Molybdate Systems for Oxidative Upgrading of Iono- and Organosolv Hard- and Softwood Lignins 1 2,3, 4, 1 4 Lucía Penín , Matteo Gigli y , Federica Sabuzi * , Valentín Santos , Pierluca Galloni , 4 1 3,5, , 2,3, , Valeria Conte , Juan Carlos Parajó , Heiko Lange * y and Claudia Crestini * y 1 Chemical Engineering Department, Polytechnical Building, University of Vigo (Campus Ourense), As Lagoas, 32004 Ourense, Spain; [email protected] (L.P.); [email protected] (V.S.); [email protected] (J.C.P.) 2 Department of Molecular Science and Nanosystems, Ca’ Foscari University of Venice, Via Torino 155, 30170 Venice Mestre, Italy; [email protected] 3 CSGI—Center for Colloid and Surface Science, Via della Lastruccia 3, 50019 Sesto Fiorentino, Italy 4 Department of Chemical Science and Technologies, University of Rome ‘Tor Vergata’, Via della Ricerca Scientifica, 00133 Rome, Italy; [email protected] (P.G.); [email protected] (V.C.) 5 Department of Pharmacy, University of Naples ‘Federico II’, Via Domenico Montesano 49, 80131 Naples, Italy * Correspondence: [email protected] (F.S.); [email protected] (H.L.); [email protected] (C.C.) Affiliated with (4) via NAST—Nanoscience & Nanotechnology & Innovative Instrumentation Center. y Received: 25 August 2020; Accepted: 13 September 2020; Published: 16 September 2020 Abstract: Recently reported acetosolv soft- and hardwood lignins as well as ionosolv soft- and hardwood lignins were transformed into monomeric aromatic compounds using either a vanadate or a molybdate-based catalyst system. Monomers were generated with remarkable, catalyst-dependent selectivity and high depolymerisation yields via oxidative exo- and endo-depolymerisation processes. Using the vanadate–hydrogen peroxide system on acetosolv pine lignin, vanillin and isovanillin were produced as main products with depolymerisation yields of 31%. Using the molybdate system on acetosolv and ionosolv lignin, vanillic acid was the practically exclusive product, with depolymerisation yields of up to 72%. Similar selectivities, albeit with lower depolymerisation yields of around 50% under standardised conditions, were obtained for eucalyptus acetosolv lignin, producing vanillin and syringaldehyde or vanillic acid as products, by using the vanadate- or the molybdate-based systems respectively. Keywords: oxidative lignin upgrade; catalytic lignin oxidation; vanadate; molybdate; organosolv; ionosolv; lignin; biomimetic 1. Introduction Lignocellulosic biomass received great interest as a sustainable and renewable source of fuel and platform chemicals in recent years [1,2]. Numerous studies target the conversion of cellulose and hemicelluloses into ethanol and other biofuels as well as platform chemicals for the chemical industries [3–5]. In sharp contrast, research on the conversion of lignin has often been limited to its removal from the other two principal biomass components either to enhance their chemical and/or enzymatic valorisation. Conversion of lignin—representing after all 30% of the weight and 40% of the energy content of lignocellulosic biomass and being isolated as a by-product in form of various technical lignins with different characteristics by cellulose-focused processes—is still a challenge [6,7]. Enzymatic and chemical reactions have been proposed for oxidative lignin valorisation. Several biocatalysts, mimics of biocatalysts, and inorganic catalysts have been studied regarding formation of aromatic Processes 2020, 8, 1161; doi:10.3390/pr8091161 www.mdpi.com/journal/processes Processes 2020, 8, 1161 2 of 13 monomers for the chemical industries. Nevertheless, both reductive and oxidative degradation methods have been studied and presented [8–12]. Mechanistic insights obtained using lignin model compounds to simulate the most abundant bonding motifs within the backbones of various technical lignins were only scarcely applicable to the complexity of a lignin oligomeric and/or polymeric material. In case of oxidative degradation, a series of oxidation products is obtained even in case of simple models, pointing at a lack of selectivity of the reactions triggered by the various catalytic sstems. As a noteworthy exception, only the methylrheniumoxide catalyst system has been reported [13,14]. Vanadium-based catalysts have been reported for lignin valorisation, interestingly both for oxidative and reductive depolymerisation approaches [8,9,11,15–20]. Molybdenum catalysts have been widely reported for the oxidative valorisation of technical lignins, especially in form of polyoxometalates (POMs) [21,22]. More recently, mixed, i.e., bifunctional catalyst systems such as copper-vanadium [23] and molybdenum-vanadium [24] systems have been reported for lignin valorisation. In case of