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Performance improvement for a fixed-bed reactor with layered loading catalysts of different catalytic properties for oxidative coupling of methane
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Performance improvement for a fixed bed reactor with layered loading catalysts of different catalytic properties for oxidative coupling of methane ⁎
Wugeng Liang , Sagar Sarsani, David West, Aghaddin Mamedov, Istvan Lengyel, Hector Perez, James Lowrey SABIC Corporate R & D, 1600 Industrial Boulevard, Sugar Land, TX 77478, United States
A R T I C L E I N F O
A B S T R A C T
Keywords: Oxidative coupling of methane Mn-Na2WO4/SiO2 catalyst Ag and Ce promoted catalysts La-Ce oxide catalyst Layered loading
Layered loading of catalysts with different catalytic properties is studied for oxidative coupling of methane reaction (OCM) in a fixed bed reactor. Layered loading of a more active catalyst Ag-Mn-Na2WO4/SiO2 in front of a more selective one Ce-Mn-Na2WO4/SiO2 compared to loading Mn-Na2WO4/SiO2 catalyst only, lowers reaction temperature and increases selectivity. Such layered loading broadens the temperature range to achieve a high selectivity, which is important for OCM reaction carried out with multi-stage adiabatic fixed bed reactor. Similarly, with a very active La-Ce oxide catalyst loaded in front of a selective catalyst Ce-Mn-Na2WO4/SiO2, the temperature needed to start the reaction is lowered significantly. Therefore, a catalyst bed such as La-Ce oxide can be used as a “pre-heater” for the high temperature OCM reaction to lower the feed temperature. The total CH4 utilization can be improved with this loading strategy.
1. Introduction As a result of recent shale gas boom, more and more natural gas is being produced. But natural gas generally is used as fuel for electricity generation. There is a strong desire to increase the value of this abundant hydrocarbon resource. Oxidative coupling of methane (OCM) is one of the way to transform methane, the principle component of natural gas, to more valuable products, such as ethylene. Although close to 40 years extensive research and development has been devoted to this technology [1], it has never been commercialized. Efficiency of transformation from methane to ethylene, even when using the best catalyst today, is still low. By using catalysts with different properties, catalyst activity or selectivity can be improved, but it is very challenging to gain activity and selectivity at the same time. As we know, OCM reaction is a multi-step process, involving consecutive reactions starting from CH4 activation to form CH3 radicals, to coupling to form C2H6, to C2H6 dehydrogenation or oxidative dehydrogenation to form C2H4 [2]. Further oxidation or reforming of the coupling products produces the unwanted CO and CO2. For example, when a fixed bed reactor is used for this reaction, there will be a profile of different product species along the reactor [3]. Therefore, it is difficult achieve the best performance by using a single catalyst, because of different properties are needed at the different sections of the bed. On this basis, one of approaches to improve performance, may be ⁎
use of catalysts with different properties, such as combination of two catalysts, one with high activity and another with high selectivity. In this work, we studied the OCM reaction with layered loading strategy in a fix bed reactor: loading catalysts with different properties in different sections in the reactor. Two cases are experimentally studied. In the first case, two catalysts with similar catalytic properties are studied with layered loading. In the second case, two catalysts with quite different catalytic properties are studied. Potential benefits for practical applications from layered loading are discussed. 2. Experimental 2.1. Catalyst preparations Four Mn-Na2WO4/SiO2 based catalysts were studied, all catalyst samples were prepared by using incipient wetness impregnation method [4]. La-Ce oxide catalyst is prepared by drying and calcining the mixture solution of raw material nitrates. 2.1.1. Catalyst 1. Mn-Na2WO4/SiO2 (1.9%Mn-5.0%Na2WO4/SiO2) 18.62 g of Davisil Grade 646 silica gel was used as catalyst support. Before use the silica gel was dried overnight. 1.73 g of Mn(NO3)2·4H2O was dissolved in 18.6 ml of DI water. The Mn(NO3)2·4H2O solution was added into silica gel dropwise to achieve a uniform distribution of the solution onto the solids. Then the mixture was dried overnight at
Corresponding author. E-mail address: [email protected] (W. Liang).
http://dx.doi.org/10.1016/j.cattod.2017.03.058 Received 27 October 2016; Received in revised form 12 March 2017; Accepted 27 March 2017 0920-5861/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Liang, W., Catalysis Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.03.058
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125 °C. 1.13 g of Na2WO4·2H2O was dissolved in 18.6 ml of DI water. The solution obtained was added into the dried Mn containing silica gel dropwise. The mixture was dried at 125 °C again overnight. Then the dried material was calcined at 800 °C for 6 h. The calcined material was ready for performance test.
were mixed and then heated at 85 °C for 2 h under agitation. The obtained mixture was then dried overnight at 125 °C to yield a dried powder, which was then calcined at 625 °C for 5 h to yield the catalyst.
2.1.2. Catalyst 2. Ag-Mn-Na2WO4/SiO2 (1.0% Ag-1.9%Mn-5.0% Na2WO4/SiO2) 15.75 g of Davisil Grade 646 silica gel was used as catalyst support. Before use the silica gel was dried overnight. 1.48 g of Mn(NO3)2·4H2O was dissolved in 15.8 ml of DI water. The Mn(NO3)2·4H2O solution was added into silica gel dropwise to achieve a uniform distribution of the solution onto the solids. Then the mixture was dried overnight at 125 °C. 0.27 g of AgNO3 was dissolved in 15.8 ml of DI water. The AgNO3 solution was added into silica gel dropwise to achieve a uniform distribution of the solution onto the solids. Then the mixture was dried overnight at 125 °C. 0.96 g of Na2WO4·2H2O was dissolved in 18.6 ml of DI water. The solution obtained was added into the dried Mn containing silica gel dropwise. The mixture was dried at 125 °C again overnight. Then the dried material was calcined at 800 °C for 6 h. The calcined material was ready for performance test.
Experiments were carried out in quartz tube fixed bed reactor of 4 mm inner diameter. Gases are fed to the reactor by the use of mass flow controllers. The reactor is heated by using conventional clamshell furnace. The furnace has three heating zones. During reaction, the top and bottom zones are set at 300 °C. Then middle zone is set at the reaction temperature. Due to the small inner diameter reactor used, there is no thermocouple inside the reactor, the temperature reported is the middle zone furnace set point temperature. We measured the temperature inside the reactor before reaction in nitrogen feed, the temperature inside is 50 °C higher than the furnace temperature. The necessary reaction temperature was settled by using the constant-temperature furnace set up described above which was varied from experiment to experiment, similar method was used in [6]. The catalyst bed is located in the center of the middle zone furnace. On the top and bottom of the catalyst bed, quartz chips are loaded to reduce the empty volume. On the both ends of the quartz chip sections, quartz wools are used to hold the catalyst bed and quartz chips to the targeted position. Catalyst performance test was carried out under atmosphere pressure. The reaction products were analyzed by using online Gas Chromatography (Agilent 6890). Two columns are used to separate the products. Molecular sieve 13X column is used for permanent gases analysis, CO, CO2, O2 and Ne. They are detected with TCD detector. HPQ-Plot column is used for hydrocarbon products analysis. Products observed are C2H4, C2H6, C3H6, C3H8, C4's and C5's. These products are detected with FID detector. C2+ selectivity reported below is the total selectivity to the hydrocarbons from C2 to C5. Catalyst loading strategy and different testing conditions used are described below.
