22942
J. Phys. Chem. B 2006, 110, 22942-22946
Homoallylic Alcohol Isomerization in Water over an Immobilized Ru(II) Organometallic Catalyst with Mesoporous Structure Hexing Li,*,† Fang Zhang,† Yin Wan,† and Yunfeng Lu‡ Department of Chemistry, Shanghai Normal UniVersity, Shanghai, 200234, People’s Republic of China, and Department of Chemical and Biomolecular Engineering, Tulane UniVersity, New Orleans, Louisiana 70118 ReceiVed: June 30, 2006; In Final Form: August 27, 2006
PPh2-functionalized SBA-15 was synthesized by co-condensation of tetraethyl orthosilicate and 2-(diphenylphosphino)ethyltriethoxysilane through prehydrolysis. The as-prepared PPh2-SBA-15 was used as the support to immobilize the Ru(II) organometallic catalyst through the strong coordination between the Ru(II) and the PPh2-ligand (Ru-PPh2-SBA-15). During 1-phenyl-3-buten-1-ol isomerization carried out in water as an environmentally friendly medium, the Ru-PPh2-SBA-15 catalyst exhibited almost the same activity and selectivity as the corresponding RuCl2(PPh3)3 homogeneous catalyst and could be used repetitively nearly 7 times. On the basis of various characterizations, the correlation of the catalytic behaviors of the Ru-PPh2SBA-15 to its structural characteristics was discussed briefly. Obviously, the high activity of the Ru-PPh2SBA-15 could be attributed to both the high surface area of the support, which ensured the good dispersion of Ru(II) active sites, and the ordered mesoporous structure, which facilitated the diffusion of organic reactants.
Introduction Most organic reactions are conducted in organic media. The use of large quantities of organic solvents for reaction and product isolation purposes eventually responds to environmental problems. A great number of attempts have been made to design new catalysts and routes so that the organic reactions can be performed in environmentally benign media. Water is the safest and cleanest solvent.1 However, due to the poor solubility of organic compounds in water, aqueous organic reactions were seldomly reported and only the homogeneous catalysts were employed.2 With the high activity of homogeneous catalysts, the product cleanup is inevitable and the catalyst cannot be used repetitively due to difficult separation, which may add to the cost and even cause environmental pollution by heavy metallic ions.3 Immobilized homogeneous catalysts can overcome the above shortcomings, but they usually exhibit much lower activity and selectivity due to the poor dispersion of the active phase and diffusion limitation, especially in the organic reactions with water as medium. Immobilization of homogeneous catalysts4-7 on the supports with mesoporous structure, such as SBA-15,8 MCM-41,9 etc., seems a promising way to develop highly active heterogeneous catalysts owing to their high surface area and large pore size.10-13 In this paper, we reported a novel Ru(II)-based organometallic catalyst immobilized onto the SBA15 support (Ru-PPh2-SBA-15). During aqueous homoallylic alcohol isomerization, the Ru-PPh2-SBA-15 exhibited matchable activity and selectivity with the corresponding RuCl2(PPh3)3 homogeneous catalyst and could be used repetitively nearly 7 times, showing a good potential in practical application. Experimental Section Catalyst Preparation. (a) Synthesis of PPh2-SBA-15. The diphenylphosphine (PPh2)-functionalized silica with a meso* Address correspondence to this author. E-mail:
[email protected]. † Shanghai Normal University. ‡ Tulane University.
