Reactant Selectivity and Regiospecificity in the Catalytic Oxidation of Alkanes on Metal-Substituted Aluminophosphates

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• 作者：Bjö ; rn Modé ; n ; Bi-Zeng Zhan ; Jihad Dakka ; José ; G. Santiesteban ; Enrique Iglesia
• 刊名：Journal of Physical Chemistry C
• 出版年：2007
• 出版时间：January 25, 2007
• 年：2007
• 卷：111
• 期：3
• 页码：1402 - 1411
• 全文大小：154K
• 年卷期：v.111,no.3(January 25, 2007)
• ISSN：1932-7455

文摘

The rate of n-hexane reactions with O₂ increased in parallel with the concentration of hexyl hydroperoxide(ROOH) intermediates and with the number of Mn_redox sites in microporous MnAPO-5 and MnAPO-18 catalysts.These data confirmed the catalytic nature of oxidation pathways and the mechanistic resemblance betweenn-alkane and cycloalkane oxidation pathways. Cyclohexane oxidation turnover rates were higher on MnAPO-5than on MnAPO-18, because small channels in the latter inhibit contact between reactants and Mn activecenters. In contrast, n-hexane oxidation turnover rates (per redox-active Mn center) were similar on MnAPO-5and MnAPO-18, because smaller n-hexane reactants diffuse rapidly and contact active sites in both microporousstructures. MnAPO-18 is able to select reactants based on their size, but no regiospecificity was detected onMnAPO-18 or MnAPO-5 for n-hexane oxidation to alkanols, aldehydes, and ketones (7-8% terminalselectivity). The relative reactivity of primary and secondary C-H bonds in n-hexane was identical on bothcatalysts (k_prim/k_sec = 0.10-0.11) and similar to that predicted from relative C-H bond energies in n-hexaneusing Evans-Polanyi relations. Spatial constraints within MnAPO-18 did not lead to any preference for terminaloxidation or to hexanoic acid as the main product, in contradiction with previous reports on materials withidentical structure. The lack of specific regioselectivity on MnAPO-18 is not unexpected, in view of its largeintracrystalline cages, of the accepted involvement of ROOH intermediates, and of the lack of diffusionalconstraints on the rates of n-hexane oxidation on MnAPO-18 catalysts.