Environ. Sci. Technol. 2009, 43, 188–194
Water-Medium Clean Organic Reactions over an Active Mesoporous Ru(II) Organometallic Catalyst H E X I N G L I , * ,† H O N G Y I N , † FANG ZHANG,† HUI LI,† YUNING HUO,† A N D Y U N F E N G L U * ,‡ Department of Chemistry, Shanghai Normal University, Shanghai 200234, China, and Chemical and Biomolecular Engineering Department, University of California, Los Angeles, California 90095
Received July 27, 2008. Revised manuscript received October 27, 2008. Accepted October 28, 2008.
Water-medium clean organic reactions including isomerization and hydrogenation were carried out over a Ru(II) mesoporous organometallic catalyst (Ru-MOC) synthesized based on surfactant-directed coassembly of a bridged Ru(II) organometallicsilane and tetraethoxysilane. Owing to the large surface area, the ordered mesoporous channels, and the highly dispersed Ru(II) active sites, the activity and selectivity of the Ru-MOC catalyst are much higher than that of the immobilized Ru(II) catalyst obtained through regular grafting technology. The Ru-MOC catalyst exhibited similar activity and selectivity to the corresponding homogeneous Ru(II) organometallic catalysts. However, it could be easily separated and recycled, thus will reduce the cost significantly and even avoid the environmental pollution caused by heavy metallic ions in industrial application. The correlation of catalytic performance to structural characteristicswasdiscussedbasedondetailedcharacterizations.
1. Introduction Most organic reactions are conducted in organic solvents, and billions of pounds of organic solvents are consumed each year in the fine chemical and pharmaceutical industries for reaction and product isolation purposes. The discharge of large quantities of organic solvents eventually adds to environmental problems (1). A notable development is the use of water as an alternative nonpolluting solvent for organic reactions since water is the most innocuous substance on Earth and therefore the safest solvent possible. Numerous biochemical organic reactions affecting the living system have inevitably occurred in aqueous media with the help of enzymes. Thus, design of powerful catalysts plays a key role in realizing the water-medium clean organic reactions (2). To date, most studies focused on homogeneous organometallic catalysts for water-medium organic reactions due to the solubility limit (3). Although homogeneous catalysts work very well, their industrial applications are limited due to the difficult separation and recycling use, which may increase cost and lead to heavy-metal pollution in water (4). Design * Address correspondence to either author: Phone: +86-2164322272(H.L.); +1-310-794-7238(Y.L.). E-Mail: HeXing-Li@ shnu.edu.cn (H.L.);
[email protected] (Y.L.). † Shanghai Normal University. ‡ University of California. 188
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of immobilized organometallic catalysts could effectively overcome those disadvantages. To obtain high catalytic efficiency matchable with that of the corresponding homogeneous catalysts, the as-prepared immobilized organometallic catalysts should maintain both the effective chemical environment and the high degree of dispersion of active sites (5, 6). Recent progress in functionlized mesoporous silica have highlighted the potential of utilizing these structurally uniform materials as a new generation of powerful heterogeneous catalysts (7-12). We previously reported the Ru(II) and Pd(II) organometallic catalysts immobilized by grafting method, i.e., by coordinating metallic ions with organic ligands originally anchored onto the mesoporous silica supports (13-15). However, these catalysts still exhibited lower activity and selectivity than the corresponding homogeneous catalysts due to the decrease in dispersion degree and the change of chemical environment. Meanwhile, the catalytic efficiency decreased gradually in consecutive recycle processes due to the leaching of active sites and/or ligands (16). Herein, we report a new approach to synthesize a mesoprous Ru(II) organometallic catalyst (Ru-MOC) based on surfactant-assembly of Ru(II) organometallic silane, which exhibits comparable activity and selectivity with the corresponding homogeneous Ru(II) organometallic catalyst during water-medium organic reactions, and it could be used repetitively,showingagoodpotentialinindustrialapplications.
