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2018 Volume 39 Issue 8
2018, 39(8):
Abstract:
2018, 39(8): 1281-1281
doi: 10.1016/S1872-2067(18)63125-0
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2018, 39(8): 1282-1282
doi: 10.1016/S1872-2067(18)63126-2
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2018, 39(8): 1283-1293
doi: 10.1016/S1872-2067(18)63032-3
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The efficient synthesis of methanol and ethylene glycol via the chemoselective hydrogenation of ethylene carbonate (EC) is important for the sustainable utilization of CO2 to produce commodity chemicals and fuels. In this work, a series of β-cyclodextrin-modified Cu/SiO2 catalysts were prepared by ammonia evaporation method for the selective hydrogenation of EC to co-produce methanol and ethylene glycol. The structure and physicochemical properties of the catalysts were characterized in detail by N2 physisorption, XRD, N2O titration, H2-TPR, TEM, and XPS/XAES. Compared with the unmodified 25Cu/SiO2catalyst, the involvement of β-cyclodextrin in 5β-25Cu/SiO2 could remarkably increase the catalytic activity-excellent activity of 1178 mgEC gcat-1 h-1 with 98.8% ethylene glycol selectivity, and 71.6% methanol selectivity could be achieved at 453 K. The remarkably improved recyclability was primarily attributed to the remaining proportion of Cu+/(Cu0+Cu+). Furthermore, the DFT calculation results demonstrated that metallic Cu0 dissociated adsorbed H2, while Cu+ activated the carbonyl group of EC and stabilized the intermediates. This study is a facile and efficient method to prepare highly dispersed Cu catalysts-this is also an effective and stable heterogeneous catalyst system for the sustainable synthesis of ethylene glycol and methanol via indirect chemical utilization of CO2.
The efficient synthesis of methanol and ethylene glycol via the chemoselective hydrogenation of ethylene carbonate (EC) is important for the sustainable utilization of CO2 to produce commodity chemicals and fuels. In this work, a series of β-cyclodextrin-modified Cu/SiO2 catalysts were prepared by ammonia evaporation method for the selective hydrogenation of EC to co-produce methanol and ethylene glycol. The structure and physicochemical properties of the catalysts were characterized in detail by N2 physisorption, XRD, N2O titration, H2-TPR, TEM, and XPS/XAES. Compared with the unmodified 25Cu/SiO2catalyst, the involvement of β-cyclodextrin in 5β-25Cu/SiO2 could remarkably increase the catalytic activity-excellent activity of 1178 mgEC gcat-1 h-1 with 98.8% ethylene glycol selectivity, and 71.6% methanol selectivity could be achieved at 453 K. The remarkably improved recyclability was primarily attributed to the remaining proportion of Cu+/(Cu0+Cu+). Furthermore, the DFT calculation results demonstrated that metallic Cu0 dissociated adsorbed H2, while Cu+ activated the carbonyl group of EC and stabilized the intermediates. This study is a facile and efficient method to prepare highly dispersed Cu catalysts-this is also an effective and stable heterogeneous catalyst system for the sustainable synthesis of ethylene glycol and methanol via indirect chemical utilization of CO2.
2018, 39(8): 1294-1302
doi: 10.1016/S1872-2067(18)63086-4
Abstract:
CoCu/TiO2 catalysts promoted using alkali metals (Li, Na, K, Rb, and Cs) were prepared by the homogeneous deposition-precipitation method followed by the incipient wetness impregnation method. The influences of the alkali metals on the physicochemical properties of the CoCu/TiO2 catalysts and the catalytic performance for CO2 hydrogenation to long-chain hydrocarbons (C5+) were investigated in this work. According to the characterization of the catalysts based on X-ray photoelectron spectroscopy, X-ray diffraction, CO2 temperature-programmed desorption (TPD), and H2-TPD, the introduction of alkali metals could increase the CO2 adsorption and decrease the H2 chemisorption, which could suppress the formation of CH4, enhance the production of C5+, and decrease the hydrogenation activity. Among all the promoters, the Na-modified CoCu/TiO2 catalyst provided the maximum C5+ yield of 5.4%, with a CO2 conversion of 18.4% and C5+ selectivity of 42.1%, because it showed the strongest basicity and a slight decrease in the amount of H2 desorption; it also exhibited excellent catalytic stability of more than 200 h.
