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An experimental study on synthesis of glycolic acid via carbonylation of formaldehyde using PTFE membrane contactor
No full textThe batch and continuous synthesis of glycolic acid via carbonylation of formaldehyde using high-temperature and high-pressure poly(tetrafluoroethylene) (PTFE) membrane contactor was demonstrated in this paper. The experiments in conventional stirred tank reactor were also conducted for comparison. The results showed that the yield of glycolic acid was significantly improved in membrane contactor system compared to the stirred tank reactor. At reaction temperature of 120 degrees C and reaction pressure of 5.0 MPa, the yield of glycolic acid was higher than 90% when the reaction time was 1 h, while it was only about 45% in the conventional stirred tank reactor. The enhancement of yield was due to the improvement of mass transfer or dispersion of CO in membrane contactor system, which could provide larger gas-liquid contact area and shorter mass transfer distance. The stability of PTFE hollow fibre membranes was also investigated by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), liquid entry pressure (LEP), bubble point (BP) and contact angle (CA) measurements. The characterizations indicated that PTFE membranes exhibited good chemical and thermal stability in this reaction system. This study provided new idea and reference for the carbonylation of formaldehyde and similar gas-liquid reactions
singleatomalloypreparationandapplicationsinheterogeneouscatalysis
No full textThere have been remarkable progresses in manipulating heterogeneous catalysts' nanostructures in the past decade. The concept of single atom alloy (SAA) was firstly proposed in 2012 when researchers successfully stabilized single Pd atoms on the Cu(111) surface. However, earlier work in 2009, which focused on replacing one Au atom with a Pd atom in thiolate protected Au25 nanoclusters, could also be considered as the pioneer work of single atom alloy. Both kinds of single atom alloys exhibited the potential of maximum utilization of scarce elements and attractive catalytic performances. The well-defined structures of SAA catalysts make accurate modeling possible, which further realizes the rational design of single atom alloy catalysts. In this review, we summarize the research trajectory of single atom alloys as well as recent achievements in this field. We also introduce several commonly adopted characterization methods for SAA catalysts such as scanning tunneling microscopy (STM), temperature programmed reaction (TPR), extended X-ray absorption fine structure (EXAFS) spectra, matrix assisted laser desorption/ionization mass spectrum (MALDI-MS) and differential pulse voltammetry (DPV). Through discussing recent progresses in SAA catalysts, we propose that future researches in this filed should be focused on exploring new kinds of metal nanocrystals and controlling the nanostructure of SAA even more precisely
钠氩混合物电离后原子发射光谱的时间分辨特性
No full text研究钠氩(Na-Ar)混合物电离后产生的原子发射光谱的时间分辨特性。氩763.5 nm光谱强度随时间演变出现2个峰,第1个峰的衰减时间为(33.3±2.3) ns,激发态钠通过碰撞传能(时间常数为(15.2±0.8) ns将氩激发到2p6能级,再由该能级的粒子快速辐射形成第1个峰;第2个峰由氩离子与电子复合产生,其衰减过程包括快过程(0.24±0.03) μs和慢过程(3.98± 1.03) μs。利用电子浓度随时间的演变关系分析了复合过程对衰减时间的影响机理,获得了电子浓度、电子温度随时间的演变关系。利用时间分辨光谱解释了钠双线辐射加宽差异的假象,其成因是Stark加宽后的氩588.9 nm谱线叠加在钠D_2线上以及钠双线的自吸收。复合后,激发态氩原子的能级间隔较小,经过级联弛豫后,2p6能级粒子数的积累时间比钠3P能级更短,氩原子发射谱线的持续时间明显短于钠原子
Lead-Free Sodium-Indium Double Perovskite Nanocrystals through Doping Silver Cations for Bright Yellow Emission
No full textLead-free halide perovskite nanocrystals (NCs) have drawn wide attention for solving the problem of lead perovskites toxicity and instability. Herein, we synthesize the direct band gap double perovskites undoped and Ag-doped Cs2NaInCl6 NCs by variable temperature hot injection. The Cs2NaInCl6 NCs have little photoluminescence because of dark self-trapped excitons (STEs). The dark STEs can be converted into bright STEs by doping with Ag+ to produce a bright yellow emission, with the highest photoluminescence quantum efficiency of 31.1%. The dark STEs has been directly detected experimentally by ultrafast transient absorption (TA) techniques. The dynamics mechanism is further studied. In addition, the Ag-doped NCs show better stability than the undoped ones. This result provides a new way to enhance the optical properties of lead-free perovskites NCs for high-performance light emitters
Hydrodynamics and local mass transfer characterization under gas-liquid-liquid slug flow in a rectangular microchannel
No full textGas-liquid-liquid three-phase slug flow was generated in a glass microreactor with rectangular microchannel, where aqueous slugs were distinguished by relative positions to air bubbles and organic droplets. Oxygen from bubbles reacted with resazurin in slugs, leading to prominent color changes, which was used to quantify mass transfer performance. The development of slug length indicated a film flow through the corner between bubbles and the channel wall, where the aqueous phase was saturated with oxygen transferred from bubble body. This film flow results in the highest equivalent oxygen concentration within the slug led by a bubble and followed by a droplet. The three-phase slug flow subregime with alternate bubble and droplet was found to benefit the overall mass transfer performance most. These results provide insights into a precise manipulation of gas-liquid-liquid slug flow in microreactors and the relevant mass transfer behavior thereof
A modeling study on reaction and diffusion in MTO process over SAPO-34 zeolites
No full textIn this work, a modeling approach based on the dual-cycle mechanism and Maxwell-Stefan diffusion theory is developed to investigate the reaction and diffusion in methanol-to-olefins (MTO) process over SAPO-34 zeolites. In this work, the effect of coke formation on diffusion and adsorption in SAPO-34 zeolites were taken into consideration. The diffusivities and adsorption isotherms of the molecules of reactant and main product were derived by experimental measurements of the uptake rate of guest molecules. This model was used to study MTO reaction over SAPO-34 zeolites with different crystal size. It is shown that the model can capture the main features of MTO reaction as obtained by experiments, for instance, the different roles of the olefins-base cycle and aromatic-base cycle at different stage of the reaction. This model can predict the evolution of activated coke and non-activated coke during the reaction process. In particular, the simulation results directly show that the formation of non-activated coke can cause the pore blocking inside SAPO-34 zeolites and constrain the diffusion of different guest molecules to various extend. This is essential for us to understand the effect diffusion on catalyst lifetime and product selectivity in MTO reaction over SAPO-34 zeolites