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MgO in the oxidative dehydrogenation of propane to olefins
JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 44, Issue 11, November 2016 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2016, 44(11), 13341340
RESEARCH PAPER
Influence of Li loading on the catalytic performance of Li/MgO in the oxidative dehydrogenation of propane to olefins WU Yi-min, LI Shuo, LI Chun-yi* State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, China
Abstract:
A series of Li/MgO catalysts with different Li loadings were prepared by incipient wetness impregnation method and
characterized by TG-DTA, N2 sorption and XRD; two modes for propane adsorption on Li/MgO were considered by calculation with Material Studio and the influence of Li loading on the catalytic performance of Li/MgO in the oxidative dehydrogenation of propane to olefins was investigated. The result indicated that with the increase of Li loading, the conversion of propane and the selectivity to C2H4, C2H6, CH4, COx increases at first, reaches the highest values at a Li loading of 3% and then decreases with further increasing the Li loading, whereas the selectivity to propene changes in an opposite trend. The adsorption and dehydrogenation of propane on Li/MgO surface are controlled by both thermodynamic and kinetic factors, whilst the dispersion of the active Li +O− sites is related to the loading of Li. Over the highly-dispersed active Li+O− sites, the dehydrogenation is thermodynamically controlled, which favors the formation of propene, whereas over the poorly-dispersed Li+O− sites, the reaction is dominated by the kinetic factor, leading to a high selectivity to ethene and other by-products. Key words:
propane; oxidative dehydrogenation; Li/MgO; olefins; Li loading; dispersion
Propene and ethene are important basic chemicals, which are now mainly produced by traditional steam cracking of petroleum[1,2]; however, the steam cracking process requires high temperature and high energy consumption, which causes severer environmental concerns[3]. With the fast development of the shale-gas exploiting technology in American in recent years, the price of propane decreases considerably, resulting in an increasing interest in the conversion of propane[4]. Although the catalytic dehydrogenation of propane (DH) has been industrialized for many years[5], the conversion of propane is strongly restricted by the thermodynamic limitation; high temperature is necessary to raise the yield of olefins, resulting in high energy consumption. In contrast, oxidative dehydrogenation of propane (ODH) can not only overcome the thermodynamic limitation, but also reduce coke deposition and inhibit the catalyst deactivation[6]. Although ODH is an attractive pathway to producing olefins, few achievements have been made in the redox-type catalysts including V and Mo, which are extensively studied. This kind of catalysts normally has high activity, but low selectivity to olefins; the yield of olefins is in general lower
than 30%, as the primary products are hard to be desorbed from the catalysts[7,8]. In comparison, Li/MgO catalyst, which was originally applied in the oxidative coupling of methane (OCM)[9–11], exhibited both high activity in ODH and selectivity to olefins[12]; however, the OCM and ethane ODH reactions were still restrained by the high reaction temperature (> 700°C) and the active component Li was easily lost at this temperature[13]. Meanwhile, it was also reported that Li/MgO performed excellently in ODH at lower temperatures (< 700°C)[14–16], especially when rare earth elements were added as the promoter; the yield of olefins could reach 50%. Differing from those over the V and Mo catalysts[5,15,17,18], the ODH reaction over the Li/MgO catalysts might follow the radical mechanism, leading to high content of ethene in the product (1 < propene/ethene < 4.5). Previously, we found that the content of Li could dramatically affect the reactivity and product distribution in ODH of propane. In this work, a series of Li/MgO catalysts with different Li loadings were prepared by incipient wetness impregnation method and characterized by TG-DTA, N2 sorption and XRD. Combining with theoretical calculation, the influence of Li loading on the
Received: 23-Jun-2016; Revised: 21-Jul-2016. Foundation item: Supported by the Key Project of National Nature Science Foundation of China Petroleum and Chemical Joint Funds (U1362201). *Corresponding author. Tel: 13225324293, E-mail: [email protected]. Copyright 2016, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
WU Yi-min et al / Journal of Fuel Chemistry and Technology, 2016, 44(11): 13341340
catalytic performance of Li/MgO in the oxidative dehydrogenation of propane to olefins was then investigated.
