Journal of Catalysis 206, 49–59 (2002) doi:10.1006/jcat.2001.3473, available online at http://www.idealibrary.com on Oxidative Dehydrogenation of Propane over Vanadia–Magnesia Catalysts Prepared by Thermolysis of OV(O t Bu) 3 in the Presence of Nanocrystalline MgO Chanho Pak, Alexis T. Bell, 1 and T. Don Tilley 1 Chemical Sciences Division, Lawrence Berkeley National Laboratory and Department of Chemistry and Department of Chemical Engineering, University of California, Berkeley, Berkeley, California 94720-1462 Received May 21, 2001; revised November 20, 2001; accepted November 20, 2001 The influence of vanadium content on the performance of V–Mg– O catalysts for the oxidative dehydrogenation (ODH) of propane was investigated. High-surface-area (380 m 2 /g) MgO was prepared by hydrolysis of Mg(OCH 3 ) 2 followed by hypercritical drying. Vana- dia was deposited on this support by thermolysis of OV(O t Bu) 3 . Catalysts prepared by this means have BET surface areas of 187–299 m 2 /g and apparent surface densities of V 2 O 5 of 1.1–10.3 VO x /nm 2 . All of the catalysts were characterized by X-ray diffrac- tion, temperature-programmed reduction, and Raman, UV–visible, and nuclear magnetic resonance spectroscopy. The environment of the V atoms depends strongly on the apparent surface density of vanadia. Isolated VO 2- 4 units are present at very low apparent sur- face densities (∼1 VO x /nm 2 ). As the vanadia density increases, mag- nesium vanadate structures are formed and above a surface density of 3.5 VO x /nm 2 well-dispersed magnesium orthovanadate domains become evident. The rate of ODH per V atom increases with in- creasing VO x surface density and reaches a maximum value at 3.5 VO x /nm 2 . Above this surface density, the rate of ODH per V atom decreases because an increasing fraction of the V atoms lie below the catalyst surface and, hence, are inaccessible. Consistent with this interpretation, the ODH activity per unit surface area reaches a plateau at a VO x surface density of about 4 VO x /nm 2 . The propane ODH selectivity of the catalysts increases with increasing VO x sur- face density and reaches a plateau of 80% for an apparent surface density of about 4 VO x /nm 2 . Rate coefficients for propane ODH (k 1 ), propane combustion (k 2 ), and propene combustion (k 3 ) were calculated for each catalyst. The value of k 1 increases with increas- ing VO x surface density, reaching a maximum at about 4 VO x /nm 2 . By contrast, the ratios (k 2 /k 1 ) and (k 3 /k 1 ) decrease monotonically with increasing VO x surface density. The observed trends in k 1 , (k 2 /k 1 ), and (k 3 /k 1 ) are discussed in terms of the surface structure of the catalyst. c  2002 Elsevier Science (USA) INTRODUCTION Considerable effort has been devoted to the identifi- cation of catalysts for the oxidative dehydrogenation of 1 To whom correspondence should be addressed. E-mail: bell@cchem. berkeley.edu; tilley@cchem.berkeley.edu. propane to propene (1–6). These studies have demon- strated that the most promising candidates contain vanadia and that the activity and selectivity of such catalysts depends on the local composition and structure of the vanadium- containing species present at the catalyst surface. Investi- gations of catalyst structure have shown that the local envi- ronment of vanadium atoms can be a strong function of the catalyst support and the vanadia loading (1–37). Vanadia supported on oxides such as SiO 2 , Al 2 O 3 , TiO 2 , and ZrO 2 is generally present as isolated vanadyl species, polymeric vanadate species, and small particles of V 2 O 5 . By contrast, when vanadia is supported on basic oxides such as CaO, MgO, and so forth, metal vanadates are readily formed. Particular attention has been given to V–Mg–O cata- lysts since they exhibit relatively high propene selectiv- ities at zero propane conversion (∼80%) and a lower loss in propene selectivity with increasing propane con- version than is observed for other vanadium-containing catalysts (18–37). Structural investigations of V–Mg–O catalysts show no evidence for either isolated vanadyl or polymeric vanadate species, or crystallites of V 2 O 5 . Instead, as the vanadia loading increases, first isolated tetra- hedral VO 4 units and then stoichiometric magnesium vana- dates are observed, the latter being principally magne- sium orthovanadate [Mg 3 (VO 4 ) 2 ] with smaller amounts of magnesium pyrovanadate [Mg 2 V 2 O 7 ]. Since the intrinsic propene selectivity at zero propene conversion increases with increasing vanadia loading, it has been proposed that the high initial propene selectivity observed for V–Mg– O catalysts is attributable to the presence of magnesium vanadate phases. While there has been some debate as to whether Mg 3 (VO 4 ) 2 or Mg 2 V 2 O 7 is more selective, com- parison of propene selectivity–conversion curves for these phases from different authors suggests that the two are very similar (27). It has also been noted that synergies between magnesium vanadate and magnesia lead to higher propene selectivities than are observed for pure magnesium vana- date phases (27). By contrast to what is known about the selectivity of V–Mg–O catalysts, relatively little has been 49 0021-9517/02 $35.00 c  2002 Elsevier Science (USA) All rights reserved.