Zirconium hydride catalysts

The zirconium hydride catalyst [Cp2Zr(H)Cl] is very effective for the high-yield synthesis of cyclotetrastibines (ArSb)4 from arylstibines ArSbH2 [eqn (2.11)]. ... 

Oxidation of waste plastics, such as polyethylene, polypropylene, polystyrene, poly (alkyl acrylates), and nylon 6,6, with NO/O2 for 16 h at 170°C, led to mixtures of carboxylic acids, for which uses would have to be developed if this method was applied to large volumes of waste plastics.180 Hydrogenation of polyethylene at 150C>C for 10 h over a sil-ica/alumina-supported zirconium hydride catalysts gave a 100% conversion to saturated oligomers.181 Polypropylene gave a 40% conversion to lower alkanes at 190°C for 15 h. It is not clear what use these materials would have, other than serving as a feedstock for a petroleum refinery. 

Comparing the product selectivity at low conversion in the hydrogenolysis of 2,2-dimethylbutane for the two catalysts is noteworthy. Zirconium hydride supported on siUca does not produce neopentane, but only isopentane (10%) as a Cs product in agreement with a /1-alkyl transfer as a key step for the carbon-carbon cleavage (no neopentane can be formed through this mechanism, Scheme 25). 

These surface hydrides are active catalysts for the hydrogenolysis of alkanes at moderate temperatures. Zirconium hydride can catalyze the hydrogenolysis of neopentane, isobutane, butane, and propane at 323 K but cannot catalyze the hydrogenolysis of ethane.259... 

Earlier transition metals, as zirconium and hafnium, are still more active in hydrogenolysis, which allows zirconium hydrides to be used in depolymerization reactions (hydrogenolysis of polyethylene and polypropylene) [89], In this case, the zirconium hydride was supported on silica-alumina. Aluminum hydrides close to [(=SiO)3ZrH] sites would increase their electrophilicity and, thus, their catalytic activity. A catalyst prepared in this way was able to convert low-density polyethylene (MW 125000) into saturated oligomers (after 5h) or lower alkanes at 150°C (100% conversion). It was also able to cleave commercial isotactic polypropylene (MW 250000) under hydrogen at about 190 °C (40% of the starting polypropylene was converted into lower alkanes after 15 h of reaction). 

Zirconium hydride, ZrH2, finds application as an antioxidant for rubber, in addition to being a moderator for nuclear fuel elements, and as a hydrogenation catalyst. Zirconium hydroxide, best formulated as Zr02 hH20 because of its variable water content, is used as a drying agent and absorbent and also has deodorant properties.19... 

Hydrides and alkyls of the titanium triad metals are polymerization catalysts, so it is understandable that there are few stable r-allyl intermediates. The only example we can locate is the reaction between the zirconium hydride XIV and butadiene, which yields the homoallyl complex XV in place of the expected t-allyl complex ... 

Ammonia Dibutyltin maleate Dibutyltin oxide Fluorosulfonic acid Phosphine Sodium ethylate Sodium hydride Tetrabutyl titanate Tetraisopropyl titanate p-Toluene sulfonic acid Zirconium butoxide catalyst, condensation reactions Dibutyltin diacetate Piperidine catalyst, conductive polymers Iron (III) toluenesulfonate catalyst, conversion of acetylene to acetaldehyde Mercury sulfate (ic) catalyst, copolymerization Di butyl ether catalyst, cracking Zeolite synthetic... 

Evidence for hydride species as the active catalysts comes from comparing the previously described non-hydride catalyst precursors with corresponding hydride complexes. For example, CpCp Zr[Si(SiMe3)3]Me and [CpCp ZrH2]2 react with a large excess of phenylsilane at comparable rates. Also, under the same conditions these two zirconium complexes produce polysilanes that have identical molecular weight properties. The reaction of CpCp Zr[Si(SiMe3)3]Me with 2 equivalents of phenylsilane allowed observation (by NMR spectroscopy) of initial conversion to... 

Recently, there have been great efforts to find catalysts which would lead to specific depolymerization. In this regard, polyethylene was depolymerized in the presence of NO, O2, and N2 (275 kPa NO, 690 kPa O2, and 3170kPa N2) to a mixture of benzoic acid, 4-nitrobenzoic acid, and 3-nitrobenzoic acid [32]. In an alternate work [33], zirconium hydride supported on silica alumina catalyst has been reported, which, in presence of hydrogen, cleaves the C—C bonds of polyethylene and polypropylene. The end products of the hydrogenolysis of these polymers have been diesel and lower alkanes and is still a subject of vigorous research. 

Hydroalumination of terminal alkenes using EtjAl as the hydride source must be carried out with titanium catalysts [24], since zirconium compounds lead to the formation of alumacyclopentanes [60, 61] (Scheme 2-11) and carbometallated products [62]. Suitable substrates for hydroalumination include styrene, allylnaphthalene and vinylsilanes. Only one of the ethyl groups in EtjAl takes part in these reactions, allowing the synthesis of diethylalkylalanes, which are difficult to obtain by other methods. 

Many of the older bis-indenyl catalysts are less selective at higher temperatures, which was ascribed initially to a lower selectivity of the insertion reaction itself. More recent work by Busico, based on deuteration studies and again based on very detailed and elegant analysis of 13C NMR spectra of the polymers, has shown that in fact epimerisation of the growing alkyl chain occurs via a series of (3-hydride eliminations and re-insertion reactions [36] involving even tertiary alkyl zirconium species. 

Catalysts employed in this study are zirconium(lV)-hydrides on oxide support (silica, silica-alumina and alumina). Their synthesis is described above. We present here some transformations or modifications of polystyrene, linear alkanes and polyethylene with Zr-H catalyst... 

The dimeric hydride acts as a single-component, regiospedfic polymerization catalyst. It catalyzes the polymerization of a-olefins RCH = CH2 (R = Me, Et, nPr) to polyolefins with M = 4000-6000 and MJMn = 1.7-2.1. As in the case of [( 5Me5)Y(ORX/i-H)]2 the polymerizations are slow as compared to single-component zirconium catalysts [59]. The proposed mechanism is illustrated in Scheme 5 [52, 60, 61]. 

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