2. Experimental Section Catalyst Preparation. Figure 1 illustrates the synthesis of the Ru-MOC catalysts by a coassembly or grafting method. First, a 3-(2-diphenylphosphino)-dichlororutheniumtriethoxysilane (Ru(II)TS) was synthesized by mixing 75 mL methanol containing 0.60 g RuCl3 · 3H2O with 5.18 g (EtO)3Si(CH2)2PPh2 at 30 °C in an argon atmosphere. Then, 0.15 g sodium borohydride (NaBH4) was added slowly to the solution until the color turned brown. After stirring for 15 h, the solvent was evaporated and the residual solid was filtered, washed thoroughly with CH2Cl2, and finally extracted with pentane. Subsequently, the Ru-MOC samples were prepared based on the coassembly described as follows. 0.50 g F127, 0.60 g 1, 3, 5-trimethylbenzene, 0.90 g CH3CN and 2.5 g KCl were mixed in 30 mL 2.0 M HCl aqueous solution. Appropriate amount of tetraethoxysilane (TEOS) was added to the solution at 30 °C and allowed for prehydrolysis for 45 min. Then, the as-prepared Ru(II)TS was added into the solution, following by vigorously stirring for 24 h and aging at 100 °C for another 24 h. The initial molar ratio of Si:F127:TMB:KCl:HCl:H2O was maintained at 1:0.004:1:1.68:6:157, where the Si refers to the total amount of TEOS and Ru-TS. The brown precipitate was filtered, followed by drying in a vacuum for 24 h. Finally, the solid was extracted with 500 mL of ethanol for 24 h to remove surfactants and other organic residues. For comparison, the grafted Ru-MOC was synthesized by refluxing a solution of Ru(PPh3)3Cl2 with functionalized mesoporous silica containing surface PPh2 ligands (13). Some physical parameters including Ru loading, BET surface area (SBET), pore diameter (DP), and pore volume (VP) were summarized in Table 1. Characterization. The carbon and hydrogen contents in the Ru(II)TS samples were analyzed using a CHN analyzer (Elementar Vario ELIII, Germany). All the samples were combusted completely at 1000 °C in pure oxygen. CO2 and H2O were determined quantitatively based on their thermoconductivities relative to the carrier gas helium. Contents of ruthenium, silica, and phosphor were determined by inductively coupled plasma spectrometer (ICP, Varian VISTAMPX) in which all the samples were dissolved in an aqueous 10.1021/es802094b CCC: $40.75
2009 American Chemical Society
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FIGURE 1. Schematic illustration of the preparation of the Ru-MOC catalyst through coassembly (top) and the grafted Ru-MOC (bottom).
TABLE 1. Structural Parameters and Catalytic Performances of the As-Prepared Catalysts in Water-Medium Isomerizationa
sample
Ru loading (wt%)
SBET (m2/g)
DP (nm)
VP (cm3/g)
conversion (%)
selectivity (%)
yield (%)
RuCl2(PPh3)3 Ru-MOC-1 Ru-MOC-2 Ru-MOC-3 Ru-MOC-4 grafted Ru-MOC Ru-MOC-3b
0.37 1.0 0.63 0.37 0.28 0.37 0.36
40 303 598 631 225 415
1.7 7.2 11 12 9.4 8.7
0.030 0.27 0.74 0.80 0.37 0.56
96 71 85 93 86 79 86
78 64 74 77 77 68 76
75 45 63 72 66 54 65
a Reaction conditions: a catalyst containing 0.0034 mmol Ru(II), 0.025 mL 1-(4-methylphenyl)-3-buten-1-ol, 5.0 mL H2O, reaction temperature ) 100 °C, reaction time ) 8 h. b After 7th recycle.
