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2018 Volume 39 Issue 7
2018, 39(7):
Abstract:
2018, 39(7): 1147-1156
doi: 10.1016/S1872-2067(18)63051-7
Abstract:
Catalysis is one of the most cross-cutting technologies in the chemical industry, intensely influenc-ing our daily society. Its practical application is closely related to the engineering disciplines. At present, the academic and industrial research on catalysis in our country has made great break-throughs in fields like hydrocarbon production, oil-quality upgrading processes, green chemical engineering, and other energy and chemical users of catalysis. In this paper, we attempt to summa-rize the industrial catalysis achievements and present a discussion on the direction and the devel-opment strategy for catalysis, based on economic and social demands.
Catalysis is one of the most cross-cutting technologies in the chemical industry, intensely influenc-ing our daily society. Its practical application is closely related to the engineering disciplines. At present, the academic and industrial research on catalysis in our country has made great break-throughs in fields like hydrocarbon production, oil-quality upgrading processes, green chemical engineering, and other energy and chemical users of catalysis. In this paper, we attempt to summa-rize the industrial catalysis achievements and present a discussion on the direction and the devel-opment strategy for catalysis, based on economic and social demands.
2018, 39(7): 1157-1166
doi: 10.1016/S1872-2067(18)63073-6
Abstract:
The electrochemical conversion of CO2 into value-added chemicals and fuels has attracted widespread concern since it realizes the recycling of greenhouse gases. Production of new materials lies at the very core of this technology as it enables the improvement of developmental efficiency and selectivity by chemical optimization of morphology and electronic structure. Transition metal-based catalysts are particularly appealing as their d bands have valence electrons which are close to the Fermi level and hence overcome the intrinsic activation barriers and reaction kinetics. The study of Mo, Fe, Co, and Ni-based materials in particular is a very recent research subject that offers various possibilities in electrochemical CO2 reduction applications. Herein, we summarize the recent research progress of Mo, Fe, Co, and Ni-based catalysts and their catalytic behavior in electrochemical CO2 reduction. We particularly focus on the relationship between structures and properties, with examples of the key features accounting for the high efficiency and selectivity of the CO2 reduction process. The most significant experimental and theoretical improvements are highlighted. Finally, we concisely discuss the scientific challenges and opportunities for transition metal-based catalysts.
The electrochemical conversion of CO2 into value-added chemicals and fuels has attracted widespread concern since it realizes the recycling of greenhouse gases. Production of new materials lies at the very core of this technology as it enables the improvement of developmental efficiency and selectivity by chemical optimization of morphology and electronic structure. Transition metal-based catalysts are particularly appealing as their d bands have valence electrons which are close to the Fermi level and hence overcome the intrinsic activation barriers and reaction kinetics. The study of Mo, Fe, Co, and Ni-based materials in particular is a very recent research subject that offers various possibilities in electrochemical CO2 reduction applications. Herein, we summarize the recent research progress of Mo, Fe, Co, and Ni-based catalysts and their catalytic behavior in electrochemical CO2 reduction. We particularly focus on the relationship between structures and properties, with examples of the key features accounting for the high efficiency and selectivity of the CO2 reduction process. The most significant experimental and theoretical improvements are highlighted. Finally, we concisely discuss the scientific challenges and opportunities for transition metal-based catalysts.
2018, 39(7): 1167-1179
doi: 10.1016/S1872-2067(18)635057-8
Abstract:
Covalent organic frameworks (COFs), established as an emerging class of crystalline porous poly-mers with high surface area, structural diversity, and designability, attract much interest and exhibit potential applications in catalysis. In this review, we summarize the use of COFs as a versatile plat-form to develop heterogeneous catalysts for a variety of chemical reactions. Catalytic COFs are cat-egorized in accordance with the types of active sites, involving single functional active sites, bifunc-tional active sites, and metal nanoparticles (NPs) embedded in pores. Special emphasis is placed on the deliberate or incidental synthesis strategies, the stability, the heterogeneity, and the shape/size selectivity for COF catalysis. Moreover, a description of the application of COFs as photocatalysts and electrocatalysts is presented. Finally, the prospects of COFs in catalysis and remaining issues in this field are indicated.
