Homoallylic Alcohol Isomerization in Water over an Immobilized Ru(II) OrganometallicCatalyst 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, andDepartment 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-(diphen-ylphosphino)ethyltriethoxysilane through prehydrolysis. The as-prepared PPh2-SBA-15 was used as the supportto immobilize the Ru(II) organometallic catalyst through the strong coordination between the Ru(II) and thePPh2-ligand (Ru-PPh2-SBA-15). During 1-phenyl-3-buten-1-ol isomerization carried out in water as anenvironmentally friendly medium, the Ru-PPh2-SBA-15 catalyst exhibited almost the same activity andselectivity as the corresponding RuCl2(PPh3)3 homogeneous catalyst and could be used repetitively nearly 7times. On the basis of various characterizations, the correlation of the catalytic behaviors of the Ru-PPh2-SBA-15 to its structural characteristics was discussed briefly. Obviously, the high activity of the Ru-PPh2-SBA-15 could be attributed to both the high surface area of the support, which ensured the good dispersionof 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. Theuse of large quantities of organic solvents for reaction andproduct isolation purposes eventually responds to environmentalproblems. A great number of attempts have been made to designnew catalysts and routes so that the organic reactions can beperformed in environmentally benign media. Water is the safestand cleanest solvent.1 However, due to the poor solubility oforganic compounds in water, aqueous organic reactions wereseldomly reported and only the homogeneous catalysts wereemployed.2 With the high activity of homogeneous catalysts,the product cleanup is inevitable and the catalyst cannot be usedrepetitively due to difficult separation, which may add to thecost and even cause environmental pollution by heavy metallicions.3 Immobilized homogeneous catalysts can overcome theabove shortcomings, but they usually exhibit much loweractivity and selectivity due to the poor dispersion of the activephase and diffusion limitation, especially in the organic reactionswith water as medium. Immobilization of homogeneouscatalysts4-7 on the supports with mesoporous structure, suchas SBA-15,8 MCM-41,9 etc., seems a promising way to develophighly active heterogeneous catalysts owing to their high surfacearea and large pore size.10-13 In this paper, we reported a novelRu(II)-based organometallic catalyst immobilized onto the SBA-15 support (Ru-PPh2-SBA-15). During aqueous homoallylicalcohol isomerization, the Ru-PPh2-SBA-15 exhibited matchableactivity and selectivity with the corresponding RuCl2(PPh3)3

homogeneous catalyst and could be used repetitively nearly 7times, showing a good potential in practical application.

Experimental Section

Catalyst Preparation. (a) Synthesis of PPh2-SBA-15.Thediphenylphosphine (PPh2)-functionalized silica with a meso-

porous structure similar to that of SBA-15, denoted as PPh2-SBA-15, was synthesized by using co-condensation of tetraethylorthosilicate (TEOS) and 2-(diphenylphosphino)ethyltriethoxy-silane (DPPTS) through prehydrolysis of TEOS according tothe following procedure: First, a certain amount of tetraethylorthosilicate (TEOS, 98%) was introduced into 75 mL ofaqueous solution containing 60 mL of 2.0 M HCl and 2.0 g ofP123 and the TEOS was allowed to be prehydrolyzed for 60min under gentle stirring at 313 K. Then, 2-(diphenylphosphino)-ethyltriethoxysilane (DPPTS, 95%) was added dropwise intothe solution, followed by rapid stirring for 20 h. After beingaged at 373 K for 24 h, the resulting white precipitate wasfiltrated and dried at vacuum overnight. Finally, the surfactantsand other organic substances in the samples were extracted andwashed away by refluxing in ethanol solution for 24 h. Theinitial 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 totalamount of TEOS and DPPTS. The PPh2 content anchored onthe SBA-15 surface was adjusted by changing the DPPTS/(DPPTS+TEOS) molar ratios in the initial mixture, from whichPPh2-SBA-15(10%) and PPh2-SBA-15(5%) were obtained byusing 10% and 5% DPPTS/(DPPTS+TEOS) molar ratios,respectively.

(b) Synthesis of Ru-PPh2-SBA-15. The Ru-PPh2-SBA-15catalyst was prepared through the coordination of the Ru(II)with the PPh2 ligand anchored on the SBA-15 support. In atypical synthesis, 1.0 g of PPh2-SBA-15 was added into 30 mLof toluene solution containing 96 mg of RuCl2(PPh3)3 and themixture was stirred for 24 h at room temperature under argonatmosphere. After Soxlet extraction in toluene solvent to removeunreacted RuCl2(PPh3)3, the Ru-PPh2-SBA-15 catalyst was driedunder vacuum for 24 h.

