Catalysis Communications 58 (2015) 132–136

Contents lists available at ScienceDirect

Catalysis Communications

j ourna l homepage: www.e lsev ie r .com/ locate /catcom

Short Communication

Large size Pd NPs loaded on TiO2 as efficient catalyst for the aerobicoxidation of alcohols to aldehydes

Kun Liu, Xiaojun Yan, Peipei Zou, Yuanyuan Wang ⁎, Liyi Dai ⁎Department of Chemistry, East China Normal University, Shanghai 200241, PR China

⁎ Corresponding authors. Tel.: +86 21 54340133.E-mail addresses: yywang@chem.ecnu.edu.cn (Y. Wan

(L. Dai).

http://dx.doi.org/10.1016/j.catcom.2014.09.0231566-7367/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 May 2014Received in revised form 5 September 2014Accepted 12 September 2014Available online 19 September 2014

Keywords:PdTiO2

Alcohol oxidationPreparation method

Anti-sintering Pd/TiO2 catalysts have beenpreparedbydifferentmethods and systematically characterized. Com-pared with the catalysts prepared by the heterogeneous deposition precipitation (HDP) method and wetnessimpregnation (WI)method, the catalyst prepared by the deposition precipitation (DP)method could give highercontent of Pd species. Pd/TiO2 prepared by DPmethod shows much higher activity than other catalysts in liquidphase oxidation of benzyl alcohol to benzyl aldehyde. The average particle size of Pd nanoparticles (NPs) is15 nm. XPS results indicate that though Pd2+ species have been reduced to Pd0 after use, the activity of catalystis not affected, which has never been reported in the literature.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The selective oxidation of alcohols to aldehydes or ketones is oneof the most important transformations in organic chemistry, whichhas wide applications in perfumery, pharmaceutical, dyestuff and agro-chemical industries [1]. In traditional methods, there are large amountsof toxic co-products produced togetherwith aldehydes or ketones usingmetal oxidants, such as dichromate and chromium trioxide [2], so themethods based on toxic or expensive inorganic oxidants to greenerand more atom-efficient methods which adopt recyclable catalyst andO2 as oxidant are exigently needed [3]. Many supported noble metalcatalysts have been explored for the oxidation of alcohols in liquid-phase [3], such as Pt [4], Au [5], Ru [6] and Pd [7]. Among these catalysts,Pd catalysts appear to be promising ones due to high selectivity andconversion could be obtained synchronously [7].

Despite extensive researches on Pd catalysts for alcohol selectiveoxidation, the active Pd species is disputed and many problems stillneed to be resolved. Firstly, the size of Pd NPs is smaller than 10 nm inorder to achieve high activity, which is liable to be sintered. Pd speciesis reconstructed after used, such as Pd/Al2O3 or Pd/TiO2 (the activePd2+ species is reduced to Pd0 after the reaction), which should be cal-cinated before the next use [7]. Furthermore, the preparation process oforganic–inorganic catalyst is intricate, such as SBA-16 supported palla-dium–guanidine complex, and the structure of mesoporous SBA-16is prone to be destroyed in acid or base conditions [8]. Thirdly, the

g), lydai@chem.ecnu.edu.cn

selectivity is lowwhen using coreshell catalysts, such as toluene, benzoicacid and benzyl benzoate are produced together using supported Pd{Au}core–shell NPs [9]. Lastly, Mondelli et al. find that the formation of car-boxylates from benzaldehyde hydration/oxidation is largely hinderedwhen using PdBi/Al2O3 as catalyst, but Pd content is 5% [10]. So it is ofgreat importance to investigate supported Pd catalysts to overcome theproblems mentioned above.

In order to solve the sintering problem, large size Pd NPs loaded onTiO2 are prepared by different methods and characterized by XRD,XPS, ICP, CO-Chemisorption and TEM. The difference between the cata-lysts and the catalytic performances is also illustrated in this paperthoroughly.

2. Experimental

2.1. Materials and methods

TiO2 was purchased from Fluka, whichwas calcined at 300 °C for 4 hbefore use. PdCl2 (A. R.), Pd (acac)2 (A. R.), benzyl alcohol (C. P.), 3-OMebenzyl alcohol (C. P.), 3-Cl benzyl alcohol (C. P.), n-butyl alcohol (C. P.),dibenzyl alcohol (C. P.) and benzyl ethanol (C. P.) were obtained fromSinopharm Chemical Reagent Co. Ltd. Broane tributylamine (A. R.) andoleylamine (A. R.) were purchased from Sigma-Aldrich. Other materialswere supplied by local companies.

