Effect of SiO2Al2O3 ratio on the activity of HZSM-5 zeolites in the different steps of methanol conversion to hydrocarbons

The conversion of methanol to hydrocarbons over HZSM-5 zeolites of different SiO₂Al₂O₃ ratio was studied at 450°C. The initial zeolite activity in the different steps of the reaction (alkene formation from methanol and dimethyl ether, alkene aromatization, coke formation) was strictly proportional to the aluminum content of the zeolites. Under reaction conditions, catalytic properties of HZSM-5 of different aluminum content were modified in the initial period of catalyst performance in different ways. Enhanced activity sites, which are generated in HZSM-5 of relatively high aluminum content, were shown to be the reason for enhanced aromatization and coking activity of these zeolites.     

27Al and 29Si n.m.r. studies of HZSM-5: Part 2. The effect of coke formation

The formation of coke in zeolite channels during the conversion of methanol to gasoline in HZSM-5 causes changes in the ²⁷Al and ²⁹Si spectra of the zeolite. Displacement of water and oxygen by bulky molecules in the channels increases the electric-field gradient at ²⁷Al, decreases the ²⁹Si relaxation rate, and eventually distorts the channel walls. Cross-polarization ²⁹Si n.m.r. was used to follow the hydrogen content of the coke, whereas water and oxygen contents were related to the amount of coke in the channels and the proportion of methyl groups.     

Novel Al2O3‐C Refractories with Less Residual Carbon Due to Nanoscaled Additives for Continuous Steel Casting Applications

A new generation of thermal shock resistant Al₂O₃‐C refractories with approximately 30% less residual carbon and functionalized due to nanoscaled additives based on carbon nanotubes (CNTs) and alumina nanosheets (α‐Al₂O₃) were developed and investigated after coking at 1000 and 1400 °C. With the aid of electron backscatter diffraction analyses (EBSD) on fracture surfaces of the carbon bonded samples, Al₃CON was identified on the nanosheet shapes already at 1000 °C coking temperature. The Al₃CON new phase based on the reaction between alumina nanosheets and CNTs offers a chemical interconnecting phase for the carbon as well as for the oxide alumina filler. The new refractory composite structure presents excellent thermo‐mechanical properties in spite the lower carbon content. In addition, due to EDS and EBSD analyses amorphous whiskers and platelets within the system of Si? O were observed in samples coked at 1000 °C, that were transformed to crystalline β‐SiC‐whiskers in samples coked at 1400 °C.     

Inactivation of external surface of mordenite and ZSM-5 by chemical vapor deposition of silicon alkoxide

The present paper describes the inactivation of the external surfaces of mordenite and ZSM-5 zeolites using chemical vapor deposition (CVD) of Si(OCH₃)₄. The external surfaces of H mordenites with different Si/Al atomic ratios, Na mordenite, and HZSM-5 were deactivated by a silica layer. The silicon concentration required for the inactivation did not depend upon the included cation and the atomic ratio of Si/Al. Si(OCH₃)₄ reacted equivalently with terminal hydroxides and acidic hydroxides.     

Post mortem study of refractory lining used in FCC units

In this work we performed a structural characterization of samples obtained at the end-of-life of refractory lining used in REGAP #2 FCC unit (Petrobras/Brazil). Samples collected from an unused lining were also used for comparison purposes. They were examined by X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray microtomography (μ-CT), mercury intrusion porosimetry (MIP), Raman spectroscopy (RS), and Fourier transform infrared spectroscopy (FTIR). We observed that alpha- and sodium beta-alumina are the major phases in the samples used in this work. SiO₂, CaAl₂O₄, and CaAl₄O₇ were observed in significant amounts in samples obtained from an unused lining. Al(OH)₃ was detected in minor amounts in these materials. CaAl₂O₄ and CaAl₄O₇ were observed as minor components in samples collected from a coked lining. Nonetheless, CaCO₃ is present in significant amounts in these materials. Raman tests revealed that the coke observed in samples collected from used linings is a carbon black-like material. It was observed by MIP and μ-CT that the presence of coke led to a partial blockage of the refractory pores. We noticed that samples collected from hot side regions usually show higher volume fractions of coke than those obtained from cold side regions.     

