H-gallosilicate (MFI) propane aromatization catalyst: Influence of Si/Ga ratio on acidity, activity and deactivation due to coking

Influence of bulk Si/Ga ratio of H-gallosilicate (MFI) on its degalliation (during calcination for removal of template and deammoniation of NH4 form), acidity/acid strength distribution (measured by stepwise thermal desorption and chemisorption of pyridine at 50-400 degrees C using GC techniques and also by acid catalyzed model reactions viz. iso-octane cracking and toluene disproportionation) and initial activity/product selectivity and catalyst deactivation in propane aromatization (at 550 degrees C) has been investigated. Framework Si/Ga ratio was measured by Si-29 MAS NMR. Information on the external acid sites is obtained by isooctane cracking. The influence of poisoning of strong acid sites (by pyridine) on acid catalyzed model reactions and the effect of deactivation on the product distribution in the propane aromatization have also been studied. The acidity and degalliation (i.e., non-FW Ga/FW Ga ratio) of the zeolite and its dehydrogenation/cracking activity ratio, aromatics/(methane + ethane) mass ratio, aromatics selectivity and deactivation rate constant in the propane aromatization are decreased to a large extent with increasing the Si/Ga ratio. The catalyst deactivation is found to be mostly due to coking. Because of the deactivation, the conversion to aromatics is decreased substantially with the time-on-stream but with little or no change in the distribution of aromatics. The product selectivity is, however, affected significantly due to the deactivation, depending upon the Si/Ga ratio. In general, the aromatics selectivity is decreased but the propylene selectivity is increased with the time-on-stream, mostly due to the decrease in the propane conversion. The shape selectivity (or para selectivity) of the zeolite is increased with increasing the time-on-stream (or deactivation). High concentration (or high density) or zeolitic acid sites is essential for obtaining high propane-to-aromatics conversion. Apart from the acidity, the non-FW gallium species formed due to the degalliation also seem to play an important role by increasing the dehydrogenation activity and consequently the aromatization activity of the zeolite. (C) 1996 Academic Press, Inc.