Department of Chemistry, Technische Universität Berlin, Straße des 17. Juni 124, D-10623 Berlin, Germany.
Dalton Trans. 2013 Sep 21;42(35):12644-53. doi: 10.1039/c3dt50611h.
The oxidative dehydrogenation (ODH) of propane to propylene by supported vanadia catalysts has received much attention in recent years, but different reactivity trends have been reported for this catalytic reaction system. In the present investigation, the origin of these differing trends are investigated with synthesis of supported V/SiO2, V/TiO2, and V/Al2O3 catalysts prepared with three different vanadium oxide precursors (2-propanol/vanadyl triisopropoxide [VO(O-Pri)3] (VTI), oxalic acid/ammonium metavanadate [NH4VO3] (AMV), and toluene/vanadyl acetylacetonate [VO(C5H7O2)2] (VAA)) in order to elucidate the influence of the precursor on supported vanadia phase and propane ODH activity. In situ Raman spectroscopy revealed that the choice of vanadium precursor does not affect the dispersion of the supported vanadium oxide phase below 4 V nm(-2) (0.5 monolayer coverage), where only isolated and oligomeric surface VO4 species are present, and only the AMV precursor favors crystalline V2O5 nanoparticle (NP) formation below monolayer coverage (8 V nm(-2)). The propane ODH specific reactivity trend demonstrated that there is no significant difference in TOF for the isolated and oligomeric surface VO4 sites. Surprisingly, V2O5 NPs in the ∼1-2 nm range exhibit anomalously high propane ODH TOF values for the supported vanadia catalysts. This was found for all supported vanadium oxide catalysts examined. This comparative study with different V-precursors and synthesis methods and oxide supports finally resolves the debate in the catalysis literature about the dependence of TOF on the surface vanadium density that is related to the unusually high reactivity of small V2O5 NPs.
近年来，负载氧化钒催化剂上丙烷氧化脱氢制丙烯引起了广泛关注，但对于这一催化反应体系，报道的反应活性趋势却存在差异。在本研究中，通过使用三种不同的五氧化二钒前体（异丙醇/氧代三异丙氧基钒（VTI）、草酸/偏钒酸铵（AMV）和乙酰丙酮氧钒/甲苯（VAA））合成负载 V/SiO2、V/TiO2 和 V/Al2O3 催化剂，研究了这些不同趋势的起源，以阐明前体对负载氧化钒相和丙烷氧化脱氢活性的影响。原位拉曼光谱表明，前体的选择并不影响负载氧化钒相的分散性，低于 4 V nm(-2)（0.5 单层覆盖率）时，只有孤立的和低聚的表面 VO4 物种存在，只有 AMV 前体有利于在单层覆盖率以下形成结晶 V2O5 纳米颗粒（NP）（8 V nm(-2)）。丙烷氧化脱氢的比活性趋势表明，孤立的和低聚的表面 VO4 位的 TOF 没有显著差异。令人惊讶的是，在 1-2nm 范围内的 V2O5 NPs 对负载氧化钒催化剂的丙烷氧化脱氢 TOF 值表现出异常高的活性。这在所有考察的负载氧化钒催化剂中都得到了证实。这项使用不同 V 前体和合成方法以及氧化物载体的对比研究最终解决了催化文献中关于 TOF 对与小 V2O5 NPs 异常高反应性相关的表面钒密度的依赖性的争论。
