La-γAl2O3 catalysts for oxidative dehydrogenation of ethane to ethylene

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VOx -Nb/La-γ Al2 O3 catalysts for oxidative dehydrogenation of ethane to ethylene AbdAlwadood H. Elbadawi, Mogahid S. Osman, Shaikh A. Razzak, Mohammad M. Hossain∗ Department of Chemical Engineering, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 12 October 2015 Revised 4 January 2016 Accepted 6 January 2016 Available online xxx Keywords: Oxidative dehydrogenation Fluidized-bed Oxygen-free environment TPD modeling Ethane conversion Ethylene selectivity

a b s t r a c t The present study deals with oxidative dehydrogenation (ODH) of ethane to ethylene in absence of gas phase oxygen using VOx -Nb/La-γ Al2 O3 catalysts. In catalyst formulation, La is used to modify γ -Al2 O3 , stabilizing its bulk phase transformation. Nb is introduced as a promoter of VOx species. The prepared catalysts are characterized using various physicochemical techniques in order to demonstrate reducibility, oxygen carrying capacity, stability, metal–support interaction and acidity of the catalysts. TPR shows that VOx -Nb/La-γ Al2 O3 is highly active and stable over repeated reduction and oxidation cycles. XRD analysis indicates that VOx appears both as amorphous and crystalline phases. The presence of Nb minimizes the formation of crystalline V2 O5 phases, which is undesirable for ODH reaction. The NH3 -TPD analysis reveals an intermediate acidity of the VOx -Nb/La-γ Al2 O3 catalysts. NH3 -TPD kinetics analysis shows that the addition of Nb increases the activation energy of ammonia desorption, suggesting increased V– support interaction and minimized V–V interaction. The ODH of ethane activity and stability is evaluated in a CREC fluidized riser simulator. The addition of Nb increased ethylene selectivity (85.7%) at 20.1% conversion comparatively at low reaction temperature (550 °C) and short contact time. Thus, Nb enhances VOx isolation and forms a secondary oxide, resulting a superior catalyst activity. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Ethylene is a starting material for several industrial syntheses. The majority of worldwide ethylene production has been consumed as a monomer to produce polyethylene [1]. Conventionally, ethylene is produced by steam cracking of petroleum hydrocarbons. Ethylene is also obtained as by product of the catalytic cracking of heavy oil. Both the steam cracking and catalytic cracking processes require high-energy consumption, which contribute to high production cost of ethylene. On the other hand, catalytic oxidative dehydrogenation (ODH) is an emerging technology that can overcome the drawbacks associated with conventional processes. A metal based ODH catalyst can play an important role achieving high ethylene selectivity by minimizing the undesired COx formation. Due to this reason, there are many research groups including the present research group have devoted efforts to develop highly active, ethylene selective and stable catalysts for ODH reactions. In selective oxidation, both the supported and unsupported metals catalysts have been studied. For example, iron phosphates FePO4 , Fe2 P2 O7 , α -Fe3 (P2 O7 ) and β - Fe3 (P2 O7 ) and FePO4 /Al2 O3 ∗

Corresponding author. Tel.: +966 13 860 1478; fax: +966 13 860 4234. E-mail address: [email protected], [email protected] (M.M. Hossain).

were found to be active as ODH catalysts [2]. The supported catalysts show several advantages over the unsupported catalysts, including better control of metal loading and metal dispersion. The supported catalysts are also flexible for adjusting their physicochemical properties. In supported catalysts, the support has an important role to determine the ultimate catalyst characteristics. For example, TiO2 supported VOPO4 catalysts gives higher ethylene selectivity than the unsupported VOx and (VO)2 P2 O7 catalysts [3,4]. Ni–Co/Al2 O3 powder catalysts are active and selective for ODH reaction with lower conversion and selectivity (less than 30%) [5]. V, Co, Mg and Mn supported on ALPO-5 structure, showed good activity at low temperature although their ethylene selectivity was within the 60–65% range [6]. On the other hand, vanadium on Ti, Sn or Zr pyrophosphates support exhibited good ethylene selectivity (approximately 90 %) at low ethane conversion level [7]. Therefore, the selection of support is an important aspect of formulating a suitable ODH catalyst. Another way to improve catalyst performance is the addition of promoters. Generally, the promoters isolate the active species and form secondary metal oxides on support surface [8,9]. For instance, phosphorous promoted V based catalysts show better performance than the un-promoted V based catalysts [8]. Cr containing pillared zirconium phosphate, synthesized by fluoro-complex, displayed good activity due to the presence of Cr oxide [9].

http://dx.doi.org/10.1016/j.jtice.2016.01.003 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: A.H. Elbadawi et al., VOx -Nb/La-γ Al2 O3 catalysts for oxidative dehydrogenation of ethane to ethylene, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.01.003

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Notation f Edes kd0 kd S Tm T rdes Vm n R VOx x

fraction of reduced vanadium energy of desorption, kJ/mol pre-exponential factor adsorption constant selectivity centering temperature, K temperature, K rate of desorption volume of ammonia desorped, ml/g order of the desorption rate the universal gas constant vanadium oxide surface species ethane conversion

Greek letters α catalyst activity function β rate of temperature increase, °C/min θ des surface coverage of adsorbed species Abbreviations ODH oxidative dehydrogenation XRD X-ray diffraction XRF X-ray Fluorescence TPR temperature-programmed reduction TPO temperature-programmed oxidation TPD temperature-programmed desorption