2.2. Catalyst testing
2.1.3. Catalyst 3. Ce-Mn-Na2WO4/SiO2 (5.0%Ce-1.9%Mn-5.0% Na2WO4/SiO2) 17.62 g of Davisil Grade 646 silica gel was used as catalyst support. Before use the silica gel was dried overnight. 1.74 g of Mn(NO3)2·4H2O was dissolved in 17.6 ml of DI water. The Mn(NO3)2·4H2O solution was added into silica gel dropwise to achieve a uniform distribution of the solution onto the solids. Then the mixture was dried overnight at 125 °C. 3.10 g of Ce(NO3)3·6H2O was dissolved in 17.6 ml of DI water. The Ce(NO3)3 solution was added into silica gel dropwise to achieve a uniform distribution of the solution onto the solids. Then the mixture was dried overnight at 125 °C. 1.12 g of Na2WO4·2H2O was dissolved in 17.6 ml of DI water. The solution obtained was added into the dried Mn containing silica gel dropwise. The mixture was dried at 125 °C again overnight. Then the dried material was calcined at 800 °C for 6 h. The calcined material was ready for performance test.
2.2.1. Testing of Mn-Na2WO4/SiO2 and promoted Mn-Na2WO4/SiO2 catalysts The first condition was used for testing Mn-Na2WO4/SiO2 and promoted Mn-Na2WO4/SiO2 catalysts with 100 mg catalyst loading. For layered loading studies, two catalysts were loaded in layers, one (with 50 mg) was loaded in the front section of the bed and the other (with 50 mg) loaded on the back. The flow rates of gases used for this condition are shown in Table 1, with Ne as the internal standard. The methane to oxygen ratio is 7.4 under this condition. By varying the reaction temperature, performances under different temperatures are obtained.
2.1.4. Catalyst 4. Ag-Ce-Mn-Na2WO4/SiO2 (1.0%Ag-5.0%Ce-1.9%Mn5.0%Na2WO4/SiO2) 17.6 g of Davisil Grade 646 silica gel was used as catalyst support. Before use the silica gel was dried overnight. 1.74 g of Mn(NO3)2·4H2O was dissolved in 17.6 ml of DI water. The Mn(NO3)2·4H2O solution was added into silica gel dropwise to achieve a uniform distribution of the solution onto the solids. Then the mixture was dried overnight at 125 °C. 3.10 g of Ce(NO3)3·6H2O and 0.32 g of AgNO3 was dissolved in 17.6 ml of DI water. This Ce(NO3)3 and AgNO3 solution was added into silica gel dropwise to achieve a uniform distribution of the solution onto the solids. Then the mixture was dried overnight at 125 °C. 1.12 g of Na2WO4·2H2O was dissolved in 17.6 ml of DI water. The solution obtained was added into the dried Mn containing silica gel dropwise. The mixture was dried at 125 °C again overnight. Then the dried material was calcined at 800 °C for 6 h. The calcined material was ready for performance test.
2.2.2. Testing of La-Ce oxide and Ce-Mn-Na2WO4/SiO2 based catalysts The second testing condition was applied for testing La-Ce oxide and Ce-Mn-Na2WO4/SiO2 system. For testing La-Ce oxide catalyst alone, 20 mg of catalyst was loaded. For layered loading, different amounts of La-Ce oxide were loaded in front of the bed with 240 mg of Ce-MnNa2WO4/SiO2 on the back. Ce-Mn-Na2WO4/SiO2 alone with 240 mg loading was also tested for comparison. The flow rates of gases at methane to oxygen ratio of 4.0 with Ne as the internal standard are
2.1.5. Comparative catalyst. Ag/SiO2 (3.0%Ag/SiO2) This catalyst is prepared for comparison. 3.00 g of Ag (15 nm) in 5% water was added into 3.00 g of silica gel which was calcined at 800 °C. After Ag addition, the mixture is dried overnight at 125 °C.
Table 1 Flow rates of feed with CH4:O2 = 7.4.
2.1.6. Catalyst 5. La-Ce oxide catalyst La-Ce oxide catalyst with La/Ce = 10 was used in this study. It was prepared using the method described in [5]. 24.06 g of La(NO3)3·6H2O was dissolved in 40 ml of DI water to yield a La(NO3)3 aqueous solution. 2.39 g of Ce(NO3)3·6H2O was dissolved in 10 ml of Di water to yield a Ce(NO3)3 aqueous solution. These two aqueous solutions 2
Gas component
Feed rate
Oxygen Methane Neon
3.9 sccm 28.9 sccm 0.5 sccm
Total flow
33.3 sccm
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Table 2 Flow rates of feed with CH4:O2 = 4.0. Gas component
Feed rate
Oxygen Methane Neon
15.8 sccm 63.4 sccm 1.0 sccm
Total flow
80.0 sccm
Fig. 3. C2+ selectivity under different temperatures with Mn-Na2WO4/SiO2, Ag-MnNa2WO4/SiO2 and Ag/SiO2.
Fig. 1. CH4 conversion under different temperatures with Mn-Na2WO4/SiO2, Ag-MnNa2WO4/SiO2, Ag/SiO2 and quartz chips only (blank test).
shown in Table 2. By varying the reaction temperature, performances under different temperatures were obtained. 3. Results and discussion Fig. 4. C2H6 selectivity under different temperatures with Mn-Na2WO4/SiO2, Ag-MnNa2WO4/SiO2 and Ag/SiO2.
3.1. Ag promotion on Mn-Na2WO4/SiO2 catalyst Mn-Na2WO4/SiO2 and Ag-Mn-Na2WO4/SiO2 catalysts each with 100 mg loading were tested with the first testing conditions shown in Table 1. The performances obtained are shown in Figs. 1–4 . Results obtained with quartz chips only are also included in Figs. 1 and 2 as blank test. From CH4 and O2 conversions shown in Figs. 1 and 2, it can be seen that the CH4 and O2 conversions obtained with only quartz chips at 800 °C are close to 0% and much lower than that obtained from catalysts, so that the contributions from the quartz chips and empty volumes in the reactor can be neglected. From CH4 and O2 conversions shown in Figs. 1 and 2, it can also be seen that higher conversions are obtained with Ag-Mn-Na2WO4/SiO2 under the same temperature than Mn-Na2WO4/SiO2; the same CH4 or O2 conversions can be achieved under a lower reaction temperature, indicating that Ag promotion improves catalyst activity. For comparison, the performances of Ag/ SiO2 obtained under the same condition are also shown in these figures. With Ag/SiO2, much lower conversion is obtained, indicating that the
improved performance of Ag-Mn-Na2WO4/SiO2 is not coming from Ag only, but the synergetic effect between Ag and Mn-Na2WO4/SiO2. The selectivity obtained under different temperatures with these catalysts is shown in Fig. 3. Comparing the selectivies obtained from Ag-Mn-Na2WO4/SiO2 and Mn-Na2WO4/SiO2, it can be seen that Ag promotion increases catalyst selectivity significantly when the reaction temperature is lower than 725 °C. This promotion effect is not clear when the reaction temperature is higher than 725 °C. The selectivity with Ag/SiO2 is shown in the same figure and it can be seen that very low selectivity is obtained. The mechanism of oxidative coupling of methane reaction has been described widely in many publications, it can be summarized into the following key steps [7–11]: [O]S + CH4 → [OH]S + CH3
(1)
2CH3 → C2H6
(2)
CH3 + O2 ↔ CH3O2
(3)
CH3 + [O]s ↔ [CH3O]s
(4)
2[OH]S + 1/2O2 → 2[O]S + H2O
(5)
The first step is the activation of methane with the participation of active oxygen sites [O]S, with the formation of methyl radical CH3 and hydroxyl group [OH]S. The gas phase reaction of coupling of methyl radicals to form the coupling product C2H6 has low activation energy, therefore does not limit reaction rate. The methyl radicals can react with gas phase oxygen to form CH3O2 (step (3)). They can also readsorb on to the catalyst surface and react with surface oxygen to form [CH3O]s (step (4)). Steps (3) and (4) are the main reactions controlling the selectivity of the different catalysts. Easy removal of methyl radicals
Fig. 2. O2 conversion under different temperatures with Mn-Na2WO4/SiO2, Ag-MnNa2WO4/SiO2, Ag/SiO2 and quartz chips only (blank test).
