No description available.

No description available.

No description available.

Meta-diisopropylbenzene is reacted with propylene over the acid form of the molecular sieves SAPO-5, mordenite, offretite, beta, hexagonal and cubic faujasite (hex and FAU), L, SAPO-37, and an amorphous silica-alumina at temperatures around 463 K in a flow-type fixed-bed reactor. A small amount of cracking is observed. The main reactions of meta-diisopropylbenzene are isomerization and alkylation. This alkylation is proposed as a new test reaction to characterize the effective size of the voids in larger pore (12 T-atom rings or above) molecular sieves by measuring the amount ratio of formed 1,3,5- to 1,2,4-triisopropylbenzene. In most cases, this ratio increases with the increasing effective void size of the molecular sieves in the order: SAPO-5 < mordenite < offretite < beta < hex ≈ FAU < L < SAPO-37 < amorphous silica-alumina. Since samples with the FAU topology show lower selectivities to 1,3,5-triisopropylbenzene than the mesoporous, amorphous silica-alumina, pore curvature has an influence on alkylation selectivity even for voids of 13 A size. / Ph. D.

Substitution of MeCN ligands in the activated cluster Os3(CO)10(MeCN)2 by the unsaturated diphosphine ligands (Z)-Ph2PCH=CHPPh2 (cDPPEn) or 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) proceeds rapidly at room temperature to furnish the ligand-bridged cluster 1,2-Os3(CO)10(P-P) (P-P represents cDPPEn or bpcd). Heating 1,2-Os3(CO)10(P-P) leads to the formation of the thermodynamically more stable chelating isomer 1,1-Os3(CO)10(P-P). Each compound of Os3(CO)10(P-P) has been characterized by x-ray diffraction, IR, 31P NMR and 1H NMR. Ligand isomerization kinetics have been investigated by UV-VIS and 31P NMR (for cDPPEn) or 1H NMR (for bpcd) spectroscopies. The isomerization mechanism is discussed based on the activation parameters and CO inhibition (for cDPPEn) or ligand trapping experiments (for bpcd). Thermolysis of 1,1-Os3(CO)10(bpcd) in refluxing toluene gives the hydrido cluster HOs3(CO)9[μ-(PPh2)C=C{PPh(C6H4)}C(O)CH2C(O)] and the benzyne cluster HOs3(CO)8(μ3-C6H4)[μ2,η1-PPhC=C(PPh2)C(O)CH2C(O)]. Photolysis of 1,1-Os3(CO)10(bpcd) using near UV light affords HOs3(CO)9[μ-(PPh2)C=C{PPh(C6H4)}C(O)CH2C(O)] as the sole product. HOs3(CO)8(μ3-C6H4)[μ2,η1-PPhC=C(PPh2)C(O)CH2C(O)] has been characterized in solution by IR and NMR spectroscopies. Furthermore its molecular structure has been determined by X-ray crystallography. Reversible C-H bond formation in HOs3(CO)9[μ-(PPh2)C=C{PPh(C6H4)}C(O)CH2C(O)] is demonstrated by ligand trapping studies to give 1,1-Os3(CO)9L(bpcd) (where L = CO, phosphine) via the unsaturated intermediate 1,1-Os3(CO)9(bpcd). The kinetics for reductive coupling in HOs3(CO)9[γ-(PPh2)C=C{PPh(C6H4)}C(O)CH2C(O)] and DOs3(CO)9[μ-(PPh2-d10)C=C{P(Ph-d5)(C6D4)}C(O)CH2C(O)] in the presence of PPh3 give rise to a kH/kD value of 0.88, whose magnitude supports the existence of a preequilibrium involving the hydride(deuteride) cluster and a transient arene-bound Os3 species that precedes the rate-limiting formation of 1,1-Os3(CO)9(bpcd). Strong proof for the proposed hydride(deuteride)/arene preequilibrium has been obtained from photochemical studies employing the isotopically labeled cluster 1,1-Os3(CO)10(bpcd-d4ortho), whose bpcd phenyl groups each contain one ortho hydrogen and deuterium atom. Equilibrium and kinetic isotope effects in the orthometallation step has been determined by 1H NMR in photochemical studies. Kinetics for the transformation from HOs3(CO)9[μ-(PPh2)C=C{PPh(C6H4)}C(O)CH2C(O)] to HOs3(CO)8(μ3-C6H4)[μ2,η1-PPhC=C(PPh2)C(O)CH2C(O)] has been studied by UV-VIS spectroscopy for which the mechanism is discussed.