Covalent organic frameworks (COFs), established as an emerging class of crystalline porous poly-mers with high surface area, structural diversity, and designability, attract much interest and exhibit potential applications in catalysis. In this review, we summarize the use of COFs as a versatile plat-form to develop heterogeneous catalysts for a variety of chemical reactions. Catalytic COFs are cat-egorized in accordance with the types of active sites, involving single functional active sites, bifunc-tional active sites, and metal nanoparticles (NPs) embedded in pores. Special emphasis is placed on the deliberate or incidental synthesis strategies, the stability, the heterogeneity, and the shape/size selectivity for COF catalysis. Moreover, a description of the application of COFs as photocatalysts and electrocatalysts is presented. Finally, the prospects of COFs in catalysis and remaining issues in this field are indicated.
2018, 39(7): 1180-1188
doi: 10.1016/S1872-2067(18)63104-3
Abstract:
Ammonia synthesis via the Haber-Bosch process, which has been heralded as the most important invention of the 20th century, consumes massive amounts of energy, around~1%-2% of the world's annual energy consumption. Developing green and sustainable strategies for NH3 synthesis under ambient conditions, using renewable energy, is strongly desired, by both industrial and scientific researchers. Artificial photosynthesis for ammonia synthesis, which has recently attracted significant attention, directly produces NH3 from sunlight, and N2 and H2O via photocatalysis. This has been regarded as an ideal, energy-saving and environmentally-benign process for NH3 production because it can be performed under normal temperature and atmospheric pressure using renewable solar energy. Although sustainable developments have been achieved since the pioneering work in 1977, many challenging issues (e.g., adsorption and activation of nitrogen molecules on the surface of photocatalysts under mild conditions) have still not been well solved and the photocatalytic activities are generally low. In this miniature review, I summarize the most recent progress of photocatalytic N2 fixation for ammonia synthesis, focusing specifically on two attractive aspects for adsorption and activation of nitrogen molecules:one is engineering of oxygen vacancies, and the other is mimicking natural nitrogenase for constructing artificial systems for N2 fixation. Several representative works focusing on these aspects in artificial systems have been reported recently, and it has been demonstrated that both factors play more significant roles in photocatalytic N2 reduction and fixation under ambient conditions. At the end of the review, I also give some remarks and perspective on the existing challenges and future directions in this field.
Ammonia synthesis via the Haber-Bosch process, which has been heralded as the most important invention of the 20th century, consumes massive amounts of energy, around~1%-2% of the world's annual energy consumption. Developing green and sustainable strategies for NH3 synthesis under ambient conditions, using renewable energy, is strongly desired, by both industrial and scientific researchers. Artificial photosynthesis for ammonia synthesis, which has recently attracted significant attention, directly produces NH3 from sunlight, and N2 and H2O via photocatalysis. This has been regarded as an ideal, energy-saving and environmentally-benign process for NH3 production because it can be performed under normal temperature and atmospheric pressure using renewable solar energy. Although sustainable developments have been achieved since the pioneering work in 1977, many challenging issues (e.g., adsorption and activation of nitrogen molecules on the surface of photocatalysts under mild conditions) have still not been well solved and the photocatalytic activities are generally low. In this miniature review, I summarize the most recent progress of photocatalytic N2 fixation for ammonia synthesis, focusing specifically on two attractive aspects for adsorption and activation of nitrogen molecules:one is engineering of oxygen vacancies, and the other is mimicking natural nitrogenase for constructing artificial systems for N2 fixation. Several representative works focusing on these aspects in artificial systems have been reported recently, and it has been demonstrated that both factors play more significant roles in photocatalytic N2 reduction and fixation under ambient conditions. At the end of the review, I also give some remarks and perspective on the existing challenges and future directions in this field.
2018, 39(7): 1189-1193
doi: 10.1016/S1872-2067(18)63077-3
Abstract:
N2O is a promising green propellant and exhibits great potential for satellite propulsion systems. It is difficult for catalytic decomposition, which is an important way to initiate the propulsion process, to occur at temperatures below 600℃ due to the high activation energy of N2O. In this work, we report an Ir supported on rutile TiO2 (Ir/r-TiO2) catalyst which exhibits a fairly high activity for high-concentration N2O decomposition. HAADF-STEM, H2-TPR, and XPS results indicate that highly dispersed Ir particles and improved oxygen mobility on the Ir/r-TiO2 could facilitate the decomposition of N2O and desorption of the adsorbed oxygen. Bridge-bonded peroxide intermediates were observed with in-situ DRIFT and herein, a detailed decomposition route is proposed.
