Spectroscopic and catalytic investigations of VxOy/SBA-15 and magnesium vanadate model catalysts for selective propene oxidation

The objective of the present work was elucidating s tructure-activity relationships concerning the individual role of vanadium sites in selective prop ene oxidation. Two suitable vanadium oxide model catalyst systems were employed: vanadium oxides sup ported on SBA-15, “VxOy/SBA-15”, and various magnesium vanadate phases. Detailed investigations on the preparation, thermal stability, structure, a nd structural evolution under reducing and propene oxi dizing condition were conducted. Various in situ methods were used, e.g. UV-Vis-DRS, X-ray absorption spectroscopy, Raman s pectroscopy and X-ray powder diffraction. These were combined with online MS gas phase analysis. Catalytic characterization of the model catalysts was performed using quantita tive GC gas phase analysis. In a first investigation magnesium vanadate phases, Mg3V10O28·28H2O, MgV2O6, monoclinic and triclinic Mg2V2O7, and Mg3V2O8, were prepared and compared with V 2O5. Besides Mg3V10O28·28H2O all oxides were stable enough to be used for reac tion tests. No significant structural changes or reduction of the bulk phase during prope ne oxidation occurred for the magnesium vanadates up to 673 K. Conversely, V 2O5 was reduced to VO 2(B) under reaction conditions and V 4O9 was formed as intermediate phase. Catalytic performance of the ma gnesium vanadates was lower compared to that of V2O5. The highest propene conversions were measured for monoclinic Mg2V2O7. Only marginal differences in selectivity towards oxygenated produ cts such as acrolein were observed for the differen t magnesium vanadate phases. Compared to V 2O5, total oxidation was increased while mainly acrole in was formed as partial oxidation product. In situ UV-Vis-DRS revealed that the catalytic performance correlated with the reducibility of the magnesium v anadate catalysts. Second part, Emphasis of the present work, was plac ed on the investigation of selective propene oxidation over VxOy/SBA-15 model catalysts. Preparation of highly disp er e VxOy/SBA-15 samples without formation of V2O5 during the preparation process was possible using a grafting anion exchange method. Three different V xOy/SBA-15 samples with vanadium loadings in the range of 1.4 wt%-7.7 wt% (0.3 V/nm – 2.4 V/nm) were synthesized. A hydrated state of V xOy/SBA-15 was present at ambient temperatures of 300 K in humid atmospheres. The hyd rate state consisted of polymerized vanadium oxide units with square pyramidal coordination of t he vanadium centers. Thermal treatment in oxygen and water saturated atmospheres resulted in dehydra tion, loss of coordinated water, and the direct formation of the dehydrated state of V xOy/SBA-15 above 440 K. The dehydrated state of V xOy/SBA-15 was described by an ordered arrangement of dimeric vanadium oxide units with tetrahedral coordination of the vanadium centers. In situ spectroscopic measurements revealed that the dehyd rated VxOy/SBA-15 structure corresponds to the vanadium oxide structu e d ring selective propene oxidation. Formation of additional V-O-Si bonds to the support during catal ytic reaction was deduced. Moreover, catalytic reaction in propene and oxygen lowered the reducibi lity of the vanadium centers in V xOy/SBA-15 using propene as reducing agent. Vanadium loading had onl y minor impact on the catalytic performance of VxOy/SBA-15. Besides formation of CO 2 and CO mainly formation of acrolein, propionaldehy de, acetaldehyde and acetic acid was observed. No signi fica t change in product selectivites was observed compared to V2O5 reference. Hence, similar active vanadium oxides c nters may be present on VxOy/SBA-15 and bulk V2O5. Conversely to V2O5, the average vanadium valence of V xOy/SBA-15 was V during propene oxidation independent of the initia l oxygen to propene ratio. Dynamic investigations in changing reducing and catalytic condition and a re ction order of zero for oxygen during propene oxidation showed that V xOy/SBA-15 exhibited a fast re-oxidation of the vanadi um centers during propene oxidation.