摘要:The activation and selective oxidation of C—H bonds under mild conditions are challenges in both academic and industrial chemistry. Single-atom catalysts have become a research hotspot in the field of catalysis due to their high atomic utilization efficiency and low-coordination environments of surface metals. ε-Keggin type zeolitic octahedral metal oxides (ZOMOs) are novel inorganic porous materials formed by the ordered assembly of polyoxometalate frameworks and transition metals. This structure can provide a large number of single-atom transition metal sites, making ε-Keggin-ZOMOs ideal catalytic materials for methane (CH4) oxidation to methanol (CH3OH). By varying molybdenum sources and transition metal sources, ten different ε-Keggin-ZOMOs materials were synthesized by hydrothermal method. The geometric and electronic structures of the materials were comprehensively analyzed by characterization techniques such as SEM, TEM and XRD. The catalytic performances of different ε-Keggin-ZOMOs materials for selective CH4 oxidation to CH3OH in H2 and O2 atmospheres under mild conditions were tested, and corresponding structure-activity relationships were studied. The results show that the morphologies and crystal structures of synthesized ε-Keggin-ZOMOs materials are highly similar, but completely different from the original oxides, and ε-Keggin-ZOMOs materials have a completely new structure. The Mo—O framework constitutes a stable skeleton of the material, but the framework structure begins to undergo progressive destruction when the calcination temperature exceeds 200 ℃ and completely collapses at 400 ℃. NH4-MM material demonstrates the best catalytic performance, with the CH3OH space-time yield of 2.65 mmol/(g·h) and the CH3OH selectivity of 93.9% (70 ℃, 3 MPa and 0.5 h). The symmetry of the electron clouds density distribution in the transition metal 3d orbitals and the atomic radius are key factors influencing the catalytic performances of ε-Keggin-ZOMOs materials.
摘要:The catalytic conversion of coal-based syngas to ethanol is one of the important pathways for the clean and efficient utilization of coal. ZnAl catalysts prepared using complete liquid-phase method can be employed for the direct catalysis of CO hydrogenation to produce ethanol. A series of ZnAl catalysts with different n(Zn)/n(Al) were prepared via the complete liquid-phase method, and their catalytic performances in syngas to ethanol were evaluated in a slurry bed reactor. The structures of the catalysts before and after reactions were characterized using XRD, N2 adsorption/desorption, FT-IR, NH3-TPD-MS and XPS. The results show that under the conditions of total syngas flow rate of 150 mL/min, V(H2):V(CO) of 2:1, temperature of 280 °C and pressure of 4.0 MPa, the catalyst with n(Zn)/n(Al) of 1.0 exhibits relatively optimal catalytic performance over 48 h to 168 h, maintaining total alcohol selectivity of approximately 23%, with the proportion of ethanol in total alcohol (mole fraction) consistently above 60%. The aluminum component content significantly influences the textural property and surface acidity of the catalyst, and adjusting n(Zn)/n(Al) can modulate the surface Zn(2 - δ)+ (0 < δ < 2) content of the catalyst, thereby affecting total alcohol selectivity.
摘要:Dry reforming of methane (DRM) can convert greenhouse gases CH4 and CO2 into high-value syngas, with dual benefits of environmental mitigation and energy upgrading. However, this reaction usually needs to be carried out above 700 ℃, leading to high energy consumption and catalyst deactivation due to metal sintering and carbon deposit. The equilibrium compositions in the DRM system were systematically evaluated based on thermodynamic analysis, and the effects of reaction temperatures, inert gas contents, reaction pressures and feed ratios of raw gas (n(CH4):n(CO2)) on the catalytic performances and carbon deposit behaviors of catalysts were discussed. The deviations between actual reaction results reported in the literature and the theoretical equilibrium predictions were also thoroughly analyzed. The results show that reducing n(CH4):n(CO2) to 1:2 can lower the complete conversion temperature by 200 ℃ and effectively suppress carbon deposit formation at above 700 °C. In addition, changing the inert gas dilution ratio n(CH4):n(CO2):n(inert gas) to 1:1:2 can remarkably suppress carbon deposit from 500 ℃ to 800 ℃. However, high-pressure operations suitable for the industrial applications are not conducive to raw material conversion and can promote carbon deposit. It is more reasonable to evaluate the actual reaction results in the low-temperature zone (< 600 ℃) and high-temperature zone (> 800 ℃) using the no carbon deposit formation model, while the actual reaction results from 600 ℃ to 800 ℃ need to be evaluated in conjunction with the carbon deposit equilibrium model. This research clearly defines the operational boundaries and suppression strategies of carbon deposit for DRM based on thermodynamic analysis, in order to provide an essential theoretical basis for catalyst design and catalytic performance evaluation and optimization.
