Thesis (MSc. (Chemistry)) -- University of Limpopo, 2015 / In this study, the use of 10-member ring zeolites with different silica-to-alumina ratios (SARs) as supports for palladium (Pd) to produce hydrocracking catalysts was investigated. The syntheses of zeolites ZSM-22, ZSM-48, ZBM-30 and ZSM-23 were carried out under hydrothermal conditions, and the resulting materials were characterised with XRD, SEM and BET surface area measurements, prior to activity tests. Successful synthesis of ZSM-48 and ZBM-30 remained elusive for the major part of this study, and these zeolites could therefore not be catalytically tested. The XRD patterns showed pure ZSM-22 materials with different SARs (60, 80 and 120) were successfully synthesised using hexamethylenediamine (HMDA) as a structure-directing agent (SDA) and a synthesis gel with pH 12. However, at synthesis gel pH 13, cristobalite and ZSM-5 impurity phases tend to form in addition to ZSM-22. Relative % XRD crystallinity of the materials prepared at synthesis gel pH 12 decreased with a decrease in SAR, and there was no specific trend for the response of a particular SAR to changes in pH from 12 to 13. SEM micrographs showed needle-shaped crystals with lengths in the range 0.6 - 1.2 μm. The BET surface area of the ZSM-22 with SAR of 60 was found to be 189 m2/g, which is around the theoretical BET surface area of ZSM-22 materials and the presence of impurities lowered the surface area of the ZSM-22 materials. The synthesis of ZBM-30 using triethylenetetramine (TETA) and a (1 TETA : 1 pyrrolidine) mixture as SDAs was also attempted. The XRD patterns showed that a completely amorphous material was obtained when using TETA as SDA and ZSM-39 was produced when using the mixture as SDA. The XRD patterns revealed that impurity-free ZSM-23 materials were successfully synthesised with SAR > 60, and that with SAR of 60, ZSM-5 was produced instead. Relative % XRD crystallinity of the impurity-free ZSM-23 materials increased with an increase in SAR from 80 to 120. SEM micrographs of the impurity-free ZSM-23 materials showed needle-shaped crystals of around 0.9 μm in length. The predominantly ZSM-5 material had the highest BET surface area compared to the impurity-free ZSM-23 materials. ZSM-48 synthesis was attempted using HMDA (produced ZSM-22), pyrrolidine (produced ZSM-23) and hexamethonium bromide (HMBr2) as SDAs. The XRD and SEM vi analysis showed only HMBr2 successfully directed the synthesis of impurity-free crystalline ZSM-48 at prolonged synthesis time of 168 h. ZSM-48 crystals were also needle-shaped and 4.2 - 11.3 μm in length. The incipient wetness impregnation method was used to achieve 0.5 wt. % Pd loadings on the catalysts. The hydrocracking of n-hexadecane (n-C16) over the catalysts was studied at conditions typical of catalytic cracking of LTFT products. At 225 oC, the Pd/ZSM-22 (80) and Pd/ZSM-23 (80) were highly selective to cracking products, with excessive secondary cracking occurring over these catalysts, as indicated by the C4/C12 ratios of 11.3 and 5.2, respectively. Excessive secondary cracking (C4/C12 = 11.7) was also observed over Pd/ZSM-23 (60). However, the Pd/ZSM-22 (60) and Pd/ZSM-23 (120) catalysts achieved a C4/C12 ratio close to 1.0, suggesting closeness to ideal hydrocracking behaviour. The Pd/ZSM-22 (60) (C4/C12 of 1.9) catalyst, was physically mixed with Pd/ZSM-5 (90) (C4/C12 = 6.4) and catalytically tested for the hydrocracking of n-C16. This Pd/ZSM-5/ZSM-22 catalyst achieved a remarkable C4/C12 = 1.1, which is less than what was achieved over the individual catalysts. On the basis of the C4/C12, this catalyst’s behaviour is close to that of an ideal hydrocracking behaviour. In summary, Pd/ZSM-22 (80), Pd/ZSM-22 (120) and Pd/ZSM-23 (80) catalysts are promising for diesel-selective catalysis and need further exploration.

