Amorphous components alumina

Weight percent profiles through first-stage (left) and second stage reactor of a) alkanes (full fines) and cycloalkanes (dashed fines) and b) aromatic components. Thick lines correspond to C23 finctions, thin lines to 23 fractions. Operating conditions p, 17.5 MPa LHSV 1.67 niL (nv hf molar H2/HC 18 Tmiei 661 K (reactor 1) 622 K (reactor 2). Catalyst NiMo on amorphous silica-alumina. 

An amorphous component such as silica-alumina is added to the catalyst, for a sort of pre-cracking of the large molecules (greater than about C25), which cannot enter the zeolite pores. The smaller fragments may then react in the zeolite. Middle distillates maximum yield is achieved by the use of dealuminated Y zeolites. 

Aluminosilicates are the active components of amorphous silica—alumina catalysts and of crystalline, well-defined compounds, called zeolites. Amorphous silica—alumina catalysts and similar mixed oxide preparations have been developed for cracking (see Sect. 2.5) and quite early [36,37] their high acid strength, comparable with that of sulphuric acid, was connected with their catalytic activity. Methods for the determination of the distribution of the acid sites according to their strength have been found, e.g. by titration with f-butylamine in a non-aqueous medium using adsorbed Hammett indicators for the H0 scale [38],... 

The main use of MTBE is as an octane booster in gasoline formulations. Table 2.1 (above) compares octane number and boiling points of some tertiary ethers and hydrocarbons. The volatility is another important property of gasoline components. In fact, the lower volatility of ETBE is an advantage with respect to MTBE. Another (smaller scale) application of MTBE is the synthesis of high purity isobutene by cracking MTBE over amorphous silica-alumina. This isobutene serves as a monomer for polyisobutene. 

Nickel and vanadium are contained within the crude oil as their respective porphyrins and napthenates (2). As these large molecules are cracked, the metals are deposited on the catalyst. Nickel which possesses a high intrinsic dehydrogenation and hydrogenolysis activity drastically increases the production of coke and dry gas (particularly H2) at the expense of gasoline. Vanadium on the other hand interacts with the zeolitic component of a cracking catalyst and leads to destruction of its crystallinity. This results in reduced activity as well as an increase in non-selective amorphous silica-alumina type cracking. Supported vanadium also has an intrinsic... 

Figure 3.22 FTIR spectra of H-MOR (Zeolyst, Si/AI = 10), up, and of an amorphous silica alumina (Strem, AI2O3 13%), after activation (full lines) and in contact with 5Torr acetonitrile vapour (broken lines) and after outgassing at room temperature 30min (point line). A, B, C denote the components associated to quasi-symmetrical hydrogen bonding.

Most modern hydrocracking processes are catalytic, and the catalyst employed is usually dual functional with both a hydrogenation component and an acidic component. Typical acidic components include amorphous silica-alumina, alumina, and a large family of zeolites. Typical hydrogenation components are noble metals such as palladium and platinum and nonnoble metals such as nickel, cobalt, tungsten, and molybdenum. The latter metals are usually in sulfided form. 

Splitting into 15 components with a splitting constant of A=0.7 mT and AH <0.3 mT is ascribed to an interaction of Cr (nuclear spin, 1=3/2) and Al (1=5/2) located close to the Cr(V) cationic species. This type of splitting does not occur when chromium ions are stabilized on the sunace of amorphous supports. Rather, it requires a particular crystallographic environment and is, therefore, not observed upon calcination of CrO, with amorphous silica-alumina. It was, however, also observed with other chromium-containing zeolites [37-38]. 

Lewis-acid sites is available. For example in dehydroxylated or acid-leached mordenite up to three types of Lewis site are reported from calorimetric studies as mentioned earlier [16(a)]. The strongest of fliese sites, with q = 170 kJ mol is said to adsorb ammonia dissociatively. It is also clear that several types of Lewis acid could arise from the various components of dislodged aluminium, from detritus silica or even from extraframework amorphous silica/alumina [39]. 

