Magnesium oxide as a catalyst for the

Arabian Journal of Chemistry (2014) xxx, xxx–xxx

King Saud University

Arabian Journal of Chemistry www.ksu.edu.sa www.sciencedirect.com

ORIGINAL ARTICLE

Magnesium oxide as a catalyst for the dehydrogenation of n-octane Elwathig A. Elkhalifa a b

a,b,*

, Holger B. Friedrich

b

Department of Chemistry, Faculty of Science, University of Khartoum, Khartoum, Sudan School of Chemistry and Physics, University of KwaZulu-Natal, Durban 4000, South Africa

Received 30 March 2014; accepted 1 October 2014

KEYWORDS Magnesium oxide; n-Octane; Dehydrogenation; Cyclization

Abstract Magnesium oxide was prepared and used as a catalyst for the dehydrogenation of n-octane. The catalyst was characterized by ICP-OES, BET, powder XRD, IR, H2-TPR and SEM. The catalyst was tested for the oxidative dehydrogenation of n-octane using a continuous flow fixed bed reactor at a GHSV of 6000 h 1 and n-octane/oxygen (O2) molar ratio of 0.8. Octenes were found to be the dominant dehydrogenation products, with 2-octenes being the dominant isomers. Among the C8 aromatics, ethylbenzene was the most-formed product, followed by styrene and o-xylene. The aromatization of n-octane was predominantly through the C1–C6 mode of cyclization. ª 2014 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

1. Introduction Magnesium oxide is one of the most important metal oxides in the field of catalysis; though commonly used as support, its employment as a catalyst was also reported, e.g. oxidative coupling of methane (Hargreaves et al., 1992; Choudhary et al., 1994), dehydrogenation–dehydration of alcohols (Aramendia et al., 1996), dehydrohalogenation of halogenated * Corresponding author at: Department of Chemistry, Faculty of Science, University of Khartoum, Khartoum, Sudan. Tel.: +249 912982467. E-mail addresses: [email protected], [email protected] (E.A. Elkhalifa), [email protected] (H.B. Friedrich). Peer review under responsibility of King Saud University.

Production and hosting by Elsevier

hydrocarbons (Mishakov et al., 2002), benzylation of aromatics (Choudary et al., 2003), synthesis of pyranopyrazole derivatives (Babaie and Sheibani, 2011), and Claisen–Schmidt condensation (Patil and Bhanage, 2013). In the dehydrogenation of short-chain alkanes, the improvement of the catalytic performance of MgO was attempted by techniques such as promotion by other metals (Morales and Lunsford, 1989) and by adding small amounts of iodine to the reactant mixture (Chesnokov et al., 2003). Due to its behaviour in aqueous solutions, MgO is usually classified as a basic support (Blasco and Lopez Nieto, 1997). Chemically, an interesting feature related to MgO is that it is an irreducible oxide, with a very electropositive cation (Mg2+), and the oxygen vacancies when formed are indeed anion vacancies with trapped electrons (Calatayud et al., 2003). Structurally, MgO is an oxide with a rock-salt structure, which means that on the surface each Mg2+ is surrounded by five O2 ions. Magnesium oxide was reported to exhibit surface defects such as edges, corners and kinks; these surface defects were believed to play a role in the splitting of

http://dx.doi.org/10.1016/j.arabjc.2014.10.002 1878-5352 ª 2014 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Please cite this article in press as: Elkhalifa, E.A., Friedrich, H.B. Magnesium oxide as a catalyst for the dehydrogenation of n-octane. Arabian Journal of Chemistry (2014), http://dx.doi.org/10.1016/j.arabjc.2014.10.002