1 1.1
Experimental section Catalyst preparation
The catalysts were prepared by incipient wetness impregnation method using aqueous solution of LiNO 3 (Shanghai Shunfeng Chemical Technology Co., Ltd., analytical grade) as the Li precursor and light MgO (Chinese Chemical Reagent Co., Ltd., analytical grade) as the support. After impregnation, the samples were kept at room temperature for 4 h, and then dried at 140°C and calcinated at 600°C for 3 h. The obtained catalysts with the Li loadings of 0, 2, 8, 30, 80, 120, 250 and 300 mmol-Li/mol-MgO are denoted as M0, M2, M8, M12, M25 and M30, respectively, whereas the corresponding un-calcined samples are recorded as m0, m2, m8, m12, m25 and m30, respectively. 1.2
Catalyst test
The propane ODH reaction was performed under atmosphere in a fixed-bed reactor, viz., a quartz tube with 6 mm internal diameter. The catalyst bed (200 mg) was packed between two quartz-sand plugs (> 180 m) to reduce any reactions in gaseous phase. It was demonstrated that the catalytic activity of Li/MgO was quite low at 550°C[19], but comparatively high at 600°C. At such a temperature, the conversion of propane in the absence of oxygen was negligible (< 1%). Before each test, the catalyst was pretreated in air flow at 550°C for 30 min; after that, two streams of gas were mixed and introduced into the reaction system, with a propane/oxygen molar ratio of 2 and a total flow rate of 30 mL/min (WHSV = 8.9 h−1). The products were analyzed with a gas chromatograph (GC 450 from Brucker). 1.3
Catalyst characterization
Fig. 1
TG-DTA analysis was conducted on a DTU-2A differential thermogravimetric analyzer. The catalyst was filled in a quartzose crucible, with another blank crucible as the reference. The catalyst sample was heated from 50 to 800°C at a ramp of 10°C/min in air (80 mL/min, 0.8 MPa). The surface area and pore structure were determined at Quadrasorb SI multi-functional adsorption apparatus by measuring the N2 adsorption-desorption isotherms at −196°C. The sample was degassed ahead of measurement at 300°C for 5 h to remove the adsorbed moisture. The surface area was calculated by the 5 point BET method. The XRD patterns were collected on an X’Pert PRO MPO diffractometer, operated at 40 kV and 40 mA using Cu K radiation with a scanning speed of 10(°)/min, from 5°to 75°. 1.4
Theoretical calculation
The size of propane was calculated with Material Studio (MS) 8.0 using module Dmol 3 based on density functional theory. The Perdew-Wang 91 (PW 91) function of the generalized gradient approximation (GGA) was employed for the Kohn-Sham exchange correlation of electrons ignoring the inner core electrons. Valence electron wave function was expanded by the double-number basis set and polarization function (DNP); for the structure optimization, the convergence criteria of energy, force and displacement were 1×10−5 Ha, 0.02 Ha/nm and 0.0005 nm, respectively.
2 2.1
Results and discussion Catalyst characterization
The TG-DTA profiles of un-calcined MgO (impregnated with water) are shown in Figure 1(a); an endothermic peak with weight loss at 417.9°C is observed, which is attributed to the dehydration of surface Mg(OH)2. Figure 1(b) shows the TG-DTA profiles of the precursor LiNO3; the endothermic peak at 267.1°C is ascribed to the fusion of LiNO3, whereas the peaks above 500°C are attributed to the decomposition of LiNO3.
TG-DTA profiles of MgO (a) and LiNO3 (b)
WU Yi-min et al / Journal of Fuel Chemistry and Technology, 2016, 44(11): 13341340
Fig. 3
Sketch of hydrolyzation, dehydration and denitrification of MgO loading with LiNO3
Fig. 2
DTA profiles of the un-calcined Li/MgO catalysts with different Li loadings a: m2; b: m8; c: m30; d: m80; e: m120; f: m250; g: m300
The DTA profiles of the un-calcined Li/MgO catalysts with different Li loadings are displayed in Figure 2. The dehydration peak shifts from high temperature (417.9°C) to lower one (309.0°C) with the increase of Li loading to 3%; after that, the Li/MgO catalysts with higher Li loadings exhibit similar DTA profiles. As illustrated in Figure 3, Li species could interact with OH− on the catalyst surface and reduce the electronic density around oxygen, thus weakening the ionic bond between OH− and Mg2+. With a Li loading below 3%, the interaction between Li species and superficial OH− was enhanced by increasing the Li loading, making the dehydration peak shift to lower temperature; however, when the Li loading was higher than 3%, Li species began to aggregate, resulting in a weakened interaction between surface OH− and Li species. Meanwhile, two endothermic peaks in the DTA profiles attributed to the decomposition of LiNO3 are observed at temperature above 500°C, which tend to shift to higher temperature with the increase of Li loading, indicating a stronger interaction between the support and LiNO3. Moreover, the highest decomposition temperature of supported LiNO3 (655.0°C, shown in Figure 2) was larger than that of pure LiNO3 (640.0°C, shown in Figure 1(b)), also suggesting a strong mutual effect between the support and LiNO3. Figure 4 shows the XRD patterns of the calcined Li/MgO catalysts with different Li loadings. Four diffraction peaks centered at 37.0°, 42.9°, 62.3°and 74.7°attributed to MgO are observed, which become shaper and narrower with the increase of Li loading to 3%, indicating an increase in the grain size of MgO. No characteristic diffraction peak of Li species is found, implying a high dispersion of Li species. However, with a Li loading above 3%, diffraction peaks of Li2CO3 appear at 21.4°, 30.6°, 31.8° and 37.0° and become more intense with the further increase of Li loading.
