Phenol synthesis from benzene

The three-step cumene process, including the liquid-phase reactions and using sulfuric acid, is energy-consuming, environmentally unfavorable and disadvantageous for practical operation the process also produces as an unnecessary byproduct acetone, stoichiometrically. Furthermore, the intermediate, cumene hydroperoxide, is explosive and cannot be concentrated in the final step, resulting in a low one-path phenol yield, ( 5%, based on the amount of benzene initially used). Thus, direct phenol synthesis from benzene in one-step reaction with high... 

Previous studies have used many oxidants for direct phenol synthesis from benzene, such as O2 [74-80], H2O2 [81-89], N2O [90-99], Hj -1- Oj [100, 101], air/ CO [102] and O2/H2O [103], Among these oxidants, the selective oxidation of benzene with economically and environmentally favorable O2 has been nominated as one of the ten most difficult challenges for catalysis [104—106] and, indeed, there have been no reports on the direct phenol synthesis with greater than 5% conversion and 50% selectivity over the last 40 years. 

In this section, we discuss the high performance of the Rejo cluster/HZSM-5 catalyst, its active structure and dynamic structural transformation during the selechve catalysis, and the reaction mechanism for direct phenol synthesis from benzene and O2 on this novel catalyst [73, 107]. Detailed characterization and determination of active Re species have been conducted by XRD, Al solid-state MAS NMR, conventional XAFS and in situ time-resolved energy dispersive XAFS, which revealed the origin and prospects of high phenol selectivity on the novel Re/HZSM-5 catalyst [73]. 

Notably, NH3 is indispensable for the catalytic phenol synthesis. In the absence of NH3, neither benzene combustion nor phenol formation occurred on the Re-CVD/HZSM-5 catalyst (Table 10.6). Other amine compounds such as pyridine and isopropylamine did not promote the catalytic reaction at aU, which indicates that the role of NH3 in the catalysis is not due to its basic function. Fe/ZSM-5 has been reported to be active and selective for phenol synthesis from benzene using N2O as an oxidant [90, 91], but N2O did not act as an active oxidant on the Re-CVD/ HZSM-5 catalyst Furthermore, no positive effects were observed by the addition of both N2O and H2O. Notably, the NH3-pretreated Re-CVD/HZSM-5 catalyst selectively converted benzene into phenol with O2 in the absence of NH3, as discussed below. 

Scheme 10.4 (a) Structural changes in the Re-CVD/HZSM-5 (19) catalyst during direct phenol synthesis from benzene and O2 and treatment with NH3 (b) proposed model structure of the N-interstitial Reio-cluster catalyst supported in the pore of HZSM-5 (calculated by DFT). 

Re species are in a dimeric form with a direct Re-Re bond. Re Lj-edge XANES in part (ii) of Figure 10.9a did not show the pre-edge peak attributed to tetrahedral conformation of Re and the edge posihon shifted to lower energy. These results indicate that the NH3 treatment reduced the Re monomers accompanied with dimerization. Negligible catalytic achvity at this stage demonstrates that small Re clusters such as dimers do not act as achve species for direct phenol synthesis from benzene and O2. 

FigurelO.lO Reaction mechanism of phenol synthesis from benzene and O2 0n a Re cluster/HZSM-5 calculated by DFT.

Direct Phenol Synthesis from Benzene and Molecular Oxygen 57... 

Direct Phenol Synthesis from Benzene and Molecular Oxygen on a Novel N-Interstitial Ren0-Cluster/HZSM-5 Catalyst... 

It was reported independently by three research groups that MFI-type zeolites selectively catalyze the reaction of N20 with benzene to give phenol C6H6 + N20 —> C6H5OH + N2 [93-96]. Fe/ZSM-5 shows remarkable performance in benzene hydroxylation to phenol with N20 as oxidant, which is the first example of a successful gas phase direct phenol synthesis from benzene [97]. No other catalysts show similar high performances to the Fe/ZSM-5 catalyst. At present, iron is the sole element capable of catalyzing the benzene-to-phenol reaction [98]. Direct oxidation of benzene to phenol by N20 has been commercialized in the so-called AlphOx process in Solutia Inc., US A, where N20 is obtained as a by-product in adipic acid production with nitric acid [97, 99, 100] a selectivity >95% to phenol is achieved at >40% conversion at around 4000 C. But the process is cost-effective only if N20 can be obtained cheaply as a by-product in adipic acid production. 