X-ray diffraction patterns (XRD) of the catalysts were performed ona Bruker D8 diffractometer with Cu-Kα radiation from 0.5° to 90° witha scan speed of 1 °/min. X-ray photoelectron spectroscopy (XPS) mea-surements were obtained on a PHI-5500 spectrometer with Al Kα X-ray radiation as the X-ray source for excitation. Transmission electronmicroscopy (TEM) was performed on a JEOL 2100F instrument

Table 2Catalytic properties of Pd/TiO2 using various substrates.

Entry Substrate Conv. (%) Sel. (%)

1 3-OMe benzyl alcohol 46 992 3-Cl benzyl alcohol 35 993 n-Butylalcohol 2 984 di-Benzyl alcohol 51 985 Benzyl ethanol 3 98

Reaction conditions: catalyst (50 mg), benzyl alcohol (46.3mmol), O2 bubbling rate (20 mL/min), and temperature (120 °C).

133K. Liu et al. / Catalysis Communications 58 (2015) 132–136

operating at 30kV. N2 adsorption–desorption isothermsweremeasuredwith a BELSORP mini II analyzer at liquid N2 temperature. Surface area(SBET) was calculated by the BET method, and average pore size(D) was determined from the desorption isotherms using the Barrett–Joyner–Halenda (BJH) method. Inductively coupled plasma atomicemission spectroscopy (ICP-AES) measurements were performed onthe Thermo Scientific iCAP 6300 instrument. Gas chromatography(GC) was performed on GC-2014 with Rtx-5 capillary column. CO-Chemisorption was performed on Autochem 2910 instrument.

2.2. Synthesis of the catalysts

2.2.1. WI methodExactly 1 g TiO2 was immersed in an aqueous solution containing

0.944mmol PdCl2 under stirring until the solution became clear. The fil-trate was filtered off and washed with water. Finally, the product wasdried at 60 °C for 10 h and calcined at 300 °C for 3 h. The catalyst wasdenoted as Pd/TiO2-WI for short [11].

2.2.2. DP methodExactly 1 g TiO2 was dispersed into an aqueous solution containing

0.944 mmol PdCl2 at 70 °C. Under stirring, 10 mL of aqueous solutioncontaining 10mmol Na2CO3was added dropwise to the suspension. Fi-nally, the catalyst was filtered off and washed with water after aging at100 °C for 6 h, which was dried at 60 °C for 10 h and calcined at 300 °Cfor 3 h. The sample was abbreviated as Pd-TiO2-DP for short [11].

2.2.3. HDP methodExactly 1 g TiO2 was added into an aqueous solution containing 10

mmol of urea and 0.944 mmol PdCl2. The suspension was transferredinto an oil bath preheated to 100 °C, and held at this temperature for6 h. The filtrate was filtered off and washed with water. Finally, theproduct was dried at 60 °C for 10 h and calcined at 300 °C for 3 h. Thesample was designated as Pd/TiO2-HDP [11].

2.2.4. Collide methodAs motioned in other literature [12], 0.25 mmol Pd(acac)2 was

mixedwith 15mL oleylamine under argon flow. The solution was heat-ed to 60 °C and 3.45 mmol broane tributylamine complex was added tothe system. The temperature was raised to 90 °C at 1 °C/min. The solu-tion was cooled room temperature and 30 mL ethanol was added, and

Table 1Oxidation of benzyl alcohol catalyzed by Pd/TiO2.

Entry Catalyst Conv. (%) Sel. (%) Metal loadings (wt%)g

1a Pd/Al2O3-WI 35 85 –

2a Pd/SiO2-WI 3 93 –

3a Pd/TiO2-WI 30 95 0.224a Pd/TiO2-DP 65 96 0.725a Pd/TiO2-HDP 34 94 0.206a Pd/TiO2-C 60 96 0.757b Pd/TiO2-DP 63 96 0.728c Pd/TiO2-DP 62 96 0.729 [13]d PdAu/MSNs 98 95 2.8810 [14]e Pd@Ni/MWCNTs 94 90 2011f Pd/TiO2-DP 64 96 0.7212 – 3 98 0

a Catalyst (50 mg), benzyl alcohol (46.3 mmol), O2 bubbling rate (20 mL/min), tem-perature 120 °C, 3 h.

b The catalyst reused.c The reused catalyst calcined in air at 300 °C.d mBzoH/mMetal = 1000/1, 90 °C, O2 pressure: 5 atm, 1 h.e Benzyl alcohol 2 mmol, K2CO3 6 mmol, catalyst (Pd: 0.2 mmol),water, H2O2 6 h, K2CO3:

100 °C, 8 h.f The catalyst was reduced with H2 at 300 °C.g The metal loading was detected by ICP-AES.

the product was separated by centrifugation. Pd NPs were dispersedinto pentane.