Thesis (PhD (Process Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Future chemical production is faced with a challenge of limited material and energy resources. However, process intensification might play a significant role to alleviating this problem. Vision of process intensification through multifunctional reactors has stimulated research on membrane-based reactive separation processes, in which membrane separation and catalytic reaction occur simultaneously in one unit. These processes are rather attractive applications because they are potentially compact, less capital intensive, and have lower processing costs than traditional processes. Moreover, they often enhance the selectivity and yield of the target product. For about three decades, there has been a great evolution in p-Xylene production technology, with many equipment improvements being instituted in the industry. Typically, these improvements bring economic as well as processing advantages to the producers. Such developments are vital, as the capital costs for process equipment to produce and separate p-Xylene from xylene isomers, especially into high purity p-Xylene, still remain very high. However, with numerous advantages of membrane-based reactive separation processes compared to the conventional processes, the research focus has been channelled toward application of MFI-type zeolite membranes for in situ separation and isomerization of xylene in extractor-type catalytic membrane reactors. To contribute to this research line, this study has focused on characterization and optimization of an extractor-type catalytic membrane reactor (e-CMR) equipped with a nanocomposite MFI-alumina membrane as separation unit for m-Xylene isomerization over Pt-HZSM-5 catalyst. Nanocomposite MFI-alumina zeolite membranes (tubes and hollow fibres) used in this study were prepared via a so-called “hydrothermal pore-plugging synthesis technique” developed by Dalmon and his group more than a decade ago. In this concept, MFI material is grown by 'pore-plugging' direct hydrothermal synthesis in a porous matrix rather than forming thin films on top of the support. The advantages of this type of architecture over conventional film-like zeolite membranes include: (i) minimization of the effect of thermal expansion mismatch between the support and the zeolite, (ii) easy to scale-up, and (iii) easy module assembly, because the separative layer (zeolite crystals) are embedded within the pores of the ceramic support, reducing the effects of abrasion and thermal shocks. After membrane synthesis, the membrane quality and separation performance of these membranes were evaluated through single gas permeation (H2), binary gas separation (n-butane/H2) and ternary vapour mixture of xylene isomers using the vapour permeation (VP) method with p-Xylene as the target product. After evaluating the xylene isomer separation performance of the membranes, the membranes were used in extractor-type catalytic membrane reactors to carry out m-Xylene isomerization over Pt-HZSM-5 catalyst with p-Xylene as the target product. This dissertation has shown that nanocomposite MFI-alumina membrane tubes and hollow fibre membranes were selective to p-Xylene from xylene isomers. The dissertation also reports for the first time in open literature the excellent xylene separation performance of nanocomposite MFI-alumina membrane tubes at higher xylene loading (or vapour pressure). Unlike their film-like counterparts, the membranes still maintain increased selectivity to p- Xylene at higher xylene vapour pressures without showing a drastic decrease in selectivity. This outstanding property makes it a promising choice for pervaporation applications where concentration profile is usually a major problem