1,721,303 research outputs found
Selectieve katalytische omzetting van cellulosefracties
No full textNowadays, there is a tremendous interest in greener alternatives to the petroleum-based production of fuels, chemicals and materials in classical refineries. The valorization of lignocellulosic biomass in so-called biorefineries could be an interesting alternative due to the renewable nature of biomass. Lignocellulose is an abundant biomaterial that can be found in cell walls of numerous plants like trees, grasses and energy crops and mainly consists of cellulose, hemicellulose and lignin, which each yield specific value-added products after downstream processing. Due to its rigid and complex nature, lignocellulosic biomass is typically first pretreated prior to its valorization in order to improve the accessibility and reactivity of cellulose and to isolate the different biopolymers and their derivatives. Although up till now research efforts mainly focused on the valorization of (hemi)cellulose, there is a recent interest in the valorization of lignin as well to improve process economics.
This doctoral research was performed within the EU project BIOCORE, in which the industrial feasibility of a biorefinery concept was investigated through an international collaboration of various companies, universities and research centers. The first main objective was to convert cellulose pulp, originating from the pilot plant-scale organosolv fractionation of wheat straw and forestry residues, into useful polyols like sorbitol, sorbitan and isosorbide using chemocatalytic conversion processes. These polyols can subsequently serve as important platform molecules for e.g. the food, cosmetic, pharmaceutical and chemical industries. The second main objective was to elucidate the role of biomass pretreatment in the chemocatalytic valorisation of cellulose, as biomass pretreatment is classically performed prior to enzymatic digestion to the make cellulose more accessible to the enzymes.
First, the role of pretreatment in the Ru/H-USY zeolite-catalyzed hydrolytic hydrogenation of cellulose to sorbitol, mannitol and sorbitan was investigated. The influence of different physical and chemical pretreatments was first investigated in the conversion of pure cellulosic substrates to identify the relative importance of key parameters determining cellulose reactivity, like degree of polymerization, crystallinity and particle size. The degree of polymerization appeared to be the most rate-determining parameter for a fast and selective conversion to hexitols, with crystallinity and particle size being more or less equally important for the conversion of microcrystalline cellulose substrates with a comparable low degree of polymerization. Next, the influence of organosolv pretreatment on the zeolite-catalyzed conversion of wheat straw and forestry residues was investigated. A thorough organosolv pretreatment and delignification appeared crucial to achieve acceptable hexitol yields from these substrates, although a complete purification of the cellulose component is not necessary to achieve hexitol yields comparable to pure cellulosic substrates. In this way, hexitol yields as high as 40% could be obtained.
Secondly, the role of organosolv pretreatment in the H4SiW12O40-Ru/C-catalyzed hydrolytic hydrogenation of cellulose to isosorbide was investigated. The bifunctional catalytic process was first optimized for microcrystalline cellulose and subsequently tested on wheat straw and forestry residues. Isosorbide yields as high as 50% were achieved from microcrystalline cellulose but where close to zero from the raw biomass substrates. However, isosorbide yields could be increased up to 60% after a thorough organosolv pretreatment and delignification, again emphasizing the important role of pretreatment in chemocatalytic biomass valorization processes.
However, upon recycling of the heterogeneous Ru/C catalyst, isosorbide yields dropped below 10% in subsequent runs. Subsequent research revealed a change in Ru/C hydrogenation activity due to the adsorption of H4SiW12O40 anions. After adsorption of heteropoly anions on Ru/C in hydrothermal conditions, sugar hydrogenation is substantially impeded, forcing formed sugars through a furan-based hydrodeoxygenation pathway which ultimately leads to the formation of liquid straight chain alkanes like n-hexane and n-pentane. Bio-derived n-hexane can be used as a technical solvent, building block for chemicals and, if sufficiently renewable and sustainable, as a precursor for greener transportation fuels. Further optimization of the adsorption conditions and reaction conditions enabled remarkable yields as 80% of n-decane soluble products, of which 50% n-hexane. This unexpected, yet innovative outcome of the doctoral research has attracted worldwide acclaim and resulted in a patent application.status: Publishe
Nieuwe metathese strategieën voor duurzame chemie
No full textWorld population growth, together with the increasing industrialization of developing countries, is associated with an expansion of raw materials exploitation to meet the demand for fuels, energy, and chemicals. As these occurrences put an immense pressure on the Earth and its resources, there is an ever-growing need for the development of sustainable chemical processes. Introducing catalysis is an important step towards sustainability.
Olefin metathesis reactions are amongst the most essential catalytic processes in organic chemistry and they have paved the way for new developments in polymer chemistry and the synthesis of fine and commodity chemicals. This powerful technique allows the efficient reorganization of olefin fragments by scission and regeneration of new carbon-carbon double bonds, catalyzed by (ruthenium-based) organometallic complexes. Notwithstanding the fact that olefin metathesis can create new molecules with high atom efficiency under mild reaction conditions with low catalyst loadings, its sustainability character can still be improved.
Previous research in metathesis chemistry has mostly focused on the transformation of petrochemical feedstocks in presence of homogeneous and heterogeneous catalysts. The latter are usually synthesized through elaborate and expensive procedures with high loss of the original catalytic activity. In this doctoral research, it was therefore decided to focus on the development of novel sustainable metathesis strategies, both in terms of the choice of feedstock (biomass derived molecules) and the type of catalyst (practical synthesis of heterogeneous catalysts). After comprehensive study of the literature, several issues have been recognized, and they formed the basis of the research objectives of this dissertation.
