Enhanced cracking activity of dealuminated Y zeolites

Cumene cracking has been studied over HY and over dealuminated Y zeolites prepared by combined silicon tetrachloride and mineral acid treatment. At 500 K, the rates of formation of propene and benzene over dealuminated zeolites were up to 100 times greater than those observed for HY zeolites of comparable proton content. The activity of HY zeolites at 640 K decayed rapidly with time-on-stream, while that of dealuminated zeolites remained effectively unchanged over a period of 10 hours. The diminished coke formation over the latter was confirmed by carbon analysis of used cataysts.     

Obtainment of hierarchical ZSM-5 zeolites by alkaline treatment for the polyethylene catalytic cracking

Great efforts have been applied in order to achieve the synthesis of hierarchical zeolites through post-synthesis alkaline treatment. In this case, the use of hierarchical ZSM-5 zeolites as acid catalysts in the catalytic cracking of polyolefins overcomes some diffusional limitations and expands the selectivity of products. Thus, the purpose of work was to promote a secondary porosity in ZSM-5 zeolite by NaOH treatment (using conventional electric and microwave heating methods), applying cetyltrimethylammonium bromide as pore-directing agent. Both heating methods generated additional secondary porosity, with microwave heating promoting a narrow pore size distribution, when compared to conventional. All hierarchical ZSM-5 have been showed low-density polyethylene conversions superior to the purely microporous ZSM-5, however the hierarchical zeolite processed by conventional electric heating reached the higher temperature reduction (42.6 K). It was evidenced through measurements of morphological and textural properties, ion exchange capacity and catalytic activity in the LDPE cracking that the CTAB cations exerts a protective effect at alkaline treatment avoiding the desilication process. The additional secondary porosity promoting the generation of compounds of lower molecular mass in LDPE catalytic cracking, by thermogravimetric analysis, proving that the alkaline treatment increases the accessibility to the active acidic sites of zeolitic catalysts for bulky molecules.     

Redox modification of high-silica zeolites with chromium via multistep cluster synthesis

This paper examines the modification of zeolites with superstoichiometric amounts of metals using multistep cluster synthesis. Ion-exchange sites in zeolite MFI-50 (SiO₂/Al₂O₃ molar ratio of 50) have been modified with hydrazonium cations. Subsequent redox reaction with chromate anions has yielded a chromium-containing zeolite. The sample prepared by three ion-exchange/redox cycles contains 10 wt % chromium (Cr-MFI-50). EPR results show that most of the chromium in Cr-MFI-50 is in the form of fine Cr₂O₃ particles. In addition, the material contains isolated Cr⁵⁺ ions and, presumably, Cr³⁺ stabilized at aluminum vacancies.     

Effect of the nonuniform dealumination on the acidity and catalytic activity of faujasite

Theoretical models are used to simulate aluminum distribution in a Y zeolite crystal when an aluminum gradient along the crystal is produced by dealumination. From these models, the theoretical distribution of different types of acid sites is calculated. The effect of the aluminum gradient along the crystal on the acid site distribution is discussed, and it can be related with the differences observed in cracking activity when the Y zeolite is dealuminated by different procedures.     

Correlation between the crystal zize and catalytic properties of ZSM-5 zeolites

The effect of crystal size on the catalytic properties of H-ZSM-5 has been studied. Good correlation has been observed between crystal size and the catalytic properties of H-ZSM-5. In contrast to large crystallites, the aggregated H-ZSM-5 consisting of small crystallites showed long catalytic life and high selectivities for hydrocarbon, aromatics and aromatics (polymethyl aromatics) in the conversion reaction of methaol to hydrocarbons (MTG reaction). Compared with large crystallites, the aggregated H-ZSM-5 had higher adsorption ability of 2,2-dimethylbutane, largr intercrystalline void space ranging from 5 nm to 20 nm, larger pore volume, and more external surface acid sites. These results indicate that the crystal size of H-ZSM-5 significantly affects the catalytic activity in the MTG reaction and that the catalytic life could be somewhat predicted from these physicochemical properties.     