porous structure similar to that of SBA-15, denoted as PPh2SBA-15, was synthesized by using co-condensation of tetraethyl orthosilicate (TEOS) and 2-(diphenylphosphino)ethyltriethoxysilane (DPPTS) through prehydrolysis of TEOS according to the following procedure: First, a certain amount of tetraethyl orthosilicate (TEOS, 98%) was introduced into 75 mL of aqueous solution containing 60 mL of 2.0 M HCl and 2.0 g of P123 and the TEOS was allowed to be prehydrolyzed for 60 min under gentle stirring at 313 K. Then, 2-(diphenylphosphino)ethyltriethoxysilane (DPPTS, 95%) was added dropwise into the solution, followed by rapid stirring for 20 h. After being aged at 373 K for 24 h, the resulting white precipitate was filtrated and dried at vacuum overnight. Finally, the surfactants and other organic substances in the samples were extracted and washed away by refluxing in ethanol solution for 24 h. The initial molar ratio in the mother solution is Si:HCl:H2O ) 0.041: 0.24:6.67, where Si refers to the total silica source, i.e., the total amount of TEOS and DPPTS. The PPh2 content anchored on the SBA-15 surface was adjusted by changing the DPPTS/ (DPPTS+TEOS) molar ratios in the initial mixture, from which PPh2-SBA-15(10%) and PPh2-SBA-15(5%) were obtained by using 10% and 5% DPPTS/(DPPTS+TEOS) molar ratios, respectively. (b) Synthesis of Ru-PPh2-SBA-15. The Ru-PPh2-SBA-15 catalyst was prepared through the coordination of the Ru(II) with the PPh2 ligand anchored on the SBA-15 support. In a typical synthesis, 1.0 g of PPh2-SBA-15 was added into 30 mL of toluene solution containing 96 mg of RuCl2(PPh3)3 and the mixture was stirred for 24 h at room temperature under argon atmosphere. After Soxlet extraction in toluene solvent to remove unreacted RuCl2(PPh3)3, the Ru-PPh2-SBA-15 catalyst was dried under vacuum for 24 h. Characterization. Ru loadings in the Ru-PPh2-SBA-15 catalysts were analyzed by inductively coupled plasma optical emission spectrometer (ICP, Varian VISTA-MPX). The X-ray powder diffraction (XRD) experiments were carried out on a Rigaku D/Max-RB diffractometer with Cu KR radiation.
10.1021/jp0641031 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/13/2006
Homoallylic Alcohol Isomerization in Water Transmission electron microscopy (TEM) studies were performed on a JEOL JEM2010 electron microscope, operated at an acceleration voltage of 200 kV. Fourier transform infrared (FTIR) spectra were collected with a Nicolet Magna 550 spectrometer by using the KBr method. For each sample, 128 scans were added to achieve acceptable signal-to-noise levels. N2 adsorption isotherms were measured at 77 K with a Quantachrome Nova 4000 analyzer. The samples were measured after being outgassed at 423 K overnight. Pore size distributions were calculated by using the BJH model. The specific surface areas (SBET) of samples were determined from the linear parts of BET plots (p/p0 ) 0.05-0.25). Solid-state 29Si MAS NMR, 13C CP MAS NMR, and 31P CP MAS NMR spectra were recorded at 79.5, 100.6, and 169.3 MHz, respectively, using a Bruker AV-400 spectrometer. Activity Test. The isomerization of 1-phenyl-3-buten-1-ol in aqueous solution was chosen as a probe to study the catalytic properties of the immobilized Ru(II) complex catalyst. In a typical reaction, 0.2 g of Ru-PPh2-SBA-15 containing 0.014 mmol of Ru or 0.014 mmol of homogeneous RuCl2(PPh3)3 was added to a well-stirred suspension containing 0.025 mL of 1-phenyl-3-buten-1-ol and 5 mL of H2O. The reaction was carried out at 373 K under vigorous stirring and reflux was used to protect the system from evaporation. After reacting for 20 h, the mixture was extracted by ether and dried by MgSO4, followed by filtration of the solid and evaporation of the solvent. The products were identified by 1H NMR spectroscopy. Quantitative analysis was performed on a high-performance liquid chromatograph (Shimadzu SPD-10AVP) equipped with a UV-vis detector and a KR100-5C18 liquid column, from which both the reaction conversion and selectivity were calculated. The reproducibility of all results was checked by repeating the results at least three times and was found to be within acceptable limits ((5%). To determine the catalyst durability, the Ru-PPh2-SBA-15 catalyst was allowed to settle down after each run of reactions and the clear supernatant liquid was decanted slowly. The residual solid catalyst was reused with a fresh charge of solvent and reactant for subsequent recycle runs under the same reaction conditions. The content of Ru species leached off from the RuPPh2-SBA-15 heterogeneous catalyst in each run was determined by ICP analysis. Results and Discussion ICP analysis revealed that the Ru loadings in both the RuPPh2-SBA-15(10%) and the Ru-PPh2-SBA-15(5%) catalysts were around 0.70 wt %, indicating that the change of the PPh2 content anchored on the SBA-15 support had no significant influence on the coordination with Ru(II), possibly owing to the large excess of the PPh2 ligand. As shown in Figure 1, the FTIR spectra revealed that, besides those observed in the pure SBA-15, the PPh2-SBA-15 displayed a number of new absorption peaks including two peaks at 2983 and 2890 cm-1 corresponding to the asymmetric and symmetric stretching modes of C-H bonds,14 one peak at 691 cm-1 characteristic of the -H out-of-plane deformation of the monosubstituted benzene ring, and a peak around 1435 cm-1 resulted from the vibrations of P-CH2.15 These results demonstrated the successful incorporation of the PPh2 groups with the SBA-15, which could also account for the abrupt decrease in the strength of the absorption peak at 3450 cm-1 characteristic of surface -OH groups, since a large number of -OH groups were substituted by PPh2 groups. The absorption peak at 1130-1090 cm-1 corresponding to the P-phenyl vibration could not be clearly
J. Phys. Chem. B, Vol. 110, No. 45, 2006 22943
Figure 1. FTIR spectra of SBA-15 and the PPh2-SBA-15 samples.
