Pyrolysis routes

Manufacture. For the commercial production of DPXN (di-/)-xylylene) (3), two principal synthetic routes have been used the direct pyrolysis of -xylene (4, X = Y = H) and the 1,6-Hofmaim elimination of ammonium (HNR3 ) from a quaternary ammonium hydroxide (4, X = H, Y = NR3 ). Most of the routes to DPX share a common strategy PX is generated at a controlled rate in a dilute medium, so that its conversion to dimer is favored over the conversion to polymer. The polymer by-product is of no value because it can neither be recycled nor processed into a commercially useful form. Its formation is minimised by careful attention to process engineering. The chemistry of the direct pyrolysis route is shown in equation 1 ... 

Acetic anhydtide is a mature commodity chemical ia the United States and its growth rate in the 1970s and 1980s was negative until 1988 when foreign demand neatly doubled the exports of 1986. This increase in exports was almost certainly attributable to the decline in the value of the U.S. doUar. Over four-fifths of all anhydtide production is utilized in cellulose acetate [9004-35-7] manufacture (see Cellulose esters). Many anhydtide plants are integrated with cellulose acetate production and thus employ the acetic acid pyrolysis route. About 1.25 kg acetic acid is pyrolyzed to produce 1.0 kg anhydtide. 

However, when it is obtained by pyrolysis of diethylmagnesium or by reaction of diethylmagnesium and LiAlH (11), it is very reactive with both air and water. This difference in reactivity mainly results from the much finer particle size of the product obtained by the pyrolysis route. 

Spray pyrolysis routes have been extensively investigated to prepare Pt-based catalysts. Typically, a liquid feed of metal precursor and carbon is atomized into an aerosol and fed into a continuous furnace to evaporate and heat-treat to form a collectable powder. The method has good control over final aggregate particle size and metal particle size distributions, as well as producing powder without further isolation or separation. Hampton-Smith et al. have reviewed efforts of Superior MicroPowder (now Cabot Fuel Cells) in this area. ... 

There have been several developments in this area since this manuscript was prepared. The heat of combustion of corannulene was determined by microbomb combustion calorimetry and its gas-phase enthalpy of formation was estimated at 110.8 kcal/mol. All anionic oxidation states of corannulene were observed by optical absorption, EPR, and NMR spectroscopies. More support for the an-nulene-within-annulene model of the corannulene tetraanion was presented. An alternative pyrolysis route to corannulene was reported, as well as some attempts toward the synthesis of bowl-shaped subunits of fullerenes. And in contrast with previous semiempirical studies," ab initio calculations predicted a general concave preference for the metal cation binding to semibuckminsterfullerene 2%. ... 

The conventional industrial method for the synthesis of a-silicon carbide is to heat silica (sand) with coke in an electric furnace at 2,000-2,500 °C. However, because of the high melting point of the product, it is difficult to fabricate by sintering or melt techniques. Thus, the discovery of a lower temperature fabrication and synthesis route to silicon carbide by Yajima and coworkers in 197526,27 proved to be an important technological breakthrough. This is a preceramic polymer pyrolysis route that has been developed commercially for the production of ceramic fibers. 

Control of Molecular Weight. Studies have been conducted on techniques for controlling the molecular weight of poly-p-xylylenes produced from di-p-xylylenes by the vacuum pyrolysis route. Earlier work by Szwarc (17), Errede (3), and Auspos (I) indicated that very reactive chain transfer agents were required to achieve a significant effect in the polymerization of p-xylylene derived from p-xylene. This general picture was confirmed in the present study. 

Recently, increased attention is being directed toward the pyrolysis route of processing coal to produce liquid and gaseous fuels, particularly when coupled to the use of char by-product for power generation.(J ) In view of the increased interest in coal pyrolysis, a better understanding of the thermal response of coals as they are heated under various conditions is needed. 

Apart from these three degradation mechanisms, rearrangements of the fractions formed may take place. A polymer does not undergo only one pyrolysis route always, but multiple routes may be taken simultaneously. The type of reaction is totally governed by the strength of bonds in the molecules. The lowest energy path will be favored. 

