Active heterogeneous catalyst

The exopolyhedral metaHacarborane complex Ti(C2B2QH22)4, which is prepared by the reaction of TiCl and 1-Li-1,2-C2B2QH22, has also been reported to be an active heterogeneous catalyst for the polymerization of olefins when supported on alumina and in the presence of (C2H3)2A1C1 co-catalyst (230). 

Active heterogeneous catalysts have been obtained. Examples include titania-, vanadia-, silica-, and ceria-based catalysts. A survey of catalytic materials prepared in flames can be found in [20]. Recent advances include nanocrystalline Ti02 [24], one-step synthesis of noble metal Ti02 [25], Ru-doped cobalt-zirconia [26], vanadia-titania [27], Rh-Al203 for chemoselective hydrogenations [28], and alumina-supported noble metal particles via high-throughput experimentation [29]. 

The liquid-phase autoxidation of cyclohexane is carried out in the presence of dissolved cobalt salts. A lot of heterogeneous catalysts were developed for this process but most catalysts lacked stability. The incorporation of cobalt ions in the framework of aluminophosphate and aluminosilicate structures opens perspectives for heterogenization of this process. CoAPO (cobalt aluminophosphate) molecular sieves were found to be active heterogeneous catalysts of this oxidation.133 Site isolation was critical to get active catalysts.134... 

Figure 14.4 Comparison between structural XAFS data measured for the active heterogeneous catalyst (a) before and (b) under the conditions of catalytic reactions (time averaged), along with respective stmctural X-ray determined bond distances (A) for the closest silsesquioxane models available [60, 62].

An important issue concerning the use of heterogeneous catalysts for biodiesel synthesis is the lack of systematic research exploring the principles of solid catalyst activity for transesterification of TGs and esterification of FFAs with alcohols. For instance, the question about the true catalytic nature of some solid bases, called heterogeneous, remains unanswered. For example, the most active heterogeneous catalysts reported to date is Ba(OH)2. However, due to its... 

The most active heterogeneous catalysts that are effective at room temperature are alkali metals supported on activated alumina. Simple C4-Cg alkenes, for example, were shown to yield equilibrium mixtures in short contact time over sodium on alumina.112116 Partial conversion of 1-butene at low temperature and in very short contact time (—60°C, 14 s) led to the stereoselective formation of m-2-butene.112 The changes in isomeric composition in the transformation of 4-methyl-l-pentene112 are as follows ... 

Mori K, Hara T, Mizugaki T et al (2004) Hydroxyapatite-supported palladium nanoclusters A highly active heterogeneous catalyst for selective oxidation of alcohols by use of molecular oxygen. J Am Chem Soc 126(34) 10657-10666... 

Estrine B, Soler R, Damez C, Bouquillon S, Henin F, Muzart J (2003) Recycling in telomerization of butadiene with methanol and phenol Pd-KF/Al203 as an active heterogeneous catalyst system. Green Chem 5 686-689... 

More recently, Corma and coworkers [64] have shown that Sn-substituted zeolite beta is a more active heterogeneous catalyst for MPV reductions, also showing high ds-selectivity (99-100%) in the reduction of 4-alkylcyclohexanones. The higher activity was attributed to the higher electronegativity of Sn compared to Ti. 

Activated carbons can be effective supports for the heterogenization of Rh complexes to produce active heterogenized catalysts for hydroformylation. 

Salen palladium(II) complexes have been reported to be effective homogeneous, or when immobilized, to be active heterogeneous catalysts for the hydrogenation of alkenes [1,2]. It is also known that platinum(O) catalysts modified with cinchona alkaloids catalyze the enantioselective hydrogenation of a-ketoesters to the corresponding a-hydroxyesters[3]. The platinum(O) catalyst is attached to AI2O3 [4] or to zeolites [5,6]. 

Polyionic gel beads consist of highly polar microenvironments, suitable for both efficient metal scavenging and active heterogeneous catalyst preparation. They display an efficient metal-soaking ability, which depends on strong non-covalent interactions between metal and ions within the polyionic gel. 

Semmelhack demonstrated that the addition of alkali metal chloride salts can sometimes markedly increase the yields of coupling products in heterogeneous Pd-catalyzed reactions, especially when the olefin component contains an amide function [15]. It has been claimed that palladium-grafted mesoporous material (MCM-41), designated Pd-TMSll, is one of the most active heterogeneous catalysts for the Heck reaction and enables C-C formation with activated and non-activated aryl substrates [16a,b]. Nanoscale particles of palladium clusters prepared by the ultrasonic reduction of Pd(OAc)2 and NR4X in THF or methanol, were also active for C-C couplings [17]. 

Dimerization of ethylene. Rhodium(IlI) chloride supported on silica gel is an active heterogeneous catalyst for dimerization of ethylene to 1-butene and then to 2-butene the transfcis ratio is 2.9. The active catalyst may have the structure depicted in (1). ... 

Numerous reports of heterogeneous catalysis active for alkylaromatic oxidations have appeared. These include an encapsulation of metal ions by zeolites or polymers [92-95]. Non-Co, Pd-based heterogeneous catalysts have been discovered by BP researchers [96-98]. Very recently, nanocrystalline ceria (Ce02) has been discovered to be a highly active heterogeneous catalyst for oxidation of pX in water to TA [99,100]. 

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