CHARACTERIZATION OF COKE ON H-GALLOSILICATE (MFI) PROPANE AROMATIZATION CATALYST - INFLUENCE OF COKING CONDITIONS ON NATURE AND REMOVAL OF COKE

Coke deposited on H-gallosilicate (MFI) in the propane aromatization a t different temperatures (400-600 degrees C), space velocities (770-82 500 cm(3) g(-1) h(-1)) and periods (7-125 or 240 h) has been characte rized by C-13 CP-MAS NMR, ESR, IR, thermal analysis, CC-MS analysis of soluble (or soft) coke and temperature-programmed oxidation (TPO). Th e nature of coke and its removal from the zeolite are found to be stro ngly 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 remova l of coke. In the high temperature (550 degrees C) aromatization at lo w space velocity, the coke formed is insoluble (or hard) and non-volat ile in nature; it consists of highly polycondensed aromatics having gr aphite-like character. Its removal is possible only by oxidative treat ment at high temperature. However, with the decrease in the coking tem perature, the coke nature is gradually changed from highly polyaromati cs to low polyaromatics. Also the coke removal by oxidative treatment becomes easier; it can be effected at lower temperatures. At the cokin g temperature of 400 degrees 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 degrees C). The high temperature (550 degrees C) coke formed at very high space velocity (82 500 cm(3) g(-1) h(-1)) is less hard than that formed at low space velocities (less than or e qual to 3100 cm(3) g(-1) h(-1)). The formation of CO and CO, upon coke oxidation is also strongly influenced by the coking conditions and he nce by the nature of the coke. (C) 1998 Elsevier Science B.V. All righ ts reserved.