Formation of clathrasil layers by secondary growth of DOH-type nuclei for gas separation applications

Abstract

Dodecasil 1H (D1H) possessing the zeolite framework type DOH is one of the all-silica zeolites and therefore offers high chemical, thermal and hydrothermal stability. D1H is a 6-ring clathrasil with small pore apertures of about 3Å and may be able to selectively transport hydrogen or helium while rejecting other gasses such as CO2. These properties make D1H an interesting microporous membrane material for precombustion applications. Moreover, it has been shown that D1H crystals can be prepared practically structure directing agent-free.Prior to the attempt to prepare complete layers of D1H crystals, the general crystallisation behaviour of D1H was investigated. To obtain optimal crystallisation conditions a large number of synthesis experiments were performed aiming, in particular, at the preparation of pure material consisting of small and very thin crystals with a reduced number of organic molecules occupying the cage-like voids of the structure. The syntheses were performed in the system aminoadamantane/silica/ethylenediamine or ammonia/water and by systematically varying synthesis parameters like temperature, gel composition, aging of the synthesis mixture, seeding and the type of agitation during the synthesis run. In some additional runs aminoadamantane was replaced by other structure directing agents.Moreover, the removal the organics filling the zeolite cages was studied by different approaches, including calcination at atmospheric pressure and by applying high pressure cycling and calcination of ball milled crystals. The obtained materials were then characterized by powder XRD, SEM, thermal analysis, chemical analysis and optical microscopy. Knowing favourable synthesis conditions, D1H layers have been prepared on a α-Al2O3 support by using secondary growth of small D1H nuclei.