Methanol conversion to hydrocarbons over WO3/HZSM-5 catalysts prepared by metal oxide vapor synthesis

Metal oxide vapor synthesis (MOVS) methodology was used to deposit WO₃ on the zeolite HZSM-5. Samples I, II, and III of various weight percentage-loaded (3.1, 5.4, and 10.8, respectively) WO₃ on the HZSM-5 support were prepared. These materials were characterized by powder X-ray diffraction pattern, sorption, chemical analysis, and scanning electron microscopy. The characterization studies showed good crystalline nature of the support, partial inclusion of WO₃ particles into the zeolite cavities, and a uniform deposition of amorphous WO₃ over the support. These samples were tested for their catalytic activity in the conversion of methanol into hydrocarbons. Results were compared with that of pure support under identical conditions. The product selectivity for aromatics over WO₃/HZSM-5 catalyst samples was negligible compared with the same selectivity observed over pure support, whereas the selectivity for olefins increased significantly.     

Coking and regeneration of fixed bed catalytic reactors

Coke concentration profiles have been determined by a non-invasive method for the catalytic reaction of xylenes over a silica-alumina bead catalyst. Coking was observed to be of the parallel type with the reactant xylene as the immediate coke precursor. Temperature profiles on regeneration of these coked reactors have been investigated experimentally and show that the maximum temperature increase occurs near the reactor inlet for this type ofcoking. The coke deposit has been analysed and found to have the composition CH_0.55; this was reasonably uniform throughout the bed. This paper is dedicated to Dr L K Doraiswamy on his sixtieth birthday.     

X.p.s. study on the effect of hydrogen treatment on the state of gallium in Ga2O3/HZSM-5 mechanically mixed catalysts

Mechanically mixed Ga₂O₃/HZSM-5 catalysts have been examined via X.p.s. and X-ray diffraction both before and after reduction with H₂. Results are consistent with the postulate that there is a strong interaction of Ga₂O₃ with HZSM-5, which results in a transfer of Ga into the zeolite upon reduction.     

Oxidation behavior of a Mo(Si,Al)2 composite at 900–1600 °C in dry air

The oxidation of a Mo(Si,Al)₂-based composite is investigated in the temperature range 900–1600 °C in dry air. Exposure time was 72 h. Comparisons are made with the oxidation behavior of a conventional MoSi₂-based material. Cross-sections are examined with scanning electron microscopy and transmission electron microscopy; the phase composition is analyzed by X-ray diffraction and convergent beam electron diffraction. The material forms a continuous external α-alumina scale throughout the temperature range. Below the scale, there is a continuous Mo₅(Si,Al)₃ layer that overlies Mo(Si,Al)₂ in the bulk. The Mo(Si,Al)₂ phase immediately beneath the Mo₅(Si,Al)₃ layer is depleted in Al. No indications of MoO₃ volatilization could be found for the Mo(Si,Al)₂ material.     

On the role of enhanced activity sites in the different steps of methanol transformation into olefins over HZSM-5

The activities of parent and mildly steamed HZSM-5 (SiO₂/Al₂O₃ = 34) were studied in the different steps of methanol-to-olefin conversion at 370°C. It was found that enhanced activity sites, which cause the increased cracking activity of the steamed zeolites, do not participate in the steps of initial CC bond formation from methanol. For the steps of methanol interaction with olefins, these sites are more active than are the normal catalytic sites of HZSM-5 (bridging hydroxyl groups).     

Formation of iron silicide nano-islands on Si substrates by metal organic chemical vapor deposition under electron beams

Electron-beam induced chemical vapor deposition (EBI-CVD) of Fe(CO)₅ was performed on both Si (111) and (110) substrates at 673–873 K inside an ultrahigh vacuum transmission electron microscope. The formation of iron silicide islands was observed on both substrates. Cubic silicide nano-rods were formed on Si(111) substrates by EBI-CVD with focused electron beams. The formation of β-FeSi₂ islands was mainly observed on Si(110) substrates by EBI-CVD when the electron beam was broadly spread. It was shown that the size and the intensity of the electron beam played a significant role in EBI-CVD and affected the CVD process extensively.     

Mechanical and chemical properties of Ni3Si and Ni3(Si,Ti) alloys multiphased by chromium addition

Abstract

The alloying behaviour and microstructure of Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys were first characterised by optical microscopy, X-ray diffraction, and scanning electron microscopy with electron probe analysis. The microstructures of the Ni–Si–Cr ternary alloys consisted of large dispersed Ni₅Si₂ phase and finely precipitated Ni₃Si phase in nickel solid solution, while the Ni–Si–Ti–Cr quaternary alloys consisted of finely precipitated Ni₃(Si,Ti) phase and nickel solid solution. Then, the high temperature mechanical properties, bend strength, and oxidation and corrosion properties of the alloys were investigated. The Ni–Si–Cr ternary alloys showed significant strengthening over a wide range of temperatures, and also large compressive plastic deformation at high temperatures. The strength and fracture toughness at ambient temperatures were correlated with the volume fraction of Ni₅Si₂ phase. The Ni–Si–Ti–Cr quaternary alloys did not show increased yield strength, but exhibited improved tensile ductility and plasticity over a wide range of temperatures. Both Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys showed substantially improved oxidation resistance in air at 1173 K, compared with Ni₃Si and Ni₃(Si,Ti) alloys. Also, the Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys showed corrosion resistance comparable to that of the Ni₃Si and Ni₃(Si,Ti) alloys.     

