Spelling suggestions: "subject:"olefin oligomerization"" "subject:"0lefin oligomerization""
1 |
The preparation and characterization of multinuclear catalysts based on novel dendrimers : application in the oligomerization and polymerization of unsaturated hydrocarbonsMalgas-Enus, Rehana 03 1900 (has links)
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.
|
2 |
SUPPORT-ENHANCED THERMAL OLIGOMERIZATION OF ETHYLENE TO LIQUID FUEL HYDROCARBONSMatthew Allen Conrad (12969596) 28 June 2022 (has links)
<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>
|
3 |
Polymerization and oligomerization reactions mediated by metallodendrimers of zinc and palladiumMugo, Jane Ngima 03 1900 (has links)
Thesis (PhD)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Please refer to full text for abstract / AFRIKAANSE OPSOMMING: Sien volteks vir opsomming
|
4 |
First Principles Analysis of Catalytic Conversion of Light Alkanes to Value-added Fuels and ChemicalsYinan Xu (12877394) 04 October 2022 (has links)
<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>
|
Page generated in 0.1113 seconds