Metal aromatic hydroxylation

About half of the dissolved organic carbon may appear in humic or fulvic acids. These are high-molecular weight organic compounds of a composition which is somewhat uncertain. They contain aromatic hydroxyl and carboxyl groups which have the ability to bind to metal ions. Rivers and estuaries typically contain 10 mg/liter of acid with an exchange capacity of 5-10 mmol/g, mainly due to carboxylic... 

Haloperoxidases are peroxidases capable of halogenating substrates in the presence of halide and hydrogen peroxide [14] or other reactions such as sulfoxidation, epoxidation and aromatic hydroxylation. Here, the halide ion is initially bound to the active site which may incorporate heme or vanadium or be metal free. The halide ion is incorporated into the substrate after electron transfer... 

A second approach to isolating redox metal ions in stable inorganic matrices, thereby creating oxidation catalysts with interesting activities and selectivities, is to incorporate them in a zeolite lattice framework. The first example of such a redox zeolite was the synthetic titanium(IV) zeolite, titanium siliealite (TS-1), developed by Enichem [30-32]. TS-1 was shown to catalyze a variety of synthetically useful oxidations with 30% H202, such as olefin epoxidation, oxidation of primary alcohols to aldehydes, aromatic hydroxylation, and ammoxi-mation of cyclohexanone to cyclohexanone oxime (see Fig. 9). 

A promising and cleaner route was opened by the discovery of titanium silica-lite-1 (TS-1) [1,2]. Its successful application in the hydroxylation of phenol started a surge of studies on related catalysts. Since then, and mostly in recent years, the preparation of several other zeolites, with different transition metals in their lattice and of different structure, has been claimed [3]. Few of them have been tested for the hydroxylation of benzene and substituted benzenes with hydrogen peroxide. Ongoing research on suppoi ted metals and metal oxides has continued simultaneously. As a result, knowledge in the field of aromatic hydroxylation has experienced major advances in recent years. For the sake of simplicity, the subject matter will be ordered according to four classes of catalyst medium-pore titanium zeolites, large-pore titanium zeolites, other transition metal-substituted molecular sieves, and supported metals and mixed oxides. 

Other manufacturers offering TS-1 for sale include PQ, which is offering its PQ-TS-1, and CleanScience PUNE, India (252). Both companies manufacture TS-1 in the form of extrudates and microspheres. The listed process apphcations are aromatic hydroxylation, oxidation of primary alcohols to aldehydes, olefin epoxidation, and alkane oxidations. Presumably, there are more companies, especially in China, producing framework metal-containing zeotype materials. 

Some newer heterogeneous catalysts which have been found effective with H2O2 work at least partly by general acid catalysis. These include layered metal phosphates (e.g. Zr, Sn) used for aromatic hydroxylation, described in section 9.5. 

Of the above two routes, the titanium silicalite appears the simpler process since the catalyst is heterogeneous. It has recently been found that several metal phosphates (e.g. Zr, Sn) are alternative heterogeneous catalysts for aromatic hydroxylation [136]. These materials have strongly... 

Taking advantage of the many aromatic hydroxyl functions in conifer bark tannins, Hartmann (83) used ground bark as a polyol for reaction with isocyanates to prepare urethane foams with particularly good flammability resistance. Most uses for conifer bark tannins that involve reactions with the hydroxyl functions center on their complexation with cations. When sulfonated, condensed tannins can also be used as water-soluble heavy metal complexes. One of the more interesting of these applications is the development of water-soluble heavy-metal micronutrient complexes that have been used to correct iron deficiency in citrus... 

Phenol is the starting material for numerous intermediates and finished products. About 90% of the worldwide production of phenol is by Hock process (cumene oxidation process) and the rest by toluene oxidation process. Both the commercial processes for phenol production are multi step processes and thereby inherently unclean [1]. Therefore, there is need for a cleaner production method for phenol, which is economically and environmentally viable. There is great interest amongst researchers to develop a new method for the synthesis of phenol in a one step process [2]. Activated carbon materials, which have large surface areas, have been used as adsorbents, catalysts and catalyst supports [3,4], Activated carbons also have favorable hydrophobicity/ hydrophilicity, which make them suitable for the benzene hydroxylation. Transition metals have been widely used as catalytically active materials for the oxidation/hydroxylation of various aromatic compounds. 

One good example of noncovalent functionalization for subsequent hybridization is the use of benzyl alcohol (BA) [118]. n-n interactions between the aromatic ring of BA and the CNT sidewalls result in a good dispersibility in ethanol. Furthermore, BA offers a well-ordered and well-distributed functionalization [119] of hydroxyl groups on the sidewalls of the CNTs that can be used to hybridize the material with a large number of metal oxides using conventional chemical methods [60]. 

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