Presence of Lithium Ions in MgO Lattice: Surface Characterization by Infrared Spectroscopy and Reactivity towards Oxidative Conversion of Propane

Abstract

The surface morphology of Li-promoted MgO catalysts prepared using the sol-gel method (sg) and wet impregnation procedure (imp), respectively, has been studied by low-temperature infrared spectroscopy of adsorbed CO molecules. The results show that step sites, as unselective catalytic centers, are the major features existing on the surface of pure MgO, and those are active toward the oxidative conversion of propane. However, the concentration of these sites is drastically reduced by the incorporation of lithium ions in the MgO lattice. In fact, the incorporated Li (+) ions tend to move into the surface region and occupy sites associated with lower coordination number (e.g., step sites). Li/MgO-sg catalysts are characterized by a higher concentration of incorporation of lithium compared to Li/MgO-imp. In the case of oxidative dehydrogenation/cracking of propane, Li/MgO-sg catalysts show higher activity and selectivity to olefins compared to materials prepared using wet impregnation. Catalytic performance differs strongly regarding (i) the amount of olefins formed, and (ii) the ratio of C(3)H(6)/C(2)H(4). It is shown that high density of active sites is essential for further oxidative dehydrogenation of propyl radicals to propylene and suppression of cracking reactions pathway.

Oxidative Conversion of Hexane to Olefins-Influence of Plasma and Catalyst on Reaction Pathways

“…The catalyst showed very good selectivity to C 2 -C 4 olefins (60 mol %) at a temperature as low as 575°C, which is much lower than temperatures used in steam crackers (T C 800°C). Similar to what is reported in literature and our earlier studies for oxidative conversion of lower alkanes (methane [3], ethane [4][5][6]12], propane [6][7][8][9][10][11] and butane [7]), we proposed hexane activation via the [Li ? O -] sites of Li/MgO abstracting H • . The hexyl radical formed then undergoes complex radical chemistry in gas phase in presence of molecular oxygen, forming the product mixture of C 1 -C 5 products, including olefins, paraffins and combustion (CO x ) products.…”

“…The existence of CH and H bands in the optical emission spectrum of C 6 H 14 -He mixture indicate the decomposition of hexane in plasma via C-H and C-C bond cleavage. CH • radicals in the presence of plasma are generally formed, as proposed in literature by Kado et al [22], through the extensive dehydrogenation of methyl radicals via multiple electron impact excitations (9-10) and/or from the coupling of atomic C and H radical (11) formed through extensive dissociation of methane (12). 3 Optical emission spectrum for a gas mixture of 10% hexane in helium in the presence of plasma at 3 W and at ambient conditions.…”

“…Recent studies on oxidic catalysts with no facile red-ox properties have shown tremendous increase in olefin yields [1,2]. Among these is the Li/MgO catalyst, which has been extensively studied in literature for the oxidative conversion of alkanes; oxidative coupling of methane [3,4] and oxidative dehydrogenation/cracking of ethane [5,6], propane and butane [7][8][9][10][11]. Unlike in the case of oxidic catalysts with red-ox properties [12], sequential combustion of olefins over Li/MgO occurs to a minimal extent, resulting in an olefin selectivity which is almost invariant with the alkane conversion levels.…”

Structure sensitivity of the oxidative activation of methane over MgO model catalysts: II. Nature of active sites and reaction mechanism

“…Computational studies indicate that CO coordination via carbon only in form of a bridged complex is more likely [14]. Trionfetti et al observed this band also after CO adsorption on Li/MgO catalysts, but with increasing Li content, the contribution of the step sites decreases [15], which is in agreement of the ''ironing'' effect of alkali additives [16].…”

“…This observation is consistent with the findings of Spoto et al [2] who argued that CO is adsorbed to pairs of Mg 2+ cations at low pressures in bridged configuration, while at higher pressure, this configuration is displaced by a 1:1 ratio between Mg 2+ centers and adsorbed CO. Features located in the range between 2180 and 2160 cm À1 are typically assigned to terminal CO adsorbed on Mg 2+ 4C centers [2,15], which occur at the surface as edges of either mono-atomic or multi-atomic steps, respectively. We therefore assign the band located at 2164 cm À1 in Fig.…”