Chen Bin, Munson Eric J
Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, USA.
J Am Chem Soc. 2002 Feb 27;124(8):1638-52. doi: 10.1021/ja010285v.
The selective oxidation of n-butane to maleic acid catalyzed by vanadium phosphates (VPO) is one of the most complex partial oxidation reactions used in industry today. Numerous reaction mechanisms have been proposed in the literature, many of which have butenes, butadiene, and furan as reaction intermediates. We have developed an experimental protocol to study the mechanism of this reaction in which (13)C-isotopically labeled n-butane is flowed over a catalyst bed and the reaction products are analyzed using (13)C NMR spectroscopy. This protocol approximates the conditions found in an industrial reactor without requiring an exorbitant amount of isotopically labeled material. When [1,4-(13)C]n-butane reacted on VPO catalysts to produce maleic acid and butadiene, the isotopic labels were observed in both the 1,4 and 2,3 positions of butadiene and maleic acid. The ratio of label scrambling was typically 1:20 for the 2,3:1,4 positions in maleic acid. For butadiene, the ratio of label scrambling was consistently much higher, at 2:3 for the 2,3:1,4 positions. Because of the discrepancy in the amount of label scrambling between maleic acid and butadiene, butadiene is unlikely to be the primary reaction intermediate for the conversion of n-butane to maleic anhydride under typical industrial conditions. Ethylene was always observed as a side product for n-butane oxidation on VPO catalysts. Fully (13)C-labeled butane produced about 5-13 times as much isotopically labeled ethylene as did [1,4-(13)C]butane, indicating that ethylene was produced mainly from the two methylene carbons of n-butane. When the reaction was run under conditions which minimize total oxidation products such as CO and CO(2), the amounts of ethylene and carbon oxides produced from fully (13)C-labeled butane were almost equal. This strongly suggests that the total oxidation of n-butane on VPO catalysts involves the oxidation and abstraction of the two methyl groups of n-butane, and the two methylene groups of n-butane form ethylene. An organometallic mechanism is proposed to explain these results.
磷酸钒（VPO）催化正丁烷选择性氧化生成马来酸是当今工业中使用的最复杂的部分氧化反应之一。文献中提出了许多反应机理，其中许多以丁烯、丁二烯和呋喃作为反应中间体。我们开发了一种实验方案来研究该反应的机理，即让（13）C同位素标记的正丁烷流过催化剂床层，并使用（13）C核磁共振光谱分析反应产物。该方案在不需要大量同位素标记材料的情况下，近似于工业反应器中的条件。当[1,4-（13）C]正丁烷在VPO催化剂上反应生成马来酸和丁二烯时，在丁二烯和马来酸的1,4位和2,3位均观察到同位素标记。马来酸中2,3:1,4位的标记重排比例通常为1:20。对于丁二烯，2,3:1,4位的标记重排比例一直高得多，为2:3。由于马来酸和丁二烯之间标记重排量的差异，在典型工业条件下，丁二烯不太可能是正丁烷转化为马来酸酐的主要反应中间体。在VPO催化剂上，乙烯始终是正丁烷氧化的副产物。完全（13）C标记的丁烷产生的同位素标记乙烯量约为[1,4-（13）C]丁烷的5-13倍，表明乙烯主要由正丁烷的两个亚甲基产生。当反应在使总氧化产物（如CO和CO2）最小化的条件下进行时，完全（13）C标记的丁烷产生的乙烯和碳氧化物量几乎相等。这强烈表明，VPO催化剂上正丁烷的总氧化涉及正丁烷两个甲基的氧化和提取，正丁烷的两个亚甲基形成乙烯。提出了一种有机金属机理来解释这些结果。
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