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from the oxygen centers will result in high C2+ selectivity; while at oxygen centers of low metal–O bond energy, oxidation of methyl radicals through [CH3O]s leads to formation of COx. Direction of step (3) depends on the temperature, while direction of step (4) depends both on temperature and on catalyst. These O-containing compounds are the precursors of deep oxidation products, such as CO and CO2. Therefore conversions of methyl radicals through steps (3) and (4) are the ones of the main selectivity loss. In addition to step (1), the activity of the catalyst is also influenced by the removal of the OH groups from the surface (step (5)), which reoxidizes the reduced sites back and complete the full cycle of the reaction. This step is usually determined by metal–O bond of the catalyst. The different functions of Ag in Ag-Mn-Na2WO4/SiO2 and Ag-SiO2 observed above can be explained by the mechanism above. In Ag/SiO2, Ag is the only active site available for OCM reaction. Since Ag is not a very good OCM catalyst by itself [12], very low activity and selectivity are obtained. In Ag-Mn-Na2WO4/SiO2, due to the superior activity of MnNa2WO4/SiO2 toward CH4 activation to form methyl radicals, there will be large amount of adsorbed OH group on the surface of the catalyst, reducing the catalyst activity. Since Ag has the function to oxidize the adsorbed OH group to water [13], Ag performs this function and re-activates the reduced activate sites, especially, under low reaction temperature. Therefore, it is highly possible that the Ag promotion in Ag-Mn-Na2WO4/SiO2 enhances the re-oxidation step of OCM reaction. With this promotion effect, the activity and selectivity are improved. The ethane selectivity obtained with these three catalysts are compared in Fig. 4. It can be seen that much higher ethane selectivity is obtained with Ag-Mn-Na2WO4/SiO2. This result can be explained by the synergistic effect proposed above as well. Ag promotion at lower reaction temperature mainly enhances re-oxidation activity, which leads to the increase of the rate of methyl radical formation, resulting in higher ethane formation rate in the products. At the same time, other functions, such as oxidative dehydrogenation and deep oxidation, do not increase proportionally with Ag promotion. Therefore, the ethane selectivity increases.
Fig. 6. O2 conversion under different temperatures with Mn-Na2WO4/SiO2, Ag-MnNa2WO4/SiO2, Ce-Mn-Na2WO4/SiO2, Ag-Ce-Mn-Na2WO4/SiO2, and layered loading of Ag-Mn-Na2WO4/SiO2 and Ce-Mn-Na2WO4/SiO2.
Fig. 7. C2+ selectivity under different temperatures with Mn-Na2WO4/SiO2, Ag-MnNa2WO4/SiO2, Ce-Mn-Na2WO4/SiO2, Ag-Ce-Mn-Na2WO4/SiO2, and layered loading of Ag-Mn-Na2WO4/SiO2 and Ce-Mn-Na2WO4/SiO2.
Ag promotion increases catalyst activity and selectivity at lower temperature, but with no effect on selectivity under high temperature as discussed before and also indicated in Figs. 5–7; Ce promotion increases catalyst selectivity but lowers catalyst activity. In order to take advantages of these two catalysts, experiments were carried out with layered loading: in the front of the bed (50% of the bed) Ag promoted catalyst was loaded and the other part of the bed (the remaining 50% of the bed) Ce promoted catalyst was loaded. The total catalyst loading in this layered loading experiment is the same as that of a single catalyst testing. Results obtained with these experiments are shown in Figs. 5–7. From Figs. 5 and 6, it can be seen that higher activity is obtained with the layered loading compared to Mn-Na2WO4/SiO2. At the same time, higher selectivity, both at lower temperature and higher temperature are obtained with the layered loading compared to Mn-Na2WO4/SiO2. Compared to Ag-Mn-Na2WO4/SiO2, layered loading has almost the same performance as Ag-Mn-Na2WO4/SiO2 at lower temperature, but higher selectivity is obtained with layered loading than Ag-MnNa2WO4/SiO2 at higher temperature. Compared to Ce-Mn-Na2WO4/SiO2, layered loading shows significantly higher performance at lower temperature, its selectivity is lower than Ce-Mn-Na2WO4/SiO2 only at very high temperature (750 °C or higher). Compared to the results obtained at total O2 consumption, the CH4 conversion is higher with layered loading than that with Mn-Na2WO4/ SiO2. This is due to the higher C2+ selectivity obtained with layered loading. With higher C2+ selectivity, less O2 is used for non-selective deep oxidation pathways under the same methane conversion, so that there are more O2 available for methane conversion. As a result, methane conversion is higher.
3.2. Layered loading of Ag and Ce promoted Mn-Na2WO4/SiO2 catalysts Promotion of Mn-Na2WO4/SiO2 catalyst by Ce is known and has been described in literature [14,15]. Ce promoted catalyst (Ce-MnNa2WO4/SiO2) which was prepared and investigated in these series of experiments shows higher C2+ selectivity (Fig. 7) compared to unpromoted Mn-Na2WO4/SiO2. But lower activity is obtained with Ce promotion, because a higher reaction temperature is needed to reach the same O2 or CH4 conversion, as indicated in Figs. 5 and 6 .
Fig. 5. CH4 conversion under different temperatures with Mn-Na2WO4/SiO2, Ag-MnNa2WO4/SiO2, Ce-Mn-Na2WO4/SiO2, Ag-Ce-Mn-Na2WO4/SiO2, and layered loading of Ag-Mn-Na2WO4/SiO2 and Ce-Mn-Na2WO4/SiO2.
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In order to further confirm the advantage of layered loading, we prepared a catalyst with composition of Ag-Ce-Mn-Na2WO4/SiO2, containing both promoters of Ag and Ce. Performances of this catalyst are also shown in Figs. 5–7. It can be seen that the activity of this catalyst is similar to that of Ag-Mn-Na2WO4/SiO2 and is more active compared to Ce-Mn-Na2WO4/SiO2. Its selectivity under high reaction temperature is the same as that of Mn-Na2WO4/SiO2 and Ag-MnNa2WO4/SiO2 and is lower than that of layered loading. From all these comparisons, it can be seen that layered loading shows enhanced performance compared to a single catalyst loading, indicating that layered loading can achieve the improved performance which a single catalyst cannot. The observed experimental results of layered loading could be explained well on the basis of reaction mechanism. As described in reaction steps (1)–(5), OCM reaction starts with methyl radical formation, coupling of which leads to the formation of ethane. Ethane is further converted to ethylene through parallel reactions of thermal dehydrogenation and catalytic oxidative dehydrogenation. Some portion of methyl radicals undergoes to deep oxidation through O-containing compounds CH3O2 and [CH3O]s as shown in steps (3) and (4). In addition to the reaction steps (3) and (4), some of the ethane and ethylene formed also undergo deep oxidation with the formation of CO and CO2 [3,16]. Due to the nature of the consecutive reactions, there is a distribution of products through contact time or through catalyst bed. On this basis, along the axial direction of a fixed bed reactor, there will be more ethane in the front of the bed and more deep oxidation products (CO and CO2) toward the end of the bed. If a more active ethane formation catalyst (like Ag-Mn-Na2WO4/SiO2) is loaded in the front of the bed, then it will enhance the ethane formation. If a less deep oxidation catalyst (like Ce-Mn-Na2WO4/SiO2) is loaded in the back of the catalyst bed, it will reduce the deep oxidation and enhance the overall selectivity. Therefore, the overall results obtained with loading of catalysts with different catalytic properties will lead to better performance than a single catalyst loading (like Mn-Na2WO4/SiO2). In summary, the layered loading is a strategy to load a specific catalyst with required performance at the specific reactor position. The strategy of catalyst layering can be extended to the case of multi catalysts loading to achieve more improved process performance.
Fig. 9. O2 conversions obtained under different temperatures with different catalyst loadings. A: La-Ce-oxide and B: Ce-Mn-Na2WO4/SiO2.
Fig. 10. C2+ selectivities obtained under different temperatures with different catalyst loadings. A: La-Ce-oxide and B: Ce-Mn-Na2WO4/SiO2.
