318 research outputs found
Astaxanthin: A Comparative Case of Synthetic VS. Natural Production
Astaxanthin, the âking of carotenoidsâ has been widely used as an animal feed additive for several decades, mainly in the aquaculture industry. Recent studies have led to its emergence as a potent antioxidant available for human consumption. Traditionally it has been chemically synthesized, but the recent market interest has generated interests in producing it naturally via yeast (Phaffia rhodozyma) fermentation, or algal (Haematococcus pluvialis) induction. This work aims to compare these production processes and their impact on the economical, environmental, and societal scale. We also look at the attempts of increasing production yields by altering various parameters during all three production processes. Ultimately, the decision of sustainable practices in producing carotenoids like astaxanthin involves sacrificing yield/potency for a greener product life cycle
Synthesis of chiral zirconium-based metal-organic frameworks as solid catalysts in asymmetric carbon-carbon coupling reactions
Comprehensive understanding of chirality has played a crucial role for ensuring safety and efficacy of drug products. In many cases, two optical configurations of a chiral molecule exhibit substantially different physiological behaviour, and thus the preparation of single enantiomers has become as an essential topic in the pharmaceutical industry.1-2 Enantiomerically pure compounds could generally be achieved by separation from racemic mixtures or direct synthesis of enantiopure molecules. Either way, chiral materials which are employed as stationary phase in chiral columns or chiral catalysis, are a basic condition to decide to enantiomeric excess of resulting mixtures. Despite obtaining high enantiomeric purity, the chiral separation of racemic mixtures is considered as an expensive and ineïŹcient approach due to undesired enantiomers, while asymmetric synthesis, which enables dominant formation of the single enantiomers, is an atom-economical method. However, the development of efficient heterogeneous chiral catalysts has been still required further investigations to provide more potential options for asymmetric organic reactions, especially carbon-carbon bond formations, which are key steps in organic synthesis.1-3
In recent years, metal-organic frameworks have emerged as one of the most intriguing solid porous materials. Together with the highly active catalytic centers, wide structural and functional variations, MOFs have been successfully employed as heterogeneous catalysts for a variety of organic transformations.4-5 However, very few achievements relating to MOFs as asymmetric catalysts have been reported to date because of their low thermal and chemical stabilities. Such solid stable frameworks, the Zr-MOFs offers great opportunities for designing novel effective asymmetric catalysts.1, 6-9 This is an interesting, but also challenging topic with many open issues:
âą How can we introduce effectively enantiopure active sites into Zr-MOFs?
âą Are there any positive or negative impacts of Zr-nets on the performance of chiral catalytic sites?
âą If any, is it possible to control these effects during the reaction phase?
âą How is the recyclability of these chiral Zr-MOFs?
Finding answers for these questions are the core of this thesis. In Chapter 3, DUT-67, an 8-connected zirconium and 2,5-thiophenedicarboxylate based MOF, was post synthetically functionalized by L-proline via solvent assisted linker incorporation to obtain a chiral base catalyst. The parent monocarboxylate could be almost completely exchanged by L-proline after 5 days of treatment. The resulting chiral DUT-67, DUT-67-Pro, was demonstrated to be a promising heterogeneous catalyst for the asymmetric Michael addition of cyclohexanone to trans-ÎČ-nitrostyrene with excellent yield (up to 96%) and enantioselectivity comparable to that of L-proline in homogeneous reaction (ee approximately 38%). The Zr-MOF could be reused at least 5 times without substantial degradation in crystallinity or catalytic activity. No leaching of catalytically active species into the liquid phase was detected over 5 cycles.
A further understanding regarding the role of catalytic active sites, including Zr-clusters and L-proline, in asymmetric aldol addition of cyclohexanone and 4-nitro-benzaldehyde is investigated in Chapter 4 to clarify the predominant formation of syn-products as well as the absence of enantioselectivity in previous catalytic systems. The presence and location of L-proline into DUT-67 was confirmed by Solid-state MAS and DNP NMR data. The chiral DUT-67-Pro catalyst exhibits an excellent catalytic activity at low temperature (298 K) with an unprecedented syn-(S,S)-product selectivity in an asymmetric aldol addition reaction of cyclohexanone to 4-nitrobenzaldehyde (yield = 95%, ee = 96%). Comparative catalytic studies using a molecular Zr6-cluster model compound indicate the Zr6-moiety to be responsible for this inverse diastereoselectivity compared to well-established L-proline organocatalysis and a mechanism is proposed to explain the Zr6-cluster-mediated syn-selectivity. Masking residual acidic active sites in the cluster of the framework was found to be a key prerequisite to achieve the high enantioselectivity. The purely heterogeneous catalytic system based on DUT-67-Pro is highly stable and can be recycled several times.
Lastly, a novel chiral diimine Zr-MOF, namely DUT-136, synthesized from one-pot reaction of ZrCl4 with 4-formylbenzoic acid, and (R,R)-1, 2-diphenylethylenediamine as an enantiopure core will be described in Chapter 5. Inspired from the versatile transformation of the C=N double bonds, a variety of post-synthetic methods, including oxidation, reduction, and metalation, was employed to modify DUT-136 for formation of the chiral amide-, amine-, and Ni-DUT-136, respectively. The catalytic behaviour of these post-synthetically modified materials was then evaluated in a wide range of asymmetric organic transformations, including the Friedel Craft alkylation, the Michael addition, the aldol reaction and the Ni-catalyzed C-C coupling.
