27A1-NMR studies

In a first part of this paper, we will discuss results of a 27A1 NMR study of the binding of Al ions on acrylamide-acrylic acid copolymers as a function of pH, at the light of a simple electrostatic model. The second part deals with the phase diagrams, physical gelation and precipitation phenomenon, for different copolymer compositions and under various conditions of concentrations, pH and salinity. 

Vucelic, M. and Jones, W. (1992). Solid state 27A1 NMR studies of LDH-intercalates in Multifunctional Mesoporous Inorganic Solids (C. A. C. Sequeira and M. J. Hudson, Eds.). Kluwer Academic Publishers, Dordrecht. 

A1 SINNMR studies of aluminum and several of its alloys have been undertaken. Since the sonication of aluminum in haloform support media results in a violent reaction occurring, the metal particles were encased in resin. Static 27A1 NMR studies of the pure metal yielded resonances with FWHM in excess of 9 kHz. These can be reduced to 700 Hz FWHM by using MAS NMR, but SINNMR revealed even narrower Knight shifted resonances with FWHM of ca. 500 Hz. 

A1 and 2,Si MAS-NMR study of the MCM-22 zeolite modified by steam and alkaline treatments... 

Although it does not relate to polytype identification, we mention a 14N static and MAS-NMR study of WZ AIN, which revealed a fortuitously small Cq < 10 kHz [167]. For comparison, the 27A1 Cq in the same compound was measured to be 1.913 MHz, and had an unusual positive temperature coefficient that has been seen for other metal nuclei in WZ structures. The 14N MAS-NMR spectrum of BN yielded a Cq of 142 kHz corresponding to an axially-symmetric tensor in the hexagonal ring structure [167]. 

The first 27A1 MAS NMR study of zeolites was carried out by Freude and Behrens (151). They measured, first, chemical shifts and half-widths of signals from stationary samples of zeolites Na-A, Tl-A, Na-Y, and Tl-Y at 16 MHz. For MAS frequencies of vR such that vR > Vq/vl the central line of the 27A1 resonance is reduced to about j of its original value. Freude and Behrens next calculated the quadrupole frequencies vQ and shifts of the center of gravity of each line due to the quadrupole interaction, i.e., vQ = (v - vL)/vL at 70 MHz. Then, apparent line positions and line widths, S and <5v 2MAS, were measured experimentally using MAS at 70 MHz. The corrected chemical shift value at 70 MHz was then calculated from the relationship <5, = 5ex — <5q. They were several ppm different from the apparent values (see Table XII). 

Freude et al. (169) carried out a systematic study of the relative amounts of four- and six-coordinated Al in thermally treated zeolite Y, using wide-line and MAS 27A1 NMR at 16 and 70.34 MHz, respectively. Table XV gives the results calculated per one faujasitic supercage ( of the unit cell). It is evident that loss of 27A1 line intensity takes place in treated zeolites in comparison with the parent material, evidently due to extra-framework Al being in an environment of low symmetry. 

It is also of interest to use MAS NMR for the study of the thermal treatment of zeolites which are not in the ammonium-exchanged form. In an X-ray study, Pluth and Smith (179) found electron density at the center of the sodalite cages in dehydrated zeolites Ca-A and Sr-A and attributed this to a partial occupancy of these sites by a four-coordinated aluminous species. No such effect was found in zeolite A exchanged with monovalent cations. Corbin et al. (180) used 27A1 MAS NMR to examine commercial samples of K-A, Na-A and (Ca,Na)-A, as received (see Fig. 41). For K-A and Na-A, only framework tetrahedral Al species were observed, with chemical shifts of 57 and 52 ppm respectively. However, in (Ca,Na)-A an additional intense resonance at 78 ppm, typical of AlfOH) but definitely not due to framework aluminum, was also found (see Fig. 41). A much weaker signal, also at 78 ppm, was detected in zeolite Sr-A its intensity increased greatly on heating the sample to 550°C. Freude et al. (183) came to very similar conclusions in their NMR study of heat-treated zeolite Ca-A. They found that maximum framework dealumination occurs at 500°C and corresponds to ac. 17% of total Al. 

Flo. 52. 27A1 MAS NMR study at 130.32 MHz of the alumination of ZSM-5/silicalite with A1C13 vapor (21 /). (a) Parent material with Si/Al > 400 (b) treated zeolite with Si/AI a 50 500 scans were acquired in each spectrum. 

While silicate and aluminate solutions have been extensively studied using 29Si and 27A1 NMR (see Sections III,B and III,I, respectively), there is only a handful of publications that investigate mixed (Si,Al)-bearing solutions by NMR spectroscopy with a view to elucidating the mechanism of zeolite synthesis. There is thus a considerable scope for further work in this important area. 

Derouane et al. (216) studied the influence of pH and the addition of Na+, Cs+, and tetrapropylammonium ion (TPA + ) on 27A1 NMR spectra of sodium silicate/sodium aluminate solutions. At pH <4 only octahedral Al(H20)g+ species were formed and the spectra were not affected by the addition of either cation. At pH7 a very broad resonance was observed at 10-25 ppm, presumably attributable to precipitated six-coordinated polymeric aluminous species. At pH 10.5 a resonance at ca. 80 ppm appeared in the absence of silicate, corresponding to A OH) it broadened on adding TPA+, but narrowed again when Na+ was also added, suggesting that Na + interacts with aluminate species in preference to TP A+. When sodium silicate was added to the solution, a broad line appeared at ca. 55 ppm corresponding... 

The 2 1 aluminium(III) complexes of azo dyes 60, 61 and 62, as environmentally safer (less toxic) dyes, have the advantage that, in addition to 15N NMR spectra, 27A1 NMR spectral behaviour can also be studied in solution and in the solid state128 from the viewpoint of coordination at the aluminium atom. 

---