3.3. Layered loading of La-Ce oxide and Ce-Mn-Na2WO4/SiO2 catalysts La-Ce oxide catalyst shows higher activity in OCM reaction compared to Mn-Na2WO4/SiO2 catalyst system, but has lower selectivity. We layered loading this catalyst with different amounts in front of CeMn-Na2WO4/SiO2 and results obtained are shown in Figs. 8–11 . Figs. 8 and 9 show the CH4 and O2 conversions under different temperatures. With La-Ce oxide loaded alone (20 mg), the reaction ignites at 450 °C and O2 conversion reaches close to 100%. At this temperature, the highest CH4 conversion (about 25%) is obtained. With
Fig. 11. Selectivity of C2H4 obtained under different temperatures with different catalyst loadings. A: La-Ce-oxide and B: Ce-Mn-Na2WO4/SiO2.
the increase of reactor temperature, due to the increase of consumption of oxygen in non-selective pathway, C2+ selectivity decreases as shown in Fig. 10, which leads to CH4 conversion decrease. Different performance was observed in the case of single loading of Ce-Mn-Na2WO4/SiO2 (240 mg loading). O2 conversion gradually increases with reactor temperature and reaches 100% at 725 °C. Due to the high C2+ selectivity obtained with this catalyst, high CH4 conversion is obtained under temperatures at and higher than 725 °C. With layered loading of 5.0 mg La-Ce oxide with 240 mg Ce-MnNa2WO4/SiO2, the reaction ignites at 550 °C, the ignition temperature is higher than that of 20 mg La-Ce oxide loading. The ignition temperature shifts to 650 °C with layered loading of 2.5 mg La-Ce oxide with 240 mg Ce-Mn-Na2WO4/SiO2. For the experiments with 20 mg, 5.0 mg and 2.5 mg La-Ce oxide loadings, the C2+ selectivity at the ignition temperatures are very close. After ignition, with increase reactor temperature, better selectivity is
Fig. 8. CH4 conversions obtained under different temperatures with different catalyst loadings. A: La-Ce-oxide and B: Ce-Mn-Na2WO4/SiO2.
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Fig. 13. Scheme of La-Ce oxide catalyst used as a “pre-heater”.
shown in Fig. 12. It can be seen that at 850 °C, layered loading shows much higher selectivity than Mn-Na2WO4/SiO2 only. This is mainly due to the less deep oxidation performance of Ce-Mn-Na2WO4/SiO2 which is loaded at the back section of the layered loading. Therefore, there is a broader temperature range to achieve, for example, 75% or higher C2+ selectivity with layered loading than with Mn-Na2WO4/SiO2 only, as depicted in Fig. 12. This result has practical significance. As we know, one practical way to carry out OCM reaction is using multi-stage adiabatic reactor [19,20]. There will be a temperature rise in each bed from the inlet of the bed to the outlet of the bed. For example, if the temperature rise in each bed is 200 °C, with inlet at 700 °C and outlet at 900 °C (the selectivity between 850 and 900 °C was predicted based on the results obtained at lower temperatures). With single catalyst loading, like Mn-Na2WO4/SiO2, the selectivity obtained will be the average of from 63% to 78%. While with layered loading of Ag-MnNa2WO4/SiO2 and Ce-Mn-Na2WO4/SiO2, the selectivity obtained will be the average of from 76% to 82%. Fig. 12 shows that the selectivity improvement is more pronounced both at the inlet and outlet sections than at the best temperature point. Therefore, the layered loading strategy provides the chance to achieve a higher overall selectivity than the loading with a single catalyst. From layered loading results of La-Ce-oxide with Ce-Mn-Na2WO4/ SiO2, at lower reaction temperature, high C2+ selectivity and higher C2H4 selectivity can be obtained in comparison with the single catalyst loading. Layered loading of a more active catalyst like La-Ce-oxide with a selective catalyst like Ce-Mn-Na2WO4/SiO2 may have other benefits as well. As we know, OCM reaction is a high temperature reaction. Feed needs to be heated to this high temperature for the reaction to take place. Normally, CH4 is used as fuel and burned to heat up the feed to the required temperature with an expensive heat exchanger. With a much more active catalyst such as La-Ce oxide loaded in front of a more selective catalyst such as Ce-Mn-Na2WO4/SiO2, the La-Ce oxide catalyst bed can be used a “pre-heater” to heat up the feed to the required temperature, as depicted in Fig. 13, if this bed is operated adiabatically. Based on the results shown above, in addition to heating up the feed, CH4 is also selectively converted to C2+, which improves the total CH4 utilization. This pre-heat strategy eliminates the need for expensive heat exchange equipment. With the results shown in Figs. 8 and 9, it can be seen that the feed temperature can be reduced to 450 °C. With other catalysts and optimized operating conditions, the inlet temperature can be reduced even lower. This makes the high reaction temperature OCM reaction easier, less costly and more efficient to operate.
Fig. 12. Comparison of the predicted selectivity changes with reaction temperature with different catalyst loading strategies.
obtained with smaller La-Ce oxide loadings under the same reactor temperature. It is apparent that when the loading of La-Ce is high, like 20 mg, almost all the oxygen conversion occurs on La-Ce catalyst; the contribution of Ce-Mn-Na2WO4/SiO2 catalyst increases with the lower La-Ce-oxide loading in front. With less La-Ce oxide loading in front, more reaction after ignition propagates on to the more selective catalyst Ce-Mn-Na2WO4/SiO2, which results in the increase of selectivity. As mentioned above, when the loading of La-Ce is high, almost of oxygen was converted over La-Ce catalyst. With the CH4 to O2 ratio of 4 and a very thin layer of La-Ce catalyst, reaction heat produced will heat up this catalyst section, it is reasonable to predict that the catalyst bed temperature will be higher than the reactor temperature [17,18]. Because of this temperature rise, there will be much more deep oxidation happening, resulting a lower C2+ selectivity. For layered loading, it is interesting to see that the C2H4 selectivity does not change with the increase reactor temperature as shown in Fig. 11, which is different from that of single loading of 20 mg La-Ce oxide catalyst. It indicates that the C2H4 obtained migrates to the CeMn-Na2WO4/SiO2 bed for the layered loading cases and further deep oxidation of C2H4 is reduced significantly over this catalyst. With further reduction of La-Ce oxide catalyst loading in front of the bed to 0.9 mg, the temperature of full oxygen consumption is increased to 700 °C, which is 25 °C lower than that of the catalyst bed comprised of Ce-Mn-Na2WO4/SiO2 only. The C2+ selectivity obtained is the same as that of Ce-Mn-Na2WO4/SiO2 only, indicating that a layered loading strategy can lower the reactor temperature without loss of selectivity. High C2H4 selectivity can be achieved with 0.9 mg La-Ce oxide layered loading at a temperature 75 °C lower than that of single loading of CeMn-Na2WO4/SiO2. The La-Ce oxide layer shifts the products toward the front of the bed and leaves more the rear section for C2H6 dehydrogenation to C2H4, so that higher C2H4 is obtained. It can be concluded that in addition to lower reaction temperature and improve C2+ selectivity, layered loading can also change the product distribution toward higher C2H4 selectivity. This result also indicates that the amount of high activity catalyst can be optimized so that the reaction temperature can be lower on one hand and there will be no selectivity loss on the other. 3.4. Potential benefits of layered loading From the results obtained from layered loading Ag-Mn-Na2WO4/ SiO2 and Ce-Mn-Na2WO4/SiO2, that is layered loading of two catalysts having close performance in OCM reaction, it is found that lower reaction temperature and higher selectivity are obtained. The selectivity at lower reaction temperature is increased significantly. In addition to the reported results in Fig. 7, layered loading of Ag-Mn-Na2WO4/ SiO2 and Ce-Mn-Na2WO4/SiO2, and Mn-Na2WO4/SiO2 only were further studied under higher reactor temperatures and results obtained (including the results obtained under lowered temperatures) were
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Performance improvement for a fixed bed reactor with layered loading catalysts of different catalytic properties for oxidative coupling of methane ⁎
Wugeng Liang , Sagar Sarsani, David West, Aghaddin Mamedov, Istvan Lengyel, Hector Perez, James Lowrey SABIC Corporate R & D, 1600 Industrial Boulevard, Sugar Land, TX 77478, United States
A R T I C L E I N F O
A B S T R A C T
Keywords: Oxidative coupling of methane Mn-Na2WO4/SiO2 catalyst Ag and Ce promoted catalysts La-Ce oxide catalyst Layered loading
Layered loading of catalysts with different catalytic properties is studied for oxidative coupling of methane reaction (OCM) in a fixed bed reactor. Layered loading of a more active catalyst Ag-Mn-Na2WO4/SiO2 in front of a more selective one Ce-Mn-Na2WO4/SiO2 compared to loading Mn-Na2WO4/SiO2 catalyst only, lowers reaction temperature and increases selectivity. Such layered loading broadens the temperature range to achieve a high selectivity, which is important for OCM reaction carried out with multi-stage adiabatic fixed bed reactor. Similarly, with a very active La-Ce oxide catalyst loaded in front of a selective catalyst Ce-Mn-Na2WO4/SiO2, the temperature needed to start the reaction is lowered significantly. Therefore, a catalyst bed such as La-Ce oxide can be used as a “pre-heater” for the high temperature OCM reaction to lower the feed temperature. The total CH4 utilization can be improved with this loading strategy.
