Hafnium, silica-supported

Incorporation of various cations into mesoporous silica often leads to the creation of Bronsted acid sites. For example, adsorption of ammonia on Ti/ MCM-41 (430,443), [A1]MCM-41 (444), and [AlJSBA-15 (182), as well as on titanium, zirconium, and hafnium species supported on mesoporous silica (445), and on Ti02-Si02 (110) leads to the appearance of bands ofpro-tonated ammonia. However, the adsorption is reversible (430) or partly reversible (182,443,445) at ambient temperature, indicating a weaker acidity of these materials in comparison to zeolites. 

We first studied group 4 metals (titanium, zirconium and hafnium) supported on a silica dehydroxylated especially at 700 °C (Table 3.8). Following the laboratory-developed strategy, surface-species have been well-characterized by classical techniques (IR, solid-state NMR gas evolvement, reactivity, etc.). Catalysis results show that titanium is the most active even if its activity is far less than that of homogeneous catalysts. In addition, an important amount of metal was lost by lixiviation even if this phenomenon seemed to stop after a certain time. 

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). 

Stabilized compounds are inherently more stable then those compounds, which contain /1-hydrogens. Therefore, catalysts where the -positions are blocked, are preferred (8). Examples are zirconium tetrakis(trimethylsilylmethyl) or hafnium tetrakis(trimethylsil-ylmethyl). Catalyst supports are porous alumina with an amount of 0.5% of silica. 

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