Catalyst carriers properties, acidity

Qynthetic and natural zeolites are becoming increasingly important as catalysts, carriers of catalysts, and adsorbents. Zeolites are especially suited to these purposes because their properties can be modified by cation exchange. The literature describes several studies which show characteristic changes in physicochemical properties resulting from cation exchange— e.g., catalytic activity (1,2), acidic properties (3), adsorption behavior (4), structure of solid (5,6), and thermal stability (7,8). 

Currently, a wide range of technological applications is based on the sorptive and catalytic properties of sepiolite [2], Sepiolite is increasingly being used as a decolorizing agent [3], as a catalyst or catalyst carrier [4-6], and as odorant adsorbents in environmental applications [7-9], Several papers have appeared recently that examine the structural, textural and sorptive properties of untreated sepiolite [10-12] and of sepiolite subjected previously to acid and/or thermal treatment [13-16], Sepiolite has also been used recently as... 

In order to compare the behaviour of different zeolites in Prins reaction a series of catalysts having various acidities and distributions of the acidic sites has been chosen. Their acidic properties are shown in Figure 1 as NH3-TPD spectra. These zeolites have been tested in standard conditions the same amount of catalyst, 30 ml/min flow rate of carrier gas, and 300°C reaction temperature. The results are summairized in Table 1. It must be mentioned, from preliminary runs, that TBA is converted promptly into isobutene and water, even at the low temperatures of 170 C, over every catalyst tested. The experiments of the methanol ccxiversion performed with aqueous methanol solution having the same concentration as those of FA, have also indicated the formation of only a very small amount of light hydrocarbons. 

Hiramatsu also prepared films on a carrier material [13]. Halogenated polymers containing noble metals were prepared on the carrier surface. These catalysts had no acidic properties they were used in hydrogen peroxide synthesis. 

Metal oxides belong to a class of widely used catalysts. They exhibit acidic or basic properties, which make them appropriate systems to be used as supports for highly dispersed metal catalysts or as precursors of a metal phase or sulfide, chloride, etc. Simple metal oxides range from essentially ionic compounds with the electropositive elements to covalent compounds with the nonmetals. However, taking into account the large variety of metal oxides, the principal objective of this book is to examine only metal oxides that are more attractive from the catalytic point of view, and most specifically transition metal oxides (TMO). In particular, TMO usually exhibit nonstoichiometry as a consequence of the presence of defective structures. The interaction of TMO with surfaces of the appropriate carriers develop monolayer structures of these oxides. The crystal and electronic structure, stoichiometry and composition, redox properties, acid-base character and cation valence sates are major ingredients of the chemistry investigated in the first part of the book. New approaches to the preparation of ordered TMO with extended structure of texturally well defined systems are also included. 

It was found in the 1960s that disperse platinum catalyst supported by certain oxides will in a number of cases be more active than a similar catalyst supported by carbon black or other carbon carrier. At platinum deposits on a mixed carrier of WO3 and carbon black, hydrogen oxidation is markedly accelerated in acidic solutions (Hobbs and Tseung, 1966). This could be due to a partial spillover of hydrogen from platinum to the oxide and formation of a tungsten bronze, H WOj (0 < a < 1), which according to certain data has fair catalytic properties. 

Catalyst characterization - Characterization of mixed metal oxides was performed by atomic emission spectroscopy with inductively coupled plasma atomisation (ICP-AES) on a CE Instraments Sorptomatic 1990. NH3-TPD was nsed for the characterization of acid site distribntion. SZ (0.3 g) was heated up to 600°C using He (30 ml min ) to remove adsorbed components. Then, the sample was cooled at room temperatnre and satnrated for 2 h with 100 ml min of 8200 ppm NH3 in He as carrier gas. Snbseqnently, the system was flashed with He at a flowrate of 30 ml min for 2 h. The temperatnre was ramped np to 600°C at a rate of 10°C min. A TCD was used to measure the NH3 desorption profile. Textural properties were established from the N2 adsorption isotherm. Snrface area was calcnlated nsing the BET equation and the pore size was calcnlated nsing the BJH method. The resnlts given in Table 33.4 are in good agreement with varions literature data. 

The catalyst activity depends not only on the chemical composition but also on the diffusion properties of the catalyst material and on the size and shape of the catalyst pellets because transport limitations through the gas boundary layer around the pellets and through the porous material reduce the overall reaction rate. The influence of gas film restrictions, which depends on the pellet size and gas velocity, is usually low in sulphuric acid converters. The effective diffusivity in the catalyst depends on the porosity, the pore size distribution, and the tortuosity of the pore system. It may be improved in the design of the carrier by e.g. increasing the porosity or the pore size, but usually such improvements will also lead to a reduction of mechanical strength. The effect of transport restrictions is normally expressed as an effectiveness factor q defined as the ratio between observed reaction rate for a catalyst pellet and the intrinsic reaction rate, i.e. the hypothetical reaction rate if bulk or surface conditions (temperature, pressure, concentrations) prevailed throughout the pellet [11], For particles with the same intrinsic reaction rate and the same pore system, the surface effectiveness factor only depends on an equivalent particle diameter given by... 

Varying the side groups X in 27b affects both the stability and selectivity of the complexes (lateral discrimination), and allows the receptor-substrate interactions in biological systems to be modelled, for instance, the interaction between nicotinamide and tryptophan [2.109b]. One may attach to 27b amino acid residues (leading to parallel peptides [2.109] as in 27c), nucleic acid bases or nucleosides, saccharides, etc. The structural features of 27 and its remarkable binding properties make it an attractive unit for the construction of macropolycyclic multisite receptors, molecular catalysts, and carriers for membrane transport. Such extensions require sepa-... 

Most of the heterogeneous catalyst which are in practical use consist of one or more catalytically active compounds which are impregnated on supporting carrier materials. This method can be chosen to immobilise acids and bases as well as salts, oxides or complexes. The major drawback is leaching of one or more component which leads to irreversible deactivation of the catalyst. Physisorption can be enhanced by choosing the appropriate porous, chemical and electronical properties. This leads to catalysts with sufficient long term stability due to e.g. ionic linkages. 

The catalyst is called bifunctional both the carrier and the metallic particles dispersed over the carrier exhibit different catalytic functions. The carrier contains chlorine ions and, as a consequence, it has acid properties and exhibits isomerization and cyclization activities. The metal particles consist of alloys of, for example, Pt/Re which exhibit hydrogenation/dehydrogenation activity. 

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