1. Introduction As a result of recent shale gas boom, more and more natural gas is being produced. But natural gas generally is used as fuel for electricity generation. There is a strong desire to increase the value of this abundant hydrocarbon resource. Oxidative coupling of methane (OCM) is one of the way to transform methane, the principle component of natural gas, to more valuable products, such as ethylene. Although close to 40 years extensive research and development has been devoted to this technology [1], it has never been commercialized. Efficiency of transformation from methane to ethylene, even when using the best catalyst today, is still low. By using catalysts with different properties, catalyst activity or selectivity can be improved, but it is very challenging to gain activity and selectivity at the same time. As we know, OCM reaction is a multi-step process, involving consecutive reactions starting from CH4 activation to form CH3 radicals, to coupling to form C2H6, to C2H6 dehydrogenation or oxidative dehydrogenation to form C2H4 [2]. Further oxidation or reforming of the coupling products produces the unwanted CO and CO2. For example, when a fixed bed reactor is used for this reaction, there will be a profile of different product species along the reactor [3]. Therefore, it is difficult achieve the best performance by using a single catalyst, because of different properties are needed at the different sections of the bed. On this basis, one of approaches to improve performance, may be ⁎
use of catalysts with different properties, such as combination of two catalysts, one with high activity and another with high selectivity. In this work, we studied the OCM reaction with layered loading strategy in a fix bed reactor: loading catalysts with different properties in different sections in the reactor. Two cases are experimentally studied. In the first case, two catalysts with similar catalytic properties are studied with layered loading. In the second case, two catalysts with quite different catalytic properties are studied. Potential benefits for practical applications from layered loading are discussed. 2. Experimental 2.1. Catalyst preparations Four Mn-Na2WO4/SiO2 based catalysts were studied, all catalyst samples were prepared by using incipient wetness impregnation method [4]. La-Ce oxide catalyst is prepared by drying and calcining the mixture solution of raw material nitrates. 2.1.1. Catalyst 1. Mn-Na2WO4/SiO2 (1.9%Mn-5.0%Na2WO4/SiO2) 18.62 g of Davisil Grade 646 silica gel was used as catalyst support. Before use the silica gel was dried overnight. 1.73 g of Mn(NO3)2·4H2O was dissolved in 18.6 ml of DI water. The Mn(NO3)2·4H2O solution was added into silica gel dropwise to achieve a uniform distribution of the solution onto the solids. Then the mixture was dried overnight at
Corresponding author. E-mail address: [email protected] (W. Liang).
http://dx.doi.org/10.1016/j.cattod.2017.03.058 Received 27 October 2016; Received in revised form 12 March 2017; Accepted 27 March 2017 0920-5861/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Liang, W., Catalysis Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.03.058
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125 °C. 1.13 g of Na2WO4·2H2O was dissolved in 18.6 ml of DI water. The solution obtained was added into the dried Mn containing silica gel dropwise. The mixture was dried at 125 °C again overnight. Then the dried material was calcined at 800 °C for 6 h. The calcined material was ready for performance test.
were mixed and then heated at 85 °C for 2 h under agitation. The obtained mixture was then dried overnight at 125 °C to yield a dried powder, which was then calcined at 625 °C for 5 h to yield the catalyst.
2.1.2. Catalyst 2. Ag-Mn-Na2WO4/SiO2 (1.0% Ag-1.9%Mn-5.0% Na2WO4/SiO2) 15.75 g of Davisil Grade 646 silica gel was used as catalyst support. Before use the silica gel was dried overnight. 1.48 g of Mn(NO3)2·4H2O was dissolved in 15.8 ml of DI water. The Mn(NO3)2·4H2O solution was added into silica gel dropwise to achieve a uniform distribution of the solution onto the solids. Then the mixture was dried overnight at 125 °C. 0.27 g of AgNO3 was dissolved in 15.8 ml of DI water. The AgNO3 solution was added into silica gel dropwise to achieve a uniform distribution of the solution onto the solids. Then the mixture was dried overnight at 125 °C. 0.96 g of Na2WO4·2H2O was dissolved in 18.6 ml of DI water. The solution obtained was added into the dried Mn containing silica gel dropwise. The mixture was dried at 125 °C again overnight. Then the dried material was calcined at 800 °C for 6 h. The calcined material was ready for performance test.
Experiments were carried out in quartz tube fixed bed reactor of 4 mm inner diameter. Gases are fed to the reactor by the use of mass flow controllers. The reactor is heated by using conventional clamshell furnace. The furnace has three heating zones. During reaction, the top and bottom zones are set at 300 °C. Then middle zone is set at the reaction temperature. Due to the small inner diameter reactor used, there is no thermocouple inside the reactor, the temperature reported is the middle zone furnace set point temperature. We measured the temperature inside the reactor before reaction in nitrogen feed, the temperature inside is 50 °C higher than the furnace temperature. The necessary reaction temperature was settled by using the constant-temperature furnace set up described above which was varied from experiment to experiment, similar method was used in [6]. The catalyst bed is located in the center of the middle zone furnace. On the top and bottom of the catalyst bed, quartz chips are loaded to reduce the empty volume. On the both ends of the quartz chip sections, quartz wools are used to hold the catalyst bed and quartz chips to the targeted position. Catalyst performance test was carried out under atmosphere pressure. The reaction products were analyzed by using online Gas Chromatography (Agilent 6890). Two columns are used to separate the products. Molecular sieve 13X column is used for permanent gases analysis, CO, CO2, O2 and Ne. They are detected with TCD detector. HPQ-Plot column is used for hydrocarbon products analysis. Products observed are C2H4, C2H6, C3H6, C3H8, C4's and C5's. These products are detected with FID detector. C2+ selectivity reported below is the total selectivity to the hydrocarbons from C2 to C5. Catalyst loading strategy and different testing conditions used are described below.
2.2. Catalyst testing
2.1.3. Catalyst 3. Ce-Mn-Na2WO4/SiO2 (5.0%Ce-1.9%Mn-5.0% Na2WO4/SiO2) 17.62 g of Davisil Grade 646 silica gel was used as catalyst support. Before use the silica gel was dried overnight. 1.74 g of Mn(NO3)2·4H2O was dissolved in 17.6 ml of DI water. The Mn(NO3)2·4H2O solution was added into silica gel dropwise to achieve a uniform distribution of the solution onto the solids. Then the mixture was dried overnight at 125 °C. 3.10 g of Ce(NO3)3·6H2O was dissolved in 17.6 ml of DI water. The Ce(NO3)3 solution was added into silica gel dropwise to achieve a uniform distribution of the solution onto the solids. Then the mixture was dried overnight at 125 °C. 1.12 g of Na2WO4·2H2O was dissolved in 17.6 ml of DI water. The solution obtained was added into the dried Mn containing silica gel dropwise. The mixture was dried at 125 °C again overnight. Then the dried material was calcined at 800 °C for 6 h. The calcined material was ready for performance test.
