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The heterogeneous metallocene catalyst is becoming a very competitive industrially due to its ability to produce tailor-made polymers. The main advantage of the metallocene polymer product is the narrow molecular weight distribution (MWD) and the systematic comonomer distribution along the polymer chains. Therefore, the metallocene polymer product has well-defined mechanical and optical properties. The aim of this thesis is to investigate the effects of the silica support on the reaction kinetics and micro properties of the heterogeneous metallocene catalyst system. These investigations include studying the influence of the pore volume, surface area, particle size distribution, and the surface chemical characteristics of silica support on the catalyst performance. The experiments showed that the silica type has an influence on the kinetic behavior. For instance, silica with a lower pore volume shows an induction period when compared with higher pore volume silicas. Moreover, the silica type has a clear influence on catalyst activity and polymer morphology. The smallest silica particles produced the highest activity among the other sizes regardless of silica type. The supported catalysts were characterized and linked to the silica type and size in terms of catalyst activity and polymer morphology. Each catalyst in terms of silica type behaved similarly regardless of type of alkylaluminum used in the formulation. The micro properties of the produced polymers, such as MWD and chemical composition distribution (CCD), were studied to understand the effects of the type and size of silica support and co-catalyst on these properties. The silica types showed no effect on the MWD, but had a slight effect on the CCD. Silica with a high pore volume had a stronger more comonomer response. However, the silica particle size had an influence on the CCD, with less comonomer incorporation observed with smaller silica particles. Finally, triethylauminum was observed to produce polymer with a different MWD when compared with other alkylaluminums. However, all alkylaluminums used in this work had no effect on the CCD of the produced polymer regardless of silica type. / Thesis (Master, Chemical Engineering) -- Queen's University, 2012-04-11 13:37:09.878

This thesis concerns the hydrothermal stability of two zeolite molecular sieves with the MFI structure, ZSM-5 and its 'aluminium free' form silicalite. Silicalite was synthesised from low pH alkali metal free aqueous gels at 95°C and characterised by XRD, SEM, DTA and TG. ZSM-5 was crystallised from similar reaction mixtures but with the addition of aluminium salts. It was found that the incorporation of aluminium into the ZSM-5 framework was favoured by higher reaction temperatures (l50°C) and the use of aluminium nitrate. Solubilities were measured for silicalite, ZSM-5, ZSM-11, ZSM-39, ZSM-48 and EU-4 at temperatures up to 95°C. The solubilities were influenced by framework structure, the presence of template within the zeolite channels and the aluminium content of the lattice. Solubility measurements over a range of liquid/solid ratios showed that calcined silicalite was contaminated with about 1% amorphous silica. Treatment with liquid water over the temperature range 95 to 230°C was used to remove amorphous silica and alnminosilicate species from silicalite and ZSM-5. This hydrothermal treatment had many other effects on the properties of these materials. The water uptake, orthorhombic to monoclinic symmetry transition, framework Si/Al ratio and thermal properties were all affected. SEM showed that the inside of the silicalite crystals was more soluble than the outer surface. The use of hydrothermally treated H-ZSM-5 as a catalyst for but-1-ene isomerisation showed that the treatment could have a marked effect on catalytic behaviour, and particularly on product selectivity.

