Activation of Mo/HZSM-5 for methane aromatization

The effect of Mo/HZSM-5 pretreatment at 973 K in inert (He), oxidizing (artificial air), and carbu-rizing (CH4/He mixture) atmospheres on its performance in non-oxidative methane dehydroaroma-tization (MDA) was investigated. The effect of post-synthesis silylation on deactivation of external acid sites was also studied. Precarburization resulted in increased aromatic selectivity and im-proved catalyst stability. The benzene selectivity was the highest for the silylated Mo/HZSM-5 cata-lyst (benzene+naphthalene selectivity after 1 h on stream was close to 100%). The deactivation of precarburized zeolites was less pronounced than that of zeolites heated in air or He. During heating in air or He, larger fractions of the molybdenum oxide species diffused into the micropores than during heating in methane. Carburization of the molybdenum oxide species in the micropores dur-ing MDA resulted in the formation of molybdenum carbide particles, and these contributed to pore blocking, making the Br?nsted acid sites inaccessible. The formation of molybdenum carbides dur-ing heating in methane resulted in a less mobile Mo phase. It is argued that the presence of molyb-denum carbide particles in the micropores contributes to rapid catalyst deactivation, in addition to the formation of hard coke on the external surface.     

介孔材料为硅源合成多级孔Mo/H-IM-5催化剂及其在甲烷无氧芳构化反应中的应用

在IM-5分子筛的合成体系中以介孔材料KIT-6作为硅源,制备出了多级孔IM-5复合分子筛(IM-5-K)。利用X射线衍射、扫描电镜、透射电镜、比表面积分析、NH3-TPD等手段对样品的物理性质和酸性进行表征。将经过钼修饰的Mo-IM-5-K催化剂应用于甲烷无氧芳构化反应中,考察其活性。催化测试结果显示,多级孔Mo-IM-5-K催化剂的最高甲烷转化率和芳烃产率为12.4%和6.9%,高于常规Mo-IM-5-C催化剂。同时,Mo-IM-5-K催化剂有更好的稳定性。催化剂反应活性和稳定性的差异是由于其具有不同的孔道结构和酸性所致。介孔的存在会影响活性Mo物种的落位及分布状态,有利于反应物与活性位的接触和芳烃产物的扩散,进而提高催化剂的活性和稳定性。     

A 13CO isotopic study on the CO promotion effect in methane dehydroaromatization reaction over a Mo/HMCM-49 catalyst

The promotion effect of CO in methane dehydroaromatization was investigated using 13CO probe molecules. By alternative injection of 13CO to the methane feed,the distribution of 13CxC6-xH6(x=0-3)products changed significantly,confirming the participation of13CO in the reaction network.The addition of 13CO did not change the conversion of CH4 but improved slightly the durability of the methane dehydroaromatization(MDA)reaction,which might be caused by the interaction of the dissociated oxygen species and the ...     

氟硅酸铵改性对HMCM-22酸性及双功能催化剂催化合成二甲醚性能的影响

用氟硅酸铵对HMCM-22分子筛进行了改性处理,并采用氨程序升温脱附和吡啶吸附红外光谱法测定了其酸性.结果表明,随着氟硅酸铵处理温度的提高,HMCM-22分子筛的B酸和L酸中心的酸量均逐渐减少.以氟硅酸铵改性的HMCM-22分子筛为甲醇脱水活性组分与铜基甲醇合成活性组分(CuO-ZnO-Al2O3)组成双功能催化剂,在连续流动加压固定床反应器上考察了其对合成气直接制二甲醚反应的催化性能.结果表明,对HMCM-22分子筛在适当温度(45℃)下进行氟硅酸铵改性,可使反应产物中烃类和CO2副产物的选择性下降,使目的产物二甲醚的选择性显著升高.当氟硅酸铵处理温度过高(85℃)时,CO的转化率和二甲醚的选择性均大幅度降低。     

