Catalyst primary particles

Such a different conclusion can be understood by considering the difficulties connected to the experimental determination and the definition of Thiele modulus parameters, such as So and D. According to Chien, S means the catalyst primary particle size with a value of about 10 cm for a-TiCIj instead, in the Multigrain model, Sp seems to correspond to the size of the whole catalyst granule. 

Therefore, it is not surprising that this cascaded process opens the door to products with combinations of properties so far not known [48-50]. It is important to underline that with this process design the polymer generated in all three polymerization reactors is finely divided in the final polymer particle. Each catalyst primary particle is enveloped with the polymer from reactors 1, 2, and 3. With this in-situ blend it is possible to obtain a homogeneous melt in the granulation facility this is necessary to exploit the full potential of the product in the solid state. Mechanical blending of such three types of polyethylenes would never lead to a homogeneous melt. 

However, when using supports with weak linkage between the primary particles of the catalyst, its splitting occurs quickly and it is unlikely to influence the shape of the kinetic curve. For example, in the case of chromium oxide catalyst reduced by CO supported on aerosil-type silica, steady-state polymerization with a very short period of increasing rate is possible (see curve 1, Fig. 1). 

Fig. 2. Models of the primary particle (a) and polymer grain (b) for the analysis of the role of monomer diffusion to tbe catalyst surface, (a) 1—catalyst 2—polymer film, (b) 1—micrograin 2—macropore.

The calculation of C according to (6) shows (95) that if the catalyst splitting results in the formation of catalyst pellets about 1000 A in size, then even under the most unfavorable conditions (the concentration of the active centers is equal to the total chromium content in the catalyst, 2r2 = ) the diffusional restriction on the primary particle level is negligible. 

The TEM images of 12 wt.% Co/MgO calcined at 873 K (Catalyst I) before and after reduction are shown in Fig. 1 (a) and (b), respectively. Although Co metal phase was detected in reduced Co/MgO by X-ray diffraction measurements (XRD) [7, 8], no Co metal particle was observed on both catalysts. EDS elemental analysis showed that primary particles contain both Mg and Co elements, whose concentrations were about the same as loaded amounts. Figure 2 shows TEM image of 12 wt.% Co/MgO calcined at 1173 K (Catalyst II). 

Interesting and interrelated with the previous case is one of enclosed partitions, when one of two partitions can be further divided into two partitions. An illustrative example is shown in Figure 9.17a. A granule of catalyst can be divided into two partitions porous aggregates (secondary particles—partition 1) and pores between the aggregates (partition 2). Partition 1 can also be divided into two partitions nonporous particles (primary particles—partition 11) and pores between particles (partition 12), excluding pores between aggregates. Another case of enclosed partitions has already been considered the case of a porous supported catalyst, which can be divided into pores and a solid phase, while the solid phase can be divided into the support and the active component. 

Although, the true density of solid phase p=m/Vp (e.g., g/cm3) is defined by an atomic-molecular structure (/ ), it has become fundamental to the definition of many texture parameters. In the case of porous solids, the volume of solid phase Vp is equal to the volume of all nonporous components (particles, fibers, etc.) of a PS. That is, Vp excludes all pores that may be present in the particles and the interparticular space. The PS shown in Figure 9.17a is formed from nonporous particles that form porous aggregates, which, in turn, form a macroscopic granule of a catalyst. In this case, the volume Vp is equal to the total volume of all nonporous primary particles, and the free volume between and inside the aggregates (secondary particles) is not included. 

Some heterogeneous catalytic reactions proceed by a sequence of elementary processes certain of which occur at one set of sites while others occur at sites which are of a completely different nature. For example, some of the processes in the reforming reactions of hydrocarbons on platinum/ alumina occur at the surface of platinum, others at acidic sites on the alumina. Such catalytic reactions are said to represent bifunctional catalysis. The two types of sites are ordinarily intermixed on the same primary particles ( 1.3.2) but similar reactions may result even when the catalyst is a mixture of particles each containing but one type of site. These ideas could, of course, be extended to crea te the concept of polyfunctional catalysis. 

Primary particles. Certain materials widely used as catalysts or supports consist of spheroids of about 10 nm (100 A) in diameter loosely cemented into granules or pellets. The texture of these resembles that of a cemented, loose gravel bed. The 10 nm (100 A) particles may be called primary particles. 

Sintering and recrystallisation. Catalysts often suffer during use from a gradual increase in the average size of the crystallites or growth of the primary particles. This is usually called sintering. The occurrence of sintering leads to... 

Figure 28.4 shows a typical image of the primary particles that make up the catalyst agglomerates. These particles are single crystals of a-Fe203 as... 

The structure of these pyrogenic silicas has been discussed by Barby [5], particularly with reference to their specific surface area. It was concluded that the initially condensed particles are only about 1 rnn in diameter and that these are so closely packed (high coordination number) to secondary particles of 10 to 30 nm that only a small amount of nitrogen can penetrate the micropores between them. Thus the secondary particles are the ones that are commonly identified in electron micrographs and which determine the specific surface area. They are the primary particles in the voluminous aggregate structure and have a low coordination number of about 3 (see Fig. 8.3). Because of the low level of impurities this type of silica is often used as catalyst support in fundamental studies. 

With this correlation we can describe the most important group of catalysts, namely all metal and metal oxide catalysts (which are produced by compressing powders at different pressures). In most cases, the primary particles are already porous themselves, so bimodal pore-size distributions are obtained. The authors recommend a value of 1.05 when m cannot be determined experimentally. For a large series containing widely varying data, values of m between 0.70 and 1.65 have been observed. 

The primary particle size of the powdered samples is to a certain extent of minor importance on the measured contact angle. The high pressure applied (10-1000 MPa) upon pelletizing causes the original particle size and shape to be distorted in such a way that a very smooth surface and a well-defined hole (advancing angle method) is obtained. This has been supported by Scanning Electron Microscopy measurements of the powders and pellets (see e.g. Fig. 3 for a Ni/AhOs catalyst). 

The magnesium chloride solids produced by the typical chlorination schemes in hydrocarbon solvents show unique morphologies. The primary particle sizes can be as small as 10 50 A in a dimension and the particle agglomerates may exhibit surface areas in excess of 400 m g. The enumerated characteristics makes these supports ideal for Ziegler-Natta catalyst active sites. The resulting supported catalysts may approach turnover rates as high as 10 mol of monomer per second per active site under commercial polymerization conditions. 

We produced two density series with two RC-ratios (ratio between resorcinol and catalyst) 800 and 1500. In tables 1 and 2 we give the important sample data. The density was calculated from volume and mass of the sample. The second column gives the mass ratio of the reactive species to the mass of the whole solution. The specific surface areas were determined by N2-adsorption (ASAP 2000 by MICROMERITICS) with data evaluation according to BET-theory. From SAXS-measurements primary particle sizes were extracted. 

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