N2O is a promising green propellant and exhibits great potential for satellite propulsion systems. It is difficult for catalytic decomposition, which is an important way to initiate the propulsion process, to occur at temperatures below 600℃ due to the high activation energy of N2O. In this work, we report an Ir supported on rutile TiO2 (Ir/r-TiO2) catalyst which exhibits a fairly high activity for high-concentration N2O decomposition. HAADF-STEM, H2-TPR, and XPS results indicate that highly dispersed Ir particles and improved oxygen mobility on the Ir/r-TiO2 could facilitate the decomposition of N2O and desorption of the adsorbed oxygen. Bridge-bonded peroxide intermediates were observed with in-situ DRIFT and herein, a detailed decomposition route is proposed.
2018, 39(7): 1194-1201
doi: 10.1016/S1872-2067(18)63095-5
Abstract:
We report a hydrogen-evolution dimerization of styrenes via the synergistic merger of Acr+-Mes photocatalyst and cobaloxime proton reduction catalysts. By utilizing this dual catalyst system, 1,2-dihydro-1-arylnaphthalene derivatives can be directly constructed from commercially available styrenes. Our reaction proceeds smoothly under mild conditions without the need for oxidants or hydrogen atom transfer reagents, and the sole byproduct is hydrogen gas. Mechanistic investigation suggests that the reaction is initiated by photoinduced electron transfer under visible-light irradiation.
We report a hydrogen-evolution dimerization of styrenes via the synergistic merger of Acr+-Mes photocatalyst and cobaloxime proton reduction catalysts. By utilizing this dual catalyst system, 1,2-dihydro-1-arylnaphthalene derivatives can be directly constructed from commercially available styrenes. Our reaction proceeds smoothly under mild conditions without the need for oxidants or hydrogen atom transfer reagents, and the sole byproduct is hydrogen gas. Mechanistic investigation suggests that the reaction is initiated by photoinduced electron transfer under visible-light irradiation.
2018, 39(7): 1202-1209
doi: 10.1016/S1872-2067(18)63102-X
Abstract:
The development of active and durable non-Pt electrocatalysts with well-defined microstructure is of great importance to both fuel cell applications and fundamental understanding. Herein, we report a surface-doping process to prepare well-defined W-doped Pd nanocubes with a tunable atomic percent of W from 0 to 1.5% by using the Pd nanocubes as seeds. The obtained 1.2%W-doped Pd nanocubes/C exhibited greatly enhanced electrocatalytic performance toward oxygen reduction reaction in alkaline media, presenting an enhancement factor of 4.7 in specific activity and 2.5 in mass activity compared to the activity of a commercial Pt/C catalyst. The downshift of the d-band center due to a negative charge transfer from W to Pd intrinsically accounts for such improvement in activity by weakening the adsorption of reaction intermediates. Also, the 1.2%W-doped Pd nanocubes/C showed superior catalytic properties for the ethanol oxidation reaction, showing great potential for serving as a bifunctional electrocatalyst in fuel cells.
The development of active and durable non-Pt electrocatalysts with well-defined microstructure is of great importance to both fuel cell applications and fundamental understanding. Herein, we report a surface-doping process to prepare well-defined W-doped Pd nanocubes with a tunable atomic percent of W from 0 to 1.5% by using the Pd nanocubes as seeds. The obtained 1.2%W-doped Pd nanocubes/C exhibited greatly enhanced electrocatalytic performance toward oxygen reduction reaction in alkaline media, presenting an enhancement factor of 4.7 in specific activity and 2.5 in mass activity compared to the activity of a commercial Pt/C catalyst. The downshift of the d-band center due to a negative charge transfer from W to Pd intrinsically accounts for such improvement in activity by weakening the adsorption of reaction intermediates. Also, the 1.2%W-doped Pd nanocubes/C showed superior catalytic properties for the ethanol oxidation reaction, showing great potential for serving as a bifunctional electrocatalyst in fuel cells.