分子筛硅／铝比对Cu-ZnO-Al2O3／HMCM-22催化合成二甲醚反应性能的影响

以六亚甲基亚胺为模板剂,采用动态水热晶化法合成了不同硅/铝比的HMCM-22分子筛,并以其为甲醇脱水活性组分与铜基甲醇合成活性组分(Cu-ZnO-Al2O3)组成双功能催化剂,在连续流动加压固定床微型反应器上考察了其对合成气直接制二甲醚反应的催化性能. 结果表明,随着HMCM-22分子筛硅/铝比的增大, CO转化率变化不大,但CO2和烃类副产物的生成量逐渐减少,从而导致目的产物二甲醚的选择性和收率均逐渐升高. 氨程序升温脱附和吡啶吸附红外光谱表征结果表明,随着分子筛硅/铝比的增大, HMCM-22分子筛的酸中心的数量逐渐减少.     

Relation between hydro-dehydrogenating and acid functions of zeolite catalysts and their effectiveness in aromatization of light paraffins

Summary Aromatization of light paraffins with Zn-containing catalysts based on the ZSM-5 zeolite is considered. The correlation between acidic and hydro-dehydrogenating functions of the catalysts, on one hand, and their activity and selectivity towards aromatics, on the other hand, is discussed. The modification of external acid sites is shown to influence significantly the catalytic properties of the prepared samples.     

Looking for a synergic effect between NHCs and chiral P-ligands

The issue of the added value of NHCs in asymmetric catalysis with respect to trusted chiral P-ligands was addressed: considering a prototypical asymmetric allylic alkylation reaction as a model, the association of a priori inhibiting and achiral NHCs with modular P-ligand resulted in enhancement of er up to 508% and increased rates.     

Investigation of the molybdenum state in the Mo(4%)/HZSM-5 catalyst for methane aromatization

The Mo(4%)/HZSM-5 catalyst for methane aromatization prepared by the impregnation of ammonium heptamolybdate followed by calcination at 700°C was studied. The formation of molybdenum ions in the oxidation state 5+ during catalysis and the presence of graphitized carbonaceous sediments on the surface of the catalyst were confirmed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), X-ray photoelectron spectroscopy (XPS), and ESR spectroscopy.     

Evidence for several species of active sites in Ziegler-Natta catalysts

The kinetics of isoprene polymerization catalyzed by VCl₃ and Et₃Al were studied by measuring fractional conversions, polymer composition, and molecular weight distributions at a series of reaction times and temperatures. The rate of polymerization plotted against temperature shows an inflection point with a minimum and maximum in the 60–90°C range. The isomeric composition of the polymer changes with temperature but not with reaction time, while the molecular weight distribution undergoes substantial change with both of these variables. The rate of polymerization at sites producing low molecular weight polymers was measured, and the activation energy calculated to be about 10 kcal/mole. The active sites were found to deactivate at different rates. The results support the hypothesis that several species of active sites are present in the system and that these exhibit characteristic polymerization behavior.     

Nature of Cu active sites in zeolite-based catalysts for selective catalytic oxidation of methane

Research on Chemical Intermediates - Though copper-containing zeolites (Cu–zeolites) have shown great potential in the direct conversion of methane to methanol under low temperature, the...     

A high-resolution MAS NMR study on the potential catalysts Mo/HBeta for olefin metathesis: The interaction of Mo species with HBeta zeolite

The interfacial interaction of Mo species with the HBeta zeolite was studied by multinuclear MAS NMR, XRD and N₂ adsorption. As proved by the quantitative ²⁷Al MAS NMR, this interaction is so strong as to dealuminate the framework of HBeta, and leads to a new peak appearing at −14 ppm, which indicates the formation of crystalline Al₂(MoO₄)₃. This can also be detected by XRD measurements when the Mo loading is as high as 9.0 wt.%. The corresponding quantitative ²⁹Si and ¹H MAS NMR spectra show that the amount of silanols and Brønsted acidic sites decrease obviously with increasing Mo loading. This also reveals an interaction between Mo species and HBeta support through an oxygen bridge resulting from condensation with the hydroxyls on the support. At higher Mo loadings, the interaction is so strong that it results in an extraction of aluminum from the zeolite framework, and subsequently appearance of Al₂(MoO₄)₃ and loss of Brønsted acidic sites. These can be correlated to the low catalytic activity of Mo/HBeta in metathesis of ethylene and 2-butylene to propylene.     

