The location of pyridine in sodium-silver-Y zeolite by powder synchrotron X-ray diffraction

The powder synchrotron X-ray diffraction pattern of a mixed sodium-silver-Y zeolite, Ag56-xNaSi136Al56O384 x approximate to 19, saturated with pyridin e, has been analysed by the Rietveld method to reveal positions for the ads orbed molecules. Cations are distributed over three sites, SII, constrained to 100% occupancy, with 17.2(1) and 14.8(1) Ag(1) and Na(1) ions per unit cell, respectively, SI' with 18.3(1) Ag(2) ions per cell, and SI with 1.8(1 ) Ag(3) ions per cell. The refinement suggests that approximately 7.5 pyrid ine molecules are adsorbed per supercage, located in two sites within the c avities of the zeolite. Pyridine(1) is in the 12-ring window connecting sup ercages. Three molecules project through each window and approach the SII c ations in a supercage, with an average Ag(1)-N distance of 3.17(2) Angstrom . An SII cation can be linked to three pyridine(1) molecules from three sep arate windows. These sites are full with 5.98(2) molecules per supercage. P yridine(2) is found in the supercage, oriented with its pi electron density towards the SII cations, with its centre at 2.87(2) Angstrom from Ag(1). T he average occupancy was fixed at 1.5 molecules per supercage, correspondin g to 20% of the pyridine content. A local ordering scheme can be postulated , whereby alternate supercages are filled by pyridine(1) and pyridine(2) mo lecules, efficiently filling the channels of the zeolite. (C) 1999 Elsevier Science B.V. All rights reserved.