solution containing HCl and HF (3/1 in volume ratio). The mesoporous structure was characterized by low-angle X-ray powder diffraction (XRD, Rigaku D/Max-RB, Cu KR radiation) and transmission electron microscopy (TEM, JEOL JEM2010, 200 kV). Thermogravimetric analysis and differential thermal analysis (TG/DTA) were conducted on a Shimadzu DT-60 to examine the thermal stability. Fine structure of the catalysts was determined by Fourier transform infrared (FTIR) spectra collected on a Nicolet Magna 550 spectrometer. N2 adsorption-desorption isotherms were measured at 77 K using a Quantachrome Nova 4000e analyzer. Specific surface area (SBET), pore size (DP), and pore volume (VP) was obtained by the BET and the BJH methods, respectively. Solid-state NMR spectra were recorded on a Bruker AV-400 spectrometer. The surface electronic states were analyzed by X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C). All the binding energy (BE) values were calibrated using the standard BE value of contaminant carbon (C1S ) 284.6 eV) as a reference. Activity Test. Water-medium organic reactions including isomerization and hydrogenation are used as probes for evaluating the performances of the as-prepared Ru-MOC catalysts. In a typical experiment, a catalyst containing 0.0034 mmol Ru(II) was mixed with 0.025 mL 1-(4-methylphenyl)3-buten-1-ol and 5.0 mL H2O in a 10 mL glass flask under vigorous stirring. After reacting at 100 °C for 8 h, the mixture was extracted using ether and dried by MgSO4. Product analysis was performed on a high-performance liquid chro-
matograph (Shimadzu SPD-10AVP) equipped with a UV-vis detector and a KR100-5C18 liquid column. Besides the target product 4-(4-methylphenyl)-3-buten-2-ol, only one byproduct, 1-(4-methylphenyl)-1-butanone was identified, which was further confirmed by the NMR analysis with the following data. 4-(4-methylphenyl)-3-buten-2-ol (product): 1H NMR (400 MHz, CDCl3, ppm), 7.12-7.27(m, 4H), 6.52 (d, J ) 16.0 Hz, 1H), 6.18-6.22 (dd, J ) 6.4, 16.0 Hz, 1H), 4.47 (t, J ) 6.4 Hz, 1H), 1.70(bs, 1H), 2.33(s, 3H), 1.36 (d, J ) 6.4 Hz, 3H); 1-(4-methylphenyl)-1-butanone (byproduct): 1H NMR (400 MHz, CDCl3, ppm) 7.25-7.85 (m, 4H), 2.92 (t, J ) 7.2 Hz, 2H), 2.40 (s, 3H), 1.76 (m, 2H), 0.99 (t, J ) 7.6 Hz, 3H). Other reaction systems together with the conditions were summarized in Table 2. The reproducibility of all reactions was demonstrated to be within acceptable limits ((5%). In order to determine catalyst durability, the catalyst was allowed to settle down after reaction, and the clear supernatant liquid was decanted slowly. The residual catalyst was reused with fresh solvent and reactant under the same reaction conditions. The leaching of Ru species was determined by ICP.
3. Results and Discussion The elemental analysis gives a composition of C (55.37) and H (6.74) in the Ru(II)TS sample, which is in good accordance with that calculated from C60H87O9Cl2P3RuSi3 (C ) 55.21 and H ) 6.53). The Ru(II) organometallic complex incorporated into the VOL. 43, NO. 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Catalytic Performances of Three Kinds of Catalysts in Different Reaction Systems
silica matrix by the surfactant-directed assembly approach was demonstrated by FTIR, solid NMR, and TG. The FTIR spectra (Supporting Information Figure S1) revealed that the surfactant remaining in the Ru-MOC catalyst was completely removed by ethanol extraction because no signals characteristic of the F127 molecule were observed. The Ru(II)TS sample displayed several peaks around 695, 1430, 1620, 2830, and 2975 cm-1 indicative of the asymmetric and symmetric stretching modes of C-H bonds, the -H out-of-plane deformation of the monosubstituted benzene ring, and the vibrations of P-CH2 (13). Most of these typical peaks could be also found in the Ru-MOC sample, indicating the successful incorporation of the Ru(II) complex into the silica network. Some peaks displayed by the Ru(II)TS could not be clearly observed in the Ru-MOC sample, possibly due to coverage by the strong absorption peak of Si-O bonds, taking into account that the Ru/Si ratio in the Ru-MOC-3 sample was much lower than that in the Ru(II)TS. The 29Si MAS NMR spectra (Figure 2a) 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)ns(OH)4-n, n ) 2∼4 and Tm ) RSi(OSi)ms(OH)3-m, m ) 1∼3. The presence of Tm peaks confirmed the incorporation of organic groups into the silica network via CsSi bonding (18) which supplied an indirect proof for incorporation of the Ru(II) 190