V?Mo?Ox/SiO2 catalysts for acrolein oxidation I. The nature of active sites

The influence of the annealing temperature and the activation of V–Mo–O_x/SiO₂ catalysts for acrolein oxidation has been studied. The catalyst activity decreases with increasing temperature of annealing due to lowering of the number of active sites of V⁴⁺ and the content of active component VMo₃O_11+x.     

Synthesis of zeolite IM-5 under rotating and static conditions and the catalytic performance of Mo/H-IM-5 catalyst in methane non-oxidative aromatization

The hydrothermal crystallization of zeolite IM-5 was investigated under rotating (denoted as IM-5-R) and static (denoted as IM-5-S) synthesis conditions. Mo-modified catalysts (Mo-IM-5-R and Mo-IM-5-S) were prepared for the methane non-oxidative aromatization. The physical properties and acidities of the samples were characterized by XRD, SEM, BET and IR spectroscopy. Compared with Mo-IM-5-S, the Mo-IM-5-R catalyst showed both a higher conversion of methane and higher selectivity to benzene in methane aromatization. A higher catalytic activity of Mo-IM-5-R may be attributed to the preferable textural properties and acidities of zeolite IM-5-R. Moreover, the catalyst prepared by the physical mixing method exhibited lower initial activity, but better stability for methane aromatization than that prepared by the impregnation method.     

Effect of the formation of secondary pores in zeolite ZSM-5 on the properties of molybdenum-zeolite catalysts for methane aromatization

A study is performed of 4% Mo/ZSM-5 (30) catalysts for methane aromatization prepared by solid-phase synthesis with mechanical mixing of a zeolite with MoO₃ followed by calcination at 550°C. Zeolite etched with sodium hydroxide solutions and dealuminated with aluminum nitrate solutions is used as a support. Catalytic studies of the catalysts are conducted. The effect of treating the initial zeolite on the properties of catalysts in methane aromatization is determined. The effect subsequently treating a zeolite support has on the acid sites of a catalyst is confirmed by means of temperature-programmed reduction and the temperature-programmed desorption of NH3. The formation of molybdenum ions in the +5 oxidation state during catalysis and the presence of graphitized carbon deposits on a spent catalyst’s surface are confirmed by EPR and temperature-programmed oxidation.     

Methane dehydrogenation and aromatization in the absence of oxygen on Mo/HZSM-5: A study on the interaction between Mo species and HZSM-5 by using Al and Si MAS NMR

By using a high-resolution solid state nuclear magnetic resonance spectrometer with ²⁷Al and ²⁹Si probes, the interaction between Mo species and HZSM-5 of frsol|Mo/HZSM-5 catalysts has been studied. The results show that there is a strong interaction between Mo species and HZSM-5 zeolite. The framework aluminum in the zeolite can be easily extracted by the introduction of Mo species. The extractability of framework aluminum by Mo species increases with increasing Mo loading and the calcination temperature. The extraction process leads to the formation of non-framework Al at first and then a new crystalline phase of Al₂(MoO₄)₃. The dealumination of the catalyst having a Mo loading of 15% and had been calcined at 973 K is so severe that all the aluminum in the framework are extracted and no framework Al could be detected by ²⁷Al MAS NMR. The catalyst, therefore, lost its catalytic activity for methane dehydrogenation and aromatization in the absence of oxygen. The Si/Al ratio measured from ²⁹Si MAS NMR further confirms the dealumination process observed by ²⁷Al MAS NMR. The MAS NMR results give us an evidence that Al₂(MoO₄)₃ crystallites are much less active for the reaction.     

