Al Ratio of Mesoporous Support on the ... - ACS Publications

J. Phys. Chem. C 2010, 114, 8523–8532

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Effect of Si/Al Ratio of Mesoporous Support on the Structure Evolution and Catalytic Performance of the Cu/Al-HMS Catalyst Anyuan Yin, Xiaoyang Guo, Wei-Lin Dai,* and Kangnian Fan Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Fudan UniVersity, Shanghai 200433, People’s Republic of China ReceiVed: February 23, 2010; ReVised Manuscript ReceiVed: March 31, 2010

Copper-containing mesoporous Al-HMS catalysts prepared via the deposition-precipitation method have been found to be highly efficient in the catalytic hydrogenation of dimethyl oxalate (DMO) to ethylene glycohol (EG). Besides the Al chemical environment, the Si/Al ratios of the mesoporous support show remarkable effect on the catalytic performance. The DMO hydrogenation activity increased with the increasing of Al content in the support, and the highest catalytic activity was obtained when the Si/Al ratio of the support reached 25. A series of catalysts with different Si/Al ratios were characterized by N2 adsorption-desorption, X-ray diffraction, temperature-programmed reduction, N2O titration, NMR, and X-ray photoelectron spectroscopy. 27Al NMR shows that the tetrahedral coordination Al species that exists in the framework of the support could enhance the catalytic performance while the extraframework Al species would decrease the catalytic properties. The structural defects produced in the Al-containing mesoporous support play an important role in improving the dispersion of active copper species and enhancing the interaction between the copper species and support. On the basis of the characterizations, the copper species on calcined CuO/Al-HMS samples and reduced Cu/Al-HMS samples were assigned. The improvement of the catalytic performance with proper Si/Al ratio may be ascribed to the increasing defect sites associated with Al cations and the electronic promotion. 1. Introduction Mesoporous materials have developed into a very promising and aesthetically appealing area of chemistry since the discovery of the M41S family in 1992.1 Due to their large surface areas and pore volumes, controlled pore sizes, and very narrow pore size distributions, ordered mesoporous materials have significant potential application in multifarious areas such as adsorption, separation, catalysis, drug delivery, photonic and electronic devices, etc.2 However, in comparison with conventional zeolites, these siliceous mesostructured materials lack active sites and have relatively low hydrothermal stability, which hinders their potential applications. Hence, many investigations have focused on the incorporation of heteroatoms into the siliceous material to improve their activities. Among them, a noteworthy example is the Al-containing mesoporous molecular sieves, such as Al-MCM-41, Al-SBA-15, and Al-HMS, which possess acidic sites and reasonably good hydrothermal stability. Among those materials, Al-HMS, due to its simple preparation method, cheap primary alkylamines, and particular wormlike pore structure, has been widely applied in many catalytic reactions.3 Chiranjeevi et al. investigated the effect of Si/Al ratio of AlHMS support on catalytic functionalities of Mo, CoMo, and NiMo hydrotreating catalysts. The activity was greatly enhanced with introduction of Al due to the improvement of the metal dispersion.4 Wang et al. observed that Au-Ag alloy deposited onto acidic supports such as SiO2-Al2O3 was found to be catalytically active for CO oxidation due to the defects in the support generated by the incorporation of Al.5 The structural defects produced in the Al-containing mesoporous support played important roles in stabilizing alloy nanoparticles and preventing them from sintering during higher temperature * To whom correspondence should be addressed. E-mail: wldai@ fudan.edu.cn. Fax: (+86-21) 55665572.

treatment. Williams et al. studied the influence of Si/Al mole ratio on the physicochemical properties of silica-alumina supported Pt catalyst. They concluded that an efficient interaction of the support with the metal particles required the presence of the accessible Lewis acid sites, and the stronger the positive effect on the metal, the higher the catalytic activity that could be obtained.6 Recently, Nares et al. investigated the hydrogenation activity of Ni/Si(Al)-MCM-41 catalysts. They found that the presence of Al in the framework of MCM-41 could delay the formation of the nickel hydrosilicate phase.7 On the basis of the results of these previous studies, pure silica did not show any catalytic activities due to the lack of active sites necessary for catalysis. Compared with the silica itself, the dispersion of active species as well as the interaction between the active species and support could be significantly improved due to the incorporation of Al into the support, which provided an extended application space for catalysis. Our previous studies showed that Cu/HMS catalyst prepared by traditional incipient wet impregnation method exhibited good catalytic performance under lower liquid hour space velocity (LHSV); however, the activity would decline rapidly under higher LHSV due to the limitation of copper species determined by the preparation method itself.8 To further improve the activity of the catalyst, Cu/HMS catalyst prepared by a one-pot synthesis method was developed to embed the Cu nanoparticles into the matrices of HMS.9 The dispersion of Cu species could be greatly improved, but the interaction between the copper species and the support was still not satisfied due to the disadvantage of the Cu/SiO2 binary catalyst. The incorporation of Cu into the HMS support can greatly create active sites for the catalytic reaction. In our previous work, we prepared Cu/HMS catalyst by the traditional incipient wet impregnation method, which exhibited good catalytic performance; however, the catalytic activity would decline under higher LHSV (higher than 0.2 h-1)

