Preparation and Catalytic Application of Pd Loaded Titanate Nanotube: Highly Selective α Alkylation of Ketones with Alcohols

Preparation and Catalytic Application of Pd Loaded Titanate Nanotube:

Highly Selective α Alkylation of Ketones with Alcohols

Jum Suk Jang,^a Min Serk Kwon,^†,a Hyun Gyu Kim,^‡ Jae Wook Park,^†,* and Jae Sung Lee^*

Department of Chemical Engineering and School of Environmental Science and Engineering,

Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea. ^*E-mail: jlee@postech.ac.kr

†Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea

*E-mail: pjw@postech.ac.kr

‡Busan Center, Korea Basic Science Institute (KBSI), Busan 609-735, Korea Received January 19, 2012, Accepted February 14, 2012

The titanate nanotube (TNT) was hydrothermally synthesized in 10 M NaOH aqueous solution at 150 ^oC for 72 h. Titanate nanotube with high surface area (292 m²/g) is a good candidate as a support for catalytic reaction or organic synthesis. Palladium nanoparticles with an average size of ca. 3 nm were well dispersed onto the surface of TNT nanotubes. Palladium loaded catalyst with high surface area shows a highly efficient α alkylation of ketones with primary alcohols.

Key Words : Hydrothermal synthesis, Titanate nanotube, Palladium nanoparticle, α-Alkylation

Introduction

Since Kasuga et al.¹ have developed the novel preparation method of titanate nanotubes by a simple hydrothermal treatment of TiO₂ powder in 10 M NaOH aqueous solution, titanium nanotube has been studied as very promising candi- date for many applications such as catalysis,^2-4 electro- catalysis,^5,6 photocatalysis,^7-12 hydrogen sensing,¹³ storage and separation,^14,15 lithium battery,^16-18 solar cell,^19,20 etc.

Recently, many groups reported titanate nanotube as the support of catalyst for the catalytic reactions.^21-25 Bavykin et al. explored the possibility of using titanate nanotubes as a support for ruthenium-hydrated oxide catalyst and studied the effect of Ru loading on the catalyst in the reaction of selective oxidation of alcohols with oxygen in the con- tinuous compact multichannel reactor.²¹ Murciano et al.

studied the selective double-bond migration reaction pro- moted by in situ generated active species of the mixed Pd(II)/Pd(0) catalyst supported on the external surface of ion-exchangeable, titanate nanotubes.²² Idakiev et al. report- ed that titanium oxide nanotubes as supports of nano-sized gold catalysts for low temperature water-gas shift reaction.²³ However, to the best of our knowledge, there are no reports available on the application of the α alkylation of ketones with primary alcohols using titanate nanotube as the support of palladium catalyst.

In this work, we synthesized palladium loaded titanate nanotube (TNT) and demonstrated a highly efficient α alkylation of ketones with primary alcohols by its use.

Experimental

Preparation of Palladium Loaded Titanate Nanotube.

To prepare amorphous precipitated powders containing ammonia and titanium aqueous ammonium hydroxide solu- tion with an ammonia content of 28-30% (99.99%, Aldrich) was slowly added drop-by-drop to 20% titanium (III) chloride solution (TiCl₃, Kanto) for 30 minutes under N₂ flow in ice bath while continuously stirring and the suspen- sion was stirred for 5h to complete the reaction.²⁶ After the completion of the reaction, the precipitates were filtered in air and washed several times with deionized water. Filtered powders were dried at 70^oC for 3-4 h in a convection oven.

The palladium nanoparticles supported on nanostructured materials were prepared from Pd(PPh₃)₄, 1-butanol, and sodium titanate through a procedure similar to those report- ed previously.²⁷ A mixture of Pd(PPh₃)₄ (200 mg, 0.17 mmol), 1-butanol (15 mL), and sodium titanate (910 mg) were added 50 mL round bottom flask equipped with condenser. The reaction mixture was stirred at 110^oC for 5 h to generate palladium nanoparticles. The black solid was filtered, washed with acetone, and dried at room temperature in the air to give 1 as black powder (900 mg, 2.0 wt % of Pd) (Scheme 1).

