Phenol sulfolane complex

Figure 13.6 Possible structure of the phenol/sulfolane complex.

This effect was confirmed by calculation of the loading of the free phenol molecule and the complex phenol-sulfolane (expressed as the number of loaded molecules in a crystal elementary unit of TS-1), using the software Sorption (Cerius 2), which turned out to be 13.6 and 0.8, respectively. Alternatively, the protective effect exerted by sulfolane can be evaluated by measuring the reaction rate, expressed as the turnover frequency (TOF moles of reacted substrate/moles of Ti per hour) for the oxidation of benzene and phenol, carried out separately in acetone and sulfolane as co-solvents. In the case of acetone, the phenol oxidation (TOF = 190) was ten times faster than that measured for benzene (TOF = 19) conversely, operating in sulfolane the rate measured for phenol (TOF = 51) was only 1.6 time higher than that measured for benzene (TOF = 31), according to the higher value of the observed selectivity. 

Recently, the use of sulfolane solvent allowed better kinetic control of the oxidation chain, with an increase of the selectivity to 80% or greater, at ca 8% benzene conversion. The by-products were catechol (7%), hydroquinone (4%), 1,4-benzo-quinone (1%) and tar (5%) [53, 54]. According to these authors, a rather stable complex, formed by hydrogen bonding with sulfolane, promoted desorption and hindered the re-adsorption of phenol, protecting it from consecutive oxidation (Equation 18.7). Actually, the rate of oxidation of phenol in the presence of sulfolane was only 1.6 times that of benzene, while it was 10 times higher in the presence of acetone. 

Why is sulfolane, and only sulfolane, so effective in improving selectivities Already in 1963, Drago suggested that sulfolane forms a complex with phenol via hydrogen bonding (Figure 13.6) [21]. 

Such a complex is not stable enough to be isolated but its occurrence was inferred by the IR spectra of the solutions. Drago s suggestion is confirmed by ah initio calculations. Therefore, the improved selectivity observed upon carrying out benzene oxidation in sulfolane may be due to the formation of this large species, which can not enter the titanium silicalite pores, thus allowing phenol to remain relatively protected against further oxidation. 

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