Structure of the two-dimensional medium-pore high-silica zeolite NU-87

Abstract

SYNTHETIC alumino-silicate zeolites are widely used in the petrochemical industry as catalysts. In particular the few with multidimensional channel systems are highly valued, as they permit more complex chemistry within the pores and are less prone to deactivation through coking than are one-dimensional systems. Materials with a high silica content have improved stability towards heat in the presence or absence of steam (conditions frequently encountered in catalytic processes). NU-87 is a recently synthesized, medium-pore high-silica zeolite¹ with promising catalytic properties². Here we describe its framework topology as deduced by electron diffraction. The data were refined by comparison with X-ray powder diffraction patterns using geometric criteria, lattice-energy minimization and Rietveld refinement. The structure is unique in having a two-dimensional channel system defined by rings of 10 oxygen atoms within the framework (10-ring windows). Adjacent one-dimensional 10-ring channels are linked by short 12-ring channels to create a two-dimensional network, but access to these bridging sections is possible only via the 10-ring windows. Although the past two decades have seen considerable advances in the synthesis of novel zeolite frameworks³, NU-87 is the first high-silica 10-ring zeolite with intersecting channels to be synthesized since the industrially important ZSM-5⁴ and ZSM-11⁵ (both of which have three-dimensional channel systems) nearly 20 years ago.