Fig. 4
XRD patterns of the Li/MgO catalysts with different of Li loadings : MgO; : Li2CO3; : Mg(OH)2
It should be noted that no peak of LiNO3 and its decomposition products (Li2O) was detected in the XRD patterns, since Li2O decomposed from LiNO3 reacted rapidly with CO2 in air to form Li2CO3 at room temperature. It was claimed that Li2CO3 could decompose into Li2O and CO2 above 500°C[19]; as a result, Li2O should be the main state at reaction temperature (600°C). Besides, the characteristic peaks of Mg(OH)2 are discovered at 18.6°, 33.0°, 38.0°, 51.0° and 58.8° with the increase of the Li loading to 8%, suggesting that the superfluous Li species might break the balance of original crystal structures, slack Mg–O bonds, and reinforce the alkalinity[20]. Hence, H2O was easier to be adsorbed on the electronegative oxygen, forming Mg(OH)2, which was further decomposed to MgO and H2O at a temperature above 500°C[21]. Although some oxygen species were generated during this process, the surface had already been covered by vast of inert Li species, which gave the poor reaction activity. As given in Table 1, the surface area and the pore volume of the Li/MgO catalysts decrease dramatically with the increase of Li loading; the Li/MgO catalyst with a Li loading of 30% only has a surface area of 1 m2/g and a pore volume of 0.01 cm3/g, suggesting the blocking of the original pores in MgO.
WU Yi-min et al / Journal of Fuel Chemistry and Technology, 2016, 44(11): 13341340 Table 1 Sample
Specific surface area and superficial density of the Li/MgO catalysts with different Li loadings
Li loading wmol/%
ABET /(m2g–1)
Pore volume
7.04×10–25
0.15
0.43
5.16×10–24
1.11
1.05
M2
0.2
71
M8
0.8
39
M80 M120 M250 M300
8 12 25 30
10 5 2 1
Atomic Li numbers per
0.60
107
16
–2
crystal lattice
0
3
Li superficial density /(molnm )
M0
M30
–1
v/(cm g ) 3
0.13 0.09 0.02 0.03 0.01
These results further confirm the strong interaction between Li species and MgO support. 2.2 Catalytic performance of Li/MgO with different Li loadings The initial performances of the Li/MgO catalysts with different Li loadings in propane ODH at 600°C are displayed in Figure 5. With the increase of Li loading from 0 to 0.2%, the conversion of propane increases dramatically from 2% to 41.4%; it reaches the highest value of 53.6% at a Li loading of 3% and decreases gradually with further increasing the Li loading. Besides, the selectivity to ethene, methane, ethane and COx shows a similar trend. On the contrary, the selectivity to propene exhibits an opposite trend. As reported in earlier works[13,22], the radius of Li+ (0.076 nm) is approximately equal to that of Mg2+ (0.072 nm), which makes it capable of being inserted into MgO crystallite, generating deficiency in MgO lattice and creating the active sites, as illustrated in Figure 6. The bond lengths and bond angles of propane were calculated by MS (Figure 6(a)), which indicates that the molecular dimension is about 0.433 (L) × 0.178 (W) × 0.262 (H) nm. According to the crystal structure of MgO, anion O2− formed surface cubic stacking and cationic Mg2+ occupied octahedral gap (Figure 6(b)); the distance between two adjacent O2− is 0.424 nm (a), which turns into 0.432 nm (a) after Mg2+ is replaced by Li+, suggesting L > a > a. To analyze the dispersion of Li species, it was assumed that all of the Li species are present in the form of Li2CO3 on the catalyst surface. On the basis of surface area (ABET), the superficial density are then determined, as given in Table 1. According to the lattice size of MgO (0.424 × 0.424 nm, Figure 6(b)), the superficial density of MgO is 9.24×10 −24 mol/nm2. The number of Li atoms per crystal lattice is far larger than 1 over Li/MgO with a Li loading of 3%, suggesting that the Li species are not distributed in monolayer on the support surface. Wang et al[22] found that the Li+O−
4.73×10 2.06×10 6.60×10 2.87×10 8.48×10
–23 –22 –22 –21 –21
10.08 42.87 134.61 550.25 1591.36
