Competitive disproportionation and demethylation of toluene on Mg-containing Y zeolites

On Mg-containing Y zeolites toluene undergoes disproportionation and demethylation. The relative importance of the two processes is discussed.

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Mg- Y . .

     

129Xe NMR study of ZSM-5 type zeolites Effect of cationic sites

A comprehensive ¹²⁹Xe NMR spectroscopy study on H-ZSM-5 zeolites having different aluminum contents and on cation-exchanged ZSM-5 zeolites is reported. The parent H-ZSM-5 zeolites were ion-exchanged with Group I–III metal ions ( K, Ca, Sr, Ba, Al, La) to varying degrees. The chemical shift of adsorbed ¹²⁹Xe is seen to be a function of the pentasil structure of ZSM-5, of the number of free Brønsted acid sites and of the number of metal cations in the framework. Differences in the chemical shift of ¹²⁹Xe are seen between cations due to their different polarizing forces against xenon. The amount of cations has also an effect on the δ_xe-xe term in Fraissard's equation that may be caused by changes in the diffusional characteristics of Xe atoms in the ZSM-5 framework.     

Temperature-programmed desorption of aromatic hydrocarbons from ZSM-5 type zeolites

Temperature-programmed desorption of C₆–C₁₂ aromatic hydrocarbons from HZSM-5 and NaZSM-5 in helium stream has been studied. The role of the length, branching and number of alkyl groups was analyzed. It was shown that adsorption of aromatic ring occurs on Na-cations.     

Catalytic properties of zeolites MCM-22 and NU-87 in disproportionation of toluene

The catalytic properties of MCM-22 and NU-87 were investigated for the disproportionation of toluene to produce benzene and xylene, and the results were compared with those obtained over mordenite, beta and ZSM-5. It turns out that dealumination of MCM-22 removes selectively the acid sites from the external surface and thus suppresses the secondary isomerization of p-xylene, enhancing the para-selectivity. This indicates that the dealuminated MCM-22 is a promising catalyst for the selective formation of p-xylene from toluene disproportionation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Selective oxidation of methane by the bis(mu-oxo)dicopper core stabilized on ZSM-5 and mordenite zeolites

This work reports on the capability of the O2-activated Cu-ZSM-5 and Cu-MOR zeolites to selectively convert methane into methanol at a temperature of 398 K. A strong correlation between (i) the activity and (ii) the intensity of the 22 700 cm-1 UV-vis band, assigned to the bis(mu-oxo)dicopper core, is found (i) as a function of the reaction temperature, (ii) as a function of the Cu loading of the zeolite, and (iii) in comparison to other Cu materials. These three lines of evidence firmly support the key role of the bis(mu-oxo)dicopper core in this selective, low-temperature hydroxylation of methane.     

Isomorphic substitution of boron in ZSM-5 type zeolites using TBP as template

A series of B-ZSM-5 samples has been synthesized using ethyl silicate (ES) ester-40, orthoboric acid and tetrabutylphosphonium (TBP) cation template and characterized by XRD, FT-IR, SEM and chemical analysis. It is observed that boron content in the initial reaction mixture influences the crystallization time and the morphology of the crystals. The migration of boron from framework upon calcination is enhanced as the boron content in the framework is increased. The test reaction of 1-hexene is under the profound influence of temperature on conversion and on the formation of primary products trans-2-hexene (t-2H), cis-2-hexene (c-2H) and 3-hexene. The ratios of the selectivity of c-2H to t-2H and 2-hexene to 3-hexene increased with decreasing temperature and decreasing boron content in the framework. Skeletal isomerization products started forming from 250°C as secondary products and increased with further increase of temperature.     

A theoretical study of hydrothermal stability of P-modified ZSM-5 zeolites

Density functional theory was employed to study the hydrothermal stability of P-modified ZSM-5 zeolites using cluster models. The calculations of hydrolysis energies indicated that the introduction of phos-phorus increases the hydrothermal stability of ZSM-5 zeolites. The initial paths of dealumination were studied with explicit water molecules. It was found that the framework Al—O coordination bond can be replaced by coodination bonds between water molecules and the aluminium. One to three water molecules ...     

