Studies of catalyst deactivation in methanol conversion with high, medium and small pore silicoaluminophosphates

The activity, stability and selectivity of a series of silicoaluminophosphate (SAPO) molecular sieves have been investigated in a flow reactor, operating at 400 degrees C. SAPO-5 and SAPO-34 catalysts exhibit higher activity than SAPO-11, SAPO-31 and SAPO-41, according to their acidic properties determined using ammonia and pyridine as probe molecules. The main products obtained with the most active solids are light olefins (SAPO-34) and C-4-C-7 hydrocarbons (SAPO-5), but their activity decreases strongly with time due to coke formation. The other catalysts give high selectivities to dimethyl ether (DME). The selectivity patterns found are well interpreted in terms of a series of reaction pathways incorporating both shape-selectivity factors and distribution of acid strength. The nature of the coke formed has been investigated by MS, FT-IR and MAS NMR and it indicates the formation of polyunsaturated long chain hydrocarbons. (C) 2000 Elsevier Science B.V. All rights reserved.