Hydroperoxides alkylaromatics, selective oxidation

Aerobic selective oxidation of alkylaromatics, including cumene (CU), ethylbenzene (EtB), and cyclohexylbenzene (CyB), to the corresponding hydroperoxides (CHPs) represents a key step for several large-scale productions, including the Hock process for the synthesis of phenol (see Chapter 2) [15] and the Shell styrene monomer/propylene oxide (SM/PO) process for the production of propylene oxide (PO) and styrene monomer (SM) [16]. 

The discovery ofmesoporous MCM-41 has opened up new avenues for the oxidation of larger substrates and the use of bulkier oxidants. The cumene hydroperoxide-based chemistry is characterized by a high selectivity and, consequently, very low by-product formation. The principal steps of the selective oxidation of propylene with cumene hydroperoxide are shown in Figure 1.7. When using mesoporous silicates with isomorphous titanium substitution, alkylaromatic hydroperoxides such as CFIP become the more attractive oxidants than TBFIP because of the ease of reuse. 

The calcined iron-grafted materials exhibit high selectivity as catalysts for oxidations of alkanes, alkenes and arenes with H2O2 as the oxidants [13a]. A similar method has been used by Tilley et al. to prepare a pseudotetrahedral (Co(II) [Co(4,4 -di Bu-bipy) OSi(0 Bu)3 2]) complex grafted onto the SBA-15 surface and subsequently use it in catalytic oxidation of alkylaromatic substrates with tert-butyl hydroperoxide [14]. Unfortunately, neither iron nor cobalt surface organometaUic compounds have been tested in the recycled catalytic system. 

In 2009, Fierro and Repsol Quimica S.A. reported the beneficial effect of combining NHPI with parts per million amounts of NaOH for the synthesis of hydroperoxides [22], with the dual effect of promoting the formation of PINO in the absence of transition metal salts and of neutralizing the acidic by-products of the reaction. This approach was particularly effective for increasing conversion and selectivity in the oxidation of secondary alkylaromatics [23]. 

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