10 mg Pd NPs were added to TiO2 and sonicated for 2 h. After evap-oration of pentane, 20 mL acetic acid was added to the mixture andstirred for 8 h. Finally, the product was filtered and dried at 60 °C for10 h, which was calcined at 300 °C for 3 h. The catalyst was denotedas Pd/TiO2-C for short.

2.3. The test of catalytic activity

The oxidation of benzyl alcohol over heterogeneous Pd/TiO2 cata-lysts was performed in 10mL bottle usingmolecular oxygen as oxidant.In each experiment, 50mg catalystwas added into 46.3mmol benzyl al-cohol. Themixture was heated to 120 °C for 3 hwith oxygen bubbled inat a flow rate of 20 mL/min. After the system cooled to room tempera-ture, the catalyst was removed from the reactionmixture by centrifuga-tion and the products were analyzed by GC. The catalyst was washedwith ethanol and dichloromethane for 3 times and dried at 60 °C in vac-uum for the next use.

3. Results and discussion

3.1. Oxidation of benzyl alcohol using Pd/TiO2 prepared by differentmethods

Catalytic performance and the amount of Pd loadings of Pd/TiO2 pre-pared by different methods were shown in Table 1. Conversion of ben-zyl alcohol and selectivity of benzyl aldehyde catalyzed by the catalystPd/TiO2, Pd/SiO2 and Pd/Al2O3 prepared by WI method were 30%, 95%,3%, 93%, 35% and 85% respectively (Table 1, entries 1–3). Given thatthe acid property of Al2O3 that may accelerate the reaction rate of sidereactions, TiO2 was chosen as support. Pd/TiO2-DP showedmuch higherconversion compared with Pd/TiO2-WI and Pd/TiO2-HDP (Table 1,entries 3–5). Pd–TiO2–C also exhibited similar conversion (Table 1,entry 6), but the preparation procedure was long and tedious. Conver-sion was unchanged whether the reused catalyst Pd/TiO2 was calcinedor not (Table 1, entries 7 and 8). Metal loadings of Pd/TiO2 preparedby the methods mentioned above were lower than that of PdAu/MSNs[13] and Pd@Ni/MWCNTs [14]. The oxygen pressure in these catalystswas 5 atm and H2O2 was used as an oxidant (Table 1, entries 9and 10). The conversion of benzyl alcohol was nearly unchangedwhen the Pd/TiO2 was reduced (Table 1, entry 11). The conversionwas low when no catalyst was added to the system (Table 1, entry

Table 3Catalytic reusability of Pd/TiO2 for the oxidation of benzyl alcohol.

Run 1 2 3 4 5

Con. (%) 65 63 65 62 66Sel. (%) 96 95 96 95 96

Reaction conditions: catalyst (50 mg), benzyl alcohol (46.3 mmol), O2 bubbling rate20 mL/min, temperature 120 °C, 3 h.

Fig. 2. XRD patterns of the catalysts. a: Pd/TiO2-WI, b: Pd/TiO2-DP, c: Pd/TiO2-HDP,d: Pd/TiO2-DP used.

134 K. Liu et al. / Catalysis Communications 58 (2015) 132–136

12). ICP experiment indicated that Pd contents in the catalyst Pd/TiO2-DP, Pd/TiO2-WI and Pd/TiO2-HDP were about 0.7%, 0.2% and 0.2% re-spectively. Since the Pd content in the catalysts was different, weadded theweight of Pd/TiO2-WI and Pd/TiO2-HDP.When thePd contentin the system was adjusted to the same level, the conversion of benzylalcohol was close to each other (ESI, Table S1). Compared with this se-ries of catalysts, Pd/TiO2-DP exhibited strong performance in the benzylalcohol oxidation reaction that may be due to the high loadings of Pdspecies.