Characterization. Ru loadings in the Ru-PPh2-SBA-15catalysts were analyzed by inductively coupled plasma opticalemission spectrometer (ICP, Varian VISTA-MPX). The X-raypowder diffraction (XRD) experiments were carried out on aRigaku D/Max-RB diffractometer with Cu KR radiation.

* Address correspondence to this author. E-mail: HeXing-Li@shnu.edu.cn.† Shanghai Normal University.‡ Tulane University.

22942 J. Phys. Chem. B2006,110,22942-22946

10.1021/jp0641031 CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 10/13/2006

Transmission electron microscopy (TEM) studies were per-formed on a JEOL JEM2010 electron microscope, operated atan acceleration voltage of 200 kV. Fourier transform infrared(FTIR) spectra were collected with a Nicolet Magna 550spectrometer by using the KBr method. For each sample, 128scans were added to achieve acceptable signal-to-noise levels.N2 adsorption isotherms were measured at 77 K with aQuantachrome Nova 4000 analyzer. The samples were measuredafter being outgassed at 423 K overnight. Pore size distributionswere calculated by using the BJH model. The specific surfaceareas (SBET) of samples were determined from the linear partsof BET plots(p/p0 ) 0.05-0.25). Solid-state29Si MAS NMR,13C CP MAS NMR, and31P CP MAS NMR spectra wererecorded at 79.5, 100.6, and 169.3 MHz, respectively, using aBruker AV-400 spectrometer.

Activity Test. The isomerization of 1-phenyl-3-buten-1-olin aqueous solution was chosen as a probe to study the catalyticproperties of the immobilized Ru(II) complex catalyst. In atypical reaction, 0.2 g of Ru-PPh2-SBA-15 containing 0.014mmol of Ru or 0.014 mmol of homogeneous RuCl2(PPh3)3 wasadded to a well-stirred suspension containing 0.025 mL of1-phenyl-3-buten-1-ol and 5 mL of H2O. The reaction wascarried out at 373 K under vigorous stirring and reflux was usedto 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 by1H NMR spectroscopy.Quantitative analysis was performed on a high-performanceliquid chromatograph (Shimadzu SPD-10AVP) equipped witha UV-vis detector and a KR100-5C18 liquid column, fromwhich both the reaction conversion and selectivity werecalculated. The reproducibility of all results was checked byrepeating the results at least three times and was found to bewithin acceptable limits ((5%).

To determine the catalyst durability, the Ru-PPh2-SBA-15catalyst was allowed to settle down after each run of reactionsand the clear supernatant liquid was decanted slowly. Theresidual solid catalyst was reused with a fresh charge of solventand reactant for subsequent recycle runs under the same reactionconditions. The content of Ru species leached off from the Ru-PPh2-SBA-15 heterogeneous catalyst in each run was determinedby ICP analysis.

Results and Discussion

ICP analysis revealed that the Ru loadings in both the Ru-PPh2-SBA-15(10%) and the Ru-PPh2-SBA-15(5%) catalystswere around 0.70 wt %, indicating that the change of the PPh2

content anchored on the SBA-15 support had no significantinfluence on the coordination with Ru(II), possibly owing tothe large excess of the PPh2 ligand. As shown in Figure 1, theFTIR spectra revealed that, besides those observed in the pureSBA-15, the PPh2-SBA-15 displayed a number of new absorp-tion peaks including two peaks at 2983 and 2890 cm-1

corresponding to the asymmetric and symmetric stretchingmodes of C-H bonds,14 one peak at 691 cm-1 characteristic ofthe -H out-of-plane deformation of the monosubstitutedbenzene ring, and a peak around 1435 cm-1 resulted from thevibrations of P-CH2.15 These results demonstrated the successfulincorporation of the PPh2 groups with the SBA-15, which couldalso account for the abrupt decrease in the strength of theabsorption peak at 3450 cm-1 characteristic of surface-OHgroups, since a large number of-OH groups were substitutedby PPh2 groups. The absorption peak at 1130-1090 cm-1

corresponding to the P-phenyl vibration could not be clearly

distinguished due to the overlap by the intense bonds at 1100cm-1 resulting from the Si-O vibration.14

The PPh2 group anchored on the SBA-15 could be furtherconfirmed by solid NMR spectra, as shown in Figure 2. The29Si MAS NMR spectra (Figure 2a) of the PPh2-SBA-15samples displayed three resonance peaks upfield correspondingto Q4 (δ ) -110 ppm), Q3 (δ ) -102 ppm), and Q2 (δ ) -92ppm), and two peaks downfield corresponding to T3 (δ ) -65ppm) 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 presenceof Tm peaks indicated the incorporation of the organic silanemoieties 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 ratiosin the initial mixture, suggesting that nearly all the DPPTSincorporated with the TEOS, i.e., the loss of DPPTS during theco-condensation could be neglected.17 The 13C CPMAS NMRspectra (Figure 2b) of the PPh2-SBA-15 samples clearlydisplayed two peaks around 10 and 58 ppm, corresponding totwo C atoms in the-CH2-CH2 group connected with the PPh2

group and one peak around 138 ppm indicative of the C atoms

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 MASNMR, (b) 13C CP MAS NMR, and (c)31P CP MAS NMR.

