Insight into the Effects of Water on the Ethene to Aromatics Reaction with HZSM-5

We report the effect of water on the conversion of ethene with H-ZSM-5 to make aromatics at 300-500 degrees C. We found that water seriously decreased the conversion of ethene and the yield of aromatics, suppressed the hydrogen transfer reaction, and changed the distribution of aromatics at low reaction temperature (300 degrees C). However, the effect of water became relatively insignificant with an increase in the reaction temperature. Characterization by TGA, in situ FTIR, and GC-MS and a simulation with DFT validated that the cofed water preferentially adsorbs at the Bronsted acid site (BAS) of the H-ZSM-5 catalysts, in comparison to ethene, leading to the formation of Z-OH center dot center dot center dot H2O hydrogen-bonded complexes and H+(H2O)(n) species. These species inside the channel of the zeolite inhibit the oligomerization of ethene, the olefin-induced hydrogen transfer reaction, and running of the hydrocarbon pool inside zeolite channels. We also validated that the conversion of ethene was recovered when the majority of H2O was desorbed from BAS, while the propagation of the HCP mechanism within HZSM-5 was still altered at higher temperature as a result of a physically adsorbed water-enhanced confinement effect in the channels. In addition, the role of water on the suppression of the amount of coke was investigated in detail.