As a significant component of lignocellulosic biomass, lignin represents a potential source of value-added aromatic chemicals. In this thesis, catalytic systems with earth-abundant metal catalysts such as molybdenum(VI) and hypervalent iodine complexes were developed to selectively break down lignin models into lower molecular weight chemicals under mild conditions. Due to the complexity of lignin, simple lignin model substrates (A to E), representing common linkages in lignin, were used to investigate the catalytic activity/selectivity of these catalysts. With the molybdenum catalysts [7– 11]/SPC/Adogen®464 system (SPC = sodium percarbonate), oxidation of simple β-1 model compound A in acetonitrile showed primarily C-H bond cleavage to form the ketone product, benzoin methyl ether, whereas the Cα-Cβ bond cleavage product, methyl benzoate, was obtained by switching the reaction solvent to benzonitrile. Preference for generating the Cα-Cβ bond cleavage product, i.e. benzaldehyde, can also be achieved with other early to middle transition metal catalysts using H2O2(aq) as the terminal oxidant. Stoichiometric amounts of hypervalent iodine/Lewis acid systems [20a-c] were able to selectively cleave Cα-Cβ bonds to aldehydes with both simple β-1 model compound A and β-O-4 model compound C. In contrast, other lignin model compounds with different linkages were unable to be oxidized to a great extent using these Mo- or iodine-based complexes. The catalytic activity and selectivity of the reported vanadium complexes, copper salts and non-metal system 1-5 on non-volatile organosolv (NVO) lignin was investigated under basic condition. Details of the depolymerisation of lignin were determined by using Gel Permeation Chromatography (GPC) and the two-dimensional NMR technique, quantitative HSQC (q-HSQC) spectroscopy. Vanadium [2] and copper systems were found to be the most active for depolymerization of NVO lignin.

Thesis advisor: Dunwei Wang / As an inedible component of biomass, lignin features rich functional groups that are desired for chemical syntheses. How to effectively depolymerize lignin without compromising the more valuable cellulose and hemicellulose has been a significant challenge. Existing biomass processing procedures either induce extensive condensation in lignin that greatly hinders its chemical utilization or focus on fully depolymerizing lignin to produce monomers that are difficult to separate for subsequent chemical synthesis. Here, we report a new approach to selective partial depolymerization, which produces oligomers that can be readily converted to chemically recyclable polymer networks. The process takes advantage of the high selectivity of photocatalytic activation of the β-O-4 bond in lignin by decatungstate catalysts (DT). In the photocatalytic system, DT works as both catalyst and oxidant, leading to two different reaction pathways, slow and steady bond cleavage pathway, and fast but limited oxidation pathway. The availability of exogenous electron mediators or external oxidants promotes cleavage or oxidation of this bond, respectively, enabling high degrees of control over the depolymerization and the stoichiometry of a key functional group, C═O, in the products. The resulting oligomers can then be readily utilized for the synthesis of polymer networks through reactions between C═O and branched −NH2 as a dynamic covalent cross-linker. Importantly, the resulting polymer network can be recycled to enable a circular economy of materials directly derived from biomass. / Thesis (PhD) — Boston College, 2024. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

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With the dwindling availability of petroleum, focus has shifted to renewable energy sources such as lignocellulosic biomass. Cellulose and hemicellulose are highly utilized components of biomass, and on the other hand, lignin is a plentiful, under-utilized component of the lignocellulosic biomass. Hence, utilization of the lignin component is necessary for the realization of an economically sustainable biorefinery model. Once depolymerized, lignin has the potential to replace petroleum-derived molecules. Further, a catalyst is capable of selectively removing the oxygen atoms without hydrogenating the aromatic components would be valuable. Bimetallic phosphides and zeolites are capable of selectively cleaving CAROMATIC–O bonds from aromatic compounds. In the present study, the applications of a bimetallic phosphides (FeMoP, RuMoP and NiMoP) for CAROMATIC–O bond cleavage and hydrogenation of C=O and C=C bond in the aromatic model compounds (Phenol, furfural, cinnamaldehyde, and CO2) were examined. The Fe:Mo ratio was varied in FeX Mo2−X P catalysts (0.88 to 1.55) to investigate the effect of catalyst acidity and hydrogenolysis capability via first principle calculations. The most acidic material was most selective for phenol to benzene. Further, combination of different transition metals with phosphorus were tested for hydrogenolysis and hydrogenation mechanism of phenol. Additionally, composition effect in RuXMo2−XP (X = 0.8, 1.0 and 1.2) have investigated for furfural and cinnamaldehyde hydrogenation. It was found that tuning in metal combination and composition results in control of binding energy and activation energy barrier which tune the selectivity for desire reaction and reaction pathway. Alternatively, highly active MWW-zeolite nanosheets have recently been explored for depolymerization in lignin. First, binding strength of different lignin dimers (phenolic and non-phenolic) was studied in terms of binding energy and binding mode over different terminated zeolite surface as a function of temperature and solvent. The optimized binding structure of lignin dimers were further considered to study the hydrogenolysis pathways over Al- and Sn-substituted MWW zeolite nanosheets. Generally, it was found that fully hydroxyl terminated surface, phenolic dimers and higher temperature in methanol pro- motes higher binding energy. Moreover, Al-substituted zeolite nanosheet resulted in lowering activation energy barriers significantly to cleave β-O-4 Linkages in Lignin dimers.

