Salah A. Hassan

The effect of different atmospheres on the sintering of PtAl2O3 reforming catalysts

Abstract The effect of different atmospheres, namely, nitrogen, oxygen and hydrogen on the sintering of diluted samples of Pt Al 2 O 3 reforming catalyst was studied in the temperature range 300–800 °C. It was found that the results of sintering in these atmospheres at temperatures below 400 °C are very similar to those previously obtained in the case of unsupported platinum. The treatment of the catalyst samples at temperatures higher than 400 °C in both N 2 and H 2 could lead to an activation; the phenomenon which was observed previously by heating in vacuo . However, the treatment in O 2 showed a continuous deactivation over the whole range of studied temperatures. Some interpretations are also given.

Catalytic Polymerization of Methyl Methacrylate in Different Media Using Supported Metal Phthalocyanines, 1. Bulk Polymerization in Relation to the Microcrystalline Structure of Supported Metal Phthalocyanines

Different catalyst samples of nickel(II) and copper(II) phthalocyanines supported over bentonite clay (Indian type) were prepared with complex loadings ranging between 0.2 and 2.2 wt.-%. Catalyst activities were investigated in the bulk polymerization of methyl methacrylate (MMA) at 80°C in the absence and in the presence of sodium bisulfite as a cocatalyst. In all cases, the catalyst efficiency was evaluated in terms of conversion yield (%), polydispersity index (M w /M n ) was determined via GPC analysis in toluene. The stereotacticity was determined from 1 H NMR spectra. Optimum polymerization conditions were determined for both catalyst systems. Samples with metal phthalocyanines loadings of 1.8% were the most active ones. The polymers produced have controlled molecular weights, which are nearly proportional to conversion, show narrow molecular weight distributions and syndiotactic-rich steric structures. Nickel phthalocyanine seemed to have more stereoregulating effect while copper phthalocyanine had a more enhancing effect for the living polymerization process. The microstructure of solid catalysts was investigated by means of scanning electron microscopy and were correlated with activity data.

Solution properties of polystyrene in cosolvent systems

At 293 K, intrinsic viscosities [η] have been measured for polystyrene samples of different relative molar mass M in mixtures of two poor solvents. These solvents were ethyl acetate and cyclohexane. Upon mixing these two poor solvents, thermodynamically better solvents could be obtained. The cosolvency was detected from the viscosity measurements. Several graphical procedures have been utilized for deriving the unperturbed dimensions of polystyrene expressed as Kθ (in the relation [η]=KθM12α13, where α is the expansion factor). It was found that the unperturbed polymer dimensions were not constant and differed from those measured in the single θ-solvent (trans-decalin) in which Kθ was found to be 81 × 10−3 dm3 kg−1.

Catalytic Behavior of Cobalt (II) Phthalocyanine Immobilized on Bentonite Clay in Bulk Polymerization of Methyl Methacrylate

The catalytic behavior of cobalt (II) phthalocyanine (CoPc) immobilized on bentonite clay, with different complex loadings ranging between 0.2 and 2.2 wt%, in the presence of n-butyl amine solvent, was investigated in bulk polymerization of methyl methacrylate without using an activator or cocatalyst. The interaction of CoPc molecules with a bentonite surface, encouraged by the amine, involved the inclined stacking model, i.e., interaction between N-atoms of the macro-ring system and OHs of the support. Two different mechanistic pathway regions could be suggested, depending on complex loadings. The first was in the range, 0.2−1.0 wt% CoPc, behaving in ionic fashion as the bare bentonite. Isolated oriented molecules probably activated bentonite through the exposure of new internal acid sites during intercalation. The second, in the range of 1.4−2.2 wt% CoPc, proceeded via combined mechanisms, ionic functioned by bentonite support and free radical functioned by complex packed oriented stacks, staggered in clay galleries to cover a fraction of internal active acid sites. The number of polymer chains formed per one CoPc center (reaction turnovers) was >2–∼10 with diluted catalyst samples. With higher loadings, one chain was formed per active site. Such behaviors, without a detectable effect on polymer characteristics or enchainment sequence, were studied in light of intercalation, orientation and accessibility models.

Effect of thermal treatment on catalytic and surface characteristics of a diluted supported CoAl2O3 catalyst

The sintering of a diluted sample of CoAl2O3 hydrocracking catalyst was investigated in the temperature range 300–450 °C. Both activity (A) and metallic surface area (Sco) measured through chemisorption of hydrogen showed essentially the same sintering trends: deactivation below 400 °C and activation above this temperature. In the temperature region below 400 °C, the activation energy of the sintering process, determined from both activity and chemisorption data, was found to be 14.70 kcal/mole. This is thought to be due mostly to surface diffusion. On the other hand, activation occurred at 450 °C as a result of the increased migration of metal atoms on the support surface. This indicates that a solid solution with lowered melting point might be formed, and the sintering step becomes rate-determining in this temperature region. Changes in the total surface area and in the pore-size distribution of the support during the heat treatment were also studied.

Can microwave assisted in situ reduction of supported Pt nanoparticles challenge the chemical method in controlling the dispersion profile-catalytic performance relationship?