2.2.1. Testing of Mn-Na2WO4/SiO2 and promoted Mn-Na2WO4/SiO2 catalysts The first condition was used for testing Mn-Na2WO4/SiO2 and promoted Mn-Na2WO4/SiO2 catalysts with 100 mg catalyst loading. For layered loading studies, two catalysts were loaded in layers, one (with 50 mg) was loaded in the front section of the bed and the other (with 50 mg) loaded on the back. The flow rates of gases used for this condition are shown in Table 1, with Ne as the internal standard. The methane to oxygen ratio is 7.4 under this condition. By varying the reaction temperature, performances under different temperatures are obtained.
2.1.4. Catalyst 4. Ag-Ce-Mn-Na2WO4/SiO2 (1.0%Ag-5.0%Ce-1.9%Mn5.0%Na2WO4/SiO2) 17.6 g of Davisil Grade 646 silica gel was used as catalyst support. Before use the silica gel was dried overnight. 1.74 g of Mn(NO3)2·4H2O was dissolved in 17.6 ml of DI water. The Mn(NO3)2·4H2O solution was added into silica gel dropwise to achieve a uniform distribution of the solution onto the solids. Then the mixture was dried overnight at 125 °C. 3.10 g of Ce(NO3)3·6H2O and 0.32 g of AgNO3 was dissolved in 17.6 ml of DI water. This Ce(NO3)3 and AgNO3 solution was added into silica gel dropwise to achieve a uniform distribution of the solution onto the solids. Then the mixture was dried overnight at 125 °C. 1.12 g of Na2WO4·2H2O was dissolved in 17.6 ml of DI water. The solution obtained was added into the dried Mn containing silica gel dropwise. The mixture was dried at 125 °C again overnight. Then the dried material was calcined at 800 °C for 6 h. The calcined material was ready for performance test.
2.2.2. Testing of La-Ce oxide and Ce-Mn-Na2WO4/SiO2 based catalysts The second testing condition was applied for testing La-Ce oxide and Ce-Mn-Na2WO4/SiO2 system. For testing La-Ce oxide catalyst alone, 20 mg of catalyst was loaded. For layered loading, different amounts of La-Ce oxide were loaded in front of the bed with 240 mg of Ce-MnNa2WO4/SiO2 on the back. Ce-Mn-Na2WO4/SiO2 alone with 240 mg loading was also tested for comparison. The flow rates of gases at methane to oxygen ratio of 4.0 with Ne as the internal standard are
2.1.5. Comparative catalyst. Ag/SiO2 (3.0%Ag/SiO2) This catalyst is prepared for comparison. 3.00 g of Ag (15 nm) in 5% water was added into 3.00 g of silica gel which was calcined at 800 °C. After Ag addition, the mixture is dried overnight at 125 °C.
Table 1 Flow rates of feed with CH4:O2 = 7.4.
2.1.6. Catalyst 5. La-Ce oxide catalyst La-Ce oxide catalyst with La/Ce = 10 was used in this study. It was prepared using the method described in [5]. 24.06 g of La(NO3)3·6H2O was dissolved in 40 ml of DI water to yield a La(NO3)3 aqueous solution. 2.39 g of Ce(NO3)3·6H2O was dissolved in 10 ml of Di water to yield a Ce(NO3)3 aqueous solution. These two aqueous solutions 2
Gas component
Feed rate
Oxygen Methane Neon
3.9 sccm 28.9 sccm 0.5 sccm
Total flow
33.3 sccm
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Table 2 Flow rates of feed with CH4:O2 = 4.0. Gas component
Feed rate
Oxygen Methane Neon
15.8 sccm 63.4 sccm 1.0 sccm
Total flow
80.0 sccm
Fig. 3. C2+ selectivity under different temperatures with Mn-Na2WO4/SiO2, Ag-MnNa2WO4/SiO2 and Ag/SiO2.
Fig. 1. CH4 conversion under different temperatures with Mn-Na2WO4/SiO2, Ag-MnNa2WO4/SiO2, Ag/SiO2 and quartz chips only (blank test).
shown in Table 2. By varying the reaction temperature, performances under different temperatures were obtained. 3. Results and discussion Fig. 4. C2H6 selectivity under different temperatures with Mn-Na2WO4/SiO2, Ag-MnNa2WO4/SiO2 and Ag/SiO2.
3.1. Ag promotion on Mn-Na2WO4/SiO2 catalyst Mn-Na2WO4/SiO2 and Ag-Mn-Na2WO4/SiO2 catalysts each with 100 mg loading were tested with the first testing conditions shown in Table 1. The performances obtained are shown in Figs. 1–4 . Results obtained with quartz chips only are also included in Figs. 1 and 2 as blank test. From CH4 and O2 conversions shown in Figs. 1 and 2, it can be seen that the CH4 and O2 conversions obtained with only quartz chips at 800 °C are close to 0% and much lower than that obtained from catalysts, so that the contributions from the quartz chips and empty volumes in the reactor can be neglected. From CH4 and O2 conversions shown in Figs. 1 and 2, it can also be seen that higher conversions are obtained with Ag-Mn-Na2WO4/SiO2 under the same temperature than Mn-Na2WO4/SiO2; the same CH4 or O2 conversions can be achieved under a lower reaction temperature, indicating that Ag promotion improves catalyst activity. For comparison, the performances of Ag/ SiO2 obtained under the same condition are also shown in these figures. With Ag/SiO2, much lower conversion is obtained, indicating that the
improved performance of Ag-Mn-Na2WO4/SiO2 is not coming from Ag only, but the synergetic effect between Ag and Mn-Na2WO4/SiO2. The selectivity obtained under different temperatures with these catalysts is shown in Fig. 3. Comparing the selectivies obtained from Ag-Mn-Na2WO4/SiO2 and Mn-Na2WO4/SiO2, it can be seen that Ag promotion increases catalyst selectivity significantly when the reaction temperature is lower than 725 °C. This promotion effect is not clear when the reaction temperature is higher than 725 °C. The selectivity with Ag/SiO2 is shown in the same figure and it can be seen that very low selectivity is obtained. The mechanism of oxidative coupling of methane reaction has been described widely in many publications, it can be summarized into the following key steps [7–11]: [O]S + CH4 → [OH]S + CH3
(1)
2CH3 → C2H6
(2)
CH3 + O2 ↔ CH3O2
(3)
CH3 + [O]s ↔ [CH3O]s
(4)
2[OH]S + 1/2O2 → 2[O]S + H2O
(5)
The first step is the activation of methane with the participation of active oxygen sites [O]S, with the formation of methyl radical CH3 and hydroxyl group [OH]S. The gas phase reaction of coupling of methyl radicals to form the coupling product C2H6 has low activation energy, therefore does not limit reaction rate. The methyl radicals can react with gas phase oxygen to form CH3O2 (step (3)). They can also readsorb on to the catalyst surface and react with surface oxygen to form [CH3O]s (step (4)). Steps (3) and (4) are the main reactions controlling the selectivity of the different catalysts. Easy removal of methyl radicals
Fig. 2. O2 conversion under different temperatures with Mn-Na2WO4/SiO2, Ag-MnNa2WO4/SiO2, Ag/SiO2 and quartz chips only (blank test).