2018, 39(7): 1210-1218
doi: 10.1016/S1872-2067(18)63089-X
Abstract:
Heteroatom-doped carbon has been demonstrated to be one of the most promising non-noble metal catalysts with high catalytic activity and stability through the modification of the electronic and geometric structures. In this study, we develop a novel solvent method to prepare interconnected N, S co-doped three-dimensional (3D) carbon networks with tunable nanopores derived from an asso-ciated complex based on melamine and sodium dodecylbenzene sulfonate (SDBS). After the intro-duction of silica templates and calcination, the catalyst exhibits 3D networks with interconnected 50-nm pores and partial graphitization. With the increase of the number of Lewis base sites caused by the N doping and change of the carbon charge and spin densities caused by the S doping, the designed N, S co-doped catalyst exhibits a similar electrochemical activity to that of the commercial 20-wt% Pt/C as an oxygen reduction reaction catalyst. In addition, in an aluminum-air battery, the proposed catalyst even outperforms the commercial 5-wt% Pt/C catalyst. Both interconnected porous structures and synergistic effects of N and S contribute to the superior catalytic perfor-mance. This study paves the way for the synthesis of various other N-doped and co-doped carbon materials as efficient catalysts in electrochemical energy applications.
Heteroatom-doped carbon has been demonstrated to be one of the most promising non-noble metal catalysts with high catalytic activity and stability through the modification of the electronic and geometric structures. In this study, we develop a novel solvent method to prepare interconnected N, S co-doped three-dimensional (3D) carbon networks with tunable nanopores derived from an asso-ciated complex based on melamine and sodium dodecylbenzene sulfonate (SDBS). After the intro-duction of silica templates and calcination, the catalyst exhibits 3D networks with interconnected 50-nm pores and partial graphitization. With the increase of the number of Lewis base sites caused by the N doping and change of the carbon charge and spin densities caused by the S doping, the designed N, S co-doped catalyst exhibits a similar electrochemical activity to that of the commercial 20-wt% Pt/C as an oxygen reduction reaction catalyst. In addition, in an aluminum-air battery, the proposed catalyst even outperforms the commercial 5-wt% Pt/C catalyst. Both interconnected porous structures and synergistic effects of N and S contribute to the superior catalytic perfor-mance. This study paves the way for the synthesis of various other N-doped and co-doped carbon materials as efficient catalysts in electrochemical energy applications.
2018, 39(7): 1219-1227
doi: 10.1016/S1872-2067(18)63094-3
Abstract:
Plasmonic photocatalysis with tunable light absorption has aroused significant attention in so-lar-to-chemical energy conversion. However, the energy conversion efficiency of plasmonic photo-catalysts is impeded by ineffective charge separation and the lack of highly active sites for redox reactions. In this work, the Au nanoparticle size and Au-TiO2 interaction of the Au/TiO2 plasmonic photocatalyst were adjusted simultaneously using a post-calcination treatment. The visi-ble-light-induced water oxidation activity exhibited a volcano-like relationship with the calcination temperature; the treated photocatalyst at 600℃ manifested the highest activity. Characterization with UV-visible spectra, XRD, SEM, and XPS revealed that the effect of the Au nanoparticle size and Au-TiO2 interaction were both responsible for the increase in plasmon-induced water oxidation activity.
Plasmonic photocatalysis with tunable light absorption has aroused significant attention in so-lar-to-chemical energy conversion. However, the energy conversion efficiency of plasmonic photo-catalysts is impeded by ineffective charge separation and the lack of highly active sites for redox reactions. In this work, the Au nanoparticle size and Au-TiO2 interaction of the Au/TiO2 plasmonic photocatalyst were adjusted simultaneously using a post-calcination treatment. The visi-ble-light-induced water oxidation activity exhibited a volcano-like relationship with the calcination temperature; the treated photocatalyst at 600℃ manifested the highest activity. Characterization with UV-visible spectra, XRD, SEM, and XPS revealed that the effect of the Au nanoparticle size and Au-TiO2 interaction were both responsible for the increase in plasmon-induced water oxidation activity.