active center was stable only at temperature over 567°C, supporting that Li can insert into MgO lattice at reaction condition in this work, forming the active sites. As Li+ and Mg2+ ions are similar in their radius, the calculated superficial density of Li species is quite reasonable. It was generally acknowledged that there were two pathways for propane ODH over the Li/MgO catalysts[23,24]. The hydrogen atom of the secondary carbon was first adsorbed on the Li+O− active site (type-Z adsorption), which was then abstracted by oxygen ions, forming hydroxyl groups and i-propyl radicals; after that, the scission of C–H bond at the -position yielded propene and H radical. The hydrogen atom of the primary carbon was adsorbed (type-B adsorption) and abstracted by Li+O−, producing hydroxyl groups and n-propyl radicals; after that, ethene and methyl radical was obtained by a fast C–C cleavage in the -position and the methyl radicals went through a series of reactions with oxygen or other radicals in gas phase, forming COx, CH4, C2H4 and so on. As the rupture of C–H was the rate-determining step, the adsorption state then determined the product distribution. Evidently, the active sites are well dispersed on Li/MgO at lower Li loading (< 0.2%). There are two types of propane adsorption on the catalyst surface; as the C–H bond energy of methene is 15 kJ/mol lower than that of methyl[25], thermodynamically, type-Z adsorption takes the superiority at low Li loading, favoring the formation of propene. With the increase of Li loading from 0.2% to 3%, the catalyst surface area is reduced and the superficial density of Li species is increased; the number of Li atoms per crystal lattice exceeds 1 for a Li loading within this range (Table 1). At this condition, the steric hindrance of type-Z adsorption is greater than that of type-B adsorption (Figure 6(c), 6(e)); two propane molecules are hard to be adsorbed on adjoining active sites via type-Z adsorption and the reaction via the type-B adsorption was much easier, viz., the reaction is dominated by the dynamic factor. As a result, the selectivity to ethane, ethene, methane and COx are enhanced over Li/MgO with a high Li loading.
WU Yi-min et al / Journal of Fuel Chemistry and Technology, 2016, 44(11): 13341340
Fig. 5
Fig. 6
Performance of Li/MgO catalysts with different Li loadings in propane ODH
Schematic diagrams of (a) propane molecule, (b) lattice structure of MgO, (c) reaction process over Li/MgO with low Li loading, (d) and (e) reaction process over Li/MgO with high Li loading
The results of N2 sorption, TG-DTA and XRD illustrate that the surface area is decreased remarkably and the Li species is no longer presented as monolayer on Li/MgO with a Li loading higher than 3%. Additional Li species can not enter the MgO lattice; what’s more, Li2O decomposed from Li2CO3 at reaction condition was nearly inert[9]. The inert Li species are then accumulated on the catalyst surface, leading to the decrease of catalytic activity and propane conversion. Additionally, the inert Li2O dispersed on the catalyst surface had a geometric effect on active site; just like the Sn-separated Pt catalyst[26], the active Li+O− site is separated by the inert component, which promotes indirectly the formation of active sites that facilitate the type-Z adsorption and then enhance the selectivity to propene.
3
Conclusions
A series of Li/MgO catalysts with different Li loadings were prepared by incipient wetness impregnation method; the
influence of Li loading on the catalytic performance of Li/MgO in the oxidative dehydrogenation (ODH) of propane to olefins was investigated. The result indicated that with the increase of Li loading, the conversion of propane and the selectivity to C2H4, C2H6, CH4, COx increases at first, reaches the highest values at a Li loading of 3% and then decreases with further increasing the Li loading, whereas the selectivity to propene changes in an opposite trend. The adsorption and dehydrogenation of propane on Li/MgO surface are controlled by both thermodynamic and kinetic factors. The dispersion of the active site Li +O−, which was created by replacing Mg with Li in the MgO lattice, is related to the loading of Li. Over the highly-dispersed active Li+O− sites, the dehydrogenation is thermodynamically controlled, which favors the formation of propene, whereas over the poorly-dispersed Li+O− sites, the reaction is dominated by the kinetic factor, leading to a high selectivity to ethene and other by-products. As a result, an increase in the surface area and improvement in the dispersion of Li species is probably
WU Yi-min et al / Journal of Fuel Chemistry and Technology, 2016, 44(11): 13341340
effective to enhance the conversion of propane and selectivity to propene for propane ODH over the Li/MgO catalyst.