at higher loading of xylene. With the use of nanocomposite MFI-alumina hollow fibre membranes, this research has demonstrated that membrane configuration and effective membrane wall thickness play a prominent role in enhancing cross membrane flux. Results presented in the study show, for the first time in open literature, that nanocomposite MFI-alumina hollow fibre membrane could enhance p-Xylene fluxes during the separation of ternary vapour mixture of xylene due to the smaller effective wall thickness of the membrane (membrane thickness <1 μm) when compared to conventional randomly oriented MFI zeolite films (membrane thickness >3 μm). During xylene isomers separation with nanocomposite hollow fibre membrane, about 30% increase in p-Xylene flux was obtained compared to the membrane tubes, operated under the same conditions. Additionally, hollow fibres offer the added advantage of membrane surfaceto- volume ratios as high as 3000 m2/m3 compared to conventional membrane tubes. Using this type of system could be instrumental in reducing both the size and cost of permeating modules for future xylene separation processes. However, obtaining high quality nanocomposite MFI-alumina membrane fibres is subject to the availability of high quality fibre supports. Regarding the application of nanocomposite MFI-alumina membrane tubes as extractor-type catalytic membrane reactors (referred to as extractor-type zeolite catalytic membrane reactor (e-ZCMR) in this study) for m-Xylene isomerization over Pt-HZSM-5, the results presented in this study further substantiate and confirm the potentials of e-ZCMRs over conventional fixed-bed reactors (FBRs). In the combined mode (products in the permeate plus products in the retentate), the e-ZCMR displayed 16-18% increase in p-Xylene yield compared to an equivalent fixed-bed reactor operated at the same operating conditions. On the basis of the high p-Xylene-to-o-Xylene (p/o) and p-Xylene-to-m-Xylene (p/m) separation factors offered by the membranes, p-Xylene compositions in the permeate-only mode (products in the permeate stream) in the range 95%-100% were obtained in the e-ZCMR. When a defect-free nanocomposite MFI-alumina membrane tube with p-Xylene-too- Xylene (p/o) separation factor >400 was used, ultra pure p-Xylene with p-Xylene purity approaching 100% in the permeate-only mode was obtained. Moreover, the e-ZCMR displayed 100% para-selectivity in the permeate-only mode throughout the temperatures tested. This is not possible with conventional film-like MFI-type zeolite membranes. Therefore, the application of nanocomposite MFI-alumina membranes in extractor-type catalytic membrane reactors could catalyse the development of energy-efficient membrane-based process for the production of high purity p-Xylene. Furthermore, in this dissertation, a report on modelling and sensitivity analysis of an e-ZCMR equipped with a nanocomposite MFI-alumina membrane tube as separation unit for m-Xylene isomerization over Pt-HZSM-5 catalyst is presented. The model output is in fair agreement with the experimental results with percentage errors (absolute) of 17%, 29%, 0.05% and 19.5% for p-Xylene yield in combined mode, p-Xylene selectivity in combined mode, p-Xylene selectivity in permeate-only mode and m-Xylene conversion, respectively. Therefore, the model is adequate to explain the behaviour of e-ZCMR during m-Xylene isomerization over Pt-HZSM-5 catalyst. The model is also adaptable to e-ZCMRs of different configurations such as hollow fibre MFI-alumina membrane-based e-ZCMRs. To gain more insight into the behaviour of the model to small changes in certain design parameters, sensitivity analysis was performed on the model. As expected, the sensitivity analysis revealed that intrinsic property