Olefin hydrogenation hydrogen activation

Hatanaka, S. Yamada, M., and Sadakane, O., Hydrodesulfurization of Catalytic Cracked Gasoline. 2. The Difference Between HDS Active Site and Olefin Hydrogenation Active Site. Ind. Eng. Chem. Res, 1997. 36 p. 1510. 

Transition metal phosphine complexes provide another important class of hydrogenation catalysts. Wilkinson s complexes, RhClfPPhjlj and RuHClfPPhjlj, well known for their olefin hydrogenation activity, were shown by Fish [77, 97] to be also good precursors for the reduction of polyaromatic substrates under mild reaction conditions (85° C, ca. 20 atm H, ). following a general activity trend consistent with a combination of electronic and steric factors ... 

Wilkinson Hyd.rogena.tion, One of the best understood catalytic cycles is that for olefin hydrogenation in the presence of phosphine complexes of rhodium, the Wilkinson hydrogenation (14,15). The reactions of a number of olefins, eg, cyclohexene and styrene, are rapid, taking place even at room temperature and atmospheric pressure but the reaction of ethylene is extremely slow. Complexes of a number of transition metals in addition to rhodium are active for the reaction. 

Olefins are hydrogenated very easily, unless highly hindered, over a variety of catalysts. With active catalysts, the reaction is apt to be diffusion limited, since hydrogen can be consumed faster than it can be supplied to the catalyst surface. Most problems connected with olefin hydrogenation involve some aspect of regio- or stereoselectivity. Often the course of reduction is influenced greatly by the catalyst, by reaction variables, and by hydrogen availability at the catalyst surface. 

The catalytic system studied by Rennard and Kokes was in fact very complex. It can be expected that the satisfactory prolongation of the reaction should, however, result in a deviation from the formulated kinetics. Unfortunately no investigation comparable to that of Scholten and Kon-valinka has been done in the case of olefin hydrogenation. Such a study of the catalytic activity of the pure /3-phase of palladium hydride in comparison with the a- or (a + /3)-phases would supplement our knowledge concerning catalytic hydrogenation on palladium. 

Reetz et al. have used N-(octyl)4Br-stabilized Pd colloids (typical size, e.g., 3nm) as precursors to form so-called cortex-catalysts, where the active metal forms an extremely fine shell of less than lOnm on the supports (e.g., AI2O3). Within the first 1-4 s, the impregnation of AI2O3 pellets by dispersed nanostructured metal colloids leads to the time-dependent penetration of the support which is complete after 10 s. Cortex catalysts were reported to show a threefold higher activity in olefin hydrogenation than conventionally prepared catalysts of the same metal loading (5% Pd on AI2O3) [388]. 

In addition to Au and noble metals, Ni-Zn nanoclusters with an amorphous structure were successfully deposited on Ti02 nanoclusters. The state of Ni was metallic. The catalytic activity of Ni-Zn/Ti02 in olefin hydrogenation was ca. 10 times higher than unsupported Ni nanoclusters. Selective deposition onto Ti02 and the addition of Zn seemed to play an important role to stabilize Ni nanoclusters and to decrease the size of Ni nanoclusters, respectively. Also, clearly Zn promoted the hydrogenation activity of Ni and inhibit the growth of the size, but did not substantially affect Ni nature itself... 

Olefins and aromatic hydrogenation reaction are undesired in gasoline HDT unfortunately, they cannot be fully inhibited. The high requirement on hydrogenolysis, but low hydrogenation activity, makes CoMo the preferred catalysts. New catalysts are being offered by the manufactures for selective HDS. Speculatively, two concepts have been used to develop new selective catalyst (i) improve thiophene HDS, or (ii) passivate olefin hydrogenation. 

New approaches to catalyst recovery and reuse have considered the use of membrane systems permeable to reactants and products but not to catalysts (370). In an attempt to overcome the problem of inaccessibility of certain catalytic sites in supported polymers, some soluble rho-dium(I), platinum(II), and palladium(II) complexes with noncross-linked phosphinated polystyrene have been used for olefin hydrogenation. The catalysts were quantitatively recovered by membrane filtration or by precipitation with hexane, but they were no more active than supported... 

Olefin hydrogenation catalyzed by Fe(CO)s normally requires somewhat severe conditions, typically 150°C and 10 atm H2 (/, p. 64). With near-ultraviolet irradiation the carbonyl becomes effective at ambient conditions for hydrogenation (and isomerization) of olefins (448, 449). Photoinduced labilization of carbonyls is thought to give tricarbonyl species as the active catalysts, e.g.,... 

Some 7r-allylmanganese(I) tetracarbonyl complexes, partly substituted by phosphites or phosphines, are apparently active for olefin hydrogenation (534). 

Metal hydrides and acyl-like CO insertion products are two types of species likely to be present in any homogeneous or heterogeneous process for the catalytic reduction of carbon monoxide. The discovery and understanding of new types of reactivity patterns between such species are of fundamental interest. As discussed elsewhere (11,22,54-57), bis(pentamethylcyclo-pentadienyl) actinide hydrides (58) are highly active catalysts for olefin hydrogenation as well as H-H and C-H activation. 

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) ... 

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)... 

Direct comparisons of the diamine system against the parent complex led to the conclusion that the effect of the diamine and KOH/i-PrOH activator decelerate olefin hydrogenation and in turn accelerate carbonyl hydrogenation. In the published report, there were no attempts to optimize turnover numbers or TOF for aldehyde hydrogenation. However, the catalyst has been shown to hydrogenate ketones with a SCR of 10000 at room temperature, which suggests that these catalysts represent the current state of the art in terms of activity and selectivity. 

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