Formation from propene of carbonaceous compounds responsible for the deactivation of a 5A adsorbent. Removal through pyrolysis and oxidative treatment

Abstract The formation of carbonaceous compounds (coke) from propene was investigated by microgravimetry at 423 K on a 5A adsorbent (80% zeolite, 20% binder) for propene pressures of 6.5, 30 and 97 kPa. First, oligomers and their position and skeletal isomers are formed on the protonic sites of the 5A zeolite. These branched compounds are trapped in the α cages. They can be removed as propene (after cracking) by simple vacuum treatment at 423 K. These compounds (reversible coke) undergo afterwards other reactions, in particular hydrogen transfer and cyclization with the formation of dienes, trienes and naphthenes, as shown by gas chromatographic-mass spectrometric (GC-MS) analysis. Part of these latter compounds can only be cracked at high temperatures leading to olefinic products which rapidly transform into aromatics. These aromatics cannot be transformed into linear products, which explains that part of the coke cannot be removed from the adsorbent by thermal vacuum treatment. All the carbonaceous compounds can be removed through treatment under oxygen at 773 K. The adsorption capacity decreases linearly with the increase in the irreversible coke content. This decrease is independent of the propene pressure used for coke formation. The total blockage of the access of nitrogen to the zeolite pores occurs for a number of coke molecules close to the number of α cages.