Photochemical Micro Reactors

The UV radiation adsorbed by chromophoric DOM stimulates production of free radical (singlet) O2. This leads to high concentrations of the free radicals within and around the CDOM, creating local conditions of high reactivity. Thus, chromophoric DOM, which tends to be HMW, can be thought of as a photochemical micro reactor in which free radical oxidations are promoted. Given its macromolecular nature, CDOM... 

Reaction 35 [R 35] Photochemical Serpentine Chip Micro Reactor... 

Drivers for Performing Photochemical Biradical Formation in Micro Reactors... 

Beneficial Micro Reactor Properties for Photochemical Biradical Formation... 

Photochemical Biradical Formation Investigated in Micro Reactors Organic synthesis 87 [OS 87] Benzopinacol formation by two-radical combination... 

Internal heat exchange is realized by heat conduction from the microstructured reaction zone to a mini channel heat exchanger, positioned in the rear of the reaction zone [1,3,4], The falling film micro reactor can be equipped, additionally, with an inspection window. This allows a visually check of the quality of film formation and identification of flow misdistribution. Furthermore, photochemical gas/liquid contacting can be carried out, given transparency of the window material for the band range of interest [6], In some cases an inspection window made of silicon was used to allow observation of temperature changes caused by chemical reactions or physical interactions by an IR camera [4, 5]. 

Using a single-channel chip micro reactor, singlet oxygen is generated by photochemical means in presence of catalytic amounts of Rose Bengal [21]. By [4 -i- 2] cycloaddition of this oxygen species to a-terpinene, the product ascaridole is obtained. 

Fukuyama, Ryu and coworkers reported intermolecular [2 + 2]-type cycloaddition of various cyclohexenone derivatives and alkenes using a micro reactor made entirely of glass, which was supplied by Mikroglas (Scheme 4.26) [39]. The device was equipped with a heat exchanger channel system through which water flowed to maintain isothermal reaction conditions. The remarkable photochemical efficiency of this device was manifested in rapid cycloaddition of vinyl acetate to cyclohex-2-enone. With this device, the desired product was obtained in 88% yield after 2 h, whereas the same reaction carried out in a Pyrex flask was very sluggish (only 8%... 

Utilizing a commercially available micro reactor, fabricated from Foturan glass, Ryu and co-workers [86] evaluated a series of [2 + 2] cycloadditions as a means of reducing the reaction times conventionally associated with the synthetic transformation. Using a high-pressure mercury lamp (300 W), the reaction of cyclohex-2-enone (155) with vinyl acetate (14-7) (Scheme 6.40), to afford the cydoadduct 156, was used to compare photochemical efficiency within the micro reactor [1000 pm (width) x 500 pm (depth)] and a conventional batch reactor (10 ml). 

Table 6.15 The photochemical [2 + 2] cycloadditions conducted in a Foturan micro reactor.

With numerous researchers investigating the advantages associated with the thermal or biocatalytic control of asymmetric reactions, Ichimura and co-workers [89] considered the potential of photochemical asymmetric syntheses performed in continuous flow reactors. To investigate the hypothesis, the authors employed the asymmetric photochemical addition of MeOH to (R)-( + )-(Z)-limonene (159) as a model reaction, comparing three quartz micro reactors, with a standard laboratory cell as a means of highlighting the synthetic potential of this approach. 

An example of a photochemical cyclization was reported by Takei et al. [92], who demonstrated the synthesis of L-pipecolinic acid (160) from an aqueous solution of L-lysine (161), as illustrated in Scheme 6.42. To achieve this photocatalytic transformation, the authors fabricated a Pyrex micro reactor in which the channel cover plate was coated with a 300nm layer of anatase Ti02 (100 nm particles), to afford a titania-coated micro reactor (TCM) the titania film was subsequently loaded with platinum (0.2 wt%), by photodeposition, to enable the TCM to be used... 

K. Jahnisch, U. Dingerdissen, Photochemical generation and [4 + 2)-cycloaddition of singlet oxygen in a falling-film micro reactor, Chem. Eng. Tech. 2005, 28, 426-427. 

Product distributions and reaction conversions of several different photochemical systems, irradiated by conventional UV source and by EDL in a MW-UV reactor (Fig. 14.5), were compared to elucidate the advantages and disadvantages of a micro-wave photochemical reactor [90], Some reactions, e.g. photolysis of phenacyl benzoate in the presence of triethylamine or photoreduction of acetophenone by 2-propa-nol, were moderately enhanced by MW heating. The efficiency of chlorobenzene photosubstitution in methanol, on the other hand, increased dramatically with increasing reaction temperature. 

Lu H, Schmidt MA, Jensen KF (2001) Photochemical Reactions and On-Line UV Detection in Microfabricated Reactors. Lab Chip 1 22-28 Manz A, Harrison DJ, Verpoorte EMJ, Fettinger JC, Ludi H, Widmer HM (1991) Miniaturization of Chemical-Analysis Systems - A Look into next Century Technology or just a Fashionable Craze. Chimia 45 103-105 McCreedy T (1999) Reducing the Risks of Synthesis. Chem Ind 15 588-590 McCreedy T (2000) Fabrication Techniques and Materials Commonly Used for the Production of Microreactors and Micro Total Analytical Systems. Trac Trends Anal Chem 19 396-401... 

Klan P, Hajek M, Cirkva V (2001) The Electrodeless Discharge Lamp A Prospective Tool for Photochemistry Part 3. The Micro-wave Photochemistry Reactor, /. Photochem. Photobiol. A Chem. 140 185-189. 

To conclude, tight and stable solids could be prepared from polysaccharides and water, and chemical reactions including electrochemical and photochemical ones could be carried out in these solids in the same way as in pure water, which offers a new concept of solution chemistry taking place in solids. In addition to its usage for conventional chemical reactions, any application to micro-scale reactors would be promising. 

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