Influence of the substituent on selective photocatalytic oxidation of aromatic compounds in aqueous TiO 2 suspensions { Giovanni Palmisano,* a Maurizio Addamo, a Vincenzo Augugliaro, a Tullio Caronna, b Elisa Garcı ´a-Lo ´pez, a Vittorio Loddo a and Leonardo Palmisano a Received (in Cambridge, UK) 9th November 2005, Accepted 4th January 2006 First published as an Advance Article on the web 23rd January 2006 DOI: 10.1039/b515853b Experimental results are reported showing that the photo- catalytic oxidation of aromatic compounds containing an electron-donor group (EDG) gives rise mainly to ortho- and para-monohydroxy derivatives while in the presence of an electron-withdrawing group (EWG) all the monohydroxy derivatives are obtained. Photocatalytic oxidation of aromatic compounds has been widely studied, 1–10 while photocatalytic syntheses represent an incoming research field. 11 In this work a systematic attempt was made to understand the influence of the substituent group on the selectivity of photo-oxidation of the aromatic molecules to hydroxylated compounds, in the presence of polycrystalline TiO 2 . Photocatalytic runs were performed in liquid–solid regime to oxidise aromatic compounds containing an EDG (phenol, phenylamine, and N-phenylacetamide) or an EWG (nitrobenzene, cyanobenzene and 1-phenylethanone). The photocatalyst used was commercial anatase TiO 2 (Merck) with an amount of 0.4 g l 21 and the pH was the natural one (ca. 6.5). The experiments lasted 1.5 h and were performed in a batch photoreactor of cylindrical shape, containing 0.5 l of aqueous suspension, with pure O 2 continuously bubbling. A 125 W medium-pressure Hg lamp (Helios Italquartz, Italy) was axially immersed within the photoreactor and it was cooled by water circulating through a Pyrex thimble. The temperature of the suspension was about 300 K. The radiation energy impinging on the suspension had an average value of 10 mW cm 22 . The initial concentration of the substrates was ca. 500 mM. The extent of adsorption in the dark was measured by monitoring the concentration of the substrates before adding the catalyst and after 1 h of contact with it, when the physical equilibrium was achieved. During the photocatalytic runs, samples for analyses were withdrawn at fixed intervals of time and the catalyst was immediately separated from the aqueous solution by filtering through 45 mm Millex Millipore filters. All the monohydroxylated species produced during the reactions were analyzed with a HPLC Beckman Coulter (System Gold 126 Solvent Module and 168 Diode Array Detector), equipped with a Luna 5 m Phenyl-Hexyl column (250 mm long 6 2 mm i.d.); the flow rate was 0.2 ml min 21 and the identification was made by comparison with authentic samples. TOC (total organic carbon) measurements were carried out by using a TOC Shimadzu 5000 A analyser. It is important to point out that two competing pathways were present from the starting of irradiation: substrate degradation that produced CO 2 and formation of monohydroxylated species released into the bulk of the solution. Particular care was devoted to determine the concentrations in the liquid phase of each hydroxylation product (ortho, meta and para). The alternative oxidation route to complete mineralization was reported in literature for toluene 6 and it should be stressed that it occurs at various extents from the beginning of irradiation when the molecules of substrates are not very big, as in our case. The evolution to CO 2 occurs through various oxidation steps involving unknown intermediates species strongly (photo)adsorbed onto TiO 2 surface without their release into the bulk of solution. An experimental evidence of the occurrence of this parallel and kinetically very fast process derives from the observation of TOC results showing a decrease from the very beginning of irradiation (see, for example, Fig. S1 in ESI{). The results obtained with an aromatic compound containing an EDG and an EWG are reported in Fig. 1(a) and (b), respectively. Fig. 1(a) reports the concentration values of nitrobenzene and its monohydroxylated products vs. irradiation time, while Fig. 1(b) reports the results obtained with N-phenylacetamide. It may be observed that the aromatic molecules were oxidised in both cases and the concentrations of their oxidation products increased with irradiation time. The whole molar yields of the three hydroxylated species (OH-derivatives produced divided by the reacted substrate) were ca. 20% and ca. 60% for nitrobenzene and N-phenylacetamide, respectively. Inside the investigated irradiation time, the monohydroxylated isomers were the main reaction products, with yields in the 50–75% range when the photocatalytic reaction was carried out by using an aromatic compound containing an EDG. On the contrary, the yields in monohydroxylated isomers were in the 20–30% range for the aromatic compounds containing an EWG. It is worth noting that oxidation of the –COCH 3 and –NHCOCH 3 substituents (although in lesser extent) could also take place during the photocatalytic process as unidentified peaks were observed in the chromatograms. Table 1 summarizes the obtained results for all of the studied compounds; it reports the amounts of substrates adsorbed in the dark, the conversions and yields in monohydroxylated isomers along with the HPLC-eluents appropriately chosen. a Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita ` degli Studi di Palermo, Viale delle Scienze, 90128, Palermo, Italy. E-mail: giovanni_palmisano@yahoo.it; palmisano@dicpm.unipa.it; Fax: +39 091 6567280; Tel: +39 091 6567246 b Universita ` di Bergamo, Facolta ` di Ingegneria, Via Marconi 5, I – 24044, Dalmine (Bergamo), Italy { Electronic supplementary information (ESI) available: TOC trend during 4 (see Table 1) photo-oxidation; graphics with the concentrations of compounds 1, 2, 5, 6 (see Table 1) and intermediates during their photocatalytic oxidations. See DOI: 10.1039/b515853b COMMUNICATION www.rsc.org/chemcomm | ChemComm 1012 | Chem. Commun., 2006, 1012–1014 This journal is ß The Royal Society of Chemistry 2006