Figure 2. NMR spectra of the PPh2-SBA-15 samples: (a) 29Si MAS NMR, (b) 13C CP MAS NMR, and (c) 31P CP MAS NMR.
distinguished due to the overlap by the intense bonds at 1100 cm-1 resulting from the Si-O vibration.14 The PPh2 group anchored on the SBA-15 could be further confirmed by solid NMR spectra, as shown in Figure 2. The 29Si MAS NMR spectra (Figure 2a) of the PPh -SBA-15 2 samples displayed three resonance peaks upfield corresponding to Q4 (δ ) -110 ppm), Q3 (δ ) -102 ppm), and Q2 (δ ) -92 ppm), and two peaks downfield corresponding to T3 (δ ) -65 ppm) and T2 (δ ) - 57 ppm), where Qn ) Si(OSi)n-(OH)4-n, n ) 2-4 and Tm ) RSi(OSi)m-(OH)3-m, m ) 1-3. The presence of Tm peaks indicated the incorporation of the organic silane moieties as a part of the silica wall structure.16 The Tm/(Tm + Qn) ratios in the PPh2-SBA-15(10%) and PPh2-SBA-15(5%) samples were determined as 10.3% and 5.2%, respectively, almost the same as the DPPTS/(DPPTS+TEOS) molar ratios in the initial mixture, suggesting that nearly all the DPPTS incorporated with the TEOS, i.e., the loss of DPPTS during the co-condensation could be neglected.17 The 13C CPMAS NMR spectra (Figure 2b) of the PPh2-SBA-15 samples clearly displayed two peaks around 10 and 58 ppm, corresponding to two C atoms in the -CH2-CH2 group connected with the PPh2 group and one peak around 138 ppm indicative of the C atoms
22944 J. Phys. Chem. B, Vol. 110, No. 45, 2006
Figure 3. TG and DTG curves of the PPh2-SBA-15 and the Ru-PPh2SBA-15 samples.
Li et al.
Figure 4. Small-angle XRD patterns of the PPh2-SBA-15 and the RuPPh2-SBA-15 samples.
SCHEME 1: A Diagram Depicting the Immobilzation of the Ru(II) Organometallic Catalyst
in the benzene ring in the PPh2 group.18,19 The small resonance peaks observed in the range of 67-77 ppm could be attributed to trace P123 surfactant remaining in the PPh2-SBA-15.20 From the 31P MAS NMR spectrum (see Figure 2c), one could see a strong peak indicative of the organic P at δ ) -10.2 ppm.21 These results confirmed that the -CH2-CH2-PPh2 groups successfully anchored on the SBA-15 surface (Si-CH2-CH2PPh2) without significant decomposition. As shown in Figure 3, the TG/DTA analysis displayed one weight loss process around 340 °C, indicating that the incorporation between the organic group and the SBA-15 (Si-CH2CH2-PPh2) was relatively stable. After immobilization of the Ru(II) complex, the original peak corresponding to the decomposition of the Si-CH2-CH2-PPh2 decreased abruptly and a new weight loss process began at 437 °C. On one hand, this demonstrated that the Ru(II) organometallic catalyst was immobilized on the support via the coordination between the Ru(II) and the PPh2 ligand, as shown in Scheme 1. On the other hand, it implied that the PPh2 ligand was much excess in comparison with Ru(II), which could account for the same Ru(II) loadings on both PPh2-SBA-15(5%) and PPh2-SBA-15(10%) regardless of the change in the PPh2 ligand content. Furthermore, one could also conclude that the immobilization of Ru(II) might stabilize the incorporation between the organic group and the SBA-15. As shown in Figure 4, the small-angle XRD patterns revealed that both the PPh2-SBA-15 and Ru-PPh2-SBA-15 samples exhibited one intense peak and two weak peaks indicative of (100), (110), and (200) reflections, suggesting that the hexagonal arrayed pore structure (p6mm) observed in the pure SBA-15 could be preserved after modification with PPh2 groups and even
Figure 5. TEM images of the PPh2-SBA-15 and the Ru-PPh2-SBA15 samples: (a and b) PPh2-SBA-15(10%) along with (100) and (001) directions, respectively; (c and d) Ru-PPh2-SBA-15(10%) along with (100) and (001) directions, respectively.