propane. Langmuir, 2008, 24(15): 8220–8228 [14] Landau M V, Kaliya M L, Oosterkamp P F V D, Bocque P S G, Herskowitz M. Produce light olefins from paraffins by catalytic
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RESEARCH PAPER
Influence of Li loading on the catalytic performance of Li/MgO in the oxidative dehydrogenation of propane to olefins WU Yi-min, LI Shuo, LI Chun-yi* State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, China
Abstract:
A series of Li/MgO catalysts with different Li loadings were prepared by incipient wetness impregnation method and
characterized by TG-DTA, N2 sorption and XRD; two modes for propane adsorption on Li/MgO were considered by calculation with Material Studio and the influence of Li loading on the catalytic performance of Li/MgO in the oxidative dehydrogenation of propane to olefins was investigated. The result indicated that with the increase of Li loading, the conversion of propane and the selectivity to C2H4, C2H6, CH4, COx increases at first, reaches the highest values at a Li loading of 3% and then decreases with further increasing the Li loading, whereas the selectivity to propene changes in an opposite trend. The adsorption and dehydrogenation of propane on Li/MgO surface are controlled by both thermodynamic and kinetic factors, whilst the dispersion of the active Li +O− sites is related to the loading of Li. Over the highly-dispersed active Li+O− sites, the dehydrogenation is thermodynamically controlled, which favors the formation of propene, whereas over the poorly-dispersed Li+O− sites, the reaction is dominated by the kinetic factor, leading to a high selectivity to ethene and other by-products. Key words:
propane; oxidative dehydrogenation; Li/MgO; olefins; Li loading; dispersion
Propene and ethene are important basic chemicals, which are now mainly produced by traditional steam cracking of petroleum[1,2]; however, the steam cracking process requires high temperature and high energy consumption, which causes severer environmental concerns[3]. With the fast development of the shale-gas exploiting technology in American in recent years, the price of propane decreases considerably, resulting in an increasing interest in the conversion of propane[4]. Although the catalytic dehydrogenation of propane (DH) has been industrialized for many years[5], the conversion of propane is strongly restricted by the thermodynamic limitation; high temperature is necessary to raise the yield of olefins, resulting in high energy consumption. In contrast, oxidative dehydrogenation of propane (ODH) can not only overcome the thermodynamic limitation, but also reduce coke deposition and inhibit the catalyst deactivation[6]. Although ODH is an attractive pathway to producing olefins, few achievements have been made in the redox-type catalysts including V and Mo, which are extensively studied. This kind of catalysts normally has high activity, but low selectivity to olefins; the yield of olefins is in general lower
than 30%, as the primary products are hard to be desorbed from the catalysts[7,8]. In comparison, Li/MgO catalyst, which was originally applied in the oxidative coupling of methane (OCM)[9–11], exhibited both high activity in ODH and selectivity to olefins[12]; however, the OCM and ethane ODH reactions were still restrained by the high reaction temperature (> 700°C) and the active component Li was easily lost at this temperature[13]. Meanwhile, it was also reported that Li/MgO performed excellently in ODH at lower temperatures (< 700°C)[14–16], especially when rare earth elements were added as the promoter; the yield of olefins could reach 50%. Differing from those over the V and Mo catalysts[5,15,17,18], the ODH reaction over the Li/MgO catalysts might follow the radical mechanism, leading to high content of ethene in the product (1 < propene/ethene < 4.5). Previously, we found that the content of Li could dramatically affect the reactivity and product distribution in ODH of propane. In this work, a series of Li/MgO catalysts with different Li loadings were prepared by incipient wetness impregnation method and characterized by TG-DTA, N2 sorption and XRD. Combining with theoretical calculation, the influence of Li loading on the
Received: 23-Jun-2016; Revised: 21-Jul-2016. Foundation item: Supported by the Key Project of National Nature Science Foundation of China Petroleum and Chemical Joint Funds (U1362201). *Corresponding author. Tel: 13225324293, E-mail: [email protected]. Copyright 2016, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
WU Yi-min et al / Journal of Fuel Chemistry and Technology, 2016, 44(11): 13341340
catalytic performance of Li/MgO in the oxidative dehydrogenation of propane to olefins was then investigated.
