The use of ZSM-5-coated ceramic foam packing in the conversion of methanol to olefins showed substantial activity and selectivity improvements as compared with conventional extruded zeolite pellets. The formation of propylene was particularly favored over the coated foams. At 380 degrees C the amount of ethylene and propylene produced per unit volume of reactor packing was 2.5 times higher over the zeolite-coated foams than over pellets. However, at a lower temperature (320 degrees C) and higher space velocities the zeolite-coated foams were less active than the zeolite pellets. This behavior could be explained by the autocatalytic hydrocarbon pool mechanism. The species involved in the autocatalytic process, which are in equilibrium with the products, were decomposed and flushed off when the space velocity was high or the characteristic size of the catalyst bulk was too small. Thus the zeolite coat has to be optimized to ensure simultaneously a high effectiveness factor and sufficient buildup of catalytic species. (c) 2005 Elsevier Inc. All rights reserved.