Nicholas M. Musyoka

Optimization of hydrothermal synthesis of pure phase zeolite Na-P1 from South African coal fly ashes

This study was aimed at optimizing the synthesis conditions for pure phase zeolite Na-P1 from three coal fly ashes obtained from power stations in the Mpumalanga province of South Africa. Synthesis variables evaluated were: hydrothermal treatment time (12–48 hours), temperature (100–160°C) and varying molar quantities of water during the hydrothermal treatment step (H2O:SiO2 molar ratio ranged between 0–0.49). The optimum synthesis conditions for preparing pure phase zeolite Na-P1 were achieved when the molar regime was 1 SiO2: 0.36 Al2O3: 0.59 NaOH: 0.49 H2O and ageing was done at 47°C for 48 hours. The optimum hydrothermal treatment time and temperature was 48 hours and 140°C, respectively. Fly ashes sourced from two coal-fired power plants (A, B) were found to produce nearly same high purity zeolite Na-P1 under identical conditions whereas the third fly ash (C) lead to a low quality zeolite Na-P1 under these conditions. The cation exchange capacity for the high pure phase was found to be 4.11 meq/g. These results highlight the fact that adjustment of reactant composition and presynthesis or synthesis parameters, improved quality of zeolite Na-P1 can be achieved and hence an improved potential for application of zeolites prepared from coal fly ash.

Synthesis of hydroxy sodalite from coal fly ash using waste industrial brine solution

The effect of using industrial waste brine solution instead of ultra pure water was investigated during the synthesis of zeolites using three South African coal fly ashes as Si feedstock. The high halide brine was obtained from the retentate effluent of a reverse osmosis mine water treatment plant. Synthesis conditions applied were; ageing of fly ash was at 47°C for 48 hours, and while the hydrothermal treatment temperature was set at 140°C for 48 hours. The use of brine as a solvent resulted in the formation of hydroxy sodalite zeolite although unconverted mullite and hematite from the fly ash feedstock was also found in the synthesis product.

In situ ultrasonic diagnostic of zeolite X crystallization with novel (hierarchical) morphology from coal fly ash.

In this paper the applicability of an in situ ultrasonic diagnostic technique in understanding the formation process of zeolite X with a novel morphology was demonstrated. The complexity of the starting fly ash feedstock demands independent studies of the formation process for each type of zeolite since it is not known whether the crystallization mechanism will always follow the expected reaction pathway. The hierarchical zeolite X was noted to follow a solution phase-mediated crystallization mechanism which differs from earlier studies of the zeolite A formation process from unaged, clear solution extracted from fused fly ash. The use of the in situ ultrasonic monitoring system provided sufficient data points which enabled closer estimation of the time of transition from the nucleation to the crystal growth step. In order to evaluate the effect of temperature on the resulting in situ attenuation signal, synthesis at three higher temperatures (80, 90 and 94 °C) was investigated. It was shown, by the shift of the US-attenuation signal, that faster crystallization occurred when higher temperatures were applied. The novel hierarchical zeolite X was comprised of intergrown disc-like platelets. It was further observed that there was preferential growth of the disc-shaped platelets of zeolite X crystals in one dimension as the synthesis temperature was increased, allowing tailoring of the hierarchical morphology.

Synthesis of Zeolite A, X and P from a South African Coal Fly Ash

Synthesis conditions for generating high quality zeolite type Na-P1, A, and X from a South African coal fly ash were identified in this study. XRF, XRD, FTIR and SEM analytical techniques were used to characterize the starting fly ash feedstock and the resulting synthesis product. Synthesis of these high quality zeolites types is expected to provide a competitive alternative for recycling the ever increasing quantity of fly ash produced in the South African coal-fired power plants. This approach will not only be environment friendly but could also be attractive from an economic point of view since the cost of disposal of fly ash could be offset by the income generated from the sale of the synthesized zeolitic materials.

In-situ ultrasound study of the kinetics of formation of zeolites Na–A and Na–X from coal fly ash

The kinetics of synthesis of zeolites Na–A and Na–X from fused South African class F coal fly ash were studied by using an ultrasound device as a real-time, in-situ diagnostic tool. Ex-situ techniques, for example XRD, ICP, and SEM, were used to complement the results of the kinetic study. Reaction rate, reaction order, and activation energy of crystallization processes in clear solution extracted from fused fly ash were calculated on the basis of ultrasound signal data recorded at different crystallization temperatures. Zeolite Na–X and zeolite Na–A crystals were both obtained without ageing. The zeolite Na–X sample showed contaminations of zeolite P and sodalite depending on the synthesis temperature. For zeolite Na–A the impact of ageing on the process of formation was also studied.

Comparison of MOF-5- and Cr-MOF-derived carbons for hydrogen storage application

AbstractNanoporous carbons which possess high surface areas and narrow pore size distributions have become one of the most important classes of porous materials with potential to be utilized for hydrogen storage. In recent times, several metal–organic frameworks (MOFs) have been shown to be promising precursors for creating nanoporous carbons due to their high surface areas and tunable pore sizes. The pore structure and surface area of the resultant carbon materials can be tuned simply by changing the calcination temperature. In this work, a zinc-based MOF (MOF-5) and a chromium-based MOF (Cr-MOF) were both used as precursors for syntheses of nanoporous carbons by the direct carbonization technique at different temperatures. The resultant carbon nanostructure from MOF-5 possessed higher surface area, higher pore volume and enhanced hydrogen storage capacity as compared to pristine MOF. Meanwhile, the derived carbons from Cr-MOF displayed lower surface areas, pore volumes and hydrogen uptake than the parent MOF due to the formation of chromium oxide and carbide species in the pores of the Cr-MOF-derived carbons.