Electrochemical study of the thionine dye incorporated into ZSM-5 and HZSM-5 zeolites

The electrochemical properties of thionine dye adsorbed into ZSM-5 and HZSM-5 zeolites (TH/ZSM-5, TH/HZSM-5) are studied in 0.5 M KCl solution. The dye is strongly retained and not easily leached from the zeolites matrix. The samples are incorporated into the carbon paste electrode (TH/ZSM-5/P, TH/HZSM-5/P) for cyclic voltammetric measurements. The redox reactions of thionine incorporated into ZSM-5 zeolite contain a quasi-reversible, two-electron one proton in the pH range 1 to 10, but thionine-loaded HZSM-5 zeolite undergoes a quasi-reversible two-electron two-protons redox reaction under acidic conditions and a one proton two-electron redox reaction takes place under basic conditions. The separation of the anodic and cathodic potentials (E _p) is high in thionine-loaded zeolites (>100) with respect to the solution of thionine (E _p = 34 for ZSM-5/P and 36 mV for HZSM-5/P), indicating that there are strong interaction between thionine molecules and the zeolites. The midpoint potentials (E _m) for TH/ZSM-5/P and TH/HZSM-5/P are −0.203 and −0.381 V, respectively. However, the midpoint potentials for the solution of thionine for the electrode system of ZSM-5/P and HZSM-5/P are −0.335 and −0.407 V, respectively. Thus, thionine dye molecules incorporated into the zeolites can be reduced more easily with respect to solution of thionine. In various electrolyte solutions, the midpoint potentials remains constant, but the midpoint potential of the thionine-zeolite electrodes depends on the solution pH. Influence of the pH of the solution on the midpoint potential of an immobilized dye reveals that thionine molecules are accessible to protons. This property is ascribed to the formation of mesopores in the structure of our zeolites suffering from a calcination step. Published in Russian in Elektrokhimiya, 2007, Vol. 43, No. 7, pp. 794–800. The text was submitted by the authors in English     

Cu+(H2) and Na+(H2) adducts in exchanged ZSM-5 zeolites

Cu(I) ions in Cu-ZSM-5 form Cu+(H2) complexes, stable at room temperature and sub-atmospheric H2 pressure, which do not have any homogeneous analogue except for matrix-isolated [Cu(eta2-H2)Cl]. Comparison with the unstable Na+(H2) adducts formed in the parent Na-ZSM-5 zeolite allow the conclusion that the Cu(I)/H2 bond is governed by sigma-pi overlap forces.     

An in situ Al K-edge XAS investigation of the local environment of H+- and Cu+-exchanged USY and ZSM-5 zeolites

Aluminum coordination in the framework of USY and ZSM-5 zeolites containing charge-compensating cations (NH4+, H+, or Cu+) was investigated by Al K-edge EXAFS and XANES. This work was performed using a newly developed in-situ cell designed especially for acquiring soft X-ray absorption data. Both tetrahedrally and octahedrally coordinated Al were observed for hydrated H-USY and H-ZSM-5, in good agreement with 27Al NMR analyses. Upon dehydration, water desorbed from the zeolite, and octahedrally coordinated Al was converted progressively to tetrahedrally coordinated Al. These observations confirmed the hypothesis that the interaction of water with Br?nsted acid protons can lead to octahedral coordination of Al without loss of Al from the zeolite lattice. When H+ is replaced with NH4+ or Cu+, charge compensating species that absorb less water, less octahedrally coordinated Al was observed. Analysis of Al K-edge EXAFS data indicates that the Al-O bond distance for tetrahedrally coordinated Al in dehydrated USY and ZSM-5 is 1.67 angstroms. Simulation of k3chi(k) for Cu+ exchanged ZSM-5 leads to an estimated distance between Cu+ and framework Al atoms of 2.79 angstroms.     

