Dichomitus squalens partially tailors its molecular responses to the composition of solid wood

White-rot fungi, such as Dichomitus squalens, degrade all wood components and inhabit mixed-wood forests containing both soft- and hardwood species. In this study, we evaluated how D. squalens responded to the compositional differences in softwood [guaiacyl (G) lignin and higher mannan content] and hardwood [syringyl/guaiacyl (S/G) lignin and higher xylan content] using semi-natural solid cultures. Spruce (softwood) and birch (hardwood) sticks were degraded by D. squalens as measured by oxidation of the lignins using 2D-NMR. The fungal response as measured by transcriptomics, proteomics and enzyme activities showed a partial tailoring to wood composition. Mannanolytic transcripts and proteins were more abundant in spruce cultures, while a proportionally higher xylanolytic activity was detected in birch cultures. Both wood types induced manganese peroxidases to a much higher level than laccases, but higher transcript and protein levels of the manganese peroxidases were observed on the G-lignin rich spruce. Overall, the molecular responses demonstrated a stronger adaptation to the spruce rather than birch composition, possibly because D. squalens is mainly found degrading softwoods in nature, which supports the ability of the solid wood cultures to reflect the natural environment.Peer reviewe

Silanes, Silicon(II) and Germanium(II) Anions Bearing Nitrogen Heterocycles: Stepping Stones towards Novel Ligands for Base-Metals

Low-valent silicon(II) compounds have recently emerged as a promising class of strong donor ligands for transition metals. N-heterocyclic silylenes (NHSis) are the heavier analogues of the widely-used N-heterocyclic carbenes (NHCs). Silylene ligands have been employed in a wide variety of catalytic reactions. In particular catalytic C–C cross-coupling reactions. Silylenes have shown to be promising alternatives to NHCs and phosphines. Related Si(II) ligands are anionic silanides (R3Si–), which are formally isoelectronic to phosphines and hence can be expected to find similar applications as stronger donor analogues. Recent studies have shown that electron-withdrawing N-heterocycles confer significant stability to the intrinsically reactive Si(II)– center by induction. In this thesis, the use of pyrrole-derivatives to stabilize silanides was investigated. Hydrosilanes bearing the dippIMP (dippIMPH = 2‑(N‑(2,6‑diisopropylphenyl)iminomethyl)pyrrole) substituent were synthesized. Somewhat surprisingly, these silanes undergo an intra‑molecular hydrosilylation reaction of the imine forming the bidentate aminomethylene (dippAMP) substituent, hampering their use as Si(II) precursors. However, this system allows for investigation of this catalyst-free hydrosilylation reaction. Detailed studies on the reaction of the monosubstituted (dippIMP)SiCl2H revealed distinct reaction steps allowing for the proposal of a reaction pathway involving a redistribution of substituents. A synthesis method that does not require hydrosilanes as a synthetic intermediate was sought in a method where the silicon atom is first bound to iron as an –SiCl3 ligand, followed by chloride substitution at silicon to form the ligand on the metal. Substitution for N-heterocycles afforded the pyr3Si–, (MI)3Si–, and (MP)2ClSi– ligands. Attempted substitution for tmim (tmimH3 = tris(3-methylindol-2-yl)methane) on Cl3Si–Fe(CO)4– and dippIMP on Cl3SiFp gave rise to unexpected complex reactions. The silanide (tmim)Si– was found difficult to access through the corresponding tetrahedral silanes (tmim)SiH and (tmim)SiCl intermediates. Alternatively, (tmim)Si– was synthesized through nucleophilic substitution for tmim on a Si(II)Cl2 precursor. (tmim)Si– undergoes direct complexation to the base-metal salts CuCl and FeCl2. The substitution on a Si(II)Cl2 precursor offers an interesting alternative method for the synthesis of free silanides and is anticipated to find more applications in this field in the future. The series (tmim)E (E = Si, P) was expanded with substitution for tmim on GeCl2·dioxane. In contrast to (tmim)Si–, (tmim)Ge– is reluctant to coordinate to FeCl2 to form the germyl dichloro iron complex. The reaction of (tmim)EK (E = Si, Ge) with Fe2(CO)9 afforded the germyl and silyl iron tetracarbonyl, akin to their phosphine analogue (tmim)PFe(CO)4. The electron donating properties of (tmim)Ge– were shown to be in between those of (tmim)P and (tmim)Si–. The knowledge obtained from this work is anticipated to further aid in the controlled synthesis of N-substituted silicon compounds. In particular the substitution on the Si(II)Cl2 precursor potentially allows for the formation of structures that would otherwise be difficult to access. The low-coordinate silyl iron chloride complex warrants research into its use in C–C cross coupling catalysis. Overall, the findings in this thesis contribute to the understanding of heavier group-14 analogues of carbenes and carbanions and promise to be instrumental for further development of this promising class of ligands

