Coking, aging, and regeneration of zeolites: VIII. Nature of coke formed on hydrogen offretite during n-heptane cracking: Mode of formation

Abstract

The carbonaceous compounds (coke) responsible for the deactivation of hydrogen offretite during n-heptane cracking at 450 °C were analyzed in detail for different reaction times. With this zeolite, which has a double pore system, n-heptane cracking occurs partly in the gmelinite cages and partly in the large channels, but in both cases through the carbenium ion chain mechanism. It is shown that the coke formation begins in the gmelinite cages starting from branched cracking products (e.g., isobutene) too bulky to be desorbed. These products undergo reactions leading successively to monoaromatic, biaromatic, and polyaromatic products. The polyaromatic products, because of their size, cannot be located entirely in the gmelinite cages and hence overflow into the large channels. With long reaction times, highly condensed polyaromatics are formed by the growth of the coke molecules in the pores near the outer surface which they cover.