Pore size determination in modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis

Elements Of X Ray Diffraction

Steam cracking of hydrocarbons has been the major source of light olefins for more than half a century. The recent studies have reported that ethylene and propylene can also be produced through the cracking of hydrocarbons over modified ZSM-5 zeolites in a considerable amount. This paper highlights the important current ideas about acid-catalyzed hydrocarbon cracking that has resulted in high yield of ethylene and propylene. Light olefin production via catalytic cracking of various industrial feedstocks, ranging from heavy hydrocarbons to ethane, over modified ZSM-5 zeolites, has been reviewed in the present paper. Furthermore, the influence of various employed promoters, i.e., alkali and alkaline earth, transition, rare earth elements, and phosphorus, on the chemical properties of the modified ZSM-5 and the performance of resulting catalyst in enhancing the selectivity to light olefins, have been addressed. Moreover, the influences of different factors, including the zeolite acidity, Si/Al ratio and the temperature, on the light olefin production and the reaction scheme have been specified. The role of incorporated element in the catalytic cracking mechanism is also summarized.

Catalytic cracking of hydrocarbons over modified ZSM-5 zeolites to produce light olefins: A review

Superionic conductors are compounds that exhibit exceptionally high values of ionic conductivity within the solid state. Indeed, their conductivities often reach values of the order of 1 Ω−1 cm−1, which are comparable to those observed in the molten state. Following Faraday's first observation of high ionic conductivity within the solids β-PbF2 and Ag2S in 1836, a fundamental understanding of the nature of the superionic state has provided one of the major challenges in the field of condensed matter science. However, experimental and theoretical approaches to their study are often made difficult by the extensive dynamic structural disorder which characterizes superionic conduction and the inapplicability of many of the commonly used approximations in solid state physics. Nevertheless, a clearer picture of the nature of the superionic state at the ionic level has emerged within the past few decades. Many different techniques have contributed to these advances, but the most significant insights have been provided by neutron scattering experiments and molecular dynamics simulations. This review will summarize the state of current knowledge concerning the crystal structures and conduction processes of superionic conductors, beginning with a comparison of the behaviour of two of the most widely studied binary compounds, AgI and β-PbF2. Each can be considered a parent of two larger families of highly conducting compounds which are related by either chemical or structural means. These include perovskite-structured oxides and Li+ containing spinel-structured compounds, which have important commercial applications in fuel cells and lightweight batteries, respectively. In parallel with these discussions, the relative importance of factors such as bonding character and the properties of the mobile and immobile ions (charge, size, polarizability, etc) in promoting the extensive lattice disorder which characterizes superionic behaviour will be assessed and the possibilities for predicting a priori which compounds will display high ionic conductivity discussed.

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Superionics: crystal structures and conduction processes

This work outlines how treatment of zeolite H-ZSM-5 (Si/Al = 46) with NaOH may improve the catalytic performance in the conversion of methanol to gasoline. The zeolite was treated with 0.05 or 0.20 M NaOH solution for 2× 4 h at 75 °C. XRD confirmed the retention of crystallinity. The Si/Al ratio of the catalyst decreased as a consequence of the treatment, in particular for the more severely treated sample, but the total acidity determined by NH 3 -TPD was not altered, showing that the treated samples contain significant amounts of Al not giving rise to acidity. The BET surface area increased from 313 to 419 m 2 /g as a consequence of the desilication, and the N 2 adsorption measurements indicated mesopore generation. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated presence of Lewis acidity in the treated samples. Methanol conversion was carried out in a fixed bed reactor at 370 °C and WHSV = 8 g g −1  h −1 . The catalyst lifetime, quantified as the total conversion capacity, increased by a factor of 3.3 as a consequence of the most severe treatment. The procedure led to a moderate increase in the initial activities. Further, the product selectivities were altered dramatically. The selectivity towards the gasoline fraction (C 5+ ) was at best increased by a factor of 1.7. Hydrogen transfer reactions became faster and led to more aromatic and paraffinic compounds in the products. Increases in the propene/ethene ratios were observed at moderate conversion. The results are well rationalized by alterations of acidic properties, mesopore formation, and improved diffusivity.

Methanol to gasoline over zeolite H-ZSM-5: Improved catalyst performance by treatment with NaOH

ZSM-5 zeolite, when doped with fluorine species in relatively low concentration using ammonium fluoride as precursor, and subsequently activated stepwise at high temperature, shows an enhanced surface acidity because of: (i) the formation of new Bronsted acid sites; and (ii) the strengthening of some acid sites of the parent zeolite. This is ascribed to the proton attack of the zeolite surface by the chemisorbed H+F− ion pair. Under these preparation conditions, the zeolite structure is fully preserved. Such enhanced acid properties are traduced by a significant increase in the yield of methyl tert-butyl ether (gas phase reaction).

ZSM-5 zeolite with enhanced acidic properties

Modification of the micropore characteristics of the desilicated ZSM-5 zeolite by thermal treatment

SnK, PbL3, and BaL3 EXAFS, X-ray diffraction and 119Sn Mössbauer spectroscopic studies of ordered MSnF4 (M = Pb and Ba) fluoride ionic conductors with the α-PbSnF4 structure

Georges Dénès

Papers

ZSM-5 zeolite with enhanced acidic properties

Modification of the micropore characteristics of the desilicated ZSM-5 zeolite by thermal treatment

SnK, PbL3, and BaL3 EXAFS, X-ray diffraction and 119Sn Mössbauer spectroscopic studies of ordered MSnF4 (M = Pb and Ba) fluoride ionic conductors with the α-PbSnF4 structure

The “bent copper tube”: A new inexpensive and convenient reactor for fluorides of metals in suboxidation states

Phase transitions and structural relationships between Ge5F12, GeF2, SnF2, and TeO2