Superheating of the catalyst

To elucidate the cause of the microwave-induced enhancement of the rate of this reaction in more detail the transformation of 2-t-butylphenol was performed at low temperatures (up to -176 °C). At temperatures below zero the reaction did not proceed under conventional conditions. When the reaction was performed under micro-wave conditions in this low temperature region, however, product formation was always detected (conversion ranged from 0.5 to 31.4%). It was assumed that the catalyst was superheated or selectively heated by microwaves to a temperature calculated to be more than 105-115 °C above the low bulk temperature. Limited heat transfer in the solidified reaction mixture caused superheating of the catalyst particles and this was responsible for initiation of the reaction even at very low temperatures. If superheating of the catalyst was eliminated by the use of a nonpolar solvent, no reaction products were detected at temperatures below zero (see also Sect. 10.3.3). 

The product distribution determined for the reactions performed over a broad temperature range (from -176 to 199 °C) under microwave heating was always more or less different from that obtained by conventional method. Thus, a vigorous formation of isobutene under reflux using microwave heating indicates superheating of the catalyst to a higher temperature. This facilitates the dealkylation reaction, which is promoted by elevated temperature. 

Temperature and solvent effects were also examined. When the reaction temperature was gradually reduced from reflux temperature (199 °C), the rate enhancement factor increased from 1.0 at 199 °C to 1.4 at 105 °C, to 2.6 at 75 °C, and to 4.1 at 24 °C. These results may indicate that superheating of the catalyst is more... 

The information on the effect of alloy formation on the catalytic activity is generally conflicting. The effect of Sc during superheating of the catalyst is interpreted as an activity increase due to reduction of SC2O3 to Sc followed by an activity decrease due to Sc + Fe alloy formation [271]. Small amounts of Co increases the rate of NH3 synthesis and N2-chemisorption [123]. Small amounts of Ni increases only the rate of NH3 synthesis, not the rate of N2 chemisorption [123]. 

Closely related to the superheating effect under atmospheric pressure are wall effects, more specifically the elimination of wall effects caused by inverted temperature gradients (Fig. 2.6). With microwave heating, the surface of the wall is generally not heated since the energy is dissipated inside the bulk liquid. Therefore, the temperature at the inner surface of the reactor wall is lower than that of the bulk liquid. It can be assumed that while in a conventional oil-bath experiment (hot vessel surface, Fig. 2.6) temperature-sensitive species, for example catalysts, may decompose at the hot reactor surface (wall effects), the elimination of such a hot surface will increase the lifetime of the catalyst and therefore will lead to better conversions in a microwave-heated as compared to a conventionally heated process. 

Recently, Hajek and coworkers have reported results on microwave-assisted chemistry performed by cooling of a reaction mixture to as low as -176 °C. Reaction rates were recorded under microwave and conventional conditions. The higher reaction rates under microwave heating at sub-ambient temperatures were attributed to a superheating of the heterogeneous K10 catalyst [44],... 

The rates of transesterification of triglycerides to methyl esters, efficiently catalyzed by boron carbide (B4C), were, on the other hand, faster under microwave conditions, probably because of superheating of the boron carbide catalyst, which is known to be a very strong absorber of microwaves [40], Scheme 10.3. Yields of methyl ester of up to 98% were achieved. 

Although all the side reactions affect the selectivity, from the point of view of the catalyst, the production of carbon (coke) is the biggest problem as it remains in situ and acts as a poison. The use of superheated steam plays a vital role here, as it helps to clean the catalyst by gasification of carbon at the working temperature (Equation 10). 

In the second step, the dioxanes are vaporized, superheated, and then cracked on a solid catalyst (supported phosphoric acid) in the presence of steam. The endothermic reaction takes place a about 200 to 2S0°C and 0.1 to OJ. 10 Pa absolute. The heat required is supplied by the introduction of superheated steam, or by heating the support of the catalyst, which operates in a moving, fluidized or fixed bed, and, in this case, implies cyclic operation to remove the coke deposits formed. Isoprene selectivity is about SO to 90 mole per cent with once-through conversion of 50 to 60 per cent The 4-4 DMD produces the isoprene. The other dioxanes present are decomposed into isomers of isoprene (piperylene etc.), while the r-butyl alcohol, also present in small amounts, yields isobutene. A separation train, consisting of scrubbers, extractors and distillation columns, serves to recycle the unconverted DMD, isobutene and fonnol, and to produce isoprene to commercial specifications. 

Chlorinations Catalyzed by Active Carbon. When carbon of different densities is subjected to superheated steam, its surface is vaporized, forming capillaries which have the property of absorbing gases and compressing them into much smaller volumes. This compression, possibly combined with the catalytic effect of the metal impurities present in the carbon, promotes reactions such as halogenation, hydrohalogenation, or dehydro-halogenation. The character of the metallic impurities, the absorption power of the carbon, the density of the carbon, and the method of capillary formation all materially influence the type of reaction and the life of the catalyst. Often materials are added to these carbons to modify their properties. 

Fresh reducing gas is generated by reforming natural gas with steam. The natural gas is heated in a recuperator, desulfurized to less than 1 ppm sulfur, mixed with superheated steam, further preheated to 620°C in another recuperator, then reformed in alloy tubes filled with nickel-based catalyst at a temperature of 830°C. The reformed gas is quenched to remove water vapor, mixed with clean recycled top gas from the shaft furnace, reheated to 925°C in an indirect fired heater, and injected into the shaft furnace. For high (above 92%) metallization a CO2 removal unit is added in the top gas recycle line in order to upgrade the quaUty of the recycled top gas and reducing gas. 

In most existing styrene processes, the catalyst is loaded into large, radial flow reactors, which are operated adiabaticaHy at low pressure and temperatures near 600°C. Heat is suppHed by superheated steam. During start-up, dehydrogenation begins slowly and accelerates as the Fe (HI) is reduced to Fe (II,III). The catalyst, which was red in color when fresh, turns to the characteristic black color of Fe O. ... 

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