2 the chemical bonds of the adsorbed molecules and, thereby, may influence the catalytic performance of the MgO (Dunski et al., 1994; Klabunde et al., 1996; Knozinger et al., 2000). Generally, the anhydrous surfaces of metal oxides possess two active sites, viz. the cations Mn+ and the anions O2 (Calatayud et al., 2003). For the adsorption of organic molecules, an important step in the catalytic activation, the cations are likely to be the active sites and the metal oxide surface is predominantly acidic (Calatayud et al., 2003). MgO was used as support for vanadium to synthesize catalysts (VMgO catalysts) that were used for the oxidative dehydrogenation of short-chain alkanes (Corma et al., 1993; Mamedov and Cortes Corberan, 1995; Blasco and Lopez Nieto, 1997; Kung and Kung, 1997). In these VMgO catalysts, the tetrahedral vanadium species are believed to constitute the active centres, with the general acceptance that the reaction mechanism takes place through the reduction–oxidation cycle between V5+ and V4+ (Mamedov and Cortes Corberan, 1995). However, there were different opinions regarding the detailed nature of these active centers; some suggested orthovanadate as active centres (Chaar et al., 1987), while others attributed the activity to the pyrovanadate phase (Siew Hew Sam et al., 1990). In spite of the different opinions regarding the detailed nature of the active sites and the phase responsible of the oxidative dehydrogenation (ODH) selectivity on the VMgO system, the supported catalysts (where vanadate and MgO coexisted) were found to be more active and selective than a single magnesium pyro- or orthovanadate phase (Chaar et al., 1987; Michalakos et al., 1993; Lemonidou et al., 1998; Oganowski et al., 1998). In our opinion, this indicates that MgO is likely to have played a role in the performance of these catalysts. In support of this, incorporation of MgO into alumina-supported vanadium catalysts improved the ODH selectivity (Shon et al., 1996; Machli et al., 2002). Moreover, when vanadium was supported on MgO, the ODH products were the dominant products, but when vanadium was incorporated in supports like Al2O3, SiO2, or TiO2, oxygenates were the dominant products (Blasco and Lopez Nieto, 1997). As mentioned above, Mg2+ is known for its low reduction potential, which makes it unlikely for MgO to get easily reduced to Mg1+ or Mg0 during the catalytic testing. Thus, an important difference between V5+ in the VMgO system and Mg2+ is that the former, unlike Mg2+, may easily be reduced to V4+ in the reduction–oxidation cycle that is believed to take place during the catalytic testing. Thus, in contrast to the VMgO system, the redox mechanism is unlikely to take place when MgO is employed as a catalyst, and as a consequence differences in the catalytic performance are expected. We have reported on the oxidative dehydrogenation of n-octane over VMgO catalysts (Elkhalifa and Friedrich, 2010, 2011, 2014) to obtain mainly octenes and aromatics. We now report on MgO as a catalyst for the dehydrogenation of n-octane in order to establish a comparison between its catalytic behaviour and that of the VMgO system.

E.A. Elkhalifa, H.B. Friedrich Thus, an aqueous solution of magnesium acetate (Sigma– Aldrich, 40 g in 700 ml) was heated to 50 °C. An aqueous solution of oxalic acid was prepared (Merck, 25 g in 700 ml); the stoichiometric amount needed for the formation of MgC2O4. This solution was then heated to 50 °C and added to the acetate solution. The resultant mixture was heated and magnetically stirred for half an hour. The resultant solid (magnesium oxalate) was filtered off using a Buchner funnel. The precipitate so-obtained was washed with cold and hot water and placed overnight in an oven operated at 110 °C. MgO was obtained by heating magnesium oxalate in a furnace operated at 600 °C for 6 h. The prepared MgO was then pelletized, crushed, and sieved to a 600–1000 lm mesh size. 2.2. Characterization

