Catalytic oxidative conversion of alkanes to alkenes has gained interest over the years for on-purpose alkene production. The main challenge is that alkenes are highly reactive and susceptible to further oxidation in the presence of gaseous oxygen via adsorption on the catalyst surface. Therefore, to maintain high alkene yields in oxidative conditions, the use of basic catalysts is essential.
Li/MgO is a promising catalyst for the oxidative conversion (dehydrogenation/cracking) of lower alkanes to alkenes [1
]. This catalyst has no formal redox character, i.e., Li+
are not susceptible to oxidation state changes during the above reactions, and together with its inherent strong Brønsted basicity, it minimizes the re-adsorption and sequential combustion of formed alkenes [1
]. Thus, 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. It has been established through the work of Lunsford [13
] on the oxidative coupling of methane that [Li+
]-type defect sites are responsible for catalytic activity. The nucleophilic [O−
] site is a strong hydrogen abstractor and initiates alkane activation via homolytic scission of the C-H bond in the alkane forming a radical. However, contradicting this, more recently Schlögl and co-workers [19
] concluded, supported by quantum chemical calculations, that [Li+
] is not the active site. Calculations on cluster models illustrated that both Li/MgO and MgO possess the same nature of active sites; i.e., low coordinated Mg2+
) at steps and corners [19
]. Thus, promotion with lithium does not introduce new active sites, but enhances the concentration of defect sites in MgO. In agreement, previous work from our lab suggests that lithium cations (Li+
) and oxygen vacancies tend to segregate at steps and corners at the MgO surface, increasing the number of low coordinated Mg2+
), hence enhancing catalyst activity and selectivity [4
]. The mechanism for oxidative conversion reaction over Li/MgO is analogous to that of the oxidative coupling of methane suggested by Schlögl and co-workers; i.e., heterolytic addition of the C–H bond of the alkane on the Mg2+
pair in MgO, leading to a surface OH group and an alkyl radical [19
]. The formed radical then undergoes a complex set of reactions in the gas phase in the presence of oxygen forming alkenes and alkanes, as well as combustion products, like H2
O and COx
Recently, we reported [6
] on the oxidative cracking of n-hexane over the Li/MgO catalyst. Our goal was to achieve higher selectivity to alkenes and lower combustion as compared to redox catalysts, which were previously attempted for the oxidative conversion of alkanes [25
]. Indeed, Li/MgO resulted in lower combustion selectivity compared to catalysts containing oxides of facile redox properties, e.g., V2
]. The low oxidation activity of the Li/MgO catalyst resulted in lower n-hexane conversions at the typical reaction temperatures (500–600 °C) studied [6
]. Furthermore, Li/MgO catalysts deactivated during time-on-stream, which was attributed to the reaction of CO2
] catalytic sites and Li2
]. Lunsford and co-workers confirmed that the presence of CO2
increases the activation energy for methyl radical generation during the oxidative coupling of methane [18
]. The same effect was reported by our group for the oxidative cracking of propane [2
]. The product CO2
interacts with [Li+
] sites, forming an intermediate Li+
, which via further reaction with another [Li+
] site is converted into the more stable Li2
In order to improve both the activity and stability of Li/MgO, we promoted the catalyst with low amounts of oxides with redox properties [6
]. Our goal was to add slight redox activity to the catalyst to enhance C–H bond scission and eventually n-hexane conversions. Of the different transition metal oxides attempted, molybdena-promoted Li/MgO showed the best alkene yields. Significantly, the presence of molybdena also prevented deactivation and catalyst stability was restored [6
Supported molybdena catalysts have been often studied for the oxidative dehydrogenation of light alkanes, e.g., ethane, propane, and butane [27
]. It has been suggested [31
] that oxidation of the C–H bond in the alkane proceeds via a Mars-van-Krevelen redox mechanism with the participation of molybdena lattice oxygen, followed by re-oxidation with gas-phase oxygen.