10.1021/jp101636e  2010 American Chemical Society Published on Web 04/20/2010

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mainly due to the limited copper loading (5 wt %). In another approach, we developed a one-pot process to directly embed the Cu nanoparticles into the matrices of HMS during its synthesis process, which showed a fairly good catalytic activity. However, the dispersion of the Cu species and the interaction between the support and the Cu species are still not satisfying due to the disadvantages of the Cu/SiO2 catalyst itself. Taking the dispersion and strong interaction between active species and the support into consideration, a novel kind of Cu/Al-HMS catalyst was designed. However, such a highly active copper catalyst is supported on acidic aluminosilicate, traditionally considered as “inert”. How the support affects the copper particle size, copper dispersion, and interaction between the copper species and the support is still not clear. In this paper, we report in detail the effect of the Si/Al ratio of the support on the structural evolution and catalytic properties of this novel catalytic material. To gain further insight into these effects, the relationship between the structure of the catalyst and the catalytic activity of different copper supported Al-HMS catalysts is evaluated in light of a systematic characterization of physicochemical properties of the catalysts by N2-physisorption, X-ray diffraction (XRD), H2-temperature programmed reduction (TPR), N2O titration, and X-ray photoelectron spectroscopy (XPS). The catalytic performance of the reduced Cu/Al-HMS catalysts was evaluated by using the selective hydrogenation of dimethyl oxalate (DMO) to ethylene glycohol (EG) as the probe reaction. 2. Experimental Section 2.1. Catalyst Preparation. 2.1.1. HMS. Mesoporous siliceous HMS was prepared according to a well-established procedure delineated by Tanev et al.10 using tetraethylorthosilicate (TEOS) as silica source and dodecylamine (DDA) as template agent. Typically, the HMS materials were prepared by dissolving 5 g of DDA in 53 g of H2O and 39 g of ethanol under stirring before the addition of 21 g of TEOS dropwise. The solution mixture was then stirred at 313 K for 0.5 h. The resulting gel was aged for 18 h at ambient temperature to afford the crystalline templated product. After that, the resulting solid was recovered by filtration, washed with deionized water, and dried at 373 K, followed by calcination at 923 K in air for 3 h to remove the residual organic template materials, yielding the final mesoporous HMS material. 2.1.2. Cu/HMS. Catalyst containing 20 wt % Cu supported on HMS was prepared by the deposition precipitation (DP) method. A typical procedure is as follows. A 3.80 g sample of Cu(NO3)2 · 3H2O was dissolved in 100 mL of deionized water, then a certain amount of aqueous ammonia was added dropwise to adjust the pH value to 8.0. Then 4 g of HMS was added into the above solution. The mixed solution was stirred at 333 K for 4 h and the resulting gel was washed three times with deionized water and once with ethanol, then dried in air overnight at 373 K, followed by calcining in air for 4 h at 723 K. The final calcined sample was designated as Cu/HMS. 2.1.3. Al-HMS. The mesoporous aluminosilicate was prepared by the sol-gel method. A typical procedure is as follows. First, 5 g of DDA was dissolved in 53 g of deionized H2O and 39 g of ethanol under vigorous stirring before the addition of 21 g of TEOS dropwise. The solution mixture was then stirred at 313 K for 0.5 h. Second, a certain amount of aluminum isopropoxide ethanol solution (0.5 M) was added dropwise into the above solution under vigorous stirring. Third, the mixed solution was stirred for 2 h at room temperature, and then the resulting gel was aged for 18 h at ambient temperature to afford the crystalline templated product. Finally, the resulting solid was recovered by filtration, washed with deionized water, and dried at 373 K,

Yin et al. followed by calcination at 923 K in air for 3 h to remove the residual organic template materials, yielding the final mesoporous Al-HMS materials. The final calcined sample was designated as Al-HMS-x where the x denotes Si/Al mol ratio. 2.1.4. Cu/Al-HMS-x. The Cu supported mesoporous aluminosilicate with 20 wt % copper loading was prepared by the DP method. The synthetic procedure is similar to the synthesis of Cu/HMS, just replacing the support HMS with Al-HMS-x. The final calcined sample was designated as Cu/Al-HMS-x, with the Si/Al ratio denoted by x. 2.2. Characterizations. Specific surface areas of the samples are measured by nitrogen adsorption at 77 K (Micromeritics Tristar ASAP 3000), using the Brunauer-Emmett-Teller (BET) method. The element analysis is determined by the inductively coupled plasma method (ICP, thermo E.IRIS). The wide-angle XRD patterns were collected on a Bruker D8 Advance X-ray diffractometer, using nickel-filtered Cu KR radiation (λ ) 0.15406 nm) with a scanning angle (2θ) of 20-80°, a scanning speed of 2 deg · min-1, and a voltage and current of 40 kV and 40 mA, respectively. The full width at half-maximum (fwhm) of CuO (111) reflection was measured for calculating crystallite sizes with use of the Scherrer equation. TEM images are obtained on a JEOL JEM 2010 transmission electron microscope. TPR profiles were obtained on a Tianjin XQ TP5080 autoadsorption apparatus. Twenty milligrams of the calcinated catalyst was outgassed at 473 K under Ar flow for 2 h. After cooling to room temperature under Ar flow, the in-line gas was switched to 5% H2/Ar, and the sample was heated to 773 K at a ramping rate of 10 deg · min-1. The H2 consumption was monitored by a TCD detector. The metallic Cu surface area was measured by decomposition of N2O at 363 K, using a pulsed method with N2 as the carrier gas.11 The consumption of N2O was detected also by a TCD detector. The specific area of metallic copper was calculated from the total amount of N2O consumption with 1.46 × 1019 copper atoms per m2. XPS spectra are recorded under ultrahigh vacuum (99%) in methanol and H2 were fed into the reactor at a H2/DMO molar ratio of 50 and a system pressure of 2.5 MPa. The reaction temperature was first set at 473 K and the room temperature LHSV of DMO was set at the range from 0.15 to 2.0 h-1. The products were condensed and then analyzed on a gas chromatograph (Finnigan Trace GC ultra) fitted with an HP-5 capillary column and a flame ionization detector (FID). 3. Results 3.1. Textural Properties of the Samples with Different Si/Al Ratios. The physicochemical properties of the calcined catalysts and the bare supports (Al-HMS) are summarized in

