Catalysis calcined

Upon calcination the template is removed and the zeolite s well-defined pores are available for adsorption and catalysis. Particularly challenging is the field of electrophilic aromatic substitution. Here often non-regenerable metal chlorides serve as the catalyst in present industrial practice. Zeolites are about to take over the job and in fact are doing so for aromatic alkylation. 

In order to get the pore system of zeolites available for adsorption and catalysis the template molecules have to be removed. This is generally done by calcination in air at temperatures up to 500 °C. A careful study (ref. 12) of the calcination of as-synthesized TPA-containing MFI-type single crystals by infrared spectroscopy and visible light microscopy showed that quat decomposition sets in around 350 °C. Sometimes special techniques are required, e.g. heating in an ammonia atmosphere (ref. 13) in the case of B-MFI (boron instead of aluminum present) to prevent loss of crystallinity of the zeolite during template quat removal. 

ATP apparatus equipped with a mass spectrometer. Right-hand side TPR patterns of silica-supported Rh, Fe, and Fe-Rh catalysts, which had been previously calcined to ensure that all metals are oxidized at the start of the measurement. [Adapted from J.W. Niemantsverdriet, Spectroscopy in Catalysis, An Introduction (2000), Wiley-VCH, Weinheim, and H.F.J. van t Blik and J.W. Niemantsverdriet, Appl. Cota/. 10 (1984) 155.]... 

Transition-metal mixed oxides active in combustion catalysis have been prepared by two main procedures i) classical coprecipitation / calcination procedures starting from metal nitrates and/ or alkoxides ii) preparation based on the supercritical drying of gels prepared from organic complexes (alkoxides, acetylacetonates or acetates), producing aerogels . Details on the second preparation can be found in Ref. 13. 

Aluminas. Aluminas, porous AI2O3, are available in many forms. They constitute the most important carrier material in heterogeneous catalysis. Alumina is amphoteric and, as a con.sequence, soluble in both acidic and basic media. Precipitation can be performed from an acid solution by adding a base or from a basic solution by adding an acid, as schematically represented in Fig. 3.18. If, for example, at a pH of less than about 3 a base is added to an aqueous solution of aluminium sulphate, a precipitate is formed. If this material is filtered, dried and calcined, an amorphous porous AI2O3 is obtained. At other pH values different porous aluminas can be synthesized. 

Fig. 3.35 shows the decrease of the specific surface area of a certain alumina as a function of calcination temperature. Apparently, the alumina is rather stable at 1000 K still over 50 % of the original surface area is retained. For most applications in catalysis the reaction temperature is far below 1000 K, and, as a consequence, the thermal stability of alumina is often sufficient. Activated carbon, which is also often used, is even more stable. 

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

In heterogeneous catalysis post-impregnation treatments such as calcination and reduction are usually performed to remove ligands or their residues. In some cases the removal of heteroatoms such as carbon, sulfur or chloride is tedious and can... 

Supported Au catalysts have been extensively studied because of their unique activities for the low temperature oxidation of CO and epoxidation of propylene (1-5). The activity and selectivity of Au catalysts have been found to be very sensitive to the methods of catalyst preparation (i.e., choice of precursors and support materials, impregnation versus precipitation, calcination temperature, and reduction conditions) as well as reaction conditions (temperature, reactant concentration, pressure). (6-8) High CO oxidation activity was observed on Au crystallites with 2-4 nm in diameter supported on oxides prepared from precipitation-deposition. (9) A number of studies have revealed that Au° and Au" play an important role in the low temperature CO oxidation. (3,10) While Au° is essential for the catalyst activity, the Au° alone is not active for the reaction. The mechanism of CO oxidation on supported Au continues to be a subject of extensive interest to the catalysis community. 

It is important to calcine at the appropriate temperature to enhance the deactivation and the catalysis for the ceria-modified HM.50,51 The catalyst calcined at 300 °C had no catalytic activity. The activity for the isopropylation of naphthalene appeared over the catalyst calcined at 450 °C and reached the maximum at 550 °C. However, the activity decreased at a temperature of 700 °C. These results show that calcination at an appropriate temperature is necessary to keep the HM pores open. The selectivity for 2,6-DIPN was as high as 70% over the catalysts regardless of their calcination temperature. These phenomena show that the isopropylation proceeds inside HM pores. This selective deactivation of HM pores was limited to modification with ceria. The modification with other rare-earth metal oxides, such as lanthanum and neodymium oxides, also deactivated the external acid sites but choked the HM pore to result in low catalytic activity. The isomerization of 4,4 -DIPB was also prevented by the ceria modification of HM in the isopropylation of biphenyl.49... 

In the preceding decade, solid-state NMR spectroscopy has provided important and novel information about the nature and properties of surface sites on working solid catalysts and the mechanisms of these surface reactions. This spectroscopic method offers the advantages of operation close to the conditions of industrial catalysis. A number of new techniques have been introduced and applied that allow investigations of surface reactions by solid-state NMR spectroscopy under both batch and flow conditions. Depending on the problems to be solved, both of these experimental approaches are useful for the investigation of calcined solid catalysts and surface compounds formed on these materials under reaction conditions. Problems with the time scale of NMR spectroscopy in comparison with the time scale of the catalytic reactions can be overcome by sophisticated experimental... 

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