Silicalite-2, a silica analogue of the aluminosilicate zeolite ZSM-11

Abstract

THE structure of silicalite, a new polymorph of silica which was first prepared by Flanigen et al.¹, is so similar to that of the silica-rich aluminosilicate zeolite ZSM-5 (ref. 2) that we suggest that it should be regarded as the aluminium-free end member of this zeolite series. Therefore it should be possible to prepare silica analogues of other silica-rich aluminosilicate zeolites such as ZSM-11 (ref. 3), ZSM-12 (ref. 4), ZSM-21 (ref. 5) and ZSM-34 (ref. 6). We report here the preparation of what we believe is the analogue of ZSM-11, which we call silicalite-2, adopting the terminology silicalite-1 for the form prepared by Flanigen et al.¹ The silicalite-1, ZSM-5, ZSM-11 and, by inference, silicalite-2 frameworks contain 4-, 5- and 6-membered rings linked to form a system of channels with 10-membered ring openings^1–3. In silicalite-1 and ZSM-5 the channels are a combination of linear and zig-zag^1,2, while in ZSM-11³ and silicalite-2 all the channels are linear. The lack of substitutional aluminium results in silicalites having no significant catalytic or cation exchange properties compared with the zeolites. However, silicalites are unusual in that they are the only known hydrophobic forms of silica and are capable of absorbing organic molecules up to ∼6 Å kinetic diameter. Thermogravimetric studies of silicalites-1 and -2 containing absorbed long straight-chain hydrocarbon compounds (C₄–C₁₆) show that degassing occurs in two stages, which we consider to be due to self-blocking of the channels at intersection points.