الفهرس 
Introduction and Object of InvestigationOnly 14 pages are availabe for public view
 |  | from | 284 |  |  |
| | | from | 284 | | |
Abstract
Catalysis is one of the largest branches in the chemical industry. One of the purposes of catalytic processes is to increase the yield of the feedstock up to a level to be used in industry using less cost and specialized catalysts.
In this way, different physicochemical techniques have been used to characterize the nature and different types of catalysts. On the basis of these as well as on adsorption studies, researches in material sciences have made possible the preparation of solid acids, based on inorganic oxides. On the top of that and in collaboration with catalytic specialists, acidity has been coupled with adequate surface and structural properties in order to optimize catalytic activity and selectivity in a large variety of reactions with organic compounds.
CuO-ZnO/Al2O3 systems, being the scope of this thesis ,bear unique characteristics that reflect interest in studying their activity .It is generally accepted , that both the structural and electronic character of a catalyst are important in determining its activity. In order to obtain a deep understanding of the mechanisms of catalysis, the relative importance of these two parameters needs to be understood.
The prevailing investigation comprises a trial of correlating the catalytic activity of the abovementioned catalysts that have the general form Cu1-xZnx/Al2O3 where 0.0≤x≤1.0 with its structure and texture towards the decomposition of ethanol. The main aim of the present study is to prepare a series of mixed oxide solid catalysts with high thermal stability, high surface area, high acidity and high catalytic activity .This was achieved by supporting such oxides onto a suitable support such as alumina. A sequential aim is to investigate the effect of alumina supported CuO or ZnO as well as mixture of these oxides on the physicochemical properties, and their catalytic activity.
The parent materials were prepared by the impregnation method followed by calcination in air atmosphere at a range of temperature 400-1000°C for five hours.
Different coordinated physicochemical techniques have been used in order to characterize the different catalysts and identify the nature of different types of acid sites. These physicochemical techniques include Thermal analysis (TGA and DTA), X-Ray diffractometery (XRD), Nitrogen Adsorption (BET) and adsorption of volatile amine such as pyridine.
Gas-Phase ethanol decomposition in flow system using gas chromatography (GC) technique attached to a flow system has been adopted to check the catalytic activity of the prepared solids.
from this study, the following conclusions have been drawn via the results presented above and discussed:
(1) γ-Al2O3 obtained by calcination at 600°C of boehmite, (γ-AlO (OH)),expose highly clean surfaces of high specific area (SBET=204 m2g-1) and low acidity values.
(2) CuO interacts with Al2O3 to produce CuAl2O4 at temperature starting from 600°C, the complete conversion of copper oxide into copper aluminate requires heating the mixture containing the solid oxides at 800-1000°C.ZnO hinders the formation of CuAl2O4 produced at 600°C and stimulates its formation at 800°C.
(3) On the other hand, ZnO reacted partially with Al2O3 giving rise to ZnAl2O4 which showed thermal stability up to 1000°C and accompanied by increase in its crystallinity upon raising the temperature
(4) Characterization with XRD, indicates that the copper oxide is present as a copper aluminate surface phase on alumina with low copper oxide content, bulk CuO are present as well.
(5) Al2O3 treatment of CuO and ZnO followed by calcination at 400-1000°C effected a measurable change in the specific surface area, acidity and catalytic activity.
(6) Specific surface area of Al2O3 was found to decrease by increasing the content of either CuO or ZnO added along the temperature range 400-1000°C.
(7) Although supporting ZnO-CuO on Al2O3 decreases its specific surface area and total pore volume, this treatment increases the surface acidity in the range of thermal treatment 400-1000°C.The decrease in the SBET values has been attributed to the phase transition, pore alteration and sintering specially at high calcined temperatures.
(8) Ethanol decomposition over Cu1-xZnxAl2O4 catalysts system was constituent with the auto- catalytic process featuring i.e. (a) A dehydration catalytic process leading to ethylene and diethyl ether; (b) the progressive dehydrogenation catalytic process leading to methane and acetaldehyde.
(9) With respect to ethanol decomposition over Cu1-xZnxAl2O4 system proceeded with:
(i) Ethylene, diethylether, acetaldehyde and methane formation represent the major products.
(ii) Dehydrogenation selectivity was strongly dependent on both the reaction temperature and ethanol conversion.
(iii) Zinc-containing catalysts especially those calcined at 600 and 700°C showed higher dehydrogenation selectivity than copper-containing ones.
(10) Regarding to the dependence of the activity of Cu1-xZnxAl2O4 catalysts on their structural properties, Our results showed that the samples containing zinc aluminate spinel phase were more active than those containing copper aluminate spinel phase (DTA, TGA and XRD analyses as well as acidity measurements vide infra).
(11) The catalytic activity of ethanol decomposition conducted at 200-450°C over Cu1-xZnxAl2O4 catalysts was found to increase progressively as a function of the calcination temperature. Moreover, the activity of the catalysts under investigation was dependent on the reaction temperature .Accordingly the optimum temperature was chosen foe each catalyst.
(12) The obtained results, demonstrate that the coexistence of CuAl2O4 and/or ZnAl2O4 as result of reaction between alumina and CuO and/or ZnO provides a synergistic enhancement of ethanol decomposition activity. Furthermore, alumina needs to be accompanied by a donor phase to achieve better performances. Such important role was played by the aluminates spinels containing the of Cu1-xZnx/Al2O3 mixtures.
(13) The dehydrogenation of ethanol was found to be dependent on the ZnO oxide content.
(14) The catalytic activity of Cu1-xZnxAl2O4 decreases with increasing the amount of ZnO present. This decrease was, however, more pronounced for the treated solids at 400°C.
(15) Loading of ZnO and CuO species, via impregnation of the alumina with aqueous solution of Zn (NO3)2 and Cu (NO3)2 and subsequent calcination at 600°C, rendered the alumina as an ethane formation selective catalyst, Also its selectivity increased towards diethylether formation.
(16) ZnO and/or CuO species at x=0.5, are converted into highly dispersed particles and, presumably, surface species dispersed in monomolecular layers onto the support surface, and/or dissolved into the support bulk. Such phases have not changed significantly the alcohol %conversion, and dehydration rate.
(17) Catalysts derived from Cu1-xZnxAl2O4 system and calcined at 600 and 700°C possess a significant total surface acidity higher than that of the corresponding calcined at 400,500,800,900 and 1000°C.The surface acidity increased with increasing calcination temperature from 400 to 600 for copper rich samples or from 400 to 700°C for zinc rich ones. Hence, catalysts were more reactive towards ethylene and diethyl ether formation.
(18) Higher copper contents were found to increase the dehydrogenation pathway from 400 to 600 for copper rich samples or from 400 to 700°C for zinc rich ones. This was attributed to suggestion that zinc cannot influence the dispersion of copper in a positive way at high copper concentrations. Also, copper forms a sort of agglomerates, at high contents which leads to a decrease in the surface area and, hence, a lower activity. The Performance of Cu1-xZnxAl2O4 system was greatly enhanced at calcinations temperature 600 and/or 700°C.
(19) The decrease in the rate of ethylene formation was attributed to the increase in the surface acidity of alumina as a result of incorporation with Zn2+ and /or Cu2+ .This explains the strengthening of the C2H5O- bond and hence a preference towards ethylene formation.
(20) Treatment of Al2O3 with CuO and/or ZnO followed by calcination at 400-1000°C did not modify the mechanism of the catalytic reaction and did not considerably decrease the concentration of surface sites without affecting their energetic nature.