Intermolecular condensation of ethylenediamine to 1,4-diazabicyclo(2,2,2)octane over H-ZSM-5 catalysts: Effects of Si/Al ratio and crystal size

The intermolecular condensation of ethylenediamine (EDA) to 1,4-diazabicyclo [2.2.2] octane (DABCO) or triethylenediamine (TEDA) was conducted over H-ZSM-5 catalysts. The effects of reaction parameters, Al content and crystal size on the EDA condensation over H-ZSM-5 have been investigated. The H-ZSM-5 catalyst with a medium Al content (Si/Al = 110) and a small crystal size (ca. 100 nm) showed 99% EDA conversion and afforded a TEDA yield as high as 74% under optimized conditions. The mechanism for the H-ZSM-5-catalyzed condensation of EDA has also been considered. The framework Al-related Brønsted acid sites were assumed to contribute to selective intermolecular condensation of EDA to TEDA. The primary intermolecular condensation of EDA to piperazine (PIP) took place mainly inside the micropores of the MFI structure, while the secondary condensation of PIP with EDA to TEDA was favored by the acid sites located near the pore entrance and on the external surfaces of crystals.