Promoting Li/MgO Catalyst with Molybdenum Oxide for Oxidative Conversion of n-Hexane

Cassia Boyadjian,Leon Lefferts

doi:10.3390/catal10030354

Abstract

In this work, molybdena-promoted Li/MgO is studied as a catalyst for the oxidative conversion of n-hexane. The structure of the catalysts is investigated with X-ray Diffraction (XRD) and Raman spectroscopy. The MoO3/Li/MgO catalyst contains three types of molybdena-containing species, the presence of which depend on molybdena loading. At low Mo/Li ratios (i) isolated dispersed [MoO4]2− anionic species are observed. At high Mo/Li ratios, the formation of crystalline lithium molybdate phases such as (ii) monomeric Li2MoO4 and tentatively (iii) polymeric Li2Mo4O13 are concluded. The presence of these lithium molybdates diminishes the formation of Li2CO3 in the catalyst. Subsequently, the catalyst maintains high surface area and stability with time-on-stream during oxidative conversion. Molybdena loading as low as 0.5 wt % is sufficient to induce these improvements, maintaining the non-redox characteristics of the catalyst, whereas higher loadings enhance deep oxidation and oxidative dehydrogenation reactions. Promoting a Li/MgO catalyst with 0.5 wt % MoO3 is thus efficient for selective conversion of n-hexane to alkenes, giving alkene yield up to 24% as well as good stability.

Highlights

Catalytic oxidative conversion of alkanes to alkenes has gained interest over the years for on-purpose alkene production
These results clearly indicate that the 0.5MoO3 /Li/MgO catalyst preserved the characteristics of the unpromoted catalyst and maintained the high alkene selectivity, even with increasing n-hexane conversion
The results presented above indicate that the promotion of Li/MgO with sub monolayer coverages of molybdena introduces structural changes, which influence both physical properties and the performance of the catalyst in the oxidative conversion of n-hexane

Summary

Introduction

Catalytic oxidative conversion of alkanes to alkenes has gained interest over the years for on-purpose alkene production. Li/MgO is a promising catalyst for the oxidative conversion (dehydrogenation/cracking) of lower alkanes to alkenes [1,2,3,4,5,6,7,8,9,10,11,12] This catalyst has no formal redox character, i.e., Li+ and Mg2+ are not susceptible to oxidation state changes during the above reactions, and together with its inherent strong. The catalyst results in high selectivity to alkenes, which is highly desirable in the oxidative conversion of alkanes This makes Li/MgO a better catalyst for oxidative reactions compared to acidic or redox-type catalysts, such as alumina or vanadia. Calculations on cluster models illustrated that both Li/MgO and MgO possess the same nature of active sites; i.e., low coordinated Mg2+ O2−

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