2. Experimental

To estimate the purity of the prepared MgO, the magnesium composition was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) using an Optima 5300 DV PerkinElmer Optical Emission Spectrometer. The BET surface area and pore volume analyses were carried out on a Micromeritics Gemini instrument. The surface areas were measured by nitrogen adsorption isotherms at 77 K using the standard multipoint method. Prior to the measurements, MgO samples were first degassed in a stream of nitrogen at 473 K for 24 h. Powder X-ray diffractograms were obtained using a Philips PW1050 Diffractometer equipped with a graphite monochromator and operated at 40 kV and 40 mA. The source of radiation was Co Ka and all data were captured by a Sietronics 122D automated microprocessor. The infrared spectroscopy was performed at room temperature using a PerkinElmer Spectrum 100 FT-IR Spectrometer fitted with a Universal ATR sampling accessory. A small amount of the powdered catalyst (MgO) was placed on top of the ATR crystal (composite of zinc selenide and diamond) and a pressure of about 120 Gauge was applied to allow for better contact between the sample and the crystal. Temperature-programmed reduction (TPR) experiments were carried out on a Micromeritics 2900 AutoChem II Chemisorption Analyzer. Before passing the reducing agent, about 0.05 g of the MgO were pretreated by being heated under a stream of argon (30 ml/ min) at 400 °C for 30 min and then cooled down to 80 °C under the same stream of argon. 5% H2 in argon was then used as a reducing agent at a flow rate of 50 ml/min. Under these reducing conditions, the temperature was ramped up to 950 °C at a rate of 10°/min. To investigate any phasic changes, powder XRD was carried out on the MgO sample after the TPR experiment, and a blank holder was used to overcome the small size of the sample. To investigate the catalyst morphology, scanning electron microscopy (SEM) was carried out using a LEO 1450 Scanning Electron Microscope. Catalyst sample for the SEM was coated with gold using a Polaron SC Sputter Coater. To determine the oxidation state of vanadium in the VMgO catalyst, X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific Multilab 2000 instrument by employing Al-Ka radiation (1486.6 eV).

2.1. Synthesis

2.3. Catalytic testing

Magnesium oxide (the catalyst) was prepared by a procedure similar to that reported by Elkhalifa and Friedrich (2010).

The catalytic testing was carried out in a fixed bed continuous flow reactor that operated in a down flow mode. The reactor

Please cite this article in press as: Elkhalifa, E.A., Friedrich, H.B. Magnesium oxide as a catalyst for the dehydrogenation of n-octane. Arabian Journal of Chemistry (2014), http://dx.doi.org/10.1016/j.arabjc.2014.10.002

Magnesium oxide as a catalyst for the dehydrogenation of n-octane tube was stainless steel with an internal diameter of 10 mm and a length of 220 mm. 1 ml of the synthesized MgO was used in the catalytic testing (pellets’ sizes were between 600 and 1000 lm). The catalyst was placed between two layers of glass wool and the rest of the reactor tube was packed with carborundum. An electrically heated block was used to heat the reactor tube. The temperature of the catalyst bed was monitored with a coaxially centred thermocouple that was placed in the middle of the catalyst’s bed. The flow rates of the oxidant (air, Afrox) and the make-up gas (nitrogen, Afrox) were controlled by ABB rotameters. n-Octane (Merck) was delivered to the reactor system using a LabAlliance Series II HPLC pump. The liquid products and the unreacted n-octane were collected in a cylindrical stainless steel vessel cooled to about 2 °C. A Ritter Drum-Type Gas Meter was used to measure the total volume of the gas that came out of the reactor. The catalytic testing was carried out at a gas hourly space velocity (GHSV) of 6000 h 1 (total flow rate of 100 ml/min), and an n-octane to oxygen (O2) molar ratio of 0.8. The products of the catalytic testing were analysed off-line using a Perkin– Elmer Clarus 500 Gas Chromatograph equipped with both an FID and a TCD. The TCD was used for the quantification of carbon oxides and the FID for the rest of the components. The FID was attached to a PONA capillary column and the carrier gas was hydrogen that flowed at 2.0 ml/min. For the capillary column attached to the TCD (Carboxen 1006 PLOT), the carrier gas was helium that flowed at 6.0 ml/min. 3. Results and discussion

3

Figure 1 XRD diffractograms of: (a) VMgO, (b) MgO, (c) MgO after the TPR experiment, and (d) blank holder.

