Mutable Lewis and Brønsted Acidity of Aluminated SBA-15 as Revealed by NMR of Adsorbed Pyridine-¹⁵N

Abstract

(1)H and variable-temperature (15)N NMR techniques have been used to study the effect of the gradual alumination of SBA-15 on the structure and adsorption properties of this mesoporous material. The interpretation of experimental spectra suggests that aluminum chlorhydrol most effectively reacts with silica surfaces in the confinement of the cavities of rough mesopore walls, instead of forming a homogeneous aluminum film. This first leads to a gradual filling of the cavities and finally results in aluminum islands on the inner surfaces of mesopores. In the sample with a Si/Al atomic ratio of 4.1, up to half of the inner surface area of the mesopores is covered with aluminum. The alumination produces Brønsted acid sites attributed to silanol groups interacting with aluminum but does not affect the proton-donating ability of isolated silanol groups. At high Si/Al ratios, the surface contains only one type of Lewis site attributed to tetracoordinated aluminum. At lower Si/Al ratios, Lewis acid sites with a lower electron-accepting ability appear, as attributed to pentacoordinated aluminum. The numerical values of the surface densities of all chemically active sites have been estimated after annealing at 420 and 700 K. We were surprised to observe that gaseous nitrogen can occupy Lewis acid sites and hinder the interaction of the aluminum with any other electron donor. As a result, aluminated surfaces saturated with nitrogen do not exhibit any Brønsted or Lewis acidity. At room temperature, it takes days before pyridine replaces nitrogen at the Lewis acid sites.

Elucidation of acid strength effect on ibuprofen adsorption and release by aluminated mesoporous silica nanoparticles

“…32 Three signals at 1545 cm À1 and 1639 cm À1 were intensied with the addition of Al, suggesting the generation of Brönsted acid sites (Si-OH-Al) in the Al-MSNs. 31 The formation of Brönsted and Lewis acid sites and their correspondence to the amount of Al graed on the surface of the MSN is illustrated in Fig. 35 A similar result was also reported for the incorporation of Al onto SBA-15, in which the Brönsted acid was generated in the aluminosilicate samples.…”

“…31 In the case of Al-loaded MSN, increasing the amount of Al reduced the Lewis acid sites. 31 In the case of Al-loaded MSN, increasing the amount of Al reduced the Lewis acid sites.…”

Natural Abundance 17O DNP NMR Provides Precise O−H Distances and Insights into the Brønsted Acidity of Heterogeneous Catalysts

“…[4,12] The 17 O{ 1 H}, DNP-enhanced SSNMR spectrum of an MCM-41-type mesoporous silica nanoparticle (MSN) sample that was acquired using the PRESTO-QCPMG technique [7b] (phase-shifted recoupling effects as mooth transfer of polarization [15] quadrupolar Car-Purcell-Meiboom-Gill [16] ), shown in Figure 1, presents abroad resonance centered at À65 ppm, as expected. This can be explained by the fact that the Al(OH)Si sites have ah igh Brønsted acidity and are deprotonated by pyridine, [18] thus eliminating their 17 O{ 1 H} PRESTOs ignal. Since this material is essentially boehmite,t he dominance of m 1 -aluminols is expected.…”

Quantification of acidic sites of nanoscopic hydroxylated magnesium fluorides by FTIR and 15N MAS NMR spectroscopy

“…1 † shows the X-ray pattern of a hydroxylated magnesium uoride sample before and aer it was calcinated at 300 C. The decrease in the peak width shows that at 300 C the crystallite size increase and probably some of the acidic sites are destroyed by surface rearrangement. 32,33 This means that all pyridine molecules are adsorbed at the catalyst surfaces. Table 2 lists the concentrations of Lewis and Brønsted sites in the four samples.…”