3.2. Oxidation of other substrates catalyzed by Pd/TiO2-DP

Table 2 illustrated catalytic results of different substrates withmolecular oxygen catalyzed by Pd/TiO2-DP. Oxidation of aromatic alco-hols, such as 3-OMe benzyl alcohol, 3-Cl benzyl alcohol and di-benzylalcohol gave well conversions to the corresponding aldehydes (99%,99% and 98%, entries 1, 2 and 4, respectively). While, aliphatic alcohols,such as benzyl ethanol and n-butylalcohol were not oxidized efficientlyunder the same conditions (Table 2, entries 3 and 5).

3.3. Catalytic reusability of Pd/TiO2-DP

Table 3 showed the catalytic recycling properties for Pd/TiO2-DP,revealing only a marginal decrease in conversion and selectivityafter 5 times. Hot filtration test indicated that the conversion wasunchanged after the catalyst separated from the system (ESI, Fig. S2).The Pd/TiO2-DP behaved as a heterogeneous catalyst for the oxidationof benzyl alcohol with O2, which could be reused conveniently afterthe reaction.

3.4. Influence of reaction time on benzyl alcohol conversion

Fig. 1 summarized the conversion of benzyl alcohol at different reac-tion times. After the conversion reached 65%, conversion and selectivitywere nearly unchanged, indicated marginal benzyl acid or benzylbenzoate produced. The kinetic data showed first order kinetics. XPSindicated that Pd2+ was reduced to Pd0. However, conversion was un-changed whether the reused catalyst calcined or not (Table 1, entries7 and 8). In order to study mass transfer limitations, we changed thestirring speed, the conversion of benzyl alcohol and the selectivity ofbenzyl aldehyde were nearly unchanged in the range of speed from500 r/min to 1700 r/min (ESI, Fig. S1). ICP experiment illustrated thatPd species was not leached from the catalyst (Table 1, entries 4 versus7). It was possible that the reaction was run under kinetic limitationsrather than mass transfer limitations [2].

Fig. 1. Effect of reaction time on catalytic performance of Pd/TiO2-DP.

3.5. Characterization of the catalysts

3.5.1. XRD patternsXRDpatterns of the catalystswere shown in Fig. 2. The fresh samples

showed intense peaks corresponding to anatase and rutile. The peakscorresponding to Pd0 or Pd2+ species were ambiguous because Pdspecies on the samples were highly dispersed or the content of Pd wasbelow the detection limit of XRD (Fig. 2a, b and c). After reaction, thepeaks corresponding to Pd0 or Pd2+ were also ambiguous and thecontents of anatase and rutile were unchanged (Fig. 2d), so the phasetransformation was excluded.

3.5.2. Nitrogen adsorption–desorption and CO-chemisorptionNitrogen adsorption–desorption and CO-Chemisorption textural pa-

rameters of the catalystswere shown in Table 4. The catalysts presenteddecreased surface area compared with TiO2, which indicated that PdNPs were adsorbed on the surface of TiO2. Surface area, TiO2 particlesize and dispersion of Pd NPs were similar to each other.

3.5.3. TEMFig. 3 showed TEM images of the catalysts. Pd NPs distributed on the

surface of anatase or rutile randomly and average particle size of Pd NPswas about 15 nm. The particle size of Pd NPs on the catalyst Pd/TiO2

reused was unchanged compared with the fresh catalyst. As we know,small Pd NPs were liable to sinter, but in our experiment, large Pd NPswere not sintered and the reused catalyst remained high catalyticactivity in alcohols oxidation reaction.

Table 4Textural parameters of the catalysts.

Catalyst SBET (m2/g)b D (nm)c DPd (nm)d DPd (nm)e Dispersionof Pdf

Pd/TiO2-WI 47.2 21.3 – 15.2 0.18Pd/TiO2-DP 45.8 21.3 – 16.7 0.16Pd/TiO2-HDP 46.7 21.3 – 14.2 0.20Pd/TiO2-DPa 43.6 21.3 – 13.5 0.15TiO2 50.1 21.3 – – –

a The catalyst reused.b Surface area (SBET) was calculated by the BET method.c Particle size of TiO2 (D) was calculated by Scherrer equation.d Particle size (DPd) was below the detection limit of XRD.e Particle size of PdNPs (DPd)was calculated by TEM(estimated bymore than 100NPs).f Dispersion of Pd species was calculated by CO-Chemisorption.