The research on synthesis of chiral Zr-MOFs and their catalytic behavior in this work are expected to provide a better understanding or at least give to other scientists open ideas for further deeper studies regarding this topic in the future
Compression vidéo basée sur l'exploitation d'un décodeur intelligent
This Ph.D. thesis studies the novel concept of Smart Decoder (SDec) where the decoder is given the ability to simulate the encoder and is able to conduct the R-D competition similarly as in the encoder. The proposed technique aims to reduce the signaling of competing coding modes and parameters. The general SDec coding scheme and several practical applications are proposed, followed by a long-term approach exploiting machine learning concept in video coding. The SDec coding scheme exploits a complex decoder able to reproduce the choice of the encoder based on causal references, eliminating thus the need to signal coding modes and associated parameters. Several practical applications of the general outline of the SDec scheme are tested, using different coding modes during the competition on the reference blocs. Despite the choice for the SDec reference block being still simple and limited, interesting gains are observed. The long-term research presents an innovative method that further makes use of the processing capacity of the decoder. Machine learning techniques are exploited in video coding with the purpose of reducing the signaling overhead. Practical applications are given, using a classifier based on support vector machine to predict coding modes of a block. The block classification uses causal descriptors which consist of different types of histograms. Significant bit rate savings are obtained, which confirms the potential of the approach.Cette thĂšse de doctorat Ă©tudie le nouveau concept de dĂ©codeur intelligent (SDec) dans lequel le dĂ©codeur est dotĂ© de la possibilitĂ© de simuler lâencodeur et est capable de mener la compĂ©tition R-D de la mĂȘme maniĂšre quâau niveau de lâencodeur. Cette technique vise Ă rĂ©duire la signalisation des modes et des paramĂštres de codage en compĂ©tition. Le schĂ©ma gĂ©nĂ©ral de codage SDec ainsi que plusieurs applications pratiques sont proposĂ©es, suivis dâune approche en amont qui exploite lâapprentissage automatique pour le codage vidĂ©o. Le schĂ©ma de codage SDec exploite un dĂ©codeur complexe capable de reproduire le choix de lâencodeur calculĂ© sur des blocs de rĂ©fĂ©rence causaux, Ă©liminant ainsi la nĂ©cessitĂ© de signaler les modes de codage et les paramĂštres associĂ©s. Plusieurs applications pratiques du schĂ©ma SDec sont testĂ©es, en utilisant diffĂ©rents modes de codage lors de la compĂ©tition sur les blocs de rĂ©fĂ©rence. MalgrĂ© un choix encore simple et limitĂ© des blocs de rĂ©fĂ©rence, les gains intĂ©ressants sont observĂ©s. La recherche en amont prĂ©sente une mĂ©thode innovante qui permet dâexploiter davantage la capacitĂ© de traitement dâun dĂ©codeur. Les techniques dâapprentissage automatique sont exploitĂ©es pour but de rĂ©duire la signalisation. Les applications pratiques sont donnĂ©es, utilisant un classificateur basĂ© sur les machines Ă vecteurs de support pour prĂ©dire les modes de codage dâun bloc. La classification des blocs utilise des descripteurs causaux qui sont formĂ©s Ă partir de diffĂ©rents types dâhistogrammes. Des gains significatifs en dĂ©bit sont obtenus, confirmant ainsi le potentiel de lâapproche
Multi-Attribute Optimization of the WEDM Process for Surface Characteristics and Productivity
Wire-cut electrical discharge machining (WEDM) process is a proficient operation for the precise manufacturing of complex profiles of difficult-to-cut materials. The purpose of the current paper is to determine optimal processing inputs, including the WEDM current (CU), WEDM voltage (DV), pulse duration (Ton), and the speed of the wire electrode (WS) to decrease the depth of the recast layer (DL) as well as the root mean square roughness (RMSR) and enhance the cutting speed (CS) of the WEDM operation. The radius basis function (RBF) approach is employed to develop the predictive models of technical responses. The non-dominated sorting particle swarm optimization (NSPSO) is applied to obtain the optimal values of processing inputs and WEDM performances measured. The findings revealed that the proposed RBF models significantly contribute to the accurate prediction for the WEDM outputs. The optimal values of the CU, Ton, DV, and WS are 5.0 A, 6.0 ÎŒs, 33.0 V, and 4 m/min, respectively.The optimized values of the RMSR and DL are decreased by 60.98% and 15.55%, respectively, while the CS is enhanced by 8.90%. This work can be listed as an alternative solution for improving the surface characteristics and productivity of the WEDM process
Fast Krasnosel'skii-Mann algorithm with a convergence rate of the fixed point iteration of
The Krasnosel'skii-Mann (KM) algorithm is the most fundamental iterative
scheme designed to find a fixed point of an averaged operator in the framework
of a real Hilbert space, since it lies at the heart of various numerical
algorithms for solving monotone inclusions and convex optimization problems. We
enhance the Krasnosel'skii-Mann algorithm with Nesterov's momentum updates and
show that the resulting numerical method exhibits a convergence rate for the
fixed point residual of while preserving the weak convergence of the
iterates to a fixed point of the operator. Numerical experiments illustrate the
superiority of the resulting so-called Fast KM algorithm over various fixed
point iterative schemes, and also its oscillatory behavior, which is a specific
of Nesterov's momentum optimization algorithms
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