Awad I. Ahmed

Surface and acidic properties of some mixed oxide catalysts in relation to their catalytic activities

Abstract A series of silica-alumina catalysts were prepared to contain different proportions of alumina. The catalysts were calcined at 773–973 K. Another series of silica-titania catalysts of different chemical compositions were prepared and calcined between 773 and 973 K. The textural properties (surface area and pore structure) of the catalysts were determined by low-temperature nitrogen adsorption at 77 K. The surface acidity (acid amount and acid strength) was determined for each catalyst by thermal programmed desorption (TPD) of pyridine. The catalytic cracking of cumene at 673–773 K and the catalytic conversion of isopropanol at 473–593 K were investigated using microcatalytic technique. Cracking of cumene is a first-order catalytic reaction which takes place on relatively strong acid sites and the reaction is structurally sensitive. Dehydration of isopropanol on mixed oxide catalysts is a zero-order reaction and proceeds on most of the acid sites. Dehydration of isopropanol is related to the surface acid density and is structurally insensitive.

Studies on the inhibition of aluminium dissolution by some hydrazine derivatives

Abstract The inhibitive effect of some hydrazine derivatives on the dissolution of aluminium in 2N hydrochloric acid solutions has been studied. The volume of the hydrogen evolved was followed as a function of time and the corresponding rates of dissolution were determined. The inhibition efficiencies of the hydrazine derivatives were evaluated as the percentage reduction in reaction rate. The obtained results are found to be in good agreement with those given by the thermometric and weight loss methods. Adsorption of the studied hydrazine derivatives occurs in one step as a monolayer of the adsorbate on the metal surface, with the exception of diformyl hydrazide. The results indicate that the adsorption of hydrazine derivatives occurs over the aluminium surface through their carbonyl group.

Effect of thermal treatment on the surface characteristics of magnesia-chromia catalysts

Abstract The adsorption of nitrogen at 77 K was measured on pure magnesia, pure chromia and magnesia-chromia containing various percentages of chromia obtained by heating in air in the temperature range 450–850 °C. The adsorption properties indicate porous structures for all the samples. The surface areas of all the samples were calculated and the pore analysis was also evaluated. With increases in the calcination temperature above 600 °C, the surface areas decrease while the pore radii increase. From the variations in surface area with the temperature, it is concluded that two opposing processes are operating during the thermal treatment. The first process is the activation process which tends to increase the surface area and which operates at low temperatures. The second process, which decreases the surface area, is the sintering process predominating at high temperatures.

Studies of dehydrogenating catalysts I: Structural and textural properties of some dehydrogenating catalysts

Abstract The structure of metal-metal oxide dehydrogenating catalysts depends on the metal content. Reduction with hydrogen is incomplete for catalysts with a high metal content and severe conditions may be required. Molybdenum metal and the lower valence state oxide Mo9O26 were detected in Mo-Al2O3 catalysts. A thin skin of aluminate is formed on Ni-Al2O3 and Co-Al2O3 catalysts; this retards their reduction with hydrogen. The deposition of the reduced metal or the formation of metal oxide or spinel on the alumina are associated with significant variations in the textural properties (surface area and pore volume distribution).

Structural and surface properties of nickel oxide

Abstract The effects of thermal treatment on both nickel hydroxide and nickel nitrate were studied by means of differential thermal analysis and X-ray analysis followed by nitrogen adsorption. The specific surface areas and pore size distributions were estimated. The surface area and pore volume increase with increasing temperature, to reach a maximum at 300 °C, and then decrease. From the effects of the thermal treatment on the surface area it is concluded that the activation process is directly followed by a sintering process. The effects of the thermal treatment on the pore size distribution are also discussed.

Structural changes and surface properties of thermally treated pure and doped cobalt oxide

Abstract Surface area measurements were carried out by nitrogen adsorption at —196 °C on cobalt oxide and on lithium- and ferric-doped cobalt oxides. Thermal products were obtained by calcination in the temperature range 250–1000 °C. Structural and phase changes were studied by differential thermal analysis (DTA) and X-ray techniques. DTA of samples showed that heating of the pure and doped cobalt hydroxide in air in the temperature range 250–700 °C yields Co 3 O 4 . On increasing the heating temperature to 800 °C CoO is formed. The dehydration products obtained at temperature less than 500 °C were amorphous to X-rays. Doping with foreign ions causes textural changes in the parent solid.