Homoallylic Alcohol Isomerization in Water J. Phys. Chem. B, Vol. 110, No. 45, 200622943

in the benzene ring in the PPh2 group.18,19The small resonancepeaks observed in the range of 67-77 ppm could be attributedto trace P123 surfactant remaining in the PPh2-SBA-15.20 Fromthe 31P MAS NMR spectrum (see Figure 2c), one could see astrong peak indicative of the organic P atδ ) -10.2 ppm.21

These results confirmed that the-CH2-CH2-PPh2 groupssuccessfully anchored on the SBA-15 surface (Si-CH2-CH2-PPh2) without significant decomposition.

As shown in Figure 3, the TG/DTA analysis displayed oneweight loss process around 340°C, indicating that the incor-poration between the organic group and the SBA-15 (Si-CH2-CH2-PPh2) was relatively stable. After immobilization of theRu(II) complex, the original peak corresponding to the decom-position of the Si-CH2-CH2-PPh2 decreased abruptly and a newweight loss process began at 437°C. On one hand, thisdemonstrated that the Ru(II) organometallic catalyst was im-mobilized on the support via the coordination between the Ru-(II) and the PPh2 ligand, as shown in Scheme 1. On the otherhand, it implied that the PPh2 ligand was much excess incomparison 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) mightstabilize the incorporation between the organic group and theSBA-15.

As shown in Figure 4, the small-angle XRD patterns revealedthat both the PPh2-SBA-15 and Ru-PPh2-SBA-15 samplesexhibited one intense peak and two weak peaks indicative of(100), (110), and (200) reflections, suggesting that the hexagonalarrayed pore structure (p6mm) observed in the pure SBA-15could be preserved after modification with PPh2 groups and even

anchoring the Ru(II) organometallic catalyst.8 The decrease ofthe peak intensity implied that the PPh2 modification and Ru-(II) immobilization might disturb the ordered mesoporousstructure to a certain degree. The TEM morphologies furtherconfirmed that both the PPh2-SBA-15 and the Ru-PPh2-SBA-15 samples displayed a two-dimensional hexagonal arrangementof one-dimensional channels with uniform size, as shown inFigure 5.

N2 adsorption-desorption isotherms revealed that both thePPh2-SBA-15 and the Ru-PPh2-SBA-15 samples exhibited thetypical IV type isotherms with a steep increase in adsorption atP/P0 ) 0.5-0.75, as shown in Figure 6. The attached pore-size distribution analysis showed a narrow range within 2 nm.On the basis of the N2 adsorption-desorption isotherms, somestructural parameters were calculated and listed in Table 1. Onecould see that modification with 5% PPh2 groups resulted in aslight increase of the pore size (dP), possibly owing to theswelling effect of the big PPh2 group. Further increase of thePPh2 content caused a rapid decrease indP, possibly due to theblockage of the pore channels by PPh2 groups. Both the surfacearea (SBET) and the pore volume (VP) decreased after modifica-tion with the PPh2 group and further decreased after im-mobilization of the Ru(II) complex. This could be attributed to

Figure 3. TG and DTG curves of the PPh2-SBA-15 and the Ru-PPh2-SBA-15 samples.

SCHEME 1: A Diagram Depicting the Immobilzation ofthe Ru(II) Organometallic Catalyst

Figure 4. Small-angle XRD patterns of the PPh2-SBA-15 and the Ru-PPh2-SBA-15 samples.

Figure 5. TEM images of the PPh2-SBA-15 and the Ru-PPh2-SBA-15 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.

22944 J. Phys. Chem. B, Vol. 110, No. 45, 2006 Li et al.

the coverage by PPh2 groups on the channel surface of the SBA-15, resulting in the increase of the wall thickness as confirmedby the calculation usinga0 - pore size (a0 ) 2d100/31/2) (seeTable 1), whered100 was calculated from thed spacings of the(100) reflection peaks.14

Isomerization of 1-phenyl-3-buten-1-ol (PBO-131) was usedas a probe to examine the performance of the as-preparedcatalysts.22 Under present reaction conditions, only two productswere detected. One was the target product, 4-phenyl-3-buten-2-ol (PBO-432), the other was the byproduct, 1-phenyl-1-butanone (PB-11). Thus, the reaction route could be simplydescribed in Scheme 2.

