Catalysts phenol hydrogenation

Besides a variety of other methods, phenols can be prepared by metal-catalyzed oxidation of aromatic compounds with hydrogen peroxide. Often, however, the selectivity of this reaction is rather poor since phenol is more reactive toward oxidation than benzene itself, and substantial overoxidation occurs. In 1990/91 Kumar and coworkers reported on the hydroxylation of some aromatic compounds using titanium silicate TS-2 as catalyst and hydrogen peroxide as oxygen donor (equation 72) . Conversions ranged from 54% to 81% with substituted aromatic compounds being mainly transformed into the ortho-and para-products. With benzene as substrate, phenol as the monohydroxylated product... 

With Binaphthol/M(OTf)3 Complexes (M = Yb, Sc) A chiral ytterbium triflate, derived from Yb(OTf)3, (R)-binaphthol, and a tertiary amine, has been applied to the enantioselective Diels-Alder reaction of cyclopentadiene with crotonoy 1 oxazolidinones. Among various tertiary amines, c/s-1,2,6-trimethyl piperidine was found to be highly effective [44] (Eq. 8 A.23). The unique structure of such chiral Yb catalysts is characterized by hydrogen bonding between the phenolic hydrogens of (R)-binaphthol and the nitrogens of tertiary amines. 

Table 5.6 Rate constants for phenol hydrogenation following the parallel (kh k3) and series reactions (kh k2) at 423 K over Pd catalyst (after Park et al. [15]).

The above kinetics is valid for small particles when the process rate is controlled by the chemical reaction at the surface. Diffusion effects should be accounted for large-size particles. Table 5.8 presents the calculation of the effectiveness factor [24] for spherical particles of 6 mm diameter and a mixture 1 3 phenol/hydrogen at 2 bar and 423 K. Other data are BET internal surface S = 40m2/g, mean pore radius 150 A, catalyst density pp = 1000kg/m3, particle void fraction = 0.3,... 

Mahata, N., Vishwanathan, V., Kinetics of phenol hydrogenation over supported palladium catalysts, Catal. Today, 49,... 

Calcined LDHs may also serve as effective catalyst supports. Davis and Derouane prepared, for example, a supported-platinum catalyst for aromatiza-tion of n-hexane, via the impregnation of a calcined MgAl-LDH with an aqueous solution of Pt(NH3)4Cl2 [203]. More recently, Narayanan and Krishna have studied a system of palladium supported on calcined MgAl-LDHs for phenol hydrogenation [204-207]. 

Narayanan, S. and Krishna, K. (1998). Hydrotalcite-supported palladium catalysts Part I preparation, characterization of hydrotalcites and palladium on uncalcined hydrotalcites for CO chemisorption and phenol hydrogenation. Appl. Catal. A 174, 221. 

Phenol Hydrogenation. In principle, appropriate lignin deconstruction processes will provide a stream of mixed phenols. Reduction of these phenols will lead to a new source of cyclic aliphatic alcohols of potential use in the manufacture of adipic acid derivatives. Several catalytic processes for these types of reductions have appeared for phenol and should be applicable to lignin-derived mixed phenols. Phenol itself is reduced to cyclohexanol in the presence of various heterogeneous catalysts based on Pd.530-535... 

Mahata, N. and Vishwanathan, V, Influence of palladium precursors on structural properties and phenol hydrogenation characteristics of supported palladium catalysts. J Catalysis 2000, 196 (2), 262-270. 

Mahata, N., Raghavan, K. V, and Vishwanathan, V, Influence of alkali promotion on phenol hydrogenation activity of palladium alumina catalysts. Appl Catalysis A-General 1999, 182 (1), 183-187. 

In the phenol hydrogenation process phenol is fed in the gas phase with hydrogen at 140-170°C through a catalyst bed at atmospheric pressure. The catalyst generally consists of 0.2-0.5 wt.% palladium on a zeolite carrier. The yield exceeds 95% at quantitative conversion. Figure 2.28 shows the flow diagram for the process. 

The unreacted cyclohexane is distilled off and recycled. The ca. 1 1 mixture of cyclohexanol and cyclohexanone is then subjected to dehydrogenation over a palladium catalyst (the same catalyst as is used in phenol hydrogenation) to give pure cyclohexanone. 

Benzene may be oxidised to phenol in an ionic liquid-aqueous biphasic system employing dodecanesulfonate salts of Fe3+, Fe2+, Cu2+, Co2+ and Ni2+ as catalyst and hydrogen peroxide as oxidant.[85] Ionic liquids with long alkyl chains like [C8Ciim][PF6] form one phase with benzene and at the same time dissolve the catalyst. As the phenol product is preferably soluble in the aqueous phase product isolation was facile. Yet, reaction rates were only moderate and after six hours at 50°C conversion of benzene was in the range of 35-55%. 

Covering a supported metal catalyst with a thin liquid film that differs from the bulk solvent affects both the reaction rate and selectivityTreating a Pd/C catalyst with 2M KOH before using it in a phenol hydrogenation with a heptane solvent gave cyclohexanone in 97% yield. Apparently, the distribution of phenol and phenolate ions between the thin aqueous film around the catalyst and the bulk hydrocarbon solvent enhanced the adsorption of the phenolate ion on the catalyst and the facile transfer of the neutral cyclohexanone to the heptane after the selective hydrogenation was completed (Figure 17.1). 

Fig. 17.1 Depiction of a thin film of alkaline water on the metal catalyst and its effect on selectivity in phenol hydrogenation. ...

As for the structure of the hetero-chiral Yb(III) catalyst, the following structure was supported (Scheme 15). Actually, the existence of hydrogen bonds between the phenolic hydrogens of (S)-BINOL and the nitrogens of (R)-MNEA was confirmed by the IR spectra of the catalyst [79,102]. 

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