Characterizations. The crystalline phases of the products were determined by powder X-ray diffraction (XRD) on a diffractometer (Mac Science Co., M18XHF) with mono- chromatic Cu Kα radiation at 40 kV and 200 mA. The morphologies of Pd loaded titanate nanotube were investi- gated by TEM (JEOL JEM 2010F) operated at 200 kV.

Mg Kα radiation (1253.6 eV) was used in the XPS measure-

aThese authors contributed equally to this work.

Scheme 1. Preparation of Pd Nanoparticles Supported on Sodium Titanate.

ment (VG Scientific, ESCALAB 220iXL) and the binding energy was performed calibrated using C1s peak as the reference energy.

To investigate the physical texture of titanate nanotube, measurements of N₂ adsorption-desorption isotherms at 77 K were performed in a constant-volume adsorption apparatus (Micrometrics ASAP 2010) at relative pressures (P/P0) ranging from 10⁻⁴ to 0.995. Before the measurement of N₂ adsorption-desorption isotherms, the samples were degassed for 4 h at 393 K under 10⁻⁴ torr. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method²⁸ and the pore size distribution (PSD) was calculated from nitrogen desorption data using the Barrett-Joyner- Halenda (BJH) method with the modified Kelvin equation.²⁹ The pore volume was assessed on the basis of the adsorbed amount at a relative pressure (P/P0) of 0.99.

Coupling of Acetophenone and Benzyl Alcohol Under Argon. Acetophenone (120 mg, 1.00 mmol), benzyl alcohol (130 mg, 1.20 mmol), 1 (1.0 mol % Pd), K₃PO₄ (636 mg, 3.00 mmol), and toluene (2 mL) was placed in a 20 mL flask and allowed to react under argon at 110 ^oC for 12 h. The catalyst was separated by filtration, and the filtrate was puri-

fied by column chromatography to give 1,3-diphenylpropan- 1-one (190 mg) in 90% yield.

Results and Discussion

Figure 1 shows XRD patterns of the titanium precipitated precursor and titanate nanotube (TNT). The precursor sample did not show any XRD peaks indicating that it was amorphous solids. When the precursor added into 10 M NaOH solution and hydrothermally treated at 150 ^oC for 72 h, the product sample exhibited characteristic peaks at around 2θ = 10, 24, 28^o which can be assigned to the diffraction of Na₂Ti₃O₇·H₂O structure.

The morphologies of titanium precursor and titanate nanotube and palladium loaded titanate nanotube samples were observed by transition electron microscopy (TEM) as shown in Figure 2(a)-(c). The titanium precursor showed an amorphous phase, which was consistent with the XRD pattern. But titanate nanotube and palladium loaded titanate nanotube showed nanotube with a narrow diameter distribu- tion and short length as shown in Figure 2(b), (c). For Pd- TNT sample, palladium nanoparticles with the average size of ca. 3 nm were well dispersed on the surface of titanium nanotube with high surface area.

N₂ adsorption/desorption isotherms are shown in Figure 3(a) for TNT sample. It exhibit similar isotherm patterns corresponding to Type IV hysteresis associated with slit- shaped pores or the space between parallel plates.³⁰ The hysteresis in the isotherm is probably caused by the pores between the nanotubes. Pore size distribution curve of the sample is shown in Figure 3(b). The TNT sample shows small pores and narrow distribution of ca. 3.8 nm in average diameter. The peak positioned around 3.8 nm is thought to correspond to the inner diameter of the nanotubes, but the broad peak didn’t be observed in our work, which caused by the aggregation of nanotubes. During crystallization process of the titanium precursor to titanate nanotube the surface area of the samples decreased from 350 (titanium precursor) to 292 (TNT) m²/g.

X-ray photoelectron spectroscopy (XPS) measurements were carried out to analyze the oxidation state of Pd in Pd- TNT sample. Figure 4(a), (b) show the core level spectra of Figure 1. X-ray diffraction patterns of (a) titanium precursor, (b)

TNT.

Figure 2. TEM images of (a) titanium precursor, (b) TNT, (c) Pd-TNT.

O 1s and Pd 3d5/2 for TNT and Pd-TNT samples. Pd in Pd- TNT sample has the binding energy of 335.3 eV, which is almost same as that of Pd metal as shown in Figure 4(a). The binding energy of Pd 3d5/2 in PdO is 336.4 eV. This result can be observed from the oxidation state of oxygen species in TNT and Pd-TNT. The core level spectrum of O 1s in TNT is consistent with that of Pd-TNT, indicating that there is no oxygen come from PdO.

