Structural and acidic properties of mordenite. An ab initio density-functional study

The effects of the Si --> Al substitution on the structure of the siliceous framework and the structure and vibrational properties of Bronsted acid si tes in mordenite have been studied using density-functional theory, both in the local-density approximation and including generalized gradient correct ions to the exchange-correlation functional. The substitution induces a sub stantial local deformation of the polytetrahedral geometry of pure-silica m ordenite. Also the counterions have a strong influence on the geometry of t he framework. Protonation of bridging (Si)-O-(Al) oxygen atoms is accompani ed by a further local distortion of the structure. Changes in the bond leng ths remain well localized to the nearest neighborhood of the perturbation c enter; O-H stretching frequencies of acidic protons were calculated in both harmonic and anharmonic approximations, indicating a rather complex relati onship among stability, frequencies, and local environment of the Bronsted acid sites.