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71.
The comparative evaluation of dehydrogenation yields and selectivities of skeletal reactions from n-hexane reactions on monofunctional Pt catalysts is a good universal indicator of catalyst deactivation, independently of the presence and the nature of the support. Four regions could be defined with increasing carbon accumulation, causing "beneficial", "selective", "nonselective" and "severe" deactivation. 相似文献
72.
73.
A conceptually new strategy for asymmetric catalysis, namely asymmetric activation, in which a chiral activator selectively activates one enantiomer of a racemic chiral catalyst, and a highly efficient screening system for finding the most effective catalysts, namely super high throughput screening (SHTS), by which the reaction can be conducted in parallel and the ee% of the product is allowed to determine within minutes, are summarized in the present account. It is reasonable to believe that SHTS technique combined with asymmetric activation or deactivation principle will provide a very powerful methodology for finding the new catalysts and the best catalyst tuning for asymmetric reactions. 相似文献
74.
利用直接焙烧法在400℃焙烧(NH4)6MoO7O24.4H2O制备了碳酸二甲酯(DMC)与乙酸苯酯(PA)酯交换合成碳酸二苯酯(DPC)反应的正交晶系MoO3催化剂,通过比表面积(BET)、X射线衍射(XRD)、X射线光电子能谱(XPS)、傅立叶变换红外光谱(FT-IR)、扫描电镜(SEM)等技术,对MoO3催化剂在失活前后的结构和形貌进行了分析,探讨了MoO3催化剂的失活行为.结果表明,正交晶系MoO3的(021)、(110)晶面有利于酯交换反应,多次使用后,催化剂表面有无定形碳的物理沉积,催化剂比表面积降低,催化剂颗粒长径比发生了改变,催化剂(021)、(110)晶面衍射峰强度减弱,进而导致了MoO3催化剂的失活. 相似文献
75.
Ali Nakhaei Pour Mohammad Reza Housaindokht Sayyed Faramarz Tayyari Jamshid Zarkesh 《天然气化学杂志》2010,19(3):333-340
A nano-structured iron catalyst for syngas conversion to hydrocarbons in Fischer-Tropsch synthesis (FTS) was prepared by micro-emulsion method. Compositions of bulk iron phase and phase transformations of carbonaceous species during catalyst deactivation in FTS reaction were characterized by temperature-programmed surface reaction with hydrogen (TPSR-H2), and XRD techniques. Many carbonaceous species on surface and bulk of the nano-structured iron catalysts were completely identified by combined TPSR-H2 and XRD spectra and which were compared with those recorded on conventional co-precipitated iron catalyst. The results reveal that the catalyst deactivation results from the formation of inactive carbide phases and surface carbonaceous species like graphite, and it will be increased when the particle size of iron oxides was reduced in FTS iron catalyst. 相似文献
76.
Monojit Das Bairagya Ryan J. Bujol Prof. Noémie Elgrishi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(18):3991-4000
Development of highly active molecular electrocatalysts for fuel-forming reactions has relied heavily on understanding mechanistic aspects of the electrochemical transformations. Careful fine-tuning of the ligand environment oriented mechanistic pathways towards higher activity and optimal product distribution for several catalysts. Unfortunately, many catalysts deactivate in bulk electrolysis conditions, diminishing the impact of the plethora of highly tuned molecular electrocatalytic systems. This Minireview covers classical and emerging methods developed to circumvent catalyst deactivation and degradation, with an emphasis on successes with molecular electrocatalysts. 相似文献
77.
