Local Environment of Phosphorus Atoms in CoAPO₄-n Molecular Sieves:  A ³¹P NMR Study

Abstract

A spin-echo mapping technique has been used to obtain the 31P NMR spectra of various cobalt-substituted aluminophosphate molecular sieves. The technique involves recording Hahn-echo spectra at different carrier frequencies in order to observe the signal within a broad spectral region. In contrast to the more commonly used magic angle spinning (MAS) or simple Hahn-echo methods, all phosphorus nuclei could be detected, even in the case of samples with high Co contents. Resonances are usually broad and range between about −500 and 10 000 ppm/H3PO4, but the resolution is sufficient to access the local structure around the 31P nuclei. The technique is particularly interesting since the presence of signals above 500 ppm is direct proof for the framework cobalt siting. Indeed, such signals were not observed for Co-impregnated samples and are thus characteristic of framework Co species. The different NMR lines of the spectra have been assigned to various P(nCo) environments in the structure, and their shift was found to be approximately proportional to the number of Co atoms in the first coordination sphere around P atoms. A good agreement was found between the composition of the framework of the molecular sieve deduced from NMR spectra and that obtained by chemical analysis. Moreover, it was possible to estimate the location of Co atoms in the structure and to propose or confirm an ordering in the unit cell for two of the studied samples.

Framework cobalt and manganese in MeAPO-31 (Me=Co, Mn) molecular sieves

“…In addition to a strong peak at about 0 ppm, there is a low and broad feature extending from about 2000 to about 5500 ppm. According to the literature [24] this is approximately the range of P(1Co, 3Al) and P(2Co, 2Al) units. Because cobalt within the cavities does not shift the phosphorus resonance, the broad shifted peak confirms cobalt incorporation into aluminum framework sites.…”

“…Measurements were carried out in a flow of He (50 cm 3 /min) with a heating rate of 10°C/min. Static 31 P NMR spectra of CoAPO-31 were obtained on a Varian Unity Inova spectrometer, operating at 242.89 MHz for phosphorus nuclei, according to the spin-echo mapping technique [23,24]. A series of Fourier-transformed echo spectra were recorded using a conventional Hahnecho sequence, by incrementing the irradiation frequency with a constant step of 34.5 kHz below and above the phosphorus resonance of H 3 PO 4 .…”

“…It was demonstrated on a number of CoAPO-n materials that, using the so-called spinecho mapping technique, one can efficiently record broad-line static 31 P spectra and prove or disprove cobalt incorporation [24]. A static 31 P spectrum of as-synthesized CoAPO-31 is presented in Fig.…”

MnOx Nanoparticles Supported on a New Mesostructured Silicate with Textural Porosity

“…For example, 31 P NMR spectroscopy was used for the analysis of the substitution of framework Al atoms with Ni, Co, Fe, and Mn atoms within aluminophosphate molecular sieves (AlPOs). [21] Recently, some studies have been published on the modification of silica-based materials with manganese. [22] As expected for a material with paramagnetic Mn centers, intense sidebands www.chemeurj.org were observed in the silicon MAS NMR spectra, and substantially accelerated relaxation of the silicon magnetization was detected.…”

STRAFI imaging of paramagnetic solids: 31P paramagnetic displacements

“…13 This method, referred to more simply as spin-echo mapping (SEM), has proved useful subsequently for 31 P studies of paramagnetic catalysts. 14,15 The STRAFI method is in fact similar to this spectroscopic work, since it deals with large bandwidths even in the case of diamagnetic samples, but differs in employing large imaging gradients. Here the pulse excites only a narrow band of resonances in a 'sensitive slice,' which is of the order of 100 µm thick, and the spatial map is constructed by stepwise translation of the sample.…”