catalysts Review One-Step Catalytic or Photocatalytic Oxidation of Benzene to Phenol: Possible Alternative Routes for Phenol Synthesis? Antonietta Mancuso 1, Olga Sacco 2,* , Diana Sannino 1 , Vincenzo Venditto 2 and Vincenzo Vaiano 1,* 1 Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II 132, 84084 Fisciano, Italy; [email protected] (A.M.); [email protected] (D.S.) 2 Department of Chemistry and Biology “A. Zambelli”, University of Salerno, Via Giovanni Paolo II 132, 84084 Fisciano, Italy; [email protected] * Correspondence: [email protected] (O.S.); [email protected] (V.V.); Tel.: +39-089-969362 (O.S.); +39-089-964006 (V.V.) Received: 9 November 2020; Accepted: 3 December 2020; Published: 5 December 2020 Abstract: Phenol is an important chemical compound since it is a precursor of the industrial production of many materials and useful compounds. Nowadays, phenol is industrially produced from benzene by the multi-step “cumene process”, which is energy consuming due to high temperature and high pressure. Moreover, in the “cumene process”, the highly explosive cumene hydroperoxide is produced as an intermediate. To overcome these disadvantages, it would be useful to develop green alternatives for the synthesis of phenol that are more efficient and environmentally benign. In this regard, great interest is devoted to processes in which the one-step oxidation of benzene to phenol is achieved, thanks to the use of suitable catalysts and oxidant species. This review article discusses the direct oxidation of benzene to phenol in the liquid phase using different catalyst formulations, including homogeneous and heterogeneous catalysts and photocatalysts, and focuses on the reaction mechanisms involved in the selective conversion of benzene to phenol in the liquid phase. Keywords: benzene; phenol; selective oxidation; homogeneous (photo)catalysts; heterogeneous (photo)catalysts 1. Introduction Phenol (hydroxybenzene) was discovered in coal tar and, under ambient conditions, appears as a white crystalline solid with a characteristic odor. It is an important industrial commodity, typically obtained from benzene. The use of phenol has been increasing due to its importance as a raw material from which to obtain other products [1]. The conversion of benzene to phenol possesses great relevance since phenol is a solvent and an intermediate of other kinds of industrial production, being a precursor of many materials and useful compounds [1]. Some examples of the usage of phenol are reported in the following: (1) PHENOLIC RESINS: by the reaction of phenol or substituted phenol with formaldehyde, phenol–formaldehyde resins or phenolic resins can be obtained. The first example was Bakelite as a commercial synthetic resin [2]. (2) POLYCARBONATES (a very pure phenol feed is required): polycarbonates are thermoplastic polymers containing carbonate groups in their chains [3]. (3) EPOXY RESINS: epoxy phenolic resins are resins modified at the phenolic hydroxyl group to include an epoxide functional group. This addition increases the ability of the resin to crosslink, creating a stronger polymer [4]. Catalysts 2020, 10, 1424; doi:10.3390/catal10121424 www.mdpi.com/journal/catalysts Catalysts 2020, 10, x 2 of 23 (3) EPOXY RESINS: epoxy phenolic resins are resins modified at the phenolic hydroxyl group to include an epoxide functional group. This addition increases the ability of the resin to crosslink, creating a stronger polymer [4]. Catalysts(4) INTERMEDIATE2020, 10, 1424 FOR CAPROLACTAM (nylon production): caprolactam is a monomer2 of 21for nylon production. Among the routes for its manufacture, one is via cyclohexanone and cyclohexanone oxime. Cyclohexanone can be prepared either from phenol or from cyclohexane. (4) INTERMEDIATEThe phenol route FOR is a CAPROLACTAM two-stage process, (nylon in wh production):ich the first caprolactam stage foresees is a monomerthe reaction for nylonamong production.phenol and Among hydrogen the in routes the presence for its manufacture, of a nickel catalyst one is viaat around cyclohexanone 180 °C to and form cyclohexanone cyclohexanol, oxime.subsequently Cyclohexanone dehydrogenated can be prepared at around either 400 °C from in the phenol presence or from of a cyclohexane.copper catalyst The to phenolyield the routecyclohexanone is a two-stage [5]. process, in which the first stage foresees the reaction among phenol and hydrogen in the presence of a nickel catalyst at around 180 ◦C to form cyclohexanol, subsequently The cumene process (also known as the Hock process) is the most important industrial process dehydrogenated at around 400 ◦C in the presence of a copper catalyst to yield the cyclohexanone [5]. for the simultaneous production of phenol and acetone starting from benzene and