The effect of underlayer-modified atomic monolayer on the mechanism of subsequent film growth

We have studied the influence of the upper Si monolayer (ML) in Fe-Si interlayers on the mechanism of subsequent growth of 7-ML-thick Cu films during molecular beam deposition. The Fe-Si interlayers and Cu films were studied in ultrahigh vacuum by the methods of electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES). The final morphology of samples was examined in air by the atomic force microscopy. It is established that the growth of a Cu film on a Si(1 ML)/Fe₂Si₃(5 ML)/Si(001) structure has a pseudolayer character (with dissolution of the uppermost Si monolayer), while the growth of Cu on a Si(1 ML)/Fe(1 ML)/Si (1 ML)/Fe(1 ML)/Si(001) structure proceeds in the form of islands. This behavior is explained by different chemical states of the uppermost Si monolayer, which is modified by Fe-Si underlayers with different structures.     

Interfacial Microstructure and Reaction Phases of Solid State Bonded at SiC/V Joints

SiC was bonded to SiC using V foil at temperatures ranging from 1473 K to 1673 K for 1.8 to 64.8 × 10³ s and 30 MPa in vacuum. Interfacial reactions and microstructures were investigated using electron probe microanalyser and X-ray diffractometer. SiC begins to react with V at temperatures above 1473 K. Granular V₂C phase was formed at the V side of the reaction zone, while a layer of V₃Si phase was formed at the interface between V₂C and SiC after bonding at 1573 K for 1.8 and 7.2 × 10³ s. The same reaction structure can be observed at 1473 and 1673 K for 1.8 × 10³ s. Hexagonal V₅Si₃Cx (X < 1) phase was formed at the interface between V₃Si and SiC after bonding at 1573 K for 14.4 and 21.6 × 10³ s, and the interface structure of the joint became SiC/V₅Si₃Cx/V₃Si/V₂C + V/V. This microstructure represents a complete diffusion path which is correlated with the corresponding Si-V-C phase diagram. At the longer bonding time of 64.8 × 10³ s, V was completely consumed, and the joint showed the layer structure of SiC/V₅Si₃Cx/V₃Si/V₂C/V₃Si/V₅Si₃Cx/SiC.     

D2H2 equilibration over γ-irradiated zeolites

D₂H₂ equilibrium was studied at 77 and 298 K over HY, AIHY, HZSM-5 and Al_xO_yHZSM-5 which had been γ-irradiated at 77 and/or 298 K. The exchange rate was found to be higher at the lower temperature regardless of the temperature of irradiation. Moreover, at 77 K the exchange rates were similar and more stable over the individual zeolites than at 298 K, thus indicating a common reaction path at 77 K. The exchange rate at 298 K depended on the zeolite type: it was more stable and higher over HZSM-5 than over HY, and extra-lattice Al increased both these properties on HY as well as on HZSM-5. The reaction mechanism is discussed in connection with the nature of defects generated by γ-irradiation.     

Silicate formation at the interface of Pr-oxide as a high-K dielectric and Si(001) surfaces

The composition and chemical bonding of the first atoms across the interface between Si(001) and the dielectric determine the quality of dielectric gate stacks. An analysis of that hidden interface is a challenge as it requires both, high sensitivity and elemental and chemical state information. We used X-ray absorption spectroscopy in total electron yield and total fluorescence yield at the Si2p and the O1s edges to address that issue. We report on results of Pr₂O₃/Si(001) as prepared by both, epitaxial growth and metal organic chemical vapor deposition (MOCVD), and compare to the SiO₂/Si(001) system as a reference. We find evidence for the silicate formation at the interface as derived from the characteristic features at the Si2p and the O1s edges. The results are in line with model experiments in which films of increasing film thickness are deposited in situ on bare Si(001) surfaces.     

