Active centers formation

So far the problem of active center formation in chromium oxide catalysts amounted mainly to a discussion of the oxidation number of chromium that is necessary for catalytic activity. As an active species chromium ions having practically every possible oxidation number—... 

The Problem of Active Center Formation during Polymerization in the Presence of Individual Organometallic Compounds... 

Moreover, we speculated the mechanism of active center formation in propylene polymerization using solid catalyst (TiClj /Support/E.D.)//co-catalyst (E.D./AIR3) system, and found polymerization characteristics of the catalyst system con well be explained by the mechanism. 

Here, we woTild like to propose the mechanism of active center formation as shown in Scheme II. 

Furthermore, we have some more expermental deta supporting the above proposed mechanism concerning the active center formation. 

Moreover, we proposed a mechanism concerning active center formation and decomposition as shown in Scheme II. 

Bruk et al. studied [712] the radiation polymerization of TFE adsorbed on highly porous substrates. It has been shown that the rate of radiation polymerization of TFE on silica is controlled by the concentration of monomer in the adsorbed layer. The polymerization rate increased with increasing concentration of PTFE grafted to silica gel as a result of an increased number of active center formation caused by a more effective energy transfer from silica gel to grafted PTFE [713]. Bruk et al. [714] further reported that... 

The oligomerization reaction of ethylene in the presence of organotitanium compounds takes places via the following steps active center formation, olefin coordination on the titanium atom and chain propagation, and termination of the chain growth. 

Examples (E) and (F) include a step of active center formation, while in the case (G) there is a decomposition of the active center. Decomposition of the intermediate is illustrated in Fig. 4.4F1. 

The mechanism of anionic polymerization of cyclosiloxanes has been the subject of several studies (96,97). The first kinetic analysis in this area was carried out in the early 1950s (98). In the general scheme of this process, the propagation/depropagation step involves the nucleophilic attack of the silanolate anion on the sUicon, which results in the cleavage of the siloxane bond and formation of the new silanolate active center (eq. 17). 

It is evident, that all of the abovementioned relates to the reaction of cycle opening type and the formation of active centers (radicals) at molecule ends ... 

A special situation is created in a polymerization of isolated dienes or similar compounds like diisocyanates. Addition of such a monomer to a growing polymeric chain leaves its second reactive unit in the vicinity of the active center. Consequently, the addition of this unit is favored to the addition of any other unit, and in fact it is governed by a unimolecular and not bimolecular kinetic law. Its addition leads to the formation of a ring, and if ring closure is... 

The following stage of the propagation center formation occurs through the reduction of Cr(VI) to the lower oxidation state. The compounds of Cr(II) seem to be active in polymerization in the solution of bis-triphenyl-silyl-chromate (109). For the formation of these compounds the following scheme taking into account the results (110) concerning the study of the reaction of bis-triphenylsilyl-chromate with olefins was considered (109) ... 

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. 

In the cases of Cr03/Si02 and Cr(7r-C3H6)3/Si02 systems a considerable part of the chromium contained in the catalyst is involved in the propagation center formation. In these catalysts all the ions of the transition metals are on the surface and the active component seems to be the main type of compounds present on the catalyst surface. 

The formation of many polymer molecules on one active center is due to regeneration reactions, e.g. after the spontaneous transfer according to the scheme ... 

Chromium compounds as catalysts, 188 Chromium oxide in catalytic converter, 62 Chromium oxide catalysts, 175-184 formation of active component, 176,177 of Cr-C bonds, 177, 178 propagation centers formation of, 175-178 number of, 197, 198 change in, 183, 184 reduction of active component, 177 Clear Air Act of 1970, 59, 62 Cobalt oxide in catalytic converter, 62 Cocatalysts, 138-141, 152-154 Competitive reactions, 37-43 Copper chromite, oxidation of CO over, 86-88... 

From Fig.2 (a), A solid phase transformation fiom hematite, Fc203 to magnetite, Fe304, is observed, indicating that the active sites of the catalj are related to Fc304. Suzuki et. al also found that Fe304 plays an important role in the formation of active centers by a redox mechanism [6]. It is also observed that the hematite itself relates to the formation of benzene at the initial periods, but no obvious iron carbide peaks are found on the tested Li-Fe/CNF, formation of which is considered as one of the itsisons for catalyst deactivation [3,6]. 

The conditions essential for the formation of this exceptionally sharp distribution are the following (1) growth of each polymer molecule must proceed exclusively by consecutive addition of monomers to an active terminal group, (2) all of these active termini, one for each molecule, must be equally susceptible to reaction with monomer, and this condition must prevail throughout the polymerization, and (3) all active centers must be introduced at the outset of the polymerization and there must be no chain transfer or termination (or interchange). If new active centers are introduced over the course of the polymerization, a much broader distribution will be produced for the obvious reason that those introduced late in the process will enjoy a shorter period in which to grow. If the chains suffer transfer, or if termination occurs with constant replenishment of the active centers by one... 

After the reaction for 5 h in a reactant stream of CH , O, and Hj (P(CHJ= 33.7, P(0,)= 8.4 and P(H2)= 50.7 kPa), the catalyst was analyzed by XRD, Mossbauer and XPS studies. As regarding the XRD and Mossbauer spectroscopic measurements, obvious changes were not observed before and after the reaction. On the other hand, a marked change was observed in the XPS spectrum of the catalyst after the reaction. As shown in Fig. 2, besides the peak at 57.7 eV, which was the only peak of Fe3p obtained for the sample before the reaction and was ascribed to Fe(III), a clear shoulder at 56.1 eV was observed after the reaction. This can be ascribed to the Fe(ll) on the catalyst surface. The same phenomenon has been reported for FeP04 catalyst [13]. Such observations suggest the occurrence of the redox of iron between Fe(Iil) and Fe(II) during the reaction. We believe that this redox plays a key role in the formation of a new active center and thus is important in the selective oxidation of CH4... 

By single-site catalysts we mean catalysts where the breaking and formation of chemical bonds occurs at isolated active centers whose chemical activity is dominated by the electronic properties of a single atomic species or of a small cluster of atoms that can act in an independent way with respect to others. 

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