The rehydration process of the zeolite brewsterite (space group P21/m, ideal formula (Sr,Ba)2Al4Si12O32.10H2O) was studied by single crystal X-ray diffraction on a sample completely dehydrated (heated at 280oC in vacuum for 24 h) and subsequently kept at room conditions for four years. The following structural modifications were observed: (1) the mineral almost regains the unit cell parameters of the untreated phase; (2) a near-complete rehydration occurs, with the water molecules reassuming the four original positions; (3) all the extra-framework cations, spread over a number of sites in the dehydrated phase, migrate to the only site originally present in the untreated phase; (4) the framework cations which migrated during dehydration to new four- and five-coordinated sites owing to the breaking of a T-O-T bridge, but which do not form new bridges, regain their original position; (5) in the case of formation of new T-O-T bridges, the return of (Si,Al) cations to the original sites occurs every time they are five-coordinated, whereas it is only partial when they are four-coordinated.
Rehydration mechanisms in zeolites: reversibility of the T-O-T breaking and of the tetrahedral cation migration in brewsterite
ALBERTI, Alberto;CRUCIANI, Giuseppe;
2001
Abstract
The rehydration process of the zeolite brewsterite (space group P21/m, ideal formula (Sr,Ba)2Al4Si12O32.10H2O) was studied by single crystal X-ray diffraction on a sample completely dehydrated (heated at 280oC in vacuum for 24 h) and subsequently kept at room conditions for four years. The following structural modifications were observed: (1) the mineral almost regains the unit cell parameters of the untreated phase; (2) a near-complete rehydration occurs, with the water molecules reassuming the four original positions; (3) all the extra-framework cations, spread over a number of sites in the dehydrated phase, migrate to the only site originally present in the untreated phase; (4) the framework cations which migrated during dehydration to new four- and five-coordinated sites owing to the breaking of a T-O-T bridge, but which do not form new bridges, regain their original position; (5) in the case of formation of new T-O-T bridges, the return of (Si,Al) cations to the original sites occurs every time they are five-coordinated, whereas it is only partial when they are four-coordinated.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
