Carbon-supported catalysts, phosphorus

Bergbreiter DE, Li J (2004) Applications of Catalysts on Soluble Supports. 242 113-176 Bertrand G, Bourissou D (2002) Diphosphorus-Containing Unsaturated Three-Menbered Rings Comparison of Carbon, Nitrogen, and Phosphorus Chemistry. 220 1-25 Betzemeier B, Knochel P (1999) Perfluorinated Solvents - a Novel Reaction Medium in Organic Chemistry. 206 61-78 Bibette J,see SchmittV (2003) 227 195-215... 

The above procedures for catalyst preparation have generally provided excellent results. Especially important are surface-sensitive reactions. With supported catalysts in which the active components have a narrow particle-size distnbution, the optimum particle size for a demanding reaction can be established. Major improvements of supported catalysts, e.g. with respect to carbon deposition and ammonia decomposition, can be achieved by preparing catalysts with a narrow par-ticle-size distribution. Also, the preparation of catalysts in which the active components have a uniform chemical composition is highly important One instance is the preparation of supported vanadium oxide phosphorus oxide (VPO) catalysts for the selective oxidation of w-butane to maleic anhydride, which has been carried out using vanadium(III) deposition onto silica [31]... 

In conclusion, a comparison of the results of this study and literature results has shown that catalytically equivalent supported catalysts can be formed either by using V0(H2P04)2 as a precursor or by adding phosphorus to PA =1 compounds to increase the P/V ratio to about 2. Characterization data suggest that these two types of supported catalysts may be identical. Finally, alkane oxidation over a bulk catalyst derived from V0(H2P04>2 may proceed via activation at the secondary carbon atoms, and small amounts of dispersed a,-VOP04 are responsible for the activation of butane. 

The addition of phosphorus to the carbon-supported Co and CoMo catalysts resulted in a decrease in the activity for HDS of thiophene.For CoP/carbon catalysts, this was attributed to the formation of an inactive metal phosphate that in the course of reaction was gradually converted to metal phos-phide. In the case of MoP/carbon catalyst, it was proposed that P was adsorbed on CUS present on the edges of M0S2. However, these experiments were conducted at a low H2 pressure. Moreover, recent information suggests that Co phosphides and Mo phosphides catalyze some hydroprocessing reactions, though at an elevated H2 pressure.At 3 MPa of H2 and 613 K, the addition of phosphorus to both NiMo/C and NiW/C catalysts enhanced the rate of HDS of thiophene and the HDN of pyridine when the mixture of these reactants was used as the feed. This was attributed to the enhanced reduci-bility of the catalyst in the presence of phosphorus. Almost certainly, different observations on the effect of phosphorus on catalyst activity can be attributed to different experimental conditions. 

The formulated mechanism is supported by the finding that no halogen from the phosphorus trihalide is transferred to the a-carbon of the carboxylic acid. For instance, the reaction of a carboxylic acid with phosphorus tribromide and chlorine yields exclusively an a-chlorinated carboxylic acid. In addition, carboxylic acid derivatives that enolize easily—e.g. acyl halides and anhydrides—do react without a catalyst present. 

XRD and LRS were used to characterize both supported and unsupported catalysts derived from V0(H2P04)2. Supported V0(H2P04)2-derived catalysts were found to be more active but slightly less selective in butane oxidation to maleic anhydride than unsupported samples, but the difference in selectivity could be eliminated by adding a small amount of phosphorus to the supported samples. For butane oxidation, the activity of the catalysts was much enhanced by the presence of a,-VOP04- The relative rates of oxidation of propane, butcme, and pentane were consistent with alkane activation occurring predominantly at the secondary carbon. 

It has generally been assumed that the bonds that link the catalyst to the polymer support are chemically stable under the reaction conditions one employs. Until recently, the literature offered little information in this regard, since lifetime studies are needed to properly evaluate stability. Recent publications have pointed out the chemical instability of the phosphorus-carbon bond of tertiary phosphine functionalized supports and the chemical reactivity of various nitrogen functionalized polymeric support materials under reaction conditions. If such chemical stability problems are present, the consequences are indeed serious. While a typical "leach" situation would necessitate a periodic reloading of the metal complex, cleavage of polymer functionality would necessitate replacement of both the metal complex and the polymer. 

The synthesis of phosphorus-containing heterocycles has been reported using a palladium-catalyzed approach (Scheme 4.172) [261]. The overall process involved two phosphorus-carbon(sp ) bond-forming reactions and started with a hydrophosphonylation reaction using a supported palladium catalyst to promote the addition. Conversion of the... 

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