Fig. 3. TEM images of the catalysts prepared by three different methods. A: Pd/TiO2-WI, B: Pd/TiO2-DP, C: Pd/TiO2-HDP, D: Pd/TiO2-DP reused.

135K. Liu et al. / Catalysis Communications 58 (2015) 132–136

3.5.4. XPSFig. 4 showed the XPS spectra of Pd 3d region of Pd/TiO2 prepared by

three different methods. The peaks of binding energy were observed at336.6 and 341.9 eV corresponding to Pd2+ 3d5/2 and Pd2+ 3d3/2 [13],respectively. Since the loading of Pd in Pd/TiO2-WI and Pd/TiO2-HDPwas much lower than Pd/TiO2-DP, Pd 3d intensity of Pd/TiO2-WI andPd/TiO2-HDPwas relatively lower. Fig. 5 showed theXPS of Pd 3d regionof reused Pd/TiO2-DP. For the fresh catalyst, most species were Pd2+,for the reused catalyst, the peaks of binding energy were observed at334.9 and 340.1 eV corresponding to Pd0 3d5/2 and Pd0 3d3/2 [15],respectively. Though Pd2+ was reduced to Pd0 in our experiment, con-version was unchanged whether the reused catalyst was calcined or

Fig. 4. XPS spectra of the catalysts prepared by different methods.

not (Table 1, entries 7 and 8) and the reduced catalyst maintained thesame activity (Table 1, entry 12).

4. Conclusions

In conclusion, we have successfully prepared a series of Pd/TiO2

catalysts by different methods and applied them in the oxidation ofbenzyl alcohol. TEM indicates that the average size of Pd NPs is about15 nm. Surface area and Pd dispersion are close to each other. However,ICP result testifies that Pd content of Pd/TiO2-DP is much higher thanthat of Pd/TiO2-WI and Pd/TiO2-HDP, which contribute to the higher

Fig. 5. XPS spectra of the fresh catalyst Pd/TiO2-DP and the corresponding catalyst reused.

136 K. Liu et al. / Catalysis Communications 58 (2015) 132–136

activity of Pd/TiO2-DP. XPS indicates that Pd species undergo recon-struction after Pd/TiO2-DP used and TEM indicates that Pd species isnot sintered. The catalytic activity is unchanged whether Pd species re-duced or not. This is the first report about alcohol oxidation reactionusing large size Pd NPs as catalyst and relevant investigation underwentin our laboratory.

Acknowledgments

The authors acknowledge the financial support from the key projectof Shanghai Science and Technology Committee (No. 11JC1403400).

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.catcom.2014.09.023.

References

[1] K. Liu, T.T. Chen, L.Y. Dai, Catal. Lett. 14 (2014) 314–319.[2] K. Liu, Z.X. Chen, L.Y. Dai, Catal. Lett. 14 (2014) 935–942.[3] G.F. Zhao, H.Y. Hu, Y. Lu, Chem. Commu. 47 (47) (2011) 9642–9644.[4] G.L. Li, L.H. Jiang, G.Q. Sun, Int. J. Hydrogen Energy 38 (2013) 12767–12773.[5] G.F. Zhao, M. Ling, Y. Lu, Green Chem. 13 (2011) 55–58.[6] C. Mondelli, D. Ferri, A. Baiker, J. Catal. 258 (2008) 170–176.[7] X.M. Wang, G.J. Wu, L.D. Li, Appl. Catal. B Environ. 115–116 (2012) 7–15.[8] H.Q. Yang, X.J. Han, F.X. Ji, Green Chem. 12 (2010) 441–451.[9] D.I. Enache, J.K. Edwards, G.J. Hutchings, Science 311 (2006) 362–365.

[10] C. Mondelli, D. Ferri, A. Baiker, J. Catal. 252 (2007) 77–87.[11] L.F. Chen, P.J. Guo, M.H. Qiao, Appl. Catal. A Gen. 356 (2009) 129–136.[12] V. Mazumder, S.H. Sun, J. Am. Chem. Soc. 131 (2009) 4588–4591.[13] X. Yang, C. Huang, L. Du, Appl. Catal. B Environ. 140–141 (2013) 419–425.[14] M.M. Zhang, Q. Sun, M. Chen, Aust. J. Chem. 66 (2013) 564–571.[15] J.F. Moulder, W.F. Stickle, K.D. Bomben, Handbook of X-Ray Photoelectron

Spectroscopy, Physical Electronics, Chanhassen, MN, 1995.