Aluminophosphates, chemical shift

Phosphorus. Previous P solid-state NMR studies of SAPO materials have detected only a single type of local environment for phosphorus T-atoms four aluminum second nearest neighbors as in a pure aluminophosphate (ALPO ) material (3,4). This environment is illustrated in Figure 1. The NMR chemical shifts from this environment t)q)ically range from -25 to -35 ppm. 

The Al MASNMR spectrum of VPI-5 has also been reported earlier (7). The spectrum contains a major resonance near 37 ppm, a neighboring resonance at 28 ppm, and a small resonance below 0 ppm. The chemical shifts of these resonances are consistent with other hydrated aluminophosphate molecular sieves that contain tetrahedrally coordinated aluminum that are linked to four phosphorus atoms through bridging oxygen atoms (10,11). 

Figure 7.20. Schematic diagram of the range of P chemical shifts in crystalline phosphate and aluminophosphate phases. The Q° range refers to the alkali and alkaline earth orthophosphates, Q denotes the end groups, the middle and ring groups and the branching groups in these compounds. The upper three bands refer to aluminophosphates, including those of the alkali and alkaline earth metals. From data of Turner et al. (1986a).

Table 1 Approximate chemical shift regions ( 5 ppm) for species in aluminophosphate glasses. Here n refers to the number of bridging oxygen atoms and m is the number of A1 atoms bonded to P...

Figure 17 (a) Dependence of catalytic activities of aluminophosphate catalysts on the P/Al ratio (I) reaction rate constant for isobutanol dehydration, (II) reaction rate for I-butene isomerization, (b) H MAS-NMR spectra of aluminophosphate catalysts with different P/Al ratios A 1.6 B 1.4 C 1.0 D 0.5 E model compound AI(H2P04)2. Spinning sidebands are indicated by asterisks. Centerband chemical shifts are indicated in the plot. (From Ref. 14.)... 

Davis and coworkers [104] studied " Xe NMR of xenon adsorbed in several SAPOs, ALPOs, and Y zeolites. From a comparison of the xenon chemical shift extrapolated to zero pressure, these authors concluded that Xe atoms feel significantly smaller electrostatic fields and field gradients in the aluminophosphates compared to aluminosilicates. The extrapolated chemical shift decreased from 97 ppm in erionite to 60 ppm in Y zeolite and to 27 ppm in AIPO4-5, with the values for SAPOs being intermediate to Y zeolites and AlPOs as would be expected from the acidity trends. They concluded as well that SAPO-37 does not contain separate aluminophosphate and aluminosilicate islands. Dumont et al. [105] also carried out xenon NMR experiments in SAPO-37. From xenon sorption capacity and the decrease in the chemical shift, their conclusion was that the framework of calcined SAPO-37 is unstable when exposed to moist air. 

Kuhl and Schmitt [107] discussed the similarities between the phosphorus-containing zeolites ZK-21 and ZK-22 and the silico-aluminophosphate SAPO-42. The phosphorous chemical shifts in all three microporous systems clearly indicate that phosphorus is in framework tetrahedral sites in each and that some... 

Xe continues to be an excellent probe in characterizing porous materials. Applying the trends predicted by Jameson and de Dios, Koskela et have interpreted the differences seen in variable temperature Xe NMR spectra of Xe adsorbed in aluminophosphate molecular sieves with circular and elliptical cross sections at two different Xe loading values. Autsbach and Zurek have repeated the calculation of the chemical shift of Xe in of a Xe atom trapped inside C60. 

MAS NMR Studies. Fluoride can play an important part in hydrothermal syntheses, where it can act as a mineralising agent in the syntheses of both silicates and aluminophosphates, and in many cases can remain attached to the framework cations such as silicon or (in the case of AlPOs, aluminium) in the as-prepared material. The F nucleus has attractive NMR properties (abundant, spin large chemical shift range) and is readily studied to give information on its presence in dilferent environments, the type of sites it adopts, and its mobility at dilferent temperatures. Examples are given in Section 5.4.2 of the study of fluorine in as-prepared pure silica polymorphs of zeolites. 

For fully connected, unsubstituted framework aluminophosphates, AIPO4, all phosphorus is tetrahedral and surrounded by four aluminium second nearest neighbours, P(OAl)4, giving P peaks in the chemical shift region -20 to -40 ppm. Any hydrogenphosphate groups that might be present, for example in interrupted frameworks or layered aluminophosphates, are readily identified on the basis of their more positive chemical shift and their enhanced... 

Zr and Mg solid-state NMR was used to examine the local environments around the metal centres in microporous zirconium silicates and an aluminophosphate incorporated with Mg. QCPMG spectra were obtained at different magnetic fields from which the quadrupolar and chemical shift parameters were extracted. ... 

P— P correlations in aluminophosphates, and Si— oi correlations in sodium borosihcate glass and silicate-1. Interestingly, they demonstrated covariance processing in double-quantum correlation experiments employing POST-C7 [71], BABA [72], and INADEQUATE [73]. In double-quantum 2D spectroscopy, time evolution along the indirect dimension is characterized by the frequencies given by the sum of the chemical shifts between the correlated spins so that horizontal pairs of correlation peaks appear as shown in Fig. 3. There, 125 data sets were acquired with a step of 12.5 ps. Thus, the coherence profile during the evolution time was truncated at 1.6 ms. 

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