铁改性的Mo/ZSM-5催化剂上NO的选择性催化还原反应

采用浸渍法制备了Mo/ZSM-5, Fe/ZSM-5和不同Fe和Mo摩尔比的Fe-Mo/ZSM-5样品, 并以氨为还原剂对其NO选择性催化还原活性以及反应条件对催化性能的影响进行了研究. 结果表明, Fe-Mo/ZSM-5样品的NOx转化率明显比单独的Mo/ZSM-5和Fe/ZSM-5的高. 当n(Fe):n(Mo)为1.5时, Fe-Mo/ZSM-5样品具有最佳催化性能, 其NOx转化率在430 ℃时达到了96%, 并且能在高空速和不同O2气浓度的条件下保持高的催化活性. 同时采用XRD和XPS技术分别对催化剂的体相结构和表面性质进行了研究, 结果表明, 当n(Fe):n(Mo)=1.5时, Fe和Mo元素之间以及与载体HZSM-5之间存在较强的相互作用, 并且其表面的Mo3d的含量最高. 这可能与其高的催化活性有关. 另外还发现, 在反应过程中Fe-Mo/ZSM-5催化剂表面的氮氧物种主要是吸附态NO, 因此可以推测NO的催化还原反应机理是, 在催化剂表面上, 吸附态NO与吸附NH3物种直接反应生成氮气, 而非经过氧化为NO2的途径.     

Methane Aromatization in the Absence of Oxygen Over Mo/H[B]ZSM-5 Catalysts

The catalytic performance of Mo/H[B]ZSM-5 has been investigated for methane non-oxidative aromatization and compared with that of a Mo/H[Al]ZSM-5 catalyst. It is found that the non-oxidative aromatization of methane proceeds only in the presence of weak acidic sites without the participation of strong acidic sites. Coke is found to be the main cause of the rapid decrease in the activity of Mo/H[B]ZSM-5 catalysts.     

Decomposition and aromatization of ethanol on ZSM-based catalysts

The adsorption, desorption, and reactions of ethanol have been investigated on pure and promoted ZSM-5 catalysts. FTIR spectroscopy indicated the formation of a strongly bonded ethoxy species on ZSM-5(80) at 300 K. TPD experiments following the adsorption of ethanol on both ZSM-5 and Mo2C/ZSM-5 have shown desorption profiles corresponding to unreacted ethanol and decomposition products (H2O, H2, CH3CHO, C4H10O, and C2H4). The main reaction pathway of ethanol on pure ZSM-5 is the dehydration reaction yielding ethylene, small amounts of hydrocarbons, and aromatics. Deposition of different additives, such as Mo2C, ZnO, and Ga2O3 on zeolite, greatly promoted the formation of benzene and toluene at 773-973 K, very likely by catalyzing the aromatization of ethylene formed in the dehydration process of ethanol. Separate studies of the reaction of ethylene revealed that the previous additives markedly enhanced the selectivity and the yield of aromatics on ZSM-5.     

A Novel Cu-Mo／ZSM-5 Catalyst for NOx Catalytic Reduction with Ammonia

The Cu-Mo/ZSM-5 catalysts with different Cu/Mo ratios were prepared by wet impregnation method, and their catalytic performance for selective catalytic reduction of NOx was studied. The results showed that Cu-Mo/ZSM-5 is a very effective catalyst for NOx catalytic reduction with ammonia, especially when Cu/Mo molar ratio is about 1.5. It not only exhibited the extremely high catalytic activity, but also showed good stability for 02. The bulk phase structure of Cu-Mo/ZSM-5 catalysts was determined by XRD technique, and the results indicated that there is a maximum dispersion for Cu species when Cu/Mo molar ratio is 1.5, and an interaction between Cu and Mo along with HZSM-5 may be present in Cu-Mo/ZSM-5, which may possibly result in a special structure favorable for the catalytic reduction of NOx over Cu-Mo/ZSM-5 catalyst.     