Avec la disparition progressive des réserves de carbone fossile, un intérêt tout particulier est aujourd’hui porté sur la valorisation de la biomasse notamment la cellulose. Elle représente une source importante (1,3 Millions de tonnes) et peu onéreuse (< 10 €/kg) de carbone renouvelable. L’utilisation de la cellulose en tant que matière première pour la chimie fine apparait comme une solution attractive tant sur le plan économique qu’environnemental. Néanmoins, la présence de liaisons hydrogène intra et extra réseau lui confère une stabilité élevée (forte cristallinité) la rendant insoluble dans les solvants organiques usuels et dans l’eau. Ainsi, l’hydrolyse de ce polymère en glucose ou oligosaccharides, en présence d’un catalyseur solide est limitée par les interactions catalyseur/cellulose. C’est pourquoi, des prétraitements de la cellulose sont souvent utilisés permettant alors d’augmenter les interactions avec les catalyseurs solides. Toutefois, les méthodes développées dans la littérature sont coûteuses ou néfastes pour l’environnement. L’objectif de cette étude est le développement d’activations physiques de la cellulose, respectueuses de l’environnement, permettant l’hydrolyse de ce polymère en présence d’un catalyseur solide. L’activation de la cellulose est effectuée par ultrasons ou par plasma atmosphérique non thermique. Ces méthodes d’activation permettent d’augmenter considérablement le rendement en glucose en modifiant i) la taille des particules et/ou ii) le degré de polymérisation et/ou iii) la cristallinité de la cellulose. Enfin, à partir des sucres issus de la dépolymérisation de la cellulose, le 5- hydroxyméthylfurfural (molécule plateforme) peut être obtenu. Cette synthèse sera étudiée et plus particulièrement la nature du solvant, qui impacte la sélectivité de cette réaction. Lors de ces travaux, un intérêt tout particulier est porté sur l’utilisation de glycérol et de carbonate de glycérol en tant que solvant. / With the depletion of fossil carbon resources, biomass (including cellulose) is widely introduced in the chemical industry, as a renewable source of carbon. Cellulose is a huge reservoir (1,3 Million tons) of cheap (< 10 €/kg) and non-edible carbon. So use cellulose as raw material has many advantages, as much as economic plan than environmental one. However, due to important inter and intra hydrogen bonds network, cellulose is highly crystalline and thus insoluble in common solvents (including water) and recalcitrant to hydrolysis by heterogeneous catalysis, due to solid/solid interactions. A preliminary step consists in the activation of cellulose to enhance the solid/solid interactions. However, the pretreatments used in the literature are limited by the cost, corrosiveness, and toxicity. The aim of this study is to develop physical pretreatments of cellulose in order to be environmentally friendly and promote cellulose/catalyst interactions. In this manuscript, two physical methods of cellulose activation will be explored. The first involves a sonic treatment and the second implies non-thermal atmospheric plasma technology. These methods lead to an increase of the glucose yield due to the change of i) the particle size, or/and ii) the degree of polymerization or/and iii) the cristallinity. From carbohydrate obtained via the depolymerisation of cellulose, 5-hydroxymethylfurfural (platform molecule) is achieved. This synthesis, including dehydration of fructose, will be studied and especially, the nature of the solvent which is a key point ofthis conversion will be discussed. In this work glycerol or glycerol carbonate-based media were studied, as co-solvent from renewable carbon.

This thesis deals with a new low-energy method of chemical recycling of poly(ethylene terephthalate) (PET) using natural oils as reagents and microwave irradiation to accelerate depolymerization. The results of experiments with PET waste and castor oil, when the reaction mixture was heated in microwave reactor, showed that a complete depolymerization of PET chain has occured. Optimal conditions for the depolymerization PET were established: wt. ratio of PET / castor oil = 1 / 9.7, when the molar ratio of ester bonds of PET / hydroxyl groups of castor oil = 1 / 2.7, catalyst : zinc acetate at wt 1% from the PET mass, reaction temperature ranging from 235 to 245řC and the reaction time 60 min. Decomposition experiments also showed, that microwave irradiation accelerated decomposition of PET. Depolymerization reactionin MW reactor was complete at 6x shorter reaction time than the decomposition in the classically heated reactor. The results of analysis showed that the resulting product,the recyclate, was composed of unreacted castor oil and polyol products, that contained partially or fully esterified structural unit of PET, which were ended by ester-linked units of castor oil.

