Selective photocatalytic oxidation of benzene for the synthesis of phenol using engineered Au–Pd alloy nanoparticles supported on titanium dioxide †

The selectivity of photocatalytic phenol production from the direct oxidation of benzene can be enhanced by fine adjustment of the morphology and composition of Au–Pd metal nanoparticles supported on titanium dioxide thereby suppressing the decomposition of benzene and evolution of phenolic compounds.

The selectivity of photocatalytic phenol production from the direct oxidation of benzene can be enhanced by fine adjustment of the morphology and composition of Au-Pd metal nanoparticles on titanium dioxide thereby suppressing the decomposition of benzene and evolution of phenolic compounds.
Phenol, as an important precursor in the production of numerous resins and drugs, is produced on a large scale by the petroleum industry and the demand for phenol keeps increasing rapidly. 1 Conventional synthesis of phenol involves the partial oxidation of isopropylbenzene (cumene) via the Hock rearrangement. 1 Though mild reaction conditions and relatively inexpensive raw materials are required to operate the synthesis, the excessive amount of acetone produced as a by-product is a problem.Since the cumene precursor is mainly produced by the Friedel-Crafts alkylation of benzene, 2 a synthesis route that directly converts benzene to phenol would be ideal to replace the indirect partial oxidation process. 3,4hilst benzene can be easily oxidised in air at high temperature even in the absence of a catalyst, the phenol produced is further oxidised to CO 2 at a much faster rate, which limits the potential conversion rate and selectivity.The utilisation of oxidants (e.g., H 2 O 2 and N 2 O) in combination with heterogeneous catalysts has shown promising conversion rates and selectivity to phenol. 3,4However, the use of such expensive oxidants has limited progress in the direct oxidation process.
Heterogeneous photocatalysis has shown huge potential in organic synthesis and can provide an alternative approach for the synthesis of phenol. 5,6The OH radicals can be generated on the surface of a photocatalyst under suitable irradiation conditions, suggesting that no additional oxidation reagents are required for the conversion of benzene to phenol.However, the photo-generated radical species generally show poor selectivity, which results in the complete decomposition of benzene to CO 2 and several unwanted phenolic compounds (e.g., hydroquinone). 7Moreover, complete decomposition of phenolic compounds is generally faster than that of phenol, which may decrease the selectivity to phenol indirectly. 8herefore, it is of great interest to promote the formation of phenol and ''switch-off'' other undesired reactions (see Scheme 1).Although the selectivity can be tuned by the addition of oxidants or tweaking the reaction conditions (i.e., solvent identity, addition of phenol), [9][10][11] tuning the surface properties of the photocatalyst seems to be the most promising and intuitive approach to improve phenol formation. 12,13Here we show that exercising a degree of control over the co-catalyst nanoparticle (NP) morphology supported on TiO 2 can significantly tune the product selectivity for photocatalytic benzene oxidation.
The co-catalysts investigated were a systematic series of metal NPs ranging from monometallic Au and Pd to random Au-Pd alloys and core-shell configurations, which were synthesised via colloidal methods and subsequently supported on a TiO 2 (Degussa P25) photocatalyst by a sol-immobilisation process. 14All the metal/TiO 2 samples have a nominal metal loading of 1 wt%, and some  14 We have carried out benzene oxidation in the liquid phase at room temperature (catalyst loading of 5 mg L À1 ) with all metal/ TiO 2 photocatalysts described above.A well-defined UV LED light source was used for all photocatalytic reactions (Fig. S3 in ESI †). 14 The evolution of liquid phase and gas phase products was analysed by in situ UV-vis and mass spectrometry, respectively, as summarised in Fig. 2 (Fig. S4-S6 in ESI †). 14 Additionally, ultra high performance liquid chromatography coupled by an electrospray ionisation inlet to a quadrupole time-of-flight mass spectrometry (UHPLC-ESI-QTOF-MS) analysis confirmed that the products were mainly phenolic compounds (phenol and hydroquinone) with short-chain carboxylic acids as minor impurities (Fig. S7 in ESI †). 14 By monitoring the sextet of peaks at l B 250 nm, it was evident that benzene oxidation took place as soon as UV irradiation was commenced for all photocatalysts (Fig. 2(a)).However, the spectra varied significantly depending on the composition and morphology of the co-catalyst NPs (Fig. S5 in ESI †). 