Calcining, with microwaves

Fig. 10.15 Kinetic variations in methanol concentration in flowing dry air at the photocatalytic reactor exit using the following photocatalysts +- P-25 x- Na bentonite caleined at 673 K - Ti02 (1 mmol/g)-pillared bentonitecalcined at 673 K A- Ti02 (10 mmol/g)-pillared bentonite calcined at 673 K and - Ti02 (10 mmol/g)-pillared bentonite calcined by microwave (Reprinted from Ref. [144], with kind permission from Springer Science + Business Media)...

The preparation of catalysts usually involves the impregnation of a support with a solution of active metal salts. The impregnated support is then dried, calcined to decompose the metal salt and then reduced (activated) to produce the catalyst in its active form. Microwaves have been employed at all stages of catalyst preparation. Beneficial effects of microwave heating, compared with conventional methods, have been observed especially in the drying, calcination, and activation steps. 

The microwave technique for drying then calcination is an excellent way of obtaining highly porous silica gel with a high surface area (as high as 635 m2 g 1) for use as a catalyst and as a catalyst support [16]. 

SEM of the calcined sample 11-IV are shown in Fig. 3. With the increase of microwave irradiation time, the morphologies shows transition from mesopore materials to the fine small particles of round-like type of silicalite-1 having ca. 3-10 pm size of cubic phase surrounding with small mesoporous particles. Fig. 3c reveals the combined assemblies of MF1 and mesoporous type materials. The silicalite-1 is partly covered with... 

Fig. 2. XRD patterns of as-synthesized micro-mesoporous composite materials prepared with increasing the time of microwave irradiation (a) sample 1, (b) sample II, (b ) calcined (b), (c) sample III, (c1) calcined (c), and (d) sample IV. See Table 1 for the notations of I, II, III and IV.

This study demonstrated that the micro-mesoporous composite materials could be synthesized with two-step treatment by microwave using two different templates system with TPABr and MTAB. This formation was controlled by the self-assembly formation of supramolecular templates between MTA micelles and SiO /TPA gels. As varying microwave irradiation time of micro-mesoporous materials, gradually transition from the mesophase to micro-mesophase was occurred. These materials have higher dm spacing of mesoporous materials and lead to transition from mesophase to micro-microphase by an increment of synthetic time, while the calcined products is formed with bimodal and trimodal pore size distribution under microwave irradiation within 3 h. From TG-DTA and PL analysis, the self-assembly formation of supramolecular templates between MTA+ micelles and SiO /TPA+ gels were monitored. 

Hydrothermal procedure was performed by heating at 90 °C for 72 h (samples denoted (Sn)MCM-41/HT) and synthesis with applying of microwave irradiation was carried out at 90 °C for 3 h (samples denoted (Sn)MCM-41/MW). The sample was transferred into a microwave digestion system (CEM Corporation, MAR-5). All solid products were filtered off, washed with deionized water, dried at 100 °C and calcined in a stream of air at 550 °C for 6 hours. The prepared samples are listed in Table 1. 

Catalytic activity in the partial oxidation (CPO) of methane of some catalysts containing Ni or Rh/Ni active metals obtained by the calcination and reduction of hydrotalcite-like compounds was investigated. In particular, two hydrotalcite-like compounds subjected to the microwave-hydrothermal method (MWHT) were studied in order to evaluate the role of the synthesis method of the precursors on the catalytic activity and catalyst stability as compared to catalysts prepared by the convention method. The tests carried out at 750°C do not evidence any difference among catalysts. However, when the temperature is reduced to 500°C a better catalytic performance is observed for the microwave-assited catalysts containing nickel, whereas for the bimetallic catalyst the best activity is achieved with the conventional method. 

Two series of HTlcs whose compositions correspond to NiioMg6iAl29-C03 and Rho.iNi5Mg66Al28.9-C03, respectively, were prepared by the coprecipitation method at constant pH [6]. The slurry obtained was subjected to two different aging treatments (i) vigorous stirring at room temperature (hereinafter called BO), and (ii) microwave-hydrothermal treatment (MWHT) at 125°C in a Milestone Ethos Plus microwave oven (hereinafter called SA). The solids were washed with distilled water imtil Na and N03 were totally absent in the washing liquids, and then dried at 40°C. The catalysts were prepared by the calcination of fresh samples at 900°C in air for 14 h. 

Microwave heating has been reported to produce materials with particular physical and chemical properties [4], Stable solid structures are formed at low reaction temperatures with unusually high surface areas, making them very useful as catalysts or catalyst supports. Calcination of solid precursors in a microwave field has significant advantages over conventional heating. The effective synthesis of the catalysts and supporting adsorbents has been reported for the examples below. 

In the microwave synthesis of zeolites, a mixture of a precursor and a zeolite support is heated in a microwave oven. The sample is then tested for its catalytic activity and the results compared with those from the sample obtained by the conventional method. Microwave irradiation at the calcination stage led to samples with more uniform partide-size distribution and microstructure and to bimetallic catalysts with different morphology. 

Microwave calcination of magnesia, alumina, and silica-supported Pd and Pd-Fe catalysts resulted in their having enhanced catalytic activity in test reactions -hydrogenation of benzene and hydrodechlorination of chlorobenzene - compared with conventionally prepared catalysts [14-16]. The greater catalytic activity was attributed to prevention of the formation of a Pd-Fe alloy of low activity, which occurs at the high reduction temperature used in conventional heating. 

In addition to previously mentioned processes, the production of N-doped HO2 nanomaterials has been reported through other methods. These processes are ball milling of Ti02 in a NH3 water solution [413], heating Ti02 under NH3 flux at 500-600°C [414, 415], calcination of the hydrolysis product of Ti(S04)2 with ammonia as precipitator, decomposition of gas-phase TiCU with an atmosphere microwave plasma torch [416], ion implantation techniques with nitrogen [417] and N2 gas flux [418] (see Table 8). 

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