Molecular selection of MoOx species during migration on Al2O3 and zeolites Y and ZSM-5

Surface migration is the basis of the solid state preparation of various materials. In the case of heterogeneous catalysts, it involves the spreading of metal oxides with low Tammann temperature, such as MoO₃, on various refractory supports like γ-Al₂O₃ and zeolites. In spite of its importance, the speciation of oxide surface migration has not been extensively studied. In this work Mo/γ-Al₂O₃, Mo/ZY and Mo/ZSM-5 were prepared from mixtures of MoO₃ and γ-Al₂O₃, or zeolites Y and ZSM-5, subjected to thermal activation. X-Ray diffraction (XRD), Raman and UV-vis diffuse reflectance spectroscopies were used to study the spread of MoO_x species on refractory metal oxide supports. It was found that during thermal activation the formation and migration of different MoO_x species depends strongly on the structure of the support.     

Fe-ZSM-5杂原子分子筛的合成与表征

采用静态水热法分别以正丁胺、四丙基溴化铵为模板剂合成ZSM-5分子筛及Fe-ZSM-5杂原子分子筛；利用XRD与FT-IR对其结构进行表征；考察了Mo改性后分子筛催化剂上甲烷脱氢芳构化的反应性能。结果表明，Fe部分进入了分子筛的骨架，导致分子筛的结晶度及表面酸强度的下降，使Mo／Fe-ZSM-5催化剂的反应性能较Mo／HZSM-5显著下降。     

Physicochemical characteristics of ZSM-5/SAPO-34 composite catalyst for MTO reaction

SAPO-34 and ZSM-5 are the most well-known catalyst for MTO reaction. A combination of ZSM-5 and SAPO-34 might give rise to optimal catalyst to meet a change of market demand for ethylene, propylene and butadiene. In this study, we have developed ZSM-5/SAPO-34 composite catalysts to control the composition of light olefins in MTO reaction. ZSM-5/SAPO-34 composite catalysts showed very different physicochemical and catalytic properties with respect to ZSM-5 and SAPO-34 synthetic procedure. The physicochemical properties of the composite catalysts have been compared by XRD, SEM, N₂ isotherm, FT-IR and NH₃-TPD. Their catalytic performances were also evaluated for MTO reaction. The series composite catalyst synthesized by successive crystallization of SAPO-34 synthetic gel after ZSM-5 crystallization exhibited relatively high catalytic performance.     

Equilibrated thermodesorption studies of adsorption of n-hexane and n-heptane on zeolites Y, ZSM-5 and ZSM-11

Gravimetric measurements of thermodesorption of n-hexane and n-heptane were performed under quasi-equilibrium conditions. Differential thermodesorption profiles for ZSM-5 and ZSM-11 showed two peaks, but for Y zeolites, only one thermodesorption peak was observed. A model function, derived from the Langmiur adsorption model, was fitted to the experimental data, and the model parameters (the adsorption entropy and enthalpy) were estimated. The two-step desorption profiles observed for ZSM-5 and ZSM-11 were attributed to the commensurate freezing effect, i.e. a transition in the adsorbed phase resulting in ordering of the adsorbed molecules in the zeolite channels. The results observed for ZSM-11 indicate that the zigzag channels typical for ZSM-5 micropore system are not necessary for this transition to occur.     

Electrical conductivity response and sensitivity of ZSM-5, Y,and mordenite zeolites towards ethanol vapor

This work is an attempt to search for highly selective sensing materials for ethanol vapor. The electrical conductivity response of ZSM-5, Y, and mordenite zeolites towards ethanol vapor have been investigated for the effects of the framework, the charge balancing cation type, and the Si/Al ratio. All zeolites were characterized using XRD, FT-IR, SEM, TGA, BET, and NH₃-TPD techniques. For the effect of the zeolite framework type, H⁺Y has a higher electrical conductivity sensitivity value than that of H⁺MOR because of a greater pore volume and available surface area. For the effect of the charge balancing cation, all NH₄ ⁺ZSM-5 zeolites (Si/Al = 23, 50, 80, 280) show negative responses, whereas the H⁺Y zeolites (Si/Al = 30, 60, 80) and the H⁺MOR zeolites (Si/Al = 30, 200) show positive responses. These differing behaviors can be traced to the electrostatic field at the cation sites in zeolite micropores, and their hydrophilic–hydrophobic character, which affect the adsorption properties of the zeolites. For the effect of Si/Al ratio, the electrical conductivity sensitivity towards the ethanol decreases with increasing Si/Al ratio or decreasing Al content, and there is a lesser degree of interaction between ethanol molecules and the active sites of the zeolites due to its higher hydrophobicity and the lower amount of cations. However, the H⁺Y (Si/Al = 5.1) and the H⁺MOR (Si/Al = 19) zeolites have lower conductivity sensitivity than those of H⁺Y (Si/Al = 30) and H⁺MOR (Si/Al = 30), respectively. The interactions between the C₂H₅OH molecules and the zeolites with respect to the electrical conductivity sensitivity were investigated and verified through infrared spectroscopy.     

