Turnover Frequency TOF

The turnover frequency TOF (the term was borrowed from enzyme catalysis) quantifies the specific activity of a catalytic center for a special reaction under defined reaction conditions by the munber of molecular reactions or catalytic cycles occuring at the center per unit time. For heterogeneous catalysts the number of active centers is derived usually from sorption methods (Eq. 1-7). 

For most relevant industrial applications the TOF is in the range 10 -10 s (enzymes 10 -10 s ). 

TOF values for the hydrogenation of cyclohexene at 25 °C and 1 bar (supported catalysts, structure insensitive reaction Table 1-2)  

---

Turnover frequency, TOF of the catalytic reaction, 4, 193 of the depletion of the promoting species, 193... 

Fig. 7 Dependence of IR band intensities on H2 partial pressure during ethene hydrogenation catalyzed by Ir4/y-Al203 at 288 K and 760 Torr (40 Torr C2H4, 50-300 Torr H2, and the balance He). The bands at 2990 (diamonds) and 2981 cnr (squares) were chosen to represent di-cr-bonded ethene and that at 1635 cnr (circles) to represent water on the y-AbOs support. These IR bands were chosen as the best ones to minimize error caused by overlap with other bands. The triangles represent the reaction rate expressed as a turnover frequency (TOF), the rate of reaction in units of molecules of ethene converted per Ir atom per second. The data indicate a correlation of the band intensities with the TOF, consistent with the suggestion that the ligands represented by the bands are reaction intermediates (but the data are not sufficient to identify the reaction intermediates) [39]...

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]. 

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).

The most important progress in the last decade has been in the design and synthesis of [RuCl2(diphosphine)(l,2-diamine)] catalysts exploiting the metal-ligand bifunctional concept developed by Noyori and co-workers.29-31 The Noyori catalysts seem to possess all of the desired properties, such as high turnover number (TON), high turnover frequency (TOF), and operationally simple, safe, and environmentally friendly reaction conditions. 

In a recent article by Botella and Najera, controlled mono- and double-Heck arylations in water catalyzed by an oxime-derived palladacycle were described [22], When the reaction was carried out under microwave irradiation at 120 °C in the presence of dicyclohexylmethylamine with only 0.01 mol% of palladium catalyst (palladium acetate or palladacycle), monoarylation took place in only 10 min with a very high turnover frequency (TOF) of > 40000 (Scheme 6.3). As regards diarylation, 1 mol% of the palladacycle catalyst and 2 equivalents of iodobenzene had to be utilized to obtain moderate to good yields of diarylated product. Whereas microwave heating at 120 °C provided a 31% yield after 10 min, a 66% isolated yield of product was obtained by heating the reaction mixture under reflux for 13 h at 100 °C. Here, the... 

The catalysis was performed batch-wise (Figure 4.36). After reaching ca. 90% conversion, the bulk phase was replaced and similar turnover frequencies (TOF) of about 25 h"1 were obtained in the following three runs 2, 3 and 4. When the catalyst capsule was removed, no further activity was detected. Furthermore, the Ru content in the bulk phase was always below the detection limit of AAS, which shows good catalyst retention by the membranes used. 

Recently, Liew et al. reported the use of chitosan-stabilized Pt and Pd colloidal particles as catalysts for olefin hydrogenation [51]. The nanocatalysts with a diameter ca. 2 nm were produced from PdCl2 and K2PtCl4 upon reduction with sodium borohydride in the presence of chitosan, a commercial biopolymer, under various molar ratios. These colloids were used for the hydrogenation of oct-1-ene and cyclooctene in methanol at atmospheric pressure and 30 °C. The catalytic activities in term of turnover frequency (TOF mol. product mol. metal-1 h-1)... 

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). 

A large number of catalysts or catalyst precursors for the reaction in Eq. (1) have been identified (Table 17.1). Almost all of these are complexes of Rh and Ru, although there are a few, less active, examples of fr, Pd, Ni, Fe, Ti, and Mo. The most active catalysts are RhCl(TPPTS)3, Rh(hfacac)(dcpb) and RuCl(OAc) (PMe3)4, with little difference between them after rough correction of the turnover frequencies (TOF) for pressure and temperature differences [38]. Musashi and Sakaki [39] argue that the barrier to C02 insertion into M-H bonds increases in the order Rh(I) < Ru(II) < Rh(I II) because Rh(lll)-formate bonds are too weak and Ru(II)-H bonds are too strong compared to the case of Rh(I). 

In order to facilitate the comparison of the effectiveness of the very diverse methods, turnover numbers (TON), and/or turnover frequencies (TOF) (if they were given by the author or could be calculated based on their data) are sum-... 

Pioneering studies on a different class of transfer hydrogenation catalysts were carried out by Henbest et al. in 1964 [15]. These authors reported the reduction of cyclohexanone (4) to cyclohexanol (5) in aqueous 2-propanol using chloroiridic acid (H2IrCl6) (6) as catalyst (Scheme 20.2). In the initial experiments, turnover frequencies (TOF) of 200 h 1 were reported. 

Analogues of BPPFA and BPPFOH have been prepared, but for many applications these two ligands still prove to be the best for enantioselective hydrogenations [125]. The introduction of another functional group into the side chain, as in 31, provided the first catalysts capable of hydrogenating the tetra-substi-tuted a,/ -unsaturated acids with high enantioselectivity, even though the activity was very low (turnover frequency, TOF, 2 h-1) [132, 133],... 

Whilst trying to be comprehensive, we have also intended to introduce a strong applied flavor to this summary. In the industrial case, catalyst performance is critically judged on overall efficiency, namely catalyst productivity and activity as well as enantioselectivity. As a result, turnover numbers (TONs) and turnover frequencies (TOFs) have been included or calculated whenever possible and meaningful. 

The first application of a heterocyclic carbenoid achiral ligand for hydrogenation of alkenes was reported in 2001 by Nolan and coworkers. Both ruthenium [36] and iridium [37] complexes proved to be active catalysts. Turnover frequency (TOF) values of up to 24000 b 1 (at 373 K) were measured for a ruthenium catalyst in the hydrogenation of 1-hexene. 

---