Thesis (PhD)--University of Stellenbosch, 2011. / In this thesis we describe the application of novel salicylaldimine and iminopyridyl nickel metallodendrimer complexes as catalysts in the transformation of á-olefins as well as in the polymerization of norbornene. New cyclic dendrimers based on cyclam as a core (L1-L8) were synthesized and characterized via FTIR and NMR spectroscopy, mass spectrometry and microanalysis. Subsequently the generation 1 cyclam-based dendrimers as well as the commercial generation 1 to generation 3 DAB-PPI dendrimers were functionalized with salicylaldimine and iminopyridyl moieties on the periphery to produce new ligands, DL1-DL10. These modified dendritic ligands were subsequently complexed to Ni salts to obtain the metallodendrimer complexes, C1-C8. The metallodendrimers were characterized by FTIR spectroscopy, mass spectrometry, microanalysis, magnetic susceptibility measurements, UV-Vis spectroscopy and thermal gravimetrical analysis (TGA). The DAB G1-G3 salicylaldimine ligands (DL1-DL3) were subjected to computational studies and the optimized structures were obtained by density functional theory (DFT) calculations. The effect of the increase in dendrimer generation on the structural arrangement of the dendrimer was also investigated. The following aspects were probed using molecular modeling: a) the possible coordination site for the Ni to the first generation dendrimer ligand, DL1, and b) the optimized structure of the first generation salicylaldimine nickel complex, C1. We subsequently evaluated catalysts, C1-C7, in the vinyl polymerization of norbornene, using methylaluminoxane (MAO) as a co-catalyst. All the catalysts were found to be active for norbornene polymerization with the weight of the polymers obtained ranging from 5.12 x 105 - 11.17 x 106 g/mol. The DAB-based iminopyridyl catalysts (C4-C6) exhibited higher activities than its analogous salicylaldimine catalysts (C1-C3) under the same reaction conditions. Also, the cyclam-based salicylaldimine nickel catalyst (C7) exhibited higher activities than the DAB-based salicylaldimine nickel catalyst, C1. A negative dendritic effect was observed for the G1-G3 DAB salicylaldimine catalysts since the optimum activity for the G3 catalyst, C3, was lower than that for the G2 catalyst, C2. These nickel complexes were also evaluated as ethylene oligomerization catalysts and were found to produce a range of ethylene oligomers (C4-C18) as well as some longer chained oligomers, when employing EtAlCl2 as a co-catalyst. We observed however that the free EtAlCl2 mediates the Friedel-Crafts alkylation of the solvent, toluene, in the presence of the obtained ethylene oligomers to give uneven carbon number products, which are mixtures of alkylated benzenes. Our metallodendrimer catalysts also isomerized and in some cases dimerized 1-pentene. In both ethylene oligomerization and 1-pentene isomerization processes, the salicylaldimine catalysts exhibited higher activity towards olefin transformation than the iminopyridyl catalysts. The cyclam-cored dendrimer catalyst again showed the highest activity. From the results obtained thus far it can be concluded that these nickel metallodendrimers exhibit great potential as catalysts in the transformation of unsaturated hydrocarbons.

<p>Thermal, non-catalytic conversion of light olefins (C2= - C4=) was originally utilized in the production of motor fuels at several U.S. refineries in the 1920-30’s. However, the resulting fuels had relatively low-octane number and required harsh operating conditions (T > 450 oC, P > 50 bar), ultimately leading to its succession by solid acid catalytic processes. Despite the early utilization of the thermal reaction, relatively little is known about the reaction products, kinetics, and initiation pathway under liquid-producing conditions. </p> <p>In this thesis, thermal ethylene conversion was investigated near the industrial operating conditions, i.e, at temperatures between 320 and 500 oC and ethylene pressures from 1.5 to 43.5 bar. Non-oligomer products such as propylene and/or higher odd carbon products were observed at all reaction temperatures, pressures, and reaction extents. Methane and ethane were minor products (< 1 % each), even at ethylene conversions as high as 74 %. The isomer distributions revealed a preference for linear, terminal C4 and C5. The reaction order was found to be 2nd order with a temperature dependent activation energy ranging from 165 to 244 kJ/mol. The importance of diradical species in generating free radicals during a two-phase initiation process was proposed. The reaction chemistry for ethylene, which has only strong, vinyl C-H bonds starkly contrasted propylene, which possesses weaker allylic C-H bonds and showed preference for dimeric C6 products over C2-C8 non-oligomers. </p> <p>Extending this work further, the thermal oligomerization of ethylene was enhanced using high surface area supports such as silica and alumina. Both supports resulted in order of magnitude rate increases compared to the gas phase reaction, however the ethylene conversion rate with alumina was superior to silica by a factor of between 100 and 1,000. Additionally, the alumina evidently confers a catalytic function, resulting in altered product distributions, notably an increase in branched products such as isobutene and isopentenes. The oligomerization chemistry with alumina appears to reflect the involvement of Lewis acid sites rather than traditional Brønsted acid or transition metal catalysis, which operate via carbenium ion and metal-alkyl intermediates, respectively. </p>