2018, 39(7): 1228-1239
doi: 10.1016/S1872-2067(18)63055-4
Abstract:
Spinel oxides containing Co and Ni are a promising substitute as a noble metal catalyst for methane combustion. Achieving a complete oxidation of methane under 400℃ remains challenging, and whether Ni3+ or Co3+ is the active center for the catalytic combustion of methane is a controversial issue. Therefore, we designed a series of spinel oxide catalysts by exposing different amounts of Ni3+ and Co3+ deposited on the surface by hydrothermal and co-precipitation methods in order to study the influence of high oxidation state (Ni3+ and Co3+) on surface and catalytic activity. The catalytic performance increased almost linearly with increasing Ni3+ + Co3+ on the surface of the catalyst. Thus, we are convinced that Ni3+ and Co3+ both act as active centers. The amount of Ni3+ + Co3+ on a hydrothermal 60 h NiCo2O4 nanosheet surface is the highest, and reveals the best catalytic performance with T50 (50% methane conversion) at about 280℃. 10 vol% H2O added to the system has little impact on activity, especially at high space velocities due to the long hydrothermal time with less absorbed oxygen species and crystal defects. Overall, these results help clarify methane activation mechanisms and aid the development of more efficient low-cost catalysts.
Spinel oxides containing Co and Ni are a promising substitute as a noble metal catalyst for methane combustion. Achieving a complete oxidation of methane under 400℃ remains challenging, and whether Ni3+ or Co3+ is the active center for the catalytic combustion of methane is a controversial issue. Therefore, we designed a series of spinel oxide catalysts by exposing different amounts of Ni3+ and Co3+ deposited on the surface by hydrothermal and co-precipitation methods in order to study the influence of high oxidation state (Ni3+ and Co3+) on surface and catalytic activity. The catalytic performance increased almost linearly with increasing Ni3+ + Co3+ on the surface of the catalyst. Thus, we are convinced that Ni3+ and Co3+ both act as active centers. The amount of Ni3+ + Co3+ on a hydrothermal 60 h NiCo2O4 nanosheet surface is the highest, and reveals the best catalytic performance with T50 (50% methane conversion) at about 280℃. 10 vol% H2O added to the system has little impact on activity, especially at high space velocities due to the long hydrothermal time with less absorbed oxygen species and crystal defects. Overall, these results help clarify methane activation mechanisms and aid the development of more efficient low-cost catalysts.
2018, 39(7): 1240-1248
doi: 10.1016/S1872-2067(18)63061-X
Abstract:
Photocatalysis driven by near-infrared (NIR) light is of scientific and technological interest for exploiting solar energy. In this study, we demonstrate a facile hydrothermal process to synthesize core-shell nanoparticles combining upconversion nanoparticles (UCNPs) and alloyed ZnxCd1-xS, which can be excited using NIR or visible light. Morphologies, phase, and chemical composition have been investigated using field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis, and atomic absorption spectroscopy. Moreover, we found that amorphous TiO2 layers existing in the final samples play an important role in formation of UCNPs@ZnxCd1-xS yolk-shell nanoparticles, which bind the as-prepared ZnxCd1-xS nanoparticles tightly to form yolk-shell nanoparticles. The chemical composition of alloyed ZnxCd1-xS can be tunable by adjusting the amount of the Cd and Zn source compounds. The photochemical reduction of Cr(VI) in water has been performed to study the photocatalytic performance under irradiation by NIR light or a simulated solar light, showing efficient photoreduction and Cr(VI) removal over the as-prepared UCNPs@ZnxCd1-xS/TiO2 yolk-shell nanoparticles. The as-prepared UCNPs@ZnxCd1-xS/ TiO2 nanoparticles show excellent production of hydroxyl radicals, which are responsible for the photochemical reduction of Cr(VI) to Cr(Ⅲ). This study will provide an alternative strategy for environmental wastewater treatment, making full use of solar energy.