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from the oxygen centers will result in high C2+ selectivity; while at oxygen centers of low metal–O bond energy, oxidation of methyl radicals through [CH3O]s leads to formation of COx. Direction of step (3) depends on the temperature, while direction of step (4) depends both on temperature and on catalyst. These O-containing compounds are the precursors of deep oxidation products, such as CO and CO2. Therefore conversions of methyl radicals through steps (3) and (4) are the ones of the main selectivity loss. In addition to step (1), the activity of the catalyst is also influenced by the removal of the OH groups from the surface (step (5)), which reoxidizes the reduced sites back and complete the full cycle of the reaction. This step is usually determined by metal–O bond of the catalyst. The different functions of Ag in Ag-Mn-Na2WO4/SiO2 and Ag-SiO2 observed above can be explained by the mechanism above. In Ag/SiO2, Ag is the only active site available for OCM reaction. Since Ag is not a very good OCM catalyst by itself [12], very low activity and selectivity are obtained. In Ag-Mn-Na2WO4/SiO2, due to the superior activity of MnNa2WO4/SiO2 toward CH4 activation to form methyl radicals, there will be large amount of adsorbed OH group on the surface of the catalyst, reducing the catalyst activity. Since Ag has the function to oxidize the adsorbed OH group to water [13], Ag performs this function and re-activates the reduced activate sites, especially, under low reaction temperature. Therefore, it is highly possible that the Ag promotion in Ag-Mn-Na2WO4/SiO2 enhances the re-oxidation step of OCM reaction. With this promotion effect, the activity and selectivity are improved. The ethane selectivity obtained with these three catalysts are compared in Fig. 4. It can be seen that much higher ethane selectivity is obtained with Ag-Mn-Na2WO4/SiO2. This result can be explained by the synergistic effect proposed above as well. Ag promotion at lower reaction temperature mainly enhances re-oxidation activity, which leads to the increase of the rate of methyl radical formation, resulting in higher ethane formation rate in the products. At the same time, other functions, such as oxidative dehydrogenation and deep oxidation, do not increase proportionally with Ag promotion. Therefore, the ethane selectivity increases.
Fig. 6. O2 conversion under different temperatures with Mn-Na2WO4/SiO2, Ag-MnNa2WO4/SiO2, Ce-Mn-Na2WO4/SiO2, Ag-Ce-Mn-Na2WO4/SiO2, and layered loading of Ag-Mn-Na2WO4/SiO2 and Ce-Mn-Na2WO4/SiO2.
Fig. 7. C2+ selectivity under different temperatures with Mn-Na2WO4/SiO2, Ag-MnNa2WO4/SiO2, Ce-Mn-Na2WO4/SiO2, Ag-Ce-Mn-Na2WO4/SiO2, and layered loading of Ag-Mn-Na2WO4/SiO2 and Ce-Mn-Na2WO4/SiO2.
Ag promotion increases catalyst activity and selectivity at lower temperature, but with no effect on selectivity under high temperature as discussed before and also indicated in Figs. 5–7; Ce promotion increases catalyst selectivity but lowers catalyst activity. In order to take advantages of these two catalysts, experiments were carried out with layered loading: in the front of the bed (50% of the bed) Ag promoted catalyst was loaded and the other part of the bed (the remaining 50% of the bed) Ce promoted catalyst was loaded. The total catalyst loading in this layered loading experiment is the same as that of a single catalyst testing. Results obtained with these experiments are shown in Figs. 5–7. From Figs. 5 and 6, it can be seen that higher activity is obtained with the layered loading compared to Mn-Na2WO4/SiO2. At the same time, higher selectivity, both at lower temperature and higher temperature are obtained with the layered loading compared to Mn-Na2WO4/SiO2. Compared to Ag-Mn-Na2WO4/SiO2, layered loading has almost the same performance as Ag-Mn-Na2WO4/SiO2 at lower temperature, but higher selectivity is obtained with layered loading than Ag-MnNa2WO4/SiO2 at higher temperature. Compared to Ce-Mn-Na2WO4/SiO2, layered loading shows significantly higher performance at lower temperature, its selectivity is lower than Ce-Mn-Na2WO4/SiO2 only at very high temperature (750 °C or higher). Compared to the results obtained at total O2 consumption, the CH4 conversion is higher with layered loading than that with Mn-Na2WO4/ SiO2. This is due to the higher C2+ selectivity obtained with layered loading. With higher C2+ selectivity, less O2 is used for non-selective deep oxidation pathways under the same methane conversion, so that there are more O2 available for methane conversion. As a result, methane conversion is higher.
3.2. Layered loading of Ag and Ce promoted Mn-Na2WO4/SiO2 catalysts Promotion of Mn-Na2WO4/SiO2 catalyst by Ce is known and has been described in literature [14,15]. Ce promoted catalyst (Ce-MnNa2WO4/SiO2) which was prepared and investigated in these series of experiments shows higher C2+ selectivity (Fig. 7) compared to unpromoted Mn-Na2WO4/SiO2. But lower activity is obtained with Ce promotion, because a higher reaction temperature is needed to reach the same O2 or CH4 conversion, as indicated in Figs. 5 and 6 .
Fig. 5. CH4 conversion under different temperatures with Mn-Na2WO4/SiO2, Ag-MnNa2WO4/SiO2, Ce-Mn-Na2WO4/SiO2, Ag-Ce-Mn-Na2WO4/SiO2, and layered loading of Ag-Mn-Na2WO4/SiO2 and Ce-Mn-Na2WO4/SiO2.
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In order to further confirm the advantage of layered loading, we prepared a catalyst with composition of Ag-Ce-Mn-Na2WO4/SiO2, containing both promoters of Ag and Ce. Performances of this catalyst are also shown in Figs. 5–7. It can be seen that the activity of this catalyst is similar to that of Ag-Mn-Na2WO4/SiO2 and is more active compared to Ce-Mn-Na2WO4/SiO2. Its selectivity under high reaction temperature is the same as that of Mn-Na2WO4/SiO2 and Ag-MnNa2WO4/SiO2 and is lower than that of layered loading. From all these comparisons, it can be seen that layered loading shows enhanced performance compared to a single catalyst loading, indicating that layered loading can achieve the improved performance which a single catalyst cannot. The observed experimental results of layered loading could be explained well on the basis of reaction mechanism. As described in reaction steps (1)–(5), OCM reaction starts with methyl radical formation, coupling of which leads to the formation of ethane. Ethane is further converted to ethylene through parallel reactions of thermal dehydrogenation and catalytic oxidative dehydrogenation. Some portion of methyl radicals undergoes to deep oxidation through O-containing compounds CH3O2 and [CH3O]s as shown in steps (3) and (4). In addition to the reaction steps (3) and (4), some of the ethane and ethylene formed also undergo deep oxidation with the formation of CO and CO2 [3,16]. Due to the nature of the consecutive reactions, there is a distribution of products through contact time or through catalyst bed. On this basis, along the axial direction of a fixed bed reactor, there will be more ethane in the front of the bed and more deep oxidation products (CO and CO2) toward the end of the bed. If a more active ethane formation catalyst (like Ag-Mn-Na2WO4/SiO2) is loaded in the front of the bed, then it will enhance the ethane formation. If a less deep oxidation catalyst (like Ce-Mn-Na2WO4/SiO2) is loaded in the back of the catalyst bed, it will reduce the deep oxidation and enhance the overall selectivity. Therefore, the overall results obtained with loading of catalysts with different catalytic properties will lead to better performance than a single catalyst loading (like Mn-Na2WO4/SiO2). In summary, the layered loading is a strategy to load a specific catalyst with required performance at the specific reactor position. The strategy of catalyst layering can be extended to the case of multi catalysts loading to achieve more improved process performance.
Fig. 9. O2 conversions obtained under different temperatures with different catalyst loadings. A: La-Ce-oxide and B: Ce-Mn-Na2WO4/SiO2.
Fig. 10. C2+ selectivities obtained under different temperatures with different catalyst loadings. A: La-Ce-oxide and B: Ce-Mn-Na2WO4/SiO2.
3.3. Layered loading of La-Ce oxide and Ce-Mn-Na2WO4/SiO2 catalysts La-Ce oxide catalyst shows higher activity in OCM reaction compared to Mn-Na2WO4/SiO2 catalyst system, but has lower selectivity. We layered loading this catalyst with different amounts in front of CeMn-Na2WO4/SiO2 and results obtained are shown in Figs. 8–11 . Figs. 8 and 9 show the CH4 and O2 conversions under different temperatures. With La-Ce oxide loaded alone (20 mg), the reaction ignites at 450 °C and O2 conversion reaches close to 100%. At this temperature, the highest CH4 conversion (about 25%) is obtained. With
Fig. 11. Selectivity of C2H4 obtained under different temperatures with different catalyst loadings. A: La-Ce-oxide and B: Ce-Mn-Na2WO4/SiO2.
the increase of reactor temperature, due to the increase of consumption of oxygen in non-selective pathway, C2+ selectivity decreases as shown in Fig. 10, which leads to CH4 conversion decrease. Different performance was observed in the case of single loading of Ce-Mn-Na2WO4/SiO2 (240 mg loading). O2 conversion gradually increases with reactor temperature and reaches 100% at 725 °C. Due to the high C2+ selectivity obtained with this catalyst, high CH4 conversion is obtained under temperatures at and higher than 725 °C. With layered loading of 5.0 mg La-Ce oxide with 240 mg Ce-MnNa2WO4/SiO2, the reaction ignites at 550 °C, the ignition temperature is higher than that of 20 mg La-Ce oxide loading. The ignition temperature shifts to 650 °C with layered loading of 2.5 mg La-Ce oxide with 240 mg Ce-Mn-Na2WO4/SiO2. For the experiments with 20 mg, 5.0 mg and 2.5 mg La-Ce oxide loadings, the C2+ selectivity at the ignition temperatures are very close. After ignition, with increase reactor temperature, better selectivity is
Fig. 8. CH4 conversions obtained under different temperatures with different catalyst loadings. A: La-Ce-oxide and B: Ce-Mn-Na2WO4/SiO2.
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Fig. 13. Scheme of La-Ce oxide catalyst used as a “pre-heater”.
shown in Fig. 12. It can be seen that at 850 °C, layered loading shows much higher selectivity than Mn-Na2WO4/SiO2 only. This is mainly due to the less deep oxidation performance of Ce-Mn-Na2WO4/SiO2 which is loaded at the back section of the layered loading. Therefore, there is a broader temperature range to achieve, for example, 75% or higher C2+ selectivity with layered loading than with Mn-Na2WO4/SiO2 only, as depicted in Fig. 12. This result has practical significance. As we know, one practical way to carry out OCM reaction is using multi-stage adiabatic reactor [19,20]. There will be a temperature rise in each bed from the inlet of the bed to the outlet of the bed. For example, if the temperature rise in each bed is 200 °C, with inlet at 700 °C and outlet at 900 °C (the selectivity between 850 and 900 °C was predicted based on the results obtained at lower temperatures). With single catalyst loading, like Mn-Na2WO4/SiO2, the selectivity obtained will be the average of from 63% to 78%. While with layered loading of Ag-MnNa2WO4/SiO2 and Ce-Mn-Na2WO4/SiO2, the selectivity obtained will be the average of from 76% to 82%. Fig. 12 shows that the selectivity improvement is more pronounced both at the inlet and outlet sections than at the best temperature point. Therefore, the layered loading strategy provides the chance to achieve a higher overall selectivity than the loading with a single catalyst. From layered loading results of La-Ce-oxide with Ce-Mn-Na2WO4/ SiO2, at lower reaction temperature, high C2+ selectivity and higher C2H4 selectivity can be obtained in comparison with the single catalyst loading. Layered loading of a more active catalyst like La-Ce-oxide with a selective catalyst like Ce-Mn-Na2WO4/SiO2 may have other benefits as well. As we know, OCM reaction is a high temperature reaction. Feed needs to be heated to this high temperature for the reaction to take place. Normally, CH4 is used as fuel and burned to heat up the feed to the required temperature with an expensive heat exchanger. With a much more active catalyst such as La-Ce oxide loaded in front of a more selective catalyst such as Ce-Mn-Na2WO4/SiO2, the La-Ce oxide catalyst bed can be used a “pre-heater” to heat up the feed to the required temperature, as depicted in Fig. 13, if this bed is operated adiabatically. Based on the results shown above, in addition to heating up the feed, CH4 is also selectively converted to C2+, which improves the total CH4 utilization. This pre-heat strategy eliminates the need for expensive heat exchange equipment. With the results shown in Figs. 8 and 9, it can be seen that the feed temperature can be reduced to 450 °C. With other catalysts and optimized operating conditions, the inlet temperature can be reduced even lower. This makes the high reaction temperature OCM reaction easier, less costly and more efficient to operate.
Fig. 12. Comparison of the predicted selectivity changes with reaction temperature with different catalyst loading strategies.
obtained with smaller La-Ce oxide loadings under the same reactor temperature. It is apparent that when the loading of La-Ce is high, like 20 mg, almost all the oxygen conversion occurs on La-Ce catalyst; the contribution of Ce-Mn-Na2WO4/SiO2 catalyst increases with the lower La-Ce-oxide loading in front. With less La-Ce oxide loading in front, more reaction after ignition propagates on to the more selective catalyst Ce-Mn-Na2WO4/SiO2, which results in the increase of selectivity. As mentioned above, when the loading of La-Ce is high, almost of oxygen was converted over La-Ce catalyst. With the CH4 to O2 ratio of 4 and a very thin layer of La-Ce catalyst, reaction heat produced will heat up this catalyst section, it is reasonable to predict that the catalyst bed temperature will be higher than the reactor temperature [17,18]. Because of this temperature rise, there will be much more deep oxidation happening, resulting a lower C2+ selectivity. For layered loading, it is interesting to see that the C2H4 selectivity does not change with the increase reactor temperature as shown in Fig. 11, which is different from that of single loading of 20 mg La-Ce oxide catalyst. It indicates that the C2H4 obtained migrates to the CeMn-Na2WO4/SiO2 bed for the layered loading cases and further deep oxidation of C2H4 is reduced significantly over this catalyst. With further reduction of La-Ce oxide catalyst loading in front of the bed to 0.9 mg, the temperature of full oxygen consumption is increased to 700 °C, which is 25 °C lower than that of the catalyst bed comprised of Ce-Mn-Na2WO4/SiO2 only. The C2+ selectivity obtained is the same as that of Ce-Mn-Na2WO4/SiO2 only, indicating that a layered loading strategy can lower the reactor temperature without loss of selectivity. High C2H4 selectivity can be achieved with 0.9 mg La-Ce oxide layered loading at a temperature 75 °C lower than that of single loading of CeMn-Na2WO4/SiO2. The La-Ce oxide layer shifts the products toward the front of the bed and leaves more the rear section for C2H6 dehydrogenation to C2H4, so that higher C2H4 is obtained. It can be concluded that in addition to lower reaction temperature and improve C2+ selectivity, layered loading can also change the product distribution toward higher C2H4 selectivity. This result also indicates that the amount of high activity catalyst can be optimized so that the reaction temperature can be lower on one hand and there will be no selectivity loss on the other. 3.4. Potential benefits of layered loading From the results obtained from layered loading Ag-Mn-Na2WO4/ SiO2 and Ce-Mn-Na2WO4/SiO2, that is layered loading of two catalysts having close performance in OCM reaction, it is found that lower reaction temperature and higher selectivity are obtained. The selectivity at lower reaction temperature is increased significantly. In addition to the reported results in Fig. 7, layered loading of Ag-Mn-Na2WO4/ SiO2 and Ce-Mn-Na2WO4/SiO2, and Mn-Na2WO4/SiO2 only were further studied under higher reactor temperatures and results obtained (including the results obtained under lowered temperatures) were
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