A multicomponent BaCl2 –TiO2 –SnO2 catalyst shows very high ethylene selectivity even at higher ethane conversion [10]. The presence of Cl− ions in the catalyst helped to minimize the COx selectivity (7%) and enhancing ethylene selectivity. The major shortcoming of the BaCl2 –TiO2 –SnO2 catalyst is its fast deactivation. La, Nd, Sm and Gd based catalysts, synthesized by modified sol–gel method, provides approximately 50% ethylene selectivity relatively at higher ethane conversion (56%) [11]. Cobalt-titania based catalysts, investigated with and without presence of phosphorous, although these catalysts were found to be not very promising with respect to ethylene selectivity [12]. Ni, Cu, and Fe treated Y-zeolites are also studied as ODH catalyst [13]. Among the three catalysts, Ni modified Y-zeolites showed superior activity and ethylene selectivity. On the contrary, the ethylene selectivity drastically dropped by 50% when only the oxides of these metals (unsupported) were used as catalysts [14]. Li, Mg, Al, Ga, Ti, Nb and Ta were used to enhance the properties of Nibased mixed metal oxides [15]. NiO and Nb–NiO nano composites were prepared based on the slow oxidation of a nickel-rich Nb–Ni gel obtained in citric acid [16, 17]. The resulting materials have higher surface areas than those obtained by the classical evaporation method from nickel nitrate and ammonium niobium oxalate. Nano-sized Ni–Zr–O catalysts prepared by sol–gel method, showed good ethylene selectivity [15]. V2 O5 /Nb2 O5 catalyst with various V2 O5 contents was studied [18]. At high ethane conversion, the ethylene selectivity remains within 40% level. Excessive use of Nb2 O5 might have contributed to the low yield with these catalysts. MoO3 –V2 O5 /Al2 O3 is an effective catalyst in ethane dehydrogenation [19]. The catalyst synthesis method also has great effects on the catalyst structure and performance. It has been reported that MoVNbTeOx catalyst gave a good results after it was post treated with oxalic acid, improving catalyst surface area, and hence product selectivity and feed conversion up to 85% and 73%, respectively [20,21].

Fig. 1. Schematic diagram of fluidized-bed oxidative dehydrogenation under gas phase oxygen free conditions.

In addition to the catalysts type, the reactor can also play an important role enhancing both ethane conversion and ethylene selectivity [12,22,23]. Most of the literature studies considered fixed bed type reactor for ODH reactions, mainly for its simplicity. In addition, the previous studies considered gas phase oxygen (from air) as oxidative agent for ODH reaction, which also favors the combustion of both the feed ethane and product ethylene. Consequently, selectivity of ethylene significantly decreased. On the other hand, membrane and fluidized bed reactor can be more interesting to improve both the ethane conversion and ethylene selectivity. The present research group has been investigating a novel gas phase oxygen free ODH using a circulating fluidized beds system [24,25]. In this approach, the ODH reactor system consist of two fluidized bed reactors called as: (i) the gas phase oxygen free dehydrogenation reactor and (ii) the catalyst oxidizer (as shown in Fig. 1). In this arrangement, ethane can be dehydrogenated in presence of the fluidizable solid oxide catalysts, which also acts as the source of lattice oxygen. The oxygen depleted catalyst can be circulated to the oxidizer to re-oxidize them by flowing air and the recycle back to the dehydrogenation reactor to maintain continuous operation. In this fashion the gas phase oxygen will never be allowed to enter into the dehydrogenation reactor, thus diminishes the possibility of complete combustion of both the feed and product. The remaining outstanding challenge for this process is to obtain a catalyst which is fluidizable, selective for ethylene formation and capable of supplying enough oxygen for dehydrogenation but limits the complete oxidation. Taking into considerations of the advantages of the above gas phase oxygen free ODH process, the present study is aimed at developing a fluidizable VOx -Nb/La-γ Al2 O3 catalyst suitable for ODH of ethane to produce ethylene. Generally, in vanadium based catalysts, the crystalline V2 O5 forms at higher vanadium loading, which is less selective to ethylene and more selective to COx . The isolated VOx species are desirable to achieve high ethylene selectivity. On the other hand, the gas phase oxygen free ODH requires appreciable oxygen carrying capacity to attain substantial ethane conversion rate. Therefore, the challenge is to develop a catalyst with highest possible amount of isolated VOx and minimum crystalline V2 O5 . In this regard, Nb is introduced as a promoter to isolate the VOx species and prevent their crystal formation. Nb also improves ethylene selectivity by enhancing the V=O and V–O–Al forms of lattice oxygen in the catalyst [26,27]. In catalyst formulation a small amount of (1 wt. %) La is employed to provide thermal stability of γ Al2 O3 .

Please cite this article as: A.H. Elbadawi et al., VOx -Nb/La-γ Al2 O3 catalysts for oxidative dehydrogenation of ethane to ethylene, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.01.003

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2. Experimental 2.1. Catalyst synthesis In catalyst preparation, γ Al2 O3 (pure alumina, surface area of 141 m2 /g) was used as support. Before metal loading, the support was calcined at 400 °C for 4 h to remove the volatile impurities. In catalyst synthesis, a successive metal loading approach, impregnation-reduction-impregnation was followed. This approach facilitates uniform metal dispersion on the support surface. In order to prevent any possible phase transformation, γ Al2 O3 was first modified with 1% La. Following the La modification, the support was dried and reduced by placing in a fluidized bed reactor at 600 °C under a gas mixture containing 10% H2 and balanced Ar using. The La modified γ Al2 O3 support was impregnated at room temperature with a solution of niobium chloride in 100 ml toluene (0.153 M). The solution was stirred for 12 h then filtered and separated from the solvent. The resulting cake was dried at 150 °C for 24 h. Catalyst precursors were reduced in the same fluidized bed reactor under hydrogen gas flow (10% H2 , 90 % Argon) at 600 °C for 6 h. Finally, vanadium was loaded on the Nb promoted La modified La-γ Al2 O3 support, following the same approach of Nb loading. In this step, V (AcAc)3 was used a precursor for vanadium. After, vanadium loading, the samples were dried and reduced by the same approach as described above. Once the desired metal loading is achieved, the reduced catalyst samples were oxidized under airflow at 600 C for 5 h. The oxidized samples appear as bright yellow color powder indicating the presence of vanadium oxide on the catalyst surface. The prepared sample were labeled with the following acronyms: V(f) for fresh calcined VOx /La-γ Al2 O3 , VNb(f) for fresh calcined VOx -Nb/La-γ Al2 O3 , V(r) for reduced VOx /La-γ Al2 O3 and VNb(r) for reduced VOx -Nb/La-γ Al2 O3 samples. 2.2. Catalyst characterization 2.2.1. X-ray fluorescence (XRF) The availability of different elements/compounds and their weight ratios in the catalyst samples were measured by XRF analysis. A Bruker Tornado M4 micro-ed XRF analyzer, equipped with a single high performance XFlash detector and 25 μm diameter spot size, was employed to carry out the XRF experiments.

