Improving deactivation behaviour of HZSM-5 catalysts

Different treatments were performed to HZSM-5 catalysts in order to improve their deactivation behaviour during the conversion of acetone/n-butanol mixtures (obtained from fermentation products) to hydrocarbons. Internal and external poisoning, selective external dealumination with silicon tetrachloride and addition of water to the feed were studied. The extent of poisoning (with pyridine or 4-methylquinoline) largely influences selectivity and deactivation behaviour of the catalyst, although no improvement is achieved since poison restricts the access to zeolitic channels also. Selective external dealumination with silicon tetrachloride leads to a slight increase in stability only for catalysts smoothly dealuminated and with indexes of specificity for external surface deacidification, SD, higher than 0.8. This is due to the fact that internal sites are mainly responsible for coke deposition. An increase in the internal deacidification degree is achieved if catalysts are severely dealuminated, thus causing a decrease in coke content; however, external coke is more toxic and deactivation becomes faster. Furthermore, damage of the zeolite structure may take place. Addition of water to the feed leads to stabilisation of aromatisation activity, since strong Bronsted sites responsible for coke deposition are deshydroxilated to Lewis sites, thus decreasing the rate of coke formation. (C) 2001 Elsevier Science B.V. All rights reserved.