Activated aluminas, description

Acetylene-substituted Si-N-P compounds, synthesis, 236-237 Activated aluminas, description, 165 Aerogels, definition, 127 Aggregation of fractals, 104,106 Alcohol-substituted polymers from aldehydes and ketones, deprotonation-substitution reactions, 249-250 Alkenylborazine copolymers, quantitative reactivity studies of copolymerization reactions, 394... 

Our efforts in this area of catalysis began in 1980. Our initial emphasis was on the preparation of supported phase transfer catalysts. We later became interested in the chemistry of anioni-cally activated alumina(25) and the reactivity of metal carbonyl anions prepared under these conditions. A brief description of our work in the preparation of these materials and their synthetic applications follows. 

Description of Equipment The major equipment pieces in the fluorination pilot plant were the fluorinator, the UO feeder, the NaF traps and the activated alumina traps. The fluorination vessel was constructed of Monel and consisted of a fluidized-bed and an expanded filter section. The expanded filter section was... 

The purpose of this paper Is 1) to describe the electrochemistry of ferrl-/ferro-cyanlde and the oxidation of ascorbic at an activated glassy carbon electrode which Is prepared by polishing the surface with alumina and followed only by thorough sonlcatlon 2) to describe experimental criteria used to bench-mark the presence of an activated electrode surface and 3) to present a preliminary description of the mechanism of the activation. The latter results from a synergistic Interpretation of the chemical, electrochemical and surface spectroscopic probes of the activated surface. Although the porous layer may be Important, Its role will be considered elsewhere. 

Different approaches to the kinetics of alcohol dehydration were attempted by two groups of authors [118,119]. In one case, it has been assumed that the active surface of alumina is formed either by free hydroxyl groups or by surface alkoxyl groups. The rate equation was then derived on the basis of the steady-state assumption a good fit to the experimental data was obtained [1118]. The second model was based on the fact that water influences the adsorption of an alcohol and diminishes the available surface. The surface concentrations of tert-butanol and water were taken from independent adsorption measurements and put into the first-order rate equation a good description of integral conversion data was achieved [119]. 

This article is focused on HDN, the removal of nitrogen from compounds in oil fractions. Hydrodemetallization, the removal of nickel and vanadium, is not discussed, and HDS is discussed only as it is relevant to HDN. Section II is a discussion of HDN on sulfidic catalysts the emphasis is on the mechanisms of HDN and how nitrogen can be removed from specific molecules with the aid of sulfidic catalysts. Before the discussion of these mechanisms, Section II.A provides a brief description of the synthesis of the catalyst from the oxidic to the sulfidic form, followed by current ideas about the structure of the final, sulfidic catalyst and the catalytic sites. All this information is presented with the aim of improving our understanding of the catalytic mechanisms. Section II.B includes a discussion of HDN mechanisms on sulfidic catalysts to explain the reactions that take place in today s industrial HDN processes. Section II.C is a review of the role of phosphate and fluorine additives and current thinking about how they improve catalytic activity. Section II.D presents other possibilities for increasing the activity of the catalyst, such as by means of other transition-metal sulfides and the use of supports other than alumina. 

In conclusion, all the available surface models for high-area aluminas are more-or-less idealized, and they describe, therefore, only roughly the geometrical and energetic situation that a molecule will see when it approaches the surface from the gas phase. The description of adsorption and active sites on the basis of such idealized models are, therefore, equally idealized. 

Descriptions of the continuous processes have been reported in some recent reviews [1,17,18]. After elimination of butadiene, the C4 stream (raffinate-1) is desulfurized with aqueous caustic soda, washed, and thoroughly dried over alumina or silica gel, and then cooled before entering a reactor which is generally of the agitated type. The activator (protogen or initiator in the Kennedy s terminology [1]) is added with the hydrocarbon feed at a concentration that is a few percent of the Lewis acid concentration. When water is used, the process does not necessarily need a specific addition, because the control of drying process can afford the required trace amounts of water for a catalytic initiation. 

The more demanding research topic will be the description of the radiolytic species surface chemistry. Owing to the very high specific surface of nanostructured materials (up to 1000 m g ), even moderate reaction rates between radiolytic species and surface may have a profound impact on the radiolytic schemes. The few studies available deal only with the surface reactivity of hydroxyl radical in gas phase and suggest a HO capture by silica and alumina. This shows that surfaces that are usually considered as inert may become active under irradiation, once more demonstrating the exceptional reactivity of radiolytic species. 

The gradient elution scheme is a scaled-up procedure originally described by Middleton (10) that has been extended to handle highly refractive materials such as coal liquids. This separation technique uses Alcoa F-20 alumina activated to a 5.5 wt % moisture level as the stationary phase. Details of this separation procedure are given elsewhere (2). This method separates SRC into 13 fractions and these fractions are listed in Table II along with some key chemical and physical descriptions of the cuts. The structural types indicated in Table II for Fractions 1-6 have been assigned based upon model compound studies and low resolution mass spectrometry (MS) (2), whereas the chemical types indicated for Fractions 7-13 are based upon IR observations and additional model compound studies. Recoveries in these separations are normally greater than 90%. 

A detailed description of a chromia-on-alumina catalyst prepared by impregnation has been given elsewhere . Another supported nonmetallic catalyst widely used commercially is cobalt molybdate-on-alumina. The preparation of this catalyst using an alumina support with controlled pore-size distribution is as follows. Silica-stabilized alumina, with greater than 50% of its surface area in 3-8 nm pores and at least 3% of the total pore volume in pores greater than 200 nm in diameter, is impregnated with an aqueous solution of cobalt and molybdenum. The finished oxysulfide catalyst was tested for hydrodesulfurization of petroleum residuum at 370°C and 100 atm for 28 days and compared with a convential cobalt-molybdate catalyst having a major portion of the surface area in 3-7 nm pores. The latter catalyst and controlled pore catalyst maintained 57 and 80% activity, respectively. 

A very same description for the acid sites in H-[Al]-MCM-41s has been proposed in more recent works by Savidha et al. [89] and Eswaramoorthi et al. [90]. Such a model for the generation of active acid sites is therefore similar to that anticipated by Uytterhoeven [91] and refined by Mortier [92] to describe the acidity of traditional amorphous siUca-aluminas. 

Weisz and Miale compared the activity for hexane cracking of a number of zeolites (Table 4.25) with a highly active silica — alumina (10% alumina). The zeolites are at least 10 times as active as amorphous silica —alumina. The catalytic process, however, cannot utilize the activity from a pure zeolite catalyst. The catalyst must be modified to decrease the acid-strength to avoid excessive formation of coke and low molecular weight gases, at the expense of gasoline. Moreover, the catalyst must be able to withstand the thermal and hydrothermal conditions experienced in regeneration. It must also withstand breakup in the mechanical circulation systems. A detailed description of the preparation of industrial catalysts is found in the literature. 

In order to propose descriptions at the molecular level for the four types of active site, use is made of the changes observed in the -OH stretching region of q-alumina as py is adsorbed and then desorbed. These are shown in Figure 7.10. 

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