0000000000868990
AUTHOR
Dirk Demuth
Orientation of the acidity probes benzene, acetonitrile and pyridine in SAPO-5 and GaAPO-5 molecular sieves: an FTIR microscopy study
The adsorption and desorption behaviour of benzene, acetonitrile (CD3CN) and pyridine for large crystals of SAPO-5 and GaAPO-5 is studied using FTIR microscopy with polarised IR radiation. From the analysis of the polarised adsorption bands it is concluded that the adsorbates are ordered in the molecular sieves at 373 K. The orientation of molecules interacting with Bronsted or Lewis acid sites and of physisorbed molecules are discussed. Benzene adsorbed at Bronsted sites is oriented with the aromatic ring parallel to the crystallographic c-axis of the molecular sieve, whereas physisorbed benzene is oriented with the aromatic ring parallel to the crystallographic a,bplane. Acetonitrile can …
Aluminum-27 double-rotation NMR investigations of SAPO-5 with variable silicon content
Summary Double-rotation NMR (DOR) studies of 27 Al species in SAPO-5, a silicon aluminophosphate molecular sieve with a one dimensional channel system, have revealed a minimum of three aluminum sites resulting from the synthesis. The DOR technique was used specifically to increase the spectral resolution by removing the broadening influences from second-order quadrupolar interactions associated with the spin 5/2 27 Al nuclei. The DOR investigations of SAPO-5 crystals with variable Si/Al ratios resulted in the identification of three aluminum species, two consistent with the reported isotropic shift values for AlPO 4 -5; however, these two resonances are only observable when small quantities…
NMR Studies of Single-File Diffusion in Unidimensional Channel Zeolites
Single-file diffusion is the restricted propagation of particles that cannot pass each other. The occurrence of this phenomenon should be reflected by a change in the time dependence of the mean particle displacement in comparison with ordinary diffusion. Although this process is considered to be the rate-controlling mechanism in a large variety of processes, so far no direct evidence of this phenomenon has been provided. Diffusion measurements made with pulsed field gradient nuclear magnetic resonance (NMR) in unidimensional pore systems (zeolites AlPO4-5 and Theta-1) reflect the expected time dependence of single-file diffusion.