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moiety since all the organic groups connected with Ru(II) in the form of RusPPh2sR, where R is the ethyl group. The 13C CP MAS NMR spectra (Figure 2b) of the Ru-MOC clearly displayed two peaks around 10 and 58 ppm, corresponding to two C atoms in the sCH2sCH2-group connected with the PPh2group. In addition, another peak around 138 ppm characteristic of the C atoms in the benzene ring in the PPh2-group (15) was also observed. The remaining peaks denoted by asterisks were rotational sidebands, which often appeared in the CP/MAS highspeed rotation process (19). From 31P NMR spetra (Figure 2c), one could see that, besides a small peak at 26 ppm resulted from the impurity in the starting material (EtO)3Si(CH2)2PPh2, the Ru(II)TS displayed three peaks at 20, 35, and 54 ppm, corresponding to three kinds of P-species due to different environments of RusP bonds (see Supporting Information Scheme S1) (20). Similarly, the Ru-MOC sample also exhibited three kind of P-signals due to different environments of RusP bonds (see Supporting Information Scheme S2) (21). These results indicated that the Ru(II) organometallic complex successfully assembled in the pore walls without significant decomposition. The position shift of P-signals in comparison with these in the Ru(II)TS could be attributed to the covalently incorporation of Ru(II)TS into mesoporous matrix, leading to the change of the P-environments (17). The TGA/DTA analysis (Supporting Information Figure S2) demonstrated that the
FIGURE 2. Solid state NMR spectra of Ru(II)TS and Ru-MOC-3 samples.
FIGURE 3. TEM images of the Ru-MOC-3 catalyst.
FIGURE 4. X-ray powder diffraction of the Ru-MOC series catalysts. weight loss (ca. 10%) due to decomposition of the organometallic complexes occurred from 350 to 450 °C, showing a relatively high thermal stability. As shown in Figure 3, the TEM images along [110] and [111] directions clearly demonstrated the ordered mesoporous structure in the Ru-MOC-3 sample. The low-angle XRD patterns (Figure 4) further confirmed that both Ru-MOC-3 and Ru-MOC-4 have a face-centered cubic structure (Fm3m) as evidenced by four well-resolved peaks of (111), (220), (311), and (422) reflections (22). The reduced peak intensity
FIGURE 5. N2 adsorption-desorption isotherms of the Ru-MOC series catalysts. The attached is the pore size distribution of the Ru-MOC-3 calculated from the adsorption branch. suggested that the ordering degree of the mesporous structure decreased while the Ru content increased. Figure 5 reveals that the Ru-MOC-2, -3, and -4 samples displayed typical typeIV nitrogen adsorption-desorption isotherms with narrow pore size distribution centered at 11 nm, corresponding to a cage-type mesoporous structure (23). However, the RuMOC-1 displayed type II nitrogen adsorption-desorption isotherm, suggesting that the mesoporous structure could not be obtained at very high Ru-TS/TEOS molar ratio in the initial mixture. The presence of a larger amount of organic molecule partially disrupted the assembly process leading to a decrease in the ordering degree of mesoporous structure. Table 1 summarized the catalytic performances of different catalysts during isomerization of homoallylic alcohol 1-(4-methylphenyl)-3-buten-1-ol in water medium. For comparison, the Ru(II) content in each run of reactions was fixed at 0.0034 mmol by adjusting the amount of the catalysts. Among the Ru-MOC series catalysts, Ru-MOC-3 (0.37 wt% Ru) exhibited the highest yield toward the target product 4-(4-methylphenyl)-3-buten-2-ol, nearly the same as the corresponding RuCl2(PPh3)3 homogeneous catalyst. To make VOL. 43, NO. 