Title : Structure and performance of Li/MgO supported molybdenum oxide for oxidative cracking of N Hexane
Abstract:
Oxidative cracking of n-hexane, as a model compound to naphtha, is studied as an alternative to steam cracking for production of light olefins. Li/MgO is a potential oxidative cracking catalyst due to its non-redox properties and high selectivity to olefins. In this work, we further study the possible enhancement of properties of Li/MgO through promotion with molybendum. Structural details of molybdenum promoted Li/MgO are investigated with XRD, XPS, TPR and Raman spectroscopy. Catalytic oxidative cracking experiments are performed in 4mm alumina reactor packed with 0.4-0.6mm catalyst particles. Reactant gas consisting of 10 mol% of hexane vapor, 8 mol% of O2 and balance helium is fed to the reactor at a rate of 100ml/min. Reactions are studied at temperature of 575 ºC and WHSV between 154 – 385h-1.
Structure-performance study of Mo-Li/MgO catalyst indicates the presence of three types of molybdate species, the formation of which depends on Mo loadings. (i) Isolated tetrahedral [MoO4]2- species, (ii) monomeric Li2MoO4 phase in which MoO4 is tetrahedrally coordinated and (iii) polymeric Li2Mo4O13 phase in which Mo is octahedrally coordinated. These phases are present in low concentrations, below the detection limit of XRD and their formation becomes significant with the increase in atomic ratio of Mo/Li. We show that these molybdate phases are inactive for C-H bond scission in the alkane, yet result in considerable improvements in both catalyst surface area and stability.
Improvement in catalyst surface area is explained by the formation of Li2MoO4 and Li2Mo4O13 phases from reaction of MoO3 with various lithium phases (Li2O and Li2CO3) during catalyst preparation reducing the amount of Li2CO3 originally present in Li/MgO and responsible for catalyst sintering when exposed to high temperatures. Catalyst stability is improved due to increased acidity of the Li/MgO upon promotion with molybdenum, preventing CO2 adsoprtion on Li+O- active sites of the catalyst, hence inhibit sintering and loss of low coordinated Mg2+LCO2-LC sites.
It is concluded that minimal Mo loadings (0.3wt%) is sufficient to bring considerable improvements in the catalyst, while high Mo loadings lead to enhanced redox activity and CO2 formation. 0.3 wt% Mo promoted Li/MgO catalyst is efficient for the selective conversion of hexane to olefins, giving olefin yield up to 24%, and very good stability with time on stream.