Metallocarbene Artificial Enzymes: Extending Transition Metal Selectivity and Protein Activity

A series of new semi-synthetic metalloprotein hybrids were created via the covalent binding of organometallic species in the active site of lipases, accordingly resulting in the first active site-directed (ASD) homogeneous artificial metalloenzymes. The use of this method promises the generation of a 2nd coordination sphere by the protein scaffold over an incorporated transition metal complex, thereby promoting stereodirecting properties, i.e. catalytic selectivity, and water solubility to the organometallic fragment, and diversifying the hosting enzyme’s activity. A review of the different metalloenzyme hybridization strategies is given in Chapter 1, with comparisons between their corresponding advantages. The ASD method in combination with N-heterocyclic carbene (NHC) ligands is proposed as a versatile strategy due to the good catalytic activity and water-tolerance of a myriad of known metal(NHC) species. In Chapter 2, two covalent ASD Rh(NHC)-lipase hybrids were created. These and other hybrids of the thesis were characterized by ESI-MS and by the resulting catalytic activity. A Rh-cutinase hybrid showed catalytic activity in the hydrogenation of the ketone acetophenone and the olefin methyl 2-acetamidoacrylate with an enhanced chemoselectivity towards the olefin promoted by the protein scaffold compared to unsupported Rh catalyst. A rhodium-CalB hybrid showed pronounced chemoselective behavior due to the deeper location of the lipase’s active pocket. In Chapter 3, Grubbs-Hoveyda II complexes were reacted with cutinase to form the first covalent ASD artificial enzymes for olefin metathesis. Important metal-protein interactions were observed, like hindered hybridization when bulky N-substituents in the NHC ligands were used and protein-metal distance influence over the catalytic activity with successfully formed hybrids. The ring-closing metathesis (RCM) of N,N-diallyl p-toluenesulfonamide and the cross metathesis (CM) of allylbenzene were achieved, the latter representing the first example of formal CM with metalloenzymes. In Chapter 4, the catalytic asymmetric allylic alkylation of allyl aryls was explored with the first covalent artificial metallo-enzyme based on palladium. PPh3 was needed as an additive to achieve catalytic activity. Its role was studied, discarding Pd(NHC)-protein cleavage and showing activation of the Pd center. Inactive in the alkylation of 1,3-diphenyl allyl acetate with malonate due to sterical hindrance, the hybrids were however active in the alkylation of the smaller 1-phenylallyl acetate. In this case, the stereodirecting influence of the protein scaffold drastically inverted the linear/branched ratio of the products in comparison with unsupported Pd catalysts. In Chapter 5, it was pursued to complement the promotion of a second coordination sphere over a Rh(NHC) hydrogenation catalyst by its compartmentalization between two enzyme hosting units, forming the first monometallic ditopic artificial enzyme. Catalytic studies and computational modeling of the hybrid showed that the catalytic center retained its racemic and chemoselective character as if it were not supported, due to large metal-protein tail allowing for flexibility in the protein-metal-protein conformation. In conclusion, the first covalent homogeneous ASD metalloprotein catalysts have been created. This hybridization method shows its effectiveness for the anchoring of a single catalytic fragment in a family of enzymes and the formation of strong protein-metal interactions, generating catalytic chemoselectivity and regioselectivity

Nickel Complexes of Diphosphine Ketone and Imine Ligands: Metal-Ligand Cooperation and Application in Hydrosilylation and Alkyne Cyclotrimerization Reactions

