A reexamination of the chemisorption of trimethylaluminum on silica

Abstract

Fourier transform infrared spectroscopy, temperature-programmed desorption, and X-ray photoelectron spectroscopy have been used to study the chemisorption and decomposition of trimethylaluminum (TMA) on silica under high vacuum. By annealing a series of silica substrates from 425 to 1573 K prior to TMA exposures at 300 K, we have examined the distributions of chemisorption products as a function of the relative concentrations of isolated hydroxyls (OHi), hydrogen-bonded hydroxyls (OHH), and siloxane bridges. The observed variation in the Si-methyl to AI-methyl population ratios supports a new chemisorption model in which a monomethylaluminum surface complex and methyl groups bonded to silicon are proposed as the majority species on the surface at 300 K. Although the initial reactive sticking probability for TMA on the silica substrates is <0.01 at 300 K, TMA chemisorption affects OHi, OHH, and siloxane bridges on the surface with equivalent probability. The common reaction probability (equivalent rate constants) implies that similar requirements may be involved in the reactive adsorption that consumes each site. Decomposition of the monomethylaluminum adsorbate begins above 373 K and increases the population of methyl groups bonded to silicon on the surface. The methyl groups react to form methane, and ethane and adsorbed hydrocarbon fragments. In addition, the methyl groups also react further with the surface to form tetramethyhane . At coverages of one-third saturation and greater, the monomethylaluminum surface complex can also react with methyf groups to yield TMA, above 500 K.

An investigation of catalyst/cocatalyst/support interactions in silica-supported olefin polymerization catalysts based on Cp*TiMe3*

“…Coordination of 6 to a nearby siloxane oxygen is likely to occur in order to maintain the preferred tetracoordination at Al. This hypothesis is nevertheless consistent with the ability of AlMe 3 /silica to adsorb Lewis bases such as Et 2 O [47] and NH 3 [43,51]. Adsorption may occur by displacement of the coordinated siloxane oxygen atom, instead of being interpreted as evidence for persistent threecoordinate Al sites [47].…”

“…Regardless of the origin of the silica, chemisorption was suggested to involve formation of isolated three-coordinate methylaluminum and dimethylaluminum sites, associated with O or Si atoms of the surface, Scheme 2. Four-coordinate Al sites, described as physisorbed, were assumed to appear only when silica is oversaturated with AlMe 3 [45,47].…”

“…In an early study, it was suggested that all the methyl groups on a trimethylaluminum-modified Aerosil surface were transferred to silicon atoms [40]. Reinterpretation of the IR spectrum led to the conclusion that (BSiO) 2 AlMe and BSiMe are the sole species on the surface of Cab-O-Sil and are present in equal amounts, as a consequence of the rearrangement of the primary grafting product (BSiO)AlMe 2 [47]. This assertion was supported by elemental analysis of AlMe 3 adsorbed on silica gel [44].…”

Reactions of Group IV Metallocene Dichlorides with Modified Silica Gel Surfaces

“…The very low levels of metal detected on 600ЊC/HMDS silica gel for all the reactants studied suggest that none react with siloxane bonds. Previous work (20,21) has shown that only the most reactive reagents, such as aluminum alkyls, react with siloxanes under similar conditions.…”

Copolymerization of polypropylene and functionalized linear α‐olefin onto glass fibers

“…Our previous work on metallocenes heterogenization onto SiO 2 surfaces offers both theoretical and experimental confirmation of this mechanism. 23,24 Additional evidence comes from the FTIR spectrum of E-glass (which contains ϳ14% Al 2 O 3 in its formulation). As shown in Figure 18c, the bands assigned to AlOOOSi and AlOO vibrations are also present.…”