1 1.1
Experimental section Catalyst preparation
The catalysts were prepared by incipient wetness impregnation method using aqueous solution of LiNO 3 (Shanghai Shunfeng Chemical Technology Co., Ltd., analytical grade) as the Li precursor and light MgO (Chinese Chemical Reagent Co., Ltd., analytical grade) as the support. After impregnation, the samples were kept at room temperature for 4 h, and then dried at 140°C and calcinated at 600°C for 3 h. The obtained catalysts with the Li loadings of 0, 2, 8, 30, 80, 120, 250 and 300 mmol-Li/mol-MgO are denoted as M0, M2, M8, M12, M25 and M30, respectively, whereas the corresponding un-calcined samples are recorded as m0, m2, m8, m12, m25 and m30, respectively. 1.2
Catalyst test
The propane ODH reaction was performed under atmosphere in a fixed-bed reactor, viz., a quartz tube with 6 mm internal diameter. The catalyst bed (200 mg) was packed between two quartz-sand plugs (> 180 m) to reduce any reactions in gaseous phase. It was demonstrated that the catalytic activity of Li/MgO was quite low at 550°C[19], but comparatively high at 600°C. At such a temperature, the conversion of propane in the absence of oxygen was negligible (< 1%). Before each test, the catalyst was pretreated in air flow at 550°C for 30 min; after that, two streams of gas were mixed and introduced into the reaction system, with a propane/oxygen molar ratio of 2 and a total flow rate of 30 mL/min (WHSV = 8.9 h−1). The products were analyzed with a gas chromatograph (GC 450 from Brucker). 1.3
Catalyst characterization
Fig. 1
TG-DTA analysis was conducted on a DTU-2A differential thermogravimetric analyzer. The catalyst was filled in a quartzose crucible, with another blank crucible as the reference. The catalyst sample was heated from 50 to 800°C at a ramp of 10°C/min in air (80 mL/min, 0.8 MPa). The surface area and pore structure were determined at Quadrasorb SI multi-functional adsorption apparatus by measuring the N2 adsorption-desorption isotherms at −196°C. The sample was degassed ahead of measurement at 300°C for 5 h to remove the adsorbed moisture. The surface area was calculated by the 5 point BET method. The XRD patterns were collected on an X’Pert PRO MPO diffractometer, operated at 40 kV and 40 mA using Cu K radiation with a scanning speed of 10(°)/min, from 5°to 75°. 1.4
Theoretical calculation
The size of propane was calculated with Material Studio (MS) 8.0 using module Dmol 3 based on density functional theory. The Perdew-Wang 91 (PW 91) function of the generalized gradient approximation (GGA) was employed for the Kohn-Sham exchange correlation of electrons ignoring the inner core electrons. Valence electron wave function was expanded by the double-number basis set and polarization function (DNP); for the structure optimization, the convergence criteria of energy, force and displacement were 1×10−5 Ha, 0.02 Ha/nm and 0.0005 nm, respectively.
2 2.1
Results and discussion Catalyst characterization
The TG-DTA profiles of un-calcined MgO (impregnated with water) are shown in Figure 1(a); an endothermic peak with weight loss at 417.9°C is observed, which is attributed to the dehydration of surface Mg(OH)2. Figure 1(b) shows the TG-DTA profiles of the precursor LiNO3; the endothermic peak at 267.1°C is ascribed to the fusion of LiNO3, whereas the peaks above 500°C are attributed to the decomposition of LiNO3.
TG-DTA profiles of MgO (a) and LiNO3 (b)
WU Yi-min et al / Journal of Fuel Chemistry and Technology, 2016, 44(11): 13341340
Fig. 3
Sketch of hydrolyzation, dehydration and denitrification of MgO loading with LiNO3
Fig. 2
DTA profiles of the un-calcined Li/MgO catalysts with different Li loadings a: m2; b: m8; c: m30; d: m80; e: m120; f: m250; g: m300
The DTA profiles of the un-calcined Li/MgO catalysts with different Li loadings are displayed in Figure 2. The dehydration peak shifts from high temperature (417.9°C) to lower one (309.0°C) with the increase of Li loading to 3%; after that, the Li/MgO catalysts with higher Li loadings exhibit similar DTA profiles. As illustrated in Figure 3, Li species could interact with OH− on the catalyst surface and reduce the electronic density around oxygen, thus weakening the ionic bond between OH− and Mg2+. With a Li loading below 3%, the interaction between Li species and superficial OH− was enhanced by increasing the Li loading, making the dehydration peak shift to lower temperature; however, when the Li loading was higher than 3%, Li species began to aggregate, resulting in a weakened interaction between surface OH− and Li species. Meanwhile, two endothermic peaks in the DTA profiles attributed to the decomposition of LiNO3 are observed at temperature above 500°C, which tend to shift to higher temperature with the increase of Li loading, indicating a stronger interaction between the support and LiNO3. Moreover, the highest decomposition temperature of supported LiNO3 (655.0°C, shown in Figure 2) was larger than that of pure LiNO3 (640.0°C, shown in Figure 1(b)), also suggesting a strong mutual effect between the support and LiNO3. Figure 4 shows the XRD patterns of the calcined Li/MgO catalysts with different Li loadings. Four diffraction peaks centered at 37.0°, 42.9°, 62.3°and 74.7°attributed to MgO are observed, which become shaper and narrower with the increase of Li loading to 3%, indicating an increase in the grain size of MgO. No characteristic diffraction peak of Li species is found, implying a high dispersion of Li species. However, with a Li loading above 3%, diffraction peaks of Li2CO3 appear at 21.4°, 30.6°, 31.8° and 37.0° and become more intense with the further increase of Li loading.