Photocatalysis driven by near-infrared (NIR) light is of scientific and technological interest for exploiting solar energy. In this study, we demonstrate a facile hydrothermal process to synthesize core-shell nanoparticles combining upconversion nanoparticles (UCNPs) and alloyed ZnxCd1-xS, which can be excited using NIR or visible light. Morphologies, phase, and chemical composition have been investigated using field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis, and atomic absorption spectroscopy. Moreover, we found that amorphous TiO2 layers existing in the final samples play an important role in formation of UCNPs@ZnxCd1-xS yolk-shell nanoparticles, which bind the as-prepared ZnxCd1-xS nanoparticles tightly to form yolk-shell nanoparticles. The chemical composition of alloyed ZnxCd1-xS can be tunable by adjusting the amount of the Cd and Zn source compounds. The photochemical reduction of Cr(VI) in water has been performed to study the photocatalytic performance under irradiation by NIR light or a simulated solar light, showing efficient photoreduction and Cr(VI) removal over the as-prepared UCNPs@ZnxCd1-xS/TiO2 yolk-shell nanoparticles. The as-prepared UCNPs@ZnxCd1-xS/ TiO2 nanoparticles show excellent production of hydroxyl radicals, which are responsible for the photochemical reduction of Cr(VI) to Cr(Ⅲ). This study will provide an alternative strategy for environmental wastewater treatment, making full use of solar energy.
2018, 39(7): 1249-1257
doi: 10.1016/S1872-2067(18)63058-X
Abstract:
A convenient, expeditious, and high-efficiency protocol for the transformation of alcohols into esters using a Co-modified N-doped mesoporous carbon material (Co-N/m-C) as the catalyst is proposed. The catalyst was prepared through direct pyrolysis of a macromolecular precursor. The catalyst prepared using a pyrolysis temperature of 900℃ (labeled Co-N/m-C-900) exhibited the best performance. The strong coordination between the ultra-dispersed cobalt species and the pyridine nitrogen as well as the large area of the mesoporous surface resulted in a high turnover frequency value (107.6 mol methyl benzoate mol-1 Co h-1) for the direct aerobic oxidation of benzyl alcohol to methyl benzoate. This value is much higher than those of state-of-the-art transition-metal-based nanocatalysts reported in the literature. Moreover, the catalyst exhibited general applicability to various structurally diverse alcohols, including benzylic, allylic, and heterocyclic alcohols, achieving the target esters in high yields. In addition, a preliminary evaluation revealed that Co-N/m-C-900 can be used six times without significant activity loss. In general, the process was rapid, simple, and cost-effective.
A convenient, expeditious, and high-efficiency protocol for the transformation of alcohols into esters using a Co-modified N-doped mesoporous carbon material (Co-N/m-C) as the catalyst is proposed. The catalyst was prepared through direct pyrolysis of a macromolecular precursor. The catalyst prepared using a pyrolysis temperature of 900℃ (labeled Co-N/m-C-900) exhibited the best performance. The strong coordination between the ultra-dispersed cobalt species and the pyridine nitrogen as well as the large area of the mesoporous surface resulted in a high turnover frequency value (107.6 mol methyl benzoate mol-1 Co h-1) for the direct aerobic oxidation of benzyl alcohol to methyl benzoate. This value is much higher than those of state-of-the-art transition-metal-based nanocatalysts reported in the literature. Moreover, the catalyst exhibited general applicability to various structurally diverse alcohols, including benzylic, allylic, and heterocyclic alcohols, achieving the target esters in high yields. In addition, a preliminary evaluation revealed that Co-N/m-C-900 can be used six times without significant activity loss. In general, the process was rapid, simple, and cost-effective.
2018, 39(7): 1258-1262
doi: 10.1016/S1872-2067(18)63045-1
Abstract:
Palladium-catalyzed carboxylative Suzuki coupling reactions of benzyl chlorides with allyl pinacol-borate were successfully conducted in the absence of any extra ligand to produce β,γ-unsaturated esters in satisfactory to good yields. The carboxylative Suzuki coupling reaction proceeded smooth-ly under mild conditions in the presence of palladium nanoparticles generated in situ through the formation of a π-benzylpalladium chloride intermediate.
Palladium-catalyzed carboxylative Suzuki coupling reactions of benzyl chlorides with allyl pinacol-borate were successfully conducted in the absence of any extra ligand to produce β,γ-unsaturated esters in satisfactory to good yields. The carboxylative Suzuki coupling reaction proceeded smooth-ly under mild conditions in the presence of palladium nanoparticles generated in situ through the formation of a π-benzylpalladium chloride intermediate.
