Silica-supported zirconium hydrides

Tetraneopentyl zirconium reacts in the same way as tetraneopentyl titanium to give, on a silica (soo), a tris(neopentyl) monografted species [32]. Treatment under H2 of this surface species yields silica-supported zirconium hydrides [33], which have been characterized as a mixture of mono- (65-70%) and bis- (35-30%) hydrides based on double quanta NMR experiments (Scheme 2.11) [34]. Interestingly, the double quantum experiment allows us to prove not only the presence of the two hydrides and the monohydride of zirconium by the presence or the absence of the double quanta correlation but also to detect the through space magnetic interaction between the zirconium monohydride and the silicon di-hydride, proving thus the spatial arrangement on the surface. This confirms the mechanism by which these hydrides have been formed on the surface. 

Sublimation of tetraneopentyl zirconium onto the surface of silica dehydroxylated at 500 °C results in the electrolytic cleavage of one Zr-C bond by a surface proton, with the formation of the tris neopentyl zirconium [Si]-0-Zr(CH2CMc3)3 grafted species. Reaction of this supported Zr alkyl with hydrogen at 150°C leads to the formation of a zirconium hydride. Interestingly, this reaction does not yield neopentane, but rather the formation of methane and ethane are observed. These products are in fact the result of the hydrogenolysis of evolved neopentane catalyzed by the silica-supported zirconium hydride. ... 

This sequence has been exploited to distinguish the surface sites of silica supported zirconium hydrides, which are known to catalyze hydro-genolysis and the homologation of the alkanes (Fig-... 

Table 3 Hydrogenolysis of alkanes catalyzed by zirconium hydride supported on silica...

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). 

V. Dufaud and J. M. Basset, Catalytic Hydrogenolysis at Low Temperature and Pressure of Polyethylene and Polypropylene to Diesels or Lower Alkanes by a Zirconium Hydride Supported on Silica-Alumina a Step Toward Polyolefin Degradation by the Microscopic Reverse of Ziegler-Natta Polymerization, Angew. Chem. Int. Ed., 37, 806-810 (1998). 

Dufaud V, Basset J-M (1998) Catalytic hydrogenolysis at low temperature and pressure of polyethylene and polypropylene to diesels or lower alkanes by a zirconium hydride supported on silica-alumina A step toward polyolefin degradation by the microscopic reverse of Ziegler-Natta polymerization. Angew Chem Int Ed 37 806... 

Fig. 7 a) Fonnation mechanism of different zirconium hydrides supported on silica b) Double quanta proton-proton multidimensional NMR allowing the identification of the two surface sites of the zirconium hydrides supported on silica, (=SiO)2Zr(H)2 (auto correlation highlighted with dotted line) and (=SiO)2Zr-H (correlation with surface SiH, full line). Adapted with permission from the authors. ... 

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. 

Thanks to recent advances in soHd-state NMR spectroscopy, the zirconium bis-hydride [(=SiO)2ZrH2] (lb) (20-35%) was found to be generated simultaneously to the major zirconium monohydride [(=SiO)3Zrft] (la) (80-65%) during hydroge-nolysis of the single site sihca or silica-alumina supported [(=SiO)ZrNp3] [14],... 

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... 

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