2.2.2. BET surface area The specific surface area of the prepared samples was determined by nitrogen adsorption/desorption data obtained from a Quantachrome ASIQwin analyzer. The isotherm experiments were conducted at –196° C and 0.04 to 1 relative pressure. For each experiment, 0.4–0.50 g of catalyst samples was used. Before analysis, the sample was degassed at 350 C for 2 h.

2.2.3. X-ray diffraction (XRD) X-ray diffraction (XRD) analysis was carried out to identify the crystallographic structure of catalyst samples. XRD patterns of the catalysts were recorded in a Rigaku miniflex diffractometer with monochromatic Cu and Kα radiation (λ = 0.154 nm, 30 kV, 15 mA), using a normal scan rate of 2°/min. X-rays were collimated by using a 1.25° divergent scattering slit, and a 0.13 mm receiving slit. Each sample was scanned within 2θ range of 20–80° with a step size of 0.005°.

2.2.4. Raman spectroscopy Raman spectra were collected by a Yvon Jobin equipment using a cooled iHR 320 Horiba spectrometer with CCD detector, which

3

removes the elastic laser scattering. The laser source was green type (λ = 532 nm), while the laser intensity was 50% at 50 to 2500 spectrum window. Powder form of catalyst sample was used to minimize the possibility of mass transfer limitations and to ensure that all catalyst particles in the cell are exposed to the flowing gases. 2.2.5. Temperature programmed reduction/oxidation (TPR/TPO) The reducibility and oxygen carrying capacity of the catalyst samples were characterized by using temperature programmed reduction/oxidation (TPR/TPO) techniques. During the high temperature reduction/oxidation cycles the metal dispersion might change, which affects the formation of VOx phases on support surface, consequently the catalyst activity [24,26,>28]. The repeated TPR/TPO cycles allow determining the catalyst stability and possibility of any metal agglomeration during consecutive reduction–oxidation cycles. The TPR/TPO experiments were conducted using a Micromeritics AutoChem II 2920 analyzer. For the TPR/TPO experiments, approximately 0.1–0.2 g catalyst sample was loaded in a “U” shaped quartz tube located inside a furnace. In order to perform TPR, the catalyst sample was first degassed by circulating argon through the U-shaped tube, while temperature was gradually increased from ambient to 300 °C for a period of 3 h. After degassing, the gas flow was switched to a mixture of 10 mol% hydrogen in argon at a rate of 50 ml/min. The catalyst sample was heated from ambient to 750 °C with a heating rate of 10 °C/min. A thermal conductivity detector (TCD) measured the concentration of the outgoing gas concentration. The TCD data was further processed to obtain the hydrogen consumption during the reduction reaction. The TPO experiments were conducted following each TPR experiments to re-oxidize the reduced sample. Before starting TPO, the system was cooled down to room temperature under helium flow. For the TPO, a stream of 5 mol% O2 and 95 mol% helium gas was circulated through the reduced catalyst bed at a rate of 50 ml/min. The bed temperature was increased from ambient to 900 °C at a heating rate of 10 °C/min. The same thermal conductivity detector (TCD) was used to measure the concentration of the outgoing gas concentration and the oxygen consumption. 2.2.6. NH3 -temperature programmed desorption (NH3 -TPD) The purpose of NH3 -TPD analysis is to determine the catalyst total acidity and acid strength of the prepared catalyst samples. The desorption kinetics analysis provides information about the metal–support interactions. Temperature-programmed desorption of NH3 (NH3 -TPD) were carried out using the same Micromeritics AutoChem-II 2920 analyzer. The catalyst sample (between 0.15 and 0.20 g) was placed in a U-shaped quartz container and degassed for 2 h at 300 °C in a flow of helium at 50 ml/min. The samples were then cooled down to 120 °C. At this stage the sample is saturated with ammonia using a NH3 /He gas mixture (4.55% NH3 ) for 1 h at a rate of 50 ml/min. Following the ammonia saturation, the sample was purged in an He stream for 2 h at 120 °C in order to remove loosely bound ammonia (i.e. physisorbed and H-bonded ammonia). The NH3 desorption experiments were conducted by raising up the sample temperature to 500°°C at different heating rates (10, 20 and 30 °C/min). As the temperature gradually increased, ammonia desorbed as it gained enough energy to overcome the desorption energy barrier. A thermal conductivity detector (TCD) measured the ammonia concentration of the outgoing gas. The TCD signal was further processed to obtain the TPD profiles and desorption data for kinetics analysis.

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2.3. Gas phase oxygen free ODH in fluidized conditions 2.3.1. Reactor set-up The catalyst were evaluated for gas phase oxygen free ODH of ethane using a fluidized CREC Riser Simulator, which can be operated under the conditions as expected in a large scale fluidized unit [24]. The CREC Riser Simulator is a bench scale mini-fluidized bed reactor consisting of (i) a 53 ml reactor basket, (ii) heaters (upper and lower), (iii) a vacuum box and (iv) an impeller. The reactor is placed between the upper and lower heaters to maintain uniform temperature of the catalyst bed. The reactor is connected to the vacuum box (10 0 0 ml) by a four port automatic control valve to ensure precise termination of the reaction following pre-specified reaction time. The vacuum box also serves as a product container and directly connected to a GC for product analysis. The impeller, placed on top of the reactor, can rotate up to 70 0 0 rpm speed. The rotation of the impeller forces the gas to flow outward of the impeller center and downwards into the reactor annulus, leading to the fluidization of the solid catalysts and ensure intimate contact between gaseous feed and the solid phase oxygen, i.e., the lattice oxygen of the catalyst. In the context of the present study, the performance (reactivity, stability, product selectivity) of the prepared catalysts were established at various temperatures and contact times. The selected temperatures for the experiments were chosen to be consistent with reduction temperatures of the catalysts, as reported in the TPR analysis. In a typical run, 0.3 g of oxidized catalyst sample was loaded into the reactor basket and the reactor was checked for potential leaks. Following the leak test, the system was purged by flowing pure Ar. The temperature program was started to heat the reactor to the desired temperature. The argon flow was maintained to keep the reactor from any interference of gas phase oxygen. Once the reactor reached to the desired temperature, the argon flow was discontinued. The reactor isolation valve was closed when it reached the desired pressure level. At this stage the vacuum pump was turned on to evacuate the vacuum box down to 20.7 kPa (3.75 psi). The catalyst was fluidized by rotating the impeller a speed of at 40 0 0 rpm. At this point, the ethane feed was injected (3 ml) into the reactor by using a preloaded gas tight syringe. The reaction continued for a pre-specified time. At the termination point, the isolation valve between the reactor and vacuum box opened automatically and transferred all the reactant and products into the vacuum box. The gas samples in the vacuum bottle were analyzed using an Agilent 7890A GC equipped with both a TCD and a FID detector. For each catalytic run, the product samples were analyzed three times to ensure the accuracy of the analysis. Finally, the product analysis data were used to calculate conversion and selectivity of various products. The following definitions were used in calculating the conversion and selectivity:

Conversion of ethane =

Moles of ethane converted × 100 % Moles of ethane fed (1)

Fig. 2. N2 adsorption/desorption isotherms for (a) VNb(f) (b) V(f) at 77.3 K.

Selectivity to product i Moles of product i = × 100 % Moles of ethane converted − Moles of product i (2) 3. Results and discussion 3.1. Catalyst characterization 3.1.1. BET surface area and XRF Fig. 2 shows adsorption/desorption isotherms of V(f) and VNb(f) catalyst samples, where volume of adsorbed/desorbed N2 was plotted against relative pressure. Both the samples display a type-iv isotherm, indicating a narrow size meso-porosity and a nitrogen monolayer adsorption on the catalyst surface. The monolayer coverage extended even beyond 0.75 relative pressures, which indicates a good dispersion of active sites [24,27]. The BET surface area of the prepared catalyst samples were determined by using N2 adsorption isotherms. It was found that the BET surface area of γ Al2 O3 (141 m2 /g) was significantly decreased (to 24 m2 /g) after impregnation of vanadium and niobium oxide species. The decrease of surface area can be attributed to the blockage of support pores by vanadium and niobium oxide species. Samples composition of various elements in the catalysts were measured by XRF analysis are within ± 2 % error margin of the targeted compositions as shown in Table 1. 3.1.2. X-Ray diffraction (XRD) The XRD patterns of V(f),V(r), VNb(f) and VNb(r) catalyst samples are presented in Fig. 3. In both catalyst samples, the peaks in 2θ range of 0–20° are due to the VOx species [24,29–35]. The peak at 2θ value between 20° and 40° represents crystalline V2 O5 phase [23,26]. According to previous studies, the crystalline V2 O5 forms only after completion of the VOx monolayer on the support surface [36]. In the context of the present study, the intensity of V2 O5 phase (at 20°) on the Nb containing sample found to be

Table 1 XRF results (within ± 2 % accuracy). Sample

Acronyms

V%

La%

Nb%

O2 %

Al%

VOx (15%)/La(1%)-γ Al2 O3 VOx (15%)-Nb(3%)/La(1%)-γ Al2 O3 VOx (15%)/La(1%)-γ Al2 O3 Reduced in TPR at 750 °C VOx (15%)-Nb(3%)/La(1%)-γ Al2 O3 Reduced in TPR at 750 °C

V(f) VNb(f) V(r) VNb(r)

13.8 14.1 13.8 14.1

0.87 0.93 0.87 0.93

3.21 3.21

45.6 43.4 -

36.52 41.64 36.52 41.64

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Fig. 3. XRD patterns of (a) V(f) (b) V(r) (c) VNb(f) (d) VNb(r) samples.

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bond and is related to isolated VOx [38-41]. It can be clearly seen in Fig. 4 that both the un-promoted and Nb promoted samples exhibit sharp peaks in the range of crystalline V2 O5 , which is mainly due to the high vanadium loading on the support. This is in agreement with the previous studies, which stated that increasing metal loading to certain level (approximately 10 wt %) lead to the formation of poly-vanadate species [24,28,36]. The active species are isolated VOx , which appears as amorphous phase and not detected in XRD. These species have been detected by Raman spectroscopy as V=O and V–O–Al bonds vibrations which are relevant to isolated VOx at 1130 cm−1 [26,36]. The peak at 1030 cm−1 is also relevant to isolated VOx species [26,36]. Surface vanadium oxide layer on oxide supports is more likely to be formed than crystalline V2 O5 due to surface mobility of vanadium oxide and the lower surface free energy of crystalline V2 O5 (8–9 × 10−6 J/cm2 ) relative to supports (Al2 O3 ∼ 68–70 × 10−6 J/cm2 ; ZrO2 ∼ 59–80 × 10−6 J/cm2 ; TiO2 ∼28–38 × 10−6 J/cm2 )[38]. Nb2 O5 was detected in the range between 700 and 740 cm−1 , however treatment of sample at temperature higher than 500 °C may produce crystal phase of Nb2 O5 [39]. Thus, Nb has two important contributions (i) isolation of VOx which helps to form their monolayer and (ii) formation of secondary oxides Nb2 O5 , which also participates in the ethane ODH reaction [26,18]. 3.1.4. Catalyst reducibility and oxygen carrying capacity As described in the introduction section, the reducibility and oxygen carrying capacity of the catalyst are the two important aspects in the context of the gas phase oxygen free ODH reaction. In this regard, temperature programmed reduction (TPR) and temperature programmed oxidation (TPO) can experimentally simulate the reduction/oxidation of the catalysts as expected during the actual ODH reaction with ethane (Eqs. 3 and 4). TPR:

V2 O5 + 2 H2 → V2 O3 + 2 H2 O

(3)

Gas phase oxygen free ODH:

CH3 –CH3 + 0.5 V2 O5 →0.5 V2 O3 + CH2 =CH2 + H2 O

Fig. 4. Raman spectroscopy of catalyst samples (a) V(f) (b) VNb(f).

