摘要:

The growing demand for new catalytic materials calls for new processes involving solid transformations. Finer control over the properties of these materials can be gained through a better understanding of the reactivity of solids. The mechanism of transformations is relevant whenever the desired solid product is not the thermodynamically stable one.

ISSN:0161-4940    

DOI:10.1080/01614948508064232

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

It has been recognized for some time that the incorporation of fluorine in oxide catalysts (for example, alumina, silica-alumina, or zeolites) enhances their activity for acid-catalyzed reactions such as cracking, isomerization, alkylation, polymerization, and disproportionation. These reactions are thought to proceed via carbocation intermediates which are formed and stabilized on surface protonic sites. The incorporation of fluorine increases the activity by enhancing the acidic properties of the catalyst. Fluorine incorporated in an oxide catalyst replaces surface O or OH, and because fluorine is very electronegative, it polarizes the lattice more than the groups it replaces, and this increases the acidity of both protonic (Brönsted) and nonprotonic (Lewis) sites on the surface. As will be seen, pure alumina is inactive or only slightly active for acid-catalyzed reactions. In contrast, it has been shown repeatedly that fluorinated alumina is a very active, selective, and stable catalyst for such reactions. The formation of fluorinated solid “superacids” which are active catalysts at low temperatures also has been reported. Very recently fluorination has been used in the modification of zeolite catalysts for better activity.

ISSN:0161-4940    

DOI:10.1080/01614948508064233

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

A class of crystalline aluminosilicates, known as the zeolites, are widely used as effective heterogeneous catalysts in the chemical industry. They possess excellent stability, activity and selectivity patterns for a wide range of hydrocarbon transformations [1–10]. X-ray diffraction studies have shown that zeolites are giant macromolecules formed by AlO4and SiO4tetrahedra joined by shared oxygens, and the structures of many synthetic and naturally occurring zeolites are now known and well documented [11]. As a simplification we can think of certain zeolites as a three-dimensional channelled network of interconnected cavities of different sizes. The excess negative charge on the framework of the zeolite, due to the AlO4tetrahedra, is either balanced by positively charged metal ions or protons which usually reside at well-defined sites in the zeolite lattice. The presence of exchangeable cations makes some zeolitic materials especially attractive in applications involving water treatment, such as domestic softeners and detergents, wastewater clean-up, and disposal and storage of radioactive elements. These subjects have been reviewed recently vis-a-vis the thermodynamics and kinetics of ion exchange in naturally occurring and synthetic zeolites [12]. This review will focus mainly on faujasite-type zeolites, one aspect of which will concern the vibrational spectra of the metal cations. The structure of zeolite Y is shown in Fig. 1, and the sites where cations can reside are indicated by Roman numerals in the accepted convention. These sites are of particular importance catalytically since they are thought to be the active centers for reaction [12–14] or the seeds of metal cluster formation [15]. Of these sites, those that can directly interact with molecules that have entered the large a-cage (supercage) at the center of the figure are more important in terms of hydrocarbon transformations. For example, to illustrate the difference between the α- and β-cavities of faujatites, even molecular oxygen with a kinetic diameter [161 of 3.54 å cannot enter through the oxygen 6–rings of the sodalite (β) cages at room temperature (see Fig. 11, but up to 6 pyridine molecules can simultaneously occupy the α-cages.

ISSN:0161-4940    

DOI:10.1080/01614948508064234

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Aliphatic amines are of considerable industrial importance and find application in almost every field of modern technology, agriculture, and medicine [1], Lower aliphatic amines (C1to C6) are important intermediates for the chemical and pharmaceutical industries. A large number of drugs, herbicides, pesticides, dyes, and other chemicals contain amino pups which originate from reactions with such intermediates, Many important applications of higher aliphatic amines (fatty amines) and their derivatives (most important derivatives are quarternary ammonium compounds) are based on their cationic surface activity. Relatively small amounts of such compounds are usually required to achieve the desired changes in surface and colloidal properties. Thus, not surprising, one of the first applications of fatty amines was in the flotation separation of nonmetallic materials such as potash, feldspar, phosphate, and mica. Today, probably the biggest demand for fatty amines lies in the production of fabric softeners. There are other important applications for aliphatic amines in the plastics and protective coat industries as emulsion stabilizers, mold release agents, pigment dispersers, and flushing agents. They are used as catalysts for polyurethane production. For granular products, alkylamines are used as anticaking and antidusting agents. In the rubber industry they are used as oxidation inhibitors and catalysts for accelerating vulcanization. Aliphatic amines find also many applications in the petroleum industry, especially as corrosion inhibitors and as components of lubricating oils, greases, and fuel oil where they act as sludge dispersants and stabilizers. They are added to gasoline as corrosion inhibitors.

ISSN:0161-4940    

DOI:10.1080/01614948508064235

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

This is a scanned image of the original Editorial Board page(s) for this issue.

ISSN:0161-4940    

DOI:10.1080/01614948508064231

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor