1,720,995 research outputs found
Synthesis and characterization of zeolites with controllable location of active sites
No full textThis bachelor thesis focusses on the synthesis, post-synthesis modification, and comprehensive characterization of two series of zeolites based on IWW and ITH topologies. The investigated zeolite structures were chosen because their frameworks contain d4r units preferentially occupied by Ge atoms, which can be substituted for various catalytically active metal sites such as Al, Ti, and Sn by post-synthesis degermanation/metallation. To study the influence of the chemical composition of the parent germanosilicate on the concentration and accessibility of the incorporated Al-, Ti- and Sn-associated acid sites, IWW and ITH were hydrothermally synthesized with different amounts of germanium in the reaction mixture (Si/Ge = 4 - 10 for IWW and Si/Ge = 10 - 30 for ITH) and used for post-synthesis Ge- for-metal substitution. X-ray diffraction (XRD) was used to verify the structure of the synthesized zeolites; chemical analysis identified their elemental composition; nitrogen physisorption was used to determine their textural characteristics (e.g., micropore volume, total pore volume pore, and external surface area), while scanning electron microscopy (SEM) was applied to visualize the shape and size of the zeolite particles. The coordination state of the incorporated Ti and Sn sites was studied using..
Synthesis of shape-selective catalysts for gas-phase m-xylene isomerization
No full textThis bachelor thesis investigates the effect of zeolite pore size and crystal morphology in gas-phase m-xylene isomerization, that ranks among the most important zeolite-catalysed reactions in the petrochemical industry. Monomolecular m-xylene isomerization produces targeted p-xylene, a key precursor to terephthalic acid and polyethylene terephthalate. In contrast, competing bimolecular side-reactions such as disproportionation and transalkylation yield undesirable byproducts, indicating the critical role of shape selectivity governed by both pore size and crystal morphology of a zeolite catalyst. To understand the relationship between zeolite pore architecture and catalytic performance, a series of zeolites with unimodal one-dimensional (1D) and three-dimensional (3D) pore systems were synthesised, characterized and tested in gas-phase m-xylene isomerization. The zeolites included 8-ring ESV and CHA, 10-ring TON and MFI and 12-ring AFI and BEA. Among them, the 10-ring zeolites TON and MFI showed the highest p-xylene selectivity, as they favour monomolecular isomerization of m-xylene and suppress bimolecular side-reactions. To investigate the combined effects of zeolite pore size and crystal morphology, zeolite IWW with its complex multimodal 3D pore network, comprising of 8-, 10- and 12-ring pores..
Study of active centers in zeolite catalysts for cascade Prins-Friedel-Crafts reaction
No full textConventional zeolites are important shape-selective heterogeneous acid catalysts due to the presence of acid sites accessible through uniform micropores. Recently designed hierarchical zeolites with combined micro-mesoporosity also contain acid sites on the external surface or in mesopores. These highly accessible acid sites are promising active centres for the transformation of bulky molecules, which cannot pass through the micropore openings of conventional zeolites. An example of a reaction that can benefit from the use of zeolites is the one-pot cascade Prins-Friedel-Crafts (PFC) reaction of an aldehyde, homoallylic alcohol, and aromatic compound, which yields valuable heterocyclic compounds containing 4- aryltetrahydropyran moieties. In this work, the acidic characteristics of a series of hierarchical aluminium- and gallium- containing MFI and MWW zeolites were evaluated by FTIR-monitored thermodesorption of probe molecules and further related to the catalytic properties of zeolites in the PFC reaction of butyraldehyde, 3-buten-1-ol, and anisole. The nature, strength, and total concentration of acid sites in the catalysts were evaluated using thermodesorption of pyridine (kinetic diameter 0.54 nm), while the characteristics of the external surface Brønsted acid sites were probed using..
Stabilization of metal nanoparticles in MWW zeolite for catalytic applications
No full textControlling both size of metal nanoparticles (MNPs) and acidobasic characteristics of the zeolite support is highly desirable for preparation of stable and active bifunctional catalysts. 2D-3D transformation of layered zeolite precursor into three-dimensional zeolite coupled with metal encapsulation is one of the most efficient synthetic strategies so far to achieve the appropriate metal dispersion and aggregative stability of MNPs within zeolite matrix. Nevertheless, the effect of support acidic characteristics on the properties of thus prepared metal@zeolite catalyst remained unrevealed, while the synthetic strategy itself requires further optimization to minimize the loss of metal component. This work addresses the influence of chemical composition of zeolite layered precursor on physical-chemical and catalytic properties of metal@zeolite catalysts prepared via 2D-3D transformation strategy, taken Pd@MCM-222D-3D system as a representative example. Both Si/Al ratio of MCM-22P layered precursor (e.g., Si/Al = 15, 20, 30) and Pd loading (e.g., 0.1, 0.3, 0.8 wt.%) were varied resulting in a set of nine Pd@MCM-222D-3D catalysts. In addition, three Pd@MCM-22impreg catalysts with the same metal loading (0.1 wt.%), but different Si/Al ratios of a support were synthesized via conventional impregnation..
Studium aktivních center v zeolitových katalyzátorech pro kaskádovou Prins-Friedel-Craftsovu reakci
Get PDFConventional zeolites are important shape-selective heterogeneous acid catalysts due to the presence of acid sites accessible through uniform micropores. Recently designed hierarchical zeolites with combined micro-mesoporosity also contain acid sites on the external surface or in mesopores. These highly accessible acid sites are promising active centres for the transformation of bulky molecules, which cannot pass through the micropore openings of conventional zeolites. An example of a reaction that can benefit from the use of zeolites is the one-pot cascade Prins-Friedel-Crafts (PFC) reaction of an aldehyde, homoallylic alcohol, and aromatic compound, which yields valuable heterocyclic compounds containing 4- aryltetrahydropyran moieties. In this work, the acidic characteristics of a series of hierarchical aluminium- and gallium- containing MFI and MWW zeolites were evaluated by FTIR-monitored thermodesorption of probe molecules and further related to the catalytic properties of zeolites in the PFC reaction of butyraldehyde, 3-buten-1-ol, and anisole. The nature, strength, and total concentration of acid sites in the catalysts were evaluated using thermodesorption of pyridine (kinetic diameter 0.54 nm), while the characteristics of the external surface Brønsted acid sites were probed using...Konvenční zeolity jsou důležité tvarově selektivní heterogenní katalyzátory díky přítomnosti jejich kyselých center, která jsou přístupná skrze stejnoměrné mikropóry. Nově navržené hierarchické zeolity kombinující mikro a mezoporozitu obsahují tyto centra kyselosti také na svém vnějším povrchu, nebo v mezopórech. Tyto vysoce přístupná centra kyselosti se jeví jako slibná aktivní centra pro transformaci rozměrnějších molekul, která nejsou schopna vstoupit do mikropórů konvenčních zeolitů. Jako příklad reakce, pro kterou by použití zeolitů bylo prospěšné, je kaskádová Prins-Friedel-Craftsova reakce aldehydu, homoallylického alkoholu, a aromatické sloučeniny, která vede k cenným heterocyklickým sloučeninám obsahující 4-aryl tetrahydopyranovou skupinu. Tato práce se zabývá analýzou kyselých vlastností série hierarchických MFI a MWW zeolitů obsahujících hliník, nebo galium metodou FTIR analyzované desorpce adsorbovaných molekul. Tyto vlastnosti byly následně uvedeny do souvislostí s katalytickými vlastnostmi těchto zeolitů v PFC reakci butyraldehydu, 3-buten-1-olu, a anisolu. Typ, síla, a celková koncentrace kyselých center studovaných katalyzátorů byla vyhodnocena termodesorpcí pyridinu (kinetický průměr 0.54 nm), zatímco vlastnosti Brønstedových center kyselosti na vnějším povrchu byly vyhodnoceny...Department of Physical and Macromolecular ChemistryKatedra fyzikální a makromol. chemiePřírodovědecká fakultaFaculty of Scienc
Syntéza a charakterizace zeolitů s kontrolovaným umístěním aktivních míst
Get PDFThis bachelor thesis focusses on the synthesis, post-synthesis modification, and comprehensive characterization of two series of zeolites based on IWW and ITH topologies. The investigated zeolite structures were chosen because their frameworks contain d4r units preferentially occupied by Ge atoms, which can be substituted for various catalytically active metal sites such as Al, Ti, and Sn by post-synthesis degermanation/metallation. To study the influence of the chemical composition of the parent germanosilicate on the concentration and accessibility of the incorporated Al-, Ti- and Sn-associated acid sites, IWW and ITH were hydrothermally synthesized with different amounts of germanium in the reaction mixture (Si/Ge = 4 - 10 for IWW and Si/Ge = 10 - 30 for ITH) and used for post-synthesis Ge- for-metal substitution. X-ray diffraction (XRD) was used to verify the structure of the synthesized zeolites; chemical analysis identified their elemental composition; nitrogen physisorption was used to determine their textural characteristics (e.g., micropore volume, total pore volume pore, and external surface area), while scanning electron microscopy (SEM) was applied to visualize the shape and size of the zeolite particles. The coordination state of the incorporated Ti and Sn sites was studied using...Tato bakalářská práce je