Table 1

Surface properties of the MgO catalyst.

Catalyst

Surface area (m2/g)

Total pore volume (cc/g)

MgO (fresh) MgO (used)

116.4 114.7

0.389 0.385

3.1. Catalyst’s characterization As shown in Table 1, the surface areas of the fresh and the used catalysts were comparable, indicating that the surface area of the MgO was not affected by the catalytic testing. In our previous study on a VMgO catalyst, a decrease in the catalyst’s surface area was observed after the catalytic testing, and that the magnitude of the surface area decrease increases parallel to the catalytic activity; this was attributed to the reduction–oxidation cycle that was believed to accompany the catalytic activity (Elkhalifa and Friedrich, 2011). It can be inferred from this that the reduction–oxidation cycle over MgO during the catalytic testing was either very minimal compared to that over VMgO or did not take place at all. As Fig. 1b shows, the XRD diffractogram of the fresh MgO catalyst shows two lines that are attributable to MgO (Chaar et al., 1987; Burch and Crabb, 1993) at 2h equal 50.2° and 43.1° (d spacings of 2.11 and 2.44 A˚, respectively; these d-spacings correspond to the (1 0 0) and (1 1 1) crystal planes). Employing the peak at 50.2° and using the Scherrer equation, the average crystallite size of the MgO was estimated as 13.3 nm. No line assignable to MgC2O4 (MgO precursor) was recorded, suggesting that MgO was the only crystalline phase. The XRD pattern of the MgO sample that was collected after the temperature-programmed reduction (TPR) experiment (Fig. 1c) shows only a diffraction line assignable to MgO at 2h equal 50.2° (d spacing of 2.11 A˚), which indicates that MgO was reluctant to reduce even under such reducing conditions under which the TPR experiment was conducted; in fact, the TPR experiment of the MgO showed no reduction peak. This is indeed consistent with the irreducibility of the

MgO (Calatayud et al., 2003). Moreover, the diffractogram of the MgO after the TPR experiment also showed two peaks that may be attributed to Mg(OH)2 at 2h equal 44.4° and 21.4° (d spacings of 2.37 and 4.83 A˚, respectively) (Corma et al., 1993). The apparent formation of the Mg(OH)2 after the TPR experiment is likely to be due to some sort of interaction between MgO and H2O as a result of the TPR experimental conditions. The XRD diffractogram of the VMgO (Fig. 1a) indicated the existence of magnesium oxide (2h equal 50°) and magnesium orthovanadate as crystalline phases. The existence of the orthovanadate phase was indicated by the lines at 2h equal 34° and 42°, which correspond to the d spacings of 3.04 and 2.5 A˚, respectively (Chaar et al., 1987; Burch and Crabb, 1993). The IR spectrum of the MgO (Fig. 2) showed three absorption bands at 836, 1411, and 3700 cm 1. The band at 836 is attributable to MgO, while the bands at 1411 and 3700 cm 1 are assignable to the O–H bending of the combined water molecule and the O–H stretching of the hydroxyl group, respectively, (Said and Abd El-Wahab, 1995; Oganowski et al., 1998). This again indicates that MgO was the only phase that existed in this catalyst, as no band attributable to the magnesium oxalate or the acetate was observed. However, the elemental analysis of the prepared MgO, by ICP-OES, showed that the percentage of the MgO in the catalyst sample was 94%; this is plausibly due to some sort of hydration and/or hydroxylation of the solid MgO, which is indeed consistent with the IR results. The MgO morphology, as shown by the


4

E.A. Elkhalifa, H.B. Friedrich 20 conversion

octenes

C8 aromatics

COx

cracking products

60

45 10 30

Conversion (mol %)

Selectivity (mol %)

15

5

15

0

0 350

400

450

500

550

Temperature (°C)

Figure 4 Conversion and selectivity for different products at GHSV of 6000 h 1.