anchoring the Ru(II) organometallic catalyst.8 The decrease of the peak intensity implied that the PPh2 modification and Ru(II) immobilization might disturb the ordered mesoporous structure to a certain degree. The TEM morphologies further confirmed that both the PPh2-SBA-15 and the Ru-PPh2-SBA15 samples displayed a two-dimensional hexagonal arrangement of one-dimensional channels with uniform size, as shown in Figure 5. N2 adsorption-desorption isotherms revealed that both the PPh2-SBA-15 and the Ru-PPh2-SBA-15 samples exhibited the typical IV type isotherms with a steep increase in adsorption at P/P0 ) 0.5-0.75, as shown in Figure 6. The attached poresize distribution analysis showed a narrow range within 2 nm. On the basis of the N2 adsorption-desorption isotherms, some structural parameters were calculated and listed in Table 1. One could see that modification with 5% PPh2 groups resulted in a slight increase of the pore size (dP), possibly owing to the swelling effect of the big PPh2 group. Further increase of the PPh2 content caused a rapid decrease in dP, possibly due to the blockage of the pore channels by PPh2 groups. Both the surface area (SBET) and the pore volume (VP) decreased after modification with the PPh2 group and further decreased after immobilization of the Ru(II) complex. This could be attributed to
Homoallylic Alcohol Isomerization in Water
J. Phys. Chem. B, Vol. 110, No. 45, 2006 22945
TABLE 1: Structural Parameters and the Catalytic Properties of Different Samplesa catalyst SBA-15 PPh2-SBA-15(5%) PPh2-SBA-15(10%) RuCl2(PPh3)3 Ru-PPh2-SBA-15(5%) Ru-PPh2-SBA-15(10%) used Ru-PPh2-SBA-15(10%)
SBET (m2/g)
VP (cm3/g)
dP (nm)
630 597 580
0.98 0.94 0.88
7.8 8.5 7.0
523 489 318
0.91 0.72 0.63
8.0 5.5 4.1
Ru content (mmol)
conv. (%)
sel. (%)
yield (%)
0.014 0.014 0.014 0.014
79 74 77 61
95 96 94 95
75 71 73 58
a Reaction conditions: catalyst containing 0.014 mmol of Ru species, 5.0 mL of H2O, 0.025 mL of PBO-131, reaction temperature ) 373 K, reaction time ) 20 h.
Figure 6. N2 adsorption-desorption isotherms of the PPh2-SBA-15 and the Ru-PPh2-SBA-15 samples The inset is their pore size distributions calculated by the BJH method.
