Small quartz/Pyrex tubes

As mentioned above, small-scale photoreactions are quite often carried out in quartz or Pyrex tubes, by external irradiation. However, this is certainly not an optimal solution for maximizing the exploitation of the emitted radiation. Internal irradiation is obviously better from the geometric point of view, but (relatively) large-scale preparations must take into account all of these factors and achieve optimal light and mass transfer. These elements are not taken into account in exploratory studies or small-scale syntheses, just as is the case for thermal reactions, where the optimization is considered at a later stage the essential requirement is that the explorative study is carried out under conditions where occurrence of the reaction is not prevented. Thus, it is important that the source is matched with the reagent absorption, the vessel is of the correct material, and the solvent does not absorb competitively (unless it acts also as the sensitizer). Figure 1.7 and Table 1.1 may help in this choice, in conjunction with the U V spectra of all of the compounds used (it is recommended that the spectra are measured on the actual samples used, in comparison with those taken from the literature, in order to check for absorption by impurities). 

Apparatus Use an apparatus consisting of a source of nitrogen (supplied through a regulator or flowmeter capable of measuring a flow rate of 1 L/min) connected to a suitable quartz combustion tube contained in a hinged furnace (Type 70 T, Arthur H. Thomas Co., or equivalent) in which the sample can be pyrolyzed at 650°. Connect the exit end of the combustion tube to the optical cell of a suitable mercury vapor meter (Beckman Model K-23, or equivalent), the microammeter of which is connected in parallel through an attenuator to a 1-mV strip chart recorder. Fit a 48.3-cm x 18.3-mm (od) quartz combustion tube at each end with Pyrex ball-joint adapters, and pack it near the exit end with 40 g of copper oxide held in place by small wads of quartz wool. Use the inlet end to hold an 88- x 12- x 8-mm combustion boat for the sample. 

The capillaries are made either of borosilicate glass or fused quartz. Pyrex glass has a cutoff at about 280 nm, so fused silica is often used. The capillary tube is coated with a polyimide to enhance flexibility by preventing the hydration of strained siloxy bonds. The columns vary in internal dia-meter from 10 to 100 pm and in length from 30 to 100 cm, with 50 to 100 cm being common. The capillary side wall is usually thick by comparison, 300 to 600 pm, to act as a heat sink. A small portion (1 to 2 mm) of the coating is burned away if a photometric detector is used. Figure 31-6 shows typical dimensions. 

Disc/flat-sheet membrane reactors are mostly applied in research work because they can be fabricated easily in laboratory with a small amount of membrane material. Figure 7.11 shows the structure of the disc/flat-sheet membrane reactor. The membrane disc is mounted between two vertical ceramic or quartz tubes, and then placed in a bigger quartz tube. Pyrex or gold gaskets are used to obtain an effective seal between the disk and the walls of the inner tubing at high temperatures by placing the assembly in... 

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