IR spectrum of MgO.

SEM micrograph (Fig. 3), consists of large aggregates. Moreover, the cubic geometry of these aggregates can also be seen.

16

20 conversion

The catalytic testing over the MgO was carried out in the temperature range of 350–550 °C, and under the same experimental conditions as were used for the VMgO catalysts (Elkhalifa and Friedrich, 2010); this was aimed at helping to establish a comparison between the two systems. To estimate the n-octane activation that was due to factors other than the catalyst, a blank experiment was carried out with the reactor tube only filled with carborundum (without MgO). In this blank experiment, a significant n-octane conversion was only obtained at 550 °C (Table 2).

2-octenes

3-octenes

4-octenes

12

15

8

10

4

5

Selectivity (mol %)

3.2. Catalytic testing

1-octene

0

Conversion (mol %)

Figure 2

0 350

400

450

500

550

Temperature (°C)

Figure 5

Selectivity for octene isomers at GHSV of 6000 h 1.

3.2.1. Activity and products distribution

Figure 3 SEM micrograph of the MgO (20,000 times magnification).

Table 2

Conversion and selectivities (mol %) at GHSV of 6000 h

As shown in Fig. 4, the n-octane conversion increases as the temperature rises. At 400, 450, and 500 °C, the n-octane conversion over MgO is noticeably low (3.8%, 9.2%, and 13.5%, respectively) when compared to that over VMgO under the same experimental conditions (12.5%, 21.0%, 25.5%, respectively, (Elkhalifa and Friedrich, 2010)). In fact, at these relatively low temperatures (400–500 °C) the catalytic activity is likely to be more affected by the properties of the catalyst rather than by merely the temperature, and hence the difference between MgO and VMgO is well-manifested at these low temperatures. As shown in Fig. 4, selectivity to octenes was the highest compared to the selectivity to C8 aromatics and cracking products (C2 to C7 alkenes). An increase in the octenes selectivity was observed at 550 °C (Fig. 4), which is probably due to the contribution of the homogeneous gas phase reactions; in

1

(blank experiment).

Temperature (°C)

Conversion

COx

Octenes

C8 aromatics

Cracking products

500 550

1.8 10.2

72.1 56.3

13.4 26.0

3.1 7.5

4.2 3.5


Magnesium oxide as a catalyst for the dehydrogenation of n-octane 3.0

20 conversion

ethylbenzene

styrene

o-xylene

2.5

1.5

10

1.0

Conversion (mol %)

Selectivity (mol %)

15 2.0

5 0.5

0.0

0 350

400

450

500

550

Temperature (°C)

Figure 6

n octane

Figure 7

Selectivity for C8 aromatics at GHSV of 6000 h 1.

dehydrogenation

octenes

dehydrocyclization

I

ethylbenzene

II

styrene

III

o xylene

Reaction scheme for the dehydrogenation of n-octane.

support of this, the increase of olefins selectivity with increasing the temperature during alkane oxidation was attributed to the heterogeneously initiated homogeneous gas phase reactions (Cavani and Trifiro, 1999). Selectivity to C8 aromatics (ethylbenzene, styrene, and o-xylene) consistently showed the lowest values; selectivity to this class of products was indeed very low, especially when compared to the values obtained over VMgO catalysts (over a VMgO catalyst, C8 aromatics selectivities of 13.8%, 19.2%, 24.7%, and 24.8% at 400, 450, 500, and 550 °C, respectively, were obtained (Elkhalifa and Friedrich, 2010), which correspond to conversions of 12.5%, 20.8%, 25.4%, and 27.6%, respectively). This indicates that the aromatization of n-octane over MgO is poor, which suggests that the reduction–oxidation cycle over MgO is slow or unfavourable; this indeed is consistent with the fact that Mg2+ is a very electropositive cation compared to V5+. Worth mentioning here is that the XPS data of the VMgO catalyst, with regard to both the peak position (binding energy) and the peak symmetry, indicated that vanadium existed as V5+ species (Bond and Tahir, 1991). In the entire temperature range of 350–550 °C (Fig. 4), selectivity to carbon oxides (combustion products) was the highest. 3.2.2. Selectivity to octenes and C8 aromatics In the temperature range of 350–550 °C, selectivity for 2-octenes was always the highest, while that for 4-octenes was constantly showed the lowest values (Fig. 5). With the exception of the selectivities at 350 and 400 °C, 3-octenes were shown to be the second highest dominant octene isomers (Fig. 5). An interesting feature related to the production of 1-octene (Fig. 5) was that the selectivity to this isomer showed a steady decrease as the temperature increased from 350 to 500 °C; the same trend was observed during the oxidative dehydrogenation of n-octane over VMgO and was attributed to that the cyclization of 1-octene to form the C8 aromatics