We first examined the oxidation of 1-phenylethanol in presence of 1 (2 mol % of Pd) in trifluorotoluene for 6 h at 50 ^oC under 1 atm O₂ and obtained acetophenone in 99%

yield.³¹ The catalyst 1 was more active than the commer- cially available Pd/C and Pd/Al₂O₃, the resin-dispersed palla- dium nanoparticles, and the magnesium oxide-supported palladium catalyst.³² Notably, 1 can be recovered simply by filtration and reused at least five times without activity

loss.³³

On the basis of this activity, we also tested the α-alkyl- ation of various ketones with alcohols under the optimized conditions (Table 1).³⁴ The catalytic activity of 1 for the coupling of acetophenone with benzyl alcohol was better than commercially available catalysts such as Pd/C, Pd/

Al₂O₃, and Pd/BaCO₃.^27c

The catalyst 1 was effective for a wide combination of ketones and primary alcohols that produced the correspond- ing α-alkylated ketone products under anaerobic conditions The coupling of acetophenone with benzyl alcohol bearing an electron-donating or an electron-withdrawing substituent also proceeded efficiently (entries 1-3). The alkylation of heteroaromatic alcohol also reacted smoothly (entry 4).

Aromatic ketone was also alkylated by aliphatic alcohol in Figure 3. (a) N₂ adsorption-desorption isotherms for TNT(,)

sample. Filled and empty symbols denote adsorption and desorption branchs of N₂ isotherms, respectively. (b) pore size distribution calculated by the BJH method from the desorption

branch of the N₂ isotherm for TNTsample. Figure 4. The XPS core-level spectra of (a) Pd 3d5/2 in Pd-TNT sample and (b) O 1s in (A) TNT and (B) Pd-TNT samples.

high yield (entry 5). Furthermore, aliphatic ketone was alkylated by an aliphatic alcohol in high yield (entry 6).

Conclusion

The TNT synthesized successfully by a hydrothermal route at 150 ^oC for 72 h showed well defined nanotube with high surface area. Palladium nanoparticles with the average size of ca. 3 nm were well dispersed on the surface of titanate nanotube. Palladium catalyst demonstrated a highly efficient α alkylation of ketones with primary alcohols.

Acknowledgments. This work was supported by National Research Laboratory, the Brain Korea 21 Project, National R

& D Project for Nano Science and Technology.

References

1. Kasuga T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K.

Lnagmuir 1998, 14, 3160.

2. Bavykin, D. V.; Lapkin, A. A.; Plucinski, P. K.; Friedrich, J. M.;

Walsh, F. C. J. Catal. 2005, 235, 10.

3. Zhu, B.; Li, K.; Feng, Y.; Zhang, S.; Wu, S.; Huang, W. Catal.

Lett. 2007, 118, 55.

4. Kleinhammes, A.; Wagner, G. W.; Kulkarni, H.; Jia, Y.; Zhang, Q.;

Qin, L. C.; Wu, Y. Chem. Phys. Lett. 2005, 411, 81.

5. Wang, Y. G.; Zhang, X. G. Electrochim. Acta 2004, 49, 1957.

6. Wang, M.; Guo, D. J.; Li, H. L. J. Solid State Chem. 2005, 178, 1996.

7. Cao, J.; Sun, J. Z.; Li, H. Y.; Hong, J.; Wang, M. J. Mater. Chem.

2004, 14, 1203.

8. Kuo, H. L.; Kuo, C. Y.; Liu, C. H.; Chao, J. H.; Lin, C. H. Catal.

Lett. 2007, 113, 7.

9. Hodos, M.; Horvath, E.; Haspel, H.; Kukovecz, A.; Konya, Z.;

Kiricsi, I. Chem. Phys. 2004, 399, 512.

10. Lee, C. K.; Wang, C. C.; Lyu, M. D.; Juang, L. C.; Liu, S. S.;

Hung, S. H. J. Colloid & Interface Sci. 2007, 316, 562.

11. Xu, J. C.; Lu, M.; Guo, X. Y.; Lia, H. L. J. Mol. Catal. A 2005, 226, 123.

12. Kim, J. C.; Choi, J.; Lee, Y. B.; Hong, J. H.; Lee, J. I.; Yang, J. W.;

Lee, W. I.; Hur, N. H. Chem. Commun. 2006, 5024.

13. Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. O.; Dickey, E.

C.; Grimes, C. A. Adv. Mater. 2003, 15, 624.

14. Bavykin, D. V.; Lapkin, A. A.; Plucinski, P. K.; Friedrich, J. M.;

Walsh, F. C. J. Phys. Chem. B 2005, 109, 19422.

15. Lim, S. H.; Luo, J.; Zhong, Z.; Ji, W.; Lin, J. Inorg. Chem. 2005, 44, 4124.

16. Kavan, L.; Kalbac, M.; Zukalova, M.; Exnar, I.; Lorenzen, V.;

Nesper, R.; Grätzel, M. Chem. Mater. 2004, 16, 477.

17. Armstrong, A. R.; Armstrong, G.; Canales, J.; Bruce, P. G. Adv.

Mater. 2005, 17, 862.

18. Gao, X.; Zhu, H.; Pan, G.; Ye, S.; Lan, Y.; Wu, F.; Song, D. J.

Phys. Chem. B 2004, 108, 2868.

19. Uchida, S.; Chiba, R.; Tomiha, M.; Masaki, N.; Shirai, M.

Electrochemistry 2002, 70, 418.

20. Ohsaki, Y.; Masaki, N.; Kitamura, T.; Wada, Y.; Okamoto, T.;

Sekino, T.; Niiharab, K.; Yanagida, S. Phys. Chem. Chem. Phys.

2005, 7, 4157.

21. Bavykin, D. V.; Lapkin, A. A.; Plucinski, P. K.; Friedrich, J. M.;

Walsh, F. C. J. Catal. 2005, 235, 10.

22. Murciano, L. T.; Lapkin, A. A.; Bavykin, D. V.; Walsh, F. C.;

Wilson, K. J. Catal. 2007, 245, 272.

23. Idakiev, V.; Yuan, Z. Y.; Tabakova, T.; Su, B. L. Appl. Catal. A:

Gen. 2005, 281, 149.

24. Sikhwivhilu, L. M.; Coville, N. J.; Naresh, D.; Chary, K. V. R.;

Vishwanathan, V. Appl. Catal. A: Gen. 2007, 324, 52.

Table 1. α-Alkylation of Ketones with Primary Alcohols Using 1

Entry Ketone Alcohol Product Time

(h)

Yield (%)^b

1 12 90

2 12 85

3 12 88

4 12 92

5^c 24 84

6^c 24 82

aA solution of ketone (1.0 mmol) and alcohol (1.2 mmol) in toluene (2 mL) was heated in the presence of 1 (1.0 mol % of Pd) and K3PO₄ (3 mmol) at 110 ^oC under argon. ^bYield of isolated product. ^c 2.0 mol % of Pd was used.

25. Bavykin, D. V.; Lapkin, A. A.; Plucinski, P. K.; Murcianob, L. T.;

Friedrich, J. M.; Walsh, F. C. Topics in Catal. 2006, 39, 151.

26. Jang, J. S.; Kim, H. G.; Bae, S. W.; Jung, J. H.; Shon, B. H.; Lee, J.

S. J. Solid State Chem. 2006, 179, 1067.

27. (a) Kim, N.; Kwon, M. S.; Park, C. M.; Park, J. Tetrahedron Lett.

2004, 45, 7057. (b) Kwon, M. S.; Kim, N.; Park, C. M.; Lee, J. S.;

Kang, K. Y.; Park, J. Org. Lett. 2005, 7, 1077. (c) Kwon, M. S.;

Kim, N.; Seo, S. H.; Park, I. S.; Cheedrala, R. K.; Park, J. Angew.

Chem., Int. Ed. 2005, 44, 6913. (d) Park, C. M.; Kwon, M. S.;

Park, J. Synthesis 2006, 22, 3790. (e) Kim, M.-J.; Kim, W.-H.;

Han, K.; Choi, Y. K.; Park, J. Org. Lett. 2007, 9, 1157. (f) Kwon, M. S.; Park, I. S.; Jang, J. S.; Lee, J. S.; Park, J. Org. Lett. 2007, 9, 3417.

