Immersion well reactors

Preparative photochemical reactions are usually carried out in an immersion-well reactor, the usual design is shown in Fig. 14.3. 

Small (1-5 g) scale UV photolysis of air sensitive compounds can be performed in quartz Schlenk tubes, or in conventional Schlenkware with the use of a UV transparent quartz stopper. The latter apparatus is easily adapted to low temperature irradiations. Large scale (10-50 g) UV photochemical reactions use quartz immersion well reactors. Medium pressure Hg-arc lamps are the preferred radiation sources for synthetic applications. ... 

Experiments were performed in an immersion-well reactor with a capacity of 125 mL. Here 200 mg of the putative catalyst was suspended in 100 mL of doubly-distilled water. The light source was a 125-W medium-pressure Hg lamp which produced more than 7 x 1018 photons s 1 cm-2 inside the immersion well. Irradiation was carried out under a stream of N2 which had been purified by passing it through a hot copper-pellet column, chromic acid solution, and potassium hydroxide solution. After irradiation the reaction mixture was distilled with base and the distillate was analyzed by the indophenol method. A portion of the mixture was centrifuged and analyzed for N03 by Cd reduction followed by colorimetric determination by the sulfanilic acid method (azo dye test) [89],... 

The activity of a-Fe203 Nd203 S04 was also tested in an immersion-well reactor. The N2-saturated aqueous suspensions of the putative catalyst were irradiated for 3h and analyzed with an NH3-selective electrode. Yields ranging from 0.35 to 1.14/xmol NH3 were obtained from 0.01 g of the putative catalyst. 

Then 200 mg of the putative catalyst was suspended in 350 mL of FI20 in a cylindrical immersion-well reactor. The pH was adjusted to 11 with NaOH, and the reactor was purged with N2 for 30 min prior to irradiation. The N2 flow rate was reduced to 10 mL min, and the reaction mixture was irradiated by a 400-W medium-pressure Hg lamp. Control experiments were performed without irradiation, or with argon substituted for nitrogen. Aliquots of solution were withdrawn at intervals and tested for N02 (azo dye test), NOj- (Cd-reduction, azo dye test), and NH3 (indophenol method). 

Photochemical syntheses, as well as photodegradation studies, are usually performed in irradiation reactors a variety of different designs are commercially available, depending on the specific application requirements. For synthetic photochemistry, immersion well reactors are commonly used (Fig. 14.4). The source, typically a low or medium pressure mercury lamp, is housed in a double-walled quartz jacket, which allows water-cooUng and/or filtering of excitation radiation. The solution to be irradiated surrounds the lamp source, enabling homogenous irradiation. 

Small scale test runs prior to preparative irradiation experiments may be carried out in tubes which are either taped to the lamp housings (immersion wells) depicted in Figures 13-1 and 13-2 or placed in turntable reactors ( merry-go-rounds ). These arrangements permit the simultaneous irradiation of several samples, but only a fraction of the available light emission is used. In Figure 13-4 a simple reactor is shown which focusses almost all the emitted light into one sample which can be scaled up also to semi-preparative volumes. In this way the necessary irradiation time can be reduced sharply. 

The conventional photolysis apparatus consists of a concentrically arranged immersion well for the lamp, which is surrounded by a cooling jacket, which is itself surrounded by the reaction vessel. If this last compartment is used for the filter solution an additional external flask for the reaction mixture has to be used. There are also photochemical reactors wherein the lamps are arranged externally around the reaction flask. 

The reaction was carried out in an immersion-well photoreactor with a medium-pressure. 80-W Hg lamp in a water-cooled double jacket (quartz and Simax glass, sintered-glass inlet at the bottom of the reactor), and equipped with a dry icc cooled spiral cooler with an hydraulic seal. A mixture of 3,4,4-trifluoro-5.5-dimethyl-4.5-dihydrofuran-2(3//)-one (3 67 mg, 40 mmol) and solvent (2 mol) was irradiated for 10 h at 18-20 C in a stream of argon. The low-boiling part of the mixture was fractionally distilled off and the product was isolated by preparative GC (polybutane-1,4-diol succinate, 500 cm, 160-200 C) vicld 50-65%. 

Figure 3.9 Left a photochemical reactor with immersed configuration with the lamp and a power source. Right detail of the immersion well. Reproduced by permission of Ace Glass Inc...

Experiments were carried out in a 500-mL photochemical reactor with a quartz immersion well. A 10 x 20 cm2 piece of film was wrapped around the immersion well and tied in place with Teflon tape. Then 350 mL of H20 was placed in the reactor and NaOH was added to adjust the pH to 12. The system was purged continuously with air or N2 and irradiated with a 400-W medium-pressure Hg lamp. After a period of time the solution was tested for NH3 by the indophenol method, and for NOJ/NOJ (combined total nitrite and nitrate) by Cd reduction, followed by colorimetric analysis by the azo-dye method [84]. The azo-dye method, sometimes known as the Griess method or the sulfanilic acid method, has been described elsewhere [87]. 

A mass of 0.200g of a putative catalyst was suspended in 300mL of deionized water in an immersion-well photochemical reactor under a slow stream of N2. The mixture was irradiated by a 400-W medium-pressure Hg lamp which produced more than 5 x 1019 photons per second. Control experiments under argon or without irradiation were performed. At the end of the irradiation, lOmL of 0.10M NaOH was added to the reaction flask. The contents were distilled into a receiving flask which contained 10 mL of HC1 and analyzed for NH3 by the indophenol method. Then 10 mL of the reactor solution was centrifuged and analyzed for N03 by Cd reduction followed by colorimetric analysis by the azo-dye method. 

The rate of photoconversion in a heterogeneous process is normally expressed in terms of a limited number of variables that should include the concentrations of the species present, the reactor volume, and the amount of catalyst irradiated in this particular volume. In the case of the immersion well-stirred photochemical reactor, currently used in photocatalytic experiments, the rate (r) expression for photoconversion is given by [199]... 

Fig. 3.19 represents an immersion-well photochemical reactor that may be employed for carrying out most of the preparative photochemical reactions. 

In order to operate an immersion-well photochemical reactor effectively, first of all the air is removed finm the solvent by slowly allowing inert nitrogen or ai on gas to bubble through it via the sintered glass-disk. It is equally important to make sure that the correct choice of lamp is made for the reactor before starting the reaction. 

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