Reactivity and Synergism of Vanadium in Microporous Supports with Copperas a Co-cation
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- Institutt for kjemi 
This study investigates three-dimensional (3-D) microporous materials as carrier systems for vanadium species for the oxidation of propene, also examining the effect of copper as a co-cation.Various 3-D systems have been studied to assess the effect of different pore characteristics and acidity on speciation and activity for the aforementioned reaction. The microporous materials used in this study were the acidic zeolites ZSM-5 and Y and neutral zeotype AlPO-5. High surface area 3-D matrix systems with shape selective properties promotes high loadings while still offering excellent speciation control. Shape selectivity played a role in zeolite Y which imposed growth limitations to the VOx species trapped inside the supercage. However the VOx species on ZSM-5 were not restricted inside the pores and sintered on the external surface. From activity studies the less acidic ZSM-5 (Si/Al=140) showed a superior selectivity towards acrolein compared to the more acidic ZSM-5 (Si/Al=15) and Y (Si/Al=15). However, the neutral AlPO-5 containing both vanadium and copper proved to be the most promising catalyst in this study. In situ X-ray Absorption Spectroscopy (XAS) is an invaluable tool for studying transition metal speciation under reaction conditions, and combined with in-house catalytic measurements, the activity can be correlated with oxidation state and local environment. From in situ XAS increased reducibility was observed in the co-cationic samples of VCu:AlPO-5 and VCu:ZSM-5 (Si/Al=140). The Cu(II)/Cu(I) redox pair initiated reduction of vanadium, while vanadium stimulated reduction of Cu(I) to metallic state in propene at 450◦C. This hyper-reduction of both metals was not observed in the monometallic analogues. In addition the size of the metallic copper clusters obtained from EXAFS suggests a fraction of the particles are small enough for the pore aperture of AlPO-5, whereas the clusters reside on the external surface of ZSM-5. To study the effect of different feed compositions, various propene (C3H6) and oxygen (O2) ratios were investigated with in situ XAS and reactivity studies, of vanadium and copper containing AlPO-5 and ZSM-5. The yield of acrolein reached a maximum with flow ratio C3H6/O2= 0.5, under these conditions copper is present as a Cu(I)/Cu(II) fraction and vanadium existed as V(IV)/V(III) fraction in VCu:AlPO-5, while VCu:ZSM- 5 consisted of Cu(I) and V(IV)/V(III) fraction. Thus the activity is strongly affected by the oxidation state of the cations and also the nature of the support, as neutral carriers are favoured. EXAFS refinements revealed a Cu/V mixing suggesting copper and vanadium are indirectly bonded via bridging oxygen (Cu-O-V) in the neutral AlPO-5 not present in ZSM-5. In addition, VCu:AlPO-5 was the only catalyst producing acrolein in oxygen deficient feed. Based on these findings it was suggested that AlPO-5 containing vanadium and copper undergoes a Mars-van-Krevelen mechanism, and the bridging oxygen partakes in the insertion step. This work contributes insight into new promising catalyst systems for selective oxidation of propene. These materials display a range of different properties when 3-D carriers containing vanadium and copper are utilised. In addition this project gives in situ proof to speciation and synergism of copper and vanadium, not previously reported.