Solid-state NMR study using trimethylphosphine as a probe of acid sites in normal and dealuminated zeolite Y

Abstract

spectra of adsorbed CO may be assessed. Magic-angle spinning yields precise isotropic shifts and thus could detect subtle changes in the properties of the adsorption site caused by, for example, promoters or coadsorbates. However, magic-angle spinning spectra of adsorbed CO should not be used to exclude the existence of some species. As noted above, the bridge-bonded species do not appear and could be overlooked if the lack of agreement between the center of mass of the broad-line and high-resolution spectra was not noted. The I3C N M R spectra of CO on Rh/silica illustrate another caveat; similar isotropic shifts of the dicarbonyl and linearly bonded CO caused two species to appear as one. To separate these components, it was necessary to decompose the spectra by using sideband intensities predicted from fits to the broad-line spectra. Also, it is tenuous to extract relative site populations from the magic-angle spinning spectra, as seen by comparing the broad-line and high-resolution results in Table I.Measuring both high-resolution and broad-line spectra aids the assignment of component peaks. For example, CO on Ru has at least three isotropic peaks in the range 200-175 ppm, which this study identifies as 199, 195, and 180 ppm. The peak at 199 ppm can be correlated to the Lorentzian peak in the broad-line spectrum by its atypical sharpness (cf. Figure 7) and its lack of sidebands. The other two peaks have sidebands whose intensities and range correspond to linearly bonded CO species. We note that these assignments are inconsistent with an earlier study of CO on Ru/Y ~e o l i t e ,~ which proposed the following peaks and assignments: dicarbonyl (1 68 ppm), linearly bonded CO (1 80 ppm), and bridge bonded (203 ppm). On the basis of the results obtained here, we propose alternate assignments: multicarbonyl (203 ppm), linearly bonded (180 ppm), and linearly bonded (168 ppm). The bridge-bonded species is predicted to lie upfield of 203 ppm and we believe was not resolved, for reasons discussed in preceding sections. V. SummaryHigh-resolution and broad-line I3C N M R spectra of CO on silica-supported Ru and Rh corroborate previous assignments and provide additional information on the adsorbed states. The high-resolution spectra obtained by magic-angle spinning reveal isotropic shifts consistent with the broad-line components proposed previously to be linearly bonded CO and multicarbonyls formed on isolated metal atoms. In addition, experimental evidence suggests two types of linearly bonded CO on Ru/silica. The sharpness of the linear peaks suggests that some Ru and Rh aggregates have negligible magnetic susceptibility, consistent with raftlike structures. Quantitative comparison of the high-resolution and broad-line spectra of adsorbed CO reveals possible pitfalls of relying on magic-angle spinning spectra alone. First, bridge-bonded CO, whose presence is indicated by the quantitatively correct broad-line spectra, is not sufficiently narrowed to be observed in magic-angle spinning spectra. If the I3C NMR spectrum o...

An Investigation of the Roles of Surface Aluminum and Acid Sites in the Zeolite MCM-22

“…As a consequence, the number of acid sites in zeolites is underestimated. This is in agreement with the quantitative TMP adsorption study carried out by Lunsford et al [28] Discussion When a basic ligand, L (including water), is adsorbed on the zeolite surface, it reacts with both the Br˘nsted, to form SiÀOHLÀAl, and Lewis acid sites. A comparison between the 27 Al MAS and CP/MAS NMR spectra of hydrated and ammonia-adsorbed zeolite samples (Figures 1 and 5) reveals that: 1) Al is invisible to 27 Al MAS NMR in the case of strictly dehydrated zeolite samples and 2) once the adsorbent is loaded, the electric-field gradient at Al is sufficiently reduced to render Al to be detectable by NMR spectroscopy.…”

Characterization of Zeolites — Infrared and Nuclear Magnetic Resonance Spectroscopy and X-Ray Diffraction

“…The chemical exchange of pyridine molecules at different adsorption sites with different cross-polarization efficiencies renders a quantitative evaluation difficult [210]. In a number of studies on zeolite acidity, the strong base trimethylphosphine (TMP) was used as a probe molecule [420][421][422]. Since the isotope 31p occurs in a natural abundance of 100 %, this method does not necessarily require the application of the CP technique.…”

“…These investigations have shown that TMP is a sensitive NMR probe molecule for acidity measurements. However, TMP is a rather large molecule and the determination of acid sites in zeolites is therefore restricted by the maximum adsorption capacity of the pore system as well as steric hindrances [421]. In another approach, Biaglow et al [423] [425], and Brunner et al [323,426].…”

Effect of acidity in TS-1 zeolites on product distribution of the styrene oxidation reaction

“…Therefore, many compounds containing 31 P have been used as probe molecules to determine the zeolite acidity. Especially, trimethylphosphine, with a pK b of 5.35, has been widely used for the characterization of acidity in zeolites such as H-Y and USY [20,21]. In this work, the acidity of TS-1 was studied using trimethylphosphine as the probe molecule.…”