Comparison of the dynamics of n-hexane in ZSM-5 and 5A zeolite structures

The translational and rotational dynamics of n-hexane adsorbed in ZSM-5 and 5A zeolites has been studied by neutron scattering and deuterium solid-state NMR, at various temperatures. The dynamics of n-hexane is quite different in the two zeolites. In the ZSM-5 structure, the molecule sits in channel segments, the energy barrier between adjacent adsorption sites is small and fast anisotropic motions are observed. In the 5A zeolite, the molecule is adsorbed in -cages; the barrier between adjacent cages is larger so that the molecule spends a longer time exploring the volume of an -cage, leading to a more isotropic motion. The diffusion coefficient of the molecule is reduced by more than 4 orders of magnitude in 5A zeolite compared with ZSM-5.Received: 1 January 2003, Published online: 14 October 2003PACS: 82.75.Jn Measurements and modeling of molecule migration in zeolites - 61.12.Ex Neutron scattering techniques (including small-angle scattering) - 76.60.-k Nuclear magnetic resonance and relaxation     

Effect of solvent and crystal size on the selectivity of ZSM-5/Nafion composite membranes fabricated by solution-casting method

Zeolite/Nafion composite membranes with high proton selectivity were successfully fabricated using the solution-casting method. The types of zeolites are nano-sized and large sized Na-ZSM-5, H-ZSM-5, and their ball-milled ones. Two different schemes of experiments were conducted depending on the type of solvent. In case of using as-received Nafion® ionomer dispersions, the experimental results clearly show that the proton conductivity of zeolite composite membrane using either H-type or Na-type ZSM-5 depends on the type of solvent. It is thought that when propanol and water as the solvents were used, more hydrophilic H-type ZSM-5 seems to have been more randomly dispersed into hydrophobic region rather than hydrophilic ionic clustered channels within Nafion. Therefore, H-type ZSM-5 existing near hydrophobic region seems to provide additional path for proton migration but weakening the mechanical strength. These composite membranes show higher water uptake than commercial Nafion® 115, strongly suggesting better water retention ability of zeolite. The most interesting result is that the methanol permeability has decreased with increasing zeolite contents even when the proton conductivity increased, and the proton selectivities of these composite membranes expressed as characteristic factor were higher than that of Nafion® 115. In case of using a mixture of high boiling point DMF and ethanol as the solvent, unlike the previous case where no DMF was used, the proton conductivity slightly dropped with increasing zeolite contents. These results should have been attributed to a blocking effect of zeolite particles surrounded by inversely oriented hydrophilic micelles of Nafion. However, the values of proton conductivity of most composite membranes were significantly higher than that of Nafion® 115, and methanol permeability also decreased with increasing zeolite contents. The significantly lower methanol permeability of the composite membrane fabricated with DMF as the solvent is probably due to the more effective blocking effect of H-ZSM-5 for ionic clustered channels as well as difficult transport of methanol through zeolite pores.In case of the composite membranes containing ZSM-5 with large crystal size, it is found that the methanol permeability has increased considerably with the increasing of zeolite contents due to void fractions between polymer phases and zeolite particles. In case of using ball-milled ZSM-5 with small crystal size, however, the value of characteristic factor tends to increase with increasing zeolite contents. Consequently, it is seen that the characteristic factor of Zeolite/Nafion composite membranes was much higher than Nafion® 115. The results obtained throughout this study strongly suggest that zeolites with small crystal size and high hydrophilicity are very prospective for composite membrane for direct methanol fuel cells in the future.     