28. Kruk, M.; Jaroniec, M.; Sayari, A. Langmuir 1997, 13, 6267.

29. Barett, E. P.; Joyner, L. G.; Halender, P. P. J. Am. Chem. Soc. 1951, 73, 373.

30. Sing, K. S. W.; Evertt, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603.

31. (a) Blackburn, T. F.; Schwartz, J. J. Chem. Soc., Chem. Commun.

1977, 157. (b) Gomez-Bengoa, E.; Noheda, P.; Echavarren, A. M.

Tetrahedron Lett. 1994, 35, 7097. (c) Aiet-Mohand, S.; Henin, F.;

Muzart, J. Tetrahedron Lett. 1995, 36, 2473. (d) Kaneda, K.; Fujii, M.; Morioka, K. J. Org. Chem. 1996, 61, 4502. (e) Kaneda, K.;

Fujie, Y.; Ebitani, K. Tetrahedron Lett. 1997, 38, 9023. (f) Peterson, K. P.; Larock, R. C. J. Org. Chem. 1998, 63, 3185. (g) Nishimura, T.; Onoue, T.; Ohe, K.; Uemura, S. Tetrahedron Lett.

1998, 39, 6011. (h) Nishimura, T.; Onoue, T.; Ohe, K.; Uemura, S.

J. Org. Chem. 1999, 64, 6750. (i) ten Brink, G.-J.; Arends, I. W. C.

E.; Sheldon, R. A. Science 2000, 287, 1636. (j) Stahl, S. S.;

Thorman, J. L.; Nelson, R. C.; Kozee, M. A. J. Am. Chem. Soc.

2001, 123, 7188. (k) Steinhoff, M. A.; Fix, S. R.; Stahl, S. S. J.

Am. Chem. Soc. 2002, 124, 766. (l) ten Brink, G.-J.; Arends, I. W.

C. E.; Sheldon, R. A. Adv. Synth. Catal. 2002, 344, 355. (m) Steinhoff, B. A.; Stahl, S. S. Org. Lett. 2002, 4, 4179. (n) Schultz, M. J.; Park, C. C.; Sigman, M. S. Chem. Commun. 2002, 3034. (o) Jensen, D. R.; Schultz, M. J.; Mueller, J. A.; Sigman, M. S.

Angew. Chem., Int. Ed. 2003, 42, 3810.

32. (a) Kwon, M. S.; Kim, N.; Park, C. M.; Lee, J. S.; Kang, K. Y.;

Park, J. Org. Lett. 2005, 7, 1077. (b) Uozumi, Y.; Nakao, R. Angew.

Chem., Int. Ed. 2003, 42, 194. (c) Pillai, U. R.; Sahle-Demessie, E.

Green Chem. 2004, 6, 161.

33. Palladium was not detected in the product by the inductively coupled plasma (ICP) analysis.

34. (a) Cho, C. S.; Kim, B. T.; Lee, M. J.; Kim, T.-J.; Shim, S. C.

Angew. Chem., Int. Ed. 2001, 40, 958. (b) Cho., C. S.; Kim, B. T.;

Kim, T.-J.; Shim, S. C. J. Org. Chem. 2001, 66, 9020. (c) Cho, C.

S.; Kim, B. T.; Kim, T.-J.; Shim, S. C. Tetrahedron Lett. 2002, 43, 7987. (d) Cho, C. S.; Kim, B. T.; Kim, H.-S.; Kim, T.-J.; Shim, S.

C. Organometallics 2003, 22, 3609. (e) Taguchi, K.; Nakagawa, H.; Hirabayashi, T. Sakaguchi, S.; Ishii, Y. J. Am. Chem. Soc.

2004, 126, 72. (f) Martínez, R.; Brand, G.-J.; Ramón, D.-J.; Yus, M. Tetrahedron Lett. 2005, 46, 3683. (g) Motokura, K.; Nishimura, D.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am. Chem.

Soc. 2004, 126, 5662. (h) Motokura, K.; Fujita, N.; Mori, K.;

Mizugaki, T.; Ebitani, K.; Kaneda, K. Tetrahedron Lett. 2005, 46, 5507. (i) Motokura, K.; Fujita, N.; Mori, K.; Mizugaki, T.; Ebitani, K.; Jitsukawa, K.; Kaneda, K. Chem. Eur. J. 2006, 12, 8228.