Polycarbonates are found in a variety of common products in daily life due to their favorable mechanical and electrical properties. In addition, they are widely used in biomedical areas due to their stability and biological inertness. Therefore, the production of polycarbonates became an important industrial process in the past decades. However, the current industrial process usually requires toxic phosgene gas as a starting material. Thus, the environmentally benign route by using metal catalyzed couplings of epoxides and CO_(2) to produce polycarbonates has received attention from researchers. In this dissertation, metal catalyzed CO_(2)/cyclic ether copolymerization, depolymerization of polycarbonates, and the equilibria between polycarbonate and corresponding six-membered cyclic carbonate will be investigated. First, the Co(III) catalyzed copolymerizations of CO_(2) and various epoxides with electron-withdrawing substituents to afford polycarbonates are examined. Comparative kinetic studies were performed via in situ infrared measurements as a function of temperature to assess the activation barriers for the production of cyclic carbonate versus copolymer involving electronically different epoxides: styrene oxide, epichlorohydrin, and propylene oxide. Thermodynamically stable cyclic carbonate byproducts are produced during the course of the reaction from the degradations of propagating polymer chains. The depolymerization reactions of several polycarbonates produced from the completely alternating copolymerization of styrene oxide, epichlorohydrin, propylene oxide, cyclohexene oxide, indene oxide, and cyclopentene oxide with carbon dioxide have been investigated. Various reaction pathways can be found under different reaction conditions, including process involving chain-end backbiting and radical intermediates. Temperature-dependent kinetic studies have provided energy of activation barriers for cyclic carbonate formation. In addition, the generated monomeric materials from the degradation of select polycarbonates show the possibility of chemical recycling of plastic waste. For the copolymers made from CO_(2) and oxetane derivatives, this study focuses on the influence of steric hindrance in the 3-position of the monomer oxetane. The (salen)CrCl/onium salt catalyzed coupling reactions of these oxetane derivatives and carbon dioxide are reported. Depolymerizations of copolymers to their corresponding cyclic carbonates were also studied. In addition, several six-membered cyclic carbonates were synthesized to examine their equilibria between monomeric cyclic carbonates and their corresponding polycarbonates.

The conversion of switchgrass lignin, a renewable source for chemicals and fuels, is investigated using reactions such as depolymerization, hydrodeoxygenation and alkylation. First, the lignin is converted into oils containing phenol, substituted guaiacols and other smaller lignin fragments using formic acid and Pt/C through a batch process. A long reaction time was observed to crucial to yield oils with the highest fraction of lower molecular weight compounds with the lowest O/C and highest H/C molar ratio. Second, the zeolite catalyzed gas phase alkylation of phenol, a model compound for the lignin oil, with propylene was investigated. Zeolite pore topology and acid strength were shown to influence the selectivity for the target product, 2-isopropylphenol. This work shows that the conversion of lignin to useful products is possible and suggests some future work to consider before it can be implemented practically.

Ces travaux de thèse visent à valoriser la lignine, biopolymère aromatique le plus abondant sur terre. Pour cela, la dépolymérisation oxydante de la lignine par voie enzymatique a été explorée afin d’obtenir des synthons aromatiques fonctionnalisés. La laccase et le système laccase médiateur (LMS) ont été sélectionnés comme système enzymatique. Dans une première partie, les paramètres réactionnels (choix du médiateurs, température, co-solvant…) de fonctionnement optimal du LMS ont été déterminés notamment via l’utilisation de molécules modèles de lignine. Ces conditions optimales ont été directement appliquées pour l’oxydation d’une lignine organosolv issue d’une plante herbacée. Une étape de fractionnement organique a été conduite sur la lignine avant oxydation afin d’éliminer les populations à l’origine de réactions de couplage. La lignine a ensuite été oxydée par le LMS en milieu biphasique, puis traitée au peroxyde d’hydrogène. Ce procédé en trois étapes a permis de générer des composés aromatiques monomères à trimères (mis en évidence par chromatographies SEC, HPLC, GC et LC-MS) et a été appliqué avec succès à une seconde lignine issue de conifère. Dans les deux cas, des rendements élevés ont été constatés comparés à ceux obtenus dans la littérature. Après isolation, les composés aromatiques produits pourraient trouver des applications comme précurseurs dans les industries de la chimie fine et des polymères. / This work aims at valorizing lignin, the most abundant aromatic biopolymer on earth. For that purpose, an enzymatic approach for the oxidative depolymerization of lignin was investigated in order to obtain fine chemicals. Laccase and the laccase-mediator system (LMS) were selected for the enzymatic oxidation. In the first part, optimal conditions (type of mediator, temperature, co-solvent…) were determined especially by studying reactions on lignin model molecules. These conditions were applied for the oxidation of an organosolv grass lignin. Prior to the oxidation, an organic fractionation was conducted on the lignin in order to remove the population responsible for radical coupling. Then, the lignin was oxidized by the LMS in a biphasic medium followed by a mild hydrogen peroxide treatment. This three-step process allowed the production of monomeric to trimeric aromatic compounds (as shown by SEC, HPLC, GC and LC-MS) and was successfully applied to a different organosolv lignin coming from hardwood. High yield were obtained in both cases as compared with literature results. After isolation, the obtained aromatic molecules could be of interest as precursors for the fine chemistry and polymer industries.

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