Recent years have witnessed the application of homogenous catalysts in many chemical transformations, impacting chemistry in both industry and academia. The sustainability of the catalysis itself can however still be a point of improvement, as many of the efficient catalytic transformations rely on expensive, rare and generally relatively toxic metals. The transition to more sustainable alternatives such as nickel can benefit from the design of new types of molecular catalysts in which an organic part (a ligand) cooperates with the metal to facilitate chemical reactions. Such metal-ligand cooperation can for example arise when relatively weakly binding π-ligands such as imines (C=N) and ketones (C=O) are covalently tethered to strongly binding phosphorus ligands. In this thesis, the utility of this kind of ligands in Ni-catalyzed reactions is investigated. The role of the side-on π-coordinated C=O and C=N sites is extensively studied by both experimental and computational analysis. First, the metal-ligand hemilability at tethered ketone π-acceptor ligands can be used as a promising strategy in nickel catalysis, improving the activity and selectivity of a particular transformation. The ketone ligand shows versatile coordination at its π-acceptor site of which the binding mode responds to the electronic properties of the metal center and the specific requirements of elementary steps. The adaptive character of the ligand provided by the π-acceptor C=O moiety opens up mechanistic pathways that lead to an enhanced catalytic performance in alkyne cyclotrimerization, as demonstrated by comparison with various related Ni–complexes for which this specific π-hemilabile reactivity is not accessible. In addition, the ability of the ligand to adapt its coordination mode according the nature of the substrate/coligand used is an interesting property to further explore for the development of catalytic protocols involving other types of substrates, potentially opening up new venues in cooperative catalysis. Secondly, the general applicability of a nickel complex of a diphosphine–imine ligand in an industrially relevant process such as alkene hydrosilylation is highlighted, showing compatibility towards a broad range of olefins containing sensitive group functionalities. Furthermore, mechanistic investigations reveal that PPh3 positively affects the selectivity of the hydrosilylation. This finding may have general implications in hydrosilylation reactions: the choice of the coligand can potentially affect the outcome of the reaction, both in term of activity and selectivity. In addition to the contribution of PPh3 in hydrosilylation reactions, the synthesis and reactivity of the diphosphine–aminosilyl derivative of the Ni–catalyst offer a better mechanistic understanding of the reaction. Remarkably, the aminosilyl unit is the reactive site in transformations involving hydrosilane substrates; it suggests that transient Si–N bond formations are of importance in the catalytic hydrosilylation performance of nickel complexes of nitrogen-containing ligands. The findings that both the aminosilyl and the PPh3 fragments plays a role in the catalytic performance of hydrosilylation processes can open up opportunities for the design of new types of metal catalysts of nitrogen-containing ligands. A careful design of organometallic compounds that incorporate both of these concepts can become key features for the optimization of metal catalysts towards a certain reactivity

Tandem catalytic aromatization of volatile fatty acids

The transition towards a circular economy requires closing the carbon loop, e.g. by the development of new synthesis routes to valuable intermediates and products from organic-rich waste streams. Volatile fatty acids (VFA) can be fermentatively produced from wastewater and serve as circular platform chemicals. We show that these VFA can be catalytically upgraded to light aromatics (i.e., benzene, toluene, ethylbenzene and xylenes, BTEX) via a tandem catalytic reaction involving TiO2-catalyzed ketonization and zeolite ZSM-5 catalyzed aromatization. Including this intermediate ketonization step is demonstrated to be much more efficient than direct VFA aromatization, as direct acid conversion mainly gave rise to short-chain olefins by decarboxylation and low BTEX yields of 1%. A one-reactor, tandem catalytic conversion instead significantly improved the yield to 45% when zeolite Ga/ZSM-5 was used. Furthermore, the effect of VFA-derived ketone composition, a process parameter set by the fermentation process, on aromatics production efficiency and product distribution was found to be very pronounced for zeolite Ga/ZSM-5, but not for non-promoted zeolite HZSM-5. This suggests a different reaction mechanism to dominate on zeolite Ga/ZSM-5, involving dehydration on the Brønsted acid sites and cyclization/aromatization on the Ga sites. Finally, water, expected to be present in the feed during VFA upgrading, caused the activity of zeolite Ga/ZSM-5 to drop reversibly, but also led to lower coke buildup. Analysis of the spent catalyst with solid-state 27Al nuclear magnetic resonance spectroscopy and temperature-programmed reduction with H2 showed that the catalyst structure remained intact, also with water present in the feed. Together, the results demonstrate that catalytic ketonization/aromatization is an attractive circular approach for converting waste-derived carboxylic acids into renewable aromatics

Recent developments in catalysis with Pickering Emulsions

Pickering emulsions (PEs), emulsions stabilized by solid emulsifiers, are already of great importance for the food, pharmaceutical and biomedical industry. More recently, PEs are also being increasingly used as advanced catalytic systems for green chemical transformations. These efforts aim to combine the green credentials of biphasic catalysis with the intrinsic advantages offered by PEs, which include increased stability and interfacial area. Here, we provide a review of the recent advances in the field of PE catalysis, emphasizing the developments in the design of (functional) solid stabilizing particles, the range of accessible catalytic reactions and reaction conditions, as well as advances in reactor engineering, such as the application of PE catalysis in continuous flow systems