Various molybdenum oxide systems have been reported, e.g., supported on MgO, ZrO2
, and SiO2
]. The presence of molybdena influences the physiochemical properties of the oxide support. In general, molybdena content, state of molybdena species on the surface of the support, and calcination temperature influence both the textural and acidic properties of the support. For example, in alumina-supported molybdena, in addition to Lewis acid sites, Brønsted acid sites are detected, the concentration of which depends on molybdena loading and calcination temperature [44
]. In zirconia-supported molybdena, the increase in acidity with an increase in molybdena loading is correlated to the formation of Mo–O–Zr surface species as precursors for crystalline Zr(MoO4
]. Moreover, molybdena enhances the surface area of zirconia through stabilizing the tetragonal zirconia phase [46
Generally, the performance of molybdena-based catalysts is related to the extent of crystallinity and chemical structure of the oxidic molybdena species on the support, e.g., free MoO3
, monomeric MoO42−
or polymeric Mo6
]. Hence, structure-performance correlations for supported molybdena catalysts have been of continuous interest. Magnesium oxide-supported molybdena (MoO3
/MgO), in particular, is reported to be an efficient and selective catalyst for the oxidative dehydrogenation of C3
alkanes to the corresponding alkenes [33
]. The activity of the MoO3
/MgO for alkane activation depends on the molybdena loading [37
]. In the oxidative dehydrogenation of propane over a MgMoOx
catalyst, for example, Yoon et al. [37
] reported that optimal molybdena loadings are necessary to maintain high propane conversions. Mg0.95
crystallites with a slight excess of molybdenum (Mg/Mo = 0.9–1.0) showed the highest activity for propane conversion (22% conversion at 515 °C). Above these optimal loadings oxidation activity became significant. The high oxidation activity in these catalysts is due to the facile redox properties of molybdena, where the cation easily undergoes a change in the oxidation state (e.g., Mo6+
In relation to the chemical structure of the molybdates on magnesium oxide, extensive literature has been published on the topic [39
]. Bare and co-workers [41
] reported that, in the case of MoO3
/MgO catalysts, the structure of molybdena species depends on the molybdenum coverage of the support. For sub-monolayer coverages, dispersed species are observed. These consist of highly distorted octahedral molybdena species, e.g., MoO6
, at low molybdenum loadings and regular octahedrally-coordinated polymolybdate species, e.g., [Mo7
, at high molybdenum loadings [41
]. For coverages exceeding the monolayer, crystalline magnesium molybdate (MgMoO4
), in which molybdenum is tetrahedrally coordinated, is observed as the dominant species. Raman spectroscopic studies [39
] showed that for magnesium oxide-supported catalysts, surface molybdena species are sensitive to hydration. Upon exposure to water, octahedrally-coordinated molybdena species transform to tetrahedrally-coordinated MoO42−
In this work, we present a detailed study on the role of molybdena in improving the performance of Li/MgO. A detailed characterization of the MoO3/Li/MgO catalysts is presented in order to identify the chemical structure of the different molybdena species (MoOx) present on surface of Li/MgO. Our objective is to determine the influence of different molybdena species on (i) n-hexane conversion, (ii) alkene versus combustion selectivity, and (iii) catalyst stability. This is expected to help establish guidelines for developing an optimal catalyst for the oxidative conversion of n-hexane.
4. Materials and Methods
Mg(OCH3)2 (6–8 wt%) solution in methanol (Sigma-Aldrich, Darmstadt, Germany), methanol (Merck, Darmstadt, Germany), and LiNO3 (assay ≥ 99.99%, Sigma-Aldrich, Darmstadt, Germany) were used for preparation of MgO and Li/MgO catalysts. Ammonium molybdate (99.98%, Sigma-Aldrich, Darmstadt, Germany) was used as the MoO3 precursor. Pure n-hexane (GC assay ≥ 99.0%, Fluka, Honeywell, USA) was used for catalytic experiments. The reference compounds Li2MoO4 (assay ≥ 99.99%, Sigma-Aldrich, Darmstadt, Germany), Li2CO3 (assay ≥ 99.0%, Sigma-Aldrich, Darmstadt, Germany), MoO3 (assay 99.99%, Sigma-Aldrich, Darmstadt, Germany), and (NH4)6Mo7O24 (assay 99.98%, Sigma-Aldrich, Darmstadt, Germany) were used as received.