A first research objective explored the practical and efficient immobilization strategy of ruthenium-based catalysts. More specifically, non-covalent immobilization of second-generation Hoveyda-Grubbs (HG2) complexes on mesoporous siliceous supports was applied according to a recent soft-immobilization invention of KU Leuven. In contrast to tedious covalent heterogeneous approaches, the proposed soft-immobilization strategy avoids complicated modifications of both the support and the catalytic complex, and is very practical to accomplish. The intense exploration revealed several important criteria with regard to the characteristics and properties of the silica support and metathesis complex for designing stable and active heterogeneous metathesis catalysis. In short, immobilization involves physical interaction of the support with the organometallic complex, through interaction with the silanols. Silica with high surface contents of silanols are therefore required. Adsorption isotherms showed a high affinity of the silica for the complex, preferably the presence of germinal silanols was beneficial. Water removal is essential as it fills the pores, hampering diffusion of organic molecules to the active supported complex, but the water removal procedure (thermal) should be carried out with care. Too high temperature treatments generate reactive siloxanes, which react with the organic complex and deactivate the complex.
Metathesis reactions are usually very fast, and therefore care has to be taken not to end up in the diffusional regime during catalytic testing. The pore structure of the silica indeed was found to have a great impact on the reaction rate. Very porous silica with 3D accessibility showed the best performance, and are able to carry high metathesis complex loadings without compromising the mass transport of molecules to and from the active sites. These micro- and macrokinetic aspects, though common in heterogeneous catalysis, have been overlooked in state-of-the-art studies on heterogeneous catalysis with immobilized complexes, but this study shows a huge impact on both observed selectivity and activity when reactions occur in the different (diffusional and chemical) regime.
After designing the optimal catalyst-support system, the heterogeneous catalyst is evaluated in several metathesis reaction types. Among them, there is one innovative synthesis of novel functional chemicals and materials from alternative feedstocks. Real compounds derived from lignin biopolymers were therefore explored to synthesize less toxic bio-based alternatives for the traditional alkylphenols. As said, methoxylated alkylphenols with an unsaturated alkyl chain (MAPs) were synthesized by cross metathesis of lignin-derived eugenol and olefins in presence of the supported metathesis catalyst. Whereas the additional presence of the methoxy group is proposed to reduce the toxicity of the alkylphenol drastically, the additional unsaturation could benefit the biodegradability of the MAP. High product yields were obtained by cross metathesis of eugenol with internal olefins, starting from a high olefin-to-eugenol reactant ratio.
Metathesis, whether applied homogeneously or heterogeneously, is an elegant reaction type to create new bio-based molecules from the carbohydrate fraction. Whereas recent literature proposed the synthesis of methyl vinyl glycolate (MVG) (a new platform molecule) from sucrose, this dissertation explored the reactivity of the vinyl substituent in this highly functional molecule for the synthesis of new C6-diacids. Self-metathesis of MVG for instance occurred in almost quantitative yields, even in absence of solvent. Hydrolysis of the MVG dimer led to the unsaturated acid 2,5-dihydroxy-3-hexenedioic acid (DHHDA), while subsequent hydrogenation gave access to 2,5-dihydroxy adipic acid. The potential of the unsaturated diacid DHHDA as a polymer building block was demonstrated by co-polymerization with lactic acid, resulting in cross-linked polylactic acid (PLA) with an improved thermal stability, compared to pure PLA. Besides, DHHDA was successfully used for the synthesis of bio-based nylon-6,6 polyamide analogs by co-polymerization with hexamethylenediamine.
The selectivity of metathesis reactions using heterogeneous catalysis is thus related to the mass transport issues and the constraints of the active site. Also the ligand structure around the metal active site may affect the selectivity outcome of the catalysis. This has been nicely demonstrated in the valorization of unsaturated polymers using metathesis as a sustainable solution to manage (industrial) waste streams. 1,4-Polybutadiene, an omnipresent polymer, was recognized here as an excellent substrate for the synthesis of large unsaturated macrocycles (≥ C16) by cyclo-depolymerization in presence of homogeneous Grubbs catalysts. Key criteria to attain high ≥ C16 cyclic oligomer yields are a low polymer concentration (< 0.2 M), a high-molecular weight polymer without vinyl impurities, but most of all the type of catalyst. First-generation Grubbs catalysts selectively synthesize the large cyclic oligomers ≥ C16, whereas second-generation Grubbs catalysts convert these primary products to the thermodynamically favored C12 cyclic oligomer. Unraveling of the reaction mechanism and study of the electronic properties of these two types of catalysts indicated a dominant effect of the electronic properties of the catalyst ligands on product selectivity, rather than catalyst deactivation. These results nicely show the potential of product selectivity control by the ruthenium ligand structures in homogeneous reactions.status: Publishe
Schors in de bioraffinaderij: implementatie van lignine-eerst principes
No full textShifting away from fossil resources constitutes an enormous challenge for our society. For energy purposes, alternatives like wind and solar power are becoming increasingly attractive options. For organic chemicals, materials and fuels, biomass could provide a renewable alternative. In this context, biorefineries are being developed, aimed at converting biomass ultimately into chemicals, materials and fuels.
Lignocellulose, the most abundant form of biomass on earth, is an interesting feedstock for a biorefinery. Lignocellulose is the structural element of plant cell walls and consists of three biopolymers, viz. cellulose, hemicellulose and lignin. It is ubiquitous in the plant kingdom (e.g. wood, grasses, shrubs) and also present in residues from agriculture or forestry. As these residues are abundantly available at low cost, they constitute a favorable feedstock for a biorefinery.