Synthesis of sulfamic acid supported on Cr-MIL-101 as a heterogeneous acid catalyst and efficient adsorbent for methyl orange dye

Typical highly porous metal–organic framework (MOFs) materials based on chromium benzenedicarboxylates (Cr-BDC) were prepared through a one-pot hydrothermal synthesis, and were then modified by loading the appropriate ratio of sulfamic acid (SA) using a simple impregnation technique. Pure and modified MIL-101 was characterized by XRD, TEM, SEM and FT-IR measurements. TEM and SEM measurements confirmed that the MIL-101 particles preserved their regular octahedral structure after loading with different weight contents of sulfamic acid. The total number of acid sites and Bronsted to Lewis acid sites ratio (B/L) were examined using potentiometric titration and pyridine adsorption. The acid strength and surface acidity of SA/MIL-101 gradually increased after the modification of Cr-MIL-101 by sulfamic acid crystals up to 55 wt%, then decreased again. The catalytic performance of the solid catalysts was confirmed in the synthesis of 14-phenyl-14H-dibenzo [a,j] xanthene and 7-hydroxy-4-methyl coumarin. In the two reactions, the sample with 55% sulfamic acid loaded on MIL-101 displayed the highest catalytic activity and acidity. The adsorption behaviors of sulfamic acid loaded on MIL-101 materials for methyl orange (MO) as an anionic dye were studied, and were exceptionally suitable for the Langmuir adsorption isotherm. All loaded adsorbents showed high adsorption capacity for methyl orange at 25 °C. The results indicate that the adsorption capacity was modified by changing the amount of sulfamic acid loaded on MIL-101.

Effects of thermal treatment and fluoride ion doping on surface and catalytic properties of NiO–ZrO2 catalysts

Abstract ZrO2 and a series of NiO/ZrO2 hydrogels (5 to 25 wt.% NiO) were co-precipitated with the aid of NaOH–Na2C2O4 solution. Two fluorinated hydrogels were also prepared by wet impregnation method. The samples were calcined in the temperature range of 550 to 850 °C. The surface properties of the samples were determined using DTA, XRD and nitrogen adsorption at −196 °C. The conversion of isopropanol was tested using microcatalytic pulse technique. DTA measurements showed that the addition of nickel oxide to zirconia influences the phase transition of ZrO2. XRD revealed that the tetragonal phase was formed at T ⩽ 650 °C, while a biphasic mixture was obtained at T ⩾ 750 °C. No spinel structure was detected by both DTA and XRD techniques and only traces of cubic NiO were detected for the samples containing ⩾ 15 wt.% nickel oxide and calcined at T ⩾ 750 °C. Significant changes in texture, surface acidity and catalytic activity were found as a result of the effects of thermal treatment and chemical composition. Incorporation of fluoride ions greatly increased the surface acidity and consequently enhanced the dehydration activity. It has been found that dehydration activity is related to the amount of surface acidity while the dehydrogenation of this alcohol is sensitive to NiO content.

Effect of gamma irradiation on the texture, acidity and catalytic activity of silica-aluminium and silica-magnesia catalysts

Abstract The textural properties of non-irradiated and γ-irradiated silica-aluminium and silica-magnesia catalysts were determined from nitrogen adsorption. The acidities of these catalysts were measured by the chemisorption of pyridine. Cracking of cumene and dehydration of isopropanol were investigated on non-irradiated and γ-irradiated catalysts. Irradiation with γ-rays enhanced sintering, i.e. decreased the surface area and increased pore size. For silica-magnesia, γ-irradiation enhanced crystallization of Forsterite and Enstatite. The surface acidity decreased upon γ-irradiation and consequently the activity of the catalysts towards cracking of cumene and dehydration of isopropanol was reduced. The former reaction is structure-sensitive, while the latter is structure-insensitive.