Amorphous catalyst

Coke deposition is essentially independent of space velocity. These observations, which were developed from the study of amorphous catalysts during the early days of catalytic cracking (11), stiU characteri2e the coking of modem day 2eohte FCC catalysts over a wide range of hydrogen-transfer (H-transfer) capabihties. 

Comparison of Yield Structure for Fluid Catalytic Cracking of Waxy Gas Oil over Commercial Equilibrium Zeolite and Amorphous Catalysts... 

Zeolites. In heterogeneous catalysis porosity is nearly always of essential importance. In most cases porous materials are synthesized using the above de.scribed sol-gel techniques resulting in so-called amorphous catalysts. Porosity is introduced in the agglomeration process in which the sol is transformed into a gel. From X-ray Diffraction patterns it is clear that the material shows only weak broad lines, characteristic of non-crystalline materials. Silica and alumina are typical examples. Zeolites are an exception they are crystalline materials but nevertheless exhibit high (micro) porosity. Zeolites belong to the class of molecular sieves, which are porous solids with pores of molecular dimensions, i.e., typically the pore diameter ranges from 0.3 to 10 nm. Examples of molecular sieves are carbons, oxides and zeolites. 

Figure 1.9 TG, DTG, and DTA profiles for an amorphous catalyst precursor obtained by coprecipitation of Fe(N03)3 and Mg(N03)2 in solution [65], This precursor is heated at high temperatures to produce a MgFe204 spinel, used for the selective oxidation of styrene. The thermal analysis reported here points to four stages in this transformation, namely, the losses of adsorbed and crystal water at 110 and 220°C, respectively, the decomposition and dehydroxylation of the precursor into a mixed oxide at 390°C, and the formation of the MgFe204 spinel at 640°C. Information such as this is central in the design of preparation procedures for catalysts. (Reproduced with permission from Elsevier.)...

Tab. 3.S. Performance of sonochemically prepared amorphous catalysts in the aerobic oxidation of cyclohexanes (Equation 3.33).

Other areas of the world which do not have such a dependency on gasoline use both zeolite catalysts and amorphous catalysts to produce a mix of gasoline and fuel oils. 

Much less studied has been the role of V species in n-pentane oxidation to MA and PA. Papers published in this field are aimed mainly at the determination of the reaction mechanism for the formation of PA (2-4,11). Moreover, it has been established that one key factor to obtain high selectivity to PA is the degree of crystallinity of the VPP amorphous catalysts are not selective to PA, and the progressive increase of crystallinity during catalyst equilibration increases the formation of this compound at the expense of MA and carbon oxides (12). Also, the acid properties of the VPP, controlled by the addition of suitable dopants, were found to play an important role in the formation of PA (2). 

A variety of solids is used as catalysts metals, alloys, clays, metal oxides, sulphides, nitrides, carbides and so on. Catalysts may be single-phase substances or multiphasic mixtures they may be crystalline, microcrystalline or even amorphous. Catalysts can be electrically insulating, semiconducting or metallic. Some examples of heterogeneously catalysed reactions are given in Table 8.1. Two aspects of catalysts are important activity and selectivity. Activity refers to the ability of catalysts to accelerate chemical reactions so that equilibrium is achieved rapidly. The degree of acceleration... 

Silica-alumina, either amorphous catalysts or zeolites, are used in several processes.261,282-285 Mobil developed several technologies employing a medium-pore ZSM-5 zeolite. They use a xylene mixture from which ethylbenzene is removed by distillation, operate without hydrogen, and yield p-xylene in amounts... 

For many reactions, especially carbonium-ion type reactions, the zeolites and the amorphous silica-aluminas have common properties. The activation energies of the processes with both types of compounds change insignificantly, and both compounds have similar responses to poisons and promotors (1, 2). In general the zeolites are far more active than the amorphous catalysts, but ion exchange and other modifications can produce changes in zeolite activity which are more important than the differences between the activities of the amorphous and zeolitic catalysts ... 

Figure 7 shows the effect of feed molecular weight on the reaction rates observed with strongly acidic hydrocracking catalysts. These data were obtained with an early version of an amorphous catalyst. They may, however, be used to illustrate general trends involving feed character and molecular weight. 

