A combination of experimental and computational methods to study the reactions during a Lignin-First approach

AbstractCurrent pulping technologies only valorize the cellulosic fiber giving total yields from biomass below 50 %. Catalytic fractionation enables valorization of both cellulose, lignin, and, optionally, also the hemicellulose. The process consists of two operations occurring in one pot: (1) solvolysis to separate lignin and hemicellulose from cellulose, and (2) transition metal catalyzed reactions to depolymerize lignin and to stabilized monophenolic products. In this article, new insights into the roles of the solvolysis step as well as the operation of the transition metal catalyst are given. By separating the solvolysis and transition metal catalyzed hydrogen transfer reactions in space and time by applying a flow-through set-up, we have been able to study the solvolysis and transition metal catalyzed reactions separately. Interestingly, the solvolysis generates a high amount of monophenolic compounds by pealing off the end groups from the lignin polymer and the main role of the transition metal catalyst is to stabilize these monomers by transfer hydrogenation/hydrogenolysis reactions. The experimental data from the transition metal catalyzed transfer hydrogenation/hydrogenolysis reactions was supported by molecular dynamics simulations using ReaXFF

Theoretical Investigations of C–O Activation in Biomass

This thesis focuses on using computational chemistry approaches to study how biobased molecules interact with both homo- and heterogeneous catalysts. The reaction mechanisms of such transformations have also been studied. The first section comprises studies of interactions between organic molecules and a heterogeneous catalyst in the palladium-catalyzed depolymerization of models of lignin derivatives. From experiments, it was proposed that a keto intermediate and its enol tautomer play a significant role in the β-O-4′ bond cleavage. The study in the first section of this thesis has been divided into three parts. First, simplified models of the keto intermediate and its enol tautomer were used to investigate the adsorption to a Pd(111) surface. By using a combination of periodic density functional theory (DFT) calculations and a constrained minima hopping method, the most stable adsorption which is the so-called global minimum, was found to be an enol adsorbed to the surface. In the second part, the study was expanded to cope with models of lignin which were used in experiments. In addition, we studied the effect of adsorbate coverage, where two different Pd(111) super cells were compared. The optimizations were performed via dispersion-corrected density functional theory (DFT-D3). The molecules were found to bind more strongly to the surface at low coverages. These results support the experimental data and show that the tautomerization has an important role during lignin depolymerization.  The third part relates to using a multilevel procedure to study the interaction of fragments derived from lignin depolymerisation with a palladium catalyst in a solvent mixture. Specifically, QM calculations and MD simulations based on the ReaxFF approach were combined to explore the reaction mechanisms occurring on Pd surfaces with lignin derivatives obtained from a solvolysis reaction. The strongest adsorptions were found to be between the aromatic rings and the Pd surfaces. The second section focuses on a Brønsted acid-catalyzed nucleophilic substitution of the hydroxyl group in alcohols. Experimentally, phosphinic acid (H3PO2) was found to be an excellent catalyst for the direct intramolecular substitution of non-derivatized alcohols proceeding with good to excellent chirality transfer. In this section, benzylic alcohols with internal O-, N-, and S-centered nucleophiles were used in the calculations. By using a hybrid functional method, we found a bicyclic transition state where the proton of the H3PO2 protonates the leaving hydroxyl group, and the oxo-group of the same catalyst partially deprotonates the nucleophile. The transition state energies for the reactions were determined computationally. The calculations support an SN2 mechanism, which corresponds to the experimental data where inversion of the stereogenic carbon was observed.At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.</p

Lignin Intermediates on the Palladium Surface : Factors for Structural and Energetic Changes

In this work, dispersion-corrected density functional theory (DFT-D3), has been used to investigate interactions between important intermediates in lignin depolymerization and a palladium catalyst. The keto structure 2-phenoxy-1-phenylethanone and its enol tautomer have been used to model reactive intermediates derived from lignin. To investigate how the adsorption energies are affected by adsorbate coverage, we have used two different Pd(111) super cells; one smaller p(6 × 4) and one larger p(6 × 6). In the gas phase, the staggered conformer of the keto tautomer is more stable than both the eclipsed form of the keto tautomer and as expected much more stable than the E-enol tautomer. However, in interaction with the palladium surface, the E-enol tautomer has a similar binding energy as the keto tautomer. Also, the eclipsed conformer of the keto tautomer is more stable than the staggered conformer of the keto tautomer when adsorbed to the palladium. We found that the coverage, that is concentration of molecules on the surface had a pronounced effect on the adsorption energies. At higher coverage, both the keto and enol models prefer to adsorb on an atop configuration to the surface. Furthermore, we found that both the keto and the enol tautomers bind strongly to the surface through their phenyl rings. Despite the strong binding of the phenyl groups, the enol adsorbs to the surface through a chemisorption by cleavage of the C═C bond, that leads to two types of di-sigma complexes depending on the position of the newly formed Pd–C sigma bonds. The generated complex is a key intermediate in the subsequent depolymerization through cleavage of a C–O bond. Our simulations show that there is an intermolecular repulsion between adsorbates on the surface, and consequently, the molecules were found to bind more strongly to the surface at low coverages (by 8-14 kcal/mol). These results are important for experimental design purposes; as previous experiments have shown that the enol form is key for an efficient β-O-4′ bond cleavage and implies that low concentration reactions are favored

