Search

Abstract: Acid leaching was investigated to remove phosphorus from the ashes of thermally-treated (gasification and combustion process) footwear leather waste in order to obtain a good quality raw material for the production of a ferrochromium alloy. The ash contained high amounts of chromium (55wt.% Cr2O3) and phosphorus (1.42wt.% P). The following key factors were selected for study: temperature, H2SO4 concentration in the leaching solution, acid solution/ash ratio and the particle size of the ash. The response was the phosphorus removal percentage and, under some experimental conditions, chromium loss. The strategy was based on a 24 full factorial design with a central point followed by the addition of star points in a face-centered central composite design. The maximum chromium loss was 1.1wt.%. The significant effects (according to variance analysis) were used for a proposed second-order response surface model. The model was useful for finding the optimal operating conditions in the experimental region by the maximization of phosphorus leaching. 99.2±2.5% phosphorus removal from the ash was achieved under the following conditions: 60°C, 13.9wt.% of H2SO4 in the leaching solution, 4.80 (ml of acid solution)(g of ash)−1, 0.0370±0.0370mm particle size. Mass balance was used as a tool to indicate that the remaining phosphorus content was acceptable for producing ferrochromium alloys. [Copyright &y& Elsevier]

Abstract: Ni–Mg–Al catalysts prepared by continuous coprecipitation were evaluated for carbon dioxide reforming of methane carried out between 500 and 700°C. The mixed oxides obtained after calcination exhibit higher surface area values and lower crystallinity. The activity results obtained with different samples demonstrated that the catalytic properties of Ni–Mg–Al catalysts are more affected by the MII/MIII ratio than the Ni/Mg ratio. For samples with constant MII/MIII ratio the best results were obtained for 5≥Ni/Mg>1 ratios. Among the thermal pretreatment effects, the reduction temperature was revealed as a strong influence on the activity and selectivity for the CO2 reforming of methane. In contrast, the catalytic properties can be considered practically independent from the calcination temperature, and the influence of this parameter is restricted to the decrease of the surface area values with the increase of the calcination temperature. [Copyright &y& Elsevier]

Abstract: Samples of zinc oxide were obtained by thermal decomposition of zinc salts (acetate and nitrate) and by precipitation (chloride and nitrate). The samples were characterized by S BET, X-ray diffraction (XRD), temperature-programmed reduction (TPR) and scanning electron microscopy (SEM). XRD showed that ZnO crystals exhibited similar morphologies, while the crystal size and the particles morphology, availed by SEM, are dependent on the preparation method and the precursor salt utilized. The catalytic properties were evaluated to sec-butyl alcohol (SBA) reaction, using nitrogen, hydrogen and synthetic air as carrier gas. The catalytic properties of ZnO surface are strongly dependent on crystal size, atmosphere of reaction and zinc salts precursor. The activity to dehydrogenation of SBA in oxidant atmosphere is strongly sensitive to crystal size, while the selectivity to methyl ethyl ketone (MEK) is slightly sensitive. The activity to dehydrogenation and dehydration of SBA increases with change of atmosphere reaction from oxidant to reducing. High reaction temperatures and reducing environment for ZnO sample with small crystal size decrease significantly the selectivity to MEK. The ZnO crystal size has an important role to establish the properties of partial reduction of ZnO and catalytic properties. [Copyright &y& Elsevier]

Zeolite dealumination and desilication are common methods, which improve accessibility of the active sites located inside the zeolite crystallites and tune the zeolite acidity. The exact mechanism of these post-synthesis zeolite treatments remains under discussion. In this paper, a series of dealuminated and desilicated ZSM-5 zeolites are prepared using single or combined post-synthesis dealumination and desilication of large zeolite single crystals with oxalic acid and sodium hydroxide. The ZSM-5 zeolite has been synthesized using silica extracted from fly ash. Zeolite desilication results in the potholes, leaf-type structures and other clearly visible defects on the surface of zeolite crystallites, while the effect of dealumination is less pronounced. The low temperature nitrogen adsorption suggests the formation of mesopores in the zeolites formed by the voids between and in the irregular zeolite crystallites produced during post-synthesis treatments. 27Al and 29Si MAS NMR in combination with XRD data are indicative of only minor modifications of the bulk zeolite structure during dealumination and desilication. The total Brønsted acidity only slightly decreases and is not affected by the type of zeolite post-synthesis treatment. After desilication of large single crystals of ZSM-5, noticeable enhancement of the Brønsted acidity on the zeolite external surface has been detected by FTIR of adsorbed collidine. The zeolites with higher acidity on the external surface have shown higher activity in the anisole acylation with hexanoic acid. A combination of characterization techniques suggests that the dealumination and desilication of the large crystals of ZSM-5 zeolite using post-synthesis treatments respectively with acid and alkali selectively starts at the zeolite external surface. Image 1 • Desilication and dealumination strongly affect the surface of zeolite crystallites. • Potholes and other defects on the zeolite external surface contribute to mesoporosity. • Zeolite bulk structure is not altered at first stages of post synthesis treatments. • Higher acidity on the external surface enhances activity in the anisole acylation. [ABSTRACT FROM AUTHOR]

Carbon membranes have emerged in the 70's and have been presenting promising results for application in processes involving gas separation because of their sieving effects. The carbon membranes are obtained by pyrolysis of a precursor polymer beyond its initial decomposition temperature under essentially inert conditions. Supported and unsupported carbon membranes can be produced, but the former are distinguished for the industrial separation of gases due to the improved mechanical strength and high chemical and thermal stability. In this context, different types of support, coating methods and pyrolysis conditions for supported carbon membranes have been reported in the literature, in order to improve the separation capability of gas mixtures in respect to permeability and selectivity. The aim of this review article is to report and discuss the evolution of supported carbon membrane in the last 10 years in respect to configuration, transport mechanisms, manufacturing processes and its main applications, highlighting the main challenges still to be overcome for this technology to be applied industrially. [ABSTRACT FROM AUTHOR]

Carbon molecular sieve membranes (CM) are typically synthesized through pyrolysis of polymers, and are suitable for gas separation under high pressure conditions due to their high selectivity and mechanical strength. There is a lack of knowledge of the mechanisms of membrane formation during pyrolysis, which would be of fundamental importance to a better understanding and precise control of the process. In this work, we investigate the process of CM formation upon pyrolysis of polyetherimide. A carbon membrane was synthesized on a ceramic support and characterized by different techniques, demonstrating the conversion of the polymeric precursor into a porous and predominantly amorphous carbon structure, containing embedded graphitic domains. The membrane showed effective molecular sieve performance as demonstrated in gas permeation tests. Reactive molecular dynamics simulations were carried out to explore the mechanisms of the polymer→CM transformation at the atomistic level. The simulations show that the continuous reaction among the reactive radicals formed upon polymer pyrolysis along with atomic rearrangements spontaneously led to the formation of a nanoporous amorphous carbon matrix, containing interconnected graphitic domains throughout the structure, as observed in the experiments. These results are expected to be of great help to the development of techniques for controllable synthesis of CMs. [ABSTRACT FROM AUTHOR]