Fig. 4
XRD patterns of the Li/MgO catalysts with different of Li loadings : MgO; : Li2CO3; : Mg(OH)2
It should be noted that no peak of LiNO3 and its decomposition products (Li2O) was detected in the XRD patterns, since Li2O decomposed from LiNO3 reacted rapidly with CO2 in air to form Li2CO3 at room temperature. It was claimed that Li2CO3 could decompose into Li2O and CO2 above 500°C[19]; as a result, Li2O should be the main state at reaction temperature (600°C). Besides, the characteristic peaks of Mg(OH)2 are discovered at 18.6°, 33.0°, 38.0°, 51.0° and 58.8° with the increase of the Li loading to 8%, suggesting that the superfluous Li species might break the balance of original crystal structures, slack Mg–O bonds, and reinforce the alkalinity[20]. Hence, H2O was easier to be adsorbed on the electronegative oxygen, forming Mg(OH)2, which was further decomposed to MgO and H2O at a temperature above 500°C[21]. Although some oxygen species were generated during this process, the surface had already been covered by vast of inert Li species, which gave the poor reaction activity. As given in Table 1, the surface area and the pore volume of the Li/MgO catalysts decrease dramatically with the increase of Li loading; the Li/MgO catalyst with a Li loading of 30% only has a surface area of 1 m2/g and a pore volume of 0.01 cm3/g, suggesting the blocking of the original pores in MgO.
WU Yi-min et al / Journal of Fuel Chemistry and Technology, 2016, 44(11): 13341340 Table 1 Sample
Specific surface area and superficial density of the Li/MgO catalysts with different Li loadings
Li loading wmol/%
ABET /(m2g–1)
Pore volume
7.04×10–25
0.15
0.43
5.16×10–24
1.11
1.05
M2
0.2
71
M8
0.8
39
M80 M120 M250 M300
8 12 25 30
10 5 2 1
Atomic Li numbers per
0.60
107
16
–2
crystal lattice
0
3
Li superficial density /(molnm )
M0
M30
–1
v/(cm g ) 3
0.13 0.09 0.02 0.03 0.01
These results further confirm the strong interaction between Li species and MgO support. 2.2 Catalytic performance of Li/MgO with different Li loadings The initial performances of the Li/MgO catalysts with different Li loadings in propane ODH at 600°C are displayed in Figure 5. With the increase of Li loading from 0 to 0.2%, the conversion of propane increases dramatically from 2% to 41.4%; it reaches the highest value of 53.6% at a Li loading of 3% and decreases gradually with further increasing the Li loading. Besides, the selectivity to ethene, methane, ethane and COx shows a similar trend. On the contrary, the selectivity to propene exhibits an opposite trend. As reported in earlier works[13,22], the radius of Li+ (0.076 nm) is approximately equal to that of Mg2+ (0.072 nm), which makes it capable of being inserted into MgO crystallite, generating deficiency in MgO lattice and creating the active sites, as illustrated in Figure 6. The bond lengths and bond angles of propane were calculated by MS (Figure 6(a)), which indicates that the molecular dimension is about 0.433 (L) × 0.178 (W) × 0.262 (H) nm. According to the crystal structure of MgO, anion O2− formed surface cubic stacking and cationic Mg2+ occupied octahedral gap (Figure 6(b)); the distance between two adjacent O2− is 0.424 nm (a), which turns into 0.432 nm (a) after Mg2+ is replaced by Li+, suggesting L > a > a. To analyze the dispersion of Li species, it was assumed that all of the Li species are present in the form of Li2CO3 on the catalyst surface. On the basis of surface area (ABET), the superficial density are then determined, as given in Table 1. According to the lattice size of MgO (0.424 × 0.424 nm, Figure 6(b)), the superficial density of MgO is 9.24×10 −24 mol/nm2. The number of Li atoms per crystal lattice is far larger than 1 over Li/MgO with a Li loading of 3%, suggesting that the Li species are not distributed in monolayer on the support surface. Wang et al[22] found that the Li+O−
4.73×10 2.06×10 6.60×10 2.87×10 8.48×10
–23 –22 –22 –21 –21
10.08 42.87 134.61 550.25 1591.36
