Effect of the organic moiety nature on the synthesis of hierarchical ZSM-5 from silanized protozeolitic units

Abstract

The synthesis of hierarchical ZSM-5 from silanized protozeolitic units has been accomplished following a three step procedure comprising gel precrystallization at 90 °C, grafting of the seed silanization agent (SSA) and hydrothermal crystallization at 170 °C. The following four organosilanes were tested and investigated as SSAs: 3-aminopropyltrimethoxysilane (APTMS), isobutyltriethoxysilane (IBTES), phenylaminopropyltrimethoxysilane (PHAPTMS) and octadecyltrimethoxysilane (ODTMS). The last agent became anchored to the zeolite seeds to a rather reduced extent (1.3%) and the obtained material presented properties very similar to those of a reference nanocrystalline ZSM-5 (S_ext ∼ 100 m² g⁻¹, S_BET ∼ 450 m² g⁻¹, crystallite size = 20–80 nm). In contrast, silanization of the protozeolitic units with APTMS or IBTES led to ZSM-5 samples having large surface areas (S_ext ∼ 200 m² g⁻¹, S_BET ∼ 500 m² g⁻¹) and exhibiting mesopores (D_p ∼ 8 nm). These materials are formed by 300–400 nm particles consisting of aggregates of ZSM-5 nanounits (10–20 nm). Seed silanization with PHAPTMS caused the greatest enhancement in surface area (S_BET = 573 m² g⁻¹, S_ext = 225 m² g⁻¹), the material so obtained being formed by the smallest nanounits (5–10 nm) within the aggregates. Moreover, this sample presented a hierarchical porosity with pores around 2.0–3.0 nm in addition to those generated by the MFI channels. The strong changes induced in the ZSM-5 crystal size and properties when using PHAPTMS as SSA have been attributed to this agent being very effective in both anchoring onto the protozeolitic units and perturbing the subsequent crystal growth step.