significantly lower (Fig. 3a) than that of the unpromoted sample (Fig. 3c), indicating a positive influence of Nb on the formation VOx monolayer by isolating them forming multilayers [36]. The peaks in the range of 22–27° are relevant to β -(Nb,V)2 O5 phase [37]. The other peaks of crystalline Nb2 O5 were not detected by XRD due to small crystal sizes as mention in literature [25,28]. The XRD pattern of the reduced sample of the un-promoted V(r) shows peaks of γ -Al2 O3 phase as indicated as indicated by the symbol () [28], and c-Al2 O3 at 78° as indicated by the symbol () [24]. On the other hand, no significant phase change was observed in the Nb promoted VNb(r) sample (Fig. 3c and d). This observation is consistent to what is reported in references [24,28]. 3.1.3. Raman spectroscopy The presence of different VOx species on the La-γ Al2 O3 support surface was examined by Raman spectroscopy analysis as shown in Fig. 4. The Raman spectra of both the un-promoted V (f) and Nb promoted VNb(f) contain different types of peaks confirming the existence of different VOx species present on support surface. The sharp peaks at 132 cm−1 are due to the vibration of V6 O13 [32]. The peaks in the range of 20 0–60 0 cm−1 represent the vibrations of crystalline VOx . The peaks at 995 cm−1 is due to V=O

(4)

One can see that both the TPR (Eq. 3) and ODH of ethane (Eq. 4) reduce V2 O5 to V2 O3 . The TPO cycle (Eq. 5) represents the catalyst regeneration cycle following the reduction in TPR. TPO:

V2 O3 + O2 →V2 O5

(5)

Considering the above, the prepared samples were characterized by temperature programmed reduction (TPR) using 10% H2 and 90% Ar gas mixture. In TPR analysis, the samples were exposed up to 750 °C to determine the reduction temperature range of the samples. Fig. 5 shows TPR profiles of V(f) and VNb(f) catalyst samples. One can see that the un-promoted (no Nb) sample reduces in the temperature range of 40 0–60 0 °C (Fig. 5a), while the Nb containing sample reduces between 500 and 700 °C (Fig. 5b). Both the samples shows multiple peaks indicating different vanadium oxides such as V2 O5 , VO4 , Nb2 O5 and β -(Nb,V)2 O5 . The presence of some of these species was also confirmed by the XRD and Raman analysis. Most of these oxide phases are responsible for the overall catalytic activity and selectivity of the catalysts [18,24,26]. However, the crystalline V2 O5 phase only has a little contribution to the catalytic activity and selectivity [24,26,31]. Therefore, it is important to ensure the minimization of the crystalline V2 O5 during the catalysis synthesis as well as during the repeated reduction (ODH) and oxidation (catalyst regeneration) cycles. In literature there are mechanisms available about the phase transformation of vanadia

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H2 Consumption (cm3/g)

6.5

note that the catalyst oxygen-carrying capacity is of great importance for the implementation of gas phase oxygen free ODH in a fluidized bed process. The oxygen-carrying capacity determines the circulation rate of the catalyst between twins fluidized bed reactors. The oxygen carrying capacity of the catalysts can be calculated by integration of the area under the TPR profile of the respective sample. It appears that the H2 consumption by the Nb containing VNb(f) sample (79 cm3 /g) is significantly higher than that of the V(f) sample. The higher hydrogen consumption of the Nb promoted sample can be attributed to the better dispersion of VOx and the Nb oxides, which contributed to the additional lattice oxygen carrying capacity [18,24]. The percentage of vanadium oxide reduction can be calculated using the following relation:

a cycle1 cycle2

6

cycle3

cycle4

5.5

cycle5 5

4.5

4 300

400

500

600

700

800

T (oC)

b Cylce 1 Cylce 2

7

Cylce 3 Cylce 4

6

% reduced =

Mwv × VH2

v × Vg × Wo

× 100

(6)

where (1) WV is the amount of reduced vanadium (g), (2) Mwv is the molecular weight of vanadium (g/mol), (3) VH2 is the volume of reacted hydrogen (cm3 at STP), (4) Vg is the molar volume of gas (mol/cm3 at STP). (5) Wo is initial weigh of vanadium (g) and (6) v is the stoichiometric number of hydrogen based on the following reaction stoichiometry. Assuming that V2 O5 is the initial reducible catalyst species present on the support [24], then the following reduction equation applies: (7)

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H2 Consumption, (cm3/g)

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Cylce 5

V2 O5 + 2H2 → V2 O3 + 2H2 O

5

(7)

800

Over the repeated TPR/TPO cycles, the percentage of total VOx species reductions was found to be 58% and 71% for V(f) and VNb(f) catalysts, respectively. The higher percentage of reduction of the Nb containing sample was due to the increased number of isolated active VOx sites [31,33].

species [31]. Usually the phase transformation occurs during the reduction steps, which create oxygen vacancies at the surface [31]. When the concentration of the oxygen vacancies surpasses a certain critical value, they aggregate into a vacancy disc, called shear plane. Part of vanadium oxide may shear so that along the shear plane the linkage between trigonal bipyramids is changed from corner-sharing into edge-sharing. Thus, another stable structure is formed, which is stoichiometrically different from original structure [31]. In order to assess the possible alumina thermal phase transformation, samples were exposed up to 750 °C in the TPR/TPO cycles. The unchanged TPR profiles in Fig. 5 confirmed that the catalyst remained stable even at high temperature (750 °C). As can be seen from Fig. 5b, the performance of sample containing Nb decreased slightly during the initial cycle and that could be due to effect of Nb by formation of Nb vanadate, which is more difficult to re-oxidize [30,31]. Other than that the repeated redox cycles are closed to each other with the same patterns. These very close TPR profiles showed a consistent metal reduction and confirmed the good thermal stability of the catalyst sample. It was hypothesized that the presence of La provides the thermal stability of the alumina support, which facilitate the stable reduction behavior of both the Nb promoted and un-promoted catalysts. Furthermore, VOx reducibility is expected to be increased since the dispersion and the isolation of VOx species enhance by Nb addition. In addition to the catalyst stability, the repeated TPR/TPO measurements also allow determining oxygen-carrying capacity and the redox properties of the catalyst under study. One should