CoCu/TiO2 catalysts promoted using alkali metals (Li, Na, K, Rb, and Cs) were prepared by the homogeneous deposition-precipitation method followed by the incipient wetness impregnation method. The influences of the alkali metals on the physicochemical properties of the CoCu/TiO2 catalysts and the catalytic performance for CO2 hydrogenation to long-chain hydrocarbons (C5+) were investigated in this work. According to the characterization of the catalysts based on X-ray photoelectron spectroscopy, X-ray diffraction, CO2 temperature-programmed desorption (TPD), and H2-TPD, the introduction of alkali metals could increase the CO2 adsorption and decrease the H2 chemisorption, which could suppress the formation of CH4, enhance the production of C5+, and decrease the hydrogenation activity. Among all the promoters, the Na-modified CoCu/TiO2 catalyst provided the maximum C5+ yield of 5.4%, with a CO2 conversion of 18.4% and C5+ selectivity of 42.1%, because it showed the strongest basicity and a slight decrease in the amount of H2 desorption; it also exhibited excellent catalytic stability of more than 200 h.
2018, 39(8): 1303-1310
doi: 10.1016/S1872-2067(18)63012-8
Abstract:
A modified rare-earth-metal catalyst system combined with quaternary ammonium salts (QASs) as cocatalysts was investigated in the alternating copolymerization of CO2/propylene oxide (PO) to produce poly(propylene carbonate) (PPC). In the presence of ZnO/SiO2, the ZnEt2-glycerine-Y(CCl3OO)3 catalyst presented higher activity for CO2/PO copolymerization, as well as a higher molecular weight of polycarbonate, while maintaining the high carbonate content originating from the neat ZnEt2-glycerine-Y(CCl3OO)3 catalyst. In the presence of QASs bearing different halide anions (F-, Cl-, and Br-), the type of the halide anion had a strong influence on the activity of the catalyst for CO2/PO alternating copolymerization. Only tetramethylammonium fluoride (TMAF) could promote the alternating copolymerization without increasing the by-product. Combined the ZnO/SiO2 catalyst and TMAF, the catalytic activity for CO2/PO polymerization increased dramatically compared to the basic ternary catalyst system. The improved catalyst system produced a polymer with a high carbonate unit level equivalent to that of the polycarbonate produced by the basic ZnEt2-glycerine-Y(CCl3OO)3 catalyst system.
A modified rare-earth-metal catalyst system combined with quaternary ammonium salts (QASs) as cocatalysts was investigated in the alternating copolymerization of CO2/propylene oxide (PO) to produce poly(propylene carbonate) (PPC). In the presence of ZnO/SiO2, the ZnEt2-glycerine-Y(CCl3OO)3 catalyst presented higher activity for CO2/PO copolymerization, as well as a higher molecular weight of polycarbonate, while maintaining the high carbonate content originating from the neat ZnEt2-glycerine-Y(CCl3OO)3 catalyst. In the presence of QASs bearing different halide anions (F-, Cl-, and Br-), the type of the halide anion had a strong influence on the activity of the catalyst for CO2/PO alternating copolymerization. Only tetramethylammonium fluoride (TMAF) could promote the alternating copolymerization without increasing the by-product. Combined the ZnO/SiO2 catalyst and TMAF, the catalytic activity for CO2/PO polymerization increased dramatically compared to the basic ternary catalyst system. The improved catalyst system produced a polymer with a high carbonate unit level equivalent to that of the polycarbonate produced by the basic ZnEt2-glycerine-Y(CCl3OO)3 catalyst system.