enzymatically mediated reactions, obtaining a large panel of different oxidation products—due to the natural evolution of phenoxy radical intermediates—is furthermore associated to a limited biocatalyst lifetime, which constitutes an additional problem for lignin valorisation. Potential ways to tackle both issues consist of supporting and/or encapsulating the enzymes or to mimic the catalytic centre of lignolytic enzymes using stable organometallic complexes that would eventually exhibit tuning possibilities towards a higher product selectivity [25–28]. In an effort to combine such an active centre-mimicking approach with our interest in evaluating the use of non-lignolytic enzymes in lignin valorisation [29], we tested a vanadate (V)-based and a molybdate (Mo)-based catalytic system [V] and [Mo], respectively, as mimics of the reactive vanadate and molybdate centres in bromide peroxidases [EC 1.11.1.18] and xanthine oxidase [EC 1.17.3.2], respectively. The vanadate and molybdate catalyst systems can, based on their reactivity, be eventually considered suitable non-lignolytic biocatalyst mimics for lignin degradation. The reactivity of the catalyst systems holds the promise that they are capable of oxidising lignin in a more selective way than the typical lignolytic enzymes, i.e., laccases or manganese peroxidases due to the different range of electric potentials [30]. The present study deals with the investigation of the oxidation potential of vanadate and molybdate catalysts toward lignin oxidation. Two hardwood and two softwood lignins, isolated from Eucalyptus nitens and Pinus pinaster, in form of both organosolv and ionosolv lignins have been selected. These lignins have been reported and characterised before [31–33] and were chosen as starting materials for this study also because the structural characterization had revealed noteworthy structural differences between the acetosolv and ionosolv lignin of each biomass. These lignins were used in this study without any further refinements or fractional purifications. We refrained in this study from following the wide-spread approach in which the catalyst activity is demonstrated using monomeric, dimeric, or sometimes trimeric lignin models. While this approach allows relatively facile mechanistic understanding of product formation, applicability of the results to whole lignin degradation is often scarce, due to the high density of reactive sites along the lignin backbone prone to undergo inter- and intramolecular reactions not delineable using lignin models. 2. Materials and Methods General information: Reagents and solvents were purchased from Sigma-Aldrich/Merck KGaA, Darmstadt, Germany and Carlo Erba, Milano, Italy, and used without further purification, if not stated otherwise. Acetosolv pine lignin (AP) and acetosolv eucalyptus lignin (AE), as well as ionosolv pine lignin (IPB) and ionosolv eucalyptus lignin IEB were produced and characterised as described elsewhere [31–33]. Ammonium vanadate (V), NH4VO3, and ammonium molybdate tetrahydrate (Mo), (NH ) Mo O 4H O were purchased from Sigma Aldrich/Merck KGaA and used without 4 6 7 24· 2 further purification. Oxidation of lignins: 50 mg of lignin were weighted in a screw cap vial and suspended in 1 mL of a pre-made stock solution containing the catalyst. Stock solutions comprised: (i) 30 µL perchloric acid; Processes 2020, 8, 1161 3 of 13 (ii) 450 µL hydrogen peroxide 30% (w/v); (iii) 2520 µL distilled water; (iv) the vanadate or molybdate catalyst at concentrations of 5%, 7.5%, or 10% (w/w) of liquid phase. Reactions were heated to 80 ◦C for 9 h while being continuously stirred and allowed to cool down to room temperature overnight. Degradation products were isolated in an extraction process. The reaction mixture was centrifuged to separate any solids, and the liquid phase was taken and mixed with an equal volume of ethyl acetate. As internal standard, 50 µL of a solution of 4-ethoxy-3-methoxy benzaldehyde in ethyl acetate at a concentration of typically 50 µm were mixed into the solution. The biphasic system was vigorously shaken for 30 s. The organic phase was separated, dried over MgSO4, and centrifuged. An aliquot was sampled for analysis by gas chromatography coupled to mass spectrometry (GC-MS) as described below. Solid lignin residues were isolated by centrifugation and washed four times using a 1 m aqueous solution of sulphuric acid. Lignin residues were dried to constant weight for 31P NMR analysis as described below. Experiments were run in duplicate by default, and selected experiments were additionally repeated for verification of results. An error margin for solid mass returns/depolymerisation yields of max 6% was encountered. ± Quantitative 31P NMR spectroscopy: In