Coking of Extruded H-ZSM-5 Zeolite Catalyst in Methanol to Gasoline

Abstract

The present study deals with the production of sustainable fuels by the methanol to gasoline (MtG) process. As the conversion suffers from catalyst coking, this work investigated the effect of local coke deposits on the MtG performance using H-ZSM-5 zeolite extrudates. Spent catalysts were taken from defined sections of an MtG pilot plant after an operation time of 1000 h. The samples were evaluated toward MtG performance and physicochemical properties employing N₂ physisorption, X-ray diffraction, temperature-programmed desorption of ammonia and isopropylamine, thermogravimetry, UV/Vis spectroscopy, Raman spectroscopy, and laser desorption and ionization coupled with mass spectrometry. The catalyst characterization showed substantial deposition of polyaromatic hard coke decreasing from 0.129 to 0.046 g/g along the catalyst bed. The strong coking at the inlet was associated with the initially high MtG rate and the recycling of short-chain olefins, enhancing the presence of coke precursors. The coke deposits blocked active Brønsted acid sites of the zeolite, which decreased from 43 to 15 µmol/g toward the inlet. Consequently, the MtG activity and selectivity were clearly affected by the axial position, with the lowest performance in the inlet section due to loss of hydrocarbon coupling activity of the zeolite.