2018, 39(8): 1311-1319
doi: 10.1016/S1872-2067(17)63005-5
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Three pillar-layered metal-organic frameworks (MOFs) based on M(HBTC)(4,4'-bipy)·3DMF (M=Ni, Co, and Zn; HBTC=1,3,5-benzenetricarboxylic acid, 4,4'-bipy=4,4'-bipyridine) were synthesized using a solvothermal method. Zn(HBTC)(4,4'-bipy)·3DMF was synthesized for the first time using both a solvothermal and microwave method, and subsequently characterized by various physicochemical methods. The structure of M(HBTC)(4,4'-bipy)·3DMF consisted of honeycomb grid layers of M2+ ions and BTC units, which were further linked by the 4,4'-bipy pillars to form a three-dimensional highly porous framework. All the MOFs displayed excellent synergistic catalytic properties with alkyl ammonium halides (TBAX) in the solventless fixation of CO2 with epoxides to produce cyclic carbonates. The catalytic activities of these MOFs followed the trend Zn > Co > Ni, which was explained by the acid-base bifunctional properties. The microwave-synthesized Zn(HBTC)(4,4'-bipy)·3DMF material exhibited physical, chemical, and catalytic properties that were similar to those of the catalyst obtained using a conventional solvothermal synthesis. The scope of various parameters, including recyclability, was studied, and a plausible reaction mechanism was suggested.
Three pillar-layered metal-organic frameworks (MOFs) based on M(HBTC)(4,4'-bipy)·3DMF (M=Ni, Co, and Zn; HBTC=1,3,5-benzenetricarboxylic acid, 4,4'-bipy=4,4'-bipyridine) were synthesized using a solvothermal method. Zn(HBTC)(4,4'-bipy)·3DMF was synthesized for the first time using both a solvothermal and microwave method, and subsequently characterized by various physicochemical methods. The structure of M(HBTC)(4,4'-bipy)·3DMF consisted of honeycomb grid layers of M2+ ions and BTC units, which were further linked by the 4,4'-bipy pillars to form a three-dimensional highly porous framework. All the MOFs displayed excellent synergistic catalytic properties with alkyl ammonium halides (TBAX) in the solventless fixation of CO2 with epoxides to produce cyclic carbonates. The catalytic activities of these MOFs followed the trend Zn > Co > Ni, which was explained by the acid-base bifunctional properties. The microwave-synthesized Zn(HBTC)(4,4'-bipy)·3DMF material exhibited physical, chemical, and catalytic properties that were similar to those of the catalyst obtained using a conventional solvothermal synthesis. The scope of various parameters, including recyclability, was studied, and a plausible reaction mechanism was suggested.
2018, 39(8): 1320-1328
doi: 10.1016/S1872-2067(18)63040-2
Abstract:
Carbon dioxide (CO2) capture and catalytic conversion has become an attractive and challenging strategy for CO2 utilization since it is an abundant, inexpensive, and renewable C1 resource and a main greenhouse gas. Herein, a novel hydrazine-bridged covalent triazine polymer (HB-CTP) was first designed and synthesized through simple polymerization of cyanuric chloride with 2,4,6-trihydrazinyl-1,3,5-triazine. The resultant HB-CTP exhibited good CO2 capture capacity (8.2 wt%, 0℃, and 0.1 MPa) as well as satisfactory recyclability after five consecutive adsorption-desorption cycles. Such a polymer was subsequently employed as a metal-free heterogeneous catalyst for the cyclo-addition of CO2 with various epoxides under mild and solvent-free conditions, affording cyclic carbonates with good to excellent yields (67%-99%) and high functional-group tolerance. The incorporation of hydrazine linkages into HB-CTP's architecture was suggested to play the key role in activating epoxides through hydrogen bonding. Moreover, HB-CTP can be reused at least five times without significant loss of its catalytic activity.
Carbon dioxide (CO2) capture and catalytic conversion has become an attractive and challenging strategy for CO2 utilization since it is an abundant, inexpensive, and renewable C1 resource and a main greenhouse gas. Herein, a novel hydrazine-bridged covalent triazine polymer (HB-CTP) was first designed and synthesized through simple polymerization of cyanuric chloride with 2,4,6-trihydrazinyl-1,3,5-triazine. The resultant HB-CTP exhibited good CO2 capture capacity (8.2 wt%, 0℃, and 0.1 MPa) as well as satisfactory recyclability after five consecutive adsorption-desorption cycles. Such a polymer was subsequently employed as a metal-free heterogeneous catalyst for the cyclo-addition of CO2 with various epoxides under mild and solvent-free conditions, affording cyclic carbonates with good to excellent yields (67%-99%) and high functional-group tolerance. The incorporation of hydrazine linkages into HB-CTP's architecture was suggested to play the key role in activating epoxides through hydrogen bonding. Moreover, HB-CTP can be reused at least five times without significant loss of its catalytic activity.
