Monitoring the location, amount, and nature of carbonaceous deposits on aged zeolite ferrierite crystals by using STEM-EELS

Abstract

By the use of electron energy-loss spectroscopy performed in a scanning transmission electron microscope (STEM-EELS), detailed spatial information is obtained concerning the amount and nature of carbonaceous deposits formed inside the crystals of the zeolite ferrierite (FER) during the reaction of n-butene to isobutene. In all cases, gradients in coke concentration over the crystal have been observed and quantified. An extensive accumulation of coke is observed at the entrance of the eight-membered-ring (MR) pores, while less coke is present at the entrances of the ten-MR channels. At a higher coke content, further filling up of the complete micropore system occurs and the eight-MR pores become fully blocked. The ten-MR channels remain partially accessible for n-butene, with alkyl-aromatic species deposited near the inlets of these channels. With regard to the selective transformation of n-butene into isobutene, this supports the view that the catalytic action takes place in the pore mouths of the ten-MR channels. Overall it is demonstrated that the major benefit of STEM-EELS is the possibility to simultaneously determine the position-resolved amount and nature of carbonaceous deposits on intact zeolite crystals.