It is extremely difficult from direct studies of hydrocarbon oxidation to unravel the contributions made to the overall mechanism at these temperatures by the additive, abstractive and pyrolysis routes. Baldwin, Walker and their co-workers have recently developed a new technique, however, which has overcome this problem to some extent and has also yielded quantitative Arrhenius parameters for many elementary reactions [108,157—161]. This technique utilizes the fact that the kinetics of slowly reacting hydrogen and oxygen mixtures in aged boric acid coated... 

The pyrolysis of hydrocarbons follows the thermal cracking mechanism (4). Apart from the pressure, the conditions in the tubular steam reformer and in the preheater are not far from that of a steam cracker in an ethylene plant. With low catalyst activity, the pyrolysis route may take over. This is the situation in case of severe sulphur poisoning or in attempts to use non-metal catalysts so far showing very low activity (1). Non metal catalysts have mainly been based on alkaline oxides being active for gasification of coke precursors. However, it has been difficult to avoid the formation of olefins and other pyrolysis products (1,2,5). In fact, it was demonstrated (2,4) that co-production of syngas and light olefins was possible from heavy gas oil and naphtha over a potassium promoted zirconia catalyst. 

Such stochastic modelling was advanced by Klein and Virk Q) as a probabilistic, model compound-based prediction of lignin pyrolysis. Lignin structure was not considered explicitly. Their approach was extended by Petrocelli (4) to include Kraft lignins and catalysis. Squire and coworkers ( ) introduced the Monte Carlo computational technique as a means of following and predicting coal pyrolysis routes. Recently, McDermott ( used model compound reaction pathways and kinetics to determine Markov Chain states and transition probabilities, respectively, in a rigorous, kinetics-oriented Monte Carlo simulation of the reactions of a linear polymer. Herein we extend the Monte Carlo... 

While commercially available titania powders, produced by a pyrolysis route from a chloride precursor, have been successfully employed, the present optimized material is the result of a procedure described by Brooks and coworkers [17]. A specific advantage of the hydrothermal technique is the ease of control of the particle size and hence of the nanostructure and porosity of the resultant semiconductor substrate. The relevant preparation flow diagram is given and the product is illustrated by the accompanying micrograph (Fig. 4 Table 1). Figure 5 presents data on the control of substrate porosity by the powder-preparation parameters. 

Later studies showed that the mechanism of reactions, in particular ionic versus free-radical, could vary. Townsend [15] has studied the reaction of a series of coal model compounds (alkyl-aryl hydrocarbons and ethers) in supercritical water. For the hydrocarbons a free-radical pyrolysis route does not take advantage of the medium. However, for the ethers enhanced rates of reaction through a hydrolysis route occurs. As a result of different possible pathways, decomposition products of some organics in supercritical water have been shown by several workers to vary with solvent strength. In the absence of water, Pr(H20) = 0, pyrolysis is dominant and yields a variety of products including polycondensates. The main products of decomposition of neat methoxy... 

Trahanovsky, W. S. Lee, S. K. A novel flash vacuum pyrolysis route to acridine. Synthesis 1996, 1085-1086. 

Polymer precursor routes based on the synthesis of polycarbosilanes have been studied most extensively. The chain backbone of these polymers contains the Si-C bond. The polycarbosilane route was used by Yajima et al. (56) to produce fibers with high Si-C content and, as outlined above, formed an important contribution to the polymer pyrolysis route for Si-based ceramics. We shall use this system as an example to illustrate some basic steps in the process. The initial step [Eq. (1.9)] is the condensation reaction between Na and dimethyldichlorosilane, (CH3)2SiCl2, in xylene to produce an insoluble poly(dimethylsilane), [(CH3)2Si] , where n 30. 

When compared to SiC, less work has been reported on the production of Si3N4 by the polymer pyrolysis route. Most efforts have focused on polymer precursors based on polysilazanes, a class of polymers having Si-N bonds in the main chain (58-61). The reactions to produce the Si-N bond in the chain backbone are based on the ammonolysis of methylchlorosilanes. A preceramic polymer can be prepared by the ammonolyis of methyldichlorosilane, followed by the polymerization of the silazane product catalyzed by potassium hydride (69) ... 

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