关键词:dry reforming of methane;thermodynamic calculation;equilibrium compositions;reaction conditions;carbon deposit
摘要:CO catalytic oxidation technology is one of the key technologies for achieving “carbon neutrality” goal. La was introduced into CeZrOx and then noble metal Pt and Pd were loaded by impregnation method to prepare Pt-Pd based bimetallic catalysts. By characterization techniques such as XRD, XPS and H2-TPR, the effects of La modification on the structures, valence states of surface elements, adsorption properties and CO oxidation mechanism of the catalysts were explored. The results show that the La forms a stable CeZrLaOx solid solution structure with CeZrOx, increasing the specific surface area and concentration of surface oxygen vacancy of the catalysts. Pt-Pd/CeZrLaOx-5% exhibits excellent catalytic performance for CO oxidation. Under the temperature from 100 ℃ to 450 ℃ and space velocity of 150000 h-1, the reaction temperatures for CO conversion rates of 50% and 90% are 165.7 ℃ and 222.0 ℃, respectively. In situ DRIFTS results indicate that on the Pt-Pd/CeZrOx, the linear adsorption of CO mainly takes place on Pd2+ sites. La enhances the adsorption capacity of Pt2+ sites to form a synergistic mechanism of Pt-Pd dual active sites, thereby promoting the rapid transformation of carbonate intermediates on the catalyst surface and improving the catalytic performances of the catalysts for CO oxidation.
摘要:Carbonyl compounds are extensively utilized in pharmaceutical and fine chemical industries. The selective oxidation of alcohols serves as an important pathway for synthesizing carbonyl compounds. However, conventional thermal catalytic methods suffer from harsh reaction conditions, low target product selectivities and environmental concerns. Heterogeneous catalysts have demonstrated unique advantages in high-efficiency selective oxidation of alcohols, and their catalytic performances depend on geometric and electronic configurations of the catalyst. Interface engineering enables precise optimization of alcohol adsorption and activation processes through modulation of interfacial active sites. The regulatory mechanisms and strategies of interface engineering in thermocatalytic heterogeneous alcohol oxidation reactions were reviewed. The mechanisms of interfacial modulation of non-homogeneous catalysts by electronic and geometrical effects were discussed and the interface-based modulation methodologies were analyzed. In addition, a brief discussion of the impact of artificial intelligence-assisted approaches on catalyst design was provided and the future development direction of designing interfacial active sites in alcohol oxidation reactions via interface engineering was prospected.
摘要:During aldol condensation to methyl methacrylate (MMA), the catalyst often has poor reaction stability due to carbon deposition. The strong acid and strong base sites on surface of the catalyst are important reasons for carbon deposition on catalyst, and the method of eliminating the strong acid and strong base sites on surface of the catalyst can be used to improve stability of the catalyst. A series of acid-base bifunctional catalysts were prepared with SiO2 as the support, Cs as the active component and Nb, V or P as the active additives. Effects of types of active additives and Cs contents (m(Cs)/m(support)) on the crystal structures, acidities and alkalinities and textural properties of the catalysts were investigated by XRD, TPD and N2 adsorption/desorption, and the catalytic performances and stabilities of the catalysts in aldol condensation to MMA were also studied. The results show that under the conditions of n(formaldehyde):n(methyl propionate) of 1:3, temperature of 340 ℃, pressure of 0.2 MPa and raw material feed rate of 0.2 mL/min for 48 h, the catalyst with Cs content of 10.0% (Cs10.0-Nb/SiO2) demonstrates excellent catalytic performance, with the product (MMA + methacrylic acid) selectivity of 93.4% and the product yield of 21.3%. After selectively covering strong acid sites with NH3 and reacting for 96 h, the product yield and product selectivity of Cs10.0-Nb/SiO2 are 20.2% and 91.5%, respectively, slightly better than the original Cs10.0-Nb/SiO2.
摘要:Converting glycerol into high-value-added acrolein is one of the effective ways to promote green and low-carbon development of chemical industry. The catalyst is the key factor influencing the production capacity of the glycerol to acrolein route. A series of copper phosphate catalysts were prepared by rotary evaporation method, and the catalysts were characterized by XRD, SEM and so on. Effects of different n(Cu)/n(P) and Cu sources on structure-activity relationships of catalysts were studied. The results show that increasing n(Cu)/n(P) can increase the specific surface area of catalyst, thereby improving the catalytic performance of the catalyst. Different Cu sources can affect the surface morphology of the catalyst, and thereby influence its acrolein selectivity. Under the conditions of reaction temperature of 300 ℃ and glycerol solution (mass fraction of 10%) feed flow rate of 4 mL/h, the catalyst prepared with n(Cu)/n(P) of 1.0 and copper nitrate as the Cu source exhibits relatively optimal catalytic performance, with glycerol conversion rate, acrolein selectivity and acrolein yield of 97%, 80% and 78%, respectively.