In this study, Pt (of 0.3, 0.6 and 0.9 wt% loadings) was supported on a mesoporous silica surface via a microwave-assisted solution (MAS) method or rotary chemical evaporation (RCE) method in the in situ reduction step. The as-synthesized Pt nanocatalysts were characterized through XRD, XRF, TGA/DSC, TEM, N2-adsorption–desorption, H2-pulse titration and electrical conductivity techniques. The samples prepared by MAS method exhibited higher surface area and a better dispersion profile of Pt NPs, of average sizes not exceeding 10 nm with increasing the Pt loading. In contrast, although RCE method showed higher efficacy in decomposing the used precursor, an uneven distribution of larger Pt nanoparticles (≥33 nm) was determined. Electrical properties in terms of AC conductivity and dielectric constant confirmed the enhancement of an even distribution of smaller Pt NPs with a higher concentration of grain boundaries effected by microwave electromagnetic radiations. Highly mobile electrons and lattice vibrations (phonons) were favored, as compared with aggregated NPs produced during RCE method. The TOF values calculated for reactions to selectively produce ethylene (from ethanol) or benzene (from cyclohexane) decreased with Pt loading on the catalyst samples synthesized by MAS method. The highly dispersed NPs (of 3–7 nm) seemed to be responsible for the activity in both reactions, probably tending to be structure insensitive. However, samples reduced by RCE method, with enlarged average sizes of surface Pt NPs (approaching 15.5 nm), exhibited increasing TOF values with Pt loading, i.e., causing the reactions most probably to be structure sensitive.

Rare earth oxides doped NiO/γ-Al2O3 catalyst for oxidative dehydrogenation of cyclohexane

Abstract The effect of rare earth oxides (RE=Ce, La, Gd, and Dy) doping of alumina support in NiO/γ-Al 2 O 3 system was investigated on its catalytic performance in oxidative dehydrogenation (ODH) of cyclohexane. The physicochemical properties of various samples were followed up through N 2 physisorption, temperature programmed reduction (H 2 -TPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and potentiometric acid-base titration techniques. In the parent NiO/γ-Al 2 O 3 catalyst, Ni species were found to be strongly interacted with alumina surface. Addition of rare earth dopants to γ-Al 2 O 3 in the catalyst system affected the nickel-alumina interaction and resulted in significant modifications in the catalytic performances in the ODH reaction. The results revealed the beneficial role of both La 2 O 3 and Gd 2 O 3 doping in enhancing the ODH catalytic activity and selectivity to cyclohexene. H 2 -TPR and XPS results indicated that majority of Ni species in NiO/La 2 O 3 modified γ-Al 2 O 3 were more weakly interacted with La 2 O 3 and alumina whereas both NiO like species and nickel aluminate were present on the surface. Doping with cerium or dysprosium increased the nickel-support interaction and led to a decrease in surface nickel concentration. In case of doping with Ce, surface concentration of cerium oxide was higher than those of the other RE oxides; the doped catalyst reached its steady state activity faster than the other catalysts. The acid-base results suggested that RE metals were interacted most likely with acidic surface hydroxyl groups. The degree of nickel-alumina interaction decreased in the following order: LaAl>GdAl>CeAl>DyAl.

A Comparative Study of Surface Characteristics of Nickel Supported on Silica Gel, γ-Alumina, Aluminosilicate

Abstract Surface characteristics of the prepared nickel catalysts containing 7, 10, and 13 wt% Ni w/w over different supports—silica gel, γ-alumina, and aluminosilicate—were investigated. Surface areas, total pore volumes, and average pore radii were determined for all catalysts. Pore analysis was discussed based on Vl-t plots and pore size distribution. The measured surface areas and pore volumes of pure supports increased in the following order: γ-alumina < aluminosilicate < silica gel. Pore analysis showed that SiO2 and Al2O3-SiO2 and their supported Ni samples were characterized by presence of narrower mesopores of ink-bottle type. Al2O3 was distinguished by presence of two distinct pore types, both showing continual increase in fraction with a shift to larger dimensions upon loading with nickel. Penetration and/or incorporation process of Ni particles took place at the expense of their interaction with Al+3. SiO2 revealed a gradual increase in surface parameters upon loading with nickel. For Al2O3-SiO2—supported samples, the result proposed the interaction of Ni with both alumina and silica contents of the support regardless of the penetration process.

Surface and catalytic characteristics of thermally and chemically activated bentonite catalysts used in the polymerization of styrene

Abstract The most active bentonite samples in the polymerization of styrene were found to be one thermally treated for 2 hr at 550 °C and one chemically treated with 35% ( w w ) H2SO4. Surface studies of various bentonite samples showed that the characteristic pore system and Lewis acid sites resulting from the removal of the adsorbed water were responsible for the catalytic activity of the thermally treated bentonite. Overheating could destroy both the catalyst activity and the exposed surface. Upon activating bentonite with H2SO4, it was found that both the produced pore system and the creation of new Bronsted acid sites could be responsible for the catalytic activity. The different factors affecting the polymerization reaction over the most active samples and the dependence of the structure of the produced polymer on the pretreatment of the bentonite were also investigated.

Redox-Initiated Bulk Polymerization of Methyl Methacrylate Using a CuO/TiO2 Catalyst System

Catalytic bulk polymerization of methyl methacrylate (MMA) has been carried out at 80°C over pure titania (anatase), and supported CuO/TiO2 catalyst system of different CuO % loadings (ranged between 0.5–20 wt%), without using an initiator or cocatalyst. The produced polymers were characterized via hydrogen nuclear magnetic resonance (1HNMR) and gel permeation chromatography (GPC). The prepared catalyst samples were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and N2-adsorption-desorption. The polymer yield (%) was shown to increase by increasing the CuO content up to 8.0 wt%, and then decreased. All polymers were of high molecular weights with low % of pentads. They were almost of syndiotactic-rich structure, produced mainly through a chain-end control mechanism. The polymerization process was suggested to be catalyst-dependent, redox-initiated and following a free radical mechanism. The sample of 8.0% CuO loading (above the monolayer coverage) was the most active; a fraction of loaded CuO existed on the titania external surface, while a larger fraction seemed to be inserted into the support pore system.