Calculated surface electrostatic potentials of molecular sieve models containing SiO4, AlO4, and PO4 units.

Surface electrostatic potentials have been computed at the ab initio HF/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 potential 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 regions, respectively, giving rise to a much more heterogeneous surface potential.