Three-coordinated magnesium in dehydrated offretite

Abstract

OFFRETITE is a rare natural zeolite whose synthetic equivalent, zeolite O, is commercially important as a molecular sieve¹. Its aluminosilicate framework is topologically related to that of erionite², an abundant natural zeolite³. The catalytic properties of molecular sieve zeolites have been ascribed to framework-bound protons, strong electrostatic fields, and oxygen vacancies in the framework⁴. As befits the small radius of Mg²⁺ ions (0.049 nm for four-coordination, 0.072 for six-coordination and 0.089 for eight-coordination)⁵, Mg normally occurs in six-coordination with oxygen and hydroxyl, though four, five and eight-coordinations occur infrequently (for example, four-akermanite: Ca₂MgSi₂O₇, Mg–O = 0.1876 nm; five-grandidierite: (Mg,Fe)Al₃O.BO₄.SO₄, 0.2176, 0.2054, 0.2040, 2×0.1970; farringtonite: Mg₃PO₄, 0.196, 0.197, 0.201, 0.206, 0.214; eight-pyrope: Mg₃Al₂Si₃O₁₂, 4×0.2196, 4×0.2342)⁶. We report here on the occurrence of three-coordinated Mg in dehydrated offretite, which should produce a strong electrostatic effect on adjacent sorbed molecules.