Sacrificial drawbacks

Iu search for efficieut aud greeuer processes over the past few years various heterogeneous catalysts such as titanium incorporated mesoporous molecular sieves [45,46], Schiff-base complexes supported on zeolite [47] and Zn(II)-Al(III) layered double hydroxide (LDH) [48], oxomolybdenum(VI) complexes supported on MCM-41 and MCM-48 [49], polyoxometallate supported materials [50], Co and Mn-AlPO s [51] etc. have been developed and studied for the catalytic epoxidatiou of a-pinene. Many of these processes suffer from drawbacks and limited applicability due to the poor conversion and also the selectivities. Sacrificial aldehydes are often used as an oxygen acceptor in such processes to achieve reasonable yield and selectivities but, this procedure leads to an increase in the E-factors and decrease in the atom economy [51]. 

This methodology, while commendable for its simplicity, has some drawbacks yields are not quantitative one has to invest i-butyraldehyde as a sacrificial reducer the resulting i-butyric acid has to be separated from the epoxide product, which detracts from the usefulness of the methodology. Nevertheless, as also shown by others recently (57), the Mukaiyama procedure can be adapted to good use. 

For undivided cells, a valid alternative is provided by the stirred tank reactor (STR), which is characterized by a high flexibility in the use of different electrode structures and shapes. The adoption of a sacrificial anode (Gennaro et al. 2004) and/or specific fluidodynamic conditions (He et al. 2004a) avoids most of the drawbacks connected with parasitic electrode reactions. 

There are a variety of materials that can be used as sacrificial cores. Inorganic sacrificial materials include Si02 and metals such as aluminum, " titanium, and nickel. Polymers such as PI, PMMA, PC, and photoresist have also been used as sacrificial materials. After deposition of the cover film, removal of the sacrificial layer can be achieved by dissolution, etching, or thermal degradation. These removal methods each have benefits and drawbacks selection of the optimal approach is specific to particular combinations of substrate, sacrificial layer, and cover film 73, 3 Recently Whitesides and coworkers " implemented a fabrication method using water-soluble sacrificial cores. Poly(acrylic acid) and dextran proved to be effective sacrificial layers that could be dissolved in water or aqueous NaCl, for making metallic microstructures by nickel electrodeposition. 

Tin-plated cans used in the food industry provide another example of electrochemical protection. Tin is likely to react with moist oxygen and so protects the surface of the iron. There is one drawback with tin plating. It is easily scratched to reveal the iron, in which case the can rusts very rapidly. What happens is that once iron is in contact with moist air, it gives sacrificial protection to the less reactive tin. Tin is used to protect cans made of iron because zinc would poison the food. 

In early studies in the 1980s [56,70,71], Ru(2N — 1)3" was used as an efficient photosensitizer and organic molecules as e -donors. The drawback was that the sacrificial reagents were more expensive than the products formed from CO2, making such systems non-viable economically. An interesting case in which a unique complex acts as sensitizer and catalyst is represented by Re(2N — 1) (CO)3X (X = Cl, Br) [70, 72] which is able to produce CO selectively. 

The main drawback of the previous approach is the use of TBHP or other radical initiator that is consumed or decomposed in the initiation steps of the chain mechanism. It would be even more convenient if no organic sacrificial initiator is added. In this regard, the pioneer work of Ishii et al. using aromatic A-hydroxylamines in combination with first-row transition metal ions such as Co ", Fe ", or Mn + has shown that by coordination of the A-hydroxylamine with these transition metals oxyl radicals are formed and these oxyl radicals can act as suitable radical initiators [40,41] (Scheme 2.12). Due to the stability of the molecule and the corresponding oxyl radical, A-hydroxylphthalimide (NHPI) has been one of the preferred organic initiators in combination with Co " or Fe " ions. 

---