过渡金属改性的ZSM-5催化剂应用于甲硫醚转化制甲硫醇

负载过渡金属的ZSM-5 催化剂用于催化甲硫醚（DMS）转化成甲硫醇（MT）的反应. 实验结果表明,催化剂的甲硫醚转化率提高和甲硫醇选择性降低的趋势一样,都是以下顺序：Co/ZSM-5>Mo/ZSM-5>Ni/ZSM-5>W/ZSM-5. 表征结果表明,由于过渡金属阳离子（W⁶⁺、Ni²⁺、Co³⁺、Mo⁶⁺）比Al³⁺活泼,而改性过程中W⁶⁺、Ni²⁺、Co³⁺、Mo⁶⁺分别代替了部分Al³⁺,使得改性催化剂对DMS和MT的化学吸附作用更强. 过渡金属的引入使得ZSM-5总酸度增强,提高了C―S键的裂解能力,从而改进了催化转化DMS的能力. 研究结果发现,在转化DMS的过程中,金属活性位和酸性位之间通过强的协同效应起作用.     

Fe-Cu/ZSM-5催化剂的NH_3-SCR脱硝性能

通过浸渍法制备了Fe和Cu含量比不同的系列Fe-Cu/ZSM-5催化剂,利用XRD、H2-TPR、NH3-TPD和原位DRIFTS等技术对催化剂进行了表征,并对其NH3-SCR脱硝性能进行了研究。结果表明,双金属改性的Fe-Cu/ZSM-5催化剂活性温度窗口拓宽,其中,Fe-Cu/ZSM-5 1∶4催化剂脱硝性能优异,250-450℃下脱硝效率均超过90%,335℃时脱硝效率达到最大值96.46%。铜和铁物种能以无定型氧化物良好分散于载体表面,双金属负载改性催化剂保留了ZSM-5的晶体结构。Fe-Cu/ZSM-5 1∶4催化剂具备丰富的酸性位、良好的氧化还原性能,一定温度条件下NH3-SCR反应过程中同时存在E-R机理和L-H机理,且E-R机理反应起始温度低于L-H机理;200℃为催化脱硝反应的起活温度。     

Active Ensembles in Methane Dehydroaromatization over Molybdenum/ZSM-5 Zeolite Identified by 2D 1H−95Mo Magic Angle Spinning Nuclear Magnetic Resonance Correlation Spectroscopy

Methane dehydroaromatization (MDA) over Mo-modified zeolite is a potential catalytic route for converting natural gas into valuable aromatics. However, the active species in this reaction are highly complex, involving diverse Mo species, acidic sites of zeolite, and organic molecules. Herein, we apply 1D ⁹⁵Mo NMR and 2D ¹H-⁹⁵Mo heteronuclear correlation solid-state NMR spectroscopy to directly observe the active ensembles in the confined channels of Mo/ZSM-5 zeolite during the MDA reaction. We monitor the evolution of the spatial correlations of Mo species with the Brønsted acid sites and organic products (olefins and aromatics) in the zeolite channels. We identified two kinds of MoO_xC_y species, with the more carbidic one (MoO_xC_y-II) exhibiting higher activity for methane activation and benzene formation. The strong spatial interactions between the active Mo species and the organic species in the Mo/ZSM-5 pores are related to the MDA activity.     

Mo/HZSM-5丙烷芳构化催化性能及其吸附量热的研究

采用等量浸渍法制得一系列不同担载量的Ｍｏ／ＨＺＳＭ－５催化剂,运用ＸＲＤ和ＦＴＩＲ方法考察了Ｍｏ物种在催化剂表面的分散状态,首次采用微分吸一热技术对Ｍｏ／ＨｚＳＭ－５催化剂的表面酸性进行表征。同时研究了催化剂对丙烷芳构化的反应活性。结果表明：对于担载Ｍｏ的ＨＺＳＭ－５分子筛催化剂,Ｍｏ物种在ＨＺＳＭ－５分子筛表面上顺序为ＨＺＳＭ－５〉１％Ｍｏ／ＨＺＳＭ－５〉２％Ｍｏ／ＨＺＳＭ－５分子筛本身表面的酸     