3.1.5. Catalyst acidity The acidity, acid strength and the metal support interactions of the V(f) and VNb(f) catalysts were assessed by NH3 -TPD analysis. Ammonia was considered as probe molecule due to its strong basicity and small molecular size. Ammonia also allows to determine the total acidity and strength of acid cites for wide range of temperatures [42–45]. Fig. 6 displays the TPD profiles of the catalyst samples at two different heating rates. One can see, both the V(f) and VNb(f) samples exhibit similar desorption profiles at deferent heating rates. One can see from the TPD profiles that the unpromoted V(f) sample gives a clear NH3 desorption peak at 195 °C and an overlapped peak (appeared as a hump) at 295 °C. The Nb promoted VNb(f) also shows a peak around at 195° but the second peak almost has disappeared. For both the samples, the first peaks (at 195 °C) indicate the availability of weak acid sites. The overlapped peak at 295 °C indicates the availability of small amount of strong acid sites, which have been diminished due to the addition of Nb. It is also clear from the TPD profiles that weak acids sites are dominant than the strong sites especially after Nb addition (Fig. 6b). This might be advantageous for the ODH catalysts given that the strong acid sites favor the undesired cracking reactions on catalyst surface. The volume of ammonia adsorbed/desorbed was calculated by integrating the area under the TPD curves using appropriate calibration of TCD signal to corresponding volume of ammonia concentration. After VOx loading, the total acidity of the alumina support dropped from 14.39 ml NH3 ml/g.cat to 9.8 ml NH3 ml/g.cat [24]. The total acidity of the catalyst sample further dropped to 8.7 ml NH3 ml/g.cat after introduction of Nb to the V (f) catalysts sample. The decreasing trend of the total acidity is mainly due to the coverage of the acid site by the metal crystals [24].

4

3 300

400

500

600

700

T (oC) Fig. 5. TPR profiles of (a) V(f) and (b) VNb(f) in repeated TPR/TPO cycles.

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7

dT =β dt





and;

dθdes dt

(10)



 =

dθdes dT



dT dt



 =β

dθdes dT

 (11)

Substituting Eqs. (10) and (11) into (8) gives:

dθdes dt



=





kdes,0 1 −Edes 1 − θ exp Vm × β des R T Tm



β

(12)

where

θdes = 1 − 

Vdes Vm

(13)

Combining Eqs. (12) and (13) results in;

dVdes dT



=

kdes,0

β



1−







Vdes 1 −Edes 1 exp − Vm R T Tm

 (14)

Eq. (14) was fitted to experimental NH3 -TPD data (at heating rates of 10 and 15 °C/min) using Mathematica parametric NonlinearModelFit build-in function. In all experiments, ammonia preadsorbed at 120 °C. It is found that the estimated activation energy of ammonia desorption of the Nb containing sample is significantly higher (171.4 kJ/mol) than the activation energy of the un-promoted sample (126.2 kJ/mol). This increased activation energy of desorption is an indication of stronger V–support interaction than V–V interaction as a consequence of the isolation of VOx species due to the presence of Nb. This observation is consistent to the XRD and N2 adsorption isotherms results, which indicate smaller amount of crystalline V2 O5 with the Nb containing sample [43,47]. 3.2. Catalyst activity for ODH of ethane Fig. 6. TPD spectra of NH3 desorption for (a) V(f) and (b) VNb(f) catalyst samples (ammonia adsorbed at 120 °C).

3.1.6. NH3 desorption kinetics The NH3 -TPD data was further analyzed to estimate desorption kinetics parameters (activation energy of desorption and frequency factors). The activation energy of desorption is important to assess the metal support–metal support interaction, which plays important role on ethane conversion and product selectivity in gas phase oxygen free ODH reactions. The model as described in [42–46] was used to estimate the desorption parameters by setting the following assumptions; I. II. III. IV.

Homogeneous catalyst surface, kd = (–Edes/RT). Ammonia doesn’t re-adsorb during experiment. Uniform adsorbate concentration in the gas flow. First order adsorption rate in surface coverage.

A high gas flow rate was maintained together with appropriate conditions to satisfy the previous assumptions, and by performing species balances on desorbing NH3 as and following same steps mention in [24], Edes and kdes,0 can be obtained;



rdes = −Vm

dθdes dt



 −E

= kd,0 θdes exp

des

R

1 T

−

1 Tm



(8)

where, (1) θ des is the surface coverage, (2) Kd , kd are the desorption constant and pre-exponential factor respectively, (3)Tm is centering temperature. The desorption is related to the heating rate (β °C/min) according to the following equation:

T = To + β t

(9)

The gas phase oxygen free ODH of ethane experiments were carried out in a fluidized CREC Riser Simulator using pure ethane (99.95% purity). Before performing the catalytic runs blanks experiments were conducted by injecting ethane in to the reactor. There was no significant ethane conversion observed. The products contain mainly ethane and a trace amount of methane most likely due thermal cracking of ethane in absence of catalyst. In the catalytic runs, the reaction temperature was varied from 525 to 600 °C while the contact time was adjusted between 10 and 50 s. After each ODH run, the catalyst was regenerated by flowing air (for 15 min) through the catalyst bed. Therefore, the alternative ODH and catalyst regeneration were achieved in a same reactor without circulating the catalyst. The product analysis of the preliminary experiments show that C2 H6 , C2 H4 , CO2 and CO are the major products of the gas phase oxygen free ODH of ethane reaction. The experimental repeats show that the results are within 3% error limit. Mass balances were established for each experimental runs and the mass balance closes consistently in excess of 95%. Based on the product analysis, one can consider the following possible reactions steps under the studied reaction conditions. Reactions involved in the ODH of ethane cycle: Desired reaction (ethane to ethylene):

C2 H6 + 0.5 V2 O5 → C2 H4 + H2 O + 0.5 V2 O5 → Hr = 58.9 kJ/mol Undesirable reactions (ethane to COx ):

C2 H6 + 3.5 V2 O5 → 2CO2 + 3H2 O + 3.5 V2 O5 → Hr °= 289.9 kJ/mol C2 H6 + 2.5 V2 O5 → 2CO + 3H2 O + 2.5 V2 O5 → Hr °= –48.1 kJ/mol

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Fig. 8. Effects of Nb loading on ethane conversion and product selectivity (w = 0.3, t = 10 s, T = 550 °C g, feed = 3 ml, ●ethane conversion, x ethylene, CO2 , ♦CO selectivities) (experimental repeats show that the results are within 3% error limit).

