NMR study of n-butane oxidation catalyzed by vanadium phosphorus oxides and other metal oxides-based catalysts.

摘要

We have developed an experimental protocol to study the mechanism of n-butane oxidation on vanadium phosphorus oxides (VPO) catalysts in which 13C-isotopically labeled n-butane is flowed over a catalyst bed and the reaction products analyzed using 13C NMR spectroscopy. When 1,4-13C 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 percentage of label scrambled maleic acid was typically in the range of 1–7%. For butadiene, the percentage of label scrambling was consistently much higher (∼40%). 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. We also found that the total oxidation of n-butane on VPO catalysts involves the oxidation and two methylene groups of n-butane form ethylene. An organometallic mechanism is proposed to explain these results.; The usability of 31P spin-echo mapping (SEM) NMR for characterization of VPO catalysts was evaluated by obtaining 31P SEM NMR data on VPO catalysts and precursors with different oxidation states of vanadium, different P/V ratios, and catalysts prepared from VOPO₄•2H ₂O using different alcohols. While certain VPO phases can be identified in some of the catalysts based on the 31P SEM NMR data on reference VPO phases, it is difficult to retrieve quantitative information on the existence of V5+, V4+ or V3+ from the 31P SEM NMR spectrum if the catalyst is completely disordered.; The protocol developed for studying the mechanism of n-butane oxidation on VPO catalysts was used to investigate the mechanism for partial oxidation of n-butane on other metal oxides-based catalyst. Similar results were obtained on many of the metal oxides-based catalysts as on VPO catalysts regarding label scrambling in the reaction products and formation of ethylene, suggesting that there is a common mechanism for the selective oxidation of n-butane on metal oxides-based catalysts.