Nature and Structure of Aluminum Surface Sites Grafted on Silica from a Combination of High-Field Aluminum-27 Solid-State NMR Spectroscopy and First-Principles Calculations

Abstract

The determination of the nature and structure of surface sites after chemical modification of large surface area oxides such as silica is a key point for many applications and challenging from a spectroscopic point of view. This has been, for instance, a long-standing problem for silica reacted with alkylaluminum compounds, a system typically studied as a model for a supported methylaluminoxane and aluminum cocatalyst. While (27)Al solid-state NMR spectroscopy would be a method of choice, it has been difficult to apply this technique because of large quadrupolar broadenings. Here, from a combined use of the highest stable field NMR instruments (17.6, 20.0, and 23.5 T) and ultrafast magic angle spinning (>60 kHz), high-quality spectra were obtained, allowing isotropic chemical shifts, quadrupolar couplings, and asymmetric parameters to be extracted. Combined with first-principles calculations, these NMR signatures were then assigned to actual structures of surface aluminum sites. For silica (here SBA-15) reacted with triethylaluminum, the surface sites are in fact mainly dinuclear Al species, grafted on the silica surface via either two terminal or two bridging siloxy ligands. Tetrahedral sites, resulting from the incorporation of Al inside the silica matrix, are also seen as minor species. No evidence for putative tri-coordinated Al atoms has been found.

Catalytic consequences of open and closed grafted Al(III)-calix[4]arene complexes for hydride and oxo transfer reactions

“…2A, and consists of a prominent resonance centered at ∼33 ppm. The chemical shift of this resonance corresponds to previously reported five-coordinate aluminum centers in oxide materials (40,41). Similar resonances are measured with spin-echo pulse sequence for methyl Al(III)-calixarene complexes before anchoring, as shown in Fig.…”

“…The center resonances of all spectra in Fig. 2 are notably shifted away from 0 ppm, the expected chemical shift for octahedral aluminum species, and are within the range known for Al(III)-alkoxides and Al (III)-phenoxides (40,41). There are no AlMe 3 species remaining after reaction as verified via complete loss of calix [4]arene OH resonance in the 1 H NMR spectra as well as lack of the characteristic 155-ppm resonance in the 27 Al NMR spectrum.…”

Well-defined silica supported aluminum hydride: another step towards the utopian single site dream?

“…The type of reaction between alkyl aluminum compounds (AlR 3 , R = Me, 29–34 Et, 35 iBu…) and silica surface still remains not elucidated despite its huge applications as a co-catalyst for olefins polymerization. 36–39 Because many aluminum alkyls exist in solution as dimers or trimers, we have chosen triisobutylaluminum (TIBA) because it is a monomer, 40 with the hope it will remain monomeric once it is grafted to the surface.…”

“…Based on the available data, the authors concluded that, under their experimental conditions, there is remarkable incorporation of Al into the framework of SBA15. 35,42 …”

Solid-state NMR: An EYE Opener in Surface Chemistry

“…The most shielded component (δ 33 ) is aligned along the p-orbital of the alkylidene ligand, as in sp 2 hybridized carbon, for weak agostic interactions; and approaches a M C alkylidyne type bond, as [18] sites formed upon Et 3 Al grafting on dehydroxlated SBA-15. [23] We reconstituted the structure of each surface site by aligning the experimental results with DFT calculations on cluster and periodic models. Two aluminum sites that correspond to a dinuclear Al species grafted either via two terminal or two bridging µ 2 -siloxy groups were identified as matching both theoretical and experimental data (Fig.…”