Catalytic Performance of the Cu/Al-HMS Catalyst

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TABLE 1: Physicochemical Parameters of the Supports and Catalysts with Different Si/Al of the Support sample HMS Cu/HMS Al-HMS-5 Cu/Al-HMS-5 Al-HMS-15 Cu/Al-HMS-15 Al-HMS-25 Cu/Al-HMS-25 Al-HMS-35 Cu/Al-HMS-35 Al-HMS-45 Cu/Al-HMS-45 a

Si/Ala SBET Vpore Dp Al(tet)/Al(oct)b (mol) (m2 g-1) (cm3 g-1) (nm) (mol/mol) ∞ 6.9 19.6 27.0 34.8 44.8

963 556 662 555 976 912 1350 1005 1111 951 1370 976

0.9 0.4 0.4 0.3 2.1 1.5 1.8 1.3 1.8 1.6 1.7 1.5

3.3 2.2 6.5 5.6 6.4 4.7 5.3 4.6 5.3 3.7 4.3 3.7

0.91 1.44 1.65 1.67 2.03

Determined by ICP-AES analysis. b Calculated from 27Al NMR.

Table 1. As can be seen, the BET surface area of the supports increased with the introduction of Al into the silica support. A moderately reduced BET surface area could be observed after depositing copper species on the supports. The BET surface area of the catalysts appeared to be a function of the content of Al. Upon increasing the Si/Al ratio, the BET surface area of the catalysts increased from 555 m2 g-1 to 1005 m2 g-1 while the pore sizes decreased from 5.6 to 3.7 nm. Compared with the support, the pore volume and pore diameter of the catalyst decreased a little, which indicated that the copper species were incorporated into the support. The total copper loading (20 wt %) and the Si/Al ratios were analyzed by the ICP method and found to be close to the nominal values within the detection error. The N2 adsorption-desorption isotherms of the pure supports and the corresponding catalysts are shown in Figure 1. It is found that the supports and the as-prepared catalysts exhibited Langmuir type IV isotherms with a H1-type hysteresis loop, corresponding to a typical mesoporous material with sizehomogeneous 1D slit channels.12 The inflection points at lower and higher partial pressure in the N2 adsorption-desorption isotherms for the pure supports and catalysts indicated that both

possessed regular mesopores. Capillary condensation of nitrogen with uniform mesopores occurred, causing an abrupt steep increase in nitrogen uptake in the characteristic relative pressure (P/P0) range of 0.2-0.4 for the samples, suggesting the presence of typical mesoporous structure with uniform pore diameters. Additionally, each sample displayed a significant N2 adsorption-desorption hysteresis at high relative pressure of P/P0 > 0.9, indicating a signature of a high degree of textural porosity.13 This observation is ascribed to the fact that in our preparation process, a rapid neutralization step was used to form the nanosized mesoporous particles. The textural macropores formed by the agglomeration of the nanosized HMS particles are thus prominent. The interparticle macropores would facilitate the transport of reactant and product molecules during the catalytic reaction, making the Cu/Al-HMS more accessible for hydrogenation, and thus allowing higher DMO hydrogenation activity to be obtained. The area of the hysteresis loop increased as the increasing of Si/Al ratio, which suggested that the Al introduction amount would greatly influence the textural porosity. This kind of loop is associated with porous materials that consist of agglomerate or compact packing of approximately regular and uniform spheres, which therefore have relatively narrow pore size distributions. The pore size distribution curves derived from the desorption branch (Figure 2) show that the average pore diameter is at ca. 3 nm. However, the fwhm of the pore size distribution increased with increasing Al content, indicating that the pore structure became more disordered with Al in the support. However, too excessive introduction of Al into the silica support would result in the destruction of the mesoporous structure. When the Si/Al ratio reaches 5, almost no mesoporous distribution could be observed from the result shown in Figure 2. The pore size distribution of the supports and the supported catalysts with different Si/Al ratios are shown in Figure 2. As can be seen, the mean pore diameter of the supports is at 2.4 nm; however, the pore diameter increased to 2.6 nm after depositing the copper particles onto the supports, which indicated that a certain amount of pores have been destroyed

Figure 1. N2 adsorption-desorption isotherms of (A) pure supports and (B) the supported catalysts.

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Figure 2. BJH pore size distribution of (A) pure supports and (B) the supported catalysts.

