Naphthols oxidation, liquid phase

Naphthols are readily dehydrated to the corresponding ethers at considerably lower temperatures (300° C) in the liquid phase in the presence of metal oxides (16, 17). 

Ti, V and Sn-modified mesoporous silicates were reported to be active in a number of liquid phase oxidation reactions. Ti-containing samples were used for the selective oxidation of large organic molecules in the presence of te/t-butyl hydroperoxide (TBHP) or dilute H2O2 [71,136,137,139-141,147,186,237]. Typical data shown in Table 5 indicate that both Ti-MCM-41 and Ti-HMS are efficient cat ysts for the epoxidation of bulky olefins such as a-terpineol and norbomene in the presence of TBHP or H2O2. Comparison with H-B indicates that the accessibility of active sites plays a critical role in the liquid phase oxidation of organic molecules. Mesoporous titanosilicates also exhibited remarkable activity in the hydroxylation of 2,6-di-rerr-butyl phenol (2,6 DTBP) [142,147] and the oxidation of cyclododecanol [147], naphthol [147] aniline [237] and chloroaniline [186]. However, they were disappointingly poor catalysts for the liquid phase oxidation of n-hexane and aliphatic primary amines, as well as the ammoximation of cyclohexanone [147,238]. 

Sayari et al. [172,174] found that like their Ti containing analogs, V-HMS and V-MCM-41 are efficient catalysts for the hydroxylation of bulky aromatic molecules such as naphthol and 2,6 DTBP. Gontier et al. [186,237] found that V-HMS has no activity in the oxidation of aromatic amines in the presence of H2O2. However, it oxidizes aniline selectively into nitrobenzene when TBHP and acetonitrile are used as oxidant and solvent, respectively. Notice that Ti-HMS is active even in the presence of H2O2 and gives a different product distribution, particularly azoxybenzene [237], Das et al. [193] also reported that Sn-MCM-41 is effective in the liquid phase hydroxylation of phenol and naphthol. 

Methyl-1,4-naphthoquinone, or vitamin K3, also known as menadione, is a member of the family of K vitamins and is used in preventing hemorrhages. The conventional method of producing menadione is by stoichiometric oxidation by Cr203 of 2-methylnaphthalene in coke-oven tar and certain petroleum fractions. An alternative two-step method (scheme E6.3.1) has been developed (Kozhevnikov, 1995) in which 1-naphthol is alkylated by methanol in the vapor phase to 2-methyl-1-naphthol (30), which then is oxidized in the liquid phase to menandione (31) by O2 in the presence of a heteropolyanion as catalyst. 

The oxidation selectivity strongly depends on the specific polyaromatic substrate. Oxidation of naphthalene with H Oj in the liquid phase over a large pore zeolite, V-NCL-1 [86a], and a mesoporous vanadium-silicate, V-MCM-41 [86b], afforded 1,4-naphthoquinone (NQ) as the main product and minor amounts of 1- and 2-naphthols at the initial stage of reaction but, at conversions greater than 5%, phthalic anhydride predominated because of susceptibility of NQ to overoxidation. In contrast to NQ, AQ is rather stable to overoxidation and high selectivities can be attained at high substrate conversions. 

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