Silica-alumina catalysts, active sites nature

Nature of the Active Sites on Silica-Alumina Catalysts... 

There are other types of active sites. In 1955 Kobozev (17) pointed out that sometimes—especially with certain metals—the catalytic properties could be due to the properties of atoms, but that in other cases—as with some metal oxides—the catalytic property might be associated with the entire crystal We showed later (j ) that in at least one case, silica-alumina catalyst, it is very easy to err concerning the nature of the site Thus, evidence for Bronsted acidity could be interpreted as evidence for the presence of aluminum which can be ion-exchanged when the catalyst is placed in salt solution Looking at all these examples, we conclude that that there are many different sources of activity on the solid catalyst surface. 

During the 1940,s, much effort was made to elucidate the nature of the active sites in amorphous silica-alumina cracking catalysts (11). By... 

In all the isomerization reactions carried out in heterogeneous conditions, the nature of the products and product ratio depended largely on the type of catalyst employed, and, moreover, in most of the cases no selectivity was found. Papers have recently appeared concerning the transformation of styrene oxide into phenyl acetaldehyde catalyzed by a series of natural silicates and amorphous silica-alumina (ref. 15) and by pentasil type zeolites (ref. 16). It is said that, in both cases, isomerization occurs on the acidic sites (si lands) of the external surface, which act as active centers even under mild experimental conditions. 

Zeolites are crystalline aluminosilicates that have exhibited catalytic activities ranging from one to four orders of magnitude greater than amorphous aluminosilicates for reactions involving carbonium ion mechanisms such as catalytic cracking (144). As a result extensive efforts have been undertaken to understand the nature of the catalytic sites that are responsible for the observed high activity. The crystalline nature of zeolites permits more definite characterization of the catalyst than is possible for amorphous acidic supports such as alumina and silica-alumina. Spectral techniques, in conjunction with structural information derived from X-ray diffraction studies, have led to at least a partial understanding of the nature of the acidic sites in the zeolite framework. 

In the early history of acid catalysis, clays had a significant utility. The acid leaching of natural smectite clays produced amorphous silica-alumina [Si-Al] materials with large surface areas and active acid sites, which were used as acid catalysts [31],... 

The principal general interest in silica and related catalysts has been attached to those reactions which are thought to be catalyzed by acid sites, cracking and isomerization of hydrocarbons. Such reactions have naturally been applied to irradiated silica and silica-aluminas. Reactor irradiation of pure silica was early found by Weisz and Swegler (10) to enhance the activity for n-hexene-1 double bond isomerization, 7.5 X IQi nvt fast neutrons giving a statistically significant increase... 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

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