Figure 3. Wide-angle XRD pattern for samples with different Si/Al mol ratios: (A) supports; (B) catalysts after calcinations; and (C) catalysts after reduction.

or filled due to the copper species deposition. This trend is in agreement with that of the average pore diameter variation listed in Table 1. 3.2. Crystalline Phase and Morphology. The powdered XRD patterns obtained for the pure supports, the calcined catalysts, and the reduced catalysts with different Si/Al ratios are presented in Figure 3. From Figure 3A, one can see that the support Al-HMS with different Si/Al ratios only showed a broad and diffuse diffraction peak at 2θ around 21.7°, which is attributed to amorphous silica. No obvious aluminum-containing phases could be observed except for the catalyst with Si/Al ratio of 5. There is only a weak broad diffuse diffraction peak at 2θ around 42° assigned to aluminum oxide, suggesting that excessive Al introduction would induce much aluminum aggregated from the skeleton. After calcinations (Figure 3B), no obvious diffraction peak assigned to copper species could be detected except for the catalyst with an Si/Al ratio of 5. This finding indicated that the proper amount of Al introduction could improve the dispersion of copper species and excessive framework-out aluminum would result in the segregation of copper

species. On the basis of the calculation from the Scherrer equation, the particle size of CuO for the Cu/Al-HMS-5 sample was 21 nm. After reduction under 5% H2/Ar at 573 K for 4 h (Figure 3C), no obvious diffraction peaks assigned to the copper species could be observed for catalysts with Si/Al ratios ranging from 15 to 45, indicating that highly dispersed copper species could be obtained via depositing the copper species onto the as-prepared supports. For Cu/Al-HMS-5, the particle size of copper was found to increase from 21 to 28 nm, as calculated from the Scherrer equation, which is also confirmed by the TEM images shown in Figure 4. The morphologies of the reduced catalysts with different Si/Al ratios are shown in Figure 4. It is observed that the copper species are distributed uniformly over the HMS support and Al-containing mesoporous support except for Cu/Al-HMS-5, suggesting that a proper amount of Al could promote the dispersion of the active species while excessive Al would result in the sintering of the active species. In addition, the average particle size decreases with increasing Al content, especially for the sample with an Si/Al ratio of 15, on which nanoparticles



Figure 4. TEM images for different copper loading catalysts after reduction with different Si/Al ratios.

with size lower than 3 nm were obtained. In contrast, on the pure siliceous mesoporous HMS support, the average particle size was found to be about 10 nm, which is in good agreement with the XRD results. Clearly, the Al-containing mesoporous support can stabilize the nanosized metal particles deposited on it, which might be related to the structural defects induced by the incorporation of Al into the framework. 27Al NMR measurement is thus performed to clarify the relationship between the chemical environment and the Al content of the support. Besides, the copper deposition did not affect the morphology of the Al-HMS support. Compared with the spherical HMS support, the incorporation of Al into the silica support induced the change of the Al-HMS support shape from sphere-like to needle-like. The copper particles were highly distributed on the surrounding needlelike supports, which might increase the interaction between the copper species and the support. The statistical results based on TEM images (not shown here) showed the mean copper particle size is about 2.4 nm. 3.3. 27Al and 29Si MAS NMR Spectra. To reveal the coordination environment of Al in the support, 27Al MAS NMR spectra were recorded and shown in Figure 5. All the Alcontaining catalysts show two NMR peaks. The strong peak positioned at around 53 ppm is attributed to the presence of aluminum in tetrahedral coordination, indicating that most of the Al species in the support are incorporated into the framework of HMS (AlO4 structural unit, Al(tet), and the aluminum is covalently bonded to four Si atoms by oxygen bridges). The weak peak at around 0 ppm is ascribed to the extraframework octahedral aluminum species, which may arise from framework dealumination during the calcinations process (AlO6 structure unit, Al(oct)).14 This result clearly shows the simultaneous presence of both tetrahedrally and octahedrally coordinated aluminum species in the Al-containing samples. The intensities of both peaks increased with the concentration of the added aluminum species. Moreover, the percentage of the tetrahedral aluminum in each catalyst could be calculated from the relative intensities of the 53 ppm line in the 27Al MAS NMR spectra, assuming that the relative content of aluminum is proportional to the intensity of 27Al MAS NMR lines. The results are presented as Al(tet)/Al(oct) in Table 1. The amount of the framework-incorporated aluminum species is gradually enhanced

Figure 5.

27

Al MAS NMR of supports with different Si/Al ratios.

from an Al(tet)/Al(oct) ) 0.91 in support Al-HMS-5 to 2.03 in support Al-HMS-45 (Table 1), as judged from the 27Al MAS NMR spectra and the peak area ratios of the tetrahedrally to octahedrally coordinated Al. In general, the homogeneity of the Si-O-Al linkages and tetrahedral aluminum sites at a molecular level is very important in catalysis for the enhanced activity, selectivity, and stability.15 The solid state 29Si NMR spectroscopy was used for the study of Al ion incorporated into the HMS framework. The NMR spectroscopy of the calcined Al-HMS supports was shown in Figure 6. All spectra showed one broad line attributable to Si(OSi)4 units (Q4 sites) and a shoulder attributed to Si(OSi)3OH units (Q3 sites).16 Also the weak shoulder peak at around -90 ppm could be observed, which was attributed to the Si(OSi)2(OH)2 (Q2 sites). This finding indicated the amorphous nature of the walls in HMS and Al-HMS materials. Pinnavaia et al.16 compared the 29Si NMR spectra of MCM-41 with TiMCM-41 and HMS with Ti-HMS material. The Ti-containing

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Figure 6.

