Bronsted acid sites in gmelinite

The structural properties of purely siliceous and Al-substituted protonated gmelinite, a zeolite with a medium-sized unit cell, are investigated by me ans of first-principles local-density functional calculations. For acid sit es introduced into an Al-free structure, optimized geometries are compared with experimental data. The substitution of a Si-O fragment by Al-O-H induc es a pronounced local deformation of the structure. Four symmetrically dist inct O sites (O-1-O-4) are classified, according to the pattern of the defo rmation, into two categories showing rather localized (sites 2 and 3), and mostly delocalized (sites 1 and 4) distortions. Relative stabilities of pro tonated structures are shown to depend on the initial geometry of the site. Larger Si-O-Si angles lead to a higher stability of the acid site. Two app roaches, a static and a dynamical one, are used to derive OH stretching fre quencies. Both of them prove a rather complex relation between the infrared (IR) frequency, the acidity, and the local environment of the Bronsted aci d site. The lowest OH stretching frequency is assigned to the O-2 site as d istinguished in the dynamical calculations. The shift occurs due to attract ive interactions of H to the framework oxygen atoms. (C) 1999 American Inst itute of Physics.