Catalysts, determination

Most Kaminsky catalysts contain only one type of active center. They produce ethylene—a-olefin copolymers with uniform compositional distributions and quite narrow MWDs which, at their limit, can be characterized by M.Jratios of about 2.0 and MFR of about 15. These features of the catalysts determine their first appHcations in the specialty resin area, to be used in the synthesis of either uniformly branched VLDPE resins or completely amorphous PE plastomers. Kaminsky catalysts have been gradually replacing Ziegler catalysts in the manufacture of certain commodity LLDPE products. They also faciUtate the copolymerization of ethylene with cycHc dienes such as cyclopentene and norhornene (33,34). These copolymers are compositionaHy uniform and can be used as LLDPE resins with special properties. Ethylene—norhornene copolymers are resistant to chemicals and heat, have high glass transitions, and very high transparency which makes them suitable for polymer optical fibers (34). 

Polypropylenes produced by metallocene catalysis became available in the late 1990s. One such process adopts a standard gas phase process using a metallocene catalyst such as rac.-dimethylsilyleneto (2-methyl-l-benz(e)indenyl)zirconium dichloride in conjunction with methylaluminoxane (MAO) as cocatalyst. The exact choice of catalyst determines the direction by which the monomer approaches and attaches itself to the growing chain. Thus whereas the isotactic material is normally preferred, it is also possible to select catalysts which yield syndiotactic material. Yet another form is the so-called hemi-isotactic polypropylene in which an isotactic unit alternates with a random configuration. 

The 300-ton inventory unit in Example 3-2 is changing catalyst type and planning to add 3.5 tons per day of new catalyst. Determine the percent of changeover after 60 days of operation. Assume a retention factor of 0.7. 

The transition state assembly 55 (Figure 3.8), that rationalizes the stereochemistry of the cycloadduct, is consistent with the structure of the chiral catalyst determined by an X-ray diffraction study. Interestingly it has been shown [58] that in the cycloadditions of maleimides 56 with 2-methoxy-l,3-butadiene, the enantioselection depends on the bulkiness of Ar and Ari groups of catalyst 54 and dienophile 56, respectively (Scheme 3.13). The importance of the bulky Ari... 

When the hydrogen pressure is 1 atm, and the temperature is 77 °K, the experimentally observed (apparent) rate constant is 0.159 cm3/ sec-g catalyst. Determine the mean pore radius, the effective diffusivity of hydrogen, and the catalyst effectiveness factor. 

Transmission electron microscopy is one of the most often used techniques for the characterization of catalysts. Determination of particle sizes or of distributions therein has become a matter of routine, although it rests of course on the assumptions that the size of the imaged particle is truly proportional to the size of the actual particle and that the detection probability is the same for all particles, independently of their dimensions. In situ studies of catalysts are of special interest and are possible by coupling the instrument to an external reactor. After evacuation of the reactor, the catalyst can be transferred directly into the analysis position without seeing air [17-19J. Numerous applications of electron microscopy in catalysis have been described in the literature, and several excellent reviews are available [2-6],... 

The atomic structure of a heterogeneous catalyst determines its chemical and phase properties, but texture determines a wide range of additional features that dictate such characteristics as adsorption and capillarity, permeability, mechanical strength, heat and electrical conductivity, etc. For example, the apparent catalytic activity,. of a grain, taking into account diffusion of reagents, depends on the interrelation between the rates of reaction and diffusion, and the latter is determined by a porous structure. 

The benzoin reaction and the reaction of Cannizzaro, which are discussed later, likewise take place because of the tendency of the aldehydes to undergo condensation. The specific catalyst determines in each case the particular way along which the condensation will proceed. 

The oligomerizahon of heavier olefins like the relatively non-valuable G5 and G6 olefins into diesel or lube products has been studied multiple times over the years, but has yet to be industrially implemented. Gonditions and catalyst determine the product selechvity for this reaction, so both need to be optimized for the particular product molecular weight desired (Table 12.7). 

Table 6.3 includes particle sizes for bimetallic catalysts determined by TEM. Figure 6.4 shows particle size distribution histograms of PtSn-BM and NiSn-BM bimetallic catalysts and the corresponding Pt and Ni monometallic ones. In all the cases, the sohds were subjected to a hydrogen reduction pretreatment. These... 

Figure 8. Temperature Programmed Desorption of methanol from the ferric molybdate (dashed line) and the manganese pyrophosphate (solid line) catalysts determined gravimetrically.