Toward Catalytic Ketonization of Volatile Fatty Acids Extracted from Fermented Wastewater by Adsorption

Volatile fatty acids (VFA) produced by fermentation of organic-rich wastewater streams can, after efficient recovery from the dilute fermentation broth, serve as a circular source of carbon and be catalytically upgraded into various valuable platform molecules. Waste-derived VFA, that is, a mixture of acetic, propionic, and butyric acids, can thus be converted into mixed ketones, which in turn are valuable intermediates for light aromatics synthesis. Here, an integrated process is presented for the recovery and in-line catalytic conversion of VFA extracted from a fermentation broth by adsorption on a nonfunctionalized resin adsorbent. Gas-phase ketonization of the VFA was studied with and without co-fed water, which is inevitably coextracted from the broth, over TiO2 anatase catalysts to assess catalyst performance, including stability as a function of time on stream. While VFA conversion over bare TiO2 at 375 °C proceeded at 90% conversion with 100% selectivity to ketones, the presence of water in the feed resulted in an activity drop to 40%. Catalyst stability toward water could be greatly improved by dispersing the titania on a hydrophobic carbon support. The carbon-supported catalyst showed superior performance in the presence of excess water, providing a quantitative yield toward ketones at 400 °C. The approach thus allows coupling of VFA recovery from a fermentation broth with successful catalytic upgrading to mixed ketones, thus providing a novel route for the production of value-added products from waste streams

Nickel-Catalyzed Alkyne Cyclotrimerization Assisted by a Hemilabile Acceptor Ligand: A Computational Study

π-coordinating units incorporated in the supporting ligand of an organometallic complex may open up specific reactive pathways. The diphosphine ketone supported nickel complex [(p-tolL1)Ni(BPI)] (p-tol1; p-tolL1 = 2,2′-bis(di-p-tolylphosphino)benzophenone; BPI = benzophenone imine) has previously been shown to act as an active and selective alkyne cyclotrimerization catalyst. Herein, DFT calculations support an adaptive behavior of the ligand throughout the catalytic cycle, several elementary steps being assisted by coordination or decoordination of the C═O moiety. A comparison with related bi- and tridentate phosphine ligands reveals the key role of the hemilabile π-acceptor moiety for the catalytic activity and selectivity of p-tol1 in alkyne cyclotrimerization

Tandem Catalysis with Antagonistic Catalysts Compartmentalized in the Dispersed and Continuous Phases of a Pickering Emulsion

Tandem catalysis combines multiple conversion steps, catalysts, and reagents in one reaction medium, offering the potential to reduce waste and time. In this study, Pickering emulsions-emulsions stabilized by solid particles-are used as easy-to-prepare and bioinspired, compartmentalized reaction media for tandem catalysis. Making use of simple and inexpensive acid and base catalysts, the strategy of compartmentalization of two noncompatible catalysts in both phases of the emulsion is demonstrated by using the deacetalization-Knoevenagel condensation reaction of benzaldehyde dimethyl acetal as a probe reaction. In contrast to simple biphasic systems, which do not allow for tandem catalysis and show instantaneous quenching of the acid and base catalysts, the Pickering emulsions show efficient antagonistic tandem catalysis and give the desired product in high yield, as a result of an increased interfacial area and suppressed mutual destruction of the acid and base catalysts

Phase-Dependent Stability and Substrate-Induced Deactivation by Strong Metal-Support Interaction of Ru/TiO 2 Catalysts for the Hydrogenation of Levulinic Acid

The choice of support type has a profound influence on catalyst performance in liquid phase hydrogenation reactions, including the catalytic hydrogenation of biomass-derived levulinic acid (LA) to γ-valerolactone (GVL). Catalytic performance, including stability, of three Ru/TiO 2 catalysts, having a similar mean Ru metal nanoparticle size but supported on three types of TiO 2 , namely P25, rutile and anatase, is evaluated by multiple reuse under batch reactor conditions. T3he catalysts’ physicochemical properties before and after recycling are characterized by XRD, STEM, TGA and FT-IR after CO stepwise adsorption. The results show that the deactivation seen for (mixed) anatase-supported catalysts in dioxane can be attributed to strong metal-support interaction (SMSI) rather than coke formation or metal sintering, with the rutile-based catalyst being more resistant against such support reduction. Notably, SMSI formation under the applied, relatively mild conditions only occurs in the presence of organic acids, such as LA or valeric acid