In these sections of our chapter, we emphasize research advances in the area of surface acidity of specific solids that have occurred during the period from 1970 to the fall of 1976. As stated earlier, the class of solids with which we are chiefly concerned are metal oxides that catalyze skeletal rearrangements of hydrocarbons via carbonium ion intermediates. However, we have included reviews of silica gel and alumina, which are relatively inactive, because the properties of these solids form a useful frame of reference. The initial sections (Sections III.A-III.D) deal predominantly with amorphous catalysts the final sections (Sections III.E and III.F), with crystalline catalysts. 

Though the relative rates were very similar, the partially amorphous catalyst exhibited the most rapid hydrogen uptake when expressed in gram of catalyst used (Table 9). 

The acid function of the catalyst is supplied by the support. Among the supports mentioned in the literature are silica-alumina, silica-zirconia, silica-magnesia, alumina-boria, silica-titania, acid-treated clays, acidic metal phosphates, alumina, and other such solid acids. The acidic properties of these amorphous catalysts can be further activated by the addition of small proportions of acidic halides such as HF, BF3, SiFit, and the like (3.). Zeolites such as the faujasites and mordenites are also important supports for hydrocracking catalysts (2). 

Catalyst Aging Effects. The yields for most of the catalysts remain relatively stable with catalyst aging. In constant conversion runs such as these, the catalyst temperature is increased to maintain conversion and yield stability can be shown by a plot of yield versus average catalyst temperature. Figure 6 shows results for two amorphous catalysts. 

This catalyst aging property does not necessarily make such a catalyst unattractive. Rather, it may limit the temperature span in which a desired product distribution can be obtained. With most amorphous catalysts, a run is usually terminated at the point at which catalyst activity and stability are sufficiently low that continuation of the run is no longer... 

The IFP hydrocracking process features a dual catalyst system the first catalyst is a promoted nickel-molybdenum amorphous catalyst. It acts to remove sulfur and nitrogen and hydrogenate aromatic rings. The second catalyst is a zeolite that finishes the hydrogenation and promotes the hydrocracking reaction. 

The hydrogenation of carbon monoxide over the amorphous Fe2oNi6oP20 and FejoZrio catalysts was carried out at atmospheric pressure and at temperatures from 220 to 370°C. The amorphous catalysts exhibited stable and high activities higher than the crystalline catalysts of the same compositions. 

As is shown in Figure 1, the production rates of the major hydrocarbons were kept constant for the amorphous catalyst. However, the crystalline FegoZrio catalyst which was prepared by heating the amorphous catalyst at 5608C for 20 hrs didn t approach any steady activity, as is shown in Figure 2. 

The BET surface areas of both the amorphous and the crystalline Fe2oNi6oP2o catalysts were nearly the same and constant during the reaction, but the surface characters of the amorphous and the crystalline phases of FegoZrio alloy were quite different with each other. For the crystalline catalyst, the BET surface area was kept constant during the reaction at about 0.25 m2/g. On the other hand, the amorphous catalyst ribbons broke into fine chips of different sizes and the BET surface area after the reaction went up to 0.9 m2/g. Since the pretreatment with a stream of hydrogen did not produce any breakage of the alloy ribbon, and also because the catalytic acitvity had been kept constant shortly after the start of the reaction, the increase of the surface area of the amorphous catalyst is considered to take place at the initial period of the reaction by carbon monoxide and hydrogen. 

With amorphous silica-alumina catalysts [5,6], the primary mode of aging involves steam-induced loss of surface area by the growth of the ultimate gel particles, resulting also In loss of porosity. While amorphous catalysts deactivate thermally as well as hydrothermally, thermal deactivation is a significantly slower process. 

The introduction of zeolites in cracking catalysts combined with various non-zeolite matrix types (a.o. higher stability silica-alumina types) certainly complicates the picture of FCC hydrothermal deactivation. Letzsch et al [7] have shown that like amorphous catalysts the zeolite is more strongly deactivated hydrothermally than purely thermally. 

The present paper describes the phenomena of shape-selective polymerization involved in converting C--C olefins over HZSM-5 to higher boiling olefins and the similarities and differences compared to amorphous silica-alumina. The channel systems of ZSM-5 (8), Figure 1, impose shape-selective constraints on the shape of the large molecules accounting for the differences with amorphous catalysts. 

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