In the mechanism illustrated in Figure 6, the combination of the redox and acid properties of the catalyst determines the relative contribution for the formation of MA and PA. It is generally accepted that the higher the crystallinity of the VPP, the more selective to PA is the catalyst (3,4,10-12,17,18). Poorly crystalline VPP, like that one formed after the thermal treatment of the precursor (especially when it is carried out under oxidizing conditions), is selective to MA, but non-selective to PA. On the contrary, a fully equilibrated catalyst, characterized by the presence of a well-crystallized VPP, yields PA with a good selectivity. The presence of dopants that alter the crystallinity of VPP may finally affect the MA/PA selectivity ratio (19). Moreover, the surface acidity also influences the distribution of products (17) an increase of Lewis acidity improves the selectivity to PA, while that to MA is positively affected by Bronsted acidity (2). 

If the aim is to explore the mechanism of the reaction and understand which are the important parameters of the catalyst determining the activity, then a micro-kinetic model is needed. A micro-kinetic model is based on a detailed mechanism and independent information about the rates of the elementary steps involved and the stability of the intermediates. The micro-kinetic model is the synthesis of all the basic knowledge about a reaction over a given catalyst. 

Fig. 32. UV-vis spectra of a 5% PdAhO catalyst determined with the equipment depicted in Fig. 31. The reference was recorded at time t = 0 during the flow of hydrogen-saturated ethanol. The three bottom spectra were recorded during the flow of hydrogen-saturated ethanol at t = 1, 15, and 30 min. The three top spectra were recorded during the flow of oxygen-saturated ethanol at t = 40, 49, and 66 min. The inset shows the absorbance at 330 nm as a function of time. The spectra represent averages of 200 scans with 100 ms accumulation time each. The flow rate was 0.3mL/min (96).

Fig. 7. Surface concentration of each metal element in the MoaBio-iCogFejO, catalyst determined by XPS analysis (41).

Fig. 12. The porosity of the catalyst determines not only its activity but also the chain length. Here melt index (MI) varies with catalyst pore volume in a series in which a common hydrogel was dried by extraction with different organic solvents to achieve variations in porosity.

Figure 8 The particle-size distribution function for a NiO catalyst determined by small-angle scattering...

Table 4 The selectivities and activity for the rotation mechanism for the different catalysts, determined at 75°C, with PD2=0.45 and PCp=0.024 atm. For a description of the different intermediates, see Figure 1 and ref. [19]...

Introduce the above value in the simulation model in which the reaction rate is expressed per mass of catalyst Determine the total number of stages needed to achieve the desired conversion. Pay attention to the profiles of temperatures, concentrations, and reaction rate. Extract liquid and gas flows, as well as fluid properties. 

Kanan MW, Yano J, Surendranath Y, Dinca M, Yachandra VK, Nocera DG. Structure and valency of a cobalt-phosphate water oxidation catalyst determined by in situ X-ray spectroscopy. J Am Chem Soc. 2010 132(39) 13692 701. 

For a given operating pressure and a desired production rate, the catalyst determines (1) the operating temperature range, (2) recycle gas flow, and (3) refrigeration requirements. It also indirectly influences the makeup gas purity requirements.74... 

Figure 2.13 The energy difference between the stereomeric forms 2.6 and 2.7 of the metallocene catalyst determines the elastomeric property of the final polypropylene. The energy difference depends on the R group and can be estimated by theoretical calculations.

Dioxolanone 33 is obtained when the unsaturated silyldiazoester 30 is decomposed by Rh2(pfb)4 in the presence of an aldehyde or of acetone (Scheme 11) [21]. The reaction sequence is likely to include formation and (probably reversible) 1,5-cyclization of carbonyl ylide 31, and Cope rearrangement of the allylvinylether 32. In analogy to carbonyl ylide 21, the SiMe3 should occupy the exo-position in 31, thereby bringing the ester carbonyl in a geometry that is favorable to the cyclization step. Again, the choice of catalyst determines the product pattern, since CuOTf catalysis affords not only 33, but also oxirane 22 and the intramolecular cyclopropanation product 34. 

As would be expected, the concentration of catalyst determines the degree to which the polymerization was affected. The data displayed in Figure 7.5 show how Mw increases with increasing acid levels. By choosing the strongest acceptable acid, the concentration required to achieve a desired effect can be minimized. 

The acid strengths shown in Table 17.3 were examined by the visual color change method using the Hammett indicators shown in Table 17.1 [43, 48]. The indicator dissolved in solvent was added to the sample in powder form in a nonpolar solvent, sulfuryl chloride [38] or cyclohexane [40]. The strength of colored materials such as S04/Fe203 and Mo03/Zr02 was estimated from their catalytic activities in comparison with those of the catalysts determined by the Hammett-indicator method. 

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