4.2. Catalyst Preparation
The MgO and Li/MgO catalysts used in this study were prepared according to the method described in detail earlier [4
]. A methanol solution containing Mg(OCH3
(0.4 M) was mixed at room temperature with another methanol solution containing water (0.8 M) to form a sol. For Li/MgO, the required amount of LiNO3
was added to the solution to obtain ~1 wt % Li. The solution was allowed to stay for gelation for 24 h. The gel formed was dried at 50 °C in vacuum for 7 h, and calcined at 600 °C in a flowing air of 50 mL/min for 1 h with a heating rate of 5 °C/min. Modified MoO3
/Li/MgO catalysts were prepared by wet impregnation of the sol–gel synthesized Li/MgO (calcined at 500 °C) using an aqueous solution of the ammonium molybdate. These were then dried at 50 °C in vacuum for 7 h and calcined at 600 °C in a flowing air of 50 mL/min for 5 h with a heating rate of 5 °C/min. Similarly, MoO3
/MgO catalysts were prepared with wet impregnation of the sol–gel synthesized MgO. Molybdena-promoted samples are denoted as xMoO3
/Li/MgO and xMoO3
/MgO, where x is wt % of MoO3
. Table 1
presents the list of the catalysts prepared.
4.3. Catalyst Characterization
The Brunauer–Emmett–Teller (BET) surface area of the catalyst was determined with nitrogen physisorption using a Micro-metrics Tristar instrument (Micro-metrics, USA).
X-ray Diffraction (XRD) patterns were recorded with a Philips PW 1830 diffractometer (Philips, Netherlands) using Cu Kα radiation, λ = 0.1544 nm. Spectra were registered in the 2θ range of 35 to 50 with step size of 0.01 and integration time of 1 s per step. Elemental composition of the catalysts was determined with atomic absorption spectroscopy (AAS). Li content in all the catalysts was 0.86 wt %. Mo loadings were determined with X-ray fluorescence spectroscopy (XRF), using Phillips PW 1480 spectrometer (Philips, Netherlands).
Temperature programmed desorption (TPD) experiments were performed to decompose the Li2CO3 inherently present in both fresh and used catalysts. A total of 100 mg of the catalyst was pretreated in O2/He at 600 °C for one hour to decompose any MgCO3 present. After cooling the catalyst to 100 °C in helium, TPD was conducted from 100 °C to 950 °C, with an increment of 10 °C/min, with helium flow of 10 mL/min as a carrier gas.
Raman spectral measurements were conducted with a SENTERRA instrument (Bruker Optics, Netherlands) equipped with a cooled charge-coupled device (CCD) detector (−60 °C). The samples were excited with a 785 nm red laser of 100 mW power. Spectra were recorded at room temperature from 100 to 1000 cm−1, at a resolution of 3 cm−1 and a 5 min integration time.
4.4. Catalytic Tests
The catalytic tests were carried out at atmospheric pressure and isothermal conditions in a fixed-bed reactor [6
]. An alumina tube reactor of 4 mm internal diameter was used. Powder catalyst was pressed, crushed, and sieved to particle size of 0.4–0.6 mm before use. A total of 10–100 mg of catalyst sample (to obtain varying contact times) was diluted in quartz particles and placed in the isothermal region of the furnace (1 cm). An alumina rod of 3 mm internal diameter was placed right below the catalytic bed to reduce the post catalytic volume in order to minimize homogenous gas-phase reactions. A Chromel-Alumel thermocouple inside a quartz tube was inserted above the catalytic bed to record reaction temperature. The temperature of the furnace was controlled by a second thermocouple placed outside the reactor tube within the isothermal zone of the tubular furnace.
Reactions were studied at 575 °C. Feed (100 mL/min) consisted of 10 mol % of n-hexane vapor, 8 mol % of oxygen, and balance helium. Similar n-hexane conversions were achieved by varying the weight hourly space velocity (WHSV). Before each catalytic test, the catalysts were pretreated at 625 °C in 50% O2/He (60 mL/min) for 1 h. For analysis of the product, samples of outlet gas stream were injected into two micro gas chromatographies (micro GCs) (Varian CP4900, Netherlands) every 30 min during a period of 6 h. The first micro GC was a quad system consisting of four channels for the separation of O2, N2, CH4 CO, CO2, H2O, C2–C4 hydrocarbons (alkanes and alkenes). The second micro GC was a dual system consisting of two channels for the separation of He, H2, and C6–C8 hydrocarbons (alkanes and alkenes).
n-Hexane conversions were calculated on carbon mol basis; i.e., (C6in moles − C6out moles) / C6in moles × 100%. The carbon balance closed between 100% and 105%. Selectivity to individual products was also calculated based on the number of moles of carbon contained in the products, divided by the total number of moles of carbon in the product mixture excluding unconverted feed; i.e., (niCi / ∑ niCi) × 100%