Nonoxidative conversion of methane into aromatic hydrocarbons on Ni-Mo/ZSM-5 catalysts

The nonoxidative conversion of methane into aromatic hydrocarbons on high-silica zeolites ZSM-5 containing nanosized powders of molybdenum (4.0 wt %) and nickel (0.1–2.0 wt %) was studied. Data on the acid characteristics of the catalysts and the nature and amount of coke deposits formed on the surface of the catalysts were obtained using the thermal desorption of ammonia and thermal analysis. The microstructure and composition of Ni-Mo/ZSM-5 catalysts were studied by high-resolution transmission electron microscopy and energy-dispersive X-ray analysis. The formation of various chemical species in the samples was detected: oxide-like clusters of Mo within zeolite channels (∼1 nm), molybdenum carbide particles (5–30 nm) on the outer surface of the zeolite, and Ni-Mo alloy particles with different compositions (under reaction conditions, carbon filaments grew on these particles). It was found that, as the Ni content was increased from 0.1 to 2.0 wt %, the rate of deactivation of the catalytic system increased because of blocking pores in the zeolite structure by filamentous carbon up to the formation of condensed coke deposits.     

FT－IR和程序升温热谱技术研究Mo／HZSM－5催化剂的制备过程

采用ＦＴ－ＩＲ和程序升温热谱技术研究了Ｍｏ／ＨＺＳＭ－５催化剂的制备过程．结果表明Ｍｏ／ＨＺＳＭ－５样品在合适的温度下焙烧一定时间，Ｍｏ物种与ＨＺＳＭ－５分子筛的酸中心（主要是强酸中心）起作用，并且一部分Ｍｏ物种会迁移到分子筛孔道内．在外表面的Ｍｏ物种和在孔道内强酸中心作用的Ｍｏ物种，可能是对甲烷活化起作用的     

Ce、Zr双组分改性对Cu/ZSM-5催化剂催化分解NO性能的影响

采用Ce、Zr双组分对Cu/ZSM-5催化剂进行改性,利用O₂-TPD和TG等手段对催化剂进行表征,考察了离子交换顺序、铈锆摩尔比以及反应条件对催化剂催化分解NO性能的影响。结果表明,铜物种是Zr-Cu-Ce/ZSM-5催化剂中催化分解NO的活性中心,是必须的催化活性组分,Ce、Zr双组分改性能显著提高催化剂在富氧条件下催化分解NO的活性,降低达到最高活性所需要的反应温度。铈锆摩尔比为1:1时,同时进行离子交换制备的Zr-Cu-Ce/ZSM-5催化剂催化分解NO的活性最好。     

Mo／ZSM-5催化剂上NO催化还原反应机理与动力学研究

采用浸渍法制备了Mo/ZSM-5催化剂样品,并以氨为还原剂对其NO选择性催化还原活性、以及NO转化的反应速率进行测定.结果表明,在Mo/ZSM-5催化剂上不会发生NO氧化成NO2的反应,也没有N2O生成,然而有少量的NO分解反应发生.在氧气存在条件下,Mo/ZSM-5催化剂上NO-NH3-O2的SCR反应遵循LH机理.NO、O2和NH3首先吸附在Mo/ZSM-5表面,吸附态NO物种与吸附NH3物种直接反应生成氮气,气相氧的作用是加强NO吸附、补充催化剂表面吸附氧物种.并由此推导出NO转化的速率方程式,分别计算和模拟了在不同O2浓度、NO浓度和反应温度条件下NO的反应速率rNO值及其变化关系.结果表明,理论模拟值能够与实验值很好地吻合,所推测的机理能够很好地描述Mo-ZSM-5催化剂上NO选择性催化还原行为.     