Leaching and antimicrobial properties of silver nanoparticles loaded onto natural zeolite clinoptilolite by ion exchange and wet impregnation.

ABSTRACT This study aimed to compare the leaching and antimicrobial properties of silver that was loaded onto the natural zeolite clinoptilolite by ion exchange and wet impregnation. Silver ions were reduced using sodium borohydride (NaBH4). The leaching of silver from the prepared silver-clinoptilolite (Ag-EHC) nanocomposite samples and their antimicrobial activity on Escherichia coli Epi 300 were investigated. It was observed that the percentage of silver loaded onto EHC depended on the loading procedure and the concentration of silver precursor used. Up to 87% of silver was loaded onto EHC by wet impregnation. The size of synthesized silver nanoparticles varied between 8.71–72.67 nm and 7.93–73.91 nm when silver was loaded by ion exchange and wet impregnation, respectively. The antimicrobial activity of the prepared nanocomposite samples was related to the concentration of silver precursor used, the leaching rate and the size of silver nanoparticles obtained after reduction. However, only in the case of the nanocomposite sample (Ag-WEHC) obtained after loading 43.80 ± 1.90 µg of Ag per gram zeolite through wet impregnation was the leaching rate lower than 0.1 mg L−1 limit recommended by WHO, with an acceptable microbial killing effect.

Metal-Organic Frameworks as Materials for Fuel Cell Technologies

The dwindling reserves of hydrocarbon fuel resources and the associated environmental impact of burning fossil fuels have led to the search for clean and sustainable energy technologies. Hydrogen is widely considered as a promising alternative to fossil-based fuels because it is clean, has high energy content, and it can potentially be derived from water, which is abundantly available. Fuel cells are a key component of the hydrogen energy value chain where their role is to convert the chemical energy of hydrogen into electrical energy. In the hydrogen energy value chain, hydrogen production and storage are critical enabling technologies for fuel cells. The future of the Hydrogen Economy faces significant challenges which must be overcome before its realisation. Therefore, considerable research is currently directed at finding efficient, safe and affordable materials for fuel cells and their enabling technologies. Metal-organic frameworks (MOFs), a class of organic-inorganic hybrid crystalline solids, are promising materials for these technologies due to their many distinct characteristics including a wide structural diversity, low weight, extraordinarily high surface areas, large free volumes, and tunable pore sizes and functionalities. This chapter presents an overview of MOFs as potential materials for fuel cell components i.e. polymer electrolyte membrane and electrocatalysts, and enabling technologies for fuel cells i.e. hydrogen production and storage. Although significant progress in research has been made there are still challenges to overcome for the practical use of MOFs in these technologies.

Synthesis of zeolite NaA membrane from fused fly ash extract

ABSTRACT Zeolite-NaA membranes were synthesized from an extract of fused South African fly ash on a porous titanium support by a secondary growth method. The influence of the synthesis molar regime on the formation of zeolite NaA membrane layer was investigated. Two synthesis mixtures were generated by adding either aluminium hydroxide or sodium aluminate to the fused fly ash extract. The feedstock material and the synthesized membranes were characterized by X-diffraction (XRD), scanning electron microscopy (SEM) and X-ray fluorescence spectroscopy (XRF). It was found by XRD and SEM that the cubic crystals of a typical zeolite NaA with a dense intergrown layer was formed on the porous Ti support. The study shows that the source of Al used had an effect on the membrane integrity as sodium aluminate provided the appropriate amount of Na+ to form a coherent membrane of zeolite NaA, whereas aluminium hydroxide did not. Morphological, the single hydrothermal stage seeded support formed an interlocked array of zeolite NaA particles with neighbouring crystals. Also, a robust, continuous and well-intergrown zeolite NaA membrane was formed with neighbouring crystals of zeolite fused to each other after the multiple stage synthesis. The synthesized membrane was permeable to He (6.0 × 106 L m−2h−1 atm−1) and CO2 (5.6 × 106 L m−2h−1 atm−1), which indicate that the layer of the membrane was firmly attached to the porous Ti support. Membrane selectivity was maintained showing membrane integrity with permselectivity of 1.1, showing that a waste feedstock, fly ash, could be utilized for preparing robust zeolite NaA membranes on Ti support.

Utilization of waste tyres pyrolysis oil vapour in the synthesis of Zeolite Templated Carbons (ZTCs) for hydrogen storage application

ABSTRACT In this study, we investigated the potential for use of waste tyre pyrolysis oil vapour as a carbon precursor in the synthesis of zeolite templated carbons (ZTC). With Zeolite 13X as the template, the ZTCs were synthesised using two methods namely: 1-step process which involved the carbonization of gaseous carbon precursor in the zeolite template (in this case, ethylene and pyrolysis oil vapour) and the 2-step synthesis method involved the impregnation of zeolite pores with furfural alcohol prior to carbonization of the gaseous carbon precursor. The replication of the zeolite 13X structural ordering was successful using both methods. The 2-step synthesized ZTCs were found to possess the highest specific surface area (3341 m2 g−1) and also had the highest H2 storage capacity (2.5 wt.%). The study therefore confirmed an additional novel strategy for value-addition of waste tyre pyrolysis by-products.