29

Si MAS NMR of supports with different Si/Al ratios.

materials were prepared with a Ti/Si molar ratio similar to that of our samples (Ti/Si molar ratio ) 0.021) and a dramatic increase in the amount of Q4 species in both Ti-containing samples was observed. This result originated from the formation of a cross-linked framework in the presence of Ti ions. Therefore, one could expect that in our samples the value of Q4 in the heteroatoms-containing samples would be higher than those of the heteroatoms-free samples, and indeed this result has been observed. Besides, the signal intensity of the Q3 species also could be increased with the presence of heteroatoms when the Al, Ti, and Zr ions are incorporated into the framework of the HMS material. In fact, the intensities of the Q4 species increased drastically in the heteroatoms-containing samples. The intensity of Q4 resonance in ours samples followed the order Al-HMS-5 > Al-HMS-15 > Al-HMS-25 > Al-HMS-35 > AlHMS-45. The values of the intensities of the Q2 and Q3 species also increase in the heteroatoms-containing samples and the last order is also valid for theses two resonances. However, the increase of the Q2 resonance was higher than the Q3 intensity value and the increase in the intensity of both Q2 and Q3 resonances values was higher than that of the increase of the Q4 resonance value in the presence of heteroatoms. Then, as a consequence, heteroatom ions incorporated into silica framework were the direct reason for the increase in the intensity value of the Q2, Q3, and Q4 resonances. This broadening could be attributed to the effect of the heteroatom sites on the chemical environment of the adjacent Si atoms. In this work, we also found that with the increases in the Al-loading, the intensities of all Qn (n ) 2, 3, and 4) resonances increased in comparison with the intensities of the resonances in the Al-free sample, but the increment in the intensities of the Q2 and Q3 resonances was higher than the increment in the intensity of Q4 resonance. In this sense, one could take the Q4/(Q2 + Q3) intensity ratio to verify the incorporation of the heteroatom ions into the silica framework. Then we can conclude that the aluminum atoms were incorporated into the framework of HMS material due to the modification in the chemical environment of the adjacent Si atoms by the presence of heteroatoms into the framework of HMS material. 3.4. Redox Properties of Calcined Catalysts. TPR measurements were carried out to investigate the reducibility of the

Yin et al.

Figure 7. TPR profiles of catalysts with different Si/Al ratios.

copper species in various Cu/Al-HMS catalysts. Figure 7 presented the reduction profiles of these samples. All the profiles exhibited a single hydrogen consumption peak at around 528 K, which indicated the highly dispersed copper species could be obtained with Al-containing HMS as support. In addition, it is worthwhile to note that the peak tends to be widened as the increasing of Al content, implying that redundant Al would be detrimental to the dispersion of copper species. These results correlate well with the metallic copper surface area measurements, which revealed the lowest dispersion in Cu/Al-HMS-5 sample. Moreover, good correlation with the XRD results is also observed, showing the presence of small CuO and Cu particles in this material. In addition, the reduction profile is narrow and almost symmetrical, indicating a narrow particle size distribution and homogeneous material. The observed shift in reduction temperature may be ascribed to the different copper particle sizes, different interactions between copper oxide and silica, and different copper oxide dispersions. Chen et al.17 reported that the lack of reduction peaks at temperatures lower than 530 K indicated the absence of oxocations (Cu-O-Cu)2+, while the lack of peaks at temperatures higher than 540 K indicated the absence of copper crystal phase. According to what has already been observed on Cu/SiO2 samples18 the unique peak observed in Figure 8 can be attributed to the reduction of well-dispersed Cu2+ species. 3.5. Chemical State of Copper and Reduction Behavior. Usually, the identification of the different oxidation states of copper in bulk samples can be easily carried out by XPS because different shapes or energy positions of the Cu 2p3/2 photoelectron and Cu Auger lines characterized the Cu+ and Cu2+ oxidation states of this element. Thus the Cu 2p spectrum in bulk CuO samples is characterized by intense satellite peaks on the high binding energy side of the main photoelectron peaks, which are absent in the spectra for Cu2O and Cu. On the other hand, although the last two oxidation states show very similar Cu 2p spectra, their Cu Auger line binding energies are shifted by 2 eV. The XPS spectra of the calcined and H2-reduced samples are illustrated in Figure 8. The intense and broad photoelectron peak at above 934.5 eV (Cu 2p3/2) along with the presence of the characteristic shakeup satellite peaks suggests that the copper oxidation state is +2 in all the calcined samples.



Figure 8. Cu 2p photoelectron spectra of catalysts with different Si/Al ratios: (A) after calcinations and (B) after reduction.

TABLE 2: Surface Cu Component Properties of the Reduced Catalysts d

catalyst

Cu 2p3/2 BEa (eV)

Cu 2p3/2 BEb (eV)

RCu/Sic

SCu (m2 g-1)

Cu/HMS Cu/Al-HMS-5 Cu/Al-HMS-15 Cu/Al-HMS-25 Cu/Al-HMS-35 Cu/Al-HMS-45

933.6 934.0 935.7 935.5 935.3 935.4

932.7 932.7 932.4 932.4 932.9 932.8

0.27 0.24 0.34 0.37 0.36 0.34

9.5 6.7 12.1 13.7 12.6 11.6

a After calcination. b After reduction under 5% H2/Ar at 573 K for 4 h. c Mole ratio of Cu to Si of the catalysts after reduction determined by XPS. d Metallic copper surface area detected by N2O titration.

