Methanol conversion to light olefins over SAPO-34: kinetic modeling of coke formation

Coke deposition during methanol conversion to light olefins over SAPO-34 has been studied in an oscillating microbalance (TEOM) reactor as a function of space velocity (57-384 g/g(cat) h), temperature (673-823 K) and methanol partial pressure (7.2-83 kPa). Two kinetic models were tested for their ability to describe the coking rate at different operating conditions. A modified Voorhies model related coke deposition to the amount of hydrocarbons formed per gram of catalyst. The model could be used to calculate the average coke selectivity and catalyst capacity for olefin formation. The average coke selectivity increased and the catalyst capacity decreased with increasing temperature, while no effect of methanol partial pressure or space velocity was observed. A kinetic model based on a mechanism with strongly adsorbed reaction intermediates as the coke precursors, which either desorb as olefins or react further into coke, was also fitted to the experimental data, and the deactivation functions were found to be linear in coke content for both olefin and coke formation. The rate of deactivation of the coke-forming reaction decreased, while the rate of deactivation of the olefin formation increased with increasing temperature, and no effect of methanol partial pressure or space velocity was observed. (C) 2000 Elsevier Science B.V. All rights reserved.