Zeolites, by being in a half way between amorphous silica-alumina and fluorinated alumina, were early recognized as potential components of bifunctional isomerization catalysts. The earliest report of zeolites being used in this application deal on Pt containing X and Y structures (3). They found that the activity increased from the Na to the Ca to the decationized forms. The residual sodium content, and therefore the final acidity of the Pt or Pd HY zeolite catalysts was critical for these type of catalysts (2,4-6). However, to make a successful commercial catalyst the following characteristic have to be accomplished by the zeolitic component ... 

The acidic function of hydrocracking catalysts can be provided by various solid acids such as amorphous silica-alumina, zeolites and doped alumina. The type of application dictates the nature of the acidic component. Hydrocracking catalysts designed for the production of naphtha require strong acidity, provided by zeolites (Y-zeolite). For special applications, such as dewaxing, where shape-selective reactions must be promoted, zeolites such as ZSM-5 and mordenite are used. 

Dupain et al. (2006). The 3%-crystalline H-ZSM-5 sample, not diluted with amorphous silica-alumina (ASA), showed high conversion activity (79 wt%), very close to that of the diluted catalyst of the crystalline H-ZSM-5. It can thus be suggested that the acid sites present in this sample are much more active for the conversion of wax compared to those of Al-MCM-41 and ASA, although the very-low crystallinity H-ZSM-5 sample consists mainly of XRD amorphous aluminosilicate phase. Fig. 18.13 shows the yields (wt% on feed) of various gasoUne components. The data in Fig. 18.13 can also be used for a qualitalive comparison of catalyst performance with regard to their selectivity toward specific gasoline components, especially in the case of H-Y- and H-ZSM-5-based catalysts, which showed similar percent conversion of wax (Fig. 18.12). The H-Y-st. catalyst presented a significant selectivity... 

The catalyst for the second stage is also a bifimctional catalyst containing hydrogenating and acidic components. Metals such as nickel, molybdenum, tungsten, or palladium are used in various combinations and dispersed on sofid acidic supports such as synthetic amorphous or crystalline sihca—alumina, eg, zeofites. These supports contain strongly acidic sites and sometimes are enhanced by the incorporation of a small amount of fluorine. 

The main components of FCC catalysts are Zeolite Y, e.g., REY orUSY as the major active component (10 to 50%), and a binder that is typically an amorphous alumina, silica-alumina, or clay material. In addition to these main components, other zeolite components, e.g., ZSM-5, and other oxide or salt components are quite frequently used additives in the various FCC catalysts available on the market. The addition of 1 to 5% ZSM-5 increases the octane number of the gasoline. ZSM-5 eliminates feed compounds with low octane numbers because it preferentially center-cracks n-paraffins producing butene and propene [14], These short-chain olefins are then used as alkylation feedstocks... 

The acid component of a hydrocracking catalyst can be an amorphous oxide, e.g., a silica-alumina ora zeolite, eg., USY. This component usually serves as a support for the metal compound responsible for the hydrogenation function. The metal compound can be a noble metal, e.g., Pt or Pd, or a mixture of sulfides, e.g., of Ni/Mo, NiAV, or Co/Mo. The relative amounts of the respective compounds have to be thoroughly balanced to achieve an optimum performance. 

The nuisance dust aspect of bauxite is in sharp contrast to the limited industrial situation where lung injury was reported in Canadian workers, who in the 1940s engaged in the manufacture of alumina abrasives in the virtual absence of fume control. Fusing of bauxite at 2000°C gave rise to a fume composed of freshly formed particles of amorphous silica and aluminum oxide. Despite the poor choice of the term—bauxite fume pneumoconiosis—sometimes used to describe the disease, scientific opinion favors the silica component as the probable toxic agent. It should be emphasized that bauxite from some sources may contain small amounts of silica. 

Allophane is a fine-grained amorphous material composed largely of silica, alumina and water. It has both a glassy and an earthy appearance. Material with this composition is clear to white in color however, because of the common presence of other ions, it can occur in a wide variety of colors. It most often forms as an alteration product and is an abundant component of many soils. Under some conditions it will crystallize into halloysite and halloysite-like clays. 

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