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Recycling test of the Ru-MOC-3 and the grafted Ru-MOC during water-medium isomerization of 1-(4-methylphenyl)-3-buten-1-ol. Reaction conditions are given in Table 1. sure whether the heterogeneous Ru(II) complex incorporated into the silica support or the dissolved homogeneous Ru(II) complex is the real catalyst responsible for the water-medium isomerization, the following procedure was carried out (24). After reacting for 4.0 h at which the conversion exceeded 45%, the reaction mixture was filtered to remove the solid catalyst and we then allowed the mother liquor to react for another 8.0 h under the same conditions. No significant change in either the conversion or the yield was observed, which confirmed that the active species were not the dissolved Ru(II) species leaching off from the Ru-MOC-3. Therefore, it is reasonable to conclude that the present catalysis is heterogeneous in nature. Accordingly, the high catalytic efficiency of Ru-MOC-3 could mainly be attributed to the high dispersion of Ru(II) active sites and the ordered mesoporous channels which facilitated the diffusion and/or adsorption of reactant molecules in aqueous medium due to the diminished steric hindrance (25). Although the RuMOC-4 with very low Ru loading (0.28 wt%) also possessed highly dispersed Ru(II) active sites and ordered mesoporous structure, together with high SBET, DP, and VP values (see Table 1), it still displayed lower activity than the Ru-MOC-3.
A possible reason was that partial Ru(II) active sites were covered by silica since TEOS was greatly in excess in comparison with Ru-TS in the initial solution (TEOS/Ru-TS molar ratio ) 83.3). The Ru-MOC-1 and Ru-MOC-1 catalysts with higher Ru(II) loadings displayed lower activity than the Ru-MOC-3 due to the decrease in dispersion degree of Ru(II) active sites, the partial blockage of mesoporous channels, and even the damage of mesoporous structure (see the SBET, DP, and VP values in Table 1). In addition, the high density of surface Ru(II) active sites might hamper the adsorption of reactant molecules due to steric hindrance. The Ru-MOC-1 exhibited much lower selectivity than the other Ru-MOC series catalysts, obviously due to the damage of mesoporous structure (see Figure 5), leading to the change of chemical environment of the Ru(II) active sites. Table 1 also revealed that the Ru-MOC-3 was much more active and selective than the grafted Ru-MOC with the same Ru loading. This could be further confirmed in other reaction systems (see Table 2, the experimental details are described in the Supporting Information). Since both two catalysts displayed monolayer dispersion of Ru(II) active sites and well defined ordered mesoporous structure, the lower activity of the grafted Ru-MOC could be mainly attributed to the blockage of the pore channels by Ru(II) organometallic complexes (see Figure 1), which could be manifested by comparing the SBET, DP, and VP values in Table 1. The narrower pore channels might limit the diffusion of organic molecules and decrease the activity. The higher selectivity for Ru-MOC-3 than the grafted Ru-MOC could be due to the difference of the chemical environment of the Ru(II) active sites. According to NMR spectra and ICP analysis, all the Ru(II) ions in RuMOC-3 were coordinated with three PPh2-ligands (Ru(PPh2)3), whereas most of the Ru(II) ions in the grafted Ru-MOC was coordinated with one PPh2-ligand and two PPh3-ligands (Ru(PPh2)(PPh3)2) (see Supporting Information Schemes S3 and S4). Further evidence is supplied by XPS spectra in the Ru2P level (Supporting Information Figure S3). In comparison with the Ru-MOC-3, the binding energy of the Ru(II) in the grafted Ru-MOC shifted negatively by 0.5 eV due to the stronger electron-donating ability of PPh3 than that of PPh2 (26). The grafted Ru-MOC exhibited lower selectivity than the Ru-MOC-3 due to the lower number of PPh2-ligands coordinating with the Ru(II) ions. In addition, the Ru(II) active sites in the Ru-MOC were present in the uniform Ru(PPh2)3.
