Laser flash pyrolysis

Starting from 27, cyclo-Cig was prepared in the gas phase by laser flash heating and the neutral product, formed by stepwise elimination of three anthracene molecules in retro-Diels-Alder reactions, was detected by resonant two-photon-ionization time-of-flight mass spectrometry [23]. However, all attempts to prepare macroscopic quantities of the cyclocarbon by flash vacuum pyrolysis using solvent-assisted sublimation [50] only afforded anthracene and polymeric material. 

There have been a number of studies of magnetic fields upon radical recombination using steady-state techniques of photolysis or pyrolysis. They have variously found large or small effects, which are not always consistent with the theoretical predictions [304—306]. However, using laser flash photolysis techniques to provide fast time resolution, Turro et al. [307] followed the combination of benzyl radicals within hexadecyltrimethyl ammonium chloride micelles in water. The combination occurs over times < 100 ns. A magnetic field of 0.04 T reduces the rate of recombination by almost a factor of two. Such a magnetic field... 

Disilene and its isomer silylsilylene were neither available by standard vacuum flash pyrolysis of precursors 59-63, nor by the more elaborate method of pulsed flash pyrolysis of 60-63, a pulsed discharge in mixtures of argon and mono- and disilane74 or by the matrix photolysis of educts 59-66 using various light sources (Hg lamps, excimer laser)69,70,72, the microwave discharge in disilane 66 or the cocondensation of silicon atoms with SiFLt. 

Finally, 2,4,6-trimethyl-l,3,5-trithiane was obtained as a minor product by trapping (CH3)2C=S by Diels-Alder cycloaddition to dimethylbutadiene <1991T4927>. 2,4,6-Trimethyl-l,3,5-trithiane has been used to supply thio-acetaldehyde during vacuum flash pyrolysis <1987JCP60> both the phosphorescence excitation and the emission spectra of thioformaldehyde were measured using a continuous wave (CW) dye laser and the transitions assigned <1987JCP60>. 

Some of the techniques that have been employed to study the flash pyrolysis of coal are (1) laser, (2) microwave, (3) flash tube, (4) plasma, (5) electric arc, (6) shock tube, (7) electric current, and (8) entraining gas (for example, see Hanson and Vanderborgh, 1979 Howard, 1981a,b Nelson et al., 1988 Mackie et al., 1990 Plotczyk et al., 1990 Maswadeh et al., 1992 Monsef-Miizai et al., 1992 Pyatenko et al., 1992). Most of the techniques can, at the present time, be classed as research tools that are useful as characterization techniques but the entraining gas technique is under serious consideration for commercialization. In fact, each particular method has some unique aspect that, because of the novelty of the concept, makes a brief discussion here. 

Greenwood, RF, Sherwood, N., Willett, G.D. (1995) Chemical examination of some petroleum source rocks by laser pyrolysis-mass spectrometry and flash pyrolysis-gas chromatography/mass spectrometry. Journal of Analytical Applications in Pyrolysis, 31,177-202. 

As an alternative to thermal decomposition, CVD, and laser-assisted H elimination, the flash pyrolysis of soluble alkylpolysilyne in a vacuum is reported recently. Unless they are under vacuum conditions, polysilanes and polysilyne may convert to silicon carbide (SiC). The flash process, which is a rapid removal of volatile organic substances and H2 gas, enables the control of the dimension of Si structure from 2D to 3D. Flash pyrolysis of alkylpolysilyne in a vacuum above 500 °C leads to the formation of poly- Si with a minimal amount of SiH termini, although the size of the crystals is limited to the range between several pm and several nm. 

Table 15 compares the results of laser irradiation of coal with flash irradiation and conventional carbonization at 1200 K. Laser irradiation yielded higher conversions of coal and greater percentages of acetylene and higher hydrocarbons than flash irradiation or carbonization. When five bursts of 10 J energy were used to irradiate a cube of coal the acetylene yield increased from 20.9 to 25.9 mole % this constituted 90% of total hydrocarbon products. The decrease in H2 (Table 15), relative to the product from the flash irradiation, is possibly related to the increase in partially saturated structures such as ethylene (4.9%) and propylene (0.7%). Further, components with molecular weights up to 130 were found in the gas from laser irradiation diacetylene and vinylacetylene recognized as pyrolysis products of acetylene accounted for 2.4% of the product. 

Figure 5 Convenient low-power laser pyrolysis system A, injection needle B, stainless steel body Cu, copper cylinder FL, flash lamp G, gas H, heater HS, heated sensor I, injector K, pyrolysis chamber LB, laser beam M, HR mirror Ma, output mirror N, Nd-GGG medium OF, optical fiber OR, O-ring P, probe PC, pumping chamber PS, power supply QC, quartz capillary S, sample Sh, shield Sw, Swagelok T, trigger V, voltage supply. (Reproduced with permission from Cecchetti W, Polloni R, Bergamasco G, et al. (1992) Journal of Analytical and Applied Pyrolysis 23 165-173 Elsevier.)...

The word flash has been used in the history of chemistry for many years. For example, flash vacuum pyrolysis [7, 8] is a well-known technique that has been used for chemical synthesis at high temperatures. Flash laser photolysis [9, 10] serves as a powerful method for generating reactive species in a very short time and has been used for mechanistic studies of extremely fast light induced chemical processes which are complete within milliseconds or less. Flash chromatography [11] is one of the most popular techniques for separating and pmifying compounds in organic chemistry laboratories. It should be noted, therefore, that flash chemistry is a new field of chemical synthesis but that the word flash is very common in chemistry. 

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