Tree bark is a major waste stream from wood processing industries, and a source of lignocellulose biomass. Currently, bark is mainly burned for energy recuperation or used as mulch in horticulture. Valorizing bark in a biorefinery thus presents an alluring possibility. In this respect, decent knowledge of the bark feedstock is indispensable. For that reason, the barks of six relevant species, viz. poplar, black locust, red oak, willow, Corsican pine and larch, were thoroughly characterized in this dissertation. Anatomical analysis illustrated the structural heterogeneity and various cell types in the different barks. Chemical compositional analyses highlighted the large differences between species. All barks had a substantial lignocellulose content, however, the fraction of (hemi)cellulosic carbohydrates was low (35-44 wt%). The lignin content was rather high (22-45 wt%), and it was found to have a low S/G ratio for hardwood barks (0.3-0.7), and a G type lignin for softwood barks. The fraction of suberin, an aliphatic polyester, was highest is black locust bark (10 wt%). Besides structural components, the studied barks had a high extractives content (14-30 wt%).
As barks generally have a smaller carbohydrate fraction, typical carbohydrate-oriented lignocellulose biorefining strategies (e.g. second generation ethanol, pulp and paper) are less suited for barks. Such lignocellulose biorefining strategies aim at removing lignin as efficiently as possible. However, as lignin is prone to undergo repolymerization reactions, the resulting isolated lignin is highly condensed and unreactive, and thus less suited for further upgrading. To tackle this, lignin-first biorefining strategies that focus on lignin conversion prior to the carbohydrate fraction, have been developed. Such lignin-oriented lignocellulose biorefining could prove very useful for bark valorization, given their typically higher lignin content. One promising example of a lignin-first biorefining strategy is the 'Reductive Catalytic Fractionation' (RCF). In an RCF process, lignocellulose is contacted with an organic solvent (mixture), like methanol, at elevated temperature in presence of an heterogeneous redox catalyst (e.g. Pd/C, Ru/C) in a reducing environment. This effectuates the solvolytic extraction of lignin fragments from the lignocellulose matrix, its further depolymerization and, at the same time, chemical stabilization of the formed intermediates through reductive catalysis. The role of the metal catalyst is hereby crucial, as the reduction of lignin fragments effectively lowers their reactivity towards repolymerization reactions. The outcome of the RCF strategy is thus a low molecular weight lignin oil, amenable for subsequent conversion into chemicals, and a (hemi)cellulose pulp suited for further valorization.
In contrast to this reductive strategy (i.e. RCF), also a review of the literature on oxidative lignin conversion was provided. Such oxidative pathways can provide an interesting tool for the formation of highly functionalized, valuable lignin products. The catalytic systems for the oxidative conversion of dimeric lignin model compounds and isolated lignins were summarized and critically discussed. Next, several challenges regarding the substrate, catalyst and operating conditions were highlighted, and a future perspective on lignin oxidation was offered. Finally, comparing reductive and oxidative lignin-first processing illustrated the advantages of the reductive process (i.e. RCF) for bark biorefining.
In this dissertation, an evaluation was made of the RCF strategy when using bark as feedstock. First, the bark of black locust (Robinia pseudoacacia) was studied. Given the substantial suberin content in this bark, focus was put on both the lignin and the suberin fraction: RCF enabled the extraction and depolymerization of both biopolymers. The thus obtained oil phase contained lignin-derived phenolic mono-, di- and oligomers, as well as suberin-derived, long-chain (bifunctional) aliphatic monomers. The process severity (i.e. temperature and reaction time) was found to govern the extend of both suberin and lignin depolymerization. Parameters involving catalytic hydrogenation (i.e. catalyst type and loading, H2 pressure) did not influence the suberin depolymerization, but affected the stabilization of the lignin fragments and consequently the phenolic monomer yield. Comparing RCF of black locust bark with black locust wood, revealed the resistance to delignification and the lesser extent of lignin depolymerization in bark.
Next, the substrate scope for the RCF biorefinery was expanded by evaluating the barks of ten different species under identical RCF conditions. By using both raw and extractive-free barks, it was found that the extractives minimally affect the lignin conversion. Between different species however, the product outcome varied strongly. The lignin monomer yield ranged from 2 to 19 wt% lignin, with a high selectivity towards 4-n-propanol-subtituted monomers. Next to lignin products, also catechols, resorcinols, pyrogallols and ring-opened flavonoids were observed, likely from the depolymerization of condensed tannins present in barks. Interestingly, the redox catalyst was found to be crucial not only for generating lignin monomers, but also for certain condensed tannins products, notably the ring-opened flavonoids.status: Publishe
Op weg naar performante polymelkzuur polymeren met nieuwe katalytische syntheseroutes
No full textRenewable polymers offer a promising alternative for certain fossil fuel derived plastics and harness potential as well in specialty applications. One of the top 3 polymers in this respect is polylactic acid (PLA). Next to its renewable origin Â# sugars, abundantly encountered in food crops but also in non-edible cellulose Â# PLA is biocompatible and biodegradable. These unique features render this polymer suitable for many custom applications, for instance in medicine (prostheses, drug delivery), next to its role as a suitable replacement for certain forms of polystyrene, polypropylene and polyethylene-terephthalate, e.g. in packaging, fibers and textiles. On the downside, two major bottlenecks threaten the worldwide megaton-scale breakthrough of PLA: i) its production cost and ii) - despite its overall decent properties Â# its brittleness, over-pronounced hydrophobicity and lack of reactive side groups. This doctoral work therefore focused on i) alternative catalytic routes for synthesizing existing PLA monomers in a more straightforward way, and ii) catalytic routes to novel alpha-hydroxy acid building blocks - preferably with reactive side groups - from available sugar