SCHEME 2: Reaction Route of the 1-Phenyl-3-buten-1-ol Isomerization
the coverage by PPh2 groups on the channel surface of the SBA15, resulting in the increase of the wall thickness as confirmed by the calculation using a0 - pore size (a0 ) 2d100/31/2) (see Table 1), where d100 was calculated from the d spacings of the (100) reflection peaks.14 Isomerization of 1-phenyl-3-buten-1-ol (PBO-131) was used as a probe to examine the performance of the as-prepared catalysts.22 Under present reaction conditions, only two products were detected. One was the target product, 4-phenyl-3-buten2-ol (PBO-432), the other was the byproduct, 1-phenyl-1butanone (PB-11). Thus, the reaction route could be simply described in Scheme 2. From Table 1, one could see that the activity, the selectivity toward PBO-432, and its yield over the Ru-PPh2-SBA-15 heterogeneous catalysts were nearly the same as those obtained over the corresponding RuCl2(PPh3)3 homogeneous catalyst. To make sure whether the heterogeneous Ru(II) complex anchored on the support or the dissolved homogeneous Ru(II) complex was the real catalyst responsible for the present isomerization reaction, the following procedure, proposed by Sheldon et al.,23 was carried out. After reacting for 10 h in which the PBO-131 conversion exceeded 45%, the reaction mixture was filtered to remove the solid catalyst, then the mother liquor was allowed to react for another 20 h at the same reaction conditions. No significant change in either the PBO-131 conversion or the PBO432 yield was observed, demonstrating that the active species were not the dissolved Ru(II) complex leached from Ru-PPh2SBA-15. Therefore it was reasonable to suggest that the present
Figure 7. Recycle test of the Ru-PPh2-SBA-15 catalysts. Reaction conditions: catalyst containing 14 mmol Ru-species, 5 mL of H2O, 0.025 mL of PBO-131, reaction temperature ) 373 K, reaction time for each run ) 20 h.
catalysis was heterogeneous in nature. The similar selectivities suggested that the active center of the Ru-PPh2-SBA-15 heterogeneous catalyst was similar to that of the RuCl2(PPh3)3 homogeneous catalyst in nature. The high activity of the RuPPh2-SBA-15 catalysts could be attributed to the high surface area of the PPh2-SBA-15 support, which could ensure an almost monolayer distribution of Ru(II) active sites. Meanwhile, the relatively large-size pore may facilitate the diffusion of reactant molecules and thus the adsorption on the active sites. Furthermore, the modification of the surface (including the pore surface) of the SBA-15 with the hydrophobic organic group might favor the adsorption of reactant molecules on the catalyst in the water as reaction medium, which could account for the slight increase of activity for the Ru-PPh2-SBA-15 catalyst with the increase of PPh2 content. Figure 7 showed the durability of the Ru-PPh2-SBA-15 in aqueous isomerization of 1-phenyl-3-buten-1-ol. Even after being used repetitively 7 times, the selectivity still remained unchanged while the activity decreased by less than 20%. According to the ICP analysis, only 5.3 ppm Ru species were detected in the solution, showing that the coordination between the Ru(II) and the PPh2 group was very strong and the leaching of Ru species could be neglected. Thus, the loss of activity could possibly be attributed to the partial destruction of the mesoporous structure of the Ru-PPh2-SBA-15, which caused the decrease in the SBET, VP, and dP, as shown in Table 1. Conclusions The above results demonstrated that the ordered mesoporous structure in the SBA-15 could be preserved after PPh2 functionalization and even after Ru(II) immobilization through the Ru(II)-PPh2 coordination. The as-prepared Ru-PPh2-SBA-15
22946 J. Phys. Chem. B, Vol. 110, No. 45, 2006 organometallic catalyst exhibited high activity and selectivity during aqueous medium isomerization of homoallylic alcohol, obviously owing to the high dispersion of Ru(II) active sites on the support and the ordered mesopores which facilitated the diffusion and adsorption of the organic reactant molecules. Besides similar activity and selectivity, the Ru-PPh2-SBA-15 displayed superiority over the RuCl2(PPh3)3 homogeneous catalyst since it could be used repetitively 7 times, possibly owing to both the strong incorporation of the Ru(II) complex with the SBA-15 support and the excellent thermal stability, especially the hydrothermal stability of the mesoporous structure. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20377031 and 20407014), Ministry of Science and Technology in China (2005CCA01100), and Shanghai Municipal Scientific Commission (06JC14060 and 03QF14037). References and Notes (1) (a) Li, C. J.; Chan, T. H. Organic Reactions in Aqueous Media; Wiley: New York, 1997. (b) Organic Synthesis in Water; Grieco, P. A., Ed.; Thomson Science: Glasgow, Scotland, 1998. (2) Li, C. J. Chem. ReV. 2005, 105, 3095-3166. (3) Sheldon, R. A. Green Chem. 2005, 7, 267-278. (4) De Vos, D. E.; Vankelecom, I. F. J.; Jabobs, P. A. Chiral Catalyst Immobilization and Recycling, Wiley-VCH: Weinheim, Germany, 2000. (5) Lu, Z.; Lindner, E.; Mayer, H. A. Chem. ReV. 2002, 102, 35433578.
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