Active sites in Fe/ZSM-5 for nitrous oxide decomposition and benzene hydroxylation with nitrous oxide

摘要

The effect of the iron content and the pretreatment conditions of Fe/ZSM-5 catalysts on the Fe speciation and the catalytic activities in nitrous oxide decomposition and benzene hydroxylation with nitrous oxide has been investigated.Iron-containing ZSM-5 zeolites with varying iron content (Fe/Al = 0.1-1.0) were prepared by solid-state ion exchange of HZSM-5 zeolite with FeCl3 followed by hydrolysis and finally calcination at 823 K.In a second step,the catalysts were treated at 1173 K in He flow.The catalysts were characterized by FT-infrared,UV-vis and Raman spectroscopy.The number of Fe~(2+) centers was determined by low-temperature nitrous oxide decomposition and the subsequent methane titration.The highest activity for catalytic nitrous oxide decomposition was achieved for catalysts with intermediate iron loading (Fe/Al = 0.66).The activity after high-temperature treatment was about three times higher than after calcination.Whereas the calcined catalysts showed negligible activity in benzene hydroxylation,high-temperature treatment resulted in dramatic improvements in activity and selectivity.The selectivity to phenol decreased strongly with increasing Fe content.The profound changes in catalytic reactivity are related to the changes in iron speciation upon high-temperature treatment.Besides considerable extraction of Al from framework positions,resonance Raman spectroscopy points to changes in the structure of the iron oxide species of low nuclearity.A useful model is the reconstruction of charge-compensating cationic iron species to iron species stabilized by extraframework Al species located in the zeolite micropores.Further analysis of spectroscopic data suggests that oligonuclear,perhaps binuclear,iron sites appear most favorable for nitrous oxide decomposition,whereas the mononuclear iron sites are active for benzene hydroxylation to phenol.