27A1 MAS NMR spectra

P and 27A1 MAS NMR spectra of aluminophosphate powders and of samples scratched from films were recorded on 600 MHz Varian NMR system equipped with a Varian 3.2 mm MAS probe. Fifty two-faced films deposited on glass slides were needed to obtain sufficient amount of material for NMR measurements. Larmor frequencies for phosphorus and aluminium at 14.1 T are 242.89 and 156.35 MHz, respectively. 

Figure 6. 27A1 MAS NMR spectra of A1SBA after different treatments ...

The 27Al MAS NMR spectra of as-synthesized Al-MCM-41 samples show a main signal of tetrahedral coordinated aluminum at ca. 53 ppm. The aluminum is nearly entirely incorporated into the framework. As generally observed, calcination leads to the formation of penta- Al[51 and octahedrally coordinated aluminum Al[61 on the expense of the tetrahedral framework Al[41. In the 27A1 MAS NMR spectra of calcined samples appear additional lines at ca 30 ppm and 0 ppm, respectively (Fig. 1). Only ca. 1/3 of the Al atoms remain in tetrahedral framework positions even after thermal treatment at 600°C. 

Fyfe et al. (155) measured high-resolution solid-state 27A1 MAS NMR spectra of a number of zeolites at 104.22 MHz. All the spectra contained one narrow peak with a chemical shift ranging, in different materials, from 51.5 to 65.0 ppm from A1(H20) + (see Table XIII). In dealuminated zeolites an additional signal was observed corresponding to six-coordinated Al in the zeolitic channels. 

A1 MAS NMR spectra of chemically untreated zeolites are thus much simpler than those of their 29Si counterparts. This is a direct consequence of the fact that while five types of Si(nAl) environments are possible for the silicon atom, only one possibility exists for the aluminum. However, while Si in zeolites is always present in four-coordination, Al can be four- or six-coordinated, and 27A1 MAS NMR is a very sensitive quantitative probe for this. In other words, 27A1 NMR is most valuable in probing the coordination, quantity, and location of Al atoms in chemically treated zeolites, but less useful than 29Si NMR for direct structural determination. 

Fig. 36. 27A1 MAS NMR spectra of zeolite H-ZSM-5 at three magnetic field strengths (15) The spectra of hydrated materials represent the following number of FID s (from top to bottom)-10,000, 130,000, and 350,000 while the spectra of the dehydrated materials needed 250 000 and 180,000 FID s. At 15.6 MHz no signal was obtained for the dehydrated material after 750 000 FID s.

The 27A1-MAS-NMR spectra of the hydrothermally treated samples consist of a sharp peak at ca. 61 ppm due to tetrahedrally coordinated Al in the zeolite framework and two signals due to octahe-drally coordinated extra-framework Al, a very broad peak of ca. 

A method has been developed to characterize the NMR-invisible Al (37), which entails the complexing of non-framework Al with acetylacetone (acac) to form Al(acac)3 complexes. Because of their near-spherical Al symmetry, these complexes yield 27A1-MAS-NMR spectra that can be used for more quantitative interpretation. 

The 27A1-MAS-NMR spectra of dealuminated zeolite Y generally consist of three signals indicative of three types of aluminum, but the intensity does not account for all the aluminum in the sample. The authors ascribe the difference to NMR-invisible Al previously determined (30) from an analysis of the 29Si-MAS-NMR data for dealuminated HY samples. The NMR resonance at 60 ppm for framework Al is relatively sharp. Octahedrally coordinated non-... 

A1 MAS NMR spectra were measured at 104.26 MHz using powerful 0.6 ps radiofrequency pulses with a 0.2 s recycle delay. Samples were spun at 3.4 kHz in an aluminium-free probehead and Vespel rotors. Chemical shifts are quoted in ppm from external A1(H20)63+. 10000 transients were acquired for each spectrum. 

Figure 2. 27A1 MAS NMR spectra. The spectra of samples 1, D-2 and R-3, but not those of the remaining samples, are given in the absolute intensity mode.

NMR. 27A1 MAS NMR spectra of the hydrated samples were obtained at room temperature using a Bruker MSL 400 NMR spectrometer. An aqueous solution of A1(N03)s was used as a reference. 

Figure 2. 27A1 MAS NMR spectra of hydrated samples A, USYA and Real A on the absolute intensity scale. 10,000 scans were acquired in each case using resonant rf pulses of 600 ns duration.

A1 MAS-NMR spectra of mesoporous, cubic Na-Al-MCM-48 molecular sieve catalyst showed tetrahedrally-coordinated aluminium in both as-prepared and calcined samples.379... 

A1 MAS NMR spectra of the pristine and modified LDH. The magnified amplitudes (x50) show the presence of tetrahedral A1 in LDH-R as compared to parent LDH. On the right-hand side, the corresponding quantification diagrams are presented. (From Vyalikh, A., et al., /. Phys. Chem. C, 113,21308,2009.)... 

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