Zeolite amorphous catalyst

The breakthrough in FCC catalyst was the use of X and Y zeolites during the early 1960s. The addition of these zeolites substantially increased catalyst activity and selectivity. Product distribution with a zeolite-containing catalyst is different from the distribution with an amorphous silica-alumina catalyst (Table 4-3). In addition, zeolites are 1,000 times more active than the amorphous silica alumina catalysts. 

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. 

The hydroamination of alkenes has been performed in the presence of heterogeneous acidic catalysts such as zeolites, amorphous aluminosilicates, phosphates, mesoporous oxides, pillared interlayered clays (PILCs), amorphous oxides, acid-treated sheet silicates or NafioN-H resins. They can be used either under batch conditions or in continuous operation at high temperature (above 200°C) under high pressure (above 100 bar). 

The tendency in the past decades has been to replace them with solid acids (Figure 13.1). These solid acids could present important advantages, decreasing reactor and plant corrosion problems (with simpler and safer maintenance), and favoring catalyst regeneration and environmentally safe disposal. This is the case of the use of zeolites, amorphous sihco-aluminas, or more recently, the so-called superacid solids, that is, sulfated metal oxides, heteropolyoxometalates, or nation (Figure 13.1). It is clear that the well-known carbocation chemistry that occurs in liquid-acid processes also occurs on the sohd-acid catalysts (similar mechanisms have been proposed in both catalyst types) and the same process variables that control liquid-acid reactions also affect the solid catalyst processes. 

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. 

Following their introduction to the refining industry in 1962, zeolite cracking catalysts, have virtually replaced the amorphous silica alumina cracking catalysts that had previously dominated the marketplace. To the rare earth industry the development of zeolite catalysts represented a new end use without precedent. Nearly all zeolite cracking... 

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... 

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],... 

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 ... 

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). 

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 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. 

Zeolites have found wide application as catalysts in the oil refining and petrochemical industry, where they have been gradually replacing amorphous catalysts. The superior catalytic performance of zeolites is related to some important properties, namely ... 

The gas phase acid-catalyzed synthesis of pyridines from formaldehyde, ammonia and an alkanal is a complex reaction sequence, comprising at least two aldol condensations, an imine formation, a cyclization and a dehydrogenation (9). With acetaldehyde as the alkanal, a mixture of pyridine and picolines (methylpyridines) is formed. In comparison with amorphous catalysts, zeolites display superior performance, particularly those with MFI or BEA topology. Because formation of higher alkylpyridines is impeded in the shape-selective environment, the lifetime of zeolites is much improved in comparison with that of amorphous materials. Moreover, the catalytic performance can be enhanced by doping the structure with metals such as Pb, Co or Tl, which assist in the dehydrogenation. 

Now zeolite catalysts have been employed by most FCCUs. Although zeolite catalysts have a much higher initial activity as compared to amorphous catalysts, coke deposit on the catalyst particles rapidly lowers their activity. As the carbon content of zeolite catalysts increases by 0.1 wt%, the activity decreases by 2-3 units. Generally the carbon content of regenerated zeolite catalysts should not be allowed to exceed 0.2 wt. %, or preferably less than 0.1 wt. % in the case of ultrastable Y zeolite (USY). Therefore, how to decrease CRC efficiently for zeolite catalysts in FCCUs has become a significant problem. 

Letzsch et al [7] have shown that like amorphous catalysts the zeolite is more strongly deactivated hydrothermally than purely thermally. 

Table-I gives the properties of different steamed catalysts used in the present study. Catalyst AM is the amorphous catalyst while all others (Z1-Z4) are zeolite based catalysts. All the steamed catalysts have been analyzed (in ASAP-2000 of Micromeritics) for the surface area of micro and macro pores. In the present study, all the pores below 30 A are considered as micro pores while those above this are lumped into macro pores. The ratio of surface areas of micro to macro pores (Sz / Sm) is used in the data analysis.

Cobalt, copper and nickel metal ions were deposited by two different methods, ionic exchange and impregnation, on an amorphous silica-alumina and a ZSM-5 zeolite. The adsorption properties towards NH3 and NO were determined at 353 and 313 K, respectively, by coupled calorimetric-volumetric measurements. The average acid strength of the catalysts supported on silica-alumina was stronger than that of the parent support, while the zeolite-based catalysts had (with the exception of the nickel sample) weaker acid sites than the parent ZSM-5. The oxide materials used as supports adsorbed NO in very small amounts only, and the presence of metal cations improved the NO adsorption [70]. 

TTeterogeneous catalysts are usually high-area porous materials which may be amorphous or crystalline. An important aspect of all such materials is the rapidity with which reactant molecules reach active sites and products leave these sites. Apart from flow in gas or liquid phase, there may be surface migration into and from micropores, whether in amorphous catalysts or in crystalline ones, such as the zeolites. It is still an open question how important such migration processes are as ratecontrolling steps. However, it seems likely that active sites deep in a porous crystal will be less important than sites near the surface because many more unit diffusion steps will be needed to transport molecules to and from deeply buried sites. As corollaries, one would expect that only a limited volume fraction of a crystal of a zeolite such as sieve Y is catalytically effective, and that for best performance crystals in the catalyst support should be well exposed and as small as possible, in order to provide the largest surface-to-volume ratio. 

The most important consequence of restricted transition state selectivity is that ZSM-5 and many other medium-pore zeolites deactivate much slower than most other crystalline and amorphous catalysts. The difference is not trivial. In most acid catalyzed reactions large-pore zeolites deactivate within minutes or in hours, whereas the activity of ZSM-5 ranges from weeks to years. Most of the coke in large-pore zeolites is formed within the pores. In ZSM-5 most of the coke is deposited on the outer surface of the crystals like an eggshell over an egg [23] because coke precursors cannot form in the pores of pentasil molecular sieves. The resistance of ZSM-5 to coking makes a number of industrial processes economical. 

With the old amorphous catalyst, the cat cracker was getting about 16 gallons of gasoline from each barrel of feed. But with the new zeolite catalyst, the same cracker squeezed out another 8 gallons of gasoline from the same barrel of feed. That s a fifty percent increase in gasoline yield — an almost unbelievable result. 

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