Fig. 7. Temperature effect on ODH reaction over (a) V(f) (b) VNb(f) (t = 10 s, w = 0.3 g, feed = 3 ml, ●ethane conversion, x ethylene, CO2 , ♦CO selectivities) (experimental repeats show that the results are within 3% error limit).

Reactions involved in the catalyst regeneration cycle:

V2 O3 + O2 → V2 O5 Hr = –324.7 kJ/mol Fig. 7 presents the ethane conversion and product selectivity at various reaction temperature and constant 10 s reaction time. Expectedly, ethane conversion increased with the increasing reaction temperature as the lattice oxygen of the catalyst activates at higher temperature (Fig. 5, TPR analysis). At all temperature levels, the Nb promoted VNb(f) catalyst gives slightly higher ethylene conversion than that of the un-promoted V(f) catalyst. The higher ethane conversion with the Nb containing catalyst is possibly due to the contribution of Nb2 O5 and β -(Nb,V)2 O5 ) as detected by Raman and XRD. Previous study also reported that Nb oxides can contribute to dehydrogenation reaction both as source of lattice oxygen and active sites for catalyzing the reaction [36]. Regarding the product selectivity, the Nb containing catalyst shows higher ethylene selectivity as compared to the un-promoted catalyst. As seen in Fig. 7, with V(f) catalyst, maximum 76% ethylene selectivity was achieved (Fig. 7a) while the ethylene selectivity have gone up to 85.7% with the Nb promoted VNb(f) catalyst (Fig. 7b). This observation is also consistent to the XRD and TPR results of the Nb promoted and un-promoted catalysts. The presence of Nb influences the VOx dispersion forming more isolated VOx species, which favors the ethylene formation and suppress the complete oxidation to COx . On the contrary, the unpromoted sample contains more crystalline V2 O5 species (XRD), which do not contribute to converting ethane to ethylene. Consequently, the ethylene selectivity with Nb promoted sample is significantly

higher than that of the unpromoted sample. Previous investigations also support the formation of isolated VOx species and their positive influence on selective oxidation process [47–49]. The higher reduction temperature of VNb(f) allows the controlled reaction between the lattice oxygen and ethylene forming ethylene and suppressing COx formation [24,25,26]. Furthermore, the increased V-support interaction with the Nb promoted sample, as revealed by the TPD kinetics analysis, also explain the control ODH reaction with the lattice oxygen of the catalyst, resulting enhanced ethylene selectivity. According to the literature, the high performance catalyst in selective oxidation has isolated active sites to minimize electron influx and to activate reactants (feed and oxygen) to activate substrate molecule [50–52]. Introduction of Nb as promoter enhanced catalyst performance as shown earlier, and it was reported that promoters enhance the ODH over VOx catalyst by two mechanisms; (1) isolation of VOx species which increase catalyst activity and (2) formation of secondary active metal oxides contributing to ODH reaction. Fig. 8 presents the effects of Nb on ethane conversion and products selectivity. With the increase of Nb, ethane conversion slightly increased but the selectivity of ethylene increased significantly, especially between 1 wt% to 3 wt% Nb. Introducing Nb to V(f) sample improved VOx isolation on support surface and enhanced the V=O and V–O–Al formation, which favored ethylene formation and suppressed COx formation. Keeping the price of Nb in mind we restricted the Nb loading at 3 wt%. Similar observation was also reported in the literature using Cr, Mo and W promoters to isolate VOx on alumina support [26,33,46]. It was confirmed that the promoted catalysts have greater intrinsic activity for ethane activation than that of the un-promoted catalysts [26,33,46]. Also, the addition of Nb also blocked the acid sites as observed in NH3 -TPD analysis and decreased the total acidity, which also have contributed to the higher ethylene selectivity by minimizing the cracking reactions. The absence of methane (the possible cracked product) in the product gas confirms the minimum cracking [6,33,46]. Table 2 presents the effects of reaction time on ethane conversion and product selectivity at constant temperature of 550 °C. As can be noticed in Table 2, both samples show increasing ethane conversion as the reaction times was increased. Ethane conversion with the V(f) sample was increased from 16.4% at 10 s to 23% when

Please cite this article as: A.H. Elbadawi et al., VOx -Nb/La-γ Al2 O3 catalysts for oxidative dehydrogenation of ethane to ethylene, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.01.003

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9

Table 2 Effects of reaction time on catalyst performance at 550 °C. Catalyst

Time(s)

Ethane conversion %

Ethylene selectivity%

CO2 selectivity %

CO selectivity %

VOx /La-γ Al2 O3

10 20 30 40 50

16.4 18.5 19.8 20.8 23.0

71.1 65.9 63.8 61.4 60.2

54 55.5 60 61.4 70.5

30.5 32.5 30.7 31.4 26.6

VOx -Nb/La-γ Al2 O3

10 20 30 40 50

20.1 21.0 25.0 28.2 34.5

85.7 71.2 65.4 62.2 64.1

52.9 54.5 61.4 68.6 75.3

23.8 30.0 28.8 26.4 21.9

Fig. 9. Effect of feed injection on ODH reaction over (a) V(f) (b) VNb(f) (T = 550 °C, t = 10 s, w = 0.3 g, feed = 3 ml).

reaction time was 50 s. With Nb addition, the sample VNb(f) gives higher ethane conversion of 20.1% at 10 s and 34.5% at 50 s. The increasing trend of ethane conversion can be attributed to production of more COx as the ethylene selectivity was not further increased. With increasing the reaction time, the product ethylene goes further oxidation producing more COx . One should remember, in the context of gas phase oxygen free ODH, the source of lattice oxygen is catalyst. On the other hand, the lattice oxygen is present in the form of nucleophilic (O2− , O− ) and electrophilic (O2 − ) form and they are selective to different oxidation products. At the early stage, the release of oxygen favors ethylene formation. With increasing reaction time, the oxygen released by the catalysts allow the formation of more COx . From kinetics viewpoint, the ODH reaction involves several steps and the rate limiting step is substrate activation. The activation of C–H bond of ethane manly depends on catalyst and oxygen active species present on the surface [27,49]. 3.2.1. Effects of feed injection and reaction/regeneration cycles The effects of the feed to catalyst ratio were studied by varying the amount of ethane injected at constant catalyst weight, temperature and reaction time. Fig. 9 shows the ethane conversion and ethylene selectivity for the V(f)and VNb(f) catalyst samples with various amount of ethylene injection. One can see that the ethane conversion is inversely proportional to the amount of feed injected. This was expected, because the excess amount of injected ethane remained unreacted due to the unavailability of lattice oxygen for the additional amount of ethane. However, ethylene selectivity was slightly affected by feed increasing (from 67.8% to 73.1%) for the

