DENSITY-FUNCTIONAL CRYSTAL VS. CLUSTER-MODELS AS APPLIED TO ZEOLITES

In order to compare solid and cluster models of zeolites, we have stud ied the substitution Si4+ --> Al3+ + H+ on the T-1 site of mordenite i n the dilute limit using a self-consistent, full potential, local dens ity functional (LDF) approach. Clusters size ranged from 9 to 105 atom s. Two crystal models with different Al concentrations were used. The first contained one substitution site per primitive cell of 72 atoms, the other one per conventional cell, containing 144 atoms. The unrelax ed substitution energies as computed with cluster and crystal models c orrespond well if the cluster results are extrapolated to infinite rad ius. Size effects are much smaller in crystal models. In addition, a s tructure relaxation (with fixed unit cell) was carried out for pure-si lica offretite, a zeolite with 54 atoms per unit cell, and pm e-silica mordenite, with 144 atoms per unit cell, starting from the low alumin um content X-ray crystallographic structure. In the offretite and mord enite optimizations full use was made of the D-3h(1)-P (6) over bar m2 and the nonsymmorphic D-2h(17)-Cmcm space group symmetry, respectivel y. (C) 1998 John Wiley & Sons, Inc.