Methane activation over Ag-exchanged ZSM-5 zeolites: A theoretical study

Methane activation catalyzed over Ag-exchanged ZSM-5 zeolites was investigated by using the density functional theory (DFT) with a cluster model. Two different pathways were taken into account in this work: the “alkyl” and the “carbenium” pathways. The activation barriers obtained are 34.09 and 66.63 kcal/mol for the “alkyl” and the “carbenium” pathway, respectively. The calculated results show that the activation barrier of the “alkyl” pathway is smaller than that of “carbenium” pathway. Consequently, the “alkyl” pathway is the preferential reaction pathway. A new mechanism of methane conversion in the presence of ethene was proposed. In the catalytic cycle, the initial step of methane activation proceeds with the “alkyl” pathway and the Ag⁺ cation acts as an acceptor of the methyl group, then ethene reacts with the Ag⁺CH₃⁻ group to form propene. In addition, it is found that the Ag⁺ cations play an important role in the methane activation, compared with the reaction of methane activation over H-ZSM-5.     

Tuning the hydrophobicity of ZSM-5 zeolites by surface silanization using alkyltrichlorosilane

ZSM-5 zeolites were modified with alkyltrichlorosilanes of various chain lengths (octyltrichlorosilane, decyltrichlorosilane, dodecyltrichlorosilane and hexadecyltrichlorosilane) and characterized by Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Thermal gravimetric analysis (TGA) and contact angle measurements (CA). The results showed that a closely packed and hydrophobic layer was presented at the particles surface and the surface wetting property varied from hydrophilic to hydrophobic, even to superhydrophobic. It was interesting to notice that the hydrophobic properties of modified ZSM-5 particles could be tuned by varying the chain length of chlorosilane and changing the pretreatment temperature before silanization. With increasing the alkyl chain length of trichlorosilane, the hydrophobicity increased. However, with an increase in the pretreatment temperature, the hydrophobicity decreased. Moreover, the relationship between the wetting properties and thermal stability was also investigated, the results showed that the modified ZSM-5 particles possessed good hydrophobicity at a temperature below 250 °C in air. These modified ZSM-5 particles may be utilized for many potential applications, such as membrane fillers, selective adsorbents, catalysts, chromatographic supports and so on.     

Synthesis of Pt modified ZSM-5 and beta zeolite catalysts: influence of ultrasonic irradiation and preparation methods on physico-chemical and catalytic properties in pentane isomerization

Influence of preparation methods and ultrasound irradiation on physico-chemical and catalytic properties was investigated by synthesizing Pt-ZSM-5 and Pt-Beta catalysts by in-situ and impregnation methods and applying ultrasound irradiation to synthesis gel mixture of ZSM-5 and Beta zeolites. It was concluded from the X-ray powder diffraction patterns of Pt-ZSM-5 and Pt-Beta zeolite catalysts that introduction of Pt by in-situ method and ultrasound irradiation did not influence the structures of ZSM-5 and Beta zeolites. Morphology of ZSM-5 and Beta zeolites were investigated by scanning electron microscopy. SEM micrographs showed that the Pt-ZSM-5-IS-US catalyst synthesized by in-situ method with ultrasound irradiation resulted in smaller crystals of ZSM-5 than Pt-ZSM-5-IS catalyst prepared without ultrasound irradiation. Furthermore SEM micrographs of Pt-Beta-11-IS-US zeolite synthesized using ultrasound irradiation showed much smaller crystals than Pt-Beta-11-IM indicating that ultrasound irradiation had a significant effect on the morphology of Beta zeolite. Conversion of n-pentane and selectivity to iso-pentane over the Pt-ZSM-5-IS-US zeolite catalysts prepared by ultrasound irradiation during in-situ introduction of platinum was higher than the catalysts prepared without the ultrasound irradiation. Furthermore ultrasound irradiated and in-situ synthesized Pt-Beta-11-IS-US catalyst also showed higher selectivity to iso-pentane than Pt-H-Beta-11-IM prepared by impregnation method.     

Optical properties of chalcogen-loaded zeolite (ZSM-5)

Sulfur, selenium, and tellurium were loaded into sub-millimeter size ZSM-5 single crystals, and the optical properties have been comparatively studied. S and Te show similar features, while Se is unique. S and Te have optical absorption edges at wavelengths of ～400 nm with transmission dips at ～450 nm, while Se has the edge at ～550 nm. The three materials provide photoluminescence at visible wavelengths, with the intensities of S and Te being stronger than that of Se by two orders. These optical properties imply that S and Te in the zeolite form small atomic units such as S₃ and Te₂, while Se condenses into single-chain structures.     