Fig. 10. Effects of reaction/regeneration on ODH reaction over (a) V(f) (b) VNb(f) (T = 550 °C, t = 10 s, w = 0.3 g, feed = 3 ml).

sample without Nb. Selectivity for sample with Nb was increased from 83.1% to 86.45% at 3 ml feed injection due to the availability of in which has more active VOx site are available of feed dehydrogenation. Fig. 10 shows the repeated ODH of ethane reaction-regeneration cycles on the ethane conversion and product selectivity. After each ODH run the catalyst regenerated in air at same reaction temperature for 15 mins. The catalyst performance slightly decreased after the first cycle, following that it remains stable in the repeated ODH/catalyst regeneration cycles. The stable ethane conversion and product selectivity are inline with the stable behavior of the catalyst as observed in TPR/TPO cycles (Fig. 5). In Addition, XRD shows no significant phase change for the sample containing Nb after ODH reaction as mentioned earlier. The regeneration in this case is oxidation of the reduced V on support surface, which depends on catalyst stability at high temperature. It is reported that certain phase transformation might occur for the support and the active phases as well where VOx isolated species agglomerate to form crystal phase which decreases catalyst performance by decreasing the total number of active sites [24,25,50]. In the context of the present study, the stable behavior of the catalyst indicates the absence of phase transformation and/or activity loss in the high temperature ODH and catalyst regeneration cycles. Table 3 shows a comparison of ethylene selectivity and yield of the present study with those reported in the literature. It is noticed the Nb–VOx /La–Al2 O3 catalyst displayed good performance as compared to the catalysts in the literature under similar conditions. Also, Nb–VOx /La–Al2 O3 catalyst shows stable

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A.H. Elbadawi et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–11 Table 3 Comparison of the performance of Nb-VOx /La-γ Al2 O3 with different catalysts in literature. Catalysts

Catalyst weight (g)

Ethane conversion (%)

Ethylene selectivity (%)

Ethylene yield (%)

References

Nb-VOx /La-γ Al2 O3 BaCl2 –TiO2 –SnO2 VOx /γ -Al2 O3 VOx /c-Al2 O3 VOx –MoOx /γ -Al2 O3

0.4 0.8 0.5 0.76 0.4

20 10 11 6.5 7.5

85 92.6 82 84.5 68.2

17.0 9.2 9.0 5.0 5.1

Present study [19] [36] [24] [25]

performance over time due to controlled oxygen release as can be seen in Fig. 10. On the contrary, the other catalysts suffer from deactivation. 4. Conclusions Nb promoted VOx /La-γ Al2 O3 catalyst have been synthesized, characterized and evaluated for ODH of ethane under gas phase oxygen free environment in a fluidized CREC-Riser Simulator. The XRD analysis of the synthesized catalyst confirmed the presence of various VOx species and (Nb,V)2 O5 . The addition of Nb helped to minimize the formation of the crystalline V2 O5 by isolating VOx species and forming their monolayer. Nb2 O5 was detected using Raman spectrum between 700 and 710 cm−1 . TPR results showed good catalyst reduction activity and re-oxidation behavior, which was further improved after Nb addition. NH3 -TPD measurements confirm that the addition of Nb decreased samples acidity which prevents cracking of the feed or the product. The desorption kinetics suggest that Nb increased the V-support interaction and minimizes V–V interaction which enhanced the monolayer formation. The ODH of ethane in free oxygen atmosphere shows that ethylene, CO and CO2 are the major products. VNb(f) catalyst gives the highest ethylene selectivity (85.7%) at 20.1% conversion. This result is in agreement with TPR analysis, which shows best reducibility (highest H2 consumption) for this catalyst. This highest ethylene selectivity was obtained at 10 s reaction time and temperature of 550 °C. Nb addition enhanced VOx species isolation on support surface and created secondary Nb2 O5 oxide, which further increased catalyst activity. The Nb also affects the metal support interaction and plays a critical role in lowering VOx reducibility, and increasing stability. The control reaction of the lattice oxygen and ethane helped to enhance ethylene selectivity. Acknowledgment The author(s) would like to acknowledge the financial support provided by King AbdulAziz City for Science and Technology (KACST) to this project under research grant number AT-32-67. The authors would also like to thank the Center of Research Excellence in Nanotechnology (CoRE-NT), Research Institute, KFUPM, for the technical supports to carry out some of the catalyst characterizations. References [1] Mulla SAR, Buyevskaya O V, Baerns M. A comparative study on non-catalytic and catalytic oxidative dehydrogenation of ethane to ethylene. Appl. Cata. A. Gen. 2002;226:73–8. [2] Loukah M, Vedrine JC, Ziyad M. Oxidative dehydrogenation of ethane on Vand Cr-based phosphate catalysts. Micropourus Materials 1995;4:345–58. [3] Ciambelli P, Galli P, Lisi L, Massucci MA, Patrono P, Pirone R, Ruoppolo G, Russo G. TiO2 supported vanadyl phosphate as catalyst for oxidative dehydrogenation of ethane to ethylene. Appl. Cata. A. Gen. 20 0 0:133–42. [4] Miller JE, Gonzales MM, Evans L, Sault AG, Zhang C, Rao R, Whitwell G, Maiti A, King-Smith D. Oxidative dehydrogenation of ethane over iron phosphate catalysts. Appl. Catal. A Gen. 2002;231:281–92. [5] Bortolozzi JP, Gutierrez LB, Ulla MA. Synthesis of Ni/Al2 O3 and Ni–Co/Al2 O3 coatings onto AISI 314 foams and their catalytic application for the oxidative dehydrogenation of ethane. Appl. Catal. A Gen. 2013;452:179–88.

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Please cite this article as: A.H. Elbadawi et al., VOx -Nb/La-γ Al2 O3 catalysts for oxidative dehydrogenation of ethane to ethylene, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.01.003