of membrane (porosity, tortuosity), membrane effective thickness and reactor size (indicated with reactor internal diameter) play a significant role on the performance of e-ZCMR during p-Xylene production from the mixed xylenes. MFI-alumina zeolite membranes with optimized parameters such as membrane porosity, membrane tortuosity, and membrane effective wall thickness might enhance transport of p-Xylene through the membrane and thus resulting in higher p-Xylene flux through the membrane. This eventually would translate into an increase in p-Xylene yield in permeate-only mode. As far as it could be ascertained, this is the first report in open literature on modelling study with sensitivity analysis of e-ZCMR equipped with nanocomposite MFI-alumina membrane tubes as separation unit for m-Xylene isomerization over Pt-HZSM- 5 catalyst. In addition, the results of this study have confirmed previous research efforts reported on the application of extractor-type catalytic membrane reactors, having MFI-type membranes as separation units, for p-Xylene production via m-Xylene isomerization over a suitable catalyst. Also, new ideas were developed, tested and proposed that now provide a solid basis for further scale-up and techno-economical studies. Such studies are necessary to evaluate the competitiveness of the technology with the traditional processes for the production of high purity p-Xylene from mixed xylene. In summary, the encouraging results, as documented in this dissertation and also communicated to researchers in the area of membrane-based reactive separation (in the form of four peer-reviewed international scientific publications and four conference proceedings), could provide a platform for developing a scaled-up membrane-based energy-efficient industrial process for producing high purity p-Xylene through isomerization. / AFRIKAANSE OPSOMMING: Die produksie van chemiese stowwe word belemmer deur die uitdaging van beperkte materiaal- en energiebronne. Prosesuitbreiding kan egter ‘n noemenswaardige rol in die verligting van hierdie probleem speel. Die moontlike gebruik van multi-funksionele reaktore in prosesuitbreiding het navorsing in membraan-gebaseerde reaktiewe skeidingsprosesse (waar membraanskeiding en die katalitiese reaksie gelyktydig in ‘n enkele eenheid plaasvind) aangemoedig. Hierdie prosesse is aantreklik omdat hulle potensieel kompak en minder kapitaal-intensief is en ook teen laer koste as tradisionele prosesse bedryf kan word. Dit is ook dikwels die geval dat die multi-funksionele reaktor die selektiwiteit en opbrengs van die gewenste produk verhoog. In die afgelope drie dekades was daar ’n sterk verandering in die tegnologie wat gebruik word in die produksie van p-Xileen, met vele verbeterings aan nuwe toerusting wat in die nywerheid in bedryf gestel is. Hierdie verbeteringe hou gewoonlik ekonomiese-, sowel as bedryfsvoordele vir die produsente in. Ontwikkelings in hierdie veld is noodsaaklik aangesien die kapitale uitgawes vir die toerusting om p-Xileen, veral baie suiwer p-Xileen, van xileenpolimere te produseer en te skei, steeds baie hoog is. Met talle voordele gekoppel aan membraangebaseerde reaktiewe skeidingsprosesse in vergelyking met normale prosesse, is die navorsing egter gekanaliseer na die gebruik van MFI-tipe zeolietmembrane vir die in-situ skeiding en isomerisasie van xileen in ekstraksie-tipe katalitiese membraanreaktore. As bydrae tot hierdie navorsingsveld het hierdie studie op die karakterisering en optimering van ‘n ekstraksie-tipe katalitiese membraanreaktor (e-KMR), toegerus met ’n nanosaamgestelde MFI-alumina membraan as skeidingseenheid vir m-Xileen isomerisasie in die teenwoordigheid van ‘n Pt-HZSM-5 katalis, gefokus. Nanosaamgestelde MFI-alumina zeolietmembrane (buise