Amorphous catalyst sites

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. 

Sets A, B, and C must occur on different sites. For amorphous catalysts activated at temperatures below 360° or so, D could be associated with the sites of set A if (4 ) is low at these sites. This could also be true for amorphous catalysts activated at 400° and for microcrystalline a-Cr203 but, owing to the different ratios of isomerization to hydrogenation, there would have to be some difference between the sites for set A on these catalysts and those activated at lower temperatures. 

Observations 1 and 3 indicate that the active sites of the catalysts are of similar nature but their numbers are different in the amorphous and crystalline states. The lack of chemisorption data renders it impossible to make an exact comparison of activities in terms of turnover frequencies. Nevertheless, the similarity of BET surface areas of the amorphous and crystalline samples points to the presence of more active sites per unit surface area on the amorphous catalysts than on the crystalline ones. However, as the observations cited below indicate, other factors also contribute to the activity difference between the amorphous and crystalline states. 

It can be seen that the activities of the amorphous alloys are lower than those of the polycrystalline catalysts. Formation of the corresponding diol was not observed on the amorphous catalysts, while the crystalline catalysts either produced the diol selectively, or a mixture of the diol and the hydroxy ketone was formed. The fundamental reason for the lower activity and higher selectivity of the amorphous alloys is their rather small surface area. Of the amorphous alloys studied, Ni-B and Ni-P alloy powders prepared by chemical reduction exhibited higher activities than those of Ni-P alloys prepared by electrolytic reduction or rapid quenching. This difference in activity can be attributed to an oxide layer covering the surface of m-P foils [Ij. It is necessary to point out, however, that the comparison of activities is based on unit catalyst weight. Obviously, this comparison does not take into account the real surface area of the nickel samples, nor active site densities. 

Figure 9.5 Catalyst site energy vs. activation energy curve for the proton transfer from an amine to an oxo ligand on amorphous carbon.

Studied [269,272,273], Both ZSM-5 catalysts emerge as the best catalysts with the highest yields of hydrocarbon products and lowest coke formation [269], The aromatics yield tends to decrease in the order ZSM-5 >H-beta>H-mordenite>H-ferrierite/HY [273], Stefanidis et al. demonstrated that, in comparison to a range of amorphous catalysts such as alumina, zirconia/ titania, and magnesium oxide, ZSM-5 is more suitable for the reduction of undesirable compounds and production of aromatics in the upgrading of pyrolysis vapors from beech wood [274], The excellent performance of ZSM-5 is attributed to the important role of its medium pore size [269], Besides, Park et al. pointed out that ZSM-5 is more efficient than Y zeolites due to the proper distribution of strong acid sites [275],... 

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. 

A major step in catalyst development was the introduction of crystalline zeolitic, or molecular sieve catalysts. Their activity is very high, some of the active sites being estimated at 10,000 times the effectiveness of amorphous silica-... 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Zeolites as cracking catalysts are characterized hy higher activity and better selectivity toward middle distillates than amorphous silica-alumina catalysts. This is attrihuted to a greater acid sites density and a higher adsorption power for the reactants on the catalyst surface. 

Plastomers represent a major advancement for polyolefins. Their success allows polyolefins to have a continuum of products from amorphous EPR to thermoplastic PE and iPP. This development coincides with the advent of single-site catalysts these are necessary for copolymers of components of widely different reactivity such as ethylene and octene. Their rapid introduction into the mainstream polymer use indicates that this spectrum of properties and the inherent economy, stability and processibility of polyolefins are finding new applications to enter. 

PMo 12-polymer composite film catalyst [9]. This demonstrates that PM012 catalyst was not in a crystal state but in an amorphous-like state, indicating that PM012 catalyst was molecularly dispersed on the PS support via chemical interaction. As attempted in this work, it is believed that heteropolyanions (PMoi204o ) were strongly immobilized on the cationic sites of the PS bead as charge-compensating components. 

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