However, the XPS peak shapes of these samples are different. Considering the asymmetry of the Cu2p3/2 envelope, the peak of the calcined Cu/Al-HMS samples can be deconvoluted into two contributions centered at around 934.5 and 933.4 eV, implying the existence of two Cu2+ species with different chemical circumstances. In general, the BE at ca. 933.4 eV was attributed to the CuO species, thus the relatively big positive BE shift of the Cu 2p core level for the calcined catalysts is indicative of a charge transfer from the metal ions toward the support matrix, that is, a strong interaction between the metal ions and the matrix.19 Therefore, the XPS results inferred the presence of well-dispersed Cu2+ ions interacting with the silica support (934.5 eV) in the calcined sample. For the reduced samples, the BE of Cu 2p3/2 core levels reduced to 932.3-932.6 eV and the satellite lines disappeared, indicating that all copper species became reduced (Cu+ or Cu0). The difference of each sample in the BE values may be due to the electron transfer between the copper species and the support. The detailed BE values of the as-obtained samples are listed in Table 2. XPS intensity ratios of the metal cations in the supported metal oxide to those in the oxide support can provide important information regarding the dispersion and crystallite size of supported particles. As listed in Table 2, the dispersion of the copper species derived by XPS results increased with the introduction of the proper amount of Al into the framework compared with the Cu/HMS catalyst. However, too excessive Al would result in the decrease of the copper dispersion due to

the coverage of aluminum on the surface of the catalyst, which is in accordance with the reported result by Agrell et al.20 The N2O titration result showed that the Si/Al ratio has great influence on the surface copper area and too excessive Al introduction would result in the sharp decrease of the Cu surface area. The maximum Cu surface area and copper dispersion could be obtained when the Si/Al ratio reached 25. 3.6. Catalytic Behavior Evaluations. The DMO hydrogenation reaction was carried out to investigate the catalytic properties of the series of Cu/Al-HMS catalysts, and their activity and selectivity as a function of LHSV were shown in Figure 9. Due to a much higher dispersion of copper species and larger Cu surface area derived by the proper incorporation of Al into the silica support, Cu/Al-HMS catalysts presented a much higher DMO hydrogenation activity and EG selectivity than that of the Cu/HMS catalyst. The DMO conversions follow an order according to the Si/Al ratio in the support: Cu/AlHMS-25 > Cu/Al-HMS-35 > Cu/Al-HMS-15 > Cu/Al-HMS45 > Cu/HMS > Cu/Al-HMS-5; and the EG selectivities follow another order according to the Si/Al ratio in the support: Cu/ Al-HMS-25 > Cu/Al-HMS-15 > Cu/Al-HMS-35 > Cu/Al-HMS45 > Cu/HMS > Cu/Al-HMS-5. With a Si/Al ratio of 25, DMO can be completely converted and the EG selectivity could be up to 98% at LHSV of 1.8 h-1. In contrast, only 70% DMO conversion and 50% selectivity could be obtained over Cu/HMS catalyst under the same reaction condition. The excellent activity could be attributed to the larger Cu surface area derived by the modification of support with Al, which exposed more active sites. This finding is in good accordance with the previous study.9 Moreover, we note that within 100 h of time on stream (not shown here), the DMO conversion over Cu/HMS rapidly decreased, while the DMO conversion over Cu/Al-HMS-25 sample remained at 100%, no deactivation could be observed, which suggested that Cu/Al-HMS is more active and stable than Cu/HMS. These results clearly demonstrated the high efficiency of the Cu/Al-HMS catalyst in the selective hydrogenation of DMO to EG. The reason for the enhanced catalytic performance as compared to Cu/HMS catalyst might originate from the good dispersion of copper species and electron promotion effect for Cu/Al-HMS catalyst, which will be discussed in the following section.

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Figure 9. (A, left) DMO conversion profiles versus LHSV (h-1) over catalysts with different Si/Al ratios. (B, right) EG selectivity profiles versus LHSV (h-1) over catalysts with different Si/Al ratios. Reaction conditions: P ) 2.5 MPa, T ) 473 K, H2/DMO ) 50 (mol/mol).

Our previous study also showed that the special pore structure of HMS provides good dispersion of copper species on the surface of the catalyst; however, the stability of the active copper species still faces a big challenge. The introduction of Al into the framework of HMS can greatly promote the dispersion of copper species and stabilize the copper species, which can open an avenue for the stabilization of active copper species under reaction conditions. 4. Discussions In the present work, the effect of Si/Al ratio on the structural evolution and catalytic performance of Cu/Al-HMS catalysts in the hydrogenation of DMO to EG was explored. For this sake, five Al-containing HMS materials with different Si/Al ratios were prepared by the sol-gel method. Characterization results showed that the chemical compositions of Al-containing materials were close to the theoretically expected ones. No significant textural structure changes could be observed in the original pore structure and the long-range periodicity order of the parent HMS sample after Al incorporation. Further characterization of Al-HMS catalysts by 27Al NMR confirmed the presence of two types of Al3+ species, namely, tetrahedrally and octahedrally coordinated ones, with the first being more abundant in all cases. These results illustrated the participation of both framework and extraframework Al species in the interaction with the support, whereas octahedral aluminum reacts with Cu to form the copper aluminate in the calcinated catalysts which could be proved from Cu 2p XPS results. The evaluation of the catalytic activity of the prepared Cu/Al-HMS-x catalysts in the hydrogenation of DMO to EG showed that the support Si/Al ratio also affected the activity and selectivity of the catalysts. The changes in the catalytic behavior observed with varying the support Si/Al ratio would be the result of two opposite effects. On the one hand, the incorporation of aluminum onto the HMS framework provides better dispersion to the deposited copper species and induces a higher active surface copper surface. On the other hand, too strong metal-support interactions, as in the case of the aluminum-rich Cu/Al-HMS-5 catalyst, make the copper aluminate species cover more active sites. This phenomenon explained the observed activity trend,