zaměřena na syntézu, postsyntetickou modifikaci a komplexní charakterizaci dvou zeolitů založených na topologiích IWW a ITH. Zkoumané zeolity byly vybrány protože jejich struktury obsahují jednotky d4r přednostně obsazené Ge. Atomy Ge lze post-syntézní degermanací/metalací nahradit různými katalyticky aktivními kovy jako jsou Al, Ti a Sn. Pro studium vlivu chemického složení výchozího germanosilikátu na koncentraci a přístupnost nahrazených kovových míst byly zeolity IWW a ITH hydrotermálně syntetizovány s různým množstvím germania v reakční směsi (Si/Ge = 4 - 10 pro IWW a Si/Ge = 10 - 30 pro ITH) a použity pro postsyntetické nahrazení Ge jiným kovem. Připravené zeolity byly charakterizovány pomocí rentgenové difrakce (XRD), chemické analýzy, fyzisorpce dusíku, skenovací elektronové mikroskopie (SEM), infračervené spektroskopie s Fourierovou transformací (FTIR) a ultrafialové viditelné spektroskopie (UV-VIS). XRD potvrdila strukturu syntetizovaných zeolitů; chemická analýza identifikovala jejich prvkové složení; fysisorpce dusíku určila jejich texturní charakteristiky (např., objem mikropórů, celkový objem pórů a vnější povrch), zatímco SEM zviditelnila tvar a velikost částic zeolitů. Koordinační stav inkorporovaných míst Ti a Sn byl studován pomocí UV-VIS spektroskopie,...Katedra fyzikální a makromol. chemieDepartment of Physical and Macromolecular ChemistryFaculty of SciencePřírodovědecká fakult
Katalýza reakcí přenosu vodíku typu Meerwein-Ponndorf-Verley pomocí zeolit-aminových komplexů
Get PDFMeerwein-Ponndorf-Verley (MPV) reduction is a selective hydrogen transfer reaction that converts carbonyl compounds to the corresponding alcohols under mild conditions, offering a safer alternative to conventional hydrogenation by avoiding the use of molecular hydrogen. Recently, Lewis acid zeolites, particularly Zr- and Sn-substituted beta zeolites, have been identified as effective catalysts. Although MPV reduction is typically highly selective, its selectivity can decrease when the substrate structure allows competing Lewis catalysed reactions. In such cases, the addition of organic bases can increase selectivity by forming zeolite-amine complexes, although the mechanism as well as the base structure-activity correlations are unknown. This study focuses on the MPV reduction of citronellal, a bio-derived aldehyde, in the presence of Zr- and Sn-beta catalysts, which suffers from side reactions like carbonyl-ene cyclization and acetalization. Small nitrogen-containing organic bases such as pyridine and N- methylpiperidine were used to increase the citronellol selectivity and to disclose the structure- activity correlations between the base and the selectivity improvement. A series of pyridine- derived bases shifted selectivity in favour of citronellol except 2,6-ditert-butylpyridine, which is...Meerwein-Ponndorf-Verleyho (MPV) redukce je selektivní reakce přenosu vodíku, která transformuje za mírných podmínek karbonylové sloučeniny na odpovídající alkoholy a nabízí tak bezpečnější alternativu ke klasické hydrogenaci, neboť se vyhýbá použití molekulárního vodíku. Účinnými katalyzátory této reakce jsou zeolity s Lewisovými kyselými centry, zejména Zr- a Sn- silikátové beta zeolity. Ačkoliv je MPV redukce obecně vysoce selektivní, její selektivita může poklesnout, pokud struktura substrátu umožňuje konkurenční Lewisovsky katalyzované reakční cesty. V takových případech lze zvýšit selektivitu přídavkem organických bází - aminů, které vytvářejí zeolit-aminové komplexy, nicméně, mechanismus tohoto jevu ani korelace mezi strukturou báze a její aktivitou při zvýšení selektivity nejsou známy. Tato práce se zaměřuje na MPV redukci citronellalu, aldehydu přírodního původu, v přítomnosti Zr- a Sn-beta katalyzátorů. Této reakci konkurují karbonyl-en cyklizace a acetalizace. Malé dusíkaté organické báze (pyridin a N-methylpiperidin) byly použity ke zvýšení selektivity tvorby citronellolu a ke zkoumání korelace mezi jejich strukturou a aktivitou. Řada bází odvozených od pyridinu posunula selektivitu ve prospěch citronellolu s výjimkou 2,6-ditert- butylpyridinu, který není schopen koordinace na...Department of Physical and Macromolecular ChemistryKatedra fyzikální a makromol. chemieFaculty of SciencePřírodovědecká fakult