FIGURE 7. A possible mechanism illustrating the isomerization of 1-(4-methylphenyl)-3-buten-1-ol catalyzed by Ru-MOC. 192
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However, besides the Ru(PPh2)(PPh3)2, trace of Ru(II) active sites in the grafted Ru-MOC were possibly present in Ru(PPh2)2(PPh3) due to the replacement of two PPh3-ligands in the RuCl2(PPh3)3 by two PPh2-ligands (Supporting Information Scheme S4), especially in the case that the PPh2ligands on the silica support were greatly exceeded. Obviously, the presence of two kinds of Ru(II) organometallic complexes in the grafted Ru-MOC was also responsible for its lower selectivity. Besides the higher activity and selectivity, the Ru-MOC-3 catalyst also displayed better durability than the grafted RuMOC. Figure 6 revealed that the yield on the Ru-MOC-3 catalyst decreased by 10% after being used repetitively seven times, whereas the yield on the grafted Ru-MOC-3 decreased by 17% after being used four times. This could be explained based on the catalytic mechanism proposed by Li and coworkers (27). As shown in Figure 7, during water-medium isomerization, the 1-(4-methylphenyl)-3-buten-1-ol first coordinated with a Ru-center; subsequent breakage of a RusP bond; and then recoordination with a PPh2-ligand in the final step for regenerating the active center. The Ru-MOC catalyst could be easily regenerated by recoordination between Ru(II) and PPh2-ligand covalently bonded onto the silica support. Thus, no significant decrease in the catalytic efficiency was observed during the second run of reactions. However, the breakage of a RusPPh3 bond in the grafted Ru-MOC catalyst resulted in the leaching of the PPh3-ligand (Supporting Information Scheme S4). Moreover, breakage of the RusPPh2 bonds may even cause the leaching of Ru(II) species from the solid catalyst into the solution. The 31P NMR spectra (Supporting Information Figure S4) confirmed that, after reaction for 8 h over the grafted Ru-MOC catalyst, phosphorus species could be detected in the solution. However, no significant phosphorus species were found in the solution when the Ru-MOC-3 was used. As a result, the catalytic efficiency of the grafted Ru-MOC decreased rapidly during the recycling test due to difficult regeneration of active sites. After being used seven times, the Ru-MOC-3 catalyst also showed lower catalytic efficiency. Since the ICP analysis demonstrated no leaching of Ru(II) active sites (see the Ru loading in Table 1), the deactivation could be mainly attributed to the damage of mesoporous structure (28), which has been confirmed by the TEM images (Supporting Information Figure S5). The N2 sorption isotherm (Supporting Information Figure S6) further evidenced that the hysteresis hoop became plainer in comparison with that from the fresh Ru-MOC-3 catalyst, indicating a decrease in the ordering degree of a mesopore structure. From Table 1, one could see that, due to the damage of mesoporous structure, the SBET, Dp, and Vp of the used Ru-MOC-3 decreased by 31, 21, and 24%, respectively, which may limit the diffusion of reactant molecules in pore channels and also hampered the adsorption on Ru(II) active sites, leading to the lower catalytic efficiency. In summary, we have developed a new approach to prepare immobilized Ru(II) organometallic catalyst (RuMOC) with highly dispersed Ru(II) active sites and ordered mesoporous structure through a surfactant-templated coassembly of the bridged Ru(II) organometallicsilane (Ru-TS) and TEOS. Such Ru-MOC exhibited comparable activity and selectivity with the corresponding RuCl2(PPh3)3 homogeneous catalyst in water-medium clean organic reactions and could be used repetitively more than seven times with only a slight decrease in activity. Other immobilized organometallic catalysts (Pd(II), Au(I), Rh(I), etc.) with controllable structure and uniform distribution of active sites could also be prepared based on the present method. This study may offer opportunities for industrial applications in green chemical synthesis.
Acknowledgments This work is supported by Science and Technology Ministry of China (2005CCA01100, 2007AA03Z339), Shanghai Leading Academic Discipline Project (No. T0402), and Shanghai Science Foundation (06JC14060).
Supporting Information Available Information about the experiment detail of catalytic reactions, FTIR spectra, TGA/DTA curves, XPS spectra, ESR spectra, 31P NMR spectra and N2 isotherm. This material is available free of charge via the Internet at http://pubs.acs.org.
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