Selective turnover frequencies

The cyclodimerization of 1,3-butadiene was carried out in [BMIM][BF4] and [BMIM][PF(3] with an in situ iron catalyst system. The catalyst was prepared by reduction of [Fe2(NO)4Cl2] with metallic zinc in the ionic liquid. At 50 °C, the reaction proceeded in [BMIM][BF4] to give full conversion of 1,3-butadiene, and 4-vinyl-cyclohexene was formed with 100 % selectivity. The observed catalytic activity corresponded to a turnover frequency of at least 1440 h (Scheme 5.2-24). 

Obtain meaningful data on the catalyst Usually for kinetic purposes is it the turnover frequency per active site (TOP) that of interest. But other parameters such as selectivity and yield are also of great importance for judging the potential of the catalyst. Instead of expressing the activity as a turnover frequency, it can also be given in terms of ... 

Reaction conditions were lOTorr CgHio, 200Torr H2, and 273-323 K for hydrogenation (400-443 K for dehydrogenation). Selectivity at 448K based on experimentally measured turnover frequencies (lOTorr CsHio, 200Torr H2). 

Attempts to determine how the activity of the catalyst (or the selectivity which is, in a rough approximation, the ratio of reaction rates) depends upon the metal particle size have been undertaken for many decades. In 1962, one of the most important figures in catalysis research, M. Boudart, proposed a definition for structure sensitivity [4,5]. A heterogeneously catalyzed reaction is considered to be structure sensitive if its rate, referred to the number of active sites and, thus, expressed as turnover-frequency (TOF), depends on the particle size of the active component or a specific crystallographic orientation of the exposed catalyst surface. Boudart later expanded this model proposing that structure sensitivity is related to the number of (metal surface) atoms to which a crucial reaction intermediate is bound [6]. 

When dispersed in a lipid hhn on a surface of a graphite electrode, the FeCu catalysts ate clearly superior to the Fe-only forms in catal5Tic selectivity, stability, and turnover frequency. 

Process Evaluation and Improvement. As homogeneous asymmetric hydrogenation processes are scaled up, one major concern is cost because the catalyst is usually expensive. Hence, several criteria for a commercially viable process (2), including selectively, conversion, catalyst loading (S/C, the molar ratio of substrate to catalyst), reaction time, and TOF (turnover frequency, the ratio of catalyst loading to reaction time), should be considered to evaluate the process and provide a guide for improvement. 

Figure 3.9. Transient C02 formation rates on Pd30 (a) and Pd8 (b) mass-selected clusters deposited on a MgO(lOO) film at different reaction temperatures [74]. In these experiments CO was dosed from the gas background while NO was dosed via a pulsed nozzle molecular beam source. The turnover frequencies (TOFs) calculated from the experiments displayed in (a) and (b) are displayed in the last panel (c). C02 formation starts at lower temperatures but reaches lower maximum rates on the larger cluster. (Figure provided by Professor Heiz and reproduced with permission from Elsevier, Copyright 2005).

Catalytic activity was measured as a function of turnover frequency [moles product/(mole catalyst) (hour)]. The standard run has a turnover frequency of 105 10. All the parameters investigated were perturbed about this standard and included the effects of catalyst, aldehyde, KOH and water concentration, initial CO pressure, and reaction time. In addition, a few selected runs were also conducted to examine the effects of hydrogen in the gas phase as well as the relative ease with which other aldehydes could be reduced. 

The catalytic system of [Ir(COD)Cl]2 with an excess of the bulky phosphine P(o-MeOPh)3 under transfer hydrogenation conditions of propan-2-ol and KOH was used successfully in the selective hydrogenation of cinnamaldehyde (Scheme 15.4) [4]. Selectivity and activity were found to increase with increasing P/Ir ratios, and complete conversion was achieved in as little as 5 minutes (turnover frequency (TOF) 6000 IT1). 

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