<p dir="ltr">Zeolites are microporous, crystalline aluminosilicates widely used in catalysis and separation. The substitution of Si⁴⁺ with Al³⁺ ([AlO_4/2]^-) creates a charge imbalance that can be compensated by a metal cation or complex (Mⁿ⁺) or a Brønsted acid proton (H⁺) within microporous voids and at external surfaces. Brønsted acid sites in aluminosilicates of diverse topologies have similar acid strength, but the diffusion of reactants and products can vary depending on the micropore size, tortuosity, and connectivity. The coupled effects of H⁺-site reactivity and diffusional constraints imposed by the inorganic zeolitic framework can be assessed by the diffusion parameter, which depends on the bulk proton density ([H⁺]) and the diffusion pathlength (L), derived from the Thiele modulus expression that relates reaction and diffusion rates within porous catalysts. This motivates synthetic approaches to control zeolite properties that influence diffusion and reactivity such as crystallite size and proton density. Prior synthetic methods have tried to minimize the diffusional constraints by decreasing the diffusion pathlength (L) by synthesizing zeolite crystallites at the nanometer scale or by increasing the effective diffusivity (D_e) by synthesizing hierarchical materials. However, these synthetic approaches may simultaneously influence multiple zeolite properties, such as the spatial distribution of acid sites throughout crystallites or at extracrystalline surfaces, convoluting the influence of these properties on the rates, selectivity, and deactivation of acid-catalyzed reactions.</p><p dir="ltr">Two types of spatial distribution of acid sites could be present within a zeolite. The first is the fraction at unconfined extracrystalline surfaces, and this property is often convoluted with the effect of crystallite size. Assuming acid sites are evenly distributed through the crystallite, as the crystallite size increases, the fraction of external acid sites decreases because the surface area-to-volume decreases. The second type of spatial distribution of acid sites is referred to as “zoning”—a concentration gradient of active sites from the external surface to its core, or vice versa. This type of spatial distribution of acid sites is challenging to quantify accurately. “Zoning” effects may also occur inadvertently during zeolite synthesize using conventional methods. In this work, we synthesize zeolitic materials (i.e., MFI) with controlled spatial distribution of acid sites independently of crystallite size and H⁺-site density to study their effects on propene oligomerization catalysis. A core@shell synthesis approach was used to passivate external MFI zeolite surfaces by an inert shell (Si-MFI) of short thickness relative to the size of the core crystallite. Although other passivation treatments can cause pore blockage or narrowing, transient sorption measurement showed no additional diffusional limitations were introduced by the growth of the Si-MFI shell. Propene dimerization rates (per H⁺, 503 K, 16 – 620 kPa C₃H₆) and transient behavior upon pressure step-changes persist reveal the influence of intrazeolite diffusional constraints on the Al-MFI core due to heavier oligomer products that accumulate inside micropores. On the contrary, dimerization rates did not reach a pseudo-steady-state on Al-MFI@Si-MFI and required high temperature caused by the formation of surface carbonaceous deposits in the absence of acid sites that otherwise assist in the cracking and desorption of coke precursor species. Thus, the passivation of the external surface imposes a transport limitation at the surface due to a carbonaceous layer that forms during the reaction, restricting the diffusion of products out of crystallites and shifting the selectivity towards a lighter product composition.</p><p dir="ltr">An inverted core@shell (Si-MFI@Al-MFI) material was also synthesized to investigate the effect of the spatial distribution of acid sites on the diffusion parameter, where the acid sites are preferentially located at the external surface and the core is inert (i.e. Si-MFI). The spatial distribution of acid sites was varied by growing an Al-MFI shell on a siliceous core and maintaining a similar bulk crystallite size. Mesitylene benzylation was used to quantify the fraction of external acid sites. Differences in measured propene dimerization rates (per H⁺) and product selectivity can be rationalized considering the thickness of the Al-rich shell in the core@shell material to an Al-MFI sample of similar crystallite size, evincing the dominant influence of the diffusion parameter on propene oligomerization catalytic behavior. Overall, this study demonstrated how zeolite synthetic methods can be used to isolate the effects of spatial distributions of Al from crystallite size and H⁺-site density and provide guidance for zeolite catalyst design efforts to control structural properties that influence reactions driven by coupled kinetic-transport phenomena.</p>

Thesis (PhD)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Please refer to full text for abstract / AFRIKAANSE OPSOMMING: Sien volteks vir opsomming

<p>      </p> <p>Full exploitation of shale resources requires new catalytic techniques to efficiently convert the methane, ethane, and propane found in shale gas to value-added fuels and chemicals. A promising process of converting ethane and propane involves catalytic light alkane dehydrogenation and the subsequent oligomerization of light alkenes. The first part of this work focuses on the examination of the mechanistic details of propane dehydrogenation on Pt-based alloy catalysts, where first principles-based free energy, microkinetic, and degrees of rate control analyses are performed to understand and rationalize the selective propane dehydrogenation using a Pt3Mn alloy. We show that only the under-coordinated, Mn-decorated Pt sites, represented by a Pt3Mn(211) surface, are selective to propylene formation, which can be attributed to several key mechanistic details: (1) facile propylene desorption and (2) hindered pathways that are inherently non-selective to propylene and lead to the formation of isomers. These kinetic details can, in turn, be interpreted using the free energy landscapes of propane dehydrogenation on the Pt3Mn(211) surface, which features a reasonably stronger binding of propylene than those of its isomers. From this study, we extract two selectivity descriptors for propane dehydrogenation: The energetics of propylene desorption versus deep-dehydrogenation, as well as the energetics of the formation of propylene versus its isomers. The properties can be used for designing further improved light alkane dehydrogenation catalysts.</p>