resources. Moreover, a preliminary evaluation of the synthesis and reactivity of novel polyesters, obtained by copolymerization of such new monomers with commercial L-lactic acid, is presented. This approach should be seen as way of creating high performance PLA based polyesters. The alternative catalytic routes to existing PLA monomers, developed within this PhD, are not enclosed due to reasons of confidentiality. After a brief introduction and scope, the doctoral manuscript presents a compilation of three peer-review-journal articles. Each chapter can be read separately, but there is a clear connection between the chapters and a logic in their appearance. Chapter 1 offers a full literature review on, and introduction to, the world of lactic acid - the basic building block of PLA. The chapter discusses the role of lactic acid as platform molecule in future bio-refineries. Emerging chemocatalytic routes to lactic acid, a three-carbon alpha-hydroxy acid (AHA), are reviewed, as well as the catalytic conversion of lactic acid to a range of useful chemicals, materials (including PLA) and fuel intermediates. A note on the racemate separation of chemically derived lactates is presented as well, in relation to the requirement for enantio-pure building blocks of PLA chemistry.,, Chapter 2 deals with the synthesis of recently discovered four-carbon alpha-hydroxy acid based esters from tetrose sugars and the section unravels the delicate mechanism of their formation. The functionalized 2-hydroxybutyrates are potential intermediates for tailored solvents and monomers for PLA resembling polyesters. We discovered the unique catalytic activity of soluble tin metal salts for synthesizing methyl vinylglycolate and methyl-4-methoxy-2-hydroxybutanoate. In situ NMR spectroscopy, deuterium labeling and control experiments with intermediates revealed the individual pathways, in which the kinetic competition between a 1,4-nucleophilic addition Â# leading to a 4-methoxy-group Â# and a 1,2-hydride shift Â# transforming glyoxal intermediates to alpha-hydroxy-carboxylates Â# is key to the product outcome. Since tetroses are a rather expensive and Â# on a large scale Â# inaccessible raw material for building block synthesis, Chapter 3 reports on the one-pot transformation of glycolaldehyde into the four-carbon AHAs catalyzed by tin halides. Glycolaldehyde, Â#the smallest sugarÂ#, is encountered in significant amounts in bio-oils, which are obtained by high temperature pyrolysis of (mainly lignocellulosic) biomass. Insight into the multitude of reactions of the complex cascade network is delivered with a focus on identifying the rate determining steps. Knowledge of the mechanism allowed for tuning the optimal Brønsted to Lewis acid ratio of the catalytic system and for unraveling the pronounced effects of the solvent on the rate and selectivity. Moreover, a first proof of concept of the great potential of the AHAs for polymer chemistry was delivered by incorporating vinyl glycolic acid via copolymerization into poly-L-lactic acid based polyesters. The vinyl side group was preserved in the polymerization and the reactive double bond proved accessible to post-synthetic modification via radical thiol-ene chemistry. The versatility of this approach to boost the performance of PLA polymers was demonstrated by grafting thiols onto the vinyls. This proved useful for rendering PLA type materials with an increased (and tunable) hydrophilicity. Moreover, the unfunctionalized copolymers possessed a lower melting point than PLLA but a higher degradation temperature. These features give this copolymer a broader temperature range for thermal processing than classic PLA.status: Publishe
Conversie van biomassa met Sn in gedealumineerde ß zeolieten
No full textThe concept of biorefinery, transforming biological feedstock into valuable chemicals, has gained in interest the last decades due to depleting cheap oil reserves, but also due to an increasing environmental awareness. An important challenge before switching to a biomass-driven society, is to transform biomolecules like triglycerides and lignocellulosics into added-value chemicals efficiently, for which new developments in the chemical processing of such biomolecules are crucial. The design of new catalytic materials, able to efficiently and selectively convert the intended reactions, is an important topic in this progress. Many new or modified catalytic materials with various catalytic entities and capabilities are being researched that can defy the new challenges associated with biomass conversion. Reagent molecules are now more polar and reactive than those we used to convert with acid/base and redox catalysis, and the new reactions are likely to be processed in aqueous conditions at elevated temperatures.
Lewis acidity has always been an important type of catalysis in the fine chemistry, but also in the petrochemistry. In the group of water stable Lewis acid catalysts, Sn-containing materials of which the acid Sn-beta (Snβ) is the most studies member receive a lot of attention currently. Snβ is capable of catalyzing several interesting biomass related reactions, including the isomerization and epimerization of monosaccharides, but is also allows the direct conversion of sugars into lactate esters and condensation C-C coupling chemistry. Furthermore, the materials also display excellent catalytic properties in more organically oriented reactions such as Baeyer-Villiger oxidations and Meerwein-Ponndorf-Verley reductions.
Though Snβ is a proven catalysts, its synthesis is arduous. Long synthesis times and hazardous chemicals are needed to incorporate Sn into the zeolite framework, impeding industrial scale use of the catalytic material. The focus of this doctoral research is to find an alternative and more sustainable synthesis procedure for the Sn-containing catalyst. Previous efforts succeeded in shortening the synthesis times by modifying the traditional hydrothermal procedure, but they still need toxic chemicals. Another approach, practiced here in this work, introduces Sn into commercial beta zeolite structures, a so-called post-synthesis procedure. This method prevents the use of hazardous chemicals and only short synthesis times are needed, but the catalytic activity of materials which have been synthesized following this approach until today, is significantly lower than its hydrothermal counterpart. The research presented in this work therefore attempts a novel synthesis method, combining both synthesis times and practical operations from the post-synthetic procedures without compromising the catalytic properties, characterized by a high activity and excellent catalyst stability.
