Characterization of coke on H-gallosilicate (MFI) propane aromatization catalyst: influence of coking conditions on nature and removal of coke

Abstract

Coke deposited on H-gallosilicate (MFI) in the propane aromatization at different temperatures (400-600°C), space velocities (770-82 500 cm³ g^-1 h^-1) and periods (7-125 or 240 h) has been characterized by ¹³C CP-MAS NMR, ESR, IR, thermal analysis, GC-MS analysis of soluble (or soft) coke and temperature-programmed oxidation (TPO). The nature of coke and its removal from the zeolite are found to be strongly influenced by its coking conditions, particularly temperature and space velocity used in the propane aromatization. However, the period of coking has only a small or no effect on both the nature and removal of coke. In the high temperature (550°C) aromatization at low space velocity, the coke formed is insoluble (or hard) and non-volatile in nature; it consists of highly polycondensed aromatics having graphite-like character. Its removal is possible only by oxidative treatment at high temperature. However, with the decrease in the coking temperature, the coke nature is gradually changed from highly polyaromatics to low polyaromatics. Also the coke removal by oxidative treatment becomes easier; it can be effected at lower temperatures. At the coking temperature of 400°C, the coke formed is completely soluble/very soft in nature and consists mainly of methyl substituted pyrenes. It is volatile in nature and can be removed simply by its desorption at high temperature (600°C). The high temperature (550°C) coke formed at very high space velocity (82 500 cm³ g^-1 h^-1) is less hard than that formed at low space velocities (≤3100 cm³ g^-1 h^-1). The formation of CO and CO₂ upon coke oxidation is also strongly influenced by the coking conditions and hence by the nature of the coke.