14 When bare TiO 2 was used for benzene oxidation, phenol (indicated by peak P, l max = 270 nm) and hydroquinone (indicated by peak H, l max = 289 nm) were the main product species detected in the liquid phase.In addition, some polymerisation of the phenolic compounds was observed, resulting in an increase of absorption at l 4 300 nm.It was found that hydroquinone formation and further polymerisation of phenolic compounds could be suppressed by using Au as a co-catalyst on the TiO 2 , but the generation of phenol was also unfortunately inhibited.Interestingly, by replacing the monometallic Au co-catalyst with Au shell -Pd core NPs, an optimum evolution of phenol was achieved while simultaneously hindering the formation of hydroquinone.Furthermore, the polymerisation processes were found to be negligible when metal co-catalysts were employed.We also compared the complete decomposition of benzene and phenol during photo-oxidation for all samples by following the CO 2 evolution, as shown in Fig. 2(b) and (c), respectively.The CO 2 evolution from benzene can be strongly suppressed by the presence of metal NPs on the TiO 2 surface (Fig. 2(b)), suggesting that the metal NPs not only facilitate the electron-hole separation but are also involved in the photo-oxidation reactions, where the partial oxidation of benzene was improved and the complete decomposition of benzene was inhibited. 14,15Surprisingly, the co-catalyst, irrespective of the variant used, has a negligible effect on reduction of the complete oxidation of phenol when compared to that of the pristine TiO 2 (Fig. 2(c)).
Fig. 3(a) and (b), based on data extracted from UV-vis spectra, depict the effectiveness of the various co-catalyst NPs in evolving phenol and hydroquinone respectively (Fig. S6 and Table S1 in ESI †). 14 Whilst monometallic Au and Pd NPs decreased the phenol evolution rates, the Au-rich and Pd-rich alloy NPs (i.e., Au 7 Pd 1 , Au 3 Pd 1 , Au 1 Pd 3 , and Au 1 Pd 7 ) exhibited enhanced phenol generation rates compared to that of pure TiO 2 .Note however that the hydroquinone evolution rates remained similar to that of pure TiO 2 for the same sub-set of photocatalysts.Surprisingly, the Au 1 Pd 1 alloy NPs supported on TiO 2 showed the worst performance in terms of generation of both phenol and hydroquinone.We also discovered that the selective oxidation of benzene depends on the nanostructure on the metal NPs.The Au shell -Pd core NPs promoted phenol production and simultaneously decreased  This journal is © The Royal Society of Chemistry 2014 the formation of hydroquinone, whereas in comparison the Pd shell -Au core NPs quenched both reactions.The inhibition of hydroquinone formation may improve the selectivity to phenol as phenolic compounds are less stable than phenol. 8We further plotted the phenol-to-hydroquinone ratio generated as a function of the co-catalyst composition/morphology (Fig. 3(c)).Noticeably, the Au shell -Pd core NPs supported on TiO 2 achieved the highest selectivity of the liquid phase products compared to the other co-catalyst formulations.Furthermore, we found that the production rate of phenol can be further improved by increasing the loading of the photocatalyst (see Fig. S8 in ESI †). 14 Although the photocatalytic conversion of benzene is still relatively low (B30%), it shows good potential for future development and use in the direct synthesis of high purity phenol.
In summary, an oxidative photocatalysis approach for the direct synthesis of phenol from benzene has been demonstrated.Furthermore, the need to engineer the metal co-catalyst composition and morphology has been shown to be critical for fine tuning the selectivity of this benzene oxidation reaction.The presence of co-catalysts has also been shown to have beneficial effects on inhibiting (i) the polymerisation of phenolic products and (ii) the complete oxidation of benzene.Specifically, the Au shell -Pd core variant NPs supported on TiO 2 exhibited the best photocatalytic performance by simultaneously increasing the phenol evolution rate and decreasing the hydroquinone generation rate.We anticipate that in-depth investigations on the role of metal co-catalysts will further boost the performance of photocatalysed selective oxidation reactions.

Scheme 1
Scheme 1 Photocatalytic oxidation pathways of benzene.The crosses indicate the unwanted reaction pathways.

Fig. 1
Fig. 1 Representative HAADF-STEM images of (a) Au, (b) Au 1 Pd 1 random alloy, (c) Pd shell -Au core and (d) Au shell -Pd core NPs sol-immobilised on TiO 2 .The Au shell -Pd core and Pd shell -Au core NPs are labelled as Au S Pd C and Pd S Au C , respectively.

Fig. 2
Fig. 2 (a) Representative UV-vis spectra time sequences recorded during benzene oxidation using pristine TiO 2 , Au/TiO 2 and Au shell -Pd core /TiO 2 , respectively, as photocatalysts.P and H denote phenol and hydroquinone, respectively.(b) and (c) Photocatalytic CO 2 evolution from both benzene and phenol oxidation using pristine TiO 2 and various metal/TiO 2 photocatalysts.The Au shell -Pd core and Pd shell -Au core NPs are labelled as Au S Pd C and Pd S Au C in all figures.