Preparation and characterization of SSZ-54: A family of MTT/ TON intergrowth materials

Abstract

In this paper we report two different methods to prepare zeolites with framework structures that are intergrowths of the MTT and TON topologies. In one route we use a single structure-directing agent ( N-isopropylethylenediamine) in hydroxide mediated syntheses. The second method uses a mixture of structure-directing agent (SDA) molecules to prepare the MTT/ TON intergrowths. In this case, one molecule alone is selective for MTT-type zeolites and the other molecule alone is selective for TON-type zeolites. By varying the ratio of the individual molecules in the syntheses, we are able to systematically control the MTT or TON character of the intergrowth product. We find that one of the SDA molecules often has a stronger structure-directing or templating effect than the other molecule in the pair. For example, for the system involving mixtures of N-isopropyl-1,3-propanediamine and 1-methylbutylamine, the ratio of N-isopropyl-1,3-propanediamine:1-methylbutylamine in the gel must be below 0.25 before intergrowths are observed; otherwise, an MTT product is observed. We refer to this ability of one SDA molecule to dominate the phase selectivity (in a mixture) as a “dominant templating effect.” We have characterized the structural nature of the MTT/ TON intergrowths by a combination of techniques: comparison of experimental powder diffraction patterns with DIFFaX simulations, observations of systematic peak broadening in the experimental diffraction patterns, comparison of experimental diffraction patterns with those of small-crystallite samples of SSZ-32 ( MTT), and direct observation of stacking faults by high resolution transmission electron microscopy.