Facile synthesis of mesoporous ZSM-5 aided by sonication and its application for VOCs capture

Application of ultrasound power to the mother liquor is popular pretreatment for zeolite synthesis which offers a simple way of accelerating crystallization process and finetuning the properties of nanocrystalline zeolites. In this work, sonication-aided synthesis of mesoporous ZSM-5 at low temperature and ambient pressure was systematically studied, in an attempt to reach efficient and benign synthesis of zeolites with hierarchical pore structure, which has wide applications as catalysts and sorbents. The effects of sonication duration, power density, sonication temperature and seeding on the crystallization of ZSM-5 were investigated. The obtained samples were characterized by XRD, SEM, BET and VOCs capture. High quality mesoporous ZSM-5 can be obtained by a facile 5 d synthesis at 363 K, much faster than conventional hydrothermal synthesis. The reduced synthesis time was mainly attributed to the enhanced crystallization kinetics caused by the fragmentation of seeds and nuclei, while sonication radiation had little impact on the nucleation process. Compared with control sample, mesoporous ZSM-5 prepared by sonochemical method had higher surface area and mesoporosity which demonstrated improved adsorption performance for the capture of isopropanol.     

氢在A、X及ZSM-5型沸石上的高压物理吸附

采用常规体积吸附装置测定了77、195、293K和7MPa的条件下氢在A、X及ZSM-5沸石上的吸附特性和吸附容量．所有的氢吸附等温线基本符合Ⅰ型等温线,但在77K,压力为2-5MPa的等温线上观察到了超临界高压吸附所特有的最大吸附量．从等温线确定了等量吸附热并讨论了其影响因素．根据骨架结构和所含阳离子类型的差异,各种沸石表现出不同的氢吸附量．其中NaX沸石在77K/4MPa下的重量储氢分数为2．55％,是该实验中所测得的最高吸附量．CaA、NaX和ZSM-5沸石的氢吸附量与其比表面积成正比,这与沸石中的可用空穴容积有关．然而NaA和KA沸石不存在这种线性关系．实验中还观察到,NaA与KA沸石间出现氢吸附量的临界值是由KA沸石中较大的阳离子堵塞效应引起的．该实验将吸附质分了的动力学直径与沸石主晶孔的有效直径之比用于判断物理吸附中的堵塞效应．     

Key factor affecting the structural and textural properties of ZSM-5/MCM-41 composite

ZSM-5/MCM-41 micro/mesoporous composite materials were synthesized by the hydrothermal technique with alkali-treated ZSM-5 zeolite as source of silica and aluminum and characterized by various physico-chemical techniques such as X-ray diffraction (XRD), nitrogen sorption at 77 K, transmission electronic microscopy (TEM), FTIR spectroscopy and NH₃ temperature programmed desorption (TPD) techniques. The effect of concentration of CTAB in the synthesis of these solids has been investigated, the mesopore volume, surface area and surface acidity decrease with increasing the concentration of CTAB. Increasing the CTAB concentration causes the recrystallization of zeolite ZSM-5 and it disadvantage the formation of mesoporous materials MCM-41. The catalytic activity of ZSM-5/MCM-41 materials has been evaluated in the Friedel–Crafts acylation of anisole with benzoyl chloride as alkylating agent. The results revealed the reaction to be influenced by surface area, pore volume and surface acidity.     

The Study of Zeolite ZSM-5 by a Dielectric Spectroscopy Method

The dielectric absorption spectrum of zeolite ZSM-5, in the range of frequencies 100 c - 10 Mc, was studied. The influence of the crystallization degree and influence of the Si/Al ratio on the amplitude of the dielectric absorption were analysed. It was observed that _max depends on the Si/Al ratio in an exponential form. An excess of Et.OH in the mixture of crystallization influences favorably the kinetics of the process.     

Characterization and catalytic performance of Zr-Mo-oxide catalysts for selective oxidation of methane to formaldehyde

The selective oxidation of methane to formaldehyde has been investigated over a series of Zr-Mo-oxide catalysts. Comparative characterization of these catalysts has been carried out by BET specific surface area (BET), X-ray diffraction (XRD), laser Raman spectroscopy (LRS), temperature programmed reduction of hydrogen (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The Zr-Mo-oxide catalyst with 12 wt% Mo is found to be highly selective and yields formaldehyde by selective oxidation of methane. The interaction between Mo and Zr induces changes of physicochemical properties such as structure, crystal size, reducible properties, charge of ion and molybdenum oxide species etc., which in turn determines the catalytic performance. These effects are a function of Mo content. Zr(MoO₄)₂in these catalysts might be responsible for selective oxidation of methane to formaldehyde.     

