The Real Structure of Intergrown ZSM‐5/ZSM‐11 Industrial Zeolite Catalysts

In some industrial-scale reactions, an intergrowth of ZSM-5 and ZSM-11 aluminosilicate zeolites is the preferred catalyst. These structures each have two intersecting channel systems with 10-ring windows, defined by 10 oxygen atoms. The channels are both straight in ZSM-11 but one is sinusoidal in ZSM-5. Parallel bands of ZSM-5 and ZSM-11 intergrown, as assumed in the literature, would lead to some straight sections along the sinusoidal [100] channels in ZSM-5. However, additional defects lead to the formation of 8-ring and 12-ring windows to replace some of the 10-ring windows along the [100] channels in ZSM-5. In turn these defects affect the adjacent, intersecting straight 10-ring channels along [010] and lead to a possible 20 geometries of the pore walls, of which about half are observed. In intergrowths with 5–20% ZSM-11, meandering antiphase boundaries dominate the defects, lattice energy calculations show correlation with numbers of 4-rings introduced, and molecular mechanics calculations suggest tetra-butyl ammonium structure-directing agents are accommodated but do not direct their formation. In catalytic use, 8-ring windows cause pore blockages while 12-ring windows create cavities that can accommodate even larger reaction intermediates than ZSM-11 or can nucleate coking. Diffusion of reactants and products along [010] is also affected.