2018, 39(8): 1329-1346
doi: 10.1016/S1872-2067(18)63100-6
Abstract:
C1 chemistry is the essence of coal chemistry and natural gas chemistry. Catalytic methods to efficiently convert C1 molecules into fuels and chemicals have been extensively studied. Syngas (CO+H2) conversion is the most important industrial reaction system in C1 chemistry, and Fe and Co catalysts, two major industrial catalysts, have been the focus of fundamental research and industrial application. In the last decade, considerable research efforts have been devoted to discoveries concerning catalyst structure and increasing market demands for olefins and oxygenates. Since the development of efficient catalysts would strongly benefit from catalyst design and the establishment of a new reaction system, this review comprehensively overviews syngas conversion in three main reactions, highlights the advances recently made and the challenges that remain open, and will stimulate future research activities. The first part of the review summarizes the breakthroughs in Fischer-Tropsch synthesis regarding the optimization of activity and stability, determination of the active phase, and mechanistic studies. The second part overviews the modulation of catalytic structure and product selectivity for Fischer-Tropsch to olefins (FTO). Catalysts designed to produce higher alcohols, as well as to tune product selectivity in C1 chemistry, are described in the third section. Finally, present challenges in syngas conversion are proposed, and the solutions and prospects are discussed from the viewpoint of fundamental research and practical application. This review summarizes the latest advances in the design, preparation, and application of Fe/Co-based catalysts toward syngas conversion and presents the challenges and future directions in producing value-added fuels.
C1 chemistry is the essence of coal chemistry and natural gas chemistry. Catalytic methods to efficiently convert C1 molecules into fuels and chemicals have been extensively studied. Syngas (CO+H2) conversion is the most important industrial reaction system in C1 chemistry, and Fe and Co catalysts, two major industrial catalysts, have been the focus of fundamental research and industrial application. In the last decade, considerable research efforts have been devoted to discoveries concerning catalyst structure and increasing market demands for olefins and oxygenates. Since the development of efficient catalysts would strongly benefit from catalyst design and the establishment of a new reaction system, this review comprehensively overviews syngas conversion in three main reactions, highlights the advances recently made and the challenges that remain open, and will stimulate future research activities. The first part of the review summarizes the breakthroughs in Fischer-Tropsch synthesis regarding the optimization of activity and stability, determination of the active phase, and mechanistic studies. The second part overviews the modulation of catalytic structure and product selectivity for Fischer-Tropsch to olefins (FTO). Catalysts designed to produce higher alcohols, as well as to tune product selectivity in C1 chemistry, are described in the third section. Finally, present challenges in syngas conversion are proposed, and the solutions and prospects are discussed from the viewpoint of fundamental research and practical application. This review summarizes the latest advances in the design, preparation, and application of Fe/Co-based catalysts toward syngas conversion and presents the challenges and future directions in producing value-added fuels.
2018, 39(8): 1347-1365
doi: 10.1016/S1872-2067(18)63090-6
Abstract:
Vanadium-titanium-based catalysts are the most widely used industrial materials for NOxremoval from coal-fired power plants. Owing to their relatively poor low-temperature deNOx activity, low thermal stability, insufficient Hg0 oxidation activity, SO2 oxidation, ammonia slip, and other disadvantages, modifications to traditional vanadium-titanium-based selective catalytic reduction (SCR) catalysts have been attempted by many researchers to promote their relevant performance. This article reviewed the research progress of modified vanadium-titanium-based SCR catalysts from seven aspects, namely, (1) improving low-temperature deNOx efficiency, (2) enhancing thermal stability, (3) improving Hg0 oxidation efficiency, (4) oxidizing slip ammonia, (5) reducing SO2 oxidation, (6) increasing alkali resistance, and (7) others. Their catalytic performance and the influence mechanisms have been discussed in detail. These catalysts were also divided into different categories according to their modified components such as noble metals (e.g., silver, ruthenium), transition metals (e.g., manganese, iron, copper, zirconium, etc.), rare earth metals (e.g., cerium, praseodymium), and other metal chlorides (e.g., calcium chloride, copper chloride) and non-metals (fluorine, sulfur, silicon, nitrogen, etc.). The advantages and disadvantages of these catalysts were summarized. Based on previous studies and the author's point of view, doping the appropriate modified components is beneficial to further improve the overall performance of vanadium-titanium-based SCR catalysts. This has enormous development potential and is a promising way to realize the control of multiple pollutants on the basis of the existing flue gas treatment system.