13C CPMAS n.m.r. study of coke formation on HZSM-5

The carbonaceous residue (coke) formed on HZSM-5 during the conversion of methanol to gasoline was studied by ¹³C n.m.r. spectroscopy using cross-polarization and magic angle spinning and by e.p.r. spectroscopy. Measurements were made on samples with different coke contents and on samples from which some external coke had been removed by treatment with an oxygen plasma. It was found that the amount of carbon visible by n.m.r. differed from that expected from the measured coke contents and that the ratio of these amounts varied with the degree of coking. We concluded that the n.m.r.-visible carbon was most probably composed of methyl-substituted polycyclic aromatic hydrocarbons, the degree of methyl substitution and ring condensation changing with the coke content. It was inferred that these methyl-substituted polycyclic aromatics were present in the channels of the zeolite whereas the coke invisible to n.m.r. could be highly condensed polycyclic aromatics or “graphitic” structures on the external surfaces of the zeolite crystals. It appears that initial coke formation is internal and that external coke forms at latter stages in the deactivation process.     

室温下MnOx/HZSM-5催化氧化甲醛的性能和机理分析

以HZSM-5分子筛、高锰酸钾和甲醇为原料制备MnOx/HZSM-5催化剂,室温催化氧化甲醛。采用X射线衍射(XRD)、拉曼光谱(Raman)、扫描电子显微镜(SEM)、透射电镜(TEM)、光电子能谱(XPS)等表征方法分析催化剂的形貌结构、化学成分,研究MnOx/HZSM-5的催化性能和再生性能,探讨其对甲醛的催化氧化机理。结果表明,MnOx/HZSM-5具有良好的催化活性和再生能力,动态测试1020min后MnOx/HZSM-5对甲醛的清除率仍然保持在90%,再生5次后,MnOx/HZSM-5对甲醛清除率仍然保持在91%,静态测试中甲醛清除率达到97%,甲醛转化率达到92%。对MnOx/HZSM-5氧化甲醛的机理分析后发现,甲醛首先被催化剂吸附至MnOx活性位点,之后被初步氧化为甲酸盐或碳酸盐等中间产物,最后被深度氧化为二氧化碳和水。     

Degradation of 4-nitrophenol (4-NP) using ZnO nanoparticles supported on zeolites and modeling of experimental results by artificial neural networks

In this paper, we report the synthesis of ZnO, ZnO/HZSM-5, ZnO/HY and ZnO/Clin by a poly acrylamide pyrolysis method for the first time. The presences of carbon network/cages in the poly acrylamide gel can effectively prevent particle agglomeration. The catalytic activity of all specimens was tested by carrying out the 4-nitrophenol degradation, used as a “probe” reaction, in the aqueous medium under ambient visible light. The prepared samples were characterized by X-ray diffraction (XRD), specific surface area (BET) and porosity determination, scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX), visible-ultraviolet diffuse reflectance spectroscopy (DRS) and Fourier transform infrared spectroscopy (FT-IR), to evaluate particle structure, size distribution and composition. The results revealed that among the catalysts, ZnO/HZSM-5 showed higher percentage of adsorption than others. The time required for complete mineralization of 4-NP under ambient visible light over ZnO/HZSM-5 was 75 min. The higher activity of ZnO/HZSM-5 is mainly due to fine dispersion of ZnO and hydrophobicity of the support. An artificial neural networks (ANNs) model was developed to predict the performance of catalytic degradation process over synthesized catalysts based on experimental data. A comparison between the predicted results of the designed ANN model and experimental data was also conducted.     

溶胶-凝胶法制备La_2Ti_2O_7/HZSM-5及其光催化活性

采用溶胶-凝胶法制备La_2Ti_2O_7/HZSM-5光催化剂,并对其光催化活性进行研究。结果表明:La_2Ti_2O_7经HZSM-5分子筛负载后,并未改变原有晶相,仍为单斜晶系钙钛矿结构。负载后的La_2Ti_2O_7分散在分子筛表面,催化剂的比表面积大幅增加并形成新的中孔结构。HZSM-5制约了La_2Ti_2O_7的生长,导致晶粒尺寸减小。材料的光谱吸收边界随负载量的减小而发生蓝移,禁带宽度增大。负载样品中La3d和O1s的电子结合能增大。La_2Ti_2O_7/HZSM-5比纯La_2Ti_2O_7具有更高的光催化活性。经紫外光照120min后,活性艳红X-3B在70%La_2Ti_2O_7/HZSM-5上的总脱色率为91.8%,而在La_2Ti_2O_7上仅为31.7%。