Phenolics synthesis

More than 95% of the cumene produced is used as feedstock for the production of phenol (qv) and its coproduct acetone (qv). The cumene oxidation process for phenol synthesis has been growing in popularity since the 1960s and is prominent today. The first step of this process is the formation of cumene hydroperoxide [80-15-9]. The hydroperoxide is then selectively cleaved to phenol [108-95-2] and acetone [67-64-1/ in an acidic environment (21). 

This phenol synthesis complements the analogous reaction (see below) from arylthallium ditrifluoroacetates (147). Although yields are only moderate, the procedure represents a viable conversion of aryl Grignard reagents to phenols. It is a practical method, however, only when the diarylthallium trifluoroacetate precursor is formed via the Grignard route the alternative synthesis via symmetrization of arylthallium ditrifluoroacetates is obviously circuitous, since the latter compounds may be converted directly to phenols. 

Weyrauch, J.P., Wolfe, M., Prey, W. and Bats, J.W. (2005) Gold catalysis proof of arene oxides as intermediates in the phenol synthesis. Angewandte Chemie International Edition, 44, 2798. 

The Phenosolvan process is used in the treatment of wastewaters from plants involving phenol synthesis, coke ovens, coal gasification, low-pressure carbonization, and plastic manufacturing. The residual phenol content after dephenolization is usually in the range 5-20 mg dm and plants treating 500 m water per day are in operation. 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

Re Clusters in HZSM-5 Pores for Direct Phenol Synthesis 401... 

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]. 

Table 10.6 shows the catalytic performances of the selective benzene oxidation on the zeolite-supported Re catalysts under steady-state reaction conditions [107]. Catalyhc activity and selectivity largely depended on the types of zeolites and the preparation methods. The Re catalysts prepared by CVD of MTO exhibited higher catalyhc achvity and phenol selechvity than those prepared by the convenhonal impregnation method as supports (Table 10.6). Physical mixing of MTO with the supports provided poor phenol synthesis.

I 70 Advanced Design of Catalyst Surfaces with Metal Complexes for Selective Catalysis Table 10.6 Performances of Re/zeolite catalysts for direct phenol synthesis at 553 K". 

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. 

The catalytic reaction rate was first order with respect to O2 pressure in the po2 range 0-12 kPa at 12kPa the phenol selectivity was maximized. The activation energy for the phenol synthesis was estimated to be 24 kj mol" . 

The Re monomer was completely inactive for the mixture of benzene and O2. NH3 treatment of the Re monomers at 553 K generated the catalyhc achvity. After around 30 min of the NH3 treatment, phenol synthesis activity appeared and the phenol formahon rate dramatically increased between 40 and 60 min of the NH3 treatment, followed by a gentle rate rise upon further treatment The reaction rate saturated at 3.75 x 10 s after 120min. Further NH3 treatment longer than 120 min did not improve the catalyhc activity. Notably, the phenol selechvity kept almost constant (90.1-93.9%) during the NH3 treatment at 553 K. 

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. 

Further treatment with NH3 at 553 K promoted neither the catalytic performance nor further growth of the Re clusters. Thus the 120 min NH3 treatment gave the best performance of the Re-CVD/HZSM-5 catalyst for phenol synthesis. DFT calculations of Rejo cluster structures embedded in the pore of HZSM-5 revealed the most stabilized structure in the pore of HZSM-5 to be as shown in Scheme 10.4b. The edge-shared Rejo structure may be due to the structure and size of the pore ofthe HZSM-5 zeolite. A broader peak at 5 3.6 ppm than that of fresh HZSM-5 in the Al solid-state NMR spectrum implies a positive interaction between the A1 sites and the Reio clusters inside the pore of HZSM-5. 

Catalytically Active Structure and its Structural Transformation during the Phenol Synthesis... 

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

Acetophenone can be hydrogenated catalytically to 1-phenylethanol. It is obtained as a byproduct in the Hock phenol synthesis and is purified from the high-boiling residue by distillation. The quantitites obtained from this source satisfy the present demand. 

Phenol Synthesis A new method for obtaining arenes from easily available furans was reported by Hashmi et al. [19]. In this first paper, AuCl3 was used to produce a highly substituted phenol without side products. 

A phenol synthesis reaction induced by gold catalysts without steric limitations for the substituents was also reported [169]. These results provided a very helpful tool for organic synthesis of a large variety of derivatives such as biaryls, iso-chromanes, benzofurans, tetrahydroisoquinolines and other natural products [133, 170, 171]. 

In a joint study by Corma and Hashmi, heterogeneous gold catalysts based on nanogold on nanocerium oxide support were employed for phenol synthesis [172]. 

The metabolism of phenolics is regulated by the activity of various enzymes. As indicated above, the main and determining enzyme of phenolic synthesis is PAL, while in oxidation processes, the enzymes involved are peroxidase (POD) and primarily polyphenoloxidase (PPO). 

Phenol synthesis.2 This metal-carbyne (or the corresponding chromium-car-byne) reacts with diynes rapidly at 25° to form phenols in 15-55% yield. Examples ... 

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