Vanadium-titanium-based catalysts are the most widely used industrial materials for NOxremoval from coal-fired power plants. Owing to their relatively poor low-temperature deNOx activity, low thermal stability, insufficient Hg0 oxidation activity, SO2 oxidation, ammonia slip, and other disadvantages, modifications to traditional vanadium-titanium-based selective catalytic reduction (SCR) catalysts have been attempted by many researchers to promote their relevant performance. This article reviewed the research progress of modified vanadium-titanium-based SCR catalysts from seven aspects, namely, (1) improving low-temperature deNOx efficiency, (2) enhancing thermal stability, (3) improving Hg0 oxidation efficiency, (4) oxidizing slip ammonia, (5) reducing SO2 oxidation, (6) increasing alkali resistance, and (7) others. Their catalytic performance and the influence mechanisms have been discussed in detail. These catalysts were also divided into different categories according to their modified components such as noble metals (e.g., silver, ruthenium), transition metals (e.g., manganese, iron, copper, zirconium, etc.), rare earth metals (e.g., cerium, praseodymium), and other metal chlorides (e.g., calcium chloride, copper chloride) and non-metals (fluorine, sulfur, silicon, nitrogen, etc.). The advantages and disadvantages of these catalysts were summarized. Based on previous studies and the author's point of view, doping the appropriate modified components is beneficial to further improve the overall performance of vanadium-titanium-based SCR catalysts. This has enormous development potential and is a promising way to realize the control of multiple pollutants on the basis of the existing flue gas treatment system.
2018, 39(8): 1366-1372
doi: 10.1016/S1872-2067(18)63103-1
Abstract:
Pt/Au/WO3 bimetallic catalysts were prepared by impregnation of Pt onto preformed Au/WO3, obtained by a hexadecyl trmethyl ammonium bromide (CTAB)-assisted one-pot synthesis method. The resulting Pt/Au/WO3 catalysts exhibited remarkable synergistic effects for selective hydrogenolysis of glycerol to 1,3-propanediol. The characterization results showed that doping of Au promoted the reduction of both Pt and W at low temperatures and uniform dispersion of Pt on the WO3 support. Furthermore, more low-valence Pt species were produced on the WO3 surface after introduction of Au. These changes in electronic properties resulted in enhancement of both glycerol conversion and selectivity for 1,3-propanediol.
Pt/Au/WO3 bimetallic catalysts were prepared by impregnation of Pt onto preformed Au/WO3, obtained by a hexadecyl trmethyl ammonium bromide (CTAB)-assisted one-pot synthesis method. The resulting Pt/Au/WO3 catalysts exhibited remarkable synergistic effects for selective hydrogenolysis of glycerol to 1,3-propanediol. The characterization results showed that doping of Au promoted the reduction of both Pt and W at low temperatures and uniform dispersion of Pt on the WO3 support. Furthermore, more low-valence Pt species were produced on the WO3 surface after introduction of Au. These changes in electronic properties resulted in enhancement of both glycerol conversion and selectivity for 1,3-propanediol.
2018, 39(8): 1373-1383
doi: 10.1016/S1872-2067(18)63106-7
Abstract:
Ti3+ self-doped anatase three-dimensional (3D) TiO2 hollow nanoboxes were synthesized via a topological transformation process involving template participation by a facile one-pot hydrothermal treatment with an ethanol solution of zinc powder and TiOF2. It is worth noting that the 3D TiO2 hollow nanoboxes are assembled from six single-crystal nanosheets and have dominant exposure of the {001} facets. It is found from EPR spectra that adding zinc powder is an environment-friendly and effective strategy to introduce Ti3+ and oxygen vacancy (Ov) into the bulk of 3D hollow nanoboxes rather than the surface, which is responsible for their enhanced visible photocatalytic properties. The photocatalytic activity was evaluated by measuring the formation rate of hydroxide free radicals using 7-hydroxycoumarin as a probe. The sample prepared with zinc/TiOF2 mass ratio of 0.25 exhibited the highest RhB photodegradation activity under visible-light irradiation with a degradation rate of 96%, which is 4.0-times higher than that of pure TiO2. The results suggest a novel approach to construct in-situ 3D hierarchical TiO2 hollow nanoboxes doped with Ti3+ and Ov without introducing any impurity elements for superior visible-light photocatalytic activity.
