Siliceous-based catalysts

As compared to conventional petrochemicals, the significant hindrance of carbohydrates induces many diffusional limitations and activity of solid catalysts is obviously strictly governed by the accessibility of the catalytic sites. In this context, the porosity of commonly used siliceous-based catalysts or metal oxides is not crucial since, because of the steric hindrance of carbohydrates, the catalytic reaction mainly takes place on the catalyst surface. In the case of organic polymers, utilization of flexible polymeric chains considerably improves the accessibility of the catalytic sites. 

An olefin epoxidation Ti-silicate-based catalyst was optimised by means of high-tbroughput experimentation for catalytic-materials synthesis, postsynthesis treatments, and catalytic testing. Soft-computing techniques for advanced experimental design and data assessment (GA, ANN) were used. The following variables were explored in the... 

TEOS) in methanol or ethanol with base catalysts, as discussed recently by Halas [27]. Other common methods of preparing symmetric silica nanowires include the condensation of silicon sources in the gas phase on nanoparticle catalysts [21, 27, 28]. The size of the siUca beads can be controlled by changing the silicates, base catalyst, and the amount of water in the system [1]. For example, the metal catalysts in the case of a nanowire can be used to control the diameter of long, straight siUca nanowires with a circular cross-section. Moreover, both catalytic and noncatalytic methods can be used to synthesize these symmetric nanosilica. 

Houdry The first catalytic petroleum cracking process, based on an invention by E. J. Houdiy in 1927, which was developed and commercialized by the Houdry Process Corporation. The process was piloted by the Vacuum Oil Company, Paulsboro, NJ, in the early 1930s. The catalyst was contained in a fixed bed. The first successful catalyst was an aluminosilicate mineral. Subsequently, other related catalysts were developed by Houdry in the United States, by I. G. Farbenindustrie in Germany, and by Imperial Chemical Industries in England. After World War II, the clay-based catalysts were replaced by a variety of synthetic catalysts, many based on alumino-silicates. Later, these too were replaced by zeolites. U.S. Patents 1,837,963 1,957,648 1,957,649. 

Octafining A process for isomerizing m-xylene to o- and p-xylene, developed by the Atlantic Richfield Company in 1960. The catalyst was originally platinum on an aluminum silicate base now a zeolite base is used. The reaction takes place in a hydrogen atmosphere. Hydrocarbon Research installed units in Argentina and the USSR. 

There are, however, two limitations associated with preparation and application of zeolite based catalysts. First, hydrothermal syntheses Umit the extent to which zeolites can be tailored with respect to intended appUcation. Many recipes involving metals that are interesting in terms of catalysis lead to disruption of the balance needed for template-directed pore formation rather than phase separation that produces macroscopic domains of zeoUte and metal oxide without incorporating the metal into the zeohte. When this happens, the benefits of catalysis in confined chambers are lost. Second, hydrothermal synthesis of zeoHtic, silicate based soHds is also currently Hmited to microporous materials. While the wonderfully useful molecular sieving abihty is derived precisely from this property, it also Hmits the sizes of substrates that can access catalyst sites as weU as mass transfer rates of substrates and products to and from internal active sites. 

Two patterns are possible in the activation mechanism by simple chiral Lewis base catalysts. One is through the activation of nucleophiles such as aUyltrichlorosilanes or ketene trichlorosilyl acetals via hypervalent silicate formation using organic Lewis bases such as chiral phosphoramides or A-oxides. " In this case, catalysts are pure organic compounds (see Chapter 11). The other is through the activation of nucleophiles by anionic Lewis base conjugated to metals. In this case, transmetal-lation is the key for the nucleophile activation. This type of asymmetric catalysis is the main focus of this section. 

Small amounts of other compounds can be added to Ni-based catalysts to improve the functional characteristics of the final catalyst. Typically, they are calcium aluminate to enhance the mechanical resistance of the catalyst pellets, potassium oxide to improve the resistance to coke formation and silica to form a stable silicate with potassium oxide [34]. Promotion with rare earth oxides such as La2C>3 also results in enhanced resistance to coking. 