The novel synthesis procedure grafts Sn precursor salts onto a dealuminated commercial Beta zeolite during reflux in dry isopropanol. Up to 2 wt% of monoatomic active Sn4+ atoms can be introduced into the zeolite framework without formation of extraframework non-catalytically active Sn-species. Essential steps in the synthesis method and requirements of chemicals and conditions were studied and discussed in this work. The catalytic properties of the material were tested in several biomass and fine chemicals related transformations. Most surprisingly, it appears that different reaction types required somewhat different Snβ catalysts for their optimal performance. Therefore, different Sn active sites, monomeric to oligomeric SnOx in nature, are possible in the same beta framework and each reaction type requires a different optimization synthesis procedure.
Biomass conversion usually requires multi-step or cascade type of reactions to form the desired product. One-pot conversions combine many different steps together, which is especially of interest when the intermediate chemicals are very reactive or hazardous. Design of multiple site catalysts is therefore paramount in biomass conversion. The great challenge is to incorporate the different active sites in balance with the requirements of the reaction kinetics. The presented novel synthesis procedure is highly versatile and therefore ideal to design such multi-active site catalysis. Partial, instead of complete, removal of Al in the original structure of Beta allows the presence of Brønsted acidity, next to the Lewis acidity of Sn. This bifunctional catalyst model catalyst showed great potential in the conversion of carbohydrate to lactate esters. In essence, this reaction requires multiple steps, while the reaction rate is determined by a dehydration step. Accelerating this step with the acidity greatly improved the reaction rate. Interestingly, the cooperative effect is only valid when the two different catalytic sites are close to each other, ie. in the same zeolite crystal. Other important reactions like glucose to HMF and levulinic acid might also benefit from the presence of the two sites, though a different balance is anticipated.
Though Snβ has already been discovered in 1997, the characteristics of the genuine active Sn site is under debate. Next to the site characteristics, there is also discussion on the importance of the active site pocket, either containing nearby silanols or not. The latter might assist in coordinating the molecules in the active site pocket or it effects the polarity, and thus the adsorption (and intrazeolitic concentration), of molecules. This work noticed that post-synthetic synthesis leads to a different active Sn site. The Sn-atom is less coordinated to the framework, but therefore shows a stronger Lewis acidity. The site appears like a coordinative binding, with separating acid – base entities due to the constraints in the active site, dictated by the rigid zeolite frame. That Sn sites in the new and old material are different might not come to a surprise, as Sn is incorporated in Al-removal sites in the post-synthesis procedure, while the incorporation of Sn in the hydrothermal synthesis runs differently. Catalytic consequences of these differences were demonstrated by detailed kinetic analysis of two reaction types for which a better activity per Sn was observed in the grafted material. In Baeyer-Villiger oxidation, both entropic (transition state stabilization) and enthalpic (increased Lewis acid strength) effects were noticed, whereas only entropic benefits (site accessibility) were found in case of a bimolecular Meerwein-Ponndorf-Verley reaction.
The new Sn catalyst presented in this work thus shows great catalytic opportunities in biomass conversion. It is not only very active, its production for large scale application is practical. Moreover, the synthesis procedure allows tailoring the catalytic properties, and can be used for other active site types like Zr, Ce and Hf, which can be of great interest in the development of various biorefinery processes.status: Publishe
Katalytische valorisatie van lignine in de bioraffinaderij
No full textLignocellulose, the most abundant biomass constituent, represents a promising renewable resource for the production of biofuels, chemicals and materials. In analogy with a petrochemical refinery, lignocellulose can be processed in a so-called biorefinery. While a large number of biorefinery strategies have been described and investigated, new ones are constantly being developed. Most of these methods enable an efficient valorization of the carbohydrates cellulose and hemicellulose, but lignin is mostly obtained as a low-value, degraded residue or precipitate, that is generally used as energy source to fuel the process. The fact that lignin is the largest renewable source of aromatics and that lignin valorization is regarded as essential for the economics of the biorefinery stimulates a higher-value use of this substrate. In this regard, the conversion of lignin to chemicals presents a very promising valorization route. However the lignin streams generated in most biorefinery schemes are not suited as feedstock for chemicals. Therefore, valorization of lignin into chemicals should be a focal point of future biorefinery research.
This doctoral research presents a lignocellulose biorefinery method that enables an efficient valorization of lignin into phenolic compounds. In this process, denoted as reductive lignocellulose fractionation, lignocellulose is treated in liquid phase under mild reducing atmosphere in the presence of a redox catalyst. The process involves the extraction and depolymerization of lignin from the lignocellulose matrix through combined solvolytic and hydrogenolytic action, yielding a lignin oil rich in phenolic monomers, next to di- and oligomers, and a solid carbohydrate pulp. The carbohydrate pulp can be further processed into chemicals or used for paper or pulp production. In order to efficiently separate the carbohydrate and lignin product families, both a high lignin removal from the pulp (delignification) and high carbohydrate retention in the pulp are important. The choice of solvent is found to significantly impact both the pulp retention and delignification efficiency. By testing a range of bio-derivable solvents with varying properties in the Pd on carbon-catalyzed reductive fractionation of birch wood, it is shown that a high solvent polarity is beneficial for delignification, but that a too polar solvent, like water, gives rise to significant carbohydrate solubilization. In order to express the efficiency of the process as a combination of delignification efficiency and pulp retention, a new empirical descriptor was introduced, denoted as ‘lignin-first delignification efficiency’ (LFDE). Methanol and ethylene glycol reached the highest LFDE values and were therefore indicated as most suitable solvents for the reductive fractionation. Processing in both solvents exhibits substantial differences in process and product characteristics, which are discussed extensively.