The extraction of geothermal heat can cause precipitation of the minerals dissolved in geothermal fluid. Their deposition on the walls of wells and above-ground plant and in pores near reinjection wells, also known as mineral scaling, is one of the main obstacles to increasing the effectiveness of utilization of the limited geothermal resources. If not controlled properly it can result in accumulation of a significant amount of scale which obstructs pipes and reinjection wells and reduces the efficacy of heat exchangers. The most abundant mineral in geothermal fluid is silica and thus its precipitation can cause the highest scaling rate. While this dissertation is devoted to the study of silica scaling the results obtained may be applicable to other minerals with similar deposition mechanism. Oversaturated silica is known to precipitate from aqueous solution either by the direct chemisorption of single silicic acid molecules (monomers) or by forming colloidal particles suspended in the solution. These particles can subsequently be transported to, and attach onto, a wall. This process of colloidal silica deposition was previously recognised to cause much faster scaling than the direct deposition of silica monomers under typical geothermal plant conditions. While the chemical kinetics of silica polymerization and colloid formation are relatively well understood, transport of these colloids and their stability, which control their aggregation and attachment rates, on the other hand are not. Previous studies of the silica scaling process have identified prominent effects of geothermal brine hydrodynamics on the scaling rate. It was found to increase with the flow rate and particle size, thus suggesting the dominance of the advective (inertial) deposition of colloidal silica. However, this conclusion contradicted the present theory of particle transport in turbulent flows which argues the dominance of the diffusive transport for the relevant range of particle sizes (<1 μm). The development and continuing improvement of the anti-scaling measures required deeper understanding of the complex combination of the phenomena involved in the process of silica scaling. This was pursued in the present study using theoretical and experimental methods. First, the rate of colloidal silica transport from a turbulent flow onto the internal surface of a circular pipe, a cylinder and a flat plate were calculated using available analytical and numerical methods. The obtained theoretical transport rate was found to be about four orders of magnitude higher than the corresponding experimental scaling rate. The latter was determined in the previous studies to be 4.2·10-8 kg/s/m2 for silica colloids of 125 nm in diameter which corresponded to the dimensionless deposition velocity (the dimensionless deposition velocity is the scaling rate normalised by the particle mass concentration and friction velocity) of 1.2·10-6 for the dimensionless particle relaxation time of 2·10-4. Next, based on the standard DLVO theory of particle interactions and in the framework of the Smoluchowski approach the probability of colloidal silica particle attachment to a wall was found to be 10-6. Therefore, the theoretical scaling rate, calculated as a product of this probability and the above-mentioned transport rate was two orders of magnitude lower than the experimental scaling rate. This suggested that the implemented theoretical approach either underestimated particle transport rate or overestimated particle stability. Both possibilities are explored in this dissertation. In addition, the silica scaling rate was measured for a range of conditions: particle size from 20 to 60 nm, particle concentration 1600-10000 ppm, friction velocity from 0.09 to 0.18 m/s (Re = 9-50·103) and ionic strength from 30 to 80 mM, pH 8.1-9.5 and temperature from 25 to 44 °C. For this, laboratory experiments were designed and progressively modified in order to improve the repeatability of the results and to study the scaling process. In these experiments colloidal silica deposition onto the walls of mild steel pipe sections was studied with a recirculating flow rig with variable (but controllable) particle size, concentration, flow rate, pH and ionic strength of the solution. In addition, a parallel plate flow test section was designed and built which will provide better capabilities for the control over the hydrodynamic and test surface conditions in future experiments. The control over the chemical conditions was achieved by the use of the synthetic colloidal solutions. Two methods of their production – hydrolysis of either sodium metasilicate or active silicic acid – were employed. The influence of the synthesis conditions, ion content and pH on the long term behaviour of these colloidal solutions was investigated. The particle size data, obtained using dynamic light scattering (DLS) and verified by electron microscopy, was analysed and compared against the predictions of the current models of nanoparticle growth and stability. The kinetic aggregation was identified to be the dominant particle growth mechanism. Experimental data collected during the long-term observations of the particle growth allowed relationships between the aggregative stability and such parameters as the particle size, ion concentration and pH of the solution to be elucidated. In particular, the aggregative stability of 10-20 nm particles was found to be 108-1010 which is 7-9 orders of magnitude higher than the corresponding DLVO stability. It was also found to decrease with the increase of the particle size. This agreed with the theory of the colloid stabilization by steric interactions. Moreover, the model of the “gel” layer was used to explain the observed “anomalies” of the colloidal silica behaviour. The deposition experiments conducted with these synthetic colloidal solutions showed that the scaling rate increased with the particle size, flow rate and ionic strength (IS) of the solution. Thus, it was measured to be 9.7·10-9 kg/s/m2 for the 45 nm particles in a solution with IS = 0.05 M, which corresponded to the dimensionless deposition velocity of 6.6·10-8 for a dimensionless particle relaxation time of 2.2·10-6. The scaling rate was calculated for these conditions by multiplying the corresponding transport rate and the actual attachment probability determined as an inverse of the experimental stability. It was found to agree with the experimental value within an order of magnitude. In addition, the observed increase of the scaling rate with the increase of particle size was explained by the compensation of the decreased rate of the particle transport by faster decrease of actual particle stability (increase in attachment probability). Therefore the contradiction between the theory and the experiment was resolved for the particles of 20 to 60 nm in diameter. Moreover, the observations of the dimensions and distribution of the scale elements formed in some of the present experiments strongly suggested the significance of the advective (inertial) mechanism of particle deposition. This and comparative analysis of other experimental and theoretical data suggested that the present theory may underestimate the convective transport of the particles onto a rough wall. Therefore, the hypothesis of the parallel-to-wall advective deposition of the nanoparticles onto the roughness/scale elements (not accounted in the current theory) was proposed. The corresponding mass transfer problem was solved analytically using experimentally found dimensions of the scale elements. The additional transport was found to decrease the above-stated discrepancy between the theoretical and experimental scaling rate for large (125 nm) particles by one order of magnitude. The remaining difference of one order of magnitude was speculated to be due to the underestimation of these particles attachment probability derived with the standard DLVO theory. The actual aggregative stability of the silica colloids larger than 60 nm in diameter and for a wider range of IS values is of interest for future experimental studies. An improved understanding of the interrelation between the chemical and hydrodynamic phenomena in the process of silica scaling and its dominant mechanisms was achieved in this dissertation. This allowed optimization of the present anti-scaling practices aimed to minimize the negative effects of mineral scaling on the operation of geothermal power stations. Besides the practical recommendations, which may ultimately help to increase the efficiency of geothermal power stations, the results of the present study may be of value in the fields of mass transfer and colloid science.