Heterogenní katalýza redoxních reakcí terpenoidů
Get PDFHeterogénne katalyzátory pre Meerwein-Ponndorf-Verley (MPV) redukciu ketónov a aldehydov sú skúmané kvôli problémom homogénnych katalyzátorov spojených s citlivosťou na obsah vody a s ťažkosťami so separáciou. Atóm vodíku je počas MPV redukcie prenesený z molekuly obetného alkoholu na karbonylovú skupinu ketónu/aldehydu, pričom vzniká príslušný alkohol. Zeolity obsahujúce cín a zirkónium sú katalyzátory aktívne v tejto reakcii, počas ktorej dochádza k prenosu vodíku. Avšak, pri používaní 3D zeolitov v redukcii objemných molekúl, napr. terpenoidných zlúčenín, môžu vzniknúť problémy s dostupnosťou aktívnych miest pre tieto objemné reaktanty. Riešením vzniknutých difúznych problémov môžu byť 2D pilierované zeolity. Piliere z oxidu kremičitého a oxidu kovu zväčšia vzdialenosť medzi jednotlivými vrstvami 2D (vrstevnatého) zeolitu a tým sa zlepší dostupnosť aktívnych miest. 2D katalyzátory boli pripravené pilierovaním materského vrstevnatého pure-Si MFI so zmesou tetraorto etylsilikátu a isopropoxidu zirkoničitého alebo isopropoxidu ciničitého, kým 3D zeolity boli syntetizované hydrotermálnou metódou. Aktivita pilierovaných zeolitov obsahujúcich Sn a Zr bola porovnaná s aktivitou 3D MFI a beta zeolitov v MPV redukcii furfuralu a citronelalu. Avšak 2D zeolity neurýchľujú reakciu tak, ako sme...Heterogeneous catalysts for Meerwein-Ponndorf-Verley (MPV) reduction of ketones and aldehydes are investigated due to issues connected with water sensitivity and separation problems of homogeneous catalysts. During MPV reduction, a hydrogen atom is transferred from the sacrificial alcohol onto the carbonyl group of the ketone/aldehyde forming corresponding alcohol. Tin and zirconium containing zeolites are active catalysts in this hydrogen transfer reaction. However, when using conventional 3D zeolites in the reduction of bulky molecules, e.g., terpenoid compounds, problems with active site accessibility for these bulky reactants may arise. The use of 2D pillared zeolites may represent a solution to the diffusion problem. Silica metal-oxide pillars increase the distance between individual layers of a 2D (lamellar) zeolite and thus improve the active site accessibility. 2D catalysts were prepared by pillaring a parent lamellar pure-Si MFI with a mixture of tetraethyl orthosilicate and Zr (IV) isopropoxide or Sn (IV) isopropoxide, while conventional 3D zeolites were synthesised hydrothermally. The activity of Sn and Zr containing pillared catalysts was compared with the activity of the 3D MFI and beta zeolites in MPV reduction of furfural and citronellal. The 2D zeolites, however, did not facilitate...Department of Physical and Macromolecular ChemistryKatedra fyzikální a makromol. chemieFaculty of SciencePřírodovědecká fakult
Design of sustainable catalysts by post-synthesis modification of germanosilicate zeolites
No full text1 Abstract Due to the presence of small structural units (e.g., D4R, D3R), the frameworks of germanosilicate zeolites are generally characterized by high pore volumes and multidimensional/extra-large pore systems, making them especially suitable in processing bulky molecules (in particular, involved in biomass-derived compounds valorization). However, weak acidity, low hydrothermal stability and high cost of Ge significantly limit the practical use of Ge-containing zeolites. This thesis is aimed at design of sustainable germanosilicate zeolite-based catalysts of modifiable chemical composition and tunable porosity for relevant acid-catalyzed reactions, such as ketalization of polyols, epoxidation of olefins, Baeyer-Villiger oxidation of cyclic ketones and Meerwein-Ponndorf-Verley reduction of aldehydes. Germanosilicate zeolites of different structures, including medium-pore ITH, large pore IWW, extra-large pore UTL and *CTH, were thoroughly characterized using a combination of techniques (i.e., XRD, physisorption, electron microscopy, chemical analysis, among others) and subjected to different post-synthesis modifications to address synthesis-structure-activity relationships in the designed germanosilicate-zeolite based catalysts. In ketalization of glycerol to solketal, weak acid centers of IWW and UTL..
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