Reductive lignocellulose fractionation enables a high yield production of phenolic monomers, which can be further upgraded to various chemicals. The monomers contain ortho-methoxy groups, next to various para-substituents, and removal of the methoxy groups (demethoxylation) is an important step in the upgrading of the phenolic monomers. In this work, the demethoxylation of a class of phenolic monomers, namely substituted guaiacols, is investigated with nickel on oxide catalysts. Two upgrading routes are studied, namely (i) demethoxylation towards substituted phenols and (ii) demethoxylation combined with aromatic ring hydrogenation towards substituted cyclohexanols.
It is shown that in the conversion of guaiacol, most nickel on oxide catalysts show aromatic ring hydrogenation activity, but only Ni on CeO2 and ZrO2 allow a high yield production of cyclohexanol. The conversion of guaiacol to cyclohexanol is demonstrated to proceed through two pathways, with 2-methoxycyclohexanol and phenol as respective intermediates. Transformation of 2-methoxycyclohexanol is the rate determining step and requires a high reaction temperature. For Ni on CeO2, the catalyst activity could be effectively increased by increasing the nickel loading. Remarkably, an increase in average nickel particle size, resulting from an increasing nickel loading, was found to enhance the activity of the accessible Ni sites, which is indicative of structure-sensitive nickel catalysis. Ni on CeO2 enables a selective conversion of various substituted guaiacols, even of real lignin-derived 4-n-propylguaiacol, to the corresponding alkylcyclohexanols. To illustrate a possible valorization of alkylcyclohexanols, the stepwise conversion to alkylcaprolactone was demonstrated, involving dehydrogenation and Baeyer-Villiger oxidation. Alkylcaprolactones can constitute novel building blocks for bio-polyesters.
Selective alkylphenol formation was only possible with Ni on a specific TiO2 support, namely anatase TiO2. Catalysts comprising a rutile or a mixed rutile:anatase TiO2 support exhibited a completely different catalytic behavior, pointing to an important role of the crystal phase in the catalytic transformation. Next to the support crystal phase, the nickel loading was also found to significantly impact the catalytic behavior and only for a well-defined range of nickel loadings, selective alkylphenol formation was observed. This range was denoted as critical loading range. When the nickel loading goes beyond the critical loading range, the product selectivity shifts towards saturated products, i.e. products with a hydrogenated aromatic ring, which implies a change in the nickel phase. Furthermore, the rate of alkylphenol formation was shown to increase with increasing specific surface area of the catalyst, but not with increasing nickel loading within the critical loading range, suggesting that the active sites for the demethoxylation are located on the TiO2 surface. These sites are proposed to be oxygen vacancy sites generated by hydrogen spillover from the nickel particles. The most remarkable finding was that demethoxylation of 4-alkylguaiacols over the Ni on TiO2 catalyst yields mainly 3-alkylphenol, instead of the expected 4-alkylphenol. It is shown that the catalyst exhibits the extraordinary activity to isomerize 4-alkylguaiacols to 5-alkylguaiacols, and convert the latter to 3-alkylphenols via demethoxylation. 3-Alkylphenols like 3-ethylphenol and 3-n-propylphenol are of special interest since they can be used as odor baits for tsetse flies, which constitute the main transmitter of the African sleeping sickness.status: Publishe
Stereocontrole en monomeersynthese voor PLA productie
No full textOligomerisation reactions of lactic acid are studied while using multifunctional catalysisstatus: Publishe
Reductieve katalytische fractionatie in de lignocellulose bioraffinaderij
No full textToday, alternative technologies are being developed to drastically reduce our society’s dependency on fossil resources. In this light, the concept of the biorefinery, in which biomass is converted to chemicals, fuels, and materials, has received a lot of attention. Lignocellulose, the most abundant type of biomass on earth, is regarded as a promising feedstock, though its complex matrix of entangled biopolymers (i.e. cellulose, hemicellulose, and lignin) impedes valorisation. Biorefineries therefore often apply a fractionation strategy to lower the complexity and to allow a tailored downstream conversion of each fraction to targeted products. This approach is for instance being used in the pulp- and paper industry and the 2nd generation bio-ethanol industry, both focused on (hemi)cellulose-based applications.
Unfortunately, lignin-containing side streams are usually regarded as a waste product and incinerated for energy recuperation or electricity production. Since lignin constitutes the largest source of aromatic compounds in nature, its full valorisation potential is nonetheless far from reached. One promising opportunity is the selective depolymerisation of lignin towards (aromatic) chemicals. A major obstacle here is lignin’s strong tendency towards irreversible repolymerisation and degradation. During processing, reactive native lignin is transformed into a condensed and unreactive lignin isolate that is unsuitable for a selective and high yield production of chemicals.
In this PhD, a lignin-first biorefinery approach has been developed that prioritises lignin valorisation at an early stage, when lignin is still in its most reactive native form. The key aspect is to create a process environment in which lignin depolymerisation can be performed without the detrimental consequences of lignin condensation. Hereto, a catalytic stabilisation mechanism in the form of a Ru/C catalyst was introduced during methanol organosolv fractionation under reductive conditions. Simultaneous depolymerisation-stabilisation of solubilised lignin intermediates resulted in a lignin oil with a near theoretical yield of lignin monomers and a high selectivity towards 4-n-propylguaiacol and 4-n-propylsyringol. Next to a soluble low MW lignin oil, reductive catalytic fractionation (RCF) also produces a delignified solid carbohydrate pulp, containing most of the (hemi)cellulose sugars. The processability of the carbohydrate pulp forms an important requisite to justify the improved lignin valorisation with RCF, and was verified by a selective conversion of the pulp to sugar polyols, hereby reusing the Ru/C catalyst in acidified water. After successfully demonstrating the RCF-concept, several key variables were investigated, highlighting the large influence of the lignocellulosic feedstock on the efficiency of lignin isolation and depolymerisation. A screening of various feedstocks (including genetically modified lines of Arabidopsis thaliana) indicated that syringyl-type lignins, as found in hardwoods (e.g., birch, poplar) are preferably used when targeting a maximal production of lignin-derived chemicals.
