Dehydration of ethanol over ZSM-5 catalysts having different Si/Al molar ratios with H3PO4 and Pd modification

Abstract

Dehydration of ethanol was studied over ZSM-5, HZSM-5 and modified HZSM-5 catalysts under atmospheric pressure and temperature range of 200 °C to 400 °C. ZSM-5 catalysts were prepared by hydrothermal method using tetraphopylammonium bromide, sodium silicate solution and aluminium nitrate nonahydrate. The results showed that the various Si/Al molar ratios led to affect the amount of acidity on the surface of catalysts. The ZSM-5(20) catalyst exhibited the highest ethanol conversion at 400 °C about 70.3 % due to its high acid sites and large surface area. The commercial HZSM-5 catalysts, which had molar ratio similar to ZSM-5 Si/Al 20 catalysts, were modified with phosphoric acid and palladium metal by incipient wetness impregnation and co-impregnation methods. The results revealed that both modifications with phosphoric acid and palladium metal had effect on acidity and physical properties such as surface area of catalysts. These modifications resulted in increased weak acid and moderate to strong acid sites indicating that the total acidity increased. Both factors led to more ethylene formation at low temperature. In addition, Pd-HZSM-5 catalyst exhibited the highest activity among modified HZSM-5 catalysts. It had ethanol conversion about 22.3% at 200 °C. For Pd-P-HZSM-5 and Pd-P-HZSM-5 (Co-impregnation), they showed ethylene selectivity equal 12.6 % and 11.4 %, respectively at 200 °C. These results were due to modification with two metals resulting in more site that was suitable for occurring ethylene. Although the synthesized HZSM-5 catalyst, which was modified from ZSM-5 Si/Al20 by ion-exchange with ammonium nitrate, exhibited higher activity than commercial HZSM-5 catalyst and Pd-HZSM-5, it had higher coke formation due to this catalysts had higher acidity and surface area.