Ti3+ self-doped anatase three-dimensional (3D) TiO2 hollow nanoboxes were synthesized via a topological transformation process involving template participation by a facile one-pot hydrothermal treatment with an ethanol solution of zinc powder and TiOF2. It is worth noting that the 3D TiO2 hollow nanoboxes are assembled from six single-crystal nanosheets and have dominant exposure of the {001} facets. It is found from EPR spectra that adding zinc powder is an environment-friendly and effective strategy to introduce Ti3+ and oxygen vacancy (Ov) into the bulk of 3D hollow nanoboxes rather than the surface, which is responsible for their enhanced visible photocatalytic properties. The photocatalytic activity was evaluated by measuring the formation rate of hydroxide free radicals using 7-hydroxycoumarin as a probe. The sample prepared with zinc/TiOF2 mass ratio of 0.25 exhibited the highest RhB photodegradation activity under visible-light irradiation with a degradation rate of 96%, which is 4.0-times higher than that of pure TiO2. The results suggest a novel approach to construct in-situ 3D hierarchical TiO2 hollow nanoboxes doped with Ti3+ and Ov without introducing any impurity elements for superior visible-light photocatalytic activity.
2018, 39(8): 1384-1394
doi: 10.1016/S1872-2067(18)63092-X
Abstract:
The one-pot synthesis of methyl isobutyl ketone (MIBK) from acetone using multifunctional catalysts is an important sustainable organic synthesis method with high atom and energy efficiency. Herein. we report a series of Pd supported on mixed metal oxide (MMO) catalysts with controllable acidic/basic/metallic sites on the surface. We study the relationship between the nature, synergy, and proximity of active sites and the catalytic performance of the multifunctional catalyst in the tandem reaction, in detail. In the existence of Lewis acid and base sites, the catalysts with medium-strength acidic/basic sites show preferred activity and/or MIBK selectivity. For multifunctional catalysts, the catalytic properties are more than just a collection of active sites, and the Pd/Mg3Al-MMO catalyst possessing 0.1% Pd loading and~0.4 acid/base molar ratio exhibits the optimal 42.1% acetone conversion and 37.2% MIBK yield, which is among the best reported so far for this tandem reaction under similar conditions. Moreover, the proximity test indicates that the intimate distance between acidic/basic/metallic sites can greatly shorten the diffusion time of the intermediate species from each active site, leading to an enhancement in the catalytic performance.
The one-pot synthesis of methyl isobutyl ketone (MIBK) from acetone using multifunctional catalysts is an important sustainable organic synthesis method with high atom and energy efficiency. Herein. we report a series of Pd supported on mixed metal oxide (MMO) catalysts with controllable acidic/basic/metallic sites on the surface. We study the relationship between the nature, synergy, and proximity of active sites and the catalytic performance of the multifunctional catalyst in the tandem reaction, in detail. In the existence of Lewis acid and base sites, the catalysts with medium-strength acidic/basic sites show preferred activity and/or MIBK selectivity. For multifunctional catalysts, the catalytic properties are more than just a collection of active sites, and the Pd/Mg3Al-MMO catalyst possessing 0.1% Pd loading and~0.4 acid/base molar ratio exhibits the optimal 42.1% acetone conversion and 37.2% MIBK yield, which is among the best reported so far for this tandem reaction under similar conditions. Moreover, the proximity test indicates that the intimate distance between acidic/basic/metallic sites can greatly shorten the diffusion time of the intermediate species from each active site, leading to an enhancement in the catalytic performance.