namely, why the catalytic activity increased with the Al loading and reached the maximum at an Si/Al ratio of 25. As is known to us all, introduction of Al into the silica framework would generate the acid sites on the surface of the catalyst, which would influence the catalytic performance of the catalysts. On the basis of the results of NH3-TPD (not shown here), almost all the catalysts generate the weak acid except the catalyst with the largest introduction of Al, and the amount of acid site calculated from the deconvolution of the desorption peak and the acid strength based on the desorption temperature is nearly the same, which indicated that they might make almost equal contributions to the effect on the catalysts with different Si/Al ratios. 4.1. Stabilizing Effect of Al on the Copper-Based Catalysts. As discussed above, the Al content of the support has a great influence on the dispersion of copper species as well as the copper particle sizes. The copper particle sizes decreased dramatically from 10 to 2.4 nm when introducing the proper amount of Al into the framework of the support. After reduction, no segregation of copper species could be observed from XRD and TEM, which indicated that the introduction of Al could obviously inhibit the segregation of copper species. A previous study showed that smaller copper particle size would be helpful to the adsorption of the substrate and the dissolution of hydrogen, and thus enhanced the catalytic performance. It should be mentioned that in our preparation procedure, the copper nanoparticles were formed after being deposited onto the mesoporous support and then calcined at high temperatures. The formed Cu nanoparticles are lower than 3 nm in size. However, after reduction under high temperature, the particle size remains essentially unchanged (∼3 nm, as estimated from TEM images). Because the incorporation of Al into mesoporous silica framework creates rich defects, especially the F+ centers (oxygen vacancy with one electron), it is believed that these defects can act as anchoring sites to make the nanoparticles less mobile during the calcination process. Probably the F+ center, which has one electron, can interact strongly with the copper species enriched on the particle surface, thus decreasing particle mobility. After subsequent reduction with H2 at high temperature, the metallic copper particles formed. This reduction



Figure 10. Schematic model of the variation of Cu species with increasing Si/Al ratio.

treatment at high temperature is the key step in activating the catalyst, although the particle size remained almost unchanged at this stage. Therefore, the support defects created by Al incorporation are favorable for obtaining smaller particles. Using DFT calculations, Lopez et al.21 found that the final size and shape of gold particles is determined by the support defects. For example, their calculations show that gold particles do not bind to a perfect TiO2 surface, but have a binding energy shift of about 1.6 eV on an oxygen vacancy in TiO2. Their conclusion supports our findings deduced above. In addition, the strong interaction between the copper species and support also improved when introducing Al into the framework of the support, which could be concluded from the shift of binding energy of copper species. This good dispersion as well as strong interaction between the copper species and the support could make a great contribution to the catalytic performance of the Al-containing catalyst. 4.2. Effect of Al Chemical Environment on the Catalytic Performance for DMO Hydrogenation. On the basis of the characterization of 27Al NMR, two kinds of chemical environmental Al could be observed. One is the aluminum in tetrahedral coordination that exists in the framework of silica; the other is the aluminum in octahedral coordination that is ascribed to the extraframework of silica. Tetrahedral Al species are more abundant in all Cu/Al-HMS-x samples. However, the relative intensity of the six-coordinate Al gradually increases with the decrease of Si/Al ratio, indicating that the proportion of extraframework aluminum atoms increases as well. The proportion of the Al(tet)/Al(oct) listed in Table 1 exhibited a linear decreasing tendency as the increase of the Al content, which indicated that an optimizing Al(tet)/Al(oct) was necessary to obtain highly efficient hydrogenation catalyst. Almost no leaching of Al was found on the catalyst with long-term reaction, which indicated the copper and the aluminum could stabilize each other during the reaction process. Besides, after the long-term reaction, the ratio of Al(tet)/Al(oct) was found to increase to 1.72, which suggested the chemical environment had changed a little. We propose that the F+ centers created by Al incorporation can facilitate the adsorption of reaction substrate. If the copper particles are near the F+ centers, then the activated substrate on the defects can spill over to the nearby copper particles to react with the activated hydrogen on Cu sites, leading to

