Kalala Jalama

Recent trends in the microwave-assisted synthesis of metal oxide nanoparticles supported on carbon nanotubes and their applications

The study of coating carbon nanotubes with metal/oxides nanoparticles is now becoming a promising and challenging area of research. To optimize the use of carbon nanotubes in various applications, it is necessary to attach functional groups or other nanostructures to their surface. The combination of the distinctive properties of carbon nanotubes and metal/oxides is expected to be applied in field emission displays, nanoelectronic devices, novel catalysts, and polymer or ceramic reinforcement. The synthesis of these composites is still largely based on conventional techniques, such as wet impregnation followed by chemical reduction of the metal nanoparticle precursors. These techniques based on thermal heating can be time consuming and often lack control of particle size and morphology. Hence, there is interest in microwave technology recently, where using microwaves represents an alternative way of power input into chemical reactions through dielectric heating. This paper covers the synthesis and applications of carbon-nanotube-coated metal/oxides nanoparticles prepared by a microwave-assisted method. The reviewed studies show that the microwave-assisted synthesis of the composites allows processes to be completed within a shorter reaction time with uniform and well-dispersed nanoparticle formation.

Thermal stability of Ti–MCM-41

Ti containing mesoporous MCM-41 materials have been synthesized through two methods: heating and non-heating [room temperature (RT)]. The synthesized materials have been characterized using X-ray diffraction, Fourier transform infrared, nitrogen sorption, and X-ray fluorescence methods and their thermal stabilities evaluated using thermogravimetric methods in inert atmosphere. The thermal stabilities have been analyzed based on the synthesis method, as well as on the amount of titanium in the MCM-41 materials. The thermal stability results suggest that uncalcined MCM-41 materials generally show higher mass loss than their calcined counterparts. Also, the RT-synthesized materials showed lower stability than the high-temperature synthesized samples for the uncalcined samples. It is also been found that MCM-41 materials show improved thermal stabilities as the amount of titanium is increased.

Carbon dioxide hydrogenation over nickel-, ruthenium-, and copper-based catalysts: Review of kinetics and mechanism

ABSTRACT This study critically reviews the mechanism of CO2 hydrogenation over Ni, Ru, and Cu, and the effect of catalyst properties and operating conditions on reaction kinetics. Most studies have reported the presence of CO and formate species on Ni-, Ru-, and Cu-based catalysts, where subsequent conversion of these species depends on the type of catalyst and the physicochemical properties of the catalyst support. Methane is the major product that forms during CO2 hydrogenation over Ni and Ru catalysts, while methanol and CO are mainly produced on Cu catalysts. A different approach for catalyst formulations and/or process development is required where long chain hydrocarbons are desired.

The effect of starch amylose content on the morphology and properties of melt-processed butyl-etherified starch/poly[(butylene succinate)-co-adipate] blends.

In this study, butyl-etherified waxy and high amylose starches were melt-processed with biodegradable poly[(butylene succinate)-co-adipate] (PBSA) to improve the long-term stability of the starch-based materials. Butyl-etherification was believed to improve the compatibility of starch with PBSA matrix. Scanning electron microscopy results demonstrated that highly branched amylopectin structures in butyl-etherified starch have better chemical interaction with the PBSA matrix compared to linear amylose structures. Thermogravimetric analysis revealed that the thermal stability of the blends decreased with increasing starch loading for all starch types with varying amylose content; however, the nature of the starch controlled the mechanical properties of the blends. Furthermore, the differential scanning calorimetric and polarised optical microscopic results demonstrated that the degree of crystallinity of PBSA decreased with increasing loading of amylopectin content in starch. Therefore, this work reveals the possibility of improving the inherent properties of starch by blending with PBSA through careful selection of the amylose content in starch.

The effect of recrystallization time on pore size and surface area of mesoporous SBA-15

Triblock copolymer, pluronic P123 (EO20PO70EO20) was used as the structure directing agent for the simple synthesis and characterization of mesoporous SBA-15 with various porosities. Extending the sample recrystallization time, after the initial synthesis and ageing, seems to have a significant effect on the pore size. It also leads to an increase in the surface area and a narrow pore size distribution. The prepared materials could find applications in areas where the diffusion of large molecules is important, and in catalysis, where greater pore accessibility would enhance the activity and selectivity of the catalyst. A possible mechanism has been proposed to describe the effect of extended recrystallization on pore sizes.