Catalytic Combustion of Methane over High Copper-Loading ZSM-5 Catalysts

Two series of Cu/ZSM-5 catalysts,loading from 5 to 20 wt% CuO,were prepared by the deposition-precipitation and impregnation methods,respectively.The catalysts prepared by the impreg- nation method showed better catalytic performances than those prepared by the deposition-precipitation method and the increase of copper loading favored methane conversion.20Cu(I)/ZSM-5 had the highest activity with T_(90%)of 746 K,and for 20Cu(D)/ZSM-5,T_(90%)was as high as 804 K.The characteriza- tion of X-ray diffraction(XRD),temperature-programmed reduction(TPR),temperature-programmed desorption(TPD),and X-ray photoelectron spectroscopy(XPS)revealed that the dispersion of cop- per species could be improved by using the deposition-precipitation method instead of the impregnation method,but the fraction of surface CuO,corresponding to active sites for methane oxidation,was larger on 20Cu(I)/ZSM-5 than 20Cu(D)/ZSM-5.The results of Pyridine-Fourier transform infrared spectrum (Py-FT-IR)showed that a majority of Lewis acidity and a minority of Brφnsted acidity were present on Cu/ZSM-5 catalysts.20Cu(I)/ZSM-5 presented more Lewis acid sites.The number of Lewis acid sites changed significantly with preadsorption of oxygen.Adsorption of methane and oxygen on acid sites was observed.The properties of Cu/ZSM-5 catalysts were correlated with the activity for methane oxidation.     

双金属Ce-Mn/ZSM-5催化剂的制备及NH3-SCR脱硝性能研究

采用浸渍法制备了铈锰复合氧化物分子筛催化剂(Ce-Mn/ZSM-5),在固定床反应器上考察不同Ce/Mn质量比对分子筛催化剂选择催化还原NO的影响,利用XRD、TEM、NH_3-TPD、H_2-TPR、in-situ DRIFTS等手段对催化剂进行了表征分析。结果表明,双金属改性的Ce-Mn/ZSM-5催化剂在NH_3-SCR反应中表现出较为优异的催化活性,具有较宽的活性温度窗口。当Ce/Mn质量比为0.4时,催化剂具有最佳的脱硝效率,在265-465℃脱硝率均可达到80%以上,在370℃时,NO的转化率最高可达97.28%。锰和铈物种高度分散于催化剂表面,未改变ZSM-5的晶体结构,且构成协同作用。0.4Ce-Mn/ZSM-5具备丰富的酸性位、良好的氧化还原性能,该配比有助于催化剂的催化活性和稳定性的提高,在NH_3-SCR反应过程同时遵循E-R机理和L-H机理。     

Confined Carbon Mediating Dehydroaromatization of Methane over Mo/ZSM‐5

Non‐oxidative dehydroaromatization of methane (MDA) is a promising catalytic process for direct valorization of natural gas to liquid hydrocarbons. The application of this reaction in practical technology is hindered by a lack of understanding about the mechanism and nature of the active sites in benchmark zeolite‐based Mo/ZSM‐5 catalysts, which precludes the solution of problems such as rapid catalyst deactivation. By applying spectroscopy and microscopy, it is shown that the active centers in Mo/ZSM‐5 are partially reduced single‐atom Mo sites stabilized by the zeolite framework. By combining a pulse reaction technique with isotope labeling of methane, MDA is shown to be governed by a hydrocarbon pool mechanism in which benzene is derived from secondary reactions of confined polyaromatic carbon species with the initial products of methane activation.     

Novel Strategy for the Synthesis of Ultra-Stable Single-Site Mo-ZSM-5 Zeolite Nanocrystals

The current energy transition presents many technological challenges, such as the development of highly stable catalysts. Herein, we report a novel “top-down” synthesis approach for preparation of a single-site Mo-containing nanosized ZSM-5 zeolite which has atomically dispersed framework-molybdenum homogenously distributed through the zeolite crystals. The introduction of Mo heals most of the native point defects in the zeolite structure resulting in an extremely stable material. The important features of this single-site Mo-containing ZSM-5 zeolite are provided by an in-depth spectroscopic and microscopic analysis. The material demonstrates superior thermal (up to 1000 °C), hydrothermal (steaming), and catalytic (converting methane to hydrogen and higher hydrocarbons) stability, maintaining the atomically disperse Mo, structural integrity of the zeolite, and preventing the formation of silanols.     

