Single-Site Vanadyl Species Isolated within Molybdenum Oxide Monolayers in Propane Oxidation

Abstract

The cooperation of metal oxide subunits in complex mixed metal oxide catalysts for selective oxidation of alkanes still needs deeper understanding to allow for a rational tuning of catalyst performance. Herein we analyze the interaction between vanadium and molybdenum oxide species in a monolayer supported on mesoporous silica SBA-15. Catalysts with variable Mo/V ratio between 10 and 1 were studied in the oxidation of propane and characterized by FTIR, Raman, and EPR spectroscopies, temperature-programmed reduction, UV/vis spectroscopy in combination with DFT calculations, and time-resolved experiments to analyze the redox properties of the catalysts. Molybdenum oxide (sub)­monolayers on silica contain mainly dioxo (Si–O−)2Mo­(O)2 species. Dilution of silica-supported vanadium oxide species by (Si–O−)2Mo­(O)2 prevents the formation of V–O–V bonds, which are abundant in the pure vanadium oxide catalyst that predominantly contains two-dimensional vanadium oxide oligomers. Existing single vanadyl (Si–O−)3V­(O) sites and neighboring (Si–O−)2Mo­(O)2 sites do not strongly interact. The rates of reduction in propane and of oxidation in oxygen are lower for single metal oxide sites compared to those for oligomers. The rate of propane oxidation correlates with the overall redox rates of the catalysts but not linearly with the chemical composition. Retarded redox behavior facilitates selectivity toward acrolein on single-site catalysts. The abundance of M–O–M bonds is more important in terms of activity and selectivity compared to the nature of the central atom (molybdenum versus vanadium).

Isolated Molybdenum(VI) and Tungsten(VI) Oxide Species on Partly Dehydroxylated Silica: A Computational Perspective

“…Consequently, A2 is energetically unfavorable, compared to the all but one dioxo models (Table 4). In fact, most sites on the dehydrated silica surface, due to geometrical constraints, are not suitable for the tetragrafted monooxo Mo(VI) species and favor the formation of the digrafted dioxo species, which therefore should be in majority, in agreement with the experimental 3,5,6,14,[22][23][24][25]28 and previous theoretical studies. 37 Finally, the stabilities of the monografted hydroxy dioxo Mo(VI) species (D1, E1, d1, e1) have been examined (Table 5).…”

“… a The frequencies are scaled by 0.9758 and 0.9804 for the periodic ( A1 , C1 , C2 , C5 , C6 ) and cluster ( a1 , a1′ , c1 , c2 , c4 , c1′ , c2′ ) calculations, respectively. b Ref . c Ref . d Ref . …”

“…It was recently experimentally shown that the symmetric O MoO stretching vibration shifts from 988 to 998 cm −1 with increasing Mo loading in the MoO x /SiO 2 systems, while no shift was observed for the asymmetric stretching vibration at 974 cm −1 . 3 The authors suggested that, based on cluster calculations, vibrational coupling of neighboring isolated metal oxide species via the support might be responsible for this blueshift of the symmetric mode. According to the results presented in this work, an alternative or complementary explanation can be proposed.…”

“…2,3,5,6,8,15,17,22−30 On this basis, there is strong evidence that under dehydrated conditions, the isolated surface Mo(VI) sites predominantly exist as four-coordinate dioxo Mo(VI) species, although minor five-coordinate monooxo Mo(VI) species can be also present (Figure 1). 3,5,6,14,[22][23][24][25]28 Besides the monomers, dimeric/oligomeric Mo(VI) oxide species or clusters at relatively low Mo loadings were sometimes suggested. 15,26,27 Crystalline MoO 3 is additionally observed for high Mo content.…”

Near 100% ethene selectivity achieved by tailoring dual active sites to isolate dehydrogenation and oxidation

“…The oxidative dehydrogenation of light alkanes to olefins (ODH) which are important building blocks for a handful of industrial processes is a well-known example of such a challenging reaction that is a promising alternative to the current industrial practice of steam cracking with no thermodynamic limitation, coke formation, and large CO 2 emission but difficult to realize commercialized utilization impeded by the liable deep oxidation [6][7][8][9] . This was because alkene is facile to be further oxidized before its desorption or re-adsorbed on the active sites of dehydrogenation (usually oxygen species) according to the Mars-van Krevelen mechanism 1,10,11 which resulted from its higher affinity and reactivity than alkane to most surfaces particularly for those of V-and Ni-based catalysts which have been studied most for ODH [12][13][14][15][16][17] . Extensive studies have contributed to manipulating the chemical environment of active oxygen species to decrease its insertion as large extent as possible concurrently with activation for C−H bond in alkane.…”