Selectivity issues in (amm)oxidation catalysis

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• 作者：Grasselli ; Robert K.
• 关键词：Selectivity ; Ammoxidation ; Propane ; Propylene ; Acrylonitrile ; Acrylic acid ; Catalytically active centers ; Site isolation ; Phase cooperation ; Symbiosis ; Reaction mechanisms ; Mo_7.8V_1.2NbTe_0.94O_28.9 (M1) ; Mo
• 刊名：Catalysis Today
• 出版年：2005
• 期刊代码：13_09205861
• 类别：ce
• 出版时间：January 15, 2005
• 卷：99
• 期：1-2
• 页码：23-31
• 文件大小：508 K

摘要

Selectivity is currently taking center stage in heterogeneous oxidation catalysis as the cost of feed materials escalates. Particularly important and imperative for commercial processes is selectivity at acceptably high conversions. Dealing with this demanding quest we proposed, some 40 years ago, the concept of site isolation, defining one of the key requirements needed to achieve selectivity in oxidation catalysis. This principle continues to be useful in the conceptual design of new selective oxidation catalysts and has successfully described the selectivity behavior of many commercial (amm)oxidation catalysts, including now the MoVNbTeO system for propane ammoxidation to acrylonitrile (or oxidation to acrylic acid). In its catalytically optimum form, this system is comprised of at least two crystalline phases, orthorhombic Mo_7.8V_1.2NbTe_0.94O_28.9 (M1) and pseudo-hexagonal Mo_4.67V_1.33Te_1.82O_19.82 (M2). The M1 phase is the key paraffin activating and ammoxidation catalyst, its active centers containing all of the key elements V⁵⁺, Te⁴⁺, Mo⁶⁺, properly arranged to catalytically transform propane to acrylonitrile, and four Nb⁵⁺ centers, each surrounded by five molybdenum-oxygen octahedra, isolating the active centers from each other, thereby preventing overoxidation and leading to the observed high selectivity of the desired acrylonitrile product. Symbiosis between the M1 and M2 phases occurs when the two phases are synthesized concurrently in one vessel; or between physical mixtures of the two separately prepared phases provided they are finally divided (≤5 μm), thoroughly mixed and in micro-/nano-scale contact with each other. This phenomenon is particularly pronounced at high propane conversion when the M2 phase begins to serve as a co-catalyst to the M1 paraffin activating phase, converting extraneous, desorbed propylene intermediate, emanating from the M1 phase, effectively to acrylonitrile in a phase cooperation mode. The M2 phase is incapable of propane activation, lacking V⁵⁺ sites, but is a better propylene to acrylonitrile catalyst than the M1 phase since it possesses a higher concentration of Te⁴⁺ sites (i.e., propylene activating sites). Reaction networks for propane (amm)oxidation are proposed for these catalysts.