Silica, Alumina, and Zeolites

In particular, emphasis will be placed on the use of chemisorption to measure the metal dispersion, metal area, or particle size of catalytically active metals supported on nonreducible oxides such as the refractory oxides, silica, alumina, silica-alumina, and zeolites. In contrast to physical adsorption, there are no complete books devoted to this aspect of catalyst characterization however, there is a chapter in Anderson that discusses the subject. 

Table 9.5. Approximate product distributions of fluid catalytic cracking for amorphous silica-alumina and zeolite catalysts.

Although beyond the scope of this book, a vast amount of work has been directed to supporting homogeneous catalysts on solid supports including silica, alumina and zeolites, and functionalized dendrimers and polymers [19]. These give rise to so-called solid-liquid biphasic catalysis and in cases where the substrate and product are both liquids or gases then co-solvents are not always required. In many ways solvent-free synthesis represents the ideal method but currently solvent-free methods can only be applied to a limited number of reactions [20],... 

In the section dealing with alumina and silica, the necessity of basic sites on the surface, which cooperate with acidic sites, has been stressed. Also, for both amorphous silica—alumina and zeolites, the simultaneous presence of acidic and basic sites has been proved and it has been suggested that OH groups act as amphiprotic centres according to the nature of the adsorbed species [49],... 

Silica-alumina and zeolites are used commercially in hydrocracking catalysts. Ni-Mo on silica-alumina is a better HDN catalyst than Ni-Mo/Al203 and is used in the first reactor of a two-stage hydrocracking process (/). 

Although a variety of amines, particularly trimethylamine and n-butylamine have widely been used as poisons in catalytic reactions and for surface acidity determinations (20), comparably few spectroscopic data of adsorbed amines are available. As with ammonia, coordinatively adsorbed amines held by co-ordinatively unsaturated cations have preferentially been found on pure oxides (176, 193-196), whereas the protonated species were additionally observed on the surfaces of silica-aluminas and zeolites (196-199). However, protonated species have also been detected on n-butylamine adsorption on alumina (196) and trimethylamine adsorption on anatase (176) due to the high basicity of these aliphatic amines. In addition, there is some evidence for dissociative adsorption of n-butylamine (196) and trimethylamine (221) on silica-alumina. Some amines undergo chemical transformations at higher temperatures (195, 200) and aromatic amines, such as diphenylamine, have been shown to produce cation radicals on silica-alumina (201, 201a). 

Some catalyst activation processes are extremely important this is the case for oxides used as catalysts and supports (AI2O3, SiC>2, TiC>2, ZrC>2, silica-aluminas), and zeolites. Extremely elaborate procedures are used. This concerns bulk, not supported systems, and is dealt with in Section A.2.1. The case ofSiC>2 mixed with active phases (e.g. in oxidation) has little relevance to the subject of the present section, as it seems that SiOj does not play the role of a real support, but rather that of a diluent or spacer. An electron microscopy study coupled with microanalysis on a typical oxidation catalyst (propene to acrolein) shows that only a small fraction of the active phases is attached to silica or is situated in its immediate proximity [69]. There are not many cases... 

Supported Platinum. Supported platinum catalysts have been prepared usually by impregnation method, using various supports such as charcoal,158 asbestos,159 silica,160 alumina,161 and silica-alumina,162 or by ion-exchange method with silica, silica-alumina, and zeolites, using cationic platinum salts.163,164... 

Selective chemisorption methods have been used with success for the determination of metal surface area and particle size in supported catalysts, and for titration of acid sites on silica-alumina and zeolite catalysts. The chemisorption methods are sometimes neglected in the quest for a more physical description of the catalyst surface, possibly with the penalty of missing an important and quantitative piece of information about the catalyst surface. 

Audiso et al. [25] has studied the catalytic degradation of polypropylene using silica, alumina and silica-alumina and zeolite catalysts in the range 200-600°C. The main products in oil production using more efficient catalyst were C5-C12 olefins. Also the... 

It has been a long time since mineral clays like montmorillonite were discovered to have strong acidity in the solid state. Moreover, clays had been utilized as catalysts for catalytic cracking to produce gasoline before amorphous silica-alumina and zeolites were invented. At the present time, besides use as catalysts, clay is being utilized in a variety of fields medicine, paint, cosmetics, detergent, and casting. 

This section has focused on liquid—liquid biphasic catalysis in which catalysts are supported in different solvents to the substrates and products. Considerable efforts have also been directed to supporting homogeneous catalysts on solid supports including silica, alumina, and zeolites as well as functionalized dendrimers and polymers.33 It has also been found that synergic effects sometimes prevail between particles embedded in the support and the tethered molecular catalyst, increasing the activity of the catalyst.34... 

The most common catalysts used in plastic cracking are acidic solids, mainly alumina, amorphous silica-alumina and zeolites. These materials are the catalysts typically used in the petroleum processing and petrochemical industries. They have very different textural and acid properties, which directly determine their catalytic activity and product selectivity. Thus, while the acidity of alumina is of Lewis type, both Brdnsted and Lewis acid sites may be present in amorphous silica-alumina and zeolites. This is an important factor because... 

Some improvements for this reaction might he expected in the presence of acidic catalysts, particulary solid acidic catalysts such as silica-alumina and zeolites. Under these conditions, an increase in the rate as well as in the yield of the reaction was immediately observed. However, smedl amounts of by-products were detected. The possible reaction pathways outlined in Scheme 2 account for all the cycloadducts identified. 

Other Materials - Degradation of polyethylene, polypropylene and polystyrene has also been carried out in the presence of other acidic materials. Trends in the product distributions are similar to those observed for silica-alumina and zeolite catalysts. Degradation studies using mesoporous materials, Lewis acids and sulfated zirconia catalysts are summarized below. 

Group 1 alkali metals (including potassium) are poisons for cobalt catalysts but are promoters for iron catalysts. Catalysts are snpported on high-surface-area binders/supports such as silica, alumina, and zeolites (Spath and Dayton, 2003). Cobalt catalysts are more active for FTS when the feedstock is natnral gas. Natnral gas has a high H2 to carbon ratio, so the water-gas-shift is not needed for cobalt catalysts. Iron catalysts are preferred for lower quality feedstocks such as coal or biomass. 

Among the different solid adds, amorphous silica-aluminas and zeolites have found wide application in oil refining, petrochemistry, chemistry, and in the production of fine chemicals. Their success is based on the ability to prepare samples with strong Bronsred acidity chat can be controlled within a certain range, and which resist high reaction temperatures and catalyst regeneration conditions. 

Note that penta-coordinated A1 species are not present in transition aluminas, so that they appear to be a feature of silica-aluminas and zeolitic systems. Simultaneously, Lewis acidity is expected. In what follows, a summary is reported on the available experimental evidence of the acidic forms of MTS. 

However, it is known for a long time that the IR spectra of silica-alumina and zeolites are strikingly different [45]. The IR spectra of a series of H-[A1]-... 

In the 1970s and 1980s already was evident how different are the spectroscopic features of silica-alumina and zeolites [211]. More recently, however, some papers reported the presence of very small bands near 3600 cm in the spectra of mesoporous silica-aluminas [212-214], supposed to be due to bridging zeolite-type sites. Theoretical works till the end of the 1990s modeled the active site for zeolites and silica-alumina in the same way, as Al-(OH)-Si bridging... 

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