2018, 39(8): 1395-1402
doi: 10.1016/S1872-2067(18)63076-1
Abstract:
Mn2O3-Na2WO4/SiO2 is considered as the most promising catalyst for the oxidative coupling of methane (OCM) process; however, it only has a better catalytic performance over 800℃. To improve its low-temperature performance, an attempt has been made to modify the Mn2O3-Na2WO4/SiO2 catalyst using TiO2, MgO, Ga2O3, and ZrO2. Among the synthesized catalysts, the TiO2-modified Mn2O3-Na2WO4/SiO2 catalyst shows markedly improved low-temperature OCM performance, achieving a high CH4 conversion of ~23% and a good C2-C3 selectivity of ~73% at 700℃ (the catalyst bed temperature), along with promising stability for at least 300 h without signs of deactivation. In comparison with the unmodified Mn2O3-Na2WO4/SiO2 catalyst, the TiO2 modification results in significant improvement in the low-temperature activity/selectivity, whereas the MgO modification has almost no impact and the Ga2O3 and ZrO2 modifications have a negative effect. The X-ray diffraction (XRD) and Raman results reveal that the formation of a MnTiO3 phase and a MnTiO3-dominated catalyst surface is crucial for the improvement of the low-temperature activity/selectivity in the OCM process.
Mn2O3-Na2WO4/SiO2 is considered as the most promising catalyst for the oxidative coupling of methane (OCM) process; however, it only has a better catalytic performance over 800℃. To improve its low-temperature performance, an attempt has been made to modify the Mn2O3-Na2WO4/SiO2 catalyst using TiO2, MgO, Ga2O3, and ZrO2. Among the synthesized catalysts, the TiO2-modified Mn2O3-Na2WO4/SiO2 catalyst shows markedly improved low-temperature OCM performance, achieving a high CH4 conversion of ~23% and a good C2-C3 selectivity of ~73% at 700℃ (the catalyst bed temperature), along with promising stability for at least 300 h without signs of deactivation. In comparison with the unmodified Mn2O3-Na2WO4/SiO2 catalyst, the TiO2 modification results in significant improvement in the low-temperature activity/selectivity, whereas the MgO modification has almost no impact and the Ga2O3 and ZrO2 modifications have a negative effect. The X-ray diffraction (XRD) and Raman results reveal that the formation of a MnTiO3 phase and a MnTiO3-dominated catalyst surface is crucial for the improvement of the low-temperature activity/selectivity in the OCM process.
2018, 39(8): 1403-1410
doi: 10.1016/S1872-2067(18)63053-0
Abstract:
Designing low-cost, highly efficient, and stable bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of vital significance for water splitting. Herein, three-dimensional lily-like CoNi2S4 supported on nickel foam (CoNi2S4/Ni) has been fabricated by sulfuration of the Co-Ni precursor. As expected, CoNi2S4/Ni possesses such outstanding electrocatalytic properties that it requires an overpotential of only 54 mV at 10 mA cm-2 and 328 mV at 100 mA cm-2 for HER and OER, respectively. Furthermore, by utilizing the CoNi2S4/Ni electrodes as bifunctional electrocatalysts for overall water splitting, a current density of 10 mA cm-2 can be obtained at a voltage of only 1.56 V.
Designing low-cost, highly efficient, and stable bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of vital significance for water splitting. Herein, three-dimensional lily-like CoNi2S4 supported on nickel foam (CoNi2S4/Ni) has been fabricated by sulfuration of the Co-Ni precursor. As expected, CoNi2S4/Ni possesses such outstanding electrocatalytic properties that it requires an overpotential of only 54 mV at 10 mA cm-2 and 328 mV at 100 mA cm-2 for HER and OER, respectively. Furthermore, by utilizing the CoNi2S4/Ni electrodes as bifunctional electrocatalysts for overall water splitting, a current density of 10 mA cm-2 can be obtained at a voltage of only 1.56 V.
2018, 39(8): 1411-1417
doi: 10.1016/S1872-2067(18)63080-3
Abstract:
Bismuth oxybromide (BiOBr) with a hierarchical microcube morphology was successfully synthesized via microwave-assisted ionothermal self-assembly method. The as-obtained BiOBr was composed of regular multi-layered nanosheets, which were formed by selective adsorption of ionic liquids on the Br-terminated surface, followed by the formation of hydrogen bond-co-π-π stacking. The synthesized BiOBr exhibited high activity, excellent stability, and superior mineralization ability in the photocatalytic degradation of organic dyes under visible light owing to its enhanced light absorbance and narrow bandgap. Furthermore, photo-generated electrons were determined to be the main active species by comparison with different trapping agents used in the photocatalytic reactions.