This acid-catalyzed cleavage of the glycosidic bonds is rather complex and often suffers from a lack of selectivity mainly due to side dehydration or recombination reactions of monosaccharides. In the existing literature, four different classes of solid catalysts are reported (1) cation-exchange resins, (2) siliceous-based materials, (3) metal oxides, and (4) sulfonated amorphous carbons. 

Landtreat is a silicate-based inorganic polymer catalyst used for the ex situ treatment of contaminated soils. The vendor claims that it acts as a catalyst to degrade halogenated compounds and organic compounds containing nitrogen and sulfur. 

A drum of styrene oxide was punctured and the spillage absorbed into an hydrated silate absorbent, the combination swept up and drummed up for disposal. The drum became hot and started emitting copious white fumes. It was not possible to duplicate this behaviour in the laboratory unless acid or base catalyst was also present [1], Absorbents may be inert, epoxides are not but contain considerable strain energy which will be liberated by autoreaction or nucleophilic substitution by, e.g. water. Only a catalyst is needed neither silicates nor floor-sweepings can be guaranteed free of these [2]. 

Derrien, A., Renard, G. and Brunei, D. Guanidine linked to micelle-templated mesoporous silicates as base catalyst for transesterification. Stud. Surf. Sci. Catal., 1998,117, 445-452. 

In general, there appears to be considerable scope for the application of ordered mesoporous structures in such fields as biochemical separations and nanocomposite materials. The potential value of MCM-41 -based catalysts has been stressed by Thomas (1994,1995). In principle, metal complexes, enzymes and other species can be attached to the channel walls to give high concentrations of structurally well-defined active sites. It should be possible therefore to take full advantage of die regularity of both the channel and the surface structure. Furthermore, the incorporation of heteroatoms such as Ti into the siliceous framework of MCM-41 should provide an elegant way of controlling catalytic selectivity (Thomas, 1995). 

Catalyst life estimation tests were also conducted on four different catalysts used for aromatization of light hydrocarbons in this system, namely H-ZSM-5, Zn/H-ZSM-5, Ga/H-ZSM-5 and H-Ga-silicate. Based on the results of characterization of the spent catalysts, the primary cause of deterioration of catalyst activity is the elimination of active metal from the zeolite framework. 

In order to synthesize gasoline effectively from carbon dioxide through one-pass reaction system, methanol synthesis catalyst was improved. Pd and Ga were added to Cu-Zn based catalyst to optimize the state of Cu during the reaction. As the result, the space-time yield (STY) of methanol from CO2 was 1,410 gd h at 270, 80 atm and SV=18,800 /h. In second stage reactor in which H-Ga or Al-silicate was packed, methanol was converted to gasoline. Maximum selectivity to gasoline fraction was 54.4 % and STY was 312 gl h at 320 C and 15 atm. 

Kubota, Y., Nishizaki, Y., Ikeya, H., Saeki, M., Hida, T., Kawazu, S., Yoshida, M., Fujii, H., Sugi, Y. Organic-silicate hybrid catalysts based on various defined structures for Knoevenagei condensation. Microporous and Mesoporous Materials 2004, 70, 135-149. 

Four precipitated iron-based catalysts were used. The first catalyst consisted of only iron. The other catalysts contained either added potassium, added silicon or both. The catalysts were designated in terms of the atomic ratios as lOOFe, 100Fe/3.6Si, 100Fe/0.71K and 100Fe/3.6Si/0.71K. The catalysts were prepared by continuous precipitation from iron (111) nitrate and concentrated ammonium hydroxide. For silica-containing catalysts, a colloidal suspension of tetraethyl ortho silicate was mixed with the iron nitrate solution prior to precipitation. Potassium was added to the catalysts in the form of potassium tertiary butoxide during the loading of the FTS reactor. 

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