Synthesis of hollow nanocubes and macroporous monoliths of silicalite-1 by alkaline treatment

A simple strategy involving desilication and recrystallization of silicalite-1 in tetrapropylammonium hydroxide (TPAOH) solution was successfully developed to prepare hollow zeolite nanocubes and three-dimensionally macroporous zeolite monoliths. Large voids were introduced to silicalite-1 crystals by controlled silicon leaching with OH^- and thin intact shells were formed by the recrystallization of silicon with TPA⁺. The size of nanocubes could be easily controlled from ∼150 nm to ∼600 nm by simply adjusting the size of parent silicalite-1. Apart from template function to increase the yield of hollow silicalite-1, TPA⁺ adsorbed on the zeolite protects the parent crystal surface where the recrystallization occurred. The size of the mesopores and/or macropores in the hollow zeolite shell can be controlled by varying the amount of competitive Na⁺ adsorbent added to the TPAOH solution. Furthermore, three-dimensional macroporous zeolite monoliths can be formed when an electrolyte, such as NaCl, was added to the TPAOH solution. When the sample was used as the support for iron-based catalyst for hydrogenation of CO₂ to hydrocarbons, both the conversion of CO₂ and the selectivity of C₅ ⁺ higher hydrocarbons were improved.