Graduation date: 1997

Structure direction in the formation of zeolitic materials has been investigated through the parallel approaches of structural and synthesis studies. The structures of gallosilicates TNU-6 and TNU-7 have been solved from powder X-ray diffraction. TNU-6 (P6₃, a = b = 10.5078(1)Å, c = 8.5277(1)Å) is found to possess a stuffed tridymite-like structure isostructural with BaFeGaO₄. Evidence from electron diffraction, single crystal and high-resolution powder X-ray diffraction suggests the presence of a √3a superstructure, analogous to the related KAlGeO₄ phase, arising from a subtle variation in tilt of tetrahedral units. The structure of TNU-7 consists of an ordered 1:1 intergrowth of alternating sheets of mazzite and mordenite (Pmmn, a = 7.5721(1)Å, b = 17.0739(2)Å, c = 25.8438(5)Å). The crystallisation field of TNU-7 is found to lie between those of mazzite and mordenite, suggesting that this is an example of a ‘boundary phase’ - phase selectivity being governed strongly by the presence and quantity of Ga in the synthesis gel. The distribution of extra-framework Na⁺ cations in the as-made material, and Cs⁺ and Sr²⁺ in ion-exchanged samples, suggests a higher extra-framework charge per T-site associated with the mazzite region of the structure, indicating the possible existence of compositional zoning, consistent with the preference to form mazzite at high Ga gel-content. The structures and synthesis conditions for both TNU-6 and TNU-7 suggests a cooperative effect between Ga and extra-framework species in directing the formation of these phases. The structure of the novel aluminosilicate TNU-9 has been confirmed by energy minimisation and powder neutron diffraction studies (C2/m, a = 28.177(2)Å, b = 20.030(1)Å, c = 19.464(1)Å, β = 92.311(4)º ). Monte Carlo-Simulated Annealing studies have been employed to investigate the favoured location of the 1,4-bis(N- methylpyrrolidinium)butane template molecule within the complex pore system. Remarkably, three (possibly four) different positions are assumed by the structure-directing agent, forming head-to-middle and head-to-head motifs between constituent framework aluminosilicate sheets. TNU-9 crystallises in a very narrow gel composition window suggesting a strong cooperative effect between organic and inorganic gel components. Structure direction of phosphate-based materials (aluminophosphates, magnesium aluminophosphates and silicoaluminophosphates) has been investigated through co-templating synthesis studies. In particular, the structure-directing activity of Cu²⁺ (and to a lesser extent Ni²⁺) complexes of the azamacrocyles cyclam and cyclen, and related derivatives, is investigated in the presence of additional amines (dipropylamine, diisopropylamine, tetraethylammonium hydroxide, diisopropylethylamine). Complexes of a selection of linear polyamines, and also cobalticinium are studied for comparison. Although added primarily as pH moderators, the additional amines are also found to influence the crystallisation and hence provide effective routes to the synthesis of transition metal-containing materials with potential catalytic application. The ‘strength’ of the structure-directing ability of the additional amines is found to vary depending on the identity of the primary structure-directing agent, with behaviour ranging from passive pH moderator to dominating structure-directing agent. The outcome of syntheses is also highly dependent on inorganic gel composition. Through appropriate combination of structure-directing agent and additional amine, mixed Cu²⁺/Ni²⁺-containing MgAPO and SAPO STA-6 (SAS) and STA-7 (SAV) materials have been synthesised for the first time, as well as a low Si form of SAPO STA-7 (with and without Cu²⁺). Also, the combination of cyclam, tetraethylammonium hydroxide and HF has been found to produce a SAPO analogue of STA-7 possessing silicate islands. In addition, transition metal-containing materials possessing the AEL, AFI, CHA, UT-6 and LEV topologies have also been synthesised via this approach. Elemental analysis, UV-Vis, ESR and NMR spectroscopy are employed to confirm the presence of the desired complex in selected samples, while X-ray diffraction and ESR spectroscopy are employed to investigate the environment of extra-framework transition metal species, post calcination, in MgAPO STA-6, SAPO STA-6 and STA-7, MgAPO-18, and MgAPO-5.