CALCULATED SURFACE ELECTROSTATIC POTENTIALS OF MOLECULAR-SIEVE MODELSCONTAINING SIO4, ALO4, AND PO4 UNITS

Surface electrostatic potentials have been computed at the ab initio H F/STO-5G level for two model systems: a fragment of a zeolite channel wall; and a fragment composed of SiO4, AlO4 and PO4 units. The potent ial above the SiO4 network is negative everywhere, despite these units commonly being regarded as electrically neutral. In marked contrast, the AlO4, and PO4 units introduce strongly negative and positive regio ns, respectively giving rise to a much more heterogeneous surface pote ntial.