Faujasite catalyst

Zeolites (section C2.13) are unique because they have regular pores as part of their crystalline stmctures. The pores are so small (about 1 nm in diameter) that zeolites are molecular sieves, allowing small molecules to enter the pores, whereas larger ones are sieved out. The stmctures are built up of linked SiO and AlO tetrahedra that share O ions. The faujasites (zeolite X and zeolite Y) and ZSM-5 are important industrial catalysts. The stmcture of faujasite is represented in figure C2.7.11 and that of ZSM-5 in figure C2.7.12. The points of intersection of the lines represent Si or A1 ions oxygen is present at the centre of each line. This depiction emphasizes the zeolite framework stmcture and shows the presence of the intracrystalline pore stmcture. In the centre of the faujasite stmcture is an open space (supercage) with a diameter of about 1.2 nm. The pore stmcture is three dimensional. 

Zeolite, or more properly, faujasite, is the key ingredient of the FCC catalyst. It provides product selectivity and much of the catalytic activity. The catalyst s performance largely depends on the nature and quality of the zeolite. Understanding the zeolite structure, types, cracking mechanism, and properties is essential in choosing the right catalyst to produce the desired yields. 

Zeolites employed in the manufacture of the FCC catalyst are synthetic versions of naturally occurring zeolites called faujasites. There are about 40 known natural zeolites and over 150 zeolites that have been synthesized. Of this number, only a few have found commercial applications. Table 3-1 shows properties of the major synthetic zeolites. 

Faujasite is a naturally occurring mineral, having a specific crystalline, alumina-silicate structure, used in the manufacturing of the FCC catalyst. Zeolite faujasite is a synthetic form of the mineral. 

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

Zeolites are used in various applications such as household detergents, desiccants and as catalysts. In the mid-1960s, Rabo and coworkers at Union Carbide and Plank and coworkers at Mobil demonstrated that faujasitic zeolites were very interesting solid acid catalysts. Since then, a wealth of zeolite-catalyzed reactions of hydrocarbons has been discovered. Eor fundamental catalysis they offer the advantage that the crystal structure is known, and that the catalytically active sites are thus well defined. The fact that zeolites posses well-defined pore systems in which the catalytically active sites are embedded in a defined way gives them some similarity to enzymes. 

The catalyst used for the conversion of methanol to gasoline is based on a new class of shape-selective zeolites (105-108), known as ZSM-5 zeolites, with structures distinctly different from other well-known zeolites. Apparently, the pore dimensions of the ZSM-5 zeolites are intermediate between those of wide-pore faujasites (ca. 10 A) and very narrow-pore zeolites such as Zeolite A and erionite (ca. 5 A) (109). The available structural data indicate a lattice of interconnecting pores all having approximately the same diameter (101). Hydrocarbon formation... 

Fischer-Tropsch synthesis could be "tailored by the use of iron, cobalt and ruthenium carbonyl complexes deposited on faujasite Y-type zeolite as starting materials for the preparation of catalysts. Short chain hydrocarbons, i.e. in the C-j-Cq range are obtained. It appears that the formation and the stabilization of small metallic aggregates into the zeolite supercage are the prerequisite to induce a chain length limitation in the hydrocondensation of carbon monoxide. However, the control of this selectivity through either a definite particle size of the metal or a shape selectivity of the zeolite is still a matter of speculation. Further work is needed to solve this dilemna. 

The catalyst is faujasite derived, with a high concentration of sufficiently strong Brpnsted acid sites and a minimized concentration of Lewis acid sites. It also contains a hydrogenation function. The process operates at temperatures of about 323-373 K with a molar isobutane/alkene ratio between 6 and 12 and a higher alkene space velocity than in the liquid acid-catalyzed processes. Preliminary details of the process concept have been described (240). 

Early attempts to utilize the high acid activity of faujasite zeolite catalysts for direct xylene isomerization suffered from low selectivity. Considerable improvement was obtained first by using a large pore zeolite (7) catalyst and subsequently in several process modifications that use ZSM-5 as catalyst (2). In the following we will show how these selectivity differences can be related to structural differences of the various zeolites. 

We find from separate e qperiments with ZSM-5 catalyst that the intrinsic rate constant for isomerization, kj, is much faster than that for disproportionation, kD at 482°C, kj/kD > 7000, i.e. kj/kD is much faster than it is for the synthetic faujasite catalyst. 

The nomenclature of zeolites is rather arbitrary and follows no obvious rules because every producer of synthetic zeolites uses his/her own acronyms for the materials. However, as mentioned before, at least the structure types of the different zeolites have a unique code. For example, FAU represents Faujasite-type zeolites, LTA Linde Type A zeolites, MFI Mobile Five, and BEA Zeolite Beta. The structure commission of the International Zeolite Association (IZA) is the committee granting the respective three-letter codes [4], Some typical zeolites, which are of importance as catalysts in petrochemistry, will be described in the following sections. 

Catalytic cracking is a process that is currently performed exclusively over fluidized catalyst beds. The fluid catalytic cracking (FCC) process was introduced in 1942 and at that time replaced the conventional moving bed processes. These early processes were based on acid-treated clays as acidic catalysts. The replacement of the amorphous aluminosilicate catalysts by Faujasite-type zeolites in the early-1960s is regarded as a major improvement in FCC performance. The new acidic catalysts had a remarkable activity and produced substantially higher yields than the old ones. 

Also, the adsorption of DMC on faujasites, has been described through two modes of interaction IR experiments indicates that DMC acts as a base to form acid-base complexes with the Lewis acidic sites of the catalyst (Scheme 4.12). 

These results represent the first ever reported evidence of strict cooperation between the steric requisites of the faujasite catalyst and the reactivity of an asymmetrical carbonate, in simultaneously inducing high methyl chemoselectivity and mono-A-methylselectivity for primary amines. 

Typically several diffraction peaks are used for comparison. ASTM method D3906 for faujasite materials uses a comparison of eight different peaks. Although the method was developed for FCC catalysts, it is also applicable to other FAU-type... 

For a zeohtic catalyst where Pt, Pd or other transition metal might be present to provide metal activity, STEM can be used to determine whether the metal is agglomerated and to what extent, whether the metal is in the zeolite or present on the geometric exterior or whether the metal is associated with the zeolite or binder. As an example of the utility of the technique. Figure 4.15 shows the growth of Pt clusters for fresh and spent faujasite zeolite catalyst. After time under reaction conditions, the Pt clusters have grown from Inm to 2nm. The clusters have remained in the channels of the faujasite. Pt agglomeration can be concluded as the deactivation mechanism. 

Kebulkova, L., Novakova,)., Jaeger, N.I., and Schultz-Ekloff, G. (1993) Characterization of nickel species at Ni/7-Al203 and Ni/faujasite catalysts by carbon monoxide adsorption. Appl Catal. A,... 

---