Hydration of AlPO4-11 studied with x-ray powder diffraction and aluminum-27 and phosphorus-31 NMR

Abstract

The influence of water adsorption in AlP04-11 has been studied with X-ray powder diffraction and 27AI and 3IP NMR. With X-ray powder diffraction, a change of the crystal symmetry from ZMA2 to PNA2, is observed upon water adsorption, resulting in a diminished unit-cell volume and more elliptical pores. N M R was used to study the hydration on an atomic scale. With 27Al double rotation (DOR) and magic angle spinning (MAS) NMR, the quadrupole coupling constants (CQ) and asymmetry parameters (7) of the different A1 sites in dry and wet AlPO4-11 were determined and correlated with the symmetry of the A1 site obtained from literature data. In contrast with earlier 27Al DOR measurements,' spectral simulations of 27Al MAS and DOR N M R spectra measured at 7.0 T prove the preferential hydration of one tetrahedral aluminum site in Alp@-1 1 (five crystallographically different sites), transforming this site reversibly into octahedrally coordinated aluminum.* From the observed correlation between the XRD data and CQ, it was concluded that the A12 site becomes preferentially octahedrally coordinated. On the basis of 27Al DORand 31P MAS NMRmeasurements performed on the partially hydrated samples, it is shown that the transformation upon hydration is a local process, comparable to the symmetry change observed upon adsorption of sorbates in

Separation of isotropic chemical and second-order quadrupolar shifts by multiple-quantum double rotation NMR

“…This process satisfies the conditions necessary to time-average both first-and second-order perturbations to the Zeeman interaction which have spatial dependences described by the second-and fourth-order Legendre polynomials, P 2 (cos b) and P 4 (cos b), respectively [1,2]. A number of studies have been reported utilizing DOR NMR for the differentiation of sodium, boron, aluminium and oxygen sites in minerals [9][10][11], materials [12][13][14][15][16] and molecular sieves, including, zeolites [17][18][19][20], sodalites [21][22][23] and aluminophosphates [24][25][26][27][28]. Recently, focus has turned to exploiting DOR for the study of oxygen sites in organic solids [29][30][31][32][33][34][35].…”

Determination of NMR interaction parameters from double rotation NMR

“…The inherently low sensitivity of NMR has also led to the application of satellite transition saturation/inversion signal enhancement techniques under DOR [16,17]. The majority of DOR NMR studies reported have been concerned with the differentiation of sodium, boron, aluminium and oxygen sites in minerals [18][19][20], materials [21][22][23][24][25] and molecular sieves, including, zeolites [26][27][28][29], sodalites [30][31][32] and aluminophosphates [33][34][35][36][37]. More recently, focus has turned to exploiting DOR for the study of oxygen sites in organic solids [17,[38][39][40][41].…”

Double rotation27Al NMR in molecular sieves

“…The different Al sites in the framework structure of calcined and hydrated AIPO4-11. [33] using five peaks with equal areas: four in the tetrahedral region and one at the octahedral position in agreement with the five distinct crystallographic sites derived from X-ray data [31]. These simulation results are not in agreement with those published by Barrie et al [34]; in that study, based on a questionable simulation of the 27A1 MAS NMR spectrum taken at 11.7 T, they concluded that all five crystallographic are partially hydrated to an equal extent.…”

“…From these results, it could be concluded that the broadening of the spectral DOR features is largely due to a dispersion in chemical shifts and neither due to heteronuclear dipolar coupling, nor to quadrupolar broadening [38]. It is important to notice that the formation of pentacoordinated aluminum upon water adsorption was not observed for fully hydrated A1PO4-11 samples [32,33,38]. From 27A1 DOR NMR experiments on dehyflrated A1PO4-5 Peeters et al [37] could detect three closely related T-sites (having quite the same CQ and [~u[ values) and so only little preferente for hydration of one particular aluminum site can be expected.…”

“…The CQ values associated with the different DOR NMR lines can be derived from the slopes of Fig. 14 or from the simulation of the DOR spectra [32,33]; these values are 2. strain parameter [Y[ of the sites All-A15, as derived from the X-ray data of hydrated A1PO4-11 [31], are respectively 0.123, 0.199, 0.116, 0.095 and 0.121.…”