From Table 1, one could see that the activity, the selectivitytoward PBO-432, and its yield over the Ru-PPh2-SBA-15heterogeneous catalysts were nearly the same as those obtainedover the corresponding RuCl2(PPh3)3 homogeneous catalyst. Tomake sure whether the heterogeneous Ru(II) complex anchoredon the support or the dissolved homogeneous Ru(II) complexwas the real catalyst responsible for the present isomerizationreaction, the following procedure, proposed by Sheldon et al.,23

was carried out. After reacting for 10 h in which the PBO-131conversion exceeded 45%, the reaction mixture was filtered toremove the solid catalyst, then the mother liquor was allowedto react for another 20 h at the same reaction conditions. Nosignificant change in either the PBO-131 conversion or the PBO-432 yield was observed, demonstrating that the active specieswere not the dissolved Ru(II) complex leached from Ru-PPh2-SBA-15. Therefore it was reasonable to suggest that the present

catalysis was heterogeneous in nature. The similar selectivitiessuggested that the active center of the Ru-PPh2-SBA-15heterogeneous catalyst was similar to that of the RuCl2(PPh3)3

homogeneous catalyst in nature. The high activity of the Ru-PPh2-SBA-15 catalysts could be attributed to the high surfacearea of the PPh2-SBA-15 support, which could ensure an almostmonolayer distribution of Ru(II) active sites. Meanwhile, therelatively large-size pore may facilitate the diffusion of reactantmolecules and thus the adsorption on the active sites. Further-more, the modification of the surface (including the pore surface)of the SBA-15 with the hydrophobic organic group might favorthe adsorption of reactant molecules on the catalyst in the wateras reaction medium, which could account for the slight increaseof activity for the Ru-PPh2-SBA-15 catalyst with the increaseof PPh2 content.

Figure 7 showed the durability of the Ru-PPh2-SBA-15 inaqueous isomerization of 1-phenyl-3-buten-1-ol. Even afterbeing used repetitively 7 times, the selectivity still remainedunchanged while the activity decreased by less than 20%.According to the ICP analysis, only 5.3 ppm Ru species weredetected in the solution, showing that the coordination betweenthe Ru(II) and the PPh2 group was very strong and the leachingof Ru species could be neglected. Thus, the loss of activity couldpossibly be attributed to the partial destruction of the meso-porous structure of the Ru-PPh2-SBA-15, which caused thedecrease in theSBET, VP, anddP, as shown in Table 1.

Conclusions

The above results demonstrated that the ordered mesoporousstructure in the SBA-15 could be preserved after PPh2 func-tionalization and even after Ru(II) immobilization through theRu(II)-PPh2 coordination. The as-prepared Ru-PPh2-SBA-15

TABLE 1: Structural Parameters and the Catalytic Properties of Different Samplesa

catalystSBET

(m2/g)VP

(cm3/g)dP

(nm)Ru content

(mmol)conv.(%)

sel.(%)

yield(%)

SBA-15 630 0.98 7.8PPh2-SBA-15(5%) 597 0.94 8.5PPh2-SBA-15(10%) 580 0.88 7.0RuCl2(PPh3)3 0.014 79 95 75Ru-PPh2-SBA-15(5%) 523 0.91 8.0 0.014 74 96 71Ru-PPh2-SBA-15(10%) 489 0.72 5.5 0.014 77 94 73used Ru-PPh2-SBA-15(10%) 318 0.63 4.1 0.014 61 95 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-15and the Ru-PPh2-SBA-15 samples The inset is their pore size distribu-tions calculated by the BJH method.

SCHEME 2: Reaction Route of the 1-Phenyl-3-buten-1-olIsomerization

Figure 7. Recycle test of the Ru-PPh2-SBA-15 catalysts. Reactionconditions: catalyst containing 14 mmol Ru-species, 5 mL of H2O,0.025 mL of PBO-131, reaction temperature) 373 K, reaction timefor each run) 20 h.

Homoallylic Alcohol Isomerization in Water J. Phys. Chem. B, Vol. 110, No. 45, 200622945

organometallic catalyst exhibited high activity and selectivityduring aqueous medium isomerization of homoallylic alcohol,obviously owing to the high dispersion of Ru(II) active siteson the support and the ordered mesopores which facilitated thediffusion and adsorption of the organic reactant molecules.Besides similar activity and selectivity, the Ru-PPh2-SBA-15displayed superiority over the RuCl2(PPh3)3 homogeneouscatalyst since it could be used repetitively 7 times, possiblyowing to both the strong incorporation of the Ru(II) complexwith the SBA-15 support and the excellent thermal stability,especially the hydrothermal stability of the mesoporous structure.

Acknowledgment. This work was supported by the NationalNatural Science Foundation of China (20377031 and 20407014),Ministry of Science and Technology in China (2005CCA01100),and Shanghai Municipal Scientific Commission (06JC14060 and03QF14037).

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