The synergic effect between Mo species and acid sites in Mo/HMCM-22 catalysts for methane aromatization

The acid properties of Mo/HMCM-22 catalyst, which is the precursor form of the working catalyst for methane aromatization reaction, and the synergic effect between Mo species and acid sites were studied and characterized by various characterization techniques. It is concluded that Br?nsted and Lewis acidities of HMCM-22 are modified due to the introduction of molybdenum. We suggest a monomer of Mo species is formed by the exchange of Mo species with the Br?nsted acid sites. On the other hand, coordinate unsaturated sites (CUS) are suggested to be responsible for the formation of newly detected Lewis acid sites. Computer modelling is established and coupling with experimental results, it is then speculated that the effective activation of methane is properly accomplished on Mo species accommodated in the 12 MR supercages of MCM-22 zeolite whereas the Br?nsted acid sites in the same channel system play a key role for the formation of benzene. A much more pronounced volcano-typed reactivity curve of the Mo/HMCM-22 catalysts, as compared with that of the Mo/HZSM-5, with respect to Mo loading is found and this can be well understood due to the unique channel structure of MCM-22 zeolite and synergic effect between Mo species and acid sites.     

Quantitative measurement of the Brönsted acid sites in solid acids: toward a single-site design of Mo-modified ZSM-5 zeolite

On the basis of our previous H/D exchange studies devoted to the quantification of the number of Br?nsted acid sites in solid acids, we report here an innovative approach to determine both the amount and the localization of Mo atoms inside the Mo/ZSM-5 catalyst, commonly used for the methane dehydroaromatization reaction. The influence of Mo introduction in the MFI framework was studied by means of BET, X-ray diffraction, 27Al magic angle spinning NMR, NH3 temperature-programmed desorption, and H/D isotopic exchange techniques. A dependence was found between the decrease of acidic OH groups and the Mo content. Depending on the Si/Al ratio of the zeolite, i.e., the proximity of two Br?nsted acid sites, the Mo atoms substitute a different number of OH groups. Consequently, a chemical structure was proposed to describe the geometry of the Mo complex in the channels of the ZSM-5 zeolite.     

Direct Imaging of Atomically Dispersed Molybdenum that Enables Location of Aluminum in the Framework of Zeolite ZSM-5

Integrated differential phase-contrast scanning transmission electron microscopy (iDPC-STEM) is capable of directly probing guest molecules in zeolites, owing to its sufficient and interpretable image contrast for both heavy and light elements under low-dose conditions. This unique ability is demonstrated by imaging volatile organic compounds adsorbed in zeolite Silicalite-1; iDPC-STEM was then used to investigate molybdenum supported on various zeolites including Silicalite-1, ZSM-5, and mordenite. Isolated single-Mo clusters were observed in the micropores of ZSM-5, demonstrating the crucial role of framework Al in driving Mo atomically dispersed into the micropores. Importantly, the specific one-to-one Mo-Al interaction makes it possible to locate Al atoms, that is, catalytic active sites, in the ZSM-5 framework from the images, according to the positions of Mo atoms in the micropores.     

Experimental and DFT study of the partial oxidation of benzene by N2O over H-ZSM-5: acid catalyzed mechanism

The reaction mechanism for hydroxylation of benzene by N(2)O has been studied on chemically modified ZSM-5 catalysts. A maximum in catalytic activity and selectivity was reached for steamed samples under mild conditions (about 30% conversion with 94% selectivity). Chemical modifications, through ion exchange (H(+) versus Na(+)), have demonstrated the importance of the presence of Br?nsted acid sites. The results obtained suggest a Langmuir-Hinshelwood mechanism between benzene and N(2)O adsorbed on two distinct active sites. A density functional theory study considering the possible reaction intermediates also confirmed the possible formation of protonated nitrous oxide, leading to a Wheland-type intermediate, thus supporting an electrophilic aromatic substitution assisted by the confined environment provided by the active zeolite framework.