Apart from effectuating reductive depolymerisation-stabilisation during RCF, the catalyst can also be used to steer the chemical functionality of phenolic side-chain groups. Here, the metal-dependent activity of C–O bond hydrogenolysis was exploited to control the removal (or retention) of the γ-OH-group on the aliphatic side-chain. A remarkable difference was demonstrated between Ru/C (vide supra) and Pd/C, producing respectively propyl- or propanol-substituted methoxyphenols in a high yield and selectivity. Moreover, the overall concentration of aliphatic OH-groups in the lignin oil (monomers, dimers, and oligomers) was almost doubled when using Pd/C instead of Ru/C. A careful selection of the catalyst therefore allows a diversification of the lignin product portfolio, leading to a broader pool of potential applications.
The issue of catalyst recuperation, related to the solid mixture of fine catalyst powder and carbohydrate pulp that is obtained after RCF, was tackled next. A solution was deduced after gaining a fundamental (experimentally-verified) understanding of the catalyst function within each step (i.e. solubilisation, depolymerisation, stabilisation) of the RCF process. It was shown that the applied solvent, being supercritical methanol, is largely responsible for the solubilisation and depolymerisation of lignin, whereas the catalyst is merely essential for the reductive stabilisation of unsaturated intermediates through hydrogenation. The latter is important to minimise lignin repolymerisation and enable high lignin monomer yields. As the catalyst is solely required to interact with soluble lignin intermediates, no physical contact is in principle needed between the lignocellulosic substrate and the catalyst. Based on this information, the use of commercial Ni-Al2O3 catalyst pellets in a reactor basket was tested with success. After performing a parameter optimisation, lignin monomer yields approached those obtained with catalyst powders, and a high selectivity towards propanol-substituted monomers was obtained. In contrast to the normally used catalyst powders, the Ni-Al2O3 pellets were easily recuperated from the reactor basket after the process. This allowed the execution of multiple recycling experiments, which demonstrated a stable catalytic performance of the Ni-Al2O3 pellets upon regeneration with a thermal H2-treatment. In addition, a catalyst-free delignified carbohydrate pulp was obtained, containing most of the hemicellulose and cellulose sugars. This pulp was subsequently converted to bio-ethanol through semi-simultaneous saccharification (accelerase trio enzyme mix) and fermentation (GSE16-T18-HAA1* yeast).status: Publishe
Gesulfoneerde mesoporeuze silica-koolstof nanocomposieten voor de conversie van biomassa
No full textOver the past decades, researchers have made great efforts in the synthesis and functionalization of ordered mesoporous carbon or silica-carbon nanocomposites. These materials have the merits of mesostructures; that is, a high surface area and large pore volume. Meanwhile, the carbon moieties in the framework provide the advantages of organic component, which is hydrophobicity and the ability to be easily functionalized. These materials are highly interesting for catalysis, adsorption, energy storage, drug delivery, sensors, etc.
Based on the mature synthesis of ordered mesoporous silica, generally two strategies, hard-template and soft-template, were applied for the synthesis of mesoporous carbon or silica-carbon nanocomposites. On one hand, the hard-template synthesis refers to nanocasting of a pre-formed hard template (such as MCM-41 and SBA-15) with a carbon precursor, which involves multi-step synthesis operations and is time-consuming. On the other hand, soft-template synthesis directly utilizes the one-pot co-assembly induced by the soft template co-polymer (such as Pluronic P123 and F127). Though the structure-directing co-polymer is essentially the same as those used for mesoporous silica synthesis, the direct synthesis from organic-organic assembly is more difficult. The introduction of a SiO2 precursor such as TEOS can assist in the co-assembly, forming well-ordered mesostructures. To avoid the macrophase separation of silica and carbon precursors and to allow for the fast constructing of a mesostructure, evaporation-induced self-assembly (EISA) approach is employed. Herein, novel silica-carbon nanocomposites with different silica-carbon ratios were synthesized and careful analysis were performed to ascertain the mesophase ordering as well as the entanglement of nano-sized silica and carbon phases in the mesopore walls. The degree of carbon may be varied between 19 to 62 wt%, and their presence creates additional microporosity in the composite material besides the mesopores, forming a very accessible hierarchical pore architecture; larger carbon contents block the mesopores.
The mesoporous silica-carbon nanocomposite precursors were subjected to pyrolysis in inert atmosphere at 400 or 550 oC. Different amounts of phenolic OH and COOH groups on the carbon surface and different cross-linking degrees of carbon were thus attained. These materials were sulfonated with concentrated H2SO4 producing 0.57-0.15 mmol/g SO3H sites, which are bound to the carbon phase, while the ordered mesoporous structure was kept intact in these circumstances due to the strengthening role of SiO2 in the framework walls.
