Hierarchical BEA Zeolite with Trimodal Micro-/Meso-/Macroporosity as a Selective and Long-Lived Catalyst for Isobutane/2-Butene Alkylation

Abstract

Isobutane/butene alkylation is an important refinery process for producing high-octane gasoline components (trimethylpentane isomers), in which highly caustic liquid acids (H2SO4 and HF) are still predominantly used as catalysts. Zeolites are promising solid acid alternatives to such liquid acids, but suffer from fast deactivation owing to the formation of bulky carbonaceous deposits within the micropores. In the present study, a series of BEA zeolites with different secondary pore structures were synthesized to investigate the effects of facilitated molecular transport on trimethylpentane selectivity and catalyst deactivation via coke formation [1]. The solely microporous BEA zeolite (MZ) was synthesized using TEAOH as a structure-directing agent (SDA) and colloidal silica as the silica precursor. Electron microscopy studies revealed that MZ was composed of bulk zeolite crystallites with particle sizes of 300–800 nm (Figures 1a and e). The BEA zeolite with bimodal micro-/mesoporosity (2MZ) was synthesized using cyclic diammonium molecule (CD) as an SDA289 and colloidal silica as a silica precursor. We previously reported that CD molecules with suppressed conformational freedom and multiple ammonium groups can strongly interact with anionic silicate species and instantaneously form a mesoporous organic-inorganic hybrid gel with a very similar composition to that of the final BEA zeolite [2]. Such a gel, which is similar to the final crystalline product both compositionally and structurally, but lacks long-range order, can be considered a pre-crystalline phase. Therefore, the gel can be rapidly transformed into a crystalline BEA zeolite without significant morphological changes, while maintaining the initial gel-like mesoporous structure. Electron microscopy images (Figures 1b and f) indicated that 2MZ consists of 15–25 nm zeolite crystallites and contains substantial intercrystalline mesoporosity. The BEA zeolite with trimodal micro-/meso-/macroporosity (3MZ) was synthesized using CD as an SDA and diatomaceous earth (kieselguhr) as a major inorganic source. Diatomaceous earth is a naturally occurring amorphous siliceous sedimentary rock (fossilized remains of diatoms) with an intrinsic macroporous structure (Figures 1d and h). The zeolite nuclei were initially formed on the surface of the macropores, where CD and silicates dissolved from the diatomaceous earth could efficiently contact. With increased crystallization time, the framework of the diatomaceous earth was fully converted to crystalline zeolite while the original macroporosity was maintained (Figure 1c). TEM images indicated that 3MZ was also composed of 15–25 nm zeolite crystallites, as in the case of 2MZ, and thus contained a substantial amount of intercrystalline mesoporosity (Figure 1g).