Mohamed Kadri Younes

Comparative Study of the Acidity of Sulphated Zirconia Supported on Alumina Prepared by Sol-Gel and Impregnation Methods

Sulphated and unsulphated alumina-zirconia with atomic ratio Zr/Al = 0.5 were prepared by sol-gel and impregnation methods. The solid prepared by the sol-gel method exhibits the higher specific surface area. The Kelvin probe shows that the value of unsulphated sample is around 400 mV. This value grows up to 1100 mV for sample prepared by impregnation of aerogel alumina by sulphated propoxide zirconium and up to 1450 mV for sulphated alumina-zirconia aerogel catalyst. The modification of the work function is probably due to the charge transfer from the zirconium and aluminium to an oxygen species, responsible for the increase of Lewis acidity. XPS results show that the aluminium and zirconium exist in oxide form as Al2O3 and ZrO2. The sulphur is present as sulphate species in the solids bonded to the Al—Zr—O framework. Furthermore, the oxygen species exist in different types created by the introduction of sulphur in the bulk of solids.Compared to the impregnated catalyst the sol-gel sulphated alumina zirconia exhibits higher activity in the isopropanol dehydration reaction in the temperature range 423 K–523 K.

Study of Acidity of Aerogels ZrO2-SO42− by Isopropanol Dehydration Reaction, Surface Potential and X-Ray Photoelectron Spectroscopy

Sulphated zirconia aerogels, with definite atomic ratio S/Zr and hydrolysis ratio (H = H2O/Zr) were prepared by the autoclave method. The addition of sulphate ions causes a decrease of the cristallinity of zirconia. XPS results show the O1s photoelectronpeak which could be decomposed in two components for the reticular oxygen of the zirconia framework and for oxygen attributed to the OH groups and/or sulphates groups, and the S2p photopeak characteristic of sulphates species. The Kelvin probe shows that the value of pure zirconia is around 200 mV. This value grows up to 1200 mV for sulphate doped catalysts. The modification of the work function is probably due to the charge transfer from the zirconium to an oxygen species, responsible for the increase of Lewis acidity. The catalysts prepared with hydrolysis ratio of H = 4 exhibit higher activities in the isopropanol dehydration reaction than those with H = 2 in the temperature range 373 K–423 K.

Effect the solvent evacuation mode on the catalytic properties of nickel-modified sulfated zirconia catalysts: n-hexane isomerization

This work studies the effect of the evacuation mode of the solvent to optimize the catalytic performances of sulfated zirconia doped with nickel prepared by the sol gel method in one step. Aerogel and xerogel catalysts exhibit different textural, structural and catalytic properties at various calcination temperatures. Aerogels, obtained by drying under supercritical conditions of solvent, exhibit a developed specific surface area and stabilize zirconia tetragonal phase before heating and even at high calcination temperature. However, xerogels obtained by ordinary drying in an oven are amorphous and has a low surface area and weak porosity. XPS spectroscopy shows that the nickel in aerogels is more reducible than those in xerogels. Aerogels exhibit higher activity than the xerogels, in the n-hexane isomerization reaction.

ZrO 2 Aerogels

Mesoporous zirconia aerogels are very interesting materials with important properties used for several applications such as ceramics and catalysis. The textural and structural properties of zirconia aerogel depend on the preparation parameters such as the hydrolysis ratio, the acid and zirconium precursor concentrations, and the gel aging. The extraction of the solvent under its supercritical conditions (high-temperature supercritical conditions) or under the supercritical conditions of CO2 (low-temperature supercritical condition) leads to aerogel zirconia with different properties. In fact, aerogel zirconia dried under high-temperature supercritical conditions is well crystallized and exhibits a large surface area. However, aerogels obtained with low-temperature supercritical conditions are amorphous and present a low surface area. Zirconia aerogels doped by metals, such as platinum, iron, and copper, or ions, in particular sulfate, phosphate, and tungstate anions, exhibit very important catalytic properties in many reactions such as n-alkane isomerization and Fischer–Tropsch synthesis. Doping zirconia with other oxides leads to materials with very interesting properties, allowing its use in fuel cells, thermal barrier coatings, oxygen sensors and many other high-temperature applications.

Characterization and reactivity of zirconia-doped phosphate ion catalyst prepared by sol–gel route and mechanistic study of acetic acid esterification by ethanol

Phosphated zirconia prepared by sol–gel method has been used as catalyst in esterification reaction of acetic acid with ethanol. Optimization of different preparation parameters on the catalyst was studied, such as the effect of molar ratio nP/nZr, surfactant assisted synthesis, calcination temperature, and the effect of the drying mode. Catalysts were characterized by N2 physisorption at −196 °C, X-ray diffraction, FTIR spectroscopy, and 31P MAS NMR spectroscopy. The obtained results show that an increase in phosphate content partially inhibits the development of tetragonal t-ZrP phase and leads to a rise of both specific area and pore size of the catalyst. Besides, the introduction of the surfactant in the preparation step develops this phase and enhances the size of pores, but decreases specific area. However, calcination of the catalyst allows the development of tetragonal ZrO2 phase and causes the disappearance of ZrP phase. The evacuation of the solvent at its supercritical conditions promotes the development of both tetragonal phase of zirconia and pore size but slows that of the phases related to the ZrP species. Catalytic properties of acid esterification by ethanol were correlated to catalyst characteristic data, suggesting that t-ZrP phase and doping agent-support interaction stabilize active sites of the catalyst. Kinetic and mechanism study shows that catalytic reaction occurs with a first order and takes place through Eley–Rideal mechanism in which adsorbed acetic acid species react with ethanol in fluid phase to form corresponding ester. Application of Eyring theory shows that the adsorption step is characterized by an endothermic character and a rapid associative mechanism occurs between adsorbed species and the second reactant.Graphical abstractEley–Rideal mechanism of acetic acid esterification by ethanol over xerogel catalyst XZrP0.05