Leah R. Kuhn Michael L. Allegrezza Nicholas J. Dougher Dominik Konkolewicz 《Journal of polymer science. Part A, Polymer chemistry》2020,58(1):139-144
Reversible deactivation radical polymerization (RDRP) techniques have become important tools for polymer chemists because they control the structure and are tolerant to functionality. Photoinduced polymerizations have seen a growing interest due to their mild conditions, as well as spatial and temporal control over the polymerization. Among these techniques, photoinduced electron/energy transfer reversible addition–fragmentation chain transfer polymerization (PET-RAFT) is one of the most widely investigated. While PET-RAFT is seen as an increasingly useful tool, there is still much to understand about the mechanism of this process. In particular, there are ongoing questions regarding the kinetic contribution of electron versus energy transfer. In order to better understand the mechanism, this work aims to use kinetic modeling along with experimental data to help determine the likelihood of the proposed mechanisms for the PET-RAFT process using the trithiocarbonate-mediated polymerization of methyl acrylate with fac-tris[2-phenylpyridinato-C2,N]iridium(III) as a photocatalyst. Simulation data show that electron transfer without a corresponding reduction pathway cannot explain the experimental kinetics, while energy transfer offers a good fit to experimental data. © 2019 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 139–144 相似文献
78.
Ali Shahrokhinia Priyanka Biswas James F. Reuther 《Journal of polymer science. Part A, Polymer chemistry》2021,59(16):1748-1786
In this review, we detail the progress throughout the years toward developing truly orthogonal polymerization mechanisms and modification procedures en route to complex macromolecular structures built from synthetic polymer materials. The orthogonal modifications of polymer side-chains and end-groups via sequential click reactions is described providing post-polymerization routes to functional materials and unique polymer topologies. Further, historical and modern orthogonal polymerization methodologies are thoroughly reviewed showing the evolution of the field through the decades long study of selective polymerization mechanisms that provide unique copolymer structures that are typically difficult to achieve. These include the combinations of reversible deactivation radical polymerization mechanisms with a variety of polymerization mechanisms including ring opening polymerizations, ring opening metathesis polymerizations, and cationic polymerizations, to name a few. 相似文献
79.
Selective Coke Combustion by Oxygen Pulsing During Mo/ZSM‐5‐Catalyzed Methane Dehydroaromatization 下载免费PDF全文
Dr. Nikolay Kosinov Ferdy J. A. G. Coumans Evgeny Uslamin Prof. Freek Kapteijn Prof. Emiel J. M. Hensen 《Angewandte Chemie (International ed. in English)》2016,55(48):15086-15090
Non‐oxidative methane dehydroaromatization is a promising reaction to directly convert natural gas into aromatic hydrocarbons and hydrogen. Commercialization of this technology is hampered by rapid catalyst deactivation because of coking. A novel approach is presented involving selective oxidation of coke during methane dehydroaromatization at 700 °C. Periodic pulsing of oxygen into the methane feed results in substantially higher cumulative product yield with synthesis gas; a H2/CO ratio close to two is the main side‐product of coke combustion. Using 13C isotope labeling of methane it is demonstrated that oxygen predominantly reacts with molybdenum carbide species. The resulting molybdenum oxides catalyze coke oxidation. Less than one‐fifth of the available oxygen reacts with gaseous methane. Combined with periodic regeneration at 550 °C, this strategy is a significant step forward, towards a process for converting methane into liquid hydrocarbons. 相似文献
80.
The oxidation of CO on planar Au/TiO2 model catalysts was investigated under pressure and temperature conditions similar to those for experiments with more realistic Au/TiO2 powder catalysts. The effects of a change of temperature, pressure, and gold coverage on the CO oxidation activity were studied. Additionally, the reasons for the deactivation of the catalysts were examined in long-term experiments. From kinetic measurements, the activation energy and the reaction order for the CO oxidation reaction were derived and a close correspondence with results of powder catalysts was found, although the overall turnover frequency (TOF) measured in our experiments was around one order of magnitude lower compared to results of powder catalysts under similar conditions. Furthermore, long-term experiments at 80 °C showed a decrease of the activity of the model catalysts after some hours. Simultaneous in-situ IR experiments revealed a decrease of the signal intensity of the CO vibration band, while the tendency for the build-up of side products (e. g. carbonates, carboxylates) of the CO oxidation reaction on the surface of the planar model catalysts was rather low. 相似文献