Structure of surface species in Mo/Al2O3 catalysts prepared through Mo(V) oxalate

EXAFS and RED structural studies of surface species of Mo/Al₂O₃ catalysts prepared through the anchoring of [Mo₂O₄(C₂O₄)₂(H₂O)]^2– anion to -Al₂O₃ have shown that these surface metal complexes preserve a binuclear structure of the Mo(V) oxalate framework fragment.

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EXAFS Mo/Al₂O₃, [Mo₂O₄(C₂O₄)₂(H₂O)₂]^2– -Al₂O₃. , (V).

     

Solid state 13C NMR studies of methane dehydroaromatization reaction on Mo/HZSM-5 and W/HZSM-5 catalysts

Methane dehydroaromatization on Mo/HZSM-5 and W/HZSM-5 catalysts was studied by solid state 13C NMR spectroscopy, both variation of the state of transition metal component and products such as ethane, benzene, ethene adsorbed on or in zeolite were observed after high temperature (900-1000 K) reaction.     

Activation and properties of Mo=O bonds in Mo/SiO2 catalysts

The existence of the charge transfer excited triplet state [Mo⁵⁺-O^-] produced by UV-irradiation of Mo/SiO₂ catalysts, and its reactivity are evidenced by experiments of photoluminescence, photoinduced metathesis, and photoreduction of CO. Mo⁵⁺ ions can be produced separately by thermal activation and O^- ions by further adsorption of N₂O on those Mo⁵⁺ ions. The latter of which are adsorbed on Mo⁶⁺ ions are found to be more reactive than O^2- of [Mo⁶⁺ =O^2-] bond. They are able either to add a molecule such as CO or C₂H₄, or to abstract hydrogen from H₂, CH₄ or trans-dicyanoethylene, or a CN group form tetracyanoethylene (TCNE). The Mo⁵⁺ ions are able to coordinate gas phase ligands when their coordination sphere possesses vacant sites. This is the case for tetracoordinated Mo⁵⁺ _4c ions arising from reduction of tetrahedral Mo⁶⁺ ions (Eq. (7)). These Mo⁵⁺ _4c ions are similar to those produced by UV-irradiaiion (Eq. (2)). In addition, if the adsorbed molecule has a sufficiently large electron affinity, such as TCNE or O₂, an electron transfer can occur (Eq. (9) and (17)). The [Mo⁵⁺-O^-] bond obtained by thermal activation is more difficult to evidence than that obtained with UV-activation because it is not detectable by EPR. However, the EPR results obtained at low temperature show that the O^- ions adsorbed on Mo/SiO₂ catalysts as well as the [Mo⁵⁺-O^-] excited triplet state obtained by UV-irradiation of 1Mo⁶⁺=O²] interact with methanol (Eq. (16)). They are consistent with the mechanism of methanol oxidation occurring at high temperature (Eq. (4)).     

The formation mechanism of Mo-methylidene species over Mo/HBeta catalysts for heterogeneous olefin metathesis: A density functional theory study

Density Functional Theory (DFT) calculations were performed to optimize the Mo active sites in HBeta zeolite catalysts as well as to locate the reaction pathways to form the Mo-methylidene species. Two different Mo active sites, i.e., the oxidized Mo^VIO₂ and its reduced form Mo^VO(OH), were developed and incorporated into HBeta zeolites by replacing a pair of Brønsted acidic sites. The Mo-methylidene species were found to be produced through two elementary reaction steps, and the Mo-oxametallacyclobutanes were identified as the intermediates. The activation barriers of the decompositions of the oxametallacyclobutane intermediates (Step 2) were estimated to be higher than those of the ethene addition on the Mo active sites (Step 1). The oxidation states of the Mo centers exerted marked influences on the stabilities of the intermediates as well as on the activation barriers and reaction heats of Steps 1 and 2, which were elucidated by the electronic properties of the O_b-ligands directly bonded to the Mo centers. Both free energy barriers and reaction heats have indicated that the whole processes of generating the Mo-methylidene species were preferred over the Mo(VI) rather than Mo(V) active site. Accordingly, the Mo(VI) active site was more efficient in catalyzing the formation of Mo-methylidene species in the heterogeneous Mo/HBeta catalytic systems.