Utilization of molecular 02 as a sole oxidant is ideal. Direct phenol synthesis from benzene with 02 has been studied extensively to date, but it shows low activity and selectivity with Cu0/A1203 [143] and Cu/ZSM-5 [144, 145], Several conditions of reactions were optimized with Cu/ZSM-5 catalysts, but the maximum yield was 4.9% with about 30% selectivity at 673 K [144, 145],... 

In conclusion no catalysts with good performances (>5% conversion and >50% selectivity, simultaneously) have been discovered to date. New selective catalysts for direct phenol synthesis from benzene with 02 are essential for the novel industrial process replacing the cumene process - there are many problems to be resolved. 

A HZSM-5 zeolite-supported novel N-interstitial Re10 cluster was found to be active for the direct phenol synthesis from benzene and 02 in the presence of NH3 [169, 170]. The acidity and pore structure of HZSM-5 led to the self-limited formation of the novel N-interstitial Re10 cluster in the pore, which cannot be produced on other oxide surfaces or in solutions, and direct phenol synthesis using 02 as a sole oxidant was achieved with impressive results (10% conversion and 94% selectivity) for the first time [169-171]. 

Table 2.5 Catalytic performance of pulse reactions on the CVD-Re/ HZSM-5 (Si02/AI203 = 19) catalyst (Re 0.58 wt%) for direct phenol synthesis from benzene and 02 at 553 K in the absence of NH3a .

The coexistence of NH3 is indispensable for selective benzene oxidation. Neither benzene oxidation nor combustion proceeded in the absence of NH3 (Table 2.5). Fe/ZSM-5 has been reported to be active and selective for phenol synthesis from benzene using N20 as an oxidant [97], but selective benzene oxidation did not proceed with N20 instead of 02. The addition of H20 to the system gave no positive effects on the catalytic performance, either. In addition, other amine compounds such as pyridine and isopropyl amine did not produce phenol. The phenol formation rate and selectivity increased with increasing NH3 pressure because the coexisting NH3 produces active Re clusters, as described below, and reached maximum conversion and selectivity at a partial pressure of NH3 of around 35—42kPa. 

Figure 2.8 Reaction mechanism for phenol synthesis from benzene and 02 on the Re10 cluster/HZSM-5 calculated by DFT. The end-on adsorbed 02 (a) dissociates to two atomic oxygen on the Re10 cluster (b), and a so-produced atomic oxygen adds to a carbon atom of benzene via a transition state (TS1),...

The dynamic XAFS study of the catalytically active structures under the reaction conditions should enable us to further develop novel catalysts for direct phenol synthesis from benzene and the efficient activation of molecular oxygen. 

Unlike liquid phase processes, air or O2 are the only oxidizing agents used in commercial processes, although recently phenol synthesis from benzene using N2O as the oxidizing agent has been developed, although it has not reached commercial viability. The process is only economical when a cheap source of N2O is available, that is, when N2O is recovered from waste streams such as in adipic acid production. 

Electrophilic mono-oxygen insertion (i) ethene epoxidation to ethene oxide using O2 and (ii) direct phenol synthesis from benzene using N2O. 

Novel Pathways and Reactants This is a very broad area. We will thus restrict discussion to few examples. The first regards the important reaction of phenol synthesis and the possibility to realize it in one step directly from benzene using molecular oxygen as the oxidant. Various aspects of direct phenol synthesis from benzene are discussed in Chapter 13. We highlight here only recent results that exemplify how starting from the previously cited activity of Re complexes in the epoxidation in homogeneous phase could lead to investigation of the behavior of Re complexes when inserted into the channels of zeolites (ZSM-5) and in gas-phase selective oxidations. This has opened a new unexpected direction. 

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