Activation of the catalyst precursor

The initial steep rise can be attributed simply to activation of the catalyst precursor. The amount of base corresponds approximately to one equivalent of KOH for the ruthenium catalyst and two equivalents for the rhodium catalyst. Activation could result from hydroxide attack as in (5) and (6) for rhodium and (10) for ruthenium ... 

In addition, careful kinetic measurements and product analysis has revealed that the activation of the catalyst precursor 26b during the induction period occurs by hydrogenation of the coordinated maleic anhydride to succinic anhy-... 

Despite the remarkable enantioselectivities observed with the Ti-ebthi catalyst for the imine and enamine hydrogenation, we consider its technical potential rather low. The ligand is difficult to prepare, the activation of the catalyst precursor is tricky, for the moment the catalytic activity is far too low for preparative purposes, and last - but not least - its tolerance for other functional groups is low. 

Kinetic studies of diallyltosylamide RCM reaction monitored by NMR and UV/VIS spectroscopy showed that thermal activation of the catalyst precursors la and Ib (25-80 °C) led to the in situ formation of a new species which could not be identified but appeared to be the active catalytic species [52]. Attempts to identify this thermally generated species were made in parallel by protonation of the catalysts I. Indeed, the protonation of allenylidene-ruthenium complex la by HBF4 revealed a significant increase in catalyst activity in the RCM reaction [31,32]. The influence of the addition of triflic acid to catalyst Ib in the ROMP of cyclooctene at room temperature (Table 8.2, entries 1,3) was even more dramatic. For a cyclooctene/ruthenium ratio of 1000 the TOF of ROMP with Ib was 1 min and with Ib and Sequiv. of TfOH it reached 950min [33]. 

Finally, a third means of ligand formation from an imidazolium cation, described by Dupont and co-workers, should be mentioned here [34]. They investigated the hydrodimerization/telomerization of 1,3-butadiene with palladium(II) compounds in [BMIM][BF4] and described the activation of the catalyst precursor complex [BMIM]2[PdCl4] by a palladium(lV) compound formed by oxidative addition of the imidazolium nitrogen atom and the alkyl group with cleavage of the C-N bond of the [BMIM] ion, resulting in bis(methyHmidazole) dichloropalladate (Scheme 5.2-5). However, this reaction was only observed in the presence of water. 

Activation of the Catalyst Precursors 515 Table 12.1 Influence of activation conditions on unpromoted V—P—O catalysts [135]. 

On the other hand, there is a need to use inhibitors of the platinum catalysts temporarily to reduce their catalytic activity in the presence of hydro- and vinyl-polysiloxanes in order to stop the curing process at room temperature, but to allow the platinum catalyst to be activated at elevated temperature. Among the principal types of compounds reported are alkenyl derivatives, esters of unsaturated acids, crown ethers, organic nitrogen compounds, phosphines, linear and cyclic vinyl-siloxanes, and poly(vinyl)siloxanes [2], and recently fumarate [44] and maleinate [33]. New co-activators of the catalysts (precursors) have been revealed in the 1990s to reduce to ppm the levels of platinum required to effect hydrosilylation curing [45, 46]. 

All these aspects contribute to the character of a metallocene catalyst. Many attempts have been made in the past to design catalysts by molecular modeling. The results were not very satisfying because there are still too many open parameters that must be considered such as the degree of activation of the catalyst precursor, the interaction between catalyst cation and cocatalyst anion, or the role of the solvent. Because of this situation, we preferred the empirical way. The author s group synthesized over 650 met-... 

Promoters are sometimes added to the vanadium phosphoms oxide (VPO) catalyst during synthesis (129,130) to increase its overall activity and/or selectivity. Promoters may be added during formation of the catalyst precursor (VOHPO O.5H2O), or impregnated onto the surface of the precursor before transformation into its activated phase. They ate thought to play a twofold stmctural role in the catalyst (130). First, promoters facilitate transformation of the catalyst precursor into the desired vanadium phosphoms oxide active phase, while decreasing the amount of nonselective VPO phases in the catalyst. The second role of promoters is to participate in formation of a soHd solution which controls the activity of the catalyst. 

Pt/H-MCM-22 catalysts for methane combustion have been prepared by ion-exchange of a highly crystalline H-MCM-22 zeolite using [Pt(NH3)4](N03)2. The activation procedure of the catalyst precursor has been optimized and all steps monitored by HRTEM, SEM and FTIR of CO adsorbed. The preliminary decomposition/calcination of the ion exchanged sample is very crucial in that influence the final properties of platinum active species. 