摘要:Alcohol-amine solvents are commonly used as CO2 chemical absorbents in industrial fields. The CO2 desorption kinetics of alcohol-amine rich solution is an important index for evaluating alcohol-amine absorbents. Studying the CO2 desorption kinetics of alcohol-amine rich solution and establishing corresponding kinetic models are of significant importance for guiding the development and optimization of alcohol-amine absorbents. Using 5 mol/L diethylene glycolamine (DGA) solution enriched with CO2 (“DGA rich solution” for short) as the target, and a continuous stirred reactor was used to determine their CO2 desorption rate. The rotating Reynolds number (Rer) and desorption temperature in the reactor were selected as the state parameters to measure the turbulence intensity and temperature in desorption process, respectively, and the kinetic equation for CO2 desorption of DGA rich solution was fitted. The results indicate that compared to Rer = 0 in reactor, increasing the turbulence intensity (Rer = 20000) can reduce the heat of macroscopic desorption reaction of DGA rich solution by 6.21% and reduce the apparent activation energy by 2.65%. In the intermittent desorption experiments, increasing the turbulence intensity can reduce the CO2 equilibrium loading of DGA rich solution by 3.82% to 9.16% (rotational speed of 200 r/min as the reference), and increasing the desorption temperature from 353 K to 372 K can reduce the CO2 equilibrium loading of DGA rich solution by 34.93% to 39.64%. In the continuous desorption experiments, increasing the turbulence intensity can increase the CO2 steady-state desorption rate from 6.46% to 79.63% (rotational speed of 200 r/min as the reference), and increasing the solution desorption temperature from 347 K to 376 K can increase the CO2 steady-state desorption rate by about 17-fold. The CO2 desorption kinetics model of DGA rich solution in the desorption temperature range of 347 K to 376 K, Rer of 846 to 7789 and CO2 loading of 1.35 mol/L to 2.68 mol/L is . Among them, r is the CO2 desorption rate, R is the ideal gas constant, Tdes is the desorption temperature, is the CO2 loading in DGA rich solution, is the CO2 equilibrium concentration in rich DGA solution, is the concentration of DGA at desorption equilibrium.
关键词:CO2 desorption kinetics;turbulence intensity;CO2 desorption rate;DGA rich solution;apparent activation energy
摘要:With the increasingly severe problem of global warming, carbon dioxide (CO2) capture technology based on solid porous adsorption materials has received widespread attention as an important carbon emission reduction strategy. A series of nitrogen-doped porous hypercrosslinked polymers (HCPs) were prepared using benzimidazole (BMZ), carbazole (CBZ), diphenyl imidazole (DMZ) and azobenzene (ABZ) as reaction monomers. Characterization methods including SEM, N2 adsorption/desorption, elemental analysis and FT-IR were employed to analyze the influence of different monomer structures on the microstructure and pore structure of the HCPs. Static adsorption, dynamic adsorption and cyclic stability tests were carried out to evaluate their CO2 adsorption performance. The results show that the HCPs exhibit a fibrous microstructure with tightly arranged pore structures on the fibers. The smaller the size of the reaction monomer, the larger the specific surface area and micropore area of the prepared HCPs. Among them, the specific surface area of HCP-ABZ reaches 875.5 m2/g. The HCPs demonstrate excellent CO2 adsorption performance. The static CO2 adsorption capacity of HCP-CBZ at 273 K reaches 4.30 mmol/g, and the saturation adsorption capacity of HCP-BMZ during the dynamic CO2 adsorption process reaches 1.13 mmol/g. After five adsorption/desorption cycles, the CO2 adsorption capacity of HCP-BMZ remains basically unchanged, indicating excellent cyclic stability. The study provides a reference for the development of efficient CO2 adsorption materials.
摘要:Hydrogen energy is one of the key clean energy sources for achieving the “carbon neutrality” goal, so developing efficient and low-carbon hydrogen production processes is of vital importance. Methanol steam reforming (MSR) emerges as a promising technique for hydrogen production, yet challenges persist in achieving both high hydrogen yield and CO2 selectivity over noble metal-based catalysts. Herein, CeO2 supports with different morphologies of nanorods (CeO2-R) and nanocubes (CeO2-C) were prepared by hydrothermal method for constructing Pt/CeO2 catalysts (Pt/CeO2-R, Pt/CeO2-C). Combined with comprehensive characterization techniques, including HRTEM, XPS, and Raman spectroscopy, the morphology-dependent catalytic behavior in MSR reactions was studied. The results show that CeO2-R with exposure of dominant (111) facet exhibits more surface oxygen vacancies, generating abundant Ptδ+-Ov-Ce3+ interfacial active sites (intrinsic active site). Under the conditions of 325 ℃, a methanol-water mixture feed flow rate of 0.02 mL/min (n(H2O):n(CH3OH) = 3:1), and space velocity of 34800 mL/(g·h), Pt/CeO2-R achieves a hydrogen production rate of 490.35 mmol/(g·h) with CO2 selectivity of 92.9%. Under the same conditions, the CO2 selectivity is only 63.0% by Pt/CeO2-C. This study can provide valuable insights for the design of new high-efficiency MSR catalysts.
关键词:hydrogen energy;methanol steam reforming;CeO2 morphology;oxygen vacancies;tandem catalytic process