active center was stable only at temperature over 567°C, supporting that Li can insert into MgO lattice at reaction condition in this work, forming the active sites. As Li+ and Mg2+ ions are similar in their radius, the calculated superficial density of Li species is quite reasonable. It was generally acknowledged that there were two pathways for propane ODH over the Li/MgO catalysts[23,24]. The hydrogen atom of the secondary carbon was first adsorbed on the Li+O− active site (type-Z adsorption), which was then abstracted by oxygen ions, forming hydroxyl groups and i-propyl radicals; after that, the scission of C–H bond at the -position yielded propene and H radical. The hydrogen atom of the primary carbon was adsorbed (type-B adsorption) and abstracted by Li+O−, producing hydroxyl groups and n-propyl radicals; after that, ethene and methyl radical was obtained by a fast C–C cleavage in the -position and the methyl radicals went through a series of reactions with oxygen or other radicals in gas phase, forming COx, CH4, C2H4 and so on. As the rupture of C–H was the rate-determining step, the adsorption state then determined the product distribution. Evidently, the active sites are well dispersed on Li/MgO at lower Li loading (< 0.2%). There are two types of propane adsorption on the catalyst surface; as the C–H bond energy of methene is 15 kJ/mol lower than that of methyl[25], thermodynamically, type-Z adsorption takes the superiority at low Li loading, favoring the formation of propene. With the increase of Li loading from 0.2% to 3%, the catalyst surface area is reduced and the superficial density of Li species is increased; the number of Li atoms per crystal lattice exceeds 1 for a Li loading within this range (Table 1). At this condition, the steric hindrance of type-Z adsorption is greater than that of type-B adsorption (Figure 6(c), 6(e)); two propane molecules are hard to be adsorbed on adjoining active sites via type-Z adsorption and the reaction via the type-B adsorption was much easier, viz., the reaction is dominated by the dynamic factor. As a result, the selectivity to ethane, ethene, methane and COx are enhanced over Li/MgO with a high Li loading.
WU Yi-min et al / Journal of Fuel Chemistry and Technology, 2016, 44(11): 13341340
Fig. 5
Fig. 6
Performance of Li/MgO catalysts with different Li loadings in propane ODH
Schematic diagrams of (a) propane molecule, (b) lattice structure of MgO, (c) reaction process over Li/MgO with low Li loading, (d) and (e) reaction process over Li/MgO with high Li loading
The results of N2 sorption, TG-DTA and XRD illustrate that the surface area is decreased remarkably and the Li species is no longer presented as monolayer on Li/MgO with a Li loading higher than 3%. Additional Li species can not enter the MgO lattice; what’s more, Li2O decomposed from Li2CO3 at reaction condition was nearly inert[9]. The inert Li species are then accumulated on the catalyst surface, leading to the decrease of catalytic activity and propane conversion. Additionally, the inert Li2O dispersed on the catalyst surface had a geometric effect on active site; just like the Sn-separated Pt catalyst[26], the active Li+O− site is separated by the inert component, which promotes indirectly the formation of active sites that facilitate the type-Z adsorption and then enhance the selectivity to propene.
3
Conclusions
A series of Li/MgO catalysts with different Li loadings were prepared by incipient wetness impregnation method; the
influence of Li loading on the catalytic performance of Li/MgO in the oxidative dehydrogenation (ODH) of propane to olefins was investigated. The result indicated that with the increase of Li loading, the conversion of propane and the selectivity to C2H4, C2H6, CH4, COx increases at first, reaches the highest values at a Li loading of 3% and then decreases with further increasing the Li loading, whereas the selectivity to propene changes in an opposite trend. The adsorption and dehydrogenation of propane on Li/MgO surface are controlled by both thermodynamic and kinetic factors. The dispersion of the active site Li +O−, which was created by replacing Mg with Li in the MgO lattice, is related to the loading of Li. Over the highly-dispersed active Li+O− sites, the dehydrogenation is thermodynamically controlled, which favors the formation of propene, whereas over the poorly-dispersed Li+O− sites, the reaction is dominated by the kinetic factor, leading to a high selectivity to ethene and other by-products. As a result, an increase in the surface area and improvement in the dispersion of Li species is probably
WU Yi-min et al / Journal of Fuel Chemistry and Technology, 2016, 44(11): 13341340
effective to enhance the conversion of propane and selectivity to propene for propane ODH over the Li/MgO catalyst.
propane. Langmuir, 2008, 24(15): 8220–8228 [14] Landau M V, Kaliya M L, Oosterkamp P F V D, Bocque P S G, Herskowitz M. Produce light olefins from paraffins by catalytic
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