On the unusual mechanism of Lewis acidity manifestation in HZSM-5 zeolites

The adsorption of CD₃CN and NH₃ on HZSM-5 zeolites containing prechemisorbed olefins were studied using diffuse reflectance i.r. spectroscopy. It is shown that in the presence of strong bases the unusual manifestation of Lewis acidity in these zeolites takes place as a result of fission of the Al←O bond in bridged alkoxyl groups by adsorbed base molecules. Nonempirical quantum-chemical calculations confirm this experimental finding and define the requirements of such Lewis acidity generation.     

Relationship between electrode size and surface cracking in the EDM machining process

This paper presents a study of the EDM machining of H13 and D2 tool steels using electrodes of different diameters. Scanning electron microscopy is employed to analyze the machined surface, and the concept of a Crack Critical Line (CCL) is introduced to explore the influence of electrode size, EDM parameters and material thermal conductivity on surface cracking. The current results reveal that the surface crack distribution is influenced by the machining parameters, the electrode diameter and the material conductivity. It is noted that cracks tend not to appear when the machining is performed with a decreased pulse current and an increased pulse-on duration. Furthermore, it is observed that changing the electrode diameter causes a parallel shift of the CCL location within the crack distribution map. The intercept of the line depends on the electrode size. When small diameter electrodes are employed in the machining process, the location of the CCL shifts upwards. This causes the no-crack zone to enlarge, and therefore permits a wider choice of machining parameters to be adopted.     

Synthesis and characterization of high-silica EMT and FAU zeolites prepared in the presence of crown-ethers with either ethylene glycol or 1,3,5-trioxane

EMT- and FAU-type zeolites were prepared by using a mixture of organic molecules, i.e., crown-ether + ethylene glycol or crown-ether + 1,3,5-trioxane. The products were characterized by elemental analysis, XRD, SEM, thermal analysis, n-hexane adsorption, and n.m.r. spectroscopy. Depending on the method used to prepare the starting gel, the solids obtained showed different characteristics. D.t.a. proved to be a good tool to determine, from the thermal decomposition of the organic templates the presence of structure defects in the EMT and FAU samples.     

The control of catalytic poisoning and stress corrosion cracking

When a structural metal is stressed in a hydrogen environment, the metal may crack at stress levels much lower than its normal strength. This embrittlement is caused by hydrogen atoms or ions rather than hydrogen molecules. The catalytic dissociation of hydrogen molecules into atoms or ions takes place at dislocation sites on a metal surface. The termini of dislocations on a metal surface are sites of localized high energy. There are “poisons” which will retard or even stop catalytic processes. The toxicity of a poison depends on the binding energy of the foreign atom to a dislocation terminus.Cracked specimens made of 4340 steel were tested in hydrogen gas and its mixture with SO₂, CS₂, CO₂, N₂ and Ar. Both SO₂ and CS₂ are very “toxic” and can stop a running crack. The toxicity of CO₂ is moderate. n₂ and Ar have no noticeable effect on a running crack.     

Relationship between microstructure and hydrogen induced cracking behavior in a low alloy pipeline steel

Hydrogen induced cracking(HIC) behaviors of a high strength pipeline steel with three different microstructures, granular bainite lath bainite(GB + LB), granular bainite acicular ferrite(GB + AF), and quasi-polygonal ferrite(QF), were studied by using corrosion experiment based on standard NACE TM0284. The HIC experiment was conducted in hydrogen sulfide(H_2S)-saturated solution. The experimental results show that the steel with GB + AF and QF microstructure present excellent corrosion resistance to HIC, whereas the phases of bainite lath and martensite/austenite in LB + GB microstructure are responsible for poor corrosion resistance. Compared with ferrite phase, the bainite microstructure exhibits higher strength and crack susceptibility of HIC. The AF + GB microstructure is believed to have the best combination of mechanical properties and resistance to HIC among the designed steels.     

Factors influencing the conversions of alkylaromatic hydrocarbons on high-silica zeolites: Part II. Presence of extralattice Al

The effect of different amounts of dislodged Al present in hydrothermally treated Y-type zeolites has been studied in the conversions of toluene, m-xylene, and alkylation of benzene with ethylene. T.p.d. of NH₃ is used to explain changes in the catalytic activity and modification of the strength and number of acid sites occurring in relation to the extralattice Al content. No indications for direct participation of the nonframework Al in the alkylaromatic conversions have been observed.