The transformation of <Emphasis Type="Italic">n</Emphasis>-alkanes on H-forms of zeolites under supercritical conditions

The skeleton isomerization of n-butane, n-hexane, and n-heptane was studied under supercritical conditions on H-forms of zeolites mordenite, beta, and ZSM-5 over the temperature and pressure ranges 260–450°C and 80–130 atm. The isomerization of n-hexane and n-heptane was accompanied by side processes such as oligomerization and cracking. The selectivity of formation of branched isomers of these hydrocarbons did not exceed 70 and 30%, respectively. The catalyst with the most stable operation was pentasil ZSM-5, but the selectivity of formation of branched isomers on it did not exceed 30% even at 260–280°C and decreased to 3% as the reaction temperature increased to 400–450°C. The fraction of aromatization products was then more than 15%. A study of the influence of C₆–C₇ n-alkane additives on the isomerization of n-pentane on mordenite in the H-form under supercritical conditions at 260°C and 120 atm showed that a gradual decrease in the activity of this catalyst in the isomerization of n-pentane was related to the formation of heavier hydrocarbons.     

Iron Species Present in Fe/ZSM-5 Catalysts – Influence of the Preparation Method

The Fe species present in Fe/ZSM-5 catalysts prepared by ionic exchange in aqueous solution or in the solid state are investigated. The samples were characterized by XRD, EPR, Mössbauer Spectroscopy (MOS-S) and chemical analysis. XRD showed that the peak intensity decreased with the increase in Fe content in the samples. EPR and MOS-S showed that the Fe atoms in all the activated samples appear as Fe⁺³, independently of the method used in the catalyst preparation. The MOS-S data also showed that Fe/ZSM-5, prepared by ion exchange in the solid state presented lower hematite content and more Fe species in charge-compensation sites, the responsible for the catalytic activity, as confirmed by the behavior in the propane oxidation.     

Characterization of Copper-Modified ZSM-5 with Sorbed Pyridine Using Nuclear Magnetic Resonance Spectroscopy

Solid-state ¹H, ¹³C and ²⁹Si MAS NMR experiments were used to detect sorbed pyridine in hydrated copper-exchanged forms of synthetic zeolite ZSM-5, which is widely used in industry and green chemistry as well. The paper follows on from the previous study of sorption of organic pollutants by copper-modified forms of ZSM-5. Multinuclear MAS NMR experiments performed on hydrated and dehydrated copper-exchanged ZSM-5 samples with sorbed pyridine indicate that dissociation of water molecules in the vicinity of copper cations results in formation of new acid sites in the zeolite channels. Through their reaction with sorbed pyridine molecules, pyridinium ions are formed. ¹³C MAS NMR spectra also confirmed the presence of pyridine molecules not involved in forming pyridinium ions in the copper-modified ZSM-5 samples. From the changes in ²⁹Si NMR spectra, some slight changes in the structure of original copper-modified ZSM-5 after pyridine sorption can be also deduced.     

Ultrasound-assisted rapid hydrothermal design of efficient nanostructured MFI-Type aluminosilicate catalyst for methanol to propylene reaction

The influence of ultrasound-assisted rapid hydrothermal synthesis of aluminosilicate ZSM-5 catalysts was examined in this work. A series of MFI-type nanostructured materials with sonochemical approach and conventional heating were synthesized and evaluated for conversion of methanol to propylene reaction. The prepared samples were tested by characterization analyses such as XRD, FESEM, BET-BJH, FTIR, TPD-NH₃ and TG/DTG. The obtained results confirmed that ultrasound treatment enhanced the nucleation process and crystal growth for ZSM-5 sample synthesized at moderate temperature of 250 °C. Therefore, it was found the formation of pure MFI zeolite with high crystallinity and improved textural, structural and acidic properties for ZSM-5(UH-250) sample compared with the other zeolites. This observation was attributed to the relationship between the perfect crystallization mechanism and catalytic properties, which led to producing an efficient MFI zeolite toward the optimal catalytic performance. In this manner, the methanol conversion and products selectivity of prepared materials were carried out in MTP reaction at 460 °C and atmospheric pressure. The ZSM-5(UH-250) zeolite with slower deactivation regime exhibited the constant level of methanol conversion (84%) and high propylene selectivity (78%) after 2100 min time on stream. Moreover, the synthesis pathway for MFI zeolite at moderate temperature and also deactivation mechanism of improved sample were proposed.