Solid-state NMR (¹³C, ²⁷Al and ²⁹Si) study of the reaction between AlMe₃ and the silica gel surface
Date
2007
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Abstract
The reaction between the silica gel surface and trimethylaluminum (AlMe3) has been studied in this thesis research. We have examined the AlMe3/silica reaction in the following stages: the initial AlMe3-reacted silica surface after it had been treated with AlMe3; the AlMe3-treated surface after it was washed with dry diethyl ether; the ether-washed surface after it was treated, in steps, with limited amounts of H2O; and finally the H2O-reacted surface after an excess-H2O workup. Solid-state NMR (13C, 27 Al and 29Si) have been used to elucidate the structures of moieties generated on the silica gel surface at each of the stages listed above. Solid-state 13C NMR showed that Al(Me)n is the major type of moieties generated on the surface in the initial AlMe3/silica reaction and Si-OMe is the second most important moiety generated. After the sample has been washed with dry diethyl ether, strong ether signals were observed by 13C NMR, which implies that diethyl ether is strongly attached to the surface, even after evacuation. There are no significant changes for the other surface moieties after the diethyl ether treatment. In the series of limited-amount H2O treatments that followed, the AlMen signal intensity decreased as more H2O was added to the surface. In the sample resulting from the final (excess H2O) work-up, AlMen and Si-OMe moieties are completely gone and peaks corresponding to Si-Me and Si(Me)2 are the only signals left in the 13C NMR spectrum. In the 29Si NMR spectra, the signal intensity of the (SiO)3Si(OH) (Q3) peak typical of silica dropped after the AlMe3 treatment. Q3 signal intensity was replaced with a broad peak centered at about-104 ppm, as expected for a conversion in which most of the Si-OH groups on the silica surface have reacted with AlMe3 and turned into Si-O-Al moieties. The formation of Si-Me, Si(Me)2 and Si(Me)3 moieties were also observed in the 29Si spectra. 29Si spectra didn't show significant changes in the sample-treatment stages that follow the initial AlMe3/SiO2 reaction. In the 27 Al spectra of AlMe3-treated silica samples, 4-, 5- and 6-coordinate Al moieties were observed. In the initial reacted sample, 5-coordinate Al moieties are the major initial products from the reaction. After the samples were washed with diethyl ether, the 5-coordinate Al moieties are still the major moieties. With limited amounts of H2O introduced onto the surface, the AlMen moieties reacted with H2O, as shown by the 13C spectra; in 27 Al NMR spectra, signal intensity of 5- coordinate Al moieties decreased, while that of 4- and 6-coordinate Al moieties increased, which implies that the 5-coordinate Al moieties turned into 4- and 6- coordinate Al moieties as a result of reaction with H2O. On the final work-up surface, the 4- and 6-coordinate Al moieties are the major Al structures remaining on the surface. This is the first observation of this kind of change of Al atom coordination on a AlMe3-reacted silica surface. The structures of surface Al moieties are much more complicated than those proposed in previous publications on AlMe3/silica reactions. In the initial reaction between AlMe3 and silica gel, we also made quantitative measurements aimed at tracking the route of methyl groups in the whole system. The methane generated during the reaction was trapped in a N2(I)-cooled trap and the volume of trapped methane was measured as a gas with the water-displacement method. Unreacted Al-Me groups in the supernatant liquid were measured by the liquid-sample 13C NMR spin-counting method. The amount of methyl groups attached on the silica surface were measured by the solid-state 13C NMR spin-counting method. The total amount of methyl groups tracked in the AlMe3/silica/toluene system is about 108% of the amount of methyl groups present in the initial AlMe3 and is about 90% for the AlMe3/silica/cyclohexane system. Relaxation studies were carried out on both the initial AlMe3-reacted and ether-washed AlMe3/silica samples using 13C CP/MAS NMR. The methyl-group proton T1 values were measured by the saturation-recovery technique and the cross polarization relaxation time (TCH) and rotating-frame proton spin-lattice relaxation time (T1p) were measured using variable-contact-time experiments. The AlMen moieties in the initial AlMe3-reacted sample showed very long (5 s ~ 7 s) proton T1 values, which implies that the AlMen moieties may be in a very restrained environment. This result supports the existence of 5-coordination Al structures indicated from 27Al results; in these structures methyl groups are bridged/shared between adjacent AlMen moieties. After the initial AlMe3-reacted silica sample was washed with diethyl ether, methyl-group proton T1 values were reduced by half, which may be due to replacing the methyl bridges with electron-rich centers consisting of the O atoms of the ether molecules introduced by the washing. This interpretation also explains why we have strong ether signals in 13C NMR spectra of the ether-washed sample and in the H2O-treated samples that followed. Overall, the moieties generated in the AlMe3/silica reaction have been characterized by solid-state NMR methods in this thesis work. And, methods were developed which quantitatively characterize the fate of all the Al-Me groups added into the reaction system.
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Nuclear magnetic resonance spectroscopy
Organoaluminum compounds -- Analysis
Silica gel -- Analysis
Solid state chemistry