Catalysts hybrid

An attractive research target is the design of heteropoly catalysts complexed with organometallics. The use of heteropoly catalysts in combination with noble metals is also promising. Research in these directions has attracted much attention recently. 

Mono-Transition-Metal-Ion-Substituted Heteropolyanions as Inorganic Synzymes 

A recent development concerns the use of polyanions of the type [XMi 039M (0H2)] . In this type, the M atom easily becomes coordinatively unsaturated by dehydration (255). The resulting dehydrated anion, [XMhOjqM ] , can be considered an inorganic metalloporphyrin analog (322, 364, 365). Oxidation catalysis by these polyanions is described in Sections VIII and IX. Here, the catalytic performance and stability are compared with that of metalloporphyrin. 

In the majority of the homogeneous oxidations of hydrocarbons by oxometal-based catalysts (including metalloporphyrins), there is appreciable decomposition of catalyst ligands by oxidation, and hence appreciable loss in activity after a few turnovers. A similar degradation of organic ligands, often hydrophobic long-chain carboxylates, is also observed in industrial processes of hydrocarbon 

TOSHIO OKUHARA, NORITAKA MIZUNO, AND MAKOTO MISONO 

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These examples are part of a broader design scheme to combine catalytic metal complexes with a protein as chiral scaffold to obtain a hybrid catalyst combining the catalytic potential of the metal complex with the enantioselectivity and evolvability of the protein host [11]. One of the first examples of such systems combined a biotinylated rhodium complex with avidin to obtain an enantioselective hydrogenation catalyst [28]. Most significantly, it has been shovm that mutation-based improvements of enantioselectivity are possible in these hybrid catalysts as for enzymes (Figure 3.7) [29]. 

Fueled by the success of the Mn (salen) catalysts, new forays have been launched into the realm of hybrid catalyst systems. For example, the Mn-picolinamide-salicylidene complexes (i.e., 13) represent novel oxidation-resistant catalysts which exhibit higher turnover rates than the corresponding Jacobsen-type catalysts. These hybrids are particularly well-suited to the low-cost-but relatively aggressive-oxidant systems, such as bleach. In fact, the epoxidation of trans-P-methylstyrene (14) in the presence of 5 mol% of catalyst 13 and an excess of sodium hypochlorite proceeds with an ee of 53%. Understanding of the mechanistic aspects of these catalysts is complicated by their lack of C2 symmetry. For example, it is not yet clear whether the 5-membered or 6-membered metallocycle plays the decisive role in enantioselectivity however, in any event, the active form is believed to be a manganese 0x0 complex <96TL2725>. 

Drew, K., Girishkumar, G., Vinodgopal, K., and Kamat, P.V. (2005) Boosting fuel cell performance with a semiconductor photocatalyst Ti02/Pt-Ru hybrid catalyst for methanol oxidation. Journal of Physical Chemistry B, 109 (24), 11851-11857. 

Wight, A.P. and Davis, M.E. (2002) Design and preparation of organic-inorganic hybrid catalysts. Chemical Reviews, 102, 3589-3614. 

Zeolite-bentonite hybrid catalysts for the pyrolysis of woody biomass... 

Hybrid catalysts consisting of a zeolite (ZSM-5 or Beta) and bentonite as a binder were prepared and characterized by XRD, pyridine FTIR and nitrogen adsorption. The hybrid catalysts exhibited similar properties as the combined starting materials. Catalytic pyrolysis over pure ZSM-5 and Beta as well as hybrid catalysts has been successfully carried out in a dual-fluidized bed reactor. De-oxygenation of the produced bio-oil over the different zeolitic materials was increased compared to non-catalytic pyrolysis over quartz sand. 

The objective of this work is to synthesize and characterize zeolite-bentonite hybrid catalysts and perform test reactions in the pyrolysis of woody biomass in a dual-fluidized bed reactor. The aim is to produce catalytic materials which have good mechanical strength and are still able to de-oxygenate the pyrolysis oil. 

Zeolite-bentonite hybrid catalysts have been prepared from ZSM-5 and Beta zeolites supplied by Zeolyst International and bentonite. The zeolite to bentonite ratio was 35/65. The Si02 to A1203 ratio of ZSM-5 and Beta was 23 and 25, respectively. The preparation of the material was conducted in the following way. The zeolite and the bentonite were bound together through mixing in a 6 1 aqueous solution at 60°C for 2h,... 

Pyrolysis and catalytic de-oxygenation of pine wood biomass was successfully carried out in a dual-fluidized bed reactor. Hybrid catalysts consisting of zeolites Beta and ZSM-5 and bentonite were used. Pure zeolites Beta and ZSM-5 and also pure bentonite were tested as bed materials in the fluidized bed reactor. 

The application of the hybrid catalysts improved the quality of the produced oil. This was noticed as an increase in the water content in the oil, implying that de-oxygenation over the hybrid catalysts was more intense than over the pure materials. 

Besides ODH processes, a few reports about non-oxidative dehydrogenation (DH) over carbon catalysts also exist. At the reaction temperature of 823-873 K, propane is reported to react to propylene and hydrogen in high yield (30-40 %) over ordered mesoporous carbon, which was shown to be much more active than graphitic and/or nanostructured carbon (CNTs) [66], On the other hand, a hybrid catalyst system for... 

