Substitution clustering in a non-stoichiometric celsian synthesized by the thermal transformation of barium exchanged zeolite X

The thermal transformation of Ba exchanged zeolite X to celsian has been studied by 27Al and 29Si MAS NMR spectroscopy. Evidence for the degradation of the zeolite framework is present in the 29Si NMR spectra after thermal treatment at 850 deg. C. Confirmation is provided by the 29Si NMR data that synthesis of celsian via the decomposition of Ba exchanged zeolite leads to a single defect phase. Clustering of the isomorphous replacement of aluminium by silicon must occur to explain the observed 29Si chemical shifts. The 27Al NMR data show distorted aluminium co-ordination sites upon the thermal transformation of Ba exchanged zeolite X. The distortions present in the amorphous matrix are greater than those present in the monoclinic and hexagonal crystalline phases of celsian. - Graphical abstract: Monte Carlo simulation of the Q 4(mAl) silicon connectivity in the α-hexagonal celsian lattice, for a Si/Al ratio of 1:1. Si atoms are shown in yellow and the Al atoms in black