2018, 39(7): 1263-1271
doi: 10.1016/S1872-2067(18)63063-3
Abstract:
Fe-containing graphitic carbon nitride (Fe-g-C3N4) materials were synthesized via one-step pyrolysis of FeCl3 and dicyandiamide. The physicochemical properties of the synthesized Fe-g-C3N4 samples were characterized by N2 adsorption-desorption, X-ray diffraction, thermal gravimetric, Fourier transform infrared, UV-vis diffuse reflectance, X-ray photoelectron spectroscopy, and transmission electron microscopy. The Fe cations were anchored by nitrogen-rich g-C3N4, whereas the graphitic structures of g-C3N4 were retained after the introduction of Fe. As heterogeneous catalysts, Fe-g-C3N4 exhibited good catalytic activity in the direct hydroxylation of benzene to phenol with H2O2, affording a maximum yield of phenol of up to 17.5%. Compared with other Fe-and V-containing g-C3N4 materials, Fe-g-C3N4 features a more convenient preparation procedure and higher catalytic productivity of phenol.
Fe-containing graphitic carbon nitride (Fe-g-C3N4) materials were synthesized via one-step pyrolysis of FeCl3 and dicyandiamide. The physicochemical properties of the synthesized Fe-g-C3N4 samples were characterized by N2 adsorption-desorption, X-ray diffraction, thermal gravimetric, Fourier transform infrared, UV-vis diffuse reflectance, X-ray photoelectron spectroscopy, and transmission electron microscopy. The Fe cations were anchored by nitrogen-rich g-C3N4, whereas the graphitic structures of g-C3N4 were retained after the introduction of Fe. As heterogeneous catalysts, Fe-g-C3N4 exhibited good catalytic activity in the direct hydroxylation of benzene to phenol with H2O2, affording a maximum yield of phenol of up to 17.5%. Compared with other Fe-and V-containing g-C3N4 materials, Fe-g-C3N4 features a more convenient preparation procedure and higher catalytic productivity of phenol.
2018, 39(7): 1272-1279
doi: 10.1016/S1872-2067(18)63064-5
Abstract:
The reaction mechanism of zeolite-or zeotype-catalyzed methanol-to-olefins (MTO) conversion is still a subject of debate. Employing periodic density functional theory calculations, the olefin-based cycle was studied using tetramethylethene (TME) as a representative olefinic hydrocarbon pool in H-SAPO-18 zeotype. The overall free energy barrier at 673 K was calculated and found to be less than 150 kJ/mol in the TME-based cycle, much lower than those in the aromatic-based cycle (> 200 kJ/mol), indicating that olefins themselves are the dominant active hydrocarbon pool species in H-SAPO-18. The similarity of the intermediates involved between the aromatic-based cycle and the olefin-based cycle was also highlighted, revealing that both cycles were pattern-consistent. The selectivity related to the distribution of cracking precursors, such as higher olefins or carbenium ions, as a result of the olefin-based cycle for the MTO conversion. The enthalpy barrier of the crack-ing step scaled linearly with the number of carbon atoms of cracking precursors to produce ethene or propene with ethene being much less favored than propene for cracking of C7 and higher pre-cursors. This work highlighted the importance of the olefin-based cycle in H-SAPO-18 for the MTO conversion and established the similarity between the olefin-based and aromatic-based cycles.
The reaction mechanism of zeolite-or zeotype-catalyzed methanol-to-olefins (MTO) conversion is still a subject of debate. Employing periodic density functional theory calculations, the olefin-based cycle was studied using tetramethylethene (TME) as a representative olefinic hydrocarbon pool in H-SAPO-18 zeotype. The overall free energy barrier at 673 K was calculated and found to be less than 150 kJ/mol in the TME-based cycle, much lower than those in the aromatic-based cycle (> 200 kJ/mol), indicating that olefins themselves are the dominant active hydrocarbon pool species in H-SAPO-18. The similarity of the intermediates involved between the aromatic-based cycle and the olefin-based cycle was also highlighted, revealing that both cycles were pattern-consistent. The selectivity related to the distribution of cracking precursors, such as higher olefins or carbenium ions, as a result of the olefin-based cycle for the MTO conversion. The enthalpy barrier of the crack-ing step scaled linearly with the number of carbon atoms of cracking precursors to produce ethene or propene with ethene being much less favored than propene for cracking of C7 and higher pre-cursors. This work highlighted the importance of the olefin-based cycle in H-SAPO-18 for the MTO conversion and established the similarity between the olefin-based and aromatic-based cycles.