Developments and structures of mesopores in alkaline-treated ZSM-5 zeolites

ZSM-5 zeolites with SiO₂/Al₂O₃ molar ratio of 24 were treated in 0.05 M aqueous sodium hydroxide solution at 325 K in different periods. The samples were characterized by means of nitrogen adsorption at 77 K, field emission scanning electron microscopy, X-ray diffractometry, and Fourier transform infrared spectroscopy. Analysis of the experimental results showed that the alkaline treatment periods have influence on the developments and structures of mesopores in the alkaline-treated ZSM-5 zeolites. Alkaline treatment initially develops mesopores mainly from the boundary portion of MFI zeolites to the bulk, while prolonged treatment destroys the mesopores, and an optimum mesoporosity is obtained by the treatment for 1.5 h. On the other hand, crystallinities and short-range order in alkaline treated zeolites have remained virtually unchanged according to the examination from X-ray diffractometry and Fourier transform infrared spectroscopy.     

Influence of Si/Al ratio on the catalytic properties of NiH mordenite in the disproportionation of toluene

The disproportionation of toluene on dealuminated NiH mordenites has been studied in order to understand the influence of the Si/Al ratio on the activity, selectivity and stability of these catalysts.Four dealuminated H mordenites were prepared by combined treatments of calcination under wet air and boiling HC1 solution; the atomic Si/Al ratio of the framework, determined by ²⁹Si NMR spectroscopy, varied from 5 to 29.3.A maximum of activity was obtained for a Si/Al ratio of ~12.6. This catalytic behaviour is explained by the influence of the Si/Al ratio on the acidity and also by strong diffusional limitations for the non-dealuminated catalyst.The most active catalyst is also the most selective in aromatic compounds. On the other hand, the non-dealuminated catalyst is the least selective, as explained by a strong hydrogenolysis activity.The decrease of selectivity in aromatics of the dealuminated NiH mordenites after ageing can be explained by the activation of some of the nickel which was not previously reduced, and also by the higher temperatures of the catalytic tests, which favour the hydrogenolysis and hydrocracking reactions.     

Temperature-programmed desorption of n-propylbenzene from HNaZSM-5 and Na(Li, K, Rb, Cs) ZSM-5 type zeolites

Adsorption of n-propylbenzene on ZSM-5 type zeolites occurs on both Brönsted and Lewis type acid sites. Alkali cations strongly bind hydrocarbons. One particle of hydrocarbon is adsorbed on one Na⁺ ion. The strength of the bond depends on the cation radius and decreases in the order: Li>Na>K>Rb>Cs.     

ZSM-5分子筛转化乙醇为乙醚的研究

用原位红外技术、程序升温热脱附和X射线光电子能谱研究了乙醇和乙醚在HZSM-5和PZSM-5上的吸附及表面状态, 并与催化乙醇为乙醚的活性和选择性相关联. 结果表明, 乙醇与质子酸中心作用形成吸附态乙醇, 该吸附态乙醇与气相乙醇反应生成吸附态乙醚, 然后从表面脱附. 磷的掺入能有效抑制积炭发生.     

A theoretical study of the alkylation reaction of toluene with methanol catalyzed by acidic mordenite.

A theoretical study of the alkylation reaction of toluene with methanol catalyzed by the acidic Mordenite (Si/Al = 23) is reported. Cluster DFT as well as periodical structure DFT calculations have been performed. Full reaction energy diagrams of the elementary reaction steps that lead to the formation of the three xylene isomers are given. The use of periodical structure calculations allows one to account for zeolite framework electrostatic contributions and steric constraints that take place in zeolitic catalysts. Especially the steric constraint energy contribution has a significant effect on the energies and bond formation paths. The activation energy barrier of p-xylene formation is found to be approximately 20 kJ/mol lower than the corresponding values for the formation of its isomers. Computed host-guest binding energies according to the DFT method need a correction due to the absence of the dispersive interaction with the zeolite wall. Apparent activation energies obtained with this correction are in good agreement with experimental data.     

Comparison of isomerization and disproportionation activity of H-clinoptilolite and other protonated zeolites

Catalytic activity of an H-clinoptilolite is compared with that of H-mordenite, HL, HZSM-5 and HY zeolites in the conversion of o-xylene and toluene. Because of the dimensions and structure of its channels, H-clinoptilolite, like H-ZSM-5, exhibits shape-selective catalysis, which enables the transformation essentially to isomerization products via a monomolecular mechanism.