The activity and selectivity of Co-ZSM-5 and Co-BEA coatings (Si/Al = 15-23) with a thickness of I pm deposited on a Mo substrate were determined in the ammoxidation of ethylene in a microreactor. Zeolite Beta coatings were more than twice as selective towards the acetonitrile formation than ZSM-5 coatings. The rate of acetonitrile formation increased by a factor of 6 by increasing the aluminum content in the coatings from a Si/Al ratio of 23 to 15. The effect of the Co/Al ratio was less pronounced. The addition of small amounts of boron decreased the selectivity to acetonitrile in the whole range of Si/Al ratios studied. A maximum TOF of 0.8 s(-1) was obtained at a reaction temperature of 500 degrees C, with a selectivity of 58% to acetonitrile. The optimized zeolitic coatings show higher reaction rates than a pelletized Co-ZSM-5 catalyst. (c) 2007 Elsevier Ltd. All rights reserved.