Lignin Intermediates on the Palladium Surface : Factors for Structural and Energetic Changes

In this work, dispersion-corrected density functional theory (DFT-D3), has been used to investigate interactions between important intermediates in lignin depolymerization and a palladium catalyst. The keto structure 2-phenoxy-1-phenylethanone and its enol tautomer have been used to model reactive intermediates derived from lignin. To investigate how the adsorption energies are affected by adsorbate coverage, we have used two different Pd(111) super cells; one smaller p(6 × 4) and one larger p(6 × 6). In the gas phase, the staggered conformer of the keto tautomer is more stable than both the eclipsed form of the keto tautomer and as expected much more stable than the E-enol tautomer. However, in interaction with the palladium surface, the E-enol tautomer has a similar binding energy as the keto tautomer. Also, the eclipsed conformer of the keto tautomer is more stable than the staggered conformer of the keto tautomer when adsorbed to the palladium. We found that the coverage, that is concentration of molecules on the surface had a pronounced effect on the adsorption energies. At higher coverage, both the keto and enol models prefer to adsorb on an atop configuration to the surface. Furthermore, we found that both the keto and the enol tautomers bind strongly to the surface through their phenyl rings. Despite the strong binding of the phenyl groups, the enol adsorbs to the surface through a chemisorption by cleavage of the C═C bond, that leads to two types of di-sigma complexes depending on the position of the newly formed Pd–C sigma bonds. The generated complex is a key intermediate in the subsequent depolymerization through cleavage of a C–O bond. Our simulations show that there is an intermolecular repulsion between adsorbates on the surface, and consequently, the molecules were found to bind more strongly to the surface at low coverages (by 8-14 kcal/mol). These results are important for experimental design purposes; as previous experiments have shown that the enol form is key for an efficient β-O-4′ bond cleavage and implies that low concentration reactions are favored

Lignin Intermediates on Palladium: : Insights into Keto-Enol Tautomerization from Theoretical Modelling

Abstract It has been suggested in the literature that keto-to-enol tautomerization plays a vital role for lignin fragmentation under mild conditions. On the other hand, previous modelling has shown that the adsorbed keto form is more stable than enol on the Pd(111) catalyst. The current density functional theory study of lignin model molecules shows that, in the gas-phase, keto is more stable than enol, but on the Pd surface, we find enol conformers that are at least as stable as keto. This supports the experimental result that the favourable reaction pathway for lignin depolymerization involves keto-enol tautomerization. An energy decomposition analysis gives insights concerning the origin of the fine energy balance between the keto and enol forms, where the moleculeâ\u80\u93surface interaction (â\u88\u927â\u80\u85eV) and the molecular strain energy (+3â\u80\u85eV) are the main contributors to the adsorption energy

Brønsted Acid-Catalyzed Intramolecular Nucleophilic Substitution of the Hydroxyl Group in Stereogenic Alcohols with Chirality Transfer

The hydroxyl group of enantioenriched benzyl, propargyl, allyl, and alkyl alcohols has been intramolecularly displaced by uncharged O-, N-, and S-centered nucleophiles to yield enantioenriched tetrahydrofuran, pyrrolidine, and tetrahydrothiophene derivatives with phosphinic acid catalysis. The five-membered heterocyclic products are generated in good to excellent yields, with high degree of chirality transfer, and water as the only side-product. Racemization experiments show that phosphinic acid does not promote S_N1 reactivity. Density functional theory calculations corroborate a reaction pathway where the phosphinic acid operates as a bifunctional catalyst in the intramolecular substitution reaction. In this mechanism, the acidic proton of the phosphinic acid protonates the hydroxyl group, enhancing the leaving group ability. Simultaneously, the oxo group of phosphinic acid operates as a base abstracting the nucleophilic proton and thus enhancing the nucleophilicity. This reaction will open up new atom efficient techniques that enable alcohols to be used as nucleofuges in substitution reactions in the future