This thesis is concerned with the formation and structure of silicified deposits in biology. The major system studied is silicified macrohairs from the lemma of the grass Phalaris canariensis L. The macrohairs consist of silica and polysaccharides. Chemical and structural studies on the mineral phase utilised electron microscopy (transmission (TEM), scanning (SEM) and ultra high resolution (HRTEM)), energy disoersive X-ray analysis (EDXA), solid state nuclear magnetic resonance ( ᷣ⁹ Si nmr), infrared spectroscopy, birefringence and nitrogen adsorption experiments. Results showed that the silica is chemically 'pure', hydrated, amorphous at a resolution of 1OÅ and a variety of structural morphologies were observed which are related to the maturity of the macrohair. Analytical studies at different times after emergence of the inflorescence utilising EDXA and scanning proton microprob eanalysis (SPM) showed that the inorganic elements Si, K, P, S and Cl are spatially organised within the macrohairs during silicification. It is proposed that the macrohairs are silicified under strict cellular control. The organic matrix in the mature macrohairs was investigated by acid hydrolysis and chromatography. The changing emphasis of polysaccharide synthesis in the macrohairs as mineralisation occurs was followed by in vivo radioactive labelling of inflorescences at different stages using ⁱ⁴C glucose and Harabinose. Analysis o fpolysaccharides synthesised involved acid hydrolysis and enzymic digestions (Amylase and Driselase), followed by paper and thin layer chromatography with scintillation counting of the products. Results showed that at the early stages of mineralisation, arabinoxylans and cellulose are the major polymers synthesised but as the macrohair matures, largely non-cellulosic glucans (as yet unidentified) are synthesised. It is proposed that the change in emphasis of polysaccharide synthesis during wall development is related to the size and ultrastructural arrangements of silica particles observed. The organic matrix was also observed to give additional order to the system, the resulting material being totally impervious. A second system, chosen for comparison, is mineralised teeth from the radula of the common limpet Patella vulgata. The mature teeth contain silica, iron oxide (goethite) and an organic matrix. Investigations on the silicified phase utilising electron microscopy revealed morphological structural variations. Analytical studies involving EDXA and SPM analysis showed that there are complex temporal and spatial variations in the inorganic composition (P, S, Ca, Fe, Si, Cu) in all regions of the teeth. It is proposed that these changes can be correlated with changes in composition of the organic matrix. A comparison is made of the silica from the two systems.

The field of mesoporous materials has been expanding rapidly in recent years, and has come to include a wide variety of different types of materials from organic to inorganic, as well as hybrid materials that encompass both worlds. The following account explores one type of mesoporous materials, specifically those consisting of silica with an attached organic group that have come to be known as periodic mesoporous organosilicas (PMOs). Much of the work here involves incorporating new types of organic groups into a mesoporous framework for the purpose of adding a useful functionality, either chemical or physical, to the material. Firstly it is shown that a borazine moiety can be successfully incorporated into a mesoporous material with a very high loading. It was further shown that once incorporated into the material many of the borazine moieties are available for further chemical reactions with acids and transition metals. Next, a new class of materials termed hybrid periodic mesoporous organosilicas (HPMOs) was developed that was able to circumvent many of the problems associated with PMO self-assembly. Now, using very simple techniques, virtually any type of silsesquioxane can be incorporated into a PMO, and the organic group can be specifically at the surface of the pores, thereby maximizing its accessibility. And finally, a PMO is made that incorporates buckyballs, and it is shown that, given the right synthetic conditions, the buckyballs are homogeneously distributed throughout the material.

The thermal plasma production of fumed silica in a transferred arc consists of the decomposition of quartz to SiO (g) and oxygen followed by an oxidizing quench back to SiO2. The particles formed have diameters of the order of 10 to 20 nm and are linked in a three dimensional branched chain aggregate. Previous work by Addona and Munz (1999) demonstrated the technical feasibility of producing fumed silica using this method, but was unable to demonstrate the special rheological properties of the powder. The most important characteristic of fumed silica is the presence of hydroxyls on the surface of the particles, in the form of isolated hydroxyl groups, hydrogen-bonded hydroxyl groups and siloxane groups. / In the present work, we studied the changes in the powder quality by varying the quench conditions used for the production of the powder and by agglomerating the obtained particles. The fumed silica was agglomerated by conveying in a length of tubing with sharp bends. The powder was characterized using BET, Viscosity tests, FT-IR, TEM, SEM and XRD. The product was compared to both a commercial product (Aerosil 200) and the material previously produced by Addona. Tests were done before and after the agglomeration experiments. / The experimental results showed that the agglomeration had no effect on the powder's rheological properties. We concluded that the smaller viscosity values obtained for the plasma produced fumed silica were due to the lack of the free hydroxyl groups from the surface of the particles.

Thesis (Ph. D.)--University of New South Wales, 2004. / Also available online.

Dissertation (Ph. D.)--University of Akron, Dept. of Polymer Science, 2008. / "December, 2008." Title from electronic dissertation title page (viewed 12/28/2008) Advisor, Roderic P. Quirk; Committee members, Scott Collins, Ali Dhinojwala, Li Jia, Mark D. Soucek; Department Chair, Ali Dhinojwala; Dean of the College, Stephen Z. D. Cheng; Dean of the Graduate School, George R. Newkome. Includes bibliographical references.