The catalytic activity of cationic rhodium precursors of formula [Rh(diene)(di-phosphine)]+ was also explored by Schrock and Osborn [28]. Halpern and coworkers made very detailed mechanistic studies of olefin hydrogenation by [RhS2(diphos)]+ species (diphos = l,2-bis(diphenylphosphino)ethane S = solvent) [31]. Significant differences have been observed in the reaction of the catalyst precursors [Rh(NBD)(PPh3)2]+ and [Rh(NBD)(diphos)]+ in methanol, as shown in Eqs. (8) and (9) ... 

Table 14.3 compares the catalyst activities of the (inactive) precursor fW(=Ar) (=CH Bu)(CH 2Bu)2], the heterogeneous catalysts obtained by grafting the precursor on Si02 7oo and on Si02 2oo, and the homogeneous silsesquioxane-based catalysts. It shows that only the siloxy alkylidene species, either molecular or surface-bound, are active catalysts. 

The rhodium complexes were prepared in situ from the rhodium precursor [Rh(nbd)2](C104) (nbd = 2,5-norbornadiene) and applied in the hydrogenation experiments under an initial hydrogen pressure of 5 bar at 35°C. The dendrimer structure had almost no effect on the activity of the catalyst in the batch-wise rhodium-catalyzed hydrogenation of dimethyl itaconate (Scheme 4). 

Allenylidene-ruthenium complex Ib readily promotes the ROMP of norbornene, much faster than the precursor RuCl2(PCy3)(p-cymene) [39] (Table 8.1, entry 1). The ROMP of cyclooctene requires heating at 80 °C (5 min), however a pre-activation of the catalyst allows the polymerization to take place at room temperature. The activation consists, for example, in a preliminary heating at 80 °C or UV irradiation of the catalyst before addition of the cyclic aikene, conditions under which rearrangement into indenylidene and arene displacement take place [39] (Table 8.1, entries 2,3). The arene-free allenylidene complexes, the neutral RuCl2(=C=C=CPh2)... 

This process was applied to prepare AlsNi precursor with 2at% Ti. The catalytic behavior was examined for the hydrogenation of acetone in an autoclave. The rate of change of hydrogen pressure with time was evaluated, i.e. the rate of decrease from 0.3MPa (initial pressure). This value is an indicator of the activity of the catalyst. It... 

An investigation of the influence of surface area on the activity of the supported catalysts has shown that the activity increases with increase in surface area, but the selectivity is virtually independent of surface area (27). This result is consistent with both mechanisms i and ii. Thus, in mechanism i the reaction takes place in the pores of the catalyst, which are sufficiently large not to impose steric demands on the reactants, so that the activity of the catalyst is dependent on the rates of diffusion of the reactants to the active site. In terms of mechanism ii, in which the supported complex acts only as a precursor of a soluble catalytically active species, the activity of the catalyst will depend on the ease with which this species is abstracted from the polymer support clearly, this will increase with increasing surface area. 

Catalyst Structure D in Fig. 6.21 represents a structurally distinct electrocatalyst compared to catalysts A-C. Unlike A-C, structure D contains no base-metal near the top layer of the smooth or high surface area catalyst. Catalysts like that in Figure 6.21D were prepared [126-129] by rapid electrochemical de-alloying (preferred leaching of the base-metal) of base-metal-rich precursor Pt alloys. The electrocatalytic ORR activities of the catalyst materials obtained after de-alloying exceeded pure Pt nanoparticle catalysts by a factor of 4-6 x. 

However the catalyst can be easily reactivated in flow of o2/Ar at 350°C (compare runs 2 and 2.1, Table 1) using the procedure reported in the previous section. Catalytic activity can be restored also by a thermal treatment in flow of He (350°C, 15 h), and this suggests that strongly adsorbed produts could be responsible for catalyst deactivation. The amount of 4-hexen-3-one converted depends on the nature of the catalyst precursor and on its thermal pretreatment. Thus, over a non activated commercial Mgo (obtained by thermal decomposition of MgC03, surface area 17 m2/g), 0.5 moles of 4-hexen-3-one/mole Mgo are converted, while when the same Mgo was activated at 350°C (surface area 34 m2/g), 2 moles of 4-hexen-3-one/mole MgO are converted. Over a high surface area Mgo (prepared by thermal decomposition of Mg(OH)2r surface area 281 m2/g) up to 5 moles of 4-hexen-3-one/mole Mgo can be converted. Conversion of 4-hexen-3-one depends also on reaction temperature 250°C is found to be the best one, since both at higher and lower temperatures side reaction are favoured (runs 2.2 and 2.3, Table 1). Since different oxides were employed, the product distributions reported in Table 1 were measured in stationary conditions after 1 hour of reaction. 

The above definition is very broad and not all possible combinations of the catalyst precursor with the activator will result in the formation of active... 

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