5

was more favourable compared to the cyclization of the other octene isomers (Elkhalifa and Friedrich, 2011). Further dehydrogenation of the formed octenes will lead to the formation of the C8 aromatics; in the oxidative dehydrogenation of n-octane, octenes are believed to be precursors to C8 aromatics (Elkhalifa and Friedrich, 2010, 2011; Dasireddy et al., 2012). As shown in Fig. 6, the formation of ethylbenzene and styrene started at 450 °C (only a small amount of styrene was formed at 400 °C). Worth mentioning is that the C8 aromatics over VMgO catalysts started to form in significant amounts as early as at 350 and 400 °C (Elkhalifa and Friedrich, 2010). Moreover, the styrene selectivity over MgO at 500 and 550 °C was around 1.3% (Fig. 6), whereas selectivity of around 14% (at a conversion of 25.4%) was obtained over VMgO under the same experimental conditions, which indicates the weak dehydrocyclization power of MgO compared to VMgO. o-Xylene only started to form at high temperatures (500 and 550 °C), and it was consistently the least-formed C8 aromatic (Fig. 6). It was suggested that the n-octane cyclization could either take place through C1–C6 closure to produce ethylbenzene and styrene or through C2–C7 closure to form o-xylene (Fung and Wang, 1991; Shi and Davis, 1995). As may be inferred from Fig. 6, the combined selectivity for ethylbenzene and styrene was greater than that for o-xylene, suggesting that the n-octane cyclization mode was predominantly C1–C6 closure rather than C2–C7 closure; the same mode of cyclization was observed for n-octane over VMgO catalysts (Elkhalifa and Friedrich, 2010). By the inspection of Figs. 4 and 6 and by taking into consideration the evolution and selectivity of the different products during the catalytic testing, the reaction scheme in Fig. 7 may be proposed. In this scheme, the dehydrogenation is favoured over dehydrocyclization, as octenes production was consistently greater than that of C8 aromatics. Also routes I and II (which produce ethylbenzene and styrene, respectively) are favoured over route III (which produces o-xylene), indicating that C1–C6 was the favoured mode of cyclization. 4. Conclusions MgO was prepared and used as a catalyst for the dehydrogenation of n-octane. The characterization showed that the MgO has a degree of hydration and/or hydroxylation, and also it was resistant to reduction during the TPR experiment. MgO was the only crystalline phase shown by the XRD. The catalytic testing showed that octenes were the dominant dehydrogenation products, whereas C8 aromatics were found to be the least-formed dehydrogenation products. Among the C8 aromatics, ethylbenzene was the dominant product, whereas styrene dominated over VMgO. Generally, selectivities for dehydrogenation products over magnesium oxide were low compared to those obtained over vanadium-magnesium oxide. No textural changes were observed after the catalytic testing, in contrast to what was observed in the VMgO systems. Acknowledgements The authors acknowledge the financial support from National Research Foundation (NRF) and THRIP, South Africa. Thanks are also extended to the University of KwaZulu-Natal (South Africa).


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