Tab. 15.3 Selected examples for nanocarbon-inorganic hybrid catalysts.

In addition to this, solid acid catalysts can also be used in the hydroisomerization cracking of heavy paraffins, or as co-catalysts in Fischer-Tropsch processes. In the first case, it could also be possible to transform inexpensive refinery cuts with a low octane number (heavy paraffins, n-Cg 20) to fuel-grade gasoline (C4-C7) using bifunctional metal/acid catalysts. In the last case, by combining zeolites with platinum-promoted tungstate modified zirconia, hybrid catalysts provide a promising way to obtain clean synthetic liquid fuels from coal or natural gas. 

The catalytic activity of a lanthanum (R)-BINOL complex tethered either on silica (62a) or MCM-41 (62b) was evaluated for the enantioselective nitroaldol reaction of cyclohexanecarboxaldehyde (Se), hexanal (Sf), iso-butyraldehyde (Sg) and hydro-cinnamaldehyde (Sh) with nitromethane inTHF (Scheme 12.22) [166]. The silica-anchored lanthanum catalyst 62a gave 55-76% e.e. and yields up to 87%, while the PMS-immobilized catalyst 62b revealed slightly higher e.e.s (57-84%) for the same aldehydes. The homogeneous counterparts showed similar catalytic performance, albeit within a shorter reaction time. The increased enantioselectivity observed for the MCM-41 hybrid catalyst 62b was explained by transformations inside the channels, which is also reflected by lower yields due to hindered diffusion. The recyclability of the immobilized catalysts 62b was checked with hydrocin-namaldehyde (Ph). It was found that the reused catalyst gave nearly the same enantioselectivities after the fourth catalytic run, although the time period for achieving similar conversion increased from initially 30 to 42 h. 

One of the most significant additions to the modern rhodium] ) catalyst ligand family was the development of the hybrid catalysts that combined the carboxamide bridging ligands with the enhanced reactivity of perfluoroalkyl substituents. In this series, rhodium] ) trifluoroacetamidate [Rh2]tfa)4] was the first described [30]. n addi-... 

Fig. 24. General scheme of the directed evolution of hybrid catalysts (22,132,202,203).

Roelfes, G. and Feringa, B.L. (2005) DNA-based asymmetric catalysis. Angew. Chem., Int. Ed., 44, 3230-3232 Kraemer, R. (2006) Supramolecular bioinorganic hybrid catalysts for enantioselective transformations. Angew. Chem., Int. Ed., 45, 858-860. 

They can be handled analogous to thermosetting resins, and thus the use of highly volatile comonomers, such as ethene or prop-ene is prohibitive. Instead, other vinyl monomers are used. A heat curable formulation uses a mixture of tetracyclododecene, 2-norbomene, 5-vinyl-2-norbomene, and divinylbenzene as reactive components (41). The mixture further contains 3,5-di-ferf-butylhy-droxyanisole as antioxidant and a hybrid catalyst system containing a zirconium based metathesis catalyst and a radical catalyst. The metathesis catalyst is benzylidene (l,3-dimesitylimidazolidin-2-yl-idene)(tricyclohexylphosphine)ruthenium dichloride and the radical catalyst is di-ferf-butyl peroxide. 

Several authors have pursued this approach and indeed observed that desulfurization of 4.6-DMDBT was increased when acidic zeolites were used in combination with conventional HDS catalysts (30, 31, 33, 137, 149-151). Figure 34 shows that there can be a great acceleration in the conversion of 4,6-DMDBT through the use of a hybrid catalyst consisting of CoMo impregnated into a composite containing 5% NiY zeolite and alumina (137). Low temperatures had to be employed, as at temperatures exceeding about 340°C, severe color fluorescence occurred in the product. 

The Direct Enantioselective Synthesis of Diols from Olefins using Hybrid Catalysts of Chiral Salen Cobalt Complexes Immobilized on MCM-41 and Titanium-containing Mesoporous Zeolite... 

The purely siliceous MCM-41 and Ti-containing MCM-41 were synthesized by the solvent evaporation method. The chiral salens were immobilized step by step on the siliceous MCM-41 by the new grafting method using 3-aminopropyltrimethoxysilane and 2,6-diformyl-4-tert-butylphenol. The enantioselective diols could be synthesized directly from olefins using the hybrid catalysts of chiral salen complexes and Ti-MCM-41. 

Asymmetric diols synthesis from olefins over the hybrid catalyst of TI-MCM4I and Co(lll) Salen complexes. 

In conclusion, the chiral salen Co(III) complexes immobilized on Si-MCM-41 colud be synthesized by multi-grafting method. The asymmetric synthesis of diols from terminal olefins was applied with success using a hybrid catalyst of Ti-MCM-41/chiral Co(III) salen complexes. The olefins are readily oxidized to racemic epoxides over Ti-MCM-41 in the presence of oxidants such as TBHP, and then these synthesized diols are generated sequentially by epoxide hydrolysis on the salen Co(lll) complexes. This catalytic system may provide a direct approach to the synthesis of enantioselective diols from olefins. 

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