Microporous pure-silica IZM-2

Abstract

Pure-silica IZM-2 was synthesized for the first time, and the concentration of sodium hydroxide used during synthesis affected the phase purity and size of crystals. Most of the micropores in calcined pure-silica IZM-2 that was synthesized in the presence of high concentrations of sodium hydroxide were inaccessible to N2 adsorption; however, the micropores could be rendered accessible by applying either of two different post-synthetic treatments. Pure-silica IZM-2 could also be synthesized without sodium ions using the hydroxide version of the template. In this case, the micropores were accessible to N2 directly after calcination. The size of pure-silica IZM-2 crystals obtained increased with the concentration of sodium hydroxide, with the highest concentrations giving spherical and micrometer-sized aggregates of pure-silica IZM-2 that consisted of intergrown particles (60–500 nm). The nature of the defects in pure-silica IZM-2 was studied with a combination of 1H, and 29Si solid-state NMR spectroscopy. As expected, direct-polarization 29Si NMR spectroscopy showed that the number of non-condensed silica groups decreased upon calcination. Calcined samples also showed broader 29Si NMR bands for the fully condensed silica moieties, which indicated a broader distribution of bond angles and/or bond lengths. The siloxy and silanol groups in calcined pure-silica IZM-2 were accessible to protonation as determined by 1H NMR spectroscopy. We could not determine the structure of pure-silica IZM-2 in its aggregated form; however, further studies of the synthetic conditions could yield larger, non-aggregated crystals that would facilitate structural determination.