Expanding the synthesis of Stober spheres: towards the synthesis of nano-magnesium oxide and nano-zinc oxide

In order to synthesise Stöber spheres of different diameters, fixed amounts of ethanol, ammonia and water were used with varying amounts of tetraethyl orthosilicate. This simple method was further applied to the synthesis of nano-ZnO and nano-MgO from their respective precursors, zinc methoxide and magnesium ethoxide. The spherical nano-SiO2, nano-ZnO and nano-MgO synthesised in this way were characterised by X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy and transmission electron microscopy. Preliminary investigations found that nano-ZnO and nano-MgO showed good catalytic activity in the trans-esterification reaction that converts vegetable oil to biodiesel.

Confinement effect of rhodium(I) complex species on mesoporous MCM-41 and SBA-15: effect of pore size on the hydroformylation of 1-octene

Rhodium(I) complexes based on triphenylphosphine (TPP) and water soluble trisodium salt of triphenylphosphine (TPPTS) as catalysts precursors were successfully anchored on mesoporous silica (MCM-41 and SBA-15) to study the effect of confinement by an inorganic support in catalysis. The performance of the immobilized complexes was investigated in the hydroformylation of 1-octene and compared with the catalytic results of homogeneous catalysts. High activities and regioselectivies towards n-nonanal were obtained for the immobilized catalysts. Overall, heterogenized catalysts offered regioselectivity towards n-nonanal in the range of 80–90% compared to their homogeneous counterparts (70–96%) in the hydroformylation of alkenes based on TPP. We found that the pore size had a remarkable effect on the in situ formation of Rh-phosphine carbonyl complexes. Furthermore, adding excess of TPP and TPPTS ligand enhances the catalytic performance of the catalyst system during hydroformylation of 1-octene. The catalysts could be reused several times without loss of activity or selectivity under identical reaction conditions.

Simulation of South African Corncob Gasification with Aspen Plus: A Sensitivity Analysis

Biomass is one of the significant fuel sources that can potentially contribute to the worlds energy demands. The most used technology that takes advantage of biomass heating value is still direct combustion which still suffers from significant inefficiencies creating therefore, the need for alternative processes to be analyzed. In this study Aspen Plus simulation package was used to develop a model for the gasification of corncob residues. The effects of equivalence ratio (ER) and steam to biomass ratio (SBR) on syngas composition and gasification temperature were investigated. The results showed that ER values of 0.34-0.35 and SBR values of 0.8 - 1 corresponding to a gasifiers temperature of 854-890°C were the optimum conditions for corncobs gasification.

Effect of Operating Pressure on Fischer-Tropsch Synthesis Kinetics over Titania-Supported Cobalt Catalyst

The kinetics for Fischer-Tropsch (FT) reaction over a titania-supported cobalt catalyst have been measured at 220 °C using a constant space velocity of 20 ml/gCat/min and total operating pressure of 1, 10 and 20 bar respectively. The catalyst was prepared by incipient wetness impregnation method and the evaluation was carried out in a fixed-bed reactor. The CO conversion increased with an increase in pressure from 1 to 10 bar and decreased when the pressure was further increased to 20 bar. The methane selectivity decreased and the selectivity toward C5+ hydrocarbons increased with increases of the total operating pressure from 1 to 10 and 20 bar. However, operating at 10 bar yielded the highest rate of C5+ hydrocarbons production.

Biodiesel production from waste vegetable oils over MgO/Al2O3 catalyst

MgO/Al2O3 catalysts with 10 and 20 wt.% MgO loadings have been prepared by incipient wetness impregnation method. A low-surface area alumina with small pore sizes was used as catalyst support to stabilise most of the MgO particles on the external surface area of the support. The prepared catalysts were subsequently tested in the conversion of a waste vegetable oil to biodiesel. The waste vegetable oil conversion was found to increase with the increase in reaction temperature, reaction time and MgO loading in the catalyst.