en hol vesels) wat in hierdie studie gebruik is, is voorberei deur die sogenaamde “hidrotermiese porie-sperring sintese tegniek” wat meer as ‘n dekade gelede ontwikkel is deur Dalmon en sy groep. In hierdie tegniek word MFI-materiaal gekweek deur direkte hidrotermiese sintese in ‘n poreuse matriks, eerder as die vorming van dun films bo-op die ondersteuningsbasis. Die voordele van hierdie ontwerp bo dié van die konvensionele filmagtige zeolietmembrane sluit in: (i) minimering van die effek van termiese uitsetting op die gaping tussen die ondersteuningsbasis en die zeoliet, (ii) die gemak van opskalering, en (iii) die gemak waarmee die modules aanmekaar gesit kan word, omdat die skeidingslaag (zeolietkristalle) binne die porieë van die keramiek-ondersteuningsbasis geleë is, wat die effek van erodering en termiese skok verminder. Ná die membraansintese is die membraankwaliteit en skeidingsvermoë geevalueer deur enkel-gas-deurdringing (H2), binêre-gas-skeiding (n-butaan/H2), en ternêre dampmengsel van xileen-isomere deur die gebruik van die damp-deurdringingsmetode met p-Xileen as die teikenproduk. Hierdie tesis het gewys dat nanosaamgestelde MFI-alumina membraanbuise en hol vesel membrane selektief was ten opsigte van p-Xileen vanuit xileen-isomere. Die tesis doen ook, vir die eerste keer in die oop literatuur verslag, oor die uitstekende p-Xileen skeidingsvermoë van nanosaamgestelde MFI-alumina buise by hoër xileenladings (of dampdrukke). Anders as hulle filmagtige eweknieë het die membrane steeds hul verhoogde selektiwiteit vir p-Xileen by hoër dampdrukke behou, sonder ‘n merkbare verlaging in die selektiwiteit. Hierdie merkwaardige eienskap maak dit ‘n belowende keuse vir pervaporasie toepassings, waar die konsentrasieprofiel (as gevolg van hoër xileenladings) gewoonlik ’n noemenswaardige probleem is. Met die gebruik van nanosaamgestelde MFI-alumina membrane het hierdie navorsing gewys dat membraankonfigurasie en –wanddikte ‘n prominente rol speel in die verbetering van vloei oor die membraan. Resultate wat in die studie voorgelê word, wys, vir die eerste keer in oop literatuur, dat hol vesel nanosaamgestelde MFI-alumina membrane die deurvloei van p-Xileen kan verbeter gedurende die skeiding van ternêre dampmengsels van xileen, as gevolg van die kleiner effektiewe wanddikte van die membraan (<1 μm) wanneer dit vergelyk word met konvensionele kansgewys-geörienteerde MFI-zeoliet films met ‘n membraandikte van >3 μm. Tydens die skeiding van xileen-isomere met nanosaamgestelde hol vesel membrane is ‘n verbetering van ongeveer 30 % in die deurvloei van p-xileen verkry, vergeleke met membraanbuise, by identiese bedryfstoestande. Hol vesels bied ook die verdere voordeel van oppervlak-tot-volume verhoudings van so hoog as 3000 m2/m3 vergeleke met konvensionele membraanbuise. Die gebruik van hierdie tipe sisteem kan deurslaggewend wees in die vermindering van die grootte en koste van deurlatingseenhede in toekomstige xileen-skeidingsprosesse. Die vervaardiging van hoë-kwaliteit nanosaamgestelde MFIalumina membraanvesels is egter onderworpe aan die beskikbaarheid van hoë-kwaliteit vessel-ondersteuningsbasisse. Wat die gebruik van nanosaamgestelde MFI-alumina membraanbuise as ekstraksietipe katalitiese membraanreaktore betref (ekstraksie-tipe zeoliet katalitiese membraanreaktor, of e-ZKMR in hierdie studie) vir m-Xileen isomerisasie in die teenwoordigheid Pt-HZSM-5, bevestig die resultate die potensiaal van e-ZKM reaktore bo konvensionele vaste-bed reaktore (VBR). In die gekombineerde verstelling (met produkte in die permeaat sowel as die retentaat) toon die e-ZKMR ‘n 16 – 18% verbetering in die opbrengs van p-Xileen vergeleke met ‘n ekwivalente VBR by dieselfde bedryfskondisies. Gegrond op die hoë p-Xileen-tot-o- Xileen (p/o) en p-Xileen-tot-m-Xileen (p/m) skeidingsfaktore wat deur die membraan gebied word, is p-Xileen-samestellings in die slegs-permeaat verstelling (produkte in die permeaatstroom) van tussen 95 en 100% in die e-ZKMR verkry. Toe ‘n defek-vrye nanosaamgestelde MFI-alumina membraanbuis met ‘n (p/o) skeidingsfaktor van >400 gebruik is, is p-Xileen met ‘n suiwerheid na aan 100% in die slegs-permeaat verstelling verkry. Die e-ZKMR het ook 100% para-selektiwiteit in die slegs-permeaat verstelling getoon by alle toets-temperature, iets wat onmoontlik is met gewone filmagtige MFI-tipe zeolietmembrane. Om hierdie rede is dit moontlik dat die gebruik van MFI-alumina membrane in ekstraksie-tipe katalitiese membraanreaktore die ontwikkeling van energie-doeltreffende membraangebaseerde prosesse vir die produksie van suiwer p-Xileen kan bevorder. Verder word daar in hierdie tesis verslag gedoen oor die modelering en sensitiwiteitsanalise van ‘n e-ZKMR wat toegerus is met ‘n nanosaamgestelde MFI-alumina membraanbuis as skeidingseenheid vir m-Xileen isomerisasie in die teenwoordigheid van ‘n Pt-HZSM-5 katalis. Die model-uitsette is redelik in ooreenstemming met eksperimentele resultate met absolute fout-persentasies van 17, 27, 0.05 en 19.5 % vir die p-Xileen opbrengs in die gekombineerde verstelling, p-Xileen selektiwiteit in die gekombineerde verstelling, p-Xileen selektiwiteit in die slegs-permeaat verstelling en m-Xileen omsetting, onderskeidelik. Om hierdie rede kan die model die gedrag van ‘n e-ZKMR verduidelik tydens die m-Xileen isomerisasie in die teenwoordigheid van ‘n Pt-HZSM-5 katalis. Die model kan ook aangepas word na e-ZKM reaktore met verskillende konfigurasies, soos hol vesel MFIalumina membraan-gebaseerde e-ZKMRe. Om meer insig te kry in die gedrag van die model op klein veranderinge in sekere ontwerpparameters, is ‘n sensitiwiteitsanalise op die model uitgevoer. Soos verwag, het die sensitiwiteitsanalise gewys dat die intrinsieke eienskappe van die membraan (porositeit, tortuositeit), die effektiewe van membraandikte en die reaktorgrootte (gemeet as die interne deursnit van die reaktor) ‘n noemenswaardige rol speel in die gedrag van die e-ZKMR gedurende p-Xileen produksie vanuit gemengde xilene. MFI-alumina zeolietmembrane met geoptimeerde parameters soos membraanporositeit, -tortuositeit, en –wanddikte mag dalk die oordrag van p-Xileen deur die membraan bevorder en sodoende ‘n hoër vloei van p-Xileen oor die membraan bewerkstellig. Dit sal uiteindelik lei tot ‘n verhoging in die opbrengs van p-Xileen in die slegs-permeaat verstelling. So ver dit vasgestel kon word, is hierdie die eerste verslag in die oop literatuur wat die modelering en sensitiwiteitsanalise van ‘n e-ZKMR, toegerus met nanosaamgestelde MFIalumina membraanbuise as skeidingseenheid vir m-Xileen isomerisasie in die teenwoordigheid van ‘n Pt-HZSM katalis, aanspreek. Verder ondersteun die resultate van hierdie studie vorige navorsingspogings op die gebruik van e-KMRe, met MFI-tipe membrane as skeidingseenhede, vir die produksie van p-Xileen deur middel van m-Xileen isomerisasie in die teenwoordigheid van ‘n geskikte katalis. Verder is nuwe idees ontwikkel, getoets en voorgestel wat dien as ’n stewige basis vir verdere opskalering- en tegno-ekonomiese studies. Sodanige studies is nodig om die vatbaarheid van die tegnologie relatief tot die tradisionele prosesse te bepaal. Ter opsomming, die bemoedigende resultate, soos in die tesis gedokumenteer (en ook gepubliseer in vier ewe-knie beoordeelde internasionale wetenskaplike joernale en vier konferensiestukke), kan as ‘n platform dien vir die ontwikkeling van ’n opgeskaleerde membraan-gebaseerde energie-doeltreffende nywerheidsproses vir die produksie van suiwer p-Xileen deur middel van isomerisasie.

