Catalytic Dehydration Of Ethanol To Ethylene Over Phosphorous-nickel Modified HZSM-5Catalysts

As the rapid development of bio-ethanol technology in recent years, the route ofethanol dehydrating to produce ethylene, which is one of the major feedstocks inpetrochemical industry, has been drawing much attention. The performance of thecatalyst employed in the process is an important factor to decide if the route will beeconomical. Among the catalysts reported, ZSM-5zeolite is a promising one and havebeen studied extensively. However, the poor hydrothermal stability and anti-carbondeposition ability inhabit its industrial application. It is necessary to modify ZSM-5and study its coking behavior in order to improve its anti-carbon deposition abilityat a low temperature.Based on the Ni-modified HZSM-5synthetized via ion-exchange method, aseries of phosphorus-nickel modified HZSM-5catalysts were prepared byimpregnating the Ni-HZSM-5with phosphoric acid aqueous solution. The existenceforms of phosphorus and effect of modification on the phase, channel structureand acid property of HZSM-5were investigated. The results revealed that the additionof phosphoric acid promoted the dealumination from framework, and thus reduced theamount and strength of strong acid sites, which resulted in the enhancement ofanti-carbon deposition ability. Phosphorus existed mainly in the form of phosphateand pyrophosphate，the former of which consisted mostly of aluminum phosphate.The results of activity test indicated that the catalyst with H3PO4content of8%(3-PN-Z) showed the best performance. The optimal values of reactiontemperature, WHSV, and ethanol concentration were260℃,1.5h-1and50%(V),respectively, and the ethanol conversion and ethylene selectivity reached99.6%and98.5%, respectively.The stability test of3-PN-Z showed that the stability of the catalyst wasimproved to some extent. Characteration of the catalysts before and after reactionrevealed that the deactivation of catalyst resulted in the cover of acid sites by cokedeposit. The carbonaceous materials consisted mainly of olefin polymers containingC=C, aromatics and some compounds containing ester group. As the reaction proceeded, the rate of coke formation decreased and the proportion of coke which canbe eliminated just at relative high temperatures increased.