propylene [6]. OxygenThe cumene from air process and small (also amounts known asof thea radical Hock initiator process) are is the also most reactants important required industrial for the process process. forThe the cumene simultaneous process productionis very complex, of phenol as it consists and acetone of several starting stages, from passing benzene through and propylene the formation [6]. Oxygenof hydroperoxide from air and (a smallvery amountsreactive substance of a radical that initiator can give are runaway also reactants phenomena), required which for the allows process. the Theindirect cumene production process is veryof phenol complex, and as acetone it consists [7]. of The several process stages, has passing three throughmain reaction the formation steps plus, of hydroperoxidefurthermore, (aa verystep reactive of concentration substance that of cancumene giverunaway hydroperoxide, phenomena), and whichthey allowsare as thefollows: indirect (i) productionproduction of of phenol cumene; and (ii) acetone conver [7sion]. The of processcumene has to threecumene main hydrop reactioneroxide; steps (iii) plus, concentration furthermore, of acumene step of concentration hydroperoxide; of cumene(iv) hydrolys hydroperoxide,is of cumene and hydroperoxide they are as follows: [1]. (i) production of cumene; (ii) conversionThe scheme of cumene of the reactions to cumene is reported hydroperoxide; in Figure (iii) 1. concentration of cumene hydroperoxide; (iv) hydrolysis of cumene hydroperoxide [1]. The scheme of the reactions is reported in Figure1. Figure 1. The reaction steps for phenol production via the cumene process. Figure 1. The reaction steps for phenol production via the cumene process. The production of cumene (isopropylbenzene) is a Friedel–Crafts reaction and occurs by the The production of cumene (isopropylbenzene) is a Friedel–Crafts reaction and occurs by the reaction between benzene and propene, using an acid catalyst [8]. reaction between benzene and propene, using an acid catalyst [8]. In one process, benzene and propene (3:1 mole ratio) are passed over an acid catalyst. The excess In one process, benzene and propene (3:1 mole ratio) are passed over an acid catalyst. The benzene acts to limit the polyalkylations and the by-reactions of the oligomerization of propene. excess benzene acts to limit the polyalkylations and the by-reactions of the oligomerization of The zeolite is more environmentally friendly than traditional acid catalysts. The problems are related propene. The zeolite is more environmentally friendly than traditional acid catalysts. The problems to selectivity because isomers can be produced with respect to cumene [8]. are related to selectivity because isomers can be produced with respect to cumene [8]. The second step, the conversion of cumene to cumene hydroperoxide, involves the use of air to The second step, the conversion of cumene to cumene hydroperoxide, involves the use of air to give the hydroperoxide in the presence of small quantities of a radical initiator (benzoyl peroxide, give the hydroperoxide in the presence of small quantities of a radical initiator (benzoyl peroxide, for example) in slightly basic conditions. for example) in slightly basic conditions. The reaction is autocatalyzed by cumene hydroperoxide. The reaction is carried out at temperatures The reaction is autocatalyzed by cumene hydroperoxide. The reaction is carried out at between 77 and 117 ◦C and 1–7 atm pressure, to hold the system in the liquid phase. temperatures between 77 and 117 °C and 1–7 atm pressure, to hold the system in the liquid phase. After the concentration of the cumene hydroperoxide, performed usually with an evaporator at After the concentration of the cumene hydroperoxide, performed usually with an evaporator at the descendent film, the third and final reaction is the decomposition of cumene hydroperoxide by the descendent film, the third and final reaction is the decomposition of cumene hydroperoxide by mixing with sulfuric acid at 40–100 ◦C to give, after neutralization, phenol and propanone (acetone). mixing with sulfuric acid at 40–100 °C to give, after neutralization, phenol and propanone (acetone). Then, the products are separated through distillation columns. Then, the products are separated through distillation columns. The economics and effectiveness of this process are related to the market of acetone, apart from The economics and effectiveness of this process are related to the market of acetone, apart from phenol. Often, much more phenol is needed than the propanone that is produced at the same phenol. Often, much more phenol is needed than the propanone that is produced at the same time. time. Moreover, this multistage process has a low overall yield (less