Cyclophilins are ubiquitous enzymes that are involved in protein folding, signal transduction, viral proliferation, oncogenesis, and regulation of the immune system. Cyclophilin A is the prototype of the cyclophilin family. We use molecular dynamics to describe the catalytic mechanism of cyclophilin A in full atomistic detail by sampling critical points along the reaction coordinate, and use accelerated molecular dynamics to sample cis-trans interconversions. At these critical points, we analyze the conformational space sampled by the active site, flexibility of the enzyme backbone, and modulation of binding interactions.We use Kramer’s rate theory to determine how diffusion and free energy contribute to lowering the activation energy of prolyl isomerization. We also find preferential binding modes of several cyclophiln A inhibitors, and compare the conformational space sampled by inhibited cyclophilin A to the conformational space sampled during wild-type interactions. We also analyze the mechanism of the next family member cyclophilin B in order to probe differences in enzyme dynamics and intermolecular interactions that could possibly be exploited in isoform-specific drug design. Our results indicate that cyclophilin proceeds by a conformational selection binding mechanism that manipulates substrate sterics, electrostatic interactions, and multiple reaction timescales in order to speed up reaction rate. Conformational space sampled by cyclophilin when inhibited and when undergoing wild-type interactions share significant similarity. Cyclophilins A and B do have notable differences in enzyme dynamics, due to variation in intramolecular interactions that arise from variation in primary structures. This work demonstrates how computational methods can be used to clarify catalytic mechanisms.

Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2006. / This study concerns the investigation of the isomerization of different metallocene catalysts in solution, and the effects thereof on the microstructure of polypropylenes prepared with these catalysts. Two C2 symmetric ansa metallocenes, ethylene-bis(indenyl) zirconium dichloride (EI) and dimethylsilyl-bis(2-methyl benzoindenyl) zirconium dichloride (MBI) were exposed, in solution, to both sunlight and UV radiation. The rac-meso isomerization of these catalysts were followed by 1H NMR spectroscopy. The reaching of a photostationary state is described, as well as the effect of isomerization of these catalysts in solution on the polymerization of propylene. Results show that metallocene structure has an effect on the isomerization rate and photostationary state. Results also show that the wavelength of light plays a role in the isomerization process. Effects on stereochemistry and molecular weight of the formed polymer as well as the catalyst activity is described and discussed. In addition the effect of activating the catalysts with MAO before exposure to light is discussed.

Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: See item for full text / AFRIKAANSE OPSOMMING: Sien item vir volteks

ABSTRACT Equilibrium unfolding, single- and double-jump kinetic studies were conducted to determine the unfolding and refolding pathway of glutaredoxin 2. Structural changes for wild-type glutaredoxin 2 were monitored by far-ultraviolet circular dichroism and intrinsic tryptophan fluorescence for equilibrium unfolding and intrinsic tryptophan fluorescence for single- and double-jump kinetics studies. Glutaredoxin 2 possesses two tryptophan residues in domain 2. In order to monitor changes in domain 1, cysteine 9 at the active site cysteines, situated in domain 1, was labelled with an extrinsic fluorophore, AEDANS, and a mutant was created (Y58W glutaredoxin 2). The AEDANS labelled protein displayed decreased alpha-helical secondary structure and conformational stability. A high degree of cooperativity and similar conformational stability was observed during the two-state transition of the urea-induced equilibrium unfolding of both the wild-type and Y58W glutaredoxin 2 proteins therefore Y58W glutaredoxin 2 could be used to assess structural changes in the local environment of domain 1 during unfolding and refolding. Two phases of unfolding, the fast and slow phase, occurred for both the wild-type and Y58W proteins. The slow phase involves structural rearrangements that expose small amounts of surface area while the fast phase represents gross structural unfolding exposing large amounts of surface area. The isomerization of the Val48-Pro49 peptide bond to the trans conformation occurs during the slow phase and this isomerization is coupled to conformational unfolding of the protein. The structural separation of these phases could be represented by two structural units (unit x and unit y), these units do not represent domain 1 and 2. The units could also result in parallel refolding pathways with the folding of the x unit involving the fast and slow refolding phases and the folding of the y unit of structure is represented by the medium phase of refolding. The fast and slow phases are further separated as the fast phase represents the gross structural folding of glutaredoxin 2 for species with the Val48-Pro49 peptide bond in the native cis conformation. The development of the slow phase after extended unfolding delay periods during double-jump refolding studies, as well as the acceleration of the rate of the phase by the peptidyl prolyl isomerase hFKBP-12 proved that the phase involves a proline peptide bond iv isomerization. This phase represents a slow isomerization coupled with conformational folding similar to the slow unfolding phase. Complex unfolding and refolding kinetics indicated the involvement of kinetic intermediates during (un)folding.