The sulfonated silica-carbon nanocomposites with accessible strong SO3H acid sites and tunable surface properties were applied for several acid-catalyzed reactions, including classic carbocation hydrocarbon chemistry and those related to biomass conversion. The catalyst performance in terms of product yields, the mechanism and kinetics, as well as stability were investigated. The sulfonated mesoporous silica-carbon nanocomposites showed high selectivity for the dimerization of styrene/α-methylstyrene, and the one with the highest mesopore volume exhibited the highest catalytic activity. However, in the ethanolysis of fructose to furans and levulinate esters, a high micropore volume appeared with the best catalytic performance. A simplified reaction scheme was proposed and fitted fairly well with the experimental results upon kinetic modeling, which deepens the understanding of the reaction cascade. Regarding the practical improvement of the synthesis, a novel rapid rotation-evaporation induced self-assembly (ROT-EISA) was proposed for the first time to avoid the laborious work of the conventional EISA via thin-film evaporation. The material synthesized from ROT-EISA resembled the counterpart from conventional EISA in the textural properties, and displayed comparably high catalytic performances for both fructose ethanolysis and sylvan condensation reactions.
In summary, a novel series of mesoporous silica-carbon nanocomposites have been synthesized successfully from tri-constituent EISA. An alternative practical synthesis method using a rotavap process, which has a better chance for upscale, is presented. The composite materials, after sulfonation, can be used in acid catalysis, and show a large potential for instance in the valorization of biomass. Besides catalysis, the highly porous nanocomposite material may also be promising in other applications.status: Publishe
Lage Temperatuur Reactienetwerk van n-Butenen over Zure Zeolieten met MFI Topologie
No full text<w:latentstyles deflockedstate="false" defunhidewhenused="true" defsemihidden="true" defqformat="false" defpriority="99" The one-step catalytic transformation of n-butenes into p-xylene would constitute the basis for a sustainable process allowing conversion of cheap gases into a high value base chemical. According to classical carbenium ion chemistry, this conversion should be possible if catalyzed by solid acids. According to literature, the transformation in presence of sterical constraints, viz. with H-ZSM-5 catalysts, still leads depending on the reaction temperature to a large variety of products, ranging from olefins over alkylnaphthenes to aromatics. The large variety of competing reactions over acid sites explains this behavior.It was the aim of the present study to disentangle the complex reaction network, by working at unusually low reaction temperatures, viz. 220 °C, and by using a typical zeolite catalyst with shape selective properties, reducing the number of molecules in the product slate. For the latter purpose, the most common modifications of the ZSM-5 zeolite framework were made. The samples obtained were then characterized by a pleiade of state-of-the-art techniques, with emphasis on the quantitative description of the acidity spectrum.First a series of ZSM-5 samples with varying number of Brønsted acid sites were prepared by changing the Si/Al ratio of the framework from 12 till 400. All framework Al atoms were distributed homogeneously across the crystals. Only one of the Al-rich samples contained significant amounts of extra-framework Al. Selected samples were subjected to a desilication treatment with an aqueous NaOH solution, showing reduced crystallinity and enhanced mesoporosity. Alternatively, specific ZSM-5 samples were prepared either with an aluminum depleted external surface or with a pronounced Al gradient in the crystals. The n-butenes were converted at 220 °C in a high throughput continuous flow reactor with 16 parallel reactors. The products in such conditions devoid of aromatics were analyzed on-line. To achieve maximum chromatographic separation power, the products were passed over a hydrogenation reactor, allowing determination of the structure of the hydrocarbon skeletons of all products. Important intermediates in the products occasionally were fed to the reactor as feedstock.The acidic properties of the H-ZSM-5 zeolites as determined by 27Al and 29Si MAS NMR, NH3-TPD, FT-IR of silanols and adsorbed pyridine, were found to be consistent among each other, namely, one framework aluminum atom yielding one (Brønsted) acid site. Despite the wide variety of ZSM-5 modifications, the carbon number product distributions can be explained by a oligomerization cracking re-alkylation mechanism. To rationalize quantitatively the selectivity to all products and the distributions in all fractions, the amount of coke remaining after reaction has to be taken into account. From thermogravimetric analysis (TGA) and FT-IR of used H-ZSM-5 catalysts, it follows that coke residues are composed of soft coke (large paraffinic residues) and of hard coke (heavy aromatic residues), bulkier than soft coke. Both specimens are desorbed at significantly different temperatures from the catalysts, the aromatic species being retained at the higher temperatures.H-ZSM-5 catalysts with a high concentration of framework aluminum atoms in the bulk systematically retain an amount of hard coke substantially higher than 3 wt%, sufficient to eliminate the microporosity determined by low temperature nitrogen physisorption. This way, access to acid sites present in the micropores is restricted. Steady state catalysis occurs therefore at the external surface, the mesopores and macropores, yielding distributions in many product fractions that match thermodynamically expected values. Despite the loss of acid sites in the micropores, these catalysts still present high reaction rates, as the aluminum concentration at the external surface is high.On the other hand, H-ZSM-5 materials, with a low concentration of framework aluminum atoms in the bulk, show partially accessible micropores. A hard coke amount lower than 3 wt% is typical for these catalysts. The reaction rate is rather low, as the aluminum concentration is proportional to the concentration of Brønsted acid sites. The selectivity to cracked products is high as a result of the ß-scission of feed oligomers located in the micropores. The product distributions are generally affected by the shape-selectivity exerted by the sites in the intracrystalline voids. Mono-methylbranched molecules with the branching at the chain end, viz. 2-methyl isomers, are abundantly present. This different behavior among the two catalyst classes is again encountered when different C8 feeds are used, the existence of pore mouth effects being more pronounced due to the large size of the feedstocks compared to that of n-butenes. Whereas Al-rich catalysts present blocked H-ZSM-5 micropores restricting the catalysis over the external surface or at the pore mouth, Si-rich H-ZSM-5 show pronounced effects of shape selectivity and consequently the presence of intracrystalline catalysis.status: Publishe
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