enhanced activity. However, it should be mentioned that the decrease in Si/Al leads not only increased defect concentration, but also increased -OH concentration on the support. However, the role of -OH (or acidity) in the enhancement of the activity is not clear yet. On the basis of the above discussion, a possible schematic mechanism was proposed and shown in Figure 10. When no Al was incorporated into the HMS support, the copper species could be dispersed very well but the interaction between the copper species and support was too weak to stabilize for a long time reaction. When Al was incorporated into the HMS support, the generated defects not only promoted the dispersion of copper species but also enhanced the interaction between the copper species and the Al-HMS support as well as the stability of the catalysts due to the electron transfer. If excessive aluminum was introduced, the aluminum oxide could segregate on the surface of the copper species, thus decreasing the Cu surface area. Hence, a much lower catalytic activity was observed for the Cu/Al-HMS catalyst with higher Al content. 5. Conclusions Al-HMS materials were shown to be promising supports for copper supported catalyst. Mesoporous aluminosilcate Cu/AlHMS-x (x ) 5-45) catalysts were successfully prepared via a facile deposition-precipitation method, and a comparative investigation of catalytic activity as well as long-term stability test in the hydrogenation of DMO to EG was carried out. Our findings indicate that the Si/Al ratio of the support has an important effect on the catalytic performance of the DMO hydrogenation reaction. The increase in dispersion of copper species and the decrease of copper particle size could be observed when introducing Al into the framework of the support. These two factors result in the increase in DMO conversion and EG selectivity. However, excessive incorporation of Al into the mesoporous framework causes some loss of structure order, leading to the decreased catalytic activity with a Si/Al ratio of 5. The defects generated by incorporating Al as those associated with tetrahedral Al not only stabilize the copper nanoparticles and thus prevent them from sintering, but also promote the electron transfer between the active copper species and the support. After optimizing the Si/Al ratio, 100% DMO hydro-

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genation activity and 98% EG selectivity could be obtained at LHSV of 1.8 h-1, which is much higher than the previously reported ones and can be served as a promising commercial candidate. In addition, the stability evaluation showed that this Al-containing catalyst could keep its original activity and selectivity for more than 100 h; however, the catalyst without Al incorporation would lose the activity and selectivity completely within 50 h under the same reaction condition, suggesting that the as-prepared Cu/Al-HMS-25 could be regarded as a promising catalyst in industry. To explore the practical use of this novel catalyst, a much longer lifetime test (∼2000 h) is under way. Acknowledgment. This work is financially supported by the Major State Basic Resource Development Program (Grant No. 2003CB615807), NSFC (Project 20973042), the Research Fund for the Doctoral Program of Higher Education (20090071110011), and the Natural Science Foundation of Shanghai Science & Technology Committee (08DZ2270500). References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Nowak, I.; Ziolek, M. Chem. ReV 1999, 99, 3603. (3) Pauly, T. R.; Liu, Y.; Pinnavaia, T. J.; Rieker, T. P. J. Am. Chem. Soc. 1999, 121, 8835. (4) Chiranjeevi, T.; Muthu Kumaran, G.; Gupta, J. K.; Murali Dhar, G. Catal. Commun. 2005, 6, 101.

Yin et al. (5) Wang, A.; Hsieh, Y. P.; Chen, Y. F.; Mou, C. Y. J. Catal. 2006, 237, 197. (6) Williams, M. F.; Fonfe´, B.; Sievers, C.; Abraham, A.; van Bokhoven, J. A.; Jentys, A.; van Veen, J. A. R.; Lercher, J. A. J. Catal. 2007, 251, 485. (7) Nares, R.; Ramırez, J.; Gutierrez-Alejandre, A.; Cuevas, R. Ind. Eng. Chem. Res. 2009, 48, 1154. (8) Yin, A.-Y.; Guo, X.-Y.; Dai, W.-L.; Li, H.-X.; Fan, K.-N. Appl. Catal., A 2008, 349, 91. (9) Yin, A.-Y.; Guo, X.-Y.; Dai, W.-L.; Li, H.-X.; Fan, K.-N. J. Phys. Chem. C 2009, 113, 11003. (10) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865. (11) Evans, J. W.; Wainwright, M. S.; Bridgewater, A. J.; Young, D. J. Appl. Catal. 1983, 7, 75. (12) Schmidt, R.; Hansen, E. W.; Stoecker, M.; Akporiaye, D.; Ellestad, O. H. J. Am. Chem. Soc. 1995, 117, 4049. (13) Pauly, T. R.; Liu, Y.; Pinnavaia, T. J.; Billinge, S. J. L.; Rieker, T. P. J. Am. Chem. Soc. 1999, 121, 8835. (14) Luan, Z.-H.; Hartmann, M.; Zhao, D.-Y.; Zhou, W.-Z.; Kevan, L. Chem. Mater. 1999, 11, 1621. (15) Yang, X.-Y.; Vantomme, A.; Lemaire, A.; Xiao, F.-S.; Su, B.-L. AdV. Mater. 2006, 18, 2117. (16) Zhang, W.; Froba, M.; Wang, J.; Tanev, P. T.; Wong, J.; Pinnavaia, T. J. J. Am. Chem. Soc. 1996, 118, 9164. (17) Chen, L.; Horiuchi, T.; Osaki, T.; Mori, T. Appl. Catal., B 1999, 23, 259. (18) Carniti, P.; Gervasini, A.; Modica, V. H.; Ravasio, N. Appl. Catal., B 2000, 28, 175. (19) Jernigan, G. G.; Somorjai, G. A. J. Catal. 1994, 147, 567. (20) Agrell, J.; Birgersson, H.; Boutonnet, M.; Melia´n-Cabrera, I.; Navarro, R. M.; Fierro, J. L. G. J. Catal. 2003, 219, 389. (21) Lopez, N.; Nørskov, J. K.; Janssens, T. V. W.; Carlsson, A.; PuigMolina, A.; Clausen, B. S.; Grunwaldt, J. D. J. Catal. 2004, 225, 86.

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