Bismuth oxybromide (BiOBr) with a hierarchical microcube morphology was successfully synthesized via microwave-assisted ionothermal self-assembly method. The as-obtained BiOBr was composed of regular multi-layered nanosheets, which were formed by selective adsorption of ionic liquids on the Br-terminated surface, followed by the formation of hydrogen bond-co-π-π stacking. The synthesized BiOBr exhibited high activity, excellent stability, and superior mineralization ability in the photocatalytic degradation of organic dyes under visible light owing to its enhanced light absorbance and narrow bandgap. Furthermore, photo-generated electrons were determined to be the main active species by comparison with different trapping agents used in the photocatalytic reactions.
2018, 39(8): 1418-1426
doi: 10.1016/S1872-2067(18)63117-1
Abstract:
Post-synthetic treatment of high-silica as-made ZSM-5 with organic template in the micropores was explored to reduce/remove the external surface acid density of ZSM-5. It is found that Na2H2EDTA treatment can selectively remove the surface Al atoms, but generates new acid sites (likely silanol nests) on the external surface. H3PO4 treatment is unable to remove surface Al atoms, while small amount of P is left on the external surface, which effectively decreases the acid density. The catalytic performance of the resultant materials is evaluated in the methanol conversion reaction. H3PO4 treatment can effectively improve both the catalytic lifetime and the stability of propene selectivity. This occurs due to a combination of the increased tolerance to the external coke deposition and the depressed coking rate (reduced side reactions). Na2H2EDTA treatment only prolongs the catalytic lifetime, resulting from the improved tolerance to the external coke deposition. Under the optimized H3PO4treatment condition, the resultant ZSM-5 gives a catalytic lifetime of about 1.5 times longer than the precursor. Moreover, the propene selectivity is improved, showing a slight increasing trend until the deactivation.
Post-synthetic treatment of high-silica as-made ZSM-5 with organic template in the micropores was explored to reduce/remove the external surface acid density of ZSM-5. It is found that Na2H2EDTA treatment can selectively remove the surface Al atoms, but generates new acid sites (likely silanol nests) on the external surface. H3PO4 treatment is unable to remove surface Al atoms, while small amount of P is left on the external surface, which effectively decreases the acid density. The catalytic performance of the resultant materials is evaluated in the methanol conversion reaction. H3PO4 treatment can effectively improve both the catalytic lifetime and the stability of propene selectivity. This occurs due to a combination of the increased tolerance to the external coke deposition and the depressed coking rate (reduced side reactions). Na2H2EDTA treatment only prolongs the catalytic lifetime, resulting from the improved tolerance to the external coke deposition. Under the optimized H3PO4treatment condition, the resultant ZSM-5 gives a catalytic lifetime of about 1.5 times longer than the precursor. Moreover, the propene selectivity is improved, showing a slight increasing trend until the deactivation.
2018, 39(8): 1427-1435
doi: 10.1016/S1872-2067(18)63107-9
Abstract:
Highly active Fe-Nx sites that effectively improve the performance of non-precious metal electrocatalysts for oxygen reduction reactions (ORRs) are desirable. Herein, we propose a strategy for introducing a carbon template into a melamine/Fe-salt mixture to inductively generate highly active Fe-Nx sites for ORR. Using 57Fe Mössbauer spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction, we studied the structural composition of the Fe and N co-doped carbon catalysts. Interestingly, the results showed that this system not only converted inactive Fe and Fe-carbides into active Fe-N4 and other Fe-nitrides, but also improved their intrinsic activities.
Highly active Fe-Nx sites that effectively improve the performance of non-precious metal electrocatalysts for oxygen reduction reactions (ORRs) are desirable. Herein, we propose a strategy for introducing a carbon template into a melamine/Fe-salt mixture to inductively generate highly active Fe-Nx sites for ORR. Using 57Fe Mössbauer spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction, we studied the structural composition of the Fe and N co-doped carbon catalysts. Interestingly, the results showed that this system not only converted inactive Fe and Fe-carbides into active Fe-N4 and other Fe-nitrides, but also improved their intrinsic activities.
