Growth mechanism of coke on HBEA zeolite during ethanol transformation

HBEA (11) zeolite was deactivated rapidly by coking during the ethanol transformation into hydrocarbons, at 623 K and 30 bar. The nature of carbonaceous deposit was studied, after zeolite dissolution by hydrofluoric acid both by gas chromatography coupled with mass spectrometry and by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS). The coke on the external zeolite surface was characterized directly on the spent catalyst by laser desorption/ionization time of flight mass spectrometry (LDI-TOF MS). The coke species were identified and classified into 17 families following their unsaturation number (4–23). The coke was polyaromatic, and it was very alkylated (methyl, ethyl, and propyl groups). It is compounded of alkylbenzenes, mainly hexamethylbenzene (HMB), and alkyl-pyrenes, located within the zeolite pore, which could migrate to the external zeolite surface and grow into polyaromatic compounds constituted up to eight aromatic rings and up to 40 carbon atoms. Pulsed electron paramagnetic resonance spectroscopy measurements and molecular simulation by using Cerius2 software have shown that HMBs are distant of 0.6 nm inside the zeolite pores.

Location and distance between coke molecules inside the HBEA zeolite at 30 bar and 623 K obtained by molecular simulation and pulsed EPR DEER experiment.Figure optionsDownload high-quality image (83 K)Download as PowerPoint slideHighlights
► HBEA (11) deactivates by coking during ethanol transformation.
► Coke molecules were characterized by several spectroscopic techniques.
► The coke was classified into 17 families following their unsaturation numbers.
► Coke molecules inside the pore are mainly hexamethylbenzene or alkyl-pyrenes.